Comparison of thermal stability of sulfur, peroxide and radiationcured NBR and SBR vulcanizates

Shamshad Ahmed1, A.A. Basfar*, M.M. Abdel Aziz2

Institute of Atomic Energy Research, King Abdulaziz City for Science and Technology (KACST), PO Box 6086, Riyadh-11442, Saudi Arabia

Received 25 June 1999; accepted 11 August 1999

Abstract

A comparison of thermal stabilities of NBR and SBR vulcanizates cured by di�erent curing agents namely sulfur, peroxide andgamma radiation was performed by thermogravimetric analysis (TGA), assessed on the basis of comparison of DTG peak maxima andtemperature for loss of 50% mass. Compared to sulfur and peroxide-cured vulcanizates, radiation-cured formulations demonstrated

much improved thermal stability. The in¯uence of the variation of the amount of coagent and other additives on the thermal stabilities offormulations of radiation-cured NBR and SBR vulcanizates was investigated. A comparison of thermal stability of various radiation-cured NBR vulcanizates with SBR counterparts was also conducted.# 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

High energy ionising radiation has recently received agreat deal of attention for many reasons, including theability to crosslink various polymers, the lower cost ofprocessing and the inherently waste-free nature of thetechnology. It is well known that exposure of cross-linking type polymers to radiation provides improvedstability and mechanical properties [1,2]. Earlier, Basfaret al. reported the unusually high improvement in ozoneresistance of styrene butadiene rubber (SBR) cured by acombination of sulfur and ionising radiation [3].As we reported previously [4], a distinct feature of the

approach adopted for assessing comparative thermal sta-bilities involves the use of the temperature for loss of 50%mass of the sample, in addition to recording DTG peakmaxima. This is especially advantageous for selecting thebest among formulations demonstrating close thermalstability due to overlap of DTG peaks. In addition, this

observation is objective and does not su�er from the sub-jective errors likely to occur in recording DTG peak max-ima. A similar approach was adopted by Smith [5] whoapplied TG-DTG techniques for the determination ofstability of polymers including PE, SBR and PVC, basedon temperatures of original weight loss.To assess the merits of radiation curing over the con-

ventional processes of sulfur and peroxide curing, a com-parison of thermal stability of sulfur and peroxide-curedformulations with their radiation-cured counterparts wasperformed. For comparison among the di�erent curingprocesses, the best thermally stable formulation wasselected. Sulfur and peroxide-cured rubber vulcanizatesof NBR and SBR were obtained by blending the elasto-mers with ®llers, antioxidants and appropriate accel-erators, followed by vulcanization at 150 to 160�C. Blendsof the same elastomers with appropriate coagents type,SR-633 and SR-517 were also cured by gamma radiationat 100 and 150 kGy.

2. Experimental

2.1. Materials

The elastomers used were commercial grade styrenebutadiene (SBR) and Acrylonitrile butadiene rubbers

0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(99 )00133-0

Polymer Degradation and Stability 67 (2000) 319±323

* Corresponding author. Tel.: +996-1-488-3648; fax: +966-1-481-

3887.1 Permanent address: Head Radiation Chemistry Group, NCD,

PINSTECH, Nilore, Islamabad, Pakistan.2 Permanent address: Radiation Chemistry Department, NCRRT,

Nasr City, PO Box 29, Cairo, Egypt.

E-mail address: abasfar@kacst.edu.sa (A.A. Basfar).

(NBR). All materials were supplied by Amiantit RubberCo., Saudi Arabia.

2.2. Compounding and vulcanization

The various formulations utilized in this study areproprietary and a general list of constituents is pre-sented in Table 1. A Brabender Plasticorder Model PL-2000 and an internal mixer Model 350 S were used. Thecompounding was carried out in accordance withASTM D-3182. A hydraulic hot press by PHI Co.,Model G236 H was used to press and vulcanize rubbersheets with an average thickness of about 2 mm inaccordance with ASTM D-3191. The formulations con-taining sulfur were vulcanized at 150�C for 30 min andthe ones containing peroxide were vulcanized at 160�Cfor 60 min.

2.3. Irradiation

The compressed sheets were irradiated in air at a doserate of 11 kGy hÿ1 to absorbed doses of 100 and 150kGy.

2.4. Thermogravimetric and derivative thermogravimetricanalysis

Throughout the work, a Perkin±Elmer TGA-7 wasused. Sample weight was generally from 8 to 13 mg. Astandard heating rate of 5�C/min was used. Generalprocedure was to program-heat from ambient to 800�Cunder a nitrogen atmosphere. Carrier gas ¯ow wasconstantly monitored with a ¯ow meter and maintainedat 40 cm3/min throughout the run.

3. Results and discussion

For comparison of thermal stabilities, the thermo-grams of sulfur, peroxide and radiation-cured rubberspyrolyzed in inert atmosphere were selected in preference

to those analyzed in static air. It is expected that oxida-tion of oils and elastomers etc. can play a predominantrole in in¯uencing the thermal degradation pattern. Byfar, the largest work on thermal analysis has been car-ried out in nitrogen, which shows purely thermal or(thermochemical e�ect) in contrast to the combinede�ect of heat and oxidation (thermooxidative) e�ect inoxygen.

