GB2316690A

GB2316690A - Lyocell fabric treatment

Info

Publication number: GB2316690A
Application number: GB9618358A
Authority: GB
Inventors: Thomas Richard Burrow; Roger Norman Ibbett; James Martin Taylor
Original assignee: Courtaulds Fibres Holdings Ltd
Current assignee: Courtaulds Fibres Holdings Ltd
Priority date: 1996-09-03
Filing date: 1996-09-03
Publication date: 1998-03-04
Also published as: EP0923657A1; DE69717828T2; PT923657E; JP3527251B2; KR20000068297A; EP0923657B1; WO1998010132A1; ES2188973T3; TR199900454T2; AU4124397A; ATE229586T1; GB9618358D0; DE69717828D1; JP2000517382A

Abstract

Fabric comprising yarns comprising lyocell fibres is treated with a swelling agent, preferably aqueous sodium hydroxide, whilst in a form in which it is unrestrained and free to shrink. The swelling agent causes the lyocell fibres to swell without destroying their structure by dissolution, the amount of swelling being in excess of the amount to which the lyocell fibres will swell during exposure to all subsequent wet processing of the fabric. The effect of the swelling agent is then diluted with water so as substantially to set the yarns in the fabric in the positions relative to one another that they have in the swollen condition. The resulting fabrics are very much less affected by creasing than untreated fabrics are and do not suffer as much from the formation of lines during subsequent wet processing.

Description

FABRICS AND THEIR TREATMENT This invention relates to fabrics and the treatment of fabrics and has particular reference to the treatment of fabrics consisting of or containing lyocell fibre.

Lyocell fibre is a cellulosic fibre formed in a process in which cellulose is dissolved in an aqueous organic solvent without the formation of a derivative and is subsequently solvent-spun.

Cellulose fibres are formed into fabrics in a number of different ways. Typically, staple lyocell fibre is spun or twisted to form yarns, which are then converted into fabrics by weaving or knitting. The yarns may be formed entirely of lyocell fibres and the fabrics may be formed entirely of lyocell yarns. However, it is possible to produce blended yarns containing less than 100% lyocell - the remaining fibre may be cotton or polyester or any other suitable fibre. Similarly, the fabric can be produced from a mixture of yarns some of which may be 100% lyocell fibre yarns or blended yarns, the remaining yarns can then be either blends or completely different fibres such as polyester or cotton.

Furthermore, the yarns may be formed of continuous filament lyocell.

Typically, lyocell fabrics, by which is meant herein fabrics at least partly formed of yarns containing or consisting exclusively of lyocell fibres, are known for their flexible drape or handle. It is also known, however, that lyocell fabrics do have a tendency to crease when being wet processed (for example in dyeing or in the enzymatic treatments used to produce the so-called "peach-skin" effect on lyocell fabrics).

A process has now been discovered which permits the treatment of lyocell fabrics so as to reduce the creasing effect during wet processing and which is capable of forming fabrics having a softer feel.

By the present invention there is provided a method of treating fabric formed of yarns wherein some at least of the fibres in some at least of said yarns are lyocell fibres, which method comprises the steps of: (i) maintaining the fabric in a form in which it is free to shrink; (ii) contacting said fabric whilst in said form with a swelling agent to cause said lyocell fibres in said fabric to swell without being dissolved to a substantial extent and said fabric to shrink, the amount of swelling of said lyocell fibres being in excess of the amount to which said lyocell fibres will swell during exposure to all subsequent wet processing of the fabric; and (iii) diluting the effect of said swelling agent with water so as substantially to set the yarns in said fabric in the positions relative, one to the other, that said yarns have in the swollen condition of step (ii) whilst shrinking the diameter of said yarns.

Any suitable swelling agent may be used. The preferred swelling agent comprises an alkali such as sodium hydroxide or lithium hydroxide or potassium hydroxide or ammonia.

Alternatively, the swelling agent may comprise a bicarbonate of an alkali metal such as sodium bicarbonate.

Alternatively, although this is less preferred, other swelling agents such as aqueous tertiary amine oxides, typically aqueous N-methylmorpholine N-oxide, may be used as the swelling agent.

The fabric may be maintained in a state free to shrink by being in woven piece-form. Alternatively, if the fabric is in the form of a garment, the fabric can be placed around a dummy or other cylindrical member so that the fabric does not have any sharp corners.

