THE UNIVERSITY OF MICHIGAN INDUSTRY PROGRAM OF THE COLLEGE OF ENGINEERING NON-FREE SULFUR CURING SYSTEMS FOR AGE RESISTANT BUTADIENE-STYRENE RUBBER L. M. Hobbs R. G. Craig C. W. Burkhart Engineering Research Institute and Michigan Memorial-Phoenix Project University of Michigan Ann Arbor June, 1957 IP-224

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TABLE OF CONTENTS Page LIST OF FIGURES.............................................. iii LIST OF TABLES............................................... iv ABSTRACT......................................................... v I. INTRODUCTION............................... II. EXPERIMENTAL PART.................................... 3 A. Materials.....3..................................... 3 B. Preparation of Vulcanizates...........*.........4.. 4 C. Physical Measurements................................. 4 III. PRESENTATION OF RESULTS.................................. 7 A. Accelerator Vulcanizing Systems...................... 7 B. Dicumyl Peroxide Vulcanizing System................... 8 C. Dicumyl Peroxide-Sulfur-Bearing Accelerator Vulcanizing Systems................................................ 11 D. Effect of Curing Time on Dicumyl Peroxide Systems..... 13 E. Aging of Selected Vulcanizates at Various Temperatures. 13 1. Aging Studies at 212~ F.......................... 14 2. Aging Studies at 250~ F......................... 14 3. Aging Studies at 300~ F......................... 15 F. Raw Materials Costs of Vulcanization Systems........... 15 IV. DISCUSSION OF RESULTS..................................... 17 A. Physical Properties of Vulcanizates.................... 17 B. Age Resistance of Vulcanizates......................... 17 C. Processing Stability of Uncured Compositions.......... 17 D. Relative Costs of Raw Materials....................... 17 AUTHORS ACKNOWLEDGEMENTS......................................... 21 BIBLIOGRAPHY..................................................... 23 ii

LIST OF FIGURES Figure Page 1 Physical Properties of SBR-1500 Vulcanizates Cured with Accelerators............................. 25 2 Physical Properties of SBR-1500 Vulcanizates Cured with Dicumyl Peroxide......................... 26 3 Tensile Properties of SBR-1500 Vulcanizates Cured with Dicumyl Peroxide-Accelerator Combinations 27 4 Effect of Curing Time on Tensile Properties of SBR Vulcanizates Cured with Systems Containing Dicumyl Peroxide......................................... 28 5 Effect of Extended Air-Oven Aging at 212~ F on the Elongation of SBR Vulcanizates..................... 29 6 Effect of Extended Air-Oven Aging at 212~ F on the Tensile Strength of SBR Vulcanizates................ 30 7 Effect of Extended Air-Oven Aging at 212~ F on the Stress at 300% Elongation of SBR Vulcanizates....... 31 8 Effect of Air-Oven Aging at 250~ F on Elongation of Selected SBR Vulcanizates......................... 32 9 Effect of Air-Oven Aging at 300~ F on Elongation of Selected SBR Vulcanizates......................... 33 iii

LIST OF TABLES Table Page I Composition of Rubber Compounds..................... 35 II Physical Properties of Selected Compositions and Vulcanizates..................................... 36 III Stability of Compounded Rubber to Heating Aira...... 37 IV Air-Oven Age Resistance of Selected Vulcanizates at 212, 250 and 300~ F................................. 38 V Cost of Materials for Selected Compositions......... 39 VI Summary of Findings............................ 40 iv

ABSTRACT The resistance to air-oven aging of styrene-butadiene synthetic rubber vulcanizates cured, (a) with accelerators, (b) with dicumyl peroxide and with phenyl P-naphthyl amine, and (c) with combinations of (a) and (b), has been studied. Attention was given to the processing stability of the uncured compositions, and limited work was devoted to the supression of the blooming tendencies of certain vulcanizates from (a). It was found that vulcanizates from all of these non-free sulfur curing systems possessed substantially greater resistance to aging than did the vulcanizate from the conventional free sulfur-accelerator combination, with vulcanizates from (c) showing the greatest resistance. The tensile properties of the unaged vulcanizates from (c) were essentially equivalent to those from the free sulfur-accelerator system, while the tensile and tear strengths of vulcanizates from (a) and (b) were somewhat less than those of vulcanizates from the free sulfur-accelerator combination. It is noteworthy that the presence of phenyl P-naphthyl amine and certain accelerators make the dicumyl peroxide vulcanizate both stronger and more age resistant. The time of resistance to scorch and thermal decomposition of the raw unvulcanized compositions at 250~F was found to be: 20-30, 30, and 30-40 minutes for compositions (a), (b), and (c), respectively, as compared with 60 minutes for the free sulfur-accelerator composition. The raw materials cost of the dicumyl peroxide-phenyl P-naphthyl system (b) is 1.8& greater, and of the other systems, (a) and (c), 5.2 - 5.8 greater than that of the free sulfur-accelerator system, as calculated for a composition containing 50 PHR carbon black. v

It is concluded that the age resistance and physical properties of vulcanizates formed by the action of dicumyl peroxide in combination with certain accelerators and phenyl P-naphthylamine (c), can justify some limitation in processing stability and the small increase in materials cost for the production of rubber articles for applications in which good age resistance is necessary. vi

