Characterization of Biodegradable Films Applicable to Agriculture with Structural Reinforcement ^†

Abstract

The agro-industrial sector generates countless food waste, causing various environmental impacts. Given this perspective, it is necessary to adopt sustainable practices. An example would be the use of natural sugar cane fibers, which are low-cost, biodegradable, and can be added to film-forming solutions. The film obtained with the addition of vegetable composites (sugarcane fibers) presents a structural network without ruptures and can be used as reinforcement in structures that compose biodegradable films for use in agriculture.

1. Introduction

With the expansion of sugarcane production, there was also an increase in the production of residues from this crop. Among the residues produced, sugarcane bagasse, from the processing of sugarcane; water, from sugarcane washing; and vinasse, which can be used in agriculture, stand out [1]. Sugarcane bagasse, as it is a waste product produced in large quantities, is commonly used in the production of reinforcement materials in starch packaging [2]. A survey by the Food and Agriculture Organization of the United Nations (FAO) shows that Brazil wastes about 27 million tons of food per year [3,4]. In view of the food waste in the country, it is necessary to adopt measures to reduce waste, enabling production areas such as agriculture [5,6]. Biodegradable films, when incorporated into the soil, are easy to biodegrade due to the action of microorganisms and solar radiation, and biodegradable starch-based films are low-cost. Furthermore, they have biocompatible and environmentally sustainable materials [7]. The film biodegrades through the presence of starch in its chemical composition and is used as a source of energy and carbon by soil microorganisms [8,9]. This decomposition occurs in three stages, namely: (1) the breakdown of carbon compounds into small molecules due to the secretion of enzymes and/or the action of the environment (temperature, humidity, and sunlight); (2) the absorption and transport of small molecules into the cells of microorganisms; and (3) the oxidation of small molecules inside microbial cells with CO₂, water, and heat [10]. The most well-known technique in the elaboration of biodegradable films is the casting technique, where, after the gelatinization process of the starch granules, amylose and amylopectin are dispersed in an aqueous solution [11,12,13].

Regarding starch from flours, the municipality of Crato, in the interior of Ceará, Brazil, has historically been known for having many flour houses; currently, there is only the Mestre José Gomes Flour House [14]. Thus, the present work aimed to obtain and characterize biodegradable films from agroindustry residues prepared using the casting technique, sugarcane fiber as a strengthening agent of the structure, and the addition of antimicrobials, potassium sorbate, and glycerol plasticizer to reduce environmental impacts.

2. Materials and Methods

Cassava flour was acquired as a product of local agro-industrialization in the city of Crato-CE and its surrounding region. Other ingredients, such as glycerol (plasticizer, Nox Solutions) and potassium sorbate (antimicrobial, Pryme Foods), were made available by the Laboratory of Food Technology and Biocomposite Characterization of the Agronomy Course-CCAB-UFCA-Ceará-Brazil. The extraction, drying, and crushing processes of sugar cane fiber follow the recommendations by Franco et al. [15]. During the preparation of biodegradable films, the casting technique was used and all determinations carried out, comparing the control film, which did not receive fiber addition (FF), and the film with sugarcane fiber addition (SFF). The preparation of the filmogenic solution (Figure S1) follows the method of Wanderley & Ribeiro, [16] (modified), consisting of mixing, cooking (at 80 and 95 °C for 15 min), spreading on plates, and drying (in an oven with air circulation at 60 °C for 24 h). The formulation and quantity of the ingredients were as follows (Table S1): starch (48 g of FF and SFF); potassium sorbate (3.2 g of FF and SFF); glycerol (14.4 g of FF and SFF); sugarcane fiber (0.0 g of FF and 2.0 g of SFF); and distilled water (1.6 mL of FF and SFF). The biofilms were characterized by thickness, grammage, density, moisture content, water vapor permeability (WVP), FTIR infrared spectroscopy, scanning electron microscopy (SEM), and soil degradability assay. The thickness was determined at 7 random points using a digital caliper (Kala Brand 6” digital). The final thickness was determined by the arithmetic mean of the measurements (seven random points) performed in each sample. The moisture content (dry base) was initially determined using the constant weighing technique (electronic analytical balance SHI-AUW-220D, Shimadzu Brazil), and the mass (g) of the film was measured and then placed in an oven with air circulation (Brand SOLAB and Model SL102/1152, Piracicaba, Brazil) at a temperature of 105 °C [17]. The determination of grammage was carried out after dehydration in a desiccator for 24 h, dividing its mass (g) by the corresponding surface area (cm²) [18,19]. Their density also influences the mechanical and barrier properties of flexible films. To determine its weight (g/cm³), divide the grammage by its respective thickness. The gravimetric determination of water vapor permeability (WVP) is based on the ASTM methodology [20], with some adaptations. Fourier transform infrared spectroscopy (FTIR) was recorded using an attenuated total reflectance (ATR) configuration in a Vertex 70 (Bruker) spectrometer (Bruker, AVANCE, Billerica, MA, USA). under vacuum, employing a Ge crystal. A total of 64 scans and a resolution of 4 cm⁻¹ were used to obtain spectra with good signal-to-noise ratios. The range used was 4000–600 cm⁻¹, and measurements were made at room temperature. The biodegradation test in the soil consisted of burying the films in pots containing soil. The experiment was maintained in the laboratory during the biodegradation tests, obeying the recommendations of the ASTM G 160-03 standard [21]. The degradation of the composites was analyzed through visual tests and the evaluation of the mass loss of the films. The results were analyzed using the Statistic program, version 7.0. The Tukey’s test (p ≤ 0.05) was used to compare the means, version 7.0 [22].

