Protective and Decorative Coatings
Often involving sophisticated chemistries, coatings play an important role in both the protection of the can from the product, and vice versa, and in imparting consumer appeal to the can on the retail shelves. They may also play a key role in the metal forming phase of the packaging.
Firstly spotlighting internal protective coatings, generally termed ‘lacquers’ in UK, these are designed firstly to withstand can/component manufacturing and subsequent use, and secondly to minimise interaction between product and can substrate over the full shelf life of the filled can – which may be up to three years. The required properties include, inertness, flexibility, freedom from taint and processibility at high temperatures (most food cans are in-can heat sterilised). Additionally for food contact applications, the formulation of the coating is obliged by national and international food laws not to prove detrimental to consumer health nor alter the organoleptic properties of the food or beverage. This is often supplemented by a ‘positive list’ of permitted starting substances which have had the necessary expert toxicological evaluation. The rigorous toxicological test requirements for alternative chemistries to those already approved inhibits the application of new chemistries into food contact – the whole proving process may take many years and a very high cost. Although non-food/beverage applications, such as general line cans (for industrial products) and aerosols, do not have these specific constraints, the general chemistries used for their internal lacquers are similar.
External coatings may be protective, decorative or a combination of both. A ‘classical’ external system would comprise a thin, clear sizecoat adjacent to the metal followed by a thick, solid coloured (often white) basecoat which would then be printed in anything up to eight colours and a clear varnish applied to impart gloss. Such a comprehensive approach may still be used for ultra high quality graphics, but improvements in technology in both the coatings themselves, and in their application methods, has meant that high quality graphics can still be obtained via lesser, cost and material saving approaches, for example by using intrinsically glossy varnishless inks.
The printing processes themselves have improved in leaps and bounds over many years. Most particular the evolution of computer-based design and the ability to read this directly onto a printing plate has reduced the time needed to proof new designs and the finalisation of design with the customer. The advent of the ‘hexachrome’ system, where most designs can be generated from a standard set of six colours, has considerably reduced make-ready times on line.
Classical coatings comprise resin systems dissolved or dispersed in organic solvents (volatile organic compounds or VOC) or DI (deionised) water. After laying down a wet film on the metal, the plate or container is then stoved at temperatures of up to 300c this both drives of the solvent (VOC or water) and cures the coating by causing chemical reaction between its functional resins. Nowadays, our industry has significant constraints through local and international environmental legislation pushing us towards significant reduction of VOC emissions. This trend is driving the wider use of 100% solid materials (electrostatically applied powder for example) or lamination of the steel or aluminium laminated with a polymer film. Another method to externally protect and decorate a container, and being very low in organic volatile content, are ultraviolet (UV) curable coatings and other radiation curable approaches, for example electron beam. The cure phase being relatively short, such curing systems may be placed in tandem with more conventional liquid coating and stoving lines to achieve two-side coverage in one pass. The reactive species which respond to the radiation curing are currently not permissible for food contact applications.
The term ‘lacquer’ normally denotes an internal coating, i.e. one that will be in contact with the packed product. Thus there needs to be a range of lacquers of various physical and chemical resistant properties to satisfy the range of needs in terms of protecting the packed product from any deterioration through interaction with the substrate metal. As well as the film-forming resins, most lacquers will contain low additions of various ‘additives’ to achieve technical effects such as substrate wetting, flow, adhesion promotion and lubricity for the manufacturing process.
Historically most of the internal lacquers in use have been epoxy based. However, the use of alternative systems, such as polyester or acrylic is now growing. Whichever chemistry is used it must achieve excellent substrate adhesion and good flexibility for metal forming and, where needed, high resistance to in-can sterilisation at high temperature normally used when packing food products.
Systems, supplied as liquid coatings, typically at less than 40% non-volatile content to preserve adequately low solution viscosities for application, represent 85 or 90% of all internal protective lacquer supplied for food and beverage cans, general line cans, aerosols and drawn aluminium containers. Other coatings can contain a pigmentation of titanium dioxide, these form the basis of most of the white internal lacquers used on the market to impart a more sanitary appearance to the can interior.
Organosol Lacquers – Comprise two phases in organic solvent – a dispersed phase of polyvinyl chloride (PVC) and a dissolved phase of thermosetting resins. The dispersion of the PVC allows comparatively high solids to be achieved (typically 50-60% non-volatile) whilst maintaining a solution viscosity low enough to apply the coating. Because of the high solids, a high film build can be achieved in a single pass, typical dry filmweights being 10 to 14 g/m2. This film is also very flexible, via the PVC in particular, allowing high levels of deformation in metal forming. Thus, organosols are typically used for such applications as easy opening ends and deep drawn cans. Sometimes a double coat system with organosol on top of an undercoat imparts further resistance properties and improves lacquer adhesion. Water-borne organosol systems are known, but have not been entirely successful, and the bulk of organosols in use still retain a 100% organic solvent volatile phase.
Polyester/Modified Polyester Lacquers – Polyester coatings, generally water-white films, are very flexible for metal forming and are suitable for both internal and external application.
Phenolic Lacquers – Very high chemical resistance but very poor flexibility. Limited application nowadays. Main use is post-sprayed to drums for chemicals.
