Lithographic Plates
Most lithographic plates have a photosensitive coating which can develop an image after being exposed to the appropriate wavelengths through a negative or positive film.
Current methods of platemaking employ aluminum and/or other metals, anodized aluminum, paper or photopolymer (plastic). Limestone plates are still used in lithography as an art form.
Lithographic plates are generally divided into two types, negative working, which are exposed through photographic negatives, and positive working, which are exposed through photographic positives.
Most plates used today are presensitized, i.e. a light-sensitive coating has been added by the plate manufacturer. A presensitized plate may be additive, i.e. a wear-resistant lacquer or pigmented resin is added to the image areas during development.
Alternatively, it may be subtractive in which case a oleophilic diazo resin is coated on the plate and hardened under exposure to light to make an ink-receptive image. The developer then removes i.e. subtracts, the coating from the nonimage area.
The plates used on our presses are Imation (formally 3M) Viking plates, which are based on a presensitized, possibly grained and anodized, aluminum surface, and are negative working and subtractive.
Current platemaking technology is moving toward faster acting plates which can be exposed by low power lasers and digitally driven. Metal based plates are usually “grained”, i.e. the surface is roughened to be more water receptive.
Roughened plates hold water better, are more durable and cause fewer problems with defects. Plates may be roughened mechanically or electrochemically.
Mechanical roughening involves brushing the surface with abrasive materials such as pumice and sand (SiO2). Electrochemical graining involves dilute acid solutions combined with an AC current between electrodes. Aluminum tends to build up a protective layer of oxide (Al2O3) on the surface.
This surface can be reinforced by anodizing. This process involves making the aluminum the anode (+) of a cell containing sulfuric or phosphoric acid.
The oxide surface is not sufficiently water receptive unless a hydrophilic polymer, such as acidified Gum Arabic, is added to the fountain solution.
The “Litho Etch” used in our lab contains Gum Arabic, which is a polysaccharide which contains ether linkages and hydroxyl groups along with carboxylic acid groups.
The carboxyl group reacts with the aluminum oxide on the plate 2RCOOH + Al2O3 -> Al2O(OOCR)2 + H2O where R denotes the polysaccharide backbone.
Since the Aluminum oxide is actually part of the solid plate this reaction actually chemically attaches the gum molecule to the plate. The ether linkages are of the form R-O-R' and alcohol groups are of the form R-O-H.
The oxygen atoms readily hydrogen bond to water which is responsible for the gum’s water solubility and the resulting water wetness of the gummed surface.
This is what really the makes the process work. The Viking Gum used with our plate processor contains Maltodextrin as a substitute for Gum Arabic.
Maltodextrin is also a polysaccharide, which is better known for its use in making beer. Other Ingredients - In addition to gum, dampening solutions contain Mg(NO3)2, which helps to buffer the solution and to reduce plate corrosion.
An alcohol such as isopropyl alcohol acts as a wetting agent (actually as a cosurfactant with the gum being the surfactant), which reduces the surface tension and helps avoid emulsification of ink into the fountain solution.
The dampening solution must be kept acid so that the carboxyl groups can bind to the plate surface. Often another acid such as phosphoric acid, lactic acid or citric acid.
Phosphoric acid also has some desensitizing properties. Usually the target pH is 3.5-4.5. Litho Etch contains Mg(NO3)2 and butyl Cellosolve (Ethylene glycol mono butyl ether). Substitute alcohols (for IPA) such as butyl Cellosolve are employed because IPA is classified as a VOC.
Conductivity -The conductivity of the dampening solution give a measure of the total ionic content, while the pH only measures the acidity.
If the conductivity is high enough and the pH is in-range, then the solution is probably buffered and will remain in the desired pH range. We have a combined pH and conductivity meter in the small press lab to aid in formulating fountain solution.