JPH0367545B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an aqueous solution of a polyacrylic acid polymer that forms a water-absorbing film on a substrate, and a coating obtained by coating and drying the aqueous solution.

Super-absorbent resins that can absorb several hundred times their own weight in water have been commercialized, and water-absorbing materials made by sprinkling resin powder onto fiber materials such as paper and pulp cotton are used in disposable diapers, sanitary napkins, and the like. In addition, various applied products such as anti-condensation wallpapers and water-absorbing paints containing super-absorbent resin powder have been put into practical use, and the range of industrial applications is wide.

Conventional technology Various types of super absorbent resins are known, but
Polyacrylic acid resins are the mainstream.

Polyacrylic acid-based superabsorbent resins are made by introducing crosslinking into acrylic acid homopolymers or copolymers whose main component is acrylic acid to make them insoluble in water, and there are many known documents regarding their production methods. It will be done.

Super absorbent resins are dispersed in powder form on fibrous materials such as paper, nonwoven fabrics, pulp cotton, etc., or mixed with plastics and rubber materials, and have the effect of imparting water absorbency to these base materials.

Polyacrylic acid-based superabsorbent resins are made by introducing appropriate cross-linking bonds into water-soluble electrolyte polymers. By applying an aqueous polymer solution to the base material before introducing crosslinking, and introducing crosslinking during the heating and drying process,
A super absorbent resin film can be formed.

JP-A-50-82143 discloses that a soluble cross-linking agent reactive with carboxyl groups contained in the polymer is added to an aqueous solution of an electrolyte polymer such as an alkali metal salt of polyacrylate, this aqueous solution is applied to a substrate, and the coating film is dried by heating. Discloses a method for forming a water-absorbing film by introducing crosslinking into a polymer during the process. As a crosslinking agent,
Compounds such as polyhaloalkanols and polyglycidyl ethers are used. In addition, an aqueous solution containing a maleic acid copolymer and a crosslinking agent reactive with the carboxyl group of areic acid is applied to a base material such as a nonwoven fabric, and crosslinking is introduced into the polymer during a dry heat treatment, making the base material water absorbent. There is also a method of assigning
58−84804).

Problems to be Solved by the Invention An aqueous solution containing a polyacrylic acid polymer and a crosslinking agent reactive with the carboxyl groups of the polymer is applied to a substrate, and crosslinking is introduced into the polymer during a dry heat treatment process. The method of forming a superabsorbent resin film belongs to a known concept, as can be seen from the above-mentioned cited examples. Here, the polyacrylic acid polymer refers to a polymer containing at least 50% by weight of acrylic acid monomer units. Polymers usually have 60 to 60 carboxyl groups.
90 mol% is neutralized as alkali metal salts.

Polyvalent epoxy compounds such as ethylene glycol diglycidyl ether, haloalkanol compounds such as epichlorohydrin, and carboxyl groups such as polyamine compounds are added to the polymer aqueous solution obtained by polymerizing an aqueous monomer solution containing acrylic acid as a main component as a crosslinking agent. and a reactive compound are added.

These crosslinking agents are known as crosslinking agents for acrylic paints and adhesives.

We investigated the known methods of applying or impregnating a polyacrylic acid polymer aqueous solution containing these known crosslinking agents onto a base material such as a nonwoven fabric, and drying and heat-treating this material to introduce crosslinking. found.

The crosslinking reaction involves a reaction between the carboxyl (carboxylate) groups in the polymer and the functional groups of the crosslinking agent. The reaction is accelerated by increasing the temperature and concentrating during the drying process.

When forming a superabsorbent resin coating on a substrate using this method, from a practical point of view it is important that the coating stock solution has sufficient storage stability and that the formation of crosslinks is short enough to be incorporated into the normal drying process. It is necessary that it be done in time. Since the base material is often made of cellulose fibers such as paper or nonwoven fabric, high temperatures exceeding 150°C should be avoided.

When using the known crosslinking agent described in the above cited example, the storage stability of the coating stock solution may be insufficient, or the introduction of crosslinking during the drying process may require a long time or a high temperature that may damage the base material. It was found that the process did not meet sufficient requirements as an industrial process. This is supported by the fact that the known technology has not been widely implemented industrially.

