GB2319495A - Method and apparatus for the manufacture of lyocell fibres - Google Patents

Abstract

In a method of manufacture of lyocell fibres having an imbibition of greater than 0.9 g water/g dried fibre, a solution of cellulose in an amine oxide solvent is forced through a spinnerette to form filaments or fibres, which are passed through an air gap and into an aqueous fluid for regeneration, the fluid being maintained at a minimum temperature of about 60{C. Also disclosed is apparatus for the production of lyocell filaments, the apparatus including an extruder for extrusion of cellulose solution through a spinnerette to form a filament, and at least one pair of opposed jets for directing opposing flows of regeneration fluid onto the filaments. The jets are spaced from the spinnerette by an air gap, and an air vent blows air across the air gap.

Description

Filaments and Fibres This invention relates to lyocell filaments and fibres.

Lyocell fibres and filaments are produced by spinning a solution of cellulose in an amine oxide solvent which is then leached into water or a dilute solution of aqueous amine oxides to produce cellulose filaments which can then be cut into fibres. The cellulose solution is forced under pressure through a spinnerette and the resulting filaments are passed through an air gap before passing into a spin bath containing the aqueous coagulation fluid.

A typical commercially produced lyocell fibre is coagulated in a spin bath the aqueous contents of which are held at ambient temperature (about 230C) . Lyocell generally has superior physical properties to cellulose fibre produced by the viscose process; however lyocell fibres generally do not absorb or imbibe as much water as viscose fibre.

The present invention seeks to provide a lyocell fibre with improved imbibition properties.

Accordingly the invention provides a method of manufacture of lyocell fibres having an imbibition of greater than 0.9 g water/g dried fibre, in which a solution of cellulose in an amine oxide solvent is forced through a spinnerette to form filaments or fibres which are then passed through an air gap and into a regeneration or coagulation fluid for regeneration, wherein the regeneration or coagulation fluid is maintained at a minimum temperature of about 600C.

The draw ratio of the spun fibres will be in the order of 5:1. The imbibition of the dried fibre can be controlled by controlling the temperature of the coagulation fluid so that a desired imbibition can be selected by selecting a particular spin bath temperature.

During the spinning process a flow of cold dry air is passed across the air gap. Preferably the air temperature is about 100C and the relative humidity of the air is about 3%. The air may be blown across the air gap at a velocity of from 1 to 4 m.s~l (meters/sec) and preferably from 2 to 3 m.s . The regeneration or coagulation fluid may flow in a substantially vertical flume fed by two opposing and substantially balanced inlet flows, the vertical height of the flume being sufficient for the filaments to be immersed in the moving fluid for a time in excess of 0.2s.

Preferably the coagulation fluid is water or an aqueous solution of an amine oxide.

The invention also provides a method of manufacture of air gap spun lyocell fibres, in which the imbibition properties and dyeability of the dried lyocell fibres are controlled by regulation of the temperature of the coagulation fluid.

In a further aspect, the invention provides an air gap spun lyocell fibre having a imbibition for dried and rewetted fibres which is greater than 0.9 g water/g dried fibre, with a tensile modulus of at least 500 cN/tex, and preferably about 1000 cN/tex.

Preferably the water imbibition is between 0.9 and 1.02 g water/g fibre, depending upon the temperature of the coagulation fluid.

The imbibition may be related to the average hydrated pore size index for the fibres. A fibre having a pore size index of about lOnm will have a water imbibition of about 0.9g/g fibre, and fibre having a pore size index of about 25nm will have a water imbibition of more than 1.00 g/g fibre.

Fibres produced in spin baths having a temperature of 600C or higher have an increased ability to accept dye, as compared with fibres produced in a spin bath at ambient temperature. An integrated strength value (as defined below) of at least 260 was obtainable using Solophenyl Green dye. The dyeability when expressed as a comparative strength, or Q value (as defined below), is about 60% higher than for standard lyocell which is manufactured by Courtaulds Fibres (Holdings) Limited and sold under the trade name TENCEL.

