Blow molding behaviour of wood fibre-reinforced polypropylene composites

Blow molding behaviour of wood fibre reinforced polypropylene composites was studied in relation to wood species, processing temperature, wood moisture content and polypropylene grade. Wood flour of varying mesh sizes (-60 to -140) with and without special additives were compounded with different grades of polypropylene in a twin screw extruder. Oven dried and undried samples were then blow molded at temperatures of 176°C, 190°C, and 204°C and behaviour was observed. Wood flour altered the behaviour of the parison; there was increased die and parison swell. Higher molding temperatures caused increased production of gases and volatiles and caused tearing and rupturing of the parison. The presence of woodflour moisture at any temperature caused bubbling and micropores to form. Low melt strength polypropylene composites were unsuitable for blow molding. Optimal parison formation occurred with oven dried pellets, processed at low temperatures with a high melt strength polypropylene compounded with fine hardwood flour.