Structural characteristics of new and conventional regenerated cellulosic fibers

ABSTRACT

Comparative investigations of the new lyocell and conventional viscose and modal fibers attempt to explain the reasons for differences in the molecular and fine structure of these fibers. This research is a systematic analysis of structural characteristics (molecular and fine structure) and their influence on fiber properties. The analysis shows that lyocell fibers consist of longer molecules, they have a greater degree of crystallinity, their slighter but rather longer crystallites are oriented in the fiber axis direction, and their void structure is similar to that of viscose fibers. Differences in the molecular and fine structure of these fibers cause different mechanical and sorption properties. Good mechanical properties are a function of the structure of lyocell fibers, especially with the highest orientation factor and crystallinity index. Sorption properties place lyocell fibers somewhere between viscose and modal fibers. Our results demonstrate that the adsorption properties of cellulosic fibers depend, except for the less ordered amorphous regions, predominantly on the void fraction.

Cellulose is a raw material with a wide variety of uses in the chemical industry for producing man-made textile fibers. Commercial methods of manufacturing man-made cellulosic fibers include viscose, cuprammonium, and several new alternative processes. Conventional regenerated cellulosic fibers are generally produced by the indirect viscose process (viscose fibers), while high-tenacity modal fibers are produced using a modification of the basic procedure. Viscose production is based on deriving cellulose with carbon bisulphide [8]. Only recently have new processes appeared, mainly due to considerable environmental problems with the viscose process. New regenerated cellulosic fibers-lyocell fibers-are produced by a more environmentally friendly procedure from a solution of nonderivative cellulose in a solvent spinning process, where the cellulose is dissolved directly in the organic solvent N-methylmorpholine-N- oxide, without the formation of derivatives [1, 3, 7, 16]. The special properties of lyocell fibers-high strength and comparatively low elongation-led to early structural tests, which have shown that these fibers are distinguished by their high degree of crystallinity and molecular orientation in comparison with viscose fibers. This special property is reflected in high wet fiber strength and, contrary to expectations, in sorption properties similar to those of viscose fibers. Different production processes and therefore different production conditions for conventional viscose, modal, and new lyocell fibers cause differences in the structure of the fibers despite the same chemical composition. Different properties of viscose and lyocell are due to structural differences, including the degree of crystallinity, crystallite dimensions, and the orientation of noncrystalline cellulose chain segments [10].

We analyze the structural characteristics of new lyocell fibers and compare them with conventional viscose and modal fibers. We investigate both the molecular structure (density, degree of polymerization, molecular mass) and the supermolecular structure (crystallinity index, crystallite dimensions, molecular orientation, structure of the voids) of regenerated cellulose fibers, and we determine the influence of their structural parameters on their mechanical properties. In addition, we determine the structural parameters (amorphous regions and void fraction) that significantly influence the water and dye adsorption properties of the fibers.

Influence of Structure on Cellulosic Fiber Properties

All regenerated cellulosic fibers have the same chemical composition, yet they differ in density, molecular mass, degree of polymerization, supermolecular arrangement, and above all, their degree of crystallinity and orientation. Thus, those portions of the less ordered amorphous regions where the adsorption processes take place must have a significant influence on the adsorption properties of these fibers. Neither water nor aqueous solutions (e.g., dyestuffs) can penetrate the crystalline regions of the fibers. The void system plays the decisive role in all cellulose heterogeneous chemical reactions. The volume and inner surface areas of the voids decisively affect the accessibility, reactivity, and adsorption properties of fibers. The void system, however, has little influence on mechanical properties; rather, it is the shape of the voids and their orientation that prove essential. The shape of voids is an innate property of the spinning process and forms during crystallization of the elementary fibrils, not later during stretching [20]. The differences between particular kinds of regenerated cellulose fibers are, above all, in the size of their crystallites and amorphous regions, amorphous and crystalline orientation, size and shape of the voids, and the number of interfibrillar lateral tie molecules.

