Introduction

New research has demonstrated the potential of nanostructured films/nanopapers from lignocellulosic materials in advanced applications of great interest to society, such as organic solar cells (Fang et al. 2014), organic sensors (Zhang et al. 2014), organic light-emitting diodes (Zhu et al. 2013), and flexible nanopaper transistors (Huang et al. 2013).

The main reasons for the application of cellulose nanofibrils (CNFs) for the production of bio-based materials are their high aspect ratio (> 100) (Iwamoto et al. 2014), crystallinity (≥ 66%) (Fonseca et al. 2021), high capacity in forming flexible films/nanopapers, low thermal expansion, high optical transparency, excellent mechanical properties (tensile strength and Young’s modulus), emulsifying potential in suspensions, and as a barrier (to oil, oxygen, and water vapor), in addition to being abundant and nontoxic. The development of new bio-based devices using nanostructures from lignocellulosic materials is a rather new but rapidly evolving research area (Siró and Plackett 2010), as their application in cementitious composites (Fonseca et al. 2021), coated papers (Mirmehdi et al. 2018), aerogels (Zhou et al. 2016), and nanostructured films (Lopes et al. 2018).

The methods commonly used for CNF production are mechanical, chemical, physical, and biological (Frone et al. 2011). Cellulose nanostructures are presented in the literature with different denominations, such as nanocrystals, nanowhiskers, nanofibrils, and microfibrillated cellulose, depending on the structure of the cellulose (Nystrom 2010). The CNFs show diameters varying from 10 to 100 nm, being attained using a specialized microfibrillator (grinder) with a mechanism consisting of forcing the cellulose fibers through an opening between a rotating stone and a static one. The mechanism generates major shearing forces that breakdown the hydrogen bonds from the multilayered cell walls of the fibers to individualized micro-/nanofibril bundles (Siró and Plackett 2010). In order to generate such nanostructures, the raw fibers should pass through various physical and/or chemical pre-treatments. Chemical pre-treatments normally start with an alkaline treatment (Rosa et al. 2010) consisting of fiber immersion in alkali solution, usually with strong basic compounds as NaOH, under heating and vigorous mechanical stirring. Strong alkaline compounds can penetrate the fiber structure and remove hemicelluloses and any other fiber components as the soluble extractives (Vardhini et al. 2016). Another widely used pre-treatment is the bleaching, which uses chlorinated compounds or hydrogen peroxide in intention to obtain pulp with greater whiteness. This process reduces or removes lignin from the pulp, possibly causing an increase in the cellulose content, chemical reactivity, dimensional stability, tensile properties, and roughness (Zuber et al. 2012). Due to pollutant production issues, chlorinated compounds are being avoided at this stage. The extent of these changes depends on the treatment time, temperature, alkali concentration, degree of polymerization, and source of cellulose (Samei et al. 2008).

Despite the enormous progress and great success in studies involving cellulose nanofibrils in the most diverse areas of science, there are still several challenges regarding the efficiency of the fibrillation in industrial scale, besides the economic aspects (Kubálek et al. 2017; Kliestik et al. 2018). Currently, the main source for CNF production has been commercial kraft pulp. The kraft pulp, especially from Eucalyptus, is the main product of planted forests for the purposes of cellulose production in Brazil. In the kraft pulping, wood in the form of chips is treated under pressure, in tanks called digesters, with sodium hydroxide (NaOH) and sodium sulfide (Na2S) in pH above 12. This chemical process aims to dissolve the lignin, preserving the fiber resistance, thus obtaining a cellulosic pulp with yield between 50 and 60% (Sixta 2006). The uses of kraft pulp range from paper for packaging products, tissue paper for personal care (toilet paper, diaper, absorbents, paper towels, and napkins), and environmental hygiene, to paper for writing and printing. The pulps from Eucalyptus are known to present short fibers, with length from 0.5 to 2.0 mm, and generally has less strength, with high softness compared to the long fibers (Alves et al. 2011). The thickness of the fiber wall ranges, on average, from 2.5 to 6.0 μm (Trevisan et al. 2017). However, the use of other vegetal fibers has also been explored, such as banana pseudostem tree fiber (Elanthikkal et al. 2010), pineapple (Abraham et al. 2011), jute (Fonseca et al. 2021), palm tree (Okahisa et al. 2018), cotton (Chen et al. 2014), sisal (Santana et al. 2017), bamboo (Guimarães Jr et al. 2018), oat straw (do Lago et al. 2020), cocoa shell (Souza et al. 2019), and red cedar bark (Zhang et al. 2019).

Brazil generates several types of wastes that could be a source of vegetal fibers, mainly at post-harvesting in large plantations. Among these wastes, those from banana cultivation can be highlighted. Banana tree wastes include the fruit skin, pseudostem, leaves, and the peduncle (Souza et al. 2010). Mitigation measures for a sustainable banana production chain should focus on the reduction of residues and on ensuring their application in other chains and products, looking for CO2 footprint and other compounds (Maroušek et al. 2020a; Stávková and Maroušek 2021).

