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Preparation of cellulose nanomaterials via cellulose oxalates
Carbohydrate Polymers 213 (2019) 208–216
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Preparation of cellulose nanomaterials via cellulose oxalates ⁎
Jonatan Henschen , Dongfang Li, Monica Ek
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T
Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56-58, SE-100 44, Stockholm, Sweden
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocellulose Cellulose oxalate Oxalic acid dihydrate Cellulose Cellulose nanofibrils Cellulose nanocrystals
Nanocellulose prepared from cellulose oxalate has been discussed as an alternative to other methods to prepare cellulose nanofibrils or crystals. The current work describes the use of a bulk reaction between pulp and oxalic acid dihydrate to prepare cellulose oxalate followed by homogenization to produce nanocellulose. The prepared nanocellulose is on average 350 nm long and 3–4 nm wide, with particles of size and shape similar to both cellulose nanofibrils and cellulose nanocrystals. Films prepared from this nanocellulose have a maximum tensile stress of 140–200 MPa, strain at break between 3% and 5%, and oxygen permeability in the range of 0.3–0.5 cm3 μm m−2 day−1 kPa−1 at 50% relative humidity. The presented results illustrate that cellulose oxalates may be a low-cost method to prepare nanocellulose with properties reminiscent of those of both cellulose nanofibrils and cellulose nanocrystals, which may open up new application areas for cellulose nanomaterials.
1. Introduction As awareness of our impact on the planet is increasing, the demand for new sustainable materials has grown rapidly in recent years, as evidenced by the UN agenda 2030 (Assembly U.G., 2015). One raw material that has the potential to be used in many new materials is wood and cellulosic pulp fibre derived from wood. By isolation of the nanostructures that make up wood fibres, it is possible to obtain materials with highly interesting properties, such as high specific strength and stiffness (Tanpichai et al., 2012), high surface area (Moon, Martini, Nairn, Simonsen, & Youngblood, 2011), biodegradability (Jung et al., 2015) and low toxicity (Alexandrescu, Syverud, Gatti, & ChingaCarrasco, 2013). The nanomaterials isolated from pulp fibres are generally categorized as either cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs). CNCs from wood are rod-like particles with a length of 50–500 nm and a width between 3 and 5 nm (Moon et al., 2011). These particles are obtained through acid hydrolysis (Rånby, 1951) of cellulose fibres to remove all amorphous and less ordered regions in the fibre, leaving only the crystalline regions of cellulose. This fabrication process produces materials with high crystallinity ranging between 54 and 88% (Moon et al., 2011). CNFs, on the other hand, contain both the amorphous and crystalline regions of the nanostructures in wood. This makes the CNFs long and flexible with a length of 500–2000 nm and a width between 4–20 nm (Moon et al., 2011). CNFs are produced through mechanical refining of pulp fibres, which is usually preceded by a chemical pre-treatment that reduces the adhesive forces between
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fibrils and reduces the energy required to liberate them. One frequently suggested application of CNFs and CNCs are as thin films. The difference in size between CNFs and CNCs makes the fabrication processes different. CNFs can be prepared either by solvent casting or by filtration over a fine membrane, while CNCs often is prepared by solvent casting. Films prepared from these materials have high transparency (Aulin, Salazar-Alvarez, & Lindström, 2012; Saito et al., 2009), high specific strength (Saito et al., 2009; Syverud & Stenius, 2008) and are good oxygen barriers. The properties of these films are however strongly affected by humidity which for some cases means that they require modification or mixing of the nanocellulose with other components to reduce this effect (Aulin et al., 2012; Zhang, Zhang, Lu, & Deng, 2012). Some suggested uses of these films are in food (Arora & Padua, 2010) and electronic (Jung et al., 2015) applications. In the pursuit of lowering production costs and changing the properties of nanocellulose, many alternative pre-treatments and methods to produce these materials have been published. CNFs have been produced with varying pre-treatments that either reduce the cellulose chain length, e.g., enzymatic pre-treatment (Henriksson, Henriksson, Berglund, & Lindstrom, 2007), or introduce electrostatic charges to cellulose, e.g., TEMPO-mediated oxidation (Saito, Kimura, Nishiyama, & Isogai, 2007), phosphorylation and carboxymethylation (Wågberg et al., 2008). CNC has been produced with varying types and concentrations of acid, including both organic and mineral acids, e.g., oxalic acid (Chen, Zhu, Baez, Kitin, & Elder, 2016; Li, Henschen, & Ek, 2017), citric acid (Spinella et al., 2016) and hydrochloric acid (Araki, Wada, Kuga, & Okano, 1998). At low concentrations, these acids
Corresponding authors. E-mail addresses: [email protected] (J. Henschen), [email protected] (D. Li), [email protected] (M. Ek).
https://doi.org/10.1016/j.carbpol.2019.02.056 Received 15 November 2018; Received in revised form 20 January 2019; Accepted 16 February 2019 Available online 18 February 2019 0144-8617/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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catalyse the hydrolysis of the amorphous regions of cellulose fibril aggregates. At higher concentrations, they can also react with cellulose to introduce new functionality. Oxalic acid has been used both at low and high concentrations to produce nanocellulose in different systems. The first publication describing the production of nanocellulose using highly concentrated oxalic acid used molten oxalic acid dihydrate (Ek et al., 2015); later publications using oxalic acid as a 70% water solution (Chen et al., 2016) and the dissolution of oxalic acid in eutectic solvents (Sirvio, Visanko, & Liimatainen, 2016) have appeared. Oxalic acid dihydrate contains 29% water of crystallization and has a melting point of 101 °C, making reactions with pulp fibres to produce cellulose oxalates without additional solvent possible. The authors have previously published a short communication where molten oxalate dihydrate was used to prepare cellulose oxalate. This material was shortly shown to be a suitable derivative to prepare CNCs at high yield using ultrasonication (Li et al., 2017). The current paper further investigates the possibility to prepare high-quality cellulosic nanomaterials using oxalic acid dihydrate from different types of pulp, reaction conditions and more sustainable washing procedures. Using microfluidization, nanocellulose was prepared with particles resembling both CNFs and CNCs. These materials characterized and found suitable to prepare films using vacuum filtration with good mechanical and oxygen barrier properties.
