Effet de lentilles gravitationnelles et polarisation du fonddiffus cosmologique

Effet de lentilles gravitationnelles et polarisation du
fonddiffus cosmologique

CELLULOSE DISSOLUTION IN ALKALINE HYDROXIDE AQUEOUS SOLUTIONS

Cellulose is not a thermoplastic polymer. Thus, cellulose melting is not possible and dissolution is a necessary step to process cellulose. Because of intra- and intermolecular hydrogen bonds, the penetration of solvent is difficult and it has to be “strong” enough to break them. Different types of solvent can be used; they are largely described and classified by Klemm et al. [KLE1998]. The oldest process to dissolve cellulose is named “viscose process”: cellulose is chemically transformed before being dissolved, this is the xanthation step. But as this process is polluting, an alternative route is to dissolve cellulose in alkali hydroxide aqueous solutions. After describing the current industrial process (part I.2.1-), we will present the first studies on this new solvent of cellulose (part I.2.2.1). Our work was based on these data concerning experimental conditions: concentrations, temperature, additives. Several authors investigated cellulose/alkali hydroxide aqueous solutions (parts I.2.2.2 and I.2.2.3) in order to understand the dissolution mechanism. Some hypotheses are given concerning the mechanism at the level of fibres and microfibrils (part I.2.2.4), but there are practically no literature dealing with the mechanism at the molecular level. Concerning additives, it will be shown that the influence of the proportion between NaOH and urea or thiourea or zinc oxide was studied in details (part I.2.3-). However, the mechanism of the action of these additives still remains unclear, despites a rather large amount of publications. Pure cellulose (Pulp, cotton) Wetted cellulose (Pulp, cotton) Water Stocked at Troom Water content = 10% Water content = 100% Steam Explosion As-steam-treated cellulose (Pulp, cotton) Water content = 200% Water content = 10% Washed steam-treated cellulose (Pulp, cotton) P = 15 bars t = X sec. Stage 1 Stage 2 Stage 3 Stage 4 Washing 

Xanthation in viscose process

The viscose process was discovered in 1892 by Cross, Bevan and Beadley and allowed spinning of viscose fibres. Still nowadays, the step of xanthation of this process is a method widely used in industry to dissolve cellulose. This process can be divided into three main chemical steps, which are identical in most of the industrial viscose processes (Fig.I.2-1). Fig.I.2-1: Scheme of viscose process (1) formation of alkali-cellulose, (2) xanthation (3) dissolution (1) Cellulose, prepared from either wood pulp or, less commonly, cotton linters, is treated with 17-20% sodium hydroxide (NaOH), at a temperature in the range of 18-25°C, to convert cellulose to alkali-cellulose. Cell-OH + NaOH → Cell-O- ,Na+ + H2O Cellulose fibres are swelling and become more accessible to other reactants. Then the alkali cellulose is aged under controlled conditions of time and temperature (between 18 and 30°C) in order to depolymerise the cellulose to the desired degree of polymerisation (DP). The molecular weight obtained determines the viscosity of viscose solution and cellulose concentration. (2) After this maturing step, carbon disulfide (CS2 ), in gas or liquid state, is added to the solution of alkali-cellulose to react with hydroxyl groups and generate the xanthation reaction: S Cell-O- ,Na+ + CS2 → Cell-O-C S- ,Na+ The resulting product is a substance called ‘cellulose xanthate’. (3) Then this alkaline cellulose xanthate is dissolved in dilute sodium hydroxide (~2.7 wt%) and forms a viscous solution called ‘viscose’. S S- ,Na+ Cell-O-C + NaOH + H2O → Cell-O-C OH + H2O S- ,Na+ S- ,Na+ (1) (2) (3)  The large xanthate substituents on the cellulose force the chains apart, reducing the interchain hydrogen bonds and allowing water molecules to solvate and separate the chains. After obtaining viscose, industrial processes vary. The next steps can be: filtration and then spinning to obtain fibres or thin films, addition of reinforcing fibres and porophores then moulding to obtain sponges (see Chapter V), etc. The last step of processes is called coagulation: cellulose is precipitating in regeneration baths (water, acid or base). The final regenerated cellulose is obtained and CS2 and sulphured compounds formed during chemical reactions are going out. However, “viscose process” involves some pollution: in the atmosphere, because of CS2 and H2S, and in water, because of the reducing agents (containing sulphur compounds) which decrease the oxygen amount in water. Optimisation and improvement in the viscose process result in a total recycling of the pollutant compounds in the aqueous phase and in a decrease of pollutant compounds in the atmosphere. Nowadays the quantity of the latter is “only” 10% of CS2 used initially. Unfortunately, technologies for pollution treatment are very expensive. Thus for economical and for environmental reasons, it is very important to find new cellulose solvents. The purpose of research laboratories is to step away from the chemical modifications of cellulose by CS2 . NMMO, for example, is a good cellulose solvent, but these solutions are not very easy to prepare and to process. That is why it is interesting to look for alternative routes.