Since these wastes are considered lignocellulosic materials, the production of CNFs from them, aiming to develop added-value products, could be a promising alternative. The novelty here is the scientific data contribution regarding the production and properties of the biodegradable films/nanopapers, mainly about their water vapor permeability, light transmittance, contact angle with water, and biodegradability of the films/nanopapers produced with banana pseudostem wastes. The insertion of polymeric additives and nanostructures in the matrix can greatly improve the optical behavior since the diameter of the nanostructures is less than the wavelength of visible light (400 to 800 nm), allowing the complete passage of the light. Examples of polymers for blending include biopolymers such as PLA (poly-lactic acid) (Gazzotti et al. 2019) and biodegradable polymers such as PVA (poly-vinyl alcohol) (Silva et al. 2020) and starch (do Prado et al. 2018). Additionally, parameters such as the greater number of passes through the grinder fibrillator, processing of suspensions with lower concentration, chemical modifications with oxidative reagents with greater selectivity, high temperature pressing, drying temperature of raw materials, and longer sonication time are examples of techniques used to obtain final products with higher transparency (Nogi et al. 2005; Nogi et al. 2009). Processes as vacuum filtration, followed by compression and vacuum drying, could also be an interesting option for production of the films/nanopapers; however, it could make the process more expensive and request greater energy consumption. It is important to highlight that depending on the application, transparent films are not required, since there are several possibilities for using the product where there is no requirement for total transparency. These are important technical-scientific knowledge that are scarce and insufficient in literature for up-scaling packaging applications, for example. Packaging industries are looking for those information and knowledge for advancement in the pre-screening of renewable raw materials for micro/nanofibril production and application in substitution of petroleum-based polymers. Biodegradable and recyclable polymers with high barrier properties are very relevant for multilayers and novel applications in cardboards, card papers, and industrial sacks in the packaging field. Therefore, the aim of the study was to evaluate films/nanopapers of cellulose nanofibrils (CNFs) produced with agro-industrial banana tree pseudostem (BTPT) wastes and Eucalyptus kraft cellulose (EKC) for their water vapor permeability, biodegradability, and light transmittance as well as their mechanical and physical properties.

Materials and methods

Obtaining the raw materials

Banana tree pseudostem (BTPT) (Musa sp.) wastes were obtained from experimental cultures from the southeastern of Brazil. The BTPT was manually cut and dried in environmental conditions (around 25 °C) to allow the evaporation of excess water. Posteriorly, the BTPT wastes were ground in a knife mill (Marconi®; SP, Brazil) to generate a sawdust, which was classified using the superposed sieves of 40 (0.420 mm) and 60 (0.250 mm) mesh. The fraction retained in 60 mesh was used for the next steps. The sawdust yield obtained was between 80 and 85%, since 15 to 20% were lost through the grinding and handling processes. A commercial bleached Eucalyptus kraft cellulose (EKC), supplied by Suzano Paper and Cellulose (Suzano, SP, Brazil), was used as a reference. The commercial pulp was obtained from kraft chemical pulping process (yields of 50 – 60%), with high brightness index of 92% ISO and viscosity 675 cm3.g−1. The pulping and bleaching processes modify the nature of the chemical constituents of fibers, vessels, and cellulosic fines. The mass loss after the commercial bleaching process is between 2.5 and 5.0%. Banana is considered a typical fruit from tropical regions, and its agricultural crops are extremely numerous and spread throughout the Brazilian territory. Considering that the processing sites and technology in nanoscale are mainly located in the south and southeast of Brazil, the costs related to the transport of the raw material tend to be low.

Chemical pre-treatments of the BTPT

The alkaline pre-treatment of the BTPT sawdust was performed following the procedures described in Yue et al. (2015), using 100 mL of solution 5% (w/v) of NaOH in macropearls (Êxodo Científica Inc.; SP, Brazil) for each 5 g of dry sawdust, for 2 h at 80°C (water bath) under mechanical stirring (1500 rpm). After the alkaline treatment, the samples were oven-dried at 50°C and forwarded to the bleaching step. The bleaching was performed using 100 mL of H2O2 (Êxodo Científica Inc.; SP, Brazil) in solution of 24% (v/v) and NaOH in solution of 4% (w/v) for 2 h at 80°C (water bath) and with mechanical stirring (1500 rpm), for each 5 g of the previously alkaline-treated sawdust samples. After the sequence of treatments, the samples were washed to remove residual reagents and oven-dried at 50°C for 24 h. The yields were 59% after alkaline treatment (from the BTPT natural sawdust to alkaline treated BTPT) and 60% after bleaching (from the alkaline treated BTPT to the bleached BTPT), resulting in a total yield of 36% (from the BTPT sawdust to the bleached BTPT). The main goals of the alkaline treatment and bleaching are to increase the brightness of the pulp and promote the removal of components such as lignin and its degradation products, extractives, metal ions, non-cellulosic carbohydrates, and other impurities. In this sense, and in accordance with the environment, the use of peroxides was proposed, instead of the chlorinated reagents (chlorine, hypochlorite, and chlorine dioxide) widely used by many pulp and paper industries, due to their lower cost and a higher yield of the final product. In addition, the reagent concentrations used in the pre-treatments are low.

Chemical analysis of the raw materials

The wastes of BTPT (natural sawdust, alkaline treated, and bleached) and the commercial EKC were analyzed according to the amount of holocellulose (cellulose + hemicelluloses; according to Browning 1963), cellulose (Kennedy et al. 1987), and hemicelluloses, obtained by the difference between the values of holocellulose and cellulose, insoluble lignin according to NBR 7989 (ABNT 2010), and ash content, according to NBR 13999 (ABNT 2003).

Water retention value of the fibers

The water retention value (WRV) of the BTPT and EKC was determined. The pulps were water-dispersed in content of 0.5% (w/t) after being boiled in water for 5 min. The suspensions were filtered with a centrifugal force of 3000 G for 15 min for dewatering, and the wet filtered pulps were weighed. The mass was measured again after being oven-dried at 110 °C for 5 h. The WRV was determined according to Eq. (1):

$$ WRV\ \left(g/g\right)=\frac{Wo-W1}{W1} $$
(1)

where W0 and W1 are the mass of the wet and oven-dried pulps, respectively.

Obtaining CNFs and the films/nanopapers

The pre-treated sawdust of both the BTPT waste and the commercial EKC were dispersed separately in 6 L of water, obtaining a suspension of 2% concentration (based in dry mass of sawdust), and stirred for 10 min (200 rpm). It is important to point out that the EKC was not subjected to any other pre-treatment after being obtained from the industry. The CNFs were obtained from each raw material following the methods suggested by Bufalino et al. (2015), using a Masuko Supermasscolloider mechanical fibrillatory (grinder) at 1600 rpm, keeping an average consumed electrical current of around 5 A. The suspensions were fibrillated in cycles of 20 and 40 passages through the Supermasscolloider. The power consumption was ~4.4 × 103 kWh/ton after 20 cycles and ~9.1 × 103 kWh/ton after 40 cycles. CNF aliquots of 40 mL of suspension with a concentration of 2% (based in dry mass of CNF) were poured on acrylic Petri dishes (15-cm diameter) for water evaporation in a conditioned room (20 ± 3°C; RH ~65%). Ten nanostructured film samples were produced from each of the raw materials (BTPT and EKC) after 20 and 40 passages, totaling 40 flexible films/nanopapers.