A physicochemical peculiarity of cellulose is its strong sorption power and high sorption capacity. The accessibility of cellulose to water, and consequently to dyes dissolved in water, varies with the size and distribution of the crystalline (ordered) and amorphous (disordered) regions and the connecting regions of low order. To characterize cellulose accessibility, interactions with water are often used because water is able to destroy weaker hydrogen bonds but cannot penetrate into the regions of high order. The water retention value, obtained by measuring the amount of liquid water retained by the swollen fiber under defined conditions, can be considered a valuable accessibility criterion for cellulosic fibers [12]. Measuring accessibility involves either measuring the equilibrium adsorption of some species (water, dye, iodine) or the rate at which cellulose is attacked by the chosen reagent.

The mechanical properties of cellulosic fibers are usually centered on criteria relevant to textile end-uses, for example tenacity or elongation at break. Tensile strength [sigma], elongation at break [epsilon], modulus, and above all the function [sigma] = [function of]([epsilon]) at a certain chemical composition of fibers, reflect the supermolecular structure. In the wet state, they reflect the influence of the aqueous medium, that is, the influence of water adsorption on the super-molecular structure during technological processes. The most important factors influencing the mechanical properties of cellulosic fibers are the molecular mass (i.e., the length of the fiber-forming linear macromolecules), the degree of lateral order or crystallinity, and the alignment of the macromolecules (i.e., their orientation with respect to the fiber axis).

Experimental

We have investigated three different regenerated cellulose fibers (viscose cv, modal CMD, and lyocell CLY), which differ structurally because of different production processes. All three are textile staple fibers produced by Lenzing AG Austria, and their specifications are presented in Table I.

TABLE I. Specifications of regenerated cellulosic fibers.

METHODS

Molecular Mass and Degree of Polymerization

The molecular mass and degree of polymerization were determined viscosimetrically after dissolving the cellulose samples in the solvent EWNN (FeTNa sodium salt of ferric tartaric acid) according to standard DIN 54 270 [21, 22]. Viscosity was measured with a modified Ubbelohde viscometer (capillary length 78 mm, capillary bore width 0.75 mm, volume of the bulb between the marks 7 cm3). The intrinsic viscosity [[eta]] was calculated from the efflux time of cellulose solution (t), the blank EWNN solution (t^sub 0^), and the concentration of cellulose in the solution (c) according to the Schulz-Blaschke equation:

where [eta]^sub rel^ is the relative viscosity and k^sub [eta]^ is the coefficient for a given polymer solvent system (0.3 < k^sub [eta]^ > 0.4). The molecular mass (M) and degree of polymerization (DP) were calculated from the most commonly used relationship between intrinsic viscosity [[eta]] and molecular mass (Kuhn, Mark, Houwink):

[[eta]] = KM^sup a^ , (2)

where K is the constant depending on the solvent at a given temperature and [alpha] is the value that describes the flexibility of the molecules (0.5 < [alpha] < 0.8) [13].

Density

The density of the regenerated cellulosic fibers was measured using a density-gradient technique in a column with a Techne Cambridge density gradient according to the standard DIN 53 497 [23]. This test method is based on observing the level to which a test specimen sinks in a liquid column exhibiting a density gradient, compared with standards of known density. The liquids were n-heptane ([rho] = 0.68376 g/cm3) and CCl^sub 4^ (p = 1.5950 g/ cm^sup 3^), and the calibrated glass floats were [rho]^sub 1^ = 1.4998 g/cm^sub 3^, [rho]^sub 2^ = 1.4770 g/cm^sup 3^, and [rho]^sub 3^ = 1.4098 g/cm^sup 3^. The crystalline weight fraction (w^sub c^) can be calculated from the density data:

where [chi]^sub c^ is the crystalline weight fraction, [rho]^sub c^ is the density of the crystalline fraction, [rho]^sub a^ is the density of the amorphous fraction, and [rho]^sub s^ is the density of the sample.

