1. Introduction -drying oils and oil films
Siccative oils are the drying oils used in artists’ oil based paints. If you paint with oils, you may have noticed that there are a large number of oils available. Some oils are from different plant sources, while others may have been treated or extracted in different processes. If you start looking through the available information on oils for oil painting, you’ll find that different oils have different properties that are important to painters. Two key properties, often discussed, are yellowing and drying time. However there is also a third important property to consider – oil film hardness (and toughness).
Yellowing in oils is complex, and involves a good bit of Chemistry – we’ll tackle yellowing in a later post with more background information covered.
Drying and film hardness are correlated to the amount and types of polyunsaturated fatty acids in the drying oil. So the next logical questions are :
What are polyunsaturated fatty acids?
What is their role in oil film formation?
Linoleic Acid: A Polyunsaturated fatty acid with 2 unsaturations. This is a molecule commonly found in drying oils, also called siccative oils.
2. Polyunsaturated fatty acid – what does this word mean?
So, what is a polyunsaturated fatty acid? It may help to take the long name apart; “polyunsaturated fatty acid” contains some smaller chemistry words that tell us what the word means.
“Poly” means many or more than one. Remember out brief introduction to polymers and polymerization in oil paint? The “poly” in polymer meant more than one unit or monomer. So polyunsaturated means we have more than one thing that is unsaturated in the key molecules of siccative or drying oils.
Unsaturated refers to the number of of hydrogens and other things attached to the carbon atoms in the molecule. When a chain of carbon atoms in a molecule contains as much hydrogen as possible attached to the carbon atoms, the molecule is called saturated. In order to be saturated, the carbon atoms have to be attached to each other by single bonds. This leaves all of the other attachments carbon can make to other atoms available to stick on as many hydrogens as possible.
When a molecule is unsaturated, there are double bonds between the carbon atoms. A carbon atom usually can make four bonds, so if two carbon atoms share a double bond, they can each attach one less hydrogen. The resulting molecule is unsaturated – it could attach more hydrogens, but some of it’s bonds are used up as double bonds between carbon instead.
3. Linoleic and linolenic acids; key components of drying oils
Below, there’s a sketch of Linoleic acid, created using Marvin free chemistry software. Linoleic acid is one of the components of linseed oil that helps it harden. It has two unsaturations in the fatty part of the molecule – two double bonds in the carbon and hydrogen long chain.
Linoleic acid (again)
The term “fatty” in polyunsaturated fatty acid refers to the long chain of connected carbons and hydrogens that make up most of the molecule. Finally the acid is a bit at the end of the molecule, a carbon atom attached to an oxygen (double bonded) and also attached to another oxygen by a single bond. The singly bonded oxygen is attached to a hydrogen on it’s other side. The chemical formula for this organic acid group is COOH. (By itself the OH part of the acid is an alcohol group, and the C=O is a carbonyl or ketone.)
So in short, a polyunsaturated fatty acid is a long string of connected carbon and hydrogen atoms with more than one double bond and an acid group at the end. You can see the bits that make up a polyunsaturated fatty acid in the sketch of linolenic acid, below.
Linolenic acid is an unsaturated fatty acid with 3 unsaturations (3 double bonds). It is a key ingredient in linseed oil.
The acid group (The part at the end with the red colored oxygen atoms) and the unsaturations are two types of functional groups within the fatty acid. These are places where a chemical reaction can readily make attachments between molecules.
4. The acid group
At an acid group attachments are usually made through condensation or through acid base chemistry. The acid will give up the hydrogen attached to the singly bonded oxygen along with the singly bonded oxygen itself. The other molecule being attached often gives up a hydrogen so that the detached atoms become an independent water molecule, but there are other possibilities. Amides and esters are two common products of organic acid reactions. Polyesters and polyamides are polymers from organic acids.
A key point to hold onto is that the acid end of a polyunsaturated fatty acid found in a drying oil can make links to other molecules.
