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Ask the Expert Question-and-Answer Archive by Ron Joseph May, 2005 Emulsion and Latex Paint Q. I would like to know alil bit more about latex acrylic and latex emulsions and the main difference between the two kinds in detail. A. Here are some definitions of emulsion. I'll give an explanation as it relates
to paints later in this reply. Liquid droplets dispersed in another immiscible liquid. The
dispersed phase droplet size ranges from 0.1 - 10 µ m. Important oil-in-water
food emulsions, ones in which oil or fat is the dispersed phase and water is
the continuous phase, include milk, cream, ice cream, salad dressings, cake
batters, flavour emulsions, meat emulsions, and cream liquers. Examples of food
water-in-oil emulsions are butter or margarine. Emulsions are inherently unstable
because free energy is associated with the interface between the two phases.
As the interfacial area increases, either through a decrease in particle size
or the addition of more dispersed phase material, i.e. higher fat, more energy
is needed to keep the emulsion from coalescing. Some molecules act as surface
active agents (called surfactants or emulsifiers) and can reduce this energy
needed to keep these phases apart. A suspension of small drops of 1 liquid in a 2nd with which the 1st will not mix. Emulsions can be formed either by mechanical agitation, or by chemical processes. Unstable emulsions will separate with time or temperature. Stable emulsions will not separate. http://www.distributiondrive.com/glossary.html Emulsion: A mixture in which two immiscible substances, like oil and water, stay mixed together thanks to a third substance called an emulsifier. The emulsifier is usually something like a soap, whose molecules have a water-soluble end and an organic-soluble end. The soap molecules form little balls called micelles, in which the water-soluble ends point out into the water, and the organic-soluble ends point into the inside of the ball. The oil is stabilized in the water by hiding in the center of the micelle. Thus the water and oil stay mixed. www.psrc.usm.edu/ An emulsion is an unstable mixture of two incompatible liquids, such as oil
and water. In the case of mayonaise, we add oil to egg yolk VERY slowly so that
the "emulsion" won't break. In the case of a latex paint we add water
to an organic resin, such as an acrylic in such a manner that the emulsion won't
break. Since the resin is an oil-based product (ultimately derived from crude
oil) we add a small amount of a coalescing agent that enables us to mix these
two unlikely components. In the case of a latex paint the coalescing agent is
compatible with the resin and is also compatible with water. For instance, household
soap is compatible with grease and water. After we have mixed the latext paint,
(which is an emulsion), we apply the paint and ideally allow the water to evaporate
off before the coalescing agent can evaporate. Eventially the co-solvent (coalescing
agent in a latex paint) evaporates and a uniform paint film is formed. Best wishes, Ron Joseph |