EP0749502A1

EP0749502A1 - Fibre production process and fibre produced thereby

Info

Publication number: EP0749502A1
Application number: EP95909899A
Authority: EP
Inventors: Jim Robert James; Phillip Ian Robinson; Alan Sellars
Original assignee: Courtaulds Fibres Holdings Ltd
Current assignee: Courtaulds Fibres Holdings Ltd
Priority date: 1994-03-09
Filing date: 1995-03-02
Publication date: 1996-12-27
Also published as: GB9404547D0; WO1995024520A1; CN1139961A; JPH09509987A

Abstract

Crimp can be generated in lyocell fibre (19) by passing the wet fibre through a compaction device such as a stuffer box (32) fed with steam (37A). The method provides some permanent crimp, which is of advantage for example during processing into spun yarn. The method can be used to introduce high levels of crimp, which is of advantage for example in the manufacture of filter rods.

Description

FIBRE PRODUCTION PROCESS AND FIBRE PRODUCED THEREBY

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the manufacture of lyocell fibres (which may also be called solvent-spun cellulose fibres) containing permanent crimp, and which may have an increased level of crimp in comparison with known lyocell fibres, and to the fibres so obtained. By the term "permanent crimp" is meant crimp which is retained when the dried crimped fibre is rewetted.

2. Description of Related Art

Lyocell fibres are known materials, and their manufacture is described for example in US-A-4,246,221. They are available commercially from Courtaulds Fibres (Holdings) Limited under the Trade Mark 'Tencel' . They are made by dissolving cellulose in an organic solvent and extruding the solution so formed through a spinnerette into a coagulating bath which serves to precipitate the cellulose into fibre and wash solvent from the fibre. This process may be called solvent-spinning, and lyocell fibres may also be called solvent-spun cellulose fibres. The cellulose is usually woodpulp. The solvent may be a tertiary amine N-oxide (amine oxide) , generally in admixture with a minor proportion of water, preferably N-methylmorpholine N-oxide, in which case the coagulating bath is preferably an aqueous bath. After precipitation in the coagulating bath, the lyocell fibres are washed and dried. The precipitated lyocell fibre is in the form of continuous filaments, and may be in the form of a tow which contains many thousands of such continuous filaments. For some end-uses, for example in the production of some types of entangled fabrics, the lyocell fibre is used in the form of such a tow. For other end-uses, for example in the production of spun yarn, the tow is first cut into short lengths to form staple fibre. A lyocell tow may be converted into staple fibre between the precipitation and washing steps, between the washing and drying steps, or after drying. In the production of spun yarn by ring spinning and suchlike processes, staple fibre is carded to form a web of parallelised fibres, the web is drawn or drafted (often in several stages) to form a fine roving, and the roving is twisted to form the yarn. The web and roving are at least in part held together by crimp in the fibre.

Solvent-spinning processes are to be distinguished from other known processes for the manufacture of cellulose fibres which rely on the formation and decomposition of a chemical derivative of cellulose, for example the viscose process. Viscose rayon fibres in general possess inherent crimp, as do natural cellulose fibres such as cotton, whereas lyocell fibres generally possess little or no crimp. Crimp is a desirable fibre property in many end-uses. Crimp provides cohesion between the fibres during and after processing into spun yarn or entangled fabric. Crimped fibres are more bulky than uncrimped fibres, and accordingly crimp provides better covering power in textile materials and increased absorbency in absorbent articles. International Patent Application WO 94/28220, published 8th December 1994, states that lyocell fibre can be crimped by passing a dried lyocell tow through a stuffer box. The crimping action can be assisted by the injection of steam, preferably dry steam, into the stuffer box. The crimped fibre may possess up to about 2.0 or 2.2, commonly 1.5 to 2.2, primary crimps per centimetre. It is believed that this crimping process is not capable of introducing significantly higher levels of crimp into the fibre. This crimp is however not permanent. It is mostly removed when lyocell staple fibre is carded and is completely removed during subsequent drafting operations. If the crimped fibre is rewetted, essentially all the crimp disappears.

