Strength and Moisture-Related Properties of Filter Paper Coated with Nanocellulose

Abstract

The aim of this study was to assess selected properties of coatings incorporating nanocellulose, with the potential of being applied as a surface modification for cellulosic and lignocellulosic materials, particularly for applications within biodegradable packaging. Cellulose nanocrystal (CNC) and cellulose nanofibril (CNF) coatings were produced and applied on both sides of pure cellulose samples in the form of filter paper with a Mayer bar. Selected organosilicon compounds, albumin, and the TEMPO reagent were used as additional modifying substances. Coating parameters were determined, such as water contact angle, water absorption, hygroscopicity, and tensile strength. The presence of the coatings resulted in a significant increase in water vapor absorption by the substrate. Nanocellulose coatings proved to be sensitive to the water vapor and showed no barrier properties against it. However, the samples coated with nanocellulose had a noticeably lower tendency to absorb liquid water. The samples coated with modifying substances had a contact angle of ≥90°, proving that such coatings were an additional barrier to the penetration of liquid water. In the case of cellulose material coated with nanocellulose (i.e., without the addition of silane), there was no barrier effect established, thereby allowing full wetting of the substrate. The nanocellulose coatings increased the tensile strength of the samples. This increase was observed for all tested coating variants. The results obtained offer several potential routes to the manufacture of more environmentally friendly coatings and packaging materials.

1. Introduction

Currently, synthetic polymer coatings dominate the packaging industry, even though there is increasing evidence in popular science of plastic-based coatings causing serious ecological problems because of their non-biodegradability. Due to environmental demands for better solid waste management, the use of biopolymers in the production of food packaging materials is increasing. Therefore, research into the development of alternative environmentally friendly packaging materials is becoming more common [1]. As a result, nanocellulosic materials are becoming more commonly used in the packaging industry [2], with many groups exploring the potential uses of this material, including their use as coatings [3,4]. However, there is still the need for further research into the development of novel coating systems incorporating nanocellulose, particularly for uses in, for example, the biodegradable packaging industry.

Nanocellulose may be divided into two groups depending on its dimensions, production method, or function: (1) nanostructured materials (cellulose microcrystals and cellulose microfibrils) and (2) nanofibers (cellulose nanfibrils, cellulose nanocrystals, and bacterial cellulose) [5,6]. Nanostructured materials will not be discussed here as we focus our attention on nanofibers. Cellulose nanocrystals (CNC) comprise highly ordered regions within cellulosic materials. They can be obtained through the removal of amorphous sections of cellulose by, e.g., acid hydrolysis, thereby removing any intrinsic plastic properties [7]. The degree of crystallinity, depending on the starting material and processing conditions, can be up to 95% [8]. CNC has a unique crystal rod-like morphology, with low specific weight and dimensions ranging between 5 and 70 nm in thickness and several hundred nm in length, depending on its source [7,9]. CNF is an aggregation of fibrils comprising both crystalline regions and amorphous regions [10]. Individual CNF fibers may have a width ranging from 5 to 30 nm, and lengths between 10 and 100 nm [11]. Due to the high degree of fibrillation, CNF has a degree of crystallinity ranging from 60% to 70% [9]. With higher fibrillation and cumulative energy applied, the ratio of amorphous to crystalline regions increases. Moreover, it reveals more of the hydroxyl groups, thereby increasing their availability for chemical reactions [12]. The presence of these amorphous regions gives CNF greater versatility compared to CNC in terms of chemical modification and flexibility [7,9]. The difference between CNC and CNF has been well described in several publications, particularly in terms of the schematic differences between the two [13]. Bacterial nanocellulose (BC), or biocellulose, is usually made by aerobic bacteria. BC is characterized by its high crystallinity (80%–90%), with a degree of polymerization between 4000 and 10,000 and good mechanical stability [7,9,14]. Unlike cellulose nanomaterials derived from wood or other plant sources, bacterial nanocellulose does not contain lignin and heterosaccharides. BC fibrils have a width of 10 to 50 nm, are very long and flexible, and have properties specific to the bacterial source. Bacterial cellulose is considered safe in contact with food [7,9]. The preparation of CNF and CNC is a top-down process, while BC preparation is a bottom-up process.