Table 1 The prepared samples, their names and reaction parameters. All reactions were performed by mixing 20 g of pulp with 77 g of oxalic acid dihydrate at 110 °C. Name
Raw material
Reaction time (min)
Washing
SWD35E SWD35A SWD60E SWD60A SWK35E SWK35A SWK60E SWK60A
Dissolving Dissolving Dissolving Dissolving Kraft pulp Kraft pulp Kraft pulp Kraft pulp
35 35 60 60 35 35 60 60
Ethanol Acetone Ethanol Acetone Ethanol Acetone Ethanol Acetone
pulp pulp pulp pulp
(2014). Initially, all samples were passed through two large chambers of 400 μm and 200 μm connected in series. Following this, they were passed multiples times through a 200 μm and a 100 μm interaction chamber connected in series. The samples were passed 1, 3 or 5 times through the small chambers, and are denoted as 1p, 3p or 5p, respectively. The pressure was set to 925 bar when passing the samples through the large chambers and 1,600 bar when passing through the large chambers. 2.2.2. Characterization Fourier transform infrared spectroscopy (FTIR) spectra of the cellulose oxalates were measured with a PerkinElmer Spectrum 2000 FTIR (PerkinElmer, USA) equipped with a heat-controlled single reflection attenuated total reflection (ATR) accessory (Golden Gate heat-controlled); 32 scans were recorded for each spectrum. The nanocellulose content, represented by the amount of colloidal particles, was calculated for SWD35A_5p and SWK35A_5p. This was done by diluting the nanocellulose to 2 g L−1 and dispersing them using a T18 UltraTurrax (IKA, Germany) at 16,000 RPM for 10 min before the suspensions were centrifuged at 2,500 RCF for 1 h. The solid content of the supernatant was determined and compared to the solid content prior to centrifugation. The total nanocellulose yield was calculated by multiplying the nanocellulose content with the gravimetric yield. The free carboxyl content (FCC) of each cellulose oxalate was determined by conductometric titration as previously described (Habibi, Chanzy, & Vignon, 2006; Li et al., 2017). A suspension was prepared by mixing 100 mg of each cellulose oxalate with 100 ml of water and 10 ml of a 0.01 M NaCl solution. The suspension was then stirred for 1 h and titrated with 0.01 M NaOH. All titrations were carried out under constant nitrogen bubbling. The calculation of the content of free carboxyl groups is based on the equation below:
2. Experimental 2.1. Materials Softwood sulphite dissolving pulp sheets with a cellulose content of ≥96 wt % were supplied by Domsjö fabriker (Aditya Birla, Domsjö, Sweden). Softwood kraft pulp sheets (Imperial Anchor) with a cellulose content of 84 wt % were supplied by Holmen AB (Iggesund, Sweden). Oxalic acid dihydrate (≥99%) was purchased from Sigma Aldrich (Merck KGaA, Darmstadt, Germany). Ethanol (96%) and acetone (≥99.5%) were supplied by VWR International AB (Stockholm, Sweden). Commercially available food-grade olive oil was acquired at a local supermarket (ICA, Stockholm, Sweden). 2.2. Methods 2.2.1. Preparation of cellulose oxalate The dry pulp sheets were torn by hand into pieces of approximately 1 × 1 cm2. An evaporation flask was filled with 20 g of the torn pulp and 77 g of oxalic acid dihydrate. To improve mixing, 30 glass balls (Ø: 15–16 mm) were added to the flask. The flask was attached to a rotary evaporator and lowered into a heating bath set to 110 °C while rotated for either 35 or 60 min. After the desired time was reached, the mixture was lifted from the oil bath and allowed to cool at room temperature while still rotating. After cooling down and solidifying, either ethanol or acetone was added to dissolve excess oxalic acid, and the solution was allowed to mix overnight. The added solvent was removed by vacuum filtration, and the remaining solids were washed further by filtering either acetone or ethanol through the material until the filtrate reached a conductivity below 10 μS cm−1. At that point, the remaining solids were collected as cellulose oxalate and allowed to dry at 40 °C in an oven. The product was then obtained as a dry powder. The different samples are denoted as seen in Table 1. The gravimetric yield of cellulose oxalate was calculated based on the dry weight of the pulp fibres and cellulose oxalate. To prepare nanocellulose out of cellulose oxalates, two of the samples (SWK35E and SWD60E) were dispersed in deionized water at a concentration of approximately 2 wt %. To fully dissociate the carboxylic acids and aid the fibrillation, the pH of the dispersions was adjusted to pH 9–10 using 0.1 M sodium hydroxide before they were mechanically disintegrated using a microfluidizer (M-110EH, Microfluidics Corp, United States)similarly as described by Khan et al.
Free carboxyl content =
CNaOH × VNaOH m
where CNaOH is the exact concentration (mol/l) of the NaOH solution, VNaOH is the exact volume (l) of the NaOH solution used for titration before the conductivity increased from the plateau of the titration curve and m is the dry weight (g) of the oxalate sample. The morphologies of SWK35E_3p and SWD60E_5p were studied by adsorbing the samples on silica wafers and imaging them using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The samples were adsorbed onto the silicon wafers using a binding layer of polyethylenimine. After washing and plasma treating the wafers they were dipped in polyethylenimine solution (1 g L−1), rinsed with deionized water, dried in a flow of nitrogen, quickly dipped in dilute suspensions of nanocellulose, rinsed with deionized waster and dried in a flow of nitrogen. Just before imaging, the samples using an S4800 field emission SEM (Hitachi, Tokyo, Japan), they were coated with a 5 nm platinum/palladium coating in a 208HR high-resolution sputter coater (Cressington, Watford, UK) to suppress specimen charging. AFM images were acquired using tapping mode on a Multimode IIIa instrument (Bruker, Santa Barbara, CA, United States). The images 209
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were recorded under ambient conditions (23 °C and 50% relative humidity) in air using Scanassyst cantilevers (Bruker, Santa Barbara, CA, United States) with a nominal resonance frequency of 70 kHz and a spring constant of 0.4 N m−1. The images captured by AFM were used to determine the dimensions of the nanocellulose. The width was measured using Nanoscope Analysis software (Bruker, Santa Barbara, CA, United States) by measuring the height of 223 (SWK35E_3p) and 307 (SWD60E_5p) particles. The length was measured using ImageJ (Schneider, Rasband, & Eliceiri, 2012) by tracking 89 (SWK35E_3p) and 59 (SWD60E_5p) particles. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA 851e to study the thermal properties of the cellulose oxalates. Approximately 4 mg of each sample was heated from 30 to 600 °C at a rate of 10 °C min−1 under N2 flow (flow rate 50 ml min−1). The data were analysed by Mettler-STARe software. X-ray diffraction (XRD) was performed on a PANalytical X’Pert PRO MRD X-ray diffractometer (The Netherlands) equipped with a PANalytical X’Celerator detector. Diffractograms were collected in a range of 2θ = 10 to 50°. Cu Kα radiation was monochromatized with a nickel filter. The crystallinity index was estimated as described by Segal, Creely, Martin, and Conrad (1959).
Fig. 1. Potential esterification reaction of the cellulose with oxalic acid dihydrate; during the reaction, the cellulose also undergoes acid-catalysed hydrolysis (not shown).
more sterically available for chemical reactions, which makes them more labile than those in cellulose in acid-catalysed hydrolysis and dehydration. By washing the acid/pulp mixture with filtration it is possible to eliminate the need for dialysis, this also facilitates recycling the acid as it will be diluted less during washing. Both high demands for chemical recovery and long washing procedures are two mayor obstacles when preparing conventional sulphuric acid based CNCs (Nelson et al., 2016). Preparation of CNCs using hydrochloric acid can also be done without the need for dialysis, unlike cellulose oxalate, the resulting material however has low surface charge and subsequently poor colloidal stability (Yu et al., 2013). Oxalic acid has the potential to participate in the simultaneous hydrolysis and esterification of cellulose (Fig. 1) (Li et al., 2017), due to the fibre structure it is difficult for the acid to fully penetrate the fibre and react with all cellulose chains. Esterification is most likely to occur on C6-OH due to its greater steric accessibility than C2-OH and C3-OH (Fox, Li, Xu, & Edgar, 2011). Esterification with a difunctional carboxylic acid can result in both the introduction of charged acid groups and crosslinking between cellulose chains. Nevertheless, our previous work has indicated that most reacted oxalic acid leaves a charged group on cellulose (Li et al., 2017). The oxalate functionality contains two carbonyl groups, which can be observed using FTIR. In all samples, a broad adsorption at 1739 cm−1 corresponding to the C]O stretching of the carbonyl group was observed. This indicates successful esterification of the cellulose oxalates. In Fig. 2, the spectra of the raw materials and SWK35E, SWK35 A, SWD60A and SWK60A are shown; spectra for all samples are found in the supplementary information (ESI). The signal of carboxyl acids (−COOH) and ester bonds (−COO−) overlaps and is difficult to differentiate in the FTIR spectra. Moreover, no clear trends were observed in the intensities of the carbonyl signals of the prepared samples. To determine the effect of the reaction time, raw material, and washing liquor on the degree of esterification, conductometric titrations were performed to quantify the free carboxyl
2.2.3. Films Films were prepared from nanocellulose suspensions (0.2 wt %) dispersed using a T18 UltraTurrax (IKA, Germany) at 14,000 RPM for 20 min before they were degassed by sonication in an ULTRAsonik 28X ultrasonic bath (NeyDental, Inc., Bloomfield, CT, USA) for 20 min. Films of ˜50 g m−2 were produced using vacuum filtration over a 0.45 μm PVDF Durapore membrane filter (Merck Millipore) in a RapidKöthen sheet former (Paper Testing Instruments, Austria). Water was drained from the suspensions until a stable gel-like material was left. At this point, a second membrane was placed on top of the gel, and both membranes were placed between two sheet-former carrier boards. The whole assembly was dried for 20 min at 93 °C and 95 kPa reduced pressure. The oxygen permeability of the above CNF films was measured using a Mocon Oxtran 2/20 (Modern Controls Inc., Minneapolis, MN) instrument equipped with a coulometric sensor. The samples were masked using aluminium foil, leaving a round area of 5 cm2 exposed for the measurement. The measurements were conducted at 23 °C and at 50% RH. Each sample was conditioned prior to the measurement. Two replicates for every sample were measured. The tensile properties of the films were evaluated on an Instron Universal testing machine 5944 fitted with a 500 N load. The initial distance between the test grips was 20 mm, and the width of each specimen was 5 mm. At least 5 specimens from each sample were tested. The separation rate was 2 mm min−1. The samples were conditioned and tested at 23 °C and 50% RH. The stress-strain curve was recorded for each test, and the data were averaged over all specimens. Young’s modulus was automatically calculated by Bluehill version 3.72 (Illinois Tool Works Inc., Illinois, USA) software. 3. Results and discussion Cellulose oxalates were prepared by mixing pulp with molten oxalic acid dihydrate. As the reaction progressed, the appearance of the pulp changed from a white to a greyish colour, followed by the torn pulp sheet pieces losing their original shape as the mixture turned into a dark yellow or brown paste. As the reaction cooled, the oxalic acid solidified, and ethanol or acetone was added to remove excess oxalic acid. For the experiments, both dissolving pulp and kraft pulp were used as raw materials (Table 1). The observed colour change was more prominent for samples prepared from kraft pulp, presumably due to its higher amount of hemicellulose. Owing to the less ordered structural conformation of hemicellulose compared to that of cellulose (Cai, Zhang, Charles, & Wyman, 2014), the hydroxyl groups of hemicellulose are
