process for the production of lagerstabilen, liquid urea formaldehyde leimharzen.

摘要

The resins (I), with molar ratio U:F 1:1.0-1:2.1 and solids content 58-75 wt.% on total wt. (I) are prepd. by 1) thin layer-evaporating UF precondensate with solids content 38-55 wt.% on wt. (I) to solids content 58-72 wt.% 2) returning 10-60 wt.% prod. to thin layer-evapn. 3) adding aq. U soln. then 4) flash-evaporating prod. directly (i.e., within interval so short that no substantial reaction occurs between U added in 3) and condensate). Opt. UF precondensate is also prepd. with melamine and/or phenol, opt. as precondensates with HCHO. Interval between 3) and 4) is 0.5-5, partic. 1.8-2.5, seconds. Thin layer evapn. is effected in one or more consecutive or parallel stages, partic. in 2 or 3 stages with prod. being returned in each stage. Average film thickness in falling film evaporator(s) is about 1.5 mm, average residence time is 1-10 minutes. Thin film evapn. is carried out with steam at 2-6 bar. Soln. is evaporated at 48-100 deg.C. Mixing is provided between 3) and 4). U soln. in 3) is 60-72 wt.%. Returned part of soln. is treated with 60-72 wt.% soln. of U before again subjecting to thin layer evapn.