1. Introduction
Food packaging can be composed of metal, plastic, glass, or paper [
1]. Plastics have a large share of the food packaging market, and there are many examples of foods traditionally packaged in glass containers that are found only in plastic packaging, such as pasteurized milk [
2]. The primary plastic materials used in the production of food packaging are low-density polyethylene (LDPE), linear low-density polyethylene (LLDPR), high-density polyethylene (HDPE), polypropylene (PP), ethylene-vinyl acetate (EVA), ethylene-vinyl alcohol (EVOH), ethylene acrylic acid copolymer (EAA), polystyrene (PS), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyamide (PA), and polycarbonate (PC) [
3].
The success of plastic is due to its low cost, lightness, flexibility, durability, corrosion resistance, and high machinability [
4], and its barrier properties are advantageous for food packaging [
3,
5]. In 2020, plastic production was 363 million tons, making up a market of USD 580 billion [
6]; by 2050, it is expected to reach 500 million tons per year [
4]. Despite the advantages of this material, it is important to emphasize the problem of plastic pollution, which increases simultaneously with plastic production [
6,
7].
Despite the availability of recycling technologies for plastic, only 9% of discarded plastic is recycled, while the remainder is incinerated (12%), deposited in the environment, or disposed of in landfills (79%) [
8]. The most used plastics are not biodegradable, remaining in the environment for a long time, often centuries [
5,
9]. This promotes the accumulation of plastics (plastic nanomaterials and plastic micromaterials) in the environment and, consequently, in living organisms through the food chain [
10]. Plastic accumulation can cause lung, intestinal, kidney, cardiac, and liver diseases, in addition to genetic mutations, nervous system degradation, and decreased fertility in humans and animals [
11,
12,
13,
14,
15].
Limiting the use of plastic is essential for natural conservation and environmental sustainability [
7]. For food packaging, cellulosic materials are suitable alternatives to plastic [
3]. Paper, the primary cellulosic material, is utilized in the food industry as bags, cartridges, cardboard boxes [
3,
16], and, more recently, bottles [
17,
18].
Paper is essentially a network of natural cellulosic fibers, usually derived from wood [
19,
20]. To produce paper, it is necessary to individualize the wood fibers through chemical or mechanical processes. The obtained fiber solution is dried in appropriate machines to form paper [
20]. Paper can be obtained from short-fibered hardwoods, producing highly malleable paper with little mechanical resistance, making it suitable for printing, writing, and tissue [
20,
21]. Softwood, on the other hand, has longer fibers that provide high mechanical strength to paper, which is suitable for packaging [
20].
The kraft pulping process involves solubilizing wood lignin through an alkaline solution of sodium hydroxide (NaOH) and sodium sulfide (Na
2S) at high temperatures and pressures [
20]. This process is responsible for more than 90% of all papers produced worldwide [
22] because it provides good mechanical strength [
20]. Kraft paper refers to the paper produced through this process, which is normally made from unbleached coniferous wood, with grammage ranging from 50 to 300 g/m
2 [
3]. This type of paper has good mechanical properties and good moisture resistance, but it is still inferior to some plastics [
23].
For a more effective use of paper in food packaging production, strategies that can improve its mechanical strength, barrier properties, and thermal properties are needed [
20]. One such strategy is the use of coatings, which form an extra layer and provide the paper with adequate characteristics [
24]. However, the coating material must also be of natural origin, biodegradable, and non-toxic, like the paper itself [
3,
25,
26]. Polysaccharides such as starch, nanocellulose, and chitosan are some examples that meet these requirements [
26,
27].
Nanocellulose refers to all cellulosic materials with at least one dimension on the nanometric scale [
20], including nanocellulose crystals (CNC), microfibrillated cellulose (CMF), and nanofibrillated cellulose (NFC) [
28]. Because nanocellulose is obtained from plant biomass, it is chemically compatible with paper [
20], and its application does not alter the paper’s characteristics, reduce its biodegradability, or make it more dangerous to human health [
20,
27,
29]. NFC exhibits hydrophobicity [
24], and when applied as a coating on paper, it enhances the paper’s barrier properties, such as its permeability to air, gas, oil, and grease [
29], and improves its thermal and mechanical properties [
3,
20,
27], making it a more suitable material for food packaging [
3].
Nanocellulose is not the only renewable compound that can improve the barrier properties of paper; starch, chitosan, alginate, and polylactic acid (PLA) are useful in this role [
27]. Furthermore, modifying the paper surface is a viable alternative, if the products of these modifications are non-toxic [
3].
Plastic remains one of the most substantial and challenging innovations to replace in the 21st century [
4]. For food packaging, paper alone cannot replace it completely, but with the use of a nanocellulose coating, the use of paper can be expanded to provide adequate protection to various foods [
3]. In this study, the performance of kraft paper with two coating thicknesses (1 and 2 mm), as well as nanocellulose films made with the same thicknesses of the coatings of papers, was evaluated. Its morphological characteristics, physical properties, barrier properties, and mechanical properties were evaluated.
2. Materials and Methods
2.1. Materials
The paper used in this study was produced in the laboratory from unbleached
Pinus spp. kraft pulp on a Rapid-Köethen paper machine (Regmed, Osasco, Brazil) with a final grammage of 60 g/m
2 [
30]. Industrial eucalyptus kraft pulp bleached with an elemental chlorine-free (ECF) bleaching sequence was used to produce NFC. An aqueous suspension of the pulp with 1.3% consistency was prepared by homogenization in a blender. Subsequently, this suspension was processed with a Masuko Sangyo Super Masscolloider MKCA6-2J (Masuko Sangyo, Kawaguchi, Japan) to obtain a gel-like suspension in water [
31]. To obtain the cellulose nanofibrils, a rotation frequency of 1500 rpm and 10 passes through the mill were adopted.
