Tread wear resistance

Tread wear resistance, abrasion resistance, traction, speed stabihty, protection to casing, rolling resistance, ice skid resistance, durability... 

In the retreading market, new silicas are expected to improve properties of retreaded tires which are currently inferior to new tires.Tread wear resistance and rolling resistance arc substantially lower with retreaded tires. 

A balanced combination of properties is the criterion of performance. In tread-wear resistance, cold butadienerstyrene tires are approximately 20-30 per cent superior to natural-rubber or hot rubber treads. However, in tire carcass or sidewalls, natural rubber exhibits superior performance, especially in truck tires, because of lowfeV heat build-up. 

There is improved abrasion resistance associated with a preferential carbon black-BR phase distribution in blends of NR-BR and SBR-BR. The first abrasion studies on the effects of carbon black phase distribution in NR-BR blends were reported by Krakowski and Tinker (1990a,b). Tread wear resistance was found to increase progressively with increasing carbon black in the BR phase, which was determined from TEM analyses. Tse et al. (1998) have shown that blends of dispersed BIMS in BR matrix failure due to fatigue can be retarded if the mean distance between the crosslinks of the BIMS is less than 60 nm. 

During the late 1940s and early 1950s interest was shown in copolymers of butadiene and methyl isopropenyl ketone, CH =C(CH3)C0CH3. The rubber was claimed to be superior to the then available butadiene-styrene rubbers in stress-strain properties, cut-growth resistance, tyre tread wear resistance and swelling in hydrocarbons. On the other hand low temperature properties and processability were said to be poorer. 

Processings and Properties. Polybutadiene is compounded similarly to SBR and vulcanised with sulfur. The high cis-1,4 type crystallizes poorly on stretching so it is not suitable as a "gum" stock but requires carbon black reinforcement. It is generally used for automotive tires in mixtures with SBR and natural mbber. Its low T (—OS " C) makes it an excellent choice for low temperature tire traction, and also leads to a high resilience (better than natural mbber) which ia turn results ia a lower heat build-up. Furthermore, the high i j -polybutadiene also has a high abrasion resistance, a plus for better tire tread wear. 

FIGURE 26.61 Log (abrasion) of an OESBR and a natural rubber (NR) tire tread compound as function of load at different slip angles at a speed of 19.2 km/h. left Abrasion loss of the OESBR compound as function of load. Right the relative wear resistance rating of natural rubber (NR) to the OESBR as function of load for different slip angles. 

The NR compound 4 is known to be better under low temperature conditions than the control, but worse under high temperature conditions. Chemically, NR has the lowest thermal stability of the polymers used for tread compounds in tire technology and it has therefore the highest temperature dependence of abrasion and wear. Thus, it is generally accepted that NR has a higher wear resistance in a moderate climate than, for instance, SBR but a much lower one in hot climates. This will be thoroughly documented below under tire wear. 

Butadiene can form three repeat units as described in structure 5.47 1,2 cw-1,4 and trans-, A. Commercial polybutadiene is mainly composed of, A-cis isomer and known as butadiene rubber (BR). In general, butadiene is polymerized using stereoregulating catalysts. The composition of the resulting polybutadiene is quite dependent on the nature of the catalyst such that almost total trans-, A, cis-, A, or 1,2 units can be formed as well as almost any combination of these units. The most important single application of polybutadiene polymers is its use in automotive tires where over 10 t are used yearly in the U.S. manufacture of automobile tires. BR is usually blended with NR or SBR to improve tire tread performance, particularly wear resistance. 

Wear Resistance in Tire Tread Stocks and The Effects of Compound Loading Variants on Wear Resistance," Preprint of Paper A-19 Presented to Inti. Rubber Conf., Prague, Czechoslovakia, Sept. 17-20, 1973. 

Blends of elastomers are routinely used to improve processability of unvulcanized rubbers and mechanical properties of vulcanizates like automobile tires. Thus, cis-1,4-polybutdiene improves the wear resistance of natural rubber or SBR tire treads. Such blends consist of micron-sized domains. Blending is facilitated if the elastomers have similar solubility parameters and viscosities. If the vulcanizing formulation cures all components at about the same rate the cross-linked networks will be interpenetrated. Many phenolic-based adhesives are blends with other polymers. The phenolic resins grow in molecular weight and cross-link, and may react with the other polymers if these have the appropriate functionalities. As a result, the cured adhesive is likely to contain interpenetrating networks. 

