Comparison of Soda, Kraft, and DES Pulp Properties of European Black Poplar

Kraft pulping as the dominant pulping method contributes to several environmental problems. To overcome these problems, environmentally friendly pulping methods have been investigated. In the last years, deep eutectic solvents (DESs) have been identified as up-and-coming reagents in the lignocellulosic material processing and they are characterized as environmentally friendly. This study investigated the use of DES in pulp production from European black poplar chips. The DES mixture was prepared from choline chloride (ChCl) and ethylene glycol (EG). In addition, traditional soda and kraft pulping methods were carried out with poplar chips for comparison with the DES pulps. It was found that pulp production from poplar chips using DES was comparable to the soda and kraft pulps in terms of pulp yield, pulp viscosity, and opacity. The DES pulps easily reached target pulp freeness levels. However, the strength properties and brightness of the DES pulps were lower than those of the soda and kraft pulps. The strength properties of DES pulps can be improved with paper strength enhancers such as starch and micro or nanofibrillated cellulose. Also, the utilization of DES in pulp production may have an important role in cleaner production and it represents a greener alternative to traditional pulp production methods.

Deep eutectic solvents (DESs) consist of a mixture of at least two components: a hydrogen-bond acceptor (HBA) and a hydrogen-bond donor (HBD) (Pena-Pereira and Namieśnik, 2014). They are non-toxic, eco-friendly, easily prepared, inexpensive, readily available, biodegradable, and recyclable green solvents. Because of these extraordinary advantages, interest in DESs continues to grow (Zhang et al., 2012). The usage potential for DESs in organic synthesis, electrochemistry, catalysis, and biology has been studied (Škulcová et al., 2016). In addition, DESs have been extensively used in the field of separation technologies (Hou et al., 2018). The solubilizing capacity of DESs on lignocellulosic biomass or its individual components such as lignins was tested by Francisco et al. (2012). Since then, studies related to biomass processing using various DESs  A number of studies have focused on DES treatment using several types of lignocellulosic biomass. De Dios (2013) studied lignin isolation from wheat straw and pine sawdust using several deep eutectic mixtures and reported that the lignin solubility was increased with the higher content of lactic acid in DES. Abougor (2014) pretreated switchgrass with ChCl/trifluoroacetamide and lignin content was reduced by 6.66 %. In addition, they noted that pretreatment did not cause a reduction in cellulose and hemicellulose content. The wheat straw was pretreated with ChCl based DESs and containing different HBDs such as urea, lactic acid, malic acid, malonic acid, and oxalic acid dehydrate by Jablonský et al. (2015). The highest lignin re-moval was 57.9 % with ChCl/oxalic acid dihydrate at 60 °C and 24 h. Kumar et al. (2016) investigated solubility of cellulose, xylan, and lignin from rice straw in DESs containing ChCl, betaine, and lactic acid. Their experiments revealed that xylan and cellulose are not soluble in DESs. The lignin solubility in DES consisting lactic acid/ChCl reached almost 100 %. The lignin removal in the lactic acid/betaine (5:1) was 38 %. Alvarez-Vasco et al. (2016) used ChCl based DESs for lignin extraction from Douglas-fir and poplar wood. The lignin amounts removed from biomass with ChCl/ lactic acid treatment were 78 % in poplar and 58 % in Douglas-fir. The isolation of willow lignin with the treatment of DESs (ChCl/lactic acid, ChCl/urea, ChCl/ glycerol) was evaluated by Li et al. (2017). Optimal DES-lignin yield (91.8 %) was obtained at a ChCl/lactic acid molar ratio of 1:10, extraction time of 12 h, and temperature of 120 °C. Pan et al. (2017) focused on the effects of DES (ChCl/urea) pretreatment on holocellulose, α-cellulose, and acid-insoluble-lignin contents of rice straw. They observed that ChCl/urea had a selective delignification. Lynam et al. (2017) noted that DESs (lactic acid/betaine, lactic acid/ChCl, lactic acid/ proline, formic acid/ChCl, and acetic acid/ChCl) were capable of selectively dissolving the lignin at 60 °C. Zulkefli et al. (2017) noted that the pretreatment of oil palm trunk with ethylammonium chloride/EG had removed 42 % lignin and 83 % hemicellulose. Hou et al. (2018) also reported that DES consisting of ChCl/urea could effectively delignify from the rice straw. Chen and Wan (2018) noted that lignin was recoverable with high purity after microwave-assisted DES pretreatment of Miscanthus, switchgrass, and corn stover. Kiliç-Pekgözlü and Ceylan (2019) extracted the Scots pine wood with several DESs. They found that DESs could be alternative solvents for organic solvents. Recently, the effects of DESs (ChCl/lactic acid and ChCl/glycerin) treatment on the chemical composition of the sapwood and heartwood of red pine were investigated by Kwon et al. (2020). They observed that the solid residue yield after DES treatment decreased with increasing HBD concentration and treatment time. In addition, the solid residue amount in the sapwood was higher compared to the heartwood.
DESs have potential applications in the pulp and paper industry. Choi et al. (2016a and 2016b, respectively) investigated the effects of DES treatment of thermomechanical pulp (TMP) and bleached chemithermomechanical pulp (BCTMP) on handsheet properties. Majová et al. (2017a) reported the effect of initial kappa number of kraft pulp on the DES pulp delignification efficiency, and determined that kraft pulp having higher kappa number was more easily delignified with DES. A recent study found that DES could be replaced by oxygen in kraft pulp delignifica-tion (Majová et al., 2017b). The hardwood kraft pulp was delignified using two different DESs (ChCl/lactic acid and alanine/lactic acid) and the effects of DES delignification on the chemical and physical properties of the kraft pulp were investigated (Jablonsky et al., 2018). The potential of potassium carbonate/glycerol (K 2 CO 3 /Gly) DES applied as a green solvent in rice straw pulping was evaluated by Lim et al. Although several DESs have been extensively studied for pretreating biomass, the literature available regarding the use of DESs in pulp production is limited. To the best of our knowledge, to date, no investigation has been carried out comparing DES pulp production with traditional pulping methods. Therefore, the aim of this study was to evaluate the usage possibilities of a DES (ChCl:EG) in pulp production from poplar wood and to compare DES pulping with traditional pulping methods. The effects of different ChCl:EG molar ratios (4:10, 5:10, 6:10) were also investigated in this study.

