Using Cellulose Nanofibril from Sugarcane Bagasse as an Eco-Friendly Ductile Reinforcement in Starch Films for Packaging

Abstract

Attempts have been made to replace conventional plastics in food packaging with biodegradable materials as a promising alternative because they are natural, renewable, and low-cost. This study aimed to develop biodegradable and resistant films from cellulose nanofibrils (CNFs) from sugarcane bagasse when used as reinforcement in starch films. Sugarcane bagasse pulps were subjected to alkaline treatment, with the residual lignin remaining. Part of the material was subjected to a bleaching process with H₂O₂. The pulps were subjected to the mechanical fibrillation process, and unbleached and bleached CNFs were produced. Percentages of 10%, 20%, 30%, and 50% CNF were added to a solution containing 2.5% starch (m/m) solids to make the films. The addition of unbleached CNF promoted an average increase in the tensile strength and Young’s modulus values, especially for films with higher percentages of CNF (30% and 50%). The contact angle values increased with the CNF concentration, with all films being classified as hydrophobic (>90°), except for the films with 30% and 50% unbleached CNF. The 50% unbleached and bleached CNF samples showed low water vapor permeability (2.17 g.mm/Kpa⁻¹ day⁻¹ m²), indicating a good vapor barrier. Although the influence of residual lignin on the test results was not identified for the other samples, treatments with 50% CNF of sugarcane bagasse (unbleached or bleached) should be highlighted among the properties evaluated for reinforcing the structure and improving the barrier properties of cassava starch-based films. Furthermore, this study proposes using sugarcane bagasse, which is a waste widely available in Brazil, placing the study in line with three Sustainable Development Goals (SDGs).

1. Introduction

Conventional plastics produced from petroleum-derived polymeric materials are widely used in food packaging due to their high durability, mechanical resistance, and low cost, and because they are easy to obtain [1]. However, using these materials results in environmental impacts since the natural degradation process can be prolonged for decades, contributing to soil and ocean pollution and the accumulation of microplastics in the environment [2,3]. Given this, the search for more sustainable alternatives is growing; biodegradable materials are an alternative that results in less environmental impact throughout their life cycle. In this context, industries worldwide have implemented economic models based on the circular economy. This system replaces the “end of life” concept with the reuse, recycling, and recovery of materials, intending to achieve sustainable development, resulting in gains in environmental quality and economic prosperity [4]. The search for biodegradable materials and the need for better uses for agricultural waste have sparked interest in developing biodegradable films and packaging [5].

The production of biodegradable films from agricultural by-products presents a promising alternative as they are materials of natural, renewable origin and they are abundant and economically viable, with cassava starch being one of the most widely used biopolymers [3]. Nevertheless, the use of starch as a membrane still has limitations regarding hydrophilicity and fragility. In this context, lignocellulosic biomass emerges as a material option with great potential for formulating biodegradable films. Lignocellulosic material is mainly composed of cellulose and lignin, and its use in the manufacture of bioplastics has attracted interest due to its structural and functional characteristics [6]. An advantage of lignocellulosic material is its large quantity and availability throughout the year, as it comes from the forestry and agricultural chains, which are very strong in Brazil. Lignocellulosic material is also attractive to microorganisms that can accelerate decomposition because they are composed of organic material, alleviating effects on the environment.

Whereas cellulose provides mechanical strength and dimensional stability to films, lignin stands out for its unique properties, such as low polarity, absorption of ultraviolet (UV) light, and antioxidant character [7,8]. These properties are particularly advantageous for biodegradable packaging as they can improve the durability and functionality of the materials. Thus, lignin molecules and cellulose nanofibrils (CNFs) have been studied for their use in producing biodegradable films [9,10]. However, there are still challenges in processing these materials, especially in the compatibility between the different components and in obtaining ideal properties for specific applications. Thus, more studies are needed to explore the potential of lignocellulose in the manufacture of biodegradable packaging.

The literature reports the potential of CNFs as reinforcement in films [7,11,12]. Nevertheless, there is a lack of scientific evidence on the influence of different pulp treatments on lignocellulosic waste that results in materials with various types and contents of chemical components and data regarding the mechanical and barrier properties of the developed membranes. This research is aligned with at least three UN Sustainable Development Goals (SDGs).

