Tailoring the properties of nanocellulose-sepiolite hybrid nanopapers by varying the nanocellulose type and clay content

As it is known, both pulp sources and nanocellulose production process determine the properties of the resulting nanocellulose samples (Jonoobi et al. 2015; Nechyporchuk et al. 2016). Hence, LCNF and CNF samples presented different properties since they were obtained through the same production process (i.e. by refining pretreatment followed by microfluidization), but using an unbleached or a bleached elm kraft pulp, respectively. The higher lignin and hemicellulose (xylan) contents of the unbleached pulp influence not only the carbohydrate-lignin content of the nanofibers but also their crystallinity and dimensions. Thus, crystallinity index was lower on LCNF sample, due to a higher presence of amorphous components (lignin and hemicelluloses) (Table 1). Moreover, both amorphous components have been reported to enhance the fibrillation process: hemicelluloses contribute to inhibit hornification and fibril coalescence (Jonoobi et al. 2015; Solala et al. 2020; Ibarra et al. 2021), and lignin antioxidant capacity stabilizes cellulosic mechano-radicals reducing crosslinking between cellulose chains (Solala et al. 2012, 2020; Rojo et al. 2015; Tyagi et al. 2021). In agreement with these statements, AFM analysis showed nanofibers with lower diameter in LCNF sample compared to CNF sample (Table 1, for more information about size distribution see Supporting Information, Fig. SI.1, SI.2 and SI.3), indicating more extensive fibrillation in LCNF sample.

As expected, when the same pulp (bleached elm pulp) was subjected to different nanocellulose production processes, significant differences in the resulting nanocellulose were observed. The replacement of the refining pretreatment by a TEMPO-mediated oxidation pretreatment significantly increases the carboxylate groups content (52 vs. 1165 µmol g^-1 for CNF and TOCNF samples, respectively) due to the selective oxidation of C6 primary hydroxyl groups on the crystalline fibril surfaces (Isogai et al. 2011). These negatively charged groups introduce electrostatic forces causing repulsion between adjacent fibrils that ease fibrillation (Isogai et al. 2011; Lavoine et al. 2012), resulting in a colloidal stable suspension of thinner and shorter nanofibers (Table 1) with a zeta potential of -70 mV (compared to -26 mV for CNF suspension). Moreover, only completely nanofibrillated fibrils were found in this sample (100 % nanofibrillation yield), in contrast to LCNF and CNF samples, in which partially fibrillated and non-fibrillated fibrils were also found (50.2–60.8 % nanofibrillation yield) (Jiménez-López et al. 2020). Nevertheless, it should be remembered that these micro-fibrils were removed by centrifugation (as indicated in Materials and Methods section) and only nanofibril fractions are considered in this work. TEMPO-mediated oxidation pretreatment also contributed to the removal of hemicelluloses and lignin due to NaClO action (Table 1), as other authors have already reported (Okita et al. 2009). Finally, no big differences in crystallinity index were observed, indicating no significant changes in cellulose I crystal structure during this pretreatment (Saito and Isogai 2004; Isogai et al. 2011).

As supposed, the biggest differences were observed when comparing nanocrystals and nanofibrils. During nanocrystals production, the strong acid treatment caused the cleavage of glycosidic bonds within cellulose chains in the amorphous domains, breaking the hierarchical structure of the fibrils into rod like nanoparticles. Thus, compared to CNFs, CNCs showed higher crystallinity index (94 % vs. 77 %) and shorter length (199 nm vs. 892 nm) but similar diameter (4.5 nm vs. 5.8 nm). This sample also presented higher cellulose content than nanofibrils samples (Table 1), likely due to the removal of other amorphous components (mainly hemicelluloses and residual lignin) during the acid treatment (Malucelli et al. 2017; Trache et al. 2017). The use of sulfuric acid introduces negatively charged sulfate ester groups on the surface of the nanocrystals (Trache et al. 2017), improving colloidal stability (zeta potential of -65 mV).

