Thermal oxidative Behaviour of Peroxide crosslinked natural ......optimum properties in natural rubber. The compounds were prepared in a two-roll mixing mill as per the ASTM D 3182-07

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Peroxide vulcanization . Natural rubber . Thermal oxidative degradation Fourier transform infrared spectroscopy . Ther-mo gravimetric analysis

The thermal oxidative behaviour of na-tural rubber cured with four different peroxides namely 2,5-dimethyl-2,5-bis(tert-butylperoxy) hexane (DHBP), 1,1’-di(tert-butylperoxy)-3,3,5-trime-thylcyclohexane (TMCH), 1,3 1,4-bis (tert-butyl peroxy isopropyl) benzene (DIPP) and dicumyl peroxide (DCP) was studied. Fourier transform infrared spec-troscopy (FTIR) and Thermo gravimetric analysis (TGA) were used to characterize the byproducts formed. Mechanical pro-perties of the vulcanizates cured with different peroxides were measured. Among the four peroxides, DHBP and DIPP registered the best mechanical properties, however unlike DCP, DHBP imparted smell free vulcanizates. Out of the four different peroxides used DIPP showed the best overall balance of pro-perties and thermal stability.

Thermisch-Oxidatives Verhal-ten von mit Peroxid vernetz-tem Naturkautschuk Peroxid Vulkanisation, Kautschuk, Ther-mische oxidativen Abbau Fourier Trans-form Infrarot Spektroskopie, Thermo gravimetrische Analyse

Es wurde das thermisch oxidative Ver-halten von Naturkautschuk untersucht, der mit vier verschiedenen Peroxiden vernetzt wurde. Bei den Peroxiden han-delt es sich um 2,5-Dimethyl-2,5-bis(tert-butylperoxy)hexan (DHBP), 1,1‘-Di(tert-butylperoxy)-3,3,5-trime-thylcyclohexane (TMCH), 1,3 1,4-Bis(tert-butylperoxyisopropylalko-hol)benzol (DIPP) und Dicumylperoxid (DCP). Fourier-transform-Infrarotspekt-roskopie (FTIR) und Thermogravimetri-sche Analyse (TGA) wurden verwendet, um die Nebenprodukte zu charakteri-sieren. Die mit unterschiedlichen Per-oxiden vernetzten Vulkanisate wurden hinsichtlich der mechanischen Eigen-schaften gemessen. Von den vier Per-oxide führen DHBP und DIPP zu den besten mechanische Eigenschaften.

Figures and Tables:By a kind approval of the authors.

IntroductionIn rubber industry, organic peroxides are useful as curing agents for both saturat-ed and unsaturated rubbers [1-7]. The reactivity of the organic peroxides de-pends on the peroxide group configura-tion and type of substituents [8, 9]. In peroxide vulcanization, the curing agent does not enter into polymer chains but form carbon-carbon linkages between adjacent polymer chains. Organic perox-ides that can be used for crosslinking polymers mainly include diacyl peroxides (benzoyl peroxide and its derivatives), dialkyl or diaralkyl peroxides (di tert bu-tyl peroxide, dicumyl peroxide, etc.) and peresters (tert- butyl perbenzoate) [10]. Diacyl peroxides and peroxyesters are more vulnerable to acid-catalyzed de-composition than dialkyl peroxides [10, 11]. The first reported organic peroxide for the vulcanization of rubber was ben-zoyl peroxide [1]. Disadvantage associat-ed with benzoyl peroxide crosslinking is the surface blooming of vulcanizates due to benzoic acid produced from the de-composition of peroxide [10]. The vul-canizates obtained have poor physical properties and poor resistance to heat ageing when compared to sulphur cured vulcanizates. Brandon and Fletcher [12] reported the use of dicumyl peroxide for the vulcanization of natural rubber. A variety of dialkyl peroxides and t- butyl perbenzoate lead to high efficiency crosslinking reactions in gum rubber but only t-butyl peroxide and dicumyl perox-ide are capable of curing compounds having reinforcing black fillers. Of these two peroxides, t-butyl peroxide is a high-ly volatile liquid and dicumyl peroxide, being a non-volatile solid are the gener-ally used peroxides [13]. Its decomposi-tion in rubber has been extensively stud-ied [14-16].

