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Article

Upscaled Multilayer Dispersion Coating Application for Barrier Packaging: PLAX and bioORMOCER®

by
Eetu Nissinen
1,*,
Adina Anghelescu-Hakala
1,
Roosa Hämäläinen
1,
Pauliina Kivinen
1,
Ferdinand Somorowsky
2,
Jani Avellan
3 and
Rajesh Koppolu
1
1
Sustainable Products and Materials, VTT Technical Research Centre of Finland Ltd., 02044 Espoo, Finland
2
Fraunhofer-Institut für Silicatforschung ISC, Neunerplatz 2, 97082 Würzburg, Germany
3
Walki Group Oy, Keilaranta 6, 4th Floor, 02150 Espoo, Finland
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(2), 214; https://doi.org/10.3390/coatings15020214
Submission received: 13 January 2025 / Revised: 7 February 2025 / Accepted: 9 February 2025 / Published: 11 February 2025
(This article belongs to the Special Issue Sustainable Coatings for Functional Textile and Packaging Materials)
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Versions Notes

Abstract

:
The shift from fossil-based packaging materials to more sustainable alternatives is driven by evolving environmental regulations aiming for enhanced recyclability and biodegradability. Dispersion coatings, as opposed to extrusion-based approaches, offer significant advantages by reducing the coat weights, but generally, multiple coating layers are needed to meet functional performance requirements. This study explores the application of upscaled multilayer dispersion coatings comprising polylactic acid-based coating (PLAX) and hybrid nanomaterial lacquer (bioORMOCER®) on commercial base papers for barrier packaging using semi-pilot reverse gravure and industrial-scale rod coaters. One multilayer structure demonstrated a low water vapour transmission rate (WVTR), achieving a WVTR of 12 g/(m2·day) under standard conditions and a 78% reduction of WVTR compared to the substrate under elevated humidity. The other multilayer structure exhibited an excellent oxygen transmission rate (OTR) of 2.3 cc/(m2·day·bar) at dry conditions, which is comparable to conventional high-performance alternatives. Both multilayer coatings enhanced the grease and mineral oil barriers significantly, as heptane vapour transmission rate (HVTR) reductions exceeded 97%. The multilayer coatings demonstrated strong potential for scalable production of sustainable, high-barrier packaging materials. These findings highlight the capability of dispersion coatings to replace traditional fossil-based barriers, advancing the development of environmentally friendly packaging solutions.

1. Introduction

Multilayer barrier packaging is a critical technology in industries such as food packaging, pharmaceuticals, and cosmetics, designed to enhance the preservation of the product by combining barrier layers with tailored mechanical and barrier properties. Multilayer barrier packaging is favoured for its ability to provide a high barrier against gases like oxygen, nitrogen and carbon dioxide, moisture and liquid barriers, sterility, flexibility, and mechanical strength, which are essential for maintaining product quality and extending shelf life [1,2,3,4,5]. However, the complexity of multilayer structures that often combine different materials poses significant challenges for recycling [6,7,8], e.g., materials like polyethylene (PE), aluminium, and polyethylene terephthalate (PET) are difficult to separate due to their strong interlayer bonds [9,10,11]. To foster a circular bioeconomy, European packaging 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), polyhydroxyalkanoates (PHA), starches, proteins, chitosan, as well as nanoclays, metal oxide nanoparticles, and nanocellulose [15,16,17,18,19,20].
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 [24,25,26].
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 provide 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 [29,30,31]. PLA-copolymer dispersions were studied by Mehtiö et al. (2016) as they formulated promising aqueous dispersions that provided fully grease resistant 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 materials 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 suspensions [36]. To mention other materials combined with PLA, Rocca-Smith et al. (2019) 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 mechanical 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 [41,42,43]. 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 processable biobased alternatives for conventional barrier coatings in packaging applications. 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 AsendoTM (A) and UPM SolideTM 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/m2, and their thicknesses were 59 ± 0.7 and 47 ± 0.3 µm, 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 µm and 100 µm 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 providing torque in the optimal range. The viscosity of the final PLAX dispersion measured with the RV04 spindle at 100 rpm was 500 mPas. The detailed steps for PLA polymer synthesis and dispersion preparation are described by Mehtiö et al. (2017) [32].
After screening several thermally crosslinking bioORMOCER® formulations and comparing application properties on the two papers, barrier properties and flexibility, one was selected that exhibits a very high degree of crosslinking. This is based on the high usage of aluminium alkoxide, which acts as a catalyst and a network former. The inorganic network is reinforced through the use of different silanes, including Tetraethylorthosilicate (TEOS) (ABCR; Karlsruhe, Germany). Hydroxypropylcellulose (HPC, Thermo Scientific) was incorporated as a biocomponent to enhance flexibility during the synthesis. The viscosity of the finished coating solution was measured at 39.7 mm/s2, with a solid content of 28%. To improve the pot life of the bioORMOCER® formulation, a multicomponent variant was also developed, which could be mixed on site.
Hereon, PLAX is referred to as ‘P’ and bioORMOCER® as ‘O’.

