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.     

Nanoindentation of regenerated cellulose fibres

Nanoindentation was performed on cross sections of regenerated cellulose fibres with different structure and properties. Same as in single-fibre tensile tests, the elastic modulus of lyocell was higher than the elastic modulus of viscose. However, in spite of its tensile strength being twice as high as viscose, the hardness of lyocell was 15% lower than viscose. The overall degree of orientation of cellulose chains being higher in lyocell than in viscose, it is proposed that reduced lateral bonding in lyocell is the reason for the low hardness measured by nanoindentation compared to viscose.     

Study of cellulosic fibres morphological features and their modifications using hemicelluloses

The microfibrilar structure and the morphology of the fibre are some of the most important characteristics that determine fibre performance during yarn making. This article is focused on understanding of morphological features of manmade cellulosic fibre and exploring an alternate way to alter fibre morphology. It is observed that though the chemical nature of different types of cellulose fibres viz. viscose, modal and lyocell is same; different processing routes lead to different cross-section and morphologies of fibres which leads to their characteristic properties and spinning behavior. A novel way is attempted to alter the fibre morphology of viscose fibre by changing the fibre regeneration kinetics and molecular weight distribution through addition of low molecular weight and branched structured hemicelluloses in spinning dope. Addition of hemicelluloses in the spinning dope prior to spinning and regeneration of viscose fibres is found to alter the morphology of the fibres without affecting tensile properties of the fibres.     

Thermal behavior of cellulose fibers with enzymatic or Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> treatment

Summary New regenerated cellulose fibers were developed during the last decades as environmentally friendly systems. In this work, three fibers: lyocell, modal and viscose were subjected to an enzymatic treatment. Likewise, different lyocell fibers were washed in a Na₂CO₃ solution under severe conditions. Analysis was performed by means of differential scanning calorimetry, thermogravimetry and scanning electron microscopy. In all samples, at low temperature, water desorption was detected. Furthermore, thermal analysis shows wide exothermic processes that began between 250 and 300°C corresponding to the main thermal degradation and it is associated to a depolymerization and decomposition of the regenerated cellulose. It is accompanied with mass more than 60% mass loss. Kinetic analysis was performed and activation energy values 152-202 kJ mol^-1 of the main degradation process are in agreement with literature values of cellulose samples.     

Controlled thermo-catalytic modification of regenerated cellulosic fibres using magnesium chloride Lewis acid

The Lewis-acid catalytic reactions of magnesium chloride with regenerated cellulosic fibres under baking conditions can be interpreted using existing semi-crystalline morphological models. Reaction at 180 °C is associated with chain scission, which takes place randomly within the accessible regions of the fibre structure. This causes a rapid reduction in the cellulose degree of polymerization, which stabilizes at a limiting value, analogous to that observed with wet-state mineral acid catalysed hydrolysis. A slower scission-reaction is also observed, believed to be due to the liberation of single glucan units from crystallite ends, again analogous to wet-state mineral acid hydrolysis. Dry-state catalysis is promoted by thermal molecular motion, allowing migration of catalyst ions and also conformational flexing of the cellulose polymer, which also induces a small amount of recrystallisation at crystallite lateral surfaces. Differences in the dry-state reaction have been observed for lyocell, viscose and modal regenerated fibres, which can be related to differences in crystallinity and resulting accessibility of the magnesium chloride catalyst. For lyocell the accessibility towards magnesium chloride is lower than found with mineral acids, which may be significant in the development of treatments to promote mechanical fibrillation, without sacrificing fibre tensile properties.     

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.     

Visualisation of the fibrillar and pore morphology of cellulosic fibres applying transmission electron microscopy

Applying transmission electron microscopy (TEM) on ultra-thin cross-sections of fibres, the main characteristics of the internal morphology of cotton and the main man-made cellulosic fibres (modal, viscose and lyocell) could be visualised. To obtain an appropriate contrast for TEM, isoprene was polymerised into the swollen fibres after a stepwise solvent exchange from water to acetone. The included polymer is stainable with osmium tetraoxide. Significant differences in distribution of pore sizes and pore arrangements in the cellulosic fibres were seen. Cotton showed very small pores in the bulk of the fibre, but drying cracks and flat pores between the sheets of the secondary wall appear as larger pores. Lyocell contains only nanopores in the bulk of the fibre with a slight gradient in pore density, and a very porous skin layer. In viscose and modal, a very wide pore size distribution from nanometer to micrometer size can be seen.     

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.     

