The Stabilization of Polypropylene and TPO: An Overview

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© 2012 BotkinChemie

THE STABILIZATION OF POLYPROPYLENE & TPO: AN OVERVIEW

James H. Botkin

dba BotkinChemie, Portland, ME USA [email protected]

Abstract

The degradation and stabilization of polypropylene and TPO are reviewed. Polypropylene-based materials degrade when exposed to heat or light by an oxidative process in which scission of the polymer chains predominates. This eventually leads to a number of undesirable effects that compromise the useful life of the polymer. Oxidation can take place during melt processing operations, at elevated temperatures during end use, or from exposure to ultraviolet light (i.e. in sunlight).

Stabilizer additives have been developed that are very effective for improving the stability of polypropylene and TPO to oxidative degradation. Most of these products act by chemically disrupting the oxidation process through the scavenging of free radical and peroxide intermediates. Examples of commonly used stabilizer additives include hindered phenols, phosphites and phosphonites, thioethers, hydroxylamines, hindered amine light stabilizers (HALS), benzoates, and UV absorbers. The mechanism of action of each type of stabilizer is reviewed and examples of commercial products are provided.

Degradation during melt processing results in melt flow shifts and discoloration. Combinations of hindered phenols and phosphites represent the state of the art for the stabilization of polypropylene and TPO under these conditions. Systems incorporating partially-hindered phenols (e.g. AO-4) and/or high performance phosphites (e.g. P-2, P-3), or phosphonites (e.g. P-4) provide the highest level of performance in terms of melt stability.

Long-term thermal exposure of a part during end use can result in the deterioration of mechanical properties as well as discoloration. Systems based on combinations of hindered phenols and either thioethers or HALS (especially HALS-5) can significantly extend the useful life of polypropylene and TPO under these conditions. Systems based on HALS-5 in combination with hindered phenols are most appropriate for use in applications where light stability is also required in the end use application.

Degradation induced by exposure to sunlight results in changes in appearance and the loss of mechanical properties. Systems based on HALS are the state of the art for the light stabilization of polypropylene and TPO. In cases where higher performance is needed, improvements can often be achieved by using synergistic combinations of HALS with other additives such as benzoates, ultraviolet absorbers, and some phenolic antioxidants (especially AO-2).

Stabilizer additives can sometimes give rise to unexpected, undesired effects such as blooming, automotive fogging, warehouse discoloration (gas fading), and interactions with automotive coatings. Guidelines are provided to avoid these problems though the proper selection of the additives.

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Introduction Polyolefin-based materials such as polypropylene and TPO are widely used in automotive components due to their excellent balance of physical properties, ease of processing, recyclability, and good cost-performance. These materials must be properly stabilized in order to survive melt processing operations as well as to meet the weatherability and long-term thermal stability requirements of automotive applications. This is normally accomplished using additives such as antioxidants and light stabilizers.

The range of available stabilizer additives is huge and formulating with them can be challenging. This paper provides an overview of some of the more common stabilization additives used in polypropylene and TPO, with an emphasis on products useful in thick-section automotive applications. Specific topics to be covered include the oxidative degradation of polyolefins, descriptions of stabilizer additives and their functions and mechanisms of action, test methods, and basic formulation guidelines to meet performance requirements and avoid common problems.

Oxidative Degradation of Polyolefins When organic materials such as polyolefins are exposed to conditions such as heat, ultraviolet light, or mechanical stress in the presence of atmospheric oxygen, free radicals are formed which initiate the oxidation process. The basic chemistry of oxidation has been known for over sixty years.[1] This process is characterized by initiation (equation 1), propagation (equations 2 & 3), chain branching (equation 4), and termination (equation 5) steps:[2]

Initiation: •!" PHP (1)

Propagation: •!!"+• OOPOP2

(2)

•+!!"!+•!! POHOPHPOOP (3)

Chain Branching: OHOPOHOPh

•+•!"" #"!!$/% (4)

Termination: 2

2 OPOOPOOP +!!!"•!! (5)

The alkoxy radical derived from the polypropylene hydroperoxide (in equation 4) readily undergoes fragmentation to give chain scission (equation 6). This is consistent with studies showing chain scission dominates over cross-linking in the oxidation of polypropylene.[2,3]

CH2

C

O

CH2

CH3

CH2

O

CH3

CH2

.

+.

(6)

Oxidation of polypropylene and TPO can take place during high temperature melt processing operations, during end use at elevated temperatures (e.g. in under-the-hood applications), or during exposure to sunlight. The damaging effects of oxidation manifest themselves somewhat differently depending on the conditions, but in all cases the changes that take place can compromise the useful properties of the material and limit the lifetime of the part.

Introduction to Stabilizer Additives Stabilizer additives function in large part by chemical disruption of the oxidation process. By reaction of stabilizers and/or their transformation products with the free radicals and hydroperoxides formed in the initiation and propagation steps (equations 1-3), oxidation can be inhibited or retarded and the useful life of a part can be significantly extended. These protective additives are characterized by their chemistry

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(e.g. hindered phenols, phosphites) and/or function (e.g. radical scavengers, peroxide decomposers, UV absorbers). Some common families of stabilizer additives used in polypropylene and TPO include:

• Hindered phenols, • Phosphites and phosphonites, • Thioethers, • Hydroxylamines, • Hindered amine light stabilizers (HALS), • Benzoates, and • Ultraviolet (UV) absorbers.

