JPS634843B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing superabsorbent polymers. More specifically, the present invention relates to a method for producing a water-absorbing material with excellent salt resistance and water absorption rate. Conventionally, various water-absorbing materials such as paper and pulp have been used in the field of sanitary materials (sanitary napkins, disposable diapers) and in the agricultural field, but these water-absorbing materials have low water-absorbing ability, and moreover, once absorbed water If pressure is applied, a large portion of it will be squeezed out. In recent years, water-absorbing materials such as hydrolysates of starch-acrylonitrile graft polymers, modified cellulose ethers, and hydrolysates of methyl acrylate/vinyl acetate copolymers have been proposed as alternatives to these materials, and further improvements have been made. ing.
However, these materials cannot be said to exhibit excellent water absorption ability and water absorption rate, and a satisfactory water absorption material has not been obtained. The present inventors have already proposed a water-absorbing material with excellent water-absorbing performance (Japanese Patent Publication No. 54-30710), and have also proposed further improved water-absorbing materials (Japanese Patent Application No. 56-30710).
43488, patent application No. 56-43489). However, since these water-absorbing materials are also polymer electrolytes, there is a problem in that their performance in salt solutions is significantly reduced, and it has been desired to develop water-absorbing materials with excellent salt resistance as soon as possible. Therefore, the present inventors conducted intensive research in order to obtain a polymer with excellent salt resistance and water absorption rate that does not have these drawbacks.As a result, the present inventors made a hydrophilic polymer and controlled the water content in the hydrous material of the polymer within a specific range. The present invention was completed based on the discovery that the polymer obtained by adding a crosslinking agent in this state is excellent in water absorption, particularly in salt resistance and water absorption rate. That is, the present invention uses a hydrophilic polymer having a carboxyl group (or carboxylate group) whose water content is adjusted to be 10 to 40% by weight.
The present invention provides a method for producing a superabsorbent polymer, which is characterized by crosslinking with a crosslinking agent having two or more functional groups capable of reacting with carboxyl groups (or carboxylate groups). Conventionally, the main method for obtaining water-absorbing polymers is to slightly crosslink a water-soluble polymer to make it water-insoluble.The crosslinking methods include (1) self-crosslinking by optimizing manufacturing conditions; (2) Crosslinking is carried out by adding a crosslinking agent during or after polymerization. is proposed. Although some of the water-absorbing polymers obtained in this manner exhibit excellent properties, they do not satisfy all required performances.
In other words, the required performances of water-absorbing polymers include (1) water absorption amount, (2) water absorption rate, and (3) gel strength. It is generally accepted that there is a relationship between water absorption rates, and conventional water-absorbing polymers have been established on the balance between these two properties, so they have been manufactured at the expense of some performance. The present invention aims to improve these drawbacks and provide a method for producing an innovative water-absorbing polymer that satisfies various performances required of super-absorbent polymers. The important points for achieving the purpose of the present invention are that the hydrophilic polymer has a carboxyl group (or carboxylate group) and that the water content of the hydrophilic polymer is within a specific range. be. The hydrophilic polymer that can be used in the present invention is not limited to any type and polymerization method as long as it has a carboxyl group (or carboxylate group) in its constituent unit. Examples of materials that can be suitably used in the present invention include Japanese Patent Publication No. 54-30710 and Japanese Patent Publication No. 56-30710.
Sodium polyacrylate obtained by the reverse-phase suspension polymerization method described in JP-A No. 55-133413, etc., and sodium polyacrylate obtained by aqueous solution polymerization (adiabatic polymerization, thin film polymerization) described in JP-A No. 55-133413, etc., Patent Publication No. 53-46199. Examples include the starch-sodium acrylate graft polymer described in . Further, when producing these polymers, the effects of the present invention are not hindered as long as a very small amount of crosslinking agent is added. Furthermore, it is desirable that these polymers be self-crosslinked. Since the present invention requires controlling the water content in the water-containing polymer, usually after synthesizing the polymer,
Requires dehydration process. Therefore, from the viewpoint of workability and the like, it is desirable to use a polymer obtained by a reversed-phase suspension polymerization method.
