JPH0713109B2 - Method for producing beaded self-crosslinking water-absorbing polymer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is excellent in water absorption, and has high water-absorbent gel strength,
The present invention relates to a method for producing a superabsorbent polymer which has a large particle size and can be easily pulverized.

[Industrial application field]

The polymer obtained by the production method of the present invention is excellent in water absorption, absorbs a large amount of water and swells, but is insoluble in water, has a high water-absorbing gel strength, and has a large particle size of the polymer itself. Since it can be easily pulverized, it can be advantageously used for the production of various absorbent materials or various materials that are used by being absorbed and swollen.

[Prior art]

Conventionally, paper, pipes, nonwoven fabrics, sponge-like urethane resins, etc. have been used as water retention agents in sanitary napkins, paper naps, various sanitary materials, various agricultural materials, and the like. However, these materials have a water absorption amount of only about 10 to 50 times their own weight, so that a large amount of material is required to absorb or retain a large amount of water, and not only becomes extremely bulky. However, there is a drawback in that water is easily separated when pressure is applied to the absorbed material.

In order to improve the above-mentioned drawbacks of this type of water absorbing material, various high water absorbing polymer materials have been proposed in recent years. For example, a starch graft polymer (Japanese Patent Publication No. 53-46199, etc.), a cellulose modifier (Japanese Patent Publication No. 50-80376, etc.), a crosslinked product of a water-soluble polymer (Japanese Patent Publication No. 43-23462, etc.), Self-crosslinking alkali metal acrylate polymers (Japanese Patent Publication No. 54-30710, etc.), etc. have been proposed.

However, the water absorption amount of these highly water-absorbent polymer materials is still insufficient, and the gel strength at the time of water absorption is small,
In some of the above publications, the polymer obtained by drying is extremely hard, difficult to pulverize easily, and requires a large mechanical pulverizing force. Therefore, there are many problems in practical or industrial production. have.

The present inventors have already proposed a method for producing a water-absorbing material that has improved the above-mentioned drawbacks of conventional water-absorbing materials (Japanese Patent Application No. 59-275308).
Issue Bulletin. Hereinafter referred to as an earlier application. However, the water absorbing material produced by the method disclosed in the above-mentioned prior application also has various drawbacks. That is, since a nonionic surfactant having an HLB of 3 to 6 is used as a surfactant in the water-in-oil type reverse phase suspension polymerization method, the obtained polymer has a particle size of 100 μm or less and is an extremely fine powder. Will be. Therefore, when handling powder, it is necessary to take measures against dust. Further, the gel strength at the time of absorbing water is still insufficient, and the emergence of a polymer having more excellent shape retention has been desired.

[Problems to be solved by the invention]

The present invention improves the above-mentioned drawbacks, further improves the gel strength at the time of absorbing water while maintaining high water absorption, and the reproducibility of a superabsorbent polymer which has an extremely large particle size of the polymer and can be easily pulverized. It is intended to provide a good manufacturing method.

[Means for solving problems]

(Structure of the Invention) The present inventors have conducted various studies to solve the above-mentioned problems, and as a result, the acrylic acid-based monomer was added in the presence of a water-soluble radical polymerization initiator, a dispersion medium, a surfactant and water. In the method of polymerizing by water-in-oil type reverse phase suspension polymerization method, a copolymer of an α-olefin having a carbon number of 16 to 60 and an α, β-unsaturated polycarboxylic acid anhydride or a surfactant thereof Using a derivative, 50 mol% or more of all the carboxyl groups of the acrylic acid monomer are neutralized with an alkali metal salt, and the acrylic acid monomer concentration in water in the polymerization reaction system is 30% by weight or more, and crosslinking By polymerizing in the absence of an agent, it has an extremely large water absorption capacity, specifically, a water absorption amount of 700 times or more of its own weight, and a large gel strength, and the particle size of the polymer is extremely large. Has an average particle size of 100μ Found that beaded self-crosslinking water-absorbing polymers that can be easily crushed can be easily obtained in the above, it has been led to completion of the present invention.

