Liquid silicone is widely used in the fields of electronics,machinery,medical treatment,automobile, etc. the research progress of addition liquid silicone rubber is introduced in this paper

Liquid silicone is widely used in the fields of electronics,machinery,medical treatment,automobile,etc. the research progress of addition liquid silicone rubber is introduced in this paper

Abstract: the advantages and applications of addition liquid silicone rubber were briefly described. By summarizing the main components of additive liquid silicone rubber, several additives used to modify the properties of additive liquid silicone rubber were introduced: adhesive additives, conductive additives, thermal conductive additives, flame retardants and special functional additives. The factors influencing the functionality of addition liquid silicone rubber were summarized. The new direction of addition curing liquid silicone rubber was introduced. Finally, the future research direction of addition liquid silicone rubber was prospected.

Key words: addition molding, liquid silicone rubber, composition, function, additive

1、 Overview of addition liquid silicone rubber

The addition liquid silicone rubber is a kind of silicone material which can be crosslinked and vulcanized at room temperature or under heating in the presence of platinum catalyst, with polyorganosiloxane containing vinyl as the basic polymer and polyorganosiloxane containing Si-H bond as the crosslinking agent. Different from condensation type liquid silicone rubber, addition type liquid silicone rubber has no by-products during vulcanization, small shrinkage, deep vulcanization and no corrosion to contact materials. It has the advantages of wide applicable temperature range, excellent chemical resistance and weather resistance, and can be easily adhered to various surfaces. Therefore, compared with condensation type liquid silicone rubber, addition type liquid silicone rubber develops rapidly. At present, it has been more and more widely used in electronic and electrical appliances, machinery, construction, medical treatment, automobile and other fields.

2、 Main components of addition liquid silicone rubber

1. Basic polymer

The basic polymers used as additive liquid silicone rubber mainly include the following two kinds of linear polysiloxane containing vinyl. Their molecular weight distribution is wide, generally from thousands to 100000 to 200000. The most commonly used basic polymer for addition liquid silicone rubber is α,ω- Divinylpolydimethylsiloxane. It is found that the molecular weight and vinyl content of the basic polymer can change the properties of liquid silicone rubber.

2. Crosslinking agent

The crosslinking agent used for addition liquid silicone rubber is organic polysiloxane with more than 3 Si-H bonds in the molecule, such as linear methyl hydrogen polysiloxane containing Si-H group, cyclic methyl hydrogen polysiloxane and MQ resin containing Si-H group. The most commonly used are linear methyl hydrogen polysiloxanes with the following structures. It is found that the mechanical properties of silicone rubber can be changed by changing the hydrogen content or structure of crosslinking agent. Lizhichao compared the effects of crosslinking agents (hydrogen containing silicone oil) with different structures and contents on the properties of addition liquid silicone rubber, and found that the hydrogen content of crosslinking agents was proportional to the tensile strength and hardness of silicone rubber. Guzhuojiang et al. Changed the synthetic process and formula to prepare hydrogen containing silicone oil with different structure, molecular weight and hydrogen content, and used it as crosslinking agent to synthesize addition liquid silicone rubber.

3. Catalyst

In order to improve the catalytic efficiency of the catalyst, researchers prepared platinum vinyl siloxane complexes, platinum alkynyl complexes and nitrogen-containing compounds modified platinum complexes. In addition to the type of catalyst, its amount will also affect the performance of liquid silicone rubber products. Yan, nianping et al. Studied the mechanism of the effect of the amount of platinum catalyst on the performance of silicone rubber, and found that increasing the concentration of platinum catalyst can promote the cross-linking reaction between methyl groups and inhibit the decomposition of the main chain.

