Known solvents for cellulose include tertiary amine N- oxides (commonly referred to as amine oxides), in particular in admixture with a minor proportion of water. A preferred class of amine oxides comprises alicyclic mono (N-methylamine N-oxide) s, one such being N-methylmorpholine N-oxide (NMMO).

Solutions containing by weight typically 5-20W cellulose, 5- 15% water and the balance NMMO have been extruded to yield shaped cellulosic articles such as fibres and films. True solutions are obtainable only within a limited region of the ternary phase diagram of these three components. In forming solutions of cellulose, it has been found convenient to mix cellulose with a solvent containing excess water (for example the 1: 1 NMMO hydrate) and to remove the excess water by evaporation. This results in more rapid and more perfect dissolution than mixing cellulose, NMMO and water in the ultimately desired proportions.

Solutions of cellulose in aqueous NMMO solidify below about 75°C (the freezing point of NMMO monohydrate), and they may solidify even at somewhat higher temperatures. These solutions tend to decompose exothermically at temperatures above about 130°C. These solutions are very viscous. There is a need for cellulose solutions based on amine oxide solvents, which solutions exhibit depressed melting point, reduced viscosity and/or increased thermal stability.

Background art US-A-3,508,941 discloses solutions of at least two different polymers, one of which may be cellulose, in solvents which comprise a cyclic mono (N-methylamine N-oxide), for

example NMMO, and optionally a viscosity-reducing diluent such as dimethyl sulphoxide, N-methylpyrrolidone or sulpholane.

US-A-4,145,532 discloses a process in which cellulose is dissolved in a solvent comprising an amine oxide and water in a very specifically defined critical range. The amount of water is specific to each amine oxide and varies accordingly within the range from about 1.4 to about 29 percent by weight based on the weight of the solution. The amount of cellulose dissolved in the solution may be varied from about 5 to about 35 percent by weight of the solution. The amine oxide may be NMMO, in which case the amount of water in the solvent is preferably in the range from about 5.5 to about 18 percent by weight. An organic co-solvent may be used in quantities up to about 25 percent by weight of the total solution as a less expensive diluent for the amine oxide or to lower the viscosity of the solution. Any aprotic organic liquid non- solvent for cellulose which does not react chemically with the amine oxide nor cause degradation of the cellulose and which has a dipole moment greater than about 3.5 Debye will be suitable as co-solvent. Examples of such non-solvents include dimethylsulphoxide, dimethylformamide, dimethylacetamide, N- methylpyrrolidinone, hexamethylphosphoric triamide, acetonitrile and sulpholane.

US-A-4,290,815 discloses cellulose solvents which comprise an amine oxide, water and as co-solvent a compound containing primary, or combinations of primary and secondary, amino groups wherein the amino groups are bonded to alkyl, alicyclic, dialkyl ether or alkyl/alicyclic radical (s) and wherein the number of carbon atoms divided by the number of primary amino groups is approximately equal to or less than 4; or wherein this ratio is greater than 4 and the number of primary amino groups divided by the number of secondary amine radicals is equal to or greater than 1. The co-solvents increase the cellulose dissolution rate; lower the temperature necessary to reasonably form a solution; lower the viscosity of the solution; provide solutions with improved colour, amine

oxide stability, and cellulose stability; provide a solution with improved flow characteristics; increase solution processability, e. g. wet spinning; permit the replacement of amine oxides by a less expensive co-solvent; and are recoverable and reusable in cellulose dissolution and processing.

US-A-4,324,593 discloses cellulose solvents which comprise an amine oxide, an alkaline compound which increases the rate at which cellulose dissolves, and preferably a small amount of water. Examples of such alkaline compounds include alkali metal hydroxides, ammonia, primary, secondary and tertiary amines, and quaternary ammonium hydroxides.

CA-A-1,251,880 discloses the use as cellulose solvents of mixtures of two or more specific amine oxides containing a small proportion of water. Cellulose solutions which utilise these solvents have relatively low melting points in comparison to solutions utilising only one amine oxide, and some such solutions can be processed at room temperature.

US-A-5,362,867 discloses the use of mixtures of NMMO and recovered caprolactam as solvents for cellulose. This recovered caprolactam is obtained by phosphoric acid-catalysed depolymerisation of waste nylon, and it may contain 25-50% of other substances, notably 6-methylvalerolactam. Comminuted cellulose is added to a mixture of NMMO hydrate (63-67t by volume NMMO, balance water) and recovered caprolactam, and solution is effected by a three-stage heat treatment involving evaporation of water, reduction of cellulose D. P., and dissolution. A preferred solution is said to comprise by weight 8-13% cellulose, 8-13% water, 47-52 NMMO and 32-37% recovered caprolactam.

