Let's talk about silicone first

Solid silica gel is made of water glass (sodium silicate) as raw material, which is hydrolyzed in acidic medium to form gel, and then it is made into silica gel through aging, washing, drying and other processes, and it is translucent or white solid according to different water content. Commercial products include irregular granular, spherical and micro-spherical silica gel, which has good wear resistance and is often used as catalyst carrier in fluidized bed operation.

As a catalyst carrier, silica gel is usually soaked in a solution containing catalytic active components, so that the solution is adsorbed in the pores of silica gel, and the active components are distributed on the surface of silica gel through drying, activation and other procedures. The pore structure of silica gel has an important influence on the performance of supported catalysts, such as pore volume and pore size distribution of silica gel. Traditionally, the average pore diameter is less than 15 ~ 20? Silica gel, called fine pore silica gel; The average pore size is greater than 40 ~ 50? , called coarse pore silica gel. Silica gel with fine pore structure has high specific surface area, which is beneficial to the dispersion of catalytically active components. However, the pore structure is not conducive to the diffusion of reactant molecules, that is, it is not easy to contact the catalytically active components buried in the pores, so the utilization rate of the inner surface of the catalyst will be reduced, and the product molecules generated in the depths of the pores are not easy to escape from the pores, which is easy to cause deep side reactions. The pore structure of silica gel is related to the manufacturing method and conditions, such as gel formation, aging, pH, temperature and time during washing. The commercially available silica gel can be reamed by reaming treatment. The common method is to put it in an autoclave and add water or saline solution (such as sodium carbonate and sodium acetate) for hot pressing. For example, after hot pressing at 320℃ and 10MPa, the specific surface area and average pore diameter can be 135m2/g and 123, respectively. Silica gel changed to 26.9㎡/g/g and 508? .

Silica Sol Besides solid silica gel, liquid silica sol is also an important carrier material. In industry, silica sol is usually prepared by removing sodium silicate by ion exchange. It is a translucent milky white liquid. The content of commercial silica is generally 20% ~ 30%, up to 50%, and the diameter of colloidal particles is 100 ~ 200? Some goods are as high as 1300? . High concentration silica sol usually maintains its stability with the help of trace sodium oxide. Silica sol becomes a porous solid after drying. For example, in the production of (phosphorus-molybdenum-bismuth-oxygen)/silica catalyst for acrylonitrile by ammoniation and oxidation of propylene, a solution containing active components is mixed with silica sol, and a microsphere catalyst is prepared by spray drying.

silicon rubber

Silicone rubber is a special synthetic elastomer, which is made of linear polysiloxane mixed with reinforcing filler and vulcanized under heating and pressure. It perfectly balances mechanical properties and chemical properties, so it can meet the requirements of many demanding applications today.

Silicone rubber performs well in the following fields:

High and low temperature stability

Inert (tasteless and odorless)

Transparent and easy to color.

Wide hardness range, 10-80 Shore hardness.

chemical resistance

weather ability

airtightness

Electrical property

resistance to compression

In addition to the above excellent properties, silicone rubber is particularly easy to process and manufacture compared with traditional organic elastomers. Silicone rubber is easy to flow, so it can be molded, calendered and extruded with low energy consumption. Easy processing means high production efficiency.

Silicone rubber can be provided in the following forms:

Rubber compound: This ready-made material can be colored and catalyzed according to your processing equipment and end use.

Substrate: This silicone polymer also contains reinforcing fillers. Rubber-based materials can be further mixed with pigments and additives to form rubber compounds to meet your color and other manufacturing requirements.

Liquid Silicone Rubber (LSR): This two-component liquid rubber system can be pumped into appropriate injection molding equipment and then thermally cured into molded rubber parts.

Fluorosilicone rubber compound and base material: fluorosilicone rubber retains many key properties of silicone, in addition, it also has excellent chemical resistance, fuel resistance and oil resistance.

