Production and Characterization of High Solid Content Cellulose Nano brils from Pretreated Fluff Pulp


 The increasing demand for cellulose nanofibrils (CNF) necessitates the development of novel processes to produce high-solid content and consistent quality nanofibrils. In this study, we investigated the combination of mechanical and chemical pretreatment methods (carboxymethylcellulose, CMC dispersion, and sodium hydroxide, NaOH swelling with ball milling) for cellulose fibers followed by high-pressure homogenization to evaluate the CNF characteristics. The carboxymethylcellulose (CMC) dispersion with 75 min ball milling and NaOH swelling with 15, 45, and 75 min ball milling of cellulose slurry reduced the fiber dimensions by up to 90% that eased the fibrillation to produce about 6% solid content CNF during high-pressure homogenization. The characterization of CNF hydrogels produced from pretreated samples revealed that they had an average fibril width of less than 30 nm with good dispersion stability. The CMC dispersion and NaOH swelling with ball milling of cellulose slurry did not significantly affect the chemical structure and the crystallinity of CNF hydrogels. On the other hand, the tensile strength of all the pretreated CNF samples was increased up to 105±14 MPa when compared with that of the control sample (58±6 MPa). NaOH treatment has slightly increased the thermal stability of CNF samples over CMC treated and control samples. In conclusion, short fibers generated by mild alkaline pretreatment with ball milling followed by high-pressure homogenization of cellulose fibers can produce the consistent quality CNF with high solid content and tensile strengths for various industrial applications.


