Carbonaceous species find widespread industrial use as carbon fibers (CF) for advanced composites, anode materials, and filtration media. In the carbon fiber industry, over 90% are derived from the air oxidation and carbonization of polyacrylonitrile (PAN) fibers.  Recent studies propose that a $5-8/lb CF would enable the growth of an emerging high-volume industrial market.  Although the conversion of low-cost polyethylene (PE) fiber to carbon fiber via the utility of concentrated sulfuric acid exists, such technology requires large-scale sulfuric acid usage and recycle.  To improve the economic position of PE to CF technology, we explored the viability of air oxidation, which could be capable of ‘dropping-in’ to existing CF plants based on PAN fibers. Above 200 °C, polyethylene will melt and subsequently oxidize to various aldehydes, ketones, alcohols, and acids, as well as experience significant radical chain-cleavage which leads to the generation of small molecular weight species, which then vaporize. In contrast, crosslinked polyethylene articles retain their shape and yield 20-40 % of black mass that is characterized as oxidized extended aromatics. After thermal treatment under inert atmosphere, 10-20 % graphitic species can be obtained.  Through exploiting the autoxidation mechanism of hydrocarbons, we sought to intercept known intermediates and prevent volatilization by introducing multi-functional species that would react with the nascent oxidized products.  This strategy is moderately successful by virtue of solid additives compounded into the resin, yielding carbonaceous products in up to 50% carbon yield. However, by introduction of ammonia into the oxidation environment, we were able to achieve up to up to 60-70 % carbon yield, with significant nitrogen incorporation.  After heating to 800 °C in an inert atmosphere, 50-65 % mass yield is obtained.  The described ammoxidation process has industrial and ecological potential to ‘up convert’ waste or low-cost polyethylene species to higher value carbonaceous products.

Shifting the oxidation pathways of polyethylene to provide carbonaceous species in high yield

2019 Fall National Meeting

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