Лако-красочные материалы — производство

Carbon black is used in large amounts for the black and gray tinting of plastics, in­cluding polyethylene, polypropylene, polyvinyl chloride, polystyrene, ABS polymers, and polyurethanes. The largest application in tonnages is in polyolefins. Pelletized black is usually employed.

Carbon black concentrations of 0.5-2% are required for full-tone tinting. For crystal-clear plastics, the addition of up to 1% carbon black is generally sufficient. Plastics with marked intrinsic color, e. g. ABS, are tinted with blacks of higher tinting

strength at concentrations of 1-2%. Transparent tints have black contents of 0.02­0.2%.

The color depth (blackness) increases as the primary particle size and the degree of aggregation decline. The blackness achieved by a given carbon black, however, also depends on the polymer. Optically critical deep black tinting is achieved with HCC and HCF gas and furnace blacks. For general black tinting, where optimal brilliance and color depth are not essential, lower-priced blacks of moderate fineness belonging to classes MCC, RCC, and MCF are employed. Coarse furnace blacks and flame black are suitable for tinting.

Most of the carbon black used in the plastics industry is produced by the furnace black process. Fine furnace blacks often appear more bluish in incident light (full — tone tinting) than coarse ones, but more brownish in transmitted light (transparent tinting) and in gray tints. By controlling the conditions of black manufacture, how­ever, substantial shifts in hue can be achieved. Bluish carbon blacks are commonly preferred to achieve the impression of greater color depth.

Blacks for tinting of products used for food packaging and storage must comply with relevant legal provisions.

Dispersion is crucial for tinting quality. In order to develop its full coloristic prop­erties, carbon black requires much greater shear forces for dispersion than other pigments. Optimal dispersion, i. e., complete disintegration of agglomerates into discrete particles (carbon black aggregates), and coating with the polymer is more difficult as the finer the black is and the lower its degree of aggregation (black struc­ture). Unsatisfactory distribution of the black has a negative effect on the tinting strength and becomes a problem, especially in gray tints and in the black tinting of opaque plastics. The depth of color in black-tinted transparent plastics, on the other hand, is only slightly affected by a small proportion of poorly dispersed black. Poor dispersion can also cause surface defects (specks) in plastic parts and mechanical defects, especially in films and fibres.

Good distribution of the black in the plastic is achieved by incorporating it in two stages. In the first step, a carbon black-plastic concentrate (master batch) is prepared, for example in kneaders. This contains approximately 20-50% carbon black, depending on the binder absorption of the black. In a second operation, the concentrate is diluted with the appropriate polymer to give the final black content. Dispersion of the black takes place during the preparation of the concentrate. The high black concentration increases the viscosity and enables the medium to transmit higher shear forces than it would be possible if 1-2% black were incorporated directly.

The carbon black-plastic concentrates are often not prepared by the plastics pro­cessor. They may be purchased, from the pigment black manufacturer in the form of chips, pellets, or powders. Black plasticizer pastes (e. g., black/DOP pastes for tinting plastisols) and water — and solvent-based dispersions (e. g., for fiber manufacture) are also commercially available.