U.S. patent application number 15/258613 was filed with the patent office on 2016-12-29 for waterless printing members and related methods.
The applicant listed for this patent is Kevin RAY, Sonia RONDON. Invention is credited to Kevin RAY, Sonia RONDON.
Application Number | 20160375712 15/258613 |
Document ID | / |
Family ID | 46201783 |
Filed Date | 2016-12-29 |
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United States Patent
Application |
20160375712 |
Kind Code |
A1 |
RONDON; Sonia ; et
al. |
December 29, 2016 |
WATERLESS PRINTING MEMBERS AND RELATED METHODS
Abstract
In various embodiments, a recording medium comprises an
oleophilic substrate and, thereover, a topmost oleophobic layer
comprising a cured polymeric silicone matrix that consists
essentially of the addition-cure reaction product of a
vinyl-functional polydialkylsiloxane and a silane cross-linking
agent. The vinyl-functional polydialkylsiloxane has a molecular
weight ranging from 30,000 to 75,000 or 110,000 to 130,000. If the
molecular weight of the vinyl-functional polydialkylsiloxane is
within the range of 30,000 to 75,000, the molar ratio of silane
groups to vinyl groups is within the range of about 11:1 to about
25:1; and if the molecular weight of the vinyl-functional
polydialkylsiloxane is within the range of 110,000 to 130,000, the
molar ratio of silane groups to vinyl groups is from about 5:1 to
about 27:1. The recording medium may be used as a lithographic
printing plate.
Inventors: |
RONDON; Sonia; (Nashua,
NH) ; RAY; Kevin; (Windham, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RONDON; Sonia
RAY; Kevin |
Nashua
Windham |
NH
NH |
US
US |
|
|
Family ID: |
46201783 |
Appl. No.: |
15/258613 |
Filed: |
September 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12970208 |
Dec 16, 2010 |
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15258613 |
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Current U.S.
Class: |
101/453 |
Current CPC
Class: |
B41C 2210/08 20130101;
B41N 1/003 20130101; B41C 2201/14 20130101; B41C 2210/262 20130101;
C08J 2367/00 20130101; B41C 1/1016 20130101; B41C 2210/24 20130101;
B41N 1/14 20130101; Y10T 428/31663 20150401; C08G 77/20 20130101;
C08L 83/04 20130101; B41C 2210/04 20130101; C08J 2483/04 20130101;
C08J 7/0427 20200101; C09D 183/04 20130101; B41C 2210/14 20130101;
C08L 83/04 20130101; C08L 83/00 20130101; Y10T 428/269 20150115;
B41C 2201/02 20130101; C08L 83/00 20130101; B41C 2210/16 20161101;
C08G 77/12 20130101; C09D 183/04 20130101 |
International
Class: |
B41N 1/14 20060101
B41N001/14; C08J 7/04 20060101 C08J007/04; C09D 183/04 20060101
C09D183/04 |
Claims
1-28. (canceled)
29. A recording medium comprising an oleophilic substrate and,
thereover, a topmost oleophobic layer comprising a cured polymeric
silicone matrix that consists essentially of the addition-cure
reaction product of a vinyl-functional polydialkylsiloxane and a
silane cross-linking agent, wherein: a) the vinyl-functional
polydialkylsiloxane has a molecular weight ranging from 110,000 to
130,000; and b) a molar ratio of silane groups in the cross-linking
agent to vinyl groups in the polydialkylsiloxane is from about 5:1
to about 27:1.
30. The recording medium of claim 29, wherein the vinyl-functional
polydialkylsiloxane is vinyl-functional polydimethylsiloxane.
31. The recording medium of claim 29, wherein the molar ratio of
silane groups in the cross-linking agent to vinyl groups in the
polydialkylsiloxane is from about 20:1 to 27:1.
32. The recording medium of claim 29, wherein the silane
cross-linking agent is a trimethylsiloxy-terminated
polymethylhydrosiloxane.
33. The recording medium of claim 29, wherein the topmost
oleophobic layer has a dry coat weight of 2 g/m.sup.2.
34. The recording medium of claim 29, wherein the substrate
comprises a polyester film.
35. The recording medium of claim 29, wherein the substrate
comprises a metal sheet having an oleophilic coating thereover.
36. The recording medium of claim 29, further comprising a
laser-ablatable imaging layer between the substrate and the topmost
layer.
37. The recording medium of claim 36, wherein the imaging layer is
ablatable upon exposure to a laser beam having a fluence of at
least 250 mJ/cm.sup.2.
38. The recording medium of claim 37, wherein the imaging layer is
a thin metal/metal oxide layer.
39. The recording medium of claim 38, wherein the imaging layer has
a thickness no greater than 50 nm.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to lithographic printing, and
in particular to waterless plate constructions imageable by
ablative discharge.
BACKGROUND OF THE INVENTION
[0002] Polyorganosiloxane compounds, or "silicones," can be
synthesized in a wide variety of forms, and are utilized in
numerous commercial applications. Silicone compounds are based on
the repeating diorganosiloxane unit (--R.sub.2SiO--).sub.n, where R
is an organic radical and n denotes the number of units in the
polymer chain. Each end of the linear chain is terminated with a
functional or non-functional end group; the chain may also be
"branched" so as to deviate from a strictly linear structure.
[0003] The physical properties of a particular silicone formulation
depend on the length of the polymer chain, the nature of the
organic functional groups bonded to the silicon atoms, and the
terminal groups (more precisely, the alpha and omega groups) at
each end of the chain. For example, the most common silicone
compounds are based on the polydimethylsiloxane unit,
--Si(CH.sub.3).sub.2O--, which, due to the relatively small organic
content of the chains, have a limited range of compatibility with
organic compounds. By contrast, silicones containing aryl
functional groups tend to exhibit properties more commonly
associated with organic materials, and such silicones are generally
miscible with a broader range of such materials.
[0004] Curing can be accomplished in a number of ways, but
generally depends on the presence of reactive functional groups on
the polymer chains that interact and bond with one another. In
"condensation cure" reactions, a small molecule is eliminated when
the two functional groups are joined. Typical condensation-cure
reactions in silicone chemistry involve combination of silanol
(--SiOH) functional groups with other such groups to produce an
oxygen linkage with the elimination of water. "Addition cure"
reactions result in no loss of species, and can involve, for
example, hydrosilylation reactions between olefinic functional
groups (such as vinyl) and hydrosiloxane groups (which have a
silicon hydride moiety).
[0005] Variations on the traditional condensation-cure reaction
include the "moisture-cure" approach, in which a precursor
functional group is first hydrolyzed to form a reactive
hydroxyl-bearing group, which then combines with a silanol group as
discussed above. Suitable precursor compounds include acetoxy,
alkoxy and ketoxime functional silanes, which form acid, alcohol
and ketoxime byproducts, respectively, upon hydrolysis.
