Extrusion coating compositions and method

Abstract

Solid extrusion coating compositions for metal substrates, a method of extrusion coating a metal substrate, and a metal article are disclosed. The extrusion coating composition is a thermoplastic material and comprises: (a) a polyester having a weight average molecular weight of about 10,000 to about 50,000, and optionally, (b) a modifying resin, such as an epoxy resin having an epoxy equivalent weight of about 500 to about 15,000. The extrusion coating composition is applied to a metal substrate in an extrusion process to provide a composition film having a thickness of about 1 to about 40 microns.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part application of U.S. patent application Ser. No. 08 / 649,480 filed May 17, 1996, pending.FIELD OF THE INVENTION [0002] The present invention relates to extrusion coating compositions for metal substrates that, after application, demonstrate excellent adhesion, weatherability, barrier properties, and flexibility, to a method of extrusion coating a metal substrate; and to a metal article, such as a metal can or container, or a material of construction, like aluminum siding, having at least one surface coated with an adherent layer of an extrusion coating composition. An extrusion coating composition comprises: (a) a polyester having a weight average molecular weight of about 10,000 to about 50,000, and optionally, (b) a modifying resin, for example, an epoxy or phenoxy resin having an epoxy equivalent weight of about 500 to about 15,000. The extrusion coating composition is applied to a metal subst...

AI Technical Summary

Benefits of technology

The solution provides a corrosion-inhibiting, flexible, and chemically resistant coating that maintains adhesion and barrier properties, suitable for both interior and exterior applications of metal containers, while avoiding environmental and toxicological issues associated with organic solvents, and enabling the use of a single coating composition for various metal articles.

Problems solved by technology

It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate.

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.

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, like beer, by imparting an off-taste to the product.

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.

To date, investigators have had difficulty in providing a solid coating composition that matches a liquid coating composition with respect to film uniformity, film appearance, and film performance.

The film laminate method is a rapid method of coating a metal substrate, but the method is expensive and the coated metal substrate does not possess all of the properties required, or desired, by can, can end, and closure manufacturers.

However, the application of a thin, uniform coating to a metal substrate; i.e., less than 40 microns, is difficult to impossible using the powder coating method.

Often, if a thin coating is applied to a metal substrate using a powder coating method, the coating has imperfections which cause the film to fail.

Such failures are impermissible in the food and beverage container industry, which further require thin coatings that can withstand shaping of a flat, coated metal substrate into a can, can end, or closure.

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 can cause a premature cure of the coating composition, especially a thermoset composition, which makes extrusion onto the metal substrate difficult due to crosslinking in the extruder and can adversely affect the performance of the composition coated on the 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.

In contrast, thermosetting compositions often provide a rigid cured film thereby making it difficult to impossible to coat the metal substrate prior to deforming, i.e., shaping, the metal substrate into a metal article, like a metal closure, can, or can end.

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