Plastic Lumber Possibilities
Traditional applications find progressive results
As a substitute for treated lumber, plastic lumber products resist insects, rot, moisture, and many chemicals. There are two different types of plastic-lumber products—the “composites” (wood products made from a mix of plastics and natural fibers) and the “wood-like” products made solely from plastics. Many of these products use recycled plastics, diverting waste from the landfills1— a potential benefit to the environment. Indeed, the plastic lumber industry largely owes its inception to finding a use for plastic scrap and post-consumer plastic waste. In some applications, these new products are now displacing traditional building materials.2 Global demand for these plastic lumber products is rising, due in part to an increase in their possible construction applications.
The difference is in the mix
Wood–plastic composites (WPCs) consist of fibers combined with some sort of polymer. For example, natural fiber composites combine plastic with flax or hemp. In Western Europe, this material first became popular with German automotive manufacturers. For companies like BMW and Daimler Chrysler, natural composites now average between 7 and 10 kg (15 and 22 lb) per vehicle in nonstructural applications such as interior panels, headliners, seat panels, parcel shelves, and acoustic panels.
Wood composites, on the other hand, are products manufactured from a combination of wood fibers and plastics. Their various qualities (including durability and weather resistance) make them suitable for exterior building applications including decking/railing systems, window/door profiles, shingles, sound barriers for roadways, and infrastructure products such as boardwalks, picnic tables, and park benches.
Both natural fiber and wood composite products can be made with either new plastic or post-consumer/industrial recycled material. Virgin polymers offer additional aesthetic options, including pigmenting choices and reproducible patterns, but can also cost more. Potentially less expensive composites incorporating recycled plastic may use specialty additives/pigments to improve aesthetics, such as color retention and ultraviolet (UV) ray resistance.
In North America, polyethylene (PE) is the most widely used polymer in composite applications. However, post-consumer supply is tightening due to increased use of the material and rising exports overseas. This, along with ongoing research into new polymer combinations and processing methods (i.e. injection molding), could increase demand for other plastic materials, such as polypropylene (PP). When this happens, these new materials could offer their own distinctive properties and characteristics, further changing how WPCs are viewed by specifiers and other design professionals. (For example, polypropylene may be oriented to provide greater stiffness.)
Durability and other characteristics
Plastic lumber products can offer several advantages over their more traditional counterparts. According to a study from the U.S. Army Corps of Engineers, composite products are durable, stable, resilient, and resistant to weather, rot, mildew, and termites (without chemical pressure-treatment). Additionally, plastic lumber products do not require regular repainting or restaining. These types of products are also workable with conventional carpentry tools and, for the most part, are low-maintenance.
The limitations of plastic lumber products include their viscoelasticity. The material’s mechanical properties are time–temperature-dependent and subject to permanent deformation (i.e. creep) under sustained loads. The rate of this creep depends on the magnitude and duration of the stress and the temperature at which it is applied. Furthermore, plastic lumber can undergo significant dimensional changes due to temperature.
Another critical issue inherent to the material has been its relatively low stiffness and flexural strength. This may explain why most of the extruded plastics or WPC boards produced have been used for deck and boardwalk surfaces, where flexibility is less important. However, recently introduced oriented wood polymer composites offer increased stiffness and flexural strength.
Single-polymer systems rely on continuously extruded, structurally foamed high-density polyethylene (HDPE). Extrusion flow molding allows the use of even heavily contaminated recycled plastics, resulting in the potential for lower raw material costs and increased benefit to the environment. New flow mold systems are being developed that promise greater potential in producing largedimension plastic timber products at high throughput.
Fiberglass-reinforced recycled composites can provide components for demanding structural applications, such as deck joists, marine break walls, and bulkheads/pilings. Vinyl extrusion profiles, on the other hand, are being used primarily in railing and deck board systems.
Increased application and awareness
Plastic lumber products have the potential to go beyond the traditional applications already mentioned. The Canadian Plastics Industry Association’s Environment, and Plastics Industry Council sponsored a study, Reviewing the Potential for Plastic Railroad Ties in Canada, exploring the development, performance, production, and pricing of plastic railway ties.3 It points to the possibility of increased demand for the material within the North American rail industry, particularly for regional and short-line railroads. Factors contributing to this potential include the growing environmental concern within the United States over the disposal of creosote-treated wooden ties.
While railway ties may seem an esoteric field, it seems more than likely that plastic lumber products will also be incorporated into a growing number of residential and nonresidential construction products. As new applications for these products continue to evolve, so will demand for the material. The material’s success lies not only in its durability and potential use of recycled materials, but also in larger numbers of architects/engineers and specifications writers (re)discovering its applications.
Indeed, plastic lumber has come a long way from the novelty material applauded by environmentalists and largely ignored by the construction industry. With a greater understanding of the material’s performance through standard test methods and specifications and the construction industry’s growing comfort with its use, it could one day be an integral part of the built environment, from landscaping components to interior applications.
Codes and Standards
One key factor driving the growth of wood plastic lumber use is the increasing number of ASTM International standards. Over the past five years, numerous standards have defined test procedures and helped engineers in designing structures. Most of the current standards were developed by the Plastic Lumber Trade Association (PLTA) and focus on single-polymer products. The following are a list of active standards pertaining to composite products:
- ASTM D 6108-03, Compressive Properties of Plastic Lumber and Shapes
ASTM D 6109-03, Flexural Properties of Unreinforced and Reinforced Plastic Lumber
ASTM D 6111-03, Bulk Density and Specific Gravity of Plastic Lumber and Shapes by Displacement
ASTM D 6112-97, Compressive and Flexural Creep and Creeprupture of Plastic Lumber and Shapes
ASTM D 6117-97, Mechanical Fasteners in Plastic Lumber and Shapes
ASTM D 6341-98, Linear Coefficient of Thermal Expansion of Plastic Lumber and Plastic Lumber Shapes between [–34.3 and 60 C] –30 and 140 F
ASTM D 6435-99, Shear Properties of Plastic Lumber and Plastic Lumber Shapes
ASTM D 6662-01, Standard Specification for Polyolefin-based Plastic Lumber Decking Boards
Further standards currently under development for plastic lumber include:
- ASTM WK 2843, Polyolefin-Based Outdoor Structural-Grade Plastic Lumber
ASTM WK 2501, Polymeric Piles
ASTM WK 1202, Guidelines for Evaluating the Mechanical and Physical Properties of Wood-Plastic Composite Products
ASTM WK 1203, Establishing Performance Ratings for Wood- Plastic Composite Deck Boards and Guardrail Systems
About the Author
Cathy Cirko is the director general of the Environment and Plastics Industry Council (EPIC) of the Canadian Plastics Industry Association (CPIA).
1Recycling is not available in all regions of the country.
2For example, Wood Composites in Decking Structures: Building of Outdoor Living Areas, a report from consulting firm, Principia Partners, predicts WPC use in deck boards and railings will double between 2000 and 2005, with annual consumption growing from approximately 27.4 million to 60.4 million meters (90 million to 198 million ft) in the United States alone.
3The report can be accessed through http://www.plastics.ca/epic by selecting Publications and Special Reports.