Preparation of Nano-sized Hematite Pigment Aqueous Dispersion by High-energy Ball Milling

Using hematite as raw material and maleic anhydride vinyl acetate copolymer PMV as dispersant, the aqueous dispersion of nano-sized hematite pigment was prepared by high-energy ball milling method. The effects of grinding time, grinding medium and dispersant dosage on the particle size and Zeta potential of the dispersion were discussed. The results show that the mass ratio of zirconium beads with diameters of 2 mm and 0.5 mm is 3 when the ball mill speed is 500 r/min, grinding time is 5 h, dispersant dosage is 0.25 gig pigment and grinding medium is 2 mm and 0.5 mm. When the particle size is 2, 230 nm hematite pigment aqueous dispersion can be obtained, which has a certain stability of particle size dispersion.、

 

Mineral pigments, as natural inorganic pigments, have a long history of application. Mineral pigments, because of the different composition of ores, will present a variety of special colors, such as brownish red, green, gray, yellow, white, etc., so in ancient times, people began to use ores for dyeing, including murals and textiles of various colors… With the development of the pigment industry, the continuous refinement of the pigment has become a trend, pigment adsorption performance, dispersion and so on are related to the particle size and particle size distribution of the powder, when the particle size of the pigment decreases, some properties of the pigment significantly improved, because of this, grinding technology has been the corresponding development. With the development of ultrafine powder, some new efficient grinding equipment and technology have emerged. In this paper, hematite as raw material, maleic anhydride vinyl acetate copolymer PMV as dispersant, using high-energy ball milling method to prepare nano-scale hematite pigment moisture dispersion, through the hematite pigment dispersion particle size and Zeta potential test. The effects of grinding conditions such as grinding time, grinding medium and dispersant dosage on the preparation of nano-sized hematite pigment aqueous dispersion by high-energy ball milling were investigated.

Ⅰ. The Experiment

1. Materials and Instruments

Material:

  • Maleic anhydride vinyl acetate copolymer PMV, self-made;
  • Hematite, industrial product.

Instruments:

  • Pulverisette 7 planetary ball mill;
  • LS13320 Laser particle size analyzer;
  • DHG electric constant heat blowing drying oven;
  • Nexus-670 Fourier transform infrared Raman spectrometer;
  • Model 501 super thermostat;
  • JS94J type microelectrophoresis apparatus (Zeta potentiometer).

2. High Energy Ball Milling of Mineral Pigments

0.5 g hematite pigment, a certain amount of dispersant, zirconia beads and 10 mL distilled water were added into the ball milling tank at a speed of 500 r/min. The average particle size, particle size distribution and dispersion stability of the dispersed particles after ball milling were determined.

3. Particle Size and Dispersion Stability Test

The particle size and distribution of hematite pigment dispersion were measured by laser particle size analyzer. According to samples from 2.0 g dry, put in with 20 mL of plug calibration tube, add distilled water to 20 mL, plus the rubber plug, scattered with ultrasonic cleaning machine at room temperature 15 rain, let stand, every once in a while by particle size analyzer to determine the particle size, a dispersion characterization system through the change of the average particle size of dispersion stability.

4. Determination of Zeta Potential

When the mass ratio of hematite pigment to disperse medium was 1/5 000(mL), Zeta potentiometer was used to test the Zeta potential of the iron ore powder after grinding. Each sample was tested 5 times and the average value was calculated.

Ⅱ. Results and Discussion

1. Influence of Dispersant Dosage on Average Particle Size

When the grinding time is 5 h and the grinding medium is 2 NLM in diameter and the mass ratio of 0.5 mm zirconia beads is 3:2, the effect of dispersant dosage on the average particle size of hematite pigment dispersion is shown in Figure 1.

Figure 1 shows that with the increase of dispersant dosage, the average particle size of hematite pigment dispersion gradually decreases. When the dispersant dosage is 0.25g pigment, the average particle size reaches less than 300 rim. When the amount of dispersant continued to increase, the average particle size remained unchanged. When there is no dispersant, the pigment particles tend to agglomerate because of the large surface energy of the pigment particles. When adding dispersant, the dispersant adsorbed to the surface of pigment particles, resulting in electrostatic repulsion. The compound stabilizing effect of steric hindrance prevents the particle attraction and reduces the chance of agglomeration between particles. However, when the concentration of dispersant reaches a certain amount, the dispersant adsorbed on the surface of pigment particles reaches saturation, so as to continue to increase the amount of dispersant, the change of average particle size of the dispersant is not obvious. Therefore, 0.25g/g pigment was selected as the amount of dispersant during pigment grinding.

2. The Effect of Grinding Time on Average Particle Size

When the dispersant dosage was 0.25g/g pigment and the grinding medium was 2 rfllTl in diameter and the mass ratio of 0.5 mm zirconia beads was 3:2, the average particle size at different grinding times was shown in Figure 2.

