Apart from chemical purity and optical properties, a broad set of features forming the physical-chemical properties of pigment (shape and distribution of sizes of particles, absorption and photocatalytic properties, pH value, specific resistance, electrokinetic potential, hardness, etc.) are of essential influence on the quality of titanium dioxide. Sometimes, some of them become of a critical character in a given application and limit the possibilities to use a specified brand of titanium dioxide.
One of the most important pigment features deciding on its pigmenting virtues is the particle size distribution. It influences in particular way the lightening power, hiding power and gloss of pigmented goods. It should be stressed that the optimum particle size distribution allowing complete utilization of pigmenting properties depends on the pigment application. In systems of relatively low concentration of pigments (e.g. coatings of high gloss) the best effects are obtained with brands of smaller size of particles (with average size of approximately 0.25 μm), whereas in systems of high concentration (e.g. matt coatings) we deal with an opposite situation. That is reason why – among other things – different brands of titanium dioxide are used despite difficulties of commercial, logistic and recipe nature etc., related to it.
Apart from the particle size distribution also the size and the character of their surfaces – that in turn determine the absorption properties, pH value, specific resistance or electrokinetic potential – play an essential role.
The absorption properties determined by the pigment’s ability for absorption of oil, solvents, water and demand for dispersing agents strongly depend on the production technology, crystallographic nature and type of surface treatment. Their knowledge allows for forecasting of the pigment wettability and demand for binder and other recipe components.
The acid-base properties of the pigment surface determined by means of pH value of water suspension influence the wettability, dispersibility and behaviour of pigment in binder. They play a great role in case of use of pigment for coatings catalysed with acid or base.
Specific resistance of water extract is the measure of content of substances soluble in water that usually disadvantageously influence the application properties of pigment (promote flocculation, intensify corrosion, disturb the process of electrophoretic coating, etc.).
The electrokinetic potential of pigment particles also called zeta potential, plays an essential role in dispersion systems by providing information on the stability of a pigmented system (e.g. susceptibility to flocculation).
Sometimes the specification of final application requires other pigment properties to be taken into consideration, e.g.:
- humidity – plastics, chemically hardened coatings;
- hardness – printing inks, paper products, artificial fibres;
- photocatalytic properties – catalytic properties, UV filters, products particularly susceptible to light (e.g. laminates), coatings cured with UV radiation.