Mylar is an extraordinarily strong polyester film that was developed in the early 1950s. During the 1960s cellophane gave way steadily to Mylar with its superior strength, heat resistance, and excellent insulating properties. The unique qualities of Mylar made new consumer markets in magnetic audio and video tape, capacitor dielectrics, packaging and batteries possible. By the 1970s, Mylar had become DuPont's best-selling film. Mylar is now a product of a joint venture, DuPont Teijin Films.