Morphology of cellulose objects regenerated from cellulose-N-methylmorpholine N-oxide-water solutions

International audienceThe precipitation in aqueous media of cellulose from solutions in N-methylmorpholine N-oxide (NMMO) hydrates is an important stage in the process of manufacturing of fibres, films and other cellulose objects. It is responsible for the formation of the structure of the regenerated object and their morphological characteristics significantly influence the properties of the final products. Regeneration of rather large cellulose objects was observed in situ by optical microscopy. It was found that all regenerated objects present an asymmetric structure composed of a dense skin surrounding a sub-layer characterised by the presence of finger-like voids. The porous texture of the cellulose parts between these voids is typical of the one obtained by spinodal decomposition. The morphologies of regenerated cellulose samples are described as a function of various parameters, initial cellulose solutions and composition and temperature of the aqueous regeneration bath. A mechanism of the structure formation during regeneration is proposed

Dissolution, crystallisation and regeneration of cellulose in Nmethylmorpholine N-oxyde

International audienceMixtures of N-methylmorpholine N-oxyde (NMMO) and water are direct solvents for cellulose, used for processing fibres (Lyocell process). The objective of this work is to study three aspects of the preparation of cellulose objects from NMMO-water solutions, the NMMO-water phase diagram in order to follow the dissolution-swelling path, the crystallisation of the cellulose solution that may occur under cooling and the regeneration of the cellulose from a solution

Current pretreatment technologies for the development of cellulosic ethanol and biorefineries

Lignocellulosic materials, such as forest, agriculture, and agroindustrial residues, are among the most important resources for biorefineries to provide fuels, chemicals, and materials in such a way to substitute for, at least in part, the role of petrochemistry in modern society. Most of these sustainable biorefinery products can be produced from plant polysaccharides (glucans, hemicelluloses, starch, and pectic materials) and lignin. In this scenario, cellulosic ethanol has been considered for decades as one of the most promising alternatives to mitigate fossil fuel dependence and carbon dioxide accumulation in the atmosphere. However, a pretreatment method is required to overcome the physical and chemical barriers that exist in the lignin–carbohydrate composite and to render most, if not all, of the plant cell wall components easily available for conversion into valuable products, including the fuel ethanol. Hence, pretreatment is a key step for an economically viable biorefinery. Successful pretreatment method must lead to partial or total separation of the lignocellulosic components, increasing the accessibility of holocellulose to enzymatic hydrolysis with the least inhibitory compounds being released for subsequent steps of enzymatic hydrolysis and fermentation. Each pretreatment technology has a different specificity against both carbohydrates and lignin and may or may not be efficient for different types of biomasses. Furthermore, it is also desirable to develop pretreatment methods with chemicals that are greener and effluent streams that have a lower impact on the environment. This paper provides an overview of the most important pretreatment methods available, including those that are based on the use of green solvents (supercritical fluids and ionic liquids)

Étude physico-chimique des solutions de cellulose dans la N-Méthylmorpholine-N-Oxyde