Fig. 2. FTIR spectra of a selection of the cellulose oxalates and raw materials. The vertical line in the inset is at wavenumber 1739 cm−1 corresponding to the C]O stretching of the carbonyl group. Spectra for all the remaining samples are available in the supplementary information (ESI). 210
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was considerably lower at 0.62 mmol g−1, corresponding to an estimated degree of substitution (DS) of 0.13-0.16. No clear trends among the FCCs of the cellulose oxalates washed with the same type of solvent could be observed despite the differences in reaction time and type of raw materials. As has been reported for the esterification of cellulose by oxalic acid dihydrate, the DS on cellulose does not change when the reaction time is prolonged from 30 to 60 min (Li et al., 2017). In our previous work, we discussed that when no procedures for cellulose activation (e.g., swelling) are carried out, substitution only occurs on the accessible surfaces of cellulose fibrils. The highest theoretical DS of cellulose in this case is 0.05 (Larsson, Hult, Wickholm, Pettersson, & Iversen, 1999; Pu, Ziemer, & Ragauskas, 2006). In the current study, all DS values exceeded this value, which indicates the occurrence of esterification on both accessible and inaccessible surfaces of the cellulose fibrils. Nevertheless, as the reaction time increased from 35 min to 60 min, esterification could not proceed further. This could be attributed to the strong intermolecular interactions among the cellulose chains, which limited the migration of molten oxalic acid further towards the crystalline regions of cellulose fibrils. Other methods to prepare nanocellulose by reacting oxalic acid with pulp fibres produced materials with FCC between 0.11 – 0.39 mmol g-1 (Chen et al., 2016; Sirvio et al., 2016), this is considerably lower than the values observed using the current procedure. It is believed that the higher FCC is obtained due to not adding water to quench the reaction. At elevated temperatures, introducing additional water in this reaction is favorable for the reverse reaction (hydrolysis on the formed ester bonds) but not the forward reaction (esterification), due to the change in chemical equilibrium. An increased FCC will facilitate the fibrillation when preparing the nanocellulose. In addition, the FCCs of samples washed with acetone were slightly higher than those of samples washed with ethanol. During washing, it is hypothesized that smaller fractions of cellulose oxalates with high DS can interact rather well with polar solvents and thus can produce smaller particles. The particles may be fine enough to either be entrapped in the filter paper or possibly pass through it. Consequently, these fractions were removed from the samples after washing. As ethanol is more polar than acetone, this is more likely to occur when
Table 2 The yield, free carboxyl content, degree of substitution and thermal degradation onset temperature for the cellulose oxalates.
SWD35E SWD35A SWD60E SWD60A SWK35E SWK35A SWK60E SWK60A
Yield [%]
Free carboxyl content [mmol g−1]
Estimated degree of substitution
Thermal degradation onset temperature [°C]
93.8 97.1 85.4 96.4 83.6 96.0 85.6 99.3
0.86 1.05 0.62 1.08 0.92 1.10 0.97 1.04
0.15 0.18 0.11 0.19 0.16 0.19 0.17 0.18
175 176 175 175 177 173 176 N/A
Fig. 3. TG (left axis) and DTG (right axis) curves for SWD60A at 10 °C min−1 under N2 flow. The curves for the other samples are similar and are available in the supplementary information (ESI).
content of each sample. As shown in Table 2, the FCC of all samples ranged between 0.86–1.1 mmol g−1, except for SWD60E, which for unknown reasons
Fig. 4. Nanocellulose suspension at approximately 1.5 wt % prepared from the cellulose oxalates passed through the small chambers in the microfluidizer for either 1 passes (top) or 3 passes (bottom). SWK35E_3p was never collected. 211
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Fig. 5. Representative SEM (top) and AFM (middle) micrographs of nanocellulose adsorbed onto silica surfaces from suspensions of SWK35E_3p (left) and SWD60E_5p (right). Distribution (bottom) width (left) and length (right) of SWD60E_5p and SWK35E_3p measured using AFM.
with ethanol resulted in lower yields in all samples, which could be attributed to the removal of small fractions of hydrolysed and esterified cellulose and hemicellulose during washing, as discussed above. Thermal degradation of cellulose oxalates was detected by TGA to determine the onset temperature and the maximum degradation temperatures of the main mass-loss regions. Three different mass-loss regions could be observed (Fig. 3). The first mass-loss region (30–130 °C) could be attributed to the loss of physically adsorbed water in cellulose oxalates (Moriana, Vilaplana, Karlsson, & Ribes, 2014). The thermal degradation onset temperatures of all the studied cellulose oxalates were 173–177 °C, which showed the beginning of the second main mass-loss region (175–280 °C). This could be related to the decomposition of chemically attached oxalate groups as well as the dehydration of cellulose. The third main mass-loss region (280–375 °C) indicated the depolymerization of cellulose (Peng et al., 2013). Considering the onset temperatures, the cellulose oxalates and
washing cellulose oxalate with ethanol. As reported earlier (Li et al., 2017), prolonging the reaction time from 35 min to 60 min when using dissolving pulp slightly decreases the yield (Table 2). This is thought to be due to increased hydrolysis with possible dissolution of some of the degradation products. This trend is not observed when using kraft pulp as a raw material; for kraft pulp, the yield increases slightly when increasing the reaction time from 30 min to 60 min. There is no clear difference in the gravimetric yield between the samples prepared from kraft or dissolving pulp, despite having different cellulose contents. The dissolving pulp has a cellulose content > 96%, while kraft pulp contains 84% cellulose. This difference indicates that the samples prepared from kraft pulp not only contain cellulose oxalate but also hemicellulose, which has been hydrolysed and esterified by oxalic acid. The yield of cellulose oxalates was affected by whether either ethanol or acetone was used to remove the excess oxalic acid. Washing 212
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aggregates present in the gel. This was determined for SWD35A_5p and SWK35A_5p. After dilution and centrifugation, the content was determined to be 93% and 98% for SWD35A_5p and SWK35A_5p, respectively. The lower content for SWD35A_5p is likely connected to the presence of larger aggregates, which reduce the viscosity and scatter light, as observed when visually comparing the gels after homogenization. The total nanocellulose yield (from raw material to dispersed nanocellulose) was determined to 90% and 94% for SWD35A_5p and SWK35A_5p, respectively. Preparing nanocellulose from dry cellulose oxalate reduces the need for biocides as it is possible to fibrillate the nanocellulose on demand. It may also be open up new processes to fibrillate nanocellulose, for example in combination with other processing steps when preparing formulations. Other studies have shown great potential to increase the yield when producing nanocellulose by first producing CNCs followed by preparing CNFs from the solid residue remaining after the reaction (Chen et al., 2016; Wang, Zhu, & Considine, 2013; Wang, Chen, Zhu, & Yang, 2017). These studies have used centrifugation followed by dialysis for washing the material, and in some cases required a substantial number of passes in a microfluidizer to prepare the CNFs. The current paper is able to simplify the washing step by using only filtration, and to prepare a nanocellulose gel with only one pass through the 200 μm and 100 μm interaction chamber of the microfluidizer. Simplifying the process and reducing the required energy to fibrillate is crucial in order to decrease the production cost in an industrial process. Samples of SWK35A_5p and SWD35A_5p were used to produce films; both of these samples formed even and transparent films with a thickness of 30 μm when they were vacuum filtered and pressed (Fig. 6). SEM images of the films (Fig. 7) show a closely packed fibril structure. The SWD35A_5p and SWK35A_5p films had crystallinity indexes of 73.6% and 78.5%, respectively (Table 3). The crystallinity indexes of the raw materials were measured to be 70.9% and 58.1% for the dissolving pulp and kraft pulp, respectively. The crystallinity of both samples increased after the reaction as amorphous regions in the cellulose were hydrolysed, which was especially evident in the sample prepared from kraft pulp. Films produced from sulphuric or hydrochloric acid CNCs are often produced using solvent casting because the particles are small enough to pass through most filter membranes. Solvent casting is a very slow process that, together with poor film properties, limits the use of films produced from CNCs. The nanocellulose prepared in the current work was prepared using vacuum filtration over a 0.43 μm membrane without any substantial material loss. Films produced from SWD35A_5p had a lower tensile strength and lower elongation at break compared to those produced from SWK35A_5p (Table 3). In the present work, the suspension of SWK35A_5p showed a higher transparency than the suspension of SWD35A_5p at the same consistency, which indicates a more efficient separation of nanosized cellulose fibrils in the former than the latter (Moser, Lindström, & Henriksson, 2015). The more efficient separation of nanofibrils in the suspension enabled the exposure
Fig. 6. Films prepared from a) SWK35A_5p and b) SWD35A_5p produced by vacuum filtration over a membrane.
