U.S. patent application number 12/748475 was filed with the patent office on 2011-09-29 for flexographic printing precursors and methods of making.
Invention is credited to Limor Dahan, Ido Gal, Ophira Melamed.
Application Number | 20110236705 12/748475 |
Document ID | / |
Family ID | 44120928 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110236705 |
Kind Code |
A1 |
Melamed; Ophira ; et
al. |
September 29, 2011 |
FLEXOGRAPHIC PRINTING PRECURSORS AND METHODS OF MAKING
Abstract
A mixture of a high molecular weight EPDM rubber with a low
molecular weight (liquid) EPDM rubber provides a highly useful
laser-ablatable flexographic printing plate precursor formulation.
This formulation is sensitive to infrared radiation by the
incorporation of an IR absorbing compound such as a carbon black.
The inclusion of the liquid EPDM rubber avoids the need for
plasticizers such as process oils during manufacturing, and
provides improved image sensitivity, print quality, and run length.
Both flexographic printing plates and printing sleeves can be made
using the mixture of EPDM rubbers.
Inventors: |
Melamed; Ophira; (Shoham,
IL) ; Gal; Ido; (Kafar-Saba, IL) ; Dahan;
Limor; (Tel Aviv, IL) |
Family ID: |
44120928 |
Appl. No.: |
12/748475 |
Filed: |
March 29, 2010 |
Current U.S.
Class: |
428/483 ;
156/307.3; 250/492.1; 264/400; 427/385.5; 428/221; 524/423;
524/426; 524/432; 524/447; 524/449; 524/526; 525/237 |
Current CPC
Class: |
B32B 15/08 20130101;
C08K 2201/001 20130101; B05D 3/02 20130101; G03F 7/00 20130101;
B32B 2274/00 20130101; B32B 2307/732 20130101; B32B 27/36 20130101;
B41C 1/05 20130101; C08K 5/548 20130101; B32B 2264/108 20130101;
B32B 5/02 20130101; B32B 25/08 20130101; C08K 3/042 20170501; B32B
7/12 20130101; B32B 27/065 20130101; B29C 43/003 20130101; B32B
27/302 20130101; B32B 27/308 20130101; B05D 5/00 20130101; B32B
2307/734 20130101; B41N 1/12 20130101; C08L 23/16 20130101; C08L
2205/025 20130101; C08K 3/041 20170501; C08K 3/04 20130101; B32B
15/20 20130101; Y10T 428/249921 20150401; B32B 2264/10 20130101;
Y10T 428/31797 20150401; B32B 2270/00 20130101; C08K 3/36 20130101;
C08L 23/16 20130101; C08L 23/16 20130101; B32B 25/16 20130101; C08L
23/16 20130101; C08L 23/16 20130101; C08K 3/04 20130101; C08L
2205/025 20130101; B32B 25/10 20130101; C08K 5/14 20130101; B32B
15/046 20130101; B32B 27/34 20130101 |
Class at
Publication: |
428/483 ;
525/237; 524/526; 428/221; 524/426; 524/423; 524/447; 524/432;
524/449; 427/385.5; 156/307.3; 264/400; 250/492.1 |
International
Class: |
B32B 27/36 20060101
B32B027/36; C08L 9/00 20060101 C08L009/00; B32B 5/02 20060101
B32B005/02; C08K 3/26 20060101 C08K003/26; C08K 3/30 20060101
C08K003/30; C08K 3/34 20060101 C08K003/34; C08K 3/22 20060101
C08K003/22; B05D 3/02 20060101 B05D003/02; C09J 5/02 20060101
C09J005/02; B29C 35/08 20060101 B29C035/08; G21K 5/00 20060101
G21K005/00 |
Claims
1. An infrared radiation ablatable flexographic printing precursor
that comprises an infrared radiation ablatable layer comprising a
mixture of a high molecular weight ethylene-propylene-diene
terpolymer (EPDM) rubber and a low molecular weight EPDM
rubber.
2. The precursor of claim 1 wherein the weight ratio of the high
molecular weight EPDM to the low molecular weight EPDM rubber is
from about 2:1 to about 10:1.
3. The precursor of claim 1 wherein the weight ratio of the high
molecular weight EPDM to the low molecular weight EPDM rubber is
from about 3:1 to about 5:1.
4. The precursor of claim 1 wherein the molecular weight of the
high molecular weight EPDM is from about 200,000 to about 800,000,
and the molecular weight of the low molecular weight EPDM is from
about 2,000 to about 10,000.
5. The precursor of claim 1 wherein the molecular weight of the
high molecular weight EPDM is from about 250,000 to about 500,000,
and the molecular weight of the low molecular weight EPDM is from
about 2,000 to about 8,000.
6. The precursor of claim 1 wherein the infrared radiation
ablatable layer further comprises a carbon black.
7. The precursor of claim 1 wherein the infrared radiation
ablatable layer further comprises a conductive carbon black.
8. The precursor of claim 1 wherein the infrared radiation
ablatable layer further comprises a conductive carbon black having
a dibutyl phthalate (DBP) absorption of less than 110.
9. The precursor of claim 1 wherein the infrared radiation
ablatable layer further comprises a non-conductive carbon
black.
10. An infrared radiation ablatable flexographic printing precursor
comprises an infrared radiation ablatable layer comprising from
about 1 to about 20 weight % of a conductive carbon black having a
dibutyl phthalate (DBP) adsorption of less than 110, and a mixture
of a high molecular weight ethylene-propylene-diene terpolymer
(EPDM) rubber and a low molecular weight EPDM rubber, wherein the
weight ratio of the high molecular weight EPDM to the low molecular
weight EPDM rubber is from about 3:1 to about 5:1.
11. The precursor of claim 10 wherein the infrared radiation
ablatable layer comprises from about 2 to about 10 weight % of the
conductive carbon black.
12. The precursor of claim 1 wherein the infrared radiation
ablatable layer further comprises a vulcanizer.
13. The precursor of claim 12 wherein the infrared radiation
ablatable layer further comprises sulfur or a peroxide as a
vulcanizer and an azo crosslinking agent, or a mixture of sulfur
and a peroxide, or a mixture of sulfur, an azo crosslinking agent,
and a peroxide.
14. The precursor of claim 1 further comprising a polyester support
upon which the infrared radiation ablatable layer is disposed.
15. The precursor of claim 1 further comprising a fabric support
upon which the infrared radiation ablatable layer is disposed.
16. The precursor of claim 15 wherein the fabric support is
disposed on a polyester support.
17. An infrared radiation ablatable flexographic printing precursor
comprises an infrared radiation ablatable layer comprising one or
more inorganic fillers, a carbon black, and a mixture of a high
molecular weight ethylene-propylene-diene terpolymer (EPDM) rubber
and a low molecular weight EPDM rubber, wherein the weight ratio of
the high molecular weight EPDM to the low molecular weight EPDM
rubber is from about 2:1 to about 10:1.
18. The precursor of claim 17 wherein the infrared radiation
ablatable layer further comprises one or more inorganic fillers
that are chosen from silica, calcium carbonate, barium sulfate,
kaolin, bentonite, zinc oxide, mica, and titanium dioxide.
19. An infrared radiation ablatable flexographic printing precursor
comprises an infrared radiation ablatable layer comprising: from
about 10 to about 35 weight % of one or more inorganic fillers and
from about 1 to about 20 weight % of a carbon black, wherein the
weight ratio of the carbon black to the inorganic filler(s) is from
about 1:50 to about 1:1.5, and a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber, wherein the weight ratio of the high
molecular weight EPDM to the low molecular weight EPDM rubber is
from about 2:1 to about 10:1.
20.-26. (canceled)
27. A method of providing flexographic printing plate or sleeve
comprising: imaging the flexographic printing precursor of claim 1
using infrared radiation to provide a relief image in the infrared
radiation ablatable layer.
28. The method of claim 27 wherein imaging is carried out using a
laser at a power of at least 20 J/cm.sup.2.
29. The method of claim 27 further comprising removal of debris
after imaging.
30. The method of claim 29 wherein debris is removed by vacuum,
compressed air, brushes, rinsing with water, ultrasound, or any
combination of these.
31. The method of claim 27 wherein imaging is carried out using a
high power laser ablating imager.
32. The method of claim 27 wherein imaging is carried out at the
same or different depths relative to the surface of the infrared
radiation ablatable layer using two or more laser diodes each
emitting radiation in one or more wavelengths.
33. A system for providing a flexographic printing plate or
printing sleeve, comprising: the flexographic printing precursor of
claim 1, a group of one or more sources of imaging infrared
radiation, each source capable of emitting infrared radiation, a
set of optical elements coupled to the sources of imaging infrared
radiation to direct imaging infrared radiation from the sources
onto the flexographic printing precursor.
34. The system of claim 33 wherein the sources of imaging infrared
radiation are laser diodes, multi-emitter laser diodes, laser bars,
laser stacks, fiber lasers, or a combination thereof.
35. An infrared radiation ablatable flexographic printing precursor
comprises an infrared radiation ablatable layer comprising a carbon
black, one or more inorganic fillers, and one or more elastomers,
wherein the weight ratio of the carbon black to the inorganic
filler(s) is from about 1:50 to about 1:1.5.
36. The precursor of claim 35 wherein the elastomers includes a
mixture of a high molecular weight ethylene-propylene-diene
terpolymer (EPDM) rubber and a low molecular weight EPDM
rubber.
