Nano ZnO as cure activator and reinforcing filler in natural rubber

Nano ZnO as Cure Activator and Reinforcing Fillerin Natural Rubber

Bindu Panampilly, Sabu ThomasSchool of Chemical Sciences, Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University,Kottayam 686 560, Kerala, India

Zinc oxide (ZnO) nanoparticles of size 20–90 nm andsurface area 9.56 m2/g were synthesized from ZnCl2and Chitosan and characterized by X-ray diffraction,high resolution transmission electron microscopy(HRTEM), and scanning electron microscopy (SEM).Natural rubber (NR) vulcanizates containing nano ZnOwas prepared by mill mixing and characterized bySEM, energy dispersive X-ray analysis (EDAX), andHRTEM. Cure characteristics, free volume studies,bound rubber, crosslink density, and dynamic mechani-cal properties were evaluated and compared with thatof NR vulcanizate containing conventional micro ZnO.Considering the cure characteristics, it was found thatNR vulcanizate with 0.5 phr (parts per 100 g rubber) ofnano ZnO showed low values of optimum cure time(t90) and very high cure rate index compared with 5 phrof conventional micro ZnO. The study shows thatmicro ZnO can be successfully replaced with nanoZnO for accelerated sulfur vulcanization process in NR,and preparation of vulcanizate containing nano ZnOwith better properties as that of micro ZnO. The opti-mum dosage of nano ZnO as a cure activator in NRvulcanization was found to be 0.5 phr compared withconventional grade micro ZnO. This will lead to sub-stantial cost reduction in the manufacture of rubberproducts and alleviate environmental pollution due toexcess ZnO in rubber compounds. POLYM. ENG. SCI.,00:000–000, 2013. ª 2013 Society of Plastics Engineers

INTRODUCTION

The elastomers or rubbers play an important role in

modern technology. The properties displayed by a particu-

lar rubber are determined by the compound composition,

proper combination of chemicals and additives, and the

vulcanization process. The vulcanization involves the con-

version of raw rubber into a network through the forma-

tion of crosslinks, a fluid material is transformed into an

elastic rubbery product [1]. When more crosslinks are

formed, the network becomes tighter and the forces nec-

essary to achieve a given deformation increase. In rubber

vulcanization process, the vulcanization agents are mostly

based on sulfur or peroxide. The rubbers containing unsat-

urated side groups or unsaturated backbone can undergo

very effective sulfur vulcanization. The type of crosslinks

formed in the case of sulfur vulcanization, largely

depends on the vulcanization system, i.e., vulcanization

chemicals which are added to the rubber. The crosslinks

may be predominantly mono-, di-, or polysulfides. The

chain length of the polysulfides in the crosslinks affects

the thermal stability and the physical properties of a par-

ticular vulcanizate [2].

Zinc oxide (ZnO) is one of the basic components of rub-

ber compounds. In natural rubber (NR), ZnO, stearic acid,

accelerators and sulfur constitute the vulcanization system.

The ZnO acts as an activator for rubber crosslinking by sul-

fur or sulfur donors [3, 4], it increases the amount of bound

sulfur and the efficiency of the crosslinking system [5]. It

can also be used as a good crosslinking agent in carboxylic

rubbers [6, 7], elastomers containing halogen groups [8],

and also used as fillers in elastomers [9]. The physical

properties of vulcanizates, namely heat build up and abra-

sion resistance are influenced by ZnO.

In the vulcanization of NR, conventional ZnO (micro)

is used as an activator and in general practice about 5

phr is the dosage. The reduction in the quantity of ZnO

is worthwhile because it is relatively expensive pigment

and has high density. Of late considering this problem,

the use of nano ZnO in NR [10–12] and neoprene rubber

was studied [13]. Some soluble zinc compounds have

been classified as toxic to aquatic species. Moreover,

Environmental Protection Agency has determined that

ZnO is a toxic chemical and lesser the amount used the

least will be the difficulty in the disposal of waste rub-

ber compounds. The European Union has also classified

ZnO as hazardous chemical for the environment and has

proclaimed that its application in rubber technology

should be reduced and controlled [14]. Considering this

ecological point of view, it is advisable to keep ZnO

content as low as possible in making the rubber products

with ZnO.

