These 2 mil Comic Book Bags are made of Archival Polyester(Mylar)an inert material of the highest clarity.Mylarwill not discolor, damage, or adhere to 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.Mylarcontains 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 be sure to only use certified acid free boards.