Structure and thermal properties of bamboo viscose,Tencel and conventional viscose fiber

Comparative investigations of new regenerated cellulosic fibers, bamboo viscose fiber and Tencel, together with conventional viscose fibers have been carried out to explain the similarity and difference in their molecular and fine structure. The analyses jointly using SEM, XRD and IR reveal that all the three fibers belong to cellulose II. Tencel consists of longer molecules and has a greater degree of crystallinity, while bamboo viscose fiber has a lower degree of crystallinty. TG-DTG-DSC study shows three fibers resemble in thermal behavior with a two-step decomposition mode. The first step is associated to water desorption, suggesting that bamboo viscose fiber holds better water retention and release ability, the second a depolymerization and decomposition of regenerated cellulose, indicating that Tencel is more thermally stable in this process than bamboo and conventional viscose fiber.     

The thermal behaviour of cellulose samples with different structure

In this work the thermal characteristics of cellulose samples with different structure were investigated. The samples were prepared by reacting the cellulose with ethanolic hydroxide solution. Depending on the time of alkaline treatment, the intensity of cellulose transformation differed. Starting from cellulose I structure, with the highest degree of crystallinity, the other samples consisted of mixed structures of cellulose I and II, or were completely transformed to cellulose II structure with the lowest degree of crystallinity. The thermal behaviour of the samples was studied by using a Perkin Elmer TGS-2 and DSC-2 instruments. The kinetic parameters of dehydration and degradation were determined from non-isothermal TG-data (Nitrogen-inert atmosphere and a heating rate of 20 deg/min). The thermal effects of water evolution (heating rate of 80 deg/min) of the cellulose samples were found to depend on the structural characteristics and the crystallinity of the samples. The activation energy and frequency factor were in correlation with the structural changes.     

Structure and properties of regenerated cellulose fibers from aqueous NaOH/thiourea/urea solution

Regenerated cellulose fibers were successfully prepared through dissolving cotton linters in NaOH/thiourea/urea aqueous solution at ?2 °C by a twin-screw extruder and wet-spinning process at varying precipitation and drawing conditions. The dissolution process of an optimized 7 wt% cellulose was controlled by polarizing microscopy and resulted in a transparent and stable cellulose spinning dope. Rheological investigations showed a classical shear thinning behavior of the cellulose/NaOH/thiourea/urea solution and a good stability towards gelation. Moreover, the mechanical properties, microstructures and morphology of the regenerated cellulose fibers were studied extensively by single fiber tensile testing, X-ray diffraction, synchrotron X-ray investigations, birefringence measurements and field-emission scanning electron microscopy. Resulting fibers demonstrated a smooth surface and circular cross-section with homogeneous morphological structure as compared with commercial viscose rayon. At optimized jet stretch ratio, acidic coagulation composition and temperature, the structural features and tensile properties depend first of all on the drawing ratio. In particular the crystallinity and orientation of the novel fibers rise with increasing draw ratio up to a maximum followed by a reduction due to over-drawing and oriented crystallites disruption. The microvoids in the fiber as analysed with SAXS were smaller and more elongated with increasing drawing ratio. Moreover, a higher tensile strength (2.22 cN/dtex) was obtained in the regenerated fiber than that of the viscose rayon (2.13 cN/dtex), indicating higher crystallinity and orientation, as well as more elongated and orientated microvoid in the regenerated fiber. All in all, the novel extruder-based method is beneficial with regard to the dissolution temperature and a simplified production process. Taking into account the reasonable fiber properties from the lab-trials, the suggested dissolution and spinning route may offer some prospects as an alternative cellulose processing route.     

Structure and properties of novel fibers spun from cellulose in NaOH/thiourea aqueous solution

Cellulose was dissolved rapidly in a NaOH/thiourea aqueous solution (9.5:4.5 in wt.-%) to prepare a transparent cellulose solution, which was employed, for the first time, to spin a new class of regenerated cellulose fibers by wet spinning. The structure and mechanical properties of the resulting cellulose fibers were characterized, and compared with those of commercially available viscose rayon, cuprammonium rayon and Lyocell fibers. The results from wide angle X-ray diffraction and CP/MAS 13C NMR indicated that the novel cellulose fibers have a structure typical for a family II cellulose and possessed relatively high degrees of crystallinity. Scanning electron microscopy (SEM) and optical microscopy images revealed that the cross-section of the fibers is circular, similar to natural silk. The new fibers have higher molecular weights and better mechanical properties than those of viscose rayon. This low-cost technology is simple, different from the polluting viscose process. The dissolution and regeneration of the cellulose in the NaOH/thiourea aqueous solutions were a physical process and a sol-gel transition rather than a chemical reaction, leading to the smoothness and luster of the fibers. This work provides a potential application in the field of functional fiber manufacturing.     

