This is a continuation of the short-term testing performed in Phase II of this study to determine the effects of adhesive pH on wood-adhesive bond durability. In this phase, the Douglas-fir block shear specimens prepared in Phase II using the nine structural adhesives, viz. four high pH phenol-formaldehyde (PF), one intermediate pH phenol-resorcinol-formaldehyde (PRF), two low pH melamine-urea-formaldehyde (MUF), and two low pH melamine-formaldehyde (MF), were tested periodically for up to 12 months under long-term vacuum-pressure / re-dry (VPD) condition. The VPD consisted of vacuum-pressure treatment followed by 0, 4, 8, and 12-month exposure durations at 50°C. The specimens were dried, in each exposure period, to their original moisture content prior to testing for shear strength and wood failure.
Indications of the extent of degradation of the wood layer, adjacent to the glue line due to pH during the long-term exposure, were also examined by the 1 % sodium hydroxide solubility test. The results indicated that the wood-layer samples closest to the glue line, which contained included-glue, showed higher solubility compared to those farther from the glue line. This suggests that wood degradation and/or potential glue decomposition occurred and is considered to be induced by the adhesive alkalinity or acidity under the long-term exposure conditions.
The PF showed the best durability performance followed by PRF and MF/MUF. The MF/MUF degraded completely after the 12-month exposure period.
For the PF, there are indications that some degree of degradation occurred in the wood layer adjacent to the bond line during the 12-month exposure period, which could be attributed to the high pH of the adhesives. This observation was not apparent for the PRF, and is considered inconclusive for the MF/MUF since they degraded during the exposure period.
Full title: Impact of extreme pH of structural adhesives on bond durability as related to development and modification of CSA O112 wood adhesive CSA standards. Part III. Evaluation of block shear properties of selected wood adhesives by long term exposure test