1. Field of the Invention
This invention relates to an improved support for a lithographic plate, and more particularly to a support for a lithographic plate comprising a plate of aluminum or an alloy thereof (hereafter referred to as aluminum) which is characterized by the depth of a grain defined by the center line averge roughness (Ra) as well as the average diameter of pits in the grain and the distribution thereof.
2. Description of the Prior Art
Heretofore, an aluminum plate has been widely used as the support of a photosensitive lithographic plate. The surface of the aluminum plate is made coarse to give better adhesion to a photosensitive layer to be disposed on the plate or to provide higher wettability (water retention) with wettening water during printing. This process of surface roughening is called graining and the rough surface obtained is a grain. Two conventional methods of graining are mechanical graining and electrochemical graining. Ball graining which is one example of the former method is a very old method which has found wide utility in small-scale graining operation. However, this method is not efficient since it does not permit continuous operation. An industrially applicable mechanical graining that is substituted for ball graining is brush graining which rotates a brush or roller having steel wires or synthetic resin hairs, optionally in the presence of a sand or abrasive, on an aluminum plate. Whether ball graining or brush graining is used, a grain having various degrees of surface roughness and shape can be produced by properly controlling the kind and shape of the sand, mesh, graining period and brush movement. However, as reported in TAGA (Technical Association of Graphic Arts) Proceedings, pp. 262-276, 1972 mechanical graining leaves residual sand on the surface of the grain as part of the grain. In addition, as shown in the picture taken by a scanning electron microscope in tha technical paper, mechanical graining of an aluminum plate provides a worked surface having a very complicated three-dimensional configuration. Therefore, if a lithographic plate is produced from a presensitized plate having a photosensitive layer disposed on a mechanically grained aluminum plate, the sensitive material penetrates too deeply into the pits in the grain to be removed easily, thus often providing a stained non-image area.
Many studies have been made on electrochemical graining. British Pat. No. 831,998 as well as U.S. Pat. No. 3,072,546 and 3,073,765 disclose an aluminum support for a lithographic plate which is grained by A. C. electrolysis using hydrochloric acid as electrolyte. British Pat. No. 1,224,226 discloses a method of A. C. electrolysis of an aluminum plate in hydrochloric acid, followed by chemical etching and anodization. Japanese Patent Publication No. 27481/71 discloses a method of A. C. electrolysis of an aluminum plate in hydrochloric acid, followed by anodization.
It is generally known that A. C. electrolysis of an aluminum plate in an electrolyte mainly consisting of hydrochloric acid or nitric acid can provide the aluminum plate with a grained surface. The grain provided by electrochemical graining which is formed of grown pits has a crater-like or honeycomb stucture and is characterized by having straight and open pits as compared with the grain produced by mechanical graining described above. Another feature of electrochemical graining is that it provides a plate having deeper pits and a coarser grain then mechanical graining. The configuration and coarseness of the grain can be controlled by selecting the electrolyte and electrolytic conditions used. West German Patent (OLS) No. 2,650,762 describes the grain provided by electrochemical graining using hydrochloric acid or nitric acid as an electrolyte. A coarse-grained surface obtained by using nitric acid or an electrolyte mainly consisting of nitric acid produces pitting having a dual structure which comprises a pit provided by electrochemical etching plus an extremely small pit formed in its surface, but the opening is generally shallow. On the other hand, the use of hydrochloric acid or an electrolyte mainly consisting of hydrochloric acid provides pitting which is generally deep but the surface of individual pits is relatively smooth without a complex configuration as achieved by use of a nitric acid based electrolyte.
Although an aluminum plate grained electrochemically has a by far a coarser surface than a mechanically grained aluminum plate, it has not yet been considered a preferred support having advantages over and replacing the conventional supports for lithographic plates. Among the problems yet to be solved with an electrochemically grained aluminum plate are its low affinity for ink, short running life (number of sheets of paper which can be printed from one plate), its low printing sensitivity and low developing speed.