JPS6399211A - Production of modified water-absorbing resin - Google Patents

Abstract

PURPOSE:To obtain the titled resin having excellent water absorption rate, hardness of gel, water retention characteristics, etc., comprising an inert part of non-rigid gel and part in the vicinity of the surface consisting of rigid gel having excellent permeability, by introducing crosslinking too the part in the vicinity of the surface of water-absorbing resin of polyacrylic acid type. CONSTITUTION:First, an aqueous solution containing a radical initiator of peroxide (e.g. potassium persulfate, etc.) is brought into contact with the surface part of water-absorbing resin of polyacrylic acid type so that the resin is made into a hydrous swollen state. Then the resin is heated to preferably 120-130 deg.C, the radical initiator is decomposed so crosslinking is introduced into polymer molecule chains in the vicinity of the surface of the resin to give the aimed resin. The aqueous solution is preferably brought into contact with the powder of the water-absorbing resin of polyacrylic acid type by a method wherein the aqueous solution is sprayed on the powder of the resin while stirring the powder.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a modified water absorbent resin. This invention relates to a novel method for improving properties such as water absorption rate.

Water-absorbing resins absorb several hundred times their own weight in water, and also have excellent absorbency for urine and blood, and are used as absorbents in disposable diapers, napkins, and the like.

BACKGROUND OF THE INVENTION Various types of water absorbent resins are known, but polyacrylic acid resins are the most popular.

As polyacrylic acid-based water-absorbing resins, starch-acrylonitrile polymer hydrolyzate, starch-acrylic acid graft cross-linked polymer, saponified vinyl acetate-acrylic acid ester copolymer, polyacrylic acid crosslinked polymer, polyacrylic Examples include acid-based crosslinked copolymers.

Polyacrylic acid-based water-absorbent resins are composed of acrylic acid monoband units as a main component, and some or all of the carboxyl groups contained therein are neutralized with alkali metal salts and are substantially insoluble in dihydrocarbons. is defined as a polymer with a high degree of stimulatory properties.

The water absorption capacity is adjusted by the density of crosslinks or bonding points, ranging from soft gels that absorb 0 water about 1000 times their own weight to hard gels that have a water absorption capacity of about 100 times, and are applied depending on the purpose. .

Water-absorbing resins are put to practical use in the form of powder or film. Performance is evaluated by water absorption capacity, water absorption rate, hardness of water absorption gel, etc. In general, soft gels with a high water absorption capacity tend to have a lower water absorption rate. The cause is thought to be due to the following phenomenon.

When soft gel particles with a low degree of t6'W come into contact with water, the surface portion absorbs water and traps water, preventing water from penetrating into the interior. On the other hand, a hard gel with a high crosslinking density has a lower water retention capacity than a soft gel A, and does not prevent water from penetrating into the inside. As a result, the hard gel has a higher water absorption rate than the soft gel.

Such a current ring is also seen in the case of particle aggregate powder6
Since the surface of soft gel cypress is covered with soft gel, it tends to form lumps, and so-called mama-ko-gen-gai can be seen. When a lump is formed, water permeation throughout the powder is hindered, and the water absorption rate is extremely reduced.

The formation and rate of water absorption are controlled by the permeability of water into the water-absorbing resin. There is an inverse relationship between permeability and water retention, and when a soft gel with low permeability and high water retention is formed on the surface, the mechanical properties of the gel particle aggregate deteriorate.

If the inside of a granular, bite-like, or fibrous water-absorbing resin is composed of a soft gel with a high water absorption capacity and the outside is a hard gel with good permeability, the water absorption rate, gel hardness, water absorption capacity, and water retention can be improved. A well-balanced and desirable product is obtained.

There has been an attempt to improve water absorption performance by forming secondary crosslinks on the surface of water-absorbing resin particles (Japanese Patent Laid-Open No. 56-131).
608.57-44627.58-42602.58-
117222 etc.).

Problems to be Solved by the Invention All of the known methods for modifying the surface of a polyacrylic water-absorbing resin by introducing crosslinking into the surface of the polyacrylic water-absorbing resin do not reduce the reactivity of the carboxyl group or carboxylate group contained in the polymer. We are using. As a crosslinking agent, an organic compound such as a diglycidyl compound represented by ethylene glycol diglycidyl ether is used.

