Aren't anime cel paints synthetic? Maybe the only thing 
cels have to worry about is sticking. From what I understand 
(hahaha) older cels (esp. Western) and cut-the-corner cheap 
quality old cels (WB, I think) may use albumen-based paints 
and those are truly fragile. Acidity is especially important 
with paper art, paintings... But, as with all fine art 
collections for long-term, better safe than sorry. Avoid 
vinyl/PVC. Avoid high acidity. Avoid lignin-containing things 
for paint-based art. Um, one might try asking a professional
animation art store/studio about these things. ^_^