Comparison of Wet- and Dry-Film Hiding-Power Tests ROSCOEH. SAWYER, Krebs Pigment and Color Corporation, Newark, N. J. Hiding power of paint or pigment is shown Paint was appIied by brush in HE hiding power of a paint each case until the design was may be determined in a to be a function of concentration of the pigment no longer visible under a 75-watt number of ways (14, 7 ) . in the film. A comparison of wet- and dry-film daylight bulb in a white conical Tests employing either wet or tests shows the two types to yield significantly reflector 3 feet (90 em.) above dry films are in common use, and digerent relations between hiding power and the test piece. Observation was definite advantages and disadpigment concentration, which are shown to be, a t about 45" and from a distance v a n t a g e s a r e attributable to of 3 t o 4 feet. Hiding power was each. in part, due to actual differences in concentration calculated in square centimeters Wet-fiIm t e s t s h a v e been of pigment in wet and dry films of the same per cubic centimeter from the favored as most easily adaptable it is unsafe, paint. The author concludes that weight of paint required to obto securing results a t so-called in general, to employ a wet-film test. literate the contrast (determined complete hiding. Results are A new dry-film test is described which gives by loss in weight of can and brush obtained as absolute values, and c o m b i n a t i o n ) and in square use of the wet paint permits hiding powers . paralleling those obtained at c e n t i m e t e r s per gram of pigspeed and fair reproducibility in either 2 per cent contrast or complete hiding in ment. testing. As commonly carried the dry film. The test is easily carried out, The dry-film tests were carried out, however, the application of has good reproducibility, and m a y be used for out on design 2, described above. the results to the practical use of comparisons at a n y contrasts or film thicknesses Three types of test were used: the paint is questionable. Paint complete hiding, spreading rate is applied a t several times the to beyond practical complete hiding to show relaa t 2 per cent contrast, and a new normal one-coat thickness over tive values of paints or pigments. method for hiding-power combackgrounds of rigorous and exparisons a t incomplete hiding. treme contrast, until these backgrounds are visually completely obliterated under the most DRY-FILMCOMPLETE H I D I ~ AND G 2 PERCENTCONTRAST. exacting conditions of observation. Perhaps the most serious Successive coats of each paint were applied to several charts objection is that the wet paint film in test is chemically and to give increasing weight of paint until a point was reached physically quite different from the dry film which actually a t which contrast was no longer visible under the conditions of observation employed for the wet-film tests. The brushrepresents the paint in use. Results a t complete hiding can also be obtained with the outs were graded by a Weston photronic cell using apparatus dry film, but methods which have been available have been so essentially as described by Kraemer and Schupp ( 6 ) . Spreaddifficult and time-consuming as to secure little recognition. ing rates a t 2 per cent contrast were read from the curves Paints are more commonly compared a t one or two normal relating contrast and film thickness for these charts. DRY-FILMINCOMPLETE HIDING. Paints were applied to coats over a contrast background for purely relative values. The advantage of this type of test is that differences between the charts a t controlled spreading rates ranging from 150 to paints may be made directly visible. The test is carried out 460 sq. em. per cc. Paint was taken up in a syringe and under conditions much more nearly comparable with practice weighed in the syringe with a brush wet with the paint. than is a wet-film test in respect to such factors as condition The paint was spread over the design with the syringe and and thickness of film, pigment concentration in the film, and then brushed out in a standard manner. Weight of paint brightness of the paint. The effect of brightness of the paint applied was determined as the loss of weight of the syringe on hiding power is pronounced (8, 8) and paints undergo and brush combination. Several charts were made of each various and, a t times, considerable changes in brightness in paint. For some of the paints series of brushouts were made drying. showing more or less uniform steps of increasing paint weight on the charts for use as arbitrary grading standards. EXPERIMENTAL Intervals of 0.5 to 1.0 cc. are most convenient. WET-FILMCOMPLETEHIDIKG. Two wet-film brush-out Gradings were carried out on a board illuminated by four tests were employed which differed only in background 200-watt daylight bulbs in industrial reflectors and mounted used. Background 1 was a checkerboard of 2.54-em. black about 3 meters from the board. Illumination was vertical and white squares printed on glossy photographic paper 27.9 t o the charts and uniform over the board a t about 16 footx 33 cm. This size was employed to make the test as nearly candles. Gradings were by four observers seated below as possible comparable with the other complete hiding tests. the lamps in the shadow and viewing the charts a t eye level Background 2 was a diamond of 1.4-cm. alternate light and and a t about 30' from the perpendicular to the charts. dark bands printed on heavy paper and machine-coated with I n grading, the brushouts from one paint were numbered nitrocellulose lacquer. The design had external dimensions consecutively and used as arbitrary standards. A brushout of 22.9 X 34 em. and was surrounded by a clear 1.4-em. from another paint was placed a t the center of the grading border to make a total painting area of 929 sq. cm. Bright- board. Consecutive brushouts of the standard were found, nesses of the areas used were: one of which showed more contrast than the sample and the other less. The position of the sample in the contrast BACEQROUND 1 2 interval was then estimated in tenths of the contrast interLight 80 75 val, Gradings made with the higher standard a t the right Dark 5 40
T
113
Vol. 6, No. 2
ANALYTICAL EDITION
114
Reproducibility is also good. An analysis of brushouts on 74 paints brushed out by five operators shows the following results: TABLEI. REPRODUCIBILITY OF BRUSHOUTS
45
Y
A
5 I 40 E
OPERATOR
PAINTS
A B C D
16 13 13 16 17
:
a"
$u I
E
Av.