37. The precursor of claim 35 wherein the weight ratio of the
carbon black to the inorganic filler(s) is from about 1:20 to about
1:5
Description
FIELD OF THE INVENTION
[0001] This invention relates to flexographic printing precursors
in the form of either plates or sleeves that contain an
IR-ablatable relief-forming layer comprising a mixture of rubbery
resins. This invention also relates to a method of preparing these
flexographic printing precursors in either plate or sleeve
form.
BACKGROUND OF THE INVENTION
[0002] Flexography is a method of printing that is commonly used
for high-volume printing runs. It is usually employed for printing
on a variety of substances particularly those that are soft and
easily deformed, such as paper, paperboard stock, corrugated board,
polymeric films, fabrics, plastic films, metal foils, and
laminates. Course surfaces and stretchable polymeric films can be
economically printed by the means of flexography.
[0003] Flexographic printing plates are sometimes known as "relief
printing plates" and are provided with raised relief images onto
which ink is applied for application to the printing substance. The
raised relief images are inked in contrast to the relief "floor"
that remains free of ink in the desired printing situations. Such
printing plates are generally supplied to the user as a
multi-layered article having one or more imageable layers coated on
a backing or substrate. Flexographic printing can also be carried
out using a flexographic printing cylinder or seamless sleeve
having the desired raised relief image. These flexographic printing
cylinder or sleeve precursors can be "imaged in-the-round" (ITR),
either by using a standard photomask or a "laser ablation mask"
(LAM) imaging on a photosensitive plate formulation, or by "direct
laser engraving" (DLE) of a plate precursor that is not necessarily
photosensitive.
[0004] U.S. Pat. No. 5,719,009 (Fan) describes elements having an
ablatable layer disposed over photosensitive layer(s) so that after
image ablation, UV exposure of the underlying layer hardens it
while non-exposed layer(s) and the ablatable mask layer are
subsequently washed away.
[0005] DuPont's Cyrel.RTM. FAST.TM. thermal mass transfer plates
are commercially available photosensitive resin plate precursors
that comprise an integrated ablatable mask element and require
minimal chemical processing, but they do require thermal wicking or
wiping to remove the non-exposed areas. These also require
extensive disposal of polymeric waste and some drying of the
processed (developed) plates.
[0006] There remains a need for a totally processless method of
producing flexographic printing plates with high throughput
efficiency. A method for forming a relief pattern on a printing
element by directly engraving (DE) with a laser is already used to
produce relief plates and stamps. However, the requirement of
relief depths in excess of 500 .mu.m challenges the speed at which
these flexographic printing plate precursors can be imaged. In
contrast to the laser ablation of the CTP mask layers atop the
photosensitive resin, which only requires low energy lasers and low
fluence, the DE of laser ablatable flexographic printing plates
requires higher energy lasers and higher fluence. In addition, the
laser ablatable, relief-forming layer becomes the printing surface
and must have the appropriate physical and chemical properties
needed for good printing. The laser engravable black mask layer is
washed away during the development and is not used during the
printing.
[0007] Flexographic printing plate precursors used for infrared
radiation (IR) laser ablation engraving must comprise an
elastomeric or polymeric composition that includes one or more
infrared radiation absorbing compounds. When the term "imaging" is
used in connection with "laser engraving", it refers to ablation of
the background areas while leaving intact the areas of the element
that will be inked and printed in a flexographic printing station
or press.
[0008] Commercial laser engraving is typically carried out using
carbon dioxide lasers. While they are generally slow and expensive
to use and have poor beam resolution, they are used because of the
attractions of direct thermal imaging. Infrared (IR) fiber lasers
are also used. These lasers provide better beam resolution, but are
very expensive. IR laser engravable flexographic printing plate
blanks having unique engravable compositions are described in WO
2005/084959 (Figov).
[0009] Direct laser engraving is described, for example, in U.S.
Pat. Nos. 5,798,202 and 5,804,353 (both Cushner et al.) in which
various means are used to reinforce the elastomeric layers. The
reinforcement can be done by addition of particulates, by
photochemical reinforcement, or by thermochemical hardening. U.S.
Pat. No. 5,804,353 describes a multilayer flexographic printing
plate wherein the composition of the top layer is different from
the composition of the intermediate layer. Carbon black can be used
as a reinforcing agent and can be present in both layers.
[0010] Flexographic printing plate precursors for near-IR laser
ablation engraving generally comprise an elastomeric or polymeric
system that is made thermosetting by a polymerization reaction and
includes fillers and infrared absorbing compounds. During recent
years, infrared laser diodes have been used for ablation of thin
layers (U.S. Pat. No. 5,339,737 of Lewis et al.) for use in offset
lithographic printing. These lasers are becoming increasingly
inexpensive and more powerful and consequently are becoming more
useful for laser ablation of thick layers such as are found in
flexographic printing precursors. Such lasers require the presence
of radiation absorbing dyes or pigments in the flexographic
printing precursors as they generally operate around wavelengths of
800 nm to 1200 nm. They have the potential to enable faster
imaging, higher print quality, and more reliable engraving than
obtained with carbon dioxide lasers. In addition, it is
advantageous to optimize imaging speed by formulating printing
plates with higher sensitivity. This will give higher productivity
in printing plate production with the potential of greater profits
for the printing houses or trade shops where the printing plates
may be produced. Imaging systems can be made by using arrays of
laser diodes. Throughput also depends on the number of laser diodes
being used and there is a balance between the cost of imaging heads
that depends on the number of diodes and their combined output
power. The requirement for high print quality has increased
considerably in recent years as flexographic printing penetrates
markets previously dominated by high quality offset lithography.
Laser engraving using infrared diodes instead of carbon dioxide
provides an opportunity for higher quality because the wavelength
of the diode radiation at 800-1000 nm is so much smaller than that
of carbon dioxide of 10.7 .mu.m.
[0011] As mentioned above, the chosen commercial means of imaging
by laser engraving has for some years been with carbon dioxide
lasers. These are capable of ablating layers to produce suitable
relief depths for flexographic printing. Such depths may be
anywhere in the range from 200 .mu.m to 5 mm. As carbon dioxide
lasers operate at a wavelength of 10.7 .mu.m, there is no need to
incorporate infrared absorbing dyes or pigments into the printing
precursors because the polymers themselves absorb at this
wavelength for ablation.
[0012] Although patents concerned with formulating laser-engraved
printing precursors may mention laser diode engraving, they have
been primarily aimed at carbon dioxide laser imaging and thus
include formulations lacking infrared absorbing materials as
described in U.S. Pat. No. 5,259,311 (MacCaughey). Formulations
designed for ablation by carbon dioxide lasers cannot be easily
modified for laser diode ablation by simply adding a suitable
infrared radiation absorbing material. For instance, infrared dyes
may react with the chemistry used to vulcanize the last-ablatable
layer, or carbon black may block UV radiation used for curing the
flexographic precursor composition.
[0013] One approach to formulation of laser-engravable flexographic
printing precursors is to produce thermoplastic formulations that
have not been crosslinked to form thermoset materials. These have
been found to be of limited suitability for laser engraving because
ablation of thermoplastic materials results in melted portions
around the ablated areas and sometimes re-deposition of ablated
material onto the ablated areas. This is because it is inevitable
that during imaging there is heat flowing to non-imaged areas that
is insufficient for ablation but sufficient for melting, as
described in U.S. Patent Application Publication 2004/0231540
(Hiller et al.).
[0014] A number of elastomeric systems have been considered for
construction of laser-engravable flexographic printing precursors.
The earliest formulations included natural rubbers (as reported in
U.S. Pat. No. 6,223,655 by Shanbaum et al. using a mixture of
epoxidized natural rubber and natural rubber). Also, engraving of a
rubber is described in "Laser Material Processing of Polymers" by
S. E. Nielsen in Polymer Testing 3 (1983) 303-310.
[0015] U.S. Pat. No. 4,934,267 (Hashimito) describes the use of
natural rubber or synthetic rubber or mixtures of both and
specifically mentions acrylonitrile-butadiene, styrene-butadiene
and chloroprene with a textile support. "Laser Engraving of
Rubbers--The influence of Fillers" by W. Kern et al., October 1997,
pp. 710-715 (Rohstoffe Und Anwendendunghen) described the use of
natural rubber, nitrile rubber (NBR), ethylene-propylene-diene
terpolymer (EPDM), and styrene-butadiene copolymer (SBR). The
article entitled "Laser Engraving of Rubbers--The Use of
Microporous Materials" by Kern et al., 1998 described the use of
natural rubber compounds and EPDM.
[0016] EP1,228,864B1 (Houstra) describes liquid photopolymer
mixtures designed for analogue UV imaging, cured with UV, and then
the resulting plates are laser engraved using carbon dioxide. Such
printing plate precursors do not contain infrared absorbing dyes or
pigments and therefore are unsuitable for use with IR absorbing
laser diode systems. U.S. Pat. No. 5,798,202 (noted above)
describes reinforced block copolymers incorporating carbon black
that is UV cured and is still thermoplastic. As pointed out in U.S.
Pat. No. 6,935,236 (Hiller et al.), such curing would be defective
due to the high absorption of UV as it traverses through the thick
precursor layer. The block copolymers described in Cushner et al.
are the basis of most commercial UV-imageable flexographic printing
precursors. Although many polymers are suggested for this use in
the prior art, only polymers that are extremely flexible such as
elastomers have been used commercially. This is because
flexographic layers that are millimeters thick need to be bent
around a printing cylinder and secured with temporary bonding tape
that must both be removable after printing and secure the printing
plate during printing.
