- Email: [email protected]
Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling
PTEC-14946; No of Pages 9 Powder Technology xxx (2019) xxx
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling Antti Korpela ⁎, Hannes Orelma ⁎ VTT Technical Research Centre of Finland Ltd., Tietotie 4 E, 02044 Espoo, Finland
a r t i c l e
i n f o
Article history: Received 12 August 2019 Received in revised form 29 October 2019 Accepted 20 November 2019 Available online xxxx Keywords: Cellulose powder Microcrystalline cellulose Crosslinking Dry milling
a b s t r a c t The present study concerns the preparation of cellulosic powders with two-stage dry milling of chemically crosslinked birch kraft pulp sheets. Chemical crosslinking of kraft pulp sheets using glyoxal with and without a catalyst (aluminium sulphate) made the pliable, tenacious kraft pulp sheets brittle. Due to the brittleness, the crosslinked pulp sheets could be disintegrated easily and rapidly using a Wiley mill. The length-weighted average fibre length of the crosslinked pulp powders (0.31–0.33 mm) was shorter than the Wiley-milled reference powder (0.44 mm). The notably higher density and less fluffy character of the crosslinked pulp powders enabled their effortless further processing with an air-flow-type ultra-fine microniser. The medium size value (D50) of the micronised crosslinked powders was around 40 μm. The study finds that chemical crosslinking pre-treatment enhances the dry milling of kraft pulps to a fine powder. Chemical crosslinking may offer a new tool for industrial cellulosic powder manufacturing. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction Cellulose is an abundant natural material that has many valuable properties, such as non-toxicity, biodegradability, and easy chemical modifiability. It has already been utilised in many commercial applications from paper to high-cost medical and cosmetic additives. Cellulose powders are used, e.g. in paints as fillers and in drug delivery as a tablet former. In those applications, the cellulose powder is typically in the micrometer size (1–100 μm). These cellulose powders are manufactured by using dry milling, acid hydrolysis, or a combination of those techniques. Acid hydrolysis is a common industrial method to produce microcrystalline cellulose powder. It is based on a sequence of acidcatalysed hydrolysis of amorphous regions of cellulosic fibres followed by neutralisation, washing, and mechanical process steps. The treated material is then evacuated from water with spray drying that produces separated fine microcrystalline cellulose (MCC) particles. The medium particle size of commercial MCC products is typically in the range of 20–100 μm. Due to the acid-catalysed hydrolysis degree of polymerisation (DP) of the remainder, MCC is typically reduced to a level of 200– 300 DP. A characteristic property of MCC is its ability to form stable dispersion and gels by vigorously mixing with water. Because of this property, MCCs are used as a thickener, a stabiliser, or an emulsifier in a variety of products, such as foodstuffs, cosmetics, and water-based coatings [1,2]. Dry MCC powders are commonly used as an excipient in pharmaceutical products. From MCC, it is possible to compress tablets ⁎ Corresponding authors. E-mail addresses: antti.korpela@vtt.fi (A. Korpela), hannes.orelma@vtt.fi (H. Orelma).
that are hard but disperse readily in water and during digestion. The main problems associated with the industrial manufacturing of MCC are the high acid consumption and high chemical oxygen demand (COD) of process effluents. In a study by Vanhatalo and Dahl, the acid hydrolysis of bleached softwood kraft pulp caused around a 15–30% yield loss, depending on hydrolysis conditions, due to the removal of the amorphous regions of the fibre [3]. Dry milling is based on the milling of dry kraft pulp fibres by using one of several types of mill, e.g. a knife mill, a hammer mill, or a ball mill [4]. The dry milling of wood fibres is energy-intensive due to the toughness and flexibility of kraft pulp fibres [5,6]. According to a review article by Mayer-Laigle et al., the high energy consumption associated with the manufacture of micron-scale biomass powders using dry milling can be partly tackled by rendering the biomass more brittle with the use of a mild pyrolysis treatment called torrefaction [6]. However, torrefaction-induced changes in the chemical composition of cellulosic fibres are not desirable in most cases. Another way to render biomass more brittle is cooling. In so-called cryogenic milling, the biomass is cooled by using liquid nitrogen. By using cryogenic milling, Hemery et al. were able to reduce the dry milling (impact mill) steps of wheat bran from 3 to 1 to reach the targeted medium 50 μm particle size [5]. Due to the high costs of cryogenic milling, the processed materials are used in the high-added value products, such as adhesives, explosives, and spices [6]. Therefore there is a need to study new pre-treatment strategies to ease the dry milling procedure of pulp fibres. The treatment of cellulosic paper with chemical crosslinking agents with relatively small molecular sizes is reported to cause fibre embrittlement and a decrease in the folding endurance of paper [7–10].
https://doi.org/10.1016/j.powtec.2019.11.064 0032-5910/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
2
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Chemical crosslinking agents of cellulose are molecules that are capable of forming covalent bridges between hydroxyl groups of adjacent cellulose molecules. Chemicals capable of crosslinking cellulose are numerous, including formaldehyde, dialdehydes, such as glyoxal and glutardialdehyde, and tricarboxylic acids such as citric acid and butanetetracarboxylic acid [11–13]. Glyoxal is also capable of crosslinking cellulose fibres, and it is a relatively safe cellulose crosslinking agent [14] that cures at a lower temperature than most other formaldehyde-free crosslinking agents, such as polycarboxylic acids. Both hemiacetal and acetal linkages can be formed in the crosslinking of cellulose with glyoxal. Hemiacetal formation is a reversible reaction, and the crosslinks are hydrolysed in water. The increase in the curing temperature and the use of catalysts favour the formation of acetal linkages, which are more water-resistant than hemiacetal bonds [7,11,15]. Due to the differing water resistance of hemiacetal and acetal bonds, glyoxal can then be used to produce either transient or more permanent effects on cellulosic material properties [7]. The most significant group of crosslinking agents are DMDHEU (1.3-dimethylol-4.5dihydroxyethylene urea) based agents in cotton fabric finishing. The crosslinks formed in cotton fibres reduce the shrinking and wrinkling of cotton textiles in use, and in the washing and drying of textiles. Cotton fabric crosslinking is conventionally performed using the so-called pad-dry-cure method, which begins with the immersion on the fabric in a water solution of the crosslinking agent and its catalyst. After immersion, the excess solution is squeezed from the fabric and subsequently the fabric is smoothed and dried. Finally the fabric carrying the crosslinking agent is cured for a few minutes at an elevated temperature (around 140–170 °C). The actual crosslinking reactions in the process take place in the curing stage [12,16]. Paper made from cellulosic fibres can be crosslinked by using chemicals following the pad-dry-cure method. Via crosslinking, it is possible to improve the stiffness and wet strength of paper by forming water-resistant inter-fibre covalent crosslinks. An unwanted consequence of chemical crosslinking in paper manufacture is an increase in the brittleness of the paper, resulting in reduced fold strength [15,17–19]. As chemical crosslinking is performed on the papermaking pulp fibres, the water retention value (WRV) of the fibres decreases
and the stiffness of the fibres increases [20,21]. The decrease in WRV, which is a measure of pulp's capability to maintain water, is probably due to the chemical crosslinks that prevent the swelling of fibre walls caused by water. Crosslinked pulps with both low WRV and high wet bulk are used as moisture distribution layers in absorption products, such as napkins [20]. In the present study we investigated if the sequential glyoxal crosslinking and dry milling technique can be utilised in the production of micrometer size cellulose powder (Scheme 1). For comparison, crosslinking of pulp sheets were also performed in the presence of aluminium sulphate, which is a known catalyst of crosslinking reactions [14,22,23]. For the crosslinking procedure, bleached birch kraft pulp sheets were first immersed in a crosslinking-agent water solution followed by drying and curing of the sheets in elevated temperature. Cured pulp sheets were dry-milled using a Wiley mill, which is a rotating blade-type cutting mill. This step was followed by further drygrinding of the powders using a decompression air-type microniser. Finally, properties, such as particle size distribution, specific surface area (BET), and bulk and tap densities of the powders were measured. The crosslinking reaction was also studied by using CPMAS 13C solid state NMR. The idea of using chemical crosslinking in order to reduce the energy consumption of dry milling cellulosic fibres to fine particles has been presented in patent application JP09392001 [24]. To the best of our knowledge, the method has not been discussed previously in scientific literature.
