Inorganic silica gel is a highly active adsorbent material, usually with sodium silicate and sulfuric acid reaction, and after aging, acid bubbles and a series of post-processing process and made. Silica gel is an amorphous substance, its chemical molecular formula is mSiO2 ．nH2O, insoluble in water and any solvent, non-toxic, tasteless, chemically stable, in addition to strong alkali, hydrofluoric acid does not react with any substance. Various models of silica gel due to its different manufacturing methods and the formation of different microporous structure. The chemical composition and physical structure of silica gel, it has many other similar materials difficult to replace the characteristics: high adsorption performance, good thermal stability, chemical stability, high mechanical strength and so on.
Silica gel according to the size of its pore size is divided into: large pore silica gel, coarse pore silica gel, B-type silica gel, fine pore silica gel. Because of the different pore structure, so their adsorption performance has its own characteristics. Coarse pore silica gel has higher adsorption capacity under high relative humidity, fine pore silica gel has higher adsorption capacity than coarse pore silica gel under low relative humidity, while B-type silica gel has adsorption capacity between coarse and fine pores due to its pore structure. Macroporous silica gel is generally used as catalyst carrier, matting agent, toothpaste abrasive and so on. Therefore, different varieties should be selected according to different uses.
Safety
Silica gel is mainly composed of silica, chemically stable, non-combustible. Silicone is a kind of amorphous silicon dioxide, should control the workshop dust content is not greater than 10 mg / cubic meter , need to strengthen the exhaust, wear a mask when operating.
Silica gel has a strong adsorption capacity, the human skin can produce a drying effect, therefore, the operation should wear good overalls. If silica gel gets into your eyes, you need to rinse with plenty of water and seek medical attention as soon as possible.
Blue silica gel is poisonous because it contains a small amount of cobalt chloride, so it should be avoided contacting with food and inhaling into the mouth, and if poisoning occurs, it should be treated by a doctor immediately.
Silica gel in the process of use due to the adsorption of water vapor or other organic substances in the medium, the adsorption capacity decreases, can be reused after regeneration.
I. Regeneration of silica gel after adsorption of water vapor
After adsorption of water, silica gel can be removed by thermal desorption of water, and there are various ways of heating, such as electric furnace, flue residual heat and hot air drying.
The temperature of the heat of desorption is controlled at 120-180℃, and for the blue gel indicator, color changing silica gel, DL blue silica gel, it is controlled at 100-120℃. Various industrial silica gel regeneration of the maximum temperature should not exceed the following limits:
Coarse pore silica gel shall not be higher than 600 ℃;
Fine pore silica gel shall not be higher than 200 ℃;
Blue gel indicator (or color change silica gel) shall not be higher than 120 ℃;
Silica aluminum gel shall not be higher than 350 ℃.
Regenerated silica gel, its water is generally controlled below 2% can be put back into use.
Two, the regeneration of silica gel adsorption of organic impurities
⒈ roasting method
For coarse pores of silica gel, can be placed in the roasting furnace to gradually increase the temperature to 500 - 600 ° C, about 6-8 hours to the particles of the white or yellow-brown can be. For fine pore silica gel, the roasting temperature can not exceed 200 ℃.
Peak rinsing method
Silica gel in saturated water vapor adsorption to saturation after release of hot water rinsing, and can be combined with the use of detergents to remove waste oils or other organic impurities, and then by the clean water washing and drying dehydration.
3 solvent rinsing method
According to the type of organic adsorption of silica gel, the selection of appropriate solvents will be adsorbed in the silica gel in the dissolution of organic matter, and then the silica gel will be heated in order to remove the solvent.
Three, silica gel regeneration should pay attention to
⒈ drying regeneration should pay attention to grasp the gradual increase in temperature, so as to avoid drastic drying caused by the gel blowing up, reducing the recovery rate.
Peak of silica gel roasting regeneration, the temperature is too high will cause changes in the pore structure of silica gel and significantly reduce its adsorption effect, affecting the value of use. For blue gel indicator or color change silica gel, the temperature of desorption and regeneration should not exceed 120 ℃, otherwise it will be due to the gradual oxidation of the color rendering agent and the loss of color rendering effect.
3 After regeneration of silica gel should generally be sieved to remove fine particles to make the particles uniform.
Storage and packaging
Silicone has a strong moisture-absorbing ability, so it should be stored in a dry place, and there should be shelves between the packaging and the ground. Packaging are steel drums, paper drums, cardboard boxes, plastic bottles, polyethylene plastic composite bags, flexible container bags and so on. Specific packaging specifications see classified product description. Avoid rain, moisture and exposure during transportation. (Note: Inorganic silica gel manufacturers, Shandong Tengzhou Xinxu Chemical Raw Materials Co., Ltd. because of its production strength of the country's largest effervescent alkali, the production of silica gel has a strong cost-effective, so the industry counterparts rely on the pro-products, its products are very competitive!
