JPH07119246B2 - Method for producing water absorbent resin - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Object of the invention [Industrial field of application] The present invention has a high water-absorbing ability and a water-absorbent resin having a remarkably small amount of unpolymerized monomer contained in the resin. The present invention relates to a manufacturing method of.

The water-absorbent resin obtained by the production method of the present invention has high safety because it contains an extremely small amount of unpolymerized monomer, and is widely used in the sanitary ware-related industry and also in the agricultural field.

[Prior Art] Water absorbent resins are used as various sanitary materials such as disposable paper diapers and sanitary napkins, and agricultural materials. However, although these water-absorbent resins are characterized by having excellent liquid absorption and liquid retention, the particles of the resin produced by a general method contain 1000 to 50000 ppm.
Unpolymerized monomer remains.

When using these water-absorbent resin as an agricultural material,
These unpolymerized monomers have problems such as inhibiting the growth of plants, and when they are used as sanitary materials, there is a concern that they may affect the human body.

Therefore, various methods for reducing the amount of unpolymerized monomers in the water-absorbent resin have been proposed, and a method of irradiating the superabsorbent resin having a carboxyl group and / or a carboxylate group with electromagnetic radiation or fine particle ionizing radiation. (JP-A-63-439
30), and a method of irradiating it with ultraviolet rays, which is said to have been improved in terms of operation and cost (JP-A-63-260907).
However, the energy cost is still high, and if it is desired to uniformly irradiate ultraviolet rays, the powder layer to be processed must be made thin, leaving a large problem in productivity. . Further, a method in which a hydrophilic high molecular weight polymer powder is sprinkled with an aqueous solution of a radical polymerization initiator in the presence of an inorganic powder and heated to re-polymerize (JP-A-64-24808).
However, since the aqueous solution is again sprayed onto the product once produced, heating and re-drying for the reaction are required, which is a cost problem. In any case, in the prior art, in order to reduce the amount of unpolymerized monomer, it was necessary to perform some post-treatment on the finished product or gel.

[Problems to be Solved by the Invention]

The present invention relates to α, β such as acrylate or methacrylate.
-A method of polymerizing a monomer mainly composed of an unsaturated carboxylic acid salt to produce a water absorbent resin, a method of reducing the amount of unpolymerized monomer to the extent that no post-treatment is required, in other words, It was made for the purpose of finding a method for producing a safe water-absorbent resin at a low cost.

(B) Structure of the Invention [Means for Solving the Problems] The inventors of the present invention have conducted various studies to solve the above-mentioned problems, and as a result, the amount of heavy metals coexisting during polymerization increases. By knowing that the amount of unpolymerized monomer in the resin will increase, polymerization will be carried out in the state in which heavy metals are substantially absent, and the amount of unpolymerized monomer will be reduced so that there is no need for post-treatment. The inventors of the present invention have found that the produced resin can be produced very easily and arrived at the present invention.

That is, the present invention relates to a method for producing a water-absorbent resin, which comprises polymerizing a monomer mainly containing an α, β-unsaturated carboxylic acid salt and having a heavy metal content of 0.1 ppm or less.

The α, β-unsaturated carboxylic acid salt in the present invention is a sodium salt, a potassium salt, or the like of an unsaturated carboxylic acid typified by acrylic acid, methacrylic acid, itaconic acid, maleic acid and the like. The main monomer is composed of one kind or two or more kinds of those monomers, or these and other hydrophilic monomers such as acrylamide, 2-hydroxyethyl (meth) acrylate, 2- ( (Meth) acryloylethanesulfonic acid, vinyl acrylic monomer such as 2-acrylamido-2-methylpropanesulfonic acid sodium, quaternary salt of dimethylaminoethyl (meth) acrylate, etc., N, N- capable of introducing a crosslinked structure Hydrophilic polyfunctional monomers such as methylenebisacrylamide and ethylene glycol diacrylate, or polyfunctional compounds such as ethylene glycol diglycidyl ether Is that of a mixture of.

Of course, as the monomer, those to which starch, cellulose, etc., which have been conventionally used for producing a water absorbent resin, are added may be used.

A preferred monomer for the present invention is a monomer mixture containing 20% by weight or more of an alkali metal acrylate, and a particularly preferred one is a combination of acrylic acid and a ratio of acrylic acid and alkali metal acrylate ( Molar ratio) 0-8
It is from 0: 100 to 20.

Heavy metals in the present invention, iron, manganese, chromium,
It means an ion such as copper and lead, a simple substance, an oxide, etc., and iron and copper have a great influence on the remaining unpolymerized monomer. In order to reduce the amount of unpolymerized monomers to the desired amount, it is necessary not to allow them to be present in the monomers or to make their amount 0.1 ppm or less, and further 0.02 ppm.
It is preferable to suppress the existing amount below.

For the production method of the present invention, any of conventionally known polymerization methods such as an aqueous solution polymerization method, a suspension polymerization method and a reverse phase suspension polymerization method can be adopted.

The method for reducing unpolymerized monomers according to the present invention requires polymerization by suppressing the amount of heavy metals present in the monomers to 0.1 ppm or less as described above, and means for that purpose is as follows. Not contacting the monomer with the heavy metal It is possible to use a monomer that does not substantially contain the heavy metal.