Materijal
European black poplar (Populus nigra L.) was chosen as the wood material because it has a rapid growth rate and provides easier delignification compared to softwoods. A 10 cm-thick wood disc was taken at breast height from a poplar log originating from Bartın Province (Turkey). This disc was debarked and subdivided into four discs (25 mm-thick). These were manually chipped, using a chisel, as homogeneously as possible to 25 mm × 15 mm × 5 mm in size for pulping.

Chemical composition and fiber morphology of poplar wood 2.2. Kemijski sastav i morfologija vlakana topolovine
The chemical analysis of poplar wood was carried out according to TAPPI T 257 cm-02. The klason lignin content (TAPPI T 222 om-02), α-cellulose content (TAPPI T 203 cm-09), and holocellulose content (Wise and Karl, 1962) of the poplar wood were determined according to the relevant methods. The cold-hot water, ethanol, and 1 % NaOH solubilities of the poplar were also determined according to TAPPI T 207 cm-99, TAPPI T 204 cm-97, and TAPPI T 212 om-02, respectively. In addition, poplar wood chips were macerated with the chlorite method (Spearin and Isenberg, 1947). After maceration, the fiber length (L) and width (D), lumen width (d), and cell wall thickness (w) of fibers were measured. The slenderness ratio (L/D), flexibility ratio [(d/D) × 100], and Runkel ratio [(2 × w)/d] were calculated from the dimensional measurements of fibers.

