Longer lasting tire curing bladders.

The use of butyl rubber for tire curing bladders has long been recognized by the rubber industry. The isobutylene based butyl rubber in combination with a poly alkyl phenolformaldehyde resin has provided very good resistance to high temperature oxidative hydrolytic conditions. However, the presence of olefinic unsaturation in the backbone adversely affects the long term oxidative resistance. As tire cure temperatures have increased and cure times have decreased, the life of bladders has decreased. The result to the tire companies has been an increase in scrapped tires due to bladder failures and an increase in press down time to change bladders.

Replacement of the butyl rubber in the bladder compound with a new isobutylene based polymer possessing a totally saturated backbone, brominated poly (isobutylene-co-paramethylstyrene), has resulted in a very significant increase in tires produced per bladder. The saturated backbone provides enhanced resistance to thermal oxidative induced mechanical failure. The presence of polymer bound bromine in combination with stearic acid and zinc oxide has eliminated the need for addition of halogenated organic or inorganic materials to activate the resin reaction. This article will describe the polymer and its optimization, discuss the compound formulation, describe the processing conditions and performance in the plant, and describe future optimization studies.

Brominated copolymers of isobutylene and PMS

Isobutylene based elastomers have historically possessed very good thermal oxidative resistance. This is due to the highly saturated nature of the polymer backbone. In butyl rubbers crosslinking is provided by the low levels of in-chain unsaturation provided by the 1,4 incorporation of isoprene.

Very significant improvements in thermal oxidative stability are realized with a new family of elastomers based upon isobutylene and para-methylstyrene (PMS). The new material possesses a totally saturated polymer chain backbone. These copolymers can be prepared in a conventional butyl type slurry polymerization process employing Lewis Acid based carbocationic catalysis over a wide range of isobutylene/PMS ratios as shown in figure 1.

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Crosslinking functionality is introduced into the copolymer by free radical initiated bromination of a fraction of the randomly distributed para-methyl benzyl units to produce monobromomethyl groups as shown in figure 2. The curable elastomer can be thought of as a terpolymer of isobutylene, paramethylstyrene and parabromomethylstyrene.

[CHART OMITTED]

The very reactivity nature of the benzyl bromide functionality permits curing to proceed by a variety of paths. The primary crosslinking reactions occur via electrophillic alkylation of aromatic rings, olefinic groups and basic heteroatoms. These reactions are catalyzed by Lewis Acids and are the mechanism by which the resin reaction proceeds.

As shown in figure 3, low glass transition temperatures can be maintained for the copolymers up to a PMS content of approximately 10 wt. %. It is therefore possible to regulate over a wide range the level of benzylic bromide cure functionality while retaining good elastomeric properties. The similar magnitude of the monomers' reactivity ratio values permits preparation of polymers across a wide Mooney range with fairly narrow molecular weight distributions and random monomer distribution.

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It is the combination of the thermal and oxidation resistant nature of the totally saturated chain backbone, and the ability to limit ultimate crosslink density by optimizing the level of benzylic bromide functionality, that recommends the new brominated polymer for tire curing bladder applications. For such uses, the brominated copolymer can be effectively cured with a modified poly methylol phenol resin recipe that minimizes post cure increases in modulus. Unlike butyl bladder formulations, compounds with the new polymer do not exhibit butyl's characteristic thermal reversion behavior on extended exposure to elevated temperatures and aggressive hydrolytic environments.

Optimization for curing bladders - polymer selection

Preliminary data generated in a dynamics application showed that high levels of PMS (10 weight % or greater) would give compounds with high compression set, shorter elongation, higher increase in hardness with aging and poorer flex life. Therefore all of the work reported here is with a 5 weight % PMS polymer (2.5 mole %).

The initial cure system selection was the typical butyl system consisting of a poly alkyl phenol-formaldehyde resin and zinc oxide so as to obtain thermally stable crosslinks. Data using this cure system showed that bromine weight % levels in the 0.50% range would give a very low state of cure and levels in the 1.10 weight % would give poor aging. Therefore a polymer featuring (table 1) 5 weight % PMS and 0.8 weight % bromine was selected. This polymer has been used for the remainder of the evaluations in this article.

Table 1 - polymer evaluated/14470

Mooney viscosity        50 [+ or -] 5
ML 1 + 8 @ 125[degrees]C
Antioxidant, wt. %      0.10 [+ or -] 0.05
Ash, wt. %              0.5 max
Water, wt. %            0.3 max
Typical inspection
Comonomer (PMS), wt. %  5
Bromine, wt. %          0.8
Calcium, wt. %          0.07
Cure characteristics(*)
ML, dN.m                20
MH, dN.m                34
Ts2, min.               7.5
T'50, min.              15
T'90, min.              21

Recipe: polymer - 100, IRB #6 black-40, stearic acid-2, zinc oxide-0.5, zinc stearate-1.0

Cure testing 160[degrees]C, 30 min., 3[degrees] arc, no preheat

Cure system selection

As previously mentioned, it was desired to continue to use the resin cure to obtain the thermally stable crosslinks that it generates. From previous studies (ref. 2) it had been demonstrated that zinc oxide and stearic acid or zinc stearate would be required to generate the Lewis acid catalysis that would initiate the alkylation reaction. Experiments showed that this system gave very poor aging due to the alkylation reaction proceeding too far. To minimize to post cure aging, sulfur and MBTS were added to moderate the resin cure. This recipe gave excellent physical properties but was quite scorchy. Triethylene glycol (TEG) was added to slow the initiation of the alkylation reaction.

