Andiroba Oil (Carapa guianensis Aubletet) as a Functionalizing Agent for Titica Vine (Heteropsis flexuosa) Nanofibril Films: Biodegradable Products from Species Native to the Amazon Region

Abstract

The diversity of species in Amazonia is exceptionally vast and unique, and it is of great interest for industry sectors to explore the potential of derivatives with functional properties for packaging applications. This study proposes the functionalization of cellulose micro/nanofibril (MFC/NFC) suspensions from Heteropsis flexuosa with andiroba oil to produce films with packaging potential. MFC/NFC was produced by using mechanical fibrillation from suspensions of H. flexuosa fibers. Proportions of 1, 3, and 5% of andiroba oil were added to make films with concentrations of 1% (m/m). Suspensions with andiroba oil provided greater viscosity, with changes in the physical properties of the films. Functionalization with andiroba oil provided films with lower degradation in water, greater contact angle, and lower wettability despite high permeability to water vapor. The films with 1% andiroba oil showed a hydrophobic characteristic (contact angle > 90°) and greater puncture resistance (6.70 N mm⁻¹). Films with 3% oil showed a more transparent appearance and high biodegradation, while 1% oil generated more opaque films with a higher thermal degradation temperature and high antioxidant activity. It was concluded that films produced from H. flexuosa fibers functionalized with andiroba oil showed packaging potential for light, low-moisture products due to their adequate thermal and barrier characteristics.

1. Introduction

Materials from renewable and biodegradable sources have gained attention from the scientific and industrial communities [1,2,3]. Conventional petroleum-based polymeric products are being replaced due to immense ecological disruption, such as that from plastic pollution [1]. Given the number of lignocellulosic materials of plant origin available, many species still need to have their potential evaluated, especially in nanotechnology. Cellulose is the most abundant natural polymer in the world. Due to its interesting chemical, physical, mechanical, and microstructural properties, it is one of the primary raw materials employed to solve these issues [4,5].

In Amazonia, a wide variety of raw materials can be better explored to obtain products with sustainable management principles. Heteropsis flexuosa (Kunth) GS Bunting, popularly known as titica vine, belongs to the family Araceae, is typical of the Amazon region, and occurs in non-flooded areas [6]. When they emerge from aerial roots flung to the ground, they are thick, woody, resilient, and long-lasting [2].

Furthermore, in the Amazon, there is also a growing interest in natural oils, such as andiroba oil (Carapa guianensis Aubl.), due to its potential for the development of phytoproducts for application in the food and pharmaceutical industries [7], cosmetics [8], and in insect repellents [9], in addition to the treatment of ectoparasites that enhance the respiratory activity of soil microorganisms, which facilitates their biodegradation [10].

The literature strongly presents the potential of different essential oils in nanocellulose matrices, which offer antimicrobial properties in food packaging, as many researchers have been committed to studying this function [11]. The characterizations of cellulose micro/nanofibril (MFC/NFC) suspensions and films functionalized with essential oils, mainly in relation to the mechanical and barrier properties of the films, seem unprecedented, but some studies have already been developed, such as the use of oil of copaíba [4], cinnamon [12], chitosan/oregano [13], thyme [14], and clove [15].

Given the importance of characterizing MFC/NFC, understanding the existing techniques for preparing bio-based films, such as casting, melt mixing (extrusion), electrospinning, and polymerization, is also necessary for industrial and scientific communities [11]. Casting has aroused more interest among scientists due to its easy handling despite not presenting a more industrial approach than the extrusion technique. The electrospinning process has performed satisfactorily due to the production of fibrous membranes that are capable of encapsulating and stabilizing bioactive molecules. However, the polymerization technique has provided less interest compared to other techniques due to its complicated conditions based on reactions and the use of toxic reagents and solvents [3].

With respect to the casting method, there is still a lack of clear scientific evidence about its role in the physical, mechanical, colorimetric, and barrier properties of films, in addition to its antioxidant activity and biodegradability, which are extremely important in packaging applications aimed at sustainability. In this context, this study proposes to promote the functionalization of the MFC/NFC suspensions from Heteropsis flexuosa with andiroba oil, an important raw material from the Amazon region, and to verify the action of the functionalizing agent on the quality properties of suspensions and films that can potentially be applied in ecological packaging.

2. Methods

2.1. Obtainment of Materials

H. flexuosa was obtained from local stores in the Amazon region, Brazil. The material was reduced in length by ~3 cm and subsequently ground and sieved using a fraction between 35 mesh (0.500 mm) and 60 mesh (0.250 mm) [16]. Andiroba oil (Carapa guianensis) (density 0.860 g cm⁻³) was supplied by a company that specializes in Amazon flora products with medicinal value. Hydroxypropyl methylcellulose (HPMC) (Celotex K60) was obtained from Aditex LTDA (São Paulo, Brazil) and used to improve the dimensional stability of the films during the casting drying process. Tween 80, used as a surfactant, was supplied by the manufacturer Labsynth (São Paulo, Brazil).

2.2. Alkaline Treatment of Fibers

The ground H. flexuosa underwent an alkaline treatment, as described in Dias et al. [5], by preparing a 5% (m/m) NaOH solution containing fibers at 5% (m/v). The fibers were kept in a water bath at 80 ± 2 °C under constant stirring at 500 rpm for 2 h and then posteriorly washed with deionized water until neutral pH was achieved.

