U.S. patent application number 13/268196 was filed with the patent office on 2012-02-23 for method for laser engraving flexographic printing articles based on millable polyurethanes.
Invention is credited to Rustom S. Kanga.
Application Number | 20120043701 13/268196 |
Document ID | / |
Family ID | 41568958 |
Filed Date | 2012-02-23 |
United States Patent
Application |
20120043701 |
Kind Code |
A1 |
Kanga; Rustom S. |
February 23, 2012 |
METHOD FOR LASER ENGRAVING FLEXOGRAPHIC PRINTING ARTICLES BASED ON
MILLABLE POLYURETHANES
Abstract
A flexographic printing sleeve or plate is made by a method that
includes providing a millable polyurethane, crosslinking the
millable polyurethane, and forming a relief by at least laser
engraving the crosslinked millable polyurethane. For example,
crosslinking may be accomplished by a peroxide-based process or by
a vulcanization process using sulfur. A relief in one example is
formed by extruding the millable polyurethane, thermally
crosslinking the polyurethane after the extrusion step and laser
engraving the crosslinked millable polyurethane. A printing article
is formed into the shape of a flat printing plate or a continuous
in-the-round printing sleeve.
Inventors: |
Kanga; Rustom S.; (Marietta,
GA) |
Family ID: |
41568958 |
Appl. No.: |
13/268196 |
Filed: |
October 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12356330 |
Jan 20, 2009 |
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13268196 |
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11813612 |
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PCT/US07/77246 |
Aug 30, 2007 |
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12356330 |
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61083327 |
Jul 24, 2008 |
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61083327 |
Jul 24, 2008 |
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Current U.S.
Class: |
264/400 |
Current CPC
Class: |
B41C 1/18 20130101; B41C
1/05 20130101; B41N 1/12 20130101; B41N 1/22 20130101 |
Class at
Publication: |
264/400 |
International
Class: |
B29C 35/08 20060101
B29C035/08 |
Claims
1. A method of making a flexographic printing article comprising:
(a) providing a millable polyurethane; (b) thermally crosslinking
the millable polyurethane to provide a laser-engravable element;
and (c) forming a relief in the element by at least laser engraving
the crosslinked millable polyurethane.
2. A method according to claim 1, wherein crosslinking the millable
polyurethane includes the step of crosslinking by a process
selected from the group consisting of a peroxide-based process and
a vulcanization process using sulfur.
3. A method according to claim 1, further comprising the step of
adding a binder selected from the group consisting of a
polyester-based polyurethane processed as a millable polyurethane,
and a polyether-based polyurethane processed as a millable
polyurethane.
4. A method according to claim 1, wherein the step of forming a
relief in the element includes the step of engraving the element
using laser radiation having a wavelength between approximately 830
nanometers and approximately 1100 nanometers.
5. A method according to claim 1, wherein the step of forming a
relief in the element includes the step of engraving the element
using laser radiation having a wavelength between approximately 830
nanometers and approximately 10,600 nanometers.
6. A method according to claim 1, wherein the step of forming a
relief includes the step of extruding the millable polyurethane
into an article selected from the group consisting of a flat
printing plate and a continuous in-the-round printing sleeve.
7. A method according to claim 1, further comprising the step of
adding an additive for increasing laser absorptivity of the
element, wherein the additive is selected from the group consisting
of nanomaterials, mica, carbon black, kaolin clay, antimony tin
oxide, and copper oxide.
8. A method according to claim 1, further comprising the step of
adding an additive for increasing heat dissipation in the element,
wherein the additive is selected from the group consisting of
metal-based nanoparticles, metal-oxide based nanoparticles,
carbotherm boron nitride platelets, carbon black, and graphite.
9. A method according to claim 1, further comprising the step of
adding an additive for reducing the density of the element, wherein
the additive is selected from the group consisting of microspheres,
boroscilicate glass bubbles, spherical porous silica, crosslinked
microspheres, and unexpanded microspheres containing liquid
hydrocarbon.
10. A method according to claim 1, further comprising the step of
adding a burn-rate modifier for decreasing the pyrolysis
temperature of the element, wherein the additive is selected from
the group consisting of ammonium perchlorate, ammonium nitrate,
potassium nitrate, iron oxide, copper oxide, copper chromate,
chrome oxide, manganese oxide, ferrocene, aluminum, boron,
magnesium powder, oxetane group energetic thermoplastic elastomers,
and azide group energetic thermoplastic elastomers.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority and is a
Divisional of U.S. Non-Provisional Application Patent Application
Ser. No. 12/356,330, filed Jan. 20, 2009, by Kanga and entitled
"Laser Engravable Flexographic Printing Articles Based on Millable
Polyurethanes, and Method". The Non-Provisional Patent Application
Ser. No. 12/356,330 is a Continuation-in-Part (CIP) of U.S.
Non-Provisional Patent Application Ser. No. 11/813,612, filed Jul.
10, 2007, by Kanga and entitled "Laser Engravable Flexographic
Printing Article." The Non-Provisional Patent Application Ser. No.
11/813,612 is a U.S. national stage application of International
Application No. PCT/US07/72246, filed Jun. 27, 2007, by Kanga and
entitled "Laser Engraveable Flexographic Printing Article." The
International Application No. PCT/US2007/072246 claims the benefit
of priority of U.S. Provisional Patent Application No. 60/816,786,
filed Jul. 27, 2006, by Kanga. This application also claims
priority to U.S. Provisional Patent Application No. 61/083,327,
filed Jul. 24, 2008, by Kanga and entitled "Laser Engravable
Flexographic Printing Article Based on Millable Polyurethanes." The
contents of the above-referenced applications are incorporated by
reference herein.
TECHNICAL FIELD OF THE INVENTION
[0002] The invention relates to an article for use in flexographic
printing, such as a plate or sleeve, and a method for laser
engraving the printing article to form a relief such that the
article can be used in flexographic printing. The present invention
also provides a method of crosslinking a Polyurethane Elastomer for
making a directly laser engravable flexographic printing article by
the use of commercially available Millable Polyurethanes (MPU). The
printing article could be either a flat printing plate or a
continuous in-the-round printing sleeve. Commercially available
MPUs can be compounded either in an extruder or a compounder such
as a Brabender using various crosslinking and laser sensitive
additives. The compounded MPU is then extruded either on a flat
carrier or a round sleeve, crosslinked either during extrusion or
thereafter using thermal energy. The extruded and crosslinked MPU
is ground or machined to the dimension required for the printing
process and is ready for laser engraving. In one embodiment of the
invention, the article does not require further processing, and as
such can be used in a "direct-to-plate" laser engraving system.
BACKGROUND OF THE INVENTION
[0003] Printing plates are well known for use in flexographic
printing, particularly on surfaces which are corrugated or smooth,
such as packaging materials like cardboard, plastic films, etc.
Typically, flexographic printing plates are manufactured using
photopolymers which are exposed through a negative, processed using
a solvent to remove the non-crosslinked areas to create a relief,
which is post-crosslinked and detackified. This is typically a very
lengthy and involved process. Recently, flexographic plates have
been manufactured using digital imaging of an in situ mask layer
which obviates the need for a negative or a photomask to make the
plate, and which has other performance benefits as well.
