JPH01292003A - Purification of water-absorbable resin - Google Patents

Abstract

PURPOSE:To improve the safety of an acrylic acid water-absorbable resin without lowering the productivity of the resin while maintaining the water-absorbing performance of the resin, by mixing the acrylic acid resin with a swelling liquid and subsequently with a dehydrating liquid to readily remove low mol.wt. components remaining in the resin outside of the resin. CONSTITUTION:A powdery polyacrylic acid water-absorbable resin (e.g., crosslinked polyethylene oxide or cellulose-acrylic acid-grafted polymer) is swollen with mixed solvents comprising water and a hydrophilic organic solvent (e.g., methanol or hexamethylene sulfamide) and subsequently dehydrated with a hydrophilic organic solvent to remove water-soluble low mol.wt. components contained in the water-absorbable resin. The ratio of the water to the organic solvent in the swelling liquid is preferably 1:9-9:1 by a weight ratio and the liquid is used in a ratio of usually 50-1000 pts.wt. based on 100 pts.wt. of the water-absorbable resin.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for purifying a water-absorbing resin, and more specifically, the present invention relates to a method for purifying a water-absorbing resin. The present invention relates to a method for purifying a water-absorbing resin for increasing the safety of the water-absorbing resin by reducing its residual concentration.

<Prior art and problems to be solved by the invention> In recent years,
Water-absorbing resins are used as water-absorbing and water-retaining materials in various fields such as sanitary products, medicine, cosmetics, agriculture, food, civil engineering and construction, and household products. Most of the water absorbent resin produced is used for materials.

Known water-absorbing resins include cross-linked polyethylene oxide, cross-linked polyvinyl alcohol, cross-linked polyacrylic acid and its salts, cellulose-acrylic acid graft polymers and their salts, and hydrolysates of starch-acrylonitrile graft polymers. However, among them, polyacrylic acid-based water-absorbing resins such as the above-mentioned cross-linked polyacrylic acid are widely used because they have excellent water absorption performance such as water absorption amount and water absorption rate, and are easy to produce industrially and are inexpensive. Most of the water-absorbing materials used in sanitary products are made up of the polyacrylic acid-based water-absorbing resin.

However, low polymerization degree components such as unreacted monomers and dimers contained in the polyacrylic acid water absorbent resin,
Since it is irritating to the skin, how to reduce the residual concentration of this low degree of polymerization component becomes a problem especially when used in the above-mentioned sanitary products, cosmetics, household products, etc.

Therefore, attempts have been made to reduce the residual concentration of low polymerization degree components, for example, by increasing the amount of polymer 13 initiator used in the polymerization step, or by conducting polymerization over a long period of time while strictly controlling the polymerization reaction. being done. However, in the former case, the overall degree of polymerization decreases, so the resulting polymer has poorer water absorption performance such as water absorption rate and water absorption amount than ordinary polyacrylic acid-based water absorbent resins, and in the latter case, Compared to conventional production methods, this method requires a long time as described above and also has problems such as increased energy consumption and poor productivity.

<Object of the Invention> The present invention has been made in view of the above circumstances, and aims to improve the safety of polyacrylic acid-based water-absorbing resin without reducing its water-absorbing performance or productivity. The purpose of the present invention is to provide a method for purifying a water-absorbing resin, which reduces the residual concentration of low-degree-of-polymerization components contained in the resin by refining an ordinary acrylic acid-based water-absorbing resin produced by the method.

Means and Effects for Solving the Problems> In order to solve the above problems, the method for purifying a water absorbent resin of the present invention is to process a powdered polyacrylic acid water absorbent resin into a mixed solution of water and a hydrophilic organic solvent. (hereinafter referred to as "swelling liquid") and then dehydration treatment in a hydrophilic organic solvent (hereinafter referred to as "dehydration liquid") to remove water-soluble low polymerization degree components contained in the water-absorbing resin. It is characterized by removing.

The present invention will be explained in detail below.