3. Results and Discussion

Table 1. Characterization of films without fibers and with sugarcane fiber.

Characterization	Films
	FF *	SFF **
Moisture content (%)	15.43 ± 0.78 b	16.58 ± 2.07 a
Thickness (mm)	0.75 ± 0.04 b	1.06 ± 0.24 a
Grammage (g/cm2)	0.08 ± 0.00 b	0.09 ± 0.00 a
Density (g/cm3))	1.12 ± 0.00 a	0.81 ± 0.00 b
WVP (g/dia.cm2)	0.005 ± 0.003 b	0.010 ± 0.007 a

* FF: fiberless film, ** SFF: sugarcane fiber film. WVP = water vapor permeability. Mean of seven repetitions ± standard deviation; letter in the same column indicates a significant difference between the samples according to the Tukey’s test (p ≤ 0.05). Source: Authors, 2022.

presents the characterization of the films in terms of moisture content, thickness, grammage, density, and permeability.

3.1. Determination of the Moisture Content in the Films Made by the Casting Technique

The values found for the moisture content resulted in 16.58% of the film with fiber and 15.43% of the film without fiber, showing a statistical difference between the FF and SFF samples, as shown in Table 1, which is similar to the study carried out by Costa et al. [23]. The research evaluated films obtained from the starch of macassar beans and found a moisture content between 14.45% and 26.50%. Therefore, one of the most important physicochemical parameters is the moisture content because the destabilization of the polymeric matrix can occur due to filmogenic solutions with a high probability of acquiring moisture [24].

3.2. Determination of Film Thickness

The film thickness results were 0.75 mm for the control film and 1.06 mm for the SFF (Table 1). Statistical analysis confirmed significant differences between them, contrasting with previous findings by Franco et al. [15], who reported thicknesses of 0.189 to 0.292 mm for starch films reinforced with sugarcane fibers. The control film’s thinner profile of 0.75 mm, lacking fiber reinforcement, contrasts with the SFF. Films reinforced with sugarcane fiber exhibited no breaks or fractures post-drying, indicating good handling. In contrast, films without fiber additions showed fractures and ruptures when handled, emphasizing the impact of reinforcement. Film thickness is influenced by drying conditions and preparation methods [25], which are crucial for assessing material uniformity, measurement repeatability, and validity of comparisons [26,27]. Thickness data provides insights into mechanical strength and water vapor barrier properties.

3.3. Determination of Film Weight

The data obtained for grammage, as shown in Table 1, were 0.08 and 0.09 g/cm², respectively. According to Costa et al. [23], larger grammage levels offer better mechanical strength and an improvement in the barrier to gases and water vapor on the material. Film grammage is defined as the mass of a given area of the material, which is directly related to the mechanical strength of the films. Greater weights offer greater mechanical strength [28].

3.4. Determination of Film Density

According to the parameters presented in Table 1, the values for density are 0.81 g/cm³ and 1.12 g/cm³ for the fiber film and for the control film, respectively, being similar to the study of Sales et al. [29], where the results obtained were 0.97 to 1.15 g/cm³ for biodegradable films of corn starch incorporated with green propolis extract. Comparing the results, it was found that the control film was denser than the film with sugarcane fiber.

3.5. Evaluation of the Vapor Permeability Presented by the Films

The water vapor permeability (WVP) parameter is crucial for analyzing the degree of inlet or outlet coating on the film. As shown in Table 1, a statistical difference was found between the samples of the control film (FF) and the film with sugarcane fiber (SFF). Studies indicate that biodegradable films, especially starch-based ones, have high water vapor permeability and low mechanical properties, requiring plasticizers and reinforcing materials [30]. Table 1 shows that films with added sugarcane fiber exhibited a good water vapor permeability, which is positive, as a higher permeability accelerates the degradation of the film in soils. Rocha et al. [31] justified the high water vapor permeability value for the Citrullus lanatus biofilm as having been influenced by glycerol. The higher the content of this plasticizer, the higher the WVP. Thus, the importance of these films produced with biodegradable polymers is highlighted, as they offer an alternative for sustainable packaging. Martins and Costa et al. [32] reinforce that the development of biodegradable materials from renewable sources and the consequent reduction in the use of synthetic plastic polymers promote environmental preservation, an essential goal of the packaging industry.

