EP1404917B1

EP1404917B1 - Dyeing and finishing of lyocell fabrics

Info

Publication number: EP1404917B1
Application number: EP02727785A
Authority: EP
Inventors: James Martin Taylor; Geoffrey William Collins
Original assignee: Lenzing AG; Chemiefaser Lenzing AG
Current assignee: Lenzing AG
Priority date: 2001-06-15
Filing date: 2002-06-14
Publication date: 2007-01-10
Anticipated expiration: 2022-06-14
Also published as: ES2279866T3; WO2002103104A3; EP1404917A2; CN1516766A; AU2002257982A1; CN1272499C; KR20040011534A; WO2002103104A2; TWI237671B; BR0210382A

Description

Field of the Invention

This invention relates to a process for the dyeing and finishing of a lyocell fabric.
In this specification, the term "lyocell fabric" means a fabric formed, particularly woven or knitted, from yarns at least some of which contain or consist of lyocell fibres. Such yarns may comprise just lyocell fibres or they may comprise a blend of lyocell fibres with one or more other fibre types, which may be cellulose-based or non-cellulosic, 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 in a mixture of organic solvent and water through a spinning jet into a coagulation bath by a process known as solvent spinning. 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 distinguished from other man-made cellulose fibres which are 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 this way.

Background art

Lyocell fibres are known to have a tendency to fibrillate during vigorous dyeing and finishing processes, and there have been a number of methods of dealing with this phenomenon. Where 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 crosslinking 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 so disfigure its appearance. On the other hand, the development of the shorter fibrils which are formed in the fibrillation process (so-called "secondary fibrillation") is encouraged. These shorter fibrils create a surface finish which is 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 this approach is described in WO-A-95/30043 and involves removing the product of primary fibrillation formed during a dyeing process by a post-treatment with an aqueous solution of an acid catalyst such as a metal salt which is a Lewis acid, used in conjunction with an optional crosslinking agent such as an N-methylol resin, and heating under conditions similar to those used to cure crosslinking resins in conventional crosslinking treatments. Another example is described in WO-A-97/30204 where, before dyeing, the lyocell fabric 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-2314568 and involves use of the dyeing machine itself, for example a jet dyeing machine, to subject the lyocell fabric to an extended treatment with an aqueous 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 which has been used successfully in commercial processing involves a post-treatment of a lyocell fabric which 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 are expensive both in terms of material costs and in terms of processing time. Moreover, cellulase enzyme treatments are not usually successful in removing the visible crease marks which are produced in the most-widely used fabric dyeing process of water-driven jet dyeing. In these jet dyeing machines, the fabric is subjected to vigorous liquid and mechanical action whilst in rope form and this produces pronounced abrasion at fold lines, which manifests itself in the dyed fabric as visible crease marks, sometimes called rope-marks.
Dyed fabrics with visible crease marks are not of commercial value, and for this reason it has not been possible to dye lyocell fabrics to a consistent commercial standard using conventional water-driven jet dyeing machines. This is a substantial commercial limitation given their widespread adoption in dyeing and finishing plants.

