francis
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Resin - charring at the end of a distillation?
Hi,
I distilled the solvent (Aromatic 150: a high-flash aromatic naptha containing only aromatic hydrocarbons) from a sample of paint waste. The
distillate was milky-white, and drying over MgSO4 caused to to clarify. On the GC-MS the solvent identity was confirmed.
But...
Once all the solvent had distilled (so far as I could tell), the temperature dropped from 180 to 140 deg C (over 10 mins) and thick gas started to
pour out of the condenser and an orange liquid emerged.
I stopped the distillation, because I thought the resin was decomposing and I was concerned about the nature of the gases.
When I ran the orange liquid on the GC-MS, it contained mostly Aromatic 150, but also, some carboyxlic acids and esters (whcih aren't part of the
solvent - I took these to be decomposition products).
Later, a different sample of resin was heated, on a sandbath, to 260 deg C. No fumes came off, and the resin simply liquified.
If the resin was thermally decomposing at between 140 and 160 deg C in the distillation, why wouldn't it do the same at 260 deg C on the sandbath?
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bbartlog
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| Quote: | | the temperature dropped from 180 to 140 deg C (over 10 mins) |
What temperature? I'm guessing that this was the temperature in the head (i.e. the measured temperature of the gas just above the distilling flask),
not the temperature of the remaining goo in the flask itself. Seeing the head temperature drop when one fraction has come over and the next one has a
much higher bp is normal. In a case where something is actually decomposing I could easily imagine a small fraction of volatile gas carrying along
condensed droplets of something with a very high nominal bp (say 300C) at which point it would be very hard to say how much heat would be getting
transferred to the head. In such cases the measured temperature there would not be very informative, and the outflow would be a mist or smoke...
What was the heat source driving the distillation in the first case? Is it plausible that the temperature in the flask in the first case was higher
than 260C?
The less you bet, the more you lose when you win.
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francis
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Quote: Originally posted by bbartlog  | | What temperature? I'm guessing that this was the temperature in the head (i.e. the measured temperature of the gas just above the distilling flask),
not the temperature of the remaining goo in the flask itself. |
Right, it was the temperature in the head.
I used a heating mantle as the temperature source.
Yeah, it's possible the temperature in the flask was over 260C.
The BP range of the orange liquid (from the chromatogram) was 110 deg C (toluene) to nonanoic acid (254 deg C).
The last two compounds off the column were unidentified, but probably had BPs a bit higher than 254.
Here's what the solvent vapours looked like:
Here's what the vapours looked like during decomposition (I added the foil about midway through the distillation because the temperature was hovering
around 150 and the remaining solvent wasn't coming off):
[URL=http://imageshack.us/photo/my-images/442/dsci0923.jpg/]
The resin that I liquified on the sandbath was obtained by dissolving the original paint waste in heptane, then decanting the heptane (this removed
the paint solvent as a heptane-aromatic hydrocarbon miture, and left the viscous paint resin).
I spread the resin over a watchglass as thinly as I could, and heated it at 110degC for 7 days to drive off any heptane.
I had just added it to the sandbath in this photo, so it's still in solid form:
Is it possible that the resin, having cured with heat and time, was more resistant to degradation than the resin in the distilling sample?
[Edited on 23-5-2012 by francis]
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