[NH2:1][C:2]([NH2:4])=[O:3].[CH2:5]=[O:6]>>[NH2:1][C:2]([NH2:4])=[O:3].[CH2:5]=[O:6].[NH2:1][C:2]([NH2:4])=[O:3]8.9 kg of urea and 20.5 kg of formalin (37%-formaldehyde aqueous solution adjusted to pH 8.5 with triethanolamine) were mixed under stirring and allowed to react with each other for 60 min. at 70° C. to form a urea resin prepolymer (formaldehyde/urea=1.8 (by mol)) liquid.
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