Changes to Corrosion Protection Coatings in Food Packaging
It may be surprising to many people, but corrosion is a considerable issue in the food packaging industry. In addition to contributing to overall aesthetics and consumer appeal, food packaging must protect its contents from internal and external contamination. Properly manufactured and sealed food packaging ensures the freshness and flavor of its contents by preventing moisture, oxygen, and microbes from reaching the product. However, many foods that are stored in bulk metal cargo containers or sold in metal cans are mildly or strongly corrosive to metal substrates. “Food packaging manufacturers need to consider the effects that food contents will have on packaging materials,” says Jonathan Mason, Sr. R&D, Dow Coating Materials. “Food packaging corrosion can lead to spoiled products, leached coating ingredients, inferior aesthetics, and packaging failure,” he adds.
Specially designed coatings are therefore required to protect metal packaging surfaces from corrosion. In fact, coatings not only prevent corrosion from contact with food and beverages, they act as a barrier between the can and the food to minimize impact on the taste profile. The quality and type of coatings used in food packaging can minimize exposure to environmental factors, helping to reduce spoilage risks and maintain food safety during food processing, transport, and storage. “While raw materials including paper and metal form the structure and shape of the packaging that contains and protects contents, barrier coatings in food packaging help ensure that food remains sterile and protected from the elements and from the packaging itself. These thin-film barrier coatings adhere to packaging and prevent interactions between food contents and the packaging,” explains Mason.
“The metal can coating plays a critical role in food packaging and must maintain its functional integrity when subjected to numerous physical and chemical stressors,” states Hitesh Soni, product manager for Specialty Epoxy Materials at Emerald Performance Materials. For example, the coating must adhere evenly and properly to the substrate and be flexible over a range of temperatures without exhibiting any cracking when applied to curved surfaces. The coating must also exhibit resistance to acids and salts that are used as preservatives and freezing point depression agents. “Failure of a coating to provide a proper barrier between the package and the food will ultimately compromise the protective film and potentially have a negative impact on food quality and safety,” he adds.
Coatings used for metal food packaging that directly or indirectly contact food have the added compliance requirement with current food-contact standards, including standards for species that can be extracted and/or migrate into the food chain from coatings or other materials (e.g., adhesives, inks, plastic, paperboard, and corrugated substrates). Specifically, food-contact compliance regulations limit the allowable levels of coating ingredient migration into food contents. “While migration testing, analysis, and monitoring occur at the local level, the implications of these nuanced regulations for coating formulation, manufacturing, and performance are widespread,” Mason observes.
Indeed, meeting these compliance requirements can be challenging, because they vary from country to country. Standards set by the U.S. Food and Drug Administration (FDA) may differ from the regulations set by U.K., European, and Chinese government agencies, with each authority permitting the use of slightly different coatings chemistries or setting different migration limits. “Given that food trade occurs on a global scale, food brands must adhere to food-contact compliance regulations across several countries to sustain global sales growth via exports of their products. As a result, food packaging coatings formulators and food packaging manufacturers must continuously work to remain aware of the changing regulatory landscape and meet increasingly restrictive regulations for food contact compliance without sacrificing performance,” notes Mason.
For corrosion protection of metal cans used as food and beverage packaging, coatings based on epoxy phenolics and epoxy esters are the most common. Both of these resin technologies are used in 1K, 75% solventborne systems (methyl ethyl ketone, alcohol, methyl cellusolve, and butyl carbitol) at room temperature in order to apply a thin, uniform film to the substrate, according to Soni.
High-molecular-weight, bisphenol A (BPA)-based epoxies are the most common due to their economical performance. However, the status of BPA as a potential endocrine disruptor is driving research efforts for the identification of alternative epoxy systems. In addition to these concerns about the potential migration of endocrine disruptors, there is also a focus on the reduction of the volatile organic compound (VOC) content of corrosion protection coatings used on the interiors of food and beverage cans, according to Soni. In line with global coatings trends across markets, coatings used in food packaging are continually shifting to waterborne formulations to address this issue. “To make this shift while maintaining performance, coatings formulators are looking for alternatives to the traditional, reliable coatings chemistries,” says Soni.
A number of new developments address these challenges. In interior can applications, for instance, Soni points to lower-viscosity epoxies that require approximately 25% less VOCs than high-molecular-weight materials. “New families of epoxy resins are also being developed that may balance that may address both the need for lower-VOC formulations and concerns about the migration of BPA, such as such as those based on isosorbide diglycidyl ether (ISDGE), which has the added benefit of manufactured from biorenewable raw materials,” Soni adds.