Splitting tendency of cellulosic fibers. Part 2: Effects of fiber swelling in alkali solutions

The splitting tendency of lyocell fiber in aqueous alkali solutions like KOH, NaOH, LiOH and TMAH was investigated. Up to 5 M concentration of alkali solutions, cation type is important on splitting of lyocell. Above 5 M concentration, cation type is no more relevant to splitting of lyocell. At 1 M of alkali solutions, alkali retention value (ARV) and split number increase in the order of KOH<LiOH<NaOH<TMAH. It was proposed that lyocell is swollen homogeneously inside fiber above 5 M alkali concentration, so that when force is applied on fiber, no split occurs in lyocell fiber since there isn’t enough internal stress inside of the fiber. Different concentrations of each type of alkali result in the same ARV for the fiber, but their alkali distribution inside fiber is the decisive point for splitting of lyocell fiber, so that for each alkali type different split numbers are observed. Depending on alkali concentration and alkali type, maximum splitting of a fiber into 15–20 fibrils could be observed.     

Splitting tendency of cellulosic fibers – Part 1. The effect of shear force on mechanical stability of swollen lyocell fibers

A procedure for splitting of a lyocell fiber into a multitude of finer fibrils was developed. Crockmeter, usually used for rub-fastness of colored textiles, was modified and used for obtaining required shear force on swollen lyocell fiber. The shear force applied on fibers, and the concentration of NaOH, which affects swelling degree of fiber, were shown to be the leading parameters determining split number of lyocell fiber. While number of shear cycles was found to be of minor relevance for fiber splitting, the applied pressure directly influences the number of splitted fibrils. For example, at a pressure of 34.8 kPa, the average split number of lyocell fiber in 2.5 M NaOH solution was observed as 15, whereas it was observed as 30 for 47 kPa and 41 for 59.3 kPa. Splitting was not observed above 5 M of NaOH solution. Analyses of fiber splitting permit new aspects to study inner structure of lyocell.     

Sorption studies on regenerated cellulosic fibers in salt–alkali mixtures

The degrees of salt sorption were determined in lyocell and viscose fibers immersed in aqueous solutions of salt–alkali mixtures with the aim of using salt sorption as an indirect measure of changes to fiber accessibility in presence of alkali. The salt–alkali mixtures used were combinations of NaOH with NaCl or NaBr, and of KOH with KCl or KBr. In general, salt sorption in fibers increased with increase in alkali concentration up to 2 mol/l, and did not change significantly thereafter. The accessibility of Br⁻ salts was greater than the Cl⁻ salts, but that of the Na⁺ salts was greater than the K⁺ salts. These trends in salt sorption indicate that salt accessibility in fibers is not influenced by the size of hydrated salt ions, but by the forces of electrostatic attraction and repulsion between the charged fiber surface and salt cations and anions.     

Fibrillation Tendency of Cellulosic Fibers. Part 1: Effects of Swelling

The fibrillation tendencies of various cellulosic fibers in aqueous solution containing alkali metal hydroxide and ethanol were evaluated with two specific parameters: the critical point of fibrillation (CPF_conc.), that is a concentration of swelling agent where the fibrillation begins, and the ratio of initial increase in fibril number to increase in concentration of swelling agent (I_i). The CPF_conc. and the I_i are defined as fibrillation stability and fibrillation sensitivity to swelling agent, respectively. Lyocell fiber (CLY1) has the smallest CPF_conc. and the largest I_i, representing the lowest fibrillation stability and the highest fibrillation sensitivity, leading to the highest fibrillation tendency in CLY1 among the fibers tested. Although crosslinking improved fibrillation stability in lyocell as compared to modal, the fibrillation stability remained higher owing to the high water capacity and the high affinity for alkali. In alkali solution at the same concentration CLY1 fibrillation increased in the order of LiOH > NaOH > KOH. However, the plot of fibril number against solvent retention value of CLY1 in different alkaline solutions gives a slope of 110 count · g/cm³ regardless of alkali type, the critical degree of swelling for CLY1 with no fibrillation was 0.62 cm³/g in alkali solutions and 0.45 cm³/g in ethanol/water mixture.     

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.     

