Лако-красочные материалы — производство

The incorporation of sufficiently large amounts of blacks can impart antistatic (re­sistivity 106-109 Q) or conductive (<106 Q) properties to plastics. The conductivity

of a plastic with carbon black filler increases with decreasing particle size and in­creasing degree of aggregation of the black, and increasing carbon black concentra­tion. Surface oxides and hydrogen-containing surface groups of the black diminish the conductivity. The distribution of the black in the plastic is also crucial for the conductivity. As the black is incorporated into the plastic, the conductivity initially increases, passes through a maximum, and, if the black is subject to excessive shear force, it decreases. Conditions are optimal when the black is uniformly distributed in the polymer but is not so widely dispersed that the black particles are completely surrounded by the medium. Only under these conditions, can bridges be formed between the particles, thus promoting the flow of current. Orientation of the black particles (e. g., in extrusion or injection molding) permits anisotropic conductivity. Conductivity also depends on the polymer system, due to variations in wettability or local concentration by partial crystallization of the polymer. For example, to obtain the same conductivity, nonrigid PVC requires roughly double the black concentration that polypropylene does.

Blacks used to produce conductive and antistatic plastics are chiefly high-structure furnace blacks with relatively fine particles and low contents of volatile components. Black concentrations for conductive systems are around 10-40%. Antistatic plastics (e. g., cable sheathing and floor coverings) contain 4-15% carbon black.

Relatively coarse-grained oxidized gas blacks with high volatile contents are es­pecially suitable for plastics with good electrical insulation (e. g., cable sheathing compounds, plastics for high-frequency welding, toners).