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Feature Story
Although their primary applications continue to be decking
Now it’s time for the “untraditional.” Making headway into the flooring, moulding, cabinet and furniture arenas are wood-plastic composites. These composites consist of a mixture of wood flour and a plastic resin, typically polypropylene, polyethylene, ABS, PVC or polystyrene. According to the research group Principia Partners, wood-plastic composites first gained recognition in the early 1950s when the automotive industry began adding wood flour to polypropylene to manufacture rear package shelves, spare tire covers and door liners. Later research in the 1980s by Mobil Oil led to the development of the Trex brand of decking. Today, decking remains the number one end use for wood-plastic composites. (See composite decking sidebar below). However, developments in the processing and machining capabilities of wood-plastic composites have resulted in a major growth opportunity for this material, notes Lou Rossi, Principia’s director of wood composites programs. Of specific interest is the growing use of decorative trim, furniture and other interior applications made from these composites. (See chart below). Making the Move Indoors
The following list compares the properties of wood-plastic composites to solid wood or wood-based composites. It is a compilation of opinions expressed by various sources cited in the accompanying articles. Pros Cons EinWood Composites is one example of a wood-plastic material being used for indoor applications such as residential flooring, cabinetry and profiles. Developed by EIN Engineering, the product is currently manufactured by licensees in Japan; the technology is available for licensing in North America by Inteque Resources. The product can be extruded or injection molded, and either pigmented or stained to look like natural wood, says Tomo Sakaguchi, Inteque manager. “EinWood composite materials, for example, are more like wood than plastic. They typically contain 55 percent wood and 45 percent plastic by weight; by volume, it is more like 80 percent wood,” Sakaguchi adds. With respect to marketing opportunities, Sakaguchi says, “We foresee an increase in volume for these types of products. Depending on the plastic mixed with the wood flour, you can have products with much harder surface properties than solid wood or other wood composites. “With flooring, for example, we mix in polypropylene, which makes a very rigid and hard final product. One of the benefits of this composite over other wood products is that it offers better anti-slipperiness under wet or dry conditions and has very low moisture absorbancy,” he adds. “I think this industry is maturing into other products. I think the window and door industry, for example, is beginning to pay more attention to it,” says Terry Laver, president of Strandex Corp., another developer of wood-plastic processing technology. “The benefit of using a wood-plastic composite is the consistent quality of the product and, in many applications, it is less expensive. If you are using a single extrusion to replace multiple wood components, you have the ability to eliminate in-process inventory for window manufacturers. The savings can be enormous,” Laver says. “Our customers are also using the products for such items as broom bases, door thresholds, door stops, vendor boards — any type of wood replacement application,” says Bill Crostic, president of Onaga Composites LLC. Onaga is a compounder which manufactures pellets of wood flour mixed with virgin polyethylene or virgin polypropylene for processing in an extruder or an injection molder. “We are approaching wood flour as an engineered filler, similar to talc or mineral, to make a wood-plastic composite engineered and tailored to the end use,” Crostic says. “The advantage of a wood-plastic composite material is in the inherent processability of plastics. You can injection mold a whole cabinet door exactly in the shape you need, without any added machining. (The product) can be turned into a sheet...everything that can be done on plastic can be done with the wood-plastic composites. Wood-plastic composites also offer an improved stiffness to weight ratio over wood,” Crostic adds. According to Crostic, the greater the wood fiber concentration, or loading, the less the ductility of the plastic. For items that require a high degree of stiffness, the wood fiber loading would typically be 50 to 70 percent of the mix. Products requiring more ductility or flexibility would have a smaller concentration of wood flour.