INTRODUCTION There is a general need to improve the age resistance of rubber vulcanizates. Non-free sulfur curing systems (1-6) have been shown to yield vulcanizates of improved age resistance. However, the uncured compositions (1, 5, 6) have been reported to be sensitive to scorching or precuring during processing, and to yield vulcanizates with poor tensile properties. Furthermore, the non-free sulfur systems comprising complex synthetic organic chemicals are more expensive than those based on elemental sulfur. The purposes of this study were to examine several non-free sulfur curing systems with the hope of identifying combinations which could be processed at reasonable cost and which would yield vulcanizates of satisfactory physical properties and age resistance. Three types of curing systems for styrene-butadiene-41.F. synthetic rubber (SBR-1500) were studied: (a) Combinations of vulcanization accelerators (b) Dicumyl peroxide with phenyl-P-naphthyl amine (c) Combinations of (a) and (b) The results presented below show that each of the three systems yield vulcanizates with substantially greater age resistance than the vulcanizates produced by the action of free sulfur-accelerator systems with vulcanizates from (c) being the most age resistant. Rubber compositions containing dicumyl peroxide and accelerators (b) could be held in air at 250~F. for 30-40 minutes without damage, and on curing for 30-40 -1

minutes at 307~F they yielded vulcanizates with physical properties essentially equivalent to those of the free-sulfur cured products. The raw materials cost of the compound containing peroxide (b) is l.8% greater, and of the other compounds 5.2-5.8% greater than the cost of the conventional free sulfur-accelerator composition. The details of the study are presented in the following sections. -12

EXPERIMENTAL PART Materials SBR-1500, styrene (30) - butadiene (70) copolymer prepared at 41~F., was used as the base rubber throughout this study. The SBR-1500 was compounded with 50 PER of medium abrasion furnace black (MAF) and with various vulcanizing agents. In the subsequent sections the particular rubber composition will be specified, simply by identifying the nature and proportions of the vulcanizing agents. In the production of SBR-1500, approximately 1.25 PHR of phenyl-p-naphthylamine (PBNA) are added by the manufacturere, however, in identifying the experimental formulations PBNA will be specified only when an additional quantity of the material was used in compositions containing dicumyl peroxide, In Table I, the compositions of the important formulations are listed. Other compositions of the same basic types were used and will be referred to by reference to the composition type and the amount of the curing agents. Compositions I and II were chosen as reference compositions for the preparation of the corresponding vulcanizates I and II; these reference' vulcanizates were used as standards for comparing the physical properties and the age resistance of experimental vulcanizates. Composition I contained a commonly used vulcanizing system consisting of free sulfur plus N-cyclohexyl-2-benzothiazyl sulfenamide (NCHBTSA) and composition II contained the sulfur-bearing accelerators dipentamethylene-thiuram-tetrasulfide (DPMTTS) and 2-mercaptobenzothiazole (MBT). This latter curing system has received considerable attention by Ossefort, Shaw and Bergstrom (6) as a non-free sulfur curing system which yields a vulcanizate with excellent age resistance. -3

Preparation of Vulcanizates All compositions were prepared on a 3" x 8" two roll mill using the ASTM mixing procedure for SBR rubber (D 15-55T). Since 200 grams of material represented the maximum amount that could be easily handled on this equipment, special care during compounding was required in order that separate batches of the same composition yielded vulcanizates having the same physical properties. The rubber compounds were placed in a single cavity (ASTM specification D 15-55) 6" x 6" x 0.075" chromium plated steel compression mold, cured for 50 minutes at 2200 psi and 307'F, and the cured sheets were quenched in cold water. The same rlot of SBR-1500 was used throughout this research with the exception of a few aging experiments which were carried out on a second lot. The curing systems consisting of free sulfur and accelerator and combinations of sulfur-bearing accelerators yielded vulcanizates having comparable physical properties with each lot of SBR-1500. However, in curing systems containing DCP-40, it was necessary to increase the amount of DCP-40 by 0.25 PER for the second lot in order to obtain comparable physical properties with the first. In order to avoid confusion, the amount of DCP-40 recorded has been on the basis of the first lot. Physical Measurements Tensile and tear specimens were cut from the test sheets by ASTM tensile die C (ASTM D 412-51T) and ASTM tear die C (ASTM D 624-54), respectively. The tensile and tear measurements were made on an automatic, recording tensile testing machine at 77~F and 50f relative humidity. The physical properties of the vulcanizates reported in succeeding tables are average values of four or more determinations. -4

Hardness values were determined according to ASTM Method D 676-55T. The scorch characteristics of the uncured compositions were determined by use of a shearing disk viscometer at 250~F according to ASTM Method D 1077-55T. Duplicate experiments were made on all compounds evaluated. The scorch test, ASTM D 1077-55T, was not entirely satisfactory for the compositions containing dicumyl peroxide since the peroxide tends to decompose without forming cross-links at temperatures approaching 250~F. Another procedure was devised to indicate the processing behavior of uncured compositions containing dicumyl peroxide in the temperature range of 230-2500F. The uncured compound was sheeted on the mill at about 0.080" thickness and 4" x 4" pads were cut out of the sheets. The pads were placed in a circulating air oven at a temperature in the range of 230-250~F for periods up to three hours. After this heating period, the pads were cured by the procedure described above for thirty minutes at 507~F and 2200 psi. The tensile properties of the cured sheets could then be used as the basis for estimating the extent of the decomposition of the peroxide due to heating of the uncured compositions. A mechanical convection oven was used for the aging experiments. The temperature variation of the oven at 212~F was within ~1~F. The conditions for most of the aging experiments were 72 hours at 212~F. However, the aging properties of selected compositions were determined for periods of 32 days at 212~F, 7 days at 250~F and for 12 hours at 300~F. Since it had been shown by Fackler and Rugg (7) that migration of compounds such as antioxidants can take place between specimens during airoven aging experiments, only similar types of vulcanizates were aged at a given time. -5