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.     

Alkali Uptake and Swelling Behavior of Lyocell Fiber and their Effects on Crosslinking Reaction

The swelling behavior of lyocell fiber in alkali solutions and the alkali uptake were investigated as well as their influences on the reaction of sodium-hydroxy-dichlor-triazine with lyocell. The uptake of NaOH was increased from 0.69 mmol/g up to 4.63 mmol/g, leading to the enhancement of fiber swelling from 1.01 cm³/g to 2.34 cm³/g when alkali concentration in preliminary alkali treatment was increased from 0.4 mol/l to 8.0 mol/l. The rise in alkali uptake heightened the crosslinking reaction. The fiber swelling was hindered by addition of acetone to alkali solution, resulting in water retention capacity of 0.64 cm³/g in the 37.5% v/v of acetone/water mixture and increase in the reaction yield. The fiber was more swollen in NaOH solution than in KOH though the uptake of NaOH was 5.7-times less than that of KOH. The reaction yield of crosslinking agent in NaOH solution was 9.9-times larger than that in KOH at the same alkali uptake. The abrasion resistance of lyocell fiber was improved by the method used in this work, causing high pilling resistance of lyocell fabric as compared to a conventional method.     

Influence of ligand type and solution pH on heavy metal ion complexation in cellulosic fibre: model calculations and experimental results

Complexation of heavy metal ions such as Cu²⁺, Zn²⁺ and Co²⁺ by cellulosic fibres cotton, lyocell and viscose was studied in the pH range from pH 7–13. Glycine and sodium D-gluconate complexes were studied. Complex formation in the cellulose matrix depends on ligand, solution pH, complex species formed and type of cellulosic fibre. Species distribution in solution was calculated using the program SPE and literature data for formation constants of M²⁺-glycine and M²⁺-gluconate complexes. The calculated data permit explanation of the experimental heavy metal uptake in the cellulose matrix. In presence of GLY lower heavy metal concentrations were observed. Heavy metal complexation decreases with increasing pH between 7 and 11. For Cu²⁺ and Zn²⁺ a strong increase in metal binding capacity in the fibres was observed at pH 13 for Cu²⁺ and pH 11–12 for Zn²⁺ respectively. Low Zn²⁺ content is analysed at pH 13 due to zincate formation. Member of EPNOE, European Network of Excellence “Polysaccharides” .     

Evaluation of new organosolv dissolving pulps. Part I: Preparation, analytical characterization and viscose processability

New acidic organosolv pulping processes, such as Acetosolv, Formacell and Milox, promise to have superior potential in terms of purification selectivity and specific investment costs. Consequently, a thorough investigation of these new acidic pulping processes in comparison to state-of-the-art acidic magnesium sulfite technology was conducted. The impact of pulping and bleaching parameters on the physical and chemical characteristics was studied to compare process efficiency and selectivity for each type of pulp made from Eucalypt wood. In addition to a detailed analysis of the chemical composition and physical properties on a molecular and supramolecular level, the TCF-bleached dissolving pulps were tested for their applicability in viscose fiber production. The influence of pulp properties as determined by standard and advanced analytical methods on process performance and selected fiber properties is emphasized.     

Lyocell blend fibers with cationic starch: potential and properties

The regeneration of cellulose from solution state offers opportunities for blending with a secondary polymer. Cellulose/cationic starch blends were spun into fibers from -methylmorpholine- oxide solution, and the fibers were characterized by moisture absorption, dye absorption, and enzymatic hydrolysis. Cellulose/starch-blend fibers with up to 30% starch content were found to retain up to three times as much water, take up to five times as much dye, and be degradable much faster by cellulase hydrolysis compared with control lyocell fiber. ID addition to starch content, the fiber's performance depended on the degree of substitution of the starch by cationic substituents.     

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.     

