WO1997038153A1

WO1997038153A1 - Method for the manufacture of lyocell filaments

Info

Publication number: WO1997038153A1
Application number: PCT/GB1997/000981
Authority: WO
Inventors: Simon Ashley Mortimer; Ian Graveson; Patricia Margaret Brownie
Original assignee: Courtaulds Fibres (Holdings) Limited
Priority date: 1996-04-10
Filing date: 1997-04-09
Publication date: 1997-10-16
Also published as: AU2303497A; GB9607456D0; ID16451A; TW339368B

Abstract

Lyocell fibres with a low tendency to fibrillation can be manufactured with good spinning stability and the avoidance of stuck filament formation by a method wherein a solution of cellulose in an aqueous tertiary amine N-oxide solvent is extruded from a spinnerette through a gaseous gap into a coagulating bath, at least one stream of gas being supplied into and caused to flow across the gap generally transversely to the filaments, the method being characterised in that in combination: (a) the length of the gaseous gap is in the range from 20 to 200, preferably from 30 to 80, further preferably from 40 to 50, mm; (b) the spinning speed of the filaments is in the range from 5 to 50, preferably from 10 to 30, m/min; (c) the residence time of the filaments in the gaseous gap is in the range from 150 to 500 ms; (d) the velocity of the stream of gas or at least one of the streams of gas supplied into the gaseous gap is in the range from 1 to 20, preferably from 2 to 10, m/s; (e) the moisture content of the stream or streams of gas referred to in (d) is in the range from 7 to 15 g/kg; and (f) the product of the said residence time in ms and the said moisture content in g/kg is in the range from 1200 to 7200.

Description

METHOD FOR THE MANUFACTURE OF LYOCELL FILAMENTS Field of the Invention

This invention relates to the manufacture of lyocell filaments, in particular to processes wherein a solution of cellulose in an aqueous tertiary amine N-oxide solvent is extruded from a spinnerette through a gaseous gap into a coagulating bath. Background art

It is known that cellulose can be dissolved in certain tertiary amine N-oxides (which may for convenience also be called amine oxides) to form a solution or dope which can be extruded by way of a die into a coagulating bath to form shaped articles such as continuous filaments, staple fibres and films. The dope generally contains a small proportion of water, and the coagulating bath is usually an aqueous bath. The coagulated article is then washed to remove residual amine oxide therefrom and dried. Such a process is an example of a solvent-spinning process, and articles so produced may be called solvent-spun cellulose articles or lyocell articles. An example of a suitable amine oxide solvent is N-methylmorpholine N-oxide (NMMO) .

US-A-4,261,943, the contents of which are incorporated herein by way of reference, describes such a solvent- spinning process wherein a solution of cellulose in aqueous NMMO is extruded by way of a spinnerette through an air-gap, for example 5 or 30 cm in length, to form filaments which then pass into a water bath, thereby producing lyocell fibre. This patent also describes a solvent-spinning process wherein the filaments in the air-gap are coated with a nonsolvent liquid for cellulose, for example water, immediately after extrusion. This is said to reduce the tendency of the filaments to fuse together in the air-gap.

O-A-93/19230, the contents of which are incorporated herein by way of reference, describes a process for producing shaped cellulose articles by shaping a solution of cellulose in a tertiary amine oxide in the warm state and introducing the shaped solution into a precipitation bath in order to precipitate the cellulose contained therein, characterised in that the warm shaped solution is cooled before introduction into the precipitation bath, the cooling taking place immediately after shaping. The cellulose items may be fibres. Cooling may be effected by a current of gas, for example air, which may be supplied essentially at right angles to the spinning direction. Examples are given in which the temperature of the cooling gas is in the range -5 to 27 °C. WO-A-95/01470 describes a similar process.

O-A-94/28218, the contents of which are incorporated herein by way of reference, describes a method for the production of cellulose filaments from a solution of cellulose in an organic solvent, which comprises the steps of extruding the solution through a die having a plurality of holes to form a plurality of strands, passing the strands across a gaseous gap into a water-containing bath to form the filaments, and providing a forced flow of gas through the gap parallel to the upper surface of the water by providing a flow of gas across the gap. The organic solvent is generally a tertiary amine N-oxide, preferably N- methylmorpholine N-oxide. The gas may be air. For optimum performance, the dew point of the air supplied to the gap should be 10 °C or less and may be in the range 4 to 10 ^°C. The temperature of the air may be in the range 5 to 30 'C.