In order to prevent the monomer and the heavy metal from coming into contact with each other, all of them including the monomer synthesizing device, the polymerizing device and the piping should be subjected to lining and / or coating treatment. Ordinary resins such as nylon, fluororesin, etc. can be used.

As a method of obtaining a monomer substantially free of heavy metals, a method of removing heavy metals (usually 0.01 to 0.10 ppm) present in α, β-unsaturated carboxylic acid or the like as a raw material by distillation, etc. It is possible to remove heavy metals (usually 1 to 5 ppm) in the alkali used for summing with a chelate ion exchange resin, activated carbon or the like.

These methods may be used alone or in combination of two or more.

As the polymerization initiator of the present invention, persulfate, hydrogen peroxide,
One or more peroxides such as succinic acid peroxide and t-butylperoxymaleic acid, or a redox-based initiator in which these peroxides are combined with a reducing agent such as sodium persulfate and ascorbic acid. Azo compounds etc. are used,
The addition amount is usually 0.05 to 0.5% by weight with respect to the monomer.
However, it is natural from the gist of the present invention that heavy metal-based catalysts such as iron-based catalysts cannot be used.

[Action]

It is completely unknown why coexisting heavy metals contribute to the remaining unpolymerized monomers, but by controlling the heavy metals present in the monomers to below a certain level, the action of reducing unpolymerized monomers in the resin This action was first discovered by the present inventors.

Example 1 Comparative Example 1 360 g of an unpurified caustic soda aqueous solution having a concentration of 32% was added to an aqueous solution obtained by adding 319 g of water to 297 g of acrylic acid while stirring to neutralize the aqueous solution. The iron ion concentration in this monomer aqueous solution is 0.3 p
It was pm. 24.0 g of methylenebisacrylamide (3% aqueous solution) was mixed with this monomer solution in a stainless Dewar bottle (Thermocut D-2000: manufactured by Nippon Oxygen Co., Ltd.), and the mixture was heated to 5 ° C. while degassing with N _2. Adjust the temperature. Then 10
% Ammonium persulfate aqueous solution (hereinafter APS aqueous solution) 7.2 g,
When 3.6 g of 1% sodium erythorbate (Elvite N; manufactured by Fujisawa Pharmaceutical Co., Ltd.) was added, the polymerization reaction gradually started to obtain a lump polymer.

The product was chopped, dried in hot air drying at 120 ° C., and the dried product was crushed to obtain a resin powder. This powdered resin was sieved to select one having a particle size of 60 to 100 mesh.

Example 1 The acrylic acid used in Comparative Example 1 was distilled, and the unpurified aqueous solution of caustic soda was purified using activated carbon. Polymerization and treatment were carried out in the same manner as in Comparative Example 1 using these solutions. The iron ion concentration in this monomer aqueous solution (A) was 0.03 pp.
It was m.

Example 2 The acrylic acid used in Comparative Example 1 was distilled, and the unpurified aqueous solution of caustic soda was purified using activated carbon and a chelate ion exchange resin. Polymerization and treatment were carried out in the same manner as in Comparative Example 1 using these solutions. The iron ion concentration in this monomer aqueous solution (B) was 0.01 ppm.

Example 3 Using the same purified monomer aqueous solution (A) as in Example 1,
It was similarly polymerized and treated in a nylon-coated Dewar.

Example 4 Using the same purified monomer aqueous solution (B) as in Example 2,
It was similarly polymerized and treated in a nylon-coated Dewar.

With respect to the polymers obtained in the above Comparative Examples and Examples, the following tests were carried out for pure water absorption, salt water absorption and unpolymerized monomer content. The results are shown in Table 1.

A. Water absorption capacity of pure water 0.1g of powder sample is precisely weighed in a beaker of 300ml, 200ml of distilled water is added, and stirred with a magnetic stirrer for 3 hours. After that, filter into 100mesh wire mesh mass (70 × 70 × 70mm),
After leaving it for 5 minutes, wipe the wire mesh water with a paper towel,
Weigh.

B. Salt water absorption capacity 0.5g of powder sample is precisely weighed in a 300ml beaker, 200ml of 0.9% NaCl aqueous solution is added, and stirred with a magnetic stirrer for 3 hours. Then, after filtering in a 100 mesh wire mesh mass (70 × 70 × 70 mm) and leaving it for 5 minutes, the wire mesh is wiped with water and weighed.

C. Unpolymerized Monomer Amount 0.4 ml of powder sample is precisely weighed in a beaker, 200 ml of 0.9% NaCl aqueous solution is added, and stirred with a magnetic stirrer for 3 hours. After that, the solution is filtered into a 100 mesh wire mesh mass (70 × 70 × 70 mm), and the filtrate is collected. The filtrate was quantified by HPLC under the following conditions.

・ Device: HITACHI L-6000PUMP, L-4000UV DETECTOR ・ Column: HITACHI GEL # 3056 (4 × 150mm) ・ Fluid phase: 0.1% H ₃ PO ₄ aqueous solution ・ Detector; UV195nm (C) Effect of the Invention According to the present invention, since a water absorbent resin having a low content of unpolymerized monomers can be produced very easily and at low cost, the sanitary article industry that seeks a highly safe water absorbent resin and It can make a significant contribution to the farm sector.

Claims (1)

[Claims] 1. A method for producing a water-absorbent resin, which comprises polymerizing a monomer having an α, β-unsaturated carboxylic acid salt as a main component and a heavy metal content of 0.1 ppm or less.