Working-Range Flow Properties of Thermoplastics F. E. WILEY
A test to measure the worlung range flow properties of thermoplastics is described, and data obtained by this test are presented. The apparatus is simple and may be readily constructed from standard laboratory equipment. No inordinate skill is required on the part of the laboratory technician in securing precise results by this method. Results can be readily correlated with those obtained on the Bakelite-Olsen flow apparatus. By this test, data on flow charadteristics of thermoplastics have been obtained that are of value in establishing correct thermal conditions in certain processes. Undoubtedly there are other processes in which such data would be useful.
Plax Corporation, Hartford, Conn.
test is capable of determining the temperature necessary for the successful forming of a plastic in many machines and processes as well as determining rheological characteristics. This method measures the coefficient of viscous traction of a thin plastic test strip subjected to high temperatures and low stresses. The viscosity of the plastic is calculated and a viscosity-temperature chart obtained.
Experimental
NE of the most important properties of a thermoplastic molding compound is the temperature at which the consistency of the material renders i t suitable for some forming process. Many tests have been devised to measure such temperature-consistency relations (1). I n this paper a simple method is described which requires a minimum of apparatus and is therefore available to any laboratory. The
0
FIGURE1. APPARATUS FOR MEASURING THE WORKINQ-RANGE FLOW PROPERTIES OF THERMOPLASTIC MATERIALS
APPARATUS.Figure 1 shows the apparatus, assembled from standard laboratory equipment. It consists of an electric hot plate, a 2-liter beaker filled to about 1 inch of the top with mineral oil1, a 500-watt thermostatically controlled immersion-type heater, a laboratory thermometer, a stop watch, and a micrometer. The measuting unit is a light metal frame consisting of a vertical member with two rigidly attached horizontal arms. The upper arm is provided with a clamp to hold the specimen. A millimeter scale is attached to the vertical member above the upper cross arm. Each cross arm has a small hole drilled vertically through its midpoint; these holes serve to guide a very light piano wire pointer. The upper end of the pointer is bent so that its position can be read conveniently on the millimeter scale. A washer is welded to the pointer at a point about 4 cm. from its lower end. By means of this washer, a load consisting of washers is bolted to the lower end of the specimen. SPECIMEN.Figure 2 shows the dimensions of standard test FIGURB2. STANDspecimens, accurately cut by a ARD SPECIMEN USED die from a compression-molded IN THERMALVISCOSITY TEST diskof the compoundto be tested. It is important that no high internal strains be created by the molding process. Standard specimens are used as a matter of convenience but are not strictly necessary. Also, specimens may be cut by hand from sheet stock or other thin material. The hole at the lower end of the specimen may conveniently be made with a paper punch. The oil bath is brought to the desired temPROCEDURE. perature, stirring being necessary to ensure uniformity which must be maintained within '1 F. The metal frame is removed from the bath, and the specimen is clamped to the
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Thermol Medium, a Btanco product, has been found satisfactory.
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cm. or more than 0.25 cm. per second. If a wide temperature spread is to be investigated, the stress may be varied in such a manner that the rate of flow falls within these limits. Most satisfactory results are obtained a t a stress between 90 and 100 grams per sq. cm. I n computing the proper load, allowance must be made for the buoyant effect of the oil bath. This effect may readily be determined experimentally and a calibration curve secured. If specimens of heavier section than the standard are used, it is advisable to hold the pointer for a few seconds to allow the specimen to heat up before the test is started.
upper arm after having been carefully measured by a micrometer to the nearest 0.0001 inch. The desired load is bolted to the specimen and the pointer, the scale reading of the pointer being noted. The frame is lowered into the bath to such an extent that the upper end of the narrow parallel portion of the specimen is slightly below the surface of the oil. The stop watch is started a t the first movement of the pointer; this usually occurs immediately upon immersion or during the first second thereafter. Positions of the pointer are read at intervals of 5 seconds until the total elongation becomes 2.5 cm. It has been found that for most plastics the data are valid when the rate of flow a t I-cm. elongation is not less than 0.01
Calculations The observed data are plotted on coordinate paper with elongation as ordinates and time as abscissas. Figure 3 shows typical elongation-time curves obtained from the test. The slope of these curves where the elongation is 1 cm. is computed as the rate of flow in centimeters per second. This rate, D,is used to compute the viscosity of the plastic, 7, by means of Equation l: 7 =
where 7
(1)
viscosity, poises
m = effective load grams g = 980 cm./sec./sec. 10 = initial length of specimen, em. I = length of specimen when rate of elongation is determined, cm. AO = initial cross-section area of specimen, sq. cm. D = rate of elongation of specimen when its length is I , cm./sec.
