I was painting a wooden basket yesterday with boilt linseed oil and thus came the inspiration for tonight’s topic. Drying oils are very important in the coatings industry, not as much as in the past but still important.
Back in the day before high quality water based paints had been developed, oil based paints were just about the only good choice except for some specialized applications. Before we go into detail, we should define some key terms regarding to paint.
The vehicle is the part of the paint that forms a tough, adherent film. In oil based paints the vehicle is generally linseed oil. In latex paints the vehicle is some type of synthetic resin.
The second component (not always in paint, but usually) is the solvent, also called the diluent. In oil paint the solvent is now usually petroleum distillates, but before oil was discovered the solvent was almost always turpentine. In latex paints the solvent is water.
The pigment is composed of inorganic powders, usually white or colorless. The pigment can add to the toughness of the film. For commercial house paints the pigment does not provide color (except for white) and usually organic dyes are added to the pigment for colors, although some other materials are also used. For art paints, many times the pigment is also the color in many cases. Pigments are similar for oil and water based paints.
There are also additives in small quantities in most paints to modify drying rate, viscosity, surface tension, and other properties. Water based paint often contains ethylene glycol as an antifreeze.
The traditional use of drying oils has been in coatings, but lots of them are used in the printing industry, because oil based inks are generally waterproof after they dry. In view of all of the printing that happens every day, it may be that that is a larger use than coatings but I have not been able to find any statistics to support this hunch.
But let us focus on the drying oils. They are called “drying” oils because they react with oxygen in the air to form the tough, durable, adherent film. This happens because they are reactive towards oxygen. They are exclusively of vegetable origin.
The reason that these oils react with oxygen is that they are highly unsaturated, i.e., contain multiple double carbon-carbon bonds. Single carbon-carbon bonds are very stable to oxygen, but double bonds are reactive. The more double bonds, the more reactive a drying oil is.
Let us look at the standard drying oil, linseed oil. Made from the seed of the flax plant (Linum usitatissimum), the same plant (but different horticultural varity) that provides the fiber for real linen. The oil varieties are usually short and highly branched and produce a lot of seed, whilst the fiber ones are taller and less branched because longer fibers make better cloth.
The oil pressed or extracted from flaxseed composes linseed oil. The composition of linseed oil is complex, but can be fairly represented by the structure shown here:
Like all vegetable oils, linseed oil is a triglyceride, that is a glycerol backbone onto which are bonded three fatty acids. In this one, the red part of the molecule represents alpha-linolenic acid, with three double bonds being extremely unsaturated. The blue part represents oleic acid (the predominant fatty acid in olive oil), and with only one double bond is much more saturated than the linolenic acid. The green represents linoleic acid, less saturated than the oleic acid but more saturated than the linolenic acid.
Note that linseed oil is NOT made up of these exact molecules, but rather a mixture of similar molecules with different amounts of the fatty acids. Linseed oil is a little over 50% alpha-linolenic acid, around 20% oleic acid, around 15% linoleic acid, and around 10% saturated acids. Thus, some molecules of linseed oil are likely to contain two alpha-linoleic acid residues, and thus contain no oleic or linoleic acid. This has another interesting result: the melting point of linseed oil is much less than that of a synthetic oil composed only of the molecules shown in the diagram. This is because, in general, mixtures of similar substances have lower melting points than those of pure substances. This helps keep linseed oil fluid at low temperatures.
Linseed oil is also a valuable nutrient, because alpha-linolenic acid is an omega-3 fatty acid. In recent years it has been found that these fatty acids are quite important in the building of nerve tissue, or that is commonly thought. There is no consensus on whether or not supplements of them should be taken (humans do not have the ability to synthesize them in the body), but I believe that it is reasonable to take a supplement unless you eat a lot of oily ocean fish. That is another whole story, because pollutants, in particular PCBs, are often found in very high concentrations in oily fish, but that is a topic for another PTG. By the way, if you take fish oil or flaxseed oil supplements, keep them in the refrigerator and tightly closed to avoid as much reaction with oxygen as possible.
There is also something represented in this structural formula worth noting. Look at the carbon-carbon double bonds in each fragment. See how the large groups are on the same side of the double bonds? This is call the cis configuration, and is thermodynamically less stable that the form where the big groups are on the opposite side of the double bond, the trans configuration. It turns out that the enzyme systems that form these fatty acids favor making the cis configuration, even though it is less stable than the trans one. It also turns out that our metabolic system works more efficiently on the cis configuration.
