Silicone thermally conductive encapsulants, thermally conductive adhesives and thermally conductive silicone greases are all composites formed by dispersing thermally conductive fillers in organopolysiloxanes. In order to improve the overall thermal conductivity of thermally conductive products, it is usually necessary to fill a large amount of thermally conductive fillers, such as zinc oxide, aluminum oxide, magnesium oxide, aluminum nitride, boron nitride, silicon carbide and so on. The density of these filler particles is relatively large, and they will be affected by gravity during storage, resulting in different degrees of sedimentation and stratification. At the same time, the van der Waals force between the filler particles will also promote the filler particles to combine with each other and gradually agglomerate into larger particles, making the Settlement stratification intensified.

Sedimentation and stratification will affect the usability of the product. For example, after the silicone thermal conductive potting compound is stored, the filler will sink to the bottom. It needs to be properly stirred before use to ensure the subsequent mixing effect. After the thermal conductive adhesive is layered, the silicone oil and additives will accumulate at the mouth of the plastic bottle. During construction, a small amount of product at the mouth of the plastic bottle needs to be extruded before normal use, which affects the continuity of the process. Therefore, how to suppress or slow down the phenomenon of settlement and stratification is a problem that needs to be seriously considered in the process of product design and development.

According to Stokes' law, the sedimentation velocity of the filler is inversely proportional to the grinding fineness, proportional to the density of the filler, and decreases with the increase of the viscosity of the system. However, the actual factors affecting the sedimentation are more complex, which can be determined from the filler, the auxiliary agent and the production process. Start with these aspects.

1. Select the appropriate filler and surface treatment of the filler
Using thermal conductive fillers with smaller particle size and increasing the viscosity of the system can delay the sedimentation of thermal conductive fillers to a certain extent, but for most applications, increasing the viscosity of the system will inevitably affect the performance. For thermally conductive encapsulant products, it is usually necessary to use fillers of different particle sizes to balance the viscosity and anti-settling properties.

At the same time, in order to further control the sedimentation of the filler, surface treatment of the filler is required to improve the stability of the system. There are many hydroxyl groups on the surface of the thermally conductive filler, which is difficult to infiltrate and disperse by the organopolysiloxane. When a large amount of filler is filled, it is easy to cohere due to the force between the particles. By surface treatment of the filler, for example, the use of silane coupling agent can form organic organic polysiloxane on the surface of the filler. Silicone coating improves the affinity of fillers and organopolysiloxanes, increases the repulsion between filler particles, and prevents filler particles from agglomerating. However, the process of surface treatment of fillers is generally complicated, which will lead to an increase in production costs.

2. Processing aids
To improve the thixotropy of the system, adding a settling inhibitor, such as fumed silica, can also get a better anti-settling effect. These sedimentation inhibitors form a reversible three-dimensional network structure in the system through physical or chemical bonding when the material is standing still, so that the thermally conductive filler is wrapped in the network and delays sedimentation. When the material is subjected to external force, such as stirring or extrusion, this network structure can be easily destroyed, meeting the requirements of the construction of the material. The addition amount of the settling inhibitor needs to be controlled more precisely. If the addition amount is too small, the anti-settling effect will not be good. If the addition amount is too large, the viscosity of the system will increase greatly, which is not conducive to the use of the product.

3. Production process
The addition amount of thermally conductive fillers in thermally conductive products is basically more than 50%, and some high thermal conductivity products are more than 90%. In actual production, it is necessary to solve the problem that a large number of thermally conductive fillers are uniformly dispersed in organopolysiloxane. By adding a small amount of time and many times, it can help the infiltration of the organopolysiloxane to the thermally conductive filler, and at the same time, it can avoid the viscosity peak caused by the excessive addition of one time, and ensure the smooth operation of the equipment. In the process of adding thermally conductive fillers, the combination between the fillers and the organopolysiloxane can also be promoted by heating up, which also has a positive effect on improving the anti-settling properties of the fillers, but heating will also lead to premature interaction of additives , resulting in a decrease in the storage period of the system, and the mixing process needs to be adjusted accordingly.