Advancements in new tire sidewalls with a new isobutylene based copolymer.

In the past, industry has resorted to chemical antidegradants to extend tire 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.

The best approach to extending sidewall life is a polymer system which is covulcanizable with other general purpose robbers (natural rubber/polybutadiene). Such systems utilizing other ozone-resistant polymers, such as halobutyl and EPDM, have provided some success but were usually limited in some property (ref. 1).

A new polymer was recently introduced based on the controlled bromination of an isobutylene/para-methylstyrene copolymer (ref. 2). This polymer, developed for tire sidewall applications, offers a polymer protection system against ozone attack and flex fatigue while eliminating the need for chemical protectants.

Preliminary assessment of this new polymer in tire black sidewall compositions revealed several attributes related to the polymer's inherent properties, ozone and flex crack resistance. In addition, early work demonstrated covulcanizability of this new functional isobutylene based copolymer with general purpose rubbers (natural rubber/polybutadiene) now used in other tire components (ref. 3).

This article will describe advancements in tire sidewalls with this new polymer, including compounding responses related to formulation variables as they affect properties important to sidewall performance. Details on factory processing and tire performance will be reviewed. This new polymer, in combination with appropriate blends of general purpose rubbers, gives a sidewall composition to meet current and future tire performance requirements.

New isobutylene-based copolymer in black sidewall compositions

Sidewall polymer blend systems exist which utilize halobutyl rubber for heat and flex resistance, along with an ethylene propylene terpolymer (EPDM) for ozone resistance. Blend systems such as these offer a polymeric protection system and eliminate the need for chemical protectants. Halobutyl and EPDM based sidewall compounds are currently in commercial use. The polymer compositions 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, oxidative and abrasion resistance.

To improve the performance of the sidewall with regard to these properties, a new isobutylene-based copolymer has been developed which combines the attributes of halobutyl and EPDM in a single polymer (refs. 3 and 4). The polymer backbone is completely saturated and can be covulcanized in blends with other general purpose rubbers with common curatives. This brominated copolymer of isobutylene and para-methylstyrene (PMS) offers improved resistance to ozone and fatigue crack growth in blends with natural rubber and polybutadiene.

Detailed information on the polymerization and bromination reactions are discussed in separate publications (refs. 2 and 3). For the purpose of this article, the brominated copolymer for tire sidewall applications will be referred to as EMDX 90-10.

Based on the results of designed experiments and several compound optimization studies, a preferred "grade" (EMDX 90-10) was produced which has a PMS level of 3.8 mol. % (7.5 wt. %) and a bromine level of 1.2 mol. % (2.0 wt. %) (ref. 2). This polymer has been determined to be the best for black sidewall applications. These early studies yielded black sidewall formulations with a 35 or 40 phr EMDX 90-10/35-50 phr BR/10-25 phr natural rubber polymer system, 40 phr HAF N351 and a sulfur/MBTS/alkyl phenol disulfide cure system. Table 1 provides a comparison of performance properties for a typical general purpose rubber based black sidewall versus a CIIR/EPDM/general purpose rubber and the EMDX 90-10 sidewall formulations. The experimental polymer formulations possessed a preferred 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 versus the GPR formulation despite having no staining/fugative chemical protectants in the experimental polymer blend compounds.

For the laboratory optimization studies discussed in this article, 35 and 40 phr EMDX 90-10 formulations (shown in table 1) were chosen because of concerns for the ability to adequately disperse the polymer at a 30 phr level. It is essential that the experimental elastomer be well dispersed (with proper phase size) throughout the sidewall composition to achieve good performance properties. In early factory evaluations, a 35 phr EMDX 90-10 black sidewall formulation was utilized. However, in subsequent evaluations the formulations contained 40 phr to ensure good polymer dispersion in a conventional internal mixer and achieve the desired sidewall performance properties. Results from these evaluations will be discussed later.