presumably the resulting nanocellulose can be used as reinforcements in thermoplastics, such as polystyrene (PS, melting temperature 74–105 °C), low-density polyethylene (LDPE, melting temperature 103–110 °C) and high-density polyethylene (HDPE melting temperature 125–132 °C). When homogenizing the cellulose oxalates, they easily pass through the 100 μm without clogging. At approximately 1.5 wt %, the samples prepared from kraft pulp resulted in thick gels, while the samples prepared from dissolving pulp were slightly less viscous and had a higher opacity (Fig. 4). Gels could be made from all samples with high viscosity after one pass through the microfluidizer. After homogenization of the cellulose oxalates, the difference in colour between the different raw materials, as described above, is clearly apparent. Micrographs of the homogenized cellulose oxalates (Fig. 5) show that the material consists of short fibrils with average lengths of 0.34 μm (SWK35E) and 0.37 μm (SWD60E) and average widths of 3.2 nm (SWK35E) and 4.3 nm (SWD60E). The width of the particles is comparable to that commonly found for CNCs and in the lower range for CNFs. The length, however, has a large range that spans lengths typical for both CNFs and CNCs. The analysed samples contain many particles that are similar to the shape and length (50–500 nm) (Moon et al., 2011) of CNCs and a considerable number of particles that are longer (up to 1.1 μm) than common CNCs and shaped similar to flexible CNFs. It should be noted that the number of long particles is likely underestimated because it is easier to adsorb, identify and measure small particles than it is long and entangled particles when analysing the images. The nanocellulose content is an indication of the amount of larger
Fig. 7. SEM micrographs of films prepared from SWD35A_5p (left) and SWK35A_5p (right). Note the difference in magnification between the two images. 213
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0.14–5.03(Aulin, Gällstedt, & Lindström, 2010, 2012; Naderi et al., 2016; Syverud & Stenius, 2008)
0.54 0.31 –
of more surface area for bonding between the nanofibrils during film formation. SWK35 A also has a higher crystallinity index, which may contribute to its higher mechanical strength. Consequently, the interfacial interactions between the nanofibrils were stronger in the film made from SWK35 A than the one made from SWD35 A, as represented by the superior tensile strength of the former relative to the latter. The mechanical strength of the prepared films is comparable with many other films prepared from CNFs, as is the strain at break and elastic modulus. In general, the films based on the nanocellulose in the current work have comparable tensile properties to those reported for neat nanocellulose films (tensile strength: 30–240 MPa; elastic modulus: 1–17.5 GPa) (Alain, Jean-Yves, & Vignon, 1997; Fukuzumi, Saito, Iwata, Kumamoto, & Isogai, 2009; Henriksson, Berglund, Isaksson, Lindström, & Nishino, 2008; Iwamoto, Abe, & Yano, 2008; Iwamoto, Nakagaito, & Yano, 2007; Iwamoto, Nakagaito, Yano, & Nogi, 2005; Leitner, Hinterstoisser, Wastyn, Keckes, & Gindl, 2007; Henriksson & Berglund, 2007; Nakagaito, Iwamoto, & Yano, 2005; Nakagaito & Yano, 2008; Nogi, Iwamoto, Nakagaito, & Yano, 2009; Rampinelli, Di Landro, & Fujii, 2010; Saito et al., 2009; Stelte & Sanadi, 2009; Svagan, Samir, & Berglund, 2007; Svagan, Hedenqvist, & Berglund, 2009; Syverud & Stenius, 2008; Yano & Nakahara, 2004), as well as higher tensile strength and elastic modulus than bio-based films based on non-cellulosic polysaccharides (e.g., starch, hemicelluloses, and pectin) (tensile strength: 9.8–74 MPa; elastic modulus: 0.8–2.4 GPa) (Cao, Chen, Chang, Stumborg, & Huneault, 2008; Edlund, Ryberg, & Albertsson, 2010; Le Normand, Moriana, & Ek, 2014; Mikkonen et al., 2010) and commercial petroleum-based polymers (e.g., polyethylene, polypropylene, polyvinyl chloride, and polyamide) (tensile strength: 8–165 MPa; elastic modulus: 0.2–4.1 GPa) (Mangaraj, Goswami, & Mahajan, 2009). The films produced from SWD35A_5p and SWK35A_5p both showed low oxygen permeability (Table 3), and the kraft-based film had slightly lower oxygen permeability than the dissolving-based film. This can be attributed to the higher degree of fibrillation and higher crystallinity index of the samples produced from kraft pulp. Fewer large aggregates increases the path required for gas molecules to travel through the film. The reported permeability is comparable to that of many others at 50% RH and is often regarded as sufficient for many applications.
2–10(Moon et al., 2011)
3.0 (+-0.4) 5.0 (+-0.6) 0.6(Reising et al., 2012)
4. Conclusions
66.2–82.1(Peng et al., 2013; Tejado, Alam, Antal, Yang, & van de Ven, 2012; Zhao et al., 2013)
The high-yield preparation of cellulose oxalate through the reaction between oxalic acid and cellulose was studied. During the reaction, simultaneous esterification and acid hydrolysis occurs, resulting in a highly charged (0.6–1.1 mmol/g) cellulose derivative with a crystallinity index of approximately 75%. The cellulose oxalate was prepared as a dry powder, after homogenization all samples produces nanocellulose gels after one pass through the 100 μm interaction chamber of the microfluidizer. The particle length of the resulting nanocellulose varies greatly and contains particles which resemble the size and shape of both CNF and CNC. It was shown that it is possible to prepare films with mechanical and barrier properties similar to many other CNFs through filtration. Using the described method it is possible to prepare nanocellulose without the need for extensive dialysis or centrifugation or extensive homogenization, and with very high yield. The nanocellulose has great potential for uses where the size of traditional CNFs may present problems, such as when the particle length results in a too high viscosity, and where CNCs are too small, such as when preparing films though filtration.
SWD35A SWK35A Typical CNC films Typical CNF films
74 79 54–88(Moon et al., 2011)
142 (+-5) 197 (+-7) 70(Reising, Moon, & Youngblood, 2012) 95–240(Moon et al., 2011)
10.6 (+-0.4) 10.2 (+-0.4) 6–14.9(Moon et al., 2011; Reising et al., 2012) 6-15(Moon et al., 2011)
Oxygen permeability [cm3 μm m−2 day−1 kPa−1] 50 % RH Modulus [GPa] Tensile strain at maximum tensile stress [%] Maximum tensile stress [MPa] Crystallinity index
Table 3 Crystallinity index, mechanical properties and oxygen barrier properties of the films prepared from SWD35A_5p and SWK35A_5p. Data are presented as the mean values. The error corresponds to the confidence interval, alpha = 0.05.
