Strength and durability of one-part polyurethane adhesive bonds to wood

One-part polyurethane wood adhesives comprise a new class of general purpos e consumer products. Manufacturers' claims of waterproof bonds brought many inquiries to the Forest Products Laboratory (FPL) from users constructing aircraft, boats, lawn furniture, and other laminated materials for outdoor use. Although FPL has technical information on several types of polyurethan e and isocyanate-based adhesives for wood, no information was available on this new class of adhesives. Four commercial polyurethane adhesives, along with a resorcinol-formaldehyde adhesive to represent a standard of performa nce, were subjected to a series of industry-accepted tests that assess vary ing levels of bond strength and durability. In bonds to yellow birch and Do uglas-fir, the polyurethanes did not differ significantly from each other i n their performance, as a group, though, their dry shear strengths showed t hat they were significantly stronger than the resorcinol. Dry wood failures by the polyurethanes were high and did not differ significantly from the r esorcinol. After three water-saturating procedures, wet shear strengths of polyurethanes and the resorcinol were statistically comparable. Wet wood fa ilures, however, were very low among polyurethanes, which is a sharp contra st to the high wood failure by the resorcinol. A moderately severe delamina tion test indicated varying levels of water resistance among the polyuretha nes, but the resorcinol was completely resistant to delamination. A very se vere cyclic delamination test caused severe delamination of polyurethane bo nds. However, a recently discovered hydroxymethylated resorcinol (HMR) coup ling agent dramatically increased delamination resistance of polyurethane a dhesives. Ln a test of resistance to deformation under static loads, polyur ethane bonds withstood extreme exposures of temperature and relative humidi ty for 60 days without deformation.