X-ray Analyses (WAXS)

The crystallinity index (CrI) and crystallite dimensions ([Lambda]*) were determined by wide-angle x-ray diffraction (2[thet\a] = 5[degrees] – 45[degrees]), using a Philips PW 1771 goniometer, Philips PW 1883 generator, and Philips detector. The x- ray analysis involved CuK[alpha] radiation ([lambda] = 0.15418 nm) and a Ni-filter. The crystallite dimensions ([Lambda]*) were calculated from the half-breadths of equatorial diffraction (002) according to the Scherrer equation. The crystallinity index (CrI) was calculated from the integral intensity of the crystalline reflections (I^sub cr^) to the total scattered intensity (I) [2, 17]:

Birefringence Measurements

Birefringence of regenerated cellulosic fibers ([Delta]n) was determined with a Leitz Laborlux 12 Pol polarization microscope equipped with an Ehringhaus compensator. The molecular orientation factor ([function of]^sub [Delta]n^) was calculated from the birefringence measurements of the fibers ([Delta]n^sub [function of]^) [11]:

where n^sub io^ is the birefringence of the perfectly oriented sample, [rho]^sub cr^ is the density of the crystalline regions, and Pf is the density of the fibers.

Size Exclusion Chromatograph

The void fraction (void diameter, void volume, specific inner surface) was determined by size exclusion chromatography. The equipment consisted of a metal HPLC column (1 = 250 mm, [phi] = 4 mm), an L-6000 Merck pumping system (flow rates 0.01-0.1 ml/min), and an LCD 202 Bishoff IR detector. Bi-distillate water (T = 20[degrees]C, flow rates 0.1 ml/min) was used as a mobile-phase, and test molecules with differing molecular sizes included methanol, ethyleneglycol, diethyleneglycol, polyethyleneglycol, and dextrin (d = 0.46-45 nm). Void diameter (d), void volume (V^sub p^), and specific inner surface (S^sub p^) were calculated from registered values of retention volume (V^sub R^) and the volume of eliminated molecules (V^sub o^) [4, 5, 6]:

Water Retention Power

We determined the water retention power of cellulosic fibers according to standard DIN 53 814 [25], which is based on determining the quantity of water the fibers can absorb and retain under strictly controlled conditions. This property is expressed as a ratio between the mass of water retained in the fibers after soaking (2 hours) and centrifuging (20 minutes) and the mass of the absolutely dry sample (105[degrees]C, 4 hours).

Dye Sorption

We evaluated dye sorption by determining the diffusion coefficient. C.I. Direct Blue 71 with a molar mass of 1029 g/mol was used for this experiment. The diffusion coefficient was determined spectroscopically on the basis of absorbency measurements of dye solution at maximum absorbency. Dyeing involved constant dyeing conditions of 30[degrees]C and a liquor ratio of 1:3000, by adding 3% dye and 20 g/l electrolyte, up to equilibrium. The equipment included a Turbomat Ahiba laboratory dyeing apparatus and a UV/VIS Lambda 2 spectrophotometer. The diffusion coefficient (D) was calculated by a simplified solution of Pick’s diffusion equation:

Mechanical Properties

We measured fiber mechanical properties (tenacity [sigma], elongation [epsilon], and modulus E) with an electronically controlled Lenzing Technik Instruments Vibrodyn 400 CRE dynamometer and Lenzing Technik Pruf-Software package, connected to the Vibroskop 400 measuring instrument for determining linear density. Measurements involved single fibers in both the conditioned and wet states according to the standard DIN 53 816 [24]. The dynamometer registered the tension inside the fiber as a function of deformation and calculated the following parameters: tenacity [sigma], elongation [epsilon], and modulus E. Measuring conditions were 20[degrees]C, 65% relative humidity, a gauge length of 20 mm, 100 mg pre-loading, and a measuring speed of 10 mm/min.

Results and Discussion

STRUCTURAL CHARACTERISTICS OF REGENERATED CELLULOSIC FIBERS

The crystalline structure of the environmentally friendly lyocell fibers differs from conventional textile viscose or modal fibers. Lyocell fibers consist of longer molecules, and compared with modal and viscose fibers, they have the highest molecular mass and degree of polymerization. Using the viscosimetric method, the average molecular mass and degree of polymerization, are given in Table II. The average molecular mass of lyocell fibers (M = 104,021) is 21% higher than the molecular mass of modal fibers (M = 82,097) and 63% higher than viscose fibers (M = 38,005). Lyocell fibers also have the highest degree of polymerization (DP = 643), modal fibers follow (DP = 507), and the degree of polymerization of viscose fibers is considerably lower (DP = 235).

