JP2984940B2 - Prevention of yellowing of thiourea dioxide. - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for preventing yellowing of thiourea dioxide, and more particularly to a method for preventing yellowing of thiourea dioxide during storage. Things.

(Prior art and its problems) Thiourea dioxide has the following structural formula, is an odorless, white powder having no oxidizing or reducing properties, but has a reducing property when dissolved in water.

Thiourea dioxide has no disadvantages of conventional reducing agents, such as instability and danger during storage and generation of sulfur dioxide gas, and has a high reduction potential in alkaline aqueous solution when used, and furthermore, Since it has excellent stability in aqueous solution, it has a feature that its intended use can be achieved with a very small amount.

Accordingly, the fields of application are reducing agents for dyeing vat dyes and sulfur vat dyes, reducing detergents for fibers dyed with disperse dyes, shrink-proofing agents for keratin fibers, bleaching agents for protein fibers and polyamide fibers, polyacrylonitrile fibers and There are a wide variety of applications in the textile industry, such as decolorizing agents in the production process of polyvinyl alcohol fibers, pulp bleaching agents, antioxidants for organic amines, polymerization catalysts, and the photo industry.

However, thiourea dioxide is a stable compound,
When stored for a long period of time, a small amount of by-products contained in thiourea dioxide is gradually decomposed and colored yellow, resulting in a decrease in pure content and a significant decrease in commercial value.

Thiourea dioxide is industrially produced by the reaction of hydrogen peroxide and thiourea, but this reaction mechanism is complicated,
It is considered that a large number of by-products are produced, and the yellowing phenomenon becomes apparent when the by-products are mixed with thiourea dioxide.

The main cause of yellowing is thought to be disulfide sulfate. For the production of disulfide sulfate, first, thiourea dioxide produced by the reaction between thiourea and hydrogen peroxide further reacts with hydrogen peroxide to produce sulfuric acid as a by-product. When the pH of the reaction solution decreases due to the formation of sulfuric acid, the oxidation of thiourea with hydrogen peroxide stops at disulfide, an intermediate of thiourea dioxide, and is not oxidized to thiourea dioxide. It is believed that the disulfide formed here reacts with the by-produced sulfuric acid to form disulfide sulfate.

As a method for preventing the yellowing of thiourea dioxide, a method for producing high-purity thiourea dioxide having a low content of disulfide sulfate causing yellowing has been proposed.

For example, a method for maintaining the reaction temperature within a certain range, a method for improving the feed method of thiourea and hydrogen peroxide, and a method for controlling the molar ratio of thiourea and hydrogen peroxide in the reaction solution within a certain range And a method of maintaining the pH of the reaction solution.

The types of solvents used in these methods can be broadly classified into non-aqueous solvents and aqueous solvents.

Carbon tetrachloride, a method for producing thiourea dioxide by reacting thiourea with hydrogen peroxide in a chlorine-based solvent such as chloroform and a non-aqueous solvent such as a lower aliphatic alcohol, German Patent No. 917,553, Italy Patent No. 57
No. 9,119 and French Patent 2,040,797. In these methods, the amount of by-products generated is large because the reaction between thiourea and hydrogen peroxide is a heterogeneous reaction, and the generated by-products do not dissolve in the solvent. There is a disadvantage that it is mixed with thiourea and causes yellowing of the product.

On the other hand, when thiourea dioxide is produced in an aqueous solvent, the raw material thiourea dissolves well in water and forms a homogeneous phase. There is an advantage that urea can be obtained, and at present, this method is exclusively used industrially, and research is being conducted on an improved method relating to this method.

U.S. Pat. No. 2,783,272 proposes a method of keeping unreacted thiourea in a reaction solution at 0.5 to 1.0%, keeping the pH of the reaction solution within a certain range, and reducing the amount of by-products produced. .

However, this method cannot produce thiourea dioxide in high purity. That is, since the reaction between thiourea and hydrogen peroxide is extremely fast and generates a large amount of reaction heat, generation of local by-products is unavoidable, and the decomposition of thiourea dioxide by these by-products takes time. Sulfuric acid is generated to lower the pH of the reaction solution, and the lowering of the pH of the reaction solution generates disulfide, which is one type of by-product. The disulfide forms a salt with low solubility with by-product sulfuric acid, and has a disadvantage that it is mixed with thiourea dioxide product to cause a decrease in product purity.