2. Experimental 2.1. Materials The pulp used in the study was dried, bleached birch (Betula bendula) kraft pulp originating from a Finnish pulp mill. The pulp was obtained as A4-sized pulp sheets (grammage 830 g/m2, air-dry, dry content 96%). The used glyoxal was obtained as a 40% water solution from Merck KGaA (USA). Aluminium sulphate tetradecahydrate (Al2(SO4)3·14H2O) was obtained from Kemira OyJ (Finland). The
Scheme 1. Schematic illustration of the studied sequential manufacturing process and possible chemical reaction pathways (hemiacetal and acetal formation) of the glyoxal between the hydroxyl groups of adjacent cellulose molecules.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Fig. 1. Produced samples. The letters A - E refer to the prepared and characterised fibre and powder samples. A: Reference pulp, B: pulp after Wiley milling (rough refining), C: Pulp after glyoxal crosslinking and Wiley milling, D: pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling, E: pulp after glyoxal crosslinking and subsequent Wiley milling and micronisation (fine refining), and F: pulp after glyoxal crosslinking with aluminium sulphate and subsequent Wiley milling and micronisation.
water used in the study was deionised water produced with a MilliQdevice. All other chemicals were laboratory grade. 3. Methods 3.1. Chemical crosslinking of cellulose papers by glyoxal Crosslinking of the birch kraft pulp sheets was carried out following the pad-dry-cure crosslinking method. A 1-kg stack of pulp sheets (dry
3
content 96%) was immersed in a water solution containing 4.0 wt% glyoxal for 60 min. Aluminium sulphate hydrate (1.33 wt%) was added in the glyoxal water solution, which was optional. After impregnation, the excess solution was drained from the pulp sheets. The dry content of the treated pulp sheets after the draining was 40%. The amount of absorbed glyoxal was 6.0%, and the aluminium sulphate hydrate was 2.0% of the sheet dry content (calculated by assuming that the percentage proportions of crosslinking chemicals in the absorbed solutions were the same as the percentage proportions of the immersion solution). The impregnated pulp sheet were first drained and then dried (the curing takes place when the water is evaporated) in a ventilated oven at 110 °C for 5 h. The sheets were set apart in an oven in order to ensure even and efficient drying and curing. After the drying and curing process, the dry content of the sheets was approximately 97%. The chemically crosslinked pulp sheets were stored at 22–23 °C in 25–35% RH. 3.2. Dry milling of glyoxal crosslinked pulp sheets The crosslinked pulp sheets were manually cut into 3 × 3 cm2 pieces, which were dry-milled using a Wiley mill (Standard model no. 3, Arthur H. Thomas Co.) The pieces were manually fed into the Wiley mill where revolving knives worked against stationary knives and crushed the material until the resulting particle size was small enough to pass through a 1 mm hole screen. Pulp powder made from the untreated reference pulp sheets was fluffy, whereas the pulp made from a crosslinked pulp sheet was denser and showed better flow properties, enabling trouble-free further processing with a decompressed air-flow-type
Fig. 2. a) A piece of untreated birch kraft pulp sheet after 20 double folds (on the left, Sample A) and chemically crosslinked sheet (glyoxal without aluminium, Sample C) after the first fold. b) cellulose samples after Wiley milling (left untreated (Sample B) and right, glyoxal crosslinked without aluminium (Sample C)), and Wiley milling plus micronisation of glyoxal crosslinked without aluminium (Sample E).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
4
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
microniser (Ultra fine mill Ceren Miller Dau MKCL8-15J DAU, Masuko Sangyo). According to the manufacturer (Masuko Sangyo), the grinding effect is comparable to the effect of jet milling. The Wiley-milled pulp was fed once through the microniser. The microniser reduced the particle size by way of strong impacts caused by air flow and a rotating impeller. The rotation speed of the impeller was kept constant at 7600 rpm, milling time was 90 s, sample feeding time 4–6 s, flushing time was 35 s, and the screw speed was 60 rpm. The sample codes of prepared powders are shown in Fig. 1.
3.3. Carbohydrate and lignin composition, and determination of ash To determine lignin and carbohydrate composition, the samples were hydrolysed using sulphuric acid (2 stages), and the resulting monosaccharides were determined using HPAEC with pulse amperometric detection (Dionex ICS 5000 equipped with CarboPac PA20 column [25]. The acid-insoluble lignin content (Klason lignin) in the kraft pulp was determined according to the TAPPI T 222 om-02 method. The acid-soluble lignin was determined using the spectrophotometric method based on the absorption of ultraviolet radiation at 215 and 280 nm and using the equation described by Sarkanen and Ludwig [26]. The ash content was determined according to ISO 1762:15. The amount of acid-insoluble lignin (Klason lignin) in the pulp was 1.3% and the amount of acid-soluble lignin was b0.1%. According to monosaccaride analysis, the carbohydrate fraction was composed of glucose (73%), xylose (25%), and mannose (0.7%) units. The percentage of ash (525 °C) in the pulp was 0.5%.
3.4. Particle size measurements using fibre analyser The fibre-length distribution of the pulp before and after dry milling via glyoxal crosslinking was measured using a Valmet F5 Fiber Image
Analyzer (Valmet, Finland) according to ISO 16065-2. All measurements were replicated. 3.5. Water retention value The WRV(ml/g) of the pulp before and after dry milling via glyoxal crosslinking was determined using the centrifugal method according to ISO 23714. All measurements were at least duplicated. 3.6. Bulk and tap density measurements The bulk density of the powders was determined by pouring a 5 g sample into a graduated 50 ml test tube and by measuring the volume of the powder. The tap density of the powders was determined by dropping (tapping) the test tube from a height of 2.5 cm onto a hard surface until no more settling of powder occurred. The Hausner ratio (H) was determined from the relation of tapping density ρT and bulk density ρB, (H = ρT/ρB). All density measurements were triplicated. 3.7. Wet and dry particle size measurements using laser diffraction particle size analyser Particle size distributions of micronised powders dispersed in water were measured using the laser light diffraction method with a Beckman Coulter LS 230 (Beckman Coulter, Inc., USA). For the measurement, 1 g powder samples were dispersed into 40 ml of milli-Q water with mixing for 15 min using a magnetic stirrer. The particle size measurements were duplicated. For comparison, the particle size distribution of powders in dry form was also measured using a Malvern Mastersizer 3000 (Malvern Panalytical Ltd., UK) laser diffraction particle size analyser equipped with standard sample tray and dispersion unit Aero S. The particle size measurements of dry powders were triplicated.
Fig. 3. FTIR a) and CPMAS 13C solid state NMR b) spectrums of pulp powders. B: pulp after Wiley milling, C: pulp after glyoxal crosslinking and Wiley milling, D: pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
5
Fig. 4. Thermogravimetric analysis of pulp powders; B: pulp after Wiley milling, C: pulp after glyoxal crosslinking and Wiley milling, D: pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling.
3.8. Fourier transformation infrared resonance (FTIR) measurements
3.10. Photomicrographing
FTIR spectroscopy was used to characterise the produced materials. The measurement was performed with a Thermo Scientific Nicolet iS50 FT-IR spectrometer with an ATR diamond (Thermo Scientific, USA). All spectra were obtained from 32 scans with a resolution of 4 cm-1 and absorption mode by using the wavelength range of 400 to 4000 cm−1. At least three repetitions were carried out for each specimen.
The unstained samples were visualised using confocal laser scanning microscopy (CLSM) equipment consisting of a Zeiss LSM 710 (Zeiss, Jena, Germany) attached to a Zeiss AxioImager.Z microscope. Fibres of each sample were separated in water and were examined on a microscope slide as sealed preparates. Samples were imaged utilising a transmitted light detector with a 10× objective (Zeiss EC Epiplan-Neofluar, numerical aperture of 0.16) and a 20× objective (Zeiss EC EpiplanNeofluar, numerical aperture of 0.30) with a resolution of 1024 × 1024 using ZEN software (Zeiss). Representative images were selected for publication.
3.9. Chemical analyses of crosslinked paper with solid state 13C CP/MAS NMR spectrometry Chemical changes in the paper caused by crosslinking with glyoxal were characterised by using a 13C cross polarisation magic angle spinning (CP-MAS) NMR spectrometer (Bruker AVANCE-III 400 MHz, Bruker BioSpin, Germany). For all samples, 20,000 scans were collected using an 8 kHz spinning frequency, 2-ms contact time, and a 5-s delay between pulses.
3.11. Thermogravimetric analysis (TGA) Thermal decomposition of the powders was studied using a Netzsch STA 449 F1 Jupiter thermal analyser (Netzsch-Gerätebau GmbH, Germany). For TGA measurements, samples of about 10 mg were analysed in air at a heating rate of 10 °C/min from 35 to 700 °C.