2.3. Density Determination and Chemical Composition of the Fibers

The basic density of the fibers was obtained following a methodology adapted from the NBR 11941 standard [17]. The total extractives of the fibers were obtained according to the standard adapted from TAPPI T 204 om-97 [18], using acetone (2:1) for 4 h in a Soxhlet extractor with a final extraction in hot water.

According to the widely accepted TAPPI T 222 om-02 [19] standard, to obtain lignin, a critical component is extractive-free samples, which were used and placed in a container with 72% H₂SO₄ (15 mL per sample) for 2 h at room temperature and then in a water bath at 103 ± 2 °C for 4 h. Finally, the samples were filtered in a crucible and taken to the oven (103 ± 2 °C).

Holocellulose was obtained according to Browning [20]. In detail, 2 g of extractive-free material was placed in a 125 mL Erlenmeyer flask along with 55 mL of water, 2 mL of a 30% (w/w) sodium chlorite solution, and 2 mL of acetic acid solution (1:5, v/v) in a water bath at 70 ± 5 °C. Acetic acid and sodium chlorite solution were added every 45 min. After 4 h, the mixture was slowly cooled and filtered through a small glass container. The resulting material was washed with deionized water.

Cellulose was obtained according to the study by Kennedy, Phillips, and Williams [21]. In an Erlenmeyer, approximately 1 g of holocellulose and 15 mL of 24% (w/w) KOH solution were added. The material was kept under stirring at room temperature for 15 h and filtered in a glass crucible. The resulting cellulose was washed with deionized water with two portions of 1% acetic acid and ethanol and then dried (103 ± 2 °C). Hemicelluloses were obtained by using the difference between holocellulose and cellulose.

Ash was obtained according to TAPPI T 211 om-02 [22]. Approximately ~2 g of samples were placed in previously calcined crucibles and transferred to a muffle furnace heated at 1.67 °C/min until it reached 525 °C for 3 h. Average values from all the analyses were performed in triplicate.

2.4. Mechanical Fibrillation of H. flexuosa

H. flexuosa was mechanically fibrillated using the SuperMassColloider Masuko Sangyo (Kawaguchi, Japan) with two stone discs. The material was processed by making 20 passes with the equipment set at 1500 rpm [23]. The initial distance between the stones was 10 μm and this was gradually modulated according to the suspension viscosity.

2.5. Energy Consumption of MFC/NFC Production

Energy consumption (EC) was recorded by considering the average amperage of each fibrillation cycle, the equipment voltage, and the fibrillation time per ton of H. flexuosa fibers processed [2]. The EC was calculated according to Equation (1).

$$EC(kWh/t)=\frac{\left(P\times t\right)}{m}$$

(1)

where P (voltage × electric current) is the equipment power (kW); t is the time taken for fibrillation (h); and m is the mass of pulp subjected to fibrillation (t).

2.6. Structural and Dimensional Analysis of the MFC/NFC

Microscope images were obtained to estimate the residual fraction and diameters of the CNFs after the mechanical fibrillation process. First, an Olympus BX41 light microscope (Tokyo, Japan) was used. The MFC/NFC suspensions were diluted in deionized water to a concentration of 0.1% (m/m). Using a Pasteur pipette, drops of the suspension were added to glass slides and stained with a 0.5% (m/m) aqueous solution of astra blue to increase the contrast with fiber fragments and aggregates, covered with a coverslip for observation, and observed under the microscope using the 10× objective at a range of 1280 × 1024 pixels.

Advanced Image J software (Version 1.54i) [24] was utilized to analyze the images and obtain the average area of the visible particles (macroscopic dimension). For each sample, five images were evaluated in this procedure. The relative frequency observed for visible particles smaller than 5 µm², between 5 and 10 µm², and greater than 10 µm² was used to calculate homogeneity. Therefore, homogeneity increases with the number of particles in the same class of dimensions [25].

Further, suspensions were observed using a TESCAN Clara ultra-high resolution FEG scanning electron microscope (Kohutovice, Czech Republic) under 10 KeV, 90 pA, and with a working distance of 10 mm. The material was metalized with gold in a sputtering device and then analyzed. Five images were obtained. The diameters were measured by recording 200 MFC/NFC information using the Image J software [24].

2.7. Stability of the MFC/NFC Suspension

The stability of the suspensions (ES) was evaluated according to the methodology presented by Scatolino et al. [4]. In test tubes, 10 mL of the MFC/NFC suspensions were added with a concentration of 0.25% (w/w) that was previously homogenized on a magnetic stirrer at 500 rpm for 1 h. The test tubes with the suspensions were kept at rest and photographed every 1 h, for 8 h, and then after 24 h. Image J software [24] was used to estimate the stability of the MFC/NFC suspensions from the images. The stability calculation was carried out according to Equation (2).

$$ES\left(\%\right)=\left(\frac{AP}{AT}\right)\times 100$$

(2)

where AP corresponds to the height of the particles suspended in the test tube, and AT is the total height of the liquid in the tube.