EP-A-0,686,712 discloses solutions of cellulose in aqueous NMMO which include an additive such as an amine, amide or other amino group-containing compound. Examples of such compounds include urea, caprolactam, aminopropanol and amino

acids. The amount of compound in the solution is preferably in the range from 4 to 75 parts by weight on cellulose. These solutions can be spun to produce flexible cellulose fibres of relatively low modulus and degree of order.

Disclosure of the invention According to a first aspect of the invention, there is provided a solution which comprises (a) cellulose, (b) a tertiary amine N-oxide solvent for the cellulose, and (c) a solution processability-enhancing amount of at least one co- solvent, miscible with the tertiary amine N-oxide, which is an amide selected from the group consisting of compounds of the formula R1CONHR2 in which either (i) R1 represents H or C.- C3 alkyl and R2 represents H or methyl or (ii) R1 and R2 together represent 1,3-propylene. If desired, the solution of the invention may additionally comprise (d) a cellulose solubility-enhancing amount of water.

Preferred examples of the tertiary amine N-oxide solvent include N, N-dimethylcyclohexylamine N-oxide, N, N- dimethylethanolamine N-oxide and in particular NMMO.

The co-solvents used in the invention are amides and are not themselves solvents for cellulose. Preferred examples of the co-solvent include acetamide, 2-pyrrolidinone, N- methylformamide, and in particular formamide. The co-solvents used in the invention exhibit good thermal stability. Some of these co-solvents, including formamide, decompose endothermically. Accordingly, the solutions of the invention may exhibit enhanced thermal stability in comparison with solutions of cellulose in aqueous amine oxides. The co- solvents used in the invention exhibit low toxicity. Use of primary amides such as formamide has the advantage that these amides scavenge nitrosating agents to produce nitrogen and the corresponding carboxylic acid, thereby reducing the risk of formation of toxic N-nitrosamines.

The solutions of the invention freeze at lower

temperatures than solutions of the same cellulose concentration in aqueous amine oxide. The solutions of the invention are often found to supercool, solidifying only after prolonged storage at room temperature or below. Addition of the co-solvent to an aqueous amine oxide in the absence of cellulose also results in depression of freezing point.

It will be appreciated that true solutions may be obtainable only in limited regions of the phase diagram of the various components of the solution, as is the case for other cellulose solvents including aqueous amine oxides. Such phase diagrams can be obtained by routine experimentation. Having regard to the foregoing comment on solution formation, as guidance the amount of cellulose in the solution may generally be in the range from 5 to 30 percent by weight, the amount of water in the solution may generally be in the range from 0 to 20 percent by weight, and the molar ratio of co-solvent to amine oxide may generally be in the range from 0.2: 1 to 1.5: 1.

The amine oxides and co-solvents may form relatively low- melting crystallisable complexes containing approximately equimolar amounts of the two components.

The solutions of the invention may include a small proportion of a stabiliser against thermal decomposition, for example a polyphenolic compound such as propyl gallate, preferably at an amount in the range from 0.01 to 0.5 percent by weight.

The solutions of the invention may be made by direct dissolution or by forming a mixture containing an excess of a volatile non-solvent for cellulose such as water and removing said excess by evaporation so that dissolution occurs.

In a second aspect, the invention provides a method for the manufacture of a shaped cellulosic article comprising the steps of:

(a) forming a solution according to the first aspect of the invention; (b) extruding the solution into a coagulating bath, thereby forming a shaped cellulosic article; (c) washing the shaped cellulosic article with aqueous liquor to remove residual organic non-cellulosic components of the solution therefrom; (d) drying the shaped cellulosic article; and (e) recovering the coagulating bath and optionally the aqueous liquor from steps (b) and (c) for reintroduction into solution-forming step (a).

The shaped cellulose article may be a fibre or a film.

The coagulating bath may be aqueous, or it may comprise or consist of the co-solvent.

The co-solvents used in the invention are less expensive than amine oxides. This is advantageous in reducing running costs; amine oxides can be recovered for re-use from coagulating baths with a high degree of efficiency, but some losses are inevitable.

The co-solvents used in the invention are of high boiling point. In processes for recovery of amine oxide and co-solvent from aqueous liquors by evaporation, water distils preferentially. This is advantageous, in that it enables uncomplicated recovery processes to be used. In contrast, dimethylformamide (DMF) distils preferentially from molten mixtures of NMMO, water and DMF.

