There are several reasons to use this particular type of sealant. First, this asphalt seal is sufficiently durable, besides being economical. Moreover, it is easy to apply and can prevent air infiltration. It also provides chemical insulation and electrical insulation, thus making the battery safe to use.
The seal on a dry cell battery can be made out of polypropylene or polyethylene plastic or a mixture of resin and wax or asphalt pitch. A dry cell battery is created in such a way so as to leave a gap or air space in between the cathode and the seal. This creates room for expansion. The asphalt seal prevents oxygen from infiltrating into the cell and causing zinc corrosion. It also prevents the electrolyte from evaporating. This increases the life of the dry cell battery.
Asphalt-pitch sealant is neutral and can therefore strongly withstand electrolytic solutions. So, they are a necessity for sealing packing entrances and caulked sections. But the presence of chains of low molecules in the pitch or asphalt decreases the sealing performance of the areas that are exposed to or are close to the electrolytic solution.
Since the trend is to manufacture smaller, thinner and lighter dry cell batteries that can deliver heavy-duty performance, manufacturers are being compelled to minimize the size of the sealing area. And yet they cannot discard the asphalt-pitch sealants because of their excellent sealing performance. Rather, battery manufacturers are tweaking the design of dry cell batteries to accommodate asphalt-pitch sealants. The latter does not minimize the battery capacity in any way while delivering higher sealing performance.