INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Table IV- Concluded Two-Lead Paint wilh X X Zinc Oxide Paint 10% 50% Carbonate XX ZINCOXIDECONTENT Titanox Weight lead, XX Zinc OB COMBINATION 45% Caradded to 50% sulfate oxide TWO-LEAD PAINT bonate lead plunger lead paint 3% Zinc 10% Zinc 45% Sulfate lead . -
50 75 100 125 150
75 60
48 43 35
Weight added to plunger
Grams 50 75 100 125 150 Weight added to plunger
50 75 100
125 150
83 59 48 41 36 X X Zinc Oxide and Barytes paint
76 70 59 54 48 46 41 40 36 35 Bar5’tes Paints
XX Zinc oxde paint
Pigment Content:
55% XX Zinc oxide 45% Barytes
73 75 57 59 48 47 42 40 37 35 Sulfate White Lead and Barytes Paints Sulfate white lead paint
Barytes paint
71 57 48 41 37
73 57 4s 42 37
72 56 47 40 35
74 58 47 39 35
Pigment Content:
50% Sulfate white lead 50% Barytes 65 53 44 38 34
A similar condition was found with respect to the red leadasbestine paint. Theoretically, 62.8 per cent pigment is required when combining the single-pigment red-lead paint and the single-pigment asbestine paint. Actually, 64.3 per cent pigment concentration was required. Single-pigment paints of red lead and asbestine adjusted t o the same consistency of mixing resulted in a thinner paint. Similar thinning was found with the sulfate white-lead single-pigment paint when mixed with varying quantities of a single-pigment paint made with XX zinc oxide. Titanox with carbonate white lead (Anaconda process) and with zinc oxide (5 per cent leaded) resulted in thinning, as did also a mixture of these three pigments. Titanox with XX zinc oxide did not result in thinning,
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although Titanox and sulfate white lead, and also XX zinc oxide with sulfate white lead thinned. A combination of these three pigments likewise thinned. As the two varieties of white lead had been found to thin with zinc, and as a mixture of lead and Titanox had been found to thin out, it seemed reasonable to forecast that a mixture of the two leads would not thin out. Mobilometer results showed that they did not. There should be some point, then, a t which a thinning of this mixture of the two leads would occur if some singlepigment XX zinc oxide paint were added. It is indicated that this point lies between a zinc oxide content of the pigment of 3 per cent and 10 per cent. Ten per cent of Titanox in the pigment content likewise produced thinning. A mixture of single-pigment paint was made to give a pigment content of 55 per cent XX zinc and 45 per cent barytes, which is a combination that has been found to give good satisfaction as a house paint. KOthinning was found. As barytes and XX zinc oxide are similar in action as regards thinning, it seemed reasonable that barytes and lead should thin out, then, similar to the way zinc and lead thinned out. Examination of the settling data, particularly of the paints which have settled to some extent, such as the barytes paint and the combination lead and zinc paint exhibit a peculiar and persistent lag in the next to the bottom opening, yet the data, read vertically, seem to give the gradual increased pigment concentration to be expected. As no evidence could be found that this was due to experimental error, further study (as yet not completed) was undertaken using a 10-gallon container with eighteen openings. As the above is only a preliminary report of the nature of the work undertaken and now under way, no deduction or no generalizations will be made at this time. Certain unexpected results have been encountered which are being further studied.
Some Theories of Pigment Settling Paul R. Croll PITTSBURGH PLATEGLASSCOMPANY, MILWAUKEE, Wis.
HE annoying defect of pigment settling so commonly found in paints and enamels is one of the most interesting phenomena among the many and varied moods of paint behs,.:or. The practical-minded paint mixer is so anxious to overcome settling difficulties that he will try almost any suggested remedy and may even give some attention to a recital of theories as to the causes of his trouble. Since all pigments have considerably greater densities than the paint liquids or vehicles, the settling problem cannot be avoided. One might imagine equally troublesome the effects of pigments lighter than paint vehicles, and then could expect hard cakes of floated pigment in cans labeled “stir well from the top down.” We could hardly hope for a choice of various colored pigments exactly matching the vehicles in density.