TABLE II. Specific viscosity [eta]^sub spec^, reduced viscosity [eta]^sub red^, intrinsic viscosity [[eta]], viscosity-average molecular mass M^sub [eta]^, and degree of polymerization DP^sub [eta]^, of regenerated cellulosic fibers.

Table III presents the results of density measurements: average values of measured levels of sample float (h), sample density ([rho]), and crystalline weight fraction ([chi]^sub c^), calculated from density data. The highest density, [rho] = 1.5205 g/cm^sub 3^, is for lyocell fibers, modal fiber density is 1.5141 g/cm^sub 3^, while viscose fibers density is the lowest, 1.5045 g/cm^sup 3^. The process of fiber formation causes differences in the density of viscose and modal fibers. In addition, the crystallinity index, calculated from density data, increases from viscose to modal to lyocell fibers. Lyocell fibers have the highest crystallinity index, calculated from the density [chi]^sub c^ = 0.36, modal fibers follow with [chi]^sub c^ = 0.27, and the smallest crystallinity index is for viscose fibers, [chi]^sub c^ = 0.16.

TABLE III. Density p and crystallinity index [chi]^sub c^ of regenerated cellulosic fibers.

From the equatorial wide-angle x-ray diffraction pattern and the position of the interference reflections, we evaluated the differences in the crystalline structure of the regenerated cellulosic fibers. Intensity reflections of (101), (101), and (002) paratropic crystallographic planes partly overlap, as respectively shown in Figures 1-3. Experimental scattering curves are separated in crystalline and amorphous scattering using Pearson VII functions. To model crystalline reflections and amorphous background, we have used the ADM Wassermann software.

FIGURE 1. Equatorial scattering curve of viscose fibers.

FIGURE 2. Equatorial scattering curve of modal fibers.

FIGURE 3. Equatorial scattering curve of lyocell fibers.

By x-ray analysis, the polymorphic cellulose II modification is observed in all three different fiber types. The position of the high intensity (002)/(101) plane reflection is located at the scattering angle 2[theta] = 17[degrees] – 25[degrees], and the position of (101) reflection is at 2[theta] = 12.2[degrees]. The intensity of the last one is essentially lower when compared to the intensities of (002)/(101) plane reflections for all these fibers.

The shape of the overlapped (002)/(101) reflection of lyocell fibers is rounded, while viscose and modal fibers are less rounded, and in the direction of lower diffraction angles, there is a slightly less intensive maximum. The conclusion is the same when analyzing the separated (002) and (101) peaks. The intensity of the (101) crystalline peak of lyocell fibers is significantly lower than the intensity of the (002) crystalline reflection. In addition, similar relations between the two peaks occur for modal fibers. However, the intensity difference between the (002) and (101) peaks is significantly lower for modal fibers, while for viscose fibers, the intensity of these two reflections remains almost the same. The intensity of the (101) plane is typically the highest on the cellulose I diffraction pattern, due to the position of the glucose rings, which are lying parallel to the (002) plane. But in the case of cellulose II, the glucose rings are slightly deflected from the (002) plane toward the (101) plane, so a higher packing density remains in the (101) plane. Deflection of the glucose rings out of the (002) plane is probably mostly present in lyocell fibers, which is shown by the highest intensity of its (101) plane reflection. These differences are additionally confirmed by the shape of the (101) plane reflection. The scattering curves of the fiber samples with different intensities differ in the initial inclination of the curves. In the case of viscose and modal fibers, the (101) plane reflection does not have a pronounced shape. In lyocell fibers, the shape of the (101) plane reflection is better expressed and more separated. The scattering diagrams of all three regenerated cellulosic fiber types indicate a typical cellulose II diffraction pattern with the lowest intensities of the (101) plane reflections. In monocline cellulose II crystalline modification, the distance between the (101) crystalline planes increases compared to cellulose I crystalline modification. Therefore, (101) planes in cellulose II are more accessible to reagents.

