Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1994-03-28
1995-09-19
Niebling, John
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
2041816, 523415, 525 65, 525107, 525438, C25D 1314
Patent
active
054513049
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the electrophoretic internal coating of metal containers for the storage of foodstuffs and beverages, using an aqueous coating composition.
The application of internal protective can coatings by means of electrophoretic deposition is known. The internal can coatings here have to meet special requirements, since they must have a very high resistance to hydrolysis and chemicals. The coatings must be resistant, for example, during storage in contact with predominantly acidic filling materials. Anodic electrodip coating is here in principle more advantageous than the cathodic variant, since the cathodically deposited films in most cases contain amino groups and thus can be less resistant on contact with acidic filling materials. Internal protective can coatings must also meet stringent regulations under food law, and the coatings must not show any hydrolysis reactions under the conditions of pasteurization and during prolonged storage.
The electrophoretic application of internal protective can coatings is advantageous since, as compared with spray application, a substantially higher coating yield and a further reduced emission of solvents can be achieved. Moreover, by means of electrophoretic deposition, the coating of very diverse can geometries is possible as a result of the effect of throwing power during electrophoretic deposition. As compared with spray application, a uniform layer thickness and good edge coverage can be achieved. Furthermore, the electrodip coating process provides the best conditions for process automation.
A large number of electrophoretically depositable coating agents for the internal coating of metal containers, which are suitable for the storage of foodstuffs and beverages (so-called "food-contacting coatings"), are known.
In many cases, these concern hydroxyester-based binders which are obtained by reacting copolymers containing carboxyl groups with epoxide resins. Such electrophoretically depositable, internal protective can coatings are known, for example, from EP-A-6334 and EP-A-6336. Other known internal protective can coatings suitable for electrophoretic application are based on polymer mixtures which are obtained by free-radical copolymerization of ethylenically unsaturated monomers, some of which contain carboxyl groups, in the presence of an epoxide resin and of peroxide initiators (see U.S. Pat. No. 4,212,781, U.S. Pat. No. 4,308,185, WO 88/1287).
Further electrophoretically depositable internal protective can coatings are described, for example, in DE-A-2,805,657, DE-A-3,601,560, U.S. Pat. No. 4,476,263 and EP-A-216,337.
Finally, U.S. Pat. No. 4,136,075 has disclosed aqueous internal can coatings which are based on acrylate resins and can contain waxes, for example carnauba wax, as additives. The coatings are not appl ied electrophoretically, but by means of spray application, dip-coating, roller-coating or brushing.
In electrophoretic deposition, the deposited wet film is subjected to a rinsing step which has a decisive influence on the quality of the dried film. The rinsing medium used is extracted ultrafiltrate, deionized water or a deionized water/solvent/amine mixture. With the coating agents, known from the state of the art, for internal can coating, the problem frequently arises that the deposited films have an inadequate stability during the rinsing step following the electrophoretic deposition, since the wet film can partially be incipiently dissolved by the rinsing medium. The thus resulting thinner coating films or, in an extreme case, complete exposure of the substrate in some areas cause high porosity values and hence unsatisfactory stability of the filling material. This sensitivity to rinsing, which occurs, of the electrophoretically deposited wet film depends on a number of factors, in particular, of course, on the nature of the coating agent, on the surface tension of the rinsing medium, ie. on the solvent content, on the neutralizing agent and coating residues, and also on application parameters, ie., for
REFERENCES:
patent: 4623860 (1986-11-01), Azarnia et al.
patent: 5322863 (1994-06-01), Figge et al.
Harris Paul
Jouck Walter
Markfort Klaus
Stenger Michael
BASF Lacke & Farben AG
Mayekar Kishor
Niebling John
Sabourin Anne Gerry
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