U.S. patent application number 09/415811 was filed with the patent office on 2002-05-16 for uv-absorbing support layers and flexographic printing elements comprising same.
Invention is credited to KANGA, RUSTOM SAM.
Application Number | 20020058196 09/415811 |
Document ID | / |
Family ID | 23647301 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020058196 |
Kind Code |
A1 |
KANGA, RUSTOM SAM |
May 16, 2002 |
UV-ABSORBING SUPPORT LAYERS AND FLEXOGRAPHIC PRINTING ELEMENTS
COMPRISING SAME
Abstract
The present invention provides a method for producing
direct-imaged flexographic printing elements such that both the
front and back exposure times are economically efficient for the
manufacturer. In one embodiment, the method comprises providing at
least one solid photocurable element. The solid photocurable
element comprises a solid photocurable material comprising an
oxygen scavenger, a support layer having an actinic radiation
absorbing compound integrated uniformly throughout such that it
absorbs at least some actinic radiation during exposure, and a
photoablative mask layer. The methods of the invention involve
creating a floor in the solid photocurable material by back
exposure through the support layer having the actinic radiation
absorbing compound, transferring a negative image directly onto the
solid photocurable material by photoablating the photoablatable
mask layer, followed by front exposure effective to cure the solid
photocurable material.
Inventors: |
KANGA, RUSTOM SAM;
(MARIETTA, GA) |
Correspondence
Address: |
John L. Cordani
Carmody & Torrance LLP
50 Leavenworth Street
P.O. Box 1110
Waterbury
CT
06721-1110
US
|
Family ID: |
23647301 |
Appl. No.: |
09/415811 |
Filed: |
October 11, 1999 |
Current U.S.
Class: |
430/270.1 ;
430/271.1; 430/273.1; 430/281.1; 430/302; 430/306 |
Current CPC
Class: |
G03F 7/091 20130101;
G03F 7/029 20130101; G03F 7/202 20130101 |
Class at
Publication: |
430/270.1 ;
430/271.1; 430/273.1; 430/281.1; 430/302; 430/306 |
International
Class: |
G03F 007/00; B41N
001/00 |
Claims
What is claimed is:
1. A printing element comprising: a support layer having an actinic
radiation absorbing compound uniformly distributed throughout said
support layer; a layer of solid photocurable material that has
first and second opposing major faces, said first opposing major
face disposed upon said support layer, wherein said layer of solid
photocurable material comprises an oxygen scavenger; and a
photoablative mask layer that is disposed on said second opposing
major face, that is substantially opaque to actinic radiation, and
is capable of being photoablated by a laser, said printing element
having a thickness of at least 0.067 inches.
2. A printing element according to claim 1 wherein said support
layer is polyethylene terephthalate.
3. A printing element according to claim 1 wherein said oxygen
scavenger comprises a phosphine compound.
4. A printing element according to claim 3 wherein said phosphine
compound is selected from the group consisting of
triphenylphosphine, triphenyl phosphite, tri-p-tolylphosphine,
diphenylmethylphosphine, diphenylethylphosphine,
diphenylpropylphosphine, dimethylphenylphosphine,
diethylphenylphosphine, dipropylphenylphosphine,
divinylphenylphosphine, divinyl-p-methoxyphenylphosphine,
divinyl-p-bromophenylphosphine, divinyl-p-tolylphosphine,
diallylphenylphosphine, diallyl-p-methoxyphenyl- phosphine,
diallyl-p-bromophenylphosphine and diallyl-ptolylphosphine.
5. A printing element according to claim 4 wherein said phosphine
compound is present at a concentration of from about 0.075 to about
0.75 weight percent of said solid photocurable material.
6. A printing element according to claim 1 wherein said solid
photocurable material comprises a plurality of layers.
7. A printing element according to claim 1 further comprising a
solid photocurable cap having first and second opposing major
faces, the first major face being disposed upon said second major
face of said solid photocurable material.
8. A printing element according to claim 1 wherein said support
layer having an actinic radiation absorbing compound uniformly
distributed throughout said support layer absorbs between about 85
and about 95 percent actinic radiation.
9. A printing element according to claim 7 wherein said solid
photocurable cap comprises an actinic radiation absorbing dye.
10. A method comprising: providing at least one solid photocurable
printing element according to claim 1; transferring graphic data to
said solid photocurable printing element by photoablating said
photoablatable mask layer with a laser, thereby providing ablated
and unablated areas forming an image, said ablated areas exposing
said second opposing major face of said solid photocurable layer;
exposing said first opposing major face of said photocurable layer
through said support layer; exposing said ablated areas of said
solid photocurable material to actinic radiation effective to cure
said solid photocurable material; and removing uncured photocurable
material and said unablated areas of said photoablatable mask layer
from said element.