Nanocellulose coatings are renewable, biodegradable, and have high mechanical strength. Initial interest into nanocellulose coatings focused on their strength properties. The individual nanocellulose crystals have modulus characteristics and a tensile strength comparable to or better than other durable materials, such as kevlar [15]. Moreover, due to the high density and extensive hydrogen bonds, the stretching of thin films based on nanocellulose may reach values similar to materials such as metals or advanced synthetic polymers. [9]. Since the hydrogen bonding present leads to a high affinity of the D-glucose particle chain to water, cellulosic materials show high sensitivity to the presence of water, both in vapor and liquid form. However, in the case of nanocellulose, this sensitivity is somewhat limited, due to the nanometric size of the primary particles. Thus, the network of polymer chains results in a higher density. Hence, coatings made of nanocellulose are characterized by higher stiffness and lower water vapor permeability compared to cellulose. Numerous studies have been carried out globally to modify nanocellulose films for further enhancing their water vapor barrier properties. High humidity causes nanocellulose coatings to lose some of their properties, such as their oxygen permeability barrier. These coatings are especially important in the food industry, particularly for food packaging [9,16]. The resistance of the material to the penetration of liquid water is an essential feature from the point of view of functional properties. The uptake of moisture by cellulose relies on several aspects, such as cellulose–cellulose, cellulose–water, and cellulose–counter ion interactions, controlled by van der Waals and Coulomb interactions. Water molecules present with cellulose hydroxyls form hydrogen bonds and saturate the sorption sites surrounding them. They also form a cluster of water molecules that consists of slow-moving bound water and fast-moving free water. Bound water is adsorbed by free sorption sites or hydroxyl groups interacting with cell wall polymers. Bound water is relatively more difficult to remove and requires more energy to dry. Free water may be found in the cell wall micropores, lumen, intercellular spaces, and small pores. There is also trapped water, which is formed when water is removed from the cellulose surface. Trapped water and bound water form “hard to remove water” (HR water) and require more energy than free water to remove from the fiber. The amount of HR water affects several physical properties, cellulose morphology, as well as the chemical reactivity of cellulose and cellulose nanofibers. Drying is accompanied by a partially irreversible closure of the pores in the fiber wall—often referred to as the collapse of the fibers. The study of fiber collapse and its correlation with water HR allows control of the collapse process by regulating the water content HR through physical or chemical modification. This can improve the physical properties of the products, water absorption and tensile strength [17]. Even though surface modification is known to be one of the critical parameters affecting the moisture uptake of cellulose and nanocellulose, these effects have not been studied in their entirety and remain insufficiently understood [18]. When cellulose films absorb water, the hydrogen bonds to adjacent cellulose chains are replaced with water molecules. This needs to be taken into account when creating a film, ensuring that the focus is not only the production of a high-density layer with a small number of pores, but also controlling the wettability of the coatings, so that the effects of water are kept to a minimum. One of the key criteria for ensuring liquid water resistance of a film is producing and maintaining high contact angle measurements [9].

The preparation of a nanocellulose coating often consists of the application of a homogeneous suspension of pure nanocellulose, or a suspension containing modifying substances, which gives specific properties to the surface of, for example, lignocellulosic materials [10]. A homogenous nanocellulose suspension is often achieved by mechanical or ultrasonic processing [19,20,21]. To improve flexibility, especially when using CNC, whose flexibility is limited due to its high degree of crystallinity, plasticizers are used to reduce the number of hydrogen bonds between the glucose chains. Among the most commonly used plasticizers are glycerol and sorbitol; their addition causes the relaxation of amorphous areas of cellulose, thereby increasing its elasticity. Studies have shown that the addition of plasticizers not only increases flexibility but also increases tensile strength and the resulting oxygen barrier [8,9,22]. Proteins and silanes are also frequently used components of such coatings. Protein coatings are hydrophilic in nature, creating a weak barrier to water vapor, though they show better oxygen and carbon dioxide barrier properties as well as mechanical properties when compared to, for example, polysaccharide coatings [23]. On the other hand, silanes, which are known for their hydrophobic properties, seem to be less desirable as they represent a synthetic component. However, there are known results demonstrating the absence of toxic effects of food packaging materials when coated with composite coatings containing silanes [24].

