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
2000-07-11
2003-02-18
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
C430S306000, C430S005000, C430S281100, C430S944000, C430S911000, C101S395000, C101S401100
Reexamination Certificate
active
06521390
ABSTRACT:
DESCRIPTION
This invention relates to a photosensitive printing element used for preparing flexographic printing plates via computer-to-plate technology with a support, a photopolymerizable layer and an IR-ablatable layer substantially opaque to actinic radiation which comprises at least one heat-combustible polymeric binder, at least one IR-absorbing material and at least one aliphatic diester. Furthermore, this invention relates to a process for making a flexographic printing plate using such an element.
In the field of flexographic printing, computer-to-plate-technology (CtP technology), also known as digital imaging technology, is becoming extremely popular. In CtP technology the photographic mask used in conventional techniques to cover such areas of a photopolymeric printing plate which should not polymerize is substituted by a mask integrated within the printing plate. Although there are several possibilities of providing such an integrated mask only two techniques are on the market, namely printing the mask on the photopolymeric plate with an ink jet printer or coating the photopolymeric plate with an IR-ablatable layer which is substantially opaque to actinic radiation and imaging such a mask with an IR laser. Such an IR-ablatable layer usually contains carbon black. Photopolymeric printing plates with such IR-ablatable layers are known in the art and disclosed for instance in EP-A 654 150 or EP-A 767 407. By means of irradiation with an IR laser the black layer is evaporated at this location and the photopolymeric layer below is bared. The laser apparatus is directly coupled with the lay-out computer system. Using this technique an image can be generated directly on the plate, which in the next step is irradiated with actinic radiation.
CtP technology not only avoids having to make separate photomasks and working with photochemicals, but also gives a much higher resolution. For a detailed discussion of the advantages of CtP technology over conventional technology see for instance “Deutscher Drucker, Nr. 21/3.6.99, pages w12 to w16”.
The crucial step in the process of making a flexographic printing plate using a photopolymeric printing element with IR-ablatable layer is the step of irradiating the element with the IR-laser The IR-ablatable layer has to fulfil several requirements in order to obtain good results. For economic reasons the time for imaging the IR-ablating layer should be as short as possible. Therefore, its sensitivity to IR radiation should be as high as possible. Furthermore, the common technology nowadays for imaging uses a laser apparatus with a rotary drum. The photopolymeric flexographic element is mounted on the cylindrical drum and irradiated with the laser beam while the drum rotates at up to 2000 rpm. For this reason, the IR-ablatable layer of the flexographic printing element also has to be of an elastomeric character in order to permit bending the flexographic printing element without the IR-ablatable layer wrinkling or ruptering.
EP-A 654 150 discloses a photopolymeric flexographic printing element with an IR-ablatable layer. The IR-ablative layer disclosed comprises an IR-absorbing material. Furthermore, it optionally comprises a polymeric binder and a wide variety of other ingredients such as dispersants for pigments, surfactants, plasticizers, or coating aids. However, the above mentioned publication does not teach anything about the sensitivity of the IR-ablatable layer. Particularly, it does not teach how to improve the sensitivity of the IR-ablatable layer and simultaneously also to improve its flexibility in order to mount it on cylindrical drums without problems.
EP-A 767 407 discloses photopolymeric printing elements with an IR-ablatable layer comprising an elastomeric, film-forming binder and carbon black. As binders polyamides (e.g. Macromelt®) or polyvinyl/alcohol/polyethylene/glycol graftpolymers (e.g. Mowiol®) are mentioned. Although such binders permit mounting the flexographic printing elements on the cylindrical drums without problems the velocity of the imaging process is insufficient.
The problem of obtaining improved photopolymeric flexographic printing plates whose IR-ablatable layers simultaneously show high sensitivity and high flexibility still remains unsolved. It was an object of the present invention to provide such photopolymeric flexographic printing elements.
In a first embodiment, this invention relates to a photosensitive printing element used for preparing flexographic printing plates via computer-to-plate technology comprising
a support,
a photopolymerizable layer comprising at least one elastomeric binder, at least one polymerizable compound, and at least one photoinitiator or photoinitiator system,
an IR-ablatable layer which is substantially opaque to actinic radiation, and
optionally a peelable flexible coversheet,
wherein the IR-ablatable layer comprises at least one heat-combustible polymeric binder, at least one IR-absorbing material and at least one aliphatic diester of the general formula R
1
(CO) [O—CHR
3
—CH
2
]
n
O(CO)R
2
where n=1-30, R
1
and R
2
are straight-chain or branched alkyl chains with 1-20 carbon atoms, and R
3
is H or methyl.
In a second embodiment, this invention relates to a process for making a flexographic printing plate using such an element.
Surprisingly, it has been found that by combination of heat-combustible polymeric binders with such special plasticizers photopolymeric flexographic printing plates are obtained whose IR-ablatable layers show an excellent sensitivity to IR-radiation, resulting in high imaging velocities and simultaneously also exhibit very high flexibility so that that the elements can be mounted on the cylindrical drum of a laser apparatus without the IR-ablatable layer wrinkling or rupturing.
Suitable supports for the photosensitive elements of the present invention are flexible but dimensionally stable materials such as films of polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate or polycarbonate that have a sufficiently high modulus of elasticity for use as a dimensionally stable support material.
The photopolymerizable layer comprises at least one elastomeric binder, at least one polymerizable compound, and at least one photoinitiator or photoinitiator system. Such photopolymerizable layers are well known in the flexographic printing art and are disclosed for instance in EP-A 767 407. Although the photopolymerizable layer is referred to herein as a single layer, it will be understood that two or more different photopolymerizable layers can be used.
The elastomeric binder can be a single polymer or a mixture of polymers. Examples of suitable binders are vinylaromatic/diene copolymers or block copolymers such as conventional S-B-S- or S-I-S- block copolymers, diene/acrylonitrilecopolymers, ethylene/propylene/diene-copolymers, ethylene/acrylic acid copolymers or diene/acrylate/acrylic acid copolymers. Suitable polymerizable compounds are conventional photopolymerizable ethylenically monounsaturated or polyunsaturated organic compounds as are used for producing photopolymeric printing plates and are compatible with the elastomeric binder. Examples of suitable monomers are conventional acrylates or methacrylates of mono- or polyfunctional alcohols, acryl- or methacrylamides, vinyl ethers, or vinyl esters. Examples of preferred monomers are butyl(meth)acrylate, 2-ethyl-hexyl(meth)acrylate, lauryl(meth)acrylate, butanediol-di(meth)acrylate, hexanediol-di(meth)crylate, ethylene glycol di(meth)acrylate or polyethylene oxide-di(methacrylates). Examples of suitable photoinitiators are aromatic keto compounds such as benzoin or benzoin derivatives. The photopolymerizable layer may additionally comprise further additives such as plasticizers, thermal polymerization inhibitors, dyes or antioxidants.
By appropriately choosing the ingredients of the photopolymerizable layer the skilled artisan can make the photopolymerizable layer soluble or dispersible in aqueous, semi-aqueous or organic developers, depending on the s
Andriessen Hieronymus
Daems Eddy
Leenders Luc
Leinenbach Alfred
Philipp Sabine
BASF Drucksysteme GmbH
Baxter Janet
Keil & Weinkauf
Lee Sin J.
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