Introduction
Nanocellulose is the nanosized polysaccharide having both amorphous and crystalline cellulose comprised of D-glucose units linked by β-1, 4 glycosidic bonds. It is one of the promising nanomaterials for the development of bio-based and environment-friendly products due to its abundant availability, biocompatibility, biodegradability, and superior material properties such as high aspect ratio, speci c area, crystallinity, and tensile strength, and ability to form hydrogen bonding with other materials ( increasing every year and is projected to be about 6 million metric tons (Cowie et al. 2014). Nanocellulose is produced in various forms and is often related to the source of biomass. Forest wood and plant biomass produce two types of nanocellulose: cellulose nano brils (CNF) and cellulose nanocrystals (CNC). In addition, the nanocellulose produced from the synthesis of bacterial microorganisms is bacterial cellulose (BC) often studied in the biomedical elds. The other forms of nanocellulose include tunicate cellulose nanocrystals (t-CNC) and algae cellulose particles (AC) which are isolated from tunicates and algal biomass (Klemm et al. 2011; Moon et al. 2011) The most common types of nanocellulose, CNF, and CNC and are predominantly produced from forest biomass. CNF/CNCs exist as micro bril bundles in the cell walls of plants and wood. The micro brils are connected by many strong inter brillar hydrogen bonds to form cellulose macro bres. Nanocellulose with 5 to 30 nm width and an aspect ratio greater than 50 are de ned as CNF, and with rod-like structure, a width of 3 to 10 nm and aspect ratio less than 50 are de ned as CNC according to the standard de nition of nanomaterials by Technical Association of the Pulp and Paper Industry (TAPPI) (TAPPI 2017). A single micron-sized cellulose ber consists of numerous CNC and CNF. They are extracted from biomass resources by chemical, mechanical and enzymatic processes. The acid hydrolysis is most widely followed for the extraction of crystalline CNCs from deligni ed cellulose pulp and cotton bers. However, the consumption of acids (64%) and low yield (~50%) poses a great challenge in the commercialization of the CNC manufacturing process (Gu et al. 2015;Peng et al. 2011).
The CNFs are manufactured from wood-based cellulose pulps through mechanical disintegration devices such as disc re ner, homogenizer, ball mill, micro uidizer, and ultra ne grinders (Cheng et  to 500 kWh t −1 (Klemm et al. 2011). However, the quality of isolated CNFs was inconsistent due to the poor dispersibility and agglomeration of cellulose in the input slurries.
The cellulose bers from wood and plant-based resources are more prone to agglomeration and formation of network structure due to more unordered cellulose and cellulose Iα on bril surface to make them stickier (Lawoko et al. 2006). The agglomerated and network-structured cellulose bers caused the clogging in the homogenizer valve and reduced brillation e ciency for increased cellulose concentration in the input slurry. The chemical pretreatment processes such as chloroacetic acid etheri cation, carboxymethylation enzymatic hydrolysis, and TEMPO oxidation were studied to modify the surface and structure of cellulose to improve ber dispersibility during CNF isolation processes (Adsul et al. 2005; Ioelovich and Morag 2011; Saito et al. 2006). These processes consumed a considerable amount of hazardous chemicals and even led to low yield, and increased greenhouse gas emissions (Arvidsson et al. 2015;Li et al. 2013).
The mechanical processes such as valley beater and PFI mill were also used to re ne and improve dispersibility (Bilodeau and Paradis 2018; Turbak et al. 1983). The cellulose slurry with the concentration of 1.0 and 1.5% was able to disperse in the PFI mill and Valley beater respectively. But, these pretreatment processes were not e cient in reducing ber width and length (Hai et al. 2013). The CNF brilization from lower concentration cellulose slurry and larger ber width and length, increased the number of passes and clogging in a homogenizer and reduced the solid content percentage in CNF hydrogel. The CNF having bril width of less than100 nm and 1% solid content was manufactured by passing cellulose slurry, 10 times through a grinder after processing 30 and 14 times in a re ner and homogenizer, respectively (Iwamoto et al. 2007;Iwamoto et al. 2005). The CNF hydrogel with lower solid content increases bulk volume and causes transportation and handling issues. Moreover, the widespread distribution of bril width attribute to inconsistent mechanical and physical properties of products produced from them (Li et al. 2015). Therefore, it is important to develop methods to produce highly dispersible, high solid content, and consistent quality CNFs for industrial applications.
The production of CNF was focused on mechanical brillation of long-ber cellulose bers into CNF (Stelte and Sanadi 2009;Turbak et al. 1983). An earlier study by Lee and Mani (2016) found that the size reduction of uff pulp by knife milling process not only reduced the ber dimensions but also improved the brillation process to produce CNF. It was hypothesized that the use of short bers will ease the mechanical brillation process and produce consistent quality CNF without ber clogging. A further reduction in ber dimension can be possible by the ball milling process and limited studies in the literature were investigated on the effect of ball milling of cellulose bers on the production of CNF. In addition, the use of carboxymethylcellulose (CMC) as a dispersing agent to minimize bril agglomeration was investigated and reported that the -CH 2 COO − groups of anionic CMCs were adsorbed into cellulose ber surface in the CMC dispersion process. More speci cally, when CMC was added, the electrostatic repulsive force was induced between cellulose bers to move them apart and exert uniformly distributed mechanical disintegration force on cellulose bers (Fras-Zemljič et al. 2006;Laine 2000). An alkaline treatment by low concentration (2%) sodium hydroxide (NaOH) was used to swell cellulose bers before the brillation process and the resultant CNF exhibited improved thermal stability and dispersibility. The  Lee et al. 2018). Therefore, the objectives of this study were to investigate the effects of ball milling with CMC as a dispersing agent and NaOH as a swelling agent followed by high-pressure homogenization of knife milled uff pulp and to determine the physical and mechanical properties of high-solid content CNFs.

Materials
The cellulose powder was manufactured from uff pulp, procured from a commercial paper mill in Georgia, U.S.A. The dried pulp sheet was reduced using the laboratory heavy-duty knife mill (Retsch SM 2000, Germany) with a 0.25 mm screen and three grinding passes. The three-pass shear cut cellulose powder was dried in an oven for 24 hours and was used as a feedstock for this study. Sodium carboxymethyl cellulose (CMC) (molecular weight (M.W) ~250,000 and degree of substitution 0.9) supplied by Sigma Aldrich was used as a dispersing agent and reagent grade sodium hydroxide (NaOH) in the form of beads, purchased from Amresco was used as a swelling agent. A standard CNF produced by the University of Maine's pilot plant was purchased and used as a reference sample (Ref.).

CNF Production
The CNFs were brillated from three-pass knife milled uff pulp by the combination of pretreatment methods and high-pressure homogenization process as shown in Fig. 1.