Examples of polymers having carboxyl groups (or carboxylate groups) in their constituent units include polyacrylic acid (and its salts) and polymethacrylic acid (and its salts), which are used in the present invention. It can be preferably used in the method. Also, maleic acid, itaconic acid, acrylic acid or methacrylic acid,
Copolymerized comonomers such as acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and 2-hydroxyethyl(meth)acrylate within a range that does not reduce the performance of the water-absorbing polymer. Polymers may also be used in the method of the invention. When adopting the reverse phase suspension polymerization method as the polymerization method,
According to the conventional method described in the above-mentioned known literature, an aqueous solution of a hydrophilic monomer having a carboxyl group (or carboxylate group) containing a water-soluble initiator such as a persulfate is prepared in a non-aqueous solvent using a protective colloid. and perform reverse phase suspension polymerization. Sorbitan monostearate is used as a protective colloid.
Sorbitan fatty acid esters such as sorbitan monolaurate, cellulose ethers such as ethyl cellulose and benzyl cellulose, cellulose esters such as cellulose acetate, cellulose butyrate, and cellulose acetate butyrate, maleated polybutadiene, maleated polyethylene,
Examples include polymeric dispersants such as maleated α-olefin, and either one or two or more of these may be used. From the viewpoint of dust control of the resulting polymer, a polymer dispersant that provides a large particle size is desirable. In addition, non-aqueous solvents used at that time include aliphatic hydrocarbons such as hexane, heptane, and octane, aliphatic hydrocarbons such as cyclohexane, methylcyclohexane, and decalin, aromatic hydrocarbons such as benzene, toluene, and xylene, chlorobenzene, and bromobenzene. , dichlorobenzene and other halogenated hydrocarbons. A particularly important requirement in the method of the present invention is the water content in the hydrophilic polymer hydrate when the crosslinking agent is added and the crosslinking reaction is carried out. Conventionally, methods for producing water-absorbing polymers in which a crosslinking reaction is performed after polymerization are known. For example, Japanese Patent Application Laid-Open No. 131608/1983 describes a method in which polyacrylates are crosslinked in a mixed solvent of water and a hydrophilic organic solvent. is written, and was published in 1987.
-28505 publication describes polyacrylic acid (or its salt)
A method for crosslinking in the presence of water is described. However, these water-containing polymers have a water content of 50% by weight or more, especially the latter have a water content of 70% by weight or more, and the effects of the present invention cannot be achieved with such a water content. Typically, hydrophilic polymers have a monomer concentration of 45% by weight
It is obtained by polymerization in an aqueous solution having a water content of 55% by weight or more. Therefore, in practicing the present invention, it is necessary to control the water content in the hydrophilic polymer product obtained by conventional methods. According to the invention, this water content is between 10 and 40% by weight.
(total amount of hydrophilic polymer hydrates) is an essential condition. More preferably, 15 to
It is 35% by weight (based on the total amount). If the water content in the hydrophilic polymer is outside the above range, the water absorption amount and/or water absorption rate will be poor, and the remarkable effects of the present invention will not be obtained. For example, in the present invention, the desired effect can be achieved by concentrating the polyacrylic acid polymer obtained by polymerization using a reversed-phase suspension polymerization method and controlling the water content within the above range. The crosslinking agent used in the present invention may be any compound having two or more functional groups capable of reacting with a carboxyl group (or carboxylate group). Examples of such crosslinking agents include polyglycidyl ethers such as ethylene glycol diglycidyl ether, polyethylene glycol glycidyl ether, and glycerin triglycidyl ether, haloepoxy compounds such as epichlorohydrin and α-methylchlorohydrin, glutaraldehyde,
Examples include polyaldehydes such as glyoxal, polyols such as glycerin, pentaerythritol, and ethylene glycol, and polyamines such as ethylenediamine. Preferably, it is a polyglycidyl ether such as ethylene glycol diglycidyl ether. The amount of crosslinking agent added varies depending on the type of crosslinking agent and the type of polymer, but it is usually 0.01% per polymer.
