In this specification, the term"lyocell fabric"means a fabric formed, particularly woven or knitted, from yarns, particularly staple fibre yarns, comprising lyocell fibres.

Such yarns may comprise lyocell fibres alone or a blend of lyocell fibres with fibres of one or more other cellulosic or non-cellulosic types, such as cotton, viscose, linen, polyester and nylon. Furthermore, the fabric may additionally include yarns which do not incorporate lyocell fibres, for example yarns of the other fibre types referred to and blends thereof.

Lyocell fibres are produced by extrusion of a solution of cellulose through a spinning jet into a coagulation bath by a process known as solvent spinning. They are thus alternatively known as solvent-spun cellulose fibres. Such a process is described in US-A-4,246, 221 and uses as the solvent an aqueous tertiary amine N-oxide, particularly N- methylmorpholine N-oxide. Lyocell fibres are thereby distinguished from other man-made cellulose fibres produced by forming the cellulose into a soluble chemical derivative and then extruding a solution of this derivative into a bath which regenerates the extrudate as cellulose fibres; viscose fibres, including the high-strength modal types, are produced in the latter way.

Background Art Lyocell fibres are known to have a tendency to fibrillate during vigorous dyeing and finishing processes, and various methods of dealing with this phenomenon have been proposed.

When fibrillation is desired to be avoided, then dyeing and finishing of lyocell fabrics is carried out using relatively gentle processing such as pad-dyeing of open- width fabric, and a resin-finishing treatment using a cross-linking agent is carried out on the dyed fabric to protect the fibres against fibrillation in subsequent laundering processes.

Another method of dealing with this fibrillation tendency of lyocell fibres is to treat the fibres so as to remove the relatively long protruding fibre ends which are formed in the first stage of the fibrillation process (so-called "primary fibrillation") and which otherwise produce a hairy effect, often matted, on the surface of the fabric and thereby disfigure its appearance. On the other hand, the development of the shorter fibrils formed in the fibrillation process (so-called"secondary fibrillation") is encouraged. These shorter fibrils create a surface finish characterised as being"clean", in the sense of being substantially free from a hairy effect, and as having a soft touch imparted by the shorter fibrils on the surface and referred to as a"soft-touch finish". When the shorter fibrils are sufficiently developed then the soft touch of the fabric surface is more pronounced and the soft-touch finish is referred to as a"peach-touch finish".

An example of such a procedure is described in WO-A- 95/30043 and involves removing the product of primary fibrillation, which is formed during a dyeing process, by a post-treatment with an acid catalyst used in conjunction with an optional cross-linking agent such as an N-methylol resin. Another example is described in WO-A-97/30204, where the lyocell fabric before dyeing is given a pre-treatment with an aqueous solution of an oxidising agent such as sodium hypochlorite or hydrogen peroxide at elevated temperatures. A further example is described in GB-A- 2,314, 568 and involves use of the dyeing machine itself, for example a jet dyeing machine, to subject the lyocell fabric to an extended treatment with a solution of a strong mineral acid such as sulphuric acid for 30 to 120 minutes before rinsing the fabric and then commencing the dyeing sequence in the machine. Each of these processes has been difficult to control to produce consistent results and none of them has been taken through into full-scale commercial operation.

One process used successfully in commercial processing involves a post-treatment of a lyocell fabric that has been subjected to a wet processing operation such as dyeing. The protruding fibre ends produced in primary fibrillation are removed in this post-treatment by applying to the fabric a solution of a cellulase enzyme. The desired secondary fibrillation is developed in subsequent processing, for example in the dyeing process itself (if later) or in subsequent washing and drying steps using rotary tumbling machines, to produce a soft-touch finish on the surface of the fabric.

Cellulase enzyme treatments are successful in removing the long fibre ends produced in primary fibrillation from the surface of the fabric, but they are expensive both in terms of material costs and in terms of processing time.