Carboxyl groups in pre-treated regenerated cellulose fibres

The influence of peroxide bleaching and slack-mercerization on the amount of acidic groups in regenerated fibres (viscose, modal and lyocell) were studied. Conductometric titration was used to determine the total content of acidic carboxylic groups. Polyelectrolyte titration was used for surface and total charge determination, and to obtain information about the charge distribution and accessibilities of charged groups. Changes in fibre crystallinity to pre-treatment processes were characterized using iodine sorption (Schwertassek method) and correlated to treatments and the amount of carboxylic groups. For all three types of fibres the amount of accessible carboxyl groups was lowered by an increase in the degree of crystallinity. Bleaching with hydrogen peroxide causes some oxidative cellulose damage and, therefore, a larger amount of carboxyl groups (presumably formed at the end of cellulose chains). Slack-mercerization did not significantly change the total amount of acidic groups in the fibres, but their accessibility to cationic polyelectrolytes, in particular to polymers with high molecular weight was substantially lowered. Lidija Fras Zemljič, Zdenka Peršin, and Karin Stana Kleinschek are the members of the European Polysaccharide Network of Excellence (EPNOE).     

The hydrolysis and recrystallisation of lyocell and comparative cellulosic fibres in solutions of mineral acid

Regenerated cellulosic fibres undergo a process described as scission-reordering during hydrolysis in solutions of mineral acid. This occurs within disordered polymer regions at lateral crystal interfaces, which are accessible to aqueous agents through the pore spaces and polymer free volume. This process is distinct from that of oligomer-solubilsation, which occurs within disordered polymer regions in series between crystal domains, where no effective template exists for recrystallisation. The degradation of series disorder will have the greatest influence on fibre tensile properties, which fall dramatically even at low levels of hydrolysis. The mechanics of fibrillation are most sensitive to the degradation of lateral disorder, which occurs at a higher rate constant. Soft-touch fabric processing may therefore be possible under conditions where there is a reduced influence on tensile performance. A kinetic model has been proposed to describe the hydrolysis and recrystallisation pathways, which shows that lyocell has longer but thinner crystal domains than viscose or modal fibres, and also a tighter distribution of lateral crystal sizes. Lyocell also has a lower proportion of series disorder and also thinner regions of lateral disorder. This is consistent with the overall greater crystallinity of the original lyocell fibre and the also of the final microscrystalline product.     

Thermal degradation of lyocell,modal and viscose fibers under aggressive conditions

Lyocell, modal and viscose fibers were subjected to mercerization or to solar degradation. The ulterior thermal degradation was analyzed by means of differential scanning calorimetry (DSC). Thermal analysis shows wide exothermic processes that began between 250 and 300°C corresponding to the main thermal degradation and are associated to a depolymerization and decomposition of the regenerated cellulose. Thermal degradation was analyzed as a function of concentration and time. Lyocell fiber is the most stable under thermal degradation conditions. Furthermore, mercerized samples are initially more degraded and present a lower thermal stability.     

Carbon-13 solid state NMR investigation and modeling of the morphological reorganization in regenerated cellulose fibres induced by controlled acid hydrolysis

CPMAS carbon-13 NMR has been used to follow structural changes affecting regenerated cellulose fibres during hydrolysis by mineral acids. The C4 envelope of regenerated cellulose was deconvoluted into separate peaks, for ordered (crystal), part-ordered (surface) and disordered (non-crystal) polymer, which allowed calculation of average crystal lateral sizes, in good agreement with WAXD data. A geometrical model has been used to describe recrystallisation at lateral crystal faces, occurring within a disordered boundary surrounding the crystal interior. A one-dimensional relaxation-diffusion model has also been constructed, appropriate to the spinodal structure of lyocell. This has provided estimates of proton T_1ρ relaxation times for pure crystalline (cellulose II) and non-crystalline cellulose, around 24 and 4.5 ms, respectively, at a 45 kHz B₁ field. From the model, crystalline and non-crystalline regions in lyocell are estimated to each be around 2.5 nm thickness for a material of 50% crystallinity, consistent with the 2–3 nm dimensions derived from C4 peak devonvolution.     

