GB2337990A - Process for shaping cellulose - Google Patents

Abstract

A process for the manufacture of shaped cellulose articles e.g. fibres from a solution of cellulose in an amine-N-oxide solvent involves the initial formation of a homogeneous cellulose dope by dissolving cellulose pulp in a mixture of a tertiary amine-N-oxide, water and an aprotic organic diluent to form a slurry. The slurry is then heated with removal of water to form the homogeneous dope. The preferred amine-N-oxide is N-methylmorpholine-N-oxide (NMMO) and the preferred aprotic diluent is N-methyl-2-pyrrolidinone (NMP). The addition of NMP decreases the time required for the cellulose to dissolve by a significant factor.

Description

2337990 Process for Shaping Cellulose This invention relates to a process

for the manufacture of a shaped cellulose article and in particular to a process in which the cellulose is dissolved in a mixed amine-N-oxide 5 and water solvent.

In US-A-4,416,698 there is described a process in which cellulose is dissolved in a solvent containing a tertiary amine-N-oxide, for example Nmethylmorpholine -N- oxide (NMMO). The solvent generally contains a proportion of water. The resultant solution may be extruded through a die across an air gap and into a regeneration bath containing water or a mixture of NMMO and water. The solution, which may contain 15-h cellulose, is extremely viscous even when processed at temperatures in the range of 100-1150C. Consequently, high pressures are required, typically 100-200 bar (1-2xl 07 N. M-2) ' to force the cellulose solution or dope through an extrusion die such as a jet.

US-A-3, 508, 941 discloses that the viscosity of a solution containing at least two different polymers can be reduced by adding a diluent. A suitable diluent, amongst others, is Nmethyl pyrrolidinone.

US-A-4,196,2082 describes solutions of cellulose in various tertiary amine-N-oxides, and includes a list of organic diluents which are non reactive with tertiary amine-N- oxides. The preferred diluents are dime thyl sulphoxide and Ndimethylformamide.

US-A-4,290,815 discloses the use of co-solvents in theformation of solutions of cellulose in amine-N-oxide. A large number of different cosolvents are disclosed, the preferred co-solvents being polyamines e.g. diethylenetriamine and hexamethylene diamine.

2 - CA-A-1, 251, 880 describes a process for spinning cellulose dissolved in mixed tertiary amine oxides directly into a regeneration bath, so called "wet spinning". The preferred amine oxides are NMMO and Ndimethyethanolamine-N-oxide 5 (DMEAO).

The present invention relates to an improved process for the manufacture of shaped cellulose articles which uses an amine oxide/water solvent in combination with a diluent.

According to the present invention there is provided a process for the manufacture of a shaped cellulose article from a solution of cellulose in an amine-N-oxide solvent, the process involving the initial formation of a homogeneous cellulose dope, wherein cellulose pulp is dissolved in a mixture of tertiary amine-N-oxide, water, and an aprotic organic diluent to form a slurry, the slurry being subsequently heated with removal of water to form said homogeneous cellulose dope.

The pref erred amine-N-oxide is N-methylmorphol ine -N- oxide (NMMO).

The pref erred aprotic diluent is N-methyl - 2 -pyrrolidinone (NMP), which may be mixed with the water and amine-N-oxide prior to addition of the cellulose pulp.

The cellulose pulp dissolves in NMMO, with excess water acting on the cellulose to assist in the dissolution process.

The addition of the non-solvent diluent NMP surprisingly decreases the time required for the cellulose to dissolve by a considerable factor as compared with the dissolution of cellulose in an NMMO and water mixture. The time required for dissolution when using NMP is less than one half of the time required with NMMO and water for similar cellulose concentrations and degrees of polymerisation.

Preferably, the mixed solvent may contain 13-18-01; water, 45-600-. NMMO and 20-50% NMP.

The premix can be mixed at lower temperatures than is conventional. Typical premix temperatures will be between 60700C. Because of the lower pending temperature, coupled with lower cellulose concentrations (about 6-%) in the NMP/NWO mixture, there is less tendency for the mixture to undergo an exothermic reaction. It has been found that it is possible to manufacture the cellulose fibres without use of propyl gallate stabiliser.

The premixed slurry may be passed into an evaporator such as a Filmtruder (TM) for removal of water under partial vacuum and the formation of the solution or dope.

As is known, the excess water is removed from the slurry during the preparation of the solution using a thin f ilm evaporator of the type shown in EP-A-0,356,419 until the water/NMMO/NMP mixture becomes a single phase with the cellulose dissolved therein - i.e. a homogeneous cellulose solution or dope.

