Ioncell-F: ionic liquid-based cellulosic textile fibers as an alternative to viscose and Lyocell

Raw materials

Eucalyptus (Eucalyptus urograndis) prehydrolysis kraft pulp ([η] = 468 ml/g, DP = 1026, M_n = 79.8 kDa, M_w = 268.6 kDa, polydispersity 3.4, Bahia Speciality Cellulose, Brazil) and birch (Betula pendula) prehydrolysis kraft pulp ([η] = 476 ml/g, DP = 1133, M_n = 65.9 kDa, M_w = 269.3 kDa, polydispersity 4.1, Enocell Speciality Cellulose, Finland) were used as cellulosic solute. The pulps were delivered in sheet form and cut to a powder by means of a Willey mill.

The commercial viscose staple fibers used in this study as a reference were kindly provided by a European viscose manufacturer and used without any additional treatment.

[DBNH]OAc preparation

[DBNH]OAc was prepared by neutralization of 1,5-diazabicyclo[4.3.0]non-5-ene, DBN, (99%, Fluorochem, UK) with acetic acid (glacial, 100%, Merck, Germany). Both components were used as received. An equimolar amount of acetic acid was slowly added under external cooling due to the exothermic nature of the reaction. After addition of the required amount of acetic acid, the solution was further stirred for 1 hour at 80℃.

Dissolution of cellulose in [DBNH]OAc

Cellulose was dissolved in [DBNH]OAc by first adding slowly the dried pulp to the IL, which was liquefied at 70℃ in a water bath, and by subsequently mixing it manually until a uniform slurry of 13 wt% cellulose consistency was obtained. The resulting slurry was then transferred to a vertical kneader where temperature, pressure and shearing rate could be controlled. The dissolution was conducted at 80℃ for 90 min at 10 rpm under reduced pressure (≈100 mbar).^14,20 To assure uniform solution quality, the resulting dope was filtrated by means of a hydraulic pressure filtration unit at 80℃ (1–2 MPa, metal filter fleece of ca. 5 µm absolute pore size, Gebr. Kufferath AG, Germany) to remove any undissolved particles or impurities. The very viscous dope was then shaped into the cylinder dimension and stored at cold temperature (5–7℃) for a few days for solidification.

Dry-jet wet spinning

Cellulosic multi-filaments were spun using a customized laboratory piston spinning unit (Fourné Polymertechnik, Germany), as depicted in Figure 1. The cylinder is first charged with the solidified and shaped IL-cellulose solution, which is then heated to 70℃ in order to obtain a homogeneous viscous spinning solution. The highly viscous dope is extruded through a multi-hole spinneret at moderate temperature into a water coagulation bath via an air gap. ¹¹ Table 1 summarizes the spinning parameters used during the three demonstration runs, named DeR1, DeR2 and DeR3. The immersion depth to the first deflection roller, the deflection angle and the retention distance of the filament bundle in the coagulation bath were kept constant throughout all spinning trials. The draw ratio D_r, given by Equation (1), was varied by keeping the extrusion velocity v_e (1.6 ml/min or 5.7 m/min) constant and by increasing the take-up velocity of the godet couple v_tu (from 60 to 80 m/min). The fibers were washed offline with hot water (70℃) for 1 hour and air dried without tension, resulting in a small natural crimp. No spin finish was applied. Due to the small production scale of this laboratory-size spinning unit, multiple runs were needed to produce the required amount of fibers for each product. Within one demo run, parameters were not changed.

D r = V tu V e

(1)

OPEN IN VIEWER

Figure 1.

Schematic illustration of the dry-jet wet spinning unit used for the production of cellulosic fibers.

OPEN IN VIEWER

Table 1.

Spinning parameters of the three demonstration runs

	Tspinning ℃	Draw ratio	Tcoag. bath ℃
DeR1	75–78	10.6	13–16
DeR2	75–80	12.4	11–17
DeR3	75–80	14.1	11–15

Spinneret geometry: 36 holes, 100 µm, length/diameter ratio of the spin capillary 0.2. Air gap distance: 10 mm. Washing temperature: 70℃. Drying condition: room temperature.

Tensile test of the spun fibers

A Vibroskop-Vibrodyn system (Lenzing Instruments GmbH & Co KG, Austria) was used for the determination of the linear density (dtex) and tenacity (cN/tex) and elongation at break (%), measured in both conditioned (23℃, 50% relative humidity (RH)) and wet states. Ten fibers per sample were tested. In the tensile measurements, the gauge length was 20 mm, pretension 5.9 ± 1.2 mN/tex and speed 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.

