PROCESS FOR THE PREPARATION OF WATER-ABSORPTIVE FIBROUS MATERIAL

Abstract: Aqueous solution of alkali or ammonium acrylate is applied to a prefabricated fibrous substrate, and the acrylate is subjected to polymerization in two steps, namely the first step in which the acrylate is polymerized by means of a radical polymerization initiator and the second step which is subsequent to the first step and in which the unpolymerized acrylate is polymerized by means of heat treatment at 100 - 250°C.

Description: BACKGROUND OF THE INVENTION Field of the Art This invention relates to a process for preparing a water absorptive composite material comprising a water absorptive polymer and a prefabricated fibrous substrate. More particularly, this invention relates to a process for preparing a water absorptive composite material in which a highly water absorptive polymer is held on a prefabricated substrate, comprising applying an aqueous solution of an acrylic acid type monomer to a prefabricated fibrous substrate, polymerizing the acrylic acid type monomer by means of an aqueous radical polymerization initiator to obtain a precursor of the composite, which is further heated to 100 - 250°C. The water absorptive composite material obtained by the process according to this invention can be advantageously used in the production of a variety of water absorptive materials, because it is excellent in water absorption properties, has an extremely low content of unpolymerized monomers and the highly water absorptive polymer is held with good stability on the fibrous substrate. Prior Art Materials such as paper, pulp, nonwoven fabric, spongy urethane resins and the like have hitherto been used as water retentive materials for a variety of sanitary goods such as a sanitary napkin, paper diaper and the like and a variety of agricultural materials. However, these materials have a water absorption capacity of no more than 10-50 times their own weight, which will cause problems that an extensively increased bulk of the material is required for absorbing or retaining a large amount of water and that water is easily released from the material in which water has been absorbed on pressing it. There have recently been proposed a variety of highly water absorptive polymer materials in order to settle the aforementioned problems of the water absorptive materials of this kind. For instance, there have been proposed a graft polymer of starch (Japanese Patent Publication No. 46199/78, etc.), a denaturated cellulose (Unexamined Published Japanese Patent Application No. 80376/75, etc.), a crosslinked water soluble polymer (Japanese Patent Publication No. 23462/68, etc.), a self-crosslinking polymer of an alkali metal salt of acrylic acid (Japanese Patent Publication No. 30710/79, etc.), and the like. However, these highly water absorptive polymer materials, while having a relatively high level of water absorption properties, are obtained as powder in most cases. Therefore, in order to use them for sanitary goods such as a sanitary napkin, paper diaper or the like, it is necessary to disperse them homogeneously on such substrates as tissue paper, nonwoven fabric, cotton or the like. However, the polymer powder having been dispersed in such a manner is difficult to be firmly held on the substrate and often agglomerate partially. Also, swollen gel after water absorption will easily move from the substrate without being held firmly on it. Therefore, if it is used for a paper diaper, for example, it will give the feeling of stiffness upon urination accompanied with the extremely uncomfortable feeling on wearing. Furthermore, in a process for obtaining an absorber by dispersing such a powdery polymer as described above on a substrate, the absorber will be very expensive because of complicated procedures for powder handling and of problems on processes for efficiently conducting uniform dispersion. As a method for dissolving these problems, there is disclosed a process for producing a water absorptive composite in which an aqueous solution of an acrylic acid type monomer is applied in a previously determined pattern to a prefabricated fibrous substrate to obtain a composite, which is then irradiated with electromagnetic radiation or corpuscular ionizing radiation to convert the acrylic acid type monomer into a highly water absorptive polymer (Unexamined Japanese Patent Publication No. 500546/82). According to this process, uniform dispersion and stable holding of the aforementioned powder on a substrate are considerably improved. However, since electromagnetic radiation or corpuscular ionizing radiation is employed for converting the monomer into the high water absorptive polymer in this process, the highly water absorptive polymer inherent to the specific monomer tends to be crosslinked excessively. As the result, the composite obtained will exhibit extremely poor properties as an absorber. Especially its water absorption capacity will be of a level of only half or less of that of the composite obtained by using the aforementioned highly water absorptive powdery polymer. More recently, Unexamined Published Japanese Patent Application No. 149609/85 discloses a process for preparing a water absorptive composite material comprising previously impregnating a water absorptive organic material with an aqueous solution of an acrylic acid type monomer and adding thereto in a mist form a water soluble radical polymerization initiator, or, a water soluble radical polymerization initiator and a water soluble reducing agent to conduct polymerization. In this process, however, the water soluble polymerization initiator is added after the water absorptive organic material has been impregnated with the acrylic acid type monomer. Thus, although the polymerization initiator is added in