ANALYSIS OF CURING PROCESS AND THERMAL-PROPERTIES OF PHENOL-UREA-FORMALDEHYDE COCONDENSED RESINS

The curing processes of resols, urea-formaldehyde (UF) resins, their m echanical blends, and phenol-urea cocondensed resins, as well as the r eaction of 2,4,6-trimethylolphenol with urea, were investigated with t he torsional braid analysis method. The thermal stabilities of these r esins after curing also were compared. The results were as follows: (1 ) In the curing process of the mechanical blends under acidic conditio ns, it was found that the self-condensation of UF resins dominates in the initial stages, and that the curing of phenolic resin is apt to be incomplete. In alkaline curing, the blended resins require higher cur ing temperatures than resol, and their heat-resistances after curing g enerally were poor. (2) It was found that the reaction of trimethylolp henol with urea involved two stages. It was suggested that the first s tage at lower temperatures is attributed to the reaction of p-methylol groups with urea residue, and that the second stage at higher tempera tures is due to the reaction of o-methylol groups and urea residue. Th is mixture gave a resin having a great heat-resistance after completio n of the reaction. (3) The curing process of the cocondensed resins al so involved two stages. It was concluded that alkaline curing is bette r than acidic curing for these resins, and also that the heat-resistan ces of the cocondensed resins are superior to those of the mechanical blended resins.