Abstract
Biodegradable polymers suffer from inherent performance limitations that severely limit their practical application. Their functionalization by coating technology is a promising strategy to significantly improve their physical properties for food packaging. In this study, we investigated the double coating technique to produce multifunctional, high barrier and heat-sealable biodegradable films. The systems consisted of a web layer, made of poly(lactide) (PLA) and poly(butylene-adipate-co-terephthalate) (PBAT), which was first coated with a poly(vinyl) alcohol based layer, providing high barrier, and then with a second layer of PLA + ethylene-bis-stereamide (EBS) wax (from 0 to 20%), to provide sealability and improve moisture resistance. The films were fully characterized in terms of chemical, thermal, morphological, surface and functional properties. The deposition of the PVOH coating alone, with a thickness of 5 μm, led to a decrease in the oxygen transmission rate from 2200 cm3/m2 d bar, for the neat substrate (thickness of 22 μm), to 8.14 cm3/m2 d bar (thickness of 27 μm). The deposition of the second PLA layer did not affect the barrier properties but provided heat sealability, with a maximum bonding strength equal to 6.53 N/25 mm. The EBS wax incorporation into the PLA slightly increased the surface hydrophobicity, since the water contact angle passed from 65.4°, for the neat polylactide layer, to 71° for the 20% wax concentration. With respect to the substrate, the double-coated films exhibited increased stiffness, with an elastic modulus ca. three times higher, and a reduced elongation at break, which, however still remained above 75%. Overall, the developed double-coated films exhibited performances comparable to those of the most common synthetic polymer films used in the packaging industry, underlining their suitability for the packaging of sensitive foods with high O2-barrier requirements.
Highlights
We investigated the double coating technique to produce multifunctional, high barrier and heat-sealable biodegradable films
Since isopropanol is able to form strong hydrogen bonds with m-polyvinyl alcohol (PVOH), it can be hypothesized that, in the conditions used for the experiments, isopropanol did not evaporate during the film formation, but remained in the coating
The m-PVOH deposition led to a decrease in the oxygen transmission rate of three orders of magnitude, from 2200 cm3/m2 d bar, for the neat substrate, to 8.14 cm3 mm/m2 d bar
Summary
Packaging, including flexible films and rigid containers, represents the largest single market for the consumption of plastics, with 23 million tons per year (and 92 million tons expected in 2050). The oxygen permeability of biodegradable polymers that are of major interest for commercial applications, i.e., PLA and its blends with PBAT, polycaprolactone (PCL) and starch, is at least 20 times higher than that of PET, and even more than two orders of magnitude larger than high-performance conventional, fossil-based materials [5–15] These limitations are critical in the case of flexible packaging, whose thickness is very small, and severely limit its application to foods with low moisture content and short shelf-life [11,16]. Polyvinyl-alcohol (PVOH), in particular, is one of the few completely biodegradable synthetic polymers showing multiple desirable features: it features good biocompatibility, excellent transparency, gas/aroma barrier, film-forming ability and chemical resistance [41–43] It is approved by the US Food and Drugs Administration (FDA) and the European Medicines Agency (EMA) for food contact applications [44,45].
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