Elsevier

Polymer

Volume 42, Issue 19, September 2001, Pages 8249-8264
Polymer

The structure and properties of cellulose fibres spun from an anisotropic phosphoric acid solution

https://doi.org/10.1016/S0032-3861(01)00211-7Get rights and content

Abstract

The structure and the mechanical properties of a newly developed highmodulus/high strength cellulose fibre spun from an anisotropic solution in phosphoric acid are discussed and compared with those of existing regenerated cellulose fibres. The crystal structure of the fibre is of the cellulose II modification, and the highly oriented and crystalline fibres have an initial filament modulus of 44 GPa, a sonic modulus of 58 GPa, and a strength of 1.7 GPa. It is shown that the mechanical properties of this fibre are well described by the continuous chain model. Lateral birefringence measurements and an electron diffraction study have established the orientation of the optical axes nα and nβ relative to the a and b axes in the crystal structure. Moreover, the most likely crack directions in the cellulose II structure have been identified.

Introduction

Regenerated cellulose fibres have long been made according to various processes yielding fibres with a wide range of mechanical properties. They include textile fibres with a low modulus and tenacity but high elongation at break, fibres for technical applications such as tire yarns with an intermediate modulus and strength, and fibres with a high modulus and tenacity but low elongation at break [1], [2], [3], [4], [5].

In the present paper, we report on the structure and mechanical properties of a highly oriented cellulose fibre spun from an anisotropic solution in phosphoric acid using an air gap [6]. A number of processes have been developed for the commercial production of cellulose yarns, both for textile and industrial applications. Yarns of these processes all have a limited tenacity. Among them are those made by the viscose, cuprammmonium, Fortisan® and N-methyl morpholine oxide (NMMO) processes [7]. Yarns made by the last three processes are not commercially available any more. Two processes from the patent literature have demonstrated the great potential of cellulose in making high tenacity, high modulus yarns. In the viscose process, cellulose xanthate (Cell–O–CS2Na) is dissolved in an alkali solution and spun into a coagulation bath of diluted sulfuric acid, during which stretching is applied, the stretch ratio depending on the desired properties of the yarn. These yarns — referred to as Enka® Viscose and Cordenka® — serve textile and industrial applications, respectively. A high modulus variant is Cordenka® EHM, now out of production, which was made by adding formaldehyde either to the spinning bath or the spinning dope. This slows down coagulation, as a consequence of which the yarn could be stretched further [2]. In the cuprammonium process, cellulose is dissolved in a mixture of copper sulfate and ammoniumhydroxide. In 1931, the possibility of spinning this solution via an air gap was already claimed in a Bemberg patent [8]. Fortisan® is a saponified cellulose acetate, which is prepared by dry spinning from a solution of cellulose acetate in acetone. Use is made of the thermoplastic properties of cellulose acetate by stretching in steam under pressure in order to improve orientation. Subsequently, the cellulose acetate is saponified in caustic soda or sodium acetate [5], [9]. In the NMMO-process, cellulose and an aqueous NMMO-solution are mixed to form a slurry. Water is then evaporated and the cellulose starts to dissolve. As an explosive mixture can be formed, the solution is stabilized by adding propyl gallate [10], [11], [12]. The solution is spun through an air gap, in which stretching is applied, into an aqueous coagulation bath. Courtaulds has commercialized the process for staple fibre production (Tencel®), while Akzo Nobel operates a pilot plant for filament yarns (NewCell®) and Lenzing for staple fibres (Lenzing® Lyocell). In patent applications by DuPont the preparation of high tenacity filaments is described [13], [14]. Cellulose acetate is dissolved in trifluoroacetic acid, to form a liquid crystalline solution, which is spun via an air gap into a methanol coagulation bath. The cellulose acetate fibre is optionally stretched in steam to improve orientation and then saponified with an alkali solution. The patent mentions filament tenacities of 2.7 GPa for a test length of one inch for a number of filaments, which indicates the high potential when a yarn is produced from an anisotropic cellulose solution. A Michelin patent application describes another process for making high tenacity cellulose yarns [15]. 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, which dissolves in phosphoric acid and the excess amount of formic acid. The resulting liquid crystalline solution is spun through an air gap, in which stretching is applied, into an acetone coagulation bath. Subsequently, the cellulose formate yarn is saponified.

Liquid crystalline solutions are known to be good precursors for high modulus/high tenacity yarns. The backbones of most of the polymers used for the preparation of these strong fibres comprise aromatic units, e.g. poly(p-phenylene terephthalamide) or PpPTA, polybenzoxazole or PBO, polybenzothiazole or PBT, and recently poly{2,6-di-imidazo[4,5-b:4′5′-e]pyridinylene-1,4-(2,5-dihydroxy)phenylene} or PIPD, but as already mentioned cellulose derivatives have also been used [16], [17]. The advantage of using a liquid crystalline solution for fibre production is that the local orientational order of the chains is already at such a level that they can be transformed into highly oriented fibres without the necessity of an after-treatment, which is required in spinning from isotropic melts or solutions of flexible polymers [18]. Reviews of mesophases of cellulose-based polymers have been presented by Gray and Gilbert [19], [20], [21]. We divide the systems into those based on derivatized and nonderivatized cellulose. Hydroxypropyl cellulose in water was the first cellulose derivative for which a mesophase was reported [22]. After this first publication the system has been studied extensively and many more mesophases of solutions of cellulose derivatives in a great number of solvents have been described [19], [20], [21]. Cellulose esters and ethers form mesophases in both organic solvents and inorganic acids [23], [24], [25]. As already mentioned, a liquid crystalline solution of cellulose acetate in, e.g. trifluoroacetic acid has been used to spin highly oriented fibres [13], [14]. The same holds for anisotropic solutions of cellulose formate in a mixture of formic acid and phosphoric acid [15]. Mesophases of nonderivatized cellulose are scarce. Reports on anisotropic solutions of cellulose have been given for NMMO/water [26], [27], trifluoroacetic acid/dichloromethane [28], DMAc/LiCl [29], [30], [31], [32], ammonia/ammonium thiocyanate [33], [34], and a specific mixture of one part sulfuric acid (SA), eight parts polyphosphoric acid (PPA), and one part water [35], [36].

