Star-branched butyl: compounding and vulcanization latitude.

Star-branched butyl: Compounding and vulcanization latitude

Butyl rubber (IIR) has been an article of commerce for over 50 years. Its halo derivative, introduced 30 years ago, has extended the product's life cycle and made significant inroads in the tire industry. The inert nature of the butyl molecule and its halogenated derivatives resist degradation on mixing and processing, so butyl has resorted primarily to Mooney and molecular weight control to achieve certain processing properties. Many times these properties were achieved at the detriment of other properties, e.g., increase in green strength with molecular weight results in longer relaxation times and vice versa.

In the spring of 1989, a new butyl polymer concept was introduced that altered the molecular architecture to achieve a unique and different response to processing for butyl and halobutyl rubbers[refs. 1 and 2]. The new polymers exhibited processing characteristics different from the parent molecule with regard to green strength and stress relaxation balance. However, the altered molecular structure also resulted in some variations in compounding and vulcanization response compared to regular IIR rubbers. This is a consequence of both very large star-like structures and the nature of the polymeric branching agent used to construct these stars. This article will detail compounding and, to an extent, vulcanization latitude of the star-branched butyls and halobutyls. Molecular parameter relationships will be described relative to compounding and vulcanization response and their impact on properties important to the rubber industry.

Experimental

Two groups of polymers were evaluated. In the first group, 50 Mooney conventional butyl (IIR 50) was compared with equivalent viscosity star-branched butyl (SBB 50) and lower viscosity (40 Mooney) star-branched butyl (SBB 40). In the second group, 38 Mooney brominated star-branched butyl (BSBB 38) was compared with low viscosity (32 Mooney) bromobutyl (BIIR 32) and high viscosity (46 Mooney) bromobutyl (BIIR 46). Process oil and carbon black effects were evaluated within each polymer utilizing a [2.sup.2] factorial experimental design with replicate center points. Process oil was varied from 10 to 30 phr (denoted as [X.sub.1]) and carbon black was varied from 40 to 80 phr (denoted as [X.sub.2]). The exact formulations and the tests conducted are available from the authors.

The [2.sup.2] factorial design supports a first order model with interactions as shown below.

Y = [B.sub.0] + [B.sub.1] [X.sub.1] + [B.sub.2] [X.sub.2] + [B.sub.12] [X.sub.1] [X.sub.2]

The regression fitted coefficients for the various tests are also available upon request. However, the experimental design used does not support a second order model. As one might expect, not all test parameters will fit a first order equation. A statistical test for curvature was performed but in some cases there was curvature in the response that was not explained by the model. However, the response was used if more than 90% of the variability was explained by the model. Additional data points would be necessary to fully define the response equations to include curvature in the model. The data averages for each parameter for the different polymers were then statistically compared (t-test at the 95% confidence level). Contour plots were also prepared for each valid response and several are included in this article to emphasize points where appropriate.

Results

Comparison of conventional butyl with high viscosity star-branched butyl A complete comparison of all of the parameters studied is shown in table 1. Only those tests showing substantial differences are discussed below.

Compound viscosity - On the average the compound viscosity of star-branched butyl is about 6% lower than the conventional butyl rubber (46 vs. 49). The contour plots for both polymers generally follow the same pattern with the difference slightly greater at low black/high oil levels and slightly less at the high black/low oil concentrations.

Green strength - The green strength of star-branched butyl is about 25% greater than that of conventional butyl. As shown in figure 2, the difference decreases slightly at the higher black levels. The green strength of the SBB 50 compound is relatively insensitive to black increases at a given oil level, while the green strength of the conventional butyl increases with increased black loading. It thus appears that the branching has a much greater effect on green strength than carbon black.

Relaxation - At any given black/oil concentration star-branched butyl exhibits approximately a 40% faster relaxation than regular butyl. The pattern of response for black and oil is similar for both polymers.

Extrusion rate - On the average, the extrusion rate of SBB 50 is about 30% faster than conventional butyl. This difference tends to decrease slightly at the higher oil levels (shown in figure 3) as the star-branched butyl is less sensitive to oil concentration than is conventional butyl.

Die swell - On the average, the SBB 50 compound exhibits about 40% less die swell relative to the IIR 50 compound. For both polymers, increased carbon black loading decreases die swell substantially, while oil concentration has only a minor effect.

Cure rate - Neither polymer exhibits a significant cure sensitivity to black/oil level. On the average this version of the star-branched butyl is about 20% faster than conventional butyl (9.4 vs. 11.5 minutes, t'90 [at] 160 [degrees] C) which is related to the contribution of the star fraction in the star-branched polymer.

Modulus - On the average, modulus is about 25% higher for SBB 50 (6.1 vs. 4.9 MPa). As shown in the contour plot in figure 1, this percentage is constant over the black/oil range studied.

Volume swell studies in cyclohexane on unfilled polymers indicates similar swell characteristics for both polymers. The increased modulus, therefore, is probably related to the star structure in the star-branched polymer.

Comparison of lower viscosity SBB with higher viscosity SBB In general, the features observed with the lower viscosity star-branched butyl are typical of what is generally seen comparing lower viscosity polymers with higher viscosity counterparts. The comparison between SBB 40 and SBB 50 is shown in table 2.

