Silicone rubber is a kind of linear polymer, which is different from other rubber in that its molecular backbone is not composed of interconnected carbon atoms, but by the silicon atoms and oxygen atoms between the composition. Silicone rubber has high thermal stability due to the high bond strength of Si-O, and it has excellent resistance to ozone aging, electrical properties and bioavailability (physiological inertness).
Silicone rubber can be divided into three categories according to its curing mechanism: organic peroxide initiated free radical crosslinking (referred to as heat vulcanization), polycondensation reaction type (referred to as room temperature vulcanization) and addition reaction type.
Heat vulcanized silicone rubber refers to the high molecular weight (40 to 70 thousand) of silicone rubber. Using the organic peroxides as a curing agent, after heating to decompose organic peroxides to produce free radicals, and it is crosslinked with the organic pendant side of the rubber to obtain vulcanized rubber. The cure mechanism is shown in pic 1:
Pic1, Peroxidation mechanism of silicone rubber
Room temperature vulcanization (condensation vulcanized) silicone rubber generally referred to as RTV, which can vulcanize silicone rubber without heating Its molecular structure is characterized by the molecular chain at both ends containing hydroxyl or acyloxy and other active functional groups, under certain conditions, through these functional groups condensation reaction, the formation of cross-linked structure can be elastomer. According to its curing mechanism and processing methods, it can be divided into single-component room temperature vulcanized silicone rubber and two-component room temperature vulcanized silicone rubber. The vulcanization mechanism shown in Pic 2, Pic 3.
Pic 2 The vulcanization mechanism of one – component room temperature vulcanized silicone rubber
Pic 3 Vulcanization mechanism of two – component room temperature vulcanized silicone rubber
Addition vulcanized silicone rubber means that the vinyl group-containing polydimethylsiloxane with a functionality of 2 is catalyzed by a platinum compound, and polyfunctional hydrogen-containing siloxanes adduction reaction, resulting in chain growth and chain crosslinking of a silicone rubber (shown in Pic 4). Due to the processing and many other reasons, the raw rubber is liquid, polymerization is more than 1000, so usually called liquid silicone rubber (LSR). Addition silicone rubber has solid type, such as the USA DowCorning company Q747 series and Germany WACKER company 400 series.
Pic 4 The Vulcanization mechanism of addition vulcanized silicone rubber
The characteristics of liquid silicone rubber
Liquid silicone rubber injection molding technology research began in the 1970s, the initial purpose is to shape some of the complex shape of the product, such as aerospace seal accessories. To the 1980’s, liquid silicone rubber is paid attention by many countries, the demand is rising rapidly. Because liquid silicone rubber injection molding not only has the advantages of low cost, low energy consumption, high production efficiency, can be automated production, and the adaptability of temperature is also very good, it can maintain good flexibility in the larger temperature range .
There are several companies that produce the liquid silicone rubber , a number of manufacturers, such as: the USA Dow Corning , GE, Toshiba silicone company, Germany WACKER , Rhodia silicone company.
For GE company, its production of liquid silicone rubber with universal grade, fast curing / high tear, low compression permanent deformation, oil proof, low viscosity, high elastic life and many other types .
According to the SRI research report , the sales volume of liquid silicone rubber is only 90,000 tons, but the average annual growth rate is more than 10% to 2003.
Liquid silicone rubber without vulcanization by-products, no pollution to the environment;
Two-component mixture can be stored at room temperature for a long time;
The compound is supplied in two components and mixed with 1: 1.
Easy to mix, fast curing, easy to operate;
Easy to color, good flexibility for product ; very short and efficient production cycle;
Less flash by controlling the mold;
Good elasticity, excellent resistance to ultraviolet light and ozone;
Low moisture resistance and good solvent resistance;
Good dielectric properties in large temperature range ;
Can not contact with N, P, S and other elements of organic matter;
Can not contact ionic compounds containing heavy metals such as Sn, Pb, Hg, Bi, As and the like;
Can not contact with alkynyl and other unsaturated bonds of organic compounds.
The basic composition of liquid silicone rubber
There are two components A and B of liquid silicone rubber , as shown in Table 1.
Table 1 Composition of liquid silicone rubber
The comparison of liquid silicone rubber with heat vulcanized silicone rubber
The comparison of performance.