Disclosure of the Invention

The present invention provides a process for producing a dyed and finished lyocell fabric having a clean, soft-touch finish by dyeing a lyocell fabric using vigorous action on the fabric so as to produce fibrillation on the surface of the fabric, and then washing and drying the dyed fabric characterised in that, before the dyeing step is carried out, the fabric is evenly impregnated with an aqueous solution of an acid or acid donor and is then heat treated in a gaseous atmosphere to activate the action of the acid or acid donor, and that at least one of the subsequent washing and drying steps is carried out in a rotary tumbling machine, whereby the dyed and finished fabric has a clean, soft-touch finish substantially free from visible crease marks normally produced on lyocell fabrics which have been subjected to vigorous action in dyeing.
The vigorous action on the fabric applied during dyeing is primarily required for the particular process function being undertaken, examples being the vigorous action imposed on the fabric when being dyed in a jet dyeing machine or in a drum dyeing machine for garments. Washing on drying the dryed fabric in rotary tumbling machines such as wet tumbling machines or tumble dryers also applies vigorous action to the fabric. This vigorous action also produces fibrillation in a lyocell fabric which, but for the control imposed by the process of the invention, would mar the surface finish of the fabric with a hairy effect and risk the formation of the visible crease marks referred to.
The ability of the process of the invention to deliver dyed lyocell fabrics having a clean, soft-touch finish, particularly a peach-touch finish, free from visible crease marks after a vigorous action dyeing step such as jet dyeing and rotary tumbling is of great commercial importance. It means that the fabric can be processed on existing equipment using normal process routines without the need for excessive care in handling or extended processing times. For example, a dyeing cycle of six hours or less may be used in a jet dyeing machine. Suitable conventional water-driven jet dyeing machines include the machines known as Thies Ecosoft, and Hisaka Circular CUT-SL. Air jet dyeing machines, which also impose a vigorous action on the fabric, may also be used. Suitable air jet dyeing machines include the machines known as Thies Airstream, Thies Luft Roto, Hisaka AJ-1, Krantz Aerodye, and Then AFS.
Conventional dyes and dye recipes for cellulosic fabrics may be used in the process of the invention, including those based on direct dyes, vat dyes, sulphur dyes and reactive dyes.
In addition to its application to woven and knitted lyocell fabrics in the length, the process of the invention can also be used in respect of the dyeing and finishing of piece goods or garments made from lyocell fabrics. Such garments can suffer from visible crease marks caused by localised abrasion during vigorous processing, such as drum dyeing or wet tumbling or tumble-drying, but this is largely avoided if the garments are subjected to the process of the invention.
The application of the solution of the acid or acid donor and the subsequent drying and heat treatment step may be carried out on the lyocell fabric prior to its being converted into piece goods or garments. However, it may also be applied to the piece goods or garments after conversion.
The avoidance of visible crease marks is the main benefit imparted by the process of the invention, but it has also been found that the clean, soft-touch finish on the product lyocell fabric is protected to some extent against further abrasion damage in subsequent processing. In particular, articles comprising the dyed fabric, such as garments or other piece goods, retain their clean, soft-touch finish through repeated laundering cycles.
The solution of the acid or acid donor 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, as by padding. 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 on weight of fabric.
Piece goods such as garments may be immersed in the solution in a vessel such as a drum washing machine.
The acid or acid donor is preferably a weak acid of the type used as a catalyst for resins used in textile finishing processes. Suitable acids or acid donors include organic acids, such as citric acid and tartaric acid, and Lewis acids. Magnesium chloride, ammonium chloride, zinc chloride, zinc fluoroborate and zinc nitrate are suitable weak acids or acid donors. Mixtures of two or more of these compounds may also be used, and, indeed, manufacturers of acid catalysts for resin finishing sometimes use mixtures in their catalyst formulations. An example of a commercial product which can be used in the process of the invention is Condensol FB (trade mark of BASF AG), an acid catalyst comprising a mixture of magnesium chloride and zinc fluoroborate.
Optimum solution concentrations of the acid or acid donor depend upon the particular acid or acid donor used; lower concentrations with highly active materials to avoid unwanted acid damage to the fabric and higher concentrations with less active materials. For commercial products, the manufacturers recommend concentrations for resin-finishing processes, and, in general, such concentrations may be used in the process of the invention. Thus, organic acids can be used in the concentration range of about 2 to 20 grams per litre in aqueous solution, for example about 4 to 6 grams per litre in the case of citric acid. Less active acids or acid donors may involve or require greater concentrations, for example up to about 40 grams per litre in aqueous solution. Unnecessarily excessive concentrations should be avoided in all cases as these can cause unwanted acid damage to the fabric. Avoidance of unnecessarily high concentrations also minimises chemical usage.
The impregnated fabric is heat-treated in a gaseous atmosphere, for example in a hot air oven. Air forms the preferred gaseous atmosphere. It may be dried in a separate drying step before the heat treatment, but preferably the drying is just the initial stage of the heat treatment step. The lyocell fabric preferably is treated in open width, desirably under tension, for example on a stenter passing through a heating oven or chamber. The temperature of the gaseous atmosphere used in the heat treatment is optimally chosen to be effective in initiating the action of the particular acid or acid donor used. The heat treatment serves to liberate acid in the gaseous phase to interact with the lyocell fibres. In general, a temperature in the range from 120°C to 220°C is suitable, more preferably from 140°C to 200°C. Again, optimum processing times depend upon the particular acid or acid donor used, but processing times are usually in the range from 30 seconds to 5 minutes. Any residual acid may be removed from the fabric by washing or scouring the heat-treated fabric and then re-drying it.
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 acid or acid donor. This desizing or scouring may be a conventional operation in which the fabric is passed through an aqueous scouring bath to remove the size or lubricant. If desired, the fabric may be singed prior to removal of size. If pre-bleaching of the fabric is required, then it is preferred that this is carried out prior to the impregnation with the acid or acid donor, suitably after the desizing step.
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 acid or acid donor and the subsequent heat treatment but before the 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-0749505, where it is described as mercerising. Causticising also enhances dyeability of the fabric which may be slightly depressed by the acid treatment, and it has the further effect that the fabric is modified in structure which reduces the propensity for shrinkage in later wet processing. After causticising, the fabric should be thoroughly rinsed with hot water and then cold water to remove residual sodium hydroxide.
After being washed to remove any unfixed dye, the dyed fabric may be given conventional finishing treatments, including application of a soft-finish as by a conventional padding operation. This may be carried out after the dyeing and washing processes without the need for any intermediate drying of the fabric. If a wet tumbling treatment is required to develop the soft-touch finish, this may be carried out in a rotary tumbling machine after dyeing, either together with or after any soft finish treatment. As with the dyeing step, the treated lyocell fabric remains free from visible crease mark damage which can otherwise occur during wet tumbling. The finished fabric may then be given a final drying, for example in a tumble-drying machine.
Thus, in one embodiment there is provided a process for, the dyeing and finishing of a woven lyocell fabric, in accordance with the present invention, which includes the following steps in the following order:

1) removing size from the fabric by means of an aqueous desizing bath,
2) padding onto the fabric an aqueous solution of an acid or acid donor,
3) drying and heat treating the fabric, for example between 120°C and 220°C, preferably between 140°C and 200°C,
4) dyeing the fabric, and
5) tumble-drying the fabric.

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 base weight 200 gsm (grams per square metre) constructed in a 2 by 1 twill weave from yarns of count 1/20s Ne comprising 100 per cent TENCEL 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 to desize it and then was dried on a stenter at a temperature of 100°C.
The desized fabric was then evenly impregnated with an aqueous solution of citric acid at a concentration of 4.0 g/l (grams per litre) 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 frame in air at a temperature of 160° C for 5.0 minutes. After thorough rinsing with water and drying, the treated fabric was then ready for dyeing.
Dyeing was carried out in a Thies Ecosoft jet dyeing machine. In addition to the treated fabric, an untreated but desized control fabric of the same specification was also dyed and finished as hereinafter set out.
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 4.0 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 fabrics were 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 tumble-dried in a fabric rope tumbler.
The dyed control fabric had a matted surface appearance with a frosty look caused by the many protruding long fibres characteristic of primary fibrillation, and it was disfigured by visible crease marks caused by the abrasive action of the jet dyeing machine on the rope of fabric being processed.
In contrast, the fabric treated according to the process of the invention had a uniformly clean surface appearance showing even dyeing, a soft-touch finish of sufficient development to be characterised as a peach-touch finish, and virtual absence of visible crease marks.

Example 2

The procedure of Example 1 was repeated using the same fabrics for both the treated and control samples but with the citric acid used in the impregnation step being replaced by CONDENSOL FB, a mixture of magnesium chloride and zinc fluoroborate sold as an acid catalyst for resin finishing of textiles, at a concentration of 20 g/l (CONDENSOL FB is a trademark of BASF AG).
The untreated control fabric was again of matted appearance with unsightly visible crease marks caused by the jet dyeing procedure. The treated fabric, on the contrary, had an attractive peach-touch finish with a clean surface free from visible crease marks.

Example 3

A double jersey interlock fabric knitted from 1/30s Ne lyocell yarns was slit to open width. Part of the fabric was kept as an untreated control and part was impregnated in a pad bath at 80 per cent wet pick-up with an aqueous solution containing 12 g/l magnesium chloride hexahydrate and 1.0 g/l Kieralon JET (Kieralon is a trademark of BASF AG), a wetting agent. The impregnated fabric was then placed on a stenter and dried in air at a temperature of 110°C before being heat-treated in air at a temperature of 190°C for 45 seconds.
The treated fabric and the control fabric were then individually subjected to the following scouring, dyeing, softening and drying regime. Scouring was carried out using an aqueous solution of a non-ionic detergent and sodium carbonate at a temperature of 90° C for 30 minutes. The fabrics were then rinsed in water before being dyed in a jet dyeing machine as described in Example 1 using the same dyebath recipe but with the dye at 3.0 per cent owf.
The dyed fabrics were treated with a soft finish, Sandoperm MEJ (Sandoperm is a trademark of Clariant), at 2.0 per cent owf from an aqueous bath at a temperature of 40°C and were then tumble-dried in a Biancalani Airo 1000 fabric rope tumbler.
The dyed control fabric had a matted surface appearance with many protruding long fibres characteristic of primary fibrillation. The fabric treated according to the invention had a soft peach-touch finish and a uniform, clean appearance free from a hairy effect and free from visible crease marks.