WO-A-95/02082, the contents of which are incorporated herein by way of reference, describes a method of manufacturing cellulose fibres by extruding a solution of cellulose in a tertiary amine N-oxide solvent through the holes of a spinnerette and passing the extruded filaments under tension through an air gap and into a regenerating bath. The method is characterised in that it is carried out in such a way that the maximum value of the mathematical expression 51.4 + 0.033D + 1937M^: - 7.18T - 0.094L - 2.50F + 0.045F- (in which D is spinnerette hole diameter in micron, M is the spinning solution throughput per hole in g/min, T is the titre of a single filament in dtex, L is the length of the air gap in mm, and F is the humidity of the air in the air gap in g of water per kg of air) is 10; provided that L is greater than 30 mm. This method is said to give cellulose fibres with a very low tendency to fibrillate. It will readily be appreciated that the value of this mathematical expression is at a minimum when the humidity of the air F is 27.8 g/kg. Examples are given relating to the applicability of the mathematical expression in the manufacture of single filaments of lyocell. Disclosure of the Invention

The present invention addresses the problem of how to manufacture lyocell filaments in the form of yarns and tows on an industrial scale, such that not only do the filaments have an inherently low tendency to fibrillation but also the formation of stuck filaments is avoided.

According to the present invention there is provided a method for the manufacture of lyocell filaments wherein a solution of cellulose in an aqueous tertiary amine N-oxide solvent is extruded from a spinnerette through a gaseous gap into a coagulating bath, at least one stream of gas being supplied into and caused to flow across the gap generally transversely to the filaments, characterised in that in combination (a) the length of the gaseous gap is in the range from 20 to 200, preferably from 30 to 80, further preferably from 40 to 50, mm; (b) the spinning speed of the filaments is in the range from 5 to 50, preferably from 10 to 30, m/min; (c) the residence time of the filaments in the gaseous gap is in the range from 150 to 500 ms; (d) the velocity of the stream of gas or at least one of the streams of gas supplied into the gaseous gap is in the range from 1 to 20, preferably from 2 to 10, m/s; (e) the moisture content of the stream or streams of gas referred to in (d) is in the range from 7 to 15 g/kg; and (f) the product of the said residence time in ms and the said moisture content in g/kg is in the range from 1200 to 7200. As used herein, the "length" of the gap is the distance through which the filaments pass in the spinning direction as they travel through the gap, usually in the vertically downward direction. The "width" of the gap is the dimension perpendicular to the length in the general direction of the transverse flow of gas across the gap. The expressions "through the gap" and "across the gap" are to be construed accordingly. The "cross-section" of the gap is measured in the plane perpendicular to the width.

The titre of the lyocell filaments produced may be in the range 0.5 to 20 decitex, often 1 to 5 decitex.

The tertiary amine N-oxide is preferably N-methylmorpholine N-oxide. The solution of cellulose preferably comprises from 3 to 25, more preferably from 4.5 to 18, percent by weight cellulose. The solution of cellulose preferably comprises from 5 to 20 percent by weight water. The average D.P. (degree of polymerisation) of the cellulose is preferably in the range from 400 to 2000, more preferably from 600 to 900. It is known from O-A-95/35400 that lyocell fibre with increased fibrillation tendency can be produced by spinning a solution containing at least 16 percent by weight cellulose of average D.P. less than about 450 in a tertiary amine N-oxide solvent, and accordingly such a combination of conditions is preferably to be avoided when it is desired to produce fibre of low fibrillation tendency. It has also been observed that spinnability tends to improve as dope viscosity is increased (within practical processing limits) . Accordingly, cellulose solutions having higher cellulose concentrations and/or containing higher D.P. cellulose within the specified ranges are generally to be preferred in the practice of the present invention.

The method of the invention is especially suited to the manufacture of large tows, for example of titre 5 ktex or greater, particularly in the range from 5 to 50 ktex. A spinnerette suitable for use in the manufacture of such tows may for example contain 5,000 to 25,000 holes. The holes in the spinnerette are preferably arranged so that there are from 10 to 60, more preferably 10 to 20, rows of holes across the width of the gap. Hole density in the spinnerette is conveniently in the range from 0.25 to 10 holes/mm^:. Hole diameter may generally be within the range from 25 to 200 micron, often 50 to 150 micron.

According to conventional practice, the take-up velocity of the lyocell filaments from the coagulation bath (the spinning speed) is higher than the extrusion velocity of the solution through the spinnerette, often by about a factor in the range 2.5 to 25 or 2.5 to 10, so as to stretch the extruded filaments with the purpose of improving their mechanical properties. It is thought that such stretching occurs almost entirely within the gaseous gap. The residence time in the gaseous gap in the method of the invention is calculated by dividing the length of the gaseous gap by the spinning speed (take-up speed) of the filaments. No account is taken in this calculation of any velocity change or stretching of the filaments which may take place in the gaseous gap. It has been found that, if the residence time of the filaments in the gaseous gap is less than 150 ms, then the fibrillation tendency of the filaments may be inclined to be undesirably high. It has further been found that, even though the fibrillation tendency of filaments spun with residence times greater than 500 ms is low, such long residence times may be associated with stuck filaments or other practical difficulties in spinning.