The development of films from sugarcane bagasse supports SDG 12 (Responsible Consumption and Production) by reducing dependence on plastics. It also contributes to SDG 15 (Life on Land) through the sustainable use of agricultural waste and SDG 9 (Industry, Innovation, and Infrastructure) by promoting innovative materials. This information could benefit packaging industries seeking to develop biodegradable formulations that align with circular economy guidelines and reduce environmental impact. Thus, this study aimed to evaluate the effect of adding fibrillated CNFs from unbleached and bleached sugarcane bagasse pulps when used as reinforcement in starch films.

2. Materials and Methods

2.1. Material

Sugarcane bagasse pulp (Saccharum sp.) and cassava starch provided by family farmers in southeastern Pará were the raw materials used to develop the films. The starch has a crystallinity index (CI) of around 45% and an amylopectin content of 85%. Sodium hydroxide (NaOH) from the company Êxodo Científica (Sumaré, Brazil) and hydrogen peroxide (H₂O₂) from the company Dinâmica Química Ltda (Indaiatuba, Brazil) were used to chemically treat the pulps.

2.2. Alkaline and Bleaching Treatments

The sugarcane bagasse pulp (5 g) was alkaline treated with a solution of 100 mL of 5% (m/v) NaOH under continuous stirring at 1500 rpm for 2 h at 80 °C. The material was then washed with deionized water until it reached pH 7.0, and excess moisture was removed by vacuum filtration. Finally, the material was dried in an oven with air circulation at 50 °C for 24 h. The pulp was bleached in an aqueous suspension in the proportion of 5 g of fibers and 100 mL of 24% (v/v) H₂O₂ and 4% (m/v) NaOH solution under continuous stirring at 1500 rpm for 2 h at 80 °C. The materials were then washed to remove all chemicals and dried in an oven (50 °C) with air circulation until they reached a constant mass.

2.3. Mechanical Fibrillation

The bleached and unbleached pulps were dispersed in suspension at a concentration of 2% (w/w) and then fibrillated using a Super Masscolloider Masuko Sangyo MKCA6-2, disk model MKGA6-80 (Tokyo, Japan), with five passes through the equipment, which was operated at 1500 rpm, according to Mendonça et al. [13].

2.4. Film Preparation

The films were prepared using the solvent evaporation (casting) method, where 1000 g of a 2.5% (w/w) cassava starch solution in a 0.6% (w/w) glycerol solution was prepared. The solution was subjected to magnetic stirring at 500 rpm at a temperature of 90 °C for 20 min (Figure 1). Different concentrations of bleached and unbleached CNF suspensions (10%, 20%, 30%, and 50%) (

Table 1. Concentrations of unbleached (UB-CNF) and bleached CNF (B-CNF) and starch for film production.

CNF (% m/m) *	Starch (% m/m)
Control **	100
B-CNF/10	90
B-CNF/20	80
B-CNF/30	70
B-CNF/50	50
UB-CNF/10	90
UB-CNF/20	80
UB-CNF/30	70
UB-CNF/50	50

* % by mass of CNF and dry mass of starch in each film; ** solution containing only starch and glycerol.

) were added to the starch solution [14,15]. A control film composed only of starch was also prepared. The suspensions were formed according to the CNF dry mass and the starch dry mass. On average, 250 g of film-forming solution was made. Then, 60 g of solution was poured into acrylic plates and left in a climate-controlled environment at 20 ± 1 °C, with 60 ± 1% relative humidity, for 10 days for drying and stabilization. Three films were produced for each film-forming solution.

2.5. Chemical Characterization of Sugarcane Bagasse

The total extractive content of the bagasse was determined according to T204 cm-17 [16], the lignin content according to T222 om-02 [17], and the ash content according to T211 om-12 [18]. The holocellulose content was obtained according to Browning [19]. The methodology of Kennedy et al. [20] was used for the cellulose content. The hemicellulose content was obtained by calculating the difference between holocellulose and cellulose. The chemical component contents were quantified in quadruplicate.