J. Henschen, et al.
Conflicts of interest The authors are shareholders in FineCell Sweden AB, a company working in commercializing nanocellulose. 214
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Acknowledgments
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Preparation of cellulose nanomaterials via cellulose oxalates ⁎
Jonatan Henschen , Dongfang Li, Monica Ek
⁎
T
Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56-58, SE-100 44, Stockholm, Sweden
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocellulose Cellulose oxalate Oxalic acid dihydrate Cellulose Cellulose nanofibrils Cellulose nanocrystals
Nanocellulose prepared from cellulose oxalate has been discussed as an alternative to other methods to prepare cellulose nanofibrils or crystals. The current work describes the use of a bulk reaction between pulp and oxalic acid dihydrate to prepare cellulose oxalate followed by homogenization to produce nanocellulose. The prepared nanocellulose is on average 350 nm long and 3–4 nm wide, with particles of size and shape similar to both cellulose nanofibrils and cellulose nanocrystals. Films prepared from this nanocellulose have a maximum tensile stress of 140–200 MPa, strain at break between 3% and 5%, and oxygen permeability in the range of 0.3–0.5 cm3 μm m−2 day−1 kPa−1 at 50% relative humidity. The presented results illustrate that cellulose oxalates may be a low-cost method to prepare nanocellulose with properties reminiscent of those of both cellulose nanofibrils and cellulose nanocrystals, which may open up new application areas for cellulose nanomaterials.
1. Introduction As awareness of our impact on the planet is increasing, the demand for new sustainable materials has grown rapidly in recent years, as evidenced by the UN agenda 2030 (Assembly U.G., 2015). One raw material that has the potential to be used in many new materials is wood and cellulosic pulp fibre derived from wood. By isolation of the nanostructures that make up wood fibres, it is possible to obtain materials with highly interesting properties, such as high specific strength and stiffness (Tanpichai et al., 2012), high surface area (Moon, Martini, Nairn, Simonsen, & Youngblood, 2011), biodegradability (Jung et al., 2015) and low toxicity (Alexandrescu, Syverud, Gatti, & ChingaCarrasco, 2013). The nanomaterials isolated from pulp fibres are generally categorized as either cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs). CNCs from wood are rod-like particles with a length of 50–500 nm and a width between 3 and 5 nm (Moon et al., 2011). These particles are obtained through acid hydrolysis (Rånby, 1951) of cellulose fibres to remove all amorphous and less ordered regions in the fibre, leaving only the crystalline regions of cellulose. This fabrication process produces materials with high crystallinity ranging between 54 and 88% (Moon et al., 2011). CNFs, on the other hand, contain both the amorphous and crystalline regions of the nanostructures in wood. This makes the CNFs long and flexible with a length of 500–2000 nm and a width between 4–20 nm (Moon et al., 2011). CNFs are produced through mechanical refining of pulp fibres, which is usually preceded by a chemical pre-treatment that reduces the adhesive forces between
⁎
fibrils and reduces the energy required to liberate them. One frequently suggested application of CNFs and CNCs are as thin films. The difference in size between CNFs and CNCs makes the fabrication processes different. CNFs can be prepared either by solvent casting or by filtration over a fine membrane, while CNCs often is prepared by solvent casting. Films prepared from these materials have high transparency (Aulin, Salazar-Alvarez, & Lindström, 2012; Saito et al., 2009), high specific strength (Saito et al., 2009; Syverud & Stenius, 2008) and are good oxygen barriers. The properties of these films are however strongly affected by humidity which for some cases means that they require modification or mixing of the nanocellulose with other components to reduce this effect (Aulin et al., 2012; Zhang, Zhang, Lu, & Deng, 2012). Some suggested uses of these films are in food (Arora & Padua, 2010) and electronic (Jung et al., 2015) applications. In the pursuit of lowering production costs and changing the properties of nanocellulose, many alternative pre-treatments and methods to produce these materials have been published. CNFs have been produced with varying pre-treatments that either reduce the cellulose chain length, e.g., enzymatic pre-treatment (Henriksson, Henriksson, Berglund, & Lindstrom, 2007), or introduce electrostatic charges to cellulose, e.g., TEMPO-mediated oxidation (Saito, Kimura, Nishiyama, & Isogai, 2007), phosphorylation and carboxymethylation (Wågberg et al., 2008). CNC has been produced with varying types and concentrations of acid, including both organic and mineral acids, e.g., oxalic acid (Chen, Zhu, Baez, Kitin, & Elder, 2016; Li, Henschen, & Ek, 2017), citric acid (Spinella et al., 2016) and hydrochloric acid (Araki, Wada, Kuga, & Okano, 1998). At low concentrations, these acids
Corresponding authors. E-mail addresses: [email protected] (J. Henschen), [email protected] (D. Li), [email protected] (M. Ek).
https://doi.org/10.1016/j.carbpol.2019.02.056 Received 15 November 2018; Received in revised form 20 January 2019; Accepted 16 February 2019 Available online 18 February 2019 0144-8617/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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catalyse the hydrolysis of the amorphous regions of cellulose fibril aggregates. At higher concentrations, they can also react with cellulose to introduce new functionality. Oxalic acid has been used both at low and high concentrations to produce nanocellulose in different systems. The first publication describing the production of nanocellulose using highly concentrated oxalic acid used molten oxalic acid dihydrate (Ek et al., 2015); later publications using oxalic acid as a 70% water solution (Chen et al., 2016) and the dissolution of oxalic acid in eutectic solvents (Sirvio, Visanko, & Liimatainen, 2016) have appeared. Oxalic acid dihydrate contains 29% water of crystallization and has a melting point of 101 °C, making reactions with pulp fibres to produce cellulose oxalates without additional solvent possible. The authors have previously published a short communication where molten oxalate dihydrate was used to prepare cellulose oxalate. This material was shortly shown to be a suitable derivative to prepare CNCs at high yield using ultrasonication (Li et al., 2017). The current paper further investigates the possibility to prepare high-quality cellulosic nanomaterials using oxalic acid dihydrate from different types of pulp, reaction conditions and more sustainable washing procedures. Using microfluidization, nanocellulose was prepared with particles resembling both CNFs and CNCs. These materials characterized and found suitable to prepare films using vacuum filtration with good mechanical and oxygen barrier properties.
