U.S. patent application number 11/150088 was filed with the patent office on 2006-12-14 for printing element with an integral printing surface.
Invention is credited to Laurie A. Bryant, Jonghan Choi, Ryan Vest.
Application Number | 20060281024 11/150088 |
Document ID | / |
Family ID | 37524460 |
Filed Date | 2006-12-14 |
United States Patent
Application |
20060281024 |
Kind Code |
A1 |
Bryant; Laurie A. ; et
al. |
December 14, 2006 |
Printing element with an integral printing surface
Abstract
A relief image printing element with an integral imageable
printing surface and a method of preparing the relief image
printing element are described. The relief image printing element
comprises a dimensionally stable base layer; a floor layer
comprised of a cured polymer selected from the group consisting of
photopolymers, and polymers with a resilience of at least 40% when
cured; and at least one layer of an imageable material. Most
preferably, the floor layer created by curing the layer through the
top of the printing element by face exposure. The printing element
may also contain a compressible layer between the base layer and
the floor layer.
Inventors: |
Bryant; Laurie A.;
(Douglasville, GA) ; Choi; Jonghan; (Smyrna,
GA) ; Vest; Ryan; (Cumming, GA) |
Correspondence
Address: |
John L. Cordani;Carmody & Torrance LLP
P.O. Box 1110
50 Leavenworth Street
Waterbury
CT
06721-1110
US
|
Family ID: |
37524460 |
Appl. No.: |
11/150088 |
Filed: |
June 9, 2005 |
Current U.S.
Class: |
430/270.1 |
Current CPC
Class: |
G03F 7/11 20130101; G03F
7/24 20130101 |
Class at
Publication: |
430/270.1 |
International
Class: |
G03C 1/00 20060101
G03C001/00 |
Claims
1-15. (canceled)
16. A method of preparing a substantially planar photosensitive
relief image printing element with an integral imageable surface,
comprising the steps of: a. providing a dimensionally stable base
layer; b. applying a floor layer comprised of a polymer selected
from the group consisting of photopolymers, and polymer with a
resilience of at least 40% when cured over the dimensionally stable
base layer; c. curing the floor layer from the top of the floor
layer; d. adjusting the floor layer to a desired thickness; e.
applying at least one imageable layer on top of the cured floor
layer; f. optionally, applying a masking layer on top of the
imageable layer; and g. optionally, applying a removable coversheet
over the at least one imageable layer or the optional masking
layer.
17. The method according to claim 16, wherein the floor layer is
fully cured before the at least one imageable layer is applied.
18. The method according to claim 16, wherein the base layer
comprises a cured polymer resin.
19. The method according to claim 16, wherein the printing element
also comprises a layer of compressible material between the
dimensionally stable base layer and the floor layer.
20. The method according to claim 16, wherein the floor layer
comprises a photopolymer.
21. The method according to claim 16, wherein the imageable
material comprises a photopolymer.
22. The method according to claim 17, wherein the base layer
comprises a fiber reinforced polymer.
23. The method according to claim 18, wherein the compressible
material has a Shore A hardness of from 30 to 70 and a resilience
of from 40 to 70%.
24. The method according to claim 19, wherein the photopolymer
comprises an A-B-A type block copolymer.
25. The method according to claim 20, wherein the photopolymer
comprises an A-B-A type block copolymer.
26. The method according to claim 21, wherein the masking layer is
selectively ablatable using laser energy at a selected wavelength
and power.
27. The method according to claim 16, wherein the floor layer also
comprises microspheres.
28. A method of doing business comprising the steps of: a.
obtaining from a printing plate manufacturer desired relief image
printing plate specifications selected from the group consisting of
thickness, resilience, hardness, durometer, and combinations
thereof; b. preparing a printing element having a pre-cured floor
layer according to said desired specifications, wherein the print
element comprises i) a dimensionally stable base layer; ii) floor
layer comprised of a cured polymer selected from the group
consisting of photopolymers and polymers, each with a resilience of
at least 40% when cured; and iii) at least one layer of an uncured
photopolymer; and c. supplying the printing element having the
pre-cured floor layer to the printing plate manufacturer for
subsequent imagewise exposure and development to produce the relief
image printing plate; wherein a back exposure step is not performed
by the printing plate manufacturer.