(Background and characteristics of the invention) The copolymer of α-olefin and polycarboxylic acid anhydride or a derivative thereof used as a surfactant in the water-in-oil type reverse phase suspension polymerization method of the present invention is JP-A-57-74
Japanese Patent No. 309 discloses it as a method for producing a bead-shaped water-soluble polymer, and this technique can be applied to the present invention.

However, the most different point between the present invention and the invention disclosed in the above-mentioned Japanese Patent Laid-Open No. 57-74309 is that the produced polymer is soluble or insoluble in water. Here, insoluble means that when a polymer is put into water and stirred at room temperature, it absorbs water and swells to form a gel, but when stirring is stopped and left to stand, fluidity is lost or phase separation occurs. Means that a uniform aqueous solution is formed by stirring, and this state continues even if stirring is stopped and the mixture is left at the same temperature.

The polymers produced according to the present invention are essentially insoluble in water and have the unique property of absorbing and retaining water. In order to have this property, it is essential in the production thereof to cause a crosslinking reaction to insolubilize the polymer. When the water-soluble polymer is mixed with the water-insoluble polymer according to the present invention, not only the water absorption performance is deteriorated, but also the water absorption gel strength is extremely decreased,
The water-soluble polymer is excluded for the purpose of the present invention because the polymer after water absorption has a slimy feel and does not give a dry feel and is difficult to handle.

In the present invention, acrylic acid or an acrylic acid / methacrylic acid mixture is used, 50 mol% or more of all the carboxyl groups are neutralized with an alkali metal salt, and the monomer concentration after neutralization with respect to water is 30% by weight. % Or more to a saturated concentration range, and when an acrylic acid / methacrylic acid mixture is used, the methacrylic acid concentration in the mixture is 20%.
It has been found that by setting the content to be not more than mol% and further using hydrogen peroxide or persulfate as a water-soluble polymerization initiator, a crosslinking reaction can be caused without using a crosslinking agent and the polymer can be insolubilized.

The polymer produced according to the present invention has characteristics that the particle size is extremely large, the water absorption is high, and the water absorption gel strength is extremely large. The water-soluble polymer having such characteristics adopts conditions that cause a crosslinking reaction in the reverse phase suspension polymerization as described above, and at the same time, α-olefin and α, β-unsaturated polycarboxylic acid as a surfactant are used. The production is possible only by using a copolymer with an acid anhydride or a derivative thereof.

Generally, the water absorption amount and the water absorption gel strength tend to contradict each other. That is, in order to increase the amount of water absorption, it is necessary to reduce the cross-linking ratio for insolubilizing the polymer as much as possible, while the strength of the water-absorbing gel is decreased to the contrary.
Therefore, in order to increase the water-absorbent gel strength, it is necessary to increase the cross-linking ratio, but in order to obtain a sufficiently satisfactory water-absorbent gel strength, the water absorption amount is usually unusable, that is, 300 times its own weight. The reality is that it will be about the following. That is, a polymer obtained by carrying out aqueous solution polymerization or solution polymerization in the presence of a bifunctional divinyl compound or a bifunctional compound capable of reacting with a carboxyl group by a method other than the method of the present invention is the amount of these crosslinking agents. Even if it is minimized, the water absorption amount is at most about 1000 times its own weight, and this product itself has a very small water-absorbing gel strength and becomes a paste, increasing the amount of cross-linking agent in order to obtain sufficient water-absorbing gel strength. In that case, the amount of water absorption will be 300 times or less of its own weight.

Further, in JP-A-56-76419, a crosslinking agent is not used,
A water-in-oil type reverse phase suspension polymerization method of an aqueous solution of an alkali metal acrylate containing 35 wt% to a saturated concentration containing hydroxyethyl cellulose in the aqueous phase has been proposed.

The water absorption of the polymer obtained by this method is 500 to 700 times its own weight, which is a relatively high value, but it has a drawback that the water absorption gel strength is extremely weak and the durability is extremely poor. Further, one of the characteristics of the polymer obtained by this method is that it has a fairly large particle size of several hundreds of μm, but it is difficult to obtain a polymer with good uniformity, and it is usually as small as about 0 μm. It shows a fairly wide distribution with a large size of about 500 μm.