FTIR spectra of NC-S hybrid nanopapers showed the typical vibration bands for cellulose rings (C-O-C) at 1164, 1111, 1060 and 1040 cm^− 1 (Castro-Guerrero and Gray 2014) in all of them (Fig. SI.4 in Supporting Information). Typical bands at 3340 and 2890 cm^− 1, corresponding respectively to ν_O−H and ν_C−H stretching vibrations, were also observed in all samples. One of the main difference between the FTIR spectra, is attributed to the presence of -COONa in TOCNF samples, indicated by the bands at 1607 cm^− 1 (assigned to ν_C=O in -COONa groups) and at 1410 cm^− 1 (assigned to ν_C−O symmetric stretching of dissociated carboxyl groups) (Fujisawa et al. 2011). This corroborates the oxidation of C6 primary OH groups in TEMPO-mediated oxidation pretreatment. Both bands can be better visualized in Fig. 2a, which shows FTIR spectra of the four nanocellulose papers (without sepiolite) in the 1700 − 1180 cm^− 1 region. The presence of sulfate groups on CNC samples can be confirmed by the bands at 1356 and 1204 cm^− 1, which are associated with S = O vibrations (Gaspar et al. 2014; Dasan et al. 2015). Finally, LCNF spectra showed a shoulder around 1510 cm^− 1 which could be assigned to the aromatic skeleton vibration in lignin (Eugenio et al. 2021).

FTIR analysis confirms the interaction between sepiolite and the different nanocellulose samples based on the observed spectral modifications. Figure 2b shows the ν_OH stretching vibrations range (3730 − 3660 cm^− 1) in the sepiolite infrared spectra, where the typical ʋ_−OH bands of both silanol (Si-OH) and hydroxyl groups bonded to magnesium atoms (Mg-OH) were observed at 3719 and 3680 cm^− 1, respectively. The first band completely disappears in the spectra of all NC-S hybrid nanopapers (Fig. 2b and Fig. SI.4), according to the hydrogen bonding interactions between the surface Si-OH of sepiolite and the C-OH groups at the nanocellulose surface (González del Campo et al. 2020; Lisuzzo et al. 2020). The ν_OH band ascribed to Mg-OH is still appreciated in the resulting NC-S nanopapers, due to the inaccessibility of the –OH groups bonded to magnesium atoms (Lisuzzo et al. 2020). In addition, the infrared spectrum of sepiolite shows bands at 986 and 472 cm^− 1, assigned to Si-O-Si siloxane bridges, and at 444 cm^− 1, assigned to Si-O-Mg bonds in the silicate network (Özcan and Özcan 2005; Perraki and Orfanoudaki 2008), which remain present in sepiolite-containing hybrid nanopapers, being especially visualized when sepiolite content increased (Fig. SI.4). Indeed, these bands appear less intense due to the dilution effect in the nanocellulose matrix but they do not modify their frequencies as they are due to vibrations of groups of atoms that are not accessible to interaction with the polymer. Finally, the intensity of the 1036 cm^− 1 band, assigned to C-O-C bridges in cellulose (Castro-Guerrero and Gray 2014), apparently increased when sepiolite content increased due to overlapping with the ν_Si−O stretching vibrations band of the clay at similar wavenumber (Perraki and Orfanoudaki 2008).

X-ray diffraction (XRD) patterns of all the NC-S hybrid nanopapers showed the typical peaks at ~ 15.1 and ~ 22.6 2θ degrees corresponding to the (\(1 \bar{1} 0\)) and (200) reflections of the crystalline planes of the I_β allomorph of cellulose (Fig. 3). A shoulder at ~ 16.4 2θ degrees can also be observed, especially in CNC patterns (Fig. 3d), corresponding to (110) crystalline plane of I_β cellulose. As mentioned above, crystallinity indexes were calculated from the diffractograms of pure nanocellulose observing higher crystallinity for CNC sample, as expected. The presence of sepiolite in the NC-S hybrids was clearly confirmed even at the lower sepiolite content (5 %) by typical peaks at 7.4, 8.8 and 26.8 2θ degrees assigned to (110), (002) and (080) reflections of crystal planes of this silicate (Tang et al. 2012; González del Campo et al. 2018; Lisuzzo et al. 2020) (Fig. 3) (X-ray diffractogram for pristine sepiolite film is shown in Fig. SI.5 in Supporting Information). A peak at 13.4 2θ degrees, corresponding to the (003) crystal plane reflection of sepiolite is also observed especially in the diffractograms of TOCNF-S and CNC-S hybrid nanopapers (Fig. 3c and d, respectively). The changes in the relative intensity of the sepiolite reflection peaks indicate changes in the orientation of sepiolite fibers on the nanopapers (González del Campo et al. 2018; Lisuzzo et al. 2020). Thus, the intensity ratio of both (110)/(080) and (110)/(003) reflections decreased from 6.4 and 17.0, respectively, in pristine sepiolite film, to 1.0-5.7 and 4.8–12.6, respectively, in NC-S nanopapers, being more severe the decrease when sepiolite content increased (except for CNF-20 S). These results suggested that sepiolite has partially lost its tendency to orientate in the film plane (González del Campo et al. 2018) induced by the interactions with nanocellulose.

