Source: https://pubs.rsc.org/en/content/articlehtml/2018/gc/c7gc02896b
Timestamp: 2019-04-21 03:03:54+00:00

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Herein, we propose a biorefinery concept for the production of man-made cellulosic fibres from waste newsprint using environmentally friendly technologies. Newsprint represents one of the most challenging ligno-cellulose substrates as it comprises mostly virtually unrefined wood pulp. Spinning dopes were prepared with pulps obtained through kraft pulping and alkaline glycerol pulping at varying intensities. The solubility of kraft deinked newsprint pulps in the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH]OAc) was not promising and resulted in solutions of poor spinnability. In contrast, dopes prepared from alkaline glycerol pulping showed promising visco-elastic properties and the spun fibres possessed high tensile properties that are comparable to commercial Lyocell fibres made from highly refined dissolving pulp. The structural features of these fibres in terms of degree of orientation, crystallinity and the moisture sorption behaviour were analyzed and are discussed.
The above mentioned drawbacks and restrictions could be bypassed through a recently developed alternative spinning technology termed Ioncell-F.18 It is based on the ionic liquid (IL) 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) as a direct solvent for cellulosic substrates allowing for the production of high-performance fibres through a Lyocell-type dry jet-wet spinning process. It involves no additional chemicals, with the ionic liquid being kept in the system through the evaporation of water from the spin bath and washing water.18–21 Furthermore, this process operates at a moderate temperature of around 80 °C and tolerates low-purity raw materials with varying amounts of non-cellulosic components. Early studies have demonstrated that not only low refined kraft pulps (containing a large amount of lignin and hemicellulose), but also recycled waste paper and board were spinnable with the resulting fibres having good to excellent mechanical and physiological properties.8 The findings from previous research also imply that the spinnability of a material depends mostly on the macromolecular integrity of the polymer matrix, and only to a minor extent on the composition of the polymer mixture.
In the study at hand, the spinnability of the newsprint was investigated for the first time. Newsprint is a complex raw material that contains a substantial amount of ink even after deinking. Moreover, the feedstock for newsprint is thermomechanical pulp (TMP).22 During the production of TMP, the wood fibre is subjected to high pressure steaming and thermomechanical stress. This may cause the lignin in the wood fibre to undergo condensation reactions in the high-temperature environment. This makes TMP more recalcitrant towards subsequent chemical refining.23 Thus, the aim of this work was to develop a suitable and environmentally friendly procedure to first convert the newsprint into a cellulosic feedstock that can be spun into high strength MMFC.
Newsprint (Helsingin Sanomat) was collected from a local recycling station in Espoo, Finland. Newsprint deinking reagents and chemicals (sodium stearate, sodium silicate, sodium chloride, sodium hydroxide and calcium chloride) were purchased from Sigma-Aldrich and used as received. Firstly, 23.88 L deionized water was preheated to 55 °C. NaCl (1 g L−1) and CaCl2 (0.1 g L−1) were added to the preheated water to enhance the water hardness and ionic strength, respectively. Subsequently, 120 g of newsprint was disintegrated by adding 4 L of preheated water (taken from the 23.88 L) in a laboratory disintegrator. After disintegration, sodium stearate (0.4 g g−1 newsprint) was added to the pulp slurry. To reach a final pH of 9.5, 1.8 g NaOH and 3 g of Na2SiO3 were added to the pulp slurry together with another 4 L of the preheated water. After mixing for 30 min, the slurry was kept without mixing for another 30 min. Finally, the slurry was added to the remaining pre-heated water and placed in a Flotation cell (Voith laboratory flotation cell Delta 25). Flotation was carried out at an air flow of 10 L min−1 and stopped when no more visual rejection could be scraped from the surface. The deinked newsprint (DNP) was then washed and air-dried for further use.
Kraft cooking of deinked newsprint (DNP) was performed in 2 L autoclaves placed in a rotary air bath digester at reaction temperatures of 130 °C and 170 °C with a liquor to wood ratio of 10 : 1. The sulphidity and alkali charge of the cooking liquor were 40% and 20%, respectively. Samples of different pulping intensities characterized by H-factors 25, 50, 500, 1000 and 1500 were taken.24 These samples will be referred to as H25, H50 etc. Detailed cooking conditions are listed in Table S1.† After kraft cooking, the black liquor was removed and the pulps were washed. The intrinsic viscosity of the pulps was determined in cupriethylenediamine (CED) according to the standard method SCAN-CM 15 : 99. The pulps were delignified, prior to viscosity measurement, using method described by Wise.25 This leads to the better dissolution of pulps in CED, which gives more reliable results.
