Coating processes – With pretreatment of the base – Metal base
Reexamination Certificate
1999-11-01
2002-07-02
Beck, Shrive P. (Department: 1762)
Coating processes
With pretreatment of the base
Metal base
C427S388100, C427S409000
Reexamination Certificate
active
06413588
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a method of producing layered coatings and more particularly relates to a simplified process that produces layered durable coatings on various difficult-to-coat substrates.
Protective coatings applied over difficult-to-coat substrates, such as aluminum, tend to be less durable than coatings applied over less difficult-to-coat substrates, such as steel, wood, or cement substrates. Difficult-to-coat substrates, generally require extensive surface preparations, such as, by corona discharge, acid etching, or sand blasting, followed by aqueous or solvent cleaning and priming before surface coating of such substrates can be attempted. Furthermore, multi-layered coating systems also generally require surface preparations, such as sanding, before subsequent coats can be applied. Finally, the durability of such coatings on the difficult-to-coat substrates is attenuated when such substrates are exposed to environmental erosion, such as that experienced by auto and truck bodies during normal course of driving conditions.
EP Patent Application 0 525 867 Al (hereafter '867 application) attempts to provide a multilayered coating system suitable for application on various substrates, particularly aluminum substrates. The '867 application involves applying a first coating of an epoxy, alkyd, polyester, polyurethane, polyacrylate or melamine-containing resins, followed by a metallized transparent powder coating layer containing mica-pigment.
However, a need exists for a process with fewer process steps that results in producing durable coatings on difficult-to-coat substrates.
STATEMENT OF THE INVENTION
The present invention is directed to a process for producing a durable coating on a difficult-to-coat substrate, said process comprising:
subjecting the surface of said difficult-to-coat substrate to an adhesion promoting treatment;
applying an intermediate layer of a flexible primer on said adhesion promoted surface of said difficult-to-coat substrate, said flexible primer comprising:
a polyester copolymer and a crosslinker, said polyester copolymer being produced through a two-stage polymerization process, said polyester copolymer having a linear segment with terminal hydroxyl groups and a GPC number average molecular weight (Mn) in the range of from 500 to 3000, and a branched segment with lateral hydroxyl groups and a GPC number average molecular weight (Mn) in the range of from 750 to 1500;
applying a mar resistant top layer of a clear coating composition on said intermediate layer to produce said durable coating on said difficult-to-coat substrate, said clear coating composition comprising:
a polyisocyanate binder, fluorinated polyisocyanate binder, an acrylic fluorocarbon binder or a combination thereof.
One of the advantage of the present invention is that it permits user to apply durable coatings on difficult-to-coat substrates with significantly fewer process steps than the steps used in conventional coating processes.
Another advantage of the present invention is that it permits the user to delay the application of the mar resistant top layer for significant time periods without undertaking cumbersome and time consuming sanding and cleaning steps.
DETAILED DESCRIPTION
As defined herein:
“Durable coating” means a coating that does not substantially chip, peel, mar, or delaminate when subjected to humidity and abrasion typically experienced by coating, particularly coatings used on auto and truck components, such as bodies, door panels, cabs, trailer bodies, airplane components, such as fuselage and wings. The chip performance is tested under a modified ASTM D3170-87 Gravelometer Test, described below.
“Clean substrate” means a substrate that has been solvent-washed or soap and water-washed but not physically abraded or chemically treated by a chemical etch, such as chromic acid, or by chemical anodization such as with phosphoric acid or chromic acid. Generally, such a cleaning step may be necessary to remove contaminants, if present, on the surface before it can be coated.
“Aluminum” means aluminum and aluminum alloys.
“Difficult-to-coat substrates” means substrates, such as, aluminum, chrome plated steel, stainless steel substrates or substrates from certain resins, such as, polyamides, polyetherimides and polyacetals.
The process of the present invention is suited for producing durable protective coatings on the difficult-to-coat substrates. The process of the present invention is particularly suited for aluminum and polyamides and more particularly suited for aluminum. However, if desired, it could be readily used for coating the less difficult-to-coat substrates.
Furthermore, the durable coatings produced by the process of present invention, in addition to being protective, can also be decorative.
It has been found that mechanical abrasion of a substrate surface is generally necessary in conventional processes to improve initial and long term adhesion of any subsequent coating on the underlying substrate surface. By mechanical abrasion is meant the use of sand paper, grit- or shot-blasting. By contrast, in the process of the present invention, it is generally not necessary to mechanically abrade the substrate surface followed by priming of the surface to improve the adhesion of subsequent coatings thereon. As a result, significant number of surface preparation steps are eliminated in the process of the present invention and a significant cost savings are achieved as a result thereof. Furthermore, as no mechanical abrasion of the surface is necessary, the structural strength of the underlying substrate is not compromised, especially when thin substrates are coated.
Unless the surface of the substrate is clean, it is desirable to remove any surface contaminants, such as dirt, airborne contaminants, oily, waxy or greasy substances from the surface, before the application of durable coatings by the process of the present invention.
In the cleaning step, in order to be sure that there is no wax, grease, silicone, dust, or other contaminants, a wax and grease remover, solvent, acid, or base wash, or soap/water can be used to clean the surface. Durable coatings do not adhere properly to a waxy surface. Depending on the size of the substrate, its surface may be immersed or wiped with a clean, dry cloth or paper towel soaked in a conventional wax and grease remover, such as 3812 S Fast-Dry Reducer or 3832 S reducer, both supplied by DuPont Company, Wilmington, Delaware. One such remover solvent includes 80 to 95 percent by weight of an aqueous medium and 5 to 20 percent by weight of an organic medium, substantially miscible with the aqueous medium. The organic medium preferably includes:
1.10 to 50 percent of an alkyl ester;
2.40 to 85 percent of a glycol ether selected from propoxy alkanol and ethoxy alkanol; and
3.2.5 to 10 percent of a conventional non-ionic surfactant. All weights being based on the total weight of the solvent.
After the foregoing cleaning step, the cleaned substrate surface is subjected to an adhesion promoting treatment to improve the adhesion of the difficult-to-coat substrate surface to the subsequent coatings thereon. Chemical treatment, such as chromic acid anodizing and phosphoric acid anodizing may be used. However, the preferred adhesion promoting treatment is described in U.S. Pat. No. 5,578,347, which is incorporated herein by reference. Such a preferred method provides for applying a thin, substantially uniform film of an adhesion promoter described below. By “thin” is meant a layer on the order of a monolayer or an amount not significantly in excess thereof, e.g., having a dry thickness less than 2,540 ANGSTROM (Å) (0.1 mil), preferably in the range of from 100 Å to 2,500 Å. The necessary thinness of the silane coating is assured by removal of excess of the adhesion promoter. This is typically accomplished by finishing with water or aqleous-organic solvent, wiping with a water-wet material or blowing with air. Residual water is removed by drying (forced or n
Pettus William W.
Tremper, III Henry Stever
Tronco, Jr. Henry A.
Beck Shrive P.
Crockford Kirsten A.
Deshmukh Sudhir G.
E. I. du Pont de Nemours and Company
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