Different families of additives are reactive with different intermediates in the oxidation process (e.g. free radicals vs. hydroperoxides), present different levels of reactivity (e.g. at different temperatures), and have different degrees of stability during exposure to environmental conditions such as high temperatures or UV light. As a result of their inherent reactivity and stability, some additives function primarily as melt processing stabilizers at high temperatures, some as thermal stabilizers but at lower temperatures (i.e. in the solid state), and still others as light stabilizers.

It is also well known that in order to provide the most effective protection against oxidation it is beneficial to combine additives acting by different mechanisms, i.e. by attacking the oxidation process at multiple steps. These combinations often exhibit a level of performance far superior to that of their individual components used alone.

Polymers and parts produced from them can be oxidized under a wide range of conditions encountered in their manufacturing and end use. Combinations of additives (often referred to as systems) are usually needed to provide effective protection.

A summary of the function of different families of stabilizer additives is presented in Table 1. In the following sections, each class will be presented in greater detail and test methods and general formulation guidelines will be provided along with illustrative examples.

Table 1. Protective Effects of Stabilizer Additives.

Stabilizer Class Reactivity High Temperature Melt Processing

Heat Stability (Solid Phase)

UV / Light Stability

Hindered phenols POO . Phosphites POOH Thioethers POOH Hydroxylamines POO . / POOH HALS P . / POO . Benzoates POO . UV absorbers hν Primary effect Secondary effect Hindered Phenols Hindered phenol antioxidants are highly effective to prevent oxidation in polyolefins both during processing in the melt and during long-term heat aging in the solid phase. These additives also have an influence on the light stability of polypropylene and TPO.

Examples of some commonly used commercial hindered phenols are shown in the Appendix. These include “fully hindered” (ortho di-tert-butyl substituted as in AO-1, AO-2, and AO-3) and “partially hindered” (ortho methyl-tert-butyl substituted as in AO-4 and AO-5) types.

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Hindered phenols function by reaction with peroxy radicals (formed in step (2) of the oxidation process) to give hydroperoxides (Scheme 1), thus terminating the oxidation chains.[4] The resonance stabilized phenoxy radical formed in this process is generally of sufficient stability not to reinitiate the oxidation process by abstraction of hydrogen from the polymer, although this can be an issue with more reactive unhindered phenols. Combinations of hindered phenols with peroxide decomposers (e.g. phosphites or thioethers) are widely employed and often provide enhanced protection during melt processing or long-term heat aging.

+ POO^ + POOH

^ ^

^

OH

R

O

R

O

R

O

R

O^

R

O^

R Scheme 1. Scavenging of Peroxy Radicals by Hindered Phenols

and Resonance Stabilization in Hindered Phenoxy Radicals. Phosphites & Phosphonites Phosphites and phosphonites are used as melt processing stabilizers in polyolefins, usually in combinations with hindered phenols or hydroxylamines. These additives function as hydroperoxide decomposers (Scheme 2).[5] They are most effective at the high temperatures encountered in melt processing operations but much less so in the solid phase at lower temperatures.

Examples of commercial phosphite stabilizers are provided in the Appendix. Phosphite P-1 is a general-purpose product very widely used in polypropylene and TPO. The “high performance phosphites” (P-2 and P-3) and the phosphonite P-4 are more reactive and tend to be used in applications employing high temperature and/or high shear processing conditions.

As esters of phosphorous acid, all phosphites are subject to hydrolysis and this can lead to problems such as black specks and equipment corrosion. When using phosphites it is important to minimize exposure of the product to moisture, keeping containers tightly sealed when not in use. The high performance phosphites P-2 and P-3 and the phosphonite P-4 tend to be more moisture sensitive than the general-purpose product P-1. Thioethers Thioether antioxidants are used to improve the long-term thermal stability of polypropylene and TPO in the solid phase. Like phosphites, thioethers function as hydroperoxide decomposers (Scheme 2),[6] but tend to be more reactive at lower temperatures (<160oC). Combinations of thioethers with hindered phenols are particularly effective.

Examples of commercial thioether stabilizers are given in the Appendix. Thioether TS-1 is a low-cost, general-purpose product that is widely used in polypropylene and TPO applications. TS-2 is a higher performance product with lower volatility, excellent compatibility, and a higher sulfur content that may be more suitable for use in demanding applications.

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+ POOH

+ POOH

+ POH

+ POH

RO P OR

OR

RO P

O

OR

OR

R

S

R R

S

R

O Scheme 2. Scavenging of Hydroperoxides by Phosphites and Thioethers.

Antagonistic interactions between thioethers and HALS are known, the effect of which is to compromise the light stabilization performance of the HALS. This effect appears to result from an interaction between the basic HALS and acidic transformation products of the thioether.[7] While it has been suggested that this effect may be overcome through the use of specific acid neutralizers,[8] combinations of thioethers and HALS are not typically recommended for the stabilization of polypropylene and TPO. Hydroxylamines Hydroxylamine antioxidants are used as non-discoloring melt processing stabilizers in polyolefins. It has been postulated that these additives function both as peroxy radical scavengers and peroxide decomposers.[2] While excellent melt processing stabilizers, hydroxylamines are not effective to prevent oxidation in the solid phase and must be used in combination with either a hindered amine light stabilizer (HALS) or a hindered phenol.[9,10]

Examples of hydroxylamine stabilizers are given in the Appendix. Note that the amine oxide H-3, while not a hydroxylamine, rearranges to form an active hydroxylamine via a Cope Rearrangement during melt processing at high temperatures. Hindered Amine Light Stabilizers (HALS) The development and commercialization of HALS in the 1970’s revolutionized the light stabilization of polyolefins. The development of these highly effective light stabilizers has been critical to the successful use of polypropylene and TPO in weatherable applications. HALS also contribute to the thermal oxidative stability of polyolefins in the solid phase, especially at lower temperatures (<120oC).

