While testing the puncture strength of various films, we have noticed that we can get different results depending on the side of the material we test. We use a method similiar to the astm puncture procedure where a piece of film is held statiorary and a probe is pulled through the film while an instron tensile testor is used to measure the force required to puncture the film. The film structure can vary, but a simplified film can be thought of as a two layer film with a nylon(polyamide) based layer on one side and a polyethylene based layer on the other. Obviously these materials have there own physical characteristics, which can be measured using monolayer films. But as a bonded whole we get these differing results based on which side of the film we test. I have some ideas which might be the solution, but I am not sure.
- Although the films are thin, 50-125 microns, the side opposite the probe may be stretched more in the axial directions, which would lead to thinning and creater weaker strengths. The amount of effect would be different for each material.
- The probe may be able to cut into the material prior to the actual puncture, leaving a weakened structure. The amount of cutting would be dependant on each material as well.
These solutions may be partly correct, or not at all. We generally have more complex film structures to complicate things and the results of going through a stronger material first does not always produce a stronger puncture nor does it always provide a weaker puncture, it varies on each structure.
Thanks in advance for any help on this topic.