Action of alkaline hydroxide solutions on cellulose fibres

For a long time, it was known that strong alkaline aqueous solutions act on cellulose. Indeed, at the end of 19th century, Mercer [MER1903] discovered that cellulose fibres are swelling in aqueous solutions of sodium hydroxide and that was drastically changing the properties of the fibres. Several years later, in 1939, Sobue showed that cellulose can be dissolved in a narrow range of temperature and concentration of NaOH [SOB1939].

Introduction on mercerisation process and cellulose dissolution

Mercerisation is a process, which John Mercer developed between 1844 and 1850. Cotton cloth is immersed in a strong caustic alkaline solution, and then washed, in order to improve the lustre and smoothness, for example. Mercerisation also increases the ability to absorb dye, improves the reactions with a variety of chemicals, the strength and elongation of fibres and the stability of form. This type of cotton is often compared to silk and is especially used to produce high-quality fabrics. But this process tended to shrink the cotton cloth. In 1889, Horace Lowe improved this technique by keeping the material under tension whilst being mercerised and he applied a more thorough washing process to remove the caustic soda. Thus the mercerisation became a viable textile process. During mercerisation, alkaline solution acts on cellulose chains by changing the fine structure, morphology and conformation of cotton. The native cellulose I crystalline form is transformed into cellulose II, resulting in a variation in fibre strength and lustre as well as adsorption properties. In addition, soda fills up almost entirely the central cavity (the lumen) of cotton fibres. So, the ribbon-like cotton fibres become perfectly cylindrical and loose their convolutions inducing a smoother and shinier texture [MAR1941]. In mercerising, a decrease in length of cotton hair is accompanied by an increase in diameter: cellulose fibres are swelling. According to Mercer [MER1903], mercerisation also occurs at low NaOH concentration (~9%NaOH) and the properties of cotton are improved. The lower the temperature, the more effectively the soda acts on the fibrous material. This reveals that the term of mercerisation was very wide and corresponded to all changes in cellulose fibres due to the action of alkaline hydroxide aqueous solutions. Several conditions of temperatures and NaOH concentrations are reported depending on material (cotton, ramie, flax, yarn or cloth…) and on process. The most often used conditions are 18-32% NaOH concentrations at 25-40°C. It is important to note that hardly few minutes (from 20s to 3min) are enough to obtain mercerisation of cellulose. Nowadays, the term of mercerisation essentially represents the action of concentrated alkali hydroxides on cellulose. The NaOH concentration and temperature conditions to swell cellulose fibres are relatively well determined and slightly change depending on cellulose origin. In 30’s, Davidson defined optimal conditions to dissolve modified cotton. As illustrated Fig.I.2-2, a decrease of temperature improves “hydrocellulose” dissolution in sodium hydroxide solutions [DAV1934] (“hydrocellulose” was obtained by cellulose hydrolysis with strong acids). Fig.I.2-2 [DAV1934]: Hydrocellulose solubility versus NaOH concentration and solution temperature The maximum solubility is 80% and it occurs at NaOH concentration of 10%, at –5°C. Moreover, the solubility increases when the chain length decreases. Consequently, authors concluded that unmodified cellulose cannot be dissolved because of the large length of the macromolecules. But, in 1939, Sobue et al [SOB1939] showed that it is possible to dissolve cellulose, from natural ramie fibres, in a narrow range of phase diagram: between NaOH concentrations of 7% and 10% at low temperature (-5°C/+1°C). In the 80’s-90’s, cellulose dissolution in 7-10%NaOH/water was largely studied. Kamide et al. [KAM1984] revealed that regenerated cellulose can be dissolved in NaOH/water and that the dissolution depends on the degree of breakdown in O3-H…O5’ intramolecular hydrogen bonds. They also showed that the cellulose solubility depends on the DP, the concentration and the crystallinity of cellulose samples. Some years later [KAM1990], the authors also compared the Xray diffraction peak intensities of cellulose I (∆I1 ) and Na-Cell I (∆I2 ) as a function of cellulose concentration in 9%NaOH/water. Fig.I.2-3 represents the results obtained. It appears that up to 9% of steam exploded cellulose (DP being ~340), there is no peak of cellulose I and no peak of Na-cell I. This means that 9%NaOH/water totally dissolves cellulose when its concentration is inferior to 9% and that the total dissolution of cellulose I (Ccell < 9%) in 9%NaOH aqueous solution gives a solution without conversion of cellulose I to Na-Cell I. 

