BACKGROUND
[0001] The present disclosure relates to the use of certain solvents in dampening fluids
used during variable data lithographic printing. This disclosure also relates to apparatuses
using such dampening fluids, and methods of using such dampening fluids, such as in
variable lithographic printing applications.
[0002] Offset lithography is a common method of printing today. (For the purposes hereof,
the terms "printing" and "marking" are interchangeable.) In a typical lithographic
process a printing plate, which may be a flat plate, the surface of a cylinder, or
belt, etc., is formed to have "image regions" formed of a hydrophobic / oleophilic
material, and "non-image regions" formed of a hydrophilic / oleophobic material. The
image regions correspond to the areas on the final print (i.e., the target substrate)
that are occupied by a printing or marking material such as ink, whereas the non-image
regions correspond to the areas on the final print that are not occupied by said marking
material. The hydrophilic regions accept and are readily wetted by a water-based fluid,
commonly referred to as a dampening fluid or fountain fluid (typically consisting
of water and a small amount of alcohol as well as other additives and/or surfactants
to reduce surface tension). The hydrophobic regions repel dampening fluid and accept
ink, whereas the dampening fluid formed over the hydrophilic regions forms a fluid
"release layer" for rejecting ink. The hydrophilic regions of the printing plate thus
correspond to unprinted areas, or "non-image areas", of the final print.
[0003] The ink may be transferred directly to a target substrate, such as paper, or may
be applied to an intermediate surface, such as an offset (or blanket) cylinder in
an offset printing system. The offset cylinder is covered with a conformable coating
or sleeve with a surface that can conform to the texture of the target substrate,
which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley
depth of the imaging plate. Also, the surface roughness of the offset blanket cylinder
helps to deliver a more uniform layer of printing material to the target substrate
free of defects such as mottle. Sufficient pressure is used to transfer the image
from the offset cylinder to the target substrate. Pinching the target substrate between
the offset cylinder and an impression cylinder provides this pressure.
[0004] Typical lithographic and offset printing techniques utilize plates which are permanently
patterned, and are therefore useful only when printing a large number of copies of
the same image (i.e. long print runs), such as magazines, newspapers, and the like.
However, they do not permit creating and printing a new pattern from one page to the
next without removing and replacing the print cylinder and/or the imaging plate (i.e.,
the technique cannot accommodate true high speed variable data printing wherein the
image changes from impression to impression, for example, as in the case of digital
printing systems). Furthermore, the cost of the permanently patterned imaging plates
or cylinders is amortized over the number of copies. The cost per printed copy is
therefore higher for shorter print runs of the same image than for longer print runs
of the same image, as opposed to prints from digital printing systems.
[0005] Accordingly, a lithographic technique, referred to as variable data lithography,
has been developed which uses a non-patterned reimageable surface that is initially
uniformly coated with a dampening fluid layer. Regions of the dampening fluid are
removed by exposure to a focused radiation source (e.g., a laser light source) to
form pockets. A temporary pattern in the dampening fluid is thereby formed over the
non-patterned reimageable surface. Ink applied thereover is retained in the pockets
formed by the removal of the dampening fluid. The inked surface is then brought into
contact with a substrate, and the ink transfers from the pockets in the dampening
fluid layer to the substrate. The dampening fluid may then be removed, a new uniform
layer of dampening fluid applied to the reimageable surface, and the process repeated.
[0006] The patterning of dampening fluid on the reimageable surface member in variable data
lithography essentially involves using a laser or some other energy source to selectively
boil off or ablate the dampening fluid in selected locations. This process can be
energy intensive due to the large latent heat of vaporization of water. At the same
time, high-speed printing necessitates the use of high-speed modulation of that energy
source, which can be prohibitively expensive for high power lasers. Therefore, from
both an energy and cost perspective, it would beneficial to reduce the total amount
of energy that is needed to achieve pattern-wise vaporization of the dampening fluid.
[0007] The essential role of the dampening fluid in both traditional offset printing and
in variable lithographic printing is to provide selectivity for the imaging and transfer
of the ink. Dampening fluid generally contains water and some additives to reduce
surface tension, such as a surfactant. The dampening fluid acts as a low viscosity
release layer film which preferentially splits at the inking nip, thus preventing
ink adhesion to the imaging member surface. In addition, the dampening fluid is to
a large degree immiscible with the ink chemistry, being oleophobic in its chemical
nature. Otherwise, the dampening fluid can break apart into small emulsified droplets
with the ink which can lead to background tinting.
[0008] As already discussed above, an additional consideration in variable lithographic
printing is the energy necessary to boil off the dampening fluid. For example, water
is a very polar molecule and has both high surface tension and a high latent heat
of vaporization, which relates to the energy required to change water from its liquid
phase to its vapor phase. The high heat of vaporization leads to high energy requirements
for the laser used to vaporize the dampening fluid.
[0009] A further consideration related to the thermal properties of the dampening fluid
is the dampening fluid boiling temperature. Too low a boiling temperature will mean
quick thinning of the fluid due to its partial pressure evaporation near room temperature.
It is desirable to have a high enough boiling temperature such that the evaporation
rate does not compete with the laser boiling process because this insures better image
definition. On the other hand, too high a boiling temperature means added laser energy
is necessary due to the specific heat of the dampening fluid required for raising
its temperature up to the boiling point, and this can therefore reduce the overall
printing speed for a given laser power.The high surface tension of water causes the
dampening fluid to tend to bead, rather than to spread evenly over the surface of
the imaging member. To reduce the surface tension, dampening fluid usually includes
another solvent which is less polar than water, such as isopropanol (IPA). However,
isopropanol is a volatile organic compound (VOC), and its emission is regulated. In
addition variable lithographic imaging typically uses an elastomeric surface, and
IPA is known to cause swelling in may many elastomeric surface materials. Other aqueous-based
surfactants tend to have high boiling points and therefore leave a residue behind
on the surface of the imaging member, compromising the integrity of the imaging member
for making images of suitable quality.
[0010] It would be desirable to provide dampening fluids that can avoid such problems for
variable lithographic printing.
