Natural rubber tread compounds

The pneumatic tire has the geometry of a thin-wallcd toroidal shell. It consists of as many as fifty different materials, including natural rubber and a variety ot synthetic elastomers, plus carbon black of various types, tire cord, bead wire, and many chemical compounding ingredients, such as sulfur and zinc oxide. These constituent materials are combined in different proportions to form the key components of the composite tire structure. The compliant tread of a passenger car tire, for example, provides road grip the sidewall protects the internal cords from curb abrasion in turn, the cords, prestressed by inflation pressure, reinforce the rubber matrix and carry the majority of applied loads finally, the two circumferential bundles of bead wire anchor the pressnrized torus securely to the rim of the wheel. 

FIGURE 26.55 Abrasion surface appearance of a natural rubber (NR) black-fiUed tire tread compound for sliding abrasion at different temperatures. 

FIGURE 26.58 Abrasion of two natural rubber (NR) tread compounds, one unprotected, the other with two parts Nonox ZA on a knurled Aluminum and a knurled steel surface, respectively, in the presence of (a) magnesium oxide (MgO) powder and (b) a dust mix of Fuller s earth and alumina powder. (Deduced from Schallamach, A.,Appl. Pol. Sci., 12, 281, 1968.)... 

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. 

FIGURE 26.64 Log (abrasion) for two tread compounds natural rubber (NR) + black and styrene-butadiene rubber (SBR) + black on two surfaces of different sharpness Alumina 60 and Alumina 180 blunt as function of log (energy dissipation). (From Grosch, K.A. and Heinz, M., Proc. IRC 2000, Helsinki, 2000, paper 48.)... 

FIGURE 26.74 Abrasion of a natural rubber (NR) and a styrene-butadiene rubber (SBR) tread compound as function of tbe tire surface temperature brought about by ambient temperature changes at a constant slip angle measured with the MRPRA test trailer. 

S. Bandyopadhyay, S. Dasgupta, N. Mandal, S.L. Agrawal, S.K. Mandot, R. Mukhopadhyay, A.S. Deuri, and S.C. Ameta, Use of recycled tire material in natural rubber based tire tread cap compound Part I (with ground cmmh rubber). Progress in Rubber, Plastics and Recycling Technology, 21(4), 299, 2005. 

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. 

Sidewall Protects the casing from side scuffing, controls vehicle-tire ride characteristics, and assists in tread support. Sidewall compounds consist of natural rubber, SBR, and BR along with carbon black and a series of oils and organic chemicals. 

The increase in natural rubber usage translates into approximately 21kg per tire for a radial construction compared with approximately 9 kg found in a bias truck tire. Natural rubber compounds also tend to find use in covers of high-performance conveyor belts where a similar set of performance parameters such as those of a truck tire tread compound are found. Low hysteretic properties, high tensile strength, and good abrasion resistance are required for both products. 

The elastomers and elastomer blends used in tire compounds, and particularly tread compounds, are thus equally important to the structural design parameters of the tire (Table 4.7). A properly designed tread compound will ensure the tire can meet its performance targets. Tread compounding materials fall into one of five general categories polymers, fillers, protectants, vulcanization system, and various special purpose additives. Elastomers for tread compounds are typically natural rubber (NR), styrene-butadiene rubber (SBR), polybutadiene (BR), and in some instances isobutylene based polymers for winter and special performance tires. 

Polybutadiene/natural rubber blends tend to find application in truck tire tread compounds and side valls. Polybutadiene is also blended vith SBR in treads. Here blend ratios tend to be optimized for factors such as vear resistance, traction requirements, rolling resistance, and damage resistance. Empirical guidelines for determining blend ratios are based on the vork of Nordsiek and co vorkers revie ved earlier [4]. Briefiy... 

Tread compounds can be based on either a single polymer or a blend of polymers. Natural rubber, emulsion SBR, solution SBR, BR, and isobutylene-based elastomers are used in such blends. The following tables illustrate tread formulations designed for specific applications ... 

Figure 3. Stress-strain response of "Green" tire tread compounds. Samples compounded with HAF black and tested at 5.0 cm/min. using ASTM D specimens. Samples are (1) Natural Rubber 1RSS, (2) IS-I-IS block copolymer with random copolymer end-blocks 60% styrene, (3) SBR (23% styrene) (4) lithium polyisoprene rubber (5) high-cis Ziegler polyisoprene rubber.

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