Dimensionally stable flexographic printing plates

Details Dimensionally stable flexographic printing plates Dimensionally stable flexographic printing plates

1994-11-08

2001-01-09

Weiner, Laura (Department: 1745)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Radiation sensitive composition or product or process of making

C430S306000, C430S011000

Reexamination Certificate

active

06171758

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to dimensionally stable flexographic printing plates.
BACKGROUND OF THE INVENTION
Flexography is a term broadly applicable to a printing format that uses a flexible substrate bearing an elastomeric or rubbery relief printing surface. The first flexographic printing plates were produced from natural or synthetic rubber compositions which were cured chemically under heat and pressure in a mold utilizing conventional rubber curatives such as mercapto compounds (
Flexography: Principles and Practices,
3rd Edition, Flexographic Technical Association, p. 158-162). More recently, photopolymer elastomeric compositions (elastomer containing compositions curable upon exposure to actinic radiation) have been used to produce relief printing plates. For example, U.S. Pat. No. 4,162,919 describes the use of a photosensitive composition containing a block copolymer as an elastomeric polymeric binder, a compatible ethylenically unsaturated monomer, and a photoinitiator. Similarly, British Pat. No. 1,454,191 describes the use of an elastomeric polyurethane based photosensitive layer. In both cases, the standard solvent wash procedure is used to develop the relief layer after exposure to actinic radiation. European Pat. No. 261,910 describes an aqueous-developable flexographic printing plate.
Both the solvent wash and aqueous wash developing systems are time consuming since drying for extended periods (1 to 24 hours) is necessary to remove entrained developer solution. In addition, these developing systems produce potentially toxic by-product wastes (both the solvent and any material carried off by the solvent, such as unreacted ethylenically unsaturated monomer) during the development process.
To avoid these problems, a thermal development process may be used. In a thermal development process, the photosensitive layer, which has been image-wise exposed to actinic radiation, is contacted with an absorbent layer at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into the absorbent material. See U.S. Pat. Nos. 3,264,103, 5,015,556, and 5,279,697.
SUMMARY OF THE INVENTION
The present invention is a flexographic printing plate having a very low degree of thermal distortion during development. Specifically, according to a preferred embodiment the invention is a flexographic printing plate comprising a dimensionally stable substrate and an image bearing relief layer, wherein the thermal distortion of the flexographic printing plate in both the machine and the transverse directions is less than 0.02% when the plate is developed at temperatures in the range from about 100° C. to about 180° C.
DETAILED DESCRIPTION OF THE INVENTION
In the development of thermally developable flexographic printing plates, we have discovered that thermal distortion may become a problem, especially when precise lines, points, and images are desired by the printers who are using the plates. In response to this newly discovered problem, we have developed printing plates that can withstand the developing temperatures without undergoing a significant amount of distortion.
“Developing temperature” is the temperature to which the imagewise exposed photosensitive layer is heated to remove the uncured portions of the layer. Although a variety of methods may be used for thermal development of flexographic printing plates, one method of development uses the apparatus disclosed in U.S. Pat. No. 5,279,697. In this method, the temperature of the developing roll which contacts the absorbent material approximates the developing temperature. The substrate, which is on the opposite side from the developing roll, does not reach the developing temperature in this embodiment. In fact, the substrate may be 15 to 30° C. cooler than the developing roll. However, if other methods of thermal development are utilized the entire plate may be heated to the developing temperature.
According to a preferred embodiment the invention is a flexographic printing plate comprising a dimensionally-stable, flexible, polymeric substrate and an elastomeric, image bearing, relief layer. The thermal distortion (includes both elongation and shrinkage) of the plate in both the machine and the transverse directions is less than 0.03%, preferably less than 0.025%, more preferably less than 0.020%, when the plate is developed at temperatures between 100 and 180° C. The distortion experienced during the development of the plate at 120 to 175° C. is preferably less than 0.015%. The machine direction is the direction that the substrate film is pulled during production. The transverse direction is perpendicular to the machine direction in the plane of the substrate. Such balanced, low distortion is critical to achieving flexographic printing plates which do not introduce distortion into the image which is to be reproduced.
According to a second preferred embodiment, the invention is a flexible plate comprising a dimensionally stable, flexible, polymeric substrate and a photosensitive elastomer layer. The polymeric substrate experiences less than 0.07% distortion, preferably less than 0.05% distortion, more preferably less than 0.03% distortion, even more preferably less than 0.025%, and most preferably less than 0.02%, in any planar direction when heated to temperatures from 110 to 180° C. The distortion is desirably less than 0.02% when the film is heated to temperatures from 140 to 150° C.
The substrate may be 0.07 to 2 mm thick and is preferably 0.1 to 1.5 mm thick. While a variety of polymeric materials may be used as the substrate, semicrystalline polymers are particularly desirable because these polymeric materials are particularly amenable to stabilization by thermal annealing. Examples of semicrystalline polymers include polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyether ketone, polytetrafluoroethylene, polyamides, syndiotactic polystyrene, and polyphenylene sulfide.
The desirability of such semicrystalline polymers arises from the discovery that dimensional stability of these polymer substrates may be controlled through a special annealing process. This annealing process comprises heating the polymer substrates to temperatures above their glass transition temperature and below their melting temperature. If this annealing occurs at low tensions, very little thermal distortion will occur when the substrate is later subjected to temperatures which are less than the annealing temperature. In fact, if the substrate is later heated to temperatures less than or equal to the annealing temperature minus about 25° C. (i.e., T
anneal
−25° C.), the thermal distortion has been found to be less than 0.05%. If the substrate is heated to temperatures less than the annealing temperature minus about 30 or 40° C. (i.e., T
anneal
−30 or 40° C.), the thermal distortion has been found to be less than 0.03%. By low tensions is meant tensions less than about 200 psi (1.4×10
6
N/m
2
), preferably less than about 150 psi (1.04×10
6
N/m
2
), and more preferably less than about 100 pounds per square inch (6.9×10
5
N/m
2
). High tension annealing causes distortions. Various annealing methods may be used including air-oven annealing, hot can annealing, annealing rolls of films, or combinations of methods.
The time required for annealing will depend upon the annealing method employed, the polymeric material of the film, film thickness, etc. With regard to the method of annealing, heat transfer by conduction, as occurs in hot can annealing, is more efficient than by convection, as occurs in air oven annealing. Thus, the annealing time for air oven annealing will be longer than that needed for hot can annealing. As an example, for annealing a 7 mil PET film in a forced air oven at 180° C., annealing times as low as 60 seconds were found to be sufficient to impart dimensional stability to the film. In general, however, for any given annealing method, the annealing time should be greater than the time requir

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