~5.0% by weight is a suitable range. If the amount of the crosslinking agent added is less than 0.01% by weight, the effect of the addition will not be sufficiently expressed, and on the other hand, if it is more than 5.0% by weight, the crosslinking density will increase, resulting in a decrease in water absorption, which does not meet the intent of the present invention. It's not the place to do it. There are various methods for adding and reacting a crosslinking agent. In other words, in the case of a polymer obtained by reverse-phase suspension polymerization, a crosslinking agent is added to a hydrophilic polymer whose water content is adjusted to the range specified in the present invention in an organic solvent. In the case of thin film polymerization, etc., the resulting polymer gel is crushed, the moisture content is adjusted by a drying process, and then the polymer is placed in a kneader and a crosslinking agent is added thereto. An example of a method is to perform a heat treatment and perform a crosslinking reaction. In order to carry out the crosslinking reaction smoothly, it is desirable to heat it.
It is preferable to carry out the reaction at a temperature in the range of 40° to 150°C. By using the method of the present invention, it is possible to obtain a water-absorbing material with excellent salt resistance and water absorption rate,
It is very advantageous for use as a water retaining agent for agriculture and a water absorbing agent for sanitary materials. The superabsorbent polymer obtained by the method of the present invention can be advantageously used particularly in the field of disposable diapers that must quickly absorb a large amount of urine and the field of sanitary napkins that must absorb blood. “Leak” and “discomfort”
It may be possible to eliminate the need to leave behind. The present invention will be specifically explained below using Examples and Comparative Examples, but the present invention is not limited to these Examples. In addition, the water absorption amount in the following examples and comparative examples is a value obtained by the following operation. That is, approximately 1 g of polymer is dispersed in a large excess of ion-exchanged water or physiological saline, allowed to swell sufficiently, and then passed through an 80-mesh wire gauze, the resulting swollen polymer weight (W) is measured, and this value is calculated from the initial This is the value obtained by dividing by the weight of the polymer (Wo). In other words, water absorption amount (g/g) = W/Wo. The water absorption rate was also expressed as the amount of physiological saline absorbed by 0.5 g of polymer in 10 minutes. Examples 1 to 3 230 ml of cyclohexane and sorbitan monostearate were placed in a 500 ml four-neck round bottom flask equipped with a stirrer, reflux condenser, dropping funnel and nitrogen gas inlet tube.
1.8g was added and the temperature was raised to 75°C. Separately, in an Erlenmeyer flask, 30 g of acrylic acid was neutralized with 13.4 g of caustic soda dissolved in 39 g of water. The monomer concentration in the monomer aqueous solution was 45% (water content 55%). Then, 0.1 g of potassium persulfate was added and dissolved. This monomer aqueous solution was dropped into the above four-necked flask under a nitrogen atmosphere for 1.5 hours, and then polymerized at 70° to 75°C.
The polymerization was completed by holding at ℃ for 0.5 hours. After this, the water content in the polymer suspended in cyclohexane was reduced to 35% by azeotropic dehydration (cyclohexane was refluxed).
%, 27%, and 20%, respectively. After this, an aqueous solution of 0.03 g of ethylene glycol diglycidyl ether dissolved in 1 ml of water was added to each at 73°C.
After maintaining this temperature for 2 hours, the cyclohexane was removed, and the polymer was dried under reduced pressure at 80° to 100°C to obtain a water-absorbing polymer. Example 4 Polymerization was carried out according to Example 1. However, 0.5 g of ethyl cellulose T-50 was used instead of sorbitan monostearate. After polymerization, the water content in the polymer was controlled to 22% by azeotropic dehydration, and 0.04g of glycerin diglycidyl ether was added.
An aqueous solution of 1 ml of water was added at 73 DEG C., and after maintaining this temperature for 3 hours, cyclohexane was removed and the polymer was dried under reduced pressure at 80 DEG -100 DEG C. to obtain a water-absorbing polymer. Example 5 Polymerization was carried out according to Example 4. However, the monomer concentration in the monomer aqueous solution is 35%, and N,
0.003 g of N-methylenebisacrylamide was added. After polymerization, the water content in the polymer is reduced by azeotropic dehydration.
