EFFECT OF SULFUR TO ACCELERATOR RATIO ON CROSSLINK STRUCTURE, REVERSION, AND STRENGTH IN NATURAL RUBBER
ABSTRACT
A new relationship among the sulfur to accelerator (S/A) ratio, the degree of reversion or the net loss of crosslinks at the prolonged cure time, and the tensile strength and crosslink structure of gum natural rubber (NR) vulcanizates is described here. To study this, N,N-dicyclohexyl-2-benzothiazole sulfenamide (DCBS), N-cyclohexyl-2-benzothiazole sulfenamide (CBS), 2,2′-dithiobisbenzothiazole (MBTS), and tetrabenzylthiuram disulfide (TBzTD) were used as accelerators. The results showed that for all four tested accelerators, the degree of reversion and tensile strength of the vulcanizates did not simply increase with increasing S/A ratios within the range of 0.26–6.67 by weight. This was because the proportion of polysulfidic linkages playing an important role on these properties was not simply proportional to the S/A ratios but turned out to pass through a maximum and then decline with further increasing S/A ratios for the DCBS, CBS, and MBTS cure systems. Nevertheless, when considering the effect of crosslink structure on the thermal and mechanical properties, it was observed that for all four tested accelerators, the increase in the extent of polysulfidic linkages gave the vulcanizate with the lower reversion resistance but the higher tensile strength. Therefore, the generalization that it is the high concentration of polysulfidic linkages in the network that causes a decrease in the reversion resistance but an increase in the tensile strength is seemingly still applicable.
INTRODUCTION
Sulfur vulcanization occurs by the formation of sulfur linkages between rubber chains. Usually, the networks are formed by a mixture of monosulfidic, disulfidic, and polysulfidic linkages, and their relative distribution depends significantly upon the sulfur to accelerator (S/A) ratio used in the vulcanizate mixture, accelerator type, curing temperature, and curing time. At the early stage of vulcanization, long polysulfidic linkages are formed. During vulcanization, there are two processes: desulfuration and thermal decomposition of the initial polysulfidic linkages. The additional crosslinkings take place during the desulfuration process, which gives a shorter sulfidic linkage, while the decreased crosslinkings are due to the thermal decomposition with a main chain modification. Once more crosslinks are being broken than formed, the net of crosslinks decreases. This is called reversion. A fair amount of research has been carried out to assess the relationship between the natural rubber (NR) formula, crosslinked structure, and final properties of different vulcanizates.1–8 The bond dissociation energy was proposed to be the underlying reason for the variation in the properties of the different vulcanizates, since it decreases as the number of sulfur atoms increases within the linkage. In general, it has been shown that vulcanizates with a high proportion of polysulfidic linkages, obtained when employing high S/A ratios (conventional vulcanization system, CV), are more susceptible to deterioration from heat.5,8 Consequently, upon over curing, such vulcanizates derived from a high S/A ratio undergo a much higher degree of reversion and show a poorer retention of their properties on aging than those with a high proportion of monosulfidic or disulfidic linkages because of the lower bond energy of polysulfidic linkages. At the same time, both the relative weakness, which can break more readily under strain, and the ability to reform the polysulfide bond render vulcanizates with a higher mechanical strength. This process permits the relief of stress concentration, which otherwise can lead to the initiation of failure, and forms a more uniform distribution of stress.6 Vulcanizates with a high proportion of monosulfidic or disulfidic linkages, successfully obtained using a low level of sulfur and a correspondingly high level of accelerator (efficient vulcanization system, EV), have a higher thermal stability, but at the expense of reduced mechanical strength.5,8
