High-Performance Bio-Based Non-Isocyanate Polyurethane Adhesive: A Solvent and Catalyst-Free Synthesis Approach

Adhesives are used to bond two surfaces together and thus find extensive applications in industry and everyday life. Most of the commercial adhesives are manufactured using formaldehyde which makes them toxic to humans and the environment. In addition, the low bonding strength of adhesives in humid environments and underwater needs to be improved for their wider applications. This work prepared and tested a green adhesive using soybean oil. The bio-based non-isocyanate polyurethane (NIPU) adhesives were synthesized by reacting carbonated soybean oil (CSBO) with different amines such as 1,2-ethylenediamine (EDA), 1,4-butylenediamine (BDA), and 1,6-hexamethylenediamine (HDA) in a solvent and catalyst-free approach. The effect of amines on the bonding characteristics such as swelling percentage, adhesion strength, and thermal properties of the adhesives were explored. The dry lap shear strength of the adhesives prepared using EDA, BDA, and HDA showed a bonding strength of 6.23, 8.26, and 7.22 MPa, respectively. Interestingly, the wet lap shear strength of HDA-based adhesive was observed to be around 7.49 MPa while it was 5.8 MPa for BDA and 2.8 MPa for EDA. The increasing chain length of diamines exhibits a decreasing trend in swelling percentage in different solvents which suggests a stronger crosslinking of NIPU adhesives. However, HDA has lower tensile strength than BDA due to its brittleness nature. The high bonding strength of soybean oil-based adhesives suggests that these green adhesives could be used to produce high-quality interior-grade plywood.