Fundamental properties of borate-modified oriented strandboard manufactured from southern wood species

Abstract

In the United States, damages to buildings from termites and decay fungi cost billions of dollars annually. As a result, there is an urgent need for building construction that will withstand the ravages of these biological pathogens. Chemical modification of building products is one of the techniques for developing durable wood-based construction. This study was conducted to examine the effects of powder zinc borate (ZB) and calcium borate (CB) on resin gel time, strength, swelling, leaching, termite, decay, and mold resistance properties of oriented strandboard (OSB). It was found that gel time of phenol formaldehyde (PF) resin decreased with increased amount of ZB, indicating interaction between the borate and the resin. The reduced gel time was partially recovered by using polyethylene glycol (PEG) in combination with ZB. Although panel stiffness was not affected by borate up to a 3.5 percent boric acid equivalent (BAE) level, ZB and CB showed a negative effect on the bending and internal bond (IB) strength. Thickness swelling (TS) of treated panels after 24-hour water soaking increased with borate level. ZB-treated OSB displayed less TS than CB-treated OSB at an equivalent BAE level. CB with a larger particle size caused significant TS. However, the chemical with a smaller particle size helped bring TS to a stable and acceptable level. A certain portion of borate leached out from OSB samples under the water-soaking conditions. The leaching rate varied with wood species, borate types, and amount. The use of borate with a smaller particle size helped reduce the leaching rate. The relationship between assayed BAE and leaching time followed a decaying exponential function for ZB and a decaying power function for CB. Laboratory termite tests showed that wood weight loss decreased and termite mortality increased with the increased BAE level. At the1% BAE or above, there was little damage on wood samples. There were significant correlations among termite mortality, weight loss, and visual damage ratings. Both borate chemicals provided an excellent decay and mold resistance for OSB. The information on various properties of borate-modified OSB is of significant value for developing durable structural panels from southern wood species.