Crosslinked low‐density polyethylene (XLLDPE) is widely used in several specialty plastics industries. However, the permanent chemical crosslinks cause high‐melt viscosity and poor processability, preventing the material from being reused and recycled effectively. This study investigates solid‐state shear pulverization (SSSP) as a continuous, commercially viable mechanochemical processing technique to initiate the decrosslinking of XLLDPE for mechanical recycling. Post‐industrial XLLDPE scrap material was processed using SSSP with a range of pulverization conditions, which were correlated with universal processing covariants of specific mechanical energy and particle size distribution. The physical properties of SSSP‐processed materials were compared to as‐received XLLDPE and uncrosslinked low‐density polyethylene. While gel content tests confirm a gradual decrease in crosslinking density with a more energy‐intensive SSSP process, melt rheology and dynamic mechanical analysis characterization revealed additional chain architecture modifications such as branching and chain scission. Based on differential scanning calorimetry and thermogravimetric analysis, the SSSP‐processed XLLDPE retained its thermal stability and crystallinity; tensile testing results showed improved stiffness, strength, and toughness. These results indicate that tunable SSSP can transform XLLDPE into a decrosslinked, branched, and melt‐processable recycled polyethylene.
Ultra‐high molecular weight polyethylene (UHMWPE) is one of the most prominent high‐performance thermoplastics for biomedical, leisure, and coating applications. Large‐scale recycling of UHMWPE is extremely difficult due to the high melt viscosity of the material as well as its exceptional chemical resistance and impact strength. There is a need for a commercially scalable methodology that can process the waste feedstock for mechanical recycling while sustaining the outstanding physical properties of the material. Solid‐state shear pulverization (SSSP) is a continuous, twin‐screw extruder‐based processing technique in which the low‐temperature application of shear and compressive forces impart changes in structure at different length scales to overcome the challenges of difficult‐to‐recycle polymers. This paper investigates the use of SSSP in mechanically recycling post‐industrial scrap UHMWPE (rUHMWPE) material from a local ski and snowboard manufacturer. The SSSP‐processed particles are flat, micron‐scale flakes with enhanced surface area, which can sinter very quickly when compression molded. The molded rUHMWPE samples in turn exhibit enhanced ductility and toughness compared to the as‐received scrap material, based on the tunable mechanochemical modification of the ethylene chains.
more » « less- NSF-PAR ID:
- 10387078
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Polymer Engineering & Science
- Volume:
- 63
- Issue:
- 2
- ISSN:
- 0032-3888
- Format(s):
- Medium: X Size: p. 319-330
- Size(s):
- ["p. 319-330"]
- Sponsoring Org:
- National Science Foundation
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Solid‐state shear pulverization (SSSP) is a continuous polymer processing methodology based on a modified twin‐screw extruder. The unique application of low barrel temperatures and mechanochemistry has contributed to the development of an extensive range of polymer‐based materials, from environmentally responsible polymer blends to nanocomposites, for more than 30 years. The complex processing‐structure‐processing relationships in SSSP can be elucidated by way of integrated, measured covariants that capture the interplay between numerous processing parameters. Residence time distribution and specific mechanical energy are evaluated in a base case polypropylene (PP) study under a full factorial experiment involving screw design, screw speed, and throughput parameters. These factors are in turn correlated to dispersion morphology and thermal property results from a parallel study based on a model PP/carbon black composite. This investigation highlights the tunability of SSSP processing parameters for tailored output with desired purposes and applications. In particular, enhanced residence distribution can be achieved with low screw speed and high throughput settings, leading to high levels of material mixing and shear. POLYM. ENG. SCI., 60:503–511, 2020. © 2019 Society of Plastics Engineers
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