This was a topic that I received many questions about over the years. In general, the lamination strength is measured by a 90° peel test, such as ASTM D3330, and the answer is simple. At some point in the laminated structure, there is a weak layer or interface where the failure occurs.
However, the result of a peel test will depend greatly on the structure of the test sample. Peel tests really are more of a total energy to failure and include the deformation of all layers in the test specimen as well as the bond strength at the failure. Therefore, it is imperative to ensure a uniform test structure for the test, especially if comparing results between labs or between manufacturing runs. Of course, it only matters commercially if the peel strength of the lamination is lower than the product specification.
If you need, or desire, to understand the cause of a delamination, the question then becomes how to determine at what point in the lamination the failure begins and what is its nature. Is the failure within a layer or at an interface between layers of the lamination? This is important in determining the ultimate cause of the failure and the robustness of the laminated product. Is the cause due to a process or material variation or contamination?
Chemical Testing of Failure Surfaces
So, if you want to determine the cause of the delamination, then you will have to do more extensive surface chemical testing of the failure surfaces. I have done this a fair number of times for various products that are experiencing a delamination failure, and the need exists to determine how to fix the problem or who to blame/charge for the failed material. To do this, what is needed is a chemical evaluation of the two failed surfaces to determine if the failure is cohesive (within one of the layers) or adhesive (between two layer surfaces) and if there is a contaminant present at the failure surface that explains the adhesive failure.
If you need, or desire, to understand the cause of a delamination, the question then becomes how to determine at what point in the lamination the failure begins and what is its nature.”
However, before testing, the samples have to be prepared and shipped to the laboratory where they will be tested. Great care must be taken to avoid contamination of the delamination surface. To ensure this, I sometimes will start the delamination or take a delaminated section and send it to the lab as is. I then will ask them to delaminate a fresh section for testing and to test both surfaces of the delaminated sample.
The two tests I generally start with are X-ray Photon Spectroscopy (XPS), with both a low- and high-resolution scan, and surface Fourier transform infrared spectroscopy (FTIR) scans, to which I often add a surface wash of each failure surface, followed by Gas Chromatography/Mass Spectra (GC/MS). The XPS gives an atomic concentration of elements on each surface, and the high-resolution scan can give some indication of the chemical functionality of the elements at the failure surface. The FTIR gives chemical functional groups and, in many cases, can identify polymers and chemical compounds from IR spectra libraries of chemical compounds. GC/MS and surface FTIR scans determine the chemical compositions of each surface to identify polymer types and any low molecular weight chemicals that might be present on the delamination surfaces.
These tests can be used to identify at which point the lamination failed (i.e., determine if the adhesive failed adhesively [only on one failure surface] or cohesively [adhesive on both failure surfaces] or if a failure occurred in a coating layer or within one of the films that are adhered together). If the surface wash determines there are smaller molecules possibly causing a weak boundary layer, I might have a Secondary Ion Mass Spectrometry (SIMS) test performed to further identify the surface contamination at the failed surface. While FTIR can identify these low molecular weight molecules, it is not always definitive and the mass spectra aids in narrowing the compositions.
From this combined information, one generally can determine the most likely cause of the delamination, such as simple overtreatment, uncured adhesive, cohesive failure in the adhesive or bond poisoning by migratory additives, etc.
It helps in the evaluation of the delamination failure if you are a chemist or remember your organic chemistry. Early in my career, before I was a chemical engineer and polymer processer, I was a synthetic organic chemist and had to identify compounds I made, so this sort of analysis is straightforward for me. If you are not a chemist, you will have to gain access to a good chemist who understands your application and component materials and can give you good insights into the mode of failure the testing is highlighting.
Eldridge M. Mount, Ph.D.
Consulting Technical Editor
585-223-3996, emmount@msn.com
ARC Member