JPH01178509A - Preparation of water-absorbing resin - Google Patents

Abstract

PURPOSE:To form a water-absorbing resin in which a monomer scarcely remains and which has little soluble part for a short reaction time, by polymerizing mildly an aq. mixture of slurry acrylic acid wherein an alkali metal salt of acrylic acid precipitated in the initial stage of polymn. is dispersed under a reduced pressure. CONSTITUTION:An aq. mixture contg. a monomer contg. at least 70wt.% acrylic acid 60-90mol.% of carboxyl groups of which are neutralized with an alkali metal salt (pref. Na salt) and a radical initiator and wherein the alkali metal salt of acrylic acid precipitated in the initial stage of polymn. is dispersed in a slurry-like state is polymerized under a standing condition without stirring under a reduced pressure of 0.1-0.9atm. Precipitates of the alkali metal salt of acrylic acid contained in the monomer mixture promote smooth generation of steam in the early stage of polymn. and are dissolved and transferred to a polymer with the progress of the reaction to form a water-absorbing resin wherein the monomer scarcely remains and which contains little soluble part for a short reaction time.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing water-absorbing resins useful as water-absorbing and water-retaining materials. Polyacrylic acid-based resins that are made water-insoluble through crosslinking and have a high water-swellability are used as water-absorbing agents for sanitary napkins, disposable diapers, and the like.

Conventional technology Polyacrylic acid water absorbent resin has 60 carboxyl groups.
It is produced by polymerizing an aqueous solution of acrylic acid neutralized with an alkali metal salt, and is swollen in water but insoluble, so that a moderate amount of crosslinking is introduced.

The key to producing polymers with excellent performance is to form uniform and moderate crosslinks in high molecular weight polymers. In order to obtain a high molecular weight polymer, it is effective to increase the monomer concentration and conduct the polymerization at a relatively low temperature.

The concentration of the aqueous solution is limited by the solubility and degree of neutralization of the alkali metal acrylate salt. Potassium salt is n) I
Although it has higher solubility than Jium salt, it is less economical, so sodium salt is usually used.

Increasing the degree of neutralization lowers the saturation concentration, but in order to neutralize the pH of the hydrogel, the degree of neutralization is usually adjusted to between 70 and 80.

A problem in the production of polyacrylic acid-based water-absorbing resins lies in the control of the polymerization reaction. Polymerization of a monomer aqueous solution with a relatively high concentration occurs vigorously, and the temperature of the system rapidly rises due to the heat of reaction, resulting in a boiling state. - Since the reaction mixture becomes gel-like due to the formation of the polymer, evaporation of water vapor is hindered, and the reaction goes out of control, resulting in the so-called popcorn phenomenon. Added to this is the gel effect, which significantly increases the polymerization apparatus due to the increased viscosity of the solution, making temperature control more difficult and making it difficult to obtain a product of desirable quality.

Introduction of crosslinking involves self-crosslinking that occurs during polymerization of a highly concentrated aqueous solution using a peroxide-based radical initiator, copolymerization of polyfunctional unsaturated monomers, and post-crosslinking using a crosslinking agent active on the functional groups of the polymer. Methods such as these are known.

The following methods have been proposed as polymerization methods. (1
) Polymerization Q reverse phase suspension polymerization method by suspending and dispersing an aqueous monomer solution in a hydrocarbon solvent (Japanese Patent Application Laid-Open No. 53-46389),
(2) A method of polymerizing an aqueous solution containing a polyhydric alcohol using a redox radical initiator (Japanese Unexamined Patent Publication No. 55-84
304), (3) A method of polymerizing a highly concentrated aqueous solution neutralized with caustic potash in a thin layer (4? 1987-49714).

□ Problems to be Solved by the Invention In the polymerization of an aqueous acrylic acid solution, methods (1) and (2) above, in which the reaction heat is removed by external cooling, do not require the use of a large amount of organic solvent or long-term polymerization using special equipment. including economically disadvantageous factors such as Further, the method (3) has problems such as the use of expensive caustic potash and the fact that it is substantially difficult to control the polymerization temperature.

As a method for removing the reaction heat of solution polymerization, the most effective method is to use the latent heat of vaporization of the solvent, and in the case of an aqueous solution, it is particularly preferable because the latent heat of vaporization of water is high, but there are difficulties as described above.

The aqueous monomer solution becomes highly viscous gel-like as the polymerization progresses, and further becomes rubber-like as it crosslinks. Therefore, stirring the mixture during the polymerization process involves equipment difficulties. Furthermore, it is known that when a rubbery mixture is kneaded under strong shear stress, the polymer collapses, significantly impairing the quality of the product.

