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IiVDVXTRIAL A N D ENGINEERING CHEMISTRY
relationship between the latter and the degree of unsaturation (Figure 1). A few examples will emphasize this fact. The two oils illustrative of minimum and maximum iodine numbers show an increase in viscosity which is approximately inversely proportional to the “constant” in question. The order of magnitude of change of this property by the non-drying oils, olive, peanut, and almond, appears to be about the same, though peanut oil, which caused the liberation of less hydrogen chloride than the others, forms the most viscous product in this group. Neat’s-foot oil forms a product less than one-half as viscous as its associate, lard oil, although its degree of unsaturation is only 8.8 points more. That suggestion of similarity which obtains among the non-drying oils is not apparent in the semi-drying group, for here cottonseed oil forms the product of minimum viscosity whereas that of maximum viscosity, or less fluidity, is formed from sesame oil, where the absence of any detectable quantity of hydrogen chloride during the reaction may have some significance. A better concordance is found among the fish and marine animal group than in the preceding,
5-01. 23, No. 6
although menhaden oil, as before, is out of line with the others. I n this group an abundant evolution of hydrogen chloride was apparent. If the oils in this series are arranged in the order of ascending viscosities, and the latter plotted against the viscosities of the treated oil (Figure 2), it will be observed that there has been a concomitant increase in this physical property of the sulfochlorinated product. Sesame and neat’s-foot oils are not far enough out of line to affect this generalization, In the light of the experimental evidence herein disclosed, it appears that the viscosity of the product resulting from the treatment of a fatty oil with approximately 7 per cent of its weight of sulfur monochloride can be predicted more satisfactorily from the original viscosity than from the degree of unsaturation of the parent substance. Literature Cited (1) Fawsitt, J. SOC.Chem. I n d . , 7, 552 (1888). (2) Harvey and Schuette, J. A m . Chem. SOC.,60, 2837 (1928) (3) Salvaterig and Suida, 2. angew. Chem., 45, 383 (1930).
Relation between Thixotropy and Leveling Characteristics of Paint’ Elliott L. McMillen THE NEWJERSEYZINCCOMPANY, PALMERTON. PA.
F
Apparatus is described which makes possible the extent of leveling is governed REUNDLICH ( I ) uses investigation of the flow properties of plastic systems by the yield value, equilibthe term “thixotropy” at exceedingly low shearing stresses of the magnitude rium being reached when the to refer to the reversinvolved in the leveling of a paint film. shear component of the surible gel-sol transformation in A method for quantitatively measuring the thisoface tension is just equal to which a solid gel is rendered tropic changes in plastic systems is described. It is the yield value of the paint. fluid or changed to the sol Waring (5) derived the folshown that the yield value of a paint is decreased by condition by mechanical aclowing mathematical expresmechanical action and increases again after the distion, such as stirring or shaksion r e l a t i n g the depth of turbance ceases. Thus we must distinguish between ing, and after the mechanical brush mark (a measure of the a temporary or transient yield value that a paint posaction ceases the sol again leveling) to the surface tensesses at a definite time after disturbance of the paint reverts to a gel. A paint a t sion, yield value, and width s h e a r i n g stresses below its and the ultimate yield value that it develops upon of the brush mark: long-continued standing. yield v a l u e must be conIt is shown that, in addition to surface tension and sidered a gel, while a t shearI t = d‘f ultimate yield value, the transient yield value and the ing stresses above the yield 8r where h = depth of brush mark thixotropic changes in the paint must also be convalue it must be considered d = width of brush mark a s a sol. Upon removal of sidered in predicting leveling ability of a paint. f = yield value the higher shearing stress, the Good leveling is obtained by proper formulation, so y = surface tension paint again reverts to the gel that the thixotropic rate of plasticity regain after The author, making reasonc o n d i t i o n . Hence paints application of the paint is fast enough to overcome the able assumptions of surface must be c o n s i d e r e d thixotendency to sag but slow enough to allow elimination tension and width of brush tropic. For the purposes of of brush marks before the paint finally sets. marks, calculates that a paint. this discussion m y p l a s t i c system which is rendered more fluid by mechanical action and having a yield value of less than 2.8 dynes per sq. cm. would regains its former plasticity after the removal of the mechani- level satisfactorily. According t o this equation, it is only necessary to measure cal disturbance will be considered thixotropic in nature. Paints possessing false body have thus long been recognized as the yield value and surface tension of a paint to predict thixotropic, although the term as such has not been applied. whether or not i t will level satisfactorily. Since in a given It has not been realized, however, that all paints exhibit class of paints surface tension does not vary greatly, a measurement of yield value alone should suffice. This yield value thixotropic phenomena to a greater or less degree. is not the one commonly obtained by extrapolation of the Previous Methods of Evaluating Leveling Ability straight portion of the flow curve to the shearing stress It is generally recognized that surface tension is the prin- axis, but it is the minimum shearing stress under which cipal force causing leveling of a paint film and that the the paint will flow-that is, the elastic limit of the paint. The microplastometer ( 2 ) has been the only instrument which Received April 9, 1931. Presented before the Division of Paint attempted to measure this yield value. This measurement and Varnish Chemistry at the 81st Meeting of the American Chemical will render an accurate measure of yield value only if the Society, Indianapolis, Ind., March 30 to April 3, 1931.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1931
