Processing stabilization of polyethylene with

traditional and natural antioxidants

Ph.D. Thesis

Dóra Tátraaljai

Supervisor:

Enikő Földes

Laboratory of Plastics and Rubber Technology

Department of Physical Chemistry and Materials Science

Budapest University of Technology and Economics

Polymer Physics Research Group

Institute of Materials and Environmental Chemistry

Research Center for Natural Sciences

Hungarian Academy of Sciences

Budapest, 2014

Contents

5

Contents

Abbreviations ............................................................................................................. 7 Chapter 1 - Introduction ............................................................................................. 9

Chapter 2 - Background .............................................................................................11

2.1. Degradation of polyethylene ...........................................................................11 2.1.1. Thermal degradation................................................................................11 2.1.2. Thermo-oxidative degradation .................................................................13 2.1.3. Photo-oxidative degradation ....................................................................15

2.2. Stabilization of polyethylene ...........................................................................18 2.2.1. Hydrogen donors and radical scavengers..................................................19 2.2.2. Hydroperoxide decomposers....................................................................21 2.2.3. Light stabilizers .......................................................................................24

Chapter 3 - Scope ......................................................................................................27

Chapter 4 - Experimental ...........................................................................................31

4.1. Materials ........................................................................................................31 4.1.1. Polymers .................................................................................................31 4.1.2. Additives ................................................................................................31 4.1.3. Reagents, solvents and gases ...................................................................32

4.2. Sample preparation .........................................................................................32 4.3. Methods .........................................................................................................33

Chapter 5 - Pipes: effect of the additive package and processing ................................39

5.1. Introduction ....................................................................................................39 5.2. Effect of soaking ............................................................................................40 5.3. General correlations, effect of processing ........................................................45 5.4. Discussion ......................................................................................................46 5.5. Conclusions ....................................................................................................49

Chapter 6 - High temperature reactions of an aryl-alkyl phosphine ............................51

6.1. Introduction ....................................................................................................51 6.2. Characterization of the nascent phosphine and its oxides .................................53 6.3. Reactions of the phosphine under thermal and thermo-oxidative conditions .....55 6.4. Reactions with carbon centered radicals ..........................................................57 6.5. Reactions with peroxy radicals ........................................................................57 6.6. Reaction with oxy radicals ..............................................................................59 6.7. Reaction with hydroperoxide ..........................................................................61 6.8. Discussion ......................................................................................................63 6.9. Conclusions ....................................................................................................63

Contents

6

Chapter 7 - Stabilization with quercetin, a flavonoid type natural antioxidant............. 65

7.1. Introduction.................................................................................................... 65 7.2. Preliminary experiments, comparison of quercetin and Irganox 1010............... 66 7.3. Effect of quercetin content .............................................................................. 68 7.4. Solubility and homogeneity of quercetin ......................................................... 71 7.5. Discussion ...................................................................................................... 74 7.6. Conclusions.................................................................................................... 77

Chapter 8 - Stabilization with a natural antioxidant, curcumin ................................... 79

8.1. Introduction.................................................................................................... 79 8.2. Stability and interactions of curcumin ............................................................. 80 8.3. Processing stabilization .................................................................................. 83 8.4. Discussion ...................................................................................................... 87 8.5. Conclusions.................................................................................................... 88

Chapter 9 - Performance of β-carotene under processing and storage conditions ....... 89

9.1. Introduction.................................................................................................... 89 9.2. Processing stability ......................................................................................... 90 9.3. Storage at ambient temperature ....................................................................... 94 9.4. Conclusions.................................................................................................... 99

Chapter 10 - Summary ............................................................................................. 101

References ............................................................................................................... 105

Acknowledgements ................................................................................................. 119

Appendix................................................................................................................. 121

List of publications .................................................................................................. 133

Abbreviations

7

Abbreviations

0 creep viscosity

max wavelength of absorption maximum in UV-VIS spectra AIBN azobisisobutyronitrile

BHT 2,6-di-tret-butyl-4-methylphenol 13C NMR carbon-13 nuclear magnetic resonance

CHP cumene hydroperoxide

DCP dicumyl peroxide

DRIFT diffuse reflectance Fourier transform infrared (spectroscopy)

DSC differential scanning calorimetry

DTBPP tris(2,4-di-tert-butylphenyl)phosphite

ESIPT excited-state intramolecular proton transfer

ET-PT electron transfer-proton transfer FT-IR Fourier transform infrared (spectroscopy) 1H NMR proton (or hydrogen-1) nuclear magnetic resonance

HAS sterically hindered amine, hindered amine stabilizer

HALS hindered amine light stabilizer

HAT hydrogen transfer from the antioxidant to the radical

HD hydroperoxide decomposer

HDPE high density polyethylene

HO hydroxil radical HPLC-MS high performance liquid chromatography coupled with a

mass spectrometer

HSQC homonuclear single-quantum correlation

HMQC heteronuclear multiple-quantum correlation

InH phenolic antioxidant, hydrogen donor IR infrared (spectroscopy)

L/D length/diameter of extruder screw

MFI melt flow index

N I Norrish I reaction

N II Norrish II reaction

NH secondary amine

NMR nuclear magnetic resonance (spectroscopy)

N-O nitroxy radical NOH hydroxylamin

NOR’ hydroxylamine ether

OIT oxidation induction time 31P NMR phosphorus-31 nuclear magnetic resonance

PE polyethylene PMP bis(diphenylphosphino)-2,2-dimethylpropane

PMP-1ox mono oxide of bis(diphenylphosphino)-2,2-dimethylpropane

PMP-2ox dioxide of bis(diphenylphosphino)-2,2-dimethylpropane

P(OAr)3 aryl phosphite

PP polypropylene

R3P phosphine

PS polystyrene

Abbreviations

8

PVC poly(vinyl chloride)

R, R’, R”, R’”, R1 alkyl group

R alkyl radical

RO oxy radical

ROO peroxy radical ROOH hydroperoxide

ROOR alkyl peroxide

SPLET sequential proton loss-electron transfer

t time TCO integrated absorption intensity of carbonyl group

TGA thermogravimetric analysis

UV-VIS ultraviolet - visible radiation

YI yellowness index

Introduction

9

Chapter 1

Introduction

Plastics represent an important part of our everyday life; they are present in

everywhere from our home to our professional activities. A huge diversity of plastics is

available on the market and their number is increasing continuously. Recently bioplastics have come into the focus of attention and their use is growing with a surpris-

ing rate, but their amount is very small at the moment compared to the total consump-

tion of plastics. Engineering and especially high temperature polymers have not met the

expectation attached to them, but their application and use maintains a constant level

and they play an important role especially in the automotive industry. The majority of

plastic products used today is still prepared from commodity plastics, from polyethylene

(PE), polypropylene PP, poly(vinyl chloride) (PVC) and polystyrene (PS) and its de-

rivatives and their position on the market will not change for many decades to come.

Commodity plastics are versatile materials produced in very large quantities and they

are very cheap as a consequence. No other structural material can compete with them at

the moment. Polyethylene occupies a special position even among commodity plastics with about 40-50 % of the total consumption.

Polyethylene is a generic name covering a wide range of materials from low

density to high density polyethylenes. These materials differ widely in the polymeriza-

tion technology, in the resulting structure and naturally in properties. Low density poly-

ethylene is produced at high temperature and pressure by radical polymerization and the

resulting polymer contains a relatively large number of branches. On the other hand,

catalytic processes usually lead to linear polymers with regular chains. The fine struc-

ture of the chains is further diversified by the type of the catalyst or the comonomers

used. The resulting polymer can be a low modulus compliant material or one with large

crystallinity and stiffness used in structural applications. The fine structure of the poly-

mer, branches, functional groups and irregularities determine its properties, but also its stability. Polyethylene, like all commodity plastics is converted to products by thermo-

plastic processing technologies during which it is exposed to the effect of heat, shear

and oxygen. In spite of its apparently neutral structure and lack of functional groups, PE

takes part in chemical reactions during processing, but also during its use, which usually

result in changes in its structure and deterioration of properties. As a consequence, pol-

yethylene must be protected against such changes by the use of stabilizers. The devel-

opment and use of efficient additive packages is one of the key aspects of the produc-

tion of a successful commodity polymer.

The degradation of PE has been studied for a long time and stabilization is a

mature technology now. Although there are plenty of unanswered questions, the pro-ducers and the scientists lost their interest gradually in research and development in this

area in the last decade. The interest has turned towards more exciting and sometimes

popular areas, like nano, bio, functional, shape memory or self healing. Both the Labor-

atory of Plastics and Rubber Technology of the Budapest University of Technology and

Economics (LPRT) and the Institute of Materials and Environmental Chemistry of the

Chapter 1

10

Hungarian Academy of Sciences (IMEC) have a longstanding tradition in the study of

the degradation and stabilization of polymers, mainly PVC and polyolefins. This activi-

ty was especially strong between 1992 and 2008, when three partners, a polymer pro-

ducer (Tisza Chemical Group Public Limited Company - TVK), one developing addi-tives (Clariant) and one doing research and development work (LPRT and IMEC)

joined forces in a very efficient cooperation. The projects pursued included a wide vari-

ety of problems from the development of additive packages, through the study of the

mechanism of degradation of stabilization, the development of new stabilizers, to the

study of the performance of polyethylene compounds in extractive media. The research

was extremely successful resulting in additive packages, which are used in industrial

practice even today, in a patent on a new stabilizer, in many BSc and MSc theses, pa-

pers and two PhD theses.

The successful cooperation on the degradation and stabilization of polyeth-

ylene ended with the decision of Clariant to stop all research and development activities

in the area of plastics additives. At this point the group had to make a decision on the future. As mentioned above, degradation and stabilization is outdated, neither industry

nor academia is interested in this area any more. The possibilities were to abandon the

field completely, continue as before, or change the focus of the research by bringing

new ideas into an old area. Ongoing projects had to be completed naturally and the

fruitful cooperation with TVK continued as well. Current projects at that time were the

study of the behavior and performance of additive packages in products exposed to the

effect of extractive media, i.e. water pipes, and the investigation of the mechanism of

phosphorous secondary stabilizers. These projects paved the way and pointed out the

new direction of our research as well. Studies carried out on water pipes indicated that

some of the transformation products of traditional phenolic antioxidants might present a

hazard to human health. The study of Brocca et al. [1] created much interest at that time and the questions raised have not been answered satisfactorily yet. Taking into account

these uncertainties, and in search of a new direction, the group turned its attention to-

wards naturally occurring compounds with antioxidant effect. Many substances and

compounds have known to be beneficial to health with proven antioxidant, antiviral and

anti-inflammatory effect in the human body. Several of these compounds are used as

additives in the food industry and the stabilizing effect in polymers was also indicated

for some of them. However, apart from Vitamin E, their possible use as stabilizers has

never been studied in detail and their advantages as well as drawbacks explored. The

contents of this PhD Thesis reflect this transitional period in the life of our group at

least in the area of polymer degradation and stabilization. It contains chapters reporting

on outgoing research (pipes, phosphorous stabilizers), but contains results on natural antioxidants as well. The success of the papers published in this new direction will de-

termine the future of the research in the area of polymer degradation and stabilization.

Background

11

Chapter 2

Background

2.1. Degradation of polyethylene

Organic materials undergo chemical changes as an effect of oxygen, heat, ag-

gressive media and/or UV irradiation. These reactions occur also in polymeric materials

and are called degradation, as they lead to the deterioration of the original properties.

The different degradation processes of polymers do not take place separately in practice. During processing the polymer is under the combined effect of heat, shear and oxygen.

In the course of application oxygen is present almost in every case; elevated tempera-

ture and/or UV irradiation or aggressive medium can enhance its harmful effect. The

stability of polyethylene is strongly affected by its inherent properties, like branching

and unsaturations, as well as the type and amount of catalyst residues.

2.1.1. Thermal degradation

The thermal degradation of polyolefins was studied the most intensively in the

1950-70’s [2-14] and the main reactions were established. The polyolefins are thermally

less stable than the saturated hydrocarbons because of the presence of some weak sites,

like unsaturated groups and branching points [3]. The thermal degradation of polyeth-

ylene is a radical process and starts with the dissociation of a C-H bond or with the

scission of the polymer chain (Reaction 2.1) which is followed by random intermolecu-

lar hydrogen abstraction and subsequent β-scission [4]. Many schemes were proposed

for the initiation step, but the origin of the primary alkyl radical R is still controversial [15]. It can be formed as an effect of heat, shear, catalyst residues, radical initiators,

and/or impurities in the monomer.

R H

R RR.

According to Holström and Sörvik [3,5-7] the thermal degradation of polyeth-

ylene starts with the scission of C–C bonds in allylic position.

CH2 CH CH2 R CH2 CH CH2 R+..

(2.2)

The primary radicals formed in the initiation reaction participate in intra-

molecular and intermolecular hydrogen transfer, as well as in depolymerization through

-scission.

Primary radicals can isomerize by intramolecular hydrogen abstraction (back-

biting) and form more stable secondary radicals. Intramolecular hydrogen transfer can

occur from the fourth or fifth carbon atom to the first one [8-14]:

(2.1)

Chapter 2

12

Primary radicals can react also with intermolecular hydrogen transfer:

R CH2 + R' CH2 R" R CH3 R' CH R"+. .

The hydrogen abstraction can be followed by β-scission. The reactions of sec-

ondary radicals lead to the formation of vinyl groups, while those of tertiary radicals

result in the formation of vinylidene and vinylene groups [3,5,7,16]:

R CH2 C R'

CH2

R''

R CH CH R' + H2C R". .

Intermolecular hydrogen abstraction followed by β-scission were considered

the most important propagation reactions in the degradation of polyethylene by

Holström and Sörvik [3]. These reactions result in a significant decrease of the average

molecular mass and yield volatile products. On the other hand Kuroki et al. [4] claimed

on the basis of activation and bond dissociation energies that in the thermal degradation

of polyethylene back-biting reactions and intermolecular radical-transfer are much more

likely to occur than depolymerization reactions.

Vinyl groups can participate in further reactions. Intermolecular hydrogen ab-straction followed by isomerization of a vinyl group results in the formation of vinylene

group [3,5,6]:

R' CH2 CH CH2 RH ++ R R' CH CH CH2.

R' CH CH CH3 + R" R' CH CH CH2

R"H

.

.

.

.

Addition of an alkyl radical to the vinyl group leads to the formation of a sec-

ondary radical [17,18]:

R' CH2 CH CH2 + R R' CH2 CH CH2 R. .

R CH2 HC

H

CH2 CH2

CH2R CH2 CH

CH2 CH2

CH3

. .(2.3)

(2.4)

R CH CH2 R' R CH CH2 + R

. .

R CH2 C CH2 R'

CH2

R"

R CH2 C CH2R'

CH2

+ R".

(2.5)

(2.6)

(2.7)

(2.8)

(2.9)

Background

13

Recombination (2.10 and 2.11) and disproportionation (2.12 and 2.13) are the

most important termination reactions of thermal degradation:

R CH2 + R' CH2 R CH2 CH2 R'. .

R CH2 + R' CH R" R CH2 CH

R'

R"

. .

R CH2 CH2 + R' CH2 R CH CH2 + R' CH3

. .

R CH2 + R' CH CH2 R".

R CH3 + R' CH CH R".

The probability of the recombination termination reaction between primary and

secondary macroradicals is 2-5 times larger than that of the disproportionation reaction

[5] but the tendency to disproportionation increases with increasing temperature [4].

Although both types of termination reactions have zero activation energy, termination

reactions are diffusion controlled in practice and the rate constant depends on the rate of diffusion of macroradicals in the media.

2.1.2. Thermo-oxidative degradation

The oxidation of hydrocarbons is a free radical-initiated autocatalytic chain reac-

tion [15]. The reaction is slow at the start and accelerates with increasing concentration

of the resulting hydroperoxides. The process can be regarded as proceeding in three

distinct steps: chain initiation, chain propagation, and chain termination.

Chain initiation is a result of the dissociation of a C-H or a C-C (chain braking) bond

(Reaction 2.1).

Chain propagation

R.

+ O2 ROO.

.+ROO

.RH ROOH + R

.+

.RH + RROHRO

.+

.RH + RHO H2O

. .+R C=C R C C

.+ R

3C

O

R1

R3

R2

CR1

R2

Oß-Scission

(2.10)

(2.11)

(2.12)

(2.13)

(2.14)

(2.15)

(2.16)

(2.17)

(2.18)

(2.19)

Chapter 2

14

.+R

FragmentationOlefin R'

.

The alkyl radicals react with molecular oxygen practically without activation

energy forming peroxy radicals (Reaction 2.14). It is a very fast reaction, the rate con-stant is around 107-109 mol-1s-1. The peroxy radicals form hydroperoxides upon abstrac-

tion of hydrogen from the polymer chain (Reaction 2.15), which requires the breaking

of a C-H bond, i.e. needs activation energy. Therefore this is the rate-determining step

in autoxidation. The rate of the abstraction reaction decreases in the following order:

hydrogen in -position to a C=C double bond (allyl) > benzyl hydrogen and tertiary hydrogen > secondary hydrogen > primary hydrogen. Primary and secondary peroxy

radicals are more reactive in hydrogen abstraction than the analogous tertiary radicals

[19,20], and the most reactive are acylperoxy radicals [21]. -scission of oxy macro-radicals yields carbonyl groups and other free alkyl radicals (Reaction 2.19) [15,22-24].

Chain branching

ROOH.

RO + OH.

.

RO +2ROOH.

ROO + H2O

Hydroperoxides decompose fast to reactive oxy and hydroxyl radicals (Reac-

tion 2.21). The rate of decomposition increases with rising temperature [15]. Metal ions

[25] and ultraviolet radiation [26] catalyze hydroperoxide decomposition. Oxy and

hydroxyl radicals formed in the decomposition of hydroperoxides are far more reactive

than peroxy radicals, and lead to the branching of the reaction chain, i.e. auto-

acceleration of the degradation process [15].

Chain termination

.ROO2 R

O

+ R

OH

+ O2

+.

ROOR ROOR.

R + R.

R R.

.ROR

.+ R RO

+.

2R

DisproportionationRH Olefin

Chain termination can occur by recombination or disproportionation reactions

of radicals. At high oxygen concentrations and moderate temperatures chain termination

proceeds by the recombination of peroxy radicals according to Reaction (2.23) [27].

However, the reaction products participate in further reactions, therefore the thermo-

oxidative reactions proceed. If the concentration of R radicals is much higher than that of peroxy radicals (characteristic for polyethylene processing), alkyl radicals are in-

volved also in the recombination reactions [Reactions (2.24) to (2.26)]. The dispropor-

(2.20)

(2.21)

(2.22)

(2.24)

(2.25)

(2.26)

(2.27)

(2.23)

Background

15

tionation of alkyl radicals according to Reaction (2.27) leads to the formation of an

unsaturated group but does not result in a decrease of molecular mass.

The melt processing of polyethylene takes place in oxygen poor environment under shear. The high mechanical forces lead to C-C chain scission resulting in

macroradicals [28]. The oxygen dissolved in the polymer reacts with the alkyl radicals

forming peroxy radicals, subsequently hydroperoxides and new alkyl radicals. These

hydroperoxides decompose rapidly to the corresponding alkoxy and hydroxyl radicals.

The latter can form inactive products (ROH and H2O) and further alkyl radicals through

hydrogen abstraction, while ß-scission leads to the scission of the macromolecule [15].

The number of weak sites in the polymer chain, the type and amount of catalyst resi-

dues, as well as the processing conditions affect significantly the rate and direction of

reactions [25,29-31]. At high concentrations of unsaturated groups (Phillips type poly-

ethylene) recombination reactions predominate during processing [15,24,30], while at

low unsaturated group contents (Ziegler-type and metallocene polyethylenes) the num-

bers of reactions leading to extension or scission of macromolecules are comparable, the overall direction of reactions depends both on the number of vinyl groups and the pro-

cessing conditions [25,31-34]. Chirinos-Padrón et al. [25] observed that 20% difference

in unsaturation has a drastic effect on the degree of cross-linking, which they explained

by thermal decomposition and subsequent reactions of allylic hydroperoxides. Johnston

et al. [35] observed that the number of unsaturated groups decreases when cross-linking

dominates and it increases when thermal scission is the main reaction. Similar reactions

were observed for vinylidene groups as for vinyl groups and they did not find correla-

tion between the formation of trans-vinylene groups and the reduction in vinyl content

during processing of polyethylene. The formation rate of trans-vinylene groups in-

creased with temperature and correlated closely with carbonyl formation. The signifi-

cant effect of unsaturated groups on the thermo-oxidative stability of polyethylene was confirmed lately by Gardette et al. [24].

2.1.3. Photo-oxidative degradation

During application and storage polymers exposed to sunlight suffer photo-

oxidative degradation. This process is initiated by the effect of photons on the polymer.

According to the basic laws only the light absorbed by the molecules results in photo-

chemical change in the molecule. Polymers without chromophoric groups do not absorb

sunlight, therefore they are not damaged in photochemically induced reactions. Polyeth-ylene belongs to the non-absorbing polymers. However, oxidized species form during

processing, like hydroperoxides or ketones, which absorb UV light and initiate photo-

chemical reactions. In some cases traces of catalysts remain in the polyolefins [36], or

atmospheric contaminants, such as polycyclic aromatic hydrocarbons, adsorb on the

surface of the polymer which can contribute to the formation of other absorbing groups

[15]. Light absorption of chromophoric defects results in the formation of radicals

which participates in different reactions: abstraction of a hydrogen atom from the mac-

romolecular chain, addition to an unsaturated group (crosslinking reaction [37]), and

reaction with oxygen.

Chapter 2

16

The main reactions of the photochemical degradation of polyethylene are

summarized in Scheme 2.1 [24]. Hydroperoxides formed as primary photoproducts

decompose by the scission of the weak O-O bond; the products are a macro-alkoxy and

a hydroxyl radicals. (The photolysis of hydroperoxyde groups by sunshine is a slow process, the average lifetime of –OOH groups is ~25 hours under constant irradiation

conditions [38].) The alkoxy macroradical is the key intermediate in the reaction. This

radical can react by several routes: a) abstraction of hydrogen results in the formation of

hydroxyls without cleavage of the polymer chain, b) β-scission of the main chain yields

aldehydes and alkyl radicals [39], c) cage reaction between radicals. The reaction be-

tween the macro-alkoxy radicals and hydroxyl radicals produces ketones. Ketones react

photochemically by Norrish type I or type II reactions which are characteristic for pho-

to-oxidation and cannot occur under the conditions of thermo-oxidation. Photolysis of

ketones by Norrish II processes results in the formation of vinyl-type unsaturations, and

produces a chain-end ketone which also reacts by Norrish II reaction and forms a vinyl

unsaturation and acetone. The photo-oxidative reactions of aldehydes, alkyl radicals and

keto radicals arisen by Norrish type I reaction result in the formation of carboxylic ac-ids, esters and lactones. Hydrogen abstraction from the carbon atom in α-position to the

vinyl group leads to the formation of trans-vinylene group through the isomerization of

an intermediate allylic radical.

While the rate of degradation depends strongly on the initial concentration of

unsaturated groups in the case of thermo-oxidation, there is no correlation between the

two parameters in photo-oxidation [24]. Vinyl groups disappear during oxidative pro-

cesses, while their concentration increases in photolysis through Norrish reactions.

Photo-oxidation and thermo-oxidation of polyethylene produces almost the same oxida-

tion products, but their relative concentrations are different. The Norrish reactions that

occur in photo-oxidation are responsible for these differences.

Background

17

PE (chromophoric defects)

h

CH2 CH2 CH CH2 CH2

O

OH

CH2 CH2 CH CH2 CH2

O2, PE

h

CH2 CH2 CH CH2 CH2

O

CH2 CH2 CH CH2 CH2

OH

CH2 CH2 C CH2 CH2

O

CH2 CH2 CH

O

CH2 CH2

OH

+

.

CH2 CH2 C

O

CH2 CH2+

O2, PE

carboxylic acids

esters

lactoneshN II

N I

CH2 CH CH2

vinyls+

CH2 CH2 C CH3

O

h, N II

CH CH2 acetone+

vinyls

P +. CH2 CH CH2 CH CH CH2 + P

C CH CH2

O

CH CH CH2

CH CH CH3

PEh

saturated ketones (vinylenes)

Scheme 2.1 Photo-oxidation of polyethylene [24]

Chapter 2

18

2.2. Stabilization of polyethylene

The thermal- and thermo-oxidative degradation of polyethylene can be hin-

dered by addition of a small amount of substances, i.e., stabilizers, which can react with

alkyl, peroxy and alkoxy radicals and/or decompose hydroperoxides. During processing

polyethylene is subjected to heat, shear and low level of oxygen. As a consequence,

several chemical processes take place simultaneously the direction and the extent of

them depending on the chemical structure of the polymer (number of unsaturated

groups, type of co-monomer, number and distribution of long chain branches), and the

processing conditions (temperature, concentration of oxygen, intensity of shear forces)

[29,30,40,41]. The role of the stabilizers is to hinder efficiently these reactions.

Stabilizers are chemical substances which are added to polymers in small

amounts (at most 1-2 w/w%) and are capable of trapping emerging free radicals or un-

stable intermediate products (such as hydroperoxides) in the course of autoxidation and

to transform them into stable end products. The possible path of the inhibition of ther-

mo-oxidative degradation is shown in Scheme 2.2 [15].

Mn+

O2,

H. Donor

RH

.OH

RO.

Complexing

agent

Radical

Scavenger

RHROOH

Hydroperoxide

Decomposer

RH

O2

ROO.

R.

Scheme 2.2 General scheme of inhibition of thermo-oxidative degradation [15]

Metallic impurities originating mainly from catalyst residues (titanium, chro-

mium or iron) are a source of the formation of alkyl radicals both under processing

conditions and during the life cycle of the end product. Suitable deactivation of the active form of these catalysts is, therefore, mandatory after polymerization. In applica-

Background

19

tion where the polymer is in direct contact with a metal (e.g., cable insulators) the use of

metal deactivators improves the lifetime of the polymer [42].

In principle, scavenging the primary macroradicals, R, would stop autoxida-tion, but it is difficult to avoid, as the reaction rate of molecular oxygen is very high.

The rate-determining step in autoxidation is the abstraction of a hydrogen from the polymer backbone by the peroxy radical which results in the formation of a hydro-

peroxide (reaction 2.15). If a substantially more easily abstractable hydrogen is offered

by a suitable hydrogen donor (InH) to the peroxy radical, then the reactions will com-

pete. H-donors are known as chain breaking donors. Suitable H-donors do not react

further by abstraction of a hydrogen from the polymer backbone. Scavenging of the RO

and HO radicals – which are far more reactive than the peroxy radicals – is practically not possible by using radical scavengers. Hydroperoxide decomposers (HD) are used as

co-stabilizers to avoid chain branching during autoxidation. They decompose hydro-

peroxides forming “inert” reaction products. Chain breaking acceptors and H-donors are

referred to as primary antioxidants, while hydroperoxide decomposers are classified as

secondary antioxidants.

2.2.1. Hydrogen donors and radical scavengers

Phenolic antioxidants, secondary aromatic amines, aromatic diamines,

sterically hindered amines (HAS) and hydroxylamines are used as hydrogen donors in

practice.

Phenolic antioxidants are the most widely used and extensively investigated

primary antioxidant for polymers [e.g., 34,43-48]. Hindered phenols are trapping peroxy

radicals efficiently. The key reaction is the formation of hydroperoxide by transfer of a hydrogen from the phenolic moiety to the peroxy radical and the formation of a

phenoxy radical according to reaction (2.28) [47,49,50].

The steric hindrance by substituents, e.g. tertiary butyl groups in the 2- and/or

6-positions, influences the stability of the phenoxy-radical or the mesomeric cyclo-

hexadienonyl-radicals. Sterically hindered phenols can be classified according to the

substituents in 2-, 4-, and 6-positions. The rate of hydrogen abstraction from phenol

increases with decreasing steric hindrance in 2- and 6- positions [51,52]. While 2,6-di-

terc-butyl phenols have a low concentration threshold-value, phenols with one or no

tertiary butyl group in those positions are efficient stabilizers already in very low con-centrations. However, physical effects have to be taken also into account, as the de-

crease in steric hindrance increases the ease of hydrogen-bonding of the phenolic hy-

droxyl groups [53]. The substituents in 2- and 6-positions influence significantly also

the reactivity of the phenoxy radicals formed in Reaction (2.28) [51,52]. Bulky substitu-

R1 R2

R3

OH

+ ROO.

+ ROOH

R1 R2

R3

O

(2.28)

Chapter 2

20

ents prevent the reaction of the phenoxy radical with the polymer and suppress dimeri-

zation of two phenoxy radicals. The sterically hindered phenols are not only effective

hydrogen donors, but undergo numerous further chemical reactions that contribute to

the inhibition of autoxidation. Sterically fully hindered phenols have tert-butyl groups in 2-, 4-, and 6-positions compared to the phenyl group, i.e. have no H-atom on the C-

atom vicinal to that in these positions. The contribution of fully hindered phenols to

stabilization is essentially the stoichiometric reaction between the phenol and the peroxy

radical. Partially hindered phenols have hydrogen atoms on the C-atom vicinal to at

least that in 4-(or 2- or 6-)position and participate in several reactions resulting in C-C

coupling products and quinone methides. The latter react with alkyl, alkoxy and peroxy

radicals [15].

Although some thermo-oxidative degradation products of polyethylene have

discoloring effect, strong discoloration of the polymer stabilized with phenolic antioxi-

dants originates mainly from the reaction products of the stabilizer. The color develop-

ment can be attributed to the formation of conjugated diene compounds [54]. The dis-coloration depends on the structure and concentration of the phenolic transformation

products. The principal contribution to polymer discoloration is due to the formation of

quinone methides.

Secondary aromatic amines and aromatic diamines react with peroxy radicals

and the primary reaction products can further react similarly to phenols [55]. Sterically

hindered amines are efficient stabilizers against thermal- and photo-oxidative degrada-

tion of polyolefins. The activity of these amines as antioxidants is based on their ability

to form nitroxy radicals (N-O). The reaction rate of nitroxy radicals with alkyl radical is only slightly lower than that of alkyl radicals with oxygen [56,57]. The mechanism of

the formation of nitroxy radicals and their reaction mechanism are controversial in the

literature [e.g., 58-60]. One of the assumed reaction paths is shown in Scheme 2.3. The

reaction of an alkyl radical with the N-O radical leads to the formation of hydroxyla-

mine ether (NOR’). This reacts with a peroxy radical (R”OO) resulting in the formation of alkyl peroxide (R’OOR”) and the reformation of the nitroxy radical [15]. It is im-

portant that the nitroxy radicals are formed in the course of polymer’s autoxidation, and HAS are not able to prevent the oxidation of hydrocarbons at high temperatures [61].

Therefore appropriate melt stabilization of the polymer is required to achieve sufficient

stability.

Scheme 2.3 Reaction mechanism of HAS compounds [15,62]

N

CH3

CH3

R1

CH3

CH3

X

N

CH3

CH3

O

CH3

CH3

X

N

CH3

CH3

O

CH3

CH3

R

X

,

ROO ; ROOH; O2

..

R'.

.R"OOR"OOR'

Background

21

Under oxygen-deficient conditions alkyl radical scavengers (chain breaking ac-

ceptors) could contribute significantly to the stabilization of the polymer, but only a few

compounds [2-tert-butyl-6-(3-tert-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl

acrylate, benzofuranone] are reported in the literature which are capable of doing that [63-67]. The reaction mechanism of 2-tert-butyl-6-(3-tert-butyl-2-hydroxy-5-methyl-

benzyl)-4-methylphenyl acrylate is shown in Scheme 2.4. The macroradical reacts with

the acrylate group by addition. Intramolecular shift of the hydrogen atom from the phe-

nol group results in the formation of a stable phenoxy radical. Such stabilizers prevent

crosslinking efficiently in styrene-butadiene and styrene-isoprene-styrene copolymers

during processing [15].

Scheme 2.4 Alkyl radical scavenging involving intramolecular hydrogen shift

2.2.2. Hydroperoxide decomposers

The hydroperoxides formed in thermo-oxidation of polyethylene and/or after

the reaction of hydrogen donors with peroxy radical (Reaction 2.28) decompose to reac-

tive RO and OH radicals resulting in autoxidation chain reactions. The rate of decom-position depends strongly on temperature. Hydroperoxide decomposers can prevent this

homolytic split of the hydroperoxide group.

Organophosphorous compounds are efficient hydroperoxide decomposers

under the processing conditions of polyolefins. Hindered aryl phosphites and phos-

phonites are used in large quantities generally in combinations with a hydrogen donor

primary antioxidant [15]. Due to the versatility of their reactions phosphines are also

promising candidates for polyolefin stabilization [68,69]. While the reaction mechanism

CH3

CH3

CH3

CH3

CH3 CH3

R1

R1

R2

OH O

CH2

O

R.

CH3

CH3

CH3

CH3

CH3 CH3

R1 R1

R2

OH O

CH

O

R

CH3

CH3

CH3

CH3

CH3 CH3

R1 R1

R2

O OO

R

CH3

CH3

CH3

CH3

CH3 CH3

R1 R1

R2

O O

OH

R

Chapter 2

22

of hindered phenols is widely investigated, less attention was paid to that of phospho-

rous stabilizers.

The stabilizing action of phosphites [P(OAr)3] and phosphonites is attributed to three basic mechanisms: decomposition of hydroperoxides, reactions with peroxy and

alkoxy radicals, as shown in Reactions (2.29) – (2.32) [70-77].

ROOH + P(OAr)3 → ROH + O=P(OAr)3 (2.29)

ROO + P(OAr)3 → RO + O=P(OAr)3 (2.30)

RO + P(OAr)3 → R + O=P(OAr)3 (2.31)

ArO + RO-P(OAr)2 (2.32)

The phosphorous compound reacts with hydroperoxides by reducing them to

alcohols in a non-radical process, while it oxidizes simultaneously to the corresponding

five-valent derivative (Reaction 2.29). Besides decomposing hydroperoxides the

organophosphorous compounds react with peroxy (Reaction 2.30) and alkoxy radicals

(Reactions 2.31 and 2.32) yielding alkoxy and alkyl radicals, respectively, as well as

oxidized and non-oxidized phosphorous derivatives. The oxidation of phosph(on)ites by

hydroperoxides is a fast reaction. The rate is affected by the chemical structure of the phosphorus compound, and it decreases with increasing electron-acceptor ability and

bulk of the groups bound to the phosphorus atom in the order: phosphonites > alkyl

phosphites > aryl phosphites > hindered aryl phosphites [70,75].

Water traces resulting from the thermal decomposition of hydroperoxides and

dehydration of alcohols formed in Reaction (2.29) may be present at high temperatures.

These reactions may lead to the hydrolysis of phosphites and phosphonites [78]:

P(OAr)3 + H2O → O=PH(OAr)2 + ArOH (2.33)

Monohydrogen phosphites and phosphonites hydrolyse further to phosphorous

acid and phosphonic acid, respectively. The contribution of this mechanism may im-prove efficiency, if the hindered phenol evolved in Reaction (2.33) can also react with

radicals [79]. However, good hydrolytic stability of phosphites does not automatically

result in poor efficiency [75]. For hindered aryl phosphites and phosphonites the ratio of

oxidation by hydroperoxides to hydrolysis depends on the oxidizability of the particular

hydrocarbon and on temperature [78]. The ratio of oxidation to hydrolysis increases

with increasing oxidizability and temperature.

Comparison of the stabilizing efficiency of an aromatic phosphite (Hostanox

PAR 24), an aromatic phosphonite (P-EPQ) and an aromatic-aliphatic phosphine (PMP)

without and in the presence of a phenolic antioxidant (I1010) revealed differences not

only in the efficiency but also in the reaction mechanism in polyethylene under the processing conditions [80,81]. Although the efficiency of the three investigated phos-

phorous stabilizers is similar at high temperature in oxygen rich environment due to

their ability to decompose hydroperoxide groups, under processing conditions (oxygen

Background

23

poor environment) both the level of oxygen and the chemical activity of the phospho-

rous stabilizer affect the mechanism of the reactions. The melt stabilizing efficiency of

the phosphonite and phosphine is larger than that of the phosphite. The former two

inhibit the recombination reactions of macroradicals with similar efficiency during processing, but the rate of consumption of the phosphonite is large, while significantly

smaller amounts are oxidized from the phosphine. Neither the investigated phenolic

antioxidant, nor its combination with the phosphite can hinder the formation of long

chain branches, which results in a gradual decrease in strength of blown films even at

high concentrations of the antioxidants. Study of the thermal- and the thermo-oxidative

stability, as well as the reactivity of the phosphite and the phosphonite by model reac-

tions revealed that diphosphonite is more reactive at high temperature in oxygen-poor

environment than the investigated phosphite and the thermal stability of the phospho-

rous compound plays also a significant role in the stabilizing efficiency [82,83]. The

phosphorous antioxidants give synergetic effect with hindered phenols which can be

attributed to a significant decrease in the consumption of each type of stabilizer during

the processing of polyethylene compared to single antioxidants [80,81].

Because of their high reactivity, phosphites and phosphonites are used as stabi-

lizers during melt processing of polyethylene (temperatures up to 300 °C). However,

their contribution to the stabilization of end products, as long term stabilizers is small.

Organosulphur compounds such as sulfides, dialkyl dithiocarbamates or thiodi-

propionates are efficient hydroperoxide decomposers under applications conditions.

[15]. Their effect is based on the ability of sulfenic acids to decompose hydroperoxides.

In the first step the sulfide reacts stoichiometrically with a hydroperoxide molecule

forming an oxide (Scheme 2.5). Sulfenic acid is formed through thermal decomposition

of the intermediate sulfoxide. Further possible reactions are the formation of sulfone or

oxidation with hydroperoxides, leading to sulfuric acid and other sulfur-containing oxidation products.

Scheme 2.5 Hydroperoxide decomposition with thiodipropionate esters [15]

CH3 CH2 O C CH2 CH2

O

Sn 2

ROOHCH3 CH2 O C CH2 CH2

O

S=On 2

+ ROH

CH3 CH2 O C CH2 CH2

O

Sn 2

+ ROHO

O

ROOH

CH3 CH2 O C CH2 CH2

O

SOHn

CH3 CH2 O C CH CH2

O

n+

(ROOH)x

e.g., SO2, SO3, H2SO4

Chapter 2

24

Intermediates with ß,ß-sulfinyldipropionate structure are particularly reactive

in the formation of sulfenic acid. Therefore compounds like di-stearyl or di-lauryl

dithiopropionate are mainly used as hydroperoxide decomposers in long time applica-

tions. They contribute to the extension of the lifetime of plastics in the course of use at temperatures up to 150 °C. During processing they do not contribute to the stabilization,

as the formation of sulfoxides and the subsequent oxidation products is a relatively slow

process [84].

2.2.3. Light stabilizers

The photooxidative degradation of polymers can be hindered by different stabi-lizers. According to their reaction mechanism the light stabilizers are designated as UV

absorbers, quenchers, H-donors, free radical scavengers, and hydroperoxyde decompos-

ers [15,85,86].

The protection mechanism of UV absorbers is based on the absorption of the

harmful UV radiation and its dissipation in a manner that does not lead to photosensiti-

zation, i.e. dissipation as heat. Besides having very high absorption themselves, these

compounds must be very stable. Hydroxybenzophenones (Structure I) and hydroxyl-

phenyl benzotriazoles (Structure II) are the most extensively studied UV absorbers. The

disadvantage of UV absorbers is that they need a certain absorption depth (sample

thickness) for good protection of plastics. The protection of polymer surfaces and of thin articles as films and fibers is only moderate.

Structure I Structure II

Quenchers are light stabilizers able to take over the energy absorbed by the

chormophores present in a plastic (e.g. the carbonyl groups formed during the thermo-

oxidation of the polymer) and to dispose of it efficiently to prevent degradation. The

energy can be dissipated either as heat or as fluorescent or phosphorescent radiation. For

energy transfer from an excited chromophore (donor) to the quencher the latter must

have lower energy states than the donor. Paramagnetic metal compounds, especially

nickel-containing light stabilizers are efficient quenchers in polyolefins. Some nickel-

containing stabilizers may also act as radical scavengers and/or as hydroperoxide de-

composers. The action of quenchers is independent of the thickness of the articles to be

protected. Therefore, they are useful for the stabilization of thin films and fibers.

O OH

O R

R = H or CH3 to C12H25

N

N

N

R1

R2OHX

X = H or Cl R1 = CH3 to C8H17 or branched alkyl

R2 = H or branched alkyl

Background

25

Hydroperoxides play a determining role in the photo-oxidative degradation of

polymers. Metal complexes of sulfur-containing compounds such as dialkyldithio-

carbamates, dialkyldithiophosphates and thiobisphenolates are very efficient hydro-

peroxide decomposers. The UV stability can be improved by combining UV absorbers with phosphites or nickel compounds due to the combined effects of hydroperoxide

decomposition and UV absorption. Apart from the absorption of harmful radiation by

UV absorbers, the deactivation of excited states by quenchers and the decomposition of

hydroperoxides by some compounds containing phosphorus and/or sulfur, the scaveng-

ing of free radical intermediates is another possibility for light stabilization, analogous

to that used in thermo-oxidative degradation.

The latest development in the field of light stabilizers is represented by the hin-

dered amine type stabilizers. Sterically hindered amines are efficient stabilizers against

thermal- and photooxidative degradation of polyolefins, as described in Chapter 2.2.1.

Therefore they are designated both as Hindered Amine Stabilizer (HAS) and Hindered

Amine Light Stabilizer (HALS). Numerous mechanisms of stabilization have been suggested for HALS [15]. Among the main mechanisms of UV stabilization, i.e., UV

absorption, excited state quenching, hydroperoxide decomposition, and free radical

scavenging, only UV absorption can be eliminated. First the stable nitroxy radical

>NO●, derived from the secondary amine >NH, was considered to account for the

photostabilizing efficiency of HALS. However, the efficiency of HALS cannot be ex-

plained only by the activity of nitroxy radicals. Then it was proposed that nitroxy radi-

cal regeneration from their reaction products, hydroxylamines >NOH and hydroxyla-

mine ethers >NOR, was an important process that increased stabilization efficiency

significantly. Further explanations for the stabilization mechanism of HALS are:

- quenching photoinitiation reactions,

- termination of oxidation chain, - decomposition of activated hydroperoxides,

- inducing termination reactions of peroxy radicals.

HALS stabilizers cannot be applied along with thiocompounds because an antagonistic

effect can occur. Sulphuric acid is formed from the thiocompounds during the decom-

position of hydroperoxydes which gives ammonium salts with the HALS compounds

[15]. The formation of ammonium salt significantly decreases the photo-stabilization

efficiency of HALS molecule.

Chapter 2

26

Scope

27

Chapter 3

Scope

As the introductory part of this thesis explained the research on the degradation

and stabilization of polymers is in a transitional state in the group at the moment. The

interest in this topic decreased considerably all over the world and less and less papers

are published in the area. On the other hand, the factors and processes determining the stability of polymer products are complex and the relationships among processing,

chemistry, structure and properties are extremely complicated. The development of new

products and especially new processing technologies with larger throughput rates puts

demands efficient stabilization which is financially viable at the same time. The experi-

ence of our group is extensive along those lines shown by the numerous papers pub-

lished and the two PhD theses completed on the processing stabilization of polyethylene

[87] and on the mechanism of phosphorous secondary stabilizers [88]. In order to con-

tinue the very fruitful cooperation with TVK and support other players of the plastics

industry if necessary, we have to preserve this knowledge and extend it even further. On

the other hand, the changing interest of the scientific community as well as other factors

force us to find a different focus for our research, bring new ideas into it in a way which uses the knowledge accumulated up to now. These very different aspects determined the

goals of this thesis. First we wanted to complete projects started earlier, compile the

results for further use and publication, if possible. Two of such projects are reported in

the thesis: the study of the behavior of pressure pipes under the effect of extractive me-

dia and the detailed investigation of a completely new, experimental secondary stabi-

lizer, an alkyl-aryl phosphine. The rest of the thesis reflects the paradigm change in our

philosophy and focuses on the use of natural compounds for the stabilization of poly-

ethylene.

As mentioned earlier, one of the main initiatives and important driving force of

our activities in the field of polymer degradation and stabilization was the cooperation

with Clariant, a leading player in the market of plastics additive78s. At a certain point of this cooperation a new project was started with the goal of exploring the behavior and

performance of polyolefin stabilizers under the effect of extractive media. Although

many products, but especially pipes are extensively used in contact with water, the

question had been neglected in spite of the known problems of hydrolysis for some

additives. The project was quite ambitious and consisted of three parts. The first focused

on the hydrolytic stability of additives with the help of model experiments. Five primary

antioxidants with very different chemical structures were hydrolyzed in beakers and

their transformation products were analyzed by HPLC-MS experiments in this part of

the project. In the second part running parallel with the first, the same stabilizers were

added to polyethylene and the samples prepared were studied with various methods.

Finally, in order to model real life conditions, pipes were produced on an industrial extruder at Pannonpipe and the effect of processing as well as storage in water on pipe

properties were studied from various aspects. Chapter 5 of the thesis reports some of

the results obtained in the study, notably the effect of additive composition, processing

and storage in water on the properties of the pipes. In order to simulate practical condi-

tions as much as possible, the pipe formulations contained industrially relevant additive

Chapter 3

28

packages based on the five primary antioxidants and five secondary stabilizers. The 25

packages covered a wide range of possible additive combinations and allowed us to

draw interesting conclusions from the study. A further challenge was to find an appro-

priate method to characterize the resistance of the pipes against mechanical loading, the adaptation of the method in our lab and the study of a large number of pipes over the

period of one year. The main conclusions drawn from the detailed analysis of the large

number of results obtained is summarized in the chapter and also closes this project at

the same time.

Another important topic of our cooperation with Clariant was the study of the

stabilization mechanism of phosphorous containing secondary stabilizers. Industrial

additive packages developed for polyethylene contain such an additive practically al-

ways, since it fulfills an important role, protect the polymer against degradation in the

last stage of production technology, during granulation. Phosphorous secondary stabi-

lizers are believed to decompose hydroperoxides during processing and prevent the

branching of the oxidation chain independently of their chemical structure. Mostly phosphites and phosphonites are used in industrial practice. A new, very promising

compound, a phosphine was also developed by Clariant in the effort of increasing effi-

ciency and decrease of the necessary amount of the stabilizer in the final product. The

studies related to the effect and efficiency of phosphorous stabilizers clearly indicated

that the mechanism of stabilization is much more complicated than claimed in text

books and depends also on the structure of the stabilizer. In a series of studies run in our

laboratory the various phosphorous stabilizers were compared to each other [89], the

respective role of the phenolic and the phosphorous antioxidant was investigated [80]

and the mechanism of stabilization of the most frequently used phosphite stabilizer was

analyzed [82]. In the last stage of this research we investigated in detail the stabilization

mechanism of the new phophine stabilizer developed by Clariant. The main results of the study are reported in Chapter 6. Model experiments were carried out to check the

reactions of the stabilizer with oxygen and with various reactive species (radicals) being

present in polyethylene during its processing. The results called the attention to the

complex nature of stabilization with phosphorous stabilizers indeed.

The rest of the thesis represents the new era and reports results on the stabiliza-

tion effect and efficiency of natural antioxidants. Several compounds with different

chemical structures may be considered as natural stabilizers in polymers and specifical-

ly in polyethylene. Alpha tocopherol, i.e. vitamin E, is a natural phenol and its effect as

antioxidant was studied in detail, among others, by Al-Malaika et al. [90-93] and it was

shown to be a very efficient stabilizer. It is already used in practice in ultra-high-molecular-weight PE for medical use [94]. Similarly, lignin also proved to have some

antioxidant effect [95], while natural oils and -carotene are expected to scavenge alkyl radicals by their unsaturated groups. Using some of these materials as stabilizers was

more or less successful, while some of the compounds proved to be inefficient or im-

practical. We selected quercetin, i.e. [2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-

chromen-4-one], a natural antioxidant as a potential stabilizer for polyolefins and car-

ried out stabilization experiments with it. The results are reported in Chapter 7 of the

thesis. This compound is found in fruits, vegetables, leafs and seeds in nature. It is a

flavonol type flavonoid, which has proven antioxidant, antiviral and anti-inflammatory

Scope

29

effect in the human body. Several groups proved already that it is an efficient stabilizer

for various polymers indeed. However, these groups always used the compound in large

amounts and did not carry out a detailed study on its effect and efficiency in a wide

composition range. Our goal was to fill this gap and offer more details on the ad-vantages, but also on the drawback of using this natural antioxidant as stabilizer. Anoth-

er natural compound which might be used as stabilizer is curcumin, 1,7-bis(4-hydroxy-

3-methoxyphenyl)-1,6-heptadiene-3,5-dione. It is the principal curcuminoid of Curcu-

ma longa rhizomes (turmeric). The curcuminoids are natural phenols that cause the

yellow color of turmeric [96]. Curcumin is an oil-soluble pigment, practically insoluble

in water at acidic and neutral pH, and soluble in alkali [97]. It is a very interesting mol-

ecule, since besides its phenolic –OH groups, also the linear linkage between the two

methoxyphenyl rings of the compound might participate in stabilization reactions. Re-

sults obtained in the stabilization experiments carried out with curcumin are summa-

rized in Chapter 8.

Although stabilization was the major goal and purpose of our investigations dur-ing the search for natural compounds which can be used as possible stabilizers we made

some interesting observations indicating a possible practical use of one of these com-

pounds. -carotene is a strongly colored red-orange pigment present in many plants and

fruits, like carrots, pumpkins, or sweet potatoes. -carotene is a precursor of vitamin A and is widely used as a colorant in foods and beverages. It has a long conjugated se-

quence of double bonds in its structure which is very reactive against many active com-

pounds. Our earlier experience proved that conjugated double bonds scavenge C cen-

tered radicals and hinder the formation of long chain branches, an important aspect of

the stabilization of Phillips type polyethylenes. -carotene revealed stabilizing effect in polyethylene in preliminary experiments, and we also observed that the originally

strong color of polyethylene fades during time, when it is exposed to light. This gave us

the idea of using an appropriately selected combination of natural antioxidants in active

packaging materials for the indication of shelf life. Chapter 9 reports the results of our

efforts to study such a possible application for an additive package containing two natu-

rally occurring compounds, -tocopherol and -carotene. As mentioned before -tocopherol or Vitamin E is a chain breaking antioxidant, which prevents the cyclic

propagation of lipid peroxidation and its stabilization effect has been studied extensive-ly. Such a combination of the two compounds for the application indicated above has

never been mentioned before in the literature and the project seemed to be fun.

In the final chapter of the thesis, in Chapter 10, we briefly summarize the

main results obtained during the work, but refrain from their detailed discussion, be-

cause the most important conclusions were drawn and reported at the end of each chap-

ter. This chapter is basically restricted to the listing of the major thesis points of the

work. The large number of experimental results obtained in the research supplied useful

information and led to several conclusions, which can be used during further research

and development related to the application of natural compounds as additives in

polyolefins, but probably also in other polymers. As usual, quite a few questions re-

mained open in the various parts of the study, their explanation needs further experi-ments. Research continues in this area in our laboratory and we hope to proceed suc-

cessfully further along the way indicated by this Thesis.

