Acceleration of the Oxidation of Drying Oils.
BY A. LIVACHE.
When a drying oil, previously treated with litharge or finely divided metallic lead is agitated with a solution of zinc sulphate, all the lead is precipitated from the oil, but the latter now holds zinc in solution. By using manganese sulphate, copper sulphate, etc., in this way, every trace of lead is removed from the oil, but the lead is replaced by manganese, copper, etc. If an oil charged with lead will dry in 24 hours when spread out in a thin layer on glass, it will dry completely in five or six hours if charged with manganese, in 30 to 36 hours with copper, zinc or cobalt, and in more than 48 hours with nickel, iron, chromium, etc.
It is more convenient to use the solid finely powdered salt in place of a solution, since the latter forms an emulsion with the oil. In fact, treatment with metallic lead and a solid sulphate may take place simultaneously, but in this case the oil will contain some lead in addition to the other metal.
Although solidification of a drying oil charged with manganese takes place in five or six hours when spread out in thin layers, the solidification of thick layers requires a long time, even though the protecting pellicle which forms on the surface is continually removed. Oxidation is more rapid at a higher temperature, but with thick layers a long time is required for complete solidification, even under these conditions.
A temperature of 50-60° accelerates the oxidation of drying oil, partly because the oil becomes more fluid, and partly because the oxygen is more active at a higher temperature.
When a manganiferous oil is dissolved in an equal volume of benzene, and agitated with air in a closed vessel, rapid absorption of oxygen takes place, especially at a temperature of 40-50°. If the air is continually renewed, so as to furnish the oxygen required for the maximum oxidation of the oil, the liquid becomes thick, and on distilling and separating the solvent a liquid is obtained which solidifies on cooling to a very dry and perfectly elastic solid. It is evident that by limiting the oxidation a series of products of varying viscosity, can be obtained intermediate between the original oil and the solid formed by maximum oxidation. The last product is characterized by its remarkable elasticity, and its absolute insolubility in water, alcohol, and ether. It is almost instantly saponified by potash in the cold, and on subsequent separation of the fatty acids it is found that the solid fatty acids have undergone no alteration, whilst the liquid fatty acid has almost completely disappeared, and has been converted into viscous products, characterized by their solubility in water and by the various salts which they form.—Comp. Rend., 97, 1311-1314; Jour. Chem. Soc., April, 1884, p. 532.