Лако-красочные материалы — производство

Many toners used in photocopiers and laser printers contain black magnetic iron oxide pigments (magnetite Fe3O4). The heart of a copier or laser printer is a cylinder coated with an organic photoconductor coating (OPC). Before the actual copying or printing process, the cylinder is electrostatically charged by a high-voltage corona, and then exposed to light from a laser or projection system (Figure 5.5). The areas of the coating subjected to light give up a varying portion of the charge, while the unexposed areas, corresponding to the black dots of the original, retain their charge. Xerography then makes use of the effect that electrostatically charged objects attract dust. What happens is that a fine, black thermoplastic powder, the so-called toner, is sprinkled over the cylinder, and the charged areas become covered by the black toner dust. This produces a mirror image of the original on the roller. The particles are then transferred electrostatically to paper and fixed by heat exposure or pressure to make the image permanent [5.45].

A magnetic brush development system serves for the transportation of the toner powder. In a Dual Component System a rotating bar magnet is covered with a mag­netizable ferrite powder, the so-called carrier (component 1), giving it the appearance of a brush. The carrier is dipped into a bath of toner powder (component 2) and

transported past the cylinder at a specifically defined distance away from it. The powder becomes attracted to the areas that are electrostatically charged.

Laser printers and many copiers work by the one-component process, which allows the design of particularly compact machines. The principle of the magnetic brush is retained, however, the toner itself is magnetizable. This can be done by dispersing ca. 30-50 wt.% of black, magnetizable iron oxide particles in thermoplastic resins. Apart from the iron oxide, various additives are generally also incorporated to control the electrostatic chargeability, the flow characteristics and the color. The particle size of these toners is ca. 5-15 pm.

5.1.6.1