2.2. Methods

The coating layers were applied on both a semi-pilot and industrial scale. The first coating layer, P, was applied at an industrial scale production line using a rod coater (Walki Group Oy; Ylöjärvi, Finland). Dispersion P was diluted to 20% due to high viscosity. Rod V5 was used, and a coating layer was applied on the precoated side in the case of base paper A and on the smooth side of base paper S. The speed of the production line was 80 m/min, and the coatings were dried at 110 °C along the whole drying section. De-curling was prevented using a steam moisturiser with a steam pressure of 0.45 bar.
The latter coating layers of O and P were applied with the pilot coating line SUTCO (Surface Treatment and Coating line) at VTT Bioruukki (Espoo, Finland). Dispersions O and P were applied using the reverse gravure coating method. A gravure roll with a surface volume of 75 cc/m2 (20 lines/cm) was used, with a coating speed of up to 20 m/min. The drying section consisted of three infrared (IR) and five air dryers. The first air dryer was set to 200 °C, and the temperature was gradually decreased to 170 °C along the line. No IR dryers were used while drying O due to the ignition risk of ethanol. Corona treatment was used to improve the adhesion of the dispersions on coated substrates. A cooling roller with a surface temperature of 10 °C was used before rewinding the coated papers. All coated samples were stored in standard laboratory conditions, 23 °C and 50% relative humidity (RH), before characterisation. Table 1 lists the coating trial points produced in this work.

2.3. Characterization

2.3.1. Coat Weight and Coating Thickness

The conditioned samples were cut with a circular sample cutter (50 cm2 area, Thwing–Albert Instrument Company; West Berlin, NJ, USA) and weighed with an analytical balance (Precisa ES220A; Dietikon, Switzerland) to calculate the grammages, reported as g/m2 as an average of five parallel samples. The coat weights were obtained by subtracting the grammages of base papers from the grammages of the coated samples. The coating thicknesses were determined using crosscut images obtained from scanning an electron microscope (SEM; Merlin, Carl Zeiss, Germany).

2.3.2. Surface Roughness and Air Permeability Measurements

Surface roughness was measured with a PPS-tester by L&W (ABB Ltd.; Zürich, Switzerland), applying the standard ISO 9791-4 to compare the surface properties of base papers. Air permeability was tested to analyse how closed the coating structure is using an L&W Air Permeance Tester (ABB Ltd.) (ISO 5636:5).

2.3.3. Imaging of Surface and Cross-Section

The coating quality was analysed by imaging the surface and cross-section of the samples using scanning electron microscopy (SEM; Merlin, Carl Zeiss). Acceleration voltage of 2 kV was used. The samples were prepared by freeze-cracking with liquid nitrogen. Prior to the imaging, a thin gold layer was sputtered on the samples.

