Method for preparing condensation resins and use thereof

A method for preparing a condensation resin and for reducing a viscosity thereof, the method comprising:

• reacting one molar part of at least one urea, from 3.8 to 9 molar parts formaldehyde, and from 3.8 to 9 molar parts of at least one CH-acidic aldehyde, at a temperature between 60 and 150° C. in the presence of an acid, a base, or any combination thereof, and optionally further in the presence of a solvent, a diluent, or any combination thereof, thereby obtaining a condensation resin;
• then
• neutralizing the acid, base, or combination thereof;
• distillatively removing a volatile constituent;
• and
• subsequently reacting the solidified condensation resin in need thereof with a monofunctional alcohol in the presence of an acid, thereby reducing a viscosity of the condensation resin,
• wherein the at least one urea is a urea of formula (I), a urea of formula (II), or any combination thereof:
• R1and R2 are each independently hydrogen or a C1-C20 alkyl radical,
• A is a C1-C20 alkylene radical,
• X is oxygen or sulfur, the at least one CH-acidic aldehyde is of formula (III):
• and
• R3 and R4 are each independently an alkyl, cycloalkyl, aryl, or alkylaryl radical, or R3 and R4 together optionally form a ring.