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Photographer's Formulary's Tween 20 , also known generically as Polysorbate 20 or sorbitan mono-9-octadecenoate poly(oxy-1,1-ethanediyl), is a surfactant and spreading agent.
First proposed for use in hand coated photographic printmaking by Dr. Michael Ware of the U.K. It is used extensively in industry but is usually confined to pharmaceuticals and other high-end applications due to its cost.
Tween 20 allows more emulsion to be put on the paper without the streaking and flaking that are normally associated with heavy coating.
Tween 20 does not "pull" the emulsion all the way through to the back side like Kodak PhotoFlo does.
It is typically used in very high dilutions. A 10 percent solution is a usual starting point, but some use it as dilute as a .1 % solution. Tween 20 almost always eliminates any need for double coating.