[Glo]

Hi, I'm new to this forum and this is my first post

I was browsing the Internet and I found a thread about polymers pyrolysis:

http://www.energeticforum.com/renewable-energy/7040-how-turn-plastic-waste-into-diesel-fuel-cheaply.html

Guy there states, that just due to temperature of about 400C polymer chains are being broken into shorter chains and he obtains hydrocarbons from diesel fuel range (10-15 carbons in chain). My question is: how is that possible, as all papers that I've read until now states, that not only high temperature is required, but also pressure of ~12MPa at least for successful decomposition of polymers. What chemical reactions are involved? It would be awesome if someone could give a short description of what happens (from a point of view of a chemist) to those polymers in the device from the link.

[Arkcon]

A quick Google confirms that pyrolysis of polythene in the range of 350 C will produce liquid fuel, and temperatures of 500-700 C produce gases that, when swept out of the reactor, produce comparable gaseous fuel, see this pdf: http://staff.aist.go.jp/tohru-kamo/FSRJ/output/7_nenkai/02/ISFR99/PP-2.pdf.  So high pressure isn't a requirement.  That forum you linked to doesn't mention the GC analysis and comparison to known fuels, like the peer-review article does, but you might be able to find a reference at a local university library, especially since the author of the forum posting mentions that there are Japanese models of his device on the market.

[curiouscat]

Guy there states, that just due to temperature of about 400C polymer chains are being broken into shorter chains and he obtains hydrocarbons from diesel fuel range (10-15 carbons in chain). My question is: how is that possible, as all papers that I've read until now states, that not only high temperature is required, but also pressure of ~12MPa at least for successful decomposition of polymers.

Devil is in the details. What conversions? What yields? etc. Maybe it works at  a lower pressure just not as well or as fast?

[fledarmus]

Most important of all, which polymers? Remember that polymers are organic compounds, albeit very, very large organic compounds. They have functional groups and undergo reactions based on those functional groups just like any other organic compounds. If you know the structure of the polymer, you can make some pretty good guesses on what types of reactions it will undergo, under what conditions.

[Glo]

Ok, I made some further reading and it seems that it in fact is not necessary to apply high pressures but it accelerates the process. In this paper:

http://www.vurup.sk/sites/vurup.sk/archivedsite/www.vurup.sk/pc/vol48_2006/issue1/pdf/PC_1_2006_Sojak.pdf

the decompostion of polyethylene with a whole list of products.
How about reacions undergoing in that process? Is it just that bonds between within the chains break due to the hight temperature or is it a more complex set of reactions? If I apply higher temperature, will I get shorter chains, as Arkcon said that 500-700C would result in gas products and in the experiment described in the paper it was about 450C?

[fledarmus]

For the most part, these are simple free-radical processes carried out under inert gas conditions so there is no oxidation of the products. C-C bonds are being broken homolytically, then the free radicals are quenched by typical H· transfer reactions. In the last paper you cited, polyethylene chains (-CH₂-CH₂)_n are broken down into mostly much smaller short-chain alkanes and terminal alkenes resulting from
 CH₃-(CH₂-CH₂)_x-CH· + CH₃-(CH₂-CH₂)_y-CH₂·    CH₃-(CH₂-CH₂)_x=CH₂ +  CH₃-(CH₂-CH₂)_y-CH₃

Polypropylene has an extra methyl group branched off of each -CH₂-CH₂- group, so the number of branched chain possibilities increases dramatically. Since chain branches also provide opportunities for more stable radicals, there are a lot of other proton and methyl radical transfer reactions that can take place, further multiplying the number of possible products.

These "cracking" reactions are typically carried out using palladium or platinum catalysts, which greatly reduce the amount of activation energy required to homolytically cleave the carbon-carbon bonds. The increase in entropy caused by the reduction of a single enormous polymeric molecule into a large number of much smaller fragments drives the reaction.