Pretreatment of knife milled cellulose
The knife milled cellulose powder was treated by two different pretreatment methods namely CMC dispersion and NaOH swelling treatment followed by ball milling at various milling times to reduce ber dimensions. For CMC dispersion treatment, the 10% (w/v) of cellulose slurry in deionized water (DI) was prepared from knife-milled cellulose powder by mixing with 2% (w/w) CMC and heated at 80°C for 2h in a hot magnetic stirrer plate. The CMC dispersed cellulose slurry cooled to room temperature before ball milling treatment. A vibratory ball mill (Retsch GmbH, Germany) having a 50 ml mill-jar and a 25 mm diameter stainless-steel ball was used in this study. About 10 g of CMC treated cellulose slurry was taken in the mill-jar along with the ball and processed at 20 Hz vibration frequency and at various ball milling times as shown in Table 1. The high impact and shear forces by the vibratory motion of mill-jar and ball broke the hydrogen bonds between the cellulose bers to reduce ber dimensions. After each treatment, the slurry samples were collected and stored at 4° C in a container for the homogenization process. The CMC treated sample without ball milling was chosen as a control sample.
For NaOH treatment, about 10% (w/v) cellulose slurry was prepared from the cellulose powder and was soaked with 2% (w/v) NaOH aqueous solutions at 4°C for 24h. After the treatment, the swollen cellulose was neutralized with acetic acid and washed in DI water to remove excess NaOH as similar to the procedure described by Lee et al., (2016). The neutralized cellulose slurry was ball milled at various ball milling times as listed in Table 1. After each treatment, the treated sample was stored at 4° C for a highpressure homogenization process. Each treatment was repeated three times. A sub-sample of the treated bers was sent for ber dimension measurement.

CNF brilization process
The pretreated cellulose slurries were homogenized using a high-pressure homogenizer (APV-1000, SPXFLOW, U.S.A) at 700 bar pressure. A positive displacement pump in the homogenizer circulated the cellulose slurry through a ceramic homogenizer valve at high pressure. The strong turbulence and cavity pressure generated at the homogenizer valve disrupted the intra and inter-molecular hydrogen bonds of micro cellulose ber bundles and caused the brillation. Preliminary studies have revealed that when the cellulose bers after CMC and NaOH treatments were subjected to ball milling, the reduction in ber dimensions enabled to increase the slurry concentration up to 3% (w/v). A further increase in slurry concentrations beyond 3% caused both clogging and ineffective brillation of bers. Thus, the cellulose slurry with 3% concentration was optimal to process in the homogenizer for all experimental conditions except for the control sample as shown in Table 1. For the control treatment, about 1% (w/v) concentration of knife milled cellulose slurry along with 2% CMC was applied. Increasing the cellulose concentration beyond 1% on the control treatment caused the clogging of the homogenizer valves and ceased the operation due to the entangled ber networks. For each treatment, about 200 ml of cellulose slurry with prede ned concentration was passed through a high-pressure homogenizer up to seven cycles to obtain a stable CNF hydrogel. The initial temperature of the cellulose slurry from 25° C reached up to 80° C at the end of the homogenization process. Finally, the homogenized hydrogel was centrifuged (5430 R Centrifuge, Eppendorf, Germany) at 6000 rpm at room temperature for 20 min to remove excess water from CNF hydrogel and the total solid content in the sample was measured. The CNF samples were stored in a refrigerator at 4° C for characterization studies. Each test was repeated three times.

Optical microscope imaging
The dimensions of control and ball mill pretreated cellulose slurries were measured on a DMLS2 optical microscope (Leica Microsystems, Germany). The samples were suspended in distilled water with 0.2% concentration and a drop was placed between the glass slide and coverslip. The images were captured for the magni cation-20x, gain-1.0, and exposure time-100 ms. The length and width of bers were measured using ImageJ software for analysis (Nechyporchuk et al. 2015;Vanderghem et al. 2012).

Total Solid content
The total solid content of the CNF was determined by drying the CNF samples in a convection oven. The oven temperature was set at 100 ± 5°C. A known amount of CNF sample was weighed ( \varvecW \varveci ) and dried in the oven for about 12 hrs or until no change in the dried sample weight.
The dried CNF sample was taken out and weighed (\varvecW \varvecf ) to determine the total solid content using the following equation (1). Each test was repeated three times.

Zeta potential
The Fourier-transformed infrared spectra (FTIR) The chemical structure of CNF samples was characterized by FTIR spectra. The CNF lms were manufactured by the lm casting method and dried in a desiccator for 24 h before FTIR spectra measurements. The FTIR spectra for each sample were collected in absorbance mode using a Nicolet 6700 VariGATRTM spectrometer (Thermo Electron Corporation, U.S.A.). The CNF lms were scanned with a resolution of 4 cm −1 in the range of wavenumbers from 4000 to 600 cm −1 and each test was triplicated.