2.8. Viscosity of the Suspensions

The viscosity of the MFC/NFC suspensions was determined according to the TAPPI T 230 om-19 standard [26]. The suspension solution was evaluated at a concentration of 0.5% (m/m) in cupric ethylenediamine (0.5 M). According to Equation (3), the measurement was obtained concerning the efflux time of the suspensions in a Cannon–Fenske viscometer number 150. Viscosity (V) analysis was carried out closer to the time of production of the films.

$$\mathrm{V}\left(Cp\right)=\mathrm{C}\times \mathrm{t}\times \mathrm{d}$$

(3)

where V is the viscosity of the MFC/NFC suspensions with cupric ethylenediamine at 25 °C (Cp); C is the viscometer constant (by calibration); t is the average efflux time (s); and d is the density of the cellulose suspension (1.052 g/cm³).

2.9. Production of the MFC/NFC Films

HPMC (5%) and Tween 80 (2%), both about the mass of the MFC/NFC, were added to allow for compatibility between the nanofibrils and the andiroba oil suspensions. The treatments were developed according to the concentration of andiroba oil in the MFC/NFC (

Table 1. Treatments evaluated with different concentrations of andiroba oil added to the MFC/NFC of H. flexuosa.

Tratament	Andiroba Oil (%)
Control	0
An1	1
An3	3
An5	5

).

Ten films were produced from each treatment by casting method using 50 g of suspension (1% m/m) in acrylic Petri dishes 15 cm in diameter (Figure 1). The samples were dried in an air-conditioned environment (20 ± 3 °C and humidity 65%) until complete detachment of the dishes occurred.

2.10. Physical and Morphological Properties of the Films

The thicknesses of 10 film samples (30 mm × 30 mm) were measured using a Mitutoyo digital micrometer with an opening between 0 and 25 mm and a resolution of 0.001 mm following the TAPPI T 411 om-15 [27]. The grammages were obtained by using the TAPPI T 410 om-08 standard [28], weighing the samples on an analytical balance (0.001 g) and measuring their diameters with a digital caliper (0.001 mm) to calculate their areas. The bulk density (ρa) of the films in g/cm³ was determined by the ratio between the grammage and thickness of each film. The porosity (Φ) was calculated using the value of the film’s bulk density (ρa) and the value of cellulose density (1.54 g/cm³), according to Desmaisons et al. [25] (Equation (4)).

$$\Phi \left(\%\right)=1-\left(\frac{\rho a}{1.54}\right)\times 100$$

(4)

The MFC/NFC film samples with dimensions of 5 mm × 5 mm were submerged in liquid nitrogen for instant freezing and then fractured. After drying, the samples were taken for metallization with gold before obtaining 5 micrographs (surface and fracture) in an ultra-high resolution FEG TESCAN Clara scanning electron microscope (Kohoutovice, Czech Republic) under 10 KeV, 90 pA, and with a working distance diameter of 10 mm.

2.11. Fourier Transform Infrared Spectroscopy (FTIR)

A Varian 600-IR FTIR spectrometer with Fourier transform was used for vibrational infrared spectroscopy analyses. It was connected to a GladiATR accessory from Pike Technologies to measure attenuated total reflectance (ATR) at 45° with a selenide zinc crystal. The 400–4000 cm⁻¹ spectral range, 32 scans, and a resolution of 2 cm⁻¹ were examined. Films made with the MFC/NFC at 1% (m/m) were evaluated from each treatment.

2.12. Light Transmission and Transparency of the Films

Following ASTM D1746 [29], the light transmittances of the films were measured with a Thermo Scientific Genesys 10S UV-Vis Spectrophotometer (Waltham, MA, USA) set to operate at 600 nm. The samples were prepared with measurements of 30 mm × 20 mm and placed in the apparatus so that the light beam could pass through them. Transparency (T600) was computed using Equation (5), adapted from Dias et al. [5].

$$T_{600}=\frac{Log\%T}{ft}$$

(5)

where %T is the percent transmittance, and ft is the film thickness (mm).

2.13. Coloration and Opacity of the Films

The functionalized films were analyzed using a Konica Minolta CM-5 colorimeter (Osaka, Japan), adjusted to a viewing angle of 10° and illuminant D65 (daylight). Five measurements were taken for each treatment to determine the color parameters (L*, a*, b*, C*, hue). Following the procedures of Fakhouri et al. [30], the apparent opacity of the sample (Y) was calculated from the relationship between the opacity of the sample placed in the black standard (Yp) and the opacity in the white standard (Yb), where Y is equivalent to the value of L* and was obtained by Equation (6), adapted by Lago et al. [31]. The results were expressed on a scale from 0 to 100%.

$$Y(\%)=\frac{Yp}{Yb}\times 100$$

(6)

2.14. Thermal Degradation of the Films

The thermogravimetric analysis (TGA) was performed using a TGA Q500 TA Instruments thermal analyzer (New Castle, DE, USA) set to a heating ramp of 10 °C/min. Samples containing 7 to 10 mg of film were cut, placed in the equipment’s support, and heated from 25 to 600 °C under a nitrogen atmosphere using a gas flow of 50 mL/min. According to Scatolino et al. [4], the intersection of the tangents to the constant mass region and the linear part of the curves after the deflection point yielded the initial degradation temperature (T onset). Similarly, the curve’s end point indicates the mass loss percentage, and the peaks in the curves derived from thermogravimetric analysis (DrTGA) indicate the maximum degradation temperature (T max).

