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A New pH Neutral Waterborne Dispersing Resin for Metallic and Organic Pigments.

Editor's Note: This paper was presented at the National Printing Ink Research Institute's (NPIRI) 44th Annual Technical Conference, Oct 17-19, 2000.

A new styrene-maleic anhydride-based dispersing resin is described and its utility in preparing water-based 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 water-based 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 reparation 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 -[greater than] [Cu.sup.+2] +2 [e.sup.-] [E.sup.[degrees]] = - 0.34 V

For comparison, actual gold has a reduction potential of-1.GSV, 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 -[greater than] [Cu.sub.2](OH)[2CO.sub.3]

In water solution, copper will reaction with trace hydroxide to form copper hydroxide, Cu(OH)2. In the presence of ammonia, this solid reacts to form a bright blue complex:

Cu[(OH).sub.2] +4 [NH.sub.3] (aq) -[greater than] [[Cu([NH.sub.3]).sub4].sup.+2] (aq) + [2OH.sup.-] (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.

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.

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.

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 ohigomer. However, this can modify the surfactant properties of the resins, as is introduces non-ionic surfactant 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.

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.

Several water-soluble resins based upon new functional chemistry have been introduced recently. For example, 2 sulfopolyester and sulfopolyester hybrid resins are soluble in water due to the presence of diethylene glycol and, especially, 5-sulfo-isophthalic 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 comparing to acrylic-based solution resins.

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 a unique combinations of properties using cost efficient building blocks.

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 waterborne 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 la). 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.

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 "finetuned" 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 ib). 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 Muller. [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.

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 possesses 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 styrene-maleic 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-carboxylato, which in water hydrolyzes to give predominately dicarboxylate functional groups.

However, if only 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:

* Are amic acids of styrene-maleic anhydride copolymers water soluble?

* 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 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 percent of the acrylic acid groups would have to be neutralized to reach a pH of 7.

A series of styrene-maleic anhydride resin amic acids was prepared to determine which exact structures gave the best combination of desired 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:

* Starting with a high-acid number styrene-maleic 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 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.

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.

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.

* Color of supernatant or change in particle surface appearance indicates tarnish of the metal.

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 80 parts

Amic Acid 1000MA (35% Solution)

33 parts

Fatty Alcohol-PEG Ester Surfactant

4 parts

Silicon-based Defoamer 2.2 parts

Water 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 made using the same ingredients except substituting an acrylic polymer/polyethylene wax blend for the SMA amic acid solution became highly tarnished and developed blue deposits.

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 60 parts

Amic Acid 1000MA (35% Solution)

25 parts

Non-silicon based Defoamer 1.7 parts

Nonylphenol Ethoxylate Surfactant

3 parts

Water 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 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 co-resins, vehicles were prepared from combinations of amic acid resin with a typical sulfopolyester, polyurethane or SB rubber.

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

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

SMA Amic Acid Solution 30 parts

Silicon-based Defoamer 2 parts

Water 18 parts

200 parts

The resin mixtures were all stable, although the ingredients for the Formulation 3 had to be pre-neutralized 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

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.

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) 97 parts

SMA Amic Acid Solution 95 parts

Emulsified Polyethylene Wax 3 parts

Silicon-based Defoamer 1 parts

Water 4 parts

200 parts

The ink made using Formulation 7 achieved 40 rubs in the Sutherland test (ink proofed on back side of Leneta 3NT3 using 165Q anilox). By comparison, the ink prepared using the sulfopolyester/SMA amic acid vehicle achieved 30 rubs in the Sutherland test.

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.

Formulation 8: Blue 15:3

Dispersion Based On SMA Amic Acid Resin

Blue 15:3 Pigment 38 parts

SMA Amic Acid Resin Solution 20.7 parts

Siloxane-based Defoamer 0.7 parts

Water 40.6 parts

100 parts

The solution viscosity of the dispersion from Formulation 8 after four 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.

Summary

The unique reactivity of the anhydride groups in styrene-maleic anhydride resins were used to 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.

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 FAA resins shown in Figure 3. Bruce McEuen is thanked for assisting in SMA amic acid preparations.

References

(1.) R.H. Petrucci; General Chemistry, 5th Ed.; Macmillan Publishing Co.; New York: 1989; pp.891-895.

(2.) T.J. DeBord, Jr., M. Schick; "Sulfopolyester Hybrids: The Next Generation of Water-Based Resins," Ink World; April 1999; pp. 47-56.

(3.) 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.

(4.) B. Mueller M. Schubert; "Corrosion Inhibition of Copper and Brass Pigments in Aqueous Alkaline Media By Copolymers," Progress In Organic Coatings; Vol. 37; 1999; pp. 193-197.

B. Muller 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. 5667-5670.

(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.

John C. Schmidhauser holds a BS degree in chemistry from the University of Southern California and a PhD in organic chemistry from Harvard University. He worked at GE's Corporate Research Center for 12 years before joining Atofina Chemicals, Inc. as a principal scientist in 1997. In 2001 he became the technical manager of the Specialty Polymers Group for Sartomer Company, where he has responsibility for new application and new product development with SMA, Poly bd and Ricon Resins.
 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
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Author:Schmidhauser, Dr. John C.
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Date:May 1, 2001
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