JPH078883B2 - Method for producing modified water absorbent resin - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a modified water-absorbent resin, and more specifically, it introduces secondary crosslinks in the vicinity of the surface of a polyacrylic acid-based water-absorbent resin. And a novel method for improving properties such as water absorption rate.

The water-absorbent resin absorbs several hundred times its own weight of water and also has an excellent absorption capacity for urine and blood, and is used as an absorbent in paper diapers, napkins and the like.

2. Description of the Related Art Although various types of water-absorbent resins are known, polyacrylic acid-based resins dominate.

As a polyacrylic acid-based water-absorbent resin, starch-acrylonitrile polymer hydrolyzate, starch-acrylic acid graft cross-linked polymer, vinyl acetate-acrylic acid ester copolymer saponified product, polyacrylic acid cross-linked polymer, polyacrylic acid Examples include cross-linked copolymers.

The polyacrylic acid-based water-absorbent resin is composed mainly of an acrylic acid monomer unit, and some or all of the carboxyl groups contained therein are substantially insoluble in water neutralized with an alkali metal salt. It is defined as a polymer with a high degree of swelling.

The water absorption capacity is controlled by the density of crosslinks or bonding points, and from soft gels that absorb about 1000 times the weight of water to hard gels with a water absorption ratio of about 100 times, they are applied according to their respective uses.

The water absorbent resin is practically used in the form of powder or coating. The performance is evaluated by the water absorption capacity, the water absorption speed, the hardness of the water absorbing gel, and the like. Generally, a soft gel having a large water absorption capacity tends to have a low water absorption rate. The cause is considered to be due to the following phenomenon.

When the soft gel particles having a low cross-linking density come into contact with water, the surface absorbs water and swells, catches water, and impedes the penetration of water into the interior. On the other hand, a hard gel having a high crosslink density has a lower water retention capacity than a soft gel and does not prevent water from penetrating inside. As a result, the hard gel has a higher water absorption rate than the soft gel.

This phenomenon is also observed in the case of aggregate powder of particles.
Since the surface of the soft gel particles is covered with the soft gel, lumps are easily formed, and a so-called lingering phenomenon is observed. The formation of moulds hinders the water from penetrating the entire powder, and the water absorption rate is extremely reduced.

As it is, its formation and water absorption rate are governed by the permeability of water into the water absorbent resin. The permeability and water retention are in inverse proportion to each other, and when a soft gel with low permeability and high water retention is formed on the surface, the mechanical properties of the gel particles as aggregates deteriorate.

If the inside of the granular, film-like or fibrous water absorbent resin is composed of a soft gel with a large water absorption capacity and the outside is a hard gel with good permeability, the water absorption rate, the hardness of the gel, the water absorption capacity, and the water retention A balanced and favorable product is obtained.

Attempts have been made to improve water absorption performance by forming secondary crosslinks on the surface of the water-absorbent resin particles (JP-A-56-131608,5).
7-44627, 58-42602, 58-117222, etc.).

Problems to be Solved by the Invention Known methods for introducing a cross-link into the surface of a polyacrylic acid-based water-absorbent resin for modification include all methods for improving the reactivity of a carboxyl group or a carboxylate group contained in a polymer. We are using. As the cross-linking agent, an organic compound such as a diglycidyl compound represented by ethylene glycol diglycidyl ether is used.

These methods have problems such as high temperature or long time required for the cross-linking reaction, residual unreacted cross-linking agent, and complicated process that burdens the manufacturing cost, and moreover, the products are often used as sanitary materials. In view of the above, it is not preferable to use an organic compound whose safety to the human body has not been confirmed. Moreover, the modifying effect is not always sufficient.

Means for Solving the Problems The present inventor previously heated an aqueous composition of a polymer composed mainly of an acrylic acid monomer unit in the presence of a water-soluble peroxide radical initiator to give a radical. A method for producing a water-absorbent resin in which crosslinking is introduced into a polymer by crosslinking has been invented (JP-A-61-296003).

The present inventor has conceived that the radical crosslinking method previously invented is used as a means for solving the above-mentioned problems, and completed the method of the present invention.

An outline of cross-linking of a polyacrylic acid polymer with a water-soluble peroxide radical initiator is shown below.

(1) In the dry state, the formation of crosslinks is hindered, and it is necessary to have an appropriate amount of water.

(2) The cross-linking efficiency of the initiator varies depending on the temperature and the water content, and each has an optimum range.

(3) A hydrophilic polyfunctional unsaturated compound is effective as a crosslinking aid.