A further benefit of the presence of NMP is that the boiling point elevation of the solvent mixture at reduced pressures for particular cellulose contents is reduced as compared with the NMMO/water mixture, allowing a homogeneous single phase more readily to be obtained.

The evaporator temperatures should be in the range 80- 1000C (typically 90-100OC), with a residence time of 6-9 minutes in the evaporator for a 6.06 by weight solution of cellulose having a DP 430, with a feed mixture comprising about 78 parts NMMO, 22 parts water and 28- 60 parts by weight of NMP.

The boiling point elevation of water/NMP/NMMO mix is such that the water may be more easily removed as compared with a typical known process. Furthermore, such is the ease of - 4 solution of the cellulose in the water/NMMO/NMP mix that it may be possible both to mix the cellulose slurry and form the solution in a single production stage in a single production vessel.

The dope is then pumped to a spinnerette for spinning into fibres either by air gap spinning or by spinning directly into a coagulant spin bath, so called "wet spinning". The spin bath contains a solution of Water/NW0/NMP, so that spin bath contains less water than a conventional spin bath for cellulose dope containing water/NMMO only. Since the spin bath contains less water than is conventional, the solvent recovery process for separating the water from the NMMO/NMP is more economic. This economy is further enhanced by the fact that the presence of the NMP reduces the boiling point elevation of the water, as described above.

The NMP/WM0/Water dopes generally have low viscosities at spinning temperatures in the range 70-900C. Typically, the viscosities are in the range of 2-30 Pa.s which makes the dopes suitable for wet spinning.

The fibre spun from the Water/NMMO/NMP dopes exhibits a resistance to fibrillation.

The invention will hereinafter be described by way of example only, with reference to the accompanying drawings in which: - Fig 1 is a schematic drawing of apparatus for carrying out a process according to the present invention; Fig 2 is a graph of Boiling Points of various solvent systems plotted against pressure; Fig 3 is a magnified picture of a conventional lyocell fibre (labelled 11Tencell' (TM)); and Fig 4 is a magnified picture of a wet spun lyocell fibre made in accordance with the present invention.

A known process and apparatus for the manufacture of a solution of cellulose in a mixture of water and tertiary-N5 amino-N-oxide is described in WO-94/28217, and such apparatus is suitable for use in the present invention. However, the process and apparatus used with the present invention will be described briefly for an understanding of the present invention. The apparatus for the production of cellulose 10 solution or dope comprises a pulp shredding device 11 and a premixer 12 which receives the shredded cellulose pulp and a mixed liquid solvent from a solvent storage tank 13. The solvent tank 13 contains a mixture of water, amine oxide, and an aprotic diluent which is a non-solvent for cellulose and 15 which does not react with the amine oxide, i.e. a cononsolvent with the water. The preferred amine oxide is a tertiary N- amine-N-oxide, preferably N-methylmorpholine-Noxide, and the preferred diluent is N-methyl-2-pyrrolidinone.

The slurry is heated in the premixer 12 at a temperature 20 in the range 40-700C for about 5-10 minutes and is then passed to an evaporator 14 which removes water from the slurry to form a homogeneous cellulose dope or solution. An apparatus suitable for use as an evaporator is a thin film evaporator of the type shown in EP-A-0,356,419 or in WO 94/06530. The 25 cellulose solution is heated to between 800-1000C in the evaporator under vacuum at a pressure of about 30 mm Hg (4 x 1 0:'N. M-2) to remove the water. The overheads removed from the evaporator 13 may pass into a solvent recovery system 20, 21. The cellulose dope is then passed into a buffer tank 15 of the 30 type described in US-A-5,358,143 before being pumped to a spinning cell 16 for spinning into fibres or filaments. Preferably the filament is spun directly into a spin bath without any air gap between the spinnerette and the regeneration liquid in the bath. The extruded filament is then passed through wash baths 17 and 18, dryer 19 and onto a collection drum, or may be cut into short lengths for use as staple fibre. The regeneration fluid may also be connected to the solvent recovery system 20, 21.

According to an embodiment of the present invention the cellulose dope has the following contents:

Example Cellulose (phs) solvent feed composition 1 15% cellulose (DP 600) 78% NMMO/22% water 2 6% cellulose (DP 15-17 mins 370) 78% NMMO 22% water 3 6% cellulose (DP 6-9 mins 430) 60% NMMO 17%water 23% NMP 15% cellulose (DP 230) 56% NMMO 15% water 29% NMP phs is parts per hundred solvent NMP 28-60% by weight Water 3-10% by weight NMMO 28-56-0 by weight Cellulose 6-15% by weight, the cellulose having a degree of polymerisation of 10 between 230-600, preferably 370-430 DP.

