Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-08-04
2002-09-17
Ashton, Rosemary (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
C430S200000, C430S302000, C430S320000, C430S327000, C430S328000, C430S944000
Reexamination Certificate
active
06451505
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an imageable element comprising a substrate and an imageable composition supported on the substrate. More particularly, the imageable composition has a first layer applied to the substrate which comprises a photosensitive composition and a photothermal converter, and an ablatable second layer contiguous to the first layer wherein the second layer is opaque to actinic radiation. This invention also relates to a printing plate prepared from such an imageable composition, a method of making such printing plates, and a method of printing using such a plate to form a desired image on a medium.
2. Background Information
The offset lithographic printing process generally utilizes a chemically developed planographic printing plate having oleophilic image areas and hydrophilic non-image areas. When a roller carrying an oil-based ink composition is passed over a dampened plate, it will be unable to ink the hydrophilic non-image areas covered by aqueous film, but will emulsify the water droplets on the water repellant oleophilic image areas, which will then take up ink. The resulting ink image is transferred (“offset”), typically onto a rubber blanket, which is then used to print onto a medium such as paper.
In one type of technology used to produce printing plates the oleophilic image areas are formed from a photosensitive layer on a substrate. In negative working compositions, the photosensitive layer is first imagewise exposed to light that cures, hardens, or decreases the rate of dissolution in developer solution of the exposed areas. Subsequently, during the developing step, only the non-exposed areas are removed from the substrate by the developer solution. In positive-working compositions, the photosensitive layer is first imagewise exposed to light that renders the exposed areas more rapidly dissolvable in a given developer than the non-exposed areas, a process called photosolubilization. Subsequently, in the developing step, only the exposed areas are removed from the substrate.
A large number of positive-working photosensitive compositions exist as derivatives of naphthoquinone diazide sulphonates of phenol- or cresol-formaldehyde condensate resins. Likewise, there are a large number of negative-working photosensitive compositions prepared as N-aryl or N-alkyl phenylaminobenzene diazonium salt condensates with formaldehyde.
A technology widely used to prepare printing plates is POLYCHROME CTX (available from Kodak Polychrome Graphics), which is a two-layer composition in which the outer surface layer is a silver halide photosensitive system that can be exposed by visible light and chemically developed to provide a surface image mask for the layer beneath, which is also photosensitive. The developed outer layer is used to mask actinic light for which the layer below is photosensitive. After exposure to such actinic radiation, removal of the mask layer and portions of the underlayer by developer solution produces a printing plate capable of offset printing in which the substrate and remaining material from the lower layer have different affinities for ink or fountain solution. Two developing steps are needed in this system; i.e. one for the mask, and one for the photosensitive layer. Thus, this system requires additional time and the use of chemicals for processing the mask layer.
U.S. Pat. No. 5,102,756 describes a two-layer composition on a substrate in which the outer mask is created by imagewise exposure to visible light that causes softening of the layer and movement of migration marking particles in the outer layer so that a mask is formed for a photosensitive layer beneath. The migration marking particles can be made from selenium and its alloys.
Advances in digital computers and thermal imaging diodes have made possible new methods to image a substrate. For example, another method of making printing plates creates an image on a substrate by thermal ablation of a layer beneath the outer layer. In these methods, an additional layer that strongly absorbs the ablating light, usually infrared (IR) light, is placed between the outer layer and the substrate. The middle layer efficiently absorbs the ablating light and is imagewise ablated, leaving the outer layer imagewise unsupported. The unsupported outer layer is then easily removed to produce the image on the substrate. For example, U.S. Pat. No. 5,440,987 claims a lithographic member in which a layer characterized by ablative absorption of imaging radiation is coated by a layer having a different affinity for ink than the substrate, or having a different affinity for ink than a third layer beneath the ablative layer.
U.S. Pat. No. 5,353,705 describes a multilayer system for lithographic printing members in which a nitrocellulose infrared-absorbing layer is ablated beneath a silicone or polyester outer layer. Additional layers may be added to the system to enhance adhesion of the photosensitive layer to the substrate, and to prevent charring of the substrate or photosensitive layer.
Ablation may also be used to form the mask in the outer layer by direct absorption of light by the outer layer. For example, U.S. Pat. No. 5,922,502 and European Patent Publication 803,770 describe multilayer systems in which an outer layer comprising carbon black or IR dye in a binder polymer is ablated with IR laser light, creating a mask in the outer layer. The exposure to IR laser light was at a scan rate of 1.1 meters per second (spot size 15 micrometers, 1050 nm wavelength).
U.S. Pat. No. 5,858,604 describes a lithographic plate made from a two-layer composition by direct ablation of an outer mask layer that is above a photosensitive layer. Alternatively, the plate is prepared by ablation of an outer mask layer by a heat-generating layer between the outer layer and the photosensitive layer that is not necessarily also ablated. The IR laser light exposure power density in the focal surface used for ablation was preferably over 200,000 W/cm
2
(not less than 500 nm wavelength).
The dose of imaging light that is needed to create the mask in these methods is critical because it limits the rate at which the printing plate can be prepared by scanning a laser light beam across the plate. Therefore, a need exists for an imageable composition that can be used to make a printing plate by exposure to a reduced dose of radiation to create a mask for an underlying photosensitive layer. Such a mask could be efficiently and rapidly created for printing plate preparation with a computer-generated and driven radiation source, such as IR radiation. Such a composition would also eliminate the additional time and use of chemicals required in conventional applications such as POLYCHROME CTX silver halide mask technology.
It is one object of this invention to provide an imageable element which increases the efficiency of use of mask-generating radiation and eliminates the need for chemicals and additional time to create the mask. The imageable element of this invention advantageously provides rapid preparation of printing plates with a reduced dose of mask-generating radiation. It is another object of this invention to provide a method of making such an imageable element. Other objects, features and advantages of this invention will be apparent to those skilled in the art from this specification and the appended claims.
SUMMARY OF THE INVENTION
The limitations of current technology described above are overcome by the imageable element of this invention, which comprises a substrate having applied thereon a first layer and a second layer contiguous to the first layer. The first layer comprises a photosensitive material and a photothermal converter. The second layer is ablatable and opaque to actinic radiation used to affect the solubility of the first layer. The imageable element is first imagewise exposed to ablative electromagnetic radiation (i.e. radiation having a wavelength different than the actinic radiation which affects the solubility of the first layer). The photothermal conv
Huang Jianbing
Merchant Nishith
Pappas S. Peter
Patel Jayanti
Shimazu Ken-Ichi
Ashton Rosemary
Baker & Botts L.L.P.
Kodak Polychrome Graphics LLC
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