Re: Is styrofoam a better insulator than a thermal bag?

Hi Sheri.

Since I don't have many details about the bag or the Styrofoam, I'd have to do the comparison experiment, and maybe you should too. The most important characteristics are the thermal conductivity of the materials, and the tightness of their seals & seams. The physical phenomena we are most concerned with are heat transfer by conduction, and mass transfer by diffusion.

Thermal insulators work by slowing the transmission of heat. See http://en.wikipedia.org/w iki/Heat_transfer for a summary of the different modes by which heat can be transferred.

A simple barrier like a plastic Tupperware box would slow the transmission of heat by convection and diffusion, but not slow conductive transfer by much. Styrofoam is a much better barrier to conductive heat transfer, a bit ironically, because it is full of air. See http://greenanswers.com/q/34763/products-shopping/manufacturing- materials/why-styrofoam-such-good-insula. Air is a very poor conductor of heat. Air circulation would, of course, speed up convection, but the air in the pockets of the Styrofoam is fairly trapped and can't circulate easily. A vacuum is an even better insulator to conductive heat transfer. That’s why in Thermos vessels, there is a double glass layer with a vacuum in the middle. Since there are so few atoms in the vacuum, there is not much way for heat to be conducted across it.

I mentioned mass transfer by diffusion. If there are any holes in your Styrofoam container or thermal bag, then the hot air from the car will be able to diffuse or “seep” right into your container. Before, I was just talking about heat transfer across a solid barrier. But a hole would mean heat transfer by mass transfer, because the barrier has been breached.

For your experiment, I would compare some different container materials and thicknesses: metal, plastic (Tupperware), maybe a double Tupperware (one inside another), Styrofoam of different thicknesses, plus your thermal bag. How you choose to measure the heat transfer will also be important. You can’t keep opening the containers because then the seals will be broken and the hot air will get in. Ideally, you would use a thermocouple and seal it tight so you could measure the temperature using a display on the outside without worrying about holes or breaches or having to open your container. If you can’t do that, I would pick a set amount of time after which you would open all the containers and see how much melting took place inside. You might have to adjust the time if you find all the lipsticks were melted (time too long) or none of them were (time too short). Finally, when you do the experiment, make sure you seal up the containers first in a cold room, then place them in the hot environment. Any initial contamination inside with hot air will ruin the experiment.

I hope all this was helpful. If you do the experiment, please report your results back to MadSci. Here are some more MadSci links on related topics:

http://www.madsci.org/posts/archives/2000- 09/969929543.Eg.r.html
http://www.madsci.org/posts/archives/2004- 02/1076960486.Ot.r.html
http://www.madsci.org/posts/archives/2000- 01/948765589.Ph.r.html
http://www.madsci.org/posts/archives/2005- 09/1126214053.Eg.r.html

-Alex Tobias