Example 4

A lyocell fabric comprised a woven fabric of base weight 150 gsm constructed in a 2 by 1 twill weave from yarns of count 1/24s Ne comprising 100 per cent lyocell fibres. The fabric was scoured in open width with an aqueous solution comprising a non-ionic detergent and sodium carbonate at a temperature of 90° C and was then dried on cylindrical heating cans at a temperature of 100°C.
The scoured and dried fabric was then padded with an aqueous solution of Condensol M of concentration 18.0 g/l at 80 per cent wet pick-up. Condensol M (which is a trademark of BASF AG) is a 32 per cent by weight solution of magnesium chloride. The impregnated fabric was dried at a temperature of 110°C and then heat treated in air at a temperature of 190°C for 45 seconds whilst on a stenter.
The treated fabric was then cut into shaped panels which were sewn together to make shirts.
The shirts were dyed in an open-pocket Tupesa Ecodye 25 garment dyeing machine using the hot exhaust migration dyeing method with the following dye recipe using a final fixation temperature of 80°C :-

Procion H-EXL at 2.0 per cent owf
Sodium sulphate at 40 g/l
Sodium carbonate at 15 g/l

The dyed shirts were rinsed with water and were then treated with a soft finish, Edunine CSA, at 2.0 per cent owf, applied from an aqueous bath at a temperature of 40°C. The shirts were then tumble-dried.
The dyed and finished shirts had an attractive peach-touch finish free from visible crease marks and long fibrils.

Example 5

A lyocell fabric comprised a woven fabric of basis weight 210 gsm constructed in a 2 by 1 twill weave from yarns of count 1/20s Ne comprising 65:35 weight blend of lyocell and cotton fibres. This fabric was scoured in open width in an aqueous solution of a non-ionic detergent and sodium hydroxide at a temperature of 90°C, bleached in a mixture of hydrogen peroxide, sodium hydroxide and peroxide stabiliser, and then steamed at a temperature of 100°C for 8.0 minutes before being washed in water and then dried on cylindrical drying cans.
The fabric was then separated into a length for a control and a length for treatment according to the invention. The fabric for treatment was padded with an aqueous solution of magnesium chloride hexahydrate in concentration 14 g/l at a wet pick-up of 80 per cent, dried at a temperature of 119°C and then heat treated at a temperature of 170° C for 3.0 minutes on a stenter.
The control and treated fabrics were dyed and finished in the same way as follows:-

Dyeing was carried out in a Thies Luft-Roto jet dyeing machine using the same dye recipe and procedure as in Example 1, followed by water rinsing, softening as in Example 1 and tumble drying in a Thies Rototumbler.
The control fabric had a matted, hairy surface and was commercially unacceptable. The treated fabric had an attractive peach-touch finish free from visible crease marks and was of a clean, uniform appearance.

Example 6

A lyocell fabric as specified in Example 4 was scoured and dried as set out in that Example. Some of the fabric was reserved as a control and the rest was impregnated with magnesium chloride hexahydrate and then dried and heat treated by the procedure described in Example 5. It was then washed in water to remove any residual magnesium chloride hexahydrate and then dried. Some of the treated fabric was reserved for processing only according to the following dyeing and finishing stages and some was first given a causticising treatment in a chainless mercerising system using an 11 per cent by weight aqueous solution of sodium hydroxide.
All three samples of fabric, the control, the acid-treated, and the acid-treated/causticised, were then dyed and finished by the same procedure. Dyeing was carried out in a Thies Ecosoft jet dyeing machine using the same dyebath recipe and procedure as in Example 1, followed by water rinsing, softening using an aqueous soft finish bath of Hansa Finish 2707 (2.0 per cent owf) and tumble drying in a Biancalani Airo 1000 fabric tumbling machine.
Examination of the three fabrics showed that the control fabric had matted primary fibrillation on its surface and had many visible crease marks showing as white lines. The treated but uncausticised fabric had no crease marks and had an attractive clean, peach-touch finish. It was of a slightly paler colour than the control fabric. The treated and causticised fabric had the same clean, peach-touch finish free from visible crease marks but had an enhanced dye yield both with respect to the uncausticised treated fabric and the control fabric.

