Sciencemadness Discussion Board

Combustion of Propylene glycol - what is the product?

RogueRose - 28-2-2018 at 17:16

I've tried to find what the product(s) are when PG is heated to it's vaporization point in devices like E-cigs, and have been unable to find accurate data on this.

I've also been researching the use of glycerine and found that this may be much worse than PG when used in the same device as it's main product is Acrolein which is a horribly toxic chemical when inhaled in this manner - repeatedly and if worry some amounts.

I've found that acetaldehyde and formaldehyde are by products of either glycerine and or PG but it doesn't state which. Neither are healthy to be inhaling.

Can anyone tell me what the product is of burning PG in an O2 rich environment of 600-1000 degrees F?

Vomaturge - 28-2-2018 at 17:27

Wikipedia says that it's boiling point (not decomposition) is just 370 F.

https://en.m.wikipedia.org/wiki/Propylene_glycol

In an oxygen rich environment at 1000 F, I expect it would burn into CO2 and H20. At 600 F you might get wider partial decomposition products without combustion.
In an Electronic cigarette, my guess is that it would heat up to 370 F, vaporize (why else would they call it a vape?:)), and the gaseous PG would move away from the element and cool. No decomposition required.

Edit: I could see a small bit of the gaseous propylene glycol being further heated to make aldehydes. To fix this, an element could be made which is entirely immersed in a thin layer of fluid, which never exceeds the boiling point. In general, E-cigarettes are better for you than the burning kind, but that doesn't mean they are healthy.

[Edited on 1-3-2018 by Vomaturge]

RogueRose - 28-2-2018 at 18:15

Quote: Originally posted by Vomaturge  
Wikipedia says that it's boiling point (not decomposition) is just 370 F.

https://en.m.wikipedia.org/wiki/Propylene_glycol

In an oxygen rich environment at 1000 F, I expect it would burn into CO2 and H20. At 600 F you might get wider partial decomposition products without combustion.
In an Electronic cigarette, my guess is that it would heat up to 370 F, vaporize (why else would they call it a vape?:)), and the gaseous PG would move away from the element and cool. No decomposition required.

Edit: I could see a small bit of the gaseous propylene glycol being further heated to make aldehydes. To fix this, an element could be made which is entirely immersed in a thin layer of fluid, which never exceeds the boiling point. In general, E-cigarettes are better for you than the burning kind, but that doesn't mean they are healthy.

[Edited on 1-3-2018 by Vomaturge]


Well I've read a number of studies that state Acetaldehyde & Formaldehyde are formed by vaping and also the temps they publish are often much lower than what the combustion chamber actually is. I know some people build their own vaping devices and they are after VERY high temps for some reason (IDK the reasoning for this) as they are searching for resistance wires that will handle 1800F and they are using ceramic tubes and ~12v Li-Po power packs for this. I know they are using this for both nicotine and CBD's and other cannabis related products.

i know that Acrolein is supposedly present in fairly high amounts in the glycerine based carrier solvent for nicotine, and don't know if this can be controlled by different temps.

I can attest that the temp in the vaping chamber for e-cigs can vary greatly between brands and even from one package to the next. I've had ones that got so hot I had a blister from touching the end while others (of the same model/brand) would only get warm, and it was being used the same way. We've also seen the cig's exploding and burning through, so that seems to suggest that there is a wide range of temps and maybe QC isn't of highest priority in these devices.

I know that these things cause MUCH worse chest congestion than any cigarette causing extreme amounts of phlem. I'm trying to see if there is a chemical that would cause this or if there is something else in the process that is adding to the end result of the feeling of a constant state of pneumonia with bronchitis when smoking a pack a day only leaves the occasional "chest clearing".

So what I found the three main compounds which may be formed are Acrolein, Acetaldehyde & Formaldehyde and if they could cause some synergistic effect to cause severe lung congestion.

I'm concerned that they are marketing these things as a safe alternative to regular cigarettes and have been HEAVILY marketing towards youth (very young ones at that) in TV, movies, magazines and everywhere online - all hidden in some way as to not be specific as to the motive of the ads.

Vomaturge - 28-2-2018 at 19:33

My guess is that the chest congestion is because a bit of vape fluid droplets or vapors deposit inside the lungs or bronchial tubes. They then absorb water from the lungs via osmosis? If that happens you'd get a water/mucus/vape mixture in you're lungs, which may be produced too quickly to be coughed up completely, and or with a "texture" that the airways aren't used to clearing out.

