Wood-Filled Composites Jump off the Deck

Wood-filled thermoplastic composites have come a long way since they were first introduced in the early 1990s. Advancements continue to be made at all levels in this dynamic market. Yet one thing is unchanged: growth potential.

According to market analyst Tonia Ferrell at The Freedonia Group LLC (Cleveland, Ohio), demand for wood-filled thermoplastic composite lumber is unprecedented. By far the biggest sector of the wood-plastic composite (WPC) industry, this segment accounts for deck board and railing, molding and trim, fencing and door and window components in the residential construction market. According to Freedonia’s Composite & Plastic Lumber report (issued in February 2006), WPC lumber production is forecast to expand nearly 14 percent per annum through 2009 to 2.7 billion lb per year.

WPC deck board, alone, accounts for about 50 percent of overall WPC volume and has staked claim to a good piece of the $4.6 billion U.S. decking market. “Wood-plastic composite has increased from 4 percent in 1996 to 14 percent in 2006 of the overall decking market,” says Ferrell. That percentage is expected to grow to 23 percent by 2011 and 32 percent by 2016. “You can’t be in the deck lumber distribution business and not be in composite decking,” says John Pruett, a principal with strategic consulting firm Principia Partners (Exton, Pa.). “Even pressure treaters distribute composite decking along with their pressure-treated woods. ‘Big box’ retailers, like Home Depot and Lowe’s, also are serious players in composite decking.”

Despite a weaker new housing market, the decking market is relatively stable because more than 85 percent of demand is generated through inherently less cyclical remodeling and repair (R&R) activity. “Approximately 88 percent of WPC decking is from R&R,” says Pruett. “Therefore, the more meaningful economic indicator for this sector is consumer confidence and its effect on outdoor living within home improvement spending.” That confidence is expected to hold: U.S. demand for decking is projected to advance 2.2 percent per year through 2011, reports Freedonia in its latest Wood & Competitive Decking report (July 2007).

Lessons Learned

One reason for consumer confidence is continuous improvement in WPC lumber performance and aesthetics. “The expectations of the composite deck consumer remain very high,” says Martin Grohman, president of Correct Building Products LLC (Biddeford, Maine). “We’ve gotten much closer, as an industry, to meeting these expectations.” In 2004, Correct Building Products conducted market research toward that end, interviewing 100 composite deck owners in Austin, Texas, Sacramento, Calif. and Richmond, Va. “They all told us pretty much the same thing,” says Grohman. “The deck boards get permanent grease stains, the color fades, and it’s hard to prevent mold and mildew.”

The answer to these flaws lies in better encapsulation of the wood. “The problem of protecting wood in our composites is as old as wood itself,” says Grohman. “Wood is great material. It looks great and it’s strong, but what it does in the tree, which is move moisture around, is not what you want it to do on the outside of your house. The best we can do is to take advantage of its strengths while protecting it from its weaknesses.”

Price also remains a factor for WPC deck board. Overall, consumers experience a 10 to 20 percent increase in cost with a composite decking system compared to pressure-treated wood. However, most composite products today are designed to compete with redwood, cedar and exotics like ipe, which have higher price points. The industry is beginning to break down into niches, says Grohman. “We see a niche at the high end, in ultralow-maintenance composite decking, which is where we’re focusing our future efforts,” he adds. Correct Building Products also has carved out a niche in WPC tongue-and-groove porch flooring.

Overall, wood-plastic composites carry a positive image in the eyes of most consumers. “Manufacturers have learned how to market their WPC products more effectively, especially in terms of recycled content,” says Ferrell.

“We used to sell primarily on looks and performance — no splinters, low maintenance — but now consumers are more concerned about the impact on the planet,” explains Maureen Murray of Trex Co. (Winchester, Va). Much of the WPC board produced today contains at least some recycled material, be it reclaimed wood, recycled plastic, or both. Trex and Springdale, Ariz.-based Advanced Environmental Recycling Technologies (AERT), which manufactures Weyerhaeuser’s ChoiceDek, are two of the bigger producers using both recycled plastic and reclaimed wood.

On the design side, nature-inspired shading, with colors like walnut, rosewood, cedar, driftwood and teak, random grain patterns, and reversible boards are common. “The natural look helps our products stand out against all-plastic and vinyl-covered options,” explains Murray.

To complete the package, the majority of deck manufacturers now offer complimentary railing systems. “Composite railing is growing faster than composite decking,” says Pruett. “All of the major manufacturers have color-matched railings, and there are at least half-a-dozen national brands of coextruded white railing now on the market,” he adds. In addition, TimberTech Ltd. (Wilmington, Ohio) recently introduced a co-extruded black railing system.

