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).     

Measuring the crystallinity index of cellulose by solid state 13C nuclear magnetic resonance

The crystallinity index of cellulose is an important parameter to establish because of the effect this property has on the utilization of cellulose as a material and as a feedstock for biofuels production. However, it has been found that the crystallinity index varies significantly depending on the choice of instrument and data analysis technique applied to the measurement. We introduce in this study a simple and straightforward method to evaluate the crystallinity index of cellulose. This novel method was developed using solid state ¹³C NMR and subtraction of the spectrum of a standard amorphous cellulose. The crystallinity indexes of twelve different celluloses were measured and the values from this method were compared with the values obtained by other existing methods, including methods based on X-ray diffraction. An interesting observation was that the hydration of the celluloses increased their crystallinity indexes by about 5%, suggesting that addition of water increased cellulose order for all the cellulose samples studied.     

Analysis of the oxidation of cellulose fibres by titration and XPS

The purpose of this study was to evaluate the effect of selective oxidation on the surface properties of cotton cellulose fibres. Four different methods to evaluate the accessibility, nature and content of ionisable acidic groups (charge) in the fibres were applied: potentiometric and conductometric titrations, polyelectrolyte adsorption and X-ray photoelectron spectroscopy (XPS). The results from this combination of methods show that two processes take place when the oxidation method is applied: elimination of low molecular mass non-cellulosic compounds and formation of new acidic groups in the cellulose chains. Which of these processes is predominating depends on oxidation time, but the first one is initially more important. Polyelectrolyte adsorption and XPS show that the surface concentration of acidic groups is considerably lower than the bulk concentration, i.e. during oxidation the content of carboxyl groups in the surface region decreases, while it increases in amorphous regions. The decrease is due to the dissolution of low molecular weight compounds; the increase is due to the formation of new acidic groups. The use of titration methods in combination with XPS appears to be a very useful tool for identification of the formation and distribution of ionic groups in cotton fibres and their surfaces.     

The influence of classical and enzymatic treatment on the surface charge of cellulose fibres

Natural cellulose fibres comprise several non-cellulose compounds and cationic trash which cause problems during different adsorption processes such as dying, printing, final fiber finishing and coating. Therefore the pre-treatment (classical NaOH or environmental friendly enzymatic treatment, demineralisation) is the most important step in cellulose textile prefinishing-cleaning. An appropriate way to describe the success of different processes in fiber pre-treatment which result in distinct surface charge is the determination of electrokinetic properties-zetapotential (ZP) of fibers and textile materials. The zetapotential was determined by streaming potential measurements as a function of the pH and the surfactant concentration in the liquid phase.Cellulose fibers in an aqueous medium are negatively charged due to their characteristic carbonyl and hydroxyl groups. The degradation and removal of specific hydrophobic non-cellulose compounds which cover the primary wall of the cellulose polymer change the surface charge.The ZP is mainly influenced by waxes, their removal decreases the negative ZP. This result is obtained by the classical chemical process as well as by an environmentally friendly enzymatic treatment.Our results indicate that the progress of textile treatment and purification is reflected by the zetapotential of the fabrics. This method enables the estimation of the process'es progress and the interaction between components of the liquid phase and the fibre surface.     

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.     

Variation of xyloglucan substitution pattern affects the sorption on celluloses with different degrees of crystallinity

The sorption of xyloglucan (XG) on cellulose is a basic feature of the supramolecular assembly of plant cell walls. The binding to cellulose of xyloglucan fractions from Rubus fruticosus suspension-cultured cells with different substitution patterns was assayed on celluloses having various degrees of crystallinity between 20 and 95%. The primary structure of XGs differing in their Xyl/Glc ratio affected their binding to cellulose. The less substituted XGs gave the highest binding yields. Selective removal of the terminal fucosyl residues of XGs differentially affected the binding depending on the crystallinity of cellulose. The results showed large variations on the way cellulose crystallinity affects the binding interaction of XGs. Interestingly, one of the highest binding capacities was exhibited by the primary cell wall cellulose isolated from the actual R. fruticosus cells which also had the lowest crystallinity. Differences in binding to primary wall cellulose appeared to be inversely related to the global substitution of the glucan main chain of XGs.     