a mist form, it is very difficult to completely polymerize the monomer because of occurrence of "uneven polymerization" and as the result the amount of the residual monomers is in a high level, which will cause problems on safety and lead to lowering of the properties of the resulting product, especially in respect of its water absorption capacity. Possible Countermeasure Under these backgrounds, the present inventors have already proposed in Japanese Patent Application No. 193403/85 a method that an aqueous solution of an acrylic acid type monomer having a monomer concentration of 25% by weight or more and either a water soluble radical polymerization initiator or a water soluble radical polymerization initiator and a water soluble reducing agent are previously mixed homogeneously and the mixture is applied in a mist form to a prefabricated fibrous substrate so that the resulting highly water absorptive polymer in the fibrous substrate will have a diameter in the range of 30 - 500 µm, followed by polymerization; in Japanese Patent Application No. 202908/85 a method that an aqueous solution of an acrylic acid type monomer containing a small amount of a crosslinking agent and either a water soluble radical polymerization initiator or a water soluble radical polymerization initiator and a water soluble reducing agent are previously mixed homogeneously and the mixture is applied in a mist form to a prefabricated fibrous substrate so that the resulting highly water absorptive polymer in the fibrous substrate will have a diameter in the range of 30 - 500 µm, followed by polymerization; in Japanese Patent Application No. 238421/85 a method that an aqueous solution of an acrylic acid type monomer containing a small amount of a crosslinking agent and an oxidizing radical polymerization initiator are previously mixed and the mixture is applied to a fibrous substrate, and then an amine or a reducing agent is added to conduct polymerization; and in Japanese Patent Application No. 238420/85 a method that an aqueous solution of an acrylic acid type monomer containing a small amount of a crosslinking agent and an amine or a reducing agent are mixed, followed by application to a fibrous substrate and then addition of an oxidizing radical polymerization initiator to conduct polymerization; and the like. It has been found according to these methods that polymerization proceeds very easily, "uneven polymerization" is appreciably reduced and a composite having a large water absorption capacity can be obtained. However, in the water absorptive composite thus obtained still remain unpolymerized monomers, thus causing problems on use for sanitary goods such as a sanitary napkin, paper diaper and the like. US-A-3 995 998 discloses a method for preparing cellulosic or synthetic fibers or fabrics having hydrophilic properties. A cellulosic or synthetic fiber or fabric is wetted with an aqueous solution comprising a water-soluble carboxylic containing acrylic or vinyl monomer and a water-soluble free-radical initiator, said aqueous solution having a pH greater than 3.6 and a water content of about 85%. The wetted fiber or fabric is then cured, and the curing step is usually effected in the range of 120 to 160°C. The monomer concentration is only about 15%. SUMMARY OF THE INVENTION Object of the Invention This invention is an improvement of water absorptive composites described in Unexamined Japanese Patent Publication No. 500546/82 and Unexamined Published Japanese Patent Application No. 149609/85 and proposed by the present inventors in Japanese Patent Application Nos. 193403/85, 202908/85, 238421/85 and 238420/85, providing a process for preparing very easily under a moderate condition a water absorptive composite material which is excellent in water absorption properties and has an extremely reduced amount of unpolymerized monomers. The Invention The present inventors have conducted an intensive research in order to solve the aforementioned problems. As the result, they have found that a water absorptive composite material, which is excellent in water absorption properties and has an extremely reduced amount of unpolymerized monomers and in which the highly water absorptive polymer is held with good stability on the fibrous substrate, can be obtained very easily at low cost by applying an aqueous solution of an acrylic acid type monomer to a prefabricated substrate to polymerize the acrylic acid type monomer and then carrying out heat treatment, and finally reached this invention. Thus, the process for preparing the water absorptive composite material according to this invention is characterized by the combination of the following steps: (A) applying an aqueous solution of a polymerizable monomer comprising as a main component acrylic acid, of which 20% or more of the carboxyl groups have been neutralized to its alkali metal salt or ammonium salt, to a prefabricated fibrous substrate, the monomer concentration of the aqueous solution of a polymerizable monomer being at least 30% by weight and the amount of the polymerizable monomer applied to the prefabricated fibrous substrate being 1 to 10,000 parts by weight per 100 parts by weight of the prefabricated fibrous substrate; (B) polymerizing the polymerizable monomers applied to said fibrous substrate by means of a water-soluble radical polymerization initiator to form a composite of a polymer derived from said polymerizable monomer and said fibrous substrate, the polymerization initiator comprising a redox initiator, and the stage of polymerizing the polymerizable monomer involving either the decomposition of said polymerization initiator while it is previously dissolved in the aqueous solution of the monomer for polymerization, or the spraying of said polymerization initiator in a solution form to the fibrous substrate before decomposition thereof; and (C) heating said composite to 100 - 250°C to obtain a water absorptive composite of which the amount of unreacted monomers is reduced as compared with said composite, wherein the composite to be heated does not contain water in a proportion of 40 parts by weight or more per 1 part by weight of the fibrous substrate. The process for preparing the water absorptive composite material of this invention is very advantageous in that most of the acrylic acid monomer applied to the prefabricated substrate are polymerized by means of a water soluble radical polymerization initiator to form a highly water absorptive polymer whereby the composite material obtained has an increased water absorption capacity, and that since said highly water absorptive polymer is subjected to heat treatment, the composite material obtained has an extremely reduced amount of unreacted monomers, and, the highly water absorptive polymer is held firmly on the fibrous substrate. Thus, a water absorptive composite material far excellent in properties as compared with those of the above mentioned prior art can be obtained easily and inexpensively. Heat treatment at the step (C) in this invention is known as a polymerization means. The effect of this invention of reducing the amount of unreacted monomers by conducting the step (C) may be explained rather easily by the known function of such heat treatment. However, it should be considered unexpected, from the known function of the heat treatment, that the effect of extremely reducing unreacted monomers without lowering the water absorption capacity of the produced composite is successfully obtained according to the invention by carrying out heat treatment in a specific temperature range, namely 100 - 250°C. EMBODIMENT OF THE INVENTION Steps (A) and (B) Monomer The monomer used in this invention contains as a main component acrylic acid, of which 20% or more, preferably 50% or more of the carboxyl groups are neutralized into its alkali metal salt or an ammonium salt. If the partial neutralization degree is less than 20%, the water absorption capacity of the resulting polymer will be remarkably decreased. In this invention, a polymer having a higher water absorption capacity may be obtained by adding in addition to the aforementioned acrylic acid and its salts one or two of the monomers copolymerizable therewith selected from the group consisting of 2-acrylamide-2-methylpropanesulfonic acid, 2-acryloylethanesulfonic acid, 2-acryloylpropanesulfonic acid, methacrylic acid and alkali metal salts or ammonium salts thereof, (meth)acrylamide, N,N-dimethyl (meth)acrylamide, 2-hydroxyethyl (meth)acrylamide, 2-vinylpyridine, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, polyethylene glycol mono(meth)acrylate, N,N′-methylene bis(meth)acrylamide and polyethylene glycol di(meth)acrylate. The term "(meth)acryl" herein used means acryl and methacryl. It is also possible to incorporate other monomers copolymerizable with acrylic acid and acrylic acid salts including water soluble monomers such as itaconic acid, maleic acid, fumaric acid, vinylsulfonic acid and alkali metal salts or ammonium salts thereof and in addition less water soluble monomers such as alkyl esters of acrylic acid, for example methyl acrylate, ethyl acrylate and the like, providing that "an aqueous solution of a polymerizable monomer" of this invention is formed. The "polymerizable monomer" of this invention comprises as a main component acrylic acid, of which 20% or more takes the salt form. Thus, the addition amount of the aforementioned copolymerizable monomer is usually less than 50 mol %, preferably 20 mol % or less. For neutralization of the aforementioned acid monomers including acrylic acid may be used a hydroxide or bicarbonate of an alkali metal or ammonium hydroxide, preferably an alkali metal oxide, specifically sodium hydroxide, potassium hydroxide and lithium hydroxide. Sodium hydroxide or potassium hydroxide is preferred from the standpoint of commercial availability, price, safety and the like. In this invention, the polymerizable monomer comprising as a main component the aforementioned acrylic acid, of which 20% or more is present in its salt form, is applied in the form of an aqueous solution to a prefabricated fibrous substrate. Any concentration of the aqueous solution may be employed as far as it is suitable for the object. Specifically, it is preferably in the range of 30% by weight or more. This aqueous solution may contain a variety of substances providing that they are not apart from the object of this invention. As an example of such substances, there is mentioned a water soluble radical polymerization initiator (described in detail hereafter). The "aqueous solution" may be the one in which a small amount of a water soluble organic solvent is also present in solution, if desired. Prefabricated fibrous substrate A prefabricated substrate to which the aforementioned aqueous solution of the polymer is applied is specifically a substrate formed by loose fabrication of fiber such as a pad, a carded or air-laid web, tissue paper, a woven fabric like cotton gauze, knitted fabric or nonwoven fabric. The term "prefabricated" fibrous substrate herein used means the substrate which requires no web forming operation, though some operations such as cutting, bonding, shaping and the like may be required for incorporating the fibrous substrate into an article. In general, absorptive fibers including cellulose fibers such as wood pulp, rayon, cotton and the like and/or polyester fibers are preferably used as a main component for the fibrous substrate. Other kinds of fibers such as those of polyethylene, polypropylene, polystyrene, polyamide, polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, polyurea, polyurethane, polyfluoroethylene, polyvinylidene