Coatings 2025 , 15 , 214
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leaks, spoilage, and a reduction in shelf life. Mineral oils, commonly found in printing inks, can migrate into food, posing potential health risks [54]. The mineral oil barrier becomes increasingly important when higher amounts of recycled fibres are desired to be used in food packaging, as recycled fibres often contain residues of mineral oils from printing inks and adhesives [55]. Grease resistance is characterised using the KIT-test, and the mineral oil barrier is evaluated by measuring HVTR. The KIT results are listed in Table 3. The base paper A performs excellently even without additional coating because of the precoating. The most important feature of the material to perform well on the KIT test is to have a closed surface with filled/covered pores. To achieve a KIT value of 12, the surface of the material should withstand the aggressive solution of heptane and toluene for 15 s. Therefore, it does not fully explain the oil and grease resistance in longer times but defines the wicking characteristics of the surface. Sample S-P suggests that just a single PLAX coating layer is sufficient to provide grease barrier properties. bioORMOCER ® and the top PLAX layers could provide grease resistance for longer periods. The HVTR results are presented in Figure 4d. The same trend appears in the KIT results, as uncoated base paper S shows a higher HVTR than base paper A due to the former’s uncoated surface. Coating layers of PLAX and bioORMOCER ® improve the mineral oil barrier by up to 97%, demonstrating a great applicability in printed food packaging and securing a utilisation of recycled fibres. High-performance mineral oil barriers are referred to as having HVTR
under 10 g/m 2 · day [55], which both S-POP and A-POP pass. Table3. KIT-test results of uncoated and coated base papers A and S.
Max. Sealing Strength (N/25mm)
Max. Sealing Strength (N/25mm)
Sample KIT Value
Sample KIT Value
BASE-S
1
- - -
BASE-A 12
- - -
S-P
12 12 12
A-P
12 12 12
S-PO
A-PO
S-POP
4.1
A-POP
7.5
3.4. Heat Sealability Heat sealability is an essential function of several packaging due to its role in ensuring product safety and maintaining freshness by enclosing the product inside a packaging. The sealing should be durable under stress throughout the whole logistic chain, from the production line to the customer. Sealing strength should be optimised depending on the end application, e.g., lower sealing strength is needed for detachable films in food packaging, and higher sealing strength is desirable for self-standing pouches. The sealing strengths of S-POP and A-POP are presented in Table 2. Both samples were heat sealable at 170 ◦ C. A-POP required a sealing pressure of 650 N and a sealing time of 1 s. A longer sealing time and higher sealing pressure, 2 s at 800 N, were needed for S-POP. The graph reveals that even with “milder” sealing conditions, A-POP offers stronger sealing properties than S-POP. With a 25 mm wide test piece, A-POP provided 7.5 N/25 mm sealing strength, whereas S-POP showed 45% lower sealing strength of 4.1 N/25 mm. The weaker sealability could potentially be explained by the foaming of dispersion. The air bubbles leave cavities on the surface, reducing the effective sealing surface area. PE is widely used commercially in packaging applications where heat sealability is required. According to published research, PE-coated (both fossil- and biobased) has imparted a seal strength of 6–15 N/25 mm [56–58]. A-POP occupies the lower part of this range and thus has the potential to be applied in heat-sealable packaging. S-POP needs further optimisation in terms of sealability. It should be retained that several parameters, such as sealing temperature, time, and dwell time,
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