Contact Compliant
In addition to good oil and grease barrier performance, the water-based coating must also be printable using flexographic, gravure or rod coating equipment. Printability means the coating must be resoluble to avoid drying on the print equipment, but should develop final properties quickly after drying on the substrate. The coating must also be easy to clean when the print job is complete and must not scuff easily or transfer to the reverse side when the coated substrate is rewound or stacked. Since the coating is applied to the inside surface of the package to prevent oil or grease from penetrating and being visible on the outside, the polymers and other raw materials used to formulate the coating must be food contact compliant.
Raw material manufacturers and coating formulators have developed laboratory testing protocols that are intended to confirm that a polymer or coating will likely meet the end-use performance and the printability requirements described.
During the development cycle, as part of the market introduction process, the raw material supplier will characterize a new polymer using established procedures including viscosity, pH, non-volatile content, glass transition temperature, minimum film forming temperature and appearance. Each test is intended to help predict how the polymer will perform in coating manufacture, during the printing process and during the lifecycle of the package. Viscosity and non-volatile content might indicate suitability for one print method or another. Glass transition and minimum film forming temperatures will indicate level of flexibility and requirement for additives to help create a continuous film on the substrate.
In most cases, the raw material supplier will complete application testing to confirm a specific parameter, such as oil, grease or liquid barrier. However, the final application work and confirmation of suitability for the end-use will be completed by the coating formulator. The formulator will use established test procedures to validate flexographic printing, resolubility, gloss, adhesion, block resistance, water and rub resistance, and barrier to oxygen, moisture, water vapor, oil and grease. Depending on test results, the formulator may decide to include modifying resins, waxes and/or additives in the coating to enhance the core performance of the selected polymer.
Performance Tests
Specific to barrier performance, testing will include various applied weights of the coating—both in a single layer and multiple layers—to ensure the targeted barrier is aligned with the desired print method and required number of print units on the equipment.
Coating weight studies will identify optimum application dosage required and provide valuable information on cost-in-use. In some cases, applying two layers of a water-based barrier coating, at the same final overall coating weight of a single layer, will provide significantly improved barrier properties.
- Conceptually, the first layer fills voids in the paper or board substrate and provides a more uniform surface for receiving the second layer
- The second layer covers pinholes or other defects in the first layer of coating. The result is better barrier performance with two coating layers when compared to a single layer of coating, both at the same final coating weight
Oil and grease barrier performance can be assessed by placing the coated substrate in contact with filter paper, saturated with a variety of oils, between two glass plates and observing over time. The sample is arranged so that the back side of the substrate, i.e. the uncoated side, is visible through the glass plate, which allows the observer to determine if the oil has penetrated the coating and the substrate.
The test can be completed with a coated substrate that is creased or folded—either at room temperature or at elevated temperatures. An acceptable coating will prevent the reagent from migrating from the filter paper, though the coating will ultimately be visible on the uncoated side of the substrate. Coatings that are unacceptable for barrier performance will allow oil and grease penetration, causing discoloration on the outside of the package. That contributes to leading the consumer to believe the product in the package is stale, expired or unfit for consumption.
Evaluation for barrier and end-use performance is typically completed with a fully formulated coating—not just the polymer developed for oil and grease barrier. While the polymer might provide excellent barrier, if it does not meet other criteria of press stability, printability and ease of conversion, market penetration will be limited. During coating development, the formulator may include other polymers or resin solutions. This is done for improved resolubility, to minimize risk of premature drying on the equipment and to enhance block resistance to ensure the dried coating does not transfer to the reverse side of the substrate.
Other ingredients might include micronized wax emulsion for improved scuff resistance and anti-foam additives to reduce foam formation while the coating is being pumped between the reservoir and print unit during application. In addition to providing desired properties and ensuring trouble-free printing, selected ingredients must also be suitable for food contact to allow use of the final coating in a wide variety of packaging application.
Grease-resistant barriers are key enablers of today’s packaging market and help support consumer preferences around snacking and convenience foods. It remains critically important for these barriers to provide effective grease resistance, maintaining package integrity and pleasing aesthetic appearance.
It is also becoming increasingly necessary for converters and paper suppliers to evaluate fluorochemical alternatives in order to meet brand owner requirements. When formulated and tested effectively, water-based coatings containing acrylic polymers provide effective grease resistance that is in line with—and in some cases better than—traditional fluorochemicals.
Over the last several decades, fluorochemicals have been used, and continue to be used, to provide excellent oil and grease barrier properties on paper and board packaging. More recently, as stakeholders became more concerned with reducing health risks associated with the entire food supply chain, including the packaging used, additional scrutiny was placed on the raw materials used to manufacture packaging.
As a result, suppliers of water-based polymers used in barrier coatings have prioritized innovation associated with products considered more sustainable, products with less environmental impact and products that will lower health and safety risks. While fluorochemicals continue to be the performance benchmark for more demanding oil and grease barriers, water-based polymers for use on paper and board food packaging—with barrier performance—like fluorochemicals, have been recently developed and introduced to the market.
Editor’s Note: Andrew Seecharan’s FLEXO Tech Talk on the subject of this article is available for listening throughout June. In July, it will be archived on FTA’s MemberConnect portal, where it can be accessed by printer/converters and their supplier partners holding FTA membership credentials.
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