Wood-plastic composite (WPC) decking has made major advances in material performance, processing and user acceptance. This paper will discuss the materials and process issues of WPC related to future recycling efforts. It will then detail how WPC have and soon may be used and later discarded based on a combination of material property, durability issues, and changing consumer needs and their perceptions on non-performance issues. We believe that a huge potential exists for the development of new markets for virgin and recycled WPC.
Though WPC’s can divert recyclable wood and plastic from the landfill and into durable building applications, additional environmental benefit could be obtained if the composites themselves are recycled at the end of their useful life. The thermoplastic nature of the waste materials used in WPCs facilitates makes this possible. However, there are a number of challenges that may impede recycling.
“Useful service life” varies depending on product and the perception of the user. For example, some studies suggest that the normal US. decking material (i.e., the deck surface, not the deck substructure) is replaced in about 8-1 4 years because of non-durability related reasons. The most often given reason was that the checking and splitting of the deck surface due to cyclic wetting and drying and poor maintenance caused significant enough aesthetic degrade that the user was no longer satisfied with its appearance and wanted to replac it. Another prime reason was related to remodeling or a change in the deck use-pattern or design resulting in deck-surface replacement.
If recycling of the base polymer used in WPC results in WPC products having less UV- or biologicalresistance or reduced structural capacities when compared to virgin polymer-based WPC, then user acceptance will lessen. Some of the primary advantages of WPC’s over treated-wood decking are its reduced maintenance and the fact that it does not check and split resulting in reduced aestheticappeal. Manufacturers of Recycled WPC must keep these issues in mind when further developing their WPC products or when modifying their processing procedures. This will become an even greater issue as WPC’s enter markets requiring even greater durability.
Raw Material Effects
Youngquist et al. examined the effect of virgin versus recycled raw materials on the properties of WPCs. There was virtually no difference in the performance values of test panels on either mechanical or physical property tests when virgin and recycled polyethylene terepthalate were compared, or when hemlock fiber was compared with demolition wood fiber. Further, demolition wood fiber performed as well as panels made from virgin hemlock fiber and compression molded WPC’s containing recycled high density polyethylene from milk bottles were equivalent to WPC’s containing virgin high density polyethylene. They found that using fiber from old newspapers as reinforcing agent provided measurable property advantages over wood flour, which is currently the most commonly used filler in commercial composites. Also, these recycled newsprint WPC systems could themselves be recycled (re-extruded and injection molded) numerous times with little or no apparent loss in mechanical properties.
Processing Effects
Processing WPCs can result in the potential for thermal degradation of each of the components. For example, exposing polyethylene to high heat during processing can result in cross-linking, or a increase in brittleness. The wood-based fiber may also degrade due to repeated heat cycle exposures. Mechanical degradation of the fiber, often seen as fiber attrition, can also be a recycling concern. Balatinecz et al. studied the effect of recycling on old newsprint filled polyethylene and polypropylene composites. They found that successive recycling resulted in a slight decrease in the mechanical properties of the boards obtained. The viscosity of neat polypropylene decreased with repeated recycling indicating some thermo-oxidative degradation and these polypropylene-based composites, showed increasing melt flow with increasing processing .
Laboratory investigations have shown that prevention of extensive fiber attrition through the use lubricants should result in significant improvements in composite properties . Other have found that to successfully recycle WPC materials and commingled plastics waste that contains paper waste, a hydrolytic treatment is needed prior to conventional processing.
Environmental Effects
How the WPC is used can also influence its suitability for recycling. Since most WPC’s are used in exterior applications, environmental exposure degrades material performance and reducing the A6 - 5 5 th Global Wood and Natural Fibre Composites Symposium April 27-28, 2004 in Kassel / Germany recycling potential. Both fungal decay and UV exposure can change the structure of WPCs during their service life.
Pendleton evaluated WPC formulations that exhibited decay and noted it was often concentrated on the exterior of the specimen. Mycelium also appeared to be concentrated in the interfacial gaps between the wood and thermoplastic component near the specimen surface. They also noted that the decay was manifested as surface erosion. In their analysis of potential treatments to address decay, they noted that zinc borate treatment would take at least 20 years to completely leach from WPC material. WPC formulations using more than 60 percent wood fiber seem to experience greater fungal decay and loss in flexural strength than those using less than 60% wood fiber (Naghipour 1997). In general, the higher the wood fiber content, the greater the potential for biological decay to occur.
Recycling and reuse of wood fiber, thermoplastics and their virgin composite products has great potential. One complication is that the application of using recycled wood fiber, thermoplastic or their composites are limited by the greater variability resulting from the myriad of recycled materials in solid waste streams. These waste streams yield materials that are more complex in that they are often more contaminated than are virgin sources of materials. Degradation of WPCs due to repeated processing cycles and environmental exposure also complicates recycling. These are not insurmountable issues, but they will require focused research. A coordinated research approach is needed to address these fundamental issues and significantly advance this science rather than piecemeal science.
