IONCELL-P&F: Pulp Fractionation and Fiber ... - ACS Publications

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IONCELL-P&F: Pulp Fractionation and Fiber Spinning with Ionic Liquids Agnes M. Stepan, Anne Michud, Sanna Hellstén, Michael Hummel, and Herbert Sixta* Department of Forest Products Technology, School of Chemical Technology, Aalto University, P.O. Box 16300, 00076 Aalto, Finland ABSTRACT: Both IONCELL-P(ulp) and IONCELL-F(iber) are emerging new cellulose processing technologies utilizing ionic liquids (ILs). IONCELL-P is upgrading paper pulp into dissolving pulp with mixtures of IL and water. IONCELL-F is an IL based cellulose fiber spinning technology. In this work, these two processes were combined using the same ionic liquid, 1,5diazabicyclo[4.3.0]non-5-ene acetate [DBNH][OAc], in both processes. In the IONCELL-P to achieve dissolving pulp quality cellulose (hemicellulose content below 5%) from birch kraft pulp, different pretreatments and optimization steps, such as targeted enzymatic predegradation of the xylans, preadjusting the viscosity of the pulp, and decreased pulp consistency (2%) in the fractionation, were implemented. From the obtained cellulose fraction containing 4.2% xylan, a dope was prepared from which high tenacity (50 cN/tex) and Young’s modulus (23.6 GPa) fibers were spun with IONCELL-F dry jet wet spinning. The produced fibers surpassed the mechanical properties of commercially available viscose and Lyocell fibers.

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INTRODUCTION One of the consequences of the digital revolution escalating in the last few decades is the slowly decreasing demand for paper in the developed countries as online news, personal devices, and e-readers take over a number of functions that paper used to fulfill.1,2 On the other hand, the population of the world is continuously increasing and has to be provided with textiles for clothing and plastics for consumables. Dissolving pulp is a high purity cellulose pulp, which is the raw material for many value added products such as textiles, hygiene products, and plastic consumables from cellulose esters and ethers, which are used to produce fibers, films, filters, etc.3,4 This decreasing need for paper pulp can be combined with the increasing demand for dissolving pulp in future biorefineries. Hitherto, multistep refining processes were needed for efficient hemicellulose removal from wood pulp, and most of them suffer from severe cellulose losses as summarized by Sixta et al. and Roselli et al.5,6 Moreover, these processes provide limited or no opportunity for hemicellulose recovery and utilization. The IONCELL-P(ulp) process is a modern fractionation method using solvents with high potential for recyclability7−10 to upgrade hemicellulose rich paper pulp to high purity dissolving pulp and coproduce polymeric hemicellulose.5,11 In the IONCELL-P, the hemicelluloses are selectively extracted from bleached paper grade pulp with an ionic liquid (IL) and water mixture, where both fractions can be recovered without yield losses or polymer degradation.5,11 This selective recovery is crucial to open new opportunities for polymeric hemicellulose applications. Several publications forecast a potentially exciting future for hemicellulose based novel materials such as barrier films, foams, and coatings.12−21 Moreover, polymeric © XXXX American Chemical Society

hemicelluloses can be readily converted to the intermediate molecular weight oligosaccharides, which can be used in pharmaceutical, food, and cosmetic applications.22,23 The advantages of the IONCELL-P compared with the currently vastly applied technologies for producing dissolving pulp have already been thoroughly summarized previously.5,11 The effect of different raw material, used IL, and applied water content on the performance of the IONCELL-P fractionation has already been published.24,25 Pretreatment of the paper pulp with enzymes such as xylanases and endoglucanases was also studied.24 It was found that the specific and targeted degradation of xylans in eucalyptus pulp increased the hemicellulose extraction efficiency of the IONCELL-P. The treatment temperature and time in the IONCELL-P processes was optimized, and to some extent the effect of the treatment suspension consistency was also studied. Higher consistencies could facilitate less IL used in the processes, making it more cost efficient. However, since different IL−water fractionation systems exhibit different rheological behaviors, increasing the polymer consistency in certain IONCELL-P systems might result in increased process time and energy demands of the fractionation. The cellulose gap is an often quoted phenomenon predicted in the near future, which refers to the increasing demand for cellulosic fibers not being met by the cotton production of the world as the world population is increasing.26 This gap is an Received: January 6, 2016 Revised: June 15, 2016 Accepted: July 7, 2016