[0004] Recently, it has been possible to laser engrave a rubber
element directly to provide the desired relief surface necessary
for flexographic printing. Laser engraving has provided a wide
variety of opportunities for rubber printing plates. Highly
concentrated and controllable energy lasers can engrave very fine
details in rubber. The relief of the printing plate can be varied
in many ways. Very steep as well as gently decreasing relief slopes
can be engraved so as to influence the dot gain of such plates.
Ethylene propylene diene monomer (EPDM) rubber can be laser
engraved to form flexographic printing plates.
[0005] The directly engraved type of flexographic printing plate is
made from vulcanized rubber. Commercial rubbers can be natural or
synthetic, such as EPDM elastomers. Lasers can develop sufficient
power densities to ablate certain materials. For example,
high-power carbon dioxide (CO.sub.2) lasers can ablate many
materials such as wood, plastic and rubber and even metals and
ceramics. Once the output from a laser is focused at a particular
point on a substrate with a suitable power density, it is possible
to remove material to a desired depth to create a relief. Areas not
struck by the laser beam are not removed. Thus, the use of the
laser offers the potential of producing very intricate engravings
in a desired material with substantial savings.
[0006] U.S. Pat. No. 3,459,733 to Caddell describes a method for
producing polymer printing plates. The printing plate is made by
exposing a layer of the polymeric material to a controlled laser
beam of sufficient intensity to ablate the polymer and form
depressions on the surface.
[0007] U.S. Pat. Nos. 5,798,202 and 5,804,353 to Cushner et al.
disclose processes for making a flexographic printing plate by
laser engraving a reinforced elastomeric layer on a flexible
support. The process disclosed in U.S. Pat. No. 5,798,202 involves
first reinforcing and then laser engraving a single-layer
flexographic printing element having a reinforced elastomeric layer
on a flexible support. The elastomeric layer may be reinforced
mechanically, thermochemically, photochemically or with
combinations of these processes. Mechanical reinforcement is
provided by incorporating reinforcing agents, such as finely
divided particulate material, into the elastomeric layer.
Photochemical reinforcement is accomplished by incorporating
photohardenable materials into the elastomeric layer and exposing
the layer to actinic radiation. Photohardenable materials include
photo-crosslinkable and photo-polymerizable systems having a
photo-initiator or photo-initiator system.
[0008] The process disclosed in U.S. Pat. No. 5,804,353 is similar
to U.S. Pat. No. 5,798,202, except that the process involves
reinforcing and laser engraving a multilayer flexographic printing
element having a reinforced elastomeric top layer, and an
intermediate elastomeric layer on a flexible support. The
elastomeric layer is reinforced mechanically, thermochemically,
photochemically or combinations thereof. Mechanical and
photochemical reinforcement is accomplished in the same manner as
described by U.S. Pat. No. 5,798,202. The intermediate elastomeric
layer may be reinforced as well.
[0009] A problem associated with elastomeric elements that are
reinforced both mechanically and photochemically is that laser
engraving does not efficiently remove the elastomeric material to
provide desired relief quality, and ultimately, printing quality.
It is desirable to use an additive in the elastomeric layer that is
sensitive to infrared light in ordeff to enhance the engraving
efficiency of the element. Photo-chemically reinforcing the element
provides the desired properties for engraving as well as in its
end-use as a printing plate. However, the presence of the additive
as particulate or other absorbing material tends to reduce the
penetration of the ultraviolet radiation required to
photo-chemically reinforce the element. If the elastomeric layer is
insufficiently crosslinked during photochemical reinforcement, the
laser radiation cannot effectively remove the material and poor
relief quality of the engraved area results. Further, the debris
resulting from laser engraving tends to be tacky and difficult to
completely remove from the engraved element. Additionally, if the
element is not sufficiently photo-chemically reinforced, the
required end-use properties as a printing plate are not properly
achieved. These problems tend to be exacerbated with increasing
concentration of the additive that enhances engraving efficacy.
[0010] U.S. Pat. No. 6,627,385 teaches the use of graft copolymers
for laser engraving. U.S. Pat. No. 6,511,784, U.S. Pat. No.
6,737,216 and U.S. Pat. No. 6,935,236 teach the use of elastomeric
copolymers for laser engraving using various infrared (IR)
additives.
[0011] Many patents in the field teach the use of typical styrenic
thermoplastic elastomers (TPEs) that have been used for
photo-crosslinking applications. One problem associated with these
non-polar TPEs is that they have limited sensitivity to laser
engraving because of their hydrocarbon backbone nature. The use of
polar TPEs such as thermoplastic polyurethanes (TPUs) thermoplastic
polyester elastomers (TPPE) and thermoplastic polyamide elastomers
(TPAE) as both laser engravable systems and as printing elements
would be desirable. However, most of the above polar TPEs on the
market would not be effective either as laser engravable systems,
or as printing plates because they are not crosslinked.
[0012] The crosslinking of the above TPEs and especially TPUs has
not been done before in flexography, and thus, TPUs have not been
used in flexography. However, polyurethanes for flexography have
been well known, particularly for liquid photopolymers. By
definition, a TPU is solid at room temperature and can be extruded,
and is workable at higher temperatures. This characteristic is due
to the presence of hard and soft segments that form a network at
room temperature, and is thus solid.
[0013] This network structure also differentiates TPUs from
traditional polyurethanes in its outstanding physical attributes
and thus offers an attractive system to be used in flexo
applications. However, most elastomers used in Flexo need to be
crosslinked to withstand the rigors of the printing process and to
minimize swells in the inks used for printing. Additionally, the
elastomers used in laser engraving have to be crosslinked.
Traditional flexo photopolymers have unsaturation in the backbone,
which allows the crosslinking with acrylate monomers and UV
photo-initiators. The TPUs on the market today do not have
unsaturation. Hence, the difficulty in UV crosslinking these for
flexo applications. Additionally, laser engraving of elastomers
with lasers lasing in the Near IR wavelengths need to be doped with
highly absorptive laser additives. This does not allow UV
crosslinking as a viable option to crosslink such elastomers.
Thermal crosslinking or vulcanization is the only feasible approach
in such applications. Millable Polyurethanes (MPUs) are a special
category of TPUs. Millable Polyurethanes, as the name suggests,
could be processed in the same way as rubber elastomers, including
the use of compounding and extrusion methods. MPUs can be thermally
crosslinked in a subsequent crosslink and post-crosslink step.
SUMMARY OF THE INVENTION
[0014] Therefore, an object of the present invention is to provide
a method for making a laser engravable flexographic printing
article.
[0015] Another object of the present invention is to provide a
reliable method for making a printing plate from crosslinking of
Millable Polyurethanes (MPUs).
[0016] These and other objects of the present invention can be
achieved in the preferred embodiments of the invention described
below.
[0017] One preferred embodiment of the invention includes a method
for making a flexographic printing article including the steps of
providing a millable polyurethane, and crosslinking the
polyurethane whereby the article can be used in a direct laser
engraving flexographic process.
[0018] According to another preferred embodiment of the invention,
the crosslinked millable polyurethane can be used in the direct
laser engraving flexographic process and in flexographic printing
without further processing.
[0019] According to another preferred embodiment of the invention,
the printing article is laser engraved by infrared laser radiation
to form a relief such that the article can be used in flexographic
printing.
[0020] According to another preferred embodiment of the invention,
the printing article can be a plate or a sleeve.
[0021] According to another preferred embodiment of the invention,
the binder is a high performance polyester-based polyurethane
processed as a millable polyurethane.
[0022] According to another preferred embodiment of the invention,
the binder is a high performance polyether-based polyurethane
processed as a millable polyurethane.
