The varying polymer structures of wood adhesive-type urea–formaldehyde resins resulting from different formaldehyde/first urea (F/U1) mole ratios used in the first step of resin manufacture were investigated using 13C. As the F/U1 mole ratio decreased progressively from 2.40 to 2.10 and to 1.80, the viscosity increase due to polymerization during resin synthesis became faster and resulted in decreasing side-chain branches and increasing free urea amide groups in the resin structure. The resultant UF resins, with the second urea added to an overall F/(U1 + U2) of 1.15, showed viscosity decreases when heated with stirring or allowed to stand at room temperature that were also characteristic with the F/U1 mole ratios used in resin synthesis. The formaldehyde emission levels of particleboards bonded with the freshly made UF resins showed relatively small but similarly characteristic variations. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2800–2814, 2001
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