FR2732367A1 - An improved method for the dyeing of polyamide materials, with good dye adhesion - Google Patents

Abstract

A new process for the prodn. of coloured articles, by dyeing with a dispersion dye followed by a simultaneous redn. and wash, and finally a fixing stage, to fix the acid dye to the nylon. Pref. the fixing is a combination of tannic acid and tartar emetic(potassium antimonlyl tartrate) or a synthetic tannin, as an aq. soln. contg. 1-6% tannic acid and 0.5-4% tartar emetic, or 1.8% synthetic tannin. A pH of 3.5-5.4 is obtd.. The redn./wash process is carried out with an aq. soln. contg. an alkali and a reducing agent, e.g. NaOH and a hydrosulphite or thiourea dioxide.

Description

PROCESS FOR PRODUCING COLORED POLYAMIDE ARTICLES
The present invention relates to a process for the production of colored articles of polyamide or of a material containing a polyamide (hereinafter abbreviated as "polyamide colored article"), which process makes it possible to obtain a colored article of polyamide whose color is very strong, even when a disperse dye is used for the dyeing operation.

 In the production of colored polyamide articles by dyeing shaped polyamide articles, such as moldings made of polyamide resin or polyamide fibers, when a disperse dye is used to dye the articles shaped polyamide, the color fastness of colored polyamide articles thus obtained is quite low. Therefore, in the dyeing of shaped polyamide articles, it is generally only in the case where it is desired to obtain a clear color that disperse dyes are used.

 In order to obtain colored polyamide articles by dyeing shaped polyamide articles such as those mentioned above, it has hitherto been customary to dye these shaped polyamide articles using a dyestuff. acid.

 There are now mixed fabrics, semi-wool fabrics and composite articles in combinations of polyamide and other synthetic resins such as polyesters. Colored polyamide articles are also produced by dyeing these combinations of polyamide and other synthetic resins such as polyesters.

 In the dyeing of articles in combinations of polyamide and other synthetic resins such as polyesters, when this dyeing is carried out using only a disperse dye, the color of the polyamide colored articles thus obtained is not very strong, as noted above. It is therefore customary to first dye the article using a disperse dye, then bleach the polyamide component of the article by removing the disperse dye, and dye the polyamide component again with the aid of an acid dye.

 However, this procedure requires two dyeing operations articles, which is inconvenient and very expensive. It is well recognized that it is desirable to be able to easily produce, using only a disperse dye, colored polyamide articles whose color is very strong.

 In recent years, there has been proposed a process, described in Japanese Patent Application Laid-open No. 5-295,676, which provides polyamide fibers of a stronger color by dyeing them with a neutral dye. containing gold and / or disperse dye, then fixing the dye on the polyamide fibers with tannin and stibic (emetic) scale, and then subjecting the dyed polyamide fibers to soaping reducer using an alkali. Despite this progress, it is still difficult to obtain colored polyamide articles with a solid color.

 The present invention aims to solve the problems encountered so far in dyeing polyamide shaped articles, problems mentioned above.

 One of the aims of the present invention is to provide a method for producing colored polyamide articles, which makes it possible to easily obtain, using a disperse dye, colored polyamide articles whose color is very solid.

 To achieve the above mentioned purpose, the process for producing colored polyamide articles, according to the invention, comprises dyeing a polyamide article shaped with a disperse dye, and then reducing and washing the polyamide dyed article at the same time, and then fixing the dye within the article using an acid dye fixing agent on nylon.

 As just indicated, according to the method of the present invention, a colored polyamide article is produced by dyeing a polyamide article shaped with a disperse dye, reducing and washing at the same time. the article dyes polyamide (reducing wash), and then fixing the dye with an acid dye fixing agent on nylon. It is therefore no longer necessary to carry out the otherwise unavoidable operation of extracting the disperse dye from the polyamide article, and the colored polyamide article thus obtained is provided with a color whose strength is improved.

 The polyamide used may be nylon 6, nylon 6-6, nylon 12, nylon 6-10, etc. The polyamide articles may be of any structure, and may be fiber articles as well as molded resin parts. Even in the case of molded resin parts which are in general difficult to color, the dyeing process described above makes it possible to obtain colored polyamide articles whose color is very solid.