TIME I N SECONDS
TIME I N SECONDS
=
rngl2/3AoEoD
FIGURE3. ELONGATION-TIVE CURVESOBTAINEDBY THERMAL VISCOSITYTEST Left, methyl methacrylate plastic: right, cellulose acetate plastic
The ratio of the force of traction per sq. cm. of cross section to the rate of elongation per cm. of length is termed the “coefficient of viscous traction”. The ratio of shearing stress to rate of shear is termed “coefficient of viscosity”. Trouton (9) established experimentally that the coefficient of viscosity is equal to one third of the coefficient of viscous traction. Burgers further substantiated this fact theoretically ( 2 ) . Lillie also uses this factor of one third in his test for the viscosity of glass (6). Hence TRACTIVE STRESS IN GRAMS/
S P . CM.
TRACTIVE STRESS IN GRAMS/
sa. CM.
FIGURE 4. CURVES FOR INVEBTIGATION OF THE PRESENCE OF A YIELD
POINT
Left, polystyrene (Bakelite XIL1S-10023); right, polyvinyl chloride-acetate (Vinylite W-lfilS)
mg12
v=aAalaD where S
=
tractive stress, dynes/sq. cm.
d = rate of elongation per cm. length,
cm./sec.
1.5
The rate of flow is always determined when the elongation is 1 cm. Hence, when a standard specimen is used a t standard stress, Equation 1 reduces to Equation 2 :
ro
y
1.0
V
E 5
4
*5
q =
K/D
(2)
L.
where K ‘0
500
1000
1500
2000
PRESSURE I N LBS/SP
2500 IN
3000 0
500
1003
I500
2OW
2500
3000
PRESSURE I N LBS/ SQ IN
FIGCRE5. PRESSURE-FLOW CCRVESOBTAINEDON BAKELITE-OLSES FLOW TESTER Left, polystyrene (Bakelite XMS-10023) : right, a cellulose acetate plastic
mg12 = - (a constant)
3Aolo
This calculated viscosity corresponds to the temperature of the test; at least four such tests are usually required to obtain a curve of viscosity us. temperature. Such a curve can
November, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
TEMPERATURE
FIGURE 6. VISCOSITY-TEMPERATURE CHARACTERISTICS OF CELLULOSE ACETATE PLASTICS OF S ~ V E R A PLASTICIZATIONS L WITH METHYL PHTHALYL ETHYLGLYCOLATE
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temperature (4). Some polyvinyl chloride-acetate compounds, however, show a small yield value. The presence of a yield point may be investigated by varying stress at constant temperature. Such curves for stress us. rate of flow are shown in Figure 4. Those for polystyrene show no yield value. Those for polyvinyl chlorideacetate show one definitely; this is the only plastic tested for which so definite a yield value could be found at these temperatures. The relation of stress vs. rate of flow is linear and remains so even at high stresses. Data obtained from Bakelite-Olsen flow tests (8) on many types of plastics (Figure 5) show this to be true, an important factor establishing the validity of flow measurements obtained at low stresses (8). Figure 6 shows the viscosity-temperature characteristics of cellulose acetate plastics of several different plasticizations with methyl phthalyl ethyl glycolate (6). The curves illustrate the change in temperature properties created by addition of plasticizer to cellulose acetate. Figure 7 is a curve of temperature us. cellulose acetate content for constant vis-
be conveniently plotted on semi-logarithmic paper as in Figure 6.
% BY WT. S A N T I C I Z E R
M-17
70
60
50
40
30
20
10
25030
40
50
60
70
80
90 100
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Peculiarities of the Test
If the specimen possesses great internal strain due to faulty preparation, and the test is run at a moderately low temperature, the specimen will a t times tend to unmold and shrink upward rather than elongate. Under these conditions the test will not be valid. Some materials possess a relatively high degree of water which, under the conditions of test, forms bubbles within the specimen. It is obvious that tests on such materials are not valid at temperatures where bubbling occurs in the specimen. I n the case of a few materials a t elevated temperatures, and with many materials at very low temperatures, the test breaks down because of a yield point, to be described later. Discussion of Data
CELLULOSE ACETATE CONTENT I N % BY W T
FIGURE7. CELLULOSE ACETATECONTENT Figure 3 (left) shows elongation-time curves for a methyl 09. TEMPERATURE NECESSARY FOR CONSTANT VISCOSITY methacrylate plastic tested over a range of temperatures. The curvature is almost entirely due to the decrease in cross-sectional area caused by elongation. Figure 3 (right) is a set of similar curves for .cellulose acetate. The rate of flow does not increase on elongation, as would be expected because .of the reduction in cross-sectional area. This indicates an increase in internal friction during flow. At higher temperatures and stresses the effect is more transient, while at lower temperatures it persists even at very high stresses. I n general, polystyrene, like methyl methacrylate, does not display to a noticeable degree any increase in internal friction at the temperature of the test. -The effect is most noticeable in highly plasticized acetate formulations. To avoid uncertainties caused by changing internal friction, the rate of flow is determined by the slope of these time-elongation curves when the elongation is 1 om. With most common thermo120%. 130 140 150 160 170 iao 190 Dlastics. this mocedure has been found very satisTEMPERATURE iactory.' It is significant that the practical or Bingham FIQURE8. VISCOSITY-TEMPERATURE RELATION OF POLYVINYL ACETATECHLORIDE, METHYLMETHACRYLATE, AND POLYSTYRENE PLASTICS yield point disappears in almost all cases at elevated
