Protecting Wood Composites
By Peter Laks
July 1995

The extensive harvesting of old growth forests over the last two centuries combined with today's conservation policies means that the average log being processed into construction materials is now a lot smaller than it used to be. The news is not all bad, however, since wood remains the classic renewable resource. Even if trees are smaller, there are plenty of them, and they are growing - or being planted - at a faster rate than they are being cut. This applies in Canada, the USA, and many of the major lumber-producing countries around the world, but it remains essential that we use sawn timber as efficiently and effectively as possible.

Wood composites, which can be made from relatively small trees and which employ much more of each tree, offer an excellent way of maximizing useful production from limited resources.

Sometimes called waferboard, oriented strandboard (OSB) is a wood composite typically made from thinnings, branches, and fast-growing species, such as eucalyptus, spruce, and aspen. Strands of wood are mixed with resin and wax, then hot-pressed to give a product which can be stronger and more consistent than sawn timber. In fact, it is often considered an engineered product without the variability of solid timber. It is also very economical in its use of the principal raw material. However it has one serious drawback. Wood composites made from fast-growing species are often prone to attack by decay fungi and insects when used outdoors or in other applications where the material may be exposed to high humidity or continued contact with water. Some form of protection is therefore needed.

Borates are good wood preservatives with characteristics that make them well suited for use in wood composites - efficacy against both fungi and insects, fire retardancy at higher loadings, low cost, ease of handling and treatment, low mammalian toxicity, and minimal environmental impact. The Michigan Tech wood preservative research group has been working with U.S. Borax for several years to develop and evaluate wood composite materials containing one of a variety of inorganic borate chemicals, but especially Tim-borŽ wood preservative, (disodium octaborate tetrahydrate) and BorogardŽ ZB fungicide (zinc borate). Materials related to OSB and containing borates may be considered for exterior siding, window frames, wall and roof sheathing in high termite hazard areas like the deep south of the United States, composite shingles, and decking. Modern resin systems also allow for ground contact applications but borates are not normally recommended for such situations.

One of our key studies compared and evaluated the properties of aspen waferboard containing either Tim-bor or Borogard at loadings of 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 5.0, 8.0, and 10.0 percent boric acid equivalent (BAE). Did either borate effect the strength of the strandboard? The borates were incorporated into the board by mixing the finely powdered chemical with the aspen wafers in a blender before addition of the adhesive and wax, and hot-pressing into the final product. An adhesive called pMDI was used in this work. The strength of the boards was not significantly affected at lower loadings. Higher loadings, especially of the Borogard, did weaken the board, but the properties could be brought back to acceptable levels by increasing the amount of adhesive.

An important difference we found between Tim-bor and Borogard is in the amount of boron that can be removed from the composite during exposure to water. As a result of its low solubility, the zinc borate is much more resistant to leaching. This may make it suitable for severe hazard situations, such as ground contact where soluble borates are not normally used.

Samples of OSB were exposed to a barrage of tests to find out how well borates protect this wood composite. As a result in the laboratory, we evaluated the fungal decay resistance of leached and unleached waferboard blocks containing 0.1, 0.2, 0.5, or 1 percent Borogard or Tim-bor using a standard procedure called the soil block test. Two typical fungal decay organisms were used: Gloeophyllum trabeum (a brown rot fungus) and Trametes versicolor (a white rot). With both organisms and both borates, there was essentially no weight loss, and therefore no fungal attack, at loadings greater than or equal to 0.45 percent BAE when the samples were not leached. This is the activity threshold, the minimum amount of preservative which is effective in preventing significant decay under the test conditions by a particular fungal species.

When the samples were deliberately water-leached before exposure to the fungi, different results were observed. As expected, the very severe leach test removed significant amounts of preservative from the sodium borate boards, and full fungal protection was therefore not maintained. However, the zinc borate boards resisted leaching to the extent that the activity threshold was still at approximately 0.5 percent BAE.

Another laboratory procedure called a fungus cellar test was used to evaluate another set of aspen waferboards containing 1, 2, 3, 5, or 8 percent BAE zinc borate. The Michigan Tech fungus cellar is a high hazard environment maintained at 27°C and 80 percent relative humidity, containing bins of soil. Small stakes are partially buried in the soil and visually evaluated for decay damage at three-month intervals. The decay rate in the fungus cellar is three to four times greater than at our field exposure site in Hawaii. The untreated control stakes began to deteriorate in a matter of weeks, and failed almost completely in six months. The zinc borate stakes sustained a very high level of protection over 12 months. The Tim-bor stakes showed a high level of protection for nine months and a moderate level at 12 months.

We currently have ground-contact field stakes and above-ground test specimens of these board types installed in our field sites near Hilo, Hawaii and Gainesville, Florida. Results from this work will be available within the next few months.

Other long-term field data is available, however, from termite testing. OSB specimens containing either Tim-bor or Borogard were exposed to the Formosan subterranean termite (Coptotermes formosanus Shiraki) in the Michigan Tech test site in Hilo, Hawaii. Results after 48 months exposure are summarized in Figure 1. An average rating of ten indicates the sample was in perfect condition, while a zero rating means the sample was totally destroyed. The higher target loadings of Tim-bor still provided some protection from termites after four years exposure, but much lower loadings of zinc borate gave almost total protection.

There are probably two factors causing the superior anti-termitic performance of the zinc borate - participation of zinc in the total anti-termitic activity of the preservative, and leach resistance.

Besides having excellent preservative properties, powdered borates are very easy to incorporate into the composite during its manufacture. Some commercial wood composites already use Borogard ZB as a preservative system, and other manufacturers have conducted successful mill trials. From the data now available to support the use of borate preservatives in wood composites, Tim-bor remains a very effective preservative for the internal construction uses for which it has always been intended, while zinc borate is remarkably leach-resistant and may be employed to protect wood composites which are used for external applications in high hazard locations. Further testing underway will prove this out.

In summary, both Tim-bor and Borogard ZB are effective preservatives for wood composites in normal use situations. In addition, Borogard has demonstrated superior performance characteristics in extreme hazard situations, which may make it suitable for ground contact applications.

Peter Laks is associate professor of wood protection at the School of Forestry and Wood Products at Michigan Technological University in Houghton, Michigan.