University of Groningen

Liquid crystalline solutions of cellulose in phosphoric acid for preparing cellulose yarnsBoerstoel, H.

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1. Introduction

1.1 Cellulose as a raw material for fibers Cellulose is probably the most extensively investigated polymer. Although its use as a raw material for fibre production started long ago the interest in it is still vivid. Its major advantage is its availability as a raw material: cellulose is a renewable and biodegradable resource. Both cellulose and starch consist of 1,4-linked anhydroglucose units. However, whereas cellulose is characterized by a P-linkage, which renders the polymer syndiotactic, starch

displays an a-linkage, giving an isotactic polymer1. This is illustrated in Figure 1.1. Whereas cellulose has an extended chain, the starch-component amylose forms a helical structure2.

cellulose

amy lose

chitin

chitosan

Figure I . I : Chemical formulae of cellulose, starch-component amylose, chitin, and chitosan. Cellulose has an extended backbone and amylose forms a helical structure.

Chitin is another natural polymer, rather similar to cellulose. It is the most abundant organic skeletal component of invertebrates, e.g., in the shells of lobsters, crabs, shrimps and prawns.

Chitin differs from cellulose only in that the C2 position is taken up by an acetarnide group in

stead of by the hydroxyl group. Chitosan is the deacetylated form of chitin. Figure 1 1 also

displays the chemical structure of chitin and ~h i tosan~-~ . The anhydroglucose units in cellulose contain 3 hydroxyl groups, which may lead to various ways of intra- and intermolecular hydrogen bonding6. The crystal structure of cellulose may vary, the most important being attached to cellulose I and I1 being the most important. Cellulose I is the kind appearing in nature, whereas cellulose I1 is present in ,e.g., man-made fibers. The diierence between these two crystal modifications is treated in Chapter 7. The degree of polymerization of cellulose (,i.e. the number of glucose-units, DP) depends on the source. Whereas in wood the DP is approximately 3000, in natural fibers like flax, ramie, and cotton the DP is even higher. Cellulose is the natural construction material in plants and trees. In wood, cellulose forms part of a fiber-reinforced composite, in which the cellulose chain molecules, organized in fibrils are held together and protected by hydrophobic lignin acting as binder and encasement7. In wood the cellulose content is approximately 50 %. In order to use cellulose as a raw material for fiber production pulping processes are necessary to isolate the cellulose. The pulping processes comprise a number of purification steps, viz. removal of lignin and hemicellulose (,i.e. lower molecular mass polysaccharides containing other sugars as monomer units in addition to glucose) bleaching, degradation and removal of

the low molecular weight cellulose. In dissolving pulp for fiber manufacturing the a-content, i.e. the pure cellulose fraction, is higher than 90%, and for some applications even higher than 95%. These days great efforts are being made in obtaining pulping processes that are less harmfUl to the environment. The three hydroxyl groups per monomeric unit can be made to react with numerous chemicals to form cellulose derivatives. The degree of substitution @S) is expressed as the number of substituents per glucose unit. There are several processes for the production of cellulose yarns, both for textile and industrial applications. The commercial types all have a limited tenacity. Among the (formerly)

commercial yam types are those obtained by the viscose, cuprammonium, Fortisan and N- methylmorpholine N-oxide (NMMO) processes. A brief review of these production processes is given below. In addition, two processes from the patent literature will be discussed, which

have shown the enormous potential of cellulose for manufacturing high tenacity, high modulus yams.

