Skins On Alkyd Paint

Q. I have a problem dealing with polymerization of an alkyd paint. The paint contains a modified alkyd resin, carbon and toluene. This coating is sprayed on a plastic substrate to manufacture variable resistors. Occasionally a rubber-like layer will form on top of the paint. When we mix in the rubber-like layer, the paint becomes useless to us because it now contains particles that clog our spray guns. One of my co-workers suggested storing the paint in a cold room, which would be expensive. I tested the shelf life of the paint by putting plastic wrap over the can to keep out air and the rubber layer did not form.

If you are familiar with these types of paints, I would certainly appreciate receiving any information as to whether or not the rubber-like layer is caused by heat, air or both. G.G.

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A. I believe you already answered your question. The right answer is that the rubber-like layer called skins or the effect that is called skinning is caused by contact with oxygen in the air. The reaction mechanism for curing alkyds resins is free radical polymerization which results from contact with oxygen. Hence, alkyds should never be stored in open containers. You can prevent skinning of these paints in closed containers by limiting the air space above the liquid level by pouring leftover paint into smaller cans or filling the cans with inert materials to raise the liquid level. Another proven method is to fill the head space with on inert gas to eliminate the oxygen.

It is well known that since the skin on these paints is polymerized, it can’t be blended or otherwise mixed into the rest of the paint. In your case, the skinning could cause other problems more serious than clogged spray guns. When paint skins, the pigment volume concentration (PVC) of the paint remaining in the container can change. With conducting paints, the conductivity or resistivity is directly related to its PVC, which determines the amount the amount of carbon present in the dry film. If the change in PVC is large enough, your variable resistors may not meet your specifications.