For the environmentallyconcerned builder or designer, recycled plastic lumber is often touted as the best thing since sliced bread. It provides a use for the millions of tons of plastic waste finding its way every year into our dumps. It offers a longer lasting, toxin-free alternative to pressure-treated lumber. And it can boast of some high-profile success stories, such as a 2,200-foot boardwalk through a bird sanctuary at Port Lavaca, Texas, and docks for a marina in Knoxville, Tennessee.
There have also been some significant failures, however. Twenty-eight thousand dollars worth of plastic lumber decking was installed at the Weston Beach Resort Hotel in Hilton Head, South Carolina, only to be torn out several months later. Mark Bailey, the hotel’s chief engineer, cited damage to the deck’s supporting structure, caused by the plastic’s high thermal expansion, and overheating of the surface, which made it too hot to walk on. The hotel managers had planned to spend ten times as much on plastic lumber installations before this failure led them to change their minds.
What went wrong in Hilton Head? How can recycled plastic lumber succeed so admirably in some applications and fail miserably in others? The answer lies in the tremendous variability in materials and manufacturing processes, and the fact that this industry is in its infancy. The term “recycled plastic lumber” does not describe one specific product, but rather a broad range of materials with various qualities and characteristics. This article and the checklist that follows will help you decide if and when to use plastic lumber and how to select the right material for your needs.
What Is Plastic Lumber?
Plastic waste that has been collected, either from end-users or from factories, can often be re-melted and given a new form. Unlike many plastic manufacturing processes that require extremely pure polymer resin, plastic lumber can be made with mixed (
commingled) and slightly contaminated material, given the right machinery. Recycled plastic lumber provides a use for waste material that would otherwise be unusable in the plastics industry.
Post-consumer plastics are generally collected and sorted alongside other municipal solid waste at a materials recovery facility (MRF). The sorted plastics are then crushed and baled for delivery to manufacturers. Bales that are cleaner and more carefully sorted are valued more highly. Because plastic lumber uses a lot of material (weighing 40-60 lbs./ft3), it generally has to make use of the less expensive—and less pure—recycled materials. Post-industrial (factory) wastes are preferred by some manufacturers because they are relatively pure and clean, but these usually command a premium price.
High-density polyethylene (HDPE) comprises the largest part of the plastics waste stream and is the resin most commonly used by plastic lumber manufacturers. Even plastic lumber manufacturers who use widely mixed (commingled) resins generally require that a large part of the mix consist of HDPE.
The recycled plastic feedstocks from the MRF or factory are granulated into small flakes. These flakes are fed into machines that melt them and then either extrude or mold the molten plastic into new shapes. Contaminants in the mix, in the form of aluminum (usually from bottle caps), cellulose from paper or wood, or plastics with higher melting points, go through the machine as whole flakes and show up in the finished product. All plastic lumber contains a small amount of such contaminants. Too much in one place, however, can cause severe structural and aesthetic defects.
Dr. Tom Nosker, of the Center for Plastics Recycling Research at Rutgers University, estimates that there are seven different types of machinery in use by the forty or more recycled plastic lumber manufacturers in North America. Much of this equipment has been purchased from Europe, where plastic recycling is far more advanced than in the U.S. and Canada. The variations in equipment affect not only the efficiency and capacity of each company, but also the type of feedstocks each can accept.
Commingled plastic product manufacturers have to deal with the inherent incompatibility of different polymer types. The molecular bonds that give homogeneous, virgin plastic its strength are not always reproducible even with a single recycled resin, and even less with commingled resins. As a result, products from unreinforced commingled plastics have significant weaknesses, including:
•low strength-to-weight ratio and poor span ratings compared to the wood they are often slated to replace.
•softening and weakening of the material at higher temperatures.
•limited color options due to the dyes in the recyclate feedstocks
•large thermal expansion
•heating and heat-transfer characteristics that can make the product uncomfortable to touch after it’s been in the sun.
Nevertheless, these products do have many appropriate (non-structural) uses, such as car-stops, landscaping timbers, and fences.
Celeste Johnson, president of Obex, Inc., uses unrefined commingled plastic waste to make a lower-end plastic product. She takes in plastic scrap directly from consumers (rather than from a MRF). Through good communication with her “suppliers,” she manages to get clean enough material that she doesn’t need to wash before processing. As a result, Obex doesn’t generate any of the waste water that is a disposal problem for some companies. Johnson also improves the quality of her material by carefully controlling the mix of plastic resins and using a different recipe for each type of product, using more polystyrene than is common in the field. Her main products are compost bins (assembled from 1x3 stock) and landscaping timbers.
Due to the limitations of simple commingled plastic, many companies have incorporated changes that improve the performance of their products. Some are mixing homogeneous HDPE recyclate with the less expensive commingled feedstock. Many use a blowing agent to “foam” the plastics, creating a lighter-weight material. Use of non-polymer additives of various types, and improved sorting and cleaning of the feedstock to reduce contamination are also increasing in popularity.
