Chemical and Physical Debonding-on-Demand of Poly(urethane urea) Thermoset Adhesives to Facilitate the Recycling of Engineered Wooden Products

Abstract

The demand for debondable adhesives is becoming more urgent, as countries begin implementing policies to enforce circularity and recyclability. Specifically in the construction industry, the switch from carbon pool-depleting materials to carbon pool-replenishing materials demonstrates the importance of circularity for the high-demand wood species used to produce engineered wood products. To this end, a dual chemical and physical debonding adhesive was prepared with the intent to debond solid wood specimens. The one-pot synthesis of a poly(urethane urea) (PUU) adhesive was prepared with a thermoresponsive furan maleimide (FM) Diels–Alder adduct capable of dissociating at temperatures above 90 °C coupled with thermally expanding microspheres (MS). First, PUU polymer films containing only FM bonds and a second control adhesive with a butyl spacer replacing the thermoresponsive FM bond were prepared and characterized. Next, simple single wooden lap specimens were prepared by using the PUU adhesives to investigate bonding and debonding via a lap shear test. Adhesives containing only FM bonds or MS were tested to investigate the effect of a chemical versus physical debonding mechanism on the bond strength of the glued specimens. Next, the combination of FM bonds and MS was incorporated to combine chemical and physical debonding mechanisms, respectively, which resulted in the most drastic reduction in tensile shear strength of the glued specimens by 71%.