what are the effects of paint binders on colour retention and viscousity of an emulsion paint?

Answer 1

"Emulsion" (in paint) refers to a mixture of 2 elements that normally do not blend together... like oil & vinegar in salad dressing. In the case of acrylic paint, Polymer resins (& other chemicals) and Water make up the emulsion, along with surfactants to keep the 2 dissimilar elements from separating. When the two are combined is Polymer Emulsion, and this is the paint's binder.

Pigment particles are suspended within the emulsion. Colour retention will depend on the clarity (when dry) and UV-resistance of the emulsion - but while it remains uncured, the emulsion is cloudy. Viscosity is reduced as the water begins to evaporate and the "binder" begins to pull the paint into an even film.

http://www.goldenpaints.com/technicaldata/faq/i_def.php
"Polymer is short for polymer emulsion, which indicates the chemistry involved in making the resins used to create paints and coatings. This is the binder or glue responsible for forming a continuous film, holding the pigments and other materials in place. A polymer is a large molecule made up of monomers, and the ones we use are called "butylacrylate" and "methylmethacrylate". In short, these polymers are floating around in water and we add thickeners and other additives to make the kind of medium, gel, paint, gesso, or whatever else we want to create. This is essentially what you have when you buy the paints or the Polymer Medium.

Once you brush some out, the water and additives start to evaporate out, and the polymers floating around start getting closer and closer. Some additives are like traffic cops and make sure everything lines up perfectly. When the film becomes dry to the touch, everything is in place. However, there is a slower curing in which the remaining water and additives leave the film and the polymers get so close they go from their round shape to a hexagonal one. Once the film reaches this, you have a strong film, and the best clarity. You may have seen this happening as the milkiness goes away and a couple of days or weeks later, the gloss and brilliance really come out."