The aim of this research was to determine selected functional properties of cellulose material coated with modified nanocellulose. Nanocellulose, which was used as a film-forming material, was modified with silanes and/or 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) or albumin. The influence of the modified nanocellulose coating on the cellulose substrate was investigated in terms of the absorption of liquid water or water vapor, along with the resulting tensile strength of the treated cellulose material. The nanocellulose was modified with TEMPO to determine the effect of TEMPO oxidation on the functionality of the coating. Research has shown that such a modification results in coatings with high transparency and low air permeability, but also with low water resistance. However, the post-oxidation of coatings of a hydrophobic nature results in coatings with low roughness and high stability [25,26,27]. Due to the extremely different physical properties of the coatings based on nanocellulose, and whether they contained silanes or proteins, it was decided to test them on a homogeneous substrate with a known structure and high affinity for coatings. This would allow a means of rapid analysis, from which further studies could be undertaken on more conventional substrates to meet commercial requirements.

2. Materials and Methods

2.1. Materials

Whatman^® filter paper (Cytiva, Maidstone, UK) grade 40, with a basis weight of 95 g/m², a thickness of 210 μm, and typical particle retention in liquid 8 µm, was used as the coated substrate. The CNC cellulose nanocrystals (7.4% by weight) were purchased from Blue Goose Biorefineries Inc. (BGB, Saskatoon, SK, Canada), produced from lignocellulosic biomass by a transition metal-catalyzed oxidation process. A 3% aqueous suspension of nanofibrous cellulose of an in-house preparation was used [28], with dispersal being achieved without any chemical agents using a high-intensity (550 W) ultrasonic generator (BRANSON 55, Emerson Electric, Brookfield, CT, USA) by sonicating for 5 h in an ice-water bath. The distance from the ultrasonic generator probe tip to the bottom of the beaker was approximately 20 mm.

Supplementary substances were also used, including glycerol as a plasticizer, hydrophobizing organosilicon compounds such as methyltrimethoxysilane (MTMOS) and tetraethylorthosilicate (TEOS), protein such as egg albumin, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) as an oxidizing reagent. The choice of silanes was determined by their behavior with the CNC and CNF. It is known that cellulose is not highly reactive towards alkoxy groups present in alkoxysilane [29]. However, the small amount of bound water present in the cellulose material can lead to a hydrolysis reaction of the alkoxy groups and thus increase the affinity of the organosilicon compounds to react with the cellulose hydroxyl groups. For the formation of bonds between the cellulose and the alkoxysilane, it must first be hydrolyzed to a silanol, which is more reactive towards the cellulose hydroxyl groups than the alkoxysilane itself.

2.2. Preparation of Nanocellulose (CNC/CNF) Film

The CNC/CNF was dispersed in distilled water to obtain a 3% concentration and mixed for 30 min with a magnetic stirrer (MS-H Pro, ChemLand, Stargard, Poland) at a speed of 500 rpm. Glycerol (4% w/w CNC or CNF) was then added, and the suspension was then mechanically homogenized with Unidrive X1000 CAT at a speed of 15,000 rpm for 8 min.

2.3. Preparation of Nanocellulose/Alkoxysilane Film

The CNC/CNF was dispersed in distilled water to obtain a 3% concentration and mixed for 30 min with a magnetic stirrer (MS-H Pro, ChemLand, Stargard, Poland) at a speed of 500 rpm. The suspension was then mechanically homogenized with Unidrive X1000 CAT at a speed of 15,000 rpm for 8 min. The MTMOS (5% w/w CNC or CNF) was added to the dispersion, and the TEOS was added to CNF dispersion. In both variants, glycerol was added as a plasticizer (4% w/w CNC or CNF). Subsequently, the solution was stirred for 30 min with a magnetic stirrer at a speed of 500 rpm at 90 °C.

