Development of high performance moisture barrier coating for PET bottles

Polyethylene Terephthalate (PET) is one of the most common polymers for packaging applications, because of its properties and versatility. The limit of the use of PET containers in packaging is its low barrier to gases, with a negative impact on the quality of the product. The project intends to improve water vapor barrier properties of PET commercial containers. Possible technological routes should be amenable to an industrial implementation, sustainable and with no impact on bottle appearance. Coating technology can satisfy all the requirements for the industrial implementation, because high performances can be achieved adding a thin barrier material, minimizing the negative impact on the recycling process. The concept at the basis of the PhD activity is the realization of a coating based on a PET compatible polymer, eventually loaded with impermeable 2D-additives, to apply by spray coating on PET bottles. The design of the coating matrix starts from prevalently amorphous vinyl alcohol polymer (HAVOH), that is a high oxygen barrier polymer. HAVOH is a water-soluble polymer, hence a chemical crosslinking agent is required to achieve the insolubility of the coating for water. Crosslinker concentration and crosslinking conditions are investigated by solubility test. WVTR tests on cross-linked HAVOH coating (35µm thickness) show undetectable WVTR (<0.005 g m-2 day-1), even at high relative humidity percentage. Coatings formula designed can be easy removed from PET washing with caustic solution (1%wt) at 85ºC, according to recycling test protocol. Graphene-based additives (graphene, graphene oxide, reduced graphene oxide) are considered as possible additives for the developed coating, to obtain undetectable WVTR, even with thinner coating. Graphene-based fillers are widely used in literature to improve gas barrier properties of a polymer matrix. They reduce gas solubility, due to insolubility of gas in the nanosheets, and diffusivity, as the gas molecules must move around the impermeable nanosheets to diffuse through the polymer. Compared to graphene and GO, RGO is the potential additive for this application. Developed coating is deposited on real PET bottles, to evaluate optimal spray coating conditions, such as air and liquid flow rate, and drying rate. To increase the wettability of the coating solution on PET, the possibility of combine coating with anti-static solvent is considered.