29. A method according to claim 28 wherein the printing element
comprises a masking layer on top of the at least one layer of an
uncured photopolymer.
30. A method according to claim 28, wherein the printing element
comprises a removable coversheet on top of the layer of an uncured
photopolymer.
31. A method according to claim 28, wherein the printing element
comprises a layer of compressible material between the
dimensionally stable base layer and the floor layer.
32. A method according to claim 29, wherein the printing element
comprises a removable coversheet on top of the masking layer.
33. A method according to claim 32 wherein the masking layer is
selectively ablatable by the printing plate manufacturer using
laser energy at a selected wavelength power.
34. A method according to claim 28 wherein the floor layer also
comprises microspheres.
Description
FIELD OF THE INVENTION
[0001] The invention reveals an improved relief image printing
element, which has a pre-formed floor layer and an integral
imageable surface thereon.
BACKGROUND OF THE INVENTION
[0002] Flexography is a method of printing that is commonly used
for high-volume runs. Flexography is employed for printing on a
variety of substrates such as paper, paperboard stock, corrugated
board, films, foils and laminates. Newspapers and grocery bags are
prominent examples. Coarse surfaces and stretch films can be
economically printed only by means of flexography. Flexographic
printing plates are relief plates with image elements raised above
open areas. Such plates offer a number of advantages to the
printer, based chiefly on their durability and the ease with which
they can be made.
[0003] Although photopolymer printing elements are typically used
in "flat" sheet form, there are particular applications and
advantages to using the printing element in a continuous
cylindrical form, as a continuous in-the-round (CITR) photopolymer
sleeve. CITR photopolymer sleeves add the benefits of digital
imaging, accurate registration, fast mounting, and no plate lift to
the flexographic printing process. CITR sleeves have applications
in the flexographic printing of continuous designs such as in
wallpaper, decoration and gift-wrapping paper, and other continuous
designs such as tablecloths, etc. CITR sleeves enable flexographic
printing to be more competitive with gravure and offset on print
quality.
[0004] A typical flexographic printing plate as delivered by its
manufacturer, is a multilayered article made of, in order, a
backing or support layer, one or more unexposed photocurable
layers, a protective layer or slip film, and a cover sheet. A
typical CITR photopolymer sleeve generally comprises a sleeve
carrier (support layer) and at least one unexposed photocurable
layer on top of the support layer.
[0005] A flexographic printing element is produced from a
photocurable printing blank by imaging the photocurable printing
blank to produce a relief image on the surface of the printing
element. This is generally accomplished by selectively exposing the
photocurable material to actinic radiation, which exposure acts to
harden or crosslink the photocurable material in the irradiated
areas.
[0006] The photocurable printing blank typically contains one or
more layers of an uncured photocurable material on a suitable
backing layer. The photocurable printing blank can be in the form
of either a flat, planar element or as a cylindrical printing
element.
[0007] The printing element is selectively exposed to actinic
radiation in one of three related ways. In the first alternative, a
photographic negative with transparent areas and substantially
opaque areas is used to selectively block the transmission of
actinic radiation to the printing plate element. In the second
alternative, the photopolymer layer is coated with an actinic
radiation (substantially) opaque layer, which is also sensitive to
laser ablation. A laser is then used to ablate selected areas of
the actinic radiation opaque layer creating an in situ negative.
The printing element is then flood exposed through the in situ
negative. In the third alternative, a focused beam of actinic
radiation is used to selectively expose the photopolymer. Any of
these alternative methods is acceptable, with the criteria being
the ability to selectively expose the photopolymer to actinic
radiation thereby selectively curing portions of the
photopolymer.
[0008] Thereafter, the imaged and exposed printing element is
developed to remove uncured photopolymer on the surface of the
printing element and thus reveal the relief image. Development may
be accomplished in a variety of ways, including water washing,
solvent washing, using an air knife, and thermally, e.g.,
"blotting."