BRIEF DESCRIPTION
[0011] Disclosed in various embodiments are dampening fluids, systems, and processes that
are useful for variable lithographic printing. Conventional offset lithographic systems
use a polar dampening fluid and a non-polar ink to form the images. In the present
disclosure, the dampening fluid is relatively non-polar and the ink is relatively
polar instead. By choosing the proper reimageable imaging member chemistry, ink chemistry,
and dampening fluid chemistry, a system can be arrived at wherein both the ink and
the dampening fluid will wet the surface of the rewriteable imaging member, and the
dampening fluid will still energetically maintain its wetting of the surface in the
presence of the ink. Such a configuration is arrived at by considering the wetting
conditions over a surface with regards to both the polar and dispersive components
of the surface tensions and energies of all three components: imaging member surface,
ink, and dampening fluid. Among other advantages, this system actually allows the
dampening fluid to clean off any small ink residues left behind by previous passes
of the imaging member when a new layer of dampening solution is applied after each
print pass.
[0012] Disclosed in embodiments is a dampening fluid for variable lithographic printing.
The dampening fluid comprises a solvent which is a volatile hydrofluoroether liquid
or a volatile silicone liquid. These classes of fluids provides advantages in the
amount of energy needed to evaporate, desirable characteristics in the dispersive/polar
surface tension design space, and the additional benefit of zero residue left behind
once evaporated.
[0013] The solvent may be a volatile hydrofluoroether liquid having the structure of Formula
(I):
C
mHpF
2m+1-p-O-C
nH
qF
2n+1-q Formula (I)
wherein m and n are independently integers from 1 to about 9; and p and q are independently
integers from 0 to 19.
[0015] The solvent may be a volatile silicone liquid having the structure of Formula (II):
wherein R
a, R
b, R
c, R
d, R
e, and R
f are each independently hydrogen, alkyl, or perfluoroalkyl; and a is an integer from
1 to about 5. In particular embodiments, the volatile silicone liquid is hexamethyldisiloxane
or octamethyltrisiloxane.
[0016] The volatile silicone liquid may alternatively have the structure of Formula (III):
wherein each Rg and R
h is independently hydrogen, alkyl, or perfluoroalkyl; and b is an integer from 3 to
about 8.
[0017] The volatile silicone liquid can be octamethylcyclotetrasiloxane or decamethylcyclopentasiloxane.
In specific embodiments, the volatile silicone liquid is a mixture of octamethylcyclotetrasiloxane
and decamethylcyclopentasiloxane.
[0018] The volatile silicone liquid can also be a mixture of hexamethylcyclotrisiloxane
and octamethylcyclotetrasiloxane. Hexamethylcyclotrisiloxane (aka D3) usually forms
a solid at room temperature but when a small amount is added to octamethylcyclotetrasilonxane,
typically less than 30% by total weight, the mixture will form a continuous fluid.
The advantage of this mixture is the boiling temperature of octamethylcyclotretrasiloxane
can be reduced thereby reducing the laser power needed.
[0019] The volatile silicone liquid may also have the structure of Formula (IV):
wherein R
1, R
2, R
3, and R
4 are independently alkyl or -OSiR
1R
2R
3.
[0020] The volatile silicone liquid may have the structure of Formula (IV-a):
[0021] The dampening fluid may have a surface tension of from about 15 to about 30 dynes/cm.
In particular embodiments, the dampening fluid has a kinematic viscosity greater than
1 centiStokes at 25°C and a surface tension of less than 72 dynes/cm at 25°C. In other
embodiments, the solvent has a heat of vaporization of less than 120 kJ/kg when measured
at 1 atmosphere and 25°C.
[0022] Also disclosed is a process for variable lithographic printing using such dampening
fluids disclosed. A dampening fluid is applied to an imaging member surface, wherein
the dampening fluid comprises a solvent which is a volatile hydrofluoroether liquid
or a volatile silicone liquid. A latent image is formed by evaporating the dampening
fluid from selective locations on the imaging member surface to form hydrophobic non-image
areas and hydrophilic image areas. The latent image is developed by applying a polar
ink to the hydrophilic image areas. The developed latent image is then transferred
to a receiving substrate.
[0023] The polar ink may comprise an acrylate monomer. Alternatively, the polar ink may
comprise a monomer containing an ester, ether, carbonyl, amino, cyano, or hydroxyl
group. In other embodiments, the polar component of the surface tension of the ink
is larger than the polar component of the surface tension of the dampening fluid.
[0024] Also disclosed in embodiments is a dampening fluid for variable lithographic printing,
which comprises a solvent having a heat of vaporization of less than 200 kJ/kg.
[0025] Also disclosed in various embodiments is a variable lithographic system comprising
an ink, a dampening fluid, and an imaging member surface. The dampening fluid has
a surface energy which is less than the surface energy of the ink and the surface
energy of the imaging member surface.
[0026] In embodiments, the dampening fluid has a total surface energy of less than 30 dynes/cm
and a polar surface energy component less than 5 dyne/cm. The imaging member surface
may have a surface energy of less than 30 dynes/cm.
[0027] These and other non-limiting aspects and/or objects of the disclosure are more particularly
described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The patent or application file contains at least one drawing executed in color. Copies
of this patent or patent application publication with color drawing(s) will be provided
by the Office upon request and payment of the necessary fee.
[0029] The following is a brief description of the drawings, which are presented for the
purposes of illustrating the exemplary embodiments disclosed herein and not for the
purposes of limiting the same.
[0030] 3
FIG. 1 illustrates a variable lithographic printing apparatus in which the dampening fluids
of the present disclosure may be used.
[0031] FIG. 2 is an alpha versus beta graph depicting the wetting conditions in the image area
of an offset plate for a conventional oil-based offset ink.
[0032] FIG. 3 is an alpha versus beta graph depicting the wetting conditions of a conventional
water-based offset dampening fluid in the non-image area of an offset plate. The dotted
circle represents the wetting condition of the dampening fluid in the presence of
ink.
[0033] FIG. 4 is an alpha versus beta graph depicting the wetting conditions obtained for variable
lithographic printing in the present disclosure, wherein both ink and dampening fluid
can both wet the surface of the imaging member simultaneously and the dampening fluid
also wets the plate in the presence of ink (seen as the dotted circle).
[0034] FIG. 5 is a view of a set of characters printed using a fountain solution of 100% Novec™
7500.
[0035] FIG. 6 is a view of the same set of characters printed using a water-based fountain solution
for comparison with
FIG. 5.
