OPTIMIZATION OF DISPERSANT AND LIQUID LEVELS IN COLOR PIGMENT CONCENTRATES

Transcription

1 OPTIMIZATION OF DISPERSANT AND LIQUID LEVELS IN COLOR PIGMENT CONCENTRATES Paula R. Cook, William L. Dechent, Robert T. Miller, Pravin Patel Troy Corporation, Florham Park, NJ 7932 Abstract The dispersion of organic and inorganic pigments for the manufacture of a color mill base (CMB) or a color pigment concentrate (CPC) requires the use of a dispersant. Much has been written about the use of specific dispersants to make CMBs or CPCs, and suppliers recommend rather wide use ranges (e.g., 2 1% dispersant based on pigment weight for carbon black). Formulators need to optimize the dispersant level to insure dispersion stability, minimum viscosity, and full color development when the CMB or CPC is added to a coating. Sufficient liquid is needed to fill the interstitial volume between dispersed pigment particles. A Liquid Dispersant Flow Point Method (LDFPM) is proposed to determine the optimum levels of dispersant and liquid for a pigment/dispersant combination. The objective is a CPC or CMB with a maximized pigment concentration. Viscosity and colorimetry are used to determine the extent of dispersion. Introduction The Liquid Dispersant Flow Point Method (LDFPM) is introduced. The method allows the optimization of the quantities of dispersant and liquid in a color mill base. This results in the maximum pigment concentration. A flow point graph is constructed using two variables: dispersant level and pigment concentration. A plot of flow points on the graph is used to determine the maximum pigment concentration. Definitions For this paper, the term color mill base refers to a formulation containing pigment, dispersant, and liquid. Synonyms of this include colorant, tinting paste, or grind. The tint base is a commercial coating containing titanium dioxide, formulated to accept a color mill base. Color coatings are often prepared by adding a color mill base to a tint base producing colors from pastels to deep tones. For colorimetry and rubup tests,.7 g of color mill base is added to 18. of a tint base and shaken for 3 minutes using a paint shaker (Harbil high-speed paint mixer model #33). Colorimetry measurements are taken with a Microflash 2d (Data Color International). Rubup tests are performed over the unsealed portion of a Leneta Penopac chart ~3 seconds after drawdown with a 3 mil Bird Film Applicator. The rubup test consists of circular rubs with an index finger over a time span of ~1 seconds. Dispersant level is a relative mass based on pigment (e.g. for a color mill base with 1 g pigment, 2% dispersant indicates.2 g of dispersant) and pigment concentration is based on total formulation mass.

2 Materials The carbon black is Raven 12 beads supplied by Columbian Chemicals. The opaque red iron oxide is R497 Kroma Red supplied by Harcros s. TRAD-1 and TRAD-2 are low molecular weight experimental dispersants for aqueous coatings. Benefit Of Dispersants The use of dispersants at optimized levels maintains separation of pigment particles providing maximum color development and lowest color mill base viscosity. The first benefit, maximum color, refers to more color strength per gram pigment--thus the dispersant improves the quality of the pigment in a color mill base. The second benefit, lowest viscosity, allows the inclusion of more pigment in the color mill base, therefore dispersants also increase the pigment quantity (more pigment per gram color mill base). Level Optimization Previous work 1 reported the use of a paint shaker method to optimize the dispersant level in a color pigment concentrate; no attempt was made to optimize pigment concentration. Hypothesis: If we determine the minimum levels of dispersant and liquid required for a color mill base, we maximize the quantity of pigment. Maximized pigment concentration allows production of larger volumes of tinted paint per batch of color mill base and reduces handling costs. Level optimization is inferred from the procedure known as the Daniel Flow Point Method (DFPM). In our interpretation of DFPM, as reported by Patton 2, both liquid and dispersant levels are optimized for a pigment mill base. LDFPM is designed to obtain the same information for a color mill base. Daniel Flow Point Method (DFPM) DFPM, as described by Patton 2, can be used to determine optimum levels for liquid and dispersant in a pigment mill base. Several titrants are prepared, each titrant containing a different concentration of dispersant in liquid. Titrations are performed by grinding a constant quantity of pigment with a spatula, and adding one of the titrants until the viscosity of the pigment/titrant mixture changes from a paste to a liquid. These titration endpoints where the viscosity changes are called "flow points" and they are plotted on a graph of titrant volume vs. titrant concentration. For DFPM, the plot of flow points (see Fig. 1) is a curve with a minimum. The data point at the minimum provides the optimized titrant concentration and a titrant volume. There are three pieces of mathematical information from DFPM: 1)pigment mass, 2)titrant concentration, and 3)titrant volume. These can be used to determine the mass ratios of pigment, dispersant, and liquid for an optimized pigment mill base. In summary, DFPM can be interpreted as a method to determine the concentration of dispersant in a liquid, such that when sufficient dispersant is added to a pigment, there is also sufficient liquid to fill the volume between dispersed pigment particles.