35
30
VOLUME P E R C E N T P I G M E N T -
MAXIMUM DEVIATION
%
%
1.62 1.30 1.97 1.36 1.74 1.6
3.76 6.58 6.66 4.73 4.76
Three brushouts were made of each paint. Deviation was from the mean of three brushouts in each case. PAINTS. The paints used were single pigment paints using a kettle-bodied linseed oil-China wood oil combination vehicle. The pigments were titanium dioxide, lithopone (enamel type), and white lead. The paints were formulated to give definite pigment binder ratios and adjusted to equal consistency by addition of volatile. The wet paints were therefore similar in general appearance.
c 5
'
AVERAQE DWIATION
DRY FILM
FIGURE1 and left were averaged. Assuming the change in contrast over the film thickness interval taken to be linear, the weight of standard paint or pigment equivalent to the known weight of the sample was calculated as weight on lower standard plus difference in weight of standards times grading in tenths.
U
b s
I
equivalent weight sample weight X specific gravity sample x 100 specific gravity standard equivalent weight (pigment) Hiding power for pigment = x 100 sample weight Then hiding power for paint
=
which are purely relative values indicating by how much the sample is better or poorer than the standard. For purposes of comparison these results were converted to square centimeters per gram or square centimeters per cubic centimeter, using values of 43.7 sq. cm. per cc. and 45.8 sq. om. per gram for lithopone paint H (Tables I1 and 111) obtained by the complete-hiding dry-film test designated as No. 2 complete. Since a wide range of hiding power was covered, several standard series were required. Intercomparisons between standards were made and the results finally calculated to a single standard. This test is simple to carry out and has the very definite advantage that a permanent record may be kept. Although four observers were used for grading, this number is not necessary. A study of agreement of observers for a group of 42 graded charts showed the average deviation of the individual grading from the mean of the group to be 1.2 per cent in terms of hiding power. The most serious individual disagreement was a deviation of 6.2 per cent.
I CK
w
2 --Ln L W
8
c
z 2
30 40 50 VOLUME P E R C E N T PIGMENT-
60
DRY FILM
FIGURE2 I n Tables I1 and I11 the fifth and sixth columns show difference in hiding power of the wet film due to differences in contrast and design. We learn from the work of Kraemer and Schupp (6) that minimum perceptible contrast is greater for the checkerboard than for the narrow band type of design. In spite of this, hiding powers are greater over the latter design because of the lesser contrast to be covered up. The absolute values obtained by these tests can serve only for vertical intercomparison to indicate relative excellence. Horizontal differences between data show differences due to method of test.