Synthetic rubber with silicon atoms and oxygen atoms alternately arranged in the main chain and organic groups on silicon atoms. The organic group in the molecule can be-CH3,-C2H3 or-C6H5, which is called methyl, vinyl or methylphenyl silicone rubber. Silicone rubber is a kind of special rubber with high and low temperature resistance (-60 ~ 250℃), ozone resistance and good electrical insulation. Silicone rubber can be divided into high temperature vulcanization type (vulcanization temperature 150 ~ 200℃), room temperature vulcanization type and addition type.

1944 American general electric company successfully developed high temperature vulcanized methyl silicone rubber. In 1952, L. Stebreton and K. Pahlman Thiel of American Tao-Corning Company developed two-component room temperature vulcanized silicone rubber. Methyl vinyl silicone rubber is made of 1953 ~ 1955; Silicone rubber and fluororubber appear in1956 ~1957; 1958 ~ 1959 successfully developed one-component room temperature vulcanized silicone rubber; After that, additive silicone rubber series products appeared. More than ten countries in the world produce silicone rubber. Since 1980s, it has been increasing at the rate of 15% every year. Since the early 1960s, various types of silicone rubber have been successfully developed in China, and now they have been mass-produced. Silicone rubber mainly has the following varieties:

High temperature vulcanized silicone rubber is colorless and transparent, plastic, with a molecular weight of 350,000 ~ 700,000, and can be dissolved in benzene and other solvents. The product has the characteristics of oxidation resistance, ozone resistance, excellent electrical insulation at high frequency, arc resistance and corona resistance, and is breathable and inert to human physiology. Widely used as wire and cable insulation layer, sealing element, conduit, moon shoes under harsh conditions, etc. Because of its non-carcinogenicity, good anticoagulation and biocompatibility, it is widely used to make internal and external materials such as catheters, intubation and artificial joints.

High-temperature vulcanized silicone rubber usually takes high-purity octamethyl cyclotetrasiloxane and cyclotetrasiloxane with groups such as-C6H5, -CH = CH2,-CH=CH2-CF3 as raw materials, and carries out ring-opening polymerization with acid-base catalyst, and then removes the catalyst and low volatile matter to obtain silicone rubber. When rubber is processed, structure control agent (diphenyl silicon glycol), reinforcing filler (fumed silica) and antioxidant (iron oxide) are added first, and then mixed in rubber mixer. After high temperature (about 200℃) treatment, organic peroxide (2,5-dimethyl-2,5-di-tert-butyl peroxide hexane) was added as vulcanizing agent. After the rubber compound enters the mold, it should be heated and pressurized. Some products must undergo secondary vulcanization after leaving the mold. The rubber mixture can be extruded into tubes, belts and covered wires and cables.

The characteristic of RTV silicone rubber is that it can be made into mixed rubber with different viscosities, can be coated at room temperature, and can be vulcanized without high temperature and high pressure. During vulcanization, low molecular products such as alcohols and acetic acid will be released. Room temperature vulcanized silicone rubber can be divided into two components and one component. Two-component room temperature vulcanized silicone rubber is used for elastic molds, dental impression materials and ablation-resistant coatings of spacecraft in precision casting. The hydroxyl-terminated low molecular weight polysiloxane in its raw material is specially prepared, usually made of ring-opening polysiloxane ring and water, and the molecular weight is controlled by the amount of water added. One-component room temperature vulcanized silicone rubber is used for sealing protection of electronic devices, and silane or siloxane with active groups such as acetoxy or ketoxime, one of its main components, also needs to be specially prepared.

Vulcanizing and addition molding silicone rubber with vinyl terminated polysiloxane and polysiloxane containing hydrogen atom as base materials and platinum compound as catalyst at a temperature slightly higher than room temperature;

“sich ch 2+HSI”——→“sich 2ch2 si”

If special silicone resin is added as reinforcing agent, transparent vulcanizate with excellent mechanical properties can be obtained. Its characteristic is that it does not release low molecular products during vulcanization, and it can be poured into deep vulcanization. It is mainly used as an infusion coating for electronic devices, as an optical fiber coating, and is also an ideal material for human soft tissue filling and facial plastic surgery.

philology

Noel, W, translated by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences: Silicon Chemistry and Technology, Science Press, Beijing, 1978. (W.Noll, Silicone Chemistry and Technology, Academic Press, new york, 1968. )