Table 1 The prepared samples, their names and reaction parameters. All reactions were performed by mixing 20 g of pulp with 77 g of oxalic acid dihydrate at 110 °C. Name
Raw material
Reaction time (min)
Washing
SWD35E SWD35A SWD60E SWD60A SWK35E SWK35A SWK60E SWK60A
Dissolving Dissolving Dissolving Dissolving Kraft pulp Kraft pulp Kraft pulp Kraft pulp
35 35 60 60 35 35 60 60
Ethanol Acetone Ethanol Acetone Ethanol Acetone Ethanol Acetone
pulp pulp pulp pulp
(2014). Initially, all samples were passed through two large chambers of 400 μm and 200 μm connected in series. Following this, they were passed multiples times through a 200 μm and a 100 μm interaction chamber connected in series. The samples were passed 1, 3 or 5 times through the small chambers, and are denoted as 1p, 3p or 5p, respectively. The pressure was set to 925 bar when passing the samples through the large chambers and 1,600 bar when passing through the large chambers. 2.2.2. Characterization Fourier transform infrared spectroscopy (FTIR) spectra of the cellulose oxalates were measured with a PerkinElmer Spectrum 2000 FTIR (PerkinElmer, USA) equipped with a heat-controlled single reflection attenuated total reflection (ATR) accessory (Golden Gate heat-controlled); 32 scans were recorded for each spectrum. The nanocellulose content, represented by the amount of colloidal particles, was calculated for SWD35A_5p and SWK35A_5p. This was done by diluting the nanocellulose to 2 g L−1 and dispersing them using a T18 UltraTurrax (IKA, Germany) at 16,000 RPM for 10 min before the suspensions were centrifuged at 2,500 RCF for 1 h. The solid content of the supernatant was determined and compared to the solid content prior to centrifugation. The total nanocellulose yield was calculated by multiplying the nanocellulose content with the gravimetric yield. The free carboxyl content (FCC) of each cellulose oxalate was determined by conductometric titration as previously described (Habibi, Chanzy, & Vignon, 2006; Li et al., 2017). A suspension was prepared by mixing 100 mg of each cellulose oxalate with 100 ml of water and 10 ml of a 0.01 M NaCl solution. The suspension was then stirred for 1 h and titrated with 0.01 M NaOH. All titrations were carried out under constant nitrogen bubbling. The calculation of the content of free carboxyl groups is based on the equation below:
2. Experimental 2.1. Materials Softwood sulphite dissolving pulp sheets with a cellulose content of ≥96 wt % were supplied by Domsjö fabriker (Aditya Birla, Domsjö, Sweden). Softwood kraft pulp sheets (Imperial Anchor) with a cellulose content of 84 wt % were supplied by Holmen AB (Iggesund, Sweden). Oxalic acid dihydrate (≥99%) was purchased from Sigma Aldrich (Merck KGaA, Darmstadt, Germany). Ethanol (96%) and acetone (≥99.5%) were supplied by VWR International AB (Stockholm, Sweden). Commercially available food-grade olive oil was acquired at a local supermarket (ICA, Stockholm, Sweden). 2.2. Methods 2.2.1. Preparation of cellulose oxalate The dry pulp sheets were torn by hand into pieces of approximately 1 × 1 cm2. An evaporation flask was filled with 20 g of the torn pulp and 77 g of oxalic acid dihydrate. To improve mixing, 30 glass balls (Ø: 15–16 mm) were added to the flask. The flask was attached to a rotary evaporator and lowered into a heating bath set to 110 °C while rotated for either 35 or 60 min. After the desired time was reached, the mixture was lifted from the oil bath and allowed to cool at room temperature while still rotating. After cooling down and solidifying, either ethanol or acetone was added to dissolve excess oxalic acid, and the solution was allowed to mix overnight. The added solvent was removed by vacuum filtration, and the remaining solids were washed further by filtering either acetone or ethanol through the material until the filtrate reached a conductivity below 10 μS cm−1. At that point, the remaining solids were collected as cellulose oxalate and allowed to dry at 40 °C in an oven. The product was then obtained as a dry powder. The different samples are denoted as seen in Table 1. The gravimetric yield of cellulose oxalate was calculated based on the dry weight of the pulp fibres and cellulose oxalate. To prepare nanocellulose out of cellulose oxalates, two of the samples (SWK35E and SWD60E) were dispersed in deionized water at a concentration of approximately 2 wt %. To fully dissociate the carboxylic acids and aid the fibrillation, the pH of the dispersions was adjusted to pH 9–10 using 0.1 M sodium hydroxide before they were mechanically disintegrated using a microfluidizer (M-110EH, Microfluidics Corp, United States)similarly as described by Khan et al.
Free carboxyl content =
CNaOH × VNaOH m
where CNaOH is the exact concentration (mol/l) of the NaOH solution, VNaOH is the exact volume (l) of the NaOH solution used for titration before the conductivity increased from the plateau of the titration curve and m is the dry weight (g) of the oxalate sample. The morphologies of SWK35E_3p and SWD60E_5p were studied by adsorbing the samples on silica wafers and imaging them using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The samples were adsorbed onto the silicon wafers using a binding layer of polyethylenimine. After washing and plasma treating the wafers they were dipped in polyethylenimine solution (1 g L−1), rinsed with deionized water, dried in a flow of nitrogen, quickly dipped in dilute suspensions of nanocellulose, rinsed with deionized waster and dried in a flow of nitrogen. Just before imaging, the samples using an S4800 field emission SEM (Hitachi, Tokyo, Japan), they were coated with a 5 nm platinum/palladium coating in a 208HR high-resolution sputter coater (Cressington, Watford, UK) to suppress specimen charging. AFM images were acquired using tapping mode on a Multimode IIIa instrument (Bruker, Santa Barbara, CA, United States). The images 209
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were recorded under ambient conditions (23 °C and 50% relative humidity) in air using Scanassyst cantilevers (Bruker, Santa Barbara, CA, United States) with a nominal resonance frequency of 70 kHz and a spring constant of 0.4 N m−1. The images captured by AFM were used to determine the dimensions of the nanocellulose. The width was measured using Nanoscope Analysis software (Bruker, Santa Barbara, CA, United States) by measuring the height of 223 (SWK35E_3p) and 307 (SWD60E_5p) particles. The length was measured using ImageJ (Schneider, Rasband, & Eliceiri, 2012) by tracking 89 (SWK35E_3p) and 59 (SWD60E_5p) particles. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA 851e to study the thermal properties of the cellulose oxalates. Approximately 4 mg of each sample was heated from 30 to 600 °C at a rate of 10 °C min−1 under N2 flow (flow rate 50 ml min−1). The data were analysed by Mettler-STARe software. X-ray diffraction (XRD) was performed on a PANalytical X’Pert PRO MRD X-ray diffractometer (The Netherlands) equipped with a PANalytical X’Celerator detector. Diffractograms were collected in a range of 2θ = 10 to 50°. Cu Kα radiation was monochromatized with a nickel filter. The crystallinity index was estimated as described by Segal, Creely, Martin, and Conrad (1959).
Fig. 1. Potential esterification reaction of the cellulose with oxalic acid dihydrate; during the reaction, the cellulose also undergoes acid-catalysed hydrolysis (not shown).
more sterically available for chemical reactions, which makes them more labile than those in cellulose in acid-catalysed hydrolysis and dehydration. By washing the acid/pulp mixture with filtration it is possible to eliminate the need for dialysis, this also facilitates recycling the acid as it will be diluted less during washing. Both high demands for chemical recovery and long washing procedures are two mayor obstacles when preparing conventional sulphuric acid based CNCs (Nelson et al., 2016). Preparation of CNCs using hydrochloric acid can also be done without the need for dialysis, unlike cellulose oxalate, the resulting material however has low surface charge and subsequently poor colloidal stability (Yu et al., 2013). Oxalic acid has the potential to participate in the simultaneous hydrolysis and esterification of cellulose (Fig. 1) (Li et al., 2017), due to the fibre structure it is difficult for the acid to fully penetrate the fibre and react with all cellulose chains. Esterification is most likely to occur on C6-OH due to its greater steric accessibility than C2-OH and C3-OH (Fox, Li, Xu, & Edgar, 2011). Esterification with a difunctional carboxylic acid can result in both the introduction of charged acid groups and crosslinking between cellulose chains. Nevertheless, our previous work has indicated that most reacted oxalic acid leaves a charged group on cellulose (Li et al., 2017). The oxalate functionality contains two carbonyl groups, which can be observed using FTIR. In all samples, a broad adsorption at 1739 cm−1 corresponding to the C]O stretching of the carbonyl group was observed. This indicates successful esterification of the cellulose oxalates. In Fig. 2, the spectra of the raw materials and SWK35E, SWK35 A, SWD60A and SWK60A are shown; spectra for all samples are found in the supplementary information (ESI). The signal of carboxyl acids (−COOH) and ester bonds (−COO−) overlaps and is difficult to differentiate in the FTIR spectra. Moreover, no clear trends were observed in the intensities of the carbonyl signals of the prepared samples. To determine the effect of the reaction time, raw material, and washing liquor on the degree of esterification, conductometric titrations were performed to quantify the free carboxyl
2.2.3. Films Films were prepared from nanocellulose suspensions (0.2 wt %) dispersed using a T18 UltraTurrax (IKA, Germany) at 14,000 RPM for 20 min before they were degassed by sonication in an ULTRAsonik 28X ultrasonic bath (NeyDental, Inc., Bloomfield, CT, USA) for 20 min. Films of ˜50 g m−2 were produced using vacuum filtration over a 0.45 μm PVDF Durapore membrane filter (Merck Millipore) in a RapidKöthen sheet former (Paper Testing Instruments, Austria). Water was drained from the suspensions until a stable gel-like material was left. At this point, a second membrane was placed on top of the gel, and both membranes were placed between two sheet-former carrier boards. The whole assembly was dried for 20 min at 93 °C and 95 kPa reduced pressure. The oxygen permeability of the above CNF films was measured using a Mocon Oxtran 2/20 (Modern Controls Inc., Minneapolis, MN) instrument equipped with a coulometric sensor. The samples were masked using aluminium foil, leaving a round area of 5 cm2 exposed for the measurement. The measurements were conducted at 23 °C and at 50% RH. Each sample was conditioned prior to the measurement. Two replicates for every sample were measured. The tensile properties of the films were evaluated on an Instron Universal testing machine 5944 fitted with a 500 N load. The initial distance between the test grips was 20 mm, and the width of each specimen was 5 mm. At least 5 specimens from each sample were tested. The separation rate was 2 mm min−1. The samples were conditioned and tested at 23 °C and 50% RH. The stress-strain curve was recorded for each test, and the data were averaged over all specimens. Young’s modulus was automatically calculated by Bluehill version 3.72 (Illinois Tool Works Inc., Illinois, USA) software. 3. Results and discussion Cellulose oxalates were prepared by mixing pulp with molten oxalic acid dihydrate. As the reaction progressed, the appearance of the pulp changed from a white to a greyish colour, followed by the torn pulp sheet pieces losing their original shape as the mixture turned into a dark yellow or brown paste. As the reaction cooled, the oxalic acid solidified, and ethanol or acetone was added to remove excess oxalic acid. For the experiments, both dissolving pulp and kraft pulp were used as raw materials (Table 1). The observed colour change was more prominent for samples prepared from kraft pulp, presumably due to its higher amount of hemicellulose. Owing to the less ordered structural conformation of hemicellulose compared to that of cellulose (Cai, Zhang, Charles, & Wyman, 2014), the hydroxyl groups of hemicellulose are
