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Silicone rubber is a high-performance material whose molecular chains possess both inorganic and organic properties. Its main molecular chain is composed of alternating silicon and oxygen atoms (—Si—O—Si—). The bond energy of the siloxane bonds is 370 kJ/mol, which is much higher than that of the carbon-carbon bonds in ordinary rubbers (240 kJ/mol); this is one of the main reasons for silicone rubber’s high thermal stability. Its molecular side chain is a hydrocarbon or substituted hydrocarbon organic group connected to a silicon atom. Initially, this group was a methyl group; later, in order to enhance the vulcanization activity of the raw rubber as well as for other modification purposes, it became common to introduce small amounts of unsaturated vinyl groups (usually not exceeding 0.5 mol%) or other organic substituents on the side chains. This low unsaturation molecular structure endows silicone rubber with excellent resistance to heat aging and weathering, as well as high stability against the effects of ultraviolet rays and ozone. The siloxane chains are arranged in a helical configuration; the molecular chains are highly flexible (more so than C–C or Si–C bonds), and the intermolecular forces between them are weak. These structural characteristics result in a vulcanized rubber that is soft and elastic, but it has poor physical and mechanical properties. When the temperature changes, the forces acting between molecules change only slightly; as a result, its various properties, especially its elasticity, do not change much. In contrast, the elasticity of ordinary rubber changes due to the significant increase in the forces between molecules when the temperature drops. Foaming involves adding a foaming agent to the silicone rubber compound, and then heating it under pressure during vulcanization to cause the rubber to foam, thereby producing silicone rubber sponge. Silicone rubber sponges are mainly used to manufacture door and window seals, gaskets, thermal insulation, and shock-absorbing materials. When preparing sponge rubber compounds using the foaming process, the following issues must be taken into account. (1) A blowing agent whose decomposition products do not affect the heat resistance of silicone rubber should be selected. Organic blowing agents such as blowing agent BN and urea are generally used, whose decomposition products can be easily removed during the two-stage vulcanization. (2) Appropriately control the amounts of vulcanizing agents and blowing agents to achieve a balance between the foaming rate and the vulcanization rate. Generally, increasing the amount of foaming agent will increase the porosity of the sponge cells and reduce its density ； Increasing the amount of vulcanizing agent reduces the porosity of the sponge pores, increases its density, and results in thicker pore walls. Furthermore, for the same rubber compound formulation, different vulcanization temperatures also have a significant impact on the porosity of the sponge. (3) The appropriate use of vulcanizing agents can effectively control the porosity and density of the sponge. Generally, using the vulcanizing agent DBPMH in combination with BP, or TBPB in combination with DCBP, yields better results. (4) Gas-phase silica No. 2, or a combination of gas-phase silica No. 2 and precipitated silica, can be used as reinforcing agents. The plasticity of the rubber compound must be strictly controlled; if its plasticity is too high, excessive expansion can occur during pore formation, resulting in a rough pore structure, and even many of the pores may break ； If the plasticity is too low, there will be insufficient pores and the product will be harder. The rubber compound using pharmacologically enhanced fillers is easier to control in terms of plasticity, and the reprocessed rubber compound should be used on the same day. (5) The blowing agent should be evenly dispersed within the rubber compound. Generally, foam agents tend to agglomerate and are difficult to disperse; therefore, a raw rubber-foam agent masterbatch (1:1) can be prepared first, followed by mixing, in order to improve dispersion. (6) For rubber compounds processed by molding, care should be taken to remove bubbles from the compound in order to prevent damage to the sponge structure. The mixing of the rubber compound for foaming can be carried out on a conventional open mill. The mixing procedure for the rubber compound in model products is as follows: raw rubber (on rolls) → vapor-phase silica → structure control agent → blowing agent → vulcanizing agent → first mixing pass → stop and let stand overnight → re-mixing → shaping into pieces. The mixing procedure for the compound of extruded products is as follows: first, prepare the blowing