Examples of commercial hindered amines are given in the Appendix. By varying attributes such as the molecular weight, linking groups between the active piperidine moieties, and substitution on the piperidine nitrogen atom, products with a range of stabilization activities and secondary properties (physical form, basicity, volatility, solubility, etc.) are obtained. HALS are conveniently classified as low- to medium-molecular weight (HALS-1 through HALS-3) or high molecular weight types (HALS-4 and HALS-5). While HALS of all molecular weights are effective as light stabilizers, only the medium and high molecular weight types are effective to improve thermal oxidative stability. It has been long recognized that hindered amines and their transformation products (i.e. nitroxyl radicals and iminoether or “NOR” derivatives) are capable of stabilizing polyolefins through multiple mechanisms involving the scavenging of free radical and peroxide intermediates in the oxidation process.[11,12] An important feature differentiating hindered amines from other classes of stabilizers is their ability to act by a regenerative mechanism, which was first proposed by Denisov and coworkers (Scheme 3).[13] While the exact nature of this regenerative mechanism is still subject to debate,[14] it is generally accepted that a single hindered amine is capable of deactivating multiple oxidation chains.[12] However, evidence also exists that the regeneration mechanism is less efficient in polypropylene than in other polyolefins.[14]

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N

H

N

O

N

OP

P

P-O-OP-O-O-P

.

.

.

Scheme 3. The “Denisov Cycle” Mechanism Proposed for Hindered Amine Light Stabilizers. Benzoates The use of derivatives of 3,5-di-tert-butyl-4-hydroxybenzoic acid (B-1, B-2; Appendix) as light stabilizers for polyolefins predates that of hindered amines. Like other hindered phenols, the benzoates inhibit the oxidative degradation process by scavenging free radicals.[15] However, compared to conventional hindered phenols, benzoates are characterized by a higher level of inherent light stability. These products also contribute to the thermal oxidative stability of polyolefins, both in the melt and at lower temperatures in the solid phase. Note that benzoates are sometimes erroneously referred to as “UV absorbers”, although they do not absorb at wavelengths longer than about 300 nm.

The photochemistry of benzoates needs to be considered when selecting the most appropriate product for use in a particular application. B-1 is well known to undergo a Photo-Fries rearrangement on exposure to light (Scheme 4),[15] giving a 2-hydroxybenzophenone derivative which is an effective UV absorber but also absorbs visible light and can be discoloring. The rearrangement can be inhibited to some degree and the amount of discoloration reduced by using B-1 in combination with an ultraviolet absorber.

O

O

OHOH

O OH

h!

Scheme 4. Photo-Fries Rearrangement of Benzoate Light Stabilizer B-1.

Ultraviolet Absorbers Ultraviolet (UV) absorbers act largely by shielding the interior of the part from damaging UV radiation. Before the advent of hindered amines, light stabilization was accomplished with systems based on UV

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absorbers in combination with other additives such as benzoates. Today UV absorbers are sometimes used in combination with HALS and other additives for the stabilization of polypropylene and TPO. They are also used in polyolefin packaging applications to protect the package content from the effects of UV radiation.

All of the compounds of this class share several general characteristics:

1. Strong UV absorption, especially in the 290 nm to 350 nm range which is very damaging to organic materials, and

2. Excited states formed upon UV absorption relax to the ground state extremely rapidly (picosecond time frame) and efficiently via radiation-less processes, which imparts high efficiency and excellent photostability.

Chemical classes of UV absorbers suitable for use in polyolefins include 2-hydroxybenzophenones (e.g. UVA-1), 2-(2-hydroxyphenyl)-2H-benzotriazoles (e.g. UVA-2, UVA-3), and tris-aryl-o-hydroxyphenyl-s-triazines (e.g. UVA-4). Chemical structures of representative UV absorbers are given in the Appendix.

Stabilizers in Action: Examples and Formulation Guidelines Melt Processing Stabilization Oxidative degradation can take place during melt processing operations associated with resin manufacturing and compounding operations as well as during the fabrication of finished parts. The degree of degradation is influenced by factors such as the processing temperature, residence time, shear, and concentration of oxygen. Melt processing stability is particularly critical in high temperature or high shear processing operations. It is also important for processes that generate large amounts of production scrap that must be recycled, such as sheet extrusion and thermoforming. In such operations, it is to be expected that a significant fraction of the polymer will be subjected to multiple heat histories in which degradation can occur.

Changes in molecular weight resulting from oxidation can affect the melt viscosity (and hence processing characteristics) and also the mechanical properties of the polymer. Chain scission processes tend to dominate in polypropylene during degradation in the melt, and thus the molecular weight tends to decrease after processing. Along with these changes, oxidative degradation during processing also results in the formation of defects (such as hydroperoxides) that can negatively affect the long-term thermal stability and the light stability of parts during their end use.

Maintaining stable color after melt processing is also important to polymer producers and processors. As a consequence of oxidation processes taking place during melt processing, hindered phenolic antioxidants are oxidized to highly colored products such as quinones that cause a visible discoloration of the polymer.[16] Other additives may either inhibit or accelerate the discoloration process. In particular, strongly basic additives such as hindered amine light stabilizers (HALS) can increase the discoloration associated with oxidation products of hindered phenols.

