OREGON STATE UNIVERSITY

Plastic barrier can double post life

08/06/1997

CORVALLIS - Researchers at Oregon State University have found that a simple plastic barrier on the below-ground portions of untreated wooden posts might effectively double their natural lifespan.

And one of the principal researchers who made these findings knows a fair amount about longevity - he's Theodore Scheffer, a retired, 93-year-old research associate in the OSU Department of Forest Products who still comes in to work regularly and clearly has not yet run out of new ideas.

"This study was based on an idea Scheffer had been kicking around for a few decades and finally insisted we test in the research laboratory," said Jeff Morrell, an OSU associate professor of forest products, international expert on wood preservation and co-author of this study just published in a professional journal.

"The results clearly show that a polyethylene wrap of wooden posts would be an alternative way to extend their lifespan," Morrell said. "It might be of particular interest to people who are reluctant to use chemically-preserved posts, or farmers and ranchers who have a lot of their own wood they would like to use."

Laboratory research on the idea was done in a "fungus cellar" designed to accelerate the natural process of wood decay from fungi, bacteria and insects.

In their study, the OSU scientists wrapped posts in customized bags made of 2-mil polyethylene film, extending to slightly above grade level. But a simple wrap of plastic material around the post, Morrell said, would probably work about as well as a bag that had a bottom in it - most decay, at least in western Oregon, happens in the upper six to 12 inches of soil.

Water trickling down inside the plastic also is not a real concern, Morrell said - simply being wet doesn't cause wood to rot unless there's a ready supply of oxygen.

In studies done with Ponderosa pine, which normally has little decay resistance, scientists found that after two years the average weight loss of an unprotected post due to decay was 12-15 percent - enough to cause an extreme loss of strength and post performance - compared to about 2 percent for posts protected by the plastic wrap.

Based on the findings, Morrell estimated that an untreated Douglas-fir post, which might now be expected to fail in five to eight years in western Oregon, could last 16-20 years with a plastic wrap.

Cedar posts, which are more naturally decay resistant, might also double their service life to almost 40 years with a plastic wrap.

Morrell said that the topical application, by brushing or dipping, of a common wood preservative such as copper naphthenate or boric acid - in combination with the plastic wrap - would help preserve the appearance of the wood, make them last even longer and give home-treated posts a functional lifespan approaching that of pressure-treated lumber.

Morrell said people who wish to use this approach for preserving posts will have the best success with naturally-resistant cedar, redwood or, to a lesser extent, Douglas-fir. The process would be far less successful with hemlock or pine. Although 2-mil plastic was used in the OSU research, a heavier plastic wrap might be less susceptible to rips or tears that would allow bacterial or fungal invasion.

"For homeowners and farmers, this really is a viable alternative to pressure-treated lumber," Morrell said. "It should work even in western Oregon, which because of its wet climate is pretty hard on wood. People should just make sure they bring the plastic wrap up an inch or so above grade level, so dirt doesn't get down inside."

For convenience, Morrell said, it would also be handy to have inexpensive plastic bags already made for this purpose and sold at building supply stores.

The process is scientifically viable and could form the basis for a modest new industry to make, sell and market such bags, he said.