Black Pigment Preparation Via Couette-Taylor Vortex for Electrophoretic Display Application

Electrophoretic display realizes the images using the movements of black and white pigments in the dielectric fluid under the external electric field. In electrophoretic display, carbon black is widely used as a black pigment. To improve the motion of black pigments in the dielectric fluid, the density and surface charge of pigments should be controlled. The most effective way to modify the property of pigments is to coat the polymer on the carbon black. The density and the surface charge are determined by the kind and the amount of coated polymer.

The polymerization reaction takes place on the surface of the carbon black as shown in Fig 1. At first, the polymerizable functional group was attached on the surface of carbon black which enabled the further polymerization. After the surface treatment, the radical polymerization was applied to grow the polymers on carbon black. However, the homogeneous polymerization, i.e. not on the surface of carbon black, can occur. If the homogeneous reaction takes place severely, the control of pigments’ properties can be failed. Thus, the selectivity of polymerization is determined by the dispersion of carbon blacks in the solution. The carbon blacks with improved dispersion provide more active surface area for polymerization step, therefore, the selectivity can be improved.

A batch reactor has been used for the polymerization on carbon black.^1,2 However, the uniformity of polymer coating is occasionally inadequate because of non-uniform and weak fluidic motion, resulting in the wide deviations of density and size of black pigments. Furthermore, the productivity of black pigments is relatively low with a batch reactor. Therefore, the preparation method or reactor should be developed to improve the property of black pigments as well as to increase the productivity of pigments.

In this study, the Couette-Taylor vortex reactor,^3,4,5 which induces the uniform shear stress and homogeneous mixing, is applied to polymerization step on carbon black. Fig. 2 shows the conventional Couette-Taylor vortex reactor, consisting of two parallel cylinders. The inner cylinder rotates, developing Couette-Taylor vortex between two cylinders. The development of Couette-Taylor vortex can be predicted based on Taylor number, which is a function of inner cylinder’s rotating speed. This vortex increases the dispersion of reactants as well as uniformity of fluidic motion. Furthermore, the continuous production is enabled by using a Couette-Taylor vortex reactor. As displayed in Fig. 2, the reactants can be injected through the inlet of reactor, and the products can be collected at the outlet. The productivity is determined by the axial flow rate, i.e. feeding rate, as indicated in Fig. 2. Because of its advantages related to the enhanced fluidic motion, it is expected that the properties and productivity of black pigments can be improved at once.

In this presentation, we studied the properties of black pigments prepared in a batch reactor, batch Couette-Taylor vortex reactor, and continuous Couette-Taylor vortex reactor. The changes in the size, density, and distribution according to the process variables were investigated. In continuous Couette-Taylor vortex reactor, the influence of the axial flow rate was also clarified. Finally, we applied three black pigments obtained from each process to electrophoretic displays with white TiO₂ particles, and the response times of electrophoretic motion under the electric field were compared.

References:

1. T. Liu, S. Jia, T. Kowalewski, K. Matyjaszewski, Langmuir, 19, 6342 (2003).

2. S. Yoshikawa, S. Machida, N. Tsubokawa, J. Polym. Sci. pol. chem., 36, 3165 (1998).

3. K. Kataoka, N. Ohmura, M. Kouzu, Y. Simamura, M. Okubo, Chem. Eng. Sci., 50, 1409 (1995).

4. M. Kristiawan, T. Jirout, V. Sobolik, Exp. Therm. Fluid Sci., 35, 1304 (2011).

5. K. Watanabe, S. Sumio, S. Ogata, J. Fluids Eng.-Trans. ASME, 128, 95 (2006).

Figure captions:

Figure 1. A schematic diagram of polymerization on carbon black.

Figure 2. A schematic diagram of the Couette-Taylor vortex reactor.