The FDA now requires the amount of trans fatty acids in foods to be on the label. Those are formed from the cis ones by treating with hydrogen under pressure in the presence of a nickel catalyst, thus the name hydrogenated oil. If hydrogen is added to all of the carbon-carbon double bonds, the resulting completely hydrogenated fats are neither cis nor trans, because all of the double bonds are gone. However, those fats are usually way too high melting to be useful for cooking, so only some of the double bonds are hydrogenated. That is done by adding less hydrogen than needed to saturate each double bond. These fats have desirable physical properties for cooking, but in the process of the hydrogenation, some of the cis double bonds are converted into the trans form (remember, I said that the trans configuration is the more stable). These partially hydrogenated oils make excellent shortening, with good melting temperatures that can be controlled by the amount of hydrogen added. However, studies show that the trans fat contribute to high blood triglyceride and bad cholesterol levels. I try to avoid them as much as possible.
Hydrogenation converts highly reactive drying oils into much less reactive oils, and that is good for making foods have a longer shelf life. However, that pretty much ruins them for paint because it is the oxygen reactivity that is necessary for them to form the tough film that is necessary for the paint to do its job for a reasonable length of time.
The reason that the highly unsaturated oils “dry” is that the react with atmospheric oxygen and crosslink at the sites of unsaturation. The mechanism is rather interesting. Here is a simplified mechanism:
Atmospheric oxygen inserts into a C-H bond of a carbon in a double bond. The resulting hydroperoxide is quite unstable and undergoes heterolytic cleavage betwixt the two oxygens, giving a free radical containing one oxygen. That radical reacts with a double bond in a nearby chain and forms an oxygen-carbon bond, crosslinking the two chains and generating yet another free radical that can react with yet another double bond in a new chain, forming a carbon-carbon bond. This continues until all (or almost all) of the carbon-carbon double bonds are crosslinked. Pure linseed oil absorbs around 17% its mass in oxygen.
These crosslinked networks are actually three dimensional, but in paint applications the film is much wider than it is deep. This is important, because for very thick coatings it takes a long time for oxygen to penetrate into the bottom. This is one of the reasons that multiple thin coats are better than one heavy coat of paint.
Even though linseed oil is quite reactive, it is usually treated to increase the rate of reaction with oxygen. The oldest was is to heat it in closed vessels, and the term boilt linseed oil was applied to it. This is now called stand oil, the term boilt linseed oil now used for a mixture of raw (untreated) oil, usually some petroleum solvent, and organometallic compounds that increase the rate of reaction with oxygen and, hence, crosslinking. Materials like lead naphthenate, manganese maphthenate, and cobalt naphthenate are commonly used.
Raw linseed oil is an excellent nutrient, but the boilt kind can be toxic because of the metals used. I like to use the boilt kind because it dries faster than the raw.
Linseed oil is by far the most commonly used drying oil, but is not the only one by any means. Lots of do it yourselfers use tung oil (from Vernicia fordii) to refinish fine furniture. Instead of relying on the three acids mentioned in linseed oil, the major fatty acid in tung oil is alpha-eleostearic acid, comprising around 80% of the fatty acid content. Here is its structural formula:
Remember, the more double bonds the more reactive the fatty acid. Linseed has only about 55% of the triply unsaturated fatty acid, but tung oil has 82% of this acid and around 9% of alpha-linolenic acid, so the content of triply unsaturated fatty acids is close to 90% in tung oil vs. 55% in linseed oil. Tung oil dries very fast, especially when metallic dryers are added. I have been using the same quart of boilt linseed oil for a three or four years now, and the oil is still quite fluid. I remember my father getting some tung oil and using a little for some such. When he went to get it to use for something else a couple of months later, the entire container was a very thick goop that could not be used!
Another drying oil that is widely used is safflower oil. From the plant Carthamus tinctorius there are two types of oil, depending on the horticultural variety. One kind in high in oleic acid, only monosaturated, and is used in the food industry. The other kind has high levels of alpha-linolenic acid and is used as a drying oil. Not much safflower oil is used as a drying oil these days, most of it being the high oleic acid kind for cooking.
Soybean (Glycine max) oil is also used as a drying oil, but most of its unsaturated fatty acids are the two double bond linoleic one. Soya oil dries much more slowly than linseed, tung, or safflower oils but due to its relative low cost is used as a drying oil, often mixed with the more expensive, more highly unsaturated drying oils. Drying accelerators are usually used with soya oil in most applications. It has only around 7% alpha-linolenic acid and around 54% linoleic acid, so both its total polyunsaturated acid level and its alpha-linolenic acid levels are much lower than for linseed oil.
