Biopolymer Paperboard Impregnation Based on Chitosan and Nanocellulose with Addition of Caffeine and Gallic Acid

Abstract

In this study, the preparation and detailed characterization of a chitosan (CHT) impregnation system modified with cellulose nanofibrils (CNFs) and enriched with bioactive compounds—caffeine (CAF) and gallic acid (GA)—applied to the surface of unbleached paperboard were described. Their mechanical properties (tensile strength, elongation at break, and bursting strength), structural features, and surface barrier parameters (water absorption) were evaluated. The antibacterial activity of the formulations comprising 1% chitosan (1% CHT), 1% chitosan with 1% caffeine (1% CHT/1% CAF), and 1% chitosan with 1% gallic acid (1% CHT/1% GA)—applied to enhance the functionality of the coated paperboard—was additionally assessed. The incorporation of cellulose nanofibrils into the coating matrix markedly improved the mechanical performance of the paperboard, particularly in terms of puncture resistance and elongation at break, while all modified coatings retained high burst strength. Impregnations containing gallic acid or caffeine showed similar mechanical characteristics but improved flexibility without compromising structural integrity. Chitosan solutions containing gallic acid and solutions containing caffeine exhibited activity against the tested Gram-positive (S. aureus, L. monocytogenes) and Gram-negative (E. coli, P. aeruginosa) bacterial strains. Antibacterial analysis showed moderate activity against Gram-positive strains and strong inhibition of Gram-negative bacteria, with the 1% CHT/1% GA impregnation giving the largest zone of growth inhibition around the sample—19 mm in the agar diffusion test—indicating the strongest suppression of E. coli. It was found that incorporation of nanocellulose into the chitosan matrix significantly reduces water uptake by treated paperboard surface, which is critical in the context of food packaging. The best result—Cobb₆₀ value of 32.85 g/m²—was achieved for the 1% CHT/1% CNF formulation, corresponding to an 87% reduction in water absorption compared to the uncoated control. The results obtained in this study indicate a promising potential for the use of these impregnation systems in sustainable packaging applications.

1. Introduction

Faced with the escalating environmental degradation resulting from the increasing use of plastic packaging, and in light of new legal regulations aimed at reducing plastic waste, efforts are underway to develop fully eco-friendly materials that can replace conventional plastic solutions. In the quest to substitute plastic products made from petroleum, scientists are continuously working on creating green food packaging. This term carries a dual meaning: on one hand, it signifies packaging that is environmentally friendly (non-polluting), harmless to both ecosystems and human health, and recyclable; on the other, it implies an energy-saving approach that excludes petroleum-based polymers in favor of sustainable development [1]. In addition, paper-based materials are emerging as a compelling substitute for plastic in the packaging sector. Their lightweight nature, biodegradability, scalability in production, safety, and non-toxicity make them attractive alternatives. As a result, these materials now account for over 40% of the overall usage among traditional packaging options, which include paper, plastic, glass, and metal [2]. Although eco-friendly packaging materials made from paper provide a green alternative, their lack of resistance to water and oil contact severely restricts their range of applications [3,4,5].

Chitosan is a polysaccharide typically produced by the alkaline N-deacetylation of chitin. It comprises β-(1→4) linkages connecting N–acetyl–D–glucosamine and D–glucosamine units, and its versatile properties arise from nucleophilic amino (–NH₂) and hydroxyl (–OH) groups capable of forming interactions and undergoing reactions under appropriate conditions. Chitosan, owing to its polycationic character and its ability to form three-dimensional, cross-linked hydrogels, has attracted considerable interest for applications in drug-delivery systems [6], water treatment technologies [7], and as a matrix in biodegradable natural composite materials [8].