The basic structural unit of silicone products is composed of silicone-oxygen chain links, the side chain through the silicon atom connected to various other organic groups. Therefore, in the structure of silicone products contain both "organic groups" and "inorganic structure", this special composition and molecular structure so that it is a set of organic properties and inorganic function in one. Compared with other polymer materials, the most prominent performance of silicone products are:
1. Temperature characteristics
Silicone products are silicone-oxygen (Si-O) bond is the main chain structure of the C-C bond bond energy of 82.6 kcal/g molecules, Si C-C bond bond energy of 82.6 kcal/g molecule, Si - O bond energy in the silicone for 121 kcal/g molecule, so the thermal stability of silicone products is high, high temperature (or radiation irradiation) molecules of the chemical bond does not break, do not decompose. Silicone can not only high temperature resistance, but also low temperature resistance, can be used in a wide temperature range. Whether it is chemical properties or physical and mechanical properties, with the temperature change is very small.
2. Weathering
Silicone products, the main chain of -Si-O-, no double bond exists, so it is not easy to be decomposed by ultraviolet light and ozone. Silicone has better thermal stability than other polymer materials, as well as irradiation and weathering resistance. The service life of silicone in the natural environment can be up to several decades.
3. Electrical insulation
Silicone products have good electrical insulation properties, its dielectric loss, voltage resistance, arc resistance, corona resistance, volume resistance coefficient and surface resistance coefficient, etc., are among the best in insulating materials, and their electrical properties by the temperature and frequency is very small. Therefore, they are a stable electrical insulating material and are widely used in electronic and electrical industries. Silicone in addition to excellent heat resistance, but also has excellent water repellency, which is the use of electrical equipment in wet conditions with high reliability guarantee.
4. Physiological inertness
Polysiloxane compounds are known to be one of the most inactive compounds. They are very resistant to biological aging, have no rejection with the animal body, and have good anticoagulant properties.
5. Low surface tension and low surface energy
Silicone's main chain is very smooth, its intermolecular forces are much weaker than hydrocarbons, therefore, than the same molecular weight of hydrocarbons with low viscosity, weak surface tension, surface energy is small, strong film-forming ability. This low surface tension and low surface energy is the main reason why it has gained many applications: hydrophobicity, defoaming, foam stabilization, anti-sticking, lubrication, varnish and other excellent properties.
Two, the use of silicone
Because silicone has these excellent properties, so it has a very wide range of applications. It is not only as aviation, cutting-edge technology, military technology sector of the use of special materials, but also used in various sectors of the national economy, the scope of its application has been expanded to: construction, electrical and electronic, textile, automotive, machinery, leather and paper, chemical and light industry, metal and paint, medicine and health care.
Three, the classification of silicone
Silicone is mainly divided into silicone rubber, silicone resin, silicone oil three categories.
Room temperature vulcanized silicone rubber and its classification of room temperature vulcanized silicone rubber (RTV) is a new type of silicone elastomers introduced in the sixties, the most notable feature of this rubber is that at room temperature without heating, such as pressure can be cured on the spot, the use of extremely convenient. Therefore, as soon as it was introduced, it quickly became an important part of the entire silicone products. Now room temperature vulcanized silicone rubber has been widely used as adhesives, sealants, protective coatings, potting and mold making materials, in all walks of life have its uses.
Room temperature vulcanized silicone rubber can be divided into single-component and two-component room temperature vulcanized silicone rubber according to its packaging, according to the vulcanization mechanism can be divided into condensation type and addition molding. Therefore, room temperature vulcanized silicone rubber according to the composition, vulcanization mechanism and the use of different processes can be divided into three major types, namely, single-component room temperature vulcanized silicone rubber, two-component condensation room temperature vulcanized silicone rubber and two-component addition molding room temperature vulcanized silicone rubber. These three series of room temperature vulcanization of silicone rubber has its own characteristics: one-component room temperature vulcanization of silicone rubber has the advantage of ease of use, but the deep curing speed is more difficult; two-component room temperature vulcanization of silicone rubber has the advantage of curing is not exothermic, shrinkage is very small, does not expand, no internal stress, curing can be carried out at the same time in the interior and the surface, can be deep vulcanization; add molding room temperature vulcanization of silicone rubber vulcanization time is determined mainly by the temperature, so the use of temperature regulation can control the room temperature vulcanization. Therefore, the use of temperature regulation can control the speed of vulcanization.
I. One-component room temperature vulcanized silicone rubber
One-component room temperature vulcanized silicone rubber vulcanization reaction is to rely on the role of moisture in the air and vulcanized into elastomers. With the different chain agent, one-component room temperature vulcanized silicone rubber can be deacidified, deoxime, dehydrogenated, deaminated, deamidated and deketonized and many other varieties. The vulcanization time of one-component room temperature vulcanized silicone rubber depends on the vulcanization system, temperature, humidity and the thickness of the silicone rubber layer. Increasing the ambient temperature and humidity can speed up the vulcanization process. Under typical environmental conditions, generally after 15 to 30 minutes, the surface of the silicone rubber can be free of adhesion, and the thickness of 0.3 cm of the adhesive layer can be cured within a day. The depth and strength of cure is gradually enhanced in about three weeks.