Figure 4 shows FE-SEM images of the upper surfaces of NC-S hybrid nanopapers without sepiolite (a-d) and with 5 % sepiolite content (e-h) at 30,000× magnification. When sepiolite is not present, thin and long nanofibers can be distinguished in the surface of LCNF and CNF nanopapers (Fig. 4a and b), which consisted in a very entangled network films. However, a flatter and more homogeneous surface is observed for TOCNF and CNC nanopapers (Fig. 4c and d). According to AFM, TOCNF sample presented thinner nanofibers compared to mechanically pretreated nanofibers, which can contribute to more compacted nanopapers. Similarly, short nanocrystals (compared to nanofibers) can also contribute to form a more compacted CNC film. Furthermore, the reduced size of the nanoparticles in TOCNF and CNC samples makes difficult to distinguish them in FE-SEM images at this magnitude. In addition, the negative charges on the surface of TOCNF and CNC contributed to more homogeneous and stable nanocellulose suspensions, resulting in more homogeneous films.

In general, when sepiolite was present in the hybrid nanopapers made with cellulose nanofibers, it can be distinguished the presence of rigid and thicker sepiolite nanofibers (diameter of 30–100 nm) (Ruiz-Hitzky et al. 2011; Lisuzzo et al. 2020) compared to the typical curved and long cellulose nanofibers (Fig. 4e-g). However, the two fibrous components are hard to distinguish in some cases due to their morphological similarity, as previously reported in similar films (González del Campo et al. 2018; Lisuzzo et al. 2020), and the entangled network that they form in the nanopapers. Conversely, in hybrid nanopapers with CNCs (Fig. 4 h), only sepiolite fibers were distinguished, due to the small size of the cellulose nanocrystals. Interestingly, a highly oriented distribution of the sepiolite fibers was found in these CNC-S hybrid nanopapers, compared to the more random orientation observed in films with cellulose nanofibers.

Independently of the nanocellulose type and the sepiolite content, all the nanopapers showed an homogeneous assembly between the organic and inorganic component, probably thanks to the sequential high-shear dispersion and ultrasound treatment applied in the preparation of precursor NC-S suspensions (González del Campo et al. 2018, 2020; Lisuzzo et al. 2020; Ruiz-Hitzky et al. 2021). As an example, FE-SEM images of the surface of nanopapers made with CNFs and different sepiolite content (0 %, 5 %, 10 % and 20 %) are shown in Supporting Information (Fig. SI.6).

Thermogravimetric parameters for NC-S hybrid samples are detailed in Table 2. TGA curves can be found in Supporting Information (Fig. SI.7: for nanocellulose samples without sepiolite and Fig. SI.8: for the NC-S hybrid with different sepiolite content). As expected, the nanocellulose samples (i.e., without sepiolite) that showed lower thermal stability (lower T_on) were TOCNF and CNC samples, due to the presence of carboxylate and sulfate groups, respectively (Jiang and Hsieh 2013; Lichtenstein and Lavoine 2017). Hence, a new degradation temperature peak was observed at 238 ºC in TOCNF sample due to the decarboxylation of sodium carboxylate groups (Lichtenstein and Lavoine 2017) and at 266 ºC in CNC sample due to the lower activation energy of decomposition from the surface sulfate groups (Lu and Hsieh 2010; Gaspar et al. 2014). TOCNF sample also showed a shift in peak at 341–345 ºC (for LCNF and CNF samples) to 297 ºC likely due to the lower nanofiber diameter of TOCNF sample, leading to higher surface area exposed to heat and promoting a faster degradation (Jiang and Hsieh 2013; Gaspar et al. 2014). Contrarily, in CNC sample this peak shifted to higher temperature (359 ºC), showing also higher T_off probably because its higher crystallinity slowed down the thermal degradation process (Henrique et al. 2015).

