DESIGN OF POLYMERIC DISPERSANTS FOR LOW AND NO VOC APPLICATIONS Jeff Norris, Tom Annable, Matt Dunn, Antonio Lopez Lubrizol Advanced Materials, Inc. USA PIGMENT DISPERSION AND STABILIZATION Polymeric dispersants (hyperdispersants) have been utilized by manufacturers of paints and coatings for a variety of reasons, all associated with improvements in the state of pigment dispersion. The benefits include: Productivity High pigment loading without increased viscosity Reduction in milling time The manufacture of millbases with broad compatibility Quality Improved viscosity control Superior color development Enhanced flocculation resistance o Stability of viscosity and particle size Polymeric dispersants can be described as having two key components in their structure, anchoring groups that adsorb on to the pigment surface and polymeric chains that provide a steric stabilization barrier around the pigment particle. Dispersants can be designed with single anchor - single chain or multi anchor multi chain structures, and a variety of possible anchor chemistries. Polymeric chains should have good solubility in the continuous phase and maintain steric stability as the ink or coating cures. The key requirement of the anchor chemistry is that a strong attachment to the pigment or particle surface is achieved and that there is a sufficient density of coverage to minimize particle to particle interaction. 1 Anchor groups have been developed for a wide variety of pigments and particles. Polymeric dispersants are not only utilized in the manufacture of pigment dispersions, but also for treatment of pigments during manufacturing to optimize dispersion in end use applications. The design and adoption of high performance low and zero VOC coating additives is a requirement for formulators to meet their sustainability goals. Pigment or particle dispersion and stabilization are critical to the design of successful coatings formulations. An ideal scenario is one in which the pigment is dispersed to the primary particle size with complete irreversible dispersant coverage and no dispersant free in solution. In aqueous and universal colorant formulations, the dispersion must be stable in a range of environments where variables such coalescents, cosolvents, ph, electrolytes, and resin chemistry must be considered. This presents particular challenges in the design and selection of dispersing systems, especially considering the range chromatic and extender pigments currently in use. By comparing the stabilization of pigment dispersions in aqueous coatings with traditional solvent borne coatings formulations, a better understanding of design challenges can be realized. Utilizing comb copolymer architecture for design of dispersants for waterborne and universal colorant allows optimal use of anchor group and stabilizing chain combinations. This can also be extended to the design of dispersants for high solids applications, which are increasingly important in the design of sustainable systems.
DISPERSION IN SOLVENT BASED AND WATER BASED APPLICATIONS Compared to traditional solvent borne systems, pigment dispersion in water borne paints and coatings using polymeric dispersants poses particular stability challenges. Table 1 lists some of the important differences. Criteria Solvent Water Anchoring Pigment Affinic Groups Pigment Affinic Groups Hydrophobic Segments Inter-particle Repulsion Soluble Steric Chains Soluble Steric Chains Electrostatic Stabilization Electro-steric Stabilization Letdown Environment Similar Soluble Polymers Colloidal phases (Other pigments & binders) Different Soluble polymers Colloidal phases Water-miscible co-solvents ph TABLE 1. COMPARISON OF DISPERSION IN SOLVENT AND WATER In dispersant design for both solvent and water based applications, the anchor group functionality determines performance with specific pigments. However, in water, the hydrophobic polymer segments used for anchoring can also self-associate in solution so anchoring on the pigment is a competitive process, subject to both kinetic and thermodynamic conditions. In solvent, anchor group attachment is much more predictable. Further complicating the situation in water, both steric and charge stabilization mechanisms are available and often utilized in combination. For maximum performance, the dispersed pigment must remain stable as the polarity of the continuous phase (binder and solvent) changes as the film cures and this is best achieved, in solvent or water, via steric stabilization. This is an especially important consideration when the stabilized pigment dispersion will be used in a latex emulsion or colloidal resin based coating. The aqueous letdown environment is more complicated, with a variety of polymeric materials and pigments stabilized themselves in a