The melt processing stability of polypropylene is often assessed in the laboratory using multiple pass extrusion experiments, in which changes in the melt flow index (MFI) and color of the polymer are measured as a function of the number of extrusion passes.[2]

The MFI of the polymer is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via a specified weight at a specified temperature. The method is described in the ASTM D1238 standard. For polypropylene, testing is conducted at 230oC using a 2.16 kg weight. MFI is regarded as an indirect measure of molecular weight, with higher values corresponding to lower molecular weights. MFI is also inversely proportional to viscosity of the melt at the conditions of the test. Mechanical properties are also affected by the molecular weight of the polymer. All other things being equal, polypropylene resins of lower MFI

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(higher molecular weight) tend to have higher mechanical strength than resins of higher MFI (lower molecular weight). In multiple pass extrusion experiments in polypropylene, the MFI tends to increase with each extrusion pass as a result of oxidative degradation. Smaller increases in MFI after multiple pass extrusion are indicative of better processing stability.

In laboratory experiments, the color of a test specimen is measured on polymer pellets or molded plaques using a spectrophotometer and is expressed as a point on a three-dimensional color space. In natural (un-pigmented test specimens), color is often expressed as Yellowness Index (YI), which is defined in the ASTM E313 (for translucent or opaque samples) or ASTM D1925 standards (for transparent samples). Higher YI values are associated with a greater perceived yellowness to the naked eye. Smaller increases in YI after multiple pass extrusion are suggestive of better processing stability.

Combinations of hindered phenols and phosphites represent the state-of-the-art for the melt processing stabilization of polypropylene and TPO. Systems based on combinations of AO-1 and P-1 (usually at a 1:1 or 1:2 ratio) are among the most widely used, and as shown in Figure 1 give excellent MFI stability under typical processing conditions.

Figure 1. Melt Stabilization of PP Homopolymer. Multiple Pass Extrusion, 260oC melt temperature.

All formulations contain 0.1% calcium stearate. Data from Reference [17]

In processes for which a higher levels of processing stability are needed, combinations of a partially-hindered phenol such as AO-4 with P-1 can provide improved melt stability (Figure 2).[18]

Alternatively, this objective can also be met through the use of AO-1 in combination with a high performance phosphite such as P-2, P-3, or the phosphonite P-4 (Figure 3). Systems based on high performance phosphites also provide reduced discoloration in some cases, which appears to be dependent on the particular resin grade used.

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Systems based on hydroxylamine stabilizers alone or in combination with other additives also provide excellent MFI stability, often with less discoloration than conventional systems based on combinations of hindered phenols and phosphites.[9,10]

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Long-Term Thermal Stabilization Polypropylene and TPO are exposed to the elevated temperatures during many end use applications. The susceptibility to oxidation is dependent on factors such as the temperature, the thickness of the part, and the processing history. Long-term thermal stability is particularly important in under-the-hood applications, interior trim components, and also in some exterior parts near the engine compartment.

Oxidation of polypropylene takes place readily at elevated temperatures and results in undesirable effects such as a deterioration of mechanical properties and discoloration. In laboratory tests, the long-term thermal stability of a material is generally assessed by heating test specimens in a circulating air oven and measuring changes in appearance or mechanical properties as a function of exposure time. The thermal degradation of polypropylene at higher heat aging temperatures (~150oC) is often characterized by a very sudden onset, and often the onset of degradation can be determined by visual examination. In these cases the formation of discolored areas on the test specimen or crumbling can be taken as the time to failure. At lower temperatures (<140oC), thermal oxidative stability is more often assessed by measurement of tensile properties.

Systems based on hindered phenols alone or in combination with other additives represent the state of the art for the long-term thermal stabilization of polypropylene and TPO. The high molecular weight hindered phenol AO-1 tends to outperform other products when used without a synergist (Figure 4).[2]

Combinations of hindered phenols and thioethers (e.g. TS-1 and TS-2) usually provide superior performance as compared to hindered phenols used alone.[2] It has been reported that the synergy is particularly pronounced in the case of partially hindered phenols such as AO-5 (Figure 5).[19]

Figure 4. Long Term Thermal Stabilization of PP Homopolymer.

Heat aging at 149oC, compression molding, thickness 1 mm. All formulations contain 0.1% calcium stearate. Data from Reference [17]

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Figure 5. Long Term Thermal Stabilization of PP Homopolymer.

Heat aging at 160oC, compression molding, thickness 1 mm. All formulations contain 0.05% calcium stearate. Data from Reference [19]

It should be noted that systems based on thioethers are not appropriate for use in combination with HALS for light stabilized applications due to interactions between the components. In these cases alternative systems based on combinations of HALS and phenolic antioxidants can be employed (Figure 6).

Figure 6. Long-Term Thermal Stabilization of PP Homopolymer.

Compression molding, thickness 2 mm. All formulations contain 0.05% AO-1 + 0.1% P-1 + 0.05% calcium stearate.

Data from Reference [2]

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High molecular weight HALS are known contribute to the thermal oxidative stability of polypropylene,[2] particularly at lower temperatures (<120oC, Figure 6). However, at higher temperatures antagonistic interactions between HALS and phenolic antioxidants (especially AO-1) are observed with some HALS (e.g. HALS-4). As a result of such interactions, the long-term thermal stability of the HALS/AO combination may in some cases be lower than when the AO is used alone. The tertiary amine HALS-5 is particularly noteworthy in terms of its strong contribution to long-term heat stability as well as its lack of interaction with phenolic antioxidants (Figures 6 & 7). Systems based on combinations of hindered phenols (especially AO-1 or AO-3) with HALS-5 are ideal for applications in which both long-term thermal stability and light stability are required. Systems based on HALS-5 also provide excellent long-term thermal stability in talc-filled substrates and in formulations containing carbon black, in some cases outperforming conventional systems based on hindered phenols in combination with thioethers.

Figure 7. Long-Term Thermal Stabilization of PP Homopolymer.