Table des matières

Résumé du Chapitre I
Etat de l’art : La cellulose et sa dissolution dans des solutions aqueuses d’hydroxyde de sodium
CHAPTER 1 STATE OF THE ART: CELLULOSE AND ITS DISSOLUTION IN ALKALINE HYDROXIDE AQUEOUS SOLUTIONS
I.1- Cellulose: structure, morphology and treatment
I.1.1- Generalities
I.1.2- Chemical structure of cellulose
I.1.3- Crystalline structures of cellulose
I.1.3.1. Cellulose I or native cellulose
I.1.3.2. Cellulose II
I.1.3.3. Cellulose III
I.1.3.4. Cellulose IV
I.1.4- Supra-molecular structure of cellulose
I.1.4.1. Micro-fibrils
I.1.4.2. Wood and cotton cellulose fibres
I.1.5- Extraction of cellulose and activation
I.1.5.1. Manufacture of cellulosic pulp
I.1.5.2. Activation of cellulose
I.2- Cellulose dissolution in alkaline hydroxide aqueous solutions
I.2.1- Xanthation in viscose process
I.2.2- Action of alkaline hydroxide solutions on cellulose fibres
I.2.2.1. Introduction on mercerisation process and cellulose dissolution .
I.2.2.2. Formation of alkali-celluloses
I.2.2.3. Hydration of alkali ions
I.2.2.4. Mechanism of mercerisation
I.2.3- Influence of additives on cellulose dissolution
I.2.3.1. Influence of zinc oxide (ZnO)
I.2.3.2. Influence of urea and thiourea
I.2.3.3. Other additives
I.3- Conclusions
REFERENCES
Résumé du chapitre II
Matériaux et méthodes expérimentales .
CHAPTER II MATERIALS AND METHODS
II.1- Materials
II.1.1- Cellulose samples
II.1.1.1. Avicel cellulose
II.1.1.2. Borregaard cellulose
II.1.1.3. Other cellulose samples
II.1.2- Solvents and additives .
II.1.2.1. Alkali hydroxides .
II.1.2.2. Additives
II.1.2.3. Surfactants
II.1.3- Other components used in sponge manufacture
II.1.3.1. Reinforcing fibres
II.1.3.2. Adhesion promoters
II.1.3.3. Na2SO4,10H2O crystals
II.1.3.4. Carrageenan
II.2- Solutions preparation
II.2.1- Drying of steam exploded cellulose
II.2.2- Dissolution procedure
II.2.3- Storage
II.2.4- Filtration
II.3- Methods
II.3.1- Differential Scanning Calorimetry (DSC)
II.3.1.1. Main principles
II.3.1.2. Temperature measurement
II.3.1.3. Melting temperature measurement
II.3.1.4. Melting enthalpy measurement
II.3.1.5. Calibration and experimental conditions
II.3.1.6. Software for peak deconvolution: “PeakFit 4.12”
II.3.2- X-ray diffraction
II.3.3- Rheology
II.3.3.1. Semi-dilute cellulose solution
II.3.3.2. Dilute cellulose solution
II.3.4- Microscopy
II.3.4.1. Optical Microscopy (OM)
II.3.4.2. Scanning Electron Microscopy (SEM)
II.3.5- Mechanical property: tensile tests
II.3.5.1. Tensile tests on films of regenerated cellulose
II.3.5.2. Tensile tests on regenerated cellulose + reinforcing fibres .
II.3.5.3. Tensile tests on sponges (regenerated cellulose + fibres + porophores)
II.3.6- Other methods
II.3.6.1. Density measurement
II.3.6.2. Water absorption
II.3.6.3. Shrinkage
REFERENCES
Résumé du chapitre III
Structure des solutions aqueuses de NaOH et de cellulose/NaOH avec et sans additifs. Limite de
