R o s c o e
H.
S a w y e r ,Krebs Pigm ent and Color Corporation, Newark, N . J.
T
H E hiding power of a paint m ay be determ ined in a num ber of ways (1-5, 7).T ests em ploying either wet or d ry films are in common use, and definite advantages and disad
v a n t a g e s a r e attrib u tab le to each.
Wet-film t e s t s h a v e b e e n favored as m ost easily adaptable to securing results a t so-called com plete hiding. R esults are obtained as absolute values, and use of th e w et pain t perm its speed and fair reproducibility in testing. As commonly carried out, however, th e application of the results to th e practical use of th e p ain t is questionable. P ain t is applied a t several tim es the norm al one-coat thickness over backgrounds of rigorous and ex
trem e contrast, until these back
grounds are visually completely obliterated under the most exacting conditions of observation. Perhaps th e m ost serious objection is th a t th e w et pain t film in te st is chemically and physically quite different from the dry film which actually represents th e pain t in use.
R esults a t com plete hiding can also be obtained w ith the d ry film, b u t m ethods which have been available have been so difficult and time-consuming as to secure little recognition.
P aints are m ore commonly com pared a t one or two norm al coats over a contrast background for purely relative values.
T he advantage of this ty p e of te st is th a t differences between paints m ay be m ade directly visible. T he te st is carried out under conditions much more nearly com parable with practice th a n is a wet-film te s t in respect to such factors as condition and thickness of film, pigm ent concentration in th e film, and brightness of th e paint. T he effect of brightness of th e paint on hiding power is pronounced (2, 8) and paints undergo various and, a t tim es, considerable changes in brightness in drying.
Ex p e r i m e n t a l
W e t - F i l m C o m p l e t e H i d i n g . T w o wet-film brush-out te sts were employed which differed only in background used. Background 1 was a checkerboard of 2.54-cm. black and w hite squares printed on glossy photographic paper 27.9
X 33 cm. T his size was employed to m ake th e te st as nearly as possible com parable w ith th e other complete hiding tests.
Background 2 wras a diam ond of 1.4-cm. alternate light and d ark bands printed on heavy paper and m achine-coated with nitrocellulose lacquer. T he design had external dimensions of 22.9 X 34 cm. and wras surrounded by a clear 1.4-cm.
border to m ake a to ta l painting area of 929 sq. cm. B rig h t
nesses of th e areas used were:
Ba c k g r o u n d
1 2
L ight 80 75
D a rk 5 40
P ain t was applied by brush in each case until the design was no longer visible under a 75-w att daylight bulb in a white conical reflector 3 feet (90 cm.) above the te st piece. Observation was a t about 45 ° and from a distance of 3 to 4 feet. H iding power was calculated in square centim eters per cubic centim eter from the w eight of pain t required to ob
literate th e contrast (determ ined by loss in weight of can and brush c o m b i n a t i o n ) and in square c e n t i m e t e r s per gram of pig
m ent.
T he dry-film tests were carried o u t on design 2, described above.
Three types of te st were used:
com plete hiding, spreading ra te a t 2 per cent contrast, and a new m ethod for hiding-power com
parisons a t incomplete hiding.
D r y - F i l m C o m p l e t e H i d i n g a n d 2 P e r C e n t C o n t r a s t .
Successive coats of each paint were applied to several charts to give increasing weight of paint until a point wras reached a t which contrast was no longer visible under th e conditions of observation employed for th e wet-film tests. T he brush- outs were graded by a W eston photronic cell using ap p aratu s essentially as described b y K raem er and Schupp (6). Spread
ing rates a t 2 per cent contrast were read from th e curves relating contrast and film thickness for these charts.
D r t - F i l m I n c o m p l e t e H i d i n g . P ain ts were applied to th e charts a t controlled spreading rates ranging from 150 to 450 sq. cm. per cc. P a in t was tak en up in a syringe and weighed in th e syringe with a brush w et w ith th e paint.