A method of polymerizing an aqueous solution by evaporating water under reduced pressure using a polymerization vessel equipped with a special stirring mechanism (#Open/Close 62-10
0507.62-227904), but requires complicated equipment and operations.

Means for Solving the Problems If the polymerization reaction is carried out under reduced pressure and smooth release of water vapor from the highly concentrated reaction system is possible, a polymer with high content and asci can be obtained.

The present inventor has discovered that by first polymerizing a slurry mixture containing pulp, the polymerization temperature can be effectively controlled due to the smooth generation of water vapor (Japanese Patent Application No. 184365/1982).
.

Polymerization conditions for obtaining a product with slight crosslinking and a small soluble portion include increasing the monomer concentration in the reaction mixture and keeping the polymerization temperature at a relatively low temperature of 100° C. or less. The soluble portion is derived from low molecular weight polymers. At the stage where the polymerization reaction progresses and a gel effect is observed, the termination reaction is suppressed and a high molecular weight polymer is produced. Low molecular weight polymers are mainly produced at the beginning of the polymerization reaction. Therefore, at the beginning of the reaction,
It is important to promote smooth generation of water vapor under reduced pressure.

Based on the above idea, research was conducted and the following remarkable facts were discovered.

When polymerizing a slurry-like high concentration mixture in which precipitates of alkali metal acrylate are dispersed at the beginning of polymerization, fine powder,
The precipitate acts like zeolite during distillation, providing smooth boiling conditions. The following comparative experiments demonstrate the features of the present invention.

(1) 42% aqueous solution of acrylic acid adjusted to a neutralization degree of 75% with caustic soda; (2) 5% concentration including precipitate.
A slurry mixture of 596 and (3) a slurry mixture prepared by adding 61 weight of purified wood pulp as a monomer thereto was polymerized using potassium persulfate, and the temperature rise of the anti-reaction system was compared. In case (1), there was a rapid temperature rise and a popcorn phenomenon, and the maximum temperature exceeded 120°C. In cases (2) and (3), water vapor was generated smoothly, with almost no temperature difference observed at around 100°C in the initial stage, and reached the maximum temperature after gelation, which was 110°C and 105°C, respectively. (2)
, (3) has a higher concentration and a higher polymerization reaction rate than (1), but the temperature is kept low due to the smooth generation of water vapor.

Although there is a difference in the maximum temperature between (2) and (3), it is a remarkable fact that there is almost no difference in the initial stage. All of the products undergo self-crosslinking and are water-absorbing resins. The water-soluble portion of the product was measured by the calcium precipitation method, and (1) >
The relationship (2) = (3) was found.

It is possible to control the polymerization temperature to a low temperature by polymerization under reduced pressure. In this case, the effect of (1) is significantly hindered, whereas (2) and (3) have a similar temperature lowering effect.

For neutralization of acrylic acid, the monomer concentration in the present invention is usually 50 to 60%, which is economical and compatible with the human body, and where precipitation occurs at the polymerization initiation temperature of 40 to 60C.

When an aqueous solution of caustic soda is added to an aqueous solution of acrylic acid with vigorous stirring, a stable slurry containing fine precipitates is obtained.

Addition of an inert solid powder is effective as an auxiliary agent that enhances dispersion stability and further promotes steam generation during polymerization. As solid powders that are inert to monomers and polymers, silica, alumina, Merck, mica, titanium oxide,
Examples include fine powders of inorganic substances such as glass and organic synthetic resins such as polyamide and polyester. The powder has a particle size of 10
A fine powder of less than a micron size is suitable. The amount added is not particularly limited, but is usually 20 parts or less per 100 parts of the polymer.

As a polymerization initiator, peroxides such as ammonium persulfate, potassium persulfate, and acid peroxide are used, and these peroxides are combined with reducing agents such as sodium sulfite and ascorbic acid. agent, azobis (4-cyanovaleric acid)
, azo-based initiators such as azobis(2-amidinopropane) hydrochloride are used, and the amount added is usually O,
OS~0.5% by weight.

Introduction of crosslinking is carried out by a known method. In the case of peroxide-based initiators, self-crosslinking occurs. Since self-crosslinking does not occur with an azo initiator, a method of copolymerizing a polyfunctional unsaturated compound such as methylene bisacrylamide or ethylene glycol diacrylate is selected.

