PEN stands for polyethylene naphthalate, a form of polyester that substitutes the terephalate group for a naphthalate group. The result is a polyester that has a higher tensile strength than PET (polyethylene terephthalate) and a higher glass transition temperature (Tg) — 121°C for PEN, and 78°C for PET. The higher Tg enables PEN to offer higher dimensional stability at elevated temperatures.