2.3.4. Moisture and Mineral Oil Barrier Measurements

Water vapour transmission rates (WVTR) were tested using the gravimetric method by applying standard ISO 2528:2017. The measurements were conducted in two conditions: 23 °C, 50% RH and 23 °C, 80% RH. The samples were cut using a circular sample mallet die punch (diameter 75 mm, Thwing–Albert Instrument Company) and conditioned in the particular condition for at least 12 h. One test set up contained cups (EZ-Cup Vapometer cup, Thwing–Albert Instrument Company), two neoprene gaskets and dry calcium chloride powder (Thermo Scientific Chemicals). A test cup was filled with CaCl2, the sample was sealed between two gaskets placing coated side upwards and tightened with a screwable lid. The test set up was kept in a conditioning chamber (CTC256, Memmert GmbH; Schwabach, Germany) for 24 h. WVTR values (g/m2/day) were calculated by weighing the increase in mass caused by the moisture that was diffused through the sample and absorbed by the salt, as an average of three parallel tests.
Heptane transmission rate (HVTR) was determined using a similar method, where CaCl2 was replaced with n-heptane, and the heptane diffusion was monitored as the mass decreased over time. The method is described by Miettinen et al. (2015) [46].

2.3.5. Oxygen Barrier Measurement

Oxygen transmission rates (OTR) were measured using OXTRAN 2/22 by Mocon (Brooklyn Park, MN, USA) (complies with the standard ASTM D3985). A 5 cm × 5 cm sample was cut and placed between two aluminium masks with a test area of 5 cm2. Masking is used to prevent the side leakage of gases, as the paper edges are not coated or sealed. Two conditions were used: 23 °C, 0% RH and 23 °C, 50% RH. The accuracy was six adjacent test points within 1%, and one cycle lasted 30 min. The sensors were flushed (ReZero) for 30 min every six cycles. The results (cc/m2/day) were reported as an average of two parallel measurements.

2.3.6. Grease Barrier Measurement

Grease resistance was tested using the KIT-test method (ISO 16532-2). The method includes 12 solutions of castor oil and two solvents, n-heptane and toluene. The solutions are ranked from 1 to 12, where 1 is the mildest one (only castor oil) and 12 is the most aggressive (mixture of n-heptane and toluene). One droplet of solution is dropped on the sample and wiped off after 15 s. The KIT value refers to the highest ranked solution that does not leave any stain on the surface. The KIT value (1–12) was reported as an average of 3 parallel tests.

2.3.7. Heat Sealability Test

Heat sealability of the coating was tested using a separate hot tack device (KOPP LABORMASTER HCT 3000), and the sealing strength was measured applying standard ASTM F88M-15 with Lloyd LS5 universal testing machine. The sealing temperatures were 170 °C for both A-POP and S-POP. A-POP was sealed for 1 s under the pressure of 650 N. S-POP needed 2 s of sealing under 800 N pressure.

3. Results and Discussion

3.1. Coating Process

PLAX and bioORMOCER® layers were coated on the substrates A and S using a roll-to-roll coating line at semi-pilot and industrial scale. The first PLAX layer was coated at an industrial scale coating line (Walki Ylöjärvi, Finland) to demonstrate the coatability of PLAX at higher speeds and volumes. The coating finish was even, matte and visually intact. bioORMOCER® and the top layer of PLAX were applied at a semi-pilot scale using a gravure coater method (SutCo: VTT, Finland) (Figure 1a,b). bioORMOCER® and PLAX were suitable for the coating method as they were picked up efficiently and transferred evenly onto the substrate. Some foaming issues were faced while coating PLAX, leaving a risk of pinholes. The coatings seemed visually even in the case of both dispersions; bioORMOCER® left a clear glossy finish (Figure 1c), and PLAX appeared as matte with a tint of beige colour (Figure 1d). No visual differences between the base papers were observed.

