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Article

Bio-Based Dual-Layer UV-Cured Oil- and Water-Resistant Paper Coating for Food Packaging Applications

by
Myungho Lee
,
Anuja P. Rananavare
and
Youn Suk Lee
*
Department of Packaging and Logistics, Yonsei University, Wonju 26393, Republic of Korea
*
Author to whom correspondence should be addressed.
Int. J. Mol. Sci. 2026, 27(5), 2210; https://doi.org/10.3390/ijms27052210
Submission received: 3 February 2026 / Revised: 24 February 2026 / Accepted: 24 February 2026 / Published: 26 February 2026
(This article belongs to the Special Issue Bioactive Molecules from Food Waste in Food Packaging Applications)
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Abstract

Fluorine-free paper coatings with water- and oil-resistance properties have gained considerable attention for sustainable food packaging applications. In this study, a dual-layer coating based on chitosan (Chi) and acrylated epoxidized soybean oil (AESO), both derived from renewable and natural resources, was applied to kraft paper. The ultraviolet-cured AESO top layer formed a dense crosslinking network, while the Chi interlayer promoted strong interfacial adhesion with the kraft paper through hydrogen bonding, effectively restricting fluid penetration. The Chi/AESO40/kraft paper showed markedly enhanced water repellency and oil resistance, with a reduced Cobb600 value of 16 g m−2 and kit rating of 12. Thermogravimetric analysis demonstrated improved thermal stability, and mechanical testing results revealed enhanced packaging-relevant strength, with the tensile strength increasing from 33 to 40 MPa and tensile index increasing from 45 to 60 kPa·m2 g−1; furthermore, the burst strength and index improved from 260 to 330 kPa and from 3.2 to 4.0 kPa·m2 g−1, respectively. Food contact tests conducted using French fries confirmed the effective barrier performance of the Chi/AESO/kraft paper, highlighting its potential for use in sustainable paper-based food packaging applications.