Dicumyl peroxide covers around 30 per cent of the total peroxide consumed by the polymer industry. One of the ma-jor disadvantages of dicumyl peroxide is the unpleasant odour of the vulcanizates due to the formation of acetophenone as a by-product. Di (tert-butylperoxyisopro-pyl) benzene can be used as an alterna-

tive to dicumyl peroxide, but the bloom-ing phenomena can take place by the formation of dihydroxy isopropyl ben-zene due its limited solubility in the rub-ber [17]. In order to overcome the diffi-culties associated with peroxide vulcani-zation several other organic peroxides and their scorch retarding grades have been developed and are commercially available [18, 19]. These new generation peroxides contain unsaturated function-al groups, in addition to peroxide group, therefore, part of the decomposition products is no longer volatile, nor will give rise to smell or blooming phenome-na [17].The use of structurally different peroxides for the vulcanization of natu-ral rubber, synthetic rubber and their blends has been investigated extensively [20 - 27]. Very little work has been done to study the effect of different peroxides on the ageing resistance and mechanical properties of peroxide cured natural rub-ber.

The objective of this study was to un-derstand the role of different peroxides in the vulcanization of natural rubber. In this study, four different peroxides, namely 2, 5-bis (tert-butylperoxy)-2, 5-dimethylhexane, 1, 1’-di (tert-butylp-

Thermal oxidative Behaviour of Peroxide crosslinked natural Rubber

AuthorsRejitha Rajan, Siby Varghese, K. N. Madhusoodanan, M. A. Fancy, Kerala,K. E. George, Cochin, India Corresponding Author:Siby Varghese Rubber Research Institute of India, Kottayam - 686 009, Kerala, India.E-Mail: [email protected]


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eroxy)-3, 3,5trimethyl cyclohexane, 1, 3 1,4-bis (tert-butylperoxyisopropyl) ben-zene and dicumyl peroxide were selected for the peroxide vulcanization of natural rubber. The thermal oxidative degrada-tion behaviour of peroxide cured vulcan-izates was characterized by Fourier trans-form infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA).

Experimental

MaterialsNatural Rubber used was ISNR 5 grade, obtained from the Pilot Crumb Rubber Factory, Rubber Board, Kottayam, Kerala, India. 2, 5-bis (tert-butylperoxy)-2, 5-di-methylhexane, 1,1’-di (tert-butylperox-y)-3,3,5-trimethylcyclohexane, 1,3 1,4-bis (tert-butyl peroxy isopropyl) benzene and dicumyl peroxide were supplied by

Arkema Peroxides India Private Limited, Cuddalore, Chennai, India. Commercial names, chemical names, structures and major decomposition products of various peroxides used were given in Table 1.

Preparation of compounds The composition of mixes used was given in Table 2. Peroxide loading is fixed after evaluating each peroxide for

optimum properties in natural rubber. The compounds were prepared in a two-roll mixing mill as per the ASTM D 3182-07.

Testing procedureCure characteristics were measured us-ing Alpha Technologies Rheometer (MDR 2000, ASTM D 5289). The com-pounds were vulcanized in a hot press at 160°C to their respective optimum cure time. Tensile and tear tests of com-pression molded samples were meas-ured according to ASTM D 412 and ASTM D 624, respectively, using a uni-versal testing machine (UTM) Zwick model 1474 (Germany) with a cross-head speed of 500 mm/min at room temperature. The mean values of tensile and tear properties taken from five specimens are reported.

1 Commercial names, chemical names, structures and major decomposition products of various peroxides used [28]Chemical/ commercial name Chemical structure Major decomposition products2,5- dimethyl-2,5- bis (tert –butyl peroxy) hexane (DHBP) Luperox 101

Ethane, methane, acetone, t-butyl alco-hol, mixture of aromatic hydrocarbons

1,1’-di(tert-butylperoxy)-3,3,5-tri-methylcyclohexane (TMCH)Luperox 231 XL

Methane, trimethyl-cyclopentane, 3,3,5, trimethyl-cyclohex-anone, CO2, acetone, t-butyl alcohol