Correspondence to: Bindu Panampilly; e-mail: bindu_patanair@yahoo.

com or [email protected]

DOI 10.1002/pen.23383

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2013 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2013

The aim of this study is to investigate the use of nano

ZnO as cure activator in NR vulcanization instead of

conventional micro ZnO. We expect that this study is an

important contribution and very scant information was

reported [10, 12] in the literature regarding the use of

nano ZnO as cure activator in the sulfur vulcanization of

NR. The morphology of NR vulcanizates was analyzed by

scanning electron microscopy (SEM) and high resolution

transmission electron microscopy (HRTEM). The dynamic

mechanical analysis (DMA), free volume measurements by

Positron Annihilation Lifetime Spectroscopy (PALS), cal-

culation of crosslink density, bound rubber content and

diffusion coefficient of vulcanizate containing micro and

nano ZnO have been carried out and presented in this arti-

cle. It is noticeable that the low value of cure time and high

value of cure rate index of vulcanizate with 0.5 phr of

nano ZnO are extremely important in the sulfur vulcaniza-

tion of NR and can be applied in the industrial scale. This

will lead to economic advantages and environmental safety

in disposing waste rubber compounds.

EXPERIMENTAL

Materials

The NR, ISNR-5 of Mooney viscosity ML (1 þ 4) at

1008C equal to 85 was obtained from the Rubber

Research Institute, Kottayam, Kerala, India. Sulfur, ZnO,

and stearic acid were obtained from Merk, Mumbai,

India, and accelerators, namely mercaptobenzothiazole

disulfide (2,20-dithiobenzothiazole, MBTS) and tetrame-

thylthiuram disulfide (TMTD) were supplied by NOCIL,

Mumbai, India. Chitosan sample was provided by M/s

India Sea Foods, Cochin, Kerala, India. Zinc chloride and

Sodium Hydroxide were supplied by M/s. S.D. Fine

Chem, Mumbai, India. Nano ZnO was synthesized from

Chitosan, Zinc Chloride, and Sodium Hydroxide.

Sample Preparation

Synthesis of Nano ZnO. Nano ZnO was synthesized

[15] from Zinc Chloride and chitosan. A 5% ZnCl2 (5 g in

100 ml 1% acetic acid) was mixed with 1% solution of chi-

tosan (1 g in 100 ml 1% acetic acid) and stirred for 21 h.,

followed by slow addition of stoichiometric amount of

NaOH (5%) with constant stirring. The whole mixture was

allowed to digest for 24 h. at room temperature. During

this time, OH2 and Cl2 ions diffused through the medium

and white gel like precipitate of Zn(OH)2 was formed. This

was filtered and washed thoroughly with distilled water to

remove unreacted chitosan and other by product such as

NaCl. This was dried at 608C and calcined at 5508C for

4 h. in a muffle furnace to get nano ZnO.

Preparation of NR Composites Containing Micro/Nano

ZnO. Dry NR (100 g) was masticated in a two-roll

mill, followed by 0.5 g nano ZnO, 2.5 g stearic acid

(co-activator) were added and again masticated. Milling was

continued by the successive addition of the accelerators

MBTS (1 g), TMTD (0.2 g), and the vulcanizing agent sulfur

(2.5 g). The NR composites with varying weight percentage

of nano ZnO, namely 1 g and 2 g were also prepared in the

same manner. The same experiment was repeated with 5 g

conventional micro ZnO instead of nano ZnO. Table 1

shows the formulation [10, 12] used for preparation.

Characterization Methods

Fourier transform infrared (FTIR; Schimadzu IR-470

IR spectrophotometer) (KBr pellet) spectrum of the nano

ZnO was recorded in the range 4000–400 cm21. X-ray

diffraction (XRD) of nano ZnO was recorded with the

help of Xpertpro-Panalytical (k ¼ 1.54060 A, Cu-Ka)machine. The surface area of nano and conventional

micro ZnO was determined by using Micromeritics-ASAP

2020 Surface Area and Porosity Analyzer (Software

V3.01G). SEM images of the ZnO nanoparticles were

recorded using FE-SEM SUPRA-25 ZEISS analyzer. The

SEM images of the rubber vulcanizates were taken with

the help of JEOL JSM 6390 Analyzer with W-filament

and secondary electron imaging. The samples were coated

with a thin film of gold. The EDS system (EDAX-

Energy Dispersive X-ray analysis) (Oxford Instruments)

with resolution at 20 kV was used. HRTEM of nano ZnO

was taken with JEOL-JEM 3010 instrument; the nano

ZnO was dispersed in acetone by ultrasonicator and a

drop of it was deposited on carbon-coated copper grid

and was analyzed under an accelerating voltage of 200

kV. The nanocomposite sample for TEM analysis was

prepared by ultracryomicrotomy with a Leica Ultracut

UCT (Leica Microsystems GmbH, Vienna, Austria).