Almost all plastic lumber manufacturers add UV stabilizers and anti-oxidants to their material. Without these additives the material would become brittle and weak after extended exposure to sunlight. Some also use heat stabilizers and inhibitors to retard degradation over time (due to causes other than UV exposure).
Additives are also used to enhance specific properties of the product. Examples of these are chlordanes, plasticizers, blowing agents (for foaming) and reinforcing fillers. Products utilizing significant quantities of reinforcing or non-reinforcing fillers are technically composites, as they incorporate both plastic and non-plastic raw materials. Mobil’s Timbrex product is a good example of a composite made with recycled polyethylene and wood fibers (see EBN
Vol. 2, No. 2). There is also a family of additives known as compatibilizers, which allow for some degree of bonding between polymers that don’t otherwise bond.
Products made from improved commingled plastics include park benches, picnic tables, and garbage can enclosures. David Grommesch, a landscape technician in the Burnsville, Minnesota Parks and Recreation Department, used lumber profiles from Bedford Industries on steel support frames to make public benches. He learned from some early failures that an 8-foot bench requires either thicker planks or additional supports. Since then he’s been thrilled with the benches’ low maintenance requirements.
Another emerging use for recycled plastic lumber is sound containment walls along highways. California’s Department of Transportation is testing material made by California Plastics Recycling in Los Angeles. The company extrudes large, hollow profiles of recycled plastic that interlock to create a structural wall. The hollow members are then filled with chipped tires to add mass and improve sound absorption. The company claims its product will cost the state 15% less than the concrete or masonry used currently. Transportation officials are most impressed, however, by its graffiti-proof surface.
Another type of sound wall has just recently been licensed to Sanders Enterprises by a German company. This innovative design uses recycled plastic to create massive planters for roadside vegetation (see photos). The living wall will absorb significant amounts of airborne pollutants, as well as sound.
The high end of the plastic lumber market is epitomized by Trimax, Inc. Trimax uses relatively pure HDPE resin (which they buy from a MRF, though until last year much of their product was made from a post-industrial product called Noryl). They add a large amount of glass fibers for strength (about 20% by weight). Most of the fibers are scrap from the fiberglass industry, but the cost is still 4 to 6 times more per pound than the cost of plastic resin. Five percent of the Trimax product by weight consists of a mix of proprietary additives. These include a foaming agent, a UV stabilizer, a chemical to absorb stray oil and soap (much of the plastic comes from motor oil and detergent containers), and a compatibilizer to bond the small amount of contaminating polypropylene to the polyethylene (HDPE) resin.
Trimax lumber is more expensive than most, but has proven itself in several large projects. Harry Weed, town manager for the village of Brightwaters, New York, has used quite a lot of it and strongly endorses the product. “We’re very, very happy with it,” he says. “If we had our way, we would do the whole marina in it right now.” The 11% additional cost over marine-grade treated wood is well worth paying, he feels, due to the longevity of the material, especially under water. Based on his existing three-year-old installation of 4,000 feet of seawall, he anticipates a useful life for Trimax that is several times that of treated wood. “You can’t tell the difference between the three-year-old stuff and what they’re putting in now, but the five-year-old CCA-treated wood is totally worm-eaten,” Weed notes.
Lumber made from recycled plastic has been billed as a solution to the solid waste problem, to the timber supply problem, and to the toxic chemicals problem of preservative-treated wood. To the extent that plastic lumber products succeed in replacing wood and concrete in high-exposure applications, these claims may in time prove valid.
Diverting millions of pounds of plastic waste from over-burdened landfills is the overriding benefit of using recycled plastic lumber. Only the cleanest, highly sorted plastic waste can be re-used to make plastic bottles and films, while contaminated and commingled wastes are what sanitation departments most often end up with.
As a replacement for treated wood, plastic lumber has a number of advantages. Preservative-treated wood uses several toxic chemicals that are processed and transported in a highly concentrated form (see EBN
Vol. 2, No. 1 for details). Even when treated with preservatives, wood is vulnerable to attack from certain marine borers. Disposal of used treated wood is a growing problem, with more and more states now requiring that it go to hazardous waste landfills. Finally, the wood resource itself is limited, and even though most of the southern yellow pine used for treated wood is plantation grown, there are serious concerns about the ecological effects of these plantations.
Concrete is another material that has its drawbacks environmentally. While there are clearly applications for which concrete is the appropriate material, there are also situations where it can easily be replaced by recycled plastic. Car-stops for parking lots are the best example of such a use. Compared with concrete, energy use in the recycled plastic industry is very low and pollution is minimal.