Although the above view is widely held, it is not universally accepted.9–13 For example, Lal9 showed that, in contrast to the above, the tensile strength of NR does not depend on the type of sulfidic crosslink. Vulcanizate compositions were based on the sulfur-diphenylguanidine system, which supposedly produces mainly polysulfidic crosslinks. The obtained vulcanizates were then reacted with triphenylphosphine under nitrogen to convert the polysulfidic links to principally disulfidic and monosulfidic linkages, but little change in the tensile strength or crosslink type was observed. This approach was preferred to a comparison based on vulcanizates prepared with different curing recipes, since the latter can lead to substantial differences in the vulcanizate network structure other than the sulfur rank. However, using a similar treatment, but with a different vulcanizing system, Bristow and Tiller5 observed a significant reduction in the tensile strength. Furthermore, our previous studies using NR formulated to the same level of crosslinks by using different ratios of sulfur to the accelerator N,N-dicyclohexyl-2-benzothiazole sulfenamide (DCBS; a delayed-action sulfenamide accelerator) revealed that vulcanizates cured with the lowest sulfur/DCBS ratio gave the highest tensile strength and resistance to growth of cuts.10 Moreover, it was also found that the relationship between the reversion resistance and sulfur/DCBS ratio was not simply linear, but rather rheographs from an oscillating disk rheometer showed that the reversion resistance of vulcanizate passed through a minimum with increasing sulfur/DCBS ratios. Brown et al.11 mentioned that the controversy over the effect of crosslink type on strength may be complicated by the strain-induced crystallization of the rubbers used. The result from their work showed that for rubbers that do not strain crystallize, polysulfidic crosslinks are more effective than other types in producing high-strength vulcanizates, and this supports the work of Bristow and Tiller, rather than that of Lal. Furthermore, Hamed pointed out that the effect of the vulcanization system on the strength of the obtained vulcanizate varies depending on the degree of crosslinking. The CV vulcanizate gives higher tear strength than the EV counterpart of the same crosslink density only at high crosslink density, but the opposite is true at low crosslink density.13
Owing to these contrasting findings, our aims here are to evaluate the reason for these discrepancies and to study in more detail the effects of the sulfur to accelerator (S/A) ratio on the properties of NR vulcanizates when different accelerators are used. To this end, in addition to DCBS, this study used three more common accelerators, N-cyclohexyl-2-benzothiazole sulfenamide (CBS), 2,2′-dithiobisbenzothiazole (MBTS), and tetrabenzylthiuram disulfide (TBzTD), which were chosen as representatives of the different accelerator classes.
The degree of crosslinking is also a very important parameter affecting the vulcanizate properties. Therefore, in this study, the crosslink densities of vulcanizates were controlled to much the same level, but with quite different crosslink structures, by changing the S/A ratio in the NR compound. The effect of the curing system used on the distribution of the crosslink structure was chemically investigated.
EXPERIMENTAL
For each of the four accelerators, a series of five NR compounds was prepared. The range of S/A (weight/weight) ratios was varied from 0.26 to 6.67. For DCBS, MBTS, and TBzTD, the same S/A ratios were used, but in the case of CBS; the ratios used were slightly different. The formulation of each of the NR vulcanizates is listed in Table I. All formulations were comprised of a common base composition of 100 phr NR, 8 phr ZnO, 1 phr stearic acid, 1 phr 1,2-dihydro-2,2,4-trimethylquinoline (TMQ), and 1 phr N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylene diamine (6PPD).
Compounding
The rubber compounds were prepared by using a laboratory Brabender mixer with a fill factor of 0.7, a temperature of 50 °C, and a rotor speed of 50 rpm. To avoid rubber scorch in the mixer, the following two stage sequential procedure was followed. NR was first masticated for 1 min before the ZnO was added and then continuously mixed for 2 min. The remaining chemicals (stearic acid, TMQ, and 6PPD) were added and mixed for 2 min. Finally, the compound was removed, giving a total heated mixing time for the first step of 5 min, and cooled down to room temperature for 16–24 h. Then, the compound was loaded back into the heated mixer, and 1 min later the curatives (sulfur and accelerator) were added and mixed with the compound for an additional 3 min, giving a total second stage heated mixing time of 4 min. Then, the compound was taken out and sheeted off with a two-roll mill to ∼5 mm thickness. These rubber sheets were stored at room temperature for 16–24 h before vulcanization.