2.15. Degradation in Water of the Films

Samples measuring 30 mm × 30 mm were dried in an oven at 105 °C, weighed, and immersed in a beaker with deionized water for 24 h. Subsequently, the water was removed, the samples were dried again at 105 °C for 24 h, and then they were weighed again. The disintegrated portion of the samples was calculated according to Equation (7), found in Silva et al. [32]. The values were obtained as an average of five samples per treatment.

$$WD(\%)=\left(\frac{Mi-Mf}{Mi}\right)\times 100$$

(7)

where WD is the degradation in water; Mi is the initial mass of the sample (g) (before the immersion and drying); and Mf is the final mass of the sample (g) (after drying).

2.16. Barrier to Water Vapor and Grease

The water vapor permeability of the films was evaluated following the permeability cell methodology described by Guimarães et al. [17], which was based on the ASTM e96-16 [33] standard. Samples with a diameter of 1.5 cm were sealed in a glass permeation cell partially filled with silica gel and placed in desiccators containing a saturated NaOH solution at 36 °C and 90% relative humidity. The capsules with the films and silica were weighed on an analytical balance (0.001 g) for 8 days. The water vapor transmission rate (WVTR) and water vapor permeability (WVP) were, respectively, calculated by Equations (8) and (9), found in Silva et al. [2].

$$WVTR=\left(\frac{M}{t\times A}\right)$$

(8)

$$WVP=\left(\frac{WVTR\times Th}{p\times (H_o-H_i)}\right)$$

(9)

where M/t is the angular coefficient of the graph obtained by the linear regression of mass gain (g) and conditioning time (days); A is the area of the sample (m²) exposed; Th is the thickness of the samples (mm); p is the vapor pressure (kPa); and (H_o − H_i) is the difference between the external and internal humidity of the capsules at 25 °C containing the samples.

The grease resistance test followed the TAPPI T559 pm-96 standard [34]. The film samples were cut with dimensions of 216 mm × 279 mm, in which drops of the solutions were applied under the surface of the films with the excess liquid being slightly removed after 15 s. The films were ranked with a solution score (1 to 12) added. The solutions were classified from 1, with low penetrating power and composed only of oil, to 12, with high penetrating power, using n-heptane and toluene.

2.17. Surface Properties of the Films

The contact angle and wettability were evaluated using the TAPPI T458 om-94 [35] standard and a Krüss DSA30 goniometer (Hamburg, Germany). The film samples, measuring 10 mm × 50 mm, were fixed on a glass slide and placed on the base of the equipment’s image acquisition system. For evaluation, deionized water was applied to the films to calculate the average contact angle between the water droplet and the surface after 5 s. The wettability (W) of the films was calculated with the average values of the contact angles measured between 5 and 55 s, according to Equation (10) as used by Mascarenhas et al. [36].

$$W \left(°/\mathrm{s}\right)=\frac{\left(A-a\right)}{55}\times 100$$

(10)

where A is the average contact angle after 5 s (°), and a is the average contact angle after 60 s (°).

2.18. Mechanical Properties

Mechanical tests were carried out using a texturometer (Stable Micro Systems, TATX2i, Godalming, UK) equipped with a load cell with a capacity of 500 N, following ASTM D 882-18 [37]. Each sample thickness was calculated by averaging ten measurements taken along the length using a digital micrometer (0.001 mm). Tensile test measurements of resistance, Young’s modulus, and elongation at break (%) were made using five samples (100 mm × 100 mm). The test speed was 0.8 mm/s, and the initial distance between the grips was 50 mm.

For the puncture resistance test, the specimens were cut with dimensions of 30 × 30 mm and adjusted to the sample support, with 5 mm of contact with the film. The distance between the puncture rod and the sample was manually adjusted, and the test speed was 0.8 mm/s.

2.19. Antioxidant Activity

The iron-reducing antioxidant activity was evaluated using the FRAP (Ferric Reducing Antioxidant Power) method. Aliquots of 9 µL of antioxidant extracts from the samples were combined with 27 µL of distilled water and 264 µL of FRAP solution TPTZ: 2,4,6-Tris(2-pyridyl)-s-triazine) with (M.W. = 312.34 a.u.m.) from Sigma, ferric chloride hexahydrate (M.W. = 270.3 a.m.), and grade P.A. acetate buffer. After incubation in a water bath at 37 °C for 30 min, the absorbances of the film samples were read at 595 nm on a CM-5 spectrophotometer. The results were calculated based on an analytical curve for ferrous sulfate (FeSO₄) (500 µM to 2000 µM) and expressed in µM ferrous sulfate (FeSO₄) g⁻¹.

2.20. Biodegradation of Films

Soil samples (50 g) were collected in places close to trees and were composed of microorganisms and insects that contributed to the biodegradation simulation of the MFC/NFC films. The soil samples were placed in transparent polypropylene plastic pots and moistened with approximately 10 mL of water during all days of the test to ensure constant soil moisture. The films were cut with dimensions of 2 cm × 2 cm, were ~0.025 mm thick, and were incubated in soil at 28 °C ± 2 °C for 28 days, according to the study by Norcino et al. [38]. For the test, 3 replications were used per treatment. The mass of the samples was meticulously obtained using an analytical balance with a precision of 0.001 g. This ensured the accuracy of the weighing process. Subsequently, images capturing the visual aspects of the films and their mass variation throughout the experiment were recorded.

