Creep Testing of One-Component Polyurethane and Emulsion Polymer Isocyanate Adhesives for Structural Timber Bonding*

Abstract

Abstract Moisture hardening one-component polyurethane (PU) adhesives and emulsion polymer isocyanate (EPI) adhesives have recently gained a considerable market share in structural timber gluing, especially in Europe. The creep behavior of bond lines between solid timber adherends with said adhesives, specifically nine PU and five EPI products, along with two phenolic resorcinol-formaldehyde (PRF) adhesives used for reference purposes, were studied. Most of the adhesives investigated are approved for gluing of structural timber components in Europe. All of the reported 34 bending creep test series (with 340 specimens), were performed with two-part laminated spruce (Picea abies) specimens according to the European structural adhesive creep test standard EN 15416‐3. The standard addresses the ratio RC of creep between PU and EPI adhesives' bonds versus the minimally creeping bond lines of matched specimens glued with a reference PRF adhesive. Mainly glue line thicknesses of 0.3 mm were investigated, with some additional tests having been performed with 0.2- and 0.5-mm-thick glue lines as well. The investigations provide a substantial insight into the creep behavior of the two regarded polyaddition adhesive families. Additionally, the evaluation proves, for PUs and EPIs for test glue line thicknesses of 0.3 mm and less, that it is well justified to reduce the presently prescribed creep periods of 26 and 52 weeks down to 13 and 26 weeks, respectively. The results will serve as a basis for a change of the European test and requirement standards on structural PU and EPI adhesives.