Structure and morphology of cellulose in wheat straw

The structure and morphology of cellulose extracted from wheat were studied. It was found that the extraction process is effective and hemicelluloses and lignin can be extracted completely. Cellulose in wheat straw was identified as cellulose I allomorph with low crystallinity and the crystallinity of cellulose from different parts of the wheat straw has little difference. There was no metastable cellulose I crystalline modification found in wheat straw; only the more stable cellulose I crystalline modification existed. Cellulose chains in the epidermis of wheat straw were observed with their orientation along the growth direction of wheat straw, while those in parenchyma were observed with almost no preferred orientation. There are two kinds of morphologies on the surface of wheat straw. One is the fiber structure with fibrils of about 5 m diameter, and the other is the fiber structure with serration morphology at the edge of the fiber, with which the fibers are connected together. The diameter of the latter one is about 10 m. The vascular bundles consist of circular rings while spiral structure cellulose backbones covered with thin cellulose film were also observed.     

Advances in the structure and microstructure determination of yttrium silicates using the Rietveld method

The Y₂O₃-SiO₂ 1:1 composition doped with a weak concentration of europium ions was prepared with the sol-gel technique and the products studied by X-ray diffraction as a function of temperature in the range from 900 to 1300 °C, using the method of Rietveld for quantitative evaluation of amorphous and crystalline evolving phases. The amorphous profile of the yttrium oxyorthosilicate glasses has been described following the “Rietveld for Disordered Materials” method and subsequently included in the patterns of semicrystalline samples that have been heat-treated for temperatures above 900 °C at 1000, 1100, 1150, 1200 and 1300 °C. The quantitative evaluation of the amorphous phase is obtainable from the Rietveld approach equivalent to the method after Ruland. This enabled us to study in fine detail the structural rearrangements and growth mechanisms that take place during the crystal-to-amorphous transformation in terms of coordination numbers, average interatomic distances, average crystallite size and microstrain and to identify the polymorphous transformation involving the Y₂SiO₅ phase from low-to-high-temperature forms, as well as some minor quantities of other phases namely α-Y₂Si₂O₇ phase, Y₂O₃ and Y_4.67(SiO₄)₃O.     

X-ray diffraction of the crystallinity of glycolysates derived from PET

Glycolysates coming from the chemical recycling of PET scraps are most often very crystalline compounds making them difficult to perform in further reactions like step growth polyaddition with diisocyanates to obtain polyurethanes. The glycolysis of PET was investigated using unusual reagents: polyesters based oligomers, bisphenol A, cycloaliphatic alcohol and aliphatic diols. The thermal properties of the glycolysates obtained were compared in terms of crystallinity with that of coming from the glycolysis with DEG. The crystallinity extents were calculated using X-ray diffraction. Surprisingly, the crystalline or non-crystalline nature of the reactants do not predict the morphology of the final glycolysates.     

Effects of temperature and molecular weight on dissolution of cellulose in NaOH/urea aqueous solution

Dissolution of cellulose having different viscosity-average molecular weight (M _η ) in 7 wt%NaOH/12 wt%urea aqueous solution at temperature from 60 to −12.6°C was investigated with optical microscope, viscosity measurements and wide X-ray diffraction (WXRD). The solubility (S_a) of cellulose in NaOH/urea aqueous solution strongly depended on the temperature, and molecular weight. Their S_a values increased with a decrease in temperature, and cellulose having M _η below 10.0 × 10⁴ could be dissolved completely in NaOH/urea aqueous solution pre-cooled to −12.6°C. The activation energy of dissolution (E_a,s) of the cellulose dissolution was a negative value, suggesting that the cellulose solution state had lower enthalpy than the solid cellulose. The cellulose concentration in this system increased with a decrease of M _η to achieve about 8 wt% for M _η of 3.1 × 10⁴. Moreover, cellulose having 12.7 × 10⁴ could be dissolved completely in the solvent pre-cooled to −12.6°C as its crystallinity (χ _c) decreased from 0.62 to 0.53. We could improve the solubility of cellulose in NaOH/urea aqueous system by changing M _η , χ _c and temperature. In addition, the zero-shear viscosity (η ₀ ) at 0°C for the 4 wt% cellulose solution increased rapidly with an increase of M _η , as a result of the enhancement of the aggregation and entanglement for the relatively long chains.     

