Materials Science Update: Plastic Wood Replacing Timber in Construction – Not Necessarily a Good Thing

June 11, 2013

Ever heard of plastic wood? It is a composite material consisting of waste wood mixed with plastic. For environmentalists it represents a dilemma. Plastic wood is a substitute for lumber so it conceivably saves trees. But plastic wood during manufacturing is emission intensive when compared to the environmental impact of harvesting natural timber, cutting it and delivering it to your local lumberyard.

In fact, collected data from the Washington State University in Seattle, suggests that greenhouse gas emissions from composites are up to 330% higher than timber harvested from redwood forests for use in decking and other construction. Currently U.S. statistics show that the plastic wood market, around for the last 20 years accounts for 10% of the total market for decking materials and is expected to grow to 32% by 2016.

What are the implications of replacing wood with plastic wood? In an article appearing in the June 5th issue of Nature, the author, Jeff Tollefson, describes how New York’s Coney Island pier is being reconstructed not with wood but recycled plastic wood over steel-reinforced concrete. So what was once a boardwalk made from wooden boards will no longer exist. Instead composite plastic wood will be underfoot.

Plastic wood is most commonly made from high-density polyethylene, polypropylene or polystyrene often recovered from waste. The plastic is combined with wood waste which can be collected from a variety of sources including sawmills and construction sites. The collected wood shavings and splinters are ground into a fine powder and mixed with the plastic. Some plastic wood manufacturers don’t even use wood. Instead they substitute agricultural fibers and in some cases combine them with fiberglass to create a lumber substitute. The market for these products includes automotive parts, pallets, playground equipment, picnic tables and benches, chairs, and substitutes for timber in landscaping, decking, docks and railroad ties.

So why is harvesting real wood a better environmental choice than manufacturing plastic wood? Because real wood from a sustainable forest is much closer to a net zero impact when measuring contributions to greenhouse gases. Wood is largely trapped carbon and sustainable forests serve as effective carbon sinks. That cannot be said about manufacturers of plastic wood products even though they are taking materials out of the waste stream and using them in a responsible way. Current manufacturing processes are net producers of CO2. Even accounting for the harvesting of timber and transportation and the emissions derived from that aspect of forestry produces far less CO2 than the plastic wood industry which also has to transport not just the raw material it needs for manufacturing, but also the delivery of its products to market.

So as Coney Island and the Jersey Shore rebuild those doing the construction need to consider the environmental impact of the materials they choose. What seemed a responsible environmental choice, the use of plastic wood, may prove not to be in the long term.

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