Table IV presents the results of wide-angle x-ray scattering analyses for regenerated cellulosic fibers-diffraction angles 2[theta] [[degrees]] of reflections (002), (101) and (101), breadth of the reflections at half maximum intensity [beta] [[degrees]], crystallite dimensions [Lambda]* [nm], and the crystallinity index CrI.

TABLE IV. Results of wide-angle x-ray scattering (WAXS) of regenerated cellulosic fibers.

We have determined the dimensions of the crystallites in the (002) direction (perpendicular to the fiber axis). As Table IV reveals, the greatest sizes of crystallites appear in viscose fibers ([Lambda]* = 5.48 nm), then modal fibers ([Lambda]* = 4.93 nm), and lyocell fibers have the thinnest crystallites ([Lambda]* = 4.73 nm). When comparing lyocell, modal, and viscose regenerated cellulose fibers, the crystallinity index decreases from lyocell fibers with a CrI of 0.44 to viscose fibers with a CrI of 0.25. The crystallinity index of modal fibers is 0.37. Basedon the high degree of crystallinity (0.44) and the thinner crystallites in the transverse direction (4.73 nm) in lyocell fibers, we can conclude that the crystallites are longer, and it will be necessary to prove this using small angle x-ray scattering measurements.

Differences in the crystalline structure, i.e., share of the crystalline phase, crystallite dimensions, and imperfections, are caused by different conditions during fiber formation. The higher degree of crystallinity in lyocell fibers is a consequence of higher orientation during stretching formation of the fibers, which accelerates crystallization. In modal fibers, produced by stretching reformation, most importantly the high stretching ratio [lambda] has a decisive influence. In viscose fibers, where the cellulose regenerates quickly and the stretching ratio is low, the influence of orientation on crystallization is small; the consequence is a smaller amount of crystallinity CrI.

Birefringence is a measure of the average orientation of macromolecules. Table V presents the birefringence results [Delta]n for the regenerated cellulosic fibers. Birefringence increases from viscose ([Delta]n = 0.0323) to modal ([Delta]n = 0.0384) to lyocell ([Delta]n = 0.0398). The orientation factor [function of]^sub [Delta]n^ calculated from birefringence is lowest in viscose fibers ([function of]^sub [Delta]n^ = 0.58), then modal fibers ([function of]^sub [Delta]n^ = 0.69), and the highest in is lyocell fibers ([function of]^sub [Delta]n^ = 0.71). The higher orientation factor of modal fibers, compared to viscose, is a consequence of slower regeneration and coagulation in the fiber formation process. This allows a higher stretching ratio and, consequently, a high orientation in the deformation direction during stretching.

TABLE V. Birefringence [Delta]n and orientation factor [function of]^sub [Delta]n^ of regenerated cellulosic fibers.

In comparison with modal, and particularly with viscose fibers, lyocell fibers have the highest orientation factor. One of the reasons for this is the difference between the fiber spinning solutions due to NMMO and the viscose spinning procedure. The “new solvents,” as used for making the lyocell fibers, probably have a different solution structure. We know that many of them can form liquid crystal mesophases, mostly of the nematic type. The solvent NMMO has been reported to form lyotropic liquid crystal solutions at concentrations of 20%. The spinning solutions used here have a cellulose concentration of about 12%, which lies below this range. We can still assume, however, that the cellulose molecules in the solution are rather extended by the stiffening action of the polymeric NMMO-hydrate [18, 19]. The new spinning mechanism and the pre-orientation of cellulose macro-molecules in the spinning solution will greatly facilitate orientation in the field of the elongation gradient in the draw zone during fiber formation. Orientation of the cellulose macromolecules is finished even before the extrudate enters the coagulation bath, and they are no longer extended after coagulation.

