Wood plastic composites (WPC) are made from plastic and, normally, a fine wood flour. Sometimes wood fibres are used, as are non-wood plants like hemp, flax, jute, Kenaf and rice husk. The flour is typically between 250 and 420µm in diameter and can be hardwood or softwood species. The wood normally makes up 50-70% by weight of a WPC. The flour is significantly cheaper than the plastic so there is a temptation to maximise the flour in the final product. The problem is that high wood loadings reduce dimensional stability and long term durability; particularly important to WPC because they are sold as low maintenance and long-term weather resistant.
Thermoplastics are used, so they can be heated and moulded into shape; polypropylene is the most common in Europe whereas polyethylene is dominant in the much larger US market. Polyvinylchloride- and polystyrene-based plastics are also used. Some WPCs are made from virgin and others from recycled plastics. Some manufacturers sell on the basis they are made from both recycled plastics and wood. Since WPCs are made from thermoplastics, they can be recycled by reheating and moulding. Wood is a hydrophilic material and the plastics used in WPCs are hydrophobic, so they do not mix very well and the bond between the plastic matrix and wood particles is limited without the addition of compatibilisers.
Many manufacturers add maleated forms of the main plastic being used, eg maleated polyethylene (MAPE) or maleated polypropylene (MAPP). The MAPE or MAPP react with the wood during moulding, thus creating a plastic, hydrophobic coating on the wood particles. Other additives include: lubricants, pigments, UV stabilisers, fire retardants, foaming agents and biocides. WPCs are more dense than solid wood, usually more than 900kg/m3. Thus manufacturers introduce holes to reduce weight. The majority of WPC products are made by extruding with a single- or twin-screw extruder, heated to melt the plastic. This causes any water in the fibre to evaporate and, unless released, the steam will cause mixing and product performance problems. Small components can be made by injection moulding. Wood based panels and WPCs do not compete directly in many markets. The main one is probably MDF mouldings.
If you would like a technical term discussed or an acronym defined then send me an email (mark.irle@ecoledubois.fr)
or a fax (+33 [0] 240 18 12 00)
Dr Mark Irle
Ecole Supérieure du Bois,
44306 Nantes, France
www.ecoledubois.fr
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