JPS6155108A - Production of highly water-absorptive resin - Google Patents

Abstract

PURPOSE:To produce economically a highly water-absorptive acrylic resin by using a specified partially neutralized acrylic acid/maleic acid mixture as a monomer mixture and thereby effecting a high-concentration rapid polymerization in good controllability. CONSTITUTION:An aqueous solution (of a concentration >=35wt%) of a partially neutralized monomer mixture of 98-85mol% acrylic acid and 2-15mol% maleic acid, wherein 60-80% of their carboxyl groups are in the form of an alkali metal salt, is placed in a continuous mixer with an open top and rapidly polymerized in the presence of a water-soluble radial initiator (e.g., potassium persulfate) without removing the heat of polymerization by external cooling, while the water in the system is being evaporated. In this way, a moderately crosslinked, highly water-absorptive resin can be produced in good efficiency. It is also possible to use 20pts.wt. of below, per 100pts.wt. said monomer mixture, polysaccharide (e.g., starch) together with said mixture.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polyacrylic acid-based super absorbent resin. Super absorbent resins have high water absorption and water retention properties, and are used in sanitary products such as disposable diapers.

The behavior of radical polymerization of a partially neutralized alkaline aqueous solution of acrylic acid using a persulfate as an initiator varies significantly depending on the degree of neutralization and concentration. The lower the degree of neutralization and the higher the concentration, the faster the polymerization will proceed, and if the temperature rises due to the heat of polymerization and exceeds the limit of control by external cooling or evaporation of water from the polymerization system, it will become explosive. A phenomenon is observed. The explosive progress of the polymerization reaction results from an abnormal increase in radical concentration due to rapid decomposition of the initiator.

For example, a concentrated aqueous solution of acrylic acid polymerizes explosively, and the product has a high crosslink density and a low water absorption capacity.

On the other hand, the polymerization of an aqueous solution of sodium acrylate is slow and the product is water-soluble.

A key point in the synthesis of polyacrylic acid-based superabsorbent resins is the control of crosslinking density. It is preferable that the superabsorbent resin is neutral from the viewpoint of use, and the neutralization point of the carboxyl group is approximately 7.
The practical neutral range is 60-80% neutralization.
within the range of Since the polymer is a gel, it is difficult to neutralize it later with acid or alkali.
It is rational to aim for neutralization during polymerization.

The polymerization behavior of a neutral aqueous solution of acrylic acid naturally varies depending on the concentration, assuming the presence of an appropriate amount of initiator to complete the polymerization. Generally, at low concentrations, the resulting polymer has a low crosslinking density and contains water-soluble parts, and on the other hand, at high a degrees,
Polymerization occurs explosively and crosslink density increases, but its control becomes difficult.

The self-crosslinking mechanism in aqueous solution polymerization of acrylic acid can be explained in a simplified manner as follows. The initiator radical reacts with the 7-mer to produce a polymer through propagation radicals, but as polymerization heat accumulates and the temperature rises, decomposition is accelerated, and the rate at which hydrogen is extracted from the polymer to produce polymer radicals increases. increases. Polymer radicals recombine and crosslinks are introduced.

In polyacrylic acid-based super absorbent resin, moderate crosslinking t
- As a means of interfering: (1) controlling self-crosslinking; C
2) A method of completely suppressing self-crosslinking and using a crosslinking agent is mentioned. Of course, it is also conceivable to use both of them together.

To suppress self-crosslinking, α) lower the monomer concentration. C2) Adopt a polymerization method that thoroughly removes polymerization heat, such as reverse phase suspension polymerization or thin layer polymerization.

However, from an industrial perspective, these methods have low productivity;
The cost disadvantage cannot be avoided. Mayu, an attempt to control self-crosslinking by reverse phase 1b85 polymerization without using any crosslinking agent and using a relatively concentrated solution (Fumitoe, JP-A-53-46389
) can also be seen.

The concentration at which a partially neutralized aqueous solution of acrylic acid in a neutral range undergoes explosive polymerization and self-crosslinking is generally 20 or more.