Cure rheometry
The cure characteristics of NR compounds were determined at 155 °C using a Monsanto Moving Die Rheometer (MDR), in accordance with the method described by ASTM D5289-95. Since the cure time (tc100) was taken as the time to reach maximum torque, and reversion refers to the loss of crosslink density associated with over curing, then the rheometer torque was used as an indicator of reversion behavior. The plateau region and the slope of declined torque refer to the reversion resistance and the reversion rate, respectively. The percentage of reversion at any cure time (t) is defined as14
where Tmax = maximum torque, Tt = torque at any cure time, and Tmin = minimum torque.
Molding
Rubber vulcanizates of 1 mm thickness were prepared with a hydraulic hot press. An unvulcanized sheet was compression molded at 155 °C for tc100. To study the effect of reversion behavior on the mechanical strength of NR, an extra rubber sheet was compression molded at tc(100) + 20 min. After molding, vulcanized sheets were immediately cooled to stop the reaction (curing) by immersing into a water bath.
Tensile testing
Tensile dumbbell specimens were cut from compression molded sheets along the milling direction using a type C die. Tensile tests were performed in accordance with ASTM D412-98a using an Instron Universal testing machine (model 5566) with a crosshead speed of 500 mm/min. The strain was monitored by an extensometer. For each rubber composition, at least five specimens were tested.
Crosslink density
Crosslink density of the rubber network can be estimated by a physical method such as swelling or stress–strain measurement. In this study, crosslink density was obtained from swelling measurement. Samples (10 mm × 10 mm × 1 mm) were immersed in n-decane and kept in the dark for 48 h to attain equilibrium swelling, ascertained and defined as no further weight increment over time. After that, each swollen sample was taken out, blotted with a paper towel, and quickly weighed to determine the swollen weight of sample (Ws). Then, the swollen sample was dried in an oven at 50°C to remove n-decane and reweighed to determine the dried weight of sample (Wd). The volume fraction of rubber in the swollen sample, 
, was calculated using the following equation:
where ρr and ρsol are the density of rubber and solvent. From the swelling result, it was possible to calculate the crosslink density or the number of moles of crosslink per unit volume (ρc, mol/cm3) using the Flory–Rehner equation:15
where VS is the molar volume of n-decane (194.92 cm3/mol), and χ is the interaction parameter (0.43 for NR and n-decane).
Crosslink distribution
The proportion of each crosslink type was determined by the use of chemical probes in conjunction with the determination of the total crosslink density described by Cunneen and Russell.16
Proportion of polysulfidic linkages
After the total crosslink density of specimens (10 mm × 10 mm × 1 mm) was determined, each specimen was put in an apparatus and swollen in n-heptane under N2 in the dark for 16 h at room temperature. Then n-heptane was run off and replaced by 10 ml of a propane-2-thiol-amine, used as a chemical probe to specifically cleave polysulfidic linkages, and agitated occasionally during 2 h under N2. After that N2 was purged through the apparatus again, and the propane-2-thiol-amine reagent was run off and replaced by 100 ml of hexane and agitated occasionally for 1 h. This hexane wash step was repeated four times, each with fresh hexane. Finally, the specimen was removed and dried overnight in vacuo at 30 °C. The treated specimen was weighed and immersed in n-decane to attain equilibrium swelling. After 2 days, the sample was taken out, excess surface and surface pore attached solvent was blotted from the surface of the specimen, and it was then weighed immediately. The proportion of polysulfidic linkages can be calculated by the difference between the total crosslink density, measured by swelling measurement, before (defined as Xtotal) and after (defined as X1) the above treatment:
where X1 is crosslink density of the treated specimen (disulfidic and monosulfidic linkages) after cleaving all polysulfidic linkages.