Degradation of viscose fibers during acidic treatment

Novel developments in the fiber sector require additional investigation of their behaviour during the production process. In the core step of the viscose process, cellulose fibres are regenerated in an acidic spinning bath. To investigate the influence of hemicellulose content and temperature on the kinetics of fiber degradation, standard and hemicellose-enriched fibers were treated in the acidic standard spinning bath for time periods up to 292 h. Viscose samples of different hemicellulose content were prepared under standardized conditions and the never dried fibres were subjected to long term degradation in the standard spinning bath at 40, 50 and 60 °C. The changes in the degree of polymerization (DP), molar mass distribution, hemicellulose and the generation of the total organic carbon in the spinning bath were monitored. Further, the changes in crystallinity and the level-off-DP of the fibers were determined to improve the accuracy of the existing degradation model. Changes in morphology of the fibers were monitored by optical microscopy and scanning electron microscopy.     

Progress in cellulose shaping: 20 years industrial case studies at Fraunhofer IAP

Cellulose, the most abundant renewable organic material on earth, exhibits outstanding properties and useful applications, but also presents a tremendous challenge with regard to economical and environmentally friendly chemical processing. The viscose process, more than 100 year old is still the most widely utilized technology to manufacture regenerated cellulose fibers and films. Viscose fibers are produced today worldwide on a 5 million ton scale with various fiber types ranging from high performance tire yarn to textile filaments and staple fibers with excellent properties close to those of cotton. At Fraunhofer IAP, the technical equipment for viscose preparation, wet spinning of fibers, hollow fibers, and tube-like films is available on a min-plant scale. Research focused on raw materials testing, process optimization with regard to economic and ecological aspects, structural analysis of cellulose during processing, and structure–property relations of fibers and films. Similar to the viscose process, cellulosic fibers can be produced via cellulose carbamate as an environmentally friendly route. In a close cooperation of Fraunhofer IAP with industrial partners, a specific process based on cellulose carbamate was developed on a pilot plant scale, giving fiber properties close to those of conventional viscose fibers. In recent decades the N-methylmorpholine-N-oxide (NMMO)-technology turned out to be a nonderivatizing commercial alternative to the still dominant viscose route. From the very beginning, Fraunhofer IAP has been engaged in investigating the structure formation of cellulose fibers precipitated from NMMO-water solution, revealing structural reasons for the fibrillation tendency of these fibers and means to overcome them. Starting from fiber formation via dry-jet wet spinning, for the first time the blown film formation and the meltblown nonwovens technology were developed for cellulosics on a pilot plant scale at Fraunhofer IAP. Based on the elastic behavior of the dope at elevated temperatures, cellulose can be processed like a melt in the air-gap, offering new possibilities of shaping cellulose like meltable mass polymers. Combining cellulose carbamate with NMMO-monohydrate as a solvent, higher polymer concentrations in the dope and outstanding mechanical properties of the resulting fibers were achieved.     

13C CPMAS NMR investigations of cellulose polymorphs in different pulps

In order to obtain information about the crystallinity and polymorphs of cellulose, and the occurrence of hemicelluloses in pulp fibers, wood cellulose, bacterial cellulose, cotton linters, viscose, and celluloses in different pulps were investigated by solid state ¹³C CPMAS NMR spectroscopy. A mixed softwood kraft pulp and a dissolving-grade pulp were treated under strongly alkaline and acidic conditions and the effect on cellulose crystallinity was studied. The presence of different crystalline polymorphs of cellulose and the amounts of hemicelluloses are considered.     

Dissolving-grade birch pulps produced under various prehydrolysis intensities: quality,structure and applications

Aqueous-phase prehydrolysis followed by alkaline pulping is a viable process to produce wood-based dissolving pulps. However, detailed characterisation of the achievable pulp quality, performance and cellulose structure is yet lacking. In this study, the production of hemicellulose-lean birch soda-anthraquinone pulps after prehydrolysis under various intensities was investigated. Increasing prehydrolysis intensity resulted in pulps of higher purity but lower cellulose yield and degree of polymerisation. Higher cellulose yield by using sodium borohydride during pulping was achieved at the expense of reducing pulp purity. Cellulose crystallinity was similar in all pulps indicating simultaneous degradation of both crystalline and amorphous cellulose regions. Reinforced prehydrolysis seemingly increased the cellulose crystal size and the interfibrillar distances. Moderate intensity prehydrolysis (170 °C) resulted in a pulp well suited for viscose application, whereas reinforced prehydrolysis favoured the production of acceptable cellulose triacetate dope. The performance of the pulps in viscose and acetate applications was strongly related to the chemical and structural properties.     

