Polyethylene Terephthalate‐Based Heat Sealable Packaging Film Without Heat Sealing Layer

ABSTRACT The food packaging industry is evolving to meet demands for circularity, cost‐effectiveness, lower carbon footprint and higher production efficiency with polyethylene terephthalate (PET) emerging as a preferred material for packaging. Despite its advantages, PET faces two main challenges in flexible barrier packaging applications, such as flow packs or form‐fill‐seal pouches. First, PET films for flexible packaging often lack adequate moisture and oxygen barriers, limiting their range of applications. Second, the high melting temperature of PET of around 250°C complicates the sealing process, causing deformation and poor‐quality seals. Traditional solutions involve multilayer films, in which PET is laminated with a superior permeation‐barrier layer and a lower melting‐temperature heat‐sealing layer. This results in nonrecyclable packaging, posing sustainability issues. In the present work, PET‐based heat sealable films were developed to address these issues. Specifically, a water‐based organic–inorganic hybrid nanocomposite sol–gel coating was applied to one side of the 23‐μm thick PET film, followed by drying and curing. The thickness of the subsequent coatings was approximately 2.3 μm. The sol–gel's rheological properties allow for a microscopically thin application that hardens into a micrometre‐thick, flexible, transparent and abrasion‐resistant coating with a decomposition temperature well above melting temperature of PET. The wettability of the coating was slightly higher than that of PET. The result of the pencil hardness test indicates tight bonding of the coating to the PET film. Mechanical tests of the coated and uncoated PET indicate that the mechanical properties of the bulk PET were retained after the application and curing of the coating, and the reinforcing effect of the coating was confirmed by the measurement of the Young's moduli and the puncture resistance test. Characterization using Fourier transform infrared spectroscopy confirms that the film contains organo‐silicon compounds and aluminium oxides. Uncoated sides of a pair of PET films were faced and heat‐welded at 250°C, demonstrating significantly reduced deformation around the welding portion. The coating reduced oxygen and water vapour transmission rates by more than a factor of 3 and 4, respectively.