Freshly sharpened glass knives with cutting edges of 458were used to obtain cryosections of about 100–150 nm

thickness at 21108C. The cryosections were collected

individually in sucrose solution and directly supported on

a copper grid of 300 mesh. Microscopy was performed

with JEOL 2100 (Japan). Transmission electron micro-

scope was operated at an accelerating voltage of 200 kV.

The cure characteristics of the NR vulcanizates were

studied using MDR-2000 Rheometer at 1508C. The cure

TABLE 1. Formulation and compound designation for NR.

Formulation Compound designation

NR–M5 NR–N0.5 NR–N1 NR–N2

NR 100 100 100 100

ZnO 5.0 0.5 1.0 2.0

Stearic acid 2.5 2.5 2.5 2.5

MBTS 1.0 1.0 1.0 1.0

TMTD 0.2 0.2 0.2 0.2

Sulfur 2.5 2.5 2.5 2.5

ZnO used Conventional

micro

Nanoparticles Nanoparticles Nanoparticles

All weights are in parts per hundred grams of rubber.

2 POLYMER ENGINEERING AND SCIENCE—-2013 DOI 10.1002/pen

time and scorch time were analyzed. The bound rubber,

RB measurements of each uncured sample were carried

out. The percentage of bound rubber content of the poly-

mer, RB, was calculated as the weight percentage of the

unsolubilized rubber on the filler surface, according to the

following equation [16]:

RB ¼ Wfg �W mf

mfþmpð Þ

� �

Wmp

mfþmpð Þ

� �� 100 (1)

where Wfg is the weight of the gel residue, mf is the

weight of the filler in the compound, mp is the weight of

the polymer in the compound and W is the initial weight

of the specimen.

For the measurement of water resistance and calcula-

tion of diffusion coefficient, circular samples of 2 cm

diameter were cut from polymer sheets by means of a

standard die. The thickness and initial weight of the sam-

ples were noted. Distilled water was allowed to diffuse

through the samples at room temperature, and weight of

the samples was checked every hour. The plot of time vs.

weight % of nano ZnO gives a measure of water uptake.

Water diffusion coefficients were also measured from the

slope of these graphs. Crosslink density values of the

samples were determined by an equilibrium swelling

method using toluene as the solvent. Circular samples

were punched from the sheet, the weighed samples were

kept in toluene until they reached equilibrium swelling,

and then the weights were taken for the calculation.

Positron annihilation lifetime spectra were used to

determine the free volume present in NR vulcanizates.

The Positron Lifetime Spectrometer consists of a fast–fast

coincidence system with BaF2 scintillators coupled with

photomultiplier tubes of type XP2020/Q with quartz win-

dow as detectors in conical shape to achieve better time

resolution. A 17lCi 22Na positron source deposited on a

pure Kapton foil of 0.0127 mm thickness was placed

between the two identical pieces of the sample under

investigation, and this was placed between the two detec-

tors of PALS to acquire lifetime spectrum. The spectrom-

eter was operated at 180 ps time resolution with 60Co as

the source, however for better count rate [17], it was

operated at 220 ps time resolution. All measurements

were performed at room temperature. In PALS analysis of

polymers, only two measured parameters are important,

namely ortho-positronium (o-Ps) lifetime (s3) which

measures the size of the free volume holes (Vf) and o-Psintensity I3 which is a relative measure of the number of

free volume sites in the polymer matrix. The free volume

cavity radius (R) is related to the o-Ps pick-off lifetime

(s3) by a simple relation. The free volume is assumed as

a spherical potential well surrounded by an electron layer

of thickness DR, and the size of the free volume holes

can be calculated using the following semiempirical

equation [18]:

