Adhesive bonding quality of cross-laminated timber made of thermally modified and unmodified wood

Abstract Thermally modified wood (TMW) is increasingly considered for outer laminations in cross-laminated timber (CLT), but its adhesive bonding quality under varying manufacturing conditions remains poorly understood and necessitates further investigation. The goal of this research was to optimize the manufacturing process for incorporating thermally modified wood into CLT, with a particular focus on adhesive bonding performance. To achieve this, the authors evaluated how different bonding pressures (0.21, 0.50, 0.70, 1.03, and 1.40 MPa) influence the quality of 1 C-PUR adhesive bonds in CLT panels made from thermally modified and unmodified yellow-poplar (YP). The specific objectives were to quantify the effects of pressure on shear strength and wood failure percentage (WFP); characterize adhesive line thickness and penetration behavior through microscopy; and identify an optimal bonding pressure suitable for commercial-scale manufacturing of CLT panels containing TMW. Hybrid CLT panels, consisting of TMW and unmodified YP layers, were prepared and compared with control panels made of unmodified wood. Block shear tests were conducted to measure shear strength and WFP, while microscopy assessed bondline quality. Unmodified samples exhibited substantially greater shear strength, approximately 1.5 to 2 times that of TMW, at 0.70 to 1.40 MPa. In contrast, TMW specimens showed predominantly wood failure (90–100% WFP), whereas unmodified wood demonstrated 60–95% WFP across 0.21 to 1.40 MPa. TMW exhibited 100% WFP at pressures from 0.48 to 1.40 MPa; however, shear strength declined with increasing pressure, indicating potential pressure-related cell damage or collapse that was not evident under light microscopy. Conversely, in unmodified wood, higher pressure resulted in increased shear strength. Unmodified wood also exhibited markedly greater adhesive penetration, approximately 5.4 to 5.6 times that of TMW, at 0.70 to 1.40 MPa. As pressure increased, the adhesive line thickness decreased while penetration increased; however, with rising pressure, the rate of maximum penetration growth slowed, whereas effective penetration continued to rise. Overall, the results identify 0.70 MPa as the optimal bonding pressure for TMW-CLT, where adhesive penetration stabilizes and shear strength remains maximized, offering a clear manufacturing parameter for commercial production.