U.S. patent application number 10/895880 was filed with the patent office on 2004-12-23 for production of flexographic printing plates by thermal development.
Invention is credited to Hiller, Margit, Schadebrodt, Jens.
Application Number | 20040259034 10/895880 |
Document ID | / |
Family ID | 31724593 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040259034 |
Kind Code |
A1 |
Schadebrodt, Jens ; et
al. |
December 23, 2004 |
Production of flexographic printing plates by thermal
development
Abstract
Flexographic printing plates are produced by thermal development
by a process in which an imagewise exposed flexographic printing
element is developed by heating and removing the softened,
unpolymerized parts of the relief-forming layer, the flexographic
printing element used comprising an olefin/(meth)acrylate copolymer
having an olefin content of from 50 to 94 mol %. The
photopolymerizable flexographic printing element comprises an
olefin/(meth)acrylate copolymer having a content of from 50 to 94
mol % of olefin monomers, from 6 to 50 mol % of (meth)acrylate
monomers and from 0 to 5 mol % of further comonomers. This
flexographic printing element is used for the production of
flexographic printing plates both by thermal development and by
development by means of washout compositions.
Inventors: |
Schadebrodt, Jens; (Mainz,
DE) ; Hiller, Margit; (Karlstadt, DE) |
Correspondence
Address: |
Herbert B. Keil
KEIL & WEINKAUF
1350 Connecticut Ave., N.W.
Washington
DC
20036
US
|
Family ID: |
31724593 |
Appl. No.: |
10/895880 |
Filed: |
July 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10895880 |
Jul 22, 2004 |
|
|
|
10414468 |
Apr 14, 2003 |
|
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Current U.S.
Class: |
430/300 |
Current CPC
Class: |
Y10S 430/145 20130101;
G03F 7/36 20130101; B41C 1/00 20130101; B41C 1/05 20130101; G03F
7/033 20130101; Y10S 430/146 20130101 |
Class at
Publication: |
430/300 |
International
Class: |
G03C 001/73 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2002 |
DE |
10241851.9 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. A photopolymerizable flexographic printing element for the
production of flexographic printing plates, at least comprising a
dimensionally stable substrate, at least one photopolymerizable
relief-forming layer, at least comprising an elastomeric binder,
monomers and a photoinitiator, wherein the elastomeric binder is an
olefin/(meth)acrylate copolymer which contains from 50 to 94 mol %
of olefin monomers, from 6 to 50 mol % of (meth)acrylate monomers
and from 0 to 5 mol % of further comonomers, based in each case on
the total amount of all monomers incorporated in the form of
polymerized units.
15. A flexographic printing element as claimed in claim 14, which
furthermore comprises a digitally imagable layer.
16. A flexographic printing element as claimed in claim 15, wherein
the digitally imagable mask is one selected from the group
consisting of IR-ablative masks, inkjet masks and thermographic
masks.
17. A flexographic printing element as claimed in claim 14, wherein
the olefin content is from 70 to 92 mol %, the (meth)acrylate
content from 8 to 30 mol % and the content of further comonomers
from 0 to 5 mol %.
18. A flexographic printing element as claimed in claim 17, wherein
the olefin content is from 80 to 90 mol %, the (meth)acrylate
content from 10 to 20 mol % and the content of further comonomers
from 0 to 5 mol %.
19. A flexographic printing element as claimed in claim 14, wherein
the elastomeric binder has a melt flow index MFI (190.degree. C.,
2.16 kg) of from 1 to 100 g/10 min.
20. A flexographic printing element as claimed in claim 14, wherein
the elastomeric binder is an ethylene/(meth)acrylate copolymer.
21. (canceled)
22. (canceled)
23. (canceled)
24. A flexographic printing element as claimed in claim 14, wherein
the olefin monomers comprise at least one olefin selected from the
group consisting of ethylene, propylene, 1-butene, isobutene,
1-pentene, 1-hexene, 1-heptene and 1-octene.
25. A flexographic printing element as claimed in claim 14, wherein
the olefin monomers are one or more olefins selected from the group
consisting of ethylene, propylene, 1-butene, isobutene, 1-pentene,
1-hexene, 1-heptene and 1-octene.
Description
[0001] The present invention relates to a process for the
production of flexographic printing plates by thermal development,
in which an imagewise exposed flexographic printing element is
developed by heating and removing the softened, unpolymerized parts
of the relief-forming layer, the flexographic printing element used
containing an olefin/(meth)acrylate copolymer having an olefin
content of from 50 to 94 mol %. The present invention furthermore
relates to a photopolymerizable flexographic printing element which
contains an olefin/(meth)acrylate copolymer containing from 50 to
94 mol % of olefin monomer(s), from 6 to 50 mol % of (meth)acrylate
monomer(s) and from 0 to 5 mol % of further comonomers, and the use
of this flexographic printing element for the production of
flexographic printing plates both by thermal development and by
development by means of washout compositions.
[0002] The most widely used process for the production of
flexographic printing plates comprises the imagewise exposure of
the photopolymerizable relief-forming layer through a photographic
or digitally prepared mask. In a further process step, the exposed
layer is treated with a suitable solvent or solvent mixture, the
unexposed, unpolymerized parts of the photopolymerizable layer
being removed while the exposed, polymerized parts are retained and
form the relief of the printing plate. However, the washout process
requires a relatively long time span. Furthermore, although the
polymerized layer components are not dissolved, they nevertheless
swell in the washout composition. The plate must therefore also be
carefully dried after washing out before it can be used for
printing. The drying process can take several hours.
[0003] As an alternative to the development by means of solvents,
U.S. Pat. No. 3,264,103, U.S. Pat. No. 5,175,072, wO 96/14603 or WO
01/88615 has proposed thermal development.
[0004] No solvent is used in the thermal development. Instead,
after the imagewise exposure, the relief-forming layer is brought
into contact with an absorbent material and is heated. The
absorbent material is, for example, a porous nonwoven, for example
of nylon, polyester, cellulose or inorganic materials. As a result
of the heating, the unpolymerized parts of the relief-forming layer
are liquefied and are absorbed by the nonwoven. The saturated
nonwoven is then removed.
[0005] Instead of using absorbent materials for removing the
liquefied material, WO 01/90818 has proposed, as an alternative,
treating the exposed flexographic printing element with a hot air
or liquid stream under superatmospheric pressure and thus removing
the unpolymerized parts.
[0006] EP-A 468 745 has proposed elastomeric polyurethanes as
preferred materials for flexographic printing elements for thermal
development, although this publication also designates some
commercially available flexographic printing elements intended for
development with solvents as being suitable in principle.
[0007] WO 01/88615 confirms that commercially available
flexographic printing elements intended for development with
solvents are frequently not suitable for thermal development and
instead proposes flexographic printing elements whose
relief-forming layer has specific dynamic mechanical
characteristics. Olefin/(meth)acrylate copolymers are not disclosed
as possible binders.
