Improved traction with BIMS.

Brominated isobutylene-co-para-methylstyrene (BIMS) is the elastomer formed by brominating poly(isobutylene-copara-methylstyrene). The para-methylstyrene monomer can be present between 2-8 moles per 100 moles of isobutylene monomer, with the bromine level ranging from approximately 20-50% of the para-methylstyrene content. BIMS, commercially known as Exxpro elastomer, has been successfully evaluated in a variety of tire compounds (ref. 1) including inner liner (refs. 2-4), black sidewall (refs. 5-11) and treads (refs. 12-16).

The tread is the wear-resistant component of a tire that comes in contact with the road. It is designed for abrasion resistance, traction, speed, stability and to protect the casing. The tread rubber is compounded for wear, traction, low rolling resistance and durability (ref. 17). For passenger tires, a blend of SBR, BR and/or NR may be used (ref. 18).

Mroczkowski (ref. 12) studied blends of BIMS/SBR/BR. Increased tangent delta values at low temperatures (-30 [degrees] C to 10 [degree] C) and decreased tangent delta values at higher temperatures ([is greater than] 30 [degrees] C) were obtained with comparable lab abrasion resistance compared to a carbon black-filled NR/BR/SBR tire tread. Zanzig and coworkers (ref. 13) evaluated BIMS blends with IBR, BR and NR with and without SBR in silane-coupled silica-filled compounds, and found increased tangent delta values at 0 [degrees] C when using BIMS. Rogers (ref. 14) reported that BIMS and silane-coupled silica increased tangent delta @ 0 [degrees] C and decreased tangent delta @ 60 [degrees] C in lab tests, with only slight reductions in tire treadwear for an eSBR/BR compound. Hojo (ref. 15) used a hydrazide compound with BIMS to lower the heat generation and improve the wet gripping of a carbon black and silane-coupled silica filled NR compound.

Previously, we (ref. 16) reported that BIMS use in an all-season BR/sSBR tread formulation improved lab dynamic properties predictive of both wet traction and rolling resistance (ref. 19). Our present investigation studies the effects of filler type, silica/black ratio and silane coupling agent level on compound performance. Particular emphasis is placed on understanding the chemistry that occurs when the BIMS elastomer is used, and improving compound abrasion resistance and dynamic properties.

Experimental

The BIMS elastomer used in this study is the Exxpro 3745 specialty elastomer that contains 7.5 weight-% para-methyl-styrene co-monomer and has 1.2 mole-% brominated para-methylstyrene. It was evaluated in the model all-season passenger tire tread compound whose formulation is shown in table 1.

Table 1 - BIMS tire tread of formulation

Mixer #1                                  Phr
   0 sec   BIMS, Exxon Exxpro 3745         20
           BR, Goodyear Budene 1207        25
           sSBR, JSR SL574                 55
      30   Silica, Huber Zeopol 8745       30
     120   Carbon black, N234              22.5
     180   Processing oil, Sundex 8125     20
           Coupling agent, Degussa X50S    4.8
           Carbon black N234               7.5
     390   Dump @ 150-160 [degrees] C

Mixer #2
       0   Masterbatch #1
      30   Antiozonant, Santoflex 13       1.5
           Antioxidant, Agerite Resin D    1
           Zinc oxide, Kadox 930C          2
     240   Dump @ 130 [degrees] C

Mixer #3
       0   Masterbatch #2
      30   Stearic acid                    1
           Sulfur                          1.2
           Accelerator, CBS                1.75
           Accelerator, DPG                1
     180   Dump @ 125 [degrees] C

Compounds were mixed in three stages using an internal mixer with the ingredients added in the order shown in table 1. Cure properties were measured using a MDR 2000 at a temperature of 160 [degrees] C and 0.5 degree arc. Test specimens were cured at 160 [degrees] C for a time corresponding to T90 + appropriate mold lag. When possible, standard ASTM tests were used to determine the cured compound physical properties. Stress/strain properties (tensile strength, elongation at break, modulus values, energy to break) were measured at room temperature using an Instron 4202. Shore A hardness was measured at room temperature by using a Zwick Duromatic. Abrasion loss was determined at room temperature by weight difference by using an APH-40 abrasion tester with rotating sample holder (5 N counter balance) and rotating dram. Dynamic properties (G*, G', G" and tangent delta) were determined using a MTS 831 mechanical spectrometer for pure shear specimens (double lap shear geometry) at temperatures ranging from -30 [degrees] C to 60 [degrees] C using a 1 Hz frequency at 0.1, 2 and 10% strains.

Compound cure, physical and dynamic property data were analyzed using SAS Institute JMP software.

