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ABSTRACT The processing and performance of multilayer films containing post‐consumer recycled polypropylene (rPP1 and rPP2) materials are investigated to understand the effect of layer multiplying elements (LMEs), die temperature, virgin polypropylene (vPP%) content, and polyethylene (PE) contamination in flexible packaging applications. Three‐layer coextruded films were created with virgin polypropylene (vPP) consistently applied as the outer layers while the core layer comprised recyclates with varying concentrations of polyethylene as an unintended contaminant to polypropylene. To enhance layer uniformity and interfacial interaction, a layer multiplying element (LME) was employed to increase the number of coextruded film layers from 3 to 9. Tensile properties (elongation at break, yield stress, and modulus) were characterized in both machine direction (MD) and transverse direction (TD); after which, multiple linear regression analyses were conducted on 45 observations to model the effect of each factor. The results indicated that the LME significantly enhanced elongation at break in TD by 1280% strain, while temperature and vPP fraction also contributed positively to ductility in TD (+341% and +2373%, respectively). However, PE contamination had a substantial negative impact on elongation in MD (−2449%) underscoring its embrittling role due to lack of compatibility with the PP matrix. Critically, LME partially mitigated the negative PE effect via an interaction term (PE*LME), improving elongation in MD by +3101%. Scanning Electron Microscopy (SEM) revealed a distinct, regular pattern of alternating polyethylene (PE) and polypropylene (PP) domains forming ribbon‐like fibrillar structures. This unique morphological arrangement suggests a self‐organizing behavior driven by immiscibility and flow‐induced alignment under extrusion conditions. The presence of regular alternating domains at near equal concentrations implies a balance among shear‐driven orientation, phase separation kinetics, and crystallization phenomena, resulting in an ordered micro‐fibrillar structure. Importantly, both monolayer and multilayer films containing rPP2 or rPP1/rPP2 blends exhibited these aligned, ribbon‐like fibrils oriented in the machine direction (MD). SEM analysis of fractured specimens further indicated that brittle failure was often associated with interfacial delamination, particularly in recyclate‐rich regions, whereas ductile failure exhibited entangled reinforcing fibrils, suggesting improved energy absorption and interlayer cohesion. Understanding and controlling this self‐organized microstructure could significantly enhance processing stability, mechanical properties, and potential applications of recycled polyolefin blends, offering novel strategies for tailoring recyclate morphology and performance. 

Abstract This study aims to establish a systematic approach for characterizing recycled polyolefins of unknown composition, with a specific focus on predicting their performance in film extrusion. We explore various characterization techniques, including differential scanning calorimetry (DSC), Fourier‐transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and rheometry to assess their effectiveness in identifying the polyethylene (PE) fractions within polypropylene (PP) recyclates. By integrating experimental data with modeling techniques, we aim to provide insights into the predictive capabilities of these techniques in determining processing behaviors. The research highlights the superior fidelity of DSC in predicting the relative fraction and type of PE in a PP recyclate. FTIR is also identified as a high‐fidelity approach, albeit requiring application‐specific calibration. TGA, capillary, and oscillatory rheometry are recognized for their ability to distinguish between grades of recycled polyolefins but provide aggregate behaviors rather than detailed constituent information. 3D flow simulation of the cast film extrusion investigated the effect of the viscosity characterization method, non‐isothermal assumption, and process settings but could not fully replicate the observed variations in the cast film processing of two industrial polyolefins with similar melt flow rates and viscosity behaviors. This underscores the practical challenge of predicting processing issues prior to actual processing, necessitating reliance on reliable instrumentation suites and human expertise for diagnosing and remedying variations. HighlightsTwo industrial recycled polypropylene materials having similar melt flow rates exhibit drastically different cast film processing behaviors.DSC and FTIR provide reasonable approaches for identifying constituent materials.Modeling of the melt viscosities characterized by capillary and parallel plate rheology suggests that viscosity variations relative to the power‐law behavior assumed in the coat hanger die design is a predominant driver of cast film instabilities. 

This article presents the first use of shape forming elements (SFEs) to produce architected composites from multiple materials in an extrusion process. Each SFE contains a matrix of flow channels connecting input and output ports, where materials are routed between corresponding ports. The mathematical operations of rotation and shifting are described, and design automation is explored using Bayesian optimization and genetic algorithms to select fifty or more parameters for minimizing two objective functions. The first objective aims to match a target cross-section by minimizing the pixel-by-pixel error, which is weighted with the structural similarity index (SSIM). The second objective seeks to maximize information content by minimizing the SSIM relative to a white image. Satisfactory designs are achieved with better objective function values observed in rectangular rather than square flow channels. Validation extrusion of modeling clay demonstrates that while SFEs impose complex material transformations, they do not achieve the material distributions predicted by the digital model. Using the SSIM for results comparison, initial stages yielded SSIM values near 0.8 between design and simulation, indicating a good initial match. However, the control of material processing tended to decline with successive SFE processing with the SSIM of the extruded output dropping to 0.023 relative to the design intent. Flow simulations more closely replicated the observed structures with SSIM values around 0.4 but also failed to predict the intended cross-sections. The evaluation highlights the need for advanced modeling techniques to enhance the predictive accuracy and functionality of SFEs for biomedical, energy storage, and structural applications.