Plate Exposure

Light Sensitive Coatings - Lithographic plates are usually imaged using photographic films. Thus, the base material must be coated with a photosensitive material. Lithographic plates are sensitive in the UV regime; ~365 nm for photopolymer and ~400 nm for diazo coatings. They may be extended into the visible regime using dye sensitized photopolymerization. The plate exposure process generally involves a photochemical reaction, i.e. one in which the reaction is initiated by the exposure to light. Most photochemical reactions exhibit a characteristic S shaped curve shown below.

There is a threshold time (during which no perceptible reaction occurs) followed by a slow increase or toe region. The toe is followed by a linear region where a reciprocity law is followed. Finally a shoulder or asymptotic region is reached where the reaction progress saturates.

Light Sources for Plate Exposure - The light that actually initiates the chemical reaction is called the actinic light. It generally consists of a narrow range of wavelengths (or frequencies) which are of the right energy to participate in the reaction. Mercury Vapor - A quartz tube with mercury and two electrodes. When current flows mercury is evaporated and ionized, which produces light. Peak wavelength are between 360 and 370 nm. Well matched with photopolymer plates.

Metal-halide - A mercury vapor lamp with silver halide or halides of iron, barium, or titanium included in the lamp. With proper choice of additive can be made to peak between 380 and 420 nm. (Typically 400-410 nm.) Often called “diazo” type because of efficiency for exposing diazo coatings because of frequency match with diazo coatings. Our nuArc plate exposure unit uses a metal-halide lamp to expose Imation Viking plates which have a diazo coating.

Diazo coatings - Used for both additive and subtractive presensitized plates. These are generally formed by polymerizing a salt of the diazonium ion. The material is polymerized with formaldehyde in the presence of zinc chloride and sulfuric acid.

When the diazo coating is exposed to light in the 400-410 nm range, the triple bonded nitrogen in the diazo group is split and released as N₂. The remaining molecule is insoluble in the developing solution. Subtractive plates employ a relatively thick layer of diazo polymer. The exposed diazo material forms a durable ink-receptive image area which is insoluble in the developer (usually a aqueous mixture of alcohols and surfactants). The developer removes (“subtracts”) the unexposed coating from the nonimage areas.

Additive plates employ a relatively thin layer of diazo polymer. These are developed with a on-step emulsion consisting of a aqueous phase (containing acidified Gum Arabic) and a solvent phase (containing an oleophilic resin and a pigment). When rubbed on the plate the emulsion breaks (separates into its component phases). The resin and pigment are deposited in the image areas and the aqueous phase dissolves the unexposed coating and deposits gum on the nonimage areas.

Photopolymer Presensitized Plates - These plates employ polymerization which is initiated by light. Both linear polymerization and cross-linking are involved. Note the diazo polymerization is linear. Photopolymer plates have attracted much current interest because of their use with laser exposure in computer to plate systems. In printing we are generally more familiar with photopolymer plates in flexography.