1. Is the speed of drying in alkyd mediums and paints due to the chemical nature of of the alkyd compounds themselves, or are driers usually added to achieve their drying speed? 2. I understand that not all alkyds are created equal, being synthesized from different oils. Do some alkyd mediums form stronger, more flexible paint films than others? (In other words, are they all equally good in forming a reliable paint film?) I'm trying to avoid excess driers, as they tend to promote cross linking of the paint film indefinitely (faster embrittlement), but wish to speed up the drying time of titanium white to be comparable to cremnitz white, which I may abandon use of because of cost. Was thinking of using a small amount of alkyd medium in a stand oil medium to compensate for the slow drying time of titanium, i.e. 10% alkyd, 30% stand oil, 60% OMS. Thank you for your insights, Richard PS I dislike being held hostage to the much higher cost of cremnitz white and am seeking the next best alternative for painting landscape on panels. I will miss cremnitz, however.
1. Quick answer: yes and yes.
Pt 1. Alkyds dry quick to the touch, compared to oils, because of the long polyester polymeric chain component to them. The chains can entangle, which helps the film stay together even if the fatty acids attached to the polyester haven’t undergone significant cross-linking yet. Alkyds polymers are long polyester polymer chains with fatty acids attached to them (covalently bonded). A simple way to see this, is to think of necklace with lots of dangly pendant things on it – so you have your long chain (the polymer), and pendants (the fatty acids). If you just had pendants in a box (so fatty acids, and oils are triglycerides, so they have glycerol with 3 fatty acids only), they pile could move around easily without getting too tangled up. If you had the chains with the pendants (so alkyds), you can only imagine how tangled that would become, and you could probably pull the mass up in one complete clump. The same thing happens on a micro-scale for alkyds.
After this first drying step, they dry much like traditional oil paints, which is by auto-oxidation and cross-linking. So, they dry to the touch faster, and can get ‘stiffer’ faster, compared to oil paints, because of the polymer component, but they eventually dry in a similar fashion to oil paints.
Pt. 2. Yes, sometimes driers are added to alkyd paints and media (that is added to oils). This is because many types of alkyds used by artists’ are derived from semi-drying oils, like soya oil. These oils have fewer points of unsaturation in them, so they need more time to dry by auto-oxidation (which is why they aren’t used in traditional oils); however, when attached to the polymer chain, you can at least take advantage of the entanglement step to form a coherent film relatively quickly. The other advantage of semi-drying oils, is that they tend to yellow a bit less, compared to say, linseed oil. The driers are added for the same reason as traditional oils, to speed up auto-oxidation.
2. The second question is a biggie. Yes, not all alkyds are created equally. You can have short, medium and long-oil alkyds, which will give you different final mechanical properties of the final film. Typically the shorter alkyds are used in industrial coatings, and long-oil alkyds are what are for artist grade alkyd materials. If we look only at long-oil alkyds, like I said above, they are often made with semi-drying oils (soya, safflower, sunflower, etc), but you can also find linseed oil ones. I imagine that the difference in mechanical properties (stronger (stiffer?) and more flexible), are due to multiple things, like drying rate (dictated by oil type and driers, as well as drying conditions) and additives. I’ve encountered artists’ alkyd paints where the titanium white of one brand remains flexible after several years of natural ageing, and some that are already brittle after only a few months. This I linked more to additives (mainly fillers) rather than the alkyd itself. The tricky (or frustrating) part of all this, is that paint manufacturers are often changing/tweaking their formulations. So there can be some variation between the paints tested by scientists, conservators and artists, and a batch a few years down the line. Some of these changes are intentional, while some are unavoidable as a distributor may change their formulation too.
As for embrittlement, unfortunately, it is in the nature of oils and alkyds to get a bit stiffer over time. Adding drier, as you’ve pointed out, only quickens the drying, and consequently also a potential increase in brittleness. In any case, all cross-linking is indefinite, and there is no reversing it, at least in the sense of returning to original properties. You can have some low molecular weight scission products forming over time, but that is not the same.
As for number three, please refer to the posts above.
Dr. Rebecca Ploeger