The optimization studies described in this article were conducted to further improve performance properties such as cured adhesion, tear strength and fatigue crack growth; develop mixing procedures which provide good polymer dispersion and carbon black dispersion/distribution; and develop formulations for specific black sidewall factory processing and tire performance requirements.

Basic sidewall studies

Using the formulations developed in earlier studies, as previously described, additional compounding refinement studies were conducted to assess the effect of compatibilizers and plasticizers (naphthenic oil) on sidewall performance properties. Table 2 contains data on the addition of a compatibilizer at 5 phr with oil levels of 12 and 14 phr. The results show significant improvement in both cured adhesion and tear properties when using a 5 phr compatibilizer/12 phr oil blend versus earlier compositions. Based on these compound refinements, black sidewall formulations were then developed for specific tire/sidewall performance requirements. The results from these studies are shown in table 3. Because of the concern for adequately dispersing the experimental elastomer in a factory mixer, the 40 phr EMDX 90-10 formulation was selected to evaluate several different carbon black types and loadings, as described in table 3. The 35 phr experimental polymer blend system will be revisited after establishing a tire sidewall performance database with the 40 phr blend system. Each formulation was developed for specific tire/sidewall processing and performance requirements, such as original equipment and high performance passenger and radial truck tires. The formulations shown in table 3 are recommended starting points and will require some minor refinements to meet specific sidewall processing and tire performance requirements.

With refinement studies on the non-productive (masterbatch) portion of the sidewall formulation complete, studies were conducted to develop curative compositions to achieve cure requirements in processing and final curing of the black sidewall. Particular attention has been given to developing systems which meet specific cure performance requirements balancing scorch safety with cure rate.

Cure studies

Having developed an optimum polymer blend ratio and cure system, factory evaluations in passenger tire black sidewalls were conducted. Details of these evaluations will be described later. However, one important result from these evaluations was that there clearly was a need for improved scorch safety and faster cure rates. Efforts to improve the cure characteristics of the compositions will be discussed in this section.

The first phase of the cure refinement studies involved evaluating several different possible retarder/accelerator combinations. These retarders/accelerators are additional ingredients in the sulfur/MBTS/alkyl phenol disulfide system. The retarder/accelerator combinations were evaluated to increase the cure rate while maintaining or improving scorch safety. With experimental polymer blend systems, the phenol disulfide cure system typically is slower in cure rate and slightly scorchy vs. a standard GPR sidewall compound.

The second approach involves the evaluation of a new curative, a derivative of a mercapto-thiadiazole, for improvements in cure characteristics of the sidewall composition.

Cure studies evaluating various combinations of retarders and accelerators are shown in tables 4 and 5. Table 4 contains data from early cure studies using DOTG, DPG, or a combination of zinc stearate and magnesium stearate as the additional accelerator with a retarder shown in the lower portion of the ingredient list. The purpose of such a composition is to reduce the [t.sub.c]90 (time to 90% max. torque) while increasing scorch times and not adversely affecting other performance properties. Compound 1 in table 4 represents the base case. Scorch times should be equal to or greater vs. compound 1, while to90 values should be no greater than 20 minutes. These target values were established by several factory processing evaluations. None of the compositions satisfy the criteria. Table 5 illustrates another approach in attempting to control the cure kinetics. This cure composition utilizes a combination of DPG and a magnesium aluminum hydroxy carbonate retarder. Compound 1 is the base case in table 5. Again, the results are similar to those in table 4.

In summarizing the results given in tables 4 and 5, screening studies conducted to improve cure performance showed:

* Common accelerators in combination with a retarder were minimally effective in improving scorch safety, while providing longer cure times.

* DPG in combination with a magnesium aluminum complex reduced scorch times with little effect on cure rate.

* Further analysis of the compositions (details of which are beyond the scope of this article) show that, while the compounds provide excellent performance properties, the modified cure systems are unable to control the cure kinetics and curative solubility with the experimental polymer.