The cellulose dopes which contain NMP have a reduced time to solution as compared with the conventional NMMO/water dopes. This can be seen from the examples in Table 1 below. The solvent compositions are the compositions on mixing the 15 slurry before removal of water.

Table 1 Time to solution for Cellulose Dopes Time to solution Time to solution Lab mix Z-blade mix at 1000C 30 rnEg vacuum 2 hours 40 mins 6 - 8 mins mins It can be seen from comparison between Examples 2 and 3 20 that the addition of NMP reduces the time to solution by about fifty percent, although the non-solvent content in the liquid phase has increased to about 400-o, and this is even with a slight increase in DP from 370 to 430. A similar effect is noted between Examples 1 and 4 (although reduced DP also has 7 a marked effect) The reduced time to solution may be due to the presence of NMP acting to aid removal of water from the solution so that the system becomes more readily homogeneous. This can be demonstrated by the effect of the presence of NMP on Boiling Point Elevation (BPE) at reduced pressure for a composition of the solvent mixture. With reference to Fig 2 there is shown curves for boiling point measured against pressure for various solvent /non -solvent combinations. For comparison the curve for water is given by W, and the curve for solvents relating to Example 1 and Example 3 above are also shown. The curves X and Y respectively relate to 83% NMMO 17% water, and 88% NMMO 120lg water.

From the curves, it can be seen that at 30 mm Hg (4 x 10 -N. M-2) reduced pressure the BPE for the 696 cellulose (430 DP) solution of Example 1 is about 700C, whereas the BPE for the 830-b NMMO: 170k water mixture (a spinnable 606 cellulose dope) of curve X is about 100C higher, and the BPE for the 88% NMMO: 12% water mixture (which equates with the spinnable solvent for a 15% cellulose dope) of curve Y is about 200 C higher. This shows that a spinnable MMO:NMP:water solution can be achieved using lower temperatures and lower processing times than for the prior NMO:water solutions.

The viscosities of the MMO:MP:water solutions are given in Table 2 below:

- 8 Table 2: Rheological data on cellulose done Dope Modulus Shear Viscosities Extensional Viscosities Example Composition G/tan L at 105-' (B) / at 20000a-l(J) at 1000s-'(J) / at 20000s-l(J) (at 10HZW) 2a 6j cell 1700 Pa/ 27 Pa.s/ at 900C 110Pa.s./140Pa.s at 901C (430 DP) at 700C 73% NMMO 1000 Pa / 600 21% water at 900C 2b 6% cell 900Pa/600 22Pa.s/1.5Pa.s at 700C 237Pa.s/258Pa.s at 701C (370 DP) at 700C 73% NMMO 250Pa/750 SPa.s/ at 900C 100Pa.s/90Pa.s at 900C 211; at 900C 2c 10t cell 2000Pa/ 40Pa.s/ at 701C (230 DP) at 701C 73% NMMO 700Pa/750 15Pa.s/ at 900C 21t water at 901C 2d 6% cell 400Pa/650 9Pa.s/ at 701C 105Pa.s/98Pa.s at 700C (430 DP) at 700C 28% NMP 6Pa.s/ at 901C 18Pa.s/30Pa.s at 901C 56% NWO 300Pa/750 10% water at 900C 2e 6% cell 100Pa/80. Thermoreversible gel at 701C (430 DP) at 900C 60% NMP 2.5Pa.s/ at 901C 29% NMMO 5% water 2f 6% Cell 1000Pa/500 30Pa.s/ at 700C (600 DP) at 700C 28% NMP 56% NMMO 1OW water 29 9% Cell 1000Pa/600 25Pa.s/ at700C (370 DP) at 700C 28% NMP 55% NMMO 8% water 2h 15% Cell Thermoreversible gel at 70OC? (230 DP) 26% NMP 750Pa/750 15Pa.s/ at 90C 49% NMMO at 900C 10% water The data shown for the low speed shear viscosity were acquired on a Bohlin rheometer, and for the high speed shear viscosity were acquired on a jet rheometer.

These viscosities are considerably reduced as compared with the commercial lyocell 15% cellulose in amine oxide/water solvent, which at 900C has shear viscosity of 1000Pa.s at 10s-1 shear rate and an extensional viscosity of 4000 Pa.s. at 1000s-1.

