Superabsorbent polymers development

In Japan, an interesting flexible waterproofing material which solidifies in water has recently been developed by Moriyoshi, et al.,1 ] and has potential as a shock-absorbing, waterproof backfill material for tunnels and dams. The material is composed of an asphalt emulsion, a superabsorbent polymer emulsion, and cement. 

Development of the internal curing of polymer-modified mortar and concrete using superabsorbent polymers [64]... 

Cassava starch-based blend films, and superabsorbent polymers were developed and their potential in specific applications was studied. Starch-konjac glucoman-... 

The most famous gel is the soft contact lens developed in Czechoslovakia in 1960 using poly(hydroxyethyl methacrylate). In the late 1970s, superabsorbent polymers were developed in the United States and their use in feminine products was worldwide. In 1980, Professor Toyoichi Tanaka discovered phase transition in gels. Today many researchers study those transitions from the basics to applications. Gel applications or functional uses due to phase transitions are found in the chapters that follow and will not be discussed here. 

Superabsorbent polymers are one of the new materials. Since 1987, when it was used for disposable baby diapers, production has increased each year. Production in 1995 reached 600,0001 [150]. Due to commercial applications like paper diapers and sanitary products, they are so familiar to us because they appear on TV almost every day. Other applications, such as a soil additive to retain moisture, materials for maintaining freshness, antiffost construction materials, gel perfumes, and controlled release drugs are under development. It seems important to measure and evaluate the superabsorbent polymers that have been used in many... 

For standardization of the water uptake test, multiple organizations conducted the round robin test using common samples and procedures. Based on their results, methods (proposal) to determine water uptake and the rate of water absorption are being developed. A written inquiry was also distributed to the member companies of the Polymer Materials Center regarding water upUdce. Currently, Ihe standard for water uptake has not been established anywhere in the world. It seems important first to establish the standardization of Ihe water uptake testing method for superabsorbent polymers and then to make it die international standard. The testing mediods (proposal) shown here are still under discussion and when it is standardized by JIS, there may be partial modification. 

This technique, developed and commercialized in Japan, currently leads the superabsorbent polymers world market. On the other hand, companies in Western countries adopted polymers without surface treatment. As the trend for reducing thickness continued in the 1990s, use of a cotton-like pulp was reduced and the amount of superabsorbent polymers increased. Under such conditions, it became apparent that surface treatment was necessary to achieve superabsorbency. As the polymer concentration increases, diffusion of urine throughout the diaper via the capillary effect of the pulp cannot be relied upon and uneven swelling results. It has become important to prevent this so as to achieve effective use of the entire diaper. Under the pressure of body weight, this phenomenon may be even fiirther magnified. Due to such needs. Western companies also developed surface treatment technologies [29-31] and today most superabsorbent polymers for diaper application have surface-treated polymers. 

The most important property of superabsorbent polymers is their ability to absorb water. Traditional water-absorbing materials such as cotton, pulp and sponges absorb water into interstices by capillary phenomenon. By contrast, superabsorbent polymer absorbs water into three-dimensional (3D) networks of a erosslinked polymer by the compatibility of polymer chains and water and by osmotic pressure. Hence, superabsorbent polymers laek absorption speed but have a much better water-absorption eapaeity eompared to cotton or pulp. Products that have a high water-absorption property include sanitary produets, such as disposable diapers, eonstmetion materials, mulch, water sealant for electric cables, freshness maintenance materials in the food industry. Of the sanitary products, of the disposable diaper is the best example of a product that has a high waterabsorbing capacity. The superabsorbent polymer is indispensable for disposable diapers, and it drastically improved the performance of disposable diapers. Here, the development and current status of disposable diapers, the structure of disposable diapers in which a superabsorbent polymer is incorporated, and advances in the development of superabsorbent polymers will be described. 

However, if the concentration of a superabsorbent polymer is further increased, the use efficiency reduces due to gel blocking. In order to solve this problem, investigations are made from the viewpoints of (i) improvement of superabsorbent polymer performance (ii) development of new materials that transport water effectively and separately and (iii) application of revolutionary product design. 

With regard to item (i), the gel strength and water absorption under stress have been improved by crosslinking the surface of the superabsorbent polymer [5]. This has made it possible to develop ultrathin... 