T
Effect of Particle Size of Pigment
The “settling force” may be defined as the weight of a unit volume of pigment minus the weight of an equal volume of vehicle. When a finely divided solid, such as a paint pigment, is immersed in a liquid, certain forces are exerted between the solid and liquid and between various particles
of the solid, the magnitude of which is proportional to the area of surface or solid-liquid interface. However, since decreasing the particle size rapidly increases surface area, these internal forces also increase. Since the practical result of increasing the interfacial forces is to overcome or decrease settling, we are led to choose finer pigments as one aid toward overcoming settling. The particle size of a pigment refers to the individual crystal and not to an aggregate of particle pressed into place. The first precaution toward overcoming settling by use of finer pigments, therefore, is to disperse the pigment mechanically in the vehicle so that each discrete particle is completely surrounded by liquid. Otherwise, the full value of the increased surface is not obtained. ~VECHANCAL DISPERSIOXIN LIquID-considering the case of an individual pigment particle of fine size immersed in a paint or enamel vehicle, the settling force of gravity will tend toward sending the particle to the bottom of the can. In falling, the particle creates a viscous sheer in the vehicle. On the assumption that the vehicle is a true liquid and not a colloidal suspension of oil or gum aggregates, the rate of fall of the particle will be affected by the viscosity of the liquid. A very viscous vehicle may slow up the rate of
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1928
settling of fine pigments to give other forces sufficient time to offset or modify the settling tendency. Paint vehicles may be so affected by heat and other treatment as not only to increase viscosity of the liquid, but also to produce in the vehicle a system of true liquid with a very thorough colloidal dispersion of molecular complexes. While true viscosity of a liquid can, theoretically, affect only the rate of pigment settling, a system of colloidal dispersion introduces a definite yield value to our paint vehicles which give the necessary suspension to fine pigments. I n a practical way it is very probable that the use of soaps, such as aluminum stearate, to avoid settling works in this manner. The aluminum stearate is very finely dispersed as a colloid in the paint vehicle, thereby giving the vehicle a gel structure and a definite yield value to oppose settling force. DEGREE OF WETTING-After perfect mechanical dispersion of a pigment in a paint vehicle, the specific degree of wetting between the particular solid and liquid evidences itself. I n a well-designed enamel the pigment and vehicle are chosen to produce the most nearly perfect degree of wetting between the two. In cases of enamels conditions of viscosity and yield value of the vehicle, and extremely fine particle sizes (large surface must be depended upon t o oppose settling force), such an enamel system in a comparatively short time shows the presence of any larger pigment particles, which settle to the bottom of the container in a very dry and often hard cake. The remaining fine pigment is held in suspension very well over long periods of time and the intermediate phase of soft settling is not experienced a t all. [END OF
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Less thorough wetting between pigment and vehicle is often accompanied by definite forces of pigment flocculation. Pigment flocculation results in grouping pigment particles in loosely held colonies which tend to settle as a unit. The force of flocculation may thus result in an actual increase in the amount of settled pigment, but this same force prevents hard, dry caking of the pigment found in thoroughly dispersed paints. Soft settling, although of larger amount, is much to be preferred to a less amount of hard, dry settling. The wetting power of a paint vehicle is usually sensitive to relatively small additions of soluble salts, driers, volatile solvents, etc. With a rather wide range of soluble products the desired effect may often be secured in several ways. Effect of Shape of Pigment Particle In addition to the size of pigment particle the shape of a particle may sometimes be employed to resist settling. Asbestine contains needle-shaped crystals of value in this respect. The list of pigments with properly shaped particles is too limited to depend very greatly on this cure for settling. Conclusion
Pigment settling is only one of many paint phenomena whose control may be more certain by a better understanding of the forces at work between solid and liquid phase. More accurate methods of measuring particle size, viscosity, and yield value, and a more thorough understanding of solidliquid interface forces will most certainly have many practical benefits. SYMPOSIVAI]
Solubility of Lubricating Oil in Liquid Carbon Dioxide' Elton L. Quinn U N I V E R S I T Y OF UTAH.
M
ANY investigators have observed that solid carbon dioxide made by expanding the liquid from an ordinary commercial carbon dioxide cylinder is nearly always contaminated with lubricating oil. If the cylinder is inverted and after standing for several days or a week is carefully drained, the carbon dioxide obtained from it will still contain oil. This oil is readily recovered by distilling the ether solution which is formed during the process of making an ordinary ether carbon dioxide freezing mixture. It is quite evident that this oil must form a solution with the liquid carbon dioxide and, in spite of the fact that liquid carbon dioxide is noted for its low solvent power, the amount of oil obtained from a 20-pound cylinder of this liquid indicates that there is a considerable solvent action in this case. This investigation was started for the purpose of determining the solvent power of liquid carbon dioxide for lubricating oil and, if possible, to determine the conditions of manufacture Ivhich would decrease to a minimum the quantity of oil reaching the plant filling-station. Most manufacturers of carbon dioxide use a high-grade lubricating oil in their carbon dioxide compressors. Glycerol has been and is being used to some extent as a lubricant, but it has few advantages over a good grade of lubricating oil, and many disadvantages. The principal advantages are that it is only slightly soluble in liquid carbon dioxide and, 1 Received
February 10,1928.
SALT
LAKECITY,
UTAH
if small quantities do reach the cylinder, it is odorless and tasteless and produces no bad results when the carbon dioxide is used for charging water for carbonated beverages. Some of its disadvantages are that its cost is high, and that it is a poor lubricant, especially if a small amount of water is present, and in case of accidental heating in the cylinders of the compressor it decomposes, producing products hp,@g a very bad odor. On the other hand, the lubricating oll nuw used is a good lubricant, has no odor or taste, but seems to form a solution with liquid carbon dioxide. The presence of oil in the liquid carbon dioxide in no way decreases its value for the manufacture of soda water, as the gas readily distils from the solution in a pure condition. When carbon dioxide is used in the refrigeration industry, however, the question of oil solubility becomes of very great importance. In this case any oil carried to the expansion coils by the liquid carbon dioxide will solidify on the inside of the pipes and prevent an efficient transfer of heat. Experimental Procedure
The solubility of lubricating oil in liquid carbon dioxide was determined in much the sameway as was the solubility of naphthalene and iodine in liquid carbon dioxide, already described by the author.2 I n this case, however, some difficulty was experienced in introducing the liquid solute into the 2
Quinn, J . A m . Chcm. Soc., SO, 672 (1928).
