Superabsorbent polymers (SAP), also referred to as superabsorbent resins, are a category of functional polymer materials capable of absorbing water in amounts ranging from several hundred to several thousand times their own weight. The main types include starch derivatives, cellulose derivatives, and polyacrylic acid-based polymers. According to their raw materials, these polymers can be classified into modified natural polymers and fully synthetic resins. They are widely used in various fields, including agricultural water retention, hygiene products such as diapers and sanitary napkins, medical dressings, food preservation, and numerous industrial applications.

Superabsorbent polymers are specialized absorbent resins characterized by their ability to absorb water at least several hundred times their own weight. As an important class of functional polymeric materials, they possess exceptional water-absorption capacity. Although they are insoluble in water, they can absorb substantial amounts of water and swell to form highly hydrated gels. Their principal properties are their outstanding water-absorption and water-retention capabilities.

Classification of Superabsorbent Polymers (SAPs)

1. Classification by Raw Material Source

Superabsorbent polymers can be divided into six main categories according to their raw material sources:

• Starch-based series: Includes starch graft copolymers, carboxymethylated starch, phosphorylated starch, starch xanthate, and other derivatives.

• Cellulose-based series: Includes grafted cellulose, carboxymethylated cellulose, hydroxypropylated cellulose, cellulose xanthate, and related derivatives.

• Synthetic polymer series: Includes polyacrylates, polyvinyl alcohols, polyalkylene oxides, inorganic polymers, and other synthetic materials.

• Protein-based series: Includes soy proteins, silk proteins, gluten, and other protein-derived materials.

• Other natural substances and their derivatives: Includes pectin, alginic acid, chitosan, heparin, and similar materials.

• Blends and composites: Includes blends of different superabsorbent polymers, as well as composites combining superabsorbent polymers with inorganic gels or organic substances.

2. Classification by Hydrophilization Method

According to the methods used to introduce or enhance hydrophilic properties, SAPs can be classified into four types:

1. Polymerization of hydrophilic monomers: Examples include polyacrylates, polyacrylamides, and acrylic acid–acrylamide copolymers.

2. Carboxymethylation or carboxyalkylation of hydrophobic or weakly hydrophilic polymers: Examples include the carboxymethylation of starch or cellulose and the reaction of polyvinyl alcohol with maleic anhydride.

3. Graft polymerization of hydrophilic monomers onto hydrophobic or weakly hydrophilic polymers: Examples include starch-graft-polyacrylate, starch-graft-polyacrylamide, cellulose-graft-polyacrylate, and starch–acrylic acid–acrylamide graft copolymers.

4. Hydrolysis of macromolecules containing cyano, ester, or amide groups: Examples include the hydrolysis of starch-graft-polyacrylonitrile, acrylate–vinyl acetate copolymers, and polyacrylamide.

3. Classification by Cross-Linking Method

Based on the cross-linking mechanism, SAPs can be categorized into four types:

1. Cross-linking using cross-linking agents: Water-soluble polymers are cross-linked with agents containing multiple reactive functional groups, polyvalent metal ions, or polymeric cross-linking agents.

2. Self-cross-linking: Certain substances, such as polyacrylates and polyacrylamides, undergo polymerization or reactions that result in self-cross-linking.

3. Radiation-induced cross-linking: Polymers such as polyvinyl alcohol and polyalkylene oxides can be cross-linked through exposure to radiation.

4. Introduction of hydrophobic groups or crystalline structures into water-soluble polymers: For example, esterifying polyacrylic acid with long-chain alcohols containing 12–20 carbon atoms can produce insoluble polymers with high water-absorption capacity.

4. Other Classification Methods

Superabsorbent polymers can also be classified according to their physical form, preparation method, and degradability.

1. By physical form: SAPs can be categorized as powders, fibers, films or sheets, and microspheres.

2. By preparation method: Classification is based on processes such as synthetic polymerization and cross-linking, carboxymethylation, starch-graft copolymerization, and cellulose-graft copolymerization.

3. By degradability: SAPs can be divided into non-degradable and degradable types. Non-degradable products include those based on sodium acrylate and methyl methacrylate, while degradable products include graft copolymers derived from natural polymers such as starch and cellulose.