Chalking of Titanium Dioxide Pigmented Exterior Finishes SYNTHETIC ENAMELS D. H. DAWSON AND R.D. NUTTING Krebs Pigment & Color Corporation, Baltimore, Md. E C E X T years have the subject. The conclusions The results of a lengthy study of the chalkshown a marked increase offered ranged a11 the way from ing characteristics of titanium dioxide pigin the use of exterior such expressions as “useless” ments in enamel vehicle systems show that enamels. This has been brought to a “fairly satisfactory correFlorida and Delaware exposures, in general, about to a considerable degree l a t i o n ” b e t w e e n accelerated undergo similar types of initial film failure. through the development of the and northern test-fence exposynthetic resins, and particusures. The degree of correlation in chalking, fadlarly to the drying-oil-modified The general conclusion, as ing, and loss of gloss between Florida and alkyds or gl y c er 01- p h t h a 1a t e stated earlier by Armstrong and Delaware exposures is such that the usually Fuller (2) is that accelerated type. faster Florida results may be accepted as T h e p i g m e n t a t i o n of such w e a t h e r i n g cycles a r e t o o representative. Differences due to variaenamels has presented definite severe. Chalking differences problems. Because of reacwere telescoped so that differtion in weather conditions during the tivity limitations, inert pigences were minimized, whereas course of the year at a given northern locaments are normally required, checking-cracking failure was tion usually are greater than between locaand in order to obtain high so greatly accelerated that sigtions. Accelerated cabinet exposures of luster, a further restriction to n i f i c a n t d i f f e r e n c e s could enamel films of the definitely chalking type the pigments of high hiding and scarcely be appreciated. opacity which will allow low I n the course of an extensive also show satisfactory correlation with pigmentation must be made. study of the chalking characactual outside exposures. In all cases, F u r t h e r , since t h e alkyd teristics of titanium dioxide however, differences due to uncontrollable enamels fail most commonly by pigments in alkyd enamel sysvariables are such that standard enamels chalking or by checking-cracktems, numerous exposure tests should be included for purposes of coming, it is important that the have been made under severe manufacturer so select a pigweather conditions in Florida parison. ment or combination of Digand in an accelerated cabinet of customary design; identical ments that these types of faiiure enamels were also placed on exposure in the milder climate are avoided or a t least minimized for a reasonable length of time. of Delaware. The chalking-type failure usually starts with a rapid decrease in the gloss of the finish, followed by a disintegration of Preparation of Exposure Panels the film into a more or less loosely adherent chalk on the The enamels used in this study were prepared by grinding enamel surface. Chalking failure is particularly undesirable the pigments in a drying-oil-modified alkyd solution having in tints because of the resultant faded appearance. The the following characteristics: chalk may generally be removed by polishing, although the ease with which the original luster can be restored by such Resin in solution, % 50 means will vary widely. Solvent % 25 P e t r d e u m spirits Checking-cracking failure is generally slower in starting, 75 Xylene Viscosity but once it has set in, the base material beneath the enamel MacMichael 3000 i s exposed to the action of the weather through myriads of zt-za G-E 3-4 Acid No. narrow but deep fissures. 0.960 Specific gravity 2-3 Color (Paint & Varnish Inst. Glrtss Stds.) Although it is of the utmost importance for the enamel manufacturer to know what types of pigments to use in order The pigments were mixed with the resin solution at a ratio of to obtain the best results, it is frequently undesirable to await pigment to nonvolatile resin of 260 to 100, and ground one pass the outcome of long-time exposure tests under the conditions through a laboratory five-roll mill at a setting of approximately of actual use. On the other hand, the paint industry, in gen0.001 inch t o return good enamel loss and fineness. The ground mill base was reduced by the afdition of more resin solueral, seems t o have reached no definite agreement concerning tion t o give the desired ratio of pigment to nonvolatile. Cobalt the usefulness of the accelerated type of exposure. naphthenate drier to t h e amount of 0.03 per cent metal on the The “Symposium on Correlation between Accelerated nonvolatile, dong with xylene-petroleum thinners, was added t o Laboratory Tests and Service Tests on Protective and Decthe reduction. The enamel was tinted gray with 5 parts of lamp black (ground separately in the resin) per 100 parts of white pigorative Coatings” (1) represented many shades of opinion on
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112
JANUARY. 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
ment. After sging overnight, the enamel was reduced with xylene to a viscosity of 18 seconds through the 10-FParlin Cup for spraying. The weight ,f to hinder wa8 at 0.75 to 1.00 for aU titanium dioxide pigments. With other less opaque pigments, the ratio of pigment to binder waa increased to give an enamel having close to the same hiding power. Where the pigmentation was principally a lowu acity igment such zu antimony oxide, equal hiding could not {e usec?hecause of the degradation of other properties, notably gloss.
