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Description  |
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BACKGROUND OF THE INVENTION
This invention is directed to a composition and method for improved aqueous
printing.
A printing process involves the distribution of ink in a uniform film, the
provision of a substrate having a uniform surface, and the transfer of the
ink to the substrate. The final transfer of ink from the printing surface
is accomplished by means of pressure; the final force on the ink film is
heavy pressure against a porous surface followed by a tension sufficient
to split the ink film.
Flexographic printing is a form of "relief" printing, in which an
impression is taken from the raised parts of a printing surface.
Flexographic printing machines are essentially high-speed web or sheet fed
rotary presses, which print with liquid inks from curved plates, usually
made of rubber or other plastic material, attached to a cylinder. An
important advantage of flexographic printing is that a uniform film of ink
can be printed even on rough papers, because the surface of the rubber
plate is sufficiently resilient to be forced into the hollows in the
paper.
Flexographic printing can be used to produce a wide variety of printed
material, including waxed or decorative wrappers, aluminum foil, plastic
films, newsprint and corrugated papers and cartons. The inking system used
in flexographic printing consists essentially of an enclosed duct to limit
the evaporation of the solvent, and a simple train of rollers. Ink control
is usually done by pressure on the end bearings of the inking rollers,
which tends to force the rollers apart at the center. Because of this,
continuous fine control and correct alignment of rollers is considered to
be essential to produce acceptable printing. Although the pressure on the
inking rollers may vary appreciably in letter press printing, in contrast,
flexographic printing requires maintenance of pressure on inking rollers
at practically zero. Because of the importance of maintaining this
critical inking roller pressure, flexographic inks must be carefully
tailored to have appropriate viscosity or fluidity. In addition the ink
must be sufficiently volatile or penetrative to dry within seconds or
less, adhere well to the printing surface, and be odorless when dry. The
ink must be free of any solvents which would attack the rubber or plastic
print plates.
In the case of pigmented inks, which are extensively used because of the
color effects and eye appeal of the printed product, the ink should be
stable and any pigment which settles out should be readily dispersible by
stirring. A great amount of research has been expended in attempting to
maintain satisfactory flow properties while at the same time maximizing
the degree of pigmentation and improving the ink transfer properties.
Another widely used printing technique is intaglio printing, which involves
taking impressions from recesses engraved or etched below the surface of a
plate or cylinder. Presently, the most important of this group is
rotogravure printing from polished copper electro-deposited on an iron,
steel or aluminum base. The design is etched into the copper by
photoengraving means. Gravure inks rely for drying mainly on solvent
evaporation, and, therefore, gravure inks usually contain large amounts of
hydrocarbon solvents. Not only are these solvents highly inflammable,
forming explosive with the atmosphere, but the vapors are also extremely
toxic. Because these undesirable properties involve significant
expenditures for flameproof electrical equipment and hoods and duct work
for containment and recovery of the solvent, an aqueous rotogravure ink
which has suitable ink transfer properties has long been an industry need.
SUMMARY
The instant invention involves the discovery that polyethylene oxide resins
can be added to aqueous printing inks to improve ink transfer properties.
Specifically, from 0.1 to 1.5 percent by weight of a polyethylene oxide
having a molecular weight in the range of from 100,000 to 350,000 is added
to an aqueous printing ink.
It is, therefore, an object of the instant invention to provide an aqueous
printing ink composition having improved transfer properties.
It is a further object of the instant invention to provide an aqueous
pigmented flexographic ink which prints out stronger, has increased
brilliance, increased intensity of color, and increased clarity.
It is still another object of the instant invention to provide an aqueous
pigmented flexographic ink which has increased scuff resistance, and has
substantially reduced mottle, feathering and striation.
Other objects and advantages will be apparent from the description which
follows. The following Examples will serve to illustrate specific
embodiments of the invention.
EXAMPLE I
An aqueous black pigmented flexographic ink was prepared by mixing together
the following ingredients; 36 percent by weight of ink varnish, 15 percent
by weight carbon black, 5 percent by weight of a combined surfactant, 10
percent by weight of a synthetic water-based resin, 1 percent by weight of
wax, and 28 percent by weight water. The aqueous ink mixture was blended
together in a mixer for approximately 20 minutes and then passed through a
sand mill to reduce any agglomerated particles and to form a smooth finish
ink. A 5 percent by weight portion of a solution consisting of 12 parts by
weight polyethylene oxide in 88 parts by water was then added to the ink.
Because the forces applied to the ink during the blending and sand-milling
operation have been found to destroy the improvement in ink transfer
properties obtained by the presence of polyethylene oxide, it is important
to add the polyethylene oxide subsequent to the sand-milling.
The ink was put on a flexographic printing press and the ink viscosity
adjusted by means of an efflux viscometer to 16 -2 Zahn seconds and a
series of corrugated cartons printed. The cartons had excellent printing
color coverage. The ink ran well and washed up well on the press.