1,3 1,4-bis (tert-butyl peroxy isop-ropyl) benzene (DIPP)Luperox F 40

Methane, acetone , t-butyl alcohol, mix-ture of aromatic hydrocarbons

Dicumyl peroxide (DCP)Luperox DCP

Methane, acetophenone, cumyl Alcohol

2 Composition of mixesIngredients A B C DNR 100 100 100 100HAF black 50 50 50 50Paraffinic oil 4 4 4 4Peroxide 1.5ai 4bi 2.6ci 2.5di

ai- 2, 5-bis (tert-butylperoxy)-2, 5-dimethylhexane, bi- 1, 1’-di(tert-butylperoxy)-3,3,5- trimethylcyclohexane, ci- 1, 3 1, 4-bis (tert-butylperoxyisopropyl) benzene, di- dicumyl peroxide



The hardness of the prepared sam-ples was measured using shore A hard-ness tester (ASTM D 2240).Compres-sion set was determined by pressing the specimen to 25 per cent of the original thickness for 22h at 70 and 100oC. The specimen was then re-moved and the permanent set was measured as a percentage of original thickness (ASTM D-395). Cured test pieces were swollen in toluene for 72h for equilibrium swelling. The crosslink density was measured using Flory- Reh-ner equation [29].

Ageing propertiesTo measure the heat ageing resistance, the samples were aged for 1, 3, 5, 7 and 14 days according to ASTM D573 - 04(2010). The aged specimens were left at room temperature for at least 16 hours before the mechanical properties are tested.

Attenuated Total Reflectance – Fourier Transform infrared spectroscopy (ATR - FTIR)The ATR – FTIR measurements were per-formed at room temperature for samples before and after ageing in a Bruker Tensor FT IR spectrometer having a resolution of 4cm-1 in the range 500 - 4000 cm-1.

Thermo gravimetric Analysis (TGA)The TGA of 10mg of natural rubber sam-ples were carried out using a Perkin Elm-er Analyzer with a temperature range of 30-600oC at a heating rate of 10°C/min.

Results and discussion

Cure characteristicsIn order to study the curing of natural rubber with different peroxides, it is nec-essary to understand the efficiency and reactivity of these peroxides. Even though thermally induced homolytic cleavage of all peroxides produces highly reactive free radicals, the efficiency of these radicals may vary considerably [8]. Free radical produced by the homolytic cleavage of peroxides may be either alkoxy radicals or alkyl radicals. The pri-mary radicals produced by thermal de-composition readily undergo rearrange-ments in order to form a stable radical [8, 30] which decreases the crosslinking effi-ciency.

Another important factor which de-termines the reaction efficiency is the energy level of radicals. The active free radical will react with polymer only if the energy level of the radicals is reduced in the process [8, 31, 32]. Reactivity of per-oxides depends on a number of factors

viz. type and concentration of peroxide, nature of the polymer, presence of other compounding ingredients in the formu-lation etc. [33]. The cure characteristics of natural rubber vulcanizates are given in Table 3.

The maximum torque (MH) generally represents the crosslink density and the minimum torque (ML) measures the vis-cosity of the vulcanizates. DCP shows the maximum torque values followed by DHBP, DIPP and TMCH whereas consider-ing the cure rate index (CRI), TMCH is the fastest followed by DCP, DIPP and DHBP. Minimum torque, ML of all compounds shows only marginal difference in the values.

TMCH yields a rapid cure under typical vulcanization conditions, but it is more prone to premature crosslinking than other peroxides. Scorch time, t2, is com-paratively high for DHBP, but requires long cure times. DIPP provides an inter-mediate cure that is less vulnerable to scorch than DCP. Crosslink density (Table 6) is highest for DCP, TMCH was found to give lowest crosslink densities whereas DHBP and DIPP show almost similar crosslink densities. Homolytic cleavage of DCP produces both cumyloxy radicals and methyl radials, both have sufficient energy to react with polymer backbone and this is responsible for the higher crosslink density. For other peroxides, alkoxy radicals are the predominant spe-cies and due to the energy considera-tions, these are less able to abstract hy-drogen from polymer backbone than cumyloxy and methyl radicals, hence low crosslink densities [7].