The viscose process Viscose Rayon was first produced more than a century ago. In 1990 the annual production of viscose fibers was 2.5 million tonss. Cellulose is swollen in an I8 % wlw- NaOH-solution to form a slurry in which alkali cellulose is produced:

Cell-OH + NaOH ++ . Cell-ONa + Hz0

The residual hemicellulose and the low molecular weight cellulose dissolves to some extent in the alkali solution. By concentrating the slurry via a filter press, part of these substances is removed. This is a very elegant way of getting rid of the components which do not contribute to the tenacity of the yarn. Thereupon, the so-called "white crumb" is allowed to ripen, which means that depolymerization occurs. Adding carbon disulfide (CS2) will result in the formation of cellulose xanthate:

For rayon production the degree of substitution (or xanthate ratio, X R ) is between 0.25 and 0.8 '. This "yellow crumb" is then dissolved in an alkali solution. After deaeration and filtration the solution is pressed through a spinneret into the coagulation bath, which comprises dilute sulhric acid, as well as a number of salts and other additives. Coagulation proceeds slowly, during which the filaments are optionally elongated. During coagulation and washing the cellulose xanthate is decomposed. Neutralizing, finishing, and drying the yarn complete the process. The degree of polymerization in the yam ranges from 300 (textile, Enka viscosen) to 600 (industrial, cordenkaa) .

A high modulus variant is Cordenka EHI@', which was made by adding formaldehyde either to the viscose or to the spinning bath. In the acid spinning bath sodium cellulose xanthate is converted into cellulose xanthic acid, which reacts with formaldehyde, in accordance with the following scheme with formation of S-hydroxy methyl cellulose xanthate.

This compound is relatively stable in the spinning bath. The newly formed filaments can be drawn further, yielding higher oriented, stronger fibers''. In the viscose process environmental issues have become increasingly important.

The cuprammonium process In 1990 the annual production of cuprammonium yarns amounted to 30,000 tonss. Cellulose, which is additionally purified, is mixed with copper sulfate and ammonium hydroxide. The solution is degassed, filtered, and pressed through a spinneret into a slightly alkaline coagulation bath. The possibility of spinning the solution via an air gap was mentioned as far back as 1931". Asahi is working on high speed spinning systems, applying spinning speeds of

1000 to 2000 m/mins. An increased recovery of chemicals and high spinning speeds will ensure its taking up a place next to viscose yams. Apart from the process being used for yarn production for textile applications, it is used for

manufacturing dialysis membranes (cuprophaneB by Akzo Nobel).

The Fortisan process Fortisan is a saponified cellulose acetate. Use is made of the thermoplastic properties of cellulose acetate. A cellulose acetate yarn prepared by dry spinning a solution of cellulose acetate in acetone, is drawn in steam, to improve orientation. Subsequently, it is saponified in caustic soda or sodium acetate to yield a highly oriented cellulose yarn12'13 The process was developed by British Celanese. The yarn is no longer produced.

The NMMO process In 1969 a patent application by Eastman Kodak first described the use of N-methylmorpholine- N-oxide (NMMO) as a direct solvent for cellulose14s8. Cellulose and an aqueous NMMO- solution are mixed. Then water is evaporated and cellulose starts to dissolve. Dissolution will only occur in a narrow range of water concentrations. Since explosive mixtures may be formed, especially in the presence of metal ions, the solution is stabilized by the addition of propyl gallate15"6. The solution is degassed, filtered and pressed through a spinneret into an air gap, which is carehlly climatized, and where the filaments are drawn. The filaments then enter a water bath, in which rapid coagulation takes place. Washing, finishing and drying complete the process. Courtaulds has commercialized the process for staple fiber production under the trade-name

~encel@. Akzo operates a pilot plant for filament yarns (~ew~el l ' ) . The staple fibers by

Lenzing are known as Lenzing ~ ~ o c e l l @ .

The DuPont process Cellulose acetate is dissolved in trifluoroacetic acid with cosolvents as formic acid or dichloromethane, to form a liquid crystalline s o l ~ t i o n ' ~ ~ ' ~ . The effects of liquid crystallinity will be dealt with later on. The solution is spun into a coagulation bath of methanol. The cellulose acetate is optionally steamdrawn to improve orientation, and subsequently saponified with a solution of sodium methoxide in methanol. For a number of filaments very impressive mecha- nical properties are found, which are indicative of the potential of a cellulose yam. However, the chemicals used are not attractive to be applied on an industrial scale.

