Ink compositions and a method for making same

UV curable ink-jet inks for continuous ink-jet printing and drop on demand (DOD) ink-jet printing are provided which are preferably applied to substrates capable of absorbing part of an ink droplet applied thereupon. The ink compositions of the present invention include a mixture of water which serves as a solvent, a water miscible polymerizable material capable of being cured upon the application of UV light, a photoinitiator, and a colorant which may be a dye or a color pigment. The ink compositions may also include a bridging fluid.

FIELD OF THE INVENTION 
The present invention relates generally to ink compositions and more 
particularly to ink compositions useful in ink-jet printing. 
BACKGROUND OF THE INVENTION 
There are known in the art two main processes of ink jet printing. In one 
process, usually called continuous ink-jet printing, a stream of ink drops 
are electrically charged and then are deflected by an electric field 
either directly or indirectly onto the substrate. Inks used in continuous 
ink-jet printing are electrically conductive and typically, are water 
based inks. The viscosity of such inks is typically 2 or 3 centipoise 
(cps) and generally does not exceed 10 cps. 
In the second process, usually called Drop on Demand (DOD) ink-jet 
printing, the ink supply is regulated by an actuator such as a 
piezoelectric actuator. The pressure produced during the actuation forces 
a droplet through a nozzle or nozzles onto the substrate. Inks for DOD 
ink-jet printing do not need to be conductive and their viscosity is 
typically between 2 and 40 cps. 
One type of prior art ink-jet ink is a water based ink-jet ink. A drawback 
of prior art water based ink-jet inks is that they dry slowly on paper. 
Another drawback of prior art water based inks is that they can be washed 
off the paper with water and they can be smudged with a wet finger. 
Another type of prior art ink-jet ink is an ultra violet (UV) curable ink. 
Typically, prior art UV curable inks include polymerizable monomers or 
oligomers or a mixture thereof, all of which are miscible in organic 
solvents and are not miscible in water. 
UV curable ink-jet inks based on organic solvents are particularly useful 
for printing on hard, non-absorbing substrates such as plastics. For 
example, U.S. Pat. No. 5,270,368 to Lent et al. describes a UV curable 
ink-jet ink for printing on printed circuit boards. Similarly, Published 
European Application No. 0,407,054 A1 describes an ink-jet ink for 
printing on plastic and metal surfaces where the ink is characterized by 
good adhesion properties. 
One drawback of UV curable ink-jet inks based on organic solvents is their 
limited usefulness in printing on absorbing surfaces, such as paper. When 
printed on paper the inks penetrate through the paper and are seen from 
the back side of the paper. This is known in the art as "strike-through". 
Another drawback of such inks is that organic solvents, in particular 
volatile organic solvents, are potentially environmentally hazardous. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide improved UV 
curable ink-jet ink compositions both for continuous ink-jet printing and 
for DOD ink-jet printing which overcome the drawbacks of prior art UV 
curable ink-jet inks. 
The ink-jet ink compositions of the present invention comprise a mixture of 
water, a water-miscible polymerizable material, a colorant and a 
photo-initiator. The composition may also include a non-volatile bridging 
fluid. 
According to one aspect of the invention, the ink compositions are 
formulated such that they are characterized by viscosities suitable for 
ink-jet printing and by good absorption properties when printed on 
substrates, such as paper, capable of absorbing fluids. 
According to another aspect of the present invention, the inks of the 
present invention may be dried by UV curing and absorption into the 
substrate, both of which occur simultaneously. 
There is thus provided, according to the present invention, an ultraviolet 
radiation curable ink composition which includes, in combination, water, a 
water miscible ultra-violet curable polymerizable material, a 
photoinitiator and a colorant. 
Further, according to the present invention, the polymerizable material is 
formed mainly of an oligomer. Alternatively, the polymerizable material 
may be formed from a mixture of an oligomer and a monomer. 
Additionally, the water content of the ink compositions may be between 10% 
and 75% by weight, the polymerizable material content may be between 20% 
and 60% by weight and the photoinitiator content is preferably less than 
12% by weight. 
