VARNISHES, CHEMISTRY OF. The drying oil most commonly used is linseed oil, obtained from the seeds of linum usitatissimum containing 35-40% of oil, cultivated in temperate zones, especially in Russia, Hungary, France, the United States, Canada and North India. This oil consists of mixed glycerides (q.v.) of linolenic, linolic, and oleic acids, with small quantities of palmitic and stearic. The glycerides are derived from open chain acids containing 18 carbon atoms which absorb oxygen in proportion to their unsaturation to give a gel (linoxyn) consisting of the oxidised glyceride in a colloidal form. Another drying oil now in demand is China-wood oil (tung oil) extracted from the fruit of Aleurites fordii or Acordate (China and Japan) contain ing the glyceride of another 18 carbon-atom open-chain acid (elaeostearic acid) isomeric with linolic or linolenic acids. The annual production of this oil is ioo,000 tons, compared with 700,000 tons of linseed oil.
In the drying of linseed or tung oil, oxygen is at first slowly ab sorbed with a considerable induction period, and subsequently more rapidly, and then progressively diminishing as the process approaches completion. At the ordinary temperature 2o-3o days are required for linseed oil with an induction period of 1-3 days. At ioo° C, 67 hours are required, with an induction period of less than a hour. The linseed oil acids behave similarly, but the oxidised films are of inferior durability. If small percentages (o.i--o.3%) of lead, manganese or cobalt as linoleates or resin ates be present, the period of oxidation is much reduced. A peroxide is formed in both absence and presence of the catalyst, and this undergoes change to a polymerised modification in which the glyceryl group remains intact but molecular rearrangement with loss of the peroxide and condensation have occurred. The metals and peroxides function as oxygen carriers, and comparison with similar systems would indicate polymerisation of a mon oxide type of glyceride or condensation according to the follow ing scheme: Treatment of Drying Oils.—The treatment of drying oils for use in paints and varnishes comprises: (I) preliminary refin ing of the crude oil to remove mucilage and part of the colouring matter, and (2) a further treatment which may either thicken the oil, or partially oxidise it or incorporate in it metallic driers (lead, cobalt or manganese) so that when applied as a film the oil will oxidise and harden rapidly. The refining may be effected
by neutralisation of the oil with sufficient alkali in the form of carbonate or caustic, whereby the soap formed carries down with it the mucilage (foots) and a fair amount of the chlorophyll colouring matter, but the use of sulphuric acid to the amount of 2% is cheaper, whereby the mucilage is coagulated in small flakes, and the oil purified by washing with water.
The thickening of drying oils may be brought about by heat (260° C) out of contact with the air, whereby association or polymerisation of the unsaturated components of the glycerides ensues. Such heat-thickened oils are known as "litho" or "stand" oils. If tung oil be heated similarly, thickening is very rapid, and unless controlled a spongy insoluble gel is produced with great evolution of heat and charring. Stand oils are still un saturated and dry to elastic and durable gels in the presence of metallic driers, yielding a more durable film than linseed oil. Sometimes air is passed into the drying oil containing small quantities of cobalt or manganese drier between 6o° and 12o° C, whereby partial oxidation takes place together with thickening, but these "blown" oils are inferior to stand oils in the durability of their films.
Boiled linseed oil is frequently preferred to raw linseed oil. It is obtained by heating the oil to about 9o° C, adding driers in the form of lead, cobalt or manganese linoleates or resinates (o.i% Pb or 0.03% Mn or Co), and raising the temperature to 120° C. Agitation with air is continued for 2-3 hours, and the oil allowed to cool. It should dry in the form of paint in hours. Extra-pale, pale boiled, and double boiled are varieties according to colour and viscosity.