The Michelin process An interesting development for making high tenacity yams is the Michelin process20. Cellulose is dissolved in a mixture of formic acid and phosphoric acid. In situ derivatization occurs, as a

consequence of which cellulose formate is formed. The excess formic acid and the phosphoric acid serve as a solvent. As is the case for the DuPont process, the solution of cellulose formate in the acid mixture is liquid crystalline. The solution is spun via an air gap spinning process. In the air gap the filaments are drawn. The filaments are coagulated in an acetone bath. The cellulose formate yarn is then saponified by using an alkali solution. This piocess produces high modulus and high tenacity yarns.

1.2 Liquid crystals 1.2.1 Liquid crystalline phases for fiber spinning High modulus and high tenacity yarns can be prepared from liquid crystalline polymer solutions. Most of the polymers used for the preparation of these strong fibers comprise aromatic units in the backbone, e.g., poly (para-phenylene terephthalarnide) (PpPTA,

tradenames: t war on@ by Akzo Nobel and ~ev la r@ by DuPont) and polybenzoxazole (PBO). Recently a new rigid rod fiber was developed by Akzo Nobel: Poly{2,6-diimidazo[4,5-b:4',5'- elpyridinylene-l,4-(2,s-dihydroxy)phenylene) (PIPD or "~5")~'. Examples of rigid chain polymers are given in Figure 1.2.

0.

PpPTA

PBO

+:la:H* N

OH

PIPD or M5 Figure 1.2: Monomeric units of st$ polymers used for spinning high modulus and high tenacity yarns from liquid crystalline solutions

An overview of the application of liquid crystalline polymer solutions is given by Northolt and sikkemaU.~he advantage of using liquid crystalline polymer solutions is that the chain conformation and the orientation are already on such a level that they can be transformed into highly oriented fibers, without the necessity of a post treatment.

1.2.2 Liquid crystalline behavior 23,24

A liquid crystalline phase is an intermediate state between the solid crystalline one and the liquid one, which is why it is also called a mesophase (meso=intermediate). Such materials can flow like a liquid, but other properties, e.g., the index of refraction, are anisotropic as in a crystal. Whereas in isotropic liquids there is only a short range order between the molecular centers of mass, in a crystal the centers of mass of the molecules are located on a three- dimensional periodic lattice, thus having a long range order. Besides, when dealing with anisotropic molecules there will also be a long range orientational order. These two long range orders generally vanish at the melting temperature, but some substances display intermediate liquid crystalline phases, characterized in that the orientational order is still present, but the positional order is reduced or has even disappeared23. This is schematically illustrated in Figure 1.3.

A liquid crystalline phase may be formed when the system contains anisometric molecules, either rodlike or disclike. For the elongated molecules the main subdivision is into nematic and smectic. In the nematic phase there is only a long range orientational order, and no long range positional order. In addition to the long range orientational order, smectic phases have a long range positional order in at least one direction. The unit vector corresponding to the direction of the preferred axis at each point in the solution is called the director.

CRYSTAL NEMATIC ISOTROPIC

Figure 1.3: A liquid crystalline phase on a temperature and concentration scale

Crystalline solid 4 L

Liquid crystalline phase

Isotropic liquid

So far only molecules with mirror symmetry have been considered. If a molecule is optically

active, a chiral nematic or cholesteric phase might be formed. Figure 1.4 gives a schematic

representation of such a phase. Locally the same order is found as that in nematics, but in

adjacent planes the locally preferred direction of alignment is slightly rotated. The distance

needed to hlly rotate the director is called the pitch. If half of the pitch is of the order of the

wavelength of visible light, this will give rise to interesting optical properties like Bragg reflections. For the solutions considered here we assume a nematic phase, whether chiral nematic or not. The solutions are also often referred to as solely anisotropic. In that case a nematic phase is meant. Two cases of liquid crystalline behavior are referred to as thermotropic, where the phase behavior is governed by temperature only, and lyotropic, where concentration is the determining factor. Both the concentration and temperature scales are indicated in Figure 1.3.