2.6. Turbidity and Stability of Suspensions

The B-CNF and UB-CNF suspensions were dispersed at a concentration of 0.1% by mass and kept under mechanical stirring for 10 min at 500 rpm [21]. Then, aliquots of the solution were taken under stirring to measure the degree of turbidity with a turbidimeter Plus Alkafit (Jundiaí, Brazil), with five replicates for each treatment. The B-CNF and UB-CNF suspensions were diluted to 0.25% (w/w), and 15 mL was transferred to test tubes to perform the stability test according to Guimarães Jr et al. [22]. Images were captured for 8 h straight. The ImageJ software (version 2024) was used to quantify the decantation of CNF, and stability was calculated according to Equation (1):

$$St (\%)=( {\displaystyle \frac{Suspension}{Total}}) \times 100$$

(1)

where St is the stability of the suspension; suspension is the height corresponding to suspended particles; and Total is the total height of the liquid in the tube.

2.7. Water Vapor Transmission Rate (WVTR) and Water Vapor Permeability (WVP)

Water vapor transmission rate (WVTR) measurements were performed using the ASTM E96/E96M-16 standard method [23]. The film samples were placed in glass cells filled with silica to approximately 1/3 of their capacity. The cell lids contained an opening where the film samples (5 cm in diameter) were placed and sealed with silicone. Posteriorly, the vials were placed in a desiccator containing a saturated sodium chloride solution. This solution promotes a relative humidity of 75%. The desiccator was subjected to a controlled temperature of 30 ± 0.5 °C for 7 days, and once a day, the vials containing the film samples were weighed. The permeability rate (WVTR) was calculated using Equation (2):

$$WVTR={\displaystyle \frac{w}{t \times A}}$$

(2)

where w is the mass gain (g) of the film; t is the time (days); and A is the area (m²). The w/t ratio was calculated by performing linear regression of the experimental points of mass gain (g) of the film as a function of time (days).

Water vapor permeability (WVP) was calculated using the value of WVTR, which was multiplied by the sample thickness (e) and divided by the water vapor pressure difference (∆P) according to Equation (3). The test was carried out by evaluating five samples per treatment.

$$WVP={\displaystyle \frac{WVTR \times e}{∆P}}$$

(3)

2.8. Contact Angle and Wettability

The contact angle and wettability of the films were measured using a Kruss Drop Shape Analyzer Goniometer—DSA25 (Hamburg, Germany), following ASTM D724–99 [24]. The dimensions of the film samples used for the test were 2.5 × 1.0 cm. A droplet of deionized water was placed on the surface of the samples using a syringe from a height of 3.2 mm at a room temperature of 25 °C. The angles were recorded from 1 to 60 s when the droplet made contact with the sample. The average recorded values for each sample were calculated and considered the contact angle values. The wettability of the films was calculated by averaging the contact angles between 5 and 55 s. In total, five samples were evaluated per treatment, with measurements taken on both sides of the sample.

2.9. Mechanical Properties

The tensile properties of the films were based on the ASTM D882-02 [25] standard, and a Stable Microsystems model TATX2i (London, UK) texturometer was used. The film specimens with nominal dimensions of 100 mm × 25 mm were pulled with a distance between grips of 50 ± 2 mm at a speed of 0.8 mm/s. The thickness of the films was determined with a Quantumike Digital External Micrometer—MITUTOYO, with ten readings taken, including two at the ends and one at the center of each sample. The tensile strength (TS) was calculated by dividing the maximum force by the initial cross-sectional area of the films. Young’s modulus, expressed in MPa, corresponds to the applied stress divided by the deformation suffered. The elongation at break, which corresponds to the ratio between the increased length and the initial length after rupture of the tested sample, was expressed in %. Mechanical properties were measured by averaging ten samples per treatment.

2.10. Microstructural Analysis of CNFs and Films

Morphological analyses of the CNFs after fibrillation were performed by transmission electron microscopy (TEM) Tecnai G2-12 (Tokyo, Japan) at an accelerated voltage of 80 kV. The films from unbleached and bleached sugarcane bagasse CNF structure were analyzed on the surface and cross-section using an ultra-high-resolution (UHR) FEG scanning electron microscope (Tescan-Clara, Kohoutovice, Czech Republic) at 10 KeV, 90 pA. The working distance was 10 mm, and the samples were subjected to “gold bath” metallization using a sputtering device (Balzers SCD 050, Oerlikon Balzers, Balzers, Liechtenstein).

3. Results and Discussion

3.1. Chemical Characterization of Sugarcane Bagasse

After alkaline treatment and bleaching, there was a considerable removal of hemicelluloses, decreasing the content from 36.28% to 7.67%, comparing the pulp “in natura” to the bleached pulp of sugarcane bagasse (

Table 2. Chemical composition of sugarcane bagasse pulp, unbleached pulp, and bleached pulp.

Initial Material	Total Extractives	Ashes	Insoluble Lignin	Hemicelluloses	Cellulose
	------------------------------------------%--------------------------------------------
Sugarcane bagasse pulp (in natura)	7.76 ± 0.04 *	1.66 ± 0.07	3.21 ± 0.45	36.28 ± 0.10	61.46 ± 0.17
Unbleached pulp	4.93 ± 0.12	1.32 ± 0.08	1.27 ± 0.31	15.23 ± 0.06	80.48 ± 0.28
Bleached pulp	3.66 ± 0.42	1.41 ± 0.07	0.83 ± 0.38	7.67 ± 0.37	90.73 ± 1.33

* Standard deviation.

). In addition, there was a significant decrease in the lignin content (from 3.21% to 1.27%), while a substantial concentration of cellulose was noted (from 61.46% to 90.73%) in relation to the sugarcane pulp. In fibrillation, lignin exerts a negative influence as it increases the energy consumption of fibrillation. Lignin is responsible for the rigidity of the cell wall; therefore, it represents an obstacle to the process, requiring the application of previous treatments.

These results corroborate those of Gilfillan et al. [14] when performing alkaline treatment on sugarcane bagasse, who obtained similar results, with an increase of 41% to 65% in cellulose and a reduction of 24% to 18% in hemicellulose. Research studies indicate that alkaline treatments are commonly employed before CNF production [26,27]. This treatment is crucial for promoting rapid swelling and improving the internal bonding of the cell wall, which leads to the partial removal of hemicelluloses and lignin [28].

The bleaching process using H₂O₂ aims to maximize lignin removal, which can be confirmed through the residual lignin content of 0.83% being observed. Similar results were obtained by Jonoobi et al. [29] when evaluating bleached and unbleached Hibiscus cannabinus pulps. They obtained bleached pulp with a 92% cellulose content and 0.5% residual lignin.

3.2. Stability and Turbidity of Suspensions

UB-CNF presented a lower turbidity index (385 NTU) than bleached pulp, indicating greater passage of light through the tube (Figure 2). Regarding B-CNF, fibrillation may have been influenced by the reduction in pulp tenacity with the bleaching treatment. Treatments on sugarcane bagasse fibers may have caused very accelerated passes of suspensions through the disks of a mechanical mill, leaving many fibrils suspended, resulting in greater turbidity of the suspension, as shown in a study by Mendonça et al. [13]. Similar behavior was observed by Banvillet et al. [30] with CNF treated with NaOH, with a value of 761 NTU, evidencing CNF aggregation.

This analysis indirectly indicates the fibrillation yield due to the scattering of light produced by large particles in suspension, which allows us to know the effectiveness of nanofibril production [31]. In UB-CNF, a higher turbidity index was expected due to the presence of lignin. This is because lignin tends to make the fibrillation and individualization of nanofibrils more complex due to its role in the cell wall of lignocellulosic materials. Nevertheless, this study observed the opposite for the suspensions’ turbidity. Similar results were reported by Espinosa et al. [31] for unbleached wheat straw nanofibrils (205.7 NTU), and a high value was reported for bleached nanofibrils (433 NTU).

The results of the sample stability show that when the nanofibril gel forms, the suspensions become more stable, causing less decantation. It is possible to verify that B-CNF presents a significant decrease in stability over the eight-hour test, evidencing unstable behavior, tending towards the regrouping of nanofibrils.

