Improving wear resistance of truck treads.

The major objective for the tire industry is to extend tire life for improving cost/performance and for reducing scrap waste, especially in truck tires. However, the safety of tires is important. Therefore, manufacturers of rubber chemicals continue to develop longer lasting antidegradants and stable crosslink accelerators and vulcanization agents to extend tire body life including carcass and wire belts. Tire durability requires heat resistance, antiflex fatigue property, antioxidation and antiozonant properties. Antidegradants improve heat aging, antioxidation, flex fatigue and antiozonant properties. A stable crosslink system, however, is important for flex fatigue and heat aged properties.

In order to produce a long lasting tire, tread wear is very important. The mechanism of tread wear may be illustrated by mechanical abrasion wear and chemical oxidation wear which occurs simultaneously during service. Several factors aid in the improvement of tread wear. These are introducing higher molecular weight polymers, polymer end modification to attach carbon black, reinforcement systems, lower glass transition polymers, liquid polymers to replace oil and a stable crosslink system.

In this article, two subjects will be discussed to improve wear resistance of truck tire tread. One approach is to develop a stable crosslink system to increase modulus without sacrificing elongation at break. Vulcanization may be defined as a reaction in the presence of heat where chemical additives (accelerators and vulcanizing agents) react with an elastomer to change it from a plastic, tacky solid to a thermoset (fixed solid with improved strength, elasticity and increased hardness).

Usually, sulfur is used as the vulcanizing agent for tire compounds. However, the sulfur may combine in many ways to form the giant crosslinked network of vulcanized rubber. As a crosslink, sulfur may be present as monosulfide, disulfide, polysulfide and cyclic sulfides. Stability of the vulcanized rubber depends upon type of crosslink. A monosulfide crosslink system provides a stable crosslink system to improve heat aging. However, flexing with monosulfide linkage is inferior to the polysulfide crosslink system. Therefore, the optimum combination level provides better heat and flex properties. These types of crosslinks can be controlled by using various types of accelerators. We will discuss tetrabenzylthiuram disulfide as a secondary accelerator to increase modulus and DIN abrasion resistance. Another approach is to replace oil with liquid polyisoprene to co-vulcanize with polymers and to increase compatibility. After vulcanization, oil can be extracted. However, polyisoprene is unable to extract from the cured rubber. We will discuss how to improve tread wear resistance and liquid polymer as a plasticizer.

Tread wear

Tread wear variables are due to tire construction/tread pattern and tread compounds. In a common tread compound, polymers, fillers (carbon black), the amount of oil, type of crosslink and crosslink density, etc. contribute to tread wear. The breaking force of tread compound is critical to wear. In order to increase the braking force, it is essential to have optimum crosslink density and to increase co-vulcanization/compatibility with liquid polymer to solid rubber by reducing/eliminating the plasticizer.

TBzTD

It was reported that tetrabenzylthiuram disulfide (TBzTD)/ sulfenamide accelerator systems offer a unique combination of characteristics which include excellent reversion resistance, efficient vulcanization with sufficient scorch delay, and increased nitrosamine safety (ref. 1). TBzTD has initially reacted with zinc oxide to generate sulfur for crosslinking, resulting in lower sulfur rank vulcanization. The formed lower sulfur crosslinks provide a thermally stable system and slightly higher modulus without sacrificing elongation at break with lower a level of TBzTD.

The proposed mechanism for TBzTD/TBBS accelerator systems was developed from the literature (figure 1). Layer, et al (ref. 2) illustrated the improved efficiency caused by the substitution of a portion of TBBS with similar or lower molar equivalents of thiuram accelerator. The overall improvement in properties offered by these systems is believed to be completed by a combination of vulcanization pathways. The mechanism depicts the speculated combinations. TBzTD can react with zinc oxide to form polysulfide moiety of the TBBS. The latter would theoretically double the efficiency of the TBZTD molecule by allowing more dithiocarbamate groups to act as sulfur donors. The similar tear strength and flex fatigue of sulfenamide systems versus lower thiuram/sulfenamide blends implies the formation of higher sulfur ranked crosslinks. The formation of high sulfur rank crosslinks is explained by the third pathway, conventional sulfenamide vulcanization.