Allthough isotropic solutions of cellulose in phosphoric acid have long been known, it was only recently discovered that anisotropic solutions can also be formed [6], [37], [38], [39]. Until then it was assume that either cellulose had to be derivatized, or that a cosolvent as sulfuric acid was needed to form anisotropic solutions in phosphoric acid [15], [24], [25], [35]. The cosolvent was said to form a complex with polyphosphoric acid. This complex was considered to account for the occurrence of an anisotropic phase. Surprisingly, it was found that neither substitution nor a cosolvent is required for the formation of anisotropic solutions in phosphoric acid [6]. It was observed that optimum results were obtained under waterfree conditions. It follows that orthophosphoric acid as such is not very suitable, because of the presence of a certain amount of water.

Phosphoric acid is a special acid in that it can form dimers, oligomers and even polymeric forms. Orthophosphoric acid can be considered as the reaction product of phosphorus pentoxide and water; pyrophosphoric acid (H4P2O7) is the dimer of orthophosphoric acid. Polyphosphoric acid as used in the work as described in this paper has the overall composition of H6P4O13. The compositions can all be characterized in terms of their P2O5-concentration. Consequently, orthophosphoric acid corresponds to a P2O5-content of 72.4%, pyrophosphoric acid of 79.6% and polyphosphoric acid (H6P4O13) of 84%. In the P2O5–water system, there is always a distribution of the various kinds of phosphoric acids, the equilibrium of which depends on the composition [40], [41].

It was found that water has a detrimental effect on the anisotropic properties of the solution, as shown by a decrease of the clearing temperature. This may be caused by a competition between water and the hydroxyls of cellulose in the interaction with phosphoric acid [42]. Water free conditions are reached for concentrations equal or larger than the composition of orthophosphoric acid; these higher concentrations can be obtained by mixing two or more of the following components: orthophosphoric acid, pyrophosphoric acid, polyphosphoric acid, phosphorus pentoxide, and water. It may take a considerable time before the new equilibrium distribution of the acids is reached, especially if the starting composition is much different from this new equilibrium distribution. Although in a wider range anisotropy was found, the optimum results were obtained when the P2O5-concentration of the solvent lies in between 72 and 76% w/w. In this range, the melting temperature of the solvent is below room temperature. However, phosphoric acid can be supercooled, and in this process it is often used in a metastable state.

Cellulose rapidly dissolves in the phosphoric acid mixture, and the solutions are already anisotropic above a polymer concentration of 8% w/w, which is extremely low, taking into account the semiflexible nature of the cellulose chain. This critical concentration is even comparable to the one for a polymer as rigid as PpPTA in sulfuric acid [18]. Therefore, this cellulose solution is eminently suited for the production of high modulus/high tenacity yarns.

Section snippets

Structure and mechanical properties of cellulose fibres

The investigation of the relation between the structure and mechanical properties of regenerated cellulose fibres has stimulated the development of the polymer fibres in general. Important contributions have been made by, among others, Meyer and Lotmar, Baule and Kratky, Ingersoll, Hermans, de Vries, Kast, Kiessig, Sprague and Noether [5], [43], [44], [45], [46], [47], [48], [49], [50], [51]. In terms of morphology and microstructure, the regenerated cellulose fibres can be positioned in

Preparation of the spinning solution

The strength of orthophosphoric acid as a solution of phosphorus pentoxide (P2O5) in water, can be expressed in a P2O5-concentration (72.4% w/w). Beyond this point waterfree conditons are reached. Such conditions can for instance be obtained by mixing two or more of the following components: orthophosphoric acid, pyrophosphoric acid, polyphosphoric acid, phosphorus pentoxide, and water. Though anisotropy was established over a wider range, the optimum results were obtained for a P2O5

Results and discussion

The modulus and tenacity of the cellulose fibre B are considerably higher than those obtained by the saponified acetate process. Sprague and Noether reported the following maximum filament values for these fibres: an initial modulus of 41 GPa, a tenacity of 1.08 GPa, and an elongation at break of 5.6% measured with a gauge length of 6 cm at 23°C and 65% RH [5]. The Fortisan® saponified acetate fibre is highly oriented and highly crystalline as shown by the well-defined spots on the X-ray

Acknowledgements

The authors wish to thank Prof. H. Chanzy of the Centre de Recherches sur les Macromolécules Végétales in Grenoble, France, for valuable advice on the preparation of longitudinal sections and the electron diffraction patterns, B. Koenders and H. Lammers for carrying out the numerous spinning experiments, Dr E. Klop and R. van Puijenbroek for small- and wide-angle X-ray diffraction characterization and the drawings of the crystal structures, Mrs Gonzalez Wientjes for optical microscopy

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