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Properties of the Constituent Components of Polysilicone Sealant

Introduction of polysiloxane sealant Polysiloxane sealant, which is suitable for curing at room temperature, is one of the important varieties in sealant. It has excellent sealing, weather resistance, temperature resistance and handling properties. It has good adhesion and sealing properties to vulcanized rubber, metal, various plastics, glass, fabrics, lacquers, etc. It is widely used in aerospace, airfield, automotive, construction, electronics, electrical, chemical, machinery manufacturing and other industrial fields.
Composition of polysiloxane sealant Polysiloxane sealant is mainly composed of silicone rubber, cross-linking agent, filler, structure control agent and tackifier.

Silicone rubber is a linear polymer elastomer obtained by polymerizing dimethylsiloxane with other silicone monomers, referred to as SIR. The macromolecular backbone is dominated by Si-O units, with monovalent organic groups as the side groups. According to the chemical structure, it can be divided into the following. Dimethyl silicone rubber, referred to as methyl silicone rubber (MQ). Methyl vinyl silicone rubber, referred to as vinyl silicone rubber (VMQ). Methylphenyl vinyl silicone rubber, referred to as phenyl silicone rubber (PVMO). Trifluoropropylmethylvinyl silicone rubber, referred to as fluorosilicone rubber (FVMQ). β-nitrile ethyl methyl vinyl silicone rubber, referred to as nitrile silicone rubber. Dimethyldiethyl silicone rubber, referred to as ethyl silicone rubber. Polydimethylsiloxane-phenylene ether-based copolymer rubber, referred to as phenylene and phenylene ether-based silicone rubber.

According to the vulcanization method, silicone rubber is divided into high temperature type hot vulcanization (HTV) and room temperature vulcanization (RTV). The room temperature vulcanized silicone rubber has a reactive active group at the molecular end, and these active end groups can react at room temperature under the action of a catalyst to crosslink into an elastomeric polymer. The molecular end of the heat-cured silicone rubber is inert. The vulcanization cross-linking must use an organic peroxide, and the silicone rubber with unsaturated side groups or reactive side groups can also be crosslinked under heating using a corresponding vulcanizing agent. The polysiloxane sealant is usually made of a room temperature vulcanized silicone rubber.

The cross-linking agent used in the one-component sealant is of the formula RSiX3. X is an easily hydrolyzable group. The cross-linking agent of the one-component sealant is divided into acetic acid type, ketone oxime type, alcohol type, amide type and amine type, and the first three applications are more. Acetic acid type sealant has a wide range of consistency, good vulcanization performance, stable storage, and good adhesion to a variety of materials. The ketone oxime sealant has good storage stability and fast surface cure, but it takes a long time to fully cure and has poor adhesion. Alcohol-type sealant cross-linking curing speed is slower than the former two. When curing under high humidity and high ambient temperature, voids may form in the sealant layer. The storage environment is stricter, and it is often required to dry and refrigerate. The use of a mixed crosslinking agent, such as methyltriacetoxysilane and di-tert-butoxydiacetoxysilane, is advantageous for improving the bonding strength of the sealant.

Commonly used crosslinking agents are methyltrimethoxysilane, methyltriacetoxysilane, methyltri(cyclohexylamine)silane, methyltris(acetone)silane, methyltris(N-methylidene) Amide) silane and tetraethoxysilane. The crosslinking agent used in the two-component sealant is an organosiloxane having Si-OR, tri-Si-H and tri-Si-OH. The organosiloxane having an acyloxy group attached to the silicon atom is tetraethyl orthosilicate, polysiloxane, methoxysiloxane, ethoxysiloxane or the like. An alkylsilane crosslinker having a hydrogen atom on a silicon atom such as methyldiethoxysilane or trioxysilane is most effective. The Si—OH type vulcanizing agent mainly contains a hydrolysis product of trimethylsiloxane and silica. The most commonly used crosslinking agents are ethyl orthosilicate (propyl ester), methyltriethoxysilane, phenyltriethoxysilane, phenyltrimethoxysilane and their oligomers and their partial hydrolysis product.

Silicone rubber does not crystallize at room temperature, but it is also non-polar molecules, and the intermolecular force is weak. The unreinforced silicone rubber has low mechanical properties and a small practical value. Selecting the appropriate reinforcing agent can greatly improve its mechanical properties. Reinforcing agents are primarily natural or synthetic silicas including diatomaceous earth, fumed silica and precipitated carbon black as well as other inorganic fillers. The best reinforcement effect is the gas phase method from carbon black. Non-reinforcing fillers are generally used as compatibilizers to reduce costs. Commonly used are diatomaceous earth, quartz sand, metal oxides (such as zinc oxide, iron oxide, titanium dioxide), calcium carbonate, clay, barium sulfate and a small amount of carbon black. Glass fiber and asbestos can also be added to reduce the relative density of the sealant, and glass or plastic beads can also be added.

In order to prevent structuring between silica and silicone rubber, promote the dispersibility of the filler in the rubber compound, and improve the mixing process and storage stability, a structure control agent is often added. Diphenylsilyl glycol in silane diol is a preferred structural control agent in view of thermal stability. Further, a silane diol having a silicon atom number of 2 to 4 (e.g., tetramethyldisilane diol) is also effective. Carboxymethyl silicone oil can also be used as a structure control agent, but it must be considered according to the hydroxyl content of the silicone oil. Too much amount will affect the performance of the rubber. When the silicone resin with a high alkoxy content is used as a structure control agent, the sealant has a long activity period and good storage stability, and can be stored for more than two years. 
Adding a tackifier can increase the adhesion, and the amount is from 0.5% to 3%. Commonly used tackifiers include various alkoxy-containing silane coupling agents, boron compounds, and the like.
In the automotive industry, polysiloxane sealants can be used as seals for gaskets, bonnet oil pans, thermostats, rear axle covers, automatic transmissions and taillights, and cab roof gaps. For example, XJ-23 type silicone sealant developed by Xiangfan City Adhesive Technology Research Institute of Hubei Province is mainly used for sealing of automobile engines, automatic pipelines, oil pans and various flat, tapered and threaded surfaces.

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