T H E GLOSS OF BAKED ALKYD UREA= FORMALDEHYDE FINISHES PIGMENTED WITH TITANIUM DIOXIDE M A R S H A L L B. A L P E R T , A R T H U R E, J A C O B S E N , ' A N D
PARKER B. M I T T O N
Technical Department, Research Laboratory, Titanium Division, National Lead Co., South Amboy, N . J . Baked alkyd urea-formaldehyde coatings pigmented with titanium dioxide can b e varied from high to low gloss. Contraction of the film on cooling produces surface irregularities above flocculated particles, with a resulting loss of gloss. In the test system gloss is increased by the formation of surface active salts from the acids of the alkyd. It i s suggested that the cation of this salt is adsorbed within the water layer present as an adsorbed film on the pigment surface. This cation confers a positive electrical charge on the pigment, while the high molecular weight anion serves as counterion. This explains the gloss increase obtained b y treatment with appropriate basic agents, particularly alumina hydrate, or by increase in relative humidity, and also the gloss decrease caused by low molecular weight acids having a strength equal to or greater than that of the alkyd acids.
urea-formaldehyde enamels pigmented with titanium dioxide can be prepared to yield baked coatings varying in appearance from essentially a matte to a high-gloss finish without change in pigment-volume concentration. The desire to understand the phenomena involved prompted this study of both the mechanical and the colloid chemical aspects of gloss development. After standard procedures for preparing, applying, and baking the finishes had been developed, consideration was given to the influence of the alkyd and urea-formaldehyde vehicles, and of the titanium dioxide pigment. Through electron microscope, hiding power, and electrophoresis studies it has been possible to develop a hypothesis relating the gloss results to vehicle interaction with the pigment causing it to cluster. This in turn is related to the nature of the ionic double layer a t the pigment surface. This hypothesis has suggested a n experimental approach emphasizing the nature and strength of the acids of the alkyd vehicle and their interaction with the pigment to yield ions that form the ionic double layer. Modified potentiometric procedures have been developed for these studies.
A
LKYD
Experimental Procedure
The conditions chosen for the preparation of the films, in particular the alkyd selected from a number of commercial vehicles and also the details of dipping and baking the panels, were designed to yield large differences in gloss between films, thereby simplifying the interpretation of the results. The alkyd selected for test purposes, except where otherwise mentioned, is a commercially available, nondrying alkyd, described by the supplier as containing 60% solids dissolved in xylene to yield a solution having a Z to 2-2 viscosity (25" C , ) , a Gardner color (1933 scale) of 2-6, and a density of 8.6 pounds per gallon. The solid resin has a minimum phthalic anhydride content of 45Yob,a minimum oil acid content of 30%, and a n acid number of 3 to 8 mg. of K O H per gram. Isolation of fractions through saponification showed the polyhydric alcohol to be glycerol and suggested that the modifying oil acid was probably derived from coconut oil. The urea-formaldehyde vehicle selected is a solution a t 50 f 2% solids in a mixture of 60% butanol and 40% xylene by weight. The Gardner color (1933) is 1 maximum, the Gardner-Holt visPresent address, 6816 Narrows Ave., Brooklyn 20, N. Y . 264
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
cosity X-Z1, and the density 8.3 pounds per gallon. The acid number of the vehicle solids is specified as 1 to 4 mg. of KOH per gram. The enamels are prepared by mixing 65 grams of pigment with 35 grams of alkyd and passing through a three-roller mill. T o a 77-gram aliquot of the paste are added 48.6 grams of additional alkyd, 25.0 grams of urea-formaldehyde vehicle, and 47.3 grams of xylene thinner. The enamels are vigorously mixed after each addition. One hour after preparation of the enamels, glass panels are coated by dipping, allowed to dry for at least 10 minutes, and then baked in a forced draft oven a t 150' C. for 30 minutes. The panels are stored overnight and their gloss is read with a 20' glossmeter. Only enamel; prepared