1. Starch super absorbent resin
Starch is a kind of polyhydroxy natural compound with a wide range of raw materials, many types and low prices. The monomers that undergo graft copolymerization with starch are mainly hydrophilic and ethylenic monomers that become hydrophilic after hydrolysis, where R is H or alkyl, and X is mainly —COOH, —COOR, —CN, —OH, —CONH2 and other groups.
In starch graft copolymerization, anionic catalysts are used to make starch undergo ionic graft copolymerization, as well as free radical graft copolymerization. At present, the synthesis of superabsorbent resins often uses free radical graft copolymerization. Free radical graft copolymerization is to generate free radicals in starch molecules. Due to the different generation methods, the grafting mechanism is also different. Using γ, α, β rays and initiators such as peroxides, azo compounds and redox, they can make the hydrogen on the carbon atom with hydroxyl group in the starch molecule be taken away to generate free radicals, and then initiate the single The body becomes a starch-monomer free radical, which continues to undergo chain-growth polymerization with the monomer, and finally chain termination occurs.
The research of superabsorbent resin started with starch grafting acrylonitrile. First, Fanta et al. of the United States used cerium ammonium nitrate as an initiator, and starch grafted acrylonitrile was successful. Gugliemelli et al. studied the saponified hydrolyzed starch-acrylonitrile graft copolymer, and Weaver et al. prepared the water-absorbing polymer "HSPAN" on this basis. The copolymer prepared by starch grafting acrylonitrile is a graft with -CN group. -The CN group is a hydrophobic group, so this compound does not absorb water. In order to make it absorb water, it must be hydrolyzed with alkali to make -CN group into -CONH2, COOH and -COOM (M is alkali metal ion) and other hydrophilic groups, so as to become a water-absorbing product.
Starch grafted acrylic acid (salt) superabsorbent resin is obtained by polymerizing starch polysaccharide and acrylic acid (salt) in the presence of a crosslinking agent. This resin has high water absorption properties and is biodegradable. When acrylic acid is used as the grafting monomer At the time of polymerization, the polymerization is carried out in water to form a white elastic gel grafted polymer. The gel is hydrolyzed with a caustic soda aqueous solution, then dried and crushed. When sodium acrylate and acrylic acid are used as graft monomers, the polymerization is carried out in an aqueous methanol solution, and the molar ratio of sodium acrylate to acrylic acid used is 80:20~70:30. If the aqueous solution of starch and monomer is dispersed in a hydrocarbon dispersion medium, a surfactant is added to stir and disperse into a suspension, and then an initiator is added, and the polymerization is heated, and a super absorbent graft product can also be obtained. This synthesis is reversed-phase suspension. (Emulsion) polymerization. The use of reversed-phase suspension polymerization can overcome the problems of high viscosity and difficult product handling caused by the graft polymerization of aqueous solution. The basic principle of starch grafting acrylamide is similar to grafting acrylonitrile and acrylic acid (salt). The difference is that the product of acrylamide grafted after starch is not an ionic product, so it does not need alkali neutralization; on the other hand, it is not like acrylonitrile grafted on starch with a lipophilic group -CN, it needs to be used The alkali saponification becomes a hydrophilic group (becomes -CONH2 and COOH), which is itself a product with a hydrophilic group -CONH2, and has strong water absorption, so it does not require saponification, and is itself a super absorbent resin. At the same time, it can also be converted into a super absorbent resin with carboxyl groups and amide groups by saponification and hydrolysis, so starch grafted acrylamide can directly obtain a super absorbent hydrogel, and this hydrogel is non-ionic, electrolyte And pH value has little effect on its water absorption performance. It can also be further hydrolyzed to prepare anionic superabsorbent resin.
2. Cellulose superabsorbent resin
Because of the emergence of starch-based super absorbent resin, people think of using cellulose as a raw material to prepare super absorbent resin. Like starch, cellulose has a wide range of raw materials and can react with a variety of low molecular weights. It is an aspect of the development of water-absorbing materials in the past decade. Cellulose has strong water absorption. On the one hand, it is a hydrophilic polyhydroxy compound; on the other hand, because it is a fibrous substance, it has many capillaries and a large surface area. Therefore, it has been widely used as a water-absorbing material. However, the water absorption capacity of cellulose is not strong. In order to improve its performance, it is mainly through chemical reactions to make it have stronger or more hydrophilic groups, but it still becomes a fiber state to maintain its large surface area and multi-capillary property. The preparation method is generally one or more of methods such as etherification, esterification, graft copolymerization, etc., to prepare a cellulose-based water-absorbing material.