Regarding the addition of sepiolite, it caused an increase of thermal stability in most NC-S hybrids, which can be ascribed to the hydrogen bonding interactions between nanocellulose and sepiolite. This effect has been previously reported for hybrid foams and/or films made with sepiolite and different types of cellulose: TEMPO-oxidized nanocellulose (Wicklein et al. 2015; González del Campo et al. 2018; Sanguanwong et al. 2021), phosphorylated nanocellulose (Ghanadpour et al. 2018b), mechanical nanocellulose (Gupta et al. 2019) and nanocellulose derived from ultrasonicated microcrystalline cellulose (González del Campo et al. 2020). The highest increase in thermal stability was found for CNC-S hybrids (increase in T_on of 61–75 ºC), while only a slight increase was observed for mechanically obtained nanocellulose hybrids (i.e. LCNF-S and CNF-S). Furthermore, both LCNF-S and CNF-S hybrids showed lower T_deg and T_on for 10 % and 20 % sepiolite content, compared to 5 %, while in TOCNF-S and CNC-S hybrids a progressive increase with the increase in sepiolite content was observed. It has been reported that negative charges on the surface of nanocellulose can positively influence the mixture of nanoclays and cellulose nanofibers (Alves et al. 2019). Hence, the negative charges in TOCNFs and CNCs (confirmed by Z-potential: -70 mV and -65 mV, respectively), enable their dispersion in water at the individual nanoelement level (due to charge repulsion effect) allowing a better interaction between nanocellulose particles and sepiolite nanofibers at the nanometric level. On the other hand, LCNFs and CNFs presented lower amount of negative charges in their surface (Z-potential of -10 mV and -26 mV, respectively), which could lead to aggregation of nanocellulose particles, reducing the surface interactions between nanocellulose and sepiolite. Thus, when sepiolite content increased from 5 % to 10 % or 20 %, LCNFs and CNFs may not dispose enough available surface area for the interaction with these higher amounts of sepiolite, causing a slight reduction in thermal stability. These different behaviors indicate the important role of the nature of nanocellulose on the clay-nanocellulose interactions, and therefore on the films preparation and final composite properties. Similar conclusion was reported by Torvinen et al. (2017), who combined different nano/micro-cellulose (TEMPO-oxidized, enzymatic, mechanical and lignocellulosic nanofibers) with kaolinite to form nanoporous composites for printed electronics.

Differences in the amount of char residue (CR) obtained after pyrolysis at 850 ºC were also found. Among the nanopapers without sepiolite, the highest CRs were found for CNC sample, which can be attributed to the dehydration effect of the sulfate groups (Jiang and Hsieh 2013; Gaspar et al. 2014). LCNF sample showed higher CR than CNF sample, probably due to the higher hemicellulose content of this sample, which could form a coal residue on the surface of the nanofibers hindering the complete cellulose degradation (Claro et al. 2019). TOCNF sample also left higher CR than CNF sample, owing to the formation of sodium carbonate from chemical reaction between the residual products of decarboxylation (carbon dioxide) and dehydration (water) (Lichtenstein and Lavoine 2017). When sepiolite was added to the nanopaper, higher CRs were found for all nanocellulose types (except for LCNF-5 S and CNC-20 S) due to increased amount of inorganic material (González del Campo et al. 2018, 2020).

According to van Krevelen (1975), the limiting oxygen index (LOI) can be estimated from the char residue upon pyrolysis of halogen-free polymers. LOI indicates the minimum volume percent of oxygen (in a mixture of oxygen and nitrogen) required to support flaming combustion. Thus, lower LOI values indicate higher flammability, and polymers with LOI values under 26 are considered flammable (Ranganathan et al. 2008). Increasing the char formation, as in the case of sepiolite addition, can limit the production of combustible carbon containing gases, decreasing the exothermicity due to pyrolysis reactions, and decreasing the thermal conductivity of the surface of burning materials (Ranganathan et al. 2008). Other authors have reported the flame retardant effect of sepiolite addition in nanocellulose-sepiolite foams (Wicklein et al. 2015; Ghanadpour et al. 2018b; Gupta et al. 2019; Köklükaya et al. 2020). This effect has been more extensively studied for lamellar clay materials such as montmorillonite in both hybrid foams (Köklükaya et al. 2017; Wang et al. 2019; Medina et al. 2019) and films (Liu and Berglund 2013; Carosio et al. 2015; Ming et al. 2017). According to Ming et al. (2017) and Darder et al. (2017), self-extinguishing behavior of hybrid films or foams improved sharply for clay content over 25–30 %, observing an immediately extinguishing of the flame after its removal. Therefore, if sepiolite content were increased over 30 % in NC-S nanopapers, even higher LOI values than those showed in Table 2 could be expected. Moreover, self-extinguishing properties of NC-S hybrid nanopapers can be tunable not only by the addition of different content of sepiolite, but also by the selection of the nanocellulose type used. Thus, NC-S hybrid nanopapers made with CNCs or TOCNFs showed higher LOI values than those made with LCNFs or CNFs.

