1. What are the molecular structural characteristics of silicone rubber? What are the main characteristics and uses?
Silicone rubber (Q) is an organic elastomer obtained by hydrolysis and condensation of various dichlorosilanes, which is a linear polymer with SiO (silicone-oxygen bond) unit as the main chain and monovalent organic groups as side groups and has both inorganic and organic properties.
2. Advantages of structure control agents
(1) has excellent physiological inertia and physiological aging
The surface energy of silicone rubber is lower than most organic materials, therefore, it has low moisture absorption, long-term immersion in water and a water absorption rate of only about 1%, physical and mechanical properties do not decline, mold resistance is good; in addition, it is not sticky with many materials, can play a role in isolation. Silicone rubber is tasteless, non-toxic, has no adverse effects on the human body, and the body tissue reaction is slight, with excellent physiological inertia and physiological aging.
(2) Excellent high and low-temperature resistance
The working temperature range is -100~-350℃, with excellent resistance to ozone aging, oxygen aging, light aging, and weathering aging. The performance of silicone rubber vulcanized rubber does not change after being placed outdoors for several years in a free state.
(3) Excellent electrical insulation performance
The electrical insulation performance of silicone rubber vulcanized rubber in moisture, frequency change, or temperature change is small, the burning of the resulting silica is still an insulator, in addition, the molecular structure of silicone rubber has fewer carbon atoms and does not use carbon black as filler, so it is not easy to occur in the arc discharge scorching, so it is very reliable in high-voltage occasions. Its corona resistance and arc resistance are very good, corona life is 1000 times that of PTFE, arc life is 20 times that of fluorine rubber.
(4) High permeability
Compared with other polymer materials, silicone rubber has excellent permeability, the permeability of nitrogen, oxygen and air at room temperature is 30~40 times higher than that of natural rubber. In addition, it also has a selection of gas permeability performance, that is, the permeability of different gases (such as oxygen, nitrogen and carbon dioxide, etc.) varies greatly, such as the permeability of oxygen is about 2 times that of nitrogen, carbon dioxide permeability is about 5 times that of oxygen.
Silicone rubber can be used in automobile parts, electronic parts, aerospace sealing products, electrical seals for adhesive joints in the construction industry, medical artificial organs, urinary catheters, etc. In textile high-temperature equipment and in alkali, sodium hypochlorite, and hydrogen peroxide concentrations of equipment for sealing materials also achieved good results.
3. Why add a structure control agent in silicone rubber?
In the rubber industry, structuring refers to the phenomenon of insoluble gel formed at the contact area after the contact between elastomeric materials and highly reactive reinforcing agents. This phenomenon is easy to occur in diolefins raw rubber, for example, the natural rubber and carbon black gel generated between the structure of the generation of structured. There must be a prerequisite for such a structured phenomenon, that is, the reinforcing agent must be highly fine particles, with large specific surface area and high activity.
Fumed silica, ultrafine precipitated silica, and silicone rubber blended rubber, often in the process of storage hardened, plasticity is reduced, the viscosity between the rubber rolling rollers rose sharply, and gradually lost refining processability of such phenomena is called “silicone rubber structured”.
The reason for the structuring is the condensation of hydroxyl groups on the surface of silica and the end groups of silicone rubber molecules, and it is also believed that it is caused by the formation of hydrogen-bonded chemisorption between the surface active groups of silica and silicone rubber molecular chains. The formation mechanism of general-purpose rubber carbon black gel is similar to the structuring of silicone rubber, but the consequences are different. In the former case, the carbon black gel does not hinder the processing but is beneficial to the performance of the product, while in the latter case, it has a negative impact on the subsequent processing of silicone rubber.
Silicone rubber structure control agents are silicone compounds, usually small molecules with hydroxyl or boron atoms. Can be summarized into the following four categories.
- Hydroxysilanes, the higher the hydroxyl content, the greater the activity, and the better the anti-structuring effect.
- Alkoxysilanes, including various alkoxysilanes and low molecular polysiloxanes.
- Siloxanes, these structure control agents are more expensive and have an odor, so they are only used in special cases.
- Boron-containing siloxanes, condensed from boric acid and various types of chlorosilanes, are characterized by the validity of up to 6 months or more.
At present, the most used and most mature is diphenylsilylene glycol, which has a good overall performance of vulcanized rubber. The product should be added to the raw rubber first, and then add silica. In order to give full play to its role, the rubber should be heat-treated before adding peroxide. In general, the dosage should be 1/100-1/5 of silica. hydroxy silicone oil is also a common structural control agent for silicone rubber, it is a colorless transparent oil with low viscosity and high hydroxyl content, which can eliminate the heat treatment process and simplify the process and improve the transparency of the product.