Application of water-swellable rubber
The water-swellable rubber is a function of utilizing the volume expansion of the rubber after water absorption, and achieves functions such as water stoppage, water shutoff, and waterproofing. Therefore, it is widely used in tunnels, culverts, subways, and the like, or in the gaps of precast concrete floors. The principle of action is that after the water-absorbent resin is added to the rubber, the rubber chain with high elasticity is expanded after being exposed to water, and its volume can be expanded to several times to several tens of times, thereby functioning as a water stop and a leak preventer.
(1) Principle
Rubber is a hydrophobic material. If a hydrophilic substance is blended, water molecules diffuse into the rubber inside through capillary siphoning and surface adsorption. An osmotic pressure difference is formed between the inside and the outside to further infiltrate the water, causing the rubber to expand and deform until it reaches equilibrium and is stable.
(2) Raw materials
1) Rubber types are preferred for rubbers that can crystallize at room temperature, such as NR, CR. Because the non-crystalline raw rubber is blended with the water-absorbing resin, cold sulfur is generated, which is detrimental to the water stop. Generally, rubber is hydrophobic and does not contain hydrophilic groups such as hydroxyl groups, mercapto groups, and ether groups. However, if these groups are introduced, the rubber can be made hydrophilic, that is, chemically modified. The rubber is hydrophilic. Similarly, chemical grafting can also be used to achieve the purpose.
2) Water-absorbent resin Any resin containing a hydrophilic group in the structure is suitable, such as starch, cellulose, polyvinyl alcohol, polyalkylene hydrazine and maleic acid liver. It is preferred to select a variety that has a high water absorption rate and is easily dispersed in rubber to ensure that it is not taken out by water when it is exposed to water. The larger the amount of the water absorbing resin, the larger the expansion ratio, and of course the premise of not affecting the performance of the rubber. The water absorbing resin requires partial crosslinking or saponification before use. Such a resin is generally in the form of a powder, and it is required to have a small particle size, otherwise it is not easily dispersed in the rubber. In addition, the rubber is separated from the resin, and the resin is extracted by water to lower the water absorption rate. The overcoming method is to use them together with water-soluble polyurethane and mix them with rubber. In doing so, it is ensured that both the water swell and the mechanical properties are taken into consideration. In addition to polyurethane, some other materials, such as partially crosslinked sodium polyacrylate, isobutylene/maleic acid liver copolymer, etc., can be used for the same purpose.
3) The main task of vulcanization system vulcanization is to maintain high strength and rigidity after the rubber absorbs water. In addition, as the degree of crosslinking increases, the water absorption will be weakened, so the degree of crosslinking should be considered again under the premise of ensuring water absorption.
4) Other additives Due to the long-term work of water-swelling rubber in wet and dark environments, it is necessary to add anti-aging agents and anti-fungal agents, which is beneficial to prolong the service life of the products.
(3) Preparation method
1) Physical method One is the dry glue method, which mechanically blends the rubber and the water absorbing resin, and then adds the resin and other auxiliaries after the plastic granules are coated. The other is a latex method in which a latex, a resin, and an auxiliary agent are added to a stirrer, and after being uniformly stirred, vacuum dehydration, ripening, injection molding, and vulcanization are carried out.
2) The hydrophilic group is grafted to the rubber macromolecule by a chemical method, and the higher the graft ratio, the better the water swelling property of the rubber.