Further, the ink composition, according to the present invention, may also 
include a bridging fluid, the content of which may be between 2% and 30% 
by weight. 
Additionally, the oligomer may be an acrylic oligomer and preferably an 
acrylic oligomer such as one selected from the group consisting of 
polyethylene glycol diacrylate, ethoxylated trimethylol propane acrylate 
and polyether acrylate. 
Additionally, the monomer may be selected from the group consisting of 2 
Hydroxy-3-methylacryloxy propyltrimethylammonium chloride, hydroxyalkyl 
acrylate and dimethylaminoethyl acrylate. 
Additionally, the photoinitiator may be selected from the group consisting 
of isopropylthioxanthone, 4-benzoyl-4'-methyl diphenyl sulphide, 
1-Hydroxy-cyclohexyl-phenyl-ketone and 
2-Methyl-1-(4-(methylthio)phenyl)-2-morpholinopropanone-1, 
1-(4-Dodecylphenyl)-2-hydroxy-2-methyl-propan-1-one, dibutoxyacetophenone 
hydroxymethyl phenylpropane-1-one and 4-benzoylbenzyl trimethylammonium 
chloride, 4-benzoyl-N, N-dimethyl-[2-(1-oxo-2-propenyloxy)-ethyl]- benzene 
methanaminium bromide and 
2-hydroxy-3-3(3,4-dimethyl)9-oxo-9H-thioxanthen-2yloxy)-N,N,N-trimethyl1-p 
ropaminiumchloride. 
Additionally, the bridging fluid may be selected from the group consisting 
of N-methyl 2-pyrrolidone, N-vinyl pyrrolidone, 2-amino ethanol. 2-butoxy 
ethanol, ethyl lactate and morpholine. 
Further, the ink compositions of the present invention have a viscosity 
between 2-40 centipoise and their colorant may be a dye. 
Further, there is provided, according to the present invention, a method of 
formulating the ink including the steps of combining at least one water 
miscible polymerizable material with water and a photoinitiator to form a 
homogeneous mixture thereof, selecting a colorant and combining the 
colorant with the mixture to receive a homogeneous ink composition 
therefrom. 
Additionally, the step of combining may include the step of combining a 
bridging fluid into the mixture before the step of selecting a colorant. 
Preferably, before a colorant is selected, the mixture is a clear 
solution. 
Further, there is provided, according to the present invention, a method of 
printing ink on a substrate comprising, in order, the steps of providing 
printing apparatus for delivering ink to a substrate, supplying an ink 
composition to the printing apparatus, delivering the ink composition to 
the printing substrate and irradiating the substrate with an ultra-violet 
light source, thereby curing the ink composition, wherein the ink 
composition includes water, a water miscible ultra-violet curable 
polymerizable material, a photoinitiator and a colorant. 
Preferably, the step of irradiating is simultaneous with the absorption of 
the water in the substrate. 
Further, the printing method includes the step of heating the ink 
composition on the printed substrate wherein the step of the heating 
includes heating by a radiation source selected of the group of IR 
radiation and microwave radiation. 
Preferably, the substrate is paper and the printing apparatus is ink jet 
printing apparatus.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
The present invention provides UV curable ink-jet inks for continuous 
ink-jet printing and drop on demand (DOD)ink-jet printing which are 
preferably applied to substrates capable of absorbing part of an ink 
droplet applied thereupon. 
Ink compositions of the present invention include a mixture of water which 
serves as a solvent, a water miscible polymerizable material capable of 
being cured upon the application of UV light, a photoinitiator, and a 
colorant which may be a dye or a color pigment. The polymerizable material 
or materials of the ink compositions may be any suitable water miscible 
oligomer or a mixture of any suitable water miscible oligomer and water 
miscible monomers, preferably, but not necessarily, water miscible acrylic 
oligomers and monomers. 
The ink compositions of the present; invention may also include a bridging 
fluid. The term "bridging fluid" refers herein to any fluid having a high 
boiling temperature and a low viscosity which has good miscibility 
properties both with water and with the polymerizable material or 
materials. 
