Viscosity of Linseed Stand Oil at High Shearing Stresses J
D. TOLLEN.;IAR .AXD H. BOLTHOF I n s t i t u u t voor Grafische Techniek, A m s t e r d a m , Hollund s t a n d oils obtained bj heating linseed oil possess colloid properties which are manifested by a decrease in viscosity with increasing shearing stress. This decrease is the result of deformation and consequent alignment of colloid particles. The decrease in viscosity of mixture4 is
greater when the average particle size of one component differs more widely from the average particle size of the other component. The measurements were carried out in a capillary viscometer over the shearing stress range of LO3 to I O 6 dynes per sq. cm.
T
shot\. normal flox. Mineral oils are slightly aiiomalous, probably because of the effect of temperature result~ingfrom the swift flow of the liquid.
HE oils used in vehicles for paints, printing inks, arid lacquers
are often considered as oleocolloids. Their viscosities are often determined in a st,andard apparatus (flov cup) but somet,irnes i n viscometers in vi-hich simpler physical conditions are realized. Practically none of these methods makes allowance for anomalous viscosity-i.e., the dependence of viscmity on shearing stresses or rate of shear. This is all right as long as the imposed shearing stresses do not lead to drtiations from true Newt.onian flow-in other words, \Then the shearing stresses are lorn. I t does not imply, however, that’ these vehicles will not show anomalous viscosity when sibjected to very high shearing stresses. In many practical applicat,ions the limiting shearing stress is never reached; for example, in brushing out paints or lacquers, there is no decrease in viscosity of the oils as a result of the imposed shearing stresses. Rut the case of printing inks is somewhat different. Here the stresses are much greater than are those of paints; a t highest pressure in the printing press they are of the order of lo’ dynes per sq. em. This led us to suppose that the viscosity anomalies, as shorn by the plasticity diagrams of printing inks (highly concentrated suspensions of pigment in oleocolloids), may be partly due to the nature of the vehicle. To verify this, it n x s necessary to obtain plasticity diagrams of the vehicles at sufficiently high shearing stresses
DEFORMATION AND ALIGN31 EST
Prolonged heating of linseed oil gives a congealed product. I t is clear that stand oil mush have colloidal properties. Since in the case of stand oils there is no question of yield stress, the presence of macromolecules can be borne out in the plasticity diagram only by deviations from true Newtonian flow. These macromolecules, which must be pictured as :t three-dimensional network (a), will generally take up random positions, and there is thus no definite alignment. Only when particle deformation occurs under the influence of shearing stress is alignment started. This then brings about a decrease in viscosity, Therefore, stand oils prepared by mixing oils of high and low viscosity will give more indications of anomalous viscosity .than monodisperse stand oil of equal initial viscosity. To verify this, let us compare the deformations per kinetic unit occurring in a liquid with normal molecules and in a liquid of the same viscosity containing macromolecules. In the former caae the repelling forces greatly increase when the molecules approach one another beyond the point of equilibrium, and there is practically no deformation. In the latter case, where there is a possibility of free rotation, the average deformation per kinetic unit is much greater. As a result the random structure breaks down and an alignment, is started. This will produce double refraction during flow (strram double refraction).