A

DOI: 10.1021/acs.iecr.6b00071 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research opportunity for the emergence of novel man-made cellulosic fibers based on less hazardous production routes such as the viscose process. Eventually, several economic and environmental factors indicate the advantages of gradually supplementing cotton based cellulosic fibers with man-made ones.27 It is not only the high quality requirements of agricultural land, necessary pesticides, and considerable irrigation demands of cotton production that make it unsustainable, but also the total carbon footprint of cotton based fibers is significantly higher compared with man-made cellulosic fibers.28 The ethics of cotton competing with food-crops for high-grade farmland also backs the ecological urge to find alternative cellulose sources for textile applications. Among the production technologies for man-made cellulosic fibers, the CS2 based viscose process is the predominant one. The N-methylmorpholine N-oxide (NMMO) monohydrate based Lyocell process is also used to produce man-made cellulosic fibers, and it is the only one of all the processes utilizing direct cellulose solvents that is scaled up to industrial level. Unfortunately the use of CS2 is not environmentally benign, and although the NMMO is a nontoxic, environmentally harmless chemical, it is unstable and tends to hemolytic and heterolytic side reactions and under certain conditions to runaway reactions. These together motivate academic and industrial researcher to seek new alternative processes to the viscose and lyocell processes.29 Spinning of cellulose−IL solutions in the so-called IONCELL-F(iber) process has been introduced as a potential alternative to the currently used viscose and Lyocell processes for the production of man-made cellulosic fibers.30−32 The IONCELL-F uses dry-jet wet spinning, where a polymer solution is extruded at elevated temperature through a spinneret via an air gap into a coagulation bath. In the air gap, the fluid filament is drawn which induces the orientation of the polymer chains and, thus, affects the properties of the resulting fibers. The final properties of the fibers are strongly influenced by the chosen spinning parameters (extrusion velocity, draw ratio, air gap), as well as the properties of the raw material (cellulose and hemicellulose contents, intrinsic viscosity, molar mass distribution).33 ILs are popular for their high potential for recyclability. However, so far the cellulose dissolving ILs are published in a broad variety of different processes and reactions.5,11,27,30,32,34−41 Eventually, it would be desired to develop such a combined process design, where one IL can be recycled from many different applications in a combined recycling system. This study strives to connect different processes through the use of the same IL for more realistic future business cases. In this work, we aim to establish a process line where two IL based processes, the IONCELL-P and IONCELL-F, are combined by utilizing the same IL for both processes, facilitating a joint recycling system for two separate steps. Fractionation with the IONCELL-P has been carried out with several different ILs and different sources of pulps, and there is a broad understanding of parameters influencing these fractionations. As for spinning man-made cellulose fibers from IL-cellulose solutions, hitherto mostly imidazolium based ILs have been tested.27 [DBNH][OAc] was a recently introduced nonimidazolium based IL published to produce strong cellulose fibers with minimal cellulose degradation.27 The spun fibers demonstrate higher tenacities than the commercial viscose and Lyocell fibers and most of the fibers spun from imidazolium-based ionic liquids.42

Therefore, [DBNH][OAc] was chosen also for the IONCELLP fractionation. Roselli et al. showed that this IL has less appealing results compared with other ILs when used in the IONCELL-P fractionation. However, the performance of this IL with water as an IONCELL-P solvent can be enhanced by the previously mentioned pre- and post-treatments, which have not been investigated earlier for this IL. In this work, we present the upgrade of birch kraft pulp to dissolving pulp by IONCELL-P and spinning it into fibers with IONCELL-F using the same [DBNH][OAc] in both process steps.

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EXPERIMENTAL PROCEDURES Materials. Birch ECF kraft pulp from Stora Enso (Imatra mill) with Cuen intrinsic viscosity (ISO 5351-1) of 1025 mL/g, Mn = 53.3 kg/mol, and PDI = 11.7 was obtained in the form of sheets and ground by means of a Wiley mill. Deionized (DI) water was used for all IONCELL-P fractionations and washing. Tap water was used in the IONCELL-F spinning regeneration bath and for fiber washing. Sulfuric acid (95−97% for analysis, Merck 210 Millipore), cupriethylenediamine (CED) from VWR (5761.5000), 1,5-diazabicyclo[4.3.0]non-5-ene (99%, Fluorochem, U.K.), and acetic acid (glacial, 100%, Merck, Germany) were used as received. Endoxylanase (Pulpzyme HC, 1000 AXU/g) was purchased from Novozymes. A syringe filter from Sigma-Aldrich (Supelco-57183, polyethylene (PE) frit, with a pore size of 20 μm) was used for filtration of smaller batches, and a dutch twilled weave metal mesh with an 8 μm geometric pore size was used for the filtrations of bigger batches. Methods. The accuracy of digital thermometers used was ±0.1 °C. The inherent error of the balances used on large and small scale experiments was ±0.01 g and ±0.001 g, respectively. Balances used for GPC and Dionex and viscosity sample preparations were ±0.1 mg accurate. The dry matter content measuring device had an accuracy of 0.01%. The inherent errors of the GPC and sugar composition analysis and viscosity measurements are much less than the random errors of these measurements. Preparation of the [DBNH][OAc]. [DBNH][OAc] was prepared by the slow and controlled addition of equimolar amounts of acetic acid to DBN, which was stirred and first cooled in the beginning at 25 °C to divert the exothermic reaction enthalpy. Then the system is heated at 70 °C toward the end of the solvent synthesis to keep the formed IL liquid while mixing to ensure the reaction runs until completion with stirring for another hour. The structure of [DBNH][OAc] is shown in Figure 1.

