METHOD OF REDUCING THE TENDENCY OF A LYOCELL FABRIC TO PRIMARY FIBRILLATION

Abstract: The tendency of a lyocell fabric to primary fibrillation after dyeing and washing is reduced by contacting the fabric, before dyeing, at elevated temperature with an aqueous solution of an oxidising agent such as a hypochlorite or hydrogen peroxide. Depending on the nature and concentration of the oxidising agent, the fabric produced may have substantially no fibrillation after subsequent dyeing and washing or may develop a peach-skin hand after subsequent dyeing and washing.

Description: Method of Reducing the Tendency of a Lyocell Fabric to Primary Fibrillation Field of the Invention This invention relates to methods of reducing the tendency to fibrillation of lyocell fabrics, that is to say fabrics which comprise lyocell fibres, either alone or in blend with one or more other types of fibre, including garments made wholly or partly from lyocell fabric. Lyocell fibres are known, and their manufacture is described for example in US-A- ,416, 698, the contents of which are incorporated herein by way of reference. Cellulose is dissolved in a solvent containing a tertiary amine N- oxide (which may also be called for brevity an amine oxide) , for example N-methylmorpholine N-oxide (NMMO) . The solvent generally also contains a proportion of a non-solvent for cellulose, for example water. The resulting solution is extruded through a die to produce fibres, which are coagulated, washed in water to remove the solvent and dried. This process of extrusion and coagulation is referred to as "solvent-spinning", and the cellulose fibre produced thereby is referred to as "solvent-spun" cellulose fibre or under the generic name lyocell fibre. It is also known that cellulose fibres can be made by extrusion of a solution of a cellulose derivative into a coagulating and regenerating bath. One example of such a process is the viscose process, in which the cellulose derivative is cellulose xanthate. Solvent-spinning has a number of advantages over other known processes for the manufacture of cellulosic fibre such as the viscose process, for example reduced environmental emissions. Lyocell fibres are known to be prone to fibrillation. Fibrillation is a phenomenon which in the main occurs when lyocell fibres are subjected to mechanical forces during wet-processing, and it results in the partial detachment of fine longitudinal fibrils from the fibres. Two types of fibrillation are recognised in lyocell fabrics, under the names primary and secondary fibrillation, as described by R Breier in a paper entitled "Die Veredlung von Lyocellfasern - Ein Ehrfahrungsbericht" in Lenzinger Berichte 9/94 at pages 99-101. Primary fibrillation results in the formation of relatively long fibrils (often 1 mm or longer) and is uneven. This leads to an undesirable unsightly appearance, particularly in dyed fabric, because primary fibrils are often found to dye to a different shade from the bulk of the fibre. In contrast, secondary fibrillation results in the formation of relatively short fibrils, typically a fraction of a millimetre long, evenly distributed over the fabric. Fabrics having only secondary fibrillation dye uniformly, and the presence of the secondary fibrils confers an attractive visual appearance and hand, often called a peach- skin finish, on the fabric. Background Art Various processes have been suggested for reducing the degree of fibrillation and/or the tendency to fibrillate of a lyocell fabric. For example, WO-A-95/30043 describes contacting the fabric with an aqueous liquor containing an acid catalyst and optionally a crosslinking agent. WO-A- 94/09191 describes contacting the fabric with a chemical reagent having at least two functional groups which are reactive with cellulose, for example vinyl sulphone groups or electrophilic three-membered heterocyclic rings. WO-A- 94/09191 also describes contacting lyocell at elevated temperature with a solution of a reagent having acrylamido groups. EP-A-538977 describes contacting lyocell with a chemical reagent having 2 to 6 groups reactive with cellulose such as a colourless dye. Disclosure of the Invention A method according to the invention for reducing the tendency of a lyocell fabric to primary fibrillation after dyeing and washing is characterised in that, before dyeing, the fabric is contacted at elevated temperature with an aqueous solution of an oxidising agent. The lyocell fabric can be a woven, knitted or nonwoven fabric and can for example be in the form of continuous fabric lengths, piece goods or garments. A woven fabric can for example be a plain weave or a twill fabric. A knitted fabric can for example be of ponti di Roma or interlock constructio . The aqueous solution of an oxidising agent is preferably an aqueous hypochlorite solution or a solution of hydrogen peroxide but solutions of other peroxy compounds such as peracetic acid can alternatively be used. The hypochlorite is preferably sodium hypochlorite but other hypochlorites, particularly