SUMMARIES OF THE INVENTION

The present invention provides, in a first aspect, a process for the manufacture of lyocell fibre characterised in that it includes the step of passing the lyocell fibre in the form of a wet tow through a compaction device thereby creating permanent crimps in the fibre. The fibre made by the process of the first aspect of the invention preferably possesses at leaet 1.0 primary crimp per centimetre. The process of the first aspect of the invention may additionally provide some temporary crimp in the fibre. The compaction device is preferably a stuffer box, although other types of compaction device, such as devices which rely upon forcing the fibre into a nip between a roller and a stationery blade or with overfeed into a nip between rollers, may also be used.

The process of the first aspect of the invention is preferably carried out on never-dried tow, further preferably on washed, never-dried tow immediately before drying. Never- dried lyocell fibre commonly contains about 60 percent by weight of water. The process of the invention may alternatively be carried out on tow which has previously been dried and has subsequently been wetted out before introduction into the compaction device. It will be understood that these alternative techniques may produce crimps of differing character, for example angularity.

The present invention provides, in a second aspect, lyocell fibre characterised in that it possesses at least 3.0 primary crimps per centimetre. The invention provides, in a third aspect, lyocell fibre characterised in that it possesses at least 1.0 permanent primary crimp per centimetre.

Lyocell fibre according to the second aspect of the invention may possess up to about 7.5 primary crimps per centimetre. It has been found that lyocell fibre which possesses more than about 6.0 primary crimps per centimetre cannot readily be opened by conventional textile processing machinery. The fibre is liable to suffer physical damage during a crimping process which introduces such a high level of crimp, whereby weak points are created in the fibre. Furthermore, sharp permanent crimps in the form of more or less right angled elbows may be formed during such a crimping process, and these elbows tend to interlock. Lyocell fibre containing high levels of crimp is cohesive and resilient, and lyocell tow of this type of fibre is useful for example in the manufacture of absorbent articles such as tampons and filters for cigarettes . Fibre according to the third aspect of the invention may possess up to about 4.0 permanent primary crimps per centimetre.

The present invention provides, in a fourth aspect, a process for the manufacture of lyocell fibre which includes the steps of:

(a) dissolving cellulose in a solvent to form a solution;

(b) extruding the solution through a spinnerette into a coagulating bath to form continuous cellulose filaments,-

(c) washing the cellulose filaments with water to remove the solvent, thereby forming a wet tow of never-dried lyocell fibre;

(d) passing the wet tow through a compaction device, thereby producing never-dried crimped lyocell fibre; and

(e) drying the never-dried crimped lyocell fibre, thereby producing dried crimped lyocell fibre.

The solvent is preferably an aqueous tertiary amine N- oxide, more preferably N-methylmorpholine N-oxide, in which case the coagulating bath is preferably an aqueous bath. The crimped lyocell fibre generally exhibits a degree of permanent crimp.

The process of the fourth aspect of the invention may additionally include the step of: (f) converting the crimped lyocell fibre into staple fibre, alternatively before or after the drying step (e) .

In a manufacturing process for lyocell fibre in which the fibre is dried as tow, the tow may be dried for example on a drum dryer or preferably a belt dryer. When a drum dryer is used, the tow is passed in extended form over one or more heated drums, which may be perforated, and the surface speed of the drums must therefore be more or less the same as the speed of extrusion through the spinnerette. This generally introduces a limitation on the maximum line speed, and therefore on the productivity of the spinnerettes. The maximum line speed may be increased by using a greater number of drums, but this is expensive. Furthermore, drying the tow in extended form involves application of tension to the tow which may remove at least some of the crimp. When a belt dryer is used, the tow may be spread out or plaited onto a mesh belt through which hot gas (normally air) is blown, whereupon the linear speed of the dryer can be a fraction of the speed of extrusion. Higher extrusion speeds can therefore be employed, so increasing the productivity of the spinnerettes. It has been found that when uncrimped lyocell tow of the prior art is dried on a belt dryer, the fibres in the tow tend to cohere. In consequence, the hot gas does not readily penetrate the tow, with the result that drying is slow and uneven, which fact must be set against the productivity advantage of belt dryers. In contrast, a lyocell tow crimped in the wet state according to the invention opens out sufficiently on a belt dryer advantageously to allow good gas penetration and rapid and uniform drying. Alternatively, a lyocell tow crimped in the wet state may be cut into staple fibre before drying on a belt dryer. It will be understood that uncrimped lyocell staple fibre cannot readily be crimped after drying.