The DNP pulp was subjected to alkaline polyol pulping (AlkaPolP)26 in oil bath reactors. Prior to cooking, glycerol and KOH were blended at 90 °C at a ratio of 10 : 1.25 to prepare the cooking liquor. This cooking liquor was added to the deinked newsprint in autoclaves. Cookings were performed at 170 °C and 180 °C. For each temperature, three treatment durations were selected: 1, 2 and 3 hours. After the treatment, the cooking liquor was removed, and the pulps were washed and air-dried at room temperature. The viscosity of the pulps was measured as described for the kraft pulps, however, without delignification.
Kraft newsprint pulps were irradiated at LEONI Studer AG, Switzerland, with a 10 MeV Rhodotron TT300 accelerator built by IBA for DP adjustment. Prior to E-beam treatment, pulp sheets (thickness is 0.15 mm for each sheet) were prepared using a laboratory sheet former. For establishing a dosage-DP relationship, the E-beam dosages were varied from 10 to 20 kGy for the different pulps.
[DBNH][OAc] was first melted at 70 °C, then blended with the air-dried pulp (ground with a Willey mill with 1 mm mesh sieves), stirred for 1.5 h at 80 °C and 10 rpm under reduced pressure (50–200 mbar) using a vertical kneader system. The polymer concentration of the dope was adjusted to 13 or 15 wt% according to the intrinsic viscosity of the pulps. The solutions were filtered through a hydraulic press filter device (metal filter mesh with 5 μm absolute fineness, Gebr. Kufferath AG, Germany) at 2 MPa and 80 °C to remove the undissolved substrate, which would lead to unstable spinning. The prepared dope was finally shaped into the dimensions of the spinning cylinder and solidified upon cooling overnight to ensure filling without the inclusion of air bubbles.
Initial attempts to dissolve and spin untreated DNP were not successful due to an inefficient dissolution of the feedstock in the [DNBH]OAc. The pronounced interactions between lignin and the polysaccharides prevent the efficient dissolution of DNP in the IL and result in an unfilterable dope with a strong gel character.27 Therefore, a suitable pretreatment is inevitable in order to disintegrate the lignin-carbohydrate matrix and increase the solubility of the individual biopolymers. As a first approach, DNP was subjected to conventional kraft pulping with a series of cooking intensities (H-factors) to identify the threshold which would allow the production of a spinnable dope. The properties of the obtained kraft DNP are summarized in Table 1.
a E-beam 20 kGy. b E-beam 10 kGy. c [η0] intrinsic pulp viscosity from delignified samples.
Both the yield and the lignin content of the kraft DNP decreased gradually as the treatment intensity increases. More pronounced delignification was observed at H500 and higher, reducing the lignin content down to 11.8%. Despite the high cooking intensity, the intrinsic viscosity of the initial kraft pulps was still too high to form spin dopes with appropriate visco-elastic properties. It should be noted that the low intrinsic viscosity of H25 and H50 DNP is an artefact attributed to the high residual lignin content which impeded efficient dissolution of the pulp in CED (solvent for the viscosity measurement). Thus, the kraft DNPs were treated with electron beam (E-beam) irradiation to adjust the degree of polymerization (DP) and the intrinsic viscosity (η0) of the kraft DNPs, respectively. E-beam irradiation has proved to be an environmentally friendly method for the DP adjustment of polymers as it prevents the utilisation of water, chemicals and avoids the high energy consumption in subsequent drying stages.28–30 Furthermore, the chemical composition remains unaltered at relatively low E-beam dosage.31 The intrinsic viscosity of all kraft DNPs after E-beam irradiation is shown in Table 1. H25 and H50 DNPs were subjected to e-beam treatment with 20 kGy. The resulting intrinsic viscosity was slightly lower than the targeted value (450 ± 40 ml g−1).19,32 Appropriate intrinsic viscosity values were obtained from H500 to H1500 DNP with an E-beam dosage of 10 kGy. The molar mass distribution (MMD) of the kraft DNPs (presented in Fig. S1 and Table S2†) was narrowed during the kraft and E-beam irradiation. Further degradation of the carbohydrates was observed during subsequent dissolution in [DBNH]OAc and regeneration of the kraft DNPs.