Caffeine (1,3,7-trimethylxanthine) is a natural purine alkaloid found in various plants, such as coffee beans (Coffea arabica), tea leaves (Camellia sinensis), cocoa beans, and kola nuts [9]. It belongs to the group of the most commonly consumed substances with a stimulating effect on the nervous system [10]. Due to its long-term and widespread use, the pharmacological and safety profile of caffeine is well established, which allows for its broad application in both consumer and therapeutic products [11]. Numerous studies have shown that caffeine exhibits antimicrobial activity, particularly against selected strains of Gram-positive bacteria (e.g., Staphylococcus aureus, Bacillus cereus) and Gram-negative bacteria (e.g., Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae) [12,13]. These properties make caffeine a promising additive in the design of bioactive materials for applications in food technology, pharmacy, and cosmetology [14,15].

Gallic acid (3,4,5-trihydroxybenzoic acid) is a naturally occurring phenolic compound found in numerous plants such as gallnuts, tea leaves (C. sinensis), sumac, and grapes [16]. It exhibits well-documented antioxidant, anti-inflammatory, cytotoxic, and antimicrobial properties [17,18]. Its efficacy against foodborne and clinical pathogens, including S. aureus, E. coli, and various Salmonella strains, has been demonstrated in both free and nanoencapsulated forms [19,20,21]. Recent studies highlight its ability to enhance the barrier, mechanical, and antimicrobial properties of bio-based materials such as chitosan films, suggesting its applicability in active and biodegradable food packaging [22,23]. Moreover, due to its strong radical-scavenging ability and environmental sensitivity, gallic acid is being explored as a natural indicator for intelligent packaging systems [24]. Its biocompatibility and renewability make it a promising candidate for food, pharmaceutical, and cosmetic applications [25].

Nanocellulose is a nanostructured material obtained from cellulose—the most abundant organic polymer on Earth [26]. As a renewable and biodegradable biopolymer, cellulose is widely available from various natural sources, including wood, agricultural residues, grasses, and certain algae [27]. Nanocellulose is generally classified into three main types: cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial cellulose (BC) [28]. CNFs consist of long, entangled fibrillar structures, whereas CNCs comprise short, rod-like particles with a high degree of crystallinity. Both forms exhibit outstanding mechanical properties, such as high tensile strength, low density, and remarkable flexibility [29]. These characteristics make nanocellulose particularly advantageous for advanced applications in diverse industrial sectors. In the packaging industry, these attributes are especially relevant, as nanocellulose can significantly enhance the mechanical durability, lightweight design, and barrier performance of sustainable packaging materials [30].

In this work, we focus on unbleached eco-kraft paperboard as the substrate. This material is renewable, recyclable, and widely used in food-packaging supply chains; however, its porous fiber network leads to high water/oil wettability and limited wet strength, constraining moisture-sensitive and direct food-contact applications [2,31]. Prior strategies to mitigate these limitations include internal/external sizing (alkyl ketene dimer, AKD; alkenyl succinic anhydride, ASA), waxes/resins, thermoplastic polymer barriers (polyethylene, PE; ethylene–vinyl alcohol copolymer, EVOH), and bio-based systems based on chitosan and/or nanocellulose. Multilayer constructs have also been explored. Each route presents trade-offs between biodegradability, adhesion to porous substrates, oil/water resistance, and process complexity [2,31,32,33,34,35]. Recent reviews and reports also document chitosan-based bioactive films with natural additives (e.g., caffeine) for food-packaging applications [36,37,38]. Building on this context, we evaluate an impregnation-type treatment with varying nanocellulose contents and the co-incorporation of caffeine and gallic acid, examining their effects on the mechanical, physical, structural, and potential antibacterial properties of paperboard. As a novel formulation, we co-load caffeine and gallic acid into a chitosan–cellulose nanofibril matrix to enhance multifunctionality. Therefore, the study supports the transition from single-use plastic packaging to more sustainable solutions and explores the potential of these systems as ecological, bioactive candidates for active food-packaging applications.