There is an attempt to produce a self-crosslinking superabsorbent resin by rapid polymerization by evaporation of water to remove polymerization heat without using a crosslinking agent (Japanese Unexamined Patent Publication No. 56-70011; 57-38809).
; 58-160302), there is also a method of adjusting the crosslinking density by adding a small amount of a polyfunctional monomer at that time (Japanese Patent Laid-Open No. 57-8210).

As mentioned above, self-crosslinking originates from an increase in the concentration of radicals as the temperature of the polymerization system increases, and the self-crosslinking density also changes depending on the degree of heat dissipation depending on the initiator concentration and the scale of the polymerization system. Therefore, the addition of a small amount of crosslinking component is one way to adjust the crosslinking density of polymer groups, and it does not constitute an essential difference.The characteristics of the rapid polymerization method described above are The advantage of this method is that it is possible to introduce crosslinking, mainly self-crosslinking, into a highly concentrated aqueous solution of partially neutralized acrylic acid without using an organic solvent as in reverse-phase suspension polymerization.

This method is attractive as a simple and inexpensive production method, but on the other hand, when carried out on an industrial scale, the polymerization reaction tends to run out of control. Even in a polymerization apparatus designed to evaporate a large amount of water in the polymerization system at once, the polymerization temperature significantly exceeds 100° C., making it difficult to control. As a result, the crosslinking density fluctuates, making it difficult to obtain a uniform high-quality product, which is a drawback.

A method is known in which acrylic acid is polymerized in the presence of polysaccharides such as starch in order to improve the performance of polyacrylic acid-based superabsorbent resins, particularly the water absorption rate.

In this case, g-shift polymerization of acrylic acid occurs in starch in an aqueous solution, and both are chemically bonded.

Polysaccharides can also be added to the rapid polymerization system to modify the product. Addition ik is acrylic acid partially neutralized product 10
A range of not exceeding 20 parts by weight relative to 0 parts by weight is suitable.

The present inventor has conducted extensive research into methods for controlling the rapid polymerization reaction of acrylic acid at high concentrations, including when polysaccharides are added, and has developed the method of the present invention.

In the present invention, acrylic acid contains 3 to 3 to 30% of the total amount of maleic acid.
The degree of neutralization of the mixture added with a composition of 15 mol is 60-80
Neutralize with alkali within the range of
% or more of the polysaccharide in the presence of a water-soluble radical initiator, with or without the addition of not more than 20 parts by weight of polysaccharide based on 100 parts by weight of the partially neutralized product. Provided is a method for producing a highly water-absorbing resin, which is characterized in that a moderate amount of crosslinking is introduced by rapidly polymerizing while evaporating water in the polymerization system without removing heat of polymerization by external cooling. do.

The present inventor has developed a method for producing a super-absorbent resin, in which a 30% or more aqueous solution of a mixture of acrylic acid and methacrylic acid added as a minor component, neutralized with caustic alkali, is rapidly polymerized in the presence of a water-soluble radical initiator. (Japanese Unexamined Patent Publication No. 59-8)
1319).

The explosive polymerization reaction of an aqueous acrylic acid solution can be controlled by copolymerizing it with a relatively small amount of methacrylic acid, but the water absorption capacity of the product cannot be controlled by, for example, acrylic acid homopolymerization by reverse-phase suspension polymerization. Slightly inferior in comparison.

The present inventors then determined that (1) the copolymerization component is physiologically harmless. (2) Excellent water absorption ability. (3) It is inexpensive. and (4) research on monomers that satisfy the conditions of controlling the polymerization reaction with the addition of a small amount, and as a result, maleic acid according to the present invention was discovered and the present invention was completed.

Maleic acid is usually added as maleic anhydride, but
In aqueous solution it is converted to maleic acid. For example, 70 moles of carboxyl groups in a mixture of 92 moles of acrylic acid and 8 moles of maleic acid are neutralized with caustic soda.
0.3 for 7 mer mixture in 1 Ca 50% aqueous solution
% of potassium persulfate is added to prepare an aqueous solution. On the other hand, a comparable comparative sample to which maleic acid was not added was prepared, and the two samples were placed side by side and heated under the same conditions to compare the behavior of the polymerization reaction.

