Stove-enamel coating media and the use thereof

Details Stove-enamel coating media and the use thereof Stove-enamel coating media and the use thereof

1998-02-10

2000-07-25

Dawson, Robert

Chemistry: electrical and wave energy

Processes and products

Electrophoresis or electro-osmosis processes and electrolyte...

204507, 523408, 523414, 524820, C25D 1306

Patent

active

060932996

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to stove-enamel coating media which crosslink by an addition reaction without elimination losses occurring from the coating film, and to the use thereof for the cathodic electro-dip coating process.
In practice, coating media which harden are produced by different types of crosslinking mechanisms. In particular, amine resin crosslinking lacquer systems which crosslink via blocked polyisocyanates or by transesterification have become outstandingly important in this respect. A common feature of these lacquer systems--which are formulated based on organic solvents or which are preferably formulated with an aqueous basis--is that they suffer elimination losses on stoving. For example, etherification or esterification alcohols, formaldehyde, or blocking agents may be eliminated, which makes it necessary to purify the exhaust air discharged from the stoving kiln and constitutes an unwanted loss of lacquer solids. Those elimination products which condense in the stoving kiln and result in contaminants there are particularly problematical.
A cathodically depositable coating medium is known from EP-A-0 356 970 which can be stoved without elimination losses and which is based on a hydroxy-functional cationic resin and a polyepoxide crosslinking agent, the epoxide groups of which are bonded alicyclically. Lacquer systems of this type exhibit stability problems, and crosslinking reactions which result in gel formation can occur in the coating medium. Polyepoxide compounds can be problematical from an industrial hygiene point of view.
EP-A-0 661 354 describes a process for cathodic electro-dip coating (CEC) using a CEC coating medium based on a binder vehicle which comprises primary amine functions and on a crosslinking agent which contains cyclic carbonate groups in its molecule. These CEC coating media exhibit stability problems, and the binder vehicles on which they are based are preferably highly neutralised in order to deal with this problem (a typical degree of cathodic electrodeposition and thus result in unsatisfactory protection from corrosion during the CEC coating of three-dimensional substrates which comprise cavities, for example, automobile bodies.
The object of the present invention is to provide a coating medium which can be stoved without elimination losses, which exhibits no stability problems and which can be neutralised to a reduced extent.
The solution which achieves this object is the provision of a stove-enamel coating medium which contains one or more binder vehicles comprising cationic groups and/or groups which can be converted into cationic groups and comprising secondary amine and/or hydroxyl groups as well as one or more crosslinking agents comprising cyclic carbonate groups or a self-crosslinking binder vehicle comprising cationic groups and/or groups which can be converted into cationic groups, secondary amine and/or hydroxyl groups as well as cyclic carbonate groups, wherein the binder vehicles are selected from amino(meth)acrylate resins and/or from customary aminoepoxide resins produced by the addition reaction of amine compounds with the epoxide groups of epoxide resins. In this connection, epoxide resins are understood to be polyepoxide compounds which are not modified at their epoxide groups, preferably the known aromatic epoxide resins, most preferably those based on bisphenol A for example. Amines or amino alcohols which contain at least one primary or secondary amine group in their molecule can be employed as the amine compounds. If primary amine groups are present in the molecule of the amine compound, these are preferably not temporarily protected, for example as ketimine groups.
According to one preferred embodiment, stove-enamel coating media are provided which contain one or more binder vehicles which are free from primary amine groups and which comprise cationic groups and/or groups which can be converted into cationic groups, as well as secondary amine and/or hydroxyl groups, and which also contain one or more crosslinking agents comprising cy

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