Titanium dioxide, colours

Many of the sohds used in a dispersed state in pol)nners have high energy, hydrophilic surfaces. Examples include titanium dioxide, coloured pigments, mica and even metal particles. Such powdered sohds can be made more compatible with polymers by coating their sm-faces with an adsorbed layer of surfactant in the form of a dispersant. Sodium di-alkyl sulphosuccinates, more commonly used as wetting agents, have been employed for this purpose. 

Since acetal resins are degraded by ultra violet light, additives may be included to improve the resistance of the polymer. Carbon black is effective but as in the case of polyethylene it must be well dispersed in the polymer. The finer the particle size the better the ultra violet stability of the polymer but the poorer the heat stability. About 1.5% is generally recommended. For white compounds and those with pastel colours titanium dioxide is as good in polyacetals as most transparent ultraviolet absorbers, such as the benzophenone derivatives and other materials discussed in Chapter 7. Such ultraviolet absorbers may be used for compounds that are neither black, white nor pastel shade in colour. 

Fillers. They are generally added to reinforce NBR adhesives. However, fillers can be added to promote tack, to increase the storage life, to improve heat resistance or to reduce cost. The most common fillers are carbon blacks. Precipitated silica can be used in applications where black colour is not acceptable, but excessive amounts tend to reduce adhesion. Titanium dioxide can be used to impart whiteness, improves tack and extend storage life. 

Pearlescent pigments give rise to a white pearl effect often accompanied by a coloured iridescence. The most important pearlescent pigments consist of thin platelets of mica coated with titanium dioxide which partly reflect and partly transmit incident light. Simultaneous reflection from many layers of oriented platelets creates the sense of depth which is characteristic of pearlescent lustre and, where the particles are of an appropriate thickness, colours are produced by interference phenomena. Pearlescent pigments are used in automotive finishes, plastics and cosmetics. 

Pigments are classified by an internationally recognised convention, published in the International Colour Index by the Society of Dyers and Colourists in Bradford, in association with the American Association of Textile Chemists and Colorists. The Index issues a generic name to chemically identical products (e.g., titanium dioxide is classified as C.I. Pigment White 6). Where structures are known, pigments are also given a constitution number (e.g., titanium dioxide has the constitution number Cl 77891).This number is reserved for the essential colorant, other ingredients, such as the carriers used in a masterbatch, do not affect this classification. 

Although, because of its cost, titanium dioxide is usually only used as a whitening agent for rubber compounds, it is in fact a reinforcing agent comparable on a volume basis with the reinforcement found with zinc oxide. There are two forms anatase, which has the greatest whitening power, and rutile, which is somewhat creamier in colour. The rutile form is more thermally stable. 

Two of the most important inorganic pigments are the white opaque titanium dioxide and carbon black. However, in this text we will only consider those pigments which are used for their colour, i.e. a specific absorption in the visible spectmm. The origin of the colour in inorganic pigments is due to one of the three following effects ... 

Filter dyes are used to protect the silver halide while it is developing in ambient light. Dyes suited to this purpose are the phthaleins, such as compounds (77) and (78) which have Amax in potassium hydroxide solution at 470 and 620 nm, respectively. These dyes become colourless as the pH of the process is lowered to about 5-8 (72USP3702245, 72USP3702244). The titanium dioxide present in the activator fluid forms a white reflective layer needed for viewing the ultimate colour print. 

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