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

The effects of posttreatments of particle-board adhesive-type urea-formaldehyde resins were studied. The resins were synthesized with formaldehyde/first urea (F/U-1) mol ratios of 1.40, 1.60, 1.80, 2.10, and 2.40 and then the second urea was added to give a final formaldehyde/urea ratio of 1.15 in alkaline pH. The resins were posttreated at 60degreesC for up to 13.5 h and the 2.5-h heat-treated resin samples were stored at room temperature for up to 27 days. Resins sampled during the posttreatments were examined by C-13-NMR and evaluated by bonding particleboards. In the posttreatments, hydroxymethyl groups on the polymeric resin components dissociated to formaldehyde and reacted with the second urea, and methylene and methyl-ene-ether groups were formed from reactions involving the second urea. Methylene-diurea and urea groups bonded to UF polymers were identified. As a result, the viscosity of the resins initially decreased but later increased along with the cloudiness of the resins. Bond-strength and formaldehyde-emission values of particleboard varied with posttreatment variables as well as with the F/U-1 mol ratios used in the resin syntheses. The results would be useful in optimizing resin synthesis and handling parameters. Various reaction mechanisms were considered. (C) 2003 Wiley Periodicals, Inc.