Two technologies merge: wood plastic composites Geoff Pritchard describes how wood and resin are being combined to make new composites which have enormous applications potential.
ood filler in plastics is nothing new. Rolls-Royce used a gear lever knob made of wood filled plastic as long ago as 1916, less than a decade after the patenting of the first completely synthetic resins. But in the last 30 years, much better materials have been developed. They have higher wood contents, better interface properties, improved processing technologies, and effective additives. Wood plastic composite (WPC) was reborn as a modern concept in Italy in the 1970s, and popularised in North America in the early 1990s. By the start of the 21st century it was spreading to India, Singapore, Malaysia, Japan and China. Universities across the globe are researching the fundamentals. According to a recent report by the Hackwell Group (found at www.woodplasticcomposites.org), the European manufacturers are still mostly small concerns, but WPC is likely to become much better known on this side of the Atlantic during the next decade, with applications in construction, infrastructure, automotive parts and furniture. WPC is an intimate mixture of wood and resin, processed by techniques familiar to the plastics industry. The term ‘extrudable wood’ has been used in a deliberate attempt to highlight the scope for producing fully finished wood-like products such as furniture, fencing, railing and skirting boards by mass production techniques. Unlike earlier attempts to mass produce furniture with plastics, WPC consists mostly of real wood. It has superior
outdoor durability and much lower maintenance costs than wood. More than half the wood from a felled tree is unusable for making planks and poles because it consists of thin branches. The idea of using them to make high quality wood substitutes is attractive because it reduces the rate at which forests need to be felled. Many countries are short of timber and have to import it.
Three quarters of all WPC worldwide is currently used in construction. History In the early 1970s GOR Applicazioni Speciali SpA developed products for automotive door panels, parcel shelves and roof panels, made of 50:50 mixtures of wood and resin, using machines from ICMA San Giorgio. By the 1990s similar products were being made on a substantial scale in the USA. Companies from the forest products sector competed with leading suppliers of polyvinyl chloride (PVC) building products to win market share. The two technologies had much to learn from each other. Louisiana Pacific, American Woodstock, Mobil Oil and Strandex were prominent in supplying products like picnic tables and industrial flooring. Decking emerged as easily the most successful application, accounting for more
than half the WPC sold in North America, where the strong summer sun combines with termites to attack outdoor timber products. Mobil Oil’s WPC division was later spun off as an independent company called Trex, now one of the market leaders.
Fabrication processes The sheets used to produce automotive panels are usually either made by extrusion or by a wet process. In the latter, wood fibres are typically generated by a rotating disc machine and mixed with hot water. Chemicals are added. The wood/water slurry is compressed in two stages to form a board between 10 mm and 20 mm thick. It is then hot pressed to bond the wood fibres. A polypropylene scrim is often used to hold the board together. In the construction products market, conventional profile extrusion remains the commonest way of manufacturing WPC, using a single or twin screw extruder, often the conical type. Only 15% by value of American WPC products are injection moulded at present, but the share will grow. Rotational moulding is under investigation. The extrusion of wood-plastics mixtures of high wood contents in the range of 50-80% by weight was not easy at first. Processors were familiar with plastics extrusion, but they knew less about the tricky rheology of wood-plastics mixtures or the temperature limitations of wood. The wood needed drying to below 1% moisture content, either in-line or 0034-3617/04 ©2004 Elsevier Ltd. All rights reserved.
Two technologies merge: wood plastic composites
before extrusion. Drying equipment adds to the costs, and fine, pre-dried wood particles are hygroscopic, a potential fire risk and a possible cause of dust explosions. Wood has a high specific heat and the cooling of WPC extruded profiles provided an extra challenge. The techniques are now established and extrusion equipment is available that can handle high wood contents consistently, even when the profile cross-section is periodically changed, along with the formulation. Cincinnati Extrusion has sold the most WPC extruders in Europe.