Fig. 5. Length-weighted fibre length distributions of reference pulp (Sample A), pulp after Wiley milling (Sample B), pulp after glyoxal crosslinking and Wiley milling (Sample C), pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling (Sample D).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
6
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Table 1 Properties of reference pulp fibres and Wiley-milled pulp fibres. Sample A
Sample B
Sample C
Sample D
Birch kraft pulp (ref.) Wiley-milled ref. pulp Glyoxal crosslinked Wiley-milled pulp Glyoxal + Al2(SO4)3 crosslinked Wiley-milled pulp Arithmetic av. fibre length, mm Length w. av. fibre length, mm Weight w. av. fibre length, mm Length b 0.2 mm, % Water retention value (WRV), g/g Bulk density, g/ml Tapping density, g/ml Hausner ratio
0.80 0.93 1.06 13.4 1.13 – – –
0.37 0.44 0.53 42.1 0.84 0.066 0.113 1.71
0.28 0.31 0.35 81.6 0.60 0.186 0.277 1.49
0.28 0.33 0.34 84.7 0.51 0.203 0.306 1.51
Fig. 6. Optical microscopy image reference pulp (Sample A), pulp after Wiley milling (Sample B), pulp after glyoxal crosslinking and Wiley milling (Sample C), pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling (Sample D), pulp after glyoxal crosslinking and subsequent Wiley milling and micronisation (Sample E), and pulp after glyoxal crosslinking with aluminium sulphate and subsequent Wiley milling and micronisation (Sample F).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
7
4. Results
4.2. Physical properties of Wiley-milled cellulose powders
4.1. Glyoxal crosslinking of paper and subsequent dry milling
The fibre lengths of the Wiley-milled crosslinked pulp powders (Samples C and D, without and with aluminium sulphate addition, respectively) were significantly shorter than those of the reference pulp powder (Fig. 5 and Table 1). Optical microscopy images (Fig. 6a-d) show that the fibre cut in the Wiley mill occurred without visible fibrillation of the fibres. Chemical crosslinking also resulted in a significantly higher amount of fibre fines (length b 0.2 mm) in the milled pulp. This is evidently a consequence of the brittleness of the crosslinked fibres. The addition of aluminium sulphate as a catalyst in the crosslinking reaction did not have any significant effect on the properties of the fibre length properties of the Wiley-milled samples. WRV is an important value for cellulose powders that are utilised in water suspensions and for drug delivery. The results in Table 1 show that Wiley milling of the reference pulp (Samples A and B) resulted in decreased WRV, from 1.13 g/g to 0.84 g/g. There is very little published information on the effect of dry milling on the WRV of kraft pulps. The results presented in US Patent 8,449,720 B2 [28] regarding the effects of Wiley milling on the WRV of softwood kraft pulp fibres are in accordance with the results of the present study. According to Ekman et al., the Wiley milling of softwood kraft pulp fibres from 2.25 mm to 0.6 mm (length-weighted averages) decreases the WRV of the pulp from 1.17 g/g to 0.92 g/g. The reason for the decrease in WRV remains unclear, but it might have been due to the mechanical impact-induced permanent compactions of fibre walls that took place in the Wiley milling. The glyoxal crosslinking (Samples C and D) further decreased the WRV values to 0.6 g/g and 0.51 g/g, respectively. This indicates that the crosslinking solidified the cell wall of the fibre fragments and, thus, decreased their ability to absorb water. The bulk and tapping densities of the Wiley-milled pulps were significantly higher than those of the reference. The Hausner ratio of both crosslinked Wiley-milled pulps decreased. The Hausner ratio is generally used in industries as an indication of the flowability of a powder; the lower value indicates better flowability [29,30].
Crosslinking treatments using glyoxal and glyoxal with aluminium sulphate caused a clear visible change in the brittleness of the pulp sheets (Fig. 2a). The crosslinked pulp sheet broke the first time it was folded into two pieces, whereas the reference pulp sheet lasted several dozen folds without breaking. Both paper sheets were pretreated using Wiley milling since the micronisation technique is not capable of handling large paper sheets. The passing time of the reference pulp (Sample B) and the crosslinked pulps (with and without aluminium sulphate, Samples C and D) in the Wiley mill, was around 45–60 s and 5–10 s, respectively. The crosslinked pulps milled fluently, whereas the reference pulp formed fluffy compactions in the mill (Fig. 2b).Thus, the processability of the crosslinked pulp from the Wiley mill was significantly better than that of the reference pulp. Chemically crosslinked Wiley-milled pulps (Samples C and D) were further processed with dry grinding using a decompressed air-type microniser. Due to fluffy character and poor flowability, the reference pulp (Sample B) was not processable with the microniser. FTIR measurements were undertaken to verify the crosslinking of cellulose with glyoxal with and without aluminium sulphate. The FTIR spectra of the reference pulp powder, Sample B, and crosslinked pulp powder, Samples C and D, are shown in Fig. 3a. The decrease in absorbance around 3300 1/cm (OH stretching of intramolecular H-bonding) is a likely consequence of reactions of glyoxal with cellulosic hydroxyl groups. An absorbance peak of 1735 1/cm may be due to unreacted glyoxal ends (C_O stretching peak). The formation of acetal and hemiacetal linkages took place within the fingerprint area of cellulose (800–1200), and thus were not visible in the spectra (Samples C and D). A quantitative analysis of the amount of acetal and hemiacetal crosslinks is difficult in practice. Schramm and Rinderer [23] have introduced a chromatographic method that can be used of quantifying the portion of glyoxal that has reacted with cellulosic material. However, the method does not deliver information on whether the crosslinks were achieved by an acetal or a hemiacetal linkage. The FTIR spectra suggest that, with the exception of the formation of acetal and/or hemiacetal bonds, no other major modifications of the cellulose and hemicellulose had taken place in the crosslinking treatments. Cellulose powder samples characterised with CPMAS 13C solid state NMR (Fig. 3b) were used to study the hemiacetal and acetal formation within the crosslinking of the cellulose fibres. As expected, the untreated powder (Sample B) showed a signal for native cellulose with crystalline cellulose I and non-crystalline cellulose [27]. Glyoxal crosslinking with and without glyoxal (Samples D and C) did not alter the carbon peak assignment or the crystallinity of cellulose compared to Sample B. However, a new broad peak arose at 94–98 ppm, which was identified as (hemi)acetal formation (acetal peak in 90–100 ppm). In a 13C NMR spectrum, hemiacetal linkages had a peak at 90–95 ppm and acetal linkages had a peak 95–100 ppm. Therefore, based on the measured spectra, it cannot be clearly verified that glyoxal formed with cellulose hemiacetal or acetal bonds. Most probably there both bond types were present after glyoxal crosslinking. In order to evaluate the thermal stability of crosslinked cellulose powder, thermogravimetric analysis (TGA) was performed (Fig. 4). The TGA curves of reference Sample B and glyoxal-crosslinked Sample C are very similar in the figure, except for a slightly higher loss of weight-% in the area of 200 °C — 300 °C. A TGA curve of cellulose powder crosslinked with glyoxal and aluminium sulphate differs more significantly from the reference powder in the figure. An examination of the reasons for the deviation was not carried out in the present study. From a powder potential application point of view, no remarkable difference in the thermal stability of the powders below 170 °C was found. This indicates that glyoxal crosslinked cellulose powder can be utilised in the same applications where regular microcrystalline cellulose is currently utilised.
4.3. Production and properties of fine micronised cellulose powders Wiley-milled cellulose powders were further processed with the micronisation technique. Micronisation clearly decreased the length of fibre segments (Fig. 6e and f), and no clear fibrillation was observed. However, unlike microcrystalline cellulose particles the appearance of which is typically grainy (Hindi 2017), the particles of both powders (Samples E and F) still look like pieces of fibre. Table 2 shows the properties of the micronised crosslinked pulp powders. Micronising resulted in powders (Samples E and F), the medium particle size value (D50) of which were close to 40 μm. Particle size distributions measured using dry and wet dispersion methods showed no major deviations (Fig. 7). Table 2 Particle properties of powders (Samples E and F) made with fine-refined crosslinked Wiley-milled birch kraft pulp (Sample B).
Size distribution, Beckman coulter, wet dispersion, D10 / D50 / D90 (μm) Size distribution, Malvern, dry dispersion, D10 / D50 / D90 (μm) Specific surface area, BET (m2/g) Bulk density (g/ml) Tapped density (g/ml) Hausner ratio
Sample E
Sample F
Glyoxal crosslinked, Wiley-milled & fine refined
Glyoxal + Al2 (SO4)3 crosslinked, Wiley-milled & fine refined
13.3 / 41.4 / 118,9
13.9 / 40.6 / 112.6
14.0 / 39.9 / 113 1.95 0.309 0.526 1.70
14.2/ 36.5 / 98.7 1.53 0.338 0.542 1.60
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
8
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Fig. 7. Particle-size distributions of dry- and wet-dispersed crosslinked pulp powders: E: pulp after glyoxal crosslinking and subsequent Wiley milling and micronisation, and F: pulp after glyoxal crosslinking with aluminium sulphate and subsequent Wiley milling and micronisation.