[0017] U.S. Pat. No. 6,776,095 (Telser et al.) lists a number of
elastomers including EPDM and U.S. Pat. No. 6,913,869 (Leinenbach
et al.) describes the use of EPDM rubber for the production of
flexographic printing plates having a flexible metal support. U.S.
Pat. No. 7,223,524 (Hiller et al.) describes the use of natural
rubber with highly conductive carbon blacks with specific
properties of structure and surface area. U.S. Pat. No. 7,290,487
(Hiller et al.) lists suitable hydrophobic elastomers with inert
plasticizers. U.S. Patent Application Publication 2002/0018958
(Nishioki et al.) describes a peelable layer and the use of rubbers
such as EPDM and NBR together with inert plasticizers such as
mineral oils. The use of inert plasticizers or mineral oils can
present a problem as they leach out either during precursor
grinding (during manufacture) or during storage, or under the
pressure and contact with ink during printing.
[0018] An increased need for higher quality flexographic printing
precursors for laser engraving has highlighted the need to increase
the desire to solve performance problems that may have been of less
importance when quality demands were less. What is especially
difficult is to simultaneously improve the flexographic printing
precursor in all directions.
[0019] For example, the rate of imaging is now an important
consideration in laser engraving of flexographic printing
precursors. Throughput by engraving depends upon printing plate
width because it is imaged point by point. Conventional printing
plates made by UV exposure followed by multiprocessing wash-out and
drying is time consuming but is independent of printing plate size,
and for the production of multiple printing plate, it can be
relatively fast because many printing plates can be passing through
the multiple stages at the same time. Throughput for flexographic
engraving is somewhat determined by the equipment that is used but
if this is the means for improving imaging speed, then cost becomes
the main factor. Improved imaging speed is related to equipment
cost. There is a limit to what the market will bear in equipment
cost in order to have faster imaging. Therefore, much work has been
done to try to improve the sensitivity of the flexographic printing
plate by various means. For instance, U.S. Pat. No. 6,159,659
(Gelbart) describes the use of a foam layer for laser engraving so
that there is less material to ablate. U.S. Pat. No. 6,806,018
(Kanga) uses expandable microspheres to increase sensitivity.
[0020] U.S. Patent Application Publication 2009/0214983 (Figov et
al.) describes the use of additives that thermally degrade to
produce gaseous products. U.S. Patent Application Publication
2008/0194762 (Sugasaki) suggests that good imaging sensitivity can
be achieved using a polymer with a nitrogen atom-containing hetero
ring. U.S. Patent Application Publication 2008/0258344 (Regan et
al.) describes laser-ablatable flexographic printing precursors
that can be degraded to simple molecules that are easily
removed.
[0021] There continues to be a need to provide improved
flexographic printing precursors that are easily manufactured
without the use of process oils that have improved sensitivity
(imaging speed) and provide improved print quality and run
length.
SUMMARY OF THE INVENTION
[0022] The present invention includes an infrared radiation
ablatable flexographic printing precursor that comprises an
infrared radiation ablatable layer comprising a mixture of a high
molecular weight ethylene-propylene-diene terpolymer (EPDM) rubber
and a low molecular weight EPDM rubber.
[0023] In some embodiments, an infrared radiation ablatable
flexographic printing precursor comprises an infrared radiation
ablatable layer comprising from about 1 to about 20 weight % of a
conductive carbon black having a dibutyl phthalate (DBP) adsorption
of less than 110, and a mixture a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber, wherein the weight ratio of the high
molecular weight EPDM to the low molecular weight EPDM rubber is
from about 3:1 to about 5:1.
[0024] In still other embodiments, an infrared radiation ablatable
flexographic printing precursor comprises an infrared radiation
ablatable layer comprising one or more inorganic fillers, an
infrared radiation absorbing material (such as a carbon black), and
a mixture a high molecular weight ethylene-propylene-diene
terpolymer (EPDM) rubber and a low molecular weight EPDM rubber,
wherein the weight ratio of the high molecular weight EPDM to the
low molecular weight EPDM rubber is from about 2:1 to about
10:1.
[0025] Other embodiments of this invention includes an infrared
radiation ablatable flexographic printing precursor comprises an
infrared radiation ablatable layer comprising a carbon black, one
or more inorganic fillers, and a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber, wherein the weight ratio of the
carbon black to the inorganic filler(s) is from about 1:50 to about
1:1.5.
[0026] Still again, other embodiments of this invention include an
infrared radiation ablatable flexographic printing precursor
comprises an infrared radiation ablatable layer comprising:
[0027] from about 10 to about 35 weight % of one or more inorganic
fillers and from about 1 to about 20 weight % of a carbon black,
wherein the weight ratio of the carbon black to the inorganic
filler(s) is from about 1:50 to about 1:1.5, and
[0028] a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber, wherein the weight ratio of the high
molecular weight EPDM to the low molecular weight EPDM rubber is
from about 2:1 to about 10:1.
[0029] This invention also provides a method of preparing the
flexographic printing plate precursor of this invention
comprising:
[0030] A) providing a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber,
[0031] B) adding optional components, and compounding the resulting
mixture in a two-roll mill,
[0032] C) applying the compounded mixture to a fabric base to
provide a continuous roll of an infrared radiation ablatable
layer,
[0033] D) causing vulcanization in the continuous infrared
radiation ablatable layer,
[0034] E) laminating a polyester support to the continuous infrared
radiation ablatable layer to provide a continuous laminated web,
and
[0035] F) grounding the infrared radiation ablatable layer.
[0036] In addition, a method of this invention for preparing the
flexographic printing sleeve precursor of this invention
comprises:
[0037] A) providing a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber,
[0038] B) adding optional components, and compounding the resulting
mixture in a two-roll mill,
[0039] C) applying the compounded mixture to a printing sleeve core
to provide an infrared radiation ablatable sleeve,
[0040] D) causing vulcanization in the infrared radiation ablatable
sleeve, and
[0041] E) smoothing the continuous infrared radiation ablatable
sleeve to a uniform thickness.
[0042] A method of this invention for providing flexographic
printing plate or sleeve comprising:
[0043] imaging the flexographic printing precursor of this
invention using infrared radiation to provide a relief image in the
infrared radiation ablatable layer.
[0044] In addition, this invention provides a system for providing
a flexographic printing plate or printing sleeve, comprising:
[0045] the flexographic printing precursor of this invention,
[0046] a group of one or more sources of imaging infrared
radiation, each source capable of emitting infrared radiation,
[0047] a set of optical elements coupled to the sources of imaging
infrared radiation to direct imaging infrared radiation from the
sources onto the flexographic printing precursor.
[0048] The present invention provides a laser engravable
flexographic printing precursor that is readily manufactured
without using process oils, and having improved image sensitivity,
print quality, and run length.
[0049] Whereas prior art researchers have used high molecular
weight EPDM rubber as well as other rubbery materials, they have
failed to appreciate that its non-polar nature has made it
particularly suitable as a basis for laser-engravable flexographic
printing precursors and superior to other materials that are
described in patents. Furthermore, we have found advantages from
the inclusion of a low molecular weight EPDM polymer with the high
molecular weight EPDM rubber, as a replacement for plasticizers.
Low molecular weight EPDM provides the benefits of process oils in
manufacture without the problems of leaching out either during
grinding, printing or precursor storage.
[0050] We have also found that the use of the low molecular weight
EPDM polymer causes an increase in crosslinking density in the
rubber mixture with consequent advantages. For example, there is an
improvement in compression set and mechanical properties such as
tensile strength and elongation to the length at which the material
breaks or snaps into at least two pieces (see ASTM D3759).
[0051] The present invention provides improved flexographic
printing precursors that can be in the form of plates or sleeves.
These precursors can be cleanly engraved using infrared radiation
(lasers) to provide very sharp features in the resulting printed
images. In addition, these precursors have improved run length and
can be used for many high quality prints without degradation. These
advantages are also provided by using a specific mixture of solid
(high molecular weight) EPDM and liquid (low molecular weight) EPDM
to formulate the infrared radiation ablatable layers.
DETAILED DESCRIPTION OF THE INVENTION
[0052] "Imaging" refers to ablation of the background areas while
leaving intact the areas of the plate precursor that will be inked
up and printed by flexography.
[0053] "Flexographic printing precursor" refers to a non-imaged
flexographic element.
[0054] The terms "laser-ablatable element", "flexographic printing
precursor", "flexographic printing plate precursor", and
"flexographic printing sleeve precursor" used herein includes any
imageable element or material of any form in which a relief image
can be produced using a laser according to the present invention.
In most instances, however, the laser-ablatable elements are used
to form flexographic printing plates (flat sheets) or flexographic
printing sleeves with a relief image having a relief depth of at
least 100 .mu.m. Such laser-ablatable, relief-forming elements may
also be known as "flexographic printing plate blanks" or
"flexographic sleeve blanks" The laser-ablatable elements can also
be in seamless continuous forms.
[0055] By "ablative", we mean that the imageable (or infrared
radiation ablatable) layer can be imaged using an infrared
radiation source (such as a laser) that produces heat within the
layer that causes rapid local changes in the infrared
radiation-ablatable layer so that the imaged regions are physically
detached from the rest of the layer or substrate and ejected from
the layer and collected by a vacuum system. Non-imaged regions of
the infrared radiation-ablatable layer are not removed or
volatilized to an appreciable extent and thus form the upper
surface of the relief image that is the printing surface. The
breakdown is a violent process that includes eruptions, explosions,
tearing, decomposition, fragmentation, oxidation, or other
destructive processes that create a broad collection of materials.
