Modeling Heat Transfer in a Chilled Twin-Screw Extruder: Towards a Better Understanding of Solid-State Shear Pulverization

Modeling Heat Transfer in a Chilled Twin-Screw Extruder: Towards a Better Understanding of Solid-State Shear Pulverization

Evan V. Miu, James E. Maneval, and Katsuyuki Wakabayashi

Department of Chemical Engineering

Bucknell University

Lewisburg, PA 17837 

The abundance of polymer-based products in modern society is due to various processing methods which can physically and chemically modify polymers and tailor them to specific applications.  Solid-state shear pulverization (SSSP) is a relatively new type of polymer processing which is gaining significant scientific and industrial traction [1].  SSSP is based on a commercial twin screw melt extruder (TSE), but applies a drastically different approach; instead of the traditional melting and compounding used in TSE, SSSP cools the material and instigates intense mechanical shearing and compression.  These characteristic solid-solid interactions have mechanochemical consequences (chain scission, chain-branching, inter-chain cross-linking, etc.), through which the material’s physical morphology and chemical structure are simultaneously altered.  A wide range of polymer-based materials can be effectively and efficiently produced [1-5].

Solid-state processing has one very important working principle: in order to induce sufficient levels of shear and compression to properly alter the material, the process must be at a low temperature, and the materials must be kept in the solid phase.  Relying on this principle is a difficult task, because the process involves an intricate balance of energy inputs and outputs.  To date, there has been little to no relevant experiments or theoretical modeling performed to study the detailed energy balance surrounding the SSSP system.  As a result, SSSP research has relied on past experience and trial-and-error work. 

Considering the potential of SSSP as a versatile technology to produce a wide range of polymeric materials, a fundamental understanding of the process is needed.  The present study aims to theoretically model the heat transfer characteristics in the SSSP system.  Fundamental chemical engineering principles of conservation and transport will be used to analyze the extruder system on different size scales.

A macroscopic energy balance has previously been developed for SSSP, and accounts for extruder power, chiller duty, and energy absorbed by the material [1].  The overall balance is applied here to analyze the effects of various material properties on energy consumption and heat generation.  Following the overall balance, a continuum mechanical approach is used to develop a better understanding of small scale transport mechanisms within SSSP.  This approach has frequently been used in the analysis of TSE, and is similarly applied here [6].  The equations of change for mass, momentum, and energy are used to provide the governing equations and relevant boundary conditions for SSSP. 

References

[1] Khait, K., & Torkelson, J. M. (1999). Polymer-Plastics Technology and Engineering, 38(3), 445-457.

[2] Lebovitz, A. H., Khait, K., & Torkelson, J. M. (2002). Macromolecules, 35(26), 9716-9722.

[3] Wakabayashi, K., Pierre, C., Dikin, D. A., Ruoff, R. S., Ramanathan, T., Brinson, L. C., & Torkelson, J. M. (2008). Macromolecules, 41(6), 1905-1908.

[4] Iyer, K. A., & Torkelson, J. M. (2014). Composites Science and Technology, 102, 152-160.

[5] Diop, M. F., Burghardt, W. R., & Torkelson, J. M. (2014). Polymer, 55(19), 4948-4958.

[6] Tadmor, Z., & Gogos, C. (2006). Principles of polymer processing (2nd ed., rev. ed.). Hoboken, N.J.: Wiley-Interscience.