Surfactants for Use as Codispersants in Architectural Coatings

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1 Surfactants for Use as Codispersants in Architectural Coatings Bruce Fillipo, Ashland Specialty Ingredients, United States Dick Henderson, Ashland Specialty Ingredients, United States Xiaochun Zhang, Ashland Specialty Ingredients, United States Courtney Usher, Ashland Specialty Ingredients, United States Kim Gaughan, Ashland Specialty Ingredients, United States Andrew Defusco, Ashland Specialty Ingredients, United States Sowmitri Tarimala, Ashland Specialty Ingredients, United States Abstract The preparation of pigment grinds for Architectural Coatings utilizes high shear dispersion techniques in combination with low molecular weight polymeric dispersants. The selection of a grind dispersant is critical to achieving optimum particle size and dispersion properties including viscosity, stability and handling as well as the final coating hiding and color characteristics. Surfactants can be used as active co-dispersants alone or in combination with low molecular weight dispersants to improve grinding efficiency and properties. Advantages of using low-voc* (Volatile Organic Compounds), non-ape (Alkylphenol Ethoxylate) surfactants in conjunction in pigment dispersants is investigated to identify surfactant functionality and dispersant combinations for improving efficiency and coating properties. Analytical techniques including rheology, microscopy and surface chemistry are investigated to evaluate and document performance and mechanisms. Introduction Titanium dioxide (TiO2) is the most important white pigment and is widely used in industries including paint, colorant, papermaking, plastics, cosmetics and pharmaceuticals. Annual worldwide TiO2 pigment production capacity is 6.55 million tons with approximately 57% consumed in paints and coatings. 1 Titanium dioxide is used in almost every paint or coating formulation for brightness, UV adsorption properties, resistance to discoloration and high refractive index, providing opacity / hiding in paints. Levels used in coating formulations vary significantly dependent on the type and total pigment loading (PVC or pigment volume concentration), but typically range from 0.05 to 0.3 kg/liter (0.5 to 2.5 lb/gal). The purpose of this study is to evaluate surfactants as active co-dispersants alone or in combination with low molecular weight dispersants to improve pigment dispersion properties. Background Titanium dioxide and pigment dispersions are produced for the manufacture of paint. Pigment concentration, level of dispersion as well as the quality and stability of the dispersion are critical for determining final coating properties. Pigments are typically dispersed by shearing (e.g. high shear mixing or milling) in a grind Figure 1: Paint manufacturing overview * low-voc: < 0.5%: Enhanced ASTM Method 6886 (GC Method)

2 tank. To facilitate dispersion, surfactants and dispersants are used to wet-out solid pigments and displace air, moisture and separate tightly packed agglomerated particles. Breaking-up the solid to primary particles is required to obtain optimum optical properties including opacity. Stability of the pigment dispersion through use of dispersants and surfactants is essential to prevent flocculation and therefore changes in opacity, color, tint strength, gloss, viscosity and separation / sedimentation. Dispersed in a liquid, pigment particles are not stationary but undergo constant thermal movement or Brownian motion. Velocity or kinetic energy of a particle in motion can be expressed by: M 2 = k T where M = mass, = velocity, T = Temperature and k = constant. As the mass of a particle increases, speed decreases at a specific temperature. As the particles move, they undergo constant random collisions with other particles. If pigment particles are not stabilized and there is no barrier to prevent contact, each particle to particle collision can result in bonding and rapid flocculation will occur: T 1/2 = 3 4 k T N where T 1/2 = time 50% particle count reduction, = viscosity and N= particle number (cm 3 ). Therefore, viscosity ( is a key parameter to increase or reduce flocculation. However, due to limitations on maximum viscosity in liquid coatings systems, it is not viable to sufficiently stabilize. Primarily three forces are involved as particles approach each other, electromagnetic (e.g. attractive London-Van der Waals), electrostatic (e.g. columbic, normally repulsive) and steric hindrance forces (adsorbed layers). Electromagnetic forces are attractive (V a ) and develop from particle interactions and must be overcome to prevent pigment flocculation. London-Van der Waals forces arise due to the influence of particle dipoles. Attractive forces increase as particles approach each other. As noted, electrostatic repulsion and steric hindrance are primary mechanisms stabilizing pigment dispersions and blocking flocculation. Electrostatic forces result from development of ions at the particle surface due to dissolution, an ionogenic source or addition of ionic dispersant or surfactant. Pigments used in coating systems normally have an anionic (-) charge. Anionic pigment particles, attract a second layer of less densely populated solution counter ions (cations, +). This system is referred to as an electrical double layer, inhibiting flocculation and maintaining the dispersion stability as the Figure 2: Electrostatic Particle Repulsion charged particles repel one another. The charge for the electrostatic double layer is highest at the particle surface and drops off rapidly in the surrounding solvent. V r / (k T) = C d ζ 2 ln(1+ exp - s / Where V r = Repulsion potential, C= constant, dielectric constant, d= particle diameter, Zeta Potential, s = distance between particle surfaces, Double Layer thickness As can be seen, particle zeta potential increases, the resistance potential increases exponentially. Large values for double layer) and (solvent dielectric constant; Air= 1, Mineral Oil ~ 2, Water =80) are also desirable for electrostatic repulsion (V r ) and dispersion stability.