The invention will hereinafter be described by way of example only, with reference to the accompanying drawings, photographs and diagrams, in which: Figure 1 shows in schematic form an embodiment of apparatus for the manufacture of lyocell according to the method of the present invention; Figure 2 is a graph of imbibition of water of dried and rewetted lyocell fibres made at different coagulation temperatures; Figure 3 is a graph of mean hydrated pore size index of lyocell fibres coagulated at different coagulation temperatures; Figure 4 is a graph of dyeability in the form of integrated strength value (as defined below), measured on a reflectance spectrometer at different coagulation temperatures; Figure 5 is a reproduction of a slide showing lyocell fibres having a tendency to fibrillate; Figure 6 is a reproduction of a slide showing lyocell fibres having substantially no tendency to fibrillate; and Figure 7 is a graph of dyeability measured as Q value or comparative strength (as defined below) versus coagulation temperature.

With reference to Figure 1, an extruder 11 contains a solution of cellulose in an amine oxide solvent, the cellulose solution comprising 15% by weight of cellulose, 10% by weight of water and 75% by weight of Nmethylmorphyline-N-oxide (NMMO). The cellulose has a degree of polymerisation of between 200 and 5000 and more typically between 400 and 1000.

The solution is forced through a spinnerette 12 to form a multi-filament tow 13 which passes across an air gap 14 and enters a flume 15 of regeneration fluid (or coagulant).

The spinnerette has some 95 jet holes, each having a diameter of 70cm. The air gap 14 is about 3cm in height.

The flume 15 comprises water, or a dilute solution of amine oxide in water, and is provided by a pair of jets 17 arranged in opposition to each other and through which the regeneration fluid is jetted onto the tow 13. The flow rates through the two opposed jets 17 should balance for a stable flume. The liquid is held by the tow 13 and passes down the tow into a reservoir 18. The tow 13 is wound onto a collection roller 19, the lower portion of which is immersed in the liquid in the reservoir 18. Excess liquid is removed from the reservoir 18 by a drain 21, and may be recirculated to the jets 17 or discharged.

A cross-draught of dry cold air is blown through air vent 22 across the air gap 14. The vent 22 is supplied by air through an inlet conduit 23.

The cellulose solution is extruded at a die temperature of about 105-1150C, preferably 1100C, and the draw ratio is about 5:1 with filament being drawn off at a rate of about 60 meters per minute. The height of the jet 17 above the reservoir 18 is between 20-30 cm so that filaments are immersed in the regeneration fluid for at least 0.2 s.

The air blown through the air vent 22 has a temperature in the range 4-12"C and more typically in the range 10-120C.

The air is preferably dry, but typically has a relative humidity of about 3t. The cross-draught velocity is typically in the range 2-3 m.s~1.

A series of different batches of filament were produced in which the temperature of the water recirculating in the flume 15 and reservoir 18 was different for the different batches. The filaments for each batch were then removed from the collection roller 19, washed three times in demineralised water and were used wet, if required, or alternatively dried at ambient temperature, and then subjected to various test procedures.

Tensile Properties The tensile properties of the dry fibres which had been coagulated in water at different temperatures are shown in Tables 1A and 1B.

Table 1A

Water Linear Breaking Breaking tenacity/ coagulation density/dtex load/cN cNtexl temp /"C Mean Mean Mean 20 1.55 5.47 35.45 60 1.81 4.59 25.53 70 1.68 4.26 25.35 80 1.86 4.55 24.64 90 1.66 3.48 21.05 Table 1B

C rl Coagulation Breaking Initial Load @ yield temp/ C elongation/% modulus point/cN /cNtex1 Mean Mean Mean 20 11.29 1029.7 1.64 60 9.10 978.4 1.81 70 10.54 936.1 1.71 80 11.41 1038.1 1.80 90 9.32 926.0 1.51 The dry initial modulus of all experimental fibres was of the order of 1000 cN/tex, which appeared to be uninfluenced by coagulation temperature. Breaking elongation also showed no clear dependence on coagulation temperature, remaining at about 10t for all samples. Dry fibre tenacity fell in an approximately linear manner with increasing coagulation temperature, reducing from about 36 to about 2lcN/tex over the full temperature range from 20 to 900C. For comparison, standard Viscose fibre has a dry tenacity of about 20-25cN/tex, with an extensibility at best of about 25%, and an initial elastic modulus of about 500cN/tex.

Measurement of Hydrated Pore Structure Both water imbibition and NMR porosity measurements have been used to characterise the hydrated structures of fibres regenerated (or coagulated) at different temperatures.