Microstructure of the raw materials and CNFs

A scanning electron microscope LEO EVO 40 XVP and typical light microscopy were used in order to observe the microstructure of the different raw materials (BTPT and EKC) and films/nanopapers. The samples were submitted to a metallization process by sputtering, depositing a gold layer on the sample surface. No tilt was applied. A carbon adhesive film was used to fix the samples on the stub. The working distance was 8.5 mm with application of high vacuum.

The structure of the CNFs after 20 and 40 passages was analyzed by scanning electron microscopy with a field emission gun (SEM/FEG). Aliquots of 0.1 mL of suspension samples were diluted in 10 mL of Milli-Q water and dispersed with a Branson ultrasound equipment 101-147-037 (1/2′′ tip diameter) operating at an amplitude of 50%, for 3 min in an ice bath to avoid the heating of the sample. From the diluted and dispersed solution, a new dilution (0.1 mL in 10 mL) and a new dispersion were prepared under the same conditions. Subsequently, a drop of the doubly diluted sample was dripped onto a silicon plate and dried at room temperature. After this procedure, the samples were fixed to a sample holder using a conductive tape (carbon) and kept for 24 h in a desiccator. A JEOL (JSM 6701) microscope equipped with a field emission gun (FEG) was used with the following parameters: work distance 3 mm, acceleration voltage 4 kV, and current 10 μA without sample coating. The software ImageJ® was used to determine the diameters of the samples and CNFs in detail. The diameter of the CNFs was determined by the average of 100 measurements proceeded in the SEM-FEG micrographs. The desired dimensions are provided by the software, proportionally to the scale (known distance) in the scanning electron microscope.

Properties of the films/nanopapers

Mechanical properties

Before the mechanical evaluations, the thickness of the samples was measured. The tensile test was carried out according to the ASTM D882-12 (2012) standard, using a TA TX 2i machine (Stable Micro Systems, England). The distance between the grips was 50 mm and the test speed was 0.8 mm/s. Five samples (25 × 100 mm) were tested for each treatment. The tensile strength (TS) and Young’s modulus were determined according to Eqs. (2) and (3), respectively.

$$ TS=M/{A}_0 $$
(2)

where TS is the tensile strength (MPa); M is the maximum load applied to the sample (N); and A0 is the initial cross-sectional area of the sample (mm2).

$$ Young^{\prime }s\ modulus=S/e $$
(3)

where S is the stress value in the elastic region (GPa) and e is the specific elastic deformation (mm/mm) corresponding to the applied stress.

Apparent density, grammage, and thickness

The apparent density of the samples can be reported as the relation between the film/nanopapers grammage and thickness, as mentioned in the TAPPI T220-om-01 (2004a) standard. The grammage corresponds to a specific mass of area (g m−2), obtained in accordance with the TAPPI T410-om-02 (2004b) standard. The thickness was directly determined by averaging six random measurements on the samples using a digital micrometer (resolution of 1 μm).

Chemical and morphological properties

The chemical groups of the BTPT and EKC films/nanopapers samples were determined by Fourier transform infrared spectroscopy, using a spectrophotometer Vertex 70 model (Bruker, Germany), operating in attenuated total reflection (ATR) mode. Spectra were recorded from 4000 to 500 cm−1 spectral ranges, at a 32-scan rate, and a 4 cm−1 spectral resolution. The effects of different passages (20 and 40×) on the surface and fracture of the samples and the presence of pores in the films/nanopapers structure were observed using a JEOL® JMS 6510 scanning electron microscope with a 10-kV voltage. The films/nanopapers were positioned on aluminum stubs and covered with gold in order to obtain conductive samples. The porosity of the films/nanopapers was calculated according to Eq. (4), proposed by Desmaisons et al. (2017).

$$ P=\left[1-\left(\frac{G}{T\ x\ {\rho}_C}\right)\right]x\ 100 $$
(4)

where P is the porosity of the film (%); G is the grammage (g·m−2); T is the thickness of the film (m); and ρC is the cellulose density (1.540 g·m−3).

Contact angle

The contact angle was evaluated by a Kruss Drop Shape Analyzer—DSA25 (Hamburg, Germany). A water drop was deposited over the sample surface through a syringe. The drop image was captured by a video camera, and the contact angle between the water drop and the sample surface was measured. The test was performed at room temperature (20°C). For each CNFs film/nanopaper, three measurements were performed after the drop stabilization (2 s).

Moisture and water absorption after 2 h of immersion

The moisture was determined using the procedures described in the TAPPI T412-om-02 (2004c) standard. Samples (diameter 30 mm) were immersed in water for 2 h for evaluation of the water absorption (WA 2h) using Eq. (5).

$$ WA\ 2h=\left(\frac{Fm-\mathit{\operatorname{Im}}}{\mathit{\operatorname{Im}}}\right)x\kern0.5em 100 $$
(5)

where Im is the initial mass of the acclimatized sample and Fm is the final mass of the sample after 2 h of water immersion.

Disintegration in water (DW)

Samples (diameter 30 mm) were kept at 65% RH and 20 ± 3°C, weighed, and immersed in 100 mL of distilled water for 24 h. The excess water was then removed, and the samples were dried at 65% RH and 20 ± 3°C, as the initial condition, being weighed again. The disintegrated portion of the samples after the immersion (DW) was calculated according to Eq. (6). The final result was obtained from the average of three measurements for each film/nanopaper.

$$ DW\ 24h=\left(\frac{Iw- Fw}{Iw}\right)x\ 100 $$
(6)

where DW 24h is the percentage of disintegrated material in water; Iw the sample initial mass; and Fw the sample final mass.