Liquid crystalline polymer solutions often display both thermotropic and lyotropic behavior.

Upon heating the birefiingent solution or melt will become optically clear, when it reaches the

isotropic state. This point is called the clearing temperature. Alternatively, it is also possible to cross-over from nematic to isotropic by changing the polymer concentration.

Figure I . 4: Schematic view of the chiral nematic (or cholesteric) phase

1.2.3 Theories on liquid crystalline phases A variety of theories on liquid crystalline systems have emerged. The main concepts will be discussed here.

The Onsager approach The first theory on the isotropic-anisotropic transition, as introduced by Onsager, describes the effect of rod-shaped particles on the state of their solution25. This is done by viewing the polymer solution as a gas of N rods and by using a potential, w, of mean force between the molecules, the size of which depends on the distance between two test rods, and on their

~r ienta t ion~~. The Helmholtz free energy A?? of the system is expressed as a hnction of the particle distribution by a virial expansion method that is truncated at the second termz6"'.

-- AF - Constant + Inn + o + bnp' NkT

where N denotes the number of rods in volume V, n the number density ( N N ) , a the entropy

loss due to orientation; the translational entropy is represented by b n p' (with p' an orientation

dependent term. Parameter b is a volume term given by ( 7 ~ / 4 ) ~ ~ d , where L is the length and d the diameter of the rod. For obtaining a stable solution Equation 1.1 is then minimized with respect to orientation. At a critical volume fraction an anisotropic phase appears. This transition is brought about by the competition between the packing entropy, favoring an ordered state, and the orientational entropy, favoring a disordered state23. At this point the isotropic and anisotropic phase coexist.

The critical volume fraction 4' for the appearance of an anisotropic phase can be expressed in terms of the axial ratio x (=Lid) ":

The solution becomes filly anistropic beyond a volume fraction #'of :

The validity of the theory is limited to the lower concentration range, which renders the appearance of an anisotropic phase rather contradictory23~2528. Furthermore, the theory is athermal, which means that the phase-transition would be independent of temperaturez4

However, the Onsager approach is appreciated in that it gives a qualitatively correct picture of

the order-disorder transition caused by the shape of the molecules.

Odijk introduced corrections for polydisperse rods and wormlike chains, and higher order virial

coefficient^^^. Also the effects of polyelectrolytes are discussed. In the event of a charge on the

rods the effective diameter increases because of the electric double layer and hrthermore the rods exert a twisting force on each other. These effects are incorporated in the description of the virial expansion by Odijk.

The Flory

In 1956 Flory introduced a lattice theory for the description of the liquid crystal phase separation in a solution of rodlike polymers with axial ratio x (=L/d). The model, which merely considers geometric contributions, is based on the Gibb's free energy of mixing of the solution.

The critical volume fraction 4' for the appearance of a stable anisotropic phase is given by:

Equation 1.4 is considered to be valid for values of the axial ratio larger than 10, and it

describes the minimum volume fraction attended with an ordered phase. For large values of x the equation reduces to:

The minimum volume fraction for a single anisotropic phase is 12.51~. The minimum value of

the axial ratio for the coexistence of two phases is 6.7. In spite of the presence of an enthalpic term in the equation for the free energy, the describtion of the critical volume fraction is reduced to a geometrical consideration.

The major drawback to this lattice theory, as for the Onsager approach, is that it is athermal, meaning that it fails to predict the occurrence of a clearing temperature.