Imbibition Imbibitions were determined after rewetting samples of dried unfinished fibre in demineralised water for at least 15 minutes, followed by centrifuging in sealed tubes at 3200 rpm for 5 minutes. Final dried sample weights were determined after oven drying to constant mass at a temperature of 1070C. Water Imbibition (W.I.) was then taken as: W.I. = (weight after centrifugation - dry weight) / dry weight The temperature of coagulation was found to have a significant effect on the volumes of water imbibed. With reference to Figure 2, the imbibition increased substantially linearly with spin bath temperature, rising from a value of 0.74 g/g fibre for material coagulated at 200C to over 1.0 g/g fibre for fibres coagulated at 900C.

The imbibition value of never-dried fibre coagulated at different temperatures is given in Table 3 below.

Hvdrated Pore Index The main size index of the hydrated pore structure for dried and rewetted fibres was determined by the water proton NMR (Nuclear Magnetic Resonance) relaxation technique using a QP20 bench spectrometer manufactured by Oxford Instruments. NMR spectroscopy detects the water situated in the pores. A sample of at least 1.5 g of the fibres was rewetted for at least 15 minutes using demineralised water.

The sample was then centrifuged at 3200 rpm for five minutes using a Centaur 2 centrifuge so as to remove interstitial water.

The NMR data were obtained on an Oxford Instruments QP20. The NMR machine was set up to subject samples to a CPMG pulse sequence and to observe the spin-spin relaxation times of water protons. Signals from the QP20 were fed to a PC on which had been installed a software package for fitting curves. The software used was MicroCal Origin (version 2.75).

Firstly, the data were fitted to an exponential curve composed of either one or two components, because it was found that no more than two components were needed to give a reasonable fit. Each exponential component could be characterised by a pre-exponential and a time constant, as follows: NMR signal intensity, I = Alexp(-t/Tl) + A2exp(-t/T2) The pore sizes of never-dried fibre coagulated at different temperatures are given in Table 3 below.

The pre-exponential is proportional to the volume of water present in the pores; the time constant of the decay is proportional to the dimensions of the pores. Hence the surface area of pores could be found from the ratio of pore volume to pore dimension.

In instances where one component's pre-exponential was less than 10% of the other's, it was concluded that the sample could be well described by a single component. In cases where two components appeared to be significant, it was concluded that the true pore-size distribution of the sample could be approximated by two sizes of pore.

The parameters produced by the graph-fitting software were entered into a Lotus 1-2-3 spreadsheet for manipulation. A series of graphs was thereby produced, showing how pore water content, pore surface area per gram of dried fibre and pore size varied with conditioning.

When two pore sizes are used to approximate the pore population, the total surface area per gram of fibre will be the sum of the surface area of 'small' pores and the surface area of 'large' pores. The surface areas are normalised with respect to dry mass.

Data for the surface area and dimensional index of an average pore were obtained by using an average decay time which was weighted in terms of the pre-exponentials of the two components produced in the fitting exercise: T2 for 'average' model = (A1tl + A2t2) / (A1 + A2) A pore size index I was then obtained by: I = T2/620 I is in nanometres and T2 is in microseconds.

This equation is derived as follows: The figure of 620 relates to the space occupied by a monolayer of water molecules. For a rough estimation of the area covered by lg of a monolayer of water, consider a water molecule to be a sphere whose area when projected onto a surface is 1.48 x l019m2. The area of lg of monolayer would be 4950m2. If the packing efficiency of water molecules is taken to be 0.8 then lg of water monolayer would cover 6.2 x 103m2.

Assuming a cylindrical pore geometry then I = 2V/S where V is the volume of water in the pores and S is the surface area of the pores.

S = mass of water at surfaces x 6.2 x 103m2 V = mass of water in pores / density The mass of water in a surface monolayer can be calculated from T2. If a water molecule is on a cellulose surface then it will 'decay' in about 200us; if it is in the interior of a pore then it will 'decay' in about 2s.

Therefore, 1/T2 = Pb/2x106 + P9/200 where Pb and P9 are the probabilities that a water molecule will be in the interior of the pore and the surface monolayer, respectively.