Outdoor durability Mechanical property changes over time are not considered a serious problem, although creep can occur under sufficient loads. The original colour of unpainted WPC characteristically fades to a driftwood appearance after about three months; many US customers are happy with this situation for typical outdoor applications like decking and fencing. Rot is not a threat for up to 25 years, except perhaps for the highest wood content products in hot, wet climates. Water absorption is much lower than that of wood. Painting is only for aesthetic reasons and not for protection against the weather; other surface treatments and veneers are also available.
Application sectors Three quarters of all WPC worldwide is currently used in construction. US suppliers are diversifying out of decking into railing, fencing, siding, window parts and doors and many other products. Steve Jones of Entek’s European office in Uttoxeter, UK, thinks Europe will also embrace decking in a big way. The second biggest application sector is in automotive applications, where real wood in WPC faces very strong competition from other natural fibres and may lose its market share. A third sector, infrastructure applications, overlaps with construction. It includes marina walkways, dockside furniture and sound reduction panels on motorways.
WPC flooring. (Picture courtesy of Ein Engineering, Japan.)
Wood is admired as a medium for craftsmen but plastic is not. Some observers see furniture as a fourth growth area, because it is intricate and the fabrication processes enable manufacturers to by-pass expensive wood machining and finishing operations. Outdoor tables, benches and chairs are already being made. There are many types of WPC, differing in resin type, whether virgin or recycled resin, tree type, geographical source, fibre geometry, fibre wood:resin ratio, additives, and process conditions. The formulation has to be matched to the application. One company, Fasalex, is different from the rest; its current products are not intended for exterior use.
Perceptions Wood is admired as a medium for craftsmen but plastic is not. Wood and resin mixed together with a majority of wood will therefore present the traditionalist
with a conundrum. If it looks like wood, WPC may be dismissed at first as a mass produced ‘fake’. But provided it is not used in applications that overstretch its mechanical properties, it will gain respect. No full life cycle analysis has yet been carried out on WPC. The new material is likely to be welcomed by Europe’s many environmentalist nongovernmental organisations (NGOs), unless of course the formulations contain mahogany. Where window and door frames are concerned, the thermal insulation properties of WPC will be an advantage. Its insulation can be further improved by foaming. A considerable amount of American WPC is made from recycled resin, usually polyolefin. In other words, transient polyethylene packaging is converted into highly durable WPC construction products every day in the USA. It can’t be bad for the public image of the synthetic resin industry. European manufacturers prefer virgin resin but say that WPC products will be recyclable at the end of their useful lives, although some observers think their re-use as fuel would make more sense. June 2004
Two technologies merge: wood plastic composites
Table 1: Some mechanical properties of WPC Wood:resin (w/w)
Tensile strength (MPa)
Flexural modulus (GPa)
Flexural strength (MPa)
*recycled resin **with compatibiliser All data taken from Technical Report 19, An investigation of the potential to expand the manufacture of recycled wood plastic composite products in Massachusetts, Chelsea Center for Recycling and Economic Development, University of Massachusetts, MA, USA, April 2000; N.M. Stark, Forest Products Journal, 49, 6 (1999) 39-46; and from J. Rettenmaier & Sohne GmbH and John Balatinecz, Toronto University.
WPC needs no treatment with toxic chemicals, and it contains no formaldehyde.
Is it really like wood? WPC is capable of being nailed, screwed, sawn, drilled and otherwise worked in the same way as wood. Nails and screws are firmly retained and dimensional stability in moist environments is good. Not all WPC looks like real wood – it depends on the composition – but it can be made to look attractive. The appearance depends on the composition and on whether the surface is painted, veneered or brushed.
Not all WPC looks like real wood – it depends on the composition. The most popular resin is polyolefin. In North America, recycled polyethylene is ahead of polypropylene, while PVC is also popular, especially in construction products. There are certain resin properties such as melt flow index (MFI) that need to be specified carefully and the MFI preferred for extrusion will not be ideal in injection moulding. Polystyrene can make a good wood substitute when filled with wood, having
an excellent appearance and better creep properties than polyolefin, but its flammability and smoke behaviour are against it. Many other resins have been tried out in laboratories, including polyurethanes and thermosetting resins. Wood darkens on heating, more rapidly at high temperatures. WPC is extruded as close to 140°C as possible. This would appear to rule out high temperature resins but ingenious solutions are being proposed to overcome this limitation.
