PIGMENTS
of the Imperial Chemical Industries, Ltd, and was first introduced to the pigment
trade under the name, c Monastral blue/ at an exhibition in London, November
1935. American manufacture and trade introduction under other names followed
early in 1936 (see first notice in Industrial and Engineering Chemistry > News
Edition [January 10, 1936]). The claim was made that it was the most important
blue discovery since those of Prussian blue in 1704 and artificial ultramarine blue
in 1824, and that in many respects it was superior as a pigment to either of these.
Copper phthalocyanine is prepared by a complex organic synthesis. Phthalic
anhydride and urea (or phthalonitrile) are fused together with copper chloride and
the product is first washed in dilute caustic soda and then in dilute hydrochloric
acid. At this stage it is copper phthalocyanine, but it is not in physical form suit-
able for pigment. It is conditioned by being dissolved in concentrated sulphuric
acid and precipitated again in excess water. After careful washing and filtering,
the resulting paste can be used directly in the preparation of lakes by adsorption
on aluminum hydrate or it can be dried for incorporation directly into non-
aqueous mediums.
M. A. Dahlen of Messrs E. I. duPont de Nemours and Co. (in an article en-
titled, 'The Phthalocyanines: A New Class of Synthetic Pigments and Dyes,'
Industrial and Engineering Chemistry, XXXI CI939lL PP« 839-847), who has
described the properties and uses as well as the method of synthesis of this new
class of pigments, says that pure copper phthalocyanine in crystalline form is
deep blue with a strong bronze reflection, but the dry, disperse form is bright
blue with little or no bronziness. It is insoluble in organic solvents, even at
high temperatures, in alkalis and in acids, except concentrated sulphuric and
phosphoric acids, and Is highly resistant to oxidizing and reducing agencies.
Dahlen says that when tested in pigment applications the phthalocyanines as a
class have good fastness to light and certain members show outstanding fastness.
It is very close to the ideal pure blue, for it absorbs light almost completely in the
red and yellow, and reflects only green and blue bands. This makes it the true
* minus red * so much needed in three-color printing. The color is very strong
tinctorially, having about twice the strength of Prussian blue and twenty to forty
times that of ultramarine. It was proposed and offered for sale under the trade
name, * Monastral blue,' and other trade names, as an artists* color very shortly
after its first commercial introduction in 1936. Other phthalocyanine pigments
were shortly introduced and more can be expected because of the current interest
in this new chromophore. Among the first of these was chlorinated copper phthalo-
cyanine which yields a green dye and pigment having properties quite similar
to those of the blue pigment, including light-fastness. It first became available
commercially at about the beginning of 1938. (See Dahlen for extended bibliog-
raphy on this class of pigments to 1938.)
Pigment is a finely divided coloring material which is suspended in discrete
particles in the vehicle in which it is used as a paint (thus being opposed to a dye