Splitting tendency of cellulosic fibers. Part 3: splitting tendency of viscose and modal fibers

The splitting tendency of viscose and modal fibers in aqueous alkali solutions of LiOH, NaOH, KOH and TMAH was investigated. The viscose fibers splitted up to 5–7 fibrils, whereas modal fibers splitted up to 2–4 fibrils depending on alkali type and concentration. The fibrillar structure of lyocell enables it to split more (15–20 fibrils) than viscose and modal fibers. Splitting occurs where internal stress of fiber is high due to different alkali or void distribution inside fiber. The splitting test couldn’t be achieved for viscose and modal fibers between 1 and 5 M concentration of NaOH and TMAH solutions due to breakage of fibers during test. Above 5 M concentration, no split can be observed due to even distribution of alkali inside fiber. Paper presented at the 7th World Textile Conference, AUTEX 2007, Tampere, Finland, 26–28 June 2007. Christian-Doppler Laboratory of Textile and Fiber Chemistry of Cellulosics is a Member of European Polysaccharide Network of Excellence (EPNOE), www.epnoe.org     

Effect of sodium hydroxide pre-treatment on the optical and structural properties of lyocell

Lyocell is a type of regenerated cellulose. Fibres spun from cellulose solution in N-methylmorpholine-N-oxide hydrate consist of crystalline cellulose II and amorphous cellulose. Lyocell fabrics were treated with aqueous sodium hydroxide solution (NaOH) to study the influence of alkali on optical and structural properties. It was observed that sodium hydroxide treatment causes the density, orientation and crystallinity of lyocell fibre to decrease with increasing sodium hydroxide concentration, a corresponding decrease in tensile strength is also observed. The greatest change in fibre properties occurs between 3.0 and 5.0 mol dm⁻³ NaOH. This is attributed to the onset of formation of Na-cellulose II at 3.0 mol dm⁻³ NaOH; a fully formed Na-cellulose II structure is expected above 6.8 mol dm⁻³ NaOH. Formation of Na-cellulose II causes plasticization of the lyocell fibres as both inter- and intra-molecular hydrogen bonds are broken by these higher sodium hydroxide concentrations.     

Thermal behaviour of lyocell fibres

Fourier-transform infrared (FTIR) spectroscopy has been applied in combination with wide-angle X-ray diffraction and measurements of strength, fluidity, yellowness, birefringence, and moisture regain to detect microstructural changes in lyocell fibres, a regenerated cellulose fibre, subjected to direct heat and annealing treatments. TMA, and SEM were used to show the effect of direct heat and annealing on lyocell fibres. The FTIR spectroscopy results show that a decrease in intermolecular hydrogen bonding occurs at 70 and 80 °C for annealed and directly heated samples, respectively. The results demonstrate increase of the intensity of O–H stretching vibrations, this associated with hydrogen bonds reforming around 130 °C. Lyocell fibres shrink with direct heating in the temperature range 130–160 °C. The crystallinity decreases gradually with increasing temperature. There is no significant change in colour of the samples annealed up to 150 °C. A continuous increase in the fluidity occurs for the annealed samples in the range 150–230 °C. The tenacity and breaking extension of heated samples decrease with increasing temperature. The lower annealing temperatures cause no observable change in the smooth and void-free surface, but in the annealing temperature range 170–230 °C, substantial non-uniformity is apparent on the surface of the fibres.     

Surface modification of lyocell fibres by graft copolymerization of thermo-sensitive poly-N-isopropylacrylamide

Thermo-sensitive poly-N-isopropylacrylamide (poly-NIPAAm) was grafted onto lyocell fibres using cerium ammonium nitrate (CAN) as initiator. The effects of initiation time, initiator concentration, monomer concentration and grafting time on the degree of grafting were investigated. A 15-60 min exposure time, 7.5 mM CAN solution concentration and a 0.5-1 mM NIPAAm monomer concentration were optimal for obtaining a maximum degree of grafting (60-70% at 24 h grafting time) of poly-NIPAAm on lyocell fibres. Higher degree of grafting was obtained increasing the grafting time, such as 120% at 72 h.The properties of the obtained poly-NIPAAm/lyocell copolymer were also investigated. Specifically, the effects of temperature and degree of grafting of poly-NIPAAm on the swelling behaviour of the copolymer were experimentally determined. Moreover, structural characterization, thermal behaviour and morphology of the poly-NIPAAm/lyocell copolymers were examined by Fourier Transform Infrared Spectroscopy (FTIR), Differencial Scanning Calorimetry (DSC) and Scanning electron microscopy (SEM) techniques, respectively.     

