Plastic/Wood Composites

Polymers in various forms continue to make inroads into traditional wood product applications.

The New Forest of Man-Made Trim, Fine Homebuilding, August/September 2004, is a full-length article covering new alternatives to traditional sawn-lumber trim. Materials discussed include PVC, wood/plastic composites, compressed wood fiber, and engineered wood products that claim greater stability, reduced maintenance, and increased longevity. Some noteworthy points:

PVC trim is manufactured by extrusion, with the final product characteristics varying with the cooling process. One process, termed "celuka", results in a material with a dense, somewhat brittle outer layer and a less homogenous core. This product type is best suited to flat trim applications. The alternative "free-form" cooling process results in a material of uniform density throughout that is more suitable to applications requiring routing, complex cutting, or other more intricate work.

Wood/plastic composites are made from the mixing of phenolic resins or PVC plastic with wood fibers. Product characteristics vary with the various manufacturing processes. Some may be homogenous and exhibit workability similar to natural wood. Others may be acrylic clad with a less dense core.

Both solid plastic and wood/plastic composite products must be installed to allow for greater thermal expansion and contraction than natural wood products.

Compressed wood fiber products are also a mixture of phenolic resins and wood fibers, but with a higher proportion of wood fiber to resin material in comparison to composites. Similar to traditional hardboard (which has a history of failures when used in exterior applications), these newly formulated products are claimed to overcome earlier limitations. In general, product characteristics are similar to traditional hardboard or medium-density fiberboard, though qualities vary with individual manufacturer's proprietary material formulations and manufacturing processes. Given the higher wood fiber content, more care must be taken to protect these products from moisture absorption at cut ends and other vulnerable locations. Some product formulations also include chemicals to prevent decay or attack by insects.

Several products consisting of plywood or OSB panel material clad with resin-impregnated waterproof paper, similar to medium density overlay plywood, are also available for use as trim material.

Finger-jointed lumber offers greater dimensional uniformity and stability in comparison to traditional sawn-lumber. It can also be manufactured from readily renewable wood species. Improvements in finger jointing technology result in joints that are invisible in finished trim.

Many proprietary trim products are also available factory-primed and/or factory-finished, offering higher finish quality and consistency, and reduced installation costs.

Wood Design & Building reports that the University of Maine's Advanced Engineered Wood Composites Center has received a patent for a reinforced building panel that offers substantial increases in structural strength in comparison to traditional structural panel products. According to the article and information provided on the University of Maine's website, fiber-reinforced polymer reinforcing at the perimeter and other nailing surfaces of plywood and OSB panels results in increases in resistance to wind and earthquake forces of up to 20 percent in traditional stick-framed construction, or more in specially engineered systems.

The Construction Specifier, July 2004, reports on the 10-year successful track record of of boardwalk decking made from recycled/reclaimed plastic and waste wood. Installed in Spring Lake, New Jersey's boardwalk after a 1992 storm damaged the previous wood deck boardwalk, the new decking material has proved successful. Claimed benefits include: freedom from rot, no risk of splinters, no periodic refinishing or sealing requirements, and elimination of popped nail heads. Reportedly, the surface is also found to be more forgiving under foot to joggers.

Bobrick, manufacturer of washroom accessories, has announced its "Siera Series" toilet partitions made from Solid Color Reinforced Composite. So-called SCRC material is made from wood fibers suspended in high-density resins, and is claimed to offer superior resistance to abuse, easier repairs, and improved surface-burning characteristics in comparison to competing products.

More Info
Plastics in Construction are discussed on pages 694 through 697 in the textbook.