Therefore, if a crosslinking method that satisfies the above-mentioned preferable conditions is found, a new water-absorbing material can be provided, and its industrial value is extremely large.

Means for Solving the Problems The present inventor first heated an aqueous composition of a polyacrylic acid polymer in the presence of a water-soluble peroxide radical initiator to form a superabsorbent resin by radical crosslinking. Invented a manufacturing method (patent application 136653/1986).
The present invention was completed based on the above-mentioned prior invention.

Specifically, the polyacrylic acid polymer includes the following polymers. Monomers copolymerized as minor components in polyacrylic acid homopolymers and copolymers include methacrylic acid, maleic acid (anhydride), fumaric acid, itaconic acid, acrylamide,
Examples include water-soluble monomers such as methacrylamide. It also includes monomers that are normally insoluble in water, such as acrylic esters, methacrylic esters, vinyl acetate, and styrene, and these monomers can be copolymerized with acrylic acid or copolymerized with acrylic esters. By hydrolyzing , a water-soluble polyacrylic acid copolymer can be obtained.

Furthermore, the copolymers also include graft or block type copolymers. For example, if a polyvinyl acetate/polyacrylic acid (ester) based graft copolymer is hydrolyzed, a water-soluble polyvinyl alcohol/polyacrylic acid based graft copolymer can be obtained.

The polyacrylic acid-based polymer of the present invention is water-soluble, usually contains at least 70% by weight of an acrylic acid monomer component, and the carboxyl groups contained in the polymer are partially neutralized as an alkali metal salt, which is preferable. The degree of neutralization is 60-90 mol%. In view of the water absorption ability of the polymer film which has been cross-linked and made into a super absorbent resin, a preferable polymer is one with 60 carboxyl groups.
Examples include polyacrylic acid with ~90 mol% neutralized to sodium salts.

Persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate are commonly used as water-soluble peroxide radical initiators, but hydrogen peroxide, peracetic acid, t-butyl peroxymaleic acid, succinic acid, etc. Also includes peroxides. These may be used alone or as a mixture of two or more. The amount of initiator added is not particularly limited, but it is usually
It is used in a range of 0.1 to 5% by weight.

The concentration of the aqueous polymer solution is adjusted to a viscosity suitable for coating or impregnating a substrate to form a film.

Since an aqueous polymer solution tends to have a high viscosity even at a low concentration, it is preferable to reduce the viscosity. An effective method for this is to add a volatile water-soluble organic solvent instead of a simple aqueous solution to make it an appropriately poor solvent. In the case of a good solvent, the viscosity of the polymer solution is high, and a method of lowering the viscosity of the solution by appropriately mixing a non-solvent to make it a poor solvent is well known for coatings.

Suitable organic solvents include alcohols such as methanol, ethanol and propanol, and ketones such as acetone and methyl ethyl ketone. Since these organic solvents are non-solvents for the polymer, they are added to the extent that the polymer does not precipitate. As a specific method, a method of adding a mixture of these organic solvents and water to an aqueous polymer solution is suitable. By such a method, an aqueous polymer solution having a relatively high concentration and good coating properties can be prepared. The concentration of polymer is usually in the range 5-20% by weight.

A stock solution prepared by adding a water-soluble peroxide radical initiator to an aqueous solution of a polyacrylic acid polymer was cast onto a flat plate, and the formation of crosslinks during the heating and drying process was investigated, and the following remarkable facts were discovered.

In order to find out the relationship between the crosslinking efficiency of the initiator and the water content, the stock solution was concentrated under reduced pressure to a predetermined water content at a temperature that suppressed the decomposition of the initiator, and then the solution was heated in a closed state covered with a polyester film. , the initiator was decomposed. The film was peeled off and the dried coating was placed in water, and the formation of crosslinks was examined from the state of swelling upon absorption of water.

