Ceiling temperature

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As a ceiling temperature of English ceiling = "ceiling", is referred to in the plastics engineering , the temperature at which the polymerization and depolymerization to proceed at the same rate and apparently no reaction takes place more. The ceiling temperature is important for most chain polymerizations .

description

The individual reaction steps in a polymerization are in most cases reversible, that is, they are reversible. The reverse reaction is depolymerization. With increasing temperature, the reverse reaction can proceed faster and faster. At a certain temperature - the ceiling temperature - both reactions take place at the same speed. For the observer, the reaction has stalled. Since it is a dynamic equilibrium , reactions are still taking place. However, back and forth reactions run at the same speed, so that the reaction has apparently come to a standstill.

Thermodynamic consideration

From a thermodynamic point of view, polymerization only takes place spontaneously - after the activation energy has been overcome - if the free enthalpy of reaction ΔG ( Gibbs energy ) is negative. This applies to polymerizations as well as to any other chemical reaction (see thermodynamics # energy calculations in thermodynamics ). ΔG results from the difference between the molar enthalpy of reaction ΔH and the product of the absolute temperature T with the molar entropy of reaction ΔS:

During polymerization, the degree of order increases as the number of molecules decreases and a phase transition to a phase with a higher state of order takes place. For example, a monomeric gas becomes a polymeric solid. The entropy thus steadily decreases during the polymerization, the value ΔS is negative, as is the value of ΔH (exothermic reaction). As long as the negative enthalpy term ΔH is greater than the positive entropy term −T · ΔS, the reaction proceeds in the direction of polymerization. If the temperature is increased, the entropy term increases. At a certain value of T, the enthalpy of reaction is equal to the entropy of reaction and ΔG becomes zero. Polymerization and depolymerization are now exactly in balance. The reaction seems to come to a standstill. If the temperature is increased further, the depolymerization (reverse reaction) predominates and the polymer formed is degraded.

The ceiling temperature can also be calculated using the following equation:

Here [M] c is the monomer concentration and R is the universal gas constant . Apart from the monomer, the ceiling temperature is primarily dependent on its concentration (see above equation). T C is therefore often related to the concentration. The ceiling temperature is proportional to the pressure at which the (de) polymerization reaction takes place. If the pressure increases, the ceiling temperature also increases.

The counterpart to the ceiling temperature is the floor temperature (from English floor = "floor"). This is the minimum temperature required for polymerization to take place. This very rare case is possible when the ΔS of a reaction is positive, i.e. the reaction is endoentropic (increasing entropy) and the reaction enthalpy has very small values ​​(positive, negative or zero). This is the case with the polymerization of sulfur rings, selenium octamers or octamethylcyclotetrasiloxane (OMS) to form the corresponding linear polymers.

Examples of the ceiling temperature of various monomers
Monomer −ΔH in kJ mol −1 −ΔS in JK −1 mol −1 T C in ° C
Isobutylene 54 165 50
α-methylstyrene 34 102 61
Tetrahydrofuran 17 to 21 71 to 84 85
Methyl methacrylate 54 110 220
Propene 71 124 300
Styrene 71 122 310
Ethene 92 137 400
Tetrafluoroethylene 193 226 580

Technical importance

The ceiling temperature is an important parameter, especially in the production of polymers. It also plays a certain role in polymer processing. If the ceiling temperature is exceeded , for example during extrusion , the polymer can be affected. However, the ceiling temperature is by no means synonymous with the maximum service temperature of a polymer. A remote from the reaction mixture polymer - this is referred to as the equivalent of a "living polymer" and a "dead polymer" ( dead polymer ) - can be stable and must not depolymerize necessarily above the ceiling temperature as long as there is no active ( " living ”) end group is formed with which a“ living ”depolymerization can start. Such end groups can easily be formed by thermal effects, photolysis ( UV light ), the biradical oxygen or other suitable chemicals. Overall, the degenerative degradation of polymers is a very complex process that is influenced by a large number of parameters. In many cases, the degeneration can also take place at temperatures below the ceiling temperature, for example through light or oxygen.

Individual evidence

  1. ^ A b c G. G. Odian: Principles of Polymerization. John Wiley and Sons, 2004, ISBN 0-471-27400-3 , pp. 79-81 ( limited preview in Google book search).
  2. ↑ a b c Georg Menges, Edmund Haberstroh, Walter Michaeli , Ernst Schmachtenberg: Material science plastics. Hanser Verlag, 2002, ISBN 3-446-21257-4 , p. 380 ( limited preview in Google book search).
  3. ^ Josef Houben, KH Büchel, Theodor Weyl, Eugen Müller, HG Padeken: Methods of Organic Chemistry (Houben-Weyl). Part 1, 4th edition, Verlag Georg Thieme, 1987, p. 5.
  4. ↑ CH Bamford, CF Tipper: Non-radical polymerization. Verlag Elsevier, 1976, ISBN 0-444-41252-2 , p. 331 ( limited preview in Google book search).
  5. ↑ JE Brandrup, EH Immergut, EA Gulke: Polymer Handbook. 3rd edition, Wiley-Interscience, 1989, ISBN 0-471-81244-7 , p. 316.
  6. ↑ CH Bamford, CF Tipper: Non-radical polymerization. Verlag Elsevier, 1976, ISBN 0-444-41252-2 , p. 103f ( limited preview in Google book search).

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