SynopsisRates of wear have been determined for several elastomer materials, using a razor-blade abrading apparatus based on one described by Champ, Southern, and Thomas. Measurements have been carried out a t different levels of frictional power input, corresponding to different severities of wear, a t both ambient temperature and a t lOO"C, and both in air and in an inert atmosphere. It is concluded that wear occurs as a result of two processes: local mechanical rupture (tearing) and general decomposition of the molecular network to a low-molecular-weight material (smearing). Marked differences were shown by different elastomers. Carbon-black-filled natural rubber, SBR (styrene-butadiene copolymer) and EPR (ethylene-propylene copolymer) were particularly susceptible to decomposition and smearing, but for natural rubber and SBR the decomposition process was not observed in an inert atmosphere. It is attributed to molecular rupture under frictional forces followed by stabilization of the newly formed polymeric radicals by reaction with oxygen, if present, or with other polymer molecules, or with other macroradicals. Cis-polybutadiene and trans-polypentenamer did not appear to undergo smearing to a significant degree. The reactive radicals formed in these materials by molecular rupture are assumed to undergo rapid addition to other molecules so that the network structure is maintained. Rates of wear have been found to increase with the applied frictional force raised to a power n. The value of n was between 2.5 and 3.5 for unfilled materials a t ambient temperature, in agreement with Champ, Southern, and Thomas, who pointed out a general correlation with mechanical fatigue. Filled materials were found to be less sensitive to the frictional force, whether wear took place by tearing or by smearing, having values of the index n of 1.5-1.8. Several observations suggest that wear, even in the absence of smearing, is not fully correlated with mechanical fatigue: the markedly lower wear rates for carbon-black-filled materials, the anomalous rankings of unfilled materials, and the relatively small effects of raising the test temperature to 100°C. It is concluded that abrasive wear by small-scale tearing is not accounted for solely by the crack growth properties of the material but involves other failure processes as well.

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elastomeric materials at a typical spacing of 10-100 uim. In carbon-blackfilled elastomers, the carbon particles themselves are found to nucleate secondary cracks profusely. Accession For NTIS CRA&I LTIC T.B Unannounomcd Justiffication-Distribution/ Avaiibi.ity Codes-%4.

Measurements have been made of the tear strength of several elastomeric materials under threshold conditions, e.g., in the swollen state and at high temperatures when dissipative contributions to the work of fracture are minimized. The materials studied were Neoprene WRT, BR, SBR, EPDM, and a castable polysulfide material, LP-32. They were crosslinked to different degrees, using a free-radical source, or sulfur, or a metal oxide as the crosslinking reagent. Values obtained for the threshold tear strength were similar in all cases, lying in the range 50–200 J/m2. They appeared to be approximately proportional to E−1/2, where E is the tensile (Young's) modulus of the elastomer, as predicted by the theoretical treatment of Lake and Thomas. Values for networks with polysulfidic crosslinks were significantly higher than with monosulfide or C—C crosslinks, by about a factor of 2. Values for carbon black-filled materials were higher also, by about the same factor, in comparison with the equivalent unfilled materials. These effects are attributed to rupture of labile crosslinks, or bonds to carbon black, prior to main-chain rupture.

SynopsisA blade abrasion device similar to that developed by Champ, Southern, and Thomas has been used to determine the wear rates of everal simple carbon black-filled natural rubber materials over a range of frictional loading at room temperature in air. The materials show two mechanisms of wear depending on the amount of frictional load applied. Smearing, the mechanochemical decomposition of the wearing surface leading to a tarlike wear product, occurs a t low frictional loads and is replaced by particulate debris created by cohesive rupture a t higher frictional loading. It was found that a representative alkyl/aryl paraphenylenediamine and hindered phenol antioxidant were only effective in reducing the rate of wear during smearing, when degradation processes are important. Furthermore, the reduction in wear rate during smearing was quite similar for these two antioxidants.