3.6. Infrared Spectroscopy Analysis—FTIR

The infrared spectra of the control film samples without fibers and the samples of the film with added sugarcane fiber were evaluated. These analyses reveal some changes that occurred in the films. Overall, the films exhibited similar behavior when subjected to FTIR analysis (Figure S2).

All film formulations presented spectra in characteristic bands: The broadband at 3336 cm⁻¹ associated with the two samples corresponds to the stretching of the O-H groups present in the starch and water molecules. It is possible to infer from the spectra obtained that the incorporation of the fibers into the polymeric matrix was sufficient to perform changes in the spectra in the 3300 cm⁻¹ region [33,34]. The biofilm showed a long vibrational band at 3272 cm⁻¹ characteristic of the O-H group, due to the hygroscopicity of the polymeric matrix. Mei and Oliveira [35] observed a band between 3300 and 3500 cm⁻¹, relative to the axial vibration belonging to the O-H group.

The 2926 cm⁻¹ band, which appears in both spectra in the range of 2908 cm⁻¹ to 2933 cm⁻¹, is attributed to the stretching of the C-H group of the aldehydes present in the polymeric matrix, being of lower intensity in the film without the use of fibers, while the 2850 cm⁻¹ band associated with the spectrum in which the fibers were added corresponds to the C=O absorption of esters [34].

The 1749 cm⁻¹ band that is observed only in the spectrum of the control film sample can be attributed to the C=O absorption of esters subject to conjugation and inductive effects, which is the name given to the transit of electrons belonging to single covalent bonds within a saturated carbon chain [36,37,38,39]. The 1646 cm⁻¹ band that is in the 1675–1645 cm⁻¹ range corresponds to the functional group C=C. The 1033 cm⁻¹ absorption band associated with the two samples corresponds to the C-O stretch of esters. Some authors, such as Bodirlau et al. [40] and Faria et al. [41], had already reported spectra of the bands very similar to those observed in this study for films containing cassava starch. Although additives are used in the formulation of the films, such as plasticizers, it is noted that there are not many modifications, like the chemical bonds of the starch that are formed in the films studied; only the displacement or different intensities are observed, which indicates greater or lesser interaction between the additional components.

In this study, we observed that bands appear in films with fibers. It is also noted that there are modifications, like the chemical bonds of the water-treated film. The displacement or intensities of different bands are also observed, which indicates greater or lesser interaction between the additional components.

3.7. Scanning Electron Microscopy (SEM)

The biofilms produced with cassava starch were micrographed by SEM to evaluate the continuity of the filmogenic layer in its entirety. In Figure 1, it is possible to verify the microscopy of the control film sample without the addition of reinforcement composites and the micrograph of the starch film sample with sugarcane fiber reinforcement (Figure S3).

Through micrography, it is possible to verify that the cassava starch film presents ruptures and undissolved starch lumps, resulting in the infeasibility of using the structure in the carried out study. Those who produced biofilms of pine nut starch with xanthan gum and glycerol prepared by the casting process observed the same behavior and found a dense structure with small cracks in the structure of the micrographs of the films. They stated that this problem may be related to the storage conditions of the samples during the drying process (the forced convection of the air or the high temperature [42]). The micrograph, referring to a sample of starch film reinforced by sugarcane fiber, showed a compact structure with no breaks, reaffirming the need to use composites to strengthen the structure.

3.8. Soil Film Degradability Test

The biodegradation test (Figure S4) in simulated soil was carried out by ASTM G 160-03 for 60 days. Irrigation was performed daily, and after 30 days, an almost complete mass loss of the films was observed. The control film exhibited a 43% mass loss during the same period, which was attributed to ruptures and undissolved starch clumps, compromising the structural viability for the study’s intended purpose, which involved the packaging of plant seedlings and soil, requiring durability during handling. In contrast, the film containing sugarcane fiber showed a 65% mass loss in 30 days, due to the higher fiber content of the raw material used. After 60 days, both films exhibited total degradation. Similar behavior was observed by other researchers analyzing biodegradability in natural soil, which contains microflora of bacteria, fungi, and protozoa, accelerating the process. Thus, the films in this study showed significant degradation, emphasizing the importance of developing biodegradable packaging to replace non-biodegradable polymeric packaging. Other tests are evaluating different irrigation periods and conditions [27,43,44].

4. Conclusions

Using flour house residue, cassava starch was employed to create biodegradable films suitable for various packaging purposes, like coriander, lettuce, and chives. Key properties considered in the production were water vapor permeability and mechanical resistance. Desirable film characteristics included low WVP, low solubility, and high mechanical strength. Control films had undissolved starch lumps and structural weaknesses. Conversely, films with added sugarcane fibers showed intact structural networks but biodegraded too quickly for mass loss calculation. Thus, sugarcane fiber from the juice industry can reinforce biodegradable agricultural films, potentially replacing plastic seedling bags.