[0009] Finally, following development, the photopolymer layer may
be post-exposed to actinic radiation to provide a more complete
cure for the photopolymer layer of the invention and thus a more
durable printing plate. The photopolymer layer may also be
subjected to a detackification step.
[0010] Historically, in flexographic printing, flat, flexible
printing plates, fabricated from photopolymers, were hand mounted
onto print cylinders by wrapping the printing plate around the
cylinder and adhering it there with using various methods such as
clamps, tape, magnets or other similar devices. This process works
well, but is labor intensive and great care must be taken to ensure
that the registration of the plate on the cylinder is accurate. In
addition, to allow for additional compression during the print
process, a compressible material may be inserted between the print
cylinder and the printing plate. A refinement of this process
provides for a compressible layer to be contained within the
flexographic printing plates
[0011] Furthermore, a floor layer, which sets the depth of relief
in the photocurable layer, has traditionally been provided in the
printing element by back exposing the photocurable layer through
the backing layer to create a cured "floor" within the photocurable
layer. However, especially in thin plate technologies, back
exposure variance can be a problem, resulting in inconsistent
imaging results that hold varying degrees of detail with the plate.
A further concern is that back exposure is an additional processing
step taking additional time to complete. Thus it would be highly
desirable to provide a floor layer in a flexographic printing
element in a more consistent manner.
[0012] U.S. Pat. No. 5,962,111 to Rach, the subject matter of which
is herein incorporated by reference in its entirety, describes a
compressible printing plate formed by casting liquid
photopolymerizable resin directly onto a compressible material. The
photopolymerizable resin is then incompletely cured by exposure to
actinic radiation. However, Rach only incompletely cures the
photopolymerizable resin to create the floor layer and does not
address the issue of back exposure variance.
[0013] The inventors of the present invention have determined that
an improved printing element can be formed with an optional
compressible layer and an integral floor layer that provides a more
consistent printing element than that of the prior art.
[0014] A key advantage of the present invention is that back
exposure is no longer required, thus eliminating problems resulting
from back exposure variance. Furthermore, there is an obvious time
savings resulting from not requiring the back exposure step.
[0015] Poor or inconsistent control of light transmittance through
the polymer during back exposure can result in the degradation of
reverse images, filling them in when compounded with the face
exposure sequence. In this instant invention, only face exposure is
needed, so the issue of pre-sensitized material or partially cured
material during the back exposure sequence causing image
degradation is eliminated, resulting in enhanced imaging of the top
layer.
[0016] Another key benefit of the present invention is the ability
to vary the composition of the photocurable layer, provided there
is good adhesion between the layers, in order to affect different
print attributes, in a manner similar to the way high density and
low density foam can affect printing today. A high durometer base,
low durometer base, various combinations of the two with the top
layer, and even compressible layers (e.g., microspheres embedded
with a styrene-isoprene-styrene/styrene-butadiene-styrene copolymer
matrix) can be formulated for good compatibility with the surface
layer.
[0017] The printing plate compositions of the invention can also be
formulated to have an extremely fast curing top layer, without
concerns for the back exposure sequence being inconsistent. In
addition, the front exposure time can be shortened, hold more
detail, compounded by the enhanced imaging benefits resulting from
the use of a front exposure-only system.
[0018] The improved printing elements of the invention also have
reduced cold flow in thicker gauges. Higher thickness materials can
flow over time, creating havoc in plates sticking to each other
during transport (worst case scenario) or small areas of thinner
calipers in the plate may stick together after processing, either
of which can hurt the print quality.
[0019] Finally, printing elements of the invention can be developed
such that the top layer itself is one layer or two and may be
either analog or digital. The floor can also be formulated to have
reduced surface tension properties, aiding in plate clean up.
[0020] Finally, exposure of the floor layer through the face and
subsequent grinding, sanding, etc. to achieve the specification has
been found to be advantageous. Moreover, using this step achieved
high adhesion to the base layer surface without the use of
primers/adhesives normally used in the art.
[0021] In the past, as noted herein, the manufacturers of printing
plates formed those printing plates by starting with a sheet of
photopolymer laminated to a stable base layer of strong polymer
resin, such as polyester. The printing plate manufacturer then
partially U.V. exposed the bottom face of the photopolymer through
the stable base layer to form a cured floor layer of photopolymer.
