flectance of pittment in the infrared region. Of course, if the coated surface may not conveniently be heat cured or subjected to solvent extraction one of the other techniques for pittmentation should be considered. Another interesting possibility that capitalizes on the unsealed porosity of solvent-extracted pittment is the slow release by volatilization of insecticides, disinfectants, deodorants, and other volatile pharmaceuticals (Seiner, 1972a-c). The extracted film will absorb 224-270 g of liquid chemical/l0 m2 of surface us. surface retention of 9-15 g by a nonabsorbent coating. Many of the liquids that one might wish to use in charging the microvoids of the coating are somewhat active as solvents for polymers. It is therefore of value to utilize the solvent resistance of the cross-linked matrix described above to contain such chemicals during their release to the immediate environment. In tests that utilized common insect repellents, 2ethyl-1,3-hexanediol and dimethyl phthalate, 90% of the liquid was released within a period of 18-28 days under laboratory temperature and humidity conditions whereas the non-pittmentized control totally released the same insect repellent in less than 3 days.
Although weight loss measurements of specimens, by which the volatilization was followed, show that coating opacity (lost upon charging the microvoids) is regained well before the chemical charge is depleted, color change of the coating could serve as notification of the need for imminent recharging of the surface. Literature Cited Dowbenko, R., McBane, B. N.,U.S. Patent 3,544.589 (1970). Judd, D. B., "Color in, Business, Science and Industry," Wiley, New York, N. Y., 1952. Judd, D. B., Wyszecki, G . , "Color in Business, Science and Industry," 2nd ed, Wiley. New York, N. Y., 1963. Seiner, J. A., U.S. Patent 3,654,193 (1972a). Seiner, J. A., U. S. Patent3,655,129 (1972b). Seiner, J. A., U.S. Patent 3,655,191 ( 1 9 7 2 ~ ) . Seiner, J. A . , Gerhart, H. L., Ind. Eng. C h e m . , Prod. Res. Develop.. 12, 98 (1973). Stieg, F. B., Jr., Paint Varn. Prod., 60, 23 (1970).
Received for review August 27, 1973 Accepted N o v e m b e r 29, 1973 Presented a t t h e D i v i s i o n of Organic Coatings a n d Plastics C h e m istry, 166th N a t i o n a l M e e t i n g of t h e A m e r i c a n C h e m i c a l Society, Chicago, Ill., A u g 1973.
Hiding Power of Microvoids in Polymer Coatings Percy E. Pierce," Simon Babil, and John Blasko PPG Industries, lnc., Coatings & Resins Division, Research & Development Center, Springdale, Pennsylvania 75 744
The Kubelka-Munk scattering coefficients and hiding power of microvoid pigmented coatings are experimentally determined. The hiding power of microvoids is compared to that of other white pigments including titanium dioxide. The microvoid scattering coefficient depends on concentration as in other white pigments. The utility of microvoids as an opacifier appears to depend on its light-scattering power, its dilution effect on TiOz, a synergistic enhancement of Ti02 and microvoid scattering, and its zero absorption coefficient. The increased whiteness of microvoid pigmented films is due to their zero light absorption rather than a high scattering power of microvoids. Microvoids potentially give the most hiding power per unit cost of any white pigment, although the most economical applications seem to require a mixture of Ti02 and microvoids.
Introduction
Experimental Section
Coatings pigmented with microvoids have been reported to give whiter, more reflective films (Seiner and Gerhart, 1972). Cost savings by replacing high-cost hiding pigment are also claimed. Coatings chemists need to know the hiding power of microvoids to develop useful coatings. Theoretical calculations of the optical behavior of microvoid coatings from the optical properties of a single microvoid suggest that the scattering power of microvoids is less than that of titanium dioxide (Seiner and Gerhart, 1972; Ross, 1971; Allen, 1972). Ross calculated a KubelkaMunk scattering coefficient of 10.2 dry milk1 PVC-I for microvoids. We experimentally determine the hiding power of coatings containing microvoids alone and in combination with Ti02 using the ASTM test method D2805-70 for hiding power of paints. The recovered Kubelka-Munk scattering coefficients and hiding power of microvoids are compared to similar data determined for other white pigments (Stieg, 1972; Mitton, et al., 1961).
The reflectances of microvoid-containing coatings were measured on black and white Carrara glass and/or Morest hiding charts. A General Electric recording reflectometer was used to measure the reflectances. For hiding power measurements the 560-nm results are used. Three systems were measured. The first two contained only microvoids. The third, an acrylic latex system, contained Ti02 and microvoids in varying proportions. The general methods of preparing these microvoid coatings were described by Seiner and Gerhart (1972). The first system based on a poly(fluorocarbon) polymer was designed to coat the interior surface of integrating spheres for spectrophotometers (Seiner, 1970). We shall refer to this coating as the sphere coating. The microvoids in the second system are prepared by a solvent precipitation process which occurs during the curing of the coating. In the third system, the latex system, the microvoids are formed by entrapment of emulsified nonsolvent in the coalescing latex film. As previously reported (Seiner and Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 1, 1974
37
Gerhart, 1972), this method may be used to produce interior and exterior wall paints by an appropriate selection of materials. The sphere and solvent precipitation process coatings were cured on black and white panels of Carrara glass. Sets of varying film thicknesses were made. The latex paints were drawn over black and white Morest charts at a wet film thickness of 3 mils. They were air-dried for 3 days before measurement. The dry film thicknesses of the various coatings were measured with a Gould 150 Surfanalyzer. At least three measurements on each film were taken and averaged. The volumes of microvoids in the films were calculated by comparing the densities of the void-containing films to the density of control films without microvoids. The microvoid volume fraction is
where d, and d, are the densities of the control and voidcontaining films, respectively. The densities were determined by weighing a definite area of measured thickness. Theory The Kubelka-Munk theory (Judd and Wyszecki, 1963) assumes a uniform paint film through which are traveling two diffuse light fluxes, one proceeding downward and one upward through the film. The average effect of the pigment particles rather than the individual scattering effects is considered. The fluxes are influenced by absorption and scattering. If the absorption and scattering coefficients of the film are known, then the reflectance of a film of specified thickness over a substrate of known reflectance can be calculated. Conversely, if the reflectance of a film of known thickness is measured over two substrates, usually white and black of known reflectance, the absorption and scattering coefficients of the film can be determined. This latter procedure was used to obtain the optical characteristics of the microvoids. A computer program was used to analyze the reflectance data. The program is based on the Kubelka-Munk equations in the form discussed by Judd and Wyszecki (1963). The procedures suggested in ASTM method D2805 were used to determine reflectivity, R,. The computer analysis gave a reflectivity value, R m ,equal to unity. This means that the film absorption coefficient is zero at 560 nm for the three coating systems examined. This is the result of using a nonabsorbing (white) vehicle as well as of the fact that Ti02 is also a pure scatterer at this wavelength. The microvoids themselves are not expected to absorb light and showed no evidence of light absorption at any wavelength examined. The Kubelka-Munk equations simplify under the condition of zero light absorption. The equation relating reflectance of a paint film, R, to the scattering coefficient of the film, S, is =
S X ( 1 - R,) S X ( 1 - R,)
+
+
R, 1
(2)
where X is the film thickness and R, is the reflectance of the substrate. The equation can be rearranged and solved for the scattering coefficient S
R - R, S = X ( l - R,)(1 - R )
(3)
The film scattering coefficient is assumed to be related to the pigment scattering coefficient, S,, by the equation
s = &Sp 38
(4)
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No, 1, 1974
A 10
t
Figure 1. A plot of RB/X us. (1 - R,) where R B is the reflectance over a black substrate and X is the film thickness in mils to determine the film scattering coefficient of the sphere coating. The film scattering coefficient is 3.0 milk1. The microvoid volume fraction in the film is 0.55
where &, is the volume fraction of pigment in the film. If several scattering pigments are present, the assumption is generally made that
s = SI@,+
S& (5) where S1 and Sz are the scattering coefficients of pigments 1 and 2 and their volume fractions are $1 and 4 2 , respectively. These relationships are only approximate and lead to scattering coefficients that vary with pigment concentration. Film scattering coefficients in this paper have units dry milk1 and pigment scattering coefficients dry milk1 PVC -1. Coatings Containing Microvoids as the Sole Opacifying Pigment The reflectance data for the sphere coating on black Carrara glass were analyzed by plotting the reflectance divided by film thickness, RBIX, us. (1 - R B ) .The data should fall on a straight line passing through the origin with slope equal to the film scattering coefficient. The value of the film scattering coefficient recovered from the data shown in Figure 1 is 3.0 milk1. Density measurements on the films gave an average volume fraction of microvoids of 0.55. The microvoid scattering coefficient is 5.5 mil-I PVC -'. To obtain complete hiding as conventionally defined, i.e., a 98% contrast ratio measured over black and a substrate of 80% reflectance, a dry film thickness of at least 4 mils is necessary. In practice a 12-mil film is specified for this material to ensure complete hiding and to provide a safety factor to ensure a high reflectivity. It is interesting to note that the whiteness or high reflectivity of this coating is due to the zero absorbance of the microvoids and low absorbance of the vehicle in addition to microvoid scattering power. The second system examined was a microvoid coating obtained by a solvent precipitation process. Analysis of the reflectance data over white and black substrates also showed a high reflectivity of the order of 100%. Thus the absorption coefficient of this system was zero within experimental error. The reflectance data over black divided by the film thickness, R B I X , were plotted us. (1 - R B ) to obtain the film scattering coefficient. These data are shown in Figure 2. The coating had a film scattering coefficient of 2.3 milk1. The microvoid volume fraction was 0.35. Thus the microvoid scattering coefficient is 6.4
Table I
I 0.8
ad a4 42
1-48 Figure 2. A plot of R B / X us. (1 - R g ) where RHis the reflectance over a black substrate and X is the film thickness in mils to determine the film scattering coefficient for a microvoid coating prepared by the solvent precipitation process. The film scattering coefficient is 2.3 mil-I. The microvoid volume fraction in the film is 0.35
milk1 PVC-l a t 560 nm. T o obtain hiding a 5-mil coating is required.
