Gap-filling properties of adhesives used for cross-laminated timber

Abstract

Cross-laminated timber (CLT) laminates layers of laminae side by side, with the fiber direction of each layer orthogonal to each other. Therefore, any error in the thickness of the laminae may cause gaps in the glue lines, leading to adhesion failure. In this study, a three-layer CLT with a thin lamina inserted in the inner layer was manufactured, and the gap-filling performance of adhesives and the allowable error for lamina thickness were investigated. The outer layer of CLT was arranged with 30.0 mm-thick laminae, and the inner layer alternated between 30.0 mm-thick lamina and thin laminae (29.9, 29.7, and 29.5 mm). One-component polyurethane adhesives (PUR), emulsion polymer-isocyanate (EPI) adhesives, and phenol-resorcinol-formaldehyde resin (PRF) were used. The glue line thickened when the lamina thinned down in PUR and EPI. By contrast, the glue line thickness at the lamina thickness of 29.5 mm in PRF was almost the same as the glue line thickness without gaps. Delamination tests and block shear test showed that the PUR had a low delamination even at 29.5 mm thickness and a high wood failure rate (WF, > 95%), indicating good adhesion. For EPI, the lamina thicknesses of 29.7 and 29.5 mm showed an increase in delamination and a decrease in WF. For the PRF, an increase in delamination and a decrease in WF were observed at a lamina thickness of 29.5 mm. These results indicated that good adhesive properties can be obtained with a lamina thickness error of 0.1 mm for EPI and 0.3 mm for PRF.