2. Materials and Methods

2.1. Materials

Chitosan of crab-shell origin (flakes, 86.0% deacetylated, Poly(D-glucosamine)); Molecular Weight: 250–300 kDa; Viscosity: 1200 cPs was obtained from ChitoLytic (Toronto, ON, Canada), and acetic acid was purchased from Avantor Performance Materials (Gliwice, Poland). Nanocellulose fibrils 3.3 w/v % in water were acquired from Cellulose Lab (Fredericton, NB, Canada), while caffeine and gallic acid monohydrate were purchased from Sigma-Aldrich (Darmstadt, Germany). The substrate for impregnation trials was unbleached eco-craft paperboard with a basis weight of 300 g/m² and a thickness of 0.338 mm. All aqueous solutions were prepared with deionized water (maximum conductivity 0.1 µS/cm) purified using a Milli-Q Ultrapure Water System (Merck Millipore, Burlington, MA, USA).

2.2. Preparation of Paperboard Impregnation Formulations

Chitosan was dissolved in 3% (v/v) acetic acid and homogenized at 200 rpm for 30 min using an Ika laboratory homogenizer (IKA Werke GmbH & Co. KG, Staufen, Germany). Caffeine and gallic acid monohydrate were also dissolved in 3% (v/v) acetic acid and added to the chitosan solution to reach a final concentration of 1% (w/v) of chitosan, caffeine, and gallic acid. The mixture was stirred for 30 min at room temperature. Finally, nanocellulose fibrils were incorporated into this chitosan–caffeine/chitosan–gallic acid mixture to obtain CNF loadings of 1.0 and 2.0% (w/v). The symbols and compositions of all impregnation formulations are presented in

Table 1. Symbols of formulation samples.

Samples	1% Chitosan	1% CNFs	2% CNFs	1% Caffeine	1% Gallic Acid
Control	—	—	—	—	—
1% CHT	✓	—	—	—	—
1% CHT/1% CNF	✓	✓	—	—	—
1% CHT/1% CNF/1% CAF	✓	✓	—	✓	—
1% CHT/1% CNF/1% GA	✓	✓	—	—	✓
1% CHT/2% CNF	✓	—	✓	—	—
1% CHT/2% CNF/1% CAF	✓	—	✓	✓	—
1% CHT/2% CNF/1% GA	✓	—	✓	—	✓

✓ = component present; — = not present. Concentrations are w/v (%). Abbreviations: CHT, chitosan; CNFs, cellulose nanofibrils; CAF, caffeine; GA, gallic acid.

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2.3. Preparation of Impregnated Paperboard

Unbleached A4 paperboard was single-sided coated with the prepared formulations using a hand-coater wire rod (24 µm, model KHC.02. S, RK Print Coat Instruments Ltd., Litlington, UK); after 1 h, the coating pass was repeated. Subsequently, the specimens were air-dried under ambient laboratory conditions for 24 h.

2.4. Antibacterial Activities

The antibacterial activity of the components used for the preparation of paperboard formulations (at a concentration of 10 mg/mL) was evaluated using the agar diffusion method. The assay was performed against selected bacterial strains relevant from a microbiological perspective, including both pathogenic and opportunistic species: Listeria monocytogenes (ATCC 19111), Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 10536), and Pseudomonas aeruginosa (ATCC 15443). Mueller–Hinton Agar plates were inoculated with standardized bacterial suspensions at a concentration of 10⁶ CFU/mL. A volume of 10 µL of each tested solution was applied to the agar surface. The prepared plates were then incubated for 24 h at 37 ± 2 °C. After incubation, the diameters of the inhibition zones were measured in millimeters using an Easy Count2 automatic colony counter (AES Laboratoire, CHEMUNEX group, Paris, France) with software for measuring inhibition zones (MultiScaneBase v14.02).