Injection molding, thickness 1.6 mm. All formulations contain 0.1% AO-3 + 0.1% P-1 + 0.2% HALS-1 + 0.1% calcium stearate.

Data provided courtesy of Sabo S.p.A. and used with permission.

Light Stabilization Exposure of PP and TPO parts to ultraviolet radiation (naturally present in sunlight) initiates degradation through a photo-oxidative process. In polypropylene this process takes place predominately in the amorphous phase of the polymer due to the higher solubility of oxygen there as compared to the crystalline phase. Degradation as a function of depth has been studied.[3] In unstabilized polypropylene, oxidation takes place most rapidly at the surface of the part. In stabilized polymers, the extent of oxidation is much reduced but is still observed even deep within the part (> 1 mm below the surface).

Photo-oxidation of the polymer is accompanied by an increase in crystallinity due to a process known as chemi-crystallization.[20] When chain scission takes place during photo-oxidation, polymer chains in the amorphous phase are freed from some of their entanglements and can achieve sufficient mobility to crystallize. It is not clear whether crystallization takes place by formation of new crystals or by addition to existing ones. In any case, chemi-crystallization results in the formation of voids that manifest as surface micro-cracks. These eventually grow large enough to be seen with the naked eye. The increased roughness of the cracked surface also scatters light, which in turn results in a loss of gloss (and

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subsequently chalking) and changes in the perceived color of the part, particularly in pigmented materials. Formation of surface cracks is also a key factor in the embrittlement of polypropylene.

The resistance of a material to the effects of weathering is typically determined by measuring changes in the appearance or mechanical properties of a test specimen resulting from natural or accelerated weathering. Changes in appearance are most often assessed using instrumentation such as a spectrophotometer or a gloss-meter. In some cases changes can be assessed visually (e.g. onset of chalking) or microscopically (e.g. the formation of micro-cracks). Alternatively the weatherability of a material can be assessed by measuring changes in a mechanical property such as elongation at break, tensile strength, or impact strength as a function of exposure time.

Natural outdoor weathering is a slow process, even when conducted in tropical or desert climates with relatively high solar irradiance and temperature. Accelerated weathering devices (often referred to as weatherometers or WOM) have been developed to provide results more quickly. These devices provide faster results than natural weathering because they can be operated continuously (i.e. samples can be exposed for 24 hours per day rather than only during daylight hours) and also because the lamps used as radiation sources provide more intense radiation than natural sunlight. They also have the advantage that the total amount of radiation received by the sample can be carefully controlled and accurately measured. The most widely used weatherometers today utilize xenon-arc lamps as the radiation source, which when properly filtered produce a spectrum of radiation that is comparable to that of natural sunlight albeit more intense. Accelerated weathering methods utilizing xenon-arc weatherometers are described and specified in industry standards such as SAE J2412 (for automotive interior trim components), SAE J2527 (for automotive exterior materials), ISO 4892, and ASTM G155.

Systems based on hindered amine light stabilizers (HALS), alone or in combination with other additives (especially benzoates and UV absorbers) represent the state of the art for the light stabilization of polypropylene and TPO.[21]

The effects of molecular weight on HALS diffusion in polyolefin matrices are well documented, as well as the importance of diffusion in the stabilization of polypropylene.[22-24] Since the regenerative mechanism of HALS is less efficient in polypropylene than in other polyolefins,[14] over the lifetime of a part deactivation of the stabilizer can occur, especially near the surface. In thick section parts containing low molecular weight HALS (HALS-1, HALS-2), the stabilizer content at or near the surface can be replenished by diffusion of the additives from the deeper layers of the part. Replenishment is also possible for medium molecular weight HALS (HALS-3), albeit at a slower rate. For high molecular weight polymeric HALS diffusion is usually negligible and there is no replenishment mechanism. Thus the low- to medium-molecular weight HALS tend to give better protection in thick section polypropylene and TPO applications than high molecular weight HALS. However, for thin-section applications like fibers and films the replenishment mechanism is less important and high molecular weight HALS (HALS-4 and HALS-5) are favored due to their resistance to migration and loss by volatilization.

Combinations of hindered amines are well known in the art and are widely used.[25] In thick-section polypropylene applications, combinations of the low-molecular weight HALS-1 and the high-molecular weight HALS-4 have been shown to provide somewhat better performance than would be expected based on the performance of the individual components (Figure 8). Systems based on the high molecular weight HALS-5 are especially useful for the stabilization of mineral-filled substrates (Figure 9), and as previously mentioned also provide excellent long-term thermal stability.

The use of systems based on HALS alone (e.g. HALS-1 or HALS-2 alone, or their combinations with HALS-4 or HALS-5) is often sufficient to provide adequate weatherability to polypropylene and TPO. However, for some applications such as automotive exterior materials, further improvements in weatherability may be required. In these cases the performance of HALS can be enhanced by use in combination with other additives such as benzoates and UV absorbers.[21]

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Figure 8. Light Stabilization of Polypropylene Homopolymer.

Outdoor weathering in Florida, injection molding, 2 mm thickness * Failure defined as < 50% retention of impact strength.


Figure 9. Light Stabilization of Calcium Carbonate Filled Polypropylene Homopolymer.

Accelerated weathering, xenon arc weather-o-meter, 0.35 W/m2 (340 nm) Injection molding, thickness 2 mm

25% CaCO3, 1% TiO2, base stabilization 0.033% AO-1 + 0.067% P-1 Data provided courtesy of Sabo S.p.A. and used with permission.

