APPLICATION BULLETIN
A New pH Neutral Waterborne
Dispersing Resin For Metallic And
Organic Pigments
Abstract
A new styrene-maleic anhydride-based dispersing
resin is described and its utility in preparing waterbased metallic and organic pigment dispersions and
printing inks is demonstrated. This resin contains
amic acid functionality, groups that are possible to
form only with anhydride-containing resins. A unique
combination of properties is exhibited by this resin,
including neutral pH, high water solubility, low
molecular weight and high acid number. The amic
acid resin functions as an efficient polymeric
surfactant in a range of water-based dispersing and
emulsifying applications. For example, it can be used
to prepare high solids, low viscosity dispersions of
organic pigments. In addition, the dispersing resin
can be combined with a co-resin or an emulsified
wax to produce a vehicle to disperse metallic
pigments. Typical formulations and their properties
are described in this paper.
Background
Dispersing resins are a crucial ingredient to waterbased ink formulations. Functions carried out by
dispersing resins include wetting the pigment
surfaces, facilitating the breakup of pigment
agglomerates to maximize color development,
stabilizing the dispersion of pigment particles and
modifying the rheological properties of the
dispersion and finished ink formulations. In addition
to carrying out these functions, the dispersing resin
must be compatible with the myriad of other
ingredients that are used in preparing dispersions
and inks. Defoamers, surfactants and emulsion
solutions all are, or contain, surface active molecules
that can potentially interact with the dispersing resin.
The challenge to select the proper dispersing resin
becomes even more difficult when the pigment,
itself, can react or deteriorate from this interaction.
Such is the case with metallic pigments, which
present special issues for the preparation of stable
water-based dispersions and inks.
Metallic pigments are finally divided powders or
platelets of a metal or a metal alloy. While metallic
pigments have a unique appearance combining color
and brilliance, they also have a unique reactivity
compared to typical organic pigments. Compared
to organic compounds, metals have a low reduction
potential. The consequence of this is that in an
oxidizing environment, the oxidant will be reduced
and the metal will be oxidized in a so called redox
couple reaction.
The redox chemistries of the metals and alloys
typically used in graphic arts are well known.1 For
example, copper, which, either by itself or as its
alloy with zinc, is the predominant metal in “gold”
inks, has a reduction potential of -0.34V:
Cu ® Cu+2 + 2 e- gE = - 0.34 V
(For comparison, actual gold has a reduction
potential of -1.68V, which is why it is a much more
stable, “noble” metal.) The low reduction potential
for copper causes it to react even under relatively
mild, ambient conditions. A shiny, new copper
surface will rapidly acquire a tarnished patina due
to reaction with atmospheric oxygen and carbon
dioxide to form a coating of copper carbonate:
2 Cu + H2O + CO2 + O2 ® Cu2 (OH)2 CO3
In water solution, copper will react with trace
hydroxide to form copper hydroxide, Cu(OH2). In
the presence of ammonia, this solid reacts to form a
bright blue complex:
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Cu(OH)2 + 4 NH3 (aq)
elements with the existing anionic functionality.
Alternatively, co-solvents, such as alcohols or
glycols, can be added to increase the water solubility
of the partially neutralized resin. This approach,
however, introduces VOC into the formulation,
contrary to the purpose of having a water-based
formulation. In addition, both strategies merely dilute
the ammonium hydroxide functionality rather than
eliminate it.
® [Cu(NH3)4]+2 (aq) + 2OH- (aq)
These are but several examples of the many
reactions that can deteriorate metallic pigments,
causing them to lose their true color and brilliance.
The rate at which metals corrode or tarnish is highly
dependent upon their surrounding environment. An
environment containing finely divided, high surface
area metal particles in an aqueous media with ionic
additives is clearly not conducive to long term
stability. Therefore, the challenge for any new
metallic pigment dispersing resin is to provide the
needed wetting and rheological properties while
maintaining or enhancing the stability and
appearance of the metal surfaces.
Water-based dispersion and emulsion resins
typically have high molecular weight and low acid
number. They consist of agglomerates of high
molecular weight acrylic or acrylic-styrene resin
stabilized by a solution resin. Due to their high
molecular weight, these resins have poorer wetting
properties, and often must be used in combination
with a low molecular weight surfactant package.