2.5. Mechanical Properties of Impregnated Paperboard

2.5.1. Tensile Strengths and Elongations at Break

The mechanical properties of uncoated and coated paperboard were evaluated using a universal testing machine (Instron, Model 5965, Instron, Norwood, MA, USA). Rectangular test specimens measuring 15 mm × 100 mm were prepared. For each sample, at least four random measurements were performed, and the mean values were calculated. Tensile strength (TS) and elongation at break (EB) were determined based on the obtained data.

2.5.2. Puncture Resistance and Burst Strength

Puncture resistance was evaluated in accordance with SIST 14477:2004 [39] using a universal testing machine (Zwick GmbH & Co.KG, 5 kN model Z005 TN ProLine, Ulm, Germany). Individual paperboard samples were positioned between the clamping rings of the test apparatus. A plunger with a diameter of 50 mm was lowered onto the sample and advanced at a constant speed until complete penetration occurred. The following parameters were recorded: puncture force (N) and puncture elongation (mm).

Burst strength was assessed according to ISO 2759:2014 [40] and expressed in kilopascals (kPa). The test measures the maximum pressure generated by a hydraulic system that forces a flexible rubber diaphragm—rigidly clamped along the sample edges—against the paperboard until rupture occurs.

2.6. Water Absorption Value

The water absorption capacity of the paperboard samples was evaluated based on Cobb₆₀ values, in accordance with ISO 20535:2022 [41]. A Cobb absorbency tester (Regmed, Osasco, Brazil) was used to expose the samples to 100 mL of deionized water for 60 s at a temperature of 20 ± 1 °C. Each measurement was performed in triplicate, and the average value was reported.

2.7. Infrared Spectroscopy

Infrared spectra of the paperboard samples were recorded using a Nicolet iS5 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an attenuated total reflectance (ATR) accessory. The ATR-FTIR spectra were collected in the range of 4000–600 cm⁻¹ with a spectral resolution of 4 cm⁻¹, averaging 32 scans per sample.

2.8. Scanning Electron Microscopy

The surface morphology of the tested materials was examined using a scanning electron microscope (EVO 10, Carl Zeiss AG, Oberkochen, Germany) equipped with a secondary electron detector. Imaging was performed in beam mode at an accelerating voltage of 30 kV.

2.9. Statistical Analysis

Statistical comparisons were carried out using one-way analysis of variance (ANOVA), followed by Tukey’s post hoc HSD test at a significance level of p < 0.05. All analyses were performed using Statistica software, version 13.3 (TIBCO Software Inc., Palo Alto, CA, USA).

3. Results and Discussion

3.1. Biological Performance of the Solution

The initial stage of the study focused on evaluating the properties of selected compounds recognized in the literature as potential bioactive agents. Three formulations were developed for this purpose: 1% CHT, 1% CHT/1% CAF, and 1% CHT/1% GA. The antibacterial efficacy of these solutions was assessed based on their inhibitory effects against representative bacterial strains.

Antibacterial Activities

The antibacterial properties of the examined compounds were tested on representative Gram-positive and Gram-negative strains. The extent of bacterial growth inhibition was evaluated by measuring the diameter of the inhibition zones, with the results presented in

Table 2. The antibacterial activity of paperboard coating constituents.

Bacterial Strains	1% CHT	1% CHT/1% CAF	1% CHT/1% GA
	Inhibition Zone (mm)
Gram-positive bacteria
L. monocytogenes	12	15	13
S. aureus	12	14	13
Gram-negative bacteria
E. coli	12	17	19
P. aeruginosa	12	17	17

Antibacterial activity was classified based on inhibition zone diameters: weak (5–10 mm), moderate (11–14 mm), and strong (>14 mm).