dissolution de la cellulose
CHAPTER III STRUCTURE OF AQUEOUS NAOH AND CELLULOSE-NAOH SOLUTIONS WITH AND WITHOUT ADDITIVES. LIMIT OF CELLULOSE DISSOLUTION
III.1- Bibliography: phase diagram of aqueous sodium hydroxide solution .
III.1.1- Phase diagrams
III.1.1.1. Binary phase diagram
III.1.1.2. Ternary phase diagram
III.1.1.3. Methods for building phase diagrams. The limits of the DSC analysis .
III.1.2- State of the art on the structure of NaOH aqueous solutions .
III.1.2.1. Pure sodium hydroxide
III.1.2.2. Sodium hydroxide hydrates
III.2- Structure of cellulose solvents: NaOH/water, NaOH/urea/water and NaOH/ZnO/water
III.2.1- NaOH/water binary system
III.2.1.1. DSC melting thermograms of NaOH/water solutions
III.2.1.2. Peak at lower temperature: melting of the eutectic mixture
III.2.1.3. Peak at higher temperature
III.2.1.4. Proportions between NaOH and water
III.2.1.5. Conclusions on the phase diagram and structure of NaOH/water solutions .
III.2.2- NaOH/urea/water ternary system
III.2.2.1. Urea/water binary phase diagram
III.2.2.2. NaOH/urea/water ternary phase diagram .
III.2.3- NaOH/ZnO/water ternary system
III.2.3.1. Viscosity of dilute NaOH/ZnO/water solutions
III.2.3.2. NMR study
III.2.3.3. DSC results
III.2.3.4. Conclusions on the NaOH/ZnO/water system
III.2.4- Conclusions on the structure of cellulose solvents: NaOH/water, NaOH/ZnO/water and NaOH/urea/water
III.3- Structure of cellulose solutions in NaOH/water, NaOH/ZnO/water and
NaOH/urea/water
III.3.1. Cellulose/NaOH/water ternary solutions
III.3.1.1. DSC results on Avicel/NaOH/water
III.3.1.2. Peak at low temperature. Limit of cellulose dissolution
III.3.1.3. Peak at high temperature .
III.3.1.4. Influence of cellulose origin on the structure of cellulose/NaOH/water solutions
III.3.1.3. Conclusions
III.3.2. Cellulose/NaOH/urea/water solutions
III.3.2.1- DSC results on Avicel/NaOH/6%urea/water
III.3.2.2. Peak of NaOH eutectic
III.3.2.3. Peak of urea eutectic
III.3.2.4. Peak of free ice melting
III.3.2.5. Conclusions
III.3.3. Cellulose/NaOH/ZnO/water solution .
III.3.4. Conclusions on the structure of cellulose solutions in NaOH/water with and without
additives
III.4- Influence of freezing on the thermal properties of the cellulose/NaOH mixtures
III.5- Conclusions
REFERENCES
Résumé du chapitre IV
Ecoulement et gélification des solutions aqueuses de cellulose/NaOH : Influence des additifs
CHAPTER IV
FLOW AND GELATION OF AQUEOUS CELLULOSE/NAOH SOLUTIONS: INFLUENCE OF ADDITIVES
IV.1. Influence of ZnO and urea on the flow behaviour of cellulose/7.6NaOH aqueous solutions
IV.1.1. Influence of the presence of ZnO or urea in the solvent on the flow of cellulose/7.6NaOH aqueous solutions
IV.1.2. Conclusions
IV.2- Gelation and dissolution of cellulose in the presence of ZnO
IV.2.1. Influence of the preparation conditions on gelation of cellulose/7.6NaOH/water
