Mylar® RL42T is a biaxially oriented polyester (OPET) film with an ethylene vinyl acetate (EVA) heat seal layer. It is used as a heat sealable lidding film in packaging frozen and refrigerated foods. Mylar® RL42T is designed to produce strong seals to polypropylene (PP). Although designed especially to seal to polypropylene, Mylar® RL42T seals to a broad range of container substrates including amorphous polyester (APET, also PETG), semicrystalline polyester (CPET), polyester coated paperboard, polyvinylchloride (PVC), polyethylene (HDPE), and polystyrene (HIPS). Mylar® RL42T has the same heat seal layer thickness as Mylar® RL32, but produces a stronger seal to polypropylene. Mylar® RL42T can produce tearing seals to polypropylene and other substrates under chilled conditions. Like the other "RL" types with EVA heat seal layer, Mylar® RL42T has a lower seal initiation temperature than lidding films with an amorphous polyester heat seal layer (e.g., Mylar® OL, OL2). This allows good seals to be made at higher line speeds (or using lower sealing temperatures).

Mylar® RL42T has corona treatment (on the opposite side of film from the heat seal layer) to enhance printing and laminating. The film is treated to an initial dyne level of 54. The dyne level of treated lidding films may decline with storage, and in-line corona treatment may be required during subsequent printing or laminating to increase the dyne level to a value adequate to get desired ink or laminate adhesion.