[0023] According to another preferred embodiment of the invention,
the millable polyurethane is extruded and thermally crosslinked
during extrusion.
[0024] According to another preferred embodiment of the invention,
the millable polyurethane is compounded in a compounder and
thermally crosslinked in a hot press.
[0025] According to another preferred embodiment of the invention,
the millable polyurethane is milled on a 2-roll mill and thermally
crosslinked in a hot press.
[0026] According to another preferred embodiment of the invention,
at least one crosslinking additive for inducing the thermal
crosslinking of the millable polyurethane is provided.
[0027] According to another preferred embodiment of the invention,
at least one laser additive comprising such as carbon black, kaolin
clay, mica, antimony tin oxide, or copper oxide is provided.
[0028] According to another preferred embodiment of the invention,
the millable polyurethane is thermally crosslinked after
extrusion.
[0029] According to another preferred embodiment of the invention,
the millable polyurethane is crosslinked for about 15-30 minutes at
about 240 to 350.degree. F., and the polyurethane is crosslinked
during the crosslinking.
[0030] According to another preferred embodiment of the invention,
the mllable polyurethane is post-crosslinked for about 8 to 12
hours at about 180-240.degree. F.
[0031] According to another preferred embodiment of the invention,
the millable polyurethane is crosslinked during crosslinking with
electron beam radiation.
[0032] According to another preferred embodiment of the invention,
the printing article is hot-pressed to a desired dimension.
[0033] According to another preferred embodiment of the invention,
the printing article is machined to a desired dimension.
[0034] According to another preferred embodiment of the invention,
the binder is millable polyurethane/rubber blend.
[0035] According to another preferred embodiment of the invention,
the binder is millable polyurethane/Energetic TPE blend.
[0036] According to another preferred embodiment of the invention,
at least one additive for dissipating heat such as metal-based
nanoparticles and/or metal oxide based nanoparticles or combination
of Graphite/Carbon Black pigment are provided.
[0037] According to another preferred embodiment of the invention,
at least one burn-rate modifier for increasing the rate of mass
transfer during laser engraving such as oxidizers, burn rate
catalysts such as Iron Oxides, Copper oxides, Copper Chromates or
burn rate accelerators such as nano aluminum, boron and magnesium
powders are provided.
[0038] According to another preferred embodiment of the invention,
microspheres for decreasing the density of the millable
polyurethane and increasing the rate of mass transfer during laser
engraving of the article are provided.
[0039] According to another preferred embodiment of the invention,
a method for laser engraving a flexographic printing article
includes the steps of providing a millable polyurethane,
crosslinking the polyurethane to form a laser engravable article,
machined to precise dimension and laser engraving the article to
form a relief such that the article can be used in flexographic
printing.
[0040] According to another preferred embodiment of the invention,
the article is engraved with a far infrared radiation laser, such
as a carbon dioxide laser (10,600 NM).
[0041] According to another preferred embodiment of the invention,
the article is engraved with a near infrared radiation laser, such
as a Yttrium-based fiber laser (1100 NM), a neodymium doped yttrium
aluminum garnet (ND-YAG) laser (1060 NM) and/or a diode array laser
(830 NM).
[0042] According to another preferred embodiment of the invention,
a method for making a flexographic printing article includes the
steps of providing a binder such as a thermoplastic elastomer from
a millable polyurethane system crosslinking the polyurethane such
that the article can be used in a direct laser engraving
flexographic process and in flexographic printing without further
processing.
[0043] In at least one embodiment of the invention, a method of
making a flexographic printing article includes providing a
millable polyurethane, crosslinking the millable polyurethane to
provide a laser-engravable element, and forming a relief in the
element by at least laser engraving the crosslinked millable
polyurethane. In at least one example, the millable polyurethane is
crosslinked by a peroxide-based process. In at least one other
example, the millable polyurethane is crosslinked by a
vulcanization process using sulfur. The relief may be formed by
lasing the element using laser radiation having a wavelength
between approximately 830 nanometers and approximately 10,600
nanometers, for example the wavelength may be between approximately
830 nanometers and approximately 1100 nanometers. In at least one
example, the article is formed as a flat printing plate, and in
another example, the article is formed as a continuous in-the-round
printing sleeve.
[0044] An additive may be added for increasing laser absorptivity
of the element. For example, an additive may be selected from
nanomaterials, mica, carbon black, kaolin clay, antimony tin oxide,
and copper oxide.
[0045] An additive may be added for increasing heat dissipation in
the element. For example, an additive may be selected from
metal-based nanoparticles, metal-oxide based nanoparticles,
carbotherm boron nitride platelets, carbon black, and graphite.
[0046] An additive may be added for reducing density of the
element. For example, an additive may be selected from
microspheres, borosilicate glass bubbles, spherical porous silica,
crosslinked microspheres, and unexpanded microspheres containing
liquid hydrocarbon.
[0047] An additive may be added for decreasing the pyrolysis
temperature of the element. For example, an additive may be
selected from ammonium perchlorate, ammonium nitrate, potassium
nitrate, iron oxide, copper oxide, copper chromate, chrome oxide,
manganese oxide, ferrocene, aluminum, boron, magnesium powder,
oxetane group energetic thermoplastic elastomers, and azide group
energetic thermoplastic elastomers.
[0048] In another embodiment of the invention, a flexographic
printing article includes a substrate, and an outer layer of a
laser-engravable cross-linked millable polyurethane applied to the
substrate. The outer layer may be crosslinked, for example, by a
peroxide-based process, or by a vulcanization process using sulfur.
The outer layer may be absorptive of laser radiation having a
wavelength between approximately 830 nanometers and approximately
10,600 nanometers, for example the wavelength may be between
approximately 830 nanometers and approximately 1100 nanometers. In
at least one example, the article is formed as a flat printing
plate, and in another example, the article is formed as a
continuous in-the-round printing sleeve.
[0049] The outer layer may include an additive for increasing laser
absorptivity of the element. For example, an additive may be
selected from nanomaterials, mica, carbon black, kaolin clay,
antimony tin oxide, and copper oxide.
[0050] The outer layer may include an additive for increasing heat
dissipation in the element. For example, an additive may be
selected from metal-based nanoparticles, metal-oxide based
nanoparticles, carbotherm boron nitride platelets, carbon black,
and graphite.
[0051] The outer layer may include an additive for reducing density
of the element. For example, an additive may be selected from
microspheres, borosilicate glass bubbles, spherical porous silica,
crosslinked microspheres, and unexpanded microspheres containing
liquid hydrocarbon.