other alkali metal hypochlorites, can be used. The pH of the hypochlorite solution is preferably in the range 7.5 to 9.5, most preferably 8 to 8.5. The temperature at which the fabric is contacted with the solution of oxidising agent, particularly hypochlorite, is preferably at least 40 or 50°C and can be any temperature up to 100°C or above if feasible in the apparatus used but is preferably in the range 60 to 100°C, for example about 80°C. The period of treatment at this temperature is preferably in the range 50 to 150 minutes, for example 30 to 90 minutes. The fabric can be brought into contact with an oxidising agent, e.g. hypochlorite, solution which is already at elevated temperature or can be brought into contact with an oxidising agent solution at ambient temperature and heated. The latter procedure is preferred for hydrogen peroxide treatment. The apparatus used for the oxidising agent treatment can in general be any of the types of apparatus used for dyeing fabrics, for example a rotary dyeing machine (Rotadyer) or a jet dyeing apparatus or a tumbling machine capable of wet processing. After the oxidising agent treatment, the fabric is dyed. Dyeing is preferably carried out using a reactive dye, although any other dyes known for dyeing cellulose fabrics can be used, for example vat dyes or sulphur dyes. Dyeing is generally carried out in aqueous solution at elevated temperature as recommended for the particular dye used. The dyeing apparatus used can be any of those used for commercial dyeing of fabrics, for example a rotary dyeing machine (Rotadyer) or a jet dyeing apparatus. For example, the fabric can be dyed as a continuous length in rope form or can be dyed in garment form. After dyeing, the fabric is preferably treated by an aqueous process in which the fabric is subjected to abrasion to remove any creasing or stresses caused by dyeing and to cause the desired hand and appearance of the fabric to develop. For example, if the fabric is in garment form it can be washed at moderate temperatures, for example at 40°C, under the degree of mechanical agitation encountered in a domestic washing machine or a commercial machine for washing garments. Pieces of fabric can be washed similarly. Continuous lengths of fabric are generally treated in rope form and can for example be finished by tumbling in a machine such as a Thies Rototumbler or a Then AFS or Bianaloni Airo tumbling machine. In these tumbling machines a jet of air throws the fabric against a deflector plate. The fabric is continually passed through the tumbling machine so that it is repeatedly impacted against the deflector plate. The tumbling machine can be operated wet (charged with aqueous liquor) or dry, typically at fabric speeds of 100-200 metres/minute wet or 500-1500 m/min dry. It may be preferred to carry out wet tumbling followed by dry tumbling. The three steps of treatment with aqueous solution of oxidising agent, dyeing and tumbling can be carried out successively in a tumbling machine if desired. Surprisingly, we have found that when using some oxidising agents such as hypochlorite the process of the invention can be used either to suppress all tendency of a lyocell fabric to fibrillate or to control the tendency of lyocell fabric to fibrillate so that only a peach-skin effect is produced, depending on the concentration of the oxidising agent solution used to treat the fabric. In general, more concentrated solutions tend to remove all tendency both to primary and to secondary fibrillation whereas less concentrated solutions tend to suppress only primary fibrillation, producing a peach-skin effect after dyeing and washing. The concentration of oxidising agent required depends also on the form in which the fabric is treated and the dyeing and washing treatments to which it is subjected. If the fabric is to be dyed in garment form and then washed in a washing machine, the concentration of hypochlorite either to produce a peach-skin effect or to remove all fibrillation is less than the concentration of hypochlorite required to produce the same effect when the fabric is to be dyed and tumbled in rope form. For example, the concentration of hypochlorite solution used is preferably in the range 0.25 to 10 grams per litre (g/L) sodium hypochlorite or an equivalent amount of another hypochlorite. When treating garments, relatively concentrated solutions, for example those of above about 1.5 g/L sodium hypochlorite or equivalent of other hypochlorite and preferably in the range 2 to 5 g/L, generally remove all tendency both to primary and to secondary fibrillation. The fabric produced after dyeing and washing has a classic, smooth, evenly dyed appearance. Less concentrated solutions of below about 1.5 g/L and preferably in the range 0.4 to 1.2 g/L sodium hypochlorite or equivalent reduce the tendency to primary fibrillation but do not suppress secondary fibrillation, producing a peach-skin hand and appearance after dyeing and washing. In both cases the effect produced is stable and is retained