3 Graphing the combined sum of electromagnetic attractive and electrostatic dispersion stabilizing (repulsive) forces (V total ) between particles as they approach each other (Figure 3) shows increasing resistance between particles to a point. Past this maximum, the repulsion decreases rapidly and attraction becomes the predominant force resulting in bonding and flocculation. Figure 3: Electromagnetic and Additionally, inorganic oxide surfaces have a surface charge electrostatic forces between particles dependent on the ph of the system. Each pigment has an isoelectric point (ph) where surface charge is zero. As the ph is adjusted away from the isoelectric ph, the charge becomes imbalanced, increasing charge resulting in increased particle repulsion. The adsorption of an ionic surfactant or dispersant to inorganic oxide alters the particle surface and its isoelectric ph. Selection of the optimum surfactant should increase and enhance charge and improve dispersion stability. Steric hindrance is the second primary mechanism for stabilizing pigment dispersions. Nonionic and ionic surfactants and dispersants adsorbing onto the pigment particle surface can provide steric hindrance and mechanically create a barrier hindering close approach of pigment particles. Additionally, when functional groups extending into the solvent come into contact with one another, osmotic pressure increases also forcing particles apart. Figure 4: Steric Hindrance Titanium dioxide particles are often coated with silica or alumina, to improve hydration and dispersion properties and prevent potential formation of radicals due to a photocatalytic reaction. Figure 5: TiO2 Surface Treated Surface active agents and polymer dispersants reduce settling in pigment dispersants due to electrostatic and steric stabilization. As a result, flocculation is reduced. Stokes Law describes the rate spherical particles fall through liquids, assuming laminar flow: 3 s p s G * d 2 )/ s = settling velocity, = density of particle and solvent, G= acceleration due to gravity, d = particle diameter, = viscosity As can be seen, if flocculation occurs and effective particle size increases, settling rates can increase exponentially. Surfactants and dispersing agents are used primarily to reduce surface tension and wet surfaces to improve the speed at which the liquid replaces the air on the pigment surface and to envelope particles with a barrier to disperse and stabilize by ionic and steric mechanisms. The generation of this envelope prevents random particle contact, adhesion and flocculation. Surfactants constitute a very important group of molecules, and are present in all types of formulations. The structure consists of a water loving hydrophilic section which can be nonionic

