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
1999-12-16
2001-12-04
Hamilton, Cynthia (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
C430S287100, C522S110000, C522S114000, C522S115000, C522S116000, C522S117000, C522S118000, C522S119000, C522S120000, C522S121000, C522S122000, C522S123000, C522S124000, C522S125000
Reexamination Certificate
active
06326127
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to photo-curable polymer compositions. In particular, it relates to photo-curable polymer compositions for use in printing plates, comprising a block copolymer having two or more polymerised monovinyl aromatic monomer blocks and one or more polymerised conjugated diene blocks, and a photo-initiator. The invention further relates to flexographic printing plate precursors containing the photo-curable polymer composition, as well as to flexographic printing plates.
BACKGROUND OF THE INVENTION
Flexographic printing plates are well known in the art and are especially useful for commercial printing on diverse products such as flexible plastic containers, cartons, plastic bags, boxes and envelopes.
For the purpose of this specification, uncured plates to be used for preparing (cured) flexographic printing plates are referred to as flexographic printing plate precursors.
The flexographic printing plate precursors typically comprise a photo-curable layer prepared from a photo-curable polymer composition, and protective layer(s) to protect the photo-curable layer from daylight. Optionally, the flexographic printing plate precursor further comprises a support. A support is often used to give strength to the flexographic printing plate (precursor).
Alternatively, the side of the flexographic printing plate precursor opposite the image (printing) side of the printing plate is cured to form a support. Typically, in the absence of a support, a flexographic printing plate is prepared by first curing the side of the photo-curable layer of the flexographic printing plate precursor removed from the printing side. As a result that side of the photo-curable layer cross-links, becomes a thermoset substantially insoluble layer and acts as a support for the uncured part of the flexographic printing plate precursor. Subsequently, the side to be used for printing is selectively cured by exposing the photo-curable layer image-wise to light, e.g. UV light. The unexposed (uncured) parts of the layer are then removed in developer baths e.g. with a solvent or water. After drying, the flexographic printing plate is ready for use.
It will be appreciated that the image-wise curing of the flexographic printing plate precursor must be done in a precise manner. Any unintentional curing may lead to an unclear image on the flexographic printing plate, and, hence unclear prints.
In order to avoid undesired scattering of light it is important that the flexographic printing plate precursor is as transparent as possible, that is, not hazy.
Moreover, there is a need for further improvement of the total combination of physical properties of such photo-curable polymer composition for use in a flexographic printing plate (precursor).
Printing plates must be soft and elastic after curing. Normally, elasticity is obtained by the presence of an elastomer in the photo-curable polymer composition. Often, the elastomer is a block copolymer having two or more polymerised styrene blocks and one or more polymerised diene blocks.
The flexographic printing plate precursor must be easy and quick to cure, and the uncured material must be easy to remove. The flexographic printing plate must be dimensionally stable during storage; must be flexible enough to wrap around a printing cylinder; strong enough to withstand the rigors experienced during a printing process; abrasion resistant; soft enough to facilitate ink transfer during the printing process; and resistant enough to the particular ink solvent to avoid blurring of the image.
It will be appreciated that the achievement of an attractive balance of all these physical properties will be difficult. It would be desirable if a photo-curable polymer composition could be found that is less hazy than conventional photo-curable polymer compositions, and, preferably, has an attractive balance of other relevant properties, such as a low Shore A hardness.
SUMMARY OF THE INVENTION
Such a photo-curable polymer composition has now surprisingly been found. Accordingly, the present invention relates to a photo-curable polymer composition comprising
(a) a first block copolymer comprising at least two blocks A of polymerised monovinyl aromatic monomer, at least one internal block B of polymerised conjugated diene monomer, and at least one tapered or random block C of polymerised monovinyl aromatic monomer and polymerised conjugated diene monomer and/or at least one block D which may be a block of polymerised conjugated diene monomer or a tapered or random block of polymerised monovinyl aromatic monomer and polymerised conjugated diene monomer, wherein block C is an internal polymer block adjacent a terminal block A or an internal block A which is adjacent to a terminal block D, and block D is a terminal block adjacent an internal block A, and, optionally, a residue of a coupling agent, wherein the residue, if present, is derived from a coupling agent containing alkoxy or epoxy functional groups; and
(b) a photo-initiator.
Further aspects of the invention relate to a flexographic printing plate precursor comprising a photo-curable layer sandwiched between two release films or a release film and a support, which photo-curable layer contains the photo-curable polymer composition. In addition, the present invention relates to a cured polymer composition, obtainable by curing the photo-curable polymer composition; to a flexographic printing plate containing the cured polymer composition, and to a process for printing on a substrate with the flexographic printing plate.
DETAILED DESCRIPTION OF THE INVENTION
Typically, and preferably, the photo-curable polymer composition further comprises an ethylenically unsaturated addition-polymerisable cross-linking agent. Such cross-linking agent is typically a component, such as a monomer or a mixture of monomers, that is compatible with the block copolymer(s). Monomers that can be used in the photo-curable polymer composition are well known in the art. Examples of such monomers can be found in U.S. Pat. Nos. 4,323,636; 4,753,865; 4,726,877 and 4,894,315 incorporated herein by reference.
The term “compatible” means that the captioned component can be mixed in a molecular-disperse fashion in the photo-curable composition or that the component has the same breaking index, does not substantially cause haze and does not separate from the photo-curable composition in the course of time, that is does not separate within 14 days, preferably not within one month.
Typically, the ethylenically unsaturated addition-polymerisable cross-linking agent has a boiling point of at least 100° C. at atmospheric pressure and a molecular weight of up to 3000, preferably up to 2000. Examples of suitable ethylenically unsaturated addition-polymerisable cross-linking agents include esters of acrylic acid or methacrylic acid, styrene and derivatives thereof, esters of maleic acid or fumaric acid, acrylamide or methacrylamide and allyl compounds. Preferably, esters of acrylic acid or methacrylic acid are used. Especially preferred monomers are 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, and hydroxyethyl methacrylate.
The first block copolymer preferably has a total polymerised monovinyl aromatic monomer content in the range from 5 to 25%wt of the block copolymer, more preferably from 10 to 22%wt, most preferably from 15 to 20%wt. The first block copolymer preferably has a total weight average molecular weight as determined by GPC in the range of from 50,000 to 500,000 g/mol, more preferably from 50,000 to 400,000 g/mol. If the first block copolymer is linear, the total weight average molecular weight is more preferably in the range from 50,000 to 300,000 g/mol, even more preferably in the range from 65,000 to 250,000 g/mol.
The weight average molecular weight can be determined with gel permeation chromatography (GPC) using polystyrene calibration standards (according to ASTM D 3536).
The weight average molecular weight of the blocks A of polymerised monovinyl aromatic monomer is typically in the range from 4,000 to 25,000 g/mo
Morren Karin Marie-Louise Renee
Muyldermans Xavier
Hamilton Cynthia
Kraton Polymers U.S. LLC
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