WO1998014648A1 - Method for the manufacture of an extruded lyocell article - Google Patents

Abstract

Cellulose is exposed to high-energy radiation at a dosage in the rampe 1-4 kGy and then dissolved in an aqueous tertiary amine N-oxide solvent. The resulting solution is extruded through a die to form an extruded precursor, which is then washed and dried to form extruded lyocell articles, such as fibres and films. In a preferred embodiment, only part of the cellulose is irradiated.

Description

METHOD FOR THE MANUFACTURE OF AN EXTRUDED LYOCELL ARTICLE

Field of the invention

This invention relates to the manufacture of extruded articles of lyocell, for example in the form of fibres or films .

Background art

Extruded articles of lyocell are known, and their manufacture is described for example in US-A-4,246 ,221, the contents of which are incorporated herein by way of reference. They are made by an organic solvent-spinning process, in which the organic solvent essentially comprises a mixture of organic chemicals and water, and in which solvent-spinning involves dissolution of cellulose and spinning without formation of a cellulose derivative. Spinning involves extrusion and precipitation of the solution into a bath which is a non-solvent for cellulose to yield a reconstituted cellulose or lyocell article. Examples of suitable organic solvents include aqueous tertiary amine N-oxides, in particular N-methylmorpholine N-oxide. Examples of non-solvent baths include water and dilute aqueous solutions of tertiary amine N-oxide. The lyocell process is to be distinguished from other known processes for the manufacture of cellulosic articles which involve the formation and subsequent decomposition of a chemical derivative of cellulose, for example the viscose process.

DE-A-2,941,624 describes a method for the manufacture of viscose in which cellulose is irradiated with an energy-rich beam to a dosage of 1 to 30 kGy and then steeped in an NaOH solution of concentration below 19 percent to form alkali cellulose. The resulting alkali cellulose can be processed into viscose in conventional manner. The method is said to permit savings in usage of the process chemicals carbon disulphide, sodium hydroxide and sulphuric acid. The reduction in carbon disulphide usage results in reduced environmental pollution both by that chemical and by hydrogen sulphide. It will be appreciated that the lyocell process does not involve the use of any such reactive process chemicals, which are necessary in the viscose process for the formation and decomposition of the water-soluble derivative cellulose xanthate.

EP-A-0, 648 , 808 discloses a method for the manufacture of extruded lyocell articles in which a solution is prepared from a first cellulose component (I) of average degree of polymerisation (D.P.) in the range from 500 to 2000 and a second cellulose component (II) of D.P. in the range from 350 to 800, the D.P. of (II) being no more than 90 percent of that of (I), the ratio by weight of (I) to (II) being in the range from 95:5 to 50:50. Use of such a mixture of celluloses is said to result in good process stability, i.e. freedom from melt flow instability and melt fracture during extrusion.

WO-A-97/11213 (published after the priority date of the instant invention) discloses a process for the manufacture of shaped cellulosic articles. A cellulose-containing material is suspended in an aqueous tertiary amine N-oxide and brought into solution by evaporation of excess water, for example in a conventional thin-film evaporator. This solution is then extruded to form the shaped cellulosic article. The cellulose-containing material is exposed to an electron beam prior to the dissolution step. This process is said to permit the use of cellulose-containing materials which do not readily dissolve in amine oxide or which dissolve to give solutions of poor spinnability. WO-A-

97/11213 discloses irradiation by an electron beam at energies of 10 or 20 kGy.

Disclosure of invention

According to the present invention, there is provided a method for the manufacture of an extruded lyocell article including the steps of:

(1) dissolving cellulose in an aqueous tertiary amine N-oxide solvent to form a solution;

( 2 ) extruding the solution through a die into a coagulating bath to form an extruded precursor;

(3) washing the extruded precursor free from solvent; and

(4) drying the washed extruded precursor, thereby forming said extruded lyocell article,

characterised in that at least part of the cellulose is subjected prior to its introduction into the dissolution step (1) to high-energy radiation at a dosage in the range from 1 to 4 kGy.

The dissolution step (1), extrusion step (2), washing step ( 3 ) and drying step ( 4 ) may be performed in any convenient conventional manner.

The solution is generally extruded through the die into the coagulating bath by way of a gaseous gap, for example an air gap, in known manner.