For some black sidewall processing and performance requirements, the previously described compositions will perform satisfactorily. However, a majority of the sidewall requirements require faster cure rates while maintaining all other performance properties, especially Mooney scorch.

In response to the requirement for better versatility in cure response, a new curative was developed which efficiently utilizes the experimental polymer's unique crosslinking functionality provided by the pendant benzylic bromide (ref. 3). More detailed information on this new curative, its description, cure activity and properties are discussed in a separate paper (ref. 6).

Optimization studies using this new curative were conducted in sidewall compositions. Results from these studies are shown in table 6. In these cure systems, the alkyl phenol disulfide is replaced with the new curative. Additional studies were conducted to develop optimum levels for sulfur and MBTS. These studies showed that the optimum accelerator combination with the new curative is DPG with higher sulfur and MBTS levels versus those described in tables 3, 4 or 5. With these results, similar formulations to those in table 3 are shown in table 6. The use of this new curative in sidewall compositions provides improved scorch safety, faster cure rate and improved cured adhesion and tensile properties versus the alkyl phenol disulfide cure system. Compounds similar to those in table 6 have been successfully evaluated in passenger tire factory trials.

Factory processing

Recipes given in tables 3 and 6 have been evaluated in black sidewall factory trials. In most cases, agreement between laboratory versus factory compounds is very good. Differences may occur with scorch times due to the heat history of factory vs. laboratory conditions with factory mixes having slightly shorter times.

Factory mixing has been conducted in conventional internal mixers (#11, two-wing rotors/F620s and F270s, two and four wing rotors). In a conventional two-step mix, a 30 second carbon black delay is recommended to allow for polymer warm-up, blending and optimum carbon black incorporation. Masterbatch dump temperatures range from 145 degrees C to 180 degrees C. Because of the excellent heat stability associated with the experimental polymers vs. halobutyls, there is no concern for dehydrohalogenation at or above 160 degrees C. Finalization dump temperatures should be at or below 100 degrees C. Like other butyl polymers, the polymer or polymer-based compound will follow cold surfaces, therefore rotor and gate temperatures should be approximately 5 degrees C higher than the mixing chamber wall temperature.

Factory mixing studies have been conducted using Y-mix procedures. This type of procedure has been successfully used to control both polymer phase morphology and carbon black distribution.

TEM images (available from the authors upon request) illustrate the effect of mixing procedures on polymer phase morphology and carbon black distribution. The effects of a factory two-step versus a Y factory mix procedure were compared. The carbon black dispersion is very good with noticeably better results using the Y-mix approach. Comparing polymer phase morphology and carbon black distribution of a two-step mix versus a Y mix made the benefits of a Y-mix evident.

Polymer phase morphology is significantly better (EMDX 90-10 phase is smaller and better dispersed) with the Y-mixes vs. the two step mix. In comparing carbon black distribution, there is more carbon black residing in the experimental polymer with the Y-mixes. These two features, phase morphology and better (balanced) carbon black distribution, are the most important results affecting the sidewall performance.

The experimental polymer black sidewall compounds have been evaluated in both hot and cold feed extruders. Although the compounds process satisfactorily in both types, because of the high cure activity, they are more ideally suited for cold feed operations. In cold feed pin-barrel extrusions, it is very important to minimize air incorporation. Stock inserted at the feed hopper should be in plied up strips "overfilling" the throat to reduce trapped air. Screw speeds should be slower vs. those used in GPR extrusions. Screw temperatures should be 5 degrees C lower than the barrel temperature. Typical internal extrudate temperatures at the extruder head range from 104 degrees C to 120 degrees C. Sufficient cooling of the extrudates is essential for preserving tack. Surface temperature of the extrudate should be 38 degrees C or lower approximately 30 meters from the extruder. In general, extruded black sidewall packages have good green/building tack. If processing conditions indicate a need for additional tack, phenolic tackifying resins can be added (1-2 phr) during finalization. These types of resins cannot be added during masterbatch preparation as they are curatives for the experimental polymers. Excellent results have been obtained in the calendering of these compounds (table 3, compounds 4 and 5). Calendered stock has been successfully used in both veneer and coverstrip applications which demonstrates the excellent covulcanization/compatibility properties.

Tire performance

As discussed earlier, various EMDX 90-10 black sidewall formulations have been evaluated in tires. These factory processing and tire evaluations were conducted in several different tire lines (OE, replacement, high performance, etc.) and constructions. Due to the complexity and number of evaluations, full details are beyond the scope of this article.

To date, approximately 13 passenger and radial truck tire factory trials have been conducted in the United States and Europe. Each of the trials involved extensive indoor and outdoor wheel testing. Because of the proprietary nature of these evaluations/testing, actual tire test data cannot be shown. However, general comparisons vs. the control can be made, as well as overall comments on performance in several tire tests.

Following is a summary of the results from these tire evaluations:

* DOT testing is comparable versus the control.

* Rolling resistance results indicate a difference of 10% or less compared to a GPR control fire.

* Early tire testing on the indoor ozone wheel showed a torque/flex cracking condition. Recent changes to a 40 phr level of the new copolymer, refinements in the mixing to better disperse the polymer and improved carbon black distribution have resolved this condition. Tire testing is in progress to confirm these recent results.

* High speed (step type - 225 kilometers/hr.) endurance/durability testing, mainly in high performance tires, shows improved performance (one full step-up in speed) versus the control.

* Significantly better curb scuffing properties (resistance and appearance - no "shaving" texture) vs. triblends; equivalent to control.

* Initiated cut-growth results show approximately a 7% increase in the length of cuts vs. the GPR control; significantly better than most triblend and biblend formulations which can average 15% or more.

* Several passenger tire and one radial truck tire fleet tests have been conducted. Results from passenger fleet testing are very good. Tires have accumulated 50,000 miles in periods of one year in high ozone level environments with no sidewall cracking. Radial truck tires have accumulated over 200,000 miles and at least one recap with no sidewall cracking. Testing is continuing in this area.

One very important result from these fleet evaluations is that the experimental tires retain their black color throughout while the chemically protected GPR sidewall stains to a reddish-brown color.

Tire factory evaluations are continuing with this new polymer as the application technology continues to evolve. Commercialization in a tire line with this new polymer is expected in the very near future.

Conclusions

This article discussed the performance of a new brominated copolymer 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. The polymerie protection of the sidewall foxmulation provides a nonstaining nondiscoloring compound throughout the service-life of the tire. Factory processing and on-tire testing have demonstrated comparable performance vs. a GPR control tire, especially in long-term fleet tests. Mixing procedures greatly affect both polymer phase morphology and carbon black distribution. A well dispersed copolymer phase along with a balanced carbon black distribution provides the optimum in performance properties such as resistance to ozone, abrasion and cutgrowth. Dynamic and static ozone resistance are improved significantly versus a chemically protected GPR black sidewall. Sidewall performance property requirements (vs. a GPR formulation) have been met or surpassed with 40 phr of EMDX 90-10.

Summary

A new polymer has been developed for tire sidewall applications which offers a polymer protection system against ozone attack and flex fatigue while eliminating the need for chemical protectants. This polymer combines the attributes of halobutyl and EPDM in a single polymer and possesses a unique yet versatile crosslinking functionality. This article detailed sidewall formulations involving filler/plasticizer blends as well as curative compositions and their effect on sidewall performance. In addition, results from factory mixing studies and tire evaluations were presented for the sidewall application.

References

1. D.G. Young, E.N. Kresge, A.J. Wallace, Rubber Chem. Technol. 55, (2) (May-June, 1982).

2. K.W. Powers and H.C. Wang, "Functionalized paramethylstyrene/isobutylene copolymers," presented to ACS Rubber Division, Toronto, Ontario, Canada (May, 1991 ).