This is distinguishable from, for example, image transfer.
"Ablation imaging" is also known as "ablation engraving" in this
art. It is distinguishable from image transfer methods in which
ablation is used to materially transfer an image by transferring
pigments, colorants, or other image-forming components.
[0056] Unless otherwise indicated, the term "weight %" refers to
the amount of a component or material based on the total dry layer
weight of the composition or layer in which it is located.
[0057] The "top surface" is equivalent to the "relief-image forming
surface" and is defined as the outermost surface of the infrared
radiation-ablatable layer and is the first surface of that layer
that is struck by imaging infrared radiation during the engraving
or imaging process. The "bottom surface" is defined as the surface
of the infrared radiation-ablatable layer that is most distant from
the imaging infrared radiation.
Flexographic Printing Precursor
[0058] The flexographic printing precursors can include a
self-supporting infrared radiation ablatable layer (defined below)
that does not need a separate substrate to have physical integrity
and strength. In such embodiments, this layer is thick enough and
laser ablation is controlled in such a manner that the relief image
depth is less than the entire thickness, for example at least 20%
but less than 80% of the entire thickness.
[0059] However, in other embodiments, the flexographic printing
precursor has a suitable dimensionally stable, non-laser ablatable
substrate having an imaging side and a non-imaging side. The
substrate has at least one infrared radiation ablatable layer
disposed on the imaging side. Suitable substrates include
dimensionally stable polymeric films, aluminum sheets or cylinders,
transparent foams, ceramics, fabrics, or laminates of polymeric
films (from condensation or addition polymers) and metal sheets
such as a laminate of a polyester and aluminum sheet or
polyester/polyamide laminates, or a laminate of a polyester film
and a compliant or adhesive support. Polyester, polycarbonate,
polyvinyl, and polystyrene films are typically used. Useful
polyesters include but are not limited to poly(ethylene
terephthalate) and poly(ethylene naphthalate). The substrates can
have any suitable thickness, but generally they are at least 0.01
mm or from about 0.05 to about 0.3 mm thick, especially for the
polymeric substrates. An adhesive layer may be used to secure the
laser-ablatable layer to the substrate.
[0060] There may be a non-laser ablatable backcoat on the
non-imaging side of the substrate (if present) that may be composed
of a soft rubber or foam, or other compliant layer. This backcoat
may be present to provide adhesion between the substrate and the
printing press rollers and to provide extra compliance to the
resulting printing plate, or to reduce or control the curl of the
printing plate.
[0061] The flexographic printing precursor contains one or more
layers. That is, it can contain multiple layers, at least one of
which is an infrared radiation ablatable layer in which the relief
image is formed. For example, there may be a non-laser ablatable
elastomeric rubber layer (for example, a cushioning layer) between
the substrate and the infrared radiation ablatable layer.
[0062] In most embodiments, the infrared radiation ablatable layer
is the outermost layer, including embodiments where that layer is
disposed on a printing cylinder as a sleeve. However, in some
embodiments, the infrared radiation ablatable layer can be located
underneath an outermost capping smoothing layer that provides
additional smoothness or better ink reception and release. This
smoothing layer can have a general thickness of from about 1 to
about 200 .mu.m.
[0063] In general, the infrared radiation ablatable layer has a
thickness of at least 50 .mu.m and generally from about 50 to about
4,000 .mu.m, and typically from 200 to 2,000 .mu.m.
[0064] The infrared radiation ablatable layer and formulation
includes one or more high molecular weight ethylene-propylene-diene
terpolymer (EPDM) rubbers. These rubbers generally have a molecular
weight of from about 200,000 to about 800,000 and more typically
from about 250,000 to about 500,000, or optimally, about 300,000.
The high molecular weight rubbers are generally solid form and the
molecular weight is at least 30 times (or even 50 times) higher
than that of the low molecular weight EPDM rubbers. The high
molecular weight EPDM rubbers can be obtained from a number of
commercial sources as the following products: Keltan.RTM. EPDM
(from DSM Elastomers) and Royalene.RTM. EPDM (from Lion
Copolymers).
[0065] In addition, this layer includes one or more low molecular
weight EPDM rubbers that are usually in liquid form, and having a
molecular weight of from about 2,000 to about 10,000 and typically
from about 2,000 to about 8,000. These components are also
available from various commercial sources, for example as
Trilene.RTM. EPDM (from Lion Copolymers).
[0066] These two essential components are present at a weight ratio
(high molecular weight EPDM rubber to low molecular weight EPDM) of
from about 2:1 to about 10:1, or from about 3:1 to about 5:1.
Higher ratios do not affect the tack of the mixture sufficiently to
permit good calendering and lower ratios give formulations that
become too tacky and are consequently hard to handle, resulting in
flexographic printing precursors that are too brittle for practical
use.
[0067] The amount of the high molecular weight EPDM in the infrared
radiation ablatable layer is generally at least 15 and up to and
including 70 weight %, based on the total dry layer weight. More
typically, the amount is from about 25 to about 45 weight %. Thus,
all components other than the two EPDM rubbers are present in an
amount of no more than 80 weight %, or typically no more than 60
weight %, based on the total dry layer weight.
[0068] The infrared radiation ablatable layer may also include
minor amounts (less than 40 weight % of the total polymers or
resins in the layer) of other "secondary" resins that are often
included in laser-ablatable layers. These materials may need the
presence of an intermediate bridging material to maintain
compatibility. Such resins can include but are not limited to,
crosslinked elastomeric or rubbery resins that are film-forming in
nature. For example, the elastomeric resins can be thermosetting or
thermoplastic urethane resins and derived from the reaction of a
polyol (such as polymeric diol or triol) with a polyisocyanate, or
the reaction of a polyamine with a polyisocyanate. Alternatively,
such polymers consist of a thermoplastic elastomer and a thermally
initiated reaction product of a multifunctional monomer or
oligomer.
[0069] Other elastomeric resins include copolymers of styrene and
butadiene, copolymers of isoprene and styrene,
styrene-butadiene-styrene block copolymers,
styrene-isoprene-styrene copolymers, other polybutadiene or
polyisoprene elastomers, nitrile elastomers, polychloroprene,
polyisobutylene and other butyl elastomers, any elastomers
containing chlorosulfonated polyethylene, polysulfide, polyalkylene
oxides, or polyphosphazenes, elastomeric polymers of
(meth)acrylates, elastomeric polyesters, and other similar polymers
known in the art.
[0070] Still other useful secondary resins include vulcanized
rubbers, such as Nitrile (Buna-N), Natural rubber, Neoprene or
chloroprene rubber, silicone rubber, fluorocarbon rubber,
fluorosilicone rubber, SBR (styrene-butadiene rubber), NBR
(acrylonitrile-butadiene rubber), ethylene-propylene rubber, and
butyl rubber.
[0071] Still other useful secondary polymers include but are not
limited to, poly(cyanoacrylate)s that include recurring units
derived from at least one alkyl-2-cyanoacrylate monomer and that
forms such monomer as the predominant low molecular weight product
during ablation. These polymers can be homopolymers of a single
cyanoacrylate monomer or copolymers derived from one or more
different cyanoacrylate monomers, and optionally other
ethylenically unsaturated polymerizable monomers such as
(meth)acrylate, (meth)acrylamides, vinyl ethers, butadienes,
(meth)acrylic acid, vinyl pyridine, vinyl phosphonic acid, vinyl
sulfonic acid, and styrene and styrene derivatives (such as
.alpha.-methylstyrene), as long as the non-cyanoacrylate comonomers
do not inhibit the ablation process. The monomers used to provide
these polymers can be alkyl cyanoacrylates, alkoxy cyanoacrylates,
and alkoxyalkyl cyanoacrylates. Representative examples of
poly(cyanoacrylates) include but are not limited to poly(alkyl
cyanoacrylates) and poly(alkoxyalkyl cyanoacrylates) such as
poly(methyl-2-cyanoacrylate), poly(ethyl-2-cyanoacrylate),
poly(methoxyethyl-2-cyanoacrylate),
poly(ethoxyethyl-2-cyanoacylate),
poly(methyl-2-cyanoacrylate-co-ethyl-2-cyanoacrylate), and other
polymers described in U.S. Pat. No. 5,998,088 (Robello et al.)
[0072] In other embodiments, the secondary polymers are an
alkyl-substituted polycarbonate or polycarbonate block copolymer
that forms a cyclic alkylene carbonate as the predominant low
molecular weight product during depolymerization from ablation. The
polycarbonate can be amorphous or crystalline as described for
example in U.S. Pat. No. 5,156,938 (Foley et al.), Cols. 9-12.
[0073] The infrared radiation ablatable layer can also include one
or more infrared (IR) radiation absorbing compounds that absorb IR
radiation in the range of from about 750 to about 1400 nm or
typically from 800 to 1250 nm as long as they do not interfere with
the vulcanization process. Particularly useful infrared radiation
absorbing compounds are responsive to exposure from IR lasers.
Mixtures of the same or different types of infrared radiation
absorbing compounds can be used if desired.
[0074] A wide range of infrared radiation absorbing compounds are
useful in the present invention, including carbon blacks and other
IR radiation absorbing organic or inorganic pigments (including
squarylium, cyanine, merocyanine, indolizine, pyrylium, metal
phthalocyanines, and metal dithiolene pigments), and metal oxides.