The medium particle size value of the prepared powders are comparable to many commercial microcrystalline celluloses, such as Avicel PH101 (D50 ~50 μm). The use of aluminium sulphate as a crosslinking reaction catalyst did not cause any clear visible differences in the appearance of the crosslinked powder particles. Apparent specific surfaces of a commercial microcrystalline cellulose, such as Avicel PH 101, determined using the BET method are reported to be in the range of 1.0– 1.5 m2/g [31]. The specific surface area (BET) of powders samples E and F were 1.95 g/m2 and 1.53 g/m2, respectively. The lower surface area of Sample F compared to Sample E is possibly a consequence of higher density and a less porous particle structure. Both the bulk and tapping density of the pulp powder increased substantially during micronisation. Bulk and tapped densities of powders samples E and F were found to be close to those of the microcrystalline celluloses Avicell PH 101, 102, and 103 reported by Rojas et al. [32]. However, the higher Hausner values for Samples E and F compared to the commercial Avicel products (1.31–1.40) indicate somewhat poorer flow properties. A clear difference between the crosslinked pulp powders (Samples E and F) and the microcrystalline cellulose Avicel PH 101 is their dispersability in water. Vigorous mixing of the Avicel in water (e.g., 10 wt% in water with Ultra Turrex using 12,000 rpm for 10 min) results in stable milky dispersions, whereas crosslinked powders start to sediment immediately after the mixing. The reason for the different behaviour of the Avicel and the crosslinked powder was not examined in the present study, but this behaviour may be related to the firmness and chemical composition of the crosslinked particles. Further research should examine the effects of different cellulose crosslinking agents, such as tricarboxylic acids, and different crosslinking conditions on the processability of cellulosic materials in dry milling and on the properties of the dry-milled kraft pulp powders. 5. Conclusions The present study examined dry milling of cellulose fibres into micro-size cellulose powder. A chemical crosslinking treatment of bleached birch kraft pulp sheets made the birch sheets more brittle and subsequently enhanced dry milling. The increased brittleness of the sheets resulted in a significantly shorter passing time for the dry pulps in the Wiley mill and a shorter average fibre length in the resultant powder. Crosslinked Wiley-milled pulp powder also showed higher density and better flow properties, thus enabling the micronisation of this powder with a decompressed airflow-type microniser. After fine grinding, the medium particle size value (D50) was around 40 μm. These findings strongly indicate that the hypothesis of the present study is correct. In addition, both FTIR and NMR spectrum analyses suggest that glyoxal with and without a catalyst (aluminium sulphate) formed hemiacetal and/or acetal linkages in the treated
birch kraft pulp sheets. Unfortunately, it is not possible to estimate the proportions of hemiacetal- and acetal linkages based on chemical analysis. Unlike some commercial microcrystalline celluloses, which have comparable particle size distribution, the crosslinked fine powders do not form stable dispersions when mixed vigorously in water. Therefore, the potential industrial uses for crosslinked kraft pulp powders are not necessarily the same as those for MCCs, but the properties of the powders may enable their use as a filler, for example, in plastics and paper products. The lower water absorbency of the powders compared to non-crosslinked cellulose powders may be an advantage in those industrial applications. Author contributions Both authors contributed equally to the manuscript, and both authors have given their approval to the final version of the manuscript. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work was part of the Academy of Finland Flagship Programme under Project Nos. 318890 and 318891 (Competence Centre for Materials Bioeconomy, FinnCERES). Our thanks go out to Mirja Muhola for the BET analysis, to Mirja Nygård for the thermogravimetric measurements, to Tommi Virtanen for NMR analysis, to Liisa Änäkäinen for photomicrographs, and to Pia Siventoinen and Markus Nikinmaa for performing the fine grinding of the cellulose powders. References [1] G. Thoorens, F. Krier, B. Leclercq, B. Carlin, B. Evrard, Microcrystalline cellulose, a direct compression binder in a quality by design environment—a review, Int. J. Pharm. 473 (2014) 64–72, https://doi.org/10.1016/j.ijpharm.2014.06.055. [2] D. Trache, M.H. Hussin, C.T. Hui Chuin, S. Sabar, M.R.N. Fazita, O.F.A. Taiwo, T.M. Hassan, M.K.M. Haafiz, Microcrystalline cellulose: isolation, characterization and bio-composites application—a review, Int. J. Biol. Macromol. 93 (2016) 789–804, https://doi.org/10.1016/j.ijbiomac.2016.09.056. [3] K. Mikael Vanhatalo, O. Dahl, Effect of mild acid hydrolysis parameters on properties of microcrystalline cellulose, BioResources. 9 (2014) 4729–4740, https://doi.org/10. 15376/biores.9.3.4729-4740. [4] K. Higashitani, H. Masuda, H. Yoshida, Powder Technology: Handling and Operations, Process Instrumentation, and Working Hazards, 2006. [5] Y. Hemery, M. Chaurand, U. Holopainen, A.-M. Lampi, P. Lehtinen, V. Piironen, A. Sadoudi, X. Rouau, Potential of dry fractionation of wheat bran for the development of food ingredients, part I: influence of ultra-fine grinding, J. Cereal Sci. 53 (2011) 1–8.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx [6] C. Mayer-Laigle, R. Rajaonarivony, N. Blanc, X. Rouau, C. Mayer-Laigle, R.K. Rajaonarivony, N. Blanc, X. Rouau, Comminution of dry lignocellulosic biomass: part II. Technologies, improvement of milling performances, and security issues, Bioengineering. 5 (2018) 50, https://doi.org/10.3390/bioengineering5030050. [7] K. Ward Jr., Chemical Modification of Papermaking Fibers, Mark Dekker, New York, 1973. [8] D.F. Caulfield, Ester crosslinking to improve wet performance of paper using multifunctional carboxylic acids, butanetetracarboxylic and citric acid, Tappi J. 77 (1994) 205–212. [9] C.Q. Yang, Y. Xu, Paper wet performance and ester crosslinking of wood pulp cellulose by poly (carboxylic acid) s, J. Appl. Polym. Sci. 67 (1998) 649–658. [10] G.G. Xu, C.Q. Yang, Comparison of the kraft paper crosslinked by polymeric carboxylic acids of large and small molecular sizes: dry and wet performance, J. Appl. Polym. Sci. 74 (1999) 907–912. [11] J. Rojas, E. Azevedo, Functionalization and crosslinking of microcrystalline cellulose in aqueous media: a safe and economic approach, Int. J. Pharm. Sci. Rev. Res. 8 (2011) 28–36. [12] V.A. Dehabadi, H.-J. Buschmann, J.S. Gutmann, Durable press finishing of cotton fabrics: an overview, Text. Res. J. 83 (2013) 1974–1995. [13] C.Q. Yang, Crosslinking: a route to improve cotton performance, AATCC Rev. 13 (2013) 43–52. [14] C.M. Welch, G. Forthright Danna, Glyoxal as a NonNitrogenous formaldehyde-free durable-press reagent for cotton, Text. Res. J. - TEXT RES J. 52 (1982) 149–157, https://doi.org/10.1177/004051758205200209. [15] G. Xu, Wet Strength Improvement of Paper Via Crosslinking of Cellulose Using Polymeric Carboxylic Acids and Aldehydes, University of Georgia, 2001. [16] K. Lacasse, W. Baumann, Textile Chemicals: Environmental Data and Facts, SpringerVerlag, 2004. [17] D.F. Caulfield, R.C. Weatherwax, Cross-link wet-stiffening of paper: the mechanism, Tappi. 59 (1976) 114–118. [18] D. Horie, C.J. Biermann, Application of durable-press treatment to bleached softwood kraft handsheets, TAPPI J. 77 (1994) 135–140. [19] P. Widsten, N. Dooley, R. Parr, J. Capricho, I. Suckling, Citric acid crosslinking of paper products for improved high-humidity performance, Carbohydr. Polym. 101 (2014) 998–1004, https://doi.org/10.1016/j.carbpol.2013.10.002.
9
[20] K. Lund, H. Brelid, 1,2,3,4-butanetetracarboxylic acid cross-linked softwood kraft pulp fibers for use in fluff pulp applications, J. Eng. Fiber. Fabr. 9 (2014) 142–150, https://doi.org/10.1177/155892501400900317. [21] A. Korpela, A. Tanaka, Bulky paper from chemically crosslinked hardwood kraft pulp fibers, JAPAN TAPPI J. 69 (2015) 747–753, https://doi.org/10.2524/jtappij.1505. [22] H. Choi, J.H. Kim, S. Shin, Characterization of cotton fabrics treated with glyoxal and glutaraldehyde, J. Appl. Polym. Sci. 73 (1999) 2691–2699. [23] C. Schramm, B. Rinderer, Determination of cotton-bound glyoxal via an internal cannizzaro reaction by means of high-performance liquid chromatography, Anal. Chem. 72 (2001) 5829–5833, https://doi.org/10.1021/ac000704r. [24] Y. Meiwa, JP pat., JP19960125098, Fine crosslinked cellulose particle and its production., JP19960125098 19960520, 1997. [25] S. Willför, A. Pranovich, T. Tamminen, J. Puls, C. Laine, A. Suurnäkki, B. Saake, K. Uotila, H. Simolin, J. Hemming, Carbohydrate analysis of plant materials with uronic acid-containing polysaccharides–a comparison between different hydrolysis and subsequent chromatographic analytical techniques, Ind. Crop. Prod. 29 (2009) 571–580. [26] K.V. Sarkanen, C.H. Ludwig, Lignins, Occurrence, Formation, Structure, and Reactions, Wiley-Interscience, New York, 1971. [27] M. Foston, Advances in solid-state NMR of cellulose, Curr. Opin. Biotechnol. 27 (2014) 176–184. [28] K. Ekman, H. Eskola, A. Korpela, US Pat., US8449720B2, Method of making paper., US8449720B2, 2013. [29] E. Ortega-Rivas, P. Juliano, H. Yan, Food Powders: Physical Properties, Processing, and Functionality, Springer Science & Business Media, 2006. [30] P.A. Kulkarni, R.J. Berry, M.S.A. Bradley, Review of the flowability measuring techniques for powder metallurgy industry, Proc. Inst. Mech. Eng. Part E J. Process Mech. Eng. 224 (2010) 159–168, https://doi.org/10.1243/09544089JPME299. [31] M. Çelik, Pharmaceutical Powder Compaction Technology, CRC Press, 2016. [32] J. Rojas, A. Lopez, S. Guisao, C. Ortiz, Evaluation of several microcrystalline celluloses obtained from agricultural by-products, J. Adv. Pharm. Technol. Res. 2 (2011) 144–150, https://doi.org/10.4103/2231-4040.85527.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling Antti Korpela ⁎, Hannes Orelma ⁎ VTT Technical Research Centre of Finland Ltd., Tietotie 4 E, 02044 Espoo, Finland