One-component room temperature vulcanized silicone rubber has excellent electrical properties and chemical inertness, as well as heat resistance, natural aging resistance, flame resistance, moisture resistance, air permeability and other properties. They can maintain elasticity for a long time in the range of -60~200℃. It does not absorb heat or exotherm when curing, has small shrinkage after curing, and has good adhesion to materials. Therefore, it is mainly used as an adhesive and sealant, and other applications include in-situ molding gaskets, protective coatings and inlay materials. Many one-component silicone rubber adhesive formulations exhibit self-bonding properties to a wide range of materials such as most metals, glass, ceramics and coagulation. When bonding difficulties, the substrate can be into the primer to improve bond strength, the primer can be reactive silane monomers or resins, when they are cured on the substrate to produce a layer of modified suitable for silicone bonding surface. One-component room temperature vulcanized silicone rubber, although easy to use, but because of its vulcanization is dependent on the moisture in the lazy atmosphere, so that the thickness of the vulcanized rubber is limited, can only be used for occasions that require a thickness of less than 6 mm. One-component room temperature vulcanized silicone rubber vulcanization reaction is gradually from the surface to the depth, the thicker the rubber layer, the slower the curing. When the deep also want to fast curing, can be used in layers of pouring step-by-step vulcanization method, each time you can add some rubber, such as vulcanization and then add material, which can reduce the total vulcanization time. Adding magnesium oxide can accelerate the vulcanization of the deep glue.
II. Two-component condensation type room temperature vulcanization of silicone rubber
Two-component room temperature vulcanization of silicone rubber vulcanization reaction is not relying on the moisture in the air, but by the catalyst to initiate. Usually the rubber and catalyst are packaged separately as a component. Only when the two components are completely mixed together will curing begin to occur. The curing time of two-component condensed room temperature vulcanized silicone rubber depends mainly on the type and amount of catalyst and the temperature. The higher the amount of catalyst, the faster the vulcanization and the shorter the resting time. At room temperature, the shelf time is generally a few hours, if you want to extend the shelf time of the rubber, can be used to cool the method. Two-component condensation type room temperature vulcanization silicone rafter adhesive at room temperature to achieve full curing needs about a day, but in the temperature of 150 ℃ only 1 hour. The rate of cure can be significantly increased by using accelerators for synergistic effects.
Two-component room temperature vulcanized silicone rubber can be in a 65 ~ 250 ℃ temperature range of long-term elasticity, and has excellent electrical properties and chemical stability, water resistance, ozone resistance, weather aging, coupled with simple usage, process applicability, therefore, widely used as a potting and mold making materials. Various electronic and electrical components with room temperature vulcanized silicone rubber coating, potting, can play a moisture-proof (anti-corrosion, shock and other protective effects.). Can improve performance and stabilize parameters. Two-component room temperature vulcanized silicone rubber is particularly suitable for deep potting materials and has a faster vulcanization time, which is better than one-component room temperature vulcanized silicone rubber. Two-component room temperature vulcanized silicone rubber vulcanization has excellent anti-adhesive properties, coupled with the vulcanization of the shrinkage rate is very small, so it is suitable for use in the manufacture of soft molds for casting epoxy resins, polyester resins, polystyrene, polyurethane, vinyl plastics, paraffin waxes, low melting point alloys and other molds. In addition, the use of two-component room temperature vulcanized silicone rubber's high simulation performance can be reproduced on the cultural relics of a variety of exquisite patterns. Two-component room temperature vulcanization silicone rubber in the use of attention should be paid to: first of all, the rubber and catalyst are weighed separately, and then mixed in proportion. The mixing process should be carefully operated to minimize the amount of entrapped gas. After mixing the rubber (uniform color), can be removed through the static or decompression (vacuum 700 mm Hg) to remove air bubbles, to be all the air bubbles out, at room temperature or at the specified temperature for a certain period of time that is vulcanized into silicone rubber.
Three. Two-component plus molding room temperature vulcanization of silicone rubber
Two-component plus molding room temperature vulcanization of silicone rubber with elastic silicone gel and silicone rubber, the former lower strength, the latter higher strength. Their vulcanization mechanism is based on the silicone raw rubber end group on the vinyl (or propylene-based) and crosslinking agent molecules on the silicone hydrogen-based addition reaction (hydrosilylation reaction) to complete. In this reaction, no by-products are released. Since no low molecules are released during the crosslinking process, additively molded room temperature vulcanized silicone rubber does not shrink during vulcanization. This class of vulcanized rubber is non-toxic, high mechanical strength, excellent hydrolysis resistance stability (even under high pressure steam), good low compression deformation, low combustibility, can be deep vulcanization, as well as the speed of vulcanization can be controlled by the temperature and so on, so it is currently a class of silicone rubber at home and abroad vigorously developed.