The service performance of rubber products can be improved by the addition of fine particle size carbon blacks or silicas. The most important effects are improvements in wear resistance of tire treads and in sidewall resistance to tearing and fatigue cracking. This reinforcement varies with the particle size, surface nature, state of agglomeration and amount of the reinforcing agent and the nature of the elastomer. Carbon blacks normally are effective only with hydrocarbon rubbers. It seems likely that the reinforcement phenomenon relies on the physical adsorption of polymer chains on the solid surface and the ability of the elastomer molecules to slip over the filler surface without actual desorption or creation of voids. 

Styrene-butadiene rubber is the largest volume synthetic elastomer commercially available. It ean be produced by free-radical emulsion polymerization of styrene and butadiene either at 50 to 60°C (hot emulsion SBR) or at about 5°C (cold emulsion SBR). The two kinds of SBR have sigmfieantly different properties. The hot emulsion SBR process, which was developed st, leads to a more branehed polymer than the cold emulsion process. Cold SBR has a better abrasion resistance and, eonsequently, provides better tread wear and dynamic properties. 

When considering only solution polymers, polymer microstructure has a greater effect on tire tread compound performance. Table 9.11 illustrates the impact on tire traction, rolling resistance, and tread wear of a polybutadiene tread on which the vinyl-1,2-butadiene level had been increased from 10% to 50% (Brantley and Day, 1986). The corresponding drop in wear and increase in tire rolling resistance are in agreement with the empirical rules presented by Nordsiek (1985), who attributed such tire property trends to the polymer Tg. 

Tread The wear resistance component of the tire in contact with the road. It must also provide traction, wet skid, and good cornering characteristics with minimum noise generation and low heat buildup. Tread components can consist of blends of natural rubber, polybutadiene (BR), and styrene-butadiene rubber (SBR), compounded with carbon black, silica, oils, and vulcanizing chemicals. 

PROPERTIES OF SPECIAL INTEREST Standard emulsion SBR is a general purpose rubber. Most widely used synthetic rubber in the world. Better tire tread-wear and aging properties than natural rubber. Good abrasion resistance and crack initiation resistance. Poor in tack and heat build-up. Physical properties are poor without reinforcing fillers. Solution SBR is a speciality rubber and more expensive than emulsion SBR. Solution SBR with high vinyl and styrene levels is used in high performance tire treads to improve wet traction. Also used as impact modifier in plastics and as thermoplastic elastomers. 

This allowed the tan d temperature curve of a tread compound run from -100 to -i-100°C to be segmented into zones which would characterize that tire tread compound s performance (Table IX). Such property targets enabled development of the concept of integral rubber i.e., a polymer can be designed to meet rolling resistance, traction, and tread wear targets without a drop in overall tire performance. 

Brantley and Day then conducted a study to compare the tire performance of emulsion- and solution-polymerized SBR [12]. The authors noted that solution-polymerized polymers, which tend to have a narrower molecular weight distribution and lower Tg than equivalent emulsion-polymerized polymers, have lower hysteretic properties. Tliey then showed that a solution SBR with the same bound styrene as an emulsion SBR will give lower rolling resistance, improved dry traction, and better tread wear. Emulsion SBR, however, tends to show better wet skid, wet traction, and wet handling performance. Kern and Futamura later elaborated on this work by evaluating the impact of... 

An example of how polymer microstructure and polymer Tg impact performance is when vinyl-butadiene is increased from 10 to 50% in polybutadiene (Table 4.5) [8]. The glass transition temperature increases from —90 to —60 °C, with a corresponding shift in the tan-delta curve. Traction performance has improved significantly, but tread wear and rolling resistance rating drop. 

Control of this phase morphology can become very important so as to prevent any deterioration in the tire tread wear performance, rolling resistance or traction and handling qualities, or loss in component to component adhesion. 

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