Лако-красочные материалы — производство

by R James, PKT Oldring, PhD BA, К O’Hara LRSC and D Fone BSc

(i) Introduction

This sector of the industiy is very important and diverse with many specialist applications. The market is constantly changing with enormous geographic differences in the types of cans required and products packed.

The past twenty-five years has seen significant changes in coatings technology for the external coatings of metal packaging. During this period DWI manufacture has come to dominate the industry, where external coatings are based on acrylic and. polyester resins. At the same time, these more sophisticated acrylic and polyester systems have gained favour in the sheet fed side of the business at the expense of the more traditional alkyd and vinyl coatings.

Before coatings for particular product lines are considered, some general comments will be made.

The term metal decorating refers to the coating and inks used to protect and decorate cans, containers, caps and closures. Normally the metal substrate is tinplate (ETP), tin free steel (TFS) or aluminium.

The finished container may be coated with the following systems:

Each process requires a stoving period prior to the application of the next stage.

The Metal Decorating industiy can be broken down in to the following segments:

• Drawn and Wall Ironed (DWI) cans for beer and beverages with a ring pull end (ETP or aluminium)

• Three piece (welded) food cans and ends (ETP bodies, ETP & TFS ends)

• Draw Redraw (DRD) food cans (ETP & TFS)

• Caps and Closures (e. g., for whisky bottles jam jars etc.) (ETP, TFS & aluminium)

• Deep and shallow drawn aluminium cans

• Shallow drawn fishcans (clubcans ) (ETP, TFS & aluminium)

• General Line — Industrial uses e. g., paint cans, film cassettes, oil filter cases, polish tins, aerosols

• Promotional uses, e. g., biscuit tins, decorative metal boxes

In each market sector coatings (metal decorating enamels) can be used on the inside and outside of the container. Always at relatively low film thickness (2-15 microns film thickness depending upon use).

The function of the internal coating is to provide a barrier between the contents of the container and the metal substrate. This will stop the product corroding through the metal and leaking out of the container, an example being soft drinks penetrating the internal lacquer through pinholes and corroding the can, eventually seeping out of the can. The internal lacquer must also prevent the product being contaminated by, for example, picking up a metallic taste from the container or taint from the coating itself.

It should be noted that some food cans are not internally lacquered. The type of food being packed will dictate the need for, and type of, internal lacquer required. For example white fruits benefit in colour and taste. Baked beans do not need an internal lacquer. In foods which generate hydrogen sulphide ( such as protein containing foods, during in-can processing [cooking] ) ETP will become discoloured due to the formation of black sulphide salts if there is not an internal lacquer present. Peas and fish are demanding products on the internal lacquers present. Rhubarb will de-tin ETP, thus TFS or lacquered ETP has to be used. Not all cans are externally coated either. Many three piece and deep drawn food cans have a paper label for product recognition and identification. In some instances the body of the can would not be coated externally. It depends upon the potential for corrosion and the manufacturer’s wishes. TFS is not as corrosion resistant as ETP, thus TFS will normally be externally coated. In many instances this leads to a three piece food can with TFS ends and ETP body with only the TFS ends being externally coated. TFS is lower cost than ETP, but cannot be easily welded to make bodies, thus ETP is used for body plate.

The external coating (and inks) fulfils many functions. They include decoration of the package as well as the promotion of the package to the consumer. A most important functional aspect of the external systems is to provide good slip and scratch resistance so that the container does not get damaged in the manufacturing and transportation process. DWI external coatings must input sufficient mobility to the can to enable it to be filled. The importance of the coating towards mobility in DWI beer and beverage cans is a factor which must not be underestimated. Without the external decoration, beverage cans could not be filled at the rate commonly practised today (up to 2000 cans per minute — cpm).

The basecoat provides a background for printing the decoration onto the can, and the overlacquer not only protects the ink, but also supphes a mar-resistant high gloss finish. In some cases an overlacquer is not used if the user is satisfied that the ink and basecoat surface provide sufficient performance for their particular requirements. There are different practices in different geographic regions. In Europe most DWI cans are not varnished and the coatings are known as NOVAR — no varnish. In the rest of the world DWI cans are varnished. Europe tends to use a white basecoat, unlike the USA where inks can be printed directly onto aluminium cans. In the USA all DWI cans are made from aluminium, a much easier substrate to coat (both from an adhesion and opacity standpoint) than ETP. In Europe the split between aluminium and ETP cans is about 50/50.

Another geographical difference is in the use of different resin systems for the same end product. An example is the use of acrylic resin based formulations for general line goods in France but not in the UK. In the Far East there are other requirements and different resin systems to those used in the USA or Europe may be used. In some Third World Countries, older and slower coating and fabrication lines may be run and these do not require the sophisticated resin systems designed for the high throughput rates of some of the latest technology in the Western World.

Most methods for making cans, closures etc., entail some form of postforming or shaping after the coating process has taken place. This means that the cured coating must have:

• excellent flexibility and extensibility. This varies depending upon the tooling demands.

• excellent adhesion to the substrate

• good mechanical resistance to marring or scratching from tooling or during processing.

even after a number of curing cycles.

Additionally, in some applications, where the can is being filled with food products, the coatings and inks must also be sufficiently resistant to withstand pasteurisation at 80°C (beer cans) or, in the case of canned meats and other food products, the coatings need to be retortable (resistant to steam under pressure in an autoclave) because many products are cooked in the can. Temperatures as high as 140°C for over 1 hour may be used, depending upon the size of the can and the type of foodstuff being processed. It is not commonly appreciated that foods and beers are heat processed after being filled in their containers rather than before.

The two most important processes used in metal decorating are sheet fed and DWI:

A. Sheet Feed Application

Flat sheets may typically be coated as follows:

a) White rollercoated basecoat 8-10mg/m dry film cured 10 minutes at 170-190°C

b) Lithographic inks cured at 10 minutes @ 140°C (or UV).

c) Clear rollercoated overlacquer 3-4mg/m dry film ( baked or UV ).

d) Internal epoxy phenolic lacquer 10 minutes @ I80°C, 3 minutes @ 240°C.

The coated and printed sheets are then formed to produce 3 piece cans, or deep drawn to produce caps, closures and some types of shallow drawn food cans.