What Is Titanium Dioxide (TiO2), and What Does It Do in a Pigment Formulation?

Before winter set in, a paint plant swapped its usual titanium dioxide for a lower-priced grade in order to bring down the formulation cost of a white emulsion paint. Whiteness on the laboratory draw-down showed no visible difference, and in fact read slightly higher, so the sample cleared internal review without trouble. Once the batch reached the job site, the applicators reported that the same spreading rate no longer covered the substrate and a further coat was needed; six months later the wall on the sunny side began to chalk and yellow.

Titanium dioxide is the industrial name for TiO2 and the white pigment with the highest refractive index and the strongest hiding power available today. In a formulation it carries three jobs at once: it provides opacity, it contributes whiteness, and it takes part in determining how the film or the finished part will weather. Crystal form and surface treatment decide what it is suited to, and we generally treat it as the backbone of the formulation, because its loading and its quality bear directly on the cost structure and the service life.

Hiding power comes from the scattering of light, and the greater the refractive index difference between a titanium dioxide particle and the medium around it, the stronger the scattering at that interface. Rutile titanium dioxide has the highest refractive index among white pigments, which is the physical basis for covering a substrate with a very thin layer. Scattering efficiency also depends on particle size: the particles have to fall in the range matched to visible wavelengths, since particles that are too fine scatter towards the blue and lose hiding power, while particles that are too coarse waste the scattering area their mass could otherwise contribute. If the titanium dioxide you buy has too broad a particle size distribution, the same loading buys lower hiding efficiency, and that loss ends up showing in spreading rate or in the wall thickness of the part.

Titanium dioxide comes in two main commercial crystal forms, rutile and anatase. Anatase has a cooler whiteness and markedly higher photocatalytic activity: under ultraviolet light it generates free radicals at the particle surface that progressively degrade the surrounding resin matrix, which shows up as chalking and loss of gloss. The rutile lattice is denser and its photocatalytic activity far lower, and combined with a silica and alumina surface coating the interfacial reaction can be suppressed further still. For exterior coatings, building profiles and automotive-related parts we recommend a fully coated rutile grade without exception. The sensible place for anatase is indoor articles, papermaking, part of the delustering of synthetic fibers and other uses that do not demand long-term weathering.

The relationship between titanium dioxide loading and hiding power is not linear. Once pigment volume concentration rises to a certain level, the spacing between particles narrows to the point where they interfere with each other's scattering, the gain in opacity from further loading falls away quickly, and the extra money no longer buys matching performance. Past the critical pigment volume concentration there is no longer enough resin to wrap every pigment particle completely, so scrub resistance, adhesion and permeation resistance of the film all drop, and plastic parts can lose impact strength. When we optimize a formulation for a customer, the first step is often to check whether titanium dioxide is being overdosed and to replace the excess with a suitable extender, which improves cost and performance at the same time.

Whether rutile and anatase can be substituted for each other directly depends on where the product is used: indoors, with no weathering requirement, it is worth considering, while outdoors the substitution does not hold, because the photocatalytic activity of anatase accelerates degradation of the matrix and leads to chalking and loss of gloss; and if the formulation was designed around rutile, hiding power and shade have to be checked again on substitution, since the two do not match in whiteness or undertone. Replacing part of the titanium dioxide with an extender to lower cost is workable, provided the substitution ratio stays within the sensible range for hiding efficiency: extenders such as calcium carbonate and barium sulfate have refractive indices close to that of the resin and contribute almost no opacity of their own, so over-substitution leaves the film compensating with added thickness; we normally measure first whether the current formulation sits in the overdosed zone and then judge how much room for substitution there is. Titanium dioxide of the same nominal description disperses very differently from one system to another because surface treatment governs the affinity between particle and medium: the inorganic coating type and the organic surface treatment agent differ for waterborne systems, solventborne systems and plastics, and poorly dispersed particles exist in practice as agglomerates, so effective scattering area falls and hiding power and gloss are both discounted.