Formosan subterranean termites pose a growing threat to all structural wood materials in residential construction. The species is one of the most aggressive and voracious insects in the world.
Chemical treatments of wood with chromated copper arsenate and borate have been effective against the termites. Structural lumber and plywood can be successfully pressure-treated with these chemicals, and both are widely used in Louisiana. Structural composite panels, such as oriented strandboard (OSB), however, cannot be pressure-treated in the same manner with these chemicals because of swelling. OSB is made of wood flakes glued with a thermal-setting resin. It is widely used as sheathing, flooring and I-joist materials in home construction, replacing plywood. Thus, alternative techniques for protecting OSB against termites must be developed.
LSU AgCenter researchers conducted a study to measure the effects of incorporating borate powder into wood flakes during the blending process on the performance characteristics of OSB. The objectives of the project were to study the effects of wood species, borate type (calcium borate or zinc borate) and borate content on panel strength, swelling, water leaching and termite resistance. This information is highly desired for developing durable structural panels for residential construction using southern wood species.
Panel Manufacturing and Testing
Fresh-cut lumber boards from eight southern species were obtained from a local sawmill in southern Louisiana. The boards were cross-cut into 6-inch-long blocks along the grain. These blocks were flaked with a laboratory flaker to produce 3-inch-long by 0.025-inch-thick flakes. The flakes were dried to 2 percent to 3 percent moisture content using a steam-heated dryer and were screened to eliminate small wood particles.
The OSB panels were then produced by blending the dry wood flakes with liquid phenol formaldehyde resin, wax and borate powder (zinc or calcium borate), forming them into loosely packed mats and then putting them through a hot press.
Samples were taken from each panel to test the borate content. The result was expressed as boric acid equivalent (BAE) (Table 1). Tests were conducted to determine each panel’s stiffness and strength, internal bond strength, thickness swelling, water leachability and termite resistance. In addition, creep tests were conducted. Creep measures how much deformation or movement wood will experience under long-term loading. For example, a beam inside a house under a heavy load (the roof) will tend to bend in the middle of the span. Creep measures how much bending will occur as a function of time. The creep data were fitted to a mathematical model, which was then used to predict the creep performance over 30 years.
Mechanical, Physical Properties
Panel stiffness and strength, which measure resistance to a bending load, were affected little by borate up to the 3.5 percent BAE level. Wood species and borate type had an insignificant influence on both properties.
The effect of borate on internal bond (IB) strength, which measures glue-bond efficiency, varied with borate type and wood species. Zinc borate showed fewer negative effects on the IB values, compared with calcium borate for both mixed hardwood and southern pine OSB. Both types of borate, however, had acceptable IB values (based on the industry standard).
Thickness swelling from 24-hour water soaking generally increased with borate content in the panel for both zinc and calcium borate OSB. Zinc borate-modified OSB had less thickness swelling than calcium borate OSB at a comparable BAE level. Borate particle size had a significant influence on the swelling properties. Calcium borate with large particle size caused significant thickness swelling at high BAE levels, but reducing the particle size of the chemical helped bring the thickness swelling to a stable and acceptable level.
A certain amount of borate leached out under the water-soaking condition. The use of borate with a smaller particle size helped reduce sample thickness swelling and leaching rate. Boron fixation with other chemical agents may be necessary for borate-modified OSB under the extreme water exposure conditions. Protection of the panel from direct water exposure can help reduce this problem.
Laboratory tests indicated that both zinc and calcium borate used in OSB resisted Formosan subterranean termites well (Table 1). As the borate loading increased, wood sample weight loss decreased and termite death rate increased. At higher borate levels, there was little damage on wood samples. The observed weight loss in these samples was caused by loss of volatile materials in the samples. Wood species showed an insignificant effect. There were strong correlations among visual damage rating, wood sample weight loss and termite death.
Long-term Structural Performance
The creep tests conducted showed that borate-modified OSB can meet the National Design Specifications for wood construction. Further testing of creep behavior of the product under combined mechanical and moisture loading conditions is ongoing.
The results of this work indicated that termite-resistant structural OSB from southern species can be successfully developed with a right combination of wood species, borate type and content and other processing variables. The study provides comparative properties between zinc and calcium borate modified OSB. This technology willhelp more OSB producers manufacture chemically modified OSB to meet increasing market demands.
This material is based on work supported by the National Science Foundation. The authors wish to thank several wood products, resin and chemical companies for their material and technical support and Dennis Ring and Gregg Henderson for their help and advice on termite tests.
(This article appeared in the summer 2002 edition of Louisiana Agriculture.)