Bonds for Strength and Resilience

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. 

German researchers noted the resilient property of 1C-PUR in a recent study of the effects of moisture and temperature fluctuations on glulam. Examining the performance of 1C-PUR-based glulam, they concluded that 1C-PUR dramatically reduced the negative effects of climate-related moisture and temperature fluctuations.^{Source 1}

What about over a longer period of time? Besides anecdotal evidence from CLT and glulam manufacturers using 1C-PUR adhesives, there are some third-party studies that show the long-term performance. For example, Swiss researchers tested glulam beams used in four pedestrian and bicycle bridges, all at least 7 years old, finding that Henkel Loctite 1C-PUR bonded beams stood the test of time in correctly installed outdoor structures.^{Source 2}

Experts from Henkel Engineered Wood, the leading supplier of PUR adhesives in CLT manufacturing, talk about ductile bonds being one of the benefits of Loctite HB S and HB X adhesive solutions. Scientists and engineers talk about PUR having ductile properties to help it withstand microscopic stresses and strains, contrasting it to more brittle legacy or conventional wood adhesives, such as melamine-urea-formaldehyde or phenol-formaldehyde. This property is just one of several that underlie Henkel Loctite 1C-PUR technology’s strength, fire safety, heat performance, environmental, and successful certifications. 

The benefits combined with manufacturing efficiency advantages of Henkel’s Loctite HB adhesive technology, such as less waste, 100% pure solids, favourable pressing-time-to-open-time ratios, flexible open time all contribute to its leadership position. Whether it is durability, toughness, aesthetics, safety, sustainability, or performance, Henkel Loctite HB technologies have a positive impact on today and tomorrow’s mass timber building materials. 

Photography Copyright: Ben Guthrie, The Guthrie Project

^{Source 1}: Mathias Schuh, Chair of Wood Science, TUM. Bausteine für die leistungsstarke Laubholzverklebung (Copywriter translation: Foundations of High Performing Hardwood Adhesives) at 4. Kooperationsforum: kleben von holz und holzwerkstoffen April 2021 Timestamp: 3m00s to 5m45s. 

^{Source 2}: “Test Report 434 929” 2004. The Swiss Federal Laboratory for Materials Testing and Research (commonly known as EMPA)