1. Introduction

Paper packaging has re-emerged as a prime eco-friendly option against petroleum-based plastic packaging given its recyclable, economic, and biodegradable nature [1]. As a type of abundant biomass, kraft paper has low production costs and can maintain desirable values of biodegradability and recyclability. Kraft paper is utilized in diverse packaging formats such as paper bags, envelopes, paper folders, gift boxes, and food packaging materials. However, their natural hydrophilicity and porous morphology result in poor resistance to water, oil, and grease, especially when used for greasy food products, which hinders their large-scale use as a substrate [2]. The barrier performance of paper-based materials has been improved by a variety of surface modification techniques developed for overcoming these constraints. Paper is inherently hydrophilic because of the abundance of hydroxyl groups in cellulose; therefore, it offers low resistance to water and oils. If untreated paper is exposed to moisture or greasy food products, it easily absorbs the liquids, causing issues such as swollen fibers, loss in mechanical strength, and deterioration of structural integrity. Consequently, improving the water and oil resistance of paper-based materials is essential for ensuring durability and suitability required in food packaging applications [3]. Water-repellent and superhydrophobic coatings are more desirable for packaging. Materials that have water contact angles (WCAs) beyond 150° and sliding angles below 10° are considered superhydrophobic materials that can strongly repel water droplets and offer low adhesion. Thus far, various methods including porosity reduction, polymer film lamination, and surface treatments using low-surface-energy materials have been adopted for improving the water and oil resistance of paper materials [4,5]. Such surfaces are created by incorporating micro–nano hierarchical structures generated using nanomaterials with materials that naturally have low surface energy, such as long-chain alkyl compounds or fluorinated materials [6,7]. Several fabrication techniques can enhance superhydrophobic properties, including sol–gel processing, spray coating, dip coating, layer-by-layer assembly, and rod coating [8,9]. However, various successful barrier treatments use low-surface-energy synthetic polymers/fluorinated chemicals, raising issues related to food contact compatibility and recyclability/environmental persistence. There appears to be growing interest in sustainable bio-based coating solutions, which can potentially deliver effective barrier properties without deteriorating environmental compatibility. Thus, developing sustainable coating technologies using bioactive and bio-based materials obtained from food-related resources is essential [10,11].
Despite continuous improvements in the water and oil resistance of paper, the current research on this topic focuses on fluorine-free materials, including those based on polysaccharides, plant oils, and nanocomposites. Polysaccharides, including chitosan, starch, and sodium alginate, exhibit good film-forming and oil-resistance properties, while the hydrophilic characteristics of polysaccharides reduce their efficiency in water-repellent applications [12,13,14]. Chitosan is a naturally renewable and biodegradable polymer and the second-most abundant natural polysaccharide after cellulose [15,16]. Structurally, it is composed of 2-amino-2-deoxy-β-D-glucose units linked through β-(1→4) glycosidic bonds; therefore, it is chemically similar to cellulose and highly compatible with paper substrates. Chitosan is industrially produced from chitin derived from crustacean shell waste, and thus, the seafood industrial side flow possesses a representative bioactive material originating from food-waste biomass. Given their excellent film-forming ability, intrinsic antibacterial activity, and strong intermolecular interactions with cellulose fibers, chitosan-based coatings have been widely investigated for food packaging applications to improve food safety and extend shelf life. However, despite these advantages, the hydrophilic nature of chitosan limits its effectiveness as a standalone barrier against water penetration [17]. Hybrid coating strategies that incorporate polysaccharides and inorganic components have also been explored. For example, Hamdani et al. developed oil- and water-resistant papers using chitosan-graft-poly(dimethylsiloxane) copolymers [13]. Selvaraj et al. fabricated superhydrophobic filter papers by coating paper substrates with inorganic–organic hybrid nanocomposites composed of zinc oxide, chitin or chitosan, and polycaprolactone using a grafting technique. The resulting chitin-ZnO-PCL and chitosan-ZnO-PCL coatings exhibited excellent superhydrophobic behavior [18]. Peng et al. further improved the hydrophobic properties of paper-based antimicrobial indicator cards by surface modification using methyltrichlorosilane and octadecyltrichlorosilane (OTS) [19]. Although these approaches demonstrate effective grease resistance, they involve complex synthesis routes or rely on nonbiodegradable or less sustainable components.
Plant oils are inexpensive and widely available, and they serve as sustainable alternatives to petroleum-based paper coatings. Plant-oil-based coatings are biodegradable [20,21], nontoxic, and environmentally friendly [22,23]. Acrylated epoxidized soybean oil (AESO) has attracted significant attention because of its ability to form dense hydrophobic polymer networks upon ultraviolet (UV) curing. Ge et al. reported that UV-crosslinked AESO coatings significantly reduced the moisture absorption and permeability of starch-based films, achieving a substantial barrier enhancement even at a low coating thicknesses because of the high crosslinking density [24]. Vijayan et al. developed a bio-based moisture-barrier paper coating using acrylated linseed oil with beeswax as a hydrophobic additive. Beeswax improved the barrier and surface properties significantly, reducing the water vapor transition rate to 20 g/m2·day·atm and increasing the WCA to 111° at 10 wt.% while maintaining durability after abrasion. The coating exhibited good transparency, thermal stability, composite composability, and repulpability, highlighting its potential for sustainable food packaging [25]. Kumar et al. developed photocurable AESO emulsions incorporating degradable crosslinkers and chemically modified epoxidized soybean oil. This approach enhanced water and oil resistance after UV curing [15]. Zhong et al. prepared fluorine-free, oil- and water-resistant coatings on kraft paper using AESO and pentaerythritol tetrakis(mercaptoacetate) via a UV-initiated thiol-ene click reaction, followed by spray coating with a silica/AESO composite layer [26]. In addition, Kuo et al. reported a nontoxic, photocurable AESO-based coating system by employing curcumin as a natural photoinitiator and investigated the effect of co-initiators on crosslinking density and coating performance [27]. These studies confirm that polysaccharide-based coatings such as chitosan provide strong adhesion and functional properties while suffering from poor water resistance; AESO-based coatings exhibit excellent hydrophobicity while offering limited interfacial compatibility when applied directly to paper substrates. Thus, designing a dual-layer composite coating film made from a combination of chitosan and AESO is considered a promising approach for synergistically improving interfacial adhesion, mechanical properties, and the water and oil resistance of kraft paper.
In this study, a novel dual-layered coating system comprising a chitosan interlayer and UV-cured AESO is presented to improve the oil resistance and durability of kraft paper. The chitosan interlayer is utilized for strong interfacial adhesion between the chitosan and kraft paper via intermolecular interactions. The AESO topcoat is cured under UV light to create a hydrophobic polymer network. The proposed double-layered coating system comprises bioactive and bio-based materials from food-related and food waste-related sources, effectively regulating the surface energy and liquid permeability. The interplay between chitosan and AESO is utilized to enhance oil resistance, mechanical strength, and moisture resistance. This study presents a novel approach for regulating intermolecular interactions and the surface polarity to create green, high-performance paper materials for food packaging applications.

2. Results

2.1. Fourier Transform Infrared Spectroscopy

Figure 1a,b presents the Fourier transform infrared (FTIR) spectra of the uncoated and coated kraft paper samples. The FTIR spectra of the kraft paper showed strong hydrogen bonding within the fibrous structure as evidenced by OH stretching vibrations in the 3000–3600 cm−1 range [28,29]. For AESO, the characteristic peaks observed in the range of 1625–1660 cm−1 were attributed to the stretching vibration of the acrylate carbon–carbon double bond (C=C); furthermore, the peak at 800 cm−1 was attributed to the vinyl part of the acrylic acid [30]. The FTIR spectrum of the chitosan solution showed broad bands at 3300 and 2880 cm−1, which can be attributed to the overlapping O–H and C–H stretching vibrations of chitosan, respectively. The peaks at 1650 cm−1 (amide I) and 1590 cm−1 (amide II) are characteristic of chitosan, while the bands in the 1150–1000 cm−1 region correspond to the symmetric C-O-C stretching vibrations of the polysaccharide structure [31]. The Chi/kraft paper showed that the stretching vibration of the amide functional group can be attributed to the distinctive absorption band observed at 1590 cm−1 because of the structural similarity between cellulose and chitosan [32]. AESO/kraft paper indicated the disappearance of the acrylated C=C bonds and the vinyl groups of acrylic acid after UV irradiation, which confirmed the complete consumption of AESO during the curing process [33]. The FTIR spectrum of the Chi/AESO/kraft paper exhibited the characteristic features of chitosan and AESO. The disappearance of the acrylate C=C and vinyl peaks confirmed the successful UV-induced polymerization of AESO. The broad O–H/N–H band was retained with reduced intensity, suggesting strong intermolecular interactions (primarily hydrogen bonding) among the cellulose, chitosan, and cured AESO network. These interactions are essential for stabilizing the dual-layer structure and enhancing coating integrity. The coexistence of chitosan amide bands around 1650 and 1590 cm−1 and the ester carbonyl peak at 1730 cm−1 confirmed the formation of a stable composite chitosan–AESO coating on the kraft paper surface [33,34].