Influence of homogenization and drying on the thermal stability of microfibrillated cellulose

Homogenization has been used to release microfibrils from cellulose fibres to produce microfibrillated cellulose (MFC). Oven drying, atomization or freeze-drying were used to dry MFC. Morphological differences were observed linked to the compaction of the system and the formation of microfibril agglomerates. Thermal stability of the dried MFC, checked by TGA, decreased after homogenization and drying. Char level at the end of the pyrolysis was higher than for cellulose fibres. Derivative TGA (dTGA) showed a shoulder around 250 °C for the dried MFC. Volatile degradation product detection by FTIR spectroscopy (FTIR) coupled to TGA and DSC showed that the shoulder corresponds to expected dehydration reactions of the cellulose. Increasing the contacts between microfibril(s) (bundles) and agglomerates of the freeze-dried MFC by compression promoted dehydration reactions. Homogenization and drying modified the thermal properties of the MFC. No significant influence of freeze-drying kinetics on the thermal behaviour of the MFC was observed.     

Characterization and pyrolysis behaviour of different paper mill waste materials

Paper industry generates a considerable amount of wastes. Their composition mainly depends on the type of paper produced and the origin of cellulose fibres. Nowadays, in Spain, 40% of solid wastes generated by the paper and pulp industry are deposited directly in landfill, 25% are used in the agriculture, 13% in the ceramic industry and 7% in the concrete production. In the last years, thermal treatment methods like combustion, pyrolysis and gasification have been widely study as alternative techniques for the valorization of different organic waste materials. The main objective of the present work is to study the pyrolysis behaviour of different paper mill waste materials. For this reason, a wide characterization of eight paper mill waste materials from different origins was performed using SEM, FTIR, DRX and thermogravimetric techniques. Paper mill sludges from recycled paper, mainly wastes obtained from deinking process, showed high CaCO₃ and clays contents. Compared with the elevated total organic matter content (TOM) of paper mill waste materials their low organic carbon content determined by Cr₂O₇²⁻ oxidation reveals the elevated chemical stability of organic matter, due to high content on cellulose fibres. Analysis of samples by SEM indicates that successive recycled processes of paper leads to paper mill waste materials with more degraded fibres. XRD analyses show as crystalline cellulose was present in reject and primary sludge from paper mills that produced paper from virgin wood. However, crystalline cellulose content significantly decreased in waste materials from recycled paper. Finally, thermogravimetric analysis indicates that presence or mineral matter and degradation of cellulose significantly influences their pyrolysis behaviour. In general, weight loss of paper mill waste materials started at lower temperatures than pure cellulose. In waste materials from recycled paper weight loss continues at temperatures highest than 500 °C due to kaolinite dehydration and carbonates decomposition.     

Fibrillation Tendency of Cellulosic Fibers. Part 2: Effects of Temperature

The influences of temperature, concentration of swelling agents and fiber materials on the fibrillation tendency in various cellulosic fibers in aqueous solutions were investigated in terms of fibrillation stability and fibrillation sensitivities to alkali and heat. The fibrillation stability and the fibrillation sensitivity to swelling agents were evaluated with a critical point of fibrillation (CPF_conc.) that is the concentration of the swelling agents where fibrillation begins, and the ratio of initial increase in fibril number to increase in concentration of swelling agent (I_i). The fibrillation sensitivity to heat was estimated with the increase in I_i against temperature. The CPF_conc. of lyocell fiber was 16.7 mol/l water in ethanol/water mixture at 25 °C and decreased to 0 mol/l at 80 °C, indicating acceleration of the fibrillation at higher temperatures. The I_i of lyocell was enhanced from 3.50 to 7.57 count l/mol. The CPF_conc. increased in the order of viscose > cross-linked lyocell > modal > lyocell while the I_i decreased in the order of viscose < modal < cross-linked lyocell < lyocell at 40 °C. The I_i of lyocell fiber increased to the greatest extent with increase in temperature as compared with the other cellulosic fibers. Lyocell fiber has the lowest fibrillation stability and the highest fibrillation sensitivities to alkali and to heat resulting in the highest fibrillation tendency.     

Single stage purification of flax,hemp, and milkweed stem and their physical and morphological properties

Agriculture biomass is an alternative possible solution for the extraction of cellulose, compared to the classical soft and hard wood. However, the valorization of cellulose is challenging for the researchers as it involves multiple steps. In the present study, the raw fibers of flax, hemp, and milkweed stem fibers were purified in single step using hydrogen peroxide in water. By this method authors successfully extracted the purified cellulose fibers without damaging the fiber length. The purified fibers were characterized to understand the thermal, functional, crystalline, and morphological properties by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The FTIR results showed the effective removal of lignin and significant improvement in thermal stability was observed by TGA. Evidently, the SEM results showed significant improvement in the morphology compared to that of the raw fibers. XRD results showed that the treatment does not affect the crystallinity of the fibers.