[0052] The outer layer may include an additive for decreasing the
pyrolysis temperature of the element. For example, an additive may
be selected from ammonium perchlorate, ammonium nitrate, potassium
nitrate, iron oxide, copper oxide, copper chromate, chrome oxide,
manganese oxide, ferrocene, aluminum, boron, magnesium powder,
oxetane group energetic thermoplastic elastomers, and azide group
energetic thermoplastic elastomers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] The subject matter that is regarded as the invention may be
best understood by reference to the following description taken in
conjunction with the accompanying drawing figures in which:
[0054] FIG. 1 provides Table 1, which lists tested MPU and MPU
blends;
[0055] FIG. 2 provides Table 2, which lists typical flexographic
printing plate physicals;
[0056] FIG. 3 provides Table 3, which lists samples used in
engraving tests with an Yttrium-based laser;
[0057] FIG. 4 provides Table 4, which lists process conditions, and
physical properties of the sample sets of Table 3;
[0058] FIG. 5 provides Table 5, which lists results of the
Yttrium-based laser engraving test of the sample sets of Table
3;
[0059] FIG. 6 provides Table 6, which lists test results of an
engraving test on MPU and MPU/Rubber blends using a CO.sub.2
laser;
[0060] FIG. 7 provides Table 7, which lists physical properties and
test results of cast polyurethanes used for an engraving test using
a CO.sub.2 laser;
[0061] FIG. 8 provides Table 8, which lists PHR values for thermal
crosslinking of TPUs during extrusion;
[0062] FIG. 9 provides Table 9, which lists a formulation for
crosslinking an MPU by peroxide and sulfur cure systems;
[0063] FIG. 10 is a graph for illustrating the theoretical concept
of balancing the physical properties and laser sensitivity;
[0064] FIGS. 11A-11F provide digital photographic images from an
engraving test on MPU and MPU/Rubber blends using an Yttrium-base
fiber laser; and
[0065] FIG. 12 is a perspective view of an engraving article.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND BEST MODE
[0066] According to a preferred embodiment of the invention, laser
engraving provides a true "direct-to-plate" technology for
flexography. The method is applied and practiced without the need
for complicated processing steps during manufacturing, resulting in
a substantial gain in productivity from laser engraving. Also, the
plates are relatively inexpensive to manufacture, obviating the
need for a sophisticated mask coating as is needed for digitally
imaged plates. Recently, there has been a decrease in flexo reliefs
with the use of thin plates (.about.45 mil) becoming more common.
This trend is very attractive and well-suited for the laser
engraving of flexo plates.
[0067] However, for laser engraving plates in the market thus far,
the image fidelity is not as good as current digitally imaged
(laser ablation of a mask) or even conventional flexo plates. This
relegates laser engraving to a niche market. Additionally, the
productivity so far has not been good. Thus, there is a market need
to improve both the two main deficiencies of engraving, compared
with mask ablation-image quality and plate making productivity.
[0068] A laser engraving article according to a preferred
embodiment of the invention comprises a flat engravable plate which
is mounted on a round cylinder during the printing step, or a
continuous "in the round" engravable sleeve. Either system
comprises a carrier on which there may be one or more binder layers
that are laser engravable.
[0069] The carrier for the laser engraving article depends on the
end product. For the flat plates a heat stabilized polyethylene
terephthalate (PET) of 5-7 mils thickness is preferred. The PET may
be corona treated to improve adhesion, and may also be primer and
adhesive coated.
[0070] For the sleeves the carrier may be a metal sleeve, typically
nickel based or a composite sleeve. The sleeve is further primer
and/or adhesive coated for improved adhesion. Often, the sleeve is
further coated with a polyurethane foam which acts as the in situ
cushion layer.
[0071] The choice of binder system for the engraving system is
governed by a combination of its performance as a printing plate
and sensitivity to or behavior in laser engraving. It is believed
that a crosslinked millable polyurethane elastomer would provide
the best performance attribute both for its printing performance
and as an engravable system.
Millable Polyurethanes
[0072] Polyurethane elastomers for direct laser engraving
applications can be divided in 3 broad classes:
[0073] 1) Thermoplastic Polyurethanes (TPU)
[0074] 2) Castable Polyurethanes (CPU) and,
[0075] 3) Millable Polyurethanes (MPU)
[0076] All of the above have been tested in direct engraving
applications but with very mixed results. Crosslinking of the
elastomer is a requirement for the material to be used as an
engravable elastomer. It was found that both the TPU and CPU
resulted in a direct laser engraving (DLE) system that was
unacceptable. The thermoplastic character in both resulted in
undesirable melting artifacts and unacceptable imaging. Only the
MPU showed acceptable engraving characteristics (clean engraving
without melting artifacts).
[0077] Usually, TPU elastomers are typically produced in a single
step with a slight excess of isocyanate (NCO). CPU elastomers are
made by reacting polyol with a surplus of isocyanate in order to be
in a liquid state during processing. Then, during final processing,
the material is mixed with chain extenders to reach stoichiometric
equivalence versus the combined OH number of polyol and chain
extender. MPU elastomers on the other hand are produced with a
final stoichiometric deficiency of isocyanates in order to obtain
the necessary millable state.
[0078] MPU rubbers can be classified in accordance to either the
chemical backbone or the type of vulcanization. As polyols, either
polytetramethylene ether glycol ethers based on
polytetrahydrofuran) or polyester. adipates (based on adipic acid
and diols like ethandiol, butanediol, methylpropanediol,
hexanediol, neopentylglycol, cyclohexanedimethanol, etc.) can be
used. The careful selection of diol/glycol. and the molar ratio of
glycol-blends influence the final properties of the MPU rubber. The
right molar ratio of glycol blends is also important. The
diisocyanate component is either aromatic diisocyanates like
methylene diphenyl diisocyanate (MDI) and toluene diisocyanate
(TDI) or aliphatic diisocyanate like dicyclohexylmethane
diisocyanates or TMXDI (tetramethylxylene diisocyanate), which is a
light stable isocyanate where the methylene groups separate the
isocyanate groups from the aromatic ring. Aromatic diisocyanates
provide excellent mechanical strength, whereas aliphatic
diisocyanates give better heat and hydrolysis resistance. Aliphatic
diisocyanates are also necessary if a light and color stable MPU is
intended to be produced. Chain extenders are of low molecular
weight like ethylene glycol, 1,4-butanediol, hydro quinone
bis(2-hydroxy-ethyl)ether, glycerolmonoallylether,
trimethylolpropane-monoallylether or water.
[0079] All polyurethane rubbers provide outstanding mechanical
strengths and a high chemical resistance. Generally, ether-based
polyurethane rubber provides excellent hydrolysis resistance, but
poorer heat resistance, while ester based polyurethanes are typical
for their outstanding oil and fuel resistance. Millable urethanes
are polymers that are known for their excellent abrasion and
strength properties, while being able to be processed on
conventional rubber equipment. Existing millable urethanes are
primarily used in applications that take advantage of these
properties.
[0080] Polyurethane rubber is a specialty rubber that finds use in
many common rubber articles such as skate wheels, conveyor belts,
rubber covered rolls and other applications where urethane is used
because of its properties. Urethane rubber compounds possess a
unique combination of excellent abrasion resistance, excellent
solvent and oil resistance, high tensile and tear properties, good
resistance to ozone and oxygen, and good low temperature
properties.
Peroxide Versus Sulfur Crosslinking of MPU
[0081] The vulcanization of polyurethane rubber leads to
crosslinking between molecular chains, which result in a network
structure. This resembles the concept of other vulcanized rubbers,
but compared to other polyurethane elastomers, to a smaller number
of urethane groups. These urethane groups form hydrogen bonds and
contribute substantially to improved mechanical strength. For this
reason, most polyurethane rubbers require the addition of active
fillers like carbon blacks or silicas, which reinforce polyurethane
rubbers in the same manner as with other rubbers. As will be seen
later, the function of the reinforcing filler (carbon black) is
also to increase the absorbance of the MPU to the lasing wavelength
and additionally act as thermally dissipative additive.
