Understanding the PLA–Wood Adhesion Interface for the Development of PLA-Bonded Softwood Laminates

Abstract

With polylactic acid (PLA) usage projected to increase in wood-based composite materials, a study comparing composite processing parameters with resulting PLA−wood adhesion and panel performance is warranted. In this study, PLA-softwood veneer laminates have been prepared and spatial chemical imaging via FTIR analysis was applied to identify PLA bondlines characterizing bondline thickness and the extent of PLA migration into the wood matrix. These PLA–wood adhesion interface characteristics have been compared with the performance of panels varying in pressing temperature, pressing time and PLA grades. For amorphous PLA, bondline thicknesses (60–120 μm) were similar, pressing at 140 °C or 160 °C, whereas with semi-crystalline PLA, the bondline thickness (340 μm) significantly reduced (155–240 μm) only when internal panel temperatures exceeded 140 °C during pressing. Internal temperatures also impacted PLA penetration, with greater PLA migration from bondlines evident with higher pressing temperatures and times with distinctions between PLA grades and bondline position. Performance testing revealed thinner PLA bondlines were associated with greater dry strength for both PLA grades. Cold-water soaking revealed laminated panels exhibit a range of wet-strength performance related to panel-pressing regimes with the semi-crystalline PLA pressed at 180 °C having similar tensile strength in dry and wet states. Moreover, an excellent correlation between wet-strength performance and bondline thickness and penetration values was evident for this PLA grade. Overall, study findings demonstrate PLA wood composite performance can be tuned through a combination of the PLA grade and the pressing regime employed.