It was found that crosslinking occurs due to decomposition of the initiator, and the crosslinking efficiency changes depending on the amount of water at that time.
Crosslinks are formed efficiently when the moisture in the mixture is in the range of 60-30%, and surprisingly, if the moisture is removed beforehand and the initiator is decomposed by heating, the formation of crosslinks is reduced. The water content of the product increases. This indicates that the presence of water is necessary for crosslink formation.

Radical crosslinking of plastics or rubbers such as polyethylene, EVA, and EPDM using organic peroxides such as dicumyl peroxide has been investigated in detail. All polymers are highly flexible, with glass transition temperatures (Tg) of 0° C. or lower, and the reaction takes place under melt-kneading. Generally, at temperatures below Tg,
It is known that the molecular motion of polymers is frozen, making chemical reactions inactive.

The polymer of the present invention, typified by polyacrylic acid sodium salt, is infusible, has an extremely high Tg, is a brittle solid even at the decomposition temperature of the initiator, and is presumed to have molecular motion in a frozen state. Water acts as a plasticizer and softens the polymer.

The fact that crosslinking is difficult to occur in the absence of water and that the presence of an appropriate amount of water is required is a novel discovery that could not have been predicted based on the knowledge of known radical crosslinking.

When a polymer stock solution is applied or impregnated onto a base material such as a non-woven fabric and heated in an atmosphere above the decomposition temperature of the initiator, water quickly evaporates from the stock solution and the temperature of the film increases to 100% due to the latent heat of vaporization. The temperature remains near ℃, and almost no decomposition of the initiator occurs. The temperature does not rise until the concentration progresses and most of the water has evaporated. In order to effectively introduce crosslinking into the polymer, it is desirable to heat the polymer to a temperature higher than the decomposition temperature of the initiator in a concentrated state in which an appropriate amount of water remains.

Regarding the decomposition properties of commonly used persulfate aqueous solutions such as potassium persulfate, the one-minute half-life is estimated to be approximately 125°C from known literature. If it can be heated to a temperature of 120℃ or higher with an appropriate amount of water remaining,
Effective crosslinks are formed in a short time. A preferred method is heating with superheated steam.

The undiluted solution coating applied to the substrate is appropriately concentrated by preliminary drying, if necessary, and then heated in a superheated steam atmosphere. Water evaporates and the water content decreases, while the temperature also rises above the decomposition temperature of the initiator and crosslinks are formed. The heating drying method using superheated steam is known as the wet curing method in resin processing of fibers, and a similar method is applied to the present invention.

Since the concentration of the aqueous polymer solution is concentrated during the process of heating and drying the coating film, the initial concentration is not limited. It is important to heat the initiator to a temperature higher than its decomposition temperature at a stage where it is appropriately concentrated.

In radical crosslinking of plastics and rubber,
It is known to use sulfur, unsaturated compounds, etc. as crosslinking aids. In the radical crosslinking of the present invention, the effects of these crosslinking aids were investigated.

As a result of selecting and examining known hydrophilic radical crosslinking aids, it was found that compounds having polymerizable unsaturated groups, especially polyfunctional unsaturated compounds having two or more polymerizable unsaturated groups in the molecule, are effective. Ivy. Examples include methylene bisacrylamide, ethylene glycol diacrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, and the like. These auxiliaries are added in the same amount as the radical initiator.

The crosslinks must be introduced at a suitable density so that the polymer has a high degree of swelling which renders it insoluble in water.
Although the objective is achieved without the use of auxiliaries, auxiliaries make it possible to reduce the amount of initiator added and promote the formation of crosslinks.

Polyacrylic acid polymers are hard and brittle in a dry state, and when paper, nonwoven fabric, or the like is used as a base material, the coating tends to lose its original flexibility. It is known that polymers can be softened by adding hydrophilic solvents.

Although the softener may be added to the coating by means such as spraying, a method of adding the softener to the stock solution in advance has been studied. Polyhydric alcohol compounds such as ethylene glycol, diethylene glycol, glycerin, and propylene glycol are examples of compounds that exhibit excellent softening effects without evaporating during the heat-drying process and without inhibiting the formation of crosslinks.