Arden, NC-based Atlas Precision has found a new niche market for its injection molding business. Using wood flour mixed with polyethylene and polypropylene pellets from compounder Onaga Composites, Atlas Precision is injection molding OEM door thresholds for sale in Lowe’s home center stores. Atlas Precision has been injection molding the thresholds for approximately eight months. Robert Bulla, sales manager, says, “We got into this because we specialize in niche markets, and we’ve seen the construction and furniture industry looking at ways to cut costs dramatically. The idea of wood-plastics has been around a long time (for structural use), but it is just now being recognized for residential use.” Areas in which Bulla foresees wood-plastic materials being used to replace all-wood or all-plastic parts in residential furniture include recliner handles, sofa legs and gliders on chairs. “By injection molding these parts using wood-plastic materials, we can offer furniture manufacturers a value-added product that you typically can’t have with regular wood or plastic. We can core the part so it is lighter in weight, but equal to or greater in strength than solid wood. The wood fiber in the mix also allows for thicker walls than you typically have with plastic unless you use gas-assist or foam,” he says. Atlas Precision has 34 Toshiba and Arburg injection molding machines, ranging in size from 38 to 720 ton. Bulla says the company typically uses a mix of 50 percent wood flour in the injection molders; the wood-plastic mixture can also be customized with regards to type and percentage of flour and plastic resin to maximize specific properties as required by the customer. Other than an investment in special blending equipment, no other changes were needed to process the wood-plastic material. “The special blending equipment allows us to tailor the blend for customers so they can have products with a certain stiffness, nailing, stapling, drilling and sawing capabilities. (Another benefit is that) color can be added in the processing, so it goes all the way through the part, not just on the surface,” Bulla adds. Atlas Precision has devised a marketing strategy to promote the injection molding of wood-plastic composites. In addition to word-of-mouth advertising, the company will be highlighting its added molding capabilities at trade shows and through direct marketing efforts. — Karen Koenig To aid new users of the material, Onaga’s plant, like many others, includes an R&D laboratory to help customers with the design, processing and finishing of wood-plastic composites. Performance Comparisons: Wood vs. Wood-Plastic The ability to manipulate a product’s stiffness is just one benefit cited by producers of wood-plastic composites. According to information by Principia Partners, wood-plastic composites possess many other characteristics not found in solid wood or traditional wood composite materials. For example, the plastic additive helps the product resist rot and insects, making it particularly suitable for items such as window and door lineals. In addition, the material absorbs less moisture than wood and often exhibits greater dimensional stability; once formed, it will not crack, warp, split or twist. “Wood-plastic composites are not inexpensive to produce, and accordingly sell for higher prices than their wood equivalents,” Rossi says. “However, the materials are generally easier to work with, and the total installed cost is often less than wood.” “Further, the material can be molded to produce complex shapes, such as cabinetry doors, which is a fundamental advantage over wood,” Rossi adds. A recent study compared the performance of wood-plastic composites and wood-based panels. Conducted by Robert H. Falk, USDA Forest Service engineer, Dan Vos, an engineer with Berger/Abam Engineers, and Steven M. Cramer, a professor in the Department of Civil and Environmental Engineering, University of Wisconsin, the information was presented at the International Conference on Woodfiber-Plastic Composites. The material they used was a standard blend, pelletized 40-mesh pine wood flour combined with a 50/50 percent weight mix of low density polyethylene and polypropylene and produced with a twin-screw extruder by North Wood Plastics Inc.. Pellet blends ranged from 20 to 60 percent wood flour by weight. The 20-inch by 20-inch by 1/2-inch panels, manufactured at the Forest Products Laboratory, were compared to plywood, OSB, particleboard, hardboard and MDF. The study found the performance of wood-plastic composites was inferior to the wood-based panels with regards to bending modulus of elasticity and bending modulus of rupture. Also rated low was the coefficient of thermal expansion. However, with regards to tensile strength, hardness load and edgewise shear strength, the wood-plastic panels were rated comparable to the wood-based panels. A superior performance rating was given to the wood-plastic composites with regards to weight change and thickness swell for a 24-hour soak (less than 1 percent water absorption compared to a high of 40 percent for wood-based panels). The study found that only the MDF panel had stability characteristics similar to those of the wood-plastic composites. For wood plastic suppliers, CLICK HERE Processing the Material Characteristics of the wood-plastic composites are often influenced by their processing method. The material is typically extruded or injection molded from compounded pellets.