cyanide and the like may be also incorporated into the prefabricated fibrous substrate. Application of an aqueous solution of the monomer and polymerization of the monomer (primary polymerization) In this invention, the aforementioned monomer solution is applied to the aforementioned prefabricated fibrous substrate, and the monomer is polymerized on the fibrous substrate by means of a water soluble radical polymerization initiator (primary polymerization). In order to apply the aqueous monomer solution to the prefabricated fibrous substrate, there may be used any means or manner suitable for the object as far as the monomer is uniformly dispersed and held on the fibrous substrate and can be subjected to polymerization. One of the typical means therefor is to impregnate the aqueous monomer solution into the fibrous substrate or to spray the aqueous monomer solution onto the fibrous substrate. For polymerizing the monomer which has been dispersed uniformly on the fibrous substrate as described above by means of a water soluble radical polymerization initiator, any method can be used as far as it is suitable for the object. Typical methods include a method wherein a radical polymerization initiator has previously been added in the aqueous monomer solution and is decomposed on the fibrous substrate, a method wherein a radical polymerization initiator is applied uniformly in the form of a separate solution from the aqueous monomer solution to the fibrous substrate, to which the aqueous monomer solution has been applied, by spraying or the like and is decomposed on the fibrous substrate and a method wherein a radical polymerization initiator is applied uniformly in the form of a separate solution from the aqueous monomer solution to the fibrous substrate and then the aqueous monomer solution is uniformly applied thereto by spraying, coating or the like. The water soluble radical polymerization initiator used in this invention is one well known in the art of polymer chemistry. There may be mentioned specifically inorganic or organic peroxides such as persulfates (ammonium salts, alkali metal salts, particularly potassium salts, or the like), hydrogen peroxide, di-tert-butyl peroxide, acetyl peroxide and the like. In addition to these peroxides, it is also possible to use such a radical polymerization initiator as an azo compound or the like, for example 2,2'-azobis(2-amidinopropane) dihydrochloride, providing that water solubility in a certain level can be obtained. The polymerization is initiated by the decomposition of the radical polymerization initiator comprising a redox initiator. Well known as a conventional means for decomposing the initiator is heating (As is often the case, when the initiator is contacted with the monomer the reaction mixture has already been raised at the decomposition temperature and thus the polymerization is initiated only by adding the polymerization initiator to the monomer without heating. This case is involved herein in the category of the decomposition by heating). Promotion of the decomposition of the polymerization initiator by means of a chemical substance is also well known in the art. When the polymerization initiator is a peroxide, a promoter of the decomposition thereof is a reducing compound (which is water soluble in this invention) such as an acidic sulfite, ascorbic acid and an amine for a persulfate, and a polymerization initiator comprising a combination of a peroxide and a reducing compound is well known in the art of polymer chemistry as "redox initiator". Thus, the term "polymerization initiator" herein used also involves initiator combined with such decomposition promoting substances, and it comprises redox initiators. The polymerization of the monomer comprising as a main component acrylic acid, of which 20% or more is in the salt form, should give in principle a non-crosslinking water soluble polymer as far as a diethylenic monomer such as N,N'-methylene bis(meth)acrylamide is not used concomitantly. However, it has been practically known that crosslinking usually occurs between acrylic acids (or its salts) or the polymers thereof or/and between those and the fibrous substrate. Accordingly, the polyacrylic acid (salt) produced in this step may be considered as highly water absorptive polymer rather than water soluble polymer. In addition, the polymerization by means of the water soluble radical polymerization initiator should be substantially aqueous solution polymerization. Accordingly, the step (B) should be conducted while avoiding the excessively dry state. The amount of the monomer applied to the fibrous substrate during the step (A) is in a proportion of 1 - 10,000 parts by weight, preferably 10 - 1,000 parts by weight per 100 parts by weight of the fibrous substrate. The monomer thus applied should be polymerized in the step (B) to an extent of 50% or more, preferably 80% or more. Rate of polymerization ordinarily reaches up to 80 - 95% in the step (B). Some of the embodiments of the steps (A) and (B) are illustrated as follows: (1) A method that an aqueous solution of an acrylic acid type monomer having a monomer concentration of 25% by weight or more and a water soluble radical polymerization initiator are previously mixed homogeneously and the mixture is applied in a mist form to a prefabricated fibrous substrate so that the resulting highly water absorptive polymer in the fibrous substrate will have a diameter in the range of 30 - 500 µm; (2) A method that an aqueous solution of an acrylic acid type monomer containing a small amount of a crosslinking agent and a water soluble radical polymerization initiator are previously mixed homogeneously and the mixture is applied in a mist form to a prefabricated