The ink varnish, along with the water-based resin used in the above
formulation, functions as a vehicle which serves as a dispersing, carrying
and binding medium for the pigment particles. The varnish and resin
provide the necessary flow, tack and plasticity to allow the ink to
distribute on the press and transfer from the rollers to the plates and in
turn, to the surface to be printed. The ink varnish used in Example I was
a maleic rosin solution, available from Filtered Rosin Co., under the
trade designation No. 5001. The synthetic waterbased resin used was a
styrenated soya type, commercially available from A. E Staley Co. under
the trade designation P3-103. Numerous ink varnishes and water-based
resins are suitable for flexographic inks, as is known in the art. The
choice of varnish and resin depends upon the speed of drying required,
gloss, hardness and adhesion requirements, economic factors and the nature
of the surface to be printed. The wax used was Fisher-Tropsch 215.degree.
F-223.degree. F M.P. commercially available from Dura Commodities Co.,
under the trade designation Microfine VI. Solid petroleum products such as
paraffin wax or micro crystalline wax are commonly added to printing ink
formulations to reduce or eliminate off-setting, to keep the ink from
penetrating too rapidly into the stock surface, and to impart a measure of
slip and scuff-resistance.
The combined surfactant included minor amounts of the following; a nonionic
surfactant, based on tetramethyl decynediol, commercially available from
Air Products and Chemicals, Inc., under the trade designation Surfynol
104; lecithin, available from Ross and Rowe, Inc., a mixed aminohydroxy
compound available from Commercial Solvent Corp., and a silicone defoamer
available from Dow Chemical Corp.
The polyethylene oxide used was a nonionic homopolymer, commercially
available from Union Carbide under the trade designation POLYOX WSR N-80.
Rheological measurements indicate that the resin has a relatively narrow
distribution of molecular weights and an average molecular weight of
200,000. The viscosity range, at 25.degree. C, using a Brookfield spindle
No. 1, 50 revolutions per minute, is given as 81-105 cps for a 5 percent
solution.
Polyethylene oxide polymers are polar linear resins characterized by a
relatively large number of polar groups in the molecule. Such polyethylene
polymers can be prepared by polymerizing ethylene oxide in contact with a
catalyst such as alkaline earth amide or hexammoniates as is well known in
the polymer art.
Polyethylene oxides which can be used in the present invention are those
which have average molecular weights in the range of 100,000 to 350,000,
preferably about 200,000. The polyethylene oxide can be present in an
amount of from 0.1 to 1.5 percent by weight of the ink. Preferably the
polyethylene oxide is present in an amount of from 0.4 to 1.0 percent and
most desirably, about 0.7 percent. Amounts greater than 1.5 percent appear
to create press problems during the printing operation.
COMPARATIVE PROCEDURE A
For purposes of comparison, but not in accordance with the instant
invention, a black pigmented aqueous flexographic ink composition was
prepared as described in Example I, except that polyethylene oxide was not
added. The ink viscosity was adjusted to 21 No. 2 Zahn seconds, and a
series of corrugated cartons printed.
A side-by-side comparison of the printed cartons indicated that the
composition containing polyethylene oxide showed an increased blackness of
print and greater sharpness. The ink containing the polyethylene oxide had
increased scuff resistance and dried faster with less smear.
EXAMPLE II
Aqueous flexographic inks having the compositions shown below were prepared
as described in Example I.
______________________________________
% %
By By
Blue Weight Red Weight
______________________________________
Ink varnish 20 Ink varnish 15
Synthetic water- Synthetic water-
based varnish
15 based varnish 14
Surfactant 2.5 Surfactant 2
*Clay pigment
15 *Clay pigment 5
Carbon black base
2.5
Titanium dioxide
16.5 Titanium dioxide
6
Phthalocyanine blue
3 Molybdate orange
20
Methyl violet base
2.5 Lithol red 12
Wax 1.0 Wax 1
Water 17.5 Water 20
**Polyethylene oxide
5 **Polyethylene oxide
5
(water solution) (water solution)
______________________________________
*The clay pigment functions as an extender pigment. A suitable extender i
commercially available from Thiele Kaolin Co., under the trade designatio
Burgess No. 80.
**Prepared as described in Example I by adding 12 parts by weight
polyethylene oxide to 88 parts by weight water.
The inks were placed on a flexographic printing press and the ink viscosity
adjusted to 16 No. 2 Zahn seconds.
Comparative tests were conducted using corresponding red and blue inks
which did not contain polyethylene oxide. The color of both the red and
blue inks containing polyethylene oxide was more intense on the cartons
than the color of the regular inks. In addition, the inks of the instant
invention eliminated the striation which was present on the cartons
printed using the regular ink.
Experimental tests were conducted to determine the effect of substituting
carboxymethyl cellulose or polyvinyl alcohol for the polyethylene oxide of
the present invention; no improvement of ink transfer properties was
obtained.
Further experimental tests indicate that the aqueous flexographic inks of
the instant invention have numerous advantages over flexographic inks
known in the art. Increased color, sharpness and intensity are obtained.
In addition, the inks provide virtual elimination of print striation,
minimize print mottle, and mask roller defects. Other advantages include
increased mileage through greater tolerance of water dilution, greater
color uniformity in a high and low viscosity range, improved printing on
rough surfaces and increased lubricity which reduces die wear.
Since rotogravure inks and flexographic inks both contain significant
amounts of solvents and have a low viscosity aqueous rotogravure inks,
containing polyethylene oxide to improve ink transfer properties and to
reduce wear on the expensive copper or chromium faced cylinders can be
prepared.
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