DES preparation 2.3. Priprema DES-a
DES was prepared by mixing ChCl with EG. All chemicals were acquired commercially (Merck) and used as received. The ChCl and EG were mixed in different mole ratios (4:10, 5:10, and 6:10) and used as DES. The solution was heated at 100 °C for 60 min. until a transparent liquid retaining no solid particles was formed. The mixture was stored in a desiccator until use after being cooled to room temperature.

DES and traditional pulping 2.4. DES i tradicionalna proizvodnja celuloze
DES and traditional pulping conditions are shown in Table 1. In DES cookings, the oven-dried (o.d.) poplar chip weight was calculated for each cooking experiment using the ChCl/EG molar ratio and cooking liquor/chip ratio. In DES-1 (4ChCl/10EG), 558.48 g ChCl (ChCl molecular weight × 4) and 620.7 g EG (EG molecular weight × 10) were used. The total weight of ChCl and EG was 1179.18 g. The o.d. poplar chip weight in the 2.5/1 cooking liquor/chip was 471.73 g (1179.18/2.5). According to the same calculation, 527.52 g and 583.37 g o.d. poplar chips were used in DES-2 (5ChCl/10EG) and DES-3 (6ChCl/10EG) cooking experiments, respectively ( Table 1).
The air-dried poplar wood chips were cooked in a rotary digester. After cooking, the DES pulps and traditional pulps were washed to remove the black liquor with tap water. The DES pulps were also washed in ethanol. All pulps were disintegrated in a laboratorytype pulp mixer with 2-L capacity. The pulps were screened with a Somerville-type pulp screen according to TAPPI T 275 sp-02. After screening, all the pulp samples were beaten to 25 °SR and 35 °SR in a Valley Beater according to TAPPI T 200 sp-15.

Statistical analysis 2.6. Statistička analiza
The data related to properties of the DES, kraft, and soda pulps from poplar chips were analyzed using analysis of variance (ANOVA) and the Duncan test at a 95 % confidence level (p <0.05). The effects of the methods and conditions of pulping on the paper properties were evaluated statistically using SPSS software. In Figures 2-8, the same letters on the columns denote no statistically significant differences between the groups. In addition, there were no significant differences among the values with the same letters in the same column of Table 3 and Table 4.

