MadSci Network: Engineering

Re: why does styrofoam work in coolers and coffee cups?

Date: Sun Sep 24 23:06:42 2000
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Engineering
ID: 969493818.Eg

The heat content of a material is the measure of the total energy contained in the motion of molecules. It is a function of the heat capacity of the material, the amount of material, and the temperature of the material. This is assuming that over the temperatures of interest there are no phase changes or chemical reactions happening. The more material, the higher the heat capacity, and the higher the temperature the more heat a given material contains.

Heat transfer between two materials depends on the amounts of each material, the temperature difference between the materials, the contact between the materials, and the thermal conductivity of the materials. It also depends on many physical and chemical factors such as specific heat, shape, mixing, reflectivity and emmissivity, the presence of chemical impurities and many other factors that must be controlled in studies of heat transfer. The net heat transfer is from Hot [High Temperature] to Cold [Low Temperature]. However, this is a dynamic process because the colder material has some molecules with more energy than some of the molecules in the hotter material.

Heat transfer happens by three methods: 1. Conduction. 2. Convection. 3. Radiation.

Conduction is when the materials are in contact and the hotter molecules collide with the colder molecules and transfer kinetic energy in the process. There is minimal material transfer in this process and the thermal conductivity is the important factor. An example of conduction is when you actually touch a warm or cold object.

Convection is especially important in liquids and gases. The surface layers of a liquid or gas that are in contact are heated or cooled. When enough material is at a different temperature from the mass, the material at a different temperature flows under the influence of gravity and moves from the surface exposing fresh material to be heated or cooled.
Convection is a mixing process. By maintaining the temperature differential at the surface it increases the efficiency of conduction. Convection is the moving streams seen in a liquid when it is heated. It is the wind. An extreme case is the wind chill factor.

Radiation is when an excited molecule emits a photon of infrared energy that can leave the material thereby lowering the temperature when the molecule is reexcited thermally. These photons can be reabsorbed, absorbed or reflected by the other material, or totally leave the system. Radiation is the heat felt at a distance from a warm body. It is why your side facing the campfire is warm while the other side is cold. It is how sunlight lights and heats the Earth.

The polystyrene cup is a good insulator; it has become the poor man's Thermos Bottle. See How Thermos Keeps Things Hot and This means that polystyrene foam must reduce heat transfer by the three methods. Polystyrene, like most plastics, has relatively low heat conduction. Polymerizing the styrene as a gel with a gas lowers this heat conduction further. This gives small granules of polystyrene foam, Styrofoam, which has extremely low heat conduction. These granules are sintered into shape which leaves more air spaces between the granules [occasionally the sintering job is poor and the cups ooze liquid]. This construction means that air molecules separate the Styrofoam granules. Air has very low heat conduction. This reduces heat transfer by conduction in the polymer. Since the gas is in the tiny microbubbles heat is not transferred by convection in the foam. The low heat conductivity is probably the most important insulating quality.

White polystyrene is a good diffuse reflector of radiation emitted by the hot liquid inside. The heat from the radiation that is absorbed remains on the surface and is partially transmitted back to the liquid. Since the outside of the cup remains cooler there is less heat loss by all three methods.

The rough surface from its semigranular nature may inhibit convection by slowing flow at both inside and outside surfaces thereby slowing heat transfer to the cup inside and from the cup outside.

Demonstration:Heat transfer is an increase in entropy. This should apply nicely to a group of children who are actively increasing the entropy of the Universe.

10-20 children - they will be molecules
2-4 Helpers - they are the surfaces
2 large clear bowls - these are the HOT and COLD materials
50-100 6 inch red construction paper squares. Write a large block letter or number on each and fold in 4 so the letter is not visible - these are heat.
10-20 red paper airplanes with tips cut off [so no one gets poked] - these are radiation.

Make a children molecular array [3x5,4x4,3x3 whatever works best]. Have each child stand on a dot on the floor. Space the kids so they can touch each others’ hands. This is a good heat conductor. Have them start vibrating by reaching out and touching their neighbors' hands. Give each child in the row nearest the hot bowl a heat particle. They want these but must quickly give them to any neighbor who doesn't have one without moving from their spot or they will be burned. This is conduction. When some reach the far side the helper puts them in the cold bowl. After several accumulate the helper gives some out from the cold bowl. Occasionally give a child an airplane [this is now an excited child uh, molecule, who will soon emit a photon] which they are to fly upwards. If any child can reach an airplane without leaving his spot he may fly it. After a short time the red squares should be moving in both directions and be distributed evenly and airplanes should be all over the room. Stop the Game and discuss what happened.

This array is a good conductor. To make a poorer conductor move the rows apart so each child has to walk to the next child and return to his spot. To make an insulator have the rows separated, slow down the initial distribution of heat and have each child open his heat, read the letter or number inside, refold it, and give it to a near neighbor. When the child has a heat particle on each side he must then deliver to the next row. Each recipient must open, read, and close his heat. Also distribute airplanes.

To demonstrate convection, use either the open or closed array and give each child in the first row a heat particle have them join hands with each other and with the next row and walk in a circle so that the rows change places. Then quickly give the cold 1st row heat while the second row conducts to the third. Then try to do both at the same time. Also distribute airplanes randomly.

Rules: A molecule can receive or give heat in any direction. Only molecules without heat can receive heat. The surfaces [helpers] can direct the heat flow by passing out more or less heat. In an open array give to a near neighbor before leaving your spot. Heat moves in all directions so molecules with no heat must try to get heat from someone else.
Heat also burns so those with heat must give it away as quickly as possible. This is a chaotic process!
Airplanes should be flown up and can be picked up and reflown only without leaving one's spot unless one encounters a plane when walking to deliver heat.
Use the good conductor as a guide for how long to run each type.

Several searches gave little useful information on Styrofoam. I hope this explains it a bit and you can simplify for your young scientists.

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