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Posted on Tung Oil

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101 posts in 5205 days

#1 posted 10-25-2007 11:03 PM

Hello Brad,

Here is a bit of detailed information to help you understand Tung Oil. Tung oil is obtained from the seed kernels of the Tung tree, Aleuritis fordii (Chinese tung oil) or Aleuritis cordata, syn. vernica and verrucosa (Japanese tung oil). The principal source of raw tung oil is China and South America. The nuts of Aleuritis montana, Aleuritis trisperma (kekunaoil) and A. moluccana or A. triloba (lumbang oil) also produce oils with properties that are similar to Chinese tung oil.

In 1298, Marco Polo reported that tung oil together with lime had been used for impregnating and sealing wooden ships. As early as 1894, tung oil was being imported into Europe and the United States as a substitute for linseed oil. Tung oil is produced by mechanical pressing, or by solvent extraction. The resulting oil is then filtered to remove any impurities.

Drying oils, including linseed and tung, can be defined as liquid vegetable oils that, when applied in thin layers to a non-absorbent substrate, will dry in the air to form a solid film. This drying is a result of polymerization by the action of atmospheric oxygen, i.e. autoxidation.

The resultant films are typically hard, non-melting and are usually insoluble in organic solvents. (This varies with the particular drying oil) Semi-drying oils, like soybean oil and some nut oils, form tacky, somewhat sticky films when dried. Non-drying oils like mineral oil undergo no marked increase in viscosity upon exposure to air.

Drying oils are typically subdivided into three main groups for classification purposes, non-conjugated, conjugated and other oils. Non-conjugated oils, such as linseed, soybean, sunflower and safflower oil, are fatty oils that contain polyunsaturated fatty acids, whose double bonds are separated by at least two single bonds (i.e. isolated double bonds make up the non-conjugated oils).

Conjugated oils on the other hand, such as tung, oiticica, dehydrated castor oil and isomerised non-conjugated oils are polyunsaturated fatty acids whose double bonds are partly or fully conjugated (i.e. alternate single and double bonds in the carbon chain are the fatty acids). Other oils include those with multifunctional fatty acids, which acquire their drying characteristics by chemical conversion, such as raw castor oil and tall oil. A simple way to classify fatty oils is by their iodine value. Drying oils have iodine values above 170, whereas semi-drying oils range between 110 and 170.

Conjugated oils like tung oil are considerably more reactive than non-conjugated oils. Conjugated double bonds favor polymerization and oxidation and dry more rapidly than non-conjugated oils, offering excellent surface-dry, through-dry and hardness. The resultant film offers a high resistance to yellowing and increased resistance to water and alkalis.

The principal drying component in tung oil is eleostearic acid, a conjugated octadecatrienoic acid. The oleic acid contained in the fatty oils and unsaturated fatty acids plays a small part in the drying process as well. The saturated fatty acids present, however, act only as plasticizers.

The drying of films typically progresses in three overlapping steps:

Induction – Through a process known as autocatalysis, the oxygen uptake, which is slow at first, steadily increases. Factors such as temperature, light and heavy metals/inhibitors in the oil affect the overall uptake rate.

Initiation – As the film continues to take up oxygen, its mass increases. The double bonds in the film begin to rearrange and polar groups such as hydroxyl and hydroperoxy develop in the film. This leads to the association of molecules, through forces such as hydrogen bonding.

Cross-Linking – As the number of double bonds in the film begins to diminish, larger molecules form, and volatile and non-volatile carbonyl compounds are generated. The exact chemical reactions, as well as the structure of the film-forming polymers, are not completely understood. The initial autoxidation step in non-conjugated oils is dehydrogenation of the unsaturated fatty acid by molecular oxygen, which forms a radical. This starts a catalytic radical chain reaction that increases incrementally with time, leading to the formation of a hydroperoxide.

At low levels, the hydroperoxides produced during autoxidation decompose to form free alkoxy and hydroxyl radicals. Higher levels of hydroperoxides form free radicals through biomolecular disproportionation. The resultant free radicals react in various ways to accelerate the autoxidation process.

The drying of tung oil varies considerably from linseed oil. Tung oil typically absorbs approximately 12% oxygen (linseed oil absorbs approx. 16%) and quickly forms a skin on the surface. Since less oxygen is absorbed, the viscosity of the oil increases at a faster rate. Unlike the hydroperoxide formation during autoxidation in linseed oil, tung oil forms cyclic peroxides. The methyl eleostearate that is formed has a higher molecular mass than linoleic acid esters.

The direct attack on the double bonds by oxygen forms cyclic peroxides. The resultant reaction of the peroxides with allylic methylene groups, leads to the formation of radicals. This creates a radical chain reaction that forms polymers. The molecular mass created during tung oil polymerization is less than that achieved through linseed oil polymerization. To speed up the film formation and curing process, manufacturers add “driers” to the oils.

Both non-conjugated and conjugated drying oils like linseed and tung can be polymerized by heating under an inert atmosphere. These polymerized oils are then referred to as “Bodied Oils.” To achieve the higher viscosities of bodied oils, non-conjugated oils are heated up to 320° Centigrade and conjugated oils are heated up to 240° Centigrade.

This increase in viscosity, or “body,” is caused from thermal decomposition of naturally occurring hydroperoxides. This decomposition yields free radicals that contribute to a limited amount of cross-linking.

The heating of tung oil must be carefully monitored, or the polymerization will lead to gelation of the oil. The viscosity can also be increased by passing air through the oil (known as Blown Oils) at high temperatures up to 150° Centigrade. Reactions similar to those observed in cross-linking cause oligomerization of the oil.

Polymerized tung and linseed oils dry faster, harder and are more durable than raw oils. In addition, polymerized oils produce a smooth glossy finish, whereas raw oils produce a matte sheen. This matte sheen is a result of the natural expansion that takes place during polymerization. This expansion creates a very finely textured surface that appears to the naked eye as a matte finish.

Some highly specialized polymerized tung oils are processed at extremely high pressures and temperatures. These are called Thermalized Tung oils and are used in the manufacture of nitro-cellulose lacquers. This process improves the drying, hardness and luster of the oil.

I hope this helps you to understand Tung oil a wee bit. Take care and all the best to you and yours!

Steve Russell
The Woodlands, Texas

-- Better Woodturning and Finishing Through Chemistry...

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