t3 ¼1

21� R

R0

� �þ 1

2p

� �sin

2pRR0

� �� 1

(2)

where t3 is the o-Ps lifetime and R is the radius of free

volume holes, which are expressed in ns and nm respec-

tively. R0 ¼ R þ DR, where DR (¼0.166 nm) represents

the thickness of the electron layer or the probability of

overlap of the Ps wave function with the electron wave

function [19]. Using this value of R, the free volume size

(Vf) was calculated as Vf ¼ (4/3)pR3. Then the fractional

free volume Fv was evaluated as, Fv ¼ CVfI3 where the

constant C is 0.0018 A23 [19].

DMA was carried out using NETZSCH DMA 242

machine in the temperature range 21408C to 1108C, at aheating rate of 28C/min and three different frequencies

10.00, 1.00, and 0.10 Hz. The sample dimensions 60 mm

3 10 mm 3 2 mm (length 3 breadth 3 thickness) were

kept in a dual cantilever mode and measurements were

performed in air atmosphere.

RESULTS AND DISCUSSION

The ZnO nanoparticles were synthesized from chitosan

and ZnCl2 [15]. The chitosan is a linear polymine with re-

active NH2, ��OH, and ��NHCO�� (��NHCO�� in par-

tially deacetylated) groups and can easily form chelates

with transition metal ions. In the present reaction chitosan

acts as a chelating agent for Zn2þ ions to form Zn–chito-

san complex.

FTIR Spectroscopy

In the IR spectrum of nano ZnO, the bands correspond

to m(ZnO) appeared at 1018 cm21 and 490 cm21 as

reported earlier [11].

FIG. 1. XRD pattern of nano ZnO.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2013 3

X-Ray Diffraction

The XRD pattern of nano ZnO was compared with the

standard values of nano ZnO from JCPDS files (JCPDS

no. 36-1451). The observed values were in good agree-

ment with the standard values, and the observed XRD

pattern (Fig. 1) was similar to the earlier report [11]. The

XRD pattern indicates the formation of hexagonal

wurtzite phase of ZnO. The sharp diffraction peaks indi-

cate good crystallinity of the prepared particles and no

evidence of bulk remnant materials and impurity. The av-

erage crystallite size was tentatively calculated from XRD

peak width of (101) based on the Scherrer equation [20].

Cs ¼ Kk/b cos y, b is the integral half-width, K is a

constant (approximately equal to unity), k is the wave-

length of the incident X-ray (k ¼ 0.154 nm), Cs is the

crystallite size, and y is the Bragg angle. The average

crystallite size calculated for synthesized nano ZnO was

23.7 nm. The crystallite size is assumed to be the size of

a coherently diffracting domain and it is not necessarily

the same as particle size [20].

Scanning Electron Microscopy

Figure 2 shows the SEM image of ZnO nanoparticles.

The individual ZnO nanoparticles have the length about

100–195 nm and the diameter 20–90 nm range.

High Resolution Transmission Electron Microscopy

The HRTEM image of nano ZnO is shown in Fig. 3.

The particle size varies from 20 to 90 nm with an

average particle size of 30 nm. The SEM and HRTEM

images show identical morphology of nanoparticles.

Surface Area Measurements

The surface area of nano ZnO and micro ZnO was

measured by Brunauer–Emmett–Teller method and it was

found to be 9.56 m2/g and 4.51 m2/g respectively. The

surface area of nano ZnO varies with the reaction condi-

tions and the various synthetic routes adopted. In the pres-

ent case, the surface area reported was small because of

agglomeration of nanoparticles.

Cure Characteristics of NR Nanocomposites

The cure characteristics of NR micro and nano ZnO

composites were done at 1508C and presented in Table 2.

FIG. 2. SEM image of nano ZnO.

FIG. 3. HRTEM image of nano ZnO.


The cure characteristics of NR vulcanizates with low

loading of nano ZnO, namely 0.25, 0.30, and 0.40 phr

were checked and was found that low loading of nano

ZnO was not effective for vulcanization. The sudden mar-

ginal decrease in the optimum cure time to 3.45 min with

the addition of 0.5 phr of nano ZnO is very interesting

and highlights the importance of the nano ZnO in the cure

chemistry of NR.

