PVC is the most common plastic used in book covering materials, and has been used for many years. Polypropylene has become more common recently. Both of these plastics have different handling characteristics. PVC is generally considered to be easier to fold and to conform to the shape of a book more readily, while polypropylene is usually a little more rigid. Sometimes PVC will stick to itself, particularly in warmer weather. Polypropylene is well known for it's non stick characteristics, in fact it is difficult to glue and even permanent marker can be removed from it, giving it superior life and resistance to damage and marking. Polypropylene is also acid free, and is considered to be an archival plastic. PVC is known to have a low level of acidity. PVC is cheaper than PP.