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1. Subtle differences in recycled polypropylene from rheology to additives impacts ease of circularity in materials extrusion additive manufacture

   https://doi.org/10.1016/j.addma.2024.104209  

   Zhu, Y; Fink, W; Yost, SF; Mather, PT; Vogt, BD (April 2024, Additive manufacturing)

   The decentralized production associated with material extrusion additive manufacture (MEX) has been proposed as an ideal path to increase the circularity of plastics through direct recycling. Although multiple studies have reported on the 3D printing of various recycled plastics, variability in recycled materials, in particular post-consumer waste, challenges the direct extension of these results into production through MEX. Here, we demonstrate filament fabrication and printing of post-consumer polypropylene (PP), where the PP is sourced from clear, cold drink cups from three large international food service and beverage retail chains to provide well defined plastic waste that is perfectly sorted for recycling. These sources for the recycled PP were selected due to their ready availability to enable the results to be directly applied for hobbyist printing, blow molded products to provide good mechanical performance, and the clarity of the PP that suggests formulation design to minimize the PP crystal size. Despite the similarities in the end use product and their physical appearance, the source for the PP impacted the mechanical properties and the visual appearance of the printed objects. These differences can be directly traced to the rheological properties and oxidative stability of the PP at conditions relevant with the print process. These results clearly illustrate differences in initial formulation design and branding details, even when the product is for the same application, impacts the performance of recycled plastics in AM. The high viscosity associated with the PP from blow molding leads to requirements for higher extrusion temperatures for printing. The combination of high temperature and shear during extrusion process of 3D printing degrades the recycled PP. For circularity with MEX with recycled PP, one needs to consider the evolution in the properties of the polymer. Rheological details of recycled plastics are critical to selection of processing conditions and performance of MEX parts. Reporting of rheological data of recycled plastics and these properties after printing is critical to enable translation towards circular 3D printing of recycled plastics. 

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2. Threading-the-Needle: Compatibilization of HDPE/ i PP blends with butadiene-derived polyolefin block copolymers

   https://doi.org/10.1073/pnas.2301352120  

   Shen, Liyang; Gorbea, Gabriela Diaz; Danielson, Evan; Cui, Shuquan; Ellison, Christopher J.; Bates, Frank S. (August 2023, Proceedings of the National Academy of Sciences)

   Management of the plastic industry is a momentous challenge, one that pits enormous societal benefits against an accumulating reservoir of nearly indestructible waste. A promising strategy for recycling polyethylene (PE) and isotactic polypropylene ( i PP), constituting roughly half the plastic produced annually worldwide, is melt blending for reformulation into useful products. Unfortunately, such blends are generally brittle and useless due to phase separation and mechanically weak domain interfaces. Recent studies have shown that addition of small amounts of semicrystalline PE- i PP block copolymers (ca. 1 wt%) to mixtures of these polyolefins results in ductility comparable to the pure materials. However, current methods for producing such additives rely on expensive reagents, prohibitively impacting the cost of recycling these inexpensive commodity plastics. Here, we describe an alternative strategy that exploits anionic polymerization of butadiene into block copolymers, with subsequent catalytic hydrogenation, yielding E and X blocks that are individually melt miscible with PE and i PP, where E and X are poly(ethylene- ran -ethylethylene) random copolymers with 6 wt% and 90 wt% ethylethylene repeat units, respectively. Cooling melt blended mixtures of PE and i PP containing 1 wt% of the triblock copolymer EXE of appropriate molecular weight, results in mechanical properties competitive with the component plastics. Blend toughness is obtained through interfacial topological entanglements of the amorphous X polymer and semicrystalline i PP, along with anchoring of the E blocks through cocrystallization with the PE homopolymer. Significantly, EXE can be inexpensively produced using currently practiced industrial scale polymerization methods, offering a practical approach to recycling the world’s top two plastics. 

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3. Block copolymer molecular design to address practical limitations to recycling polyolefin blends

   https://doi.org/10.1073/pnas.2508921122  

   Cui, Shuquan; Jeong, Daun; Shi, Yukai; Jahan, Nusrat; Lodge, Timothy P; Bates, Frank S; Ellison, Christopher J (July 2025, Proceedings of the National Academy of Sciences)

