Field of the invention

This invention relates to a method for the manufacture of elongate lyocell members such as fibres and film, wherein a solution of cellulose in an aqueous tertiary amine N-oxide solvent is extruded through a die by way of a gaseous gap into an aqueous coagulating bath to form an elongate member which is thereafter washed and dried.

Background art Elongate lyocell members such as fibres and film are known. The manufacture of lyocell fibres is described for example in US-A-4,416 ,698, the contents of which are incorporated herein by way of reference. Cellulose is dissolved in a solvent containing a tertiary amine N-oxide (which may also be called for brevity an amine oxide), for example N-methylmorpholine N-oxide (NMMO). The solvent generally also contains a proportion of a non-solvent for cellulose, for example water. The resulting solution is extruded through a suitable die to produce fibres, which are coagulated, washed in water to remove the solvent, and dried. This process of extrusion and coagulation is referred to as "solvent spinning", and the cellulose fibre produced thereby is referred to as "solvent-spun" cellulose fibre or as lyocell fibre. It is also known that elongate cellulose members can be made by extrusion of a solution of a cellulose derivative into a regenerating and coagulating bath. One example of such a process is the viscose process, in which the cellulose derivative is cellulose xanthate . Solvent spinning has a number of advantages over other known processes for the manufacture of elongate cellulose members such as the viscose process, for example reduced environmental emissions.

Elongate lyocell members, particularly fibres, are known to be prone to fibrillation. Fibrillation is a phenomenon which in the main occurs when the lyocell member

is subjected to mechanical forces during wet-processing, and it results in the partial detachment of fine longitudinal fibrils from the elongate member. Efforts have been made to reduce or eliminate fibrillation tendency by chemical aftertreatments , such as those described in US-A-5,310,424, or by suitable choice of spinning parameters, as described for example in WO-A-95/02082. It is an object of the present invention to provide a method of manufacturing elongate lyocell members, particularly low-decitex fibres, in which spinning stability (i.e. freedom from breakdowns) is good and in which the elongate members thereby produced have a low fibrillation tendency.

Disclosure of invention

According to the invention there is provided a method for the manufacture of an elongate lyocell member, wherein a solution of cellulose in an aqueous tertiary amine N-oxide solvent is extruded through a die by way of a gaseous gap into an aqueous coagulating bath to form an elongate member which is thereafter washed and dried, characterised in that in combination (1) the concentration of cellulose in the solution is in the range from 2.0 to 4.5, preferably from

2.0 to 4.0, percent by weight, (2) the ratio of the velocity of the elongate member as it is removed from the coagulating bath to the velocity of the solution as it is extruded through the die is in the range from 1.0 to 2.0, preferably from 1.0 to 1.5, and (3) the width of the die is in the range from 40 to 80, preferably from 60 to 80, further preferably from 70 to 80, micrometre.

The ratio of the velocity of the elongate member as it is removed from the coagulating bath to the velocity of the solution as it is extruded through the die may conveniently be referred to as the draw ratio.

The elongate member may take the form of fibre, for example the form of continuous filament yarn or of tow intended to be cut into staple fibre, or it may take the

form of film in flat or tubular form.

In the case of the manufacture of fibre, the "width" of the die is defined as being the cross-sectional diameter of the individual holes in the die (which may also be called a spinnerette or jet). In the case of dies for fibre manufacture which comprise slot or multilobal holes, the "width" of the die is defined as the width of the slot or lobes. In the case of the manufacture of film, the "width" of the die is defined as being the narrow cross-sectional dimension of the generally rectangular or circular slot through which the solution is extruded. The width of the die is measured at the face thereof.

The tertiary amine N-oxide is preferably N- methylmorpholine N-oxide. Mixtures of tertiary amine N- oxides may be used. The solution may also comprise one or more water-miscible non-solvents for cellulose in known manner. The concentration of tertiary amine N-oxide in the solution is preferably in the range from 83 to 90 percent by weight. The solution may also comprise one or more dispersed substances, for example a pigment such as titanium dioxide, and one or more other dissolved substances, for example a stabiliser such as propyl gallate, in known manner. The temperature of the solution supplied to the die is preferably in the range from 80 to 120 "C.

Any suitable dissolving-grade cellulose may be used to form the solution. High D.P. (degree of polymerisation) cellulose, for example cotton linters, may be preferred. The D.P. of such cotton linters or other cellulose used may be in the range from 1500 to 5000. Mixtures of cellulose from different sources may be used.

