Note: Descriptions are shown in the official language in which they were submitted.
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OFFSET LITHOGRAPHIC PRINTING PROCESS
FIELD OF THE INVENTION
The invention relates to an offset lithographic
printing process utilizing a water-based ink composition.
The process is conducted out at a relative humidity of
75-100% and preferably at a temperature of 10 to 30 C.
The ink composition will typically contain water, one or
more macromolecular binders, a pigment and a re-wetting
agent.
BACKGROUND OF THE INVENTION
In an attempt to eliminate VOC's in the pressroom,
water-based alternatives are being sought for ink
formulations. Water-based printing inks for use in
flexographic printing processes are known in the prior
art. This type of printing process utilizes printing
plates wherein the printing images stand up in relief,
i.e. the areas to be printed are raised above the non-
printing areas. Printing by the flexographic procesr
requires relatively low pressure - sufficient pressure is
applied to transfer the ink from the face of the image
carrier to the surface of the substrate. Examples of
useful water-based flexographic printing inks are
disclosed in US Patent 4,173,554 and The Printinq Ink
Manual, edited by R.H. Leach and R.J. Pierce, pages 571-
576, 5th edition, (Blueprint, 1993).
Water-based inks for gravure printing are also well
known. In the gravure process, the printing image is
engraved into a cylinder in the form of cells which
become filled with ink. Printing is achieved by passing
the substrate between the gravure cylinder and impression
roller under pressure. Examples of useful water-based
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gravure printing inks are disclosed in US Patents
4,954,556 and 5,098,478.
The offset lithographic printing process presents
unique challenges to ink formulators since such process
utilizes a planographic printing plate, i.e. the image
and non-image areas are in the same plane on the image
carrier, and two fluids are concurrently utilized.
It is fairly simple to define an image area by
raising it above the background as iri the case of the
flexographic printing plate or lowering it as in the case
of the gravure printing plate; avoidance of ink adhering
to the non-image area is not too difficult to achieve.
However, when all areas are on the same level, techniques
must be utilized to insure that ink adheres only to the
image area, and not to the non-image area.
In conventional offset lithographic printing
processes, the plate is damped before it is inked with an
oil-based ink. Typically, the damping process utilizes a
fountain solution such as those described in US patents
3,877,372, 4,278,467 and 4,854,969. Water will form a
film on the hydrophilic areas (i.e. the non-image areas)
of the printing plate, but will contract into tiny
droplets on the oleophilic areas (i.e. the image areas).
When an inked roller containing the oil-based ink is
passed over the damped plate, it will be unable to ink
the areas covered by the water film (the non-image
areas), but will emulsify the droplets on the water-
repellant areas (the image areas) and these will ink up.
Such process is called offset lithography because the
inked image on the plate does not directly print onto the
paper substrate, but is first "offset" onto a rubber
blanket, and transferred therefrom onto the paper
substrate.
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As mentioned above, conventional offset lithographic
printing processes entails the use of oil-based inks and
water-based fountain solutions. The ink/water balance is
critical and is quite demanding of the pressman's skills.
This issue is one of the several disadvantages associated
with such printing processes as compared to flexographic
and gravure printing processes. Moreover, the oil-based
inks and aqueous fountain solutions typically employed in
conventional offset lithographic printing processes
contain fairly high levels of undesirable volatile
organic compounds ("VOCs").
U.S. Patent 3,356,030 discloses the use of a water-
based printing ink in respect to a method of planographic
printing utilizing a lithographic printing plate whose
non-image areas are coated with a cured coating of a
thermosetting silicone resin. However, the patented
method also entails the use of a volatile hydrocarbon
fountain solution which will coat the non-image areas and
which is re-applied between successive printings. Of
course, the use of a volatile hydrocarbon fountain
solution undermines the principal purpose of the offset
lithographic printing process of the present invention,
i.e. the avoidance of the use of volatile organic
compounds ("VOCs") during the printing process. Indeed,
the offset lithographic printing process utilizes water-
based ink compositions without any fountain solution
whatsoever.
In the 1980s, a resurgence of interest occurred in
respect to "waterless" lithographic printing processes.
Both positive and negative waterless planographic
printing plates are commercially available from Toray
Industries of Japan. The image area of a waterless
planographic plate is a photopolymer similar to that
employed for the image area of a conventional plate.
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However, the non-iiaage area is coated with a polymer such
as a silicone which is ink repellent. Further information
about waterless printing plates and processes may be found
in U.S. Patents 5,370,906 and 5,417,749.