The comparative sample without maleic acid undergoes explosive polymerization with intense heat generation after the start of the reaction, and the reaction ends in an extremely short time. With the addition of maleic acid, rapid polymerization occurs in the sample of the present invention while releasing water vapor, but the reaction continues without runaway and eventually terminates.The product is both hydrated and then a gel, but the weight is Then, the sample of the present invention is much lighter. This means that more water was evaporated in the maleic acid copolymerized sample by the heat of polymerization.

Both samples were washed with water, the remainder was weighed, and after drying, the following was found: 7? ,. Although acrylic acid homopolymer contains a large amount of water-soluble portion, maleic acid copolymer is almost completely converted into a water-retaining gel that is insoluble in water, and also has a much higher water absorption capacity.

My friend explained the effect of maleic acid using an actual example, and after conducting many experiments under various conditions, he came to the following conclusion.

In a rapid polymerization reaction in which polymerization occurs rapidly while water in the polymerization system is evaporated without attempting to remove the heat of polymerization by external cooling, (1) the polymerization rate is high. (2) The generation of polymerization heat and the evaporation of water are balanced, and the reaction continues without runaway. (3) Without leaving any water-soluble parts in the product,
Complete polymerization. (4) It is important that the product has high water absorption and water retention capacity, and the method of the present invention satisfies all of these requirements.

Although these requirements are met to a large extent by the copolymerization of methacrylic acid according to the prior invention of the present inventors, the following drawbacks can be cited as compared to the case of maleic acid according to the present invention. (
1) It is necessary to add a large amount to control the polymerization reaction. (2) Water-soluble portion tends to remain, (3) Water absorption and water retention ability are poor, η
) It is disadvantageous in terms of cost.

Of course, a method of using maleic acid and methacrylic acid in combination is also conceivable, but as is clear from the above comparison, such a method is naturally included in the scope of the present invention as a bypass method.

The copolymerization composition of maleic acid is from 2 to 15 mol, and the effect is exhibited by adding a small amount, but if it is less than 2 mol, it is not sufficient, and if it is more than 15 mol, the reaction rate decreases and the self-crosslinking density decreases. As a result, a water-soluble portion remains. The crosslinking density can be adjusted by adding a crosslinking agent, and examples of the crosslinking agent include polyfunctional compounds such as N,N'-methylenebisacrylamide and ethylene glycol di(meth)acrylate. However, the use of these crosslinking agents is not preferred from a hygienic standpoint in product use, and is also disadvantageous in terms of cost.

The method of the present invention is characterized in that it is possible to adjust the self-crosslinking density without originally using a crosslinking agent, but in some cases, it may be preferable to use a small amount of a crosslinking agent as an auxiliary. . However, this is an auxiliary means and can be eliminated by selecting conditions such as composition 1 maleic acid polymerization initiator addition amount and monomer concentration. In this sense, a method of adding a trace amount of a crosslinking component to the polymerization system is clearly within the scope of the present invention.

Partially neutralized to the neutral range, the concentration of the aqueous solution of the nine monolithic mixture is 3
It is 5% or more, and this is based on the economical reason of making drying easier by lowering the moisture content of the produced polymer as much as possible, and the reactive factor of effectively forming self-crosslinking.

The concentration is usually 40% or more, particularly in the range of 50 to 60%, and the lower limit is set at 35%.

Examples of polysaccharides selectively used in the present invention include various starches, modified starches, cellulose u-conductors, and the like. Preferred polysaccharides are starches, including corn, corn, and starch. Starch such as wheat is used. Also preferred are soluble starches and α-starch. Lots of 8!
The effect of adding jM is as described above.

Examples of water-soluble radical initiators include inorganic compounds such as persulfates and hydrogen peroxide, and water-soluble organic peroxides such as peracetic acid and t-butyl hydroperoxide, but water-soluble azo compounds are unsuitable. It is. This is because the initiator radical does not have the ability to polymerize monomers, but the ability to extract hydrogen atoms from polymer molecules is necessary for self-crosslinking.

