It is generally customary to employ metal catalysts for the crosslinking of CEC coating layers in order to reduce the stoving temperature. In this connection, organotin catalysts have generally been used. The use of dialkyltin oxide is desirable, and the use of dibutyltin oxide (DBTO) is particularly desirable due to its ready availability and favourable price. However, in order that it can successfully be used in CEC coatings, DBTO requires conditioning. This conditioning may consist of a separate synthesis step, i.e. in the chemical conversion of the DBTO into a dibutyltin dicarboxylate for example.
Thus CEC coating media which crosslink by means of blocked isocyanates are known from EP-A-0 509 437. These contain dialkyltin dicarboxylates derived from aromatic carboxylic acids as crosslinking catalysts, in addition to bismuth or zirconium compounds as additional catalysts. Bismuth hydroxide, trioxide, nitrate, benzoate, citrate, oxycarbonate and silicate can be used as the bismuth compounds.
DE-A-39 40 781 describes a process for producing catalyst pastes containing dibutyltin oxide by the preliminary dispersion of DBTO with organic solvent and 0.05 to 5 moles, per mole of DBTO, of an organic acid which is customary for the neutralisation of cathodically depositable electro-dip coatings, at a water content of up to 5% by weight with respect to the sum of DBTO, solvent, acid and water, and the subsequent dispersion and comminution of the mixture obtained with grinding binder vehicles and water. Preliminary dispersion is preferably effected within a temperature range from 50 to 80.degree. C. During the production of CEC coating media, the catalyst pastes drag in organic acid. However, low acid contents are wanted in CEC coating media, since they promote good wrap-around behaviour and low current consumption during cathodic lacquer deposition.