Silicone rubber has the characteristics of slow combustion, no droplet, non-toxic gas release, and is widely used in the field of fire-resistant and flame-retardant materials. Ceramic silicone rubber is a new type of fire-proof material. Its performance at room temperature is the same as that of ordinary rubber material, but it can form a dense and hard ceramic body at high temperature to prevent the spread of flame. In recent years, the research of ceramic silicone rubber has attracted the attention of many experts and scholars.
The ceramic polymer material (EPDM in the video) can still maintain complete structure after combustion
(from cerampolymerik)
Fig. 1 during combustion, the filler will produce eutectic phenomenon with the pyrolysis product silica of the matrix, forming a hard ceramic layer
At present, there are many kinds of ceramic polymers, but the most successful one is ceramic silicone rubber. Because the silicone rubber itself has good thermal stability, and the Si-O structure will change into a continuous, oxidation-resistant and insulating network of silica ashes covered on the surface during combustion, which can not only prevent further ablation, but also provide material basis for the subsequent ceramics.
Characteristics of ceramic silicone rubber
One
When the rubber material containing inorganic filler is exposed to the flame, the rubber will degrade or burn, and the inorganic residue or ash with less strength will be left after the degradation or combustion products volatilize. These residues are usually not agglomerated or self-supporting, or even broken into particles or powder, so there is no fire protection effect. When the ceramic rubber with special design formula is exposed to the flame, depending on the rubber variety and formula adjustment, the ceramic can be produced in the range of 350-800 ℃ or above, and sintered into porous self-supporting ceramic body, which can maintain the structural integrity in the high temperature flame of 650-1000 ℃ for a certain period of time (0.5-2h), and can play the role of "passive fire protection" well, so as to ensure the fire safety All win precious time.
Two
It has a certain strength and good thermal shock resistance. After vitrified, the sintered body is relatively hard. According to coating on-line, the bending strength of ceramic silicone rubber is much higher than that of ordinary silicone rubber, and it increases obviously with the increase of temperature. The sintered body of ceramic silicone rubber does not crack when the high temperature ablated specimen is sprayed with water during the simulated fire fighting process, which shows good thermal shock resistance.
Three
Halogen free, low smoke, low toxicity, self extinguishing, environmentally friendly ceramic silicone rubber can achieve the effect of self extinguishing without adding halogen flame retardant. The flame retardant UL94V-0 and oxygen index can be as high as 38. Under the ablation of the flame, the smoke will be completely cut off after being burned for several minutes. In the next ablation process, no smoke will be generated. Ceramic silicone rubber produces carbon dioxide, water and silicon dioxide by combustion, which are non-toxic substances.
Four
The electrical properties of the ceramic silicone rubber with good electrical properties and optimized formula: the volume resistivity before sintering shall not be less than 1015 Ω? Cm. With the increase of sintering temperature, the volume resistivity decreases. After combustion at 1000 ℃ for 30 minutes, the volume resistivity decreases to 107 Ω? Cm. The volume resistivity of ordinary silicone rubber before and after sintering shall not be less than 1015 and 107 Ω? Cm respectively.
Five
Easy to process rubber material preparation and product production process are similar to ordinary silicone rubber, which can be produced using conventional rubber processing equipment (extruder, flat vulcanizer, injection machine, etc.). The production process of fire-resistant cable is simplified and the cost is reduced.
2、 Formula of ceramic silicone rubber
One
Raw rubber mainly uses methyl vinyl silicone rubber (such as domestic 110-2 silicone rubber). High temperature vulcanized vinyl terminated methyl vinyl silicone rubber can also be used, that is, dimethylsiloxane methyl vinyl siloxane copolymer (such as domestic 110-4s silicone rubber). This kind of silicone rubber can be vulcanized with addition vulcanization system. The vulcanizate has high tear strength and low compression set, which is suitable for fire-resistant silicone rubber sealing strip. If a material with better ablative resistance is needed, phenylsilicone rubber or methylphenylvinyl silicone rubber (such as 120-2 phenylsilicone rubber made in China) can be selected. The ablation rate of phenyl silicone rubber is about 1 / 2 of that of vinyl silicone rubber, which is only used for special purpose due to its high price.
Two
Silica is used as reinforcing filler of ceramic silicone rubber, which is mainly fumed silica and precipitated silica. The strength and hydrophobicity of silicone rubber vulcanizate can be effectively improved by silica treated with silane coupling agent and octamethylcyclotetrasiloxane.
Three
Incremental fillers and incremental fillers only play a weak reinforcing role in silicone rubber. They are generally used in combination with silica to adjust the hardness of silicone rubber vulcanizate, improve the technological performance of the compound (such as extrusion performance) and reduce the cost. It mainly includes quartz powder, diatomite, zinc oxide, titanium dioxide, talcum powder and calcium carbonate. The silica powder treated with silane coupling agent can not only improve the reinforcement, but also play the role of sintering binder and contribute to the ceramics.
Four
In order to restrain the "structure" of silicone rubber compound reinforced by fumed silica and ensure the processing performance, structural control agent needs to be added. Generally, hydroxy silicone oil is used (hydroxy content is 0.06-0.12).
Five
Heat stabilizers generally include selenium oxide, selenium hydroxide or octanoate, iron oxide or octanoate, etc.
Six
Peroxide vulcanizing agent is used in the vulcanization system of methyl vinyl silicone rubber and methyl phenyl vinyl silicone rubber. The variety depends on the vulcanization process and equipment of the products.