The above description was developed for rodlike molecules, which means that the molecular weight should be sufficiently low3'. At higher molecular weights the contour length exceeds the persistence length and the chain becomes semiflexible. Flory incorporated this into the theory by considering the macromolecule as a chain of freely jointed rigid segments3'. Instead of the contour length, the length of the rigid block is the determining parameter then. Flory

suggested that the persistence length would be a good estimate of this length. The axial ratio would then be based on this persistence length. Ciferri pointed out that by using the persistence

length there is a discrepancy between theory and experimental observations, which would

vanish when taking the Kuhn length, i.e. twice the persistence length, rather than the

persistence

Using the length of a rigid segment offers the possibility of incorporating thermal effects. By

rendering the Kuhn length, or the persistence length temperature dependent, thermotropic

behavior is introduced, in which case the phase transition is governed by both temperature and concentration. It was found experimentally that the persistence length L, depended on

temperature T as:

with for cellulose (derivatives) the value of n ranging from 1.6 to 3.7, whereas for aramids this value is found to be In later versions of the lattice theories orientation-dependent

interactions were taken into account, based on anisotropy of the polarizability34. Cifem and Marsano have evaluated the applicability of the virial expansion method and the

lattice theory for cellulose derivatives31. Khokhlov and Semenov combined aspects of both the Onsager and the Flory approach. They incorporated attractive forces in their so-called universal method. Various descriptions of a

polymer chain, e.g., a wormlike chain or freely jointed rigid segments, gave rise to different answers2'.

The Maier-Saupe theory35'36'23'24 This kind of approach is widely used for describing low molecular weight nematics and it

considers the effect of attractive dispersion forces. Hence, in contrast to the virial expansion and lattice theories, enthalpic effects are considered, rather than entropic ones. The need of incorporating enthalpy terms for low molecular weight substances is evident. For, the excluded

volume theories (the Onsager and Flory approaches) fail to predict anisotropy at low axial ratios23. The Maier-Saupe model describes the orienting effect on a molecule in the mean field of its neighbors. The effective potential that leads to macroscopic orientational order is related

to the orientational order via a coupling constant E, which in the basic concept corresponded to the anisotropy of the polarizability23. A first-order phase transition is predicted from an

anisotropic to the isotropic phase24. A drawback to this approach is the fact that geometric contributions have been neglected. However, the Maier-Saupe model appears to give a fairly good description of a number of properties of the nematic phase, e.g. the orientation in the nematic phase, and the prediction of a clearing Later on the original meaning of the coupling constant was abandoned and the model was

therefore turned into a phenomenological one, with the coupling constant determining the strength of the orienting Picken made use of the presence of this coupling

constant to incorporate influences of concentration and molecular weight and by taking the influence of temperature on the persistence length into a c c o ~ n t ~ ~ , ~ ~ , ~ ' . For a sufficiently high molecular weight the persistence length L, is used rather than the contour length. The temperature dependence was considered to be of the form of Equation 1.6. The clearing temperatureT, was considered to be related to the temperature as:

where the change of persistence length with temperature is incorporated in the parameter n. Thus a description could be given of the phase behavior of aramid solutions for fiber spinning.

1.3 Liquid crystalline solutions of cellulose-based polymers 1.3.1. General Gray has formulated some rules of thumb for the formation of mesophases in solution of cellulose-based polymers38. Highly polar or acidic solvents generally initiate the formation of a liquid crystalline phase at lower concentrations than simple organic solvents. Cellulose derivatives with a high degree of substitution and large substituents easily form mesophases in a wide variety of solvents. However, the less substituted derivatives, or those with smaller substituents only form mesophases in specific solvents and not in others. Gilbert gives a review of mesophases of cellulose-based polymers39*40. Some highlights will be given below.