Surface modification of cellulose with plant triglycerides for hydrophobicity

Hydrophobic cotton was achieved by surface modification of the cellulose with triglycerides from several plant oils including soybean, rapeseed, olive and coconut oils. These oils were delivered to the cellulose substrates in homogeneous solutions of ethanol or acetone as well as aqueous emulsions. Surface modification was facilitated by solvent evaporation followed by heating between 110 and 120 °C for 60 min. All oils, except for coconut, produced hydrophobic and less water-absorbing cotton, supporting the desirable role of higher unsaturation in the fatty acids to achieve crosslinked network. The most hydrophobic surfaces were obtained by the reaction with 1% soybean oil in acetone. On both bleached and scoured cotton, a water contact angle of 80° and water absorption value of 0.82 μL/mg were achieved. The acquired hydrophobicity was not only retained after water washing but also improved with subsequent exposures to elevated temperatures. The surface tension of scoured cotton cellulose was lowered from 63.81 mJ/m² to 25.74 mJ/m² when modified by soybean oil delivered in acetone, which is lower than that of poly(ethylene terephthalate). An aqueous emulsion of soybean oil also rendered the scoured cotton hydrophobic, which shows promise for a green chemistry and bio-based approach to achieve water repellency on cellulosic materials.     

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.     

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)     

Cellulose crystallinity and ordering of hemicelluloses in pine and birch pulps as revealed by solid-state NMR spectroscopic methods

Solid-state ¹³C NMR spectroscopy was used to determine the degree of cellulose crystallinity (CrI) in kraft, flow-through kraft and polysulphide–anthraquinone (PS–AQ) pulps of pine and birch containing various amounts of hemicelluloses. The applicability of acid hydrolysis and the purely spectroscopic proton spin-relaxation based spectral edition (PSRE) method to remove the interfering hemicellulose signals prior to the determination of CrI were also compared. For softwood pulps, the spectroscopic removal of hemicelluloses by PSRE was found to be more efficient than the removal of hemicelluloses by acid hydrolysis. In addition to that, the PSRE method also provides information on the associations between cellulose and hemicelluloses. On the basis of the incomplete removal of xylan from the cellulose subspectra by PSRE, the deposition of xylan on cellulose fibrils and therefore an ordered ultrastructure of xylan in birch pulps was suggested. The ordered structure of xylan in birch pulps was also supported by the observed change of xylan conformation after regeneration. Similarly, glucomannan in pine pulps may have an ordered structure. According to the ¹³C CPMAS measurements conducted after acid hydrolysis, the degree of cellulose crystallinity was found to be slightly lower in birch pulps than in the pine pulps. Any significant differences in cellulose crystallinity were not found between the pulps obtained by the various pulping methods. Only in pine PS–AQ pulp, the degree of cellulose crystallinity may be slightly lower than in the kraft pulps containing less hemicelluloses.     

Microfibril diameter in celery collenchyma cellulose: X-ray scattering and NMR evidence

Cellulose isolated from celery collenchyma is typical of the low-crystallinity celluloses that can be isolated from primary cell-walls of higher plants, except that it is oriented with high uniformity. The diameter of the microfibrils of celery collenchyma cellulose was estimated by three separate approaches: ¹³C NMR measurement of the ratio of surface to interior chains; estimation of the dimensions of the crystalline lattice from wide angle X-ray scattering (WAXS) measurements using the Scherrer equation; and the observation that microfibrils of this form of cellulose have the unusual property of packing into an irregular array from which small angle X-ray scattering (SAXS) shows features of both form and interference functions. The interference function contributing to the SAXS pattern implied a mean microfibril centre-to-centre distance of 3.6 nm, providing an upper limit for the diameter. However modelling of the scattering pattern from an irregular array of microfibrils showed that the observed scattering curve could be matched at a range of diameters down to 2.4 nm, with the intervening space more or less sparsely occupied by hemicellulose chains. The lateral extent of the crystalline lattice normal to the 200 plane was estimated as a minimum of 2.4 nm by WAXS through the Scherrer equation, and a diameter of 2.6 nm was implied by the surface: volume ratio determined by ¹³C NMR. The WAXS and NMR measurements both depended on the assumption that the surface chains were positioned within an extension of the crystalline lattice. The reliability of this assumption is uncertain. If the surface chains deviated from the lattice, both the WAXS and the NMR data would imply larger microfibril diameters within the range consistent with the SAXS pattern. The evidence presented is therefore all consistent with microfibril diameters from about 2.4 to 3.6 nm, larger than has previously been suggested for primary-wall cellulose. Some degree of aggregation may have occurred during the isolation of the cellulose, but the larger microfibril diameters within the range proposed are a consequence of the novel interpretation of the experimental data from WAXS and NMR and are consistent with previously published data if these are similarly interpreted.     