The water adsorption isotherms of NC-S hybrid nanopapers are shown in Fig. 5, while adsorption-desorption isotherm curves can be found in the Supporting Information (Fig. SI.9). A sigmoidal or S-shape profile can be observed for all NC-S hybrids and pristine sepiolite, indicating their hydrophilic character. As expected, differences between the pure nanocellulose samples were found, observing higher water uptake for TOCNF sample, due to the presence of carboxylate groups with strong water affinity (Guo et al. 2018; Jiménez-López et al. 2020) and lower hydrophilic character for CNC sample, due to its higher crystallinity (Table 1) (Guo et al. 2017). When sepiolite (with higher hydrophilic character than nanocellulose) was introduced in the nanopapers, no significant changes in water sorption properties were found for LCNF-S, CNF-S and TOCNF-S hybrids. However, in the case of CNC-S hybrids, a progressive increase in hydrophilic character was found while increasing the sepiolite content. Similarly, González del Campo et al. (2020) reported an increase in hydrophilicity due to the sepiolite content in nanopapers made of nanocellulose from ultrasonicated microcrystalline cellulose and sepiolite. This effect could be due to a progressive reduction in the apparent bulk density of the CNC-S hybrid nanopapers while increasing the sepiolite content (1.65, 1.53, 1.49 and 1.28 g cm^− 3 for CNC-0 S, CNC-5 S, CNC-10 S and CNC-20 S, respectively). A reduction in apparent bulk density is related to an increase in porosity, which can result in an increase of water adsorption (Tyagi et al. 2021), since it depends on both surface energy and pore structure near the surface. Lower differences in apparent bulk densities were found in the NC-S nanopapers prepared with cellulose nanofibers and varying sepiolite content (1.50–1.33, 1.47–1.39 and 1.63–1.46 g cm^− 3 for LCNF-S, CNF-S and TOCNF-S nanopapers, respectively) explaining the lack of this effect on water adsorption for these samples. Contrarily, González del Campo et al. (2018) reported a reduction in hydrophilicity for nanopapers made with TEMPO-oxidized nanocellulose and 2–10% sepiolite, attributing this effect to the blocking of available sorption sites due to hydrogen bonds between nanocellulose and sepiolite fibers and to an increase in surface roughness. However, this effect was not observed herein for TOCNF-S hybrids.

Table 3 shows the water vapor permeability (WVP) for the different NC-S hybrid nanopapers, calculated from the water vapor transmission rate (WVTR) measurements and the film thickness. TOCNF nanopapers presented the highest WVP values (lowest barrier properties), due to the presence of carboxylate groups on the surface. These groups not only increased the water affinity of the nanofibers, as mentioned above, but also reduce the interfibrillar hydrogen bonds between nanofibers (Isogai et al. 2011), contributing to lower compaction of the film and easier swelling at high relative humidities (Jiménez-López et al. 2020). When 5–10 % sepiolite content was introduced in the nanopapers, no significant changes were found in permeability, indicating a good interaction between nanocellulose and sepiolite. However, when sepiolite content increased to 20 %, more significant increases in water vapor permeability were found, independently of the involved nanocellulose type. In agreement with these results, González del Campo et al. (2020) reported that sepiolite should be kept lower than 20 % in order to preserve the barrier properties of nanocellulose-sepiolite films. Similar results were found when sepiolite was added to other polysaccharides films such as alginate or arabinoxylan (Sárossy et al. 2012; Cheikh et al. 2020). Finally, it should be mentioned that although WVP of NC-S nanopapers (1.6–3.6·10^− 11 gm^− 1 Pa^− 1s^− 1) was higher than that of traditional plastic (low-density polyethylene (LDPE): 0.014 − 0.007·10^− 11 gm^− 1 Pa^− 1s^− 1; high-density polyethylene (HDPE): 0.002–0.004·10^− 11 gm^− 1 Pa^− 1s^− 1; polyethylene terephthalate (PET): 0.002–0.023·10^− 11 gm^− 1 Pa^− 1s^− 1; polyvinyl alcohol (PVOH): 0.343·10^− 11 gm^− 1 Pa^− 1s^− 1) (Bastarrachea et al. 2011), it was similar to that reported for other bioplastic such as polylactic acid (PLA): 1.9–2.5·10^− 11 gm^− 1 Pa^− 1s^− 1 (Dadashi et al. 2014; Li et al. 2017), polylactic acid/poly(3-hydroxybutyrate) composites (PLA/PHB): ~4.8·10^− 11 gm^− 1 Pa^− 1s^− 1 (D’Amico et al. 2016) or trilayer policaprolactone/methylcellulose: 3.9·10^− 11 gm^− 1 Pa^− 1s^− 1 (Takala et al. 2013).