[0008] Apparatuses suitable for carrying out thermal development
have been proposed by EP-A 469 735 and WO 01/18604 and are also
commercially available under the name Cyrel.RTM. Fast.
[0009] In spite of the basic suitability of the thermal development
for the production of flexographic printing plates, a person
skilled in the art is faced with a multiplicity of problems in
implementing this concept, so that the thermal development has to
date played only a minor role on the market and has by no means
been able to date to replace the development by means of solvents.
In particular, the production of high-resolution plates of constant
quality and the production of plates having large relief heights
continue to present problems.
[0010] In the thermal development, the unpolymerized material
should be liquefied as thoroughly as possible in order to permit
efficient and complete removal. Deposits remaining on the printing
relief lead at least to reduced resolution and to a poorer-quality
printed image which is not crisp. Under certain circumstances, fine
relief elements are severely deformed or not formed at all.
[0011] Here, a person skilled in the art is in a typical dilemma.
On the one hand, the good liquefaction of the material to be
removed is of course promoted by higher temperatures. The lower the
viscosity, the more readily and the more rapidly is the liquefied
polymeric material absorbed by the nonwoven. The dimensional
stability of the substrate film is adversely affected by increasing
temperature. Distortion of the substrate film results in poorer
register of the printing plate. Furthermore, in particular fine
relief elements can be deformed or destroyed by excessively high
temperature. Moreover, while the nonwoven is being pressed on, the
printing plate is also subjected to a certain mechanical load
which, in combination with the thermal load, can considerably
damage the printing relief.
[0012] As a solution to this problem, WO 96/14603 has proposed for
example, using films having a particularly good dimensional
stability, for example those comprising PEN. However, such films
are expensive and moreover have no effect on the loading of the
relief elements themselves.
[0013] Furthermore, a person skilled in the art cannot use more
readily softening polymers without limits for facilitating the
liquefaction, since of course the elastic and the printing
properties of the printing plate are also influenced by the type of
polymer. In addition, very soft, plastic relief printing plates are
unsuitable for flexographic printing.
[0014] Furthermore, customers frequently demand that flexographic
printing elements be suitable both for thermal development and for
development by means of solvents.
[0015] It is an object of the present invention to provide a
process for the production of flexographic printing plates by means
of thermal development and flexographic printing elements suitable
for this purpose, which process does not have the above
disadvantages. The thermal development should be capable of being
carried out at temperatures as low as possible which should
nevertheless be sufficiently rapid and give a high-quality printing
relief. Furthermore, the flexographic printing element should be
capable of performing a dual function, i.e. of being capable of
being developed in comparable quality both thermally and by means
of conventional organic washout compositions.
[0016] We have found that this object is achieved by a process for
the production of flexographic printing plates by thermal
development, in which the starting material used is a
photopolymerizable flexographic printing element which at least
comprises, arranged one on top of the other,
[0017] a dimensionally stable substrate,
[0018] at least one photopolymerizable, relief-forming layer, at
least comprising an elastomeric binder, monomers and a
photoinitiator,
[0019] the process comprising at least the following steps:
[0020] (a) imagewise exposure of the photopolymerizable
relief-forming layer by means of actinic radiation,
[0021] (b) heating of the exposed flexographic printing element to
a temperature of from 40 to 200.degree. C.,
[0022] (c) removal of the softened, unpolymerized parts of the
relief-forming layer with formation of a printing relief,
[0023] the elastomeric binder being an olefin/(meth)acrylate
copolymer having an olefin content of from 50 to 94 mol %, based on
the total amount of all monomers incorporated in the form of
polymerized units.
[0024] In a second aspect of the present invention, a
photopolymerizable flexographic printing element was found, whose
relief-forming layer comprises an olefin/(meth)acrylate copolymer
having an olefin content of from 50 to 94 mol %, from 6 to 50 mol %
of (meth)acrylate monomers and from 0 to 5 mol % of further
comonomers, based in each case on the total amount of all monomers
incorporated in the form of polymerized units.
[0025] In a third aspect of the present invention, the use of this
flexographic printing element for the production of flexographic
printing plates, in particular by thermal development or
development with the aid of a solvent, was found.
[0026] Surprisingly, it was found that flexographic printing plates
of outstanding quality are obtained by the novel process. The
unpolymerized material can be removed thermally in an outstanding
manner without residues remaining behind on the relief elements or
relief elements being damaged. The thermal loading of the element
in the course of the process is so low that good results are
obtained even with the use of conventional PET films. The novel
flexographic printing elements can furthermore be equally well
developed thermally and by means of washout compositions.
[0027] Regarding the present invention, the following may be stated
specifically:
[0028] Examples of suitable dimensionally stable substrates for the
photopolymerizable flexographic printing elements used as starting
material for the process are sheets, films and conical and
cylindrical sleeves of metals, such as steel, aluminum, copper or
nickel or of plastics, such as polyethylene terephthalate (PET),
polyethylene naphthalate (PEN), polybutylene terephthalate,
polyamide or polycarbonate, and, if required, also woven fabrics
and nonwovens, such as glass fiber fabrics, and composite
materials, for example comprising glass fibers and plastics.
Particularly suitable dimensionally stable substrates are
dimensionally stable substrate films, for example polyester films,
in particular PET or PEN films, or flexible metallic substrates,
such as thin metal sheets or metal foils of steel, preferably of
stainless steel, magnetizable spring steel, aluminum, zinc,
magnesium, nickel, chromium or copper.
[0029] The flexographic printing element furthermore comprises at
least one photopolymerizable, relief-forming layer. Said layer can
be applied directly to the substrate. However, other layers, for
example adhesion-promoting layers and/or resilient lower layers,
may also be present between the substrate and the relief-forming
layer.
[0030] The photopolymerizable relief-forming layer comprises at
least one elastomeric binder, ethylenically unsaturated monomers, a
photoinitiator or a photoinitiator system and optionally further
components.
[0031] The at least one elastomeric binder is an
olefin/(meth)acrylate copolymer which has an olefin content of from
50 to 94 mol %, based on the total amount of all monomers
incorporated in the form of polymerized units. Preferably, the
olefin content is from 70 to 92, particularly preferably from 80 to
90, mol %.
[0032] The melt flow index MFI (190.degree. C., 2.16 kg), measured
according to ASTM D 1238, of the olefin/(meth)acrylate copolymer is
as a rule from 0.5 to 100 g/10 min, preferably from 1 to 100 g/10
min. In particular cases, however, satisfactory results can also be
obtained outside this range. The MFI (190.degree. C., 2.16 kg) is
particularly preferably from 2 to 50, very particularly preferably
from 3 to 10, g/10 min.
[0033] The copolymer preferably comprises, as olefinic monomer
building blocks, olefins of 2 to 8 carbon atoms, for example
ethylene, propylene, 1-butene, isobutene, 1-pentene, 1-hexene,
1-heptene or 1-octene. Of course, mixtures of different olefins may
also be present. Olefins of 2 to 4 carbon atoms are particularly
preferred, ethylene being very particularly-preferred. The
particularly preferred ethylene/(meth)acrylate copolymers may be
those which contain exclusively ethylene as the olefinic monomer.
