Oil oxidation

Ordinary linseed oil is characterized by hydrophilicity and poor resistance to light and oxygen, which manifests itself in rapid yellowing and decomposition of the membranes of this binder.
A measure of the deterioration of oil coatings is the oxygen number (especially for undeveloped oils).
Concentrated oils have a low oxygen number.
Oxidized oil must be dehydrated or treated with sulfur chloride or another product that causes cross-linking of the internal structure of the binder.
Oxidized but further undehydrated oil contains a significant number of hydroxyl groups, which cause moisture sensitivity.
Oxidation makes the oil chemically resistant to oxygen and weak alkaline solutions and water, and increases the spreadability, gloss and smoothness of the paint film.

Oxidation process

Air is passed through the oil at room temperature or 100 ÷ 150°C. Oxidation of the oil's glycerides then occurs, increasing its specific gravity and thickening it.
Such oil is harder and shinier when drying and, unlike ordinary oil, gives a permanent emulsion with water.
Oil thickened by heat dries more quickly than oil thickened at room temperature.
Oxidation is accompanied by the release of heat. If this process is not artificially inhibited, a solidified gel, prone to spontaneous combustion, is obtained.
The transformation of oil into a gel cannot be completely inhibited even after only a short oxidation, because due to the presence of peroxide groups in the oil, the thickening process continues to advance during prolonged storage of such oil.
The difference between the process of artificial oxidation and spontaneous drying of oil is only the difference in the degree of oxidation and polymerization of the final product.
Spontaneous desiccation of oil leads to the formation of linoxin, a product with eucoloid characteristics (only poppyseed oil shows a deviation in this respect). Artificial oxidation, on the other hand, produces peroxy groups that can react with the double bonds of the same or another oil molecule to form a dioxohexane ring, which either thickens the molecule internally or links two oil molecules to each other through their acid chains.
The bonding of individual linseed oil molecules can take place via their peroxy groups, which are formed by the action of oxygen from the water that is released.