Our analyses of voids using size exclusion experiments show that void diameter, volume, and specific inner surface are largest for viscose fibers (Figure 4). Lyocell fibers are very similar to viscose in their void structures. The average diameter of the voids decreases from viscose fibers with d = 3.1 nm to lyocell fibers with d = 3.0 nm, and the voids of modal fibers are essentially smaller with d = 2.4 nm. The trend is the same for the volume of voids V^sub p^ (CV = 0.68 cm^sup 3^/g, CLY = 0.62 cm^sup 3^/g, CMD = 0.49 cm^sup 3^/g), while the inner surface of the voids of modal fibers (S^sub p^ = 409 m^sup 2^/g) is insignificantly lower compared to the other two fiber types (CLY = 432 m^sup 2^/g, CV = 439 m^sup 2^/g). The greater inner surface and small volume of the voids in modal fibers indicate different void distributions for viscose and lyocell fibers, which have voids of greater diameter. All three fiber types have similar inner surface areas. This can be explained by fewer voids with a greater volume for viscose and lyocell fibers compared to modal fibers.

FIGURE 4. Void diameter d, void volume V^sub p^, and specific inner-surface S^sub p^ of voids.

INFLUENCE OF STRUCTURAL CHARACTERISTICS ON FIBER PROPERTIES

Our analyses of structural parameters have shown that the three regenerated cellulosic fibers differ in their crystalline area structures, amorphous areas and void structures. The degree of polymerization, density, crystallinity index, and molecular orientation factor are higher for lyocell fibers. The amorphous regions are smaller, and the void fraction of lyocell fibers, above all, the number of voids, is similar to that of viscose fibers. Figure 5 presents the accessible regions (less ordered amorphous regions and void fraction) in the fiber structures of the different regenerated cellulose fibers schematically.

FIGURE 5. Accessible regions in structures of different kinds of regenerated cellulosic fibers: (a) viscose fibers, (b) modal libers, (c) lyocell fibers [14].

Differences in the molecular and fine structure of fibers yield different sorption properties. We evaluated sorption properties using a method for determining water and dye adsorption. Table VI presents the water retention values of our cellulosic fibers, obtained by measuring the amount of liquid water retained by the swollen fibers under defined conditions. Viscose fibers have the highest water retention value, followed by lyocell. The water retention values decrease from viscose (84.9%) to lyocell (72.8%) to modal (57.8%). The water retention value of modal fibers is 15% lower than that of lyocell and 32% lower in comparison to viscose fibers.

TABLE VI. Water retention value and dye adsorption of regenerated cellulosic fibers.

Table VI also shows the results of our dye adsorption measurements: initial absorbency A^sub 0^, absorbency at equilibrium A^sub [infinity]^, dye concentration on the fibers at equilibrium [c^sub f^]^sub [infinity]^, and the diffusion coefficient D of C.I. Direct Blue 71. The diffusion coefficient is the lowest in modal fibers, D = 0.95 10^sup -11^ cm^sup 2^/s, in lyocell fibers it is D = 1.06 10^sup -11^ cm^sup 2^/s, and it is highest for viscose fibers, D = 1.24 10^sup -11^ cm^sup 2^/s. In addition, dye concentration on the fibers at equilibrium [C^sub f^]^sub [infinity]^ increases from modal (2.32 10^sup -3^ mol/kg) to lyocell (4.39 10^sup -3^ mol/kg) to viscose fibers (5.81 10^sup -3^ mol/ kg). Viscose fibers, with the lowest degree of crystallinity, the greatest portion of amorphous regions, volume, and inner surfaces of voids, adsorb the most dyestuff. Lyocell fibers have a relatively greater specific inner surface and void volume (similar to those of viscose fibers), and so the highest diffusion coefficient and dye concentration on the fibers are expected. The influence of the higher degree of crystallinity (smaller amorphous regions, few accessible -OH groups) of lyocell fibers can be seen in comparison to viscose fibers. Figure 6 shows the correlation between the structural characteristics (V^sub p^, CrI, f^sub [Delta]n^) of the fibers and their adsorption properties (water and dye adsorption).

