Question: We convert silicone-coated PET, and test it to make sure we are working with the coated side. At present we’re testing with 30 dyne and 50 dyne inks. Any comments on this, and do you have a test procedure for this application?
Answer: First, the test procedure, which covers virtually all polymers, is available at https://www.accudynetest.com/qctest.html.
Untreated, uncoated PET generally has a surface energy of about 43 dynes/cm. As such, if the PET surface has not been modified, I would expect the 50 dyne/cm test fluid to bead more or less instantaneously when applied to the material. The 30 dyne/cm test fluid would wet out for a long period of time – permanently as likely as not.
For silicone-coated PET, it is obvious the 50 dyne/cm test fluid should bead instantly, as silicone compounds have surface energies ranging in the 20s. In most cases the 30 dyne/cm test fluid would do the same, but some formulations may result in a surface slightly higher than 30 dynes/cm, so some short-lived wetting may be observed.
I would suggest testing at 36 dynes/cm. Print-primed PET has a surface energy of about 38 dynes/cm; virgin PET is at 43, and surface-treated PET should be at 48 dynes/cm or higher. All these surfaces should show wetting for at least 4 seconds, and perhaps even permanently, when tested at 36 dynes/cm. The presence of any silicone compound would cause immediate beading at this dyne level.