Silica leads to structuring of silicone rubber Structural and Thixotropic Effects of Silica on Silicone Rubber

Silica leads to the structuring of silicone rubber, which is mainly based on the huge amount of hydrogen bonding between silica and high molecular weight polysiloxane. This conclusion has long been drawn. The thixotropic contribution of silica to silicone rubber (including coating systems in coatings) is mainly based on the interaction between silica and silica.

So, the problem is simplified:

structure: the interaction between silica and raw silica gel (massive hydrogen bonds), the entanglement of polysiloxane polymer chains, the combined result of these two effects;
Thixotropy: Silica ------ interaction between silica, hydrogen bonds, and van der Waals forces are combined.

1. Structural breakdown

There are three silanol groups on the surface of silica, which should be the isolated and highly reactive silyl group that leads to structuring (hydrogen bonding). The huge amount of hydrogen bonds here is caused by nano-scale powder. If it is micro-scale silicon powder, the number is too small and there is almost no structuring effect. When the silanol groups of silica are blocked or isolated in some way, the structuring will be reduced. If the blocking or isolation is sufficient, the structuring will not occur. This process is usually the so-called silica treatment, and the product is called "hydrophobic silica". Although the term hydrophobic silica is also used commercially, this title is not necessarily correct, at least unscientific. The reason is that the hydrophobic silica (which does have external hydrophobicity) uses the same hydrophilic silica as the raw material, but with different treatments, although the degree of hydrophobicity can be exactly the same, when it is added to the silicone rubber, it does not necessarily have the same hydrophobicity. Anti-structuring effect. That is to say, it does not mean that as long as hydrophobic silica is used, anti-structuring products can be obtained.

Structure can be controlled by blocking hydroxyl groups, isolating or adding suitable additives. Completely blocking the hydroxyl group with a chemical reaction can indeed eliminate the structuring phenomenon. Isolation, such as adding lots of white oil, can also remove structuring. Additives, such as low-molecular-weight hydroxysilicone oils, can delay structuring (adding in sufficient amounts can also be unstructured for a long time). For these measures, choose one or a combination of them according to product requirements.

In industry, adding silazane during rubber mixing is an effective measure for sealing silanol groups. To achieve complete sealing, a large amount of addition is required, the cost is high, the post-processing is troublesome, and the mechanical properties of the rubber may be affected. have negative effects. Adds a lot of white oil, only suitable for certain products. At present, it is more economical to add low molecular weight hydroxy silicone oil (why is it short chain? Usually it is 5 chain units, this question is very interesting, you can try it yourself), or the hydrolyzate of alkoxysilane, diphenyldihydroxysilane Wait. Even, with the addition of water, alcohol has some anti-structuring effect.

There is a saying that structuring may contain chemical bonds, which is based on the phenomenon that although the structured glue can continue to be used after retraining, its performance has a certain gap compared with that before structuring. After structuring, due to the huge amount of hydrogen bonds between silica and polysiloxane and the entanglement of polymers between polysiloxanes, retraining requires great shear force and a certain temperature, and local huge friction It is not surprising that the force (local high temperature) can cause the polysiloxane chain to break, degrade, and change in properties. Although structuring is inevitable during rubber mixing, the structuring that we usually focus on refers to the phenomenon that occurs within a few months of storage under normal temperature, normal pressure and very mild conditions. From the structure of raw rubber, which group has the possibility of chemical reaction with silica under normal temperature, normal pressure and mild conditions? You can say that silica contains chlorine and is acidic, so as long as a very small amount of amine is added, structuring should not occur, but so far, no one has obtained anti-structuring by adding amines or adjusting silica to neutrality. chemical products, so the structuring can only be the effect of massive hydrogen bonds.

Low-molecular polysiloxane (here does not refer to the hydroxyl or alkoxy-terminated kind, but only methyl or vinyl that is inert to silica), no structuring phenomenon can be observed, or the degree is very light, very It is obviously related to the entanglement of polymer chains.

Therefore, the structuring is firstly the effect of silica and raw rubber, and secondly, the macromolecule entanglement of raw rubber strengthens this effect.