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Saturated Fatty Acids
The main additives using carboxylic acid functionality as an anchor group are the saturated fatty acids. These are relatively inexpensive, are widely available products, and are the classic noncoupling workhorse surface treatment used on calcium carbonate and other basic fillers.
Fatty acids are usually derived from natural sources, where they occur as complex mixtures in both animal and vegetable matter. Separating out pure components from these mixtures is expensive, and economics dictates that blends of acids are normally used industrially. These blends are still quite complex, containing a number of different chain lengths and even small amounts of unsaturated acids and nonacid material. The composition of the blends used in precoating fillers is rarely specified and can vary markedly. Some of the minor components can cause problems in certain final applications and need to be minimized. Unsaturated acids (such as oleic) are a case in point and can adversely affect final properties such as color and heat stability.
Blends approximating to 18 carbon atoms are usually the least expensive and are those most often used for filler coating. They are often referred to as stearic acid or stearate coatings although this specific acid may be less than 50% of the overall composition.
Fatty acids are usually precoated onto fillers, and two methods of coating are generally employed: wet coating and dry blending. In the wet coating process, an aqueous solution of salt of the fatty acid is added to an aqueous slurry of the filler. If an alkali metal salt of the acid is used, then the coproduct containing it has to be washed out from the system. This can be circumvented by using the ammonium salt.
In the dry coating procedure, filler and fatty acid are usually reacted together in a high-shear mixer. Most fatty acids are solids and have to be melted before spreading and reaction can take place. In some cases, the shear action of the mixer generates enough heat, but external heating can also be used.
Very little information is available about the wet coating process although it has some commercial significance, especially in precipitated calcium carbonate production.
More information is available about dry coating methods, although it is still not extensive. Fekete and coworkers (1990) have shown that the adsorption and reaction are very dependent on the type of mixer and the exact conditions used.
While there has undoubtedly been much industrial interest in alternatives to the “stearic” acid blends, not much of this work has been published, and there is little evidence for the commercial use of alternatives. Iso-stearic acid is sometimes employed, probably because it is a liquid and easier to use in dry coating processes.
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