Fibre which possesses low levels of crimp is generally treated with a cohesive spin-finish to assist processing through operations such as carding and yarn spinning. Cohesive spin finishes generally impart a harsh handle to the fibre. It is an advantage of the invention that the generation of crimp in the fibre means that less cohesive spin finishes can be used, thereby providing lyocell fibre with a desirable soft handle.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more particularly described, by way of example, with respect to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of the various stages in the manufacture of crimped lyocell fibre according to the invention;

Figure 2 is a diagrammatic representation of a crimping process according to the invention in which lyocell fibre is passed through a stuffer box;

Figure 3 is a diagrammatic representation of the steam supply to the stuffer box,- and

Figure 4 is a diagrammatic representation of an alternative type of compaction device utilising a Vickery blade.

DESCRIPTION OF PREFERRED EMBODIMENTS

In Figure 1 is shown a mixer 10 with inlets 11 and 12 to receive shredded cellulose and an aqueous amine oxide solvent respectively. The cellulose dissolves in the solvent to form a solution. A hot solution from the mixer 10 is pumped, via a metering pump 13, to a spinnerette 14 through which the solution is extruded to form a continuous tow of fibres.

As the hot solution leaves the spinnerette 14 in fine streams 14a it is passed through a coagulating bath 16 in which a mixture of water and amine oxide is recirculated. Bath 16 may contain about 25% by weight amine oxide. Cellulose is precipitated from the solution to form solid filaments 15a and solvent is washed out from the filaments. From coagulating bath 16, the multiplicity of cellulose filaments in the form of a tow 15 is passed via roll 17 through a water bath 18. The tow passing through the water bath may be, for example, up to 30 to 35 cms (12 to 14 inches) wide. More than one such bath may be used. In the water bath or baths, the remaining amine oxide is washed out of the filaments and the tow 19 emerging from the water bath or baths is of lyocell. The titre of the filaments in the tow may for example be in the range 1 to 10 decitex, often 1.5 to 5 decitex.

From the water bath 18 the tow (now numbered 19) is passed through a finishing stage 19A where the filaments are lubricated using spin finishes well known in the art.

The wet tow is then passed into a nip defined by rolls 21 and 22 from which it is fed into a stuffer box 23. Steam is fed into the stuffer box via inlets 24 as generally indicated by the arrows.

It should be understood that alternatively the wet unfinished tow may be fed into the stuffer box and the spin finish applied to the fibre by injecting it into the stuffer box or by spraying it onto the tow after emergence therefrom. It has been found that application of the spin finish to the fibre before passage into the stuffer box may assist in the crimping operation and this method may be preferred.

The degree of crimp applied in the stuffer box may in general be in the range l.0 to 7.5 primary crimps per centimetre. (It will be appreciated that fibres with primary crimp 34A as represented in Figure 2 may be either straight or of wavy formation, i.e. they may also have secondary crimp 34.) If the fibre is intended for textile end-uses, the degree of crimp is preferably in the range about 1.5 to about 3.0 primary crimps per centimetre. As the tow emerges from the stuffer box 23, it may have been compressed in width, and it may be allowed to widen as it passes to a dryer. The crimped tow 25 is then passed through a drying oven 20 maintained at elevated temperature. The drying oven is preferably of the perforated belt type but may alternatively be of the can, calender or perforated drum type. The temperature of the drying oven depends on its design and configuration, but is generally in the range 60 to 180^*C.