Once the fabric has been treated with a swelling agent such as sodium hydroxide, it is then treated with water so as to dilute the effect of the swelling agent (e.g. sodium hydroxide). Subsequently, the fabric may be dried for manufacture into garments and subsequent wet processing such as dyeing. Alternatively, the fabric may go straight from the diluting step into a dyeing or other wet processing finishing process.

Preferably the fabric is a woven fabric, but the fabric may alternatively be a knitted fabric, for example a knitted single jersey fabric, for which the setting process may reduce curl in the fabric. The fabric may alternatively be a nonwoven fabric, for example a hydroentangled or needlepunched or bonded nonwoven fabric.

Typically, the swelling agent may comprise sodium hydroxide at a concentration in water in the range 3 to 10 per cent, preferably in the range 5 to 9 per cent, further preferably in the range 7 to 8 per cent, by weight. The swelling agent may be at ambient temperature or at elevated or reduced temperature. The swelling agent should not dissolve the lyocell fibres to such an extent that their structure is destroyed. Higher concentrations of swelling agent increase the dissolution of the cellulose and it is preferred that the cellulose should not be dissolved to a substantial extent, for example 10%.

The fabric may be processed continuously in piece-form by being passed through a shrinking bath containing the swelling agent and then through an aqueous diluting bath.

It has been found that the method produces fabrics which are very much less affected by creasing and which in particular do not suffer as much on subsequent wet processing from the formation of lines during such wet processing, for example dyeing processes.

The present invention further provides a dyed fabric comprising yarn comprising lyocell fibre, said fabric having a structure produced by the steps of: (i) swelling the lyocell fibre-containing yarns of an undyed fabric in a substantially unrestrained condition by an amount in excess of that occurring when fabric is exposed to wet processing such as dyeing conditions, (ii) exposing said swollen fabric to the diluting effects of water, whereby said fabric is permitted to relax whilst in an unrestrained condition, and (iii) subsequently dyeing said fabric under wet processing conditions.

The present invention further provides a lyocell fabric wherein said fabric has a uniform separation of fibres said uniform separation having been obtained by pre-swelling the lyocell fibres in said fabric in an unrestrained condition and setting said pre-swollen fabric.

The present invention further provides a method of treating fabric comprising yarns comprising lyocell fibres, said fabric being intended for subsequent wet processing, which method comprises the steps of: (i) exposing said fabric in a substantially unrestrained condition to a swelling agent so as to cause the lyocell fibres and yarns containing them in said fabric to swell without destroying their structure; and (ii) diluting the swelling agent with water, whereby said fabric has the position of the swollen yarns within said fabric set in position, and optionally drying said fabric.

The present invention further provides a method of processing lyocell fabric containing lyocell fibres which comprises the steps of: (i) passing said fabric in a substantially unrestrained condition through a swelling agent which causes the lyocell fibres to swell in diameter by an amount greater than any swelling of said fibres during wet processing in stage (iii) below; then (ii) passing said fabric through a water-containing bath so as to dilute the effect of said swelling agent; and (iii) subsequently wet processing said fabric, there optionally being a drying stage after said diluting stage (ii) but before said wet processing stage (iii).

The fabric material subjected to the subsequent wet processing may have been made up into the form of garments prior to the subsequent wet processing if it was not in such form prior to pre-swelling.

Fabrics of lyocell fibre, whether said fabrics comprise 100 per cent lyocell fibre or a lesser amount of lyocell fibre, have proved to be extremely popular. Much effort has been invested in developing novel treatments for lyocell fabrics, in particularly treatments to produce a so-called "peach-skin" effect on the fabric. These types of treatments typically involve extensive wet processing of lyocell fabrics. It has been found that the wet processing conditions must be very carefully chosen to avoid excessive creasing of lyocell fabrics and the formation of lines on dyed lyocell fabrics resulting from an optical effect relating to dye up-take in creased regions of the fabric or garments. The present invention provides a method of producing fabrics and treating fabrics which reduces this effect.

Essentially, according to the invention the material, in fabric form, is treated with a swelling agent, typically sodium hydroxide, so as to cause swelling of the lyocell fibre in the fabric. The amount of sodium hydroxide is so chosen as to result in a swelling in excess of that which occurs during normal wet processing of the fabric but not so high as to destroy the fibres by excessive dissolution. It has been found that, if lyocell fabric is swollen with a swelling agent such as sodium hydroxide and the swelling agent is then diluted, the fabric takes on a set whereby the yarns in the fabric are fixed in a position relative to one another which reduces the creasing in subsequent wet processing.

