Let's first start with "what" a varnish is. The definition that most consistently applies is that a varnish is a protective coating comprised of one or more resins dissolved into a solution which can easily be applied to a surface. There are two main categories of coatings that all types of coatings fall under, albeit with some caveats and footnotes; evaporative and reactive coatings are these main two categories (in the coatings industry, non-convertible and convertible respectively are often used, but the preceding terms are in more common use and are easier to explain/understand). Evaporative coatings like shellac are deposited via solvent/carrier evaporation. Reactive coatings cure by chemical reaction, oxygenation, moisture, radiation, etc. Evaporative coatings, like shellac, can be reversed at any time by simply using one of the main solvents originally used to keep the coating in a workable, liquid state. Reactive coatings, which include oil-based varnishes, must be removed with strong solvents after they are cured, i.e. with paint remover.
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 (autooxidation). 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 favor 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 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 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.
The exact chemical reactions in these steps, as well as the structure of the film-forming polymers, are not fully understood. The initial autooxidation step in nonconjugated oils (Linseed), is dehydrogenation of the unsaturated fatty acid by oxygen, which forms a radical. This starts a radical chain reaction that increases incrementally with time, leading to the formation of a hydroperoxide.
At low levels, the hydroperoxides produced during autooxidation decompose to form free alkoxy and hydroxyl radicals. Higher levels of hydroperoxides form free radicals through bimolecular disproportionation. The resultant free radicals react in various ways to accelerate the autooxidation 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 autooxidation 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 autooxidation, 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 colored 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 colored product by oxidation, or by condensation in the presence of formic acid. Colored 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!
As I mentioned, Linseed Oil is obtained from Linseed or Flax (Linum usitatissimum). It is obtained by various methods including preexpelling, followed by hexane extraction of the press cake. The resultant oil is refined to remove phosphatides and gums, which naturally occur in the oil. (Post-desliming with sulfuric acid and phosphoric acid, yields oils with virtually no traces of phosphatides or gums).
Further post-treatments include lye neutralization and earth bleaching, which yield very light drying oils. The natural odor of Linseed oil is removed through a deodorization process (if desired), as a final step in the refining process.
Therefore, "Raw Linseed Oil" (RLO) is simply, Linseed oil that has been extracted and packaged without any additional additives. Raw Linseed Oil takes significantly longer to dry than "Boiled Linseed Oil" (BLO) (RLO-needs a few weeks to cure, vs. 36-48 hours for BLO to cure).
So called "Boiled Linseed Oil" is Linseed Oil that has been altered through the addition of chemical drying accelerators, i.e. solvents and siccatives/driers. BLO is frequently mixed with 10-15% Stand Oil (SO's). SO's are drying oils of increased viscosity, which is achieved through heating the oil in the absence of oxygen.
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 catalyze throughout the film and are known as "through driers". Calcium salts help to reduce the amounts of other driers that may be needed. Various other compounds may also be present in some BLO's including but not limited to: Beryllium, Cadmium and Nickel.
As for whether or not you should use any Linseed Oil for a preservative on an exterior children's picnic table... That really depends on what you prefer. :-) There are many exterior finishes that will outperform both raw and boiled Linseed Oil. Other negative factors to consider when considering Linseed Oils for exterior applications (boiled or raw) include: 1.) Lack of any UV inhibitors. 2.) On surfaces where abrasion will be frequently encountered, Linseed Oil may not harden sufficiently, causing frequent repair. 3.) Linseed Oil can support the growth of mildew.
Many people use Linseed Oils on exterior applications. However, if I wanted to insure greater protection and longevity of the finish, I would chose another product. There are many other products that will offer superior overall protection, UV resistance and durability. This is however, a personal preference type of decision, that you will no doubt hear dissenting opinions on from other woodworkers.
I hope this does not put you to sleep... Both nonconjugated and conjugated drying oils can be polymerized by heating under an inert atmosphere. These polymerized oils are then referred to as "Bodied Oils". To achieve the higher viscosity's of bodied oils, nonconjugated oils are heated up to 320 degrees Centigrade and conjugated oils are heated up to 240 degrees Centigrade.
The 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 (Blown Oils) at high temperatures (up to 150 degrees Centigrade). Reactions similar to those observed in cross-linking cause oligomerization of the oil.
To put it into simpler terms...
Pure Tung Oil cures through polymerization and oxidation. To speed the drying process further, manufacturer's heat the Tung Oil (now called Polymerized Tung Oil) which completes polymerization portion of the drying process. This speeds the drying time, because half of the process has already been completed. Polymerized Tung Oil (PTO's) when applied, cures through oxidation and evaporation of any solvents.
Polymerized Tung Oils dry faster, harder and are more durable than "pure" Tung oils. In addition PTO's produce a smooth glossy finish, whereas Pure Tung Oils produce a matte sheen due to the 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 specialized Polymerized Tung oils are processed at very high pressures and temperatures. These are called "Thermalized Tung Oils" (TTO's) and are used in nitro-cellulose lacquers. .
Steven D. Russell operates Eurowood Werks Woodturning Studio in The Woodlands, Texas