Yet it’s not easy to make a good railing system. “Structural load requirements are much greater for railing systems,” explains Michael Wolcott, research director for Washington State University’s Wood Materials & Engineering Laboratory (WMEL, Pullman, Wash.). Consequently, materials with higher structural properties, such as polyvinyl chloride (PVC), are playing a bigger role.

“We’re also seeing a lot of ASA [acrylic-styrene-acrylonitrile] and rigid PVC cap stocks,” says Tom Brown, marketing manager, Milacron Extrusion Systems (Cincinnati, Ohio). Designed to better protect the wood-plastic core against UV light and water absorption, capstocks are also becoming more common on deck boards. CertainTeed (Valley Forge, Pa.) recently added a PVC-ASA capstock to its Boardwalk composite decking series, which is made from the company’s patented EcoTech material — a blend of PVC and recycled natural fibers. “The capstock maintains the rich, natural colors of the product line as well as provides superior stain, scratch and fade resistance,” says Patti Pellock, marketing manager for CertainTeed’s Fence, Railing, and Deck Div.

In addition to Trex, AERT, TimberTech and CertainTeed, the big WPC decking producers include Composatron Mfg. (Toronto, Ontario, Canada), Fiber Composites LLC (New London, N.C.), Royal Crown Ltd. (Milford, Ind.) and Universal Forest Products (Grand Rapids, Mich.). In all, more than 30 manufacturers currently compete in the composite decking market.

Beyond the Deck

A success in the deck market, WPC is finding applications elsewhere. “Wood-plastic composites can be used for just about anything that’s on the outside of the house,” says Al England of Strandex Corp. (Madison, Wis.), a developer of composite wood extrusion technology. Bayport, Minn.-based Andersen Corp. was one of the first firms to incorporate WPCs into window and door profiles. Anderson’s patented Fibrex material is a proprietary blend of wood and PVC. Other products include siding and siding accessories — which England believes will be “the next big thing” — and roofing.

“The biggest current area of commercial development for the WPC industry is fencing,” says Wolcott. “But you can’t just make a deck board and hang it on a fence.” Heavy self-weight is the biggest issue, and the answer could be foaming.

“Foaming can effectively remove from 5 percent to more than 30 percent of a board’s weight,” explains Brown. Foamed WPCs are not only lighter, but the internal pressures generated during the foaming process reportedly result in a texture and appearance that is more like wood.

The technology sounds relatively straightforward — introduce a foaming agent into the wood/plastic matrix, apply a rapid pressure drop to induce foaming and then rapidly cool the part to support bubble formation. However, the fact that the plastic is dispersed throughout the mix, which can have up to 60 percent wood content, makes process control a challenge. “It’s more difficult than foaming pure plastics because you have such a high loading of wood flour, and you can’t foam wood flour,” quips Brown. And because foaming weakens overall mechanical properties, the manufacturer must find a weight/performance balance.

In development for more than four years, Strandex’s Strancel foaming technology is used by Composatron to produce the latter’s new Fensations wood composite fencing, as well as its UltraCell composite deck board. The wood flour-filled, high-density polyethylene (HDPE) board is reportedly 33 percent lighter than comparable composite board.

Many companies now offer products specifically designed for waterfront and structural applications. One such material, Universal Marine Materials (UMM), was developed under the direction of the Office of Naval Research through the combined efforts of WMEL, Strandex, McFarland Cascade (Tacoma, Wash.), Penn State University and the Naval Facilities Engineering Service. UMM is designed to replace treated lumber in piers, docks and footbridges, which require long spans and load-bearing construction. Coextruded with a PVC cap, this wood/PVC composite has built-in connection detailing, a patented fastening system and is engineered to handle high loads, says WMEL’s Wolcott.

“These products are enormous in size compared to a standard deck board,” adds England. “A 4-inch by 12-inch [101 mm by 305 mm] UMM board weighs more than 18 lb/linear ft [26.8 kg/m], which is no more than the wood it’s replacing.” Boards are available in lengths up to 24 ft/3.72m.

UMM was recently used to surface a 90-ft/27.4m footbridge in the Rattlesnake Wilderness and National Recreation Area (Missoula, Mont.), and it also makes up a 300-ft/91.4 wave screen that shelters a marina at the U.S. Naval Academy in Annapolis, Md.

More Material Choices

Virgin and recycled PE (both high- and low-density) remains the most common thermoplastic used in the WPC industry, while the use of PVC continues to grow as more structurally enhanced products emerge. To conserve material, many producers have begun removing material from the underside of boards that will be viewed from only one side, creating a scooped out appearance without affecting overall structural integrity. For materials exposed on both sides, some manufacturers extrude hollow profiles. “PVC allows for hollow profiles with thinner walls,” says Wolcott.

