Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2000-09-01
2002-03-12
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S311000, C430S330000, C430S944000, C430S945000, C101S463100
Reexamination Certificate
active
06355398
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the imaging of a plate or board having a coating imageable by ultraviolet or visible radiation and more specifically to actinic imaging with simultaneous heating to improve the imaging process. The invention is particularly directed to the imaging of lithographic printing plates.
One type of imageable lithographic printing plate or other imageable plate is a negative-working, actinic plate which has a resin coating normally soluble in a developer and which is rendered insoluble when exposed to radiation, usually in the ultraviolet range. The plate is imaged by exposing the coating to the radiation in those areas corresponding to the image to be printed with those areas becoming insoluble and ink receptive. Another type of imageable lithographic printing plate is a positive-working plate which is also actinically imaged. This type of plate has a resin coating normally insoluble in a developer which is rendered soluble when exposed to the radiation.
Actinic imaging, whether it be by ultraviolet radiation or by visible radiation, can be accomplished by one of two techniques. One technique is to expose the printing plate through a film negative. The other approach is to serially scan the plate with small image spots or areas. This latter approach can be accomplished by digital laser imaging or by a method referred to as digital screen imaging which will be explained later. Although digital screen imaging is not digital imaging in the strictest sense of that term as will be explained, the term “digital” will be used herein to encompass both the digital laser imaging and the digital screen imaging which both involve imaging by serially scanning the plate with small areas or spots of the imaging radiation.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved actinic imaging method for either negative-working or positive-working plates or boards such as lithographic printing plates or printed circuit boards. The invention involves the imaging of the plate to cause a reaction in the coating in the exposed areas by actinic imaging using either ultraviolet or visible radiation in combination with infrared radiation. The reaction may either solubilize or insolubilize the coating depending on whether the plate is positive- or negative-working. The infrared radiation operates to increase the localized temperature of the coating to a level at which the rate of the actinic imaging reaction is increased. Either the ultraviolet/visible radiation or the infrared radiation can be image modulated. The relative areas covered by the ultraviolet/visible radiation and by the infrared radiation can be varied such that the areas are superimposed or that the area of the ultraviolet/visible radiation closely trails the infrared area.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention relates to the imaging of plates or boards with imageable coatings such as lithographic printing plates and circuit boards. The invention will be described with particular reference to lithographic printing plates but it is to be understood that the description also applies to such other products. Lithographic printing plates with coatings which are imageable by ultraviolet or visible radiation wherein the radiation causes an insolubilizing reaction in the coating for negative-working plates and a solubilizing reaction for positive-working plates are well known. Typically, the insolubilizing reaction for negative-working plates is a crosslinking or photopolymerization reaction but other chemical changes may also insolubilize the coating and are within the scope of the invention. The insolubilized imaged areas become the ink-receptive plate areas and the non-imaged areas are removed with a typical developer solution. The coatings for such plates are well known in the art and are typically a diazo resin having reactive sites which are capable of being chemically altered by the radiation. A suitable diazo resin is the condensation product of 3-methoxy-4 diazo-diphenylamine and paraformaldehyde. Other suitable diazo compounds are described in a variety of prior patents including U.S. Pat. Nos. 5,998,095; 4,956,261; 3,406,159; 3,277,074; 3,311,605; 3,163,633 and 3,050,502. For positive-working plates, the coatings are normally insoluble in the developer solution and are solubilized in the areas exposed to the imaging radiation. Such coatings are well known and usually comprise a diazide, such as diazonaphthoquinone derivatives, mixed with or reacted with a phenolic resin. For a specific disclosure of diazonaphthoquinone derivatives, see U.S. Pat. No. 5,858,626. The mechanism of such coatings is described in the article, “The Molecular Mechanism of Novolak Resins” by Arnost Reiser appearing in the Journal of Imaging Science and Technology, Volume 42, No. 1, January/February 1998 on pages 15 to 22.
Lithographic printing plates may be imaged by digital laser imaging and by digital screen imaging. The invention will first be described with respect to digital laser imaging and the digital screen imaging will be described later. Also, the invention will be described with respect to negative-working plates with coatings which are insolubilized by the radiation but it is to be recognized that the invention applies equally to positive-working plates with coatings which are solubilized by the radiation. Furthermore, the invention is applicable to imaging radiation with a wavelength shorter than infrared. For convenience purposes, ultraviolet radiation or devices will be referred to but it is to be understood that the imaging radiation of the invention includes visible as well as ultraviolet radiation. For example, blue or violet diode lasers or a double frequency YAG laser at 532 nm can be used. Like most chemical reactions, the rate at which the solubility conversion takes place for the actinic digital imaging of a lithographic plate is dependent on the temperature. The rate of reaction is slower at ambient conditions than at elevated temperatures. By elevating the temperature at which the imaging is done, the speed of the imaging process can be increased. It is problematic to increase the temperature of the entire plate by elevating the temperature of the imaging platen or drum. The thermal expansion of the components of the imager and the dynamics of managing the heat flow to the plate in a timely and uniform manner make this approach impractical.
According to the present invention, the temperature of the resin coating is increased to enhance the reaction but it is only locally and momentarily heated and heated generally simultaneously with the imaging. The invention involves spot heating the coating with an infrared laser and subjecting the coating while it is heated to the ultraviolet radiation to react the coating at the elevated temperature. The coating is not reacted or otherwise imaged by the infrared radiation but the infrared radiation interacts with the coating and/or substrate merely to heat the coating. Either the ultraviolet radiation or the infrared radiation can be image modulated as discussed below. If desired, an infrared absorbing dye may be incorporated into the coating to facilitate the infrared absorption and heating. These dyes are well known in the art and include materials such as squarylium, croconate, cyanine, phthalocyanine, merocyanine, chalcogenopyryloarylidene, orzindollizine, puinoid, indolizine, pyrylium, thizine, azulenium and xanthene dyes. The infrared heating of the coating can also be effected by using a substrate having infrared absorption characteristics. For example, an anodized aluminum substrate which has been rotary brush grained with calcined alumina so as to embed the graining particles to cover a portion, perhaps 10%, of the surface of the substrate will effect the rate of heat absorption by the substrate and the conduction of heat away from the spot by the aluminum. The power and intensity of the infrared laser as well as the dye in the coating and the nature of the substrate can be selected to i
Fromson Howard A.
Rozell William J.
Alix,Yale & Ristas, LLP
Fromson Howard A.
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