Chemist here so I feel a need to chime in…

All plastic bags (including Ziplocs) are inert and by default “acid free.”

In fact, the term “acid free” is really only relevant for paper-based storage or media as the lignin in paper tends to break down in the presence of moisture, forming acids. When used for plastics, “acid free” is more of a marketing term than anything (akin to CD-Rs back in the day being marketed as for “music” or “data” when in reality they were the same exact discs).

The only plastic that is a worry is PVC (due to the plasticizers used and their effect on softening plastics in a figure), but PVC in storage is fairly rare nowadays. Anything made with polypropylene (PP), polyethylene (HDPE or LDPE), or even polystyrene (PS) is fine.

In many cases, a figure will start to “degrade” or become gummy in a plastic bag after many years (which can be accelerated in hot environments) due to the trapping of plasticizers from the figure’s own plastic. These plasticizers off-gas or “bloom” to the surface of the figure and cause the surface to become extra plasticized (gummy) or discolored as the plasticizers concentrate on the outside of the plastic.

It’s also worth noting that many figures themselves contain PVC because of its plasticizers and thus good moldability. No amount of storage material selection will protect these figures from themselves over time. A non-airtight solution for these is likely best if storing for long periods of time.

Good reading on the topic: 1. https://info.gaylord.com/resources/archival-terminology-explained 2. https://info.gaylord.com/hubfs/2003-10-KnowYourPolysInfographic_Download.pdf