Recent publications have labeled copolymeric dispersants as “high molecular weight” (HMW) and other low-molecular-weight dispersants as “traditional.”^2-4 The exact molecular weights of commercial HMW dispersants are proprietary, but the literature has focused on the molecular weight differences to distinguish HMW dispersants from traditional dispersants.

Color pigments may be dispersed directly into a coating or dispersed as a color concentrate that is stored and later used to tint a coating. This article focuses on the dispersion of color pigment to make color pigment concentrates, hereafter referred to as color concentrates.

Pigment Dispersion

Pigment agglomerates are broken apart using techniques such as ball milling, small media milling and high-speed dispersing. The separation of pigment particles and the desorption of gas and liquid residues from the pigment surface exposes new pigment surface. Pigment wetting is achieved when dispersants and solvents adhere to exposed pigment surfaces. Dispersants sustain pigment particle separation by maintaining a minimum safe distance between two pigment particles: ~10 nm if the dispersant is a resin, and ~5 nm if the dispersant is a surfactant.^5,6 If the distance between pigment particles is less than the minimum safe distance, attractive forces will bring particles together, forming flocculates. Flocculation leads to noticeable changes to pigment properties such as color strength, gloss and viscosity.

Dispersants maintain pigment separation by two mechanisms: electrostatic stabilization and steric hindrance. For electrostatic stabilization, a double layer of ionic charges surrounds each pigment particle. For example,¹ a pigment preferentially adsorbs anions [–], and is subsequently surrounded by a second layer of cations [+]. When two pigment particles approach each other, coulombic repulsive forces dominate, changing the trajectory of each particle to prevent close contact. The electrostatic stabilization mechanism requires a polar solvent, such as water. Water ionizes to H3O+, and OH–, and thus can displace conjugate ions, allowing cation [+] / anion [–] dissociation.

For steric hindrance, the bulk of a dispersant chain is the barrier to close contact between pigment particles. When chains intermingle, osmotic pressure builds up and drives solvent between the chains. The steric hindrance mechanism works in both polar (e.g., water) and nonpolar (e.g., mineral spirits) solvents.

Degree of Dispersion

• Hegmann grind measures the size of the larger pigment particles. A well-dispersed pigment gives a higher Hegmann grind reading. Two coatings with the same Hegmann grind reading may give different degrees of opacity and/or color development.

• Colorimetry measures the color strength quantitatively as vectors in color space. For this article, color strength values are obtained by adding color concentrates to a white tint base and measuring reflectance values from the dried films in CIELAB L* (Commission Internationale d’Eclairage 1976 L*a*b*). Colorimetry has been called the most effective evaluation method for extent of dispersion.⁷

• Particle size analysis provides information about the pigment particle size, including both the range of particle diameters and relative pigment population as a function of particle diameter.

• Brookfield viscosity quantifies the difference between a color concentrate with a poorly dispersed pigment and a color concentrate with a well-dispersed pigment.

The following method uses quantitative colorimetry measurements to judge the degree of dispersion. Literature was researched for theories and/or test methods regarding optimum pigment dispersion, and one detailed procedure was found, the Daniel Flow Point Method.¹

The Shaker Test

Method

1. Set the total mass for the color concentrate (excluding glass beads), i.e., 70 g.

2. Set the pigment concentration based on total mass. For example, 21% carbon black=14.70 g carbon black [70*(21/100)=14.70].

3. Set the dispersant concentration based on carbon black mass. For example, 18% dispersant=2.65 g dispersant [14.70*(18/100)=2.65 g].

4. Add defoamer (0.10 g used for all samples).

5. For the remaining mass, the 5 parts by weight water:1 part by weight propylene glycol mixture is added [70 – (14.70 + 2.65 + 0.10) = 52.55 g water/propylene glycol].

6. Add a specified mass of glass beads [e.g., 84 g of glass beads].

The color concentrate mass ratios may change with a change of the pigment being tested.

The lid of the glass jar is secured with electrical tape. Up to four glass jars are placed in a wooden holder (see photo, page 34) to prevent glass jars from shaking free from the parallel plates. The wooden holder contains four cylindrical insets into which the bottoms of the glass jars will fit. The jars are shaken for a total time of 12 minutes in three-minute intervals.

At the conclusion of the shaker test, 1.25 oz glass jars are filled with 18.00 g of a commercial white tint base (100%-acrylic, interior pastel base). The color concentrate is added, 0.75 g, and the jars are sealed. The glass jars are placed between the parallel plates, and shaken for three minutes. A drawdown, using a 3 mil Bird Film Applicator, is made on a Leneta™ Penopac chart. A rubout is performed on the unsealed portion of the chart, with 50 circular rubs ~30 seconds after the time of the draw down. The drawdowns are air dried at room temperature, and colorimetry data is measured. Color measurements are made with a Microflash® 200d (Data Color International).

Laboratory Small Media Mill

Experimental - Verification of the Shaker Test

Figure 2 / Color Strength vs. Time (Shaker-Carbon Black)

Experiment 1: Does the method provide pigment dispersion?

This yields a slurry with 27% dispersant on pigment and 21% pigment on total mass. The jar is sealed and shaken for three-minute intervals taking 0.75 g aliquots at each interval until eight aliquots were collected. Samples were prepared from these aliquots for colorimetry measurement by mixing with white tint base, as previously established in the shaker test. The reflectance value, reported as CIELAB L*, is measured and the rubout is observed. L* decreases with increasing color development of the carbon black pigment. Data is shown in Figure 2.

Figure 3 / Shaker Test Reproducibility/Carbon Black

Experiment 2: Is the data reproducible?

