The Use of Comb-Branched Copolymers as Pigment Dispersants in Inks
Dry pigments are available commercially in the form of solid agglomerates. By definition, they are insoluble in their carrier medium, be it aqueous or solventborne. Without appropriate dispersion techniques, pigment agglomerates may settle out of pigment dispersions and/or the final pigmented coating composition. Dispersion techniques typically include a media milling/grinding process in which the pigment agglomerates are broken down into smaller agglomerates and/or individual particles that are wetted by the carrier medium. Pigment dispersants are provided in the pigment dispersion to facilitate the wetting process. In addition, they are important to the suspension and homogeneous dispersion of the pigment particles in the carrier medium as they are being milled and subsequently during storage. The function of the pigment dispersant is to therefore wet, disperse and stabilize the pigment dispersion in its carrier solvent and to maintain stable particle distributions in the finished letdown formulation.
A particularly challenging application for pigments is in inkjet inks. While dyes have been successfully formulated into inkjet inks with reliable printhead performance and excellent optical properties, image deficiencies have been experienced with respect to durability, water resistance and lightfastness. As a result, the technology is trending towards the development of pigmented inkjet inks. A challenge to using pigments is to stabilize a fine particle size distribution. Otherwise, particle agglomeration can lead to degradation of properties or even catastrophic failures in the small channels and nozzle orifices of an inkjet printhead. Moreover, it is important to control rheology, maintain high filterability and develop appropriate surface tension. Thus, compatibility between the stabilized pigment concentrate and other inkjet ink ingredients (humectants, surfactants, cosolvents) is critical.
Mechanical and chemical approaches to stabilization of pigment dispersions are practiced. A common approach is through the use of polymeric dispersants. This report highlights the properties of aqueous pigment dispersions based on unique polyether/acrylic comb-branched copolymers.
DispersionWe consider the case where the dispersion medium is mainly water. Water-insoluble pigments, like organic pigments or carbon black, are mechanically milled to produce a hydrophobic colloid. The dispersant assists with multiple functions that occur simultaneously: wetting, dispersion and stabilization (against particle agglomeration).1 Wetting involves preferential replacement of gases, liquids or other contaminants from the pigment surfaces with molecules comprising the aqueous medium. In this process, the dispersant may function as a surfactant by reducing the contact angle at the pigment surface. The dispersion process, by media milling for example, includes mechanical action and consequent particle size reduction to create new pigment surfaces that require wetting that is aided by the dispersant polymer. As the total pigment surface area increases, the volume concentration of free water decreases, the viscosity increases and the tendency of pigment particles to agglomerate increases. This leads to the most critical function of the dispersant copolymer - stabilization. The colloidal particles are under the influence of attractive and repulsive forces. The magnitudes of these forces determine the stability. Stabilization requires complete wetting and separation of the primary pigment particles, their homogeneous distribution in the medium and the prevention of pigment particle flocculation and/or aggregation. The dispersant polymer acts by adsorbing onto the particle surfaces, encapsulating the pigments and thereby providing a barrier against particle association.
Stabilization may be electrostatic and/or steric. Electrostatic stabilization2 is a result of preferential adsorption of the charged dispersant molecules onto the surface of the pigment particle and the thickening of the charge layer within the immediately surrounding medium. The dispersant therefore influences the magnitude and shape of the repulsive electronic potential energy as a function of interparticle separation. The magnitude of the electronic potential is also a function of the ionic character of the medium. Therefore, charge-stabilized colloids are vulnerable to variations in pH, electrolyte concentrations, and are less effective in systems containing mixed pigments with different zeta potentials.
Steric3 stabilization is a result of a comb or block copolymer dispersant structure in which one functional moiety (‘anchoring group') has a strong affinity for the pigment surface and extending into the solvent from the anchoring group are solvated polymeric stabilizer chains (‘pendant groups') that prevent the pigment particles from associating. Steric stabilization is due to configurational restrictions of the solvated pendant groups as interparticle distance decreases. Solvated chains lose their degrees of freedom as particles approach leading to a decrease in entropy. Therefore, the driving force to overcome this entropy change is repulsive. Steric stabilization is insensitive to electrolytic environment.
Acrylic/Polyether Comb-Branched CopolymersThe acrylic/polyether comb-branched copolymer dispersants are water-soluble or water-dispersible comb-branched copolymers with a basic structure represented in Figure 1. Both electrostatic and steric mechanisms contribute to the development of stabilized pigment dispersions. Electrostatic stabilization is dependent upon the formation of a repulsive-charged double layer potential due to the acrylate "anchor" groups on the backbone of the comb-copolymer. Extending into the solvent from these anchor groups are pendant polyether macromonomers. Steric stabilization is related to configurational restrictions of these pendant chains as particle surfaces approach each other. Thermodynamically, the free energy of the system increases as the particles approach one another (increase in DG due to decrease in DS), which provides the energy of repulsion.
