Optical properties The term of „optical properties” consists of a series of different pigment features, such as: scattering power, whiteness, lightening power, shade or hiding power, often supplemented by properties of pigmented coatings: gloss and haze of gloss (indicator of gloss ‚purity’).
More or less, all of them depend on the crystallographic structure, chemical composition (type of surface treatment and impurities present), particle size distribution and pigment dispersibility. From among pigments and fillers, titanium dioxide is characterised by the highest index of refraction, and thus the highest light scattering power. This allows for making the pigmented products almost ideally white and non-transparent. Thanks to these unique optical properties, titanium dioxide is named „the best white pigment”. In many cases, titanium dioxide is the only pigment allowing for obtaining of adequate colour and non-transparency of a product.
Titanium dioxide reflects light with almost equal high efficiency within the whole range of the visible light (thus exhibits high whiteness). Only in the violet part of the spectrum it shows slight absorption (stronger in case of rutile type). This results in yellowish shade becoming deeper along with increase of level of impurities in a pigment.
In contrast to whiteness that – to a great degree – depends on the pigment purity, scattering power or lightening power depend mainly on the particle size distribution of pigment (measurement of lightening power and undertone is the most broadly used as indirect method of evaluation of particle size distribution of pigment). The particle size distribution of pigment is also of essential influence on gloss and haze of gloss of coatings. Along with decrease of pigment particle size the gloss increases, and – at the same time – the haze of gloss decreases, which contributes to improvement of the coating appearance.
Due to the above reasons, proper progress of pigment dispersion process plays and extremely essential role, leading to fragmentation of agglomerates into elementary particles and their homogenous distribution in the pigmented system. Sometimes this process is carried out by the recipients of pigment in a way that is insufficiently efficient. It does not allow for full development of potential pigment properties, and thus for obtaining maximum efficiency in a given application.
At selection of pigment grade one should take into consideration also nature of a pigmented system, e.g. type of binder, volume concentration of pigment and fillers, method of preparation of the coating and its application. Numerous tests prove that in coatings with Pigment Volume Concentration (PVC) below critical concentration (most often having high gloss) the best results are assured by pigments of finer particles, whereas in coatings of PVC above the critical (most often matt ones), deeply surface treated pigments with greater particles are better. This results from slightly different mechanism of light scattering taking place in these both systems. In the first one, the light is scattered by evenly distributed pigment particles surrounded by binder; in the second one by particles of pigments and fillers, often in reciprocal contact, only partly surrounded by binder. The second case of light scattering is defined with term of so called “dry coating”. This is – among other things – due to the above differences, why continuous search for a ‘universal’ brand of titanium dioxide does not bring fully expected results.
Taking into consideration the specification resulting form different contents of pigment in coatings, Grupa Azoty Zakłady Chemiczne „POLICE” offers TYTANPOL® R-001, R-003 and R-210 for low-pigmented coatings (of high gloss), and deeply treated brands of TYTANPOL® R-211 for coatings of high PVC (matt).
To sum up, optimum selection of physical-chemical and optical properties of pigment according to the specification of the final application is an essential condition for full utilisation of its pigmenting properties.