The gas supplied into the gaseous gap is preferably air, although other inert gases or gas mixtures, for example nitrogen, may also be used. The temperature of the gas supplied to the gap is often in the range from -10 to 50 °C, frequently 10 to 40'C. It will be appreciated that the constraints on the moisture content of the supplied gas may set limits on the lowest temperature that can be used in order to avoid supersaturation and/or condensation.

The temperature of the solution of cellulose supplied to the spinerette is commonly in the range of about 80 to 125 'C, and the gas flow accordingly serves to cool the extruded filaments in the gaseous gap.

The present inventors have found that the fibrillation tendency of the lyocell filaments is inclined to be excessively high if the moisture content of the gas stream or at least one of the gas streams supplied into the gap is less than 7 g/kg. The present inventors have further found that excessive numbers of stuck filaments may be formed if the moisture content of the supplied gasin this stream or streams is more than 15 g/kg. Furthermore, the present inventors have observed that when the moisture content of the supplied gas is high, such as 20 g/kg or more, the freshly-spun lyocell filaments absorb water vapour from the gas, with the consequence that the moisture content of the gas falls as it passes across the gap. As a result of this, filaments adjacent to the gas supply experience atmospheric conditions in the gap different from filaments more remote from the gas supply, which can lead to undesirable variations in physical properties between filaments across the width of the gap and to reduced spinning stability. This effect may be particularly noticeable in the manufacture of large tows, and may be found to be almost independent of the velocity of the gas. By contrast, when gas of lower moisture content, in the range from 7 to 15 g/kg, is supplied into the gap, atmospheric conditions are more constant across the width of the gap and filaments of desirably uniform properties can be produced.

The gas is conveniently supplied into the gap through a nozzle or nozzles which directs the gas across the gap, transversely to the spinning direction. Gas is generally discharged from the side of the gap opposite to the supply, and is preferably extracted by means of a nozzle or nozzles situated on the other side of the gap in opposition to the supply nozzle(s). The supply and extraction nozzles may extend over the whole or part of the length of the gap, and they are preferably rectangular in cross-section. Different qualities of gas may be supplied into different regions of the gap along its length, as described in our copending International Patent Application PCT/GB96/00030 (published as 096/21758. According to Application PCT/GB96/00030, relatively dry air is supplied into a first region of the gap adjacent the face of the spinnerette and relatively moist air into a second region more remote from the face of the spinnerette. When this technique is employed in conjunction with that of the present invention, it is not essential that the moisture content and velocity of the gas supplied into both regions complies with the requirements of the present invention, but only that the quality of the air supplied into the second region should do so.

Lyocell filaments produced by the method of the invention may subsequently be washed, dried and collected, and if desired cut to form staple fibre, in known manner. Brief Description of Drawing

The method of the invention provides a means for spinning on an industrial scale lyocell filaments with good resistance to fibrillation while avoiding the formation of stuck filaments and providing good spinning stability.

The invention will now be more particularly described with regard to the accompanying Figure, which is a schematic illustration of apparatus suitable for carrying out the invention.

Referring to the Figure, a solution of cellulose in aqueous amine oxide is fed by way of a gear pump 1 to a spinnerette 2. The cellulose solution (dope) may for example contain from 3 to 25% by weight cellulose, from 65 to 90% by weight NMMO and from 5 to 20% by weight water together with a small proportion of a thermal stabiliser such as propyl gallate, and the temperature of the dope may be in the range 80 to 125 ^°C The dope is extruded downwardly through the holes in the spinnerette 2 into an air gap 3 maintained at a temperature below that of the dope, where it solidifies to form a bundle of filaments 4. The filaments 4 then pass into an aqueous coagulating bath 5, pass around part of the circumference of a roller 6 and are withdrawn for washing, drying and other conventional processing operations . The surface speed of the roller 6 is higher than the velocity of the dope issuing through the holes of the spinnerette 2 so as to stretch the filaments 4. Stretching of the filaments occurs largely within the air gap 3.