In U.S. Pat. No. 3,355,486, the reaction time between thiourea and hydrogen peroxide is set to 7 minutes or less to minimize the production of by-product sulfuric acid, complete the reaction before sulfuric acid is produced, Has been proposed to keep the pH of 3-7. However, even if the reaction time is shortened, the reaction between thiourea and hydrogen peroxide is extremely fast and generates a large amount of heat of reaction as described above, so that local by-products cannot be avoided.

In Japanese Patent Publication No. 45-17665, after the reaction is completed, the reaction solution from which the crystals are separated is treated with a carbonate or hydroxide of an alkaline earth metal to remove sulfuric acid which inhibits the reaction, and then the next reaction solution is added. Is proposed to be reused as
In Japanese Patent No. 62934, it is pointed out that the method of Japanese Patent Publication No. Sho-sho reduces the purity of the product.

Japanese Patent Publication No. 58-39155 proposes a method in which ammonium bicarbonate is continuously added during the reaction between thiourea and hydrogen peroxide to neutralize and remove sulfuric acid produced as a by-product. In this method, sulfuric acid produced as a by-product is successively neutralized and removed, so that sulfuric acid does not accumulate in the reaction solution, and contamination by disulfide sulfate in thiourea dioxide is considerably improved. However, yellowing of the product due to long-term storage is inevitable.

Therefore, it is difficult to prevent yellowing of thiourea dioxide by long-term storage by the conventional method, and it is often unsatisfactory.

Japanese Patent Publication No. 52-20971 proposes a method of adding a chelating agent to thiourea dioxide to prevent yellowing of thiourea dioxide during storage. However, even with this method, satisfactory yellowing cannot be prevented.

It is an object of the present invention to provide a more effective method of preventing thiourea dioxide from yellowing.

Means for Solving the Problems In view of the prior art as described above, the present inventors have proposed to improve the reaction method between thiourea and hydrogen peroxide to prevent yellowing during storage of thiourea dioxide. Was considered difficult, and as a result of various studies on the method of preventing yellowing by adding an additive to thiourea dioxide, it was found that adding ultrafine silica to thiourea dioxide caused yellowing during storage of thiourea dioxide. Can be effectively prevented.

That is, the present invention is a method for preventing thiourea dioxide from yellowing, which comprises adding ultrafine silica to thiourea dioxide.

As the ultrafine silica used in the present invention, there are a dry ultrafine silica obtained by hydrolyzing silicon halide and a wet ultrafine silica obtained by acid decomposition of sodium silicate. The average particle diameter of the ultrafine silica is usually 20 μm or less, preferably 5 μm or less.

The addition amount of the ultrafine silica is 0.01% to 1.0%, preferably 0.05% to 0.5%, based on the weight of thiourea dioxide. Usually, the ultrafine silica is added to powdered thiourea dioxide and mixed uniformly.

The thus obtained thiourea dioxide powder can significantly prevent yellowing even when stored for a long time.

Next, examples will be described. The following examples illustrate embodiments of the present invention and do not limit the present invention.

(Examples) The table below shows the results obtained by storing thiourea dioxide to which ultrafine silica was added at 40 ° C. for 6 months and measuring the whiteness and yellow coloration of thiourea dioxide.

The whiteness is measured by a whiteness meter (model KR) manufactured by Kumagai Science & Industry Co., Ltd.
-III), the yellow coloring degree (b value) is Nippon Denshoku Industries Co., Ltd.
It measured using the color difference meter (model 1001DP) made from each.

Aerosil-200: Dry-process ultrafine silica manufactured by Nippon Aerosil Co., Ltd. Syloid-244: Wet-process ultrafine silica manufactured by Fuji Devison Chemical Co., Ltd. It is understood that it is done.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-55-28954 (JP, A) JP-A-55-17339 (JP, A) JP-A-48-46577 (JP, A) (58) Field (Int. Cl. ⁶ , DB name) C07C 381/00 REGISTRY (STN) CA (STN)

Claims (2)

(57) [Claims] 1. A method for preventing thiourea dioxide from yellowing, comprising adding ultra-fine silica to thiourea dioxide. 2. The method for preventing yellowing of thiourea dioxide according to claim 1, wherein the addition amount of the ultrafine silica is 0.01 to 1.0% based on the weight of thiourea dioxide.