It will be appreciated that while the term "water miscible" is used herein, 
any of the water miscible compounds may be also water soluble. 
The water content of the inks of the present invention may be between 10% 
and 75% by weight. More preferably, the water content of the ink is 
between 20% and 60% by weight. 
The polymerizable material is preferably an oligomer or a combination of a 
monomer and an oligomer. Preferably, the content of the polymerizable 
material is between 20% and 60% by weight. The polymerizable material may 
be any water miscible oligomer or a combination of an oligomer and a 
monomer, all of which polymerize when exposed to UV light. 
Preferably, acrylic oligomers, such as the poly-acrylate CN 435, 
manufactured and sold by Cray Valley Products of Exton, Pa., USA, are used 
to form the ink compositions of the present invention. Preferred oligomers 
are oligomers which include ethoxylated groups derived from polyethylene 
glycols. Examples of such materials are polyethylene glycol diacrylate 
sold by UCB as Ebecryl 11, Photomers 4155 and 4158--ethoxylated 
trimethylol propane acrylate sold by Henkel, Sartomer 344--a polyethylene 
glycol 400 diacrylate sold by Cray Valley and CN 435--a polyether acrylate 
(ethoxylated trifunctional). These materials are commercially available as 
oligomers. However, other polyol acrylates are commercially available as 
monomers because of their relatively low molecular weight and their 
inability to form films without the presence of an oligomer. 
Preferably, the amount of oligomer used is typically between 20% and 60% 
since in this range the oligomer produces sufficient film-forming 
properties to effectively dry during UV curing and the viscosity of the 
ink formulation is within the desired range for ink-jet printing. 
According to the present invention, monomers may be added as additives to 
the oligomer. This may be done to increase the water solubility of the 
polymerizable material. An example of such a material is 2 
Hydroxy-3-methylacryloxy propyltrimethylammonium chloride, sold by Servo 
Delden BV as a 50% aqueous solution. Further examples are hydroxyalkyl 
acrylate and dimethylaminoethyl acrylate. 
The ink compositions of the present invention include a photoinitiator 
which absorb the UV light and initiates, for example by producing free 
radicals, the polymerization of the oligomer or the oligomer and the 
monomer. Examples of suitable photoinitiators are isopropylthioxanthone, 
4-benzoyl-4'-methyl diphenyl sulphide, 1-Hydroxy-cyclohexyl-phenyl-ketone, 
2-Methyl-1-(4-(methylthio)phenyl)-2-morpholinopropanone-1, 
1-(4-Dodecylphenyl)-2-hydroxy-2-methyl-propan-1-one, dibutoxyacetophenone 
hydroxymethyl phenylpropane-1-one, 4-benzoylbenzyl trimethylammonium 
chloride, 4-benzoyl-N, N-dimethyl-[2-(1-oxo-2-propenyloxy)-ethyl]-benzene 
methanaminium bromide, 2-hydroxy-2-methyl-1-phenylpropane-1-one and 
2-hydroxy-3(3,4-dimethyl)9-oxo-9H-thioxanthen-2yloxy)-N,N,N-trimethyl-1-pr 
opaminium chloride. 
The total amount of initiator in the system may be as high as 12% because 
of the quenching effect of the dye colorant. High photoinitiator 
concentrations are preferred when black dyes are used. Preferably, liquid 
photoinitiators may be used because they contribute minimally to 
viscosity. Preferably, the content of the photoinitiator exceeds 2% by 
weight. 
The ink compositions may also include synergists and stabilizers. 
According to the present invention, the ink compositions may also include a 
bridging fluid. Examples of such materials are N-methyl 2-pyrrolidone, 
N-vinyl pyrrolidone, 2-amino ethanol, 2-butoxy ethanol, ethyl lactate and 
morpholine. Preferably, the content of the bridging fluid may be between 
2% and 30% by weight. 
