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
1998-01-20
2003-06-17
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
C430S283100, C430S285100, C430S273100, C430S905000, C430S907000, C430S912000, C522S109000, C522S142000, C522S144000
Reexamination Certificate
active
06579661
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to a photopolymerizable composition comprising an elastomeric binder, a photopolymerizable ethylenically unsaturated monomer and a photoinitiator; a photopolymerizable printing plate comprising at least one photopolymerizable layer consisting of such a composition; and a flexographic printing form made from the photopolymerizable printing plate.
2. Description of Related Art
It is known to use photopolymerizable printing plates for the preparation of flexographic printing forms, whereby the printing surface is produced by imagewise exposure of a photopolymerizable layer with actinic radiation, and subsequent removal of unexposed, non-photopolymerized areas of the printing plate. Examples are disclosed in the following patents: DE-C 22 15 090, U.S. Pat. Nos. 4,266,005; 4,320,188; 4,126,466 and 4,430,417. Such photopolymerizable printing plates usually consist of a support, an optional adhesive or other auxiliary layer, one or more photopolymerizable layers, optionally an elastomeric layer, and a cover layer.
A preferred process of making multilayer, photopolymerizable printing plates is one in which a previously extruded photopolymerizable composition is fed into the nip of a calender and is calendered between a support and a cover element to form a photopolymerizable layer. EP-B 0 084 851 discloses a process for making multilayer, photopolymerizable printing plates having a further elastomeric layer between the flexible tear-resistant polymeric layer and the photopolymerizable layer.
The photopolymerizable layers contain polymeric binders, photopolymerizable monomers, photoinitiators and further additives like plasticizers, fillers, stabilizers, etc. Usually thermoplastic, elastomeric block copolymers are used as polymeric binders, like those described in DE-C 22 15 090. These are normally A-B-A block copolymers with thermoplastic blocks A and elastomeric blocks B, especially linear and radial block copolymers with polystyrene end blocks, i.e. polystyrene-polyisoprene-polystyrene, (polystyrene-polyisoprene)
4
Si or the corresponding butadiene polymers.
Often, printing forms made according to the state of the art do not meet all requirements. Especially in corrugated board direct printing, problems occur again and again. The deep relief printing forms used for corrugated board printing have to show a high cohesion in the photopolymer to prevent break out of halftone dots, especially of isolated dots, and to anchor them securely on the printing form. The same is true for fine lines, which additionally are not allowed to be reproduced in waves, i.e., have the form of waves. Simultaneously, the printing forms have to be soft enough to avoid the so-called “wash-board effect” for solids. A varying compressible wave profile and very rough print materials, like often used recycling papers, further complicate a uniform color transfer. The production of such printing forms, which show this combination of deep relief, stable dots and lines, good color transfer and required softness for good solids, is often expensive, and the reproducibility of plate properties is not very good. The printing plates of the state of the art especially show cold flow which makes storage and handling more difficult.
SUMMARY OF THE INVENTION
The present invention provides flexographic printing forms, especially for corrugated board direct printing, which do not show the disadvantages of printing forms according to the state of the art. The photopolymerizable printing plates for production of such flexographic printing forms show higher photosensitivity, without other essential properties of the photopolymerizable printing plates or of the flexographic printing forms being negatively influenced.
This objective was surprisingly solved by a photopolymerizable composition for a photopolymerizable printing plate comprising at least one elastomeric binder, at least one photopolymerizable, ethylenically unsaturated compound, and at least one photoinitiator or photoinitiator system, characterized in that the composition contains at least one radial (polystyrene-polybutadiene)
n
X block copolymer, with X=Sn or Si and n=2 or 4, with an average molecular weight (M
w
) of 80,000-300,000, a molecular weight distribution (M
w
/M
n
) of 1.00-1.40, a content of diblock copolymers of less than 15% by weight, which is extended with up to 50% by weight of a paraffinic oil. The invention also comprises a flexographic printing form made from such a photopolymerizable composition.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Surprisingly, the production process of photopolymerizable printing plates is simplified by the use of the polymers according to this invention. Products which are more stable and with better reproducibility of properties of the printing plates and forms, which are essential for corrugated board direct printing, are provided. Furthermore, the photosensitivity of photopolymerizable printing plates is enhanced so that shorter exposure times for imagewise main exposure as well as for overall back exposure are possible. A special advantage of the materials according to the invention is a high Shore A hardness of the non-photopolymerized printing plates and simultaneously a low Shore A hardness of the photopolymerized printing forms when compared to state-of-the-art printing plates. Therefore, the printing plates of the invention show lower cold flow than printing plates of the state-of-the-art without loss of the necessary softness for printing forms. The Shore A hardness of the non-photopolymerized composition or printing plate is ≧20, the Shore A hardness of the photopolymerized composition or printing form is ≦40, preferably ≦38, and the difference between the Shore A hardness of the photopolymerized material and the Shore A hardness of the non-photopolymerized material is ≦22, preferably ≦20. Printing forms made of the materials according to the invention also show good color transfer.
Radial (polystyrene-polybutadiene)
n
X block copolymers, with X=Sn or Si and n=2 or 4, are used as polymers essential for the invention, preferably (polystyrene-polybutadiene)
4
Si block copolymers are used. These polymers have an average molecular weight (M
w
) of 80,000-300,000, preferably 100,000-250,000, and a molecular weight distribution (M
w
/M
n
) of 1.00-1.40, preferably 1.00-1.25. The polymers essential for the invention contain less than 15% by weight of di-block copolymers, preferably less than 10% by weight. Furthermore, the polymers are extended with up to 50% by weight, preferably 25-40% by weight, of one or more paraffinic oils. These paraffinic oils preferably contain less than 10% by weight of aromatic compounds. The polystyrene content of the polymers of the present invention is 10-40% by weight, preferably 15-35% by weight. Especially (polystyrene-polybutadiene)
4
Si block copolymers with an average molecular weight (M
w
) of 150,000-200,000, a content of di-block copolymers of less than 10% by weight, which are extended with 25-35% by weight of a paraffinic oil, are preferred.
Preferably, the block copolymers of the invention are used as the sole binders of a photopolymerizable composition for a photopolymerizable printing plate. But they can also be used as mixtures with other thermoplastic, elastomeric block copolymers, if special requirements to plate properties demand this. In this case, the amount of polymers essential for this invention is not below 30% by weight, preferably greater than or equal to (≧) 50% by weight, especially preferred greater than or equal to (≧) 70% by weight, based on the total amount of polymeric binders.
Especially suitable as further block copolymers are those disclosed in DE-C 22 15 090; U.S. Pat. No. 4,320,188; U.S. Pat. No. 4,197,130; U.S. Pat. No. 4,430,417 or U.S. Pat. No. 4,162,919. Especially linear and radial block copolymers with polystyrene end blocks, i.e., polystyrene-polybutadiene-polystyrene, (polystyrene-polybutadiene)
4
Si or the corresp
Kraska Ursula
Simon Reimund
E. I. du Pont de Nemours and Company
Lee Sin J.
Magee Thomas H.
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