The 135-degree peel test (sometimes also called '45-degree peel test') is preferred over 90-degree and 180-degree peel tests when it is desirable to replicate how a user opens a package diagonally in a real-life situation. It is typically used where a flexible material is bonded to a more rigid substrate and the contents are liquid or semi-solid e.g. a yoghurt container comprising an aluminium foil lid bonded to a thermoformed plastic pot.
The 135-degree peel test requires a dedicated fixture to ensure the peel angle remains constant. This usually entails securing the more rigid substrate (e.g. a pot) in a clamping fixture on the base of the tester and the flexible material in an upper vice clamp which is attached to the moving crosshead of the test stand.
For thin-walled flexible packaging, it is often preferable to use a vacuum clamp on the base, which does not adversely distort the package thus causing the lid to deform and thereby influencing the peel force. To maintain the constant peel angle as far as possible, it is best to use a flexible link between the upper vice clamp and the moving crosshead of the test stand and to keep the link length as long as practically possible.
As the crosshead moves upwards at a constant speed, the flexible material is peeled from the semi-rigid material whilst maintaining a constant 135-degree angle between the two pieces.