Water vapor permeability

The analysis of water vapor permeability of the films/nanopapers was carried out following the permeability cell methodology described in Guimarães Jr et al. (2015). This method determines the amount of water vapor that passes through a known area of sample, induced by the vapor pressure difference between two specific points on the exterior and interior of the permeability cell. Samples with known thickness were sealed in a glass permeation cell containing silica gel (relative humidity 0%; with no water vapor pressure), placed in a desiccator and kept at 25°C and relative humidity 100%. The film/nanopaper was positioned in the cap of the glass bottle, so that it formed a membrane between the exterior and interior of the permeability cell (Figure 1).

Fig. 1
figure 1

Scheme of permeability cell methodology for evaluation of the permeability of the films/nanopapers

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The mass of the permeability cell was measured daily for 10 consecutive days. Five samples per treatment were evaluated. The values of permeability were provided in the water vapor transmission rate (WVTR; g·day−1·m−2), calculated by Eq. (7) according to ASTM E 96-00 (2000).

$$ WVTR=\frac{G}{dxA} $$
(7)

where WVTR is the water vapor transmission rate; G/d is the angular coefficient from the graph obtained by the linear regression of the mass gain (g) versus conditioning time (days); and A is the permeation area of the sample (m2).

Light transmittance of the films/nanopapers

The light transmittance of the samples was measured in a Bel Spectro S-2000 spectrophotometer (Monza, Italy) operated at 600 nm to measure the percent transmittance (%T), according to ASTM D1746-03 (ASTM 2003). The films/nanopapers were cut into 3 × 1 cm pieces and positioned in the equipment to allow the spectrophotometer beam to pass through the sample without any obstacles. Three measurements were performed for each treatment.

Biodegradability

The biodegradability test was performed by measuring the mass loss of the films/nanopapers when incubated in soil. It was evaluated according to the procedures described by Bardi and Rosa (2007). The samples were buried in simulated soil prepared with cattle manure (23%), soil (23%), sand (23%), and water (31%) (m/m), which resulted in a final moisture of around 90%. The test was carried out in a room with a monitored temperature (around 23°C). Each sample (diameter 10 cm) was buried separately in a soil container, and their masses were monitored for 18 weeks. Five samples were evaluated per treatment. After this time, the samples were removed from the simulated soil, and each sample was subjected to visual inspection by light microscopy. Visual parameters that could indicate possible degradation, with or without the direct action of the microorganisms, were evaluated, such as the presence of small cracks, stains, any pore formation, fragmentation, changes in color, and the formation of biofilm on the film/nanopaper surface.

Results and discussion

Mechanical properties of the films/nanopapers

There was an increase in TS and Young’s modulus with the increased number of passages through the grinder fibrillator, independently of the raw material used (Table 1). This was assumed to be due to the dense hydrogen-bonding network formed by the larger surface area of the CNF obtained (Spence et al. 2010) and the diameter reduction caused by the fibrillation cycles (Potulski et al. 2016). The average in diameter ranged from 20 ± 5 to 15 ± 5 nm for the BTPT CNF with the increase in the number of passages from 20 to 40. Zuluaga et al. (2009) found diameters ranging between 40 and 60 nm for CNF bundles isolated from banana rachis, values superior to those found in this research. On the other hand, Velásquez-Cock et al. (2016) found values for diameter of nanocellulose from BTPT ranging between 15 and 20 nm after 30 cycles of mechanical fibrillation, which were close to those found in this research. The average diameter ranged from 31 ± 23 to 21 ± 9 nm for EKC, showing that the sequence composed of pre-treatments and mechanical treatment was effective for generating structures in nanoscale (Table 2), indicating suitable nanofibrillation performance. The lower diameter of the BTPT CNFs is the consequence of the weakness of these non-woody fibers that are easily deconstructed with the shearing forces during the grinding.

Table 1 Average values of tensile strength (TS) and Young’s modulus of the nanostructured films/nanopapers
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Table 2 Average and standard deviation of the CNF diameter and the maximum (Max) and minimum (Min) values measured for each fiber source
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The smaller diameters may explain the better performance of TS for the BTPT films/nanopapers. BTPT presented longer starting fibers (~3.4 mm) (Figure 4) compared to EKC pulp (~0.6 to 0.9 mm) (Figure 5). According to Stelte and Sanadi (2009), fibrillation occurs more rapidly for long fiber species and may be obtained with lower energy consumption. The thickness of the cell wall can also result in easier fibrillation, the cell walls with lower thickness being more favorable to mechanical treatment. Ogunsile and Oladeji (2016) found 5 μm for the cell wall thickness of BTPT. Additionally, the lignin content of these films/nanopapers (Figure 2) did not have a negative effect on the tensile strength. Commercial pulp EKC presented 15% of hemicellulose, whereas for BTPT, the value found was 11%. This is indicative of a greater facility to fibrillation for EKC pulp. Pulps with higher hemicellulose content show a tendency to easier fibrillation, as well as the fibrillation yield decreases as the content of hemicellulose is getting down (Chaker et al. 2013).

Fig. 2
figure 2

Chemical composition of the BTPT and EKC; *ND, not detected; n, natural; at, alkaline treated; b, bleached

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The TS values of films produced with BTPT in the present work were higher than those of synthetic plastics such as linear low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP), used in most packages, with about 37, 7–16, 17, and 35 MPa, respectively (Auras et al. 2004; Avérous 2004; Liu et al. 2012), and papers produced with bleached eucalyptus pulp nanofibers, with TS of around 35 ± 10 MPa (Malucelli et al. 2019). Li et al. (2017) produced papers using CNFs of pine bleached pulp, and vacuum filtering, followed by compression and vacuum drying in the process. For papers produced with partially fibrillated cellulose, they found TS of approximately 26 MPa. Both the EKC and BTPT films/nanopapers obtained the highest values of Young’s modulus with 40 passages of fibrillation.

The significant reduction of film thickness that occurred with the increase of the number of passages from 20 to 40 (Table 3) may also have influenced their mechanical properties. More passages may have resulted in the increase of apparent density, having a positive impact on the mechanical properties. According to Potulski et al. (2016), the decrease of fiber dimensions after the mechanical fibrillation process allows greater bonding and rearrangements of the filaments, forming a more homogeneous and compact structure, and then reducing the thickness. The more compact structure is provided by improved CNF entanglement and greater interaction between them when they show a more fibrillated structure (Lavoine et al. 2012), as verified after 40 passages through the grinder fibrillator.

Table 3 Average and standard deviation values of thickness, grammage, and apparent density of the films/nanopapers
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The presence of fibers with ineffective deconstruction by the fibrillation process favors failures and internal defects such as pores and microcracks, which act as stress concentration spots, reducing the mechanical strength of the samples. Parameters such as thickness and grammage require more detailed studies and with more replications in order to obtain a conclusion about their real trend regarding the number of fibrillation cycles. The morphological structures of the BTPT (Figure 3) showed a more cohesive film/nanopaper with few non-fibrillated fiber structures. On the other hand, the EKC films/nanopapers (Figure 4) presented a significant content of bundles of intact fibers of greater dimensions observed on the sample surface, which may have contributed to the lower TS. The porosities calculated were similar for BTPT films/nanopapers with 20 and 40 passages and EKC with 40 passages. Films/nanopapers from EKC CNFs with 20 passages presented the highest porosity among the treatments.

Figure 3
figure 3

Typical scanning electron microscopy (SEM) images of BTPT films/nanopapers: a) and b) surface and fracture view, respectively (20 passages); c) and d) surface and fracture view, respectively (40 passages); e) and f) SEM/FEG micrographs of the CNFs suspension, respectively, after 20 and 40 passages

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Fig. 4
figure 4

Typical scanning electron microscopy (SEM) micrographs of EKC films/nanopapers: a) and b) surface and fracture view, respectively (20 passages); c) and d) surface and fracture view, respectively (40 passages); e) and f) SEM/FEG micrographs of the CNFs suspension, respectively, after 20 and 40 passages

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The scanning electron microscopy (SEM) micrographs showed some fibers not fully fibrillated after 20 passages, mainly in the EKC films/nanopapers. The amount of non-fibrillated fibers may have contributed to the greater porosity for EKC films/nanopapers (see Table 3). Films/nanopapers produced with EKC CNFs after 20 cycles of fibrillation presented 61.2% of porosity along the structure, the highest value found among the treatments. Non-fibrillated fibers can be a source of defects in the samples due to the pores they cause in the microstructure. The CNF suspensions showed a slightly heterogeneous aspect after 20 passages, possibly containing non-fibrillated and long fiber fragments (Siró and Plackett 2010). As the number of passages increased, there was a decrease of internal pores caused by a more compact and denser structure. This may have promoted a greater number of hydrogen bonds due to the greater specific contact area between the CNFs and a higher content of bonding clusters (Zimmermann et al. 2010). The images show greater individualization of the CNFs as the number of passages increases. Mechanical disintegration through the grinder resulted in fibrillar structures with diameters below 100 nm.

Physical properties of the films/nanopapers

The contact angle obtained for the BTPT and EKC films/nanopapers confirms the surface structure aspects of the samples observed by SEM. The greatest angles were observed for the BTPT films/nanopapers after 20 and 40 passages, with average angles of 81° and 99° after 2 s, respectively (Figure 5). For the EKC film/nanopaper, the greatest average angle with the surface (65°) was obtained after 40 cycles of fibrillation. Denser micro-/nanostructure of the films with high number of passages led to higher contact angles with water. The chemical composition of BTPT used for production of CNF may also have significantly influenced the surface characteristics of the film/nanopaper, as they have a greater amount of lignin compared to EKC (see Figure 2).

Fig. 5
figure 5

Average contact angle (after 2 s) of water with the BTPT and EKC films/nanopapers obtained after 20 and 40 passages through the grinder fibrillator

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Table 4 shows that more cycles of fibrillation produced less hydrophilic films/nanopapers. The samples of both raw materials absorbed a smaller amount of water in 2 h when comparing the 20 and 40 passages, with reduced absorption by 10% for BTPT and 27% for EKC. This occurred due to the more compact organization, denser structure, and less porous morphology, which impairs the penetration of water through the internal structures (Dufresne 2012). More cycles of fibrillation can provide structures in nanoscale with the possibility of forming a more strongly connected network of nanofibrils when compared to fewer fibrillation cycles (Scatolino et al. 2018). Two additional factors that can also influence the resistance to water penetration are the porosity and roughness. The more homogeneous and smoother surface as verified through the morphological analyses of the films/nanopapers after 40 passages may be an effect of the increase in density. In addition, lower values of water absorption indicate stronger cohesion between the CNFs after 40 passages. This is of fundamental importance, since the possible applications for these materials (films for multilayer paper packaging, substrates for electronic devices, solar cells, sensors, loudspeaker membranes, displays, among others) may require reduced values of this parameter.

Table 4 Average and standard deviation of disintegration in water after 24 h (DW 24h), moisture and water absorption after 2 h of immersion (WA 2h)
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The disintegration in water (DW 24h) for all the films ranged from 2.3 to 7.6% and showed the same behavior of the water absorption after 2 h. The BTPT films/nanopapers reduced by 5% the disintegration in water after 40 passages. For EKC, the reduction was 17%. Regarding the type of raw material, the lower disintegration in water found for the EKC films/nanopapers was probably because the raw material came from a commercial source, where they are generally subjected to controlled processes and treatments in the industry, such as reagents for hydrophobization, specific reagents for cellulose purification, and drying. In this sense, the performance of commercial pulps can generally have some advantages when compared to those from agro-industrial wastes. Disintegration in water of films/nanopapers composed by CNFs is mainly related to leaching out of soluble aggregates (hemicelluloses, residual extractives, soluble lignin, etc.) and debonding of the residual fibers and CNFs of the surface, instead of the solubility of CNF components. The water retention value (WRV) results for BTPT and EKC were 1.61 and 1.43, respectively, clearly indicating that the agricultural waste has a greater tendency to absorb water. Additionally, the BTPT presented higher content of non-cellulosic chemical components as lignin and hemicelluloses (see Figure 2), resulting in some difficulty of packing the micro-/nanofibrils and lignin/hemicelluloses fragments.

