Classification of oils

Vegetable oils are triglyceride esters of fatty acids of chain length C12 to C24 but mainly C18. These fatty acid chains can contain up to 6 carbon-carbon double bonds and each fatty acid has its own name. The common ones found in the vegetable oils that we supply are as follows:-

C14:0 Myristic acid
C16:0 Palmitic acid
C16:1 Palmitoleic acid
C18:0 Stearic acid
C18:1 Oleic acid
C18:1 OH Ricinoleic acid
C18:2 Linoleic acid
C18:3 Linolenic acid
C18:3 CONJ Eleostearic acid
C20:0 Arachidic acid
C22:1 Erucic acid
Oil and Seeds

These fatty acids are bonded in groups of three linked by a glycerine molecule to form a triglyceride as shown below:-
Chemical structure of triglyceride
Where RI, RII, RIII represent fatty acids which may or may not be the same.

If an oil consists of triglycerides with a high percentage of unsaturated fatty acids such as linolenic acid then that oil will be reactive and is known as a drying oil. This means that when applied to a non-porous surface it will dry to a film in a certain time. For example linseed oil contains 50 - 60% Linolenic acid (carbon 18 chain with 3 double bonds, C18:3) and will usually dry in less than 4 days without additional processing or the incorporation of driers.

Towards the other end of the scale is rapeseed oil. This contains 52 - 62% oleic acid, C18:1 (carbon 18 chain with just 1 double bond, C18:1). Therefore the oil is not as reactive so will not dry to a film when exposed to air. It is consequently classed as a non-drying oil.

A measure of unsaturation is iodine value. The higher the iodine value, the higher the level of unsaturation and more reactive the oil is. Non-drying oils have low iodine values (less than 125) whereas drying oils have high iodine values (more than 150). Oils with an IV between 125 and 150 are known as semi-drying oils.

The table below shows how our oils are classified:

Oil Main fatty acid present   Iodine value Classification
Linseed Linolenic (51 - 60%) C18:3 175 - 195 Drying oil
Wood/Tung Eleostearic(78 - 82%) C18:3 Conj 158 - 180 Drying oil
Safflower Linoleic (70 - 80%) C18:2 135 - 148 Semi-drying oil
Soya bean Linoleic (50 - 56%) C18:2 129 - 143 Semi-drying oil
Sunflower Linoleic (56 - 70%) C18:2 112 - 138 Semi-drying oil
Rapeseed L/E Oleic (56 - 62%) C18:1 100 - 125 Non-drying
Rapeseed H/E Erucic (45 - 55%) C22:1 100 - 120 Non-drying
Arachis Oleic (35 - 45%), Linoleic (35 - 45%) C18:1 C18:2   85 - 105 Non-drying
Castor Ricinoleic (83 - 90%) C18:1 OH   82 - 90 Non-drying

Drying and semi-drying oils find uses in surface coating applications where drying properties are essential, eg paints, resins, printing inks.

Non-drying oils, generally, find uses in applications where oxidation is undesirable, eg lubricants, leather dressings.