Proportion of monosulfidic and disulfidic linkages
The specimen was placed into the apparatus, purged with N2, and treated as in the above step, except hexane-1-thiol-amine reagent was used (for 48 h) in place of propane-2-thiol-mine, so as to specifically cleave both polysulfidic and disulfidic linkages. Finally, after completion of the swelling measurement, the concentration of monosulfidic linkages was obtained (defined as X2), allowing the proportion of disulfidic linkages to be calculated by the following equation.
RESULTS AND DISCUSSION
The effect of the sulfur to accelerator (S/A) ratio on the properties of vulcanizates cured with different accelerators was evaluated as described in the methods section. The change in the S/A ratio is expected to change the vulcanizate network consisting of various sulfur linkage types. However, the change in the S/A ratio also affects the degree of crosslinks, which is another factor having a large impact on the strength of vulcanizates. So to obtain vulcanizates with the same crosslink level, the rubber composition must be carefully formulated. The crosslink densities from the swelling testing and the delta torques from the MDR rheographs for the rubber compositions in Table I are summarized in Tables II and III, respectively. The results showed that for each accelerator only a slight difference in the crosslink densities and delta torques was observed, and so any differences in the observed properties of the vulcanizates should be attributed mainly to differences in the crosslink structures.
Effect of the s/a ratio on the vulcanizate reversion behavior
Representative MDR rheographs showing the time dependent torque of NR compounds cured with different accelerators and at various S/A ratios at 155 °C are depicted in Figure 1. The change in torque indicates the different state of vulcanization, where a higher torque is directly related to a higher level of crosslinks.
Citation: Rubber Chemistry and Technology 89, 3; 10.5254/rct.16.85963
From the MDR rheographs shown in Figure 1, after attaining a maximal torque, this then decreased with further heating time, which indicates the onset of a reversion process. There was a different rate of the progressive decrease in the cure plateau among the different accelerators. For some compounds, their torque was held constant for a while before the onset of the decrease in the torque was observed. On the other hand, reversion of some compounds started immediately after their maximum torque was attained. Among the four tested accelerators, TBzTD (classified as a secondary accelerator, in general, giving a short scorch time and an ultra fast cure rate) caused the fastest reversion. This is indicated by the shortest plateau region (time at which the maximal torque retains constant) and highest reversion rate (slope of the declined torque) as shown in Table III. The degree of reversion (%) as a function of cure time was calculated from the MDR rheographs using Eq. 1, and the results are shown in Figure 2.
Citation: Rubber Chemistry and Technology 89, 3; 10.5254/rct.16.85963
Figure 2 shows that the magnitude and the onset timing of reversion depend not only on the accelerator type but also upon the S/A ratio. For clarity of presentation of these two factors on reversion behavior, the plateau region, the reversion rate, and the degree of reversion (%) at 20 min above tc100 were used as an indicator of the reversion resistance. The results are given in Table III. The percentage of reversion at 20 min above tc100 was plotted against the S/A ratios in Figure 3.
Citation: Rubber Chemistry and Technology 89, 3; 10.5254/rct.16.85963
Previous studies have reported that an increase in the S/A ratio results in vulcanized rubber with a higher reversion.17–18 Indeed, this belief that a low S/A ratio cure system gives a network with a high reversion resistance is widely held because a low sulfur level with a correspondingly high accelerator level forms linkages with fewer sulfur atoms and so a greater thermal stability. However, unexpectedly, the results presented here revealed that for the DCBS, CBS, and MBTS accelerator cure systems, the degree of reversion passed through the maximum value and then declined with further increases in the S/A ratio, which suggests that the above generalization about the effect of the S/A ratio on reversion behavior cannot be extended to the whole spectrum of S/A ratios, at least in the S/A range studied here, and thus is an oversimplification. Rather, there was only a certain range of S/A ratios at which the generalization was applicable. Moreover, for rubber cured with TBzTD, the effect of the S/A ratio on the reversion behavior of the NR vulcanizate was different since the increase in the degree of reversion behavior was not directly proportional to the increase in the S/A ratio, but rather it initially increased with increasing S/A ratios and then leveled off. For vulcanizates cured with TBzTD, a maximum point of reversion as a function of the S/A ratio was not observed, at least not in the range of S/A ratios studied here. In addition to the degree of reversion, it was also observed here that the other two reversion parameters, including the plateau region of MDR rheographs representing the resistance to reversion and slope of the declined MDR torque representing the reversion rate, did not directly vary with the S/A ratio. Mostly, the former one passed through a minimum, whereas the later one passed through a maximum with the increasing S/A ratio. Clearly, the general statement that reversion increases with the increase of S/A ratios is not always applicable.
