Tetrafluoroethylene

Explosiveness

Description of key information

The minimum Ignition temperature of explosive decomposition (MITD) was determined to be 205°C at 30 bara for stabilised TFE, 230° at 20 bara for non-stabilised TFE, and 220°C at 20 bara for air contaminated TFE.

Additional information

Tetrafluoroethylene is a chemically unstable gas which can decompose explosively, without any addition of air or other oxidisers. Because of its highly dangerous nature, in many countries, it is not allowed to fill TFE at pressures of more than 0.5 MPa (5 bar) in gas cylinders. For storage and transport of TFE a stabiliser, often a-pinene, a radical interceptor, is added.

Lisochkin and Pozynak (2006) in their paper on the explosivity of TFE in the absence of air or oxygen state that there have been no reported TFE explosions in industry at pressures below 0.6 MPa (6 bar), whereas the explosion probability at pressures above 1 MPa (10 bar) is rather high. However, in experiments with high power igniters, these authors observed TFE deflagration at pressures down to 0.1MPa (1 bar). They suggest there are no industrial explosions at pressures < 0.6 MPa (6 bar) because the probability of emergence of ignition sources sufficient for initiating TFE deflagration under these pressures in processing equipment is almost zero.

The dimerisation reaction of TFE to c-octafluorobutane plays an important role as an initiation reaction for the explosive decomposition (Beckmann-Kluge et al, 2008).

C₂F₄→0.5 c-C₄F₈   

ΔH_R= -103 kJ mol^-1(Dimerisation)

C₂F₄→CF₄+C  

ΔH_R= -257 kJ mol^-1(Decomposition)

The Federal Institute for Materials Research and Testing, Berlin, Germany (BAM) (2005), in a study sponsored by Plastics Europe, investigated the Minimum Ignition temperature of decomposition (MITD) - the lowest temperature of a hot surface e.g. the vessel walls, at which self-ignition for the decomposition of a chemically unstable gas or gas mixture occurs.

The dimerisation kinetics were determined by help of pressure-time curves at isothermal conditions in the pressure range 1 bara to 20 bara. The second order Arrhenius constants for the dimerisation were found to have activation energies and frequency factors that are reported in Table A below.

K = A.e^-EA/RT

Table A: Kinetic constants of the second order exothermic dimerisation reaction

TFE	A cm^3mol^-1s^-1	EAkJ mol^-1
Stabilised	8.3 x 10E10	105
Non-stabilised	9.5 x 10E10	106
Air contaminated	3.0 x 10E10	101

Minimum ignition temperature values of decomposition (MITD) were determined in heatable empty 3 dm³vessels for stabilised, non-stabilised, and air contaminated TFE at initial pressures up to 30 bara. The temperature increments were 10 K. To confirm the MITD at least 3 tests were executed at the last temperature increment below the MITD.

Table B: Minimum Ignition Temperatures of Decomposition (MITDs) of TFE measured in an empty 3-dm³vessel.

Initial pressure (bara)	MITD stabilised TFE (°C)	MITD non-stabilised TFE (°C)	MITD air- contam TFE (°C)
2.5	350	380	390
5.0	310	310	310
10.0	270	270	270
20.0	240	230	220
30.0	205	n.m.	n.m.

n.m. = not measured.

According to the simplified Semenov theory the pressure influence on MITD can be calculated as follows:

MITD_stabTFE  =  

5980 / (ln p + 8.673 ) - 273.15  

MITD_non-stabTFE  

=  4545 / (ln p + 6.050) - 273.15

MITD_TFE-air  

=  4079 / (ln p + 5.276) - 273.15  

where MITD is given in °C and p bara.

The presence of air contamination and the stabiliser α-pinene seems to be of little importance for hot surface ignition processes.

The minimum Ignition temperature of explosive decomposition (MITD) was determined to be 205°C at 30 bara for stabilised TFE, 230° at 20 bara for non-stabilised TFE, and 220°C at 20 bara for air contaminated TFE.

References:

Lisochkin YA, Poznyak VI, 2006. Explosive hazard estimates for several fluorine-containing monomers and their mixtures, based on the minimum ignition pressure with a fixed igniter energy. Combustion, Explosion, and Shock Waves 42(2), 140 -143

Beckmann-Kluge M, Krause H, Schröder V, Acikalin A, Steinbach J, 2008.Study of a self-heating process of tetrafluoroethylene by the exothermic dimerization reaction to octafluorocyclobutane. Proceedings of the COMSOL Conference 2008, Hanover.

Justification for classification or non-classification

The minimum Ignition temperature of explosive decomposition (MITD) was determined to be 205°C at 30 bara for stabilised TFE, 230° at 20 bara for non-stabilised TFE, and 220°C at 20 bara for air contaminated TFE. At lower pressure (2.5 bara) the MITD rose considerably e.g. 350°C for stabilised TFE. In the absence of air or oxygen there have been no reported TFE explosions in industry at pressures below 0.6 MPa (6 bar). In conclusion, elevated temperature and pressure are required to initiate explosive decomposition of TFE and thus classification as Explosive according to EU Directive 67/548/EEC and EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation EC No. 1272/2008 is not warranted.