Figure 1. Structure of [DBNH][OAc].

Enzymatic Pretreatment. The enzymatic pretreatment was performed based on the publication of Gehmayr et al. and Roselli et al.5,43 The enzyme was applied in 2000 U/g pulp as described by Roselli et al. The pulp was stirred in a buffer of 9 mM Na2HPO4 and 11 mM NaH2PO4 at pH 7 and 60 °C for 120 min. B


C

sample name

IONCELL-P X X·IONCELL-P IONCELL-P A A·IONCELL-P Xd Xd.A XA·IONCELL-P Xd Xd.IONCELL-P X·IONCELL-P·A X X·A X·A·IONCELL-P A

starting material

birch pulp birch pulp birch pulp X birch pulp birch pulp A birch pulp Xd X·A birch pulp Xd X·IONCELL-P birch pulp X X·A birch pulp

None 15% water + [DBNH]OAc pH = 7; 2000 U/g Pulpzyme 15% water + [DBNH]OAc 15% water + [DBNH]OAc 180 mmol/L H2SO4 15% water + [DBNH]OAc pH = 7; 2000 U/g Pulpzyme 150 mmol/L H2SO4 15% water + [DBNH]OAc pH = 7; 2000 U/g Pulpzyme 15% water + [DBNH]OAc 20 mmol/L H2SO4 pH = 7; 2000 U/g Pulpzyme 150 mmol/L H2SO4 15% water + [DBNH]OAc 75 mmol/L H2SO4

treatment 180 120 180 180 60 180 120 60 180 120 180 60 120 60 180 60

time (min) 60 60 60 60 90 60 60 90 60 60 60 90 60 90 60 90

temp (°C) 5 3 5 2 3 3 3 3 2 3 2 3 3 3 2 3

consistency (%) 25.86 7.53 19.28 5.23 6.52 24.5 5.82 18.26 16.79 4.36 18.26 5.10 4.75 20.16 19.36 4.23 25.11

xyl 74.14 92.47 80.72 94.77 93.48 74.57 94.18 81.74 83.21 95.64 81.74 94.90 95.25 79.84 80.64 95.77 74.89

cell

cellulose sample

27.54

72.46

55.29

70.20

44.71

29.80

24.77

12.69 4.63

87.31 95.37

75.23

9.45

cell

90.55

xyl

hemicellulose samples

99.3 95.5 100.0 96.7 96.8 e 91.0 85.08 94.4 91.0 82.8 82.0 95.1 95.8 89.9 93.25

tot. grav. yield (%) 1025 e 1045 1135 1226 363 453 1065 353 421 1065 e e 1012 372 452 439

visc (mL/g)

12.89

4.18

5.63

10.11c

3.59 1.59

3.10

dissolved cell. (%)

93.3

81.9

61.8

73.1

95.5 96.7 96.8

99.3

comb. yieldsb (%)

Abbreviations: expt, experiment number; X, xylanase treatment as described in Methods; A, acid treatment as described in Methods. Experiments 7 and 8 are performed on large scale with 75 g of pulp as starting material. bCombined process yields per experiment. cBased on calculated yield. dThe same xylanase treatment was performed for both experiments 5 and 6. eNot available.

a

8

7

6

5

3 4

1 2

exp.

Table 1. Pretreatments and IONCELL-P Process

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Industrial & Engineering Chemistry Research

