First, what is low molecular polysiloxane
The main raw material of organosilicone polymer is organosiloxane, which can be prepared by ring-opening polymerization under the action of alkali or acid catalyst. The commonly used cyclic siloxanes mainly include dimethylcyclosiloxanes, methylvinylcyclosiloxanes, methylphenylsiloxanes, diphenylcyclosiloxanes, methyltrifluoropropyl cyclosiloxanes and methylhydrocyclosiloxanes. Various polyorganosiloxanes can be synthesized by combining these cyclosiloxanes with different types of end-group functional groups.
The polymerization of polysiloxane is a ring-opening reaction under the action of base catalyst, which belongs to the equilibrium reversible reaction. For polysiloxanes, when the polymerization reaction reaches equilibrium, there is still unfinished cyclosiloxanes in the equilibrium reaction system, and at the same time, the catalyst will also break the main chain of silicone rubber to form a small molecular weight polysiloxanes (linear bodies) at a higher temperature. Therefore, low molecular polysiloxane refers to low molecular cyclic siloxane (D3~D20) and linear bodies. In order to facilitate expression, it is generally synthesized for low molecules, and in the polysiloxane raw material industry, volatile substances (VOC) are commonly used to represent, and REACH in 2018 restricted D4, D5, D6 (suspected carcinogens), and now also increased the test of cyclosiloxane (generally required to provide D3~D10 data).

Second, the harm of low molecular polysiloxane
Usually after the polymerization is completed, most of the low molecular polysiloxane in the system can be removed by vacuum deoxidation treatment, but the amount of polysiloxane elimination is limited, and the content of low molecular polysiloxane in the market is usually about 1%. The residual low molecular polysiloxane will cause some adverse effects on the application of silicone rubber products, such as the failure of micro-motor operation and the fuzzy fogging of optical devices, the reduction of electrical insulation performance and the bad phenomenon of the coating process.

On the one hand, the pollution of low molecular polysiloxane on the electrical contact is directly due to its low surface tension and the electrical contact surface creep, on the other hand, the indirect pollution comes from the volatilization of polysiloxane itself and the deposition on the contact surface. In addition, the low molecular weight polysiloxanes remaining in polymers such as silicone rubber will gradually evaporate. Due to the above reasons, these low molecular ring siloxanes are filled with electrical contact space due to volatilization, and electrical contact often produces arc and spark, low molecular polysiloxanes will react to produce Si02 and SiC and other substances under the action of arc. In long-term use, these si02 and SiC will be deposited in the conductive contact site to form an insulating layer. Resulting in electrical contact failure.

For the pouring and sealing of electrical, electronic, communication equipment, instruments and optical components used in aviation and aerospace as packaging materials for insulation, shockproof, moisture-proof and conductive connections, the "three defense" coating adhesive on the PCB board must strictly control the content of low molecular polysiloxane in silicone rubber. In order to avoid the use of open electrical contacts or closed, semi-closed electrical equipment, microelectronics systems, optical systems, due to the volatilization of the low molecular polysiloxane contained in the contact failure, optical device pollution and PCB coating defects.