The development of waterborne radiation curable ink formulations for ink jet applications is challenged by the need for hydrolytic stability. Most conventional waterborne inks are formulated in the basic pH range (pH ≥ 8). In this case, stable waterborne pigment dispersions are formulated with conventional anionic and nonionic pigment dispersants.
However, if these same dispersions are used in combination with water-soluble acrylate monomers and oligomers, the basic pH often causes hydrolysis of the acrylic esters. The result is a decrease in pH, changes in viscosity and poor overall stability.
Conversely, if the basic pigment dispersions are added to the acid stable radiation curable monomer and oligomers, the dispersions become unstable, resulting in pigment flocculation and changes in viscosity.
To address this dilemma, a novel radiation curable pigment dispersant and a co-functioning surfactant are introduced that yield acid stable pigment dispersions that remain stable when added to the water-soluble acrylate monomers and oligomers. The utilization of these additives is demonstrated in red, yellow, blue and black waterborne radiation curable ink jet formulations.
There are an increasing number of radiation curable materials being used to develop jettable ink formulations. These materials react with radiant light to become part of the polymeric film and are 100 percent solids, which allows formulators to avoid using solvents and the subsequent regulatory concerns with volatile organic compounds.
In order to achieve the low viscosities needed to jet the inks through the tiny orifices within the print head, many low viscosity, mono-functional monomers are used. While these materials effectively reduce the viscosity of the inks, they often are slower curing, have high Draize values and skin sensitization ratings that make them more challenging to handle safely.
One way to avoid using higher amounts of these lower viscosity, slower curing and more hazardous materials is to use water to reduce ink viscosity. By using water-soluble monomers, oligomers, photoinitiators and pigment dispersions, jettable ink formulations can be made that are less hazardous to make and use.
Waterborne radiation curable inks have successfully been developed for flexographic and gravure applications. However, one of the most challenging aspects of making these formulations is gaining the necessary shelf stability. The balance of electric charge of the various formulation components and the final pH of the system can be challenging.
With radiation curable acrylates, this is made especially difficult since most conventional waterborne pigment dispersions are formulated in the basic pH range. In this pH range, water-soluble acrylate materials are susceptible to hydrolysis. When this occurs, drastic changes in viscosity can result. In addition, pigment dispersions can become unstable, resulting in flocculation and/or thixotropic rheology.
This study introduces cationic radiation curable pigment dispersants for waterborne ink jet inks. Pigment dispersions and ink jet formulations will be shown that demonstrate that formulating inks with pH less than 7 provides much improved stability compared to inks that are formulated in the basic pH range.
The radiation curable, cationic CN3230 dispersant, PRO6134 surfactant, polyethylene glycol diacrylate monomer, and 15 mole ethoxylated trimethylolpropane triacrylate monomer were from Sartomer Company, Inc. The pigments used in this study were as follows: Phthalocyanine Blue 15:3 from Sun Chemical; Arylide Yellow 74 from Magruder Color; Quniacridone Magenta, Red 122 from Magruder Color, and Carbon Black 7 from Cabot. The defoamer Dee-Fo PI-35 from Ultra Additives, and the 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one photoinitiator from Ciba Specialty Chemicals were used in the ink formulations. The anionic styrene acrylate copolymer dispersion resins were from Johnson Polymer.
Viscosities of the inks were measured using a Brookfield DVII+ viscometer with a #19 spindle at 100rpm, at 25°C. PH measurements were performed on a Titralab Tim900 Titration Manager using a PHG201 electrode.
Pigment Dispersion Preparation
The cationic pigment dispersions were prepared using the formulations in Table 1, and the anionic pigments dispersions were prepared using the formulations in Table 2. The liquid ingredients were mixed together in a blender for one minute on low speed. In the same blender, at medium mix speed, the pigments were individually added in three increments. The mixer was then turned to high speed for 15 minutes.
High shear dispersing was done by charging 25 percent of the pigment concentrate and 75 percent 0.8mm YTZ media into an Eiger mill. The mill was then run for 20 minutes at 5000 rpm in re-circulation mode, with a residence time of approximately 10 minutes.
Pigment Dispersion Stability Testing
Each of the cationic and anionic pigment dispersions was stored at 25°C, and the viscosities were measured periodically. When stored for 130 days at 25°C, no change in viscosity and no pigment settling were observed. The cationic dispersions were also stored in a 49°C oven, and the viscosity was observed over time. The black and blue compounds were stable for 45 days, the red compounds were stable for 37 days, and the yellow compounds were stable for 22 days before the dispersions became too thick to measure viscosity.
The cationic inks were prepared using the formulations in Table 3, and the anionic inks were prepared using the formulations shown in Table 4. In both cases the monomers were blended together with additional dispersant, and water at low shear. The dispersions were then slowly added, and then the inks were mixed for one hour.
Each sample was stored at 25°C, and the viscosity was measured periodically. The inks were considered to be stable as long as the viscosity did not change by more than 10cps, and/or the ink began to phase separate. The stability for each of the cationic inks, along with the mode of failure is shown in Table 5.
In a similar manner, the stability of the anionic inks was measured. The duration and the mode of failure are noted in Table 6.
Ink Film Testing
Films of the inks were drawn down with a number five Mayer rod on both glossy Lanetta Charts and on flat paper. The films were then cured using a Fusion 600 Watts/inch H bulb system. The lamp power output was tested using an EIT Power puck, and the energy needed to cure each color ink film is shown in Table 7.
Gloss measurements were made at an 85-degree angle on each of the films using a BYK-Gardener micro-TRI-Gloss Meter. The values for each color on glossy Lanetta Charts and on flat paper are shown in Table 8.
Both the anionic dispersions, which were formulated with typical styrene acrylate resins, and cationic dispersions, which were formulated with the radiation curable cationic dispersant, were quite stable when tested at room temperature. Both became less stable when tested at 49°C.
However, a very significant difference was observed when the dispersions were formulated into inks. While the cationic inks were very stable for 47 to more than 110 days, depending on the color, the anionic inks became unstable within several hours.
As a result, it was not possible to evaluate the film properties of the anionic inks. Conversely, the cationic inks produce bright colored ink films that vary in gloss depending on the substrate on which they were made.
A novel cationic radiation curable pigment dispersant has been developed that yields stable pigment dispersions. These dispersions can then be formulated into cationic radiation curable ink jet inks.
These inks are much more stable than inks formulated with anionic pigment dispersions. The cationic inks were stable for 47 to 110 days, depending on the color, whereas the anionic inks became unstable within hours after being made. Hence it is now possible to formulate radiation curable waterborne ink jet inks.