Process for flame-retardant finishing of cotton

The present invention relates to a process for flame- retardant finishing of cotton. The substantive process proceeds in particular by effecting the fixing of flame retardants on the cotton at elevated temperature and in the presence of sufficient residual moisture on the part of the cotton.

Processes for flame-retardant finishing of cotton, which is to be understood as meaning in particular cotton fibres, regenerated cellulose fibres such as for example viscose, solution-spun cellulosic fibre products such as, for example, lyocell, and also textiles comprising same, such as protective clothing, children's clothing, drapes, carpets and upholstery, are known that give permanent, i.e. washing-durable, and non-permanent results. Flame-retardant finishes which are not durable to washing are based in particular on inorganic salts, such as ammonium hydrogenphosphate, ammonium sulphate, borax and boric acid, which are additionally combined with organic nitrogen bases if appropriate. Permanent flame- retardant finishes involve either (I) addition of a flame-retardant additive to an appropriate polymer melt or polymer spinning solution, or

(II) application of a reactive flame-retardant component to the fibre or textile material in the form of an actual finishing operation.

Permanent flame-retardant effects according to (I) utilize antimony halogen compounds, organophosphorus compounds and nitrogen-sulphur-containing additives. Finishes according to (II) mainly utilize flame retardants which are based on phosphorus or phosphorus- nitrogen and which are applied to the fibre either directly or as a monomer or precondensate to be cured on the fibre in the form of a polymer. For example,

precondensates of tetrakishydroxymethylphosphonium chloride and urea are applied to the fibre followed by a treatment with ammonia and then with hydrogen peroxide - see Melliand Textilberichte 3/1990, 219-224. This process and similar processes are technically inconvenient and also lead to a fabric having a relatively stiff handle. Dialkylphosphonocarboxamides and melamine resins likewise give a good flame- retardant finish on cotton, but only at the cost of a relatively high reduction in rub-fastness.

Further disadvantages of the known flame-retardant finish effected in the form of an actual finishing operation are the high emissions to air and water. The fixing of, for example, dialkylphosphonocarboxamide- melamine flame retardants is carried out at about 140 ⁰ C for several minutes in a stenter in a dyeing and finishing facility. These conditions can generate high emissions of compounds of toxic potential in some cases, examples being formaldehyde and organophosphorus compounds. It is estimated that more than half of the dialkylphosphonocarboxamide does not become fixed to the cellulosic fibre material and ends up in the wastewater in the course of a subsequent washing operation. It is likely that the ongoing development of appropriately validated analytical techniques for determining flame retardants and scissioning products in air and water will lead to serious curtailments and restrictive measures on the part of the legislator with regard to the application of known flame retardants. DE 10038100 describes a process using specific cyanuric chloride derivatives. One disadvantage of this process is that the cyanuric choride derivatives described therein are very difficult to apply to the fibre in aqueous solutions, so that they are not very suitable, particularly for the commercially typical application of the flame retardant by means of a pad-mangle as part of a continuous operation.

Swiss patent No. 370 384 describes a process for finishing textiles with salts of dihydroxymonochloro- triazine (DHCT) to improve the wet crease resistance and the rotting resistance. The preparation of the DHCT and the application to cotton is described, and even fixing by steaming is mentioned, but only dry fixing is exemplified and the reported amount applied can, from a single application, achieve at most a nitrogen content on the fibre of 1.29% of N2 (assuming a 100% yield). BP 896,814 is primarily concerned with the production of DHCT. The application to cotton is also described in principle, although only 7.5 g of DHCT are applied per 100 g of cotton, which can lead to maximally 1.75% of nitrogen on the fibre and the yield of fixation is not reported (content similar to above) . The use/properties with regard to flame retardancy are not mentioned. EP 0 616 071 likewise describes finishing cotton with triazine hydrolysates (NHDT for example) to achieve a formaldehyde-free crease-resist finish at temperatures <150°C. The amounts applied give a maximum nitrogen content of 1.6% (assuming a 100% yield), fixing is carried out dry only, and nothing is said about the yield of fixation. The use in a combination with a phosphorus-containing flame-retardant finish is mentioned.

DE 10155066 claims a process for permanent flame- retardant finishing of cellulose fibres. This process was intended to lead to flame-retardant or self- extinguishing cellulosic fibres, or articles comprising same, having an ASTM 02863-77 limiting oxygen index

(LOI) of respectively 24 and 27 or higher without the breaking strength of the fibre being significantly affected. In this process, the cotton is always swollen with caustic soda prior to fixation. Nitrogen contents of <2% by weight are achieved.

It is an object of the present invention to provide a permanent flame-retardant finish on cotton with an

environmentally friendly and simple application process to obtain a flame-retardant cotton having good processing properties, being skin friendly and also satisfying Oeko-Tex standard class I (ref: www.oeko- tex.com) . More particularly, the process should make it possible to ensure a high yield of fixation coupled with at least equivalent, if not superior material properties (softer handle) . In addition, the process should also be more economical than those of the prior art.

We have found that this object is achieved as claimed.

A process for flame-retardant finishing of cotton, wherein an aqueous solution of dihydroxychlorotriazine

(DHCT) is applied to cotton without prior causticization thereof, the fixing of the dihydroxychlorotriazine is effected at a residual moisture content of 5 - 100% and at a temperature of 80° - 160 ⁰ C, and the cotton is subsequently washed and dried, is a completely surprising yet very simple way to achieve the stated object. High yields of fixation of above 2% can be achieved when the cotton to be treated is precisely not pretreated with a caustic prior to fixing and fixing is effected within a certain residual moisture content at elevated temperature. It is very surprising that the prior art swelling of the cotton in a caustic can be omitted before fixing while nonetheless achieving high nitrogen contents of >2% by weight, based on the cotton, from a single application. It is particularly preferred for the fixing to be effected at residual moisture contents of 20 - 100%, based on the cotton, more preferably at 40 - 100% by weight residual moisture.

