I got the information from the plethora of battery patents, where the chemicals, seals etc. are discussed, and taking apart old alkaline batteries that never leaked after 10 years.
It's common asphalt or tar pitch and now could be nylon, styrene hot glue, epoxy etc.
I find the leaks start at the -ve end and after a while include the +ve end.
"... The closure members, since they are usually exposed to the elements contained within the cell are required to be substantially inert to such elements. As a result it has been preferred to utilize plastic e.g. nylon, polyethylene, polysulfone, and other generally inert materials for such members. In many cases, however the aforementioned materials have shortcomings such as cold flow under temperature cycling and imperfections in the surface area thereof. The metallic surface areas in contact therewith generally have imperfections as well. Thus, a cell electrolyte such as the common alkaline KOH utilized in many commercial cells (an aggressive leaking material) may leak, over a period of time, through such imperfections. Such leakage is in fact exacerbated by the hydrogen gas evolution common in alkaline cells which tends to push the electrolyte through the path of least resistance, i.e. the seal."
"... The aliphatic or fatty polyamides are coated on sealing members such as grommets by means of a relatively expensive and complicated spraying procedure."
"alkaline electrolyte battery container, cover, etc. tends to crawl the wall by capillary action to wet the sealing piece has the property that oozes to the outside from the sealing part of the battery. This behavior metal surface is connected to the negative electrode, i.e. it appeared strongly to the negative terminal side of the sealing portion, prone to leakage. This is commonly known as liquid leakage due to electrical capillary action..."
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