Destructive distillation of polypropylene

Some years ago I collected together a large quantity of waste polypropylene (PP) plastic in the form of yoghurt pots for the purpose of pyrolysis. I had discovered a book section^[1] which detailed the products of thermal decomposition of polymers. This stated that the products of thermal decomposition of polypropylene were "...pentane (24 percent), 2 methyl-1-pentene (15 percent), and 2–4 dimethyl-1-hep-tene (19 percent)." It was on this basis that I planned to pyrolyse a quantity of polypropylene followed by fractionation for these high-value products. It seems, however, from my own experiments that the result under my conditions more closely resembled crude oil.

Earlier this month I finally got around to shredding about a third of them into small enough pieces to fit through the neck of a bottle.
I bought a cheap 500 mL stainless-steel water bottle and filled it as full as I could with chopped PP, stoppered it with a sparkling wine cork and set up for distillation with a gas burner. Once the charge of plastic had been exhausted, the cooled steel bottle appeared empty without any char, fortunately. I then refilled it, and repeated several times until I had destructively distilled all 660 g of chopped plastic. I found that cork was a poor choice of stopper for the high temperatures involved, and I turned to a sacrificial rubber bung instead. After about half a dozen runs (the whole lot), the rubber stopper was in no fit state for anything, and was thrown away.

My yield of pyrolysis oil was 650 mL, with a density of 0.701 g/mL at about 22°C (est.).

This oil was then simply distilled, and crudely fractionated into six fractions:
rt 2 L (est.)
35-85°C 50 mL
85-135°C 50 mL
135-155°C 70 mL
150-250°C (est.) 400 mL
250-300°C (est.) 40 mL
300+°C (est.) 40 g

My thermometer does not go above 150°C, hence estimated temperatures based on vegetable oil viscosity of the penultimate fraction. The fractions were dried over calcium chloride, with the higher boiling ones taking much longer to dry than the lightest. The final result was five crystal-clear liquids, darkening to olive-oil coloured for the 250-300°C fraction. The final "fraction" was the residue left in the flask, and is the colour of black treacle with a consistency of hard vaseline.

The first fraction to boil out of solution was not condensed by an ice-water-cooled double-surface Leibig condenser, I concluded that this was petroleum gas, comprising (presumably) ethylene, propane, propylene, butane, and butenes. I was not expecting this, so I cannot confirm or deny if alkenes were present in this gas.

Based on these results, I would propose that the pyrolysis oil of polypropylene serves a remarkably cheap and safe alternative to crude oil for educational purposes, and contains a full range of crude oil fractions, without carcinogenic aromatics like benzene. This later assertion is based on the C:H ratio in the feedstock plastic compared to the C:H ratio of aromatic compounds. A considerable amount of hydrogen would have to be released for this class of compound to be formed, which is unlikely under the conditions.

[1] C. L. Beyler and M. M. Hirschler, Chapter 7, SPE Handbook of Fire Protection Engineering, 2001, pp. 110–131.

Attachment: Beyler, Hirschler - 2001 - Thermal Decomposition of Polymers(2).pdf (165kB)
This file has been downloaded 474 times

My set-up for pyrolysis:

In my case, they're polypropylene:



I've done destructive distillation of polystyrene before, and that produces a completely different product with a very different and distinctive smell. The pyrolysis oil described here has a petroleum odour, similar to fuel (fuel meaning petrol, diesel, kerosene, etc.)

[Edited on 26-8-2019 by 12thealchemist]

Quote: Originally posted by 12thealchemist

contains a full range of crude oil fractions, without carcinogenic aromatics like benzene. This later assertion is based on the C:H ratio in the feedstock plastic compared to the C:H ratio of aromatic compounds. A considerable amount of hydrogen would have to be released for this class of compound to be formed, which is unlikely under the conditions.