A process was developed by the present applicant to overcome these deficiencies and is described in WO-A- 02/103104. It comprises a pre-treatment of a lyocell fabric carried out before the normal fibrillation-inducing steps of dyeing (such as jet-dyeing), washing and drying, using vigorous action on the fabric in at least one of those steps so as to produce fibrillation on the surface of the fabric, and it involves evenly impregnating the fabric with an aqueous solution of an acid or acid donor before heat- treating it in a gaseous atmosphere to activate the action of the acid or acid donor. The temperature ranges quoted for heat treatment are 120 to 220OC, preferably 140 to 200oC. This process is able to produce dyed and finished lyocell fabrics having a clean, soft-touch finish substantially free from visible crease marks.

A further process has now been devised by the applicant which allows some or better flexibility to fabric finishers in the way fibrillation is controlled to produce the desired clean, soft-touch finish, and which is especially useful in the finishing of fabrics in the form of garments or piece goods.

Disclosure of the Invention The present invention provides a process for producing a dyed and finished lyocell fabric having a clean, soft-touch finish, comprising subjecting a lyocell fabric to a wet processing treatment using mechanical action to induce fibrillation on the surface of the fabric and subsequently giving the fabric a tumbling treatment to develop the clean, soft-touch finish, characterised in that, at a stage prior to the tumbling treatment, the fabric is subjected to impregnation with a solution comprising a mixture of a multi-functional carboxylic acid and a Lewis acid, followed by a heat-treatment in a gaseous atmosphere at a temperature of 60°C to below 120°C.

The process of the invention may be applied to woven and knitted lyocell fabrics in the length. It may also be applied to garments and piece goods made from lyocell fabrics, for which it is especially useful.

The steps of impregnation with the mixed acids solution followed by the gaseous heat treatment are preferably carried out as a pre-treatment before the fibrillation- inducing step. Surprisingly, it is also possible to achieve effective results by carrying out these steps after the fibrillation-inducing step. The fibrillation- inducing step may be a dyeing process involving mechanical action on the fabric, one example being a jet-dyeing process in which the fabric, in fabric rope form, is propelled through a dyeing machine under the influence of jets of air or liquor and is thereby subjected to vigorous action, as by bending and twisting forces and impact and abrasive forces against machine surfaces.

The effect of the steps of mixed acid impregnation and gaseous heat-treatment seems to be one of weakening the surface fibres of the lyocell fabric so that the relatively long fibres formed during the primary fibrillation phase are more easily removed in the normal course of the subsequent process steps. The use of the mixed acids is surprisingly more effective at giving rise to a clean, soft-touch finish after a tumbling treatment than using the individual carboxylic or Lewis acids on their own or a mixture of Lewis acids in corresponding amounts, and it allows the use of temperatures in the gaseous heat-treatment step of below 120°C, in particular below 1150C, more particularly below 110°C and preferably no greater than 100°C.

This is of importance in commercial finishing operations because many finishing plants do not have the hot-air stenters or curing ovens normally required to heat-treat fabrics at temperatures above 120°C. This is particularly the case for garment or piece goods finishers, where the available heat-treatment equipment such as a tumble-drier tends to operate at temperatures below 100°C.

The solution of the mixed acids is preferably an aqueous solution. The fabric may be evenly impregnated with this solution using any of the conventional techniques for applying liquids to fabric. Fabric in the length may be passed in open width through a pad bath of the aqueous solution, usually with a wet pick-up of the solution in the range 65 to 80 per cent by weight owf (on weight of fabric). Piece goods or garments may be immersed in the solution in a vessel such as a drum washing machine. In this case, the wet pick-up is lower than with the padding method, usually about 45 to 55 per cent by weight owf.

Immersion methods also require a hydro-extraction step such as spin-drying before the following heat treatment.

The multi-functional carboxylic acid preferably is citric acid or tartaric acid. The Lewis acid preferably is magnesium chloride, zinc chloride, zinc fluoroborate or zinc nitrate.

