JPH051821B2 - - Google Patents

Description

[Detailed description of the invention] Industrial applications The present invention relates to a new water absorbent material.

Polyacrylic acid-based superabsorbent resins are used in powdered form as water-absorbing materials for sanitary materials such as sanitary napkins and disposable diapers.

Conventional technology Polyacrylic acid-based super absorbent resins are well known;
There are many patent applications related to the manufacturing method. The structure of the polymer is an alkali metal salt of a polyacrylic polymer made insoluble in water by crosslinking, and the degree of neutralization is adjusted so that the pH of the hydrated gel is in the neutral range.

The water absorption performance of a resin is expressed by the balance of water absorption amount, water absorption rate, and mechanical strength of the hydrated gel. These properties are primarily controlled by crosslink density. As the crosslinking density increases, the mechanical strength of the hydrated gel increases, but the water absorption decreases. The water absorption rate is basically proportional to the surface area, but it tends to decrease as the crosslinking density decreases and the hydrated gel becomes softer.

Since crosslinking is introduced to make the polymer insoluble in water, it is desirable that the crosslinking be uniformly distributed in the polymer. If the crosslinking is nonuniform, not only will the water absorption performance deteriorate, but some water-soluble portions will remain, imparting stickiness to the hydrated gel, impairing its feel, and lowering the product value.

Polyacrylic acid polymers are produced by using acrylic acid as the main component, and selectively adding water-soluble comonomers such as methacrylic acid and maleic anhydride, and water-soluble polymers such as starch and cellulose derivatives. A radical initiator is added to an aqueous solution neutralized with caustic alkali and polymerized.

The polymerization reaction occurs rapidly at concentrations above 20%, which is the practical concentration range, and is accompanied by intense heat generation, so controlling the reaction is the key to the production method. As a known polymerization method, the most common method is reverse-phase suspension polymerization, in which a stock solution is suspended and dispersed in an organic solvent. There are known methods in which polymerization is carried out while suppressing the polymerization and crosslinking of the resulting polymer, or thin layer polymerization methods in which the heat of polymerization can be easily removed.

Known methods for introducing crosslinking include (1) self-crosslinking during polymerization, (2) copolymerization of a crosslinkable monomer during polymerization, and (3) post-crosslinking of the polymer.

The following methods have been proposed for post-crosslinking polymers.

(1) Use a crosslinking agent that has two or more functional groups that react with functional groups such as carboxyl groups of the polymer. For example, crosslinking is introduced by reacting a polyhydric alcohol such as glycerin or a polyhydric epoxy compound such as ethylene glycol diglycidyl ether with a polyacrylic acid polymer (Japanese Patent Laid-Open No. 57-44627
Such).

(2) A polymerizable unsaturated monomer having a functional group that reacts with the functional group of the polymer is added to the polymer and polymerized to introduce crosslinking. For example, in the case of glycidyl acrylate, this polymer has many glycidyl groups in its side chains, which react with the carboxyl groups of the polymer to form crosslinks (JP-A-58
−79006 etc.).

(3) Introducing crosslinking by irradiating a water-absorbing polymer with radiation such as an electron beam (JP-A-59-12923260-
32830).

Polyacrylic acid-based superabsorbent resin is applied as a powder to paper, pulp cotton, etc., and provides these base materials with a high degree of water supply and water retention. In the case of powder dispersion, there is a drawback that the hydrated gel separates from the base material when water is absorbed and adheres to the skin, giving an unfavorable feel.

A crosslinking agent is added to a polymer solution containing carboxyl or hydroxyl groups, which is a precursor of superabsorbent resin, and the mixture is applied to a porous substrate such as a nonwoven fabric, and crosslinks are formed by heating during drying. There is also a method of imparting water absorbency (Japanese Patent Application Laid-open No. 1983-
84804). In this case, the resin adheres closely to the base material and separation of the hydrated gel is prevented when water is absorbed, but it is difficult for a high viscosity polymer solution to penetrate uniformly into the base material, and the water absorption capacity tends to be poor.