Germany), described elsewhere.27,32 After loading the solid dope into the spinning cylinder, the dope was melted to obtain a completely homogeneous spinning solution. The spinning temperature was chosen according to the rheological behavior of each polymer solution to reach the optimum window for successful spinning, as reported earlier.33 The chosen temperature was thus different for each blend but in the same range from 65 to 73 °C. The take-up velocity was the only parameter varied during the spinning process in order to determine the maximum draw ratio applicable on the fibers. The extrusion velocity (1.6 cm3/min to 5.7 m/min), spinneret (36 holes, diameter of 100 μm, and capillary length of 20 μm), air gap distance (1 cm), coagulation bath temperature (10−15 °C), and immersion depth in the coagulation bath were kept constant. Analytical Methods. The intrinsic viscosity measurements were performed according to the standard method SCAN-CM 15:99 with the cupriethylenediamine (CED) solution of the samples in a calibrated capillary viscometer. Samples were prepared in duplicate and each measured 2 times. The molecular weight characterization was performed by gel permeation chromatography (GPC) with the method described by Borrega et al.46 Briefly, the samples were subjected to a solvent exchange sequence before being dissolved into a lithium chloride−DMAc solution and analyzed with a Dionex Ultimate 3000 system. Pullulan standards (343 Da to 708 kDa, Polymer Standard Service GmbH, Mainz, Germany) were used to calibrate the system, and the molar masses of the standards were converted based on the calculations suggested by Berggren et al.57 Sample was measured twice for the GPC analysis. The sugar composition and gravimetric yield of each sample was determined according to standard NREL/TP-510-42618. The sample material was acid hydrolyzed to carbohydrate monomers and analyzed by high performance anion exchange chromatography equipped with a pulse amperometric detector (HPAEC-PAD) (Dionex ICS-3000). The polymer content was then calculated according to the Janson formula.47 Each sample was measured twice, and the average standard deviation of sugar composition measurements was less than 0.2%. The inherent errors for data in Table 1 can be calculated based on the variables m, weight of a component (cellulose or xylan), and W, mass of dry matter. The errors

Pulp Viscosity Adjustment. Oven dry pulp was stirred for 60 min at 90 °C and 3% consistency in 20, 75, 150, or 180 mM H2SO4 (depending on the target and history of the specific samples) and then washed 3−4 times with hot DI water. These parameters were chosen each time based on a series of different times and temperatures tested. IONCELL-P Fractionation. The simple experimental setup of the IONCELL-P process is presented in more detail by Froschauer et al. and Roselli et al.5,11 In short, the birch pulp is suspended in the [DBNH][OAc]−15% water mixture and agitated in a water bath at a moderate temperature of 60 °C for 3 h. The experiments were performed with 1 or 2 g of pulp at 5 and 2 wt % pulp consistency in 50 mL Falcon tubes on small scale, sealable 100 mL jars on medium scale, and a 2.5 L stirred jacketed sealed glass reactor when treating 75 g of pulp. The extraction was then followed by filtration of the cellulose/IL−water mixture. For small samples, a syringe filter of 20 μm porosity was used. The samples fractionated at lower consistencies required a larger capacity filtration unit (200 mL), and a custom constructed nitrogen pressurized steel filtration unit with a metal sieve with a cutoff size of 5−6 μm was used. The 75 g batches were filtered with a 1 L capacity hydraulic press filtration device (1−2 MPa, dutch twilled weave metal mesh with an 8 μm geometric pore size supported with a stainless steel woven wire cloth, Gebr. Kufferath AG, Germany) in batches. After adequate washing steps as described in literature, the residual cellulose fraction was air-dried at room temperature, and the precipitated hemicellulose fractions were freeze-dried for molar mass distribution (MMD) measurements, sugar composition analysis, viscosity measurement, and further processing. The gravimetric yields of both the undissolved cellulose and dissolved hemicellulose fractions were determined. The reproducibility of IONCELL-P fractionations is typically very good. In previous studies, the average standard deviation of the cellulose or xylan content of pulps produced in triplicate fractionation experiments on small scale was 0.2% (unpublished results). Based on the previously confirmed consistency of the IONCELL-P, the experiments were all single runs. Results of large scale fractionations have been also consistent with those obtained in small scale. Dissolution of Pulp in [DBNH][OAc]. [DBNH][OAc] was first liquefied at 70 °C by means of a water bath and then mixed with the air-dried cellulose pulp to a final concentration of 13 wt %. The sticky pulp−IL mixture was then transferred to a vertical kneader system, which has been described in detail elsewhere.44 The mixture was stirred at 80 °C and 10 rpm under reduced pressure (50 mbar), and complete dissolution was observed in less than 90 min.44,45 The solution was then filtrated by means of a hydraulic press filtration device (1−2 MPa, fiber-wire composite mesh with 5−6 μm absolute fineness, Gebr. Kufferath AG, Germany) to ensure constant solution quality throughout the spinning trials. The solution was shaped to the dimensions of the spinning cylinder and solidified within 1 to 3 days after preparation. IONCELL-F (Dry-Jet Wet Spinning of Cellulose−IL Solutions). Spinning parameters were chosen based on the work of Michud et al. summarizing the influence of the MMD of cellulose on the spinability of cellulose−[DBNH]OAc and on the mechanical properties of the regenerated cellulose fibers.33 Cellulosic multifilaments were spun using a customized laboratory piston spinning unit (Fourné Polymertechnik,