Sulfur-free dissolving pulps and their application for viscose and lyocell

In this study, the concept of multifunctional alkaline pulping has been approved to produce high-purity and high-yield dissolving pulps. The selective removal of hemicelluloses was achieved by either water autohydrolysis (PH) or alkaline extraction (E) both applied as pre-treatments prior to cooking. Alternatively, hemicelluloses were isolated after oxygen delignification in a process step denoted as cold caustic extraction (CCE). Eucalyptus globulus wood chips were used as the raw material for kraft and soda-AQ pulping. In all process modifications sulfur was successfully replaced by anthraquinone. By these modifications purified dissolving pulps were subjected to TCF bleaching and comprehensive viscose and lyocell application tests. All pulps met the specifications for dissolving pulps. Further more, CCE-pulps showed a significantly higher yield after final bleaching. Morphological changes such as ultrastructure of the preserved outer cell wall layers, specific surface area and lateral fibril aggregate dimension correlated with the reduced reactivity towards regular viscose processing. The residual xylan after alkali purification depicted a lower content of functional groups and a higher molecular weight and was obviously entrapped in the cellulose fibril aggregates which render the hemicelluloses more resistant to steeping in the standard viscose process. Simultaneously, the supramolecular structure of the cellulose is partly converted from cellulose I to cellulose II by the alkaline purification step which did not influence the pulps reactivity significantly. Nevertheless, these differences in pulp parameters did not affect the lyocell process due to the outstanding solubility of the pulps in NMMO. Laboratory spinning revealed good fiber strength for both, regular viscose and lyocell fibers. The high molecular weight xylan of the CCE-treated pulps even took part in fiber forming.     

Nonalkali swelling solutions for regenerated cellulose

Swelling of regenerated cellulose in nonalkali aqueous solutions containing lithium chloride and urea (LiCl/urea/water) was examined. The effect of solution concentration on fiber properties was studied using microscopy, weight gain (swelling), and mechanical strength tests. The regenerated cellulose samples included lyocell fibers, viscose fibers, and fibers spun from alkali. The change in the mechanical properties of treated fibers was smaller than that of fibers treated with alkali to the same level of swelling. The degree of swelling in these solutions was related to the propensity for the formation of Li–cellulose coordination complexes, and these were enhanced by reductions in both urea and water content.     

Investigations on the structure of regenerated cellulose fibers; Herrn Professor Janeschitz-Kriegl zum 70. Geburtstag mit den besten Wünschen gewidmet

A review is given on comparative investigations onto the structure of regenerated cellulose fibers of the regular viscose type (modal) and of solvent spun fibers of the lyocell type, namely the NMMO fiber spun from a solution of cellulose in N–methylmorpholine N–oxide and water. It was found that in the lyocell fiber the mechanical properties in the wet state detoriate less. This is explained by an increased length of the crystallites, less clustering of the crystalline regions, and a shorter and better oriented amorphous portion. These structural features could be caused by a different spinning mechanism due to a preordered spinning solution in which the stiff complexed cellulose molecules are oriented lengthwise in the spinning direction. This will greatly facilitate their orientation during fiber formation in the elongtional deformation velocity field of the draw down zone.     

Crosslinking and pyrolytic behavior of natural and man-made cellulosic fibers

Pyrolysis rates, energies of activation, and DSC data were obtained for cellulosic fibers crosslinked with increasing amounts of formaldehyde. Pyrolysis rates are affected by the reduction in degree of polymerization, the breaking of intermolecular hydrogen bonds, and the introduction of covalent linkages that accompany the crosslinking process. Thermal stabilization of cellulose is related mainly to the formation of interchain crosslinks. The influences of the crystallinity and orientations of the polymers upon changes in thermal stability and pyrolytic behavior due to crosslinking are demonstrated.     