Cellulose is a natural linear polymer that does not melt below its degradation temperature. It can be processed by means of various techniques, more or less complex and polluting. Among numerous cellulose solvents, N-methylmorpholine-N-oxide (NMMO) is the only solvent used in industrial way for fibres spinning. Despite the fact that this process has been used for nearly fifteen years, it remains empirical. The scientific objective of our work was to clarify various questions concerning the stages of dissolution, forming and regeneration. The main characteristic of this process is the continuous change of NMMO/water mixture composition. Thus, a complete NMMO/water phase diagram was built. The existence of the NMMO monohydrate with a melting temperature of 80°C was confirmed, the existence of the dihydrate (Tm=40°C) instead of the 2,5 hydrate was proposed and a new hydrate with 8 water molecules per NMMO molecule with a melting temperature of - 45°C was found. The investigation of the interactions between cellulose and NMMO/water mixtures in the whole concentration range showed that cellulose II swells and dissolves more readily than cellulose I. For NMMO/water mixtures with high water contents, swelling affects only amorphous regions; it is accompanied by the organisation of the amorphous phase when water content is between 28% and 50%. The predominant role in the crystallisation process of the cellulose/NMMO/water solutions belongs to the solvent. The crystallisation velocity does not depend on the viscosity for given temperature and cellulose concentration. It depends on the properties of the solvent - its amount and water content, and on the dispersion of cellulose in solution. This dispersion depends on the cellulose origin. When the cellulose crystallinity is high, the dispersion of cellulose chains is low. This leads to a high crystallisation velocity of the solvent. The precipitation of cellulose from cellulose/NMMO/water solutions occurs during solution immersion in the water bath. The precipitation velocity is defined by the diffusion of the solvent (NMMO) from the cellulose/NMMO/water solution to the water bath and by the diffusion of the non-solvent (water) from the bath to the cellulose solution. The diffusion of the non-solvent is affected by NMMO content in the water bath while the diffusion of the solvent is also influenced by cellulose concentration of the cellulose/NMMO/water solution. We demonstrated that the diffusion of the non-solvent is ten times more rapid than the diffusion of the solvent. This result was linked to the particular morphology of cellulose solutions regenerated in molten state.La cellulose, polymère naturel linéaire non fusible en dessous de sa température de dégradation, peut être mise en forme moyennant des procédés plus ou moins complexes et polluants. Parmi des nombreux solvants connus pour la cellulose, le seul, à ce jour, à être utilisé industriellement pour le filage des fibres, est la N-méthylmorpholine-N-oxyde (NMMO). Le procédé NMMO, bien que utilisé depuis une quinzaine d'années, est encore sujet à une utilisation assez empirique. L'objectif scientifique de notre travail était de lever un certain nombres de questions relatives aux diverses étapes du procédé : dissolution, mise en forme et précipitation. La variation de la fraction NMMO/eau à ces différentes étapes qui est caractéristique au procédé a fait émerger la nécessité de la construction d'un diagramme de phases complet du système NMMO/eau. Nous avons construit un tel diagramme. Dans ce cadre, nous avons pu confirmer l'existence d'un composé monohydraté (1H2O-NMMO) ayant une température de fusion de 80°C et nous avons montré l'existence d'un composé à 2 molécules d'eau par molécule de NMMO ayant une température de fusion de 40°C (certains auteurs penchaient en faveur d'un composé 2,5H2O-NMMO. De plus, nous avons montré la possibilité de formation d'un autre composé hydraté, à 8 molécules d'eau par molécule de NMMO ayant une température de fusion de - 45°C. L'étude des interactions de la cellulose avec les mélanges NMMO/eau dans toute la gamme des concentrations a permis de montrer que la cellulose II présente les cinétiques de gonflement et de dissolution plus rapides que la cellulose I. Pour les mélanges à forte teneur en eau le gonflement n'affecte que la phase amorphe et s'accompagne d'une structuration de celle-ci lorsque la teneur en eau est entre 28% et 50%. Le rôle prédominant dans la cristallisation des solutions cellulose/NMMO/eau appartient au solvant. La vitesse de cristallisation, à température et concentration de cellulose données, n'est pas fonction de la viscosité de la solution, mais dépend des propriétés du solvant - sa quantité, sa teneur en eau et de l'état de dispersion de la cellulose en solution. Cet état de dispersion dépend de l'origine de la cellulose. Lorsque le taux de cristallinité de la cellulose est élevé, la dispersion des chaînes est faible et la vitesse de cristallisation du solvant est élevée. La vitesse de précipitation de la cellulose à partir des solutions cellulose/NMMO/eau dans un bain aqueux est définie par la vitesse de diffusion du solvant (NMMO) de la solution vers le bain et la vitesse de diffusion du non-solvant (eau) du bain vers la solution. La diffusion du non-solvant est influencée par la teneur en NMMO du bain d'eau tandis que la diffusion du solvant est affectée en plus par la concentration de cellulose dans la solution. Nous avons montré que la diffusion du non-solvant est dix fois plus importante que la diffusion du solvant et nous avons relié ce résultat à la morphologie particulière des solutions régénérées à l'état fondu

