Mylar® RL4T 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® RL4T is designed to seal to a broad range of container substrates including amorphous polyester (APET, also PETG), semicrystalline polyester (CPET), polyester coated paperboard, polyvinylchloride (PVC), polyethylene (HDPE), polypropylene (PP), and polystyrene (HIPS).

The thickness of the heat seal layer and the heat seal strength of Mylar® RL4T is intermediate between Mylar® RL31 and Mylar® RL33. Mylar® RL4T and Mylar® RL32 produce similar heat seal strengths to most substrates, but Mylar® RL32 has a greater tendency to produce tearing seals while Mylar® RL4T tends to produce more peelable seals. Mylar® RL4T 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® RL4T 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.