Well, it's me again. Here's an article I came across on using Tung and Linseed oils that might be of interest.
Tung & Linseed Oils
By: Steven D. Russell
Eurowood Werks Woodturning Studio
The Woodlands, Texas U.S.A.
Email: benzer@flash.net
Posted with permission of the Author
There have been some threads over the last few months, that have debated the relative merits of Tung oil and Linseed oil. In order to shed a bit of light on the complex drying process of these two oils, the following is a general introduction to both oils.
Linseed oil is made from the seeds of the flax plant (Linum usitatissimum L.) Tung oil (aka wood oil, chinawood oil) is made from the seed kernels of the Tung tree (Aleurites fordii). Drying Oils can be defined as liquid vegetable oils that, (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 (autoxidation). Two common drying oils are Tung oil and Linseed Oil. The resultant film formed is typically hard, non melting and usually insoluble in organic solvents (this varies with the particular oil).
Nonconjugated oils, such as Linseed oil, are fatty oils that contain polyunsaturated fatty acids, whose double bonds are separated by at least two single bonds. Conjugated oils on the other hand, such as Tung oil, are polyunsaturated fatty acids whose double bonds are partly or fully conjugated.
The place of cultivation and its climate can alter the fatty acid spectrum of a drying oil. The high proportion of linolenic acid in nonconjugated oils (like Linseed oil), affects its drying characteristics. High concentrations of linolenic acid can result in rapid drying, yellowing and brittleness.
Conjugated oils like Tung oil, are considerably more reactive than nonconjugated oils (like Linseed oil). Conjugated double bonds favour polymerization and oxidation and dry more rapidly than nonconjugated oils. The resultant film offers a high resistance to yellowing and increased resistance to water and alkalies.
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 act only as plasticizers.
The drying of films typically progress in three overlapping steps:
At low levels, the hydroperoxides produced during autoxidation, decompose to form free alkoxy and hydroxyl radicals. Higher levels of hydroperoxides form free radicals through boimolecular 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 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 is less than that achieved through Linseed oil polymerization. To speed up the film formation, manufacturers add driers to the oils.
Driers are oil soluble metal salts of organic acids. When these driers are dissolved in aliphatic or aromatic hydrocarbons, they are known as siccatives. When driers are added to drying oils, they are known as Boiled Oils. In order to increase the viscosity of the Boiled Oil, air is sometimes "blown" through the oil at 60-100 degrees Centigrade.
Rags soaked with drying oils (treated with siccatives) present a significant danger of exothermic autoxidation, which could lead to spontaneous combustion. Therefore, when working with boiled oils (or other oils treated with driers or siccatives) you must pay careful attention to the safe disposal of any oil soaked rags. The rags should never be folded, crumpled or otherwise compressed, until the oil has fully dried. Proper disposal according to manufacturer's recommendations i.e. fireproof containers, should be used if possible.
The yellowing of Linseed oil is caused when conjugated unsaturated hydroperoxides are converted into conjugated unsaturated ketones. These unsaturated ketones can produce long-chain coloured polyenes. Additionally, if 1,4-diketones are formed during the drying, enol tautomers can react with trace amounts of atmospheric ammonia.
This produces a substituted pyrrole, that can be converted into a coloured product by oxidation, or by condensation in the presence of formic acid. Coloured metal siccatives can also contribute to discoloration and/or yellowing.
While this can be a complex subject, if broken down into steps, it becomes much easier to understand. The plethora of reactions and changes that occur during the drying process, turn a liquid oil into a solid film. A bit of magic to be sure!
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Tung & Linseed Oils
By: Steven D. Russell
Eurowood Werks Woodturning Studio
The Woodlands, Texas U.S.A.
Email: benzer@flash.net
Posted with permission of the Author
There have been some threads over the last few months, that have debated the relative merits of Tung oil and Linseed oil. In order to shed a bit of light on the complex drying process of these two oils, the following is a general introduction to both oils.
Linseed oil is made from the seeds of the flax plant (Linum usitatissimum L.) Tung oil (aka wood oil, chinawood oil) is made from the seed kernels of the Tung tree (Aleurites fordii). Drying Oils can be defined as liquid vegetable oils that, (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 (autoxidation). Two common drying oils are Tung oil and Linseed Oil. The resultant film formed is typically hard, non melting and usually insoluble in organic solvents (this varies with the particular oil).
Nonconjugated oils, such as Linseed oil, are fatty oils that contain polyunsaturated fatty acids, whose double bonds are separated by at least two single bonds. Conjugated oils on the other hand, such as Tung oil, are polyunsaturated fatty acids whose double bonds are partly or fully conjugated.
The place of cultivation and its climate can alter the fatty acid spectrum of a drying oil. The high proportion of linolenic acid in nonconjugated oils (like Linseed oil), affects its drying characteristics. High concentrations of linolenic acid can result in rapid drying, yellowing and brittleness.
Conjugated oils like Tung oil, are considerably more reactive than nonconjugated oils (like Linseed oil). Conjugated double bonds favour polymerization and oxidation and dry more rapidly than nonconjugated oils. The resultant film offers a high resistance to yellowing and increased resistance to water and alkalies.
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 act only as plasticizers.
The drying of films typically progress in three overlapping steps:
- Induction - through a process known as autocatalysis, the oxygen uptake 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 begin to diminish, larger molecules form and volatile and non-volatile carbonyl compounds are generated.
At low levels, the hydroperoxides produced during autoxidation, decompose to form free alkoxy and hydroxyl radicals. Higher levels of hydroperoxides form free radicals through boimolecular 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 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 is less than that achieved through Linseed oil polymerization. To speed up the film formation, manufacturers add driers to the oils.
Driers are oil soluble metal salts of organic acids. When these driers are dissolved in aliphatic or aromatic hydrocarbons, they are known as siccatives. When driers are added to drying oils, they are known as Boiled Oils. In order to increase the viscosity of the Boiled Oil, air is sometimes "blown" through the oil at 60-100 degrees Centigrade.
Rags soaked with drying oils (treated with siccatives) present a significant danger of exothermic autoxidation, which could lead to spontaneous combustion. Therefore, when working with boiled oils (or other oils treated with driers or siccatives) you must pay careful attention to the safe disposal of any oil soaked rags. The rags should never be folded, crumpled or otherwise compressed, until the oil has fully dried. Proper disposal according to manufacturer's recommendations i.e. fireproof containers, should be used if possible.
The yellowing of Linseed oil is caused when conjugated unsaturated hydroperoxides are converted into conjugated unsaturated ketones. These unsaturated ketones can produce long-chain coloured polyenes. Additionally, if 1,4-diketones are formed during the drying, enol tautomers can react with trace amounts of atmospheric ammonia.
This produces a substituted pyrrole, that can be converted into a coloured product by oxidation, or by condensation in the presence of formic acid. Coloured metal siccatives can also contribute to discoloration and/or yellowing.
While this can be a complex subject, if broken down into steps, it becomes much easier to understand. The plethora of reactions and changes that occur during the drying process, turn a liquid oil into a solid film. A bit of magic to be sure!
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