It is also believed that the polyhydric alcohol acts as a crosslinking agent that reacts with the carboxyl groups of the polymer to form crosslinks. Therefore, as an example, crosslinking was compared using a stock solution in which propylene glycol was added at a ratio of 20% by weight to the polymer and a stock solution without the additive. The polymer films produced by casting the stock solution onto a hot plate and heating and drying exhibited almost the same water absorption capacity, and it was found that crosslinking due to polyhydric alcohol could be ignored under the conditions of the present invention.
The above result is natural since radical crosslinking occurs in the presence of water, while crosslinking by polyhydric alcohol occurs at a high temperature of 150° C. or higher after water has been distilled off.

The film to which polyhydric alcohol was added remained flexible even after drying, while the film to which no polyhydric alcohol was added was hard and brittle. The amount of polyhydric alcohol added is usually 5 to 20% by weight based on the polymer to give preferable results.

The substrate to which the aqueous polymer solution is applied or impregnated is not particularly limited, but it must not contain components that inhibit the formation of a crosslinked polymer film. It is necessary that the raw solution film does not contain any substances such as inorganic salts that are eluted into the film or radical inhibitors that inhibit the action of the initiator.

This is because multivalent ions such as calcium form salts with the polymer and impair water absorption.

Cotton-like or paper, non-woven fabrics made of natural fibers such as wood pulp, linder pulp, and cotton, synthetic fibers such as polyester, acrylic, and nylon,
Sheet-like fiber materials such as woven fabric, felt, and mat are preferably used. In these porous substrates,
Although the product tends to have a superabsorbent resin film formed all the way to the inside, the permeation of the solution can be adjusted freely depending on the solution viscosity and coating amount. Porous inorganic granules such as pumice are also used, and the product is used as a water-absorbing and water-retaining material, for example by being mixed with soil, for agricultural and horticultural purposes.

The substrate is not limited to porous materials. By forming a water-absorbing film on a base material having a smooth surface, it is also possible to impart dew condensation prevention properties, for example.

The method of applying the aqueous solution of the present invention is carried out in the same manner as the application of paints and adhesives.

Function A highly water-absorbent film is formed by applying the aqueous solution of the present invention to a base material and introducing crosslinking into the polymer during the heat-drying process. Cross-linked polyacrylic acid polymers are hard and brittle, so they are unsuitable as films or fibers by themselves, but by coating them on a substrate, they can impart high water absorption and water retention properties to the substrate. .

Various types of water-absorbing and water-retaining materials are provided by coating systems of polyacrylic acid-based superabsorbent resins.

Examples 1-2 Preparation of stock solution An aqueous solution in which 72 parts of acrylic acid and 33 parts of caustic soda (purity 93%) were dissolved in 250 parts of water, and 0.2 parts of potassium persulfate was added thereto, was stirred in a nitrogen atmosphere and the temperature was lowered.
Adjust the temperature to 70~80℃, polymerize for 4 hours, and reduce the concentration.
A 25% aqueous polymer solution was obtained. Add 1.2 parts of potassium persulfate and 220 parts of water to 150 parts of this aqueous solution to give a concentration of 10%.
A stock solution was prepared. Separately, a stock solution was prepared by adding 1.2 parts of potassium persulfate dissolved in a mixed solution of 120 parts of water and 100 parts of methanol to 150 parts of an aqueous solution. The stock solution showed significantly lower viscosity than the stock solution. Next, the stock solution and the stock solution were applied to a pulp-based nonwoven fabric with a thickness of about 1 mm using a roll coater, and the mixture was heated and dried for 6 minutes in a superheated steam blowing dryer heated to 130°C. The resin application amounts were determined to be 22 g/m ² (undiluted solution) and 20 g/m ² (undiluted solution), respectively, from the weight increase of the product.

The obtained resin-coated nonwoven fabric was cut into a size of 20 cm x 10 cm, and water was sprayed onto it to achieve saturated absorption. Weight is 152g (undiluted) and 146g respectively
(undiluted solution). An untreated nonwoven fabric of the same size weighed 1.2 g and had a saturated water absorption of 7.5 g. From this, the water absorption capacity (water absorption capacity) per gram of resin is
326g/g (undiluted solution) and 340g/g (undiluted solution)
It is calculated as follows.