The gas in the gaseous gap is preferably air, although other inert gases or gas mixtures may also be used. The length of the gaseous gap (i.e. the distance between the face of the die and the surface of the coagulating bath) is

preferably in the range from 10 to 100 mm, more preferably 20 to 40 mm. The ability to use a relatively short gaseous gap is an advantage of the invention, in that the use of long gaps may result in spinning instability or (in the case of fibre) stuck filaments. Nevertheless, longer gaps may, if desired, be used in the spinning of continuous filament yarn. Particularly in the case of fibre, gas (e.g. air) iε preferably supplied into and extracted from the gaseous gap generally transversely to the direction of travel of the elongate member therethrough, as described in O-A-94/28218. The velocity of the supplied gas (e.g. air) is preferably in the range from 1 to 10 m/s. The temperature of the supplied gas is preferably in the range from 0 to 30 °C. The relative humidity of the supplied gas is preferably in the range from 0 to 60 percent. The absolute humidity of the gas (e.g. air) is preferably in the range from 0 to 20, more preferably from 6 to 15, g/kg.

The coagulating bath preferably comprises from 0 to 70, more preferably from 20 to 40, percent by weight of the same tertiary amine N-oxide as in the solution of cellulose that is extruded.

The velocity of the elongate member as it is removed from the coagulating bath (the take-up velocity) is preferably in the range from 20 to 150 m/min.

Washing and drying of the coagulated elongate member and optional operations such as bleaching and cutting fibre to staple length may be carried out in known manner.

The decitex of fibre produced by the method of the invention may be in the range from 0.5 to 10 dtex, often from 1 to 2.5 dtex. The method of the invention is particularly suited to the manufacture of low-decitex fibres of the latter kind.

It has been found that conventional solutions of

cellulose in amine oxides, which typically contain about 10 to 20 percent by weight cellulose, cannot readily be spun at the low draw ratio required by the method of the invention. This would require the use of very small spinnerette holes, which are difficult both to manufacture and to spin through. Use of conventionally-sized holes of 40-80 micrometres, as in the method of the invention, with such conventional solutions but with low draw ratio permits the manufacture of fibre only of relatively high decitex. Furthermore, it has been found that under these latter conditions filament tensions during spinning are very low, leading to instability of spinning. In contrast, the combination of conditions specified for the method of the invention exhibits good spinning stability and ease of use, and it results in fibres with an inherently low tendency to fibrillation.

Fibrillation tendency of lyocell fibres can be assessed by the following Test Method.

Test Method (Sand Test)

A small tuft of fibre containing 100 to 200 filaments is cut to 5 mm length. These short fibres are placed in a

20 ml phial containing 4 g of glass micro-beads, and 8 ml water is added. The phial is securely stoppered and shaken on a Stewart flask shaker at 2800 cycles/min for 20 min.

A few fibres are removed and placed on a microscope slide. The fibrillation index (C _f ) is calculated from optical micrographs of the fibrillated fibres. The total lengths of the fibrils, f, attached to a length of fibre, L, are measured. The fibrillation index is given by the equation: C _f = Σf/L

This operation can be carried out manually or by image analysis. Alternatively, a set of standard micrographs can be set up for comparison. Trained fibre technologists have

been found to be consistent in their assessment with this method. In practice, it is impossible to measure a fibrillation index above about 30, owing to the difficulty in seeing the large number of fibrils. Data are measured on the middle of the 5 mm length of fibre and on the end. Experience shows that the result for the fibre end correlates best to fabric performance, and only this is the figure quoted herein as Fibrillation Index.

The invention is illustrated by the following Examples, in which parts and proportions are by weight unless otherwise specified.

Example 1

A solution comprising 3.5% cellulose (Buckeye M-155-07 1AY cotton linters of average DP 3970), 9% water and 87.5% N-methylmorpholine N-oxide (NMMO) (solution S.G. 1.25) was extruded at a rate of 240 mg/min through a spinnerette having a single hole 70 micrometre in diameter, corresponding to an extrusion velocity of 50 m/min. The extruded filament was passed by way of an air gap 30 mm long into an aqueous coagulating bath at 20°C comprising 20% NMMO. Air (24'C, 10% relative humidity, 1.9 g/kg absolute humidity) was supplied into the gap transversely to the filaments at a velocity of 2 m/s. The filament was taken up from the coagulating bath by means of a reel with surface velocity 63 m/min, washed free of NMMO and dried. The ratio of take-up velocity to extrusion velocity was therefore 1.26. The fibre had the following properties: titre 1.4 dtex, air-dry tenacity 26 cN/tex, air-dry extensibility 9.5%, wet tenacity 24 cN/tex, wet extensibility 14%, initial wet modulus (2% extension) 170 cN/tex. Fibrillation was induced and assessed in a sample of this fibre and in a control sample of commercial lyocell fibre by the Sand Test. The Fibrillation Indexes of the two samples were 2.5 and 7 respectively, demonstrating that the fibre made according to the invention had a lower fibrillation tendency than the control.

Example 2

Example 1 was repeated, except that the solution comprised 4.2% cellulose (2.0% HV woodpulp (D.P. approximately 850, available from International Paper Co.), 1.0% Buckeye MVE cotton linters of average DP 4550 and 1.2% 1AY Buckeye cotton linters of average DP 3970). Similar results were obtained.