PRESENTATION OF RESULTS Accelerator Vulcanizing Systems As mentioned in the Introduction, combinations of the accelerators (which are frequently used with free sulfur) were employed as the total vulcanizing system in the absence of added free sulfur. Three systems were studied: (a) 1 PER Dipentamethylene-thiuram-tetrasulfide (DPMTTS) 1 PER 2-Mercaptobenzothiazole, (MBT) (b) 1 PHR Dibenzothiazole-dimethylthiol urea (DBTDMTU) 1-3 PHR Tetramethyl-thiuram-disulfide, and (TMTD) (c) 1 PHR 2-Mercaptobenzothiazole 1-2 PER Tetramethyl-thiuram-disulfide. Also, as previously mentioned, the first composition (a) was selected as a reference standard (reference composition II). The ultimate tensile and elongation values of the vulcanizates prepared from the second and third compositions, (b) and (c), are plotted in Figures la and lb. The values of the physical properties after airoven aging are also shown on the graph. It may be seen from these curves that the optimum compositions are approximately: (III) 1.75 PHR TMTD 1 PHR DBTDMTU, and (IV) 1.75 PHR TMTD 1 PHR MBT. The physical properties of vulcanizates prepared from the two optimum compositions, III and IV, as well as those of the reference vulcanizates I and II are listed in Table II. Vulcanizates from compositions III and IV possessed lower values of tensile strength, stress at 3004 elongation (300p modulus), and hardness, about the same tear resistance and considerably higher elongation values than reference vulcanizate I which was cured with -7

sulfur plus NCHBTSA. Vulcanizates III and IV had physical properties which were generally comparable to the non-free sulfur cured reference standard II, although the elongation values of vulcanizates III and IV were somewhat higher, When the physical properties of the aged samples in Table II are compared, however, it is apparent that vulcanizates from compositions. III and IV were somewhat more resistant to aging than the reference vulcanizate II and they were substantially more resistant than reference vulcanizate I. The scorch resistance of compositions III and IV and reference compositions I and II, as measured by a shearing disk viscometer, are also reported in Table II. Compositions III and IV were considerably more scorch resistant than reference compositions II but less than reference composition I. Vulcanizates III and IV exhibited a tendency to bloom, the blooming. properties of the former being the more pronounced. It was learned (8) that the addition of low molecular weight polyethylene waxes to rubber compositions might hinder or completely prevent blooming. A few vulcanizates of the composition III (TMTD with DBTDMTU) were prepared with 2 PER of the polyethylene wax. These products had slightly lower tensile strengths and slightly higher per cent elongations and no bloom. While this study was limited to a few mixes of only one type of composition, the results do offer encouragement for the possibility of better control of bloom. Dicumyl Peroxide Vulcanizing System The improved aging qualities of vulcanizates produced in the absence of added free sulfur, such as presented in the preceding section, have increased interest in curing systems containing no sulfur at all. -8

Much of the early work on various types of peroxides (1,4,9) did not indicate much promise for commercial applications. During the early stages of the present investigation, however, the commercial availability of dicumyl peroxide (10) was announced and the first information indicated that it possessed promising qualities for rubber vulcanization. Thus, it was of considerable interest to examine this material. Also, during the course of this work successful vulcanizations of Hevea and nitrile rubbers with dicumyl peroxide were reported (5,11). The ultimate tensile and elongation values of vulcanizates cured with various amounts of dicumyl peroxide are shown in Figures 2a and 2b. The vulcanizates used to obtain the curve in Figure 2a were prepared from rubber compounds which contained only the PBNA originally present in the SBR-1500 (approximately 1.25 PHR) while those in Figure 2b contained one additional PHR of PBNA. It should be noted that the concentration of DCP-40 corresponding the maximum tensile value shifted from approximately 1.5 to 2.25 PHR of DCP-40 by the addition of 1 PHR of PBNA. Thus, it was apparent that portions of the amine antioxidant and the dicumyl peroxide reacted. The physical properties of the optimum vulcanizates V and VI cured with DCP-40 are recorded in Table II. These vulcanizates had somewhat low values of tensile strength, per cent elongation, stress at 300% elongation and tear resistance in comparison with those of the standard NCHBTSA-sulfur cured vulcanizates while the hardness values were comparable. These peroxide cured vulcanizates also had lower per cent elongation and tear resistance values than the DPMTTS-MBT reference vulcanizate II but otherwise the original physical properties were similar. After air-oven aging, however, the values of the tensile properties of the DCP-40 cured vulcanizates were higher than those of the reference vulcanizates I and II. -9

The Mooney scorch resistance of composition VI containing 2.25 PER of DCP-40 appeared to be comparable to that of the reference composition I which contained free sulfur and NCHBTSA. This conventional scorch test, however, was not entirely meaningful for the peroxide compositions since some peroxide decomposed without causing vulcanization during the test (12). Therefore, the supplementary test described in the Experimental Part was used which involved heating sheets of the compounded rubber, as obtained directly from the mill, in a circulating air-oven for periods up to three hours at temperatures from 230 to 250~F, followed by curing at 307~F for 30 minutes, and finally determination of the physical properties. The physical properties of vulcanizates from compositions heated at 2300~, 240~, and 2500F and containing originally 2.25 PHR of DCP-40 and one additional PER of PBNA are summarized in Table III. It may be seen that the compositions withstood heating in air for about 30 - 40 minutes at 250~F and somewhat longer periods at the lower temperatures before changes in the physical properties of the corresponding vulcanizates were substantial. Thus, the safe processing time is judged to be approximately 30 - 40 minutes rather than 54 minutes as measured by the Mooney scorch test. Since the supplementary test seemed more realistic than the Mooney scorch test for evaluating the processing safety of rubber compounds containing dicumyl peroxide, it was of interest to determine the safe processing times of compositions III and IV by this heating procedure. Table III contains the results of these experiments on compounds containing accelerator combinations. These data indicate that periods in which the raw compositions can be held at 250~F without causing substantial changes in the tensile properties of the final vulcanizates are in the range of 20 - 30 minutes. The heating of the compositions for periods of 30 - 60 -10

