Oil paint can take a long time to dry. When it's thinned with turpentine the turpentine evaporates quickly and the paint "dries", but sometimes that layer can still be smudged. What is it about oil and oil paint that makes it stay "wet" and liquid for a long time and then - voila! It starts to thicken and then dries to a hard film.
Think about the liquids you use everyday; water, isopropanol (rubbing alcohol), possibly even solvents like acetone and ethyl acetate. Most of these liquids evaporate fairly quickly. Even a drop of water doesn't take days and days to dry. When these everyday pure liquids dry *poof* they're gone.
2. Typical solutions and their drying
Of course it's possible to take something solid, for example salt or sugar, and dissolve it in a liquid like water. When the liquid water evaporates, solid salt or sugar is left behind. In contrast, when oil paint is dissolved or thinned with turpentine or another solvent, the solvent evaporates and the pasty paint is left behind -- and then the paint slowly hardens over time. Step back and think about this - oil paint has an unusual property.
|Solid particles (gray balls) dissolved in a solvent (blue "water molecules"). Instead of particles you can also imagine that the gray balls are molecules of something that would be a solid without the water dissolving it.|
|In a solution of a solid in a liquid, salt water for example, the liquid solvent can slowly evaporate and disappear into the air. The solid cannot evaporate, and is left behind.|
3. Oil paint - drying then hardening
If oil paint is mixed with turpentine or another solvent, it can be applied in thin films. When the turpentine evaporates and disappears into the air, the oil paint paste is left behind as a very thin film. Very thin films have a special property. The outside of a very thin film of wet paint on a canvas - the part touching the air - contains a lot of the molecules that make up the film. In a thicker film, more of the molecules are hiding inside the film, away from the air.
If air is responsible for some of the chemistry that hardens oil paint, thin films will dry more quickly (and they do). There seems to be a fairly general agreement that oils that harden into paint films have a particular set of chemical properties.
4. After Evaporation, polymerization (hardening)
Drying oils can polymerize. A molecule that can polymerize is a molecule that can form a chain. Plastics are all made of polymers plus additives. Have you ever made a chain using paperclips? Each paperclip in the chain is attached to two other paperclips. A molecule that can polymerize must be able to attach to two other molecules or no chain forms. The distinct parts of a molecule in chemistry are called "functional groups" (or sometimes "moeities"). If a molecule has at least two functional groups that can polymerize, then it can react with it's neighbors to form long rope like polymer molecules.
As the molecules polymerize and join up into long rope-like polymers, the oil becomes thicker. Bigger molecules like polymers are harder to move around. The increase in the molecular weight of the oil increases its viscosity. If the polymers grow long enough the oil film will become waxy and then solid.
5. Polymerization ... and crosslinking?
The Nerdly Painter has found wide agreement that the oils in oil paint do polymerize. Polymerization is not difficult to measure in the lab. The next question is does oil paint do more than polymerize? Does it also form crosslinks? (and why is this question important?)
A polymer is like a long rope or chain formed by many little molecules stuck together through the magic of Chemistry. A crosslinked polymer network is like a mesh of chains all woven together. Woven threads produce a fabric that is stronger than each individual thread. Crosslinked polymers can be very strong and very resistant to solvents.
There are some features of "best hardening oils" that suggest that the oils in oil paint do crosslink. The best hardening oils also have the most extra polymerizable functional groups. Two of these groups form the oil polymer "rope", and the extras are available to attach different polymer ropes together and form a mesh.
6. Why is crosslinking desirable?
A crosslinked polymer can also resist solvents and humidity better than an uncrosslinked polymer. To dissolve a polymer each of the long twisty polymer molecules must be surrounded by solvent molecules and then carried away into the solution. As you can imagine, as the polymer gets larger and larger it becomes slower and more difficult to surround the long polymer with solvent molecules and bring it into solution.
A single crosslink between two polymer chains connects them and makes them act like one super-sized molecule twice the size of each original polymer. Crosslinking multiplies the size of the polymer molecules. Sometimes there are enough links to connect all of the polymer molecules into a single mesh. When this level of crosslinking happens, there is no longer a way to dissolve the polymer molecules - they all go into the solvent together in one lump or they don't go at all.
Humidity damage often involves swelling of a paint film by moisture. The water molecules start to surround the components of the paint and make it softer. When the film becomes softer, the components of the film can move around and the film itself can be pushed and mushed into new shapes. Most of these new shapes are not intended, and they tend to be weaker than the original film. Crosslinks set a limit on how much water can get into a film. As water moves in between crosslinked polymer molecules it pushes them apart. The crosslinks and mesh structure prevent too much of this pushing apart - the mesh will only stretch up to a point. Without crosslinks the polymer molecules can be pushed apart a lot further.
7. Crosslinking in oil paint - breaking down the questions
There are some references to crosslinking in oil paint films, but there are also a number of questions.
1. Do drying oils really crosslink? Do they crosslink enough to become insoluble in all common solvents?
2. Do all of the oils crosslink? Equally well?
3. Do the alkyd and other media also crosslink? Or do they simply mix into oil and harden because a solvent evaporates?
4. What types of chemistry speed up and enhance crosslinking? For example, what is really happening when cobalt colors dry faster than other colors?
I'm looking through a big pile of technical papers for answers. If the answers aren't in papers, they are in the lab. Crosslinking can be examined using thermal analysis and mechanical analysis, both of which allow me to use the very nice big machines sitting and waiting for an experiment. But first, we need to understand a little bit more about drying or siccative oils.
The technical description for the hardening properties of painting oils is that drying (or "siccative") oils contain a high proportion of a "polyunsaturated fatty acid". In the next post we'll go into a little more chemical detail about what a "polyunsaturated fatty acid" actually is, and why and how it helps oil to dry.
Next up: The Chemistry of siccative oils