Surface acidity carbon black

Figure 4. Neutralization of carbon black surface acids on black pearls (2) in aqueous sodium hydroxide. Key 1.0...

Before the 1970s, carbon black reinforcement of elastomers was generally considered chemical by nature (Wiegand, 1925). It was supposed that carbon black surface acidic groups were reacting with natural rubber basic moieties conducting to a strong covalent bond that was responsible for carbon black reinforcement ability. 

It was then obvious that chemical reaction of carbon black surface acidic groups with natural rubber basic moieties was not responsible for reinforcement. So the newly discovered mechanism of molecular slippage, proposed by Dannenberg and based on molecular adsorption, was quickly and fully adopted (Dannenberg, 1975 Dannenberg and Brennan, 1966 Dannenberg, 1960). 

Both types of surface oxides are found on technical products. Rubber grade carbon blacks are produced in different processes. Channel blacks are made by cooling a flame on iron plates, the so-called channels. The resulting carbon blacks are acidic in character because an excess of air is present (25). In the production of furnace blacks, the fuel, mostly oil or natural gas, is burned with a limited supply of air. Thermal blacks are obtained by thermal cracking of the gas, which sometimes is diluted with hydrogen. In consequence, both types show weakly basic reaction in aqueous suspension. 

Another attempt by Tricas et al. to modify the surface of carbon black was by the plasma polymerization of acrylic acid [34]. Treatment with acrylic acid made carbon black hydrophilic. Plasma-coated carbon black was mixed with natural rubber and showed increased filler-filler interaction. The bound rubber content was reduced after the surface treatment of the filler. The authors also concluded that the surface of the carbon black was completely covered by the plasma polymer film, preventing the carbon black surface from playing any role in the polymer matrix. 

The oldest method for the modification of carbon black surface chemistry is oxidation. Common oxidants include air, hydrogen peroxide, hypochlorites, nitric acid, nitrogen dioxide, ozone and persulfates. Each reagent produces a mixture of oxygen functional groups on the surface, with the distribution depending on the oxidant. Materials that disperse in water can be produced with sufficient oxidation, and hypochlorites and persulfates have been used to make water dispersible carbon blacks for inkjet inks. 

It is worth noting that the model for the carbon black surface deduced from these observations possesses a limited predictive capability for other materials systems than those studied herein. The current viewpoint that polymer interactions may be discussed in terms of Lewis acidity and basicity associated with particular molecular groups comprising the polymer(44-46) coincides with the present description of the origin of carbon black activity. Specifically BPL, which contains localized Lewis acid sites, can be expected to interact readily with polymer sites that are capable of acting as a Lewis base towards the carbon sites. On the other hand Graphon, which lacks these localized Lewis acid sites, is predicted to interact weakly with the same polymer sites. Contact charge injection experiments (3 3) provide a particularly sensitive probe of the carbon-polymer interaction and may supply the best means to test such model predictions. 

This observation allows us to believe that acidic groups on carbon blacks are mainly produced by surface oxidation in the production process, probably during drying following pelletization. Therefore, acidic groups could be considered an alteration of carbon black surface. 

Oxygenated Functions. Oxygenated functions on carbon black surface were observed in the early 1950s [70] and completely characterized by H. P. Boehm in the 1960s [71]. At this time, interaction between carbon black and natural rubber was considered the consequence of chemical reactions between the carbon black surface s acidic groups and basic moieties present in the natural rubber structure [71a]. 

The carbon black surface function characterization consists of suspending a given amount of carbon black in solutions of known normality of basis of different strength NaHCOs, Na2C03, NaOH in water, and EtONa in ethanol [72]. Then carbon black is filtered and the number of reacted acidic groups obtained by titrating the remaining basis in filtrate (Fig. 2). 

Some examples of these applications include studies on p-hydroxybenzoic acid-6-hydroxy-2-naphthoic acid copolyester-based adhesives [269] and further miscellaneous studies on adhesion [270, 271]. West [272] has characterised medical polymers using XPS and ToF-SIMS. These two techniques have also been used to characterise carbon black surfaces [273] and carbon fibres [274]. Other workers have reviewed various aspects of the application of ToF-SIMS to polymer surface studies [237, 275-277] (See also Section 3.11.1). 

Among recent references to cationic polymerization in the presence of carbon black is the report of a graft polymerization of poly(lV-vinyl-2-pyrrolidone) (NVP) onto a carbon black substrate. Typical carbon black surface structures include carboxylic acid groups. From the dependence of polymerization rate on the concentration of these groups and the effects of surface treatments with basic compounds the inference is drawn that the carboxylic acids initiate cationic polymerizations. In the case of NVP grafting the mechanism would presumably involve proton initiation followed by termination on the residual anionic surface groups. 

The introduction of azo groups onto the carbon black surface was readily achieved by the reaction of 4,4 -azobis(4-cyanopentanoic acid) with surface isocyanate groups, which were introduced by the reaction of phenolic hydroxyl and carboxyl groups on the carbon black surface with tolylene-2,4-dusocyanate (TDI) as shown in Scheme 2 (Tsubokawa et al., 1990). 

However, the opposite effect of the carbon black with highest volatile content on thermal stability was observed. This phenomenon is attributed to the adsorption of antioxidant by the carbon black surface or to the sensitization of thermal oxidative reactions by the surface oxygenated groups present. It has been proved that different behaviour is related to the volatile content, due to the presence of carboxylic and sulphonic acid groups. Not only are the temperature and rate of decomposition influenced by embedded carbon black particles, but also the decomposition products. The presence of 1-alkene oligomers with 3n C atoms is reduced, while 2-alkenes and 1-alkenes with 3n+l C atoms are increased. Carbon black promotes chain scission and participates in the radical transfer reactions. Incorporated particles of carbon black, especially those with small particle size, improve UV durability. ... 

These effects can be illustrated more quantitatively. The drop in the magnitude of the potential of mica with increasing salt is illustrated in Fig. V-7 here yp is reduced in the immobile layer by ion adsorption and specific ion effects are evident. In Fig. V-8, the pH is potential determining and alters the electrophoretic mobility. Carbon blacks are industrially important materials having various acid-base surface impurities depending on their source and heat treatment. 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Substitution of some of the alkoxy groups on the polytitanoxanes with glycols, P-diketones or P-ketoesters, fatty acids, diester phosphates or pyrophosphates, and sulfonic acids gives a group of products that are very effective surface-treating agents for carbon black, graphite, or fibers (32). 

---