On the can: non-toxic coating alternatives15 December 2017
Epoxy-based can coatings are gradually being replaced by alternatives because of toxicological evidence, public discussions and recent regulatory decisions. Birgit Geuke, scientific officer at the Food Packaging Forum, looks at what the alternatives are and how they perform.
Why are coated cans coming under increased levels of scrutiny? Cans preserve the taste and nutritional values of their filling for several years. As a consequence of such long storage times, the interactions between the packaging and the contents need to be minimised. Cans are typically coated with a layer that prevents chemical reactions between the can metal and its contents. To fulfil technical and legal requirements, can coatings should be able to withstand production and sterilisation processes; be universally applicable for all food and beverage types; prevent chemical migration into food in quantities that endanger human health; adhere to the can even after unintentional deformation; resist aggressive food types and protect the metal of the cans; and preserve the food and maintain its organoleptic properties over several years.
More than 300 billion beverage cans are produced globally each year. In 2014, 90% were made of aluminium, with the remaining 10% consisting of steel. Recent estimates put the global production capacity of can coatings at 800,000t, which corresponds to a market value of €2.8 billion.
Alternatives to epoxy coatings
Many different can coatings are available commercially, but most of them are based on a limited number of chemical functionalities. Coatings contain different additives; for example, agents to increase surface slipping, as well as abrasion and scratch resistance of can coatings, lubricants, anti-foaming agents, adhesives, scavengers for hydrochloric acids, and pigments.
Epoxy-based coatings have the largest market, accounting for more than 90% of the sector. However, manufacturers and food companies have started to replace BPA-based epoxy coatings with alternatives as a consequence of toxicological evidence, public discussions and recent regulatory decisions. Acrylic and polyester coatings are currently used as first-generation alternatives to epoxy coatings and, more recently, polyolefin and non-BPA epoxy coatings have been developed. Further inventions include BPA-capturing systems and top coatings. Most of these alternatives are more expensive than epoxy coatings, and may not yet display the same array of characteristics with respect to stability and universal applicability:
Epoxy coatings: in the 1950s, epoxy resins were introduced as coatings for aluminium and steel cans. Their stability, protective function and technical properties made them the most commonly used coating material. Most epoxy coatings are synthesised from bisphenol A (BPA, CAS 80-05-7) and epichlorohydrin, forming bisphenol A-diglycidyl ether epoxy resins. Many different blends of epoxy coatings were developed, with epoxy-phenolic coatings being the most important subgroup. Other blended resins include epoxy amines, acrylates and anhydrides.
Oleoresins: early can coatings were made of oleoresins, which are mixtures of oil and resin extracted from plants. Oleoresins are rather flexible and easily applied, but do not adhere well to metal surfaces, have a limited corrosion resistance, and need long curing times. Furthermore, they may change the organoleptic properties of food.
Vinyl: vinyl coatings are synthesised from vinyl chloride and vinyl acetate. They are highly flexible and stable under acidic and alkaline conditions, but they do not adhere well on metal and do not withstand high temperatures. Vinyl coatings need plasticisers and stabilisers, and are often blended with other resins. Vinyl organosols are prepared from suspensions of resin in organic solvent. Organosols offer comparably higher chemical resistance, thermal stability and adhesion properties than vinyl coatings.
Polyester: isophthalic acid (IPA) and terephthalic acid (TPA) are the main carboxylic acids used in polyester coatings. Polyester resins are easy to handle during the production process and adhere well to the metal surface, but they are usually not stable under acidic conditions and have poor corrosion resistance. Alternatively, polyethylene terephthalate (PET) coatings are used to laminate beverage cans, but adhesives are needed to bind the PET onto the metal.
US and European regulations
Polymeric and resinous coatings are covered under US federal regulation 21 CFR 175.300. This code lists permitted starting substances, and specifies test conditions and migration limits. Can coatings meeting these specifications are compliant with the law. In May 2015, California’s Office of Environmental Health Hazard Assessment added BPA to the list of chemicals known to cause reproductive harm under Proposition 65. Manufacturers, distributors and retailers now have to inform the consumers of BPA-containing products with a clear and reasonable warning regarding the chemical hazards, as the FPF reported.
Can coatings are not regulated by EU-wide legislation, but national measures are in place in Belgium, the Czech Republic, France, Greece, Italy, the Netherlands, Slovakia and Spain. Harmonised regulations for specific chemicals known to migrate from can coatings exist for bisphenol A diglycidyl ether (BADGE) and its derivatives (European Commission regulation EC 1895/2005) and for inorganic tin (regulation EC 242/2004). A draft of a regulation on the use of BPA in varnishes and coatings currently suggests a specific migration limit of 0.05mg/kg food. In France, the use of BPA is prohibited in FCMs including all packaging, containers and utensils intended to come into direct contact with food.
Migration, exposure and biomonitoring
Most studies investigating chemical migration from cans focused on BPA, BADGE and their derivatives. The amount of data for BPA, particularly, provides a good basis for exposure estimates. However, the total migrates from cans may also contain oligomers, catalysts, reaction accelerators, epoxidised edible oils, amino resins, acrylic resins, various esters, waxes, lubricants and metals. Furthermore, non-intentionally added substances (NIAS) such as impurities, reaction by-products and degradation products generally constitute part of the migrate. Exposure estimates for these often-complex mixtures are much more difficult or even impossible to calculate, because many NIAS are unknown or unidentified substances.
A correlation exists between human exposure to BPA and the consumption of canned food and, to a far lower extent, canned beverages. In 2012, a study showed that BADGE and its derivatives were detected in all test samples from the US and China and that urinary concentrations exceeded those of BPA by three to four times.
Can coatings generally release complex chemical mixture into the food and only a few of the migrants were thoroughly tested. Extensive toxicity data exists for BPA, covering many different end points, such as reproductive and developmental effects as well as neurological, immune-modulatory, cardiovascular and metabolic effects. In 2004, BADGE was judged to not raise concern for genotoxicity, carcinogenicity, reprotoxicity and developmental toxicity. However, more recent studies indicated some effect on reprotoxic and developmental end points. Many migrating substances are completely unknown, but they may strongly contribute to the toxicity of the migrate. In 2006, cytotoxic effects of migrates from epoxy and polyesterbased coatings were tested using a series of assays. The results of one of these assays showed that only about 0.5% of the cytotoxic effects measured in the migrate from epoxy coatings could be traced back to the amount of BPA, BADGE and BADGE H²O. This example illustrates the importance of tests targeting the final migrate and not just single substances during risk assessment.
This article has been abridged from the original to focus on beverage can coatings.