How does the ice in coolers stay cool?

When ice melts, heat must be absorbed from the surroundings. This is because energy is required to break the hydrogen bonds that pin the water molecules in place in ice. If you stop heat from being absorbed, the ice won't melt.

Coolers are just containers made of a material which conducts heat very poorly. Styrofoam (polystyrene extruded under high pressure) is often used. Two things make Styrofoam an excellent insulator:

1. Trapped gases. Gases are much poorer thermal conductors than solids or liquids. Air, for example, has a thermal conductivity of about 1/2000 that of metals and about 1/27 that of glass. So one way to improve the insulating properties of a solid material is to blow it into a foam. Styrofoam is a network of of plastic polystyrene chains with many tiny bubbles of trapped gas. The gas can't be air, since blowing air through the hot, flammable molten plastic in the manufacturing process is not a good idea. Originally, CFCs (compounds of chlorine, fluorine, and carbon) were used because of their low flammability and low toxicity. This made early Styrofoam manufacture environmentally unfriendly, since CFCs catalyze the destruction of ozone in the stratosphere. The HCFCs (compounds of chlorine, fluorine, carbon and hydrogen) used to blow Styrofoam today have a much lower impact on the ozone layer.
2. Large internal surface area It's quicker to walk straight across a lawn than it is to follow a winding sidewalk. Heat travelling along the polymer chains must go a longer distance to penetrate the Styrofoam if the chains make a large number of detours around trapped gas pockets.

These two qualities have been pushed to extremes in a newly developed material called aerogel. Aerogel is a solid network of silica that traps a large amount of air or other gases. Aerogels are sometimes called 'frozen smoke' because they are hazy, glasslike solids that are only a few times denser than air. The internal surface area of aerogels can be as much as 600 to 1000 square meters per gram; a pound of aerogel has an internal surface area of about 90 acres! Aerogels insulate so well that a 1 inch thick aerogel plate will protect a human hand from a blowtorch flame.

Visit the NASA/Marshall Space Science site to learn more about aerogels.

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/thermo/faq/thermal-insulators.shtml