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

[0031] In accordance with an important feature of the present invention, an extrusion coating composition includes one or more thermoplastic polyesters in a total amount of about 50% to about 100%, by total weight of the composition. Preferably, an extrusion composition includes from about 55% to about 90%, by total weight of the composition, of a polyester. To achieve the full advantage of the present invention, an extrusion coating composition includes about 60% to about 85% of a polyester, by total weight of the composition. As used here and hereafter, the term “polyester” refers to a single polyester or to a mixture of two or more polyesters.

[0032] The polyesters are prepared from a dicarboxylic acid, preferably an aromatic dicarboxylic acid, and an aliphatic diol. These ingredients are interacted to provide a polyester having an M of about 10,000 to about 50,000, preferably of about 15,000 to about 40,000, and to achieve the full advantage of the present invention, about 20,000 to about 35,000. Alternatively stated, the polyesters have a number average molecular weight (Mn) of about 5,000 to about 30,000. Accordingly, the polyesters are considered high molecular weight polyesters. The polyesters have an acid number of about 0 to about 150 mg KOH / g, and preferably about 5 to about 100 mg KOH / g. The polyesters have a hydroxyl number of 0 to about 150 mg KOH / g, and preferably about 5 to about 100 mg KOH / g.

[0033] Useful polyesters also possess properties that allow the polyester to be blended with the optional modifying resins and other composition components, to be extruded onto a metal substrate, and to provide an extruded coating composition having the necessary adhesion and flexibility to be applied to a metal substrate prior to shaping the metal substrate into a metal article. The polyester also is sufficiently nonreactive such that, when the extrusion composition is melted prior to and during extrusion, the polyester does not enter a crosslinking reaction with the optional modifying resin or other composition components.

[0034] A polyester suitable for use in a present extrusion coating composition, provides an extruded coating composition having good film tensile strength, good permeation resistance, retortability, and good barrier properties. The polyester, and the extrusion coating composition, therefore, have a softening point of 140° C. or greater, as measured using the procedure set-forth in DIN 52011. Preferably, the polyester and extrusion coating composition have a softening point of 120° C. to about 200° C. Above about 200° C., the polyester and extrusion coating composition lose flexibility, and subsequent shaping of a coated metal substrate into a metal article can cause film failure. Below 120° C., the polyester and extrusion coating composition is too soft to withstand the pasteurization and processing temperatures used by food packagers when packaging food in a metal container.

[0035] Similarly, the polyester has a Tg of about −30° C. to about 120° C., and preferably about 15° C. to about 100° C. To achieve the full advantage of the present invention, the polyester has a Tg of about 20° C. to about 80° C. In this Tg range, a polyester is sufficiently flexible to permit deformation of an extruded coating composition without forming cracks, and is sufficiently hard to exhibit excellent chemical and mar resistance. If the Tg of the polyester is below about −30° C., an extruded coating composition is too soft to provide effective chemical resistance and mar resistance. If the polyester has a Tg above about 120° C., an extruded coating composition lacks sufficient flexibility.

[0036] Useful polyesters also exhibit a melt viscosity of about 10 to about 100 Pa·s (Pascal seconds), and preferably about 20 to about 100 Pa·s, at 200° C., or about 25 to about 200 Pa·s, and preferably about 40 to about 175 Pa·s at 240° C. The melt flow index (MFI), as measured using DIN 53735, of a useful polyester is about 20 to about 800, and preferably about 25 to about 600 g / 10 min at 200° C.

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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