Silanol-functional silicones and mixtures of silanol-functional
silicones with silicones containing acetoxy, alkoxy or ketoxime
groups are relatively stable so long as moisture is excluded; this
is particularly true for silicone polymers having appreciable
molecular weights. Obtaining useful reaction rates generally
requires a catalyst such as a metal carboxylate compound.
[0006] Silanol groups also react with hydrosiloxane species to
liberate hydrogen and produce the silicon-oxygen-silicon linkage
characteristic of the condensation cures. Use of a metal salt
catalyst (such as dibutyltindiacetate) is generally necessary to
achieve useful reaction rates. Because it proceeds rapidly when
catalyzed, this reaction is widely used for silicone coating
formulations applied on a coating line to a web substrate.
[0007] Silicone rubber coatings have been adopted by some
manufacturers of planographic printing plates. Planographic
printing, as contrasted with letter-press and gravure printing,
relies on plate constructions in which image and non-image areas
lie substantially in the same plane. The plate is prepared by
altering the affinities of different areas of the plate for ink.
Depending on the type of plate imaging system employed, non-image
plate areas become (or remain) oleophobic, or ink-repellent during
printing, while image areas remain (or become) oleophilic, or
ink-accepting. Ink applied to the plate surface (e.g., by a roller)
will adhere to the oleophilic image areas but not the oleophobic
non-image areas. The inked plate is then applied to the recording
medium (in direct printing) or to an intermediate "blanket"
cylinder which then transfers the image to the recording medium (in
offset printing).
[0008] Manufacturers of planographic printing plates often employ
silicone rubber compositions as plate coatings due to their low
surface energies, which render them oleophobic and therefore
suitable for "dry" or "waterless" plates. In contrast to the
traditional "wet" plate, which requires application of a fountain
or dampening solution to the plate prior to inking in order to
prevent ink from adhering to and transferring from non-image areas,
the non-image material of waterless plates is itself sufficiently
ink-repellent that no fountain solution is necessary.
[0009] Silicone compositions used as coatings for planographic
printing plates typically include two basic constituents: a primary
polyorganosiloxane base-polymer component and a smaller
cross-linking component. The base component is usually a linear,
predominantly polydimethylsiloxane copolymer or terpolymer
containing unsaturated groups (e.g., vinyl) or silanol groups as
reactive centers for bonding with the cross-linking molecules.
These groups are commonly situated at the chain termini, although
it is possible to utilize a copolymer incorporating the reactive
groups within the chain, or branched structures terminating with
the reactive groups. It is also possible to combine linear
difunctional polymers with copolymers and/or branch polymers.
[0010] The cross-linking component is generally a multifunctional,
monomeric or oligomeric compound of low molecular weight, which is
reacted with the first component to create connections among the
chains thereof. The curing reaction generally requires some type of
catalyst, either chemical or physical, to produce favorable
kinetics. Platinum metal complexes (such as chloroplatinic acid)
are often employed to facilitate addition cures, while metal salt
catalysts (such as a dialkyltindicarboxylate) are frequently used
in conjunction with condensation cures.
[0011] If the functional groups of the cross-linking component are
situated at the chain termini, cross-linking molecules will form
bridges among the base-polymer molecules (particularly if the
latter have functional groups distributed along the chains). On the
other hand, if the cross-linking component contains functional
groups distributed along its length, each molecule will form
numerous points of attachment with the base-polymer molecules.
Typically, this type of cross-linking molecule is combined with
base polymers having chain-terminal functional groups in order to
maximize the number of different base-polymer molecules attached to
each cross-linking chain.
[0012] In addition-cure systems, the hydrosilylation reaction
provides fast-curing silicones that can be tailored to display
strong bonding to hydroxyl-rich surfaces such as metal substrates.
Silicone coating properties such as mechanical and adhesion
performance depend on the extent of crosslinking and, therefore,
the molar ratio of crosslinker to vinyl functional groups. The
addition-curing chemistry involves at least three curing reactions
indicated below at (1), (3) and (4). The primary reaction, (1),
occurs between a vinyl-functional silicone polymer and a
silicon-hydride (--SiH) functional group in the presence of a heavy
metal catalyst compound, usually platinum or rhodium. Slower
secondary reactions may occur, especially when silicones are
formulated with an excess of crosslinker: the first of these is the
catalyzed hydrolysis of silicon hydride groups to form --SiOH
(reaction (2)). The newly formed silanol groups catalytically react
with remaining --Si--H groups to form --Si--O--Si-- bonds (reaction
(3)). Finally, in the slowest condensation reaction, two silanol
groups form an --Si--O--Si-- linkage (reaction (4)). Reaction (1)
is referred to as the "crosslinking" or "cure" reaction while
reactions (2) through (4) are "post-cure" reactions.
##STR00001##
[0013] When preparing silicone printing plates, the presence of a
substrate interface increases the possibility of competing
side-reactions. It is known that crosslinker silicon hydride groups
and hydrolyzed silanol groups may react with substrate-borne
hydroxyl groups. Silicone molecules attach to a substrate via two
mechanisms: mechanical interlocking and chemical reaction.
Mechanical interlocking occurs when silicone is applied to
semi-porous substrates such as paper, but it cannot account
exclusively for long-term stable anchorage of silicones to metal
and polymer surfaces. The strength of silicone bonding depends
mainly on chemical interactions between the silicone and a
substrate. Metal surfaces exposed to air (or passivated metal)
contain hydroxyl groups that interact with silicon hydride and
silanol moieties forming a silyl ether linkage. FIG. 1 suggests a
mechanism for the adhesion of silicone to metal substrates. These
side reactions account for long-term adhesion of silicone to metal
substrates, and illustrate the importance of using a large excess
of crosslinker when coating addition-cured silicones on metals.
Similarly, the adhesion of silicone to a polymer surface occurs
when silicone crosslinker reacts not only with unreacted surface
hydroxyls but also with available carboxyl groups.
[0014] A silicone coating composition is applied to a plate
substrate using any of a variety of well-known coating techniques.
The choice of technique is critical not only to the ultimate
performance of the plate, but also to the efficiency and
reliability of the overall platemaking process. Typical coating
techniques include roll coating, reverse-roll coating, gravure
coating, offset-gravure coating, slot coating, and wire-wound rod
coating. The coating procedure must be rapid enough to achieve a
satisfactory production rate, yet produce a highly uniform, smooth,
level coating on the plate. Even small deviations in coating
uniformity can adversely affect plate performance, since the
planographic printing process depends strongly on coplanarity of
image and non-image areas; in other words, the printing pattern
reproduced by the plate must reflect the configuration of
oleophilic and oleophobic areas impressed thereon, and remain
uninfluenced by topological characteristics of the plate
surface.
