There are many different types of plastic, but it sounds like your tupperware is made of the porous kind, which will hold on to food particles and explains your note.  Heat will expedite the leaching of chemicals into your food, so you should use the containers only for storage, and heat up and eat food from an actual plate or bowl.  If you have to continue using them, you could always wrap your food in paper to minimize contact.

Here's some more info:
http://articles.mercola.com/sites/articles/archive/2011/11/16/practical-options-to-store-your-food-without-contaminating-them-with-plastics.aspx