3 Cure characteristics of the mixes at 160°CSample MH, d.Nm ML, d.Nm t90, min t2, min CRI, %A(DHBP) 16.08 1.59 33.26 1.29 3.13B(TMCH) 11.55 1.86 2.00 0.22 56.17C(DIPP) 13.9 1.65 27.38 1.38 3.84D(DCP) 18.82 1.48 16.26 1.02 6.57

Fig. 1: FT IR spectra of natural rubber cured with DHBPbefore and after thermal ageing.

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Fig.2: FT IR spectra of natural rubber cured with TMCH before and after thermal ageing.

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Fourier transform infrared spectroscopy studiesAn attempt was made to study the oxi-dative degradation behaviour of natural rubber cured with four different perox-ides by analyzing the FT IR (ATR) spec-trum of samples. FT IR spectra of vulcan-izates cured with different peroxides be-fore and after ageing (14 days) at 70°C were shown in Figures 1 to 4.

One of the major decomposition products of all the peroxides used in this study is an alcohol (Table 1), therefore, FT IR spectra of all the unaged vulcanizates contains a broad absorption band in the range 3583-3018 cm-1 due to –OH stretching vibration. The FT IR spectra of natural rubber cured with DHBP (Figure 1) showed that after 14 days of ageing, there is a considerable increase in the intensity of absorption due to –OH stretching vibrations with a significant reduction in the intensity of absorption due to –CH3 asymmetric stretch (2962, 2956 cm-1). Chain scission was further confirmed by the broad and intense ab-sorption band at 1500-1650 cm-1 due to C=O stretching mode. In addition, strong bands in the range 1428-1369 cm-1 are attributed to chain scission (C-H bond) [35].

Natural rubber vulcanizates cured with TMCH exhibits a different behav-iour towards oxidative degradation (Fig-ure 2). Here, upon ageing, the intensity of absorption band due to

–OH stretching vibrations is consider-ably reduced without any change in the absorption band due to –CH3 asymmet-ric stretch. After ageing, chain scission occurs as evident from the considerable decrease in the intensities of the absorp-tion band at 1660 cm-1 (C=C stretch) and also at 1428-1368 cm-1 (scission of C-H bond).

The oxidative degradation behaviour of DIPP cured natural rubber vulcanizate is still different. The narrow absorption band at 3273 cm-1 (-OH stretch) in the unaged vulcanizate was converted into a less intense broad peak in the absorption

spectrum of vulcanizate after ageing (Figure 3). Further, there is no change in the intensity of absorption band due to –CH3 asymmetric stretch (2962, 2956 cm-1). However, chain scission can be confirmed by the broad absorption bands in the range 1490- 1666 cm-1 and 150-1650 (C=O stretching mode).

For DCP, intensity of absorption band for –OH is enhanced while with –CH3 is considerably reduced (Figure 4). For all vulcanizates, especially after ageing, the absorption band in the range 1800-1600 cm-1 (corresponding to C=O group) was enhanced due to the oxidation of main polymeric chain.

Thermogravimetric AnalysisThe thermal stability of natural rubber vulcanizates cured with four different

Fig. 5: TGA and DTG cur-ves of natural rubber vulca-nizates curedwith different peroxides

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Fig. 3: FT IR spectra of natural rubber cured with DIPP before and after thermal ageing

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Fig. 4: FT IR spectra of natural rubber cured with DCP before and after thermal ageing

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4 Ti and Tmax of peroxide cured natural rubber vulcanizatesSamples Ti, °C Tmax, °CA 314.15 383.78B 316.20 383.78C 316.85 384.43D 293.80 384.43



peroxides were analyzed by thermo-gravimetric analysis (TGA). The TGA (mass loss) curves and derivative curves (DTG) were shown in Figure 5. The onset decomposition temperature was calcu-lated from the TGA curve by extrapolat-ing the initial mass loss of the natural rubber vulcanizates. The maximum de-composition temperature was deduced from the derivative thermogravimetry curves.