The p ain t was spread over th e design w ith th e syringe and then brushed out in a standard m anner. W eight of p ain t applied was determ ined as th e loss of w eight of th e syringe and brush com bination. Several charts were m ade of each paint. F or some of the paints series of brushouts were m ade showing more or less uniform steps of increasing p ain t weight on th e charts for use as arbitrary grading standards.
Intervals of 0.5 to 1.0 cc. are m ost convenient.
Gradings were carried o u t on a board illum inated by four 200-w att daylight bulbs in industrial reflectors and m ounted about 3 m eters from th e board. Illum ination was vertical to the charts and uniform over th e board a t about 16 foot- candles. Gradings were by four observers seated below th e lam ps in th e shadow and viewing th e charts a t eye level and a t about 30° from th e perpendicular to th e charts.
In grading, th e brushouts from one p ain t were num bered consecutively and used as arb itra ry standards. A brushout from another paint was placed a t th e center of th e grading board. Consecutive brushouts of th e stan d ard were found, one of which showed more contrast th a n th e sample and th e other less. T he position of th e sample in th e contrast interval was then estim ated in te n th s of th e contrast inter
val. Gradings m ade with the higher sta n d ard a t th e right H id in g pow er o f p a in l or p ig m e n t is show n
to be a fu n c tio n o f concentration o f the p ig m en t in the film . A com parison o f wet- a n d dry-film tests shows the two types to y ie ld sig n ific a n tly d ifferent relations between h id in g pow er a n d p ig m en t concentration, w hich are show n to be, in p a rt, due to actual differences in concentration o f p ig m en t in wet a n d d ry fd m s o f the sam e p a in t. T he author concludes that it is unsafe, in general, to em ploy a w et-film lest.
A new dry-film lest is described which gives hid in g powers paralleling those obtained at either 2 p er cent contrast or complete h id in g in the d ry film . T he lest is easily carried out, has good reproducibility, a n d m a y be used fo r com parisons a t a n y contrasts or film thicknesses to beyond practical complete h id in g to shew rela
tive values o f p a in ts or p ig m en ts.
113
114 A N A L Y T I C A L E D I T I O N YoL 6, No. 2 plue difference in weight of standards tim es grading in tenthE.
At. 1.6
T hree brushouts were m ade of each p a in t. D eviation was from th e m ean of th ree brushouts in each case.
Pahsts. T he p ain ts used "were single pigm ent p ain ts nning a kettle-bodied linseed oil-CM na wood oil combination vehicle. T he pigm ents were titan iu m dioxide, lithopone H I} obtained b y th e complete-hiding dry -fim te st designated as No. 2 complete.
Since a wide range of hiding pow er was covered, several standard series were required. Intercom parisons between standards were m ade and th e results finally calculated to a single standard.
T his te s t is simple to carry o u t and has th e very definite advantage th a t a perm anent record m ay be k ept.
Although four observers were used for g rad in g th is num ber is n o t necessary. A stu d y of agreem ent of observers for a group of 42 graded charts showed th e average deviation of th e individual grading from th e m ean of th e group to be 1 2 per cent in term s of hiding power. T he m ost serious indi
vidual disagreem ent was a deviation of 6.2 p er cent.
F i g o t e 2 vertical intercom parison to indicate relative excellence.
H orizontal differences betw een d ata show differences due to
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 115 While th e two wet-film tests tend to give generally par
allel results, these results are quite different from the results by dry-film te sts which also give parallel results among them selves. In Figures 1 to 6 hiding power of paint or pig
m ent is shown as a function of volume concentration of p ain t in th e film. H ere are plotted all d ata over th e gray and vrhite diamond background. F or simplification in plotting, th e actual values obtained a t 2 per cent contrast have in each case been divided b y 2 .
Figures 1 to 3 show pain t hiding powers. Significant differences between w et- and dry-film hiding powers are noted. T he trends of changes in hiding power as pigm ent concentration increases are different. T he d ry film indicates a point of inflection a t interm ediate pigm ent concentration which is entirely missed in th e w et film. In th e case of titanium dioxide (Figure 3) this inflection is so extreme th a t th e hiding power of th e p ain t actually is im paired by the addition of pigm ent over a portion of th e range. Although it is possible th a t a case of this sort is unusual, nevertheless it represents a situation which requires th e use of a dry-film hiding-power te st. A wet-film te s t would be an unsafe guide in p ain t form ulation.
