Water-swellable rubber (WSR) is a new type of specialty rubber that is made by physically mixing or chemically grafting hydrophilic components (or hydrophilic groups) and a rubber matrix under multiple expansion. It is known as a "super sealant" due to its dual sealing function of elastic waterproofing and swelling waterproofing. It is widely used as waterproofing material, sealant, filler, etc. in civil engineering. The high elasticity and good mechanical strength of rubber allow rubber to expand from several to hundreds of times after absorbing water, which can overcome failure defects of waterproofing caused by permanent deformation, creep and shrinkage of filling material. The mechanism of action is that when rubber is in contact with water, water molecules continuously penetrate into water-absorbing swelling rubber through physical effects such as diffusion, capillary and surface adsorption, and form a strong affinity for hydrophilic groups in rubber. causing rubber to swell Deformation, when strain stress and osmotic pressure difference are balanced, water-swellable rubber maintains a relatively stable water stop and blockage effect.
1. Principle of water swelling in WSR
Moisture absorption of WSR is mainly due to added hydrophilic components or grafted hydrophilic groups. When WSR is in contact with water, water molecules diffuse into colloid by adsorption on surface of colloid and capillary adsorption, water molecules entering rubber form a strong interaction with hydrophilic component or hydrophilic group, and water is retained in rubber. rubber causes expansion and deformation of colloid, and elastic contraction force of rubber increases, and resulting difference in osmotic pressure causes outer water molecules to penetrate further into rubber. When difference in osmotic pressure is equal to anti-deformation force of colloid itself, water absorption reaches a balance. At this time, expansion ratio of rubber is regarded as maximum expansion rate of hydrostatic water. It is generally believed that this process is caused by diffusion of water molecules and relaxation of rubber molecular chains. If water-swellable rubber is used in a closed environment, swelling rate after contact with water cannot reach maximum hydrostatic expansion rate, so a contact pressure is generated between swelling rubber and holding body. Due to this contact pressure, water-swellable rubber can act as a sealant for water.
2. WSR classification
In terms of vulcanization, it can be divided into unvulcanized water-absorbent swell rubber and vulcanized water-absorbent swell rubber; according to hydrophilic component, it can be divided into polyacrylic acid, grafted maleic anhydride, modified high-acid bentonite, white carbon and polyethylene glycol, hydrophilic polyurethane prepolymer, etc.; According to degree of expansion after water absorption, it can be divided into low expansion (50% ~ 200%), medium expansion (200% ~ 350%), high expansion rate. There are three expansion ratios (>350%); According to preparation method, it can be divided into physical mixing and chemical inoculation.
3. Preparation of grafted WSR
Compared with physical mixing method, water-swellable chemical grafting elastomer has advantages of good microscopic compatibility, high strength and good repeatability. However, grafting reaction is complex and process is cumbersome. Since expansion rate of material depends on number of hydrophilic groups or hydrophilic chain segments, it is difficult to carry out industrial production, so current industrial production of WSR is mainly based on mixing.
4. Preparing mixed WSR
Physical mixing can be divided into two methods: mechanical mixing and emulsion mixing. The mechanical mixing method is to first mix rubber in rerubber mixer with a roller to coat raw rubber, and then add various additives and water-absorbent components to mixture, so that water-absorbent components are macroscopically and uniformly dispersed in rubber, and finally mix rubber Vulcanize. The emulsion mixing method is to uniformly mix rubber matrix, corresponding various additives and water-absorbing components in a mixer, then vacuum dewatering, maturation and vulcanization. The resulting products have higher dispersibility and uniformity than mechanical mixing method. has a smaller domain size.
WSR used in different working conditions, has different performance requirements (high temperature resistance, oil resistance, aging resistance, fire resistance, high strength, etc.), mechanical properties such as rubber matrix elasticity determine physical properties of WSR . , and The hydrophilic component or hydrophilic group (hydrophilic segment) determines WSR's ability to swell in water. Based on created matrix and hydrophilic components, addition of various additives (reinforcing, flame retardant, anti-aging, etc.) can improve performance of IDT or endow IDT with some special properties. Therefore, matrix, hydrophilic components and additives are main factors influencing characteristics of HRV.
5. WSR Matrix
Currently, rubber matrices used to make WSR are mainly Chloroprene Rubber (CR), Natural Rubber (NR), Styrene Butadiene Rubber (SBR), Nitrile Rubber (NBR), Ethylene Propylene Diene Rubber (EPDM). etc. There are also reports of using chlorinated polyethylene (CPE) and polyvinyl chloride (PVC) as a matrix or two rubbers mixed in a certain proportion as a matrix. Matrix selection is mainly considered from several aspects such as physical and mechanical properties, special rubber characteristics and compatibility with hydrophilic components. For example, NR has excellent physical and mechanical properties, but poor compatibility with hydrophilic components and a slow expansion rate; while NBR with high oil resistance and strong polarity has good compatibility with hydrophilic components; EPDM has good tensile properties and aging resistance.
6. Hydrophilic component
The water-absorbent swelling property of water-absorbent swelling material is due to hydrophilic component, and swelling and water-stopping function can only be achieved by above-mentioned water absorption principle. The choice of hydrophilic components mainly requires high water absorption, good water holding capacity, heat resistance and good matrix compatibility. There are many types of superabsorbent resins, and current research mainly includes starch, cellulose, and synthetic superabsorbent resins.absorbent resins. A comparison of characteristics of these three types of super absorbent resins is shown in following table:
The following water-absorbent resins are commonly used in production:
①Starch such as saponified product of starch-acrylonitrile graft polymer, starch-acrylic acid graft polymer, etc.
② Cellulose such as graft polymer of cellulose and acrylonitrile, cross-linked product based on carboxymethyl cellulose, etc.
③ Polyvinyl alcohols such as crosslinked polyvinyl alcohol products, saponified acrylonitrile vinyl acetate copolymer products, etc.
④Acrylic acid such as polyacrylate (mainly sodium salt) (PNaAA), saponification product of methyl methacrylate-vinyl acetate, etc.
⑤ Polyalkylene ethers such as cross-linked products from polyvinyl alcohol and dipropylene ether, etc.
⑥Maleic anhydrides such as isobutylene-maleic anhydride alternating copolymers, anionic polyacrylamide (PHPAM-1), etc.
As we all know, different water absorbent resins have different water absorption principles, so monomeric composition of synthetic resins is main factor affecting water absorption performance of materials. In addition, method of polymerization of monomer and selected crosslinking agent are important influencing factors.
The physical mixing method is characterized by simple process, low equipment requirements, high water expansion rate, etc., and easy industrial production. However, commonly used water-absorbent resin has strong polarity and high cohesion, and is unevenly distributed in rubber matrix. After soaking in water, it is easily separated from matrix and released into environment, resulting in high weight loss. a speed that is not conducive to reuse. Therefore, it is necessary to add additives such as tackifying resins, adhesives or compatibilizers to improve dispersion of water-absorbent resin and improve compatibility with rubber matrix, thereby improving various properties of WSR. Of course, it is also necessary to select a hydrophilic component with good compatibility with matrix. The compatibilizer molecular chain usually has same segment as matrix or water absorbent resin, or a functional group that can react with it, while some amphiphilic compatibilizers are grafted onto matrix molecular chain to interact with water absorbent resin. resin Segments with same structural features. In addition, surfactants also belong to class of compatibilizers. The compatibilizer mainly plays a binding role, establishes a bond between matrix and hydrophilic component, reduces interfacial tension and improves compatibility of mixed components.
8. Strengthening agent
As water-swellable rubber is used more and more, corresponding WSR performance requirements are also getting higher and higher. In sealing and plugging tasks of some large-scale projects such as engineering expansion joints, dam seals and construction joints, WSR used must have a certain strength, especially strength after water absorption and swelling. At present, strength of known rubber matrix is low, and HRV based on PVC and other resins has disadvantages of slow water absorption, low expansion, and poor elasticity of matrix, so it is difficult to achieve a swelling effect. water blocking. When WSR absorbs water and swells, water acts as a plasticizer and strength of material is reduced. Therefore, as reinforcing additives (reinforcing additives), it is necessary to add high-strength inorganic fillers, such as soot, silica, clay, etc., which makes it possible not only to increase strength of material, but also to reduce its cost. In recent years, research and application of nanomaterials has become widespread in various fields. Nanomaterials used in rubber industry are usually referred to as nanofillers. Due to their small size and surface effect, distance between particles is reduced, interaction between particles is enhanced, which in turn exhibits a good reinforcing effect.
Due to carbon black contamination, interaction between silica without surface modification and rubber is weak, and rubber filled with silica has a long curing time. The large particle size and low surface activity of clay makes reinforcing effect very weak, so typically clay is organically modified to widen interlayer spacing of clay or even become a delaminated structure.
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