Chapter 3

30

Experimental

31

Chapter 4

Experimental

4.1. Materials

4.1.1. Polymers

In the work ethylene/1-hexene copolymers were investigated which were pol-

ymerized by Phillips catalysts and provided by TVK, Hungary in additive-free powder form. In Chapter 5 the Tipelin PS 380 pipe grade (melt flow index of the powder: 0.3-

0.4 g/10 min at 190 °C, 2.16 kg; nominal density: 0.938 g/cm3), while in Chapters 7, 8

and 9 the Tipelin FS 471 film grade (melt flow index of the powder: 0.3 g/10 min at 190

°C, 2.16 kg; nominal density: 0.947 g/cm3) were studied.

4.1.2. Additives

In Chapter 5 the additive packages contained five phenolic antioxidants and

five application stabilizers. Hostanox O10 and Hostanox O3 (both from Clariant) are

ester type antioxidants, while Ethanox 330 (Albemarle) and Cyanox 1790 (Cytec Indus-

tries) were used for comparison, since they do not contain such groups and should be

stable in water. The fifth antioxidant used was Hostanox O310, which is the 1:1 mixture

of Hostanox O3 and O10. The application stabilizers included Hostanox SE4 (Clariant), an organosulphur compound, and Hostavin N30 (Clariant), Tinuvin 622 and

Chimassorb 944 (both BASF Schweiz AG), which are relatively large molecular mass

hindered amines with various structures. The fifth application stabilizer was Tinuvin

783 (BASF Schweiz AG), which is the 1:1 mixture of Tinuvin 622 and Chimassorb 944.

The commercial and chemical names, the supplier, abbreviation, molecular mass and

chemical structure of the stabilizers are compiled in the Appendix in Tables A1 and A2.

All five antioxidants were used in combination with the five application stabilizers, thus

altogether 25 packages were created. Packages prepared with the SE4 application stabi-

lizer contained 1500 ppm of the antioxidants, while all the other packages 1000 ppm.

The amount of the application stabilizer was always 2500 ppm. Additionally, all pack-

ages contained also 1500 ppm calcium stearate acid scavenger and 1000 ppm Sandostab

P-EPQ (P-EPQ, Clariant; Appendix Table A3) phosphonite processing stabilizer.

In Chapter 6 the model reactions of bis(diphenylphosphino)-2,2-dimethyl-

propane (PMP; PEPFINE) alkyl-aryl phosphine were investigated. The antioxidant, as

well as its mono- (PMP-1ox) and dioxides (PMP-2ox) were produced and provided by

Clariant (Appendix Table A3).

In Chapter 7 the melt stabilizing efficiency of quercetin (Sigma-Aldrich; Ap-

pendix Table A4) was studied. The polymer was stabilized with 1000 ppm quercetin

and with a mixture of 1000 ppm quercetin and 2000 ppm Sandostab P-EPQ, or with

Irganox 1010 (I1010, BASF Schweiz AG; Appendix Table A1) alone and in combination

with P-EPQ for comparative measurements. For the determination of the effect of addi-

Chapter 4

32

tive concentration on the stability of polyethylene, quercetin was added in various

amounts (5, 10, 15, 20, 50, 100, 250, 500 and 1000 ppm) in combination with 2000 ppm

P-EPQ phosphonite secondary stabilizer. A sample without any additive and one con-

taining only 2000 ppm P-EPQ were also prepared for comparison.

In Chapter 8 the polymer was stabilized with 1000 ppm curcumin ( 65 % from Curcuma Longa [Turmeric], Sigma-Aldrich; Appendix Table A4), and with a

mixture of 1000 ppm curcumin and 2000 ppm Sandostab P-EPQ. Samples processed

with the same amount of Irganox 1010 and its combination with P-EPQ were used as

references.

In Chapter 9 the polymer was stabilized with 500 ppm -tocopherol (Ronotec 201; Hoffmann-La Roche; Appendix Table A4), 2000 ppm Sandostab P-EPQ, and dif-

ferent amounts of -carotene (purum, 97 %, UV, Sigma; Appendix Table A4) ranging from 0 to 2000 ppm.

4.1.3. Reagents, solvents and gases

In the model experiments (Chapter 6) PMP was reacted with cumene

hydroperoxide (CHP; Luperox CU90) of 88 % purity, dicumyl peroxide (DCP; Luperox

DCP) of ≥99 % purity and azobisisobutyronitrile (AIBN) of 98 % purity. The reagents

were purchased from Aldrich. The chemical structures of the materials used in the ex-

periments are summarized in the Appendix in Table A5. Freshly opened deuterated

chloroform-d1 (99.96 % D, Aldrich) was used for solution-state NMR spectroscopy.

Argon of 99.999 %, nitrogen of ≥99.5 %, and oxygen of ≥99.5 % purity were applied in

the model reactions and analysis.

4.2. Sample preparation

In the experiment presented in Chapter 5 the polymer powder and the additives were homogenized in a Henschel FM 40D/FM 40 AK high speed mixer at 300 rpm for

10 min. The dry-blend was pelletized using a Davo Viscosystem 1.30.02 type single

screw extruder at 170-180-185-190-195-200-205 °C zone temperatures and 90 rpm at

TVK. The processing stabilization efficiency of the packages was compared by multi-

ple, degradative extrusions. The granules were extruded using a Rheomex S 3/4" 25 L/D

single screw extruder attached to a HAAKE Rheocord EU-10 V driving unit. The ex-

truder was equipped with a screw of constant pitch and 3:1 compression ratio. The die

was fitted with a single orifice of 4 mm diameter. The temperature of all zones was set

to 260 C in all extrusion steps. Samples were taken for testing after each extrusion. Pressure pipes with 32 mm outer diameter and 3 mm wall thickness were produced

from the pellets by using a Szvogép SZ-63 extruder at 180-183-183-187 °C barrel and

190-194-195 °C die temperatures and 100 rpm at Pannonpipe Ltd. Pieces of 350 mm

length were cut from the extruded pipes and stored in water at 80 °C at TVK. Samples were taken after 0, 3, 5, 7 and 12 months and submitted to various tests.

Experimental

33

In the studies described in Chapters 7, 8 and 9 the polymer and the additives

were homogenized in a high speed mixer (Henschel FM/A10) at a rate of 500 rpm for

10 min. In the case of quercetin and -tocopherol the necessary amount of antioxidant was dissolved in 200 ml acetone and the solution was added to the PE powder in the

mixer and the powder was dried overnight to remove acetone. The powder with the

additives was pelletized using a Rheomex S ¾” type single screw extruder attached to a Haake Rheocord EU 10V driving unit at a screw rate of 50 rpm. The compositions pre-

pared with quercetin (Chapter 7) and curcumin (Chapter 8) were processed by six con-

secutive extrusions at barrel temperatures of 180, 220, 260, and 260 °C. Samples were

taken after each extrusion step. Polyethylene powder mixed with -carotene and antiox-idants (Chapter 9) was pelletized at barrel temperatures of 180, 190, 190, and 190 °C.

From each pellet films of about 100 µm and 1 mm were compression molded for further

studies at 190 °C and 5 min using a Fontijne SRA 100 machine.

4.3. Methods

The chemical structure of polyethylene and the concentration of the phospho-

rous stabilizer were determined by Fourier transformed infrared (FT-IR) spectroscopy

using polymer films of 100 µm. A Matson Galaxy 3020 (Unicam) spectrophotometer

(Chaper 5) and a Tensor 27 (Bruker) spectrophotometer (Chapters 7, 8, and 9) were

applied and the measurements were carried out in the wavenumber range of 4000-400 cm-1 at 2 cm-1 resolution with 16 scans on 5 parallel samples. The spectrum of the nas-

cent polymer powder was recorded by Diffuse Reflectance Fourier Transform Infrared

(DRIFT) spectroscopy [98]. The concentration of methyl [99], vinyl [100,98], t-

vinylene [100,98] and carbonyl groups [101] was determined from the spectra according

to the methods described in the respective references. In the presence of phosphonite

antioxidant the amount of vinylidene groups cannot be determined because their ab-

sorbance (888 cm-1) overlaps with that of the P(V) transformation products of P-EPQ

(890 cm-1). The chemical structure of additives and that of the reaction products of

phosphine (Chapter 5) was determined according to the physical state of the substance.

2-3 mg sample was mixed with 780 mg of KBr powder then compressed to wafer, or it

was spread onto the surface of a KBr wafer.

The thermal and thermo-oxidative characteristics of polyethylene and additives

were studied using the Mettler TA 4000 Thermal Analyzer. Differential scanning calo-

rimetric (DSC) measurements were carried out in the DSC-30 cell under nitrogen (50

ml/min) and oxygen (100 ml/min) at heating and cooling rates of 10 °C/min.

Thermogravimetric (TGA) measurements were run in the TG-50 cell in nitrogen and

oxygen gas flow of 100 ml/min, as well as in air without gas flow using a heating rate of

3.5 °C/min. The melting and crystallization characteristics of PE were measured under

nitrogen between -50 and 200 °C on 7-8 mg samples cut from the pipes in two heating

and a cooling cycles (Chapter 5). The characteristics of phosphine were determined by

both DSC and TGA methods under the conditions described above (Chapter 6). The melting characteristics and thermal stability of quercetin and curcumin, as well as their

mixtures with P-EPQ were studied by DSC in nitrogen atmosphere between -50 and 300

°C according to the method given above (Chapters 7 and 8).

Chapter 4

34

The melt flow index (MFI) of the granules and the pipes was determined ac-

cording to the ASTM D 1238-79 standard at 190 °C with 2.16 kg load using a Göttfert

MPS-D apparatus. Five parallel measurements were carried out on each sample. Creep

curves of the polymer were recorded at 190 °C with 500 Pa mean stress and 300 s creep/600 s recovery phase times using a Rheoplus/32 V3.40 type (Anton Paar) Univer-

sal Dynamic Spectrometer (Chapter 8). Creep viscosity (0) was derived from the com-pliance measured in the creep phase. The residual thermo-oxidative stability of polyeth-

ylene was characterized by the oxidation induction time (OIT) measured at 200 °C in

oxygen atmosphere using a Perkin Elmer DSC 2 equipment. Three parallel measure-

ments were carried out on granule samples and pieces cut from the pipes. The color of

the polymer was characterized by the yellowness index (YI D1925). The measurements

were carried out by a Hunterlab Colorquest 45/0 apparatus.

The resistance of pipes against mechanical failure was characterized by the rate

of slow crack propagation (Chapter 5). The measurement was carried out according to

the ISO 13480:1997 standard on 100 mm long pipes notched to 10 mm length (Fig 4.1)

in 5 % polyglycol ether surfactant (NP-10 type Tergitol, Sigma-Aldrich) solution at 80 °C. The rate of crack propagation was followed by daily measurements.

Fig. 4.1 Crack propagation in a prenotched pipe.

The reaction mechanism of the phosphine antioxidant (PMP) was studied by

model experiments (Chapter 6). High temperature reactions were carried out with 1 g of

PMP in a round bottom flask equipped with a magnetic stirrer, a reflux condenser, and a

gas inlet. The flask was heated by an oil bath. The conditions of the various reactions

are summarized in Table 4.1. The reaction was carried out with continuous stirring and the flask containing the reactant(s) was purged with argon or oxygen before and during

the experiments.

Experimental

35

Table 4.1 Reaction conditions of the model experiments

Reagent Reaction conditions

Type

Amount

(mol/mol

PMP)

Gas Temperature

(°C)

Time

(min)

Reactant

- - Ar 200 1 Heat

- - Ar 240 1 Heat

- - O2 200 1 Oxygen

- - O2 240 1 Oxygen

AIBN 0.514 Ar* 200 1 Carbon centerd radical

AIBN 0.065 O2 200 1 Peroxy radical

DCP 0.326 Ar 200 1 Oxy radical

DCP 0.326 O2 200 1 Oxy radical

CHP 0.552 Ar 200 3 Hydroperoxide

CHP 0.552 Ar 23 24 h Hydroperoxide

* Cleaned and dried

The thermal stability and the reaction of PMP with molecular oxygen were

investigated in argon and oxygen atmosphere, respectively, at 200 and 240 °C by im-

mersing the reaction flask into the heated oil bath and holding it there for 1 min. The

argon of 99.999 % purity was further cleaned from oxygen by bubbling through

pyrogallol and then drying on CaCl2 for the reaction of the phosphine with carbon cen-

tered radicals. 1 mol of PMP was mixed with 0.514 mol of AIBN and purged with the

purified argon for 10 min before gradually heating up the flask to 200 °C where the

reaction was continued for 1 min. The reaction of the phosphine with peroxy and oxy

radicals was also studied at 200 °C. The mixture of 0.065 mol of AIBN and 1 mol of PMP were heated up to the reaction temperature under oxygen and then reacted there

for 1 min. 1 mol phosphine was reacted with 0.326 mol DCP in argon and oxygen, re-

spectively, by immersing the flask into the heated oil bath and holding there for 1 min.

The reaction of 1 mol of phosphine with 0.552 mol of CHP was carried out by two

methods: 1) CHP was added slowly to the phosphine under argon at 200 °C (total reac-

tion time: 3 min); 2) the components were mixed at ambient temperature in air then left

standing without mixing at 23 °C for 24 h. After the high temperature reactions the

product was cooled down to ambient temperature under the corresponding gas by re-

moving the flask from the oil bath and then analyzed by FT-IR spectroscopy without

dissolution and also by solution-state Nuclear Magnetic Resonance (NMR) spectrosco-

py.

Solution-state NMR spectra were obtained using a Varian Unity INOVA spec-

trometer operating at the 1H frequency of 400 MHz with a 5 mm inverse detection

probe. The samples were dissolved in deuterated chloroform (10-15 mg sample in 0.6

ml solvent) directly before the measurement to prevent unwanted reactions with the

solvent or with air. No change was detected in the proton spectra during the solution-

state NMR measurements (max. 5 hours), so we can eliminate the possibility of any side

reactions which could influence our results. Here we have to note, that traces (< 1%) of

Chapter 4

36

oxidative products were detected in the solution of PMP in chloroform-d1 stored at -20

ºC for 10 days. The signals of the solvent (7.26 ppm on the 1H and 77.0 ppm on the 13C

scale) were used as reference for chemical shifts. The temperature of the measurements

was 25 ºC in all cases. 16 s delay time and 4 s acquisition time were used to get accurate integrals of the proton spectra. Two-dimensional 1H-13C correlation spectra were rec-

orded for the assignation and to map the connections in a molecule. Homonuclear Sin-

gle-Quantum Correlation (HSQC) [102] and Heteronuclear Multiple-Quantum Correla-

tion (HMQC) [103,104] measurements were carried out under standard conditions with

2 s delay time to identify directly bonded C-H pairs and longer connections (2-3 bonds),

respectively. The assignation of 1H and 13C signals is given in the Appendix.

The dispersion of the quercetin in polyethylene (Chapter 7) was checked by

light microscopy using a Zeiss Axioscop equipped with a Leica DMC 320 digital cam-

era. Micrographs were recorded with the Leica IM 50 software on samples compressed

between glass plates.

The light stability of curcumin (Chapter 8) was studied by aging the material in

powder form and in different solutions (ethanol, methanol, acetone, acetonitrile, toluol,

chloroform) in natural light (behind a window) and in dark with periodical sampling for

5 months. 5 mg of the sample was dissolved in 100 ml solvent and analyzed by a HP

8452A type (Hewlett-Packard) UV-VIS spectrophotometer in the wavelength range of

190-820 nm.

The concentration of β-carotene in polyethylene processed with α-tocopherol/

phosphonite/β-carotene additive combinations was determined by UV-VIS spectroscopy

(Chapter 9). The spectra of 100 m thick films were recorded in the wavelength range of 200-600 nm using a Unicam UV 500 type spectrophotometer. The absorption spectra

are complex, as showed by Fig. 4.2. The absorption band between 200 and 300 nm can

be assigned mainly to α-tocopherol, although β-carotene dissolved in cyclohexane has also a small band in this region. The main absorption band of β-carotene is located be-

tween 400 and 600 nm (blue-green region), and the band between 300 and 400 nm orig-

inates also from β-carotene; it is the cis-band region [105, 106]. The concentration of

residual β-carotene in polyethylene films was calculated from the absorption intensity at

456 nm after calibration in cyclohexane solutions. This method yields approximate

values for the concentration of β-carotene in polyethylene due to the effect of different

factors (e.g., isomerisation, oxidation) on the UV-VIS spectra.

Experimental

37

200 300 400 500 6000

1

2

3

Rel

ativ

e ab

sorb

ance

Wavelength (nm)

456 nm

273 nm

345 nm

-tocopherol

-carotene

cis--C

Fig. 4.2 UV-VIS absorbance spectra of polyethylene processed with -tocopherol/

phosphonite antioxidant package and -carotene of 0 (—), 250 (– –), 500 (····), 1000

(··–··), 1500 (----), and 2000 (–·–) ppm. Absorption intensities are related to 100 µm film thickness.

Chapter 4

38

Pipes: effect of the additive package and processing

39

Chapter 5

Performance of PE pipes under extractive conditions: effect of the addi-

tive package and processing

5.1. Introduction

The service life of plastic pipes and the factors influencing it have been the

subject of considerable interest for some time. Pipes are used in the most diverse appli-

cation areas including water, waste water and sewer pipes, floor, wall and ceiling heat-

ing systems, warm and hot water solar systems and natural gas supply [107]. Their

guaranteed service life is 50 years in most cases. The lifetime of pipes is usually pre-

dicted by using internal pressure tests, in which the pipe is subjected to different internal

stresses and the time to rupture is measured [108,109]. In accordance with most litera-

ture sources, Gedde and Ifwarson [110] claim that chemical processes play a role only in the last stage of lifetime, when the complete consumption of stabilizers leads to brit-

tle fracture. The detailed study of the failure of pipes in a pressure test showed that

many different mechanisms contribute to rupture, e.g. the diffusion of additives and

oxygen, degradation reactions, etc. [111-114]. These processes depend on the polymer,

the additive package, the surrounding environment and other conditions [115-121].

Evaporation, leaching by the surrounding media or chemical reactions result in

the loss of stabilizers during the lifetime of the pipes. According to Gedde et al. [122-

125] the consumption of stabilizers by chemical reactions can be neglected, the largest

loss is usually caused by leaching. Pfahler [126,127] also showed that in the presence of

water migration and loss depends on the chemical structure of the stabilizer; the use of additives with smaller molecular mass often leads to shorter lifetimes, because of faster

evaporation or leaching [128,129]. Gedde et al. [124,125,130] divided the time depend-

ence of stabilizer loss and chemical degradation into three stages: the precipitation and

segregation of the additive, leaching, and the autoxidation of the polymer. Although

segregation might occur indeed, Dörner et al. [131] did not find a stepwise decrease in

stabilizer content and OIT with time, and Billingham [132] could not prove the exist-

ence of additive droplets in the polymer. Nevertheless, it is clear that the hydrolytic and

chemical stability, solubility and diffusion of additives are crucial factors determining

the lifetime of polyolefin pipes especially when they are in contact with extractive me-

dia. At the beginning chemical reactions are not supposed to play any role in the failure

of the pipe. However, it is known that oxygen, stabilizers and the polymer always react with each other leading to stabilizer consumption and to the modification of the chain

structure of the polymer.

Numerous literature sources including those cited above prove that considera-

ble work has been done on the degradation and failure of polyethylene pipes. Neverthe-

less, several questions are completely unclear or have not been dealt with at all. For

example, only limited attention has been paid to the hydrolytic stability of antioxidants

and its possible effect on the lifetime of products in contact with extractive media [133].

Moreover, industrial additive packages used in pipe production contain several compo-

Chapter 5

40

nents including a primary antioxidant, a processing stabilizer and additives to protect

the pipe during its application. These components may have different influences on the

lifetime of the pipes and they might even interact with each other resulting in synergistic

or antagonistic effects [75,134]. In the framework of a larger project, we prepared polyethylenes with a wide variety of additive packages and extruded pipes from them

under industrial conditions. We stored them in water at 80 °C and followed changes in a

number of properties including the chemical structure of the polymer, additive loss,

color, MFI and slow crack propagation. The goal was to identify the main factors de-

termining the behavior of pipes during their use in the presence of water. The effect and

efficiency of the various additive packages are also compared and the role of processing

versus soaking in the determination of pipe performance is analyzed in the final section

of the chapter.

The results are presented in three sections. First the effect of storage in water

on various pipe properties is shown then the influence of granulation and pipe produc-

tion on the structure of the polymer is analyzed. General correlations are discussed in a final section including the role of the additive package on the processes taking place

during production and storage.

5.2. Effect of soaking

One would expect a number of changes to take place in the physical and chem-ical structure of PE pipes during their storage in water at 80 °C. The stabilizers may

react with dissolved oxygen and water, with each other, and the functional groups of the

polymer may also take part in chemical reactions. Soaking leads also to the transfor-

mation of crystalline structure, to recrystallization and the perfection of the lamellae. All

these changes should appear in the measured properties of the pipes. Rather surprisingly

the functional group content of the polymer changed only slightly or practically not at

all during one year soaking in water. The limited changes occurring were determined

primarily by the application stabilizer, but the primary antioxidant had some effect as

well. Vinyl group content is plotted against soaking time in Fig. 5.1, as an example, for

additive packages containing E330 and the various application stabilizers. The lines

drawn in Fig. 5.1, and in fact in all subsequent figures, are not based on models, but are

included to indicate trends and to guide the eye. Vinyl group content seems to decrease slightly at the beginning of soaking in the case of some additive combinations (C944,

T783 and N30), while it is constant for the other two additives (T622, SE4). The differ-

ences are difficult or impossible to explain, since T783 also contains the Tinuvin 622

stabilizer. The comparison of the effect of the additive packages studied showed that the

vinyl group content of the polymer is more or less constant. Similar conclusions can be

drawn from the analysis of changes in the concentration of the other functional groups

as well, i.e. the chemical structure of the polymer practically does not change during

soaking. On the other hand, some migration or leaching of the additives was indicated

by the decrease in the intensity of methyl and carbonyl absorptions in the FTIR spectra.

As consequence, it is not surprising that the MFI of the polymer does not change much

either during soaking. Melt flow index is plotted against soaking time in Fig. 5.2 for packages containing the C1790 stabilizer. The viscosity of the polymer changes in a

very limited extent indeed.

Pipes: effect of the additive package and processing

41

0 2 4 6 8 10 120.75

0.80

0.85

0.90

0.95

1.00

N30

SE4

T783

Vin

yl/

10

00

C

Soaking time (month)

T622

C944

Fig. 5.1 Independence of vinyl content of the time of storage in water. Effect of the

application stabilizer. Primary antioxidant Ethanox 330. Symbols: ( ) SE4, ( ) N30,

( ) T783, ( ) C944, ( ) T622.

0 2 4 6 8 10 120.19

0.21

0.23

0.25

0.27

MF

I (g

/10

min

)

Soaking time (month)

SE4N30

T783

T622

C944

Fig. 5.2 Effect of soaking on the melt flow index of pipes prepared with various addi-tive packages. Primary antioxidant C1790. Symbols: ( ) SE4, ( ) N30, ( ) T783,

( ) C944, ( ) T622.

Chapter 5

42

Although soaking time does not have a significant influence on the properties

of the polymer, based on Figs. 5.1 and 5.2 it is clear that the type of the additives, espe-

cially that of the application stabilizers, do. Compounds form groups, and the pro-

cessing of PE in the presence of the various application stabilizers results in products with quite different properties. The vinyl group content of the polymer is the smallest

when packages containing N30 and SE4 are used, while considerably larger in the pipes

produced with the other three additives. Melt flow index changes in the opposite direc-

tion; the largest MFI is measured in the presence of SE4, N30 and T783, while signifi-

cantly smaller values are obtained with T622 and C944. Some correlation seems to exist

between vinyl content and MFI, but the relationship is not straightforward shown by the

effect and performance of the T783 additive. Although general correlations are clear

(groups of additives, relationship between MFI and vinyl) further measurements and

analysis are needed to explain small differences and the specific effect of the individual

additives satisfactorily.

The results presented above might easily lead to the conclusion that chemical reactions do not take place during soaking or at least they do not influence the proper-

ties of the pipes. However, Fig. 5.3 strongly contradicts this conclusion. The photograph

of two sets of pipes is shown in the figure indicating very strong discoloration and dif-

ferent colors. The combination of HO10 and N30 results in strong dark color, whereas

yellow color develops gradually during soaking in pipes containing E330 and C944. All

pipes show some discoloration, the weakest is observed in the presence of the HO3

antioxidant. A relatively weak yellow color was observed in its combination with all

five application stabilizers, but color was always considerably stronger than without any

stabilizers. The color developed and the extent of discoloration depended very much on

the combination of the primary antioxidant and the application stabilizer also in all other

cases changing from very weak to the strong color shown in Fig. 5.3.

Fig. 5.3 Effect of soaking time on the color of the pipes. Left: HO10/N30, Right: E330/C944. Soaking time: 0, 3, 5, 7 and 12 months from top to bottom.