. Regarding the Gram-positive bacterial strains, L. monocytogenes and S. aureus exhibited moderate susceptibility to chitosan (1% CHT), with inhibition zones measuring 12 mm. A slight increase in antibacterial efficacy was observed upon the incorporation of caffeine (CAF) or gallic acid (GA), with the 1% CHT/1% CAF formulation demonstrating the highest activity—15 mm against L. monocytogenes and 14 mm against S. aureus. A more pronounced antibacterial effect was observed for Gram-negative bacteria. In the case of E. coli, the inclusion of GA led to a marked enhancement of activity, resulting in an inhibition zone of 19 mm, indicative of strong antibacterial potential. Furthermore, both 1% CHT/1% CAF and 1% CHT/1% GA formulations exerted superior effects against P. aeruginosa (17 mm) compared to pure chitosan (12 mm). The 12–19 mm range obtained here aligns well with previous reports on comparable biopolymer systems. Stefanowska et al. documented a 1% chitosan film containing 1% caffeine that produced a 19 mm inhibition zone against P. aeruginosa [36]. Similarly, Yu et al. observed that GA-loaded chitosan hydrogels generated inhibition zones of 21 mm for E. coli and 16 mm for S. aureus [42]. Incorporating gallic acid into chitosan films is reported to enhance antimicrobial activity in a dose-dependent manner, showing the strongest effects against Gram-positive bacteria such as Bacillus subtilis and Listeria innocua [37]. These values confirm that incorporating caffeine or gallic acid into a chitosan matrix effectively broadens the antimicrobial spectrum and enhances potency, particularly against Gram-negative bacteria. Our results align with Stefanowska et al. [36], who reported ≈19 mm for P. aeruginosa with a 1% chitosan/1% caffeine film, and with Yu et al. [42], who found 21 mm (E. coli) and 16 mm (S. aureus) for GA-loaded chitosan hydrogels.

3.2. Mechanical Parameters

3.2.1. Tensile Strengths and Elongations at Break

The impregnating treatment reduces apparent surface porosity and bridges adjacent fibers, which rationalizes the changes observed in TS/EB and Cobb₆₀. The paperboard treated with the addition of chitosan, nanocellulose, and bioactive compounds was characterized by varied mechanical parameters, namely, tensile strength and elongation at break, as presented in Figure 1. The uncoated sample (control) exhibited the highest TS value (≈85 MPa). Coating the paperboard with chitosan and nanocellulose—either alone or combined with gallic acid (GA) or caffeine (CAF)—lowered the tensile strength relative to the uncoated control. No significant differences were detected among the CNF-containing treatments, whereas the 1% CHT film maintained a slightly higher tensile strength than all CNF variants, indicating that neither CNF concentration nor the addition of GA or CAF improved tensile strength within the tested formulations. This outcome can be explained by the inability of thin coatings and their penetration into the porous substrate to form a continuous CNF network for stress transfer. In addition, CNF aggregation, charge neutralization, or interactions between GA and CAF with chitosan may reduce network continuity or stiffness, offsetting potential reinforcement. Similar outcomes were reported by Costa et al. [38], who observed a significant increase in TS only when 50 wt% of cellulose nanocrystals was incorporated into the chitosan matrix in films. The concentration range used in the present study (1–2% CNFs) may therefore have been insufficient to induce reinforcing effects. Talebi et al. likewise reported that improvements in mechanical performance became evident in films only at CNF contents above 5–7% [43].

Regarding elongation at break (EB), chitosan and CNFs showed a favorable influence. Most modified samples exhibited higher EB values than the control, with the highest averages (~16.5%) observed for paperboards coated with 1% CHT/2% CNF and 1% CHT/2% CNF/1% CAF formulations (Figure 1b). These findings suggest that CNFs may improve material elasticity, likely by promoting a more entangled internal network within the chitosan matrix. Interestingly, while GA- and CAF-containing formulations demonstrated comparable EB performance to coatings with CNF alone, no improvement in TS was noted. Previous studies indicate that polyphenol–polymer interactions may enhance flexibility through intensified hydrogen bonding, without necessarily improving strength. Effective reinforcement of chitosan- or carboxymethyl-cellulose (CMC) composites occurs only when a well-dispersed nanofiller exceeds its percolation threshold. Below this level, the filler behaves chiefly as a plasticizer, increasing ductility without enhancing strength. Conversely, excessive loading promotes agglomeration and ultimately reduces tensile strength and/or elongation at break [44]. In TPS/BNC films, incorporation of gallic acid slightly increased tensile strength, most notably at 5–10 wt % BNC [45]. Polyphenolic additives such as gallic acid and caffeine can further enhance the flexibility of films by introducing additional hydrogen bonding. Zarandona et al. demonstrated that incorporating gallic acid into chitosan films produced a statistically significant increase in both tensile strength and Young’s modulus relative to the pure chitosan [46].