2.2. Appearance and Scanning Electron Microscopy

Figure 2a visually compares the surfaces of the uncoated and coated kraft paper samples. The uncoated kraft paper exhibited a typical matte and fibrous texture. After chitosan coating, the kraft paper surface became smoother and more uniform, indicating an effective film formation on the cellulose fibers. In addition, the kraft paper coated only with AESO showed improved surface gloss and roughness with increasing AESO concentration, thereby confirming the poor adhesion of the coating on the kraft paper. In comparison, the surfaces of the samples coated with the two-layer coating of Chi/AESO appeared smoother and more compact, demonstrating the contribution of the chitosan layer to the coating.
The surface structures of the uncoated and coated kraft papers and their cross-sectional structures are presented in Figure 2b–g. The uncoated kraft paper has a porous network of randomly oriented cellulose fibers (Figure 2b), while the chitosan-coated kraft paper has a smoother surface (Figure 2c), as confirmed by a thin compact film on the cross-section. The AESO10/kraft paper shown in Figure 2d has a higher coating coverage, while the AESO40/kraft paper shown in Figure 2e has a thicker film coverage, leading to a smoother surface. The Chi/AESO10/kraft paper shown in Figure 2f has small particles dispersed in the composite structure, as well as a layered cross-section. The Chi/AESO40/kraft paper shown in Figure 2g exhibits a continuous structure with minimal fiber exposure; additionally, small voids can also be seen inside the film coating. These findings prove that the coating composition affects the internal structure and surface of kraft paper.

2.3. Grammage, Thickness, Coating Weight, and Coating Thickness

Table 1 summarizes the structural properties of uncoated and coated kraft paper samples. The uncoated kraft paper exhibited the lowest grammage and thickness, which confirms the absence of a surface layer. Chitosan coating moderately increased grammage, total thickness, coating weight, and coating thickness, suggesting the formation of a uniform film. AESO-only coatings showed limited thickness at lower loadings, while higher AESO content resulted in increased coating weight and thickness. The Chi/AESO composite coatings displayed a gradual increase in all parameters with increasing AESO content, which reflects the effective coating deposition and layer build-up on the paper surface.

2.4. Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was performed to study the thermal stability and degradation characteristics of the uncoated and coated kraft paper samples, and the results are presented in Figure 3a,b and Table 2. The onset degradation temperature of the uncoated kraft paper sample was 270.03 °C, at which the sample lost 5% and 10% weight at 322.00 and 337.33 °C, respectively. The final degradation temperature occurred at 409.01 °C with a char yield of 10.49%. The Chi/kraft paper sample degraded at a lower onset temperature of 241.40 °C, indicating the early degradation of chitosan. The char yield increased significantly to 16.78% for the Chi/kraft paper sample. The AESO/kraft paper samples had higher end set degradation temperatures above 460 °C for both the AESO10/kraft and AESO20/kraft paper samples. This suggested that the polymer coating imparted higher high-temperature stability. The onset degradation temperature of the AESO/kraft paper samples was slightly lower than that of the neat kraft paper sample.
Among the AESO-coated samples, the char yield decreased with increasing AESO loading. However, the char yield decreased to 5.81 wt.% for the AESO40/kraft paper, which can be attributed to the increased proportion of aliphatic components in AESO. For the Chi/AESO/kraft papers with a double-layer configuration, temperatures for the start of degradation ranged between 218.58 and 236.97 °C. These samples exhibited higher char yields of ~13.76–15.97 wt.% compared to those of AESO/kraft paper, presenting a synergistic effect of the dual-layer structure in promoting char formation. Specifically, the char yield increased from 5.81 wt.% for AESO40/kraft paper to 13.76–15.97 wt.% for the Chi/AESO-coated samples (Table 2). The TGA results confirm that bio-based coatings slightly reduce the initial thermal stability of kraft paper, and the UV-cured AESO and dual-layer Chi/AESO systems significantly enhance the high-temperature resistance and char yield, which is beneficial for paper-based food packaging applications requiring improved thermal robustness.

