Sept,,
1920
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
quired for each square centimeter of section which is measured. We prefer t o report the reciprocal of this quantity, i. e., the square centimeters of surface which one gram of pigment is capable of covering. This quantity is called the diffusing power of the pigment, and, while expressed in square centimeters per gram, i t would be equally satisfactory if expressed in square feet per pound. As a measure of fineness i t is probably even better t o express t h e results on a volumetric basis, i. e., as square centimeters per cubic centimeter of pigment, or square feet per cubic foot. The diffusing power of a pigment is a function of the completeness with which it is dispersed in the fluid employed. I n the investigation linseed oil has usually been used, t h e pigment ground by hand in a small amount of oil, and gradually diluted with more oil, till a suitable suspension was obtained. For most pigments the diffusing power becomes practically constant after a half hour’s grinding. Only clear liquids should be used; bleached linseed oil, for instance, eliminates any effect of turbidity of the oil itself. T h a t the diffusing power increases with time of,grinding is shown by Table I. TABLE I-CARBON Time of Grinding Minutes 0
10 20 30 45
S9P
the same pigment dispersed i n water in the same way , gave a diffusing power of only 104 Diffusing power. must not be confused with coveiing power, nor with t h e go-called “hiding po’wer.’’i The covering power of pigments is unquestionakdy influenced by their light absofption capacity, a factor which is entirely eliminated in the determination of diffusing power. The latter is, however, a very direct measure of the fineness of a given pigment as suspended in the liquid employed. Everyone experienced in the determination of covering power is familiar with t h e difficulties which develop in testing colored pigments This method of determining diffusing power entirely eliminates difficulty. The color of t h e pigment has appa no influence on the result obtained. The m can be applied t o all pigments, whether white, or colored, independent1 gloss, and other factors which so seriou he, determination ows the diffusing of covering power. T power of samples of certain of the more irnportgnt, pigments. TABLE 111-PIGMENTS GRO
BLACKIN LINSEED OIL Diffusing Power Sq. Crn,/G. 343 1416 1760 2220 2180
Table I1 indicates t h a t t h e results are independent of the concentration of the pigment in the diffusing liquid. TABLE11-WHITE LEADIN LINSEEDOIL-NO GRINDING Concentration Diffusing Power Grn./1000 Cc. Oil Sq. Cm./G.
0.834 1.21 1.45 1.53
The diffusing power of pigments can be determined
as square centimeters per pound. The determination simple apparatus, and resu
224 212 209 198
The diffusing power is profoundly influenced by the dispersing liquid employed; thus for a certain white lead dispersed in linseed oil without grinding, a diffusing power of 2 1 2 sq. cm. per gram was obtained, whereas
pigment, and as such is of obvious interest and value t o the rubber trade. 1 A. H. Pfund,
