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
2000-12-26
2003-08-12
Baxter, Janet (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
C430S944000, C430S178000, C430S302000
Reexamination Certificate
active
06605407
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to the field of lithography and in particular to imaging materials for digital-on-press technology
BACKGROUND OF THE INVENTION
At present, virtually all commercially printed copy is produced through the use of three basic types of printing. One type is a relief plate that prints from a raised surface. Another type is gravure that prints from a depressed surface. The third, namely lithographic printing is planographic and is based on the immiscibility of oil and water wherein the oily material or ink is preferentially retained in the image area of a printing plate and the water or fountain solution retained by the non-image area. A widely used type of lithographic printing plate has a light sensitive coating applied to a hydrophilic base support, typically made from anodized aluminum. The coating may respond to the light by having the portion that is exposed becoming soluble so that it may be removed by a subsequent development process. Such a plate is said to be positive working. Conversely, when the area that is exposed remains after development and the unexposed areas are removed instead, the plate is referred to as a negative working plate.
In the production of the bulk of standard commercial lithographic printing plates of this nature, a hydrophilic support is coated with a thin layer of a negative-working photosensitive composition. Typical coatings for this purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids, and a large variety of synthetic photopolymers. Diazo-sensitized systems in particular are widely used.
Imagewise exposure of such imagable light-sensitive layers renders the exposed image insoluble while the unexposed areas remain soluble in a developer liquid. The plate is then developed with a suitable developer liquid to remove the imagable layer in the unexposed areas.
A particular disadvantage of photosensitive imaging elements such as those described above for making a printing plate, is that they work with visible light and have to be shielded from normal room lighting. Furthermore, they can have the problem of instability upon storage.
One approach that has been extensively followed in recent times is to laser ablate either a hydrophobic or hydrophilic coating layer to reveal a surface of the opposite character. An example is provided by Lewis et al in U.S. Pat. No. 5,339,737. This process, while simple, has the drawback of generating ablative debris and dust. This dust and debris may accumulate on sensitive optical components of the system and affect performance. It may also find its way onto the printing surface and generate unwanted artifacts on the printed copies.
Methods have been known since the 1960's for making printing plates involving the use of imaging elements that utilize heat-driven processes rather than direct photosensitivity. This allows processing without the need for photographic darkrooms and makes possible the concept of on-press processing. In view of this benefit, as well as the limitations of direct photosensitive plates described above, the trend towards these heat-based printing plate precursors is to be anticipated and is, in fact, reflected in the market.
In 1964 Vrancken in U.S. Pat. No. 3,476,937 described a basic heat mode printing plate or thermal printing plate precursor in which particles of thermoplastic polymer in a hydrophilic binder coalesce under the influence of heat, or heat and pressure, that is image-wise applied. The fluid permeability of the material in the exposed areas is significantly reduced. This approach forms the basis of heat-based lithographic plates that are developed using various aqueous media. In the later U.S. Pat. No. 3,793,025 Vrancken describes the addition of a pigment or dye for converting visible light to heat, after which essentially the same process is followed as in the earlier disclosure. In U.S. Pat. No. 3,670,410 interlayer structures based on the same principles are presented. In U.S. Pat. No. 4,004,924 Vrancken describes the use of hydrophobic thermoplastic polymer particles in a hydrophilic binder together with a material to convert visible light to heat. This combination is employed to generate printing masters specifically by flash exposure.
This early work of Vrancken has formed the basis of commercial lithographic products. However, this work did not address the inherent problems associated with the use of lithographic plates sensitive to visible wavelengths of light under the practical conditions of commercial printing. This early work was performed at a time when digital-on-press technology had not yet been developed. The patents therefore did not anticipate many of the considerations typical of this newer technology wherein data is written point for point directly to the imaging surface by a point light source or combination of such sources such as laser arrays, and the imaging surface is developed on-press.
There is a fundamental principle to take note of in comparing photographic and thermal media. In the case of photographic media the image is produced by a photochemical effect and the imaging process is driven directly by the light-sensitivity of the photosensitive material. In the case of thermal media, the coagulation or coalescence of the hydrophobic polymer particles is a process driven by heat. These media, in typical formulations available at this time, therefore also contain an element that converts electromagnetic radiation to heat. The choice of this converter material determines the range of electromagnetic wavelengths to which the media will respond.
Recently the use of infra-red wavelengths of light generated either by YAG lasers or, more recently, 800-900 nm radiation from high power Group III-V laser diodes and diode arrays has increased radically. By employing these infrared wavelengths of light, the need for dark room handling of undeveloped plates is obviated as described earlier. The choice of infrared wavelengths of light, however, is not to be confused with the fact that this light also has to be converted to heat in order to drive the thermal process that leads to the coalescence of polymer particles. The terms “thermal plates” or “heat mode plates” therefore refer to the conversion mechanism by which the hydrophilicity of the surface of the plate is changed, and does not refer to the wavelength of the light being employed. Products that function on the basis of this principle are today on the market. One example is the Thermolite product from the company Agfa of Mortsel in Belgium.
Since the basic offset printing process requires fountain solution to wet the printing surface before inking, much effort has been put into ensuring that on-press media may be developed using the same fountain solution or at least an aqueous liquid. There is, however, a trade-off between durability of the imaged printing surface and its developability. If the surface is easily developed, it is often not very durable. This durability limitation is thought to be due to the abrasive action of the pigments employed in offset inks coupled with the physical interaction between the blanket cylinder and the plate master cylinder that results in relatively rapid wear of the oleophilic image areas of the printing plate.
As pointed out by Vermeersch in U.S. Pat. No. 6,001,536, these newer technological issues were addressed to some degree by Research Disclosure No. 33303 of January 1992. This document discloses a heat-sensitive imaging element comprising, on a support, a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the surface of the imaging element at these areas ink accepting without any further development. A disadvantage of this method is that the printing plate so obtained is easily damaged since the non-printing areas may become ink-accepting when some
Christall Keith
Emans John
Goodin Jonathan William
Yu Yisong
Baxter Janet
Creo Inc.
Oyen Wiggs Green & Mutala
Walke Amanda C.
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