Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1989-02-14
1991-03-05
Nielsen, Earl
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
523412, 2041816, 427386, C08L 6300, C09D 300
Patent
active
049978659
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an aqueous coating agent obtained from an epoxy resin, ethylenically unsaturated monomers, some of which contain carboxyl groups, a peroxide initiator, a crosslinking agent, a neutralizing agent, organic solvents, and, if appropriate, further conventional additives, such as plasticizers, stabilizers, wetting agents, dispersion auxiliaries, catalysts and pigments.
High-molecular-weight epoxy resins are suitable, in particular, for internal protective lacquers for tinplate packaging. The crosslinking agents used are, for example, phenol-formaldehyde, melamine and urea resins. Due to the prespecified application viscosity, such coating agents based on solvents have a solvent content which is usually between 70 and 60%. If--as in the coating of two-part drink cans--it is necessary to carry out the application of the coating by spraying, a further increase in the solvent content usually results, which has the consequence of great pollution through solvent emissions.
In contrast to this, the advantages of aqueous coating systems are to be seen in a markedly reduced solvent emission. In this connection, the application of aqueous synthetic resin dispersions by means of electrocoating is particularly advantageous, since a virtually 100% coating yield and a further reduced emission of solvents can be achieved using this method. In addition, it is possible to coat a very wide variety of can geometries using electrophoretic coating through the effect of the throwing power of electrodeposition coatings, a uniform coating thickness, and thus also good edge coverage, being achieved, in contrast to coating application by spraying. In addition, the electrocoating process offers the best prerequisites for process automation, this process additionally offering the opportunity for savings besides the reduced material requirement.
As is known, electrocoating can be employed both for anionic and for cationic binder systems. However, in the case of contact with foodstuffs, for example for internal coatings of cans, it must be remembered that internal protective coatings must meet strict legal requirements regarding foodstuffs. In addition, such coatings must be stable on storage in contact with the contents, which are mainly acidic to neutral. Taking into account these requirements, anodic electrocoating is, in principle, more advantageous than the cathodic version since cathodically deposited films usually contain amine groups and can thus give rise to stability weaknesses on contact with acidic contents.
Solvent containing internal protective coatings for cans which are based on combinations of epoxy resins and phenol-formaldehyde resins or amino resins and which have good properties have long been known for coating cans. In particular, epoxy resins which are based on bisphenol A and have average molecular weights of more than 3,000 g/mol give rise to very resistant coatings, whereas phenolformaldehyde resins make a decisive contribution to stability against acidic and sulfur-producing contents.
In order to use such systems in an aqueous medium, the epoxy resin must be modified by incorporation of solubilizing groups in a fashion such that a water-soluble or water-dispersible system is produced. Cationic, aqueous systems can be obtained in a known fashion by reaction of epoxy resins with amines. For the preparation of anionically dissolved synthetic resins, a carboxyl functionality is usually introduced. To this purpose, the epoxy resin, as described, for example, in U.S. Pat. No. 3,862,914, is converted into a carboxyl-functional polymer by means of a reaction with polycarboxylic anhydrides. Such systems, in which polycarboxylic acids are bound to polymers via monoester functions, are however extremely susceptible to hydrolysis, which causes the storage stability of the corresponding aqueous dispersions of such polymers to be too low (E. T. Turpin, J. Paint Technol., Vol. 47, No. 602, page 40, 1975). Hydrolysis-stable attachment of the carboxyl functionality to the epoxy resin can be achieved accordi
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patent: 4302373 (1981-11-01), Steinmetz
patent: 4423165 (1983-12-01), Harper et al.
patent: 4444923 (1984-04-01), McCarty
patent: 4476262 (1984-10-01), Chu et al.
Holscher Hans-Jorg
Reichelt Uwe
Reiter Udo
Scherping Karl-Heinz
BASF Lacke & Farben AG
Krass Frederick
Nielsen Earl
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