However, they may also be those in which ethylene forms the main
component of the olefins, and other olefins, in particular said
C.sub.2- to C.sub.8-olefins, are also incorporated in small amounts
as polymerized comonomers.
[0034] Acrylic esters and/or methacrylic esters are used as
comonomers for the olefins. Particularly suitable are
(meth)acrylates having straight-chain or branched C.sub.1- to
C.sub.12-alkyl radicals, preferably C.sub.1-- to C.sub.8-alkyl
radicals. Examples include methyl, ethyl, n-propyl, isopropyl,
n-butyl, isobutyl, n-hexyl, 2-ethylhexyl, n-octyl, 2-ethyloctyl,
n-decyl and n-dodecyl radicals. The alkyl radicals may also be
further substituted and/or may have functional groups, provided
that the properties of the relief-forming layer are not adversely
affected thereby. Functional groups which have proven useful are,
for example, epoxy groups. Glycidyl radicals are particularly
suitable as functionalized alkyl radicals.
[0035] Preferred comonomers are methyl (meth)acrylate, ethyl
(meth)acrylate, n-butyl (meth)acrylate, ethylhexyl (meth)acrylate
and glycidyl (meth)acrylate. Mixtures of a plurality of
(meth)acrylates can of course also be used as comonomers. For
example, a mixture of glycidyl (meth)acrylate with methyl, ethyl or
butyl (meth)acrylate may be used.
[0036] The amount of (meth)acrylate comonomers is as a rule from 6
to 50, preferably from 8 to 30, particularly preferably from 10 to
20, mol %, based on the total amount of all monomers incorporated
in the form of polymerized units in the copolymer.
[0037] For fine control of the properties of the copolymer used
according to the invention, further comonomers having ethylenically
unsaturated groups may also be present in addition to the olefins
and (meth)acrylates, provided that the properties of the
relief-forming layer are not adversely affected thereby. Examples
of suitable comonomers include maleic anhydride, (meth)acrylic
acid, maleic acid and its (mono)esters, fumaric acid and its
(mono)esters, vinyllactams, vinylamides, vinyl halides, vinyl
esters, vinyl ethers, vinylsilanes and vinylsiloxanes. A
particularly preferred further comonomer is maleic anhydride.
However, the amount of such comonomers should as a rule not exceed
5 mol %, based on the total amount of all monomers contained in the
olefin/(meth)acrylate copolymer.
[0038] The ethylene/(meth)acrylate copolymers are preferably random
copolymers. They are commercially available, for example, under the
name Lotryl.RTM. or Lotader.RTM.. Mixtures of two or more different
ethylene/(meth)acrylate copolymers can of course also be used. In
addition to the at least one ethylene/(meth)acrylate copolymer, the
relief-forming layer can optionally also have one or more secondary
binders. Such secondary binders can be used by a person skilled in
the art for the fine control of the properties of the subsequent
printing plate. For example, the resilience or the ink acceptance
of the printing plate can be influenced. Examples of suitable
secondary binders include ethylene/propylene/diene terpolymers,
ethylene/octene copolymers, ethylene/vinyl acetate copolymers,
nitrile rubber, natural rubber, butyl rubber, polyisobutylene,
polyisoprene, polybutadiene, polychloroprene, styrene/diene block
copolymers, hydrogenated styrene/diene block copolymers,
polyvinylbutyral and styrene/butadiene emulsion copolymers. The
choice of secondary binders is in principle not limited, provided
that the properties of the relief-forming layer are not adversely
affected thereby. In particular, the type and amount of secondary
binders should be chosen so that the relief-forming layer has
sufficient transparency. However, the amount of secondary binders
should as a rule not exceed 20, preferably 10, particularly
preferably 5, % by weight, based on the total amount of all binders
used.
[0039] The total amount of binders, i.e. olefin/(meth)acrylate
copolymers and any secondary binders present together, is usually
from 40 to 95, preferably from 50 to 85, particularly preferably
from 60 to 80, % by weight, based on the sum of all components of
the relief-forming layer.
[0040] The photopolymerizable relief-forming layer furthermore
comprises polymerizable compounds or monomers in a known manner.
The monomers should be compatible with the olefin/(meth)acrylate
copolymers and should have at least one polymerizable,
ethylenically unsaturated double bond. Esters or amides of acrylic
acid or methacrylic acid with mono- or polyfunctional alcohols,
amines, aminoalcohols or hydroxyethers and hydroxyesters, esters of
fumaric or maleic acid or allyl compounds have proven particularly
advantageous. Examples of suitable monomers are butyl acrylate,
2-ethylhexyl acrylate, lauryl acrylate, 1,4-butanediol diacrylate,
1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate,
1,9-nonanediol diacrylate, trimethylolpropane tri(meth)acrylate,
dioctyl fumarate and N-dodecylmaleimide. The amount of monomers in
the relief-forming layer is as a rule from 4.9 to 30, preferably
from 4.9 to 20, % by weight, based on the amount of all
components.
[0041] The photopolymerizable relief-forming layer furthermore has
a photoinitiator or a photoinitiator system in a manner known in
principle. Examples of suitable initiators are benzoin and benzoin
derivatives, such as methylbenzoin or benzoin ethers, benzil
derivatives, such as benzil ketals, acylarylphosphine oxides,
acylarylphosphinic esters, polynuclear quinones and benzophenones.
The amount of photoinitiator in the relief-forming layer is as a
rule from 0.1 to 5% by weight, based on the amount of all
components of the relief-forming layer.
[0042] The relief-forming layer can optionally comprise a
plasticizer. Mixtures of different plasticizers may also be used.
Examples of suitable plasticizers include modified and unmodified
natural oils and natural resins, such as high-boiling paraffinic,
naphthenic or aromatic mineral oils, synthetic oligomers or resins,
such as oligostyrene, high-boiling esters, oligomeric
styrene/butadiene copolymers, oligomeric
.alpha.-methylstyrene/p-methylstyrene copolymers, liquid
oligobutadienes, in particular those having a molecular weight of
from 500 to 5 000 g/mol, or liquid oligomeric
acrylonitrile/butadiene copolymers or oligomeric
ethylene/propylene/diene copolymers. Vinyl-rich polybutadiene oils,
high-boiling aliphatic esters and mineral oils are preferred.
High-boiling, substantially paraffinic and/or naphthenic mineral
oils are particularly preferred. For example, paraffin-based
solvates and special oils under the names Shell Catenex S and Shell
Catenex PH are commercially available. In the case of mineral oils,
a person skilled in the art distinguishes between technical white
oils, which may still have a very low aromatics content and medical
white oils, which are substantially free of aromatics. They are
commercially available, for example under the names Shell Risella
(technical white oil) and Shell Ondina (medical white oil).