As described above, the method for purifying a water absorbent resin of the present invention includes a swelling treatment step of the water absorbent resin using a swelling liquid and a dehydration treatment step using a dehydration liquid. The swelling treatment step is, for example,
This is done by putting the above-mentioned swelling liquid and water-absorbing resin into a treatment tank and stirring for a certain period of time if necessary. Through this treatment, the water-absorbing resin absorbs the water in the swelling liquid and becomes swollen. Become. Then, the water-soluble low degree of polymerization component contained in the water-absorbing resin is eluted into the water absorbed by the resin.

Next, the dehydration treatment step using a dehydration solution is performed by adding a dehydration solution to the water-absorbing resin that has become swollen through the above-mentioned swelling treatment, and through this treatment, the water-absorbing resin is dissolved in the low-polymerization degree component. Water is expelled and the resin contracts, resulting in a reduction in the residual concentration of low polymerization degree components in the resin. Note that when adding this dehydrating liquid, excess swelling liquid may be removed in advance, but the water-absorbing resin can be dehydrated in the same way even if the dehydrating liquid is added without removing the swelling liquid. , there is an advantage that work efficiency can be improved because the step of removing the swelling liquid can be omitted. Further, the water-absorbing resin after the dehydration treatment is suitably used in the form of a powder from which residual organic solvent and water have been removed by means such as filtration and drying.

As mentioned above, according to the illegal method, ordinary polyacrylic acid-based water-absorbent resins manufactured with high productivity by conventional methods can be mixed with swelling liquid and dehydration liquid sequentially to reduce the residual amount in the resin. Since the concentration of the polymerization degree component can be reduced, it is possible to maintain the water absorption performance of the resin and increase the safety of the resin without reducing productivity.

Note that the polyacrylic acid-based water absorbent resin purified by the present invention refers to a polymer or copolymer containing at least a monomer unit of acrylic acid or an acrylate in the main chain, such as polyacrylic acid, polyacrylic acid, etc. Preferred examples include acid salts, acrylic acid-acrylate copolymers; partially neutralized products of starch-acrylic acid graft polymers; cellulose-acrylic acid graft polymers and their salts; or crosslinked products of these resins. be done. Further, the polyacrylic acid-based water-absorbing resin may be a copolymer with acrylamide, low vinyl pyrrolidone, 2-hydroxyethyl methacrylate, etc., if necessary, in order to improve properties such as hydrophilicity.

The above-mentioned polyacrylic acid-based water-absorbent resin is manufactured by a so-called dipolymerization method in which other copolymerizable monomers, polymers such as starch, crosslinking agents, etc. are added as necessary to an aqueous solution of acrylic acid or acrylate. Aqueous solution polymerization is common, and in this production method, when water is removed after polymerization, the resin is obtained in the form of a block, so it is ground into a powder with a grinder to an appropriate particle size. The particle size is between 5 and 400 mesh on a Tyler standard sieve, more preferably between 1O and 2
00 mesh is preferably used in the present invention. Water-absorbent resins can also be produced by emulsion polymerization or suspension polymerization, in which case the resin is obtained in the form of small spheres.
It may be used as is, but it can also be further pulverized.

In addition, in order to make the water-absorbent resin into a crosslinked product, in addition to the method of adding a crosslinking agent that can react with the acrylic acid monomer during polymerization as described above, known crosslinking methods such as self-crosslinking and crosslinking with a catalyst are used. can. In addition, it is preferable that the above-mentioned water-absorbing resin is one in which the surface of the powder particles is intensively cross-linked in terms of water-absorbing properties and gel strength.In order to uniformly cross-link the particle surface, for example, Preferred is a method in which a polyhydric resin is crosslinked using an aluminum compound or an organic crosslinking agent having two or more functional groups in the presence of a hydrophilic organic solvent such as a polyhydric alcohol or water.

The swelling liquid used to swell the polyacrylic acid-based water-absorbing resin is a mixed solution of water and a hydrophilic organic solvent, as described above. Examples of the hydrophilic organic solvent include methanol, ethanol, isopropylene Examples include polar solvents such as Patul, acetone, methyl ethyl ketone, N,N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, hexamethylene sulfonamide, etc. Among them, those that are easily volatile and dry are
It is preferably used because the solvent can be easily removed after treatment. The above polar solvents can be used alone or in combination of two or more. Moreover, as water, ion exchange water, distilled water, tap water, etc. can be preferably used.