Fig. 2. FTIR spectra of a selection of the cellulose oxalates and raw materials. The vertical line in the inset is at wavenumber 1739 cm−1 corresponding to the C]O stretching of the carbonyl group. Spectra for all the remaining samples are available in the supplementary information (ESI). 210
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was considerably lower at 0.62 mmol g−1, corresponding to an estimated degree of substitution (DS) of 0.13-0.16. No clear trends among the FCCs of the cellulose oxalates washed with the same type of solvent could be observed despite the differences in reaction time and type of raw materials. As has been reported for the esterification of cellulose by oxalic acid dihydrate, the DS on cellulose does not change when the reaction time is prolonged from 30 to 60 min (Li et al., 2017). In our previous work, we discussed that when no procedures for cellulose activation (e.g., swelling) are carried out, substitution only occurs on the accessible surfaces of cellulose fibrils. The highest theoretical DS of cellulose in this case is 0.05 (Larsson, Hult, Wickholm, Pettersson, & Iversen, 1999; Pu, Ziemer, & Ragauskas, 2006). In the current study, all DS values exceeded this value, which indicates the occurrence of esterification on both accessible and inaccessible surfaces of the cellulose fibrils. Nevertheless, as the reaction time increased from 35 min to 60 min, esterification could not proceed further. This could be attributed to the strong intermolecular interactions among the cellulose chains, which limited the migration of molten oxalic acid further towards the crystalline regions of cellulose fibrils. Other methods to prepare nanocellulose by reacting oxalic acid with pulp fibres produced materials with FCC between 0.11 – 0.39 mmol g-1 (Chen et al., 2016; Sirvio et al., 2016), this is considerably lower than the values observed using the current procedure. It is believed that the higher FCC is obtained due to not adding water to quench the reaction. At elevated temperatures, introducing additional water in this reaction is favorable for the reverse reaction (hydrolysis on the formed ester bonds) but not the forward reaction (esterification), due to the change in chemical equilibrium. An increased FCC will facilitate the fibrillation when preparing the nanocellulose. In addition, the FCCs of samples washed with acetone were slightly higher than those of samples washed with ethanol. During washing, it is hypothesized that smaller fractions of cellulose oxalates with high DS can interact rather well with polar solvents and thus can produce smaller particles. The particles may be fine enough to either be entrapped in the filter paper or possibly pass through it. Consequently, these fractions were removed from the samples after washing. As ethanol is more polar than acetone, this is more likely to occur when
Table 2 The yield, free carboxyl content, degree of substitution and thermal degradation onset temperature for the cellulose oxalates.
SWD35E SWD35A SWD60E SWD60A SWK35E SWK35A SWK60E SWK60A
Yield [%]
Free carboxyl content [mmol g−1]
Estimated degree of substitution
Thermal degradation onset temperature [°C]
93.8 97.1 85.4 96.4 83.6 96.0 85.6 99.3
0.86 1.05 0.62 1.08 0.92 1.10 0.97 1.04
0.15 0.18 0.11 0.19 0.16 0.19 0.17 0.18
175 176 175 175 177 173 176 N/A
Fig. 3. TG (left axis) and DTG (right axis) curves for SWD60A at 10 °C min−1 under N2 flow. The curves for the other samples are similar and are available in the supplementary information (ESI).
content of each sample. As shown in Table 2, the FCC of all samples ranged between 0.86–1.1 mmol g−1, except for SWD60E, which for unknown reasons
Fig. 4. Nanocellulose suspension at approximately 1.5 wt % prepared from the cellulose oxalates passed through the small chambers in the microfluidizer for either 1 passes (top) or 3 passes (bottom). SWK35E_3p was never collected. 211
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Fig. 5. Representative SEM (top) and AFM (middle) micrographs of nanocellulose adsorbed onto silica surfaces from suspensions of SWK35E_3p (left) and SWD60E_5p (right). Distribution (bottom) width (left) and length (right) of SWD60E_5p and SWK35E_3p measured using AFM.
with ethanol resulted in lower yields in all samples, which could be attributed to the removal of small fractions of hydrolysed and esterified cellulose and hemicellulose during washing, as discussed above. Thermal degradation of cellulose oxalates was detected by TGA to determine the onset temperature and the maximum degradation temperatures of the main mass-loss regions. Three different mass-loss regions could be observed (Fig. 3). The first mass-loss region (30–130 °C) could be attributed to the loss of physically adsorbed water in cellulose oxalates (Moriana, Vilaplana, Karlsson, & Ribes, 2014). The thermal degradation onset temperatures of all the studied cellulose oxalates were 173–177 °C, which showed the beginning of the second main mass-loss region (175–280 °C). This could be related to the decomposition of chemically attached oxalate groups as well as the dehydration of cellulose. The third main mass-loss region (280–375 °C) indicated the depolymerization of cellulose (Peng et al., 2013). Considering the onset temperatures, the cellulose oxalates and
washing cellulose oxalate with ethanol. As reported earlier (Li et al., 2017), prolonging the reaction time from 35 min to 60 min when using dissolving pulp slightly decreases the yield (Table 2). This is thought to be due to increased hydrolysis with possible dissolution of some of the degradation products. This trend is not observed when using kraft pulp as a raw material; for kraft pulp, the yield increases slightly when increasing the reaction time from 30 min to 60 min. There is no clear difference in the gravimetric yield between the samples prepared from kraft or dissolving pulp, despite having different cellulose contents. The dissolving pulp has a cellulose content > 96%, while kraft pulp contains 84% cellulose. This difference indicates that the samples prepared from kraft pulp not only contain cellulose oxalate but also hemicellulose, which has been hydrolysed and esterified by oxalic acid. The yield of cellulose oxalates was affected by whether either ethanol or acetone was used to remove the excess oxalic acid. Washing 212
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aggregates present in the gel. This was determined for SWD35A_5p and SWK35A_5p. After dilution and centrifugation, the content was determined to be 93% and 98% for SWD35A_5p and SWK35A_5p, respectively. The lower content for SWD35A_5p is likely connected to the presence of larger aggregates, which reduce the viscosity and scatter light, as observed when visually comparing the gels after homogenization. The total nanocellulose yield (from raw material to dispersed nanocellulose) was determined to 90% and 94% for SWD35A_5p and SWK35A_5p, respectively. Preparing nanocellulose from dry cellulose oxalate reduces the need for biocides as it is possible to fibrillate the nanocellulose on demand. It may also be open up new processes to fibrillate nanocellulose, for example in combination with other processing steps when preparing formulations. Other studies have shown great potential to increase the yield when producing nanocellulose by first producing CNCs followed by preparing CNFs from the solid residue remaining after the reaction (Chen et al., 2016; Wang, Zhu, & Considine, 2013; Wang, Chen, Zhu, & Yang, 2017). These studies have used centrifugation followed by dialysis for washing the material, and in some cases required a substantial number of passes in a microfluidizer to prepare the CNFs. The current paper is able to simplify the washing step by using only filtration, and to prepare a nanocellulose gel with only one pass through the 200 μm and 100 μm interaction chamber of the microfluidizer. Simplifying the process and reducing the required energy to fibrillate is crucial in order to decrease the production cost in an industrial process. Samples of SWK35A_5p and SWD35A_5p were used to produce films; both of these samples formed even and transparent films with a thickness of 30 μm when they were vacuum filtered and pressed (Fig. 6). SEM images of the films (Fig. 7) show a closely packed fibril structure. The SWD35A_5p and SWK35A_5p films had crystallinity indexes of 73.6% and 78.5%, respectively (Table 3). The crystallinity indexes of the raw materials were measured to be 70.9% and 58.1% for the dissolving pulp and kraft pulp, respectively. The crystallinity of both samples increased after the reaction as amorphous regions in the cellulose were hydrolysed, which was especially evident in the sample prepared from kraft pulp. Films produced from sulphuric or hydrochloric acid CNCs are often produced using solvent casting because the particles are small enough to pass through most filter membranes. Solvent casting is a very slow process that, together with poor film properties, limits the use of films produced from CNCs. The nanocellulose prepared in the current work was prepared using vacuum filtration over a 0.43 μm membrane without any substantial material loss. Films produced from SWD35A_5p had a lower tensile strength and lower elongation at break compared to those produced from SWK35A_5p (Table 3). In the present work, the suspension of SWK35A_5p showed a higher transparency than the suspension of SWD35A_5p at the same consistency, which indicates a more efficient separation of nanosized cellulose fibrils in the former than the latter (Moser, Lindström, & Henriksson, 2015). The more efficient separation of nanofibrils in the suspension enabled the exposure
Fig. 6. Films prepared from a) SWK35A_5p and b) SWD35A_5p produced by vacuum filtration over a membrane.