from rutile pigments are considered here. For most of this study a t least two pigments have been employed. Pigment A is a commercial pigment, Titanox-RA, which has had a hydrous alumina (postcalcination) treatment and has excellent gloss-producing properties. Pigment B has had its hydrated alumina chemical activity somewhat impaired and in the test procedure described above yields a relatively low gloss. Two additional pigments have been employed for some of the tests to extend the available gloss range. Pigment C is a commercial pigment, Titanox-RA-IO, containing little hydrous alumina and having a n essentially unchanged rutile surface. Pigment D was prepared in the laboratory by simply dry-milling rutile calciner discharge, a process intermediate (7) that has no alumina hydrate on it. There has been some variation in gloss levels between sets of measurements a t different times; for this reason pigment A has been used as a standard of reference in each group of measurements. Mechanical Aspects of Gloss Development
Irregularities which project above the film surface are effective in scattering light and hence in decreasing the almost ideally specular reflectance of a high gloss surface. These irregularities appear to contain small groups of pigment particles. Electron micrographs of titanium dioxide-pigmented film surfaces exhibiting low and high gloss and of the diluted enamels corresponding to the films are shown in Figure 1. Electron microscopic examination of the diluted enamels has demonstrated that those yielding low gloss films showed considerable clustering of the pigment particles, while enamels yielding high gloss films showed good pigment dispersion. As confirmatory evidence that low gloss in these enamels is related largely to pigment clustering, it has been found that paints with lower gloss also have lower hiding power (Table I).
Figure 1. Upper.
Electron micrographs of titaiiium dioxide-pigment ec ^. , - ~ ~
Surface replicas of film
lower.
Diluted points
MECHANISM FOR PROJECTING PIGMENT PARTICLE GROUPS ABOVE GENERAL FILM SURFACE.When panels are removed from the baking oven they have a high gloss; if low gloss is to develop, the gloss is observed to decrease while the panel is cooling. For example, one panel showed a gloss of 71 within 20 seconds after removal from the oven, but this value fell to 46 upon cooling. Reheating the panel in the oven brought the gloss back to 71 and rhe gloss cyde was repeated upon cooling. I t appears that the decrease in gloss on cooling is due to the thermal contraction of an elastic film. A 25-micron thick film attached rigidly to a base having a relatively low coefficient of thermal expansion will contract about 0.5 micron in thickness when the film is cooled from 150' C. to room temperature. If the individual pigment particles were free to move with the shrinking elastic film, relatively high gloss would result. How-
.
r.
..
.
Proposed Mechanist
.-
-..".-..-.-.-
Since flocculation ...-... of low gloss, an explanation for the variation in the degree of flocculation must he sought. More specifically, microscopic examination has indicated that flocculation in the finished enamels studied here closely parallels that in dispersions prepared from the alkyd vehicle alone. Hence, the problem essentially centered on the question of the mechanism causing flocculation o r dispersion of the pigment in a n alkyd vehicle. CHEMICAL NATUREOF PIOMENT-ALKYD SYSTEM.An alkyd is the reaction product of glycerol, fatty acid, and phthalic anhydride. It contains some titratable acid which, from results reported in this paper, has approximately the same strength as a monoesterified phthalic acid. Commercial rutile pigments have in most cases received a treatment (7) consisting of hydrated alumina, and in some cases of other additives, to control the reactivity and wetting behavior of the pigment. Ay.6b.,
~
T o study electrical charges on pigments in I.v..~.,IyIy..~ persionsaU-tuhecell has heendesigned withadiameterofabout 1 cm. and fitted a t the bottom with a funnel and stopcock arrangement to allow formation of a sharp boundary by flowing a dispersion under the clear dispersing medium. The platinum disk electrodes are mounted in standard ground-glass joints to permit maintenance of constant cell geometry on reassembly. The electrical path distance between the electrodes, computed from the cross-sectional area of the cell and the electrical conductance of the cell when containing a liquid of known conductivity, is about 7 cm. A variable d.c. power supply, 0 to 2200 volts, allows development of convenient boundary velocities for most organic dispersions. VOL. 2