2.21. Data Analysis

The F test was applied to the collected data for the assessed parameters at a probability level of 5%. The Scott–Knott test was used at a 5% probability level when differences in at least one pair of means between treatments were indicated. The statistical software package SISVAR (Version 5.6) was used to conduct the tests [39].

3. Results and Discussion

3.1. Chemical and Physical Characterization of the Fibers

In the chemical constitution of H. flexuosa fibers, cellulose values were increased after alkaline treatment (NaOH). The content of lignin, hemicelluloses, and extractives showed a significant reduction, demonstrating the effectiveness of the treatment. As for ash, a slight increase in values was observed (Figure 2). This effect of alkalinity is reported in the literature for different lignocellulosic materials, corroborating that of the present study [2,4,23].

Since these polysaccharides prevent the MFC/NFC from coalescing, hemicellulose concentration reductions significantly aid in mechanical fibrillation and cell wall breakdown [5]. Even though alkaline treatment is known to be effective, lignin was not eliminated. It is an amorphous polymer that releases monomers when dissolved in an alkaline solution. The monomers may be rearranged and condensed under different chemical bonds after the system cools, possibly resulting in the formation of distinct lignin–phenolate complexes [40].

In contrast to the function of hemicelluloses, lignin can help prevent cell wall disintegration by increasing its resistance, which lowers the cost of the nanofibrillation process. However, following the mechanical fibrillation process, a systematic decrease in fibril diameter and an increase in residual lignin were noted in the study by Rojo et al. [41]. The amorphous nature of lignin and its plasticization during hot pressing of the nanopapers allowed for a bonding effect, which filled the spaces between the nanofibrils, decreased porosity, and smoothed the surfaces of the nanopapers, according to the same authors.

The low content of extractives, a non-cellulosic component, can favor the deconstruction of the cell wall and enhance the optical properties of the films/nanopapers of potential products generated with the MFC/NFC, such as transparency. Conversely, an increase in cellulose content enhances the final yield of particles on a micro- and nanometric scale. As for ash content, this slight increase is associated with the presence of residues from the alkaline treatment, which are often not completely removed during the fiber washing step after the chemical procedure [32]. Forest Products Laboratory [42] classified H. flexuosa as medium density (0.500–0.720 g cm⁻³) based on their basic density of 0.540 g cm⁻³. The raw material depends on the resistance provided by its cell wall to further aid fibrillation. The cell wall’s thickness is the key to increasing the fibers’ basic density. As a result, there will be more microfibril bundles, which will provide increased resistance to breaking by the mechanical ultra-refiner discs. This could hinder the production of the MFC/NFC and result in higher energy consumption [2].

3.2. Energy Consumption

In producing the MFC/NFC from H. flexuosa, energy consumption was close to 25,000 kWh/t for 20 passes in the fibrillator, in which the gel aspect was formed on the 12th pass (Figure 3). This result indicates that although the alkaline treatment provided improvements in chemical properties (see Figure 2), it was not enough to reduce energy consumption, making it necessary to analyze other means of treatment to improve this parameter, as this is one of the biggest challenges to the production of MFC/NFC on an industrial scale.

The high energy consumption for H. flexuosa may be associated with the slow delamination of the cell wall of the fibers, which reduces the individualization of microfibrils in each pass and minimizes water retention in the suspension, requiring a greater number of passes in the fibrillator to achieve the gelatinous consistency, viscosity, and formation of the NFC network in the cellulose suspension [43].

3.3. Microstructure of the Suspensions

Particle homogeneity was high in the class with dimensions < 5 µm², totaling 54.1% (Figure 4). Therefore, the presence of nanometer-scale fibers was adequate to reduce the surface area of the MFC/NFC and increase the stability of the suspension through homogeneity, achieving this study’s objective.

The morphology of the MFC/NFC was investigated by using SEM to measure the diameters (Figure 5). It was possible to observe a structure of fibril elements with the general appearance of a tightly woven MFC/NFC network with aggregations, a characteristic obtained by mechanical nanofibrillation where shear forces delaminate the fibrils to free and individualize them [44].

The highest proportions of more homogeneous MFC/NFC were in the diameter ranges of 45–60 nm (30%). According to Balea et al. [45], cellulose nanostructures with these dimensions (diameters) are potentially helpful as reinforcing agents. On the other hand, only 7.5% of all the measured structures had diameters > 90 nm, indicating that H. flexuosa has potential as a raw material for producing cellulose nanofibrils. According to Scatolino et al. [4], MFC/NFC with smaller diameters permits better intertwining of the structures because of their larger surface area. This can result in higher densities and improved mechanical, physical, and barrier qualities for the films and nanopapers.

3.4. Stability of the Suspensions

Stability was analyzed from 0 to 8 h and after 24 h, showing decreasing behavior over time (Figure 6a). In the first hours (0–2 h), the state of dispersion remained constant. After 3 h, fragments of the MFC sedimented, with this behavior being visible at the bottom of the tubes from 4 h to 24 h (Figure 6b). Overall, the suspension resulted in ~90% stability, confirming the high amounts of NFC.

The sedimentation after 3 h can be attributed to the presence of non-fibrillated matter in the suspensions. During mechanical fibrillation, some fibers may cling to the equipment’s stone discs, becoming detached at the end of the process without necessarily being fibrillated. This fact can also explain by the large proportion of MFC over NFC in the suspensions. This sedimentation reached a final proportion of approximately 9% after 24 h. A high-molecular-weight constituent such as lignin can also contribute to the precipitation of material at the bottom of the tube. The same behavior can be seen in Silva et al. [2] for the same lignocellulosic material.

