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The onset of thermal degradation dictates the maximum temperature at which a polymer can be used. It is an important limitation in how the polymer is manufactured and processed. For instance, polymers become less viscous at higher temperatures which makes injection moulding easier and faster, but thermal degradation places a ceiling temperature ...
These temperatures vary strongly with the type of polyethylene, but the theoretical upper limit of melting of polyethylene is reported to be 144 to 146 °C (291 to 295 °F). Combustion typically occurs above 349 °C (660 °F).
As polymers approach their ceiling temperature, thermal degradation gives way to complete decomposition. Certain polymers like PTFE , polystyrene and PMMA [ 44 ] undergo depolymerization to give their starting monomers, whereas others like polyethylene undergo pyrolysis , with random chain scission giving a mixture of volatile products.
HDPE is known for its high strength-to-density ratio. [4] The density of HDPE ranges from 930 to 970 kg/m 3. [5] Although the density of HDPE is only marginally higher than that of low-density polyethylene, HDPE has little branching, giving it stronger intermolecular forces and tensile strength (38 MPa versus 21 MPa) than LDPE. [6]
] Pyrolysis gas has a high percentage of heavy tar fractions, which condense at relatively high temperatures, preventing its direct use in gas burners and internal combustion engines, unlike syngas. The process is used heavily in the chemical industry , for example, to produce ethylene , many forms of carbon , and other chemicals from petroleum ...
Another example of thermal decomposition is 2Pb(NO 3) 2 → 2PbO + O 2 + 4NO 2. Some oxides, especially of weakly electropositive metals decompose when heated to high enough temperature. A classical example is the decomposition of mercuric oxide to give oxygen and mercury metal.
In other cases, such as polyethylene, depolymerization gives a mixture of products. These products are, for polyethylene, ethylene , propylene , isobutylene , 1-hexene and heptane . Out of these, only ethylene can be used for polyethylene production, so other gases must be turned into ethylene, sold, or otherwise be destroyed or be disposed of ...
Ceiling temperature is a measure of the tendency of a polymer to revert to its constituent monomers. When a polymer is at its ceiling temperature, the rate of polymerization and depolymerization of the polymer are equal.