While Mylar® is a legal trademark of DuPont Teijin films. Their main archival grade polyester films are Mylar® type D and Melinex® 516. There are other companies that make polyester film also known as BoPET (biaxially-oriented polyethylene terephthalate) is a such as the Imperial Chemical Industries in England.
Uncoated archival quality polyester film encapsulation is commonly known as Mylar Sleeves, for most comic collectors. These sleeves are at present the most logical form of long term storage, that is in direct contact with your comic books. These Polyester sheet usually come in the form of sleeves that are made in 1 to 4 mils in thickness. There is another product called Melinex which used for encapsulation, but not usually seen in the comic world. It should be note the Type D is considered best.
This is another archival polyester product. This product is useful but no as easy to find as Mylar in terms of the various book book sizes. This vendor has good break down of various Melinex products. Click Here
Note: For really good overview of these two products as related to comics see: comic-book-collection-made-easy.com page on:
NOTE: Mylar can crack and tear.
- The thicker Mylar usually is the one that may crack if its too dry and hot.
- Thinner mylar may tear, if not handled with care.
Another source for a break down of plastic types used in storage: Guide to storage products