2.5. Water Contact Angle

The WCA behaviors of the coated and uncoated kraft papers at 0 and 300 s are shown in Figure 4a. The hydrophilic cellulose surface of the uncoated kraft paper induced quick soaking, as indicated by the low starting WCA of 62.7°, which decreased to 35.7° in 300 s. The Chi/kraft paper indicated WCAs of 67.6° at 0 s and 42.1° at 300 s, suggesting that the chitosan layer was insufficient to prevent the spread of water. However, AESO/kraft paper showed significantly higher WCAs because of the development of a continuous UV-cured coating with decreased surface polarity, surpassing 96° at the beginning. The stability of the water droplet was enhanced by increasing the AESO concentration; after 300 s, the AESO40/kraft paper maintained a WCA of 90.1°. The most stable water-resistant behavior was demonstrated by the dual-layer Chi/AESO/kraft papers, especially Chi/AESO40/kraft paper, which maintained WCAs of 100.4° at 0 s and 92.3° at 300 s. This improved performance can be explained by the efficient surface coverage of the AESO layer and the limited water penetration of the underlaying chitosan layer, which produces long-lasting water resistance. A comparative bar diagram is shown in Figure 4b.

2.6. Air Permeability

Figure 4c reveals that the uncoated kraft paper showed high air permeability, with a value of ~380 mL min−1 because of the highly porous fibrous structure. However, the Chi/kraft paper decreased air permeability to nearly 140 mL min−1, filling up the pores to some extent. With the AESO/kraft paper, the values reduced further (from 230 to 90 mL min−1) depending on the concentration because of the compact layer developed after the UV-curing process. The dual-layer Chi/AESO/kraft papers demonstrated the lowest value of air permeability, which dropped below 60 mL min−1 to 10–20 mL min−1, indicating that the pores are properly sealed with a dense layer suitable for food packaging with the aim to improve barrier properties [35].

2.7. Water and Oil Resistance

Figure 5a shows the uncoated and coated kraft paper samples and the evaluated water resistance values reported as Cobb600 values. Untreated kraft paper showed a high Cobb600 value of ~83 g/m2. According to the Cobb600 values, an almost 50% reduction in water absorption was observed for the paper coated with the chitosan solution in comparison with the uncoated kraft paper. This reduction was attributed to the pores of the paper being covered by the coating, which impeded the passage of water to some extent [36]. AESO/kraft paper exhibited a greater reduction, with the water uptake lowered by nearly 40–70% based on the coating level, which reflects the formation of an effective hydrophobic barrier after UV curing. Dual-layer Chi/AESO/kraft papers displayed the strongest performance, with a reduction of ~70–80% compared to that of the uncoated kraft paper, which is ~16 g/m2. This pronounced decrease highlights the combined role of pore sealing achieved through the chitosan layer and surface energy reduction provided by the AESO coating in limiting water penetration, which is suitable for moisture-resistant food packaging applications.
The oil-resistance properties of the uncoated and coated kraft papers were tested using a kit test method. As shown in Figure 5b, the results confirm that the chi/kraft paper has better oil-resistant properties that that of the uncoated kraft paper, which has poor oil-resistant properties. The Chi/kraft paper has a high Kit rating value of ~11, which can be attributed to the filling effect of the chitosan polymer. The AESO/kraft papers have better oil-resistant properties. The AESO20/kraft paper has a Kit rating value of ~7, which can be attributed to the formation of a hydrophobic polymer layer. As depicted in the abovementioned results, the Chi/AESO/kraft papers have the maximum Kit rating value of 12. This can be attributed to the ability of coated kraft papers to prevent oil migration caused by the effective sealing properties of the chitosan polymer [26,33].
Figure 5c presents the digital photographs of French fries placed on the surfaces of uncoated and coated kraft papers for 30 min. Substantial oil absorption was observed on the surface of the uncoated kraft paper, which resulted in a clearly visible oil stain. In contrast, the AESO10/kraft paper exhibited only slight oil staining over a limited area. Both Chi/kraft and Chi/AESO/kraft papers showed no visible oil stains on either side of the paper after 30 min, indicating excellent resistance to oil penetration. This enhanced oil barrier performance can be attributed to the formation of a dense and continuous polymeric network formed by chitosan and AESO on the kraft paper surface, effectively inhibiting oil permeation from oil-rich foods such as French fries.

2.8. Tensile and Burst Strengths

Figure 6 shows the tensile strengths and indices for the uncoated and coated kraft paper materials. The lowest tensile strength and index were observed in the uncoated kraft paper material, as seen in Figure 6a; the tensile strength was ~33 MPa and the tensile index was ~45 kPa·m2g−1, indicating low interfiber bonding in the cellulose material. The tensile strength increased to ~37 MPa for the chitosan-coated paper samples, whereas the tensile index increased to ~55 kPa·m2g−1.
The AESO-coated kraft papers exhibited better tensile properties with increased AESO loading. The tensile strength increased from 31 MPa up to 37 MPa. The tensile index was normalized considering the grammage effect on the tests increased from 35 to 45 kPa·m2g−1. The normalized tensile index increased with tendencies similar to those of the tensile strength. This consistency between strength and index values confirms that the observed mechanical improvements are not solely due to thickness or grammage variations but primarily arise from coating-induced structural reinforcement. In other words, enhanced tensile properties can be attributed to better bonding. The combination of two papers with Chi and AESO layers, i.e., Chi/AESO-coated kraft papers, recorded a high tensile strength of 40 MPa and tensile index of 60 kPa·m2g−1 [37,38].
Figure 6b illustrates the burst strength and index of the uncoated and coated variants of the kraft papers. The graph indicated that the uncoated variant of the kraft paper had the lowest burst resistance; the burst strength and index were ~260 kPa and 3.2 kPa·m2g−1, respectively. The chitosan coating of the kraft paper resulted in a stronger matrix, and the burst strength and index were ~300 kPa and 3.6 kPa·m2g−1, respectively. The AESO coating samples exhibited improved burst resistance as the burst strength values increased with AESO incorporation, in the range of ~275–315 kPa, while the burst index values varied over the range of 3.0 to ~3.8 kPa·m2g−1. Furthermore, burst index measurements were conducted to normalize the effect of grammage on the paper properties, which indicated a close correlation with the trend observed over the burst strength measurements, validating the optimization effect on the structure. The correlation between burst strength and burst index trends further indicates that thickness effects are not the dominant factor governing the mechanical enhancement. The highest values measured were ~330 kPa as well as ~4.0 kPa·m2g−1, as observed for the double-layer Chi/AESO coating samples on the kraft paper. This enhancement improved the usability of the packaging material [39].

