INDUSTRIAL AND ENGINEERING CHEMISTRY
540
VOL. 32, NO. 4
Photomicrographs of acicular and nonacicular zinc sulfides are shown in Figure 6. D is a commercial zinc sulfide of uniform particle size that develops high hiding power. I is a very acicular zinc sulfide of high hiding power. Wurtzite has a birefringency of 0.022 with two indices of refraction of 2.356 and 2.378, compared with 2.368 for sphalerite. As might be expected, Sphalerite and wurtzite give essentially the same hiding power and whitening strength when the crystals are round and have the same size.
OF PARTICLE SIZEOF WURTZITE( Z ~ r i c TABLE I. EFFECT
SULFIDE)
Particle Size, Micron 0.13 0.20 0.35 0.50 0.60
ON
OPACITY"
Relative Whitening Strength 95 100 87 65 67
Relative Hiding Power 95 100 89
74 68
Theae samples were all of about the same brightness (equal t o t h a t of a standard commercial zino oxide). 0
X-ray analysis has shown that wurtzite is hexagonal (sphalerite is cubic), and that the unit cell has almost exactly the same dimensions as zinc oxide. This explains why it will crystallize in rounds or needles like zinc oxide and in mixed crystals of zinc oxide and zinc sulfide. Wurtzite is known to be so unstable that it decomposes even on grinding. Accordingly, it has not been thought practical to use it as a pigment. The ability of the wurtzite (1) shown in Figure 61 to resist even grinding is probably due to less than 0.1 per cent of stabilizing zinc oxide. (This acicular zinc sulfide is not available commercially a t present.) When round particles of wurtzite (zinc sulfide) are reheated, they grow into larger rounds like zinc oxide. Whitening strength and opacity tests on several of these round zinc sulfides of different particle size, made by reheating a zinc sulfide of relatively fine particle size, are given in Table I. These relative data on hiding power and whitening strength have been reduced to a curve, and the value (115) for the acicular zinc sulfide has been added (Figure 7). The curve (Figure 7) for zinc sulfide taken from Table I is similar t o the
.2
.4
PARTICLE
.6
1.0
SIZE, MICRON
FIGURE 7 . EFFECTOF PARTICLE SIZE A N D SHAPEON THE WHITENISGSTRENGTH OF ZINCSULFIDE PIGMENTS
curve in Figure 1for zinc oxide. The hiding power in absolute units is much higher, however, for the zinc sulfide as compared to the zinc oxide. The similarity of the relative data for zinc oxide, zinc sulfide, and white lead suggests that the other white pigments in acicular form will also develop higher whitening strength and hiding power than they possess in their present nonacicular form.
Literature Cited (1) Depew, H. A., a n d M a i d e n s , W. T., U. S.P a t e n t 2,140,658 (Deo. 20, 1938). (2) Eide, A. C., a n d Depew, H. A , , Paint Oil Chem. Rev., 98, No. 8, 40 (1936). (3) G e h m a n , S. D . , a n d Morris, T. C., IND. EXQ.CHEM.,Anal. E d . , 4, 157 (1932). (4) Kekwick, L. O., a n d Pass, A , J. Oil Colour Chem. Assoc., 21, NO. 215, 118-39 (1938). ( 5 ) Stutz, G. F. A., a n d P f u n d , A. H., IND. ENQ.CHEM.,19, 51 (1927).
PRESENTED before the Diviaion of Paint and Varnish Chemistry a t the 97th Meeting of the American Chemical Society. Baltimore. M d .
Viscositv Variations in Methacrylic Ester Polymer Solutions J
D. E. STRAIK E. I. du Pont de Nemours & Company, Inc., Wilmington, Del.
V
ARIOUS recent publications' indicate the growing com-
mercial importance of the methacrylic ester polymers as protective coatings. Films of these ester polymers are water-white, nonyellowing, and tough, and have very good resistance to water and aqueous solutions. The various comdu Pant de Nemours, E. I., 8~ CO., INn. E N C .CHEM..28, 1160 (1936); Pearce, W.D.,Paint Oil Chem. Rev., 101, 22 (1939); IND.ENG.CHEM.,as, 635 (1936); Strain, D. E . , Kennelly, R . G . , and Dittmar, H. R., I b i d . , 31, 382 (19391.
mercially available esters afford a wide selection with respect t o hardness and flexibility. These resins exhibit exceptional stability on outside exposure and are gaining wide acceptance as metal protective finishes. In using these products in lacquers, it is important to have information on the influence of Jarious solvents on the viscosity, so that high solids can be obtained at working viscosities in order to effecteconomies in required for costs and the number Of given build.
APRIL, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
541
It is generally known that a given percentage of a highmolecular-weight resin or cellulose derivative dissolved in a series of solvents may give solutions with widely different viscosities due to the nature of the specific solvents. This same effect is observed with respect to polymethacrylic esters as illustrated by data in Table I for the viscosity of 15 per cent methyl methacrylate polymer solutions in a number of organic solvents. From these data it is obvious that toluene gives lower viscosity solutions of this polymer than any other single solvent tested.
