Overview of liquid silicone rubber

?According to its commodity form, silicone rubber is divided into two categories: mixed silicone rubber and liquid silicone rubber. Mixed silicone rubber, usually also known as hot vulcanized silicone rubber, is a linear high polymerization degree (5000-10000 siloxane structural units) polyorganosiloxane with reinforcing fillers, incremental fillers and additives to give various properties of the base material, when used, with vulcanizing agent, by forming, vulcanization into products, belongs to free radical polymerization.

Liquid silicone rubber is different from compound rubber in that it is a self-leveling or thixotropic base formulated with a linear polysilicone oxane with a medium degree of polymerization, adding crosslinking agents, reinforcing fillers and other compounding agents, and separating it into initiators or vulcanized crosslinked elastomers with water in the air. Liquid silicone rubber can be divided into condensation type and additive molding system according to its vulcanization mechanism.

1. Condensation type one-component liquid silicone rubber

Condensation type one-component liquid silicone rubber is made of terminal hydroxyl polydimethylsiloxane (viscosity 3000-100000 mPa.s) as the base polymer (commonly known as base rubber) and crosslinking agent, catalyst, filler and other characteristic additives in the moisture isolation mixer after mixing evenly, packaged in a sealed cylinder or hose, pressure extrusion when used, contact with the moisture in the air vulcanization into elastomer, in this process neither heat absorption nor heat release. The vulcanization time depends on the temperature, humidity, and thickness of the silica gel layer of the vulcanization system.

Condensation type one-component liquid silicone rubber is widely used as adhesive, sealant and coating material due to its adhesion. As a one-component liquid silicone rubber used as an adhesive, the vulcanized performance should have good adhesion to the substrate contacted, no corrosion, and at the same time have good physical and mechanical properties, electrical properties, so it is best to choose non-corrosive dealcoholized or acetone one-component silicone rubber. When selecting one-component silicone rubber sealant, the modulus size after complete vulcanization, adhesion to the contact substrate, dynamic durability, surface weather resistance, corrosiveness to the substrate, pollution to the surrounding area and other factors are mainly considered, among which, modulus is a parameter that characterizes the sealing agent's ability to move, measured by the fixed tensile stress of the sealant, and is also the grade mark of the sealant, among which the typical variety characteristics are introduced as follows:

(1) One-component high modulus type: is the most widely used variety, and its curing system includes deacetic acid type, deketoxime type, and dealcoholization type;

(2) One-component low modulus type: the adhesion is worse than the above varieties, and the surface of the contact substrate must be primed when used;

(3) Plastic sealant: dealcoholization curing system, used for transparent plastic plate instead of inorganic glass complement joint sealing;

(4) Madi lipid type: this sealant only cures on the surface, so it can be regarded as a very ultra-low modulus type, which is used for difficult to bond and brittle substrate joints such as plexiglass and plastic;

(5) Structural sealant: used for the bonding of glass curtain wall of high-rise buildings, its adhesion, durability, fatigue resistance, curing speed, etc. have extremely high requirements, so as to adapt to the wind pressure resistance, seismic resistance of the components and prevent the danger of glass damage or falling in extreme cases.

2. Condensation type two-component liquid silicone rubber

Condensation type two-component liquid silicone rubber is a base rubber, filler, crosslinking agent as one component, catalyst as another component, the two are mixed in a certain proportion, vulcanization can be carried out simultaneously inside and on the surface. The crosslinking agent is usually ethyl orthosilicate, methyltriethoxysilane, and the catalyst is often organotin compound. The vulcanization rate is related to the type of cross-linking agent, catalyst, dosage and ambient temperature, and in the case of the vulcanizing machine and its dosage, increasing the temperature and increasing the ambient humidity can accelerate the vulcanization speed and shorten the vulcanization time. According to the type of crosslinking agent, it can be divided into dealcoholization condensation, deoxime condensation, dehydration condensation, and dehydrogenation condensation.

Its applications are mainly divided into four categories:

(1). Electrical and electronic equipment pouring packaging, insulation sealing

(2). Reproduction of cultural relics and handicrafts, artificial leather surface embossing models, dental impressions

(3). Flame retardant, heat insulation, cushioning foam

(4). Caulking sealant

3. addition molding liquid silicone rubber

Under the action of platinum catalyst, the liquid silica gel is directly vulcanized into elastomers or gels of various cross-linking densities by liquid state at room temperature or heat. The vulcanization rate can be freely adjusted by the catalyst type and inhibitor. However, during the storage vulcanization process, it is not possible to contact organic matter containing N, P, S and other elements and heavy metal ionic compounds containing Sn, Ph, Hg, Bi, As and so on, which will be catalyst poisoning and hinder vulcanization.

4. the outlook

With the continuous improvement of people's requirements for silicone rubber products, liquid silicone rubber is also improving product performance and process improvement. Compared with condensed liquid silicone rubber, additive molding liquid silicone rubber (LSR) has the advantages of no by-products in the vulcanization process, minimal shrinkage, and deep vulcanization, and LSR has developed particularly rapidly and has rapidly penetrated into various industries.

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