New tire black sidewall composition.

New tire black sidewall composition

Modern pneumatic tires are laminated structures engineered for customer comfort. Each unit of construction provides a specific contribution to overall performance. This article focuses on new compositions for the tire sidewall.

The sidewall is an important component for structural integrity and protects the outer surface of the tire. It also protects the fabric ply and contributes to the aesthetics of the tire surface. Basic properties for a good performing sidewall have been age/ozone, flex/crack and fatigue-resistance while retaining good appearance. The current trend toward high performance passenger-car tires demands even longer life, improved durability and improved sidewall appearance (ref. 1). Truck and off-the-road tires continue their trend to radials which places even greater demands on sidewall performance.

A summary of trends in tire performance requirements includes:

* Increased long-term durability and nonstaining.

* Improved appearance and uniformity.

* Improved abrasion resistance.

* Reduced rolling resistance.

* Increased traction and wet grip.

Increased tread life resulting from radialization and new polymer/compounding improvements continues to increase tire life. The desire for multiple retreading capability, an economic driving force in truck tires, puts even greater pressure on sidewall life (ref. 1). Achieving extended tire life requires sidewalls with improved age/ozone and flex crack/fatigue resistance. In the past, industry has resorted to chemical antidegradants to extend sidewall life. These protectants are often used in combination with waxes to control the blooming process for the antidegradant to provide its protection. This protection has a finite life depending on rate of leaching of the chemical protectant. The most effective antidegradants are phenylated amines, which lend a reddish-brown color to the sidewall surface when blooming to afford protection. Some progress has been made with nondiscoloring antidegradants, but they are not very cost-effective. Furthermore, the desire to protect our environment by minimizing the leaching of chemicals from tire components is a new and welcome trend.

The longer-life tire sidewall needs are:

* Excellent ozone resistance.

* Longer life/durability and weather resistance.

* Improved original and aged properties.

* Nonstaining and nonleaching.

The best approach is a polymer system that can satisfy these needs and still be compatible with other general-purpose rubbers used in the tire composition. Such systems utilizing other ozone-resistant polymers, such as halobutyl, EPDM, polychloroprene and chlorosulfonated polyethylene, have provided some success but were usually limited in some property (ref. 2). This article describes a new isobutylene-based polymer with improved functionality, completely saturated to provide ozone, oxidation and flex fatigue resistance. This new polymer, in combination with appropriate blends of general purpose rubbers, gives a sidewall composition to meet current and future needs while retaining all other basic performance requirements for the tire sidewall.

Role of halobutyl in tire black sidewalls

Tire sidewalls are subjected to extensive flexing. This, as indicated previously, requires good ozone resistance, high cured adhesion to adjacent components and low heat build-up. In addition to the requirements, there is an increasing demand for a non-staining non-discoloring black sidewall compound which has a shiny and bloom-free appearance throughout its service life (ref. 3). Removal of the staining and fugitive chemical protectants places a heavy demand on the polymer system of the sidewall compound to resist oxidative degradation to avoid the formation and growth of flex fatigue cracks.

Sidewall polymer blend systems exist which utilize halobutyl rubber for heat and flex resistance, along with an ethylene propylene terpolymer for ozone resistance. This composition offers a polymeric protection system and eliminates a need for chemical protectants. Table 1 provides a comparison of performance properties for a typical general purpose rubber-based (natural rubber/polybutadiene) black sidewall versus a chlorobutyl/EPDM/general purpose rubber sidewall formulation (ref. 2).

As shown, there are significant benefits for using CIIR and EPDM in the blend polymer system versus the GPR formulations:

* Elimination of chemical protectants - by using a polymeric system which provides continuous protection throughout service without surface bloom and discoloration.

* Superior dynamic and static ozone resistance - to improve sidewall performance against ozone attack, a polymer protection system is used. Good dispersion in mixing is critical in order to achieve the desired performance.

* Equivalent fatigue crack growth - by using the polymeric system with the proper filler and cure system, crack growth resistance can be maintained despite the higher tearing energy (modulus) of this blend.

Halobutyl-based sidewall compounds similar to that in table 1 are currently in commercial use, and are continuing to be evaluated for a range of passenger sidewall applications. However, because of the demands placed on sidewalls for extended service life, additional improvements are still needed in the areas of fatigue crack growth and abrasion resistance.