[0043] Furthermore, certain synthetic plasticizers, for example
peralkylated aromatics, which are free of aromatics harmful to
health, are also suitable as plasticizers. Such synthetic
plasticizers, frequently extended with white oils, are available,
for example, under the name Kettlitz Mediaplast.
[0044] The amount of an optionally present plasticizer is
determined by a person skilled in the art according to the desired
properties of the layer. However, it should as a rule not exceed
40, preferably 30, % by weight, based on the sum of all components
of the photopolymerizable relief-forming layer. If a plasticizer is
present, amounts of from 5 to 30, preferably from 10 to 25, % by
weight have proven useful.
[0045] The relief-forming layer can optionally comprise further
components, for example thermal polymerization inhibitors, dyes,
pigments, photochromic additives or antioxidants. As a rule,
however, not more than 10, preferably not more than 5, % by weight,
based on the sum of all components of the layer, should be
used.
[0046] The photopolymerizable relief-forming layer may comprise a
plurality of photopolymerizable layers one on top of the other,
which have an identical, almost identical or different composition.
A multilayer structure has the advantage that the properties of the
surface of the printing plate, for example ink transfer, can be
changed without influencing the typical flexographic printing
properties of the printing plate, for example hardness or
resilience. Surface properties and layer properties can thus be
changed independently of one another in order to achieve an optimum
printed copy.
[0047] The flexographic printing elements may optionally also
comprise further layers in addition to the relief-forming
layer.
[0048] Examples of such layers include an elastomeric lower layer
comprising another formulation, which is present between the
substrate and the relief-forming layer or layers. By means of such
lower layers, it is possible to change the mechanical properties of
the flexographic printing plates without influencing the properties
of the actual printing relief layer.
[0049] Resilient substructures which are present under the
dimensionally stable substrate of the flexographic printing
element, i.e. on that side of the substrate which faces away from
the relief-forming layer, serve the same purpose.
[0050] Further examples include adhesion-promoting layers, which
bond the substrate to layers present thereon or bond different
layers to one another.
[0051] The thickness of the relief-forming layer or layers is
determined by a person skilled in the art according to the desired
intended use of the flexographic printing plate and is as a rule
from 0.5 to 7 mm, preferably from 0.5 to 5 mm, particularly
preferably from 0.7 to 2.5 mm.
[0052] The photopolymerizable flexographic printing element may
furthermore have a transparent, nontacky release layer. They
facilitate the removal of any protective film present before the
use of the flexographic printing element and thus avoid damage to
the relief-forming layer. They furthermore make it easier to lay on
and remove the photographic negative for imaging. Release layers
are formed by a polymer forming strong films and any additives
contained therein. Examples of suitable polymers forming strong
films are polyamides, completely or partly hydrolyzed polyvinyl
acetates and polyethylene oxide/vinyl acetate graft polymers. In
general, the release layers are from 0.2 to 25, preferably from 2
to 20, .mu.m thick.
[0053] The flexographic printing element used as a starting
material can optionally also be protected from damage by a
protective film, for example a PET protective film, which is
present on the respective uppermost layer of the flexographic
printing element, i.e. as a rule on the release layer. If the
photosensitive flexographic printing element has a protective film,
this must be peeled off before the novel process is carried
out.
[0054] The production of the novel flexographic printing element
has no peculiarities at all and can be carried out by the methods
known in principle to a person skilled in the art, for example by
kneading the components and forming the layer by molding, by means
of extrusion and calandering between substrate film and cover sheet
or by casting the dissolved components of the layer onto the
dimensionally stable substrate.
[0055] The flexographic printing element just disclosed is intended
for conventional imaging by means of photographic masks. In a
further embodiment of the present invention, it may also be a
digitally imagable flexographic printing element. Here, the
flexographic printing element has an additional digitally imagable
layer. This may be present on the transparent release layer but,
when digitally imagable layers are present, the release layer can
also be dispensed with.
[0056] The digitally imagable layer is preferably a layer selected
from the group consisting of the IR-ablative layers, inkjet layers
and thermographic layers.
[0057] IR-ablative layers or masks are opaque to the wavelength of
actinic light and usually comprise a film-forming thermally
decomposable binder and at least one IR absorber, for example
carbon black. Carbon black also ensures that the layer is opaque.
Suitable binders are both binders such as polyamides or
nitrocellulose, which are soluble in an organic medium, and binders
such as polyvinyl alcohol or polyvinyl alcohol/polyethylene glycol
graft copolymers, which are soluble in an aqueous medium. In the
IR-ablative layer, it is possible to write into a mask by means of
an IR laser, i.e. the layer is decomposed and removed in the areas
where the laser beam is incident on it. Imagewise exposure to
actinic light can be effected through the resulting mask. Examples
of the imaging of flexographic printing elements using IR-ablative
masks are disclosed, for example, in EP-A 654 150 or EP-A 1 069
475.
[0058] In the case of inkjet layers, a transparent layer writable
using inkjet inks, for example a gelatin layer, is applied. Said
layer can be printed by means of inkjet printers using opaque inks.
Examples are disclosed in EP-A 1 072 953.
[0059] Thermographic layers are transparent layers which contain
substances which become black under the influence of heat. Such
layers comprise, for example, a binder and an inorganic or organic
silver salt and can be provided with an image by means of a printer
having a thermal printing head. Examples are disclosed in EP-A 1
070 989.
[0060] The digitally imagable layer may also be a peel-off layer,
as disclosed, for example, in EP-A 654 151.
[0061] In a preferred embodiment, the digitally imagable layers are
soluble in water or predominantly aqueous solvent mixtures.
[0062] The digitally imagable layers can be cast onto the
photopolymerizable layer or the release layer in a manner known in
principle.
[0063] For carrying out the novel process, the flexographic
printing element is used as a starting material. If the
flexographic printing element comprises a protective film, this is
first peeled off. In the first process steps, the flexographic
printing element is exposed imagewise and then thermally developed
in further process steps.
[0064] In process step (a), the photopolymerizable relief-forming
layer is first exposed imagewise by means of actinic radiation. The
imagewise exposure can be effected by means of the methods known in
principle.
[0065] In the conventional method, a photographic mask is placed on
top for imaging the relief-forming layer in process step (a). The
flexographic printing element is then exposed to actinic light
through the mask placed on top.
[0066] Suitable actinic, i.e. chemically effective, light is known
to be, in particular, UVA or UV/VIS radiation. As a result of the
irradiation, the photopolymerizable layer is crosslinked in the
parts which are not covered. To ensure that the photographic
negative is placed on top in a trouble-free manner, the exposure
can be carried out in a known manner using a vacuum printing frame
or under a glass plate.
[0067] If the dimensionally stable substrate is transparent, the
flexographic printing element can optionally be exposed to actinic
light from the back in a process step upstream of (a). By means of
such a step, the relief height can be determined, and said step
helps to achieve better adhesion of the relief elements.
[0068] In process steps (b) and (c) of the novel process, the
imagewise exposed flexographic printing element is thermally
developed. The two process steps can be carried out in succession
or simultaneously.
