Hi guys. just thought I'd add my two cents.
Most plastics, including pvc, ps and abs are formed with large hydrocarbon chains (hydrogen and carbon atoms). Both these atoms love combining with oxygen, as in water (h2o) and carbon dioxide (co2).
Over time this will cause structural degredation to almost all plastics. Heat and UV light will speed up this process. The addition of other chemicals (flame retarding and colourings) will also speed up this process. This is the major cause of "yellowing". The process can be retarded or slowed by addition of a UV resistant, WATER based clear coating or varnish.
As for different grades of plastics contacting one another, this will, over time, definately cause degradation and in extreme cases, different plastics will "weld" themselves together. I've noticed this on my own 25 year old plastic model kits.