11 октября, 2015 
Malyar by Andrew Hoggan BSc, David Fone BSc, Peter Oldring PhD BA,
Kevin O’Hara LRSC and R James
The principles of the manufacture of cans, caps, closures and other items generally coated with metal decorating products, have already been outlined in the solvent based application section.
Many users of waterborne products expect to replace solvent based products with minimal, if any, changes to conditions of application or final film properties. Thus, water based formulations are expected to have the robustness of solvent based systems which have been developed and ‘fine tuned’ over many years. Water also has the major disadvantage that it is very poor at wetting. In most, if not all, applications the sheet or coil has a surface layer of lubricant present to facilitate ease of handling. This is normally a relatively low viscosity grade of a mineral oil.
Water is very poor at displacing this oil which can form very thin films, even down to mono-molecular layers, on the surface of the steel (ETP — electro-tinplate or TFS — tin free steel, sometimes referred to as ECCS) or aluminium. This is in direct contrast to solvent based systems which readily displace any oil present. Thus, the wetting of the substrate is a major problem with water based systems. The fact that water based systems are run successfully day in, day out, on DWI fines producing 1500-2000 cans per minute, per fine, illustrates that these wetting problems can be overcome.
Water based systems for metal decorating are in essence only used for two main areas, namely:
• DWI externals
• DWI internals
Systems for waterborne DWI externals are based upon either polyesters or acrylics (the same as for solvent based systems) and DWI internals are based upon epoxy modified acrylics.
Currently there is a limited amount of sheet fed work based upon waterborne systems. There are some other applications, particularly in the USA, where water based systems are used (aluminium coil for DWI easy open ends applied by electrodeposition being one such example). However, these other water based systems are insignificant when compared to the numerous solvent based systems being run world-wide. Many companies are evaluating water based systems in order to comply with forthcoming environmental legislation. However, reducing VOC’s may not necessarily reduce odour to an acceptable level and incineration may still be required. It is more economic to run
an incinerator when 100% solvent based systems are being used than where, say, 50% of the systems are water based. This is because the solvent has a calorific value and when water is present, rather than solvent, additional expensive fuel oil is required to run the incinerator.
The major difference between the resins used for DWI water based externals and solvent based ones, whether polyester or acrylic, is the acid value of the resin. With very few exceptions, water based metal decorating resins are based upon inverted dispersions. Very few true acrylic emulsion resins are used in metal decorating. A relatively high acid value resin (50-80 mg KOH/g) is prepared in solution in a water soluble solvent. This is then neutralised with an amine, normally but not always, dimethylethanolamine (DMAE) and this neutralised resin is then dispersed/inverted into the water phase. For internal systems DMAE is normally used as the neutralising amine.
The other common alternatives to DMAE are triethanolamine and triethylamine. In contrast to many other water based systems, the amine does not need to evaporate at ambient temperatures. Indeed it is undesirable for this to happen. Metal decorating has the advantage, compared to many applications, that coatings are stoved at relatively high temperatures (typically up to 200°C). This allows lower volatility amines to be used.
The majority of internal lacquers for metal decorating are based upon either epoxy or vinyl systems. The latter are all solvent based. Epoxy phenolic lacquers for food cans are generally solvent based. Epoxy resins are modified with acrylic resins or comonomers to make them water dispersible for use in internal lacquers for DWI spray applications.
The first consideration for the formulator must be the application technique to be used, and the substrate to be applied. This will determine the resin chemistry, and the physical specifications of the formulation. All two piece beverage can fabricators currently use airless spray techniques of application of internal lacquer, whilst the three piece food can industry use roller coated sheet or coil to fabricate food cans. Three piece beverage cans, still used in some parts of the World, use roller coated sheet or coil. Some non-carbonated dinks, (such as fruit juices) which are particularly popular in the Far East, are not suitable for packing in DWI cans, because they are not pressurised, thus three piece cans have to be used.