3.2.2. Puncture Resistance and Burst Strength

The mechanical properties of uncoated and treated paperboard samples were assessed in terms of puncture force, puncture elongation, and burst strength, as summarized in

Table 3. Mechanical properties of treated and uncoated paperboard samples.

Type of Coating	Puncture Force [N]	Puncture Elongation [mm]	Burst Strength [kPa]
Control	27.98 a, b ± 2.15	2.78 a, b ± 0.10	369.77 b ± 28.17
1% CHT	26.05 b ± 2.19	2.75 a, b ± 0.13	466.87 a ± 22.42
1% CHT/ 1% CNF	30.78 a ± 1.42	2.88 a, b ± 0.05	431.14 a, b ± 33.31
1% CHT/ 1% CNF/ 1% CAF	30.35 a, b ± 1.79	2.88 a, b ± 0.10	417.93 a, b ± 41.36
1% CHT/ 1% CNF/ 1% GA	25.88 b ± 1.64	2.70 b ± 0.08	397.34 a, b ± 16.94
1% CHT/ 2% CNF	29.88 a, b ± 1.03	2.98 a ± 0.10	440.07 a, b ± 28.17
1% CHT/ 2% CNF/ 1% CAF	29.08 a, b ± 0.99	2.85 a, b ± 0.13	438.37 a, b ± 49.60
1% CHT/ 2% CNF/ 1% GA	26.78 a, b ± 3.23	2.83 a, b ± 0.19	425.83 a, b ± 16.60

Values are mean ± SD (n = 4). Within each column, identical superscript letters indicate no statistically significant differences between means (p > 0.05), as determined by Tukey’s HSD post hoc test. a—means belonging to homogeneous group a; b—means belonging to homogeneous group b.

. The control sample exhibited a puncture force of 27.98 N and a burst strength of 369.77 kPa. Coating the paperboard with 1% chitosan led to a notable increase in the paperboard’s burst strength to 466.87 kPa, while changes in puncture resistance were minimal. Paperboard coated with cellulose nanofibrils demonstrated further enhancements in mechanical performance. The paperboard treated with 1 wt% chitosan and 1 wt% CNF exhibited the highest puncture force (30.78 N). Samples containing 2 wt% CNF showed the greatest puncture elongation (up to 2.98 mm) while retaining high burst strength (e.g., 440.07 kPa for the 1% CHT/2% CNF treatment). The incorporation of bioactive additives—caffeine (CAF) and gallic acid (GA)—had only a limited effect on the mechanical performance. Although slight differences were observed in the puncture force and fracture strength of the tested paperboards, the paperboard coated with 1% CHT/1% CNF/1% GA had a lower puncture force (25.88 N); however, these differences were not statistically significant compared to the paperboards treated with the remaining tested formulations (except for the paperboards coated with 1% CHT/1% CNF/1% GA). Importantly, all modified paperboards preserved high burst strength, exceeding 397 kPa. According to Priyadarshi and Negi, chitosan films demonstrate enhanced mechanical performance due to the reinforcing effect of nanoscale materials within the polymer matrix, achieved through the incorporation of nanoparticles [47].

