Development of Foam Silicone Sealant

Introduction Silicone rubber is composed of silicone with a silicon-oxygen bond as the backbone, and has good weatherability, durability, heat resistance, cold resistance and electrical properties. Silicone sealant is one of the main types of sealants and is widely used in construction, electronics, electrical appliances, automobiles, aerospace and other industrial sectors. In addition to the high and low temperature resistance of silicone sealants in general, foam silicone sealants also have low density, good thermal insulation, insulation, moisture resistance, and shock resistance (especially good vibration resistance at high frequencies). It is an ideal lightweight packaging material. Fosil silicone sealants from Nusil, 3M, and Dow Corni- ng, USA are usually platinum-catalyzed or tin-catalyzed, and vulcanization is completed within a few minutes at room temperature. The density of the sealant is 0.16-o. 45 g·m-3, mainly used for fireproof sealing of wire and cable passing locations (such as holes in roofs, walls, buildings, etc.), and has a service life of up to 50 years. It has been widely used in nuclear power stations, computer centers, and offshore oil production. Equipment and other harsh environments or places where fire protection requirements are particularly high. DC3-65482 components were mixed at equal mass ratio, rapidly expanded at room temperature for 1-5 min and vulcanized into foamed elastomer. Room temperature vulcanized foam silicone sealant has become a leader in new type of silicone sealant because of its advantages of convenient use, easy repair, and excellent overall performance. Generally, according to the foaming and vulcanization mechanism of the sealant, it can be divided into two types: one is the addition of foaming agent when the foam material is formed, and the foaming agent is not involved in the vulcanization; the second is that there is no external foaming agent, and the vulcanizing agent or catalyst is also developed. Foam, foaming and vulcanization are carried out simultaneously. We developed a new type of foamed silicone sealant using hydrogenated silicone oil, a self-developed elemental organic compound as a catalyst, capable of generating small molecules of gas during the vulcanization process. The sealant has a wide range of active periods. Can meet the long-term use in -76 °C and 300 °C high and low temperature environment, the material has been widely used in aviation, aerospace, electronics and electrical appliances and other high-tech fields.
1 Experimental section
1.1 Raw material liquid hydroxyl-terminated polydimethylsiloxane, Beijing Chemical Industry Co., Ltd.; liquid hydroxyl-terminated polydimethylmethyl phenylsiloxane, imported; liquid hydroxyl-terminated polydimethyl diphenyl silicon Oxane, Shanghai Resin Factory; vulcanizing agent, methyl hydrogen-containing silicone oil, Beijing Chemical Co., Ltd.; catalyst, homemade.
1 Experimental method (1) Mix a certain proportion of liquid hydroxyl-terminated siloxane with silicon dioxide or metal oxide, grind it on a three-roll mill and mix evenly into a base paste.
(2) Under certain conditions of temperature and humidity, a certain proportion of base paste, vulcanizing agent and catalyst are mixed uniformly, poured into a certain mold, and made into a sample.
1.3 Performance Tests (1) Apparent Density The three components were uniformly mixed and poured into a mould made of polyethylene plastic with a diameter of approximately 22 mm and a height of approximately 50 mm. The added sealant dose did not exceed 35% of the height of the container. , Curing for 72 h or more under standard conditions of temperature and humidity, and removing the surface until the viscosity disappears. The surface of the sample should be flat and free of defects such as porous structures. The apparent density of the foam sealant is calculated using the following formula:

(2) Absorption of water The mass m1 of the sample is first measured in air and is accurate to 0.01 g (the same below). Then it is immersed in water for l0 h and then removed. The water droplets on the surface are immediately removed with a filter paper, and the mass is then weighed. M2. The water absorption was calculated according to the following formula, and the average of at least 3 test results was taken.