 The disperse dye used in the present invention may be any of the known disperse dyes. By way of examples, mention may be made of the disperse dyes belonging to the series of anthraquinone, azo and methine dyes, as well as disperse dyes of the nitrodiphenylamine, aminoketone and ketoimine type. It is particularly suitable to use a disperse dye which disperses easily in water and has a high affinity for polyamide articles.

 For dyeing a polyamide article shaped with a disperse dye such as those mentioned above, one can for example implement the known method of dip dyeing.

 The treatment, indicated above, of reducing the polyamide article dyed with a disperse dye is carried out using an alkali such as caustic soda or sodium carbonate, in combination with a reducing agent such as a hydrosulfite or dioxide of thiourea. In general, a detergent surfactant is also used. It has been found inappropriate to use, for this washing and reducing operation, a combination of an acid such as acetic acid or formic acid and sodium formaldehyde sulfoxylate because this combination causes dye removal during the reducing wash.

 When carrying out this reducing wash using an aqueous solution containing an alkali and a reducing agent such as those mentioned above, the amounts of alkali and reducing agent to be used are as follows: alkali, from 0.5 to 2 g / l of caustic soda or from 1 to 4 g / l of sodium carbonate, and for the reducing agent from 1 to 4 g / l of hydrosulfite or 0.2 at 0.5 g / l of thiourea dioxide.

During the reducing wash operation, the temperature is set to a value in the range of 600 ° C. to 900 ° C., and the duration of this operation is approximately 15 to 60 minutes.

 The polyamide dyed article, after having been simultaneously reduced and washed in the manner described above, is subjected to the dye fixing operation, performed using an acid dye fixing agent on nylon.

As a fixing agent, it is possible to use a combination of a tannin, such as Chinese tannin, and stibnate tartar (potassium tartrate and antimony), or a synthetic tannin.

 As concrete examples of synthetic tannin that can be effectively used herein, there may be mentioned phenol-sulfonic acid derivatives, aromatic sulfonic acid condensation products, aromatic macromolecule derivatives, oxysulfonate condensation products. , polyphenol derivatives, condensation products of aryl sulfonates and formaldehyde, and aromatic macromolecular sulfonic acids.

 When treating previously dyed polyamide articles, made for the purpose of fixing the dye with an acid dye fixing agent on nylon, such as those mentioned above, if using a In combination of tannin and tartar, it is desirable that the amounts of tannic acid and tartar, expressed as the solids content of the solution, are respectively 1 to 6% by weight and 0.5 to 5% by weight. 4% by weight. If only synthetic tannin is used, it is appropriate to use it in an amount corresponding to a solids content of 1 to 8% by weight.

 During the fixing treatment, it is also desirable that the pH of the solution containing the fixing agent be from 3.5 to 4, that the temperature at which this treatment is carried out is in the range of from 50 ° C. to 800C, and that the duration of this treatment is worth 15 to 30 minutes.

 The use of the method of the present invention to obtain a colored article of polyamide makes it possible to avoid the operation, if not essential, which consists in eliminating the disperse dye from the polyamide article, and makes it possible to obtain a colored polyamide article. whose color is very strong, using a disperse dye.

 Even in the case of an article made of a mixture of polyamide and another synthetic resin, such as a polyester, the method of the present invention makes it possible to obtain a colored article in such a mixture, also having a color. very solid, simply dyeing, using a disperse dye, an article made in such a mixture. The method of the present invention thus eliminates the need for a complicated procedure of dyeing an article with a disperse dye, then bleaching the polyamide fraction by extracting the disperse dye, and then dyeing the dye again. polyamide fraction using an acid dye, as has been practiced so far, and it therefore makes it easy to produce such colored articles.

 The method of the present invention, a process for producing a colored polyamide article, using operating examples and comparative examples, which are given here in order to demonstrate that the protocols will now be described more precisely. operating examples make it possible to produce colored polyamide articles having a very solid color.

Example 1 and Comparative Example 1
In Example 1 and Comparative Example 1, slide fasteners consisting of polyamide, nylon 6 molded coupling elements and polyethylene terephthalate stitching tapes and sewing threads are prepared for dyeing. ) containing 4.8% by weight of polyethylene glycol nO 600, incorporated by copolymerization.

In Example 1, the zipper was dyed at 1100C for 60 minutes, with an aqueous dispersion of the Miketon Red Polyester Red 3B-SF
Mitsui Toatsu Dye Co., Ltd.), selected as a disperse dye and used in a proportion of 3% by weight based on the weight of the fibers or fabric.