As mentioned previously, the reversion behavior is related to type as well as the number of crosslinks formed, due to the different types of crosslinks having different thermal stabilities. So, to elucidate why the degree of reversion initially increased and then decreased with increasing S/A ratios, the actual crosslink type distribution was determined for each of the NR vulcanizates, and the results are shown in the next section (Figure 4).
Citation: Rubber Chemistry and Technology 89, 3; 10.5254/rct.16.85963
effect of the s/a ratio on the proportion of the different crosslink types
Figure 4 shows the proportion of the three different types of crosslink structure in NR vulcanizates cured with each of the four different accelerators at various S/A ratios. Interestingly, it was found here (Figure 4a) that for the DCBS and MBTS accelerator cure systems, the percentage of polysulfidic linkages in the network was not proportional to the S/A ratio, even though it has been reported to be so in many publications,1–5,8 but rather the S/A ratio giving the highest polysulfidic linkages was toward the lower end of the tested S/A range (0.26 to 6.67) at 1.17. For rubber cured with TBzTD, the percentage of polysulfidic linkages increased with increasing S/A ratios, as expected from previous reports, while for CBS the observed trend was more of an inverse exponential dose-dependency, reaching the asymptote at a S/A ratio of 3.2 to 6. Moreover, for the CBS accelerator cure system, after treatment with hexane-1-thiol-amine, no further swelling was noted, suggesting that vulcanizates cured with the CBS accelerator consisted of only polysulfidic and disulfidic linkages no matter which S/A ratio was used. It is worth noting here that Kim and Lee12 also found, when using only CBS as an accelerator, the monosulfidic linkage was not observed. Figure 4b sequentially shows the proportion of the crosslink structure in the above NR vulcanizates with the extended cure time of 20 min. Both the reduction in crosslink density (as seen in Table II) and the over vulcanizing led to the change in the proportion of the crosslink structure. During the prolonged heating, the linkages with two or more sulfur ranks broke down, and some of them may reform into new crosslinks of lower sulfur rank. As seen in Figure 4b, the increment of proportion of monosulfidic linkages was clearly observed.
From the early days, the application of sulfur vulcanization to NR has always been accompanied by the use of an accelerator, since they speed up the action of sulfur on the rubber, leading to much shorter cure times, and they improve the efficiency of sulfur vulcanization by decreasing the number of sulfur atoms in the linkages. Together, these allow for commercially and technologically viable production levels. However, the results presented here suggest that there is a certain optimal S/A ratio from which the concentration of polysulfidic linkages decreases with changes either way in the S/A ratio.
The effect of the S/A ratio on the reversion behavior is evaluated here with respect to the proportion of polysulfidic linkages, by plotting the degree of reversion (%) at time 20 min above tc100 against the S/A ratio (Figure 5). It was found that the S/A ratio that gave the highest proportion of polysulfidic linkages exactly matched that which caused the highest degree of reversion (%). Moreover, from Table III, it was also observed that the S/A ratio that gave the highest proportion of polysulfidic linkages was very close to that which gave the shortest plateau region and fastest reversion rate. Therefore, the generalization that it is the high concentration of polysulfidic linkages in the network that causes a decrease in the reversion resistance is seemingly still applicable.
