The effect of humidity on crosslinked and entanglement networking of formaidehyde-based wood adhesives

Abstract

Listen

Translate article

Thermomechanical analysis (TMA) tests on joints bonded with synthetic phenol-formaldehyde (PF) resins and urea-formaldehyde (UF) resins have shown that frequently the joint increase in modulus does not proceed in a single step but in two or more steps, yielding an increase of modulus first derivate curve presenting two (or more) rather than a single peak This behaviour has been found to be due to the initial growth of the polycondensation polymer leading first to linear polymers of critical length for the formation of entanglement networks. Two or more modulus steps and two or more first derivate peaks then occur, and when two only occur the first is due to the formation of linear polymers entanglement networks and the second to covalent cross-linked networks. The faster is the reaction of a phenolic resin for any reason, such as decreasing water content of the resin, or the higher is the reactivity of a PF resin the earlier and at lower temperature the entanglement network occurs and the more important is its modulus value in relation to the final, cross-linked resin modulus. The accepted method of calculating the gel point and gel temperature of a polycondensation resin from the single peak of the first derivate curve is still acceptable in resins where the entanglement effect is small or is not present? In resin systems where the entanglement effect is instead of importance, the question of what is the gel point in such systems had to be addressed, and gel temperature and gel point must be obtained from the modulus and its first derivate curve in a different manner, which is presented. Similar findings are shown as regards UF resins and the influence of water and of wheat flour extender on their networking during curing. The formation and disappearance during thermosetting adhesives curing of entanglement networks has not been recognized before as a determining occurence in the mechanism of hardening of wood adhesives. These entanglement networks can contribute strongly, or can detract according to conditions, to both the green and the initial strength of a wood joint bonded with a thermosetting adhesive resin by altering considerably its rheology before and during hot-pressing. They can also contribute to the final strength and performance of the joint when the entanglement network still exists after resin hardening, as it is the case for wheat flour extension of a UF resin.