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Why is Polystyrene a good insulator

  1. Mar 6, 2004 #1
    Im doing this coursework about what material can keep heat the longest. My test results show that Polystyrene was the best material, but why ? Why is polystyre a good insulator?
     
  2. jcsd
  3. Mar 6, 2004 #2

    marcus

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    hello Musse and welcome!
    there might be several explanations
    and I can't be sure what I am going to tell you is the best, but I can get the ball rolling in any case with a partial answer.

    Heat gets transferred by radiation, conduction, and convection.

    Dry air is a poor conductor of heat----it moves heat by convection.
    In a large air gap, like between the double walls of a house,
    convection currents will start up that circulate the air from contact with the warm inner wall to contact with the cold outer wall, so the circulating air physically carries the heat

    Most solids are better conductors of heat than still dry air. The thing with air (or some other gas) is you have to immobilize it, trap it in little bubbles or pockets, so it doesnt get started circulating in convection currents.

    So people use FOAMS because they break up the air into trapped
    pockets which obstructs convection. Or they stuff the air-gap between the walls with fluffy fiber, which also interferes with convection.

    And they try to keep out moisture because moist air is not such a good insulator (condensation and evaporation dont help matters)

    So I suspect you could go a long way towards explaining polystyrene as an insulator if you cut a sample open and looked at it under magnification. Is it a foam? If so how big are the bubbles?
    Is it low density (high proportion of dry trapped gas as compared to solid)?

    Another thing you might want to do is get some solid, unfoamy,
    polystyrene plastic and test it as an insulator compared with other materials.

    Even among solids there are differences in heat-conductivity---metal conducts better than glass and so on. It might be that even solid unfoamy polystyrene is comparatively good insulation.

    though I suspect that the main insulating effect comes from the gas bubbles in the foam.

    btw do you know what the gas in plastic foam is? is it nitrogen or air or carbondioxide or what? maybe someone here knows, I dont.
     
  4. Mar 6, 2004 #3

    FZ+

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    http://www.eere.energy.gov/consumerinfo/refbriefs/ed3.html

    A variety of gases are used. CO2, CFC, HCFC, air....
     
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