Example 3 0.9 parts of ammonium persulfate and 0.2 parts of N,N'-methylenebisacrylamide were dissolved in 210 parts of water in 100 parts of the aqueous solution of polyacrylic acid sodium salt (neutralization degree 75 mol%) with a concentration of 25% as in Example 1. and add concentration 8
% stock solution was prepared.

Polyester spunbond nonwoven fabric (basis weight 50g/
m ² ) in the same manner as in Example 1, and was heated and dried for 5 minutes in a superheated steam blowing dryer at a temperature of 135°C. Based on the weight of the product, the amount of resin applied is 18
g/ ^m2 .

The obtained resin-coated nonwoven fabric was cut into a size of 20 cm x 10 cm, and water was sprayed onto it to achieve saturated absorption. The weight was 105g. The untreated nonwoven fabric absorbs almost no water; the water absorption is due to the swelling of the resin.
The resin equivalent water absorption capacity is 290g/g.

Example 4 0.75 parts of ammonium persulfate and 3 parts of propylene glycol were dissolved in 105 parts of water and added to 100 parts of the aqueous solution of polyacrylic acid sodium salt (degree of neutralization 75 mol%) with a concentration of 25% as in Example 1 to give a concentration of 12 % stock solution was prepared.

Apply the undiluted solution to a pulp-based nonwoven with a thickness of about 1 mm,
A resin-coated nonwoven fabric was obtained by heating and drying in the same manner as in Example 1. The resin coating amount was 21 g/m ² . Next, this material was cut into a size of 20 cm x 10 cm, and water was sprayed to achieve saturated absorption. The water absorption weight was 132g.

This resin-coated nonwoven fabric has a softer feel compared to the product of Example 1.

Example 5 Preparation of stock solution 57.6 parts of acrylic acid, 14.2 parts of acrylamide, and 27.2 parts of caustic soda (93% purity) were dissolved in 245 parts of water.
An aqueous solution to which 0.2 part of potassium persulfate was added was stirred in a nitrogen atmosphere while controlling the temperature to 70 to 80°C, and polymerized for 4 hours to produce an aqueous polymer solution with a concentration of 25%. Next, 2.2 parts of potassium persulfate, 3.75 parts of glycerin, and water were added to 100 parts of this aqueous solution to dilute the concentration to 10% to prepare a stock solution.

Next, apply the stock solution onto a paper towel using a roll coating machine, cut the coated material into 10cm x 10cm pieces, stack the 5 sheets with Teflon film in between, and place in a household microwave for 4 minutes. Heat and dry. The average amount of resin applied was 26 g/m ² . The sheet was sprayed with water to saturate absorption and weighed 65 g. An untreated paper towel of the same size had a water absorption weight of 4.2 g. The product does not lose its flexibility upon drying.

Effects of the Invention As shown in the above examples, the aqueous polymer solution of the present invention has good coating properties on various substrates,
By heating and drying, a suitable amount of crosslinking is introduced into the polymer, and as a result, a high degree of water absorption and water retention is imparted to the base material.

Claims (1)

[Scope of Claims] 1. A water-soluble peroxide radical initiator and a crosslinking co-agent in an amount necessary to introduce crosslinking into the polyacrylic acid polymer and coating film to make the polymer insoluble in water through heat drying. 2 polymerizable unsaturated groups in the molecule as
1. An aqueous solution for forming a water-absorbing film comprising a hydrophilic polyfunctional unsaturated compound having at least 100% hydrophilic polyfunctional unsaturated compound and a polyhydric alcohol compound as a softening agent for the dry film. 2. The aqueous solution according to claim 1, wherein 60 to 90 mol% of the carboxyl groups contained in the polyacrylic acid polymer are in the form of an alkali metal salt. 3. The aqueous solution according to claim 1, wherein the aqueous solution contains a volatile water-soluble organic solvent and has a lower viscosity than the aqueous solution. 4. The aqueous solution according to claim 1, wherein the crosslinking aid is at least one selected from the group consisting of methylene bisacrylamide, ethylene glycol diacrylate, polyethylene glycol diacrylate, and polypropylene glycol diacrylate.