Lithographic Plate Composition

When positive-working diazo presensitized plates are made, surface treatments may not be necessary; fine graining and /or cleaning usually precede the application of positive-working diazos.

Anodizing. Most high-quality plates are anodized after graining. Aluminum anodizing is a process by which a thin, uniform layer of extremely hard aluminum oxide is produced electrolytically on the grained aluminum. This anodic layer has many extremely small pores, similar to a honeycomb. The anodic layer must be sealed before the photosensitive coating is applied. Usually, hot solutions of sodium silicate are used to treat the anodized layer, making it highly water-receptive. This process also prepares the plate surface to receive the light-sensitive coating. Furthermore, the anodic layer is hard, abrasion-resistant, and highly durable.

Diazo sensitizers. Both water and solvent-soluble diazos are commonly sued as a sensitizing agent for plates. Plsitive-acting plates are coated with diazo oxides or quinone diazides. Exposure to UV light converts negative-acting diazos directly to insoluble resins that have good ink receptivity and durability for printing. Positive-acting diazos decompose upon exposure to light and become soluble in the developer while the unexposed diazo remains on the plate, forming the image areas.

Photopolymer sensitizers. A thin polymer coating makes an excellent oleophilic surface. A number of different reactive polymers can be sensitized with a suitable photoinitiator for use as a plate coating. On exposure to light, the exposed parts of the coating become insoluble in the same solvents that dissolve the unexposed portions of the coating. Thermal plates often employ photopolymer coatings that harden when exposed to light. The resultant images are tough, and the plates generally withstand long runs. Most photopolymer coatings are aqueous (water-developable).

Silver-halide sensitizers. Silver-halide coatings have been used to sensitize graphic arts films for decades. Manufacturers are now using silver-halide as a sensitizer for plates. The light-sensitive siver-halides are suspended in an emulsion layer, which lies on top of what is called the nuclei layer. A barrier separates these two layers. T he laser in the platesetter exposes the nonimage areas of the plate, reducing the silver-halides. During the processing, the unexposed silver-halides diffuse through the barrier layer into the nuclei layer, where it is reduced to molecular silver, which is oleophilic in nature. A second processing step removes the exposed silver and corresponding nuclei layer from the plate surface.