Birefringence measurement

The orientation of the spun fibers was determined using a polarized light microscope (Zeiss Axio Scope) equipped with a 5λ Berek compensator. The birefringence Δn of the fibers was obtained by dividing the retardation of the polarized light by the thickness of the fiber, which was calculated from the linear density using a cellulose density value of 1.5 g/cm³. The total orientation factor f_t was determined by dividing Δn by the maximum birefringence of cellulose 0.062. ²¹

Scanning electron microscopy

Scanning electron microscopy (SEM) imaging of the Ioncell fibers and viscose/Lyocell fibers were taken with a Zeiss Sigma VP with variable pressure at 3 kV operating voltage. The cryo-fracture was performed by dipping a fiber bundle into liquid nitrogen, fracturing it and gluing it vertically onto the conductive support. The samples were sputter-coated with a gold layer to enhance the electric conductivity.

Yarn production

The spun endless filaments were cut to staple fibers of 37 mm for DeR1 and 41 mm for DeR2 and DeR3, opened and conditioned at 21℃ and 62% humidity for 72 hours for DeR1 and at 23℃ and 50% for 72 hours for DeR2 and DeR3. The conditioning of the fibers is essential to minimize electrostatic charges, which can cause a lack of inter-fiber cohesion from the action of repulsion of electrostatic forces during yarn processing. The conversion of staple fibers into two-ply yarns was carried out at the Swedish School of Textiles using a Mesdan lab mini spinning line comprising a Laboratory carding machine, a Stiro-Roving-lab, and a Ring-Lab Fibres. The plying and winding of the yarn were performed with an AGTEKS DirectTwist-2A. Two rounds of carding were performed, differing by the orientation of the fiber/sliver feeding. The second carding is done by feeding back the folded fiber sliver, which was collected from the cylinder, to the card perpendicular to the first round. The sliver was afterwards rolled to form a tube and fed to a drafting machine. The sliver was first drafted once, then doubled and drafted again. Finally two of the double-drafted slivers were combined, drafted together and false twisted to form a roving, which was fed to a ring spinning unit. A single-Z twist direction yarn was spun by using a 35 mg ring and a two-ply yarn (ply S-on-Z) was subsequently produced by inter-twisting two yarns. Table 2 provides the fiber batch size and the ring spinning and plying parameters.

OPEN IN VIEWER

Table 2.

Yarn spinning parameters for viscose and Ioncell fibers

		Viscose	DeR1	DeR2	DeR3
Batch size (g)		25	25	25	38
Ring spinning	Preliminary drafting	2.4	2.4	1.5	1.5
	Total draft	36.6	32.2	26.3	40.1
	Z-Twist/m	596	595	597	602
Plying	S-Twist/m	250	250	250	300
Yarn count	tex	31.5 tex × 2	27.0 tex × 2	25.5 tex × 2	49.0 tex × 2

For reference purposes, a yarn from commercial viscose fibers was spun with the same equipment in order to compare the performance of the IL-based fibers during yarn manufacture and the final yarn properties to commercial fibers.

Tensile test of the spun yarns

The yarn count was determined by measuring the weight of two-ply yarn samples. The tensile parameters have been determined by means of a Mesdan lab electromechanical tensile testing machine with 100 N load cell and pneumatic grips at 23℃ and 50% RH. Yarn strength was measured by using the ISO 2062 tensile protocol. An elongation rate of 250 mm/min and a preload of 0.25 cN/tex were applied on ten 250 mm samples per yarn. The coefficient of variation of mass CV in percentage was also determined.

Yarn dyeing

The produced yarns were dyed before the knitting and weaving process. Remazol reactive dyes manufactured by Dystar were used for this purpose. The yarns were dyed in a batch process at 40℃ in an alkali solution, including the dissolved dye, sodium sulfate and sodium carbonate. Afterwards, the yarns were rinsed with cold water, neutralized at pH 6 using acetic acid and vinegar and finally boiled to wash away the dye remains. The dyed yarns were finally dried at room temperature.

