Effect of Shading Principle and dispersant on Low Titanium Dioxide

Shading principle

Hiding power or opacity is the ability of a coating film to hide the color of the surface of the substrate and is the result of the pigment's absorption, scattering and reflection of light. The hiding power of dark paints comes from the absorption of light by dark or black pigments, and the hiding power of white paints comes from the light scattering of titanium dioxide particles. Among various pigments, titanium dioxide (TiO2) has a high refractive index, has a series of advantages such as high hiding power, strong decolorizing power, and large lightness value. It is a key component for paints and coatings to show white and provide hiding power. Due to its outstanding light scattering properties, chemical stability and non-toxicity, rutile titanium dioxide has been recognized as a good and widely used white pigment. The shortage and rising price of titanium dioxide has prompted paint manufacturers to actively look for its alternatives, or other technologies that can help reduce the amount of titanium dioxide used. Since titanium dioxide is the largest component of the raw material cost of architectural coatings, this impact and impetus is particularly significant for the architectural coatings industry.

The role of dispersants

In addition to the difference in refractive index with the surrounding medium, the ability of titanium dioxide to scatter light in a coating also depends on its particle size and the distance between the particles. Dispersants can not only help it wet and make it easier to disperse, but also stabilize the dispersed opacifying pigments. Therefore, the importance of the choice of dispersant goes without saying. In the production process of latex paint, the aggregates and enriched bodies of pigments and fillers are depolymerized into the original particle state under the action of mechanical force, but after the crushed solid powder is dispersed in the water phase, it tends to coalesce and sink. The main function of the paint is to surround and shield the dispersed pigment particles, make use of the repulsion between the particles to make them stably suspended in the liquid, and prevent the pigment from enriching during the drying process of the paint film. The optical efficiency of titanium dioxide is directly related to its degree of dispersion: the better the pigment dispersion, the stronger the light scattering and the better the hiding power.

Dispersants are of great significance to the production, storage and use of latex paints. Dispersants use repulsive forces to prevent random contact between pigment particles, thereby preventing flocculation and delamination. Electrostatic repulsion and steric hindrance are two important mechanisms for stabilizing pigment particles in water-based formulations. Ionic dispersants and steric dispersants stabilize the dispersion system through ionic repulsion and steric hindrance, respectively. In order to reduce the photocatalytic activity and optimize the dispersion, TiO2 crystals are generally surface-modified by metal oxides, and most inorganic oxides have surface charges in the aqueous phase due to the influence of pH; therefore, the stability of the dispersion depends on at the pH of the system. Pigments and fillers have a pH at which the surface potential of the particles is zero, and this pH is called the isoelectric point. Near the isoelectric point, the charge repulsion is small and the stability of the dispersion system is poor. Polyacrylate and acrylate copolymers are commonly used ionic dispersants in water-based coatings, because of the high content of acrylic acid in the polymer chain, this type of dispersant has good adsorption on the surface of inorganic pigments, and because polyacrylate is in water Dissociation to form an electric double layer protection for the pigment. Therefore, the ionic dispersant adsorbed on the surface of the particles can change the isoelectric point of the oxide, strengthen the electrostatic repulsion of the surface, and increase the stability of the pigment. The repulsive potential energy (VR) between two similarly charged particles can be estimated by Equation (1):

In the formula, C is a constant; ε is the dielectric constant; d is the particle diameter, μm; ζ is the Zeta potential, mV; s is the distance between two charged particles, Å; δ is the thickness of the electric double layer, Å; k is Boltzmann's constant, J/K; T is the thermodynamic temperature scale, K.

According to formula (1), at a fixed separation distance, the particle charge characterized by Zeta potential energy has the greatest influence on the repulsion force, and a thicker electric double layer (large δ value) is also conducive to the stability of the dispersed system. For the pigment particles that have been separated by the ionic dispersant, the steric dispersant can further increase the thickness of the steric barrier, and maintain the spatial distance and good spacing of the shading pigment until the paint film dries. The composition of sterically hindered dispersants usually includes a water-soluble polymer main chain and a binding group with a strong force on the surface of the pigment. Therefore, the non-ionic dispersant can be adsorbed around the pigment particles to form a polymer shell. When the pigment particles Closer to each other, the polymer diffusion layers begin to overlap causing steric hindrance. Not only the dispersion of titanium dioxide particles in the liquid phase needs to be optimized, but also a certain degree of stability in the coating film is required to prevent enrichment and affect the hiding power. During the drying process of the coating film, the distance between the particles becomes closer, and the steric hindrance dispersant can maintain the spatial distance of the particles more effectively.

Effect of Air Porosity on Shading

The air pores in the dry coating can also act as light scattering centers. If the size is right, the air pores in the coating can provide additional hiding power, so the amount of titanium dioxide can be reduced. There are two approaches to introducing air voids into paints: (1) formulating paints above the critical pigment volume concentration (CPVC); (2) using hollow sphere opaque polymers. Both can achieve good dry film optical properties and reduce the effect of titanium dioxide dosage. But the limitations of the former are obvious to all: Although the pores in the dry coating are good for light scattering, they also increase the porosity, weaken the integrity of the coating structure, and make some important coating properties such as scrub resistance, durability, etc. Contamination and durability etc. are affected. Relatively speaking, the use of opaque polymers is relatively simple and direct, and has little impact on the performance of coatings, so it has received widespread attention.

relative tinting strength

Rich colors are a major feature of latex paint. It is estimated that colored latex paint accounts for more than 80%. Relative tinting strength is a measure of how effectively a colorant changes the color of a paint by adding a specified amount of colorant to the absorption characteristics of a standard white paint. Tinting strength has an important economic value, which determines the amount of titanium dioxide and colorant added to the formulation to achieve the desired depth of color. Therefore, the selection of color paste and the accurate control of color matching have become an important link from the production of latex paint to retail sales. In the test, relative tinting strength compares the color difference between the test paint and the reference paint using the same resin and single color pigment, and the desirable situation is that the test paint is the same color as the reference paint. Paint manufacturers generally list relative tinting strength as a quality inspection item, and control the variance of each batch of paint produced within 2%. Relative tinting strength, like hiding power, is one of the optical properties that characterizes the light-scattering efficiency of white pigments. The gloss or lightness of colored coatings is usually expressed in Y%. The whiter the color, the higher the Y% value. The lighter the test paint, the white base paint can use less titanium dioxide to maintain the same dry/wet hiding power and brightness as the reference paint. Therefore, if the formula of the base paint can be readjusted, the cost of the white paint and the colored paint after tinting will be reduced due to the reduction of the amount of expensive titanium dioxide.

For more information, please refer to the full text of "Professional Covering Polymer and New Dispersant Technology to Optimize Covering Power and Reduce Titanium Dioxide Consumption" (Wu Wenjun, Xiong Xizhu, Christine Fortener, Medhi Bouzid, Mike Kaufman, Liu Lili)