3.2. Coat Weights and Coating Thicknesses

The weight of the different layers of the coat is presented in Figure 2. The function of the bottom layer of PLAX was to provide a smooth closed surface and thus to prevent the penetration of low viscous bioORMOCER® into the paper, enabling better coating hold-out, while the top PLAX layer protects the bioORMOCER® layer and provides a water vapour barrier. A difference of the coat weights of the first PLAX layer was observed between substrates A and S, as approximately twice as much of coating was applied on the substrate A compared to substrate S. Following factors were identified to affect the coat weights: (1) surface roughness (Base papers A has rougher surface than S, 2.7 μm vs 1.5 μm); (2) coating method (rod coater meters more coating on rougher surface); (3) changes in suspension during the process (agglomeration, foaming, evaporation) and (4) swelling of base paper causing uneven surface for coating application. The coat weights of bioORMOCER® show that it was applied uniformly on both substrates using the reverse gravure coating method. The surfaces of substrates were smoothened with PLAX precoating, providing even coatability for the bioORMOCER® layer. The cross-cut images (Figure 3) display that the bioORMOCER® coating layer of A-POP and S-POP adheres well with PLAX. The foaming problem of PLAX was detected in the SEM-image of A-POP. Air cavities cause deviations in coatings, which deteriorate the uniformity of coating quality and thus affect barrier performance. The issue could be resolved by collapsing the air bubbles either by vacuum or by using defoamers. In an industrial process, the latter is preferable. The overall coating quality of base papers after three coating layers was demonstrated by testing the air permeabilities of A-POP and S-POP. Both of them were fully covered as they showed air permeabilities of 0.003 µm/Pa∙s, which is the minimum value that the device can detect.

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 °C and 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/m2 with a coat weight of 8–12 g/m2 [26,47]. PLA-based dispersion coatings have shown WVTR values of 36 and 23 g/(m2∙day) with coat weights of 10–15 g/m2 and PLA-based extrusion coating WVTR of 57 g/(m2∙day) with 23 g/m2 coat weight while A-POP and S-POP resulted to have WVTR values of 12 and 15 g/(m2∙day) with coat weight of 13–14 g/m2 [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 leading 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/(m2∙day∙bar) and 3.8 cc/(m2∙day∙bar), and of A-POP 29 cc/(m2∙day∙bar) at the mentioned conditions, respectively. 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/(m2∙day∙bar) with 3 µm coating thickness [44]. S-POP has demonstrated an oxygen barrier comparable to conventional alternatives, such as nanocellulose films (1–3 cc/(m2∙day∙bar)) [50,51] and metal oxide coatings (1–2 cc/(m2∙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/(m2∙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/(m2∙day)) and oxygen (OTR of 1 cc/(m2∙day)). Biobased alternatives, such as CNF, provide an excellent barrier against oxygen (OTR of 4.8 cc/(m2∙day)) but are extensively sensitive to humidity. The comparison remarks the difficulty of achieving combined barrier properties with such low coat weights and emphasises the potential of S-POP and A-POP in the barrier packaging industry.
Food packaging should incorporate effective barriers against grease, oil, and mineral oil to ensure product safety, maintain product quality, and protect consumer health. Grease and oil from food products can cause the structural weakening of packaging, leading to 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/m2·day [55], which both S-POP and A-POP pass.

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 [56,57,58]. 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, affect the sealing strength, and therefore, a sealing process should be optimised thoroughly for each material.