3. Discussion

The improved performance of Chi/AESO-coated kraft paper was attributed to the synergistic effect of its dual-layer coating system, which combined both adhesion and barrier properties. The FTIR spectra confirmed that strong hydrogen bonding occurred between cellulose, chitosan, and cured AESO, ensuring effective interfacial compatibility between hydrophilic kraft paper and hydrophobic AESO films. The presence of a chitosan interlayer facilitated a molecular bridge for effective and defect-free AESO film formation. Therefore, this compact and defect-free coating structure effectively sealed capillary paths, ensuring improved water resistance and hydrophobicity for the kraft paper, while the dense crosslinked AESO network provided excellent oil and grease resistance. These improved mechanical properties and thermal stability were attributed to the enhanced bonding between fibers and synergistic stabilization during thermal degradation processes. Thus, this study demonstrated a simple and effective fluorine-free and bio-based dual-layer coating technology that effectively eliminated the shortcomings of single-layer coatings for kraft paper food packaging materials.
The two-layer coating approach presented in this study aligns well with coating technologies commonly used in industrial applications. Standard methods in the paper industry, such as bar coating, blade coating, and spray coating, are appropriate for depositing chitosan-based layers. In addition, ultraviolet curing, which is widely implemented in industrial processes, is compatible with the AESO-based coating system. The simple processing and curing conditions employed indicate that the method can be adapted beyond laboratory-scale experiments. Although the coating system shows potential for industrial implementation, further optimization may be necessary for continuous production processes.

4. Materials and Methods

4.1. Materials

Chitosan, characterized by a degree of deacetylation ≥ 95% and a viscosity of 100–200 MPa (derived from shrimp shells), was provided by Aladdin Reagents Co., Ltd. (Shanghai, China). The AESO containing 4000 ppm of the monomethyl ether hydroquinone inhibitor was obtained from Sigma-Aldrich (St. Louis, MO, USA). 2-hydroxy-2-methylpropiophenone purchased from TCI Co., Ltd. (Tokyo, Japan) was used as the photoinitiator. The solvents and reagents including glacial acetic acid and ethyl acetate were supplied by Duksan Chemicals Co., Ltd. (Ansan, Republic of Korea). Kraft paper (100 g/m2) was obtained commercially in Korea, and distilled water was used for all preparations.

4.2. Preparation of the Chitosan Solution

A 2% (w/v) chitosan solution was prepared by dissolving 4 g of chitosan in 200 mL of 2% (v/v) aqueous acetic acid solution. The mixture was stirred continuously at 60 °C for 3 h using a magnetic stirrer to ensure complete dissolution and homogeneity.

4.3. Preparation of Acrylated Epoxidized Soybean Oil Solution

AESO solutions were prepared at concentrations of 10, 20, and 40% (w/v) by diluting AESO in ethyl acetate. The mixture was stirred at 30 °C for 15 min. Subsequently, a photoinitiator (4 wt.% based on the weight of AESO) was added to the solution, followed by additional stirring for 15 min to obtain a homogeneous coating.

4.4. Fabrication of Dual-Layer Coating on Kraft Paper (Chi/AESO/Kraft Paper)

Three types of coated papers are fabricated: chitosan single-layer (Chi), AESO single-layer, and chitosan/AESO dual-layer coated papers (Figure 7).
  • Single-layer Chi/kraft paper samples were prepared by applying the chitosan solution onto kraft paper using a bar-type automatic film coating apparatus (KIPAE E&T Co. Ltd., Hwasung, Republic of Korea) equipped with a rod bar (#44) at a coating speed of 3.0 mm/s, followed by drying in a convection oven at 60 °C for 12 h.
  • For the AESO/kraft paper single-layer samples, the AESO coating formulations were applied directly onto the kraft paper by brush coating to ensure uniform coverage. Subsequently, the coated samples were cured for 5 min using a UV-curing system (UVGO, China) equipped with a 1600 W UV LED lamp (365 nm) at a distance of 15 cm.
  • The Chi/AESO/kraft paper dual-layer coated papers were fabricated by applying the AESO coating solution onto the surface of the previously dried chitosan-coated paper using the same brush coating and UV-curing conditions. All coated samples were conditioned at 23 ± 1 °C and 50% ± 5% relative humidity for at least 24 h prior to characterization.
For clarity, the prepared samples were designated as follows: uncoated kraft paper (KP), chitosan-coated kraft paper (Chi), AESO-coated kraft paper with AESO concentrations of 10, 20, and 40 wt% represented as (AESO10, AESO20, and AESO40, respectively), and chitosan/AESO dual-layer coated kraft paper with AESO top-layer concentrations of 10, 20, and 40 wt% represented as (Chi/AESO10, Chi/AESO20, and Chi/AESO40, respectively).

