Polytetrafluoroethylene thermal degradation

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

It should not be thought, however, that perfluorocarbons are completely inert toward combustion. Even the very inert perfluorocarbon polymer polytetrafluoroethylene [PTFE, Du Pont s Teflon F(CF2CF2)nF] is thermodynamically unstable in oxygen with respect to CO2 and CF4 (Exercise 12.6) and can burn in a 95% 02/5% N2 mixture at 0.1 MPa, although combustion is hard to initiate because of the nonvolatility of PTFE and the resistance of the thermal degradation products to oxidation. Conflagrations involving more reactive, volatile fluorocarbons such as perfluoro-toluene have been reported.15... 

The thermal stability of polytetrafluoroethylene oxide and PTFE have been compared under the same conditions by Donato et al. [263] between 450 and 600°C. The decomposition rate has a maximum at 628°C for the oxide and at 568°C for PTFE. The activation energy for the first-order degradations are 98 kcal mole"1 between 8.5 and 85% for the oxide polymer and 85 kcal mole-1 between 523 and 571°C for PTFE. The rate of weight loss is less than 1.2% per min for both polymers below T = 550°C for the oxide and T = 590°C for PTFE. The oxide, however, loses weight below 390° C whereas PTFE does not. The main components of the volatile material are trifluoroacetyl fluoride, carbonyl fluoride and tetrafluoroethylene. An end-initiated thermal degradation with small zip length is proposed. 

Thermal decomposition of a y-irradiated poly( vinyl fluoride) occurred in two main steps firstly, elimination of hydrogen fluoride and secondly, main chain scission to yield unsaturated hydrocarbons." Polytetrafluoroethylene and the copolymer of tetrafluoroethylene and hexafluoropropene were degraded in various atmospheres and the decomposition products analysed." - For an inert atmosphere over twenty different fluorinated products were identified. For air the major products were COF, CF , and COj with minor amounts of fluorocarbons. The gaseous and solid decomposition products have also been analysed from the thermal degradation of poly(carbon monofluorides) containing different proportions of fluorine. Kinetic data were also obtained. 

Materials were not directly tested for their hazardous effects but it is assumed that their toxicity is similar to component materials which under normal conditions of use are not hazardous ingredients. It should be considered that polytetrafluoroethylene emits highly toxic fumes when thermally degraded at high temperatures. 

---