throughout subsequent washes under conditions typically used in a domestic washing machine. When treating long, for example continuous, lengths of fabric in rope form, hypochlorite solutions of concentration in the range 1 to less than about 5 g/L sodium hypochlorite or equivalent generally reduce the tendency to primary fibrillation but do not suppress the secondary fibrillation which leads to a peach-skin hand and appearance after dyeing and washing or hot wet treatment in a tumbling machine. Concentrated solutions of above 5 g/L sodium hypochlorite or equivalent, for example more than 5 up to 10 g/L, generally lead to an absence of both primary and secondary fibrillation after dyeing and washing or hot wet treatment in a tumbling machine. When using hydrogen peroxide as the aqueous oxidising agent, an attractive peach-skin fabric showing secondary fibrillation with no primary fibrillation can be achieved, but it is not usually possible to produce a fabric developing neither primary nor secondary fibrillation. The concentration of hydrogen peroxide used is generally in the range 1 to 30 g H 2 0 2 per litre (3 to 90 g/L of a commercial 33% solution) . When treating garments, treatment with a solution containing 2 g H 2 0 2 up to 10 g H 2 0 2 per litre is preferred to produce a peach-skin hand and appearance after dyeing and washing. When treating long, for example continuous, lengths of fabric in rope form, treatment with a solution containing 3 g H 2 0 2 up to 20 g H 2 0 2 per litre generally produces a peach-skin hand and appearance after dyeing and washing or hot wet treatment in a tumbling machine. The invention is illustrated by the following Examples. In all cases, the lyocell fabrics used consist solely of lyocell fibres available from Courtaulds Fibres (Holdings) Limited under the Trade Mark TENCEL. Example 1 Sodium hypochlorite was dissolved in water to form a 3 g/L solution of pH 8.1-8.4. The solution was heated to 80°C in a Rotadyer. Pieces of plain-weave lyocell fabric were introduced into the Rotadyer and treated with the hypochlorite solution at 80°C for 60 minutes. After removal from contact with the hypochlorite, the fabric was rinsed. The fabric thus treated was dyed by reactive dyes at 80°C for 60 minutes in a Rotadyer. The dyed fabric was then washed in a domestic washing machine at 40°C and dried. The washed fabric had a smooth hand with no apparent fibrils and an evenly dyed appearance. The tensile strength of the fabric after hypochlorite treatment, dyeing and washing was 70% of its strength before these treatments. The degree of polymerisation of the cellulose of the fibre was unchanged by the treatment. The fabric was washed a further 5 times in a domestic washing machine at 40°C. The hand and appearance of the fabric remained unchanged. Example 2 The process of Example 1 was repeated except that the concentration of the sodium hypochlorite solution was 0.75 g/L. The washed fabric had a peach-skin hand and appearance with uniform secondary fibrillation and no apparent primary fibrillation. The tensile strength of the fabric was 85% of its strength before hypochlorite treatment, dyeing and washing. The fabric retained the peach-skin hand after further washing in a domestic washing machine. Example 3 The process of Example 1 was repeated using a Roaches 300 L jet dyer for both hypochlorite treatment and dyeing of a length of plain-weave lyocell fabric in rope form. After dyeing, the fabric, wet from the dyeing process, was treated in a Thies Rototumbler in place of the washing step. The fabric was initially tumbled wet in a 4 g/L aqueous lubricant solution at 80°C and 150 m/min for one hour. A silicone softener was added. The aqueous liquor was then drained, and the tumbling machine was run using air at 120°C with the speed gradually increasing from 150 to 1000 m/min over one hour. The fabric was then substantially dry and was further treated in the tumbling machine at 500 m/min using air at 120°C for a further hour. The fabric after tumbling had a peach-skin hand and appearance which was retained after forming the fabric into garments and washing them. Example 4 Using the process and apparatus of Example 1, a ponti di Roma knitted lyocell fabric was treated with a 3.5 g/L sodium hypochlorite solution at 80°C for 60 minutes and was then dyed and washed. The resulting fabric had no apparent fibrillation and an improved appearance compared to ponti di Roma lyocell fabric which had not received the hypochlorite treatment. The strength loss of the fabric after the hypochlorite treatment, dyeing and washing was less than 10%. The fabric retained its hand and appearance after subsequent washing. E am le 5 The process of Example 4 was repeated using an interlock knitted fabric. The fabric produced had no apparent fibrillation and a strength loss of less than 10%. Example 6 A Thies Rototumbler was charged with a 4 g/L aqueous solution of conventional lubricant containing conventional stabiliser and sequestering agent. The solution was heated to 50°C