Thermal directed metal oxidation

In some cases, a pigment s thermal and chemical resistance can be improved by the encapsulation of the pigment particles by an iasoluble, colorless layer of metal oxide or oxide—hydroxide, eg, siUca, Si02. The function of such a shell is to prevent direct contact and reaction between the pigment surface and the organic matrix ia which the pigment is dispersed (11). 

In 1990, Schroder and Schwarz reported that gas-phase FeO" " directly converts methane to methanol under thermal conditions [21]. The reaction is efficient, occuring at 20% of the collision rate, and is quite selective, producing methanol 40% of the time (FeOH+ + CH3 is the other major product). More recent experiments have shown that NiO and PtO also convert methane to methanol with good efficiency and selectivity [134]. Reactions of gas-phase transition metal oxides with methane thus provide a simple model system for the direct conversion of methane to methanol. These systems capture the essential chemistry, but do not have complicating contributions from solvent molecules, ligands, or multiple metal sites that are present in condensed-phase systems. 

It was concluded that the reaction mixture was thermally unstable if subjected to thermal initiation, and that temperatures of over 300°C were capable of attainment, when direct reaction between the metal oxides amd the chlorinated paraffins or their degradation products were possible. The origin of the heat energy necessary to increase the temperature of the milled batch from 55 to 91°C (the DSC-identified exotherm teperature) could not be identified [1],... 

CIR-FTIR spectroscopy provides a direct technique for studying in situ hydrous metal oxide surfaces and molecules adsorbed on these surfaces (37). By itself, FTIR spectrometry is a well established technique which offers numerous advantages over dispersive (grating) IR spectrometry (1) improved accuracy in frequency measurements through the use of a HeNe laser (2) simultaneous frequency viewing (3) rapid, repetitive scanning which allows many spectra to be collected in a small time interval (4) miriimal thermal effects from IR beam and (5) no detection of sample IR emissions (38). 

Wet mixes are usually dried before calcination. Calcination is performed continuously in rotary or tunnel kilns, or batchwise in directly fired drum or box furnaces. The temperature at which the mixed metal oxide pigments are formed can be reduced by adding mineralizing agents [3.75]. In the case of chromium rutile pigments, addition of magnesium compounds [3.81] or lithium compounds [3.80] before calcination improves thermal stability in plastics. 

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