2 Results and Discussion
2.1 Vulcanization and Foaming Mechanism Foam silicone rubber is based on condensation type hydroxyl-terminated silicone raw rubber, hydrogen-containing silicone oil is a foaming agent, and self-made elemental organic compounds are used as catalysts, which are foamed and vulcanized at room temperature. A sponge-like elastomer with micropores. The reaction formula is as follows:

Under the action of complex organic compounds, the silicon-hydrogen bond in the vulcanization foaming agent molecule undergoes a condensation dehydrogenation reaction with the hydroxyl end group of the silicone rubber, and the generated hydrogen gas accumulates in the sealant, and the catalyst seals the same. The component of the raw rubber molecule extending chain forms the skeleton of the supporting foam material quickly in the body, so that the gas generated by the vulcanization is not broken and collapsed, and a foam material with satisfactory apparent density and comprehensive performance is obtained.
The amount of blowing agent hydrogenated silicone oil affects the reaction rate and structure of the foam. Too much amount will cause more open pores on the surface of the foam, which will accelerate the vulcanization rate. When the amount is small, the volumetric expansion will be small and the apparent density of the product will increase.
2.2 The effect of the type of silicone rubber on the performance of the sealant. Hydroxy-terminated dimethyl silicone rubber (107 dimethyl silicone rubber), hydroxyl-terminated dimethyl diphenyl silicone rubber (108 bis phenyl silicone rubber), The hydroxydimethyl methyl phenyl silicone rubber (monophenyl silicone rubber) was subjected to a comparative test. The test results are shown in Table 1. The three kinds of raw rubber can all be foamed at room temperature and vulcanized into a sponge-like elastomer with the same vulcanization. In the catalytic system, the resulting silicone foam sealant has a relatively high foaming rate, the methyl phenyl silicone rubber has a higher density of the raw rubber sealant, the 107 rubber has the lowest density and thermal conductivity, and the raw rubber price is the lowest. Foam silicone sealants made from phenyl silicone rubber have the best resistance to high and low temperatures.

2.3 Influence of Environmental Conditions on the Foaming and Curing Speed ​​of Sealants The matching of the foaming and vulcanization speeds is the key to achieving a predetermined target for the room-temperature vulcanized foam sealant. The encapsulant catalyst system must function in the presence of trace amounts of water to match the vulcanization and foaming reactions. In the low temperature and low humidity conditions in winter, the vulcanization and foaming reaction of the sealant become slower, and the two speeds do not match, the vulcanization speed is obviously slowed down, the pot life is prolonged, the viscosity of the rubber material increases slowly, and early generation of hydrogen and ammonia The gas accumulates and escapes, so that the bottom of the foamed elastomer becomes a thick hard glue layer without cells, and the foaming coefficient is low and the density is increased. However, under high-humidity environment, the catalyst is easily hydrolyzed, and even the catalyst is hydrolyzed during storage, and the catalytic effect is lost, so that the sealant is not vulcanized completely. Under high-temperature and high-humidity conditions, it is beneficial to increase the collision probability of -SiH and raw rubber-oH, accelerate the formation of transition state of catalyst and active groups, accelerate the foaming and sulfuration reaction, shorten the pot life, and increase the foaming coefficient. . The influence of different ambient temperature and humidity on the vulcanization foaming reaction is shown in Table 2.

The experimental results in Table 2 indicate that the physical properties and process performance of the sealant are closely related to the construction environment. When the temperature and humidity are low, the apparent density is large and the pot life is increased; the temperature and humidity are increased, the apparent density and the pot life are reduced. small. At 20 to 30°C and RH 42% to 68% under environmental conditions, the foam sealant can generally match the foaming and vulcanization rates. Therefore, the environmental temperature and humidity of the vulcanized foam must be controlled.
2.4 Sealant Adhesion Properties _
The destruction test was performed on the sealant and aluminum alloy, stainless steel, silver, titanium alloy, copper, polyvinyl chloride, glass, and ceramic, and both were 100% cohesive failure. The results show that the sealant has good adhesion to metal and non-metal materials.
3 Conclusions
Room-temperature vulcanized foam silicone sealants made from hydroxyl-terminated polydimethylphenylsiloxane, hydrogen-containing silicone oil, and self-prepared organic composite compound compounds, in a range of ambient temperature and humidity, foaming and vulcanization of sealants By matching the speed, a new type of foam functional sealant with excellent resistance to high and low temperatures, thermal insulation properties and adhesive properties can be obtained.