Plywood Wood Based Composites Using Crystalline/Amorphous PLA Polymer Adhesives

Tailored crystalline and amorphous powdered polylactic acid (PLA) bio-renewable green polymers were studied for their use as adhesive for the joining of 2 and 3-ply veneers to produce and test plywood panels for strength, substrate failure and water soak resistance properties per ANSI/HPVA standards. For 3-ply construction Douglas Fir (DF) was used for the core whereas, Southern Pine (SP) veneer was used for the faces per sponsor specifications. It was found that heat or ionizing radiation tailored crystalline and amorphous type PLA-based adhesive resin formulations can both offer adequate strength as measured by the attained shear stress [ranging through ~3.4 MPa (500 psi)], and degree of substrate failure (50–90%) for a range of adhesive loadings ranging from 85 g/m2 to 300 g/m2. Highest shear strength values approaching 500 psi were obtained with emulsion spray based application using amorphous PLA in the 125–250 μm range. For 2-ply samples prepared with either crystalline or amorphous PLA adhesive, both offered satisfactory strength and soak resistance. Successful dry shear strength and 3-cycle soak water resistance under standard conditions were possible to achieve with amorphous PLA formulation for both 2-ply and 3-ply plywood samples. 3-ply tests with panels using crystalline form PLA adhesive formulation did not meet 3-cycle resistance metrics even with 300 g/m2 adhesive loading; although, 1-cycle soak resistance was possible. The superior performance of the amorphous formulation is deemed to arise out of improved wetting and substrate penetration due to its x10 higher melt index and x3 lower relative viscosity as compared with the crystalline form polymer adhesive. Plywood samples prepared with PLA formulations (as yet without cross-linking or strength tailoring), did not survive boiling water testing metrics as would be required of Type I (exterior grade) plywood. However, the research results indicate that ANSI/HVPA metrics for Type II (interior grade) plywood are attainable using the as-formulated amorphous form PLA based adhesive and the associated application methods described in the paper.