The copolymer as shown in Figure 1 is composed of acrylic acid and polyether macromonomer units. Other ethylenically unsaturated comonomers and macromonomers may be incorporated into the structure. Other possible structural variations include the density of the polyether pendant group in the molecule and its solubility. This latter property is a result of the molecular weight and the level and type of alkoxylation. In addition, the molecular weight and polydispersity of the polyether/acrylic comb-branch copolymer is a controllable property. A high degree of structural flexibility for the copolymer allows for optimization as a function of the pigment type, the carrier medium and the final application.
Results and DiscussionAqueous dispersions of phthalocyanine blue pigment (CI 15:3) containing acrylic/polyether comb-branched copolymer dispersant were milled in a small media mill (Netzsch Minizeta) for 60 minutes using a 200 ml charge of 0.800 to 1.000 mm zirconium/silica beads. A premix was first produced by mixing the materials on a high-shear blade disperser to produce a homogeneous mixture before transferring to the mill. A series of dispersions were made using 40 wt. % pigment and a range of dispersant concentrations. Figure 2 illustrates the effect of dispersant concentration on particle size. The optimum level of dispersant is indicated by the minimum in the average particle size as shown in the figure. This level of dispersant also corresponds to the most narrow and monodisperse particle size distribution.
Aqueous Phthalo Blue Pigment Dispersions
Rheological flow curves were obtained for the dispersions represented in Figure 2. The viscosity of the dispersions over a range of shear rates is illustrated in Figure 3. Non-Newtonian (shear thinning) rheology and elevated viscosity are evidence of flocculation instability above and below the optimum dispersant concentration. At the optimum dosage, Newtonian flow behavior indicates a stable dispersion. Flocculation instability at low dispersant concentrations (below 17 wt. %) is due to pigment agglomeration. At dispersant overdoses, flocculation is due to interparticle bridging via the pendant polyether groups of the acrylic/polyether comb-branched copolymer.
The inks were also evaluated for gloss. The effect of the properties of the pigment dispersion on gloss is evident from the increasing values for gloss as shown in Table 1. Smaller stable pigment particles provide more light-reflecting surfaces and hence contribute more to gloss than do large, poorly dispersed, aggregated pigment materials with an associated reduced surface area.
Aqueous Carbon Black Pigment Dispersions for Inkjet InksAqueous dispersions of carbon black (low structure, 254 m2/g) were milled using a 50 ml capacity horizontal bead mill (Eigermill M50) charged with 35 ml of zirconium silicate beads for 40 minutes at 3000 rpm. A pre-mix was first produced on a high shear blade stirrer to produce a homogeneous mixture before transferring to the mill.
The testing of each dispersant/pigment combination consisted of making a series of dispersions with fixed pigment loading (10 and 15 wt. %) and varying the level of dispersant. All dispersant levels are expressed as percent solids on pigment. Viscosities were used initially to find the optimum dispersant level - a minimum viscosity corresponding to the optimum level.
Sedimentometry testing was then carried out to confirm the optimum level and characterize the effect of the dispersant on stability. Material contained in a sample tube was placed vertically in the apparatus and a near-infrared light beam was scanned along the tube taking transmission and backscattering readings at 40-micron intervals. Any particle size variations will increase or decrease the amount of backscatter along the whole length of the sample tube. Repeat scans of a sample over time will show particle migration and any particle size variation (coagulation, re-flocculation). Particle migration/settling appears as a stepwise pattern due to an increase or decrease in backscatter where clarification has occurred. The sample meniscus can be seen as a drop in backscatter. In general, it was found that at below optimum levels some pigment settlement occurred. At the optimum level, there was less (or no) pigment settlement or particle size changes. Above optimum levels there was no improvement in the amount of settlement and the excess dispersant resulted in bridging flocculation (viscosity and particle size increases).
The ability to control the structure and distributions of the acrylic/polyether comb-branched copolymer affects the sensitivity of the dispersion rheology to dispersant loading. Table 2 summarizes the optimum dispersant loadings for four different comb-branched copolymers and the viscosities of 10 wt. % pigment dispersions at these loadings. Differences in the viscosity vs. % dispersant profiles were also observed for the different copolymer dispersants. All curves showed sharp reduction in viscosity with increasing dispersant approaching the optimum loading. With increasing dispersant dosage above the optimum, different rates of viscosity increase indicated different strengths of particle interaction. The shape of the viscosity/demand curves for Dispersants B and C suggest significant particle interaction (bridging flocculation) at the higher levels. This is consistent with the fact that those dispersants have higher molecular weight than Dispersants A and D. It is interesting that Dispersants B and C are more efficient stabilizers.