The zipper thus dyed is then subjected to a simultaneous reduction and washing treatment, with an aqueous solution containing caustic soda, a hydrosulfite and a detergent (product of Dai-ichi Kogyo
Seiyaku Co., Ltd., marketed with the brand "Amiradin
D "), all in a concentration of 2 g / l.

 The zipper is then subjected to the fixing treatment, carried out at 600 ° C. for 30 minutes, using an aqueous solution of an aromatic sulphonic acid derivative (a synthetic tannin produced by Ipposha K.K.

and marketed with the trademark "Nylox 1500"), selected as an acid dye fixing agent on nylon and used in a proportion of 8% based on the weight of the fabric or fibers.

 In contrast, in Comparative Example 1, the zipper was subjected to the dyeing and reducing wash operations following the protocol of Example 1, but the fixing treatment was not performed.

The slide fasteners obtained according to Example 1 and according to the Example are then examined.
Comparative 1, described above, to determine how far the dye can migrate. For this purpose, the slide fasteners obtained according to Example 1 and Comparative Example 1 were clamped together between a sheet of polyvinyl chloride and a piece of polyester fabric, and then pressurized, under a load of 4.5 kg, between two sheets of plastic 12 cm x 5 cm, and placed in a desiccator filled with water, we put the desiccator and its contents in an oven and they are left there at 40 ° C for 48 hours.

Next, the polyvinyl chloride sheet and the piece of polyester fabric between which the tinted zippers of Example 1 and Comparative Example 1 were examined were examined to see if the dye had migrated. or not, from the elements of the zippers, comparing the sheet and the piece of fabric with the gray scale. The results obtained are shown in Table 1 below.

Table 1

<tb><SEP><SEP> Sheet of <SEP> Poly (Chloride <SEP> Tissue <SEP> of
<tb><SEP> of <SEP> vinyl) <SEP> polyester
<tb><SEP> Example <SEP> 1 <SEP> 4-5 <SEP> 4-5
<tb><SEP> Example <SEP> 2-3 <SEP> 3
<tb> Comparative <SEP> 1
<Tb>
From these results, it is clear that, in the case of the sample of Example 1, the dye has only migrated slightly from the polyamide elements to arrive at the polyvinyl chloride sheet and on the polyester fabric, compared to what happened with the sample of Comparative Example 1. This indicates that the color imparted to the polyamide is stronger.

Example 2 and Comparative Example 2
In Example 2 and Comparative Example 2, slide fasteners consisting of polyamide (nylon 6-6) molded coupling members and polyethylene stitching tapes and yarns are prepared for dyeing. terephthalate).

In Example 2, the zipper was dyed at 1300C for 60 minutes, with an aqueous dispersion of the red dye Dianix Red KBN-SE (Mitsubishi product).
Chemical Industries, Ltd.), selected as a disperse dye and used in a proportion of 4% by weight based on the weight of fabric or fibers.

 The zipper thus dyed is then subjected to a simultaneous washing and reduction operation carried out with the aid of an aqueous solution containing 2 g / l of caustic soda and 0.7 g / l of thiourea dioxide. (product of Tokai Electrochemistry KK, marketed with the trademark "Tecklight"), and 2 g / l of detergent (product of Dai-ichi Kogyo Seiyaku Co., Ltd., "Amiradin D").

The zipper is then subjected to a fixing treatment, carried out at 600 ° C. for 30 minutes, using an aqueous solution of a condensation product of oxysulphonate (synthetic tannin, product of Shichifuku).
Chemicals KK), marketed with the brand "Nylon Sticker
KB Soft ™), chosen as an acidic dye fixing agent on nylon and used in a proportion of 8% by weight based on the weight of fabric or fibers.

 In contrast, in Comparative Example 2, the zipper was subjected to dyeing and reductive washing according to the protocol of Example 2, but the fixing treatment was not performed.

 Then, in the same manner as in Example 1 and Comparative Example 1, the slide fasteners of Example 2 and Comparative Example 2, obtained in the manner described above, were examined to see to what extent the dye migrates from the zipper elements onto the polyvinyl chloride sheet or onto the polyester fabric as compared to the gray scale. The results obtained are shown in Table 2 below.