3. D.D. Flowers, J.V. Fusco, L.J. Gutsky and D.G. Young, "New tire black sidewall composition," presented to ACS Rubber Division, Toronto, Canada (May, 1991).

4. K.W. Powers and H.C. Wang, European Patent Publication No. 0344021 (11/29/89).

5. J.V. Fusco and D.G. Young, "Isobutylene-based polymers in tires - status and future trends," presented to ACS Rubber Division, Washington, D.C. (October, 1990).

6. I.J. Gardner, R.F. Ohm, J.V. Fusco and D.D. Flowers, "New curative for brominated poly (isobutylene-co-paramethylstyrene)," presented to ACS Rubber Division, Orlando, Florida (October, 1993).

  Table 1 - comparison of GPR vs.
 CIIR/EPDM/NR/BR and EMDX 90-10
nonstaining black sidewall compounds
                                          1      2      3      4
EMDX 90-10                               --     --     35     40
NR(1)                                    50     20     15     10
BR(2)                                    50     35     50     50
CIIR(3)                                  --     35     --     --
EPDM(4)                                  --     10     --     --
N351                                     --     40     40     40
N660                                     50     --     --     --
Naphthenic 641                           12     12     12     12
Phenolic tackifying resin(5)              5      5     --     --
Hydrocarbon resin(6)                     --     --      5      5
Hydrocarbon wax(7)                        1     --     --     --
Quinoline(8)                              1     --     --     --
PPD(9)                                  2.5     --     --     --
Stearic acid                              1      1      1      1
Sulfur                                    2    0.2    0.3    0.3
Zinc oxide                                3      5      1      1
Alkyl phenol disulfide(10)               --    0.5    0.8    0.8
MBTS                                     --      1    0.8    0.8
BBTS(11)                                  1     --     --     --

Mooney scorch (MS)
 @ 135 degrees C, t5                   16.4   13.8   12.0   11.4

Mooney viscosity,
ML 1 +4' @ 100 degrees C                 48     43     42     43

Rheometer @ 170 degrees C, 3 degrees arc
ML, dN-M                               13.0   12.4   11.3   11.2
MH, dN-M                               69.9   43.2   46.7   46.4
ts2, min.                               2.5    2.6    2.7    2.8
tc90, min.                              6.8   12.4   13.1   12.9

Press cure tc90 @ 170 degrees C
300% Modulus, MPa                       5.4    3.8    4.6    4.9
Tensile, MPa                           13.8   11.6   11.8   11.6
Elongation, %                           620    710    570    580
Hardness, Shore A                        54     53     53     54

Die B tear @ 100 degrees C, kN/m       31.5   21.9   29.1   28.7

Self adhesion @ 100 degrees C, kN/m    32.9   18.7   20.4   21.1

Dynamic ozone, 100 pphm,
25% extension, 40 degrees C
Hours to crack                           94   >576   >576   >576

Bent loop static ozone, 100
pphm, 40 degrees C
Hours tocrack                           120   >576   >576   >576

Dynamic properties
MTS<100 Hz., 60 degrees C
[+ or -]5% strain
Tan delta                             0.106  0.123  0.128  0.130

Fatigue crack growth @ 50 degrees C
FCG, nm./cyc. @ 15% strain             12.3   14.9   11.6   12.0
FCG, nm./cyc. @ 30% strain              460    780    380    310
Tearing energy, J/[M.sup.2] @ 15%       655   2055    620    570
 strain
Tearing energy, J/[M.sup.2] @ 30%      1700   2136   2045   2000
 strain

Pico abrasion                           100     90    111    108
ASTM D 2228
4.5 Kg. load
80 Rev. @ 1 Hz.

(1) SMR 20           (2) Budene 1207     (3) Chlorobutyl 1066
(4) Vistalon 7500    (5) SP 1077         (6) Escorez 1102
(7) Sunolite 240     (8) Flectol H       (9) Santoflex 13
(10) Vultac #5       (11) Santocure NS

[Tabular Data Omitted]