[0069] In process step (b), the flexographic printing element is
heated so that the unpolymerized parts of the relief-forming layer
soften, liquefy or melt. A person skilled in the art is aware that
the term melt with regard to the material comprising polymer,
monomer, photoinitiator and, if required, plasticizer and other
additives cannot be defined as precisely as in the case of pure,
low molecular weight substances. What is meant here is that the
viscosity of the material is to be reduced to such an extent that
it can be absorbed by a nonwoven or removed in another manner in
process step (c).
[0070] The flexographic printing element is heated in step (b) to a
temperature which is sufficiently high to liquefy the unpolymerized
layer parts to a sufficient extent, but without damaging the
polymerized parts of the layer. In general, a temperature of from
40 to 200.degree. C. is required for this purpose. The flexographic
printing element is preferably heated to a temperature of from 60
to 160.degree. C.
[0071] The heating can be effected, for example, by exposure to a
heat source, for example by means of an IR lamp. Further examples
include immersion in hot baths, heating by means of hot air or
liquid streams or bringing the flexographic printing element into
contact with hot surfaces, without it being intended to restrict
the invention thereto. A combination of a plurality of methods ay
also be used. Preferably, the flexographic printing element is
heated from the front. Thus, the surface of the relief-forming
layer usually has a higher temperature than the parts of the layer
which are located below the surface.
[0072] In step (c), the softened, unpolymerized parts of the
relief-forming layer are removed. The polymerized parts remain
behind on the substrate, forming the printing relief.
[0073] Step (c) can be carried out, for example, by bringing the
heated relief-forming layer into contact with an absorbent
material. The heated, liquefied, unpolymerized parts of the
relief-forming layer are thus absorbed by the absorbent material.
In order to achieve very efficient absorption, very intimate
contact between the absorbent material and the surface of the
flexographic printing element should be established. For example,
the absorbent material can be placed on the surface and then
pressed down. After saturation of the absorbent material with the
polymeric material, the absorbent material is removed from the
still warm flexographic printing plate. To ensure as complete
removal as possible of the softened material, it is usually
advisable to repeat this process with fresh absorbent material
until all liquefied material has been removed. Suitable absorbent
materials are suitable porous materials, for example nonwovens of
nylon, polyester or cellulose. Further details on the procedure of
step (d) are disclosed in wO 01/88615, page 15, line 6, to page 17,
line 2, which is hereby expressly incorporated here by
reference.
[0074] In an alternative embodiment for process step (b), the
removal is effected by processing the heated flexographic printing
element using a hot air or liquid stream under superatmospheric
pressure. For example, a jet of steam under superatmospheric
pressure can be directed at the top of the flexographic printing
element. The hot steam stream ensures, on the one hand, heating of
the flexographic printing element and softening of the
unpolymerized parts of the relief-forming layer. The mechanical
energy of the jet ensures separation of the liquefied material from
the polymerized parts. Further details are disclosed in wO 01/90818
page 3, lines 5 to 16.
[0075] The printing plate obtained by thermal development can
optionally also be aftertreated. It can, for example, be
postexposed uniformly to actinic light. It can furthermore be
rendered nontacky on the surface by means of Br.sub.2 solution or
by means of exposure to UV-C light, in a manner known in
principle.
[0076] The novel process with the use of flexographic printing
elements having digitally imagable layers is very similar to that
described above. Instead of using a photographic mask, in process
step (a), the digitally imagable layer is imaged by means of the
technique required in each case and a mask is thus produced more or
less in situ on the relief-forming layer.
[0077] With the use of IR-ablative masks, the IR-ablative layer is
removed imagewise with the aid of an IR laser. Those parts which
are subsequently to be crosslinked and form the relief elements are
bared. With the use of inkjet layers or thermographic layers, the
digitally imagable layer is printed by means of inkjet or
thermographic printers in those parts which are not to be
crosslinked in the course of the irradiation.
[0078] After production of a mask from the digitally imagable
layer, exposure to actinic light is effected as with the use of a
photographic mask. A vacuum printing frame for exposure is not
necessary. Exposure is preferably effected by means of a flat-bed
exposure unit in air.
[0079] The exposed flexographic printing elements can be used as
such, i.e. including the digitally imagable layer or the residues
thereof, for the thermal development.
[0080] In a preferred embodiment, the digitally imagable layer is
removed in a process step upstream of process step (b). The prior
removal of the digitally imagable layer saves time during the
thermal development and prevents contamination of the thermal
processing apparatus with components of the mask layer. The
digitally imagable layer can be removed, for example, by peeling
off or by detaching by means of a suitable solvent. In a
particularly preferred embodiment, a water-soluble or at least
water-swellable digitally imagable layer is used. After imaging and
exposure to actinic light, the digitally imagable layer or the
residues thereof can be detached by means of water or predominantly
aqueous solvents. The detachment can optionally be supported by
gentle mechanical treatment, for example by brushing. Since the
olefin/(meth)acrylate binders used according to the invention are
as a rule soluble only in organic solvents or solvent mixtures, the
unpolymerized parts of the relief-forming layer are not dissolved,
nor are the polymerized parts swollen. The flexographic printing
element pretreated in this manner can therefore be further
processed directly thereafter without drying being necessary.
[0081] The novel flexographic printing elements are equally
suitable both for thermal development and for conventional
development by means of washout compositions. In the conventional
development, after the imagewise exposure of the flexographic
printing element, the exposed flexographic printing element is
developed in a known manner using an organic solvent or solvent
mixture.
[0082] The novel flexographic printing elements can furthermore be
used as starting materials for the production of flexographic
printing plates by means of direct laser engraving. A flexographic
printing element which has no digitally imagable layer is used for
this purpose. This is crosslinked uniformly by means of actinic
light, i.e. without placing a mask on top, in a first process step.
The printing relief can then be engraved directly into the
uniformly crosslinked relief-forming layer by means of a CO.sub.2
laser. The crosslinked layer is thermally decomposed in the parts
where the laser is incident on it. Further details of the direct
laser engraving technique for the production of flexographic
printing plates are disclosed, for example, in EP-A 1 136 254 and
U.S. Pat. No. 5,259,311.
[0083] The novel flexographic printing elements have a number of
advantages over conventional flexographic printing elements. They
have a particularly good ozone tear resistance, low swelling even
in UV inks, high resilience, low plastic deformation, constantly
good adhesion to the substrate even during long print runs and
particularly high transparency of the relief-forming layer.
Furthermore, they can be produced particularly economically by
means of extrusion at relatively low temperatures.
[0084] The examples which follow illustrate the invention.
