Improving the storage stability of cold-setting MUF resins using amine stabilizers

Cold-setting melamine-urea-formaldehyde (CS-MUF) resins are used as adhesives for laminated timber products such as glued-laminated timber (glulam) and cross-laminated timber (CLT). However, these resins exhibit poor storage stability due to extensive hydrogen-bonding networks. This study aims to improve the storage stability by introducing several amine compounds into the CS-MUF resin via post-addition and in-situ addition. In the post-addition method, amine basicity weakens the adhesion performance by interfering with the curing reaction, and diethanolamine (DEA) is identified as the most effective stabilizer. Therefore, only DEA is used for in-situ addition. Unlike the post-addition, the in-situ method mitigates the pH-induced deterioration of adhesion performance and widens the window of DEA contents in which the trade-off between storage stability and adhesion performance occurs. This is further expanded to low-condensed CS-MUF resins, thereby indicating that the in-situ addition to low-condensed CS-MUF resin is the most suitable approach for storage stabilization. The results of gel permeation chromatography (GPC), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and differential scanning calorimetry (DSC) indicate that the aging of CS-MUF resin is dominated by hydrogen-bond-related physical changes rather than chemical reactions. Amine addition also lowers the free formaldehyde content. Overall, amines introduced via both modification methods disturb hydrogen bonding, suppress physical gelation, and partially react with free formaldehyde and hydroxymethyl groups, thereby improving the storage stability for up to one month. This study elucidates the interaction mechanism between CS-MUF resin and amine stabilizers and proposes effective stabilization conditions that offer essential guidance for designing high-value resins with industrial applicability.