The discoloration of the pipes clearly indicates that chemical reactions take

place during their soaking. These reactions do not influence the structure of the poly-

Pipes: effect of the additive package and processing

43

mer, but change the amount of active stabilizer shown by the decrease of oxidation

induction time (Fig. 5.4). The residual stability of the pipes decreases gradually to very

small values, at some additive combinations (E330/C944) to zero, in one year. Although

loose correlation exists between discoloration and the decrease of stability, strong devia-tions can be observed from the general tendency as well (e.g. HO3). The comparison of

OIT with carbonyl concentration and yellowness index unambiguously shows that the

loss of stability does not result only from chemical reactions including hydrolysis, but

also from the leaching of the stabilizer. Similarly to the functional group content of the

polymer and the color of the pipes, residual stability is also strongly affected by the

chemical structure of the application stabilizer.

0 2 4 6 8 10 120

20

40

60

80

100

120

140

OIT

at

20

0 o

C (

min

)

Soaking time (month)

Fig. 5.4 Decrease of residual stability (OIT) with soaking time for pipes prepared with

HO10. Symbols: ( ) SE4, ( ) N30, ( ) T783, ( ) C944, ( ) T622.

Besides chemical reactions one would expect changes also in the crystalline

structure of the polymer as an effect of soaking at 80 °C. Crystallinity is plotted against

storage time in Fig. 5.5 showing an increase, which is relatively fast at the beginning

then levels out at the end of storage. Quite unexpectedly, significant differences can be

seen also here in the effect of the various additives, especially in that of the application stabilizers. Similarly to other properties, pipes containing C944 and T622 form one

group and the remaining three another; the degree of crystallinity is smaller in the pres-

ence of the first group and larger when a stabilizer from the second group is used. A

possible reason for the different degree of crystallinity might be the grafting of some of

the stabilizers (C944, T622) onto the polymer chains that changes their regularity [135].

Mechanical properties and strength are important attributes of pipes. They, including

crack propagation, should be determined by the crystalline structure of the polymer. As

a consequence, a similar dependence on storage time is expected as in the case of

crystallinity.

Chapter 5

44

0 2 4 6 8 10 1245.0

47.5

50.0

52.5

55.0

57.5

60.0

Cry

stal

lin

ity

(%

)

Soaking time (month)

SE4, N30, T783

C944, T622

Fig. 5.5 Changes in the crystallinity of PE pipes as a function of storage time at 80 °C.

Primary antioxidant Hostanox O10. Symbols are the same as in Fig. 5.4.

The rate of slow crack propagation is plotted against soaking time in Fig. 5.6.

Contrary to the expectation crack propagation rate changes only slightly as a function of

time, and definitely not according to the function as crystallinity. The lack of close

correlation between the two parameters (compare Figs. 5.5 and 5.6) indicates that not

crystallinity, but some other factor determines the fracture resistance of the pipes (see

section 5.3.). The different effect of the two groups of application stabilizers is clearly

visible in this case too. Some of the primary antioxidants, particularly when SE4, N30 and T783 are used as secondary stabilizer, also influence crack propagation rate, but in a

lesser extent than the application stabilizers.

The analysis of the results showed that the majority of polymer and pipe prop-

erties are independent of storage time. The occurrence of chemical reactions including

hydrolysis is unambiguously proved by the strong discoloration of the pipes and the

decrease of OIT. Leaching, especially after hydrolysis, contributes also to the decrease

of residual stability. The crystalline structure of the polymer changes, crystallinity in-

creases as a result of soaking, but this is not the main factor determining mechanical

properties, including the rate of crack propagation of the pipes. The type of the applica-

tion stabilizer plays a crucial role in the determination of properties, which do not de-

pend on the time of soaking, but on some other factors (see section 5.3.).

Pipes: effect of the additive package and processing

45

0 2 4 6 8 10 120.3

0.6

0.9

1.2

1.5

Cra

ck p

rop

agat

ion

rat

e (m

m/d

ay)

Soaking time (month)

SE4, N30, T783

C944, T622

Fig. 5.6 Effect of storage time and additive package on the rate of slow crack propaga-

tion in pipes produced with Hostanox O10. Symbols are the same as in Fig. 5.4.

5.3. General correlations, effect of processing

The effect of storage time on properties was analyzed in the previous section in accordance with the goals of the study. Storage time influences some properties only in

a limited extent and definitely did not account for differences in the effect of the various

combinations of additives. Obviously, some other factors determine the properties of the

pipes and we have not considered the effect of processing, i.e. pelletization and pipe

production yet. The effect and efficiency of additive packages are routinely character-

ized by multiple extrusion runs; all 25 compounds produced underwent the process. The

results of such an experiment are presented in Fig. 5.7 showing the effect of the number

of extrusions on the vinyl content of the polymer. According to the figure vinyl content

decreases with increasing processing history and the packages can be divided into two

groups just as before. Large differences can be observed in the initial vinyl content of

the polymer containing the two groups of application stabilizers, and this difference

does not change during multiple extrusions, the correlations run parallel to each other. Most of the changes in the structure of the polymer take place during the first extrusion

and these changes depend very much on the type of the application stabilizer present.

The structure developed in the first extrusion step does not change much during further

processing, and definitely not during soaking. As a consequence, the chemical structure

of our polymer is determined by reactions occurring in the first processing step.

One would expect that the rate of crack propagation should depend on the

physical structure of the polymer. We expected crystallinity to play an important role in

the determination of crack propagation rate, but the comparison of Figs. 5.5 and 5.6

shed some doubt on the existence of a close correlation between the two quantities. In

order to check the relationship more explicitly, the rate of crack propagation was plotted

Chapter 5

46

against crystallinity in Fig. 5.8. Although some tendency can be observed on the plot,

we cannot talk about strong correlation at all. Crystallinity influences crack propagation,

but some other factor or factors influences it more strongly. The rate of crack propaga-

tion is plotted against the vinyl content of the polymer in Fig. 5.9. A much closer corre-lation is obtained between a quantity related to the chemical structure of the polymer

and crack propagation than with crystallinity. Since most chemical reactions taking

place during processing and storage in water were shown to be closely related to each

earlier [136], we can conclude that the final properties of the pipes including crack

propagation are determined by a few chemical reactions taking place during processing,

and mainly in the first extrusion step. The direction and extent of these reactions are

strongly influenced by the type of the application stabilizer used.

0 1 2 3 4 5 60.9

1.0

1.1

1.2

Vin

yl/

10

00

C

No of extrusions

C944, T622

SE4, N30, T783

Fig. 5.7 Changes in the vinyl content of the polymer stabilized with HO3 in multiple

extrusion experiments. Symbols: ( ) SE4, ( ) N30, ( ) T783, ( ) C944, ( ) T622.

5.4. Discussion

The rather surprising fact that basically the first extrusion of the polymer de-termines the final properties of polyethylene pipes needs further considerations. Similar-

ly, the lack of change in some properties and considerable variation in others merits

additional thoughts. We tried to summarize the factors and processes of granulation,

pipe extrusion and soaking in a scheme presented in Fig. 5.10. Chemical reactions take

place during extrusion among oxygen, the polymer and the stabilizers. These reactions

change the structure of the polymer mostly resulting in long chain branches, and in the

transformation and loss of the stabilizers. The reactions of the stabilizers are usually

accompanied by discoloration, which is, however, much weaker than the color change

observed during soaking [136].

Pipes: effect of the additive package and processing

47

47 48 49 50 51 52 53 54 55 56 57

0.4

0.8

1.2

1.6

2.0

Rat

e o

f cr

ack

pro

pag

atio

n (

mm

/day

)

Crystallinity (%) - first heating

Fig. 5.8 Lack of correlation between the crystallinity (first heating) of the pipes after

soaking and the rate of crack propagation for the entire set of samples. Symbols:

HO10 and: ( ) SE4, ( ) N30, ( ) T783, ( ) C944, ( ) T622;

HO3 and: ( ) SE4, ( ) N30, ( ) T783, ( ) C944, ( ) T622;

HO310 and: ( ) SE4, ( ) N30, ( ) T783, ( ) C944, ( ) T622; E330 and: ( ) SE4, ( ) N30, ( ) T783, ( ) C944, ( ) T622.

C1790 and: ( ) SE4, ( ) N30, ( ) T783, ( ) C944, ( ) T622.

0.75 0.80 0.85 0.90 0.95 1.000.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Rat

e o

f cr

ack

pro

pag

atio

n (

mm

/day

)

Vinyl/1000 C

Fig. 5.9 Effect of the chemical structure of the polymer chains (vinyl content) on the

rate of crack propagation in pipes stabilized with all 25 additive packages. Symbols are

the same as in Fig. 5.8.

Chapter 5

48

Fig. 5.10 Scheme outlining the main processes occurring during the processing and

soaking of PE pipes and the factors determining their properties.

Various processes take place also during storage in warm water. Stabilizers

may react with each other, with oxygen dissolved in the water, and they also hydrolyze

[133]. These reactions and the leaching of the stabilizer or its small molecular mass

transformation products formed by hydrolysis result in discoloration, additive loss and

the decrease of residual stability (Figs. 5.3 and 5.4). The polymer basically does not take

part in these reactions; its structure does not change significantly as shown by the con-

stant value of functional groups (Fig. 5.1) and MFI (Fig. 5.2). On the other hand, the

physical structure of the polymer changes, its crystallinity increases (Fig. 5.5) and most probably other features of crystalline structure are also modified (e.g. lamella thickness,

Pipes: effect of the additive package and processing

49

number of tie molecules). Nevertheless, not these physical changes, but the chemical

structure of the polymer developed in the first extrusion step seems to determine the

mechanical properties of the pipes. Slow crack propagation rate measured on pipes

depends much more strongly on the chemical structure of the polymer (Fig. 5.9) than on crystallinity (Fig. 5.8). The reactions of the application stabilizers play a crucial role in

the determination of the chemical structure of the polymer, but their type and reactions

are important also during soaking, in the loss of additives and the decrease of OIT. The

mechanism of crack propagation and the influence of the fine structure of the polymer

chain on it need further study.

5.5. Conclusions

Pipes prepared with a wide range of additive packages were stored in warm

water for a year and the study of their properties showed that chemical reactions take

place both during processing and soaking. The chain structure of the polymer seems to

be modified only during processing, but not during storage, at least in the time scale of

the study. The direction and extent of changes are determined mainly by the type of the

application stabilizer, but primary antioxidants also influence them to some extent.

Soaking modifies the physical, but not the chemical structure of the polymer. On the

other hand, the chemical reactions of the additives determine color and stabilizer loss

thus the residual stability of the polymer. The chemical structure of the polymer has a

larger effect on final properties, on the rate of slow crack propagation and failure than the physical structure of the pipes. As a consequence, the application stabilizer plays an

important role in the determination of the performance of the pipes. The exact chemical

reactions taking place during processing and soaking, the mechanism of crack propaga-

tion and the influence of the chain structure of the polymer on it need further experi-

mentation and study.

Chapter 5

50

High temperature reactions of an aryl-alkyl phosphine

51

Chapter 6

High temperature reactions of an aryl-alkyl phosphine, an exceptionally

efficient melt stabilizer of polyethylene

6.1. Introduction

Usually the combination of a hindered aryl phosphite or phosphonite and a

hydrogen donor primary antioxidant is used as processing stabilizer for polyolefins in

practice [15]. Due to the versatility of their reactions phosphines are also promising

candidates for the stabilization of polyolefins. Phosphines had not been investigated as

polyolefin stabilizers before patenting PMP [68] and the related studies of our laborato-

ry carried out in cooperation with Clariant [69,80,81,89]. The results showed that the

stabilizing effect of the phosphine, bis(diphenylphosphino)-2,2-dimethylpropane, is

similar to that of the phosphonite, Sandostab P-EPQ, in polyethylene. On the other hand, the phosphine is efficient already in small concentrations and its consumption rate

is significantly slower than that of the phosphonite, when it is used in combination with

a phenolic antioxidant.

The reactions of various phosphines have been studied extensively in solution.

Trisubstituted phosphines react with hydroperoxides yielding phosphine oxides and the

corresponding alcohol [137-141]:

R3P + R’OOH R3P=O + R’OH (6.1)

The mechanism of this reaction is similar to that of trisubstituted phosphites

with hydroperoxides and is described by the attack of the phosphorous compound on the

hydroxyl oxygen of the hydroperoxide resulting in the formation of a four-coordinate phosphorus-centered intermediate, which yields the products by a simple proton transfer

[137]. Trialkyl phosphites are oxidized by a direct reaction with peroxy radicals to give

the corresponding phosphate and an alkoxy radical [72]. The same mechanism was

established for the reactions between trisubstituted phosphines and peroxy radicals by

Ingold [138].

Buckler [142] investigated the reaction of tributylphosphine with molecular

oxygen in different solvents in the temperature range of 20 and 80 °C using different

rate of air or oxygen flow. He concluded that autoxidation of the phosphine is a rapid,

clean, exothermic reaction. Tertiary phosphine oxides and phosphinic acid esters are the

major products. Changes in the oxygen concentration in the gas stream, the flow rate, the initial phosphine concentration and the temperature do not affect significantly the

relative amounts of the major products. The medium in which the process is carried out

plays the determining role. The ratio of phosphine oxide to phosphinic acid ester in-

creases steadily as the solvent becomes more polar. Small amounts of diphenylamine

and hydroquinone inhibit efficiently the autoxidation of tributylphosphine and similar

tertiary phosphines. On the basis of the experimental results the author proposed a reac-

tion mechanism, according to which the oxygen reacts with a hydrocarbon radical rather

Chapter 6

52

than directly with the phosphorus compound through a radical chain mechanism. The

relative amount of phosphine oxide and phosphorus ester produced is determined by

competitive reactions. The author concluded that the polarity of the attacking radical

and the substituents of the phosphorus compound determine the direction and rate of these reactions.

The oxidation of phosphines and phosphites by alkoxy radicals occurs by oxy-

gen-transfer through the formation of a common four-coordinate phosphorus-centered

intermediate [18,143,144]:

R3P + R'O

.R P

.O

R

R

R'

a b

a

b

R.

.R'

+

+

R POR'2

R P=O3 (6.2)

The configuration, configurational stability, and lifetime of the intermediate

radicals depend on the nature of the substituents of the phosphorus compound and on

the configurational requirements imposed by these substituents [143]. The ratio of -

scission [Reaction (6.2a); displacement] to -scission [Reaction (6.2b); oxidation] of the intermediate radical is delicately balanced and varies greatly with the nature of R and R’

[145]. To the first approximation the relative strength of R’-O and P-R bonds in the

radical intermediate determines the ratio of - to -scission [146,147]. Kochi and Krusic [144] observed that the reaction of trialkylphosphites with alkoxy radicals differs

from that of trialkylphosphines. In the first case -scission predominates, while dis-placement is the characteristic reaction of trialkylphosphines. According to the authors, the P-O bond breaks when the resonance stabilization of the displaced oxy radical is

possible.

Trialkyl phosphines and trialkyl phosphites are effective reducing agents for

peroxides [18,148-149]. According to Walling et al. [17,18] the reaction of di-terc-butyl

peroxide with triphenylphosphine follows the same path as the reaction with phosphites.

The peroxide undergoes thermal dissociation and the radicals react with the phosphorus

compound. On the other hand, Greatrex and Taylor [148] concluded that the trivalent

phosphorus atom of triphenylphosphine inserts into peroxide bonds yielding reactive

phosphorane intermediates, which undergo nucleophilic substitution or elimination

reactions.

All these works reveal that phosphines react with reactive oxygen containing

compounds and radicals, which promote the degradation of polyolefins during pro-

cessing. The reaction of phosphine with molecular oxygen in solvents was explained by

the formation of alkylperoxy radicals, which oxidize the phosphorus compound. The

mechanism of the reaction of phosphines and phosphites with hydroperoxides and per-

oxide radicals was found similar. Differences were observed in the reactions of

phosphites and phosphines with oxy radicals. In solution phosphines yield esters in

larger amounts than phosphites due to the domination of displacement reactions. Ac-

cording to Buckler’s reaction mechanism [142] phosphorus acid esters can react further

with alkyl peroxy and alkoxy radicals before oxidation to the five-valent derivative.

High temperature reactions of an aryl-alkyl phosphine

53

Most reactions of phosphines have been investigated at ambient or elevated

temperatures in solution. To explore the differences in the stabilization mechanism of

phosphites, phosphonites and phosphines during the processing of polyethylene, high

temperature model reactions were carried out. The phosphorous compounds were reacted directly with substances accelerating the degradation of polyethylene without

using any solvent, as the degradation of polyolefins is a heterogeneous local process

[150-154]. In this chapter the reactions of the phosphine, bis(diphenylphosphino)-2,2-

dimethylpropane (PMP), used as an antioxidant in the earlier studies [69,80,81,89] is

presented. Two problems were encountered during the analysis of the reaction products:

1) oxidized PMP dissolves only in a very limited number of solvents; 2) PMP oxidizes

fast in most of the solvents of oxidized PMP. Chloroform was found to be the best

solvent for the analysis, but it is not suitable for liquid chromatography. Therefore

solution-state Nuclear Magnetic Resonance (NMR) spectroscopy was selected for the

identification of the reaction products.

6.2. Characterization of the nascent phosphine and its oxides

The FT-IR spectra of crystalline PMP, PMP-2ox and a mixture of PMP with

PMP-1ox and PMP-2ox measured in KBr wafer are shown in Fig. 6.1. The phosphine

has a P(III)-Ar absorption band at 513 cm-1 [155] which disappears upon its oxidation.

The absorption of P(V)=O groups gives several bands in the regions between 1220 and

1045 cm-1 with two characteristic maximums at 1175 and 1117 cm-1, as well as between 600 and 480 cm-1 with peak maximums at 570 and 550 cm-1. The intensity of the band

at 570 cm-1 is small in the spectrum of the oxidized crystalline substances, while it in-

creases considerably when the material is in amorphous state or dissolved.

1500 1000 500

PMP-2ox

Mixed

PMP

P(V)=O

Ab

sorb

an

ce

Wavenumbers (cm-1)

Fig. 6.1 FT-IR spectra of phosphine (PMP), di-oxidised phosphine (PMP-2ox), and a mixture of PMP with mono-oxidised phosphine (PMP-1ox) and PMP-2ox (Mixed)

measured in KBr wafer.

Chapter 6

54

The methyl (1.0-1.2 ppm) and methyn (2.3-2.7 ppm) signals were used to de-

termine the relative amount of PMP, and its mono- and dioxidized forms by 1H solu-

tion-state NMR (Figure 6.2). The methyn signals are split to doublets due to the 1H-

31P

J-coupling. In the carbon spectra (not shown) all the 13C signals are split by 13C-31P couplings, while 31P couplings can be observed only on the methyl protons. The small

peaks between 1.5 and 2.0 ppm in the 1H NMR spectrum belong to water and/or OH

signal. Evaluation of the spectrum of the nascent phosphine reveals that it contains 1.5

mol % of ethanol (most probably originating from production); its signals appear at

1.24(t) and 3.72 (q) ppm (Fig. 6.2a). Note: the aromatic region is composed of highly

overlapped signals, so it is useless for our analysis; therefore this region is not shown in

Fig 6.2. Oxidation of the phosphorus atom results in stronger coupling on the methyn

resonance (10.8 Hz for oxidized and 3.0 Hz for not oxidized 31P) and larger chemical

shift (Fig. 6.2b). According to the spectrum shown in Fig. 6.2c the mixture of the refer-

ence materials consists of ~30 mol % of PMP, ~9 mol % of PMP-1ox and ~61 mol % of

PMP-2ox.

Fig 6.2 1H solution-state NMR spectra of PMP (a), PMP-2ox (b) and a mixture of PMP,

PMP-1ox and PMP-2ox (c). The aromatic region is not shown.

P

CH3CH3

P

P

CH3CH3

P

O

O

P

CH3CH3

P

O

O

P

CH3CH3

P

O

+

High temperature reactions of an aryl-alkyl phosphine

55

6.3. Reactions of the phosphine under thermal and thermo-oxidative con-

ditions

The composition of the reaction products formed during the thermal and ther-

mo-oxidative degradation of PMP is listed in the Appendix Table A6. The phosphine

melts at 94 °C and does not show any thermal reaction up to 190 °C in nitrogen atmos-phere (Fig. 6.3). TGA measurements reveal that the thermal decomposition of PMP

starts only above this temperature and proceeds with a slow rate up to 250 °C in inert

atmosphere (Fig 6.4). A small mass loss at around 75 °C indicates the presence of some

impurity in the material, which is in accordance with the conclusion drawn from the 1H

NMR investigation of the nascent phosphine (1.5 mol % ethanol). Heat treatment in

argon at 200 and 240 °C confirms that PMP possesses proper thermal stability in the

temperature range of polyethylene processing. The solid reaction product consists prac-

tically only of non-reacted PMP and does not contain any side products. Oxidized de-

rivatives are present in a very small amount of <0.1 mol % (Table A6) and their for-

mation may be attributed to the presence of ethanol in the nascent additive.

-50 0 50 100 150 200 250

En

do

H

eat

flo

w

Ex

o

nitrogen

oxygen

0.5 mW/mg

Temperature (°C)

Fig. 6.3 DSC traces of PMP measured under nitrogen and oxygen at a heating rate of 10

°C/min.

The DSC trace of the phosphine shows that in pure oxygen exothermic reac-

tions start at around 140 °C (Fig. 6.3). Oxidation is accompanied by a mass increase

(Fig. 6.4) revealing that molecular oxygen reacts directly with PMP. According to the

TGA analysis the oxidation and decomposition processes equilibrate at 220 °C, at high-

er temperatures decomposition dominates. Direct oxidation of PMP with molecular

oxygen is proved also by heat treatment carried out at 200 and 240 °C in oxygen atmos-

phere. The characteristic absorption bands of P(V)=O group can be detected in the FT-

IR spectrum taken after the reaction (Fig. 6.5). The results of the NMR investigations

show that the amount of oxidized phosphine increases with increasing temperature. It is

Chapter 6

56

worth to mention that the amount of PMP-1ox is three or four times larger in the reac-

tion mixtures than that of PMP-2ox (Table A6). At 240 °C a small amount (<1 mol %)

of side products are also formed, which yellow slightly the crystalline reaction mixture.

These products can result from thermo-oxidative decomposition reactions.

0 50 100 150 200 250 300-15

-10

-5

0

5

nitrogen flow

oxygen flow

static air

Mas

s ch

ang

e (%

)

Temperature (°C)

Fig 6.4 Mass change of PMP measured by TGA at a heating rate of 3.5 °C/min under

different conditions.

2000 1500 1000 500

Reacted

P(V)=OP(V)=O

PMP-2ox

PMP

Ab

sorb

an

ce

Wavenumbers (cm-1

)

Fig 6.5 FT-IR spectra of phosphine (PMP), oxidized phosphine (PMP-2ox) and the

reaction product of PMP heat treated at 240 °C in oxygen atmosphere (Reacted), meas-

ured in KBr wafer.

High temperature reactions of an aryl-alkyl phosphine

57

The TGA measurement run in static air, when gaseous reaction products were

not removed from the system, indicates some mass increase at around 125 °C followed

by decomposition reactions above 200 °C (Fig. 6.4). The comparison of the reactions of

the phosphine under different conditions reveals that its behavior at high temperatures differs essentially from that of the phosphite studied in a previous work [82]. The

phosphite does not react directly with molecular oxygen and is thermally unstable above

its melting temperature (200 °C), which leads to various decomposition and recombi-nation reactions. PMP has sufficient heat-stability and oxidizes directly with molecular

oxygen at high temperatures. Side products form only in traces. The rate of thermal

decomposition of PMP is moderate even above 200 °C (the mass loss is less than 20

w/w% at 300 °C) and is not accelerated considerably by volatile reaction products.

6.4. Reactions with carbon centered radicals

Carbon centered radicals can be produced by heating of AIBN, since in oxy-

gen-free environment it decomposes to cyanoisopropyl radicals [72]:

AIBN N2 + 2(CH3)2C–CN (6.3)

The reaction of PMP with these carbon centered radicals was studied at 200

°C. A yellowish amorphous material was obtained in the reaction. The results of the

NMR measurements reveal that 98 mol % of PMP did not react at all (Table A6). Alt-

hough the reaction system – including the glassware and the reagents – was purged

continuously with oxygen-free argon before and during the experiment and then the

reaction mixture was held under argon, both the FT-IR (Fig. 6.6b) and the 1H NMR (Fig. 6.7a) spectra indicate some oxidation of the phosphine. 2 mol % of PMP-1ox

formed besides a very small amount (< 0.1 mol %) of PMP-2ox. Under these conditions

the partial oxidation of the phosphine can be attributed to the presence of ethanol in the

nascent material. 3-5 mol % of side products originating from cyanoisopropyl deriva-

tives was also found in the reaction mixture. We assume that these substances discolor

the reaction product and hinder the crystallization of PMP. We can conclude from these

results that PMP does not react with carbon centered radicals, but it oxidizes easily even

in the presence of a small amount of oxidizing agent.

6.5. Reactions with peroxy radicals

PMP was mixed with AIBN and heated up to 200 °C in oxygen in order to

study its reactions with peroxy radicals. The reaction started at 120 °C with the evolu-

tion of bubbles and a dark-yellow honey-like product was obtained as the final product.

In oxygen rich environment the cyanisopropyl radicals formed in the thermal decompo-

sition of AIBN oxidize to peroxy radicals, which react with PMP as revealed by the FT-

IR (Fig. 6.6c) and 1H NMR (Fig 6.7b) spectra. Quantitative analysis (Table A6) showed

that 90 mol % of PMP remained non-reacted. At high temperatures two peroxy radicals form from each of the AIBN molecules (6.5 mol) added to 100 mol of phosphine at the

start of the experiment. The amount of PMP-1ox (8.6 mol %) and PMP-2ox (1.1 mol %)

in the reaction mixture indicates that most of the peroxy radicals reacted with PMP.