11. A method according to claim 10 wherein said support layer is
polyethylene terephthalate.
12. A printing element according to claim 10 wherein said support
layer having an actinic radiation absorbing compound uniformly
distributed throughout said support layer absorbs between about 85
and about 95 percent actinic radiation.
13. A method according to claim 10 wherein said oxygen scavenger
comprises a phosphine compound.
14. A method according to claim 13 wherein said phosphine compound
is selected from the group consisting of triphenylphosphine,
triphenyl phosphite, tri-ptolylphosphine, diphenylmethylphosphine,
diphenylethylphosphine, diphenylpropylphosphine,
dimethylphenylphosphine, diethylphenylphosphine,
dipropylphenylphosphine, divinylphenylphosphine,
divinyl-p-methoxyphenylphosphine, divinyl-p-bromophenylphosphine,
divinyl-p-tolylphosphine, diallylphenylphosphine,
diallyl-p-methoxyphenyl- phosphine, diallyl-p-bromophenylphosphine
and diallyl-ptolylphosphine.
15. A method according to claim 13 wherein said phosphine compound
is present at a concentration of from about 0.075 to about 0.75
weight percent of said solid photocurable material.
16. A method according to claim 10 wherein said solid photocurable
material comprises a plurality of layers.
17. A method according to claim 10 wherein said solid photocurable
element further comprises a cap layer upon which said photoablative
mask layer is disposed.
18. A printing element comprising: an inherently UV-absorbing
support layer; a layer of solid photocurable material that has
first and second opposing major faces, said first opposing major
face disposed upon said support layer, wherein said layer of solid
photocurable material comprises an oxygen scavenger; and a
photoablative mask layer that is disposed on said second opposing
major face, that is substantially opaque to actinic radiation, and
is capable of being photoablated by a laser, said printing element
having a thickness of at least 0.067 inches.
19. A printing element according to claim 18 wherein the inherently
UV-absorbing support layer is polyethylene naphthalate.
20. A printing element according to claim 19 wherein the
polyethylene naphthalate support layer is from about 3 to 5 mils
thick.
21. A method comprising: providing at least one solid photocurable
printing element according to claim 19; transferring graphic data
to said solid photocurable printing element by photoablating said
photoablatable mask layer with a laser, thereby providing ablated
and unablated areas forming an image, said ablated areas exposing
said second opposing major face of said solid photocurable layer;
exposing said first opposing major face of said photocurable layer
through said support layer; exposing said ablated areas of said
solid photocurable material to actinic radiation effective to cure
said solid photocurable material; and removing uncured photocurable
material and said unablated areas of said photoablatable mask layer
from said element.
22. A method according to claim 21 wherein the support layer is
polyethylene naphthalate.
23. A printing element according to claim 22 wherein the
polyethylene naphthalate support layer is from about 3 to 5 mils
thick.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to flexographic printing
elements having light-attenuating support layers, to the formation
of relief images on direct-image flexographic printing elements
and, more particularly, to methods for achieving a uniform floor in
the manufacture of such direct-image flexographic printing
elements.
BACKGROUND OF THE INVENTION
[0002] Relief image printing plates are used in both flexographic
and letterpress processes for printing on a variety of substrates,
including paper, corrugated stock, film, foil, and laminates.
Relief elements typically include a support layer and one or more
layers of photocurable polymer in the form of solid sheets. The
printer typically peels a cover sheet from the element to expose
the photocurable polymer and places a silver halide photographic
negative or some other masking device upon the photopolymer. The
printer exposes the negative-bearing element to ultraviolet (UV)
light through the negative, thereby causing exposed areas of the
element to harden, or cure. After the uncured areas of the element
are removed, cured polymer remains as the relief printing
surface.
[0003] The negatives used in such processes typically are costly
items, and the time required for their preparation can be
considerable, particularly in those print shops that are not
capable of preparing negatives in-house. Moreover, any negative
which is used for printing must be nearly perfect. Even minor flaws
will be carried through onto each printed item. As a consequence,
effort must be expended to ensure that the negative is precisely
made. In addition, the negative is usually made with silver halide
compounds which are costly and which are also the source of
environmental concerns upon disposal.
[0004] In the art of flexographic printing, processes have been
developed to eliminate the use of the negative, thereby offering
significant advantages over previous methods such as, for example,
cost efficiency, environmental impact, convenience, and image
quality. Many such processes are referred to as direct-to-plate
(DTP) processes. One DTP process is disclosed in U.S. Pat. No.
5,846,691 to Cusdin, et al., herein incorporated by reference,
which describes formation of a computer-generated negative on a
photosensitive printing element by ejecting a negative-forming ink
from an ink jet print head directly onto the surface of the
printing element. Another such process is disclosed in U.S. Pat.
No. 5,925,500 to Yang, et al., herein incorporated by reference,
which describes a method of making a laser-imaged printing plate by
modifying the slip film with a UV absorber and employing a laser to
selectively ablate the slip film. In such methods, the slip film,
in effect, becomes the negative as only the areas of the
photopolymer to be cured are exposed to actinic radiation. Yet
another DTP process is disclosed in U.S. Pat. No. 5,262,275 to Fan,
herein incorporated by reference, in which a layer of
laser-ablatable infrared radiation sensitive material is disposed
upon the surface of the printing element.
[0005] DTP technology is significantly different than the
conventional plate making technology in a number of respects. DTP
plates, for example, typically have a photoablative mask directly
on the plate. Also, in DTP technology, face exposure, i.e., a
blanket exposure to actinic radiation of the photopolymerizable
layer on the side that does (or, ultimately will) bear the relief,
is done in air (in the presence of oxygen), whereas, with
conventional plates, exposure is typically done in vacuum.
[0006] Because face exposure is conducted in the presence of
oxygen, there is the potential for excessive exposure of the
photocurable layer to oxygen in areas where the masking layer has
been removed. This can present problems because the
photopolymerization kinetics of many materials in the presence of
oxygen are very different from those observed in the absence of
oxygen because oxygen is a known free radical scavenger. Hence,
oxygen has the effect of inhibiting polymerization of the
photocurable material, thus requiring longer exposure times. In
addition, oxygen could potentially act as a UV screening agent,
resulting in attenuation of the actinic radiation.
[0007] Generally, this phenomenon is referred to as "oxygen
inhibition." Oxygen inhibition is typically compounded when
so-called "capped" photocurable printing elements are used. Capped
photocurable elements have a thin photocurable cap disposed upon
the main body of the photocurable material. Typically, with such
elements, the relief image formed includes photocurable material
from the cap layer. Capped printing elements typically have several
significant advantages over uncapped elements in DTP processes. For
example, the cap typically has a rough surface that can act as an
ink receptive layer, resulting in higher ink densities on the
printed substrate. Also, capped plates, because of their longer
exposure times, typically enable the user to modify dot shape,
resulting in smaller, but more robust dots. The cap layer can also
include an image contrast (e.g. green) dye which aids in the
inspection of the registered image. The cap itself, however, due to
the presence of the dye (and other components), acts as an actinic
radiation absorbing layer. Thus, the phenomenon of oxygen
inhibition is amplified when imaging capped photocurable printing
elements to the extent that relatively long front exposures may be
required to hold fine detail dots (i.e., 1% dots on a 150
line).
[0008] To decrease front exposure times when processing printing
elements with DTP technology such that such times are comparable to
those of conventional printing elements, the photo speed (i.e., the
speed of photopolymerization) typically is increased to counter the
effects of oxygen inhibition. One way to do this is to incorporate
oxygen scavengers such as, for example, triphenylphosphine and
triphenylphosphite, into the polymer formulation. The addition of
oxygen scavengers to the polymer formulation, however, not only
decreases the front exposure time, but, also decreases the back
exposure time as well.
[0009] As used herein, "back exposure" is a blanket exposure to
actinic radiation of the photopolymerizable layer on the side
opposite that which does (or, ultimately will) bear the relief.
This is typically done through a transparent support layer. Such
exposure is used to create a shallow layer of polymerized material,
herein referred to as a "floor," on the support side of the
photopolymerizable layer. The purpose of the floor is generally to
sensitize the photopolymerizable layer and to establish the depth
of the relief. Typically, it is desired to have back exposure times
greater than 15-30 seconds. In DTP technology, however, increasing
the photo speed as described above often results in a back exposure
time of less than 30 seconds. Such short back exposure times are
undesirable because, for reasons discussed in detail below,
variations in the thickness of the floor are typically observed. In
turn, a non-uniform floor typically contributes to uneven printing
due to variation in the relief across the plate.
[0010] Back exposure times can be increased in DTP systems by
applying a thin, i.e., 1-2 microns, coating of a UV-absorbing
compound between the photopolymerizable layer and the support, or
backing, layer. This approach, however, is problematic, as it is
difficult to apply the UV-absorbing coating uniformly. This, of
course, also creates variations in the thickness of the floor.