   Plastics offer innumerable societal benefits but simultaneously contribute to persistent environmental pollution, dominated by polyethylene (PE) and isotactic polypropylene (iPP). Melt blending and reformulating postconsumer PE andiPP into useful materials presents a promising recycling approach. However, such repurposed plastics are generally mechanically inferior due to an inability to efficiently separate polyolefins in mixed waste streams; phase separation of PE andiPP results in brittleness as a consequence of poor interfacial strength. Recently, we demonstrated that a small amount (1 wt%) of a poly(ethylene)-block-poly(ethyl ethylene-ran-ethylene)-block-poly(ethylene) (EXE) triblock copolymer, synthesized by low-cost anionic polymerization of 1,3-butadiene followed by solution hydrogenation, restores tensile toughness to levels equivalent to virgin polyolefins. Unfortunately, low-temperature solvent insolubility of EXE, driven by crystallization of the E blocks containing 1.5 ethyl branches per 100 backbone repeat units (EB), presents a challenge for industrial hydrogenation. Comparable toughness (ca. > 400% strain at break) was achieved in the present work with 1.5 ≤ EB ≤ 6.5, accompanied by reduced EXE crystallinity and dissolution in cyclohexane down to room temperature at the highest EB content. This remarkable toughening behavior is attributed to a synergy between chain entanglements between the E end blocks and semicrystalline PE homopolymer and formation of E block “crystal nodules” that prevent chain pullout, along with topological constraints between the X loops and semicrystallineiPP. Our findings overcome barriers to commercial production of EXE with existing industrial facilities, providing a cost-effective strategy for recycling PE andiPP. 

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4. Functionalization and Repurposing of Polypropylene to a Thermoset Polyurethane

   https://doi.org/10.1021/acsmacrolett.4c00505  

   Monegro, Ronard Herrera; Krishnamoorti, Ramanan; Robertson, Megan L (October 2024, ACS Macro Letters)

   Developing effective recycling pathways for polyolefin waste, enabling a move to a circular economy, is an imperative that must be met. Post-use modification has shown promising results in upcycling polyolefins, removing the limitation of inertness, and improving the final physical properties of the recycled material while extending its useful lifetime. Grafting of maleic anhydride groups to polypropylene is an established industrial process that enhances its reactivity and provides a convenient route to further functionalization and upcycling. In this work, maleic anhydride grafted polypropylene (PPgMAH) was hydroxylated, and subsequently cured with a diisocyanate to form a thermoset polyurethane (PU). The crystal structure (unit cell and lamellar structure) of the polypropylene (PP) was preserved in the PU. At room temperature, the PU showed high modulus due to the crystallization behavior of the PP; upon increasing the temperature above the melting temperature, the modulus decreased to a rubbery plateau, consistent with formation of a network. The resulting PU showed higher glass transition temperature and lower degree of crystallinity than its PP predecessor due to the crosslinked nature of the polymer. The mechanical integrity of the PU was maintained through several reprocessing cycles due to the melt processability enabled by the presence of a urethane exchange catalyst. This functionalization and upcycling route thus offers a promising alternative to repurposing PP waste, in which the creation of melt-processable thermoset polymers expands applications for the materials. 

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5. Impact of salt and fillers on the rheological properties of polymer composites

   https://doi.org/10.1002/pc.29409  

   Teymoory, Parya; Driscoll, Stephen_Burke; Zhao, Jingzhou; Shen, Caiwei (December 2024, Polymer Composites)

   Abstract Polymer composites with salts or conductive fillers are promising for various solid‐state energy storage applications, where processability is often determined by their rheological properties. This study investigates the effect of lithium salts and conductive fillers on the rheological behavior of polylactic acid (PLA)‐based composites. We specifically examine how these additives influence complex viscosity and the interactions between the salt, fillers, and polymer. Our findings reveal that adding salt to the polymer reduces its viscosity, whereas adding conductive fillers imparts a shear‐thinning property, which is advantageous for thermal processing methods like thermal drawing, injection molding, or 3D printing. The combination of salt and conductive fillers results in multifunctional electrode‐electrolyte composites with enhanced shear‐thinning behavior and improved storage modulus. Characterizations through x‐ray diffraction, electrical measurements, and transmission electron microscopy link the electrical properties and morphology with rheological behavior. The formation of a robust filler network in these composites ensures stable viscoelastic behavior across a range of temperatures and frequencies, indicating their suitability for efficient manufacturing of polymer‐based solid‐state electrode‐electrolyte composites via thermal processing. HighlightsShear‐thinning behavior enhanced by conductive fillers.Viscosity increased with CB and CNT fillers, forming robust networks.Salt reduced viscosity but filler networks dominated flow behavior.Filler combinations led to stable viscoelastic properties across temperatures.Polymer electrolyte–electrode composites improved processability and storage modulus. 

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