The concentration in the treatment solution of the carboxylic acid is suitably in the range 2 to 30 g/l (grams per litre), preferably 3 to 12 g/l, and the concentration of the Lewis acid is suitably in the range 2 to 45 g/l, preferably 8 to 30 g/1. As mentioned, the wet pick-up of the treatment solution varies according to the method of application and this will affect the concentration of each of the acids to be used. With the padding method used on fabric in the length (wet pick-up 65 to 80 per cent owf), the respective concentration ranges are suitably 2 to 20 g/l, preferably 3 to 8 g/l, for the carboxylic acid and 2 to 30 g/1, preferably 8 to 20 g/1, for the Lewis acid. With the immersion method used on garments and piece goods (wet pick-up 45 to 55 per cent owf), the respective concentration ranges are suitably 3 to 30 g/l, preferably 4 to 12 g/1, for the carboxylic acid and 3 to 45 g/l, preferably 11 to 30 g/l, for the Lewis acid.

The heat-treatment of the impregnated fabric preferably is carried out in hot air as the gaseous atmosphere. At the specified temperatures of 60°C to below 120°C, in particular below 115°C, more particularly below 110oC, treatment times depend upon the temperatures or acid concentrations being used, longer times being needed at the lower ends of the specified ranges. For example, at a temperature of 100°C, the treatment time may be in the range 2 to 15 minutes.

At a temperature of 80°C, which is typical for garment or piece goods treatment in a tumble-drier, the treatment time required may extend to 45 to 60 minutes. At lower temperatures still, several hours of heat-treatment may be required.

If the yarns of the lyocell fabric have been sized or lubricated to facilitate weaving or knitting, then the fabric preferably is subjected to a desizing or scouring operation, usually carried out prior to the impregnation with the mixed acids solution. This desizing or scouring may be. a conventional operation in which the fabric is passed through a scouring bath to remove the size or lubricant. If pre-bleaching of the fabric is required, then it is also preferred that this is carried out prior to the impregnation with the mixed acids solution.

Another possible treatment of the fabric is a so-called causticising treatment with an aqueous sodium hydroxide solution. If this is carried out, it is preferably carried out after the impregnation with the mixed acids solution and the subsequent heat treatment but before a dyeing step.

Causticising is often carried out to improve the flexibility of the fabric in the wet state. A process for its application to lyocell fabrics is described in EP-A- 0,749, 505, where it is described as mercerising.

Causticising also enhances dyeability of the fabric which may be depressed by the acid treatment and it has the further effect that the fabric is tightened (modified) in structure, which helps to reduce any propensity for shrinkage in later wet processing. After causticising, the fabric should be thoroughly rinsed with hot and then cold water to remove residual caustic soda.

As mentioned, the acid impregnation and gaseous heat- treatment steps are preferably (but not necessarily) carried out as a pre-treatment before the fibrillation- inducing step. The latter step is a wet-processing step in which mechanical action exerted on the wet fabric causes fibrillation to occur on the fabric surface. This fibrillation is of the so-called primary fibrillation type in which relatively long protruding fibre ends are produced to give what would be an unsightly, hairy effect on the fabric surface if these long fibre ends were to remain in place. The effect of the gaseous heat treatment of the fabric impregnated with the solution of mixed acids is to substantially weaken the strength of these fibre ends, so that they are removed from the fabric surface during processing.

It appears that many of the fibre ends are removed during the fibrillation-inducing step itself, with any remaining ones being rubbed off the fabric surface during the subsequent processing steps.

The fibrillation-inducing step preferably is also the dyeing step, using a dyeing process which exerts the requisite mechanical action on the fabric. In the case of the treatment of fabric in the length, this may be a jet- dyeing operation using commercial water-driven jet-dyeing machines or air jet-dyeing machines. In the case of treatment of garments or piece goods, this may be a rotary drum dyeing operation.

Conventional dyes and dye recipes for cellulose fabrics may be used in the process of the invention, including those based on direct dyes, vat dyes, sulphur dyes and reactive dyes.

After being washed to remove any unfixed dye, the dyed fabric may be given one or more conventional finishing treatments including application of a soft-finish. These may be carried out after the dyeing and washing processes without the need for any intermediate drying of the fabric.

At this stage, the surface of the dyed fabric does not yet have the desired clean, soft-touch finish. It remains flat and uneven in appearance, possibly with some adherent lint.