DETAILED DESCRIPTION
[0036] A more complete understanding of the processes and apparatuses disclosed herein can
be obtained by reference to the accompanying drawings. These figures are merely schematic
representations based on convenience and the ease of demonstrating the existing art
and/or the present development, and are, therefore, not intended to indicate relative
size and dimensions of the assemblies or components thereof.
[0037] Although specific terms are used in the following description for the sake of clarity,
these terms are intended to refer only to the particular structure of the embodiments
selected for illustration in the drawings, and are not intended to define or limit
the scope of the disclosure. In the drawings and the following description below,
it is to be understood that like numeric designations refer to components of like
function.
[0038] The modifier "about" used in connection with a quantity is inclusive of the stated
value and has the meaning dictated by the context (for example, it includes at least
the degree of error associated with the measurement of the particular quantity). When
used with a specific value, it should also be considered as disclosing that value.
For example, the term "about 2" also discloses the value "2" and the range "from about
2 to about 4" also discloses the range "from 2 to 4."
[0039] FIG. 1 illustrates a system for variable lithography in which the dampening fluids of the
present disclosure may be used. The system
10 comprises an imaging member
12. The imaging member comprises a substrate
22 and a reimageable surface layer
20. The surface layer is the outermost layer of the imaging member, i.e. the layer of
the imaging member furthest from the substrate. As shown here, the substrate
22 is in the shape of a cylinder; however, the substrate may also be in a belt form,
etc. The surface layer
20 is typically a silicone (e.g. a methylsilicone or fluorosilicone), which may have
carbon black added to increase energy absorption of the surface layer.
[0040] In the depicted embodiment the imaging member
12 rotates counterclockwise and starts with a clean surface. Disposed at a first location
is a dampening fluid subsystem
30, which uniformly wets the surface with dampening fluid
32 to form a layer having a uniform and controlled thickness. Ideally the dampening
fluid layer is between about 0.15 micrometers and about 1.0 micrometers in thickness,
is uniform, and is without pinholes. As explained further below, the composition of
the dampening fluid aids in leveling and layer thickness uniformity. A sensor
34, such as an in-situ non-contact laser gloss sensor or laser contrast sensor, is used
to confirm the uniformity of the layer. Such a sensor can be used to automate the
dampening fluid subsystem
30.
[0041] At optical patterning subsystem
36, the dampening fluid layer is exposed to an energy source (e.g. a laser) that selectively
applies energy to portions of the layer to imagewise evaporate the dampening fluid
and create a latent "negative" of the ink image that is desired to be printed on the
receiving substrate. Image areas are created where ink is desired, and non-image areas
are created where the dampening fluid remains. An optional air knife
44 is also shown here to control airflow over the surface layer
20 for the purpose of maintaining clean dry air supply, a controlled air temperature,
and reducing dust contamination prior to inking. Next, an ink is applied to the imaging
member using inker subsystem
46. Inker subsystem
46 may consist of a "keyless" system using an anilox roller to meter an offset ink onto
one or more forming rollers
46A, 46B. Ink is applied to the image areas to form an ink image.
[0042] A rheology control subsystem
50 partially cures or tacks the ink image. This curing source may be, for example, an
ultraviolet light emitting diode (UV-LED)
52, which can be focused as desired using optics
54. Another way of increasing the cohesion and viscosity employs cooling of the ink.
This could be done, for example, by blowing cool air over the reimageable surface
from jet 58 after the ink has been applied but before the ink is transferred to the
final substrate. Alternatively, a heating element
59 could be used near the inker subsystem
46 to maintain a first temperature and a cooling element
57 could be used to maintain a cooler second temperature near the nip
16.
[0043] The ink image is then transferred to the target or receiving substrate
14 at transfer subsystem
70. This is accomplished by passing a recording medium or receiving substrate
14, such as paper, through the nip
16 between the impression roller
18 and the imaging member
12.
[0044] Finally, the imaging member should be cleaned of any residual ink or dampening fluid.
Any dampening solution residue can be easily removed quickly using an air knife
77 with sufficient air flow. Removal of any remaining ink can be accomplished at cleaning
subsystem
72.
[0045] The role of the dampening fluid is to provide selectivity in the imaging and transfer
of ink to the receiving substrate. When an ink donor roll in the ink source of
FIG. 1 contacts the dampening fluid layer, the layer splits so that ink is only applied
to areas on the imaging member that are dry, i.e. not covered with dampening fluid.
[0046] As discussed above, water is usually the majority component of the dampening fluid
(by weight). Water itself has a high latent heat of vaporization, which leads to high
energy requirements at the imaging station.
[0047] In addition, water has a high surface tension of around 70 dynes/cm. This reduces
the ability of the dampening fluid to quickly form a thin film on the imaging member
surface. One conventional solvent that is sometimes also added to reduce surface tension
is isopropyl alcohol (i.e. isopropanol). However, isopropanol is a volatile organic
compound (VOC), and environmental regulations typically require lower emissions. For
example, the state of California requires that the amount of isopropanol either be
less than 5%, or that printing production equipment have solvent reclaim systems to
capture the emissions. At this level of isopropanol, the surface tension is still
too high to give good performance.
[0048] Thus, to reduce surface tension further, surfactants are typically added to the water
to lower the surface tension to around 20-30 dynes/cm. However, such surfactants usually
consist of copolymer molecules with hydrophilic heads and hydrophobic tails which
must be long enough to adequately wet both the surface of the imaging member and the
dampening fluid, and these copolymer molecules tend to have high boiling points above
200°C. As a result, these copolymer molecules can plate out non-uniformly as a residue
when the water is evaporated, leading to variability in the printing process as well
as dampening fluid layer ghosting, which occurs when the residue on the surface slightly
affects the thickness of a new layer of dampening fluid that is subsequently laid
down. A dampening fluid that produces little or no residual surfactant residue is
needed to provide precision control over the thickness of the dampening fluid.
[0049] In addition, water has a low kinematic viscosity of about 1 centiStoke (1 mm
2/sec). Generally, the dampening fluid must have a positive spreading coefficient so
that it can adequately wet the surface of the imaging member when initially laid down.
However, during the imaging process where dampening fluid is heated and evaporated
to form the latent image, the adjacent dampening fluid is also partially heated. This
partial heating further lowers the kinematic viscosity of the adjacent dampening fluid,
allowing it to either spread or pull back, depending on the geometry of the latent
image formed. For sharp convex corners in letters such as "W" or "V", a pull-back
effect occurs that leads to a rounding of concave corners in the ink when printing.