The ratio of water to organic solvent in the swelling liquid is not particularly limited, but the ratio W (water):S (organic solvent) of both is within the range of 1:9 to 9:1, more preferably 5 : Preferably around 5.

When the ratio W:S of both is less than 1:9, the concentration of water in the swelling liquid is too low, and the water absorbent resin cannot be sufficiently swollen, and the effect of eluting low polymerization degree components from the resin is insufficient. There is a possibility that this will happen. In addition, if the above ratio Was exceeds 9:1, the water-absorbing resin powder aggregates during swelling and creates bonding sites, which is a so-called mamako phenomenon, which tends to occur, and the resin is not swollen uniformly, resulting in low polymerization components. There is a risk that it will not be able to elute sufficiently.

In addition, the amount of the swelling liquid used is 100f of water-absorbing resin! It is preferable that the amount is in the range of 50 to 1000 parts by weight. If the amount of the swelling liquid is less than 50 parts by weight per 100 parts by weight of the water-absorbing resin, all of the water-absorbing resin will not be able to come into sufficient contact with the swelling liquid, and there is a risk that the swelling process will not be uniform. If the amount of the swelling liquid exceeds 1,000 parts by weight per 100 parts by weight, the recovery work of water and organic solvent after treatment will be extensive, resulting in poor productivity and, due to the use of organic solvents, it will not be industrially safe. It is difficult to manufacture and there is a risk that the cost will be high.

As the dehydrating liquid for dehydrating the water-absorbing resin that has been swollen with the swelling liquid, the above-mentioned hydrophilic organic solvents are preferably used. The organic solvent used as the dehydrating liquid and the organic solvent contained in the swelling liquid may be the same or different, but in consideration of recovery and reuse after treatment, they must be the same. is preferred. Also,
The amount of the dehydrating liquid used is preferably in the range of 50 to 1000 parts by weight per 10 parts by weight of the water absorbent resin.

If the amount of dehydration liquid is less than 50 parts by weight based on 100 parts by weight of the water-absorbing resin, the water concentration in the tank cannot be sufficiently reduced, depending on the amount of water present in the treatment tank, and the water is removed from the water-absorbing resin. Water-absorbing resin 10 that may not be able to dehydrate sufficiently
If the amount of dehydrated liquid exceeds 1000 parts by weight relative to 0 parts by weight, as in the case of swelling liquid, the recovery work after treatment will be extensive, productivity will be poor, and it will be difficult to manufacture safely industrially. There is a possibility that the cost will be high.

<Example> Examples of the present invention are shown below. In addition, in the following, 9
6 indicates weight % unless otherwise specified.

Preparation of test water absorbent resin Prepared by mixing 75 parts by weight of 80% acrylic acid, 48.0 parts by weight of 48.6% sodium hydroxide, and 48.3 parts by weight of ion-exchanged water, with a degree of neutralization of 70%. 1028 g of an acrylate aqueous solution (neutralized solution) was placed in a sealable container, the inside of the container was replaced with nitrogen, and then 14.4 g of a 40% glyoxal aqueous solution was diluted 50 times with water.

38 g of 1% potassium pyrosulfite (K2 S208) aqueous solution and 2% potassium pyrosulfite (K2 S208)
S20s) 21.8 g of the aqueous solution was added to the above neutralized solution and mixed. Then, the obtained mixed liquid was
The mixture was poured into a 7 cm shallow vat whose inner surface was coated with Teflon, and heated and polymerized in a hot air circulation dryer at 42° C. for 20 minutes to obtain a hydrous gel with a thickness of 5 to 6 mm. Next, put this hydrogel into a drum dryer,
After completing the polymerization and drying at the same time to obtain a flaky resin, this resin is crushed with a pin mill, and most of the resin is 16
Powdered water absorbent resin (-
I obtained the following product.