However, the surface term dominates. Hence, 1/T2 > P200 = mass in surface layer/total mass of water in pores. Therefore, S = 200 T2 x (total mass of water in pores) x 6.2x103. After inserting these results into the equation for I and simplifying by cancellation, I = T2/620 The average pore size index for the different batches coagulated at different temperatures is shown in Figure 3.

An increase in average hydrated pore size index was observed to correspond with an increase in coagulation temperature for the fibres, although this appeared to be non-linear, with the changes enhanced at higher temperatures. The size index rose from a value of 7nm, which is typical for ambient temperature coagulated lyocell, up to almost 24nm for the fibre coagulated at 900C.

Dyeability The test requires 6g of sample and 6g of standard lyocell fibre (Tencel (Trade Mark) fibre) manufactured using ambient temperature spin bath for each sample. The fibre was chopped to lengths of 40mm and opened by hand carding.

Procedure 1. Put sample and standard fibres in a Rotadyer tube separated by a gauze.

2. Add to the tube 240ml of a solution containing 0.05% Solophenyl Green direct dye (approximately 1% on weight of fibre) and 1% NaC1.

3. The tube is put into a Rotadyer at 500C. The temperature of the bath is then ramped up to 1000C over 30 minutes and held at 1000C for 45 minutes.

4. The fibres are then thoroughly washed and then dried.

5. After being twice passed through a fibre blender to smooth any unevenness, the dyed filaments are then ready for the spectrometer.

The depth of dyeing was measured using a Minolta reflectance spectrometer. The spectrometer measures the intensity of light reflected from the fibre sample across the full visible spectrum, referenced against a standard white tile. The 'integrated strength' was calculated for each sample, which is the sum of the differences between the tile and the sample at all wavelengths.

The comparative Q-value was taken as: Q = (integrated strength - integrated strength x 100% ( of sample standard Tencel fibre) integrated strength of standard Tencel fibre The effect of spin-bath temperature on rate of dye uptake was emphasised when the integrated strength, and Qvalues of the fibres were plotted against coagulation temperature as shown in Figures 4 and 7 respectively. An increase from 0 to 60% in Q value was observed over the entire temperature range, suggesting that the control of spin-bath temperature could be a critical factor in adjusting fibre dyeability, especially where differences in Q-value of only 5% can lead to drifts from a colour specification.

Fibrillation A sample of 100 filaments of 5mm length was shaken for 20 minutes in a 2 x 1 inch (5.08 x 2.54cm) stoppered glass tube containing 8ml 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 to 10, 0 being no fibrillation and 10 being substantive fibrillation, according to their appearance after being abraided. For example the standard lyocell fibre is a highly crystalline lyocell filament and has a high tendency to fibrillate as shown in Figure 5, and typically would be assigned a Fibrillation index of 7. A fibre that has substantially no tendency to fibrillate would be assigned a fibrillation index of zero, such a fibre being shown in Figure 6.

Table 2 The Fibrillation ProDerties of Tencel Fibre Coagulated at Different Temperatures

Coagulation Sand Test temperature /OC fibrillation index 20 7 60 7 70 6 80 6 90 4 It can be seen that the tendency to fibrillate reduces with increasing coagulation temperatures.

The apparatus of Figure 1 allows stable spinning at coagulation temperatures of up to 900C.

Water vapour from the hot spin-bath must be removed efficiently from the filament region. This was achieved by use of the cold dry cross-draught.

Variations in lyocell filament properties were experienced over a range of coagulation temperatures. The effects were most significant above temperatures of 600C, beyond which point there were marked changes in fibre porosity and mechanical behaviour. Table 3 summaries the properties of fibres coagulated at ambient temperature, 600C, and under the more extreme conditions at 900C.

Comparative data for Viscose are also included.

Table 3 Typical Properties of Standard Viscose Fibre. Compared with Lvocell Fibre Coagulated at 200C. 600C and 900C

20"C 600C 900C viscose Tenacity cN/tex 35 25 20 20-25 Initial dry modulus cN/tex 1000 1000 950 500 Rewetted Water Imbibition g/g 0.75 0.9 1.0 1.0 Rewetted Pore Size Index nm 7 12 24 15 Never-dried Imbibition g/g 2 2.5 3 3 Never-dried Pore Size nm 20 55 85 60 Comparative Strength from Q- +2 +35 +60 +110 test a Thus, the lyocell fibre produced at the higher temperatures has water imbibition properties comparable with viscose fibres, whilst retaining the tensile modulus of typical lyocell fibres.

