Clever coatings lift metal appeal

Packaging News
October 2007

Metal packaging manufacturers are looking to develop 'cleaner' coatings to improve container performance and compete in new markets. Sally Cousins reports

Metal cans offer affordable long-term storage for a wide range of foodstuffs, and coatings play an intrinsic protective role, inside and out.

Protective internal coatings for beverages and solid foodstuffs have for decades been based on a relatively small number of basic chemistries for regulatory and performance reasons, according to Metal Packaging Manufacturers Association (MPMA) technical manager David Smith.

Smith says: "Epoxy-based internal coatings have been the mainstay for use in contact with both food and beverage products for decades because of their workability, substrate adhesion, product resistance and freedom from taint qualities."

One way to improve the perception of coated metal packaging for the future could lie in the formulation of 'cleaner' coatings. Smith says this doesn't mean coatings haven't always been clean: "It has to be stressed that 'traditional' coatings have been 'clean' in the regulatory and consumer safety senses, but even cleaner coatings could raise the image of metal packaging onto a higher plane."

The findings of a three-year Defra-funded study could point the way for the future development of coatings. The collaborative study, carried out by the Central Science Laboratory (CSL), Leeds University, coatings manufacturer Valspar, metal packaging specialist Impress and Heinz, which also acted as project manager, looked in detail at the issue of migration and how it can be minimised in future coating development.

The main focus of the study was on new formulations for epoxy phenolics and polyester polyurethane coatings. The project studied the way different coating formulations act as a protective barrier and the potential for new cleaner coatings.

According to project manager Julian Stocker from Heinz, coatings science and technology have historically been driven mainly by technological demands, such as faster manufacturing speeds, lightweighting, more user-friendly cans and lower solvent emissions in oven curing.

So why was it the right time to conduct the project? Stocker says: "The protection offered has been so successful that there has been little investment into fundamental research of the interactions between coatings and food. The time was right to do more basic research towards identifying and understanding the factors affecting levels of migration in order to accelerate the development of reduced migration coatings."

What did the research involve?
During the study, the University of Leeds carried out kinetic tests of chemical migration from coatings, and the interactions between food and coatings. Coating parameters that may influence migration rates into food were also investigated. CSL provided the chemical analysis to evaluate the prototype cleaner coatings as well as establishing test protocols for the evaluation of current coatings.

Influences on migration
According to the results of the study there are three key influences on the migration of chemicals into can contents: the rate of migration is related to curing, film thickness and food temperature; waxes, which are incorporated to lubricate the coating, can influence migration; and the age of the coatings (both wet and plate) also affects migration.

As a result of the study, project partners have developed a range of coatings with very low migration levels with compliance to EU and member state legislation being a 'given'. These coatings will be made commercially available by Valspar.

An approach to further reaffirm the image of canned foodstuffs was also needed. Stocker says: "The protocols established for screening analysis and for measuring targeted migrants can be used by anyone for future coatings development."

Stocker hopes these tools will aid the future development of new, improved and cleaner can coatings. He adds: "The industry is in a significantly stronger position to assure its customers and consumers that the coatings used on its cans are not only safe, but can release even lower levels of migrants in the future."

MPMA's Smith feels that coating innovation could open up new markets for metal packaging. "In the general sense an improved reputation and image should influence packaging buyers' decision making," he says. "However, with a few notable exceptions, such as polymer-coated metal, it has been coating improvement chasing new metal packaging concepts rather than the other way around."

Protact, a polymer-coated steel, was developed by Corus Packaging Plus in response to the demand for metal packaging with improved performance. The company feels that the combination of steel with food grade PET sets a new standard in the food industry.

Protact protects against corrosion and is used for paint and chemicals tins, aerosols, food packaging, and peel-off rings, where the 100% PP bond between the Protact ring and the membrane is said to produce a superior seal integrity to that of heatseal lacquers and can reduce the heat-sealing time.

Crown Holdings produces food cans for a variety of sectors. Executive vice president for technology and regulatory affairs Dan Abramowicz says that although existing coatings perform perfectly well, there is the potential to make them better.

Market expansion
He feels that although the coatings available today allow most foodstuffs to be packed in metal, advanced coatings could allow light fruits such as bananas and pears, which previously suffered from discolouration in a two-piece container, to be packed in cans. "Oxygen scavengers placed in the coatings of the future could prevent this," says Abramowicz. "These are already used in some packaging applications, such as in the liner of the beverage crowns for use with oxygen-sensitive products like beer. I am not aware of such scavengers directly in can coatings. Such scavengers would need to be quite special - able to handle thermal treatment and to be 'turned on' at the correct time, such as right before filling."

Using different curing or 'clever' epoxy coatings could, according to Abramowicz, be two ways to reduce migration even further. He says: "This is all consistent with the move for more pure ingredients as a society and what we can do to make the packaging more consistent with that trend."

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