Citation: Rubber Chemistry and Technology 89, 3; 10.5254/rct.16.85963
effect of the s/a ratio on the vulcanizate tensile strength
The tensile properties of rubber sheets compression molded at either tc100 or tc100 + 20 min are shown in Figure 6, with data from the tc100 + 20 min sheets being used to determine the effect of reversion on the strength of the vulcanized NR. The modulus, at a certain extension, is also proportional to the total crosslink density of the vulcanizate. Thus the data derived from analysis of the comparable 100% modulus also supports that the NR vulcanizates were formulated to have a similar crosslink density (Figure 6). As reported earlier, vulcanization of rubbers by sulfur alone takes a long time to be completed, thus leading to the oxidative degradation of the rubber. Moreover, the reaction forms the crosslink with 40 to 55 sulfur atoms. Therefore, the product possesses inadequate mechanical and thermal properties. The presence of an accelerator expedites the reaction and forms the crosslinks with the lower sulfur ranks. The accelerator type and the accelerator dosage play an important role on the properties of the obtained vulcanizates. When a relatively high ratio of the sulfur concentration to the accelerator concentration was used, the obtained vulcanizate consists mainly polysulfidic linkages. Consequently, the strength of the obtained vulcanizate is supposedly high due to the ability of S–S bonds to break reversibly, thereby locally releasing high stresses that could initiate failure. From this study, it was found that the dependence of the tensile strength on the S/A ratio between each of the four tested accelerators was different. However, for all four accelerators used the highest tensile strength was not obtained when the highest S/A ratio was used. This might be because the network of vulcanizates cured with the highest S/A ratio did not have the highest proportion of polysulfidic linkages as shown in Figure 4a. Furthermore, it was found that for the range of S/A ratios studied here, the tensile strength of vulcanizates cured with CBS, MBTS, and TBzTD passed through a maximum, while that of vulcanizates cured with DCBS decreased linearly as the S/A ratio increased.
Citation: Rubber Chemistry and Technology 89, 3; 10.5254/rct.16.85963
When considering the relationship between crosslink structure and tensile strength of the vulcanizates as shown in Figure 7, it was observed for DCBS, CBS, and TBzTD cure systems that the vulcanizates with the lowest proportions of polysulfide linkages gave the lowest tensile strength. In the case of the vulcanizate cured with MBTS, the vulcanizate containing the second lowest content of the polysulfidic linkages gave the lowest tensile strength. However, the difference in the proportion of polysulfidic linkages between the vulcanizates containing the first and second lowest polysulfidic linkages was very small. The results here indicated that the amount of the polysulfidic linkages has an effect on tensile strength. Moreover, it was found that there were two cure systems (MBTS and CBS accelerators) at which the vulcanizate with the highest proportion of polysulfidic linkages gave the highest tensile strength. In the case of the vulcanizate cured with DCBS, although the vulcanizate containing the highest proportions of polysulfide linkages did not give the highest tensile strength, the one containing the third highest proportions of polysulfide linkages did. In the case of the vulcanizate cured with TBzTD, the result showed that the tensile strength of the vulcanizates passed through a maximum and decreased with the increase in the proportion of polysulfidic linkages. In addition to the formation of sulfur linkages, sulfur vulcanization is also inevitably accompanied with side reactions including cyclic sulfide, pendant sulfide, and conjugated diene formation along the main chains. The increase in the S/A ratio shifts the curing system toward the conventional system that forms more side reactions. In the case of the NR vulcanizates, the strength depends not only on the crosslink distribution but also on the main chain modification. Interruptions in the stereoregularity of NR chains lead to the reduction of strain-induced crystallization, and, as a consequence, the strength of the NR vulcanizate is reduced. In this study, for all four tested accelerators, only vulcanizates cured with TBzTD showed that the increase in the S/A ratio gave the vulcanizate with the higher proportion of polysulfide linkages. The reason that the vulcanizate containing the highest proportion of polysulfide linkages did not show the highest tensile strength might be because the vulcanizate was formed by the use of the highest S/A ratio that can also generate more main chain modification. Consequently, the vulcanizates cured at the S/A ratios of 3.2 and 6.6 gave lower strength than the ones cured at the S/A ratio of 1.91 in spite of containing the higher proportion of the polysulfide linkages. However, from Figure 7, it can be observed that the change in the tensile strength was related to the change in the crosslink distribution. With very few exceptions, the vulcanizate containing higher proportions of the polysulfide linkage gave higher tensile strength, and vice versa.
