[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]A 30%-urea aqueous solution (11) at 15.2 kg/h and formalin (37%-formaldehyde aqueous solution adjusted to pH 8.0 by addition of 20%-triethanolamine aqueous solution) (12) at 11.1 kg/h were supplied to a resin prepolymer reaction vessel (13), and were caused to reside therein for 70 min. at 70° C. under stirring to continuously produce a urea resin prepolymer (formaldehyde/urea=1.8 (by mol)) liquid.
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