Seven
The fire-resistant silicone rubber products of the fire-retardant system are often designed with fire-retardant properties, and the commonly used halogen-free flame retardants are aluminum hydroxide, magnesium hydroxide and platinum flame retardants. Aluminum hydroxide and magnesium hydroxide release water vapor at high temperature to generate alumina and magnesium oxide. The evaporation latent heat of water reduces the combustion temperature. The combination of aluminum hydroxide and magnesium hydroxide (1 / 9-6 / 4) has the characteristics of promoting ceramics, as shown in Table 1.
Table 1 Effect of flame retardant
Eight
Ceramic system ceramic system is the key component of ceramic silicone rubber formula, mainly including ceramic filler and flux. At present, the ceramic fillers added to the ceramic silicone rubber are generally layered silicate mineral fillers with crystal structure, high melting point (high fire resistance) and high sintering degree, and excellent electrical insulation performance. The representative varieties are as follows:
① Mica
Mica is a kind of complex silicate with many kinds of layered crystals and various crystal systems. Its melting point is about 1800 ℃, or even higher. Mica Used for ceramic silicone rubber mainly includes muscovite and phlogopite. Muscovite is a double octahedral alkaline aluminum silicate. It is a layered structure of aluminum silicate sheets weakly bonded to each other through potassium ion layer. Its composition is kal3si3o10 (OH) 2. Phlogopite is a triple octahedral alkali Sexual aluminum silicate is the layered structure of magnesium aluminum silicate sheets weakly bonded to each other through potassium ion layer. The composition is kmg3alsi3o10 (OH) 2? Muscovite with good electrical insulation and fire resistance. Compared with phlogopite, the sinter has better dimensional stability and electrical performance, as shown in Table 2. Because Muscovite is commonly used in electrical products such as this cable. But the strength of gold mica sintered at 1000 ℃ is higher than that of muscovite.
Table 2 Effect of muscovite and phlogopite on electrical conductivity of ceramic silicone rubber
Note: 1) the mass fractions of peroxide and silicone rubber are 0.02 and 0.98 respectively;
2) The contents of peroxide, muscovite and silicone rubber were 0.02, 0.30 and 0.68 respectively;
3) The mass fractions of peroxide, phlogopite and silicone rubber are 0.02, 0.30 and 0.68 respectively.
Fig. 2 SEM photos of sintering products of silicone rubber with Muscovite powder
② Wollastonite
Calcium silicate fillers are also important ceramic fillers for silicone rubber, mainly including wollastonite, xonotlite, tobermorite, Sodalite, manganese wollastonite, etc. among them, wollastonite is the most widely used one? Wollastonite is natural calcium metasilicate (CaSiO3), triclinic system, melting point is 154 ℃? Wollastonite used for ceramic silicone rubber is α - CaSiO3, suitable for the ratio of length to diameter of 2:1 ~ 5:1 (extruded when it is too high Will the surface lose smoothness?
Fig. 3 SEM picture of sintering product of silicone rubber with Wollastonite
③ Kaolin
Kaolin is a kind of clay with natural kaolinite as its main component. Its crystal is pseudohexagonal. Its melting point is about 1735 ℃? It is more advantageous to use calcined kaolin with surface treatment, which can improve physical strength and electrical insulation performance?
Fig. 4 SEM photos of sintering products of silicone rubber with kaolin
In the environment of high temperature or open fire ablation, the porous ceramic body formed by silicon rubber sintering at the earlier lower temperature has the function of heat insulation, which can effectively prevent the spread of flame; when the temperature is further increased, the ceramic body becomes hard and dense, which can better protect the inner coating, so as to achieve excellent fire-retardant and fire-resistant effect.
The melting point of ceramic fillers used in ceramic silicone rubber is very high, and the ceramic temperature after combustion is also very high. In order to be able to "burn at low temperature", that is to say, to start ceramic at a lower temperature, to obtain a certain strength of ceramic body, to better play the fire-proof function of ceramic silicone rubber, a flux is added to the rubber formula. Flux generally refers to a substance that can reduce the softening, melting or liquefying temperature of other substances. At present, glass additives, boron compounds and zinc oxide are the main fluxes for ceramic fillers.
Nine
Other additives can be added as needed, such as colorant and foaming agent.
application area
The main application of ceramic silicone rubber is the fire-resistant wire and cable of medium and low voltage, in order to ensure the power on safety in the process of fire. Therefore, it is not difficult to understand the enthusiasm of the cable industry for the urgent development and application of new fire-resistant insulating material ceramic silicone rubber. The production equipment of ceramic silicone rubber compound is the same as that of ordinary silicone rubber. The compound has good extrusion and molding properties. It can be extruded and vulcanized into wires and cables directly by using silicone rubber wire and cable equipment. Without adding equipment, the production cost can be greatly reduced. Ceramic silicone rubber has rubber elasticity at room temperature, and will not be brittle and fall off like mica tape after burning. It can be tested by spray and vibration. Considering that in recent years, many major fires in many major civil buildings are related to wires and cables, and the demand for fire-resistant cables in civil buildings is very large, at present, ceramic silicone rubber medium and low-voltage fire-resistant cables will have a great development.
When ceramic silicone rubber is used as a firewall, it can form a fire-resistant ceramic layer under the high temperature flame to prevent the flame from penetrating. When flame-retardant ceramic silicone rubber firewall is used in aircraft, it can prevent the flame from spreading in the engine room. In addition, it also has other very wide applications, such as fire doors, fire boards, building fire coatings, foam fire-resistant high-temperature treatment materials, high-temperature fire insulation layer of rocket launch platform, etc.
Source:
Ceramic silicone rubber Xie Zhonglin
Study on properties of ceramic silicone rubber Huang Tao et al
Research progress of porcelain flame retardant silicone rubber Ji chentao et al
Cerampolymerik official website, powder circle, Baidu Encyclopedia, etc