variety of ways in addition to the presence of co-solvents and other additives. Compare this to the solution resin based environment of the typical solvent based coating, where solubility and resin compatibility during cure is are primary factors. Compatibility of the pigment dispersion in the water based coating must take many more factors into consideration, which requires careful design and selection of anchor groups and stabilizing chains for robust performance. DISPERSANT DESIGN Designing the optimal dispersant for a given combination of pigment or particle and the dispersion vehicle (solvents and resins) is a search for the best combination of specific anchor and stabilizing chain chemistries. Anchor group chemistry and functionality must correspond to the surface chemistry of the pigment, particularly the functionality present as the dispersion process creates de-agglomerated particles and fresh surface area is exposed to the dispersion vehicle. Typical anchor group functionalities include anionic (carboxylic, phosphate, sulfonate), cationic (amine or quaternary), and nonionic (aromatic, hydrophobic) groups. These can be
present as single or multiple anchor configurations depending on the overall polymer architecture. Ionic anchoring is predominantly utilized for stabilization of inorganic pigments and mineral fillers, while hydrogen bonding mechanisms are most effective with particles with lower levels of ionic functionality such as organic pigments and carbon black. Multi-functional anchor configurations are also commonly encountered as more dispersants are developed for broad applicability. For many polymeric dispersants (Mw > ~1000), starting point dosages are generally determined from the published or estimated pigment surface area which provides the percentage of active dispersant on weight of pigment rather than on total formula weight. Since the dispersant anchor group always has some degree of functionality, avoidance of an overdose is important for target coating durability and performance. To properly stabilize the anchored pigment, the polymer chain or chains attached to the anchor must be appropriately compatible in the continuous phase, i.e. fully solvated and extended to provide maximum steric stability at the minimum effective dose. Figure 1 demonstrates the importance of chain length / molecular weight (and distribution) in developing an effective steric barrier to flocculation. FIGURE 1. EFFECT OF THE STABLIZING CHAIN LENGTH The potential for the stabilizing chain to pull the anchor off the pigment surface and cause flocculation depends not only on the chain length but also on the strength of the anchor to pigment interaction. Therefore, much investigation and development effort has gone into designing polymeric dispersants based on multi-anchor configurations that offer stronger anchoring than simpler, single anchor systems. In comparison to linear block copolymers (including AB, ABA, and BAB structures), the use of comb copolymers offers a variety of methods to create high performance multi anchor / multi chain dispersants. Figure 2 provides a visual description of the benefits of comb copolymer architecture in the design of polymeric dispersants with improved viscosity and particle size stability.
FIGURE 2. LINEAR BLOCK COPOLYMER AND COMB COPOLYMER DISPERSANTS The higher molecular weight and often multifunctional pigment anchor of the comb copolymer dispersant interacts more strongly with the particle surface compared to the single anchor structure. A stronger anchor also allows more flexibility in modifying the composition of the stabilizing chains, providing the potential for broader system compatibility. In this way, improved dispersants for low VOC universal colorants, both solvent based industrial and for architectural / decorative colorant applications have been developed. Thus, comb copolymer dispersant technology now is used in solvent, water borne and 100% solids applications. OPTIMIZING DISPERSANT ANCHOR AND STABILIZING CHAIN COMBINATIONS Finding the optimal combination of pigment anchor stabilizing chain composition and molecular weight depends on development of effective applications screening tests. In the development of an improved dispersant for low VOC dispersions of inorganic pigments, a series of high solids water based dispersions of alumina treated titanium dioxide (with an overall basic surface) were prepared using acid functional dispersants with an increasing number of hydrophilic steric stabilizing chains and measuring the resulting dispersion properties. The goal was to compare steric and electrostatic stabilization in the formulation of > 60% solids dispersions with improved long term particle size and viscosity stability compared to commercially available pigment slurries.
