These 4 mil Comic Book Bags are made of Archival Polyester an inert material of the highest clarity. Mylar will not discolor, damage, or adhereto items placed inside it. It is a sturdy film that beautifully displays, timelessly protects and gives extra support to fragile documents. Resistance to moisture, insect attack, fungus, mold, mildew, acid, oils, grease, and solvents. Mylar contains no dangerous plasticizers, slip additives, surface coatings, antioxidants, acid-hydrolysis. This bag is a semi-rigid bag and does not require the use of a backer board. But if you do use a backer board or extenders be sure to only use certified acid free boards.