The top face of the photopolymer layer was then imagewise exposed
to U.V. in order to create the relief desired. Thus printing plate
manufacturers were forced to use at least two U.V. exposures to
create the printing plate, a back exposure to create the floor and
a frontal imagewise exposure to create the desired imagewise relief
structure. The back exposure is troublesome from at least two
points of view. First it is a separate manufacturing step. Second,
because of variability in the U.V. transparency of the stable base
layer and the need to expose through this layer, non-uniformity in
curing the floor layer and variations in floor layer thickness were
experienced.
[0022] The practice of this invention allows for the use of a new
business practice, namely supplying printing plate manufacturers
with solid sheet photopolymer with a pre-cured floor layer. This
allows the printing plate manufacturer to skip the back exposure
step and allows for a more uniform and exacting floor layer to be
formed.
SUMMARY OF THE INVENTION
[0023] The present invention is directed to an improved relief
image printing element with an integral imageable printing surface.
The improved relief image printing element of the invention
comprises the following elements: [0024] a. a dimensionally stable
base layer; [0025] b. a floor layer comprised of a cured polymer
selected from the group consisting of photopolymers, and polymers
with a resilience of at least 40% when cured; and [0026] c. at
least one layer of an imageable material.
[0027] Optionally, but preferably, the printing element of the
invention may also comprise a layer of compressible material
between the dimensionally stable base layer and the floor
layer.
[0028] In addition, the relief image printing element may also
comprise a masking layer on top of the at least one layer of
imageable material. Finally, the printing element may comprise a
removable coversheet on top of the at least one layer of imageable
material or the on top of the masking layer (if used).
[0029] A method of making the improved printing element of the
invention is also provided. The method generally comprises the
steps of: [0030] a. providing a dimensionally stable base layer;
[0031] b. applying a floor layer comprised of a polymer selected
from the group consisting of photopolymers, and polymers with a
resilience of at least 40% when cured over the dimensionally stable
base layer; [0032] c. curing the floor layer; [0033] d. applying at
least one imageable layer on top of the cured floor layer; [0034]
e. optionally, applying a masking layer on top of the imageable
layer; and [0035] f. optionally, applying a removable coversheet
over the at least one imageable layer or the optional masking
layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The following is a brief description of the drawings which
should be read in conjunction with the detailed description of the
invention:
[0037] FIG. 1 illustrates a cross section of one embodiment of the
relief image printing element of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] The present invention is directed to an improved relief
image printing element with an integral imageable printing surface.
The improved relief image printing element of the invention
comprises the following elements: [0039] (1) a dimensionally stable
base layer 10 made of a high strength polymer resin, which is
optionally fiber reinforced; [0040] (2) optionally, but preferably,
a layer of compressible material 12; [0041] (3) a floor layer 14
comprised of a layer of photopolymerized photopolymer or other
polymerized resilient material. Optionally, layers 12 and 14 may be
one and the same layer; [0042] (4) an imageable layer 16 comprised
of at least one layer of an imageable (preferably, photoimageable)
material; [0043] (5) optionally, a masking layer 18 which absorbs
radiation at the wavelengths used to polymerize the photoimageable
material but is selectively removable by laser ablation or other
equivalent means; and [0044] (6) optionally, a removable coversheet
20.
[0045] Thus in practicing this invention, a supplier of
photopolymer, particularly solid sheet photopolymer, to the
printing plate industry can supply the foregoing printing element
with a pre-cured floor layer to the manufacturer of printing
plates. The floor would be pre-cured by the photopolymer supplier
to the specifications of the printing plate manufacturer as to
thickness, resilience and hardness. The pre-curing of the floor can
most efficiently be done by the photopolymer supplier through
frontal face exposure prior to the application of the imageable
layer, thus eliminating the need for back exposure and the quality
variations caused thereby. Upon receipt of the printing plate
element of this invention, the printing plate manufacturer is
merely required to imagewise expose the imageable layer (possibly
through a mask or the selectively ablated masking layer) and then
develop away the uncured photopolymer to create the relief
image.