Δm = ±0.01 (large scale),

Δm = ± 0.001 (small scale)

ΔW = 0.01%W

Calculated error of the portion for large scale experiments (largest errors), Δ

0.01 g × W + m × 0.01%W m ΔmW + mΔW = = 2 W W W2 0.01 g + 0.01%m = W

Shear rheology of all solutions was measured on an Anton Paar MCR 300 rheometer with a plate−plate geometry (25 mm plate diameter, 1 mm gap size).33 The viscoelastic domain was determined by performing a dynamic strain sweep test and a strain of 1%, which fell well within the linear viscoelastic regime, was chosen for the frequency sweep measurements. The edges were sealed with paraffin oil for the time of the measurements to prevent interaction of the test specimen with the surrounding humidity that could potentially affect the D


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mixture in previous publications.11,25 Performing an optimized IONCELL-P fractionation on this birch pulp using [DBNH][OAc] results in 7.5% residual xylan in the pulp fraction (Table 1, experiment 1). Multiple options were considered to improve the purity of the pulp, such as previously published fractionation enhancing pre- and post-treatments. The IONCELL-P process is a selective extraction of xylan from the pulp, and the rate of the extraction is determined by the diffusion of xylan to the liquid phase. Since the extraction is done in batch process, increasing the volume of the extraction liquid compared with the pulp (thus decreasing the consistency of the fractionation) would result in a higher Fickian driving force (csaturation − ci) for diffusion of the hemicelluloses, while also facilitating better mixing due to more favorable rheology at lower consistency. This is one option to improve the fractionation but consequently requires an undesirable increase in IL consumption. As an alternative to changing the consistency, Roselli et al. reported improved selectivity when applying a targeted enzymatic degradation of the hemicellulose in the pulp.5 In the same report, the necessary acid treatment to adjust the pulp viscosity also seemed to decrease the residual hemicellulose content of the pulp. First, to avoid the increased amount of IL used in the process, an enzymatic pretreatment was performed prior to the fractionation in experiment 2. The same Pulpzyme xylanase enzyme cocktail was used as in the work of Roselli et al.5 In the enzymatic treatment of this study the enzyme and doses were adopted from the mentioned study, and although Roselli et al.5 did not observe any significant decrease in enzyme performance between 500 U enzyme per gram of pulp and 2000 U/g of pulp doses, in the present study 2000 U/g of pulp was more efficient and achieved a notably decreased residual xylan content after the fractionation. Although not reaching the benchmark of less than 5% residual xylan in the pulp after fractionation, the residual xylan content was as low as 5.2% when applying this pretreatment. The compositions of fractions obtained from IONCELL-P combined with different pretreatments are presented in Table 1. In experiment 3, a single fractionation was performed at a lower consistency of 2% compared with the standardized 5% to justify the gain of such a modification to the IONCELL-P itself. The results clearly showed a decreased xylan content after fractionation with only 6.5% left in the pulp (Table 1). However, realistically the used IL should be kept at a minimum to avoid increased energy consumption and costs of recycling larger volumes of IL. These individual results support the idea that the combination of enzymatic predegradation of the xylan and decreased consistency in the IONCELL-P should result in sufficiently low residual xylan content. Before spinning the pulp into fibers, the viscosity of the pulp has to be adjusted to approximately 400−500 mL/g for ideal spinability and to achieve high tenacity fibers from the spinning process. As previously mentioned, a clear trend toward the reduction of the xylan content in the pulp was found upon sulfuric acid treatment to adjust the pulp viscosity. Thus, experiment 4 was designed to potentially avoid the use of costly enzyme treatments by predecreasing the viscosity of the pulp with sulfuric acid prior to an IONCELL-P performed with a low consistency (Table 1). There are data published showing the decreased selectivity resulting from a predegraded pulp’s IONCELL-P treatment.24 However, it would save a process step, thus saving time, energy, and even the cost of enzymes. When performing an acidic viscosity adjustment of the pulp before an IONCELL-P, the viscosity should be adjusted below

measurements. The samples were subjected to a dynamic frequency sweep at 65, 70, 75, 80, and 85 °C over an angular velocity range of 0.1−100 s−1 (expt 8 was not measured at 85 °C). Regenerated Fiber Characterization. The regenerated fibers were characterized for their linear density, mechanical properties, and orientation in less than a week after drying and conditioning at 23 °C and 50% relative humidity in a climate chamber. A Vibroskop 400 was used to determine the linear density (dtex) and a Vibrodyn 400 for measuring the conditioned and wet tenacities (cN/tex) and elongation at break (%). Both devices were from Lenzing Instrument and were located inside the climate chamber. The tensile measurements comprise a gauge length of 20 mm, pretension of 5.9 ± 1.2 mN/tex, and a speed of 20 mm/min according to DIN 53816. The Young’s modulus of the spun fibers was calculated from the slope of the entire elastic region of the stress−strain curves with a Matlab script (according to ASTM standard D2256/D2256M); 10 specimens were tested per sample and averaged. The level of orientation in the dry conditioned fibers was determined through birefringence measured with polarized light microscopy (Zeiss Axio Scope equipped with a 5λ Berek compensator). The birefringence Δn of the fibers was calculated by dividing the retardation of the polarized light by the thickness of the fiber, which was obtained from the linear density using a cellulose density value of 1.5 g/cm3. The total orientation of the fibers was established by dividing Δn with the maximum birefringence of cellulose 0.062.48 Three specimen were analyzed per sample, each 3 times.

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RESULTS AND DISCUSSION The main goal of this work was to utilize [DBNH][OAc] for both the IONCELL-P fractionation of birch kraft pulp and as a spinning solvent in the IONCELL-F process to produce high tenacity man-made cellulose fibers out of paper pulp. Connecting these two IL based processes harbors the potential to link the paper industry to the textile industry through a novel technology and environmentally friendly, recyclable solvent. The large scale feasibility of such processes is only realistic if the recycling of the used ILs is accounted for. Therefore, it is critical to strive for an interconnected process system where the same IL can be used throughout, thus a joint recycling system can be designed to recycle only one type of IL from both processes. IONCELL-P Fractionation. IONCELL-P fractionation studies revealed that several ILs show fractionation potential but differ in their efficiency depending on the pulp type and source.24 [DBNH][OAc] was found to have comparably moderate extraction efficiency. However, since the IONCELL-F process for the production of high-performance cellulosic fibers is based on [DBNH[OAc],27 this IL was chosen in this study as the IL to combine IONCELL-P and F. Another strong reason for using this IL is that the recyclability of this IL has been already addressed49 and is continuously being developed in recent and ongoing studies, while several publications only speculate on the recyclability of most of the research stage ILs.7−10,50,51 In this research work, we aim to decrease the residual xylan content of the birch kraft pulp below 5% to meet the standards of dissolving pulp using [DBNH][OAc] in the IONCELL-P process. The IONCELL-P process has been optimized for treatment time, temperature, and water content of each solvent E


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Industrial & Engineering Chemistry Research the final target viscosity, since the viscosity of the pulp always increases when performing an IONCELL-P due to the removal of the short chained hemicelluloses. Unfortunately, the viscosity being adjusted before the IONCELL-P requires higher concentration of acid (increased treatment time or temperature is also an option) mainly due to the presence of hemicelluloses, which protect cellulose against degradation. On the other hand, this also supports the empiric observation that performing the viscosity adjustment before the IONCELL-P breaks up cellulose−cellulose and also cellulose−xylan bonds and weakens their interaction, making the IONCELL-P itself less time and energy consuming. Incorporating this viscosity pretreatment has led to a successful decrease in the residual xylan content down to 5.8% after the IONCELL-P, which is still not sufficient without enzymes to qualify as dissolving pulp. As a conclusion, all three tested process optimizing steps (xylanase pretreatment, acidic predegradation of the pulp, and lower fractionation consistency in the IONCELL-P) should be included in the process to ensure sufficiently low average residual xylan content. When multistep process experiments were performed, noticeably lower yields were obtained due to smaller sample sizes. Small scale experiments were run with different order of treatments: In experiment 5, where preadjusting the viscosity was performed right after the enzymatic pretreatment, prior to the IONCELL-P, resulted in additional dissolved cellulose, showing up as a pulp yield loss and reduction in the purity of the recovered hemicelluloses. This should not necessarily be considered a drawback when considering the applications of the dissolved fraction, since Sundberg et al.52 and Laine et al.53 published cellulose and hemicellulose blend films performing outstandingly compared with other polysaccharide films or pure hemicellulose films. Also, the achieved residual xylan content was sufficiently low, reaching 4.4%. By connecting the processes in such order eventually some drying steps can be avoided, thus saving energy and time while preventing yield losses from additional handling of the pulp. In practice, it would not be economically feasible to include an acid degradation stage between IONCELL-P and F stages (experiment 6), even though the selectivity of the fractionation would be higher. If the IL needs to be removed prior to the IONCELL-F, then the process has very low chances of becoming economic. In practice, on industrial scale the viscosity adjustment of the pulp could be incorporated in the pulp production with reinforced cooking and bleaching stages. However, this way one is dependent on the pulp supplier. In this work, based on the small scale results, the process was scaled up using 75g of pulp as starting material with the process order of enzymatic treatment followed by acid treatment and last the IONCELL-P using 2% pulp consistency (experiment 7). Figure 2 shows the GPC curves of the pulp sample in each process step. The xylanase treatment is easy to see as the separation between the larger right peak of the cellulose and the smaller left peak of hemicellulose is becoming more distinct with the targeted xylan chopping (Figure 2a). It is then evident from Figure 2b why the acidic pretreatment is disadvantageous for the IONCELL-P because the molecular weight range that is susceptible to the IL-based extraction contains degraded cellulose chains. However, as previously discussed, the acid also has a beneficial effect of further disrupting the cellulose− hemicellulose bonds and also decreasing the hemicellulose molecule degree of polymerization. Finally, Figure 2c presents