It can be observed that (Figure 5) all the vulcanizates undergo two step de-composition process. The first decompo-sition corresponds to the degradation of natural rubber and the second one is due to the loss carbon black (filler). Further, since all the peroxide cured vulcanizate produce thermally stable C-C crosslinks, thermal stability of all the vulcanizates are similar. Table 4 shows the initial (or onset, Ti) and the final (maximum) de-

composition temperatures (Tmax) of the vulcanizates. The initial decomposition temperature is used to predict the ther-mal stability of rubber vulcanizates. DCP is found to have slightly lower Ti than other peroxides. The maximum decom-position temperatures of all the vulcani-zates were similar as they contain the same type of elastomer and the natures of crosslinks are also same. A better un-derstanding of the thermal stability of the vulcanizates can be deduced from the percent of weight retained at differ-ent temperatures.

The thermal decomposition behav-iour of peroxide cured vulcanizates was shown in Table 5. It can be seen that at 450°C, among the four different types of peroxides used for the study, the thermal stability follows the order DIPP> TMCH> DHBP> DCP.

Mechanical propertiesOne of the major disadvantages of per-oxide cure is the poor mechanical prop-erties of the vulcanizates owing to the rigid carbon - carbon crosslinks. Mechan-ical properties can be increased to great extent by the addition of fillers. The ten-sile properties of filled vulcanizates are generally governed by filler dispersion, particle size or specific surface area of filler and rubber -filler interaction [38, 39]. The tensile strength values of all vulcanizates are greatly improved by the addition of HAF black. The unaged me-chanical properties of vulcanizates are given in Table 6. The rate of decomposi-tion of organic peroxide depends on the strength of peroxide (-O-O-) bond, which in turn depends on the chemical nature of the alkyl group and hence the free radicals derived from it. The resonance stabilization, inductive effect and steric effect of free radicals play an important role in determining the final vulcanizates properties [22, 38].

The major decomposition products, which are indicative of the types of radi-cals generated form the thermal decom-position of various peroxides are shown in Table 1. DHPB, DIPP and DCP are di-alkyl peroxides whereas TMCH is a per-oxy ketal. DHBP, TMCH and DIPP primar-ily produce highly reactive t- butyloxy radicals, which further decomposes in to methyl radicals. However, the amount of t – butyl and methyl radicals vary among peroxides. In the case of DCP, since natu-ral rubber is a highly unsaturated poly-mer, the approach of least sterically hin-dered methyl radical is preferred than t

6 Physical properties of natural rubber vulcanized with different peroxidesParameters DHBP TMCH DIPP DCPTensile strength, MPa 23.6 19.9 23.2 22.1Elongation at break, % 368.5 350.8 398.6 320100% modulus, MPa 2.1 1.78 1.84 2.92200% modulus, MPa 7.38 6.68 6.60 11.36300% modulus, MPa 16.1 14.18 13.68 19.8Hardness, Shore A 51 50 52 58Compression set 22h at 70°C,% 5.99 15.55 8.77 5.66Compression set 22h at 100°C,% 14.46 52.27 18.97 16.34Crosslink density, x10-5 mol/cm3 5.34236 3.38295 4.89557 10.4925

5 Thermal decomposition characteristics of peroxide cured vulcanizates

Temperature, °CWeight retained, %

A B C D100 99.13 99.37 99.56 98.91200 97.75 97.76 98.09 96.77300 93.16 93.53 93.96 91.71350 88.09 88.73 89.01 86.06400 48.84 50.09 50.26 46.51450 30.50 32.06 32.38 29.08500 29.33 30.90 31.28 28.01550 28.72 30.41 30.82 27.27

600 28.00 29.74 30.31 26.51

Fig. 6: Tensile strength values of NR vulcanizates before and after ageing at 70°C.

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Fig. 8: Modulus (100%) values of NR vulcanizates before and after ageing at 70°C

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–butyloxy or cumyloxy radicals. Among the samples, highest tensile strength (Table 6) is observed for DHBP followed by DIPP, DCP and TMCH. The variation in tensile strength of vulcanizates can thus be accounted by the type and amount of reactive radicals generated during vul-canization.

Elongation is defined as the length at the breaking point expressed as a per-centage of its original length. Usually, for peroxide cured vulcanizates suffers from lower elongation at break (EB) due to the strong and rigid carbon-carbon bond be-tween the polymer chains. EB depends on the peroxide loading and hence crosslink density of rubber vulcanizates. As the peroxide loading increases more crosslinks are formed and EB decreases. Among the four different peroxides used, the vulcanizates cured with DCP showed the lowest EB owing to its high crosslink density. Among other peroxides, vulcani-zates cured with DIPP shows a relatively higher value compared to others. From Table 6, it can be seen that both moduli (100, 200 and 300% elongations) and hardness followed the same trend as that of crosslink density.