Examples include RAVEN.RTM. 450, RAVEN.RTM. 760 ULTRA.RTM.,
RAVEN.RTM. 890, RAVEN.RTM. 1020, RAVEN.RTM. 1250 and others that
are available from Columbian Chemicals Co. (Atlanta, Ga.) as well
as N 330 and N 772 that are available from Evonik Industries AG
(Switzerland). Carbon blacks and especially conductive carbon
blacks (described below) are particularly useful.
[0075] Useful IR radiation absorbing compounds also include carbon
blacks that are surface-functionalized with solubilizing groups are
well known in the art. Carbon blacks that are grafted to
hydrophilic, nonionic polymers, such as FX-GE-003 (manufactured by
Nippon Shokubai) are also useful. Other useful carbon blacks are
Mogul L, Mogul E, Emperor 2000, and Regal.RTM. 330, and 400, all
from Cabot Corporation (Boston Mass.). Other useful pigments
include, but are not limited to, Heliogen Green, Nigrosine Base,
iron (III) oxides, transparent iron oxides, magnetic pigments,
manganese oxide, Prussian Blue, and Paris Blue. Other useful IR
radiation absorbing compounds are carbon nanotubes, such as single-
and multi-walled carbon nanotubes, graphite, grapheme, and porous
graphite.
[0076] Conductive carbon blacks can also be used in the practice of
this invention. Such carbon blacks can be acidic or basic in nature
and can have a dibutyl phthalate (DBP) absorption value less than
110 (110 ml/100 g), as opposed to conductive carbon blacks having
high DBP absorption values that are generally known for commercial
conductive carbon blacks. Lower DBP absorption values are desirable
because they provide lower viscosity of the infrared radiation
ablatable layer formulations, making easier the manufacture of the
flexographic printing precursors. Useful conductive carbon blacks
can be obtained commercially as Ensaco.TM. 150 P (from Timcal
Graphite and Carbon), Hi Black 160 B (from Korean Carbon Black Co.
Ltd.), and N 293 (from Evonik Industries).
[0077] Electrically conductive carbon blacks with low DBP
absorptions (as measured using ASTM D2414-82 DBP Absorption of
Carbon Blacks) or low BET surface area (BET nitrogen surface area
as measured by ASTM D 3037-89) are preferred. High DBP absorption
or high surface area blacks produce formulations with too high a
viscosity and cause handling problems during manufacture.
[0078] A finer dispersion of very small particles of pigmented IR
radiation absorbing compounds can provide an optimum ablation
feature resolution and ablation efficiency. Particularly suitable
particles are those with diameters less than 1 .mu.m.
[0079] Dispersants and surface functional ligands can be used to
improve the quality of the carbon black or metal oxide, or pigment
dispersion so that uniform incorporation of the IR radiation
absorbing compound throughout the infrared radiation ablatable
layer can be achieved.
[0080] The IR radiation absorbing compound(s), such as carbon
blacks, are present in the infrared radiation ablatable layer
generally in a total amount of at least 1 weight % and up to and
including 20 weight %, and typically from about 2 to about 10
weight %, based on the total dry weight of the layer.
[0081] It is also possible that the infrared radiation absorbing
compound (such as a carbon black) is not merely dispersed uniformly
within the infrared radiation ablatable layer, but it is present in
a concentration that is greater near the bottom surface than the
image-forming surface. This concentration profile can provide a
laser energy absorption profile as the depth into the infrared
radiation ablatable layer increases. In some instances, the
concentration change is continuously and generally uniformly
increasing with depth. In other instances, the concentration is
varied with layer depth in a step-wise manner. Further details of
such arrangements of the IR radiation absorbing compound are
provided in copending and commonly assigned U.S. Ser. No.
12/581,926 (filed Oct. 20, 2009 by Landry-Coltrain, Burberry,
Perchak, Ng, Tutt, Rowley, and Franklin).
[0082] Thus, some embodiments of the present invention infrared
radiation ablatable flexographic printing precursors comprise an
infrared radiation ablatable layer comprising from about 1 to about
20 weight % (or from about 2 to about 10 weight %) of a conductive
carbon black having a dibutyl phthalate (DBP) adsorption of less
than 110, and a mixture a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber, wherein the weight ratio of the high
molecular weight EPDM to the low molecular weight EPDM rubber is
from about 3:1 to about 5:1. Non-conductive carbon blacks are also
useful.
[0083] The infrared radiation ablatable layer can further comprise
a carbon black and one or more inorganic fillers. Useful inorganic
fillers include but are not limited to, silica, calcium carbonate,
magnesium oxide, talc, barium sulfate, kaolin, bentonite, zinc
oxide, mica, and titanium dioxide, and mixtures thereof. Thus,
useful inorganic filler particles are silica and alumina, such as
fine particulate silica, fumed silica, porous silica, surface
treated silica, sold as Aerosil from Degussa and Cab-O-Sil from
Cabot Corporation, micropowders such as amorphous magnesium
silicate cosmetic microspheres sold by Cabot and 3M Corporation,
calcium carbonate and barium sulfate particles and microparticles.
Particularly useful fillers are zinc oxide, calcium carbonate,
titanium dioxide, and silicas. The amount of inorganic fillers is
generally at least 5 and up to and including 50 weight %, based on
the total dry layer weight. However, more typically, the amount of
inorganic fillers is from about 10 to about 35 weight %.
[0084] Contrary to the teaching in the prior art, for example,
"Laser Engraving of Rubbers--The Influence of Fillers" by W. Kern
et al., October 1997, 710-715 (Rohstoffe Und Anwendendunghen)
describing EPDM formulations we have found that the use of
inorganic fillers do not adversely affect sensitivity. This may be
due to the presence of the lower molecular weight EPDM in the
infrared radiation ablatable layer. Some fillers can also improve
the mechanical properties of the precursor.
[0085] If both a carbon black and inorganic filler are present, the
weight ratio of the carbon black to the inorganic filler(s) is from
about 1:50 to about 1:1.5, or from about 1:20 to about 1:5. We have
found that these ratios are particularly useful in the preparation
of flexographic printing plate precursors even when the precursors
have infrared radiation ablatable layers that are not based on EPDM
elastomers.
[0086] It is also desirable that the infrared radiation ablatable
layer further comprises a vulcanizer (or crosslinking agent) that
can crosslink the EPDM rubbers and any other resins in the layer
that can benefit from crosslinking Useful vulcanizers include but
are not limited to, sulfur and sulfur-containing compounds,
peroxide, hydroperoxides, and azo crosslinking agents. A mixture of
sulfur and a peroxide can also be used, or a mixture of sulfur, a
peroxide, and an azo crosslinking agent can be used. The amount of
vulcanizer that can be present in the layer is at least 0.5% and up
to and including 5 weight %, based on the total dry layer weight.
Useful sulfur-containing compounds include but are not limited to,
zinc dibutyl dithiocarbamate (ZDBC), tetramethylthiuram disulfide
(TMTD), and tetramethylthiuram monosulfide (TMTM). Useful peroxides
include but are not limited to, di(t-butylperoxyisopropyl)benzene,
2,5-dimethyl-2,5 bis(t-butyl)peroxy)hexane, dicumyl peroxide, and
di(t-butyl)peroxide, and any others that can react with single
carbon-carbon bonds and thus produce a higher curing density and
better compression set. Compression set is important because it
represents the resistance to changes in printing by the printing
plate being impacted on each printing impression followed by a
brief recovery between printing. Peroxide vulcanization gives less
odor than sulfur vulcanization.
[0087] There are certain materials, however, that should be avoided
in the practice of this invention. Plasticizers such as mineral
oils have been found to cause various problems. They tend to come
to the surface during grinding and thus block the grinding medium.
They also provide printing plates that may swell or lose material
during long run printing and may sweat out during long term storage
of the flexographic printing precursors. This sweating out of the
plasticizer can reduce the adhesion between the infrared radiation
ablatable layer and a polyester substrate causing debonding or
delamination either during printing or when the printing plate is
removed from the press after printing.
[0088] In some embodiments, microcapsules are dispersed within the
infrared radiation ablatable layer. The "microcapsules" can also be
known as "hollow beads", "hollow spheres", "microspheres",
microbubbles", "micro-balloons", "porous beads", or "porous
particles". Such components can include a thermoplastic polymeric
outer shell and either core of air or a volatile liquid such as
isopentane and isobutane. These microcapsules include a single
center core or many voids within the core. The voids can be
interconnected or non-connected. For example, non-laser-ablatable
microcapsules can be designed like those described in U.S. Pat. No.
4,060,032 (Evans) and U.S. Pat. No. 6,989,220 (Kanga) in which the
shell is composed of a poly[vinylidene-(meth)acrylonitrile]resin or
poly(vinylidene chloride), or as plastic micro-balloons as
described for example in U.S. Pat. No. 6,090,529 (Gelbart) and U.S.
Pat. No. 6,159,659 (Gelbart).
[0089] The amount of microspheres that may be present is from about
2 to about 70 weight % of the total dry layer weight. The
microspheres can comprise a thermoplastic shell that is either
hollow inside or enclosing a hydrocarbon or low boiling liquid. For
example, the shell can be composed of a copolymer of acrylonitrile
and vinylidene chloride or methacrylonitrile, methyl methacrylate,
or a copolymer of vinylidene chloride, methacrylic acid, and
acrylonitrile. If a hydrocarbon is present within the microspheres,
it can be isobutene or isopentane. EXPANCEL.RTM. microspheres are
commercially available from Akzo Noble Industries (Duluth, Ga.).