2.4. Preparation of Nanocellulose/Alkoxysilane/TEMPO Film

2,2,6,6-Tetramethyl-1-piperidine-1-oxy (TEMPO) crystals were dissolved in distilled water to obtain 0.05% TEMPO concentration; then, 1% (w/v) of nanocellulose (CNC or CNF) was added. Due to the TEMPO treatment, the concentration of nanocellulose had to be lowered to 1%. Subsequently, the dispersion was mixed for 30 min with a magnetic stirrer (MS-H Pro, ChemLand, Stargard, Poland) at a speed of 500 rpm and mechanically homogenized with Unidrive X1000 CAT at a speed of 15,000 rpm for 8 min. Next, the MTMOS (5% w/w CNC or CNF) was added to the dispersion, and the TEOS was added to CNF dispersion, as well as glycerol as a plasticizer (4% w/w CNC or CNF) in both variants. The solution was stirred for 30 min with a magnetic stirrer at a speed of 500 rpm at 70 °C.

2.5. Preparation of Nanocellulose/Albumin Film

Albumin was dissolved in distilled water with the addition of sodium hydroxide. Subsequently, 1% CNC or CNF and 4% (w/w CNC or CNF) glycerol was added to the albumin dispersion and stirred at 500 rpm with a magnetic stirrer (MS-H Pro, ChemLand, Stargard, Poland) for 30 min. The preparation of the albumin coating required the use of more distilled water, hence the lower content of nanocellulose. The suspension was homogenized using a homogenizer at 15,000 rpm for 8 min, followed by further stirring at 500 rpm for 30 min.

2.6. Application of Nanocellulose Films

Dispersions were applied to a cellulose substrate in the form of Whatman^® filter paper, with samples coated on both sides using a Mayer bar. The Mayer bar allowed a coating thickness of less than 30 μm to be applied. The scheme of coating application on a cellulose substrate with a Mayer bar is presented in Figure 1. Films were dried and conditioned at 50% RH at 18 °C before being tested. Prior to drying the nanocellulose/albumin films, a heat treatment was applied to denature the protein. The nanocellulose/albumin film samples were dried at 50 °C for 15 min. The choice of the substrate was conditioned by its predominantly homogeneous structure, which allowed the properties of the coatings to be observed.

2.7. Hygroscopicity Test

For hygroscopicity tests, 5 replicate samples of Whatman^® paper of dimensions 30 × 30 mm coated with each variant of the coating were prepared. Control samples of uncoated Whatman^® paper of the above-mentioned dimensions were also prepared. The samples covered with nanocellulose coatings that had previously been dried and conditioned at 18 °C and 50% humidity were further dried to a constant weight at a temperature of 70 ± 2 °C for 48 h. After cooling to room temperature, they were placed in sealed containers over a supersaturated solution of ammonium monophosphate to ensure a relative air humidity in the range of 90%–95%. Determination of the water vapor absorption by the samples coated with nanocellulose was carried out by measuring the weight gain, which was recorded at the following time intervals: 0.25, 0.5, 0.75, 1, 2, 4, 8, 24, 48, and 72 h.

2.8. Water Absorption Test

In total, 5 samples of Whatman^® paper (dimensions 30 × 30 mm) coated with each variant of the coating, along with uncoated controls, were prepared. In order to test the free water absorption of the coated samples, successive immersions in a container of distilled water were undertaken, with mass measurements determined after 0.5, 1, 3, and 5 min.

2.9. Water Contact Angle Test

Measurements were made on a DSA 25 KRÜSS goniometer (Kruss Scientific GmbH, Hamburg, Germany), which allows precise drop shape analysis of the water droplet and measuring of the surface tension. The device was used to analyze the water contact angle. The ADVANCE (version 1.9) program allowed processing of the images and measuring the contact angles, as shown in Figure 2.

2.10. Tensile Strength Test

The tensile strength test was carried out on the basis of the TAPPI T494 standard [30] using a Zwick/Roell Z005 testing machine (Zwick GmbH & Co. KG, Ulm, Germany) for testing corrugated boards using the edge crush test (ECT), flat crush test (FCT), and box crush test (BCT). Five replicates of the coated papers and a control sample measuring 2 × 15 cm were respectively prepared. Samples were tested to breaking point whilst embedded in the clamping jaws, with the distance between the jaws set at 11.5 cm. The tensile strength value was calculated from the following formula:

$\mathrm{TS}=\frac{\mathrm{Maximum} \mathrm{breaking} \mathrm{strength} \left[\mathrm{N}\right]}{\mathrm{Sample} \mathrm{thickness} \left[\mathrm{m}\right]\times \mathrm{Sample} \mathrm{width} \left[\mathrm{m}\right]} \left[\mathrm{MPa}\right]$

2.11. Data Analysis

The figures in the article were created using Excel 2022. GNU PSPP software (version 1.6.2) was used to conduct the statistical and variance analysis. A one-way ANOVA study and the Fisher’s Least Significant Difference (LSD) test with a confidence level of 95% (p < 0.05) were used to compare the significance of the difference between the obtained results. ANOVA analysis showed significant differences between the obtained data. The Fisher LSD test was performed to determine where these differences existed.

3. Results and Discussion

3.1. Hygroscopicity Test

The dependence of the water vapor absorption on the exposure time of the samples up to 72 h in a relative humidity in the range of 90%–95% was determined (Figure 3 and Figure 4). The most dynamic increases in moisture content for all samples were recorded during the first two hours of the study.

The non-coated samples, after 72 h in the conditions of high relative humidity, were found to absorb the least water, with a moisture content of about 9.6% being noted. The presence of the coatings resulted in an increase in water vapor absorption. Protein films have high permeability to polar substances such as water vapor. The highest humidity of 21%–23% was recorded for samples containing albumin as a component of the coating (CNC + A and CNF + A). The moisture content of the remaining samples, irrespective of additional substances (silanes and/or TEMPO), after 72 h from the beginning of the experiment, was in the range of 11%–13%. The results of these studies turned out to be surprising. A previous publication shows that the presence of nanoparticles in the composite lowers the water vapor permeability, which is related to an increased tortuous path of the water molecules to penetrate through nanocomposite matrices [31]. The deterioration of the water vapor barrier properties could be caused by the lack of resistance of nanocellulose coatings or their components to hygroscopic water and liquid water [9,16], whilst the TEMPO oxidation has been suggested to result in the oxidation of glucosyl units within microfibrils. The reduction of barrier properties could also be a direct result of the presence of a water-soluble plasticizer, which was glycerol. The research indicates that the presence of a plasticizer and the cross-linking of the coating may increase the water vapor absorption by the sample. Rodríguez and colleagues observed an increase in the moisture content of the samples from 16% to 52% after cross-linking and adding sorbitol to the coatings [8]. A similar situation occurred when sorbitol was added as a plasticizer, which resulted in the moisture contents of samples coated with three layers of nanocellulose increasing from 22% to 32%, and those coated with six layers from 16% to 26% [27].

3.2. Water Absorption Test

The weight gains of the samples during soaking in distilled water were also determined (Figure 5 and Figure 6). The highest weight gain was observed for all tested samples in the initial period of the experiment, i.e., during the initial 30 s of immersion in water.

Figure 5 and Figure 6 show a noticeably lower tendency for free water absorption by samples with the nanocellulose coating compared to the non-coated samples. In the CNF and CNC variants, an improvement in the resistance to liquid water absorption was determined to be approx. 33%–41%. Jannatyha et al. observed a reduction of moisture content in samples containing nanocellulose. They indicated that this decrease in moisture content may be related to the reduction in the number of hydroxyl groups reacting with water molecules [31]. Despite the natural hydrophilicity of proteins, coatings containing albumin (CNC + A and CNF + A) reduced the samples’ absorption tendency by approx. 50%.

The CNF + TEOS sample, after 5 min of soaking with distilled water, had a moisture of 139%. This result is comparable to the silane-free CNF (146%) and CNC (138%) sample. Previous studies found that there were four chemical environments for the sol-gel process: high and low water content, and high and low pH [29]. It was assumed the reason for the high absorbability of CNF + TEOS samples was the lack of a factor regulating the pH of the reaction medium. The studies herein show the potential water absorption–reducing properties of samples containing silanes and silanes with the TEMPO reagent (CNC + MTMOS, CNF + TEOS, CNC + MTMOS + TEMPO, CNF + TEOS + TEMPO).