Table 2

<tb><SEP><SEP> Sheet of <SEP> Poly (Chloride <SEP> Tissue <SEP> of
<tb><SEP> of <SEP> vinyl) <SEP> polyester
<tb><SEP> Example <SEP> 2 <SEP> 4 <SEP> 4
<tb><SEP> Example <SEP> 2-3 <SEP> 3
<tb> Comparative <SEP> 2
<Tb>
From these results, it is clear that in the case of the sample of Example 2, as well as for the sample of Example 1 mentioned above, the dye has only slightly migrated since polyamide elements on the polyvinyl chloride sheet and on the polyester fabric, compared to what happened with the sample of Comparative Example 2. This indicates that the color imparted to the polyamide is more solid.

Example 3 and Comparative Example 3
In Example 3 and Comparative Example 3, slide fasteners consisting of polyamide, nylon 6 molded coupling elements and polyethylene terephthalate stitching tapes and sewing threads are prepared for dyeing. ) containing 4.8% by weight of polyethylene glycol nO 600, incorporated by copolymerization.

In Example 3, the zipper was dyed at 1100C for 60 minutes, with an aqueous dispersion of the Miketon Red Polyester Red 3B-SF
Mitsui Toatsu Dye Co., Ltd.), selected as a disperse dye and used in a proportion of 3% by weight based on the weight of the fibers or fabric.

The zipper thus dyed is then subjected to a simultaneous reduction and washing treatment, with an aqueous solution containing caustic soda, a hydrosulfite and a detergent (product of Dai-ichi Kogyo
Seiyaku Co., Ltd., marketed with the brand "Amiradin
D "), all in a concentration of 2 g / l.

 Then, to treat this zipper in order to fix the dye, a combination of tannin and stibnate tartar is chosen as the acid dye fixing agent on nylon. The zipper is first treated at 60 ° C for 30 minutes, using an aqueous solution which contains 8% tannin, by weight with respect to the fibers or fabric, and the pH was adjusted to 4 with The zipper is then treated at a temperature of 60 ° C. for 20 minutes, using an aqueous solution containing 1.8% of stibnated scale, by weight relative to the fabric or fibers, and the pH of which is was adjusted to 4 with acetic acid, to fix the dye.

 On the contrary, in Comparative Example 3, the zipper was subjected to the dyeing and reductive washing operations following the protocol of Example 3, but the fixing treatment was not performed.

 Then, in the same manner as in Example 1 and Comparative Example 1, the slide fasteners of Example 3 and Comparative Example 3, obtained in the manner described above, were examined to see to what extent the dye migrates from the zipper elements onto the polyvinyl chloride sheet or onto the polyester fabric as compared to the gray scale. The results obtained are shown in Table 3 below.

Table 3

<tb><SEP><SEP> Sheet of <SEP> Poly (Chloride <SEP> Tissue <SEP> of
<tb><SEP> of <SEP> vinyl) <SEP> polyester
<tb><SEP> Example <SEP> 3 <SEP> 4-5 <SEP> 4-5
<tb><SEP> Example <SEP> 2-3
<tb> Comparative <SEP> 3
<Tb>
From these results, it is clear that, in the case of the sample of Example 3, as well as for the sample of Example 1 mentioned above, the dye has only slightly migrated since the polyamide elements on the polyvinyl chloride sheet and the polyester fabric, as compared to the sample of Comparative Example 3. This indicates that the color imparted to the polyamide is more solid.

Example 4 and Comparative Example 4
In Example 4 and Comparative Example 4, dyed composite yarn fabric consisting of 25% nylon 6 and 75% poly (ethylene terephthalate) containing 4.8% by weight polyethylene glycol was prepared for dyeing. No. 600, incorporated by copolymerization.

In Example 4, the fabric was dyed at 110 ° C for 60 minutes, with an aqueous dispersion of the red dye
Miketon Polyester Red 3B-SF (product of Mitsui Toatsu Dye
Co., Ltd.), selected as the disperse dye and used in a proportion of 3% by weight based on the weight of the fibers or fabric.

The composite dyed yarn thus dyed is then subjected to a simultaneous reduction and washing treatment, with an aqueous solution containing caustic soda, a hydrosulfite and a detergent (product of Dai-ichi Kogyo
Seiyaku Co., Ltd., marketed with the brand "Amiradin
D "), all in a concentration equal to 1 g / l.