The present invention has a large water-absorbent gel strength that solves the conventional problems as described above, and further, without reducing the water absorption amount, specifically, has a water-absorption capacity of 700 times or more of its own weight, and its average particle size is The present invention provides a method for producing a superabsorbent polymer having a particle size of 100 μm or more and a narrow distribution, which has the greatest feature of the present invention.

(Detailed Description of the Invention) (1) Monomer The monomer used in the polymerization reaction of the present invention is acrylic acid or a mixture of acrylic acid and methacrylic acid containing 20 mol% or less of methacrylic acid, and all the carboxyl groups thereof. 50 mol% or more, preferably 60 mol% or more is an acrylic acid-based monomer partially neutralized with an alkali metal salt. And in the case of a mixture of acrylic acid and methacrylic acid, it is possible to contain methacrylic acid in an amount of up to 20 mol% as described above, but when the amount of methacrylic acid is large, the water absorption becomes significantly small and the solubility is high. Therefore, the content of methacrylic acid is preferably 10 mol% or less, because the content of methacrylic acid increases.

The degree of partial neutralization of such acrylic acid or a mixture of acrylic acid and methacrylic acid (hereinafter, these may be collectively referred to as “acid monomer”) is 50 mol% or more of all the carboxyl groups, as described above. It is preferably in the range where 60 mol% or more is an alkali metal salt. If the degree of partial neutralization becomes too low, the water absorption will decrease significantly,
The strength of the obtained water-absorbent gel is also extremely weak.

To neutralize the acid monomer, alkali metal hydroxides and bicarbonates can be used, but alkali metal hydroxides are preferable, and specific examples thereof include sodium hydroxide,
Examples include potassium hydroxide and lithium hydroxide. Sodium hydroxide is most preferable from the viewpoint of industrial availability, price, and safety.

(2) Water-soluble radical polymerization initiator The radical polymerization initiator used in the production method of the present invention is a persulfate such as potassium persulfate or ammonium persulfate, or hydrogen peroxide. These water-soluble radical polymerization initiators may be used as a mixture, or may be used as a redox type polymerization initiator by combining a reducing substance such as sulfite and amines. .

The amount of these persulfates and hydrogen peroxide used is 0.01 to 5% by weight, preferably 0.
It is 1 to 1% by weight.

In addition, other water-soluble radical polymerization initiators such as hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide, and 2,2′-azobis (2-amidilipropane) dihydrochloride When the azo compound is used, the polymer obtained becomes water-soluble and the object of the present invention cannot be achieved.

(3) Surfactant The surfactant used in the production method of the present invention is α
A copolymer of an olefin and an α, β-unsaturated polycarboxylic acid anhydride or a derivative thereof. The α-olefin has 10 to 100 carbon atoms, preferably 16 to 60 carbon atoms.
Examples of α, β-unsaturated polycarboxylic acid anhydrides include maleic anhydride, citraconic anhydride, and itaconic anhydride, with maleic anhydride being preferred. The derivative of these copolymers is a partially esterified product or partially amidated product of the copolymer. Examples of the partially esterified product include copolymer monomethyl ester, monoethyl ester, and monobutyl ester. Examples of the partially amidated product of the copolymer include monoethylamide, monopropylamide and monobutylamide of the copolymer.

The molecular weight of the above copolymer or its derivative is 5,000 to 100,00.
It is 0, preferably 10,000 to 50,000.

Further, in the present invention, the α-olefin / α, β-unsaturated polycarboxylic acid anhydride copolymer may be in an acid anhydride state at the time of use or in a partially or fully ring-opened state. Good.

The amount of these surfactants used is 0.01 to the dispersion medium.
It is 10% by weight, preferably 0.05 to 5% by weight.

(4) Dispersion medium As the dispersion medium used in the present invention, in principle, any liquid can be used as long as it does not participate in the polymerization and is not mixed with water. For example, benzene, ethylbenzene, toluene, aromatic hydrocarbons such as xylene, cyclohexane, methylcyclohexane, cyclooctane, alicyclic hydrocarbons such as decalin, hexane, pentane, heptane, aliphatic hydrocarbons such as octane, chlorobenzene, Examples thereof include halogenated hydrocarbons such as brombenzene and dichlorobenzene.
Among these, alicyclic hydrocarbons such as cyclohexane and methylcyclohexane, and aliphatic hydrocarbons such as hexane and heptane are particularly preferable. Further, these dispersion media may be used alone or in combination of two or more kinds as appropriate.