Dualite and Micropearl polymeric microspheres are commercially
available from Pierce & Stevens Corporation (Buffalo, N.Y.).
Hollow plastic pigments are available from Dow Chemical Company
(Midland, Mich.) and Rohm and Haas (Philadelphia, Pa.). The
microspheres generally have a diameter of 50 .mu.m or less.
[0090] Inert microspheres can be hollow or filled with an inert
solvent, and upon laser imaging, they burst and give a foam-like
structure or facilitate ablation of material from the infrared
radiation ablatable layer because they reduce the energy needed for
ablation. Inert microspheres are generally formed of an inert
polymeric or inorganic glass material such as a styrene or acrylate
copolymer, silicon oxide glass, magnesium silicate glass,
vinylidene chloride copolymers.
[0091] The amount of inert particulate materials or microspheres
that can be present is from about 4 to about 70 weight % based on
the total dry layer weight.
[0092] Optional addenda in the infrared radiation ablatable layer
can include but are not limited to, dyes, antioxidants,
antiozonants, stabilizers, dispersing aids, surfactants, and
adhesion promoters, as long as they do not interfere with ablation
efficiency or vulcanization.
[0093] An infrared radiation ablatable flexographic printing
precursor comprises an infrared radiation ablatable layer
comprising a carbon black, one or more inorganic fillers, and one
or more elastomers, wherein the weight ratio of the carbon black to
the inorganic filler(s) is from about 1:50 to about 1:1.5 (or from
about 1:20 to about 1:5). The elastomers can include but are not
limited to, copolymers of butadiene and styrene, copolymers of
isoprene and styrene, styrene-diene-styrene block copolymers such
as polystyrene-polybutadiene-polystyrene,
polystyrene-polyisoprene-polystyrene, and
polystyrene-poly(ethylenebutylene)-polystyrene. Elastomers also
include non-crosslinked polybutadiene and polyisoprene, nitrile
elastomers, polychloroprene, polyisobutylene and other butyl
elastomers, chlorosulfonated polyethylene, polysulfide,
polyalkylene oxides, polyphosphazenes, elastomeric polymers and
copolymers of acrylates and methacrylates, elastomeric
polyurethanes and polyesters, elastomeric polymers and copolymers
of olefins such as ethylene-propylene copolymers and
non-crosslinked EPDM, and elastomeric copolymers of vinyl acetate
and its partially hydrogenated derivatives. In particular, the
elastomers include a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber, as described above.
[0094] The flexographic printing precursors of this invention can
be prepared in the following manner:
[0095] The mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber is formulated with a desired weight
ratio as described above. Additional components (such as inorganic
fillers or a carbon black) can be added and the resulting mixture
is then compounded using standard equipment for rubber processing
(for example, a 2-roll mill or internal mixer of the Banbury type).
During the mixing, the temperature can rise to 90.degree. C. due to
the high shear forces in the mixing apparatus. This process takes
from 5 to 30 minutes depending upon the batch size, amount of
inorganic fillers, type of rubber resin, and other factors known to
a skilled artisan. Alternatively, it is desirable to incorporate a
small quality of the inorganic filler(s) and a small quantity of
the low molecular weight EPDM, until all of the ingredients are
mixed with the high molecular weight EPDM rubber. The compounded
mixture with the two rubbers and any other components (such as
antioxidants, inorganic fillers, vulcanizers, carbon black), in
their un-vulcanized state, are fed into a calender where a
continuous sheet of rubber is deposited onto a carrier base (such
as a fabric web) and wound into a continuous roll of infrared
radiation ablatable layer on the fabric base.
[0096] Controlling the rubber sheet thickness is accomplished by
adjusting the pressure between the calender rolls and the
calendaring speed. In some cases, where the rubbery mixture does
not stick to the calender rolls, the rolls are heated to improve
the tackiness of the rubber and to provide some adhesion to the
calender rolls. This continuous roll of calendered material can be
vulcanized in an autoclave under desired temperature and pressure
conditions. For example, with a sulfur vulcanization system, the
curing conditions are generally about 140.degree. C. for up to 6
hours. Shorter times can be used if higher than atmospheric
pressure is applied in the process. For peroxide curing systems,
for example with Parkadox 14/40 (Kayaku Akzo), the curing
conditions would be about 175.degree. C. for up to 6 hours.
[0097] The continuous infrared radiation ablatable layer is then
laminated to a suitable support, such as a polyester film. The
continuous infrared radiation ablatable layer can be ground using
suitable continuous grinding apparatus to provide a uniform
thickness in the continuous web. The web can then be cut to size to
provide suitable flexographic printing precursors.
[0098] The process for making flexographic printing sleeves is
similar but the compounded mixture is applied to a printing sleeve
core to provide an infrared radiation ablatable sleeve. This sleeve
is then vulcanized in a suitable manner, and can be ground to a
uniform thickness using suitable equipment.
[0099] The flexographic printing precursor can also be constructed
with a suitable protective layer or slip film (with release
properties or a release agent) in a cover sheet that is removed
prior to ablation imaging. Such a protective layer can be a
polyester film [such as poly(ethylene terephthalate)] forming the
cover sheet. A backing layer on the substrate side opposite the
infrared radiation ablatable layer can also be present.
Laser Ablation Imaging
[0100] Ablation energy is preferably applied using an IR radiation
emitting diode but carbon dioxide or YAG lasers can also be used.
Ablation to provide a relief image with a minimum depth of at least
50 .mu.m is desired with a relief image having a minimum depth of
at least 100 .mu.m or a typical depth of from 300 to 1000 .mu.m or
up to 600 .mu.m being desirable. The relief image may have a
maximum depth up to about 100% of the original thickness of the
infrared radiation ablatable layer when a substrate is present. In
such instances, the floor of the relief image may be the substrate
(if the ablatable layer is completely removed in the imaged
regions), a lower region of the infrared radiation ablatable layer,
or an underlayer such as an adhesive layer or compliant layer. When
a substrate is absent, the relief image may have a maximum depth of
up to 80% of the original thickness of the original ablatable
layer. An IR diode laser operating at a wavelength of from about
700 to about 1400 nm is generally used, and a diode laser operating
at from 800 nm to 1250 nm is useful for ablative imaging.
[0101] Generally, ablation imaging is achieved using at least one
infrared radiation laser having a minimum fluence level of at least
20 J/cm.sup.2 at the element surface and typically infrared imaging
is at from about 20 to about 1000 J/cm.sup.2 or typically from 50
to 800 J/cm.sup.2.
[0102] A suitable laser engraver that would provide satisfactory
ablation is described in WO 2007/149208 (Eyal et al.) that is
incorporated herein by reference. This laser engraver is considered
to be a "high power" laser ablating imager or engraver and has at
least two laser diodes emitting radiation in one or more
wavelengths so that imaging with the one or more wavelengths is
carried out at different depths relative to the precursor surface.
For example, the multi-beam optical head described in this
publication incorporates numerous laser diodes each having a power
in the order of at least 10 Watts per emitter width of 100 .mu.m.
These lasers can be modulated directly at relatively high
frequencies without the need for external modulators.
[0103] Thus, ablative imaging can be carried out at the same or
different depths relative to the surface of the infrared radiation
ablatable layer using two or more laser diodes each emitting
radiation in one or more wavelengths.
[0104] Other imaging (or engraving) devices and components thereof
and methods are described for example in U.S. Patent Application
Publications 2008/0153038 (Siman-Tov et al.) describing a hybrid
optical head for direct engraving, 2008/0305436 (Shishkin)
describing a method of imaging one or more graphical pieces in a
flexographic printing plate precursor on a drum, 2009/0057268
(Aviel) describing imaging devices with at least two laser sources
and mirrors or prisms put in front of the laser sources to alter
the optical laser paths, and 2009/0101034 (Aviel) describing an
apparatus for providing an uniform imaging surface, all of which
publications are incorporated herein by reference. In addition,
copending and commonly assigned U.S. Ser. No. 12/502,267 (filed
Jul. 14, 2009 by Matzner, Aviel, and Melamed) describes an
engraving system including an optical imaging head, a printing
plate construction, and a source of imaging radiation, which
copending application is incorporated herein by reference.
Copending and commonly assigned U.S. Ser. No. 12/555,003 (filed
Sep. 8, 2009 but Aviel and Eyal) describes an imaging head for 3D
imaging of flexographic printing plate precursors using multiple
lasers, which copending application is also incorporated herein by
reference.
[0105] Thus, a system for providing flexographic printing plates or
sleeves include one or more of the flexographic printing precursors
described above, as well as one or more groups of one or more
sources of imaging infrared radiation, each source capable of
emitting infrared radiation (see references cited above). Such
imaging sources can include but are not limited to, laser diodes,
multi-emitter laser diodes, laser bars, laser stacks, fiber lasers,
or a combination thereof. The system can also include one or more
sets of optical elements coupled to the sources of imaging infrared
radiation to direct imaging infrared radiation from the sources
onto the flexographic printing precursor (see references cited
above for examples of optical elements).
[0106] Ablation to form a relief image can occur in various
contexts. For example, sheet-like elements can be imaged and used
as desired, or wrapped around a printing sleeve core or cylinder
form before imaging. The flexographic printing precursor can also
be a printing sleeve that can be imaged.