The curing characteristics were expressed in terms of

the vulcanization parameters such as, tS2 (scorch time), t90(optimum cure time), and cure rate index CRI, expressed

as CRI ¼ 100/(t90 2 tS2) were compiled in Table 2. The

optimum cure time, t90, was reduced by the incorporation

of low amount (0.5 phr) of nano ZnO, showing accelerated

vulcanization process with respect to that of NR vulcani-

zate with micro ZnO. It was deduced that the nano ZnO

behaves as an effective vulcanizing agent and cure activa-

tor for NR. These results were confirmed by the cure rate

index values, CRI, which show a significant increase with

addition of nano ZnO instead of conventional micro ZnO.

Furthermore, these results suggest that the NR became

more crosslinked in the presence of nano ZnO.

Here, it was found that nano ZnO, combined with ste-

aric acid, reduces the vulcanization time and improves

rubber properties. The stearic acid helps to solubilize the

Zinc into the system and liberate Zinc ions free to form

complexes with accelerators [21]. The accelerators

(Fig. 4) used were MBTS and TMTD, which contain one

or two sulfur atoms between a pair of organic end groups.

Structurally, all accelerators contain a common function-

ality, N¼¼C��S. When nano ZnO was used as the cure ac-

tivator, due to its large surface area the reaction between

nano ZnO and stearic acid became easy. Thus, large

amount of Zinc Stearate molecules were formed, i.e. the

interfacial area for reaction is increased and the diffu-

sional resistance is reduced by the smaller interparticle

distances. Hence, vulcanization became easy in the pres-

ence of nano ZnO. When 0.5 phr nano ZnO was used, the

particles were better dispersed within the matrix and as a

result the process can be easily activated by the formation

of Zinc Stearate.

It was assumed that when nano ZnO was used as the

cure activator, the mechanism of vulcanization is the

same as that of micro ZnO. Because the surface area of

nano ZnO is twice that of micro ZnO, its reaction with

stearic acid and accelerators are very fast and hence

accelerates the formation of Zinc accelerator complex

which is the key step in the vulcanization process [6, 22].

This complex is more reactive than the free accelerator

and allows sulfur insertion to occur more rapidly than in

the case of micro ZnO [23]. The Zn-accelerator complex

interact with sulfur or sulfur donors to form the very

active sulfurating agent, which reacts with allylic sites of

rubber chains to form increased number of crosslink pre-

cursors [24]. The crosslink precursors can react with

another polymer chain to generate increased number of

crosslinks. It was assumed that the high surface area of

nano ZnO accelerates the adsorption of accelerators, sul-

fur, and stearic acid on its surface and hence can act both

as a reactant and a catalytic reaction template. From this

point of view, the dispersion of nano ZnO in the elasto-

mer matrix is the most important parameter. The less cure

time and very high cure rate index of NR vulcanizate

containing nano ZnO indicate the high efficiency of nano

ZnO as an effective vulcanizing agent and cure activator

for NR. This point helps to highlight the importance of

nano ZnO in the vulcanization of NR and can be applica-

ble to rubber industries.

Morphology

The phase morphology of vulcanizates with micro and

nano ZnO was taken. Figures 5a–c and 6a and b show the

SEM images and EDAX of rubber vulcanizates containing

nano and micro ZnO. The ZnO nanoparticles appear as

white dots. From the SEM image, it was clear that the

TABLE 2. Cure characteristics at 1508C of NR vulcanizates containing

micro and nano ZnO.

Composition

Optimum cure

time*, t90 (min)

Scorch time,

ts2 (min)

CRI ¼ 100/(t90 2 ts2)

(min21)

NR–N0.5 3.45 2.59 116.28

NR–N1 4.31 3.12 84.03

NR–N2 6.31 5.00 76.34

NR–M5 7.06 4.21 35.09

*Is the time to achieve 90% cure. At this state, crosslinks are

formed, the vulcanized compound becomes tighter and the forces (stress)

necessary to achieve a given deformation increase. The most useful

information obtained from the rheometer curve is at ‘‘t90’’.

FIG. 4. Structure of accelerators.


nano ZnO was embedded within the matrix. The EDAX

confirms the presence of nano ZnO embedded in the rub-

ber matrix. The samples were coated with a film of Au

before electron beam irradiation so the presence of Au

peak in the EDAX. The SEM image of NR–N0.5 (Fig.

5a) exhibits uniform distribution of nano ZnO particles in

the NR matrix, whereas the SEM of NR–N2 (Fig. 5b)

exhibits slight agglomeration of nano ZnO particles in the

NR matrix; the agglomeration of nanoparticles decrease

the rate of adsorption of accelerators, sulfur, and stearic

acid on its surface and further formation of Zinc Sulfurat-

ing complex hence the low cure rate index for NR–N2.

The HRTEM images of NR–N0.5 and NR–N2 were

presented in Fig. 7a and b, respectively. The dark spots in

the TEM images show the dispersion of nano ZnO in the

rubber matrix. It can be seen that, when nano ZnO

content is 0.5 phr, it is well dispersed in the matrix

compared with 2 phr. When the concentration of nano

ZnO is 2 phr, there is agglomeration of the particles.

Water Resistance

The diffusion of water is strongly influenced by the

microstructure of the nanocomposites. Water absorption

FIG. 5. SEM images of(a) NR–N0.5, (b) NR–N2, and (c) EDAX of

NR–N0.5.

FIG. 6. SEM image of (a) NR–M5 and (b) EDAX of NR–M5.


of NR vulcanizates was plotted as a function of time in

Fig. 8 and was observed that the water uptake of NR–

N0.5 and NR–M5 vulcanizates exhibit a similar trend.

Water uptake ð%Þ ¼ Wt �W0ð ÞWw

� 100 (3)

where Wt is the weight of the sample at time (1 h, 2 h,

. . .), W0 is the weight of the sample at zero time, and Ww

is the weight of the sample at infinite time.

Water diffusion coefficient (D) of the nanocomposites

can be calculated using the following equation [25]:

D ¼ phy4Mw

� �2

(4)

where D is the water diffusion coefficient, h is the thick-

ness of each sample, y is the slope of the linear portion

of the curve, and Mw is the weight of each sample at

infinity. The water diffusion coefficients of NR–N0.5 and

NR–M5 vulcanizates were calculated and compared; the

values are 1.32 � 1022 cm2 s21 and 1.42 3 1022 cm2

s21, respectively.

Bound Rubber

Bound rubber (RB) measurements were carried out

(Table 3) to understand rubber–filler interactions. Bound

rubber is the rubber that is trapped by the filler aggregates

after mixing. The rubber chains are attracted either physi-

cally or chemically to form a rubber shell on the surface

of the nanofiller particles. The bound rubber fraction of

an uncured compound is the amount of rubber that is not

extracted when it is exposed to a good solvent. The bound

rubber determination is based on the assumption that the

fraction of rubber, which does not interact or is not bound

to the nanofiller is soluble in toluene. The bound rubber

content of NR–N0.5 is slightly higher than that of NR–

M5, due to the high surface area and uniform dispersion

of nano ZnO particles in the matrix. So, the higher the

bound rubber content, the higher is the rubber–filler inter-

action [26].

FIG. 7. TEM images of (a) NR–N0.5 and (b) NR–N2.

FIG. 8. Effect of time on the % of water uptake of NR vulcanizates.

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]


Crosslink Density

The crosslink density values can be calculated from

swelling method using the equation [27] and reported in

Table 3.

n ¼ 1

2Mc

(5)

where Mc is the molar mass between crosslinks of

dynamically vulcanized samples. Mc is calculated by equi-

librium swelling in toluene using the Flory–Rehner equa-

tion [28],

Mc ¼�rpVsV

1=3r

lnð1� VrÞ þ Vr þ wV2r

(6)

where rp is the density of the polymer, Vs is the molar

volume of the solvent, w is the rubber solvent interaction

parameter, and Vr is the volume fraction of swollen rub-

ber which is given by Ellis and Welding equation [29],

Vr ¼ðd � f1wÞr�1

p

ðd � f1wÞr�1p þ Asr�1

s

(7)

where d is the deswollen weight of the sample, f1 is the

volume fraction of the insoluble component, w is the ini-

tial weight of the sample, rs is the density of the sample,

and As is the amount of the solvent absorbed. The interac-

tion parameter is given by [30],

w ¼ bþ Vs

RTds�dp� �2

(8)

where Vs is the molar volume of the solvent, ds and dpare the solubility parameters of the solvent and polymer

respectively, R is the universal gas constant, T is the

absolute temperature and b is the lattice constant.