Solutions prepared from E-beam treated H25 and H50 kraft DNP behaved similarly to the untreated DNP. The pulp did not dissolve completely and the resulting dopes were not filterable which impeded subsequent spinning. By contrast, kraft DNP obtained with pulping intensities of H500, 1000 and 1500 dissolved fully in [DBNH]OAc at 13 wt% concentration. The resulting dopes exhibited different rheological properties (Table 2), even though they all had a similar average DP (Table 1). The zero shear viscosity of the solutions prepared from H500, 1000 and 1500 DNP decreased drastically with increasing treatment intensity. The dope prepared from H500 possessed the highest zero shear viscosity and also the rheological properties were outside the parameter range that was identified as optimum for spinning in several previous studies (zero shear viscosity 25 000–50 000 Pa s, dynamic moduli 3000–5000 Pa cross-over point of moduli 0.5–2 s−1).19,32 As a result, the dope was not spinnable and the filaments broke immediately upon any attempt to stretch them in the air gap. Both H1000 and H1500 DNP dopes showed more promising rheological properties. However, only the solution prepared from H1500 DNP exhibited good spinnability that is the filaments could withstand a high draw ratio (DR 10.6). The results suggest that the high amount of lignin in tight interaction with the carbohydrate matrix leads to an aggregated structure of the polymers in the IL-solution, causing unfavourable rheological properties. Despite the good spinnability of the E-beam treated H1500 sample, the tensile properties of the resulting fibres were only moderate, similar to viscose fibres, but not comparable to commercial Lyocell or standard Ioncell-F fibres. Thus, kraft pulping did not seem suitable for the pretreatment of waste newsprint for processing into high-quality MMCFs. Moreover, kraft pulping is not an environmentally benign process. Therefore, a greener pretreatment technology was sought in order to convert newsprint into a suitable feedstock for continuous filament spinning.
In the study at hand, DNP was first subjected to AlkaPolP treatments for preliminary assessment for 1, 2, and 3 hours at 170 °C and 180 °C, respectively. Table 3 presents the characterization of the AlkaPolP DNPs. The results clearly showed that alkaline glycerol pulping is efficient and promising for the delignification and DP adjustment requirements of the Ioncell-F process. The lignin content could be reduced to 4.3% at 170 °C and 3 h. Meanwhile, the DP was lowered to the exact value (reflect by the intrinsic viscosity, 469 ml g−1) that is suitable for the Ioncell-F process. The molar mass distributions of AlkaPolP DNPs are illustrated in Fig. S4, S5† and summarized in Table S7.† When increasing the temperature to 180 °C, the optimal pulp properties were already obtained after 1 h. Table 3 also shows that the hemicellulose content of the treated DNP was altered only a little throughout the alkaline glycerol treatment even after 3 hours which also explains the high yield at a relatively low lignin content.
These preliminary trials indicated that DP adjustment and delignification could be accomplished in only one step. Based on these results large batch cookings (at 170 °C 3 h and 180 °C 1 h) were performed to prepare suitable amounts of substrates for subsequent spinning trials. Table 4 summarizes the properties of the AlkaPolP DNPs and fibres from the selected conditions. Subsequently, the two AlkaPolP DNPs were dissolved in [DBNH]OAc to prepare the spinning dopes. A 13 wt% solution was prepared with 170 °C 3 h AlkaPolP DNP. Due to the slightly higher intrinsic viscosity, the dope concentration for 180 °C 1 h AlkaPoIP DNP was reduced to 12 wt% in order to obtain the desired rheological properties. Fig. 1 illustrates complex viscosity and dynamic moduli as a function of angular frequency. The zero shear viscosities of both dopes were higher and outside the optimal range, and the dynamic moduli and their crossover points (COP) were slightly lower than the optimum described in previous studies (η0 = 30 000 Pa s, COP = 3000–5000 Pa at 0.8–1.5 1 s−1).19,32 Thus, spinning was performed at a temperature higher than 70 °C (74 °C for the dope from 170 °C 3 h AlkaPolP DNP and 76 °C for that from 170 °C 3 h AlkaPolP DNP) to achieve the best spinning performance.
Fig. 1 Rheological properties of dopes prepared from AlkaPolP DNPs at 70 °C.
Both solutions showed an excellent spinning performance. Draw ratios (DRs) of 18 were reached without any instabilities over a long period of time during spinning. These high DRs led to microfibres with a linear density of 0.87 dtex (170 °C, 3 h, ∼8.6 μm) and 0.76 dtex (180 °C, 1 h, ∼8.0 μm), respectively. Fig. 2 demonstrates the tenacity and elongation of the spun fibres as a function of draw ratio in both conditioned and wet states. The numerical results at the max. DR are listed in Table S8.† Despite the considerable amount of hemicelluloses and residual lignin, the mechanical performance of the fibres from AlkaPolP DNP was superior to the kraft DNP fibres and comparable to those of commercial Lyocell fibres spun from a highly refined dissolving pulp. The general development of tenacity and elongation of fibres spun from both AlkaPolP DNPs is similar. However, the tenacity of fibres from 170 °C AlkaPolP DNP starts to level off at DR 5 and a slight drop was observed after DR 10. In the case of fibres from 180 °C AlkaPolP DNP, there was a significant increase in the tenacity after DR 10. The higher DP of the cellulose chains in 180° C AlkaPolP DNP, which improves the fibre tenacity even at higher DRs, could explain this. The wet tenacity was high for both fibres and followed almost the same trend as observed in the dry state. These results demonstrate that newsprint waste can be pretreated in an environmentally benign way to serve as a feedstock for high-performance man-made cellulosic fibres suitable for a wide spectrum of applications.
Fig. 2 Tenacity and elongation of AlkaPolP DNP fibres in both conditioned and wet states. Upper left: Conditioned tenacity; upper right: wet tenacity; lower left: conditioned elongation; lower right: wet elongation.
Fig. 3 shows a comparison of the stress–strain curves and the elastic moduli of the fibres from kraft DNP, AlkaPolP DNP, Ioncell-F with pre-hydrolysis kraft birch pulp (cellulose 92.4%, hemicellulose 6.3% and lignin 1.3%) and commercial viscose and Lyocell fibres. As mentioned above, the tensile strength of the AlkaPolP DNP fibres is comparable to Lyocell fibres, but with slightly lower Young's modulus. Ioncell-F fibre from birch PHK pulp reveals the best tensile strength and elastic modulus due to its high cellulose content and the perfect visco-elastic properties of the dope. The poor spinnability of kraft DNP results in both low tensile properties and elastic modulus. Because of the difference in the spinning technology (wet spinning), the mechanical strength of the viscose fibre is lower compared to those of the Ioncell-F and Lyocell fibres. However, viscose fibres are characterized by a substantially higher elongation to break, of which dry-jet wet spun fibres fall short.
Fig. 3 The tensile properties of fibres from Kraft DNP H1000 (DR 10), H500 (DR 5), AlkaPoIP DNP (DR 12), Ioncell-F from pre-hydrolysis kraft birch pulp (DR 12), and commercial viscose and Lyocell fibres. Upper: Stress strain curves. Lower: Elastic modulus.
The tensile properties of the regenerated fibres are directly connected to the orientation of cellulose in the fibre matrix, which can be assessed by determining the fibre birefringence.15,43 When the liquid filament is drawn in the air gap, extensional stress is exerted which leads to the alignment of the polymer chains along the fibre axis, resulting in an increase in the total orientation. Fig. 4 shows the total orientation (f_tot) of the spun fibres as a function of draw ratio. The degree of orientation increases quickly between DR 1 and DR 5 after which it levels off and remains at the same high value of ca. 0.70. This trend is in agreement with previous studies.43,44 The values are slightly lower than those of the Ioncell-F fibres made from dissolving pulps (ranging from 0.674 to 0.816 at DR 0.9 to DR 14) due to their high hemicellulose contents. Nevertheless, the tensile properties of the fibres can be clearly reflected by the development of the f_tot.
Fig. 4 The total orientation of fibres spun from AlkaPolP DNP.
The crystallinities of the TMP, DNP and the AlkaPolP DNP fibres were determined by XRD measurement and crystallite sizes from 110, 1–10 and 020/200 directions were calculated using Scherrer equations.45 The XRD spectra of the investigated substrates are presented in Fig. S6.† The results indicate that the crystallinity follows the course of the orientation of the regenerated cellulose fibres. The crystallinity of the spun fibres was larger than that of the DNP as a result of the alkaline glycerol pulping and the IL treatment. The crystallinity of the fibres has already reached a high level (50%) at DR 1. Only minor increases were observed following the extensional stretching. Table 5 shows the crystallinity and crystallite size of the raw materials and spun fibres. Generally, the crystallite size reflected from the 020 direction is more precise owing to the clear sharp peak ranging from 22 to 25° with a high intensity in the XRD diffractograms. The fibres from 170 °C 3 h AlkaPolP showed a larger crystallite size than that from 180 °C 1 h AlkaPolP. The crystallinity and the crystallite size at the 020 position of the AlkaPolP fibres are clearly higher than has been reported previously for the Ioncell-F fibres. A possible reason for this is a change in the amorphous component, which is modelled by the scattering from sulphate lignin. The sulphate lignin generally describes the scattering contribution of amorphous cellulose and hemicellulose well.46 The crystallinity is determined with the amorphous fitting method47 adapted for cellulose II. The amorphous fitting method is generally a reliable and robust method which gives crystallinity values that agree with NMR values for cellulose I samples, but its goodness depends on the choice of the amorphous background.47 A possible explanation for the deviation in crystallinity values is, therefore, a change in the scattering pattern of the amorphous components during the processing of the samples.