Next, put the above-mentioned product 2000i into a super mixer with an internal volume of 20g, and mix it with 200g of water while stirring.
- A solution in which 40 g of butanediol and 2 g of ethylene glycol diglycidyl ether were dissolved was added by spraying over a period of 2 minutes. After the addition of the above solution was completed, stirring was continued for another 5 minutes, and then the jacket of the super mixer was put under pressure of 2
kg/c+a of water vapor was introduced and heated and dried for 10 minutes to crosslink the resin surface and complete a sample water absorbent resin product.

Example 1 A swelling liquid consisting of ethanol and water (
After adding 400 g of the water-absorbing resin product (ethanol:water-4:6) and stirring for 5 minutes, 1 kg of ethanol as a dehydrating liquid was added while stirring, and stirring was continued for another 10 minutes.

After stirring, water and ethanol were removed by filtration, and further 1
The powdered water absorbent resin was purified by drying at 00° C. for 60 minutes.

Example 2 Powdered polyacrylic acid-based water absorbent resin 50K, which was collected by disassembling a disposable disposable diaper (manufactured by Daio Paper ■, trade name: Elle Air Friend), was added to a swelling liquid (made by acetone and water).
After stirring for 10 minutes with 300 g of acetone:water (6:4), add 200 g of acetone as a dehydrated liquid while continuing to stir.
was added and stirring was continued for an additional 30 minutes. Next, water and acetone were removed by filtration, and then dried under the same conditions as in Example 1 to complete purification of the powdered water absorbent resin.

The amount of residual low degree of polymerization components of the powdered water absorbent resin used in each of the above Examples before and after purification was determined by gas chromatography, and also by the methods shown in a) and b) below.
The water absorption amount (times) of the powdered water absorbent resin before and after purification and the gel strength upon water absorption were measured.

The results are shown in the table.

In addition, in the quantitative determination using the gas chromatography method described above, it was almost impossible to detect components with a low degree of polymerization other than the heptamer both before and after purification.
Only 1 (%) was presented. In addition, this residual monomer amount (
The monomer reduction rate (%) was calculated from the following formula (Q% before purification, q% after purification). The results are also shown in the table.

a) Measurement of water absorption amount Powdered water absorbent resin (Xg) was placed in a folded commercially available coffee filter, and after immersing it in 0.9% saline for a certain period of time, the total weight (Y + r) including the coffee filter was calculated. It was measured. On the other hand, the weight (Zg) of only the water-absorbed coffee filter was measured, and the water absorption amount (times) was determined from the following formula.

Water absorption amount (times) - (Y-Z)/X b) Measurement of gel strength 0.9% saline in a 200 ml beaker 97.
5 g was added thereto, and while stirring with a magnetic stirrer, 2.5 g of powdered water-absorbing resin was added to form a gel.

After leaving the generated gel for 24 hours, JIS standard ball bearing steel balls whose diameter increased from 3 to 16 inches in increments of 1/16 inches were placed on top of the gel in order from the smallest to the smallest, and the steel balls settled in the gel. I observed whether or not.

The maximum diameter of the steel balls that did not settle in the gel was defined as the gel strength. Note that during the above measurements, the steel balls that did not settle were removed before the next steel ball was placed.

From the results in the table, in Example 1.2, the amount of residual monomer could be significantly reduced compared to before purification while maintaining water absorption performance such as water absorption amount and gel strength at the level before purification.
We were able to improve safety.

Effects> According to the water absorbent resin purification method of the present invention having the above configuration, the low degree of polymerization component remaining in the acrylic acid water absorbent resin produced with high productivity by a conventional method is removed by the swelling liquid and Since it can be removed from the resin by sequentially mixing it with the dehydration solution, it is possible to improve the safety of the acrylic acid-based water-absorbing resin while maintaining its water-absorbing performance and without reducing productivity. It becomes possible.

Patent applicant Sekisui Plastics Co., Ltd. (and one other person)

Claims (1)

[Claims] 1. Powdered polyacrylic acid water absorbent resin was swollen with a mixed solution of water and a hydrophilic organic solvent. After water treatment, dehydration treatment with a hydrophilic organic solvent water contained in the water-absorbent resin. It is characterized by removing low polymerization degree components from the solution. A method for purifying water-absorbing resin.