The two piece food cans which use water based lacquers are either sprayed after fabrication or for shallow draw applications, pre-coated substrate (roller coated sheet or coil) is used. Current water based internal lacquers do not have the same can fabrication properties as solvent based lacquers which can be used for deep draw applications.
The substrates available to the fabricators are of two metals, representing three substrate types, aluminium, electro-tin plated steel (ETP), and tin free steel (TFS). Tin free steel is used for two and three piece food cans and some closures. There are many different grades of substrate and, equally, different surface treatments on the metal. The coating must perform no matter which type of substrate is chosen by the customer.
Acrylic resins are not used in internal lacquers for metal decorating. However, epoxy modified acrylic resins are extensively used in DWI internals. These systems are waterborne and the methods used to make the resins waterborne have been reviewed in great detail in the epoxy volume of this series of books.
Two piece cans are used throughout the world to distribute beer and soft drinks. Against the threats of glass and polyester containers, two piece cans have the advantage of fitting the requirements and also being cheaper overall, hence they hold the greater part of a growing market. Currently, the developed market in the West is growing at 2-3%, whilst the newer developing markets in the East are growing at a projected rate of 11-12% per annum. The internal organic coating is an integral part of the two piece can. Without the thin organic film, the product packed in the can would spoil or at the very least become contaminated. The can would suffer from corrosion problems, due to acid attack from soft drinks, such as Colas. Therefore it is necessary to use organic coatings to protect both the can and the packed beverage. The standard organic solvent based coating within the two piece can industry was the epoxy-phenolic, or epoxy-urea interior spray coating. This type of coating has now been almost completely superseded by waterborne systems.
Environmental legislation passed in the early 1970’s (e. g. California’s Rule 66) prohibited the use of specific organic solvents and restricted the use of others, and demanded that coatings with lower solvent emission be developed. In the late 70’s and early 80’s, ecological pressure demanded that manufacturers cease using only organic solvents within their products. This has led to a reduction in the VOC of polymeric coatings within the industry, especially in the high volume consumer markets like the can coatings industry. The result has been the development and expansion of waterborne coatings technology including, of course, waterborne epoxy acrylic resins.
The level of solvent is dictated by the required VOC. It should be remembered that the amine contributes to the VOC in addition to the solvent. The definition and requirement of VOC’s differ between Europe and the USA. The simplest definition of VOC often used in Europe is :
VOC = (100 — solids — water)/solids x 1000 g/kg
In the USA a mass per volume (lbs/gal) definition is used and there is no easy direct correlation or conversion between the two. The density of the coating has a significant effect. White basecoats have a much higher density than a clear varnish. The solids are measured at 110°C for 60 minutes in accordance with the standard ASTM test. The water content can be measured by either a Karl Fischer titriometric technique or by gas chromatography using an ion flame detector. Values from different techniques may well differ.
Water based acrylics, for external applications, would be polymerised in solution in a similar manner to those intended for solvent based applications. Similar comonomers would be used except that a high level of either acrylic or methacrylic acid would be copolymerised with a resulting change in the ratio of the ‘soft’ comonomers. The solvent
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DWI cans are fabricated as follows. A coil of substrate is lubricated and fed through a press where shallow cups are punched out. The cups are then thinned and formed by drawing through a series of progressively smaller dies (rings) until the can shape is formed. The can is then trimmed to the correct height, and passed through a series of washes (aluminium is acid etched at this stage) and rinsed before being dried, externally decorated, and spray coated internally. The main difference, but not exclusively so, is after the can has received the external decoration. At this point, steel cans are generally (though not always) passed through a necking process before receiving the internal coating. Aluminium cans, however, are washed, decorated externally, and coated internally before the necking process. The slightly different fabrication route between the two substrates is due to the lack of lubrication in aluminium cans causing die wear. The necking process for aluminium cans relies on the external and internal coatings to provide die protection. The tin coating of the ETP steel provides sufficient lubrication to prevent pronounced wear of the necking dies.
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