One method of formulating the ink compositions of the present invention may 
involve the use of oligomers, monomers or both. The desired viscosity of 
the ink compositions of the present invention is typically achieved by 
dilution with water or also with water and bridging fluids. Therefore the 
ink compositions of the present invention are generally more 
environmentally friendly than those prior art ink compositions which 
include volatile organic solvents. 
According to a method of formulating the ink compositions of the present 
invention, the uncolored ingredients are mixed together and then a 
colorant is added to the mixture. Preferably, the mixture of the uncolored 
ingredients is a stable solution and not an emulsion. 
According to a preferred method of printing using the inks of the present 
invention, UV curing is used simultaneous With drying by absorption. 
According to one preferred method of printing, the ink is applied to an 
absorbing substrate, such as paper, by any suitable ink-jet printer. The 
UV radiation is then applied and it may increase the viscosity of the ink 
by cross linkage which immobilizes the ink and thereby reduces set-off of 
ink from one sheet to the next sheet of paper. The cross-linkage also 
reduces strike-through and the spread of ink into the paper. 
Simultaneously, with the UV radiation, part of the water is absorbed by 
the paper. 
It is a particular feature of the present invention that ink compositions 
need not be heated to drive off the water before UV curing. However, 
according to one preferred method of printing, the ink compositions may be 
heated by any suitable form of radiation such as IR radiation, heat or 
microwave radiation, before or after curing. 
According to another preferred method of printing, the application of the 
UV light follows some absorption of the ink in the substrate. 
It will be appreciated that ink compositions of the present invention may 
be formulated with a wide range of surface tensions to suit the particular 
ink jet system being used. Since water has a high surface tension and 
non-volatile bridging fluids typically have relatively low surface 
tensions, mixtures can be made with intermediate surface tensions. 
It will also be appreciated that the presence of water permits the 
introduction of a wide range of ionic salts into the ink compositions. 
Therefore, the ink compositions of the present invention may be made with 
conductivities suitable for continuous ink-jet printing. 
The ink compositions of the present invention may be formulated with a wide 
range of colorants which include water soluble as well as water insoluble 
dyes. 
The present invention will be understood and appreciated more fully from 
the following non limiting examples of ink compositions produced in 
accordance with the present invention. All parts are by weight. Viscosity 
measurements were made with a cone and plate Brookfield DVIII Programmable 
Rheometer at a shear rate of 75sec.sup.-1. Surface tension measurements 
were made using a Fisher Scientific Surface Tensiometer 20 with a platinum 
ring. The inks were cured with a 400 W Dymax 5000-EC light source which is 
a medium pressure mercury vapor lamp giving a broad spectrum of UVA, UVB 
and UVC light at 100mW/cm.sup.2. 
EXAMPLE I 
470 parts of the polyethylene glycol diacrylate oligomer, sold as Ebecryl 
11, were mixed with 390 parts of deionized water and the mixture was 
stirred until a completely clear light brown liquid was obtained. 117 
parts of hydroxymethyl phenylpropane-1-one, sold by Ciba Geigy as Daracur 
1173, were slowly added in conditions of subdued light and stirred in so 
that the liquid remained clear. 27 parts of an alcohol soluble nigrosine 
were stirred in. The resulting ink was suitable for use in piezo ink jet 
heads. It had a measured :viscosity of 16.0 centipoise at 16.degree. C. 
and a surface tension of 38.5 dynes per cm. Tests showed that after 
illumination with the UV lamp the inks were cured. Prints on paper were 
not smeared by rubbing even with a wet finger. 
COMISON EXAMPLE 
An attempt to formulate a water miscible UV curable ink was made with the 
formulation described in Published European Application No. 0,407,054 A1. 
200 parts of N-vinyl pyrollidone were mixed in with 600 parts of GAFGARD 
233 to give a clear homogenous solution. 150 parts of water were added 
gradually with stirring. The clear solution gradually became opaque and 
white as the water was added. It was evident that an emulsion had been 
formed. The mixture was left to stand for a day, after which it could be 
seen that water had separated out on the surface of the mixture and the 
composition could not have been used as an ink. 