4.5. Grammage and Thickness

The grammage of the samples was measured in accordance with TAPPI T 410. The paper was cut into 10 × 10 cm2 sections, and the weight of the paper was recorded before and after the coating. The grammage (g/m2) was calculated as
Grammage (g/m2) = weight (g)/area (m2)
where the weight and area are expressed in g and m2, respectively. The coating load (g/m2) was calculated using
Coating load (g/m2) = grammage (coated − uncoated)
The thickness of the sample (μm) was recorded using a digital thickness gauge (ID-C112XBS, Mitutoyo Co., Tokyo, Japan) at five different locations. The final reported values corresponded to the average of these measurements.

4.6. Scanning Electron Microscopy

The surface morphology of the cross-section and surface of the paper before and after coating was observed through a field emission scanning electron microscope (FE-SEM, JEOL JSM-IT800, Tokyo, Japan) operated at an accelerating voltage of 5 kV. Prior to the investigation, the sample was sputter-coated with a thin layer of gold to prevent charging.

4.7. Fourier Transform Infrared Spectroscopy

FTIR was performed on a spectrometer (Spectrum 3, PerkinElmer Inc., Waltham, MA, USA) in the attenuated total reflection mode in the range of 4000–400 cm−1 with 16 scans and a resolution of 4 cm−1.

4.8. Thermogravimetric Analysis

The TGA of the paper samples was performed using a thermogravimetric analyzer (TGA 4000, PerkinElmer Inc., Waltham, MA, USA) under a nitrogen atmosphere at a flow rate of 20 mL/min. A sample of ~10 mg was scanned between 30 and 700 °C at a heating rate of 10 °C/min.

4.9. Mechanical Test, Tensile Strength/Index, Burst Strength/Index

The tensile properties of the paper sample were measured using a universal testing machine (TA1, Lloyd Instruments Ltd., Bognor Regis, UK) in accordance with TAPPI T 494. The samples were precision cut into strips with a 20 mm width and length suitable for gauge length. The initial gauge length was set to 100 mm, and the crosshead speed was maintained at 12.5 mm/min. To normalize the effect of the weight, the tensile index (N∙m/g) was calculated as
Tensile Index (N∙m/g) = Tensile strength (N/m)/Grammage (g/m2)
The burst strength was determined in accordance with TAPPI T 403. The measurements were performed using a digital Mullen-type bursting tester (SJTM-003, Sejin Tech Co., Siheung, Republic of Korea) by applying increasing hydraulic pressure to the sample until rupture. To normalize the strength relative to the grammage, the burst index (kPa∙m2/g) was calculated as
Burst Index (kPa∙m2/g) = Burst strength (kPa)/Grammage (g/m2)

4.10. Air Permeability

The air permeability of the paper samples was measured using a Gurley densitometer (SJTM-018D, SAMJEE Tech Co., Seoul, Republic of Korea). All tests were performed in accordance with TAPPI T 460. A Gurley tester was used to measure the air resistance of the materials to determine the time taken by 100 mL of air to pass through 6.45 cm2 of the material. The results were recorded in seconds per 100 mL. Samples exceeding a measurement time of 3600 s (1 h) were designated as “impermeable”.

4.11. Water and Oil Resistance Test

The water resistance values of the paper samples were determined as Cobb600 values according to TAPPI T 441. The paper samples were cut into squares (12.5 × 12.5 cm2) and a specific area of the sample (100 cm2) was exposed to 100 mL of water for 600 s (5 min) using the Cobb test. The Cobb600 values were calculated as
Cobb value600 (g/m2) = (m2 − m1)/Area
where m1 and m2 represent the weights of the coated paper samples before and after contact with water, respectively.
The oil resistances of the papers were measured according to TAPPI T 559. Castor oil, toluene, and n-heptane were mixed in specific ratios to obtain a series of solutions with kit numbers ranging from 1 (least aggressive resistance to the coated surfaces) to 12 (strongest resistance to the coated surfaces). The ratio of toluene to n-heptane increased with an increase in the kit number of the solution, which resulted in reduced surface tension and greater penetration capability of the paper. A droplet of the solution was applied to the paper surface ~13 mm above the sample. After 15 s, the droplet was quickly cleaned with tissue paper, and the surface was examined for spots (normally darkened spots or regions). The solution (with the highest number) that did not stain the coated surface was reported as the “kit rating” for that sample.

4.12. Contact Angle

The WCAs of the materials were measured using a contact angle analyzer (SmartDrop, Femtobiomed Co. Seongnam, Republic of Korea). A single droplet of 3 μL distilled water was dropped on the surface of the paper sample, and the contact angle of the droplet was measured after 300 s (5 min) with an average of at least three measurements being performed on the same specimens at different spots.