3.5. Viscosity of the Suspensions

When the oil was introduced in different proportions, the mixture exhibited no difference in viscosity (Figure 7). The difference in polarity of the components and non-immediate interactions between the andiroba oil and the hydrogen bonds of the MFC/NFC could cause differences in the viscosity of the suspensions. However, the addition of 3 and 5% oil, combined with the addition of HPMC and surfactant, may have resulted in a more homogeneous mixture, with viscosity values between 9.96 and 10.14 cP, respectively.

3.6. Physical and Morphological Properties of the Films

Andiroba oil improved the films’ physical properties, according to the addition percentages (

Table 2. Film thickness, grammage, bulk density, and porosity.

Treatments	Thickness (µm)	Grammage (g m−2)	Bulk Density (g cm−3)	Porosity (%)
Control	43.0 ± 0.9 * a	33.7 ± 1.9 a	0.82 ± 0.01 a	46.7 ± 2.0 b
An1	48.8 ± 1.3 c	41.8 ± 1.4 b	0.85 ± 0.02 b	44.6 ± 2.3 b
An3	46.0 ± 1.9 b	42.3 ± 1.3 b	0.92 ± 0.05 c	40.1 ± 3.4 a
An5	49.6 ± 0.7 d	45.5 ± 1.2 c	0.93 ± 0.02 c	39.3 ± 1.4 a

* Standard deviation; averages followed by the same letter in the same column do not differ by the Scott–Knott test at a 5% probability level.

). The An3 and An5 treatments presented more significant values of grammage and density, with low porosity (40.1 and 39.3%, respectively). Consequently, these results are also related to the thickness of the films, which was increased from 43.0 µm (Control) to 48.8 (An1), 46.0 (An3), and 49.6 µm (An5). Thickness variations are explained by the casting method, which does not apply a vacuum in the film production. This increase in thickness with additives can also be seen in the study by Scatolino et al. [4] on NFC films from commercial eucalyptus and açaí pulps with copaíba oil and vegetal tannins.

The highest bulk density for the produced films is reflected in their surface characteristics. SEM microscopy showed that the films with higher densities (An3 and An5) had more regular surfaces and fewer imperfections (Figure 8). However, the control and An1 treatments provided a more heterogeneous appearance due to the presence of MFC/NFC clusters and cell wall fragments. Another fact is the polarity relationship between the added oil and NFC, which may have contributed to fewer flaws on the biofilm surface.

3.7. Fourier-Transformed Infrared Spectrometry (FTIR)

The spectra showed a difference in transmittance between the film treatments. However, the peaks in the spectral profile were similar to each other (Figure 9). The adsorption at the wavelength close to 3336 cm⁻¹ could have originated from the O–H stretching vibrations of cellulose hydroxyls, cellulose molecules, and intermolecular and intramolecular hydrogen bonds [32]. On the other hand, the low intensity of this band can be attributed to the formation of strong hydrogen bonds between the drying nanofibril bundles, which result in the loss of hydroxyl groups in the form of water, which in turn evaporates in the processing of the films, resulting in a decrease in the amount of accessible OH groups [46]. The 2900 cm⁻¹ band is related to the symmetric and asymmetric C-H stretching of cellulose, as is typical of lignocellulosic materials [23].

The intensity close to 1635 cm⁻¹ corresponds to the angular deformation of the OH groups from the water absorbed in the MFC/NFC [47]. The bands at ~1320 cm⁻¹ are attributed to the in-plane bending vibration mode of methyl and/or C–O stretching, which are characteristic signatures for the presence of the acetyl group [48], while the ~1025 cm⁻¹ band is attributed to the C-O-C vibration of the cellulosic chain [49] and the -C-O stretching of andiroba oil [50].

3.8. Transmittance and Transparency of the Films

In transmittance, the An3 and control achieved the highest result, with transparencies of 26.3% and 24.9%, respectively, at a wavelength of 600 nm (Figure 10). In general, all the films presented low transparency, but it is possible to observe writings throughout all the films produced. The addition of oils without characteristic color was not expected to generate significant changes in transparency in the films, with the different values obtained due to the arrangement of oil and NFC on the plate at the time of film formation or due to drying stresses during casting. Although the raw material evaluated in this study contained non-cellulosic components, the film structure enabled the passage of light and visibility. Light transmittance is influenced by several factors, such as the fibril diameters, dispersion, hemicelluloses content, lignin content, suspension homogeneity, and surface roughness [51]. According to Qing et al. [52] and Okahisa et al. [53], light dispersion is directly related to transparency since MFC/NFC has fibrous fragments that scatter light. As the size of the fragments is reduced, the films tend towards transparency.

Guimarães et al. [23] obtained 65% for transmittance in films from bleached banana tree pseudostem NFC and 58% for commercial eucalyptus pulps without oil addition. Similarly, films produced with NFC from bleached mandacaru cactus were evaluated by Dias et al. [5], who found values of 38% for transparency. The literature shows that the addition of oil to suspensions can reduce the transparency of packaging films. This property is important in the application of films in industry, especially in the food sector, as it allows for visualization of the conservation status of the packaged product. Furthermore, it helps with light dispersion, which gives the food a longer shelf life [54].