3.3. Water Contact Angle Test

The water contact angle (WCA) measurements after water droplet addition to samples coated with the nanocellulose coatings under investigation were determined after 1 and 5 min (see Figure 7 and Table 1). Additionally, Table 1 contains the relative standard deviation. For the pure nanocellulose coating, the contact angle was not measured with the goniometer, since the applied drop of distilled water absorbed into the samples almost immediately. This was also the case for both CNC- and CNF-coated samples. The surface of the samples was completely wetted, which confirmed the results and observations of other researchers [29,32].

The hydrophobic properties, and thus the highest contact angle, were recorded for the variants with silanes and the TEMPO reagent. CNC + MTMOS + TEMPO samples showed a WCA after 1 min of 136°, whilst the value for CNF + TEOS + TEMPO was 128°. The contact angle for the coatings containing albumin (CNC+ A and CNF + A) was 90° and 94°, respectively, after 1 min of the drop being applied. However, any hydrophobic nature disappeared after 5 min (WCA after this time was recorded at 76° and 84°, respectively), which indicated the hydrophilic nature of the coating. In the case of coatings with silanes (CNC + MTMOS and CNF + TEOS), the contact angle was 121° and 124°, respectively. From the results presented in Figure 7 and Table 1, it can be seen that the silane-containing coatings showed greater durability of the hydrophobic effect. In the case of these variants, the WCA measurement taken after 1 min did not differ significantly from the result recorded after 5 min. The above observation turned out to be consistent with the results and conclusions of the authors of other works [31]. The literature shows that it is possible to create silane-containing coatings with even higher contact angles. The paper by Chen et al. shows that nanocellulose-silica coatings can exhibit superhydrophobic properties (WCA > 150°) [32,33].

3.4. Tensile Strength Test

Figure 8 and Table 2 show the results of the tensile strength (TS) test of tissue samples. For all samples covered with a nanocellulose coating, an increase in strength was observed compared to the control samples. Additionally, Table 2 shows the relative standard deviations.

The TS value of the uncoated control samples was 7.8 MPa on average. The highest increase in tensile strength, of between 167% and 181%, was achieved in variants containing protein (CNC + A and CNF + A). The improvement in tensile strength may be related to the inherent strength and stiffness of the CNC filler chain, and high intra- and intermolecular hydrogen bonds between NCC molecules and the protein network [34]. The addition of silanes and the TEMPO reagent did not affect the tensile strength. The TS value of the other variants fluctuated in the range of 10–12 MPa (representing an increase in tensile strength of 37%–50%). A lower strength index of coatings containing 3% nanocellulose than those coatings containing 1% nanocellulose (CNC + A, CNF + A, CNC + MTMOS + TEMPO, CNF + TEOS + TEMPO) was also observed. A similar phenomenon was observed by Reddy and Rhim in their research [35]. They created an agar film with different CNC percentages (1%, 3%, 5%, and 10%), with results indicating there was an optimal amount of nanocellulose that caused the maximum increase in tensile strength. The effect of nanocellulose concentration on the tensile strength values of coatings was also observed in research on pectin coatings, where the maximum TS increase was obtained at 5% CNC concentration and then decreased with increasing CNC concentration [19].