The amount of these dispersion mediums used is 0.5 to 10 times by volume, preferably 1 to 10 times the amount of the aqueous monomer solution in order to make the water-in-oil type of the polymerization reaction system and from the viewpoint of removing the heat of the polymerization reaction.
It is desirable to make the amount 5 times.

(5) Water Water and the amount of acrylic acid monomer are allowed to be present in the polymerization reaction liquid of the present invention in the ratio as described above (this point will be described in further detail later).

Further, the water and the dispersion medium, as described above, the dispersion medium is an aqueous phase, that is,
It is preferably used in a ratio of 0.5 to 10 times, preferably 1 to 5 times the volume of the aqueous monomer solution. If the ratio of water to the dispersion medium is too large, it becomes extremely difficult to remove the heat of polymerization, and a stable water-in-oil type suspension system cannot be obtained. Further, if the ratio of water to the dispersion medium is too low, the monomer concentration per unit batch will be low, resulting in poor productivity and being economically disadvantageous.

In addition, as for the addition of water to the reaction system, it is generally and preferably added in the form of water which dissolves the neutralizing agent for neutralizing the acid monomer as described above, but a part of the water is separately added by an appropriate method. It is also possible to add in. In addition, water is partially generated by the neutralization reaction between the acid monomer and the neutralizing agent.

In the present invention, the acrylic acid-based monomer detailed above is used as a water-soluble radical polymerization initiator, a dispersant, a surfactant,
Polymerization is carried out by a water-in-oil type reverse phase suspension polymerization method in the presence of water and water, and as described above, the concentration of the acrylic acid monomer relative to water present in the polymerization reaction system at that time is neutralized and the monomer concentration after neutralization is reached. As a range from 30% by weight or more to a saturated concentration,
This is one of the important constituent features of the present invention. That is, when the monomer concentration after neutralization is 30% by weight or less,
The obtained polymer has a large amount of water-soluble portion and the obtained water-absorbent gel strength is too small to attain the object of the present invention.

(6) Polymerization conditions Typical embodiments of the polymerization reaction of the present invention are as follows. That is, a polymerization initiator is added and dissolved in a previously neutralized aqueous monomer solution, and an inert gas such as nitrogen is introduced to perform deaeration.
On the other hand, a surfactant is placed in a dispersion medium, if necessary heated slightly to dissolve it, and an inert gas such as nitrogen is introduced to degas. The above monomer aqueous solution is poured into this and heated to a predetermined temperature. During this period, the aqueous solution portion of the reaction system becomes minute droplets and is dispersed and suspended in the dispersion medium. After the initiation of polymerization, cooling or heating is appropriately performed depending on the heat generation state.

The polymerization reaction temperature of the present invention is 60 to 100 ° C, preferably 60 to 8
It is 0 ° C. If the reaction temperature is too low, the soluble parts of the produced polymer will increase, and the water absorption performance will decrease.If it is too high, the water absorption performance will vary greatly, and not only will a homogeneous polymer not be used, but a high boiling point solvent will be used. The need arises,
It has the disadvantage of requiring high temperatures for solvent recovery and the like.

The polymerization reaction of the present invention is carried out in a water-in-oil type suspension polymerization system. For that purpose, as described above, a copolymer of α-olefin and α, β-unsaturated polycarboxylic acid anhydride or The purpose can be achieved by using the derivative, combining the means such as appropriately adjusting the ratio of the dispersion medium and water, and further performing appropriate stirring.

Stirring of the polymerization reaction system in the present invention is not only important for maintaining the desired stable water-in-oil type suspension system of the polymerization reaction system, but also for improving the properties of the produced polymer. But it's important.

That is, if the stirring is too strong, the primary particles of the fine hydrogel of the produced polymer are aggregated and agglomerated, or the crosslinked structure is broken to produce a partially water-soluble polymer, so that the performance of the polymer is deteriorated. The variation increases.
On the other hand, if the stirring is too weak, a stable dispersion system is not formed, abnormal polymerization occurs, and beads are not formed, and the water absorption performance of the obtained polymer is significantly deteriorated. Therefore, it is necessary to perform appropriate stirring, but when referring to the case of using a polymerization reaction device with a usual stirrer for carrying out this kind of suspension polymerization, 100-600 rpm, preferably 200-400 rpm by stirring, A bead-like polymer which is excellent in water absorption and water absorption gel strength and is easy to pulverize can be obtained again with good properties.

