Wood and plastic are best friends these days. They can be combined to give the aesthetics of wood with the added durability of plastic. Termed as wood/plastic composites - WPCs' are a relatively new family of thermoplastic composites based on wood-fibres and the commodity thermoplastics. The polymers used for WPCs' are the high volume, low cost, commodity thermoplastics - polyethylene, polypropylene and PVC.
How it all began
The use of wood fiber started a few years ago when Mobil undertook research efforts to find a way to recycle polyethylene grocery bags. Their team used recycled polyethylene (PE) and cellulose fibres, such as obtained from sawdust or newspaper, to make a composite that they formed into deck boards. The story did not end there. The group that developed this technology organized a management buyout in 1996 and formed Trex, a company based in Winchester, Virginia.
Major North American players
With funding from green fund sources and a superb marketing effort and a network of over 2700 lumberyards, Trex has built their company to the stage that in the year 2003 alone they sold over $200 million of WPCs' that went primarily into the decking market. Trex has lead the pack of companies that now manufacture WPCs', which are mainly sold in the construction market.
Other large US manufacturers of WPC decking are Timbertech owned by Crane (TimberTech), US Plastic Lumber (SmartDeck), AERT (ChoiceDeck) which is distributed by Weyerhaeuser, and Louisiana-Pacific (WeatherBest) who all use polyethylene; CertainTeed (Boardwalk), who uses PVC; and Correct Deck (CorrectDeck), who uses polypropylene.
In Canada, WPC decking manufacturers include Nexwood (Nexwood) in Brampton, Composite Building Products (Xtendex) in Barrie, and Brite Manufacturing (Life Long) in Bolton. These companies are three of the founding members of the Canadian Natural Composites Council (CNCC) that was formed in 2002/3 as the newest Council of the Canadian Plastics Industry Association (CPIA).
Processing of WPC and inherent challenges
The dominant manufacturing technology has been extrusion and this has been supplemented by injection moulding. The biggest challenge in processing WPCs' is that the cellulose fibre, whether it be wood fibre (pine or maple saw dust), recycled newsprint, hemp, sisal, kenaf, jute, flax or rice hulls, is removing the moisture and keeping the processing temperature below the decomposition temperature of the cellulose fibre being used. Wood, in its green condition contains 20% moisture. In its air-dried condition, wood contains about 12% moisture. To process wood fibre in extrusion processes, the moisture content must be below 0.5%. Another problem is that that when you dry wood to a very low moisture content, it is very hygroscopic and wants to add back the water that was removed. Therefore, once the wood fibre is dried, it should be used immediately so that it will not pick up moisture. Above 200°C (400°F), the wood fibre starts to decompose. This adversely affects the colour and the physical properties of the part being made.
For extrusion of WPCs', there are choices to be made. Wood fibre can be used directly with a twin-screw extruder to make a final part, or wood fibre can be used with a twin-screw extruder to make a wood fibre concentrate. The wood fibre concentrate can then be used in a single screw extruder to make the final part. Wood fibre concentrate can also be purchased from a compounder, such as Onaga Composites. There are other choices to be made. All wood is not the same. Which cellulose fibre to use? Wood fibre and rice hulls are common for decking products. With a wood fibre choice, there are still more selections to be made. Hardwood (oak or maple) or softwood (spruce, pine, fir) are the first choices. Mesh size of the wood fibre is next. 40, 60 and 100 meshes are common choices. Then there is the question of how much wood fibre to use in the finished part. Common choices range from 30% to 65%. Needless to say, there are quite some differences in processing this wide a range of filler loadings.
For injection molding, the only practical choice at this time is the use of wood fibre concentrate. The other choices, outlined in the extrusion section, still apply though.
And so, what physical properties are obtained from WPCs'? Again there is a difference between extrusion and injection molding. In extrusion, there is now first generation technology and second-generation technology. In injection molding, there is only first generation technology. What is first generation and second-generation technology? First generation technology refers to the simple processing of the WPC - that is, just extrusion or injection molding. Second generation technology involves orientation of the polymer molecule after it has been formed into a shape by the extrusion process. This orientation process, similar to die drawing in metal forming technology, increases the strength and stiffness of the WPC. It can also lower the density.
With first generation technology, as one increases the wood fibre content, the strength of the WPC decreases and the stiffness (modulus) increases. While with second-generation technology, the initial strength and stiffness of the polymer is increased six-fold and four-fold respectively. When wood fibre is added, the strength and stiffness are both lowered, but from a much higher level. If a die-drawing process is used for orientation instead of a ram-extrusion process, the wood fibres and the polymer pull apart giving a composite of a much lower density. Typical numbers are a density of 1.0 g/cc for a ram-extruded 30% wood fibre/oriented polypropylene in contrast to a density of 0.5 g/cc for a die-drawn 30% wood fibre/oriented polypropylene composite, both with a draw ratio of about 10. This orientation technology is not available to injection molding at this time. Some developmental work has also been using rotational moulding.
Exciting new applications, combination of wood/plastic properties and rapid growth are fueling the WPCs market. The decking market has been the first one to be addressed in a major fashion. Other construction market applications are now coming to the fore. Railing to go with the decking has been the next for most companies. Some players have pursued window and door profiles. The fencing market, which is quite large, is seeing a major effort as several companies commercialize new systems. The $8 billion North American siding market is seeing its first prototype products now. This should be available commercially in the next couple of years. Other construction markets where products will soon emerge include shingles and roofing tiles. For WPCs, the field is wide open.
Frank Maine, PSA Composites Inc., 40 Taggart Street, Units 3 & 4, Guelph, Ontario N1H 6H8 Canada
Dr. Maine is an organic chemist with a B.Sc. and an M.Sc. in Engineering Chemistry from Queen's University and a Ph.D. in organic chemistry from Cambridge University. He held various positions in government laboratories and in industry in various areas of plastics engineering. He was Manager of Research and Development at Fiberglas Canada.
Dr. Maine is actively involved in the commercialization of oriented plastics. Currently, he is the Chairman of PSA Composites Inc. and is working with a group of companies that are developing and commercializing oriented thermoplastic and composite products including woodfibre/plastic composites. He has given numerous presentations and is often sought speaker in WPC technology.
Previously, he was the Member of Parliament (Canada) for the Guelph riding of Wellington from 1974 to 1979.
Frank Maine recently published an update for this article. Please click here.