Examination of selected synthesis and room-temperature storage parameters for wood adhesive-type urea-formaldehyde resins by C-13-NMR spectroscopy. IV

Typical particleboard wood-adhesive urea-formaldehyde (UF) resins, synthesi zed with formaldehyde/first urea (F/U-1) mol ratios of 1.80, 2.10, and 2.40 and the second urea added to an overall F/U ratio of 1.15, in weak alkalin e pH, were allowed to stand at room temperature over a period of 50 days. C -13-NMR of time samples taken over the storage period showed gradual migrat ion of hydroxymethyl groups from the polymeric first-urea components to the monomeric second-urea components and also an advancing degree of polymeriz ation of resins by forming methylene and methylene ether groups involving t he second urea. These phenomena that varied with the F/U-1 mol ratios used in the resin syntheses due to the varying polymer branching structures resu lted in the first step of resin synthesis. Varying viscosity decreases and increases of the resins also occurred. Due to these chemical and physical c hanges, the particleboards that bonded with the sampled resins showed varyi ng bond strength and formaldehyde-emission values, indicating process optim izations possible to improve bonding and formaldehyde-emission performances . (C) 2001 John Wiley & Sons, Inc.