a r t i c l e
i n f o
Article history: Received 12 August 2019 Received in revised form 29 October 2019 Accepted 20 November 2019 Available online xxxx Keywords: Cellulose powder Microcrystalline cellulose Crosslinking Dry milling
a b s t r a c t The present study concerns the preparation of cellulosic powders with two-stage dry milling of chemically crosslinked birch kraft pulp sheets. Chemical crosslinking of kraft pulp sheets using glyoxal with and without a catalyst (aluminium sulphate) made the pliable, tenacious kraft pulp sheets brittle. Due to the brittleness, the crosslinked pulp sheets could be disintegrated easily and rapidly using a Wiley mill. The length-weighted average fibre length of the crosslinked pulp powders (0.31–0.33 mm) was shorter than the Wiley-milled reference powder (0.44 mm). The notably higher density and less fluffy character of the crosslinked pulp powders enabled their effortless further processing with an air-flow-type ultra-fine microniser. The medium size value (D50) of the micronised crosslinked powders was around 40 μm. The study finds that chemical crosslinking pre-treatment enhances the dry milling of kraft pulps to a fine powder. Chemical crosslinking may offer a new tool for industrial cellulosic powder manufacturing. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction Cellulose is an abundant natural material that has many valuable properties, such as non-toxicity, biodegradability, and easy chemical modifiability. It has already been utilised in many commercial applications from paper to high-cost medical and cosmetic additives. Cellulose powders are used, e.g. in paints as fillers and in drug delivery as a tablet former. In those applications, the cellulose powder is typically in the micrometer size (1–100 μm). These cellulose powders are manufactured by using dry milling, acid hydrolysis, or a combination of those techniques. Acid hydrolysis is a common industrial method to produce microcrystalline cellulose powder. It is based on a sequence of acidcatalysed hydrolysis of amorphous regions of cellulosic fibres followed by neutralisation, washing, and mechanical process steps. The treated material is then evacuated from water with spray drying that produces separated fine microcrystalline cellulose (MCC) particles. The medium particle size of commercial MCC products is typically in the range of 20–100 μm. Due to the acid-catalysed hydrolysis degree of polymerisation (DP) of the remainder, MCC is typically reduced to a level of 200– 300 DP. A characteristic property of MCC is its ability to form stable dispersion and gels by vigorously mixing with water. Because of this property, MCCs are used as a thickener, a stabiliser, or an emulsifier in a variety of products, such as foodstuffs, cosmetics, and water-based coatings [1,2]. Dry MCC powders are commonly used as an excipient in pharmaceutical products. From MCC, it is possible to compress tablets ⁎ Corresponding authors. E-mail addresses: antti.korpela@vtt.fi (A. Korpela), hannes.orelma@vtt.fi (H. Orelma).
that are hard but disperse readily in water and during digestion. The main problems associated with the industrial manufacturing of MCC are the high acid consumption and high chemical oxygen demand (COD) of process effluents. In a study by Vanhatalo and Dahl, the acid hydrolysis of bleached softwood kraft pulp caused around a 15–30% yield loss, depending on hydrolysis conditions, due to the removal of the amorphous regions of the fibre [3]. Dry milling is based on the milling of dry kraft pulp fibres by using one of several types of mill, e.g. a knife mill, a hammer mill, or a ball mill [4]. The dry milling of wood fibres is energy-intensive due to the toughness and flexibility of kraft pulp fibres [5,6]. According to a review article by Mayer-Laigle et al., the high energy consumption associated with the manufacture of micron-scale biomass powders using dry milling can be partly tackled by rendering the biomass more brittle with the use of a mild pyrolysis treatment called torrefaction [6]. However, torrefaction-induced changes in the chemical composition of cellulosic fibres are not desirable in most cases. Another way to render biomass more brittle is cooling. In so-called cryogenic milling, the biomass is cooled by using liquid nitrogen. By using cryogenic milling, Hemery et al. were able to reduce the dry milling (impact mill) steps of wheat bran from 3 to 1 to reach the targeted medium 50 μm particle size [5]. Due to the high costs of cryogenic milling, the processed materials are used in the high-added value products, such as adhesives, explosives, and spices [6]. Therefore there is a need to study new pre-treatment strategies to ease the dry milling procedure of pulp fibres. The treatment of cellulosic paper with chemical crosslinking agents with relatively small molecular sizes is reported to cause fibre embrittlement and a decrease in the folding endurance of paper [7–10].
https://doi.org/10.1016/j.powtec.2019.11.064 0032-5910/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
2
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Chemical crosslinking agents of cellulose are molecules that are capable of forming covalent bridges between hydroxyl groups of adjacent cellulose molecules. Chemicals capable of crosslinking cellulose are numerous, including formaldehyde, dialdehydes, such as glyoxal and glutardialdehyde, and tricarboxylic acids such as citric acid and butanetetracarboxylic acid [11–13]. Glyoxal is also capable of crosslinking cellulose fibres, and it is a relatively safe cellulose crosslinking agent [14] that cures at a lower temperature than most other formaldehyde-free crosslinking agents, such as polycarboxylic acids. Both hemiacetal and acetal linkages can be formed in the crosslinking of cellulose with glyoxal. Hemiacetal formation is a reversible reaction, and the crosslinks are hydrolysed in water. The increase in the curing temperature and the use of catalysts favour the formation of acetal linkages, which are more water-resistant than hemiacetal bonds [7,11,15]. Due to the differing water resistance of hemiacetal and acetal bonds, glyoxal can then be used to produce either transient or more permanent effects on cellulosic material properties [7]. The most significant group of crosslinking agents are DMDHEU (1.3-dimethylol-4.5dihydroxyethylene urea) based agents in cotton fabric finishing. The crosslinks formed in cotton fibres reduce the shrinking and wrinkling of cotton textiles in use, and in the washing and drying of textiles. Cotton fabric crosslinking is conventionally performed using the so-called pad-dry-cure method, which begins with the immersion on the fabric in a water solution of the crosslinking agent and its catalyst. After immersion, the excess solution is squeezed from the fabric and subsequently the fabric is smoothed and dried. Finally the fabric carrying the crosslinking agent is cured for a few minutes at an elevated temperature (around 140–170 °C). The actual crosslinking reactions in the process take place in the curing stage [12,16]. Paper made from cellulosic fibres can be crosslinked by using chemicals following the pad-dry-cure method. Via crosslinking, it is possible to improve the stiffness and wet strength of paper by forming water-resistant inter-fibre covalent crosslinks. An unwanted consequence of chemical crosslinking in paper manufacture is an increase in the brittleness of the paper, resulting in reduced fold strength [15,17–19]. As chemical crosslinking is performed on the papermaking pulp fibres, the water retention value (WRV) of the fibres decreases
and the stiffness of the fibres increases [20,21]. The decrease in WRV, which is a measure of pulp's capability to maintain water, is probably due to the chemical crosslinks that prevent the swelling of fibre walls caused by water. Crosslinked pulps with both low WRV and high wet bulk are used as moisture distribution layers in absorption products, such as napkins [20]. In the present study we investigated if the sequential glyoxal crosslinking and dry milling technique can be utilised in the production of micrometer size cellulose powder (Scheme 1). For comparison, crosslinking of pulp sheets were also performed in the presence of aluminium sulphate, which is a known catalyst of crosslinking reactions [14,22,23]. For the crosslinking procedure, bleached birch kraft pulp sheets were first immersed in a crosslinking-agent water solution followed by drying and curing of the sheets in elevated temperature. Cured pulp sheets were dry-milled using a Wiley mill, which is a rotating blade-type cutting mill. This step was followed by further drygrinding of the powders using a decompression air-type microniser. Finally, properties, such as particle size distribution, specific surface area (BET), and bulk and tap densities of the powders were measured. The crosslinking reaction was also studied by using CPMAS 13C solid state NMR. The idea of using chemical crosslinking in order to reduce the energy consumption of dry milling cellulosic fibres to fine particles has been presented in patent application JP09392001 [24]. To the best of our knowledge, the method has not been discussed previously in scientific literature.