Problems to be Solved by the Invention Super absorbent resins are put to practical use in the form of powder or impregnated into fibers. Most of its uses are in sanitary materials, where it is used as a water-absorbing material in sanitary napkins, disposable diapers, and other products.

In order to obtain a practical product using the conventionally known method described in the previous section, a crosslinking agent is used to introduce crosslinking. That is, crosslinking is introduced by copolymerization of a polyfunctional comonomer such as methylene bisacrylamide or by reaction with a post-crosslinking agent such as ethylene glycol diglycidyl ether.

Sodium polyacrylate is approved as a food additive and its safety has been confirmed.
Therefore, in the production of polyacrylic acid-based superabsorbent resins, it would be possible to obtain resins with excellent water absorption capacity without using crosslinking agents whose safety has not been confirmed.
Its practical value is great. Furthermore, the conventionally known production methods include factors that are undesirable in terms of production cost, such as the use of organic solvents, suppression of crosslinking during the polymerization process due to a reduction in the concentration of the stock solution, and thin layer polymerization. All of these elements are necessary for removing polymerization heat in the polymerization reaction.

There is a need for a process that effectively controls intense polymerization reactions, has high production efficiency, and has low manufacturing costs.

Furthermore, it would be of great value if a new and effective method could be found to replace dispersion of resin powder or coating of polymer solution in order to impart high water absorbency to textile products such as paper and nonwoven fabrics.

Means for Solving the Problems The present inventor first impregnated fibers such as pulp with an aqueous stock solution for producing a polyacrylic acid-based superabsorbent resin to smoothly dissipate water vapor generated by polymerization heat to the outside of the system. invented a method of polymerization (patent application 1983-
186581, 60−27570).

In the present invention, cellulose is produced by adding a water-soluble polymerization initiator to an aqueous solution containing an alkali metal acrylic acid salt as a main component, containing no crosslinkable polyfunctional comonomer, and having a pH in the neutral range. The resulting polymer is heated and reacted in the presence of a water-soluble peroxide radical initiator to form a polymer component by radical crosslinking. The present invention relates to a water-absorbing material comprising a close composition of a water-absorbing resin made by insoluble in water and cellulose fibers.

The above-mentioned stock solution is characterized in that it does not contain a polyfunctional copolymerizable monomer component that generates crosslinks during the polymerization process. The concentration of the stock solution is usually 20% or more, and the above-mentioned known knowledge is applied to small amounts of components other than acrylic acid, such as monomers and water-soluble polymers. A typical example is one that does not contain these minor components.

Cellulose fibers are selected depending on the purpose of the product, such as pulp, absorbent cotton, cellulose fiber web, mat, paper, nonwoven fabric, and woven fabric.

For example, to obtain a powdered product, a concentrated stock solution is impregnated into a small amount of pulp and polymerized. Due to the presence of pulp fibers, water vapor generated by polymerization heat is smoothly diffused out of the system, making it possible to effectively control the reaction. In this case, it has been found that the amount of pulp fiber required for reaction control can be significantly reduced by suppressing the formation of crosslinks during the polymerization process without copolymerizing the crosslinkable polyfunctional monomer. Polymerization products often lack crosslinking density for practical use, so the polymerization product is heated and reacted in the presence of a water-soluble peroxide radical initiator to introduce appropriate crosslinking through radical crosslinking.

Radical crosslinking is a familiar crosslinking means in the rubber industry, and organic peroxides such as dicumyl peroxide are used.

The present inventor conceived of a method of radically crosslinking an aqueous composition of a polymer, which is a polymerization product, using a water-soluble radical initiator such as a persulfate, and completed the present invention as a result of intensive research.

Avoiding the use of crosslinkable polyfunctional comonomers has the advantage not only of safety as described above, but also of reducing the amount of cellulosic fibers required to control the polymerization reaction.