INDICATIONS OF ANOMALOUS VISCOSITY
The technical literature does not report many instances of anomalous viscosity behavior of stand oils. According to Nisizawa ( 4 ) nonpolymerized fatty oils do not behave like true Newtonian liquids at very low rates of shear. Ostwald, Trakas, and Kohler (6) pointed out that in this case the dimensions of the capillary were partly responsible for the high initial values. Thus, it is uncertain whether the anomalies shown by these oils are due to the properties of the materials themselves or to the apparatus. These investigators did find, however, that linseed stand oil and Volta oil (a mixture of mineral and vegetable oil) showed anomalous viscosity a t very low shearing stress. Freundlich and Albu ( 1 ) examined air-blown linseed oil, as me11 m airblown linseed oil with drier, in a Couette torsion viscometer and found no viscosity anomaly. They probably did not reach the low shearing stress of the viscometer used by Nisizawa. Wacholtz ( 7 ) experimented with stand oils prepared either by heating linseed oil in the usual way or by diluting highly viscous stand oil with untreated linseed oil. His data make it clear that shearing stresses of approximately lo5 dynes per sq. cm. were reached. This investigator found also that stand oils showed definite anomalous viscosity and that, given the same initial viscosity, this phenomenon was more pronounced when the material contained more viscous oil. At lower rates of shear, and hence also a t lower shearing stresses, even the highly viscous stand oils
INSTRUMENT
The choice of viscometer was governed mainly by the requirement that shearing stresses of lo7 dynes per sq. cm. should be attainable. Rotation viscometers are unsuitable as too much heat is generated under such conditions. A capillary viscometer has the drawback that the shearing stress is not the same a t all points of the capillary diameter. The measurements plotted in our plasticity diagrams represent the rate of shear as a function of the shearing stress a t the wall, 7.. This constitutes no great difficulty, since Saal and Koens’ formula (6) makes it possible to calculate from these data the viscosity a t any given shearing stress. For the development of the present theory this calculation is not essential, and it has been omitted. The construction of the viscometer was briefly as follows: A reservoir containing the material can be kept a t constant pressure up to 70 atmospheres by means of nitrogen gas from two cylinders. The liquid is then successively pressed through three capillaries nhich are fixed in mide, short channels. Preceding each capillary, a side branch is connected to a manometer. The capillaries are interchangeable, and for each shearing stress the correct capillary can be chosen from a series. The entire apparatus is suspended in a
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TEMPERATURE ERROR
One of the main sources of error, peculiar to the capillary viscometer for high pressures, is the decrease in viscosity as a result of the increase of temper:iture during the swift flow through the capillary, Wacholtz ( 7 ) experienced similar difficulties with his viscometer, and perhaps t,he anomalous viscosity behavior of mineral oils in his experiments can be ascribed to this . . .... . . ..-... . .... I effect. *\* In the construction of our apparatu much allowance as possible was made for this phenomenon. The metal capillaries I1 were fitted in solid brass frames, which quickly removed the heat evolved by friction. If all the potential energy of the liquid is converted into heat tlnring flow,thih 1 leads to a heat development, of 106 ergs pcr ml. or 0.025 calorie per atmosphere drop i n pressure. Given a specific heat of 0.5, tliis mrrcsponds t o a temperature increase of 0.05' C. per atmosphere drop in pressure. Assuming the initial viscosity at low shearing stresses to be 150 poises and taking into account ii decrease of viscosity at room temperature or 5 poises per ' C. rise in temperature (3),a drop in pressure of 40 atmospheres produccs an error entirely as a result of generated heat of friction, of 10 poises; this is more t,han the 5% allowed for such measurements. I n order to avoid this error, for each capillary the measuring range was deterI I _ _ _ ~ ~ mined empirically on a mineral oil, 3 4 5 since there had never been any indiLOG SHEARING STRESS C LOGTwl- DYNE/SQ.CM cation of anomalous viscosity with these Figure 1. Viscosity of Mineral Oils (I1 and 111) and Linseed Stand Oils So long as no noticeable heating oils. (H and €16) as a Function of Shearing Stress occurs during the flow through the capillary. we may consider the viscosity of the mineral oil to be independent of the shearing stress. If ~e graduwater thermostat, the temperature of which is kept constant ally