[0015] Blank dry plates are subjected to an imaging process that
removes the silicone coating from image areas to reveal an
oleophilic surface. Imaging can be accomplished in a number of
ways. In traditional photosensitive plates, exposure of a
photoresist material in the plate structure to actinic radiation
alters the solubility or anchorage properties of the silicone. In
typical commercial plates, exposure to light results either in firm
anchorage of the silicone coating to the plate (in positive-working
plates) or in destruction of the existing anchorage (in
negative-working plates). Depending on the process chosen, the
plate is first exposed to actinic radiation passing through a
positive or negative rendition of the desired image that
selectively blocks transmission of the radiation to the plate.
After this exposure step, the plate is developed in chemical
solvents that either anchor the exposed silicone or remove it to
produce the final, imaged plate.
[0016] To circumvent the cumbersome photographic development,
plate-mounting, and plate-registration operations that typify
photoexposure printing technologies, practitioners have developed
electronic alternatives that store the "imagewise" pattern in
digital form and impress the pattern directly onto the plate.
Plate-imaging devices amenable to computer control include various
forms of lasers. Current laser-based lithographic systems often
rely on removal of an energy-absorbing layer from the lithographic
plate to create an image. Exposure to laser radiation may, for
example, cause ablation--i.e., catastrophic overheating--of the
ablated layer, which de-anchors the silicone and facilitates its
removal (along with remnants of the ablated "imaging" layer).
[0017] This imaging process and the rigors of commercial printing
impart significant stress to the silicone rubber layer of a
lithographic printing plate, and indeed, at the coating weights
typically employed for economic reasons as well as to promote easy
handling, such layers are inherently fragile and prone to
scratching. Waterless printing plates fail when the silicone rubber
loses ink repellency and non-image areas accept ink. Accordingly,
there is a need for silicone coatings with improved robustness,
leading to printing plates with enhanced durability and
correspondingly longer run lengths.
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
[0018] Although laser-imageable lithographic printing plates using
addition-cured silicone top layers are known, it has been found,
surprisingly, that the durability of a finished coating depends
strongly on certain formulation parameters--in particular, when
combining a vinyl-functional polydialkylsiloxane with a silane
cross-linking agent, the molecular weight of the vinyl-functional
polydialkylsiloxane and the molar ratio of silicon hydride
functional groups to vinyl groups are both critical and
interrelated. (As used herein, the term "silane cross-linking
agent" means a polydialkylhydrosiloxane polymer having multiple
(e.g., terminal) silicon hydride functional groups.) Specifically,
the vinyl-functional polydialkylsiloxane should have a molecular
weight ranging from 30,000 to 75,000 or 110,000 to 130,000. (The
molecular weights given herein are weight average molecular
weights.) If the molecular weight of the vinyl-functional
polydialkylsiloxane is within the former range, the molar ratio of
silane groups to vinyl groups should be within the range of about
11:1 to about 25:1; and if the molecular weight of the
vinyl-functional polydialkylsiloxane is within the latter range,
the molar ratio of silane groups to vinyl groups should be from
about 5:1 to about 27:1.
[0019] Typically, the silicone layer is formed from an addition
cure, hydrosilylation reaction using a platinum catalyst such as
elemental platinum, platinum chloride, chloroplatinic acid, olefin
coordinated platinum, an alcohol modified complex of platinum, or a
methylvinyl polysiloxane complex of platinum. The vinyl-functional
polydimethylsiloxane may be vinyl-terminated, and the
polymethylhydrosiloxane polymer may be trimethylsiloxy-terminated.
The silicone constituents may be applied to a plate precursor in a
solvent or solvent mix carrier.
[0020] Accordingly, in a first aspect, the invention relates to a
recording medium comprising an oleophilic substrate and, thereover,
a topmost oleophobic layer comprising a cured polymeric silicone
matrix that consists essentially of the addition-cure reaction
product of a vinyl-functional polydialkylsiloxane and a silane
cross-linking agent. The vinyl-functional polydialkylsiloxane has a
molecular weight ranging from 30,000 to 75,000 or 110,000 to
130,000. If the molecular weight of the vinyl-functional
polydialkylsiloxane falls within the range of 30,000 to 75,000, the
molar ratio of silane groups to vinyl groups falls within the range
of about 11:1 to about 25:1; and if the molecular weight of the
vinyl-functional polydialkylsiloxane is within the range of 110,000
to 130,000, the molar ratio of silane groups to vinyl groups is
from about 5:1 to about 27:1.
[0021] In various embodiments, the vinyl-functional
polydialkylsiloxane is vinyl-functional polydimethylsiloxane. In
one formulation, the vinyl-functional polydialkylsiloxane has a
molecular weight ranging from 30,000 to 65,000, while in another
formulation, the vinyl-functional polydialkylsiloxane has a
molecular weight ranging from 110,000 to 130,000 and the molar
ratio of silane groups to vinyl groups is from about 20:1 to 27:1.
The silane cross-linking agent may be, for example, a
trimethylsiloxy-terminated polymethylhydrosiloxane.
[0022] The silicone layer may have a dry coating weight of, for
example, 0.5 to 2.5 g/m.sup.2, with the range 1 to 2.5 g/m.sup.2
being particularly preferred for typical commercial applications.
The substrate may be or comprise a polyester film, or a metal sheet
having an oleophilic (e.g., polyacrylate) coating thereover, or
other suitable constructions presenting an oleophilic surface. In
various embodiments, a laser-ablatable imaging layer is disposed
between the substrate and the topmost layer. In particular, the
imaging layer may be ablatable upon exposure to a laser beam having
a fluence of at least 250 mJ/cm.sup.2. For example, the imaging
layer may be a thin (e.g., 50 nm or thinner) metal/metal oxide
layer.
[0023] In another aspect, the invention pertains to a method of
imaging a lithographic printing member. In various embodiments, the
method comprises the steps of, first, providing a printing member
comprising an oleophilic substrate, an ablatable imaging layer
thereover, and a topmost oleophobic layer comprising a cured
polymeric silicone matrix that consists essentially of the
addition-cure reaction product of a vinyl-functional
polydialkylsiloxane and a silane cross-linking agent. In
particular, the vinyl-functional polydialkylsiloxane has a
molecular weight ranging from 30,000 to 75,000 or 110,000 to
130,000, and if the molecular weight of the vinyl-functional
polydialkylsiloxane is within the range of 30,000 to 75,000, the
molar ratio of silane groups to vinyl groups is within the range of
about 11:1 to about 25:1; whereas if the molecular weight of the
vinyl-functional polydialkylsiloxane is within the range of 110,000
to 130,000, the molar ratio of silane groups to vinyl groups is
from about 5:1 to about 27:1. The printing member is exposed to
imaging radiation so as to ablate the imaging layer in an imagewise
pattern, following which the topmost layer is removed where the
printing member received radiation, thereby creating an imagewise
lithographic pattern on the printing member.
