Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-11-17
2002-08-20
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
active
06436260
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for electrocoating bulk articles, such as screws, that are placed in a container and then electrocoated with an electrocoating composition.
Electrocoating is a process that is used for the coating of industrial bulk goods, in particular, used in the coating of metallic substrates in order to protect them against corrosion. In this case, the electrocoating process generally is limited to a process for priming the metallic substrates with a protective coating.
When bulk articles, for example screws, are electrocoated, no additional coatings are applied. A single layer of an electrocoating composition generally is sufficient to protect the bulk articles from corrosion, in particular against the effects of weather and humidity.
To increase the corrosion protection, an additional layer of an electrocoating composition can be deposited on the electrocoated bulk articles. This can readily be accomplished by making the initial layer of the electrocoating composition electrically conductive. EP-A 421 247 describes the double-coating of metallic substrates, such as metallic fastening elements, by applying an anodic coating, thermally curing the coated elements and then applying a cathodic coating and curing that coating. The use of conductive carbon blacks in double coating of articles using an electrocoating process is described in F. Beck and H. Guder, “Elektrotauchlackierung mit rubgefallten Systemen”, Farbe+Lack, 93. Jahrgang July 1987, S. 539 ff. but does not show the coating of bulk articles that require the use of a receiving container in the electrocoating process.
In a conventional electrocoating process for coating bulk articles, the bulk articles are placed in basket-like conductive construction (receiving container), preferably, made of metal. In the electrocoating process, an electrical contact is made with the receiving container and electrical current is passed through the bulk articles in the container that are in direct contact with the container and each other. During electrocoating, the bulk articles and the receiving container are coated. Then, the receiving container and the coated bulk articles are place in an oven and baked to form a coating layer or film on the bulk articles. However, a coating layer also is formed on the receiving container and this layer substantially reduces the electrical conductivity of the receiving container. In order to use this receiving container in a subsequent electrocoating process, the coating layer must be removed so that there is an electrical contact between the container and the bulk articles to be coated. Such removal requires a special chemical or solvent treatment that incurs considerable time and expense. There is a need for a process that will eliminate or substantially reduce this removal or cleaning operation that is currently required by conventional electrocoating process used to coat bulk articles.
SUMMARY OF THE INVENTION
This invention is directed to a process for electrocoating bulk articles to provide a corrosion protective coating and comprises the following steps:
(1) the bulk articles to be coated are placed in an electrically conductive receiving container and the container and bulk articles are electrocoated with an aqueous electrocoating composition containing a film forming binder and electrically conductive components;
(2) baking the coated bulk articles and the receiving container to form an electrically conductive coating on the bulk articles and the container and removing the coated bulk articles from the container; and
(3) repeating steps (1) and (2) at least once without any intermediate cleaning of the receiving container having an electrically conductive coating.
The advantage of the novel process of this invention is that the receiving container can be used at least twice and up to six times without any intermediate cleaning or removal of the electrocoating layer that has been applied since the coating is electrically conductive and allows for further electrocoating of bulk articles. This has not been possible with prior art processes.
DETAILED DESCRIPTION OF THE INVENTION
In the process of this invention, an electrocoating composition containing electrically conductive components is deposited on bulk articles and then the coated articles are baked at an elevated temperature. An electrically conductive coating layer is formed on the coated bulk articles and on the receiving container holding the coated bulk articles.
After the electrocoated and baked bulk articles have been removed from the receiving container, the container can be used again without removing any coating deposited on the container and the container is filled with bulk articles that are to be coated and these articles then are electrocoated in the receiving container. The conductive coating layer on the receiving container imparts sufficient electrical contact to the bulk articles to be coated so that they can be electrocoated even though the receiving container has already been coated. This makes it possible to use the receiving container in the electrocoating process for up to six times in a row without intermediate removal of the coating with solvents, chemicals or by mechanical means, such as brushing.
The electrical resistance of the coating deposited on the receiving container is generally slightly higher than a container without the coating but this can be compensated for by adequately controlling the deposition voltage and deposition time in order to obtain the same required coating thickness on the bulk articles.
Anodic or cathodic deposition electrocoatings that are known in the art can be used as the electrocoating composition used in the process of this invention. These coating compositions contain components that impart a sufficiently low specific electrical resistance to the coating. The resulting coating, after baking at an elevated temperature, has the required level of electrical conductivity.
Suitable electrocoating compositions useful in the process of this invention can be either anodic or cathodic electrocoating compositions. These compositions are based on the anionic or acid film forming binders or cationic or alkaline film binders that are conventionally used in electrodeposition processes. Examples of such binders are: epoxy resins, acrylic resins, polyester resins, maleic oils, polybutadiene oils, polyurethane resins, and polybutadiene resins.
In these electrocoating compositions, at least one part of the binders carries ionic substituents and/or substituents able to be converted into ionic groups and optionally, chemically cross-linkable groups.
The ionic groups or the groups able to be converted into ionic groups of the binder can be anionic groups or groups able to be converted into anionic groups, i.e. acid groups, such as COOH, FO
3
H, PO
3
H and the corresponding anionic groups neutralized with a base. They may also be cationic groups or groups able to be converted into cationic groups, for example, alkaline groups, preferably nitrogenous alkaline groups. These groups may be present in quaternized form, or they can be converted into ionic groups with a conventional neutralizing agent, for example an organic monocarboxylic acid such as, formic acid or ethanoic acid. Examples of such groups are amino, ammonium, phosphonium and/or sulfonium groups.
Examples of anodic electrocoating binders are binders based on polyesters, epoxy resin esters, (meth)acrylic copolymer resins, maleic oils or polybutadiene oils having an average weight molecular mass (Mw), for example of 300 to 10 000 and an acid value, for example of 35 to 300. These binders carry, for example, COOH—, SO
3
H— and/or PO
3
H
2
-groups. The resins may be converted into the aqueous phase after neutralization of at least some of the acid groups.
Examples of cathodic electrocoating binders are resins containing primary, secondary and/or tertiary amino groups having amine values, for example of 20 to 250, and an average weight molecular mass (Mw) of the resin, for example of 300 t
Buettner Gabriele
Klein Klausjoerg
Lorenz Klaus-Peter
Deshmukh Sudhir G.
E. I. du Pont de Nemours and Company
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