The waterless printing process solved two issues: VOCs
emanating from the fountain solutions and control of the
ink/water balance :by the pressman. However, the
difference in surface energy between the image and non-
image areas of the conventional offset lithographic
printing plate is typically 4 x 10-4 N/cm (40 dynes/cm),
and is dramatically reduced to 2 x 10-4 N/cm (20 dynes/cm)
in the case of the waterless printing plate. Therefore,
the latitude between scumming and poor print density is
considerably narrowed and the issue of VOCs (emanating
from the oil-based ink) still remains in respect
of waterless printing.
German Offenlegungsschrift DE 41 19 348 Al pertains
to a moistureless offset printing method and a water-
based printing ink. The ink described therein is one
which will adhere to hydrophilic materials, but not to
hydrophobic materials, and contains a dye, water, 5-50%
water-soluble macromolecular binder and a hygroscopic
liquid, preferably a multihydric alcohol.
OBJECT OF THE INVENTION
It is an object of the present invention to
eliminate the principal disadvantages of conventional
offset lithographic printing processes, viz. high levels
of VOCs emanating from the oil-based ink and the aqueous
fountain solution and the difficulty in controlling the
ink/water balance, while preserving the principal
advantage of the offset lithographic printing process,
i.e., high surface energy differential between the image
and non-image areas of the printing plate.
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Such object-has been achieved by means of the offset
lithographic printing process of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The offset lithographic printing process of the
invention is conducted at a relative humidity of 75-100*,
preferably 80-85%. Desirably, the process is conducted
at a temperature of 10 to 30 C, preferably 15 to 25 C.
The process will utilize a water-based printing ink
without the need for any accompanying fountain solution.
5 The water-based ink for the offset lithographic
printing process of the invention may be one which
contains water, one or more macromolecular binders, a
pigment and a rewetting agent.
The printing plates for use with the offset
lithographic printing process of the invention should be
such that the image areas thereof are hydrophilic in
nature, while the non-image areas are hydrophobic in
nature. An example of a suitable printing plate is the
"waterless" Toray type discussed above. However, the
image area of the plate need not contain a photopolymer.
The image area of the plate may comprise, e.g. a grained
aluminum surface which has no coating thereon, but is
hydrophilic in nature. The non-image area of the plate
must, of course, be hydrophobic in nature. However, the
non-image area may be covered with any type of
hydrophobic material, provided that such hydrophobic
material adheres to the non-images area of the plate
during the printing process.
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A useful water-based printing ink for the offset
lithographic printing process of the invention may
comprise the following components:
(a) 25 to 60 wtA, preferably 35 to 50 wtA, based
on the weight of the ink, of water which is
present as a continuous phase in the ink;
(b) 10 to 70 wtA, preferably 30 to 60 wtA, based
on the weight of the ink, of a macromolecular
binder comprising:
(i) macromolecular binders which are soluble
in the water phase regardless of the pH of
the water phase, present in an amount of
0-5 wtA, based on the weight of the ink;
and
(ii) macromolecular binders which are soluble
in the water phase only at a pre-
determined pH value (preferably in the
range of about 7.5 to about 10), present
in an amount of 10-70 wtA, based on the
weight of the ink; and
(iii) macromoZecular binders comprising aqueous
emulsions, present in an amount of 0-20
wt.%, based on the weight of the ink;
(c) a pigment, present in the amount of 2-30 wtA,
preferably 5-20 wtA, based on the weight of
the ink; and
(d) a re-wetting agent, present in the amount of
0.5-10 wtA, based on the weight of the ink.
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Examples of suitable macromolecular binders which
are soluble in the water phase of the ink regardless of
the pH of the water phase include: carboxymethyl-
cellulose, hydroxyethylcellulose, hydroxypropyl-
cellulose, hydroxybutylmethylcellulose, poly(C1-C4)
alkylene oxides, polyethyleneimine, polyvinyl alcohol,
polyvinyl acetate, polyvinylpyrollidone, polyvinyl-
oxazolidone and polyacrylamide polymers.
Preferably, the macromolecular binders present in
the ink are only those macromolecular binders which are
soluble in the water phase only at a pre-determined pH
value. Such pH value may be acidic, e.g. about 2.5 to
about 6.5, but is preferably in the range of about 7.5 to
about 10. Suitable examples of such macromolecular
binders include methacrylic resins; styrene-acrylic
resins; rosin salts; and polystyrenesulfonic acid and its
salts. Ammonia or an organic amine such as monoethanol-
amine or N,N-diethanolamine may be added to the water
phase in order to adjust the pH to the preferred value (a
mineral acid or an organic acid such as acetic acid may
be used to adjust the pH to a value in the range of about
2.5 to about 6.5).