Of course Irritating or poisonous compounds could also play a role, too. It would make sense that if the vapors are passed over too hot of an element, some would be decomposed, burned, and/or dehydrogenated. I. could see aldehydes, including the ones you mentioned, from PG. The effect could probably be minimized by designing a burner that only heats the solution as a liquid. For glycerol on the other hand, the boiling point is higher, and closer to the temperature where acrolein forms. So designing a liquid-phase heating coil (wrapped in a wick?) and also using PG fluid might lead to less decomposition. While this may not make a difference for chest congestion, it would still be worthwhile: IIRC, acetaldehyde, formaldehyde, and acrolein are all carcinogenic.

If I had to guess, people make the high temperature coils in hopes of getting more output of a stronger output really, they'd get more vapors, healthier, and maybe even stronger or better smelling and tasting ones, by delivering a LOWER powered heat source into a tiny amount of liquid. That also uses the energy only to bring it up to 370F, and to overcome the heat of vaporization. After that, no further heating is needed. The hard part would be keeping a small amount of liquid around the heater at all times.

In any case, underhanded marketing of nicotine (or other addictive substances) to children is NOT cool.

[Edited on 1-3-2018 by Vomaturge]

Bert - 28-2-2018 at 20:49

I recall some of the early adopters of biodiesel had the idea to use the excess glycerin they were left with from their operation as fuel in oil burners for process heat and space heating in their buildings.

I also recall that ending very badly for some when the burner was not perfectly adjusted/combustion gasses were not 100% vented away from people at their workshop and they were exposed to acrolein from combustion gasses. Can't find the reference right now.

https://www.atsdr.cdc.gov/MMG/MMG.asp?id=552&tid=102
Quote:

Acute Exposure

The mechanism by which acrolein produces toxic symptoms is not known, but the compound is highly reactive, cross-links DNA, and inhibits the activities of some enzymes (including cytochrome P450 and glutathione-S-transferase) in vitro by reacting with sulfhydryl groups at the active sites. It has also been shown to suppress pulmonary antibacterial defenses, to release oxygen radicals, and to react with proteins. Onset of irritation is immediate, but pulmonary edema may be delayed and respiratory insufficiency may persist for up to 18 months after exposure.

unionised - 1-3-2018 at 01:44

Quote: Originally posted by RogueRose  


Can anyone tell me what the product is of burning PG in an O2 rich environment of 600-1000 degrees F?

At the top end of that temperature range, carbon dioxide and water.
At the lower end, a lot of things are possible and none of them would be good for you.

clearly_not_atara - 1-3-2018 at 06:15

I was concerned about acrolein production as well, but the thing is that acrolein has such a strong odor and lachrymation that it would be impossible to "accidentally" inhale even a small amount of it. Acrolein is not some silent poison. Other compounds in that series, such as allyl alcohol, glycidol, acrylic acid, propanal etc are also unmistakeably foul. The fact that using an e-cig is not unbearably painful demonstrates by itself the absence of acrolein.

[Edited on 1-3-2018 by clearly_not_atara]

unionised - 1-3-2018 at 06:24

Well...
https://en.wikipedia.org/wiki/Bronchiolitis_obliterans

Diacetyl isn't likely (too many carbons) but pyruvaldehyde is a definite possibility and I' be worried about breahting a molecule that similar to diacetyl in large quantities.

Morgan - 1-3-2018 at 07:06

I wonder if the other substances in E-cig concoctions could in any way react with the glycol when heated, forming new compounds in trace amounts?

phlogiston - 1-3-2018 at 13:33

An interesting paper in this regard is "Solvent Chemistry in the Electronic Cigarette Reaction Vessel" by Jensen PR et al.
It is open access: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5307352/

All kinds of oxidation products form. The ones mentioned already, and numerous other aldehydes, ketones, diketones.

@Morgan, yes, hemiacetals are known to form from the reaction between aldehydes and the propylene glycol and glycerol.

Also, certain nitrosamines are known to form when NO2 reacts with nicotine.

@clearly_not_atara, consider that many e-cigarette users are former smokers and are used to inhaling irritating acrid fumes. Also, the concentrations of acrolein are very low, and may not even be noticeable, or may even be desirable to some extent to users that are seeking a flavour that is similar to that of an analog cigarette.

clearly_not_atara - 1-3-2018 at 17:00

Tobacco smoke is not actually that irritating. I did not cough at all the first time I smoked a cigarette, which is in stark contrast to eg marijuana smoke or for that matter DMT. Marijuana by contrast still made me cough even after the thousandth time. The infamous "smoker's cough" appears only after tobacco has done significant damage to the lung. Additionally cigarette-naive users do not complain of irritation using e-cigarettes, generally.

I do not know if low, subthreshold doses of acrolein are toxic. Such concentrations would have to be very low indeed. I've never heard of acrolein as a major constituent of cigarette smoke; it is too large to be entropically favorable and too unstable to be energetically favorable, in combustion.

Diacetyl seems magnificently unlikely. Methylglyoxal, perhaps, but lactate would be a much more favorable 4e oxidation product.