Evaluation of block shear properties of selected extreme-pH structural adhesives by short-term exposure test

摘要

Nine structural adhesives with varying pH were selected to examine the effect of adhesive pH on wood-adhesive bond quality. The adhesives evaluated included four highly alkaline phenol-formaldehyde, one intermediate pH phenol-resorcinol-formaldehyde, two acidic melamine-urea-formaldehyde, and two acidic melamine-formaldehyde resins. Block shear specimens were prepared using Douglas-fir and black spruce wood. The adhesive performance was evaluated by measuring the shear properties (strength and wood failure) of the specimens tested at the dry and vacuum-pressure-redry (VPD) conditions. Adhesive pH, test condition, and wood species showed significant effects on shear properties. The different adhesives performed differently at the dry and VPD conditions. The high-pH adhesives (phenol-formaldehyde and phenol-resorcinol-formaldehyde) showed similar high wood failures at both test conditions and performed better than the low-pH adhesives (melamine-formaldehyde and melamine-urea-formaldehyde), especially after the VPD conditioning. The low-pH adhesives showed high wood failure at the dry condition, but wood failure decreased significantly after VPD conditioning for both species, indicating that the low-pH adhesives were less durable than the high-pH adhesives. High-pH adhesives did not have a negative impact on the strength of the bonded specimens.