Improved rubber properties created with sulfenimide accelerators.

The rubber industry is looking for alternative systems to replace many commonly used accelerators based on secondary amines because of the related potential for nitrosamine generation. Several options (refs. 1-3) are available to replace secondary amine based sulfenamides including:

* N-cyclohexyl--2-benzothiazyl-sulfenamide (CBS) or

* N-tert-butyl-2-benzothiazyl-sulfenamide (TBBS), or

* CBS or TBBS with N-cyclohexylthio-phthalimide CTP).

The last approach can provide comparable scorch safety and in all three approaches the cure rate is faster and modulus development higher in most compounds.

Another alternative is to use N,N'-dicyclohexyl-2-benzothiazyl-sulfenamide (DCBS). Although containing a secondary amine, accelerators such as DCBS have been considered as a safe accelerator and can be used accordingly to give longer scorch safety, slower cure rates, and lower modulus development than the primary amine based sulfenamides in natural rubber compounds. There is also an increasing need to improve reversion resistance in natural rubber compounds used for heavy duty applications.

The introduction of N-tert-butyl-2-benzothiazyl-sulfenimide (TBSI), in 1991, provided a new alternative to obtain long scorch safety combined with a moderately slow cure rate and good physical properties along with improved reversion resistance.

Experimental details

Standard ASTM sample preparation and test methods were used in all of the comparisons. Rubber masterbatches were prepared on a laboratory size "00" internal mixer. The sulfur and accelerator were added to the masterbatch on a heated (50[degrees]C), 12 inch lab mill.

Test Methods

Processing characteristics were measured on the Monsanto Mooney Viscometer (ASTM D-1646-89). Curing properties were determined on the Monsanto Rheometer model MDR-2000 (ASTM D-5289-93). Test specimens were cured to T90 at temperatures specified in the data. Stress-strain properties were measured according to ASTM-D412-87. Goodrich Heat Buildup data were obtained using the Goodrich Flexometer (ASIM D-623-88).

Improved reversion resistance and reduced heat-buildup

Maintaining properties and performance of rubber products on extended cure (e.g. thick cross-section articles) is a major technical need of the rubber industry. Traditionally, the use of more efficient vulcanization systems has been one method to improve reversion resistance of rubber compounds. Through the reduction of sulfur content in these efficient vulcanization systems, more heat stable cross links are formed, resulting in an improved heat and reversion resistant vulcanizate. The introduction of N-t-butyl-2-benzothiazyl sulfenimide (TBSI) accelerator provides another alternate to achieve reversion resistant compounds. Figures 1-3 compare accelerator TBSI to several common sulfenamide accelerators in a 100% NR compound with the formula used in table 1.

Table 1 - natural rubber formula

Masterbatch                     PHR
SMR CV-6                        100.0
N-330 carbon black               50.0
Napthenic oil                     5.0
Zinc Oxide                        5.0
Stearic acid                      2.0

Final batch
Master batch                    162.0
Sulfur               0.5-3.5 variable
Accelerator TBSI     0.5-3.5 variable

Rheometer cure characteristics

The new sulfenimide and all the sulfenamide accelerator types tested (ref. 4) in NR are compared at equal phr in figure 1. The level of sulfur and TBSI used is commonly known as a semi-efficient vulcanization cure system that is sometimes employed for thick cross section compounds and where reversion resistance is important.

Reversion resistance - rheometer method

A measure of reversion resistance that many people use is the rheometer torque drop on extended cure. In figures 2 and 3 the data are normalized where the time of maximum cure equals zero minutes, and the maximum torque equals 100. This gives a standard basis of comparison between the various curves that have different maximum torques and different times to maximum torque. The TBSI curve shows the least drop in torque, seen in figure 2. This means that TBSI has more reversion resistance as compared to the other accelerators. The mixtures of TBSI+TBBS in figure 3 uniformly decrease in reversion resistance as the level of TBBS increases .

Figure 4 gives the following additional reversion resistant properties of TBSI in a NR/BR (75/25) blend. Laboratory data are given in table 3.

* Superior reversion resistance with accelerator TBSI as compared to accelerator TBBS (stocks 3 vs. 1).

* Improved reversion resistant systems are shown with blends of TBSI/TBBS (stock 4) and semi-EV system (stocks 2 and 5).

The heat build-up of rubber compounds and the effect of performance is a major concern for the rubber industry. Earlier work (ref. 5) has shown that heat build-up can be reduced by decreasing carbon black and oil loadings and losses in physical properties can be compensated by modifying the vulcanization system.

Recent work at our lab indicates that sulfenimide accelerator TBSI can provide rubber compounds with reduced heat build-up properties. Figures 5-7 show the following heat buildup properties of TBSI. Laboratory data are given in tables 2-4.