4. Conclusions

Preliminary studies were carried out to investigate the potential of various nanocellulose coatings on cellulose paper. In subsequent hygroscopicity tests, samples without the coating absorbed the least water vapor. The presence of the coatings resulted in a significant increase in the absorption of water vapor. The nanocellulose coatings were sensitive to the presence of water vapor and showed no barrier properties against it. However, the samples coated with the nanocellulose coating had a noticeably lower tendency to absorb liquid water when tested for water absorption. Thus, all variants of coatings investigated constituted a good barrier to liquid water and reduced the absorbability of the samples. Furthermore, water absorption was the lowest in the case of coatings containing silanes and the TEMPO oxidant (CNC + MTMOS + TEMPO and CNF + TEOS + TEMPO). Samples coated with CNF + TEOS were characterized by high water absorption compared to the coatings with the addition of MTMOS silane. After 5 min of soaking with distilled water, the sample with CNF + TEOS had a moisture content of 139%, whilst treatment with CNC + MTMOS was 82.5%. The probable reason for the high absorption levels of CNF + TEOS samples was the lack of factors regulating the pH of the reaction medium. The samples with the coating containing albumin, silanes, as well as silanes + TEMPO, had a contact angle of ≥90°. This proves that these coatings were an additional barrier to the penetration of liquid water. In the case of cellulose material coated with nanocellulose (i.e., without the addition of silane), there was no barrier effect established, allowing full wetting of the substrate. All the nanocellulose coatings realized an increase in the tensile strength of the treated samples. The greatest increase in strength was obtained in the variants of samples containing albumin (where an increase in the range of 167%–181% was observed). The tensile strength of the remaining coated samples increased by 37%–50%. The maximum increase in breaking strength was observed at a concentration of nanocellulose equal to 1%. In conclusion, there is an optimal amount of nanocellulose that will produce the maximum increase in tensile strength. The results herein suggest several promising routes to biobased coatings for a range of applications, particularly for more complex substrates. Further research will be carried out to better understand the complex hygroscopic properties of nanocellulose coatings. Research on cellulose coatings will be continued with the hope that they will be applied in various industries, especially in the biodegradable packaging industry.

Author Contributions

Conceptualization, B.M. and K.T.; methodology, B.M. and K.T.; software, K.T.; validation, B.M. and D.J.; formal analysis, K.T.; investigation, K.T.; resources, B.M.; data curation, all authors.; writing—original draft preparation, K.T.; writing—review and editing, all authors; visualization, K.T.; supervision, B.M.; project administration, B.M. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Nanofiber cellulose (CNF) dispersion was prepared in cooperation with NanoBioMedical Centre, Adam Mickiewicz Univeristy Poznań, Poland.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Scheme of coating application on a cellulose substrate with a Mayer bar.

Figure 2. Example of an image from the measurement of the water contact angle in ADVANCE software (version 1.9).

Figure 3. Moisture content (%) of tested CNC samples. * The relative standard deviation (RSD) (%) values are shown in parentheses.

Figure 4. Moisture content (%) of tested CNF samples. * The relative standard deviation (RSD) (%) values are shown in parentheses.

Figure 5. Water absorption (%) of tested CNC samples. * The relative standard deviation (RSD) (%) values are shown in parentheses.

Figure 6. Water absorption (%) of tested CNF samples. * The relative standard deviation (RSD) (%) values are shown in parentheses.

Figure 7. Water contact angle (WCA) of samples 1 and 5 min after drop application.

Figure 8. Tensile strength (MPa) of samples.

Table 1

Statistical parameters of the water contact angle (°) of samples.

Variant	Statistical Parameters
	After 1 min				After 5 min
	Minimum (°)	Mean (°)	RSD *	Maximum (°)	Minimum (°)	Mean (°)	RSD *	Maximum (°)
CNC + A	84	90	6.5	97	72	76	3.4	79
CNF + A	87	94	6.1	103	73	84	8.4	93
CNC + MTMOS	117	121	2.3	124	114	119	2.8	122
CNF + TEOS	121	124	1.6	127	119	122	2.3	126
CNC + MTMOS + TEMPO	132	136	1.9	139	130	135	2.2	138
CNF + TEOS + TEMPO	115	128	7.0	137	114	125	8.6	137

* The relative standard deviation (RSD) (%).

Table 2

Statistical parameters of the tensile strength (MPa) of samples.

Variant	Statistical Parameters
	Minimum (MPa)	Mean (MPa)	RSD *	Maximum (MPa)
Control Samples	7.7	7.8	1.1%	7.9
CNC	10.8	11.6	6.2%	12.8
CNF	10.7	10.9	1.8%	11.2
CNC + A	20.5	21.0	2.5%	21.7
CNF + A	21.1	22.2	3.5%	23.0
CNC + MTMOS	11.4	11.8	3.5%	12.5
CNF + TEOS	9.9	10.8	6.7%	11.8
CNC + MTMOS + TEMPO	11.1	12.2	5.2%	12.7
CNF + TEOS + TEMPO	11.0	11.7	4.8%	12.5

* The relative standard deviation (RSD) (%).

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