For lines without curvature, the dampening fluid exhibits a tendency to spread and
fill in evaporated areas, especially for thicker layers of dampening fluid (~ 2 µm
thickness). Over millisecond time scales, water-based dampening fluid will spread
somewhat even when the surface of the imaging member has a proper level of surface
roughness to help pin the dampening fluid in place. Sometimes arabic gum is added
to dampening fluid to increase the viscosity. However, arabic gum also has a high
boiling point and can leave residue behind.
[0050] It should be noted that one advantage of a water-based dampening fluid is that water
has a high polar component to its surface tension (surface tension can be separated
into two components, a polar component and a dispersive component). This helps the
dampening fluid reject ink, which tends to have a low polar surface tension and high
dispersive surface tension. As a result, the interfacial surface energy between the
ink and the dampening fluid remains high, and they mutually do not wet one another.
[0051] As a result, a desirable solvent for a dampening fluid should be a liquid having
a low heat of vaporization, low surface tension, and high kinematic viscosity. Unfortunately,
liquids having these three components tend to have a high dispersive surface tension
component and a low polar surface tension component. In a conventional lithographic
printing system, these liquids would tend to mix more readily with a polymer/oil-based
ink, which also tends to have high dispersive surface tension and low polar surface
tension. This would lead to background effects in the non-imaging areas (where ink
is not applied) and tinting in the imaging areas.
[0052] It should be noted that silicones or fluorosilicones are considered desirable and
useful materials for the surfaces of an imaging member because they have low surface
free energy and excellent ink release properties under a pressurized nip. Such materials
typically have a siloxane backbone with methyl (-CH
3) or trifluoromethyl (-CF
3) side chains. However, it is well known that many liquids cause elastomeric materials
to swell. It would be preferable to use a liquid for the dampening fluid solvent that
does not act as a plasticizer or solvent to these rubbery materials to provide for
long surface lifetime without wear. Some solvents have such a low molecular weight
that they inevitably cause some swelling of silicones and fluorosilicones depending
upon the degree of fluoro substitution. Wear can proceed indirectly under these swell
conditions by causing the release of near infrared laser energy-absorbing particles
at the imaging member surface, such as carbon black. These particles then act as abrasive
particles. Desirably, the dampening fluid liquid/solvent should have a low tendency
for swell or elastomer penetration.
[0053] If possible, it would be desirable for the dampening fluid to be easily recyclable.
This is useful both in reducing total waste and in lowering the overall cost of the
system. It would be helpful if the dampening fluid liquid had a density that was very
different compared to water, so that the liquid can be more easily separated if recondensed.
The liquid is also desirably non-toxic, exempt from VOC regulations, have a low global
warming potential, have low to no ozone depletion potential, and easily handled /
transported.
[0055] According to such surface energy models, the surface tension of any fluid or the
surface energy of an imaging surface can be represented in air as being primarily
composed of dispersive and polar components according to the relationship of equation
(i):
where γ is the total surface energy (or surface tension for a liquid) in units of
J/m
2 or more commonly given in dynes/cm. The total surface energy is composed of two orthogonal
components, the dispersive component γ
d and polar component γ
p, which act to a large degree independently. The polar and dispersive components of
surface tension for a liquid or surface energy for a solid can be calculated from
tensionmeter or contact angle measurements known in the art using commercial scientific
equipment provided by several equipment companies including, for example, First Ten
Angstroms, Inc located in Portsmouth, VA, Diversified Enterprises located in Claremont,
NH, or Biolin Scientific Inc. located in Linthicum Heights, MD. Alpha and beta of
equation (i) are further defined according to equations (ii) and (iii):
[0056] Alpha and beta are useful in describing molecular surface interactions acting across
molecular distances. Hydrogen bonding components of the liquid surface tensions are
usually not included in these models because they give rise to small effects but it
should be noted they cannot be neglected in determining the chemical compatibility
and miscibility of the dampening solution and ink chemistries, i.e. the diffusion
of one fluid into another.
[0057] The total surface energy γ of equation (i) applies to surfaces as measured in vacuum
or in dry air. However, the measured values for surface tension often change when
two fluids or a fluid and solid surface come in contact. Generally, the surface tension
that develops between any two constituents a and b (in the absence of air) is often
referred to as γ
ab. Thus, the surface tension between ink (i) and dampening fluid (f) is often denoted
as γ
if. Similarly, the surface tension between ink (i) and the imaging surface (s) is often
written as γ
is.
[0058] It has been found both empirically and theoretically that this interaction energy
can be estimated to a good degree by the Fowke's model for the interaction energy
of equation (iv):
[0059] From these simple models, various spreading coefficients can be calculated from an
understanding of Young's equation which describes the equilibrium behavior of a fluid
over a surface and dynamic interactions associated with the spreading. The condition
for the dampening fluid (f) to energetically spread uniformly over an imaging plate
(s) in a non-image area is given by a positive spreading coefficient S
f > 0 of the dampening fluid over the imaging surface in the presence of air,as shown
in equation (v):
where the term
r is an enhancement factor if the surface has microroughness or microtexture which
tends to increase the effective interfacial energy provide air is not trapped within
the texture.
[0060] Similarly, the condition for the ink (i) to uniformly wet the imaging plate (s) in
an image area is given by a positive ink spreading coefficient S
i >0 of the ink over the imaging surface in the presence of air, as shown in equation
(vi):
[0061] The condition for the dampening fluid (f) to reliably wet the imaging plate (i) in
the non-image area and reject ink (i) in the non-image area (when an ink roller is
presented over a layer of dampening fluid) is given by a positive spreading coefficient
of the dampening fluid in the presence of ink S
fi > 0 as shown in equation (vii):
[0062] Often the surface energies of the ink, dampening fluid, and printing surface are
plotted on a graph of alpha (y-axis) versus beta parameters (x-axis), which is useful
in illustrating the wetting phenomenon. Referring now to
FIGs. 2-4, on such graphs the solution for each component wherein the spreading coefficient
S is equal to zero is graphically represented by a circle also known as the wetting
envelope. For the spreading coefficient of a liquid over a surface in the presence
of air, these circles have their center at the origin. When one considers the spreading
coefficient of a fluid in the presence of another fluid besides air, such wetting
envelopes no longer have their centers at the origin. Fluids having alpha- beta surface
tension parameters that lie inside these circles typically wet a surface and have
a positive spreading coefficient, whereas fluids having alpha-beta parameters outside
these circles typically do not spread over the surface, but form droplets with a wetting
contact angle given by Young's equation.
