Development of superabsorbent polymers

The synthesis of the first water-absorbent polymer goes back to 1938, when AA and divinylbenzene were thermally polymerized in an aqueous medium. In the late 1950s the first generation of hydrogels appeared, mainly based on hydroxyalkyl methacrylate and related monomers, and with swelling capacity up to 40-50%. These hydrogels were used in the development of contact lenses that revolutionized ophthalmology. 

The first commercial SAP was produced through alkaline hydrolysis of starch-graft-polyacrylonitrile, and was developed in the 1970s at the Northern Regional Research Laboratory of the US Department of Agriculture. Expense and inherent structural disadvantages of this product, such as a lack of sufficient gel strength, are seen as the major factors in its early market defeat. 

Commercial production of SAPs began in Japan in 1978 for use in feminine napkins, and further developments led to SAP materials being used in baby diapers in Germany and France in 1980. In 1983 low-fluff diapers containing 4-5 g SAPs were marketed in Japan, followed shortly by the introduction of thinner superabsorbent diapers in other Asian countries, the US, and the Europe. By 1990 the world production of SAP resins exceeded 1 million tons. 

According to the European Disposables and Nonwovens Association, the total production of SAPs in 2005 approached 1,483,000 tons, with 623,000 tons reported in Asia (mostly by Nippon Shokubai, San-Dia Polymers, and Sumitomo Seika Chemicals), 490,000 tons in North America (by Degussa, BASF, Dow, and Nippon Shokubai), and 370,000 tons in Europe (mostly by Degussa and BASF). The SAPs are mainly made from partially neutralized, lightly cross-linked polyacrylic acid, which has been proven to give the best performance versus cost ratio. The polymers are synthesized, dried, and milled into granular white solids. When placed in water they swell to a rubbery gel that in some cases can contain up to 99 wt% water. 

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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. 

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

Fushitni, T. (1990). Development of superabsorbent polymers. Collection of Application Ideas, Kogyo Chosa-kai, pp. 29-31. 

A joint research farm was established in the desert development area in the Bustan district located approximately 150 km northwest of Cairo, Egypt. Water-saving agriculture and deforestation experiments were carried out for three years, from 1990 to 1993, under the leadership of the Dry Land Investigation Center, Tottori University in Japan. The Society for Research and Development of Superabsorbent Polymers, established primarily by the makers of superabsorbent polymers, assumed responsibility for the development of the superabsorbent composite. The water-saving effect using superabsorbent composite was demonstrated, and a pilot plant for the composite with the capacity of 2t/day was constmcted and transferred to Egypt. 

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. 

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. 

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. 

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. 

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. 

China is one of the major agricultural countries in the world, it has a population of 1.2 billion, and it is experiencing rapid economic development. Although China possesses vast farmlands, approximately 40% of the country is semidry land. In these areas, the dry season is from early spring to early summer, which is the time of seeding and transplantation of seedlings. Hence, the use of superabsorbent polymers might prove effective. 

As absorbent products for personal care evolve to even thinner designs, the cellulose will play a decreased role for the initial absorption. Additionally, when an application requires a faster rate of absorption, such as adult incontinent devices, we will need to understand how to alter superabsorbent polymer to endow it with faster absorption rate. This work develops simple viscometric tests for measuring the absorption rate and elucidates the important variables influencing the absorption rate of superabsorbent polymers. 

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]... 

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... 

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