The cellulose dopes containing NMP as a co-non-solvent have a maximum extrusion ratio of up to 1.5 and are suitable for wet spinning applications and can be spun at jet pressures of between 25-75 psi (1.7-5.5 bar; 1.7-5.5x105N. M-2) at 60-700C directly into a spin bath at a temperature of 200C (room temperature), containing 22-43% NMMO, 17-50% NMP and 20-50% water, dependent upon the composition of the cellulose dope. In the spin bath, the NMMP:WP ratio should lie between 1:2 and 2:1, for example the dope 2d can be spun into a spin bath containing 33-6-43-. NMMO, 17%-22-06 NMP and 35%-50% water, and dope 2e can be spun into a spin bath containing 22-27% NMMO, 43-54.06 NMP and 20-35% water.

Preferably, the NMMO:WP ratio by weight in the spin bath is substantially equal to the NMMO:WP ratio in the extruded cellulose dope.

Mechanical Properties of the fibres wet spun at a temperature of 60-700C from dopes containing NMP have been measured according to tests based on BSEN/ISO 5079:1996.

The results are given in Table 3 below:- - 10 Table 3 - Mechanical Properties Dope System Tenacity dtex Extension Modulence Decitex % CN/tex 2e 26 7 1040 2.9 19 12.5 900 2.9 2d 19-20 8-9 800 1.9 6% cellulose (370DP) 30-0 NMP 17 8 750 1.2 60-0. NMMO 10% water The fibre orientation and crystallinity lie between that of viscose cellulose and cellulose regeneration from NMMO/water solvents. The fibres also show a greater resistance to fibrillation than the conventional lyocell fibres.

is The fibrillation tests were conducted by the following procedures. A sample of 100 filaments of Smm length was shaken for 20 minutes in a 2 x 1 inch (5.08 x 2.54 cm) stoppered glass tube containing 8m1 of distilled water and 4g of glass microspheres. The tube was shaken at 1800-1900 rpm clamped to the 15cm arm of a scientific shaker.

The fibrillation index is a subjective index in which the filaments are viewed on a glass slide under a microscope after removal from the tube, and are assigned a rating from 0-10 (0 representing no fibrillation) according to their appearance after being abraded. For example a commercial fibre sold under the Trade Mark Tencel made from 15% cellulose NMMO/water dope has a highly crystalline lyocell filament, with a high tendency to fibrillate, as is illustrated in Figure 3 of the drawings, and has been assigned a Fibrillation Index of 7.

A fibre made from the dope composition of Example 2d using wet spinning techniques shows a substantially reduced tendency to fibrillate. This is shown in Figure 4 and has been assigned a fibrillation index of between 1-2. A fibre made from a dope composition of Example 2a which is outside of the scope of the invention, will have a fibrillation index in the range 3-4.

11 -

Claims (12)

1. A process for the manufacture of a shaped cellulose article from a solution of cellulose in solution in an amine-N- oxide solvent, the process involving the initial forTnation of a homogeneous cellulose dope, wherein cellulose pulp is dissolved in a mixture of a tertiary amine-N-oxide, water and an aprotic organic diluent to form a slurry, the slurry being subsequently heated with removal of water to form said homogeneous cellulose dope.

2. A process as claimed in claim 1 wherein the aprotic organic diluent is N-methyl-2-pyrrolidinone (NMP).

is

3. A process as claimed in claim 2, wherein the mixture contains 21-60% by weight of NMP.

4. A process as claimed in any one of claims 1 to 3, wherein the cellulose pulp has a degree of polymerisation in the range 200-600, preferably in the range 230-430.

5. A process as claimed in any one of claims 1 to 4, wherein the water content of the solvent mixture is in the range 13-18% by weight.

6. A process as claimed in any one of claims 1 to 5, wherein the amine-Noxide is N-methylmorphol ine-N- oxide (NMMO).

7. A process as claimed in any one of claims 1 to 6, wherein the water is removed under conditions of reduced pressure of at least 300 mm Hg 4 X 105N.M-2 at a temperature of about 901C

8. A process as claimed in any one of claims 1 to 7, wherein the dope has a viscosity in the range 2-30 Pa.s at a shear rate of 105-' within the temperature range of 70-900C.

9. A process as claimed in claim 8, wherein the cellulose dope is extruded directly into a liquid regeneration bath 12 containing at least two non-solvents for the cellulose material, in order to remove amine-N-oxide from the dope.

10. A process as claimed in claim 9 when dependent upon claim 2 and claim 6, wherein the regeneration bath composition comDrises a mixed solution of NMMO, NMP and water in which the water content is between 20-50% by weight, and the NMMO:NMP ratio by weight is substantially equal to the NMMO:NMP ratio in the cellulose solution prior to its extrusion into said bath.

11. A process as claimed in claim 10, wherein the ratio of NMMO:NMP by weight is between 1:2 and 2:1.

12. A cellulose fibre made by a process as claimed in any one of claims 1 to 11.

is