Extruding composites of wood and plastic has long been a "black art" practiced by a few specialists. But in the past year, this "hidden" market has burst into the sunlight as new producers and new applications sprout up all over.

Wood and plastic are best friends these days. Composites of wood flour and PP, PE, or PVC are the buzz in the building-products business.

Dozens of firms are seeking to exploit these materials for manufacturing tough, lightweight window profiles, plastic lumber, and interior auto panels. "Per-pound cost is extremely low, throughput rates are extremely high, scrap is minimal," says James Pratt, v.p. of Comptrusion Corp., a two-year-old company that makes wood-filled window profiles.

The groundswell of new activity is impressive--especially considering the technical difficulties involved in handling and processing combustible wood flour safely and productively. Industry estimates peg total production of wood-plastic compounds at nearly 300 million lb last year, up from an estimated 100 million lb two years earlier.

'Tip of the iceberg'

Louisiana-Pacific Polymers Inc. took 18 months to reach commercial production last October of a new 50/50 wood/polypropylene sheet. It's being used for interior door panels of 2001-model cars and trucks. The company is a partnership between forest-products giant Louisiana-Pacific and auto-parts supplier G&L Industries. The venture is using G&L's hardboard scrap to make composite sheet for G&L to thermoform.

Meanwhile, Mikron Industries started up in December a new plant that will eventually house eight extrusion lines making window profiles out of thermoplastic resin alloys with 10-70% wood fiber.

Trex Co. broke ground in February on a new plant near Reno, Nev., that will make 50% wood-filled polyethylene decking. Trex is putting two extrusion lines in Nevada to supplement its eight existing lines in Virginia in order to meet sharply increased demand for its product. Sales grew from $3.5 million in 1993 to $34.1 million in '97. The Reno plant will buy used stretch wrap from local warehouses to use as raw material.

"That's just the tip of the iceberg. There are a lot of projects in the works sampling wood flours, but it takes time to qualify new parts to meet building and automotive codes," says Don Murray, industrial sales manager of American Wood Fibers, a commercial supplier of wood flour.

Old forest-products companies are being transformed into plastics-composite manufacturers. They see plastics as a way to make new construction materials with attributes that wood lacks--like moisture, rot, and insect resistance. The composites are less expensive than board lumber, and they recover higher value from their own wood scrap. In addition, the composite's screw-holding power is far higher than that of pine lumber.

On the plastics side, processors see wood as a readily available, inexpensive filler that can lower their resin costs, add stiffness, and improve wear resistance of products like tool handles, stair railings, door sills, and siding on hot tubs. Wood fillers also increase profile extrusion rates because wood cools faster than plastic. "There's no need for calibrating to shape a part. It's cooled strictly with chilled water," says Computrusion's Pratt. Wood also imparts natural uv resistance and causes less abrasion to extrusion machinery than do mineral fillers.

On the negative side, handling wood flour adds a fire hazard and risk of explosion. So processors may need to alter their plants to add spark detection, upgrade sprinkler systems, and build firewalls.

Woodstock 25 years later

Wood-filled plastics go back 25 years to a proprietary process, called Woodstock, for mixing 50% wood flour in PP. Woodstock was patented by extrusion machinery builder ICMA San Giorgio in Italy and processor G.O.R. Applicazioni Speciali SpA. (They later transferred the rights to resin supplier Solvay S.A. in Belgium.) The process went into production in 1971 at G.O.R. for Fiat cars. Materials were premixed, cram-fed into ICMA's counter-rotating twin-screw, and extruded in-line into sheet for use in door panels and other interior parts. Lear Corp.'s American Woodstock plant licensed this process in 1983 and still uses it today to make automotive panels.

Eight years ago, ICMA patented a corotating twin-screw approach to wood-filled composites. "A corotating twin allows separate feeding of wood and polymer, with side feeding of the wood flour instead of preblending," says Marco Rezzonico, commercial director of ICMA. One producer in Spain uses another ICMA patent for coextrusion of a three-layer sheet, called Naturcore, using the corotating twin-screw process. Naturcore is used to make crates for vegetables and fish. It has wood filler in the middle between cap layers of unfilled PP.

Next, Bausano Group in Italy came up with an approach of adding molten polymer to dry wood flour, instead of vice versa. Bausano uses two counter-rotating twin-screw extruders, one feeding into the side of the other. A large 35:1 L/D twin-screw dries the wood flour by drawing off moisture through two barrel vents, the second of which uses vacuum. A smaller 24:1 twin-screw melts and mixes PP and stabilizers and feeds them into the side of the larger extruder two-thirds of the way down the barrel, after the second vent. Louisiana-Pacific Polymer's new plant uses this technology.

Putting wood fillers into plastic lumber and window profiles started in the early '90s. Andersen Corp. launched the first wood-fiber reinforced PVC profiles, called Fibrex, in '93. Andersen started with patio door sills and has since expanded Fibrex to include custom replacement windows.