* Reduced Goodrich heat build-up and permanent set in TBSI accelerated compounds with conventional sulfur vulcanization systems are shown in figure 5 (stocks 1 vs. 2), figure 6 (stocks 1 vs. 3 and 4) and figure 7 (stocks 1 vs. 3, 4 and 5).

* Similar Goodrich flexometer data are shown between a semi-E.V. cure based on TBSI and the two following systems:

* A semi-E.V. based on silane coupling agent Si-69 - figure 5 (stocks 3 vs. 4).

* A semi-E.V. system based on sulfur donor DTDM - figure 6 (stocks 2 vs. 5).

[TABULAR DATA 2-4 OMITTED]

Mechanistic considerations of sulfur vulcanization accelerated by TBSI

The improved physical properties observed in vulcanizates accelerated by TBSI suggest significant 2differences from conventional sulfenamide cure chemistry. TBSI differs from TBBS in steric effects and mercaptobenzothiazole and amine stoichiometric differences. These structural and reactivity differences result in changes in the kinetics of some reactions occurring during vulcanization. Several changes in the mechanistic details are expected. The extended scorch delay observed in TBSI has been attributed to its inherent stability. TBSI exhibits excellent resistance towards hydrolysis. The extended scorch safety and hydrolysis stability have been attributed to the steric hindrance provided by the two benzothiazole rings and the t-butyl group. This steric hindrance has been suggested to be the cause of the slow reaction of the TBSI molecule with MBT, an important step in the scorch delay sequence of reactions. In fact it has been shown that TBSI reacts slower with MBT than does its counterpart TBBS. The scorch delay behavior of TBSI is only the first distinction from its analogue TBBS. In addition to the longer scorch delay, slower cure rates and slower reversion rates provide vulcanizates having lower heat build-up.

Hann et al, pointed out that many of the intermediates and kinetic profiles identifiable by high performance liquid chromatography formed during vulcanization accelerated by TBSI or TBBS are similar. In fact it was shown that TBBS is produced from TBSI during the vulcanization reaction. The fate of the TBBS formed in situ is the same TBBS in accelerated sulfur vulcanization. The major difference is that MBT reacts with the TBBS faster than with the TBSI. Hence, TBBS concentrations reach a low level steady state' concentration early in the vulcanization reaction.

However, even though the sequence of reactions is similar for TBBS and TBSI, the kinetic efficiency by which these reactions occur are significantly different. The related differences in cure kinetics and reversion reaction rates must be attributable to the different benzothiazole to amine ratios in the sulfenimide and sulfenamide accelerators. The sulfenamide accelerators likely form zinc accelerator complexes having the proposed structure shown in figure 8 (ref. 8). Indeed semi-empirical quantum mechanical calculations have shown that scorch delay, cure times (t90 - t2) and maximum rates of vulcanization correlate well with the observed zinc to nitrogen bond distances (ref. 9). The bond lengths observed in the complex determine the extent of electron donation the zinc ion receives from the amine molecules.

On the other hand, the intermediate formed in vulcanization catalyzed by TBSI would not have the same proposed structure due to the stoichiometric differences in amine and benzothiazole. In fact one would expect only half the level of amine to be present in the complex shown in figure 8. While zinc is predominantly tetravalent (ref. 10), the most probable complex formed would be similar to that shown in figure 9. In figure 9, L is an amine molecule derived from the sulfenimide molecule and L' may be hydroxide or carboxylate ion. Semi-empirical molecular orbital calculations suggest L' = carboxylate ion. Characteristics of the complex using L' = carboxylate ion can be correlated to vulcanization characteristics (ref. 11). The different ligands in the accelerator zinc complex are most likely to cause the differences observed in the rates of crosslinking and the rates of reversion observed. The slower crosslinking rates and rates of reversion would be expected to produce a different distribution of network sulfur rank when compared to conventional sulfenamide accelerators. In fact, it has been shown that TBSI is more efficient at forming more mono and disulfidic crosslinks than the corresponding sulfenamide accelerators (ref. 12). The combination of a stable network (more mono- and disulfidic crosslinks) coupled with the slower rates of reaction (both crosslinking and reversion) provide a more heat and fatigue resistant vulcanizate. The low heat build up and good compression set properties are also observed in the flexometer results for TBSI accelerated vulcanizates.

Design of experiment

The statistically designed experiment used natural rubber and is based on a central composite type of design as described in the literature (ref. 9). This gives a wide basis of comparison for the accelerator and sulfur ratios that can add to understanding in a process for determination of an optimized system. Contour plots are generated to compare trends in: physical properties, aging properties, reversion resistance and Goodrich heat buildup properties over a wide range of accelerator and sulfur concentrations.

The formula base for the design of experiment is given in table 1.