Kinetics of precipitation of cellulose from cellulose-NMMO-water solutions

International audienceThe regeneration of a solid, crystallized cellulose solution in a N-methylmorpholine-N-oxide (NMMO)-water mixture was studied by measuring the diffusion coefficient of both the water uptake from the regenerating bath and the NMMO outflow to this bath. The diffusion coefficient of water going to the cellulose solution is about 10 times larger than the diffusion coefficient of NMMO leaving the solution. This difference expresses the strongly hygroscopic character of NMMO. None of these coefficients depends on cellulose molecular weight showing that no major rearrangement of cellulose chains occurs at the beginning of the regeneration. The diffusion coefficient of water is not influenced by the cellulose concentration, whereas the diffusion coefficient of NMMO decreases strongly when the cellulose concentration increases. Extrapolating the diffusion coefficient of NMMO versus cellulose concentration to zero shows that the maximal concentration of cellulose in NMMO-water is about 15%. Above this value, undissolved cellulose should be present. From the influence of the NMMO content in the water regenerating bath, it is possible to see that NMMO is removed from the solution if the bath has a NMMO content lower than 60%, to be compared with the 80% NMMO concentration in the solution. © 2005 American Chemical Society

Phase diagram of a cellulose solvent: N-methylmorpholine-N-oxide-water mixtures

International audienceThe phase diagram of the N-methylmorpholine-N-oxide-H2O mixtures from 0 to 100% has been determined. Three crystalline hydrates have been identified, the already known monohydrate, a dihydrate and a hydrate composed of 8 water molecules per NMMO. The melting temperature of the 8H(2)O-NMMO hydrate is -47degreesC with a melting enthalpy of about 80 J/g. The region between 25 and 55% of water does not show any crystallisation, but a glass transition around - 60 to - 100degrees C

Crystallisation of cellulose/N-methylmorpholine-N-oxide hydrate solutions

International audienceN-methylmorpholine-N-oxide (NMMO) hydrates are direct solvents for cellulose used commercially in the preparation of cellulose spinning dopes for fibre and film manufacturing. The fact that the cellulose/NMMO/water solutions can crystallise under cooling is important in the process of preparing fibres and films and in their structure formation. It is shown here that the major difference with classical polymer solutions is that the crystallisation of cellulose/NMMO/water solutions is only due to the crystallisation of the solvent, not of the cellulose. The reason for the decrease in crystallisation velocity with the increase in the cellulose concentration is the reduction in the crystallisable part of the solution. The concentration of water in solutions with the same cellulose content is found to strongly influence the crystallisation velocity and the morphology of crystallised solutions. The variation of the crystallisation velocity values with the type of cellulose can be explained by different amounts of free water bound to NMMO, that depend on the cellulose origin

Influence of the lyocell fibers structure on their fibrillation

International audienceThe objective of this work is to link structure and processing conditions to the fibrillation of Lyocell fibers. Two main types of Lyocell fibers were studied, standard with various processing conditions and crosslinked. Standard fibers have a skin-core structure with a more amorphous skin of 35-80 nm. Crosslinked fibers are resisting much more to fibrillation in the wet state. The crosslinked layer is thicker that the skin. The diameter distribution of fibrils is very wide and ranges from a few tens of nanometers to several micrometers. It depends on the type of fiber and its processing conditions. The higher the wet abrasion resistance is, the larger the number of fibrils is and the finer their diameter is

Small-angle scattering of polarized light .V. Liquid crystalline droplets in an isotropic polymer

International audienceThe light scattering properties of spherical droplets of a nematic low molar mass liquid crystal dispersed either in a polymer matrix or in water have been investigated. The size of the droplets varied from 0.5 mu m to 100 mu m. Only a radial arrangement of the director in spherical droplets was considered. Up to ten orders HV scattering patterns were experimentally measured. Calculations have been conducted using the anomalous diffraction and Rayleigh-Gans-Debye theories. Particles larger then 2 mu m show an extremely good agreement with the anomalous diffraction theory. Particles smaller than 2 mu m show no agreement with either theory. This may be due to a more complex director arrangement than the radial distribution assumed here