The unique properties and intentions of silicone rubber
Silicone rubber polymer molecules are chain-like structures connected by Si-O bonds. The bond energy of Si-O is 443.5kj/mol, which is much higher than the bond energy of c-c (355Kj/Moi). Therefore, silicone rubber has better stability than other organic rubbers. Generally speaking, silicone rubber has much better heat resistance, electrical insulator, chemical stability, etc. than other organic rubbers.
Typical silicone rubber polydimethylsiloxane. It has a helical molecular structure, and its division force is small, so it has good resilience, compressibility, and excellent frost resistance. At the same time, the methyl groups pointing out of the helix can rotate freely, thus giving the silicone rubber unique surface properties, such as water-enhancing and surface stickiness.
Conditions for the Vulcanization Reaction of Silicone Rubber
1. Vulcanization and its standard conditions:
1. What is “one-time sulfur addition”?
The process of putting the material into the mold force of the hydraulic press and turning it into a product under the conditions of the specified pressure, temperature, and time. It’s called “sulfur”.
2. Three standard conditions that must be met for one-time vulcanization:
(1) Pressure: Pressure is the function of making the material flow fully, filling the mold, and solidifying the mold. Under normal circumstances, the pressure must be below 2000 Kg. If the pressure is not enough, the front weight will be high. If the pressure is too high, the front weight will be low.
(2) Temperature: Temperature is a necessary condition for the vulcanization of the material. Generally, it is 150-170℃. If the temperature is too low, the product will not be able to complete the vulcanization.
(3) Vulcanization time: Under the above two conditions, after a certain period of time, the material will be completely vulcanized, reducing the vulcanization time will cause the product to be covered with air; black particles will fall off and other adverse effects on the service life of the product. The product is deformed and difficult to disassemble, and the keys are resilient. The curing time of each model is different, and the production must strictly adhere to the tolerance range on the “Molding Standard Card”.
3. In the actual process of silicone rubber products, in addition to the above standard conditions, the following two conditions must be met at the same time:
① Vulcanizing agent:
The commonly used vulcanizing agent for silicone rubber is organic peroxide, which uses the free radical formed by high-temperature decomposition to crosslink the organic group of the side bond of the silicone rubber molecule. The most commonly used vulcanizing agents are 2.5-dimethyl-2.5-di-tert-butyl peroxyhexane and dicumyl peroxide and other low-activity vulcanizing agents. The vulcanization temperature is 150-170℃.
② Exhaust
In the process of silicone rubber products, the number of times of exhaust is usually 1-4 times. The function of exhaust is to make the filling of the rubber mold appropriate for the number of times of exhaust and the size of the opening (Sec). Otherwise, the product will be deformed, and the hemp bread will be bad.
the role of secondary vulcanization
Silicone rubber products are mainly composed of two processes primary vulcanization (molding) and secondary vulcanization.
1. The product is further vulcanized.
The product after one-time vulcanization may not be fully vulcanized. (Only T90) To make the product fully vulcanized, secondary vulcanization is necessary.
2. Removal of peroxide decomposition products, moisture, and other organic low molecular weights.
3. Removal of low molecular cyclosiloxane
Because low-molecular-weight cyclosiloxane is an insulating material, it decomposes into a gas under the action of an arc and is adsorbed on the surface of the conductive inner particle, causing black particles or dot printing ink to fail to conduct, resulting in button failure.
The secondary vulcanization conditions greatly influence the final low molecular cyclosiloxane content of the product. The conditions include temperature, time, ventilation conditions, etc. The higher the temperature, the better the effect of eliminating low molecular rings, but if the temperature is too high, it will affect the performance of the product. Therefore, it is generally around 180°C – 200°C.