High-performing synthetic polyurethane (PUR) adhesives are used by many of the world’s leading cross laminated timber (CLT), and glued-laminated timber (glulam) manufacturers. A PUR adhesive has chemical and physical properties that make it more resilient than brittle legacy adhesives.
In real life, it means that modern PUR adhesives work with the wood rather than against it, all the while maintaining strength, form, and functionality over time. CLT assemblies move in two directions at different rates towards natural moisture equilibrium. And glulam assemblies all move in the same direction. The bond lines in both types of assembly need to be able to redistribute natural moisture-related strains and stresses without introducing adhesive-related cracking, deformation, failure, or delamination.
In finished buildings, there are audible hints of it. There used to be a lot of loud brittle crackling and popping sounds during the settling period of a new mass timber building as the wood reached moisture equilibrium with the surrounding air. These days, with the increasing use of modern one-component polyurethane (1C-PUR) adhesives, especially in CLT structural elements, it is a lot less noisy, according to Chris Whelan, Business Development Director, Henkel Engineered Wood Adhesives.
A PUR bond provides aesthetic benefits too. Since it acts like a flexible backbone, there is less chance for the kinds of cracks that wood develops as moisture conditions stabilize. While fans of weathered wood and timber like that look, too much of it is not desirable. The more intact the wood remains, the fewer points of entry for ambient moisture to penetrate the wood. In this respect, 1C-PUR adhesives serve to protect the wood.