The anisotropic structure and mechanical behavior of films extruded from recycled polypropylene

Published

January 26, 2026

Author(s)

Beijun Shen, McKenzie Coughlin, Sixtus Nzeh, Kalman Migler, David Kazmer, Thao Nguyen

Abstract

The properties of recycled polyolefins vary widely because postconsumer and postindustrial feedstocks originate from diverse sources and contain contaminants that degrade mechanical performance. Here, we investigate the process-structure relations that govern the anisotropic microstructure and mechanical behavior of films extruded from real-world polypropylene (PP) recyclates containing varying levels of polyethylene (PE) contamination. Two recycled PP sources were analyzed: rPP1 with negligible PE content, and rPP2 containing approximately 40% by mass of high-density PE, quantified by differential scanning calorimetry. A layer multiplying element (LME) was introduced upstream of the die to enhance redistribution of the PE and PP phases and to influence the development of striated morphologies during extrusion. Scanning electron microscopy revealed distinct elongated domain structures (ribbon- and thread-like striations) produced by shear-driven deformation, which reduced elongation at break in the transverse direction from about 1700% at a PE level near 0.3% to roughly 300% as the PE content increased to about 40%. LME processing improved the uniformity of these striations, and enhanced interfacial integrity and failure resistance in the transverse direction, with the largest improvements of about 100% observed at higher PE contents near 40%. Rheometry confirmed that viscous stresses dominate over elastic contributions under processing conditions, with a capillary-to–end-effect ratio near 20, supporting theoretical models of striation formation through shear-driven deformation. Small-angle x-ray scattering indicated limited lamellar alignment (Herman's orientation factor < 0.05) despite pronounced domain orientation, indicating that the anisotropy of mechanical failure arises from the striated two-phase domain structures rather than from crystalline lamellar. Notably, recycled PP films with optimized striations exhibited mechanical performance comparable to virgin materials of similar PE content, underscoring their practical utility. Moreover, elongation at break in the transverse direction remained above 1000% when PE content was below approximately 20%, suggesting a threshold for maintaining desirable ductility. These findings advance the understanding of striation formation and its relationship to processing conditions and mechanical properties, providing practical guidance for enhancing the consistency and performance of recycled polyolefin films.

Citation

Polymer Journal

Pub Type

Journals

Download Paper

https://doi.org/10.1002/pen.70353

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Keywords

recycled polypropylene, polyethylene impurities, layer multiplying element, striations, anisotropic mechanical behavior

Citation

Shen, B. , Coughlin, M. , Nzeh, S. , Migler, K. , Kazmer, D. and Nguyen, T. (2026), The anisotropic structure and mechanical behavior of films extruded from recycled polypropylene, Polymer Journal, [online], https://doi.org/10.1002/pen.70353, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=960031 (Accessed January 31, 2026)