Rubber wear resistance

Carbon blacks are the most widely used fillers for elastomers, especially vulcanised natural rubber. They cause an improvement in stiffness, they increase the tensile strength, and they can also enhance the wear resistance. Other particulate fillers of an inorganic nature, such as metal oxides, carbonates, and silicates, generally do not prove to be nearly so effective as carbon black. This filler, which comes in various grades, is prepared by heat treatment of some sort of organic material, and comes in very small particle sizes, i.e. from 15 to 100 nm. These particles retain some chemical reactivity, and function in part by chemical reaction with the rubber molecules. They thus contribute to the crosslinking of the final material. 

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 wear resistance of rubber compounds is of great practical importance for tires, but the mechanisms involved in tire wear are multiple and may vary depending on the vehicle, the driving conditions, and other extraneous variables. Many attempts have been made to develop laboratory test methods to simulate tire wear under various conditions. Whilst none can fully replicate road wear, various laboratory abrasion tests can be used to provide an indicator of wear resistance of tires under certain conditions. 

It has been well established that wear resistance of filled rubber is essentially determined by filler loading, filler morphology, and polymer-filler interaction. For fillers having similar morphologies, an increase in polymer-filler interaction, either through enhancement of physical adsorption of polymer chains on the filler surface, or via creation of chemical linkages between filler and polymer, is crucial to the enhancement of wear resistance. In addition, filler dispersion is also essential as it is directly related to the contact area of polymer with filler, hence polymer-filler interaction. 

The characteristic property of elastomers is their rubber-elastic behavior. Their softening temperature lies below room temperature. In the unvulcanized state, i.e. without crosslinking of the molecular chains, elastomers are plastic and thermo-formable, but in the vulcanized state—within a certain temperature range — they deform elastically. Vulcanization converts natural rubber into the elastic state. A large number of synthetic rubber types and elastomers are known and available on the market. They have a number of specially improved properties over crude rubber, some of them having substantially improved elasticity, heat, low-temperature, weathering and oxidation resistance, wear resistance, resistance to different chemicals, oils etc. 

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. 

The part now played in chemical industry by sulphur and its wide variety of compounds scarcely needs emphasis. The astonishing increase in the production of sulphur and sulphuric acid during the last two decades, made possible by the discovery of the vast deposits of sulphur in Louisiana, is itself indicative of widespread applications. Many of the compounds of sulphur are notable for possessing valuable properties, and their use extends into almost every branch of- modern industry their chemical investigation has contributed much to the advancement of science and to the amenities of domestic life. Probably in no direction has sulphur more widely and directly influenced the general public than in connection with methods of transport, which have been revolutionised by the introduction of sulphur-vulcanised rubber as a wear-resisting, shock-absorbing material for the equipment of wheels of motor vehicles. 

Rubber wear products for the mineral processing, fertilizer and materials handling industries, such as scrubber linings, rubber screening panels, wear resistant sheets, etc. are well recognized and eminently suitable materials of construction. 

Mining and Ore processing Abrasion resistant linings for hoppers, chutes, cyclones, slurry pipelines, separators etc. Also bolt-in rubber wear plates for chutes and hoppers and conveyor rollers. Soft natural rubber for abrasion resistance. Hardness range 40 to 70°A... 

Marine Industry Corrosion resistant linings for fume scrubbers and cooling pipelines. Wear resistant linings for dredging pipelines. Ebonite or hard rubber for corrosion resistance. Soft natural rubber for wear resistance. 

Power Generation Corrosion resistant linings for water treatment plant and cooling pipelines. Wear- resistant linings for coal handling plant, pulley rubber lagging for conveyor rollers etc. Soft natural rubbers and other synthetic rubbers... 

In other areas, POD has been used to improve the wear resistance of a rubber latex binder by incorporation of 25% of Oksalon fibers. Heat-resistant laminate films, made by coating a polyester film with POD, have been used as electrical insulators and show good resistance to abrasion and are capable of 126% elongation. In some instances, thin sheets of PODs have been used as mold release agents. For this application a resin is placed between the two sheets of POD, which is then pressed in a mold, and the sheets simply peel off from the object and mold after the resin has cured. POD-based membranes exhibit salt rejection properties and hence find potential as reverse osmosis membranes in the purification of seawater. PODs have also been used in the manufacturing of electrophotographic plates as binders between the toner and plate. These improved binders produce sharper images than were possible before. 

Wear resistance and tensile strength of rubbers and polymers are increased by the introduction of silica in the organic network. In order to preserve these improved qualities in wet conditions, however, the silica surface has to be modified to assure interfacial bonding.32... 

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. 

Elastomers require, in most applications, to be reinforced by fillers in order to improve their mechanical properties. Carbon black and silica have been used for a long time in the rubber industry to prepare composites with greatly improved properties such as strength, stiffness and wear resistance. These conventional fillers must be used at high loading levels to impart to the material the desired properties (1). The state of filler dispersion and orientation... 

Other than the wear problems, actual in-plant maintenance usually involves removal of wood, pieces of blasting wire, and other trash from the ports. When a reagentized feed is used, layers of oily reagents can build up on the spiral surface and sometimes require scrubbing for removal. With feeds containing oily reagents that attack rubber, abrasion-resistant alloy spiral sections are used. 

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. 

Polymers, as well as elastomers, are reinforced by the addition of small filler particles. The performance of rubber compounds (e.g. strength, wear resistance, energy loss, and resilience) can be improved by loading the rubber with particulate fillers. Among the important characteristics of the fillers, several aspects can be successfully interrogated by AFM approaches. For instance, the particle and aggregate size, the morphology, and in some cases the surface characteristics of the filler can be assessed. 

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