FIGURE 6. Correlation between structure characteristics and adsorption properties (water and dye adsorption) of regenerated cellulosic fibers.

When considering water and dye adsorption, the new lyocell fibers are similar to the less density, low crystalline, and least oriented viscose fibers. Adsorption properties place lyocell fibers between the most hydrophilic viscose fibers and the least hydrophilic modal fibers. The portions of amorphous regions and void fractions significantly influence fiber adsorption properties, where the adsorption processes take place. Contrary to expectations, sorption phenomena in the aqueous medium do not conform to the basic structural data, i.e., degree of crystallinity and molecular orientation. For the sorption properties of regenerated cellulosic fibers, except for the fraction and orientation of amorphous areas, the structures of voids (especially their diameter and volume, and less their inner surface) are more important than other structural characteristics. Despite the highest degree of crystallinity and orientation, lyocell fibers have excellent sorption properties, similar to those of viscose fibers.

The structural characteristics of lyocell fibers reveal their good mechanical properties. A high degree of crystallinity and, above all, a high degree of molecular orientation, as well as high molecular mass enable lyocell fibers to achieve high tenacity and elasticity modulus values. Void numbers have little influence on the mechanical properties (Figure 7).

FIGURE 7. Influence of structural characteristics on mechanical properties.

As shown in Table VII, modal and lyocell fibers reach relatively high tenacity and wet modulus in the conditioned state [sigma] (CMD = 31.0 cN/tex, CLY = 31.7 cN/tex) and E (CMD = 349 cN/tex, CLY = 354 cN/tex) when compared with viscose fibers [sigma] (cv =18.1 cN/tex) and E (cv = 248 cN/tex). Stronger stretched modal fibers have, in comparison to viscose fibers, a higher degree of orientation, and their tenacity increases with increased molecular orientation f^sub [Delta]n^ (cv 0.58, CMD 0.69, CLY 0.71). The elongation increases from lyocell fibers ([epsilon] = 12.5%) to modal ([epsilon] = 16.5%) and then to viscose fibers ([epsilon] = 17.6%). Mechanical properties deteriorate in all three fiber types in the swollen state. For viscose fibers, breakage has already occurred at a tenacity of 10 cN/tex and a high elongation of 21.6%; wet tenacity decreases by 43% and elongation increases by 23%. For modal fibers, breakage occurs at a tenacity of 15.2 cN/tex and an elongation of 18%; wet tenacity decreases by 51% and elongation increases by 11%. The w\et modulus at 1% extension for viscose fibers decreases by 71% (E = 71 cN/tex) and in modal fibers by 78% (E = 75 cN/tex). The decreased modulus and tenacity, as well as increased elongation in the wet state are also true for lyocell fibers, but the influence of wetting is not as obvious as it is for viscose and modal fibers. We have confirmed this by investigating the modulus, which decreases by 62% (from 354 to 135 cN/tex). The tenacity of wet lyocell fibers decreases by only 9% to 28.8 cN/tex, while the elongation increases by 25% to 15.2%. A greater reduction in the tenacity of the viscose and modal fibers in the wet state compared to lyocell fibers is a consequence of essential differences in the fiber structures. Decreased tenacity upon wetting can be explained by the structure- loosening effects of swelling and by a reduction of linkages between cellulose molecules in the aqueous medium. This is a reflection of smaller inner tensions and a consequence of the circumstances under which the molecular orientation occurs.

TABLE VII. Mechanical properties of regenerated cellulosic fibers in conditioned and wet state.