3 Fig. 1 Textbook Example of Daniel Flow Point Method Graph Liquid Dispersant Flow Point Method (LDFPM) For LDFPM, flow points are plotted on a graph with Dispersant Level as one axis, and Concentration as the other. All color mill bases on a graph have the same pigment mass (e.g. 7. g pigment). For the discussion of LDFPM, the word liquid is used instead of solvent, because the color mill base formulation may use water or a low molecular weight polymer instead of a solvent. LDFPM separates the components of the color mill base into three categories: 1)pigment, 2)dispersant, 3)liquids (see Fig. 2). Therefore, the LDFPM definition for liquid includes all materials (solvent, water, additives, etc.) that fill the volume between dispersed pigment particles. Fig. 2 Formulation with pigment (P), dispersant (D), and liquid (L) Qualitative viscosity determinations are used to determine a flow point, with two designations: low viscosity and high viscosity. If a color mill base has a low viscosity similar to water (it will easily flow from a plastic transfer pipette, and it will drip from the end of a spatula), then the pigment is well dispersed and the color mill base is labeled pass. If the viscosity is high (similar to a paste or a crayon), the color mill base is labeled fail. Colorimetry and rubup are used to confirm that the pigment of the mill base is well dispersed. Paint Shaker A paint shaker is used to prepare color mill bases., dispersant and liquid are combined in a glass jar, along with glass beads. As one example, a 4 oz glass jar is filled with g of color mill base (pigment + dispersant + liquid) and 1 g of 1.2 mm glass beads. The glass jar is shaken in 3 minute intervals for a total shake time of 12 minutes. From previous work 1 with the shaker, there were three

4 conclusions: 1) The shaker produces color development equivalent to the color obtained with a laboratory small media mill; 2) The shaker test results are reproducible; 3)The shaker test is considerably faster than equivalent work with a laboratory small media mill. The LDFPM Graph The vertical axis of the graph is pigment concentration and the horizontal axis of the graph is dispersant level. mass is constant for all color mill bases. The first part of the experiment creates a horizontal segment of points (see Fig. 3a) by holding the pigment concentration constant and increasing the dispersant level until several color mill bases with low viscosity are obtained. Fig. 3a. First Segment of LDFPM Fig. 3b. Adding Second Segment of LDFPM Colorimetry is used to confirm the optimum dispersant level. All low viscosity color mill bases are added to white tint bases and tested for color development using CIELAB values (L*, a*, b*). The color coating with the strongest color corresponds to the color mill base with the best pigment dispersion. 1 The second part of the experiment creates a vertical segment of points (see Fig. 3b) by holding the dispersant level constant, and increasing the pigment concentration until a color mill base with a high viscosity is obtained. When dispersant is held constant, increases to pigment concentration result in decreases to liquid level, so this second part of the experiment optimizes (minimizes) the liquid level. Colorimetry is not used because pigment concentration varies. Rubup is used to confirm the extent of dispersion. The objective to maximize pigment concentration is complete. Results Optimizing Dispersant Level For TRAD-1/Carbon Black For the work with the waterborne dispersant, TRAD-1, a fixed pigment mass of 7. g was used for each color mill base. For the first segment (see Fig. 4), the pigment concentration was fixed at 2% and dispersant levels were increased in % increments from 1% to 4%. The first three dispersant levels (1, 1 and 2% dispersant) produced color mill bases with high viscosity, and were labeled fail. The remaining six levels (2, 3, 3, 4, 4 and % dispersant) produced color mill bases with low viscosity and were labeled pass.

