1、 What is low molecular weight polysiloxane
The main raw material of organosilicon polymers is organosilicon cyclosiloxane, which can be synthesized by ring opening polymerization under the action of alkali or acid catalysts. The commonly used cyclic siloxanes include dimethyl cyclosiloxane, methyl vinyl cyclosiloxane, methyl phenyl siloxane, diphenyl cyclosiloxane, methyl trifluoropropyl cyclosiloxane, and methyl hydrogen cyclosiloxane. By combining these cyclosiloxanes with different types of terminal functional groups, various polyorganosiloxanes can be synthesized.

The polymerization reaction of polyorganosiloxane is an equilibrium reversible reaction that occurs under the action of an alkali catalyst. For polyorganosiloxanes, when the polymerization reaction reaches equilibrium, there is still unfinished cyclosiloxane in the equilibrium reaction system. At higher temperatures, the catalyst also causes the main chain of the silicone rubber to break and form small molecular weight polysiloxanes (linear bodies). Therefore, low molecular weight polysiloxanes refer to low molecular weight cyclosiloxanes (D3~D20) and linear compounds. For the sake of convenience in expression, it is generally synthesized as low molecular weight, while in the polysiloxane raw material industry, volatile matter (VOC) is commonly used to characterize. In 2018, REACH restricted D4, D5, and D6 (suspected carcinogens), and now it has added testing for cyclosiloxanes (usually requiring data from D3 to D10).

2、 The Hazards of Low Molecular Weight Polysiloxane
Usually, after the polymerization is completed, most of the low molecular weight polysiloxanes in the system can be removed through vacuum reduction treatment. However, the amount of elimination for polysiloxanes is limited, and the content of low molecular weight polysiloxanes in the market is usually around 1%. The residual low molecular weight polysiloxane can have some adverse effects on the application of silicone rubber products, such as the failure of micro motor operation, blurred fogging of optical devices, reduced electrical insulation performance, and poor coating process.

The pollution of low molecular weight polysiloxane on electrical contacts is directly caused by its low surface tension, which leads to creep on the surface of electrical contacts. On the other hand, indirect pollution comes from the volatilization of polysiloxane itself and its deposition on the contact surface. In addition, low molecular weight polysiloxanes remaining in polymers such as silicone rubber will gradually evaporate. Due to the above reasons, these low molecular weight cyclosiloxanes are filled with electrical contact space due to volatilization, and electrical contact often generates arcs and sparks. Under the action of arcs, low molecular weight polysiloxanes will react to generate substances such as SiO2 and SiC. In long-term use, these SiO2 and SiC will deposit on the conductive contact area, forming an insulation layer, leading to 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 proofing" coating adhesive on PCB boards must strictly control the content of low molecular weight polysiloxane in silicone rubber to avoid use near open electrical contacts or in closed or semi closed electrical equipment, microelectronic systems, and optical systems, Due to the volatilization of low molecular weight polysiloxane contained in it, contact failures, optical device contamination, and PCB coating defects are caused.