Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonmetal coating
Reexamination Certificate
2000-03-30
2001-12-11
Dawson, Robert (Department: 1712)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonmetal coating
C205S317000, C205S320000, C106S287230, C106S287320
Reexamination Certificate
active
06328873
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to electrodeposition coating compositions (i.e., coating compositions that can be applied by cathodic electrodeposition (“CED”) process) as well as to a process for the CED coating of substrates having a galvanized surface.
CED is a process in which coatings are applied to substrates where the substrate acts as the cathode. When substrates having an at least partially galvanized surface undergo CED coating, surface defects frequently form within the CED coating layer deposited due to the formation of fine hydrogen blisters on the substrate. The hydrogen blisters, particularly after the CED coating layer has been stoved, are visible as pinholes or so-called zinc cratering. The pinholes are usually still visible after subsequent coatings are applied. Thus, individual pinholes in the CED coating layer can only be eliminated by post-working such as sanding and post-coating with suitable repair coating agents. In this case, not only are quality deficiencies of the CED coating possible, but in particular the productivity of the CED coating process suffers. If the CED coating layer has a very large number of pinholes, post-processing is no longer possible. The defectively coated substrate then becomes scrap.
The problem of pinholes as outlined above becomes more pronounced as the deposition voltage is increased, in other words the number of pinholes per unit of area increases with the deposition voltage. Pinholes thus become a significant problem when coating substrates at high deposition voltage. CED coating of three-dimensional substrates such as, for example, automobile bodies, takes place at the highest possible deposition voltage in order to achieve the highest possible throwing power. Throwing power is understood to mean that ability of an electrodeposition coating agent to be deposited within voids of a three-dimensional substrate which is significant for effective corrosion protection. Accordingly, there is a need for CED coating compositions that produce substantially pinhole-free coatings, even at high deposition voltages.
SUMMARY OF THE INVENTION
The present invention provides a cathodic electrodeposition coating composition which produces substantially pinhole-free coatings on galvanized surfaces, said composition comprising 0.1 to 5% by weight, based on the weight of resin solids in the composition, of at least one sulphonamide.
In another aspect, the invention provides a process for coating substrates having an at least partially galvanized surface, said process comprising the electrodeposition on said substrate of a cathodic electrodeposition composition of this invention.
In a further aspect, the invention provides a process for making substantially pinhole-free coatings on substrates having an at least partially galvanized surface, the process comprising applying a cathodic electrodeposition coating composition of this invention to galvanized surface of the substrate.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The CED coating compositions according to the invention produce substantially pinhole-free coatings on substrates having an at least partially galvanized surface. By “substantially pinhole-free” we mean that the number of pinholes in the coating is kept to a minimum. However, as is demonstrated in the examples, the number of pinholes is a function of the sulphonamide content and deposition voltage. Thus, the term “substantially pinhole-free” should be understood to mean, e.g., no more than 5 pinholes visible to the unaided eye in a square meter area.
The CED coatings of this invention are aqueous coating compositions having a solids content of, for example, from 10 to 30 wt-%. The total solids content of the CED coating compositions consist of resin solids, pigments, extenders, other additives conventionally used in coating compositions and the sulphonamide(s). The resin solids of the CED coating compositions consist of binder(s) and any optional cross-linking resin(s) used in the composition. Pigment paste resins optionally contained in the CED coating compositions are included with the binders. The binders may be self-cross-linking or externally cross-linking. If externally cross-linking binders are used, the CED coating compositions would also contain cross-linking agents.
At least some of the binders carry cationic substituents and/or substituents able to be converted into cationic groups. The cross-linking agents may also contain cationic groups. Examples of cationic groups include basic groups, preferably nitrogen containing basic groups, which may be present in quaternized form. In lieu of or in addition to cationic groups, groups which may be converted to cationic groups (“cationic convertible groups”) may be used. Cationic convertible groups are those which form cationic groups when reacted with a neutralizing agent, such as organic acids, for example, formic acid, acetic acid, lactic acid or methane sulphonic acid. Examples of suitable groups include primary, secondary or tertiary amino groups and ammonium groups, e.g., quaternary ammonium, phosphonium and/or sulphonium groups. Cationic convertible groups may be present in completely or partially neutralized form.
The CED coating compositions according to the invention contain binders depositable by cathodic electrodeposition, preferably resins containing primary, secondary and/or tertiary amino groups and having amine values of around 20 to 250 mg KOH/g. The weight average molecular weight (M
W
) of the binders is preferably around 300 to 10000. The resins may be converted into the aqueous phase after the quaternization or neutralization of at least some of the cationic convertible groups. Examples of such CED coating binders are aminoepoxy resins, aminoepoxy resins having terminal double bonds, aminoepoxy resins having primary OH groups, aminopolyurethane resins, amino group-containing polybutadiene resins and/or modified epoxy resin-carbon dioxide-amine reaction products as well as amino(meth)acrylic resins.
The CED coating binders carry functional groups, in particular hydroxyl groups, for example corresponding to a hydroxyl value of 30 to 300, preferably 50 to 250 mg KOH/g, which are able to cross-link chemically. The CED coating binders may be self-cross-linking, or they are used in a mixture with known cross-linking agents. Examples of such cross-linking agents are amino resins, blocked polyisocyanates, cross-linking agents having terminal double bonds, polyepoxy compounds or cross-linking agents containing transesterifiable groups.
For the preparation of the CED coating compositions, the cationic binders may be used as a CED coating binder dispersion which may optionally contain cross-linking agent. CED coating binder dispersions may be prepared by the synthesis of CED coating binders in the presence or absence of organic solvents and conversion into an aqueous dispersion by diluting with water the CED coating binders which have been neutralized with neutralizing agent. The CED coating binder(s) may be present in a mixture with one or more suitable cross-linking agents and be converted together with the latter into the aqueous dispersion. Organic solvent, where present, may be removed, for example by vacuum distillation, before or after conversion into the aqueous dispersion until the desired content is achieved.
The subsequent removal of solvents can be avoided, for example, if the CED coating binders are neutralized in the low-solvent or solvent-free state, for example as a solvent-free melt at temperatures of, for example, up to 140° C., with neutralizing agent and are afterwards converted with water into the CED coating binder dispersion. It is likewise possible to avoid the removal of organic solvents if the CED coating binders are charged as a solution in one or more olefinically unsaturated monomers which are polymerizable by free radical polymerization, or the binder synthesis is carried out in, as the solvent, one or more monomers (for example styrene) which are polymerizable by free radical polymerization, and the solution is afterwards
David Norbert
Katzmann Heinz-Peter
Klein Klausjoerg
Dawson Robert
E. I. Du Pont de Nemours and Company
Feely Michael J
Tesari Joseph A.
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