Polymers, burning combustion region

To understand the burning of polymers better, the combustion region may be divided into five zones, as shown in Figure 5.1. The first zone is defined by the polymer layer where pyrolysis takes place due to heat produced in the flame, but very little oxidation takes place. Thermal oxidation takes place in the second superficial zone. In the third zone, which is a gaseous one, low-molecular-weight products formed in these two zones are mixed with heated air and are decomposed or oxidized by oxygen or by radicals coming out of the flame. 

FIGURE 5.1 Schematic representation of the combustion region of a burning polymer. 2006 by Taylor Francis Group, LLC... 

Azide polymers such as GAP and BAMO are also used to formulate AP composite propellants in order to give improved specific impulses compared with those of the above-mentioned AP-HTPB propellants. Since azide polymers are energetic materials that burn by themselves, the use of azide polymers as binders of AP particles, with or without aluminum particles, increases the specific impulse compared to those of AP-HTPB propellants. As shown in Fig. 4.15, the maximum of 260 s is obtained at (AP) = 0.80 and is approximately 12 % higher than that of an AP-HTPB propellant because the maximum loading density of AP particles is obtained at about (AP) = 0.86 in the formulation of AP composite propellants. Since the molecular mass of the combustion products. Mg, remains relatively unchanged in the region above (AP) = 0.8, decreases rapidly as (AP) increases. 

On the basis of theoretieal equilibrium studies, phosgene formation from the burning of poly(vinyl chloride) is considered most favourable when the polymer combustion takes place in a limited excess of oxygen in an intermediate temperature region (with a lower limit of 800 K), and the gases of combustion are maintained between 600 and 850 K for the longest possible period [202]. 

---