Then, in order to carry out the fixing treatment on the composite yarn fabric, an acidic dye fixing agent on nylon is chosen, an aromatic sulfonic acid derivative (synthetic tannin, product of
Ipposha KK, marketed with the brand "Nylox 700"), is adjusted to pH 4, with acetic acid, an aqueous solution containing 8% of this fixing agent, by weight relative to the fibers or the fabric, and uses this solution to treat the composite yarn at a temperature of 600C and for 30 minutes to fix the dye.

 In contrast, in Comparative Example 4, the tissue was subjected to dyeing and reductive washing according to the protocol of Example 4, but the dye fixing treatment was not performed.

 The dyed pieces of fabric obtained according to Example 4 and Comparative Example 4, described above, are then examined to determine the extent to which the dye can migrate. To do this, the dyed fabrics are clamped together between a piece of nylon fabric and a piece of polyester fabric, and then put under pressure, under a load of 4.5 kg, between two sheets of plastic 12 cm x 5 cm, and placed in a desiccator filled with water, the desiccator and its contents are placed in an oven and left at 40 ° C. for 48 hours. of nylon between which the dyed fabrics had been enclosed to see if the dye has migrated or not, from the tissues, comparing with the gray scale.

The results obtained are shown in Table 4 below.

Table 4

<tb><SEP><SEP> Fabric of <SEP> Polyester <SEP><SEP> Fabric of <SEP> Nylon
<tb><SEP> Example <SEP> 4 <SEP> 4-5 <SEP> 4-5
<tb><SEP> Example <SEP> 3 <SEP> 2-3
<tb> Comparative <SEP> 4
<Tb>
From these results, it can be seen that in the case of polyamide-containing composite yarn-dyed fabrics obtained according to Example 4, the migration of the dye onto the polyester fabric and the nylon fabric is quite insignificant. comparison of the case of the composite yarn dyed composite containing polyamide obtained according to the Example
Comparative 4, which indicates that the color of the polyamide part is stronger.

Example 5 and Comparative Examples 5 and 6
In Example 5 and Comparative Examples 5 and 6, slide fasteners made of nylon extrusion molding coupling elements 6, and poly-tapes and sewing threads are prepared for dyeing (FIG. ethylene terephthalate) containing 4.8% by weight of polyethylene glycol nO 600, incorporated by copolymerization.

In Example 5, a zipper was stained with a 1/30 bath ratio at 110 ° C. for 60 minutes using an aqueous dispersion containing, as disperse dyes, 2, 1% of the red dye
Miketon Polyester Red 3B-SF (product of Mitsui Toatsu Dye
Co., Ltd.), 0.005% of Foron Brilliant Blue blue dye
SE-CTLY (a product of Sandoz Corp.) and 0.93% of the orange dye Dianix Orange G-SE (a product of Mitsubishi Chemical
Industries, Ltd.), the proportions indicated being based on the weight of fibers or fabric.

 The zipper thus dyed is then subjected to a simultaneous reduction and washing treatment, at 800 ° C. for 30 minutes, with an aqueous solution containing caustic soda, a hydrosulfite and a detergent (product of Dai-ichi Kogyo Seiyaku Co. , Ltd., marketed with the trademark "Amiradin D"), all in a concentration of 2 g / l.

The dye-fixing treatment is then carried out on the zipper, first using an aqueous solution containing 6%, based on the weight of the fibers or fabric, of tannic acid (tannin extract, product of Dainippon Pharmaceutical Co., Ltd.) and adjusted to pH 4 with acetic acid, at 600C and for 30 minutes, and then using an aqueous solution containing 6%, based on the weight of the fibers or the tissue, of stibnate scale ( product of Dainippon Pharmaceutical Co.,
Ltd., marketed with the "New Poweron" brand) at 600C for 30 minutes.

 On the contrary, in Comparative Example 5, the zipper was dyed and reductive washed according to the protocol of Example 5, but the fixing treatment was not performed.

 In Comparative Example 6, the reducing wash and the fixing treatment were carried out, but in the reverse order of that of Example 5. The zipper was dyed with the same disperse dyes as in Example 5 then, the tinted zipper is first subjected to the fixing treatment mentioned above, and then to a reducing wash as described above.