(7) Separation of Polymer The polymer obtained by the production method of the present invention is composed of wet, bead-shaped particles, and can be easily separated from the dispersion medium by decantation or evaporation operation. Then, the separated wet polymer is, for example,
When dried at a temperature of 120 ° C. or lower, a powdery polymer or a powdery polymer of easily crushable mass is obtained.
The polymer thus obtained usually has a diameter of 100 to 500.
It is a particle having a size of about μm and slightly containing a spherical primary particle whose surface is covered with a surfactant or a secondary particle in which they are partially secondary-aggregated. The secondary particles can also be easily pulverized with a slight mechanical force. This has great advantages in terms of polymer production and use.

[Effects of the Invention]

The polymer obtained by the production method of the present invention, as described above, the water absorption performance is as high as 700 times or more of its own weight, and the water absorption gel strength is large, or the polymer itself is a powder from the beginning,
Alternatively, it can be easily made into a powder by an extremely simple pulverization operation.

Therefore, the polymer obtained by the production method of the present invention can be advantageously used for the production of sanitary napkins, paper naps, and other sanitary materials by utilizing its excellent water absorption performance.

Further, by utilizing its excellent water absorption performance and gel strength, it is also used for the production of various materials for gardening or agriculture such as a soil improving agent and a water retention agent, which have recently attracted attention. be able to.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

The pure water absorption capacity and the water absorption gel strength described in these examples show the results measured by the following test methods.

A. Pure water absorption capacity About 0.5 g of polymer and about 1 of pure water were weighed and put into a beaker having a pure water absorption capacity of 1 and mixed, and then left for about 60 minutes to swell the polymer sufficiently with water. Then, after draining with a 100-mesh sieve, the amount of excess liquid is weighed, and the pure water absorption capacity is calculated according to the following formula.

B. Water absorption gel strength Add 200 times the amount of pure water to the polymer to absorb water,
The strength of the water-absorbent gel was examined by pressing the elasticity of the water-absorbent gel with a finger and evaluated according to the following criteria.

Example 1 A 500 ml four-necked round-bottomed flask equipped with a stirrer, a reflux condenser, a thermometer, and a nitrogen gas inlet tube was charged with cyclohexane 1
Add 85 g and add and dissolve 1.8 g of α-olefin and maleic anhydride copolymer (Mitsubishi Kasei Co., Ltd., trade name "Diakarna 30", molecular weight about 10,000), and dissolve at 65 ° C under nitrogen gas atmosphere. And

Separately, to a conical flask having a capacity of 200 ml, 12.9 g of caustic soda in which 49 g of water was dissolved was added while cooling 30 g of acrylic acid from the outside, and 77.4% of the carboxyl vessel was neutralized. The monomer concentration with respect to water in this case corresponds to 40% by weight as the monomer concentration after neutralization.

Next, 0.1 g of potassium persulfate was added to and dissolved in this.

The contents were added to the 200 ml flask to the contents of the four-necked round bottom flask, and polymerization was carried out at 65 to 70 ° C. for about 1 hour with stirring. The stirring was performed at 300 rpm.

When the stirring was stopped after the reaction for 1 hour, the wet polymer particles were settled on the bottom of the flask and could be easily separated from the cyclohexane phase by decantation. The separated wet polymer was transferred to a vacuum dryer and heated at 80 to 90 ° C. to remove the attached cyclohexane and water.

The resulting dry polymer was a powder containing a free-flowing, easily crushable mass.

Example 2 Except that the diacarna 30 used in Example 1 was treated with a large excess of methanol under reflux for about 8 hours, and excess methanol was removed under reduced pressure as a surfactant.
Polymerization and post-treatment were carried out with the same formulation as in Example 1.

The resulting polymer was a powder containing a free-flowing, easily crushable mass.