TABLE11. HIDINGPOWEROF PAINTS HIDINQPO WE^
P ~ CENT R PIQMENT PAINT
A B C D E
PIQM~NT BCWL BCWL BCWL BCWL Lithopone Lithopone Lithopone Lithopone TiO: Ti01 Ti02 Ti02
(VOLUME)
Wet
Dry
33 15 .. 03 669.7 0.2 26.3 39.7 2 9 .6 20.6 36.4 69.7 32.6 49.9 F 39.8 G 28.0 H 22.1 29.7 69.8 I 28.8 49.8 26.6 J 24.0 39.8 K 29.7 L 20.3 M Ti01 16.2 21.6 N TiOn 11.6 14.3 Black and white cheokerboard, wet-film complete hiding. b Qray and white chart wet-film complete hiding Qray and white chart: dry-film complete hiding:
1"
WET BILM
DRY FILM
2b
NEW"
2, Completec 2,2%d T18T Sa. c m h . Sq. cm./g. Sq. cm./g. Sp. cm. /Q. Sq. cm./g. 43.2 43.9 74.6 48.8 34 70 .. 67 42.0 33.9 66.6 36.8 34.6 36.6 36.6 64.6 37.1 29.7 29.4 29.0 67.4 29.9 63.0 61.6 66.2 109.0 60.1 61.3 60.4 62.6 107.7 60.1 42.7 63.0 60.6 103.1 48.6 39.6 45.9 43.7 96.6 43.7 96.2 115.8 110.2 273.6 142.3 87.6 106.8 88.6 183.1 91.6 82.9 102.1 70.9 158.5 79.0 82.6 99.1 74.5 179.9 56.9 63.8 86.6 77.3 186.5 88.8 60.1 78.3 66.3 168.3 77.6 d Qray and white chart dry film 2 per cent contrast. ' Qra and white chart: dry film graded at incomplete hiding and oonverted to complete hi&ng value. I
March 15, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
While the two wet-film tests tend to give generally parallel results, these results are quite different from the results by dry-film tests which also give parallel results among themselves. I n Figures 1 to 6 hiding power of paint or pigment is shown as a function of volume concentration of paint in the film. Here are plotted all data over the gray and white diamond background. For simplification in plotting, the actual values obtained at 2 per cent contrast have in each case been divided by 2. Figures 1 to 3 show paint hiding powers. Significant differences between wet- and dry-film hiding powers are noted, The trends of changes in hiding power as pigment concentration increases are different. The dry film indicates a point of inflection at intermediate pigment concentration which is entirely missed in the wet film. In the case of titanium dioxide (Figure 3) this inflection is so extreme that the hiding power of the paint actually is impaired by the addition of pigment over a portion of the range. Although it is possible that a case of this sort is unusual, nevertheless it represents a situation which requires the use of a dry-film hiding-power test. A wet-film test would be an unsafe guide in paint formulation.
140
cl NY
5
120
I p: LI1
2 100
0
u
z
P
*b-
BO
f p4 60
eo 30 40 V O L U M E P E R C E N T PIGMENT
-
50
60
D R Y FILM
FIGURE 3 The data are even more striking in Figures 4 to 6, showing pigment hiding powers. Here the wet-film hiding powers are plotted against actual concentration in the film tested and also as a dotted line against concentration in the wet film. The latter relation is shown for easy comparison of hiding powers obtained by wet- and dry-film tests on the same paint. It is obvious that there can be no direct relation between wetand dry-film hiding power of the pigment if hiding power is a function of concentration of pigment in the film, unless the paint contains no volatile thinner. The data clearly show hiding power of the pigment to be a function of the concentration. Since the presence of volatile in the wet film has pronounced effect on concentra-
LO
115
I
I
I
30
40
50
60
VOLUME P E R C E N T PlGMENT
FIGURE 4 tion of pigment in the film, we expect and find no definite relation between wet- and dry-film hiding powers of the same paint. Because of low specific gravity, volatile thinner occupies considerably more volume than its weight per cent in the paint appears to indicate. Pigment hiding power increases as concentration of the pigment decreases from a moderately high value, and represents an increase of efficiency of utilization of the pigment that is probably primarily due to increased separation of particles and consequent improved dispersion. At comparable concentration pigment hiding power is lower in a wet film than in a dry film in the cases presented here. The lower refractive index of a vehicle containing volatile probably is insufficient to make up for poorer dispersion. One very important use made of hiding-power data a t the present time is for the comparison of pigments, for which ratios of hiding powers rather than single absolute values are of primary significance, Since pigment hiding power is dependent upon concentration of pigment in the paint film, it follows that no single vaIue can be assigned to a pigment without a recognition of the limitations of its meaning. This is particularly true of the high hiding pigments such as titanium dioxide. Several reports on hiding power investigations in the literature have shown titanium dioxide to have about twice the hiding power of lithopone. In the group of paints here reported titanium dioxide shows from 1.7 to 4.9 times the hiding power of lithopone and 3.5 to 11.7 times the hiding power of white lead. This is a wide range, but the upper value is by no means a limiting one. The two to one relation frequently reported is based upon a comparison of paints of equal mobility in which the binder-volatile ratio is constant. Such paints are grossly different in commercial value and do not represent a practical basis for comparison. A more practical comparison S o b tained in paints of equal hiding power, where tests will in general show titanium dioxide to have a t least four times the hiding power of lithopone.
TABLE111. HIDINQPOWER OF PIGMENTS PAINT
PIGMENT
PERCENTPIGMENT (VOLUME) Wet Dry
HIDINGPOWER1
WET FILM
Sq. cm./o.
A
B
C D E F G H I J K L
M N
BCWL BCWL BCWL BCWL Lithopone Lithopone Lithopone Lithopone. Ti02 Ti02 TiOn Ti02 TiOr Ti02
35.3 31.0 26.3 20.5 35.4 32.6 28.0 22.1 28.8 26.5 24.0 20.3 16.2 11.5
59.7 50.2 39.7 29.6 59.7 49.9 30.8 29.7 59.8 49.8 39.8 29.7 21.6 14.3
2 Sp. cm./o.
-
2, Complete
2, 2%
NEW TEST
Sq. cm./g.
