Free radicals in the gas phase. ESR investigation of flame and microwave plasma of atomized gases. A contribution to flame retardation and polymer erosion mechanisms

Abstract

Various experimental techniques based on electron spin resonance (ESR) are presented for the detection of paramagnetic species directly in the ‘hot’ H 2/O 2 flame and in the ‘cold’ plasma produced by microwave radiation. The concentration of atomic hydrogen in the H 2/O 2 flame was traced by shifting the burner in the ESR cavity or by exhausting the H· atoms from different positions in the flame. In this way the ‘steady-state’ radical concentration during the change from the diffuse to oscillating and finally to a premixed flame is quantitatively determined and the optimal position for the most effective flame retardation with halogenated flame retardants is located. Diatomic gases, H 2, N 2 and O 2 under reduced pressure, and volatilized molecules of H 2O, D 2O, H 2O 2 and NH 3, are effectively dissociated to their atoms when they are exposed to microwave radiation of 2450 MHz. At constant power of microwave radiation (5 to 100 W) the highest concentration of atomized gases, 2–3 × 10 16 spin/cm 3, is reached in the vacuum interval from 0·8 to 1·5 Torr. With increasing pressure the atomized species gradually disappear as a consequence of recombination. The presence of HO· and HOO· radicals and the effect of the applied magnetic field on gas phase radical reactions were ascertained by introducing the spin-trapping technique, transforming the primary unstable radicals of the plasma to stable nitroxy spin-adducts of DMPO (5,5-dimethyl-1-pyrroline- N-oxide). The concentration relationship between the highly reactive HO· and HOO· radicals as products of the reaction ·O· + HOH → 2HO· was measured after freezing the radicals from the flowing plasma on the internal cryostat situated in the ESR cell and cooled with liquid nitrogen. Using a cross-flow experimental method, the kinetics of the rapid reaction between the colliding atomized gases with different molecules, introduced into the ESR cell from the opposite direction to the plasma flow, was studied. The reactivity of atomized oxygen with solid targets was measured directly in the ESR cavity. In this way the one-electron transfer from chelated cobalt(II) with a 3 d 7 unpaired electron to atomized oxygen to form the complex Co(III)O • was proved. Atomized oxygen can initiate surface crosslinking after its addition to a double bond when it is in contact with exposed films of natural rubber or polyisoprene.