Mass spectrometric study of combustion and thermal decomposition of GAP

Abstract

Glycidyl azide polymer (GAP) is an active energetic binder in rocket propellants. The main objective of this research was to study the decomposition and combustion chemistry of thoroughly characterized GAP samples to develop a model for the combustion of GAP and propellants based on GAP. The combustion characteristics (burning rates, temperature profiles) and kinetic parameters (order of reaction, activation energy, pre-exponential factor of rate constants) for the thermal decomposition of GAP together with the composition of the products of both the combustion and decomposition of uncrosslinked GAP (with a molecular weight of 350 or 2000) and cured GAP were studied. The flame and thermal decomposition of GAP, as well as the composition of the products were studied using molecular-beam mass-spectrometry (MBMS). The final temperature of a flame of GAP was measured as 1000 to 1100 K. About half of the mass of the combustion products involves large fragments of a polymer without its azide groups. For this reason the mass spectrum obtained on direct MBMS sampling of a flame burning GAP could not be completely interpreted. However, ∼47% of the mass of the combustion products was found to be the volatile gases N2, H2, CO, CO2, CH4, C2H4, C2H6, NH3, H2O, acetonitrile, acrylonitrile, and furane, as obtained by mass-spectrometry using freezing/thawing in a liquid nitrogen trap.The thermal decomposition of thin films of GAP at 1 bar was done in a flow reactor with Ar flowing through it. A tungsten plate was used as a sample heater; its temperature was controlled using a chromel-copel (copel is an alloy of 56.5% Cu, 43.0% Ni and 0.5% Mn) or Pt-PtRh (10%) thermocouples. The thermal decomposition of GAP was studied at a high heating rate over a wide temperature range in three ways: (1) the heating rate was changed from the maximal to the minimal one in the course of decomposition (400–100 K/s); (2) at the linear heating rate (50–400 K/s); (3) fast heating (∼400 K/s) to the given temperature and subsequently maintained isothermal. Three stages of thermal decomposition were found. The first stage (yield of nitrogen is ∼15%) is a first order reaction. The second stage (yield of N2 is ∼25%) is an autocatalytic one; the third stage is first order and is a weakly exothermic one, with a yield of nitrogen of ∼60%. Kinetic parameters (activation energy and pre-exponential factor of rate constants) were found for each stage. The results for both the combustion and thermal decomposition of GAP were compared with literature data and it was concluded that the results strongly depend on the conditions of the experiment and on the source of the GAP.