X-ray diffraction (XRD)
The crystalline structure of CNF samples was studied by the D8 Advance model XRD system (Bruker, U.S.A) having X-ray source-Co tube and wavelength-1.79037 Å

Thermal degradation analysis
The thermogravimetric analysis (TGA) was performed to study the thermal degradation behavior of CNF samples. The SDTA851e thermogravimetric analyzer (Mettler Toledo, U.S.A.) and STARe data analysis software were used to determine the thermal degradation temperature of each sample. The CNF samples were heated in the range of 25 to 600°C with a rate of 10°C/min under an inert atmosphere of nitrogen and with a gas ow of 50 mL/min. The sample mass between 5 to 9 g was used.
Tensile test The tensile properties of CNF lms were determined by the lm cast method. The dried CNF lms were conditioned at 23° C and 50% RH for 24 h in a desiccator before tensile testing. The tensile tests were performed according to the ASTM D882 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting). Five replications from each CNF sample were tested. An AGS-X tensile tester (Shimadzu, Japan) with a 1 kN load cell was used at 50 mm/min crosshead speed. The tests were performed at ambient temperature. The ultimate tensile strength and Young's modulus were recorded for the analysis.

Statistical analysis
The effect of ball milling time on ber dimension reduction, solid content percentage, and bril width was studied by the one-way ANOVA method. The multiple comparison method was also performed to determine which sample mean was different from others. The MATLAB software was used for statistical analysis and tests were conducted for the signi cance level of 5%.

Results And Discussion
Effects of pretreatment methods on cellulose ber dimensions The initial dimensions of both untreated and pretreated cellulose powder were measured using the optical microscope images (Fig. 2). The untreated cellulose powder had an average ber width of 33±6 µm and length of 309±201 µm. The increase in ball milling time reduced the ber dimensions with CMC and NaOH treatment as shown in Figs The one-way ANOVA test showed that there was a signi cant reduction in cellulose ber dimensions by the combination of both the pretreatment methods (See the supplementary document -Tables S1 and S2). The multiple comparison test also con rmed that there was a signi cant change in ber dimensions between the control and the ball-milled samples (see the supplementary document -Figs S1 and S2). The mean ber length of pretreated samples was signi cantly different among all combinations of pretreatments except for the 15MC sample (CMC dispersed and 15min ball milling). The CMC dispersion with ball milling did not signi cantly change the mean ber width for 15-min and 45-min ball milling times. However, there was a signi cant reduction in the mean ber length of all treatments of CMC dispersed cellulose with ball milling. The hydrogen bonds of cellulose networks were mainly affected by ball mill treatment in the CMC dispersion-ball milling pretreatment method. The swelling of cellulose ber with NaOH followed by 15 min ball milling signi cantly reduced the ber length and width. The cell wall reduction by NaOH swelling made the cellulose bers more susceptible to weakening the intramolecular hydrogen bonds, thus reducing the ber width and length even at lower ball milling time. Most cellulose ber bundles were cleaved by ball milling impacts as shown in Fig. 2c. and 2d to enable e cient dimensional reduction.

