Improvement of storage stability of UF resins by adding caprolactam

Abstract During storage, the structure of urea-formaldehyde (UF) resins suffers modifications due to reactions between monomers, oligomers, polymer and free formaldehyde, leading to increase in viscosity and decrease in pH. Eventually, viscosity reaches a value that renders the resin unusable, and it must be disposed off. This aging process is accelerated if storage temperature increases. The aim of this work is to obtain UF resins with long storage stability, even when exposed to relatively high temperatures, such as 40 °C. The main strategy adopted was the addition of a chain growth blocker, caprolactam. This monofunctional compound reacts with end groups, blocking them and therefore reducing the polymer's reactivity. In addition, a weak base was added to adjust the pH value, instead of the traditional strong base, sodium hydroxide, therefore hindering the Cannizzaro reaction. The storage stability of UF resins with formaldehyde to urea molar ratio of 1.6–2.0 was monitored by pH and viscosity measurements. Caprolactam was added in different amounts and at different reaction stages. It was found that 10% addition at the beginning of condensation led to the best results, giving a much higher storage stability at 40 °C (2 months when compared to 4 days for a commercial UF resin with low F/U molar ratio). As expected, the resin reactivity decreased with caprolactam addition, demanding for longer pressing times for wood-based panel manufacture. These verified the internal bond strength specification for EN 312 - P2 standard class. Formaldehyde content in the panels was above the E1 class limit when fresh or one month old modified resins were used, implying addition of formaldehyde scavengers. The resin stored for 2 months allowed producing panels within E1 limit. These preliminary results demonstrate the concept that addition of an end-group blocker during UF synthesis is an effective strategy for improving storage stability, encouraging future work on alternative compounds and synthesis conditions optimization.