agent masterbatch and the vulcanizing agent masterbatch, then carry out the mixing process. The order of adding materials is: raw rubber → blowing agent masterbatch → fumed silica → structure control agent → vulcanizing agent masterbatch → light mixing → resting → re-mixing → shaping. For the rubber compound, the number of thin-passes should be 10; it is advisable to wait 24 hours after one thin-pass before proceeding with the second one. The rubber compound used for molding sponge products, after being mixed into sheets, should be cut to size according to the mold specifications, and a release agent should be applied to its surface prior to vulcanization in the mold. Talc is generally used as a release agent, and silica gel can sometimes be used as well. There are two methods for the curing and setting of silicone rubber sponge mold products: one is the one-step method, in which the rubber compound forms a sponge of a certain shape and size through cell formation directly in the mold, with no further cell formation occurring during the two-stage curing process ； Another method is the two-step approach: first, the rubber compound is vulcanized for a short period of time in a mold, which allows it to develop pores initially and form a surface layer; thereafter, it is placed in an oven to further develop pores until it reaches the desired shape and size. For sponge sheets, the vulcanization time for the former type is usually 15–20 minutes, while that for the latter type is generally within 5 minutes. When using a two-step vulcanization process, two sets of molds with different sizes are required. The size of the first set of molds should be determined based on the initial pore expansion rate, while the size of the second set of molds should be determined according to the size of the final product. For easier demolding, the mold can be coated with diluted soapy water to prevent sticking ； At the same time, the mold should be designed with a certain slope to allow the products to be removed quickly when the mold is opened. For sponge rubber sheets using the vulcanizing agents BP and DBPMH together, in the case of finished products with a thickness of 9 mm, when vulcanization is carried out in two steps, the initial swelling rate due to pore formation is approximately 30–40%. For the two-step legal vulcanization conditions, when the vulcanizing agents BP and DBPMH are used together, the temperature for the first step of vulcanization is 125±2°C; the time required varies depending on the thickness of the product – for a thickness of 8–16 mm, 2–3 minutes is an appropriate duration ； The vulcanization temperature in the second step is 150–160°C, with a time of 10–15 minutes. The curing of silicone rubber sponge extruded products generally involves first using hot air for continuous curing to initially form the pores in the sponge, followed by high-temperature curing in an oven to fully develop the pores and finalize the shaping, which constitutes a two-stage curing process. The extruded semi-finished product can also be subjected to thorough cell formation and shaping vulcanization using direct steam in a vulcanization tank, followed by a second-stage vulcanization in an oven. Recently, a new process for vulcanization using the salt bath method has emerged, in which vulcanization is carried out in a metal tank. The groove contains a mixed salt solution of potassium nitrate, sodium nitrate, and sodium nitrite as a heat carrier. The weight composition of the mixed salt solution is: potassium nitrate 53, sodium nitrate 7, sodium nitrite 40. After being extruded, the rubber compound goes directly into a metal tank filled with salt bath for shaping and vulcanization. The vulcanization temperature is generally between 155 and 165°C. The vulcanization time depends on the specific formula and the specifications of the product; it can be adjusted by changing the length of the metal tank or the extrusion speed, and is usually around 2 minutes. Before secondary vulcanization, it is necessary to wash repeatedly with cold and hot water to remove the salts adhering to the surface of the product. The cleaning process must be continued until the surface of the product no longer shows an alkaline reaction when tested with phenolphthalein. The advantages of sulfurization using the salt bath method are rapid and uniform heating of the rubber compound, a uniform cellular structure in the sponge-like material, good elasticity, a smooth surface, simple processing, and the ability to achieve continuous sulfurization. The downside is that the equipment is prone to corrosion and damage. This article is from: Rubber Technology Network. Please indicate the source when reproducing it; the full address of this article is: http://www.sto.net.cn/bbs/thread-3892-1-1.html