Can coatings

The term ‘can coating’ embraces paints for use on a vast variety of decorated metals for packaging purposes.

This includes cans of all kinds, including food cans and aerosol cans, collapsible tubes and caps and closures of many kinds. Cans can be considered as a single material, consisting of a metal substrate with an organic lacquer.

Container coatings were historically called metal decorating coatings, since a major portion of the business was in coating flat sheets, followed by lithographic printing and a finishing varnish (clear top coat) to protect the ink.

The field now usually referred to as container or can coatings. Most cans are food or beverage containers, and one of the key requirements is that there be no possibility of introducing toxic compounds into the foods or beverages.

The interior coating of cans is very important because it prevents the metal from reacting with the ingredients of the filling goods.

The can exterior is painted to prevent corrosion, but also for decorative reason. Accordingly, the requirements for can coatings are extreme in many ways. They include printability and block and scratch resistance yet sufficient elasticity to permit forming and drawing without damage to the paint.
Can coatings

Three piece cans

Three piece cans consisting of a can body and two end pieces, are used to seal heat-sterilized foods hermetically as well as for other food products like powders, syrups and cooking oils.

The can wall is formed by welding. Three piece welded cans are only constructed from steel as aluminum is not suitable for welding.

As the material for the can wall, lightly coated Sn-plated steel sheets and Ni-plated steel sheets may be employed.

Two main types of base steel are commonly used in can manufacturing type L and type MR. Type L is very corrosion-resistant and is used in canning of very corrosive products e.g., apple juice, berries, prunes and pickles.

Type MR is more suitable for canning moderately to mildly corrosive products, e.g. grapefruit, peaches, peas, and corn.

In both the two-piece can and the three-piece can the outside surface of the can is provided with printing, in order to appeal to consumers for commercial value of the canned goods.

On the other hand the inside surface of the can is coated with resin so as to ensure the corrosion resistance of the can body.

The coatings and ink are normally dried by passing the sheets through a thermally heated oven where the temperature is in the range 150–205 °C.

Three piece cans usually are delivered to the packer with one end in place and the other shipped separately. After filling, the second end is double-seamed by closing machines, which can change the headspace air to a vacuum or inert gas, if required.

Presently, the three-piece cans are being widely used and other cans like two-piece cans, aluminum cans, and other flexible containers are slowly replacing them.
Three piece cans

Epoxy in food packaging

Epoxy resins are characterized by the presence of at least two epoxy (or oxirane) groups within their molecular structure.

The group is composed of a planar, three-membered ring comprising two carbon atoms and one oxygen atom.

Epoxy resins containing Bisphenol-A or BPA are used as the inside coating of almost all food and beverage cans. The function of epoxy resins are to provide a protective lining on the interior surface of food and beverage cans to prevent the cans’ contents from contacting and reacting with the metal walls, ensuring sterility.

Many types of internal enamel coatings are available for food containers including epoxy-phenolic, epoxy-amino, epoxy-acrylate, epoxy-anhydrite, vinyl organosol, thermoset polyester, thermoset polymer coated, phenolic, and oleoresinous.

The original can coatings were based on oleoresinous products that include all those materials made by fusing material gums and rosins and blending them with drying oils such as linseed or tung (Chinese wood oil).

Epoxy-phenolic are the commonly used lacquer. They are epoxy resins based on epichlorohydrin and bisphenol.

They are combined with phenolic resins, which re produced by the reaction of phenol with formaldehyde.

This combination gives a gold lacquer with good flexibility, adhesion and chemical resistance.
Epoxy in food packaging