113
gree of contrast between the adhering particles of white chalk and the black velvet was used to establish a scale, practicdy identical with the fading scale, for estimating the stage of chalking to which the panel had progressed. Figure 1 illustrates this scale in various stages of chalking failure.
Results in Alkyd Enamels The degree of correlation afforded by ex-
Slight
Conriderable
Bad
Very bad
The enamel was sprayed on 4 X 12 inch steel panels primed with a commercial short-od alkyd rimer surfacer and well sanded and tacked before application. $wo full coats of enamel, with sanding between costs, were applied at intervals of several days to allow thorough drying. The panels for accelerated exposure measured 2.75 X 5.75 inches and were prepared in an identical manner.
and normal exposure conditions is not so good (Figure 3). Here the Florida exposures indicate a distinct sunerioritv for “Ti-Pure” Y-CR over a 50/50 conbinatirk of titanium dioxide and antimony oxide, and a slight advantage during the early stages of exposure over a 25/75 mixture. These results arc
Eztreme
F1
”::;
Exposure of Panels
8 - N
The prepared panels were exposed at an angle of 45’ to the south in both Delaware and Florida. I n the accelerated exposure cabinet the panels were exposed alternately to the light from a carbon arc filtered through a Pyrex No. 9200 P X globe a t a distance of 15 inches and to a mild mistlike water spray. At intervals a heavy water spray, simulating rain, was applied to the panels. A “day” in the accelerated cahinet was dividod into the following operations:
C-
G
S-
1. Seventeen hours of exposure to the carbon arc; for 90 seconds in every 20 minutes the panels were wet with the fine spray of water. 2. Three hours of exposure to a heavy water spray without the carbon are. 3. Four hours for examinationof the panels, etc.
The carbon arc was operated a t 14-17 amperes and 220 volts, and approximately 53 kilowatt-hours were consumed in one exposure day. The spectral distribution of the light reaching the panels is compared with June sunlight at Washington, D. C., in the following table:
FIGURE
2.
FADING
RATESOF
RAP1nI.Y
ALXYDENAMELS
CHALKING
at variance with the more normal exposures in Delaware which indicate early equality between “Ti-Pure’’ Y-CR and the 50/50 mixture of titanium dioxide and antiWave length. m y 270zoo- 320- 360- 480- 6 0 ~ 4 ~4ZoO-~ - mony oxide. It is somewhat surprising, per290 320 360 480 600 1400 4200 12,000 haps, that in this series of exposures the accelb r o light, filtered through Pyrex clobe 0.05 4.6 6.2 13.0 8.3 14.9 21.3 30.65 crated cabinet results fall more nearly in line June sunlight ”.’ 0.4 with those obtained from the panels exposed in Delaware than from the onesexposed inFlorida. All panels were examined frequently, and their condition In spite of certain discrepancies, we have found the accelwas noted. The degree to which the panel had faded was erated cabinet exposures and Florida exposures to be of definite estimated by polishing a section and observing the contrast value, particularly in our search for a definitely chalk-resistant in whiteness between the faded and polished portions. A type of titanium dioxide. Figure 4 represents a number scale of gradings was set up, designated as slight, considerable, of experimental pigments (C, E, G , and I) which were varibad, very had, and extreme fading. ous steps in the development of the chalk-resistant titanium I n order to estimate the amount of chalk held on the surdioxide, “Ti-Pure” 0. Although several of the panels in the face of the enamel film, a narrow piece of black velvet was accelerated cabinet started checking after 50 to 60 days, all rubbed firmly across the exposed face of the panel. The deof the panels failed eventually by chalking when exposed t o
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114
E
1
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l [ACCFL. CABINET]
I
I d 3 4
JAN
MAR
I
5
l
6
l
7
MOMHS OF T H E YEAR
9' l b I? I!? SEPT. D LC.
FIGURE 6. EFFECT OF TIMEOF EXPOSURE os RAPIDITY OF FADING IN DELAWARE
FIGURE 3. FADING RATESOF RELATIVELY CHALK-RESISTANT ALEYDENAMELS 1
/
BAKED
FIGURE7. EFFECTOF AIR DRYINGAND BAKINGON FADING
FIGURE 4. FADING RATESOF A GROUPOF EXPERIVENTAL AND COMMERCIAL PIGMENTS WHICH ILLUSTRATE THE UTILITYOF ACCELERATED DEVICESFOR PREDICTION OF PIGMENT PERFORMANCE
I
S
3
FIGURE 5 . FADING RATESOF IDENTICAL ENAMELS
EXPOSED IN DELAWARE AT DIFFERENT SEASONS
4
FIGURID 8. FADING RATESOF COMMERCIAL PIGMENTS IN A MODIFIED PHENOLIC VARNISH
IADLSTRIAL A h D ENGINEERING CHERlISTRY
JAkNUARY,1940
the weather, both in Florida and in Delaware. In spite of this irregularity, we were able to eliminate from further consideration, on the basis of cabinet exposures, pigments C, E , and G, since they were not so effective as the 25/75 mixture of titanium dioxide and antimony oxide. The inclusion of
115
is much more fade resistant than “Ti-Pure” T-CR, which is here about equal to the 50/50 mixture during the early stages of failure.
Results in Vehicles Other Than Alkyds Attention has been directed solely to finishes of the alkyd type. However, the same general agreement appears between pigments having various degrees of chalk resistance in other enamel vehicles, such as a modified phenolic type in Figure 8 and a long-oil alkyd marine type in Figure 9.
Effect of Pigment Combinations
’]&$To MONTHS
6
6
FIGURE 9. FADING RATESOF COMPIGMENTS IN A LONGER OIL
MERCIAL
MARINE-TYPE ALHYD
Four titanium dioxide pigments, varying in their resistance to chalking in the following order of increasing resistance, have been discussed: “Ti-Pure” LO, “Ti-Pure” CR, “TiPure” Y-CR, and “Ti-Pure” 0. By a suitable combination of these pigments it is possible to cover practically all the variations in the degree of chalking and fading required by enamel manufacturers. Figure 10 represents a family of curves obtained by mixing varying amounts of “Ti-Pure” Y-CR and “Ti-Pure” 0. The percentage of “Ti-Pure” Y-CR in each mixture is indicated by a number adjacent to the appropriate curve. The fading re-
M I X T U R E S O F T I P U R E Y-CR
this control pigment assured us that the checking was probably illusory, for i t was known that this mixture fails eventually by slow chalking and not by checking in the formulation used.
Importance of Control Panels The necessity of including control enamels in every series of exposures is emphasized by this group. To emphasize further the importance of including controls, Figure 5 shows the 25/75 titanium dioxide/antimony oxide exposure curves in Delaware, taken from a large number of different exposure series. Depending upon the time of the year a t which the exposures were started and upon the character of the weather during the life of the panel, wide variations in fading characteristics are obtained. It is apparent that these variations are greater than those between accelerated and normal exposures, although the reversal of relationships between pigments previously noted has never been found to occur. It is interesting to note that there is usually a 5-6 month “incubation” period before chalking begins. Figure 6 shows that exposures started in December, having the full quota of summer weather, fail much more rapidly than exposures started in ilugust; the latter must wait until February for the end of the “incubation” period and several months longer, of course, until the more severe chalking weather sets in.
Effect of Film Application Variations in film application may also notably affect the manner in which various pigmented enamels behave on exposure. I n Figure 7 the two groups of pigments mere ground in the same enamel liquids in identical fashion, the only difference being that one group was applied as an air-dry finish to the panels, whereas in the other group the enamels were formulated without drier and were baked. I n the baked enamel “Ti-Pure” Y-CR appears to definite advantage over the 25/75 mixture of titanium dioxide/antimonp oxide in the early stages of exposure and is decidedIy superior to the 50/50 mixture. In the air-dry formulation, however, the 25/75 mixture
f
l
0
T
I
2 3 4
5 6MONTHS I 2 3 4
5 5
7 8 9
FIGURE IO. FADING RATESOF MIXTURES OF COMMERCIAL TITANIUM DIOXIDEPIGMENTS
sistance increases regularly as the percentage of “Ti-Pure” Y-CR in the mixture is decreased and the amount of “TiPure” 0 is increased. The ratios of “Ti-Pure” Y-CR to “Ti-Pure” 0 used in formulating these enamels were: 100/0, 70/30, 50/50, 30/70, 10/90, and 0/100. These results are in good correlation in all three types of exposure. Because of their superior hiding power, it is now possible to utilize the special chalk-resistant grades of “Ti-Pure” to effect substantial savings in pigment costs and a t no sacrifice in the chalk and fade resistance of the finished enamel. By eliminating much or all of the less opaque pigments, lower ratios of pigment to binder can be utilized to effect substantial improvements in gloss. The savings in pigment cost through the use of the chalkresistant grades of “Ti-Pure” are estimated a t between 20 and 50 per cent and pigmentation (ratio of pigment to binder) can be reduced in the same order and a t the same time equal film hiding power and equal resistance to chalking and fading can be maintained.
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Summary Exknded exposure tests show that within the field of alkyd enamels: 1. Accelerated cabinet exposures show excellent correlation with Florida and Delaware exposures in rapidly chalking enamel films. 2. In more chalk-resistant films of the type most widely used commercially, the correlation between accelerated cabinet and normal exposures is definitely poorer; however, the technique may still be used to estimate gross chalking differences with proper precautions. 3. In the more chalk-resistant films, the accelerated cabinet cycles used in this study yielded definitely misleading results with respect to checking type failure. 4.
Even normal exposures may show reversals between pig-
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ments in chalking rate, depending on whether the film is air-dried or baked, and in some cases depending on location. The necessity for reporting rates in relative rather than absolute terms is emphasized by the large differences obtainable a t varying periods with the same enamels. The relation between the chalking rates of the new chalkresistant types of titanium dioxide and other available pigmentations has been discussed and illustrated.
Literature Cited (1) Am. Soc. Testing Materials, 48th Ann. Meeting, New York, June 29, 1937. (2) Armstrong, M. S., and Fuller, W. B., 1930 meeting of SubCommittee 7, Committee D-I, A.S.T.M. PRESENTED before t h e Division of Paint and Varnish Chemistry at the 98th Meeting of the American Chemioal Society, Boston, Mass.
Sorption of Water Vapor by Vermiculite and Its Silica L. A. HANSEN, W. S. SAMUEL, JR., AND P. A. FORK1 Rensselaer Polytechnic Institute, Troy, N. Y. ERMICULITE (1) is one of the comparatively rare nonmetallic minerals whose commercial possibilities are just being realized. It is classed as one of the micas. Vermiculite occurs in broad crystals or crystalline flakes of yellowish brown, greenish black, or greenish yellow color. The mineral has a soapy, graphitic feel. The chief deposits (6) are in Montana and Colorado. The structural formula is (OH)z(Mg, Fe)3(Si, Al, Fe)4010.4Hz0,and the molecular formula is 22Mg0.5 A1203.Fez03.22Si02.40Hz0. According t o Gruner (2), the structure consists of sheets of (OH)rMge(Si, A1)8020 with alternate layers of 8Hz0. Half of the water can be driven off a t 110' C. with no change in structure. The average analysis of seven vermiculites reveals the following composition (6):
V
B v . Analysis, yo
Theoretical Composition
Total
99.91
100.00
The most remarkable property of vermiculite is its great expansion when heated ( I ) . On exfoliation the expansion occurs only a t right angles to the cleavage planes, and the sheets spread apart into an accordionlike structure, the color meanwhile changing to a silvery or golden brown (4). The freshly exfoliated vermiculite has been reported as being a powerful desiccant (5). According to Guthrie and Wilbor (S), a silica of excellent sorbing properties can be prepared from the mineral. Silica can be separated from the other oxides by treating vermiculite with acids such as sulfuric and hydrochloric. After the reaction is complete, the remaining silica is washed thoroughly and dried. The yield of silica was found to be 99 per cent of the theoretical. The silica prepared from ex-
foliated vermiculite was a chalky white material; that prepared from the unexfoliated vermiculite had a silvery white color. The density of the silica is about 0.1. There is apparently little, if any, information in the literature on the desiccating or adsorbing properties of either exfoliated vermiculite or its silica. This report consists of the results of a preliminary study of these properties. A more thorough investigation is under way. The silica was prepared from both the exfoliated and the unexfoliated vermiculite by treatment with hot dilute sulfuric acid (1:3) for about 3 hours. The silica was filtered in a Buchner funnel, washed with dilute sulfuric acid and finally with distilled water, and dried in an oven a t 105' C. The apparatus (8) used is sketched in Figure 1. Air obtained from the laboratory line was first passed through tube 1 containing fuller's earth. The pressure was regulated by means of tube 2 containing water, the height of which was controlled by the position of bottle 3. The air next passed through saturators 4, 5, and 6, the latter having a sintered glass dispersing disk to ensure saturation with water vapor. The saturated air then passed through tube 7 containing the sorbent and through tubes 8 and 9 containing phosphoric anhydride. The volume of air was determined by allowing water t o flow from bottle 10 into container 12 and keeping the water in the two limbs of manometer 11 a t the same height. The volume of air corresponds to the volume of water displaced from bottle 10. The saturators and the tube containing the sorbent were immersed in a constant temperature bath held a t 25" C. The rate of flow of satura,ted air was usually adjusted so as to be between 0.5 and 1.0liter per hour per gram of desiccant. To determine the amount of water adsorbed, the tube of desiccant was weighed before and after a run, as was also the first tube containing phosphoric anhydride. The gain in weight of the two tubes measured the total amount of water vapor. Measurements were made only a t 100 per cent relative humidity. The results for exfoliated vermiculite are given in Table I.