New isobutylene-based co-polymer for sidewalls

To improve the ozone and flex resistance of tire sidewalls, a new isobutylene-based polymer has been developed which combines the attributes of halobutyl and ethylene propylene terpolymers (EPDM) in a single polymer (ref. 4). This brominated co-polymer of isobutylene and paramethylstyrene (PMS), offers improved resistance to ozone and fatigue crack growth in blends with natural rubber and polybutadiene. For the purpose of this article the brominated co-polymer will be referred to as brominated (Br) XP-50. (The designation XP-50 represents "experimental polymer-fifty years" and commemorates the 50th anniversary of the invention of butyl rubber). Additional designations are used (EMDX #) to identify the various experimental grades of this polymer. Details of these polymers are discussed in later sections. The polymer backbone is completely saturated and can be covulcanized in blends with other general purpose rubbers with common curatives. This new polymer is also very similar to polybutadiene (BR) in flex crack growth (FCG) resistance at low strains. Figures 1 and 2 provide schematics of the polymerization and bromination processes, respectively. More detailed information on the polymerization and bromination reactions are available (ref. 5).

To summarize, this new polymer combines the attributes of halobutyl and EPDM which include the following:

* Complete ozone resistance

* Dynamic properties

* Heat resistance

Additional benefits include other characteristics important in a tire sidewall compound:

* Improved flex and crack growth resistance versus HIIR sidewall compounds

* Single polymer to replace HIIR and EPDM

* Wide range in choice of curatives

* Increased reactivity versus HIIR

Both para-methylstyrene and bromine content can be controlled to obtain the desired performance properties in the sidewall formulation. Typical para-methylstyrene levels range from 5.0-10.0 wt. % while total bromine content ranges from 1.0-2.0 wt. % (0.7 - 1.4 mol. %). Details on the effects that PMS and bromine content have on sidewall performance properties will be described later.

Basic sidewall studies

Initially, the first prototype brominated XP-50 polymer (EMDX 89-1) was

evaluated as a direct replacement for the halobutyl and/or EPDM rubber in a state-of-the-art passenger tire triblend compound. As shown in table 2, ozone resistance and fatigue crack growth resistance were equal to or better than the triblends. Compounds 1 and 2 represent the chlorobutyl and bromobutyl triblends, respectively.

In compound 3 the halobutyl is replaced with an equivalent amount of EMDX 89-1 and in compound 4 both the EPDM and halobutyl are replaced with EMDX 89-1. In the two compounds with the new polymer, static ozone resistance was increased to 300 plus hours (vs. 160 hours for the triblends) and dynamic ozone resistance to 288 plus hours (vs. 168 hours for the triblends). Modulus was similar for all of the compounds. In addition, resistance to fatigue crack growth was about 25-50% better for EMDX 89-1 relative to the bromobutyl triblend. In all cases the need for improvements in adhesion was apparent.

EMDX 89-1 levels ranging from 30 phr to 55 phr were studied in an attempt to optimize the level of brominated XP-50 relative to ozone resistance, flex resistance and adhesion, as shown in table 3. In this study, about 40 phr of brominated XP-50 appeared to be required for dynamic ozone and fatigue crack growth resistance (compound 4). However, at levels of 40 phr and above, adhesion values were reduced (compounds 1 and 2) probably because of the reduced natural rubber levels. As anticipated, heat buildup (tan delta) decreased with decreasing brominated XP-50 content (increased natural rubber) and tensile strength increased with decreasing brominated XP-50 (increased natural rubber). Other properties were not significantly affected by EMDX 89-1 concentration.

Table 4 contains data on replacing half of the natural rubber with polybutadiene in the compound containing 40 phr EMDX 89-1. Compound 2 with polybutadiene has a tan delta value about 40% lower (0.096 vs. 0.143) than compound 1 with all natural rubber. In addition, adhesion to a commercial carcass compound was improved with polybutadiene as plucking and stock tear was noted at the interface as compared to interfacial separation with the natural rubber/brominated XP-50 blend.

Based on a balance of heat buildup, adhesion, ozone resistance, tensile strength, modulus and cure, a system based on 3 phr zinc oxide, 0.4 phr sulfur, 1.5 phr alkyl phenol disulfide and 1.7 phr MBTS was selected for a designed experiment to optimize the cure system for a NR/BR/Br XP-50 blend. The results show there was good confirmation between the predicted values based on the response equations generated and the actual properties from a laboratory mix.

Having selected an optimum polymer blend ratio and cure system, a study was conducted to optimize the comonomer and bromine levels of the brominated XP-50. As shown in table 5, polymers containing 5 and 10 wt. % and 1 and 2 wt. % bromine were evaluated. The data illustrate the effect of PMS level and bromine content on sidewall performance properties. Little or no difference is observed in cure and physical properties among the three polymers. However, EMDX 90-2 with 10 wt.% PMS and 2 wt. % bromine exhibits a significant improvement in dynamic ozone resistance (216 plus hours relative to 72 hours for EMDX 89-1). The relatively low value for EMDX 89-1 compared to the earlier work was a result of a new ozone generating chamber and more precise control of ozone levels. In summary, the performance properties which are affected by PMS levels and bromine content are:

* Mooney scorch - decreases with increasing bromine level. This is expected, since higher bromine content increases cure activity.

