Field of the Invention

This invention relates to a process involving the cross-linking of man-made cellulose fibres and the subsequent dyeing of fabrics made from such fibres.

5 Man-made cellulose fibres are produced by taking a naturally occurring source of cellulose, such as wood pulp, converting it into a spinnable solution and spinning fibres from that solution. Lyocell fibres are man-made cellulose fibres produced by extrusion of a solution of cellulose 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 o 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 to regenerate the extrudate as cellulose fibres. Viscose fibres, including the high strength modal types, are made by forming a 5 soluble derivative in this way.

Background Art

Cross-linking treatments of cellulose fabrics are well known for imparting crease- resistance, durable-press or wash- wear qualities to such fabrics. A general description of such treatments is found in Kirk-Othmer's Encyclopaedia of Chemical Technology, o third edition, Volume 22 (1983), Wiley- Interscience, in an article entitled "Textiles (Finishing)" at pages 769-790, and by H. Petersen in Rev. Prog. Coloration, VoI 17 (1987), pages 7-22. Crosslinking agents may sometimes be referred to by other names, for example crosslinking resins, chemical finishing agents and resin finishing agents. Crosslinking agents are small molecules containing a plurality of functional ₅ groups capable of reacting with the hydroxyl groups in cellulose to form crosslinks. One class of crosslinking agents consists of the N-methylol resins, that is to say small molecules containing two or more N-hydroxymethyl or N-alkoxymethyl, in particular N-methoxymethyl, groups. N-methylol resins are generally used in conjunction with acid catalysts chosen to improve crosslinking performance. In a typical process, a 0 solution containing about 5-9% by weight N-methylol resin crosslinking agent and 0.4-3.5% by weight acid catalyst is padded onto dry cellulosic fabric to give 60-100%

by weight wet pickup, after which the wetted fabric is dried and heated to cure and fix the crosslinking agent.

It is known that crease-resistant finishing treatments embrittle cellulose fibre and fabric with a consequent loss of abrasion resistance, tensile strength and tear strength. A balance has to be kept between improvement in crease resistance and reduction in those other mechanical properties.

It is also known that such cross-linking treatments reduce the dye affinity of cellulose fabrics. This is regarded as a serious disadvantage which can rule out the use of such treatments, especially when the dye affinity falls below about 50 to 60 % of the dye affinity of the equivalent non-cross-linked cellulose fabric. Attempts have been made to limit this reduction in dye affinity by the inclusion of a linear polymer which becomes bound to the cellulose by the cross-linking agent. These attempts have had a degree of success.

For example, US-A-4,780,102 describes a process for cross-linking cotton fabrics to impart wash-wear qualities by padding the fabrics with an N- methylol cross-linking agent, an acid catalyst for the cross-linking reaction and polyethylene glycol. The fabric is dried and heated to effect cross-linking before being dyed. The inclusion of the polyethylene glycol allows the cross-linked fabric to be post-dyed with a dye normally used to dye cellulose whilst achieving the desired wash-wear quality imparted by the cross-linking treatment. The level of dyeing achieved in this process is still reduced compared with non-cross-linked cotton fabrics. This is put to advantage by applying the cross-linking chemicals as a print formulation to achieve differential dyeing between the printed and unprinted areas of the fabric when it is post-dyed.

Another process aimed at balancing the benefits of cross-linking with retention of dye affinity is described in US-A-5, 580,356. In this process, cross-linking is carried out to reduce the fibrillation tendency of lyocell fibres. A flexible linear polymer, for example polyethylene glycol, is included with the cross-linking agent in order to impart dye affinity to the cross-linked lyocell fibres in a post-dyeing operation.

Further dyeing processes on cellulose containing fabrics are described in US-A-4, 629,470 and US-A-5,298,584.

The present invention is also concerned with using a cross-linking process of the type described but for the purpose of overcoming a problem caused by the reduced dye uptake of cotton compared with man-made cellulose fibres such as lyocell and viscose fibres.

Disclosure of the Invention

The present invention provides a process for producing an evenly-dyed fabric comprising both cotton fibres and man-made cellulose fibres, in which a fabric is manufactured from both said fibres and is dyed, characterised by impregnating the man-made cellulose fibres, prior to manufacture of the fabric, with a water-soluble, flexible linear polymer and a cross-linking agent reactive with cellulose, and, at a stage of the process prior to dyeing of the fabric, effecting a cross-linking reaction between the man-made cellulose fibres and the cross-linking agent, thereby producing a reduction in the dye affinity of the man-made cellulose fibres to a level more proximate to the dye affinity of the cotton fibres.

The process enables the even dyeing of the fabric to a solid shade of colour, in contrast to the skittery, marl or speckled effects that are usually obtained when fabrics made from such fibre blends are dyed.

The dye affinity of a fibre is a relative value, which is measured by a test in which a weighed sample of fabric is dyed using a standardised dyebath and procedure. The colour strength obtained is measured by a spectrophotometer and compared with the value obtained when a control fabric of identical construction and weight is dyed and measured by the same procedure. The value given to the control is usually expressed as 100 per cent and the value given to the fabric being compared to the control is expressed as the ratio of the measured colour strengths times 100 per cent. The dye affinity test specified for use to determine the dye affinity of the fibres for compliance with the process of the present invention is described later in this specification.

The evenness of dyeing required to obtain a solid shade of colour is usually judged by eye. The standard of evenness required will vary according to the proposed end-use of the dyed fabric and according to the dye colour used. In the latter case, for example, yellow dyes disguise skittery dyeing more easily than dark blue dyes. Bearing in mind these qualifications, we have found that, in general, even-dyeing to a solid shade of

colour may be obtained using the process of the invention if the dye affinity of the man-made cellulose fibres is reduced to within about 15 per cent of (either side of but, more usually, above), the dye affinity of the cotton fibres, taking the latter at the 100 per cent value.

The dye affinity of cotton fibres varies according to type, source and treatment but, in general, taking the cotton fibres at the control value of 100 per cent, standard viscose fibres have a dye affinity of about 130 per cent and standard lyocell fibres have a dye affinity of about 140 per cent. Despite this original disparity in dye affinity between man-made cellulose fibres and cotton fibres, evenly dyed blend fabrics can be obtained using the process of the invention.

The man-made cellulose fibres preferably are lyocell fibres, but the process is also effective using other man-made cellulose fibres such as viscose fibres.

The impregnation of the man-made cellulose fibres with the cross-linking chemicals may be carried out on the fibres in fibre form or in yarn form. It is preferably carried out on the fibres before they are spun into a yarn and, more preferably, by applying the flexible linear polymer and the cross-linking agent as a solution to the man-made cellulose fibres whilst they are in the form of a never-dried tow, i.e. just after the fibres have been wet-spun but before they have been dried for the first time.

The cross-linking agent may in general be any of those known in the art for crease- resistant finishing of cellulose but is preferably an agent classed as a zero- formaldehyde or low- formaldehyde cross-linking agent, preferably used in conjunction with a cross-linking catalyst. The cross-linking agent is preferably a zero- formaldehyde cross-linking agent when the method of the invention is carried out on fabric.

One class of low- formaldehyde cross-linking agents consists of the N-methylol resins. Examples of suitable N-methylol resins are those described in the abovementioned articles in Kirk-Othmer and by Petersen. Examples of such resins include 1,3- dimethylolethyleneurea (DMeEU), 1,3-dimethylolpropyleneurea (DMePU) and 4,5- dihydroxy-l,3-dimethylolethyleneurea (DHDMeEU). Other examples include compounds based on urones, triazinones and carbamates. Another example of a suitable cross-linking agent is melamine.

Of the zero-formaldehyde cross-linking agents, a preferred class consists of compounds based on l,3-dialkyl-4,5-dihydroxy(alkoxy)ethyleneurea, for example l,3-dimethyl-4,5-dihydroxyethyleneurea (DMDHEU). Another example of a suitable zero-formaldehyde cross-linking agent is butanetetracarboxylic acid (BTCA). Cross-linking agents for crease-resistant finishing of cellulose are generally used in conjunction with a catalyst for the cross-linking reaction, commonly an acid catalyst. The method of the invention preferably utilises such a catalyst when recommended for use with the chosen cross-linking agent. For example, N-methylol resins and 1,3- dialkyl-4,5-dihydroxy(alkoxy)ethyleneureas are preferably used in conjunction with an acid catalyst, for example an organic acid such as acetic acid or a latent acid such as an ammonium salt, amine salt or metal salt, e.g. zinc nitrate or magnesium chloride. Mixed catalyst systems may be used.