4 (ethoxylates), anionic (phosphate esters and sulfonates), cationic (quaternary amines) or amphoterics (betaines) with an oil loving or hydrophobic group (e.g. alcohols and fatty acids) in the same molecule. The type and balance between the two groups is decisive for properties including wetting and dispersing. Surfactants by nature are surface active, moving to interfaces where the hydrophobic section orients itself away from the water and hydrophilic section protrudes into the water phase thus disrupting hydrogen bonding at the water interface, reducing surface tension. Experimental Titanium dioxide dispersions were prepared with the formulation summarized below using a commercial surface treated (2.5 %Alumina + 3%Silica) rutile TiO2 (93% minimum) with a mean particle size of µ. The initial dispersion was prepared with a high shear Dispermat equipped with a cowles bladethe consistency of the initial dispersion (no dispersant or surfactant) was a soft paste. Water DI 33.1% Titanium Dioxide 66.6% Foam Control Additive 0.3% Titanium dioxide dispersant demand curves were generated using 350g aliquots of the prepared TiO2 paste. 15% active solutions of dispersant and/or surfactant (wt/wt) were fed to the paste over mins using a syringe pump. The dispersion was mixed at 500rpm using a HAAKE Viscotester 550 with a 2 inch (5cm) marine prop blade as the surfactant / dispersant solutions were fed. Motor torque was measured by the HAAKE Viscotester 550, a Searle type viscometer. For this testing we fitted the HAAKE with an agitator blade in-place of a spindle to measure relative motor torque. Rotational speed was set to 500rpm. The measured dispersion flow resistance (torque) to maintain the constant mixing speed is proportional to the viscosity. Solution feed and torque measurement was initiated as soon as readings stabilized. ph of the final dispersions remained between Additives evaluated included a low molecular weight polyacrylate (MW ~5000) and a diisobutylene maleic anhydride copolymer with a crosssection of nonionic surfactants including tridecyl alcohol ethoxylates and anionic phosphate and sulfate surfactants. Materials included in this paper are summarized in Table 1. Figure 6: HAAKE 550 Characterization of dispersion and films included: Table 1: Surfactants and Polymers evaluated 1. Rheology: Stress Strain G was measured using ARG2 rheometer, TA Instruments 2. Surface Tension: TiO 2 dispersions were diluted by 50% (~33% solids) and the surface tension measured using pendent drop method. 3. Surfactant Adsorption: Spectroscopic Ellipsometry (Horiba Smart SE) was used to measure the thickness of the adsorbed thin surfactant films. Flat Al2O3 and SiO2 (glass) crystal substrates were used as models for the Al2O3 and SiO2 coated TiO2 surface. These substrates were immersed in 3% surfactant solutions for 48hrs and rinsed with DI water and dried with N2

5 4. Cryogenic Scanning Electron Microscopy: A sample was flash frozen in liquid ethane. The frozen sample under vacuum at -180 C was fractured and coated with Au-Pd. SEM was held at -190 C and sample surface examined in SEI (Secondary Electron Imaging) mode. 5. Atomic Force Microscopy (AFM): Images acquired in AC (tapping) mode using an Asylum Research MFP-3D SA atomic force microscope. Rectangular silicon cantilevers were used (Budget Sensors Tap300Al-G, typical spring constant 40 N/m). Images were obtained at 0.5Hz scan rate, 512x512 (line) resolution. Results and Discussion Polymer and Surfactant Demand Curves Dispersion Demand Curves were generated for Titanium dioxide dispersions using the HAAKE Viscotester 550 as specified in the Experimental section. Two polymers were included, a low molecular weight polyacrylate and a diisobutylene maleic anhydride copolymer, with a crosssection of nonionic surfactants including tridecyl alcohol ethoxylates and anionic phosphate surfactants. Additives were fed 1.4 g (0.4%) to the paste over 45 mins. A sampling of results of additives on torque / viscosity is summarized in Figure 7. Mixer Torque is shown on the Y axis versus the grams of active dispersant added. Feeding was initiated with a stable reading of ~ Torque platue for the polymers occurred at ~3000. Torque read-out for water using the same marine prop blades is 870. Results showed both of the low molecular weight dispersants evaluated were the more efficient than the evaluated surfactants in reducing motor torque and therefore viscosity. Torque plateau was reaching at < 0.1% actives. None of the nonionic surfactants (low to high HLB) had a significant impact on dispersion viscosity, all of the torque results were in a narrow range. However, the anionic surfactants evaluated had significant impact on rheology. Phosphate esters F5, T1 and O1 were effective in reducing dispersion viscosity. F5 achieved similar torque to the polymeric Figure 7: TiO2 Dispersant Demand Curves 350g 2:1 in Water dispersants at ~0.2% and T1 and O1 at ~0.4%. All three of these phosphate esters are ethoxylated and hydrophilic. Hydrophobic Phosphate Esters K, Coester and Z impact on rheology was highly dependent on concentration. At a low concentration K had a significant reduction on motor torque but after addition of < 500ppm torque increased steadily contrasting with Phosphate Ester Z which exhibited an initial increase in torque but after addition of ~ 0.1% exhibited a rapid decrease. As can be seen the impact on viscosity / torque of the phosphate ester surfactants were not consistent varied depending on functionality. Generally, the evaluated hydrophilic phosphate ester surfactants predictably reduced dispersion viscosity. The more hydrophobic anionic surfactants required generation of a demand curve to understand impact on system viscosity.