Sq. cm./o.
Sq. cm./g.
31.0 31.5 36.3 42.2 72 70 87 100 244 177 169 22s 295 352
20.5 17.6 20.9 22.0 39.2 35.5 40.3 45.8 125 88 85 111 146 173
DRY FILM
18.4 16.1 20.0 21.2 36.0 37.2 41.9 45.8 102 86 77 99 127 152
ANALYTICAL EDITION
116
E:
W
N\
5 I K
w
3
?L W
z
e
I Iw
-0z. cl
20
30 40 50 VOLUME P E R C E N T PIGMENT
60
FIGURE 5
The spreading rate a t 2 per cent contrast is very nearly twice the absolute hiding power a t complete hiding as determined in this group of tests. This value, however, is probably closer to the practical value of the paint than is the complete hiding value, because under practical conditions paint is rarely required to cover up a background so easily seen through the film. Kraemer and Schupp (6) find that the least perceptible contrast increases considerably when the Contrast line is not sharp and may reach a value of nearly 2 per cent under the conditions they studied. For comparing paints or pigments it should be desirable to approximate the least perceptible contrast under practical rather than extreme conditions. Although the hiding powers reported have been determined a t one-coat thickness by the new incomplete hiding method described, the method has also been employed for comparing paints a t two or more coats and UT) to complete hiding over
Vol. 6, No. 2
actually to see differences in paints over the d e s i g n used at, contrasts beyond complete hiding for less rigorous b a c kgrounds. This is, in principle, similar to a method described by Haslam (6) except that v i s i b i l i t y beyond “ c o m p l e t e ” hiding is secured by use of a more easily 20 30 40 50 60 distinguished backVOLUME P E R C E N T PIGMENT ground rather than FIGURE 6 by alteration of the color t e m p e r a t u r e characteristics of the source of illumination. ACKNOWLEDGMENT The writer acknowledges the criticisms and contributions of J. E. Booge, who suggested the study, and M. L. Hanahan, and the assistance of J. C. Knochel, R. S. Emslie, and J. W. Dunn in making the measurements for this paper. LITERATURE CITED (I) Bruce, Bur. Standards, Tech. Paper 306 (1926). (2) Gardner, Sward, and Levy, Am. Paint Varnish Mfrs.’ Assoc., Soi. Sect., Circ. 362 (1930). (3) Hallett, PTOC. Am. Soc. Testing Materials, 30,S95 (1930). (4) Haslam, IND. ENO. CHEM., Anal. Ed., 2, 69 (1930). (5’1 Ibid.. 2. 319 (1930). (6) Kraemer and‘schupp, “Determination of Hiding Power of White Paints,” presented before the 85th Meeting of the American Chemical Society, Washington, D. C., March 26 to 31, 1933. ),( Pfund, J. Franklin Inst., 188,675 (1919). (8) a d . . 196,69 (1923).
Cornparative Clarification of Sugar Solutions Copper Reduction and Iodine Titration Methods L. G. SAYWELL AND E. P. PHILLIPS Fruit Products Laboratory, College of Agriculture, University of California, Berkeley, Calif.
T
H E recent use of the iodine reduction method in the analysis of honey (8) and in plant materials (IO,12) raises the question of the effect of various clarification and clearing methods on the determination. The use of the iodine method for dextrose broadens the problem of clearing solutions for sugar analysis. Many studies have been reported on the various methods of clarification and subsequent clearing. Englis and Tsang (4) reported that with clarification with basic lead acetate the deleading with disodium phosphate appears to be most satisfactory. The work of Loomis ( 7 ) confirms that of Bryan (8) showing that the use of basic lead acetate results in the loss of some of the reducing sugars, probably through the formation of a less soluble lead oxide-sugar combination. This loss did not result with the use of neutral lead acetate. On the basis of the work of Sawyer (ii), Eynon and Lane (5),and Mead and Harris (9), Loomis (7) used potassium oxalate as the deleading agent and found it convenient and satisfactory. Sodium oxalate instead of potassium oxalate may be preferable because of its lower solubility. Sodium
oxalate has been tentatively adopted by the Association of Official Agricultural Chemists (1, 15). Inasmuch as Englis and Tsang (6)studied the effect of the various deleading agents with basic lead acetate only, and Loomis (‘7) in comparing basic and neutral lead acetates used potassium oxalate only, it appeared desirable to compare the use of disodium phosphate, potassium oxalate, and sodium oxalate with both the basic and neutral acetates. The previous studies have employed only copper reduction methods of estimating the reducing sugars. Consequently a comparison was made of these several factors with both the copper reduction and iodine titration under the same laboratory conditions.
EXPERIMENTAL All determinations were made on a standard invert sugar solution.