Suitable examples of the macromolecular binders
comprising aqueous emulsions include acrylic or vinyl
emulsion polymers prepared from monomers selected from
the group consisting of acrylic acid esters, methacrylic
acid esters, acrylic acid esters of polyhydric alcohols,
methyl methacrylate, styrene, vinyl styrene and vinyl
acetate.
The pigment may be any of those which are suitable
for formulating offset lithographic printing inks such as
CI Pigment Yellows 1, 3, 4, 5, 12, 13, 14, 17, 55, 65,
73, 83, 97 and 98; CI Pigment oranges 13, 16 and 46; CI
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0 4 MAR 1996
Pigment Reds 2, 3, 4, 10, 12, 48, 48:1, 48:2, 53, 57:2,
81, 104, 146, 170 and 176; CI Pigment Greens 2, 7 and
36;CI Pigment Blues 1, 15:1, 15:2, 15:3, 15:6, 16, 29,
56 and 61; CI Pigment Violets 3, 23 and 37; CI Pigment
Blacks 6 and 7; and CI Pigment Whites 6, 7, 18 and 26.
Suitable examples of rewetting agents include urea,
thiourea, hydroxyethylethylene urea, glycerol, sorbitol,
ethylene glycol and butyl carbitol.
Preferably, the water-based printing ink will
include a nonionic surfactant, present in the amount of
1s up to 5 wtA, based on the weight of the ink. Suitable
examples of the surfactant include acetylenic glycols,
ethoxylated glycols and sorbitan esters.
If desired, the usual adjuvants such as waxes,
antifoam agents, biocides, corrosion inhibitors, etc.
may be incorporated in the water-based printing ink.
Offset lithographic printing presses having the controls
for-achieving such desirable relative humidity and
temperature levels during the printing process are not
within the purview of this invention.
A printing press which could efficiently carry out
the offset lithographic printing process of the
invention might have the following features:
a) an inking mechanism for applying the ink to the
print cylinder;
b) a housing surrounding the inking mechanism and
the print cylinder so as to define an enclosed
space;
133M Q3Ot3M
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c) a cooling mechanism for cooling the atmosphere
within the enclosed space;
d) a humidifier for controlling the humidity of the
atmosphere within the enclosed space; and
e) means for introducing a chemical substance such
as ammonium hydroxide or anhydrous ammonia or an
amine such as ethanolamine into the enclosed
space in order to insure that the pH is
maintained at a desired value of about 7.5 to
about 10 (if an acidic pH value is desired, the
chemical substance may be a mineral acid or an
organic acid such as acetic acid); maintenance
of the pH within such range will minimize the
likelihood that the water-based ink described
above will dry out on the rollers during
operation of the press (the water-based inks
described above contain macromolecular binders
which are predominantly or wholly of the type
which are soluble in the water phase of the ink
only at a pre-determined desired pH value).
A printing press having the features such as those
set forth above would likely also contain sensors and
valving mechanisms to insure that the desired
temperature, humidity and pH value of the ink are
continuously monitored and adjusted as may be necessary
throughout the course of the printing operation.
The offset lithographic printing process of the
present invention are further illustrated by the
following non-limiting examples in which all parts and
percentages are by weight, unless otherwise indicated.
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Example 1
A water-based ink was prepared from the components
indicated below. The water phase of the ink was supplied
by the water present in the acrylic resin latex, hydroxy-
propyl cellulose, hydroxyethylethylene urea and the
maleated rosin ester:
Com,ponent Amount, wtA
Styrene/maleic anhydride resin 12
Phthalocyanine Blue pigment 12
Acrylic resin latex (50 wtA solids) 5
Hydroxypropylcellulose (3 wtA solids) 10
Hydroxyethylethylene urea (70 wtA solids) 8
Monoethanol amine 2
Polyethylene Wax 2
Ethoxylated acetylenic diol surfactant 2
Maleated rosin ester (50 wt.% solids) 47
Total 100
The printing ink was printed using a Didde press
whose printing units, printing plate and printing
blankets were enclosed within a chamber in order to
control the relative humidity and temperature during the
press run.
The printing plate was obtained from Toray
industries; the image area of the aluminum oxide
substrate was coated with a photopolymer whose surface
was hydrophilic in nature, while the non-image area was
coated with a silicone polymer. The press run was
carried out at a relative humidity in the range of 85-90%
and a temperature in the range of 15-20 C and the press
speed was 1,000 feet/minute (about 500 cm/second).
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The prints obtained from this press run were clear
and sharp. The print density was 1.5 and the dot gain
was 15%. There was no discernible toning in the non-
image area.
4
