Hot vulcanized silicone rubber is a kind of products with large output and wide application in silicone products. Silicone rubber has excellent high and low temperature resistance, can be used for a long time in the range of -60 ~ 250℃, and has excellent electrical insulation performance, ozone resistance, climate aging resistance, water hate, moisture, and good physiological inertia.

In the hot vulcanized silicone rubber raw rubber to add reinforcing filler, vulcanizing agent (or not add first) and other additives, after mixing is mixed rubber, can be molded, extrusion and other general rubber processing methods processed into a variety of silicone rubber products.

The mixed silicone rubber can be prepared in the general rubber mixer, and the mass production is carried out in kneaders or closed mixing equipment. The following is the basic matching process:

Mixing:

When mixing with open mixer, it is best to equip dust or air pumping device above the roller of open mixer to reduce the flying of white carbon black. No other impurities or colloidal particles shall be mixed in the mixing process. The temperature shall be controlled below 40℃ and the cooling water shall be fully opened. The mixing time of each batch of rubber material is about 20 ~ 40min. The mixing machine can not only improve the production efficiency and reduce the labor intensity, but also reduce the flying of white carbon black and improve the operating environment. The mixing sequence of internal mixer is similar to that of open mixer, but the interval time is not strict. The mixing time of each batch of rubber material is about 8 ~ 18min, and the filling coefficient of the mixer should be controlled in the range of 0.7 ~ 0.75. The discharge temperature is related to the packing type and should be controlled in the range of 50℃ ~ 70℃.

Heat treatment:

The structure control agent must be added to the compound when the white carbon black without surface modification is used as reinforcing agent. Therefore, the heat treatment of the compound has become a necessary process. The main purpose of heat treatment is to further combine the structure control agent with silica surface (Si-OH) and eliminate the low molecular volatiles in the adhesive. The general heat treatment conditions are as follows: under the protection of nitrogen or in a kneader under decompression at 160℃ ~ 200℃ for 1 ~ 1.5h.

Filtering:

For extruded products, calendered products, coated products and film products, mechanical impurities and undispersed compound particles are one of the main factors leading to product quality problems. Therefore, the glue used in the above products must be filtered. Filter can be used glue filter, can also be used to replace the ordinary extruder. The temperature of the barrel and screw should be controlled to the lowest, and the filter size should be 120 ~ 200 mesh.

Return tests:

After parking, the gel content increases and the plasticity decreases, so it must be refined when used. The return of rubber material should be enough, the return of insufficient, poor flexibility of rubber material, the surface is not smooth; And return excessive, the surface of the compound will be sticky, is not conducive to operation.

The composition of the compound

Raw rubber:

Silicone rubber rubber types: dimethyl silicone rubber (referred to as methyl silicone rubber), methyl vinyl silicone rubber (referred to as vinyl silicone rubber), methyl phenyl vinyl silicone rubber (referred to as phenyl silicone rubber) and fluorine silicone, nitrile silicone rubber, etc. Most of the hot vulcanized silicone rubber is methyl vinyl silicone rubber, especially the end of the rubber molecular chain with vinyl dimethyl siloxane (vinyl double head) chain end type is the main product type of the market.

Packing:

Adding filler can improve the mechanical properties of silicone rubber, and the amount of filler has a great influence on the electrical and mechanical properties of silicone rubber. Fillers are generally inorganic substances, mainly silica, titanium dioxide, quartz powder, kaolin, micronized talc and precipitated calcium carbonate and so on. The reinforced filler used in silicone rubber is mainly silica, including gas phase method and precipitation method silica. Due to the low intermolecular cohesive energy density of silicone rubber raw rubber, the tensile strength of vulcanized rubber is only about 0.35mpa, almost no use value, need to use gas phase method or precipitation method of silica black reinforcement to improve the mechanical properties of vulcanized silicone rubber. With the increase of silica content, the tear strength and tensile strength of the vulcanized silicone rubber increased, the hardness gradually increased and then decreased, and the volume resistivity gradually decreased.

The silicone compound itself has good insulation, the addition of filler will affect the molecular chain, making the resistivity of silicone rubber after vulcanization decreased. But because the filler has the reinforcing effect, so as to improve the mechanical properties of silicone rubber, so as to improve the hardness of silicone rubber, but if the filler is added too much, it will affect the cross-linking between polymer molecular chains, so that the hardness of silicone rubber is reduced.

Structure control agent

The silicone rubber compound with gas phase white hair and black reinforcement will harden during storage, reduce plasticity, and gradually lose processing performance, which is called "structured" effect. The results show that the hardening degree of the compound is related to the specific surface area of silica and the isolating hydroxyl group on the surface of silica.

The complexes added to prevent and reduce this tendency to "structure" are called "structure control agents". Structure control agent can prevent or delay the structural effect, because the active group of these substances and the active hydroxyl on the surface of silica condensation reaction, so as to prevent the effect between silica and raw glue.

The most commonly used antistructural agent is (terminal) dihydroxyl oligomeric dimethylsiloxane, or low molecular hydroxyl silicone oil. Hydroxyl content is the main performance index, the higher the hydroxyl content, the better the anti-structural, the less the dosage. Use the hydroxyl silicone oil for structure control agent has good resistance to structural, easy to use, the advantages of mixing glue is transparent, but there is a single function, poor stability (often with dehydration condensation reaction), dosage is bigger, easy to make the rubber is tacky, heating or a bad smell, vulcanization molding will reduce dosage a bit more break strength, increase the pull a permanent deformation and resilience.

Release agent

Internal and external release agent. Most of the internal release agent will reduce the transparency of the mixing glue, can be added in the return of thin pass.