4. Conclusions

The present study demonstrates the performance and potential of PLAX and bioORMOCER® multilayer coatings using pilot- and industrial-scale reverse gravure and rod coaters on two commercial base papers for food packaging applications. Both coatings exhibited strong barrier properties, significantly reducing water vapour and oxygen transmission rates as well as a mineral oil barrier. Sample A-POP, with its precoated base paper, generally provided barrier (WVTR of 12 g/(m2∙day) at 23 °C and 50% RH) and sealing characteristics, highlighting its suitability for applications requiring higher moisture resistance. The smooth surface and dense fibre structure of substrate S facilitated even coating distribution for sample S-POP, which was shown as an effective oxygen barrier performance at dry (2.3 cc/(m2∙day∙bar)) and standard conditions (3.8 cc/(m2∙day∙bar)), though further optimisation may enhance its heat sealability. A grease barrier was achieved with both samples, and the KIT value was evaluated at 12. The multilayer coating structure provided a high mineral oil barrier on both the substrates, allowing the use of recycled fibres, including minor contaminants. This study was executed utilising coating lines at a higher technical readiness level, demonstrating the potential of producing high-performance barrier packaging materials at a larger scale. Overall, the multilayer coatings applied on both substrates contribute to a more sustainable, functional packaging solution, offering promising alternatives to conventional fossil-based barriers. These findings provide insights for future packaging development, aiming to enhance packaging integrity, prolong shelf life, and reduce the environmental impact of packaging materials. In the future, the end-of-life of the material should be investigated to confirm the biodegradability and recyclability of the multilayer structure. Additionally, testing the barrier properties at higher humidities and applying a creasing would demonstrate the performance and the convertibility of the material better.

Author Contributions

Conceptualization, E.N., A.A.-H., F.S., J.A. and R.K.; methodology, E.N., A.A.-H., R.H., P.K., F.S., J.A. and R.K.; formal analysis, E.N., R.K. and J.A.; investigation, E.N., A.A.-H., R.H., P.K., F.S., J.A. and R.K.; data curation, E.N. and R.K.; writing—original draft preparation, E.N.; writing—reviewing and editing, E.N., A.A.-H., R.H., P.K., F.S., J.A. and R.K.; supervision, R.K. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement N°952972.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Acknowledgments

We would like to thank following people who were involved in the work and gave their valuable contribution: Panu Lahtinen for microfluidisation of PLAX-dispersion, Katja Pettersson, Ulla Salonen and Timo Kaljunen for contribution in coating application and characterisation, Mari Leino for acquiring SEM-images, Walki Group Oy for providing the base papers and Project coordinators, and Ulla Forsström and Ilona Leppänen for reviewing the article.

Conflicts of Interest

Authors Eetu Nissinen, Adina Anghelescu-Hakala, Roosa Hämäläinen, Pauliina Kivinen and Rajesh Koppolu were employed by the company VTT Technical Research Centre of Finland Ltd. Author Jani Avellan was employed by the company Walki Group Oy. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