The water-soluble, flexible linear polymer is preferably a wholly aliphatic polymer, preferably unbranched. It may have terminal functional groups, for example hydroxyl or amino groups, and it is possible that these take part in the reaction with the cross- linking agent. The effect of the flexible linear polymer is to moderate the depression of the dye affinity of the man-made cellulose fibres by the cross-linking resin so that the dye affinity becomes more proximate to that of cotton. It appears to achieve this effect by preventing excessive collapse of the cellulose fibre structure during the cross-linking reaction, so that the structure retains a degree of openness which allows ingress of sufficient dye to produce the desired level of dyeing. After dyeing, the flexible linear polymer has no further function and will tend to be washed out of the fibres to the extent that it is not held in place by any involvement of its functional groups in the cross-linking reaction.

Preferred types of flexible linear polymer include polymerised glycols such as polypropylene glycol (PPG) and in particular polyethylene glycol (PEG). Amine- tipped derivatives of such polymerised glycols may be used. It will be understood that such flexible linear polymers are generally mixtures of molecules having a range of chain lengths and are characterised in terms of their average molecular weight and chain length. For example, chain lengths may range from about 5 to 150 atoms. A preferred example of a flexible linear polymer is PEG having an average molecular weight in the range 200 to 2000.

The cross-linking agent, flexible linear polymer and any catalyst are preferably applied to the man-made cellulose fibres from solution, preferably an aqueous solution. Polymerised glycols such as PEG and PPG are generally soluble in water. The solution may be applied to the fibres by known methods, for example the solution may be padded onto a tow of the fibres, preferably a never-dried tow, or a tow of fibres may be passed through a treatment bath of the solution. It is also possible to treat the fibres in staple form. A tow of never-dried fibres may have a moisture content of about 45-65%, often around 50%, by weight, after application of the solution. The treatment solution may contain 0.5 to 15%, preferably 1.5 to 5%, by weight cross-linking agent (expressed on a 100% activity basis). The solution preferably contains 0.1 to 5% by weight flexible linear polymer. When a catalyst is used, the solution may contain 0.1 to 5%, preferably 0.25 to 2.5%, by weight catalyst. The solution may contain one or more additional substances, for example a soft finish for the fibre.

The treated man-made cellulose fibres preferably contain 0.2 to 5%, more preferably 1 to 4%, by weight cross-linking agent calculated owe (on weight of cellulose) and 0.1 to 3% by weight flexible linear polymer calculated owe.

The fabric made from the impregnated man-made cellulose fibres and the cotton fibres may be a knitted, woven or non- woven fabric. In the case of knitted and woven fabrics, the fibres are first spun into yarns. A preferred method is to blend the impregnated man-made cellulose fibres and the cotton fibres and to spin yarns from the blend. An alternative method is to make yarns individually from the respective fibres and to combine the yarns in making the fabric. Any blend ratio suitable for the desired end-use may be used, common blend ratios with cotton being in the range

70:30 to 30:70 by weight, with a 50:50 blend being most common.

The cross-linking, or curing, step is carried out before dyeing. If there is any other wet treatment to be carried out before dyeing, then cross-linking should be effected before that other wet treatment in order to avoid loss of the cross-linking chemicals from the treated yarns during that wet treatment step. Preferably, cross-linking is carried out on the fabric or on the yarn. When the intended fabric is a knitted fabric or a non-woven fabric, cross-linking preferably is carried out on the fabric. When the intended fabric

is a woven fabric, however, it is preferred to carry out the cross-linking on the yarn, because the warp sizing operation that is usually preliminary to weaving can lead to loss of the cross-linking chemicals from the yarns, if they are not already cross-linked. Cross-linking at the fibre stage is also a possibility but is not preferred because the cross-linked fibres can become hairy and difficult to process through the yarn- spinning and fabric-making stages.

A preferred process comprises impregnating the man-made cellulose fibres with the flexible linear polymer and the cross-linking agent, blending the impregnated fibres with the cotton fibres, making a yarn from the blended fibres, manufacturing a fabric from the yarn, and effecting the cross-linking reaction on the yarn or on the fabric, preferably on the yarn in the case of a woven fabric.

Cross-linking may be effected by a heating step at a temperature and for a time appropriate to the cross-linking agent and any catalyst employed. When cross-linking is carried out on the fabric, the fabric may be passed through a hot air oven on a stenter. Suitable cross-linking conditions comprise a temperature in the range 14O ⁰ C to 200 ⁰ C for a period in the range 30 seconds to 5 minutes, with higher temperatures as a rule involving appropriately shorter times than lower temperatures.