Adsorption of cationic surfactants and subsequent adsolubilization of organic compounds onto cellulose fibers

Cationic surfactants with different hydrophobic chain length were adsorbed onto cellulose fibers in an aqueous medium. The adsorption isotherms exhibited three characteristic regions which were interpreted in terms of the mode of aggregation of the surfactant molecules at the solid–liquid interface. The hydrophobic layers were used as a reservoir to trap various slightly water soluble organic molecules. A quantitative study of these phenomena suggested typical partition behavior of the organic solutes between the aqueous phase and the surfactant layer. The surfactant chain length (from C12 to C18) was shown to play an important role in terms of the capacity to retain the organic solute and the capacity increased with the number of carbon atoms.     

Topochemical modification of cotton fibres with carboxymethyl cellulose

Adsorption of carboxymethyl cellulose (CMC) as a method to introduce charged (ionizable) groups onto cellulose cotton fibre surfaces was investigated. The method was based on application of a previously published method used for wood fibres. The amount of adsorbed ionizable groups was determined indirectly by analysis of CMC in solution by the phenol–sulphuric acid method and directly by conductometric titration of the fibres. Results from the two methods correlated well. The molecular weight and purity of the CMC had an influence on its adsorption onto cotton; high molecular weight CMC was preferentially adsorbed. The adsorbed charge correlated linearly with the amount of CMC adsorbed. The total charge of the cotton fibres could be increased by more than 50% by adsorption of CMC. It is expected that this modification procedure can be used in a wide spectrum of practical applications. Lidija Fras Zemljič and Karin Stana-Kleinschek are the members of the European Polysaccharide Network of Excellence (EPNOE).     

Modified cellulose fibres for adsorption of dissolved organic solutes

Cellulose fibres were grafted with aliphatic anhydrides having C6, C8, C12 and C16 chain length using a heterogeneous solvent exchange acylation procedure. The ensuing materials were fully characterised by FTIR, solid state ¹³C-NMR, Wide-angle X-ray scattering and contact angle measurements. These techniques showed that the chemical coupling has indeed occurred. The prepared modified fibres appeared to be efficient to trap different organic molecules dissolved in water. Recycling tests revealed that the saturated substrates could be regenerated tens of times without loosing their capacity of absorption of organic contaminants.     

Morphology and crystallinity of the two diastereomer racemates of triglycidyl isocyanurate (TGIC)

Triglycidyl isocyanurate [2451-62-9] is a trifunctional monomer containing epoxy groups, and is presently used mainly for cross-linking carboxyl-functional polyester resins in powder coatings. There are three chiral carbon atoms in its molecule, and due to its symmetrical structure four antipodes exist according to the configuration of the chiral carbon atoms; namely RRR, SSS, RRS and SSR. Therefore the synthesized or commercially available TGIC is a physical mixture of two diastereomer racemates, i.e. beta-TGIC or RRR/SSS and alpha-TGIC or RRS/SSR. The physical and chemical properties of the two diastereomer racemates of TGIC are different. Their morphology and crystalline structure have been determined by polarizing microscopy and X-ray diffraction analysis. Beta-TGIC has hexagonal symmetry and belongs to the space group R3 or R3m. The cell parameters are: a₀ = 14.037 Å, c₀ = 11.55 Å, and cell volume: V₀ = 1970.88 Å³. Alpha-TGIC has orthorhombic crystallinity and belongs to the space group Pna2₁. Its cell parameters are: a₀ = 9.29 Å, b₀ = 9.48 Å, c₀ = 15.66 Å and cell volume: V₀ = 1379.16 Å³.     

Surface characterization of cellulose fibres by XPS and inverse gas chromatography

Inverse gas chromatography (IGC) was used to determine the dispersive component of the free energy as well as the acid-base properties of cellulose fibre surfaces, before and after modification by corona treatment. It was found that the corona treatment increases both the dispersive contribution to surface energy and its acidic character, whereas only a slight increase in its basicity was observed. It was also found that some chemical degradation of the surface occurs at high corona currents. The extent of modification of the surface properties, as revealed by IGC, was correlated to the surface chemical composition deduced from XPS analysis as well as with the electrical conductance and the pH of the water suspensions of the cellulose fibres.