Results and discussions

Physical properties:

Tensile or ultimate strength is one physical measure of a rubber compound,s properties that may change during extended heavy duty service or in an overcure situation as on the outer layers of the tire during the original vulcanization. Figures 10 through 12 compare the original tensile to five and ten times the optimum cure as defined by the T90 (90% of maximum) based on the Monsanto moving die rheometer (MDR-2000) at 150[degrees]C. In figures 10A and 10B the tensile response surface for the central composite - design of experiment is shown. Figure 10B is included as the TBSI - sulfur axis aligns with the following figures for easy comparisons. Figure 11 shows the tensile at ten times overcure as compared to the T90 optimum cure in figure 10B. Figure 11 shows what may occur on the outer layer of the tire which is usually overcured during the vulcanization process. At optimum T90 cures, levels of 2 phr TBSI and 3.5 phr sulfur create the highest level of tensile at T90. At overcure conditions higher levels of both sulfur and TBSI give a decline in tensile. Figure I 1 shows that a balanced level of 2 phr of each TBSI and sulfur may give reasonable tensile even with moderate overcure. In the actual use of these types of compounds the addition of compound modifiers such as antidegradants would be expected to shift the retained tensile higher and permit the use of slightly higher levels of TBSI and sulfur if required to achieve the desired rubber strength. The reason behind this is that normal aging is a combination of both thermal and oxidative effects.

Another measure of retention of physical properties shown to give a good indication of desirable aging resistance is the function of tensile times elongation at break (ultimate stress x strain). This was identified as the TE factor ref 14).

Figure 12 shows these results for the TE at the T90-150[degrees]C cure. Figure 13 shows the percent change in the TE Factor at 10 times T90 overcure. The result is the highest tensile for the T90 cure at 150[degrees]C is in the 3 phr sulfur and 1.50 to 1.75 phr TBSI range. With the 10 times overcure the higher the levels of both sulfur and accelerator the greater the drop in retained tensile times elongation (TE) factor.

Modulus is a measure of how the compound will perform in service. As the levels of both sulfur and of accelerator increase, the change in both 100% and 200% modulus also changes more drastically on overcure. is expected as there are more and longer sulfur chains in the polymer matrix. Figures detailing this are available from the authors.

Goodrich heat buildup

The Goodrich heat buildup test is a widely accepted way of comparing various compounds for service in thick cross section rubber parts. The level of sulfur and TBSI required to give the lowest heat buildup is approximately at 2 phr for both ingredients.

Reversion resistance

Figures 2 and 3 in the beginning of this study showed that TBSI and TBBS both had the lowest reversion rate of any of the sulfenamide series as a percent of rheometer torque drop over a period of time for the maximum torque level. The mixtures of sulfur and TBSI can give various results for each particular mixture. A study of the Newton-meter (dNm) drop from the time of maximum torque showed the longer the time the slower the rate of reversion. At 150[degrees]C cure the point of slowest reversion is the highest level or TBSI with the lowest sulfur. The point of fastest reversion corresponds with the area of a conventional cure system of 2.25-3.25 phr of sulfur and 0.5 - 1.0 phr of accelerator.

Conclusions

Sulfenimide accelerators give reversion resistance and heat build-up in natural rabber and NR blends. This study shows that the sulfenimide accelerator, N-t-butyl-2-benzothiazyl sulfenimide (TBSI), also provides the long processing safety and slow cure rate that are characteristic of secondary amine based sulfenamide accelerators. There may be continued concerns over potential health hazards from stable N-nitrosamines which are produced by secondary amines in some sulfenamides. The sulfenimide accelerator such as TBSI with a primary amine is a viable replacement candidate.

Through statistically design experiments, the effects of a wide range of sulfenimide accelerators and sulfur concentrations were studied. This result shows the flexibility compounder may have to select the vulcanization system based on sulfenimide which best fits the cost and performance requirements.

[Figure 1 to 11 ILLUSTRATION OMITTED]

References

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A.Y. Coran Rubber Chem. and Technol. 38, 1 (1965) Milligan, B.J. Chem. Soc., Part I, 34, (1966). (9.) F. Ignatz-Hoover and G. Kuhls, Rubber Chemistry and Technology, paper No. 3, ACS Rubber Division Chicago, Spring 1994. (10.) McCleverty, Jon A., in "Sulfur, its significance for chemistry, for the geo-, bio-, and cosmo-sphere " A. Muller and B. Krebs (Eds. , Studies in Inorganic Chemistry, Yol. 5, p. 31-329 (1984). (11.) Monsanto Company (F. Ignatz-Hoover) unpublished results. (12.) Monsanto Company (Shirley Lee) unpublished results. (13.) Box, Hunter & Hunter, J. Wiley & Sons, NY, NY 1978pp. 510-539. (14.) Lederer, D.A. and Helt, W.F., III, "Comparison of the effectiveness and durability of amine-based antidegradants, " paper 80 presented at Houston 1983 ACS Rubber Division Meeting.