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demonstrated the compatibility of PLA and wheat gluten protein by forming a trilayer laminate, where wheat gluten improved both oxygen and water vapour barriers relative to PLA film [37]. Nanoclay was blended into tri-layered PLA film by Scarfato et al. (2017), reducing oxygen permeance [38]. Another approach to achieve an oxygen barrier involves the use of hybrid inorganic- organic dispersions or lacquers, such as ORMOCER ® (a registered trademark of Fraunhofer– Gesellschaft), which is made from ceramics, glass, and organic polymers [39]. These dispersions offer strong adhesion, transparency, chemical and mechanical stability, and easy processability, thus making them ideal for coating applications [40]. The mechan- ical and barrier properties can be tailored by varying the organic-to-inorganic ratio or adding functional groups. ORMOCER ® coatings have been used to improve the barrier properties of polymer films [41–43]. By substituting organic polymers with biobased ones, the resulting lacquer, bioORMOCER ® , and coated paper can become biodegradable [44]. Additionally, its low viscosity and high solid content make it compatible with standard coating equipment. Both PLA-based dispersions and bioORMOCER ® represent high-performing pro- cessable biobased alternatives for conventional barrier coatings in packaging applica- tions. Additionally, both PLA-based material and bioORMOCER ® have been confirmed to be biodegradable [44,45]. This study proposes a novel multilayer barrier coating of crosslinkable PLA copolymer (PLAX) and bioORMOCER ® on two paper substrates for packaging applications. The papers were precoated with PLAX, continued with a layer of bioORMOCER ® and another layer of PLAX on top. The coating quality, barrier properties and heat sealability were characterised to confirm the suitability of the barrier coated papers for packaging applications. The goal was to demonstrate sustainable multilayer packaging material by applying minimum coat weight but aiming for a combination of barrier properties against oxygen, water vapor, and grease, as well as heat sealability.
2. Materials and Methods 2.1. Materials
Two commercial paper substrates were used in this work: UPM Asendo TM (A) and UPMSolide TM Lucent (S) (supplied by UPM; Valkeakoski, Finland). Substrate A is a precoated barrier paper with medium barrier properties suitable for dry foods. Substrate S is a calendered, uncoated barrier base paper with a smooth surface and a dense fibre structure. The reason behind the selection of base papers was to investigate which type of substrate surface is optimal to achieve even coating quality and thus high-barrier properties. The basis weights of substrates A and S were 64 ± 0.3 and 61 ± 0.4 g/m 2 , and their thicknesses were 59 ± 0.7 and47 ± 0.3 μ m, respectively. Two types of dispersions were used to produce the multilayer coated papers: PLAX and bioORMOCER ® . PLAX polymer was copolymerised using 90% L-(+) lactic acid solution (Thermo Scientific; Leicestershire, UK) and itaconic acid (Acros Organics, Novasol Chemicals; Fair Lawn, NJ, USA), 2,3-butanediol (Thermo Fisher Scientific) as initiator and Sn(II) 2-ethylhexanoate (Sigma-Aldrich; St. Louis, MI, USA) as catalyst. The aqueous dispersion of PLAX was prepared using the thermomechanical method with a commercial polyvinyl alcohol (PVA) as a dispersion stabiliser. The PLAX dispersions were homogenised using a high-shear mechanical treatment with a Microfluidics high-pressure microfluidiser type MF7125-30. The PLAX dispersions were microfluidised twice at 1800 bar and processed through the 400 μ m and 100 μ m chambers. The solid content of the final dispersion measured with an IR-35 Moisture Analyser (Denver Instrument; Bohemia, NY, USA) was 24%. The viscosity was measured using a Brookfield DV-III ULTRA Programmable Rheometer (Brookfield Engineering Laboratories Inc.; Stoughton, MA, USA) with a spindle
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