Mitigating Extrusion Instabilities and Enhancing Mechanical Performance of Industrial Recycled Polyolefins via Layer Multiplying Elements
Description
This study investigates the structural and mechanical performance of multilayer films containing recycled polypropylene (PP) materials, with a focus on understanding how layer multiplication and blend composition influence properties relevant to flexible packaging applications. A three-layer coextruded film structure was used as the base, with virgin polypropylene (vPP) consistently applied as the skin layer, while the core layer composition was varied using either 100% recycled PP2, PP1, or 50/50 blends of recycled PP2 and recycled PP1. To enhance layer uniformity and interfacial interaction, a Layer Multiplying Element (LME) was employed to increase the number of layers from 3 to 9. Comprehensive tensile testing followed by multiple regression analysis revealed that the use of LME significantly improved mechanical properties, particularly elongation at break and modulus in the transverse direction (TD). The interaction between LME and PP2 was statistically significant and contributed positively to ductility. Temperature and vPP content were also found to positively influence stress and modulus. However, the presence of recycled PP2 in the core generally reduced ductility and strength, highlighting the trade-offs when incorporating post-consumer materials. Scanning Electron Microscopy (SEM) of fractured specimens showed that brittle failure was often associated with interfacial delamination, particularly in recycled-rich regions, while ductile failure exhibited entangled layer structures, indicating better energy absorption and interlayer cohesion. Importantly, both monolayer and multilayer films with recycled content displayed ribbings or striations aligned in the machine direction (MD), particularly in areas rich in recycled PP2 or PP1/PP2 blends. These features suggest uneven melt flow and domain elongation due to molecular inhomogeneities in the recycled polymers. Visual inspection with an online camera system during film processing further supported these findings. Films processed without LME exhibited surface perforations and non-uniform coloration, indicating poor material distribution and potential phase incompatibility. In contrast, films processed with LME were visually uniform and defect-free, highlighting the LME’s role in promoting layer refinement, better dispersion, and optical consistency.
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Recycled PP was co-extruded with vPP as the skin layer. Layer multiplying element was used to increase the number of layers from 3 to 9. To investigate and quantify the effects of layer multiplying element (LME), die temperature, virgin polypropylene content (vPP%), and recycled polypropylene content (PP2%) on the mechanical properties of the films, multiple regression analyses were conducted 45 observations. The dependent variables in the analysis were elongation, stress at yield, and modulus, measured in both the machine direction (MD) and transverse direction (TD)
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Funding
U.S. National Science Foundation
2118808