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The synergy between benzoates and HALS in the stabilization of polypropylene has been known for some time.[26-28] As shown in Figure 10, the combination of HALS-2 with B-2 clearly gives better light stability than HALS-2 used alone. Synergistic behavior is also known for combinations of other HALS with either B-1 or B-2. The synergistic performance suggests that the benzoates may be more effective scavengers of certain types of free radicals involved in the degradation process than are HALS.

Figure 10. Light Stabilization of Black Reactor TPO.

Accelerated weathering, xenon arc weather-o-meter, SAE J1885 (auto exterior cycle). Data from Reference [28]

It has also been reported that like benzoates, the conventional hindered phenol AO-2 exhibits a synergy with HALS,[29,30] unlike the similar antioxidant AO-1 (Figures 11 & 12). This effect is not surprising given that an early mechanistic study found AO-2 to provide performance comparable to the benzoate B-2 for the stabilization of polypropylene films.[15] The positive effect for AO-2 as compared to AO-1 is possibly linked to the ability of the lower molecular weight AO-2 to migrate to layers near the surface where photo-degradation is occurring.[29-31]

Systems based on combinations of HALS with UV absorbers are also sometimes used for the stabilization of polypropylene and TPO. Because the screening effect of UV absorbers is directly dependent on the path length, they provide little or no protection at or near the surface of a part, where the path length is very short. However, for the deeper layers (> 1 mm below the surface) of thick section parts, all of the UV radiation can be blocked with incorporation of ~0.1% UV absorber and photo-degradation in these deeper layers is effectively prevented. This effect may serve to improve retention of mechanical properties after exposure. As illustrated in Figure 13, the combinations of HALS and UV absorbers provide some improvement in physical property retention in polypropylene after exposure but no improvement in surface protection (exposure time to cracking) as compared to HALS used alone.

UV absorbers can also improve surface protection indirectly if other stabilizers are able to migrate from the protected deeper layers to the layers closer to the surface in which photodegradation is taking place.[31]

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Figure 11. Light Stabilization of Polypropylene Homopolymer. Effect of Phenolic Antioxidant.

Accelerated weathering (carbon arc). Data from Reference [29]

Figure 12. Light Stabilization of Pigmented (Dark-Gray) High Impact Propylene/Ethylene

Copolymer (14% Ethylene). Effect of Phenolic Antioxidant.

Accelerated weathering (xenon arc, ASTM G26). Injection Molding, 3.2 mm thickness.


Figure 13. Light Stabilization of PP Homopolymer: Effect of UV Absorbers. Accelerated weathering (Ultra-Vitalux lamps), injection molding, 1 mm thickness.

* Failure defined as < 50% retention of tensile strength. Data from Reference [32]

> >

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Other formulation components such as pigments can also perform the function of blocking the penetration of UV radiation. In this situation the UV absorbers would not be expected to be effective as light stabilizer synergists. Generally the benefits of using a UV absorber are most prominent in natural and lightly pigmented parts. However, in the case of some organic pigments with limited lightfastness the use of UV absorbers as part of the formulation can be beneficial.[9] Here the UV absorber may be acting to improve the lightfastness of the organic pigments by quenching of their excited states or by another unknown mechanism.

Synergistic combinations of HALS, benzoates, and UV absorbers have also been reported.[33,34] To understand the reason for this three-component synergy, one has to consider that benzoates (unlike HALS) do absorb short-wavelength UV radiation (λ < 300 nm) and are vulnerable to photo-degradation themselves. Since the incorporation of a UV absorber into the formulation prevents photo-degradation at the deeper levels of the part, low molecular weight additives such as B-1, B-2, and AO-2 are able to migrate from these protected deeper layers to layers closer to the surface in which photo-degradation is taking place.[31]

Common Problems Associated with Stabilizer Additives While the use of stabilizer additives is essential to protect polypropylene and TPO during processing and end-use, they can give rise to unanticipated problems during part manufacturing or end use. Potential pitfalls include problems associated with migration (such as blooming and fogging), unexpected discoloration (warehouse discoloration or gas fading), and interactions with automotive coatings. Through proper selection of the additives, these problems can largely be avoided. Blooming and Fogging Excessive migration of additives is associated with problems such as blooming and fogging. Blooming (or exudation) of additives on part surfaces can be a problem after storage, and sometimes during end use. Low molecular weight additives can migrate to the surface of part where they crystallize as a separate phase, forming the powdery or chalky deposit on the surface commonly referred to as bloom.[35] Additives present in these surface deposits can often be identified using FT-IR spectroscopy.

In particular, the problem of blooming is known to be associated with the low molecular weight HALS-1,[28,36] which is thought to be due to its high diffusivity and relatively low solubility in polyolefin substrates. Besides being unsightly, exudates containing HALS-1 can present a safety concern due to its known skin irritation and sensitization effects. This problem can be easily eliminated by substitution of a more soluble low molecular weight HALS such as HALS-2.

Fogging is the condensation of volatile materials on the windshield, which originate from interior components. The tendency of plastic additives to contribute to this phenomenon is related to both the inherent volatility of the additive and its compatibility with the polymer matrix. The fogging behavior of plastics is assessed using standard test methods such as SAE J1756 in which polymer pellets are heated and volatile components collected on a cooled cover plate. HALS-1 has been found to be associated with automotive fogging.[36] As in the case of blooming, it is expected that this problem can be reduced by substitution of a more compatible low molecular weight HALS such as HALS-2 or a non-migratory high molecular weight HALS. Warehouse Discoloration or Gas Fading Oxidation products formed from hindered phenols can produce discoloration in plastic parts.[16] For example, a pink or yellowish discoloration is sometimes observed when parts are exposed to nitrogen oxides (NOx, formed during combustion of natural gas) during storage, which is referred to as “gas-

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fading” or “warehouse discoloration”.[9] This phenomenon is a particular problem in natural or light-colored parts, and appears most frequently in the winter due to the use of natural gas heaters and less efficient ventilation. The problem can also be aggravated by the presence of HALS in the formulation, especially secondary amine types such as HALS-1, HALS-2, and HALS-4.