These methods have problems such as requiring high temperatures or long periods of time for the crosslinking reaction, leaving unreacted crosslinking agent behind, and requiring complicated processes that burden manufacturing costs. From this point of view, it is not preferable to use organic compounds whose safety for the human body has not been confirmed. Furthermore, the reforming effect is not necessarily sufficient.

Means for Solving the Problems The present inventor first heated an aqueous composition of a polymer composed of acrylic acid monomer units as a main component in the presence of a water-soluble peroxide radical initiator to generate radicals. Invented a method for producing water-absorbent resins that introduces crosslinks into polymers (
JP-A-6L-296003).

The present inventor conceived the idea of using the previously invented radical crosslinking method as a means to solve the above-mentioned problems, and completed the method of the present invention.

The outline of crosslinking of a polyacrylic acid polymer using a water-soluble peroxide radical initiator is shown below.

(1) In a dry state, the formation of crosslinks is inhibited, and the presence of an appropriate amount of water is necessary.

(2) The crosslinking efficiency of the initiator varies depending on the temperature and moisture content, and each has an optimal range.

(3) Hydrophilic polyfunctional unsaturated compounds are effective as crosslinking aids.

Polyacrylic acid-based polymers, especially in the case of metal salts, do not have a melting point and are thought to have an extremely high secondary transition point (Tg), which is said to be the boundary between molecular motion and reactivity. It is explained that water acts as a plasticizer for the polymer, inducing molecular movement and reactivity.

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

As a means for uniformly permeating a relatively small amount of the aqueous solution into the surface of the resin powder particles, a mixed solvent containing a volatile water-soluble solvent such as methanol, ethanol, acetone, or methyl ethyl ketone may be used as the solvent. It is preferable to spray the aqueous solution while stirring the resin powder.

As peroxide radical initiators, in addition to persulfates such as ammonium persulfate, potassium persulfate, and sodium persulfate, and inorganic compounds such as hydrogen peroxide, organic acid peroxides such as acetic acid and oxalic acid are used. . Also, as a crosslinking aid, N
, N-methylenebisacrylamide, ethylene glycol bis(meth)acrylate, and polyethylene glycol(meth)acrylate. The action of the crosslinking aid is considered to be similar to that in the case of crosslinking of ethylene polymers using known organic peroxides.

The amount of initiator used is 0.01 to 10% by weight based on the resin.
However, preferably a range of 0.1 to 2% by weight usually gives preferable results.

Water is pttlV! It is necessary to swell the 8r-s surface part and to infiltrate the initiator, and in this case, the resin may be in a water-containing state in advance to the extent that it does not block. The amount of water added to the resin as an aqueous solution 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 necessarily 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 volatile organic solvents such as acetone are distilled off before the reaction, the amount used is not particularly limited. Water holder K Engineering F Ren Recol, Flopylene Recol.

Polyhydric alcohol compounds such as glycerin also act as plasticizers for the resin, but since these compounds are non-volatile, they must be chosen to be inert to the initiator.

For the purpose of introducing crosslinking into the resin surface and improving water absorption performance, the thickness and crosslinking density of the crosslinking layer are important, and the composition and amount of the aqueous solution are adjusted respectively.

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

Water-absorbing resin particles having crosslinks introduced into the vicinity of the working surface have different crosslinking densities and different water absorption behavior between the inside and outside. Particles in which the outside of a soft gel with a high water absorption capacity is coated with a hard gel with good water permeability exhibit fast water absorption and large water absorption capacity either by themselves or as an aggregate. Furthermore, since water uniformly permeates into the interior of the particles at the initial stage of water absorption, the surface feels dry and the aggregate feels hard. the result,
It exhibits desirable properties as an absorbent for sanitary materials such as disposable diapers. Similar effects can be obtained in the case of a film or a fibrous water-absorbing resin.

Example 1: Production of water-absorbing resin powder An aqueous solution KN was prepared by mixing 66.6 parts of a caustic soda aqueous solution with a concentration of 48X, 72 parts of acrylic acid, and 85 parts of water.