REZULTATI I RASPRAVA
The results of the chemical composition analysis and fiber morphology of the Populus nigra wood are presented in Table 2. These results are similar to those of Populus tremula.
The pulp properties of DES, soda, and kraft pulps are presented in Table 3. The screened yield of the DES pulps was higher than that of the soda and kraft pulps. The highest screened yield was obtained from DES-3 pulp. The screened yields of the DES pulps after washing with ethanol were similar to those of the traditional pulps ( Table 3).
The effect of ChCl molar ratio on kappa number of DES pulp was insignificant (p >0.05). The kappa numbers of DES pulps were higher than for the traditional pulps (p <0.05). This result can be ascribed to the insufficient delignification of DES pulping compared to the traditional pulping methods. Alvarez-Vas-   The tensile index of the unbeaten and beaten DES pulps was significantly lower (p <0.05) than that of the traditional pulps ( Figure 2). In the unbeaten, 25 The stretch values of the unbeaten DES pulps were higher than those of the traditional pulps, whereas the stretch values of the beaten DES pulps were lower than those of the traditional pulps (Figure 3, p <0.05). In the unbeaten pulps, the highest and the lowest stretch values were obtained from DES-2 pulp and soda pulp as 1.37 % and 0.85 %, respectively. On the other hand, the stretch values of the unbeaten and 25 °SR DES pulps were irregularly affected by the ChCl amount in the DES cooking liquor. In the 35 °SR pulps, the effect on stretch of the ChCl amount in the DES cooking liquor was statistically insignificant (p >0.05). Stretch values of the DES pulps, as for the traditional pulps, were significantly increased with beating (p <0.05) ( Table 4).
In the unbeaten pulps, the highest TEA value was 28.86 J/m 2 (DES-2 pulp). In the 25 °SR and 35 °SR  (Table 4). However, the effect of beating on TEA was more pronounced in the traditional pulps.
In the unbeaten and beaten pulps, the tear index values of the DES pulps were lower than those of the soda and kraft pulps (p <0.05). The highest tear index values of the DES pulps were determined in DES-3 pulp samples. At all pulp freeness levels, the effect on the tear index of the ChCl amount in the DES cooking liquor was statistically insignificant ( Figure 5, p >0.05). In terms of the tear index, the response of DES pulps to beating was different from that of traditional pulps. The tear index increased when the traditional pulps were beaten up to 25 °SR. With increasing beating levels, their tear index values decreased. In contrast, the tear index values of the DES pulps regularly decreased with increasing beating levels ( Table 4).
The burst index values of the unbeaten samples of DES-2, soda, and kraft pulps were 1.54, 1.38, and 1.62 kPa·m 2 /g. The burst index values of the DES pulps were lower than those of the soda and kraft pulps except for the unbeaten DES-2 pulp ( Figure 6). The burst index values of the unbeaten and 35 °SR pulps varied irregularly with increasing ChCl amounts in the cooking liquor. However, the decrease in the burst index with increasing ChCl amount was insignificant (p >0.05). The relationship between the burst index of DES pulps and increasing pulp beating levels was linear (p <0.05) ( Table 4). However, the tensile index increases after pulp beating were more pronounced in the traditional pulps. At all pulp freeness levels, the DES pulp had lower brightness values compared to soda and kraft pulps ( Figure 7) because of the higher kappa numbers and insufficient delignification of the DES pulps (Table  3). Pulp brightness was significantly reduced with the increase of ChCl in the DES (p <0.05). In addition, the brightness of DES-1 pulp was reduced with beating, whereas for DES-2 and DES-3 pulps, the changes were irregular ( Table 4). As expected, the brightness of kraft and soda pulps was reduced with beating. This result can be explained by the homogeneous lignin distribution of the DES fibers in the cell walls.
In the unbeaten and beaten samples, the DES pulp exhibited higher opacity values compared to the soda and kraft pulps (Figure 8). The effect of the amount of ChCl in DES on pulp opacity was statistically insignificant (p >0.05). Although the opacity of the traditional pulps changed with beating (p <0.05), the opacity of the DES pulps did not change (p >0.05) ( Table 4). This experiment demonstrated that, compared to traditional pulping methods, DES pulping had a negative effect on pulp brightness, although it had a positive effect on pulp opacity. Choi et al. (2016a) noted that lactic acid and betaine DES treatment had no effect on the optical properties of TMP pulp. An in-  applications in the pulp and paper industry. The novelty of this study is the utilization of DES in pulp production and comparison of traditional pulps and DES pulps. The results of this study showed that the use of DES was an effective method for the pulping of poplar lignocellulosic biomass (Populus nigra L.). The DES formed by ChCl and EG (molar ratios = 4:10, 5:10, 6:10) applied at 190 °C for 3.5 h enabled pulp production from poplar chips. The DES pulps were comparable to those produced by traditional pulping methods in terms of pulp yield, pulp viscosity, and opacity. The DES-1 pulp exhibited the best cooking conditions in terms of total pulp yield. On the other hand, the beata- *There were no significant differences among the values with the same letters in the same column. / Nema značajnih razlika među vrijednostima s istim slovom unutar istog stupca.

ZAKLJUČAK
bility of the pulp was positively affected by the DES pulping. However, the strength properties and brightness of the DES pulps were lower than those of the traditional pulps. In the unbeaten and beaten DES pulps, the highest strength values were obtained from the DES-2 and DES-1 pulps. This study demonstrated that DES composed of ChCl and EG can be used as green solvent for pulp production from biomass. It can be readily applicable to pulp production. The DES pulping process is an alternative to traditional pulping due to its low-environmental-impact. Inexpensive and biodegradable DESs, used in pulping, are characterized as economically and environmentally viable solvents. These solvents could offer unique opportunities for cleaner pulp production. Therefore, in order to reveal true potential of DESs and to improve pulp properties, further research is needed on the use of DESs as green solvents in pulping.