Building & construction boom?

Dow has brought out a new grade of its high-temperature-resistant PE, described as a one-step, cost-effective alternative to cross-linked PE (XLPE) pipes for hot-water transport. Dowlex 2344E, a bimodal grade, reportedly offers advantages over XLPE such as flexibility, easy welding, possibility to extrude in multilayer structures, and high hoop strength. RC

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Wood composites continue to build a marketplace

Technical advances in machinery and materials have coupled with regulatory changes to present new opportunities.

Voluntary elimination of wood that is pressure-treated with chromated copper arsenate preservatives—by 2005 in Canada and at the end of 2003 in residential construction in the U.S.—has opened wider the door for a composite lumber industry (featuring extrusions of PVC or polyethylene compounded with wood flour) that was already enjoying accelerated growth.

Pressure treatment is used to extend a wood product''s life, increase its strength, and ward off mold and rot. But the chemicals are thought to increase the likelihood of lung or bladder cancer.

Wood-composite lumber is therefore making greater inroads due to its relative environmental friendliness, while still offering resistance to the elements. And many manufacturers, like market leader Trex, use recycled plastics and wood. Between two existing manufacturing facilities in the U.S., with a third under construction, Trex processes more than 100,000 tonnes/yr of wood and reclaimed polyethylene film.

Peter Simko, a senior engineer with Carney Timber (Barrie, ON), works with that company''s line of extruded composite decking and railing, as well as molded fixtures sold under the brand name Xtendex. The company loads virgin HDPE with 50% organic fibers in the formulation of its product.

Organic fillers like rice husks, flax, hemp, other plant derivatives, and wood flour are the norm in composite decking, offering an attractive price and lending added mechanical functionality to the boards. But they do pose some processing challenges.

Mike Millsaps, sales and marketing manager for JSW''s extrusion machinery in North America, says moisture content can pose big processing hangups. JSW has patented a degassing extruder to handle the problem. Softer woods like pine have more moisture than harder woods, but processors working with wood flour that has moisture content of 7% or less should consider themselves lucky, Millsaps says.

"You have to keep the wood flour under a certain temperature to prevent burning and oxidation of the polymer, but you have to [have it high enough to] drive out the water," he says.

A number of processors use a separate twin-screw compounding extruder to feed several single-screw lines profiling the lumber. "You''re only going to make a mess on that section of your plant," Millsaps explains. The alternative is to pre-dry the material, using dedicated furnaces, or to try inline compounding.

The Europeans are coming

Krauss-Maffei has not been active in extrusion equipment for wood composites until now, but in June it said it was "almost ready" to enter the U.S. market with a machine developed specifically for that sector. A prototype has already been tested, based on a KMD 130 twin-screw counter-rotating machine, capable of a throughput up to 700 kg/hr.

The equipment uses a cascade configuration, with an initial, twin-screw corotating section designed to pre-dry the material before feeding it to the hopper of the second section. It can accept compounds with 6% to 8% moisture.

Cincinnati Extrusion (Vienna, Austria) has made several advances in its vented, conical, twin-screw extruders for wood composites since it ran a line at K 2001. The company''s Fiberex machine is scheduled to produce polypropylene-based material at K 2004 at a line speed of 3 m/min—10 times faster than three years ago.

The machine uses a new three-zone screw design, as well as vacuum calibration. It applies water-, rather than air-cooling, of the profile, which allows speeds up to 5 m/min, even for profiles with asymmetric and complex geometries. The line employs gravimetric dosing of wood flour, additives, and resin.

In spite of these and other gains, the products still face one important hurdle compared to traditional lumber. "The major stumbling block in the market is still price," Millsaps says.

"It''s two to three times the price of pressure-treated wood, depending on what you get. But they''ve come a long way in appearance, so they''re getting those issues worked out." TD

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