Even though fibrous wood filler is known to be effective for improving mechanical and water-resistant properties of wood plastic composite (WPC), it has not been commercially adopted because of its high cost and less handling ability. In the present study, with an expectation of similar positive effect of fibrous wood filler, we carried out a fibrillation of a surface of practical wood flour in pre-mixed compound, containing wood flour, polypropylene, and compatibilizer, for producing WPC with high filler content by means of wet disk-milling. Scanning electron micrography revealed a micro-fibrillation on the surface of wood flour in the ground pre-mixed compound. The size of the surface micro-fiber could be controlled by disk-milling conditions. The formation of the micro-fibrous structure affected the melt viscosity of the compounds and improved mechanical properties of molded samples. The positive effect of the surface micro-fiber formation can be attributed to an interaction among the micro-fibers such as hydrogen bonding and entanglement. The observed reinforcement effect by the surface fibrillation was essentially identical to that of wood fiber although fibrillation was attained just in the surface of wood flour.
