Co-extrusion is a widely used processing technique for combining various polymers with different properties into a tailored multilayer product. Individual melt streams are combined in a die to form the desired shape. Under certain conditions, interfacial flow instabilities are observed; however, fundamental knowledge about their onset and about critical conditions in science and industry is scarce. Since reliable identification of interfacial co-extrusion flow instabilities is essential for successful operation, this work presents in situ measurement approaches using a novel co-extrusion demonstrator die, which is fed by two separate melt streams that form a well-controlled two-layer co-extrusion polymer melt flow. An interchangeable cover allows installation of an optical coherence tomography (OCT) sensor and of an ultrasonic (US) measurement system, where the former requires an optical window and the latter good direct coupling with the cover for assessment of the flow situation. The feasibility of both approaches was proven for a material combination that is typically found in multilayer packaging applications. Based on the measurement signals, various parameters are proposed for distinguishing reliably between stable and unstable flow conditions in both measurement systems. The approaches presented are well suited to monitoring for and systematically investigating co-extrusion flow instabilities and, thus, contribute to improving the fundamental knowledge about instability onset and critical conditions.

When two compatible polymer melts are brought into contact, a diffuse interphase is formed that governs the adhesion between them. The degree of interdiffusion is defined mainly by the processing conditions and the physico‐chemical properties of the materials. Despite the great industrial relevance of co‐extrusion in manufacturing multilayer structures, little research has dealt with interdiffusion under these processing conditions, where polymer‐polymer interfaces are subject to shear load. In this work, procedures for investigating interdiffusion both under static and real co‐extrusion flow conditions were developed. Using various grades of poly(methyl methacrylate) and styrene‐co‐acrylonitrile copolymer, we studied both theoretically and experimentally the effects of interfacial contact time, temperature, material compatibility, and interfacial shear stress on interdiffusion. Spectroscopic analysis of the interfaces using confocal Raman microscopy showed that the general relationships between interdiffusion, time (Fick's law) and temperature (Arrhenius relationship) are identical under both static and co‐extrusion conditions. However, the interdiffusion rate is significantly higher under shear influence. Since the prediction of the interdiffusion process supports material selection and process design in multilayer manufacturing, a simple analytical model was developed to quantify the effects of aforementioned material properties and processing conditions on the apparent mutual interdiffusion coefficient.

In the co‐extrusion of plastics, pressure‐throughput behavior, layer distribution, and residence time are crucial parameters, modeling of which contributes to manufacturing high‐quality products at optimized process efficiency and significantly shortens development times for die systems. In previous work, we have presented symbolic regression models to predicting the (i) pressure‐throughput behavior, (ii) position of the interface, (iii) interfacial shear stress, (iv) ratio of volume flow rates, and (v) interfacial flow velocity for isothermal two‐layer co‐extrusion flows through rectangular ducts. These regression models are mathematically simple and capable of capturing the shear‐thinning nature of polymer melts without the need for numerical methods. Here, we present an experimental study validating the proposed models against co‐extrusion process data and comparing them to existing theories. To this end, a two‐layer co‐extrusion demonstration die instrumented with an optical coherence tomography sensor for detecting the interfacial position was used. To accurately set up and evaluate the die flows, the overall co‐extrusion process was represented by means of a digital process twin. Industrially relevant combinations of materials were tested under a wide range of processing conditions. Comparisons of pressure losses and interfacial positions to the predictions showed excellent agreement and the results outperformed the concept of representative viscosity.

Co-extrusion is commonly used to produce polymer multilayer products with different materials tailoring the property profiles. Adhesion between the individual layers is crucial to the overall performance of the final structure. Layer adhesion is determined by the compatibility of the polymers at the interface and their interaction forces, causing for example the formation of adhesive or chemical bonds or an interdiffusion layer. Additionally, the processing conditions, such as temperature, residence time, cooling rate, and interfacial shear stress, have a major influence on the interactions and hence resulting layer adhesion. Influences of temperature and residence time are already quite well studied, but influence of shear load on the formation of an adhesion layer is less explored and controversially discussed in existing literature. In this work, we investigated the influence of different processing conditions causing various shear loads on layer adhesion for a two-layer co-extruded polymer sheet using a polypropylene and polypropylene talc compound system. Therefore, we varied the flow rates and the flow geometry of the die. Under specific conditions interfacial flow instabilities are triggered that form micro layers in the transition regime between the two layers causing a major increase in layer adhesion. This structure was analyzed using confocal Raman microscopy. Making use of these interfacial flow instabilities in a controlled way enables completely new opportunities and potentials for multi-layer products.

Polymer melts that are diluted, that is, to which low-molecular-weight diluents have deliberately been added to modify them, have attracted growing interest due to their ability to significantly reduce melt viscosity, thereby improving processability and enhancing extrusion efficiency. We present a novel experimental setup based on slit rheometry that integrates a slit rheometer with a single-screw extruder, a gear pump, and a high-pressure liquid chromatography pump for diluent injection. A distinctive feature of this setup is its adjustable diluent injection point, which allows systematic investigation of the impact of injection location on in-line mixing, diluent incorporation, and the resulting rheological behavior under high-pressure processing conditions. In order to mitigate diluent evaporation or bubble nucleation, the Antoine equation is used to determine the pressure under which the setup operates. Experimental results demonstrated that adding n-decane to high-density polyethylene substantially reduced viscosity, with direct injection into the extruder yielding lower measurement variability and a higher saturation threshold than injection downstream of the melt pump. This advanced method offers a robust framework for determining rheological properties of diluted melts to enhance process design and optimization.

Accurate prediction of viscosity reduction is a prerequisite for optimizing advanced polymer recycling and processing technologies, particularly, when dealing with complex fluids such as postconsumer (PCRs) or diluted melts. This study presents a comprehensive rheological framework characterizing the flow behavior of high-density polyethylene (PE-HD) and low-density polyethylene PCR diluted with $$$ n $$$-decane (C10), a specialty recycling solvent from the CreaSolv-technology (CS), and supercritical carbon dioxide ( $$$ {\mathrm{CO}}_2 $$$) using an in-line slit-die rheometer. By systematically evaluating three modeling approaches of increasing physical depth, we bridge the gap between fundamental thermodynamics and practical process control. While parallel-plate rheometry confirmed the absence of wall slip in the investigated shear rate regime, the modeling analysis revealed distinct trade-offs between physical fidelity and predictive capability. A semiempirical model utilizing an exponential (Arrhenius-like) concentration term demonstrated good performance for the observed diluent concentration regime, achieving mean relative errors for all diluent systems below 5%. In contrast, a fundamental approach coupling the Sanchez–Lacombe Equation of State (EOS) with the Kelley–Bueche theory successfully validated the functional form of dilution but consistently underestimated the magnitude of viscosity reduction. Ultimately, this work derives a physically validated, four-parameter semiempirical model that eliminates the need for complex molecular weight data as needed for the reptation-based Schausberger model, enabling the direct integration of rheological predictions for variable PCR feedstocks into digital twins and real-time process optimization tools.