Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2001-01-03
2002-08-20
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
Reexamination Certificate
active
06436478
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to a process for coating thermoplastic substrates with a coating composition that contains a non-aggressive solvent that does not attack the substrates and the process provides a finish that is smooth, even and free of defects and covers defects in the substrate. 2. Description of the Prior Art
Thermoplastic parts of ABS (acrylonitrile/butadiene/styrene), polyethylene, polypropylene, chlorinated polyolefins, polycarbonates, polyvinyl chloride and the like are widely used in automobiles and trucks and particularly in the interior of automobiles and trucks. Conventional coating compositions contain solvents such as toluene, xylene, ethyl acetate, butyl acetate, methyl ethyl ketone, propylene glycol monomethyl ether acetate and the like, which attack the surface of such thermoplastic parts. These conventional coating compositions when cured will telegraph any defects in the thermoplastic part or substrate. To hide such defects, a sealer is first applied and at least partially dried before a topcoat is applied. Another techniques that is used is to apply a layer of a conventional solvent based coating composition, allow it to cure, scuff or sand the resulting coating and then apply a second coating which usually hides the defects of the thermoplastic substrate.
Water borne coating compositions can be used which will not attack the thermoplastic substrate and when cured will not telegraph the defects of the plastic substrate through the coating as occurs with solvent based coating compositions in which the solvent attacks the plastic substrate. Typically, waterborne coatings do not provide the same high level of performance as do solvent based coatings.
It would be very desirable to utilize a solvent based coating composition to coat, thermoplastic substrates that cures at ambient or relatively low temperatures in which the solvent of the composition will not attack the thermoplastic substrate and still be an excellent solvent for the coating composition and preferably not a regulated solvent which contributes towards air pollution problems.
SUMMARY OF THE INVENTION
A process for coating a thermoplastic substrate which comprises applying a solvent based coating composition to the substrate and curing the coating composition at ambient temperature to 125° C. to form a uniform smooth film on the substrate; wherein the coating composition contains about 45-80% by weight of a film forming binder and 20-55% by weight of an organic liquid carrier; wherein the binder contains
(A) 40-90% by weight, based on the weight of the binder, of a polymer selected from the following: acrylic polyol, polyester polyol, polyether polyol or a polyurethane polyol; and
(B) 10-60% by weight, based on the weight of the binder, of an organic polyisocyanate crosslinking agent; and
wherein the organic liquid carrier used in the coating composition comprises at least 50% by weight, based on the weight of the organic liquid carrier used in the coating composition, of tertiary butyl acetate.
The process of this invention also can be used to apply lacquers, i.e., coating composition that do not contain a crosslinking agent such as a polyisocyanate.
DETAILED DESCRIPTION OF THE INVENTION
The process of this invention provides for the use of a solvent based coating composition to coat plastic substrates that cures at ambient temperatures and does not attack the substrate and hides certain defects in the substrates. The coating composition contains at least 50% by weight, based on the total weight of organic liquid carrier used in the composition, of tertiary butyl acetate and up to 50% by weight of a compatible organic solvent. Preferably, the composition contains 75-90% by weight of tertiary butyl acetate and 10-25% by weight of other compatible organic solvents. The composition contains 45-80% by weight, based on the total weight of the composition, of film forming binder and correspondingly 20-55% by weight of organic liquid carrier. The film forming binder is composed of 40-90% by weight, based on the weight of the binder, of an acrylic polyol, a polyester polyol, a polyether polyol or a polyurethane polyol and 10-60% by weight of an organic polyisocyanate crosslinking agent.
Typical plastic substrates that are used to make a wide variety of parts that can be coated by the process of this invention are polyolefins such as polyethylene, polypropylene, chlorinated polyolefins; polycarbonates, polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride, polyesters such as polybutylene terephthalate, polyethylene terephthalate, polyethylene terephthalate/polybutylene terephthalate blends, acrylic polymers such as poly(ethylene/methyl acrylate/glycidyl methacrylate), poly(ethylene/butyl acrylate/glycidyl methacrylate), ABS, blends of polymers like polycarbonate/ABS, reinforced polymers like polypropylene/glass fibers, polycarbonate/glass fibers, polyamide/glass fibers and thermoplastic elastomers like polyester elastomers.
The principal solvent used in the organic liquid carrier of the coating composition that will not attack these plastic substrates and help the coating composition hide a variety of surface defects, such as splay, in the substrate is tertiary butyl acetate. T-butyl acetate can readily be incorporated into isocyanate containing coating compositions and will not react with the isocyanate constituents of the coating. Also, it is a good solvent for most polymers such as acrylics, polyesters, polyethers, polyurethanes and is compatible with many solvents and other constituents that are typically used in coating compositions. In particular, t-butyl acetate is readily volatilized at ambient temperatures and above and coatings formulated therewith dry rapidly. Up to 50% by weight of other compatible solvents can be used such as glycol ethers such as n-propoxy propanol, alcohols such as methanol, ethanol, isopropanol, butanol, and high molecular weight esters such as 2-ethyl hexyl acetate. By compatible is meant solvents that will blend with tertiary butyl acetate and will be a solvent for the binder used in the coating composition.
The following is a list of aggressive solvents that can not be used since these solvents attack one or more of the aforementioned plastic substrates or cause the coating formed on these substrates to allow the defects of the substrate to show through the resulting finish on the substrate: ketones such as methyl ethyl ketone, methyl isobutyl ketone, methyl amyl ketone, 1,4 pentanedione, low molecular weight esters such as ethyl acetate, hexyl acetate, and surprisingly the isomers of t-butyl acetate, i.e. n-butyl acetate secondary butyl acetate and isobutyl acetate; and aromatic hydrocarbons such as toluene and xylene. Depending on the plastic substrate and how it was manufactured under some circumstances up to 30% by weight, based on the weight of organic liquid carrier in the composition, of these aggressive solvents can be used but it is preferable not to use these solvents in the coating composition.
The coating composition used in the process of this invention primarily is used as a pigmented base coat or monocoat and contains solid color pigments or metallic flake pigments or mixtures thereof and is applied to a plastic substrate and if desired, then can be top coated or over coated with a clear coating composition. The coating composition can be applied as a pigmented monocoat. Also, the coating composition used in the process of this invention can be applied with conventional spray equipment or electrostatic spray equipment and cured at ambient temperatures and temperatures up to 125° C. to decrease drying time depending upon the plastic substrate used.
Under some circumstances, the process of this invention is used to apply a clear coat directly to the plastic substrate. For example, if the plastic substrate has the desired color, a clear coat is applied directly to the substrate and splay marks or defects in the substrate do not show through.
Acrylic polyols used in the coating compositio
Cameron Erma
Deshmukh Sudhir G.
E. I. de Pont de Nemours & Company
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