[Nova]

Hello,

So this is primarily a materials question. Which polymer would be the best to contain an exothermic reaction lasting 5 hours at a temperature of roughly 80°C. It would need to have a low friction constant, but cheap (expendable) and have a high melting point with great flexibility. Also which exothermic reaction would be best.

[Enthalpy]

Hi Nova, welcome!

Which exothermic reaction: no idea, there are so many. Do you have more requirements on it? Nontoxic, cheap, self-regulating temperature...? A worry is that most reactions give a quantity of heat, at a poorly predictable pace, and the temperature depends on how well the heat is evacuated, so +80°C could also mean "between +30°C and +120°C" or even "some location see +180°C" and then it isn't the same difficulty at all for the polymer. In that aspect, a solidification would be better than a reaction.

80°C for 5 hours is rather easy. The cheap polypropylene could still work, it's more or less its limit. PVC should work and has all materials and tools to assemble by hot glue, but isn't very flexible. Among the more expensive polymers, nearly everyone accepts +80°C.

Then, friction... It depends brutally on the temperature. (Expensive) PTFE for instance has a low coefficient but over some +50°C it climbs to banal 0.3-0.5. Polypropylene has a low coeff at room temperature.

Maybe you can cover your polymer with a sheet of well-gliding material? Films exist (Du Pont and others) with an adhesive layer and a PTFE or FEP layer for instance. Or try to find a spray.

http://www.bpf.co.uk/Plastipedia/Default.aspx
http://www.matbase.com/
http://www.matweb.com/
check the "deflection temperature" or "operating temperature", not the fusion temperature.
If you read German, a fantastic compensium is
Kunststofftabellen, by Bodo Carlowitz

[Enthalpy]

Solidification is a good way to obtain heat at a predictable temperature. Among the compounds, paraffins are safer, cheap, efficient, and offer a broad choice. They're well-known and broadly used.

Hey, will you improve the hot-water bottle? More heat capacity or a smaller bottle would be nice! Just mind the expansion at fusion.

[Enthalpy]

A first means to limit the paraffin's temperature when melting it in the hot-water bottle: take a compound or mixture that melts below +100°C and heat it in boiling water.

Second means, adequate for a microwave oven: mix a conductive (alloy) or semiconductive (graphite) powder in the paraffin. Upon melting, the paraffin expands a lot, interrupts the contact between the grains, the resistivity rise at once, the paraffin gets no hotter. This spreads the temperature evenly over the paraffin.

Silicone packages exist already for hot water bottles. Accommodate the expansion of melting paraffin.

Marc Schaefer, aka Enthalpy

[hollytara]

There is a reason they make Silicone Cookware....