Mechanical properties of the diverse NC-S hybrid nanopapers were determined based on tensile test (Table 3). All the nanopapers presented a basis weight of 10.4 ± 1.3 g m^-2, a thickness of 7.0 ± 1.1 μm and an apparent bulk density of 1.48 ± 0.11 g cm^-3. When pure nanocellulose samples were compared, higher mechanical properties were found for mechanically pretreated nanofibrillated cellulose (LCNF and CNF samples), especially for bleached nanofibers (higher tensile index and elongation at break, and only slightly lower specific Young’s modulus than LCNF sample). Despite the controversial results published about the effect of lignin on mechanical properties of lignin-containing nanocellulose (Ferrer et al. 2012; Rojo et al. 2015; Solala et al. 2020; Jiménez-López et al. 2020), most of the works agree that unbleached and bleached nanopapers showed comparable mechanical properties, although a loss of ductility (lower elongation at break) could be associated with the lignin content (Rojo et al. 2015). When the TEMPO-mediated oxidation was applied as pretreatment in nanocellulose production, resulting TOCNF nanopaper showed lower mechanical properties, due to lower hydrogen bonding between nanofibers caused by the presence of carboxylate groups (Isogai et al. 2011). Finally, CNC nanopaper presented the lowest mechanical properties, as expected, due to the lower length of nanocrystals compared to nanofibers (199 nm vs. 595–892 nm, respectively) yielding lower crosslinking into the film.

The addition of sepiolite caused an increase in specific Young’s modulus, tensile index and elongation at break in most of the nanopapers (especially those made with TOCNFs and CNCs). In line with these results, several authors have reported an increase in Young’s modulus and tensile index related to the increase in sepiolite content not only in composite films made of nanocellulose (González del Campo et al. 2018, 2020) but also made of other polysaccharides or cellulose derivatives (such as alginate, starch, pectin, xanthan, chitosan, hydroxypropylmethylcellulose or carboxymethylcellulose, among others), even with high loading of fibrous clay (Darder et al. 2006; Chivrac et al. 2010; Alcântara et al. 2014, 2016; Cheikh et al. 2020). This behavior could be attributed to a good dispersion in the hybrid suspension (especially those made with negatively charged nanocellulose: TOCNFs and CNCs) and to strong interactions at the nanometer scale between the fibrous clay and the biopolymer (Alcântara et al. 2014, 2016).

In spite of the general positive effect of sepiolite on mechanical properties, some reductions in specific Young’s modulus, tensile index and/or elongation at break were found for LCNF-S and CNF-S hybrid nanopapers, especially in CNF-10 S. These results could indicate a weaker interaction between sepiolite and mechanically pretreated nanocellulose, which was also suggested by the lower thermal stability of LCNF-S and CNF-S nanopapers with 10–20 % sepiolite content. Regardless of these reductions, LCNF-S and CNF-S hybrid nanopapers still showed higher mechanical strength than TOCNF-S and CNC-S hybrid nanopapers, due to the most outstanding properties of pure LCNFs and CNFs. These results, along with their better barrier properties (lower WVP) make LCNF-S and CNF-S hybrid nanopapers good candidates for packaging applications.

