Enhancing the performance of cold-setting phenol–resorcinol–formaldehyde resin adhesives through condensation extension

Phenol–resorcinol–formaldehyde (PRF) are the most essential adhesives for laminated timber products. PRF resins are known for their excellent strength and durability. However, when cold-pressed, PRF resins have various issues, including free phenol in the resin solution, which cannot react with formaldehyde. Aside from decreasing reactivity, excessive amounts of free phenol will render PRF resins hazardous. In addition, using hardeners containing significant amounts of paraformaldehyde might result in high formaldehyde emissions if the resin and hardener do not react completely during curing. Therefore, this research aims to reduce the possibility of high levels of free phenol and formaldehyde in PRF resins by synthesizing the resins with various condensation times and using an oxazolidine hardener to replace paraformaldehyde. PRF resins' properties and adhesion performance with various condensation times were evaluated. The results showed that PRF resins' viscosity and solids content increased when the condensation time increased, whereas their gelation time reduced. PRF resins with a longer condensation time had a higher proportion of methylene linkage and greater reactivity, as evidenced by lower apparent activation energy during curing. Furthermore, increasing the condensation time of PRF resins enhanced the adhesion strength of glue-laminated timber products bonded with these resins. The results showed that increasing the condensation time of PRF resins significantly impacts their reactivity and performance.