[0063] It is necessary, but not sufficient, that all three of these spreading conditions
described by the conditions (v, vi, and vii) above be satisfied in the imaging system
in order to achieve good print quality. In the past, these three spreading conditions
have been satisfied in traditional offset printing by patterning the imaging plate
to have two separate regions, an imaging region with surface energy γ
s1 and a non-imaging region with surface energy γ
s2. These regions preferentially accept either the dampening fluid or the ink, but not
both. In other words, the plate is composed of hydrophilic/oleophobic non-image areas
and hydrophobic/oleophilic image areas which have mutually exclusive wetting characteristics
such that different surface energies applies for simultaneously solving equations
(v), (vi), and (vii). These conditions can be graphically plotted in two separate
alpha beta plots as shown in
FIG. 2 and
FIG. 3, which correspond to imaging and non-imaging areas of the plate. For these plots we
have assumed r ~ 1, but the interpretation of these plots do not change if r assumes
higher values.
[0064] Referring to
FIG. 2, the solid line circle represents an enclosed alpha-beta region of solutions which
will wet the imaging area on a plate. The position of the diamond on the circle corresponds
to the surface energy of the imaging area on the plate, i.e. the alpha-beta coordinates
of the plate's surface energy. Similarly, the ink surface tension is represented by
the dotted circle and is inside the offset plate's circle, indicating that the ink
will wet the surface of the offset plate. The inverted triangle (beta~7, alpha~4)
represents water, and indicates that water alone will not wet the surface of the offset
plate, due to its high surface tension. However, when a surfactant is added to the
water to form a fountain solution (FS), the effective beta component (i.e. polarity)
decreases dramatically resulting in a new position on the alpha beta plot (beta~3,
alpha~4) within the surface wetting circle. This new position is represented by the
upright triangle. The dashed circle connecting the diamond and the dotted circle represents
the wetting envelope where wetting will occur on imaging areas of the surface in the
presence of ink (condition vii, when S
fi=0). The fact that the fountain solution is outside the dashed circle indicates that
the surface of the imaging area is preferentially wetted by the ink.
[0065] FIG. 3, in contrast, presents the situation over the non-imaging areas of a plate. Here the
plate surface energy has a much higher polar component and therefore the wetting circle
indicated by the solid line circle is much larger. This time, the fountain solution
surface tension (represented by dotted circle) falls within both the solid line circle
wetting envelope and within the dashed circle wetting envelope. This time the fountain
solution is much more closely coupled to the actual surface energy of the non-imaging
area. The fact that the dotted circle falls within the dashed circle indicates preferential
wetting of the fountain solution over the surface in the presence of an inking roller.
In other words, because the fountain solution lies within the dashed circle, the fountain
solution will robustly reject ink in the non-imaging area. Note the arrangement of
these curves and points is for illustrative purposes only and does not suggest that
other configurations do not exist.
[0066] Unlike for traditional offset printing, all three spreading conditions must be satisfied
over the single unique surface of the imaging member for variable lithographic printing.
Such a solution has not been explored in the past due to the fact that traditional
offset plates have two separate surfaces, so that the material and chemical properties
of the ink, dampening fluid, and imaging surface were not so mathematically constrained.
Thus, it is desirable for variable lithographic printing, where only a single imaging
surface is used, to provide a dampening fluid wherein all three spreading conditions
can be satisfied using only one surface (i.e. on one alpha-beta graph, not two) with
a range of ink chemistries in order to provide robust printing quality.
[0067] The present disclosure contemplates a system where the dampening fluid is hydrophobic
and the ink somewhat hydrophilic (having a small polar component). This system can
be used with an imaging member surface which has low surface energy which is mainly
dispersive in character. Thus it can work with an imaging member that is a silicone,
fluorosilicone, or Viton® based elastomer, which offers high temperature wear robustness
to the laser energy used in variable lithographic printing. An ink / dampening fluid
/ surface system of the present disclosure is representatively illustrated in
FIG. 4. Here, the solid line circle represents the wetting conditions over a perfectly smooth
surface. However, if a surface is micro-textured, the radius of the wetting condition
of the surface can be effectively enlarged in the alpha beta space and the true wetting
circle is represented by the dotted circle. This is due to the fact that the effective
fractal surface area of the surface is increased, i.e. the factor
r becomes larger than 1 and increases the effective wetting energy space allowable.
This allows both the ink (represented by the triangle) and the dampening fluid (represented
by the square) to fall within the textured wetting envelope of the surface. Therefore
the surface energy of the surface (represented by the diamond) no longer falls directly
on top of its own wetting envelope (the dotted circle). Note that the dampening fluid
represented by the square is within the dashed circle formed by the surface energy
of the plate (diamond) and the triangle (representing the ink) and this indicates
that the surface is preferentially wetted by the fountain solution, even when ink
is present.
[0068] When an energy source (such as a laser) is used to create a pattern in the dampening
fluid layer, it is also desirable that the edges of the pattern remain stable over
time. The surface texture which enhances the ink wetting also helps pin the dampening
fluid in place to maintain the pattern. In addition, by using a dampening fluid surface
tension that falls close to the smooth surface wetting curve, the spreading coefficient
is minimized at the contact line (which does not see the effects of the microtexture
roughness), thereby reducing the surface tension promoting spreading or pullback of
the dampening fluid. Therefore by careful choice and design of the ink and dampening
fluid chemistries and the re-imagable surface energy and texture, image quality can
be improved both in terms of background tinting and image edge quality
[0069] In such a system, the ink should have an appreciable polar surface tension and low
dispersive surface tension. For example, ultraviolet (UV) offset inks based on acrylate
oligomers and monomers have a recognizable polar surface tension component as well
as having many other desirable characteristics. For example, UV-based lithographic
printing has been used in packaging and sheetfed offset printing due to both environmental
concerns and lower total cost of ownership. Smell issues that used to exist with such
UV offset inks have generally been eliminated with the use of monomers having a higher
boiling point. Indeed, some companies have an approved list that includes only UV
offset inks, as other inks (e.g. cobalt cured or solvent-based inks) are sometimes
considered to have greater risks in the packaging industry. The polar component of
the surface tension of the UV offset ink can be tuned and controlled by choice of
the proper monomer and oligomers for the ink, as well as by adding surface leveling
agents to the ink. In other embodiments, the ink may comprise a monomer containing
an ester (-COO-), ether (-O-), carbonyl (-CO-), amino (-NRR'), cyano (-CN), or hydroxyl
(-OH) group. Exemplary monomers that can be used in a UV offset ink contemplated by
the present disclosure include acrylates like methyl methacrylate or t-butyl acrylate;
acrylonitriles, acrylamides, vinyl alcohol, etc.