thixotropic nature of paint is well understood. It has been generally considered that yield value is a constant. Since paints are thixotropic, it is obvious that the yield value is not constant, but depends upon the previous mechanical disturbance of the paint. Thus we must distinguish between a transient variable yield value which a paint will possess a t a certain time after disturbance and the ultimate yield value that it will develop upon long standing. 3
Rate of Shear
cmhec /cm 2
I
S h e a r ~ n qJtreJs D
0
126
677
istics of certain classes of paints, nevertheless paints can be formulated with widely differing leveling characteristics and the instrument will not detect any differences. In addition, the data from this instrument do not add in the least to our theoretical conception of the mechanism of leveling. Williamson ( 6 ) has developed a method for evaluation of leveling properties of paint based upon the manner in which apparent fluidity varies with rate of shear a t low rates of shear. A modified Stormer viscometer was used for the measurements. Unfortunately, the Stormer instrument, owing to friction, does not allow accurate measurements of apparent fluidity at the low rates of shear and low shearing stresses involved in leveling, so that the method amounts to an extrapolation into a region of shearing stresses in which it is impossible to obtain measurements when using this instrument. In general, the use of this method evaluates differences in consistency which are responsible for differences in leveling characteristics provided the differences are great enough, but paints can be formulated which will differ in ability to level out whose flow characteristics will appear almost identical (Curves 1 and 3) by the use of this method. This method still gives no real insight into the reason one paint will level while another one will not.
Dynes / c d a ,so
The previously mentioned equation relating leveling to surface tension and yield value of the paint was developed upon the idea of a constant yield value, but it is equally valid as far as equilibrium conditions are concerned even though the yield value is variable. Leveling will cease when the variable transient yield value becomes as great as the shear component of surface tension, but the speed of leveling, as well as the magnitude of the transient yield value a t equilibrium and thus the extent of leveling, will be governed by the fluidity of the paint during the time leveling is proceeding. The equation is thus inadequate as i t fails to take into account the kinetic factors governing the speed of leveling.
The use of the dun'ouy tensiometer has been recommended (3) to evaluate the leveling characteristics of paints, since this instrument apparently measures, not the surface tension of a plastic system, but rather snme factor associated with the consistency. Although the tensiometer detects differences between various classes of paints, such as enamels and flat wall paints, and is useful in predicting leveling character-
A factor closely associated with leveling which has received scant attention is the tendency for paints t o sag or run. It is possible to formulate two paints whose consistencies, as measured by the Stormer instrument or any other consistency instrument, are practically identical (Curve 2), and both paints will level perfectly when applied to horizontal surfaces. If these two paints are applied to vertical surfaces, one will level out and remain in place, while the other will level but will, in addition, run or sag. The latter paint is, of course, unsatisfactory. This tendency of a paint to sag must be considered in connection with its leveling characteristics. The failure of capillary consistency instruments to allow measurements a t the low rates of shear and low shearing stresses involved in leveling is due to the excessive length of time required to make a measurement and the inaccuracy that is introduced as a result of the thixotropic nature of the paint. The failure of rotation viscometers of the Stormer type to yield accurate consistency measurements a t these low shearing stresses and rates of shear has been due mainly to friction in the instrument but partly to the failure to take into consideration the thixotropic nature of paint. I n this paper the adaptation of a viscometer to the measurement of flow characteristics of paints a t low shearing stress
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INDUSTRIAL AND ENGINEERING CHEMISTRY
will be described and the relation of these characteristics to the leveling properties of the paints will be discussed. Apparatus
A modification of the new Kampf (4) viscometer, of the rotation type, which is practically frictionless, was used in this work, making possible consistency measurements a t shearing stress less than 1 dyne per sq. cm. and a t rates of shear as low as reciprocal seconds. Figure 1 shows the details of the instrument, while Figure 2 shows the complete apparatus. The instrument makes use of a partial vacuum to lift all rotating parts so that their weight is not upon any bearing.