Also, since the same type of solution resin that is
used to make pigment dispersions is often used to
stabilize the emulsion latex, in a sense, use of these
resins in metallic formulations again introduces
ammonium hydroxide functionality in a dilute form.
Current Water Soluble, pH Neutral Resins
A variety of different water soluble resins are
currently used to prepare metallic pigment
dispersions and inks. While, as a group, these resins
offer a spectrum of performance vs. cost
possibilities, these limited choices do not provide a
universal solution to issues encountered when
formulating.
Several water-soluble resins based upon new
functional chemistry have been introduced recently.
For example2, sulfopolyester and sulfopolyester
hybrid resins are soluble in water due to the presence
of diethylene glycol and, especially, 5-sulfoisophthalic acid monomer units. The sulfonic acid
groups have different pKas and different surfactant
properties compared to carboxylic acid functional
groups, and consequently the resins exhibit an
interesting combination of properties. However,
these resins are relatively expensive compared to
acrylic based solution resins.
Solution dispersing resins are most commonly low
molecular weight copolymers of acrylic acid. They
have high acid numbers and are solubilized in water
by neutralizing their carboxylic acid groups, most
commonly with ammonium hydroxide. They form
true solutions, with limited self-agglomeration, a very
small particle size and a clear appearance. These
solutions have very good wetting properties, and
are a common ingredient in many organic pigment
dispersions. However, the ammonium hydroxide
carboxylic salt functionality which provides the water
solubility contributes to poor stability with metallic
pigments.
While there are a number of water soluble resins
that are currently used in preparing metallic pigments
and inks, there are still needs for new resins that
combine unique combinations of properties using
cost efficient building blocks.
Different approaches have been used to reduce the
concentration of carboxylate salt functional groups
while maintaining the solution resin’s water solubility.
These include partially esterifying the acid groups
with an alcohol which has hydrophilic
characteristics, such as an ethylene oxide oligomer.
However, this can modify the surfactant properties
of the resins, as is introduces non-ionic surfactant
Styrene-Maleic Anhydride Resins: Platforms
For Polymeric Surfactants
Styrene-maleic anhydride copolymers (SMA®
Resins) have a long history of use in the graphic
arts industry as dispersing resins and additives in
2
water-borne formulations. Low molecular weight
base resins are commonly available with styrene/
maleic anhydride ratios ranging from 1/1 to 4/1. The
styrene-maleic anhydride resins have been routinely
solubilized in water by reacting with an excess of
alkali or amine base. In the presence of excess base
(greater than 2 moles of base for each mole of
anhydride groups in the resin) the anhydride rings
react to give dicarboxylic acid salt functional groups
(see Figure 1a). Therefore, the higher the anhydride
content of the resin (or lower the styrene/maleic
anhydride ratio), the higher the acid number of the
resin, and the higher the solubility of the base
hydrolyzed product in water.
Figure 1. Hydrolysis reactions of styrene-maleic anhydride resins in excess base, (a) hydrolysis of base
resins, (b) hydrolysis of ester resins.
O
CH
CH 2
x
O
CH
CH
C
C
O
CH
n
CH 2
x
O
O
C
C
CH
CH
z
O
O
CH
CH
C
C
OR
OH
y
n
O
X+ OH +
X OH
-
O -X+ O -X+
O
CH
CH 2
x
O
CH
CH
C
C
O
CH
n
CH 2
x
O - X+ O - X +
(a)
C
C
CH
CH
O
z
O
CH
CH
C
C
OR
O -X+
y
n
O
(b)
Hydrolyzed styrene-maleic anhydride resins
perform as classic anionic polymeric surfactants,
combining hydrophobic (styrene) and hydrophilic
(carboxylic acid ammonium salt) structural units
along a common backbone. Changing the styrene/
maleic anhydride ratio in the base resin will have an
obvious impact on the balance of hydrophobic/
hydrophilic properties. In addition, this balance can
be “fine-tuned” by esterification of the base resin to
produce partial monoesters. Variables introduced
using this reaction include the type of alcohol used
and the extent of the esterification reaction. The
esterified resins can also be solubilized in water by
reacting with excess base (see Figure 1b). In these
reactions, the carboxylic acid groups of the partial
esters are converted to their carboxylate salts, while
the residual anhydride groups give dicarboxylate salt
groups. Thus, between polymerization chemistry and
post reactions, it is a simple matter to prepare series
of resins varying by systematic structural changes.
