| Résumé : |
The term emulsion, as applied to coating binders as is known today, is, in fact, a misnomer since by definition an emulsion is a mixture of two or more immiscible liquids, and binders are a mixture of an immiscible solid and a liquid. The correct terminology should thus be a resin dispersion but, for the sake of convenience, will always be referred to as "emulsion".
Aqueous polymers exist as three specific types. These types vary very significantly in both physical and mechani-cal properties. These three types are aqueous dispersions or emulsions, colloidal dispersions and true solutions.
Aqueous dispersions, or emulsions as they are commonly known, can be defined as discrete particles of high molecular weight polymer dispersed in an aqueous medium. The solid polymer is known as the disperse phase, while the water is the disperse medium.
Colloidal dispersions can be considered to be ultra-fine particles or molecular weight intermediates between that of aqueous dispersions and true solutions. The polymers contain polar groups, either acidic or basic, which do impart some degree of solubility. The colloidal dispersions can also be considered as hybrids of dispersions and true solutions, in that they possess properties which are common and characteristic of both types. True solutions, or water reducible resins, are polymers in which polymerisation reactions are performed in water miscible organic solvents such as esters and alcohols, or solubilised by the addition of alkali, i.e. neutralisation. Solubility is also attributed to polar groups on the polymer side chains. The polymers are thus totally soluble in the medium water.
The above generalisation will thus suffice to explain the varions "states" in which aqueous polymers are available. Emulsions find application in the function as binders or topcoats for leather finishing, while colloidal dispersions are used to a smaller extent as thickeners for the finish mixes and more commonly, as pasting adhesives. Water reducible urethane lacquers are also the colloidal dispersions type. True solutions are not currently used in leather finishing but are in the paint industry, particularly the automotive and appliance industries. |
Aller sur edunet
Accueil
The effect of particle size on the characteristics and performance of emulsion binders and topcoats in JOURNAL OF THE SOCIETY OF LEATHER TECHNOLOGISTS & CHEMISTS (JSLTC), Vol. 62, N° 4 (07-08/1978)