3.9. Opacity and Color of the Films

Additive proportions in film An1 and An5 generated more opaque films (

Table 3. Opacity and color of the films.

Treatments	Opacity (%m−1)	L*	C*	hue
Control	0.987 ± 0.03 * b	40.78 ± 2.0 a	11.23 ± 2.64 a	91.34 ± 0.72 c
An1	1.220 ± 0.02 d	44.92 ± 1.1 c	15.11 ± 0.79 b	89.53 ± 1.50 b
An3	0.875 ± 0.01 a	40.46 ± 1.1 a	16.07 ± 0.98 c	86.49 ± 1.80 a
An5	1.013 ± 0.08 c	42.59 ± 0.7 b	16.69 ± 1.20 c	87.68 ± 1.53 a

* Standard deviation, averages followed by the same letter in the same column do not differ by the Scott–Knott test at a 5% probability level.

).

The film An3 had the lowest opacity. In contrast, isolated agglomerates were observed in the control and An1 treatments, culminating in voids on the surface of the films (see Figure 8). These voids allow light to pass more easily than spaces filled with MFC/NFC, which explains the smaller opacity values. It is worth noting that despite the increase in opacity after the addition of andiroba oil, these values remain so as not to compromise the visibility of the product to be packaged. This is particularly important for materials used as edible coatings, as a high opacity can interfere with the original color of the coated product, potentially reducing its acceptance [55]. Additionally, the darkening of the film surface and the increase in the intensity of the yellowish color may be attributed to the higher concentration of andiroba oil, as indicated by the values of L*, C*, and hue.

3.10. Thermal Degradation of Films

The treatments showed thermal resistance up to 250 °C, with a rapid initial mass reduction due to the evaporation of surface moisture. Then, degradation occurred as the temperature increased, with a maximum peak at 500 °C (Figure 11). This trend is better visualized in Figure 8b with the data treated by the first derivative.

The degradation profiles were similar for all treatments. Nonetheless, An5 presented a higher thermal degradation temperature (T_max), while the An3 films had a lower initial temperature (T_onset). The composition of andiroba oil may be one of the factors that culminated in the increase in the thermal degradation temperature of the H. flexuosa films. Andiroba oil is composed of stearic, oleic, linoleic, linolenic, and other fatty acids, and these elements may have decomposed between 460 and 500 °C [56]. Silva et al. [50] suppose that this rise in degradation temperatures is connected to the barrier effect of polymer chains on andiroba oil molecules, which prevents volatile compounds from diffusing and raises the thermal stability of hybrid materials.

3.11. Degradation in Water of the Films

The An5 treatment also presented the best degradation in water (2.69%), with the lowest values found for the control (5.01%). This confirms the effectiveness of the addition of andiroba oil mainly because it is a non-polar substance (Figure 12). As a result, An5 films are more durable in water-saturated conditions, allowing the material to be applied to high-value packaging.

The low degradation in water is related to the stability of the films in high humidity conditions. According to the present study, MFC/NFC films from Heteropsis flexuosa with andiroba oil can mainly be applied for contact with products that have a high water activity, such as edible coatings for fruits and vegetables, for example, as well as coatings for packaging frozen foods, which are subjected to storage under conditions of high humidity [32].

3.12. Barrier Properties and Grease Resistance

In the water vapor transmission rate (WVTR) analysis, the An1 films presented the lowest result (

Table 4. Water vapor transmission rate (WVTR), water vapor permeability (WVP), and grease resistance.

Treatments	WVTR	WVP	Grease Resistance Score
Control	710.3 ± 7.7 * b	2.84 ± 0.03 a	12
An1	686.9 ± 11.7 a	3.35 ± 0.07 c	12
An3	729.4 ± 20.9 b	3.17 ± 0.09 b	12
An5	754.5 ± 11.3 c	4.60 ± 0.16 d	12

* Standard deviation, averages followed by the same letter in the same column do not differ by the Scott–Knott test at a 5% probability level.

). In contrast, in water vapor permeability (WVP), the control obtained the most considerable value, although in the physical properties (bulk density, thickness, and grammage) (see Table 2), the treatments with andiroba oil reached significant values. The WVTR and WVP values must be minimal when applying films to packaging, enabling greater product longevity by reducing moisture transfer between the food and the atmosphere [31].

In the grease resistance test, all the films with MFC/NFC from H. flexuosa showed resistance to the 12 oil solutions due to the most “aggressive” solution with the classification no. 12 being composed of toluene and n-heptane. Similar results were found for different lignocellulosic fibers [2,4], demonstrating their potential for use in packaging materials. The degree of fibrillation explains this blocking of water and fats. The number of pores in the films decrease as this rises [57].

3.13. Surface Properties of the Films

The An1 treatment obtained the highest angle (93.07°), while An5 achieved lower wettability (~0.04°/s) (Figure 13). This highlights that the treatment applied to the MFC/NFC suspension improved the films’ surface parameters, adding a less hydrophilic character with the addition of andiroba oil.

According to Sahraee et al. [58] and Abbasi et al. [59], a contact angle between 0 and 90° is designated for surfaces susceptible to wettability, while values greater than 90° indicate that the material is hydrophobic. Thus, An1 films are hydrophobic, and the others are classified as hydrophilic. Despite this, the increase in the concentration of andiroba oil contributed to reducing the wettability of the surfaces, indicating greater stability in contact with water, as demonstrated in the water degradation test.