In a further preferred embodiment, the fixing is effected within a time interval of 5 min to 60 min,

preferably 5 min - 30 min and most preferably 10 min - 20 min.

The fixing in accordance with the present invention is effected at a temperature of 80 - 160 ⁰ C, more preferably 110 - 150 ⁰ C and most preferably at around 130°C.

One advantage of the process according to the present invention is that fixing can be effected before or after or else in combination with other finishing operations (see DE 10155066). It is particularly advantageous for other compounds, for example dyes or other flame retardants, to be also applied to the cotton simultaneously with the fixing. This makes it possible to combine operations and thereby enhance the efficiency and hence the economics of the substantive process .

To achieve very high nitrogen contents (>3% by weight) on the fibre, it can be advantageous to carry out the process repeatedly. A person skilled in the art is free to determine how often the process of the present invention is carried out consecutively. It is advantageous for the process to be carried out up to 5 times, preferably 4 times and more preferably 3 times consecutively.

The process of the present invention is preferably carried out as follows. Cotton staple, yarn or fabric is treated with an aqueous DHCT solution having a content of 1-20% by weight (preferably 10-15%), a pH of 10 - 14 (preferably 11-13) and a temperature of 10 - 50 ⁰ C, preferably 15-25°C), so that the cotton is uniformly wetted with the solution. Application can be effected for example via a dipping operation, via pad- mangling or by spraying. Fixing is carried out with the fibre having a certain residual moisture content which

remains constant throughout the entire fixing time, i.e. fixing is preferably carried out in a steamer in which a defined residual moisture level can be set. The temperature in the steamer is adjusted appropriately. Subsequently, the cotton is washed, preferably with boiling water, and dried at 60 - 140 ⁰ C, preferably 80 - 120 ⁰ C and most preferably at around 100°C. In accordance with the present invention, the fixing of DHCT is effected without prior causticization of the cotton (in contradistinction to DE 10155066), which leads to distinctly superior material properties (softer handle) and is also more economical. The application of the DHCT solution can - as stated - also be repeated a number of times if necessary in order that the nitrogen content may be further increased.

The yield of fixation is determined via the nitrogen content, while the flame-retardant properties can be detected via the LOI or a flame test. [LOI: to DIN 18128, flame test: to prEN 14878 (final draft of December 2005) having regard to DIN EN 6941 (05.04) and 1103 (03.06) ] .

The flame-retardant finish can be applied not only to the final fabric, to the yarn but also to the cotton staple.

The process of the present invention makes it possible to achieve up to 90% yields of fixation and nitrogen contents of up to 4% by weight.

Example 1 :

0.36 g of dry untreated cotton staple is drenched with a 16% aqueous DHCT solution adjusted with NaOH to pH 12, and squeezed off. The add-on is 900%. The sample is dried in a drying cabinet at 130 ⁰ C for 40 min. Samples are taken after various drying times, washed 3 times with boiling water, dried and subsequently analysed.

The example shows that nitrogen fixation on the staple only starts at a residual moisture content of about 200%; at higher residual moisture contents, only DHCT hydrolysis takes place.

Example 2 :

0.36 g of untreated woven cotton fabric (144 g/m2) is drenched with a 16% aqueous DHCT solution adjusted with NaOH to pH 12, and squeezed off. The add-on is 200%. The sample is dried in a drying cabinet at 100 ⁰ C for 30 min. Samples are taken after various drying times, washed 3 times with boiling water, dried and subsequently analyzed.

The woven fabric takes up less solution than cotton staple, so that the initial add-on is 200%. The example shows that nitrogen fixation takes place in the moisture content range of 200 - 0% on the woven fabric also. The hydrolysis of DHCT is distinctly slower in this moisture range. Owing to the better penetration of the fibre and the lower hydrolysis, higher nitrogen contents can be achieved on staple from a single application than on woven fabric.

Example 3 :

6 g lots of untreated cotton knit (156 g/m2) are drenched with a 10% aqueous DHCT solution adjusted with NaOH to pH 12, and squeezed off. The add-on is 120%. The samples are fixed in a steamer at 130 ⁰ C and various moisture contents for 15 min, subsequently washed with boiling water, dried and analysed.

The experiments show that optimum yields of fixation are achieved at a residual moisture content of 60 - 100% during fixing.

Example 4:

204 g of untreated cotton knit (156 g/m2) are drenched with a 10% aqueous DHCT solution adjusted with NaOH to pH 12, and squeezed off. The add-on is 120%. The

samples are fixed in a steamer at 130 ⁰ C and 50% moisture for 15 min, subsequently washed with boiling water, dried and analysed. This treatment is repeated a number of times.

The experiments show that the nitrogen content can be further increased by a multiple treatment, although the yields of fixation decrease with every further step, since fewer and fewer free bonding sites are available.

Example 5 :

Samples with different nitrogen contents, treated with DHCT according to Example 3 and 4, were investigated for their flame-retardant properties:

N2 LOI Burn time to Classification Oeko-Tex content burnthrough of standard 3rd marking thread*

% sec

1 50 20.0 23 B class 1

1 84 21.0 24 B class 1

2 10 21.1 40 B class 1

2 99 23.2 23 B class 1

3 68 25.1 41 B class 1

* to prEN 14878 (final draft of December 2005; having regard to DIN EN 6941 (05.04) and 1103 (03.06)