It is unfortunate that objects may have had adverse experiences that are not visible at the time they enter your care which may impact on their future. Good care will, however, slow down most forms of degradation.
Plastics differ from each other in their care needs. The exact recipe of each plastic, including its range of additives, influences how it will age. Even the pigment used to colour an otherwise identical object can cause objects to age differently. That said, most plastics are relatively stable if looked after appropriately but degradation, when it does occur, is irreversible making good care in the first place vital.
There are four plastics that are especially problematic. These are cellulose acetate, cellulose nitrate, polyvinyl chloride and polyurethane. Objects made of these materials should be identified and managed separately, according to their special needs. For more information please go to Problem plastics.