Figure 2. Molar mass distributions of pulp before and after treatments and fractionation.

the pure cellulose fraction with no apparent hemicellulose peak after the IONCELL-P fractionation. The low yields of the scaled-up and modified IONCELL-P experiments were due to limitations in the laboratory infrastructure. This final pulp had a 4.2% residual xylan content and a higher amount of cellulose in the dissolved hemicellulose fraction. IONCELL-F Fiber Spinning. The cellulose fraction from the scaled up process series (entry 7, Table 1) was dissolved in [DBNH][OAc] as described in the experimental section and after assessment of the rheological properties of the cellulose dope, the sample was spun into fibers. The rheological behavior of the dope was measured as a function of temperature, which provided a guideline to the optimum spinning temperature range as well (Table 2). The rest of the spinning parameters were adapted from the optimum conditions used for spinning of eucalyptus and birch prehydrolysis kraft pulp described earlier.32,33 The optimum viscoelastic properties of the spinning dope were achieved at 73 Table 2. Rheology for 13% Cellulose Dope from Expt 7 and Expt 8 temperature (°C) expt 7

expt 8

F

zero shear viscosity (Pa s) G″ = G′ (Pa) ω (s−2) zero shear viscosity (Pa s) G″ = G′, (Pa) ω (s−2)

65

70

75

80

85

35691

26382

19494

15268

11650

4027 0.77 16851

4210 1.150 11753

4243 1.64 8182

4442 2.29 5771

4563 3.1

2350 0.96

2533 1.600

2602 2.4

2670 3.48


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Table 3. Mechanical Properties of the Fibers Spun from the IONCELL-P Treated and Nontreated Birch Kraft Pulp with IONCELL-F conditioned

sample expt 7 (spun at 73 °C)

expt 8 (spun at 65 °C)

a

draw ratio

average titera (dtex)

calcd diamb (μm)

12.4 14.1 15.9 17.7 5.3 9.0 10.6 12.5

1.8 1.5 1.3 1.2 3.6 2.3 2.2 1.7

13.8 12.7 11.9 11.2 19.6 15.7 15.3 13.4

elongation (%) 9.5 9.2 8.6 8.4 9.8 9.6 9.1 7.4

± ± ± ± ± ± ± ±

0.6 0.7 0.7 0.8 1.2 0.7 0.6 0.9

tenacity (cN/tex) 46.8 47.6 50.7 49.7 34.8 35.9 35.6 33.6

± ± ± ± ± ± ± ±

2.9 2.5 3.4 3.1 3.1 1.5 2.7 3.3

wet Young’s modulus (GPa) 21.8 22.5 23.6 22.7 15.0 13.9 15.8 14.9

± ± ± ± ± ± ± ±

1.6 2.0 2.0 3.6 1.6 2.5 2.3 2.8

elongation (%) 10.8 11.0 9.8 10.4 10.5 10.1 9.7 9.9

± ± ± ± ± ± ± ±

1.0 1.2 0.7 0.9 1.0 0.6 0.6 2.8

tenacity (cN/tex)

total orientation (%)

xylan content (%)

± ± ± ± ± ± ± ±

0.71 0.73 0.74 0.74 0.69 0.68 0.66 0.71

3.9 3.5 3.5 c 21.7 21.7 21.4 21.2

41.7 42.0 46.5 47.5 25.7 28.1 28.5 30.8

3.1 3.2 3.6 4.6 3.4 2.4 2.0 2.3

Average titer is calculated as an average of wet and conditioned titer. bCalculated fiber diameter. cNot applicable.

Figure 3. Schematic illustration of interconnected IONCELL-P&F with some possible IL recycling routes.