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Electrophoresis by a moving boundary procedure in this cell showed the pigment in a n alkyd urea-formaldehyde enamel to be positively charged; the mobility was of the order of 1 X sq. cm. set.-' volt-' when the paint viscosity was about 0.5 poise. More precise measurements on alkyd enamels, in this case highly diluted with xylene or benzene, show a positive zeta potential that may be estimated in the case of well dispersed pigments to be greater than 100 mv. This provided a clue to the nature of the surface chemistry of the alkyd vehicles, since T i 0 2 pigments dispersed in aromatic solutions of anionic wetting agents-e.g., sodium dioctylsulfosuccinate (Aerosol 0T)-also show a highly positive zeta potential. I n other words, alkyd dispersions resemble closely the type of dispersion produced by a n anionic wetting agent. Since it was found experimentally that pigments having little or no hydrated alumina postcalcination treatment exhibit poorer dispersion and yield low alkyd urea-formaldehyde gloss, it was clear that the alumina must be closely involved if a n anionic wetting agent type of behavior is to be obtained. Specifically the following hypothesis was developed, which appears capable of describing the additional experimental results presented in this paper. It is considered that the acids of the alkyd react to some extent with the hydrated alumina of the postcalcination treatment to form a n aluminum (or basic aluminum) salt of the alkyd acids; this salt is adsorbed a t the pigment-vehicle interface. The aluminum ion is absorbed within the adsorbed moisture layer that is already present a t the interface between pigment particle and vehicle, while the organic anion, if its molecular weight is sufficiently large so it is not tightly held by the carboxyl group to the adsorbed water layer, serves as a counterion layer within the organic phase. I n this mechanism the water layer plays an important role both in solvating the cation and in tending to reduce ion pair formation between cation and anion because of its high dielectric constant. The situation is shown diagrammatically in Figure 2. The anion is considered essentially to be a monoesterified phthalate ion where the ester radical is basically the remainder of the high molecular weight alkyd molecule. T h e anion must be free to move with the organic phase, if adequate protection is to be provided by the charge mechanism against the flocculating van der M'aals forces between pigment particles. If the anion were bound closely to the particle surface-e.g., because of adsorption a t the water surface, ion pair formation, or insoluble salt formation-it would essentially neutralize the equivalent amount of cation and would not prevent close approach and hence flocculation of pigment particles by attractive van der Waals forces. From basic electrostatic theory, a spherical shell will have an external electrostatic field identical to that produced by a point charge of the same total magnitude located a t the center of the shell. Thus specifically if the charge of the cation layer were $Q and there were a rigidly adsorbed anion counterion layer of - q, the particle would have no external electrostatic field and would not repel another such particle until both particles were practically in contact a t their adsorption films. If this occurred, there would be but a few Angstrom units separating the two particles and the particles would tend to adhere because of attractive van der Waals forces. O n the other hand, with a diffuse counterion layer extending outward a distance of several hundred Angstrom units o r more ( 4 ) the approach of two particles would distort each other's counterion layer with the development of increasingly great repulsive forces, the nearer the distance of approach. T o estimate the double layer thickness in this system, d.c. 266
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
PIGMENT PARTICLE A-
AA-
P
i
WATER
A-
A-
A-
COO-
ALKYD PHASE Figure 2.