[0024] In still another aspect, the invention relates to a method
of forming an imageable lithographic printing member. In various
embodiments, the method comprises the steps of applying, to an
ink-receptive layer, an imaging layer ablatable by exposure to
imaging radiation; and applying, to the imaging layer, a coating
composition comprising a vinyl-functional polydialkylsiloxane and a
silane cross-linking agent. In particular, the vinyl-functional
polydialkylsiloxane has a molecular weight ranging from 30,000 to
75,000 or 110,000 to 130,000, and if the molecular weight of the
vinyl-functional polydialkylsiloxane is within the range of 30,000
to 75,000, a molar ratio of silane groups to vinyl groups is within
the range of about 11:1 to about 25:1; whereas if the molecular
weight of the vinyl-functional polydialkylsiloxane is within the
range of 110,000 to 130,000, the molar ratio of silane groups to
vinyl groups is from about 5:1 to about 27:1. The coating
composition is cured into a silicone matrix, e.g., by an addition
cure. The addition cure may be performed with a platinum
catalyst.
[0025] It should be stressed that, as used herein, the term "plate"
or "member" refers to any type of printing member or surface
capable of recording an image defined by regions exhibiting
differential affinities for ink and/or fountain solution. Suitable
configurations include the traditional planar or curved
lithographic plates that are mounted on the plate cylinder of a
printing press, but can also include seamless cylinders (e.g., the
roll surface of a plate cylinder), an endless belt, or other
arrangement.
[0026] Furthermore, the term "hydrophilic" is used in the printing
sense to connote a surface affinity for a fluid which prevents ink
from adhering thereto. Such fluids include water for conventional
ink systems, aqueous and non-aqueous dampening liquids, and the
non-ink phase of single-fluid ink systems. Thus, a hydrophilic
surface in accordance herewith exhibits preferential affinity for
any of these materials relative to oil-based materials.
[0027] "Ablation" of a layer means either rapid phase
transformation (e.g., vaporization) or catastrophic thermal
overload, resulting in uniform layer decomposition. Typically,
decomposition products are primarily gaseous. Optimal ablation
involves substantially complete thermal decomposition (or
pyrolysis) with limited melting or formation of solid decomposition
products.
[0028] The terms "substantially" and "about" mean .+-.10% (e.g., by
weight or by volume), and in some embodiments, .+-.5%. The term
"consists essentially of" means excluding other materials that
contribute to function. For example, a cured polymeric silicone
matrix that consists essentially of the addition-cure reaction
product of a vinyl-functional polydialkylsiloxane and a silane
cross-linking agent may include modifiers added to alter physical
properties, such as adhesion or rheology, of the finished coating;
and/or colorants, in the form of dyes or pigments, to facilitate
quality-assurance evaluation or monitoring of the imaging
process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The foregoing discussion will be understood more readily
from the following detailed description of the invention when taken
in conjunction with the accompanying drawings, in which:
[0030] FIG. 1 illustrates an adhesion mechanism between a silicone
layer to a metal underlying layer or substrate.
[0031] FIGS. 2A-2C are enlarged sectional views of a printing
member in accordance with the invention and show the results of
imaging the printing member.
[0032] The drawings and elements thereof may not be drawn to
scale.
DETAILED DESCRIPTION
1. Lithographic Printing Members
[0033] FIG. 2A illustrates a negative-working printing member 100
according to the invention that includes a substrate 102, an
imaging layer 105, and a topmost layer 110. Layer 105 is sensitive
to imaging (generally IR) radiation as discussed below, and imaging
of the printing member 100 (by exposure to IR radiation) results in
ablation of layer 105. The resulting de-anchorage of topmost layer
110 facilitates its removal by rubbing or simply as a result of
contact during the print "make ready" process. The surface of
substrate 102 exhibits a lithographic affinity opposite that of
topmost layer 110. Consequently, ablation of layer 105, followed by
imagewise removal of the topmost layer 110 to reveal an underlying
layer or the substrate 102, results in a lithographic image. Most
of the films used in the present invention are continuous, meaning
that the underlying surface is completely covered with a uniform
layer of the deposited material.
[0034] Each of these layers and their functions is described in
detail below.
[0035] a. Topmost Layer 110
[0036] The topmost layer participates in printing and provides the
requisite lithographic affinity difference with respect to
substrate 102. In addition, the topmost layer may help to control
the imaging process by modifying the heat dissipation
characteristics of the printing member at the air-imaging layer
interface. The topmost layer 110 remains bonded to the imaging
layer and rejects ink.
[0037] In accordance herewith, topmost layer 110 comprises or
consists essentially of a cured polymeric silicone matrix that
itself consists essentially of the addition-cure reaction product
of a vinyl-functional polydialkylsiloxane and a silane
cross-linking agent. The vinyl-functional polydialkylsiloxane has a
molecular weight ranging from 30,000 to 75,000 or 110,000 to
130,000. If the molecular weight of the vinyl-functional
polydialkylsiloxane is within the range of 30,000 to 75,000, the
molar ratio of silane groups to vinyl groups is within the range of
about 11:1 to about 25:1; and if the molecular weight of the
vinyl-functional polydialkylsiloxane is within the range of 110,000
to 130,000, the molar ratio of silane groups to vinyl groups is
from about 5:1 to about 27:1.
[0038] In various embodiments, and as illustrated in the examples
below, the vinyl-functional polydialkylsiloxane is vinyl-functional
polydimethylsiloxane. In one implementation, the vinyl-functional
polydialkylsiloxane has a molecular weight ranging from 30,000 to
65,000, while in another implementation, the vinyl-functional
polydialkylsiloxane has a molecular weight ranging from 110,000 to
130,000 and the molar ratio of silane groups to vinyl groups is
from about 20:1 to 27:1. The silane cross-linking agent may be, for
example, a trimethylsiloxy-terminated polymethylhydrosiloxane.
Layer 110 may have a dry coat weight of, for example, 2
g/m.sup.2.+-.0.2 g/m.sup.2.
[0039] b. Imaging Layer 105
[0040] The imaging layer 105 can consist of a polymeric system that
intrinsically absorbs in the near-IR region, or a polymeric coating
into which near-IR-absorbing components (usually near-IR dyes
and/or carbon) have been dispersed or dissolved. Alternatively,
layer 105 can be a metal layer thin enough to support imaging by
absorbing, rather than reflecting, IR radiation. To perform an
absorptive function, a metal layer 105 should transmit as much as
70% (and at least 5%) of the IR radiation incident thereon; if
transmission is insufficient, the layer will reflect radiation
rather than absorbing it, while excessive transmission levels
appear to be associated with insufficient absorption.
