(141d) Solid-State Shear Pulverization (SSSP): An Investigation into Thermoplastic Types and Properties

Polymers used in high-end applications like pharmaceutical testing, cell culture, and cancer research are still adapting to the changing industrial needs, including cost reduction, customizability, and environmental sustainability. While twin screw extrusion (TSE) is one of the most versatile methods of polymer processing, new and modified TSE-based methods continue to develop to allow researchers to tailor polymeric materials to specific properties and performance.

One technique to make polymer processing more sustainable is solid-state shear pulverization (SSSP).^1–9 SSSP is a modified twin screw extruder with chilled barrels that subject materials to high shear and compressive forces. The material travels down the barrel and undergoes fragmentation and fusion, often achieving morphological changes via mechanochemical reactions. These effects include chain scission, free-radical formation, chain branching, and inter-chain cross linking. Consequent macroscopic morphological changes include immiscible phase compatibilization, increased filer exfoliation and blended phase dispersion. SSSP is a continuous and commercially scalable process that does not rely on heat, solvents, monomers, or additives.

Previous SSSP studies compatibilized immiscible blends,^2,5,6 fabricated nanocomposites,^3,4,7 effectively recycled co-mingled plastics,⁸ and modified homopolymers.⁹ In most of these studies, the base polymer was almost always polyolefins, because they respond well to the SSSP process. However, an in-depth understanding of why is unknown. Therefore, this study aims to conduct a direct comparison of how 10 different thermoplastics respond to SSSP.

In this study, pure polymer processing and model nanofiller compounding were performed on a Berstorff 25mm SSSP instrument. As-received graphite was used for the nanofiller at 2 vol% and analyzed over multiple passes through the SSSP. Processing parameters such as screw harshness, barrel temperature, screw speed, and mass flow rate were controlled, and process covariants such as specific mechanical energy and screw temperature were measured to characterize the process. Output was analyzed through scanning electron microscopy (SEM). Mechanical testing consisted of tensile and dynamic mechanical analysis (DMA). Thermal characterization was determined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and thermal conductivity. Relative molecular weight and viscosity was determined by melt rheology.

(1) Khait, K.; Carr, S. H.; Mack, M. H. Solid-State Shear Pulverization; CRC Press, 2001.

(2) Lebovitz, A. H.; Khait, K.; Torkelson, J. M. Stabilization of Dispersed Phase to Static Coarsening: Polymer Blend Compatibilization via Solid-State Shear Pulverization. Macromolecules 2002, 35 (23), 8672–8675.

(3) Wakabayashi, K.; Pierre, C.; Dikin, D. A.; Ruoff, R. S.; Ramanathan, T.; Brinson, L. C.; Torkelson, J. M. Polymer−Graphite Nanocomposites: Effective Dispersion and Major Property Enhancement via Solid-State Shear Pulverization. Macromolecules 2008, 41 (6), 1905–1908.

(4) Wakabayashi, K.; Brunner, P. J.; Masuda, J.; Hewlett, S. A.; Torkelson, J. M. Polypropylene-Graphite Nanocomposites Made by Solid-State Shear Pulverization: Effects of Significantly Exfoliated, Unmodified Graphite Content on Physical, Mechanical and Electrical Properties. Polymer 2010, 51 (23), 5525–5531.

(5) Tao, Y.; Lebovitz, A. H.; Torkelson, J. M. Compatibilizing Effects of Block Copolymer Mixed with Immiscible Polymer Blends by Solid-State Shear Pulverization: Stabilizing the Dispersed Phase to Static Coarsening. Polymer 2005, 46 (13), 4753–4761.

(6) Lebovitz, A. H.; Khait, K.; Torkelson, J. M. Sub-Micron Dispersed-Phase Particle Size in Polymer Blends: Overcoming the Taylor Limit via Solid-State Shear Pulverization. Polymer 2003, 44 (1), 199–206.

(7) Masuda, J.; Torkelson, J. M. Dispersion and Major Property Enhancements in Polymer/Multiwall Carbon Nanotube Nanocomposites via Solid-State Shear Pulverization Followed by Melt Mixing. Macromolecules 2008, 41 (16), 5974–5977.

(8) Miu, E. V.; Fox, A. J.; Jubb, S. H.; Wakabayashi, K. Morphology and Toughness Enhancements in Recycled High-Density Polyethylene (RHDPE) via Solid-State Shear Pulverization (SSSP) and Solid-State/Melt Extrusion (SSME). J. Appl. Polym. Sci. 2016, 133 (10), 43070.

(9) Ganglani, M.; Torkelson, J. M.; Carr, S. H.; Khait, K. Trace Levels of Mechanochemical Effects in Pulverized Polyolefins. Journal of Applied Polymer Science 2001, 80, 671–679.