The drying of the lyocell fibres causes some irreversible collapse of the pore structure, although some of the benefits from increased porosity can still be retained on rewetting. Dye molecules or other fabric treatment reagents would be expected to diffuse more easily into the rewetted fibre, as already demonstrated by the Qtest. Absorbency will also be enhanced, although the contribution from the increased internal rewetted pore volume will only account for a part of the total liquid uptake.

In general, both fibre pore size and volume show a very clear relationship with fibre coagulation temperature.

The speed of dye uptake is also strongly influenced by fibre coagulation temperature, since diffusion processes are highly dependent on the sizes and shapes of pores within water-filled morphologies. The dyeing behaviour of cellulosic fibres suggests that pore volume may be proportional to the diffusion coefficient of direct dyes, and the relationship between Q-value and fibre imbibition in Figure 4 bears this out. Thus, the manipulation of lyocell spinning conditions can have a predictable effect on fibre morphology, and which in turn can be used to induce a predictable effect on the commercially important fibre dyeability.

Claims (16)

1. A method of manufacture of lyocell fibres havinc an imbibition of greater than 0.9 g water/g dried fibre, ir which a solution of cellulose in an amine oxide solvent is forced through a spinnerette to form filaments or fibres which are passed through an air gap and into an aqueous fluid for regeneration, wherein the fluid is maintained at a minimum temperature of about 600C.

2. A method of manufacture as claimed in claim 1, wherein the imbibition of the dried fibres is controlled by selecting a desired temperature for the regeneration fluic in the range of 60"C to 970C.

3. A method of manufacture as claimed in claim 1 oi claim 2, in which air flows across the air gap, the aii having relative humidity of about 3%.

4. A method of manufacture as claimed in any of claims 1 to 3, wherein the temperature of the air flow ic about 100C.

5. A method as claimed in any one of claims 1 to 4, wherein the regeneration fluid flows in a substantialli vertical flume fed by two opposed and substantially balancec inlet flows, the vertical filaments being immersed in the moving fluid for a time in excess of 0.2s.

6. A method as claimed in claim 5, wherein the filaments are wound onto a storage or take off roller which is partially immersed in a bath into which the flume of regeneration fluid flows.

7. A method as claimed in any one of claims 1 to ( wherein the filaments are drawn as they pass through the aii gap and the draw ratio being in the order of 5:1, to produce an oriented fibre.

8. In a method of manufacture of air gap spun lyocell fibres, the imbibition properties and dyeability of the dried lyocell fibres are controlled by regulation of the temperature of the coagulation bath.

9. Apparatus for the production of lyocell filaments wherein the apparatus includes an extruder for extrusion of cellulose solution through a spinnerette to form filaments which are drawn vertically downwards, and at least one pair of opposed jets for directing opposing flows of regeneration fluid onto the filaments emerging from the spinnerette, the fluid running down the filaments to a collecting trough and the jets being spaced from the spinnerette by an air gap, with an air vent for blowing air across the air gap.

10. An air gap spun lyocell fibre having a water imbibition for dried and rewetted fibres which is greater than 0.9 g/g dried fibre, and a tensile modulus of at least 500 cN/tex dry.

11. Lyocell fibre as claimed in claim 10 wherein the water imbibition is between 0.9 and 1.02 g/g dried fibre.

12. Lyocell fibre as claimed in claim 10 or claim 11, wherein the fibres have an average hydrated pore size index of between 10 nm for an imbibition value of about 0.9 g/g and 25 nm for imbibition value of greater than 1.00 g/g.

13. Lyocell fibre as claimed in any one of claims 10 to 12 wherein the fibres have a dyeability (measured as integrated strength, as defined herein) of at least 250.

14. Lyocell fibre as claimed in any one of claims 10 to i3, wherein the tensile modulus of dried fibre is about 1000 cN/tex.

15. Lyocell fibres as claimed in any one of claims 10 to 14, wherein the fibres after drying have a breaking elongation of about 10%.

16. Lyocell fibres made by a method as claimed in any one of claim 1 to 8, or by means of apparatus as claimed in claim 9.