Processless digitally imaged printing plate using microspheres

Details Processless digitally imaged printing plate using microspheres Processless digitally imaged printing plate using microspheres

2002-03-25

2004-10-19

Huff, Mark F. (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Microcapsule, process, composition, or product

C430S273100, C430S281100, C430S286100, C430S306000, C430S309000, C430S348000, C430S394000, C430S494000, C430S944000, C430S945000, C101S453000, C101S463100

Reexamination Certificate

active

06806018

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to photosensitive elastomeric compositions used to prepare digitally imaged relief-printing plates without the need for an interim process step.
BACKGROUND OF THE INVENTION
Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only be means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing plate as delivered by its manufacturer, is a multilayered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. The backing layer lends support to the plate. It is typically a plastic film or sheet about 5 mils or so thick, which may be transparent or opaque. Polyester films, such as polyethylene terephthalate film, are examples of materials that can be suitably used as the backing. When only a single photocurable layer is present, it may be anywhere from about 25-275 mils thick, and can be formulated from any of a wide variety of known photopolymers, initiators, reactive diluents, etc. In some plates, there is a second photocurable layer (referred to as an “overcoat” or “printing” layer) atop this first, base layer of photocurable material. This second layer usually has a similar composition to the first layer, but is generally much thinner, being on the order of less than about 10 mils thick. The slip film is a thin (0.1 to 1.0 mils) sheet, which is transparent to UV light, which protects the photopolymer from dust and increases its ease of handling. The cover sheet is a heavy, protective layer, typically polyester, plastic, or paper. Typical prior art methods for making flexographic printing plates may be found, for example, in U.S. Pat. Nos. 4,045,231, 5,223,375 and 5,925,500, the teachings of which are incorporated by reference herein in their entirety.
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of plates in developing relief images, in order to go from plate to press more quickly. An early attempt to reduce solvents, and the inherently longer drying required for solvent developing was the aqueous developable flexographic printing plate as taught in U.S. Pat. Nos. 4,177,074, 4,517,279, 5,364,741 and 6,017,679, the teachings of which are herein incorporated by reference in their entirety. However, the use of water to develop relief is still a “processing” step. In addition, water developable printing plates have inherent disadvantages, such as limited print performance and the generation of wastewater.
Thermal mass transfer plates, such as DuPont Cyrel® FAST™, are gaining popularity because they are chemical free. In the case of the FAST™ approach, the thermal process of removing the uncured non-image areas of the photopolymer is carried out after cross-linking the image areas of the plate. This approach is demonstrated in U.S. Pat. No. 6,171,758, and in Patent Nos. WO0118604 and WO0188615, the teachings of which are herein incorporated by reference in their entirety. Since the photopolymer is “dense”, removing of the uncured non-image areas takes a substantial amount of time to achieve. Customers must also invest in a special proprietary processor.
Laser-engraving systems from Fulflex and BASF (called LEP) are also process-free. An example of this technology is found in Patent No. EP0982124A2 the teachings of which are herein incorporated by reference in their entirety. In the BASF and ZED/Fulflex approach, the photopolymer/rubber is cured or cross-linked prior to the engraving step. Once again, because of the high density of these materials, the thermal engraving step is long and tedious. Additionally, high resolution is difficult to achieve. Thus, the disadvantage of prior art engraved plates is a combination of limited resolution and throughput.
Directly engraving a relief plate with a laser is a highly desirable concept. However, CO
2
engraving lasers lack beam resolution and cause anomalies due to heat dissipation. The resolution of such systems is limited to well below 133 lines per inch (LPI) on a practical basis. Infrared (IR) lasers such as Nd-YAG lasers are extremely high in resolution and are precisely controlled. However, these lasers lack the necessary power and reactivity to engrave conventional photopolymers and may be too slow due to mass transfer limitations in dense “cured” photopolymer or rubber systems.
A solution to the problem may lie in the use of a UV-curable thermoplastic elastomer that contains micro-bubbles. The composition is essentially a photocurable elastomeric uncured foam that is laden with a dye that is both IR absorbing and UV transmissive. As the IR laser strikes the dye, it transfers IR energy into heat, causing “laser collapse” of the micro-bubbles or microspheres. Because the photocurable elastomeric material consists of foam cells which are only microns in size, the ablation-to-depth process can occur much more quickly, using much lower energy than is required in true mass transfer systems such as mask ablation or polymer engraving. In addition, the lower density and the corresponding lower heat energies involved in this process act to prevent conductance of heat energy to adjoining cells, thus limiting thermal damage and having the potential for higher resolution than traditional laser engraving. After all of the non-printing (relief) areas have been laser collapsed, there may be an additional process step to laser collapse the top layer to form a denser printing surface. This denser printing surface can also be created by a “bump” UV-exposure in concert with the regular exposure. A “bump” or “flash” exposure refers to a quick exposure, generally of less than about 1 second. The photopolymer is then flood UV-exposed to cross-link the formed image for enhanced physical properties. Finally, the process may contain a detacking step.
The advantage of this “low density” approach is that it may be used in any of the conventional plate-setters in the industry, with only a change in the software that is used to control the energy density; no major investment in hardware is needed. The disadvantage of UV-imaging through a “foam” is obviated because the imaging is done by the interaction of the IR laser with the microspheres. UV-curing is used simply to set the image in place. Furthermore, in using this process, one avoids the washout process step, and hence has the workflow advantage of going from the plate to press much more quickly than in conventional flexographic printing plates, while at the same time reducing solid waste generation.
U.S. Pat. No. 6,159,659 and U.S. Pat. No. 6,090,529, both to Gelbart, the teachings of which are incorporated herein by reference in their entirety, disclose methods for directly creating a raised image on a flexographic printing surface. These patents disclose laser ablation of an intermediate layer that comprises an elastomer and a high concentration of plastic or glass microballoons, in order to form recessed areas on the surface. In addition, these patents disclose controlling the intensity of the laser beam and the dwell time of the laser beam in each spot so that the laser power applied to each part of the surface is sufficient to cause localized melting of the intermediate layer. The dwell time is sufficiently long so as to produce viscous flow of the melted material, while the laser intensity is insufficient to cause complete ablation of the intermediate layer. In one example, the printing plate is made from a closed-cell black polyurethane foam, where the foam has a density of about 10% that of s

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