Consequences of the nanoporosity of cellulosic fibers on their streaming potential and their interactions with cationic polyelectrolytes

Electrokinetic tests, based on the streaming potential method, were used to elucidate interactions between cationic polyelectrolytes and cellulosic fibers and to reveal aspects of fibers’ nanoporosity. The fibrillated and nanoporous nature of bleached kraft fibers gave rise to time-dependent changes in streaming potential, following treatment of the wetted fibers with poly-diallyldimethylammonium chloride. Electrokinetic test results were consistent with an expected longer time required for higher-mass polyelectrolytes to diffuse into pore spaces, compared to lower-mass polyelectrolytes. Further evidence of the relative inability of polyelectrolyte molecules to diffuse into the pores of cellulose was obtained by switching back and forth between high and low ionic strength conditions during repeated measurement of streaming potential, after the fibers had been treated with a moderate amount of cationic polymer. By changing the concentration of sodium sulfate it was possible to switch the sign of streaming potential repeatedly from positive to negative and back again. Such results imply that a continuous path for liquid flow exists either in a fibrillar layer or within the cell walls. The same concepts also helped to explain the dosages of high-charge cationic polymer needed to achieve maximum dewatering rates, as well as the results of retention experiments using positively and negatively charged microcrystalline cellulose particles.     

Preparation and characterization of electrospun fibers based on poly(L-lactic acid)/cellulose acetate

PLLA/CA mixtures of different compositions were successfully electrospun to obtain composite nanofibrous membranes.The microstructures of the membrances changed from homogeneous to heterogeneous with the addition of CA, which was observed by FE-ESEM.The PLLA/CA fabric membranes were characterized by mechanical testing,DSC and contact angle measurements.The tensile stress of the composite fibrous membranes increased obviously with the increase of CA content.DSC results indicated that the CA component was the main factor for the changes of enthalpies in the composite fibers.Contact angle measurements showed the hydrophilicity of the electrospun nanofiber membranes was improved with the addition of CA.     

Interpretation of relaxation and swelling phenomena in lyocell regenerated cellulosic fibres and textiles associated with the uptake of solutions of sodium hydroxide

The uptake of solutions of sodium hydroxide by lyocell fibre results in a phenomenon in textiles described as swelling–shrinkage. The response of woven fabrics in a tensile stress–relaxation experiment shows two time-dependent processes, corresponding to different mechanisms of pressure development. Rapid diffusion has been assigned to osmotic swelling through the interconnected pore structure of the fibre (D = 6–15 × 10⁻¹² m²/s), which is influenced by the extent of ionization of hydroxyl groups at the pore surfaces. A ratio for the cellulose and water dissociation constants (K_cell/K_w) of 70 provides best agreement with experimental data. A second slower diffusion process (D = 2–10 × 10⁻¹⁴ m²/s) is assigned to transport through the cellulose polymer structure, associated with the Na-cellulose transition. This can be modeled assuming an ion-exchange equilibrium, where the cellulose gel converts reversibly between compact hydrogen and expanded sodium forms, with K = 1.04 × 10¹⁴, in favour of the hydrogen form. The model successfully predicts the concentration dependence of the transition and the movement to higher concentration with external constraint. The slow diffusion process only becomes apparent at high alkali concentrations, as the pores in the fibre collapse due to the expansion of the gel. Continued gel-diffusion is only possible through the polymer phase, which then dominates over fast pore-diffusion.     

Swelling and dissolution of cellulose,Part III: plant fibres in aqueous systems

Raw and refined flax, hemp, abaca, sisal, jute and ramie fibres are dipped into N-methylmorpholine N-oxide (NMMO)–water with various contents of water and into hydroxide sodium (NaOH)–water. The swelling and dissolution mechanisms of these plant fibres are similar to those observed for cotton and wood fibres. Disintegration into rod-like fragments, ballooning followed or not by dissolution and homogeneous swelling are all observed as for wood and cotton fibres, depending on the quality of the solvent. Balloons are not typical of wood and cotton and they seem to be present in all plant fibres. Another interesting result is that the helical feature seen on the balloon membrane is not related to the microfibrillar angle. Plant fibres are easier to dissolve than wood and cotton. This is not related to the molar mass of the cellulose chain. Raw plant fibres keeping most its non-cellulosic components do not show the formation of balloons. Patrick Navard is a Member of the European Polysaccharide Network of Excellence (EPNOE)     

Electrospun cellulose acetate fibers: effect of solvent system on morphology and fiber diameter

