Wood flour filled PP composites:: adhesion, deformation, failure

Poly(propylene) (PP) wood composites were prepared in a wide composition range from 0 to 70 wt% wood content. Matrix/wood adhesion was improved by the introduction of two maleinated polypropylenes (MAPPs) with different molecular weights and functionality. MAPP/wood ratio changed from 0 to 0.25 in 0.05 steps. Mechanical properties of the composites were characterized by tensile testing, while their fracture resistance was determined with instrumented impact measurements. Micromechanical deformation processes were followed by acoustic emission and volume strain measurements, which were supported by scanning electron microscopy done on the broken surface of fractured samples. The results show that stiffness increases with wood content and it does not depend very much either on the type or the amount of the functionalized polymer used. However, ultimate tensile properties are strongly influenced by the amount and properties of MAPP; larger molecular weight and smaller functionality are more advantageous both for strength and impact resistance. The optimum MAPP/wood ratio was found to be around 0.05 in accordance with some literature data. Because of their large size, wood particles debond very easily from the matrix leading to volume increase and catastrophic failure at small deformations. When adhesion is improved by the introduction of MAPP, large wood particles fracture thus also contributing to the failure of the composite. At large wood content considerable aggregation of the particles may take place leading to inferior strength. Copyright (c) 2006 John Wiley & Sons, Ltd.