increase the pressure difference in this cdpilary, the dewithin 0.1" C. From the volume of liquid passing through per crease in viscosity as a result of heating will gradually become unit of time V / t (determined by weighing), the pressure drop in noticeable. This divergence in the first approximation is proporeach capillary, and the dimensions of the capillary, the viscosity tional to the imposed shearing stress. When this shearing stress can be calculated by means of Poiseuille's law: hm increased so that the error is about 5%, the limit for that V_ = TPR' particular capillary has been reached. This limit is evident in the . t &l of ?I us. log T~ diagram. As the specific heat, the thermal plot t Thus q=PpC1 conductivity, and the dependence on temperature of viscosity are approximately of the same order in mineral and fatty oils, the where P = pressure, atm./sq. cm. eapillaiy has the same measuring range for both oils. It follow that, to determine the instrument constants C1 and Cn, the meas106R' urements must lie in this range. For the capillaries a t our dirC1 = -, hence CI is dependent only on the dimen81 C ) C the ~ limiting value \vas T~ = 106dynes per sq. cm. sions of the capillary. . ,
_
By accurate measurements of length (to 0 *0.0001 cm.), it was pojsible after calibrating with a viscous liquid (a mineral oil of known viscosity) to calculate C1 from which R may bc derived. By substituting I? in the formula rw = PR/21, the shearing stress a t the wall of the capillary can be expressed by tlie relation, T - = PC2. I n this way the following instrume~ltconstaxits for the capillaries were obt'ained: Capillary NO. I b KO. IC S O . 2b No. 2 c
KO. 3 b
1 . cm. 0.519 0.534 0.531 2.033 42
R , CIII. 0 . O d 11 0,0245 0.0520 0.0578 0.25
CI 5.24 0.308
1.9;
5.32 2.11
4.9 x 10' 1 . 4 2 X 10'
32.3
c y2
4
0.29
x x
x
10' 101
101
I
EXPERIIIENTAL CURVES
The measurements were carried out on two mineral oils (I1 and 111) and two linseed stand oils (H and H6). Mineral oil I1 w:w obtained by intensive mixing of 877, highly viscous oil (506 poises at 30.3" C.) T7it.h 13% spindle oil at 150" C. At 26" C. tlie viscosity, determined in the capillary vijcometer, was 140 poistas (Figure 1). hlineral oil I11 consisted of 807, oil of 506 poises (at 30.3' C.) and 207, spindle oil. With both mineral oils the viscosity appeared to be independelit of the applied shearing stress. Stand oil H was prepared from bleached linseed oil. Heating took place in a flask (Jena glass) at 290" C. while nitrogen gas \vas passed through. This process W:IP
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continued for about 20 hours and produced a stand oil of 138 poises at 30" C., measured in a Hijppler viscometer. Stand oil R swas a mixture of oil H (SO\%) with bleached linseed oil (2070), obtained by heating and stirring for a short time a t 176" C. As Figure 1 shows, both stand oils decrca.ed in viscosity at high $hearingstresses. 1
+d
0
853
EFFECT O F MIXLNG
Similar measurements were carried out on mixtures of staiid oils and unpolymerized linseed oil (Figure 2). Stand oil G n'ab prepared by heating bleached linseed oil for a long time in a glass receiver at 290' C. while nitrogen gas rn as passed through Stand oil Fc was prepared by heating bleached linseed oil for 3 hours at 290" in a tin-coated copper vessel while nitrogen gas was passed through (viscosity, 297 poises at 30" C.). The resulting stand oil was mixed with 31y0 linseed oil to form oil F,. The curves show that oils G, He, and Fd have the same viscosities at low shearing stresses. \i7ith a greatw variation in the viscosities of the components, there is a. greater decrease in viscosity at increasing shearing stress. A stand oil giving minimum decrease of viscbosity at increasing shearing stress must be prepared by heating -t:il Smith, L. B. P e t t . and Esther Phipard.
prevention of rancidity, and formation of dark pigments, would result in superior products. Such superiority was reflected in the increased acceptance, which stimulated research and led to development of ultimate quality in dehydrated foods previously unassociated with them. The growth of the dehydration industry during the last three years is indicated by the following figures: The production of white potatoes during the period 194243 was about 50 million pounds (dry n eight) ; the 1944-45 production rose to 129 million pounds. Figures for the corresponding years in the case of beets are 3 million and 7 million pounds, for cabbage 7 million and 10 million pounds, onions 6 million and 21 million pounds, and w e e t potatoes 8 million and 16 million pounds. 16 the evaluation of dehydrated foods, the retention of nutritive value is an obvious criterion. Paradoxically, the literature on this phase of the subject is not veiy extensive. The few pub-