[0070] By choosing the proper chemistry it is possible to devise a system where both the
ink and the dampening fluid will wet the imaging member surface, but the ink and the
dampening fluid will not mutually wet each other. The system can also be designed
so that it is energetically favorable for dampening fluid in the presence of ink residue
to actually lift the ink residue off of the imaging member surface by having a higher
affinity for wetting the surface in the presence of the ink. In other words, the dampening
fluid could remove microscopic background defects (e.g. < 1 µm radius) from propagating
in subsequent prints.
[0071] Generally speaking, the variable lithographic system can be described as comprising
an ink, a dampening fluid, and an imaging member surface, wherein the dampening fluid
has a surface energy alpha-beta coordinate which is within the circle connecting the
alpha-beta coordinates for the surface energy of the ink and the surface energy of
the imaging member surface. In particular embodiments, the dampening fluid has a total
surface tension greater than 15 dynes/cm and less than 30 dynes/cm with a polar component
of less than 5 dynes/cm. The imaging member surface may have a surface tension of
less than 30 dynes/cm with a polar component of less than 2 dynes/cm. For example,
the imaging member surface may be made of a silicone, fluorosilicone, or fluoroelastomer.
[0072] The dampening fluid of the present disclosure is useful for meeting all of the conditions
listed above for digital variable lithographic printing. The dampening fluid comprises
a solvent which is either a volatile hydrofluoroether (HFE) liquid or a volatile silicone
liquid. The hydrofluoroether and silicone are liquids at room temperature, i.e. 25°C.
[0073] In specific embodiments, the volatile hydrofluoroether liquid has the structure of
Formula (I):
C
mH
pF
2m+1-p-O-C
nH
qF
2n+1-q Formula (I)
wherein m and n are independently integers from 1 to about 9; and p and q are independently
integers from 0 to 19. As can be seen, generally the two groups bound to the oxygen
atom are fluoroalkyl groups.
[0074] In particular embodiments, q is zero and p is non-zero. In these embodiments, the
right-hand side of the compound of Formula (I) becomes a perfluoroalkyl group. In
other embodiments, q is zero and p has a value of 2m+1. In these embodiments, the
right-hand side of the compound of Formula (I) is a perfluoroalkyl group and the left-hand
side of the compound of Formula (I) is an alkyl group. In still other embodiments,
both p and q are at least 1.
[0075] In this regard, the term "fluoroalkyl" as used herein refers to a radical which is
composed entirely of carbon atoms and hydrogen atoms, in which one or more hydrogen
atoms may be (i.e. are not necessarily) substituted with a fluorine atom, and which
is fully saturated. The fluoroalkyl radical may be linear, branched, or cyclic.
[0076] The term "alkyl" as used herein refers to a radical which is composed entirely of
carbon atoms and hydrogen atoms which is fully saturated and of the formula -C
nH
2n+1. The alkyl radical may be linear, branched, or cyclic. It should be noted that an
alkyl group is a subset of fluoroalkyl groups.
[0077] The term "perfluoroalkyl" as used herein refers to a radical which is composed entirely
of carbon atoms and fluorine atoms which is fully saturated and of the formula - C
nF
2n+1. The perfluoroalkyl radical may be linear, branched, or cyclic. It should be noted
that a perfluoroalkyl group is a subset of fluoroalkyl groups, and cannot be considered
an alkyl group.
[0079] Of these formulas, Formulas (I-a), (I-b), (I-d), (I-e), (I-f), (I-g), and (I-h) have
one alkyl group and one perfluoroalkyl group, either branched or linear. In some terminology,
they are also called segregated hydrofluoroethers. Formula (I-c) contains two fluoroalkyl
groups and is not considered a segregated hydrofluoroether.
[0080] Formula (I-a) is also known as 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane
and has CAS# 132182-92-4. It is commercially available as Novec™ 7300.
[0081] Formula (I-b) is also known as 3-ethoxy-1,1,1,2,3,4,4,5,5,6,6,6-dodecafluoro-2-(trifluoromethyl)hexane
and has CAS# 297730-93-9. It is commercially available as Novec™ 7500.
[0082] Formula (I-c) is also known as 1,1,1,2,3,3-Hexafluoro-4-(1,1,2,3,3,3-hexafluoropropoxy)pentane
and has CAS# 870778-34-0. It is commercially available as Novec™ 7600.
[0083] Formula (I-d) is also known as methyl nonafluoroisobutyl ether and has CAS# 163702-08-7.
Formula (I-e) is also known as methyl nonafluorobutyl ether and has CAS# 163702-07-6.
A mixture of Formulas (I-d) and (I-e) is commercially available as Novec™ 7100. These
two isomers are inseparable and have essentially identical properties.
[0084] Formula (I-f) is also known as 1-methoxyheptafluoropropane or methyl perfluoropropyl
ether, and has CAS# 375-03-1. It is commercially available as Novec™ 7000.
[0085] Formula (I-g) is also known as ethyl nonafluoroisobutyl ether and has CAS# 163702-05-4.
Formula (I-h) is also known as ethyl nonafluorobutyl ether and has CAS# 163702-06-5.
A mixture of Formulas (I-g) and (I-h) is commercially available as Novec™ 7200 or
Novec™ 8200. These two isomers are inseparable and have essentially identical properties.
[0086] It is also possible that similar compounds having a cyclic aromatic backbone with
perfluoroalkyl sidechains can be used. In particular, compounds of Formula (A) are
contemplated:
Ar-(C
kF
2k+1)
t Formula (A)
wherein Ar is an aryl or heteroaryl group; k is an integer from 1 to about 9; and
t indicates the number of perfluoroalkyl sidechains, t being from 1 to about 8.