[0046] The dimensionally stable base layer 10 can be fabricated
using the materials and methods applicable to producing printing
elements of the prior art. However, it is preferred that
dimensionally stable base layer 10 be fabricated from a polymer
resin reinforced with a fibrous material.
[0047] The fibrous material typically contains fibers of glass,
carbon, metal, ceramic, aramid or any other synthetic long fibers
that increase the stability, stiffness, and rigidity of the base
layer 10. Preferably, the fibrous material is a fiberglass cloth.
The fibrous material content of the base layer 10 is preferably
from about 20 to about 70 by weight.
[0048] Preferred resins useful in fabricating the base layer 10
include polyester resins, phenolic resins and epoxy resins, with
epoxy resins being most preferred. Suitable resins include the
following commercially available resins: Epoxical resin and
Epoxical hardener from Epoxical Inc, of Minnesota and other Epoxy
resins from JeffCo and Bakelite, Augusta, Ga. In one embodiment,
the base layer 10 is substantially uniformly transparent or
translucent such that actinic radiation can be transmitted through
the surface of the base layer 10 to the imageable layer 16.
[0049] One preferred method of fabricating the base layer 10 is to
directly cast the resin and fibrous material to form the required
base layer thickness. The base layer 10 is then cured using heat,
actinic radiation, and/or radiant energy. The base layer 10 is
finished to specification by being sanded, ground or otherwise
machined to size. The base layer 10 typically ranges from about 10
mils to 100 mils in thickness.
[0050] Once base layer 10 is fabricated, a layer of compressible
material 12 is optionally, but preferably applied to the top
surface of base layer 10. The compressible layer can take a number
of forms. In one embodiment, the compressible material 12 consists
of a layer of solid foam material, which is adhesively bonded to
the top surface of the base layer 10. Alternatively, and
preferably, the layer of compressible material 12 may be formed by
uniformly mixing hollow microspheres with an uncured photocurable
or thermally curable resin formulation. The resin/microsphere
mixture is then applied to base 10 in a layer or layers using a
knife or extrusion to provide uniform thickness. The resin
microsphere layer is cured using heat, actinic radiation, and/or
radiant energy as appropriate. Preferably, electron beam curing is
advantageously used for curing the microsphere compressible foam
layer. In a third alternative, a soft resilient polymer such as a
polyurethane, or softer version of the Styrene-isoprene-styrene or
styrene-butadiene-styrene photocurable layer may be used as the
compressible material. In this case, the uncured material is
similarly applied using a knife or extrusion to ensure uniform
thickness and then cured in place. After application and
photocuring, the compressible layer is preferably further grounded
or sanded to achieve the desired thickness.
[0051] The thickness of the layer of compressible material can vary
depending upon the material being used and the particular printing
application. Generally, if a layer of compressible material is
used, the thickness of the layer may range from about 20 mils to 40
mils. This thickness of the compressible layer assures wide
latitude of approximately 20 mils impression during the printing
without significant dot gain. The cured layer of compressible
material can be sanded, ground, or otherwise machined to
specification.
[0052] Next, whether or not the layer of compressible material 12
is used, a floor layer 14 comprised of one or more layers of
photopolymer or other resilient polymeric material is applied to
the top surface of base layer 10 or, if used, to the top surface of
the layer of compressible material 12. It is entirely possible that
the compressible layer 12 and the floor layer 14 are one and the
same. If the floor 14 and compressible 12 layers are the same,
microspheres are preferably included in the polymer/photopolymer
composition to increase its compressibility. In a preferred
embodiment, the floor layer 14 comprises a photopolymer, which is
subsequently cured before the imageable layer 16 is applied. The
photocuring of the floor layer 14 is achieved by face exposure of
the floor layer 14 to actinic radiation for a certain length of
time through the front. The floor layer 14, after curing, is
preferably sanded, ground, or otherwise machined to
specification.