3.1. Thermal stabiliteis of radiation-cured NBRformulations containing di�erent concentrations ofcoagent type SR-517 and carbon black or silica as ®llers

It is common practice in industry to add to the poly-mer, polyfunctional monomers containing two or moreethylenic bonds to enhance radiation crosslinking [6±8].Use of crosslinking agents can help reduce the doserequired for crosslinking [9,10] and the detrimentalin¯uences associated with high dose irradiation can beavoided. For this study, the in¯uence of addition ofvarying amounts of coagent SR-517 was investigated bycomparing DTG peak maxima (see Table 2). In for-mulations of NBR containing variable concentrationsof coagent type SR-517, loaded with silica and irra-diated to 100 kGy, an optimal amount of 2% appears toresult in a formulation with the highest thermal stabilityin the group. Excess amounts above this concentrationlead to formulations with lower thermal stability hintingat the detrimental in¯uence of the excess of the coagent.This is expected, as the excess of unreacted coagent islikely to generate free radicals on radiolysis. The inter-action of these radicals with the elastomer presumablyresults in retardation of curing. However, no clear cor-relation between the amount of coagent and the thermalstability could be established. Oxidation of elastomers isaccelerated by a number of factors including heat, heavymetal contamination, sulfur, light, moisture, swelling inoil and solvent, dynamic fatigue, oxygen and ozone.One variable in the compound formulation that can beoptimized to inhibit oxidative degradation is judiciouschoice of an antidegradant system. As shown in Table 2,

Table 1

Formulations of di�erent rubber vulcanizates

Notation Ingredients (phr)

Elastomer Carbon

black/silica

Oil ZnO Steraic

acid

Anti-oxidants

+additives

Anti-ozonant Curing agent and coagent

S-cured SBR 100 35 10 4 1.5 7 ± Sulfur

S-Cured NBR 100 56 8 5 1 7.5 1 Sulfur

Peroxide-cured SBR 100 35 10 4 1.5 7 ± Peroxide

Peroxide-cured NBR 100 56 8 5 1 7.5 1 Peroxide

Radiation-cured SBR 100 45 ± ± ± 2.5 1 Radiation/coagent SR 633a

Radiation-cured NBR 100 45 ± ± ± 2 0.5 Radiation/coagent SR 517b

a SR-633 (zinc diacrylate).b SR-517 (trimethacrylic ester).

320 S. Ahmed et al. / Polymer Degradation and Stability 67 (2000) 319±323

antiozonants, IPPD and para�n wax, TMQ (anti-oxidant) were added to assess their in¯uence on thermalstability. No signi®cant improvement in the thermalstability was observed.Fillers or reinforcement aids such as carbon black,

clays and silica are added to rubber formulations tomeet material property targets such as tensile strengthand abrasion resistance etc. [3]. Addition of silica to arubber compound o�ers a number of advantages suchas improvement in tear strength, reduction in heat buildup and increase in the compound adhesion in multi-component products such as tires [11]. It was consideredimportant to explore the in¯uence of carbon blackloading on the thermal stability of NBR.In general, replacement of silica by carbon black

resulted in formulations with lower thermal stability(see Table 3). An optimum amount of 2% of coagentappears to achieve a formulation with the highest ther-

mal stability in the group. Amounts in excess of thisconcentration seem to in¯uence the thermal stability inan unfavorable manner for the reasons stated earlier.Further, addition of antioxidants and antiozonants doesnot seem to a�ect the thermal stability in any appreci-able manner.

3.2. Thermal stabilities of SBR formulations containingdi�erent concentrations of coagent type SR-633 andcarbon black or silica as ®llers

The role of coagent type SR-633 containing two dou-ble bonds on the thermal stability of the SBR formula-tion cured by radiation was investigated. Informulations of SBR containing variable concentrationsof coagent SR-633 and loaded with silica, it is observedthat the formulation having no coagent appears to bethe most thermally stable (see Fig. 1). Concentrations ofcoagent above 1.5% caused a decline in the thermalstability. In general, a progressive decrease in tempera-tures for the DTG peak maxima and temperature forthe loss of 50% mass is observed with correspondingincrease in the amount of the coagent. However, whensilica is replaced by carbon black as a ®ller, the for-mulation containing 1% coagent appears to be the mostthermally stable (see Fig. 2). Although the temperaturefor the DTG peak maxima are very close, on the basisof a higher temperature for loss of 50% mass, the for-mulation with 1 phr of coagent may be considered themost thermally stable. In general, thermal stability ofthese formulations is marginally lower than the coun-terpart formulation loaded with silica.