Vol. 23, No. 6
tion is so low that the weight of the thread alone is sufficient to cause continuous rotation even though a highly viscous oil is in the viscometer. In using the instrument to study thixotropy of paint-that is, the change in consistency with time after mechanical disturbance of the paint-at low shearing stresses, it is necessary to time the speed of rotation over short intervals and for only a portion of a revolution. This is accomplished with a telescope and scale arrangement utilizing a small mirror mounted upon the rotating shaft of the viscometer. In this way it was possible t o have a determination of the apparent fluidity of a paint as quickly as 20 seconds after thoroughly stirring it. Experimental Results
In the application of a paint by brushing, the paint is subject to relatively high shearing stresses. The leveling or elimination of brush marks depends, then, upon the flow characteristics of the paint immediately following the removal of the high shearing stresses of brushing. To duplicate the shearing stresses of brushing the paints tested were thoroughly stirred, after which the change in their flow characteristics was studied for a considerable period of time. Table I shows the thixotropy and leveling ability of several paints. Table I-Thixotropy
and Leveling Ability of Several P a i n t s
APPARENT FLUIDITY AT SHRARING STRESSOP
0.287 DYNEPER
PAINT
GRADING
1
The vacuum which is regulated by the water column, A , and indicated on the manometer, B , is slightly more than sufficient to lift all the rotating parts, so that the only contact between the stationary and moving parts is a t point C. The clearance of flange D is just a few thousandths of an inch, so that the flange acts as a piston under the action of the vacuum, thus lifting the rotating parts of the instrument. The sample of paint whose consistency is t o be measured is contained in cup E, which is always filled to the same point and raised to the same height upon the stand. Thus the rotating cylinder is always immersed to the same depth in the paint. The thread which supports the weight passes over pulley F , having jeweled bearings, and is wound on the drum about point C by turning knurled nut G by hand. When using the instrument as a viscometer, the weight is allowed approximately 0.5 meter's fall to bring the rotation to a constant speed and then timed for the next meter's fall. I n testing the instrument straight-line flow curves (reciprocal of time for 1 meter fall vs. weight) through the origin were obtained with four different oils, one of which, of known viscosity and supplied by the Bureau of Standards, was used to calibrate the viscometer in absolute viscosity units. Fric-
S Q . CM.
LEVELING
2 3 4 5 6
Poor Poor Good Good Good Good
7
Sags
T I M E AFTER STIRRING
I
20 Seconds
2 Minutes
Rhes.
Rhes. 0.000283 0.000744 0.000708 0.00177 0.00141 0,00233 0 0116
20 Minutes
Rhes.
It is seen that immediately after stirring (or brushing) the fluidity of all paints is quite high and of approximately the same order of magnitude, but decreases very rapidly, as shown by the values for 2 minutes after stirring has ceased. Exactly how long the process of leveling proceeds is not definitely known, but it appears that most of it occurs in less than 2 minutes. However, in some cases it seems certain that some leveling goes on for a considerably longer time. Paint 1 is a typical poor leveler, paint 4 a perfect leveler, while paint 7 typifies the sagging paints. Paint 2 (classed as a poor leveler) and paint 3 (classed as a good leveler) are both very close to the line of demarcation between the two classes, paint 3 probably owing its superiority to retaining more fluidity a t 20 minutes after stirring and being an example of a case where leveling probably proceeds for several minutes. It is apparent that paint 7 sags because it retains a much higher fluidity after brushing ceases than do the others. Paints 1 and 5 have set to solid gels in 20 minutes, so that they may be said to have yield values greater than 0.28i dyne per sq. cm. a t 20 minutes after stirring, although immediately after stirring their yield values are much smaller. We have thus a picture of the paint becoming less fluid and its yield value growing with time after application. It is not necessary that a paint have a yield value of 2.8 dynes per sq. cm., as declared by Waring (j),in order not to level because, even though the yield value may be much smaller, the paint attains viscosities of 10,000 to 1,000,000 poises in a short time after application, and this high viscosity may hinder leveling until surface drying sets it. Thus, even though we cannot assign definite yield values t o certain paints, we must assume that as their viscosities approach a million poises they behave essentially like solids.