solutions in the presence of ZnO
IV.2.2. Influence of ZnO concentration on gelation and cellulose dissolution
IV.2.3. Dissolution of cellulose in a solvent with a constant molar ratio ZnO:NaOH=1:10
IV.2.4. Influence of the presence of ZnO on gelation kinetics
IV.2.5. Influence of ZnO on cellulose intrinsic viscosity
IV.2.6. Discussion on the influence of ZnO on gelation of steam exploded cellulose in 7.6NaOH aqueous solutions
IV.2.7. Conclusions
IV.3. Gelation and dissolution of cellulose in the presence of urea; comparison between urea and ZnO
IV.3.1. Influence of the preparation conditions on cellulose dissolution and gelation in the presence of urea and of urea concentration
IV.3.2. Influence of cellulose DP and origin on the dissolution and gelation in the presence of urea and ZnO
IV.3.3. Discussion on the influence of urea on gelation and dissolution of steam exploded
cellulose in 7.6NaOH aqueous solutions and comparison with the role of ZnO
IV.3.4. Conclusions
IV.4. Influence of freezing on rheological properties of cellulose/NaOH aqueous solutions
IV.5- Tests on gelation and dissolution of cellulose in other alkali solvents: KOH and NaOH with other additives
IV.5.1. Testing cellulose dissolution in potassium hydroxide
IV.5.2. Addition of surfactants
IV.5.3. Other additives: salts and oxides
IV.5.3. Conclusions
IV.6. Conclusions
REFERENCES
Résumé du chapitre V
Propriétés mécaniques d’objets régénérés préparés à partir de solutions aqueuses de cellulose/NaOH
CHAPTER V. MECHANICAL PROPERTIES OF REGENERATED OBJECTS PREPARED FROM CELLULOSE-NAOH AQUEOUS
SOLUTIONS
V.1. Summary on sponge preparation: viscose and NaOH processes
V.1.1. Viscose process
V.1.2. NaOH process
V.2. Properties of sponges prepared with the NaOH process
V.2.1. Wet samples made from regenerated 5cellulose/7.6NaOH aqueous solutions, without
reinforcing fibres and porophores
V.2.1.1. Tensile stress results
V.2.1.2. Influence of regeneration bath parameters
V.2.1.3. Influence of solution thermal treatment
V.2.1.4. Influence of additives
V.2.1.5. Conclusions
V.2.2. Wet samples made from regenerted 5cellulose/7.6NaOH aqueous solutions with
reinforcing fibres without porophores
V.2.2.1. Influence of added reinforcing fibres
V.2.2.2. Influence of fibres treatment on the rupture stress
V.2.2.3. Influence of freezing on rupture stress and adhesion between fibres and matrix
V.2.2.4. Influence of the thermal history of fibre treatment with 8%NaOH aqueous solution on the mechanical properties of wet regenerated samples
V.2.2.5. Conclusions
V.2.3. Wet samples made from regenerated 5cellulose/7.6NaOH aqueous solutions with
reinforcing fibres and porophores
V.2.3.1. Rupture stress
V.2.3.2. Absorption
V.2.3.3. Shrinkage
V.2.4. Conclusion on the role of the freezing step
V.3. Conclusions
REFERENCES

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