minutes resulted in products which, after curing, possessed higher tensile strength than those which had not been pre-heated. It is interesting that after heating the raw composition III for 2 hours at 2500F, the formulation could still be compression molded at 307~F to yield a sound sheet. These results emphasize the arbitrariness associated with the rating of general processing stability from specific laboratory tests. Dicumyl Peroxide - Sulfur-Bearing Accelerator Vulcanizing Systems The third type of vulcanizing system comprised the use of accelerators in combination with dicumyl peroxide and PBNA. They included the compounds in the following groups: 1) Zinc dibutyl-dithiocarbamate (ZDBDTC) Tetramethyl-thiuram-disulfide (TMTD) Tetrabutyl-thiuram-monosulfide (TBTM), 2) 2-Mercaptobenzothiazole (MBT) Diphenylquanidine (DPG), and 3) 2,4-Dinitrophenyl-dimethyl-dithiocarbamate (DMPDMDTC). Vulcanizates cured with combinations of DCP-40 and members of the first group generally possessed the highest values of physical properties, those formed from the use of the second group of accelerators appeared to have some less desirable properties, and those prepared by use of DCP-40 and the compound of the third group had the least desirable physical properties. The ultimate tensile and elongation values for vulcanizates cured with various amounts of DCP-40 with TMTD, and with ZDBDTC are shown in Figures 3a and 3b. Typical sets of physical properties for the vulcanizates cured with combinations of DCP-40 and members of the three groups mentioned above are listed in Table II (composition numbers VII - XII). It was found necessary to add 1 PHR of stearic acid and 3 PER of -11

zinc oxide to the compositions containing DCP-40'plus TMTD or ZDBDTC as curing agents in order to obtain vulcanizates with good physical properties. The optimum composition VII contained 5.75 PER of DCP-40, 0.75 PHR of TMTD, 1 PHR of PBNA, 1 PER of stearic acid and 3 PHR of zinc oxide. When the properties of the vulcanizate from this compound are compared with those of the optimum vulcanizate VI, containing DCP-40 as the only curing agent (Table II), it is apparent that the use of TMTD with DCP-40 has caused some increase in tensile strength and a large increase in elongation, with little effect on the other physical properties and the age resistance. Similar and somewhat more pronounced changes resulted from the use of DCP-40 with ZDBDTC and PBNA. The optimum composition VIII as determined from Figure 3b was 3.75 PHR of DCP-40, 1 PHR of ZDBDTC, 1 PHR of stearic acid and 3 PHR of zinc oxide, and it can be seen in Table II that the resulting vulcanizate had considerably higher tensile and elongation values than the DCP-40 cured vulcanizate with equally good resistance to aging. The values of the physical properties of the unaged vulcanizates of composition VIII are therefore similar but not identical with those of the vulcanizates from the free sulfur-accelerator combination. It should be noted, however, that the concentration of DCP-40 required to yield a product possessing maximum tensile strength is greater than the concentration necessary to give the product with maximum tear strength. The tensile values for the aged vulcanizates in Figures 3a and 3b were higher than those of the comparable unaged samples, the effect being the more pronounced for the products formed in the presence of DCP-40 and TMTD. The reasons for this effect are not completely clear but must be due at least in part to additional curing during the aging process. This effect -12

will be discussed further in the following section. Mooney scorch values for compositions VII and VIII were 26 and 40 minutes, respectively; and the time of heating of composition VII, at 2500F, which did not produce changes in the physical properties of the vulcanizate was 30 minutes. Effect of Curing Time on Dicumyl Peroxide Systems As anticipated above, it was of interest to examine the effect of curing time at 307~F on the unaged physical properties of vulcanizates cured with dicumyl peroxide alone and in combination with accelerators. In Figures 4a, 4b and 4c, variations of tensile properties with curing time are shown for vulcanizates cured with DCP-40 and with combinations of DCP-40 and TMTD or ZDBDTC. Figures 4a and 4c show that the tensile properties of the vulcanizates have reached or are rapidly approaching limiting values at curing times of 30 to 35 minutes at 307~F. The tensile properties of the DCP-40 - TNTD cured samples (Figure 4b), however, appear to be rising at the 40 minute point. It seems likely, therefore, that postcuring during aging may account, at least in part, for the large increase in tensile strength after air-oven aging shown in Figure 3a. Aging of Selected VulcFnizates at Various Temperatures The vulcanizates III to VIII were resistant to air-oven aging at 212~F during 72 hours. To gain further information about their age resistance under more stringent conditions, vulcanized test specimens were subjected to air-oven aging at 212~F for periods up to 32 days, at 250~F for periods up to 15 days, and at 300~F for periods up to 12 hours. The effects of extended aging at 212~F on ultimate elongation, tensile and stress at 3004 elongation are summarized in Figures 5, 6 and 7. Figures 8 and 9 show the effect of air-oven aging at 2500 and 500~F on the ultimate elongation of the various vulcanizates. A brief summary of these data is reported -13

in Table IV. Aging Studies at 212~F On inspection of the curves shown in Figures 5, 6 and 7, it can be seen that the per cent elongation of the vulcanizates varied more regularly with time of aging than did the stress at 300% elongation or the ultimate tensile strength. Thus, for purposes of discussion, attention may be concentrated on Figure 5. It is interesting that the vulcanizates from the peroxide containing compositions' VI, VII and VIII, showed the greatest resistance to change in per cent elongation throughout the 30 day period at 212~F. However, vulcanizates III and IV, from the sulfur-bearing accelerator combinations, showed equally good resistance after the first 10 days. Vulcanizate V, produced by the action of 1.5 PER DCP-40 (without additional PBNA), showed relatively poor resistance to aging, but vulcanizate VI formed from 2.25 PHR of DCP-40, with one addition PHR of PBNA showed excellent resistance. The age resistance of the vulcanizate formed from 3 PFR DCP-40 with 2 additional PHR of PBNA (not shown on the graph) was somewhat greater than that of vulcanizate VI; however, the product from 3.75 PHR DCP-40 with 3 additional PHR of PBNA (not shown on the graph) gave no further improvement in age resistance. Aging Studies at 250~F The per cent elongation values of the selectedvulcanizates after air-oven aging for various times at 250~F are shown in Figure 8, and the tensile, elongation, stress at 300% elongation and hardness after aging fbr 7 days at 250~F are reported in Table IV. The data indicate that 250~F represents severe aging conditions, with the per cent elongation for all the aged vulcanizates being essentially zero after 11 days at that temperature. -14