The crimped dried tow (now numbered 29) emerging from the dryer 20 is passed via a roll 26 to a cutter 27 where it is cut to form staple fibre. The length to which the staple fibre is cut in the cutter 27 will depend on the intended end use of the staple fibre. Thus, for example, lengths of 3 to 6 mm may be required for paper making, 6 to 15 mm for non-woven fabrics, 30 to 60 mm for use in cotton-type yarns and carded non-woven fabrics and 60 to 150 mm for use in worsted-type yarns. Such different end-uses generally require different filament titres, as is well known in the art. Filament titre may generally be in the range 1 to 30 decitex, typically 1 to 3 decitex for cotton-type applications and 2 to 8 decitex for worsted-type applications. The crimped staple fibre is collected in a carton 28. The crimped dried tow may alternatively be collected as tow, with the omission of the cutting step. Such a tow may be useful for the manufacture of certain types of absorbent articles, for example cigarette filters.

There may be, as shown, a single tow emerging from the spinnerette 14 and this may contain, for example, up to 20,000 filaments and may be of basis weight, for example, 30 to 65 ktex, i.e. 30 to 65 g/metre, after the drying stage. A tow useful for the manufacture of cigarette filters may have a basis weight in the range 5 to 8 ktex. Alternatively, the tows emerging from several spinnerettes may be combined into a single large tow, containing for example 0.2 to 1 million filaments, before passage through the compaction device. Cigarette filters in which the body of the filter contains or consists of a fibre tow, particularly a cellulose acetate fibre tow, are well known. The tow is disposed parallel to the longitudinal axis of the filter (or cigarette) . The body of such a known filter may consist solely of such tow, in the so-called 'mono' construction. The body of such a known filter may alternatively comprise an outwardly disposed filter element of such a tow and an inwardly disposed filter element of compressed pleated paper, in the so-called 'dual' construction. The body of such a known filter may further alternatively comprise an outwardly disposed filter element of such a tow, a middle filter element of activated carbon, and an inwardly disposed filter element of compressed pleated paper, in the so-called 'triple' construction. Paper filter elements have a high filter efficiency in comparison with acetate tow filter elements, but are prone to become soggy and easily compressible when the cigarette is smoked and impart an undesirable 'papery' taste to the cigarette.

The invention further provides a cigarette filter characterised in that the body of the filter contains or consists of crimped lyocell tow. It is thought that such filters possess a high filter efficiency in comparison with known cellulose acetate tow filters. This is advantageous in view of the trend towards low tar cigarettes. Such filters may be made by methods generally known for tow filters. The invention further provides a cigarette containing such a filter.

The crimping stage of the process is shown in more detail in Figure 2.

The tow 19 is passed into a nip 30 between rolls 21 and

22 and is fed from nip 30 into the stuffer box 23 at a rate sufficient to fill passageway 31 of the stuffer box, which is defined between plates 32 and 33 and extends from the exit of the nip 30 to the exit 35 of the stuffer box, whereby creasing or crimping of the filaments at regular longitudinal intervals is caused. Liquor is expelled from the tow by the pressure exerted by the rolls 21 and 22. The crimped tow is compressed between plates 32 and 33 in the stuffer box as shown to form a series of loops 34 (shown spaced out in Figure 2 for clarity) and emerges at exit 35 in permanently crimped form. Steam is injected into the stuffer box as indicated by the arrows and passes through holes 36 in the plates 32 and 33 to contact the tow as it passes along the passageway 31.

If it is desired to make fibre for textile processing possessing about 2.0 to 2.5 primary crimps per centimetre, the pressure between rolls 21 and 22 should be about 0.3 to 0.5 bar. Top plate 32 of the stuffer box is moveable and is retained in place by applying external pressure, preferably about 0.0 to 0.2 bar. Higher pressures applied to the plate 32 result in higher levels of crimp.