Results and discussion

Cure chemistry

A statistical design of silane (5.4-7.2 phr), sulfur (1.0-1.3 phr) and sulfenamide accelerator (1.0-1.4 phr) levels was studied in a BIMS/NR/sSBR compound which was generated upon substituting BIMS for an equal amount of sSBR. BIMS is a saturated-backbone hydrocarbon elastomer and cannot sulfur vulcanize. Instead, it can crosslink via a Friedels-Craft alkylation reaction involving two brominated-methylstyryl groups catalyzed by zinc oxide/stearic acid (ref. 20). Experiments in a 50 phr silica-filled, 100 phr BIMS compound showed that adding 0.25 phr of sulfur did not affect cure or physical properties. Thus, it is necessary to reduce both the amounts of sulfur and accelerator(s) used in the initial compound being modified in order to avoid potential over-curing of the diene elastomer domains.

Results of the statistically designed study showed that linearly increasing elongation to break and linearly decreasing 300% modulus values were obtained upon reducing the levels of sulfur, of sulfenamide accelerator and/or of silane coupling agent. Lab abrasion index values were slightly improved, but did not show a correlation. Figure 1 is a plot of indexed compound properties relative to the initial NR/sSBR compound, with property improvements shown as positive values relative to the control, which is assigned a value of 100. The G" @ 0 [degrees] C values, lab predictor of wet traction (ref. 19), and G* @ 60 [degrees] C values, lab predictor of cornering coefficient (ref. 19), are significantly increased using BIMS (figure 2), in agreement with previous results (ref. 16). These dynamic properties are not significantly affected by the statistical reductions in silane, sulfur and accelerator levels. This result is to be expected since the BIMS elastomer has a saturated-backbone and does not crosslink by sulfur vulcanization nor can Si69-treated silica couple to the BIMS backbone. Figure 2 shows that tangent delta @ 60 [degrees] C (rolling resistance predictor) is increased when the level of silane coupling agent used is reduced.

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Cure studies

For our model tread compound studies, curative levels for the BIMS/BR/sSBR model all-season tread compound whose formula is shown in table 1 were established using cure, physical and dynamic property data obtained from statistically designed studies. Five materials were studied in a series of rotatable central composite designs including zinc oxide, stearic acid, sulfur, and DPG and sulfenamide accelerators. Levels were selected in order to afford a [T.sub.90] cure time at 160 [degrees] C of less than ten minutes with acceptable scorch safety and no reversion, while also optimizing DIN abrasion resistance index and G" @ 0 [degrees] C. Figure 3 is the cure curve of compound 1, which was mixed according to procedures outlined in table 1. Compound 1 shows improved dynamic properties compared to compound 2, which has the identical formulation except that it does not contain 20 phr of BIMS, instead having 75 phr of sSBR. Wet traction properties of compound 1 are potentially improved based on increased lab dynamic values of G" @ -30 [degrees] C and @ 0 [degrees] C, and tangent delta @ -30 [degrees] C and @ 0 [degrees] C. Rolling resistance is also potentially improved based upon a reduced tangent delta @ 60 [degrees] C value. However, elongation at break, tensile strength, energy to break and abrasion resistance index are notably decreased, and modulus values at higher strains are increased when using BIMS (table 2).

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Table 2 - characteristics of BIMS tread
compound (ref. 16)

Compound                       1(*)    2(#)

[M.sub.L], dN.m                2.87    2.36
[M.sub.H], dN.m               16.43   16.81
[M.sub.H]-[M.sub.L], dN.m     13.55   14.44
[T.sub.s]2, min.               2.26    1.45
[T.sub.25], min.               2.57    1.75
[T.sub.50], min.               3.05    2.31
[T.sub.75], min.               3.72    3.54
[T.sub.90], min.               5.07    5.99
Peak rate                      7.60    7.00

Shore A hardness                58      58
Elongation at break, %        376     478
Tensile strength, MPa         18.7    21.7
20% Modulus, MPa               0.94    0.92
100% Modulus, MPa              2.64    2.19
300% Modulus, MPa             13.97   11.55
Energy to break, J             9.76   15.2
DIN abrasion index              138     161

G"@-30 [degrees] C, MPa       2.582   2.188
G*@-30 [degrees] C, MPa       6.349   6.321
Tan delta @ -30 [degrees] C   0.445   0.362
G"@-0 [degrees] C, MPa        0.744   0.723
G*@-0 [degrees] C, MPa        3.257   3.337
Tan delta @ 0 [degrees] C     0.235   0.222
G"@30 [degrees] C, MPa        0.339   0.422
G*@30 [degrees] C, MPa        2.399   2.393
Tan delta @ 30 [degrees] C    0.142   0.179
G"@60 [degrees] C, MPa        0.235   0.308
G*@60 [degrees] C, MPa        1.963   1.963
Tan delta @ 60 [degrees] C    0.121   0.159

(*) Compound 1 formula shown in table 1

(#) Compound 2 contains 75 phr sSBR and 25 phr BR

BIMS/coupling agent study

A statistical design of BIMS (10-20 phr), Si69 silane coupling agent (5-8% of silica) and DCBS accelerator (1.76-2.04 phr) levels was undertaken to establish the value of each ingredient in the model all-season tread. Again, since the BIMS elastomer is a saturated backbone hydrocarbon, it is unable to react with a silane-treated (hydrophobated) silica via the sulfur coupling reaction, hence the level of silane can potentially be reduced when using the BIMS elastomer.

