Metallized PET starts as a polyester film base, which already offers good tensile strength, heat resistance, and dimensional stability, and adds a vacuum-deposited aluminum layer only microns thick. That thin metal layer dramatically improves moisture barrier, light barrier, and gas barrier performance relative to unmetallized PET, while keeping material cost and weight far below true aluminum foil laminates.
The metallized layer's barrier performance depends heavily on metal coating integrity and adhesion to the PET substrate, which is why it is typically laminated to a sealant layer like polyethylene rather than used bare, protecting the metal layer from abrasion that would otherwise create barrier-breaking microcracks. Metallized PET is a common choice when a brand wants the visual shine and light-blocking properties associated with foil, along with genuine barrier improvement, without foil's cost, weight, or microwave incompatibility.