6 Figure 8: TiO2 Co-Dispersant Demand Curves350g, 2:1 in Water Based on results for individual LMW 450ppm (active) + Surfactants additives, demand curves were generated incorporating 450ppm low molecular weight polyacrylate. The concentration of 450ppm is short of the viscosity plateau measured for individual additives and chosen to assess impact of surfactant additives in a polymer starved system. The same cross-section of surfactants was evaluated; however, additives 0.7g (0.2%) were fed to the paste over 30 mins. An overview of demand curves / torque results with different additives is summarized in Figure 8. Results are consistent with the earlier addition of individual constituents. The low molecular weight dispersants evaluated were more efficient than the evaluated surfactants in reducing torque. Again, none of the nonionic surfactants (low to high HLB) had a significant impact on dispersion viscosity where all torque results were in a narrow range and close to the starting torque (~6000). The impact on system rheology from the anionic surfactants were similar and variable to the evaluation of individual additives where the more hydrophilic phosphate esters were effective in reducing dispersion viscosity and the hydrophobic phosphate ester impact was dependent on functionality and concentration. Increasing the level of low molecular weight polymer (in this case, diisobutylene maleic anhydride copolymer) above the plateau concentration, to 1200ppm, was evaluated. As expected, the starting torque readings were at the minimum ~3000 before starting the addition of surfactant and most of the surfactants had no significant impact on motor torque (Figure 9). The surfactant additives were fed up to 0.7g (0.2%) to the TiO2 dispersion over 30 mins. All of the hydrophilic phosphate esters maintained a flat line torque reading as well as the hydrophilic nonionic (HLB=16). Again, the more hydrophobic anionic and nonionic surfactants impact on rheology was dependent on functionality and surfactant concentration elevating torque readings up to Viscosity Stress / Strain Curves All of the TiO2 samples are highly non- Newtonian and shear thinning. Dispersion Torque data was validated by testing samples for dynamic mechanical analysis. Stress / Strain (G ) Storage Modulus (e.g. Elastic Figure 9: TiO2 Co-Dispersant Demand Curves 350g, 2:1 in Water 1200 ppm active Polymer Dispersant + Surfactants Figure 10: Stress Strain G Storage Modulus Elastic response / solid-like behavior

7 response / solid-like behavior) is reported for several evaluated samples. The more hydrophilic anionic phosphate esters F5, T1 an O1 had lower G values with F5 exhibiting lowest viscosity and yield point. The nonionic surfactant profiles were similar to the control containing no surfactant. Dispersion Stability As expected, due to differences in surfactant charge, molecular weight and viscosity, significant variation was observed in TiO2 dispersion stability. Samples were stored for approximately 3 weeks at ambient and examined for separation, consistency (fluid or gelled), hard packed deposit and ease of redispersibility. There was a correlation between the Viscosity Stress / Strain Curves and sample separation. Samples with low G (e.g. < 1000 Pa) and low yield point exhibited water separation (2% - 40%) after three weeks of ambient storage. Samples prepared only with dispersant showed formation of a water layer >30% by volume and hard-packed sludge whereas surfactant stabilitzed dispersions exhibited varying degree of separation and redispersibility, Hydrophilic anionics also showed separation but generally easily reconstituted. Table 2: Dispersion Stability Combinations of polymeric dispersants and surfactants exhibited the best overall storage characteristics. As can be seen, the hydrophobic anionics in combination with the polymeric dispersants showed the lowest level of separation and best recoverability. Surface Tension The surface tension of the titantium dioxide dispersions and surfactant solutions (0.1 and 0.4% in water) was measured to assess relative level of free surfactant. Free surfactant reduces surface tension aiding in wetting, facilitating Table 3a: Dispersion surface tension (33% solids) incorporation of pigments and extenders. Generally, the surface tension of the surfactant solutions and TiO2 dispersion + surfactant showed similar trends with only small differences. This trend is true for the nonionic surfactants (low to high HLB) and the hydrophilic phosphate esters. However, a notable exception is the more hydrophobic Phosphate Coester based dispersion where dispersion surface tension was approximately 10 dynes higher. Little change between solution and dispersion surface tensions indicates systems are above the critical micelle concentration and most of the more hydrophilic surfactants are more mobile in the dispersion. The Phosphate Coester may be more tightly bound to the TiO2 surface due to its

