Kazuyuki Hayashi, in Nanoparticle Technology Handbook (Third Edition), 2018
2.1 Improved Dispersibility of Nanosized Iron Oxide Red Particles
Iron oxide has been used in many kinds of industrial applications because it is a rich element on the Earth and a multifunctional material. α-Fe2O3 (hematite) is the most popular material and it is often called “Bengara” or iron oxide red. Bengara has a long history from ancient wall painting . α-Fe2O3 particles are manufactured by various procedures today. An example of manufacturing process is shown in Fig. 38.1. One of the most popular methods is wet synthesis, where iron sulfate and sodium hydrate are added to neutral reaction and oxidized to get iron oxide precursors such as Fe3O4 and α-FeOOH. Then the precursors are heated to derive α-Fe2O3 particles. Final particle size and distribution are almost decided by precursor's characteristics. Nanosized α-Fe2O3 particles have lower hiding power and can be applied for colorant particles as trans-iron oxide red, which have higher light transparency in coated films. Especially, a suitable trans-iron oxide red particle could be derived from nanosized acicular α-FeOOH precursors.
Particularly, UV light is absorbed by iron oxide red–coated film. The relationship between wavelength and light transparency in trans-iron oxide red–coated film is shown in Fig. 38.2. The light transparency of UV light is lower; however, IR light easily passes through the coated film. The relationship between particle size of iron oxide red particles and light transparency at λ = 700 nm is shown in Fig. 38.3. When the particle size is finer than 100 nm, light transparency becomes larger. Then, iron oxide pigment can be applied as transpigment for transparency film with a function of UV absorbent.
The preparation procedure of trans-iron oxide red particles is mentioned as follows: iron sulfate solution and sodium carbonatesolution are mixed and aged in the reactor with N2-gas bubbling; then, the oxidation reaction occurs with aeration and nanosized α-FeOOH particles are synthesized in the reactor. α-FeOOH particles are washed and dried to derive α-FeOOH powder with acicular shape and particle size of 80 nm as precursor particles of iron oxide red pigment. α-FeOOH particles are heated at 250–400°C in the oven and dehydrated to α-Fe2O3 particles, and then nanosized iron oxide red particles are derived as shown in Fig. 38.4. Nanosized iron oxide red particles are hard to disperse because particle size is so small. It seems that they tend to coagulate together as shown in TEM photograph. It is necessary to introduce a surface treatment onto particles for easy dispersion. We recommend silicone coating onto iron oxide red particles to reduce their coagulation force between particles. Silicone additive is coated on nanosized iron oxide red particles in amounts such as 1, 1/2, 1/4, 1/8, 1/16, and 1/32-layer equivalent. If coagulation force between particles was reduced, particles would be dispersed well in the lacquer, and light transparency becomes higher in coloring film. Silicone-coated iron oxide red particles with 1/4-layer equivalent coating are shown in Fig. 38.5. It is found that particles are dispersed well and pulverized to almost primary particles. The results concerning lacquer dispersibility and transparent film are described in Table 38.1. The small amount of silicone surface treatment is effective for the dispersibility improvement of nanosized iron oxide red particles. Then, the lacquer viscosity is reduced and light transparency becomes higher. It is found that 1/8-layer equivalent silicone coating is enough for the practical use of nanosized iron oxide red particles. The appropriate surface treatment such as silicone coating is very effective for the practical use of nanosized iron oxide red particles.
|trans-Iron Oxide Red Nontreatment||Run 1|
|Particle size (nm)||70||69||69||70||69||68||68|
|Specific surface area (m2/g)||195.8||183.8||174.4||165.9||161.9||149.1||123.4|
|Si content (wt%)||0.03||0.24||0.45||0.87||1.71||3.40||6.77|
|Lacquer viscosity (D = 1.92 s−1, cP)||5120||4100||3330||2970||2560||2250||1990|
|Light transparency (λ = 700 nm, %)||67.5||68.2||69.7||71.8||72.3||72.0||72.1|
Pigment/binder ratio = 1/2.7, Film thickness = 17 μm.