[0107] During imaging, products of ablation can be gaseous or
volatile and readily collected by vacuum for disposal or chemical
treatment. Any solid debris from ablation can be collected and
removed using suitable means such as vacuum, compressed air,
brushing with brushes, rinsing with water, ultrasound, or any
combination of these.
[0108] During printing, the resulting flexographic printing plate
or printing sleeve is inked using known methods and the ink is
appropriately transferred to a suitable substrate such as paper,
plastics, fabrics, paperboard, or cardboard.
[0109] After printing, the flexographic printing plate or sleeve
can be cleaned and reused and a printing cylinder can be scraped or
otherwise cleaned and reused as needed. Cleaning can be
accomplished with compressed air, water, or a suitable aqueous
solution, or by rubbing with cleaning brushes or pads.
[0110] The present invention provides at least the following
embodiments and combinations thereof:
[0111] 1. An infrared radiation ablatable flexographic printing
precursor that comprises an infrared radiation ablatable layer
comprising a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber.
[0112] 2. The precursor of embodiment 1 wherein the weight ratio of
the high molecular weight EPDM to the low molecular weight EPDM
rubber is from about 2:1 to about 10:1.
[0113] 3. The precursor of embodiment 1 or 2 wherein the weight
ratio of the high molecular weight EPDM to the low molecular weight
EPDM rubber is from about 3:1 to about 5:1.
[0114] 4. The precursor of any of embodiments 1 to 3 wherein the
molecular weight of the high molecular weight EPDM is from about
200,000 to about 800,000, and the molecular weight of the low
molecular weight EPDM is from about 2,000 to about 10,000.
[0115] 5. The precursor of any of embodiments 1 to 4 wherein the
molecular weight of the high molecular weight EPDM is from about
250,000 to about 500,000, and the molecular weight of the low
molecular weight EPDM is from about 2,000 to about 8,000.
[0116] 6. The precursor of any of embodiments 1 to 5 wherein the
infrared radiation ablatable layer further comprises a carbon
black.
[0117] 7. The precursor of any of embodiments 1 to 6 wherein the
infrared radiation ablatable layer further comprises a conductive
carbon black.
[0118] 8. The precursor of any of embodiments 1 to 7 wherein the
infrared radiation ablatable layer further comprises a conductive
carbon black having a dibutyl phthalate (DBP) absorption of less
than 110.
[0119] 9. The precursor of embodiments 1 to 6 wherein the infrared
radiation ablatable layer further comprises a non-conductive carbon
black.
[0120] 10. An infrared radiation ablatable flexographic printing
precursor comprises an infrared radiation ablatable layer
comprising from about 1 to about 20 weight % of a conductive carbon
black having a dibutyl phthalate (DBP) adsorption of less than 110,
and a mixture of a high molecular weight ethylene-propylene-diene
terpolymer (EPDM) rubber and a low molecular weight EPDM rubber,
wherein the weight ratio of the high molecular weight EPDM to the
low molecular weight EPDM rubber is from about 3:1 to about
5:1.
[0121] 11. The precursor of embodiment 10 wherein the infrared
radiation ablatable layer comprises from about 2 to about 10 weight
% of the conductive carbon black.
[0122] 12. The precursor of any of embodiments 1 to 11 wherein the
infrared radiation ablatable layer further comprises a
vulcanizer
[0123] 13. The precursor of any of embodiment 12 wherein the
infrared radiation ablatable layer further comprises sulfur or a
peroxide as a vulcanizer and an azo crosslinking agent, or a
mixture of sulfur and a peroxide, or a mixture of sulfur, an azo
crosslinking agent, and a peroxide.
[0124] 14. The precursor of any of embodiments 1 to 13 further
comprising a polyester support upon which the infrared radiation
ablatable layer is disposed.
[0125] 15. The precursor of any of embodiments 1 to 14 further
comprising a fabric support upon which the infrared radiation
ablatable layer is disposed.
[0126] 16. The precursor of embodiment 15 wherein the fabric
support is disposed on a polyester support.
[0127] 17. An infrared radiation ablatable flexographic printing
precursor comprises an infrared radiation ablatable layer
comprising one or more inorganic fillers, a carbon black, and a
mixture of a high molecular weight ethylene-propylene-diene
terpolymer (EPDM) rubber and a low molecular weight EPDM rubber,
wherein the weight ratio of the high molecular weight EPDM to the
low molecular weight EPDM rubber is from about 2:1 to about
10:1.
[0128] 18. The precursor of any of embodiments 1 to 17 wherein the
infrared radiation ablatable layer further comprises one or more
inorganic fillers that are chosen from silica, calcium carbonate,
barium sulfate, kaolin, bentonite, zinc oxide, mica, and titanium
dioxide.
[0129] 19. An infrared radiation ablatable flexographic printing
precursor comprises an infrared radiation ablatable layer
comprising:
[0130] from about 10 to about 35 weight % of one or more inorganic
fillers and from about 1 to about 20 weight % of a carbon black,
wherein the weight ratio of the carbon black to the inorganic
filler(s) is from about 1:50 to about 1:1.5, and a mixture of a
high molecular weight ethylene-propylene-diene terpolymer (EPDM)
rubber and a low molecular weight EPDM rubber, wherein the weight
ratio of the high molecular weight EPDM to the low molecular weight
EPDM rubber is from about 2:1 to about 10:1.
[0131] 20. A method of preparing the flexographic printing plate
precursor of any of embodiments 1 to 19 comprising:
[0132] A) providing a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber,
[0133] B) adding optional components, and compounding the resulting
mixture in a two-roll mill,
[0134] C) applying the compounded mixture to a fabric base to
provide a continuous roll of an infrared radiation ablatable
layer,
[0135] D) causing vulcanization in the continuous infrared
radiation ablatable layer,
[0136] E) laminating a polyester support to the continuous infrared
radiation ablatable layer to provide a continuous laminated web,
and
[0137] F) grounding the infrared radiation ablatable layer.
[0138] 21. The method of embodiment 20 wherein the infrared
radiation ablatable layer is ground in the continuous laminated web
to a uniform thickness.
[0139] 22. The method of embodiment 20 or 21 wherein the mixture of
high molecular with EPDM and low molecular weight EPDM further
comprises a carbon black in an amount of from about 1 to about 20
weight %, and the weight ratio of the high molecular weight EPDM
rubber to the low molecular weight EPDM rubber is from about 2:1 to
about 10:1.
[0140] 23. The method of any of embodiments 20 to 22 wherein the
mixture of high molecular with EPDM and low molecular weight EPDM
further comprises one or more inorganic fillers, a vulcanizer, or
both an inorganic filler and a vulcanizer.
[0141] 24. The method of any of embodiments 20 to 23 wherein the
continuous laminated web further comprises a fabric layer between
the polyester support and the continuous infrared radiation
ablatable layer.
[0142] 25. A method of preparing the flexographic printing sleeve
precursor of any of embodiments 1 to 19 comprising:
[0143] A) providing a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber,
[0144] B) adding optional components, and compounding the resulting
mixture in a two-roll mill,
[0145] C) applying the compounded mixture to a printing sleeve core
to provide an infrared radiation ablatable sleeve,
[0146] D) causing vulcanization in the infrared radiation ablatable
sleeve, and
[0147] E) smoothing the continuous infrared radiation ablatable
sleeve to a uniform thickness.
[0148] 26. The method of embodiment 25 wherein the mixture of high
molecular weight EPDM and low molecular weight EPDM further
comprises a carbon black, and optionally one or more inorganic
fillers and a vulcanizer
[0149] 27. A method of providing flexographic printing plate or
sleeve comprising:
[0150] imaging the flexographic printing precursor of any of
embodiments 1 to 19 using infrared radiation to provide a relief
image in the infrared radiation ablatable layer.
[0151] 28. The method of embodiment 27 wherein imaging is carried
out using a laser at a power of at least 20 J/cm.sup.2.
[0152] 29. The method of embodiment 27 or 28 further comprising
removal of debris after imaging.
[0153] 30. The method of embodiment 29 wherein debris is removed by
vacuum, compressed air, brushes, rinsing with water, ultrasound, or
any combination of these.
[0154] 31. The method of any of embodiments 27 to 30 wherein
imaging is carried out using a high power laser ablating
imager.
[0155] 32. The method of any of embodiments 27 to 31 wherein
imaging is carried out at the same or different depths relative to
the surface of the infrared radiation ablatable layer using two or
more laser diodes each emitting radiation in one or more
wavelengths.
[0156] 33. A system for providing a flexographic printing plate or
printing sleeve, comprising:
[0157] the flexographic printing precursor of any of embodiments 1
to 19,
[0158] a group of one or more sources of imaging infrared
radiation, each source capable of emitting infrared radiation,
[0159] a set of optical elements coupled to the sources of imaging
infrared radiation to direct imaging infrared radiation from the
sources onto the flexographic printing precursor. 34. The system of
embodiment 33 wherein the sources of imaging infrared radiation are
laser diodes, multi-emitter laser diodes, laser bars, laser stacks,
fiber lasers, or a combination thereof.
[0160] 35. An infrared radiation ablatable flexographic printing
precursor comprises an infrared radiation ablatable layer
comprising a carbon black, one or more inorganic fillers, and one
or more elastomers, wherein the weight ratio of the carbon black to
the inorganic filler(s) is from about 1:50 to about 1:1.5.
[0161] 36. The precursor of embodiment 35 wherein the elastomers
includes a mixture of a high molecular weight
ethylene-propylene-diene terpolymer (EPDM) rubber and a low
molecular weight EPDM rubber.