The behavior of UB-CNF in this study may have been caused by the antioxidant action of lignin, which prevented the reconnection of covalent bonds that had already been broken [32], or by the percentage of hemicelluloses (15.23%) present in this sample and its structural properties, such as amorphous character, macromolecular branching, and high hydrophilicity. A high hemicellulose content may have acted as a physical barrier to keep the cellulose nanofibrils away from each other, thus preventing their aggregation [33]. Conversely, Guimarães Jr et al. [22], when evaluating the stability index of bleached and unbleached bamboo pulp CNFs, found results indicating greater stability for the bleached suspensions. Figure 3a and b show the nanofibrillated morphology structures of unbleached and bleached sugarcane bagasse fibers, validating the CNFs in the nanoscale.

The morphology of the starch films and starch films with unbleached and bleached CNFs was investigated using scanning electron microscopy (SEM). The SEM micrographs provided evidence of strong adhesion of cassava starch with the CNF due to good dispersion. The starch film presented a homogeneous structure without phase separation, and fine cracks were observed (Figure 4a,b). This may have been caused by the progress of crystallization due to the greater molecular mobility associated with water vapor diffusion to the film surface [34]. Starch is known for having high hydrophilicity, factors that can favor the movement of vapors, and the appearance of various cracks and imperfections [22].

Films with concentrations of 20% and 30% unbleached CNFs (Figure 5a–d) presented a homogeneous surface without bubbles or cracks, showing only some accumulations of unsolubilized starch, indicating that the presence of lignin did not cause any change in the morphology. Films with higher concentrations, namely 50% of unbleached CNFs (Figure 5e,f), presented a rougher surface with the visible presence of CNFs. The film with 30% and 50% bleached CNFs (B-CNF/30 and B-CNF/50) (Figure 6c–f) presented the same surface type.

The micrographs of the film with 50% unbleached CNFs exhibit a rough and irregular surface with some agglomerates. The greater or lesser incidence of nanofibril agglomerates on the surface strongly influences the mechanical and barrier properties of the films, as observed throughout this study. This condition may be caused by inter-nanoparticle interaction due to the magnitude of hydrogen bonds and Van der Waals forces. In the case of unbleached CNFs, the amount of lignin may also have interfered with the quality of the film surface. A higher concentration of CNFs also represents a greater possibility of fibril and lignin agglomerations. In the films with 30% and 50% bleached CNFs (Figure 6c–f), a more heterogeneous surface was observed, with some isolated agglomerates (highlighted by the arrows). It is understood that these bleached nanofibril films established stronger bonds with the starch molecules, which is why they formed more homogeneous surfaces completely filled with CNFs. Grainy surfaces do not hinder the application of films for packaging. In some cases, irregular surfaces may actually enhance the use of adhesives and paints depending on the packaging’s purpose and the conditions of use.

3.3. Barrier Properties and Wettability

It is observed that the WVP values increased with the addition of nanofibrils in the film since the control presented a lower WVP value (

Table 3. WVP, contact angle, and wettability of cassava starch films with different concentrations of bleached and unbleached sugarcane bagasse CNFs.

	WVP (g mm/kPa−1 day−1 m2)	Contact Angle	Wettability (°s−1)
Control *	0.352 ± 0.07 **c	82.75 ± 2.06 b	0.11 ± 0.02 a
UB-CNF/10	3.085 ± 0.81 a	91.16 ± 4.09 a	0.10 ± 0.03 a
B-CNF/10	3.212 ± 0.31 a	93.85 ± 3.24 a	0.05 ± 0.02 b
UB-CNF/20	2.788 ± 0.29 a	92.36 ± 2.23 a	0.09 ± 0.07 a
B-CNF/20	2.621 ± 0.07 a	96.42 ± 7.18 a	0.07 ± 0.005 b
UB-CNF/30	2.862 ± 0.63 a	86.16 ± 0.10 b	0.07 ± 0.02 a
B-CNF/30	2.467 ± 0.21 b	97.28 ± 5.10 a	0.04 ± 0.005 b
UB-CNF/50	2.050 ± 0.18 b	83.68 ± 4.08 b	0.06 ± 0.002 b
B-CNF/50	2.178 ± 0.08 b	93.08 ± 2.08 a	0.02 ± 0.002 b

* Starch and glycerol; ** standard deviation. Averages followed by the same letter in the same column are statistically equal according to the Scott-Knott test at 95% probability.

