Composites Do Wood One Better

Wood-filled/thermoplastic composites, first introduced to consumers in the early 1990s, may represent the largest single consumer growth market for composite materials. Extruded or compression-molded wood-filled/thermoplastic lumber for outdoor decks — the "killer app" according to John Pruett, marketing manager for CertainTeed Corp.'s (Valley Forge, Pa., U.S.A.) Boardwalk composite lumber product — has grown 35 to 40 percent per year over the last five years. Today, at least 50 companies offer composite products of wood-reinforced polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC) for decks, docks, door and window trim (both interior and exterior) and automotive interiors.

The current U.S. market value for this sector is estimated at more than $600 million, representing more than 1 billion lb of material says a recent market study by Principia Partners (Exton, Pa., U.S.A.). Principia has pegged the total demand for wood-filled/thermoplastic materials in North America and Western Europe at $2 billion by 2007. Spurring the growth trend is a U.S. Environmental Protection Agency announcement that pressure-treated wood containing chromated copper arsenate (CCA), traditionally used by homeowners for exterior construction, will be banned for residential use after Jan. 1, 2004.

The term "composite" in the building materials industry is often used rather loosely to describe any wood material adhesively bonded together — capturing plywood, oriented strandboard, particleboard, fiberboard and other types of wood panels. This article focuses more narrowly on products made from wood or other natural fiber fillers in a continuous matrix of thermoplastic resin. (A third group of wood composites, which combine solid wood with fiberglass or carbon fiber reinforcements wet out with thermoset resins, was covered in "Engineering Insights," CT March/April 2001, p. 40.)

"These products are so popular that producers can't make them fast enough," says Al England of Strandex Corp. (Madison, Wis., U.S.A.), a developer of composite wood extrusion technology. "The biggest advantage is that they require no maintenance." Composite wood is uniform and dimensionally stable, impervious to rot and insect damage. Unlike wood, it doesn't take up moisture and swell. But it can be sawn conventionally and joined with ordinary nails and screws. And it's "green" — made with recycled products and recyclable. While some early products suffered from fading, new pigments and formulations are much more color stable.

Despite its advantages, current composite lumber doesn't have the load-bearing strength of wood and can't be used as joists or structural members, although some companies offer stronger glass-reinforced products. And, the upfront materials are costly. At present, composite products retail for more than two times the cost of real wood, but have lower life-cycle costs when the expense of cleaning, painting, staining and preserving wood are figured into the equation.

MATERIALS AND PROCESS

The principal filler in wood composites is wood flour made from recycled sawdust, planer shavings, sanding dust and scraps, purchased by wood recyclers from a variety of industries. The collected scraps are hammermilled to form a very fine powder, classified by the standard sieve mesh size that it can pass through. Flour that passes through a 40-mesh sieve (with 40 openings per square inch) is termed "40-mesh," made up of particles 420 microns (0.42 mm) in diameter or smaller with an aspect ratio of 2:1 to 4:1. Most wood-filled thermoplastic manufacturers specify flour in the 30- to 80-mesh range. Some manufacturers also use recycled papermaking waste, containing individual cellulose fibers with an aspect ratio of between 10:1 and 20:1.

John Youngquist, a retired U.S. Forest Service Forest Products Laboratory employee (Madison, Wis., U.S.A.), says that wood flour or fiber filler has the advantage of light weight and low cost — about $0.50/lb — and it does not abrade molding equipment or dies. In reality, any of the agri-fibers like coir, kenaf, rice hulls, corn and flax can also be used to make composites, says England. Japanese composite lumber maker Eidai Kako Co. Ltd. has even tried ground bamboo in its products, with some success, says Eidai spokesman Frank Sugiyama.

"The two materials have great synergy — plastics expand and contract due to temperature changes and wood expands and contracts with humidity changes," says Brent English, also of the Forest Products Laboratory. "Adding wood to plastic significantly decreases thermal linear expansion, often by 50 percent or more, and the plastic mitigates moisture movement into the part," notes English.

A handful of wood recycling companies provide the majority of the wood flour to the industry, including P.J. Murphy Forest Products Corp. (Montville, N.J., U.S.A.), American Wood Fibers (Shofield, Wis., U.S.A.) and Southern Wood Services LLC (Macon, Ga., U.S.A.). Some manufacturers, like sawmill operators who have added composites to their product line, make their own. "The system works like scrap resin collection," says Fred Faehner, president of P.J. Murphy. "We take only species-specific wastes without impurities like waste glue so that we can ensure product quality to our customers." Faehner says his facility processes the flour to drive off more water, to get to a uniform moisture content of 3 percent. The dried flour material is blown into truck freight trailers and hauled to producer facilities.

"Scrap from secondary wood products like furniture or cabinet makers is preferred," says Doug Gardner of the Advanced Engineered Wood Composites Center at the University of Maine (Orono, Maine, U.S.A.). "The wood has already been kiln dried to an ambient moisture content of about 8 to 10 percent — drying fresh sawdust from green wood is very energy-intensive." Low moisture content is key, since drier material mixes better and is less likely to burn at processing temperatures. Martin Grohman, president of Correct Building Products LLC (Biddeford, Maine, U.S.A.) says, "To get the best product, you want a plastic molecule to be anywhere there was a water molecule in the wood material — to do that you have to drive off all of those water molecules. We've spent a lot of development time on drying technology."

Hardwood species are preferred by most manufacturers over softwood like Southern pine, because they contain less sap and thus fewer volatile aromatic hydrocarbon compounds that can react with polymer chemistry and off-gas during processing, which creates voids and affects surface quality. Wood species also can affect final color and processability, because of the wood's natural pigmentation and "reactivity," that is, how it interacts with the resin chemistry and other additives like stabilizers or lubricants. For example, the tannins present in oak can react with formulation chemistry.

"Typical additives in wood flour/polymer formulations include processing aids like lubricants, antioxidants and acid scavengers and property enhancers, like biocides, coupling agents, inorganic fillers, fire retardants, UV stabilizers and pigments," says Gardner. Foaming agents like carbon dioxide gas are sometimes employed to reduce the density of the final product. Typical wood loadings range from 20 to 60 percent.

The most common thermoplastic resins used are PE, PVC and PP, supplied by large commodity resin makers like Dow Chemical Co. (Midland, Mich., U.S.A.) and BP Solvay Polyethylene (Houston, Texas, U.S.A.). If viewed as a hierarchy, PE is typically the cheapest resin and has excellent toughness and impact strength, but the lowest in-service temperature. PVC is the strongest resin with the highest use temperature, but is the most brittle, measured by percent elongation. PP has good use temperature characteristics and the lowest density of the three. Wood burns at 225°C/437°F so processing temperature is critical. Polycarbonate, for example, can't be used for this application because its melt temperature is too high. "Wood is very heat and shear sensitive," explains Grohman. "If time in the extruder is more than 45 seconds to a minute, the wood will start to thermally decompose and darken."

Composite lumber is produced with single-screw extruders, counter-rotating twin-screw extruders and custom extruder setups. A single-screw system is the least expensive type, but typically uses pre-compounded pellets (containing the plastic resin, the wood flour and additives), an added processing step which adds cost, and overall output is lower. Because the polymer is melted with the fiber in the single screw, there is a greater risk of thermal decomposition of the wood.

Twin-screw extruders are preferred for wood products because the mixing action blends the polymer and the wood filler together in a single process, with low screw speed and low-shear mixing, which keeps temperatures down, says Gardner. Using resin with a higher melt flow index (MFI) (i.e., lower viscosity) also will help to decrease the heat within the screw by cutting down on the shear forces created by the fluffy and difficult-to-mix wood filler. This type of system requires upstream material blending and flour drying equipment, as well as vents to remove the water vapor.

England and his partners, as well as other machinery makers, have developed production technologies, using equipment customized for wood composites. The Advanced Engineered Wood Composites Center's Woodtruder, a counter-rotating twin extruder machine with a separate single-screw extruder, keeps wood feeding and drying separate from polymer melting until well down the barrel. This strategy limits damage to the wood filler and ensures that filler is completely encapsulated in polymer. The Woodtruder is available for product development, prototyping and testing. England's patented Strandex system has been licensed to nine companies to date, including Kroy (described below), Eidai Kako and Louisiana-Pacific Corp. (Portland, Ore., U.S.A.) and incorporates some custom touches for cost-effective wood processing.

The continuous extrusions are cut to length as they come out of the die. Most profiles are solid, but a few are complex profiles with hollow space. Solid profiles weigh more than a hollow profile, about the same as green hardwood, which increases shipping costs. Some products are embossed or patterned while still warm to improve their resemblance to wood.

A BEVY OF BUILDING PRODUCTS

Italian extrusion machinery maker ICMA San Giorgio (San Giorgio su Legnano, Italy) was an early developer of wood-reinforced polymer products, with partner G.O.R. Applicazioni Speciali SpA. Their 1974 Woodstock process mixed polypropylene (PP) flakes with wood flour to make sheet material for auto interiors, still used today by several licensees. Anderson Corp. (Bayport, Minn., U.S.A.) was one of the first U.S. companies to adopt wood flour/thermoplastic extrusions for its window and door profiles and holds 30 patents for its wood/PVC processes.

A composite lumber product called Rivenite, a solid wood/PE profile, was developed in the late 1980s. Mobil Chemical Co. purchased Rivenite in 1992 from developer Roger Wittenberg and formed a composite products division to market the product, with considerable success. In 1996, a group of Mobil executives, including Wittenberg, purchased the division's assets and formed Trex Co. (Winchester, Va., U.S.A.). Today, Trex is the largest composite building products manufacturer (based on deck board sales), with multiple extrusion lines in Virginia and Fernley, Nev., U.S.A.