Pigmentation – Internal lacquers may be pigmented for aesthetic reasons (e.g. sanitary white appearance as previously mentioned) and/or as inhibitory or masking effect against sulphur staining of the metal substrate by certain meats and vegetables which generate sulphur compounds during in-can sterilisation. The is achieved by inclusion of fine aluminium powder dispersed in the resin (physical barrier plus masking effect) or by inclusion of zinc oxide or zinc carbonate powder which chemically absorbs the sulphur compounds, thereby preventing a potentially offensive odour when the can is first opened, in addition to preventing brown or black under-film staining of the metal substrate.
The word ‘Coatings’ can be used generically to apply to any film which is not ink, but the more specific meaning in this case is an overall coating, pigmented or not, applied to the external surface of the packaging for visual and/or protective effect. The most common example would be a heavy, white pigmented coating applied as a basis for a printed design. As with lacquers, the formulations can contain low levels of additives to enhance substrate wetting, flow, gloss and lubricity. The lubricity aspect can be particularly important for the efficient and damage-free passage of cans or components through high speed can making and filling lines. On the other hand, and where subsequent printing may be involved, additives, particularly lubricants, require to be carefully selected and incorporated so as not to lead, for example, to dewetting of the inks. A typical pigmented coating dry filmweight would be 10 to 18 g/m2. The most commonly used chemistries are:
Acrylic – Produces a high scuff-resistant water white film which pigments well. Availability of a large range of acrylic monomers allows the production of a large range of polymers tuned to end requirements. Acrylics form thermosetting systems normally cross-linked with amino resins such as to preserve their colourlessness. With the correct polymer design, stable liquid acrylic coatings can be produced in a high-water content volatile phase.
Polyester – Again the availability of many starting monomers (organic polyols and polyacids) allows formulation flexibility. Functional polyesters are often cross linked with amino resins to the same purpose as for acrylics. Generally carry pigments well. With the correct polymer design, stable liquid polyester coatings can be produced in a high-water content volatile phase.
Alkyd – Long standing approach using naturally occurring unsaturated oils (e.g. linseed, castor, soya) and/or equivalent fatty acids reacted with glycerol into triglycerides. Ambient oxygen is responsible for natural cross linking but the drying can be enhanced by the addition of further external cross linkers, amino resins for example. Alkyds pigment moderately well. At the elevated stoving temperatures applied, or over a period of time, there is a tendency for the film to yellow naturally, generally restricting the use of alkyds to lower grade decorative work.
Miscellaneous – ‘Coating’ can also be used to describe the application of lacquers, such as epoxy phenolic, to the external surface, for example of ECCS, where resistance to ambient conditions is necessary. Another miscellaneous external coating operation is that applied to the base rim of both steel and aluminium DWI beverage cans with the dual purpose of improving line mobility and inhibiting secondary or ‘outside-to-in’ corrosion which can occur, and produce a cascade effect, if there are leakers in pallets of filled cans. Chemistries used in this case are normally water-borne.
The normal prerequisite with varnishes is a completely colourless appearance so that print and coating underneath show their full visual effect. Most overprint varnishes are covered by the acrylic, polyester and alkyd approaches described under ‘Coatings’, of course without any pigmentation. An additional chemistry used is epoxy ester which combines epoxy resin, fatty acid and cross linker. Varnishes may need an enhanced level of lubricity and will most often be designed for high gloss. However, this is not exclusively so and such finishes as ‘matt’ or ‘crackle’ (like an aged painting) are sometimes designed in where such special effects are desired.
Polymer Coated Metal
100% non-volatile approaches are finding increasing usage as can and component manufacturers look to alternative ways of reducing their emission of volatile organic compounds (VOC) to meet environmental legislation.
Polymer coated metal is most normally ECCS or aluminium and the manufacturing process thereof involves single pass, two side coating of the metal either by heat sealing of a previously formed film or direct extrusion of a polymer film onto the metal. Normally both sides of the substrate are coated in one pass. The functional film is currently either polyethylene terephthalate (PET) or a polyolefin such as polypropylene (PP), and can be pigmented or clear. Proponents of the technology claim that comparative life cycle analysis demonstrates an overall energy saving in addition to environmental benefit over conventional liquid coating. However the technology is currently limited to ‘niche’ applications whilst the industry utilises its existing heavy investment in liquid coating and stoving equipment, often already including abatement equipment to deal with the volatile organic compounds.
Powder Coating / Side-striping
Powder coating, another 100% non-volatiles approach, finds its main application currently in the internal side-striping of three-piece cans as an alternative to the spray or roller application of liquid coating. Sidestriping covers the ‘step’ formed, including bare steel cut edge, when a rectangular body blank is formed into a cylinder, overlapped slightly and welded. Where a high level of protection is needed, a high film build is necessary, and powder coating permits this. Formulations are normally thermoplastic, i.e. contain little or no cross linker, and are often based on polyester resins, relying on a thick film to achieve the necessary product and process resistance. Internal powder sidestripes are most normally pigmented white for sanitary effect. Side-stripe curing, both powder and liquid coatings, normally involves a short bake of between 8 and 30 seconds, using flame, hot air or induction heating, in line with the can bodymaker. In the case of powder, this is primarily to fuse the fine powder particles into an even and continuous film.
Wider uses of powder coating are under experimentation, including their use for total internal protection of a preformed can. Progress will be strongly influenced by economics, therefore the ability to deposit relatively thin and even films. Current heavy investment in equipment to apply and stove conventional liquid coatings, in some cases already inclusive of ‘end-of-pipe’ VOC, abatement will also prove inhibitory to progress.