Re: BLO, Tung oil, darkening, safety
Steven Russell
>Hello,
Your questions require an indepth answer... In the past I have been hammered for my long posts by a few individuals, so I will try and give you the nickle tour to appease those who prefer one paragraph or less answers.
Here is a bit of an overview on Linseed and Tung Oil, excerpted from volume 1 of my educational CD-Rom titled "Woodturning with Steven D. Russell"... To understand the colour changes that occur, you must first understand the basics of how the oil is produced, refined and the process it undergoes to cure into a hard film. Some of this may be a bit heavy for you, but bear with me for a wee bit.
Linseed oil is derived from the seeds of the flax plant (Linum usitatissimum L.) and is the oldest cultivated plant in Europe. Linseed oil is obtained by various methods including pre-expelling, followed by hexane extraction of the resulting press cake. The oil is refined to remove phosphatides and gums, which naturally occur in the oil. Subsequent refining through post-desliming with sulfuric acid and phosphoric acid yields an oil with virtually no traces of phosphatides or gums.
Further post-treatments include lye neutralization and earth bleaching, which yields a very light drying oil. The natural odour of linseed oil is removed through vacuum steam distillation, which removes lingering odorous volatile compounds like aldehydes and keytones.
Dewaxing the oil is the last step in the refining process, which removes the thin layer of wax that covers each linseed. Dewaxing is accomplished by cooling the oil to 4� Centigrade. The cooled Linseed oil is then held at this temperature until the waxes have crystallized. The resulting crystallized wax is then removed by mechanical filtration.
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 (kekuna oil) and A. moluccana or A. triloba (lumbang oil) also produce oils with properties that are similar to Chinese tung 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 polymerisation 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, nonconjugated, conjugated and other oils.
Nonconjugated 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 nonconjugated oils). Conjugated oils on the other hand, such as tung, oiticica, dehydrated castor oil and isomerised nonconjugated 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.
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. Oils with low or no linolenic acid, like soybean and safflower oil, obtain their drying characteristics from high levels of triglycerides, which contain linoleic acid. The drying of these oils produces flexible films with very little yellowing.
Conjugated oils like tung oil are considerably more reactive than nonconjugated oils. Conjugated double bonds favour polymerisation and oxidation and dry more rapidly than nonconjugated 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: 1.) 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.
2.) 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.
3.) 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 nonconjugated 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 polymerisation is less than that achieved through linseed oil polymerisation. To speed up the film formation and curing process, 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.Traditionally, driers contained combinations of oil-soluble metal salts like Cobalt and/or Manganese with Zirconium, Lead or Calcium salts of 2-ethylhexanoic acid or naphtenic acids. Cobalt and Manganese salts act as surface driers and aid in the drying of the film on the surface, where oxygen concentrations are the highest.
Lead and Zirconium salts catalyse throughout the film and are known as through driers. To avoid the use of Lead, which is highly toxic, modern siccatives employ blends of Cobalt and Zirconium. This combination reduces surface drying speed, promoting even drying throughout the substrate.
Calcium salts are sometimes used as well, mainly to reduce the amounts of other driers that may be needed. Various other compounds may also be present in some siccatives including Beryllium, Cadmium and Nickel.
Both nonconjugated and conjugated drying oils like linseed and tung can be polymerised by heating under an inert atmosphere. These polymerised oils are then referred to as �Bodied Oils.� To achieve the higher viscosities of bodied oils, nonconjugated 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 polymerisation 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.
Polymerised tung and linseed oils dry faster, harder and are more durable than raw oils. In addition, polymerised 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 polymerisation. This expansion creates a very finely textured surface that appears to the naked eye as a matte finish.
Some highly specialized polymerised 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 lustre of the oil.
The yellowing of linseed oil is thought to be 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, which 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 the discoloration and/or yellowing of linseed oil. To alleviate the yellowing, saturated aliphatic aldehydes may be added to the oil.
Ok, you now have a good idea of how the oils are produced, refined and the process through which they polymerise. Many factors can influence the colour of the oils, including driers which can also impact any post application colour changes.
Linseed oil in particular has a nasty habit of darkening after application. To date, no chemical treatment can reverse, or forestall the darkening of Linseed oil. Over time, the dark colour may eventually turn to a near black colour. Many factors can influence this colour change and the total amount of discolouration that will occur. For this reason, I prefer to limit use of Linseed based finishes in my studio.
Your saftey concerns are real when working with or applying finishes that contain toxic solvents or driers. Your skin is a poor protective layer when using these finishes. The toxic effects of solvents vary between human, animal and plant organisms.
Many factors can influence the nature, severity and probability of toxic injury. These include, but are not limited to, the number of exposures, magnitude of exposure, route of exposure, time of dosing, formulation and impurities present in the toxic substance, as well as individual metabolic differences.
Acute damage may result from short-term exposure to high solvent doses, whereas the absorption of smaller doses over longer periods can lead to chronic damage and sensitisation. Chronic damage is often accompanied by an acquired tolerance for the solvent, which can lead to late detection of the damaging effects of the solvent.
Most cases of solvent poisoning are related to inhalation of solvent fumes. Inhaled vapours pass via the lungs and blood, where they accumulate in high lipid content tissues like brain, nerve, bone marrow, liver and kidney tissues.
The damage may occur in two forms. The cells can be damaged directly by the solvents, or indirectly by their decomposition products. In addition to inhalation, solvents may enter the body through cutaneous, or gastrointestinal absorption.