2. Experimental 2.1. Materials The pulp used in the study was dried, bleached birch (Betula bendula) kraft pulp originating from a Finnish pulp mill. The pulp was obtained as A4-sized pulp sheets (grammage 830 g/m2, air-dry, dry content 96%). The used glyoxal was obtained as a 40% water solution from Merck KGaA (USA). Aluminium sulphate tetradecahydrate (Al2(SO4)3·14H2O) was obtained from Kemira OyJ (Finland). The
Scheme 1. Schematic illustration of the studied sequential manufacturing process and possible chemical reaction pathways (hemiacetal and acetal formation) of the glyoxal between the hydroxyl groups of adjacent cellulose molecules.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Fig. 1. Produced samples. The letters A - E refer to the prepared and characterised fibre and powder samples. A: Reference pulp, B: pulp after Wiley milling (rough refining), C: Pulp after glyoxal crosslinking and Wiley milling, D: pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling, E: pulp after glyoxal crosslinking and subsequent Wiley milling and micronisation (fine refining), and F: pulp after glyoxal crosslinking with aluminium sulphate and subsequent Wiley milling and micronisation.
water used in the study was deionised water produced with a MilliQdevice. All other chemicals were laboratory grade. 3. Methods 3.1. Chemical crosslinking of cellulose papers by glyoxal Crosslinking of the birch kraft pulp sheets was carried out following the pad-dry-cure crosslinking method. A 1-kg stack of pulp sheets (dry
3
content 96%) was immersed in a water solution containing 4.0 wt% glyoxal for 60 min. Aluminium sulphate hydrate (1.33 wt%) was added in the glyoxal water solution, which was optional. After impregnation, the excess solution was drained from the pulp sheets. The dry content of the treated pulp sheets after the draining was 40%. The amount of absorbed glyoxal was 6.0%, and the aluminium sulphate hydrate was 2.0% of the sheet dry content (calculated by assuming that the percentage proportions of crosslinking chemicals in the absorbed solutions were the same as the percentage proportions of the immersion solution). The impregnated pulp sheet were first drained and then dried (the curing takes place when the water is evaporated) in a ventilated oven at 110 °C for 5 h. The sheets were set apart in an oven in order to ensure even and efficient drying and curing. After the drying and curing process, the dry content of the sheets was approximately 97%. The chemically crosslinked pulp sheets were stored at 22–23 °C in 25–35% RH. 3.2. Dry milling of glyoxal crosslinked pulp sheets The crosslinked pulp sheets were manually cut into 3 × 3 cm2 pieces, which were dry-milled using a Wiley mill (Standard model no. 3, Arthur H. Thomas Co.) The pieces were manually fed into the Wiley mill where revolving knives worked against stationary knives and crushed the material until the resulting particle size was small enough to pass through a 1 mm hole screen. Pulp powder made from the untreated reference pulp sheets was fluffy, whereas the pulp made from a crosslinked pulp sheet was denser and showed better flow properties, enabling trouble-free further processing with a decompressed air-flow-type
Fig. 2. a) A piece of untreated birch kraft pulp sheet after 20 double folds (on the left, Sample A) and chemically crosslinked sheet (glyoxal without aluminium, Sample C) after the first fold. b) cellulose samples after Wiley milling (left untreated (Sample B) and right, glyoxal crosslinked without aluminium (Sample C)), and Wiley milling plus micronisation of glyoxal crosslinked without aluminium (Sample E).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
4
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
microniser (Ultra fine mill Ceren Miller Dau MKCL8-15J DAU, Masuko Sangyo). According to the manufacturer (Masuko Sangyo), the grinding effect is comparable to the effect of jet milling. The Wiley-milled pulp was fed once through the microniser. The microniser reduced the particle size by way of strong impacts caused by air flow and a rotating impeller. The rotation speed of the impeller was kept constant at 7600 rpm, milling time was 90 s, sample feeding time 4–6 s, flushing time was 35 s, and the screw speed was 60 rpm. The sample codes of prepared powders are shown in Fig. 1.
3.3. Carbohydrate and lignin composition, and determination of ash To determine lignin and carbohydrate composition, the samples were hydrolysed using sulphuric acid (2 stages), and the resulting monosaccharides were determined using HPAEC with pulse amperometric detection (Dionex ICS 5000 equipped with CarboPac PA20 column [25]. The acid-insoluble lignin content (Klason lignin) in the kraft pulp was determined according to the TAPPI T 222 om-02 method. The acid-soluble lignin was determined using the spectrophotometric method based on the absorption of ultraviolet radiation at 215 and 280 nm and using the equation described by Sarkanen and Ludwig [26]. The ash content was determined according to ISO 1762:15. The amount of acid-insoluble lignin (Klason lignin) in the pulp was 1.3% and the amount of acid-soluble lignin was b0.1%. According to monosaccaride analysis, the carbohydrate fraction was composed of glucose (73%), xylose (25%), and mannose (0.7%) units. The percentage of ash (525 °C) in the pulp was 0.5%.
3.4. Particle size measurements using fibre analyser The fibre-length distribution of the pulp before and after dry milling via glyoxal crosslinking was measured using a Valmet F5 Fiber Image
Analyzer (Valmet, Finland) according to ISO 16065-2. All measurements were replicated. 3.5. Water retention value The WRV(ml/g) of the pulp before and after dry milling via glyoxal crosslinking was determined using the centrifugal method according to ISO 23714. All measurements were at least duplicated. 3.6. Bulk and tap density measurements The bulk density of the powders was determined by pouring a 5 g sample into a graduated 50 ml test tube and by measuring the volume of the powder. The tap density of the powders was determined by dropping (tapping) the test tube from a height of 2.5 cm onto a hard surface until no more settling of powder occurred. The Hausner ratio (H) was determined from the relation of tapping density ρT and bulk density ρB, (H = ρT/ρB). All density measurements were triplicated. 3.7. Wet and dry particle size measurements using laser diffraction particle size analyser Particle size distributions of micronised powders dispersed in water were measured using the laser light diffraction method with a Beckman Coulter LS 230 (Beckman Coulter, Inc., USA). For the measurement, 1 g powder samples were dispersed into 40 ml of milli-Q water with mixing for 15 min using a magnetic stirrer. The particle size measurements were duplicated. For comparison, the particle size distribution of powders in dry form was also measured using a Malvern Mastersizer 3000 (Malvern Panalytical Ltd., UK) laser diffraction particle size analyser equipped with standard sample tray and dispersion unit Aero S. The particle size measurements of dry powders were triplicated.
Fig. 3. FTIR a) and CPMAS 13C solid state NMR b) spectrums of pulp powders. B: pulp after Wiley milling, C: pulp after glyoxal crosslinking and Wiley milling, D: pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
5
Fig. 4. Thermogravimetric analysis of pulp powders; B: pulp after Wiley milling, C: pulp after glyoxal crosslinking and Wiley milling, D: pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling.
3.8. Fourier transformation infrared resonance (FTIR) measurements
3.10. Photomicrographing
FTIR spectroscopy was used to characterise the produced materials. The measurement was performed with a Thermo Scientific Nicolet iS50 FT-IR spectrometer with an ATR diamond (Thermo Scientific, USA). All spectra were obtained from 32 scans with a resolution of 4 cm-1 and absorption mode by using the wavelength range of 400 to 4000 cm−1. At least three repetitions were carried out for each specimen.
The unstained samples were visualised using confocal laser scanning microscopy (CLSM) equipment consisting of a Zeiss LSM 710 (Zeiss, Jena, Germany) attached to a Zeiss AxioImager.Z microscope. Fibres of each sample were separated in water and were examined on a microscope slide as sealed preparates. Samples were imaged utilising a transmitted light detector with a 10× objective (Zeiss EC Epiplan-Neofluar, numerical aperture of 0.16) and a 20× objective (Zeiss EC EpiplanNeofluar, numerical aperture of 0.30) with a resolution of 1024 × 1024 using ZEN software (Zeiss). Representative images were selected for publication.
3.9. Chemical analyses of crosslinked paper with solid state 13C CP/MAS NMR spectrometry Chemical changes in the paper caused by crosslinking with glyoxal were characterised by using a 13C cross polarisation magic angle spinning (CP-MAS) NMR spectrometer (Bruker AVANCE-III 400 MHz, Bruker BioSpin, Germany). For all samples, 20,000 scans were collected using an 8 kHz spinning frequency, 2-ms contact time, and a 5-s delay between pulses.
3.11. Thermogravimetric analysis (TGA) Thermal decomposition of the powders was studied using a Netzsch STA 449 F1 Jupiter thermal analyser (Netzsch-Gerätebau GmbH, Germany). For TGA measurements, samples of about 10 mg were analysed in air at a heating rate of 10 °C/min from 35 to 700 °C.
Fig. 5. Length-weighted fibre length distributions of reference pulp (Sample A), pulp after Wiley milling (Sample B), pulp after glyoxal crosslinking and Wiley milling (Sample C), pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling (Sample D).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
6
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Table 1 Properties of reference pulp fibres and Wiley-milled pulp fibres. Sample A
Sample B
Sample C
Sample D
Birch kraft pulp (ref.) Wiley-milled ref. pulp Glyoxal crosslinked Wiley-milled pulp Glyoxal + Al2(SO4)3 crosslinked Wiley-milled pulp Arithmetic av. fibre length, mm Length w. av. fibre length, mm Weight w. av. fibre length, mm Length b 0.2 mm, % Water retention value (WRV), g/g Bulk density, g/ml Tapping density, g/ml Hausner ratio
0.80 0.93 1.06 13.4 1.13 – – –
0.37 0.44 0.53 42.1 0.84 0.066 0.113 1.71
0.28 0.31 0.35 81.6 0.60 0.186 0.277 1.49
0.28 0.33 0.34 84.7 0.51 0.203 0.306 1.51
Fig. 6. Optical microscopy image reference pulp (Sample A), pulp after Wiley milling (Sample B), pulp after glyoxal crosslinking and Wiley milling (Sample C), pulp after glyoxal crosslinking with aluminium sulphate and Wiley milling (Sample D), pulp after glyoxal crosslinking and subsequent Wiley milling and micronisation (Sample E), and pulp after glyoxal crosslinking with aluminium sulphate and subsequent Wiley milling and micronisation (Sample F).
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
7
4. Results
4.2. Physical properties of Wiley-milled cellulose powders
4.1. Glyoxal crosslinking of paper and subsequent dry milling
The fibre lengths of the Wiley-milled crosslinked pulp powders (Samples C and D, without and with aluminium sulphate addition, respectively) were significantly shorter than those of the reference pulp powder (Fig. 5 and Table 1). Optical microscopy images (Fig. 6a-d) show that the fibre cut in the Wiley mill occurred without visible fibrillation of the fibres. Chemical crosslinking also resulted in a significantly higher amount of fibre fines (length b 0.2 mm) in the milled pulp. This is evidently a consequence of the brittleness of the crosslinked fibres. The addition of aluminium sulphate as a catalyst in the crosslinking reaction did not have any significant effect on the properties of the fibre length properties of the Wiley-milled samples. WRV is an important value for cellulose powders that are utilised in water suspensions and for drug delivery. The results in Table 1 show that Wiley milling of the reference pulp (Samples A and B) resulted in decreased WRV, from 1.13 g/g to 0.84 g/g. There is very little published information on the effect of dry milling on the WRV of kraft pulps. The results presented in US Patent 8,449,720 B2 [28] regarding the effects of Wiley milling on the WRV of softwood kraft pulp fibres are in accordance with the results of the present study. According to Ekman et al., the Wiley milling of softwood kraft pulp fibres from 2.25 mm to 0.6 mm (length-weighted averages) decreases the WRV of the pulp from 1.17 g/g to 0.92 g/g. The reason for the decrease in WRV remains unclear, but it might have been due to the mechanical impact-induced permanent compactions of fibre walls that took place in the Wiley milling. The glyoxal crosslinking (Samples C and D) further decreased the WRV values to 0.6 g/g and 0.51 g/g, respectively. This indicates that the crosslinking solidified the cell wall of the fibre fragments and, thus, decreased their ability to absorb water. The bulk and tapping densities of the Wiley-milled pulps were significantly higher than those of the reference. The Hausner ratio of both crosslinked Wiley-milled pulps decreased. The Hausner ratio is generally used in industries as an indication of the flowability of a powder; the lower value indicates better flowability [29,30].
Crosslinking treatments using glyoxal and glyoxal with aluminium sulphate caused a clear visible change in the brittleness of the pulp sheets (Fig. 2a). The crosslinked pulp sheet broke the first time it was folded into two pieces, whereas the reference pulp sheet lasted several dozen folds without breaking. Both paper sheets were pretreated using Wiley milling since the micronisation technique is not capable of handling large paper sheets. The passing time of the reference pulp (Sample B) and the crosslinked pulps (with and without aluminium sulphate, Samples C and D) in the Wiley mill, was around 45–60 s and 5–10 s, respectively. The crosslinked pulps milled fluently, whereas the reference pulp formed fluffy compactions in the mill (Fig. 2b).Thus, the processability of the crosslinked pulp from the Wiley mill was significantly better than that of the reference pulp. Chemically crosslinked Wiley-milled pulps (Samples C and D) were further processed with dry grinding using a decompressed air-type microniser. Due to fluffy character and poor flowability, the reference pulp (Sample B) was not processable with the microniser. FTIR measurements were undertaken to verify the crosslinking of cellulose with glyoxal with and without aluminium sulphate. The FTIR spectra of the reference pulp powder, Sample B, and crosslinked pulp powder, Samples C and D, are shown in Fig. 3a. The decrease in absorbance around 3300 1/cm (OH stretching of intramolecular H-bonding) is a likely consequence of reactions of glyoxal with cellulosic hydroxyl groups. An absorbance peak of 1735 1/cm may be due to unreacted glyoxal ends (C_O stretching peak). The formation of acetal and hemiacetal linkages took place within the fingerprint area of cellulose (800–1200), and thus were not visible in the spectra (Samples C and D). A quantitative analysis of the amount of acetal and hemiacetal crosslinks is difficult in practice. Schramm and Rinderer [23] have introduced a chromatographic method that can be used of quantifying the portion of glyoxal that has reacted with cellulosic material. However, the method does not deliver information on whether the crosslinks were achieved by an acetal or a hemiacetal linkage. The FTIR spectra suggest that, with the exception of the formation of acetal and/or hemiacetal bonds, no other major modifications of the cellulose and hemicellulose had taken place in the crosslinking treatments. Cellulose powder samples characterised with CPMAS 13C solid state NMR (Fig. 3b) were used to study the hemiacetal and acetal formation within the crosslinking of the cellulose fibres. As expected, the untreated powder (Sample B) showed a signal for native cellulose with crystalline cellulose I and non-crystalline cellulose [27]. Glyoxal crosslinking with and without glyoxal (Samples D and C) did not alter the carbon peak assignment or the crystallinity of cellulose compared to Sample B. However, a new broad peak arose at 94–98 ppm, which was identified as (hemi)acetal formation (acetal peak in 90–100 ppm). In a 13C NMR spectrum, hemiacetal linkages had a peak at 90–95 ppm and acetal linkages had a peak 95–100 ppm. Therefore, based on the measured spectra, it cannot be clearly verified that glyoxal formed with cellulose hemiacetal or acetal bonds. Most probably there both bond types were present after glyoxal crosslinking. In order to evaluate the thermal stability of crosslinked cellulose powder, thermogravimetric analysis (TGA) was performed (Fig. 4). The TGA curves of reference Sample B and glyoxal-crosslinked Sample C are very similar in the figure, except for a slightly higher loss of weight-% in the area of 200 °C — 300 °C. A TGA curve of cellulose powder crosslinked with glyoxal and aluminium sulphate differs more significantly from the reference powder in the figure. An examination of the reasons for the deviation was not carried out in the present study. From a powder potential application point of view, no remarkable difference in the thermal stability of the powders below 170 °C was found. This indicates that glyoxal crosslinked cellulose powder can be utilised in the same applications where regular microcrystalline cellulose is currently utilised.
4.3. Production and properties of fine micronised cellulose powders Wiley-milled cellulose powders were further processed with the micronisation technique. Micronisation clearly decreased the length of fibre segments (Fig. 6e and f), and no clear fibrillation was observed. However, unlike microcrystalline cellulose particles the appearance of which is typically grainy (Hindi 2017), the particles of both powders (Samples E and F) still look like pieces of fibre. Table 2 shows the properties of the micronised crosslinked pulp powders. Micronising resulted in powders (Samples E and F), the medium particle size value (D50) of which were close to 40 μm. Particle size distributions measured using dry and wet dispersion methods showed no major deviations (Fig. 7). Table 2 Particle properties of powders (Samples E and F) made with fine-refined crosslinked Wiley-milled birch kraft pulp (Sample B).
Size distribution, Beckman coulter, wet dispersion, D10 / D50 / D90 (μm) Size distribution, Malvern, dry dispersion, D10 / D50 / D90 (μm) Specific surface area, BET (m2/g) Bulk density (g/ml) Tapped density (g/ml) Hausner ratio
Sample E
Sample F
Glyoxal crosslinked, Wiley-milled & fine refined
Glyoxal + Al2 (SO4)3 crosslinked, Wiley-milled & fine refined
13.3 / 41.4 / 118,9
13.9 / 40.6 / 112.6
14.0 / 39.9 / 113 1.95 0.309 0.526 1.70
14.2/ 36.5 / 98.7 1.53 0.338 0.542 1.60
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
8
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx
Fig. 7. Particle-size distributions of dry- and wet-dispersed crosslinked pulp powders: E: pulp after glyoxal crosslinking and subsequent Wiley milling and micronisation, and F: pulp after glyoxal crosslinking with aluminium sulphate and subsequent Wiley milling and micronisation.