A stock solution was prepared by adding 0.08 part of N'-methylenebisacrylamide and 0.1 part of potassium persulfate. The stock solution was placed in a container as a layer with a thickness of about 1 cIrL, and polymerization was initiated by heating to about -50°C in a nitrogen atmosphere. The reaction temperature was controlled so as not to exceed 80° C. by cooling the container, and a rubbery polymer was obtained after 10 minutes. The polymer was dried and ground into powder. The powder was sieved to collect resin powder with a particle size of 32 to 200 mesh.

The water absorption capacity was determined by heating the hydrogel swollen in pure water and 0.9% saline for 60 minutes on a wire mesh of 80 mesh and measuring the weight, and expressed as the amount of water absorbed per resin II. The water absorption capacity was 610 times that of pure water and 59 times that of saline water (0.99).

8': While stirring 50 parts of the resin powder, 5 parts of an aqueous solution of ammonium persulfate at a concentration of 62 was sprayed onto the powder to uniformly impregnate the surface of the resin powder. Next, the impregnated material was heated and dried for 6 minutes in a superheated steam blowing dryer at a temperature of 130°C. The water content of the resulting resin was 5%.

The water absorption rate of the resin powder was measured by the following method. Diameter 15
Add 0.9% saline to a 1.5α thickness in an α petri dish, place a polyurethane sponge with a diameter of 3 cm and a height of 3 crn, and place a glass filter plate (positive 2) with a diameter of 30 on top of it. so that the liquid reaches the surface of the glass filter. 0.0 resin powder on top of the glass filter.
3 g was placed on it, and the amount of water absorbed 10 minutes later was measured and compared.

The water absorption capacity of the crosslinked sample is 560 for pure water.
It was 54 times that of 0.9% saline. On the other hand, the amount of water absorbed (water absorption capacity) expressed as water absorption rate converted into resin 1y was 33 times that of the treated sample, compared to 10 times that of the untreated sample. The unprocessed sample remains as it is due to this formation.
This is because uniform swelling of the powder was hindered. 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% potassium persulfate aqueous solution and 8 parts of acetone was sprayed to impregnate the resin powder.

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 550 times that of pure water, 0
．． It was 52 times that of 9% saline. On the other hand, the water absorption rate expressed as the water absorption rate was 38 times.

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

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

After evaporating the ketone from the impregnated material, it was heated and dried for 10 minutes in a steam-blown dryer at a temperature of 9120°C. The moisture content of the produced resin was 6 ta. The water absorption capacity of the cross-layered sample was 530 times that of pure water, and 0
．． It was 51 times that of 9% saline. On the other hand, the water absorption capacity expressed as the water absorption rate was 40 times. Example 4 A commercially available water absorbent resin of the starch/polyacrylic acid graft type (Sanyo Kasei: Sunwet 1Ni-300) was used.

The water absorption capacity was measured to be 670 times that of pure water and 64 times that of 069% saline.

While stirring 50 parts of the resin powder, a mixture of 4 parts of a 7% potassium persulfate aqueous solution (with 6 parts of acetone added thereto) was sprayed to impregnate the resin powder.

Examples 1 and 11 after evaporation of the acetone from the impregnation]
I heated and dried it in one way.

3. The water absorption capacity of the 8 physical samples is pure water; (590
The water absorption rate, expressed as water absorption rate, was 37 times higher than that of the untreated sample, compared to 15 times that of the untreated sample.

As shown in the effect examples, by the method of the present invention, the water absorption rate of the water-absorbing resin powder with the bridge introduced in the vicinity of the surface is significantly increased, the formation of this structure is prevented, and the hardness of the water-absorbing gel is improved. It also improves your mood. In the case of a film or fibrous resin, the same effect as in the case of powder is exhibited.

Claims (4)

[Claims] (1) The surface of a polyacrylic acid-based water-absorbent resin is brought into contact with an aqueous solution containing a peroxide radical initiator to form a hydrated and swollen state, and the resin is heated to cause the radical initiator to decompose, thereby decomposing the surface of the resin. 1. A method for producing a modified water-absorbing resin, which comprises introducing crosslinks into the molecular chains of a polymer. (2) The method according to claim 1, wherein 60 to 90 mol% of the carboxyl groups contained in the polyacrylic acid-based water-absorbing 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) Claim 1, in which the surface of the polyacrylic acid-based water-absorbing resin is brought into contact with an aqueous solution containing a peroxide radical initiator to form a hydrated and swollen state, and the resin is heated in a superheated steam atmosphere. the method of.