Understanding the development of interfacial bonding within PLA/woodbased thermoplastic sandwich composites

Poly(lactic acid) wood veneer sandwich composites have been prepared and tested in situ across a range of PLA processing temperatures to provide new information of bondline morphology and composite strength. Mechanical property analysis revealed no distinctions betwee use of semi-crystalline or amorphous PLA polymer grades, but greater strength was observed using maple compared to beech wood. Composites also exhibited greater tensile strength when tested below 60 °C compared to>80 °C which was related to PLA softening above its glass transition temperature. Quantification of interfacial strength revealed composites prepared at>160 °C had higher bondline tensile strength than those formed at 140 °C. Microscopy analysis revealed that this higher tensile strength was associated with a thinner bondline and greater PLA migration away from the bondline consistent with a lower melt viscosity. This deeper PLA migration contributed to a better physical interlocking of the PLA matrix within the wood ultrastructure and associated increases in interfacial strength. Complementary DMTA analysis confirmed the thicker PLA bondlines produced at 140 °C when heated above the PLA Tg had a significant reduction in composite strength. In contrast, the thinner bondlines formed at 200 °C, were less influenced by PLA properties and at elevated temperature retained greater storage modulus.