The resistance of the films/nanopapers to disintegration in water may be an important issue since it can determine their final applications, as previously stated. Total disintegration in water can be required in some cases, such as in semi-finished products for cooking (Fakhouri et al. 2007), or for increasing the integrity and resistance of the coating (Gontard et al. 1994). Overall, the disintegration of the samples was low, since cellulose is insoluble in water due to the strong internal structural arrangement.

More cycles of fibrillation resulted in lower WVTR for the films/nanopapers produced from EKC. This behavior was similar to that found for WA 2h, DW 24h, and moisture, which showed better performance after 40 passages. The films/nanopapers produced from BTPT did not show significant differences between 20 and 40 cycles of fibrillation for WVTR due to the overlapping standard deviations. Table 5 presents the WVTR found in this study and some other results found in the literature for several raw materials.

Table 5 Average values of water vapor transmission rate (WVTR) of the films/nanopapers of this study and some values reported in the literature
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The porosity values presented by the films/nanopapers (see Table 3) confirm that greater number of fibrillation cycles of the raw material tended to generate films/nanopapers with lower WVTR. The EKC films/nanopapers showed a reduction of about 8% in the WVTR with greater number of passes through the fibrillator. This was assumed to be due to the increase of individualized CNF, after 40 passages, with a consequent increase of the surface area. The compact and dense three-dimensional network formed by hydrogen bonds did not allow the transport of water vapor through the film/nanopaper, since there were no carbonyl and hydroxyl groups available to make bonds with the water molecules. Therefore, the absence or low amount of empty spaces between the cellulose fibrils hinders the diffusivity of water vapor. The improvement of barrier properties with the increase of the degree of fibrillation is strongly associated with the decrease of the diffusion coefficient, caused by the strong entanglements between the cellulose nanofibrils (Kaushik et al. 2010). Scatolino et al. (2017) found lower results for the properties of permeability and water disintegration when evaluating nanocellulose films produced with Amazonian wood species, after more fibrillation cycles.

The films/nanopapers produced from BTPT and EKC obtained lower values of WVTR when compared to other nanostructured materials from vegetal sources reported in the literature; in particular, almost half of the WVTR values for films are composed of potato starch. Films produced with starch require improvements due to the hydrophilic structure resulting from the presence of amylose and amylopectin in its composition (Romero-Bastida et al. 2015). Excessively high values of WVTR can be explained by the existence of larger pores in the microstructure of the films. The films/nanopapers from bleached wood as the raw material also obtained higher values of WVTR compared to those produced in this study. The ideal structure of CNF networks is a compact complex form, presenting an obstacle to the water vapor diffusion. These are potential results for packaging applications and advancement in the pre-screening of renewable raw materials for substitution of petroleum-based polymers applied in multilayer packaging. Biodegradable and recyclable polymers with high barrier properties are very important for application as novel layers or in composite mixtures in cardboards, card papers, and industrial sacks for packaging. Films produced from biopolymer poly-lactic acid (PLA) and the biodegradable poly-vinyl alcohol (PVA) still had WVTRs dramatically inferior to those produced in this study, and their use in the formulation of composite mixtures has great potential. Biodegradable polymers such as the abovementioned have several established applications, ranging from plastic bags and cups and small household items to materials for electrical insulation.

Chemical structures of the films/nanopapers

The chemical structures (functional groups) of BTPT and EKC films/nanopapers were compared by FTIR and the spectra indicated the expected similarities in chemical composition for all samples. In general, the appearance or disappearance of peaks was not observed in the spectrum analysis in the different raw materials used. An absorption band was observed at the onset spectrum (Figure 6) with a peak at 3300 cm−1, corresponding to the free OH groups (Silverstein and Webster 2000) and to the intermolecular hydrogen bonds. It was also observed that the increase in the fibrillation passages through the grinder led to increased intensity of this band for BTPT, especially for the films/nanopapers produced with CNFs with 40 passages, suggesting more exposed hydroxyl groups in the cellulose structure in comparison to the EKC films/nanopapers after 40 passages. A greater number of –OH bonds indicates a greater interaction between fibers and consequently better mechanical performance for these samples, as previously observed in Table 1. The BTPT film/nanopaper after 40 passages shows greater amounts of hydrogen bonds and, consequently, inter- and intramolecular bonds between fibers, forming a kind of organized dense network. This leads to a greater amount of homogeneous fibers in nanoscale for BTPT films/nanopapers.

Fig. 6
figure 6

Typical FTIR spectra for BTPT and EKC films/nanopapers obtained after 20 and 40 passages through the fibrillator

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The vibrations around 2900 cm−1 are attributed to the absorption of C–H symmetrical and asymmetrical stretching originating from cellulose clustering, which is typical of organic materials (Silverstein and Webster 2000). A minimal change in this region, with the increase of the number of passages, was observed for the different films/nanopapers obtained.

The peaks observed at 1598 and 1446 cm−1 are attributed to C=C axial deformations of lignin aromatic rings (Alemdar and Sain 2008; Thomas et al. 2015), demonstrating the presence of lignin in the BTPT films/nanopapers (see Figure 2) and lower intensity for EKC. A little content of 8% of lignin was found for the BTPT samples, while for EKC, no lignin content was detected.

The band at 1022 cm−1 was attributed to –C–O–C– pyranose ring vibration (Elanthikkal et al. 2010). The crystalline regions existing in the CNF are directly related to the quality and/or quantity of cellulose present in their bonds, and probably the greatest intensity of this band occurred due to a greater reorganization after 40 passages, when followed by drying of the fibrils that compose the BTPT films/nanopapers, as reported in the methodology section. This may have occurred due to the characteristics reported in the previous sections related to the production process, such as greater surface area, smaller diameter, greater aspect ratio, and greater homogeneity. This phenomenon was not observed for the EKC films/nanopapers, however. Instead, a reduction of the intensity of these bands was observed, supposedly due to some increase of the amorphous region with the increased fibrillation after 40 passages.

