Synthesis and characterization of a novel eco-friendly adhesive for plywood composed by vinyl acetate-ethylene copolymer emulsion and polyvinyl alcohol

In the plywood industry, formaldehyde-based adhesives are commonly used, but the health risks associated with them are a significant concern. As a result, there has been a growing emphasis on developing formaldehyde-free alternatives. This study presents an eco-friendly adhesive formulated from vinyl acetate-ethylene (VAE) copolymer emulsion and polyvinyl alcohol (PVA). The synthesis conditions, curing behavior, synthesis mechanisms and curing mechanisms were further investigated. The optimal preparation conditions were as follows: VAE emulsion and 4 % PVA1788 were synthesized at 90 °C for 2 h, and the solid content was adjusted to 50 % by adding distilled water. Subsequently, 5 % pMDI was incorporated. The wet shear strength of the resulting adhesive, which is 0.93 MPa, met the requirements of the Chinese national standard GB/T 9846-2015 (≥0.7 MPa) for Class II plywood. The curing behavior of the VAE-PVA adhesive was investigated by measuring the insoluble mass proportion and thermal analysis. The results showed that when the adhesive was heated at 130 °C for 10 min, the insoluble mass proportion reached 81.16 %, and an endothermic reaction occurred at approximately 100 °C. To investigate the synthesis mechanism of VAE-PVA adhesive, FT-IR analysis revealed that esterification occurred between the VAE emulsion and the hydroxyl groups in PVA. Additionally, FT-IR analysis, XPS analysis, and Py-GC/MS analysis were used to explore the curing mechanism of the VAE-PVA adhesive. The results indicated that during the curing process, pMDI polymerized with the vinyl groups in the VAE emulsion and the hydroxyl groups in PVA, forming a three-dimensional network of cured polymer.