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may be prepared. Copolymerization of this material with TABLE11. INFLUENCE OF TEMPERATURE ON RATEOF CONVER- purified water-white vinyl acetate, styrene, or methyl methSION OF VIKYLACETATE-DIETHYLENE GLYCOL MALEATE SOLU- acrylate yields water-white resins. From tests on a FadeTIONS ometer these castings have been found to be almost com--Conversion Rate, Hourspletely light-fast. 390 c. 50' C. 64" C. The Rockwell hardness of copolymerized vinyl acetatePartiai gel 100 ... ... Complete gel 150 ... ... glycol maleate ranges from L 50 to L 110, which is comparable Hard gel 240 20 ... Soft resin ... 70 20 to commercially available cast phenolic resinoids as well as to Hard resin ... 140 80 the vinyl, acrylic, and methacrylic resins. The greatest commercial virtue of these copolymerized resins lies in their rapidity of cure and ability to be made in light colors or of converted vinyl acetate-glycol maleate mixtures containing water-white, as desired. With relatively nonvolatile vinyl more than 32 per cent of vinyl acetate are explainable in derivatives such as styrene, molding compositions have been the light of the foregoing. When the concentration of vinyl made which exhibit curing rates comparable to those of urea acetate is low, a8 in mixtures containing less than 32 per cent resin compositions. Molding temperatures of 130 140' C. vinyl acetate, reactions b and c would predominate; but in were employed with pressures of 2000 pounds per square view of the greater activity of vinyl acetate, reaction c would inch to give set-up periods of 15 seconds and curing times of 2 probably occur most readily. minutes as compared to 30-second set-up and 2-minute cure Vinyl derivatives other than vinyl acetate show different for urea resin compositions under similar conditions. limits of compatibility in the cured resin. Styrene, for instance, has a limit about that of vinyl acetate, whereas a Acknowledgment mixture of equal parts of vinyl acetate and styrene will yield The author wishes to thank the Ellis-Foster Company for clear, cured masses when present in amounts up to 40 per their kind permission to publish these results. cent of the casting solution. Methyl methacrylate, on the other hand, is compatible in all proportions. Literature Cited
Miscellaneous Properties of Glycol Maleate Copolymers By the purification of maleic anhydride and diethylene glycol on vacuum distillation and reaction under controlled oxygen-free condition, water-whi te diethylene glycol maleate
(1) Bradley, T. F., Kropa, E. L., Johnston, W . B., IND.ENG.
CHEM.,29, 1270-1276 (1937). (2) Dykstra, H. B., U.S. Patent 1,945,307 (Jan. 30, 1934). (3) Vincent, H. L . , IND. ENG.CHEM.,29, 1267-9 (1937). PRESENTED before the Xorth Jersey Section of the American Chemical Society.
1
Explosibility of Aluminum Powder-Silica Dust Clouds 0
RALPH B. MASON AND CYRIL S. TAYLOR Aluminum Research Laboratories, New Kensington, Penna.
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HE discovery that aluminum powder is an effective agent in the prevention of silicosis (1) has raised a number of questions regarding its application in practice. For example, does the introduction of the necessary amount (1 per cent) of aluminum powder into the silica dust created in mine atmospheres by drilling and blasting create an explosion hazard? Fortunately, the answer is no. Although the explosion-preventing effect of the silica dust on concentrated suspensions of aluminum dust could easily be predicted from the well known effect of rock dust in coal mines (S), it was thought worth while to determine the facts experimentally. Previous work from this laboratory ( 2 ) showed that the lower explosive limit of mixtures of aluminum powder and dry air was approximately 40 mg. of aluminum powder per liter of air (40 ounces per 1000 cubic feet). That work also showed that aluminum dust cloud explosions could not be initiated when the air was so diluted with either carbon dioxide or nitrogen that the oxygen content was below 10 per cent. A study has now been made of the effect of silica dust upon the explosibility of aluminum powder-silica dust clouds in air. The apparatus for this investigation and the details of procedure were given in the previous paper ( 2 ) . The method
consists essentially in raising, with a measured puff of air, a uniformly distributed dust cloud in a closed glass tube. An igniter is then fired a t the proper moment, and the pressure produced is recorded on a pressure recorder. The air used in the present experiments was not dried, as other experiments indicate that the presence of normal amounts of moisture has no effect on the explosive limits. The aluminum powder used in these experiments was the extremely fine, light, fluffy powder designated in the previous paper as powder B, which had an average flake thickness of about 0.14 micron. The silica used was dust collected from the girders in the crushing plant of the McIntyre-Porcupine Mines, Ltd., and was furnished through the kindness of J. J. Denny. It passed easily through a 325-mesh screen. I n one series of experiments aluminum dust was used in such an amount that the concentration obtained in the explosion chamber was approximately 106 mg. per liter, or over twice the minimum amount required to produce an explosive mixture in air. This is more than enough t o produce a sharp explosion in the absence of an inhibiting agent. The addition of an equal weight of silica dust (106 mg. per liter) practically prevented an explosion; with double that weight of