Experiments

Experimental studies to evaluate combination of TBzTD with N-t-butyl-2-benzothiazole sulfenamide (TBBS) were taken in a model truck tread with natural rubber containing levels of TBzTD from 0 to 1.0 phr with 1 phr of TBBS. The first group of compounds contain 2.0 phr of tire sulfur, and the other group has reduced levels of sulfur from 2.0 to 1.0 phr for replacing the identical amount of sulfur with TBzTD. Masterbatches of each group were mixed in a laboratory 13 liter internal mixer (14 kg) to eliminate experimental errors in carbon black dispersion in polymer phase and levels of ingredient variation. The same masterbatch was used to mix with curatives for each group. The completely mixed compounds were cured to optimum physical properties, Mooney viscosity, Mooney scorch at 132[degrees]C, cure meter at 145[degrees]C and 160[degrees]C, and stress/strain properties were measured initially. Cured samples were aged at 100[degrees]C for three days and at 70[degrees]C for two weeks prior to testing physical properties. A DIN abrasion test was carried out to predict wear resistance cured at 145[degrees]C and 160[degrees]C, respectively. All cured samples were tested for crosslink density and Monsanto flex fatigue failure.

Results and discussion

Constant sulfur

A natural rubber compound which contains 55 phr of carbon black, 4 phr of zinc oxide, 2 phr of stearic acid, 7.5 phr of aromatic oil, 2.5 phr of 6PPD and 1 phr of blended wax was evaluated with TBBS/TBzTD and sulfur (table 1).

All individual stocks from A-1 through F-1 were prepared using the same masterbatch. No differences in Mooney viscosity were observed among the six batches. However, an excessive amount of TBzTD reduces scorch from 17.5 minutes to as low as 12.3 minutes. The maximum torque in cure meter was increased as the TBzTD was increased (table 1). The completely mixed compounds were cured to optimum properties and tested in accordance with ASTM standard test method for determining stress/strain, flex to fatigue failure and DIN abrasion. Also, viscoelastic properties, tangent delta, elastic modulus, loss modulus, complex modulus, elongation at break, tear Die C and trouser tear were significantly reduced with more than 0.5 phr of TBzTD. This is a clear indication that TBzTD donates sulfur for monosulfide crosslink which may lead to tight cure. Slight increase in 300% modulus and Shore A hardness were observed in table 2.

A DIN abrasion test was carried out by curing samples at 14[degrees]C and 160[degrees]C, respectively. TBzTD at 0.25 phr provided the best wear resistance, shown in figure 2. This result explains that the combination of monosulfide with polysulfide crosslink systems provides not only better aged physical properties, but also better wear resistance. Monsanto flex fatigue was measured by curing samples at 160[degrees]C. The conventional cure system with TBBS/sulfur provides the best flex fatigue properties as predicted. The polysulfide crosslink system has better flex fatigue properties compared to the monosulfide crosslink system (figure 3). Also, tangent delta and loss modulus with 0.25 phr of TBzTD have the lowest value for lower heat build-up (figures 4 and 5). Crosslink density measurements were made on samples cured at 160[degrees]C. The crosslink density is directly proportional to the amount of TBzTD, which is shown in figure 6.

Replacement of sulfur with TBzTD

We previously evaluated TBzTD with 2 phr of sulfur. Crosslink density measurements and physical properties, especially elongation at break, indicated that TBzTD is not only a secondary accelerator, but also a vulcanizing agent (lower sulfur crosslink). Therefore, it is essential to replace the amount of sulfur with TBzTD. Experimental studies were done to evaluate TBzTD at 0 to 1.0 phr to replace the same amount of sulfur. The NR model recipes are shown in table 3. Because stocks were prepared with the same masterbatch, we do not believe that the variation due to experimental error should be minimized. Slight reduction in Mooney scorch was measured with more than 0.5 phr of TBzTD. However, similar torque values with A to F-2 stocks were obtained at 145[degrees]C and 160[degrees]C, respectively, shown in table 3. Physical properties were tested in accordance with ASTM test method D-412 for determining stress/strain, die C tear, trouser tear and Shore A hardness. All samples were cured at 145[degrees]C and 160[degrees]C, using optimum cure time. No significant variations between the stocks were measured. Aged results also indicated that all the variations were within experimental error ranges. Significant reduction in DIN abrasion with 0.25/1.75 TBzTD/sulfur was attained, cured at 145[degrees]C. However, the samples, which were cured at 160[degrees]C, clearly indicate that the combination of TBzTD with TBBS is much better in DIN abrasion resistance than the TBBS system (figure 7). This result illustrates that the type of crosslink and crosslink density provides better wear resistance (approximately 12%). Unaged Monsanto flex fatigue to failure with TBBS cure is superior to TBBS/TBzTD cure system. However, aged flex fatigue with TBBS/TBzTD is much better than the conventional cure system with TBBS (figure 8). Cured at 160[degrees]C, 0.25/1.75 TBzTD/sulfur ratio, the viscoelastic property measurement gives the lowest tangent delta and more elastic property (figures 9 and 10). The crosslink density was highest for samples cured at 160[degrees]C, 0.5/1.5 TBzTD/sulfur. The optimum wear resistance would be achieved between approximately 1.9-2.0 x [10.sup.4] crosslink density. Also, the combination of monosulfide and polysulfide crosslink system is essential to achieve stable crosslink and better wear resistance.