Conclusions

The properties of fibers with similar chemical structures, in this case fibers from regenerated cellulose, differ because of different molecular and supermolecular arrangements. The main differences in structure, and consequently in fiber properties, originate from variations in production processes. The first difference that has a definite influence on properties is the degree of polymerization in terms of molecular mass. These properties influence mechanical properties in both the conditioned and wet state. The average molecular mass of lyocell fibers is 21% higher than that of modal fibers and 63% higher in comparison to viscose fibers. The degree of crystallinity of lyocell fibers is 16% higher when compared with modal fibers and significantly higher (43%) compared to viscose fibers. Molecular orientation [function of]^sub [Delta]n^ in lyocell fibers is the highest, and it exceeds the orientation factor of viscose fibers by 18% and the relatively high [function of]^sub [Delta]n^ of modal fibers by 3%. A certain amount of void volume is certainly a prerequisite for a good fiber. Spinning conditions are obviously a major influence, and like other features, a fiber’s inner structure, will depend strongly on its spinning conditions. The void fraction diminishes with increased stretching of the fibers, so the amount of void fraction (diameter, volume, and specific inner surface of voids) is largest in viscose fibers, lyocell fibers follow, and the smallest voids have the most stretched modal fibers.

Sorption properties of conventional and new regenerated cellulose fibers are, in addition to size and orientation of amorphous regions in the fiber, influenced above all by void fractions, their diameter, volume, and specific inner surface. The ordered regions (crystalline) do not contribute significantly to water and dye adsorption. The voids of lyocell fibers are similar to those of viscose fibers, and so are the swelling and dyeing properties in an aqueous medium.

The structural characteristics of lyocell fibers are also responsible for their superior mechanical properties. Lyocell fibers have a high degree of crystallinity and orientation, as well as a high average molecular mass and degree of polymerization. This enables lyocell fibers to reach high tensile strength and modulus. The influence of wetting on the mechanical properties of lyocell fibers is significantly lower in comparison with viscose and modal fibers. In the wet state, lyocell fibers keep over 90% of their tensile strength in the conditioned state, modal fibers approximately 50%, and viscose fibers around 60%.

Textile Res. J. 73(8), 675-684 (2003)

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15. Kreze, T., and Malej, S., Influence of Structure on Mechanical Properties of Regenerated Cellulose Fibers, Tekstil 49, 681-687 (2000).

16. Kruger, R., Cellulosic Filament Yarn From the NMMO Process, Lenz. Ber. 74, 49-52 (1994).

17. Sfiligoj, S. M., and Zipper, P., WAXS Analysis of Structural Changes of Poly(ethylene terephalate) Fibers Induced by Supercritical-fluid Dyeing, Colloid Polym. Sci. 276, 144-151 (1998).

18. Schurz, J., Was ist neu an den neuen Fasern der Gattung Lyocell?, Lenz. Ber. 74, 37-40 (1994).

19. Schurz, J., and Lenz, J., Investigations on the Structure of Regenerated Cellulose Fibers, Macromol. Symp. 83, 273-289 (1994).

20. Schurz, J., Lenz, J., and Wrentschur, E., Inner Surface and Void System of Regenerated Cellulose Fibres, Angew. Makromol. Chem. 229, 175-184 (1995).

21. Standard DIN 54 270 Bestimmung der Grenzviskositat von Cellulosen, Teil 1 Grundlagen.

22. Standard DIN 54 270 Bestimmung der Grenzviskositat von Cellulosen, Teil 3 EWNN, Verfahren.

23. Standard DIN 53 479 Bestimmung der Dichte.

24. Standard DIN 53 816 Einfacher Zugversuch an einzelen Fasern in klimatisiertem oder nassem Zustand.

25. Standard DIN 53 814 Bestimmung des Wasserruckhalte-vermogens von Fasern und Fadenabschnitten.

Manuscript received November 20, 2000; accepted November 22, 2002.

TATJANA KREZE1

Institute of Textile Chemistry, Ecology and Colorimetry, University of Maribor, SI-2000 Maribor, Slovenia

SONJA MALEJ

Department of Textiles, University of Ljubljana, SI-1000 Ljubljana, Slovenia

1 To whom correspondence should be addressed: Laboratory for Characterization and Processing of Polymers, Institute of Textile Chemistry, Ecology and Colorimetry, Faculty of Mechanical Engineering, University of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia, telephone: +386 2 220 7890, fax: +386 2 220 7990, e-mail: tanja.kreze@uni-mb.si.

Copyright Textile Research Institute Aug 2003