Diagram of alkyd wetting
conductivities of a few enamels and the unpigmented vehicle were found to range from 2.0 to 2.7 X ohm-' cm.-' a t a kinetic viscosity of 1.2 stokes, equivalent to 2.4 to 3.2 X lo-' ohm-' cm.-' a t 1 cs. Although ionic conductivities are not known for the system, it is likely that "hydrogen ion" carries much of the current. The concentrat'on of dissociated salt may be estimated as -10-6 molar from aqueous values. In the present discussion, therefore, the double layer thickness is assumed to be large (Debye length -7OOA.) and not a significant variable. The development of the theory of the double layer (6) and its application to the stability of colloids in organic systems (4) have been discussed. Since the positive zeta potential and the function of alumina hydrate in obtaining good dispersion and high gloss were consistent with this picture, it was accepted as a working hypothesis; the experiments described below were intended both to confirm this hypothesis and to extend its application to the particular variables of the alkyd urea-formaldehyde finishes. Nature of Alkyd Acid
T h e hypothesis of salt formation as given above required that acids be present in the alkyd having at least sufficient strength to avoid hydrolysis of, for example, a n aluminum salt a t the film of water adsorbed a t the pigment-vehicle interface. Attention was therefore directed to the determination of the acid content of the alkyd and its relative strength. The procedure was essentially a potentiometric titration of the alkyd (10gram samples were convenient) dissolved in 100 ml. of a solution of 1-butanol and xylene (3 to 1 ratio by volume) with 10 grams per liter of lithium chloride being added to this solvent to provide the necessary conductivity. The titration with a 0.1N solution of potassium hydroxide in 3 to 1 butanol-xylene employed a commercial p H meter with glass and calomel
electrodes. A titration curve for a typical alkj-d sample is presented in Figure 3. The meter readings are reported as "pH*" because no attempt has been made to correct them for change from aqueous to organic medium. T h e end point is determined from the maximum in the differential curve as with the usual aqueous titrations. For the general ionization reaction HA
F-'
H + f A-
one may write, assuming activity coefficients are constant, the expression for the ionization constant
where parentheses represent the concentration of the ion involved. When half the volume of base required to reach the end point has been added, the amount of acid ion is approximately equal to the amount of un-ionized acid, assuming the salt formed is not associated. Therefore one may estimate the relative acid strength from the expression
By definition p K would be the negative logarithm of the ionization constant. Because of the approximations involved, a n asterisk is used to emphasize that while p K * values may be compared when obtained from the same titration procedure they are not the thermodynamic values. Data on the alkyd compared to certain pure acids are shown in Table 11. T h e alkyd contains relatively strong acids; the acid is stronger than benzoic acid o r a typical aliphatic acid such as lauric acid. Indeed, the acid strength is comparable to that of a monoesterified phthalic acid and only slightly weaker than some other ortho-substituted benzoic acids such as salicylic acid or mandelic acid. Data on the acid content of the urea-formaldehy. de vehicle, also included in Table 11, are considered later.
Table II.
Comparative Data on Acids of Test Vehicles
(Butanol-xylene titration) Acid -1-0,
.tiaterial Alkyd vehicle Urea-formaldehyde vehicle Mandelic acid Salicylic acid Mono-n-butyl phthalate Phthalic acid
-\.19.
...
4.0
...
4.5
. . ,
Benzoic acid Lauric acid Typical aliphatic acids a
pK
p K * from K O H j G . .Vidpt. pH* 4.0 4.5 1.0 7.7 ... 3.7
Aqueous" ...
...
3.37 2.97
{i:!:
... 5.6 6.1
...
... ...
4.20
4.76'tO 4.87
, . .
From (3).
T ' f
6
t
I
I
Interaction of Alumina-Treated Pigment with Acid TI KOH(V)
The hypothesis of aluminum salt formation further requires that the alumina hydrate present as a postcalcination treatment interact with the acid of the alkyd to form a salt. This demanded experimental verification. If a n acid solution of volume V and normality N , is added to a volume, VO, of neutral organic solvent, the p H * values fall with increasing volume of acid. Since the concentration of acid anion must equal the concentration of the hydrogen ion, one has from Equation 1 2pH* Z pK*
- log ( H A )
Then assuming V to be much smaller than to be largely un-ionized 2pH* S pK* - log V
Figure 3. Titration curve of alkyd vehicle with 0.0903N potassium hydroxide
(3)
V,,and
A' - log VO
the acid HA
(4)
I n other words, a semilog plot of the p H * values against the volume of acid solution will be approximately linear. If pigment is present \vhich interacts with acid to reduce the acid content by reaction to form a salt, or adsorption, or both, the line will be displaced to higher p H * values. Such plots have been obtained for a number of acids comparable in strength to the alkyd acids. Experimentally 10gram pigment samples were slurried into about 100 ml. of butanol-xylene-LiC1 solvent and increments of the acid, dissolved in the same solvent, were added with vigorous stirring. Most directly of interest are the results of these titrations with a solution of alkyd vehicle itself serving as acid, presented in Figure 4. T h e results of the titrations with a few acids are
I
OS
,
1
8
8
GI
1 05
. I
10
,
I
I
I l l 1 1 5
I IO
1 53
&LI(IC S3LUTION ( m i l
Figure 4. Titration of pigments with alkyd vehicle in butanol-xylene solvent Acid concentration 0.0374N VOL 2
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Table 111.
Gloss and Acid Titration Data for Rutile Pigments Level of Acid Titration Curves Mono-nAlkyd Salicylic butyl soln., acid, phthalate, Pi.cment Gloss 0.03741’ 0. IN 0. IN 71 1 1 1
A B C Blank (no pigment during acid titration)
30 14
2 3
2 3
2 3
..
4
4
4
given in Table 111. Here the pigment which reacted with the most acid, the one whose curve was the highest, is numbered 1 and so on down. From data o n the gloss values for these pigments, also included in Table 111, it is seen that the relative degree of interaction with the various acids correlates well with the gloss. Treatment of Alkyd Vehicle with Bases
Since the formation of the aluminum salt of the alkyd acid appeared essential to obtain good dispersion, the effect of treating the alkyd vehicle separately with various basic substances, including alumina hydrate, to form the desired anionic “dispersing agent” prior to preparing the gloss enamel was explored. The usual result, as might be expected, was a very marked increase in gloss. Hydrated aluminas were given primary experimental attention, since they were known to be effective as a postcalcination treatment in obtaining high gloss. For this it was convenient to use a chromatographic grade of alumina hydrate having a large particle size (Fisher adsorption alumina, grade A-540) because this material could be mixed with the alkyd vehicle and the excess largely removed by gravitational settling. Typical data are exhibited in Table IV. Treatment of the alkyd lowered the acid content and increased the gloss values for the pigments studied. Even pigment D yielded a fairly high gloss in the treated alkyds, despite its relatively high degree of agglomeration resulting from inadequate laboratory milling. More detailed examination of the potentiometric titration curve for a treated alkyd compared to the original alkyd shows
Table IV.
Wt. Fraction Alumina 0 0.1 0.4 Table V.
Acid
Gloss
None Sulfuric acid Salicylic acid Phthalic acid Phthalic anhydride Benzoic acid Lauric acid
72 17 25 17
46 70 69
that not only is the total amount of acid decreased on treatment but also the initial portion of the curve is considerably flattened. T o illustrate this from the data of Table IV, the pH* value a t the start of the titration (“initial” pH*) was 2.1 for the original alkyd but increased to 3.5 for the alkyd treated with 0.1 weight fraction of alumina. On the other hand the p H * value a t the midpoint of the titration has changed only 0.1 unit on treatment of the vehicle, as indicated by the equivalent p K * values. In other words, the result of the alumina treatment a t least in part is to buffer the acid of the alkyd; this is additional evidence that a soluble aluminum salt has been formed as a result of the treatment. Other alumina hydrates prepared in the laboratory from such sources as aluminum chloride, aluminum isopropoxide, and sodium aluminate have also been used to treat the alkyd vehicle. Although they were less completely removed from the vehicle than the chromatographic grade material, the results were generally the same-viz., a decrease in the acid content of the vehicle and a n improvement in the gloss level yielded by the treated vehicle. An exception was a n alumina hydrate prepared from strong aluminum sulfate solution so that a large amount of sulfate ion (S04/.41203 ratio of 0.32) was present even after prolonged washing. Treatment with this alumina actually increased the total amount of acid present in the alkyd and decreased gloss values. A number of other hydrous metal oxides and other basic materials were also tried as treatments for the alkyd vehicle. Typica, data are shown in Table V. While the actual degree of effectiveness of these agents is somewhat hard to predict, there was a general trend toward increased gloss after treatment.