and a woven lyocell fabric was loaded. The solution was heated to 60°C at 2°C/min. Hydrogen peroxide (commercial 33% solution) was added at 15 g/L (5g/L H 2 0 2 ) and the mixture was heated to 80°C at 2°C/min. The machine was run for 60 minutes at 80°C, then cooled to 50°C and drained. The treated fabric was rinsed, then dyed in the Rototumbler with a reactive dye at 80°C for 60 minutes. After drying, the fabric was treated wet with a silicone sof ener as described in Example 3. The Rototumbler was then run using air at 115°C until the fabric was substantially dry, and the fabric was then tumbled at 500 m/min using air at 115°C for 60 minutes. The resulting fabric had a peach-skin hand which was retained after washing. Example 7 A woven lyocell fabric was treated with hydrogen peroxide oxidising agent as described in Example 6, except that wet processing was carried out on a Then AFS - air jet tumbling machine and hydrogen peroxide was applied at 20 g/L of the 33% solution because mechanical action of the Then AFS is less than that of the Thies Rototumbler. After treatment of the fabric with hydrogen peroxide and reactive dyeing on the Then AFS, silicone softener was applied to the fabric by padding. The resulting fabric was dried and tumbled on a Bianalani Airo high speed drying/tumbling machine using the conditions of Example 6. A peach-skin fabric was obtained which retained its hand and appearance after washing. Example 8 Interlock knitted fabric pieces were treated with hydrogen peroxide on a Planet garment wash machine using the times and temperatures described in Example 6 with only 10 g/L peroxide because of the aggressive action of the garment wash machine. The fabric pieces were reactive dyed and softened in the Planet wash machine, then dried in a tumble drier. The dried fabric had a peach-skin hand and appearance which was retained even after washing.

Claims: CLAIMS 1. A method of reducing the tendency of a lyocell fabric to primary fibrillation after dyeing and washing, characterised in that, before dyeing, the fabric is contacted at elevated temperature with an aqueous solution of an oxidising agent. 2. A method according to claim 1, characterised in that the oxidising agent is a hypochlorite. 3. A method according to claim 1, characterised in that the oxidising agent is hydrogen peroxide. 4. A method according to any of claims 1 to 3, characterised in that the temperature at which the fabric is contacted with the oxidising agent solution is in the range 60 to 100°C. 5. A method according to any of claims 1 to 4, characterised in that the lyocell fabric is in the form of a garment. 6. A method according to claim 5, characterised in that the garment is contacted with an aqueous hypochlorite solution having a concentration of hypochlorite equivalent to more than 1.5 up to 5 grams per litre sodium hypochlorite, so that the fabric has substantially no fibrillation after subsequent dyeing and washing. 7. A method according to claim 5, characterised in that the garment is contacted with an aqueous hypochlorite solution having a concentration of hypochlorite equivalent to at least 0.4 but less than 1.5 grams per litre sodium hypochlorite, so that the fabric has a peach-skin hand after subsequent dyeing and washing. 8. A method according to claim 5, characterised in that the garment is contacted with an aqueous solution of 2 to 10 grams hydrogen peroxide per litre so that the fabric has a peach-skin hand after subsequent drying and washing. 9. A method according to any of claims 1 to 4, characterised in that the lyocell fabric is contacted with the oxidising agent solution in the form of a continuous length of woven or knitted fabric and is subsequently dyed in rope form and subjected to hot wet treatment in a tumbling machine. 10. A method according to claim 9, characterised in that the fabric is contacted with an aqueous hypochlorite solution having a concentration of hypochlorite equivalent to more than 5 up to 10 grams per litre sodium hypochlorite, so that the fabric has substantially no fibrillation after dyeing and treatment in the tumbling machine. 11. A method according to claim 9, characterised in that the fabric is contacted with an aqueous hypochlorite solution having a concentration of hypochlorite equivalent to at least 1 but less than 5 grams per litre sodium hypochlorite, so that the fabric has a peach-skin hand after dyeing and treatment in the tumbling machine. 12. A method according to claim 9, characterised in that the fabric is contacted in rope form with an aqueous solution of 3 to 20 grams hydrogen peroxide per litre so that the fabric has a peach-skin hand after dyeing and treatment in the tumbling machine.

Inventors: COOPER KAREN JANE, TAYLOR JAMES MARTIN, COOPER, KAREN, JANE, TAYLOR, JAMES, MARTIN
Applicants: COOPER KAREN JANE, COURTAULDS FIBRES HOLDINGS LTD, TAYLOR JAMES MARTIN, COOPER, KAREN, JANE, COURTAULDS FIBRES (HOLDINGS) LIMITED, TAYLOR, JAMES, MARTIN
Application year: 1997
Main classification: D06M11
IPC codes: D01F2, D06M11