). Other authors have reported opposite behavior, such as do Lago et al. [15] and Rani et al. [35], but this does not mean that the results cannot be considered satisfactory since films that are very permeable to water vapor may be suitable for packaging fresh food products, while a slightly permeable film may be ideal for dehydrated food products [36]. For films with lower WVP values (50% unbleached and bleached CNFs), this result may be related to the crystalline domains with a high degree of cellulose orientation that, when spread uniformly throughout the polymer matrix, act as a labyrinth, increasing the tortuosity and length of the diffusion path, consequently preventing the diffusion of water vapor [37]. Mascarenhas et al. [7], studying CNFs from Cofeea arabica wood, reported that alkaline treatment and bleaching can increase the crystallinity index of lignocellulosic fibers in relation to in natura fibers, which can improve barrier properties.

The average thickness of the films ranged from 0.065 to 0.121 mm (control to B-CNF/50). Although the results were not statistically significant, an increase in thickness was noted when comparing the UB-CNF to the B-CNF films. The sample containing 50% CNFs showed distinct behavior, with the bleached NFC film exhibiting the greatest thickness at 0.074 mm. The thickness results are consistent with the findings in the literature regarding the casting method [22]. However, contrary to what other authors have reported, this study did not observe a significant increase in thickness with higher amounts of CNF.

A lower WVP means less water vapor permeating the film, indicating a more substantial water vapor barrier. It is possible to observe that the control film (starch) obtained a lower permeability value (0.352 g mm/kPa⁻¹ day⁻¹ m²) in relation to the films with the addition of CNF, possibly due to cracks in the film surface (see Figure 3a), which facilitates the permeation of water vapor. This result corroborates that obtained by Luchese et al. [36] when performing an analysis on cassava starch films with a 2% mass of starch and 30% of glycerol, obtaining a WVP of 0.22 ± 0.02 g mm/h⁻¹ m⁻² kPa⁻¹.

The samples with 30% bleached CNFs (B-CNF/30) and both unbleached and bleached samples with 50% CNF (UB-CNF/50 and B-CNF/50) obtained lower WVP values. The higher CNF content in the mentioned treatments may hinder the mobility of water vapor during passage through the sample. In addition, a higher CNF content indicates a higher residual lignin content, which may also have hindered water vapor access. The treatments with lower CNF contents (10% and 20%) obtained higher water vapor permeability values.

Considering contact angle values superior to 90°, most films can be classified as hydrophobic [38], except for the treatments with 30% and 50% UB-CNF and the control, which presented angles slightly below the mentioned value. The three treatments mentioned presented lower averages for the contact angle and did not present statistical differences among them. Regarding the treatment UB-CNF/50, this behavior may be related to the presence of lignin in the fibers, which may have caused an irregular surface in the films, as can be confirmed by SEM (see Figure 3 and Figure 4). In the case of UB-CNF/30, these results may be correlated with many factors, such as CNF agglomeration in the starch solution, possible agglomerations of lignin granules, or conditions that favored agglomeration or cracks during the film’s drying.

It is possible to observe that as the CNF content increases in the composite films, a greater contact angle occurs, and the same behavior was observed in the literature [39]. However, it presents decreases of 30% and 50% in UB-CNF. These results indicate that there is a strong interaction between starch and CNF. A similar result was obtained by Ma et al. [40] when they observed a decrease in the contact angle with the increase in the content of 30% lignin and 60% CNFs in the production of films. For treatments containing 10% and 20%, both for unbleached and bleached fibers, no significant differences in contact angles were observed, all being statistically superior to the control. The treatments containing 30% and 50% bleached CNFs were statistically equal to each other and to the treatments with 10% and 20% CNFs. Hydrogen bonds (starch and CNFs) reduce the interaction between water and the composite films. Therefore, the behavior observed for the contact angle analysis corroborates that obtained for the water vapor permeability analyses, which indicates an improvement in the barrier properties by adding both bleached and unbleached CNFs. Wettability is a parameter that provides information about the surface properties of the films and can be determined by measuring the contact angle [41]. In this case, Table 3 highlights in seconds the time taken for the droplet to spread on the film surface. Although the 20% and 30% B-CNF samples presented the most significant angles, they presented fast scattering on the surfaces of 0.07 and 0.04°s⁻¹, respectively.