June, 1931
INDUXTRIAL AND ENGINEERING CHEMISTRY Discussion
thixotropy Of plastic systems has been known for some time and the degree of thixotropy has been estimated by rough empirical tests, such as failure to pour from an inverted test tube. the mesent use of the KamDf viscometer represents the first attempt a t a quantitati;e evaluation of the thixotropic changes in consistency in terms of absolute viscosity or fluidity units. This instrument makes possible the investigation of thixotropy in plastic systems not previously suspected of being thixotropic. The use of the apparatus and methods described should ultimately lead to a clearer understanding of the phenomena to thixotropy and it5 relation to plasticity. It is planned to show in a future
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publication that the thixotropic change in consistency with time follows a definite course and can be expressed by means of an empirical mathematical equation, the constantsof which completely describe the thixotropic nature of the plastic Literature Cited (1) Freundlich and Bercumshau, Kolloid-Z,, 40, 19 (1926), ( 2 ) Green, IND. END. CHEM.,I T , 726 (1925). (3) Haslam and Grady, I b i d . , Anal. Ed., 2, 66 (1930). KoJzord-Z**61* 165 (lQ30) (4) (5) Waring, Paper presented before Society of Rheology, Easton, Pa., Dec., 1930. (6) Williamson, Patterson, and Hunt, IND. END. CHEM, 21, 1111 (1929).
Asphalt from the Cracking Process' Gustav Egloff and Jacque C. Morrell UNIVERSAL OIL PRODUCTS COMPANY, 310 SOUTHMICHIGAN AvE., CHICAGO, ILL.
ARCUSSON (3) conThe production of asphalts from liquid residue deis present. They postulated siders the acidic or rived from the cracking process is reported herein. that oxidation of the sulfur s a p o n i f i a b l e subThe cracking reaction can be controlled so as to procompounds causes a simulduce essentially gas, gasoline, and asphaltic substances. taneous oxidation of the other stances present in asphalts as The quality of the asphalts produced may be so diconstituents. For example, asphaltogenic acids (these are rected as to be useful for paving, roofing, shingle the addition of 8 per cent sulconverted by heat into the anhydrides). the substances saturants, coating asphalts, flooring, and emulsions. fur to a residue from iMexican oil yielded, after blowing with adsorbable' b y fuller's earth from a petroleum-ether solution of the asphalts as resins, and air for 31 hours a t 225" C. (437"F.), a product with a softenthe insolubles in petroluem ether as asphaltenes. He states ( 2 ) ing point of 185" C. (365" F.) and a penetration of 7 mm. a t that the distillate from natural asphalts contains considerable 25' C. (77' F.), while under similar conditions of treatment organic acids, whereas very little acid passes over in the the addition of 12 per cent sulfur gave a product with a softening point of 200" C. (392' F.) and a penetration of 5 case of petroluem pitches. The generally accepted classification of the components in mm. The time required for a pronounced hardening by asphalt includes petrolenes extractable with petroleum ether oxidation may be reduced from 96 hours to 14 hours by the (hexane), asphaltenes extractable with carbon tetrachloride, addition of from 3 to 5 per cent sulfur. I n the cracking process, however, where no oxygen is presand carbenes extractable after above operations in cold ent and sulfur is practically absent when oils such as those carbon bisulfide. from Pennsylvania are treated, the above four chemical Chemistry of Asphalts processes for the formation of asphalt are reduced to polymerization and condensation. Very little is known about the chemistry of asphalts. Asphalt produced by cracking has a smaller percentage of They have been generally considered as being formed by hydrogen than the original charging stock or any fraction chemical processes such as oxidation, sulfuration, polymeriza- thereof; hence dehydrogenation is one of the chemical proction, and condensation. The term "condensation" as used esses by which such asphalt is formed. The resultant ashere refers to the combination of unlike molecules as dis- phaltic compounds may not be due t o direct dehydrogenation, tinguished from polymerization which refers to the com- but may form in several steps comprising dehydrogenation, bination of like or identical molecules. polymerization, condensation, intermolecular rearrangement, One plausible theory about the formation of asphalts by or combinations thereof. The course of the reactions is oxidation is that polycyclic compounds, acidic in character, suggested below: are formed during the intermediate stages of oxidation which Paraffins upon further heating change to anhydrides such as those of the polynaphthenic acids, with progressive condensation / \ Olefins4Naphthenes and polymerization. I I I It is well known that asphalt has been made by the reaction of sulfur and hydrocarbons in the absence of oxygen. This is probably brought about by a series of dehydrogenaI tions, forming hydrogen sulfide and hydrocarbon compounds of sulfur of a highly condensed character. Aromatics The presence of sulfur promotes oxidation. Brooks and Humphrey (1) stated that hydrocarbons of the saturated Condensation may take place between any of these prodand unsaturated type are more readily oxidized when sulfur ucts. It is shown by the present work that polymerization and 1 Received February 26, 1931. Presented before the Division of condensation accompanied by dehydrogenation may account Industrial and Engineering Chemistry at the 81st Meeting of the American for the formation of asphalts in the cracking process. Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931.
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