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silica dust no appreciable increase in pressure was obtained when ignition of the suspension was attempted. Apparently some aluminum dust, within the zone of high temperature produced by the ignition, burned and added its increment of heat, but the blanketing effect of the silica prevented propagation of the ignition and thus prevented any explosion. In another series of experiments, the concentration of the aluminum dust in the explosion chamber was maintained a t approximately 212 mg. per liter, or more than five times the amount required to produce an explosive mixture with clean air. This, of course, would produce a violent explosion if ignited in the absence of any inhibiting material. The addition of silica dust in the concentration of 106 mg. per liter (one half the concentration of the aluminum powder) did not prevent an explosion; but when the silica content was increased to approximately 160 mg. per liter, the reaction was less violent and only a moderate increase in pressure was recorded. Upon increasing the silica dust content to 424 mg. per liter (twice the concentration of the aluminum powder), no appreciable increase in pressure was recorded when an attempt was made to ignite the suspension. It was practically impossible to produce a uniform dust cloud when attempts were made to suspend larger amounts of aluminum dust and silica dust in the explosion chamber. Therefore recourse was had to qualitative experiments. As a qualitative experiment, a mixture of one part silica dust and one part aluminum powder was blown through the flame of a Fisher burner; it ignited with a vivid flash. When, however, two parts of silica dust and one part of aluminum
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dust were used, the mixture barely burned in the flame. In blowing the material through a flame, the actual concentrations of dust obtained are naturally many times those used in the explosion chamber experiments. I n actual practice, we understand that an ounce of aluminum powder has been found to be enough to insolubilize the silica in a dust cloud of 8000 cubic feet volume, produced by a blast a t a mine face. In order to produce an explosive mixture, this ounce of aluminum dust would have to be concentrated in a volume of not more than 25 cubic feet of clean air. Silica dust present in the atmosphere effectively reduces the explosibility of aluminum powder a t even higher concentrations. With large proportions of silica dust, such as would be present when the aluminum dust was used to prevent silicosis, it would be impossible to initiate an explosion, even if the concentration of the aluminum dust were accidentally increased until it passed the normal lower explosion limit. This effect is obviously similar to that of rock dust in preventing coal dust explosions, and presumably results from the absorption of radiated heat by the inert mineral powder.
Literature Cited (1) Denny, J. J., Robson, W. D., and Irwin, D. A., Can. Med. Assoc. J., 37, 1-11 (1937); 40, 213-28 (1939); 2nd. Med., April, 1939; J . Am. Med. Assoc., 112,2093 (1939). (2) Mason, R. B., and Taylor, C . S., IND.ENQ.CHEM.,29, 626-31 (1937). (3) Price, D. J., and Brown, H. H., "Dust Explosions", p. 197, Boston, National Fire Protection Association, 1922.
Flow Characteristics of Lime-Base Greases J. F. T. BLOTT AND D. L. SAMUEL Asiatic Petroleum Company, Ltd., London, England
HE physical nature of petroleum greases constitutes a problem to which no agreed solution has yet been found. Greases are described vaguely as colloidal dispersions of soaps in oils, or as water in oil emulsions stabilized by soap. Lawrence (6) classifies them as ( a ) true gels, in which class he places lime-base greases, and ( b ) pseudo gels or pastes of fully crystalline soaps suspended in oils, in which he includes soda- and aluminum-base greases. However greases may be classified, there is no doubt that they belong t o the category of plastic materials. These are bodies which may have a yield value and whose viscosities are dependent on the shearing stress applied to them. Yield value, which may be defined as the minimum tangential force per unit area necessary to cause flow, is possessed to some degree by the majority of commercial greases. If a force greater than this critical value is applied, the lubricant will flow, but the rate of flow will not be directly proportional to the applied stress. When grease is applied to a bearing, it is subjected to a shearing stress which will depend on the speed of rotation and the clearances involved. Under these conditions it will flow and its viscosity will become an important factor in the satisfactory running of the moving parts.
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Some attempts to measure the viscous properties of greases have been made in recent years. Porter and Cruse (8) made use of a modified form of Bingham and Green plastometer to examine the flow under stress of a number of cup greases. They measured a yield value and mobility as defined by Bingham (3). A consistometer designed to measure the flow of plastic materials under stress was developed by Bulkley and Bitner (4) in 1930. This apparatus was restricted to measuring comparatively low rates of flow under pressures not exceeding one atmosphere. Later, in 1932 Arveson ( 1 ) carried out a number of experiments with cup greases in his constant-shear viscometer in which the material was forced through a capillary at a constant speed and the pressure drop through the capillary was measured. In 1934 Arveson (2) published a pager in which the effect of temperature was taken into consideration. For the calculation of his results Arveson used the Poiseuille equation :
where
?la: = apparent viscosity, poises
P = pressure in dynes/cm.2 R = radius of capillary, cm. L = length of capillary, cm. Q = rate of efflux, cc./sec. F = shearing stress at walls = (PR/SL) dynes/cm.* S = rate of shear = (4&/rR*)sec.-l