[0085] The following materials were used for the examples:
[0086] A) Binder
[0087] Lotader AX8900: Random ethylene/methyl acrylate/glycidyl
methacrylate copolymer (25% by weight of methyl acrylate, 8% by
weight of glycidyl methacrylate, MFI=6 g/10 min according to ASTM D
1238, melting point 65.degree. C., from ATOFINA)
[0088] Lotryl 35MA05: Random ethylene/methyl acrylate copolymer
(35% by weight of methyl acrylate, MFI=4.5-5.5 g/10 min according
to ASTM D 1238, melting point 50.degree. C., from ATOFINA)
[0089] Lotryl 29MA03: Random ethylene/methyl acrylate copolymer
(29% by weight of methyl acrylate, MFI=2-3.5 g/10 min according to
ASTM D 1238, melting point 61.degree. C., from ATOFINA)
[0090] Lotryl 28MA07: Random ethylene/methyl acrylate copolymer
(28% by weight of methyl acrylate, MFI=6-8 g/10 min according to
ASTM D 1238, melting point 65.degree. C., from ATOFINA)
[0091] Lotryl 35BA40: Random ethylene/butyl acrylate copolymer (35%
by weight of butyl acrylate, MFI=35-40 g/10 min according to ASTM D
1238, melting point 67.degree. C., from ATOFINA)
[0092] Lotryl 24MA07: Random ethylene/methyl acrylate copolymer
(24% by weight of methyl acrylate, MFI=6-8 g/10 min according to
ASTM D 1238, melting point 73.degree. C., from ATOFINA)
[0093] Lotryl 30BA02: Random ethylene/butyl acrylate copolymer (30%
by weight of butyl acrylate, MFI=1.5-2.5 g/10 min according to ASTM
D 1238, melting point 78.degree. C., from ATOFINA)
[0094] Lotryl 20MA08: Random ethylene/methyl acrylate copolymer
(20% by weight of methyl acrylate, MFI=7-9 g/10 min according to
ASTM D 1238, melting point 80.degree. C., from ATOFINA)
[0095] Lotryl 17BA07: Random ethylene/butyl acrylate copolymer (17%
by weight of butyl acrylate, MFI=6.5-8 g/10 min according to ASTM D
1238, melting point 91.degree. C., from ATOFINA)
[0096] Kraton D-1161: styrene/isoprene/styrene block copolymer (15%
by weight of styrene units, two-block fraction=17%, from
KRATON)
[0097] Kraton D-1102: Styrene/butadiene/styrene block copolymer
(30% by weight of styrene units, two-block fraction=17%, from
KRATON)
[0098] B) Further Components of the Relief-Forming Layer
[0099] Ondina 934: Paraffinic mineral oil plasticizer (from
SHELL)
[0100] Hallstar BST: Butyl stearate (from C. P. HALL)
[0101] Nisso PB-B 1000: Polybutadiene oil having an average
molecular weight of about 1 050 g/mol (from NIPPON-SODA)
[0102] Laromer HDDA: 1,6-Hexanediol diacrylate (from BASF)
1,6-HDDMA: 1,6-Hexanediol dimethacrylate (from DEGUSSA/ROEHM)
[0103] Lauryl acrylate 1214: Mixture of n-dodecyl acrylate and
n-tetradecyl acrylate (from BASF) Lucirin BDK Benzil dimethyl ketal
(from BASF)
[0104] Benzophenone 99% benzophenone (from ALDRICH)
[0105] Kerobit TBK 2,6-di-tert-Butyl-p-cresol (from RASCHIG)
[0106] Toluene Toluene (from BASF)
EXAMPLE 1
[0107] Production of a photopolymerizable flexographic printing
element of novel composition comprising an ethylene/methyl
acrylate/glycidyl methacrylate copolymer (ethylene content=87.3 mol
%, melting point (measured by means of DSC)=65.degree. C.) as a
binder.
[0108] 33.23 g (66.46% by weight) of Lotader AX8900 are
preplasticated in a 50 ml laboratory measuring kneader at an
initial temperature of 140.degree. C. for about 2 minutes. A
solution of 1.0 g (2.00% by weight) of lauryl acrylate 1214, 4.0 g
(8.00% by weight) of Laromer HDDA, 1.0 g (2.00% by weight) of
Lucirin BDK, 0.25 g (0.50% by weight) of benzophenone, 0.5 g (1.00%
by weight) of Kerobit TBK and 20 mg (0.04% by weight) of a red dye
is then added. As soon as a homogeneous melt has formed again, 10.0
g (20.00% by weight) of Ondina 934 are also added. After a few
minutes, the melt has become homogeneous again.
[0109] The elastomeric mixture obtained is divided into relatively
small pieces. A 1.27 mm thick flexographic printing element is
produced by pressing between a 5 .mu.m thick PET protective film
with a polyamide substrate and a PET substrate film coated with a
mixture of adhesive-forming components, at 140.degree. C. and 200
bar, in a commercial rubber press. The cold flexographic printing
element is homogeneous and flexible. By repeating the process
described, further examples of the flexographic printing element
are produced. Before the further processing, the flexographic
printing elements are stored for 3 days.
EXAMPLE 2
[0110] Production of a flexographic printing plate of novel
composition comprising an ethylene/methyl acrylate/glycidyl
methacrylate copolymer (ethylene content=87.3 mol %, melting
point=65.degree. C.) as a binder.
[0111] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 3
[0112] Production of a flexographic printing plate of novel
composition comprising an ethylene/methyl acrylate/glycidyl
methacrylate copolymer (ethylene content=87.3 mol %, melting
point=65.degree. C.) as a binder.
[0113] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 4
[0114] Production of a flexographic printing plate of novel
composition comprising an ethylene/methyl acrylate copolymer
(ethylene content=85.1 mol %, melting point=50.degree. C.) as a
binder.
[0115] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 5
[0116] Production of a rigid flexographic printing plate of novel
composition comprising an ethylene/methyl acrylate copolymer
(ethylene content=88.3 mol %, melting point=61.degree. C.) as a
binder.
[0117] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 6
[0118] Production of a rigid flexographic printing plate of novel
composition comprising an ethylene/methyl acrylate copolymer
(ethylene content=88.8 mol %, melting point=65.degree. C.) as a
binder.
[0119] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 7
[0120] Production of a flexographic printing plate of novel
composition comprising an ethylene/butyl acrylate copolymer
(ethylene content=89.5 mol %, melting point=67.degree. C.) as a
binder.
[0121] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 8
[0122] Production of a rigid flexographic printing plate of novel
composition comprising an ethylene/methyl acrylate copolymer
(ethylene content=90.7 mol %, melting point=73.degree. C.) as a
binder.
[0123] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 9
[0124] Production of a rigid flexographic printing plate of novel
composition comprising an ethylene/butyl acrylate copolymer
(ethylene content=91.4 mol %, melting point=78.degree. C.) as a
binder.
[0125] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is homogeneous and flexible.
EXAMPLE 10
[0126] Production of a flexographic printing plate of novel
composition comprising an ethylene/methyl acrylate copolymer
(ethylene content=92.5 mol %, melting point=80.degree. C.) as a
binder.
[0127] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. The flexographic printing
element obtained is relatively rigid and not very resilient. The
amount and type of the components used are shown in table 1.
COMPARATIVE EXAMPLE A
[0128] Production of a flexographic printing plate having a
composition not according to the invention and comprising an
ethylene/butyl acrylate copolymer (ethylene content=95.7 mol %,
melting point=89.degree. C.) as a binder.
