this is the question that comes to mind recently, so I looked it up a little bit. It seems that few people have written popular science articles, but there are a lot of patents _ (: patent) ∠) _

first of all, this thing has a name, it is called "follow-up sealant", generally called "ink follower" in English.

the function of follow-up sealant is easy to understand, mainly to prevent the ink of the neutral pen from leaking from behind the refill and to prevent the ink from volatilizing. It is very sticky, but it can flow with the use of ink to avoid negative pressure on the sealed part and affect normal use. In order to achieve this goal, you need a follow-up sealant: ① is very thick; ② is stable, for example, it cannot be left dry, nor should it suddenly become smaller and leak out after a little shock, and so on; ③ can move forward smoothly with the ink.

as for the composition of follow-up sealant, there are actually more versions, but the general principle is a combination of several things:

① base oil, which is generally a liquid that has a certain viscosity, is difficult to volatilize and will not be miscible with neutral pen ink, such as silicone oil, polybutene, liquid paraffin and so on.

② thickener can thicken the base oil and form a gel-like structure through intramolecular or intermolecular interaction. For example, the thickener can be nano-sized silica.

③ surfactants and other additives to adjust the fluidity and mixing state.

an example of a follow-up sealant formula in the patent:

feels that the formula is quite fastidious, after all, there is a contradiction between high viscosity and fluidity, so it has to go through a lot of adjustments to make it easy to use.

several related patents (though probably no one wants to see them):

https://patents.google.com/patent/US6227738

https://patents.google.com/patent/CN101280173B/zh

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