Also, the coating could interact with the laser and create problems
of adhesion.
[0011] Accordingly, there is a need in the art for an improved
method to produce direct-imaged capped and uncapped flexographic
printing plates.
BRIEF DESCRIPTION OF THE INVENTION
[0012] The present invention provides methods for producing
direct-imaged flexographic printing elements such that both the
front and back exposure times are economically efficient for the
manufacturer. The present invention provides a solid photocurable
element that comprises a layer of solid photocurable material
containing an oxygen scavenger disposed on a support layer. The
support layer has an actinic radiation absorbing compound
integrated uniformly throughout such that it absorbs at least some
actinic radiation during exposure. The solid photocurable element
also comprises a photoablative mask layer disposed on the solid
photocurable layer. The mask is substantially opaque to actinic
radiation and is capable of being photoablated by a laser.
[0013] The methods of the present invention comprise transferring
graphic data from a computer to the solid photocurable element
described above by photoablating selected areas of the
photoablatable mask layer using a laser that is in communication
with the computer, thus providing ablated and unablated areas
forming an image. The ablated areas expose the solid photocurable
layer which ultimately becomes the relief. A "floor" is also
established by exposing the photocurable layer through the support
layer. The solid photocurable material that is exposed through the
ablated areas of the photoablatable mask layer are then exposed to
actinic radiation effective to cure the solid photocurable material
and leave solid photocurable element underneath the unablated areas
uncured. The uncured solid photocurable material and the unablated
areas of said photoablatable mask layer are then removed.
[0014] In another embodiment of the present invention, the solid
photocurable printing element further comprises a solid
photopolymerizable cap layer. In this embodiment the photoablative
mask layer is disposed directly onto the cap layer and the method
is performed accordingly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The numerous objects and advantages of the present invention
may be better understood by those skilled in the art by reference
to the accompanying non-scale figures, in which:
[0016] FIG. 1 is a cross-sectional view of a printing element
according to the invention; and
[0017] FIG. 2 is a cross-sectional view of another embodiment of a
printing element according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] As used herein, the term "photocurable material" refers to a
solid composition which undergoes polymerization, cross-linking, or
any other curing or hardening reaction in response to actinic
radiation with the result that the unexposed portions of the
material can be selectively separated and removed from the exposed
(cured) portions to form a three dimensional or relief pattern of
cured material.
[0019] The separation and removal of the unexposed portions can be
accomplished using a jet of air ("air knife"), brushing, selective
solubilization or dispersion in a suitable developer solvent or
detergent solution, a squeegee, a combination of the foregoing, or
other suitable development means.
[0020] As shown in FIGS. 1 and 2 wherein like elements have like
numerals, a preferred photocurable element 10 to be used in a DTP
imaging process comprises a support layer 12, at least one
photocurable layer 14, an optional second photocurable layer 16,
and a photoablatable mask layer 18. The preferred photocurable
element can also comprise a "cap" layer 17 (shown in FIG. 2). The
photocurable elements 10 shown in FIGS. 1 and 2 can also comprise
an adhesive layer between photocurable layer 14 and the support
layer 12 (not shown). The photocurable elements according to the
present invention preferably are substantially planar solid
elements having a thickness of at least about 0.067 inches.
[0021] Typically, in DTP technology, the cover-sheet (not shown) is
removed, thus exposing the photoablatable mask layer. A computer
then transfers digital information to the photoablative mask layer
via a laser that is in communication with the computer that ablates
those areas of the photoablative mask layer that have to cure,
i.e., those areas that ultimately become the relief layer. The
plate is then back-exposed to build the floor, face exposed through
the in-situ mask, and processed in a solvent processor. The area of
the mask that was not ablated prevents the underlying photopolymer
to cure and is removed during the processing step. That area where
the mask was removed is cured and becomes the relief area. The
plate is then dried and post-exposed and de-tacked as usual.
[0022] The photocurable layers 14,16 of the photocurable element
can include any of the known photopolymers, monomers, initiators,
reactive diluents, fillers, and dyes.
[0023] Preferred photocurable materials include an elastomeric
compound, an ethylenically unsaturated compound having at least one
terminal ethylene group, and a photoinitiator. Exemplary
photocurable materials are disclosed in European Patent
Applications 0 456 336 A2 (Goss, et al.), and 0 640 878 A1 (Goss,
et al.), British Patent No. 1,366,769, U.S. Pat. Nos. 5,223,375
(Berrier, et al.), 3,867,153 (MacLahan), 4,264,705 (Allen),
4,323,636 (Chen, et al.), 4,323,637 (Chen, et al.), 4,369,246
(Chen, et al.), 4,423,135 (Chen, et al.), 3,265,765 (Holden, et
al.), 4,320,188 (Heinz, et al.), 4,427,759 (Gruetzmacher, et al.),
4,622,088 (Min), and 5,135,827 (Bohm, et al.), which are
incorporated herein by reference.
