Application of Western red cedar (Thuja plicata) tree bark as a functional filler in pMDI wood adhesives

In this study, Western red cedar (Thuja plicata) tree bark was used as a reactive functional filler (oven-dry particles, <0.1 mm) in polymeric diphenylmethane diisocynate (pMDI) wood adhesives at four different loading levels (5, 10, 15 and 20 wt.%) to improve the bondline formation at the adhesive-substrate interface. Adhesion performance and curing behavior of the bark containing pMDI (B-pMDI) were investigated. B-pMDI adhesive had a higher viscosity than pMDI control and the viscosity value varied according to the bark/pMDI weight ratio. Compared to the pMDI control wood adhesive without bark, B-pMDI showed comparable dry bonding strength and enhanced wet bonding strength for wood substrates. SEM images indicated that B-pMDI formed a thicker and more consistent bondline than the pMDI control due to bark particles effectively bridging the gap between the wood substrates. Based on both Kissinger and isoconversional model-free methods, cure kinetics revealed that oven-dry bark particles chemically reacted with pMDI. Moreover, it was found that a lower activation energy and a higher reaction rate were observed for pMDI curing with moist bark than with oven-dry bark. FTIR characterization suggested the existence of urethane and urea linkages in the cured adhesives.