8 Table 3b: Surfactant surface tension (0.4% and 0.1% in water) hydrophobic functionality. As concentration increases, the more hydrophobic characteristic can depress the hydration layer result in increased dispersion system torque and viscosities. Adsorbed Layer Persistency and thickness of surfactant films was evaluated by ellipsometry. As Table 4: Surfactant Films by Ellipsometry noted, the TiO2 is surface treated, therefore flat SiO2 and Al2O3 crystals were used as model surfaces of the surface treated titanium dioxide and evaluate adsorption. Crystals were exposed to 3% surfactant solutions for 48hrs and rinsed with DI water; dried. Generally, adsorbed layers were more significant on the aluminum oxide versus silica, where all of the surfactants evaluated developed a measurable film on the aluminum oxide but the higher HLB nonionics and Phosphate Ester Z showed no film persistency on silica. The Low HLB Nonionic and Phosphate ester O1 developed the thickest films on aluminum oxide and silica crystals respectively. Cryogenic Scanning Electron Microscopy In most cases, SEM particle size measured nm but some variation noted between different surfactant systems. Typical examples are shown below at 20000x magnification. The most unusual was regions within the diisobutylene maleic anhydride copolymer. Phosphate Ester F5 contains abundant, fine, spherical to rounded particles ranging from approximately nanometers in diameter. Some agglomerates are visible in other anionic and noionic surfactants indicating some flocculation. Figure 11: Ti02 dispersion films cryogenic SEM Atomic Force Microscopy (AFM) Draw down of Titanium dioxide dispersion were dried at +25C onto a glass slide in air before imaging. Variation in structure of dried films was observed with the different surfactant and polymer systems. Examples are illustrated in Figure 3. The most ordered film structures were observed with the Phosphate Ester 5, the Low HLB nonionic and the copolymer. There appears to be some flocculation in the Phosphate Ester O1, Z and Coester.

9 Figure 12: Ti02 dispersion films AFM Low PVC Coatings Screen. TiO2 Dispersions were formulated into a low PVC coating to evaluate the surfactant impact on TiO2 dispersion on coating properties such as opacity and hiding. An example of a draw-down is attached (Figure) showing increased opacity when using the Hydrophilic Phosphate Ester F5 versus the copolymer dispersant. AFM images of the coating show TiO2 agglomeration when using only the dispersing copolymer elucidating the difference in opacity. Conclusions Phosphate Ester Diisobutylene maleic F5 anhydride copolymer Figure 13: 15 PVC VAE coating Ti02. Opacity is 1.5% higher with Phosphate Ester F5 versus the copolymer dispersant. AFM shows more agglomeration with the copolymer dispersant in the dry coating Impact on dispersion rheology with anionic phosphate ester surfactants varied depending on functionality, hydrophobicity and surfactant concentration. Evaluation of surfactant demand curves is critical to understand impact and optimize dispersion properties. Generally, the hydrophilic phosphate ester surfactants predictably reduced dispersion viscosity whereas the

10 more hydrophobic anionic surfactants required analysis of demand curves to understand impact on system viscosity. Nonionics generally did not have a significant impact on dispersion viscosity. Dispersant Polymers were most efficient in reducing viscosity of pigment dispersions but also exhibited significant separation and potential for hard pack deposits while low viscosities were still obtained with some anionic surfactants and lower levels of separation observed. The use of anionic surfactants in particular more hydrophobic PE s with polymeric dispersants gave the best dispersion properties. Incorporation of dispersants and surfactants into the pigment grind can have an impact on final coating opacity and hiding where surfactants can improve results. Several analytical characterization tools were found to be useful in understanding dispersion properties including Atomic Force Microscopy, Surface Tension Analysis, Ellipsometry and Rheology. Acknowledgements Margaret Oplinger: Ashland Specialty Ingredients Coatings Research and Development Sharon Wilson: Ashland Specialty Ingredients Coatings Research and Development Scott Bradley: Specialty Ingredients Analytical Microscopy Michael Mroz: Ashland Specialty Ingredients Coatings Technical Service Eric Dickson-Peppler: Ashland Specialty Ingredients Coatings Research and Development References: 1. U.S. Geological Survey, Mineral Commodity Summaries, January Plaint Flow and Pigment Dispersion, Temple C. Patton, Stokes Law - Trinity College 2001