[0024] "Cap" layer 16 generally comprises photocurable material
which is the same as or similar to the photocurable material
present in the photocurable layer. Suitable compositions for the
cap layer are those disclosed as elastomeric compositions in the
multilayer cover element described in U.S. Pat. Nos. 4,427,759 and
4,460,675 (Gruetzmacher, et al.), both of which are incorporated
herein by reference. Additional components present in the cap layer
include: a coating solvent, optionally but preferably a
non-migrating dye or pigment which provides a contrasting color
with any colorant or dye present in the photocurable layer.
Optionally, cap layer 16 can also include one or more ethylenically
unsaturated monomeric compounds, and/or a photoinitiator or
initiator system. The contrast dye can be Acid Blue 92, or other
dyes disclosed in, for example, U.S. Pat. No. 3,218,167,
incorporated herein by reference. In general, the thickness of the
cap is in the range of from about 0.00001 to 0.003 inches.
Preferably, the thickness of the cap is from about 0.000015 to
about 0.0025 inches. An example of a capped photopolymer element as
described is FLEXLIGHT EPIC.RTM. (commercially available from
Polyfibron Technologies, Inc., Atlanta, Ga.).
[0025] The photocurable materials of the invention should
cross-link (cure) and, thereby, harden in at least some actinic
wavelength region. As used herein, actinic radiation is radiation
capable of effecting a chemical change in an exposed moiety.
Actinic radiation includes, for example, amplified (e.g., laser)
and non-amplified light, particularly in the UV and infrared
wavelength regions. Preferred actinic wavelength regions are from
about 250 nm to about 450 nm, more preferably from about 300 nm to
about 400 nm, even more preferably from about 320 nm to about 380
nm.
[0026] As described above, longer front exposure times are
typically required for the transfer of fine detail images onto the
photocurable element due to the presence of oxygen in DTP
technology. Thus, it is preferable to include, for example, oxygen
scavengers into the photocurable material to counter the effects of
the oxygen, thereby decreasing the exposure time (i.e., increasing
the photospeed of the photopolymer).
[0027] Preferably, the oxygen scavenger is a phosphine compound.
Representative phosphine compounds include triphenylphosphine,
tri-p-tolylphosphine, diphenylmethylphosphine,
diphenylethylphosphine, diphenylpropylphosphine,
dimethylphenylphosphine, diethylphenylphosphine,
dipropylphenylphosphine, divinylphenylphosphine,
divinyl-p-methoxyphenylp- hosphine, divinyl-pbromophenylphosphine,
divinyl-p-tolylphosphine, diallylphenylphosphine,
diallyl-p-methoxyphenylphosphine, diallyl-p-bromophenylphosphine
and diallyl-p-tolylphosphine.
[0028] Triphenylphosphine is particularly preferred.
[0029] Preferably, the phosphine compound is present in the solid
photocurable compound at from about 0.01 to about 2.0 weight
percent of the solid photocurable material, more preferably from
about 0.05 to about 1.0 weight percent of the solid photocurable
material, and most preferably from about 0.075 to about 0.75 weight
percent of the solid photocurable material.
[0030] Additional ways to decrease the exposure times include
increasing the intensity of the actinic radiation. High intensity
lamps, however, typically generate excessive heat which can create
problems such as plate warping and image deterioration.
[0031] By employing any of these methods for decreasing the front
exposure time, the back exposure time required to build a floor of
a particular thickness also should decrease. For example, in a
conventional flexographic printing element manufacturing process
(i.e., in a vacuum and without oxygen), a 0.067 inch element will
have a floor thickness of about 0.029 inches. The back exposure
time required to build a floor of this thickness is typically about
15-60 seconds. For the same element processed with conventional DTP
technology (i.e., ablation in the presence of oxygen and where the
photocurable element is doped with oxygen scavengers), the back
exposure time required to build a floor of the same thickness is
typically about 1 to 5 seconds.