The unsaturation – the double bonds between carbons – can be visualized as carbon atoms holding hands and/or feet. I should elaborate. A carbon atom has enough unpaired electrons to make four bonds to other atoms. Imagine that the carbon is a little monkey with two hands and two prehensile feet. (anyone remember the barrel full of monkeys game?) In the fatty part of the polyunsaturated fatty acid, each carbon has to attach to two other carbons or the chain falls apart. The only exceptions are carbons at the ends of the chain. So each carbon atom within the chain, each little “monkey” in the interior of the chain of monkeys, is already holding on to its neighbors with both feet. The doubly bonded carbons are also holding hands with a neighbor. That leaves only one hand free to hold a hydrogen. The singly bonded carbons have two free hands to hold hydrogen atoms.
What if the doubly bonded carbons – the monkeys holding onto the chain with both feet and one hand – were to let go of each other’s hands? What if the double bond were “opened”? Each of those carbons would now have a free hand to grab onto something else – without breaking the chain. The double bonds are also potential sites to attach molecules together, they just have to be opened up.
6. Opening the double bond
There are a variety of chemical reactions that open double bonds. Oxidation is the word that comes up most frequently in searches of technical articles on the reactions that harden siccative oils. Oxidation is related to acid base chemistry. The “simple” (uncatalyzed) oxidation reactions for opening a double bond involve ozone, O3, or a weird little molecule called a percarboxylic acid, which is an acid with an extra singly bonded oxygen. The chemistry of double bond opening and oxidation is fairly complex – this is another area to revisit, since it may also provide some insight into why different pigments have different effects on oil drying.
Halogens, atoms like bromine, iodine, and chlorine can also open up double bonds. This chemistry is also interesting and worth a deeper look after we go through pigment chemistry. There are, for example, phthalo blue pigments that are heavily chlorinated – do they make oil dry faster?
Catalyzed reactions also open double bonds. A catalyst is a chemical that helps a reaction along without getting used up itself. It doesn’t become permanently attached to and incorporated within the new molecules formed by chemical reactions. In organic and polymer chemistry catalysts that work with double bonds are often organometallic. Theres a metal atom in the center surrounded by complex carbon-based organic rings. A number of paint pigments look a lot like common catalysts – do they make the oil harden faster by catalyzing double bond opening?
Clearly there is a lot to be understood about the chemistry behind oil drying, especially when we consider all of the different chemicals in paint, or even in painters’ drying oils, that might be having an effect on oil film formation. This is a topic I expect we’ll revisit repeatedly with more knowledge in hand on the other components of oil.
7. Why are functional groups and chemical reactions important for oil hardening?
As noted in an earlier post, oil paint dries in two stages. First the solvents and volatiles in the paint dry off, leaving behind a viscous paste or soft solid. Then the paint slowly hardens over time. The hardening of paint is due to polymerization and crosslinking of the oil. A simplified schematic of crosslinking is shown below.
A polyunsaturated fatty acid (simplified). This is a key component of siccative, or “drying” , artists oils such as linseed oil. The ability of an oil to dry is related to the amounts of these molecules in the oil, especially the molecules with more double bonds or unsaturation.
A group of molecules, each with two reactive spots – two functional groups that can form attachments to other molecules (bonds) can each link up with two neighbors. Molecules with this property can form long chains of linked bonded molecules called polymers.
If the molecules each have more than two places where they can link up, the polymer can bond back to itself through crosslinks, to form a molecular mesh. Meshes with more crosslinks tend to make harder materials, while meshes with longer lengths of molecule between crosslinks can be tougher.
Linseed oil contains a high proportion of linolenic acid, a polyunsaturated fatty acid with 3 carbon carbon double bonds. It forms a hard film. Poppy oil contains more linoleic acid, which has only 2 double bonds. It forms a softer film. Additives and components that affect crosslinking and polymerization will have an effect on the hardness and toughness of oil films. However there are also other factors, such as how the film behaves as a composite material that contribute.
A simplified cartoon of the key ingredients of paint. Modern oil paint also contains waxes and fillers. The waxes a longer versions of the long molecules in the picture, with different functionalities, while most of the fillers are particles like pigment.