[0041] Imaging layer 105 may, for example, be a thin metal layer
having a high extinction coefficient in the imaging wavelength
range (e.g., 600-1200 nm). In general, layer 105 is very thin
(e.g., no thicker than 50 nm) so as to be be ablatable upon
exposure to a laser beam having a fluence of at least 250
mJ/cm.sup.2. A metal layer may be applied using physical vapor
deposition methods (PVD) such as magnetron sputtering deposition,
thermal evaporation, or laser vapor deposition. A preferred metal
is titanium, but alternative metals include alloys of titanium,
aluminum, alloys of aluminum, nickel, iron, chromium, and others
exhibiting the required optical densities and adequate radiation
absorption. Bilayers as described in U.S. Ser. No. 12/405,732,
filed on Mar. 17, 2009 (the entire disclosure of which is hereby
incorporated by reference) may also be used. Also suitable are
polymeric imaging layers, e.g., nitrocellulose materials, polymers
such as polyester loaded with radiation-absorptive pigments (such
as carbon black), conductive polymers (such as the ICP-117
polypyrrole-based conductive material supplied by Polaroid Corp.
Commercial Chemicals, Assonet, Mass., or Americhem Green #34384-C3,
a proprietary polyaniline-based conductive coating supplied by
Americhem, Inc., Cuyahoga Falls, Ohio), or polymers containing
nigrosine in particulate or solubilized form. See, e.g., U.S. Pat.
No. 5,704,291 (the entire disclosure of which is hereby
incorporated by reference).
[0042] More generally, in polymeric embodiments, layer 105 can be
any polymer capable of stably retaining, at the applied thickness,
an IR-absorptive pigment dispersion (generally nonconductive carbon
black) adequate to cause ablation of the layer in response to an
imaging pulse. Layer 105 should exhibit good adhesion to the
overlying layer 110, and resistance to age-related degradation may
also be considered. In general, pigment loading levels are at least
25 wt %, and the coating is applied at a dry weight of at least 0.2
g/m.sup.2, or at least 0.4 g/m.sup.2, or at least 0.8 g/m.sup.2, or
at least 1.0 g/m.sup.2, or in some embodiments, at least 1.5
g/m.sup.2. Representative materials include BAKELITE (phenol
formaldehyde) and other phenolic resins, vinyl chloride resins,
acrylic resins, and/or polyvinyl butyral. Other suitable materials
include polymers formed from maleic anhydride and one or more
styrenic monomers (that is, styrene and styrene derivatives having
various substituents on the benzene ring), polymers formed from
methyl methacrylate and one or more carboxy-containing monomers,
and mixtures thereof. These polymers can comprise recurring units
derived from the noted monomers as well as recurring units derived
from additional, but optional, monomers (e.g., (meth)acrylates,
(meth)acrylonitrile and (meth)acrylamides). The carboxy-containing
recurring units can be derived, for example, from acrylic acid,
methacrylic acid, itaconic acid, maleic acid, and similar monomers
known in the art. Other suitable materials include polymer binders
having pendant epoxy groups. Particularly useful polymers of this
type have pendant epoxy groups attached to the polymer backbone
through a carboxylic acid ester group such as a substituted or
unsubstituted --C(O)O-alkylene, --C(O)O-alkylene-phenylene-, or
--C(O)O-phenylene group wherein the alkylene has 1 to 4 carbon
atoms. Preferred ethylenically unsaturated polymerizable monomers
having pendant epoxy groups useful to make these polymer binders
include glycidyl acrylate, glycidyl methacrylate,
3,4-epoxycyclohexyl methacrylate, and 3,4-epoxycyclohexyl acrylate.
The epoxy-containing polymers can also comprise recurring units
derived from one or more ethylenically unsaturated polymerizable
monomers that do not have pendant epoxy groups including but not
limited to, (meth)acrylates, (meth)acrylamides, vinyl ether, vinyl
esters, vinyl ketones, olefins, unsaturated imides (such as
maleimide), N-vinyl pyrrolidones, N-vinyl carbazole, vinyl
pyridines, (meth)acrylonitriles, and styrenic monomers. Of these,
the (meth)acrylates, (meth)acrylamides, and styrenic monomers are
preferred and the styrenic monomers are most preferred. For
example, a styrenic monomer could be used in combination with
methacrylamide, acrylonitrile, maleimide, vinyl acetate, or N-vinyl
pyrrolidone.
[0043] Still other useful materials include polyvinyl acetals,
(meth)acrylic resins comprising carboxy groups, vinyl acetate
crotonate-vinyl neodecanoate copolymer phenolic resins, maleated
wood rosins, styrene-maleic anhydride co-polymers, (meth)acrylamide
polymers, polymers derived from an N-substituted cyclic imide, and
combinations thereof. Particularly useful materials include
polyvinyl acetals, and copolymers derived from an N-substituted
cyclic imide (especially N-phenylmaleimide), a (meth)acrylamide
(especially methacrylamide), and a (meth)acrylic acid (especially
methacrylic acid). The preferred polymeric materials of this type
are copolymers that comprise from about 20 to about 75 mol % and
preferably about 35 to about 60 mol % of recurring units derived
from N-phenylmaleimide, N-cyclohexylmaleimide, N-benzylmaleimide,
or a mixture thereof; from about 10 to about 50 mol % and
preferably from about 15 to about 40 mol % of recurring units
derived from acrylamide, methacrylamide, or a mixture thereof; and
from about 5 to about 30 mol % and preferably about 10 to about 30
mol % of recurring units derived from methacrylic acid. Other
hydrophilic monomers, such as hydroxyethyl methacrylate, may be
used in place of some or all of the methacrylamide. Other
alkaline-soluble monomers, such as acrylic acid, may be used in
place of some or all of the methacrylic acid.
[0044] Further suitable polymeric materials include resins having
activated methylol and/or activated alkylated methylol groups. Such
resins include, for example, resole resins and their alkylated
analogs, methylol melamine resins and their alkylated analogs
(e.g., melamine-formaldehyde resins), methylol glycoluril resins
and alkylated analogs (e.g., glycoluril-formaldehyde resins),
thiourea-formaldehyde resins, guanamine-formaldehyde resins, and
benzoguanamine-formaldehyde resins. Commercially available
melamine-formaldehyde resins and glycoluril-formaldehyde resins
include, for example, CYMEL resins (Dyno Cyanamid) and NIKALAC
resins (Sanwa Chemical). The resin having activated methylol and/or
activated alkylated methylol groups is preferably a resole resin or
a mixture of resole resins. Resole resins are well known to those
skilled in the art. They are prepared by reaction of a phenol with
an aldehyde under basic conditions using an excess of phenol.
Commercially available resole resins include, for example, GP649D99
resole (Georgia Pacific).