In addition, methods have been developed to screen
the relative effectiveness of these resins to wet and
bind to pigment surfaces.3
One example of the studies that are possible by
varying the structures of maleic anhydride/styrene
resins is found by the work of Müller.4 Several
papers describe the corrosion inhibition properties
of a series of dimethylethanol amine salts of
copolymers of maleic anhydride/styrene/acrylic
esters when used with aluminum, copper or brass
pigments.
3
Until now, all water-based formulations using
styrene-maleic anhydride resins have solubilized the
resins by converting to their carboxylic acid salts.
Therefore, while these resins can furnish unique
properties, they also contain the same structural
elements that contribute to tarnish and instability of
metallic pigment dispersions and finished inks.
However, the anhydride functional group posses
unique reactivity compared to carboxylic acid
groups, and this has furnished a way to prepare
water soluble resins with unexplored properties.
SMA® Amic Acids: A New Class Of Polymeric
Surfactants
A closer look at the base hydrolysis of anhydride
functional groups suggests a practical route to a new
type of polymeric surfactant derived from styrenemaleic anhydride resins. In water solution
anhydrides rapidly react with ammonia or primary
or secondary amines to form a monoamide,
monocarboxylic acid group, which is commonly
referred to as an amic acid (see Figure 2).5 If an
excess of amine is used, this intermediate reacts
further to form an amide-carboxylate, which in water
hydrolyzes to give predominately dicarboxylate
functional groups.
Figure 2. The reaction of anhydrides with amines
R
CH
O C
CH R
C O
NH2R
R
CH
O C
O
OH
Anhydride
CH R NH2R R CH
C O
O C
NHR
O-
Amic Acid
Heat
-H2O
R
CH R
CH
O C
CH R
C O
NHR
+NH3R
Amide-Carboxylate
H2O
R CH
CH R
O C
C O
-
OO
+NH3R +NH3R
Dicarboxylate
C O
N
R'
Imide
However, if only one equivalent of amine is reacted
with an anhydride, the reaction stops at the amic
acid, as a stable product. In fact, most commercial
manufacture of thermoplastic polyimide resins is
based on a two step process where a dianhydride
is reacted with an aromatic diamine at low
temperature to form a poly amic acid. The amic acid
is then heated to eliminate water to form the
polyimide. While the chemistry of amic acids is not
new, applying it to form water soluble polymeric
surfactants which can be used in graphic arts
formulations is. Questions that had to be addressed
included:
-
-
Do these amic acid polymers behave as
polymeric surfactants in dispersing applications?
-
Is the amic acid functionality stable with the
metal pigments used in graphic arts
formulations?
The acid-base titration curves for styrene-maleic
anhydride polymers gave the first indications that it
should be possible to form amic acid derivatives of
these resins. In Figure 3 the titration curve of a SMA®
resin is compared with that of a polyacrylic acid
polymer. The curve for the SMA® resin consists of
two “plateaus”, corresponding to the opening of the
anhydride ring followed by neutralization of the
second carboxylic acid. The break in the curve
indicates that these two reaction steps should occur
Are amic acids of styrene-maleic anhydride
copolymers water soluble?
4
sequentially, and that the intermediate, the amic acid,
should have a pH of approximately 7. In comparison,
the titration curve for the PAA polymer is a
continuum, with no distinct break point that would
indicate a discrete intermediate. The curve also
shows that roughly 80% of the acrylic acid groups
would have to be neutralized to reach a pH of 7.
properties. Properties that were evaluated included
solubility in water (maximum percent solids), solution
viscosity and pH. Variables that were investigated
included SMA® structure (S/MA® ratio), amine
structure and process conditions (order of addition,
reaction time and temperature, etc.). Conclusions
from this matrix of experiments were:
Figure 3. The acid-base titration behavior of a
styrene-maleic anhydride resin and a polyacrylic acid
resin.
