These 2 mil Comic Book Bags are made of archival polyester which is an inert material of the highest clarity. Mylar will not discolor, damage, or adhere to items placed inside of it. It is a sturdy film that beautifully displays, timelessly protects, and gives extra support to fragile documents. Polyester is resistant to moisture, insect attack, fungus, mold, mildew, acid, oils, grease, and solvents. Mylar contains no dangerous plasticizers, slip additives, surface coatings, antioxidants, and is not subject to acid- hydrolysis. This bag does not require the use of a backing board. However, if you choose to use a backing board be sure to use only certified acid-free boards.