Mg/PTFE

Ladouceur, H.D. (2005) An overview of the known chemical kinetics and transport effects relevant to Mg/PTFE combustion. 2nd International Workshop on Pyrotechnic Combustion Mechanisms, Pfinztal Germany, June... 

Classified work by Ladouceur on the reaction kinetics of Mg/PTFE has been summarized by Douda in 1991 [9]. Ladouceur concluded that CF2 is the main oxidising species in Mg/PTFE flames formed by fast dissociation of tetrafluoroethylene (TEE) (Table 5.2) [10]. He also found that neither E nor E2 play a significant role in the combustion process as there is no efficient and fast path providing either species. Table 5.2 gives three alternative thermokinetic coefficients for decomposition reactions of TFE and coefficients for reaction of either TFE or CE2 with oxygen. 

For the ternary system Mg/PTFE/Viton the adiabatic combustion temperature is depicted in Figure 5.7. The maximum temperature coincides with the basehne of the binary Mg/PTFE system at 32wt% Mg. Subsequent substitution of PTEE with Viton successively lowers the combustion temperature, by about 1000 K at 75 wt% Viton. 

Figure 5.8 Adiabatic combustion temperature of Mg/PTFE/air at 0.1 MPa calculated with NASA CEA [8].

There is good reason to assume that fluoro-Grignards form as part of the PIR in the condensed phase with Mg/PTFE and Mg/PMF (polycarbon monofluoride) [9, 29]. Samples of both Mg/PTFE and Mg/PME heated just above their observed PIR-onset temperature (500 and 520°C) show signals in the FTIR spectrum (Figures 6.2 and 6.3), which can be assigned to a C-Mg-F units. After further exposure of the samples at T > 700 °C, these stmctures disappear and characteristic vibrations for MgF2 are seen in both samples. 

Figure 6.2 FTIR spectra of Mg/PTFE pyrolant and residues at 600 and 700°C each [29].

Figure 6.7 DSC for Mg/PTFE pyrolant (30/70) with 45 pm Mg particles and 5 pm PTFE particle size, at 0.1 MPa pressure.

Figure 6.9 Thermal conductivity of Mg/PTFE based on either the Maywell or Rayleigh model [33].

Figure 6.11 Influence of pressure on gas-phase temperature of Mg/PTFE above surface. (After Ref [34].)...

The ignition of Magnesium/Teflon/Viton (MTV) by radiative or conductive heat transfer has been investigated in order to understand processes in both bulkhead and laser igniters (Table 7.1). Lombard appears to be the first to have tested radiative ignition of MTV. He used a solar furnace that allowed a maximum irradiance, H = 100 W cm [2]. Mg/PTFE (polytetrafiuoroethylene) samples of unknown stoichiometry were tested at = 20 W cm. The ignition temperature was determined from radiometric measurements and was found to be quite low, 217 °C. This corresponds to 29 s irradiation at the reported irradiance level and translates into a heating rate of 6 K s. ... 

An effect on the ignition delay is also affected by the inclusion of amorphous boron [9] (see Figure 7.4), which is known as poor thermal conductor X(B) = 27Wm A.(Mg) = 171 Wm K [11]. Figure 7.5 shows the change of thermal diffusivity as a function of the substitution of Mg by boron in Mg/PTFE. 

Figure 7.5 Influence of boron content in Mg/PTFE on thermal diffusivity of pyrolant [9].

The combustion behaviour of fuel-lean Mg/PTFE pyrolants (23/77) at reduced pressure (2-26kPa) indicates an incomplete reaction [12]. From another study,... 

The authors argue that the lower bum rate in air is due to competing oxidation that is less exothermic than fluoridation and, hence, influences the heat balance at the surface of the pyrolant. At higher pressures, these effects are compensated by better heat transfer as can be seen by the convergent lines in Figure 8.8 [13, 14]. Ladouceur explained the lower burn rate with preferred CF4 formation in oxygeneous atmosphere. CF4 reacts slower with Mg than that with CF2 and, thus, is held responsible for the observed lower burn rate [15]. The pressure dependence of a stoichiometric binary Mg/PTFE (32/68) formulation made with fine particles... 

