The liners are made of a good chemical-resistant plastic (PET) that is already completely recycled. Even common 2 component paints and lacquers you should be able to just work in them. If you work with very special coatings and still have doubts, try it with a small amount first.
You can reuse a liner at least 2 to 4 times, even with a different color of paint, provided it dries properly. Some painters use a liner as many as 10 times, after emptying and drying it thoroughly. Replace your liner in any case when paint particles start coming loose, or preferably before.