Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1986-04-30
1988-11-01
Ivy, C. Warren
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
523409, 523410, 523415, C25D 1320
Patent
active
047818085
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an aqueous cathodic electrocoating material which contains a synthetic resin binder, which contains amino and/or hydroxyl groups and can be rendered water-soluble by protonation with acids, and a mixture of crosslinking agents, as well as pigments, fillers, corrosion inhibitors, surface coating assistants, if necessary catalysts, and organic solvents in an amount of up to 15% by weight, based on the total weight of the coating material.
Over the past few years, cathodic electrocoating has increasingly replaced anodic electrocoating, particularly for priming car bodies. For this purpose, a large number of binder systems are available on the market. The binder systems are either self-crosslinking or externally crosslinking. The externally crosslinking systems have become more important since they have a wider range of applications. Such externally crosslinking binder systems are described in, for example, German Laid-Open Application DOS No. 2,701,002, EP-A No. 12,463 and German Pat. No. 3,108,073. Thus, for example, German Laid-Open Application DOS No. 2,701,002 describes a water-dispersible cationic synthetic resin and its use for coating an electrically conductive substrate which has been made the cathode. The synthetic resin used is the ungelled reaction product of a polyepoxide, a secondary amine and an organic polyol having at least two primary hydroxyl groups.
The coating materials contain crosslinking agents or curing agents. Blocked polyisocyanates are preferred externally crosslinking crosslinking agents. They react with the reactive amino or hydroxyl groups only when the coated article has been heated to a fairly high temperature which is sufficient to eliminate the blocking of the isocyanate. Mixtures of different polyisocyanates are also used for this purpose.
EP-B No. 4 090 likewise describes an aqueous cathodic electrocoating material which consists of a synthetic resin binder which possesses amino groups and can be rendered water-soluble by the addition of an acid, and of a crosslinking agent containing ester groups, which react with the primary and/or secondary amino groups of the synthetic resin binder in a basic medium at temperatures above 140.degree. C. with amide formation. In order to vary the performance characteristics, further modifying resins, e.g. blocked isocyanates, are, if appropriate, also incorporated into the emulsion in a small amount. With the aid of such externally crosslinking binder systems, deposited films having an acceptable surface can be produced. Nevertheless, there is still an urgent need for these surfaces to be improved substantially so that, where necessary, a two-coat process can be used for the total coating. A frequent cause of the surface defects in films is that roughness of the sheet metal substrate to be coated is visible through the baked film and appears at the surface of the latter. This also results in a top coat with a defective surface. Thus, the substrate roughness is projected upward.
It is therefore the object of the invention to avoid reproducing the substrate defects in the baked film and to provide an electrocoating material which leads to better surface quality.
Surprisingly, it has been found that this object is achieved by an electrocoating material which contains an externally crosslinking synthetic resin binder and a mixture of crosslinking agents which have different reactivities. The present invention therefore relates to an electrocoating material of the type stated at the outset, wherein the mixture of the crosslinking agents contains at least two crosslinking agents which possess different reactivities and are based on activated esters and/or on polyisocyanates blocked with different blocking agents, and different reactivities means that, when the mixture is heated, the crosslinking reaction begins at temperatures at least 10.degree. C. apart, and the crosslinking agent which reacts first is present in an amount of from 15 to 60 mol %, based on the mixture of the crosslinking agents.
Differential scanning c
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PCT preliminary examination report PCT/DE85/00261 dated Sep. 25, 1986.
Dobbelstein Arnold
Geist Michael
Ott Guther
Schon Georg
BASF Lacke & Farben Aktiengesellschaft
Ivy C. Warren
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