Earlier drying treatments, for example the drying of the tow after the application of the cross-linking chemicals, should be carried out under conditions, mainly lower temperatures, at which premature cross-linking does not occur. For example, the temperature of the fibres themselves (as distinct from the air temperature) is preferably kept below HO ⁰ C during drying. Also, the fibres preferably are not dried to a moisture level below about 7 per cent by weight on weight of fibres.

Dyeing of the fabric may be carried out using dyes and methods conventionally used for dyeing cellulose fabrics. Suitable dyes include direct dyes, vat dyes, sulphur dyes and reactive dyes. Commercial dyeing machines may be used, including water-driven jet dyeing machines, for example the machines known as Thies Ecosoft, Gaston

County Futura and Hisaka Circular CUT-SL, and air jet dyeing machines, for example the machines known as Thies Airstream, Thies Luft Roto, Hisaka AJ-I, Krantz Aerodye and Then AFS.

The invention includes a first embodiment of undyed fibrous product, comprising both cotton fibres and man-made cellulose fibres, in which the man-made cellulose fibres are impregnated with a water-soluble, flexible linear polymer and a cross- linking agent reactive with cellulose, the impregnated man-made cellulose fibres not yet being cross-linked but having the potential of a reduced dye affinity more proximate to the dye affinity of the cotton fibres upon the effecting of a cross- linking reaction between the man-made cellulose fibres and the cross-linking agent prior to dyeing of the fibrous product.

This first embodiment of undyed fibrous product may be an undyed fibre blend of the cotton fibres and the impregnated man-made cellulose fibres, yarn made from such a fibre blend, or a fabric made from such yarn. It may also be a fabric made from cotton yarn and yarn comprising the impregnated but non-cross-linked man-made cellulose fibres.

The invention further includes a second embodiment of undyed fibrous product, comprising both cotton fibres and man-made cellulose fibres, in which the man-made cellulose fibres are impregnated with a water-soluble, flexible linear polymer and with a cross-linking agent which has reacted with the cellulose of the man-made cellulose fibres in a cross-linking reaction which has reduced the dye affinity of the man-made cellulose fibres to a level more proximate to the dye affinity of the cotton fibres.

This second embodiment of undyed fibrous product preferably is a yarn or a fabric. It may also be a fabric made from cotton yarn and yarn of the cross-linked man-made cellulose fibres.

Preferably, the dye affinity of the cross-linked cellulose fibres is at a level within about 15 per cent of the dye affinity of the cotton fibres as measured by the test specified herein, using cotton as the 100 per cent control value.

The invention further includes the cross-linked fibrous product of the second embodiment in the form of a fabric which is evenly dyed to a solid shade of colour.

The dye affinity test specified for employment to determine compliance with the process of this invention uses an aqueous dyebath incorporating 0.05 per cent Solophenyl Green 27 (a direct dye) and 10 g/1 (grams per litre) sodium chloride. Test

fabric samples of 5 g (grams) weight are dyed at a liquor-to-fabric weight ratio of 20:1, with dyeing being carried out at a dyebath temperature of 95 ⁰ C for 45 minutes, before the dyebath is cooled and the fabric sample is rinsed and dried.

The colour strength of each dyed sample is measured using a Minolta CM-3300D spectrophotometer, and all readings are made relative to a sample of dyed control fabric, which is rated at the 100 per cent figure. In the case of this invention, the control fabric is taken as a cotton fabric because it is the dye affinity of cotton that is being matched in order to obtain even dyeing of blend fabrics.

Parity between the tested fabrics is maintained by making them all from the same count of yarn (20 Tex), the same fabric (plain knit) and the same sample weight (5g). Before dyeing, all fabric samples are scoured in an aqueous scour bath containing 2 g/1 soda ash and 2 g/1 Zetex HPLFN for 30 minutes at a temperature of 70 ⁰ C.

The invention is illustrated by the following Examples:

Examples 1 and 2

Lyocell fibres were spun by a process based on a commercial process for spinning lyocell fibres of 1.4 dtex by spinning a solution of cellulose in an aqueous solvent of N-methylmorpholine N-oxide through a spinning jet into an aqueous coagulating bath to form fibres in the form of a tow of filaments, followed by washing of the tow.

The washed, freshly spun tow of filaments, usually referred to as being never-dried, was passed through an aqueous pad bath at a temperature of 55°C and containing 30 g/1 DMDHEU zero-formaldehyde resin, a concentration of PEG (average molecular weight 400) specified below and 4 g/1 magnesium chloride as cross-linking catalyst.