[0032] Forming a uniform floor with a back exposure time of less
than about 15 to 20 seconds is often very difficult primarily
because the fluorescent lamps that are used typically have a
significant variation in intensity across the bank of lights, and
often have a significant variation in intensity across any given
light in the bank due to variations in the filament. This
non-uniformity in the actinic radiation intensity translates
directly to non-uniformity of the floor build-up during back
exposure. If the back exposure times are too short, as observed
with plates processed with conventional DTP technology, this
problem is more severe. If the times are longer then the problem is
less pronounced. A non-uniform floor build-up results in
non-uniform printing because printing presses typically are
adjusted for a certain relief. Those areas having shallower relief
will print bold. Those having deeper relief may print with poor
quality and distortion. As described herein, a modification to the
support, or backing, layer, will allow printers to better control
floor-formation in DTP technology.
[0033] The support layer 12 of the photocurable element is
preferably formed from a variety of flexible, transparent
materials. Examples of such materials are cellulose films, or
plastics such as, for example, PET (polyetheylene terephthalate),
PEN (polyethylene naphthalate), polyether, polyethylene, polyamide
(Kevlar) or nylon. Preferably, the support layer is formed from
polyethylene terephthalate (PET). The support layer can be from
about 0.001 to about 0.010 inches thick. Preferably, the support
layer is about 0.005 inches thick. More preferably, if the support
layer is a polyester film such as, for example, PET, the support
layer is typically about 0.005" for 0.067" and thicker plates.
[0034] According to the present invention, the support layer 12 is
UV-absorbing to counter the increased photo-speed that results from
the use of oxygen scavengers or other means used to counter the
effects of oxygen inhibition in DTP technology. As described in
detail below, this can be accomplished either by forming the
support layer 12 from a material that is inherently UV-absorbing,
i.e., attenuates actinic radiation itself, or by adding a dopant to
the material forming the support layer 12.
[0035] In one embodiment of the present invention, the support
layer 12 is formed from a material that is inherently UV-absorbing.
Of the above-mentioned materials that are preferably used to form
the support layer, only PEN (for example, Kaladex 1030 and Kaladex
2000 commercially available from DuPont PET, Hopewell, Va.) is
inherently UV absorbing. The inventors have found that, when an
inherently UV-absorbing support layer is used, the percent of
actinic radiation that is absorbed is a function of the thickness
of the support layer. The inventor has found that, for example, a
PEN support layer having a thickness of about 5 mils absorbs about
97 percent of actinic radiation; a PEN support layer having a
thickness of about 3 mils absorbs about 95 percent of actinic
radiation.
[0036] According to another embodiment of the present invention,
the support layer 12 comprises a UV-absorbing material to counter
the increased photo-speed that results from the use of oxygen
scavengers or other means to counter the effects of oxygen
inhibition in DTP technology. This can be accomplished by adding a
UV-absorbing dopant to the support layer during manufacture.
[0037] Transparent materials that are not inherently UV-absorbing
need to be doped with a UV-absorber when made into the support
layer 12 according to the present invention. The UV-absorbing
dopant should be uniformly distributed throughout the support layer
12. This can be accomplished if, for example, the UV-absorbing
material is soluble in the support layer or evenly dispersed
throughout during the process of manufacturing the support layer
12. As used herein, the term "soluble" refers to the capability of
one compound of being dissolved. The term "dispersed" refers to one
substance being evenly distributed throughout another. In
commercial processes, a uniform distribution of the dopant
throughout the support layer 12 is typically achieved during the
manufacturing process as the PET, for example, is stretched both in
the transverse and machine directions so that the UV absorber is
distributed uniformly throughout the PET.
[0038] The commercially available UV absorbing PET products known
to the inventor are Melinex 943 (DuPont PET, Hopewell, Va.), Skyrol
Polyester Type TU84B (SKC LTD, Suwon, S. Korea), Teijin Teonex Type
Q51 (Teijin, Japan), and Eastman PET 9921 G0071 (Eastman Chemicals,
Kingsport, Tenn.).
[0039] The spectral range of the flood-exposure lamps used in most
applications is about 300-400 nm. Therefore the UV absorbing dopant
typically should be active in this range.
[0040] Preferably, the presence of the UV absorber changes a
normally UV transparent support layer into an attenuation tool that
absorbs at least a portion of UV radiation that passes through it.
Preferably, the support absorbs between about 80 to about 99%, more
preferably between about 85 to about 95%, and most preferably about
88% of actinic radiation.
[0041] The intensity of flood exposure lamps used in the curing of
flexographic printing plates is typically in the range of about
5-25 milliwatts/cm.sup.2, but intensities can be as high as 50
milliwatts/cm.sup.2. Therefore, the support layer should be capable
of absorbing irradiated light of such intensities from the UV flood
lamps.