Recycled plastic lumber tends to be somewhat more expensive than treated wood, though prices vary considerably. Maintenance costs are negligible, however, as no painting, staining, or re-treating is needed. Plastic products tend to be graffiti-proof (in that paint comes off easily with a light solvent), and much more resilient to common abuse and vandalism than wood. Although actual life-expectancy for plastic lumber is only guesswork at this stage, indications are it will last significantly longer in exposed conditions than treated wood. (Dr. Charles Kibert of the University of Florida estimates that plastic lumber will last 400 to 600 years.) Thus, when the lifecycle economics are considered, plastic lumber has significant advantages.
Environmental concerns with plastic lumber are minimal, but they do exist. If waste water from washing the plastic feedstock is generated, it has to be disposed of properly or it can contaminate surface and ground water. Solid waste is not a problem, as almost all plastic lumber manufacturers will take back scraps and off-cuts to feed back into their production process, but you should check this out before making a purchase. Material that has been chemically cross-linked is not recyclable into lumber or any other material, according to Rutgers’ Dr. Nosker. (Only one or two manufacturers use cross-linking in their process.)
Additives are another possible concern, as a wide range of additives of various types are used in making plastic lumber. Most of the additives are commonly used in the processing of virgin plastics. “Based on the application of some of these same things in a lot of other materials, I think that these additives are fairly benign, and furthermore they’re used in very minimal quantities within the material,” says Richard Lampo, a materials engineer with the U.S. Army’s Construction Engineering Research Lab (CERL). Others in the field confirmed this position, though manufacturers consider details about the additives they use to be proprietary, so these claims are difficult to verify.
Cadmium—from the dyes used in brightly colored plastic containers—has been found to leach out in small quantities from most plastic lumber. While many states are outlawing the use of cadmium in dyes, it will be several years before it has worked its way through the waste stream. The concentrations of cadmium are such that recycled plastic products are unacceptable as containers for drinking water, but they are not considered a threat to groundwater.
The lack of common testing standards is emerging as a huge impediment to more widespread use of plastic lumber products. Without standards from recognized organizations, code officials are reluctant to approve their use in most applications. Engineers and designers cannot accurately compare data on competing products, and companies tend to release only data that make their products look good.
At present there are no agreed-upon test procedures or industry standards by which recycled plastic lumber products can be compared. Dr. Nosker explains that existing tests for virgin plastic materials are not applicable to recycled plastic, because a high degree of uniformity and homogeneity is assumed in the tests. The inconsistency caused by contaminants and imperfectly mixed feedstocks make these tests inappropriate for plastic lumber. Furthermore, many plastic lumber products are not manufactured as a uniform material but as a structural element, with core properties that differ from the surface properties.
Dennis Hurley, a product engineer at Trimax, suggests that existing tests for structural wood members are the most appropriate for recycled plastic lumber, because most of the new material’s applications replace wood products. While Trimax uses wood testing standards for evaluating their product, most other companies do not, perhaps because the plastics engineers are not familiar with tests for wood.
Over the next few years that should change, however, as research progresses and the young industry learns the need for cooperation among competitors. Rutgers’ Center for Plastics Recycling Research has just started a program in cooperation with the U.S. Army’s Construction Engineering Research Lab in Champaign, Illinois to establish useful and reliable testing procedures for the material. A third party, the Port Authority of New York and New Jersey, withdrew from the project but is considering ways to rejoin, according to a senior engineer.
Richard Lampo of CERL chairs the American Society for Testing and Materials (ASTM) task force for section D20-95 on recycled plastic products. At a meeting in Pittsburgh this July, Lampo hopes the task force will become an official subcommittee of ASTM, with the long-term goal of adopting test procedures from the current Rutgers research on ASTM standards. It is likely to be several years before such standards are in place, however. Lampo foresees the possibility of some type of grading system: “We might have material standards, much like you’d have with wood, in that we’ll have grades of material.… Then as a specifier, you would select a material not based on composition, but on a performance range.”
Meanwhile, an effort is underway to establish a trade association of plastic lumber manufacturers. Spear-headed by Alan Robbins, president of The Plastic Lumber Company, Inc., a group of manufacturers will be meeting on July 28 in Pittsburgh (in conjunction with the ASTM meeting) in the hopes of establishing an association. Hurley (of Trimax) hopes that existing wood testing standards will be accepted by the industry as interim standards, pending the development of new ASTM-approved test procedures. Other companies may not be eager to go along with such a plan, however.
Recycled plastic is an exciting new material that offers the construction industry an alternative to some rather undesirable products. As with any new material, it should be used conservatively and with caution until reliable standards are established and problems are worked out. While almost any plastic lumber product can replace wood or concrete for certain limited applications, only the higher-end products are appropriate for more widespread use. The following checklist provides some guidelines to consider when specifying recycled plastic lumber.