Wood Plastic Composite Processing
Wood fibres and waste plastic alone would not make a particularly useful composite. Additives include lubricants, fire retardants, coupling agents to increase the bonding between plastic matrix and fibre, UV stabilisers and pigments. The exact specification of the additives is proprietary, as are other elements of the production process, such as coating. Lignin, for example, can be used as a bonding agent for wood fiber-filled polystyrene composites (Maldas et al, 1994).
Compounding and Extrusion
The raw materials are mixed either in batches or by a continuous process, and the mixture forced through a die (sheet or profile extrusion), injection moulded or compression moulded. Most WPCs are profile extruded, ranging from solid sections in common sizes, to hollow profiles. The processing temperature depends on the plastic used in the mix but is typically around 150 deg C. Although standard extrusion equipment can be used for RPL sections (costing in the region of £5000 to £8000) for WPCs quite complex computer controlled mixing, extrusion and coating equipment is required. A complete production line including twin screw mixer/extruder, heated die, coating unit, water tank, feeder and automated sawing machine costs in the region of £300,000. In addition to this a wood fibre drier may be required. The feeder unit is needed to draw the extrusion through the water tank to prevent buckling. To produce a grain effect of the WPC the soft extrusion is passed under a patterned roller.
Advances in Wood Plastic Composite Manufacture
Researchers at the University of Toronto (Forcht, 1992) have developed a self-reinforced plastics composite made from recycled industrial and consumer waste. By using a unique thermo-kinetic (high temperature) mixing process followed by a solid phase extrusion technique (which processes polymers at or below their softening temperature) ordinary plastics and wood fibres are converted into components that are many times stronger and stiffer than normally processed wood plastic composites. During the process, the plastic polymer chains are unravelled and purposely aligned with wood fibres to maximise strength.
Nogellova (1998) reported on the use of peroxide as an agent to cross-link LDPE and wood flour, resulting in substantial increase in bending stiffness.