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d
?! 3 8 0
2
"0
'6,
I e
340
300
&
z
E 260 220 260 300 340 TEMP.IN'F. FOR FLOWOFI IN.IN2MIN. UNDER 1500LB./SQ.IN. IN BAKELITE-OLSEN
FLOW TESTER
FIGURE9. CORRELATION CURYEOF THERMAL VISCOSITY TESTWITH BAKEFLOW TESTFOR CELLULORE LITE-OLSEN ACETATEP L l s T I C S
cosity. The relation is nearly linear in the range 50 to 80 per cent. Figure 8 gives the temperature-viscosity relation of certain polyvinyl acetate-chloride, methyl methacrylate, and polystyrene plastics. This viscosity test may be correlated with standard flow tests on similar groups of materials and serves as an inexpensive method of flow determination. A typical correlation curve for cellulose acetate is shown in Figure 9. The curve is linear and nearly a t a 45" slope, which indicates that the viscosity curves are nearly parallel over the temperature range involved, However, as Bakelite-Olsen tests are carried out a t lower temperatures, there are some inversions due to the differing rates of decrease in viscosity with increase in temperature of some groups of plastics (7).
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Experience has shown that for each forming machine or process there is a characteristic viscosity which a plastic must possess if optimum results are to be obtained. Thus when the best operating temperatures for several different plastics are found by experiment on a machine under similar production rates, reference to a viscosity-temperature chart of those plastics will give a fairly definite viscosity which may be considered as characteristic of the machine. TT'hen once the characteristic viscosities of several machines or processes are known, a laboratory determination of the viscosity-temperature relation will prophesy all of the various operating temperatures required for a plastic. I n experimental work, a known operating temperature in conjunction with a stability temperature will render valuable savings in many instances. I n large high-speed machines much material may be wasted in determining operating temperatures by cut-and-try methods. With a laboratory guide, this waste may be considerably reduced.
Acknowledgment The writer wishes to acknowledge the assistance of James Bailey and the Plax laboratory staff, Literature Cited (1) Bender, Wakefield, and Riley, A. S. T. M.Symposium on Consistency, p. 67 (1937). (2) Burgers, J. &I.,1st Rept. on Viscosity and Plasticity, pp 67-72, Amsterdam, Aoad. of Sei., 1935. (3) Gloor, W. E., private communication, Nov. 9, 1940. (4) Houwink, "Elasticity, Plasticity and Structure of l l a t t e r " , p . 12, London, Cambridge Univ. Press, 1937. (5) Ibid., p. 144. (6) Lillie, H. R , J . Am. CeTam. Soc., 14, 502 (1931). (7) Meysr, L. W. A., AIodern Plastics, 18, No. 4, 59 (1940). (8) Penning and Meyer, A. S. T. M . Symposium on Plastics, p. 23 (1938). (9) Trouton, F. T., Proc. Roy. Soe. (London), A77, 426-40 (1906).
Heat Requirements for Steam Distillation of Turpentine Gum E. L. PATTON AND R. A. FEAGAN, JR. Naval Stores Station, Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Olustee, Fla.
I
N MODERN gum naval stores plants turpentine gum, either crude or refined, is converted by distillation into products long familiar to commerce-turpentine and rosin. Heat may be supplied to the still by direct fire, by hot oil, or other heat-conveying liquid in coil or jacket, or by a condensing vapor in coil or jacket. The direct-fired still is always hazardous, even when properly designed, and since steam-heated stills are easily controlled and are relatively free from fire hazard, the use of the steam still will undoubtedly become more common. In the development work of the Kava1 Stores Station we had occasion to determine the steam consumption of a tur-
pentine still, and the data are presented here for the benefit of those wishing to design steam-heated kettles for the batch distillation of turpentine gum. The still was a copper, pot-bellied kettle containing about 100 square feet of coil area and was insulated with a 2-inch layer of 85 per cent magnesia lagging. In the distillation of turpentine gum, water or open steam is added to the charge to start the distillation of the turpentine a t a temperature below its own boiling point. The rate of addition of water or open steam (or both) is controlled so that the temperature of the charge rises gradually and all the turpentine is off when the charge temperature reaches the turning-out point (300' to 330" F.).