Especially in the case of metal salt, the polyacrylic acid polymer has no melting point, and the second transition point (Tg), which is said to be the boundary between molecular motion and reactivity, is considered to be extremely high. Water is explained to act as a plasticizer for the polymer, inducing molecular motion and reactivity.

When the aqueous solution containing the peroxide radical initiator is brought into contact with the polyacrylic acid-based water-absorbent resin particles, the particles absorb the aqueous solution to form an aqueous composition containing the initiator in the vicinity of the surface.

As a means for uniformly permeating a relatively small amount of an aqueous solution into the surface of the resin powder particles, a mixed solvent prepared by adding a volatile water-soluble solvent such as methanol, ethanol, acetone or methyl ethyl ketone can be used as a melt. The aqueous solution is preferably sprayed while stirring the resin powder.

As peroxide radical initiators, in addition to persulfates such as ammonium persulfate, potassium persulfate and sodium persulfate, inorganic compounds such as hydrogen peroxide, organic acid peroxides such as acetic acid and oxalic acid are used. . Further, as a crosslinking aid, N,
Hydrophilic polyfunctional unsaturated compounds such as N'-methylenebisacrylamide, ethylene glycol bis (meth) acrylate, polyethylene glycol (meth) acrylate and the like can be mentioned. The action of the crosslinking aid is considered to be the same as in the case of crosslinking the ethylene-based polymer with a known organic peroxide.

The amount of the initiator used is in the range of 0.01 to 10% by weight with respect to the resin, but usually in the range of 0.1 to 2% by weight, a preferable effect is provided.

Water is necessary for swelling the resin surface and for penetrating the initiator, and the resin may be in a water-containing state in advance so that it does not block. The amount of water added to the resin as an aqueous solvent is not particularly limited, but is usually in the range of 2 to 20% by weight based on the resin.

Although a crosslinking aid is not always required, it is effective in increasing the crosslinking efficiency of the initiator depending on the reaction conditions. The amount used is generally in the same range as the initiator.

Since the volatile organic solvent such as acetone is distilled off before the reaction, the amount used is not particularly limited. In addition to water, polyhydric alcohol compounds such as ethylene glycol, propylene glycol, and glycerin also act as plasticizers on the resin, but since these compounds are non-volatile, those that are inactive to the initiator are You have to be chosen.

The thickness and crosslink density of the crosslink forming layer are important for the purpose of introducing crosslinks into the resin surface to improve the water absorption performance, and the composition and amount of the aqueous solution are adjusted respectively.

The crosslinking reaction occurs by radical decomposition of the initiator. From the decomposition rate of persulfate such as ammonium persulfate, which is a typical initiator, in an aqueous solution, it is estimated that the 1-minute half-life temperature of the initiator in the reaction mixture in a water-containing swollen state is about 120 ° C.
From this, it is considered that the appropriate reaction temperature is 120 to 130 ° C.
Since the water absorbent resin strongly adsorbs water, it does not easily dry even at a high temperature of 100 ° C. or higher, but it is necessary to maintain the water content during the reaction. As a preferred heating method,
Heating and drying in a superheated steam atmosphere can be mentioned.

Water-absorbent resin particles with cross-linking introduced near the working surface have different cross-linking densities inside and outside, and also different water-absorption behavior. The particles obtained by coating the outside of a soft gel having a large water absorption capacity with a hard gel having a good water permeability show fast water absorption and a large water absorption ability as such or as an aggregate. Further, since water evenly penetrates into the inside of the particles at the initial stage of water absorption, the surface gives a dry feel and a hard feel as an aggregate is obtained. As a result, it exhibits preferable properties as an absorbent for sanitary materials such as disposable diapers. In the case of a film or a fibrous water-absorbent resin, the same effect is exhibited.

Example 1. Production of water-absorbent resin powder 66.6 parts of a 48% strength aqueous caustic soda solution, 72 parts of acrylic acid,
A stock solution was prepared by adding 0.08 part of N, N'-methylenebisacrylamide and 0.1 part of potassium persulfate to an aqueous solution prepared by mixing 85 parts of water. The stock solution was put into a container as a layer having a thickness of about 1 cm, and heated to about 50 ° C. in a nitrogen atmosphere to start polymerization. The container was cooled and adjusted so that the reaction temperature did not exceed 80 ° C., and a rubbery polymer was obtained after 10 minutes. The polymer was dried and ground into a powder. The powder was sieved to collect a resin powder having a particle size of 32 to 200 mesh.

The water absorption capacity was obtained by separating the hydrogel swollen in pure water and 0.9% saline solution for 60 minutes on a wire mesh of 80 mesh and weighing, and expressed by the amount of water absorption per 1 g of resin. The water absorption capacity was 610 times for pure water and 59 times for 0.9% saline.