Besides 2-3 mol % of cyanoisopropyl derivatives only traces of side products formed

Chapter 6

58

from PMP. These results indicate that some volatile products left the reaction flask with

the N2 gas developed from AIBN.

2000 1500 1000 500

b)

P(V)=OP(V)=O

c)

a)

Ab

sorb

an

ce

Wavenumbers (cm-1

)

Fig. 6.6 FT-IR spectra of PMP taken before (a) and after reaction with AIBN under

argon (b) and under oxygen (c) at 200 °C, measured in KBr wafer.

Fig. 6.7 1H solution-state NMR spectra of PMP reacted with AIBN under argon (a) and

oxygen (b) at 200 °C. The peak at 1.54 ppm belongs to a cyanoisopropyl derivative.

High temperature reactions of an aryl-alkyl phosphine

59

6.6. Reaction with oxy radicals

The reaction of the phosphine with oxy radicals was studied by two methods:

PMP was reacted with DCP at 200 °C in 1) argon and 2) in oxygen atmosphere. At high

temperatures the peroxides decompose first to oxy radicals [156,157], which react fur-

ther depending on their chemical structure and the environment [157-160].

A white crystalline material was obtained in the reaction of PMP with DCP in

argon atmosphere. The FT-IR spectrum of the product shown in Fig. 6.8b reveals the

partial oxidation of the phosphine. According to 1H NMR 84.6 mol % of PMP remained

non-reacted, while 7.2 mol % of PMP-1ox and 8.2 mol % of PMP-2ox formed in the reaction (Table A6). Besides the main components, an intense peak also appears at 1.56

ppm in the aliphatic region of the 1H NMR spectrum (Fig. 6.9a). Analysis by 1H-13C

correlation (HSQC, HMQC) reveals that it belongs to a methyl group attached to a

compound containing an etheric type oxygen atom. The methyl group is connected to a

quaternary carbon atom attached also to an aromatic ring. Accordingly, this signal can

be assigned to a derivative of DCP. This product does not show any 31P splitting either

on the proton or on the carbon signals clearly demonstrating that it is not a derivative of

PMP.

1500 1000 500

b)

c)

a)

P(V)=OP(V)=O

Ab

sorb

an

ce

Wavenumbers (cm-1

)

Fig. 6.8 FT-IR spectra of PMP taken before (a), and after reaction with DCP under

argon (b) and oxygen (c) at 200 °C. Spectra a) and b) measured in KBr, c) spread on

KBr wafer.

The phosphine oxidized in a larger extent in the reaction with DCP in oxygen

atmosphere than under argon, and a yellow, honey-like reaction product was obtained

under the former conditions. The FT-IR spectrum of the product is shown in Fig. 6.8c.

NMR investigations reveal that the reaction mixture consists of 41.7 mol % non-reacted

PMP, 39.1 mol % PMP-1ox, and 19.2 mol % PMP-2ox as the main products (Table

Chapter 6

60

A6). Besides, a small amount of various side products also formed neither of them

showing any 31P splitting. Fig. 6.9b shows the 1H NMR spectrum of the reaction mix-

ture, while its HMQC spectrum can be seen in Fig. 6.9c. The derivatives of DCP are

labeled with roman numbers.

Fig. 6.9 1H solution-state NMR spectra of PMP reacted with DCP under argon (a) and

oxygen (b), and HMQC spectrum of the reaction product formed under oxygen (c).

Derivatives of DCP are signed with roman numbers.

I and II contain oxygen atom, while III does not. Additional NMR study re-

vealed that compound I corresponds to 2-phenyl-2-propanol. The decomposition of all DCP molecules (32.6 mol) mixed with 100 mol of PMP at the start of the experiment

could yield a maximum of 65.2 mol oxy radicals. Accordingly we can conclude that a)

the majority of cumene oxy radicals oxidized PMP; b) PMP oxidized not only in reac-

High temperature reactions of an aryl-alkyl phosphine

61

tions with cumene oxy radicals but also with molecular oxygen and/or some reaction

products of DCP. The side products are non-volatile derivatives of cumene. Phosphorus

ester cannot be identified among them indicating that the dominating reaction of the

phosphine with oxy radicals corresponds to reaction (6.4) and the displacement reaction (6.5) does not occur at high temperature in the absence of solvents.

R’O + R3P R’ + R3P=O (6.4)

R’O + R3P R + R2POR’ (6.5)

The absence or presence of oxygen influences the degree of oxidation of PMP

and the reactions of DCP, the source of oxy radicals. While the reaction carried out in

argon yielded only one derivative of DCP, more derivatives formed in the reaction done

in oxygen atmosphere. The detailed investigation and identification of these products is

out of the scope of the present work.

6.7. Reaction with hydroperoxide

The reaction of PMP with cumene hydroperoxide was investigated in argon

atmosphere at 200 °C for 3 min (including gradual addition of CHP) and at 23 °C for 24

h. A light yellow honey-like substance was obtained in the high temperature reaction,

while a transparent amorphous substance formed in the reaction run at ambient tempera-

ture. The FT-IR spectra of the reaction products are shown in Fig. 6.10. They reveal that

PMP oxidized extensively in both reactions. According to the 1H NMR results (Table

A6) approximately half of the original phosphine (~47 mol %) oxidized mainly to diox-

ide (~42 mol % PMP-2ox) in the longer experiment carried out at ambient temperature. In the shorter, high temperature experiment ~66 mol % of PMP remained non-reacted,

while ~11 mol % oxidized to PMP-1ox and ~23 mol % to PMP-2ox.

After the reaction run at 23 °C, a broad intense absorption band can be ob-

served in the FT-IR spectrum of the products between 3700 and 3200 cm-1 (Fig. 6.10b)

assigned to the O-H vibration of a hydroxyl group bound to carbon atom. This band is

missing from the spectrum of the high temperature reaction product (Fig. 10c). Besides

the characteristic peaks of PMP and its oxidized derivatives, an additional peak can be

observed at 1.59 ppm in the 1H NMR spectra of the reaction products shown in Fig.

6.11. Additional analysis of 13C and 1H-13C correlation spectra identified this compound

as 2-phenyl-2-propanol. The amount of this compound is significantly larger after the

reaction run at 23 °C than in the product formed at high temperature. The formation of 2-phenyl-2-propanol corresponds to reaction (6.1), and the smaller amount in the high

temperature reaction mixture can be explained with its thermal decomposition and re-

combination reactions [161,162]. The occurrence of these reactions is confirmed by the

presence of a small amount (<2 mol %) of various derivatives of CHP which can be the

source of the light yellow color observed. The identification of these molecules is not

relevant in this work, because the attention is focused on the reactions of PMP. The

phosphine does not take part in any side reactions with the derivatives of CHP, as no 31P

coupling can be detected in the 1H or 13C spectra of the compounds formed.

Chapter 6

62

4000 3500 3000 1500 1000 500

b)

a)

c)

P(V)=O

Ab

sorb

an

ce

Wavenumbers (cm-1

)

Fig. 6.10 FT-IR spectra of PMP taken before (a), and after reaction with CHP in air at

23 °C (b) and under argon at 200 °C (c); spread on KBr wafer.

Fig. 6.11 1H solution-state NMR spectra of PMP reacted with CHP under argon at 23

°C for 24 h (a) and at 200 °C for 3 min (b).

O

P

CH3CH3

P

O

P

CH3CH3

P

High temperature reactions of an aryl-alkyl phosphine

63

6.8. Discussion

The aim of the present study was to explore the reaction mechanism of

bis(diphenylphosphino)-2,2-dimethylpropane which is an extremely efficient melt stabi-

lizer of polyethylene even in small concentrations. The main question was how a small

amount (200 ppm) of this phosphine could prevent the degradative processes and long

chain branching of polyethylene during processing. Contrary to the effect of PMP long

chain branching occurs when tris(2,4-di-tert-butylphenyl)phosphite (Irganox 168,

Hostanox Par 24, DTBPP) is used as a secondary antioxidant even at high concentra-

tions. A previous study [82] showed that DTBPP does not react with molecular oxygen,

it is unstable at the temperatures of polyethylene processing, and besides its known reactions it takes part in different thermal decomposition and recombination reactions.

The results of the experiments presented above reveal that PMP behaves com-

pletely differently than DTBPP at high temperatures. First of all, PMP has higher ther-

mal stability in the temperature range of polyethylene processing than DTBPP. It starts

to decompose at around 200 °C but the rate of decomposition is much slower than that

of the phosphite, and is not accelerated significantly by volatile decomposition products.

Secondly, PMP oxidizes directly with molecular oxygen at high temperatures without

the formation of reactive side products. This result is important from two aspects. Stud-

ying the reactions of tributylphosphine with molecular oxygen in different solvents

below 100 °C Buckler [142] arrived to the conclusion that oxygen reacts with a hydro-carbon radical rather than directly at phosphorus. All the experiments presented above

prove that at high temperatures PMP reacts directly with molecular oxygen. This reac-

tion explains the strong melt stabilizing efficiency of the phosphine. Polyethylene is

processed in oxygen-poor environment. Under these conditions the phosphine does not

react with alkyl radicals, but oxidizes with molecular oxygen and decreases the proba-

bility of oxidation of macroradicals formed from polyethylene. In other words the phos-

phine hinders the initiation reaction of the autocatalytic thermo-oxidative degradation of

polyethylene. Although the exact mechanism of stabilization is not known, it is worth to

note that the amount of monooxidized PMP is three-four times larger than that of

dioxidized PMP after the reaction of the phosphine with molecular oxygen at 200 and

240 °C.

As expected, PMP reacts readily with hydroperoxides, peroxy and oxy radicals.

In these reactions oxidation of the phosphine dominates, the formation of phosphorus

esters was not detected, and side products of PMP remained under the detection level (1

mol %) of NMR measurements. All these factors contribute to the exceptionally high

melt stabilizing efficiency of PMP in polyethylene already in small concentrations.

6.9. Conclusions

The thermal- and thermo-oxidative stability and reactions of an aryl-alkyl

phosphine, bis(diphenylphosphino)-2,2-dimethylpropane (PMP), were studied at the

temperatures of polyethylene processing in the absence of solvents. The phosphine was

reacted with carbon centered, peroxy and oxy radicals, as well with hydroperoxide. The

results proved that the investigated phosphine has high heat-stability; its thermal- and

Chapter 6

64

thermo-oxidative decomposition is relatively slow above 200 °C and influenced only

slightly by the presence of volatile reaction products. Any oxidizing agent, including

molecular oxygen, reacts with PMP. Phosphorous esters did not form in reactions run at

200 °C, and side products of PMP were found only in traces. The high melt stabilizing efficiency of PMP in polyethylene during processing can be explained by its sufficient

heat-stability and fast oxidation. Its reaction with molecular oxygen always present in

small concentrations in the processing machine has vital importance, because it hinders

the initiation reaction of the auto-catalytic thermo-oxidative degradation of the polymer.

Stabilization with quercetin, a flavonoid type natural antioxidant

65

Chapter 7

Stabilization of PE with quercetin, a flavonoid type natural antioxidant

7.1. Introduction

Synthetic antioxidants are used for stabilization of polyethylene in industrial

practice, but about a decade ago questions emerged regarding the effect of the reaction

products of phenolic antioxidants on human health [1]. Most of these questions have not

been answered yet. As a consequence, more and more attention is paid to the potential

use of natural antioxidants for the stabilization of food, but also of polymers [90-

94,163-166]. These compounds usually can be found in vegetables and fruits, are often

used as spices and have known beneficial effect on human health. Several compounds

with different chemical structures may be considered as natural stabilizers in polymers

and specifically in polyethylene.

Quercetin, i.e. [2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one],

a natural antioxidant might potentially be used for the stabilization of polyolefins. It is

found in fruits, vegetables, leafs and seeds in nature. The compound is a flavonol type

flavonoid, which has proven antioxidant, antiviral and anti-inflammatory effect in the

human body. Some research groups have attempted already to use quercetin as an anti-

oxidant in polyolefins [167,168], while others tried to apply it as a component of active

packaging material [169-172]. In the latter case mostly the migration and dissolution of

the antioxidant was studied in various solvents modeling food. Koontz et al. [167], for

example, homogenized quercetin and its complex with cyclodextrin with PE in a twin-

screw extruder and investigated homogeneity as well as stability. The authors did not find any aggregates in the polymer, stability increased considerably, while oxygen per-

meation decreased simultaneously. Samper et al. [168] studied the thermo- and photo-

oxidative stabilization of polypropylene (PP) with different flavonoids and found that

flavonol type compounds, like quercetin, are the most efficient antioxidants. Lopez de

Dicastillo and co-workers [170] stabilized successfully an ethylene vinyl alcohol copol-

ymer with quercetin.

Although the studies cited above prove that quercetin is a very efficient stabi-

lizer of various polymers, several questions remain open. Quercetin is a polar compound

with a melting temperature of around 320 °C, thus its solubility in the polymer must be

limited. Its efficient homogenization is also questionable under the usual conditions of polyolefin processing, since the additive does not melt. In most studies quercetin was

added at concentrations of 2-3000 ppm [167,168] or in even larger quantities, which

raises again the question of solubility, but also price, since the compound is rather ex-

pensive. As a consequence, the goal of the present work was to study the stabilization

effect of quercetin in a wide concentration range not investigated before. The effect of

this compound was compared to a frequently used commercial antioxidant in an addi-

tive package routinely used in industrial practice in order to determine the possibility as

well as the benefits and drawbacks of using this natural antioxidant in polymer stabiliza-

tion.

Chapter 7

66

The results are presented in several sections. First those of the preliminary ex-

periments are reported which were designed to compare traditional antioxidants with the

natural one. The effect of additive content on efficiency is shown next, followed by the estimation of the solubility of quercetin in PE. The mechanism of stabilization and im-

pact on practice are discussed in the last section of the chapter.

7.2. Preliminary experiments, comparison of quercetin and Irganox 1010

Before launching a larger project we wanted to check, if quercetin stabilizes

polyethylene at all, and in the case of a positive answer we intended to determine its efficiency compared to an industrial antioxidant used routinely. Although all previous

reports indicated considerable stabilization effect, they did not answer the second ques-

tion, efficiency. Four compounds were prepared for this comparative purpose, PE con-

taining the two antioxidants, quercetin and I1010 at 1000 ppm and their combination

with 2000 ppm P-EPQ phosphonite processing stabilizer. The melt flow index of the

polymer processed in the presence of the four additive combinations is presented as a

function of the number of extrusions in Fig. 7.1.

0 1 2 3 4 5 6 70.11

0.13

0.15

0.17

0.19

0.21

MF

I (g

/10

min

)

No of extrusions

I1010

quercetin

with 2000 ppm P-EPQ

Fig. 7.1 Comparison of the effect of 1000 ppm quercetin and 1000 ppm Irganox 1010

on the melt flow index of Phillips polyethylene. Symbols: ( ) quercetin, ( ) I1010,

( ) quercetin with 2000 ppm P-EPQ, ( ) I1010 with 2000 ppm P-EPQ.

Considerable differences are shown in stabilization efficiency when the antioxidants are

used alone, without the processing stabilizer. The MFI decreases in the presence of

I1010 with increasing number of processing steps, while it increases when quercetin is

used as antioxidant. This latter behavior is quite strange and needs explanation. It is

known that Phillips type polyethylenes contain a chain-end double bond on each mole-

Stabilization with quercetin, a flavonoid type natural antioxidant

67

cule, which reacts during processing forming long chain branches. Increasing MFI indi-

cates fewer branches with increasing number of extrusions. Possible explanations might

be the dissimilar stabilization mechanism of quercetin compared to traditional hindered

phenol stabilizers. Hindered phenols are known to react mainly with oxygen centered radicals. Increasing MFI indicates the lack or at least the decrease of the number of

reactions leading to long chain branching, i.e. reaction also with alkyl radicals. The

mechanism of stabilization and this tentative explanation naturally needs further study

and proof. Large efficiency and resupply of reacted stabilizer in subsequent processing

steps due to its limited solubility might also contribute to the increase in MFI observed.

The solubility of most antioxidants is rather small in polyolefins. Irganox 1010, for

example, dissolves in amounts less than those used in industrial practice [173].

Quercetin being more polar than I1010 is expected to dissolve in PE in even lesser

amounts. As a consequence, the stabilizer is dispersed as a separate phase in the form of

droplets, particles or crystals in the polymer. Dissolved stabilizer molecules react during

processing thus preventing long chain branching, oxidation or other degradation pro-

cesses. Reacted molecules are replaced or resupplied from dispersed stabilizers particles in each subsequent step of multiple extrusions, i.e. molecules dissolve from the separate

phase into the polymer matrix. This resupply process appears as increased stability, i.e.

larger MFI and OIT (see later). However, an increase in MFI can occur only, if alkyl

radicals are absent either because their formation is prevented or because they react with

the stabilizer. In the presence of the secondary stabilizer the efficiency of the two stabi-

lizers is practically the same.

The large efficiency of the natural antioxidant is reflected even better in the

residual stability of the polymer. The OIT is plotted against the number of extrusions for

the four compounds in Fig. 7.2. The stability of PE containing quercetin exceeds several

times that of the compounds stabilized with I1010. The effect is especially strong when quercetin is combined with P-EPQ, the phosphorous secondary stabilizer. One may

speculate about the large difference in efficiency which tentatively might be explained

with the dissimilar functionality of the two compounds. However, a simple comparison

based on molar number of functional groups does not make much sense, since we do

not know almost anything about the mechanism of stabilization, and the five functional

groups must have different roles and functions. Literature references indicate indeed

that one quercetin molecule can react only with two radicals and not with five [174].

Increasing stability with increasing number of processing steps also merits some atten-

tion since it implies that these systems become more stable with increased processing.

This is indeed very unusual and requires an explanation as one would normally expect a

decrease in the stability of the system with increased processing due to the reduction in the unspent stabilizer concentration, formation of hydroperoxide species, etc. However,

increasing stability was observed before [175], which could be explained with the poor

solubility of the stabilizer and the resupply of consumed molecules as well as with im-

proving homogeneity with each subsequent processing step. The effect must be even

more pronounced in the case of quercetin, which is more polar than I1010, the stabilizer

used before. Nevertheless, the preliminary study led to the conclusions that quercetin is

a very efficient antioxidant in polyethylene and it is worth to study its effect and effi-

ciency further.

Chapter 7

68

0 1 2 3 4 5 6 70

30

60

90

120

150

OIT

(m

in)

No of extrusions

+ P-EPQ

+ P-EPQ

quercetin

I1010

Fig. 7.2 Effect of the natural antioxidant and I1010 on the residual stability of polyeth-

ylene in the presence and absence of P-EPQ. Symbols are the same as in Fig. 7.1.

7.3. Effect of quercetin content As mentioned in the introductory part, the stabilizing effect of quercetin was

always studied at quite large additive contents at around and above 2000 ppm

[167,168]. Figs. 7.1 and 7.2 indicate that quercetin is more efficient than I1010 already

at 1000 ppm, thus it seemed to be obvious to carry out further investigations in a range

between 0 and 1000 ppm quercetin content. Several groups showed that degradation is

accompanied mainly by the formation of long chain branches and a decrease of MFI in

Phillips polyethylene [176-178]. The related reactions go through the chain-end vinyl

group of the polymer, thus changes in vinyl content indicate sensitively both the modi-

fication of the chemical structure of the polymer and the efficiency of the stabilizer

package used. The vinyl group content of polyethylene studied in this experiment is

plotted against the number of extrusions at selected quercetin contents in Fig. 7.3. All

packages contained 2000 ppm P-EPQ as well. The number of vinyl groups decreases quite rapidly with increasing processing history at small additive concentrations, but the

slope of the decrease becomes smaller as the quercetin content increases. The depend-

ence of vinyl content on the number of processing steps is quite small above 100 ppm

antioxidant content. Obviously quercetin hinders very efficiently the reactions of the

vinyl groups and prevents the formation of long chain branches.

Stabilization with quercetin, a flavonoid type natural antioxidant

69

0 1 2 3 4 5 6 70.70

0.72

0.74

0.76

0.78

0.80

0.82

Vin

yl/

10

00

C

No of extrusions

500,1000 ppm

5

20

100

250

Fig. 7.3 Changes in the vinyl group content of polyethylene as a function of quercetin

content and processing history. Symbols: ( )5, ( )20, ( )100, ( )250, ( )500,

( )1000 ppm quercetin with 2000 ppm P-EPQ.

The effect is clearly visible in Fig. 7.4 presenting the effect of processing histo-

ry on viscosity at various quercetin contents. The correlations are very similar to those

shown in Fig. 7.3 for vinyl groups. The MFI decreases with an increasing number of

extrusions at small concentrations and practically does not change at large quercetin

contents. The MFI is more or less constant already at and above 50 ppm quercetin con-

tent showing the extreme efficiency of this natural antioxidant as a stabilizer in PE. The OIT proportional to the residual stability of the polymer is plotted against the number of

processing steps in Fig. 7.5. The increase in stability with increasing number of extru-

sions is unusual and can be explained by a dissimilar stabilization mechanism, but espe-

cially by the limited solubility of the additive, the resupply of active molecules and

improved homogeneity during subsequent processing steps, as discussed above. Alt-

hough OIT also improves with increasing quercetin concentration, the composition

dependence of this property is slightly different from that of the other two shown in

previous figures; OIT increases more or less linearly with increasing additive content

and at least 250 ppm is needed to achieve practically acceptable levels of stability. We

must keep in mind, though, that different products require different levels of stabiliza-

tion. One way packaging must be protected only during processing and 50 ppm

quercetin is sufficient, while pipes must usually have an OIT of at least 20 min, thus 200-250 ppm is needed in this case. However, the composition dependence of all prop-

erties indicates unambiguously that already much smaller amounts of quercetin stabilize

PE efficiently than those reported before [167,168].

Chapter 7

70

0 1 2 3 4 5 6 70.00

0.05

0.10

0.15

0.20

0.25

MF

I (g

/10

min

)

No of extrusions

0

5

10

15

20

50-1000 ppm

Fig. 7.4 Effect of the number of extrusions and quercetin concentration on the melt flow

index of PE. Symbols: ( )0, ( )5, ( )10, ( )15, ( )20, ( )50, ( )100,

( )250, ( )500, ( )1000 ppm quercetin and 2000 ppm P-EPQ.

0 1 2 3 4 5 6 70

30

60

90

120

150

5-20

50-100

250

500

1000 ppm

OIT

(m

in)

No of extrusions

Fig. 7.5 Changes in the residual stability of polyethylene as a function of quercetin

content and processing history. Symbols are the same as in Fig. 4.

The color of the polymer is plotted against the number of processing steps at

various quercetin contents in Fig. 7.6. The main drawback of this additive is shown

clearly by the figure. Quercetin is inherently yellow and colors also products prepared

with it; discoloration is quite strong at large quercetin contents. It is interesting to note

Stabilization with quercetin, a flavonoid type natural antioxidant

71

that with increasing number of extrusions the yellowness of the samples decreases at

small additive contents, it is more or less constant at 500 and increases at 1000 ppm.

Although the effect is slight, it appears to be consistent and must have some reason. We

may assume that the reaction of the antioxidant yields colorless products and the deple-tion of the stabilizer results in the decrease of yellowness at small quercetin contents.

Obviously, the resupply of active, dissolved molecules from dispersed particles cannot

keep up with their consumption in stabilization reactions at small quercetin concentra-

tions. On the other hand, increasing color at large additive contents indicates that homo-

geneity improves with each processing step; either the particles become smaller or more

molecules are dissolved due to the changing chemical structure (increasing number of

functional groups, reaction products) of the PE compound. The dependence of color on

processing history is in complete agreement with the change of OIT and MFI and indi-

cates the strong effect of solubility in stabilization.

0 1 2 3 4 5 6 720

40

60

80

100

120

Yell

ow

ness

in

dex

No of extrusions

5

20

100

250

500

1000 ppm

Fig. 7.6 Dependence of the color of PE on quercetin content. Effect of the number of

extrusions. Symbols are the same as in Fig. 7.3.

7.4. Solubility and homogeneity of quercetin

Besides color the major problem of the application of quercetin as stabilizer is

its high melting temperature and polarity, which results in limited solubility in apolar

polymers. Although Koontz et al. [167] claimed complete homogeneity at 2000 ppm,

one has slight doubts. Experimental results and the blooming of the additive indicate

that the solubility of Irganox 1010 is around or less than 1000 ppm in polyethylene

[173], thus complete homogeneity or even solubility of quercetin at larger concentra-

tions would be quite surprising. The MFI of the polymer is plotted against quercetin

content in Fig. 7.7 for samples taken after the first and sixth extrusions. After the first

extrusion the MFI of PE containing only 5 ppm quercetin is considerably larger than

that of the material prepared with P-EPQ alone. The MFI reaches a maximum between

Chapter 7

72

15 and 20 ppm quercetin content then decreases slightly to a value which does not

change appreciably above 50 ppm. After six extrusions the MFI of the polymer does not

show the small maximum, but the steep increase and leveling off can be observed as a

function of quercetin content. Differences at small quercetin contents and the composi-tion dependence of MFI indicate that branching reactions control viscosity at small

quercetin concentrations, while solubility plays an important role at larger additive

contents.

0 50 100 150 200 2500.00

0.04

0.08

0.12

0.16

0.20

0.24

MF

I (g

/10

min

)

Quercetin content (ppm)

extrusion 6

extrusion 1

Fig. 7.7 Changes in the melt flow index of polyethylene with quercetin content after the

1th and 6th extrusions. Symbols: ( ) one, ( ) six extrusions.

0 200 400 600 800 10000

20

40

60

80

100

120

Yell

ow

ness

in

dex

Quercetin content (ppm)

15 ppm

Fig. 7.8 Effect of the amount of quercetin on the color of PE after the first extrusion.