[0045] c. Substrate 102
[0046] The substrate 102 provides dimensionally stable mechanical
support to the printing member 100. The substrate 102 should be
strong, stable, and flexible. One or more surfaces (and, in some
cases, bulk components) of the substrate may be hydrophilic. The
topmost surface, however, is generally oleophilic. Suitable
materials include, but are not limited to, polymers, metals and
paper, but generally, it is preferred to have a polymeric
ink-accepting layer (e.g., applied over a metal or paper support).
As used herein, the term "substrate" refers generically to the
ink-accepting layer beneath the radiation-sensitive layer 105,
although the substrate may, in fact, include multiple layers (e.g.,
an oleophilic film laminated to or coated on a metal support, or an
oleophilic coating over a paper support).
[0047] Substrate 102 desirably also exhibits high scattering with
respect to imaging radiation. This allows full utilization of the
radiation transmitted through overlying layers, as the scattering
causes back-reflection into layer 105 and consequent increases in
thermal efficiency. Polymers suitable for use in substrates
according to the invention include, but are not limited to,
polyesters (e.g., polyethylene terephthalate and polyethylene
naphthalate), polycarbonates, polyurethane, acrylic polymers,
polyamide polymers, phenolic polymers, polysulfones, polystyrene,
and cellulose acetate. A preferred polymeric substrate is
polyethylene terephthalate film, such as the polyester films
available from DuPont-Teijin Films (Hopewell, Va.) under the
trademarks MYLAR and MELINEX, for example. Also suitable are the
white polyester products from DuPont-Teijin such as MELINEX 927W,
928W 329, 329S, 331.
[0048] Polymeric substrates can be coated with a hard polymer
transition layer to improve the mechanical strength and durability
of the substrate and/or to alter the hydrophilicity or
oleophilicity of the surface of the substrate. Ultraviolet or
electron-beam cured acrylate coatings, for example, are suitable
for this purpose. Polymeric substrates can have thicknesses ranging
from about 50 .mu.m to about 500 .mu.m or more, depending on the
specific printing member application. For printing members in the
form of rolls, thicknesses of about 200 .mu.m are preferred. For
printing members that include transition layers, polymer substrates
having thicknesses of about 50 .mu.m to about 100 .mu.m are
preferred.
2. Imaging and Printing
[0049] An imaging apparatus suitable for use in conjunction with
the present printing members includes at least one laser device
that emits in the region of maximum plate responsiveness, i.e.,
whose .lamda..sub.max closely approximates the wavelength region
where the imaging layer 105 absorbs most strongly. Specifications
for lasers that emit in the near-infrared (IR) region are fully
described in U.S. Pat. No. Re. 35,512 ("the '512 patent") and U.S.
Pat. No. 5,385,092 ("the '092 patent"), the entire disclosures of
which are hereby incorporated by reference. Lasers emitting in
other regions of the electromagnetic spectrum are well-known to
those skilled in the art.
[0050] Suitable imaging configurations are also set forth in detail
in the '512 and '092 patents. Briefly, laser output can be provided
directly to the plate surface via lenses or other beam-guiding
components, or transmitted to the surface of a blank printing plate
from a remotely sited laser using a fiber-optic cable. A controller
and associated positioning hardware maintain the beam output at a
precise orientation with respect to the plate surface, scan the
output over the surface, and activate the laser at positions
adjacent selected points or areas of the plate. The controller
responds to incoming image signals corresponding to the original
document or picture being copied onto the plate to produce a
precise negative or positive image of that original. The image
signals are stored as a bitmap data file on a computer. Such files
may be generated by a raster image processor ("RIP") or other
suitable means. For example, a RIP can accept input data in
page-description language, which defines all of the features
required to be transferred onto the printing plate, or as a
combination of page-description language and one or more image data
files. The bitmaps are constructed to define the hue of the color
as well as screen frequencies and angles.
[0051] Other imaging systems, such as those involving light valving
and similar arrangements, can also be employed; see, e.g., U.S.
Pat. Nos. 4,577,932; 5,517,359; 5,802,034; and 5,861,992, the
entire disclosures of which are hereby incorporated by reference.
Moreover, it should also be noted that image spots may be applied
in an adjacent or in an overlapping fashion.
[0052] The imaging apparatus can operate on its own, functioning
solely as a platemaker, or can be incorporated directly into a
lithographic printing press. In the latter case, printing may
commence immediately after application of the image to a blank
plate, thereby reducing press set-up time considerably. The imaging
apparatus can be configured as a flatbed recorder or as a drum
recorder, with the lithographic plate blank mounted to the interior
or exterior cylindrical surface of the drum. Obviously, the
exterior drum design is more appropriate to use in situ, on a
lithographic press, in which case the print cylinder itself
constitutes the drum component of the recorder or plotter.
[0053] In the drum configuration, the requisite relative motion
between the laser beam and the plate is achieved by rotating the
drum (and the plate mounted thereon) about its axis and moving the
beam parallel to the rotation axis, thereby scanning the plate
circumferentially so the image "grows" in the axial direction.
Alternatively, the beam can move parallel to the drum axis and,
after each pass across the plate, increment angularly so that the
image on the plate "grows" circumferentially. In both cases, after
a complete scan by the beam, an image corresponding (positively or
negatively) to the original document or picture will have been
applied to the surface of the plate.
[0054] In the flatbed configuration, the beam is drawn across
either axis of the plate, and is indexed along the other axis after
each pass. Of course, the requisite relative motion between the
beam and the plate may be produced by movement of the plate rather
than (or in addition to) movement of the beam. Examples of useful
imaging devices include models of the TRENDSETTER imagesetters
(available from Eastman Kodak Company) that utilize laser diodes
emitting near-IR radiation at a wavelength of about 830 nm. Other
suitable exposure units include the CRESCENT 42T Platesetter
(operating at a wavelength of 1064 nm, available from Gerber
Scientific, Chicago, Ill.) and the SCREEN PLATERITE 4300 series or
8600 series plate-setter (available from Screen, Chicago,
Ill.).
[0055] Regardless of the manner in which the beam is scanned, in an
array-type system for on-press applications it is generally
preferable to employ a plurality of lasers and guide their outputs
to a single writing array. The writing array is then indexed, after
completion of each pass across or along the plate, a distance
determined by the number of beams emanating from the array, and by
the desired resolution (i.e., the number of image points per unit
length). Off-press applications, which can be designed to
accommodate very rapid scanning (e.g., through use of high-speed
motors, mirrors, etc.) and thereby utilize high laser pulse rates,
can frequently utilize a single laser as an imaging source.
[0056] FIGS. 2A-2C show the consequences of imaging the printing
member 100. As illustrated in FIG. 2B, the exposed area of the
layer 105 absorbs the imaging pulse and converts it to heat. The
heat builds up until the layer 105 ablates, i.e., undergoes either
rapid phase transformation (e.g., vaporization) or catastrophic
thermal overload. This process is mainly attributed to the
contribution of an explosive mechanism generated in the image areas
of the plate by exposure to laser radiation.