After controlling the concentration to 27%, an aqueous solution of 0.15 g of polyethylene glycol diglycidyl ether (n=9) dissolved in 1 ml of water was added at 90°C, and after keeping at this temperature for 3 hours, the cyclohexane was removed and the polymer was heated to 80°C. It was dried at 110°C under reduced pressure to obtain a water-absorbing polymer. Example 6 Caustic soda with 30 g of acrylic acid dissolved in 39 g of water
It was neutralized with 13.4 g, and the monomer concentration in the monomer aqueous solution became 45%. Then, 0.1 g of sodium persulfate was added and dissolved. This monomer aqueous solution was poured between two Teflon plates to form a thin film, and was held at 65°C for 3 hours to polymerize. 3mm of the generated polymer gel
After cutting into pieces, they were dried in a hot air dryer until the moisture content was 30%. This material was placed in a kneader, and an aqueous solution of 0.03 g of ethylene glycol diglycidyl ether dissolved in 1 ml of water was sprayed on it, and after being held at 70°C for 1 hour, it was dried under reduced pressure at 70° to 80°C to remove the resulting polymer. It was pulverized to obtain a water-absorbing polymer with a center particle diameter of 100 to 250 μm. Comparative Example 1 Polymerization was carried out according to Example 1. However, 0.03 g of ethylene glycol diglycidyl ether was added to the aqueous monomer solution to effect crosslinking simultaneously with polymerization. After the polymerization was completed, cyclohexane was removed, and the polymer was dried under reduced pressure at 80° to 100°C to obtain a water-absorbing polymer. Comparative Example 2 Polymerization was carried out according to Example 1. However, 0.03g of ethylene glycol diglycidyl ether is added to 1 part of water.
ml of the aqueous solution was added after the polymerization was completed (water content of the hydrous polymer was 55%) and maintained at 73°C for 1 hour. After the crosslinking reaction was completed, cyclohexane was removed, and the polymer was dried under reduced pressure at 80° to 100°C to obtain a water-absorbing polymer. Comparative Example 3 Polymerization was carried out according to Example 1. However, 0.03g of ethylene glycol diglycidyl ether is added to 1 part of water.
ml of the aqueous solution was added when the water content in the polymer was controlled to 45% by azeotropic dehydration, and maintained at 60°C for 2 hours. After the crosslinking reaction was completed, cyclohexane was removed, and the polymer was dried under reduced pressure at 80° to 100°C to obtain a water-absorbing polymer. Comparative Example 4 After polymerization was carried out according to Example 1, cyclohexane was removed and the mixture was dried under reduced pressure at 70° to 80°C.
The moisture content of this polymer was 7%. This polymer was again dispersed and suspended in cyclohexane and 0.03% of ethylene glycol diglycidyl ether was added.
Add an aqueous solution of 1 ml of water and stir at 70°C.
Holds time. Then remove the cyclohexane,
The polymer was dried under reduced pressure at 80° to 100°C to obtain a water-absorbing polymer. Table 1 shows the particle size, water absorption amount, and water absorption rate of each water-absorbing polymer obtained in Examples 1 to 6 and Comparative Examples 1 to 4.

【table】

[Table] It is clear from Table 1 how the polymer obtained by the present invention exhibits excellent absorption performance in salt resistance and water absorption rate.

Claims (1)

[Scope of Claims] 1. A hydrophilic polymer having a carboxyl group (or carboxylate group) whose water content is adjusted to be 10 to 40% by weight is reacted with a carboxyl group (or carboxylate group). 1. A method for producing a superabsorbent polymer, which comprises crosslinking with a crosslinking agent having two or more functional groups that can bind to water. 2. The hydrophilic polymer according to claim 1, wherein the hydrophilic polymer is a hydrophilic polymer obtained by reverse-phase suspension polymerization of an aqueous solution of a hydrophilic monomer having a carboxyl group (or carboxylate group) containing a water-soluble initiator. A method for producing superabsorbent polymers. 3 Carboxyl group (or carboxylate group)
3. The method for producing a superabsorbent polymer according to claim 1 or 2, wherein the hydrophilic polymer having the following is a polymer or copolymer of acrylic acid (or an alkali salt of acrylic acid). 4. The method for producing a superabsorbent polymer according to claim 1, 2, or 3, wherein the crosslinking agent is polyglycidyl ether. 5 A hydrophilic monomer aqueous solution in which the hydrophilic polymer has a carboxyl group (or carboxylate group) containing a water-soluble initiator is dispersed and suspended in a non-aqueous solvent in the presence of a polymeric dispersant that is a protective colloid. The method for producing a superabsorbent polymer according to claim 1, which is a hydrophilic polymer obtained by polymerization.