[0087] The term "aryl" refers to an aromatic radical composed entirely of carbon atoms and
hydrogen atoms. When aryl is described in connection with a numerical range of carbon
atoms, it should not be construed as including substituted aromatic radicals. For
example, the phrase "aryl containing from 6 to 10 carbon atoms" should be construed
as referring to a phenyl group (6 carbon atoms) or a naphthyl group (10 carbon atoms)
only, and should not be construed as including a methylphenyl group (7 carbon atoms).
[0088] The term "heteroaryl" refers to a cyclic radical composed of carbon atoms, hydrogen
atoms, and a heteroatom within a ring of the radical, the cyclic radical being aromatic.
The heteroatom may be nitrogen, sulfur, or oxygen. Exemplary heteroaryl groups include
thienyl, pyridinyl, and quinolinyl. When heteroaryl is described in connection with
a numerical range of carbon atoms, it should not be construed as including substituted
heteroaromatic radicals. Note that heteroaryl groups are not a subset of aryl groups.
[0089] Hexafluoro-m-xylene (HFMX) and hexafluoro-p-xylene (HFPX) are specifically contemplated
as being useful compounds of Formula (A) that can be used as low-cost dampening fluids.
HFMX and HFPX are illustrated below as Formulas (A-a) and (A-b):
It should be noted any co-solvent combination of fluorinated damping fluids can be
assumed to help suppress non-desirable characteristics such as a low flammability
temperature.
[0090] Alternatively, the dampening fluid solvent is a volatile silicone liquid. In some
embodiments, the volatile silicone liquid is a linear siloxane having the structure
of Formula (II):
wherein R
a, R
b, R
c, R
d, R
e, and R
f are each independently hydrogen, alkyl, or perfluoroalkyl; and a is an integer from
1 to about 5. In some specific embodiments, R
a, R
b, R
c, R
d, R
e, and R
f are all alkyl. In more specific embodiments, they are all alkyl of the same length
(i.e. same number of carbon atoms).
[0091] Exemplary compounds of Formula (II) include hexamethyldisiloxane and octamethyltrisiloxane,
which are illustrated below as Formulas (II-a) and (II-b):
[0092] In other embodiments, the volatile silicone liquid is a cyclosiloxane having the
structure of Formula (III):
wherein each Rg and R
h is independently hydrogen, alkyl, or perfluoroalkyl; and b is an integer from 3 to
about 8. In some specific embodiments, all of the Rg and R
h groups are alkyl. In more specific embodiments, they are all alkyl of the same length
(i.e. same number of carbon atoms).
[0093] Exemplary compounds of Formula (III) include octamethylcyclotetrasiloxane (aka D4)
and decamethylcyclopentasiloxane (aka D5), which are illustrated below as Formulas
(III-a) and (III-b):
[0094] In other embodiments, the volatile silicone liquid is a branched siloxane having
the structure of Formula (IV):
wherein R
1, R
2, R
3, and R
4 are independently alkyl or -OSiR
1R
2R
3.
[0095] An exemplary compound of Formula (IV) is methyl trimethicone, also known as methyltris(trimethylsiloxy)silane,
which is commercially available as TMF-1.5 from Shin-Etsu, and shown below with the
structure of Formula (IV-a):
[0096] Any of the above described hydrofluoroethers / perfluorinated compounds are miscible
with each other. Any of the above described silicones are also miscible with each
other. This allows for the tuning of the dampening fluid for optimal print performance
or other characteristics, such as boiling point or flammability temperature. Combinations
of these hydrofluoroether and silicone liquids are specifically contemplated as being
within the scope of the present disclosure. It should also be noted that the silicones
of Formulas (II), (III), and (IV) are not considered to be polymers, but rather discrete
compounds whose exact formula can be known.
[0097] In particular embodiments, it is contemplated that the dampening fluid comprises
a mixture of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5).
Most silicones are derived from D4 and D5, which are produced by the hydrolysis of
the chlorosilanes produced in the Rochow process. The ratio of D4 to D5 that is distilled
from the hydrolysate reaction is generally about 85% D4 to 15% D5 by weight, and this
combination is an azeotrope.
[0098] In particular embodiments, it is contemplated that the dampening fluid comprises
a mixture of octamethylcyclotetrasiloxane (D4) and hexamethylcyclotrisiloxane (D3),
the D3 being present in an amount of up to 30% by total weight of the D3 and the D4.
The effect of this mixture is to lower the effective boiling point for a thin layer
of dampening fluid.
[0099] The volatile hydrofluoroether liquids and volatile silicone liquids of the present
disclosure have a low heat of vaporization, low surface tension, and good kinematic
viscosity. For reference, Table 1 below compares their properties with that of water:
Table 1.
Compound |
Heat of Vaporization at boiling point (kJ/kg) |
Surface Tension at 25°C (dynes/cm) |
Kinematic Viscosity at 25°C (cSt) |
Vapor Pressure at 25°C (mmHg) |
Density (g/mL) |
Solubility in Water (ppm) |
Water |
2257 |
72 |
1 |
23.8 |
1 |
-- |
(I-a) |
101.7 |
15 |
0.71 |
44.9 |
1.66 |
0.586 |
(I-b) |
88.5 |
16.2 |
0.77 |
-- |
1.61 |
<3 |
(I-c) |
115.6 |
17.7 |
1.07 |
-- |
1.54 |
<10 |
(I-d)/(I-e) |
-- |
13.6 |
-- |
202 |
1.52 |
12 |
(I-f) |
142 |
12.4 |
0.32 |
484.5 |
1.40 |
60 |
(I-p)/(I-h) |
125.5 |
13.6 |
-- |
109 |
1.43 |
<20 |
(II-a) |
200.8 |
15.9 |
0.65 |
35 |
0.79 |
-- |
(II-b) |
158 |
17.4 |
1 |
4 |
0.82 |
-- |
(III-a) |
133 |
18.4 |
2.3 |
1.5 |
0.96 |
-- |
(III-b) |
157 |
18.0 |
3.9 |
1 |
0.96 |
-- |
(IV-a) |
- |
16.8 |
1.5 |
<10 |
0.85 |
-- |
[0100] An examination of Table 1 indicates that these liquid compounds all have much lower
heats of vaporization, which reduces the amount of energy that needs to be provided
at the imaging station to form the latent image. In addition, the liquid compounds
have a much lower surface tension, such that surfactants may not need to be added
at all. Many of the densities of these compounds also differ significantly from water,
and they are generally insoluble with water as well.