The impregnated tow was then dried to a moisture level of 7 per cent by weight owf

(on weight of fibre) by being passed through an oven at a temperature of 100 ⁰ C for 2 minutes.

For Example 1 the specified concentration of PEG in the pad bath was 5 g/1, and for Example 2 it was 10 g/1. The levels of PEG on the fibres were 0.5 per cent by weight owf for the fibres of Example 1 and 1.0 per cent by weight owf for the fibres of Example 2.

In order to measure the dye affinities of the fibres of Examples 1 and 2, samples were made up for testing by the specified method. For this purpose, staple fibres of 38 mm staple length were cut from each of the respective dried, impregnated tows and spun into respective yarns of count 20 Tex. Plain knit fabrics were made from these respective yarns and cross-linking of the resin and polymer on each of the fabrics was effected by heating the fabrics at a temperature of 14O ⁰ C for 10 minutes, hi fact, cross- linking would be effected in a shorter time than this but an excess time was used to be certain of full curing. Samples of these cross-linked fabrics weighing 5 g were tested for dye affinity using the specified test method, in comparison with identical fabric samples wholly of cotton and wholly of standard lyocell fibres, respectively. The results are set out in the following table:

Table

The dried, impregnated tows produced according to Examples 1 and 2 were then tested for evenness of dyeing of the fibres in blends with cotton. Again, staple fibres of 38 mm staple length were cut from the respective tows, and, in each case, these were blended with combed cotton fibres in a 50:50 weight blend. The respective blends of fibres were each spun into yarn of count 30s Ne, and these respective yarns were knitted into respective fabrics of a double jersey interlock construction of basis weight 200 gsm (grams per square metre). Cross-linking of the resin on the respective

fabrics was effected by heating the fabrics in a stenter oven at a temperature of 170 ⁰ C for 1 minute.

As a control for comparison purposes, a fabric of the same construction was made from yarn of the same count spun from a 50:50 weight blend of combed cotton fibres and lyocell staple fibres which had not been treated with the cross-linking resin and PEG.

Each of the fabrics was slit and pre-set at a temperature of 17O ⁰ C for 1 minute before being re-sewn into tubular form and loaded into a jet dyeing machine. In this machine, the fabrics were first scoured in an aqueous bath containing 2 g/1 A-lube P60 (a lubricant), 2 g/1 Sandoclean PCT (a detergent) and 2g/l soda ash for 30 minutes at a temperature of 8O ⁰ C. The scoured fabrics were then successively rinsed in hot and then cold water before being dyed brown using hot-dyeing reactive dyes by the hot migration method. The aqueous dyebath comprised (percentages by weight):

0.1 per cent Procion Dark Blue H-EXL 0.2 per cent Procion Crimson H-EXL

4.0 per cent Procion Amber H-EXL

4 g/1 A-lube P60

3 g/1 Ludigol (anti-reductant)

2 g/1 Depsodye LD-VRD (levelling agent) 60 g/1 Glauber's Salt

20 g/1 Soda Ash.

The bath was set with the dyeing auxiliaries at a temperature of 50 ⁰ C. The dyes were then added to the bath whilst raising the bath temperature to 95 ⁰ C at a rate of 2°C per minute. The dyebath was held at this temperature for 30 minutes before being cooled to a temperature of 80 ⁰ C. The soda ash was then added to the dyebath in two successive portions (one third and then two thirds) over 15 minutes, after which the machine continued to run for a further 60 minutes. The dyebath was then cooled to a temperature of 50 ⁰ C before being dropped from the machine. The dyed fabrics were then rinsed and boiled off in the machine.

Whilst still in the dyeing machine, the fabrics were given a softening treatment for 20 minutes using an aqueous soft-finish bath at a temperature of 40 ⁰ C and comprising (percentages by weight):

1 ml/1 (millilitre per litre) Acetic Acid (40 per cent) 0.5 per cent Hansa Fin 2707 (silicone micro) 1 per cent Edunine CSA (polyethylene).

On removal from the dyeing machine, the fabrics were hydro-extracted and then dried in a relaxed condition in ambient air.

Both of the fabrics made according to Examples 1 and 2 became evenly dyed to a solid shade of brown, being free from any skittery or marl appearance. In contrast, the control fabric became dyed to a shade of brown having an uneven mottled appearance arising from the differential dyeing of the lyocell and cotton components.