5 Fig. 4. First segment for TRAD-1/Carbon Black It was known from Fig. 4 that the optimum dispersant level occured between 2 and 2%. A second series of levels (see Fig. ) was tested: 21, 22, 23 and 24% dispersant. All color mill bases provided low viscosities. Fig.. First Segment of TRAD-1/Carbon Black with more data points The CIELAB coordinate L* can be used to compare the effectiveness of the dispersant for carbon black mill bases with identical pigment concentrations. Lower L* values indicate better pigment dispersion. In Fig. 6 there was a significant change in L* between 2 and 21% dispersant. All color mill bases with greater than 2% dispersant showed no rubup. Colorimetry and viscometry identified 21% as the optimum dispersant level for a color mill base with a pigment concentration level of 2%. Fig. 6 Colorimetry for 2% pigment concentration color mill bases of TRAD-1 L*

6 Optimizing Liquid Level For TRAD-1/Carbon Black For the second segment of the flow point graph (see Fig. 7), dispersant mass was held constant at 21% and liquid mass was reduced. Liquid mass reductions were calculated to yield integer values (e.g, 2, 2 and 3% pigment concentrations). The example below shows the math to calculate an integer value for pigment concentration: Example The pigment mass is 7. g for all color mill bases. Because pigment mass is constant, pigment concentration is varied by changing the total mass of the color mill base. If a pigment concentration of 3% is desired, then the pigment mass is held constant then the total mass is changed to 2 g (7./.3=2). concentrations were tested in increments of % from 2 to %. Flow points were observed for pigment concentrations of 2, 3, 3, and 4%. These color mill bases provided good color with no rubup. High viscosity was observed for 4% and %. These color mill bases gave poor color and rubup. Colorimetry data was not taken because color mill bases that vary in pigment content are not comparable. Fig. 7. Second Segment of TRAD-1/Carbon Black Colorimetry and viscosity indicate that the maximum pigment concentration is 4% with a dispersant level of 21%. Optimizing Dispersant Level For TRAD-2/Carbon Black For the work with the waterborne dispersant, TRAD-2, a fixed pigment mass of 7. g was used for each color mill base. For the first segment (see Fig. 8), pigment concentration was held constant at 2% and dispersant level was increased from 1 to 3% in increments of %. Flow points were observed for the last two color mill bases: 3% and 3% dispersant.

7 Fig. 8. First Segment for TRAD-2/Carbon Black Dispersant levels between 26 and 29% dispersant were tested (see Fig. 9). All color mill bases gave low viscosity, suggesting optimum dispersant level at 26%. Fig. 9 First Segment for TRAD-2/Carbon Black with more data The colorimetry of the color mill bases for TRAD-2 at 2% pigment concentration indicated that better color was achieved when the dispersant level was increased from 26 to 28%. The colorimetry of 28 and 29% is equivalent. All samples with disperant levels above 27% showed no rubup. Fig. 1 Colorimetry for 2% pigment concentration of TRAD-2/Carbon Black L* Optimizing Liquid Level For Trad-2/Carbon Black Colorimetry indicated optimum dispersant level at 28%. For the second segment (see Fig. 11), the dispersant level was held constant at 28% and pigment concentrations were varied from 2% to 4% in increments of %. Low viscosities and no rubup were observed for color mill bases with 2, 3, and

