#2257
Author: Charles A. Bishop, C.A.Bishop Consulting Ltd.
Email:
Subject: There are potentially some properties that can be mismatched and cause a problem.
Others may well be better qualified to answer this question however it did occur to me that there are potentially some properties that can be mismatched that could cause a problem.
I am assuming that you are using biaxially oriented PET & PP. The production of the PET & PP may be different one may be oriented using a stenter & the other using an expanding bubble process. Even if the processes are identical the settings for the orientation may be different with different draw ratios. The result of this is that the residual stress profile of each material is likely to be different. The amount of residual stress may also be different this may be reflected in the shrinkage values quoted. It is common for there to be ?x% shrinkage @ 100 Deg C quoted for the material. This shrinkage will start at a lower temperature. The different Tg vales for the two polymers will also mean that the starting temperature for this relaxation will be different.
The net result of this may be a differential shrinkage between the two materials depending upon the residual stress profile mismatch & the differential shrinkage. This does also assume that the laminate is seeing a temperature of the order 70 - 100 Deg C during processing &/or use.
Early indications might be the laminate showing some curl which would indicate a differential shrinkage.
If one of the substrates is not biaxially oriented but cast or uniaxially oriented this could exacerbate the problem.
Another possible source of problems could be the surface energy &/or contamination of one or more of the surfaces.
Virtually all polymer surfaces are treated in some way to help make them manageable or to prime them for printing or vacuum deposition. The polymer surfaces without treatment do not easily slip against each other & the rolls would block and hence the surfaces often have coatings or treatments to help ease this problem. The polymer surfaces will have some unpolymerised monomer on the surface that is known as low molecular weight material as well as dust/debris attracted by electrostatic charges. These contaminants may reduce the surface energy & make adhesion to the surface poor. Thus for many coatings the polymers will be treated by flame, corona or vacuum plasma treatments to raise the surface energy and increase the adhesion.
Thus it would be worth checking the surface energy values for the polymer surfaces in case there is some residual low molecular weight material left causing a weak boundary layer.
There would still need to be something to trigger the delamination, some differential stress caused by bending or change in temperature causing differential stress relaxation, etc. such that the increased stress cannot be transferred across this weak boundary layer and delamination is the preferred low energy result.
Back to top
|