Andersen coextrudes a smooth color layer of neat PVC capstock over the fiber-filled core. Competitor Certainteed Corp. does the reverse: A neat PVC core gets a cap layer of wood-filled PVC so that it can be stained or painted like wood.

An even newer market for wood fillers is commercial pellet compounds. "That market is still in its infancy," says Michael Ford, marketing v.p. for Natural Fiber Composites, which compounds up to 60% wood filler into custom formulations. Big commercial compounders have yet to embrace wood flour, but virtually all have sampled it. "Messiness has been one of the drawbacks," Ford says. "It's dusty, and it can be volatile. A lot of color compounders work in clean-room environments, so it's easier for them to subcontract out the wood-filled compounds." But when wood-filled compounds become a proven commercial success, the major compounders will want a piece of the action, Ford predicts.

All wood is not the same

Commercial wood flour comes in mesh sizes of 20 to 100, but most thermoplastic applications fall in the 40 to 80 mesh range. "Fiber" has an aspect ratio (length ÷ diameter) of 10:1 or 20:1, while "flour" generally has an aspect ratio of 1:1 to 4:1. Shore Pacific makes compounds with pulverized flour as fine as 200 mesh. Dura Products International makes pallets with coarse 20-mesh fiber.

Fine-mesh flour adds stiffness but hurts impact strength. Longer wood fibers contribute strength and light weight but are trickier to bind into the composite.

Besides mesh size and uniformity, it also makes a difference what species of wood the flour or fibers come from. American Wood Fibers offers 15 grades based on hardwood or pine. P.J. Murphy Forest Products offers materials from pine, spruce, hemlock, maple, oak, and birch. Ellingers Wood Flour sells 10 grades from two wood species. "Hardness of the wood and cell structure probably have an effect on the hardness and flex modulus properties of the composite," notes Fred Faehner, president of P.J. Murphy. Wood species also affects color.

It's interesting to note that one Japanese processor of wood-flour/PE composites is experimenting with bamboo. Osaka-based Eidai Kako Co. says it has not yet achieved consistent quality with this approach.

Learn to handle it right

Wood flour is very sensitive to moisture and temperature. Temperatures over 375 F begin to degrade it. Moisture levels above 0.5-0.7% in the flour will make the composite foam in the extruder. (However, some processors deliberately combine chemical foaming with wood filler.) Makers of wood/plastic composites specify maximum moisture levels for incoming wood flour. Andersen tolerates no more than 8% moisture in its in-house scrap. LP Polymers allows up to 6%. But Dura Products accepts sawdust from lumber mills with up to 20% moisture.

Plastic and wood don't mix easily, though some polymers are more compatible than others. PVC, which is polar like wood, reportedly bonds to the filler without special alloying or coupling agents. Polyolefins adhere less well to wood, so they are more likely to need modification. In the case of one patented process (developed and licensed by Strandex Corp.), a pinch (2-3%) of thermoset resin helps bind wood to either polyolefins or PVC.

A taste of the action

Primary growth areas for wood/plastic composites are still window profiles, sheet for car interiors, and plastic lumber for decking. Commercial compounds and masterbatches are also growing, but more slowly. Here's a rundown of who's doing what:

AERT Corp. (Advanced Environmental Recycling Technologies) in Springdale, Ark., was one of the first to launch wood-filled PE profiles for windows and doors in the early '90s. It still holds one of the earliest and broadest materials patents on wood-plastic composites.

Andersen Corp., Bayport, Minn., launched Fibrex profiles with 60% PVC and 40% wood fiber in 1993 for patio door sills and later for replacement windows. Both the PVC and wood flour come from scrap generated in making its other products. Andersen first pelletizes the mix on a twin-screw compounder with a single-screw hot-melt extruder, then extrudes the profiles on Cincinnati Milacron conical twins. Andersen's patents say the wood fibers are aligned during extrusion, which reportedly increases tensile modulus to 0.7-1.1 kpsi from 0.43 kpsi for neat PVC.

B&F Plastics in Richmond, Ind., makes thick structural sheet out of highly filled polyolefins. The five-year-old company's products include compounds of wood flour, recycled plastic, and recycled tires. B&F's first compound, called Tireplast, uses 70% recycled tire crumb and 30% polyolefin. Its second product, WoodPlast, commercialized two years ago, uses up to 50% wood flour and 50% recycled HDPE. B&F pelletizes the mix on a corotating twin-screw compounder from ICMA San Giorgio and extrudes sheet on six single-screw extruders.

Certainteed Corp., Valley Forge, Pa., makes wood-composite PVC window profiles, called CertaWood, in Grinnell, Iowa. The product was launched last year for Haverford-brand replacement windows and New Haven-brand windows for new construction.

