What is the relation between ceiling temperature and degradation temperature?
Ceiling temperature (Tc) is a measure of the tendency of polymers to revert to their monomers. When a polymer is at its ceiling temperature, the rate of polymerization and depolymerization of the polymer are equal. Polymers with high ceiling temperatures are often commercially useful. While degradation temperature is the temperature at which the polymers chains start the thermal pyrolysis and lower molecular weight products are formed. Both these transitions are important. The former helps in polymerization while the later helps in deciding the processing and use temperature of the polymers.
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Temperature of decomposition (Td) and ceiling temperature (Tc) both have to do with the thermal stability of polymers. At Tc, rate of polymerization and depolymerization are equal - so below Tc polymer is thermodynamically stable, while above Tc the polymer is thermodynamically unstable. This does not mean that polymer cannot exist above Tc - certain poly(olefin sulfones) used as electron beam resists take advantage of this. The polymer is prepared at low temperatures (-77 C). Room temperature is above Tc, but depolymerization requires a chain scission to begin - thermally this does not happen until 200 - 300 C (that would be the Td). But if a chain break is created by electron beam radiation, the chain rapidly depolymerizes to the highly volatile monomers (for example, 1-butene and SO2).
Td is more commonly used - in TGA it is the temperature at which decomposition starts. There is some dependence on rate of heating. Most organic polymers decompose between 200 and 300 C, but a few are stable up to 500 C.
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