* Ozone resistance - polymers containing the highest bromine levels give the best ozone protection in blends without significant changes in other performance properties. In addition, polymers with the highest PMS levels provide the greatest latitude for achieving the desired bromine concentration. This effect will again be demonstrated with flex properties discussed later in this article.

* Modulus - results have generally shown that increasing bromine will increase cure activity and provide directionally higher states of cure even though the data indicate minor differences.

Compound refinement studies

Using the formulation developed with the prototype polymer EMDX 89-1, as previously described, additional refinement studies were conducted to assess the effect of comonomer (PMS) level and bromine concentration on important sidewall performance properties. This was accomplished through mixing and basic screening studies using polymers with a range of PMS and bromine levels.

In the laboratory, it is recommended that a conventional mixing cycle be used with a 30 second carbon black delay and finalization on a mill. Variations in the lab mixing procedure were evaluated (internal mixer vs. mill finalization and carbon black charge times) and found to offer no significant benefits.

Based on the results previously presented, and using the optimization response equations discussed in the last section, the Br XP-50 sidewall formulation was further refined with regard to the following performance properties:

* Fatigue crack growth (FCG)

* Tear strength

* Modulus

* Cured adhesion to self and carcass

* Dynamic properties

* Scorch safety

* Ozone resistance

This was achieved by evaluating the effects of polymer blend ratios, carbon black type and loading and cure system refinements on sidewall performance properties. The following compound variations were considered in these studies:

* EMDX 90-2 - 30 to 40 phr

* Natural rubber and polybutadiene - 60 to 70 phr (total amount)

* Carbon black

(HAF) N351 - 40 phr

(GPF) N660 - 50 phr

In summarizing the results given in table 6, screening studies conducted in preparation for a designed experiment showed:

* Formulations (#1,2,3) with 40 phr EMDX 90-2/50 phr N660 performed better than or equal to the GPR BSW in ozone, Die B tear at 100 [degrees] C and outdoor flex testing.

* Both Pico abrasion and cured adhesion at 100 [degrees] C were poor (vs. GPR BSW), primarily due to the low reinforcement characteristics of N660 carbon black.

* All of the formulations have higher tan delta levels at 60 [degrees] C than does the GPR BSW.

* Replacement of N660 with N351 (#5,6,7) significantly improves Pico abrasion, cured adhesion and outdoor flex properties.

* Formulation #5 provides the best balance of critical performance properties. Figures 3-5 provide fatigue crack growth properties on selected formulations versus the CIIR and EPDM "triblend" and GPR compounds previously discussed in table 1. The performance of compound #5 is confirmed by these results which also supports the outdoor flex data. This formulation provides crack growth properties very similar to those of the GPR sidewall and better than the triblend compound.

TEM images shown in figures 6-10 illustrate the effect of polymer blend ratios on phase morphology. The black areas of the photos represent of GPR/carbon black loaded phase and the white areas are the brominated XP-50. Figures 6-9 show the effects of increasing the polybutadiene/natural rubber ratio and total content. The reduction of the white areas corresponds to reductions in ozone and flex resistance. The benefits of HAF versus GPF carbon black in promoting a co-continuous phase are evident in comparing figure 8 versus 10. The only difference is that the compound in figure 10 contains N351. The greater reinforcement and higher shear provided by the N351 have yielded a co-continuous morphology which is desirable for fatigue and ozone resistance.

Performance property optimization

Previous screening and optimization studies yielded a black sidewall formulation with a 35 phr brominated XP-50/40 phr BR/25 phr NR polymer system, 40 phr HAF N351 and a sulfur/MBTS/alkyl phenol disulfide cure system. This formulation possessed a balance of critical properties such as heat buildup, cured adhesion, tear strength and Pico abrasion. Fatigue crack growth and ozone resistance were equal to or superior to a GPR formulation despite having no staining/fugitive chemical protectants in the brominated XP-50 blend compound. From the base of these studies, a designed experiment was conducted to further refine the formulations and all compounding constituents without compromising performance.

Table 7 summarizes the outline of the designed experiment. The design consists of several quadratic, linear and factorial "sub-models" all combined into one large model to account for all the main and interaction effects. Results from the experiment are available and show significant effects for each response variable and an R-squared value. The R-squared indicates the degree in which the response equation (using this model/data) can accurately predict the property; values closer to 1.0 indicate better prediction or fit with respect to the data. Overall, the model does an excellent job in prediction responses for each variable. Due to the complexity and number of terms in the response evaluations used in this design, full details are beyond the scope of this article.

Some important observations which can be made from this study are:

* Both the model and input data indicate that a 35 phr brominated XP-50 sidewall formulation is feasible.