Comparison of brominated SBB with high viscosity and low viscosity bromobutyl A complete summary of all of the parameters studied comparing the brominated star-branched butyl with low and high viscosity bromobutyls is shown in table 3. Again, only those parameters showing significant differences are discussed below.

Compound viscosity - On the average, the compound viscosity for the star-branched bromobutyl compound is equivalent to the lower viscosity bromobutyl even though polymer viscosity is about 6 points higher. The pattern of response to black and oil is also similar for both materials as is observed with the non-brominated polymers. In addition, the polymer viscosity for BSBB 38 is about 8 points lower than the high viscosity bromobutyl, but 10 points lower in compound viscosity.

Green strength - For equivalent compound viscosity the green strength of the BSBB 38 compound is about 20% greater than the compound with the low Mooney polymer. In contrast, the BSBB 38 compound had, on the average, equivalent green strength to the high Mooney bromobutyl even though compound viscosity is 10 points lower. As shown in figures 4 and 5 the BSBB 38, like its non-brominated cousin, is not sensitive to black loading. In addition, although it is possible to increase the green strength of the conventional bromobutyl with carbon black, one cannot attain the green strength of brominated star-branched butyl with the low viscosity bromobutyl within practical black levels (figure 4).

Relaxation - On the average, BSBB 38 relaxes about 20% faster than BIIR 32. As seen in figure 6, as black increases and oil decreases the differences become less pronounced and the contours tend to converge. At the higher oil/lower black level the contours tend to diverge. Relative to BIIR 46 (figure 7), the BSBB 38 is about 50% faster in relaxation with the same pattern as the lower viscosity polymer.

Extrusion rate - On the average, BSBB 38 has an equivalent extrusion rate to that of the low viscosity bromobutyl and is about 20% faster than the high viscosity polymer.

Die swell - As anticipated, the die swell for BIIR 32 is about 20% lower than BIIR 46. The BSBB, however, is about 50% lower than the BIIR 46.

Cure rate - This version of the brominated SBB is slightly faster curing than either of the conventional bromobutyls evaluated (13.5 vs. 15 minutes, t'90 [at] 160 [degrees] C).

Modulus - The response of the brominated star-branched butyl relative to modulus is similar to the non-brominated material. On the average, modulus is about 15-20% higher for BSBB 38. In addition, the percentage difference is relatively constant over the black/oil range studied.

Discussion of results

The differences shown in the response to processing (green strength, stress relaxation, die swell and extrusion) and vulcanization (modulus) of the star-branched butyls resides in the bimodal distribution of the molecular constituents of SBB compared to conventional butyl.

The higher green strength and modulus observed with the star-branched polymer is related to the star-like structure in the polymer. We postulate that the large overlapping chains of the star structure contribute to its green strength while the creation of tight networks at the junction point of the stars contributes to its higher cured modulus.

The linear fraction of the distribution has a correspondingly lower peak molecular weight than the conventional butyl to compensate for the high molecular weight star fraction in order to obtain the desired polymer Mooney viscosity. The linear fraction is also, by far, the majority of the chains in the polymer and is the phase that accounts for most of the stress relaxation and die swell characteristics we observe.

Compound viscosity is a strong function of the wide molecular weight distribution and of the compound microstructure. Adding the star fraction and lowering the molecular weight of the linear fraction increases shear thinning, which is manifest in a lowering of the compound Mooney. Also, the low shear viscosity was found to be greater for the SBB 50 compound and is reflected in its higher green strength. The increase in extrusion rates that we also see for SBB is a clear consequence of the higher shear thinning for this material.

Conclusion

The results of this study indicate that the star-branched butyls generally have significant benefits in green strength, stress relaxation and extrusion characteristics over a wide range of black and oil levels.

The only exception was the higher green strength of the high viscosity bromobutyl (BIIR 46) relative to the intermediate viscosity BSBB 38 at high black levels. This occurred because, surprisingly, the green strength of the star-branched polymers has a relative flat response to carbon black loading.

For the most part, however, the star branched polymers exhibit the green strength benefits normally attributed to high viscosity polymers while maintaining the fast stress relaxation characteristics associated with low viscosity polymers.

Modulus is also higher for the star-branched polymers over the black and oil concentrations studied and, as indicated earlier, is probably related to the creation of dense networks at the star junctions in SBB. For most of the parameters studied, the pattern of the response to black and oil (contours) of the star-branched polymers is similar to those obtained with conventional butyl and bromobutyl, albeit at different levels. [Figures 1 to 7 Omitted] [Tabular data 1 to 3 Omitted]

References

[1]H.C. Wang, K.W. Powers, J.V. Fusco, presented at a meeting of the Rubber Division, American Chemical Society, Mexico City, Mexico, May 9-12, 1989. [2]I. Duvdevani, L. Gursky, I.J. Gardner, presented at a meeting of the Rubber Division, American Chemical Society, Mexico City, Mexico, May 9-12, 1989.

L. Gursky, J.V. Fusco, I. Duvdevani and T. Takeda, Exxon Chemical, Polymers Group