2.2. Coating of Papers and Films Production
The NFC coatings were deposited on the kraft paper through the laminar method on a flat surface at 1 and 2 mm of NFC suspension thickness and previously dried in an oven at 50 °C for 20 min, then dried in a Rapid-Köethen sheet dryer [
30] at 90 °C and a pressure of 80 kPA until completely dry (8% humidity). Films without paper were also made with NFC suspension thicknesses of 1 mm and 2 mm, following the same steps as for the coatings.
In total, five treatments were analyzed in 10 replications and named as follows:
Uncoated kraft paper (sample called control KP);
Kraft paper with deposition of 1-mm of NFC suspension (sample called K1);
Kraft paper with deposition of 2-mm of NFC suspension (sample called K2);
NFC 1-mm film (sample called F1);
NFC 2-mm film (sample called F2).
Before all analyses described below, the papers, paper coatings, and films were previously packed in a climate-controlled room with a temperature of 23 ± 2 °C and a relative humidity of 50 ± 2% [
32].
2.3. Morphological Analysis
A morphological analysis of the cellulose nanofibrils (NFC) was performed via transmission electron microscopy (TEM) using a JEOL JEM 1200EXII Transmission Electron Microscope (600×) (Jeol, Peabody, MA, USA). In this analysis, the NFC suspension was diluted to 5 ppm in deionized water and dripped onto the surface of a 400-mesh screen. The samples were then allowed to dry at room temperature. To check the size of the fibrils, three diameter measurements were taken for each NFC micrograph.
To analyze the morphology of the papers with and without the application of the NFC coating, SEM images were obtained using a HITACHI scanning microscope (model TM-1000) (HITACHI, Tokyo, Japan) and a Philips microscope (model XL 30 series) (Philips, Eindhoven, The Netherlands). For this analysis, the samples were metallized using an electron beam to better identify their structures.
2.4. Physical Tests
The physical characterization of the papers and films was performed by considering their apparent density [
33] and water absorption using the Cobb method [
34] and the airflow resistance using the Gurley method [
35]. In the case of the water absorption of the NFC-coated papers, both sides of the papers were evaluated (coating side and paper side).
In addition, surface wettability was characterized by determining the apparent contact angles of the samples. A Data Physics OCA goniometer was used to apply the sessile drop method at room temperature (20 ± 2 °C). Three droplets of deionized water (5 µL) and three droplets of glycerol (5 µL) were deposited in three different samples, totaling nine replicates for each treatment and solvent. To ascertain the wettability kinetics, measurements were performed at the moment of droplet contact with the sample (0) and after 5, 15, and 30 s of contact between the droplet and sample surface. In this analysis, both sides of the coated paper were evaluated.
The nanocellulose-coated papers had their grammage determined before and after coating. Based on these values and the apparent density of the films, the dry thickness of the coating was determined.
2.5. Mechanical Tests
Tensile strength was determined with an adaptation of TAPPI T 494 om-13 [
36], where the distance between the claws was 50 mm. This analysis was performed using a dynamometer. The index was calculated using the relationship between tensile strength and grammage. The burst resistance was measured according to TAPPI T 403 om-15 [
37]. It is expressed in kPa, and its index is calculated as the ratio between the breaking strength and grammage. The tear strength was determined using the T 414 om-98 [
38] standard of the Elmendorf Pendulum equipment and expressed in mN. Its index was calculated by the ratio between tear strength and grammage, expressed in mN·m
2/g.
2.6. Thermal Stability
A thermogravimetric analysis (TGA) of the nanocellulose was performed using a Setaram Setsys Evolution TGA/DSC (Setaram, Lyon, France). The analyses were performed under an argon gas atmosphere at a constant flow rate of 40 mL·min−1, using approximately 5 mg of the sample. The ramp adopted was 10 °C per minute and the temperature ranged from 35 °C to 650 °C. A thermogravimetric (TG) curve was obtained to evaluate mass loss as a function of temperature. The first derivative of the mass loss was used to generate the second curve (DTG) to determine the onset, maximum, and end set temperatures for thermal degradation. The TG curves were used to calculate the mass loss in the following temperature ranges: 100–200 °C, 200–300 °C, 300–400 °C, 400–500 °C, and 500–600 °C. The initial, maximum, and final thermal degradation temperatures were determined.
2.7. Statistical Analysis
The following tests were performed for the statistical analysis: the Grubbs test for outliers, the Shapiro–Wilk test for data normality, Levene’s test for homogeneity of variance, and the analysis of variance (ANOVA). Tukey’s mean comparison test was performed when the equality hypothesis was rejected. All tests were performed using the Statgraphics Centurion XV Program (Statgraphics Technologies Inc., The Plains, VA, USA) at a 95% probability.
4. Conclusions
As shown above, based on the analysis results of the morphological, physical, mechanical, and thermal properties, the use of nanofibrillated cellulose (NFC) as a coating can improve the performance of kraft paper for food packaging. With the application of the coatings, the porosity of the kraft papers decreased and the density increased. The water absorption on the coating side decreased, whereas that on the paper side increased.
The bulk density, water absorption and final dry thickness were not affected by the deposition thickness of the NFC suspension.
The NFC films absorbed less water, and the different thicknesses did not influence the apparent density or water absorption. Uncoated kraft paper was the only air-permeable material.
The NFC coatings improved the mechanical properties of the paper, with a coating thickness of 2 mm increasing the bursting resistance and tear resistance and a coating thickness of 1 mm improving the tensile strength.
All the evaluated materials demonstrated the minimum thermal stability required in the production of food packaging, with the onset of thermal degradation occurring at temperatures greater than 220 °C.