Symptoms of acute solvent poisoning may include dizziness, drowsiness, severe headache, loss of consciousness and other narcotic effects on the central nervous system. Chronic poisoning symptoms may be initially undetectable, but subsequently cause significant organ damage according to the particular solvent involved.
Chronic exposure to organic solvents has been associated with numerous neurotoxic effects, including permanent brain and nervous system damage in some studies.
Solvents dissolve the natural fatty layer of the skin (known as defatting), allowing cracks, microorganisms and dirt to penetrate the skin easier, causing infection. In addition, solvents can cause inflammation, dermatitis, burning or blistering of the skin. Allergic reactions can also occur after sensitisation of the skin, but vary widely depending upon individual tolerances or susceptibilities. Turpentine oil in particular can trigger allergic hypersensitivity reactions of an allergic nature in some individuals.
Gum Turpentine: (aka: Spirit of Turpentine, Oil of Turpentine, Wood Turpentine) This is pure essential plant oil, which is obtained by distillation of the resin exudates (balsam or turpentine) from living trees in the genus Pinis. Contains Terpenes.
The resin exudates are distilled at temperatures up to 180� Centigrade. It is a high quality paint solvent, which is stronger than paint thinner and dries at a slightly faster evaporation rate. Turpentines are colourless to pale yellow liquids, with a low viscosity and a characteristic odour. Turpentines are miscible in most organic solvents, but are immiscible in water.
Turpentine is used primarily to thin oil based paints, stains and varnishes to improve levelling, reduce application viscosity or increase penetration. If added in excessive amounts, it will reduce the overall lustre level. Turpentine is also a good solvent for hydrocarbon resins, waxes, fats and oils. Turpentine accelerates the drying of coatings and paints (particularly drying oils) by peroxide formation.
It is miscible with ethanol, aliphatic and aromatic hydrocarbons. Its dissolution properties are better than white spirit, and it is frequently used in addition to white spirit in alkyd resin and oil-based coatings.
Health Concerns:
Eye: Contact may cause irritation and severe burns.
Skin: Causes skin irritation. May cause sensitisation, an allergic reaction, which becomes evident upon re-exposure.
Ingestion: Causes gastrointestinal irritation with nausea and vomiting. May cause central nervous system depression, headache, dizziness, drowsiness, nausea, collapse, unconsciousness and possible death from respiratory failure. Aspiration into the lungs may lead to pulmonary edema, or pneumonitis.
Inhalation: May cause respiratory tract irritation. Aspiration may cause respiratory swelling and pneumonitis (inflammation in the lungs).
Chronic Exposure: May cause kidney and bladder damage.
Odourless Mineral Spirits: This is a specially refined, low odour version of traditional mineral spirits. Contains Stoddard Solvent. Odourless Mineral Spirits are used to thin oil base paints, enamels, stains, varnishes and polyurethane�s to improve levelling, reduce application viscosities and increase penetration. Odourless Mineral Spirits is an excellent solvent for oil and wax.
Health Concerns: Same as Mineral Spirits
Using Odourless Mineral Spirits exposes you to benzene, which is known to cause cancer, and toluol, which is known to cause birth defects and other reproductive harm.
Mineral Spirits: (aka: Stoddard Solvent, Paint Thinner, Varsol, Solvasol) Mineral Spirits are clear, colourless liquids used to thin oil base paints, stains, varnishes and polyurethane�s to improve levelling, reduce application viscosities and increase penetration. Contains Stoddard Solvent.
Health Concerns:
Eye: Causes redness, swelling and irritation. May cause chemical conjunctivitis and corneal damage.
Skin: May be harmful if absorbed through the skin. May cause irritation, redness, drying, defatting and dermatitis. May cause cyanosis of the extremities.
Ingestion: Causes dizziness, drowsiness, cough, sore throat, headache, gastrointestinal irritation, nausea and vomiting. May cause central nervous system depression, collapse, unconsciousness, coma and possible death. Aspiration into the lungs may lead to pulmonary edema, or pneumonitis.
Inhalation: Vapours are harmful. May affect the brain or nervous system causing headache, dizziness, nausea, fatigue and loss of consciousness. Causes nose and throat irritation. High vapour concentrations can cause drowsiness, suffocation, or a burning sensation in the chest.
Chronic Exposure: Repeated skin contact can cause dermatitis. May cause liver damage.
Using Mineral Spirits exposes you to benzene, which is known to cause cancer, and toluol, which is known to cause birth defects and other reproductive harm.
The solvents and chemicals woodturners regularly use can range from somewhat benign compounds, to highly toxic compounds with known carcinogenic (cancer causing), mutagenic (damage to DNA that is heritable), teratogenic (causing structural or functional abnormalities in the fetus or embryo), or neurotoxic effects. Do not take chances with your health! You should be wearing the appropriate protective gloves when using finishes and products that contain harmful solvents or driers.
Humm... It seems as if I have given you the 25-cent answer, instead of the nickle answer, please accept my apologies. Some things cannot be answered in one paragraph. I better stop here or I'm gonna hear from the "Quickie Answer Only" gurus.
Better Woodturning and Finishing Through Chemistry...
Steven D. Russell
Eurowood Werks Woodturning Studio and Advanced Research Laboratory
The Woodlands, Texas
Email: benzer@flash.net
