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Surface Modifier Types
The vast majority of surface modifiers fall into two chemical types depending on the anchor group. The first type uses carboxylic acid groups or their precursors for anchoring, with fatty acids being the prime example. These react well with amphoteric and basic surfaces, but not with acidic ones (in reality fillers generally have both basic and acidic sites present, but in most cases one type dominates). The other type uses metal alkoxy groups to provide the anchoring, especially silicon alkoxide groups. These react well with surfaces containing free hydroxyls, but not with carbonates, sulfates, and similar surfaces. Neither type is effective on carbon blacks, but fortunately, their surface is sufficiently reactive to achieve good interaction with many polymers.
Carboxylic Acids and Related Compounds
For a recent review of this subject, see Rothon (2010).
Carboxylic acid groups provide a convenient method for anchoring organic species to basic and amphoteric fillers, as they can react with the surface to produce carboxylate salts. Additives containing them are widely used as surface modifiers, especially with calcium carbonates. In addition to the free carboxylic acid, it is possible to use a precursor, such as an acid anhydride. Carboxylate salts can also be used as additives themselves in some cases. It should be noted that the term “stearate” is used loosely in the literature and can mean stearic acid, sodium stearate, calcium stearate, zinc stearate, or ammonium stearate. Each of these is chemically unique and will not perform in exactly the same way.
The carboxylic acidic groups can be part of simple monomeric species or attached to a polymeric backbone. Finally, the additives can be either noncoupling or coupling, depending on the groups attached to them.
Fig. 4 Idealized representation of the different states for a fatty acid modifier on a typical mineral filler with divalent metal ions (e.g., calcium carbonate)
Care has to be taken with the terminology and with the completely separate use of carboxylic acid salts as processing aids (e.g., internal and external lubricants). The following states can be distinguished in polymer composites containing carboxylic acid derivatives and fillers:
Free salt of the acid (i.e., not chemically attached to the filler surface)
Bound salt of the acid, where the metal ion is still part of the filler surface
This is illustrated in Fig. 4.
The aim of surface modification is to maximize the bound salt. The importance of the other two types varies significantly with the system. Free acid is often detrimental, while the free salt may act as a processing aid (indeed, such salts are frequently added for this purpose).
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