1.3.2 Cellulose derivatives Although Panar and Willcox filed a patent application on many liquid crystalline solutions of cellulose esters and ethers in various organic solvents before Werbowyj and Gray published their results on mesophases of hydroxy propyl cellulose (HPC) in water, the paper by the latter authors was published earlier and HPC in water is therefore considered as the first mesophase of a cellulose (deri~ative)~"~~. After this first publication the system was studied extensively. Overviews of various liquid crystalline solutions are given by Gray and by ~ i l b e r t ~ ~ ~ ~ . We only mention a selection here. Asahi filed a patent application on various cellulose esters and ethers in inorganic In a DuPont patent application anistropic solutions of cellulose acetate in trifluoroacetic acid are used to spin high tenacity A Michelin patent application describes anisotropic solutions of cellulose formate in a mixed solvent of formic acid and phosphoric acid. Cellulose formate yarns are spun and saponified to high tenacity cellulose yarns20.

1.3.3 Non-derivatized cellulose Chanzy and Peguy studied anisotropy in solutions of cellulose in a mixture of N-

methylmorpholine N-oxide (NMMO) and water4'. Solutions appeared to be birefringent in a polarization microscope above a cellulose concentration of 20 % wlw. Parameters determining

the appearance of a mesophase were cellulose concentration and DP, molar ratio of water to NMMO and temperature. Whereas oriented films were prepared from birefringent solutions only, they did not succeed in inducing orientation in isotropic solutions. Navard and Haudin determined phase-transitions rheologically and optically. In capillary flow they found a jump in viscosity and in the polarization microscope the solution became optically clear at the transition

temperature46. Coulsey and Smith stated that at the high concentrations needed for mesophase formation the solution still has such a high viscosity that preparing and handling would be impractical and that commercial processes would therefore operate under isotropic conditions4'. Patel and Gilbert reported on anisotropic solutions of cellulose in mixtures of trifluoroacetic

acid (TFA) and dichloromethane (DCM). It was found that whereas a solution of 20 % w/v cellulose in the solvent at a ratio of TFA/DCM of 70130 was birefringent, solutions below a concentration of 15% wlv were isotropic48. In the review by Gilbert it is discussed whether or

not derivatization occurs in this system39.

Anisotropic phases of cellulose in dimethyl acetamide (DMAc)/ Lithium chloride (LiCI) have

been However, before a hlly anisotropic system was reached insolubility

occurred upon further increasing the concentration. Precipitation also occurred when the birefringent phase was separated from the isotropic one. It was shown by means of light

scattering that aggregation of molecules occurs and it was concluded that this must play a role in the anisotropic phase. Chen and Cuculo describe the formation of a mesophase of solutions of cellulose in ammonia1 ammonium t h i ~ c ~ a n a t e ~ ~ . For a degree of polymerization of 450 a single anisotropic phase was formed above a concentration of 13 % wlv. Below these concentrations biphasic solutions

appeared. Isotropic solutions became turbid and gel-like upon ageing, and spherulites were

formed, the nature of which was not elucidated. Miyamoto, Okajima and Kamide prepared anisotropic solutions of cellulose in a mixture of sulfiric acid (SA), polyphosphoric acid (PPA), and water (w) using one specific composition of the solvent, viz. SA/PPA/w=1/8/1. Birefringence was observed above a cellulose concentration of 16 % wlw.

1.4 Phosphoric acid

1.4.1 General Phosphoric acid is a special acid in that it can form dimers, oligomers and even polymeric

forms. Orthophosphoric acid is the reaction product of phosphorus pentoxide and water:

Pyrophosphoric acid ( w 2 o 7 ) is the dimer of orthophosphoric acid. Polyphosphoric acid as used in this thesis has the overall composition of &P4Ol3. The compositions can all be reduced to a P205 concentration. Thus orthophosphoric acid corresponds to a P205 content of 72.4 %

WIW, pyrophosphoric acid of 79.6 % wlw, and polyphosphoric acid of 84 % wlw. Analogously, concentrations can be expressed as orthophosphoric acid concentrations. The concentration of orthophosphoric acid is 100 % wlw, of pyrophosphoric acid 109.9 % wlw and of polyphosphoric acid 116.0 % wlw.