[0053] The floor layer 14 is preferably applied as a liquid
extrudate polymer, using a knife or extrusion to ensure uniform
thickness. Once applied, the layer is cured preferably from the
front using heat, actinic radiation and/or radiant energy to form a
substantially continuous layer over the top of base layer 10 or
compressible layer 12 if used. If necessary, the floor layer 14 can
then be sanded, ground or otherwise machined to the proper
thickness. The thickness of the floor layer may range from about 5
mils to about 134 mils. When cured, the floor layer 14, depending
on the thickness and type of photopolymer, should have a resilience
of from 40% to 70% and a hardness of from 30 to 70 Shore A as
measured by ASTM D2240.
[0054] The purpose of the floor layer 14 is to ensure that the
imageable layer 16 has excellent adhesion and remains firmly
attached to the base layer 10 or optional compressible layer 12,
even when very fine isolated structures are developed in the
imageable layer 16. The floor layer 14 also gives physical support
to fine dots and lines during the printing process. This is
especially critical for constructions where the total thickness of
the imageable photopolymer 16 and floor 14 layers is relatively
thin (.about.28-30 mil total). The floor layer 14 as used herein,
with face exposure from the front, also assures extreme uniformity
of the relief layer, which is necessary for high-end printing
applications where floor variation can create degradation of image
fidelity. Also, since the floor layer 14 can be completely cured
from the front, it assures a very dry surface after the processing
step and post-exposure/detack steps. It is also preferable for the
floor layer 14 to comprise the same photopolymer as is present in
the imageable layer 16. Moreover, it is important for the floor
layer 14 to exhibit good adhesion to the layer below and to the
imageable layer 16. In this regard, photocurable styrenic block
copolymers generally work well in providing an adequate floor
layer.
[0055] Imageable layer 16 is formed from a material which can be
imaged, either mechanically, optically, via heat or differential
melting and/or chemically. Preferably, imageable layer 16 comprises
a photocurable or photopolymerizable material. Typically, the
photocurable material is applied to the floor layer 14 by
spreading, dipping, casting, extruding or molding. The thickness is
controlled either by using a knife, die or mold as appropriate. If
necessary, the precise thickness of imageable layer 16 can be
adjusted via grinding, sanding or other machining. If desired, more
than one imageable layer can be sequentially applied.
[0056] The photocurable material for use in fabricating the floor
layer 14 and the imageable layer 16 generally comprises binder(s),
monomer(s) and photoinitiator(s). The binder preferably comprises
an A-B-A type block copolymer where A represents a non-elastomeric
block, preferably a vinyl polymer or most preferably polystyrene,
and B represents an elastomeric block, preferably polybutadiene or
polyisoprene. Preferably, the non-elastomer to elastomer ratio is
in the range of from 10:90 to 35:65. Photopolymer compositions
useful in the practice of this invention include those described in
U.S. patent application Ser. No. 10/353,446 filed Jan. 29, 2003,
the teachings of which are incorporated herein by reference in
their entirety.
[0057] The photocurable material also comprises at least one
monomer, which should be an ethylenically unsaturated compound.
Suitable monomers include multi functional acrylates,
multifunctional methacrylates and polyacryloyl oligomers. Examples
of suitable monomers include one or more of ethylene glycol
diacrylate, hexanediol diacrylate, diethylene glycol diacrylate,
glycerol diacrylate, trimethylol propane triacrylate, hexane diol
dimethacrylate, glycerol triacrylate, trimethylolpropane
triacrylate, ethylene glycol dimethacrylate, 1,3-propanediol
dimethacrylate, 1,2,4-butanetriol trimethacrylate, and
1,4-butanediol diacrylate. The photocurable material should also
have at least one photoinitiator. Any of the known classes of
photoinitiators, particularly free radical photoinitiators such as
quinones, bemzophenones, benzoin ethers, aryl ketones, peroxides,
biimidazoles, diacyliodoniums, triacylsulfoniums, phosphoniums, and
diazoniums. In addition to the binder, monomer, and photoinitiator,
the photocurable composition may also comprise other additives
known in the art such as plasticizers, anti-oxidants, oxygen
scavengers, flow modifiers, colorants and fillers.
[0058] Although the photocurable material noted above is preferred
for fabricating the floor layer 14 and the imageable layer 16, any
photocurable material useful in fabricating flexographic printing
plates may be used. For additional examples of photocurable
materials useful in this regard, please refer to U.S. Pat. Nos.