[0082] Sulfur vulcanization requires unsaturated components to be
built into the structure of polyurethane rubbers. This is done by
using OH functional compounds with a double bond as chain
extenders, for example glycerolmonoallylether (GAE) or
trimethylpropane-monoallylether (TMPMAE). Of all isocyanates, only
MDI hard segments are suitable co-reactants for peroxide
crosslinking. With MDI we get stabilized diphenylmethane radical
formation through the central methylene group which then results in
crosslinking of the MPU. MPUs based on other isocyanates, i.e.,
aliphatic isocyanates, require unsaturation for peroxide
crosslinking. Unlike sulfur vulcanization, only small amounts of
unsaturation are sufficient.
[0083] Peroxide crosslinking gives the best heat resistance and the
lowest compression set. On the other hand, Sulfur vulcanization
allows a wide processing flexibility. Overall, both peroxide and
sulfur crosslinks can be used very effectively to crosslink solid
urethane rubber compounds. Rubber compounding almost always
involves some compromise, and decisions on what properties are most
important are necessary to make educated decisions on ingredient
types and amounts to use.
Millable Polyurethanes and Blends
[0084] Since MPUs are processed similar to rubber compounds, they
could be blended together with rubber compounds such as Natural and
NBR rubbers. This gives the advantage of having blends with
synergistic properties. Blending MPU with rubber compounds also
allows value added benefits such as superior swell resistance, cost
reduction and other improvements in engraving speeds etc. A new
class of TPEs called Energetic TPEs (ETPE) could also be blended to
accelerate the burn rate during laser engraving.
Commercial Sources of Millable Polyurethanes and Blends
[0085] Millable Polyurethanes and their blends with Natural and NBR
rubbers are commercially available from TSE Industries in USA
(named Millathanes) and Rhein Chemie in Europe (named Urepan).
Table 1 (FIG. 1) lists the various backbone structures available,
either aliphatic, MDI or TDI or Polyester or Polyether based
polyurethanes.
Advantages of MPU for Laser Engraving Applications for Flexo
[0086] MPUs allow the use of main chain backbones not available
with TPUs, CPUs, and liquid photopolymers. This versatility allows
the choice of backbone chemistries having desirable attributes for
both laser engraving and for flexographic printing.
[0087] MPUs are commercially available and can be suitably
optimized with additives to function as a direct laser engravable
system. Commercially available manufacturing processes can be
efficiently utilized.
[0088] MPUs allow versatility of crosslinking with both peroxides
and sulfur. A crosslinked elastomer is a requirement for clean
engraving of fine details.
[0089] High resolution engraving of small details is entirely
possible using MPUs. Higher resolution and Screen Counts (5000 DPI,
200 LPI) are possible as compared to Rubber.
[0090] Further advantages achieved by using MPU in direct laser
engraving include:
[0091] Well known process for crosslinking or vulcanizing MPUs,
similar to rubber
[0092] Well known process for manufacture of plates and sleeves
from MPUs
[0093] MPUs allow formulatory versatility. Since MPUs are processed
similar to rubber elastomers, MPUs allow mixing and blending of
various additives typically required in Direct Laser Engraving with
ease
[0094] Outstanding Physical properties required for a Flexo
printing plate, yet better ink transfer and better print quality
than rubber elastomers
[0095] MPUs allow blending of other co-binders such as rubber, ETPE
etc to give other benefits such as cost savings and improvement in
engraving speeds etc
[0096] Millable Polyurethanes (MPU) Compared to Cast Polyurethanes
(CPU) and Thermoplastic Polyurethanes (TPU)
[0097] As seen from the above, MPU systems are ideal as a laser
engravable elastomer and also as a flexographic printing plate. A
comparison was made between MPU and CPU and TPU as a direct laser
engravable flexographic system. CPU is made into a crosslinked
thermoset during the manufacturing process. TPUs may be thermally
crosslinked as well. Both CPU and TPU showed properties acceptable
as a flexographic printing plate. However, both these systems did
not perform adequately as an engraving elastomer. Both these
elastomers showed severe melting artifacts from the engraving
process, which resulted in undefined and blurred images resulting
in unacceptable imprint quality. For the CPU the engraved residues
were liquid in character and very tacky to touch, which is
undesirable as well. The TPU also showed severe melting artifacts
during the engraving process.
[0098] It was surmised that although the CPU are crosslinked and
supposedly a thermoset in nature, there is still significant
thermoplastic character or perhaps low crosslink density resulting
in the heat from the laser undesirably melting rather than
resulting in clean and sharp ablation or engraving. A like
conclusion can be drawn for the crosslinked TPU. This is in
contrast to MPUs, which did not show such melting artifacts, and
showed a clean and sharp engraving profile.
Printing Plate/Sleeve
[0099] A preferred printing plate/sleeve has the approximate
physical properties as provided in Table 2 (FIG. 2) below. These
physical properties can serve as a guideline for a system behaving
as a printing plate. Other attributes such as ink transfer are not
reflected here. It is possible that systems having physical
characteristics outside these parameters may also behave as a
satisfactory printing plate. Many of these properties are
interlinked. Thus, a high Shore A implies a high Modulus by nature.
These physical characteristics can be easily measured on an Instron
and may be a good starting point to consider when designing an
elastomer for a laser engravable printing plate or sleeve. Another
characteristic to consider is compatibility with inks. Thus, it is
entirely possible to have more than one polymer system depending on
its end use.
Laser Engraving
[0100] For the binder to be efficient in laser engraving, the main
chain needs to have labile hetero bonds which have sensitivity at
the respective lasing wavelengths of the lasers in the field such
as carbon dioxide laser (10,600 NM) or a Yttrium-based fiber laser
(1100 NM) or a neodymium-doped yttrium aluminum garnet (ND-YAG)
laser (1060 NM) or a diode array laser (830 NM). , For the lasers
lasing in the near-IR regime such as Yttrium-based fiber, ND-YAG
and Diode Array lasers, doping the binder elastomer with pigments
such as carbon black is required to have absorption in that
wavelength range. This interaction allows conversion of laser
photons to heat efficiently and the elastomer is engraved when
exposed to a laser beam of adequate intensity. The layer is
preferably evaporated, or thermally or oxidatively decomposed in
the process without melting, so that its decomposition products are
removed from the layer in the form of hot gases, vapors, fumes or
small debris particles. The gaseous products of decomposition are
rapidly ejected from the surface at the speed of sound.
[0101] In general, thermoplastic elastomers (TPEs) based on Kraton
polymers currently used in typical printing plates and other carbon
based polymers such as polyolefins will not be efficient as
engravable binders. Hydrophilic polymers mentioned above, such as
polyurethanes, polyesterspolyamides, and polyvinylalcohol should
function adequately as an engravable system. In particular,
thermoplastic polyurethanes (TPUs) are suitable as laser engravable
systems. However, the TPUs need to be crosslinked before they can
adequately function both as a printing plate and as an engravable
system. Use of polyurethanes based on Millable Polyurethanes allows
both processing as TPUs and crosslinking similar to rubber.
[0102] Table 9 (FIG. 9) provides two formulations for crosslinking
a typical MPU utilizing peroxide (Sample A) and sulfur (Sample B)
as the crosslinking agent.
[0103] The final process for flexographic sleeves is as
follows:
[0104] 1. Formulation of MPUs with the curatives, co-agent, laser
additives, and other additives was carried out in a compounder such
as a Brabender. A 2-Roll mill may also be used in this step.