Claims

A process for producing a dyed and finished lyocell fabric having a clean, soft-touch finish by dyeing a lyocell fabric using vigorous action on the fabric so as to produce fibrillation on the surface of the fabric, and then washing and drying the dyed fabric, characterised in that, before the dyeing step is carried out, the fabric is evenly impregnated with an aqueous solution of an acid or acid donor and is then heat treated in a gaseous atmosphere to activate the action of the acid or acid donor, and that at least one of the subsequent washing and drying steps is carried out in a rotary tumbling machine, whereby the dyed and finished fabric has a clean, soft-touch finish substantially free from visible crease marks normally produced on lyocell fabrics which have been subjected to vigorous action in dyeing.
A process according to claim 1, characterised in that the lyocell fabric is dyed in a jet dyeing machine.
A process according to claim 1, characterized in that the lyocell fabric is dyed in the form of piece goods such as garments in a drum dyeing machine.
A process according to claim 3, characterized in that the impregnation and heat-treatment steps are carried out on the lyocell fabric before it is formed into piece goods such as garments.
A process according to any preceding claim, characterized in that the acid or acid donor is used in concentration of up to 40 grams per litre in aqueous solution.
A process according to any preceding claim, characterised in that an aqueous solution of an acid donor used selected from magnesium chloride, ammonium chloride, zinc chloride, zinc, fluoroborate, zinc, nitrate and mixtures of two or more of these compounds.
A process according to any of claims 1 to 5, characterised in that an aqueous solution of an acid which is an organic acid is used.
A process according to claim 7, characterised in that the organic acid is used in a concentration of 2 to 20 grams per litre in aqueous solution.
A process according to claim 7, characterized in that the organic acid is citric acid.
A process according to claim 9, characterised in that the citric acid is used in a concentration of 4 to 6 grams per litre in aqueous solution.
A process according to any preceding claim, characterised in that the impregnated fabric is heat-treated in a gaseous atmosphere at a temperature of 120°C to 220°C.
A process according to claim 11, characterised in that the gaseous atmosphere is at a temperature in the range of 140°C to 200°C.
A process according to any preceding claim, characterised in that the gaseous atmosphere used for the heat treatment is air.
A process according to any preceding claim, characterised in that the lyocell fabric is a woven or knitted fabric.
A process according to any preceding claim, characterised in that the lyocell fabric is subjected to a scouring operation prior to the impregnation with the acid or acid donor.
A process according to any preceding claim, characterised in that the lyocell fabric is subjected to a causticising treatment after the impregnation with the acid or acid donor and the subsequent heat treatment step but before the dyeing step.
A process according to any preceding claim, characterised in that the dyed lyocell fabric is treated with a soft finish before final drying.
A process according to claim 1, which includes the following steps in the following order:
1). removing size from a woven lyocell fabric by means of an aqueous desizing bath;

2). padding onto the fabric an aqueous solution of an acid or acid donor;

3). drying and heat treating the fabric;

4). dyeing the fabric; and

5). tumble-drying the fabric,
thereby producing a dyed lyocell fabric having a peach-touch finish.
A process according to claim 18, which further includes the step of singeing the fabric prior to removing size from the fabric.
A process according to claim 18 or claim 19 which further includes the step of bleaching the fabric between the desizing step (1) and the acid-padding step (2).
A. process according to any of claims 18 to 20, which further includes the step of causticising the fabric after the acid-padding step (2) but before the dyeing step (4).
A process according to any of claims 18 to 21, which further includes the step of padding a softening agent onto the fabric after the dyeing step(4) but prior to drying.
A process according to any of claims 18 to 22, in which the dyeing step (4) is carried out in an air-jet machine.
A process according to any of claims 18 to 23, in which the drying and heat treating step (3) is carried out at a temperature of 140°C to 200°C.
A process according to claim 24, in which the drying and heat treating step (3) is carried out under tension.