1.  Field of the Invention
The present invention relates to a planographic printing plate precursor and in particular to a planographic printing plate precursor capable of printing with an infrared laser for direct plate-making from digital signals in computers etc.  Further, the invention relates to a laminate in which the planographic printing plate precursor capable of printing with an infrared laser is laminated on an inserting paper contacted with the outermost surface of the precursor at the side of a negative image recording layer, to protect the negative image recording layer etc. 
2.  Description of the Related Art
Development of lasers is remarkable in recent years, and in particular, high power output and miniaturization of solid lasers and semiconductor lasers having an emission range in the near infrared to infrared range are being developed.  Accordingly, these lasers are very useful as light source used in exposure for direct plate-making from digital data in computers etc. 
A negative planographic printing plate precursor for infrared laser, using an infrared laser having an emission region in the infrared ray region as light source used in exposure, utilizes a recording system where crosslinking reaction by an acid is caused by an acid generated as an initiator by light or heat to cure a recording layer on the region exposed to light, to form the image region, or polymerization reaction is caused by radicals generated as an initiator by heat to cure a recording layer on the region exposed to light, to form the image region, or when a dye is used as an infrared ray absorbing agent, polymerization reaction is caused by radicals generated as an initiator by transfer of electrons from the dye having absorbed infrared rays, to cure a recording layer on the region exposed to light thus forming the image region. 
Because such negative image forming materials are inferior in image formability to those of positive type causing solubilization of the recording layer by the irradiation energy of an infrared laser, heat treatment called burning treatment or baking treatment is generally conducted after the development step in order to promote curing reaction such as crosslinking or polymerization to form a strong image region. 
However, when an aluminum substrate is used in the planographic printing plate precursor provided with the recording layer, heat is diffused significantly into the highly thermally conductive substrate, so the energy of infrared rays irradiated is diffused into the substrate and not utilized for initiation and promotion of crosslinking or polymerization reaction for forming an image, thus failing to achieve sufficient sensitivity and strength of the image region. 
To transfer a plurality of planographic printing plate precursors, use is made in some cases of a means of forming a laminate thereof by alternately laminating planographic printing plate precursors on inserting papers such that each inserting paper is contacted with a recording layer of the planographic printing plate precursor thereby protecting the recording layer.  If the mechanical strength of the image region on the negative image recording layer is insufficient as described above, the friction between the recording layer and the inserting paper can cause damage to the surface of the recording layer upon transfer of the laminate with an auto-loader or upon removal of the inserting paper from the laminate.  If the recording layer is damaged, an undesired curing reaction is initiated from the damaged region, resulting in the problem of staining on the non-image region. 
In the planographic printing plate precursor provided with a negative image recording layer forming an image region by polymerization reaction, the negative image recording layer is provided in some cases with an overcoat layer having a function of shielding the recording layer against the air inhibiting the polymerization reaction.  When a laminate made of the planographic printing plate precursors having such a structure and inserting papers is transferred, the friction between the overcoat layer and the inserting paper can cause damage to the surface of the overcoat layer.  If the overcoat layer is damaged, the function of shielding the recording layer against oxygen in the damaged region is lost thus causing polymerization to be inhibited, resulting in occurrence of white mars on the image region.