Compression set is the measure of the vulcanizates ability to recover its original dimensions after being compressed un-der specified conditions. As peroxide vul-canization produces covalent crosslinks (C - C), which is thermally stable, and do not undergo any cleavage under com-pressed state. Therefore, peroxide cure results in vulcanizates with improved set resistance compared to suphur or other vulcanizing systems. At low temperature say 70°C, DHBP and DCP shows lowest set values followed by DIPP. TMCH shows highest set value. At high temperature (100°C), lowest set values were observed for DHBP followed by DCP, DIPP and TMCH. Further, the set value shown by TMCH is abnormally high. The variation in compression set values can be ac-counted by the nature and type of free radicals generated by thermal decompo-sition of peroxides. Therefore, for vulcan-izates, were compression set is a critical parameter, DCP can be replaced with DHBP. Further, vulcanizates cured with DHBP has comparatively higher tensile strength and elongation at break than those cured with DCP, but it requires pro-longed cure time (Table 3). As the CRI of TMCH is very high and optimum cure time and crosslink density are very low, therefore, generally it is not recommend-ed for curing NR.

Mechanical properties after ageingAgeing is the process of deterioration of desirable properties during storage or on service. The service life or shelf- life of rubber compounds is usually determined by ageing the samples at an elevated temperature and measuring the me-chanical properties there after [39]. Here an attempt was made to study the ther-mal oxidative behaviour of the vulcani-zates by analyzing the mechanical prop-erties. The tensile strength, elongation at break and modulus (100% elongation)

values of natural rubber cured with four different types of peroxides before and after ageing for 1, 3, 5, 7 and 14 days at 70°C are shown in Figure 6, Figure 7 and Figure 8. All the properties were found to decrease gradually with increasing age-ing periods. As all these properties are related to crosslink density of the vulcan-izates, the decrease can be attributed to the chain scission during ageing in pres-ence of oxygen. The percentage reten-tion in properties before and after ageing is shown in Table 7. It can be seen that

Fig. 7: Elongation at break values of NR vulcanizates before and after ageing at 70°C

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7 Percentage retention in properties after ageing for 14 daysParameters* DHBP TMCH DIPP DCPTensile strength, % 61.01 71.85 67.67 65.61Elongation at break, % 29.34 34.71 33.64 41.56100% Modulus, MPa 27.14 34.44 26.31 11.42* after 14 days of ageing

among the four peroxides used, the highest percentage retention in tensile strength is observed for TMCH followed by DIPP, DCP and DHBP.

ConclusionThe thermal oxidative behaviour of nat-ural rubber cured with four different peroxides viz 2,5– dimethyl-2,5-bis (tert -butylperoxy) hexane (DHBP), 1,1’-di(tert-butylperoxy)-3,3,5-trimethyl-cyclohexane (TMCH), 1,3 1,4-bis (tert-bu-tyl peroxy isopropyl) benzene (DIPP) and dicumyl peroxide (DCP) were studied. The reactivity of the decomposition products of peroxides formed during vul-canization and accordingly the crosslink-ing efficiency of various peroxides was rated. Mechanical properties like tensile strength, elongation at break, modulus and compression set (70° and 100°C) of the vulcanizates cured with different peroxides were measured. The FTIR data revealed clear understanding of the na-ture of the by-products formed during vulcanization. Thermogravimetric stud-ies revealed that the degradation behav-iour of all vulcanizates were similar. The choice of the peroxide for natural rubber depends on the requirements of the ap-plication. Among the four peroxides, DH-BP and DIPP registered the best mechan-ical properties, however the prolonged vulcanization time of DHBP is a con-straint. However unlike DCP, DHBP im-parted smell free vulcanizates. Out of the four different peroxides used DIPP showed the best overall balance of prop-erties and thermal stability.

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Thermal oxidative Behaviour of Peroxide crosslinked natural ......optimum properties in natural rubber. The compounds were prepared in a two-roll mixing mill as per the ASTM D 3182-07