While polypropylene (PP) also offers enhanced properties, its high melt temperature poses a processing challenge. Correct Building Products remains one of the few WPC manufacturers to use PP. Its decking, railing and dimensional lumber products combine virgin and recycled PP and recovered Maine hardwood. “PP has more stiffness, less creep, higher HDT than PE or PVC and is halogen-free,” explains Grohman.

Although most WPCs still incorporate wood flour, WPC manufacturers today have filler alternatives that permit more targeted products in terms of performance. Wood flour, derived from reclaimed pine, maple and oak sawdust, planer shavings, sanding dust and milled scraps, hits the “sweet spot” between performance and cost, according to Wolcott. The smaller the flour size, the better the properties. “You get a higher surface area, which provides a better mix between the wood and plastic,” he explains. (For the basics about WPC materials and processes, see “Composites Do Wood One Better,” in “Related Content,” at left.) However, pulp cellulose fiber, which is generally reclaimed from post-consumer waste paper, is another option.

“During production, chemical volatiles like lignin and tannin are removed to reveal pure cellulose,” explains Edward Schut, director of marketing for Creafill Fibers Corp. (Chestertown, Md.), which mills pulp cellulose and agricultural fibers. “Pulp cellulose fiber has zero coefficient of thermal expansion,” he claims, “and, because it has no lignin, the fiber doesn’t photo-degrade like wood flour.”

To enable extrusion, Creafill’s low-density, ribbon-like fibers are coated with coupling agents and dispersants and are compressed into friable pellets that are customized for a variety of resin types, including nylon, PE, PVC and PP. Royal Crown Ltd., for example, uses a proprietary blend of pulp cellulose and PVC named Celucor, coextruded with a vinyl cap, for its decking materials.

Although interest grows in agricultural fibers, Schut notes, “There just isn’t enough volume being produced in the U.S. and Canada to supply the plastics industry on a large scale.” Instead, Stephen Faehner, VP of industrial sales and business development at American Wood Fibers (AWF, Columbia, Md.), sees “the potential to supplement what we do in wood with some kind of suitable, sustainable ag fiber,” he says. “The main driver for such a blend, from our perspective, is sustainability,” he adds. “We want to ensure that we’ll have the fiber available to allow our customers to grow.”

For many manufacturers, the potential for enhancing WPC physical properties is a motivation for incorporating natural fiber. “Flax, jute, hemp and kenaf have strength-to-weight ratios that are far higher than glass fiber,” Wolcott maintains. “Since they have similar surface functionality and handling characteristics as wood, they can be mixed into a wood composite to regulate properties, much like adding carbon to a glass-reinforced layup.” (To read more about agricultural fibers in composites, click “Natural Fibers Slowly Take Root,” in “Related Content,” at left.)

Processors who use an ag fiber, however, will face challenges related to growing cycles. “Because of its seasonality, there is a limited window of harvest,” adds Faehner. “And then you have the issue of maintaining product integrity during the rest of the year.”

“It’s just a matter of economics,” says England. What is the cost of procuring the fiber, drying it and storing it?”

Moreover, agricultural and other alternative fillers present some processing difficulty. “The challenge of using a fiber with great structural properties is feeding it into the equipment,” says Faehner. Greg Phillips, product line manager for McFarland Cascade, illustrates why, with a promising alternative: “Rice hulls are inexpensive and abundant, but they’re very dense and cause significant wear on equipment,” he explains. Still, rice hulls have high silicate content. “Silicate does not absorb water like wood fiber,” he explains. “Therefore, it eliminates many of the issues that plague wood-based products today.”

McFarland Cascade offers two rice hull/HDPE deck boards — one hollow and one solid. However, the company recently closed its Canadian board plant. “We couldn’t generate enough sales in North America to sustain production,” says Phillips. “The consumer has not yet been fully educated on the benefits of higher quality, more expensive materials,” he maintains, but adds, “We still have a viable market in Europe for these products, and we’re experimenting with outsourcing production to a toll plant.”

JER Envirotech International Corp. (Delta, British Columbia, Canada) is targeting injection molding and a host of new applications for its WPC formulation — a patented combination of plastic, wood waste, and other organic material, such as rice hulls and palm fiber, in pellet form, which can be extruded, injection molded or thermoformed. Possible areas of application include a compression-molded WPC panel board that can be used as a “substitute for standard or marine-grade plywood.”

New Processing Options

In terms of manufacturing, extruded lineal products continue to drive the North American WPC industry. Injection molding and compression molding together account for less than 10 percent of the overall market. The picture, however, is slightly different in Europe where injection molding is being adopted in a host of applications.