Figure 4 / Color Strength vs. Time (Laboratory Small Media Mill-Carbon Black)

Experiment 3: Is the data comparable to established laboratory dispersion methods?

Use of the Shaker Test

• Traditional dispersants — TRAD-1, TRAD-2 and TRAD-3 at 100% active; and TRAD-4 at 70% active

• High-molecular-weight dispersants — HMW-1 at 52% active, a block copolymer dispersant

• Pigment — Raven 1200 beads, a general-purpose carbon black supplied by Columbian Chemicals.
  
  
  

  Figure 5 / Dispersant Efficacy

  Experiment 4: Evaluating dispersant efficacy

  Four experimental traditional dispersants, TRAD-1, TRAD-2, TRAD-3 and TRAD-4, were evaluated using the shaker test to determine efficacy in the dispersion of a carbon black pigment. Sixteen different color concentrates were used: four dispersants, each at four dispersant concentrations. Using the color concentrate formulation used in Experiment 1, the dispersant concentrations were varied. The dispersant concentrations were 12%, 15%, 18% and 21% on pigment weight. Four samples at each dispersant concentration were shaken simultaneously for 12 minutes in three-minute intervals with aliquots taken at 12 minutes. Samples were prepared from these aliquots by mixing with white tint base as previously established in the shaker test. For the colorimetry test, the reflectance values, reported as CIELAB L*, are measured and the rubout is observed. The process was repeated for each dispersant concentration. From the graph of the colorimetry data, efficacy can be identified by decreasing L* values (TRAD-2 and TRAD-4), as shown in Figure 5.
  
  
  

  Figure 6 / Optimizing Dispersant Concentration (Carbon Black)

  Experiment 5: Optimizing dispersant concentration for a carbon black pigment

  Based on results of Experiment 4, TRAD-4 was selected for optimization of carbon black pigment. Four color concentrates were prepared. Using the color concentrate formulation used in Experiment 1, the dispersant concentrations were varied. The dispersant concentration values were 18%, 21%, 24% and 27% on pigment weight. Samples were shaken simultaneously for 12 minutes in three-minute intervals with aliquots taken at 12 minutes. Samples were prepared from these aliquots by mixing with white tint base, as previously established in the shaker test method. For the colorimetry test, the reflectance values, reported as CIELAB L*, are measured and the rubout is observed. The data is shown in Figure 6.
  
  
  

  Figure 7 / Equal Weight Substitution

  Experiment 6: Importance of dispersant optimization

  An experimental dispersant, Trad-4, optimized at 21-24% dispersant per weight of carbon black was compared to a commercial high molecular weight dispersant, HMW-1, requiring 65-80% dispersant (manufacturers recommendation) per weight of carbon black. Four color concentrates were prepared. Using the color concentrate formulation used in Experiment 1, the dispersant concentrations were varied. Two color concentrates used Trad-4 at a dispersant concentration of 21% and 80% on pigment weight. Two color concentrates used HMW-1 at a dispersant concentration of 21% and 80% on pigment weight. Samples were shaken simultaneously for 12 minutes in three- minute intervals with aliquots taken at 12 minutes. Samples were prepared from these aliquots by mixing with white tint base as previously established in the shaker test. For the colorimetry test, the reflectance values, reported as CIELAB L*, are measured and the rubout is observed. The data is shown in Figure 7.
  
  
  

  Figure 8. Below Optimum Concentration vs. Optimized Concentration
  Rubouts can be used as a means of judging dispersion. The color contrast between the rubout area and the undisturbed film of the draw down shows if the pigment has maintained dispersion. When there is little or no difference between the rubbed and undisturbed film, dispersion stability has been maintained. The importance of dispersant optimization is pointed out by comparing the rubouts of two color concentrates, made at equal dispersant levels, added to a white tint base. Figure 8 shows the comparison of HMW-1 at 21% (below manufacturers recommended concentration) vs. TRAD-4 at 21% dispersant concentration (optimized concentration).
  
  

  Conclusion

  The shaker test method can be used to determine dispersant efficacy and optimum dispersant concentration for a carbon black pigment. The method was evaluated for effectiveness and reproducibility and compared to an established laboratory small media mill method. The experiments show that the proposed shaker test is effective for the determination of dispersant efficacy and the determination of the optimum dispersant concentration for a particular carbon black pigment. In our tests, dispersants with efficacy were identified by substantial decreases in L* values with increasing dispersant concentration.

  The proposed shaker test allows the comparison of four dispersants or dispersant levels at one time. This provides four samples, for comparison, with the exact same time and energy of dispersing action. Experiments show that the shaker test is reproducible from one sample to another and results matched those obtained using a traditional laboratory small media mill. The ability to test four samples at one time and the small amount of clean up required for the shaker test, results in a time saving of over 80% when compared to the time required using the traditional small media method.

  The importance of optimizing dispersant concentration for a particular pigment is pointed out in Experiment 5. The comparison of an experimental traditional dispersant (Trad-4) to a high-molecular-weight dispersant (HMW-1) showed that the HMW-1 dispersant was not efficacious when used at the optimized level of the experimental traditional dispersant, 21% on carbon black pigment weight. Increasing the level of the HMW dispersant to 80%, the manufacturer’s recommended use level, gave results comparable to the traditional dispersant at 21%. This was shown by the L* values and comparing color acceptance using the rub up test.

  This paper was presented at the Waterborne, High-Solids and Powder Coatings Symposium on March 1–3, 2000, in New Orleans.

  For more information on pigment dispersions, contact Troy Corp., 8 Vreeland Road, Florham Park, NJ 07932.