Fabric manufacture

The resulting viscose and Ioncell yarn from DeR1 and DeR2 were knitted with a Stoll CMS 340 TC-L multi-gauge flatbed knitting machine at the Department of Design of the School of Arts, Design and Architecture, Aalto University, into a scarf and a dress, respectively. The yarn from DeR3 was woven into a bowtie and pocket handkerchief by means of a handloom Toika. The scarf (DeR1) and the bowtie/pocket handkerchief (DeR3) were designed by two textile design students from Aalto University, while the dress (DeR2) was designed by Tuula Pöyhönen from the Finnish design company Marimekko®.

Spun fiber properties

Multi-filament bundles of eucalyptus (DeR1) and birch (DeR2 and DeR3) prehydrolysis kraft pulp solution were extruded at 75–80℃, collected from the take-up godet, cut into staple fibers, washed at 70℃ and dried at room temperature. Contrary to the second and third runs, the fibers were cut after drying at room temperature in the first demonstration run. Upon drying, the fibers gain a slight natural crimp. Thus, cutting before drying reduced the variation in length notably. Figure 2 shows the linear density and the tensile properties of the spun fibers from the three demonstration runs, as well as of commercial viscose fibers. Figure 3 depicts the measured stress–strain curves and demonstrates the strength superiority of the Ioncell fibers over the commercial viscose fibers. The process stability from the first to the third demonstration run could be improved notably and enabled the increase of the draw ratio from 10.6 to 14.1. This was mainly a result of the optimization of parameters and procedures associated with the laboratory scale of the spinning unit. Charging the cylinder with the dope as one piece avoided the intrusion of air bubbles and improved spin stability. Lowering the deflection roll gave the filament slightly more time to solidify and gain strength before meeting the first point of stress. This increased the spinnability and maximum draw. Consequently, a reduction in the linear density and a slight increase of the tenacity of the resulting fibers were reached. This is explained by the orientation of the cellulose molecules, which is promoted by increasing the draw of the filaments. By aligning cellulose chains along the longitudinal axis of the fibers, the molecules are packed closely, maximizing cohesive forces between the cellulose chains. The increased crystallinity results in higher tenacity and smaller elongation, which decreases as the molecules are not able to slide alongside each other anymore. ²² The Ioncell fibers present superior tenacities and thus lower elongation than the commercial fibers with tenacities over 45 cN/tex and elongation around 10%. The Young’s modulus, which is a measure of the stiffness of a material, is defined as the resistance of a fiber against deformation. The Ioncell fibers demonstrate Young’s moduli three times higher than the viscose fibers, as shown in Table 3. The orientation of the cellulose chains in the Ioncell fibers was quantified by birefringence measurements. The birefringence values and the calculated total orientation of viscose and Ioncell fibers are summarized in Table 4. The high values of the Ioncell fibers explain the higher tenacity and Young’s modulus. The cross-sectional structure of the spun Ioncell fibers has been characterized by SEM and is illustrated in Figure 4. The observed morphology is similar to the Lyocell-type fiber that is represented by a smooth fiber surface, round cross-section, and homogeneous and dense fibrillar structure. ⁸ Furthermore, it is expected that the Ioncell fibers would behave similarly toward mechanical abrasion as Lyocell fibers and tend to fibrillate due to the high orientation of the cellulose chains. However, this structural aspect of the Ioncell fibers will not be addressed in this study. OPEN IN VIEWER

OPEN IN VIEWER

Figure 3.

Stress–strain curves of viscose and Ioncell fibers.

OPEN IN VIEWER

Figure 4.

Scanning electron microscopy images of viscose (a), Lyocell (b) and Ioncell-fiber from DeR1 (c). The insets in the left corners show a 2.8 and 2.5 times magnification of the body of the viscose and Ioncell fiber, respectively.

OPEN IN VIEWER

Table 3.

Properties of the Ioncell and commercial viscose fibers

	Viscose	DeR1	DeR2	DeR3
Linear density (dtex)	1.8 ± 0.1	1.9 ± 0.3	1.7 ± 0.2	1.5 ± 0.2
Cond. a tenacity (cN/tex)	20.7 ± 1.5	46.3 ± 3.2	48.3 ± 4.6	45.6 ± 4.7
Cond. elongation (%)	19.2 ± 2.2	9.4 ± 1.1	8.8 ± 1.0	8.9 ± 1.7
Cond. Young’s modulus (GPa)	6.9 ± 0.5	23.4 ± 3.5	20.9 ± 3.1	20.6 ± 3.7
Wet tenacity (cN/tex)	7.5 ± 1.3	42.6 ± 3.6	44.1 ± 4.6	42.4 ± 3.6
Wet elongation (%)	16.5 ± 1.3	11.7 ± 1.3	11.6 ± 1.2	11.3 ± 0.7
Fiber length (mm)	40 ± 1.0	37 ± 1.0	41 ± 1.0	41 ± 1.0

a

Conditioned.

