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
2001-11-26
2004-05-18
Huff, Mark F. (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
C430S281100, C430S306000, C430S944000, C430S945000, C430S913000, C430S915000
Reexamination Certificate
active
06737216
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to a flexographic printing element and a method for engraving the printing element by laser radiation to form a relief so that the element is useful as a flexographic printing plate.
2. Description of Related Art
Printing plates are well known for use in flexographic printing, particularly on surfaces which are corrugated or smooth, such as packaging materials, e.g., cardboard, plastic films, etc. One type of flexographic printing plate is made from vulcanized rubber. Commercial rubbers can be natural or synthetic, such as ethylenepropylenediene monomer (EPDM) elastomers.
Lasers can develop sufficient power densities to ablate certain materials. Lasers such as high-power carbon dioxide lasers can ablate many materials such as wood, plastic and rubber and even metals and ceramics. Once the output from a laser is focused at a particular point on a substrate with a suitable power density, it is possible to remove material in depth to create a relief. Areas not struck by the laser beam are not removed. Thus, the use of the laser offers the potential of producing very intricate engravings in the proper material.
Recently, it has been possible to laser engrave a rubber element directly to provide the desired relief surface necessary for flexographic printing. Laser engraving has provided a wide variety of opportunities for rubber printing plates. Highly concentrated and controllable energy lasers can engrave very fine details in rubber. The relief of the printing plate can be varied in many ways. Very steep as well as gently decreasing relief slopes can be engraved so as to influence the dot gain of such plates. EPDM rubber can be laser engraved to form flexographic printing plates.
U.S. Pat. No. 3,459,733 issued to Caddell describes a method for producing polymer printing plates. The printing plate is made by exposing a layer of the polymeric material to a controlled laser beam of sufficient intensity to ablate the polymer and form depressions on the surface.
U.S. Pat. No. 5,798,202 and U.S. Pat. No. 5,804,353 disclose processes for making a flexographic printing plate by laser engraving a reinforced elastomeric layer on a flexible support. The process disclosed in U.S. Pat. No. 5,798,202 involves reinforcing and laser engraving a single-layer flexographic printing element comprised of a reinforced elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, or thermochemically, or photochemically or combinations thereof. Mechanical reinforcement is provided by incorporating reinforcing agents, such as finely divided particulate material, into the elastomeric layer. Photochemical reinforcement is accomplished by incorporating photohardenable materials into the elastomeric layer and exposing the layer to actinic radiation. Photohardenable materials include photocrosslinkable and photopolymerizable systems having a photoinitiator or photoinitiator system.
The process disclosed in U.S. Pat. No. 5,804,353 is similar to U.S. Pat. No. 5,798,202 except that the process involves reinforcing and laser engraving a multilayer flexographic printing element comprised of a reinforced elastomeric top layer, and an intermediate elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, or thermochemically, or photochemically or combinations thereof. Mechanical and photochemical reinforcement is accomplished in the same manner as described by U.S. Pat. No. 5,798,202. The intermediate elastomeric layer may be reinforced as well.
A problem associated with the elastomeric elements which are reinforced both mechanically and photochemically is that laser engraving does not efficiently remove the elastomeric material to provide desired relief quality, and ultimately printing quality. It is desirable to use an additive in the elastomeric layer which is sensitive to infrared light in order to enhance the engraving efficiency of the element. Photochemically reinforcing the element provides the desired properties for engraving as well as in its end-use as a printing plate. However, the presence of the additive as particulate or other absorbing material tends to reduce the penetration of the ultraviolet radiation required to photochemically reinforce the element. If the elastomeric layer is insufficiently cured during photochemical reinforcement, the laser radiation cannot effectively remove the material and poor relief quality of the engraved area results. Further, the debris resulting from laser engraving tends to be tacky and is difficult to completely remove from the element. Additionally, if the element is not sufficiently photochemically reinforced the required end-use properties as a printing plate are not achieved. These problems tend to be exacerbated with increasing concentration of the additive that enhances engraving efficacy.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the disadvantages of the prior art and to provide an elastomeric element which can be laser engraved to form a relief surface suitable for use as a flexographic printing plate.
In accordance with this invention there is provided a laser-engravable flexographic printing element comprising a support; and at least one laser engravable reinforced elastomeric layer of an elastomeric composition comprising an elastomeric binder, at least one monomer, a photoinitiator system that decreases in UV absorbance as polymerization proceeds, and at least one additive containing Si—O functionality or P—O functionality which absorbs infrared radiation, preferably at 9 to 12 micrometers.
In accordance with another aspect of this invention there is provided a method for forming a flexographic printing plate from the above flexographic printing element comprising reinforcing the elastomeric layer of the elastomeric composition by blanket exposure to ultraviolet radiation and exposing the reinforced element imagewise with infrared laser radiation at 9 to 12 micrometers to engrave the elastomeric layer.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The term “laser engravable” as used herein refers to materials capable of absorbing laser radiation such that those areas of the materials which are exposed to a laser beam of sufficient intensity become physically detached with sufficient resolution and relief depth to be suitable for flexographic applications. By “physically detached”, it is meant that the material so exposed is either removed or is capable of being removed by any mechanical means such as by vacuum cleaning or washing or by directing a stream of gas across the surface to remove the loosened particles.
The present invention involves a flexographic printing element which includes on a support at least one reinforced elastomeric layer of an elastomeric composition comprising an elastomeric binder, at least one monomer, a photoinitiator system that decreases in ultraviolet (UV) absorbance as polymerization proceeds, and an additive sensitive to infrared radiation. The flexographic printing element is engravable with laser radiation to physically detach the reinforced elastomeric layer and form relief depth in the areas irradiated with the laser. The elastomeric layer formed from the elastomeric composition is reinforced both by photochemical means in that the composition is photosensitive and hardens upon exposure to actinic radiation and by mechanical means in that the composition includes at least one additive.
Suitable elastomeric binders should be chosen so that the resulting element can be laser engraved as discussed below. In addition, the resulting plate should have the characteristics associated with flexographic printing. These characteristics include flexibility, resilience, Shore A hardness, ink compatibility, ozone resistance, durability and resolution. It is also preferred, but not essential, that such materials do not incorporate halogens or heteroatoms such as sulfur so as to avoid any toxic gases being emitted during the laser engraving process. Thus, either a
Kannurpatti Anandkumar Ramakrishnan
Simmons, III Howard Ensign
E.I. du Pont de Nemours and Company
Huff Mark F.
Magee Thomas H.
Walke Amanda C.
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