Extrusion foaming of polyolefin/wood-flour composites

The chemical reactions between cellulosic materials (wood flour and cotton cellulose) and functionalized polyethylene coupling agents (acrylic acid-functionalized and maleic anhydride-functionalized) were studied. The experimental results indicate that chemical bonds between hydroxyl groups of cellulosic materials and functional groups of the coupling agents have occurred through esterification reactions.; Although the tensile and flexural strength properties of polyolefin/wood-flour composites were enhanced with the addition of coupling agents, the impact resistance of the composites did not improve and the high density and brittleness of these composites were not reduced. Extrusion foaming technology was applied to address some of these problems.; The polyolefin/wood-flour composite foams were developed through a continuous extrusion process using factorial and central composite designs. The effects of material compositions and processing parameters including wood flour content, wood flour moisture content, chemical foaming agent (CFA) concentration, CFA form (with vs. without polymer carrier), CFA type (endothermic vs. exothermic), extruder die temperature, and extruder screw speed, together with the effect of coupling agent on the void fraction and average cell size of foamed polyolefin/wood-flour composites were examined. The experimental results indicate that a large amount of gas molecules available for cell growth is not the only requirement for the production of PP/wood-flour composite foams with a high void fraction. A gas containment limit was observed for neat HDPE foams, whereas the average cell size achieved in the HDPE/wood-flour composite foams remained insensitive to the CFA content, irrespective of the CFA type. The experimental results also indicate that the use of coupling agent in the formulation is required to achieve HDPE/wood-flour composite foams with a high void fraction. When the wood flour contains 12% moisture, HDPE/wood-flour composite foams with the highest void fraction can be achieved at high extruder screw speed (120 rpm) and low extruder die temperature (170°C) without the use of chemical foaming agent.; Finally, the effect of void fraction on the mechanical properties of HDPE and HDPE/wood-flour composite foams was examined. (Abstract shortened by UMI.)...