[0057] After imaging, the topmost layer 110 is degraded and/or
de-anchored in the areas that received imaging radiation. The
exposed areas that contain ablation debris are either purged of the
debris prior to printing or during print "make ready." In some
embodiments, the printing member can be used on press immediately
after being imaged without the need for a post-imaging processing
step. More typically, however, the de-anchored overlying layer and
the central layer are removed (at least partially) by a
post-imaging cleaning process--e.g., rubbing of the plate with or
without a cleaning liquid--to reveal the third layer (typically an
oleophilic polymer, such as polyester). In a preferred embodiment,
the printing member is cleaned with a cleaning liquid that is
aqueous in nature and free of organic solvent. For example, the
cleaning liquid may consist essentially of water, e.g., plain tap
water, and may be warmed to a temperature of 32-50.degree. C. The
cleaning liquid may be applied manually, using a cloth and/or
brush, or mechanically using commercial plate-cleaning equipment.
For example, many platemakers contain internal rotary scrub rollers
that clean the plate following imaging.
[0058] Printing with the printing member includes disposing ink on
at least a portion of the printing member, preferably the
oleophilic exposed areas. The ink is transferred in the imagewise
lithographic pattern (created as described above) to a recording
medium such as paper. The inking and transferring steps may be
repeated a desired number of times, e.g., the approximately 5,000
to approximately 50,000 times in a low to medium printing run.
EXAMPLES
Examples 1-3 and C1
[0059] These examples describe negative-working waterless printing
plates that include a highly crosslinked silicone layer disposed
above a thin titanium imaging layer that is itself disposed on a
polyester substrate. A preferred substrate is a 175 .mu.m white
polyester film sold by DuPont-Teijin Films (Hopewell, Va.) under
the name MELINEX 927W.
[0060] The imaging layer of the printing member is a thin titanium
layer disposed on the polymer substrate by DC magnetron sputtering
of a titanium target using ultrahigh purity argon as the sputtering
gas. Prior to the metal deposition, the sputtering chamber is
evacuated to base pressures below 3.times.10.sup.-5 Torr and
back-filled with argon up to a pressure of 5 mTorr. The sputtering
process yields a layer of mixed metal/metal oxide composition with
total thickness of 25 nm which, upon exposure to air, reacts with
water molecules yielding a surface rich in hydroxyl functional
groups. The latter act as the bonding sites for the adhesion of the
polymer top-silicone layer of the plate structure.
[0061] The silicone layers used in the present invention may have a
highly crosslinked network structure produced via the addition or
hydrosilylation reaction between the vinyl groups (SiVi) of
vinyl-terminated functional silicones and the silicon hydride
groups of trimethylsiloxy-terminated poly(hydrogen methylsiloxane)
crosslinker, in the presence of a Pt catalyst complex and an
inhibitor.
[0062] The vinyl polymers employed in the following examples have a
nominal chemical formula of
[CH.sub.2.dbd.CH--Si(CH.sub.3).sub.2--O--[{Si(CH.sub.3).sub.2--O}.sub.n---
Si(CH.sub.3).sub.2--CH.dbd.CH.sub.2] with an average degree of
polymerization, n, between 840 and 1580 and molecular weight
ranging from 62,000 to 117,000 g/mol. The below table details the
properties of a series of vinyl-terminated polydimethylsiloxanes
(PDMS) manufactured by Gelest Inc. (Morrisville, Pa.) used in the
ensuing examples. A higher-molecular-weight example is also
included for comparative purposes.
TABLE-US-00001 Product Molecular Weight Viscosity Density Weight %
name (g/mol) (cps) (g/l) Vinyl DMS-V41 62,700 10,000 0.97 0.08-0.12
DMS-V42 72,000 20,000 0.98 0.07-0.09 DMS-V46 117,000 60,000 0.98
0.04-0.06 DMS-V52 155,000 165,000 0.98 0.03-0.04
[0063] Silicone formulations of the following composition, by
weight, were prepared with each of the vinyl-functional PDMS
examples listed in the previous table.
TABLE-US-00002 Component Parts Vinyl Silicone 12.40 DC Syl Off 7367
Crosslinker 0.53 CPC 072 Pt Catalyst 0.17 Heptane 86.9
[0064] The DC SYL OFF 7367 is a trimethylsiloxy-terminated
poly(hydrogen methylsiloxane) crosslinker manufactured by Dow
Corning Silicones (Midland, Mich.) which is supplied as a 100%
solids solution containing about 30% 1-ethynylcyclohexane
[CH.ident.CH--CH(CH.sub.2).sub.5], which functions as catalyst
inhibitor. This crosslinker has a nominal formula of
[(CH.sub.3).sub.3Si--O--{SiH(CH.sub.3)--O}.sub.m--Si(CH.sub.3).sub.3]
with degree of polymerization, m, between 30 to 32, and average
molecular weight of about 2000 g/mol.
[0065] The CPC 072 is a 1,3
diethyenyl-1,1,3,3-tetramethyldisiloxane Pt complex catalyst
manufactured by Umicore Precious Metals (South Plainfield, N.J.)
which is supplied as a 3% xylene solution. The formulation solvent,
heptane, is supplied by Houghton Chemicals (Allston, Mass.).
[0066] Silicone formulations were prepared by mixing of catalyst
and vinyl PDMS in heptane, followed by addition of a heptane
solution of the crosslinker. The formulations were applied to the
metalized substrate with a wire-round rod, then dried and cured at
138.degree. C. for one minute to produce uniform silicone coatings
of 1.1 g/m.sup.2.
[0067] The printing members of these examples were evaluated by two
different methods to obtain information on the effect of the
changes in silicone formulation on plate durability. First, plates
were tested in the laboratory by assessing adhesion and solvent
resistance with methyl ethyl ketone (MEK). A MEK resistance test is
performed on pieces of the plate examples having lengths of about
20 cm by applying, in a reciprocating mode, MEK double-rubs using a
five-pound load. The cycle is repeated to the point of visual
evidence failure, i.e., marring of the surface or loss of silicone
adhesion.
[0068] These plate examples were also imaged and run on-press to
determine their press durability. The plates were imaged on a
Presstek DIMENSION 425 plate setter at a power of 300 mJ/cm.sup.2
and manually cleaned to remove the loosened silicone debris left on
the plate after imaging. The cleaning involves a two-step
procedure: first, a dry rubbing the surface with a dry cotton
towel, and second, wet rubbing with a cotton towel saturated with
isopropanol.
[0069] The press durability of the printing members was evaluated
on a GTO Heidelberg press using black ink (Aqualess Ultra Black MZ
waterless ink, Toyo Ink America LLC, Addison, Ill.) and uncoated
stock (Williamsburg Plus Offset Smooth, 60 lb white, item number:
05327, International Paper, Memphis, Tenn.). The plates were run
until signs of background wear were observed on the printing
sheets. The number of print sheets at which background wear starts
to appear is reported as the press durability of these printing
members under the described printing conditions.
[0070] The following table shows the results of the laboratory and
press evaluation of different printing plate examples.
TABLE-US-00003 Vi-PDMS Molecular Weight Press Example (g/mol)
SiH:SiVi MEK Rubs Durability Example 1 62,700 13.3:1 15-20-25 5000
to 6000 Example 2 72,000 15.2:1 20-25-30 5000 to 6000 Example 3
117,000 24.7:1 15-20-25 5000 to 6000 Example C1 155,000 32.8:1 5-10
3000
[0071] Both the MEK solvent resistance and press testing
demonstrate that printing members made with vinyl-functional PDMS
resins of molecular weight from 62,700 to 117,000 g/mol and having
high SiH:SiVi ratios between 13:1 and 25:1 display at least twice
the durability of the comparative example made with a
high-molecular-weight vinyl silicone.
Examples 4-6 and C2
[0072] Printing members similar to those described in Examples 1-3
were made with a thicker silicone top layers to create more durable
printing members. In these examples, the same formulations were
applied to the metalized substrate with a wire-round rod, then
dried and cured at 138.degree. C. for one minute to produce
coatings of 2.0 g/m.sup.2.
[0073] The printing members were imaged, cleaned, and tested as
described in Examples 1-3. The performance of these examples is
summarized in the following table:
TABLE-US-00004 Vi-PDMS Molecular weight Formulation Press Example
(g/mol) SiH:SiVi MEK Rubs Durability Example 4 62,700 13.3:1 30-35
12,000 to 14,000 Example 5 72,000 15.2:1 30-35 13,000 to 15,000
Example 6 117,000 24.7:1 30-35 15,000 to 16,000 Example C2 155,000
32.8:1 10-15 8000
[0074] Consistent with the results obtained in the previous
examples, the results show that printing members made with the
intermediate-molecular-weight silicone resin exhibit improved
durability, which is at least twice that of the comparative Example
C2 made with the same silicone layer thickness. Furthermore,
doubling the coating weight leads to at least a two-fold increase
in durability compared to the above examples made with the 1.1
g/m.sup.2 silicone layer.
Examples 7 and C3-C5
[0075] Silicone formulations of the following composition, by
weight, were made with the vinyl-functional PDMS formulations
described above. These examples contain about half the
concentration of the crosslinker used in Examples 1-3.
TABLE-US-00005 Component Parts Vinyl Silicone 12.40 DC Syl Off 7367
Crosslinker 0.27 CPC 072 Pt Catalyst 0.17 Heptane 87.16
[0076] The formulations were applied to the metalized substrate
with a wire-round rod, then dried and cured at 138.degree. C. for
one minute to produce coatings of about 1.1 g/m.sup.2.
TABLE-US-00006 Vi-PDMS Molecular weight Formulation Example (g/mol)
SiH:SiVi MEK Rubs Example C3 62,700 6.6:1 5-10 Example C4 72,000
7.6:1 5-10 Example 7 117,000 12.4:1 15-20 Example C5 155,000 16.4:1
5-10
[0077] The reduction of the crosslinker concentration leads to
considerable loss of solvent resistance by the printing members of
Examples C3 and C4 made with the lower-molecular-weight resins
(compared to Examples 1 and 2). The change may be attributed to the
reduction of crosslinker molecules available for crosslinking and
bonding to the metal surface. Note that these two formulations have
a much lower SiH:SiVi ratio than that used in the earlier
examples.
[0078] The decrease in crosslinker density has slight or no effect
on the solvent resistance of Example 7 (compared to Example 3) made
with the higher-molecular-weight resin. The formulation of Example
7 has a relatively high (>10:1) SiH:SiVi ratio, which appears
sufficient to achieve formation of a highly crosslinked silicone
network and provide adhesion to the metal substrate.
Examples 8 and C6-C8
[0079] Silicone formulations of the following composition, by
weight, were prepared with each of the vinyl-functional PDMS
formulations described above. These examples contain one-fourth the
concentration of the crosslinker used in Examples 1-3.
TABLE-US-00007 Component Parts Vinyl Silicone 12.40 DC Syl Off 7367
Crosslinker 0.13 CPC 072 Pt Catalyst 0.17 Heptane 87.30
TABLE-US-00008 Vi-PDMS Molecular weight Formulation Example (g/mol)
SiH:SiVi MEK Rubs Example C6 62,700 3.3:1 0-5 Example C7 72,000
3.8:1 0-5 Example 8 117,000 6.2:1 15-20 Example C8 155,000 8.2:1
10-15
[0080] Comparative Examples C6 and C7 present very low solvent
resistance. This is consistent with the observations made regarding
Examples C3 and C4, and shows that further reduction of SiH:SiVi
ratio to values lower than six causes complete loss of the MEK
resistance of these silicone coatings. An important observation is
that Example 8, made with the 117,000 g/mol vinyl PDMS, still
displays good solvent resistance. Note that Example 8 has
relatively high (>6:1) SiH: SiVi ratio.
Examples C9-C12
[0081] Silicone formulations of the following composition, by
weight, are prepared with each of the vinyl-functional PDMS
described above. These examples contain twice the concentration of
the crosslinker used of Examples 1-3.
TABLE-US-00009 Component Parts Vinyl Silicone 12.40 DC Syl Off 7367
Crosslinker 1.07 CPC 072 Pt Catalyst 0.17 Heptane 86.36
[0082] The formulations were applied to the metalized substrate
with a wire-round rod, then dried and cured at 138.degree. C. for
one minute to produce coatings of 1.1 g/m.sup.2.
TABLE-US-00010 Vi-PDMS Molecular weight Formulation Example (g/mol
SiH:SiVi MEK Rubs Example C9 62,700 26.5:1 0-5 Example C10 72,000
30.4:1 0-5 Example C11 117,000 49.4:1 0-5 Example C12 155,000
65.5:1 0-5
[0083] These comparative examples show that an excess of
crosslinking molecules causes complete loss of the solvent
resistance of the printing members made with the
intermediate-molecular-weight PDMS resins. The formulations of
these four examples would lead to the formation of silicone
coatings with large amounts of loose residual crosslinker.
[0084] It will be seen that the foregoing techniques provide a
basis for improved lithographic printing and superior plate
constructions. The terms and expressions employed herein are used
as terms of description and not of limitation, and there is no
intention in the use of such terms and expressions of excluding any
equivalents of the features shown and described or portions
thereof. Instead, it is recognized that various modifications are
possible within the scope of the invention claimed.
* * * * *