Mechanism of Swelling
The mechanism of swelling rubber is that when the rubber is in contact with water, water molecules continuously enter the water-absorbing swelling rubber through physical effects such as diffusion, capillary and surface adsorption. When the stress resisting deformation and the osmotic pressure difference reach to a balance, the water-absorbing swelling rubber maintains a relatively stable waterstop and leak plugging effect.
Water swelling rubber is mainly composed of rubber matrix and hydrophilic components. Elasticity and strength of the rubber matrix determine the physical properties of the water-swelling rubber. Therefore, rubbers with large elasticity and good adhesion are generally selected, such as chloroprene Rubber (CR)， styrene butadiene rubber (SBR) and Chlorinated Polyethylene （CPE）.
Appropriate hydrophilic materials are the key to the preparation of water swelling rubber. Hydrophilic components mainly include natural polymers, hydrophilic polymers, superabsorbent resins and some water-absorbent inorganic substances. Among them, super absorbent resins are widely used due to their high water absorption rate, high water swelling rate, and strong water retention performance.
There are many types of super absorbent resins.The 5 main types are:
(1) Starch super absorbent resin. Starch molecules contain a large number of hydroxyl groups. Various starch derivatives can be prepared through different reactions of hydroxyl groups.
(2) Cellulose super absorbent resin. It is a high molecular compound formed by β-d-glucose. It can carry out grafted copolymerization with low-molecular hydrophilic unsaturated substances
(3) Synthetic super absorbent resins. It mainly includes vinyl acetate and methyl acrylate copolymers. For example: polyacrylamides, PAANa cross-linked products, polyvinyl alcohol-acrylic acid graft copolymers, and so on. Because of the fast water absorption rate, the ionic super absorbent resin is widely used to prepare super absorbent resin by copolymerization of ionic and non-ionic monomers.
(4) Biodegradable super absorbent resin. Polylactic acid, sodium alginate, polyamino acid, chitosan and other water-absorbing resins are environmentally friendly and degradable.
(5) Organic-inorganic composite super absorbent resin. In recent years, the commonly used inorganic composite materials include kaolin, bentonite/montmorillonite, talc, attapulgiteand diatomite, etc.
Because water swelling rubber can absorb tens or even hundreds of times of its own mass of water while maintaining high elasticity and strength, it is widely used in waterproof sealing materials, medical and sanitary products, toys, anti-condensation wall materials, etc. Among them, the water swelling rubber can solve the sealing problems of permanent deformation of rubber and cement materials, and the waterproof and leak-proof caused by material shrinkage. Since water-swelling rubber has good wettability and adhesion with polar surface materials such as asphalt, plastic and rubber, and non-polar surface materials such as cement mortar and reinforced cement, it is suitable for tunnels, subways, culverts, construction, underground engineering. One of the most ideal caulking materials for engineering facilities such as offshore oil production. It is also an ideal waterproof and moisture-proof sealing packaging material for automobiles, containers, precision instruments, medicines, and food. In addition, water-absorbing swelling rubber can also be used as a soil conditioner and water-retaining agent, especially in dry seasons and arid areas. Water-absorbing swelling rubber has a high sensitivity to temperature, pH and salt concentration. Therefore, water-sensitive sensors have broad potential application in medical and physiological aspects.
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