EFFECT OF EXTREME PH ON BOND DURABILITY OF SELECTED STRUCTURAL WOOD ADHESIVES

摘要

This is the second part of a two-part study aimed at examining the effect of extreme adhesive pH on bond durability. The first part dealt with short-term exposure and this second part dealt with long-term exposure. This part also included an examination of wood degradation by adhesive pH. Nine structural wood adhesives (four high-pH phenol-formaldehyde [PF], one intermediate pH phenol-resorcinol-formaldehyde [PRF], two low-pH melamine-formaldehyde [MF], and two low-pH melamine-urea-formaldehyde [MUF]) were studied in terms of their pH effect on wood-adhesive bond durability using Douglas-fir wood substrate with specimens tested in block shear. The block shear specimens were initially subjected to vacuum pressure treatment under water, followed by exposure, while wet at 50 degrees C for 0, 4, 8, 12, and 17 mo. There were indications that the wood layer closest to the bond line, which contained included glue, had higher solubility compared with those farther from the bond line. This suggests that wood degradation and/or adhesive decomposition occurred and was considered to be induced by the adhesive alkalinity or acidity under the long-term exposure conditions. The PF showed the best durability performance followed, in decreasing order, by PRF and MF/MUF. The latter adhesives degraded completely after an exposure period of 8-17 mo. The four PF adhesives passed the shear strength and wood failure requirements of the well-known North American structural wood-adhesive standards indicating that their high pH had no significant detrimental effect on the wood-adhesive bond durability after the 17-mo exposure period despite being subjected to multiple cyclic tests. This observation was not apparent for the PRF, and the pH effect was considered inconclusive for the MF/MUF since they degraded during the exposure period. The results of this study provide support to wood-adhesive standards that do not impose restriction on the upper spectrum of the pH range, and would be useful to adhesive standard developers. These results also serve as background information for adhesive companies in their formulation of wood adhesives as well as for bonded wood product manufacturers in their use of adhesives and for builders in their use of bonded wood products.