Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1999-03-16
2001-08-07
Short, Patricia A. (Department: 1712)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S482000, C428S035800, C428S418000, C428S457000, C525S176000, C525S177000, C525S438000, C525S444000
Reexamination Certificate
active
06270855
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to powder coating compositions for metal substrates that, after application, demonstrate excellent adhesion, weatherability, barrier properties, and flexibility; to a method of powder coating a metal substrate; and to a metal article, such as a metal can or container, or a material of construction, such as aluminum siding, having at least one surface coated with an adherent layer of a powder coating composition. A powder coating composition comprises: (a) a first polyester having a weight average molecular weight of about 10,000 to about 80,000 and a glass transition temperature (Tg) of greater than 45° C. to about 100° C., (b) a second polyester having a weight average molecular weight of about 10,000 to about 70,000 and a Tg of about −10° C. to about 45° C., and optionally, (c) a modifying resin, for example, an epoxy or phenoxy resin having an epoxy equivalent weight of about 500 to about 15,000 or an end-capped epoxy resin having a weight average molecular weight (M
w
) of about 300 to about 10,000, wherein the Tg of the first and second polyester differ by about 5 C° to about 60 C°. The powder coating composition is applied to a metal substrate as a film having a thickness of about 1 to about 200 microns.
BACKGROUND OF THE INVENTION
It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate, or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules, and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible, and more adhesive coating, but often, improving one advantageous property is achieved at the expense of a second advantageous property.
In addition, practical considerations limit the thickness, adhesive properties, and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing, and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal container is able to withstand the solvating properties of a product packaged in the metal container. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, such as beer, by imparting an off-taste to the product.
Conventionally, organic solvent-based coating compositions were used to provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and thereby effectively resist the solvating properties of water-based products packaged in the metal container. However, because of environmental and toxicological concerns, and in order to comply with increasingly strict governmental regulations, an increasing number of coating compositions are water based. The water-based coating compositions include ingredients that are water soluble or water dispersible, and, therefore, cured coatings resulting from water-based coating compositions often are more susceptible to the solvating properties of water.
In addition, water-based coating compositions do not completely overcome the environmental and toxicological problems associated with organic solvents because water-based compositions typically contain two or more pounds of organic solvent per gallon of coating composition. The organic solvent is a necessary ingredient to dissolve and disperse composition ingredients, and to improve the flow and viscosity of the composition. Therefore, in order to entirely avoid the environmental and toxicological problems associated with organic solvents, investigators have sought solid coating compositions that can be applied to a metal substrate. In attempts to find a useful solid coating composition, investigators have tested powder coatings, laminated film coatings, radiation cure coatings, and extrusion coatings.
Solid powder coatings have been used to coat a metal substrate with a coating composition. Solid coating compositions have been extruded onto a metal substrate, for example, as disclosed in European Patent No. 0 067 060, PCT publication WO 94/01224, Smith et al. U.S. Pat. No. 5,407,702, and Jones et al. U.S. Pat. No. 5,736,086. The extrusion coating of a solid composition onto a metal substrate is complicated by the fact that the solid composition must be heated sufficiently to melt the composition for flow through the extrusion apparatus. The heating step either can alter the chemical make-up of the coating composition or can cause a premature cure of the coating composition, especially a thermoset composition, which changes the properties of the coating on the metal substrate or makes extrusion onto the metal substrate difficult due to crosslinking in the extruder.
In order to overcome the problem of premature curing, investigators have attempted to extrude thermoplastic coating compositions onto a metal substrate. These investigators also encountered serious problems, such as composition components having either too high of a molecular weight for easy, economical extrusion, or too low of a molecular weight thereby providing an extruded film that is too soft for many practical applications, such as on the interior or exterior of a food or beverage container.
Investigators, therefore, have sought a solid coating composition for use on the exterior and interior of food and beverage containers that exhibits the advantageous properties of adhesion, flexibility, chemical resistance, and corrosion inhibition, and that is economical and does not adversely affect the taste or other esthetic properties of sensitive foods and beverages packaged in the container. Investigators especially have sought useful powder coating compositions in order to reduce the environmental and toxicological concerns associated with organic solvents. In particular, investigators have sought a solid, powder coating composition for food and beverage containers (1) that meets increasingly strict environmental regulations, (2) has corrosion inhibition properties at least equal to existing organic solvent-based coating compositions, and (3) is easily applied onto a metal substrate, by powder coating techniques, as a thin, uniform film. Such a powder coating composition would satisfy a long-felt need in the art.
A present powder coating composition comprises: (a) a first polyester, (b) a second polyester, and optionally, (c) a modifying resin, wherein the Tg of the first polyester differs from the Tg of the second polyester by about 5 C° to about 60 C°. A present powder coating composition is a thermoplastic composition and can be applied as a powder coating onto a metal substrate. A crosslinking step after application of the composition onto the metal
Jung Rolf
Schmid Christian
Marshall O'Toole Gerstein Murray & Borun
Short Patricia A.
The Valspar Corporation
LandOfFree
Powder coating compositions and methods does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Powder coating compositions and methods, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Powder coating compositions and methods will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2445974