Light transmittance of the films/nanopapers

The fibrillation altered the average diameter of the CNFs, as well as the light transmittance of the films/nanopapers (Figures 7 and 8). The higher the number of passages and the degree of fibrillation, the lower is the diameter of CNF, which varied from around 20 ± 5 to 15 ± 5 nm for BTPT and from 31 ± 23 to 21 ± 9 nm for EKC, demonstrating the effectiveness of the mechanical fibrillation in the process to modify the raw materials from micro- to nanoscale. Smaller diameters were observed for the BTPT CNF.

Fig. 7
figure 7

Typical images of the banana tree pseudostem (BTPT) structure and films/nanopapers: (a) SEM micrographs of the fibers, (b) SEM/FEG micrographs of the CNFs 20×, and (c) CNFs 40×; (d) visual aspect and light transmittance (T) of the films/nanopapers; Md, average diameter; Sd, standard deviation

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Fig. 8
figure 8

Typical images of the Eucalyptus kraft cellulose (EKC) structure and films/nanopapers: (a) light microscopy image of the fibers, (b) SEM/FEG micrographs of the CNFs 20×, and (c) CNFs 40×; (d) visual aspect and light transmittance (T) of EKC films/nanopapers; Md, average diameter; Sd, standard deviation

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The results demonstrated that the increase of fibrillation decreased the optical barrier for all the samples, allowing the passage of greater amount of light. The transmittance was increased by around 5% for BTPT films/nanopapers (from 65 to 68%) and 12% for EKC films/nanopapers (from 58 to 65%) with more cycles of fibrillation. The high-density values of the BTPT films/nanopapers, as well as the lower diameter of their CNFs, are probably important factors that made them less opaque in relation to the EKC samples, although lignin could be detected in the bleached fibers of the BTPT. More compact films/nanopapers with thinner CNFs do not scatter light inside (Okahisa et al. 2018; Qing et al. 2015), allowing greater passage of light. In this sense, Nogi et al. (2013) reported that the dispersion of light was increased by wider CNFs or by lower-density films/nanopapers. Such information can be confirmed by the values obtained in film/nanopaper porosity (see Table 3). As discussed in the previous section, porosity was lower for BTPT nanopapers, which may have avoided light retention through the structure. The considerable amounts of non-fibrillated fibers in the EKC films/nanopapers (see Figure 4), as well as the higher contents of hemicelluloses reported in the previous section, contributed to the greater opacity. The high content of hemicelluloses in the CNF is assumed to interfere with the complete dispersion in water, providing lower light transmittance to the films/nanopapers (Nogi et al. 2009). Although the two raw materials evaluated in this study presented contents of non-cellulosic components in their structures, the structure of the films from both raw materials enabled the passage of light, besides to allow the visibility through its structure (Figures 7d and 8d), especially the films/nanopapers produced with 40 passages.

Several factors, such as the fibril diameters, dispersion, hemicelluloses content, lignin content, suspension homogeneity, and surface roughness, influence the light transmittance (Miri et al. 2015; Abral et al. 2020). An alternative that could improve the optical performance is the mixture of CNFs with other agents for production of blends. Another potential example of treatment which could be used in intention to improve the transparency of the films is the use of specific enzymes on the raw material (Long et al. 2017), which can result in high quality of fibrillation, as well as chemical pre-treatments, besides the advantage of being environmentally friendly. However, enzymatic treatment is limited due to its sensitivity to different temperature ranges, pH, and prolonged conditions. Transparency and diameter can be used to indirectly assess the degree of fibrillation of the CNFs. The diameter measurements performed on the CNFs after 20 and 40 passages indicate the efficiency of the fibrillation process; that is, their values were 20 ± 5 and 15 ± 5 nm for BTPT and 31 ± 23 and 21 ± 9 nm for the EKC samples, respectively. In addition to the abovementioned properties, there was an improvement in the mechanical properties of tensile strength, density, water absorption, contact angle, water vapor transmission rate, as well as the large amount of hydroxyl groups on the CNFs surface (seen in the FTIR) and film compaction (seen with SEM-FEG). All these factors enable to infer that there was an increase in the degree of fibrillation of CNFs with more passages through the grinder.

The carboxyl content may have increased after alkaline treatment and after the passages through the mechanical fibrillator. These initial processes have promoted an increase in the amount of hydroxyl groups on the surface of the CNFs due to oxidation and the increase of the surface area of the fibrillated material (deconstruction of the cell wall). Probably, the degree of polymerization may have reduced after these steps, increasing the cationic demand of the suspension. The literature shows a certain linear relation between the increase in the tensile strength and rupture stress with the number of carboxylic groups on the fiber surface (Serra et al. 2017); that is, the increase in the number of hydroxyls and carboxylate groups provides the improvement in the tensile properties of films/nanopapers. According to Zarna et al. (2021), it has been expected that nanocellulose with suitable fibrillation degree and an adequate surface chemical bonds would be beneficial for improving the mechanical properties of a given biocomposite material. In addition, the strength of plain paper depends, among others, on the number of hydrogen bonds that form between cellulose fibers when water is removed from the fibrous suspension. It is then expected that CNFs, with a larger surface area and a greater number of carboxyl and hydroxyl groups (negative charges) on their surface, will be able to form great contents of hydrogen bonds between them.

Biodegradability of the films/nanopapers

The mass loss after soil incubation is a commonly used parameter for measuring changes caused by the microbial attack on polymers (Flemming 1998). The mass loss increased with the increase of incubation time, independent of the raw material (Figure 9A). The total mass losses after 18 weeks (126 days) for 20 and 40 passages were ~40% and ~46% for the BTPT films/nanopapers and ~26% and ~24% for the EKC samples, respectively. Signs of degradation such as cracks, color change, roughness, and the presence of stains on the surface of the samples were observed (Figure 9B). Other factors such as the type of microorganisms and the pH of the soil also interfere in the biodegradability process (Doi et al. 1992; Flemming 1998). A great decrease of mass was observed until the first 4 weeks (28 days) of evaluation.