4. Reaction inhibitor

Fang Jiafeng et al. Found that the mass ratio of alkynol inhibitor to platinum catalyst would affect the non isothermal vulcanization kinetic behavior of addition liquid silicone rubber system. The higher the mass ratio of ethynol inhibitor to platinum catalyst, the higher the vulcanization characteristic temperature of the addition liquid silicone rubber system. Liu Jin et al. Prepared alkoxysilylated alkyne hydrosilylation inhibitors, which can not only prolong the scorch time of addition liquid silicone rubber, but also make the surface of sulfide smoother. Leiweihua et al. Found that the liquid addition silicone rubber has the effects of prolonging the inhibition time, shortening the curing time, significantly increasing the torque and faster vulcanization speed by properly compounding dimethylhexynol and vinyl cyclosiloxane inhibitors.

3、 Additive liquid silicone rubber functional additive

1. Conductive additive

Liaobo et al. Used liquid silicone rubber as insulating matrix and nano conductive carbon black particles as filling materials to make composite materials with excellent conductivity. Through experimental tests, the resistivity of composite materials with different carbon black content was obtained. When the carbon black mass fraction was greater than 4%, better conductivity could be obtained. Zou Rui et al. Prepared conductive silicone rubber with vinyl liquid silicone rubber as matrix and nickel plated graphite as conductive filler. It was found that when the amount of nickel plated graphite was 280 phr, the volume resistivity of the composite could be reduced to less than 0.1 Ω· cm.

2. Thermal conductive additive

Additive liquid silicone rubber is widely used in electronic and electrical industry because of its good thermal stability and thermal conductivity. As the heat dissipation material of electronic devices, addition liquid silicone rubber can effectively transfer heat, improve heat dissipation and prolong the service life of electronic devices. However, the thermal conductivity of the addition liquid silicone rubber without thermal conductive additives is very small and cannot be produced in industry. Common thermal conductive additives include AG, Cu, CA, Al2O3, AlN, BN, etc. The type, content, particle distribution and surface modification of thermal conductive additives can affect the thermal conductivity of liquid silicone rubber.

3. Special functional additives

In addition to the above additives for addition molded liquid silicone rubber, researchers have prepared special functional additives to give special properties to addition molded liquid silicone rubber. Liu Tian et al. Added cinnamon oil, thyme oil and oregano oil into the addition liquid silicone rubber to solve the problem that algae microorganisms are easy to grow on the surface of silicone rubber, resulting in the decrease of hydrophobicity on the surface of the material. It is found that adding three kinds of plant essential oils can improve the algae resistance of the addition liquid silicone rubber. Xie, Chixin et al. Prepared fluorosilane grafted silica (f-SiO2) as an additive for addition liquid silicone rubber. It was found that f-SiO2 delayed the occurrence of strong arc discharge and weak arc discharge on the surface of addition liquid silicone rubber. The anti plasma irradiation and anti tracking ability of the addition liquid silicone rubber are obviously enhanced.

4、 New direction of addition curing of addition molded liquid silicone rubber

As mentioned above, the vulcanization mechanism of the traditional addition liquid silicone rubber is the hydrosilylation reaction between the basic polymer containing vinyl and the polymer containing silicon hydrogen bond. The traditional addition liquid silicone rubber usually needs a rigid mold to manufacture the final product, but this traditional manufacturing technology has the disadvantages of high cost, time-consuming and so on. Products are often not suitable for electronic products. The researchers found that the liquid silicone rubber added by mercaptan double bond can be used to prepare a series of silicone rubber with better properties by new curing technology. Its excellent mechanical properties, thermal stability and light transmittance can be used in more new fields. Zhao, Tingting et al. Prepared silicone elastomers with adjustable hardness and mechanical properties based on the sulfhydryl ene bond reaction between branched chain mercaptan functionalized polysiloxanes and vinyl terminated polysiloxanes with different molecular weights. Printing elastomers show high printing resolution and excellent mechanical properties. The elongation at break of silicone elastomer can reach 1400%, which is much higher than the reported UV curable elastomer, and even higher than the most stretchable heat curable silicone elastomer. Then the super stretchable silicon elastomer was applied to the hydrogel doped with carbon nanotubes to prepare stretchable electronic devices. Printable and processable silicone rubber has broad application prospects in the fields of soft robots, flexible actuators, medical implants and so on.