Talc, surface chemistry

The origin of the talc ore used in polypropylene will have a significant influence on the color and long-term oven aging performance of the composite. Numerous tests have been performed to determine the variables in talc that contribute to this difference. It appears that talc surface chemistry can lead to talc-polymer and talc-stabilizer interactions that can cause discoloration and depolymerization under long-term heat (150-160°C) aging. High iron content in the talc can also adversely influence the level of performance. 

In a large variety of applications, the surface of a solid plays an important role (e.g., active charcoal, talc, cement, sand, catalysis). Solids are rigid structures and resist any stress effects. Many such considerations in the case of solid surfaces will be somewhat different for liquids. The surface chemistry of solids is extensively described in the literature (Adamson and Gast, 1997 Birdi, 2002). Mirror-polished surfaces are widely applied with metals, where the adsorption at the surface is much more important. Further, the corrosion of metals initiates at the surfaces, thus requiring treatments based on surface properties. As described in the case of liquid surfaces, analogous analyses of solid surfaces can be carried out. The molecules at the solid surfaces are not under the same force field as in the bulk phase (Figure 5.1). 

Fig. 6 Influence of surface chemistry of pigments on theophylline release from pellets coated with Eudragit RS 30 D containing dibutyl phthalate as a plasticizer. (V) Titanium dioxide as fine platelets ( ) mica (x) talc (O) red iron oxide fine platelets. (From Ref. f)...

The surface chemistry of talc is not well understood. Reactive groups that form upon talc s fractured surface are (a) weaMy acidic terminal hydroxyl groups (HO—Si —> + [0—Si] ) (b) sites ofproton release through polarization of water... 

The surface chemistry of talc is not fully understood. It is theorized that when the talc crystal is fractured, certain reactive groups are formed at the edge, perpendicular to the plate, that can potentially react with the polymer or deactivate stabilizers. The theorized surface chemistry of a talc crystal is shown in Fig. 8.4. When slurried with water, talc has a pH of 9-9.5, yet most proposed reactive sites are acidic in nature. The reversible nature of these surface reactions is probably what leads to the basic response in the water slurry. 

Most talc sold to paper, ceramics, and other industrial customers is manufactured to specifications agreed to between the producer and consumer. In paper, properties such as color, abrasion, surface area, and tint ate most important, whereas in ceramics, oxide chemistry, fired color, pressing characteristics, and alkaH metal content ate mote important. There ate some military specifications for talc used in corrosive coatings (6) and for cosmetic talc products used for cleaning of personnel in chemical warfare zones (7). 

According to these approaches, one assumes a given chemistry and/or morphology of the solid surface. Yet, the same function cannot simultaneously describe a macro-heterogeneous surface solid such as talc, clay minerals or crystalline silicas, having basal and lateral surfaces and a micro-heterogeneous one like the surface of an amorphous silica. Therefore many authors tried to develop resolution methods without making any assumption for the shape of the distribution function. 

An effective surface treatment chemistry for talc has eluded scientists for decades because the talc platelet faces are inert and only the edges contain reactive sites. The rheological study confirmed that talc did not respond to any of the usual dispersant types, but, surprisingly, trichlorosUane was seen to be very efficacious. This could pave the way for effective dispersants and coupling agents for talc. 

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