An experimental study of polymer-polymer interdiffusion under co-extrusion processing conditions

• Published in: AIP conference proceedings Vol. 3158; no. 1
• Main Authors: Hammer, Alexander, Leimhofer, Claudia, Roland, Wolfgang, Ehrmann, Timo, Hild, Sabine, Berger-Weber, Gerald
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Melville American Institute of Physics 08.05.2024
• Subjects: Coextrusion; Contact melting; Contact stresses; Copolymers; Design of experiments; Diffusion coefficient; Extrusion dies; Extrusion rate; Interdiffusion; Interfacial shear stresses; Melt temperature; Polymers; Polymethyl methacrylate; Shear stress; Stratified flow; Styrene acrylonitrile resins; Transport phenomena
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/5.0205411

Interdiffusion – defined as the mutual interpenetration of macromolecules to form a diffuse interphase – is observed at polymer-polymer interfaces in a wide range of plastics processing techniques. This transport phenomenon has been studied extensively on static interfaces (i.e., the polymers and, in particular, the interface are not subject to deformation), for instance, in plastics welding, and consolidation of thermoplastic UD tapes. In contrast, interdiffusion under pressure-flow conditions – as present in stratified flows during co-extrusion – has been studied scarcely to date. In particular, the correlation between the magnitude of interfacial shear load and the rate of interdiffusion is unknown. In this work, a two-layer co-extrusion demonstration die and the material combination poly(methyl methacrylate) (PMMA) and styrene-co-acrylonitrile copolymer (SAN) were used to study the effect of the interfacial shear stress on the rate of interdiffusion. The implementation of a model-based digital process twin enabled to accurately adjust the co-extrusion processing conditions. Employing a partial factorial experimental design, the magnitude of interfacial shear stress was varied at certain levels of interfacial contact time and melt temperature. The diffuse interphase of the calibrated two-layer sheets was then characterized by means of confocal Raman microscopy and an apparent mutual diffusion coefficient was determined. It is shown that already low levels of interfacial shear stresses (e.g., 1,000 Pa) significantly boost the rate of interdiffusion compared to static conditions. Though, a gradual increase of the interfacial shear stress level up to approximately 40,000 Pa revealed no further impact on the mutual interdiffusion coefficient.