“Injection molding is a frontier that hasn’t really been tapped in the North American WPC market,” says Faehner. “It can be a challenging process because wood will begin to degrade at temperatures above 200°C [392°F], and as you load up with wood, you dramatically affect your melt flows.”

“The lion’s share of the machines in operation today are 80-mm to 86-mm counter rotating, conical twin-screw extruders with maximum throughput rates of 1,000 lb/hr and 1,700 lb/hr respectively,” says Milacron’s Brown. Because of the large feed zone, cone-shaped screws offer a big advantage for wood composites, which are relatively low in bulk density. Vent zones and vacuum venting systems are needed to draw out the moisture in the fiber, says Brown, who adds that while it is possible to run wet wood fiber, he doesn’t recommend it. “If you want to run at maximum rates and make the best products, take the moisture out of the wood before you extrude it,” he says.

Adding Quality

Additives are playing a key role in enhancing product characteristics, particular-ly in the area of weatherability. More than 50 companies currently supply the market with a variety of additives, including lubricants, UV stabilizers, flame retardants, antimicrobials, color concentrates, coupling agents, foaming agents, dispersion agents, mineral fillers, antioxidants and compatibilizers.

It’s a big change from just five years ago, says Jim Krohn, business development manager, Equistar Chemical Co. (Houston, Texas). “Lubricants, colorants and mineral fillers (primarily talc) were once the common additives, but today, there is a broader use of maleic anhydride-based coupling agents to improve dispersion and adhesion between the wood and plastic, as well as tannin inhibitors, antifungal biocides, and other mineral fillers.”

Better dispersion and adhesion between the wood and polymer have a direct affect on product durability, reducing the rate of moisture absorption and improving strength properties. “Coupling agents hook into the plastic matrix and then attract the wood,” explains Krohn.

Equistar’s Integrate series of polyethylene- and polypropylene-based coupling agents are polyolefins that are chemically modified to provide polar anhydride functionality on the polymer backbone. There is a wide assortment of coupling agents available, including Fusabond from DuPont Co. (Wilmington Del.), Polybond from Chemtura Corp. (Middlebury, Conn.), and Eastman G polymers from Eastman Chemical Co. (Kingsport, Tenn.).

A new compatibilizer developed by Oregon State University’s (OSU) Department of Wood Science and Engineering promises the ability to mix wood with nylon — typically not used in WPCs because nylon has a higher melt temperature than the wood degradation temperature. The hope is that this new technology will allow the use of inexpensive waste products, such as old nylon carpet fibers, that typically go into landfills. Further testing is being done using an E-Max 27mm twin-screw, corotating extruder donated to OSU by Entek Extruders (Lebanon, Ore.).

Colorants remain a pivotal additive, especially in the battle against color fade. Research conducted by the USDA Forest Service’s Forest Products Laboratory (FPL, Madison, Wis.) shows that the addition of a pigment in a carrier wax could provide some protection against the fading experienced by WPCs when exposed to UV light and water spray, which can degrade not only aesthetic appeal but mechanical properties as well.

To provide more long-lasting, realistic color, suppliers like PolyOne (Cleveland, Ohio), which recently introduced OnColor, a color and additive concentrate specifically designed for WPC, and Clariant Masterbatches Div. (Winchester, Va.) have turned to more colorfast and highly reflective pigments. Many colorant suppliers now offer customized packages that combine pigments with lubricants and other necessary additives. Lubricants, which improve surface appearance and processibility, help drive down cost through increased throughput.

Many producers add zinc borate to WPCs to ward off fungal decay and inhibit the growth of mold and mildew. Correct Building Products, for example, now offers its CorrectDeck CX brand WPC with trademarked Microban antimicrobial product protection. Commercial sources of antimicrobials include Ciba Specialty Chemicals (Tarrytown, N.Y.), Rohm and Haas Co. (Philadelphia, Pa.) and U.S. Borax (Greenwood Village, Colo.).

With greater emphasis on additive-driven quality, WPC formulations are getting more complex. A rough formulation today might be 55 percent wood flour, 5 percent talc, 4 percent lubricant, 2 percent colorant, 2 to 3 percent other additives such as coupling agents and/or mold and tannin inhibitors and 33 percent resin. “As you add lubricants or coupling agents, you back off on your plastic in the overall ratio, and when you add a mineral filler like talc, you back off on your wood fiber,” explains Krohn. The result is a more difficult balancing act, but one producers are willing to perform to earn consumer confidence.

“Everybody in this industry wants to make their existing product better,” says England. “They want better durability, better color retention, higher strength-to-weight ratios, better dimensional stability [and] higher processing rates.”