4,045,231; 4,323,636; 5,223,375; and 5,290,633, the teachings each
of which are incorporated herein by reference in their entirety.
The imageable layer usually ranges from about 15 mil to 35 mil
depending on the printing application.
[0059] On top of the imageable layer is optionally, but preferably,
a masking layer 18. The purpose of the masking layer 18 is to allow
for the selective polymerization of the imageable layer. Thus, the
masking layer 18 must be made to be removed or become transparent
to actinic radiation in areas where the imageable layer 16 is to be
polymerized, but at the same time block actinic radiation in areas
where the imageable layer is to remain unpolymerized and developed
away to create the relief image necessary for flexographic
printing. In the alternative, the imageable layer 16 can be
selectively exposed to actinic radiation through a negative or a
mask as is well known in the art.
[0060] Preferably, the masking layer can be selectively ablated
using laser radiation in the pattern of the image desired. In the
case of laser ablation, the masking layer generally comprises an
ultraviolet radiation absorbing material, an infra-red radiation
absorbing material and a binder. Dark inorganic pigments such as
carbon black or graphite can function as both the ultraviolet
radiation absorbing material and infra-red radiation absorbing
material. Polyamides or cellulosic polymers are suitable binders.
Suitable masking layers are described in U.S. Pat. Nos. 6,605,410
and 6,238,837, the teachings each of which are incorporated herein
by reference in their entirety.
[0061] The masking layer can be disposed on the imageable layer 16
using several methods. It can be spread directly on the imageable
layer 16 and dried. It can be separately spread on a plastic cover
sheet and the coversheet laminated to the imageable layer 16 with
the masking layer 18 between the imageable layer 16 and the
coversheet. In this case, the coversheet is stripped away before
use. The masking layer must be removable using the development
means used to develop away (remove) the uncured portions of the
imageable layer. The masking layer usually ranges from about 1
.mu.m to about 10 .mu.m and having an optical density of from 2.5
to 4.5.
[0062] Finally, a removable coversheet may be applied on top of the
masking layer to protect the flexographic printing element during
handling. If used, the coversheet is removed either just before or
just after the selective exposure to actinic radiation.
[0063] The printing plate of the invention can be utilized in the
following manner: [0064] 1. If used, the masking layer 18 is
selectively exposed to laser radiation at a wavelength and power
such that the portions of masking layer 18 contacted with the laser
radiation are ablated away without damaging the underlying
imageable layer 16. Preferably the laser is computer controlled to
scan the surface of the masking layer 18 according to the image
desired. In this regard, please refer to U.S. Pat. No. 5,760,880,
the disclosure of which is incorporated herein by reference in its
entirety. In the alternative the imageable layer 16 is covered with
a negative and is selectively exposed to actinic radiation through
the negative as is well known in the art. [0065] 2. Next, the
surface of the printing plate is exposed to actinic radiation such
that the portions of the imaging layer 16 that have been exposed as
a result of the ablation of portions of the masking layer 18 above
are polymerized, but the portions of the imaging layer 16 that
remain covered by the masking layer 18 remain unpolymerized. [0066]
3. The printing plate is then subjected to development using heat
and/or chemicals such that the masking layer and the unpolymerized
portions of the imaging layer are removed leaving behind the
polymerized portions of the imaging layer standing out in relief in
the image desired. Preferably, the printing plated is subjected to
thermal development. [0067] 4. Optionally, the printing plate is
subjected to further post-curing and detacking the remaining
imaging layer using actinic radiation or heat. [0068] 5. The
printing element is then mounted onto a printing cylinder using
conventional means. [0069] 6. Finally, the printing element of the
invention is mounted in a flexographic printing press to begin
printing.
[0070] The printing plates of the invention are particularly suited
for thermal development. Printing elements of the present invention
can be processed down to the cured floor without concern over not
removing any uncured photopolymer. This results in a more
consistent relief, which is a traditional issue with thermal
processing. A higher melt flow top layer can also be formulated
without concern for processing issues due to the flow of the
partially cured material during processing. This is a concern with
dot structure, line structure, and plate to plate registration
issues.