It is a feature of the treatment with the swelling agent that the fabric as a whole shrinks during the swelling process. Because the fabric is in an unrestrained condition this shrinkage can occur. By an unrestrained or substantially unrestrained condition as used herein is meant a condition in which shrinkage of the fabric is permitted.

In other words the fabric, although under a small amount of tension in some cases, can still shrink. The shrinkage is believed to result from the lyocell fibres forming a crimp along their length. When the fabric having swollen lyocellcontaining yarn is washed with water as a diluting agent, the effect of the swelling agent is removed. This means that the fibres shrink. However, the fabric itself does not expand to its original condition so that the fibres and yarns within the fabric are in a relatively spaced state.

It has been found that when this fabric is subsequently wet processed, for example during dyeing or enzymatic treatment, the fabric does not produce creases or a creasedyed effect to anywhere near the extent found hitherto.

By way of example, embodiments of the present invention will now be described with reference to the accompanying drawing, which is a schematic view of an apparatus for carrying out the process of the present invention.

Referring to the drawing, this shows a fabric 1 passing around a roller 2 and through the nip of a pair of rollers 3 and being pushed down into a bath 4 containing 7 per cent by weight aqueous sodium hydroxide solution 5 at room temperature. The fabric runs around a pair of rollers 6,7 and up through the nip of a pair of rollers 8. The fabric then passes around guide rollers 9 and 10 before being passed through a bath 11 containing water 12. The fabric then passes through the nip of a pair of rollers 13 before passing around a guide 14 for subsequent processing. In practice, the rollers 3 run more quickly than the rollers 8.

This means that the fabric passing through the bath 5 is in a substantially unrestrained condition, so that it can shrink as it passes through the sodium hydroxide solution.

This causes the lyocell fibres in the fabric to swell. The fabric is then passed through bath 11, being pulled by the rollers 13 in a substantially unrestrained condition, so as to dilute the sodium hydroxide by passing the swollen fabric through the water bath 11. This causes the lyocell fibres to shrink in diameter but the fabric itself does not increase in length, thus forming a slightly looser fabric which can be subsequently processed in wet processing, so as to give less creasing and less creased dyeing.

Optionally, the fabric at point 15 can go direct to wet processing such as piece dyeing and/or enzyme treatment.

Alternatively, the fabric at point 15 can be dried and made into garments prior to wet processing.

Rather than treating the fabric as fabric in continuous lengths, it is possible to take lyocell fabric straight from the woven condition, to form it into garments such as skirts and then to put the skirts through the process of swelling and diluting whilst the skirts are on mannequins or dummies or other cylinders. The fabric need not be kept completely flat as such but it does have to be in an unrestrained condition, sufficiently unrestrained to permit the fibres to swell and the fabric to shrink during the swelling process.

By way of a comparative test, two samples of fabric formed of 100 per cent woven lyocell were prepared. Each sample was a 3X1 twill at a weight of 170 g/m2. Each sample was constructed from 45ns/cm of 50 number metric yarn and 23 pic of 34 number metric yarn.

The first sample of fabric was immersed in a solution of 80 g/litre sodium hydroxide in water (8% by weight sodium hydroxide). The sample was then washed and dyed with a reactive dye in an alkaline solution containing 3g/litre sodium hydroxide (0g3% by weight sodium hydroxide). The treatment in 8% sodium hydroxide was at room temperature.

The fabric was then subjected to a wet softness test.

The actual test comprised a bending length test. In this test a wet strip of fabric is pushed up an incline of 450 until the end of the strip of fabric passes the upper end of the incline. Initially, the fabric maintains the 450 angle until eventually the weight of the fabric over the end of the incline causes the fabric to bend. A measure of the length of the fabric which has passed the end of the incline gives an indication of the softness of the fabric. Clearly the softer the fabric, the smaller the amount which can support itself beyond the end of the incline before bending.

In the case of wet dyed fabric which has been treated with 8% sodium hydroxide as set out above, 2cm of fabric passed the end before the fabric bent. In a comparative example which was treated identically in dyeing, but which had not been given the 8% sodium hydroxide treatment, 5.5 cm of the wet fabric had to pass the end of the incline before the fabric bent under its own weight.

It is clear from the above results that the invention is capable of producing a fabric having a much softer handle when wet. This is in comparison to the techniques described in published PCT Application W095/24524, which requires fabric to be treated with a cellulase to produce a softening of the mercerised lyocell.