[0129] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1. After cooling, an oily film
caused by exudation of incompatible formulation components forms on
the surface of the kneaded material. Further processing of the
kneaded material to give a flexographic printing element is no
longer possible. In addition, the kneaded material is very hard and
not very resilient after cooling.
COMPARATIVE EXAMPLE B
[0130] Production of a flexographic printing plate having a
composition not according to the invention and comprising an SIS
block copolymer (styrene content=15% by weight, two-block
fraction=17%) as a binder.
[0131] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1.
COMPARATIVE EXAMPLE C
[0132] Production of a flexographic printing plate having a
composition not according to the invention and comprising an
SBS-block copolymer (styrene content=30% by weight, two-block
fraction=17%) as a binder.
[0133] Analogously to example 1, a flexographic printing element is
produced by kneading the starting components and pressing the
kneaded material between two PET films. The amount and type of the
components used are shown in table 1.
1TABLE 1 Amount of the components used Monomers Photoinitiator and
Additives Lauryl antioxidant Plasticizer Dye, acrylate Laromer
Lucirin Kerobit Ondina Nisso Hallstar UVC 1214 HDDA 1,6-HDDMA BDK
TBK 934 PB-B 1000 BST initiators Ex. Binder [% by [% by [% by [% by
[% by [% by [% by [% by [% by No. [% by wt.] wt.] wt.] wt.] wt.]
wt.] wt.] wt.] wt.] wt.] Binder = Lotader AX8900 Ethylene content:
87.3 mol % 1 66.46 2.00 8.00 -- 2.00 1.00 20.00 -- -- 0.54 2 66.46
2.00 8.00 -- 2.00 1.00 -- 20.00 -- 0.54 3 62.46 2.00 8.00 -- 2.00
1.00 -- -- 24.00 0.54 Binder = Lotryl 35MA05 Ethylene content: 85.1
mol % 4 73.45 -- 6.00 2.00 2.00 1.00 15.00 -- -- 0.54 Binder =
Lotryl 29MA03 Ethylene content: 88.3 mol % 5 73.46 -- 6.00 2.00
2.00 1.00 15.00 -- -- 0.54 Binder = Lotryl 28MA07 Ethylene content:
88.8 mol % 6 73.46 -- 6.00 2.00 2.00 1.00 15.00 -- -- 0.54 Binder =
Lotryl 35BA40 Ethylene content: 89.5 mol % 7 73.45 -- 6.00 2.00
2.00 1.00 15.00 -- -- 0.54 Binder = Lotryl 24MA07 Ethylene content:
90.7 mol % 8 73.46 -- 6.00 2.00 2.00 1.00 15.00 -- -- 0.54 Binder =
Lotryl 30BA02 Ethylene content: 91.4 mol % 9 73.46 -- 6.00 2.00
2.00 1.00 15.00 -- -- 0.54 Binder = Lotryl 20MA08 Ethylene content:
92.5 mol % 10 73.46 -- 6.00 2.00 2.00 1.00 15.00 -- -- 0.54 Binder
= Lotryl 17BA07 (Comparative example) Ethylene content: 95.7 mol %
A 73.46 -- 6.00 2.00 2.00 1.00 15.00 -- -- 0.54 Binder = Kraton
D-1161 (Comparative example) SIS block copolymer B 73.46 -- 6.00
2.00 2.00 1.00 15.00 -- -- 0.54 Binder = Kraton D-1102 (Comparative
example) SBS block copolymer C 73.46 -- 6.00 2.00 2.00 1.00 15.00
-- -- 0.54
[0134] Production of Flexographic Printing Plates by Thermal
Development by Means of the Novel Process
[0135] A commercial exposure unit or flexographic printing plates
(nyloflex.RTM. F III exposure unit, BASF Drucksysteme GmbH) is used
for exposing the flexographic printing elements.
[0136] First, the back of the flexographic printing plates is
exposed uniformly to UV/A light without reduced pressure for 15
seconds. Thereafter, the transparent PET protective film is removed
and a test negative is placed on the front of the flexographic
printing plate. Exposure is effected through a test negative for
various main exposure times from 6 to 20 minutes in steps of 2
minutes under reduced pressure. For this purpose, a vacuum film is
drawn over negative and flexographic printing plate and a vacuum
pump connected to the exposure unit is activated. The reduced
pressure is about 800 mbar).
[0137] After the exposure, the printing plate is thermally
processed with the aid of an experimental apparatus. For this
purpose, pieces of the exposed printing plate which have maximum
dimensions of 15 cm.times.15 cm are fixed to a motor-driven metal
roll having a diameter of 20 cm with the use of double-sided
adhesive tape. A commercial polyamide nonwoven is passed over a
heatable metal roll having a diameter of 5 cm. An infrared lamp for
relatively rapid heating of the printing plate is fastened above
the roll carrying the printing plate. After the infrared lamp has
been switched on and the heatable metal roll heated to 150.degree.
C., the metal roll carrying the printing plate is caused to rotate
slowly (about 2 rpm) by means of the motor. The nonwoven is
conveyed past over the heatable roll under a defined contact
pressure of about 2.5 bar. One complete revolution of the roll
carrying the printing plate is defined as one cycle. Altogether,
different numbers of cycles are carried out for processing the
printing plate, the number being stated in the evaluation under the
parameter Cycles.
[0138] Finally, the printing plate is released from the
double-sided adhesive tape, postexposed uniformly for 10 minutes to
UV/A light without reduced pressure and rendered nontacky by
exposure for 20 minutes in a UV/C exposure unit.
[0139] The reproduction quality of the thermally processed
flexographic printing elements is assessed by means of the same
subject elements as in the method for the conventional processing.
Here, the exposure time in which all positive elements are formed
without errors is stated for the positive elements (LEL=lower
exposure limit).
[0140] Background (=uniformity and cleanness of the relief
background)
[0141] Edge crispness (=crispness of edges of the elements, freedom
from melt residues)
[0142] Table 2 lists the corresponding features determined for
examples 1, 3, 6 and 8 and for comparative examples C and D.
2TABLE 2 Results of the thermal development Negative Negative
Example Cycles PT* Hardness.sup.# Relief height LEL dot line Edge
No. [ ] [ ] [.degree. Sh A] [.mu.m] [min] [min] [min] Background
crispness Binder = Ethylene/Methyl acrylate/Glycidyl methacrylate
copolymer (Lotader AX8900) 1 6 175 65 740 16 >20 >20 good
very good 3 6 240 56 840 16 >20 >20 very very good good
Binder = Ethylene/Methyl acrylate copolymer (Lotryl 28MA07) 6 6 118
70 780 14 >20 >20 very very good good Binder =
Ethylene/Methyl acrylate copolymer (Lotryl 24MA07) 8 6 213 74 805
12 20 >20 very very good good Binder = SIS block copolymer
(Kraton D-116) (Comparative example) B 10 180 60 460 14 16 14 good
moderate Binder = SBS block copolymer (Kraton D-1102) (Comparative
example) C 10 205 82 630 >20 18 10 poor moderate *Pendulum tack,
measured similarly to DIN 53157 using a modified pendulum
.sup.#Hardness measured on the completely processed printing
plate
[0143] Table 2 clearly shows that flexographic printing elements of
novel composition can be thermally processed in an excellent manner
since, with a reduced number of cycles or reduced duration of the
thermal processing, well formed positive elements can be reproduced
in conventional exposure times.
