Friction of nonwoven wood-polypropylene fiber mats on heated steel platens

Pultrusion is a composites manufacturing process where a polymer-impregnated fiber mat or roving is pulled through a stationary die. The manufacture of wood-thermoplastic composites utilizing pultrusion requires knowledge of the composites' frictional properties. Many processing variables (e.g., normal stress, die temperature, sliding velocity, and moisture content) influence the frictional properties of synthetic and natural polymers. The coefficient of dynamic friction (μ_k) was determined for the contact of wood fiber-polypropylene (PP) composites against a smooth heated steel surface. The μ_k was found at varying PP fiber percentages, applied normal stresses, platen temperatures, and dwell times. A decrease in μ_k was observed with increasing applied normal stress and dwell time, with normal stress having the greatest influence on frictional properties. An increase in μ_k was observed with increasing PP fiber content and surface temperature above the melt temperature (T_m) of PP. Assuming an adhesion-controlled friction mechanism, the contact area and interfacial shear stress played an important role in property development.