[0042] The photoablative mask layer 18 can be any photoablative
mask layer known in the art. Such mask layers include those that
can be ablated by any type of laser known to those skilled in the
art such as, for example, UV-type Eximer lasers typically operating
at wavelengths of about 300 to 400 nm; IR-type lasers such as, for
example, CO.sub.2 lasers typically operating at a wavelength of
about 10,640 nm; Nd-YAG lasers typically operating at a wavelength
of about 1064 nm; and diode array lasers typically operating at a
wavelength of about 830 nm. Examples of such photoablative mask
layers are disclosed in, for example, U.S. Pat. No. 5,925,500 to
Yang, et al., herein incorporated by reference, which discloses
slip films modified with a UV absorber as the mask layer, thus
employing a laser to selectively ablate the modified slip film; and
U.S. Pat. No. 5,262,275 to Fan, herein incorporated by
reference.
[0043] Generally, the methods of the invention involve transferring
an image to the surface of the photocurable elements 10 without the
use of phototools or photomasks such that both the front and back
exposure times are economically efficient for the manufacturer of a
printing plate. This typically is accomplished by providing at
least one solid photocurable element 10. The solid photocurable
element comprises at least one solid photocurable material 14,16, a
solid photopolymerizable cap layer 17 (if applicable), a
photoablative mask layer 18, and a support layer 12 having an
actinic radiation absorbing compound integrated uniformly
throughout such that it absorbs at least some actinic radiation
during exposure. According to the present invention, the
photocurable layers 14, 16 and also the cap layer (if used) contain
oxygen scavengers to counter the longer exposure times that occur
as a result of oxygen inhibition.
[0044] When the solid photocurable printing element is to be used,
a laser is employed to selectively ablate, or remove, the
photoablative mask layer such that the areas where the
photoablative mask layer was ablated will cure, or harden, upon
exposure to the UV light and the areas where the photoablative mask
layer was not ablated will remain uncured. A floor is created in
the solid photocurable material by back exposure through the
UV-absorber-doped support layer. The uncured plate is then
front-exposed to UV light in the usual fashion effective to cure
the solid photocurable material. There are many devices that can be
used to perform this so-called "front" exposure of the photocurable
elements, including FLEXLIGHT.RTM. brand UV modules (Polyfibron
Technologies, Inc.), as well as those manufactured by Anderson
& Vreeland (Bryan, Ohio).
[0045] Following front exposure of the exposed areas of the
photopolymer, uncured photopolymer, i.e., the photopolymer under
the areas of the photoablative layer that were not laser-ablated,
is removed from the mounted photocurable elements, typically by
washing the elements with (and/or in) an organic and/or aqueous
solvent in which the photocurable material is at least somewhat
soluble. This solvent wash step typically is accompanied or
preceded by brushing, wiping, or some other mild, non-destructive
abrasion of the elements. Useful washing devices include those
commercially available from Polyfibron Technologies and Anderson
& Vreeland.
[0046] Additional objects, advantages, and novel features of this
invention will become apparent to those skilled in the art upon
examination of the following examples thereof, which are not
intended to be limiting.
EXAMPLES
Example 1
Preparation of an Uncapped Flexo DTP Plate
[0047] A Flex-Light Atlas 0.067" plate (Polyfibron Technologies,
Inc., Atlanta, Ga.) was modified in the following way: The slip
film was carefully removed using isopropanol. The plate was then
dried and then laminated with a carbon black (CB) based mask. The
CB mask comprised of a) CB pigment, b) a binder for its film
forming property and c) a self-oxidizing binder to increase
sensitivity to ablation.
[0048] After lamination, the coversheet was removed and discarded.
The plate was mounted on a commercially available flexo
plate-setter such as Misomex's OmniSetter 4000 or Creo's Thermoflex
5280. The digital file from the computer was transferred onto the
CB mask through an ablative mechanism. The plate was then back
exposed for 17 seconds, face exposed for 18 minutes to hold 1% dots
at 133 LPI. The face-exposure time of 18 minutes was deemed to be
too long for an uncapped DTP plate.
Example 2
Preparation of an Uncapped Flexo DTP Plate with Lower Face-exposure
Times
[0049] A FLEX-LIGHT ATLAS.RTM. 0.067 inch plate was doped with 0.1%
triphenylphosphine. Plate construction for DTP application was same
as in Example 1. However, the following back exposure and face
exposure times had to be used to hold the same level of detail:
[0050] Back Exposure: 3 to 5 seconds
[0051] Face Exposure: 5 minutes
[0052] Although, the face exposure times were acceptable, the back
exposure time was deemed too short for a 0.067" flexo plate. Floor
build-up was found uneven with such short exposure times.