Persulfates such as potassium persulfate and ammonium persulfate are preferred.

Polymerization methods for carrying out this invention can take a variety of forms. The key to a polymerization reactor is to balance the reaction heat generated during rapid polymerization with the water evaporation rate. Although the reaction rate can be adjusted by copolymerization, a design that avoids excessive accumulation of polymerization heat is required. For example, a speed reading mixer with an open top is suitable. Although the polymerization reaction is preferably initiated in a nitrogen atmosphere, the inhibiting effect of oxygen is prevented by the water vapor generated as the reaction progresses. The reaction usually completes in a short time, within 15 minutes, and a considerable amount of water is evaporated and removed during the reaction process. The product is made into a product through grinding and drying processes.

Example 1 A mixture of 36 parts of acrylic acid and 4 parts (7.4 mol) of maleic anhydride was mixed with 16.5 parts of caustic soda (purity 93%).
Dissolve in an aqueous solution containing 4 parts, and add water to make the total 100 parts (partially neutralized monomer concentration 49 parts). Add 170.15 parts of persulfuric acid to this aqueous solution to form a circle.

The aqueous solution was added to a flat-bottom reaction vessel with an open top, and the liquid layer had a thickness of 25 cm.The inside of the reaction vessel was purged with nitrogen, and heated from the bottom of the reaction vessel until the polymerization reaction started. . Polymerization was initiated at a temperature of about 70° C., heating was discontinued, and the reaction occurred almost adiabatically with the evolution of water vapor. The reaction was substantially completed in about 15 minutes after the start of the reaction. The temperature inside the reaction system rose to a maximum of 112°C. The product was hydrated and formed into a gel, which was ground and dried in a hot air circulation dryer at 120°C.

Measurement of water absorption capacity is carried out as follows.

Place the sample in a 0.55"t beaker, add 500 mj of ion exchange water or 100 mj of 0.9 saline solution,
After the gel was left to stand for the processing time, the gel was filtered through an 80-mesh wire mesh and weighed to determine the amount of water absorbed per 1f of polymer.

The water absorption capacity of the polymer obtained in the above experiment was 870% for ion-exchanged water and 98% for 0.9% saline. The temperature compound aqueous solution is placed in a reaction vessel with an open top and a liquid thickness of z5c.
It was confirmed that the reaction could be controlled without runaway by adding 1r1 and rapidly polymerizing while evaporating water without removing the heat of polymerization by external cooling.

For comparison, when a corresponding aqueous solution of acrylic acid alone was polymerized under similar conditions, the reaction occurred explosively and the contents were scattered. The remaining part was treated in the same manner and the water absorption capacity was measured, and it was found to be 320 points for ion-exchanged water and 37 points for 0.9 saline solution. In addition, the presence of a considerable amount of water-soluble portion was observed in this product.

Example 2 Acrylic acid, maleic anhydride,
A sample water bath solution was prepared by adding methacrylic acid and soluble starch gold. Common conditions are that 70% of the carboxyl groups in the monomer mixture are neutralized with caustic powder, and the concentration of the partially neutralized monomer mixture is approximately 50%.

The potassium persulfate concentration is 0.3 factors (relative to the partially neutralized monomer mixture).

The aspect of the polymerization reaction was almost the same as in Example 1.

The composition and water absorption capacity of the samples are shown in Table 1.

Table 1

Claims (1)

[Claims] A partially neutralized mixture consisting of acrylic acid and maleic acid in which 60 to 80% of the carboxyl groups are alkali metal salts is 35% by weight or more, and the composition of maleic acid is in the range of 2 to 15 mol%. In a certain aqueous solution, 10 of the partially neutralized product
Heating in the presence of a water-soluble radical initiator with or without addition of polysaccharide in an amount not exceeding 20 g per 0 parts by weight, and removing the heat of polymerization by external cooling. A method for producing a super-absorbent resin, which is characterized in that a suitable amount of crosslinking is introduced by rapidly polymerizing while evaporating water in the polymerization system.