The resistance of a material to this type of discoloration is typically assessed by exposing parts to controlled levels of NOx in an atmospheric fume chamber operated according to a test method such as AATCC 23. Color change in the parts is measured using a spectrophotometer as a function of exposure time.

Hindered phenols AO-1 and AO-2 are particularly prone to this problem, but as shown in Figure 14, other products can be substituted (AO-3, AO-4) that are non-discoloring under these conditions.[18] The partially hindered phenol AO-5 has also been reported to have excellent resistance to gas fade discoloration.[37] Processing stabilizer systems based on hydroxylamines are also inherently non-discoloring and can be used when long-term thermal stability requirements are modest.

Figure 14. Gas Fade Discoloration in LLDPE.

All formulations contain 0.15% HALS Data from Reference [18]

Coating Adhesion Many HALS are strongly basic, and this can lead to undesired interactions with acid-catalyzed thermoset acrylic automotive coatings used on TPO exterior parts.[9,36,38]. These interactions lead to effects such as incomplete cure of the coating and reduced adhesion between the coating and the plastic substrate. The interaction of low molecular weight, basic HALS (especially HALS-1, and possibly HALS-2 as well) can be particularly problematic.

The effects of HALS on coating adhesion are assessed by tests on painted plastic panels containing HALS. Abrasion tests on the painted panels (e.g. Taber Scuff, GM 9911P) give an indication of effects on coating cure, while tape peel tests (with or without crosshatch scoring of test specimens) are used to investigate adhesion between the substrate and coating.

For paintable applications, systems based on high molecular weight HALS in combination with non-basic NOR HALS have been developed and give good results.[9,38] Alternatively, systems based on

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combinations of benzoates with high molecular weight HALS (especially HALS-5) would also be expected to be non-interacting and also to provide excellent weatherability performance. Conclusions Polypropylene degrades when exposed to heat or light by an oxidative mechanism in which chain scission predominates. Oxidative degradation leads to a number of undesirable effects that compromise the useful life of the polymer.

Stabilizer additives are very effective for improving the stability of polypropylene and TPO to heat and/or light induced degradation. Most of these products act by scavenging the radical and peroxide intermediates in the oxidation process. Examples of commonly used stabilizer additives include hindered phenols, phosphites and phosphonites, thioethers, hydroxylamines, hindered amine light stabilizers (HALS), benzoates, and UV absorbers.

Degradation during melt processing results in melt flow shifts and discoloration. Combinations of hindered phenols and phosphites represent the state of the art for the stabilization of polypropylene and TPO under these conditions. Systems incorporating partially-hindered phenols (e.g. AO-4) and/or high performance phosphites (e.g. P-2, P-3, P-4) provide the highest level of performance in terms of melt stability.

Exposure to heat during end use in the solid phase eventually results in a loss of useful mechanical properties as well as discoloration. Systems based on combinations of hindered phenols and either thioethers or HALS (especially HALS-5) can significantly extend the useful life of polypropylene and TPO under these conditions. Systems based on HALS-5 in combination with hindered phenols are most suitable for use in applications where light stability is also required.

Light-induced degradation results in changes in appearance and a loss of mechanical properties. Systems based on HALS represent the state of the art for the light stabilization of polypropylene and TPO. When higher performance is needed, improvements can often be achieved by using synergistic combinations of HALS with other additives such as benzoates, ultraviolet absorbers, and some phenolic antioxidants (especially AO-2).

Stabilizer additives can sometimes give rise to unexpected, undesired effects such as blooming, automotive fogging, warehouse discoloration (gas fading), and interactions with automotive coatings. Through the proper selection of additives, these problems can largely be avoided.

References 1. Bolland, J., & Gee, G., Trans. Faraday Soc., 1946, 42, 236-243.

2. Schwarzenbach, K. et al., in “Plastics Additives Handbook”, 5th ed., H. Zweifel, Ed., Hanser Publishers, Munich, 2001, Ch. 1.

3. White, J., Shyichuk, A., Turton, T., Syrotynska, I., Poly. Deg. and Stab., 2006, 91, 1755-1760.

4. Pospisil, J., in Developments in Polymer Stabilisation, G. Scott, Ed., Elsevier, 1979, Vol. 1, Ch. 1.

5. Denney, D., Goodyear, W., Goldstein, B., J. Am. Chem. Soc., 1960, 82, 1393-1395.

6. Armstrong, C., Husbands, M., Scott, G., Eur. Poly. J., 1979, 15, 241-248.

7. Kikkawa, K., Poly. Deg. and Stab., 1987, 18, 237-245.

8. Yagi, M., Nishina, T., Sugibuchi, K., U.S. Patent No. 5,081,170, January 14, 1992.

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9. Botkin, J., Solera, P., “Stabilizer Selection Guidelines for Automotive TPO Applications”, presented at the Society of Plastics Engineers Global Automotive TPO Conference, Dearborn, MI, October 2001.

