Cationic polyurethane-based polymer dispersions

Details Cationic polyurethane-based polymer dispersions Cationic polyurethane-based polymer dispersions

1997-06-23

1999-07-27

Niland, Patrick D.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

4273722, 4273855, 524507, 524839, 524840, 525123, 525455, C08J 300, C08K 320, C08L 7500, C08F 830

Patent

active

059291613

DESCRIPTION:

BRIEF SUMMARY
The invention relates to dispersions of cationically solubilized polyurethanes, to a process for their preparation and to their use as an additive in cationic electrodeposition coating materials.
GB-A 2 068 391 describes a process for preparing polymer latices, where aqueous emulsions of polymerizable monomers are polymerized in the presence of an amine salt of a polyurethane prepolymer. The polyurethane prepolymer is prepared in four stages by reacting a hydrophilic polyol with an excess of a polyisocyanate, blocking the isocyanate groups with a ketoxime, reacting the prepolymer, which is terminated with capped isocyanate groups, with a polyfunctional amine, and finally adding acid to form salts.
DE-A 37 18 520 describes the use of the emulsion polymers disclosed therein as paper sizing agents. Cationically modified polyurethanes are employed as emulsifier.
EP-A 0 334 032 describes a process for preparing dispersions of polymers by polymerization of olefinically unsaturated monomers in water in the presence of oligourethanes having a branched molecular structure and hydrophilic groups which, exclusively or predominantly, have been incorporated terminally in the oligourethane. The dispersions are used as coating compositions.
The technique of electrodeposition coating has been found advantageous in connection with the coating of metal panels. In connection with the coating of edges, especially sharp edges, however, only low coat thicknesses can be obtained. Although increasing the pigment concentration improves coating in the vicinity of edges, this is at the cost of an increase in surface roughness. Substantially increasing the pigmentation is undesirable on economic grounds as well. The presence of microgels in the binder also improves the coating of edges. Such techniques are described in, for example, US-A 4,788,246, EP-A 0 463 474, EP-A 0 282 000 and DE-A 42 04 581.
The preparation of these microgel-containing binders requires additional reaction stages. The microgel particles must be to a large degree chemically similar to the binder in order to avoid separation. The difference in density between microgel and the bath liquid must not be too large, so as to prevent sedimentation.
Microgel particles having a core-shell structure have been proposed (European Coatings Journal, (1), p. 6ff, 1991). Ionic, preferably cationic, groups in the material of the shell improve the hydrophilicity and the stability of the deposition bath containing microgel particles. These microgel particles are prepared by adding water to a resin mixture to form a dispersion in water of particles having a core-shell structure. In this system, one of the resins of the mixture forms the shell, the other the core. These primary particles are crosslinked by heating at from 80 to 90.degree. C. for a period of from 1 to 7 hours. The crosslinked dispersion is added to the deposition bath. In this process, the two resins must first of all form a homogeneous mixture, and following the addition of water the difference in hydrophilicity must lead to formation of a core-shell structure. As a consequence, there are limitations on the combination of materials.
An alternative solution is described in EP-A 0 249 884: a mixture of capped isocyanates is employed as hardener, the dissociation temperature of one isocyanate component being between 60 and 120.degree. C. and that of the second component being 40.degree. C. higher. The blocked isocyanate that dissociates at the lower temperature provides early thickening of the binder and prevents it flowing away from the edge. Because of the heightened reactivity of one isocyanate component, these binders have a tendency to form sediments in the bath as a result of premature setting, thereby adversely affecting the useful life of the bath. The edge protection effect is dependent on the particular amounts of isocyanates in the bath liquid and cannot be varied in a closely controlled manner.
The object of the invention therefore, is to improve the edge protection afforded in cationic electrodeposition coa

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Tsuchiya et al; "Corrosion protection at sharp edges"; European Coatings Journal; pp. 6,7,10-12.

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