This paper reports an investigation of the effects of solvent system, solution concentration, and applied electrostatic field strength (EFS) on the morphological appearance and/or size of as-spun cellulose acetate (CA) products. The single-solvent systems were acetone, chloroform, N,N -dimethylformamide (DMF), dichloromethane (DCM), methanol (MeOH), formic acid, and pyridine. The mixed-solvent systems were acetone–DMAc, chloroform–MeOH, and DCM–MeOH. Chloroform, DMF, DCM, MeOH, formic acid, and pyridine were able to dissolve CA, forming clear solutions (at 5% w/v), but electrospinning of these solutions produced mainly discrete beads. In contrast, electrospinning of the solution of CA in acetone produced short and beaded fibers. At the same solution concentration of 5% (w/v) electrospinning of the CA solutions was improved by addition of MeOH to either chloroform or DCM. For all the solvent systems investigated smooth fibers were obtained from 16% (w/v) CA solutions in 1:1, 2:1, and 3:1 (v/v) acetone–DMAc, 14–20% (w/v) CA solutions in 2:1 (v/v) acetone–DMAc, and 8–12% (w/v) CA solutions in 4:1 (v/v) DCM–MeOH. For the as-spun fibers from CA solutions in acetone–DMAc the average diameter ranged between 0.14 and 0.37 μm whereas for the fibers from solutions in DCM–MeOH it ranged between 0.48 and 1.58 μm. After submersion in distilled water for 24 h the as-spun CA fibers swelled appreciably (i.e. from 620 to 1110%) but the physical integrity of the fibrous structure remained intact.     

Swelling and dissolution of cellulose, Part V: cellulose derivatives fibres in aqueous systems and ionic liquids

The swelling and dissolution mechanisms of several cellulose derivatives (nitrocellulose, cyanoethylcellulose and xanthate fibres) are studied in aqueous systems (N-methylmorpholine-N-oxide—water with various contents of water, hydroxide sodium—water) and in ionic liquids. The results are compared with the five modes describing the swelling and dissolution mechanisms of cotton and wood cellulose fibres. The mechanisms observed for the cellulose derivatives are similar to the ones of cotton and wood fibres. Swelling by ballooning is also seen with cellulose derivatives, showing that this phenomenon is linked to the fibre morphology, which can be kept after undergoing a heterogeneous derivatisation. Patrick Navard and Thomas Heinze—Members of the European Polysaccharide Network of Excellence (EPNOE),     

Synthesis of a novel antimicrobial-modified starch and its adsorption on cellulose fibers: part II––adsorption behaviors of cationic starch on cellulose fibers

The adsorption of guanidine polymer modified starches on cellulose fibers was investigated along with the systematic studies on various influencing factors including temperature, pH, ionic strength and charge density of the starches. The AFM results revealed the relationship between the adhesion force and adsorption capacity. The adsorption capacity is not necessarily proportional to the adhesion force. The conditions for achieving the maximum adsorption were: temperature, 40 °C; pH, 6; C_NaCl, 0 mM and charge density, 0.4 meq/g. The corresponding the normalized adhesion force is approximate 1 mN/m. In terms of the surface roughness determined by AFM, it has been proved that adsorbed starches of high charge density tend to form train structure, whereas those of low charge density tend to form tails and loops. Due the comb molecular structure, the adsorption capacity of the novel cationic starch reaches 124.3 mg/g, which is much greater than those reported previously.     

Extraction and characterization of new cellulosic fibers from Indian mallow stem: An exploratory investigation

Natural fibers are one of the good alternative sources for replacing synthetic fiber and reinforcing polymer matrices because of their eco-friendly nature. This investigation deals with the extraction and characterization of new natural fiber from Indian mallow plant stem. The physico-chemical, thermal, and mechanical properties of Indian mallow fibers (IMFs) were reported and compared with other natural fibers for the first time. Cellulose (78.22%), wax (0.47%), density (1.33 g/cm³), and tensile strength (979.83 MPa) were recognized in IMFs. Fourier transform-infrared spectroscopy, X-ray diffraction, and thermo-gravimetric analysis confirmed that IMFs are rich in cellulose content and thermally stable with a crystallinity index of 72%.