Comptrusion Corp., Toronto, a Strandex licensee for two years, makes custom window and door profiles. It uses 40-60 mesh hardwood and softwood flour in PE and PVC. Most profiles are coextruded with a cap layer. Comptrusion has three conical twin lines from Cincinnati Milacron.

Crila Plastics Inc. in Mississauga, Ont., makes foamed PS lumber called Extrudawood, some formulations of which contain up to 10% wood flour. Crila uses pelletized commercial masterbatches of wood flour to provide additional stiffness.

Crane Plastics in Columbus, Ohio, developed a wood-filled composite called TimberTech over three years ago. It is used to make decking, office-system components, and parts of windows and doors. Crane uses several Cincinnati Milacron conical twin-screws to make 50% wood-filled virgin HDPE profiles. Crane will increase capacity this year.

C.W. Ohio in Conneaut, Ohio, buys compounded wood-filled pellets and extrudes deck and porch railings, posts, and other components.

Dura Products International of Etobicoke, Ont., makes 70% wood-filled profiles into Duraskid pallets at two plants. The compound isn't pelletized but is batch-fed to the extruders. The plastic comes from recycled HDPE bottles; sawdust comes from sawmills and commercial wood-flour suppliers. Dura's Etobicoke plant has 3600 lb/hr of compounding capacity and fourteen 4.5-in. Davis-Standard extruders. A joint venture, Duraskid New England in Andover, Mass., is being set up with 6000 lb/hr of compounding capacity and 15 extruders.

Eaglebrook Products in Chicago (which just became a division of U.S. Plastic Lumber Corp.) has been a Strandex licensee for three years. It makes decking and railing profiles called Durawood EX. They combine 70% hardwood flour with recycled HDPE.

EC Polymers, a sister company of Shore Pacific in Mira Loma, Calif., compounds proprietary polymer alloys for injection molding. For the past two years, EC has offered compounds with 5% to 50% wood filler of up to 100 mesh.

Fiber Composites Corp. in New London, N.C., owned by BB&S Treated Lumber, has operated for 18 months. It extrudes wood-filled decking and railings based on recycled and virgin PE.

Formtech Enterprises Inc., Stow, Ohio, extrudes PVC custom profiles with wood fillers for uses other than window lineals.

Hoff Forest Products in Boise Idaho, is a Strandex licensee that has extruded wood-replacement profiles and architectural millwork for a year. Hoff gets wood flour from its own scrap.

Lear Corp. inherited the American Woodstock operation in Sheboygan, Wis., when it acquired Automotive Industries in 1995. The operation runs three ICMA counter-rotating twin-screw extruders to make sheet for car interior panels and trim.

Louisiana-Pacific Polymers, Chesterfield, Mich., opened a new plant last August that uses Bausano's dual counter-rotating twin-screw system with one extruder side-feeding the other. The firm extrudes wood-filled sheet, laminated on both sides, for automotive applications and is developing sheet for building products. It uses hardboard scrap generated by its partner and next-door neighbor, G&L Industries.

Mikron Industries, Kent, Wash., set up a new plant for eight conical twin-screw lines to extrude wood/plastic composites and other products. Its MikronWood includes foamed composites and nonfoamed hollow composite profiles. Four Milacron lines are installed so far.

Natural Fiber Composites, Baraboo, Wis., is a two-year-old compounder of custom wood-filled PP, PS, HDPE, and ABS pellets for extrusion and injection molding. Its capacity is about 10 million lb/yr. The company has one Davis-Standard corotating twin-screw compounding extruder and runs around the clock, five days a week. It will soon expand to seven-day operation and expects to add a second line later this year.

North Wood Plastics Inc., Sheboygan, Wis., is a custom compounder with capacity for 10 million lb/yr, primarily of wood-filled masterbatches. It makes pellets with 10-60% wood in PE, PP, and PS. North Wood uses an ICMA corotating twin-screw.

Phoenix Color & Compounding Inc. in Sandusky, Ohio, makes up to 60% wood-filled masterbatches and 30-40% wood-filled compounds of PE, PP, PS, and ABS.

Polywood Products (formerly Pacific Southeast Forest Products) in Diamond Springs, Calif., started six years ago to transform itself from a lumber company into a maker of plastic composites. It applied for materials and process patents in 1992. It compounds up to 50% wood-filled pellets and also uses wood-filled masterbatches. It hires custom extruders and injection molders to make wood-substitute millwork and mouldings.

Shore Pacific LLC, Mira Loma, Calif., is a commercial compounder that has offered custom compounds with wood for the past two years. It pulverizes some of its own flour, putting 5-60% in polyolefins and alloys of ABS and other resins.

Star Guard Inc., Lancaster, S.C., combines wood flour and post-consumer recycled plastics to extrude edge reinforcements for boxes and pallets.

Trex Co., Winchester, Va., the industry giant, uses its own process to combine coarse wood fiber in a single-screw extruder with recycled PE from grocery sacks and stretch wrap. Trex makes 50/50 wood-filled lumber for decks and railings.