Pat, it has nothing to do with the "partial pressure of glycol at the hot surface" here, it's simply the concentration of the flammable vapour in air that is important and an ignition source. You're trying to make things too complicated...
Originally Posted by gainer
I know you love MSDS sheets, you may want to look closer at one for propylene glycol. Try this one: http://www.jtbaker.com/msds/englishhtml/p6928.htm Look in Section 5 - fire fighting info. You will see right after the flashpoint and the autoignition point values, it gives the info for explosive mixtures of propylene glycol in air. It says, "Flammable limits in air % by volume: lel: 2.6; uel: 12.5" That's a lower explosive limit (lel) of 2.6% propylene glycol in air, and an upper explosive limit of 12.5%. Not enough vapour in the air and there cannot be a fire or explosion, between the lel and uel and you can get a fire/explosion, and above the uel and there cannot be an fire/explosion.
PE's concern is you were not concerned about heating propylene glycol in a microwave to temperatures above the flashpoint. You said, "I question whether the vapor seen at about 250 degees is propylene glycol. The boiling point of the glycol is much higher than that. I think it is likely that the glycol had some water in it which distilled out." http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00Bhk2 You may want to think about that statement some more.
Flashpoint is a test used to measure the temperature at which a material will form an explosive mixture in air. I've done many flashpoint measurements of both waste materials and commercial products. I can tell you that when a sample has enough flammable material in it to give a strong flashpoint, you get flames several inches long coming from the closed cup tester. It's a lot of fun when they go. You'd like it.
The vapour from propylene glycol heaed to 250F is most likely from the glycol, and not water as you proposed in the thread I linked to above. Think about those flashpoint temps that you quoted above. "Thhe flash point of propylene glycol is 210 F by the closed cup method and 224 F by the open cup method." There must be at least 2.6% propylene glycol vapor present in the flash point apparatus in order for the glycol to flash. The liquid does not have to be anywhere near its boiling point. And do you have any suggestions as to why the closed cup flashpoint is lower than the open cup flash? Perhaps is has to do with the propylene glycol vapour being more concentrated at a lower temperature in the closed cup test vs. the open cup test, which as the names imply, the open cup test is done with a confined area above the material that is being tested while the open cup is open to the air.
So there can be enough propylene glycol vapour present at the temperature of 210 in a closed cup in order for the vapor to catch fire or even explode given a suitable ignition source, like a spark or an open flame. (And yes, I agree with you that the glycol will not autoignite at the flashpoint temp and that it does need an ignition source.)
Anyway, heating propylene glycol in a microwave does present a hazardous condition similar to that in the closed cup flashpoint tester and it should not be recommended due to safety concerns. We can't know that there is internal sparking in the microwave that could present an ignition source. (And I have had a microwave melt down and catch fire breifly due to failed electronics. I'm glad it did not contain a flammable vapour in it as well when it failed.)
And I'm glad that you did eventually agree that propylene glycol did not need to be heated to such high temperatures in the p.net thread - even though you agreed because you decided that such high temperatures were bad for dissolving ascorbic acid, and not because it was a safety issue...
PS - it would be fun to see a microwave blow itself up by heating a flammable material above the flashpoint. Perhaps we can get the Mythbusters guys on this one.
Last edited by Kirk Keyes; 01-03-2008 at 02:28 AM. Click to view previous post history.