As the water-soluble peroxide radical initiator, inorganic peroxides such as potassium persulfate, ammonium persulfate, and hydrogen peroxide are preferably used. A radical initiator is added to the polymerization product, and the mixture is heated and reacted while being uniformly kneaded in a kneader such as a kneader to perform radical crosslinking. In this case, if the water content in the aqueous polymer composition is large, the crosslinking efficiency of the initiator (the number of moles of crosslinking bonds generated from 1 mole of initiator) will decrease, so the water content should be 60% by weight or less. desirable. Therefore, the polymerization product is concentrated in advance if necessary.

As a convenient method, an amount of initiator required for radical crosslinking can be added to the polymerization stock solution in advance, and post-crosslinking can be carried out using the initiator remaining after the polymerization reaction. In this case, the initiator is the same as the polymerization initiator, so
Temperature control of the polymerization reaction is essential. If the temperature of the polymerization system increases beyond the limit, most of the initiator will decompose, causing the polymerization reaction to run out of control and also reducing the quality of the polymer. The presence of cellulosic fibers in the polymerization system effectively removes the heat of polymerization through evaporation of water and controls the temperature of the polymerization system, making this convenient method possible. For example, the amount of initiator required for a polymerization reaction is usually 0.1% or less of the stock solution, but even if 0.5% of initiator is added, by controlling the polymerization temperature, the remaining initiator can generate radicals. It has been found that post-crosslinking is carried out effectively. Radical crosslinking is carried out during heating drying of the polymerization product.

For example, when 0.5% potassium persulfate is added to a 40% concentrated stock solution and the mixture impregnated into pulp fibers is polymerized while stirring, the product partially dissolves in water and crosslinking is incomplete. However, by drying the product at a temperature of 100°C or higher, it becomes insoluble in water and exhibits a high water absorption ability. By this method, the polymerization reaction can be effectively controlled even if the pulp content in the dry composition is reduced to 6%, and a powdered product with a high water absorption capacity can be obtained by grinding the dry composition. .

For the purpose of obtaining a fibrous water-absorbing material, it is desirable that the amount of resin in the polymeric composition be smaller than the amount of fiber. When the amount of resin is large, the polymer composition is hard and difficult to dry-open, and even when wet-open, a hydrated gel is generated, making processing difficult.

The forms of cellulose fibers vary depending on the intended product, but to speed up understanding, an example of water-absorbing absorbent cotton will be used for explanation.

Absorbent cotton itself has about 10 times the water absorbency, but if absorbent cotton can be obtained that has two to three times the water absorption and water retention ability of ordinary absorbent cotton, for example, it would have great practical value. This is accomplished by impregnating the cotton wool with a few percent of the resin. As a result of various studies, it has been found that a product prepared by opening a composition with a resin content of about 20% and mixing it with absorbent cotton to dilute the resin content to several percent is suitable for this purpose.

The resin-impregnated absorbent cotton obtained in this way does not separate the hydrated gel from the fibers when water is absorbed, and has an excellent feel. It has high practical value as a blood-sucking material.

Spray the polymerization stock solution uniformly onto absorbent cotton to impregnate it.
Polymerization is carried out in a polymerization vessel purged with nitrogen. in this case,
Due to the large amount of fiber, the heat of polymerization is easily removed by evaporation of water. The initiator necessary for radical crosslinking is added to the polymerization stock solution in advance. The polymerization product is fibrous, and after pre-drying at a temperature that does not decompose the remaining initiator to reduce the moisture content, radical cross-linking is introduced at the same time as drying at a high temperature.
A super absorbent resin is produced that is uniformly impregnated into the fibers.
When the resin content is approximately 20%, the dried product is passed through a fiber opening machine, and at the same time as it is opened, it is uniformly mixed with untreated absorbent cotton to obtain the desired water-absorbing absorbent cotton.

