More profile extruders explore wood/polymer composites.

Wood/polymer composite technology is providing profile extruders with more opportunities to penetrate markets dominated by wood. In decks, doors, and other building products, a handful of processors are introducing new plastic/wood formulations that provide a wood-like appearance without its maintenance drawbacks.

Profile extruders that want to start making these types of wood substitutes have two ways to go. They can take a cue from companies like Crane Plastics of Columbus, Ohio, and Andersen Corp. of Bayport, Minn., which recently developed their own technologies. Or they can do as a growing number of firms have done and license existing technology. That way, they get not only production-proven formulation and processing know-how, but tooling as well. Currently, the only known licensor of wood/polymer composite technology is Strandex Inc., Madison, Wis.

Specifics of these processes remain proprietary. But they generally involve mixing wood flour with HDPE or PVC plus binders and a variety of other additives in a low-intensity mixer. After initial mixing, the powder blend is processed in a twin-screw extruder.

REACTIVE PROCESSING

The Strandex process utilizes up to 70% wood content. Strandex's formulation also includes an undisclosed thermoset material that reactively couples the wood and plastic.

Strandex licensees extrude the material through a stranding die that produces a honeycomb-shaped profile cross section. The honeycomb design of the die internals serves to raise melt temperature by exposing the material to shear stress. That's what allows the thermoset binder in the formulation to react with the wood, according to Al England, Strandex's executive v.p.

The Strandex process requires neither calibration tools nor pullers, apparently owing to the stiffness of the profile as it exits the die.

Two of Strandex's more recent licensees are Eaglebrook Products in Chicago and Comptrusion, based in Toronto. For about a year, Eaglebrook has been making Strandex door and window components using HDPE bottle reclaim from its Eaglebrook Plastics recycling operation as a feedstock. It has two 80-mm twin screws from Milacron.

Comptrusion was formed about a year ago to supply Strandex-type custom profiles and window lineals. It will be running 55- and 80-mm twins from Milacron, using both HDPE and PVC as matrix resins.

England notes that Strandex's formulations can be injection molded as well as extruded. He says Strandex currently has five licensees, all for extrusion. Strandex also has a formulation for PVC, he adds. He describes the volume of wood currently used by plastic processors as "miniscule," but maintains that the market potential is enormous.

INDEPENDENT DEVELOPMENTS

Crane, which was the first Strandex licensee back in 1995, more recently developed its own wood/HDPE composite process, called TimberTech. Decking is the biggest end-use application, but Crane also fabricates profile components for doors and other construction-related products. The company makes the composite on five Cincinnati Milacron twin-screw lines at its Columbus plant and plans to install five more at another plant in Nacogdoches, Texas. Crane designs its own dies and calibration tools in-house.

Crane varies the wood content to control moisture absorption, says Steve Cork, v.p. of sales. That's important in outdoor applications because excess moisture in the part will cause it to expand and perhaps buckle.

Andersen Corp. is another processor that developed its own product, called Fibrex. Andersen's proprietary process combines 40% wood and 60% PVC into complete window systems.