Many new materials have been developed for transporting water effectively and separately [6-9], including crosslinked cellulose called curly fibers, thermally fused PE/PP fibers, and a special nonwoven rayon cloth. These new materials absorb a liquid quickly and distribute it to the superabsorbent polymer. In addition, these new materials show good integrity to moisture. In particular, there is no corruption of the structure even after wetting, allowing the maintenance of chaimels through which the liquid ean be transferred. 

Superabsorbent polymers were developed in 1974. Initially, they were mostly graft copolymers of polysaccharides such as starch and cellulose. However, the majority of currently used superabsorbent pol3miers are lightly crosslinked acrylic acid polymers. 

In the early 1990s, composting and recycling of disposable diapers were actively considered in the United States, Canada and Europe. For composting, a biodegradable component such as pulp and superabsorbent polymer are used after removing the plastic back panel and tape. The compost has excellent water retention and can be used for soil improvement. Techniques to remove those plastics are now under development. 

First, high-performance superabsorbent polymers must be developed. Such performance includes high gel volume, high absorbency under load, low extractable level, and high permeability. Furthermore, in addition to the traditional role of water absorption and holding, it is desirable that superabsorbent polymers perform the same as pulp in transport and distribution. 

Major application areas for superabsorbent polymers (SAP) are disposable sanitary products, which include sanitary napkins, and disposable diapers for children and adults. Of these sanitary products, superabsorbent polymers were first used in sanitary napkins. Later, superabsorbent polymers were adopted for disposable diapers for children and adults. Accordingly, the performance of sanitary products has been dramatically improved. At the present time, more than 90 % of superabsorbent polymers produced are considered to be used for sanitary products. Here, the history, current status and recent trends and advances in the sanitary napkin will be given, and development of new superabsorbent polymers for sanitary napkin applications will be described. 

Approximately 10 % of the users have experienced some type of leakage problems resulting in soiled elothes. This indicates that leakage protection, the most fundamental funetion of sanitary napkins, is not yet sufficient. Thus it is neeessary to improve the absorbency of the absorption core to improve the protection fimction. A superabsorbent polymer that has a high absorbency towards blood, the so-called menses-specific superabsorbent polymer, is essential for a high-performance core and its development specific is strongly desired. 

Development of Superabsorbent Polymers for Sanitary Napkins 29 Table 3 Absorptivity of commercial superabsorbent polymers. 

Because porous superabsorbent polymers possess large surface areas as well as chaimels and capillaries for liquid diffirsion, high blood-absorption rates can be expected. As shown in Fig. 4, the lower the bulk density (the property that relates to porosity) of superabsorbent polymers, the higher the blood-absorption rate. Thus, various porous superabsorbent polymers have been developed and their absorbencies are under investigation. 

Fibrous superabsorbent polymers have larger surface areas than particulate ones. Furthermore, they have excellent diffusion and permeation of liquid by capillary effect and thus high absorbencies are expected. These fibers can be manufactured with ordinary structural fibers such as cellulose fibers or synthetic fibers by the airlaid technique, and the potential for further development is high. 

Quasifibrous superabsorbent polymers are a composite of ordinary fibers and superabsorbent polymer powder that is adhered to the fiber by a binder. Various quasifibrous superabsorbent polymers are being investigated. For example, a composite made of a cellulose fiber and a superabsorbent polymer has already been developed [22]. Polycarboxylic acids, polyamines or polydiols can be used as a binder. The interaction between the cellulose fiber and the binder or the superabsorbent polymer and the binder is hydrogen bonding or covalent bonding. Quasifibrous superabsorbent polymers do not exhibit gel blocking and a capillary effect caused by the fiber networks, thus providing favorable conditions for blood absorption. 

Superabsorbent polymer sheets have also been developed. If a particulate superabsorbent polymer is crosslinked between particles by a cationic polymer, for example, an amine-epichlorohydrin adduct, a superabsorbent polymer sheet can be prepared [23]. A particulate superabsorbent polymer, cellulose acetate and a plasticizer are dispersed in an appropriate solvent followed by casting onto a nonwoven cloth substrate. Upon irradiation by ultrasound, a superabsorbent sheet can also be prepared [24]. However, such material has poor compatibility with blood and lacks capillary channels. Thus, further improvement is necessary. Accordingly, various new superabsorbent polymers have been developed. Unfortunately, these polymers show insufficient property improvement or are too costly to be commercially available. 

The development of sanitary products for pets influenced strongly the generation of pet products. Of particular significance to this growth of the market is the application of superabsorbent polymers (hereinafter abbreviated as SAP). In the following, sanitary products for pets will be described with respect to the application of SAP. 

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