A first supply of air is blown into the air gap 3 from a blowing nozzle 7 and extracted from the air gap 3 by a suction nozzle 8, passing through the air gap 3 transversely to the direction of travel of the filaments 4. The nozzles 7, 8 are so arranged that this procedure serves to maintain the temperature and humidity of the atmosphere in a first region 9 of air gap 3 which lies adjacent the face 2a of spinnerette 2 at desired values. A second supply of air is similarly blown into air gap 3 from a blowing nozzle 10 to a suction nozzle 11. The nozzles 10, 11 are so arranged that this procedure serves to maintain the temperature and humidity of a second region 12 of the air gap 3 situated between the first region 9 and the coagulating bath 5 at desired values according to the invention. The nozzles 7 and 10 extend to supply air across the entire bundle 4 of filaments. The moisture content of the air supplied to blowing nozzle 7 is lower than that of the air supplied to blowing nozzle 10. The temperatures of the two supplies of air may be the same or different.

The invention is illustrated by the following Examples, in which parts and proportions are by weight unless otherwise specified:- Example 1

A solution of cellulose in aqueous NMMO (13% cellulose with D.P. 800, circa 75% NMMO, balance water) was extruded through a spinnerette containing a rectangular array of 16700 holes each 70 micrometer in diameter through an air gap 45 mm long to form 1.7 dtex filaments which were coagulated, washed, dried and collected in conventional manner. Spinning speed was 15 /min and residence time in the air gap 180 ms . Moist air (7.9 g/kg water) was blown into and extracted from the uppermost 5 mm region of the gap at 8 m/s transversely to the spinning direction. A separate supply of moist air (9 g/kg water) was blown into and extracted from the remaining lower region of the gap at 8 m/s. The collected lyocell filaments exhibited a very low fibrillation tendency and contained no stuck filaments .

Example 2

Example 1 was repeated, except that the solution contained 15% cellulose of D.P. 600; a spinnerette containing 95 holes was used; air gap length was 80 mm (residence time 320 ms) ; and the moisture content of the air blown into the upper and lower regions of the gap was 5.8 and 7.65 g/kg respectively. Similar results were obtained.

Example 3

Example 1 was repeated, except that the solution contained 15% cellulose of D.P. 700; a spinnerette containing 6130 holes of 80 micrometer diameter was used; spinning speed was 25 m/min; air gap length was 80 mm (residence time 192 ms) ; and the moisture content of the air blown from a single supply was 14 g/kg. Similar results were obtained. Example 4

Example 2 was repeated, except that the moisture content of the air blown into the lower region of the gap was 13.6 g/kg. Similar results were obtained.

Claims

1. A method for the manufacture of lyocell filaments wherein a solution of cellulose in an aqueous tertiary amine N-oxide solvent is extruded from a

5 spinnerette through a gaseous gap into a coagulating bath, at least one stream of gas being supplied into and caused to flow across the gap generally transversely to the filaments, characterised in that in combination (a) the length of the gaseous gap is in the range from 20 to 200, preferably from

10 30 to 80, further preferably from 40 to 50, mm; (b) the spinning speed of the filaments is in the range from 5 to 50, preferably from 10 to 30, m/min; (c) the residence time of the filaments in the gaseous gap is in the range from 150 to 500 ms; (d) the velocity of the stream of gas or at

15 least one of the streams of gas supplied into the gaseous gap is in the range from 1 to 20, preferably from 2 to 10, m/s; (e) the moisture content of the stream or streams of gas referred to in (d) is in the range from 7 to 15 g/kg; and (f) the product of the said residence time in ms and the 0 said moisture content in g/kg is in the range from 1200 to 7200.

2. A method according to claim 1, further characterised in that the tertiary amine N-oxide is N- methylmorpholine N-oxide.

5 3. A method according to claim 1 or claim 2, further characterised in that the concentration of cellulose in the solution of cellulose is in the range from 3 to 25, preferably from 4.5 to 18, percent by weight.

4. A method according to any one of the preceding 0 claims, further characterised in that the solution of cellulose contains from 5 to 20 percent by weight of water.

5. A method according to any one of the preceding claims, further characterised in that the average degree of polymerisation of the cellulose is in the range from 400 to 2000, preferably from 600 to 900.

6. A method according to any one of the preceding claims, further characterised in that the number of holes in the spinnerette is in the range from 5000 to 25,000.

7. A method according to any one of the preceding claims, further characterised in that the density of the holes in the spinnerette is in the range from 0.25 to 10 holes/mm-.

8. A method according to any one of the preceding claims, further characterised in that the gas is air.

9. A method according to any one of the preceding claims, further characterised in that the temperature of the gas in the stream or streams supplied into the gaseous gap is in the range from -10 to 50, preferably from 10 to 40, °C.

10. A method according to any one of the preceding claims, further characterised in that the lyocell filaments have a titre in the range from 0.5 to 20, preferably from 1 to 5, decitex.