Trex manufactures its decking and railing products from both low-density and high-density recycled PE obtained from recycled grocery shopping bags and pallet stretch wrap film collected primarily at supermarket chains. The material, commingled and baled by the supermarkets, is supplied through a well-established collection network, says Trex spokesperson Maureen Murray. Off-spec PE is sometimes used along with the recycled material. Wood flour is mostly hardwood, from furniture operations and recycled pallets. The approximately 50 percent wood/50 percent PE profiles are produced in five different colors with a combination of single- and twin-screw equipment. No pattern is embossed on the finished product.

The solid Trex material can be cut, routed, sanded and fastened like wood. "Dealers set their own prices, but our product compares to a higher-grade redwood decking product, at between $3/ft2 and $3.40/ft2," says Murray.

CertainTeed, a division of material supplier giant Saint-Gobain, is a major building products producer, with 50 manufacturing facilities in North America. The company, which has been making PVC extrusions for windows and siding for decades, began producing wood-reinforced composite products in 2000. Boardwalk composite decking and railing is a patented 40 percent oak flour/60 percent PVC material called EcoTech. Profiles are made with multiple extrusion lines at CertainTeed's Jackson, Mich., U.S.A. facility, says Pruett. The process uses PVC from the company's extruded vinyl profiles and recycled, broker-supplied wood flour.

CertainTeed prefers to offer solid profiles over hollow because they are more impact resistant and have lower thermal expansion properties, which translates to better performance during winter's freeze/thaw cycles in cold climates, says Pruett. "PVC is a slightly more expensive resin, but it has better properties," he says. "Boardwalk composite deck planks and railing components are stiffer and more durable than hollow extrusions." Some weight is saved by the inclusion of a proprietary foaming additive, which creates microscopic openings that decrease the material density.

Correct Building Products, which began making CorrectDeck material in 1999, was the first producer to use PP, and very few competitors have followed suit. PP has better properties than PE, but the issue, says Correct's Grohman, is that it processes at a temperature 50° to 75° higher than PE.

"There's a very small overlap between the melt temperature of the PP and the thermal decomposition temperature of the wood flour," explains Grohman. "We've had to work hard to make the process work."

Correct uses only white hardwood flour (birch, red maple and beech) for color control in the final product. While suppliers like P.J. Murphy do provide wood to the company, Correct also tries to use scrap from local Maine industries like wood dowel makers, who use very dry and dimensionally stable wood. Correct further dries the wood flour to a moisture content of 0.5 percent before gravimetrically feeding it into Milacron counter-rotating conical twin-screw extruders. The CorrectDeck product is thinner than Trex or Boardwalk, at 7/8-inch, which is possible because of the higher stiffness of PP, says Grohman.

Kroy Building Products Inc. (York, Neb., U.S.A.) produces its wood flour/PE Timberlast deck profiles and related accessories under a Strandex license. Timberlast is an engineered hollow profile, with fluted edges, installed with a patented clip system, says composites product manager Ben Kaczmarek. Kroy considers the profile concept preferable to a solid board because of a better weight to performance ratio. It's lighter, easier to install and can be produced with a textured surface, which addresses the most frequent consumer request, says Kaczmarek.

Strandex is a twin-screw process, in which the wood and polymer materials are mixed together upstream and then fed gravimetrically to the extruder. England says the deep flight screws mix the materials very efficiently and speed output rate. The key to the process is a "stranding plate" midway down the extruder through which the molten composite passes. "Pushing the material through the holes in the plate helps reduce shear stresses in the die and maintains better uniformity in the product," he explains.

The process eschews the pullers, calibrators and cooling jackets that are normally used in the production of extruded profiles, to keep the profile shapes intact while cooling. Strandex employs a simpler water spray system to quickly cool the extrusion, keeping tooling costs down, says England. The profiles are run through a brushing tool while still warm to achieve a textured appearance.

Like competing solid profiles, Kroy's Timberlast is not designed for structural applications. But Kaczmarek points out that many companies are working on stronger, structural composite lumber. "The top of the deck, the walking surface, represents only one-third of a total deck system — the substructure also represents significant market opportunities," says Kaczmarek. U.S. Plastic Lumber Ltd. (Boca Raton, Fla., U.S.A.) is currently producing Carefree Xteriors structural lumber products made with fiberglass/high-density PE suitable for joists, beams and posts.

Many more companies make similar products. Big producers include Advanced Environmental Recycling Technologies (AERT, Springdale, Ark., U.S.A.), maker of the ChoiceDek product distributed by lumber giant Weyerhauser; Crane Plastics (Wilmington, Ohio, U.S.A.), the source of TimberTech decking; Louisiana-Pacific Corp. (Portland. Ore., U.S.A.), maker of WeatherBest decking; and Nexwood Industries Ltd. (Brampton, Ontario, Canada). Several producers are even compression molding wood composite products.

Although, according to some industry observers, there is likely to be some shakeout, the boom in wood composite decking shows no signs of slowing, and some say that wood composite pallets could be the next "killer app."

"The plastics industry has traditionally been fast-moving and dynamic, while lumber businesses are somewhat traditional and staid," says Grohman. "It's interesting to be in a business that straddles both worlds."