8 3% pigment concentrations. High viscosity and rubup was observed for the 4% pigment concentration color mill base. Fig. 11 Second Segment for TRAD-2/Carbon Black Colorimetry and viscosity indicate that the maximum pigment concentration is 3% with a dispersant level of 28%. Daniel Flow Point Efforts were made to carry out a Daniel Flow Point experiment for carbon black, using 1) a spatula and a glass plate and 2) a mortar and pestle. In both cases, the methods yielded liquid levels and dispersant levels that were below the minimum levels determined using the paint shaker. When these color mill bases were added to white paint, poor color development and a high degree of rubup was observed. Optimizing Dispersant Level For TRAD-1/Red Iron Oxide A fixed pigment mass of 7. g was used for each color mill base. For the first segment, the pigment concentration was held constant at 4% and dispersant levels were tested from to 1% in intervals of 1%. All levels provided low viscosity. Fig. 12a. TRAD-1/Red Iron Oxide First Segment Fig. 12b. TRAD-1/Red Iron Oxide Second Segment

9 Colorimetry was necessary to determine the optimum dispersant level. The CIELAB coordinates a* and b* can be used to compare the effectiveness of the dispersant for red iron oxide mill bases with identical pigment concentrations. Increasing a* values and increasing b* values indicate better pigment dispersion: the increase to a* corresponds to increasing strength of the red color; the increase to b* corresponds to the increasing strength of the yellow tone of red iron oxide. A significant increase to both a* and b* was observed at 2% dispersant. Severe rubup was observed for and 1%. Slight to moderate rubup was observed for levels from 3 to 1%. The best results were observed for 2%. Fig. 13 Colorimetry of opaque red iron oxide (4% pigment) dispersed with TRAD-1 2 a* b* 2 Colorimetry Optimizing Liquid Level For TRAD-1/Red Iron Oxide For the second segment (Fig. 12b), dispersant level was held at 2% and pigment concentration was varied in % increments from 4% to 6%. The 4% color mill base provided low viscosity and no rubup. The color mill bases %, %, and 6% provided high viscosity with rubup. Colorimetry and viscosity indicate that the maximum pigment concentration is 4%, with a dispersant level of 2%. Stability Tests ( C/ 1 month) The Liquid Dispersant Flow Point Method (LDFPM) is a screening method that provides optimized dispersant level and maximum pigment concentration for a color mill base (CMB). Some applications require long-term storage so stability testing at C for 1 month was done. CMB s at the maximum pigment concentration showed viscosity increases. Additional testing showed that by increasing the dispersant level and decreasing the pigment level, long-term viscosity stability can be achieved.

10 Conclusions Concentration Optimized The Liquid Dispersant Flow Point Method (LDFPM) can be used to maximize the pigment concentration of a color mill base by first determining the optimum dispersant level, and then minimizing the liquid level. concentration was used to chose one experimental dispersant over another, given that both dispersants provided optimum color with no rubup at low CMB viscosity, and passed the 1 month/ C oven age stability test. Importance Of The Paint Shaker The optimum pigment concentration can be determined with the preparation of about 2 samples. Fewer samples are necessary if the required accuracy is within %; more samples are necessary if the required accuracy is within 1%. The ability of the shaker to reduce the throughput time by a factor of five (relative to a laboratory small media mill) saves a significant amount of time (several days of work per dispersant). Colorimetry Colorimetry is used to confirm the optimum dispersant level prior to the maximization of the pigmen concentration. Colorimetry is particularly important for identifying the optimum pigment concentration if the color mill base with % dispersant gives a low viscosity. Daniel Flow Point Method Our lab reported better results for color mill bases prepared with the paint shaker over color mill bases prepared with either a spatula and a plate of glass, or a mortar and pestle. The improvement to the extent of dispersion is attributed to the much higher shear forces produced by the paint shaker. References R. T. Miller, P. C. Cook, P. Patel, W. L. Dechent, Proc. 27 th International Waterborne, High-solids, & Powder Coatings Symp. 394 (21) T. C. Patton, Paint Flow and Dispersion, 2 nd Ed. Wiley: New York (1979)