Low Shrink Polyester Films and Method of Making

A method for making dimensionally stable polyester films comprising the steps of:

• (a) providing a treatment assembly comprising a plurality of layer packets, each layer packet comprising a polyester film, the polyester film comprising one or more semicrystalline polyesters having a Tg and a Tm, having first and second major surfaces and a release layer having first and second major surfaces, the first major surface of the release layer being attached to the second major surface of the polyester film, wherein the layer packets are arranged in a stack such that the second major surface of the release layer of an overlying layer packet is separably attached to the first major surface of the polyester film of an underlying layer packet;
• (b) bi-axially orienting the polyester films by heating the treatment assembly such that the polyester films reach a temperature of at least TOrient wherein TOrient is above the Tg of the polyester film and biaxially stretching the treatment assembly while the layer packets are at a temperature of at least TOrient to yield an oriented assembly;
• then
• (c) heat setting the polyester films by heating the oriented treatment assembly to a temperature TSet wherein TSet is above TOrient and below Tm while maintaining the orienting configuration and dimensions so as to raise the crystallinity of the polyester component to yield a heat set assembly;
• and then
• (d) heat relaxing the heat set treatment assembly by heating to a temperature TRelax wherein TRelax is below TSet and above TOrient while being substantially unrestrained in the x and y axial dimensions to yield a heat relaxed assembly.