Acid No., pK* from Gloss Mg. Midpt. “Initial” Pigment Pigment Pigment KOHIG. pH* pH* A B D
3.9 2.3 1.o
4.4 4.5 4.9
2.1 3.5 4.4
53 79 77
18
77 74
14 55 63
Effect of Treating Alkyd Vehicle with Basic Agents
Agent
A1 isopropoxide Butyl titanate
268
Effect of Acids on Gloss
Acid added in amount to double acid content of alkyd)
Treatment of Alkyd Vehicle with Adsorption Grade Alumina
wt.
KzC03
Table VI.
(Pigment A.
Fraction Agent
0 0,024 0.029 0,010 0.012 0.011
Acid No. Mg. KOH/G.
4.3 3.8
3.4 3.6 4.1
0.2
Pigment A
62 66 65 66 69 67
Gloss Pigment B
23 51 66 57 72 62
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
20
TEMPEWTUQE
tEPl
Figure 5. Effect of thermal treatment on gloss of laboratory-finished rutile pigments
Addition of Acids to Control Gloss Yielded by Alkyd Vehicle G. per Gloss Kg. PigPigAdditive Alkyd ment A ment B Alkyd
Table VIII.
Table VII.
Regular test alkyd Insensitive alkyd
None None Salicylic acid &Mandelicacid
...
... 3.25 1.25
60 60 59 58
26 65 29 25
Effect of Humidity on Gloss
0% R H 5
%
moistured Gloss 0 59 0 23 a Pigment vacuum-dried. saturated ( I V H ~ ) ~ Ssolution. O~ dried pigment sample to dejined moisture could be removed from
Pigment A Pigment B
Preconditioning Approx. 50% R H b
9%
81% RHc
%
moistured
Gloss
moistured
0.40 0.21
62 30
0.60 0.33
Gloss 66
54
IVormal room conditions. c Exposed over d From weight change on exposing vacuum humidity conditions. Approximately 0.770 vacuum-dried samples by drying at 160’ C.
Effect of Acids on Gloss
The effect of relatively low molecular weight acids on gloss when added to the system may be easily summarized: Acids of a strength equal to or greater than the acids of the alkyd vehicle caused a marked decrease in gloss, while weaker acids were ordinarily without effect (Table VI). Acids causing a decrease in gloss values include mineral acids, such as sulfuric acid, and most ortho-substituted benzoic acids, such as salicylic acid, mandelic acid, and monoesterified phthalic acids. When a weak acid, such as lauric acid, was added to the pigment before addition of alkyd vehicle, so that it reached the pigment surface before the stronger alkyd acid, the resulting enamel showed lowered gloss immediately after preparation, but successive films prepared from the enamel showed a rapid and continual increase in gloss; kvithin a few days the gloss values were substantially identical to those of the control enamel. Phthalic acid and phthalic anhydride also acted as glossdecreasing additives. It is strongly suspected that free phthalic anhydride or acids of low molecular weight, such as free phthalic acid, represent components in the alkyd that differentiate between a n alkyd that yields relatively loiv gloss values and one that yields relative1)- high gloss values. This suggested that addition of strong acids to a n alkyd which yields relatively high gloss values with the titanium dioxide pigments (termed here a n “insensitive” alkyd) lvould result in a n alkyd showing a spread in gloss values for pigments, as does the one regularly employed in this study. Mandelic and salicylic acids were selected for this purpose because of their ready solubility. Data presented in Table VI1 demonstrate that addition of these acids in suitable amount results in gloss values substantially identical to those yielded by the same pigments in the regular test alkyd. The degree of pigment flocculation in the alkyd urea-formaldehyde enamels appears to be similar to that of the same pigment dispersed in the alkyd vehicle alone. I n view of the relative weakness of the acid present in the urea-formaldehyde vehicle as shown in Table 11, one would not expect the addition of this urea-formaldeh>.de vehicle to the alkyd to change significantly the degree of flocculation induced by the alkyd vehicle alone. Variation in Alumina Hydrate Activity