-
Starting with a high-acid number styrenemaleic anhydride resin gives an amic acid
product with a higher water solubility.
-
A monoalkyl amine with a short alkyl group
(methyl or ethyl) gives an amic acid product
with a higher water solubility.
-
The viscosity of the amic acid product is
dependent upon the reaction conditions. A
higher reaction temperature favors a lower
viscosity product solution.
Based on these experimental results, attention was
focused on the SMA® amic acid prepared from a
styrene-maleic anhydride resin with a S/MA® ratio
of 1 (acid number of 475) and methyl amine. The
structure of this new resin, named 1000MA, and
the properties of a typical water solution are given
in Figure 4.
A series of styrene-maleic anhydride resin amic
acids was prepared to determine which exact
structures gave the best combination of desired
Figure 4. Structure of the styrene-maleic anhydride resin amic acid 1000MA, and typical properties of a
water solution of this resin.
CH CH2
CH CH
1
n
O C
C O
O
Appearance:
% Solids:
pH:
Solution Viscosity:
Molecular Wt. (Mw):
CH CH2
CH CH
1
n
O C
C O
OH NHCH3
Clear, Yellow Solution
35 wt. % in Water
6.8
300 mPa*s
5,000
5
A key difference between the SMA® amic acid water
solution and traditionally used ammonium salt
solutions of acrylic or SMA® resins is that the amic
acid solutions contain no free amine or ammonium
ion. While amine is used in the preparation of the
amic acid resin, all the amine becomes covalently
bound in the amide groups of the product. Therefore,
there is no trace of amine odor to the water solutions,
and no possibility of amine generation during routine
processing of the amic acid solutions. Most
importantly of the formulations with metallic
pigments, the amic acid functional groups should be
non-reactive with the commonly used metals, so that
tarnish and corrosion would be expected to be
greatly reduced. However, the ability of this new
resin to function as a polymeric surfactant had to
be demonstrated.
-
To test the dispersing and stabilizing properties of
the SMA® amic acid resin, it was used to prepare a
paste of a common bronze powder (copper/zinc
alloy) with a pigment solids level of 40 wt. % and a
pigment/dispersing resin ratio of 6/1:
Formulation 1:
Bronze Powder
Amic Acid 1000MA (35% Solution)
Fatty Alcohol-PEG Ester Surfactant
Silicon-based Defoamer
Water
As another example of the utility of SMA® amic acid
dispersing resin, a paste was prepared from a
copper flake pigment:
Formulation 2:
Copper Flake
Amic Acid 1000MA (35% Solution)
Non-silicon based Defoamer
Nonylphenol Ethoxylate Surfactant
Water
The performance of a dispersing resin in stabilizing
a metallic pigment dispersion can be evaluated by
monitoring several properties:
Evidence of crusting indicates insufficient metal
wetting, since surface bound air bubbles
transport unwetted pigment particles to the
surface, where they dry to form a crusty layer.
-
Degree of settling (and consistency), since all
metallic pigments are high density and will tend
to settle. Use of proper levels of dispersing
resin will keep this to a minimum and produce
a “soft” settle which is easily stirred back to a
uniform fluid.
80 parts
33 parts
4 parts
2.2 parts
80.8 parts
200 parts
This paste had a uniform consistency, with no
evidence of a crust and no “hard” settle. The
supernatant had a faint green appearance, but the
metal powder retained its original color and
brilliance. No change in properties occurred over
30 days of aging. For comparison, a paste was
made using the same ingredients except substituting
an acrylic polymer/polyethylene wax blend for the
SMA® amic acid solution which became highly
tarnished and developed blue deposits.
Metallic Pigment Dispersions Based Upon
SMA® Amic Acid Resins
Aqueous pigment dispersions can be stabilized using
resins that contribute to either a charge double layer
mechanism or a steric repulsion mechanism.6 Of the
two, the approach using charged stabilizing resins
is much more common in waterborne formulations.
The pH dependence of the conformational
properties of charged resins, such as acrylic acid
polymer ammonium salts, and their influence on
surface binding and stabilization properties is well
documented.7 However, since the SMA® amic acid
resins do not contain carboxylate salts, they would
have to contribute to the stability of a pigment
dispersion by the steric repulsion mechanism.
