The formation of a three dimensional network structure in cured urea-formaldehyde oligomers

Abstract

The IR spectra of a number of cured urea-formaldehyde oligomers have been studied. It was found that in curing of samples with urea: formaldehyde ratios of 1 : 2 and 1 : 1·8, in addition to intermolecular condensation of methylol and imine grups, intramolecular interaction of methylol groups occurs, with formation of urone rings in the polymer chain. Increase in the proportion of urea brings about more complete usage of the functional groups in curing. The existence of a transition region in the strain-temperature curves of this series of resins was discovered. The glass temperature T g is directly proportional to the network segment molecular weight M c as calculated according to the proposed structure of the fragment and IR spectroscopic determination of conversion of the functional groups during curing. The variation in breaking stress in this series of samples, is in agreement with the conclusion that increase in the proportion of urea brings about an increase in network density.