In order to develop the desired finish, the fabric is given a tumbling treatment, sometimes referred to as a beating treatment, which raises the relatively short fibres of so- called secondary fibrillation in an even nap over the fabric surface. This treatment preferably is carried out as a dry treatment, i. e. without added liquid, and may be the step in which the fabric is given its final drying.

Garments and piece goods may be given a tumbling treatment in a rotary tumbling machine such as a tumble-drier.

Tumble-driers usually operate at temperatures of less than 100°C, often no greater than 85°C. Fabric in fabric rope form may be given a tumbling treatment in a fabric rope tumbler such as a Biancalani Airo tumbling machine.

Tumbling treatment, times for a fabric rope are usually no longer than 30 to 50 minutes using air temperatures of 50°C to 150°C.

The dry lyocell fabric produced by the process of the invention is a dyed fabric having a clean, soft-touch finish. The uniform, soft nap on the fabric surface imparts an attractive handle to the fabric and a subtle appearance, sometimes referred to as a frosted effect.

When the nap is developed to a higher level, then the fabric surface may be characterized as having a peach- touch finish.

The invention is illustrated by the following Examples. In all of the Examples the lyocell fibres comprising the lyocell fabric processed were produced by Tencel Limited under the Trade Mark TENCEL.

Example 1 A lyocell fabric comprised a woven fabric of basis weight 180 gsm (grams per square metre) constructed in a 3 by 1 twill weave from yarns of count 1/30s Ne comprising 100 per cent lyocell fibres. The fabric was scoured in open width using an aqueous scour bath containing a non-ionic detergent and sodium carbonate and at a temperature of 90°C and then was dried at a temperature of 100°C.

The desized fabric was then evenly impregnated with an aqueous solution comprising magnesium chloride hexahydrate at a concentration of 10.0 g/1 (grams per litre) and tartaric acid at a concentration of 4.0 g/l by uniformly squeezing the solution into the fabric using a pad mangle at an expression of 80 per cent wet pick-up. The impregnated fabric was dried and heat-treated on a stenter at an air temperature of 100°C for 15 minutes. After thorough rinsing with water, the treated fabric was then ready for dyeing.

Dyeing was carried out in an Alliance Colora jet dyeing machine. The dyeing method used was a hot exhaust migration dyeing method carried out over a period of six hours using an aqueous dyebath containing:- Procion Navy H-EXL dye at 3.0 per cent owf (on weight of fabric) (Procion is a trademark of Dystar AG) Sodium sulphate at 60 g/l Soda ash at 20 g/l The dyed fabric was then treated with a soft finish, Edunine CSA (Edunine is a trade mark of Uniqema), at 2.0 per cent owf applied from an aqueous bath at a temperature of 40°C before being tumbled in a Thies Rototumbler HT fabric rope tumbling machine. The rope speed used was 300 metres per minute and the tumbling treatment was carried out in air at a temperature of 80°C for 45 minutes.

The treated fabric had a uniformly clean surface appearance showing even dyeing and a soft-touch finish of sufficient development to be characterised as a peach-touch finish.

Example 2 A lyocell. fabric comprised a fabric knitted in an interlock construction from yarn of count 1/40s Ne comprising 100 per cent lyocell fibres.

The fabric was evenly impregnated with an aqueous solution comprising 10 g/1 magnesium chloride hexahydrate and 4 g/l citric acid. Impregnation was achieved by soaking the fabric for 10 seconds in a bath of the solution and then removing excess solution by hydro-extraction in a spin- drier for 1 minute. The solution pick-up on the fabric was 60 per cent owf.

The damp fabric was dried and heat-treated in a tumble- drier at a temperature of 80°C for 1 hour.

Pieces of the treated fabric were then dyed in an open- pocket Tupesa Ecodye 25 garment dyeing machine using the hot exhaust migration dyeing method with the following dye recipe at a final fixation temperature of 80°C : Procion H-EXL at 2 per cent owf Sodium sulphate at 40 g/l Soda ash at 15 g/l The dyed fabric pieces were given a soft-finish treatment as described in Example 1 and were then tumbled for 1 hour in a tumble-drier at a temperature of 80°C. At the end of this tumbling and drying treatment, they were seen to have a uniformly clean surface appearance, were evenly dyed and had a peach-touch finish.