Comonomers that may be added as minor components of the monomers in an amount of less than 300% by weight include acrylamide,
Includes methacrylic acid, maleic anhydride, fumaric acid, etc. However, the desired water-absorbing resin product can usually be obtained using acrylic acid alone.

The slurry mixture prepared in the above manner is placed in a layer in a polymerization vessel, and polymerized by heating under reduced pressure without stirring. As a continuous polymerization method, a belt conveyor type can be adopted. It is preferable to control the polymerization temperature to 100°C or less, and usually to 60 to 80°C, so that the polymerization is carried out under a reduced pressure of 0.4 to 0.8 atmospheres. The polymerization reaction is generally substantially complete within 10 to 20 minutes.

The precipitated alkali metal acrylate salt contained in the active monomer mixture plays a role in promoting smooth generation of water vapor at the initial stage of polymerization, and as the reaction progresses, it dissolves and converts into a polymer, and the monomer Almost no residue is observed, and a water-absorbing resin with a small soluble portion is produced in a short reaction time. Polymerization is carried out essentially in a static state, does not require expensive and special equipment, and is carried out with simple operations.

Example 1 An aqueous solution of 72 parts of acrylic acid and 14.5 parts of water was neutralized by adding 62.5 parts of a 48% aqueous solution of caustic soda with vigorous stirring. The product is a 55 part slurry mixture with finely divided precipitates of sodium acrylic acid salt dispersed therein. 0.1 part of potassium persulfate was added to the mixture, and the mixture was placed in a layer with a thickness of 2 cr 1M in a polymerization vessel, and after purging with nitrogen, the pressure was adjusted to 0.5 atm, and polymerization was carried out by heating from the bottom.

The temperature of the system rose to 8°C, and polymerization was completed in about 10 minutes. The product was chopped, dried and ground to obtain resin powder.

Powder sample 0.5 was mixed with pure water 1j> and 0.91 saline 200
- respectively, and after leaving it for 1 hour, add 80% of humanologel.
It was weighed in a furnace on a mesh wire mesh, and the water absorption capacity was 540 times that of pure water and 58 times that of 0.91 saline water.

The soluble portion was measured for Solution F of 0.91 saline in accordance with the method for measuring low polymers of sodium polyacrylate for food additives, and the precipitate formed by the addition of an aqueous calcium chloride solution was weighed. The welded part was determined within a few minutes.

Example 2 Methylenebisacryla<r in the O and OS parts was added to the monomer mixture of Example, and polymerized in the same manner as in Example 1.
A resin powder was obtained. Water absorption capacity is 380 times that of pure water.
45 times as much as 0.996 saline, and the soluble part is 2.1
It was regrettable.

Example 3.4 Finely divided silica (Aerosil,
5 parts of mica fine powder (average particle size 0.8 microns) (Example 3) and 5 parts of mica fine powder (average particle size 0.8 microns) (Example 4)
were added to each. The dispersion stability of the slurry was significantly improved by the addition of these solid powders, and no sedimentation was observed even after standing for a long time. Polymerization was carried out in the same manner as in Example 1, and the following results were obtained.

(Example 3) Water absorption magnification: 520 times (pure water), 56 times (0
, 996 saline solution, soluble portion 4.61) (Example 4) Water absorption capacity 530 times (pure water), 55 times (0
, 9 saline solution, soluble part 4.86) Comparative Example 1 Aqueous solution containing 72 parts of acrylic acid and 87 parts of water, concentration 48
Add the aqueous solution of caustic soda 'j, neutralization degree 75, concentration 4.
A 0% aqueous solution was obtained. 1 part of persulfuric acid (130%) was added to this, and polymerization was carried out in the same manner as in Example 1 to prepare a resin powder. The water absorption capacity was 580 times that of pure water, 60 times that of 0.996 saline, and the soluble portion was 19 times higher.

Claims (4)

[Claims] (1) Contains a monomer containing at least 70% by weight of acrylic acid in which 60 to 90 mol% of the carboxyl groups are neutralized with an alkali metal salt, and a radical initiator, and the alkali metal acrylic salt precipitated at the start of polymerization is A method for producing a water absorbent resin, which comprises polymerizing an aqueous mixture in the form of a dispersed slurry under reduced pressure of 0.1 to 0.9 atmospheres without stirring, and introducing crosslinking into the polymer. (2) The method according to claim 1, wherein the alkali metal is sodium. (3) The method according to claim 1, wherein the aqueous mixture further contains an inert solid powder as a dispersant. (4) The method according to claim 1, wherein the aqueous mixture contains a hydrophilic polyfunctional unsaturated compound as a crosslinking agent.