Stabilization with quercetin, a flavonoid type natural antioxidant

73

Yellowness index measured after the first extrusion is plotted in Fig. 7.8 in the

same way. Similarly to MFI, or even more so, the slope of color change differs drasti-

cally at small and large additive contents. It seems to be obvious that the dissolved yel-

low molecules of quercetin discolor the polymer much more than dispersed crystals, thus the change in slope must be related to the solubility of the additive in PE. The sol-

ubility value indicated by the arrow in Fig. 7.8 is around 15 ppm. A further proof for the

very limited solubility of quercetin in PE is supplied by Fig. 7.9 showing the optical

micrograph of a PE film containing 1000 ppm quercetin. The needle like crystals of the

additive are clearly visible in the photo. Their presence is not so obvious at smaller

concentrations, most probably because of the limited resolution of optical microscopy.

The consequence of limited solubility is demonstrated well by Fig. 7.10 presenting the

dependence of residual stability on quercetin content after the 1st and 6th extrusions. The

OIT is considerably larger after the 6th processing step because of the resupply of react-

ed quercetin and the better homogeneity of the additive with increasing processing his-

tory. Continuously increasing OIT seems to contradict the composition dependence of

MFI and color and also the effect of limited solubility. We must consider here, however, the different conditions of processing and OIT measurement. Only dissolved molecules

are effective during processing by reacting with the limited amount of oxygen and/or

radicals generated by it. On the other hand, during the OIT measurement oxygen con-

centration is large with constant resupply, thus stabilization reactions must proceed also

on the surface of dispersed particles by scavenging all kinds of radicals and preventing

extensive oxidation. In fact the increase of homogeneity with increasing number of

processing steps may result in the finer dispersion of smaller particles, larger surface

and better stability explaining the increase of OIT in Figs. 7.2 and 7.5. These results

confirm our assumption that the solubility of quercetin is very limited in PE and it must

be considered during the application of this compound as a stabilizer.

Fig. 7.9 Optical micrograph of a polyethylene film containing 1000 ppm quercetin.

Needle crystals of the additive.

Chapter 7

74

0 200 400 600 800 10000

20

40

60

80

100

120

extrusion 1

Ox

idat

ion

in

du

ctio

n t

ime

(min

)

Quercetin content (ppm)

extrusion 6

Fig. 7.10 Effect of quercetin content and processing history on the residual stability of

polyethylene stabilized with quercetin. Symbols: ( ) one, ( ) six extrusions.

7.5. Discussion

The results presented in previous sections unambiguously verify the strong antioxidant effect of quercetin. Its positive effect has been known for the human body

for some time and indicated also for polymers, but its efficiency at very small concen-

trations has not been demonstrated before. The comparison of the effect of Irganox

1010, a traditional phenolic antioxidant, and that of quercetin clearly shows the superi-

ority of the latter. The larger efficiency might be explained by the larger molar func-

tionality of quercetin, but the reactivity of its five hydroxyl groups is not the same, not

all of them participate in stabilization reactions. Previous results showed that ring B (see

Fig. 7.11) is responsible for the antioxidant effect of quercetin, especially if it contains

the catechol moiety [179]. High efficiency probably results from the electron donating

effect of the –OH group located in position 3 in ring C [180, 181]. According to the

latest results, unlike most flavonoids, one quercetin molecule can react with two radi-

cals, which may partly explain its efficiency [174].

Fig. 7.11 Molecular structure of quercetin.

Stabilization with quercetin, a flavonoid type natural antioxidant

75

Polyphenols, like quercetin, may react with radicals according to four mecha-

nisms: hydrogen transfer from the antioxidant to the radical (HAT) [182,183], electron

transfer-proton transfer (ET-PT) [180], sequential proton loss-electron transfer (SPLET)

[184,185] and adduct formation [186]. In apolar solvents, and probably also in polyeth-ylene, HAT is expected to be the dominating mechanism, but the electron affinity of the

radical also influences the actual mechanism. Both carbon and oxygen centered radicals

are present in polyethylene during its processing. The scavenging of the first prevents

the formation of long chain branches, and viscosity can even decrease during pro-

cessing. At least a few long chain branches always form if the stabilizer preferably re-

acts with oxygen centered radicals. In such cases close correlation is expected between

the vinyl group content of Phillips polyethylene and its MFI. The two quantities are

plotted against each other in Fig. 7.12.

0.63 0.66 0.69 0.72 0.75 0.78 0.81 0.840.00

0.05

0.10

0.15

0.20

0.25

MF

I (g

/10

min

)

Vinyl/1000 C

Fig. 7.12 Correlation between the vinyl group content of the polymer and its melt flow

index. Different effects and mechanisms independently of the number of extrusions.

Symbols: ( )Neat, ( )1000 ppm I1010, ( )1000 ppm I1010 and 2000 ppm PEPQ,

( )1000 ppm quercetin, ( )0, ( )5, ( )10, ( )15, ( )20, ( )50, ( )100,

( )250, ( )500, ( )1000 ppm quercetin and 2000 ppm PEPQ; each symbol is related

to results generated in six consecutive extrusion steps, i.e. the effect of processing can

be followed by moving from right to left along the points.

The concentration of vinyl groups decreases gradually during multiple extrusions in an extent depending on the amount of quercetin used for stabilization (Fig. 7.3). The MFI

changes as well and relatively close correlation can be found between the two character-

istics independently of the number of extrusions indeed, which confirms the role of long

chain branching in the determination of MFI. On the other hand, the existence of the

two correlations indicates differences in stabilization mechanism and/or efficiency. All

the points for compounds containing the natural antioxidant fall onto a different correla-

tion as those for the neat polymer, for the compounds containing only P-EPQ, and those

Chapter 7

76

obtained on PE stabilized with I1010 with or without the secondary antioxidant. The

difference appears already at 5 ppm quercetin content. The dissimilar efficiency of the

two additives, i.e. quercetin and I1010, respectively, may be explained with the known

threshold concentration of hindered phenol stabilizers, which does not exist for the non-hindered phenols [53] like quercetin. The larger reactivity of the natural antioxidant

may allow reactions with some of the C-centered radicals as well. However, further,

more detailed study of the stabilization mechanism of quercetin is needed to explain the

reason for the existence of the two correlations in Fig. 7.12.

The increased efficiency of quercetin in the presence of P-EPQ also needs ex-

planation. The small solubility of the antioxidant in PE and its high melting temperature

indicates strong interaction among quercetin molecules. The P-EPQ may interact with

quercetin in one way or other, helping homogenization and increasing reactivity. Some

indication of the existence of such an interaction is offered by Fig. 7.13, in which the

melting traces of the two additives, i.e. quercetin and P-EPQ, is compared to that of

their mixture containing the same amount of each component. The very sharp melting peak of quercetin appearing at around 320 °C becomes diffuse and shifts to a much

lower temperature, to about 235 °C in the mixture. The interaction might influence

solubility, but the composition dependence of color contradicts this, since it indicated a

solubility limit of only 15 ppm (Fig. 7.8). Obviously further study is needed to explain

satisfactorily the stabilization mechanism of quercetin in PE and its interaction with the

phosphonite secondary antioxidant. Nevertheless, we may conclude that the natural

antioxidant studied in this work is a very efficient stabilizer also in polyethylene and

merits further consideration and study.

-50 0 50 100 150 200 250 300 350-5

-2

-1

0

1

quercetin

P-EPQ

quercetin+P-EPQ

En

do

Hea

t fl

ow

(W

/g)

E

xo

Temperature (°C)

quercetin

Fig. 7.13 DSC melting traces of quercetin, the phosphonite secondary stabilizer, and

their mixture containing equal amounts of the two additives. Heating rate: 10 °C/min.

Stabilization with quercetin, a flavonoid type natural antioxidant

77

7.6. Conclusions

The study of the melt stabilization effect of quercetin, a flavonoid type natural

antioxidant, showed that it is a very efficient stabilizer in polyethylene. Quercetin pre-

vents the formation of long chain branches already at a concentration as small as 50

ppm and its dosage at 250 ppm renders the polymer sufficient residual stability. The

efficiency of quercetin is considerably larger than that of Irganox 1010, the hindered

phenolic antioxidant used as reference stabilizer. The difference in efficiency might be

explained with the dissimilar number of active –OH groups on the two antioxidant mol-

ecules, but the stabilization mechanism of quercetin might be also different from that of

I1010. Quercetin interacts with the phosphonite secondary stabilizer used, which pre-sumably improves dispersion and thereby increases efficiency. The mechanism of stabi-

lization and interactions need further study in the future. Besides its advantages,

quercetin has also some drawbacks, it has a very high melting temperature, its solubility

in polyethylene is very small and it gives a strong yellow color to the product. These

disadvantages might be overcome by the use of colorless derivatives and the use of a

solid support to help dosage.

Chapter 7

78

Stabilization with a natural antioxidant, curcumin

79

Chapter 8

Processing stabilization of PE with a natural antioxidant, curcumin

8.1. Introduction

In this chapter the study of the melt stabilizing efficiency of another natural

antioxidant, curcumin is described. Curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-

1,6-heptadiene-3,5-dione, is the principal curcuminoid of Curcuma longa rhizomes

(turmeric). The curcuminoids are natural phenols that cause the yellow color of turmeric

[96]. Curcumin is an oil-soluble pigment, practically insoluble in water at acidic and

neutral pH, and soluble in alkali [97]. Curcumin can exist in several tautomeric forms,

including a 1,3-diketo form and two equivalent enol forms (Table A4). In solid state and

in many solutions the planar enol form is more stable than the nonplanar diketo form

[187], while the keto form predominates in acidic and neutral aqueous solutions and in the cell membrane [188]. Curcumin has superb antioxidant activity which was found to

be stronger for the enol isomer than for the keto form [189]. This antioxidant has radical

and hydrogen peroxide scavenging, as well as metal chelating activities [e.g., 190-193].

Besides scavenging oxygen centered radicals curcumin also scavenges nitrogen cen-

tered free radicals [194,195]. The antioxidant and antifungal activities of curcumin is

enhanced when used in combination with ascorbic acid [196]. Curcumin is not particu-

larly stable to light, especially in solutions [97]. A cyclic as well as other decomposition

products, such as vanillic acid, vanillin, and ferulic acid were detected in its solutions

after photo-irradiation [97,197,198]. The photosensitization of curcumin requires oxy-

gen [199]. In solid state the intramolecular hydrogen bonding provides a channel for fast

deactivation of excited states [200]. In solution the photochemical properties are strong-ly influenced by the type of solvent. In protic solvents, like ethanol, both parts of the

curcumin molecule are involved in the excited singlet state. Curcumin photogenerates

singlet oxygen, superoxide, as well as some carbon-centered radicals in this solvent

[200].

Powdered turmeric rhizomes is used as food colorant, spice and food preserva-

tive. The medical activity of curcumin has been known since ancient times. As a conse-

quence, the different effects and reactions of curcumin have been the subject of investi-

gation in the fields of biology, medicine, pharmacology, as well as in the food industry

for many years yielding a large number of publications. However, the actual reaction

site and the mechanism of free radical scavenging have not been clarified yet. H-atom donation was found as a preferred reaction of curcumin at pH ≤ 7 and in nonprotic sol-

vents [188]. Barclay et al. [201] observed that the antioxidant activities of o-methoxy-

phenols decreased in hydrogen bond accepting media. According to some authors the

OH groups on the two phenyl rings participate in the reactions [e.g.,189,190,201], oth-

ers claim that the β-diketone moiety is responsible for the antioxidant action [188],

while other publications [202,203] indicate that the strong antioxidant activity of

curcumin originates mainly from the phenolic OH groups, but a small fraction is due to

the >CH2 site in the β-diketone moiety. Not only the site but also the reaction mecha-

nism of antioxidant activity is controversial. Some authors [e.g., 188,202,204] proposed

Chapter 8

80

H-atom transfer (HAT), some others [205] suggested single electron transfer (SET)

mechanism, while Litwinienko and Ingold [206,207] elaborated the Sequential Proton

Loss Electron Transfer (SPLET) theory.

The antioxidant activity of curcumin is usually studied in solution at ambient

temperature. The question is whether this natural antioxidant can keep its superb stabi-

lizing efficiency under the conditions of polyolefin processing (190-260 °C, shear). We

could not find any publication dealing with this aspect. In this work the stabilization

effect of curcumin was studied in polyethylene processed by multiple extrusions. The

natural antioxidant was investigated as a single stabilizer and in combination with a

phosphorous secondary antioxidant, which plays an important role in the processing

stabilization of polyethylene [80,81]. The stabilizing efficiency of curcumin was com-

pared to that of a commonly used synthetic phenolic antioxidant.

8.2. Stability and interactions of curcumin

We made an attempt to prove the H-bonding capability of curcumin by an FT-

IR study carried out in methanol solution. Unfortunately we failed completely, because

of the intense absorption of methanol made the determination any shift in absorption

bands impossible. However, thermal analysis offered an indirect way to prove the exist-

ence of such molecular interactions. The melting traces of curcumin, P-EPQ, and the 1:1

mixture of curcumin and P-EPQ (Fig. 8.1) were recorded under nitrogen after melting the materials in Ar atmosphere at 220 °C for 1 min followed by cooling to ambient

temperature.

-50 0 50 100 150 200 250 300

En

do

H

eat

flo

w

Ex

o

0.5 W/g

Curcumin + P-EPQ

P-EPQ

Curcumin

Temperature (°C)

Fig. 8.1 Thermograms (melting) of curcumin, P-EPQ, and a 1:1 mixture of curcumin

and P-EPQ measured under nitrogen after melting in Ar atmosphere at 220 °C for 1 min

and then cooling down to ambient temperature. Heating rate: 10 °C/min.

Stabilization with a natural antioxidant, curcumin

81

The blend of the two additives, i.e. curcumin and P-EPQ was studied in a similar way.

Equal amounts of the two powders were thoroughly mixed and melted under the condi-

tions indicated above (220 °C, Ar). We assumed that after melting the mixture became

homogeneous. Curcumin fuses at 179 °C then starts to decompose above 190 °C with an exothermal peak at 277 °C. P-EPQ does not crystallize, it goes through glass transition

with an overheating peak at 69.5 °C. The heat flow does not change up to 265 °C at

which the substance starts to decompose giving an exothermal peak with a maximum at

around 293 °C. Interaction between curcumin and P-EPQ in the mixture is indicated by

the shift of the glass transition of P-EPQ to 66 °C and the disappearance of the fusion

peak of curcumin. The mixture of the two additives is thermally less stable than the

components themselves. These phenomena could be the result of H-bonding or other

intermolecular interactions. The reactions resulting in exothermal heat flow start at

around 140 °C and show a maximum at 207 °C.

Curcumin was aged at ambient temperature in powder form and in methanol

solution in dark and light for 147 days. The UV-VIS absorption spectra shown in Fig. 8.2 reveal that the chemical structure of the antioxidant does not change significantly

when it is stored in powder form either in dark or in light for five months, which, ac-

cording to literature references, can be explained with intramolecular hydrogen bonding

that provides a channel for fast deactivation of excited states [200]. The oxygen penetra-

tion into the crystals is also slower in the solid resulting in slower decomposition, be-

cause of the absence of oxygen. Similarly, only a small change can be observed when

curcumin is dissolved in methanol and stored in dark (Fig. 8.2). On the other hand,

photochemical decomposition is revealed by the UV-VIS spectra when the antioxidant

is stored in methanol solution in light (Fig. 8.3). The fragmentation of the molecule

[198] is proved by the fast decrease in the intensity of the absorption bands appearing at

424 and 262 nm and the increase in those detected at 232 and 200 nm wavelengths.

In order to confirm the specific effect of H-bonding on the stability of

curcumin, we carried out an ageing study in various solvents. Fig. 8.4 shows that in the

two solvents which can form strong H-bonds with curcumin, i.e. in ethanol and metha-

nol, degradation is much slower than in any of the other solvents thus proving the im-

portant role of specific interactions on curcumin stability. These results are in accord-

ance with the work of Nardo et al. [208]. Electronically excited β-diketones can deacti-

vate in the fastest way by excited-state intramolecular proton transfer (ESIPT). Intermo-

lecular H-bond formation heavily perturbs intramolecular H-bonding. The higher the

ESIPT rate is the tighter the intramolecular H-bond is between the enol and the keto

moiety.

Chapter 8

82

200 300 400 500 6000.0

0.2

0.4

0.6

0.8

Ab

sorb

an

ce

Wavelength (nm)

Fig. 8.2 UV-VIS spectra of curcumin recorded before aging (), and after aging at

ambient temperature for 147 days in powder form in dark (− − −) and light (−· ·), as

well as in methanol solution in dark (····).

200 300 400 500 6000.0

0.2

0.4

0.6

0.8424

262

232

~200

Ab

sorb

an

ce

Wavelength (nm)

Fig. 8.3 UV-VIS spectra of curcumin measured before aging (), and after aging in

methanol solution in light at ambient temperature for various times: − − − 17, −· · 28,

−·· ·· 56, and ···· 147 days.

Stabilization with a natural antioxidant, curcumin

83

0 10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

1.2

Rel

ativ

e ab

sorb

ance

, A

42

3

Ageing time (day)

Fig. 8.4 The stabilization effect of H-bonds on the light stability of curcumin in solu-tion. Solvents: () ethanol, () methanol, () acetone, () acetonitrile, () toluol,

() chloroform

8.3. Processing stabilization

The vinyl group concentration of the nascent polymer powder decreases the

most significantly (from 0.983 to 0.805 vinyl/1000 C) in the first extrusion then the change becomes smaller in further processes (Fig. 8.5).

0 1 2 3 4 5 6 70.74

0.76

0.78

0.80

0.82

0.84

Vin

yl/

10

00

C

Number of extrusions

Fig. 8.5 Effect of the number of extrusions on the vinyl group content of polyethylene

stabilized with 1000 ppm curcumin (○), 1000 ppm curcumin + 2000 ppm P-EPQ (●),

1000 ppm I1010 (□), and 1000 ppm I1010 + 2000 ppm P-EPQ (■).

Chapter 8

84

The total decrease in the vinyl group concentration is only 0.025 vinyl/1000 C in the

five consecutive extrusions. The composition of the stabilizer system does not influence

significantly the vinyl group content of the polymer. This result means that curcumin

does not change the reactions of the vinyl groups during processing compared to the effect of I1010.

Curcumin hinders the oxidation of polyethylene during processing more effi-

ciently than I1010 (Fig. 8.6). Carbonyl groups do not form in its presence or when its

combination with P-EPQ is used, while the oxidation of the polymer stabilized with the

synthetic phenolic antioxidant is proved by the carbonyl concentration which is larger

than the one caused by 1000 ppm I1010 (0.1 mmol CO/mol PE). The degree of oxida-

tion is smaller when I1010 is combined with the phosphonite antioxidant.

0 1 2 3 4 5 6 70.0

0.1

0.2

0.3

0.4

C

=O

(m

mo

l/m

ol

PE

)

Number of extrusions

Fig. 8.6 Effect of the number of extrusions on the carbonyl group content of polyeth-

ylene stabilized with 1000 ppm curcumin (○), 1000 ppm curcumin + 2000 ppm P-EPQ

(●), 1000 ppm I1010 (□), and 1000 ppm I1010 + 2000 ppm P-EPQ (■).

Similarly to vinyl group content, the melt flow index of the polymer decreases

the most significantly in the first extrusion, in an extent depending on the amount of P-

EPQ (from 0.32 g/10 min to 0.17 and 0.20 g/10 min at 0 and 2000 ppm P-EPQ, respec-tively), as shown in Fig. 8.7. The addition of the phosphonite enhances the melt stabiliz-

ing efficiency of both antioxidants, while the type of the phenol derivative does not

influence the change significantly. In further extrusion processes the change in MFI is

much smaller and the type of the phenol plays an essential role in its direction. In the

presence of I1010 the decrease in MFI continues, while some increase can be observed

with curcumin. This latter result indicates that besides chain extension processes (char-

acteristic for the degradation of Phillips type polyethylenes) chain breaking also takes

place. Similar conclusions can be drawn from the changes in creep viscosity plotted as a

function of the number of extrusions in Fig. 8.8. 0 increases considerably when I1010

Stabilization with a natural antioxidant, curcumin

85

is applied as a phenolic antioxidant, while it increases only slightly when the polymer is

stabilized with curcumin or does not change when the curcumin/P-EPQ combination is

used.

0 1 2 3 4 5 6 70.10

0.15

0.20

0.25

0.30

0.35

MF

I (g

/10

min

)

Number of extrusions

Fig. 8.7 Effect of the number of extrusions on the melt flow index of polyethylene stabi-

lized with 1000 ppm curcumin (○), 1000 ppm curcumin + 2000 ppm P-EPQ (●), 1000

ppm I1010 (□), and 1000 ppm I1010 + 2000 ppm P-EPQ (■) Neat powder (▲).

0 1 2 3 4 5 6 70

1

2

3

4

Cre

ep

vis

co

sity

,

0 x

10

-5 (

Pa s

)

Number of extrusions

Fig. 8.8 Effect of the number of extrusions on the creep viscosity of polyethylene stabi-

lized with 1000 ppm curcumin (○), 1000 ppm curcumin + 2000 ppm P-EPQ (●), 1000

ppm I1010 (□), and 1000 ppm I1010 + 2000 ppm P-EPQ (■).

Chapter 8

86

The residual thermo-oxidative stability of the polymer characterized by the

oxidation induction time is influenced by both types of antioxidants (Fig. 8.9). Without

phosphonite the phenolic antioxidants provides only moderate thermo-oxidative stabil-

ity to the polymer, although stability is independent of the number of extrusions. OIT increases considerably when phenol/phosphonite antioxidant combinations are applied.

With increasing number of extrusions the phosphonite is consumed and the thermo-

oxidative stability of the polymer decreases. Fig. 8.9 also reveals that the same amount

of curcumin is somewhat more efficient oxidative stabilizer than I1010, either alone or

in combination with P-EPQ.

0 1 2 3 4 5 6 70

10

20

30

40

50

60

OIT

at

20

0 °

C (

min

)

Number of extrusions

Fig. 8.9 Effect of the number of extrusions on the residual thermo-oxidative stability of

polyethylene stabilized with 1000 ppm curcumin (○), 1000 ppm curcumin + 2000 ppm

P-EPQ (●), 1000 ppm I1010 (□), and 1000 ppm I1010 + 2000 ppm P-EPQ (■).

The yellowness index of the samples is plotted as a function of the number of

extrusions in Fig. 8.10. I1010 discolors the polymer in a given degree, as known, and

yellowness index increases with increasing number of extrusions. The addition of P-

EPQ to I1010 improves the color stability of the polymer, but some gradual yellowing

cannot be avoided during multiple extrusions even in this case. Due to its original yel-low color, curcumin discolors the polymer quite strongly. The addition of P-EPQ to

curcumin does not change yellowness index. In contrast to the effect of I1010, the YI of

the samples stabilized with curcumin slightly decreases with increasing number of ex-

trusions indicating that the heptadienone linkage between the two methoxyphenol rings

of curcumin also participates in the chemical reactions taking place during the pro-

cessing of polyethylene.

Stabilization with a natural antioxidant, curcumin

87

Fig. 8.10 Effect of the number of extrusions on the yellowness index of polyethylene

stabilized with 1000 ppm curcumin (○), 1000 ppm curcumin + 2000 ppm P-EPQ (●),

1000 ppm I1010 (□), and 1000 ppm I1010 + 2000 ppm P-EPQ (■).

8.4. Discussion

The effect of curcumin on the processing stability of polyethylene can primari-

ly be explained with its ability to scavenge free radicals. The oxidation of the

macroradicals formed as an effect of heat and shear cannot be hindered in the first ex-

trusion of the polymer powder, as it contains absorbed oxygen at a relatively large con-

centration [40]. As a consequence, the reactions of the polymer are controlled primarily

by the number of unsaturated groups and the efficiency of the hydroperoxide decompos-

ing phosphorous secondary antioxidant [80,81]. In further processes, in which thermal

reactions dominate, the reaction mechanism of the phenolic antioxidant plays an im-

portant role. In the case of I1010 the abstraction of the H-atom from the phenolic OH

group proceeds slower or with similar rate as the addition of macroradicals to vinyl groups resulting in the formation of long chain branches, an increase of viscosity and

decrease of MFI. Due to its complex structure curcumin may participate in several types

of reactions. It can donate H atoms not only from the phenolic OH groups but also from

the linear linkage between the two methoxyphenyl rings. β-diketone radicals are as-

sumed to form by H atom donation from the CH2 group of the heptadienone linkage

[202] and they may react with the vinyl groups of the polymer and/or with

macroradicals. Further radicals can form also by the thermal decomposition of

curcumin, which is not the subject of investigation in the literature, since degradation is

studied practically always in solution, generally at ambient temperature [198,209]. Fur-

ther possibility is the addition of macroradicals to the unsaturated groups of the linear

linkage of curcumin. The reactions of the radicals derived from curcumin through reac-tions with the vinyl group of the polymer, and the addition of macroradicals to the dou-

ble bonds of curcumin can hinder the formation of long chain branches. In this case the

Chapter 8

88

molecular mass of the polymer decreases, which results in an increase of MFI and de-

crease of discoloration. The exact processing and thermo-oxidative stabilization mecha-

nism of curcumin in polyethylene needs further studies. Model experiments are in pro-

gress, and our first results show that curcumin is able to scavenge alkyl radicals indeed, while Irganox 1010 cannot. This additional capability may result in the larger efficiency

of curcumin compared to that of I1010.

