Polyethylene Terephthalate, or “polyester” film, is a chemically inert thermoplastic material, in large part due to the biaxial orientation process (stretched in the machine and transverse directions), during manufacture. This property, while a desired benefit for many high performance differentiated, specialty, and industrial applications, can make polyester film challenging to print.
Mylar® Specialty Films (formerly DuPont Teijin Films™) adds chemical primers to Melinex® and Mylar® polyester film to enhance printability and improve ink adhesion. Printers are able to achieve success by using a primed polyester base film, in conjunction with formulated ink systems for a given set of printing and curing conditions.
One commonly asked question at Tekra related to secondary printing or processing of polyester film is, “How do I tell the presence of a primer?” Oftentimes, this question pertains to either a one-side primed, or non-primed polyester film. Knowing the presence of a primer is the first step to achieving a successful outcome.
Below are some general suggestions we offer our customers for determining the presence of a primer system:
1. For clear film, uncoated polyester is almost always tackier than a primer treated side. Fold film in half, so that you are rubbing the same side against itself between your fingers; you will notice it is much easier to move the primed film against itself than the non-primed film.
2. For clear films and low to mid haze films, swipe one side of film with MEK or acetone. If it is primer treated, you will see a coating layer removed with the solvent. You should notice a clear difference (i.e. hazy, white deposit resulting from the attack on the primer by the solvent). If you do not see a difference, it is most likely not primer treated.
3. Testing surface tension by various methods is reliable only when you have a reference with a target given primer on a target film type; surface energy of primed film is slightly more (2-3 dynes) than non-primed polyester.
4. The use of dyne pens is not reliable, due to relatively short shelf life of the pen.