-
Color of supernatant or change in particle
surface appearance indicates tarnish of the
metal.
60 parts
25 parts
1.7 parts
3 parts
60.3 parts
150 parts
Again, this paste, or dispersion, exhibited uniform
consistency and excellent retention of color and
brilliance over a 30 day time period.
Metallic Inks Based Upon SMA® Amic Acid
Resins
While SMA® amic acid resins can be used to make
stable dispersions of metallic pigments, since they
are low molecular weight resins, they can not
6
provide all the physical properties that would be
required in a finished ink. Properties such as rub
resistance and water resistance would be provided
by a high molecular weight co-resin and/or wax
additives. To test the compatibility of the amic acid
resins with the high molecular weight co-resins,
vehicles were prepared from combinations of amic
acid resin with a typical sulfopolyester, polyurethane
or SB rubber.
The resin mixtures were all stable, although the
ingredients for the Formulation 3 had to be preneutralized with amino-methyl propanol to prevent
resin shocking, or “kick-out” from occurring. These
vehicles, and also a commercial vehicle based on
acrylic resin, were, in turn, used to prepared finished
metallic inks by combining with bronze paste
described in Formulation 1.
Formulation 6: Finished Inks From Based On Vehicles
Vehicle (From Formulation 4, 5 or 6)
95 parts
Bronze Paste (From Formulation 1)
97 parts
Water
7 parts
Silicon-based Defoamer
1 parts
200 parts
Formulation 3: Sulfopolyester-SMA Amic Acid Vehicle
Sulfopolyester Resin Solution
35 parts
SMA ® Amic Acid Solution
163 parts
Silicon-based Defoamer
2 parts
200 parts
The four finished ink formulations were evaluated
based upon their relative viscosities, printability/
transfer and amount of tarnish. The observed
properties are summarized in Table 1, with a rating
of 1 being the best and 4 being the worst for each
property.
Formulation 4: Urethane-SMA Amic Acid Vehicle
Urethane Resin Solution
157 parts
SMA ® Amic Acid Solution
35 parts
Silicon-based Defoamer
2 parts
Water
6 parts
200 parts
Formulation 5: SB Rubber-SMA Amic Acid Vehicle
SB Rubber Emulsion
150 parts
®
30 parts
SMA Amic Acid Solution
Silicon-based Defoamer
2 parts
Water
18 parts
200 parts
Table 1. Comparison of metallic ink properties.
Property
Solution Vehicle Resin
Viscosity
Printability
Tarnish
Sulfopolyester / Amic Acid
3
1
1
Urethane /Amic Acid
2
2
3
SB Rubber / Amic Acid
4
2
2
Acrylate
1
2
4
Finally, an “All-SMA® Amic Acid” bronze ink was
prepared by combining the amic-acid based paste
of Formulation 1 with more amic acid resin and an
emulsified wax:
Formulation 7
Bronze Paste (From Formulation 1)
SMA® Amic Acid Solution
Emulsified Polyethylene Wax
Silicon-based Defoamer
Water
7
97 parts
95 parts
3 parts
1 parts
4 parts
200 parts
The ink made using Formulation 7 achieved 40 rubs
in the Sutherland test (ink proofed on back side of
Leneta 3NT-3 using 165Q anilox). By comparison,
the ink prepared using the sulfopolyester/SMA®
amic acid vehicle achieved 30 rubs in the Sutherland
test.
prepare a new class of polymeric surfactant, the
SMA® amic acid resin. Important properties of this
new resin include:
- High water solubility at neutral pH
- Low solution viscosity
- No free amine or ammonium ion
Organic Pigment Dispersions Based Upon
SMA® Amic Acid Resins
Based upon the successful results obtained when
the SMA® amic acid resins were used to disperse
metallic pigments, several screening experiments
were performed to determine if these resins could
also be used to disperse organic pigments.