The medium particle size value of the prepared powders are comparable to many commercial microcrystalline celluloses, such as Avicel PH101 (D50 ~50 μm). The use of aluminium sulphate as a crosslinking reaction catalyst did not cause any clear visible differences in the appearance of the crosslinked powder particles. Apparent specific surfaces of a commercial microcrystalline cellulose, such as Avicel PH 101, determined using the BET method are reported to be in the range of 1.0– 1.5 m2/g [31]. The specific surface area (BET) of powders samples E and F were 1.95 g/m2 and 1.53 g/m2, respectively. The lower surface area of Sample F compared to Sample E is possibly a consequence of higher density and a less porous particle structure. Both the bulk and tapping density of the pulp powder increased substantially during micronisation. Bulk and tapped densities of powders samples E and F were found to be close to those of the microcrystalline celluloses Avicell PH 101, 102, and 103 reported by Rojas et al. [32]. However, the higher Hausner values for Samples E and F compared to the commercial Avicel products (1.31–1.40) indicate somewhat poorer flow properties. A clear difference between the crosslinked pulp powders (Samples E and F) and the microcrystalline cellulose Avicel PH 101 is their dispersability in water. Vigorous mixing of the Avicel in water (e.g., 10 wt% in water with Ultra Turrex using 12,000 rpm for 10 min) results in stable milky dispersions, whereas crosslinked powders start to sediment immediately after the mixing. The reason for the different behaviour of the Avicel and the crosslinked powder was not examined in the present study, but this behaviour may be related to the firmness and chemical composition of the crosslinked particles. Further research should examine the effects of different cellulose crosslinking agents, such as tricarboxylic acids, and different crosslinking conditions on the processability of cellulosic materials in dry milling and on the properties of the dry-milled kraft pulp powders. 5. Conclusions The present study examined dry milling of cellulose fibres into micro-size cellulose powder. A chemical crosslinking treatment of bleached birch kraft pulp sheets made the birch sheets more brittle and subsequently enhanced dry milling. The increased brittleness of the sheets resulted in a significantly shorter passing time for the dry pulps in the Wiley mill and a shorter average fibre length in the resultant powder. Crosslinked Wiley-milled pulp powder also showed higher density and better flow properties, thus enabling the micronisation of this powder with a decompressed airflow-type microniser. After fine grinding, the medium particle size value (D50) was around 40 μm. These findings strongly indicate that the hypothesis of the present study is correct. In addition, both FTIR and NMR spectrum analyses suggest that glyoxal with and without a catalyst (aluminium sulphate) formed hemiacetal and/or acetal linkages in the treated
birch kraft pulp sheets. Unfortunately, it is not possible to estimate the proportions of hemiacetal- and acetal linkages based on chemical analysis. Unlike some commercial microcrystalline celluloses, which have comparable particle size distribution, the crosslinked fine powders do not form stable dispersions when mixed vigorously in water. Therefore, the potential industrial uses for crosslinked kraft pulp powders are not necessarily the same as those for MCCs, but the properties of the powders may enable their use as a filler, for example, in plastics and paper products. The lower water absorbency of the powders compared to non-crosslinked cellulose powders may be an advantage in those industrial applications. Author contributions Both authors contributed equally to the manuscript, and both authors have given their approval to the final version of the manuscript. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The work was part of the Academy of Finland Flagship Programme under Project Nos. 318890 and 318891 (Competence Centre for Materials Bioeconomy, FinnCERES). Our thanks go out to Mirja Muhola for the BET analysis, to Mirja Nygård for the thermogravimetric measurements, to Tommi Virtanen for NMR analysis, to Liisa Änäkäinen for photomicrographs, and to Pia Siventoinen and Markus Nikinmaa for performing the fine grinding of the cellulose powders. References [1] G. Thoorens, F. Krier, B. Leclercq, B. Carlin, B. Evrard, Microcrystalline cellulose, a direct compression binder in a quality by design environment—a review, Int. J. Pharm. 473 (2014) 64–72, https://doi.org/10.1016/j.ijpharm.2014.06.055. [2] D. Trache, M.H. Hussin, C.T. Hui Chuin, S. Sabar, M.R.N. Fazita, O.F.A. Taiwo, T.M. Hassan, M.K.M. Haafiz, Microcrystalline cellulose: isolation, characterization and bio-composites application—a review, Int. J. Biol. Macromol. 93 (2016) 789–804, https://doi.org/10.1016/j.ijbiomac.2016.09.056. [3] K. Mikael Vanhatalo, O. Dahl, Effect of mild acid hydrolysis parameters on properties of microcrystalline cellulose, BioResources. 9 (2014) 4729–4740, https://doi.org/10. 15376/biores.9.3.4729-4740. [4] K. Higashitani, H. Masuda, H. Yoshida, Powder Technology: Handling and Operations, Process Instrumentation, and Working Hazards, 2006. [5] Y. Hemery, M. Chaurand, U. Holopainen, A.-M. Lampi, P. Lehtinen, V. Piironen, A. Sadoudi, X. Rouau, Potential of dry fractionation of wheat bran for the development of food ingredients, part I: influence of ultra-fine grinding, J. Cereal Sci. 53 (2011) 1–8.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064
A. Korpela, H. Orelma / Powder Technology xxx (2019) xxx [6] C. Mayer-Laigle, R. Rajaonarivony, N. Blanc, X. Rouau, C. Mayer-Laigle, R.K. Rajaonarivony, N. Blanc, X. Rouau, Comminution of dry lignocellulosic biomass: part II. Technologies, improvement of milling performances, and security issues, Bioengineering. 5 (2018) 50, https://doi.org/10.3390/bioengineering5030050. [7] K. Ward Jr., Chemical Modification of Papermaking Fibers, Mark Dekker, New York, 1973. [8] D.F. Caulfield, Ester crosslinking to improve wet performance of paper using multifunctional carboxylic acids, butanetetracarboxylic and citric acid, Tappi J. 77 (1994) 205–212. [9] C.Q. Yang, Y. Xu, Paper wet performance and ester crosslinking of wood pulp cellulose by poly (carboxylic acid) s, J. Appl. Polym. Sci. 67 (1998) 649–658. [10] G.G. Xu, C.Q. Yang, Comparison of the kraft paper crosslinked by polymeric carboxylic acids of large and small molecular sizes: dry and wet performance, J. Appl. Polym. Sci. 74 (1999) 907–912. [11] J. Rojas, E. Azevedo, Functionalization and crosslinking of microcrystalline cellulose in aqueous media: a safe and economic approach, Int. J. Pharm. Sci. Rev. Res. 8 (2011) 28–36. [12] V.A. Dehabadi, H.-J. Buschmann, J.S. Gutmann, Durable press finishing of cotton fabrics: an overview, Text. Res. J. 83 (2013) 1974–1995. [13] C.Q. Yang, Crosslinking: a route to improve cotton performance, AATCC Rev. 13 (2013) 43–52. [14] C.M. Welch, G. Forthright Danna, Glyoxal as a NonNitrogenous formaldehyde-free durable-press reagent for cotton, Text. Res. J. - TEXT RES J. 52 (1982) 149–157, https://doi.org/10.1177/004051758205200209. [15] G. Xu, Wet Strength Improvement of Paper Via Crosslinking of Cellulose Using Polymeric Carboxylic Acids and Aldehydes, University of Georgia, 2001. [16] K. Lacasse, W. Baumann, Textile Chemicals: Environmental Data and Facts, SpringerVerlag, 2004. [17] D.F. Caulfield, R.C. Weatherwax, Cross-link wet-stiffening of paper: the mechanism, Tappi. 59 (1976) 114–118. [18] D. Horie, C.J. Biermann, Application of durable-press treatment to bleached softwood kraft handsheets, TAPPI J. 77 (1994) 135–140. [19] P. Widsten, N. Dooley, R. Parr, J. Capricho, I. Suckling, Citric acid crosslinking of paper products for improved high-humidity performance, Carbohydr. Polym. 101 (2014) 998–1004, https://doi.org/10.1016/j.carbpol.2013.10.002.
9
[20] K. Lund, H. Brelid, 1,2,3,4-butanetetracarboxylic acid cross-linked softwood kraft pulp fibers for use in fluff pulp applications, J. Eng. Fiber. Fabr. 9 (2014) 142–150, https://doi.org/10.1177/155892501400900317. [21] A. Korpela, A. Tanaka, Bulky paper from chemically crosslinked hardwood kraft pulp fibers, JAPAN TAPPI J. 69 (2015) 747–753, https://doi.org/10.2524/jtappij.1505. [22] H. Choi, J.H. Kim, S. Shin, Characterization of cotton fabrics treated with glyoxal and glutaraldehyde, J. Appl. Polym. Sci. 73 (1999) 2691–2699. [23] C. Schramm, B. Rinderer, Determination of cotton-bound glyoxal via an internal cannizzaro reaction by means of high-performance liquid chromatography, Anal. Chem. 72 (2001) 5829–5833, https://doi.org/10.1021/ac000704r. [24] Y. Meiwa, JP pat., JP19960125098, Fine crosslinked cellulose particle and its production., JP19960125098 19960520, 1997. [25] S. Willför, A. Pranovich, T. Tamminen, J. Puls, C. Laine, A. Suurnäkki, B. Saake, K. Uotila, H. Simolin, J. Hemming, Carbohydrate analysis of plant materials with uronic acid-containing polysaccharides–a comparison between different hydrolysis and subsequent chromatographic analytical techniques, Ind. Crop. Prod. 29 (2009) 571–580. [26] K.V. Sarkanen, C.H. Ludwig, Lignins, Occurrence, Formation, Structure, and Reactions, Wiley-Interscience, New York, 1971. [27] M. Foston, Advances in solid-state NMR of cellulose, Curr. Opin. Biotechnol. 27 (2014) 176–184. [28] K. Ekman, H. Eskola, A. Korpela, US Pat., US8449720B2, Method of making paper., US8449720B2, 2013. [29] E. Ortega-Rivas, P. Juliano, H. Yan, Food Powders: Physical Properties, Processing, and Functionality, Springer Science & Business Media, 2006. [30] P.A. Kulkarni, R.J. Berry, M.S.A. Bradley, Review of the flowability measuring techniques for powder metallurgy industry, Proc. Inst. Mech. Eng. Part E J. Process Mech. Eng. 224 (2010) 159–168, https://doi.org/10.1243/09544089JPME299. [31] M. Çelik, Pharmaceutical Powder Compaction Technology, CRC Press, 2016. [32] J. Rojas, A. Lopez, S. Guisao, C. Ortiz, Evaluation of several microcrystalline celluloses obtained from agricultural by-products, J. Adv. Pharm. Technol. Res. 2 (2011) 144–150, https://doi.org/10.4103/2231-4040.85527.
Please cite this article as: A. Korpela and H. Orelma, Manufacture of fine cellulose powder from chemically crosslinked kraft pulp sheets using dry milling, Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.064