The extruded lyocell article generally takes the form of a fibre or film. The method of the invention may be used to make fibre in the form of continuous filament yarn, tow or staple fibre, or it may be used to make film in flat or tube form.

Irradiation of cellulose results in the production of free radicals, which may be long-lived. For example, a hardwood dissolving pulp irradiated according to DE-A-2, 941,624 may exhibit electron spin resonance signals corresponding to such radicals for a year or more after irradiation. It is known that substances such as transition metal ions which contain unpaired electrons can catalyse the exothermic decomposition of solutions of cellulose in tertiary amine N-oxides and should be rigorously excluded from the solvent system in a lyocell process. We have surprisingly found that solutions of cellulose prepared in accordance with the method of the invention have similar stability to those prepared from unirradiated cellulose.

Convenient sources of high-energy radiation suitable for use in the method of the invention include electron beams and gamma rays. The energy of the radiation source is preferably in the range from 200 keV to 1 MeV. An energy source of about 10 MeV may be found to generate X-rays and undesirably large numbers of free radicals.

We have found that use of a high radiation dosage can result in a marked reduction in the alpha-cellulose content and in a marked increase in the copper number of dissolving-grade cellulose. For example, exposure of a hardwood pulp to a radiation dosage of 15 kGy resulted in a fall in alpha-cellulose content from 92 to 88 percent and in an increase in copper number from 1.0 to 2.0 or 3.0 or higher. In contrast, exposure in accordance with the invention to a dosage of 4 kGy resulted in a reduction in alpha-cellulose content of only 0.5 percent and in an increase in copper number of only 0.3.

The method of the invention permits precise control of the D.P. of the cellulose introduced into a lyocell process. This is desirable in order to achieve precise control over the process and precise control of the properties of extruded lyocell articles produced thereby. The method of the invention can be used to reduce within-batch and between-batch variation in cellulose D.P. The method of the invention can also be used if it is desired to reduce the D.P. of cellulose to be introduced into a lyocell process in controlled manner. It will be appreciated that, in contrast to the viscose process, there is no convenient way in conventional lyocell processes of reliably and reproducibly effecting a reduction in cellulose D.P.

We have found that irradiation of cellulose generally tends to reduce the spread of D.P. of the polymer molecules of which cellulose consists. This can be shown for example by gel permeation chromatography. In the manufacture of extruded lyocell articles, it is generally desirable to use cellulose of relatively broad D.P. spread, as described in EP-A-0, 648, 808 and in copending unpublished International patent application PCT/GB97/02173. In a preferred embodiment of the invention, only part of the cellulose introduced into the lyocell process is irradiated as specified and part is not subjected to irradiation prior to its introduction into the dissolution step. Mixtures of irradiated and unirradiated celluloses can be used. A single supply of cellulose may be taken and part of it irradiated in order to increase its D.P. spread. Alternatively, a single supply of cellulose may be irradiated under non-uniform non-isodose conditions, for example by irradiation of the surface layer of a thick bed of cellulose, in order to increase its D.P. spread, and introduced into the dissolution step (1).

Claims

1. A method for the manufacture of an extruded lyocell article including the steps of:

(1) dissolving cellulose in an aqueous tertiary amine N-oxide solvent to form a solution;

( 2 ) extruding the solution through a die into a coagulating bath to form an extruded precursor;

(3) washing the extruded precursor free from solvent; and

(4) drying the washed extruded precursor, thereby forming said extruded lyocell article,

characterised in that at least part of the cellulose is subjected prior to its introduction into the dissolution step (1) to high-energy radiation at a dosage in the range from 1 to 4 kGy.

2. A method according to claim 1, further characterised in that part of the cellulose is not subjected to irradiation prior to its introduction into the dissolution step (1).

3. A method according to claim 2, further characterised in that a single supply of cellulose is split into two parts, that one only of these two parts is subjected to irradiation, and that the two parts are recombined within the dissolution step (1).

4. A method according to claim 2, further characterised in that a single supply of cellulose is irradiated under non-uniform non-isodose conditions and introduced into the dissolution step (1).

5. A method according to any one of the preceding claims, further characterised in that the energy of the radiation source is in the range from 200 keV to 1 MeV.

6. A method according to any one of the preceding claims, further characterised in that the extruded lyocell article is lyocell fibre.