EXAMPLE II 
407 parts of a polyether acrylate (ethoxylated trifuntional), sold by Cray 
Valley as CN 435, were added to 185 parts of ethyl lactate and stirred to 
give a completely clear homogenous solution. 301 parts of deionized water 
were slowly added by stirring in subdued light, followed by 83 parts of 
hydroxymethyl phenylpropane-1-one. The resulting solution was completely 
clear. It is especially suitable for colored inks where the purity of 
color is important (for example, process colored inks). In this particular 
example, for the purposes of testing in the most exacting conditions, 24.9 
parts of an alcohol soluble nigrosine dye was added. Black dyes tend to 
make U.V. curing difficult, but as in Example I, the ink was successfully 
UV cured. The ink had a viscosity of 17.5 centipoise at 25.degree. C. and 
a surface tension of 42 dynes per cm. 
EXAMPLE III 
312 parts of Ebecryl 11 were added to 121 parts of n-methyl pyrrolidone 
with stirring until a homogenous solution was achieved. 432 parts of 
deionized water were then slowly added by stirring in subdued light, 
followed by 104 parts of hydroxymethyl phenyl propane-1-one. 30.5 parts of 
naphthol blue black were then mixed in to give an ink suitable for 
drop-on-demand ink jet printing. The ink had a viscosity of 12 centipoise 
at 25.degree. C. and a surface tension of 42.5 dynes per cm. The ink was 
successfully UV cured. 
EXAMPLE IV 
The method of mixing and use was as for EXAMPLE I. The ingredients were as 
follows; 
______________________________________ 
266 parts Ebecryl 11 
103 parts n-methyl pyrrolidone 
521 parts deionized water 
84 parts 2-hydroxy-2-methyl-1-phenylpropane-1-one 
26 parts naphthol blue black 
______________________________________ 
The resulting ink had a viscosity 8.1 centipoise and a surface tension of 
43 dynes per cm. The ink was successfully UV cured. 
EXAMPLE V 
The method of mixing was as for EXAMPLE I. The ingredients and quantities 
are listed below. This ink may be used both for continuous jet and 
drop-on-demand. 
______________________________________ 
262 parts Ebecryl 11 
109 parts n-methyl pyrrolidone. 
507 parts deionized water 
12.6 parts sodium chloride 
84 parts 2-hydroxy-2-methyl-1-phenylpropane-1-one 
25 parts alcohol soluble nigrosine 
______________________________________ 
The resulting ink had a viscosity of 8.6 cps, a surface tension 43.3 
dynes/cm and a conductivity of 52 milliSiemans/cm. The ink was 
successfully UV cured. 
EXAMPLE VI 
273 parts of Ebecryl 11 were mixed with 114 parts of Sartomer 344 until a 
homogenous mixture was obtained. 496 parts of deionized water were stirred 
in, followed by 91.5 parts of hydroxymethyl phenyl propane-1-one. 25.6 
parts of naphthol blue black were then added with stirring. The resulting 
ink had a viscosity of 11.6 centipoise and a surface tension of 41.3 dynes 
per cm. When deposited on papers, whether hard surface or absorbent, it 
U.V. cured in less than a second. 
EXAMPLE VII 
To 338 parts of Ebecryl 11, 132 parts of n-methyl pyrrolidone were added 
followed by 5.7 parts of 1-hydroxy-cyclohexyl-phenyl-ketone, 12.7 parts of 
2-methyl-1-[4-(methylthio)phenyl-2-morpholinopropanone-1] and 12 parts of 
benzildimethylketal. The mixture was warmed to 400.degree. C. and 470 
parts of deionized water were added by stirring. Finally, 27.7 parts of 
alcohol soluble nigrosine were added. The resulting ink jet ink had a 
viscosity of 6.7 centipoise and surface tension of 43.2.dynes per cm. The 
ink was successfully UV cured. 
It will be appreciated by persons skilled in the art that the present 
invention is not limited by what has been particularly shown and described 
herein above. Rather the scope of the invention is defined by thee claims 
which follow: