OH MY GOD!!!! are you wanting to be next in line for the Darwin awards!!!!
Gainer's hypothesis isn't entirely flawed. I see one oversight though. The water heating scenario makes the incorrect assumption that microwave radiation heats similarly to convection--that is to say evenly. In fact, radiation tends to heat very unevenly, leaving hot-spots and cool-spots in the subject medium. Water tends to distribute heat very evenly through it's volume. When water is placed inside of a microwave and exposed to radiation, the exposure does not create lasting hot spots. Hot-spots ARE created, but they rapidly dissipate, especially on removal of the radiation.
I'm sure we've all experienced putting a delectable food item into the microwave and guessing at the time. When the item is removed, you must check it to make sure that it is evenly heated, particularly with frozen items, there will be a few spots that have thawed, but others that are still frozen solid. The new microwaves with the turn tables help, but it's still an issue.
I am not sure of the precise properties of propylene glycol, but I'm willing to bet that if one were to put it in a microwave for very long at all, hot-spots would quickly develop. Although the average temperature of the substance might not reach the flashpoint, different areas within the container would rapidly approach them.
I would venture to say that using a microwave would be more dangerous than an open flame, with a microwave, it's much more difficult to gage whether or not the propylene glycol is about to boil over, or to get any real indication of how hot it might be. Again, even if one was to measure the temperature of the propylene glycol while it was in the microwave, one could only get an average temperature.
I've exploded a number of things in a microwave, mainly peeps, grapes, and hot dogs. Propylene glycol would definitely be ranked low on my list of things to try.. right next to gasoline.
Last edited by jacobj; 01-07-2008 at 05:40 AM. Click to view previous post history.
Reason: Typo's galore
Also, you can buy an electric hotplate at Walmart for $20 which will warm at gentle temperature. With a little diligence you can buy a nice heated Corning stirrer on eBay for a fair price and it is a wonderful luxury for mixing chemicals...EC
Originally Posted by Photo Engineer
When I mix Pyrocat-HD in PEG, I just mix in a bottle and heat in tap water. Seems like not much is needed if I just can wait a while. I swirl it around gently and don't shake it, not to introduce too much air into the liquid.
So your advice would be not to use the microwave to reheat a pizza or sub or to cook anything that might be flammable. I'm surprised that you do not know the definition of flashpoint. Don't tell me that having used the test apparatue to measure flashpoint shows that you know what it means. What, for instance, is the flashpoint of diesel fuel? Is that the temperature required to ignite a fuel-air mixture in the cylinder by the heat of compression?
Originally Posted by Kirk Keyes
Heating water was not for the purpose of learning what happens to the liquid, but to show that the microwave walls, floor and ceiling were not heated in the process of heating a liquid to the flashpoint of glycol, and demonstrating it safely.
The flashpoint of propylene glycol is close to the boiling point of water. Its autoignition point is 700 F. If there is no external ignition source, it will not ignite until it reaches that temperature. It boils at 372 F, at which point the vapor pressure at the liquid surface equals atmospheric pressure. It is a little lower here as I am at about 900 ft. altitude. That means that in order for autoignition to occur, the vapor must be at 700 F or must come in contact with an object at that temperature. Furthermore, there IS such a thing as partial pressure of glycol vapor in the atmosphere. It is dependent on the concentration of glycol vapor in air, and the ignitability of that vapor by a spark or open flame will depend on that concentration. There is a partial pressure gradient from the surface of the glycol upward. That is why the spark or open flame must be near the surface if it is to ignite the vapor at the flashpoint. The gradient is diferent in the closed cup test than in the open cup test. The situation in the microwave is that of the open cup tester. If there is a spark somewhere in the microwave, what conditions will allow it to ignite the surface of the container of glycol? The partial pressure of glycol vapor will have to be the same as it is at the surface of the glycol at its flashpoint. In order for that to happen, the glycol will have to be at a much higher temperature than the flashpoint.
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Of course a microwave manual is written to avoid unsafe conditions, misuse, etc., but they usually say not to run one empty (or with paper towels). I think it may have something to do with the empty oven space functioning as a microwave 'cavity' and is part of the RF load on the magnetron.
A friend (now an electrical engineer) once heated a jar of caramel ice cream topping and didn't realze it had a foil label. He came back to find a shattered jar & a mess to clean up.
Certain he was smarter than the average microwave, he soaked the next jar in water to remove the label. This one he observed thru the glass door. It started to boil over, then developed a purple corona then shattered again.
I'm reminded of the fun we used to have experimenting at school with the student union microwaves.
We submitted a variety of metallic and food items to irradiation endurance tests.
We found that metallic items close to the walls didn't create fireworks (forks, thin flat foil ashtrays), but all hell broke loose with larger items close to the center of the the cavity (cafeteria napkin holder).
Something to do with standing wave behavior at the walls. Not sure how many conclusions can be drawn from heating behavior at the bottom of the microwave.
Canada Mints were fun (kind of like those 4th of July 'snakes') and relish packets would clear the building.
Regarding glass, ceramic molecule size, I always associated microwave 'safeness' of vessels to be an issue with how 'lossy' a material they were made of. I don't know if this is related to molecule size or something else. I have had some ceramics that said "microwave safe", but the contents didn't heat as well and the ceramic came out hotter than other vessels. (Clay mineral content?)
Similarly, some polymers are terrible insulators at certain frequencies and melt in certain installations (nylon at vhf and higher for example, in antenna structures). Is a nylon molecule smaller than a Teflon one...maybe. Does the polymer chain length matter?
Last edited by Murray@uptowngallery; 01-07-2008 at 11:29 AM. Click to view previous post history.
Peeps in the microwave are a thing to behold.
If the vapour comes in contact with an object at or above its flashpoint, it can ignite. So, your statement above that "the vapour must be at 700F or must come in contact with an object at that temperature" is in error as both Kirk and I have repeatedly pointed out to you in posts.
The lightbulb in my microwave could reach the flash point temperature during the heating process! IDK, but I don't want to run that test.
I have boiled most organic solvents over my years in the lab, and I always used a steam bath or insulated electric hot cup to keep from igniting vapors. I also had a condenser over the flask in most cases to insure that there was no vapour escaping from the flask or container while heating. In spite of all of this, I have had at least two serious fires in the lab. I don't recommend that experience for anyone.
Originally Posted by pesphoto