In order to evaluate optical properties of NC-S nanopapers, total transmittance and transmission haze were determined between 200 and 900 nm (Fig. SI.10 and SI.11 in Supporting Information). Most significant optical properties are shown in Table 4. All 100 % cellulose nanopapers presented high transparency: total transmittance higher than 86 % and opacity lower than 11 a.u. mm^− 1 at 550 nm, indicating a dense packaging of small nanofibers/nanocrystals without voids between them able to cause light scattering (Hsieh et al. 2017). Interestingly, LCNF samples showed a relative UV-protection indicated by a low total transmittance at 280 nm (36 %, compared to 72–79 % for the other nanocellulose types), due to the presence of phenolic structure in lignin able to absorb UV light (Sadeghifar et al. 2017). Sepiolite content also improved UV-protection, as previously reported by Gonzalez del Campo et al. (2018). On the contrary, only a slight progressive reduction in transparency (550 nm) was found when sepiolite was incorporated in the nanopapers, being slightly more significant this effect in CNF-S hybrid nanopapers. Nevertheless, all NC-S nanopapers with up to 10 % sepiolite content presented very high transparency (total transmittance at 550 nm higher than 84 %) and only LCNF-20 S and CNF-20 S nanopapers presented transmittances lower than 80 % (75.5 % and 73.1 %, respectively) and opacity higher than 13 a.u. mm^− 1 (14.8 a.u. mm^− 1 and 16.7 a.u. mm^− 1, respectively). Thus, when NC-S hybrid nanopapers were placed right on a printed paper, the letters can be clearly read (Fig. 6). The small changes in transparency and opacity indicated that NC-S hybrid nanopapers were densely packed together, although a more heterogeneous structure when sepiolite content increased over 10 % could promote light dispersion. In this context, other authors have found high transparency in cellulose nanopapers with layered-clays, reporting total transmittances of 72–92 % even with 50 % clay content (Wu et al. 2014; Ming et al. 2017; Liu 2018; Qin et al. 2019). However, other authors observed a significant reduction in transparency when the fibrous or layered clays content increased (Aulin et al. 2012; Wu et al. 2012; González del Campo et al. 2018).

Optical transparency, combining with high mechanical strength and flexibility, is of interest in applications such as optoelectronics and barrier coating (Liu 2018). Furthermore, tunable transmission haze, i.e. the percentage of the transmitted beam of light that deviates from the incident beam by more than 2.5º due to light scattering, is also very important in optoelectronic devices (Jiang et al. 2018). In this regard, different approaches have been proposed such as varying the nanofibers/microfibers rate (Fang et al. 2014; Jiménez-López et al. 2020), the nanofibers/carboxymethyl cellulose (CMC) rate (Kim et al. 2021) or changing the dispersion solvent (Jiang et al. 2018). In this work, we propose to tune haze properties by changing the sepiolite content of the NC-S hybrid nanopapers, since a significant increase in transmission haze was observed for increasing sepiolite content. Thus, when the NC-S hybrid nanopapers were placed 3.5 cm separated from a printed paper, the letters become fuzzy when increasing sepiolite content (except for CNC hybrid nanopapers) because the light through the hybrid nanopaper becomes scattered by its higher haze (Fig. 7). This effect is probably due to the higher size of sepiolite fibers compared to nanocellulose particles. Nevertheless, low haze was observed in all CNC-S hybrid nanopapers, probably due to the smaller length of nanocrystals, compared to nanofibers, and the stronger intimate interaction between nanofibers and sepiolite, as suggested by the resulting thermal stability and mechanical properties. It is worth mentioning that although other authors have achieved higher optical haze for cellulose nanopapers, LCNF-20 S, CNF-20 S and TOCNF-20 S hybrid films presented high haze per unit thickness (5.1–5.5 % µm^-1). These values are comparable to those reported by Li et al. (2020) for highly transparent cellulose nanopapers with varying hemicelluloses and lignin content (3.55–5.5 % µm^-1) and by Kim et al. (2021) for all cellulose transparent films with different CNF/CMC ratio (6.1–8.9 % µm^-1). These authors claimed the outstanding optical properties of their films compared to other cellulose paper/nanopapers previously reported (0.05–2.28 % µm^-1) (Kim et al. 2021).

Taking into account these results, NC-S hybrid nanopapers made with nanofibers and high sepiolite content (especially TOCNF-20 S), which presented high transparency and transmission haze, could be applied in devices such as solar cells (providing diffuse scattered light) and LED lighting or backlight units in Liquid Crystal Display (LCD) (rendering uniform light distribution). LCNF-S, CNF-S and TOCNF-S with low sepiolite content, could be used in outdoor devices such as touch panels, since moderate haze would contribute to anti-glaring, providing comfortable visibility. Finally, CNC-S hybrid nanopapers, which presented high transparency and low transmission haze, could be applied in displays requiring high clarity with vivid and clear images (Fang et al. 2014).