Formulations were carried out using a
phthalocyanine blue 15:3 pigment and pigment/
binder ratios of 5/1. The viscosities of the
dispersions were pH dependent, with a drop in
viscosity when the pH was increased. The pH of
the dispersions could be varied by adding either
ammonium hydroxide or AMP (amino methyl
propanol). Dispersions with solids levels as high as
44 wt. % were prepared, and were found to be
viscous, but not gelled. To compare properties with
dispersion based on commonly used acrylic
dispersing resins, dispersions were made at 38%
pigment solids.
The utility of the SMA® amic acid resin was
demonstrated by using it as a dispersing resin to
prepare metallic pigment pastes and finished inks.
General conclusions from this formulation work
includes:
- SMA® amic acid formulations exhibit
reduced tarnish compared to acrylic resin
based formulations
- Good compatibility with sulfopolyester
resins and acrylic resins, some compatibility
with urethane and SB rubber resins
- Excellent stability for SMA® amic acid resin /
wax emulsion formulations
Finally, preliminary experiments indicate that SMA®
amic acid resins can be used to prepare very high
solids, low viscosity dispersions of organic pigments,
such as blue 15:3.
Acknowledgments
We would like to thank Lisa Hahn (Flexo Tech, Inc.)
for preparing the pigment pastes, dispersions and
inks. Dr. Cristophe Dumousseaux (ATOFINA
Chemicals, Inc., CAL Development Lab, Paris)
provided the titration curves for SMA® and PAA
resins shown in Figure 3. Bruce McEuen is thanked
for assisting in SMA® amic acid preparations.
Formulation 8: Blue 15:3 Dispersion Based On SMA®
Amic Acid Resin
Blue 15:3 Pigment
38 parts
®
20.7 parts
SMA Amic Acid Resin Solution
Siloxane-based Defoamer
0.7 parts
Water
40.6 parts
100 parts
The solution viscosity of the dispersion from
Formulation 8 after 4 days of aging was 210 mPa*s
(Brookfield, at 60 rpm). This compares with a
viscosity of 580 mPa*s for an analogous dispersion
prepared using an acrylate dispersing resin.
Experiments to determine the generality of these
encouraging results are planned.
References
1.) R.H. Petrucci; General Chemistry, 5th Ed.;
Macmillan Publishing Co.; New York; 1989;
pp.891-895.
2.)
Summary
The unique reactivity of the anhydride groups in
styrene-maleic anhydride resins were used to
8
T.J. DeBord, Jr., M. Schick; “Sulfopolyester
Hybrids: The Next Generation of
Water-Based Resins”; Ink World;
April 1999; pp. s47-56.
3.)
4.)
J.C. Schmidhauser, R. Lewis, L.M. Hahn;
“Comparative Analysis of Dispersant Polymer
Adsorption On Organic Pigment Surfaces
Using NMR Spectroscopy”, presented at the
43rd NPIRI Technical Conference; October,
1999.
B. Müller, M. Schubert; “Corrosion Inhibition
of Copper and Brass Pigments in Aqueous
Alkaline Media By Copolymers”; Progress In
Organic Coatings; Vol. 37; 1999; pp. 193197.
B. Müller, A. Paulus, B. Lettmann, U. Poth;
“Amphiphilic Maleic Acid Copolymers as
Corrosion Inhibitors for Aluminum Pigment”;
Journal of Applied Polymer Science; Vol. 69;
1998; pp. 2169-2174.
5.)
R. Kluger, J.C. Hunt; “Aminolysis of Maleic
anhydride. Kinetics and Thermodynamics of
Amide Formation”; Journal of the American
Chemical Society; Vol. 106; 1984; pp. 56675670.
6.)
H.J. Spinelli; “Polymeric Dispersants in Ink
Jet Technology”; Advanced Materials; Vol. 10;
1998; pp. 1215-1218.
7.)
M. Kardan; “Effect of Charged Resins on
Waterborne Coating and Adhesive
Properties”; Coatings World; September
1999; pp. 42-47.
The information in this bulletin is believed to be accurate, but all recommendations are made without warranty since the conditions of use are beyond Cray Valley Company's
control. The listed properties are illustrative only, and not product specifications. Cray Valley Company disclaims any liability in connection with the use of the information,
and does not warrant against infringement by reason of the use of its products in combination with other material or in any process.
9