Heterogeneous combustion polymers

Rastogi and Bisht (Ref 3a) made combustion studies on hybrid propints consisting of o-s m-and p-toluidine nitTates with aniline-formaldehyde polymer as solid fuels, and red fuming nitric acid as oxidizer. They found that the results fitted a burning rate equation of the type, = a (G)v, where a and v are constants and G is the mass velocity. The authors conclude that the heterogeneous combustion reaction is diffusion controlled, and its rate is dependent on particle size... 

As noted in Section 2.1, polymeric materials may be burned under various conditions which affect the location of the reaction responsible for heat generation during combustion. Heterogeneous combustion of polymers is also possible. 

HMX and RDX are energetic materials that produce high-temperature combustion products at about 3000 K. If one assumes that the combustion products at high temperature are HjO, Nj, and CO, rather than COj, both nitramines are considered to be stoichiometricaUy balanced materials and no excess oxidizer or fuel fragments are formed. When HMX or RDX particles are mixed with a polymeric hydrocarbon, a nitramine pyrolant is formed. Each nitramine particle is surrounded by the polymer and hence the physical structure is heterogeneous, similar to that of an AP composite pyrolant... 

One of the authors of this review observed heterogeneous ignition and combustion in air of the highly reactive polyvinylene, a polymer with an acyclic system of conjugated —CH = CH— bonds. After only minor heating, this originally black polymer becomes red and finally transforms into a light-yellow nonvolatile oxidized product. 

Moreover, the sensitivity of the effect of a flame retardant to the ambient pressure should also be taken into account. Flame retardants that are active only in the gas phase usually fail to affect the composition of the volatile pyrolysis products and the coke yield. In this case whatever the nature of the polymer, the flame retarding element is released into the gas phase during combustion the type of oxidant (O /Nj, N O/N ) strongly affects the flammability. On the other hand the effect of flame retardants active in the solid phase depends on the polymer nature, but is independent of the nature of the oxidant. Variations of the pressure of the oxidative environment affect the rates of gas-phase as well as heterogeneous (interfacial solid-gas) reactions. 

Certain polymers are able to bind metal ions by chelating. The heterogeneous oxidation during combustion is thereby affected. An example is the polymer formed... 

The available data concerning the effect of metal compounds on the pyrolysis and combustion of polymeric materials show that there is a great potential in the control of these materials, This field is still in its embryonic stage. Much is yet unclear. In many cases the analysis is rendered difficult because of the dual function of metal compounds (especially of the d- and f-types) in redox reactions. Their ability not only to inhibit but also to promote polymer decomposition manifests itself most clearly in concentration effects The specific effects of certain metal compounds on the ignition and combustion rate of polymers indicate that heterogeneous oxidation... 

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