Integrity of Melamine Formaldehyde Bonds in Ponderosa Pine Cross-Laminated Timber: Isolating Adhesive Compatibility Effect

The integrity of melamine formaldehyde bonds in prototype cross-laminated timber (CLT) specimens was tested as part of a project on utilization of ponderosa pine (Pinus ponderosa) from forest restoration programs in the western United States. Bond integrity tests, block shear, and cyclic delamination are prescribed by ANSI/APA PRG-320 (ANSI/APA 2019) and ANSI 405 (ANSI 2018) to qualify new products. Of these, the cyclic delamination criterion is particularly challenging for layups developed in research labs and pilot plants. Delamination is often blamed on poor compatibility between adhesive and wood species, clamping pressure, or distribution of adhesive, neglecting other potential factors. One of the study objectives was to separate the effect of adhesive compatibility from other potential factors affecting bond integrity in CLT. Bonding integrity tests were conducted on prototype specimens bonded with melamine formaldehyde adhesive. Three types of specimens were studied: (1) specimens harvested from panels fabricated in an industrial CLT plant, (2) specimens harvested from panels fabricated in a pilot-line, and (3) short blocks cross-laminated from 102 by 102 -mm sections. The short blocks included sections with juvenile wood and blue stain on bonded surfaces. All samples passed the PRG-320 block shear criteria. All short blocks passed the delamination criterion, demonstrating sufficient adhesive compatibility with the surfaces regardless of heavy presence of blue stain and juvenile wood. Specimens harvested from panels did not meet the delamination criterion. Delaminations developed near preexisting interlaminar gaps observed in prototype panels, which may be related to thickness variation or to inconsistent clamping pressure.