The invention is illustrated by the following Example, in which parts and proportions are by weight unless otherwise specified:-

Example Cellulose solvents may be investigated on small scale in preliminary fashion by mixing together cellulose, amine oxide hydrate and any co-solvent, and storing the resulting mixture under vacuum at about 90°C so that excess water evaporates and, desirably, dissolution occurs. The possibility of direct dissolution in a solvent may be investigated by mixing amine oxide hydrate and co-solvent, removing excess water by evaporation under vacuum, and then adding cellulose to the concentrated mixture. Better results can be obtained by using finely-divided cellulose and by subjecting cellulose- containing mixtures to high shear. Preliminary experiments may be designed to test mixtures containing about 1 g cellulose and of 10 g total weight.

Rotary evaporation of equimolar amounts of NMMO monohydrate and formamide yielded a concentrate containing about 0.25 mole water per mole amine oxide or amide. Rotary evaporation of equimolar amounts of NMMO monohydrate and N- methylformamide yielded a concentrate containing about 0.35 mole water per mole amine oxide or amide. Cellulose was mixed into these concentrates for a few minutes at 45-50°C, and the resulting mixtures were stored at 70°C. The bulk of the cellulose dissolved within three hours without stirring, showing that these concentrates are direct solvents for cellulose. Improved solution was obtained by heating to 90°C or by evaporation. These solutions were difficult to crystallise and were fibre-forming. Preliminary experiments indicated that the formamide-containing solution exhibited similar rheological properties at 90°C to those of a conventional solution containing the same percentage of cellulose in aqueous NMMO at 105°C.

Similar results were obtained using concentrated 1: 0.5 and 1: 1.5 molar mixtures of NMMO monohydrate and formamide, although solution formation was generally less satisfactory.

When a concentrated 1: 2 molar mixture was used, cellulose was

slow to dissolve. A 1: 1 molar mixture of NMMO and acetamide swelled cellulose less rapidly than the 1: 1 mixture of NMMO and formamide and was generally a less satisfactory solvent.

A concentrated 1: 1 molar mixture of NMMO monohydrate and N- methylformamide swelled cellulose less rapidly than the 1: 1 mixture of NMMO and formamide, but it was a good cellulose solvent, and the resulting solution appeared less viscous.

Concentrated 1: 1.5 and 1: 2 molar mixtures of NMMO and N- methylformamide gave similar results to the corresponding formamide-containing mixtures.

With 2-pyrrolidinone, concentration of a 1: 1 molar mixture with NMMO resulted in precipitation of a high-melting solid, difficult to redissolve, possibly anhydrous NMMO.

Similar phenomena were observed on occasion with other N- methyl amides. This phenomenon was not observed if a slight excess of 2-pyrrolidinone was used; the concentrate melted below 60°C and was a moderate cellulose solvent. Mixtures richer in 2-pyrrolidinone were higher melting and less effective solvents. Almost all the water could be evaporated from a mixture containing a slight excess of NMMO, yielding an excellent cellulose solvent; the cellulose solution could readily be drawn.

Solutions containing over 20% cellulose could be obtained using the small scale technique, although they were of high viscosity: liquid in the technical sense at room temperature, but rubbery.

No evidence was seen of the co-solvent reacting with cellulose or decomposing at temperatures up to about 130°C.

A concentrated 1: 1 molar mixture of NMMO and formamide was assessed as cellulose solvent on the 1 kg scale, using a jacketed Z-blade mixer. Much of the cellulose dissolved within a few minutes, by the time the temperature had been raised to 50-55°C. Solution was virtually complete within 2 hours, by which time the temperature was 90°C. It is thought that even

more rapid solution could have been achieved with a more efficient mixer. The solution contained 15% cellulose, 62% NMMO and 23% formamide and was fibre-forming. Differential Scanning Calorimetry (DSC) revealed a small exothermic peak at 170°C and exothermic runaway at about 185°C. For comparison, DSC of a conventional solution of cellulose in aqueous NMMO containing a small amount of propyl gallate stabiliser revealed exothermic peaks at 140°C and 175°C (more pronounced than in the solution of the invention) and exothermic runaway at 190°C. The solution of the invention appeared to decompose less violently and less completely at the runaway exotherm than did the comparative solution. The solution of the invention may therefore be processable at higher temperatures than such comparative solutions.

Cellulose solutions of the invention such as those described in general or specifically above can be extruded through spinnerets into spin baths to form cellulosic fibres in a manner similar to known techniques.

Cellulosic films could be cast from the solutions of the invention by immersing in water glass plates coated with the solutions.