4.13. Statistical Analysis

Data are presented as the mean ± standard deviation of at least three replicates. SPSS (version 24.0; Chicago, IL, USA) was used to analyze the data using one-way analysis of variance. Duncan’s multiple range test was used to determine significant differences in the mean values (p < 0.05).

5. Conclusions

In this study, a fluorine-free dual-layer coating based on chitosan and UV-cured AESO was applied successfully to kraft paper for improving its water and oil resistance. The development of a compact, crosslinked AESO layer on the paper surface, assisted by the chitosan interlayer, markedly enhanced the surface wettability and suppression of liquid penetration. The optimized Chi/AESO40/kraft paper exhibited stable WCAs of 100.4° at the initial contact and 92.3° after extended exposure, accompanied by a substantial reduction in the Cobb60 value from 83 to 16 g m−2 and the highest Kit rating of 12. Thermal analysis indicated increased thermal stability and char formation; the tensile strength increased from ~33 to 40 MPa, while the tensile index increased from 45 to 60 kPa·m2 g−1. The burst properties improved from 260 to 330 kPa and from 3.2 to 4.0 kPa·m2 g−1. In addition, food contact tests using French fries confirmed the effectiveness of the coating in limiting the oil migration. These results demonstrate that the developed coating strategy offers a practical pathway toward replacing conventional fluorinated barrier coatings. This bio-based fluorine-free coating system shows strong potential as a sustainable solution for high-performance paper-based food packaging.

Author Contributions

Conceptualization, M.L.; Methodology, M.L.; Investigation, M.L.; Formal analysis, M.L. and A.P.R.; Data curation, M.L. and A.P.R.; Validation, A.P.R. and Y.S.L.; Writing—Original draft, M.L. and A.P.R.; Writing—Review and editing, A.P.R. and Y.S.L.; Supervision, Y.S.L.; Resources, Y.S.L.; Project administration, Y.S.L.; Funding acquisition, Y.S.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by “Cooperative Research Program for Agriculture Science and Technology Development (RS2023-00231209)” Rural Development Administration, Republic of Korea.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author (please specify the reason for restriction, e.g., the data are not publicly available due to privacy or ethical restrictions).

Acknowledgments

The authors would like to thank SeoHee, Jang and SungHyun, Kang for their assistance through the experiment and data collection.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ChiChitosan solution
AESOAcrylated epoxidized soybean oil
UVUltraviolet
ZnOZinc oxide
PCLPolycaprolactone
FTIRFourier-transform infrared spectroscopy
SEMScanning Electron microscope
WCAWater contact angle
TGAThermogravimetric analysis