(Table 3) demonstrate comparable values with fibers spun with DR 17.7 considering their standard deviations, attesting that exceeding a certain draw there is no significant further development in the respective fiber properties.33 The final fibers spun from the pulp of experiment 7 with DR = 15.9 had as low as 3.5% residual xylan, which is slightly lower than the xylan content of the pulp produced in experiment 7 (4.2%). This is because a small fraction of xylan, depending on the molecular mass, is not regenerated during spinning. The phenomenon of xylans or lignins staying in solution of the spinning bath was observed previously by Rogers et al.55 The spinning of birch (hardwood) paper pulp into fibers has not been published before and spinning the original birch ECF kraft paper pulp directly from IL without the discussed critical pretreatments (xylanase treatment and IONCELL-P) was a relevant benchmark for this spinning experiment. The acidic viscosity adjustment of the paper pulp is necessary to have a spinable dope though. The spinning temperature was chosen always based on the rheological behavior of the individual dopes to spin in optimum conditions. The rest of the parameters (spinneret geometry, extrusion velocity, air-gap

°C, which allowed a draw ratio (DR) of 15.9 resulting in a tenacity of 50.7 cN/dtex in the conditioned state and a Young’s modulus of 23.6 GPa (Table 3). The spinability of the prepared polymer solutions and the properties of the resulting fibers are comparable to the results obtained with solutions prepared from commercial dissolving pulps.27,33 The spun fibers’ properties were comparable to the previously published fiber properties when spinning from this IL solution of cellulose, demonstrating higher tenacities than the commercial viscose and Lyocell fibers and most of the fibers spun from imidazolium-based ionic liquids.42 The increase of the draw ratio results in fibers with a lower linear density and favors the alignment of the cellulose chains along the fiber axis. Failure of the filament can occur in the coagulation bath when the tension generated within the filament, due to the increase of the draw ratio, exceeds the resistance through the elongational viscosity.54 Spinning with a draw ratio of 17.7 was not fully stable and fibers could be collected only for a few seconds. By contrast, stable conditions were observed at DR 15.9. Eventually the mechanical properties (tenacity and elongation at break) and total orientation of the fiber spun from DR 15.9 G



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ACKNOWLEDGMENTS Stora Enso is kindly acknowledged for their financial support and for the supply of birch ECF kraft pulp. The authors would like to thank Kaarlo Nieminen for his valuable comments and consultation on the data evaluation of this research work.

length, coagulation bath temperature) were synchronized with the spinning trials of the refined pulp. Table 3, summarizing the fiber spinning parameters and fiber properties, shows that the highest possible DR achieved with the untreated paper pulp was 12.6, lower than the maximum DR of the IONCELL-P purified samples. This is one reason for the fibers’ lower orientation and resulting lower strength, barely reaching 36 cN/tex in conditioned state. This is in good agreement with previously published data on spinning high hemicellulose content cellulose.56 The hemicelluloses are also shorter polymers with less tendency to orient compared with cellulose and comprise a higher fraction of the pulp now. Hemicelluloses will also readily regenerate as a blend with cellulose from ILs52 and disrupt the packed regeneration and alignment of regenerating cellulose. The relevance of removing hemicellulose from wood based pulp to achieve high quality and strong fibers is a wellestablished and critical in production of man-made cellulose fibers.56 Eventually the benefits of removing hemicellulose from cellulose pulp were confirmed, and with this experiment series, for the first time the IONCELL-P and IONCELL-F ionic liquid based processes were connected by using the same IL in both processes. The presented process concept brings opportunity for a more energy efficient recycling of the ILs. For example, the IL− water mixture in the precipitation bath of the spinning can be dewatered only to 15% water content where it can be streamed back directly to an IONCELL-P fractionation before or after purification if necessary (Figure 3).

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REFERENCES

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CONCLUSIONS The essence of this work is to report the IONCELL-P and IONCELL-F being connected for the first time by using the same IL, namely, [DBNH][OAc]. This study lays out a practical business case, where a single IL can be used in two consecutive processes, closing the loop via a joint IL recycling system. The challenges of the less favorable fractionation capacity of the [DBNH][OAc] in IONCELL-P were overcome by introducing enzymatic and acid pretreatments lowering the residual xylan content of birch kraft pulp from 26% to 4.2%, qualifying it as dissolving pulp. The final fibers spun from this pulp with IONCELL-F technology had as low as 3.5% residual xylan with tenacity of 50.7 cN/tex with 23.6 GPa Young’s modulus, which is comparable to the quality of fibers spun from commercial prehydrolyzed kraft pulps and superior to the commercial viscose and Lyocell fibers. These high tenacity and highly oriented cellulose fibers could be suitable for producing yarn for textiles or nonwovens for composites. The significance of this work lies in the potential of scaling up ionic liquid processes and outlining a new direction for forestry based textile industry.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: herbert.sixta@aalto.fi. Telephone: +358 50 3841746. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. H


Article


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