fibrous substrate so that the resulting highly water absorptive polymer in the fibrous substrate will have a diameter in the range of 30 - 500 µm; (3) A method that an aqueous solution of an acrylic acid type monomer containing a small amount of a crosslinking agent and an oxidizing radical polymerization initiator are previously mixed, the mixture is applied to a fibrous substrate and an amine or a reducing agent is added to form a redox system thereby initiating polymerization (see Japanese Patent Application No. 238421/85); (4) A method that an aqueous solution of an acrylic acid type monomer containing a small amount of a crosslinking agent and an amine or a reducing agent are mixed, followed by application to a fibrous substrate and then addition of an oxidizing radical polymerization initiator to form a redox system thereby initiating polymerization (see Japanese Patent Application No. 238420/85); and (5) A method that an aqueous solution of an acrylic acid type monomer is previously impregnated into a fibrous substrate and then a water soluble radical polymerization initiator is added in a mist form. Step (C) Heat treatment (secondary polymerization) The composite comprising the highly water absorptive polymer obtained as above and the prefabricated fibrous substrate is then subjected to heat treatment. The heat treatment is conducted by heating the composite up to a temperature of 100 - 250°C, preferably 150 - 200°C. If the temperature exceeds 250°C, the water absorption capacity of the resulting composite material is remarkably lowered though the amount of unreacted monomers may be effectively reduced. If it is less than 100°C, it takes quite a long time to reduce unreacted monomers, which is unfavorable in the light of industrial operation. The treatment time to be adopted may vary depending upon the type of the apparatus used, heating method, etc., and is generally from a few second to two hours, preferably from a few second to 30 minutes. If the treatment time is exessively long while the treatment temperature is relatively high, the water absorption capacity of the resulting composite material is remarkably lowered. If the treatment time is too short, it is difficult to effectively reduce unreacted monomers. For carrying out the heat treatment, any method or means can be used as far as it is suited for the object. Specific methods include, for example, a batch-wise heating method using a boxy reactor whose inner temperature is maintained at a predetermined level and a continuous heating method wherein the composite is continuously contacted with a roller whose surface temperature is maintained at a predetermined level using steam or the like. As the water content of the composite in this case is adopted a content in a proportion of 40 parts by weight or less, preferably 20 parts by weight or less per 1 part by weight of the fibrous substrate. If the water content exceeds 40 parts by weight, it is not preferred because of necessity for quite a long heating time. The heat treatment of the aforementioned composite according to this invention may be conducted under vacuum, in the presence of an inorganic gas such as nitrogen, argon, helium or the like, and preferably in air. EXAMPLES Example 1 In a 100 cm³ conical flask, 13.1 g of sodium hydroxide (purity: 95% by weight) was placed and dissolved in 39.0 g of pure water under ice cooling. The aqueous solution was neutralized by slowly adding 30 g of acrylic acid under ice cooling. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 45% by weight. 0.1 g of N,N'-methylene bisacrylamide as a crosslinking agent was added and dissolved in the aqueous monomer solution, and the mixture was heated to 50°C. 0.2 g of potassium persulfate as a radical polymerization initiator was also dissolved in the above mentioned mixture. Separately, 0.1598 g of a polyester nonwoven fabric was provided, and the whole surface of the nonwoven fabric was coated and impregnated with the above mentioned raw material, and the coated nonwoven fabric was maintained at a temperature of about 50°C in a constant temperature bath. The amount of the monomer thus impregnated was 7.0 times the weight of the nonwoven fabric. Next, an aqueous solution of 5% sodium hydrogen sulfite as a reducing agent was sprayed on the whole surface of the above mentioned nonwoven fabric. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized sodium acrylate crosslinked with N,N'-methylene bisacrylamide was firmly held on the polyester nonwoven fabric was obtained. Next, the composite was adjusted to a water content of 20% by weight and was placed in a boxy oven whose inner temperature was maintained at 170°C and left there for 15 minutes to obtain a water absorptive composite material. Example 2 In a 100 cm³ conical flask, 30 g of acrylic acid was placed and 16.9 g of pure water was added to and mixed with it. The mixture was neutralized by slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice cooling. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 65% by weight. 0.1 g of N,N'-methylene bisacrylamide as a crosslinking agent was added to and dissolved in the above mentioned monomer solution, and the mixture was heated to 40°C. 0.4 g of 31% aqueous hydrogen peroxide as a radical polymerization initiator was dissolved in the mixture. 0.1869 g of a polyester nonwoven fabric was provided, and the whole surface of the nonwoven fabric was coated and impregnated with the aforementioned raw material, and the nonwoven fabric thus treated was maintained at a temperature of 40°C in a constant temperature bath. The amount of the monomer thus impregnated was 5.8 times the weight of the nonwoven fabric. Next, an aqueous solution of 5% L-ascorbic acid was sprayed through a spraying nozzle onto the whole surface of the above mentioned nonwoven fabric. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized potassium polyacrylate crosslinked with N,N'-methylene bisacrylamide was firmly held on the polyester nonwoven fabric was obtained. Next, the composite (having a water content of about 19% by weight) was heated by contacting it with a steam roller whose surface temperature was maintained at 160°C with steam to obtain a water absorptive composite material. Example 3 In a 100 cm³ conical flask, 30 g of acrylic acid was placed and 16.9 g of pure water was added to and mixed with it. The mixture was neutralized by slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice cooling. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 65% by weight. 0.1 g of N,N'-methylene bisacrylamide as a crosslinking agent was added to and dissolved in the above mentioned monomer solution, and the mixture was heated to 30°C. 0.2 g of L-ascorbic acid as a radical polymerization initiator was dissolved in the mixture. 0.2582 g of a polyester nonwoven fabric was provided, and the whole surface of the nonwoven fabric was coated and impregnated with the aforementioned raw material, and the nonwoven fabric thus treated was maintained at a temperature of 30°C in a constant temperature bath. The amount of the monomer thus impregnated was 6.2 times the weight of the nonwoven fabric. Next, 10% aqueous hydrogen peroxide was sprayed through a spraying nozzle onto the whole surface of the above mentioned nonwoven fabric. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized potassium polyacrylate crosslinked with N,N'-methylene bisacrylamide was firmly held on the polyester nonwoven fabric was obtained. Next, the composite (having a water content of about 25% by weight) was heated by contacting it with a steam roller whose surface temperature was maintained at 165°C with steam to obtain a water absorptive composite material. Example 4 A water absorptive composite material was obtained in the same manner as in Example 2 except that the mixture of 30 g of acrylic acid and 3.5 g of acrylamide was used in place of the acrylic acid in Example 2. Example 5 A water absorptive composite material was obtained in the same manner as in Example 2 except that the mixture of 30 g of acrylic acid and 5 g of 2-acrylamide-2-methylpropanesulfonic acid was used in place of the acrylic acid in Example 2. Comparative Examples 1 - 8 The composites of Comparative Examples 1 to 5 are prepared as follows: Comparative Example 1 In a 100 cm³ conical flask, 13.1 g of sodium hydroxide (purity: 95% by weight) was placed and neutralized by slowly adding 30 g of acrylic acid under ice cooling. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 45% by weight. As a radical polymerization initiator, 0.05 g of potassium persulfate was added to and dissolved in the aqueous solution, and deaeration was conducted using N₂. Separately, 0.1569 g of a polyester nonwoven fabric was provided, and the whole surface of the nonwoven fabric was coated and impregnated with the above mentioned monomer solution. The weight of the monomer thus impregnated was 6.2 times the weight of the nonwoven fabric. The nonwoven fabric was placed in a constant temperature reaction bath which had preliminarily been deaerated with N₂ and heated to 90°C. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized self-crosslinked sodium polyacrylate was firmly held on the polyester nonwoven fabric was obtained. Comparative Example 2 In a 100 cm³ conical flask, 30 g of acrylic acid was placed and 9.3 g of pure water was added to and mixed with it. The mixture was neutralized by slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice cooling and maintained at a temperature of 50°C. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 74% by weight. Separately, as a radical polymerization initiator, 0.05 g of potassium persulfate was dissolved in 1 g of water and the aqueous solution was coated on the whole surface of 0.0985 g of a rayon nonwoven fabric. Thereafter the monomer solution as the raw material was rapidly coated on the whole surfaces of the nonwoven fabric, the fabric was placed in a reaction bath which had been preliminarily deaerated with N₂ and maintained at a temperature of 90°C. The amount of the monomer thus impregnated was 10 times the weight of the nonwoven fabric. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized self-crosslinked potassium polyacrylate was firmly held on the rayon nonwoven fabric was obtained. Comparative Example 3 In a 100 cm³ conical flask, 13.1 g of sodium hydroxide (purity: 95% by weight) was placed and dissolved in 39.0 g of pure water under ice cooling. The aqueous solution was neutralized by slowly adding 30 g of acrylic acid under ice cooling. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 45% by weight. 0.005 g of N,N'-methylene bisacrylamide as a crosslinking agent and 0.1 g of 2,2'-azobis(2-amidinopropane) dihydrochloride as a radical polymerization initiator were dissolved in the aqueous monomer solution, and deaeration was conducted with N₂. Separately, 0.1505 g of a polyester nonwoven fabric was provided, and the whole surface of the nonwoven fabric was coated and impregnated with the above mentioned raw material. The amount of the monomer thus impregnated was 7.5 times the weight of the nonwoven fabric. The nonwoven fabric was placed in a constant-temperature reaction bath which had preliminarily been deaerated with N₂ and heated to 90°C. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized sodium acrylate crosslinked with N,N'-methylene bisacrylamide was firmly held on the polyester nonwoven fabric was obtained. Comparative Example 4 In a 100 cm³ conical flask, 30 g of acrylic acid was placed and 9.3 g of pure water was added to and mixed with it. The mixture was neutralized by slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice cooling and maintained at a temperature of 70°C. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 74% by weight. Separately, as a radical polymerization initiator, 0.2 g of potassium persulfate was dissolved in 3 g of water. 0.5869 g of a polyester nonwoven fabric was provided and maintained at a temperature of about 70°C in a constant temperature bath. The aqueous radical polymerization initiator solution was mixed with the aqueous monomer solution mentioned above, and the mixture was immediately sprayed through a spraying nozzle onto the above mentioned nonwoven fabric. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized self-crosslinked potassium polyacrylate was firmly held on the rayon nonwoven fabric was obtained. The amount of the monomer thus coated was 12 times the weight of the nonwoven fabric, and the highly water absorptive polymer had a particle diameter in the range of 100 - 300 µm. Comparative Example 5 In a 100 cm³ conical flask, 26.9 g of 25% aqueous ammonia was placed and neutralized by slowly adding 30 g of acrylic acid under ice cooling and heated to a temperature of 70°C. The aqueous solution exhibited a neutralization degree of about 95% and a monomer concentration of about 65% by weight. Separately, 0.2 g of potassium persulfate as a radical polymerization initiator was dissolved in 3 g of water. 0.4695 g of a polyester nonwoven fabric was provided and maintained at a temperature of about 70°C in a constant temperature bath. The aqueous radical polymerization initiator solution was mixed with the aqueous monomer solution mentioned above, and the mixture was immediately sprayed through a spraying nozzle onto the above mentioned nonwoven fabric. Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized self-crosslinked ammonium polyacrylate was firmly held on the rayon nonwoven fabric was obtained. The amount of the monomer thus coated was 8 times the weight of the nonwoven fabric, and the highly water absorptive polymer had a particle diameter in the range of 100 - 250 µm. The precursors obtained in Examples 1 - 3, that is the composites before the heat treatment are herein regarded as the composites in Comparative Examples 6 ∼ 8, respectively. Comparative Example 9 In a 100 cm³ conical flask, 30 g of acrylic acid was placed and 16.9 g of pure water was added to and mixed with it. The mixture was neutralized by slowly adding 20.6 g of potassium hydroxide (85% by weight) under ice cooling. The aqueous solution exhibited a neutralization degree of about 75% and a monomer concentration of about 65% by weight. 0.3852 g of a polyester nonwoven fabric was provided, and the whole surface of the nonwoven fabric was coated and impregnated with the above mentioned raw material. The amount of the monomer impregnated was 7.5 times the weight of the nonwoven fabric. Next, the nonwoven fabric having been impregnated with the aqueous solution of the partially neutralized potassium acrylate monomer was irradiated with electron beam at a dose of 20 Mrad by means of an electron beam generating apparatus equipped with an accelerator (DYNAMITRON). Polymerization started immediately and a composite in which a highly water absorptive polymer comprising a partially neutralized self-crosslinked potassium polyacrylate was firmly held on the polyester nonwoven fabric was obtained. For the water absorptive composite materials obtained in Examples and the composites obtained in Comparative Examples, the following tests were carried out to evaluate physiological saline absorption capacity and unpolymerized monomer concentration. The results are shown in Table 1. A. Physiological saline absorption capacity About 0.5 g of the composite or water absorptive composite material and about 200 g of a saline solution having a concentration of 0.9% by weight were precisely weighed, respectively and charged in a 300 ml beaker. The beaker was left standing for about 4 hours to swell the polymer satisfactorily with the solution. The beaker content was filtered through a 100-mesh sieve, and the amount of the filtrate is weighed and the physiological saline absorption capacity is calculated according to the following equation: B. Unpolymerized monomer concentration 0.5 g of the composite or water absorptive composite material was precisely weighed and added to 1 liter of ion exchanged water in a 2 liter beaker to swell sufficiently with stirring for about 10 hours. The swollen polymer gel was filtered through a 200-mesh sieve and the filtrate was analyzed by a high speed liquid chromatography. Separately, standard monomer solutions having determined concentrations were prepared to make a calibration curve, and the absolute monomer concentration of the filtrate was determined with consideration for the degree of dilution (1/2000). The results are shown in Table 1. The water absorptive composite material obtained by the process of this invention, as apparent from the results shown in Table 1, has high water absorption capacity, and has an extremely low content of unpolymerized monomers and thus possessing very high safety, as compared with those in prior art. Further, the composite material handles easily because of its sheet form as compared with conventional powdery water absorptive resins, so that they can be used advantageously for the production of a variety of sanitary goods such as a sanitary napkin, paper diaper and the like. The water absorptive composite material according to this invention, taking advantage of its excellent water absorption capacity and easy handling, can be also used for the production of a variety of materials for gardening and agriculture such as a soil conditioner and a water retaining agent which have recently attracted public attention.

Claims: A process for preparing a water absorptive composite material, which comprises the combination of the following steps of: (A) applying an aqueous solution of a polymerizable monomer comprising as a main component acrylic acid, of which 20% or more of the carboxyl groups have been neutralized to its alkali metal salt or ammonium salt, to a prefabricated fibrous substrate, the monomer concentration of the aqueous solution of a polymerizable monomer being at least 30% by weight and the amount of the polymerizable monomer applied to the prefabricated fibrous substrate being 1 to 10,000 parts by weight per 100 parts by weight of the prefabricated fibrous substrate; (B) polymerizing the polymerizable monomers applied to said fibrous substrate by means of a water-soluble radical polymerization initiator to form a composite of a polymer derived from said polymerizable monomer and said fibrous substrate, the polymerization initiator comprising a redox initiator, and the stage of polymerizing the polymerizable monomer involving either the decomposition of said polymerization initiator while it is previously dissolved in the aqueous solution of the monomer for polymerization, or the spraying of said polymerization initiator in a solution form to the fibrous substrate before decomposition thereof; and (C) heating said composite to 100 - 250°C to obtain a water absorptive composite of which the amount of unreacted monomers is reduced as compared with said composite, wherein the composite to be heated does not contain water in a proportion of 40 parts by weight or more per 1 part by weight of the fibrous substrate. A process according to claim 1, wherein the polymerizable monomer comprises acrylic acid of which 20% or more of the carboxyl groups have been neutralized to its alkali metal salt or ammonium salt. A process according to any one of the preceding claims, wherein the polymerizable monomer contains up to 20 mol% of at least one of the monomers selected from the group consisting of 2-acrylamide-2-methylpropanesulfonic acid, 2-acryloylethanesulfonic acid, 2-acryloylpropanesulfonic acid, methacrylic acid and alkali metal salts or ammonium salts thereof, acrylamide, methacrylamide, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide, 2-hydroxyethyl acrylamide, 2-hydroxyethyl methacrylamide, 2-vinylpyridine, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate, N,N'-methylene bisacrylamide, N,N'-methylene bismethacrylamide, polyethylene glycol diacrylate and polyethylene glycol dimethacrylate. A process according to any one of the preceding claims, wherein the fibrous substrate formed in step (A) comprises as a main component a cellulose fiber or/and a polyester type fiber. A process according to any one of the preceding claims, wherein the fibrous substrate formed in step (A) is a pad of loose fabric, a carded web, an air-laid web, a paper, a nonwoven fabric, a woven fabric or a knitted fabric. A process according to any one of the preceding claims, wherein the stage of applying the aqueous solution of the polymerizable monomer in the step (A) to the prefabricated fibrous substrate comprises spraying of said aqueous solution to said fibrous substrate or impregnation of said fibrous substrate with said aqueous solution.

Inventors: ITOH, KIICHI MITSUBISHI PETROCHEM. CO., LTD., SHIBANO, TAKESHI MITSUBISHI PETROCHEM. CO., LTD.
Applicants: MITSUBISHI PETROCHEMICAL CO., LTD., UNI-CHARM CORPORATION
Application year: 1987
Main classification: D06M14
IPC codes: D06M101, C08F2, C08F120, D06M13, C08F20, D06M14, D21H17