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The most commonly used material for punched pockets is polypropylene. However, there are some punched pockets made of polyethylene, cellophane, or other plastics. These may include recycled and biodegradable plastics (as defined by ASTM D5511) as well as plastics with antimicrobial additives. The clarity of the plastic will vary with the thickness of the pocket due to the semicrystalline nature of the polymers and Light scattering by particles. Typically, biaxially-oriented polypropylene (BOPP) is used to improve clarity of punched pockets. Using the customary unit of plastic thickness, the mil, punched pockets are produced in various thicknesses for different uses, and may or may not cover both sides of the document completely. The thinnest pockets may be less than 2 mil, while the thickest are over 4 mil.