Process for preparing condensation resins and the use thereof

A process for preparing a condensation resin, the process comprising:

• reacting (a) at least one selected from the group consisting of a urea of formula (I) and a urea of formula (II):
• wherein R1 and R2 are each independently a hydrogen atom or a C1-C20 alkyl radicals radical, A is a C1-C20 alkylene radical, and X is oxygen or sulfur,
• (b) formaldehyde, and
• (c) at least one CH-acidic aldehyde of formula (III):
• wherein R3 and R4 are each independently an alkyl, a cycloalkyl, a aryl, or a alkylaryl radical, or R3 and R4 together form a ring,
• wherein
• the reacting is carried out at a temperature in a range between 60 and 150° C. in the presence of an acid or a base, at least partly in the presence of at least one compound (IV) comprising two nucleophilic groups selected from the group consisting of a hydroxyl group (—OH), a primary amino group (—NH2), a secondary amino group (—NH—), and a thiol group (—SH), and optionally in the presence of at least one selected from the group consisting of a solvent and a diluent.