Effects of pretreatment methods on the homogenization of cellulose bers.
A high-pressure homogenizer was used to brillate pretreated cellulose slurries into CNF hydrogels. Each pretreatment method determines the concentration of knife-milled cellulose slurries sent to the homogenizer. About 1% knife-milled cellulose slurries with NaOH treatment and without ball milling clogged the homogenizer due to swollen cellulose bers. However, the CMC dispersed cellulose slurry without ball mill pretreatment (control sample), was able to pass through the homogenizer with a maximum concentration of 1%. Increasing the cellulose beyond 1% caused pressure uctuation and clogging of the homogenizer valve. On the other hand, all the pretreated and ball-milled bers were processed with up to 3% concentration of cellulose slurry in a homogenizer. The turbulence, shear, and cavitation pressures exerted on the cellulose bers effectively brillated the bers into CNF hydrogels. Increasing the slurry concentration beyond 3%, decreased the pumping ability of the homogenizer and reduced the brilization after a few passes of all test samples. It was observed that the increases in ball milling time from 15 min to 75 min did not in uence the input slurry concentration beyond 3%.
After each treated sample was homogenized, the CNF hydrogel was centrifuged to remove free water and tested for total solid content ( Table 2). The control sample (0MC) produced ~4% solid content CNF due to improved dispersibility during homogenization. The pretreated samples showed a gradual increase in the total solid content of CNF due to reduced ber dimensions. The CMC treated samples with 15 min and 45 min ball milling (15MC & 45MC) produced only ~4% solid content CNF due to minimal reduction in ber dimensions (~20%). When the ball milling time was increased to 75 min on the CMC treated ber (75MC), the total solid content was increased to ~6% due to a drastic reduction in ber dimensions (~80%). When the cellulose powder was treated with a combination of NaOH and ball milling, the CNF solid content has distinctively changed. The CNF solid content for the treatments 15MN and 45 MN was increased to ~5% due to the ber dimension reduction of 60%. A further reduction in ber dimensions (~80%) achieved by the treatment condition-75MN produced a CNF with ~6% solid content. The reduced ber dimensions achieved by severe ball milling treatment, especially 75MC, and 75MN samples decreased the entangled structure of CNF and reduced the water holding capacity of hydrogel ( The one-way ANOVA test con rmed that the CNF solid content was signi cantly different between different treatment methods and the control sample (see the supplementary document - Table S3). Furthermore, the multiple comparison test showed that the CNF solid contents from the treatments 15MC and 45MC including the control sample (0MC) were signi cantly different from the rest of the samples (75MC, 15MN, 45MN, and 75MN) (see the supplementary document - Fig. S3). Therefore, a short ber length (<50 µm) with mild alkaline treated cellulose can produce consistent quality and high solid content CNF using a high-pressure homogenizer.

Dispersion stability
The dispersion stability of CNF depends on the surface charge density of CNF emulsion. The strong repulsive forces generated by electric charges of CNF emulsion in water prevented bril agglomeration (Chami Khazraji and Robert 2013; Nishiyama 2018). The surface charges of CNF samples are usually measured and reported as zeta potential (mV). The observed mean zeta potentials of all CNF samples were close to 30 mV and were 60% higher than that of the Ref. sample as shown in Table 2. The higher electric charges of the cellulose chains indicated the higher Coulomb repulsive forces between cellulose molecules that prevented the agglomeration and improved the dispersion stability. When the zeta potential values are higher than the absolute value of 15 mV, the CNF suspensions in an aqueous solution were considered stable (Dukhin and Goetz 1998;Khouri 2010;Mohaiyiddin et al. 2016). The dispersion of cellulose in water is due to the interaction of cellulose chains and water molecules through electrostatic forces. The electrostatic forces appear between hydrogen atoms with δ + charge and a cellulose chain carrying δ-charge (Chami Khazraji and Robert 2013; Rizzato et al. 2010). The one-way ANOVA test on the zeta potential of CNF from different treatments con rmed that there were no signi cant differences among the treatments and the control sample (see the supplementary document - Table S4). The increased ber surface area during the ball milling treatment generated higher surface changes on the CNF hydrogel with stable dispersion stability (Jiang and Hsieh 2013). Therefore, short and uniform ber dimensions facilitated effective brillation of cellulose bers to produce highly dispersible CNF hydrogel.

FTIR analysis
The chemical structure of all the CNF samples was veri ed by Fourier-transformed infrared (FTIR) spectra as shown in Fig. 3. There were no signi cant changes in the FTIR spectra among all CNF samples.

Crystalline structure
The crystallinity of CNF samples was studied from X-ray diffractograms (XRD) as shown in Fig. 4. It shows the peaks at 2θ = 17 to 18° and 26 to 27° corresponding to cellulose crystallographic planes 1 0 1 and 0 0 2 de ned by International Center for Diffraction Data -ICDD (Morais et al. 2013b). It also implies that the cellulose type I did not change into type II allomorph by any of the treatment processes.
The one-way ANOVA and multiple comparison tests inferred no signi cant differences among the crystallinity index of all treated CNF, control, and Ref. samples (see the supplementary document - Table   and Fig. S5). The crystallinity index was about 72% for control (0MC), Ref. and certain treated samples (15MC, 45MC, and 15MN). It was slightly increased by up to 5% for the treated samples-75MC, 45MN and 75MN. The impact and shear forces applied over cellulose bers during ball milling were effective to isolate bers from the bundle and to reduce amorphous regions without affecting the crystalline region (Nuruddin et al. 2016;Yu and Wu 2011;Zhang et al. 2015). The NaOH treated samples showed a slightly increased crystallinity index than that of the CMC dispersed samples. The ball milling of NaOH treated bers exhibited a marginal reduction in the amorphous regions of cellulose causing a slight increase in crystallinity index (Fattahi Meyabadi et al. 2014;Zhao et al. 2006). However, the effect of ball milling was not severe to reduce the crystalline region as reported elsewhere (Amidon and Houghton 1995;Zhao et al. 2006;Lee 2016;Lee et al. 2018). Table 2 The total solid content, zeta potential, and the crystallinity index of various pretreated CNF samples 1 Data presented after the symbol -"±" are standard deviations with n = 9 c Data presented after the symbol -"±" are standard deviations with n = 20 d Data presented after the symbol -"±" are standard deviations with n = 3 1 Mean values under each column with the same type of symbols are not signi cantly different at a 5% con dence interval

