Water uptake and mechanical characteristics of wood fiber-polypropylene composites

The influence of mixing process (in a two-roll mill, high-speed mixer, or twin-screw extruder) oil the strength properties of polypropylene/wood fiber composites was studied. The best results were obtained for composites compounded in a twin-screw extruder. The water uptake and the influence of moisture on the flexural strength (sigma(fl)) and modulus (E-fl) were studied by immersion of the composites in water at 20, 50, and 90 degrees C. Most strongly the moisture affected the value of E-fl, but the degree of water uptake and the change in sigma(fl) and E-fl also depended on temperature and the presence of a modificator - maleated polypropylene (MAH). MAH improved the strength properties of the composites both in dry and wet states and also decreased the extent of water uptake and swelling in cyclic (soaking/drying) tests.