Presensitized printing plate with pigmented back coating

Details Presensitized printing plate with pigmented back coating Presensitized printing plate with pigmented back coating

2001-06-19

2003-12-30

Hamilton, Cynthia (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Radiation sensitive composition or product or process of making

C430S276100, C430S056000, C430S496000, C430S950000, C430S271100, C430S531000

Reexamination Certificate

active

06670097

ABSTRACT:

The present invention relates to a recording material for the production of offset printing plates having a dimensionally stable support, a radiation-sensitive layer located on the front of the support, and a layer which comprises an organic polymeric material and which is resistant to processing chemicals located on the back of the support.
Recording materials for the production of offset printing plates (also known as “presensitized printing plates”) are usually supplied in stacks of 20 units or more. Extended storage times, the action of pressure and/or elevated ambient temperatures frequently result in the plates adhering to one another. On removal of individual plates from the stack, scratches may then form on the front and/or back. The problem of undesired adhesion can be substantially eliminated with the aid of separating paper. The paper is particularly necessary in the case of recording materials having an aluminium support with an uncoated back. However, the separating paper results in new problems. The recording materials are frequently produced in in-line finishing plants, in which the plates are automatically cut to the desired size and packed. The separating paper is likewise inserted automatically. However, this step is relatively slow and susceptible to faults. In addition, the paper in some cases affects the radiation-sensitive layer and adversely changes its properties. This may result in discoloration of the layer, due to a change in the pH, a drop in its light sensitivity or rapid ageing. With surface-sealed papers, the interaction between paper and radiation-sensitive layer can be reduced; however, such papers are significantly more expensive. In relatively large print shops, the plate stacks provided with separating paper are generally processed in automatic plants, with the paper usually being blown out. This operation is again relatively slow and susceptible to faults. In addition, the paper cannot be recycled and has to be disposed of.
The recording material described in JP-A 02/040657 manages without separating paper. A UV-cured layer produced from a photopolymerizable material is located on the back of its aluminium support. In addition to monomers, the composition used for the production of the back coating may also comprise photosensitizers, binders, fillers, inhibitors for preventing thermally induced polymerization of the monomers and other additives.
JP-A 06/202312 discloses a recording material for the production of offset printing plates whose aluminium support is likewise coated on the back with an organic polymer, such as polyethylene, polypropylene, polybutadiene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polystyrene or a methacrylate resin. The back coating reduces attack by the aqueous-alkaline developer on the aluminium support. The light-sensitive layer in this recording material comprises from 1 to 10% by weight of a compound which is insoluble in the developer.
A recording material having an anodized aluminium support, a photopolymerizable layer on the aluminium oxide layer produced by anodization, and a back coating with a thickness of from 0.1 to 8.0 &mgr;m is disclosed in JP-A 09/265176. This coating consists of a saturated copolymerized polyester resin, a phenoxy resin, a polyvinyl acetal or a vinylidene chloride copolymer, each of which has a glass transition temperature T
g
of 20° C. or above. This is intended to prevent scratching of the plates during transport in the stack and delamination of the radiation-sensitive layer due to excessive adhesion to the back of the overlying plate.
A recording material for the production of offset printing plates which can be stacked without separating paper is also described in EP-A 528 395. It comprises a support (made of aluminium), a layer of an organic polymeric material having a glass transition temperature of not less than 20° C. with a thickness of from 0.01 to 8.0 &mgr;m on the back of the support, and a light-sensitive layer on the front of the support. A discontinuous matting layer consisting of particles having a mean diameter of not greater than 100 &mgr;m and a mean height of not greater than 10 &mgr;m is in turn located on the light-sensitive layer. The weight of the matting layer is from 5 to 200 mg per square meter. The matting layer enables the air between the master and light-sensitive layer in the vacuum contact copying frame to be pumped out more quickly. The matting layer can be produced, for example, by spraying-on a solution of a methyl methacrylate-ethyl acrylate-acrylic acid terpolymer, some of whose carboxyl groups are in salt form, in an electrostatic field with the aid of a spray bell rotating at about 25,000 revolutions per minute. In general, the matting layer is soluble in water or aqueous alkali. However, matting layers, in particular those comprising a material having a low glass transition temperature, tend to stick to the back of the overlying plate in the stack. This may cause relatively large areas of the radiation-sensitive layer to be delaminated, meaning that the recording material can then no longer be used further.
EP-A 490 515 relates to a presensitized printing plate which, after imagewise exposure, is developed using an aqueous alkali metal silicate solution. In order to prevent the developer from dissolving aluminium out of the back of the plate, this is provided with an organic polymeric coating which is insoluble in the developer.
The coating comprises polymers such as polyethylene, polypropylene, polybutene, polybutadiene, polyamide, polyurethane, polyurea, polyimide, polysiloxane, polycarbonate, epoxy resins, polyvinyl chloride, polyvinylidene chloride or polystyrene. It may also comprise a thermally or photochemically curing component.
DE-A 199 08 529, which is not a prior publication, proposes a recording material having a support which has on the back a layer comprising an organic polymeric material having a glass transition temperature of 45° C. or above, and a pigmented light-sensitive layer located on the front of the support. If polymers of low T
g
are used in the back coating, sticking to the radiation-sensitive layer of the underlying recording material may then occur.
The object was still to provide a radiation-sensitive recording material for the production of planographic printing plates which can be stacked without separating paper. The type of radiation-sensitive layer in this material should not play a particular role. It may be positive- or negative-working. Even after extended storage, even at elevated temperature, and after extended transport, it should be possible to remove the plates from the stack without damage. Sticking of the plates to one another should be reliably prevented. On development of materials having an aluminium support, the aqueous-alkaline developer should in addition only be loaded to a small extent with aluminium hydroxide. This is particularly important if a strongly alkaline developer (pH>12) is employed.
It has now been found that the said objects can be achieved by means of a recording material which has a pigmented back coating comprising an organic polymeric material having a T
g
of at least 35° C.
The present invention thus relates to a recording material for the production of offset printing plates having a dimensionally stable support, a radiation-sensitive layer located on the front of the support, and a layer which comprises an organic polymeric material and which is resistant to processing chemicals located on the back of the support, where the recording material is characterized in that the glass transition temperature of the organic polymeric material is 35° C. or above and that the layer located on the back is pigmented.
The layer covers the entire back of the support, i.e. forms a continuous layer. The pigment particles incorporated therein generally have a mean particle size of from 0.1 to 50.0 &mgr;m, preferably from 0.5 to 30.0 &mgr;m. The particles themselves consist of a sufficiently hard inorganic and/or organic material. Preferred pigmenting agents on

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