Photopolymer sachet

Details Photopolymer sachet Photopolymer sachet

2001-11-01

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

C430S138000, C430S284100, C430S286100, C430S287100, C430S288100, C430S302000, C430S306000, C430S309000, C430S322000, C430S325000, C430S394000, C430S434000, C101S114000, C252S299010, C428S034100

Reexamination Certificate

active

06737219

ABSTRACT:

The present invention relates to a method for forming a sachet containing a curable liquid polymer from which a printing plate is to be formed and to a photopolymer package comprising a sachet.
The normal technique for making photopolymer printing plates for hand stamps or flexography is as follows: a masking element (in practice a photographic negative) is placed on a horizontal glass support of a plate-making machine (i.e. a device for irradiating the photopolymer with curing radiation). A transparent plastics cover sheet is placed over the masking element and, in turn, a containment wall is put on the cover sheet to form a reservoir or tray for liquid photopolymer.
Liquid photopolymer is then poured into the tray defined by the containment wall to substantially fill the reservoir to its top. Indeed, it is preferably filled sufficiently for a convex meniscus to rise above the containment wall as any overfilled polymer will be forced over the containment wall, whereas an underfilled reservoir would produce a thin plate of poor image. A semi-rigid backing sheet is then carefully placed over the liquid. An upper radiation source (the “lid” of the plate making machine) is closed down the backing plate and the remaining air is evacuated from under the cover sheet which forms the bottom of the liquid reservoir. The prepolymer is then irradiated from above, to form a continuous cured polymer layer supporting the printing surface and from below, for imagewise curing of the prepolymer to form the printing surface. The backing sheet has a surface-treatment so that cured polymer layer adheres firmly to it.
The cured plate is removed and uncured resin is washed out from the unexposed areas, whereupon the plate is ready for use.
The use of a backing plate which is semi-rigid is regarded as indispensable for two purposes:
a) to exclude air from the reservoir, since it would be difficult or impossible to do this reliably with a fully flexible thin sheet; and
b) to provide the plate, when cured, with the necessary strength and stability to stop premature disintegration during its use as a hand stamp plate or a flexographic printing plate.
The technique of pouring liquid into a reservoir is universally used, notwithstanding that it is difficult to fill the reservoir correctly and that the use of such liquid is difficult, messy and can represent a health and environmental hazard. It is not surprising that attempts have been made to provide the liquid resin in a sealed package, therefore, but none of these attempts has met with success.
JP 61041148, published on Feb. 27, 1986, addresses the problem of the intrusion of air bubbles into the resin and the difficulty of pouring the correct amount of resin into the reservoir. JP 61041148 proposes to solve these problems by sealing a prescribed amount of a photosensitive resin inside a flat container which has transparent walls. The specification alleges that this eliminates concern regarding the presence of air bubbles.
The container of JP 61041148 contains two walls: the first of these comprises a transparent flexible synthetic resin film with an outer face which is contacted with an imaging medium, such as a photographic negative, and which is designed to be detachable from the imaging medium, and whose inner face is arranged to be detachable from the sealed photosensitive resin following curing by exposure; the second of the container walls, on the other hand, is formed from a transparent flexible synthetic resin film which is designed to be adhesive to the cured photosensitive resin. The films forming the two walls are formed to the desired size, then superposed to form the container, a suitable inlet being retained. The required amount of the photosensitive resin is then poured into the container by means of the inlet, a reduced pressure filling device being used to ensure the absence both of air bubbles and of any air gap between the container wall and the resin. The inlet edge of the container is then tightly sealed by melt deposition to form the desired resin holder, which may then be irradiated through a photographic negative to cure the resin in the image areas.
It is noted that the method of JP 61041148 incorporates the use of a container wall comprising a synthetic resin film which is specifically designed to adhere to the cured photosensitive resin so that, following removal of the uncured resin in the non-irradiated areas, the plate comprises the imaged, cured resin attached to the adherent film which, thereby, serves as a backing sheet for the printing plate. This is very much in accordance with the accepted practice in the platemaking art, wherein it is universal for a stiff backing sheet to be used, the backing sheet being caused to adhere to the photopolymer when it is cured to provide a support for the plate formed on curing the photopolymer.
In the Japanese patent, the film which forms the backing sheet comprises a chlorinated, or chlorosulphonated, polyalkylene resin. The use of such materials would, however, be impractical due to their high cost and difficulty in manufacture. Furthermore, in the technique of JP 61041148 it would be expected that further problems would arise as a consequence of the differences in flexibility, tensile strength, and thermal and other physical properties between these resins and the resins forming the opposing walls of the containers.
However, apart from the particular difficulties associated with a backing sheet in the form disclosed in JP 61041148, there are clearly potential disadvantages generally associated with the use of such components in platemaking. It is necessary, for instance, that they should be selected such that adhesion occurs between backing sheet and resin after curing, and that the adhesion should be strong enough to withstand the harsh conditions associated with printing operations without risk of detachment. Furthermore, the requirement for the backing sheet to display a certain level of strength and rigidity has clear implications in terms of cost.
The technique described in JP 61041148 has apparently never been commercialised. In this respect, the skilled person would have considered it to suffer from two technical problems. Firstly, the specification asserts that the method “eliminates” air bubbles. It is known in the liquid packaging industry that it is difficult to eliminate air bubbles unless the packaging is overfilled or steps are taken to avoid contamination with air bubbles, for example via the use of a vacuum system; the claimed technique appears to offer no advantages over the known art in this respect. Secondly, the method involves the thermosealing of dissimilar plastics sheets, one of which is a relatively expensive chlorinated film.
The thermosealing of dissimilar sheets is always avoided wherever possible because the different coefficients of thermal expansion will give rise to a somewhat distorted product. Moreover, the different physical properties of the two films are liable to result in differential stretching and flexing of the opposed faces, again leading to distortions in the finished package and hence likely distortions in the cured plate.
Another attempt to form a sealed photopolymer sachet is disclosed in EP-A-607106 (De Caria), which concerns a process wherein a photosensitive composition is introduced into a flat pocket or bag made from transparent thermoplastic material, and its drawings show that it relies essentially on the established technology of pouring liquid into a tray. When filled, air is evacuated from the assembly, and the whole is then thermosealed to provide a bag which is laid on a plane surface and pressed by means of a parallel upper plane surface. The specification proposes use of a relatively stiff backing sheet, without which the skilled person would not have contemplated performing the invention. When the bag is placed between plane surfaces which comprise transparent plates, a photographic negative may be interposed to allow imagewise irradiation of the bag such that curing of the photosensitive composition occurs. Removal

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