Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics. Such compositions are generally provided as imageable layers.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
It is known that many imageable elements that are imageable using thermal means such as lasers are insufficiently stable to white light for handling. As a result, such elements cannot be readily handled in room light without fogging or toning in the imageable layer. This instability can be caused by the presence of initiator components that absorb in the spectral region of from 300 to 500 nm. To alleviate this problem, the elements may be handled under “yellow” light or exposed to white light for very short periods of time. These restrictions are not desirable for various manufacturing and printing operations.
It is known that white light stability can be improved by adding a filter dye to layer disposed on the imageable layer. For example, the filter dyes can be incorporated into topcoats (overcoat) or layers between the imageable layer and a topcoat. See for example, EP Publications 1,152,294A1 (Murota et al.), 1,445,655A1 (Takamuki et al.), and 1,832,928A1 (Yamamoto et al.), U.S. Pat. No. 6,420,089 (Baumann et al.), and WO 2009/015467 (Nguyen et al.).
A filter dye is incorporated into an imageable layer in UV-sensitive elements described in EP Publication 1,906,245 A1 (Kawauchi) and the filter dyes have an absorption wavelength that is +50 nm of the sensitizing dye used for UV irradiation.
This is a need to provide white light stability in negative-working imageable elements without the need for an overcoat containing filter dyes since the removed filter dyes contaminate prewash baths and cause an environmental concern in waste water. It is further desired that this white light stability be achieved without a loss of other imaging properties (for example, photospeed).