Latex Coating Containing Ti02 and Microvoids A series of latex coatings pigmented with various levels of titanium dioxide and microvoids was prepared. The coatings were drawn down on Morest charts and the scattering coefficient was determined from reflectance measurements on the white and black portions of the chart. The measured scattering coefficients are listed in Table I as a function of the volume fraction concentration of titanium dioxide and microvoids. The volume fractions of microvoids and Ti02 are based on the theoretical volume solids of the compositions. A regression analysis was made on the data to determine the contribution of the titanium dioxide and microvoids to the scattering coefficients. The data had a strong concentration dependence. The following equation was found to fit the results S = 53.78d1 13.5342 - 79.9641’ -
+
+
23.18~~5~~31.874,G2 (6) In the absence of microvoids the data suggest a Ti02 scattering coefficient that depends on concentration of the form
S,= 53.78
-
79.964,
(7)
At a volume fraction of 0.20 this equation gives a scattering coefficient of 37.8 mil-1 PVC-l for Ti02 alone. The average value of the scattering coefficient over the range of 0.0-0.30 volume fraction of T i 0 2 is 41.8 milk1 PVC-I. These values are close to those typically given for Ti02 (Stieg, 1972; Mitton, et al., 1961). The scattering coefficient for microvoids alone is given by
S=
13.53
- 23.184,
(8)
At a volume fraction of microvoids of 0.30 the microvoid scattering coefficient is 6.68 milk1 PVC -I. The average value of the microvoid scattering coefficient over the concentration range 0-0.50 volume fraction is 7.74 milk1 PVC-I. The microvoid scattering coefficient in this system is slightly greater than the microvoid scattering coefficient obtained with the sphere and solvent precipitation coatings in the previous section. The concentration dependence of Kubelka-Munk scattering coefficients is well known for Ti02 and other white pigments (Mitton, et al., 1961). Thus the concentration
Scattering coeff, dry mil -
Ti02 vol fraction
Microvoid vol fraction
1.64 1.80 4.48 5.29 8.36 8.78 9.07 9.09 10.7 12.2
0 0 0.120 0.0690 0.215 0 ,2910 0.0725 0.354 0.153 0.248
0.430 0.345 0 .oo 0.424 0 .oo 0 .oo 0.529 0 .oo 0.291 0.297
dependence of microvoid scattering coefficients is not unexpected. In the range of concentrations typically used in coatings the scattering power of the T i 0 2 is 5-6 times greater than that of microvoids. This ratio is somewhat smaller than Ross’s theoretical analysis which did not include the effect of concentration but is in accord with Ross’s general conclusion that microvoid scattering must be considerably less than that of TiOz. The measured microvoid scattering coefficients in general agreement with Ross’s estimate except for their concentration dependence. The cross term in eq 6 which depends on the product of concentrations of the Ti02 and microvoids is positive indicating that the combination of Ti02 and microvoids give a slightly enhanced scattering. This effect contributes an increase in scattering of about 17% over the separate contribution of microvoids and Ti02 a t pigment concentrations of 0.30 and 0.20, respectively. A further beneficial effect of microvoids is their dilution effect on the TiOz. Because a given opacity can be obtained a t a lower volume fraction of TiOz, the Ti02 is used more efficiently. This is due to the fact that the scattering coefficient of Ti02 increases as the volume fraction of Ti02 in the dry film decreases. The four effects-(a) scattering power of microvoids, (b) dilution of TiOz, (c) synergistic enhancement of TiOzmicrovoid mixture scattering, and (d) the zero microvoid absorption coefficient of all wavelengths-combine together to produce the optical benefits observed when microvoids are used together with TiOz.