Here is a simple experiment that you can do safely at home. Take a tablespoon or two of soya oil (make sure that it is the kind with no added antioxidants, read the label) and put it into a small glass. Do the same with linseed (flaxseed) oil from the nutritional supplement aisle. Leave both glasses (be sure that you label them) out on the counter for a few days to a several weeks, depending on the temperature. Take a sniff every few days. The linseed one will begin to go rancid faster than the soya one, because the oxidation is faster. It will also form a semisolid film much faster than the soya one. For an even more striking demonstration, do a third test with canola or peanut oil. Those will stay “good” even longer than the soya oil because both of them have only about 32% of their fatty acids polyunsaturated.
At the other end of the spectrum are oils that do not react with oxygen much at all, at least under ambient conditions. Before the advent of petroleum based products those were sort of rare, the oil from sperm whales (Physeter macrocephalus) was one of the best and was used by clockmakers to lubricate delicate works because it tended to form less gum than other oils. Sperm oil is actually predominately a liquid wax, not a fat. The difference is that a fat is by definition a molecule composed of one glycerol molecule bonded to three fatty acids (hence the term triglyceride) and a wax is composed of one alcohol molecule bonded to one fatty acid. Sperm oil is no longer available, and with good reason.
There are a few plant oils that are not very drying. A notable example is coconut oil, which is 91% saturated. However, since it has a softening point of around 76 degrees F, it is not suitable as a lubricant in cold climates. That is why sperm oil was so popular.
Oils from petroleum have almost completely supplanted natural sources for nondrying oils. They are inherently more stable than natural oils, since they are not fats at all but mostly hydrocarbons, and hydrocarbons are quite stable (unless gotten to hot that they catch fire). I have an antique German wall clock that my father gave me. When I first tried to use it, it would run for an hour or two at most and then stop. I took out the works and discovered that they were completely covered with a gummy film, doubtless from repeated applications of clock oil (even sperm oil reacts slowly with atmospheric oxygen). I soaked the works (after removing anything that could be damaged) in acetone for a couple of days, then rinsed them in fresh acetone and allowed them to dry.
Then I used Rem Oil (a trade name for a highly refined, petroleum based lubricant for firearms) to lubricate each and every moving part, including the two spring drums. I let it penetrate for a day or two, then wiped the excess off of the works and reassembled the clock. After getting the pendulum set correctly, this clock keeps time to within about a minute or less a month, and I lubricate it only about every two years now. The nice thing about Rem Oil is that it not only does not react with atmospheric oxygen, but it also contains tiny granules of polytetrafluoroethene (Teflon, another trade name) that continues to lubricate even after the petroleum components have evaporated, since it is nonvolatile.
In contrast to coatings and inks, oils in lubricants that are exposed to oxygen HAVE to be quite inert to it, or they would just turn to glop like what happened to my clock. In the covered wagon era, axles were lubricated usually with grease from cattle rendering, but those were very low temperature applications compared to automobile applications. Petroleum oils and greases are the only cost effective lubricants now used for most mechanized applications (do not be fooled by the term synthetic oil; the starting materials are mostly from petroleum). A notable exception is lithium grease, which is a mixture of petroleum oils in a true soap (a soap is a combination of one or more fatty acid molecules with a metal ion). Sometimes natural products are still excellent.
Now to debunk the myth of “oil soaked rags” starting fires. In almost no instance is that the case these days. Almost all oil that we think of except for cooking is petroleum based, and you can pour motor oil on as many rags as you want and expose them to the air and they will not become alit. However, if you do the same with drying oils, it IS possible to start a fire. Here is what happens:
Go back to the crosslinking scheme earlier. When the oxygen forms the hydroperoxide, and the hydroperoxide cleaves, energy is released. This energy is in the form of heat, and if there is an arrangement of rags such that the heat is pretty much trapped, the temperature can rise enough for spontaneous combustion to occur. By the way, the absorbent paper that I used when I coated the basket with linseed oil went outside to oxidize in a single layer, safely. There is more danger involved with putting paper towels in a trash bag after a large kitchen spill of soya oil than there is in motor oil starting a fire, since soya oil is a drying oil. To be on the safe side, spread out such towels in a single layer or put them in a fireproof can outside.
That about does it for tonight. I hope that this was informative and interesting. I shall be here all evening since I have no where else to go to field comments, questions, corrections, and other feedback. Tips and recs are always welcome! I shall also return around 9:00 Eastern tomorrow evening for Review Time.
Doc, aka Dr. David W. Smith