The precipitation phenomenon and the semicrystallization characteristics of the deposition appearing in the urea/formaldehyde polycondensation were described for the first time. Quantitative analysis revealed that the deposition decreased with the amount of formaldehyde increasing, based on the reaction condition of the urea/formaldehyde molar ratio of 1.0, however, the amount kept a constant with that of the urea increasing. The percentage yield of the resulting deposition achieved 100% approximately, calculated by the reaction describing the formation of a linear urea-formaldehyde resin molecule. The similar quantitative analytic result could also be obtained as varying the reaction conditions of the acidity or the total reactant amount. FT-IR characteristics of the deposition varied with the reaction conditions. Intensity of the characteristic absorption of each functional group in the deposition molecules was weakened sharply with the amount of formaldehyde increasing in the reaction, while the absorption enhanced slightly with that of the urea doing. The similar effect was also obtained as the acidity or the total reactant amount varying. The XRD results were in agreement with the FT-IR characteristics, suggesting that the precipitation process in the urea-formaldehyde polycondensation reaction be caused by the semicrystallization of the linear polymeric molecules ([NHCONHCH2]n), induced by the hydrogen bonding interaction between them.

Key words: urea-formaldehyde resin, precipitation phenomenon, semicrystallization, linear molecule, hydrogen bonding