The composition, performance, processing technology and other aspects of comparison of liquid silicone rubber (LSR) and heat vulcanized silicone rubber (HTV) shown in Table 2.
As shown in Table 3, the performance of liquid silicone rubber and heat vulcanized silicone rubber at room temperature is not very different, the oil resistance of liquid silicone rubber is also quite good (for the same heat vulcanized silicone rubber). Due to the advantages of liquid silicone rubber, it is determined that liquid silicone rubber will have a tendency to gradually replace heat vulcanized silicone rubber.
Table 2 The comparison of liquid silicone rubber with heat vulcanized silicone rubber
Table 3 Physical properties of liquid silicone rubber
Comparison of process performance
Liquid silicone rubber, A, B two components after mixing injection molding by the feed pump, this type of processing reduces the production cycle from a few minutes to a few seconds, which can achieve lower production costs and simplify the purpose of the operation process. And the product’s surface smooth, there is no need for trimming this follow-up process.
The vulcanized silicone rubber is pre-machined and is manually molded by the operator. The automatic production of this process is relatively low, the production cycle is long and the mold releaseability is relatively poor. The molded product needs to be repaired.after molding process. Of course, hot vulcanized silicone rubber can also be processed by rubber injection molding vulcanizing machine, but this method requires a higher production equipment, the need for a dedicated injection system.
Therefore, for the molding process of liquid silicone rubber, there are many advantages of the molding process, following operation, application.
Liquid silicone rubber is now widely used in injection mold, fabric coating, immersion or extrusion coating, food and other rubber products. Because viscosity of liquid silicone rubber is relatively low, So it has the characteristics of fast dispersion and non-stickiness to glass and other substrates. Although the liquid silicone rubber raw materials prices is slightly higher than ordinary silicone rubber, but the cost is lower than ordinary silicone rubber, for small product, the superiority is more obvious.
Liquid silicone rubber products of high purity, good transparency, volatile matter content is low, oil proof, chemical resistance, the choice of a wide range of colors, anti-aging, light resistance, insulation, insulation is particularly prominent. Due to the advantages of the liquid silicone rubber, it is widely used in automotive, construction, electronic power, health care, mechanical engineering and other fields.
Liquid silicone rubber is widely used in powder field. Such as insulators, cable joints, arrester and so on. The following application is introducing the application of insulators, cable joints.
Post composite insulator
Post composite insulator are suitable for power station facilities. It has good hydrophobic, anti-aging, anti-trapping and erosion resistance, with high tensile strength, bending strength, and high mechanical strength, impact resistance, shock and anti-fracture performance, Light weight, easy installation.( Pic 5).
(Pic 5 Post composite insulator)
The product has been previously made of ceramic, hot vulcanized silicone rubber. However, the ceramic products fragile, heavy weight, the installation of inconvenience are fund during using, etc. Heat vulcanized silicone rubber products, with the higher cost, longer production cycle, the molding process is relatively complex and so on. The liquid silicone rubber products, with short production cycle, the molding process is relatively simple, low production costs, without post-processing, so liquid silicone rubber products will replace ceramic products and heat vulcanized silicone rubber products.
This product is a new type of wall bushing, using the new insulation materials for internal insulation, using high-performance high-temperature vulcanized silicone rubber for external insulation, with excellent resistance to pollution and explosion-proof performance, that conform to the power sector without oil and development trend, it is new generation of high-pressure products ( Pic 6).
Composite suspension insulator
This product is particularly suitable for polluting areas, high mechanical tensile load, large span and compact line. With light weight, small size, not easily broken, bending, high torsional strength, strong explosion-proof. The product diagram shown in Pic 7.
Pic 7 Composite suspension insulator
As well known, heat vulcanized silicone rubber is a linear rise in the application in the automotive industry is a linear rise in the application, heat vulcanized silicone rubber is made of various types of seals due to good heat resistance and ease of handling. However, due to the high cost, low production efficiency, vulcanization byproducts and other shortcomings of these products, it is being gradually replaced by liquid silicone rubber products At the same time, in the mechanical engineering, health care and other industries, the traditional hot vulcanized silicone rubber products are also replaced by liquid silicone rubber.
Liquid silicone rubber has good market, there is a gradual replacement trends of heat vulcanized silicone rubber, owing to the advantages of liquid silicone rubber, so that the prospects for liquid silicone rubber will be very good.
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