[0071] Thermal development processes are highly desirable as such
processes eliminate the need for chemical processing of printing
elements in developing relief images, in order to go from plate to
press more quickly. Processes have been developed whereby
photopolymer printing plates are prepared using heat and the
differential melting temperature between cured and uncured
photopolymer is used to develop the latent image. The basic
parameters of this process are known, as described in U.S. Pat.
Nos. 5,279,697, 5,175,072 and 3,264,103, in published U.S. patent
publication Nos. U.S. 2003/0180655, and U.S. 2003/0211423, and in
WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of
which are incorporated herein by reference in their entirety.
[0072] These processes allow for the elimination of development
solvents and the lengthy plate drying times needed to remove the
solvent. The speed and efficiency of the process allow for use of
the process in the manufacture of flexographic plates for printing
newspapers and other publications where quick turnaround times and
high productivity are important. In thermal development, the
composition of the photopolymer is such that there exists a
substantial difference in the melt temperature between the cured
and uncured polymer. It is precisely this difference that allows
the creation of an image in the photopolymer when heated. The
uncured photopolymer (i.e., the portions of the photopolymer not
contacted with actinic radiation) will melt or substantially soften
while the cured photopolymer will remain solid and intact at the
temperature chosen. Thus the difference in melt temperature allows
the uncured photopolymer to be selectively removed thereby creating
an image.
EXAMPLE I
[0073] The improved printing element of the invention may be
prepared as follows:
[0074] An epoxy resin and hardener (Ratio approximately 3.3:1) is
mixed by hand for 1-2 minutes until it thickens. This
resin-hardener mixture is then cast on a suitable substrate as the
base layer. Type 106-glass fabric pre-cut into 4-inch width is then
coated on the resin-hardener mixture to be sure that the fabric is
totally wetted out. The polymer-glass composite is then allowed to
crosslink or gel by applying heat for about 30 minutes. The
printing element base is then baked for 4 hours at 120.degree. F.
After the baking step the printing is further machined or ground to
the specified gage (16.+-.1/2 mil thickness).
[0075] A floor layer 16, as set out in Table I, is then extruded
onto either the base layer 12 or the compressible layer 14. The
floor layer 16 is then cured using either UV actinic radiation or
electron beam radiation, and is then ground to the specified gage
required for the floor (at least about 20-40 mils thick). The
photopolymer imageable layer 18, as set out in Table II is next
extruded on top of the floor layer, and is ground to the specific
gage required (usually 20-25 mils). A mask layer is further applied
on top of the photopolymer using a roller coating operation.
TABLE-US-00001 TABLE I Composition of Floor Layer INGREDIENT WEIGHT
% Kraton .RTM. D1102 .sup.1 57.4 Shellflex .RTM. 6371 .sup.2. 21.2
HDDA .sup.3. 5.30 TMPTMA .sup.4. 5.30 Irgacure .RTM. 651 .sup.5.
3.30 BHT .sup.6. 2.27 Irganox .RTM. 1010 .sup.7. 0.03 Calcium
Stearate 0.13 Tinuvin .RTM. 1130 0.01 Microspheres 5.00 100.0
.sup.1 Available from Kraton Polymer Company .sup.2. Available from
Shell Chemical Company .sup.3. Hexane dioldiacrylate .sup.4.
Trimethylolpropane trimethacrylate .sup.5. Available from Ciba
Specialty Chemicals .sup.6. Butyleted hydroxytoluene .sup.7.
Available from Ciba Specialty Chemicals .sup.8. Available from Ciba
Specialty Chemicals
[0076] TABLE-US-00002 TABLE II Composition of Imageable Layer
INGREDIENT WEIGHT % Kraton .RTM. D1102 60.46 Shellflex .RTM. 6371
22.33 HDDA 5.58 TMPTMA 5.58 Irgacure .RTM. 651 3.48 BHT 2.39
Irganox .RTM. 1010 0.03 Calcium Stearate 0.14 Tinuvin .RTM. 1130
0.01 100.0
* * * * *