[0101] In embodiments, the liquid solvent used in the dampening fluid has a heat of vaporization
of less than 200 kJ/kg, or less than 120 kJ/kg when measured at 1 atmosphere and 25°C.
[0102] One result of using these liquids for the dampening fluid is that the dampening fluid
can have a surface tension of from about 15 to about 30 dynes/cm. This low surface
tension aids in spreading of the dampening fluid to wet the imaging member surface.
In other particular embodiments, the dampening fluid has a kinematic viscosity of
greater than 1 centiStokes at 25°C and a surface tension of less than 72 dynes/cm
at 25°C.
[0103] There are at least three conditions which desirably are met for a robust digital
offset imaging system. First, the dampening fluid has a slight positive spreading
coefficient so that the dampening fluid wets the imaging member surface. In addition,
it is often still necessary that the dampening system used to coat the imaging surface
member be as uniform and reproducible as possible. Initially, the dampening fluid
spreads rapidly above the micro-roughness of the imaging member surface due to the
positive spreading coefficient. This could be considered a "self-leveling" process
which occurs passively and spontaneously. Secondly, the dampening fluid maintains
a spreading coefficient in the presence of ink, or in other words the dampening fluid
has a closer surface energy value to the imaging member surface than the ink does.
This causes the imaging member surface to value wetting by the dampening fluid compared
to the ink, and permits the dampening fluid to lift off any ink residue and reject
ink from adhering to the surface where the laser has not removed dampening fluid.
Third, the ink should wet the imaging member surface in air with a roughness enhancement
factor (i.e. when no dampening fluid is present on the surface). It should be noted
that the surface may have a roughness of less than 1 µm when the ink is applied at
a thickness of 1 to 2 µm.
[0104] In addition to these three conditions, it is desirable that the dampening fluid should
not wet the ink in the presence of air. In other words, fracture at the exit inking
nip should occur where the ink and the dampening fluid interface, not within the dampening
fluid itself. This way, dampening fluid will not tend to remain on the imaging member
surface after ink has been transferred to a receiving substrate. In practice, this
condition is difficult to achieve and small amounts of dampening fluid may need to
be removed from the inking system by using an air knife to selectively evaporate away
the dampening fluid from the inking system. Alternatively it may be acceptable to
allow a small equilibrium build up of dampening fluid to emulsify within an inking
subsystem.
[0105] Finally, it is also desirable that the ink and dampening fluid are chemically immiscible
such that only emulsified mixtures can exist. Though the ink and the dampening fluid
may have alpha-beta coordinates close together, often choosing the chemistry components
with different levels of hydrogen bonding can reduce miscibility by increasing the
difference in the Hanson solubility parameters.
[0106] Other additives may also be present in the dampening fluid. Such additives may include
a biocide, a sequestrant, a corrosion inhibitor, and a humectant.
[0107] A biocide impedes the growth of or destroys any fungus or microorganisms that may
be present in the dampening fluid. Exemplary biocides include sodium benzoate, phenol
or derivatives thereof, formalin, imidazole derivatives, sodium dehydroacetate, 4-isothiazolin-3-one
derivatives, benzotriazole derivatives, derivatives of amidine and guanidine, quaternary
ammonium salts, derivatives of pyridine, quinoline and guanidine, derivatives of diazine
and triazole, derivatives of oxazole and oxazine, bromonitropropanol, 1,1-dibromo-1-nitro-2-ethanol,
and 3-bromo-3-nitropentane-2,4-diol. The biocide can be used in an amount of from
about 0.001 wt% to about 1 wt% of the dampening fluid.
[0108] A sequestrant, or chelating agent, is used to chelate dissolved ions that may be
present in the dampening fluid to prevent their reaction with other ingredients in
for example the ink. Exemplary sequestrants include organic phosphonic acids and phosphonoalkanetricarboxylic
acids, such as ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic
acid, triethylenetetraminehexaacetic acid, hydroxyethylethylenediaminetriacetic acid,
nitrilotriacetic acid, 1-hydroxyethane-1,1-diphosphonic acid, aminotri(methylenephosphonic
acid), and salts thereof. The sequestrant can be used in an amount of from about 0.001
wt% to about 1 wt% of the dampening fluid.
[0109] A corrosion inhibitor protects the associated components of the imaging member from
corrosion. Exemplary inhibitors include sodium nitrate, sodium phosphate, benzotriazole,
5-methylbenzotriazole, thiosalicylic acid, and benzimidazole.
[0110] A humectant prevents the dampening fluid from drying too rapidly, which can cause
some problems with the final printed product. Exemplary humectants include ethylene
glycol, glycerin and propylene glycol.
[0111] The dampening fluid of the present disclosure is based on a nonpolar solvent. It
is contemplated that the dampening fluid would be used in a system in combination
with an ink having a polar component.
[0112] Aspects of the present disclosure may be further understood by referring to the following
examples. The examples are illustrative, and are not intended to be limiting embodiments
thereof.
EXAMPLES
[0113] FIG. 5 is a view of a set of characters printed using a dampening fluid of 100% Novec™ 7500.
FIG. 6 is a view of the same set of characters printed using a dampening fluid containing
90 wt%water, 8 wt% isopropanol, and 2 wt% SILSURF surfactant. Remember that the ink
is laid down after the fountain solution. Comparing the arrows (a) in each Figure
at the top of the "p", the corner is sharp in
FIG. 5 and not rounded as in
FIG. 6, indicating less pull back by the dampening fluid in
FIG. 5. Similarly, comparing the arrows (b) in each Figure at the top left of the "a", the
edges here are sharp in
FIG. 5 and rounded in
FIG. 6, again indicating less pull back by the dampening fluid in
FIG. 5. The same effect is seen at the bottom right of the "a", which is marked with arrows
(c).
[0114] The present disclosure has been described with reference to exemplary embodiments.
Obviously, modifications and alterations will occur to others upon reading and understanding
the preceding detailed description. It is intended that the present disclosure be
construed as including all such modifications and alterations insofar as they come
within the scope of the appended claims or the equivalents thereof.