Thermodynamic effects occurring during swelling of the

elastomer chains were analyzed (Table 3), for understand-

ing the rubber–filler interaction in the nanocomposite.

Swelling of a sample depends on the crosslink density and

the solvent used. The expansion of the rubber in the pres-

ence of a solvent will significantly modify the conforma-

tional entropy (DS) and the elastic Gibbs free energy (DG).The elastic Gibbs free energy can be determined from the

Flory–Huggins equation [31],

DG ¼ RT ln 1� Vrð Þ þ Vr þ wV2r

� �(9)

From the statistical theory of rubber elasticity, DS can be

obtained from the equation, DG ¼ 2TDS, which indicates

that no change in internal energy of the network occurs on

stretching. The DS and DG, of the vulcanizates are reportedin Table 3. It should be noted that bound rubber content,

crosslink density and DS values of NR–N0.5 are slightly

higher than that of NR–M5, this is due to the high surface

area and uniform dispersion of nano ZnO particles in the

matrix. It is assumed that DG is closely related to the elas-

tic behavior of the material, i.e., the elasticity of NR–N0.5

is slightly higher than that of NR–M5. The results can be

attributed to better compatibility between nano ZnO and

rubber.

PALS Analysis

Free volume is a frequently used microstructure param-

eter that can reflect the motion of polymer chain seg-

ments, and thus plays an important role in deciding the

mechanical properties, diffusion of small molecules

through polymer, and so on. PALS is based on the fact

that the positronium (Ps) atoms formed in polymers are

predominantly localized in the atomic scale and nanoscale

holes [32], and is an important method to detect nano-

scale holes in polymers. The purpose of this research is

mainly to investigate the effects of nanodispersed ZnO on

the free volume characteristic of NR–N0.5.

The diffusion of permeant through polymeric mem-

branes can be described by molecular and free volume

theories. According to free volume theory, the diffusion is

not a thermally activated process as in a molecular model,

but it is assumed to be the result of random redistribu-

tions of free volume voids within a polymer matrix [33]

and free volume provides the pathways for the permeant.

The effect of nano ZnO and micro ZnO on o-Pslifetime (s3), o-Ps intensity (I3, %), and fractional free vol-

ume (%) of NR–N0.5 and NR–M5 are presented in Table

4. It can be seen from the table that the free volume cell

size and fractional free volume are slightly less for NR–

N0.5 when compared with NR–M5. The reason for the

slight decrease in free volume cell size of NR–N0.5 was

due to the restricted mobility of the chain segments in the

presence of nano ZnO which establish more crosslinks,

thereby increasing crosslink density, and it also reduces

permeation by virtue of increasing diffusion path length.

TABLE 3. Variation in crosslink density, molar mass between

crosslinks, bound rubber and thermodynamic parameters DG and DS of

NR vulcanizates.

Composition

Bound

rubber

(%)

Crosslink

density

m 3 1025

(mol/g)

Molar mass

between

crosslinks

Mc (g/mol)

DG(J/mol)

DS 3 1022

(J/mol)

NR–N0.5 3.39 5.60 8928.57 216.34 5.32

NR–M5 3.24 5.24 9541.98 215.01 4.89

TABLE 4. PALS measurement data of NR vulcanizates.

Composition

o-Ps

lifetime,

s3 (ns)6 0.02

o-Ps

intensity,

I3 (%)

6 0.13

Free

volume

cell size,

Vf (A3)

Fractional

free volume,

Fv (%)

NR–N 0.5 2.42 13.67 140.05 3.45

NR–M 5 2.45 14.40 142.20 3.69


The contact surface area between the filler and the rubber

matrix was higher in NR–N0.5 due to its high aspect ratio,

which in turn reduces the free volume concentration (Fv).

However, the data indicate that the nature of crosslinks and

their restricted mobility of the chain segments in NR–M5

and NR–N0.5 is slightly different and addition of 0.5 phr

of nano ZnO, not only accelerates and catalyze the sulfur

vulcanization process but also brings better rubber–filler

interaction in NR–N0.5 compared with NR–M5.

Dynamic Mechanical Analysis

The effect of nano ZnO on the dynamic mechanical

property of NR–N0.5 and NR–M5 was analyzed by

DMA. The DMA of NR–N0.5 and NR–M5 was done in

the dual cantilever mode at three different frequencies,

namely 10.0, 1.0, and 0.1 Hz. The storage modulus (E0),loss modulus (E00), and tan d (¼E00/E0) were characterized

as functions of temperature and the plot of storage modu-

lus vs. temperature was shown in Fig. 9. The storage

modulus (E0) is the measure of stiffness where the elastic

property is higher. The higher the E0 value, the higher

will be the stiffness and load bearing capability of any

material. Comparing NR–N0.5 and NR–M5, it was found

that incorporation of nano ZnO increases the storage mod-

ulus, loss modulus and exhibits the best mechanical stiff-

ness compared with NR–M5. The reinforcement at the

nano level was the highest thereby restricting the mobility

of the chain segments. The plot of log E0 vs. temperature

(Fig. 9) gives an information about the increase in

the storage modulus at the high temperature region, i.e.

25–808C for NR–N0.5. The reinforcing effect is higher

for NR–N0.5 in the region above the glass transition tem-

perature of the matrix.

Finally, the optimum loading of 0.5 phr of nano ZnO

for the superior properties can be explained as follows. It

was clear from cure characteristics, free volume studies,

bound rubber, crosslink density, diffusion studies, and

dynamic mechanical properties that the high surface area

and nano level dispersion of nano ZnO accelerates vul-

canization process, reduces vulcanization time, and

improves the properties of the resulting vulcanizate. The

reinforcement at nano level was very high, hence restricts

the mobility of the rubber chains and brings high rubber–

nanofiller interaction in NR–N0.5 vulcanizate compared

with NR–M5. It was also found that further loading of

nano ZnO causes aggregation of nanoparticles in the rub-

ber matrix and thereby reduces rubber–nanofiller interac-

tion and this adversely affects the properties of the vul-

canizate.

CONCLUSIONS

Based on the above results and discussions, it has been

observed that the addition of 0.5 phr nano ZnO in NR

decreases cure time and increases cure rate index. How-

ever, bound rubber content, crosslink density, and DS val-

ues are slightly higher, diffusion coefficient, free volume

cell size, and fractional free volume are slightly lower for

NR–N0.5 compared with NR–M5. When the nano ZnO

was introduced into rubber, they occupy the holes/pores

within the matrix. The nano ZnO particles interact with

the matrix, and the interaction is maximum when they are

uniformly dispersed within the matrix. This attributes to

the high storage modulus due to restricted chain mobility.

The DMA studies revealed that the storage modulus (E0)increases when nano ZnO was used instead of micro

ZnO.

Generally, in the NR system, ZnO acts as the cure acti-

vator. It reacts with stearic acid, forms Zinc Stearate,

which activates the vulcanization process. In this study,

we expect that when nano ZnO was used as the cure acti-

vator, the surface area of nano ZnO is higher than that of

micro ZnO, its reactions with stearic acid and accelerators

are very fast and hence accelerates the formation of Zinc

accelerator complex, and further the formation of highly

reactive Zinc Sulfurating complex, which accelerates the

whole vulcanization process; hence vulcanization became

easy in the presence of nano ZnO. In this study, we could

reduce the use of micro ZnO (5 phr) by replacing it with

0.5 phr of nano ZnO which could contribute enhanced

sulfur vulcanization. Finally, the optimum dosage of

nano ZnO as cure activator in sulfur vulcanization of NR

was found to be 0.5 phr. Further addition of nano ZnO in

NR causes aggregation of nanoparticles. Finally, it is

important to add that this study will lead to economic

advantages for the processing of rubber goods and

environmental safety in disposing of waste rubber

compounds.

ACKNOWLEDGMENTS

One of the authors Dr. P. Bindu thanks the Department

of Science and Technology (DST), New Delhi, India, for

FIG. 9. The temperature dependence of the storage modulus of NR vul-

canizates. [Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]


the award of Young Scientist Fellowship and financial

assistance for carrying out the research project.

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Nano ZnO as cure activator and reinforcing filler in natural rubber

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