Other plant-based products Some countries are short of wood, but they have plenty of other plant-based products that can be used instead. The mechanical properties turn out broadly similar. Even in the West, the automotive industry is using flax, hemp, jute, sisal and kenaf, despite the fact that wood is cost-competitive. The usual thermoplastic binder is polypropylene or polyurethane. Natural materials offer zero net carbon dioxide release, 40% lighter weight than glass fibres, and they use only 20% of the energy needed for glass fibre production. Some market research studies count all these materials as WPC, and the term ‘wood’ has sometimes been used loosely, but this article uses the word ‘wood’ in the plain sense.
Additives Pigments are widely used in WPC, as are lubricants, because they increase throughput rates and ease the problems of extruding viscous mixtures. Flame retardants have not yet been very widely adopted and would need to treat the wood as well as the resin. Biocides are rarely thought necessary, but they could be useful for prolonged contact with moisture, especially if the wood particles are not completely encapsulated. Compatibilisers improve bonding between polyolefins and wood (PVC is not such a problem because of its greater polarity). They transform both the mechanical properties and the price.
Mechanical properties Yes, WPC is a form of reinforced plastics. Wood fibres do increase the modulus of the resin, but not by much. The critical strain energy release rate (GIc) of uncompatibilised WPC is not much better than that of the base resin. Comparing the mechanical properties of wood with a material of variable composition like WPC is difficult; the properties of wood change with moisture content and those of WPC depend a great deal on the manufacturing procedure. Without a compatibiliser, flexural strength and modulus of WPC are inferior to wood, and if the wood content is low,
Two technologies merge: wood plastic composites
thermoplastic resins creep. On the other hand the hardness, abrasion resistance, shear and compressive strength can surpass those of wood. Figures given in Table 1 are merely examples; compositions differ considerably. Polyolefin WPC flexural strength values typically lie in the wide range 15-75 MPa, whereas wood (containing normal moisture) falls mostly between 60-90 MPa. Just as thermoplastics shouldn’t imitate light alloys, WPC products should not imitate wooden ones. They have a low impact strength with currently available formulations, and thin sections are best avoided. On the other hand intricacies such as stiffening ribs and channels that would be expensive and time-consuming with wood can easily be introduced. Hollow shapes are commonplace with extruded profiles, saving material costs and reducing weight. WPC can be foamed, halving its density and improving thermal insulation.
Suppliers and users There are upwards of 30 significant companies in Europe engaged in the manufacture of WPC, and numbers are expected to increase. Most have employees in single figures. Ploma AS is geared to supplying vehicle builders; OFK Plast AB targets the construction market. Well known continental companies include Fasalex (Austria), Tech-Wood (Netherlands) and Haller Formholz. Betoratio of Germany is developing WPC for the formers used in casting concrete. Strandex Europe licenses WPC technology and is based in the UK. One of its licensees (Silvadec) is based in France. The UK has more WPC manufacturers than most European countries, including Knotwood, Timbaplus, and Ecodek. WTL provides woodflour and compounds. WPC will certainly improve. Cheaper compatibilisers will improve the mechanical properties, and in the medium term, incorporating small quantities of nanofillers could also upgrade
mechanical strength and stiffness, as well as helping with flammability. New fabrication processes will be refined, starting with the more widespread adoption of injection moulding. One company is developing strong corner joints. The difficulty will be to realise all the potential advantages of WPC in a single formulation. ■ Geoff Pritchard; e-mail: [email protected]
Ploma; website: www.ploma.cz. OFK Plast AB; website: www.ofk.se. Fasalex; website: www.fasalex.com. Tech-Wood; website: www.tech-wood.com. Haller Formholz; website: www.hallerformholz.de. Betoratio; website: www.betoratio.de. Strandex; e-mail: [email protected]
Knotwood; website: www.knotwood.co.uk. Timbaplus; website: www.timbaplus.co. uk. Ecodek; website: www.ecodek.co.uk. WTL; website: www.wtl-int.com.