[0162] 37. The precursor of embodiment 35 or 36 wherein the weight
ratio of the carbon black to the inorganic filler(s) is from about
1:20 to about 1:5
[0163] The following Examples are provided to illustrate the
present invention and are not to be limiting in any manner.
EXAMPLES
[0164] Comparisons of commercially available laser-ablatable
flexographic printing plate precursors were made to flexographic
printing plate precursors of the present invention as described
below. The comparison method was to measure the depth of engraving
under standard conditions using a laser diode engraver with a
constant drum angular velocity of 100 rpm and constant laser power
of 4.5 watts. Typical engraving depths for the commercial printing
plate precursors were compared to the precursors made according to
Invention Example 1 and the data are as follows:
TABLE-US-00001 Printing Plate Precursor Engraving depth (.mu.m)
Invention Example 1 64 Comparative Example 1 (Bottcher 47 SBR)
Comparative Example 2 (Printec) 45
[0165] Surface Evenness:
[0166] The demands on achieving high surface evenness have
increased because of the need to accurately reproduce delicate half
tones. Thus, for instance, U.S. Pat. No. 5,798,202 (noted above)
specifically teaches away from the use of vulcanized rubbers
because the resulting flexographic printing plate precursors must
be ground down to a uniform surface. Grinding is time-consuming.
However, we have found that it is not possible to reach the high
level of surface evenness without grinding and have found a method
of doing so in a continuous manner to continuous forms, thus
minimizing the time necessary for grinding and thereby making it
commercially viable.
[0167] In addition to the production of even surfaces,
investigation has also determined that the unground surface of the
flexographic precursor tends to be smooth and shiny due to the
formation of a continuous surface film over any pigment in the
outer layer. Grinding has the effect of disrupting this film so
that the oleophilic pigment such as carbon black, which is present
in more completely exposed regions, imparts improved ink accepting
properties to the flexographic precursor. In addition, grinding
produces a very accurate flexographic precursor thickness that
improves printing performance. We found it necessary to provide
laser-engravable formulations that will have optimized printing
properties when the printing surface has been ground. Also, we
found that successful grinding is dependent upon the
laser-engravable formulation. For instance, the presence of
plasticizers, such as process oils (mineral oils) that are regarded
in the rubber industry as essential for calendering to produce
standard sheets of rubber, causes severe problems if the sheets
must be ground, as is the case for the high quality flexographic
precursors of the present invention. During grinding, flexographic
precursors containing mineral oils produce a sticky mixture of oil
and ground rubber that clogs the medium used for grinding, making
continuous grinding impossible.
[0168] Print Quality:
[0169] While print quality can be qualitatively assessed overall
and quantitatively measured by certain parameters, it is difficult
to predict how a given laser-engravable formulation may affect
this. Optical density of solid print areas is an important
parameter. Sharpness is partially defined by distinguishable line
width and the comparative examination of 2-point text under a
low-powered microscope.
[0170] Means of Flexographic Printing Precursor Production:
[0171] Top quality printing requires high quality flexographic
plate and sleeve production, and thus methods of production
described in the art are not sufficiently reliable for use in the
present invention. For example, production by laying down multiple
layers from solvents would be too prone to the incidence of solvent
bubbles during the drying process in which the solvent is removed
from the coating. Whereas vulcanization is a proven method that may
involve extrusion and calendering without the danger of clogging
during compounding, catalysis of reactions such as urethane
formation during compounding and possible extrusion can cause
blocking of the production equipment. U.S. Pat. No. 5,796,202
describes a variety of methods for flexographic printing precursor
production but does not appreciate that the combination of
extrusion, calendering, and grinding is the only method that will
produce sufficient flexographic precursor quality.
[0172] In some prior art, processing oils are used in
laser-ablatable flexographic printing plates. However, such
formulations cause problems during continuous grinding procedures.
On the other hand, processing oils are needed for the production of
EPDM sheets. Without such oils, EPDM is too dry and hard and does
not adhere sufficiently to the calendar rollers for proper
calendering. The processing oils promote good calendering by
improving tack and by lowering the viscosity. However, the
processing oils introduce other problems and inhibit long print run
capability. The present invention solves this problem of
calendering, grinding, and run length by including a low molecular
weight EPDM in the laser-engravable formulation.
[0173] Long Run Length Capability:
[0174] The use of flexographic printing in the packaging industry
frequently requires long printing runs for consumer products such
as packaging of food-stuffs. Many known laser-engravable
flexographic printing plate precursors include plasticizers. The
printing process utilizes inks based on solvent mixtures or water.
During long printing runs, plasticizers may be extracted by the
inks solvents and the ink solvents absorbed. Due to these effects,
flexographic printing precursors will change in durometer hardness
and may either swell or shrink. Any changes will affect the quality
of prints, making it variable. Consequently, the choice of
materials in the laser-engravable layer (such as the elastomer) is
important. Historically, this importance is not been fully
appreciated in the art and the inclusion of plasticizers show a
lack of appreciation for the highest standards of retention of
integrity of the formulation during the print run. We found that
such plasticizers leach out during long print runs as well as
during the process of grinding when the precursors are being
prepared.
Invention Example 1
[0175] One hundred parts by weight of an EPDM rubber was masticated
in a two roller mill. The grade of EPDM as based on ethylidene
norborene and was the commercial grade KEP240 (sold by Kumho).
Mastication was continued until the shapeless lump placed in the
mill had been formed into a semi-transparent sheet. This sheet was
rolled up and fed into a Banbury mixer operating at between 70 and
80.degree. C. During this mixing, the following components (parts
by weight) were added individually in the order shown below.
TABLE-US-00002 Stearic Acid 1.0 part Zinc oxide 6.25 parts Carbon
black 12.0 parts
[0176] The following two ingredients were then added, approximately
one third at a time, firstly one third of the silica, one third of
the liquid EPDM rubber, and then the next third of the silica and
so on until the quantities had been completely added
TABLE-US-00003 Silica 30 parts Trilene 67 EPDM 20 parts
Trilene 67 is a liquid EPDM rubber sold by Lion Polymers and has a
molecular weight of approximately 7700 and a Brookfield viscosity
at 100.degree. C. of 128,000.
[0177] The entire mixture was mixed for approximately 20 minutes in
the Banbury mixer until a constant stress reading could be observed
on the Banbury mixer. The resulting material was removed from the
Banbury mixer as a homogenous lump that was fed onto a two roller
mill and the following materials added were then added:
[0178] 6 parts by weight of a silane coupling agent,
bis[3-(triethoxysilyl)propyl]polysulfide,
[0179] 10 parts by weight of di-(t-butylperoxyisopropyl)benzene,
and
[0180] 1.5 parts of 2,4,6-triallyoxy-1,3,5-triazine co-agent.
[0181] The Mooney viscosity of the resulting mixture was about 53.
Mooney viscosities need to be between 30 and 80 or more preferably,
between 40 and 60. Higher and lower viscosities than these values
will not allow processability on a two roller mill.
[0182] The milled material was then fed through a calendar at a
temperature of 30-80.degree. C. together with a fabric base. The
calendar gap was pre-set to the thickness requirements. The
resulting roll of laminated rubber and fabric was fed into an
autoclave at 135.degree. C. for a period of time. After cooling the
roll to room temperature, it was laminated to a 125 .mu.m
poly(ethylene terephthalate) film and post cured in an autoclave at
120.degree. C.
[0183] The completed flexographic printing plate precursor was
continuously ground on the non-polyester side to a uniform
thickness by a buffing machine.
[0184] The precursor was cut to an appropriate size and placed on a
laser ablating plate imager where excellent sharp deep relief
images were produced that were used on a flexographic printing
press to produce hundreds of thousands of sharp, clean impressions.
The compression set for this printing plate precursor was measured
according to ASTM D 395 Method B and found to be 13%.
Comparative Example 1
[0185] Invention Example 1 was repeated but a mineral oil was
substituted for the liquid EPDM rubber. Although the formulation
was processable to form flexographic printing precursor sheets,
they could not be ground to give good evenness in the
laser-ablatable surface.
[0186] In addition, when the sheets were soaked in a solvent
mixture of ethyl acetate (20%) and isopropanol (80%) for 24 hours,
we determined that the mineral oil leached out with a weight loss
of 1.5% compared to 0% loss from the flexographic printing plate
precursors of Invention Example 1. Measurements of swelling showed
that the Invention Example 1 flexographic printing precursor
swelled by 3.2% while the Comparative Example 1 precursor swelled
by 4.6%.
[0187] Comparative printing tests showed that the Invention Example
1 flexographic printing plates provided better ink transfer, less
dot gain that can be seen in smaller sharper 50% dots, thinner 50
.mu.m lines, and more constant print quality for the 100.sup.th
impression as compared to the 10,000.sup.th impression.
[0188] The Invention Example 1 precursor had a Taber abrasion
weight loss per cycle of 0.350 mgr compared to the Comparative
Example 1 precursor value of 0.447 mgr. The compression set for
this printing plate precursor was measured according to ASTM D 395
Method B and found to be 19%.
Comparative Example 2
[0189] Invention Example 1 was repeated, but SBR 1502 rubber was
substituted for the EPDM rubber mixture. The SBR 1502 rubber
required no addition of mineral oil to make it manufacturable but
it exhibited a swelling of 9.9% using the test described in
Comparative Example 1.
[0190] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *