What is the plastic material of the CD cases?
These are called slimline jewel cases.

From the Wikipedia article on Optical Disc Packaging:


  The bulk of slimline cases are made with translucent or transparent polystyrene, and are available in multiple colors. A stronger alternative is made from semi-opaque, semi-flexible polypropylene which is strong enough to protect the disc, but flexible enough not to break easily. 


Here is an image of a polystyrene case to compare to yours: