The reinforcement of rubber compounds with short fibers has, at times, become necessary in many product applications. Particularly compounds with relatively low fiber content have proven successful in improving hose and belt performance. This is mostly due to an increase in composite stiffness without a great sacrifice of basic processability characteristics of the compound. Too large a fiber content becomes a primary source of difficulties during manufacture and/or product performance. Therefore, an understanding of how various composite properties depend upon fiber and matrix properties, as well as on fabrication methods, will help design better products. The mechanical properties, such as modulus, strength, and ultimate elongation depend upon fiber orientation, aspect ratio, and adhesion between fiber and matrix compound. Unfortunately, the degree and type of adhesion cannot be estimated quantitatively at present even though its importance in the improvement of composite properties is well recognized. Aspect ratio is another parameter which can be used in improving composite properties. As a rule, a higher aspect ratio gives higher composite stiffness. During processing, fibers are buckled and crimped under large deformations, which results in a distribution of fiber lengths, rather than a constant length as before mixing, as shown, for example, in Figure 1. One can, therefore, expect to achieve the same composite properties regardless of the initial fiber length (up to, say, 15 mm) or fiber length distribution. Of the three parameters, fiber orientation affects composite properties the most. During processing (milling, extrusion, etc.) of rubber composites, the fibers tend to orient along the flow direction, causing mechanical properties to vary in different directions. Therefore, by changing or suitably controlling the flow direction, optimum properties can be generated for a given product. A good example is the balanced fiber orientation in a hose which gives optimum design strength. Milling or calendering is perhaps the most commonly used processing method in which fibers tend to orient along the mill direction. Since each mill or calender differs from any other in size, roll speed, and other characteristics, it is essential to determine the influence of these parameters on composite properties. Results of a systematic study to identify significant mill parameters which influence the composite properties are presented here.
