Substrate Secrets

In an earlier post, I talked about the need to have coextruded layers which are compatible in terms of orientation behavior.    The figure attached to this post shows a little bit of what I mean.  In this instance a PP film was being produced with an EVOH skin layer.  In this case the EVOH should stretch well with the PP core layer, but because to orientation temperatures were not optimized the skin was found to fibrillate on orientation.  The picture is a micrograph of the stained EVOH layer on the film surface.

Now the question that arises with this picture is why didn’t the EVOH stretch as it should have?  Is it the orientation temperatures or the morphology of the EVOH skin which is causing the layer fracture? 

If it was orientation temperatures you might expect to see the defect after the tenter alone.  But in this case the fracture was beginning to form in the MD orienter in the form of stress whitening edges and in extreme instances in the EVOH building up on the stretch rolls.  This suggested that the morphology of the EVOH was not optimum.  The stress whitening suggests the presence of internal crazing of the layer usually due to polymer crystallinity (morphology).  In this case the corrective action was to modify the casting temperatures to modify the crystalline form of the EVOH (most likely a different crystallite size or perhaps percent crystallinity).  This took the form of increasing the quenching temperature on the casting drum.  A more definitive answer on the exact morphology change is not possible as no analytical measurements of the EVOH, such as DSC, were made.