Figure 8.9 Pressure dependence of combustion rate Mg/PTFE 32168) [4],...

Figure 8.10 Combustion rate of Mg/PTFE as a function of stoichiometry at 5.1 MPa [4],...

If the porosity increases, the permeability of the grain for gaseous reaction products increases as well. Cudzilo investigated the effect of density on the burn rate of both binary Mg/PTFE pyrolants Mg4Ah/PTFE and could demonstrate a significant effect of porosity on the combustion rate (see below) [22, 23],... 

Figure 9.1 depicts a combustion flame and typical morphology of a Mg/PTFE (polytetrafluoroethylene) flame. The luminous cone designated a is dominated by continuum radiation and both fluorocarbon species. The outer aerobic combustion zone b is less optically dense and shows mainly molecular radiation of MgF, MgO, C2, CO and CO2. 

Figure 9.1 Morphology of Mg/PTFE combustion flame (A) and colour photograph (B). (Reproduced with kind permission from Volker Weiser.)...

Griffiths et al. have been the fost to investigate the UV-VIS spectra of Mg/PTFE flames at various stoichiometries and various pressures [12]. They identified Mg(g), MgF(g), MgO(g) and C2 species in these flames depending on the pressure regime. Table 9.2 shows the signals found for a stoichiometric Mg/PTFE composition (32/68 wt%) at different pressures. 

Table 9.2 Spectroscopic properties of Mg/PTFE (32/68 wt%) at different pressures.

Species Wavelength (nm) Mg/PTFE (32/68) MgH2/PTFE (34/66) MgBj/PTFE (48/52) Mg3N2/PTFE (57/43) Mg2Si/PTFE (44/56)... 

The IR emission spectrum of combustion of Mg/PTFE (5/95) under pressurized oxygen atmosphere (200-300 kPa) is depicted in Figure 9.32 (H. D. Ladouceur,... 

The midwave ambient atmosphere and pressure IR spectrum of a Mg/PTFE/Viton combustion flames is dominated by a strong carbon continuum superimposed from a series of molecular emitters such as H2O, HF and CO2 both in emission and absorption (Figure 9.33). As carbon dioxide emission is always partially absorbed by the cooler combustion gases, the typical so-called blue spike and red wing appear. At greater distances between flare and spectrometer, the absorption of radiation because of the atmospheric constituents becomes relevant as can be seen from the overall spectrum depicted in Figure 9.34, which has been recorded in 10 m distance to the flare. 

Figure 9.32 FTIR spectrum of a Mg/PTFE = 5/95 wt% flame in pure O2 at 200-300 kPa. (After data obtained from H. D. Ladouceur, personal communication. Ref [29].)...

Figure 9.35 Ciose-range (1 m) CVF-iR spectrum (2.4-6 nm) of combustion of Mg/PTFE, MgH2/PTFE, MgB2/PTFE, Mg3N2/PTFE and Mg2Si/PTFE in air. (Reproduced with kind permission from Voiker Weiser and Eveiin Roth [19].)...

In another study, binary Mg/PTFE (45/55) has been modified with unspecified amounts of various substances leading to higher radiant intensity (Table 10.6) [35],... 

In a parametric study, the effect of barium peroxide, Ba02, on binary pyrolant based on Mg/PTFE has been tested for its effect on radiant intensity and bum time [36]. 

Figure 10.23 Comparison of standard MTV decoy flare composition in 2-2.6 pm band with extruded Mg/PTFE/PS/DMPP composition [41].

Figure 10.25 Burn rate and normalized radiant intensity of binary Mg/PTFE (Fluon Gl) compositions [43],...

A comparison of binary Mg/PTFE and Mg Als/PTFE formulations is given in Table 10.9 [47]. Although the spectral efficiency is similar to that of Mg-based payloads, the higher burn rate leads to higher pointance. 

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