An example of water-absorbent nonwoven fabric is given below.

The pulp sheet is uniformly impregnated with the polymerization stock solution and polymerized. The method of radical crosslinking is the same as that for absorbent cotton. The resin-impregnated pulp sheet is opened, for example, in a methanol-water mixed solvent. The swelling properties of the sheet can be freely adjusted depending on the composition of the mixed solvent. At the same time as opening, it is uniformly mixed with untreated pulp or rayon fibers at a predetermined ratio, and after forming a web, it is processed into a nonwoven fabric. Non-woven fabrics with a resin content of several percent are 2 to 3 times cheaper than regular products.
It has twice the water absorption and water retention capacity. The fibers to be mixed are not limited to cellulose fibers, and for example, polyolefin fibers, nylon fibers, and polyester fibers are also selected depending on the purpose of the product.

As understood from the above description, the water-absorbing material of the present invention can take various forms such as powder, cotton, web, mat, paper, and nonwoven fabric, and can be manufactured depending on the purpose.

Effect The water-absorbing material of the present invention is in the form of powder or fibers, and the polyacrylic acid-based superabsorbent resin and fibers are uniformly integrated and strongly bonded, so it is significantly more effective than conventional products. have characteristics. Existing polyacrylic acid-based superabsorbent resin products are in powder form, and products in which water absorption is imparted by scattering powder onto a fiber base material are known. In this case, there is a drawback that the hydrated gel separates from the fibers upon water absorption.

The fibrous water-absorbing material of the present invention does not contain any cross-linking agent compound and has been confirmed to be safe.There is no separation of the hydrated gel when it absorbs water, there is no sticky feeling to the touch, and it has a high degree of water absorption and water retention. has.

Example 1 A stock solution was prepared by adding 72 parts of acrylic acid to an aqueous solution of 34 parts of caustic soda dissolved in 116 parts of water, and further adding 0.1 part of potassium persulfate. Add flocculent pulp 3 to the stock solution
1 part and heated while stirring to polymerize. Polymerization was completed in about 5 minutes with smooth dissipation of water vapor. The product is partially water soluble. To this was added 1 part of potassium persulfate dissolved in a small amount of water, and the mixture was kneaded in a kneader while raising the temperature to 120° C. for about 30 minutes to react. The reactant was chopped, dried at a temperature of 110°C, and ground to obtain a powdered product.

Add 0.5g of the product to 200c.c. of water, leave it for 1 hour, and then
The hydrated gel produced by passing through a mesh wire mesh was weighed and weighed 110 g. The water absorption capacity was 260 times.

0.5g of product to 0.9% saline (physiological saline) 100c.c.
In addition, the hydrated gel was weighed in the same manner after being left for 1 hour. The water absorption rate was 42 times.

Example 2 A stock solution was prepared by adding 72 parts of acrylic acid to an aqueous solution of 34 parts of caustic soda dissolved in 116 parts of water, and further adding 1.2 parts of potassium persulfate. Add flocculent pulp 3 to the stock solution
The polymerization was carried out in the same manner as in Example 1. the product
It was dried at a temperature of 120°C and ground to obtain a powder.

The water absorption capacity was measured in the same manner as in Example 1, and the water absorption capacity for water was 230 times, and the water absorption capacity for physiological saline was 41 times.

For comparison, polymerization was carried out by heating without adding pulp to the stock solution. The heavy reaction went out of control like a popcorn.

Example 3 A stock solution was prepared by adding 72 parts of acrylic acid to an aqueous solution of 34 parts of caustic soda dissolved in 190 parts of water, and further adding 1.6 parts of potassium persulfate.

33.5 parts of the stock solution was uniformly sprayed onto 90 parts of absorbent cotton to impregnate it, and the mixture was heated to 100° C. in a nitrogen atmosphere and polymerized for 10 minutes. The weight of the product was 114 parts. this thing
It was dried at a temperature of 110°C and weighed 100 parts. The resin content is 10%.