Table 2

Thermal stabilities of NBR formulations irradiated to 100 kGy con-

taining variable concentrations of coagent type SR-517, 45 phr silica

and other additives (antiozonants and antioxidants)

Concentration of

coagents (phr)

Onset of

degradation

(�C)

DTG peak

maxima

(�C)

Temperature for

loss of 50% mass

(�C)

0.5 439 517 501

1 441 514 499

2 445 522 508

3 398 514 500

4 437 514 502

2a 440 515 499

2b 436 514 500

a Formulation contains in addition 1 phr IPPD, 1 phr TMQ and 0.5

phr para�n wax.b Formulation contains in addition 1 phr IPPD, 1 phr TMQ and 1

phr para�n wax.

Fig. 1. Thermal stabilities of SBR formulations containing variable

concentrations of coagent type SR-633 ®lled with 45 phr silica and

irradiated at 150 kGy.

Table 3

Thermal stabilities of NBR formulations irradiated to 100 kGy con-

taining variable concentrations of coagent type SR-517, 45 phr carbon

black and other additives (antiozonants and antioxidants)

Concentration of

coagent (phr)

Onset of

degradation

(�C)

DTG peak

maxima

(�C)

Temperature for

loss of 50% mass

(�C)

0.5 440 510 496

1 441 510 496

2 442 513 495

3 435 508 495

4 428 505 491

0.5a 432 506 492

0.5b 436 510 497

a Formulation contains in addition 1 phr IPPD, 1 phr TMQ and 0.5

phr para�n wax.b Formulation contains in addition 1 phr IPPD, 1 phr TMQ and 1

phr para�n wax.

S. Ahmed et al. / Polymer Degradation and Stability 67 (2000) 319±323 321

When comparing the thermal stability of various for-mulations of SBR, the one loaded with silica and con-taining no coagent (and irradiated to 150 kGy) wasfound to be the most thermally stable in the group (seeFig. 2).

3.3. Intercomparison of thermal stabilities of sulfur,peroxide and radiation-cured NBR and SBRformulations

Peroxide vulcanization tends to perform best forreversion resistance as a result of the absence of sulfurand formation of carbon±carbon crosslinks [11]. Con-ventional cure systems which involve the high sulfurlevel and low accelerator concentration show poor heatand oxidation resistance because the polysul®dic cross-links are thermally unstable and readily oxidized. How-ever, as seen in Table 4, slight improvement in thermalstability, as demonstrated by increase in DTG peakmaxima, was observed in case of SBR. In case of NBR,a decline in thermal stability was observed.When compared with their sulfur or peroxide-cured

counterparts, radiation-cured formulations demon-strated much higher thermal stability both in case ofSBR and NBR (see Table 4). This is quite evident fromthe much higher temperatures observed individually incase of DTG maxima, the temperature for the loss of50% mass and the temperature for onset of degradationfor the radiation-cured formulations. This may beattributed to the existence of more uniformly dis-tributed crosslinks coupled with enhanced crosslinkdensity in the radiation process as compared to theperoxide process.

4. Conclusions

The following conclusions can be drawn:

1. While developing formulations for radiation cur-ing, selection of optimum amount of coagent isquite important as excessive amounts lead todecline in thermal stability.

2. Radiation-cured NBR and SBR demonstratemuch higher thermal stabilities compared to sulfuror peroxide-cured counterparts. Both in case ofSBR and NBR, replacement of carbon black bysilica, in general, leads to formulations with mar-ginally higher thermal stabilities.

3. NBR formulations ®lled with 45 phr of either silicaor carbon black in the presence of 2 phr of cross-linking coagent and irradiated to 100 kGy possessthe best thermal stability among the investigatedformulations.

4. SBR formulation ®lled with 45 phr of silica, con-taining no coagent and irradiated to 150 kGy isthe most thermally stable among the investigatedformulations.

5. SBR formulation ®lled with 45 phr of carbonblack and irradiated to 150 kGy in presence of 1phr of crosslinking coagent possesses the bestthermal stability among investigated formulations.

Acknowledgements

The authors would like to express their sincereappreciation to King Abdulaziz City for Science andTechnology (KACST) for funding the project, which ledto this publication. Also, thanks are extended to Mr.FadelAl-Fadel,Mr.KhalidAl-Showish andMr.HaithamAl-Githmi for compounding and irradiating samples. We

Fig. 2. Thermal stabilities of radiation-cured SBR formulations con-

taining variable concentrations of coagent type SR-633 ®lled with 45

phr HAF carbon black and irradiated at 150 kGy.

Table 4

Comparison of thermal stabilities of radiation-cured vulcanizates of

SBR and NBR with sulfur and peroxide-cured counterparts

Rubber

type

DTG peak due

to elastomer,

Tmax (�C)

Temperature for

50% weight loss

(�C)

Temperature for

onset of degradation

(�C)

Sulfur-cured vulcanizates

SBR 474 479 444

NBR 480 494 274

Peroxide-cured vulcanizates

SBR 481 478 313

NBR 475 479 300

Radiation-cured vulcanizates

SBR 532 538 480

NBR 522 508 445

322 S. Ahmed et al. / Polymer Degradation and Stability 67 (2000) 319±323

also appreciate the assistance of Mr. Hammad Al-Orainifor performing the thermal stability measurements.

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