Comparison of TiOz, Microvoids, and Other White . Pigments The preceding analysis suggests that microvoids have a practical scattering coefficient in the range 5-7 mi1-I PVC-l over concentration levels that might be used in practice. If we select a value of 6 mil-I PVC-l as typical, we can compare the hiding power of microvoids to other white pigments. Stieg has tabulated some comparisons for selected pigments. We shall compare microvoids against his data. In Table I1 we list the hiding power in ft2/gal and scattering coefficients for a number of white pigments. The highest hiding pigment when compared on a volumetric basis is rutile titanium dioxide. Anatase Ti02 and the extended rutile calcium bases follow with zinc sulfide intermediate in hiding between the 50 and 30% rutile calcium bases. The remaining white pigments are grouped rather closely together. Although microvoids lie a t the bottom of the table, the volumetric scattering coefficient is quite comparable to zinc oxide and lithopone. Here the volumetric superiority of Ti02 as an opacifying pigment is clearly evident. For coatings applied a t low film thickness (many industrial products) or in applications requiring a minimum pigment loading (gloss enamels), Ti02 would be the pigment of choice. Ind. Eng. Chert., Prod. Res. Develop., Vol. 13, No. 1. 1974
39
Table I1
Table IV
Pigment
Hiding power, ft2/gal
S, dry mil - 1 PVC - 1
Rutile Ti02 Anatase Ti02 50% rutile calcium base Zinc sulfide 30% rutile calcium base Antimony oxide Basic carbonate white lead Lithopone Zinc oxide Microvoids
5140 3740 2380 1970 1540 1040 1000 970 940 820
38 27 17 14 11 7.6 7.3 7.1 6.8 6
Table 111
Pigment
Hiding power, ft2/lb
Rutile Ti02 Microvoids Anatase TiOl 50% rutile calcium base Zinc sulfide 30% rutile calcium base Lithopone (29% ZaS, 71% BaS04) Antimony oxide Zinc oxide Basic carbonate white lead
147 124 115 82 58 57 27 22 20 18
When viewed on a weight basis the comparison gives a different ordering. In Table I11 the white pigments are arranged in order of their power on a ft2/lb basis. Rutile Ti02 is still a t the top of the table followed closely by microvoids, which lies just above anatase TiO2. The remaining pigments have significantly lower hiding power on a weight basis. This ordering is heavily influenced by the density of the pigments. Since the microvoid precursor has a much lower density than the inorganic white pigments, the lower scattering power per unit volume is more than compensated for by its low density. In Table IV the hiding powers of white pigments are compared on the basis of ft2/$. Here the potential value of microvoids shows itself quite clearly. Microvoids give almost twice the hiding power per dollar offered by 30% rutile calcium base. This pigment was the lowest cost hiding pigment available before it was commercially discontinued. The high ranking of microvoids despite their lower volumetric scattering coefficient is a result of the low pound cost of the hydrocarbon microvoid precurser and the low
40
Ind. Eng. Cham., Prod. Res. Develop., Vol. 13, No. 1, 1974
Hiding power, ft2,{$
Pigment Microvoids 30% rutile calcium base 50% rutile calcium base Rutile T i 0 2 Anatase TiOs Lithopone Zinc sulfide Zinc oxide Basic carbonate white lead Antimony oxide
1180 626 580 524 442 277 181 105 82 41
density of the hydrocarbon. Since microvoids offer twice the hiding power per dollar of rutile TiO2, they would appear to be the most economical hiding pigment. Because the volumetric scattering power of microvoids is less than that of TiO2, thicker films are required to obtain the hiding of conventional paint systems when microvoids are used alone as the sole opacifying pigment. Except in special circumstances when the whiteness and nonabsorbing qualities of microvoids are essential, more economical coatings can be prepared by using microvoids with TiO2. This is especially the case for most interior coatings which are usually adjusted to a 90% or less reflectivity. Latex wall paints which require extender pigments are products that can be modified to achieve lower cost while maintaining adequate performance properties. Replacement of all or part of the extender pigments with microvoids permits a lowering of Ti02 level with no loss in hiding power. Since microvoid hiding costs less than half the Ti02 hiding, a net cost saving is obtained. The vehicle required is, of course, not changed.
Literature Cited Allen, E., paper presented at the 50th Federation of Societies for Paint Technology Meeting, Atlantic City, N. J., Oct 1972. Judd, D. B., Wyszecki, G., "Color in Business, Science, and Industry," 2nd ed, Wiley, New York, N. Y . , 1963. Mitton. P. B., Vejnoska. L. W., Frederick, M., Off. Dig., Fed. SOC.Paint Techno/., 33, No. 441, 1264 (1961). Ross, W. D., J. Paint Techno/., 43, No. 563, 50 (1971). Seiner, J. A., Canadian Patent 847,936 (July 28, 1970). Seiner, J. A.. Gerhart, H. L., FATlPEC Congr., 11, 127 (1972). Stieg, F. B.. J. Paint Techno/., 44, No. 565, 63 (1972).
Received for reuieu August 27, 1973 Accepted November29, 1973
Presented a t the Division of Organic Coatings and Plastics Chemistry, 166th National Meeting of the American Chemical Society, Chicago, Ill., Aug 1973.
