Reinforcement of Rubber through Short Individual Filaments

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Abstract
Short, individual filaments of seven organic fibers, along with glass and wire, were mixed into several types of rubber stock, mainly on a rubber mill. Both the glass and the wire filaments broke into fragments. A high degree of orientation was achieved with most organic fibers. The presence of these filaments caused, in the direction of orientation, increased green strength (up to 37×) and exceptionally high stress at low elongations (e.g. 5.8 MPa at 100% with only 5 phr fiber). This was accompanied by moderate reduction in uncured yield elongation and by substantial decrease in cured elongation at break. Hardness, both uncured and cured, increased moderately, while heat buildup rose considerably in the presence of fibers. Penetration resistance, measured by a new laboratory test, increased two- to fourfold. The magnitude of these effects was directly proportional to the amount of well-dispersed and oriented fiber. No adhesive coating or special treatment was needed at the aspect ratios studied (> 180) to achieve tensile reinforcement with the organic fibers. The use of an adhesive dip on nylon actually reduced the level of reinforcement. The effects of staple length were insignificant on cured modulus, elongation at break, heat buildup, or penetration resistance; minor on green strength; moderate on Mooney viscosity. The most promising reinforcing filaments were the aramids, nylon, poly(vinyl alcohol), and polyester with aspect ratios between 180 and 3100.

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