Example 3 Diacarna 30 used in Example 1 was treated with a large excess of butylamine under reflux at 70 ° C. for about 8 hours, and the excess butylamine was removed under reduced pressure. Polymerization and post-treatment were carried out with the same formulation as in Example 1.

The resulting polymer was a powder containing a free-flowing, easily crushable mass.

Example 4 Instead of acrylic acid in Example 1, 28 g of acrylic acid
And 2 g of methacrylic acid were used, and the other reaction was performed in the same manner as in Example 1 and post-treatment was carried out.

The polymer obtained was a powder containing a free-flowing, easily crushable mass.

Example 5 The amount of water used for dissolving caustic soda in Example 1 was 6
A polymerization reaction was performed in the same manner as in Example 1 except that the amount was changed to 3.1 g, and the same post-treatment was performed. In this case, the monomer concentration with respect to water in the reaction system is 35% by weight as monomer agriculture after neutralization.

The resulting polymer was a powder containing a free-flowing, easily crushable mass.

Example 6 A polymerization reaction was carried out in the same manner as in Example 1 except that the amount of caustic soda in Example 1 was changed to 10 g and the amount of water used for dissolving the caustic soda was changed to 48.8 g, and the same post-treatment was performed. . In this case, 60 mol% of acrylic acid was neutralized, and the monomer concentration with respect to water in the reaction system was 40% by weight as the monomer concentration after neutralization.

The resulting polymer was a powder containing a free-flowing, easily crushable mass.

Comparative Example 1 Polymerization was carried out by the same formulation and operation as in Example 1 of JP-A-56-76419 to obtain a dry polymer. That is, a stirrer,
500cc4 with a reflux condenser, dropping funnel, and nitrogen gas inlet pipe
Hexane 230 ml sorbitan monostearate 1.8 g was placed in a one-necked flask, nitrogen gas was blown thereinto to expel dissolved oxygen, and then the mixture was heated to 60 to 65 ° C. Separately, 30 g of acrylic acid was dissolved in 49 g of water while cooling with ice from the outside in a beaker 13.4 g
78% of the cartoner was neutralized with 98% caustic soda.
The monomer concentration in the aqueous phase was 40% by weight. Next, 1.8 g of hydroxyethyl cellulose (degree of etherification 0.8, number of added moles: 1.9) was added and dissolved, and then nitrogen gas was blown into the solution to remove dissolved oxygen. Put the contents in the beaker above 4
The mixture was added to a one-necked flask and dispersed to maintain the internal temperature at 60 to 65 ° C, and stirring was continued for 3 hours. Hexane was distilled off under reduced pressure,
The remaining swollen polymer portion is dried under reduced pressure at about 80 ° C,
A powdery polymer was obtained.

Table 1 shows the results of measuring the pure water absorption capacity, the water absorption gel strength and the particle size of the dried polymers obtained in Examples 1 to 6 and Comparative Example 1 above.

As is clear from this result, the polymer obtained by the method of the present invention has a large water absorption capacity, a large water absorption gel strength, a large particle size, and a relatively narrow particle size distribution.

Claims (3)

[Claims] 1. An acrylic acid-based monomer comprising acrylic acid or a mixture of acrylic acid and methacrylic acid and an alkali metal salt of acrylic acid or a mixture of acrylic acid and methacrylic acid, a water-soluble radical polymerization initiator, a dispersion medium, In the method of polymerizing by water-in-oil type reverse phase suspension polymerization method in the presence of a surfactant and water, carbon number as a surfactant
Using a copolymer of 16 to 60 α-olefin and α, β-unsaturated polycarboxylic acid anhydride or its derivative, 50 mol% or more of all carboxyl groups of the acrylic acid-based monomer are in the alkali metal salt. It is characterized that the concentration of acrylic acid-based monomer relative to water present in the polymerization reaction system is in the range of 30% by weight or more to the saturated concentration after neutralization, and the polymerization is performed in the absence of a crosslinking agent. And a method for producing a beaded self-crosslinking water-absorbing polymer. 2. The production method according to claim 1, wherein the methacrylic acid in the mixture of acrylic acid and methacrylic acid is 20 mol% or less. 3. The water-soluble radical polymerization initiator is hydrogen peroxide,
The production method according to claim 1, which is one or more selected from the group consisting of persulfates.