While stirring 50 parts of the resin powder, 5 parts of an aqueous solution of ammonium persulfate having a concentration of 6% was sprayed onto this to uniformly impregnate the surface part of the resin powder. Next, the impregnated material is heated with superheated steam.
It heat-dried for 6 minutes in a 130 degreeC dryer. The water content of the produced resin was 5%.

The water absorption rate of the resin powder was measured by the following method. 0.9% saline solution is added to a 1.5 cm thick petri dish with a diameter of 15 cm, a polyurethane sponge with a diameter of 3 cm and a height of 3 cm is placed on it, and a glass filter plate (No. 2) with a diameter of 3 cm is placed on it. ,
Allow the liquid to reach the surface of the glass filter. 0.3 g of resin powder was placed on the glass filter, and the water absorption amount after 10 minutes was measured and compared.

The water absorption capacity of the cross-linked sample is 560 against pure water.
It was 54 times that of 0.9% saline. On the other hand, the water absorption amount (water absorption ratio) converted into 1 g of the resin, which was expressed as a water absorption rate, was 33 times in the treated sample, compared with 10 times in the untreated sample. This is because the formation of the untreated sample left the uniform swelling of the powder prevented. On the other hand, the treated sample swelled uniformly.

Example 2. The resin powder prepared in Example 1 was used.

While stirring 50 parts of the resin powder, a mixture of 4 parts of a 7% aqueous potassium persulfate solution and 8 parts of acetone was sprayed and impregnated.

After evaporating acetone from the impregnated material, it was heated and dried in the same manner as in Example 1.

Water absorption capacity of cross-linked sample is 550 times that of pure water, 0.9%
52 times that of saline. On the other hand, the water absorption capacity expressed as the water absorption capacity was 38 times.

Example 3 The resin powder prepared in Example 1 was used.

While stirring 50 parts of the resin powder, 5 parts of an aqueous solution of ammonium persulfate having a concentration of 4% was added to N, N'-methylenebisacrylamide.
A mixture of 0.1 parts and 5 parts of methyl ethyl ketone was sprayed and impregnated.

After evaporating methyl ethyl ketone from the impregnated material, it was heated and dried for 10 minutes in a superheated steam blowing type dryer at a temperature of 120 ° C. The water content of the produced resin was 6%.

The water absorption capacity of the crosslinked sample is 530 times that of pure water, 0.9%
It was 51 times that of saline. On the other hand, the water absorption ratio expressed as the water absorption rate was 40 times.

Example 4 A commercially available water-absorbent resin of starch / polyacrylic acid graft type (Sanyo Kasei: Sunwet IM-300) was used. The water absorption capacity was measured to be 670 times for pure water and 64 times for 0.9% saline solution.

While stirring 50 parts of the resin powder, a mixed solution of 4 parts of potassium persulfate aqueous solution having a concentration of 7% and 6 parts of acetone was sprayed and impregnated.

After evaporating acetone from the impregnated material, it was heated and dried in the same manner as in Example 1.

The water absorption capacity of the crosslinked sample is 590 times that of pure water, 0.9%
It was 59 times that of saline. On the other hand, the water absorption ratio, which was expressed as the water absorption rate, was 37 times that of the untreated sample, which was 15 times.

As shown in the Examples, by the method of the present invention, the water-absorbent resin powder in which cross-linking has been introduced in the vicinity of the surface has a significantly increased water-absorption rate, and this generation is prevented as it is. Also improves. In the case of a film or fibrous resin, the same effect as in the case of powder is exhibited.

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Claims (4)

[Claims] 1. A surface of a polyacrylic acid-based water-absorbent resin is brought into contact with an aqueous solution containing a peroxide radical initiator to bring it into a water-containing swelling state, and the resin is heated to decompose the radical disclosing agent. A method for producing a modified water absorbent resin, which comprises introducing a crosslink into a polymer molecular chain in the vicinity thereof. 2. The method according to claim 1, wherein 60 to 90 mol% of the carboxyl groups contained in the polymer of the polyacrylic acid-based water-absorbent polymer are in the form of an alkali metal salt. 3. The method according to claim 1, wherein the aqueous solution contains a hydrophilic polyfunctional unsaturated compound as a crosslinking aid. 4. A method of heating a resin in a superheated steam atmosphere by bringing an aqueous solution containing a peroxide radical initiator into contact with the surface of the polyacrylic acid-based water-absorbent resin to bring the resin into a water-containing swelling state. Method described in section.