Aqueous Carbon Black Pigmented Inkjet InksThe formulation of inkjet inks from the dispersions consisted of letting down with water and adding various co-solvents, humectants and ink additives such as surfactants and defoamers. These materials can strongly influence the stability of the pigments through chemical and/or physical effects. Compatibility with a range of co-solvents/humectants and additives was first assessed by addition of each material individually and characterization of the physical properties. Based on the compatibility testing a range of ink-jet inks were formulated and their stability and physical properties were characterized.
Three different inkjet inks were prepared from an aqueous carbon black dispersion based on the acrylic/polyether comb-branched dispersant (see Table 3). The inks demonstrated suitable physical properties (viscosity, surface tension, filterability) for printing and have the same if not better stability (based on sedimentometry) as the dispersions.
Printer Trials of Aqueous Carbon Black Pigmented Inkjet InksInks were loaded and printed through an inkjet print head. The Ink A formulation (Table 3) was slightly modified to produce an ink based on the acrylic/polyether comb-branched copolymer dispersant that performed well in the thermal inkjet printer (HP 720C) used for testing. The adjusted formulation is given in Table 4.
Testing conducted on this ink included cartridge loading, ease of priming and printing of multiple pages of solid color to assess the continuous ability of the ink to fire. This testing would highlight any common failures such as faceplate wetting, starvation or kogation. Decap/dwell/recoverability studies were also carried out. Image quality analysis equipment and a droplet imaging system were used to help study the inkjet printing properties and performance. Wetfastness and long-term stability of the ink were also assessed.
After loading the cartridge and inserting it into the printer, the ink worked immediately on all nozzles. The ease of loading indicated good wetting of the ink path inside the print head.
Ten-page solid blocks were printed, which equates to about 14 ml of ink (a third of a cartridge). No nozzles were lost during the printing of 10 pages, and there was no reduction in optical density from the first to tenth page. This result indicates that there is no starvation or kogation, and inspection of the faceplate also showed there to be no faceplate wetting. Analysis of image quality was also used to assess whether there was any deterioration in image quality during the lifetime of a cartridge. This was done by filling a cartridge with the ink and comparing initial images with images printed after the cartridge was almost empty. Between the initial and final images the ink was printed continuously (25 pages of solid color). There was no deterioration in print quality, which is further evidence that there is no faceplate wetting, starvation or kogation. Kogation is a failure phenomenon associated with thermal inkjet printheads. The firing process (bubble formation and droplet ejection) causes instability (thermal degradation) of the materials in the ink. The degraded materials deposit onto the resistor in the printhead, and subsequent firings result in smaller droplets. Finally, the print fades to nothing.
A high magnification lens and camera system was used to observe the print quality of a printed image. Figures 5a and 5b show images of dots and lines when printed onto three different types of media. Images obtained using the OEM ink (for the HP 720C) are shown for comparison.
Waterfastness was assessed by placing a printed sample in water for a set period of time (1, 5 and 30 minutes) and measuring any decrease in optical density. No change in optical density was determined. The longer-term stability of the ink and the benchmark inks was studied by aging the inks at elevated temperature. The inks were filtered to 1mm, placed in an oven at 45 _C, and their physical properties and stability assessed weekly. Filtration, viscosity and surface tension were measured and their stability assessed by sedimentometry. Table 5 shows the properties before and after aging. At the weekly intervals, there was no visible settling, the ink filtered easily through 1mm with no residue present, there was no change in viscosity or surface tension, and sedimentometry showed the ink to be stable.
SummaryMany forces in coating and ink technology have driven the need for high-performing pigment dispersants. The trend towards high-solids formulations and low-VOC, waterborne coatings requires lower-molecular-weight binder resins with lower functionality. Thus, high-molecular-weight, high-functionality binder resins that were effective as grinding resins are less available. In inkjet ink printing, there has been a strong trend away from soluble dyes towards pigments based on the need to produce more lightfast images with improved durability and water resistance. In this paper, we have introduced a new dispersant technology based on versatile comb-branched copolymer structure and have demonstrated its efficacy in different pigment systems and applications. The ability to modulate copolymer composition and molecular distribution allows for optimum stabilization and performance in pigmented systems.
For more information, visit www.lyondell.com, e-mail steve.schwartz@ lyondell.com, or call 888/777.0232.
1 Parfitt, G.D. Dispersion of Powders in Liquids, 3rd Edition, Applied Science, England, 1981.
2 Verwey, E.J.W.; and Overbeek, J.Th.G. Theory of the Solubility of Lyophobic Colloids, Elsevier Pub. Co., Amsterdam, 1948.
3 Napper, D.H. Polymeric Stabilization of Colloidal Dispersions, Academic Press, 1983.
The authors gratefully acknowledge Netzsch USA and Xennia Technology for their contributions to this paper.