3.4. The Mechanical Properties of the Films

There was an increase in tensile strength (Figure 7) for all films with unbleached CNFs compared to the control film (4.6 MPa). The films with the addition of unbleached CNFs tended to present higher values, with emphasis on the samples with 20%, 30%, and 50% CNFs showing tensile strengths of 15.7, 13.4, and 15.5 Mpa, respectively. Especially for CNF contents of 20% and 30%, the performance of unbleached sugarcane bagasse was superior to that of bleached material with the same percentages.

A similar analysis was performed by Guimarães Jr et al. [22] when adding 1.5% CNF in starch films; they observed that there was a decrease in the tensile strength, probably due to CNF aggregation into a discontinuous form and heterogeneous phase, which consequently decreased the mechanical properties of the films. The results indicate that the presence of lignin increased the tensile strength of the films since the unbleached bagasse presented 1.27% of lignin (see Table 2). A certain content of lignin is reported to increase the film toughness, tensile index, and Young’s modulus [42]. Mascarenhas et al. [7], studying CNFs from Cofeea arabica wood, reported that alkaline treatment and bleaching can increase the crystallinity index of lignocellulosic fibers in relation to in natura fibers, which can improve the mechanical properties and film’s resistance. Commercial polymers frequently used in the production of flexible packaging for the food and agricultural sectors were previously studied by Auras et al. [43] and Tanpichai et al. [44], and they found values of 37 MPa for linear low-density polyethylene and 35 MPa for polypropylene, which were superior compared to those presented in this study. On the other hand, the abovementioned authors obtained values of 6.9–16 MPa for low-density polyethylene, 17 MPa for polyesteramide, and 8.5–10.5 MPa for highly branched low-density polyethylene, which makes the films in the present study interesting for this application.

De Souza Fonseca et al. [45] found improvements in the tensile strength and Young’s modulus in films with a lignin content below 10%. Zhang et al. [46] confirmed this idea, citing that small amounts of lignin can enhance the mechanical strength of materials. However, when the lignin content is too high, the mechanical strength will decrease, as well as the flexibility. The shape and surface chemistry of lignin particles may explain this fact. Österberg et al. [47] declared that spherical lignin particles with well-defined surface chemistry and morphology have great potential to improve the properties of renewable and biodegradable composites, be part of greener adhesives, and decrease the need for synthetic emulsifiers. Conversely, a high content of lignin means less fibrillated fibers, which can adversely affect the mechanical properties by weakening the bonds between CNFs. Zhang et al. [48] reported that the use of starch composites is still limited in industrial applications owing to their low resistance to mechanical stresses and humidity.

It is observed that samples that obtained higher values for Young’s modulus were those with 50% UB-CNF, followed by samples with 20% UB-CNF (Figure 8).

Adding 10% bleached and unbleached CNFs did not cause significant changes in the MOE. This behavior occurred due to the greater presence of starch in relation to 10% CNFs. The films with 20%, 30%, and 50% bleached CNFs also did not differ statistically from the control. Guimarães Jr et al. [22] obtained a Young’s modulus of 359 MPa for starch films with the addition of 4.5% bamboo CNFs. When the CNF content increased to 6.5%, the property value reduced to 319 MPa. Different results were found by Amini et al. [49] when comparing CNF films with and without residual lignin; they observed that the tensile strength and Young’s modulus were higher for the CNF films without residual lignin. The authors pointed out that the presence of lignin in the CNF structure makes it difficult, to a certain extent, to form direct hydrogen bonds between cellulose molecules. Several authors have already reported an increase in film stiffness due to the presence of CNFs, which can be attributed to the nanoscale of cellulose, which allows for the formation of strong hydrogen bond networks with the polymer matrix [15]. Regarding unbleached CNFs, the presence of hemicellulose may have played the role of a coupling agent for lignin with the starch matrix to improve film strength [14].

Overall, the addition of CNFs increased the elongation of the samples in all treatments. Adding 10% and 20% CNFs increased elongation regardless of whether unbleached or bleached material was used. Unbleached CNFs presented lower elongation values since the addition of 30% and 50% unbleached CNFs presented deformations statistically equal to the control treatment (Figure 9). In this case, it can be seen that adding a greater volume of CNFs from sugarcane bagasse with residual lignin did not effectively contribute to a greater elongation at break. This can be assumed to be due to the greater interaction in hydrogen bonds between the unbleached CNFs and the starch, which may have contributed to the fragility of the film obtained from the combination. Furthermore, the presence of more residual lignin may have contributed to the lower flexibility of films with a higher concentration of unbleached CNFs.

Starch films are known to be fragile and brittle [50]. Maniglia et al. [51] reported that starch films are rigid because of strong amylose–amylose or amylose–amylopectin interactions in the polymer matrix. Additive (as CNF) effects depend on size, molecular mass, and compatibility with the polymer matrix. The elongation at break values demonstrate a different behavior, indicating the films’ flexibility.

The packaging industry increasingly demands materials with quality and resistance, and currently, aspects that bring sustainability into the debate. Replacing plastics with biodegradable materials is a sure shot in the attempt to alleviate these problems that affect society. The transition of the pulp from the micro scale to the nanoscale requires energy. The energy needed for the process comes precisely from the deconstruction of the cell wall by the action of the microfibrillator mill. Without this process, it is not possible to obtain nanocellulose films, which have superior characteristics to those presented in existing papers. In addition to packaging, nanocellulose has several other applications, such as in adhesives, electronic equipment, fabrics for controlled drug release, and the automotive and aviation industries. Therefore, to achieve these objectives, high energy consumption is justified. Chemical treatments work by reducing the fibrillation energy of the mill. Nowadays, these treatments are performed with chlorine-free chemicals, which are more environmentally friendly. Bleached pulps undergo one more chemical treatment than unbleached pulps. Comparing the two types of pulps can help us determine which is more efficient in packaging development. If unbleached pulp is more successful, preparing the raw material would require one less treatment. All research involving this objective should be valued and verified with interest. Furthermore, this research is aligned with at least three UN Sustainable Development Goals (SDGs). The development of films from sugarcane bagasse supports SDG 12 (Responsible Consumption and Production) by reducing dependence on plastics. It also contributes to SDG 15 (Life on Land) through the sustainable use of agricultural waste and SDG 9 (Industry, Innovation and Infrastructure) by promoting innovative materials.

4. Conclusions

In this work, cassava starch films were successfully molded with the addition of unbleached and bleached CNFs of sugarcane bagasse pulp. It was found that residual lignin in the case of unbleached CNFs can significantly affect the properties of CNF–starch films. Unbleached CNFs showed greater stability (97%) and lower turbidity (385) compared to bleached CNFs. The samples with 50% unbleached and bleached CNFs showed low water vapor permeability, indicating that the films have a stronger water vapor barrier. The contact angle of the bleached samples was >90°, which is considered hydrophobic. The wettability of the bleached films was also lower. Incorporating unbleached CNFs into the films resulted in improvements of 20%, 30%, and up to 50% in the Young’s modulus, with a maximum increase of 100%. Additionally, films with 20% and 50% CNF additions demonstrated a remarkable 300% increase in tensile strength values. On the other hand, the elongation at break was notably higher in films that included bleached CNFs, with increases approaching 100%. Treatments with 50% CNFs of sugarcane bagasse (unbleached or bleached) should be highlighted among the properties evaluated. Considering that sugarcane bagasse has a significant place in Brazil’s agroindustry, the option of using its waste to develop films and packaging becomes an interesting and attractive topic. Furthermore, this research aligns with three crucial Sustainable Development Goals (SDGs).