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3.3. Barrier Properties Barrier properties of food packaging are essential to secure food safety, extended shelf life, and better quality of food. Barrier properties against water vapour, oxygen, grease, and mineral oil were determined to evaluate the applicability of coated samples A-POP and S-POP for food packaging applications. Excellent water vapour barrier in varying conditions is a foundation for both dry and fresh food packaging to control the moisture transmission and thus prevent unwanted hydration/dehydration and microbial growth of the product. Figure 4a,b presents the WVTR values for the substrates and coated papers at standard laboratory conditions (23 ◦ C and 50% RH) and elevated humidity (23 ◦ Cand 85% RH). The first PLAX layer did not provide a major reduction in WVTR at either of the conditions due to low coat weight. Sample S-POP showed enhanced water vapour barrier, as the WVTR was reduced approximately 30% at standard conditions and 35% at elevated humidity relative to the base paper S. A-POP showed even better barrier against moisture than S-POP, especially at higher humidity, because the base paper A was initially precoated with a light layer of barrier coating. Water vapour transmission was accelerated at higher humidity due to the hydrolytic degradation of PLA. The presence of hemicellulose biopolymer in bioORMOCER ® is also contributing to the higher WVTR. These results were competitive compared to the data on fossil-based dispersions and PLA-based coatings that had been published before. Fossil-based latexes have shown WVTR values of 20–40 g/m 2 with a coat weight of 8–12 g/m 2 [26,47]. PLA-based dispersion coatings have shown WVTR values of 36 and 23 g/(m 2 · day) with coat weights of 10–15 g/m 2 and PLA-based extrusion coating WVTR of 57 g/(m 2 · day) with 23 g/m 2 coat weight while A-POP and S-POP resulted to have WVTR values of 12 and 15 g/(m 2 · day) with coat weight of 13–14 g/m 2 [29,32,34]. Theoretically, applying thin separate coating layers, in contrast to individual thicker layers, delays the diffusion of water vapour by increasing the amount of coating layer interfaces (more tortuosity) and thus enhances the moisture barrier properties [48,49]. An oxygen barrier is essential for food packaging to prevent oxidation reactions lead- ing to spoilage, microbial growth, and off-flavours and -tastes. OTR values of S-POP and A-POP at zero humidity (23 ◦ C and 0% RH) and standard laboratory conditions (23 ◦ C and 50% RH) are presented in Figure 4c. The OTRS of S-POP were 2.3 cc/(m 2 · day · bar) and 3.8 cc/(m 2 · day · bar), and of A-POP 29 cc/(m 2 · day · bar) at the mentioned conditions, respec- tively. The difference in OTR between S-POP and A-POP was due to the characteristics of the base papers. Base paper S is highly calendered paper with a dense fibre structure and an extremely smooth surface for coatings. The smooth surface facilitates, especially thin, coating layers to be evenly thick with less variation and defects. The OTRs were measured at 0% RH to evaluate the maximum oxygen barrier potential of bioORMOCER ® . No drastic change at 50% was observed, as the PLAX layer was acting as a protective shield against the moisture. A significant improvement on the oxygen barrier was achieved compared to single coated bioORMOCER ® on polyolefin, which exhibited an OTR of 25 cc/(m 2 · day · bar) with3 μ m coating thickness [44]. S-POP has demonstrated an oxygen barrier comparable to conventional alternatives, such as nanocellulose films (1–3 cc/(m 2 · day · bar)) [50,51] and metal oxide coatings (1–2 cc/(m 2 · day · bar)) [52,53]. Moisture and oxygen barrier properties are parallelly compared to other commonly used barrier packaging materials in Table 2. Both S-POP and A-POP stand out due to their combined barrier performance. Among the conventional barrier materials, low- density ethylene (LDPE) shows a comparable WVTR value of 16 g/(m 2 · day) but lacks oxygen barrier, whereas metallisation exceeds both S-POP and A-POP showing excellent barrier properties against both moisture (WVTR of 0.35 g/(m 2 · day)) and oxygen (OTR of 1 cc/(m 2 · day)). Biobased alternatives, such as CNF, provide an excellent barrier against oxygen (OTR of 4.8 cc/(m 2 · day)) but are extensively sensitive to humidity. The comparison
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