Treatment in Swelling Solutions Modifying Cellulose Fiber Reactivity – Part 2: Accessibility and Reactivity

The reorganization of cellulose fibers by swelling treatments in alkali solutions results in numerous changes to fiber structure, causing changes of chemical reactivity in the fiber-solution heterogeneous system. An important part of the change in chemical reactivity is the change of fiber accessibility because it results in exclusion of chemicals such as reagents or catalysts from the fiber. In the second of a two-part series of papers, we examine the influence of changes in fiber accessibility and/or reactivity due to treatment in swelling solutions on the performance or behavior of substrates during and after chemical finishing treatments. Changes in fiber accessibility due to alkali treatments are visualized with fluorescence microscopy. The effect of alkali treatments on enzymatic hydrolysis and pad-dry-cure crosslinking treatments of cellulose substrates are discussed as representative examples to demonstrate the effects of swelling processes on fiber reactivity and accessibility. Model calculations indicate that a considerable redistribution of chemicals in substrates occurs during dry-cure operations resulting from molecule-specific exclusion effects. Pilling tests on lyocell knit-fabrics show the impact of preceding alkali processes on the final physical performance of textile fabric highlighting the importance of correct selection of alkali processes to achieve desired behavior.     

Novel Fibers Prepared from Cellulose in NaOH/Urea Aqueous Solution

Summary: Novel regenerated cellulose fibers have been successfully spun from the cellulose dope in NaOH/urea aqueous solution, which could rapidly dissolve cellulose. The fibers possess circular cross‐sections as well as relatively high molecular weight, and a crystallinity index with cellulose II family crystal structure, leading to good mechanical properties. This technology is simple, cheap, and environmentally friendly, promising to substitute for viscose rayon production having hazardous byproducts.

SEM micrograph of the cross‐section of the novel cellulose fibers generated here.     

Effect of alkaline ultrasonic pretreatment on crystalline morphology and enzymatic hydrolysis of cellulose

In this study, ultrasound-assisted alkaline pretreatment is developed to evaluate the morphological and structural changes that occur during pretreatment of cellulose, and its effect on glucose production via enzymatic hydrolysis. The pretreated samples were characterized using scanning electron microscopy, infrared spectroscopy, and X-ray diffraction to understand the change in surface morphology, crystallinity and the fraction of cellulose Iβ and cellulose II. The combined pretreatment led to a great disruption of cellulose particles along with the formation of large pores and partial fibrillation. The effects of ultrasound irradiation time (2, 4 h), NaOH concentration (1–10 wt%), initial particle size (20–180 μm) and initial degree of polymerization (DP) of cellulose on structural changes and glucose yields were evaluated. The alkaline ultrasonic pretreatment resulted in a significant decrease in particle size of cellulose, besides significantly reducing the treatment time and NaOH concentration required to achieve a low crystallinity of cellulose. More than 2.5 times improvement in glucose yield was observed with 10 wt% NaOH and 4 h of sonication, compared to untreated samples. The glucose yields increased with increase in initial particle size of cellulose, while DP had no effect on glucose yields. The glucose yields exhibited an increasing tendency with increase in cellulose II fraction as a result of combined pretreatment.     

Changes of supramolecular cellulose structure and accessibility induced by the processive endoglucanase Cel9B from Paenibacillus barcinonensis

A newly identified cellulase with a high polysaccharide degrading potential and a processive mode of action, has been evaluated on cellulose fibers. Cellulase Cel9B from Paenibacillus barcinonensis is a modular endoglucanase with the domain structure GH9-CBM3c-Fn3-CBM3b, consisting of a family nine catalytic module GH9, an auxiliary module CBM3c, a fibronectin-like module Fn3, and a functional cellulose binding module CBM3b. The whole cellulase Cel9B (E1) and two truncated forms of the enzyme that consist of the catalytic module linked to the auxiliary module, GH9-CBM3c (E2), and of the cellulose binding module of the enzyme, CBM3b (CBD), were applied to softwood dissolving pulp. The changes in the supramolecular structure and morphology of the fibres after the enzymatic treatment were evaluated by viscosimetry, X-ray diffraction (XRD), thermogravimetric analysis, differential scanning calorimetry and scanning electron microscopy (SEM). XRD studies provided the crystallite size, interplanar distances and crystallinity index of the samples before and after the enzymatic treatment. The treatment with cellulases E1 and E2 decreased the degree of polymerization and increased the crystallinity index of the pulp. Both E1 and E2 had a pronounced capacity for removing fuzz and improved the smoothness and surface appearance of the fibers, as shown by SEM. On the other hand, CBD proved to be less effective under the tested conditions. Moreover, the solubility of dissolving pulp in alkaline solutions has been evaluated as an indirect measure of cellulose accessibility. A notable enhancement in alkaline solubility of the samples treated with the cellulases was observed.     

