Investigating the influence of fabrication parameters, flax fibre reinforcement, and ageing on interlaminar shear strength in thermoplastic-bonded wood veneers

This study investigates the suitability of two thermoplastic polymers that are non-toxic and environmentally friendly, namely polylactic acid (PLA) and recycled maleic anhydride-grafted polypropylene (rMAPP), as potential alternatives to formaldehyde-based adhesives in plywood production. Two types of rotary cut wood veneers, beech and Douglas fir, are tested. The performance of interfaces is evaluated using interlaminar shear strength tests and compared to those obtained with a benchmark polyvinyl glue. This study examines the manufacturing process settings on interlaminar shear strength, as well as the influence of incorporating plant fibre reinforcement into the adhesive. It also evaluates the effects of accelerated ageing on the shear strength. The results indicate that manufacturing parameters tested within the specified range have a limited impact on shear strength. Both rMAPP and polyvinyl glue exhibit similar performance. This strong adhesion obtained with rMAPP is attributed to the formation of covalent bonds between the maleic anhydride (MA) and the hydroxyl groups within the amorphous constituents of the wood cell wall and to mechanical interlocking resulting from the polymer’s efficient penetration into the various wood pore structures, including cell lumens and lathe checks. The incorporation of flax fibres enhances interface performance under ambient conditions but has a negative effect in the case of hygro- and hydrothermal accelerated ageing. The results with PLA adhesive show more varied outcomes, with lower shear strength when manufactured via vacuum bagging technique. Furthermore, the performance of PLA adhesive does not meet plywood ageing standards due to its moisture sensitivity and susceptibility to hydrolysis degradation.