The preferred range for the sodium hydroxide concentration in the pre-treatment and swelling step is 3% to 10% by weight. As the sodium hydroxide concentration increases, the amount of cellulose dissolved increases and at 10% by weight sodium hydroxide 8% of the cellulose is dissolved. This is about the maximum acceptable for this technique, and the preferred range of 7 to 8% sodium hydroxide gives the optimum combination of improved softness together with a reduction in crease marking and damage marking, whilst having acceptably low levels of cellulose dissolved.

The invention is of further particular interest in the treatment of non-woven fabrics. Firstly, the swelling treatment and subsequent dilution treatment 'sets' the structure of the nonwoven fabric so that the fabric is stronger in subsequent wet processing such as dyeing and the fibres in the fabric are less likely to fall out. Although the nonwoven fabric is stronger, it is still softer compared to nonwoven fabrics which have not been given the swelling technique of the present invention. The nonwoven fabric is preferably a hydroentangled fabric.

Claims

1. A method of treating fabric formed of yarns wherein some at least of the fibres in some at least of said yarns are lyocell fibres, which method comprises the steps of: (i) maintaining the fabric in a form in which it is free to shrink; (ii) contacting said fabric whilst in said form with an aqueous solution containing from 3 to 10 per cent sodium hydroxide by weight as swelling agent to cause said lyocell fibres in said fabric to swell and said fabric to shrink, the amount of swelling of said lyocell fibres being in excess of the amount to which said lyocell fibres will swell during exposure to all subsequent wet processing of the fabric; and (iii) diluting the effect of the sodium hydroxide swelling agent with water so as substantially to set the yarns in said fabric in the positions relative, one to the other, that said yarns have in the swollen condition of step (ii) whilst shrinking the diameter of said yarns.

2. A method of treating fabric comprising yarns comprising lyocell fibres, said fabric being intended for subsequent wet processing, which method comprises the steps of: (i) exposing said fabric in a substantially unrestrained condition to a swelling agent so as to cause the lyocell fibres and yarns containing them in said fabric to swell without destroying their structure by dissolution; and (ii) diluting the swelling agent with water, whereby said fabric has the position of the swollen yarns within said fabric set in position, and optionally drying said fabric.

3. A method of processing fabric containing lyocell fibre, which comprises the steps of: (i) passing said fabric in a substantially unrestrained condition through a swelling agent which causes the lyocell fibres to swell in diameter by an amount greater than any swelling of said fibres during wet processing in stage (iii) below, then (ii) passing said fabric through a water-containing bath so as to dilute the effect of said swelling agent; and (iii) subsequently wet processing said fabric, there optionally being a drying stage after said diluting stage (ii) but before said wet processing stage (iii).

4. A method of treating fabric formed of yarns wherein some at least of the fibres in some at least of said yarns are lyocell fibres, which method comprises the steps of: (i) maintaining the fabric in a form in which it is free to shrink; (ii) contacting said fabric whilst in said form with a swelling agent to cause said lyocell fibres in said fabric to swell without being dissolved to a substantial extent and said fabric to shrink, the amount of swelling of said lyocell fibres being in excess of the amount to which said lyocell fibres will swell during exposure to all subsequent wet processing of the fabric; and (iii) diluting the effect of said swelling agent with water so as substantially to set the yarns in said fabric in the positions relative, one to the other, that said yarns have in the swollen condition of step (ii) whilst shrinking the diameter of said yarns.

5. A method as claimed in any one of claims 1 to 4 in which the swelling agent is an aqueous alkali.

6. A method as claimed in claim 5 in which the alkali is sodium hydroxide.

7. A method as claimed in claim 6 wherein the sodium hydroxide is present in an amount of 3 to 10 per cent, preferably 5 to 9 per cent, further preferably 7 to 8 per cent, by weight.

8. A method as claimed in any one of claims 1 to 7 in which the fabric is either (a) in woven piece form or (b) in garment form so held as not to have sharp corners.

9. A method as claimed in any one of claims 1 to 7 in which the fabric is in continuous piece form.

10. A dyed fabric comprising yarn comprising lyocell fibre, said fabric having a structure produced by the steps of: (i) swelling lyocell fibre-containing yarns of an undyed fabric in a substantially unrestrained condition by an amount in excess of that occurring when the fabric is exposed to wet processing dyeing conditions, (ii) exposing the swollen fabric to the diluting effects of water, whereby said fabric is permitted to relax whilst in an unrestrained condition, and (iii) subsequently dyeing said fabric under wet proces sing dyeing conditions.

11. A lyocell fabric, wherein said fabric has a uniform separation of fibres, said uniform separation having been obtained by pre-swelling the lyocell in said fabric in an unrestrained condition and setting said pre-swollen fabric.