Liquid polyisoprene

Liquid polyisoprene has been used for tire compounds to improve processing reinforcement with vulcanization and high hardness compounds. There are, however, several other liquid polymers compatible with tire compounds which contain natural rubber. There are two mechanisms to extend tread life by using liquid isoprene instead of oil. Most major truck tread manufacturers have tried to eliminate/reduce oil to improve tread wear. Higher modulus would also give better tread wear resistance. However, without oil in the tread compound, it is very difficult to mix and extrude tread compounds. Therefore, in order to obtain higher modulus, liquid polyisoprene can be used as a processing plasticizer and reactive plasticizer in truck tread compounds. Another mechanism to improve tread wear is to vulcanize with natural rubber for better reinforcement. Oil can be extracted from the cured rubber, but liquid isoprene cannot be extracted. Therefore, overall tread performance can be improved with liquid polyisoprene.

Experiments

The best cure system, which has been illustrated in the previous TBzTD section, has been employed to evaluate liquid polyisoprene in a natural rubber truck tread. The NR truck model recipe contains 7.5 phr of aromatic oil. One of the compounds was replaced with 5% of oil and the other has replaced oil with liquid polyisoprene (table 4). The mixed compounds were measured for Mooney viscosity at 100[degrees]C, Mooney scorch at 132[degrees]C, cure meter at 145[degrees]C and 160[degrees]C. All the stocks were cured to the optimum physical properties at 160[degrees]C. Stress/strain, die C tear, Shore A hardness, Monsanto flex to fatigue, DIN abrasion and viscoelastic properties were tested (table 4).

Results and discussion

There were no significant differences in Mooney viscosity for all three stocks. However, a slight reduction in Mooney scorch was observed with liquid polyisoprene. As expected, a slight increase in the maximum torque was attained. No differences in tensile, modulus and Shore A hardness were observed. However, a slight increase in elongation with liquid polyisoprene was obtained (table 4). A DIN abrasion test was carried out to predict wear resistance and realized an approximate 10% improvement in wear resistance. Monsanto flex to fatigue values indicated a slight improvement with liquid isoprene. The variation of this test is so large, however, that these differences are insignificant. Also, a slight reduction in tangent delta with liquid polyisoprene was achieved.

Conclusions

The optimum cure system which contains monosulfide and polysulfide crosslink systems provides approximately 14% better wear resistance than the conventional cure system in natural rubber truck tread.

The replacement of oil with liquid polyisoprene provided a 10% reduction in DIN abrasion.

All the above results are based on the DIN abrasion test. Tire testing requires the correlation of laboratory test with actual fleet testing.

Using TBzTD truck tread, cure cycle can be reduced without sacrificing scorch safety below a 0.25 phr level.

References

[1.] Ferrandino, M.A., Sanders, JA. and Hong, S. W., "Tetrabenzylthiuram disulfide: A secondary accelerator for stable crosslink systems in tire applications," presented at a meeting of the Rubber Division, ACS, Philadelphia, PA, May 2-5, 1995.

[2.] Layer, R.W., "A study of thiocarbamyl sulfide/zinc oxide and TMTD/zinc oxide sulfurless cure systems," Rubber Chemistry and Technology 36, 1993, p. 513.

[3.] Hong, S. W., "Polymer blends for improved tire tread performance," presented at the 36th IUPAC International Symposium on Macromolecules, Seoul, Korea, August 4-9, 1996.

[4.] Hong, S. W., "Predicting tire performance using dynamic viscoelastic properties," presented at the International Rubber Conference, Moscow, Russia, September 27-October 1, 1994.