As can be seen from Figure 2, the average particle size of the dispersion decreases with the extension of grinding time. When the grinding time is longer than 5 h, the average particle size changes to a stable level with the extension of grinding time, which remains around 230 nm. Therefore, 5 h was chosen as the grinding time. Because in the grinding process, the collision between the grinding medium and the hematite pigment inclusions collide with each other to make hematite pigment refinement, average particle size is reduced, at the same time, due to the dispersant adsorption on the surface of pigment particles, prevent agglomeration; When grinding to a certain time, the average particle size of pigment dispersions hematite reduce to a certain degree, continue to extend the milling time, due to the grinding media and pigment than remain unchanged, and after the surface of the paint can is too big, grinding media it may increase the collision between the reunion, which eventually led to the average particle size of basically the same.

The Zeta potential is one of the major interactions between particles. When the grinding time is 5 h, the absolute value of Zeta potential approaches 40 mV. Particles of the same charge with a higher Zeta potential, whether positive or negative, will repel each other. In general, both positive and negative charge symbols can form high Zeta potentials, i.e. – 30 mV > All +30 mV will be considered as high Zeta potential. For molecules and particles small enough and low density to remain in suspension, a high Zeta potential implies high stability, i.e. the solution or dispersion will be resistant to condensation. With the extension of grinding time, the particle size of hematite pigment dispersion becomes smaller, and the dispersant adsorb to the surface of pigment particles. Due to electrostatic steric hindrance – steric hindrance, particles are difficult to aggregate, and the absolute value of Zeta potential also increases.

3. Influence of Grinding Medium on average particle size

When the dispersant dosage was 0.25g/g pigment and the grinding time was 5 h, the influence of zirconia beads with different diameters and different amounts on the particle size of the abrasive pigment dispersion was shown in Table 1.


It can be seen from Table 1 that the average particle size of dispersion is relatively large when zirconia beads with a diameter of 0.5 mm and 2 mm are used for grinding alone. The average particle size of hematite pigment dispersion is relatively small when the zirconium beads with different diameters are used for grinding, especially when the mass ratio of zirconium beads with diameters of 2 mm and 0.5 mm is 3:2, the average particle size of pigment is only 234 nm. This is because when the diameter of the grinding medium is larger, the impact force on the pigment is larger, and it is easier to crush the pigment particles. However, the filling between the grinding medium is not dense enough to crush the smaller particles between the interspaces of the grinding medium, so the particle size of the pigment dispersion can not be too small; Because the hardness of hematite pigment is large, the diameter of the grinding medium is small, the impact strength of the pigment is not enough to fully grind and crush the pigment, and the average particle size of the dispersion is large. Therefore, the use of larger diameter zirconium beads more, less diameter of the ratio of less zirconium beads grinding grinding effect is better. The selection of the diameter of zirconia beads depends on the particle size of pigment before grinding and the particle size of pigment dispersion after grinding. In this experiment, the particle size of pigment before grinding is 2 ~ 4um, and the particle size of pigment dispersion after grinding needs to reach about 200 nm, so the particle size of zirconia beads is 2 mm and 0.5 mm.

4. Dispersant Stability

If the dispersion of particles in the liquid is good, the wettability of the liquid to the particles must be good, so the particles are not easy to aggregate and bond, and the particles are easy to arrange tightly in the settlement, so the particle size change is small, and the settlement speed of the suspension with good dispersion is slow, and the particles of the suspension are gradually thickened from top to bottom. On the contrary, if the dispersion of particles is not good, the particles will contact and attach to each other irregularly, and there are more gaps between particles due to bridging. When settling, it is easy to form loose precipitated polymers, so the particle size changes greatly. Figure 3 shows the stability test of the lapping iron yellow pigment dispersions with different amounts of dispersants when the grinding time is 5 h and the mass ratio of zirconia beads with diameters of 2 mm and 0.5 mm is 3:2.

As can be seen from Figure 3, there was little difference in the stability of different amounts of dispersants in the first week, but after 20 days, the dispersants with more dispersants had smaller particle sizes and higher stability.

5. Influence of Dispersant on Average Particle Size

Dispersant dosage was 0.25 g/g pigment, grinding time was 5 h, grinding medium was 2 nlin in diameter and mass ratio of 0.5 mm zirconia beads was 3:21.

As can be seen from Table 2, the particle size of the dispersion obtained by grinding with commercial dispersant No. 1 and PMV is relatively small, ranging from 200 nm to 300 nm, while the dispersion effect of acrylamide acrylate copolymer is poor. The reason is that acrylic acrylamide copolymer is too hydrophilic, as a dispersant, easy to fall off from the surface of the pigment, so the pigment dispersion size is larger; However, PMV saponification products have hydrophilic groups and hydrophobic groups, which can better disperse pigment particles, and the obtained pigment dispersions have smaller particle size.

Ⅲ. Conclusion

Using hematite as raw material, the nano-sized aqueous dispersion of hematite pigment was prepared by high-energy ball milling method. The optimal technological conditions were as follows: the dosage ratio of zirconia bead diameter of 2 mm to 0.5 mm was 3:2, the rotation speed of ball mill was 500 r/min, the grinding time was 5 h, and the dosage of dispersant was 0.25 g/g pigment. Under these conditions, 230 am hematite pigment aqueous dispersion can be obtained, which has a certain stability of particle size dispersion.

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