Continuous Processing of High Thermal Conductivity Polyethylene Fibers and Sheets

A Lightweight Material for Heat Exchange Applications

Introduction Thermal conductivity is an important consideration in choosing materials for different manufacturing applications. For example, materials used in heat exchangers require high thermal conduc- tivity because heat must be transferred between substances located at a distance from one another. Historically, heat exchangers have been manufactured from metal because metals are effective in conducting thermal energy. Plastics, although cheaper, lighter, and less energy intensive to make than metals, are much Illustration of continuous processing of UHMW-PE sheets (or fibers) with highly aligned less efficient in conducting heat than molecule chains. UHMW-PE nanofibers are extruded through a template to form a continuous sheet (or fiber) and are stretched (not shown) further to align the molecules. metals. This project will combine the Graphic courtesy of MIT. valuable performance characteristics of both metals and plastics by developing a continuous production process for The sheet will then be reheated and stretched to enhance the molecular chain orientation ultra-high molecular weight polyethylene and create high thermal conductivity. A combination of process modeling and experi- (UHMW-PE) fiber and sheet production mental trials will be used to identify conditions that yield plastic sheets with high that will result in a thermally conductive thermal conductivity. plastic material. The thermal conductivity of the fibers and sheets is expected to be about two orders of A number of lab-scale studies have magnitude higher than bulk polymers and at least twice that of currently available fibers. shown that thermal conductivity is im- proved when polymer fibers are stretched Benefits for Our Industry and Our Nation mechanically. This increase is believed to Benefits are expected to be realized in both fabrication and end use. Energy intensity be a direct result of the higher molecular will be significantly lowered during material synthesis and processing because lower chain alignment achieved by stretching. process heating temperatures will be used. The result will be that production plants The aim of this work is to transform these for these materials will generate less pollution compared to metal processing plants. early laboratory results into a scalable These polymers could be a game changer for many heat exchangers, especially in fins production process. that require directionally high thermal conductivity values. They also could be used The proposed production process will in thermal management systems in microelectronics, where an electrical insulating involve using a spinneret specially de- material is preferred. High thermal conductivity sheets could significantly expand the signed for sheet formation to extrude an potential application for plastics and could help the U.S. plastics industry to expand its UHMW-PE gel at a chosen temperature competitive position internationally. From an end use perspective, polyethylene is 35% and shear rate. The resultant gel then will less dense than aluminum, making the material lighter per unit volume. For example, be exposed to a controlled temperature using these polyethylene sheets in automotive applications would reduce vehicle weight environment to partially evaporate the and improve fuel economy. solvent and solidify the gel sheet.

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Applications in Our Pathways Commercialization Nation’s Industry The primary task involves manufacturing There is strong interest from major This process could provide the plastics platform design and fabrication. The con- chemical and polymer producers re- industry a low-energy, cost-effective cept is based on the gel-extrusion process garding this high thermal conductivity method of producing high thermal and hot drawing. Efforts will focus on polyethylene material. The Massachusetts conductivity polyethylene material for fabricating and assembling the process- Institute of Technology (MIT) intends further use as a lighter and less expen- ing apparatus as well as the spinneret. to secure intellectual property for this sive alternative to metals in various heat Process modeling and optimization will technology, then determine whether to transfer applications. Process integration be conducted to guide the manufacturing license it directly to other companies or is simplified by existing knowledge and process considering four key variables: pursue another commercialization option. widespread use of the primary material mechanical strength of the spinneret, flow As has been the case for numerous MIT input (polyethylene). of the polymer through the spinneret, technology developments, there is also a polymer drying, and the optimal stretch strong possibility that a start-up company Project Description ratio. could be formed by project participants to further advance the technology. Project tasks will focus on manufacturing Researchers will characterize the platform design and fabrication, process molecular orientation of the polymer modeling and optimization, materials chains within the finished product using Project Partners characterization, and property charac- techniques such as X-ray diffraction, Massachusetts Institute of Technology terization and modeling. The project polarized microscope, scanning electron Cambridge, MA objective is to develop and validate a microscope, and transmission electron Principal Investigator: Gang Chen continuous manufacturing process for microscopy. Email: [email protected] polyethylene fibers and sheets yielding a thermal conductivity value greater than Researchers will also undertake prop- For additional information, 60 Watts per meter-Kelvin (W/m.K). erty modeling to determine the thermal conductivity of the polymer sheets, please contact specifically focusing on discovering the Bob Gemmer Barriers heat transfer characteristics between the Technology Manager • Lack of scalable processes for manu- polyethylene chains. These models will U.S. Department of Energy facturing high thermal conductivity also provide guidance for structure and Advanced Manufacturing Office polyethylene sheets. process optimization. Phone: (202) 586-5885 Email: [email protected] • Uncertainties associated with mo- lecular-scale chain manipulation and Milestones modeling. This project began in 2012.

• Understanding of the optimum condi- • Achieve the capability to fabricate 1x5 tions for proper polymer flow through square centimeter (cm2) polymer sheets the spinneret to limit or eliminate by means of the proposed gel-extrusion clogging. and hot drawing process (2013).

• Reach a thermal conductivity greater than 30 W/m.K for the 1x5 cm2 sheets (2014).

• Reach a thermal conductivity greater than 60 W/m.K for the 1x10 cm2 sheets (2015).

For more information, visit: manufacturing.energy.gov

DOE/EE-0869 • Updated September 2014