Re: Plastics are poor conductors of heat so why do they melt when they are heat

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Have you ever heard the expression "you are comparing apples and oranges?" That is the case with this question. Let's consider the observations behind this question:
1. Plastics conduct heat poorly. (This is true; plastics in general conduct heat more poorly than metals)
2. Plastics melt at low temperatures. (This again is true; compared with most metals, many plastics melt at relatively low temperature, although some plastics do not melt at all.)

Now, the problem is if we attempt to connect these two observations. The statement "if something does not allow heat to pass through it, does it melt at low temperature?" is a hypotheses. There are two ways that we might examine a hypotheses to see if it may be correct. First, we might make a few observations to see if there is data that supports our hypotheses. Second, we might examine the theory or underlying mechanism to see if there might be a theoretical basis to support our hypotheses.

So lets make a few observations. Although the initial question was posed about plastic, there is an implication that it should generally apply. Do all materials that conduct heat poorly have a low melting point, and do all materials that conduct heat well have a high melting point?

Lets look at aluminum metal. It has quite good thermal conductivity (around 236 W/mK), and a melting point of 660C. Aluminum oxide, Al2O3, has a thermal conductivity of around 2 W/mK, and a melting point of 2072C. I found this data in the CRC Handbook of Chemistry and Physics. So we see that aluminum oxide has around 100 times poorer thermal conductivity than aluminum metal, but a melting point that is over 1400 degrees higher than aluminum metal. This is one example that clearly refutes our initial hypotheses.

Lets look at the mechanisms of melting and the mechanisms of heat transfer. A solid material is held together by the chemical, physical, and electrical bonds between adjacent atoms and molecules. Some materials have strong bonds between atoms. A diamond, for example, is essentially a single carbon molecule, since every carbon atom is chemically bonded to its neighbors. Because of the strong bonds between the carbon atoms, diamond, and essentially all forms of carbon, much reach very high temperatures (above 3000C) in order to melt or convert directly to gas. A polymer consists of very long chains of hydrocarbon molecules. In some plastics (such as epoxies), there is chemical bonding between chains. These types of plastics will not melt at all; when heated they just chemically break down to form a black mass of mostly carbon. Other plastics, such as polystyrene (which we use to make pill bottles) and polyethylene (used in milk jugs and hot melt glue) do not have any chemical bonding between chains. The long chains are attracted by hydrogen bonding, which is not very strong. When heated, polyethylene and polystyrene turn into very thick liquids. Polyethylene is interesting; after it has been melted and then cooled, several minutes pass between the time that the polyethylene becomes solid and its eventual conversion to a translucent, crystalline material. Most polymers have thermal conductivities in the range of 0.1 to 1 W/mK.

Heat is transferred in a gas largely by a hotter, faster-moving molecule hitting a cooler, slower-moving molecule. The hotter molecule transfers some energy, and cooler molecule adsorbs some energy, and they both come closer to the same temperature. Heat transfer in solids is not easy to explain, since there appear to be several mechanisms. There is, to some extent, the same transfer of kinetic energy between adjacent molecules. There may also be heat transfer through free electrons. http://www.lytron.com/support/heat_transfer.htm provides a easy, quick overview of heat transfer. There are also subatomic particles called phonons that may also be involved in heat transfer. Frankly, the exact mechanisms of heat transfer in solids is the subject of ongoing research and discussion. However, it appears clear that the mechanisms of melting and heat transfer are not closely related.

So, heat transfer is oranges, melting point is apples. And it appears that they are not closely related.