What is it about?

This paper explores the use of bromine as a novel oxidant to functionalize polyethylene prior to carbonization, leading to efficient cyclization and carbonization on heating. Through a process of low-temperature photo-induced liquid-phase bromination followed by dehydrobromination at moderate temperature and subsequent carbonization to 800 °C, a carbonaceous product was obtained in high carbon yield (>90%). The effects of a range of process conditions, as well as the use of different types of polyethylene and polyethylene blends, were explored.

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Why is it important?

Carbon fibres can have high stiffness, high tensile strength, chemical resistance, lightweight, temperature tolerance and low thermal expansion. Hence, they are valuable as reinforcements in composites for demanding applications, where lightweight and high strength are needed. The current production of carbon fibres is dominated by the use of special grades of solution spun (non-melting) polyacrylonitrile (PAN) fibres as the resin source. However, due to the high cost of PAN and the associated complex solvent spinning manufacturing process, the production costs of polyacrylonitrile-based carbon fibres are high, with a current market price of around US $21.50/kg. Polyethylene as a polymeric precursor for carbon fibre production has attracted much attention over recent years. Not only is it a low-cost commodity polymer, it has a higher theoretical carbon yield (at 100%) than polyacrylonitrile (at 67%) and is very readily melt processed into fibres. The challenge is to efficiently dehydrogenate and graphitize polyethylene fibres.

Perspectives

PE has excellent physical and mechanical properties at low density and it is melt-processable. Its properties have been well studied for more than 50 years. Based on the results obtained, this strategy of preparing brominated polyethylene will be an efficient way of producing carbon from a commodity polymer polyethylene and has the potential of recycling polyethylene into value-added carbon fiber. Future work should focus on understanding the mechanisms involved in the dehydrobromination and subsequent pyrolytic processes of these brominated polyethylenes, so as to optimize the overall process. A detailed analysis of the kinetics of this process using in-situ mass spectroscopy would be of value.

Dr Pratheep K Annamalai
University of Queensland

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This page is a summary of: Pyrolysis of brominated polyethylene as an alternative carbon fibre precursor, Polymer Degradation and Stability, February 2020, Elsevier, DOI: 10.1016/j.polymdegradstab.2019.109057.
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