Nine structural adhesives with varying pH were selected to examine the effect of pH on wood-adhesive bond quality. These included 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) adhesives. Block shear specimens were prepared with these adhesives using Douglas fir and black spruce. The adhesive performance was evaluated by measuring the shear properties (strength and wood failure) of the specimens tested at the dry and vacuum-pressure / re-dry (VPD) conditions.
Adhesive pH, test condition, and wood species showed significant effects on the shear properties. Douglas fir yielded about 40% higher shear strength at the dry condition compared to the VPD condition. Black spruce showed smaller difference in shear strength between the dry and the VPD conditions, the difference being only about 6%.
The different adhesives performed differently at the dry and VPD conditions. The high pH adhesives showed similar wood failures at both test conditions. On the other hand, the low pH adhesives showed high wood failure at the dry condition, but dropped significantly at the VPD condition for both species. This indicates that the low pH adhesives were less durable than the high pH adhesives.
Some correlation was observed between shear properties (strength and wood failure) and cured adhesive pH in the VPD condition, but not in the dry condition. Such a correlation was stronger in Douglas-fir than in black spruce.
Full title: Impact of extreme pH of structural adhesives on bond durability as related to development and modification of CSA O112 wood adhesive standards. Part II. Evaluation of block shear properties of selected wood adhesives by short term exposure test