10. Botkin, J., Stadler, U., U.S. Patent No. 7,291,669 B2, November 6, 2007.

11. Klemchuk, P. P., Gande, M. E., Poly. Deg. and Stab., 1988, 22, 241-74.

12. Allen, N. S., Chirinis-Padron, A., Henman, T. J., Poly. Deg. and Stab. 1985, 13, 31-76.

13. Shilov, Yu. B., Battalova, R. M., Denisov, E. T., Doklady Akademii Nauk, SSSR, 1972, 207, 388.

14. Gryn’ova, G., Ingold, K., Coote, M., J. Am. Chem. Soc., Articles ASAP (As Soon As Publishable), Publication Date (Web): April 11, 2012 (Article), DOI: 10.1021/ja3006379.

15. Allen, N., Parkinson, A., Loffelman, F., Susi, P., Poly. Deg. and Stab., 1983, 5, 241-266.

16. Klemchuk, P., Horng, P.-L., Poly. Deg. and Stab., 1991, 34, 333-346.

17. Gugumus, F., in “Plastics Additives”, 3th ed., Gachter, R., Muller, H., Eds., Hanser Publishers, Munich, 1990, Ch. 1.

18. Vulic, I., Vitarelli, G., Zenner, J., Macromol. Symp., 2001, 176, 1-15.

19. Yachigo, S., Sasaki, M., Kojima, F., Poly. Deg. and Stab., 1992, 35, 105-113.

20. White, J., Rabello, M., Polymer, 1997, 38, 6379-6387.

21. Botkin, J., “Light Stabilization of Polypropylene: An Independent Perspective,” presented at the Society of Plastics Engineers International Polyolefins Conference, Houston, TX, February 2007.

22. Malik, J., Hrvik, A., Tomova, E., Poly. Deg. and Stab., 1992, 35, 61-66.

23. Gugumus, F., Poly. Deg. and Stab., 1999, 66, 133-147.

24. Tobita, E., Fukushima, M., Funamizu, T., Zingde, G., Goman, P., “Effect of HALS on Stabilization of Filled and Pigmented Polypropylene”, presented at the SPE Global Automotive TPO Conference, Dearborn, Michigan, October 2003.

25. Gugumus, F., Poly. Deg. and Stab., 2002, 75, 295-308.

26. Mathis, R., U.S. Patent No. 4,035,323, July 12, 1977.

27. Nagasaki, H., Yachigo, S., Takata, T., Yamamoto, H., Takahashi, Y., U.S. Patent No. 4,985,479, January 15, 1991.

28. Davis, L., Zenner, J., Stretanski, J., “Extending the Useful Lifetime of Pigmented PP and TPO”, SPE International Polyolefins Conference 2006, Houston, March 2006.

29. Kikkawa, K., Poly. Deg. and Stab., 1995, 49, 135-143.

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31. Shlyapintokh, V., Pure and Appl. Chem., 1983, 55, 1661-1668.

32. Disteldorf, J., Haage, H.-J., Libera, H., U.S. Patent No. 4,986,932, January 22, 1991.

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35. Calvert, P., Billingham, M., J. Appl. Poly. Sci., 1979, 24, 357-370.

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36. Goldstein, S., Coughlin, G., “Recent Advances in UV Stabilization Systems for Automotive TPO’s”, presented at the SPE Automotive TPO Global Conference, Sterling Heights, Michigan, October 2007.

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38. Lau, E., Edge, D., U.S. Patent No. 5,733,956, March 31, 1998.

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Appendix: Some Commercially Available Stabilizer Additives Chemical Structure CASRN

AO-1

O

OO

O

O

O

O

OH

OH

OH

OH

O

6683-19-8

AO-2

OH

O

OC18H37

2082-79-3

AO-3

N

N

N

OH

OH

OH

O

OO

27626-62-6

23


AO-4

N

N

NCH

2

CH2

CH2

OH

CH3

CH3

O

OH

CH3

CH3

O

OH

CH3

CH3

O

40601-76-1

AO-5 OH

O

O

O

O

O

O O

O

CH3

CH3

CH3

CH3

CH3

OH

CH3

90498-90-1

P-1

OPO

O

31570-04-4

P-2

O P

O

O

O

O

P O

26741-53-7

P-3

O P

O

O

O

O

P OCH3

CH3

80693-00-1

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P-4

PP

O

O

O

O

119345-01-6

TS-1

S

O

O C18H37

OH37C18

O

693-36-7

TS-2

O

O

O

O

S

S

C12H25

C12H25

O

O

O

O

S

SH25C12

H25C12

29598-76-3

H-1 R

N

OH

R

R = C14-18

143925-92-2

H-2 N

OH

621-07-8

H-3 N

+

R

R

CH3

O R = C14-24

204933-93-7

HALS-1

O

O

O

O NH

NH

52829-07-9

HALS-2 R = C11-20predominantly C16-18N

O

O

R

H

167078-06-0

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HALS-3

O

O

OO

NCH

3

NCH3

OO

N

CH3

O O

N

CH3

91788-83-9

HALS-4

N

N (CH2)6

N

NN

NN

NH

N

N

(CH2)6

N H

N

H

C8H17

HHHHn

71878-19-8 70624-18-9

HALS-5

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

NBu

Bu

H

N

Bu

N

Bu N

CH3

N

CH3

NCH3

N Bu

N

CH3

N

Bu

N

Bu

N CH3

N

CH3

N

CH3

H

N

Bu

N

CH3

106990-43-6

UVA-1 O OH

OC8H17

1843-05-6

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UVA-2

NN

N OH

25973-55-1

UVA-3

NN

N OH

CH3

Cl

3896-11-5

UVA-4

N N

N

OH

OC8H17

CH3

CH3

CH3

CH3

2725-22-6

B-1

OH

O

O

4221-80-1

B-2

OH

O

OC16H33

67845-93-6

The Stabilization of Polypropylene and TPO: An Overview

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