The wetting behaviour is a very important property of the fibre, in particular for textile applications. The water sorption depends on different factors, e.g. morphology, degree of orientation and chemical compositions.48–52 Thus, the fibres from both AlkaPolP DNPs were subjected to dynamic vapour sorption (DVS) measurement. Fig. 5 illustrates the moisture sorption and desorption and the corresponding hystereses of fibre samples at DR 1, 10 and 18. The moisture sorption behaviour of the AlkaPolP DNP fibres is in line with Lyocell and classical Ioncell-F fibres.48,53 No clear differences were observed between the fibres of AlkaPolP DNP from 170 °C and 180 °C. The water sorption of DR 1 fibres was slightly more pronounced than for fibres spun at a higher DR. This can be explained by the high specific surface area and low degree of orientation of the fibres. At an identical DR, the hysteresis of fibres from 180 °C was lower than that from 170 °C, especially at low relative humidity (less than 30%) at which the monolayer sorption and desorption are dominating.
Fig. 5 Equilibrium moisture sorption and desorption (upper) and their hysteresis (lower) of the fibres at DR 1, 10 and 18.
To further discuss the wetting properties of AlkaPolP DNP fibres, the monolayer hydration of the spun fibres was calculated using the Hailwood–Horrobin (HH) model.54,55 It has been shown that the main factors governing the water sorption and desorption are the lignin and hemicellulose content as well as the degree of orientation of the regeneration fibres.56 According to the previous study, the water sorption slightly increases with increasing hemicellulose content, whereas fibres are more resistant to water when the lignin content and degree of orientation increase. Table S9† presents the water sorption/desorption (presented as ML water sorption, desorption and their hysteresis), the chemical compositions and the degree of orientation of the fibres from 3 selected DRs (1, 10 and 18) of each fibre sample. In agreement with the previous work, the orientation has a significant influence on the wetting of the fibres. It was observed that the fibres from DR 1 (which represents the lowest degree of orientation) from each sample show the highest ML sorption and desorption compared to the higher DRs. The effect of lignin on the ML hydration was also reflected while comparing the two samples. The fibres from 180 °C AlkaPolP DNP, which contains a higher lignin content (4.7%), demonstrated a lower hydration rate than that from 170 °C (2.9% lignin).
Fig. 6 Yarns produced from 50% AlkaPoIP DNP fibres and 50% commercial viscose fibre. The average titer is 93.78 tex.
The Ioncell-F process is a promising technology for the conversion of low refined waste cellulosic materials to high-quality MMCFs. For the first time, the utilization of waste newsprint for the production of MMCF was successfully demonstrated. Since the pre-treatment by kraft pulping did not lead to a readily spinnable cellulose solution, a novel organosolv process, an alkaline polyol treatment, denoted as AlkaPolP, was adapted as a viable fractionation method of waste newsprint. Different to kraft pulping, both the activation of the substrate by partial lignin removal and the DP adjustment could be accomplished by AlkaPolP in just one single step, which improved the overall process economy. The resulting well-dissolved cellulose dope revealed excellent viscoelastic properties and thus spinnability. Considering the high proportion of non-cellulosic constituents, the spun fibres exhibited excellent tensile strengths both in conditioned and wet states, thus comparable to commercial Lyocell fibres made from a high-purity dissolving pulp.
DVS measurements confirmed the favourable interaction of Lyocell fibres with water. As expected, the water sorption slightly increases with increasing hemicellulose content while it decreases for fibres with a higher degree of orientation and an increasing lignin content. The capability of converting MMCFs spun from a recycled newsprint solution in [DBNH][OAc] to high-quality textile yarns could be demonstrated.
The Ioncell-F (fibre) innovation opens up new possibilities for the production of regenerated cellulose fibres of the highest quality, taking the principles of green chemistry into account.
This study is part of the “Design Driven Value Chains in the World of Cellulose” project funded by the Finnish Funding Agency for Innovation (TEKES). The authors would like to thank Rita Hataka for performing carbohydrate and molar mass distribution analyses. The authors also appreciate Pirjo Kääriäinen and Marja Rissanen for coordinating and performing the yarn processing.
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