3.3. Water Absorption Performance

The water absorption capacity of the paperboard samples was evaluated using the Cobb₆₀ method. Previous studies have consistently shown that chitosan–CNF impregnations improve the barrier properties of porous cellulose-based substrates [31]. As illustrated in Figure 2, the uncoated control group exhibited the highest water uptake, with a mean Cobb₆₀ value of 254.5 g/m². In contrast, all treated formulations significantly reduced the water absorption of paperboards. Although the paperboard treated with 1% CHT/ 1% CNF showed the lowest Cobb₆₀ value (32.85 g m⁻²; 87.1% lower than the control), this reduction was not statistically different from the values obtained for the other paperboards coated with chitosan-based formulations. Hassan et al. likewise reported that CNF films alone lowered the water absorption value by ~33 %, while a CNF layer containing chitosan nanoparticles achieved a similar ~31 % decrease, underscoring the effectiveness of nanocellulose-based barriers [34]. Similarly, paperboard treated with 1% CHT and 2% CNF, or with additional active compounds, maintained Cobb₆₀ values below 45 g/m², reflecting water uptake reductions of between 82.2% and 84.9%. The sample coated with chitosan alone (1% CHT) exhibited a moderate reduction, reaching 66.1 g/m², which still represents a 74.0% reduction in water absorption compared to uncoated paperboard. This finding is consistent with data in the literature, which indicate that the incorporation of chitosan into coatings based on xylan derivatives enables a significant reduction in water absorption, reaching approximately 25% [35]. Similarly, data in the literature indicate that incorporating cellulose nanofibrils into chitosan- and alkyl ketene dimer (AKD)-based multilayer coatings can result in a significant improvement in water absorption, with Cobb₆₀ values reaching as low as 5.7 g/m² [32]. Interestingly, increasing the CNF content from 1% to 2% did not result in a further decrease in Cobb₆₀ values, suggesting that the water barrier effect of CNF may reach a saturation point at lower concentrations. This improvement in water resistance is attributed to the strong intermolecular interactions between chitosan and CNF, which promote the formation of a compact and cohesive film structure. Additionally, the polycationic nature of chitosan enhances electrostatic interactions with the negatively charged cellulose surface, improving adhesion and limiting water penetration. Statistical analysis showed that the Cobb₆₀ values of the uncoated (control) paperboard were significantly higher than those of every treated sample. Moreover, the paperboard treated with chitosan alone (1 wt% CHT) differed significantly from the more complex coatings containing CNF and/or bioactive additives, whereas no statistically significant differences were detected among those CNF-containing/bioactive formulations themselves. These results confirm that the developed coating systems—particularly those containing CNFs—markedly improve the water barrier performance of the paperboard.

3.4. Structural Characteristic

3.4.1. Infrared Spectroscopy

FTIR spectra recorded for uncoated paperboard and for samples after different treatments are shown in Figure 3. These spectra revealed characteristic absorption bands associated with the functional groups of chitosan and nanocellulose. In all treated samples, broad bands in the range of 3290 cm⁻¹ were detected, corresponding to the stretching vibrations of amine (N–H) and hydroxyl (O–H) groups—typical for both chitosan and cellulose-based materials [33,47,48]. The treated samples also exhibited absorption bands associated with the polysaccharide backbone, including those at 1160–1260 cm⁻¹ (C–O, C–O–C symmetric stretching, and C–N stretching) [38]. Additionally, bands were observed near 1550 cm⁻¹ (amide II, N–H bending) and 1640 cm⁻¹ (amide I, C=O stretching), confirming the presence of amide groups derived from chitosan [49]. Minor shifts in band positions and intensities were noted between the bioactive-enriched coatings (CAF, GA) and the control chitosan–CNF films. These changes may suggest hydrogen bonding or other non-covalent interactions between the added active compounds and the polymer matrix [18]. Nonetheless, no new signals were detected, indicating that the primary structure of the polysaccharide backbone remained unaffected [50].

