Carbon black surface acid groups

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). 

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. 

Experimental. Characterizations of a heterogeneous surface by means of surface group titration utilizing visible and ultraviolet chemical indicators to define the titration end point have frequently been employed with white solid catalysts(7-12), (17-20). Aspects of the surface acid group distribution have often correlated with the catalytic activity of the solid(2-9), (21-25). An adaptation of the technique appears to be suitable for studying the interactions between the surface acid groups in mixtures of carbon black and white reference solids. 

Advantages and disadvantages of the titration technique are discussed in detail elsewhere(26) It suffices here to state that the surface acid group distributions on the white reference solids are sufficiently well defined by the experiment to provide a gauge by which to measure the interaction of carbon black with the white solids in binary mixtures. 

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. 

Carbon blacks are manufactured by three production processes, furnace black, lamp black and gas black. Each process gives a range of surfaces with gas blacks having acidic groups while furnace blacks are weakly basic. 

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. 

In the 1970s, furnace gradually replaced channel technology. But even if furnace carbon blacks present 10 times less surface acidic groups, their reinforcing ability remains unchanged or increased. On the other hand, synthetic elastomers, which obviously have more basic moieties than natural rubber, were also perfectly reinforced by carbon black. In addition, the preparation of surface-oxidized carbon or grafted blacks (Le Bras and Papirer, 1983) leads to a decreased reinforcement ability. 

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). 

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. ... 

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). 

The physicochemical properties of carbon are highly dependent on its surface structure and chemical composition [66—68], The type and content of surface species, particle shape and size, pore-size distribution, BET surface area and pore-opening are of critical importance in the use of carbons as anode material. These properties have a major influence on (9IR, reversible capacity <2R, and the rate capability and safety of the battery. The surface chemical composition depends on the raw materials (carbon precursors), the production process, and the history of the carbon. Surface groups containing H, O, S, N, P, halogens, and other elements have been identified on carbon blacks [66, 67]. There is also ash on the surface of carbon and this typically contains Ca, Si, Fe, Al, and V. Ash and acidic oxides enhance the adsorption of the more polar compounds and electrolytes [66]. 

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