Carbon black rubber-grade

Table 7. Typical Properties Rubber-Grade Carbon Blacks ...

For very many years it has been common practice to improve the electrical conductivity of plastics and rubbers by the incorporation of certain additives like special grades of carbon black. Such materials were important, for example, in hospital operating theatres where it was essential that static charges did not build up, leading to explosions involving anaesthetics. 

Carbon blacks are the most widely used fillers for elastomers, especially vulcanised natural rubber. They cause an improvement in stiffness, they increase the tensile strength, and they can also enhance the wear resistance. Other particulate fillers of an inorganic nature, such as metal oxides, carbonates, and silicates, generally do not prove to be nearly so effective as carbon black. This filler, which comes in various grades, is prepared by heat treatment of some sort of organic material, and comes in very small particle sizes, i.e. from 15 to 100 nm. These particles retain some chemical reactivity, and function in part by chemical reaction with the rubber molecules. They thus contribute to the crosslinking of the final material. 

The effects of carbon black morphology on dispersibility described above have been borne out by practical experience. Higher surface area and lower-structure blacks are known to be more difficult to disperse. Traditionally, carbon blacks with surface areas higher than 160 m /g and CDBP lower than 60 mL/100 g cannot be sufficiently well dispersed using normal dry-mixing equipment, so they are not considered rubber grades. Figure 33.4 shows the ASTM carbon black spectrum used in the mbber industry, expressed by compressed DBPA versus surface area. 

FIGURE 33.4 Surface area versus structure for rubber grades of carbon black with some nonrubber grades. 

Rubber-grade carbon blacks, 4 775 classification, 4 777 composition, 4 765t properties of, 4 778t spectrum of available products, 4 779 uses of, 4 793-796, 794t Rubber industry... 

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. 

The tinting strength of rubber-grade carbon blacks shows a linear relationship with D s shown in Figure 5. Since performance characteristics are known to depend on aggregate volume, surface area, and bulkiness, it appears that the D s values combine the effects of all these factors. As such, it is a valuable addition to carbon black characterization methodology. 

Fig. 11. Electron micrographs of rubber-grade carbon blacks where (a) is N110, (b) is N220, (c) is N550, and (d) is N762.

Another standard industry method for surface area is based on the adsorption of cetyhrimethylammonium bromide (CTAB) from aqueous solution. This is ASTM method D3765-85 (2). This method measures the specific surface area of carbon black exclusive of the internal area contained in micropores that are too small to admit the large CTAB molecules. For rubber-grade nonporous blacks the CTAB method gives excellent agreement with nitrogen... 

The increasing demand led to new production processes. The most important process today is the furnace black process. It was developed in the United States in the 1930s and substantially improved after World War II. It is a continuous process, which allows the production of a variety of carbon black types under carefully controlled conditions. Nearly all rubber grades and a significant part of pigment-grade carbon blacks are now manufactured by the furnace black process. Nevertheless, other production processes, such as gas black, lamp black, thermal black, and acetylene black processes, are still used for the production of specialties. 

In the past decades the rapidly expanding automobile industry required increasing numbers of tires with various characteristics. This led not only to the development of new rubber grades, but also to the development of new carbon blacks required by the increasingly refined application processes and to the development of a new and better manufacturing process, the furnace black process. Unlike the old channel black process, this process allows the production of nearly all types of carbon black required by the rubber industry. It also meets the high economic and ecological requirements of our times. 

For a deeper understanding of structure-property relationships it is useful to consider the effect of carbon black grade and concentration as well as polymer type on the dielectric properties more closely. In Fig. 29 the real part of the a.c.-conductivity o at 20 °C of a series of rubber composites, consisting of the more polar statistical co-polymer NBR and the fine black N220, is depicted for various filler concentrations in the high frequency regime up to 1 GHz. For the lower carbon black concentrations, a power law behavior with exponent around 0.6 is observed, while the highly filled com-... 

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