As shown in Figure 3 the viscosity of the resulting dispersions decreased as the number of stabilizing chains was increased. In this case, charge stabilization alone is not the most efficient stabilizing mechanism for achieving highest pigment loadings at lowest viscosity. In fact, with this dispersant structure, steric stabilization was the dominant mechanism for achieving not only viscosity minimums but Newtonian rheology as well. The lowest viscosity dispersions also had the desired long term viscosity and particle size stability, providing equal or better gloss and opacity in formulated coatings compared to commercial benchmarks, as is expected of dispersions with effective steric stabilization. FIGURE 3. INFLUENCE OF STERIC CHAINS ON DISPERSION & RHEOLOGY The acid functional dispersant that was used in the titanium dioxide study is not an effective choice for acid functional or low functionality pigments, for example organic colorants and oxidized carbon blacks. In another dispersant study, quinacridone red and magenta, (C.I. Pigment Violet 19 and Red 122), were used to screen combinations of a proven multifunctional anchor with different steric stabilizing chain compositions and dispersant polarities In this case, the dispersions were evaluated for stability with increasing ph and separately with increasing concentrations of 2-butoxyethanol. These challenges evaluated both anchor strength and optimal chain composition. Available dispersants could make very good zero VOC water based dispersion with good ph tolerance but relatively poor performance in colorants and coatings containing moderate levels of glycol ethers, which are increasingly encountered in the conversion to water reducible coatings from higher VOC solvent borne systems. Modifications that had an overall lower HLB (fewer hydrophilic chains) performed better in the co-solvent containing coatings but had a negative impact on ph stability. In the end, as shown in Figure 4, a new comb copolymer dispersant was developed via modification to the anchor group which
strengthened attachment to the pigment, and improved co-solvent compatibility while maintaining ph stability. FIGURE 4. OPTIMIZATION OF ANCHOR AND STABILIZING CHAINS VOC COMPLIANT APPLICATIONS BEYOND WATER BASED SYSTEMS As mentioned previously, comb copolymer dispersants find use across the entire spectrum of colorant applications solvent, water, and 100% solids. In response to ever restrictive Volatile Organic Content (VOC) regulations, comb copolymer dispersant technology provides enabling technology to industrial and architectural paint and colorant formulators. To address reduction of overall coating solvent content, polymeric dispersants and comb copolymer dispersants in particular provide the ability to maximize pigment loadings in production millbases or in-plant colorants at minimal viscosity, thus reducing solvent normally needed for viscosity adjustment. This is particularly important in higher resin solids systems and even 100% solids applications, where the use of polymeric dispersants is growing. In fact, 100% solids, liquid dispersants developed using comb polymer technology are particularly effective in these applications. The ability to develop improved performance 100% solids, liquid polymeric dispersants with improved viscosity and color development performance was the result of the modification of stabilizing chain copolymer composition which not only changed the physical form (significantly reduced melt points) but also broadened the solubility and compatibility range. This expanded compatibility includes the variety of exempt organic solvents that are increasingly being used to address regional VOC regulations.
The ability to modify copolymer composition of the stabilizing chains and to also introduce mixed polarity chains has also been important to the development of new dispersants for solvent based and architectural / deco universal colorants. Figure 5 demonstrates a few of the modifications of comb copolymer dispersant technology for use in 100% solids and universal colorants. FIGURE 5. STABILIZING CHAIN OPTIMIZATION FOR 100% SOLIDS AND UNIVERSALS CONCLUSION Design and selection of polymeric dispersants for use in solvent and water based applications requires special considerations. Formulation of colorants and pigment dispersions for water based paints and coatings require dispersants that can tolerate the changing polarity and ph that occurs during film formation. Differences in the compositions of the continuous phases of solvent (simpler) and water (complex) creates challenges in the design of dispersants for water. The optimization of dispersant anchor chemistry is pigment dependent. Steric stabilizing chain composition can be varied in a number of ways to create high performance dispersants and pigment dispersions. Polymeric comb copolymer dispersants combine strong pigment anchoring with versatile steric stabilization which leads to high pigment loading at low viscosity and broad compatibility. Developments in comb copolymer dispersants cover the breadth of paint and coatings applications solvent, water and 100% solids and find increasing use in the formulation of low and no VOC applications. Examples of the polymeric dispersant technologies described here are available from Lubrizol Advanced Materials.
REFERENCES 1. J. D. Schofield, Handbook of Coatings Additives Vol. 2, (Marcel Dekker; NY, 1992), Chapter 3. The information contained herein is believed to be reliable, but no representations, guarantees or warranties of any kind are made as to its accuracy, suitability for particular applications or the results to be obtained. The information is based on laboratory work with small scale equipment and does not necessarily indicate end product performance. Because of the variations in methods, conditions and equipment used commercially in processing these materials, no warranties or guarantees are made as to the suitability of the products for the applications disclosed. Full-scale testing and end product performance are the responsibility of the user. Lubrizol Advanced Materials, Inc. shall not be liable for and the customer assumes all risk and liability of any use or handling of any material beyond Lubrizol Advanced Materials, Inc. s direct control. The SELLER MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Nothing contained herein is to be considered as permission, recommendation, nor as an inducement to practice any patented invention without permission of the patent owner.