The vulcanizates VII and VIII (cured with DCP-40 plus TMTD or ZDBDTC and including additional PBNA) had the greatest resistance to aging at 250~F. The order of decreasing age resistance of the remaining vulcanizates was: l) accelerator cured reference vulcanizate II, 2) dicumyl peroxide cured vulcanizate VI, and 3) the sulfur plus accelerator cured standard vulcanizate I. Aging Studies at 300~F The curves in Figure 9 represent the change in elongation with the time of air-oven aging at 3000F, while Table IV lists the tensile, elongation, stress at 300% elongation and hardness after aging 12 hours at 300~F. Again, the vulcanizates cured with dicumyl peroxide plus accelerator combinations, VII and VIII, exhibited the greatest age and/or temperature resistance. These vulcanizates showed little change in elongation until after 9 to 12 hours in air at 300~F. Beyond that point the elongation decreased regularly to a value of approximately 150'k at 20 hours. Raw Materials Costs of Vulcanization Systems In Table V the relative costs of the compositions I - VIII (excluding V) including the indicated vulcanizate agents and 50 PHR MAF carbon black are listed. It may be seen that raw materials costs of vulcanizate VI is 1.%8 greater, and the other compositions are 5.2 - 5.8* greater than the cost of the sulfur-accelerator system. -15

DISCUSSION OF RESULTS The results of this study are summarized in a qualitative fashion in TableVI, and are briefly discussed in the following paragraphs. Physical Properties of Vulcanizates It has been seen that physical properties of the unaged vulcanizates prepared by use of the peroxide-accelerator-PBNA system are essentially equivalent, though not identical, to those of the products cured with the free sulfur-accelerator system. Vulcanizates prepared by the action of dicumyl peroxide without accelerators, and vulcanizates produced by the accelerators combinations possessed somewhat lower tensile properties. However, heating of the uncured accelerator compositions for 30 minutes at 250~F followed by compression molding at 3070F for 50 minutes yielded sheets with high tensile strengths. The vulcanizates which had been cured with accelerator combinations showed a tendency to bloom. The presence of a small amount of polyethylene wax eliminated bloom in the one case tried. Vulcanizates from the peroxide systems showed no bloom. Age Resistance of Vulcanizates The age resistance of the vulcanizates produced from all of the non-free sulfur curing systems was substantially greater than those from the free sulfur-accelerator combination. The vulcanizates produced by the action of dicumyl peroxide in combination with accelerators were the most resistant to air-oven aging. By reference to Table VI, a rough parallelism may be noted between the age resistance of vulcanizates and the strength of the probable crosslinks as listed by Dogadkin and Torasova (15). On that basis it would appear -17

that C-C crosslinksshould yield the strongest and most age resistant rubber. Then the question arises regarding the contribution of the accelerators (which contain sulfur and nitrogen) to the dicumyl peroxide vulcanization. Three possible functions of the sulfur-nitrogen bearing accelerators to the peroxide vulcanization are suggested: 1) the accelerators induce the decomposition of the peroxide allowing the process to take place at a lower activation energy and resulting in a more even distribution of crosslinks; (2) the decomposition products of the sulfur-nitrogen bearing accelerators function as antioxidants; and (3) the accelerators act as dispersing agents for the peroxide in the rubber. While the results of this study do not afford a decision regarding the relative merits of these suggestions, they do emphasize that further study of dicumyl peroxide vulcanization systems should include attention to other reducing and dispersing agents. Processing Stability of Uncured Compositions The results listed in Table VI show that compounds containing the conventional free sulfur-accelerator combination remain the most resistant to precuring or decomposition during processing. Compounds with the peroxide combinations are stable in air for 350 - 40 minutes at 250~F and for longer periods at lower temperatures. The accelerator compositions gave scorch values of 20 - 30 minutes. However, the heating of the uncured accelerator formulations from 0.5 to 2.0 hours a;t 250F did not prevent curing by compression molding at 307~F. Relative Costs of Raw Materials The raw materials costs for compositions containing 50 parts of carbon black per hundred parts of rubber and the specified vulcanizing agents -18

are surprisingly similar. The DCP-40 + PBNA composition is 1.8% greater and the others are 5.2 - 5.8o greater than the cost of the free sulfuraccelerator combination. Thus it is concluded that vulcanization systems for SBR based on dicumyl peroxide in combination with accelerators and phenyl-p-naphthylamine yield vulcanizates with satisfactory physical properties and excellent aging qualities, but they are somewhat more expensive and are less stable to processing than the conventional free-sulfur accelerator curing systems. -19

ACKNOWLEDGMENTS We express our appreciation to the Ordnance Corps, Department of the Army, for financial support of this work, to Messers A. C. Hanson, R. Shaw, and Z. T. Ossefort of the Rock Island Arsenal Laboratories for their help and suggestions, and to Messers J. E. Marberry and R. N. Wetterholt for assistance in the experimental work. -21

Bibliography 1. T. Alfrey, J. G. Hendricks, R. M. Hershey and H. Mark, India-Rubber World, 112, 577 (1945); ibid. 113, 653 (1946). 2. D. J. Metz and R. B. Mesrobian, J. Polymer Sci., 11, 83 (1953)3. A. L. Pedersen, Aging Properties of Low Sulfur Rubber Vulcanizates, a communication from Northern Cable andWire Works, Ltd., Copenhagen (1954). 4. M. Braden, W. P. Fletcher and G. P. McSweeney, I.R.I. Trans., 30, 44(1954). 5. M. Braden and W. P. Fletcher, I.R.I. Trans., 31, 155(1955). 6. Z. T. Ossefort, R. F. Shaw and E. W. Bergstrom, Rubber World, 135, 867 (1957); ibid, 136, 65 (1957). 7. M. B. Fackler and J. S. Rugg, Anal. Chem., 23, 1646 (1951). 8. Private communication from Semet-Solvay Petrochemical Division, Allied Chemical and Dye Corp., New York, New York. 9. I. I. Ostromuisslenskii, J. Russ. Phys.-Chem. Soc., 47, 1467 1885 (1915). 10. Hercules Powder Company, New Product Data Sheet No. 4-5-55. 11. C. H. Lufter, Rubber World, 133, 511 (1956). 12. Hercules Powder Company, New Product Data Sheet No. 5-2-55 135. B. A. Dogadkin and Z. N. Tarasova, Rubber Chem. and Technol. 27, 883-(1954). -23

3500 1 3500 Ultimate Tensile(Original) -0Ultimate Tensile (Aged 72 hrs( 212~F) --- Ultimate Elongation (Original) Ultimate Elongation (Aged 72 hrs(C2 12~F) ----- 3000 0 - 3000 0 0/ 2500 / 700- 2500 Qw 2000- \ T r ~ -2000 %E H I l -500- / \ / I 500 - j / 1 500 I 400- / \I / 1000 A 300 —1000 loooF \I PI 4oo- -3002 3 2 PHR TMTD(+I PHR DBTDMTU) PHR TMTD (+ I PHR MBT) (a) (b) Fig. 1 Physical properties of SBR-1500 vulcanizates cured with accelerators.

Ultimate Tensile (Original) - ----- 1 Ultimate Tensile (Aged 72 hrs(i) 212~F) — 0 — Ultimate Elongation (Original) ----- Ultimate Elongation (Aged 72 hrs C 21 2~F) — --- 600 - 2500 -2500 2000 I11 1 \2 200 04 P D 40+1400 U)( ( nF P p U)( 1500 -': 00- 1500 200- 1000- 000 I 2 3 4 5 I 2 3 45 PHR DCP-40 PHR DCP- 40 (+1 PHR QF PBNA) (a) (b) Fig. 2 Physical properties of SBR-1500 vulcanizates cured with dicunyl peroxide.

Ultimate Tensile (Original) - Ultimate Tensile (Aged 72hrs ( 212~F) ---- Ultimate Elongation (Original) --- 3000 Ultimate Elongation (Aged 72 hrs@2120F) — 0 — 3000 /040 2500 o'"' -70 c r700- / 2500 I500 (n ( ~-~ ~ ~ i. 3 qqepKoete / SE-10 vlaite cure -ihicmlerx acceleratQUoo1) 600- - T-400- W / ^^ 1500 4 ~000 1000- / -— 5 —- ^ --- i --- ii1 --- ^ --- ^ --- i --- k - ^500 2 3 4 5 2 3 4 PHR DCP - 40 (+0.75TMTD,3ZnOISTEARIC ACID) PHR DCP - 40 (+1 ZDBDTC)3 ZnO, I (a) STEARIC ACID) (b) Fig. 5 Tensile properties of SBR-1500 vulcanizates cured with dicumyl petoxideaccelerator combinations.

l I I..... I I I I I 1 I I 2.25 DCP-40+ IPBNA 3.75 DCP-40 +0.75 TMTD 3.75 DCP-40+ + IPBNA + 3ZnO + IStearicacid I ZDBDTCt IPBNA + 3ZnO+ I Stearicacid 2500 -2500-600- 600 400sile -- ST-R-ES % ^ —'^ /.....~'~' 0. -. —~ ^''. / 20 3 - 50025, )2COO 500 50 v, \3 f" —-o — - o^~%lw onC%E T^'" — 0"-40O- -- 400 i^ 1500- -1500Tensile -200- 200 25 30 35 40 25 30 35 40 20 3040 5060 300 La) (b) (C) CURE TIME (MIN ~ 3070F) Fig. 4 Effect of curing time on tensile properties of SBR vulcanizates cured with systems containing dicumyl peroxide.

~I~I I~I I O — 0 — 1.5DCP-40 -— 0 —- 2.25DCP-40+ IPBNA — t —- 3.75 DCP-40 - 0.75 TMTD+ IPBNA — 0 — 3.75 DCP-40+ I ZDBDTC+I PBNA - 0 —- INCHBTSA+ 1.75 Sulfur 600 - --- DPMTTS + I MBT -0 > -- - a 1.75TMTD+ I MBT - ------ 1.75 TMTD + I DBTDMTU 500 — - - _ - -- -—..I. - 100 _ ~ 1 0 I R\) 300 0 200 100o 0 0 2 4' 6 8 I0 20 30 40 DAYS AT 212~ F Fig. 5 Effect of extended air-oven aging at 212~F on the elongation of SBR vulcanizates.

3000 1500 \ 1.5DCP-40 \W ---- --- 2.25 DCP - 40 +1 PBNA \ o --— j —-- 3.75 DCP-40 +0,75 TMTD + IPBNA.N... ---------- 3.75 DCP-40 + I ZDBDTC +1 I PBNA ^ 500 0 I NCHBTSA+ 1.75 Sulfur ------ I DPMTTS + I MBT --— i- 1.75 TMTD + I MBT ~ 1.75 TMTD+IDBTDMTU I I I I I I I I 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 DAYS AT 212 ~F Fig. 6 Effect of extended air-oven aging at 212~F on the tensile strength of SBR vulcanizates.

-— 0 — 1.5 DCP- 40 -— 0 — 225 DCP-40 -I PBNA ----- 3.75DCP-40 +0.75TMTD + IPBNA — ~ — 3.75DCP-40- + I ZDBDTC+IPBNA -0 — I NCHBTSA + 1.75 Sulfur -- -- I DPMTTS + I MBT --- 1.75 TMTD +I MBT 4 —-- 1.75 TMTD + I DBTDMTU 2000 -- z 0 z _J w ^! / / / ( 1500' / -0 C/) Hn 1- 00 0/ 2 4 6 8 10 20 30 40 Vef#~~~ ~~DAYS AT 212~F Fig. 7 Effect of extended air-oven aging at 212~F on the stress at 300% elongation of SBR vulcanizates.

600 c600c - -- -- - - -- - - - I I I I I I - I- I | i l 0 1.5 DCP-40 A 2.25 DCP- 40 + I PBNA 3.75 DCP-40 +0.75TMTD + I PBNA 50O ~ 3.75DCP-40+ I ZDBDTC + IPBNA ^\ ^~~~, \ ^^^~ x~ 1X I NCHBTSA+ 1.75 Sulfur \ I DPMTTS - IMBT 400 z 0 z 300 0 LUI ~~IV~~~~~ ~ 0 200 100 1 2 3 4 5 6 7 8 9 10 I 1 12 13 14 15 DAYS AT 250 F Fig. 8 Effect of air-oven aging at 250OF on elongation of selected SBR vulcanizates.

0 —- -- 1.75 SULFUR + INCHBTSA -0 — 2.25 DCP-40 + I PB,NA ---- -- 3.75 DCP -40 + 0.75 TMTD + I PBNA. - 3.75 DCP-40+ IZ DBDTC + IPBNA 600 100 2 4 6 8 10 12 14 16 18 20 22 24 (_9 z 0 Le 300 200 o00 0 0 2 4 6 8 10 12 14 16 18 20 22 24 HOURS AT 300 OF Fig. 9 Effect of air-oven aging at 300 F on elongation of selected SBR vulcanizates

COMPOSITIONS OF RUBBER COMPOUNDS Parts by Weight Ingredient Code Name I II III IV V VI VII VIII IX X XI XII SBR 1500a 100 100 100 100 100 100 100 100 100 100 100 100 MAF carbon. blackb 50 50 50 50 50 50 50 50 50 50 50 50 Zinc oxide 3 3 3 3 3 3 3 3 3 3 Steraric acid 1 1 1 1 1 1 1 1 1 Phenyl-p-naphthylamine PBNA 1 1 1 1 1 1 1 1 1 1 Sulfur 1.75 N-cyclohexyl-2-benzothiazyl sulfenamindec NCHBTSA 1 Dipentamethylena-thiuramtetrasulfide DPMTTS 1 2-mercaptobenzothiazolee MBT 1 1 0.4 0^~ ~ ~ Dibenzothiazyl-dimethylthiol ureaf DBTDMTU 1 Tetramethyl-thiuram-disulfideg TMTD 1.75 1.75 0.75 Dicumyl peroxide-40%-calcium carbonate 60%h DCP-40 1.5 2.25 3.75 3.75 3.75 3.74 3.75 3.75 Zinc dibutyl-dithiocarbamatei ZDBDTC 1 Diphenylquanidinej DPG 0.20 2,4-dinitrophenyl-dimethyldithiocarbamate - DNPDMDTC 075 Tetrabutyl-thiuram-monosulfide1 TBTM 0.75 aexas-U.S. Chemical Co., and Goodrich-Gulf Chemicals Inc. bPhilblack A, Phillips Chemical Co. hMethyl Tuads, R. T. Vanderbilt Co. CSantocure, Monsanto Chemical Co. hDiCup 4oC, Hercules Powder Co. dTetrone A, E.I. du Pont de Nemours and Co.'Butyl Zimate, R. T. Vanderbilt Co. eCaptax, R. T. Vanderbilt Co. Monsanto Chemical fEl Sixty, Monsanto Chemical Co. kSafex, Naugatuck Chemical 1Pentex, Naugatuck Chemical

TABIE II PHYSICAL PROPERTIES OF SEIECTED COMPOSITIONS AND VULCANIZATES Mooney Composition Curing Tensile, Elongation, Stress at 300% E., Tear, Durometer Scorch, Number Systema psi % psi lbs/in. Hardness m. Unaged Agedb Unaged Agedb Unaged Aged Unaged Aged Unaged Agedb t5 t1 I 1.75 Sulfur + 2450 2550 480 260 2150 --- 260 170 68 74 61 67 1 NCHBTSA II 1 DPMTTS + 2100 2400 440 380 1340 1890 280 210 65 67 7 8 1 MBT III 1.75 TMTD + 2180 2680 650 590 950 1290 290 280 65 65 19 22 1 DBTDMTU IV 1.75 TMTD + 2360 2720 610 550 1020 1440 280 270 66 66 23 28 1 MBT 1 MBAT V 1.5 DCP-40 2220 1970 420 400 1550 1450 220 220 65 66 VI 2.25 DCP-40++ 2280 2170 450 44o 1430 1680 230 260 65 65 54 70 1 PBNA OC VII 3.75 DCP-40 + 2330 2570 550 560 1170 1290 210 210 62 66 26 33 0.75 TMTD + 1 PBNA VIII 3.75 DCP-40 + 2540 2630 470 450 1480 1610 180 200 65 66 40 58 1 ZDBDTC + 1 PBNA IX 3.75DDCP-40 + 2280 2200 310 310 2090 2010 110 110 68 69 0.4 MBT + 1 PBNA X 3.75 DCP-40 + 2360 2250 330 320 2060 1980 16o 160 69 70 0.2 DPG + 1 PBNA XI 3.75 DCP-40 + 1360 1720 570 550 690 890 210 270 62 66 0.75 DNPDMDTC + 1 PBNA XII 3.75 DCP-40 + 2460 2480 480 460 1480 1500 190 210 65 67 0.75 TMTM + 1 PBNA a The rubber contained 1o25 PHR of PBNA. Only concentration of additional PBNA is shown in formulations. b Samples were aged in an air oven at 2120F. for 72 hours c t5 and t10 represent the number of minutes required for a five and ten unit rise over the minimum.

TABIE III STABILITY OF COMPOUNDED RUBBER TO HEATING IN AIRa Tensile, psi Elongation, % Stress at 300%.E., psi Durometer Hardness Composition Curing Temp. Hours Hours ours Hours Number System ~F. 0 1/2 1 2 3 0 1/2 1 2 3 0 1/2 1 2 3 VI 2.25 DCP-40 + 250 2380 2300 1770 1010 -- 490 490 610 680 -- 1240 1190 710 380 -- 65 65 62 60 -- 1 PBNA i" 240 2270 2230 2140 1610 1670 400 420 460 510 510 1760 1570 1340 870 950 67 6 65 63 63 7 a ^230 2130 2120 2080 1500 1510 500 520 560 590 600 1280 1130 950 780 740 63 63 63 63 62 III 1.75 TMTD + 250 2150 2520 2340 2120 -- 650 640 650 64 -- 1300 1450 1400 1400 -- 65 65 65 69 1 DBTDMTU IV 1.75 TMTD + 250 2300 2480 2400 2110 -- 6co 580 580 570 - 1430 1580 1620 1570 -- 65 65 65 67 1 MBT VII 3.75 DCP-40 + 250 2350 2310 2060 1980 -- 510 550 580 500 -- 1410 1250 1200 1170 -- 65 65 65 69 0.75 TMTD + 1 PBNA aThe compounded rubbers were heated in air at the designated temperatures and times after which the compounds were cured for 30 minutes at 307~F. and the physical properties of the resulting vulcanizates were measured.

TABIE IV AIR-OVEN AGE RESISTANCE OF SELECTED VULCANIZATES AT 212, 250 and 300~F. Tensile, psi Elongation, % Stress at 300% E., psi Durometer Hardness Unaged Aged Unaged Aged Unaged Aged Unaged _ Aged 32 days 7 days 12 hours 32 days 7 days 12 hours 32 days - 7 days 12 hours 32 days 7 days 12 hours Composition Curing System at at at at at at at at at at at at Number 212~F. 250~F. 300~F. 212~F. 250~F 300~F 212~F. 250~F. 300~F. 212~F. 250~F. 300~F. I 1.75 Sulfur + 2450 800 1180 560 480 20 4o 70 2150 -- -- - 68 89 83 77 1 NCHBTSA II 1 DPMITS + 2100 1330 1480 -- 440 180 110 -- 1340 1960 1710 -- 65 72 70 1 MBT J III 1.75 TMTD + 2180 1890 - - 650 310 -- - 950 1590 -- - 65 71 Oo 1 DBTDMTU IV 1.75 TMTD + 2360 1950 - -- 610 300 -- -- 1020 1910 -- - 66 70 1 MBT V 1.5 DCP-40 2220 610 410 -- 420 50 6o 70 1550 1640 1190 -- 65 79 82 VI 2.25 DCP-40 + 2280 1760 480 990 450 310 70 310 1730 1510 1300 870 65 69 77 66 1 PBNA 2330 2270 1820 1690 550 340 270 410 1170 1490 1270 970 62 70 71 66 VWII 3.75 DCP-40 + 0.75 TMTD + 1 PBNA VIII 3.75 DCP-40 + 2540 1810 660 2070 470 280 150 430 1480 1620 1560 1300 65 70 71 66 1 ZDBDTC + 1 PBNA

TABIE V COST OF MATERIALS FOR SELECTED COMPOSITIONS Materials Cost,a Per cent Cost Composition Curing $/100 lbs. Increase Over Number System Compounded Rubber Composition I I 1.75 Sulfur + 1 NCHBTSA 19.05 II 1 DPMTTS + 1 MBT 20.05 5.2 III 1.75 TMTD + 1 DBTDMTU 20.16 5.8 IV 1.75 TMTD + 1 MBT 20.04 5.2 VI 2.25 DCP-40 + 1 PBNA 19.40 1.8 VII 3.75 DCP-40 + 0.75 TMTD + 20.04 5.2 1 PBNA VIII 3.75 DCP-40 + 1 ZDBDTC + 20.15 5.8 1 PBNA The values listed were estimated from market prices listed in Rubber Age, 79 870 (1956). -59

TABLE VI SUMMARY OF FINDINGS Vulcanizates Crosslinks Bond Safe Relative Composition Tensile Strength Processing Time Materials No. Type Properties Bloom Age'Resistance Type k cal 2500at Min. Cost I Free Sulfur + Excellent No Poor -c-sm-c- 27.5 60 100.0 o~ E~NCHBTSA II-IV Accelerator Combinations Fair Yes Good -c-s-s-c- 27.5 20-30 105.2-105.8 -c-s-c- 54.5 VI DCP-40 + PBNA Good No Very Good -c -c- 62.7 30 101.8 VIII DCP-40 + ZDBDTC + Excellent No Excellent? 40 105.8 PBNA