Statistically significant observations were that using higher levels of sulfenamide accelerator increased the abrasion resistance index. Use of higher silane coupling agent levels afforded the expected decreased rolling resistance based on lower tangent delta values @ 60 [degrees] C, but decreased wet traction based on lower G" and tangent delta @ 0 [degrees] C values.

Use of the BIMS elastomer in a low silane compound:

* Increases wet traction based on increased values of G" @ 0 [degrees] C and tangent delta @ 0 [degrees] C;

* reduces rolling resistance (tangent delta @ 60 [degrees] C); and

* increases cornering coefficient based on increased G* @ 60 [degrees] C values.

Figures 4-6 are respective surface contour plots of wet traction, rolling resistance and cornering coefficient based on the statistical analysis of the lab dynamic data. As also can be seen in figures 4-6 for the 20 phr BIMS level, increasing use of the silane coupling agent from 5% to 8% of the silica level does not result in any further reductions in tangent delta @ 60 [degrees] C, or increases in G" @ 0 [degrees] C. This result may also be expected since the saturated-backbone BIMS elastomer cannot couple to Si69-treated silica. There were no statistically significant BIMS-silane coupling agent interaction terms identified.

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Carbon black/silica study

The effect of filler type, N234 carbon black versus silane-coupled highly-dispersible precipitated silica, and the carbon black/silica ratio were examined in the model all-season tread compound with BIMS used at 0 phr and 20 phr. At 60 phr total filler, N234 black was reduced from 60 phr to 0 phr in 15 phr increments by substituting with precipitated silica. As the amount of silica in the tread was increased, the amounts of silane coupling agent (8% of silica) and DPG accelerator (0.5 phr/15 phr silica) were increased proportionally. In some cases 6.5% silane was also examined.

For all ratios of carbon black/silica, BIMS compounds always afforded higher G" values @ -30 [degrees] C and 0 [degrees] C, higher tangent delta values @ -30 [degrees] C and @ 0 [degrees] C, and lower tangent delta values @ 60 [degrees] C. These results indicate potentially improved winter and wet traction properties without a rolling resistance trade-off (figures 7-9). However, for all BIMS compounds, abrasion resistance was decreased (figure 10).

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For 75 phr silica-filled compounds modeled after an all-season, high-performance passenger tire tread (ref. 21), use of 20 phr BIMS as a direct replacement for 20 phr of sSBR broadened the tangent delta curve (DMTA) (figure 11). The result is increased tangent delta values in the -35 [degrees] C to +20 [degrees] C temperature range, with decreased values in the +40 [degrees] C to 100 [degrees] C temperature range, again indicating the potential for improved winter and wet traction and reduced rolling resistance.

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Summary

A model all-season tread formulation containing brominated isobutylene-co-para-methylstyrene blended with BR and sSBR (20/25/55) affords improved lab dynamic properties compared to the BR/sSBR (25/75) control. The increased values of G" @ -30 [degrees] C, G" @ 0 [degrees] C, tangent delta @ -30 [degrees] C and tangent delta @ 0 [degrees] C afford lab evidence indicating the potential improvement in tire winter and wet traction properties using the BIMS elastomer. In addition, the reduced tangent delta @ 60 [degrees] C indicates a potentially lower rolling resistance using BIMS. However, abrasion resistance is also reduced. These improved traction and reduced rolling resistance properties are obtained for all carbon black/coupled-silica ratios studied at the 60 phr total filler level. Similar traction and rolling resistance improvements were obtained for a 75 phr silica-filled compound modeling an all-season, high-performance passenger tire tread.

Since the BIMS elastomer has a totally saturated hydrocarbon backbone, it does not crosslink by sulfur vulcanization. Zinc oxide/stearic acid catalyze the crosslinking of BIMS via a Friedels-Craft alkylation reaction. As a result, it is necessary to reduce the sulfur and/or accelerator levels when using BIMS to replace a diene elastomer in an existing formulation in order to avoid over-curing of the diene phases. Similarly, the saturated backbone prevents BIMS from coupling to the sulfur of Si69-silane-treated silica. This allows for a potential reduction in the amount of silane coupling agent used in a tread compound containing BIMS. The improved lab dynamic properties obtained for tread compounds using the BIMS elastomer are not significantly affected by the reductions in sulfur, accelerator or silane coupling agent levels. Thus, compound cure and mechanical properties can be optimized by adjusting these additives, without sacrificing the improved dynamic properties obtained when using the BIMS elastomer.

Acknowledgements

"Improved traction with BIMS" is based on a paper given at the February, 2000 meeting of the Akron Rubber Group.

"Highly dispersible silica in non-tire formulations" is based on a paper given at the June, 2000 meeting of the Latin American Society of Rubber Technology.

"An overview of tire technology" is based on a paper given at the April, 2000 meeting of the Rubber Division.

"Design of EPDM for blends with NR/BR for tire sidewalls: Influence of molecular structure and carbon black distribution on properties" is based on a paper given at the September, 1999 meeting of the Rubber Division.

References

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