Wood-plastic composites can be mixed and saturated with plastic in an extrusion machine and die-formed into a profile. The low-maintenance composite is stiffer than pure plastic products and can span the same centers as wood. It often works and cuts like wood; can be glued; and can be fastened with screws, nails or staples. Critical factors in processing include tailoring the extruder’s screw geometry to the compound, matching the temperature to the speed and balancing the flow from the tips of the screws into the die. Regardless of the material’s formulation, the functions of conveying, melting, mixing, devolatilization, homogenization and pressure generation must be in sync for the processed material to be delivered through the shape die without defects. One extruder technology for wood-plastics processing is the counter-rotating, conical, twin-screw machine. Conical twin-screw extruders are designed for conveying and pumping “fluffy” feedstocks, such wood fiber, due to the seal developed in the screw intermesh and the compression. Screw to screw, the flights have the same geometry and are slanted in the same direction in the nip region, reducing the open spaces between the flights and minimizing leakage from one screw channel to the next. Incoming material is conveyed away from the nip of the screws, around the flights, and converges at the bottom of the screws. The natural compression of a conical twin aids in effective wetting-out of the wood fiber in the polymer system. With the wood fully encapsulated in the plastic, maximum vacuum can be drawn at the vent zone to effectively devolatilize the melt without drawing dry wood fiber through the vacuum system. The above information was supplied by Chris Weinrich, ExtrusionTek Milacron. For information, contact him at (513) 536-2428 or visit www.milacron.plastics.com. In a typical extrusion process, a concentration of wood flour is combined with melted plastic pellets at a high viscosity. A reinforcing agent is mixed into the compound, binding the plastic and wood flour together. The mixed compound is then pushed through a steel mold in the extruder. (Also see sidebar on extruding wood waste) One general concern with processing the material concerns possible inconsistencies in the product. “When you have two different types of materials together such as wood pulp and plastic, without making the necessary accommodation you can get inconsistencies,” says John Shea, president of Canon & Shea Assoc., which represents industry suppliers. “Sagging, warping and twisting can occur because the extruded product isn’t held consistent during production.” Added tooling in the extrusion process can help. “Guill’s Equaflow tool (for example) has a split flow head design which balances the materials equally on both sides for conformity,” says Shea. In many instances the processing technology is proprietary, with licenses sold to manufacturers. For example, EinWood’s composite technology calls for drying the wood to 0 percent moisture content prior to processing. It also involves blending and fusing a small amount of plastic with a large amount of wood powder, then using a thixotropy extrusion process to produce panels at high-production rates. The Strandex process on the other hand, Laver says, was developed around the use of a unique die system and process formulation. This formulation contains high levels of wood or other cellulose fiber, thermoplastic resin and other materials which enhance physical properties and process advantages. Machining Considerations According to John Quarmley, director at Principia Consulting, one of the beneficial characteristics of these composites is that they machine similarly to wood. “You can saw, nail, screw, drill and plane the material very much like a piece of wood,” Quarmley says. “Much of the R&D work that wood-plastic composite companies are doing today on the formulation is to get them to act even more like wood. One of the things that they are doing now is putting in blowing agents that will make the core of the board be of a very high density foam with air pockets within the product, so that it will nail, screw and saw even more like wood then ever before.” Generally speaking, the same cutting tool materials that can be used on wood or plastic — carbide, high-speed steel or diamond — can also be used successfully on wood-plastic composites. “You can rout it to create some nice design edges,” says Maureen Murray, a spokesperson for Trex Co. “Because of the high wood flour content, when you fabricate the material with a router, you can smell the wood just like you would with a (regular) wood product,” Sakaguchi adds. Products such as EinWood composites, he adds, can be sanded, stained, painted and finished with the same tools used on natural wood products.