Morphological Structure
The SEM images of CNF lm samples were used to estimate the average width of cellulose brils as shown in Fig. 5. A strong cellulose microstructure network with entangled brils was observed in the SEM images. The surface morphology of the brils was smooth in all the samples. The CMC dispersion, NaOH swelling, and ball milling treatments affected the CNF bril width. The control CNF sample (0MC) had an average brils width of about 43±19 nm. The average width of brils from 15MC and 45MC samples was in the range of 20 to 30 nm and was less than 20 nm for the rest of the samples (75MC, 15MN, 45MN, and 75MN). The reduction of cellulose ber width and length to less than ~5 and 50 µm by pretreatment methods caused smaller bril width in 75MC, 15MN, 45MN, and 75MN CNF samples.
A one-way ANOVA analysis of bril width on various pretreatment methods indicated that there was a signi cant difference between the treated and the control CNF samples (see the supplementary document - Table S6). It was also found from multiple comparison tests that the mean bril width was signi cantly different from all pretreated samples

Thermal degradation analysis
The thermal degradation behavior of CNF was studied from the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves presented in Fig. 6. In all samples, an initial weight loss of 5% was observed between 40 to 100°C due to absorbed moisture. The second stage of weight loss was due to the thermal degradation of cellulose. The increased tensile properties of pretreated CNF samples indicated the presence of rigid nano bril network structure in CNF lms. The CNF brilization process also increased the surface area of bers for hydrogen bonding. The hydrogen-bonded bril network with reduced porosities increased the tensile strength of lms prepared from pretreated CNF. It was evident from SEM images that a combination of CMC dispersion/NaOH swelling and ball milling effectively liberated nanostructured brils having a width up to 30 nm. The brils with reduced width exposed more hydroxyl groups for hydrogen bonding and enhanced entanglement of the network to resist deformation by higher tensile loading. The loosely packed network due to larger width brils resulted in lower tensile strength in control samples.

Conclusions
The cellulose powder treated by CMC dispersion with 75 min ball milling and NaOH swelling with 15-75 min ball milling achieved more than 80% ber dimension reduction. This reduction in cellulose ber dimensions by various pretreatment methods increased the CNF solid content by up to 6%. It also enhanced the dispersion abilities and prevented the clogging of cellulose micro bundles during the homogenization process. Among the pretreatment methods, the NaOH-15 min ball mill treatment was the most effective pretreatment that used the least ball milling time for the larger ber size reductions.
However, the CMC/NaOH treatment with the longest ball milling time of 75 min produced the highest solid content CNF during high-pressure homogenization. The pretreatment method did not in uence the chemical structure and dispersion abilities. The crystallinity was increased by 4 to 5% for CMC dispersion with 75 min ball milling and NaOH swelling with 45 and 75 min ball milling pretreatments. The thermal degradation study revealed a marginal improvement in thermal stability of all CNF samples. The tensile strength of CNF lms manufactured from all pretreated samples was between 80 and 105 MPa. Overall, short cellulose bers with mild alkaline pretreatment can produce consistent quality and highly dispersible CNF hydrogel with high solid content for industrial applications.
Declarations Figure 1 CNF production from pretreated cellulose powder Figure 2 Fiber dimensions and optical microscope images of ball mill treated cellulose bers. a Comparison of cellulose ber width to different pretreatment conditions. b Comparison of cellulose ber length to different pretreatment conditions. c Optical microscope image of a cellulose ber after CMC dispersion-75 min ball milling pretreatment. d Optical microscope image of a cellulose ber after NaOH swelling-75 min ball milling pretreatment.