* Optimum formulations using a 42 ML polymer with PMS levels of 7.5 or 10 wt. % were generated by the model.

* The level of brominated XP-50 is significant for several responses:

Response Sign of coefficient

Mooney viscosity (compound)

  Cure rate                                    +
  300% modulus                                    +
  Tear resistance                                 -
  Dynamic ozone                                   -
  Cured adhesion to carcass                       +
  Fatigue crack growth rate (15 and 30% strain)   -

Increasing the level of brominated XP-50 will increase compound viscosity, cure rate and dynamic ozone resistance while reducing the others. Laboratory evaluations are underway to confirm these results and further refine the existing formulation to enhance the performance properties.

Based on the results of the designed experiment and several parallel studies, a preferred grade of brominated XP-50 (EMDX 90-10) was produced which has a PMS level of 7.5 wt % and a bromine level of 2 wt. %. This polymer is expected to be the best for black sidewall applications. Table 8 contains data on EMDX 90-10 in the sidewall formulation previously discussed. Formulations containing 35 phr EMDX 90-10 perform equally well, especially in the case of fatigue crack growth and ozone resistance. Compounds similar to that in table 8 have been successfully evaluated in passenger tire factory trials. More complete information on the factory evaluations is beyond the scope this article.

Conclusions

This article discussed the performance of a new brominated co-polymer in tire black sidewall formulations. This new polymer combines the attributes of halobutyl and EPDM in a single polymer. It eliminates the need for fugitive and staining chemical protectants without significantly compromising sidewall performance properties, and allows potential for sidewall gauge reduction:

* The polymeric protection of the sidewall formulation provides a non-staining non-discoloring compound throughout the service-life of the tire.

* Dynamic and static ozone resistance are improved significantly versus a chemically protected GPR black sidewall.

* Fatigue crack growth properties of the new sidewall composition are equivalent to a GPR sidewall formulation.

* Polymer phase morphology is greatly affected by both polybutadiene and brominated XP-50 levels as well as the type of carbon black employed. A co-continuous phase with the natural rubber and polybutadiene is possible. This provides the optimum in performance properties such as resistance to ozone and crack growth.

* Sidewall performance property requirements (versus a GPR formulation) have been met or surpassed with 35 phr brominated XP-50.

Summary

A new polymer has been developed for tire sidewall applications which offers a polymeric protection system against ozone attack and flex fatigue while eliminating the need for chemical protectants. This polymer, a brominated co-polymer of isobutylene and para-methylstyrene, combines the attributes of halobutyl and EPDM in a single polymer an offers improved resistance to ozone and fatigue crack growth in blends with natural rubber and polybutadiene.

This article detailed the molecular parameters of the new polymer which is tailored to have increased functionality specifically for tire black sidewalls. In addition, vulcanization and compound optimization studies were presented for the sidewall application. [Tabular data 1 to 8 omitted] [Figure 1 to 10 omitted]

References

[1]W.W. Barbin, "Trends in tire and rubber technology - five-year outlook," presented to IISRP (August, 1990). [2]D.G. Young, E.N. Kresge, A.J. Wallace, Rubber Chem. Technol. 55, (2) (May - June, 1982). [3]J.V. Fusco, D.G. Young, "Isobutylene-based polymers in tires - status and future trends," presented to ACS Rubber Division, Washington, D.C. (October, 1990). [4]K.W. Powers and H.C. Wang, European patent publication No. 0344021 (11/29/89). [5]K.W. Powers and H.C. Wang, "Functionalized paramethylstyrene/isobutylene copolymers," presented to ACS Rubber Division, Toronto, Ontario, Canada (May, 1991).

Appendix A - Ingredients List

SMR 20 and SMR 5 - Standard Malaysian Rubber Budene 1207 - Goodyear Tire & Rubber Chemical Div. Chlorobutyl 1066 - Exxon Chemical Co. Bromobutyl 2233 - Exxon Chemical co. Vistalon 7500 - Exxon Chemical Co. Vistalon 6505 - Exxon Chemical Co. EMDX 89-1 (Br XP-50) - Exxon Chemical Co. EMDX 90-1 (Br XP-50) - Exxon Chemical Co. EMDX 90-2 (Br XP-50) - Exxon Chemical Co. EMDX 90-10 (Br XP-50) - Exxon Chemical Co. Sunolite 240 Wax - Sun Oil Company Flexon 641 - Exxon Chemical Co. Wood Rosin FF - Harwick Chemical Corporation SP 1077 - Schenectady Chemicals, Inc. Escorez 1102 - Exxon Chemical Co. Flectol H - Monsanto Chemical Co. Santoflex 13 - Monsanto Chemical Co. Vultac #5 - Pennwalt Corporation, Rubber Chemicals Dept. Santocure NS - Monsanto Chemical Co.