The recent decision by the U.S. Environmental Protection Agency to phase out the use of chromated copper arsenate pressure treated lumber over the next two to three years is just one of the factors propelling the growth of alternative composite decking materials to an anticipated 488 million board feet by 2005. Overall, U.S. demand for decking is projected to increase 1.6 percent per year through 2005, to 5.1 billion board feet. According to information in the Wood & Competitive Decking study by The Freedonia Group, advances will be particularly strong for alternative decking materials such as wood-plastic composites, vinyl and polyethylene, which will increase their combined share of the market to over 10 percent by 2005. Despite the EPA’s decision, wood should continue to dominate the decking market through 2005, making up nearly 90 percent of overall sales. According to Robyn Margulies, Freedonia analyst, the demand for wood decking will stem from the familiarity of builders and consumers with the material, as well as from its high tensile strength and good aesthetics. Although wood will probably lose some of its market share to wood-plastic composite materials, Margulies says that substitutes for CCA in treated lumber should be in place to offset any large changes. From Decks to Chairs The Deck Yard and Country Gardens in St. Charles, IL, was founded in 1992 by Ted Palpant, who specializes in making furniture ideas into a reality for everything outside the home. Palpant uses Trex Wood Polymer lumber composites in his products. “The (wood-plastic) material works almost the same as any wood you would use,” says Palpant. “It routs, sands, cuts and shapes — everything we can do with wood we can do with composites — and just as easily.” Palpant says the material’s water absorption ratio is negligible, which makes it ideal for exterior applications. “You don’t get some of the cracking and warping that you might get with wood,” says Palpant. “For instance, we do a lot of projects for parks, swimming pools and campgrounds. A lot of furniture that is constantly sitting in wet environments.” The idea of having maintenance-free furniture goes a long way for his customers, who purchase cupolas, planters, benches, swings, rockers, chairs, tables and bridges. “I don’t think people want to be out maintaining public furniture,” says Palpant. “Another reason the swimming pool people like it is because it is heavier. This makes it less likely for people to slide it around and toss it into the swimming pools.” Fit to Finish Due to their plastic properties, wood-plastic materials will accept colorants during the processing phase, which often eliminates the need for additional finishing. Otherwise, experts note, the finishing process is similar to that of wood. Because a lot of the ends of the wood fibers are exposed on the composite material, these products can be stained and painted. “You’re going to use the same tools and materials to finish this, especially if the composite is 50 percent or greater loading of wood fiber,” Crostic says. “Unlike plastic, which doesn’t paint very easily,” Quarmley says, “the wood plastic composites accept paint quite well for interior applications and can also be pigmented and supplied in a wide range of colors. The pigments are susceptible to sunlight and over time do fade. However, some of the major chemical companies are coming up with different additives that would make the colors more light-stable so they have better color for a longer period of time,” he adds. These recent developments in finishing and processing technologies are giving wood-plastic composite producers a bright outlook for future growth. “We do feel with the harder look at alternatives, the future looks bright,” says Murray. “We think we have a lot of room for growth — we’ve only reached the tip of the iceberg.” Wood Flour: The Basic Ingredient in Wood-Plastic Composites Wood flour is added to plastic materials, such as polypropylene, polyethylene, polystyrene, ABS or PVC, to manufacture flooring, decking, mouldings and other products. According to Fred Faehner, president of P.J. Murphy Forest Products Corp., wood flour is defined as a finely-ground wood cellulose with a mesh size above 20 mesh or below 850 microns. The mesh size, Faehner explains, is determined by the number of openings in the screen, measured to the linear inch. As the mesh size increases the particles become smaller. Typically, flours made from hardwoods are preferred when manufacturing wood-plastic composites, Faehner says. “The reason is that most softwoods have aromatics in them — organic chemicals which can be inflammatory and can ignite in the extruder.” Pre-drying the flour to under 1 percent moisture content helps the processing, he says. Additional hints provided by P.J. Murphy Products Corp. include: • Always keep process temperatures as low as possible. Temperatures below 395F (200C) are recommended to keep wood flour from burning. • Always allow for adequate venting. The moisture driven off by the extrusion process must be able to escape the extruder, thereby cutting down on both temperature and pressure. • Increasing the resin MFI will decrease the heat within the screw by cutting down on the shear from the wood flour. Also, allow for two or three mixing zones in the screw configuration. The more uniform the mixing, the lower the process temperature will be. • Twin screw extruders seem to work better than single screw extruders for wood filled plastics, Faehner says. However, if using a single screw extruder, he suggests using a compounded pellet. Wood-Plastic Conference Slated Sponsored by the USDA Forest Service, the 7th international conference will explore a range topics such as: composite processing, material requirements and market opportunities. For more information contact Julie Lang, USDA Forest Service conferences director, at (608) 231-1361, ext. 208. Click here to go back to the PMF feature articles.
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