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Coatings 15 00214 g001
Figure 1. Images from the coating process. (a) bioORMOCER® is poured into the coating station. (b) PLA-X dispersion at the coating station. (c) Visual appearance of A-PO. (d) SutCo-line during the coating run of S-POP.
Figure 1. Images from the coating process. (a) bioORMOCER® is poured into the coating station. (b) PLA-X dispersion at the coating station. (c) Visual appearance of A-PO. (d) SutCo-line during the coating run of S-POP.
Coatings 15 00214 g001
Coatings 15 00214 g002
Figure 2. Coat weights and thicknesses of coating layers of A-POP and S-POP.
Figure 2. Coat weights and thicknesses of coating layers of A-POP and S-POP.
Coatings 15 00214 g002
Coatings 15 00214 g003
Figure 3. SEM cross-cut images of the samples (a) A-POP and (b) S-PO with 5000x magnification.
Figure 3. SEM cross-cut images of the samples (a) A-POP and (b) S-PO with 5000x magnification.
Coatings 15 00214 g003
Coatings 15 00214 g004
Figure 4. Barrier performance of coated samples. (a) Comparison of water vapour transmission rates (WVTR) of coated samples related to base papers at 23 °C and 50% RH and (b) at 23 °C and 80% RH. (c) Oxygen transmission rates (OTR) of the samples S-POP and A-POP at 23 °C and 0/50% RH. (d) Heptane vapour transmission rates (HVTR) of the samples S-POP and A-POP and base papers at 23 °C and 50% RH.
Figure 4. Barrier performance of coated samples. (a) Comparison of water vapour transmission rates (WVTR) of coated samples related to base papers at 23 °C and 50% RH and (b) at 23 °C and 80% RH. (c) Oxygen transmission rates (OTR) of the samples S-POP and A-POP at 23 °C and 0/50% RH. (d) Heptane vapour transmission rates (HVTR) of the samples S-POP and A-POP and base papers at 23 °C and 50% RH.
Coatings 15 00214 g004
Table 1. The list of samples produced in this work.
Table 1. The list of samples produced in this work.
Base Paper1st Layer2nd Layer3rd LayerAbbreviation
Solide LucentPLAX--S-P
Solide LucentPLAXbioORMOCER®-S-PO
Solide LucentPLAXbioORMOCER®PLAXS-POP
AsendoPLAX--A-P
AsendoPLAXbioORMOCER®-A-PO
AsendoPLAXbioORMOCER®PLAXA-POP
Table 2. Comparison of water vapour transmission rates (WVTR) and oxygen transmission rates (OTR) at 23 °C and 50% RH between the samples S-POP (blue) and A-POP (green) and other common barrier packaging materials (white) [36]. The WVTR and OTR values of reference materials (white) are normalised to the thickness of 10 µm, which is comparable to the coating thicknesses in this work.
Table 2. Comparison of water vapour transmission rates (WVTR) and oxygen transmission rates (OTR) at 23 °C and 50% RH between the samples S-POP (blue) and A-POP (green) and other common barrier packaging materials (white) [36]. The WVTR and OTR values of reference materials (white) are normalised to the thickness of 10 µm, which is comparable to the coating thicknesses in this work.
SampleWVTR g/(m2∙Day)OTR cc/(m2∙Day)
S-POP153.8
A-POP1229
CNF 2204.8
Chitosan1800200
LDPE1625,000
PET149110
PP6011,000
EVOH7504.5
Metallised PET0.351
Table 3. KIT-test results of uncoated and coated base papers A and S.
Table 3. KIT-test results of uncoated and coated base papers A and S.
SampleKIT ValueMax. Sealing Strength
(N/25 mm)		SampleKIT ValueMax. Sealing Strength
(N/25 mm)
BASE-S1-BASE-A12-
S-P12-A-P12-
S-PO12-A-PO12-
S-POP124.1A-POP127.5
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MDPI and ACS Style

Nissinen, E.; Anghelescu-Hakala, A.; Hämäläinen, R.; Kivinen, P.; Somorowsky, F.; Avellan, J.; Koppolu, R. Upscaled Multilayer Dispersion Coating Application for Barrier Packaging: PLAX and bioORMOCER®. Coatings 2025, 15, 214. https://doi.org/10.3390/coatings15020214

AMA Style

Nissinen E, Anghelescu-Hakala A, Hämäläinen R, Kivinen P, Somorowsky F, Avellan J, Koppolu R. Upscaled Multilayer Dispersion Coating Application for Barrier Packaging: PLAX and bioORMOCER®. Coatings. 2025; 15(2):214. https://doi.org/10.3390/coatings15020214

Chicago/Turabian Style

Nissinen, Eetu, Adina Anghelescu-Hakala, Roosa Hämäläinen, Pauliina Kivinen, Ferdinand Somorowsky, Jani Avellan, and Rajesh Koppolu. 2025. "Upscaled Multilayer Dispersion Coating Application for Barrier Packaging: PLAX and bioORMOCER®" Coatings 15, no. 2: 214. https://doi.org/10.3390/coatings15020214

APA Style

Nissinen, E., Anghelescu-Hakala, A., Hämäläinen, R., Kivinen, P., Somorowsky, F., Avellan, J., & Koppolu, R. (2025). Upscaled Multilayer Dispersion Coating Application for Barrier Packaging: PLAX and bioORMOCER®. Coatings, 15(2), 214. https://doi.org/10.3390/coatings15020214

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