Example 3 The same construction of knitted fabric as was used in Example 2 was impregnated with an aqueous solution comprising 10 g/1 magnesium chloride hexahydrate and 2.6 g/1 citric acid. Impregnation was carried out by padding at 100 per cent pick-up owf. The wet fabric was dried and heat-treated on a pin stenter at an air temperature of 100°C for 5 minutes.

Pieces of the treated fabric were dyed and then tumbled according to the procedures described in Example 2.

The resulting fabric pieces had an attractive, clean surface. with a peach-touch finish.

Example 4 The same construction of woven lyocell fabric as was used in Example 1 was scoured and dyed as described in that Example but without the intermediate steps of impregnation and heat-treatment.

The dyed fabric was seen to have an unsightly hairy and uneven surface caused by primary fibrillation induced during the jet dyeing process.

This fabric was cut into pieces which were impregnated with an aqueous solution comprising 20 g/1 magnesium chloride hexahydrate and 8 g/1 tartaric acid. Impregnation was effected by soaking the fabric pieces for 10 seconds in a bath of the solution and then removing excess solution by a 1 minute hydro-extraction in a spin-drier to give 60 per cent pick-up owf.

The damp fabric pieces were tumbled in a tumble-drier at a temperature of 80°C for 1 hour. The hairy, uneven surface appearance of the fabric pieces had been replaced by a uniform, clean surface appearance with a peach-touch finish.

Examples 5&6 The procedure of Example 3 was repeated using the same fabric construction but using an aqueous impregnating bath comprising: in the case of Example 5,4 g/1 zinc chloride and 2. 5 g/l tartaric acid, and in the case of Example 6,17 g/1 zinc fluoroborate and 4 g/l tartaric acid.

The final fabric pieces produced, both for Example 5 and for Example 6, had a clean, peach-touch finish.

Examples 7&8, and Comparative Examples 9,10, 11 and 12 These Examples were carried out to demonstrate the effectiveness of the process of the invention using a mixture of carboxylic and Lewis acid (Examples 7&8) as compared with using an individual carboxylic acid or Lewis acid, at an appropriately higher concentration (Comparative Examples 9,10, 11 and 12).

A lyocell fabric knitted in a 22 gauge interlock construction from yarn of count 1/50s Ne comprising 100 per cent lyocell fibres was used in each case. Individual samples of the fabric were evenly impregnated (to a pick- up of 95 to 99 per cent) with the respective acid or acid mixture solution by padding the fabric sample in a bath of the solution at the specified concentration for 10 seconds. Excess solution was then removed by hydro- extraction in a spin-drier for 1 minute.

The respective acid and acid mixtures used in the various impregnating solutions were as follows: Examples 7&8 (according to the invention) 10 g/l magnesium chloride hexahydrate and 4 g/l tartaric acid . Comparative Examples 9&10 15 g/1 magnesium chloride hexahydrate 'Comparative Examples 11&12 10 g/l tartaric acid The impregnated samples were individually heat treated according to one of two methods as indicated in the Table below: Method A Drying and heat treatment of the impregnated fabric sample on a stenter in a heated stenter oven at an air temperature of 100°C for 11 minutes.

Method B Tumble drying in a tumble-drier at an air temperature of 80°C for 60 minutes.

After the respective heat treatment, each of the fabric samples was dyed, treated with a soft-finish and tumbled in a tumble-drier, all as described in relation to the equivalent steps in Example 2, in order to develop or to try to develop the desired fabric surface finish. The results are shown in the Table below: Table Heat Example treatment Comments Method 7 A The fabric surface had a uniform, clean appearance and a peach-touch finish 8 B The fabric surface had a uniform, clean appearance and a soft-touch finish which was less developed than that of the fabric of Example 7 Comparative Example 9 A The fabric surface did not have a soft-touch finish but was unevenly matted and hairy because of the adherent product of primary fibrillation 10 B The fabric surface was similar to that of the fabric of Comparative Example 9 11 A The fabric surface was similar to that of the fabric of Comparative Example 9 12 B The fabric surface was similar to that of the fabric of Comparative Example 9