Coatings 2025 , 15 , 214
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markets are currently challenged by new directives and legislation, mainly the Single-use Plastics (SUP) directive and Packaging and Packaging Waste regulation (PPWR) proposed by the European Commission [12]. The goal is to reduce packaging waste by fostering reusable or refillable packaging, aiming for fully recyclable packaging, and demanding the use of more recycled materials [13,14]. The changes are directly related to both recycling systems and packaging design. Consequently, packaging material research is progressively focused on recyclable, compostable, and biodegradable alternatives in order to achieve the set goals in the future. Thus, the use of biopolymers and biobased nanomaterials has been extensively investigated in the last decades, e.g., polylactic acid (PLA), polyhydroxyalka- noates (PHA), starches, proteins, chitosan, as well as nanoclays, metal oxide nanoparticles, and nanocellulose [15–20]. Conventionally, extrusion coating has been the most utilised coating method in the barrier packaging sector, using common fossil-based thermoplastics, like PE, polypropylene (PP), and PET, with relatively high coating thicknesses. These thermoplastics provide barrier properties against moisture and grease, sealability, and durability, but they hinder the recycling of fibres and are not biodegradable [21]. Biobased thermoplastics, such as PLA and PHAs, are being actively researched, but they share the same disadvantages as their fossil-based counterparts when it comes to end-of-life performance. One of the key disadvantages of extrusion coated structures, especially using bio-polyesters, is that the coating thicknesses have to be high in order to avoid thermal degradation during the coating process [22,23]. These higher thicknesses pose challenges during recyclability and biodegradability tests. Dispersion coating has been considered a more sustainable coating method, as it can achieve much lower coat weight compared to traditional extrusion coating, particularly for applications requiring high-barrier properties, without deteriorating the recyclability and compostability of packaging [24–26]. PLA is a biodegradable thermoplastic, which is typically synthesised from plant-based carbohydrates. It is a renewable alternative for barrier application due to its ability to pro- vide a barrier against water vapour and sealability [15,27,28]. PLA has also been dispersed into water-based coating emulsions aiming to provide sufficient barrier performance with lower coat weights [29–31]. PLA-copolymer dispersions were studied by Mehtiö et al. (2016) as they formulated promising aqueous dispersions that provided fully grease re- sistant coating and improved water vapour barrier by 50% with reduced coat weight compared to corresponding extruded PLA-coated paperboard [32]. In their study, the PLA copolymers were produced from oil-based D,L-lactic acid, and the coated PLA dispersions contained additional additives (thermal or a combination of thermal and UV crosslinking agents). Crosslinking of PLA copolymer after dispersion preparation enhanced polymer performance by increasing the molecular weight and thus providing better mechanical and barrier properties [32]. The drawback of PLA is that it is prone to hydrolysis under high humidity and has inadequate oxygen barrier properties [33]. To tackle the disadvantages of PLA, recent studies have focused on PLA-based materi- als for multilayer structures to enhance the performance of PLA. Cellulose nanomaterials, such as cellulose nano/microfibrils (CNF/CMF) and cellulose nanocrystals (CNCs), have been extensively explored as biomaterials for high oxygen barrier applications aiming also for recyclability and biodegradability. Koppolu et al. (2019) applied nanocellulose coating followed by PLA extrusion coating on a paperboard substrate and achieved a 98% lower oxygen transmission rate compared to PLA-coated paperboard [34]. PLA-based dispersion was also proved to protect nanocellulose coating, improving both water vapour and oxygen barrier at standard conditions [35]. However, scaling up nanocellulose coatings remains challenging due to the high viscosity, high cost, and low solid content of CNF/CNC sus- pensions [36]. To mention other materials combined with PLA, Rocca-Smith et al. (2019)
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