Toda United Indusrial (Zhejiang) Co., Ltd.

Transparent iron oxide pigments, traditionally used to produce silver and gold effects, create a sense of prestige among consumers. In current automotive styling formulations, depth of shade and transparency are of critical importance. Not only can transparent iron oxides meet these coloristic requirements, but also they are weatherable, low in cost and offer attractive UV absorption characteristics.

The attributes of enrichment, beautification and protection are all essential in automotive finishes. The synthetic manufacture of low-cost, transparent iron oxide pigments ensures that the color shades are clean and consistent. The inherent property of iron oxide to absorb ultraviolet radiation along with its ability to make the product a colored transparent material makes it of particular importance for the coloration of automobiles. In this article, we will address each of these roles as well as dispersion characteristics and styling.

Pigmentary Features

The manufacturing processes of transparent iron oxide pigments allow for the control of physical and surface chemistry properties of the primary particles. The particle size is optimized to ensure that minimal light interference occurs, thus maximizing transparency. However, a consequence of the small particle size is high surface area and oil absorption values, similar to those of organic pigments.

The demands of the different systems and complimentary pigments tend to require different surface chemistries. Therefore, the surface chemistry of the transparent iron oxide particles is manipulated within the manufacturing process, allowing for different compatibility and stability in coatings formulations. For example, acidic grades are not recommended for use with waterborne systems as the chemical balance can be disrupted and the resin precipitated. Conversely, in solventborne formulations, the highly charged acidic pigments tend to yield superior performance. This results in a range of colors with different performance characteristics. In light of this, it's important to understand the application so that proper pigment selection can be made.

Dispersion Processes

To achieve the nano particle size of transparent iron oxides, a manufacturing process is used that results in increased aggregation and agglomeration. To fully disperse the pigment, a high level of shear and energy is required. For relatively low viscosity systems, a bead mill containing glass, steel or zirconia media is preferred, although ball mills or attritors may also be used. Where high viscosity formulations are required (e.g., pastes or highly pigmented concentrates), two or three roll mills may be required. High-speed mixing with a Cowles-type blade will not provide sufficient shear to disperse transparent iron oxide pigments.

Although most automotive coatings are produced in-house, the time and difficulty associated with dispersing transparent iron oxides has led to these dispersions often being produced by specialty dispersion houses. Once the pigment is fully dispersed, the particle size is such that the resultant colloidal suspension is stable and good in-can stability is achieved. This is not the case with larger, dense particles, which often need additional stabilization through the use of rheology and/or charge modifiers in the formulation.

This article comes from pcimag edit released