VOLUME P E R C E N T P I G M E N T — D R V F IL M Fi g u r e 3
T he d a ta are even more striking in Figures 4 to 6, showing pigm ent hiding powers. H ere th e wet-film hiding powers are p lotted against actual concentration in th e film tested and also as a d o tte d line against concentration in th e wet film.
T he la tte r relation is shown for easy comparison of hiding powers obtained b y w et- and dry-film te sts on th e same paint.
I t is obvious th a t th e re can be no direct relation between wet- and dry-film hiding power of th e pigm ent if hiding power is a function of concentration of pigm ent in th e film, unless th e pain t contains no volatile thinner.
T he d a ta clearly show hiding power of th e pigm ent to be a function of th e concentration. Since th e presence of volatile in th e w et film has pronounced effect on
concentra-30 40 50 60
V O L U M E P E R C E N T P I G M E N T Fi g u r e 4
tion of pigm ent in th e film, we expect and find no definite relation between w et- and dry-film hiding powers of the same paint. Because of low specific gravity, volatile t hinner occupies considerably more volum e th a n its w eight p er cent in th e p aint appears to indicate.
Pigm ent hiding power increases as concentration of th e pigm ent decreases from a m oderately high value, and repre
sents an increase of efficiency of utilization of th e pigm ent th a t is probably prim arily due to increased separation of particles and consequent im proved dispersion. A t com
parable concentration pigm ent hiding power is lower in a w et film th a n in a d ry film in th e cases presented here. T he lower refractive index of a vehicle containing volatile prob
ably is insufficient to m ake up for poorer dispersion.
One v ery im p o rtan t use m ade of hiding-power d a ta a t th e present tim e is for th e comparison of pigments, for which ratios of hiding powers ra th e r th a n single absolute values are of prim ary significance. Since pigm ent hiding power is dependent upon concentration of pigm ent in th e p ain t film, it follows th a t no single value can be assigned to a pigm ent w ithout a recognition of th e lim itations of its meaning.
This is particularly tru e of th e high hiding pigm ents such as titan iu m dioxide. Several reports on hiding power investi
gations in th e literatu re have shown titanium dioxide to have about twice th e hiding power of lithopone.
I n th e group of paints here reported titan iu m dioxide shows from 1.7 to 4.9 tim es th e hiding power of lithopone and 3.5 to 11.7 tim es th e hiding power of w hite lead. This is a wide range, b u t th e u pper value is b y no m eans a lim iting one. T he two to one relation frequently reported is based upon a comparison of paints of equal m obility in which th e binder-volatile ratio is constant. Such paints are grossly different in commercial value and do n o t represent a practical basis for comparison. A m ore practical com parison is ob
tained in paints of equal hiding power, w here te sts will in general show titan iu m dioxide to have a t least four tim es th e hiding power of lithopone.
Ta b l e I I I . Hi d in g Po w e ro f Pi g m e n t s
Pa j s t
A B C D EF G H I J K LM N
PlOHENT B C W L BCTPTL B C W L B C W L L ithopone l ith o p o n e L ithopone lith o p o n e T iO , TiO i TiO , TiO . TiO .
■no.
Pe r Ce n t Pigm ent — Hid in g Po w e b---
-f Vo lum e) WET FILM DBT FILM
2. 2 %
NEW
W et D ry 1 2 2 , C om plete TEST
S 3, cm ./g . Sq. cm./Q. Sq. c m ./g . S 3. e m ,/g . Sq. c m ./g .
