Barrier coatings
To ensure that polymer films have sufficient barrier properties in relation to their application, it is often more efficient to use a thin layer of a high barrier polymer rather than to increase the monolayer thickness of the structural polymer. These functional coatings can impart specific barrier features to the film, while maintaining high product-to-package ratios and enabling downgauging.
A "coating" is a thin covering applied to the surface of a substrate for functional purposes, such as gas barrier function, adhesion, wettability, or corrosion resistance. Lacquers are coatings that are largely used in the packaging industry to improve the barrier properties of the plastic film substrate. Commonly used coatings act as barriers to oxygen, moisture/water vapor or CO2 in order to prevent the penetration or loss of the specific gas. Other barrier functionalities are against UV, to protect the packaged content from light, or against volatile organics, to preserve aroma.
As an example of coating usage, polypropylene, PP, due to its average gas barrier properties, is often used in combination with coatings. Polyvinylidene chloride, PVDC, displaying excellent moisture- and gas-barrier properties, is commonly applied as a coating to materials such as PP. If the combined film is used in food packaging, the reduced permeability to atmospheric oxygen protects the food inside the package and results into an extension of its shelf life. The major disadvantage of PVDC is that it undergoes degradation at temperatures very near to the processing temperatures, this causing a significant amount of product loss in the manufacturing process.
Ethylene vinyl alcohol, EVOH has excellent gas barrier properties and is commonly used as oxygen-barrier coating layer in food packaging. Due to the complexity of its industrial production process it results more expensive compared to other plastic resins used in packaging. EVOH strong barrier properties against oxygen are lost when the film is exposed to moisture. For that reason, it is preferably coupled with films that have good moisture barrier properties, such as polyethylene.
The most modern and innovative coating materials are designed to overcome the drawbacks of existing solutions and to pay more attention to sustainability, in order to guarantee the recyclability or compostability of the final packaging product, and to enable the film downgauging while keeping high gas barrier properties.
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