Plasticizers do not melt plastic. They are added to polymer materials to achieve certain flexibility and texture. Melting a polymer material with a substance generally means a reaction has occurred. When plasticizers are leached from a plastic, it will become brittle and crack. It does not melt the plastic, it is added when the plastic is made and it simply inserts itself between polymer chains.
This observation of plastic melting is very concerning. A substance that can melt a plastic is probably very bad to inhale. I am leaning toward the culprit being citrus oil, which is commonly limonene. This will melt polystyrenes. Other oils, such as clove oil (eugenol) will etch certain polycarbonates. Try grinding al bunch of cloves in a coffee grinder with a shiny plastic lid...it won't be shiny after that. Citrus and orange oils are used to cut grease, as in orange cleaners. These are powerful surfactants, and will be rather disruptive to the delicate balance of surfaces in the lungs.
Not saying it is citrus oil, or that the caps are polystyrene, but clearly the cap is a canary in the coal mine, so to speak. Whatever is doing this, do not vape it!
I am a long term fan of TPA blueberry, but I've not noticed this before...then again, I have not been using caps on cartos in a very long time. The soft caps are most likely silicone, so not an issue. Not sure what the hard caps are, but they do not seem to be particularly resilient plastic, and I guess with most juices they don't need to be.