1 g of the dried product was added to 100 c.c. of water, and after standing for 1 hour, it was filtered on a wire mesh, and the weight of the water-containing product was 31 g. The weight of water absorbed by 1 g of untreated absorbent cotton was 11 g.

The above fibrous product was opened and mixed uniformly with an equal amount of untreated absorbent cotton. The weight of water absorbed by 1 g of mixed cotton was 23 g.

Example 4 100 parts of a pulp sheet was uniformly impregnated with 73 parts of the stock solution of Example 3, and polymerized in the same manner as in Example 3. The weight of the product was 148 parts. This product was dried at 110°C and weighed 122 parts. Resin content is 18%
It is.

Next, 10 parts of the resin-containing pulp and 20 parts of the untreated pulp were added to 150 parts of water to swell them, and then 300 parts of methanol was added and opened to obtain a slurry. A web was created from this slurry. The web was press dried to produce a nonwoven fabric.

The water absorption capacity of the nonwoven fabric was 24 parts. Moreover, the water absorption capacity of a nonwoven fabric similarly produced only from untreated pulp was 9 times.

Effects of the Invention As shown in the above examples, the water absorbent material of the present invention has excellent water absorption and water retention properties in both powdered products and fibrous products. The hydrated gel, which is a powdered product, exhibits not only good water absorption ability but also good hardness and texture, and has excellent performance as a water absorbing material for sanitary napkins and disposable diapers.

Examples of water-absorbing absorbent cotton and non-woven fabric are shown in the examples as fibrous water-absorbing materials, and as can be seen from these examples, the resin and fibers are tightly integrated,
The hydrated gel does not separate from the fibers when water is absorbed.
Provides excellent water absorption/retention properties and a good feel.

The fibers used for mixing are not limited to cellulose fibers, but synthetic fibers such as polyolefin fibers, nylon, polyester, acrylic fibers, and mixtures thereof can also be used, and products with their respective characteristics can be obtained.

Claims (1)

[Scope of Claims] 1 A water-soluble polymerization initiator is added to an aqueous solution containing an alkali metal acrylic acid salt as a main component, containing no crosslinkable polyfunctional comonomer, and having a pH in the neutral range. A partially water-soluble polymer produced by impregnating cellulose fibers with the stock solution and heating the impregnated product to polymerize monomer components is heated and reacted in the presence of a water-soluble peroxide radical initiator, A water-absorbing material made of a tight composition in which a water-absorbing resin made by making a polymer component insolubilized in water through radical crosslinking and cellulose fibers are integrated and bonded. 2. A stock solution made by adding a water-soluble polymerization initiator to an aqueous solution containing an alkali metal acrylic acid salt as a main component, containing no crosslinkable polyfunctional comonomer, and having a pH in the neutral range, is applied to cellulose fibers. The partially water-soluble polymer produced by impregnating and heating the impregnated material to polymerize the monomer components is heated and reacted in the presence of a water-soluble peroxide radical initiator, and is polymerized by radical crosslinking. A powdery water-absorbing material obtained by pulverizing a tight composition in which the combined components are insolubilized in water and contains more water-absorbing resin than cellulose fibers, in which both are integrated and bonded. 3. A stock solution made by adding a water-soluble polymerization initiator to an aqueous solution containing an alkali metal acrylic acid salt as a main component, containing no crosslinkable polyfunctional comonomer, and having a pH in the neutral range, is applied to cellulose fibers. The partially water-soluble polymer produced by impregnating and heating the impregnated material to polymerize the monomer components is heated and reacted in the presence of a water-soluble peroxide radical initiator, and is polymerized by radical crosslinking. A fibrous water-absorbent product obtained by making the combined components insoluble in water and opening a tight composition in which the two are integrated and bonded, containing more cellulose fiber than the water-absorbent resin, and mixing it with fibers. material.