Brand owners have long recognized that consumers make choices based on visual response.
Advertisers are adept at using flashy graphics, bright colors and catchy names in order to increase consumer demand. Issues that the consumer views as suboptimal, such as bland graphics or designs, can cause them to look for alternative products. This is especially true when it comes to packaging issues which convey a sense of damage to, or inferiority of, package contents when viewed on the store shelf.
How often have you consciously chosen a package which was torn or damaged in some way, rather than the pristine container right next to it? Now think about whether you would choose a package which was stained or leaking. Preference becomes more obvious when the packaging in question contains food.
Since many of the foods we like to eat are oily and greasy, the materials used for packaging have been designed to limit the transmission of these substances through the containment vehicle. As consumers, we enjoy protection from grease while eating things like popcorn and fast food—and so do our clothes, furniture and car interiors. One class of chemicals successfully utilized in many of these paper-based food packaging applications has been fluorochemicals.
Wet or Stain?
Though there are many things that impact whether a liquid will wet or stain a surface, the surface energy of the substrate and the surface tension of the liquid are the main drivers. The surface tension of the liquid must be lower than that of the surface for it to wet. By comparing the surface tension of the solid, that of the grease or oil, and the interfacial tension between them, the tendency for the liquid to spread can be estimated:
- If the surface tension of the liquid is substantially higher than that of the solid surface, the liquid will bead up and not spread
- When compared to water, the surface tension of most food oils is low, causing them to readily spread and stain fiber-based packaging
The addition of fluorochemicals lowers the surface energy of the paper, allowing it to repel the flow of the liquid through it. This can be likened to the effect of water beading up on a freshly cleaned and waxed car.
While fluorochemicals are effective at repelling oils and greases in paper-based food packaging applications, they have come under increasing regulatory scrutiny and proposed bans. In February 2019, the EPA issued its first ever PFAS action plan, referring to these products by their chemical name: Per- and polyfluoroalkyl substances (PFAS). These chemistries, which have been around since 1940, are used in a variety of industries and application areas.
In its action plan, the EPA mapped out short- and long-term goals to limit the impact of PFAS on consumers and the environment. Many state governments have followed suit and begun to impose their own restrictions and regulations on the use of these chemicals. As this regulatory focus has increased, brand owners have increasingly sought out effective alternatives, and in many applications, they have been successfully replaced by water-based barrier coatings.
Resist vs. Repel
Water-based barrier coatings can also effectively protect the packaging from oil and grease, though they perform in a much different manner. They are used to create a physical barrier, and rather than repel the liquid, they resist it from passing through. While this can be quite effective, these barriers can be easily compromised by coating imperfections and non-uniformity. Additionally, they are subject to cracks formed during creasing and folding of converting operations. Cracks create weak points in the coating that allow the liquids to pass through and can quickly wick through the fiber structure, causing significant staining to the package.
While fluorinated chemicals are successfully used to treat paper and board in order to provide oil and grease resistance for food packaging, consumers and brand owners are requesting alternatives, such as water-based coatings with similar barrier properties. Fluorochemicals are typically applied in the wet-end or size press during the paper-making process to ensure good fiber saturation and high barrier performance, including edge wicking barrier.
Water-based coatings are applied to the paper surface during the printing stage and may not penetrate significantly into the paper or board substrate. Therefore, there may be some applications where current water-based polymers or coatings may need further development to completely match the oil and grease barrier performance of fluorochemicals.
At a minimum, and in order to be considered suitable alternatives, water-based coatings must provide barrier performance both at room temperature and at elevated temperatures up to 60 degrees Celsius. Resistance at elevated temperatures is required, since many food products are packaged soon after cooking, while still warm. Also, since the paper or board package is usually folded or creased during manufacture and as part of normal use, the water-based coatings must be highly flexible to avoid cracking that could detract from barrier performance.