Table 1.1 : Percentage contents of individual polyphosphoric acids in a concentrated polyphosphoric acid mixture as a firnction of P2O5 content. The percentages refer to the fraction of phosphorus present in the form of the acid indicated. In the heading are the numbers o f ~ a t o m s ' ~ .

* trace

In Ullmann an overview is given of the PzO5-water system. Table 1.1 shows a part of that54.

It appears that there is always a distribution of acids. Above a P205 concentration of 68.7 % wlw orthophosphoric acid dimerizes and water is split off.

Table 1.1 shows that the phosphoric acid system is a homologous series of acids of the general formula H(P03H),0H. There is always a mixture of acids, the distribution of which is depen- dent on the PzOS content. The equilibrium distribution might be temperature dependent and non-equilibrium situations might also be possible.

Figure 1.5: Melting temperatures of the phosphoric acid ~ y s t e m ' ~ ' ~ ~

Because of the complexity of the P205 /water system it also has a rather complicated phase- diagram. The melting temperature as a fbnction of the P205 concentration is displayed in Figure 1 . ~ ~ ~ ~ ~ ~ . Maxima appear at concentrations corresponding to the homologs of the acid, such as orthophosphoric acid, pyrophosphoric acid. A maximum even appears at a lower concentration than that corresponding to orthophosphoric acid, and is typical of the hemihydrate of orthophosphoric acid (&Pod. 112 H20).

So far we have discussed the equilibrium conditions of the phosphoric acid system, but it seems worthwhile to also look into the rate at which equilibrium is reached. In the kinetics of the phosphoric acid system various influences can be distinguished5'. The most important parameters that influence the hydrolization rate of pyrophosphoric or polyphosphoric acid are

temperature and pH.

1.4.2 Phosphoric acid as a solvent for cellulose (derivatives) Cellulose has long been known to be soluble in phosphoric acid. As far back as in 1925 a patent application was filed by British Celanese on the solubility of cellulose in phosphoric acids6. In the patent it is described that solutions containing 4 % wlw of cellulose (Cotton Linters) could be prepared in aqueous phosphoric acid, the solvent having a concentration of 85-88 % wlw &Po4. In 1980 Turbak mentions phosphoric acid as an interesting solvent for cellulose. However, the drawback he sees is that once dissolved in phosphoric acid, the cellulose is difficult to isolate5'. In the mid 1980s there are two Russian patent applications on

the matter. In that case cellulose is dissolved in 80-85 % wlw orthophosphoric None of the publications of solutions of non-derivatized cellulose in phosphoric acid make mention of anisotropy. However, there are a number of papers on anisotropic solutions of derivatized cellulose in phosphoric acid. In 1979 an Asahi patent application describes anisotropic solutions of cellulose esters and cellulose ethers in inorganic acids, e.g., cellulose

acetate in aqueous orthophosphoric acid43.44. In a Michelin patent application filed in 1984 phosphoric acid is used in a solvent mixture. Due to in situ derivatization cellulose formate dissolves in a mixture of formic acid and phosphoric

acid to form a liquid crystalline solution. The solutions were used for spinning high tenacity cellulose yarns20. Finally, an Asahi-patent application and a relevant paper describe the formation of an anisotropic solution of underivatized cellulose in a specific mixture of sulhric acid (SA), polyphosphoric acid (PPA) and ~ a t e r ~ " ~ ' . Cellulose having a DP of 331 was added in an amount of 5 % wlw to various mixtures of SA, PPA, W, with the solvent composition being varied very systematically. They found an area in which cellulose is soluble and rather stable, in the sense that there occurs no unduly fast degradation. In that area of solubility solutions of 18 % wlw cellulose were prepared; and it was found that an anisotropic phase is formed only in the case of one specific solvent composition. The cellulose concentration had to exceed 16 %

