Regardless of whether it is treated or not, silica has good thixotropy in many systems. Of course, it also needs nano-scale to have obvious effects, and micro-scale silicon powder is much worse. Even, a similar thixotropy phenomenon can be observed in the silica itself: the silica treated with silane is shaken, and the silica is as fluid as water. After standing for a while, the silica seems to form agglomerates. , and then shake, and immediately have the same fluidity as water. This process can be repeated. Here, when the silica is shaken, high-voltage static electricity is generated, so the powders have good fluidity, and the time for the agglomeration phenomenon depends on the time when the static electricity is derived. When white carbon black is added to other powders in a small amount, it can improve the fluidity, which is also related to the static electricity generated by the friction of white carbon black. Especially after treatment, the hydrophobic white carbon black has better effect.

Treated silica that does not have the ability to structure silicone rubber still has good thixotropy and is even better used in non-polar coating systems. Therefore, it can be predicted that the factor that causes the thixotropy of silica should be the van der Waals force. Of course, the hydrogen bond will not be useless, because the untreated hydrophilic silica seems to be more thixotropic when it is well dispersed. powerful. So I think it's a combined result, but with van der Waals forces dominant.

Since the hydrogen bond strength is far greater than the van der Waals force - which seems to be a contradiction, why is the thixotropy dominated by the van der Waals force? In fact, it can be understood that only a few nanometers of fumed silica will not exist in a separate way, but in the form of agglomeration. The agglomerated silica is contained, of course, only due to the limitation of solids, the agglomeration can only reach a certain degree, into the micron level. When the rubber is mixed, the agglomerated particles are broken up and a large number of silanol groups are exposed, which creates an extremely favorable condition for structuring. Glue also has an important contribution - as can be seen with non-silicon polymers.

However, even if the maximum shear force is applied and the affinity of silica gel is used, in view of the huge specific surface area of nano-silica and the strong hydrogen bonding of silanol groups, it is almost impossible to disperse into single particles. is not possible.

So far, based on the understanding of nano-powder agglomeration, and looking back at structuring, do you have a new understanding of the following phenomena?

1. The structuring progresses slowly over time. It is likely that the agglomerated powder slowly exposes more silanol groups, giving the impression that anti-structuring agents such as hydroxy silicone oil have lost their effect.

2. Although more silicon hydroxyl groups are exposed, the hydrogen bond between silica is stronger, and the raw rubber or additives are not enough to disperse silica more effectively (so it will not be more transparent after structuring, or even Transparency decreased), which can be regarded as a small "turn" on the surface particles of the agglomerated silica.

3. The next conclusion is that it is difficult to eliminate the silanols by in-situ treatment (usually adding silazane). It is also understandable that professional silica treatment may not be able to deal with it in place, purely considering treatment The bond energy of the agent is also insufficient.

Looking back at thixotropy, it can naturally be understood as an effect dominated by van der Waals force between silicas. Of course, the effect of hydrogen bonds cannot be ruled out. Therefore, as long as a small external force is required, the system can flow, almost Infinitely reversible, fast to build, and effective in non-silicon systems.