presumably the resulting nanocellulose can be used as reinforcements in thermoplastics, such as polystyrene (PS, melting temperature 74–105 °C), low-density polyethylene (LDPE, melting temperature 103–110 °C) and high-density polyethylene (HDPE melting temperature 125–132 °C). When homogenizing the cellulose oxalates, they easily pass through the 100 μm without clogging. At approximately 1.5 wt %, the samples prepared from kraft pulp resulted in thick gels, while the samples prepared from dissolving pulp were slightly less viscous and had a higher opacity (Fig. 4). Gels could be made from all samples with high viscosity after one pass through the microfluidizer. After homogenization of the cellulose oxalates, the difference in colour between the different raw materials, as described above, is clearly apparent. Micrographs of the homogenized cellulose oxalates (Fig. 5) show that the material consists of short fibrils with average lengths of 0.34 μm (SWK35E) and 0.37 μm (SWD60E) and average widths of 3.2 nm (SWK35E) and 4.3 nm (SWD60E). The width of the particles is comparable to that commonly found for CNCs and in the lower range for CNFs. The length, however, has a large range that spans lengths typical for both CNFs and CNCs. The analysed samples contain many particles that are similar to the shape and length (50–500 nm) (Moon et al., 2011) of CNCs and a considerable number of particles that are longer (up to 1.1 μm) than common CNCs and shaped similar to flexible CNFs. It should be noted that the number of long particles is likely underestimated because it is easier to adsorb, identify and measure small particles than it is long and entangled particles when analysing the images. The nanocellulose content is an indication of the amount of larger
Fig. 7. SEM micrographs of films prepared from SWD35A_5p (left) and SWK35A_5p (right). Note the difference in magnification between the two images. 213
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0.14–5.03(Aulin, Gällstedt, & Lindström, 2010, 2012; Naderi et al., 2016; Syverud & Stenius, 2008)
0.54 0.31 –
of more surface area for bonding between the nanofibrils during film formation. SWK35 A also has a higher crystallinity index, which may contribute to its higher mechanical strength. Consequently, the interfacial interactions between the nanofibrils were stronger in the film made from SWK35 A than the one made from SWD35 A, as represented by the superior tensile strength of the former relative to the latter. The mechanical strength of the prepared films is comparable with many other films prepared from CNFs, as is the strain at break and elastic modulus. In general, the films based on the nanocellulose in the current work have comparable tensile properties to those reported for neat nanocellulose films (tensile strength: 30–240 MPa; elastic modulus: 1–17.5 GPa) (Alain, Jean-Yves, & Vignon, 1997; Fukuzumi, Saito, Iwata, Kumamoto, & Isogai, 2009; Henriksson, Berglund, Isaksson, Lindström, & Nishino, 2008; Iwamoto, Abe, & Yano, 2008; Iwamoto, Nakagaito, & Yano, 2007; Iwamoto, Nakagaito, Yano, & Nogi, 2005; Leitner, Hinterstoisser, Wastyn, Keckes, & Gindl, 2007; Henriksson & Berglund, 2007; Nakagaito, Iwamoto, & Yano, 2005; Nakagaito & Yano, 2008; Nogi, Iwamoto, Nakagaito, & Yano, 2009; Rampinelli, Di Landro, & Fujii, 2010; Saito et al., 2009; Stelte & Sanadi, 2009; Svagan, Samir, & Berglund, 2007; Svagan, Hedenqvist, & Berglund, 2009; Syverud & Stenius, 2008; Yano & Nakahara, 2004), as well as higher tensile strength and elastic modulus than bio-based films based on non-cellulosic polysaccharides (e.g., starch, hemicelluloses, and pectin) (tensile strength: 9.8–74 MPa; elastic modulus: 0.8–2.4 GPa) (Cao, Chen, Chang, Stumborg, & Huneault, 2008; Edlund, Ryberg, & Albertsson, 2010; Le Normand, Moriana, & Ek, 2014; Mikkonen et al., 2010) and commercial petroleum-based polymers (e.g., polyethylene, polypropylene, polyvinyl chloride, and polyamide) (tensile strength: 8–165 MPa; elastic modulus: 0.2–4.1 GPa) (Mangaraj, Goswami, & Mahajan, 2009). The films produced from SWD35A_5p and SWK35A_5p both showed low oxygen permeability (Table 3), and the kraft-based film had slightly lower oxygen permeability than the dissolving-based film. This can be attributed to the higher degree of fibrillation and higher crystallinity index of the samples produced from kraft pulp. Fewer large aggregates increases the path required for gas molecules to travel through the film. The reported permeability is comparable to that of many others at 50% RH and is often regarded as sufficient for many applications.
2–10(Moon et al., 2011)
3.0 (+-0.4) 5.0 (+-0.6) 0.6(Reising et al., 2012)
4. Conclusions
66.2–82.1(Peng et al., 2013; Tejado, Alam, Antal, Yang, & van de Ven, 2012; Zhao et al., 2013)
The high-yield preparation of cellulose oxalate through the reaction between oxalic acid and cellulose was studied. During the reaction, simultaneous esterification and acid hydrolysis occurs, resulting in a highly charged (0.6–1.1 mmol/g) cellulose derivative with a crystallinity index of approximately 75%. The cellulose oxalate was prepared as a dry powder, after homogenization all samples produces nanocellulose gels after one pass through the 100 μm interaction chamber of the microfluidizer. The particle length of the resulting nanocellulose varies greatly and contains particles which resemble the size and shape of both CNF and CNC. It was shown that it is possible to prepare films with mechanical and barrier properties similar to many other CNFs through filtration. Using the described method it is possible to prepare nanocellulose without the need for extensive dialysis or centrifugation or extensive homogenization, and with very high yield. The nanocellulose has great potential for uses where the size of traditional CNFs may present problems, such as when the particle length results in a too high viscosity, and where CNCs are too small, such as when preparing films though filtration.
SWD35A SWK35A Typical CNC films Typical CNF films
74 79 54–88(Moon et al., 2011)
142 (+-5) 197 (+-7) 70(Reising, Moon, & Youngblood, 2012) 95–240(Moon et al., 2011)
10.6 (+-0.4) 10.2 (+-0.4) 6–14.9(Moon et al., 2011; Reising et al., 2012) 6-15(Moon et al., 2011)
Oxygen permeability [cm3 μm m−2 day−1 kPa−1] 50 % RH Modulus [GPa] Tensile strain at maximum tensile stress [%] Maximum tensile stress [MPa] Crystallinity index
Table 3 Crystallinity index, mechanical properties and oxygen barrier properties of the films prepared from SWD35A_5p and SWK35A_5p. Data are presented as the mean values. The error corresponds to the confidence interval, alpha = 0.05.
J. Henschen, et al.
Conflicts of interest The authors are shareholders in FineCell Sweden AB, a company working in commercializing nanocellulose. 214
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Acknowledgments
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