Hiding Power, Transparency

It has often been observed that the coloristic properties of an organic pigment are a function not only of the size of particles but also of their shape. This is due to the anisotropy of the optical properties in different crystallographic directions within the crystal forms of a pigment. In 1974 [5, 6], it was demonstrated that of the equally sized but differently shaped particles of beta copper phthalocyanine blue, the almost completely cubic, i.e., more or less isometric form produces greenish blue shades, while acicular forms are responsible for reddish blue hues. The optical behavior of ordered pigment particles in systems has been reported in the literature [7, 8]. 

7 Particle Size Distribution and Application Properties of Pigmented Media 

However, there is no exact quantitative method which makes it possible to calculate the critical particle size that will produce an optimum in hiding power in a colored pigment formulation. Although the basic theory behind this phenomenon has been treated extensively, it is still most advantageous for practical R D purposes to experimentally determine the particle size that affords a maximum in hiding power. A more approximate than quantitative rule has been established by H.H. Weber, which makes it possible to estimate the particle size that will afford a maximum in scattering  

There are several techniques which determine the hiding power of a pigmented medium for practical application. Colored systems are typically tested by measuring their ability to entirely cover up a standard black and white checkerboard substrate. Parameters used as variables are the pigment concentration and/or the 

It is possible to colorimetrically determine the ability of a pigmented layer to scatter light above a black substrate i.e., its transparency (Sec. 1.6.1.3) by finding the normal value Y or the distance A ab on the DIN 6174 Color Chart. This method may also be used to compare increasingly thick layers. To quantitatively describe the transparency of a system, the so-called Transparency Number / lias been introduced. It is defined as the inverse of the distance AE ah between colors 

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P.O.36 is a reddish, somewhat dull orange pigment which provides very good lightfastness and weatherfastness. It is a highly important pigment. The commercially available types differ considerably in their hiding power/transparency. Different types vary even in terms of shade - the opaque version is redder and noticeably cleaner - and other properties, as well as fastness properties. 

Significant pigment attributes are tinctorial strength, durability (photochemical stability), hiding power, transparency, and heat and solvent resistance. Other properties include brightness (saturation), gloss, rheol-ogy, crystal stability, bleed resistance, flocculation resistance, and other properties associated with specialized applications. 

An elegant and economical one-step synthesis of P.Y. 139 is described in l As initially obtained, the pigment is in an unsatisfactory physical, or cmde form. It is crystallized therefore in aqueous suspension in a finishing process to develop the required physical properties such as crystal shape and size, crystal phase and parti-de size distribution. This determines important application properties like hue, color strength, rheological behavior, hiding power, transparency and light and weather fastness. 

If sample 2 is used as a standard with a hiding power of 100, then the relative opacity of the type with a coarse particle size equals 275, while the transparent variety with a fine particle size has a relative hiding power of 30. 

Finishing not only improves the application properties of a pigment, such as hue, tinctorial strength, brilliance, transparency/hiding power, dispersibility, and flow behavior, but also considerably enhances its lightfastness and weatherfastness and its solvent and migration resistance. 

Disazopyrazolone pigments, for instance, can be tailor-made for various applications in order to provide features such as high transparency or good hiding power, easy dispersibility, and high tinctorial strength. 

They can be used as transparent pigments, but to develop hiding power they have to grow to a larger size. In the growth tank, the reaction is carried out at 75-90 °C, i. e. 

OPACITY. The optical density of material, usually a pigment the opposite of transparency. A colorant or paint of high opacity is said to have a good hiding power or covering power, by which is meant its ability to conceal another tint or shade over which it is applied. See also Paints and Coatings. 

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