Reactive Functionalized Multilayer Polymers in a Coextrusion Process: Experimental and Theoretical Investigations of Interfacial Instabilities

Coextrusion technologies are commonly used to produce multilayered composite sheets or films for a large range of applications from food packaging to optics. The contrast of rheological properties between layers can lead to interfacial instabilities during flow. Important theoretical and experimental advances regarding the stability of compatible and incompatible polymers have, during the last decades, been made using a mechanical approach. However, few research efforts have been dedicated to the physicochemical affinity between the neighboring layers. The present study deals with the influence of this affinity on interfacial instabilities for functionalized incompatible polymers. Polyamide (PA6)/polyethylene grafted with glycidyl methacrylate (PE-GMA) was used as a reactive system and PE/PA6 as a non reactive one. Two grades of polyamide (PA6) were used in order to change the viscosity and elasticity ratios between PE (or PE-GMA) and PA6. It was experimentally confirmed, in this case, that weak disturbance can be predicted by considering an interphase of non-zero thickness (corresponding to an interdiffusion/reaction zone) instead of a purely geometrical interface between the two reactive layers. According to rheological investigations from previous work, an experimental strategy was here formulated to optimize the process by listing the parameters that controlled the stability of the reactive multilayer flows. Plastic films with two layers were coextruded in symmetrical and asymmetrical configurations in which PA6 was the middle layer. Indeed, for reactive multilayered systems, the interfacial flow instability could be reduced or eliminated, for instance, by (i) increasing the residence time or temperature in the coextrusion bloc (for T above the reaction temperature T = 240°C), and (ii) reducing the total extrusion flow rate. The reaction rate/compatibilization played a major role that must be taken into account. Furthermore, the role of the viscosity ratio, elasticity ratio, and layer ratio of the stability of the interface were also investigated coupling to the physicochemical affinity. The results show that it is necessary to obtain links between the classic factors that are introduced in the evaluation of the theoretical, given by linear stability analysis/longwave asymptotic investigations, and its corresponding experimental stability charts. Hence, based on this analysis, guide-lines for a stable coextrusion of reactive functionalized polymers can be provided.