Table 2 shows the recipes used in a [2.sup.6-2] face centered statistical design to optimize the levels of these ingredients. The data for the control and center point formula are shown in table 3. The data show good unaged physical properties and a significant improvement in aged properties (table 4). Hot flex resistance showed a significant improvement in tearing energy as well as the point of catastrophic failure, but the compound was still too scorchy to process in a factory.

Table 2 - statistical study recipe (phr)

14470           100
N330             55
Castor oil        5
SP1045         2-10
Zinc oxide      2-5
Stearic acid    0-1
Sulfur          0-3
MBTS            0-3
TEG             0-1

Design was a [2.sup.6-2] face-centered design. Three masterbatches were blended to make finals.

Table 3 - statistical study summary

                                            B268          14470
                                                   Center point
Rheometer @ 190[degrees]C, 1[degrees] arc
ML, dN.m                                       9             10
MH, dN.m                                      27             23
Ts2                                          3.3            1.6
T'90                                        23.5           26.0
Cure T'90 + 2 @ 190[degrees]C
Shore A hardness                              66             66
Tensile, MPa                                13.7           14.5
300% Mod., MPa                               4.4            4.3
% Elongation                                790            860
300% Tension set                             6.2            6.2

Table 4 - statistical study aged properties

                                    B268        14470
                                           Center point
Cure T'90 + 2 @ 190[degrees]C
Air age 48 hr. @ 177[degrees]C
Shore A hardness                      88             80
Tensile, MPa                          6.7          10.3
300% Mod., MPa                        --            7.7
% elongation                         240            430
300% tension set                   Broke           15.6
Cure T'90 + 2 @ 190[degrees]C
Steam age 48 hr. @ 170[degrees]C
Shore A hardness                      72             60
Tensile, MPa                        13.9           13.7
300% Mod., MPa                       7.9            5.2
% Elongation                         550            680
300% Tension set                     9.3            9.3

To further improve the scorch, the TEG was replaced by magnesium aluminum hydroxy carbonate (DHT4A2). This material is an effective acid scavenger. Its function may be to slow down the solubilization of the zinc oxide by competing for the stearic acid.

Another statistical study was run to optimize the level of DHT4A2 and MBTS. Data from this study (table 5) show DHT4A2 at 0.8 phr and MBTS at 1.4 phr gave good scorch safety and maintains the outstanding aged properties (table 6).

Table 5 - 14470 vs. B268 in bladders

Initial properties                       14470   B268
Scorch time, t5 at 135[degrees]C, min.      32     37
T'50 at 190[degrees]C, min.                4.2    5.2
T'90 at 190[degrees]C, min.                7.6     13
Hardness, Shore A                           48     51
100% modulus, MPa                          1.1    1.3
Tensile strength, MPa                     15.8   13.0
Elongation, %                              750    740
Tension set, 300%                          6.4   12.0
Tension set, 50%                           4.2    7.0

Table 6 - 14470 vs. B268 in bladders

Aged properties                    14470   B268
4 days at 180[degrees]C in air
Shore A hardness                      71     75
100% modulus, MPa                    2.6    2.7
Tensile, MPa                         7.4    3.0
Elongation, %                        330    150
Tension set, 50%, %                   12   18.4
4 Days at 180[degrees]C in steam
Shore A hardness                      51     63
100% modulus, MPa                    1.5    2.3
Tensile, MPa                        14.2   13.5
Elongation, %                        485    550
Tension set, 50%,                     36   34.6

Plant processing

The recipe shown in table 7 has been used for the majority of the plant trials. The black levels and types as well as the castor oil levels are proprietary but in each case has been the same as that used with the normal butyl bladder recipe. Data shown in table 8 are typical factory data. Good processing safety, faster cure rates, reduced growth as measured by tension set and improved retention of properties with aging was attained in each case.

Table 7 - factory trial recipes

14470             100
Black          Proprietary
Castor oil     5-10 Range
ZnO                 2
Stearic acid      0.5
MBTS              1.4
Sulfur           0.75
SP 1045             5
(*)DHT4A2         0.8

(*)Magnesium aluminum hydroxy carbonate

[TABULAR DATA OMITTED]

Factory mixing utilized a conventional two or three step mix except that the first step dump temperature was lowered to accommodate the brominated polymer.

Whether a two step or three step mix was used was dictated by available mixing equipment and the types of black employed but in each case, the number of steps and cycle times have been the same as for a normal butyl compound.

Extrusion guidelines should be followed closely. The tendency of the polymer to follow cold metal requires the screw should be set cooler than the barrel. Extrusion speeds and temperatures should be set to obtain a maximum internal slug temperature of 110[degrees]C. Because of the very fast cure rate, the slugs must be cooled to room temperature as rapidly as possible and storage of the slugs should be limited to two weeks or less.

Curing of the slugs in factory trials has ranged from 180[degrees]C to 215[degrees]C. The optimum cure temperature appears to be in the range of 190-195[degrees]C. This temperature gives the best mold flow with the fastest cure. Cure times are normally 10-20% less than for regular butyl.

Because of the proprietary nature of our factory evaluations, we cannot show actual factory data of bladder types and heat life improvements. We can state though that several thousand bladders of almost all sizes and types have been tested and the average heat life, when the compound has been processed properly, has been increased by 30% to 100%.

Conclusions

The use of brominated poly (isobutylene-co-4methylstyrene) in place of butyl rubber and the modified cure system has significantly improved air and steam aging resistance, improved flex crack resistance, reduced hot growth and provided significantly longer bladder life.

Future work

Future work will concentrate in the areas of: further improvements in scorch safety; improved molding for giant bladders; improved extrusion rates and a non-resin based cure system.

References

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

(2.)I.J. Gardner, H.C. Wang, J.V. Fusco, Paper #7, presented to ACS Rubber Division, Louisville, May 19, 1992.

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