3 5 .3 5 9 .7 17.1 18.1 18.4 3 1 .0 2 0 .5
3 1 .0 50 .2 17 .8 20.0 16.1 3 1 .5 17 .6
2 6 .3 3 9 .7 1 9.5 20.6 20.0 3 6 .3 2 0 .9
2 0 .5 2 9 .6 2 1 .9 2 1 .7 2 1 .2 4 2 .2 2 2 .0
3 5 .4 5 9 .7 3 4 .4 4 0 .3 3 6 .0 72 3 9 .2
3 2 .6 4 9 .9 36 .2 4 2 .9 3 7 .2 76 3 5 .5
2 8 .0 3 9 . S 3 5 .8 4 4 .0 4 1 .9 87 4 0 .3
2 2 .1 2 9 .7 4 1 .1 4 8 .2 4 5 .8 100 4 5 .8
2 8 .8 5 9 .8 85 104 102 244 125
2 6 .5 4 9 .8 84 103 86 177 88
2 4 .0 3 9 .8 88 110 77 169 85
2 0 .3 2 9 .7 104 126 99 228 1 1 1
1 6.2 2 1 .6 104 136 127 295 146
11.5 14.3 137 177 152 352 173
116 A N A L Y T I C A L E D I T I O N Vol. 6, No. 2
V O L U M E P E R C E N T P I G M E N T Fi g u r e 5
V O L U M E P E R C E N T P I G M E N T F i g u r e 6
The spreading ra te a t 2 per cent contrast is very nearly twice th e absolute hiding power a t complete hiding as de
term ined in this group of tests. This value, however, is probably closer to th e practical value of th e paint th a n is th e complete hiding value, because under practical conditions paint is rarely required to cover up a background so easily seen through th e film. K raem er and Schupp (6) find th a t the least perceptible contrast increases considerably when the contrast line is not sharp and m ay reach a value of nearly 2 per cent under the conditions they studied. F or comparing paints or pigments it should be desirable to approxim ate the least perceptible contrast under practical rath er than extreme conditions.
Although the hiding powers reported have been determined a t one-coat thickness by the new incomplete hiding method described, th e m ethod has also been employed for comparing paints a t two or more coats and up to complete hiding over this background. I t is easy to distinguish small differences in hiding power a t contrasts between 1 and 2 per cent and even to make gradings a t contrasts below 1 per cent and thus
actually to see differ
ences in paints over the d e s ig n used a t c o n t r a s t s b e y o n d com plete hiding for less rigorous b a c k grounds. T his is, in principle, sim ilar to a m e th o d d e s c r i b e d by H aslam (5) except th a t v i s i b i l i t y be
yond “ c o m p l e t e "
hiding is secured by use of a more easily distinguished back
ground ra th e r than b y alteration of the color t e m p e r a t u r e characteristics of th e source of illumination.
Ac k n o w l e d g m e n t
T he w riter acknowledges th e criticisms and contributions of J . E . Booge, who suggested th e study, and M . L. H ana- han, and th e assistance of J . C. Knochel, R . S. Emslie, and J . W . D unn in m aking th e m easurem ents for th is paper.
Li t e r a t u r e Cit e d (1) Bruce, Bur. Standards, Tech. P aver 306 (1926).
(2) G ardner, Sward, and Levy, Am. P a in t V arnish M frs.’ Assoc.*
Sci. Sect., Circ. 362 (1930).
(3) H allett, Proc. A m . Soc. Testino Materials, 30, 895 (1930).
(4) Haslam, I n d . E n g . C h e m ., Anal. E d . , 2 , 69 (1930).
(5) Ibid., 2, 319 (1930).
(6) K raem er and Schupp, “ D eterm ination of H iding Pow er of W hite P ain ts," presented before the 85th M eeting of the American Chemical Society, W ashington, D. C., M arch 26 to 31, 1933.
(7) Pfund, J . Franklin Inst., 188, 675 (1919).
(8) Ibid., 196, 69 (1923).
Re c e i v e d Ju n e 21, 1933. P resen ted before th e D ivision of P a in t and V arnish C h em istry a t th e 8 5 th M eeting of th e A m erican C hem ical Society, W ashington, D. C ., M arch 26 to 31, 1933.