Fig. 9
figure 9

A Mass loss of the films/nanopapers along with the biodegradation and visual aspect of the samples before and after 18 weeks in simulated soil: from (a) to (d) BTPT films/nanopapers (20 and 40 passages) before and after biodegradation; from (e) to (h) EKC films/nanopapers (20 and 40 passages) before and after biodegradation. B Typical optical microscopy images of the samples after 18 weeks after soil incubation. Red arrows show pores and stains. From (a) to (d) BTPT nanostructured films/nanopapers (20 and 40 passages) before and after biodegradation; from (e) to (h) EKC nanostructured films/nanopapers (20 and 40 passages) before and after biodegradation

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The first days of incubation correspond to the abiotic phase of biodegradation, in which the macromolecules suffer hydrolysis. The mass reduction remained stable between the 5th week (35 days) and 10th week (70 days), strongly decreasing again between the 11th (77 days) and 16th (112 days) weeks. The decomposition was observed especially for BTPT films/nanopapers. In part, this could be due to the pores observed in their cross section (see Figure 3), which facilitated the dispersion of degrading enzymes after the microorganism’s attack. A consortium of various aerobic bacteria and fungi working cooperatively degrades the cellulose to glucose and cellodextrins (Chandra and Renu 1998).

Biomasses are known to show recalcitrance characteristics to enzymatic deconstruction, which even includes hampering on the development of biomass-based fuels and chemicals (Weiss et al. 2017; Dias et al. 2019) and refers to compositional and structural features that provide more resistance against microbial decomposition (Sollins et al. 1996). This recalcitrance is due to a number of both chemical and structural factors and is the result of millions of years of parallel evolution of plants and plant degraders (Himmel et al. 2007; Durães et al. 2020). The samples show remarkably different recalcitrance depending on the characteristics of the raw material, including biomass porosity, cellulose accessibility, degree of cellulose polymerization, lignin/hemicelluloses contents, and microstructural aspects (Meng et al. 2017; Lu et al. 2019).

Lignin is considered to provide a physical barrier that protects the cellulose and hemicelluloses against degradation agents (Bianco et al. 2021; Şenol 2021). Despite the mechanical barrier provided by lignin, the fibrillation may result in the depolymerization (Widsten et al. 2004), softening, and redistribution of fragments of this structure (Hietala et al. 2011), allowing the higher biodegradation of BTPT films/nanopapers even with a higher content of lignin.

For some authors, the removal of hemicelluloses is considered the factor with the most impact on the accessibility of cellulose in relation to delignification (Haverty et al. 2012; Pei et al. 2012). Hemicelluloses can act as a physical barrier that hinders enzymatic hydrolysis because they are between and surrounding the cellulose microfibrils in the secondary cell walls (Zhu et al. 2011). The presented biodegradation test provides an excellent basis for further and more detailed analyses of the effect of the soil on the biodegradation process of CNF films/nanopapers produced with lignocellulosic biomasses. The incubation process in the simulated soil creates a humid and dark environment, which favors the reduction of the recalcitrance characteristics of the lignocellulosic biomass components. It can be said that BTPT films/nanopapers presented higher biodegradation in comparison to EKC films/nanopapers, which could in part be due to the reduced recalcitrance of the BTPT.

The results obtained in this research provide insights about the management of the lignocellulosic wastes obtained mainly from the processing of the banana cultivation. Such sources of lignocellulose allow reducing the cost of inputs and at the same time eliminate any ethical criticism regarding the possible threats to food sources (Maroušek et al. 2015; Peters et al. 2020). Furthermore, there are still some unresolved issues in the field of green entrepreneurship (Muo and Adebayo Azeez, 2019). The share of renewables is rising rapidly, especially in developed countries (Maroušek et al. 2020). Further insights are required for improvement of the banana production chain efficiency, reducing residual biomass wastes and CO2 footprint with their possible application in other chains and products. This research indicated the possibility of producing nanostructured materials with high mechanical qualities using residual biomass, similar to films/nanopapers produced with commercial wood pulps. Additionally, based on the attractive properties of strength and biodegradability, it is suggested that the potential of cellulose nanofibrils combined with polymers should be evaluated for purposes such as emulsifier agents, coating layers on commercial papers and cardboards, or the production of functionalized CNFs with chelating agents for the treatment of wastewater and absorption of heavy metals. The applications of these biomass wastes for the production of biodegradable films and products contribute to reduce CO2 footprint of banana production chain, reducing environmental problems and generating economic benefits for agro-industry.

Conclusions and future prospects

The study demonstrated the potential of BTPT wastes for the production of nanostructured films/nanopapers through the evaluation of their biodegradability, physical (relations with water), barrier, and mechanical properties. The increase in the number of passages through the grinder fibrillator reduced the CNF diameter. Also, predominance of smaller empty spaces led to a greater apparent density and greater transparency. In general, the physical and mechanical properties were improved with the increase of cycles of fibrillation. The BTPT films/nanopapers showed also the greatest mechanical properties after 40 passages, with a Young’s modulus and tensile strength of around 2.4 GPa and 51 MPa, respectively. Films/nanopapers produced after 40 fibrillation cycles tended to show lower WVTR, especially those from the EKC. The total mass loss after 18 weeks of soil incubation for 20 and 40 passages were ~40% and ~46% for the BTPT films/nanopapers, showing higher biodegradation in comparison to EKC. Further research is required to look for alternative and eco-friendly pre-treatments or fibrillation methods that are cost-effective for upscaling their application and efficient for complete fibrillation of the fiber cell wall in nanoscale. Combined with other biodegradable polymers and chelating agents, the CNFs could be used for promising purposes, such as coating layers on sack papers, cardboards, card, and multilayered papers, as emulsifier agents, and the production of special membranes for the absorption of heavy metals. Films/nanopapers with some lignin content are expected to be used in a variety of fields, such as biodegradable plastics, filtration media, and packaging materials. Application of BTPT CNF in other chains and products may reduce CO2 footprint in the banana production chain and may support environmentally conscious decision-making by stakeholder companies, consumers, and professionals.