[0105] 2. The compounded system is then extruded in a single screw
extruder directly on the composite or Nickel sleeves, which has the
adhesive or primer coating. A twin screw extruder may also be
employed to do both the mixing and extrusion functions. The key
obviously is to keep the extrudate temperature below the
crosslinking temperature of the Peroxide or Sulfur crosslink
system. Alternately, a "light crosslink" may also be possible at
the tail end zones of the extruding cycle to allow forming the
polymer and to minimize "cold flow" during the crosslink step.
[0106] 3. The extruded sleeve is then wrapped with a nylon or Mylar
webbing to minimize oxygen inhibition similar to "Roll
crosslinking" operations used currently in rubber roll
applications. A post-crosslink may also be necessary. Table 4 (FIG.
4) summarizes the process conditions used during the crosslink and
post-crosslink steps: Obviously several sleeves could be
crosslinked together for workflow reasons.
[0107] 4. The surface of the sleeve is then ground or machined to
bring the sleeve to the final gage. The MPU sleeve is now ready for
laser engraving.
[0108] 5. After the engraving step a simple water or detergent wash
is all that is required to remove the residual debris.
[0109] 6. The sleeve is now ready for the press.
[0110] The final process for flat flexographic plates is as
follows:
[0111] 1. Formulation of MPUs with the curatives, co-agent, laser
additives, and other additives was carried out in a compounder such
as a Brabender. A 2-Roll mill may also be used in this step.
[0112] 2. The compounded system is then extruded in a single screw
extruder directly between 2 polyester sheets. The top sheet acts as
the protective coversheet. The bottom acts as the backing sheet
preferably with an adhesive or primer. A twin-screw extruder may
also be employed to do both the mixing and extrusion functions. The
key obviously is to keep the extrudate temperature below the
crosslinking temperature of the Peroxide or Sulfur crosslink
system. Alternately, a "light crosslink" may also be possible at
the tail end zones of the extruding cycle to allow forming the
polymer and to minimize "cold flow" during the crosslink step. The
extrusion takes place between nip rolls of a calendar, in order to
control the thickness.
[0113] 3. Alternately, and preferably, the compounded MPU system is
hot pressed between the coversheet and backing polyesters to bring
it to the precise gage required in the flexo process.
[0114] 4. This sandwich structure is then crosslinked in a press
typically used for rubber crosslinking making sure that the PET
sheets are not warped during the crosslink step. A post-crosslink
may also be necessary. Table 4 (FIG. 4) summarizes the process
conditions used during the crosslink and post-crosslink steps.
Obviously several plates could be crosslinked together for workflow
reasons.
[0115] 5. The crosslinked MPU plate is now ready for laser
engraving after removing and discarding the top PET coversheet.
[0116] 6. After the engraving step a simple water or detergent wash
is all that is required to remove the residual debris.
[0117] 7. The plate is now ready for the press.
[0118] For test purposes the compounds listed in Table 3 (FIG. 3)
were also hot-pressed and crosslinked in a typical commercially
available rubber crosslinking press at elevated temperatures and
pressures (160.degree. C., 60 PSI). The plaques were mounted on the
mandrel of the Yttrium based fiber laser. Table 5 (FIG. 5)
summarizes the laser conditions used during the engraving tests and
also the results of the engraving test carried out on the sample
set from Table 3 (FIG. 3). Only a detergent and water wash rinse
steps were required to clean the debris.
Additives
[0119] Most of the MPUs need to be further modified or compounded
to be functional as a laser engraving system. The choice of
additives will be dependent on the proposed effect. Additives can
be classified under the following categories.
Additives to Increase Laser Sensitivity
[0120] Additives to increase laser sensitivity increase the
absorptivity of the polymer at the lasing wavelengths. There are
two areas that can be used as a resource for laser additives: Laser
Marking and Solar Absorbing Glass used in automotive and greenhouse
applications. Both of these use a strong IR absorber additive,
which acts to convert IR photons to heat. Since many of these
additives are nanomaterials, uniformly and molecularly dispersing
these in the binder of choice presents a challenge. Laser
masterbatches are available for ease of incorporation in the binder
system. There are additives that are selective for both lasers
lasing in the far IR range (e.g. CO.sub.2 10,600 NM) and those
lasing in the near IR range for (830-1100 NM). These additives are
available from a number of sources depending on the IR regime, such
as Engelhard, Sumitomo Metal Mining and Clariant, among others.
[0121] The mica additives are well known for this function. The
most common additive used in laser engraving applications is carbon
black pigment, which also acts as a reinforcing filler.
Additives for Heat Dissipation
[0122] The incorporation of certain additives for charge
dissipation in coating systems, films or composites can reduce the
buildup of static charge. Typically these are used in the
electronics industry to avoid destructive discharges that can harm
electronic components or, in hazardous operations, where it may act
as an ignition source. In addition, these conductive additives have
also been used in films used to produce conductive display screens,
such as for interactive touch screens, eliminating the need to use
expensive sputtering technologies. Some of these additives can also
be incorporated in. our engraving polymer systems to dissipate the
heat buildup in the MPU elastomers, which are known poor conductors
of heat. Use of heat dissipative or heat conductive additive would
allow engraving at very high resolutions, up to 5080 DPI and allow
Screen rulings of up to 200 LPI (80 LPC). At such high resolutions
there is tremendous heat buildup from the laser. In addition the
dots and lines at such high resolutions are of very small dimension
(<10 .mu.m). These fine structures will have a propensity to
degrade or melt if the heat generated is not removed efficiently.
Use of heat dissipative additives allows engraving of such fine
structures.
[0123] The most promising additives for heat dissipation during
engraving are available from companies such as Nanophase
Technologies. Nanoparticles based on metals, such as silver and
copper, can be used as heat dissipaters. Nanoparticles based on
metal oxides such as Indium-Tin-Oxide and copper oxide, have shown
high propensity of heat dissipation when used in small amounts.
Nano copper oxide is the most cost effective in this
application.
[0124] Other additives that may be used are Carbotherm Boron
Nitride platelets available from Saint-Gobain Advanced Ceramics.
There are some grades of carbon black pigment and graphite
platelets which function as both the laser wavelength absorptive
and as heat dissipative or conductive additives.
[0125] Additives for Density Reduction of the Elastomeric
Composition
[0126] Since laser engraving is a mass transfer phenomenon, it is
believed that if the bulk density of the polymer were reduced
without affecting the integrity of other physical attributes, it
would aid in increasing the productivity in laser engraving of the
printing plate--a current shortcoming in laser engraving systems.
The extreme case is, of course, the difference in engraving
sensitivity and power requirements of steel versus a rubber, all
else being equal.
[0127] Additives that can be advantageously used in density
reduction are Microspheres from Akzo Nobel, Borosilicate glass
bubbles from 3M and spherical porous silica. Microspheres decrease
the density of the polymer and increase the rate of mass transfer
during laser engraving. There are various microspheres, but the
most promising are the unexpanded microspheres and crosslinked
nanospheres. The former has liquid hydrocarbon encapsulated in a
thermoplastic polymer shell, which expands during the extrusion
process causing a drop in bulk density from .about.1.0 to
.about.0.2. FIG. 10 indicates theoretically the concept of
balancing the physical properties and laser sensitivity
(productivity) which run counter to each other: Borosilicate Glass
bubbles have adequate "crush strength" to survive the various
extrusion and mixing processes.