[0144] Furthermore, in the case of flexographic printing elements
of novel composition, negative elements, measured for comparative
examples C and D, for up to substantially longer exposure times are
sufficiently deep to enable subjects which contain such elements
also to be printed without the danger of losses of quality. On
visual assessment, too, the flexographic printing plates produced
by means of thermal processing and of novel composition appear to
have very crisp edges and to be uniform which guarantees a
constantly high print quality even during relatively long print
runs.
[0145] Production of flexographic printing plates (conventional
processing by means of a washout composition).
[0146] The flexographic printing elements are exposed imagewise as
described for the thermal development.
[0147] After exposure, the printing plate is washed out in a
commercial continuous washer for flexographic printing plates (type
Variomat VF2, from BASF Drucksysteme GmbH) using the washout
composition (nylosolv II, from BASF Drucksysteme GmbH). The washout
rate is set at 200 mm/min and the brush feed setting is 0 mm in all
cases.
[0148] Thereafter, the plate is dried for 2 hours at 65.degree. C.
in a through-circulation drying oven, postexposed uniformly to UV-A
light for 10 minutes without reduced pressure and rendered nontacky
by exposure for 20 minutes in a UV/C exposure unit.
[0149] For assessing the reproduction quality achievable with the
novel flexographic printing elements, the test negative contains
various fine subject elements which can be optically detected or
measured and evaluated. Specifically, these are the following
elements, which are shown together with the parameters used for the
quality assessment:
3TABLE 3 Explanation of the measured values in table 4 Element name
Element dimension Parameter determined Unit Relief Solid area
Height of the printing .mu.m height background relief Positive dot
Diameter 200 .mu.m Exposure time to min complete formation Positive
Line width 100 .mu.m Exposure time to min line complete formation
Grid Width of the grid lines Exposure time to min 55 .mu.m complete
formation 2% screen 2% dot area with Exposure time to min 60 l/cm
complete formation Negative dot Diameter 400 .mu.m Exposure time
with the min height still > 70 .mu.m Negative Line width 2000
.mu.m Exposure time with the min line height still > 500
.mu.m
[0150]
4TABLE 4 Results of the development by means of washout
compositions Washout Relief Positive Positive Screen Negative
Negative Example rate PT* Hardness.sup.# height dot line Grid 2%
dot line Nr. [mm/min] [ ] [.degree. Sh A] [.mu.m] [min] [min] [min]
[min] [min] [min] Binder = Lotader AX8900 MFI .apprxeq. 6 g/10
minEthylene content = 87.3 mol % 1 160 57 63 780 8 8 12 8 >20
>20 2 160 18 74 810 8 8 8 8 >20 >20 3 140 83 65 920 10 12
20 12 >16 >20 Binder = Lotryl 35MA05 MFI .apprxeq. 5 g/10
minEthylene content = 85.1 mol % 4 140 52 59 940 12 12 12 12 16
>20 Binder = Lotryl 29MA03 MFI .apprxeq. 3 g/10 minEthylene
content = 88.3 mol % 5 120 69 70 780 18 16 16 16 18 >20 Binder =
Lotryl 28MA07 MFI .apprxeq. 7 g/10 minEthylene content = 88.8 mol %
6 140 49 68 930 16 16 16 16 20 >20 Binder = Lotryl 35BA40 MFI
.apprxeq. 40 g/10 minEthylene content = 89.5 mol % 7 140 34 65 950
8 8 10 12 >20 >20 Binder = Lotryl 24MA07 MFI .apprxeq. 7 g/10
minEthylene content = 90.7 mol % 8 70 105 74 1 050 16 16 16 12 18
20 Binder = Lotryl 30BA02 MFI .apprxeq. 2 g/10 minEthylene content
= 91.4 mol % 9 50 95 76 40 n.d. n.d. n.d. n.d. n.d. n.d. Binder =
Lotryl 20MA08 MFI .apprxeq. 8 g/10 min Ethylene content = 92.5 mol
% 10 50 108 82 10 n.d. n.d. n.d. n.d. n.d. n.d. Binder = Lotryl
17BA07 (Comparative example) Ethylene content = 95.7 mol % A ---
Plate production not possible --- Binder = Kraton D-1161
(Comparative example) B 180 230 59 850 10 >20 >20 12 12
>20 Binder = Kraton D-1102 (Comparative example) C 180 250 79
960 8 >20 >20 10 14 >20 *Pendulum tack, measured similarly
to DIN 53157 using a modified pendulum .sup.#Hardness measured on
the completely processed printing plate, n.d.: not determined
[0151] Table 4 shows that the flexographic printing elements of
novel compositions can be readily processed conventionally, i.e. by
means of a solvent-based washout process and subsequent drying, and
printing plates of high quality are obtained. In particular,
negative elements have a height of above-average magnitude even in
the case of relatively long exposure times which has an
advantageous effect on the print quality.
[0152] The production of a flexographic printing element of novel
composition is possible only if the ethylene content of the binders
is not greater than 94 mol %. Otherwise, components usually used in
flexographic printing plates, such as monomers and plasticizers,
are not compatible with the binder.
[0153] With decreasing ethylene content, the solubility of the raw
layer in conventional flexographic printing washout compositions
increases. In the case of binders having an ethylene content of
from 92 to 94 mol %, conventional processing is possible but
relatively slow. Such flexographic printing elements are still
relatively hard and not optimally flexible.
[0154] Optimum properties of the novel flexographic printing
elements are obtained with ethylene contents below 92 mol %. These
flexographic printing elements can be readily washed out, and the
finished printing plates are soft and flexible. The ink transfer of
such flexographic printing plates with water-based, alcohol-based
and UV-curable printing inks is excellent without there being any
defects in the print in the course of the printing process.
[0155] Printing Tests
[0156] Three flexographic printing elements having a composition
according to example 1 are produced by extrusion in a twin-screw
extruder and calandering between substrate film and cover sheet,
exposed with a test subject, developed at a washout rate of 160
mm/min in a flexographic washer operated with nylosolv II and dried
for 2 hours at 65.degree. C. Following the aftertreatment
(postexposure to UV/A light for 10 minutes and elimination of tack
by means of UV/C light for 15 minutes), one printing plate each is
used for proof printing on a proof press using a water-based, an
alcohol-based and a UV-curable printing ink. The ink transfer, the
crispness of the printed image and the uniformity of the print
quality over a run time of 3 hours in each case are excellent.
* * * * *