OPEN IN VIEWER

Table 4.

Birefringence measurements of commercial viscose fibers and Ioncell fibers

	Viscose	DeR1	DeR2	DeR3
Δn	0.0264 ± 0.0033	0.0419 ± 0.0026	0.0453 ± 0.0020	0.0420 ± 0.0048
ftotal	0.425 ± 0.054	0.676 ± 0.042	0.730 ± 0.035	0.677 ± 0.078

Yarn spinning

Ring spinning

For successful yarn spinning, certain specifications are required such as the yarn count, which is the weight per unit length of yarn, and the level of twists. Prior to twist insertion, the roving was drafted once again in two phases to straighten and arrange the staple fibers with a high degree of parallelism. The position of the roving had to be optimized to be fed to the machine at constant speed and, thus, avoid roving breaks. Due to the sleekness of the Ioncell fibers, the Ioncell roving had the tendency to fall off from the bobbin during DeR1. The drafting zone had to be shortened for the Ioncell fibers in DeR1 compared to the viscose fibers, which were longer. Furthermore, the total draft applied to the viscose and Ioncell fibers was different (see Table 2) to compensate the thickness variation of the roving after carding and drafting. Thicker roving requires higher draft. A single-twist Z-direction was applied to the Ioncell and viscose fibers during the ring process spinning and the number of twists inserted per meter was about 600 (see Table 2). The ply-twist S-direction was applied when plying two yarns and the twist per meter was between 250 (viscose, DeR1 and DeR2) and 300 (DeR3). Rigidity of the fibers and cohesion of the fiber network are important properties for successful ring spinning. The two-ply yarns’ characteristics were first evaluated by determining their mechanical properties, which are summarized in Table 5. The tensile properties of yarns are highly dependent on the length, fineness, strength and extension of the fibers. As shown in Table 3, the Ioncell fibers are twice as strong as the viscose fibers, but show greater brittleness with an elongation at break of less than half the value of viscose fiber. A similar trend is therefore observed between the Ioncell and viscose yarn, which demonstrates half of the strength of the IL-based fibers. Moreover, an estimation of fiber count in yarns was calculated by dividing the linear density of the yarn by the linear density of the staple fibers. The slight increase in the fiber count per cross-section between DeR1 and DeR2 seems not to enhance the resulting tenacity, as mentioned in the literature. ²³ The yarn produced in DeR3 was intended for weaving. The linear density of the two-ply yarn was doubled in order to resist the substantial tension applied to the yarn during the weaving process. The coefficient of variation (CV, %) gives an idea on the irregularity of the spun yarn, which is greater at higher CV. However, since the yarns differ in fiber count, the irregularity index I, given by Equation (2), allows for a better comparison.

I = CV CV lim

(2)

with CV_lim being the limit coefficient of variation according to the Martindale formula:

CV lim = 1 n * 100 %

(3)

with n being the fiber count.

OPEN IN VIEWER

Table 5.

Properties of the Ioncell and commercial viscose two-ply yarns

	Viscose	DeR1	DeR2	DeR3
Yarn count (tex)	31.5 tex × 2	27.0 tex × 2	25.5 tex × 2	49.0 tex × 2
Tenacity (cN/tex)	17.3 ± 1.6	34.4 ± 3.8	33.2 ± 5.1	28.5 ± 2.8
Elongation (%)	18.2	7.4 ± 0.7	7.5 ± 0.7	7.9 ± 0.4
CV (%)	9.1	13.6	15.4	9.8
Fiber count/yarn cross-section	418	286	299	661
Irregularity index I	1.86	2.29	2.66	2.51

It should be noted, though, that this index does not take into account the different fiber length or cross-section. Yarn irregularities come from the variation of the number of fibers in the yarn cross-section and fiber fineness or length distribution. The higher CV and I of the Ioncell yarn can be explained by the greater variation of the fiber fineness arising from the processing fluctuation in laboratory scale and by the fiber length variation due to the cutting of the filament to staple fibers done by hand.