Burn-rate modifiers
[0128] Additives from the fields of propellants and rocketry
dealing in burn-rate modification can be used to decrease the
pyrolysis temperature of the MPU elastomers during laser engraving,
giving work flow advantage by increasing the engraving speed. Since
laser engraving is a "mass transfer" process, the efficiency of
engraving can be improved by the use of suitable oxidizers and
burn-rate modifier described below.
[0129] These additives need to be stable at the process conditions
used for crosslinking or vulcanization of the elastomers (160C and
60+ PSI). Common oxidizers are Ammonium Perchlorate, Ammonium
Nitrate and Potassium Nitrate. Common burn rate catalysts, which
can be employed, are various oxides of metals such as Iron Oxides,
Copper oxides, Copper Chromates, Chrome Oxides, manganese oxides
etc and organic derivatives such as Ferrocene. Typical fuels or
burn rate accelerators used could be aluminum, boron and magnesium
powders especially as nanoparticles from the field of
nanotechnology. A combination of the above additives can be used to
accelerate the burn rate of our elastomer composition during laser
engraving.
[0130] Recent emerging field of "nanoenergetics" can also be used
advantageously. Nanoenergetic materials can store higher amounts of
energy than conventional energetic materials and one can use them
in unprecedented ways to tailor the release of this energy so as to
increase the burn rate of our elastomers resulting in higher
productivity during the engraving process.
[0131] Energetic Thermoplastic Elastomers (ETPE) are another class
of polymer compounds that can be blended in with the MPUs. ETPEs
are thermoplastic elastomers with an energetic content in their
backbone that is released during the engraving process resulting in
lowering the pyrolysis temperature of the overall formulation and
accelerating the burn rate. Examples of ETPE are polymers based on
oxetane groups (PolyNIMMO-Poly(3-nitratomethyl-3-Ethyl Oxetane) and
Azide groups (GAP-Glycidyl Azide Polymer). ETPEs are available
allowing process temperatures below the vulcanization temperature
of the MPUs (160C).
Lasing Wavelength
[0132] Much of the efforts in the industry have been focused on
CO.sub.2 lasers because such lasers are mature technology. CO.sub.2
lasers typically have a spot size of around 40 .mu.m. Thus, it is
difficult to achieve image fidelity higher than 100-125 LPI. The
advantage is that the lasing wavelength (10,600 NM) allows a wide
use of elastomers due to their absorptivity. Near IR lasers,
particularly Yttrium-based fiber (1100 NM) and ND-YAG lasers (1060
NM), have a significantly lower spot size (.about.10 .mu.m)
allowing resolutions of 125-200 LPI. The problem is that the lasing
wavelength (1060 NM) makes the choice of a binder difficult since
most binders do not absorb at that wavelength. Additionally,
historically, these Near-IR lasers do not have adequate power for
engraving so productivity was not good. These shortcomings can be
overcome by a judicious choice of binder and additive (carbon black
pigment). Recently, however, the Yttrium-based fiber laser and the
ND-YAG lasers have shown advances where the power required for
elastomer engraving is adequate. Diode array lasers in the near IR
(830 NM) are also available and increasing in power capacity.
[0133] Crosslinking of TPUs and TPEs may be inefficient, resulting
in unsatisfactory engravability (see Comparative Example 4 below).
The crosslink density of TPUs considered was not high enough to
allow sufficient engraving. Engraving wavelength is a key
consideration in the preparation of a good engraving plate. Near IR
wavelengths (830-1110 NM) are preferred over the far IR (10,600 NM)
for high-resolution engraving. Additives such as Carbon Black may
be needed to make it absorptive. This disallows the use of UV
crosslinking (UV curable TPUs, TPEs and liquid photopolymers).
Liquid photopolymers that are also polyurethanes are discussed in
U.S. Pat. No. 7,029,825 to Asahi.
[0134] There are two key properties that essentially characterize
laser light, namely lasing wavelength and beam quality. Typical
lasers used in Graphic Arts imaging work in the infrared range:
GaAs: 864 nm, Nd-YAG: 1060 nm, Yttrium fiber laser: 1110 nm,
CO.sub.2: 10600 nm. In addition to wavelength; the beam quality is
also a key characteristic of the particular laser type. The ideal
laser beam has a radially symmetric Gaussian intensity
distribution. The beam quality is defined in the form of e.g. beam
quality coefficient M.sup.2. The ideal laser has an M.sup.2 of 1.
It is close to 1 for fiber lasers, approximately 5 for YAG lasers
and approximately 15 for diode lasers. Both the wavelength and the
beam quality have a direct influence on the image quality. They
define the resolution and depth of focus of the write beam. The
resolution is determined by the spot size of the laser beam (beam
diameter in focus). The smaller the beam when focused on the
printing plate, the higher the resolutions achieved. Typically,
with all else equal, the spot size is a function of wavelength and
depth of focus.
[0135] The productivity of a laser is particularly important when
it comes to direct engraving. The imaging and engraving times
depend essentially on two factors:
[0136] 1. The laser power available on the material. The only
aspect determining the productivity of a laser is the power
actually applied to the surface of the engraving article.
[0137] 2. The sensitivity of the material being processed. This is
specified in J/cm.sup.2 or Ws/cm.sup.2. Direct engraving uses a
variable depth approach. It is therefore logical to define the
sensitivity of the material as the energy per quantity of
material.
[0138] In conclusion, the CO.sub.2 laser is a very mature
technology but has only limited usage for high resolution flexo
direct laser engraving applications. The Yttrium fiber laser and
the ND-YAG lasers are increasing in their power capacity and
applicability for engraving at resolutions in excess of 4000 DPI
and at Screen counts approaching 200 LPI. Preferred embodiments of
the invention are further explained and exemplified below.
[0139] Example 1
Laser Direct Engraving of Millable Polyurethanes using a Near IR
(Yttrium-Based Fiber) Lasing at 1100 NM
[0140] A number of MPU and MPU/Nitrile Butadiene Rubber Blends
summarized in Table 1 (FIG. 1) were included in this engraving
test. Table 3 (FIG. 3) summarizes the sample set that was tested as
a direct laser engravable system in the Yttrium based fiber laser.
As can be seen from Table 3 (FIG. 3) the test matrix included 3
different types of hard segments (Aliphatic Isocyanate, MDI and
TDI) and 2 different types of soft segments (Polyester and
Polyether) from the options available in the Millable PU range. In
addition there were 2 different types of thermal curatives:
Peroxide crosslinking and Sulfur vulcanization. Some curatives were
specific to the type of PU. There are significant advantages and
disadvantages of each curative.
[0141] Table 4 (FIG. 4) summarizes the crosslinking conditions and
the physical properties of each formulated system used for the
engraving test from Table 3 (FIG. 3). It is seen that most samples
show excellent physical properties within the range described in
Table 2 (FIG. 2) from above. Thus, most of the formulated MPUs will
function as an excellent flexographic printing system.
[0142] Formulation of MPUs with the curatives, co-agent, laser
additives, and other additives was carried out in a compounder such
as a Brabender. A 2-Roll mill may also be used in this step. The
compounded system is then extruded in a single screw extruder
directly on the composite or Nickel sleeves, which has the adhesive
or primer coating. A twin screw extruder may also be employed to do
both the mixing and extrusion functions. It is important to keep
the extrudate temperature below the crosslinking temperature of the
Peroxide or Sulfur crosslink system. Alternately, a "light
crosslink" may also be possible at the tail end zones of the
extruding cycle to allow forming the polymer and to minimize "cold
flow" during the crosslink step. The extruded sleeve is then
wrapped with a nylon or Mylar webbing to minimize oxygen inhibition
similar to "Roll crosslinking" operations used currently in rubber
roll applications. A post-crosslink may also be necessary. Table 4
(FIG. 4) summarizes the process conditions used during the
crosslink and post-crosslink steps. Obviously several sleeves could
be crosslinked together for workflow reasons. The surface of the
sleeve is then ground or machined to bring the sleeve to the final
gage. The MPU sleeve is now ready for laser engraving. After the
engraving step a simple water or detergent wash is all that is
required to remove the residual debris. The sleeve is now ready for
the press. In the case of flat plates a similar process as above is
employed except the compounded MPU system is hot pressed between a
coversheet and backing polyesters. The precise thickness or plate
gage is thus achieved during the hot-press process.
[0143] For test purposes the compounds listed in Table 3 (FIG. 3)
were hot-pressed and crosslinked in a typical commercially
available rubber crosslinking press at elevated temperatures and
pressures (160.degree. C., 60 PSI). The plaques were mounted on the
mandrel of the Yttrium based fiber laser. Table 5 (FIG. 5)
summarizes the laser conditions used during the engraving tests and
also the results of the engraving test carried out on the sample
set from Table 3 (FIG. 3). Only a detergent and water wash rinse
steps were required to clean the debris.
[0144] As mentioned before, since the MPU has very little
absorbance at the lasing wavelength of the Fiber laser, all of the
samples were doped with an absorbing CB pigment. The entire sample
set showed adequate sensitivity to the Yttrium fiber laser, as seen
in Table 5 (FIG. 5). FIGS. 11A-11F shows digital photographs of the
laser imaging studies from Table 5 (FIG. 5) on the Yttrium-based
fiber laser. As can be seen from pictures, the resolution of fine
images achieved in this test was excellent, with imaging of very
fine and sharp dots and deep reverses being achieved.
[0145] As can also be seen from Table 5 (FIG. 5), most samples had
relief depth of around 450 .mu.ms. There was very little undercut
seen indicating that artifacts such as melting of fine dots are not
an issue with MPUs. Although the resolution employed for this test
was 2540 DPI and screen ruling of 125 LPI, it may be possible to
use up to 5080 DPI and allow screen rulings up to 200 LPI, which is
only achievable in Digital flexo plates.
[0146] Several other factors were considered such as level of
coagent, level of plasticizer, type and level of other laser
additives discussed before etc.
[0147] Several conclusions were noted from these tests: [0148]
Ester vs. Ether Soft Segments: Between Ester and Ether soft
segments, the esters seem to give sharper image. However, this was
not conclusive. [0149] TDI-ester vs. MDI-Ester Hard Segments: From
this preliminary study TDI/Ester seems to give better results than
MDI/Ester.
Example 2
Laser Direct Engraving of Millable Polyurethanes using a Far IR
(CO.sub.2) Laser Lasing at 10,600 NM
[0150] Similar to Example 1 above, MPU and MPU/Nitrile Butadiene
Rubber Blends summarized in Table 1 (FIG. 1) were included in this
engraving test. Table 6 (FIG. 6) summarizes the various samples
that were tested as a direct laser engravable system. Various
commercially available MPU and MPU/Rubber blends were used. In
addition, two different types of thermal curatives were used:
Peroxide and Sulfur (vulcanization). Unlike the sample set for the
Yttrium laser, several samples were included without the use of a
Carbon Black pigment dopant. These clear samples imaged as well as
their doped counterparts. This is the major advantage of the
CO.sub.2 lasers lasing in the far IR regime-elastomers without
absorbing additive can be used for laser engraving.
[0151] The process conditions used, method of compounding,
crosslinking and laser engraving was similar to what is described
in Example 1. The physical properties of the sample set were also
similar to what was described in Table 4 (FIG. 4) from the above
Example 1. As before, it was seen that most samples show excellent
physical properties and would thus function as an excellent
flexographic printing system. This was true even for the MPU/Rubber
blends. This is one of the major advantages of MPU--the versatility
and flexibility to allow blends of other elastomers, TPEs etc for
value added.
[0152] Once again other factors tested were the type and level of
coagents used. Sample sets with higher loading of TMPTMA and lower
loadings were briefly studied. Higher coagent level, not
surprisingly gave faster crosslinking and higher crosslink density
and sharper image fidelity after the engraving step
[0153] As seen from Table 6 (FIG. 6), most all of the crosslinked
Millable PU plaques showed clean and sharp engraving, even the
samples without the Carbon Black additive. Only a detergent and
water wash rinse steps were required to clean the debris.
[0154] Additionally, the entire sample regime in Table 6 (FIG. 6)
showed adequate sensitivity to the CO.sub.2 laser as seen from the
"depth" of the trough engraved (18-20 mils at 100% Power and 6-8
mils at 50% Power). The initial engraving test was positive enough
for us to attempt some rudimentary imaging. The image fidelity, for
the most part, was acceptable, but not as good as those from the
Yttrium fiber laser.
Comparative Example 3
Laser Direct Engraving of Castable Polyurethanes using a Far IR
(CO.sub.2) Laser Lasing at 10,600 NM
[0155] Cast Polyurethanes were made using typical commercially
available processes. The raw materials were available from Anderson
Development Company. The physical properties and test results from
the engraving tests are summarized in Table 7 (FIG. 7). The
physical properties were acceptable as a Flexo plate. However, the
elastomers showed severe melting artifacts that resulted in
undefined and blurred images and unacceptable imprint quality. The
engraved residues were undesirably liquid in character and very
tacky to touch. It appears that although the Cast Polyurethanes are
crosslinked, there is still significant thermoplastic character or
low crosslink density resulting in the heat from the laser
undesirably melting rather than resulting in clean and sharp
ablation or engraving. This is in contrast to MPUs, which did not
show such melting artifacts.
Comparative Example 4
Laser Direct Engraving of Thermoplastic Polyurethanes using a Far
IR (CO.sub.2) Laser Lasing at 10,600 NM
[0156] Table 8 (FIG. 8) teaches the use of a crosslinked TPU for
laser engraving. The crosslinking was carried out during the
extrusion step. The TPU was compounded with the additives and
extruded in a TSE (or single screw) keeping manufacturer
recommended extrusion temperatures (see Samples 8A-8J). The article
was then laser engraved on a CO.sub.2 laser lasing at 10,600 NM
commonly available in the market. Just like for the, CPU in
Comparative Example 3 above, the crosslinked TPU showed severe
melting artifacts, with undefined and blurred images resulting in
unacceptable imprint quality. It was surmised that although the
TPUs are crosslinked, there is still significant thermoplastic
character or low crosslink density resulting in the heat from the
laser melting rather than creating clean and sharp ablation or
engraving. This is in contrast to MPUs, which did not show such
melting artifacts.
[0157] FIG. 12 is a perspective view of an engraving article 100
having an inner composite or nickel sleeve 102 having a thickness
of approximately 7 to 10 mils, and an outer MPU engraving rubber
surface 104 having a thickness of approximately 67 to 125 mils.
[0158] While specific embodiments of the present invention have
been described, it will be apparent to those skilled in the art
that various modifications thereto can be made without departing
from the spirit and scope of the invention. Accordingly, the
foregoing description of the preferred embodiment of the invention
and the best mode for practicing the invention are provided for the
purpose of illustration only and not for the purpose of
limitation.
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