8.5. Conclusions

Curcumin, a natural antioxidant, is stable as a crystalline solid at ambient tem-

perature in air. It keeps its stability in different kind of solutions when stored in dark, but decomposes in light. Its rate of decomposition depends on the solvent used.

Curcumin is thermally stable up to 190 °C, but the interaction with a phosphonite stabi-

lizer results in some decrease in the starting temperature of thermal decomposition.

Curcumin is a more efficient melt and thermo-oxidative stabilizer of polyethylene than

the synthetic phenolic antioxidant used as reference (I1010), and the addition of a

phosphonite secondary stabilizer improves its efficiency further. Although the number

of vinyl groups participating in chemical reactions during processing is similar in the

case of the two phenolic derivatives, changes in the rheological properties and color of

the polymer reveal that the mechanism of their stabilization reactions differs. The syn-

thetic phenolic antioxidant used cannot hinder reactions leading to the formation of long

chain branches. In the presence of curcumin the melt flow index of the polymer increas-es and its color decreases with increasing number of extrusions. These changes indicate

that besides the phenolic OH groups also the linear linkage between the two methoxy-

phenyl rings of curcumin participates in the stabilization reactions.

Performance of β-carotene under processing and storage conditions

89

Chapter 9

Performance of -carotene in PE under processing and storage conditions

9.1. Introduction

Polyethylene is one of the main components of modern food packaging materi-

als used for various purposes. Active packaging films can be produced by incorporating

natural antioxidants into the polymer matrix resulting in quality improvement in differ-

ent ways. Some examples: 1) The shelf-life of oxygen-sensitive products can be pro-

longed by using radical scavenging natural antioxidants [163,210-212]. 2) The migra-

tion of antioxidants (like vitamins, flavonoids) from the packaging material to the pack-aged goods may be also beneficial. 3) Most of the natural antioxidants are colorful sub-

stances. Intelligent packaging materials may be produced with their application, as the

loss in color can indicate the expiration of guarantee for the product.

-Carotene is one of the colorful carotinoids with antioxidant activity. It is a strongly-colored red-orange pigment present in many plants and fruits, like carrots,

pumpkins, or sweet potatoes. -Carotene is the pro-vitamin of vitamin A and is widely used as a colorant in foods and beverages. It is a non-polar highly conjugated lipophilic

hydrocarbon compound (Table A4). The eleven conjugated C=C double bonds serve as

a chromophore moiety in the molecule, which is responsible for the light absorption and

the color of -carotene [213]. Accordingly, -carotene has a characteristic UV-VIS

spectrum, in which the position of the principal absorption maximum (max) depends on the configuration [105,106] and oxidation of the molecule [214,215], as well as on the

type of the solvent used [214,216] according to the changes in transition energies be-

tween the ground and excited states.

The melting point of -carotene is higher than 180 °C, but melting is not re-

versible and the true melting point cannot be determined [214]. Pure crystalline -carotene (purified by fractional dissolution) is black but undergoes fast direct oxidation

by air in the crystal phase even at low temperatures (-80 °C) in dark. Oxidation results in a change of color to orange or red, the powder becomes similar in appearance and

composition to laboratory samples available commercially [214]. The oxidation of -carotene with molecular oxygen is an autocatalytic free radical process which leads to

the formation of different epoxycarotenoids, apocarotenals, small molecular mass prod-

ucts, and some oligomeric/polymeric materials, some of them representing the key aro-

ma compounds in certain flowers, vegetables, and fruits [217-222]. The initial site of the

oxygen attack on the β-carotene molecule occurs at the terminal double bonds resulting

in predominant formation of epoxides [219,223]. These and long chain β-apocrotenals

formed in the first stage of oxidation decompose to lower molecular mass carbonyl-

containing compounds in further oxidation reactions [219]. The pathway of cleavage of

the molecule (central or random) is affected by the surrounding medium (type of sol-

vent), as well as the absence or presence of a phenolic antioxidant [224]. Phenolic anti-

oxidants, like BHT and α-tocopherol, inhibit the oxidation of β-carotene [219,224]. -carotene is more resistant to heat than to oxygen [225,226]. Besides isomerization of

Chapter 9

90

cis- to trans-isomer, a few high molecular mass components are formed at high tempera-

tures in nitrogen atmosphere, while polymerization and decomposition reactions take

place simultaneously in air [217,225]. As an effect of UV irradiation the C=C and C-C

bonds of the hydrocarbon chain break leading to a loss of color [213]. The UV stability

of -carotene can be improved by the addition of a photostabilizer which absorbs light

below 465 nm [213,227].

-Carotene is considered as a chain-breaking antioxidant, as it exhibits high reactivity towards carbon-centered, peroxy, alkoxy, and NO2 radicals [228]. It can even

prevent the photosensitization of human skin [229]. According to Ozhogina and

Kasaikina [220] -carotene reacts primarily by the addition of free radicals to its conju-gated system. The radical-trapping behavior of β-carotene depends on the partial pres-

sure of oxygen [230,231]. It proved to be a good radical-trapping antioxidant at partial

pressures lower than the pressure of oxygen in normal air. At higher oxygen pressures,

β-carotene loses its antioxidant activity and shows an autocatalytic, prooxidant effect,

particularly at relatively high concentrations. The mixtures of -carotene, -tocopherol, and ascorbic acid provide better protection against protein oxidation than any of the

components alone [231,232].

Although the antioxidant activity of -carotene has been investigated mainly in food and medical sciences, some experiments have already been carried out also in

polymeric materials. Abdel-Razik [233] found that -carotene increases the thermo-oxidative stability of ABS copolymer and explained the effect by the formation of

biradicals which react rapidly with oxygen molecules inhibiting the formation of alkoxy

and peroxy radicals from the polymer. López-Rubio and Lagaron [234] observed the

UV stabilizing effect of -carotene in biopolyesters [poly(lactic acid), polycaprolactone,

and polyhydroxybutyrate-co-valerate]. -carotene plasticize these biopolymers already

at small concentrations (0.4 w/w %) and a part of the effect is retained even after UV-VIS irradiation.

Preliminary experiments run in our laboratory revealed that -carotene im-proves the processing stability of polyethylene. The positive effect is further enhanced

by the addition of a phosphonite secondary antioxidant; the efficiency of this additive

combination proved to be similar to that of the synthetic phenolic antioxidant package

frequently used in industrial practice. Gradual loss of the original orange color of films

stored in an artificially lighted room was observed in the experiment, which was at-

tributed to the decomposition of -carotene. As α-tocopherol inhibits oxidation of β-carotene [219,224], and a phosphonite antioxidant enhances its stabilizing efficiency,

the effect of β-carotene on the characteristics of polyethylene was investigated in the

presence of these additives in this work. Two main questions were studied: 1) the effect

of the concentration of β-carotene on the processing stability of polyethylene; 2) chang-

es in polymer characteristics during storage at ambient temperature.

9.2. Processing stability

The polymer and the additives participate in different chemical reactions dur-

ing the processing of polyethylene. These reactions are influenced significantly by the

Performance of β-carotene under processing and storage conditions

91

antioxidant combination and the initial concentration of β-carotene. Without any antiox-

idant the vinyl content of the nascent polymer powder drops from 0.91 to 0.66 vi-

nyl/1000 C after processing. The protecting effect of the -tocopherol/phosphonite antioxidant pair is well reflected in the reduced change to 0.88 vinyl/1000 C at 0 ppm β-

carotene. With the addition of -carotene to the antioxidant package the concentration

of vinyl groups decreases slightly reaching a value of 0.84 vinyl/1000 C at 2000 ppm -carotene concentration. The residual concentration of phosphonite antioxidant decreas-

es, as well. Similarly to our earlier studies on polyethylene stabilization [81], the num-

ber of vinyl groups changes linearly with the residual amount of phosphonite. β-Carotene also participates in the chemical reactions taking place during the processing

of polyethylene. The loss in -carotene derived from the absorption intensities at 456 nm in the UV-VIS spectra (Fig. 4.2) increases linearly with its initial concentration and

it is around 50-60 wt % at each additive package. Figure 9.1 shows that the reactions of

vinyl groups and phosphonite are directly related to those of -carotene.

Fig. 9.1 Effect of the reactions of -carotene on the consumption of phosphonite antiox-idant (●) and the decrease in the vinyl group concentration of polyethylene (■) during

processing at 190 °C.

The melt flow index of the nascent polymer powder (0.32 g/10 min) decreases

to 0.13 – 0.14 g/10 min independently of the -carotene content after processing (Fig. 9.2). This is a smaller change than without any stabilizer where MFI drops to 0.06 g/10

min. The independence of MFI from -carotene concentration indicates that the reac-tions of vinyl groups do not always lead to the chain extension of the polymer, though

the latter is characteristic for the degradation of Phillips type polyethylenes [81]. This

can be attributed to the radical trapping ability of -carotene at low oxygen concentra-

tions [220,230,231]. -carotene protects the polymer from oxidation during processing. In the absence of β-carotene the polymer oxidizes slightly (0.042 mmol C=O/mol PE),

0 200 400 600 800 1000 12000.02

0.04

0.06

0.08

0.10

0.12

0.02

0.04

0.06

0.08

0.10

0.12

Rea

cted

vin

yl

gro

up

/10

00

C

Rea

cted

ph

osp

ho

nit

e x

10

-4 (

pp

m)

Reacted -carotene (ppm)

Chapter 9

92

but carbonyl groups are not formed when the concentration of reacted -carotene is

200 ppm. These results can be explained by the reactivity of -carotene towards oxy-gen and different radicals (carbon-centered, oxy) [219,220,228]. β-Carotene reacts fast

with molecular oxygen present in small concentrations in the extruder and hinders the

formation of peroxy macroradicals which are the precursors of oxidized derivatives. The

reactivity of -carotene towards oxygen leads to an opposite effect at high oxygen con-centrations. The OIT measured at 200 °C in pure oxygen atmosphere decreases with

increasing concentration of unreacted -carotene (Fig. 9.3), which confirms that -carotene looses its antioxidant activity and has a prooxidant effect at large oxygen con-

centrations [230,231].

0.84 0.86 0.88 0.90 0.920.10

0.15

0.20

0.25

0.30

0.35

MF

I (g

/10

min

)

Vinyl/1000C

Increasing concentration

of -carotene

Nascent

PE powder

Fig. 9.2 Correlation between the vinyl group concentration and the melt flow index of

polyethylene before and after processing with additive packages containing -tocopherol/phosphonite antioxidant package and different amounts of β-carotene.

-carotene colors polyethylene already at small concentrations due to its eleven conjugated C=C double bonds absorbing light in the blue-green region. Although the

absorption intensity at 456 nm increases linearly with the amount of -carotene added to the polymer (Fig. 4.2), the yellowness index determined from the color coordinates

changes non-linearly (Fig. 9.4). 250 ppm -carotene increases the yellowness index from 2.5 to 135 and turns the visible color from colorless to orange-yellow. With further

addition of -carotene the color deepens gradually and turns from orange-yellow to red-orange. At the same time yellowness index increases more moderately and reaches a

value of YI = 175 at a -carotene concentration of 2000 ppm. This character of the

yellowness index indicates limited solubility of -carotene in polyethylene. From the concentration dependence of YI we can deduce that the solubility is below 200 ppm.

Performance of β-carotene under processing and storage conditions

93

0 200 400 600 800 100010

20

30

40

50

OIT

at

20

0 °

C (

min

)

Residual -carotene (ppm)

Fig. 9.3 Correlation between the residual concentration of -carotene after processing

and the thermo-oxidative stability of polyethylene measured at 200 °C in oxygen.

0 200 400 600 800 10000

50

100

150

200

Yel

low

nes

s in

dex

Residual -carotene (ppm)

Fig. 9.4 Effect of the concentration of -carotene on the color of polyethylene pellets

stabilized with -tocopherol/phosphonite antioxidant combination; yellowness index and visual color.

Summarizing the results we can conclude that -carotene in combination with

-tocopherol and phosphonite antioxidants inhibits the oxidation of polyethylene, does not affect its melt flow index, increases the number of phosphorous stabilizer molecules

participating in chemical reactions and colors the polymer to orange during processing.

Chapter 9

94

9.3. Storage at ambient temperature

The compressed film and sheet samples were stored at ambient temperature in

dark and in light for various lengths of times. UV-VIS spectra and yellowness index

were measured regularly as a function of storage time.

Some of the UV-VIS spectra taken from the film (100 m thick) containing

1500 ppm -carotene stored in dark for different times are shown in Fig. 9.5. In the first

period (8 days) isomerization of -carotene is revealed by the decrease in the intensity of the absorption band between 300 and 400 nm (characteristic of the cis-isomer) and

the increase of the one appearing between 400 and 600 nm. The maximum of the main

band shifts from 456 nm to 460 nm. This introduces some uncertainties in the determi-

nation of the concentrations of -carotene from the UV-VIS spectra but deviations do

not exceed 10 %. In the further period of storage in dark slow loss in the -carotene concentration occurs which is less than 150 ppm after 133 days. The occurrence of

some reactions of -tocopherol is indicated by the increase in the intensity of the ab-sorption between 250 and 300 nm and the shift of the peak maximum from 272 nm to

270 nm.

200 300 400 500 600

0.5

1.0

1.5

2.0

Ab

sorb

ance

Wavelength (nm)

Fig. 9.5 UV-VIS spectra of polyethylene films of 100 m thickness stabilized with 500

ppm -tocopherol + 2000 ppm phosphonite + 1500 ppm -carotene stored at ambient temperature in dark for 0 (—), 8 (– –), 20 (–·–), and 64 (–··) days.

The yellowness index values of 1 mm thick plates stored in dark are shown as a

function of time in Fig. 9.6. An increase of YI can be observed even for the sample

stabilized with -tocopherol/phosphonite antioxidant combination without -carotene.

In this latter case yellowing of polyethylene can be explained by the reactions of -

Performance of β-carotene under processing and storage conditions

95

tocopherol, as its transformation products are more colored than the parent antioxidant

[48].

0 50 100 150 2000

5

140

160

180

200

Yel

low

nes

s in

dex

Time of storage (day)

Fig. 9.6 Discoloration of 1 mm thick polyethylene sheets processed with -tocopherol/

phosphonite antioxidant package and -carotene of 0 (), 250 (), 500 (∆), 1000 (○),

1500 (), and 2000 (□) ppm during storage at ambient temperature in dark.

0 200 400 600 800 100040

60

80

100

120

Ind

uct

ion

tim

e o

f Y

I (d

)

Residual ß-carotene after processing (ppm)

Fig. 9.7 Effect of the concentration of -carotene on the time elapsing before accelerat-ed change in the yellowness index (induction time) of 1 mm thick polyethylene plates

stored at ambient temperature in dark (▲) and light (■).

Chapter 9

96

YI values of the polymer processed with -carotene increase sigmoidally with storage time indicating autocatalytic reactions. The time elapsed before the start of ac-

celerated change in YI (induction time) increases with increasing residual concentration

of -carotene measured after processing (Fig. 9.7). The differences in YI measured

before and after storage in dark for 182 days are ∆YI ≈ 25-30 at -carotene concentra-

tions below 600 ppm and decrease at higher -carotene contents. The shift of the main

UV-VIS absorption peak of -carotene to higher wavelengths and the increase in YI can be attributed to the reaction of β-carotene with oxygen. As the terminal double bonds of

-carotene are less stable than the central one (because of resonance) [235] and the C-H bond is the weakest at the α-position next to the double bond [236], we may assume that

the oxidation starts there. One or two peroxy radicals then hydroperoxide groups are

formed which participate in further reactions. The reaction products formed in dark do

not contain carbonyl groups. The changes in color characteristics confirm that the reac-tions take place in the terminal rings of the molecule and may originate from an increase

in the number of conjugated double bonds in the reaction products or from a decrease in

solubility and separation of reacted molecules from the solution state. The decrease in

YI with increasing concentration of -carotene above 600 ppm suggests higher proba-bility of the latter case. Further experiments are in progress to explore the mechanism of

reactions in dark.

The chemical structure and the melt flow properties of the polymer do not

change significantly during storage in dark for 8 months. Although vinyl groups partici-

pate in some reactions resulting in a slight decrease in their concentration (-cvinyl= 0.03-0.07 vinyl/1000 C) but the polymer does not oxidize and its melt flow index re-

mains practically constant (MFI=0.01 g/10 min). From these results we can conclude that the reactions of additives leading to some deepening of color during storage in dark

do not affect the stabilizing efficiency of antioxidants which controls the characteristics

of polyethylene.

The degradation of -carotene in light is fast, occurs heterogeneously, and

results in fading of polyethylene. The time of the complete loss of color depends on the

concentration of -carotene and the thickness of the sample. Fig. 9.8 shows some UV-

VIS spectra of 100 m thick films processed with 1500 ppm -carotene and stored in

light at ambient temperature for different times. First (8 days of storage) isomerization

of -carotene occurs revealed by the decrease in the intensity of the absorption band in the region of 300-400 nm (cis-isomer) and the increase of the band appearing between

400 and 600 nm. The maximum of the main absorption band shifts from 456 nm to 460

nm and then to 462 nm during further storage. Fast decomposition of -carotene is re-vealed by the decrease and finally the complete disappearance of the absorption band

between 300 and 600 nm in 25 days. The maximum of the absorption band of -tocopherol shifts gradually from 272 nm to 267 nm indicating its participation in chemi-

cal reactions. The concentration of -carotene determined from the intensity of the main absorption peak is plotted as a function of storage time in Fig. 9.9. We can see that the

reactions of -carotene are autoaccelerated and the increase in the initial concentration results in an increase of decomposition rate.

Performance of β-carotene under processing and storage conditions

97

200 300 400 500 600

0.5

1.0

1.5

2.0

Ab

sorb

ance

Wavelength (nm)

Fig. 9.8 UV-VIS spectra of 100 m thick polyethylene films processed with -

tocopherol/ phosphonite antioxidant package and 1500 ppm -carotene stored at ambi-ent temperature in light for 0 (—), 8 (– –), 15 (··–··), 20 (····), 22 (----), and 25 (–·–)

days.

0 5 10 15 20 25 300

200

400

600

800

1000

-c

aro

ten

e co

nce

ntr

atio

n (

pp

m)

Time of storage (day)

Fig. 9.9 Residual concentration of -carotene in 100 m thick polyethylene films pro-

cessed with -tocopherol/phosphonite antioxidant package and -carotene of 250 (), 500 (▲), 1000 (●), 1500 (▼), and 2000 (■) ppm, stored at ambient temperature in light.

Chapter 9

98

Changes in the yellowness index of 1 mm thick plates confirm the autoaccel-

erated nature of -carotene decomposition in light (Fig. 9.10).

0 50 100 150

0

50

100

150

200

Yel

low

nes

s in

dex

Time of storage (d)

Fig. 9.10 YI of 1 mm thick PE plates contain -carotene of 0 (), 250 (), 500 (∆),

1000 (○), 1500 (), and 2000 (□) ppm, stored at ambient temperature in light.

YI of the polymer stabilized with -tocopherol/phosphonite antioxidants increases

gradually as a function of storage time due to the reactions of -tocopherol. In the pres-

ence of -carotene, following an induction time the yellowness index drops unexpected-

ly fast and almost independently of the concentration of -carotene to a value corre-

sponding to the polymer processed without this additive. Two points must be empha-sized here. The length of the induction period, as well as the overall decomposition time

of -carotene do not depend only on its concentration measured before storage but also

on the thickness of the polymer sample. The time of complete decomposition of -

carotene in light is around 30 days in 100 m thick films independently of the initial concentration (Fig. 9.9), while it changes from 80 to 170 days with increasing initial

concentration in 1 mm thick plates (Fig. 9.10). It is also remarkable, that the length of

the induction time of YI is similar in dark and light as shown in Fig. 9.7. This reveals

that the reactions of -carotene are governed primarily by the composition of the addi-tive package and the thickness of the sample, and depend less on the presence of light in

the induction period. As we deduced above, peroxy radicals and hydroperoxide groups

in the allylic position to the terminal double bonds can be expected to form in the induc-

tion period of oxidation. In this stage the rate of oxidation must be determined primarily

by the concentration of oxygen which decreases with increasing film thickness, i.e.,

with the decrease in the surface/volume ratio. The eleven conjugated double bonds of -carotene absorb a part of visible light and get into excited state. In this state the mole-

cule becomes more sensitive to oxygen attack. After the induction period the rate of

oxygen uptake accelerates and decomposition becomes the main direction of degrada-

tion in light.

Performance of β-carotene under processing and storage conditions

99

The radiation of light does not influence only the reactions of the additives but

also the characteristics of polyethylene measured after storage for 8 month. The concen-

tration of vinyl groups decreases more (-cvinyl= 0.12-0.14 vinyl/1000 C) than in dark

and melt flow index decreases significantly (MFI = –0.11 g/10 min) independently of

the concentration of -carotene. This change can be attributed to the reactions of vinyl groups leading to extension of polymer chains after total consumption of the phos-

phorous antioxidant. Carbonyl groups form, their concentration increases with β-

carotene content. Fig. 9.11 reveals that the formation of carbonyl groups is accompanied

by a decrease in the number of vinyl groups participating in reactions. This and the

independence of MFI from the concentration of -carotene indicate that -carotene oxidizes primarily and protects the polymer from degradation in some extent.

0.06 0.08 0.10 0.12 0.140.1

0.2

0.3

0.4

0.5

0.6

C

O (

mm

ol/

mo

l P

E)

Reacted vinyl group/1000C

Increasing concentration

of -carotene

Fig. 9.11 Correlation between the concentration of reacted vinyl groups and carbonyl

groups formed in polyethylene processed with -tocopherol/phosphonite/-carotene additive packages and stored in light for 8 months.

We can conclude that storage in light results in reactions of the unsaturated

groups of polyethylene leading to long chain branching. After an induction time -carotene decomposes in oxidation processes which protect the polymer from degrada-

tion in some extent and result in fast fading of polyethylene.

9.4. Conclusions

Studies of the effect of -carotene in polyethylene stabilized with a combina-

tion of -tocopherol and phosphonite reveals that the additive with its eleven conjugat-ed double bonds colors the polymer significantly but does not reduce the stabilizing

efficiency of antioxidants either during processing or in the course of storage at ambient

temperature. -Carotene reacts with molecular oxygen both at high and ambient tem-

Chapter 9

100

peratures. It hinders the oxidation of polyethylene completely under the processing

conditions which cannot be avoided with the usually applied phenolic/phosphorous

antioxidant packages. The direction of reactions of -carotene at ambient temperature is controlled by the absence or presence of light, as well as the concentration of oxygen.

The molecular structure of polyethylene is primarily determined by the stabilizing effi-

ciency of antioxidants and is not affected significantly by the reactions of -carotene during storage at ambient temperature. The color of the polymer depends on the nature

of the reaction products of -carotene. Deepening of orange color occurs in dark, while

the polymer fades rapidly in light after an induction period. This special character of color changes could be utilized in developing functional packaging materials with indi-

cator function.

Summary

101

Chapter 10

Summary

Our laboratory has been working on the degradation and stabilization of poly-

mers for decades. The work and our commitment to the topic were strongly supported

by the three-way cooperation among TVK, Clariant and our lab. However, the situation

changed drastically at the beginning of the previous decade when Clariant decided to

stop doing research and development work on plastics additives. As a consequence, we

had to decide on the future of research in this area. Ongoing projects had to be complet-

ed obviously, and we decided to focus our attention on natural based antioxidants. The

decision was justified by the possible health hazard of synthetic phenolic stabilizers and

also by the general interest in raw materials from renewable resources. Accordingly, the

third thesis of our group on the degradation and stabilization of polymers shows this duality, traditional topics together with new ideas and goals. The change in direction

was successful and interesting results were obtained in various studies; several papers

were published already in leading journals of the field. This thesis reports the progress

that we achieved during the last few years. The conclusions drawn from the research are

summarized in this chapter. The most important new findings are compiled in six thesis

points at the end of the chapter.

In one of the traditional projects, pipes prepared with a wide range of additive

packages were stored in warm water for a year and the study of their properties showed

that chemical reactions take place both during processing and soaking. The chain struc-

ture of the polymer seems to be modified only during processing, but not during stor-age, at least in the time scale of the study. The direction and extent of changes are de-

termined mainly by the type of the application stabilizer, but primary antioxidants also

influence them to some extent. Soaking modifies the physical, but not the chemical

structure of the polymer. On the other hand, the chemical reactions of the additives

determine color and stabilizer loss thus the residual stability of the polymer. The chemi-

cal structure of the polymer has a larger effect on final properties, on the rate of slow

crack propagation and failure than the physical structure of the pipes. As a consequence,

the application stabilizer plays an important role in the determination of the perfor-

mance of the pipes.

In another project the thermal- and thermo-oxidative stability and reactions of a

new phosphorus containing secondary stabilizer, an alkyl-aryl phosphine, bis(diphenyl-phosphino)-2,2-dimethylpropane, were studied at the temperature of polyethylene pro-

cessing in the absence of solvents. The phosphine was reacted with carbon centered,

peroxy and oxy radicals, as well with hydroperoxides. The results proved that the inves-

tigated phosphine has excellent heat-stability; its thermal- and thermo-oxidative decom-

position is relatively slow above 200 °C and influenced only slightly by the presence of

volatile reaction products. Any oxidizing agent, including molecular oxygen, reacts with

the compound. Phosphorous esters did not form in reactions run at 200 °C, and side

products of the phosphine were found only in traces in the reaction mixture. The large

melt stabilizing efficiency of the new stabilizer in polyethylene during processing can

be explained by its sufficient heat stability and fast oxidation. Its reaction with molecu-

lar oxygen always present in small concentrations in the processing machine has vital

Chapter 10

102

importance, because it hinders the initiation reaction of the autocatalytic thermo-

oxidative degradation of the polymer.

The study of the melt stabilization effect of quercetin, a flavonoid type natural antioxidant, showed that it is a very efficient stabilizer in polyethylene. Quercetin pre-

vents the formation of long chain branches already at a concentration as small as 50

ppm and its dosage at 250 ppm renders the polymer sufficient residual stability. The

efficiency of quercetin is considerably larger than that of Irganox 1010, the hindered

phenolic antioxidant used as reference stabilizer. The difference in efficiency might be

explained with the dissimilar number of active –OH groups on the two antioxidant mol-

ecules, but the stabilization mechanism of quercetin might be also different from that of

I1010. Quercetin interacts with the phosphonite secondary stabilizer used, which im-

proves dispersion and increases efficiency. However, besides its advantages, quercetin

has also some drawbacks, it has a very high melting temperature, its solubility in poly-

ethylene is very small and it gives strong yellow color to the product.

Curcumin, another natural antioxidant, is stable as a crystalline solid at ambient

temperature in air. It keeps its stability in different kinds of solutions when stored in

dark, but decomposes in light. The rate of decomposition depends on the solvent used.

Curcumin is thermally stable up to 190 °C, but the interaction with a phosphonite stabi-

lizer results in some decrease in the starting temperature of thermal decomposition.

Curcumin is a more efficient melt and thermooxidative stabilizer of polyethylene than

the synthetic phenolic antioxidant used as reference (Irganox 1010), and the addition of

a phosphonite secondary stabilizer improves its efficiency further. Although the number

of vinyl groups participating in chemical reactions during processing is similar in the

case of the two phenolic derivatives, changes in the rheological properties and color of

the polymer reveal that the mechanism of their stabilization reactions differs. The syn-thetic phenolic antioxidant used cannot hinder reactions leading to the formation of long

chain branches. In the presence of curcumin the melt flow index of the polymer increas-

es and its color decreases with increasing number of extrusions. These changes indicate

that besides the phenolic OH groups also the linear linkage between the two methoxy-

phenyl rings of curcumin participates in stabilization reactions.

The study of the stabilization effect of β-carotene, α-tocopherol and a phos-

phonite antioxidant in polyethylene revealed that all components participate in chemical

reactions during processing at 190 °C. The loss in -carotene content and the consump-tion of the phosphorous stabilizer increase with increasing initial amount of β-carotene.

Changes in the vinyl group content of polyethylene are related to the consumption of

the phosphorous stabilizer, but the reactions do not always result in long chain branch-

ing because of the reactivity of -carotene towards molecular oxygen. This large reac-

tivity also prevents the formation of carbonyl groups above 500 ppm -carotene con-tent. The additive colors the polymer strongly as it absorbs light in the blue-green re-

gion. The colorless polymer turns orange-yellow at 250 ppm -carotene content and

then the color deepens with increasing concentrations and turns gradually red-orange. -carotene participates in various chemical reactions during the storage of polyethylene at

ambient temperature. The direction and the rate of color change depend on the amount

of -carotene, the presence or absence of light, as well as on the thickness of the poly-

Summary

103

mer. Deepening of orange color occurs in dark, while the polymer fades rapidly in light

after an induction period. The induction time of autoaccelerated chemical reactions of β-

carotene determined from changes in yellowness index is similar in dark and light; it

depends primarily on the additive package and on the thickness of the sample, i.e., oxy-gen diffusion. The additive packages investigated protect polyethylene from degradation

during storage in dark for eight months, but oxidation reactions and chains extension

occurs in light. This behavior opens up several possibilities to develop active packaging

materials with indicator functions.

The most important conclusions of the research can be summarized briefly in

the following thesis points:

1. As a result of the study of a large number of industrially relevant additive combina-

tions we found that the chain structure of the polymer is modified only during pro-

cessing, but not during storage in the time scale of the study. Contrary to expecta-

tions, the direction and extent of changes is determined mainly by the type of the

application stabilizer and not by the primary antioxidant (Chapter 5). 2. We pointed out the first time the surprising fact that the chemical structure of the

polymer has a larger effect on the final properties, on the rate of slow crack propa-

gation and failure of PE pipes stored in water than their crystalline structure. Chem-

ical structure is determined by reactions occurring in the first processing step and

depends mainly on the type of the application stabilizer used (Chapter 5).

3. In the study of the stabilization mechanism of a new, experimental phosphine sec-

ondary stabilizer we proved that its large melt stabilizing efficiency in polyethylene

during processing is the result of its good heat stability and large reactivity. Its reac-

tion with molecular oxygen hinders the initiation reaction of the autocatalytic

thermooxidative degradation of the polymer (Chapter 6).

4. In the study of the melt stabilization effect of quercetin, a flavonoid type natural antioxidant we showed that it is a very efficient stabilizer in polyethylene. It pre-

vents the formation of long chain branches already at much smaller concentration

than reported in the literature and than that of synthetic phenolic antioxidants used

in industrial practice. The drawback of the additive (color, high melting tempera-

ture, solubility) must be overcome before practical use (Chapter 7).

5. We used curcumin another natural antioxidant for the stabilization of polyethylene

for the first time. We showed that the melt flow index of the polymer increases and

its color decreases with increasing number of extrusions in the presence of

curcumin and concluded that besides the phenolic OH groups also the linear link-

age between the two methoxyphenyl rings of curcumin participates in stabilization

reactions (Chapter 8).

6. We used the first time the combination of the natural compounds of -tocopherol

and -carotene together with a phosphonite secondary stabilizer for the stabilization of polyethylene. The combination hinders the formation of long chain branches be-

cause of the reactivity of -carotene towards molecular oxygen, which also pre-vents the formation of carbonyl groups. The additive combination opens up several possibilities to develop active packaging materials with indicator functions (Chap-

ter 9).

Chapter 10

104

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Acknowledgements

119

Acknowledgements

First of all, I would like to thank my supervisor Enikő Földes for her continu-

ous help and attention during the years we worked together including also my graduate

period. I would like to thank to Béla Pukánszky for his patience, encouragement and the

fruitful discussions during my PhD years. This thesis could not have been written with-

out the help of my colleagues. I would like to thank to Monika Meskó, Judit Szauer,

Erika Selmeci, Ede Tatay and László Cseke for their technical assistance. I am grateful

for their enormous work. I should express my gratitude for my undergraduate students,

Luca Major and Balázs Kirschweng for their work in the carotene and the curcumene

projects. I am grateful to all of my colleagues for the nice atmosphere, I am lucky to work and "live" at such a good workplace like the Joint Laboratory.

I am indebted to the Hungarian Academy of Sciences for giving me the oppor-

tunity and financed my PhD work for three years as a young scientist. The National

Research Fund of Hungary (OTKA K 77860 and K 101124) is greatly acknowledged

for the financial support of the research. I should express my thank to the Tisza Chemi-

cal Ltd. for the nascent HDPE powder and some additives. I also thank for Clariant Ltd.

for the financial support of the phosphine and pipe project. The help of Pál Tóth in the

extrusion of pipes is also appreciated very much.

Finally I would like to thank my family for their support, for the strong back-ground and encouragement of my parents and my husband and I also would like to

thank my baby daughter for sleeping some time when I had to finish my thesis.

Acknowledgements

120

Appendix

121

Appendix

Table A1 Phenolic antioxidants (Chapter 5,7,8,9)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure

Molecular mass

(g/mol)

HO10

I1010

Hostanox O10 in Chapter 5

(Clariant)

Irganox 1010 in Chapter 7,8,9 (BASF Schweiz AG)

Pentaerythritol tetrakis[3-

(3,5-di-tert-butyl-4-

hydroxyphenyl)propionate]

OH O

O

O O H

4

C

1178

HO3 Hostanox O3 (Clariant)

Bis[3,3-bis-(4’-hydroxy-3’-

tert-butylphenyl)

butanoicacid]-glycol ester OH O O

O

OH

OH

O

OH

794

HO310 Hostanox O310 (Clariant) 1:1 mixture of HO10 and

HO3 1:1 mixture of HO10 and HO3 -

Appendix

122

Table A1 Phenolic antioxidants (Chapter 5,7,8,9) (continued)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure

Molecular mass

(g/mol)

E330 Ethanox 330 (Albemarle)

1,3,5-trimethyl-2,4,6-

tris(3,5-di-tert-butyl-4-

hydroxybenzyl)benzene

CH3

CH3

CH3

OH

OH

OH

775

C1790 Cyanox1790

(Cytec Industries)

1,3,5-tris(4-tert-butyl-3-

hydroxy-2,6-dimethyl ben-

zyl)–1,3,5-triazine-2,4,6-

(1H,3H,5H)-trione

OH

N

N

N

O

O

OH

OH

O

699

Appendix

123

Table A2 Application stabilizers (Chapter 5)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure

Molecular

mass

(g/mol)

SE4 Hostanox SE4

(Clariant)

Distearyl-3,3'-

thiodipropionate

S

O

OC

18H

37

O

OC

18H

37

683

N30 Hostavin N30

(Clariant)

Polymer of 2,2,4,4-

tetramethyl-7-oxa-3,20-diaza-

dis-piro[5.1.11.2]-

heneicosan-21-on and

epichlorohydrin

NH

O

C

O

NH

OCl

n

+

>1500

T783 Tinuvin 783

(Ciba SC)

1:1 mixture of T622 and

C944 1:1 mixture of T622 and C944 -

Appendix

124

Table A2 Application stabilizers (Chapter 5) (continued)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure

Molecular

mass

(g/mol)

C944 Chimassorb 944

(Ciba SC)

Poly[[6-[(1,1,3,3-tetramethylbutyl)amino]-

1,3,5-triazine-2,4-

diyl][(2,2,6,6-

tetramethyl-4-

piperidinyl) imino]-1,6-

hexanediyl[(2,2,6,6-

tetramethyl-4-

piperidinyl)imino]])

C(C8H

17)3

(CH2)6

(CH2)6

NH

N HH

NH

N H

NH

NH

N

N

N

NH

H

NH

NH H

n

~2550

T622 Tinuvin 622

(Ciba SC)

Butanedioic acid,

dimethylester, polymer with 4-hydroxy-2,2,6,6-

tetramethyl-1-piperidine

ethanol

O

O

CCH2 CH

2 CH2

n

O

NCH2

O

COCH3

H

H

3100 - 4000

Appendix

125

Table A3 Phosphorus derivatives (Chapter 5,6,7,8,9)

Abbreviation

Commercial name

(Producer) Chemical name Chemical structure

Molecular

mass

(g/mol)

P-EPQ

San

dost

ab P

-EP

Q (

Cla

riant)

Di-

phosphonite

>70 %

[4-[4-bis(2,4-di-tert-

butylphenoxy)phosphanylphenyl]phenyl]-

bis(2,4-ditert-butylphenoxy)phosphane

1035

Mono-

phosphonite ~20%

2,4-di-tert-butylphenyl (2,5-di-tert-

butylphenyl) [1,1'-biphenyl]-4-phosphonite

595

Phosphite

<10 % [tris(2,4-di-tert-butylphenyl)]phosphite

647

P P

O

O O

O

P

O

O

P

O

OO

Appendix

126

Table A3 Phosphorus derivatives (Chapter 5,6,7,8,9) (continued)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure

Molecular mass

(g/mol)

PMP PEPFINE

(Clariant)

Bis(diphenylphosphino)-

2,2-dimethylpropane P

CH3CH3

P

1

2

3

4

5

67

440.5

PMP-1ox Monooxidized PEPFINE

(Clariant)

Bis(diphenylphosphino)-

2,2-dimethylpropane

monooxide

P

CH3CH3

P

O

1

2

3

4

5

67

1'

3'

4'

5'

6' 7'

456.5

PMP-2ox Dioxidized PEPFINE

(Clariant)

Bis(diphenylphosphino)-

2,2-dimethylpropane

dioxide

P

CH3CH3

P

O O

1

2

3

4

5

67

462.5

Appendix

127

Table A4 Natural antioxidants (Chapter 7,8,9)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure

Molecular

mass (g/mol)

-

Quercetin

(98 %; Sigma-Aldrich)

2-(3,4-dihydroxyphenyl)-3,5,7-

trihydroxy-4H-chromen-4-one O

O

OH

OH

OH

OH

OH

302

-

Cu

rcum

in f

rom

Curc

um

a L

onga

[Turm

enic

]

(Sig

ma

-Ald

rich

)

Curcumin

65 %

(1E,6E)-1,7-Bis(4-hydroxy-3-

methoxyphenyl)-1,6-heptadiene-3,5-

dione

O OH

OH OH

OO

CH3 CH3

O O

OH OH

OO

CH3 CH3

368

Demetoxi

curcumin

(1E,6E)-1-(4-hydroxy-3-

methoxyphenyl)-7-(4-hydroxyphenyl)-

1,6-heptadiene-3,5-dione O OH

OH OH

O

CH3

338

Bis-demetoxi

curcumin

(1E,6E)-1,7-Bis(4-hydroxyphenyl)-1,6-

heptadiene-3,5-dione O OH

OH OH

308

Appendix

128

Table A4 Natural antioxidants (Chapter 7,8,9) (continued)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure

Molecular mass

(g/mol)

- -carotene

(purum, 97 %, UV, Sigma)

1,3,3-Trimethyl-2-

[3,7,12,16-tetramethyl-18-(2,6,6-trimethylcyclohex-

1-en-1-yl)octadeca-

1,3,5,7,9,11,13,15,17-

nonaen-1-yl]cyclohex-1-

ene

537

- -tocopherol

((Ronotec 201; Hoff-mann-La Roche)

(2R)-2,5,7,8-Tetramethyl-

2-[(4R,8R)-(4,8,12-

trimethyltridecyl)]-6-

chromanol

OH

O

431

Appendix

129

Table A5 Reagents used for model reactions (Chapter 6)

Abbreviation Commercial name

(Producer) Chemical name Chemical structure Molecular mass

(g/mol)

CHP Cumene hydroperoxide

(Luperox CU90; 88 %;

Aldrich)

1-methyl-1-phenylethyl

hydroperoxide

CH3

CH3

O OH

152

DCP Dicumylperoxide

(Luperox DCP; ≥99 %;

Aldrich)

Bis(1-methyl-1-

phenylethyl) peroxide

CH3

CH3

O O

CH3

CH3

270

AIBN Azobisisobutyronitrile

(98 %; Aldrich) 2,2’-azobis(2-

methlypropionitrile) CC

CH3

CH3

N

NC

CH3

CH3

CN

N

164

Appendix

130

Table A6 Composition of the reaction products of PMP formed under different

conditions determined by 1H NMR (Chapter 6)

Reagent Gas T

(°C)

Composition of major

products (mol %) Side products

PMP PMP-

1ox

PMP-

2ox

of PMP

(mol%)

Other

Type Amount

(mol %)

PMP 100 0 0 0 Ethanol 1.5

PMP-2ox 0 0 100 0 -

Mixture of PMP, PMP-

1ox and PMP-2ox 29.8 9.3 60.9 0 -

– Ar 200 100 < 0.1 < 0.1 0 -

– Ar 240 100 < 0.1 < 0.1 0 -

– O2 200 99.2 0.6 0.2 0 -

– O2 240 96.7 2.6 0.7 < 1 -

AIBN Ar 200 98.0 2.0 < 0.1 < 1

Cyano-

isopropyl

derivatives

3-5

AIBN O2 200 90.3 8.6 1.1 < 1

Cyano-

isopropyl

derivatives

2-3

DCP Ar 200 84.6 7.2 8.2 < 1 DCP

derivative

DCP O2 200 41.7 39.1 19.2

< 1

3 different

types of

DCP

derivatives

CHP Ar 200 65.8 10.9 23.3 < 1 CHP

derivatives

CHP Ar 23* 52.7 5.5 41.8 < 1 CHP

derivatives

*Reaction time: 24 h

Appendix

131

Assignation of 1H and

13C signals of PMP and its oxides (Chapter 6)

PMP: 1H NMR (CDCl3) δ 1.03(s; 6H; H1), 2.32(d; 4H; JHP 3.0Hz; H2, 7.26-7.31(m;

12H; H6, H7) ), 7.42(ddd; 8H; JHH 1.8, 7.4, 7.6Hz; H4). 13C NMR (CDCl3) δ 30.3(t; 2C;

JCP 9.1Hz; C1), 35.1(t; 1C; JCP 14.3Hz; C2), 44.1(dd; 2C; 9.1, JCP 16.7 Hz; C3), 128.2-

128.4(m; 12C; C6, C7), 133.0(d; 8C; JCP 20.0Hz; C5), 140.0(d; 4C; JCP 11.7Hz; C4)

PMP-1ox: 1H NMR (CDCl3) δ 1.11(s; 6H; H1, H1’), 2.42(d; 2H; JHP 3.0Hz; H3’),

2.52(d; 2H; JHP 10.8Hz; H3), 7.26-7.32(m; 6H; H6’, H7’), 7.39-7.47(m; 10H; H5’,H6,

H7), 7.72(ddd; 4H; JHH 1.3, 8.3, 11.2Hz; H5). 13C NMR (CDCl3) δ 30.7(dd; 2C; JCP 6.5,

8,.9Hz; C1,C1’), 35.8(dd; 1C; JCP 4.2, 14.8Hz; C2), 41.8(dd; 1C; JCP 9.8, 70.1Hz; C3), 44.5(dd; 1C; JCP 8.1, 15.8Hz; C3’), 128.2-128.6(m; 10H; C6, C6’,C7’); 130.5(d; 4C; JCP

9.0Hz; C5), 131.2(d; 2C; JCP 2.7Hz; C7); 133.0(d; 4C; JCP 19.6Hz; C5’); 135.3(d; 2C;

JCP 97.2Hz; C4), 139.6(d; 2C; JCP 11.6Hz; C4’)

PMP-2ox: 1H NMR (CDCl3) δ 1.13(s; 6H; H1), 2.80(d; 4H; JHP 10.8Hz; H3), 7.39-

7.47(m; 12H; H6, H7), 7.79(ddd; 8H; JHH 1.4, 8.3, 11.3Hz; H5). 13C NMR (CDCl3) δ

31.6(t; 2C; JCP 7.3Hz; C1), 36.0(t; 1C; JCP 4.6Hz; C2) 40.8(dd; 2C; 5.3, JCP 69.9Hz;

C3), 128.5(d; 4C; JCP 11.5Hz; C6), 130.6(d; 8C; JCP 11.5Hz; C5), 131.3(d; 4C; JCP

2.2Hz; C7), 135.0(d; 4C; JCP 97.3Hz; C4)

Appendix

132

List of publications

133

List of publications

Papers:

1. Pénzes, G., Domján, A., Tátraaljai, D., Staniek, P., Földes, E., Pukánszky, B.,: High

temperature reactions of an aryl-alkyl phosphine, an exceptionally efficient melt stabilizer of polyethylene, Polym Degrad Stab 95 (2010) 1627-1635 IF:2.594; C:2]

2. Tátraaljai D., Vámos M., Staniek P., Földes E., Pukánszky B.: Az adalékrendszer

hatása a polietilén csövek szerkezetére és tulajdonságaira, Műanyag és Gumi 47

(2010) 31-34

3. Pataki P., Tátraaljai D., Kovács J., Földes E., Pukánszky B.:A lignin és a karotin

hatása a polietilén feldolgozási stabilitására, Műanyag- és Gumiipari Évkönyv 2011

42-53

4. Tátraaljai D: Természetes antioxidánssal a polimer védelméért, Kémiai Panoráma 4 (2012;1) 26-29

5. Tátraaljai D., Földes E., Pukánszky B.: Polietilén feldolgozási stabilizálása

kvercetin természetes antioxidánssal, Műanyag és Gumi 50 (2013) 374-378

6. Tátraaljai, D., Kirschweng, B., Kovács, J., Földes, E., Pukánszky B.: Processing

Stabilisation of PE with a Natural Antioxidant, Curcumin, Eur Polym J 49 (2013)

1196-1203 [IF:2.562; C:1]

7. Tátraaljai, D., Vámos, M., Orbán-Mester, Á., Staniek, P., Földes, E., Pukánszky,

B.:Performance of PE pipes under extractive conditions; effect of the additive

package and processing, Polym Degrad Stab 99 (2014) 196-203 [IF:2.77; C:0]

8. Tátraaljai, D., Major, L., Földes, E., Pukánszky, B.: Study of the effect of natural

antioxidants in polyethylene: Performance of β-carotene, Polym Degrad Stab 102

(2014) 33-40 [IF:2.77; C:0]

9. Tátraaljai, D., Földes, E., Pukánszky, B.: Efficient melt stabilization of polyeth-

ylene with quercetin, a flavonoid type natural antioxidant, Polym Degrad Stab 102

(2014) 41-48 [IF:2.77; C:0]

List of publications

134

Conference presentations:

1. Tátraaljai, D., Vámos, M., Kriston, I., Kovács, J., Staniek, P., Földes, E.,

Pukánszky, B.: Effect of Additive Combinations on the Processing and Hydrolytic Stability of Polyethylene Pipes, European Weathering Symposium, Budapest,

Hungary, September 16-18, 2009 (poster)

2. Tátraaljai, D., Vámos, M., Kriston, I., Kovács, J., Staniek, P., Földes, E.,

Pukánszky, B.: Effect of additive combinations on the processing and hydrolytic

stability of polyethylene pipes, Polymer Degradation Discussion Group Confer-

ence 2009, Sestri Levante, Italy, September 6-10, 2009 (lecture)

3. Tátraaljai D., Kovács J., Földes E., Pukánszky B.: Természetes anyagok hatása a

polietilén feldolgozási stabilitására XIII. Doki Suli 2010, Balatonkenese 2010.

április 21-23 (lecture)

4. Tátraaljai, D., Pénzes, G., Domján, A., Staniek, P., Földes, E., Pukánszky, B.,:

Exceptional melt stabilizing efficiency of aryl-alkyl phosphine in polyethylene;

mechanism of stabilization, Modification Degradation Stabilization Conference

2010, Athens, Greece September 5-9, 2010 (lecture)

5. Tátraaljai, D., Kovács, J., Pataki, P., Földes, E., Pukánszky, B.,: The effect of natu-

ral additives on the processing stability of polyethylene, Modification Degradation

Stabilization Conference 2010, Athens, Greece September 5-9, 2010 (poster)

6. Tátraaljai D., Kovács J., Földes E., Pukánszky B.: Természetes anyagok hatása a

polietilén feldolgozási stabilitására Szemináriumi előadás MTA-KK-AKI 2010. október 12 (lecture)

7. Tátraaljai, D., Pataki, P., Kovács, J., Földes, E., Pukánszky, B., The crutial role of

phosphorous antioxidants in the processing stabilization of polyethylene and the

potential use of natural compounds in stabilizer packages , Advances in Polymer

Science and Technology Conference 2011, Linz, Austria September 27-30, 2011

(lecture)

8. Tátraaljai, D., Pénzes, G., Kovács, J., Földes, E., Pukánszky, B., Aspects of the

stabilization of PE with lignin, Polymer Degradation Discussion Group Confer-

ence 2011, Sestri Levante, Italy, September 5-7, 2011 (lecture)

9. Tátraaljai, D., Pénzes, G., Janecska, T., Orbán-Mester, Á., Suba, P.,Földes, E.,

Pukánszky, B,: Chemical reactions during the storage of polyethylene pellets at

ambient and elevated temperature, Polymer Degradation Discussion Group Con-

ference 2011, Sestri Levante, Italy, September 5-7, 2011 (poster)

List of publications

135

10. Tátraaljai, D., Pénzes, G., Pataki, P., Janecska, T., Orbán-Mester, Á., Suba,

P.,Földes, E., Pukánszky, B.: Study of degradation processes taking place in poly-

ethylene pellets stored under different conditions, Interfaces’11 Conference 2011, Sopron, Hungary September 28-30, 2011 (lecture)

11. Tátraaljai D., Kirschweng B., Kovács J., Földes E., Pukánszky B.: A kurkumin

hatása a polietilén feldolgozási stabilitására Kutatóközponti Tudományos Napok

2011, Budapest, MTA-KK, 2011. november 24. (lecture)

12. Tátraaljai, D., Kirschweng, B., Kovács, J., Földes, E., Pukánszky, B.: The Effect

of Curcumin on the Processing Stability of Polyethylene, International Conference

on Bio-Based Polymers and Composites 2012, Siófok, Hungary, May 27-31, 2012

(poster)

13. Tátraaljai, D., Kirschweng, B., Pénzes, G., Szabó, P.T., Földes, E., Pukánszky, B.: Natural antioxidants – study of the processing stabilizing efficiency and mecha-

nism of curcumin in polyethylene, Modification Degradation Stabilization Confer-

ence 2012, Praha, Czech Republic, September 2-6, 2012 (lecture)

14. Tátraaljai, D., Vágó, B., Kovács, J., Földes, E., Pukánszky, B.: Study of the effect

of quercetin on the melt stability of polyethylene, Modification Degradation Stabi-

lization Conference 2012, Praha, Czech Republic, September 2-6, 2012 (poster)

15. Major, L., Tátraaljai, D., Földes, E., Pukánszky B.: Controlled loss of natural anti-

oxidants in polyethylene for functional packaging materials, Polymer Degradation

Discussion Group Conference 2013, Paris, France, September 1-4, 2013 (lecture)

16. Polyák, P., Vágó, B., Hári, J., Tátraaljai, D., Földes, E., Pukánszky, B.: Halloysite

Nanotubes as Support for Quercetin, a Novel Natural Antioxidant for PE, Polymer

Degradation Discussion Group Conference 2013, Paris, France, September 1-4,

2013 (lecture)