 Then, in order to examine the dyed zippers obtained in the manner described above in Example 5 and Comparative Examples 5 and 6, in order to study the migration of the dyes, the closures were clamped together. slideway dyed between two sheets of poly (vinyl chloride), the whole is pressed between two plastic sheets of 12 cm x 5 cm, under a load of 4.5 kg, and placed in a thermostatically controlled enclosure, set at 37.5 ° C and a constant relative humidity of 90%, and left there for 48 hours.

The polyvinyl chloride sheets in which the tinted slide fasteners of Example 5 and Examples were examined were then examined.
Comparatives 5 and 6, to see if the dye has migrated or not from the elements of the zippers, compared with the gray scale. The results obtained are shown below in Table 5.

 The ribbons are then subjected to slide fasteners of Example 5 and Comparative Examples 5 and 6, dyed as described above, have been tested for color fastness to light according to Japanese industry standard JIS-L. 0842, and also subjected to washing and bleaching tests according to JIS-L 0844 A-2 as indicated below.

Color fastness to light
According to JIS-L 0842, the test article and a blue scale are exposed to the light of a charcoal arc lamp for 20 hours and then left for at least 2 hours in the dark. The color fastness of the tested article is assessed by comparing the degree of discoloration of the test article with that of the blue scale.

Color fastness during washing
According to JIS-L 0844 A-2, a composite sample is prepared by sewing a piece of undyed white fabric on one side of the test article, and the whole is treated at 50 ° C. for 30 minutes. in an apparatus for testing, with a solution containing 5 g / l of soap, then rinsed with water and dried. The color washing strength of the tested article is estimated by comparing the degree of discoloration of the test article with the degree of coloration of the white fabric, respectively with a gray scale for discoloration and with a gray scale for coloring. .

 The results obtained are also shown in Table 5.

Table 5

<tb><SEP> Element <SEP> | <SEP> Ribbon
<tb><SEP> Sheet <SEP> of <SEP> Strength <SEP> to <SEP> Decolo- <SEP> Barbouil
<tb><SEP> poly- <SEP><SEP> light <SEP> ration <SEP> at <SEP> time <SEP> at
<tb><SEP> (chloride <SEP> wash <SEP> wash
<tb><SEP> of <SEP> vinyl
<tb><SEP> Example <SEP> 5 <SEP> 4-5 <SEP> 4 <SEP> 4-5 <SEP> 4-5
<tb><SEP> Example <SEP> 2-3 <SEP> 4 <SEP> 4-5 <SEP> 4-5
<tb> Comparative <SEP> 5
<tb><SEP> Example <SEP> 3 <SEP> 4 <SEP> 4-5 <SEP> 4-5
<tb> Comparative <SEP> 6
<Tb>
From these results, it can be seen that in the case of the sample of Example 5, the dye has only slightly migrated from the polyamide elements on the polyvinyl chloride sheets, compared with that we observe in the case of samples of the Examples
Comparatives 5 and 6, and therefore, the color fastness of the polyamide is markedly improved in the sample of Example 5, compared to the samples of Comparative Examples 5 and 6. It is also seen that There is no difference in the strength of the color of the zipper tape between the sample of Example 5 and the samples of Comparative Examples 5 and 6, which indicates that the dyeing of the tape is not adversely affected.

 Certain specific embodiments of the invention have now been described, purely by way of illustration and not limitation.

Claims (7)

 A method of producing colored polyamide articles, characterized in that it comprises dyeing a polyamide article shaped with a disperse dye, and then simultaneously operating a reduction and washing the dyed polyamide article, and then attaching the dye to the article using a fixing agent for acid dye on nylon.  2. Method according to claim 1, characterized in that said fixing agent is a combination of tannic acid and stibic scale.  3. Process according to claim 1, characterized in that said fixing agent is a synthetic tannin.  4. Process according to claim 2, characterized in that said fixing agent is an aqueous solution containing from 1 to 6% by weight of tannic acid and from 0.5 to 4% by weight of stibnated scale.  5. Process according to claim 3, characterized in that said fixing agent is an aqueous solution containing from 1 to 8% by weight of a synthetic tannin.  6. Method according to one of claims 1 to 5, characterized in that said fixing agent has a pH of 3.5 to 4.  7. Process according to one of Claims 1 to 6, characterized in that the reduction and washing of the article are simultaneously carried out with the aid of an aqueous solution containing an alkali and a reducing agent.