Isolation of cellulose fibers from kenaf using electron beam

Cellulose fibers were isolated from a kenaf bast fiber using a electron beam irradiation (EBI) treatment. The methods of isolation were based on a hot water treatment after EBI and two-step bleaching processes. FT-IR spectroscopy demonstrated that the content of lignin and hemicellulose in the bleached cellulose fibers treated with various EBI doses decreased with increasing doses of EBI. Specifically, the lignin in the bleached cellulose fibers treated at 300 kGy, was almost completely removed. Moreover, XRD analyses showed that the bleached cellulose fibers treated at 300 kGy presented the highest crystallinity of all the samples treated with EBI. Finally, the morphology of the bleached fiber was characterized by SEM imagery, and the studies showed that the separated degree of bleached cellulose fibers treated with various EBI doses increased with an increase of EBI dose, and the bleached cellulose fibers obtained by EBI treatment at 300 kGy was separated more uniformly than the bleached cellulose fiber obtained by alkali cooking with non-irradiated kenaf fiber.     

Crystalline structure of developing cotton fibers

The crystalline structure of dried cotton fibers at varying development stages has been investigated using wide angle x-ray diffraction (WAXS) techniques. The cellulose I crystalline structure has been confirmed on dried SJ-2 Acala cotton fibers collected at varying developmental stages and at maturity. The cellulose I crystalline structure is clearly evident at the early developmental stage of 21 days postanthesis (dpa). The crystal system remains unchanged during the cotton fiber biosynthesis and at maturity. The degree of crystallinity and crystallite dimensions in the cotton fibers increase with cell development. The most significant increments are observed between 21 and 34 dpa (i.e., during the first half of the secondary wall thickening process). The unit cell sizes slightly decrease and thus the crystal density increases with fiber development. The alignment of the glucosidic rings in respect to the 002 planes improves with fiber cell development. © 1996 John Wiley & Sons, Inc.     

Crystallinity changes in lyocell and viscose-type fibres by caustic treatment

One of the most important treatments performed on cellulosic fibres to improve properties such as dimensional stability, tensile strength and lustre, is mercerisation. The aim of this work was to study the crystallinity, accessibility and unit cell structure changes occurring in three types of regenerated cellulose fibres (lyocell, modal and viscose) that were mercerised with caustic soda solutions of different concentrations. Differences were observed between the behaviour of the viscose type fibres (viscose and modal) and that of the lyocell fibres. For the viscose type fibres, the proportion of crystalline regions increased at low alkali concentrations, while for lyocell fibres a decrease in crystallinity was observed. In all three fibres there was a transformation from cellulose II to amorphous cellulose. While for lyocell the transformation was partial, the modal and in particular the viscose fibres showed a complete transformation, and the swelling agent caused the fibre to dissolve at high caustic concentrations.     

Water Accessibilities of Man-made Cellulosic Fibers – Effects of Fiber Characteristics

The dynamic vapor water sorption and desorption experiments were performed on cellulosic fibers with different characteristics. The hysteresis between moisture sorption and desorption cycle at 10% relative humidity (RH) was independent on the total moisture regain and approximately 45% for all materials except for viscose fibers. Brunauer–Emmett–Teller surface volume (V_m) for moisture sorption and retention capacity of liquid water (WRV) were also measured. The V_m and WRV increase in proportion to the total amount of moisture sorption (M_inf(total)) in all specimen except in poplar fiber. The coefficients of parallel exponential kinetics (PEK) were estimated by the curve-fitting of experimental data of the moisture regain, and the influences of the fiber characteristics on the PEK coefficients, the moisture regain, the hysteresis, V_m and WRV are discussed. The total equilibrium moisture content in the viscose fibers was higher but the moisture uptake and release rate was slower than the lyocell and poplar fibers. The cationization and the modification of shape of cross section accelerated the total equilibrium moisture content in the viscose fiber. A drying process at low temperature enhanced both the equilibrium moisture content and the moisture uptake and release rate in lyocell fibers while a spin finish retarded them. The total equilibrium moisture content was heightened by the crosslinking of the fiber, however, no obvious effect of the crosslinking on the moisture uptake and release rate was found. Effects of the type of the specimen and linear density on the moisture accessibilities are also discussed.