Example 3
Construction of an Uncapped Flexo DTP Plate Using
Polyethylenenaphthalate Backing
[0053] The doped FLEX-LIGHT ATLAS.RTM. 0.067 inch plate of example
2 was next constructed with polyethylene naphthalate (PEN), an
inherently UV absorbing backing material as the UV-absorbing
backing layer. The PEN was 5 mil Kaladex 1030 commercially
available from DuPont PET (Hopewell, Va.). The rest of the plate
construction, laser imaging, and plate processing conditions were
identical to the plate used in Example 2. In this example, the UV
absorbing backing allowed reasonable back exposure times. The back
exposure was now 120 seconds. The face exposure was still 5
minutes. The floor of this plate was very even. Although the back
exposure times were acceptable for an uncapped DTP, it was desired
to achieve back exposure times normally used for 067 plates, around
15-30 seconds. This was accomplished as described in the next
example.
Example 4
Final Construction of an Uncapped Flexo DTP Plate.
[0054] The doped FLEX-LIGHT ATLAS.RTM. 0.067 inch plate of example
2 was next constructed with a UV absorbing PET commercially
available from DuPont Polyester (Hopewell, Va.). The UV-absorbing
PET was 500 gage Melinex 943. The rest of the plate construction,
laser imaging, and plate processing conditions were identical to
the plate used in Example 2. However, in this case the UV absorbing
PET allowed reasonable back exposure times. The back exposure was
now 20-22 seconds. The face exposure was still 5 minutes. The floor
of this plate was very even. Thus, the times were acceptable for an
uncapped DTP.
Example 5
Preparation of a Capped Flexo DTP Plate
[0055] The slip film of a Flexlight EPIC 0.067" plate (Polyfibron
Technologies, Inc., Atlanta, Ga.) was carefully removed leaving the
green cap on the photopolymer base. A carbon black based mask on a
coversheet was then laminated onto the dried plate so that the cap
was now in intimate contact with the mask. The coversheet was
removed and discarded. The plate was mounted on a commercially
available flexo plate-setter such as Misomex's OmniSetter 4000 or
Creo's Thermoflex 5280 and laser imaged. The laser removed the mask
in selective regions. Hence, the digital file from the computer was
transferred onto the CB mask through an ablative mechanism.
[0056] The plate was then back-exposed for 20 seconds, face exposed
for 60 minutes to hold 1% dots at 133 LPI. The face exposure time
of 60 minutes was deemed too long for a capped plate.
Example 6
Preparation of a Capped Flexo DTP Plate with Lower Face-exposure
Times
[0057] The Flexlight EPIC 0.067" plate of example 5 (both the
photo-polymer as well as the cap) was doped with 0.1%
triphenylphosphine (TPP), a known oxygen scavenger. It was
necessary to keep the same level of TPP in both the cap as well as
the underlying photopolymer.
[0058] The "doped" photopolymer was extruded onto the "doped" cap.
The CB mask was laminated on the green cap. The rest of the imaging
and processing steps were as described in Example 5. Here, however,
the exposure times were dramatically different as shown below:
[0059] Back exposure: 3 to 5 seconds
[0060] Face exposure: 15 minutes
[0061] The above exposure, times were required to hold the same
level of detail and a similar floor thickness as the plate from
Example 5. Although now the face exposure was deemed acceptable for
a capped plate, the back exposure resulted in uneven floor
thickness. Thus, the back exposure times were too short to get a
consistent floor.
Example 7
Final Construction of a Capped DTP Flexo Plate
[0062] As seen from Example 6, the capped DTP flexo plate met all
possible requirements except for the back exposure time, which was
too short. To increase the back exposure times it was necessary to
use a UV absorbing PET commercially available from DuPont PET
called Melinex 943 (500 gage). The rest of the capped plate
construction, i.e., the "doped" photopolymer, the "doped" cap, and
the CB mask, was identical to the plate from Example 6. The laser
imaging and subsequent processing steps (except for the BEX time)
were also identical. The UV absorbing PET yielded a reasonable back
exposure of 20-25 seconds. The face exposure was still 15 minutes.
The floor of the plate was very consistent. Thus, the exposure
times and all process conditions were acceptable for a capped DTP
plate.
[0063] Those skilled in the art will appreciate that numerous
changes and modifications may be made to the preferred embodiments
of the invention and that such changes and modifications may be
made without departing from the spirit of the invention. It is
therefore intended that the appended claims cover all such
equivalent variations as fall within the true spirit and scope of
the invention.
* * * * *