|MadSci Network: Chemistry|
Dear Jen, Clearly I can feel with you the frustration experienced on searching for the secrets behind an everyday article like paint. One reason is the enormous variety in COATING formulations that exists today. Trade secrets also are important in this area, which developed largely by empirical methods rather than theory. The reason why paint sticks to a substrate has to do with ADHESION (Did you search the archives with this keyword?). Here paint chemistry borders on the science of "glues", which has been amply reviewed by MADSCIentists, see e.g. at http://www.madsci.org/posts/archives/jul99/932360244.Ch.r.html The references mentioned there are also useful for you. Many of the POLYMER types used in adhesives (e.g. epoxy, urethane, acrylics) are also used in paints. There, they function as BINDERS, i.e. upon drying of the paint film, they prevent the powdery PIGMENTS from blowing away at the slightest breeze. Polymers also make the finished paint film look glossy, if desired. To be able to apply the paint in the first place, it has to be liquid: this is the task of the SOLVENT (except in the case of "powder coating", which is applied in a molten state). These 3 components, pigment (white or colored), binder(s) and solvent, are present in every typical paint. In water-based paints, EMULSIONS are present in place of solutions. Historically, "drying oils" like linseed oil, had a double function of solvent and binder, since they transform into a crosslinked solid by oxydation in air. Artists' oil colors work that way. Other binders crosslink by radical polymerisation mechanisms (e.g. UV curing) or isocyanate reacting with hydroxyl groups to form urethane. Paint films that are crosslinked cannot be dissolved (only swelled, ie. weakened) by purely physical means (solvents). If the bonds are ester or amide bonds, they can be hydrolysed e.g. by ammonia. I never heard of tomato as a PAINT REMOVER (see that keyword in the archives). The coating (LINER) on the inside of canned tomato cans certainly has to resist their attack... Best regards Werner Sieber
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