Лако-красочные материалы — производство

For a long time, characterization of carbon blacks was a question of determining different shades of black with the human eye. Precise data on the reinforcing effect were available only to a limited degree.

The composition described below refers to all carbon black grades, regardless of the production method. Process-related variances have already been addressed in the description of the various methods used today for the manufacture of carbon black. The specific surface area of carbon black is a function of its primary particle size. The structure designates the three-dimensional arrangement of primary particles in the aggregates. Extensive interlinking or branching characterizes a “high-structure”, whereas less pronounced interlinking or branching indicates a “low-structure”. The structure of a carbon black particle can only be determined indirectly. The most commonly accepted method is based on DBP (dibutyl-phthalate) absorption. DBP is added to a specified quantity of carbon black in a Brabender kneader. The mea­surement consists of registering the amount of added DBP to reach a predetermined level of torque generated by the kneading machine during titration.

The method is based on the premise that all interstices, known as void volumes, in the carbon black aggregates are filled with DBP. The surface of the carbon black is considered to be “wetted” and this new surface state is translated into a change in the torque required for the kneading machine. The DBP requirement thus allows conclusions on the degree of aggregation of individual carbon blacks. The rule: the greater the DBP absorption, measured in ml g-1, the higher the carbon black structure.

High-resolution electron microscopy shows the primary particles of carbon blacks in detail. This technology, combined with X-ray structural analysis, shows that the primary particles consist of concentrically arranged, graphite-like crystallites. By par­tially fusing together, the graphite layers are often twisted into each other, exhibiting a non-ordered state. One single primary particle can contain up to 1500 of such crystallites.

Scanning tunneling microscopy leads to the supposition that the primary particles consist of superimposed, scale-like layers of graphite. For this reason, the L-values determined by X-ray diffraction have to be regarded as a median overlap surface of graphite layers and not as the average size of individual crystallites.

The chemical analysis of carbon blacks detects in many cases minute quantities of oxygen, hydrogen, nitrogen and sulfur besides the carbon. Carbon blacks also contain traces of metals, the amounts and type being feedstock dependent. The type and the content of metals in pigment blacks can be summarized as follows:

Most of these elements are concentrated on the surface of the carbon black. Re­moval of traces of organic elements is possible with the use of special solvents. The carbon black extraction based on toluene, the toluene extract, results in values of mostly less than 0.1%. Further analysis of the material shows, among other sub­stances, minute quantities of PAHs (polycyclic aromatic hydrocarbons), which are tightly bound to the carbon black surface.

Hydrogen is, to a certain degree, directly fused to the carbon. However, together with oxygen, another part forms surface-bound functional groups that can be iden­tified by analysis, both qualitative and quantitative. Here, carbonyl, carboxyl, py — rone, phenol, quinone, lactol and ether groups have been identified as the oxygen — containing groups bound to the surface of the carbon black particles. Heating the substance to 950 °C in the absence of oxygen, however, results in separation. This explains their designation as “volatile matter”.

Oxygen-containing functional groups on the carbon black surface can also be cre­ated through specific oxidative aftertreatment. Oxygen content levels of 15% and higher are possible.

Sulfur is present in a variety of forms, in its elementary form, as a bound molecule, but also in an oxidized state. High sulfur contents import certain acidity to industrial carbon blacks. Nitrogen, when present, is usually included in the graphite grid. One thing to bear in mind is that sulfur and nitrogen content are contingent primarily on feedstock type and quality.

Among the physical properties of carbon blacks, the density and the electrical conductivity are the most important.

4.5.1