Citation: Rubber Chemistry and Technology 89, 3; 10.5254/rct.16.85963
Furthermore, when the cure time was extended 20 min from tc100, the reduction of the 100% modulus was observed for all vulcanizates (Figure 6), indicating the decreased proportion of the crosslink density and resulting in a decrease of the tensile strength. The reduction of tensile strength of the NR cured with tc100 + 20 min may also be due to the decrease of polysulfidic linkages that are labile and reversible crosslinks.
CONCLUSIONS
The effects of the S/A ratio on the reversion behavior, tensile properties, and crosslink distribution of NR vulcanizates cured with four different accelerators (DCBS, CBS, MBTS, and TBzTD) were studied. Unlike previous reports, this study revealed that the degree of reversion and tensile strength of the NR vulcanizates was not directly proportional to the S/A ratio. After evaluating the crosslinking distribution, it was found the discrepancy occurred because within the certain range of the S/A ratios studied here when using DCBS, CBS, MBTS, but not TBzTD, the amount of polysulfidic linkages of the NR vulcanizates playing an important role for those two properties was not directly proportional to the S/A ratios. In fact, it passed through a maximum and then declined with increasing the S/A ratios. Only the vulcanizates cured with TBzTD showed that the amount of polysulfidic linkages increased with increasing S/A ratios. However, for all four accelerators, the vulcanizates with the highest polysulfidic linkages gave the highest reversion. With a few exceptions, it was observed that the reduction of tensile strength concurrently occurred with the decline in the proportion of polysulfidic linkage. Therefore, the generalization that it is the high concentration of polysulfidic linkages in the network that causes a decrease in the reversion resistance but an increase in the tensile strength is seemingly still applicable.
Representative MDR rheographs of NR compounds cured with various S/A ratios at 155 °C using (a) DCBS, (b) CBS, (c) MBTS, and (d) TBzTD as the accelerator.
The degree of reversion (%) as a function of the cure time of NR compounds cured with various S/A ratios at 155 °C using (a) DCBS, (b) CBS, (c) MBTS, and (d) TBzTD as the accelerator.
The degree of reversion (%) at 20 min above tc100 of NR compounds cured with various S/A ratios at 155 °C using (a) DCBS, (b) CBS, (c) MBTS, and (d) TBzTD as the accelerator.
Relative proportion of the three different types of crosslinks in NR compounds cured (a) at tc100, (b) tc100 + 20 min with various S/A ratios at 155 °C.
The degree of reversion (%) at tc100 + 20 min, and the corresponding percentage of polysulfidic linkages, of NR compounds cured with various S/A ratios at 155 °C using (a) DCBS, (b) CBS, (c) MBTS, and (d) TBzTD as the accelerator.
Tensile properties of NR compounds cured with various S/A ratios at 155 °C using (a) DCBS, (b) CBS, (c) MBTS, and (d) TBzTD as the accelerator. The filled symbols indicate tensile properties of vulcanizates cured at tc100, while open symbols represent tensile properties of vulcanizates cured at tc100 + 20 min.
The tensile strength and the corresponding percentage of polysulfidic linkages of NR compounds cured with various S/A ratios at 155 °C using (a) DCBS, (b) CBS, (c) MBTS, and (d) TBzTD as the accelerator.
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