The nature of alumina hydrates, including their composition, their crystal structure, and their surface area, depends greatly upon the method of preparation ( 5 ) . The reactivity of the alumina hydrate with the acids of the alkyd vehicle may also be altered. Since the nature of alumina hydrates is markedly influenced by the temperature to which they have been exposed ( Z ) , this represented a convenient means of varying continuously the activity of alumina hydrates. Therefore, the effect of the temperature to which a pigment had been heated on gloss was
considered. Filter cake from a rutile slurry that had been treated with 1% alumina hydrate (A1203 on a pigment basis) was vacuum-dried in the laboratory; samples were exposed to various temperatures for one hour, and then finished by dry milling. Gloss data for the resulting pigments expressed as per cent loss in gloss (based on pigment finished a t room temperature) are presented in Figure 5. Clearly, increasing temperature has progressively impaired the activity of the postcalcination alumina hydrate and consequently progressively decreased the gloss level. Effect of Moisture
Finally. the hypothesis to account for the variation of pigment dispersion and hence of gloss in this system emphasizes the importance of the adsorbed water film a t the pigment surface both to solvate the cations of the double layer and to prevent excessive ion pair formation with the organic counterions of relatively high molecular \veight. At normal room humidities the rutile pigments under consideration contain approximately 0.2 to 0.4YG of physically adsorbed moisture. An additional portion of tht: Lvater, hoLvever, is more tightly bound-e.g., as part of the hydrous oxide postcalcination treatment. The moisture content of the pigment may be varied by exposing the pigment to varying relative humidity. The effect of humidity on the gloss of two pigments is demonstrated in Table VIII. Higher relative humidities resulted in a marked improvement in gloss, especially \vith pigments tending otherwise t o sholv low gloss. The vehicles themselves also contain some moisture, of the order of 0.2%. Since even vacuumdried pigment Lrould adsorb some moisture from the vehicle, the results do not extend to an ideally dried s)-stem. Acknowledgment
The assistance of W. Lasko in preparing the electron micrographs is gratefully ackno\vledged. literature Cited
(1) “Encyclopedia of Chemical Technology,” Vol. 14, pp. 22330, Interscience Encyclopedia, New York, 1955. (2) Gregg, S. J., TVheatley, K. H., J . Chem. SOL.1955, 3804. (3) “Handbook of Chemistry and Physics,” 41st ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1959. (4) Koelmans? H., Overbeek, J. Th. G., Discussions Faraday SOC. 18, 52 (1954). (5) Russell, .4.S., Gitzen, LV. H., Newsome, J. W., Riker, R. W., Stowe, V. M., Stumpf, H. D., \$’all, J. R., TVallace, P., ‘‘Alumina Properties.” Alcoa Research Lab., Aluminum Co. of America, Pittsburgh, Pa., Tech. Paper 10 (1956). (6) Verwey, I:. J. LV., Overbeek, J. Th. G., “Theory of the Stability of Lyophobic Colloids,” Elsevier, New York, 1948.
RECEIVED for review March 12, 1963 ACCEPTEDJune 10, 1963 Division of Organic Coatings and Plastics Chemistry, 145th Meeting, ACS, New York, N. Y., September 1963. VOL. 2
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