The curves obtained by plotting viscosity against the per cent solvent (other than toluene) are decidedly irregular for the various types of modifying solvents and diluents. The aliphatic alcohols, ethylene chlorohydrin, cyclohexanone, and methyl and ethyl acetates give well-defined minima. The curve for dioxane gives a maximum followed by a minimum; a practically horizontal line is obtained for chlorobenzene; the n-butyl acetate and isobutyl propionate curves go higher in viscosity as increasing amounts of toluene are replaced. The ester solvents as a class show an interesting gradation in their effect on viscosity. Methyl acetate, a very low-molecular-weight member of the series, gives about a 30 TABLE 1. VISCOSITY O F 15 I'ER CENT METHYL METHACRYLATE per cent reduction in viscosity which holds over a wide range of solvent concentration. With ethyl acetate only a small POLYMER SOLUTIOXS drop in viscosity is obtained. In the case of n-butyl acetate Viscosity a t Viscosity a t Solvent 25' C., Poises Solvent 25' C., Poises and even more with isobutyl propionate (higher molecular 3.8 Dioxane 22.0 Toluene weight esters) the viscosity does not drop but actually in9.0 Ethyl acetate 40 0 Ethylene dichloride Acetone 20.0 Butyl acetate >I50 creases rapidly as the per cent ester in the solvent mixture is increased. Data in Tables I11 and IV indicate that alcohols used along with toluene are also effectire in reducing the viscosities of 12Some unexpected results have been obtained from a study propyl methacrylate polymer and n-butyl methacrylate polyof the viscosity of methacrylic ester polymers dissolved in mer, but in the case of these higher ester polymers the change mixed solvents. Data are given in Table I1 for the viscosity in viscosity is less marked. of 20 per cent methyl methacrylate polymer solutions in solvents containing toluene and various percentages of other solvents or diluents. TABLE111. CHANGE IN VISCOSITY RESULTING FROM PARTIAL The pronounced effect of the lower aliphatic alcohols REPLACEMENT OF TOLUENE IN 44 PER CENTPROPYL METH(which are nonsolvents for the polymer) on the viscosity of ACRYLATE POLYMER SOLUTIONS toluene solutions of this polymer is clearly demonstrated. Viscosity in Poises (25' C.) with the Following ReplaceThe minimum viscosity in each case was obtained when apment of Toluene by Modifying Solvent: proximately the following percentages of toluene were reSolvent 0% 10% 20% 30% 40% 50 % 29.6 27.5 Methanol 42.7 27.6 33.7 ca.52.5 placed by the alcohol: 30.6 31.4 95'%ethanol 42 7 34.1 41.8 Isobutanol
22 25 13 20
Methanol 95% ethanol Isobutanol Amyl alcohol
Methanol is the most effective of the alcohols, the viscosity of a 20 per cent solution of methyl methacrylate polymer in a solvent composed of 80 per cent toluene and 20 per cent methanol being only about 15 per cent as great as that of a 20 per cent solution of this polymer in toluene. Ethylene chlorohydrin, methyl acetate, and methyl ethyl ketone also effect considerable reductions in the viscosity of methyl methacrylate polymer solutions.
42.7
37.2
35.5
ca.9l.O 0~.70.0
ca. 5 4 . 0
42.0
TABLE IV. VISCOSITY O F 49.5 PERCENTn-BUTYL METHACRYLATE POLYMER IS TOLUENE-METHANOL MIXTURES Methanol in Solvent Mixt.,
%
0 10
20
Viscosity a t 250
c.,
Poises
Methanol in Solvent Mixt.,
44.0 29.4
27.0
%
30 40 50
Viscosity a t 250 c., Poises 27.2 30.1 40.4
From the data above, toluene may be recomniended as a most effective single solvent for methacrylic ester polymers. However, much lower viscosities can be obtained a t the same solids content using toluene-alcohol mixtures as the solvent. TABLE 11. CHANGEIN VISCOSITY RESULTING FROM PARTIAL REPLACEMENT OF TOLUENE IN 20 PER CENT METHYL METHACRYLATE POLYMER SOLUTIONS
Solvent Methanol 95% ethanol Isobutanol Amyl alcohol Methyl acetate Ethyl acetate n-Butyl acetate Isobutylpropionate Methyl ethyl ketone Cyclohexanone Ethylene chlorohydrin Monochlorobeneene Dioxane Methyl isobutyl ketone Isopropyl ether Cyclohexanol B-22 alcohols" B-23 slcohols' B-24 E I C O ~ O I I I ~ Tetralin Xylene
Viscosity in Poises (25' C.)with the Following Per Cent Replacement of Toluene by Modifying S o lren t : 0 5 10 15 20 25 30 3 5 40 50 60 41 . . 10 .. 6.5 .. 7 . 16 21 70
41 41 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44
..
16 10.5 20 31 39 46
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14 12 14 30 41
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27 41
50
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46 36
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40
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52
..iB . . 12 . . 13 . . 10
, , ,, ,,
27 32 41
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25 28
26 26 27 50
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35
75
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46 55
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45
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.
51
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31
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. . . 2B . . . . 27
14 20
20
25
,,
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46 46
..
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,, 54 . . 4 6
a Mixtures of primary and secondary branched-chain alcohols obtained in the high-pressure hydrogenation of carbon monoxide. The boiling range of B-22 alcohols is 130-150° C . , of H-23 150-160° C., and of B-24 160-200° C.
VI~ZE-WATER RESERVOIR
Courtesy, C I B A
