Controlling the stability and adhesion performance of cold-setting phenol–resorcinol–formaldehyde resin adhesives through methanol addition

Abstract

Phenol–resorcinol–formaldehyde (PRF) resin adhesives are commonly used in the manufacture of laminated timber products, and they are cold-set at room temperature rather than hot-pressed to generate a three-dimensional polymer structure in the products. However, their excellent reactivity often leads to poor storage stability and a short pot life. This study examines the influence of methanol addition on the stability and adhesion performance of PRF resins. These resins were synthesized by various levels of methanol addition (5 %, 10 %, and 15 %), and their structures were investigated using Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies. Furthermore, differential scanning calorimetry (DSC) was used to analyze their thermal behavior, and their cure kinetics were analyzed with two isoconversional analyses: Kissinger–Akahira–Sunose (KAS) and Vyazovkin (VYZ) method. The adhesion performances of PRF resins were evaluated for glue-laminated timber (GLT). The results demonstrated that the viscosity and solids content of PRF resins decreased while their gelation time increased as the methanol addition level increased. Methanol addition has also improved the resin stability over time. The chemical structure of PRF resins did not change considerably with an increase in the methanol level. However, methanol interaction with formalin in a hardener presumably hindered the reactivity of PRF resins. This was supported with an increase in the apparent activation energy of the curing process of PRF resins as the methanol level increased. In addition, decreasing the methanol levels from 15 % to 5 % enhanced the adhesion performance of GLT bonded with PRF resins. The results indicated that adding methanol to PRF resins improved its stability while simultaneously weakened its performance. However, 5 % methanol level is recommended for PRF resins.