Controlling interfacial instabilities in PP/EVOH coextruded multilayer films through the surface density of interfacial copolymers
Résumé
Co-extruded polypropylene/tie/ethylene vinyl alcohol/tie/polypropylene (PP/tie/EVOH/tie/PP) films often exhibit optical defects which appear as randomly distributed scattering objects, in the sub-millimeter range. These defects may strongly affect the film transparency and prevent their practical use in packaging. Based on an objective optical test aimed at quantifying the film transparency, and on a systematic analysis, through optical microscopy, of transverse cuts of films obtained in various co-extrusion conditions, the nature of the defects could be identified as resulting from a modulation of the thickness of the inner EVOH Layer, with no variation in the overall thickness of the multilayer films. Thanks to a recently developed method to dose the surface density of interfacial copolymers, a clear correlation between the amplitude of the thickness modulation of the inner EVOH layer and the density of co-polymer molecules formed in situ at the EVOH/tie layer interface during the co-extrusion process was established. These results open the way to a better design of tie layers composition to avoid these kinds of defects. KEYWORDS: Controlling interfacial instabilities in PP/EVOH coextruded multilayer films through the surface density of interfacial copolymers.
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