For food preservation and protection, these materials typically need a more hydrophobic quality when used for packaging. Food and perishables are the main products that have storage issues when packaged with materials that are hydrophilic (<90°) [32]. Furthermore, the high concentration of hydroxyl groups in the surface layer may be the cause of the hydrophilicity of the films [60].

3.14. Mechanical Properties of the Films

The control treatment showed higher tensile strength (63.59 MPa), although the An3 treatment required a higher elasticity value (3.74 GPa) (Figure 14). The use of andiroba oil may have produced a discontinuous matrix with larger pores and/or cavities throughout the films than those found in the control films, which could account for the observed decrease in tensile strength.

Another explanation is that the oil may have acted as a plasticizing agent, attenuating the bonds between the NFC molecules and making the material more malleable, which could be attractive for industrial applications. This same behavior is corroborated by Scatolino et al. [4], who used copaíba oil and tannin in açaí films, with a 10 and 25% reduction, respectively.

Another factor is that andiroba oil can stop MFC/NFC entanglement. The stress–strain curves highlight that the films from the control treatment were superior in resistance among all treatments. The puncture force An1 obtained 6.70 N mm⁻¹, the most satisfactory value. On the other hand, the casting method presents limitations such as variation in the thickness and weight of the films, humidity gradients, roughness, and an accumulation of more viscous suspensions at specific points of the sample-forming plates, which can provide low mechanical resistance [36].

3.15. Antioxidant Activity of Films

Figure 15 shows the antioxidant activity of the MFC/NFC films with different percentages of andiroba oil based on the FRAP (Ferric Reducing Antioxidant Power) procedure.

A significant increase was observed in the treatments with andiroba oil concerning the control. The explanation for this behavior is that the porous structure throughout the samples with andiroba oil allows for a greater release of antioxidant compounds trapped in the MFC/NFC. Although increased permeability leads to greater antioxidant activity, this also reflects a greater exposure of the material, contributing to the protective action of food against external agents [61].

3.16. Biodegradability of Films

The influence of incorporating andiroba oil on the biodegradation of films was investigated. The results obtained over 28 days show that the four treatments analyzed underwent aerobic biodegradation in the soil (Figure 16).

Two distinct analyses can be established: mass loss analysis and visual analysis of the samples. The An0 (control) and An3 treatments showed more pronounced degradation, especially after 14 days of testing, with a remaining mass of 44.7 and 43.1%, respectively. On the other hand, the An1 and An5 films showed lower percentages of degradation. When visually analyzing the samples, the An1 films had a better or more preserved appearance after biodegradation. This suggests that treatments (3 and 5%) with higher concentrations of andiroba oil did not prevent microbial activity from being exacerbated on the film structure. After fibrillation and alkaline treatment, the NFC still contained a high content of hemicelluloses (~20%) (see Figure 2), which acts as a physical barrier to cellulose and can hinder the interaction between the oil and NFC. Bardi and Rosa [62] mention that biodegradation depends on the degree of interaction between the blend’s components, in addition to the morphological arrangement. Microorganisms affected the surface and depth of the films. As a result of the biodegradation process, the control sample broke into small pieces after 28 days of exposure to the soil. Guimarães et al. [23] evaluated the biodegradation of NFC films comprising banana tree pseudostem and found a remaining mass of ~90% after 28 days. The same authors stated that pores throughout the film may have contributed to the dispersion of enzymes following the microorganism attack. Biodegradation is the breakdown of polymers or other materials by microorganisms, and it results in the production of CO₂, methane, water, cellular biomass, and energy [63]. Moreover, it is affected by crystallinity because amorphous domains in polymers are the targets of enzyme attacks. Due to their inadequate compacting, the amorphous regions of the MFC/NFC are more prone to deterioration [64].

4. Conclusions

In this study, H. flexuosa MFC/NFC films were functionalized with andiroba oil to verify their barrier, thermal degradation, and biodegradation properties. Andiroba oil added to the generated MFC/NFC films caused less degradation in water, greater contact angle, and lower wettability despite the high permeability to water vapor. The An1 films presented the suitable characteristics of a contact angle (>90°) and greater puncture resistance. The An3 films exhibited a more transparent appearance concerning the other treatments and high biodegradation. At the same time, An1 generated more opaque films with a higher maximum thermal degradation temperature (Tmax) and high antioxidant activity. Adding andiroba oil reduced the tensile strength properties of the MFC/NFC films compared to the control. However, the films produced in this study from H. flexuosa fibers and functionalized with andiroba oil showed packaging potential for lightweight, low-moisture products due to their adequate thermal and barrier characteristics. Work with NFC from banana pseudostem, sugarcane bagasse, jute fibers, and mandacaru cactus has already been carried out with promising results similar to those obtained for titica vine. The film production method can be improved using vacuum filtration, extrusion processes, UV drying, etc. Other additives can be tested, such as flavored oils, dyes with antimicrobial properties, and other compounds that can offer more functionality to packaging. Furthermore, studies should be carried out on the methodology of alkaline treatments, such as the use of biodigesters with pressure and temperature in the preparation of waste fibers before fibrillation to reduce energy consumption and improve the properties of the films.