wlw for anistropy to appear. This specific solvent composition was SAPPAIW = 11811. No other composition of the solvent was found to be attended with anisotropy, not even when the cellulose concentration was increased to 20 % wlw. It was observed that in the case of this specific composition a white precipitate was formed in the solvent at -10 "C. Based on this

observation and on Raman spectra of the solution it was concluded that in this particular solvent a complex was formed, with sulfuric acid being built in a matrix of polyphosphoric

acid. Thus account is given of this composition inducing liquid crystallinity, upon cellulose being dissolved. We now have found that anisotropic solutions can be formed when dissolving cellulose in phosphoric acid in a specific concentration range, without the aid of sulhric acid and without derivatization, contrasting with the patent applications by Asahi and Michelin, r e ~ p e c t i v e l ~ ~ ~ ' ~ ~ . The solution can be prepared within a few minutes, which indicates the power of the solvent. The quick dissolution of cellulose in this solvent is very advantageous for processing. It was found that water plays a detrimental role in the solvent quality. The problem then is, however, that anhydrous orthophosphoric acid is a solid with a melting point of 42°C. If cellulose is to be dissolved therein, the temperature has to be raised. Under such conditions rapid degradation occurs. Apart from that there is always still about 1-2 % w/w water in the acid, and approximately 5 % w/w water in the cellulose, which might both account for the relatively poor solubility of cellulose. Both problems can be overcome by increasing the orthophosphoric acid concentration to over 100 % wlw, which will be elucidated in the following chapters.

1.5 Outline of the present thesis The present thesis describes a new process for the preparation of high modulus and high tenacity cellulose yarns. To that end use is made of liquid crystalline solutions of cellulose in anhydrous phosphoric, also referred to as superphosphoric acid. Chapter 2 describes liquid crystalline solutions of cellulose acetate in phosphoric acid. By determining the clearing temperature, the role of the degree of substitution and of the solvent quality governed by the amount of water in the solvent were studied separately. Chapter 3 deals with liquid crystalline phases of solutions of cellulose in phosphoric acid. The clearing temperatures of the solutions are compared with those of the cellulose acetate solutions. An attempt is made to compare the experiments with theory. A solution was analyzed displaying the optical effects typical of a chiral nematic phase.

In order to be able to transport the solutions exhibiting a peculiar flow behavior through the spinning machine, an extensive study has been made of the rheology. Chapter 4 deals with the response of the solution to shear flow. The cellulose concentration at which the phase transition occurs in flow is compared with that at rest. An analysis has been made of a flow induced phase transition. Chapter 5 gives a method for the determination of the elongational viscosity. Velocity profiles during drawing in the air gap were determined by means of laser Doppler anemometry. The elongational viscosity could be determined by force measurements after the coagulation bath as a fknction of the air gap length. The experimental values are compared with the Doi theory. Chapter 6 describes the development of the orientation in the air gap monitored by an on-line birefringence measurement. It was found that two different deformation mechanisms manifest

themselves both in the die swell area and at draw ratios larger than unity. A description for the

die swell area is proposed. Chapter 7 presents a structural and mechanical characterization of the new yarns. A comparison is made with existing yarns, e.g. those spun by several viscose processes.

1.6 References 1. G. Odian, Principles of Polymerization, 3rd ed., Wiley, New York (1991) 2. L. Vollbracht, Cherniefasern 39, 935 (1991) 3. Kirk-Other, Encyclopedia of Chemical Technology vol3, 4'h ed. pp 430 (1993)

4. R.A.A. Muzzarelli, Carbohydrate Polymers 3, 53 (1983) 5. H. Stryszcyk, 0. Kivekaes, British Polymer J. 23, 261 (1990)

6. L.M.J. Kroon-Batenburg,J. Kroon, M.G, Northolt 27, 290 (1986) 7. Ullmann's Encyclopedia of Industrial Chemistry, vol 5, pp375, VCH Weinheim (1991) 8. Kirk-Other Encyclopedia of Chemical Technology vol 10 4"ed., pp 696 New York

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