3.4.2. Scanning Electron Microscopy

Scanning electron microscopy (SEM) was employed to investigate the surface morphology of uncoated paperboard and samples functionalized with chitosan and 1 wt % cellulose nanofibers, with or without the incorporation of 1 wt % caffeine or gallic acid, as depicted in Figure 4. The uncoated control exhibited a characteristic cellulose fiber network, consisting of randomly oriented, loosely packed fibers with wide inter-fiber voids and pronounced surface roughness. Application of a 1 wt % chitosan treatment led to the formation of a thin, continuous polymer layer that partially covered the cellulose fibers. This treatment reduced the apparent porosity and bridged adjacent fibers; however, some residual voids remained, indicating incomplete sealing of the substrate. SEM micrographs of these variants (Figure 4B−E) enable a direct assessment of how each additive influences the surface microarchitecture of the paperboard. The addition of 1 wt % cellulose nanofibrils to the chitosan matrix resulted in a significantly denser and more uniform paperboard surface morphology. A similar trend was reported by Ruberto et al., who found that slot-die-coated CNC/chitosan layers produced an even smoother, well-adhered, and uniformly dried coating across the paper surface [48]. The nanofibrils occupied microvoids and created an interpenetrating fibrous mesh, effectively sealing the surface and substantially reducing porosity. This observation is consistent with the markedly reduced Cobb₆₀ values measured for the paperboard coated with these formulations. The addition of 1 wt % caffeine or gallic acid produced a homogenous and defect-free paperboard surface. The micrographs further indicate that the gallic acid-containing coatings look unchanged, confirming that this substance did not aggregate during solvent evaporation. These findings are consistent with the earlier observations made for thermoplastic starch (TPS) films incorporating GA, as described by Almeida et al. [45]. Sun et al. likewise reported—on the basis of SEM micrographs—that glycerol-plasticized chitosan films tolerate up to 1 wt % gallic acid without disrupting their continuous, pore-free morphology, whereas higher loadings create voids that weaken the coating’s barrier performance [37]. The absence of visible aggregates suggests that caffeine and gallic acid were well dispersed within the chitosan–CNF network and successfully embedded in the polymer matrix without disturbing structural coherence. SEM imaging confirmed that the chitosan–CNF system forms an effective barrier layer by sealing the porous structure of the paper substrate. Similarly, SEM analysis of AKD-CNF-chitosan coatings reported in recent studies confirms that chitosan contributes to pore sealing due to its excellent film-forming ability, leading to a smoother and more compact surface morphology [32]. These results establish a clear laboratory-scale baseline for unbleached eco-kraft paperboard impregnated with 1% chitosan and 1–2% CNF (fixed GA/CAF levels) under a single, well-controlled set of processing parameters, enabling unambiguous structure–property insights. The deliberately focused scope provides practical guidance for formulation selection and process tuning. Building on this foundation, future work will extend to broader substrate grades and composition ranges, assess long-term and additional barrier performances, and validate scalability under pilot-scale conditions.

4. Conclusions

The tested paperboard-treatment formulations, based on chitosan reinforced with cellulose nanofibrils and enriched with caffeine and gallic acid, exhibited improved functional properties, particularly with respect to their use as eco-friendly, paper-based packaging materials. The addition of CNF contributed to increased puncture resistance and flexibility, and all modified paperboards demonstrated a significant reduction in water absorption. The most effective barrier performance was observed for the 1% CHT/1% CNF formulation, which achieved the lowest Cobb₆₀ value. The analytical approach applied in this study does not fully exhaust the possibilities for evaluating bioactive coating systems. Further research is required to determine the optimal ratios of chitosan, CNFs, and bioactive additives to maximize mechanical properties. It can be assumed that the effectiveness of such systems depends not only on the type of additives but also on their interactions with the paperboard matrix. Based on the obtained results, the proposed impregnations show strong potential as biodegradable alternatives to conventional packaging materials.