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Ijms 27 02210 g001
Figure 1. (a) FTIR spectra of the uncoated kraft, AESO, AESO10/kraft, AESO20/kraft, and AESO40/kraft papers. (b) FT-IR spectra of uncoated kraft, AESO, Chitosan solution, Chi/kraft, Chi/AESO10/kraft, Chi/AESO20/kraft, and Chi/AESO40/kraft papers.
Figure 1. (a) FTIR spectra of the uncoated kraft, AESO, AESO10/kraft, AESO20/kraft, and AESO40/kraft papers. (b) FT-IR spectra of uncoated kraft, AESO, Chitosan solution, Chi/kraft, Chi/AESO10/kraft, Chi/AESO20/kraft, and Chi/AESO40/kraft papers.
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Figure 2. (a) Visual appearance of the effect of chitosan and AESO coatings on the kraft paper surface (sample size: 2 cm × 2 cm). SEM micrographs and cross-section of the (b) kraft, (c) Chi/kraft, (d) AESO10/kraft, (e) AESO40/kraft, (f) Chi/AESO10/kraft, and (g) Chi/AESO40/kraft papers.
Figure 2. (a) Visual appearance of the effect of chitosan and AESO coatings on the kraft paper surface (sample size: 2 cm × 2 cm). SEM micrographs and cross-section of the (b) kraft, (c) Chi/kraft, (d) AESO10/kraft, (e) AESO40/kraft, (f) Chi/AESO10/kraft, and (g) Chi/AESO40/kraft papers.
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Figure 3. (a) TGA and (b) DTG analysis of kraft, Chi/kraft, AESO10/kraft, AESO20/kraft, AESO40/kraft, Chi/AESO10/kraft, Chi/AESO20/kraft, and Chi/AESO40/kraft papers.
Figure 3. (a) TGA and (b) DTG analysis of kraft, Chi/kraft, AESO10/kraft, AESO20/kraft, AESO40/kraft, Chi/AESO10/kraft, Chi/AESO20/kraft, and Chi/AESO40/kraft papers.
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Figure 4. (a) WCA images of uncoated and coated kraft paper samples measured at 0 and 300 s. The samples include pristine kraft, Chi/kraft, and AESO-coated kraft papers at different coating levels, and dual-layer Chi/AESO-coated kraft paper. The corresponding contact angle values are indicated in each image. (b) Comparative bar chart of the WCA values of the uncoated and coated kraft paper samples measured at 0 and 300 s. (c) Bar diagram illustrating the air permeation of the coated and uncoated kraft paper samples.
Figure 4. (a) WCA images of uncoated and coated kraft paper samples measured at 0 and 300 s. The samples include pristine kraft, Chi/kraft, and AESO-coated kraft papers at different coating levels, and dual-layer Chi/AESO-coated kraft paper. The corresponding contact angle values are indicated in each image. (b) Comparative bar chart of the WCA values of the uncoated and coated kraft paper samples measured at 0 and 300 s. (c) Bar diagram illustrating the air permeation of the coated and uncoated kraft paper samples.
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Figure 5. (a) Cobb600 values of uncoated and coated kraft paper samples. (b) Kit ratings of uncoated and coated kraft paper samples. (c) Digital photos showing the oil resistance of uncoated and coated kraft papers in a French fry test after 30 min.
Figure 5. (a) Cobb600 values of uncoated and coated kraft paper samples. (b) Kit ratings of uncoated and coated kraft paper samples. (c) Digital photos showing the oil resistance of uncoated and coated kraft papers in a French fry test after 30 min.
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Figure 6. (a) Tensile strength of uncoated and coated kraft paper samples. (b) Burst strength of uncoated and coated kraft paper samples. (Note—Total weight (W) and coating thickness (T)).
Figure 6. (a) Tensile strength of uncoated and coated kraft paper samples. (b) Burst strength of uncoated and coated kraft paper samples. (Note—Total weight (W) and coating thickness (T)).
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Figure 7. Schematic of dual-layer Chi/AESO coating on the kraft paper.
Figure 7. Schematic of dual-layer Chi/AESO coating on the kraft paper.
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Table 1. Structural characteristics of the control and coated paper samples, including grammage, total thickness, coating weight, and coating thickness.
Table 1. Structural characteristics of the control and coated paper samples, including grammage, total thickness, coating weight, and coating thickness.
SampleGrammage
(g/m2)		Total Thickness
(µm)		Coating Weight
(g/m2)		Coating Thickness
(µm)
Kraft paper78.93 ± 1.52104.8 ± 1.40--
Chi/kraft paper83.63 ± 1.11119.0 ± 3.274.70 ± 1.4014.20 ± 3.68
AESO10/kraft paper93.15 ± 1.80105.4 ± 0.9714.23 ± 1.630.60 ± 1.43
AESO20/kraft paper100.80 ± 2.99119.7 ± 5.7621.88 ± 4.2414.90 ± 3.14
AESO40/kraft paper124.00 ± 2.49134.2 ± 3.2745.08 ± 2.3429.40 ± 3.66
Chi/AESO10/kraft paper89.43 ± 0.78126.0 ± 2.5810.50 ± 1.2521.20 ± 2.90
Chi/AESO20/kraft paper90.45 ± 2.11134.2 ± 2.7411.53 ± 1.5729.40 ± 2.91
Chi/AESO40/kraft paper100.98 ± 4.56137.9 ± 3.7322.05 ± 3.4433.10 ± 3.14
Note: Grammage and coating weights were measured in quadruplicate (n = 4), while the total thickness and coating thickness were measured at ten random positions on each sample (n = 10). All results are reported as the mean ± standard deviation.
Table 2. Thermal decomposition temperatures and char yield of the kraft and coated paper samples.
Table 2. Thermal decomposition temperatures and char yield of the kraft and coated paper samples.
SampleTonset (°C)T5 (°C)T10 (°C)T20 (°C)Tendset (°C)Char Yield (wt.%)
Kraft paper270.03322.00337.33354.00409.0110.49
Chi/kraft paper241.40303.67323.33345.00416.2416.78
AESO10/kraft paper261.46318.00335.00353.33463.3310.06
AESO20/kraft paper245.92318.33336.00353.67470.6710.01
AESO40/kraft paper218.06318.33338.00356.00469.005.81
Chi/AESO10/kraft paper235.83298.67318.67340.00417.2415.57
Chi/AESO20/kraft paper236.97301.00322.33342.67423.4113.76
Chi/AESO40/kraft paper218.58290.67312.33335.00472.2015.97
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Lee, M.; Rananavare, A.P.; Lee, Y.S. Bio-Based Dual-Layer UV-Cured Oil- and Water-Resistant Paper Coating for Food Packaging Applications. Int. J. Mol. Sci. 2026, 27, 2210. https://doi.org/10.3390/ijms27052210

AMA Style

Lee M, Rananavare AP, Lee YS. Bio-Based Dual-Layer UV-Cured Oil- and Water-Resistant Paper Coating for Food Packaging Applications. International Journal of Molecular Sciences. 2026; 27(5):2210. https://doi.org/10.3390/ijms27052210

Chicago/Turabian Style

Lee, Myungho, Anuja P. Rananavare, and Youn Suk Lee. 2026. "Bio-Based Dual-Layer UV-Cured Oil- and Water-Resistant Paper Coating for Food Packaging Applications" International Journal of Molecular Sciences 27, no. 5: 2210. https://doi.org/10.3390/ijms27052210

APA Style

Lee, M., Rananavare, A. P., & Lee, Y. S. (2026). Bio-Based Dual-Layer UV-Cured Oil- and Water-Resistant Paper Coating for Food Packaging Applications. International Journal of Molecular Sciences, 27(5), 2210. https://doi.org/10.3390/ijms27052210

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