As illustrated in Figure 3, steam from a source indicated generally as 37A at, for example, around 5.7 bar (85 p.s.i) is injected into and passed through a drier 37 comprising a series of baffle plates 38. The dry steam is then passed through a pressure reduction valve 39, e.g. a simple diaphragm reducing valve, where its pressure is reduced to, for example, 4.3 bar or 3 bar (65 p.s.i. or 45 p.s.i., respectively). The reduced pressure steam is then fed into the stuffer box 23 where it passes through the holes in the plates 32 and 33 to contact the crimped tow in the passageway 31 as described above. Additional soft finish, preferably a synthetic finish rather than a soap finish, may be injected through the holes in the plates 32 and 33 to lubricate the fibre.

Referring to Figure 4, a washed and lubricated tow of lyocell fibres 40 is fed through a primary input nip 41 between a main roll 42 and a first roll 43. After passage around the main roll 42, for somewhat under one quarter of a revolution, the tow 40 travels through a control nip 44 between the main roll 42 and a control roll 45. Shortly thereafter, the tow 40 is stripped from the main roll 42 by a scraper 46. The scraper 46 is preferably in grazing contact with the surface of the main roll 42 and is arranged at an acute angle of 70° or thereabouts to the surface thereof to provide a nip. Tow 40 then passes through a box 50 formed by a box top 48 and a box bottom 47, the latter being an extension of the scraper 46. The combination of main roll 42, scraper 46 and the box 50 acts as a compaction device. The width of the box 50 may be 80 cm, the depth and length of this box being chosen to provide the desired degree of crimp in the fibre. On emerging from between box top 48 and box bottom 47, the tow 40 is deposited upon a conveyor belt 49 from which it is delivered to a dryer (not shown) .

The number of primary crimps in lyocell fibre can be measured as follows. Six strands of fibre 12-15 cm in length are cut from six random positions within a i m length of dry tow. The strands should not be taken from the very edge of the tow. If filament titre is about 1 to 3 decitex, a strand should contain about 1000 to 1500 filaments (around 200 tex) , although it will be appreciated that the preferred size of strand depends to some degree on filament titre. Each strand is then clipped by one end to a board and gently extended to pull out all the crimp. If all the crimp cannot be pulled out, the sample is probably too large and one containing fewer filaments should be chosen. The point on the strand 10 cm from the clip in this extended configuration is then marked. The strand is allowed to relax and the number of crimps between the clip and the mark is counted. Division of this number by the extended length of the sample (10 cm) gives the number of primary crimps per centimetre. The results on the six strands are averaged.

Claims

1. A process for the manufacture of lyocell fibre, characterised in that it includes the step of passing the lyocell fibre in the form of a wet tow through a compaction device, thereby creating permanent crimps in the fibre.

2. A process according to claim l, characterised in that the compaction device is a stuffer box.

3. A process according to claim 1, characterised in that the wet tow is washed never-dried tow.

4. A process according to claim 2, characterised in that the wet tow is washed never-dried tow.

5. A process for the manufacture of lyocell fibre including the steps of:

(a) dissolving cellulose in a solvent to form a solution;

(b) extruding the solution through a spinnerette into a coagulating bath to form continuous cellulose filaments;

(c) washing the continuous cellulose filaments with water to remove the solvent, thereby forming a wet tow of never-dried lyocell fibre;

(d) as characterising step, passing the wet tow of never-dried lyocell fibre through a compaction device, thereby producing never-dried crimped lyocell fibre,- and

6. A process according to claim 5, characterised in that the drying step is performed on a belt dryer.

7. A process according to claim 5, characterised in that it additionally includes the step of converting the crimped lyocell fibre into staple fibre, alternatively before or after the drying step (e) .

8. A process according to claim 6, characterised in that it additionally includes the step of converting the crimped lyocell fibre into staple fibre, alternatively before or after the drying step (e) .

9. Lyocell fibre, characterised in that it possesses at least 1.0 permanent primary crimp per centimetre.

10. Lyocell fibre, characterised in that it possesses at least 3.0 primary crimps per centimetre.

11. A cigarette filter, characterised in that the body of the filter contains or consists of crimped lyocell fibre in tow form according to either of claims 9 or 10 or prepared by the process of any one of claims 1 to 6.

12. A cigarette, characterised in that it contains a cigarette filter according to claim 11.