Rheological Pro erties of Polystyrene below 80"C. d
d BRYCE 3IAXWELL AND L. F. RAHRI
Plastics Laboratory, Princeton University, Princeton,
T h e rheological properties of polystyrene below 80' C . have been studied by means of stress relaxation tests at various elongations and temperatures and by creep tests at various loads and temperatures. The results indicate that there are two mechanisms of stress relaxation and creep in this material. The first is the slipping of chain segments and the second is crazing. A new method of measuring crazing is described, and detailed studies of the relation of crazing to time, temperature, stress, and strain are reported. I t is concluded that a specific critical elongation must be reached before crazing takes place. This elongation is independent of temperature but is affected hy certain organic solvents.
T
HE properties of polystyrene make it a n outstanding elec-
N.J .
random orientation when the sample is unstressed. No cross links or primary chemical bonds are thought t o exist between these chains, The only forces preventing one chain from slipping past another are the relat'ively weak secondary or van der Waals forces and the mechanical interferences that are caused by the bulky side groups and branch chains. When a sample of this polymer is elongated, the 1engt.h of the sample may be increased in several ways. The first is by bending thc bonds along the chains. This elongation is small and not dependent on t,ime. The second is the uncoiling of t,he chains from their relaxed, coiled configuration, and does depend on time. The third is the chain-chain slipping which is also dependent on time. Stress relaxation tests may be used to study the last two mcchanisms. When the sample is elongakd, the chains uncoil. The resulting stress depends on the amount of uncoiling. A relaxation of stress with time then takes place as the chains slip over one another t o reform in their relaxed configuration.
trical insulator. LON power factor and dielectric constant together with good dimensional stability and low water absorption favor its use in electrical components wherever light weight, ease of manufacture, and low loss are important considerations. S T R E S S RELAXATION The material has also found extensive application in household items, toys, and photographic products. Studies of the meThe equipment generally used for relaxation of rubbers ( 7 ) and other high polymers is not suitable for polystyrene because chanical properties show that i t excels in all but two respects. First, the impact strength is rather low (0.25 to 0.6 foot-pound per of t h e very small elongations involved. A4method was devised for conducting these tests in a 60,000-pound Southwark-Emery inch of notch); second, under certain conditions the material develops cracks and a surface blush known as crazing. The Universal testing machine. Samples of standard A.S.T.M. contour ( 2 ) were prepared and studies reported here were undertaken to gain fundamental annealed in a n air oven at 175' F. (79.4' C.) for 16 hours to reknowledge of the structure of polystyrene in general and the move fabricating strains. The sample was placed in a thermomechanism of crazing in particular, with the objective that this statically controlled oven in the testing machine. After the knowledge may be of value in improving these properties. sample was allowed to come t o temperature equilibrium with the The rheological properties of polystyrene above its heat distortion point have been the subject of recent investigations (4,6). test chamber, it was elongated at the rate of 0.5 inch per minute. Upon reaching the desired length, the elongation was mainAt approximately 80 C . polystyrene exhibits a gradual transitained manually by slight adjustments of the loading and untion from a soft, rubbery material of high elongation t o a hard, loading valves within 1-0.0002 inch on a 2-inch gage-length brittle material with an ultimate elongation of approximately 1.25%. The methods of determining the rheological properties above the 1.0 transition point are not applicable a t lower temperature. 0.9 Rheology is the science of flow or 0.8 deformation. Many factors affect the rate at which deformation takes place; 0.7 among them are temperature, time, stress, and previous strain history. 0.6 The relation of these factors to strain VI 2 0.5 may be studied either by stress relaxa0) tion tests ( I ) or creep tests. I n the 0.4 former the sample is held a t constant 6I -I : strain and the decay of stress is studVI 0.3 W ied as a function of time. In the z t 0.2 latter the stress is held constant and w the strain-time relation studied. Both K 0.1 methods were used in this investigation. 0.0 10 100 IO00 l0000 Polystyrene may be considered t o TIME (SEC.) be composed of long chain molecules normally coiled and intertwined in a Figure 1. Stress Relaxation of Compression-Molded Polystyrene
1988
September 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
RELAXATION OF POLYSTYRENE AT 50. C. FOR VARIOUS % ELONGATION
0 -0.90 % Q -0.75 % 0 -0.50% d 0.25
-
-
1989
tive stress quickly. However, in, the tests a t lower elongations crazing was not so rapid in the early part of the test; thus the relative stress a t the beginning was high and decayed later to even lower values than those found for high elongations, after crazing took place. It is impossible to separate the contribution of crazing to the stress relaxation curve from the contribution of the chain slipping mechanism. Both tend to reduce the stress, and therefore each masks the effect of the other. For this reason a separate stJudy of crazing and its relation to stress, strain, time, and temperature was undertaken. MEASUREMENT OF CRAZING
Many theories have been proposed to explain the mechanism of crazing. Among TIME-SECONDS them are the following suggested causes: immrities (including excess monomer. solFigure 2. Stress Relaxation at Various Elongations vents, oxygen, catalyst, or inhibitor), depolymerization due to ultraviolet light, irregular molecular weight distribution, residual strains (caused by machining, molding, improper annealing, or extensometer. Taking zero time as the time when the desired thermal shocks). No attempt will be made here to evaluate elongation was reached, readings of stress at various time inall of these possible causes. tervals up to 5 hours were taken. The authors believe that crazing may be due to any of these Figure 1 shows the results of relaxation tests on compression causes, but that in each case the effect on the material is the molded samples a t various temperatures a t 0.7570 elongation same-namely, a stress will be developed in the material (either as relative stress us. log time; relative stress is the stress a t time locally or over the entire piece) which will result in a local strain 1 divided by the stress a t time 0. The stress relaxation curves indicate that a t low temperature (30' C.) the relaxation of stress greater than the material can stand. Therefore a study of the over a period of 5 hours is very slight (approximately 1570). relation of mechanically applied stress and the resulting strain to the rate of crazing should indicate the fundamental nature of We may conclude that some strong force is preventing the chains from slipping over one another in this temperature range. As crazing, whether the material crazes in use because of thermal stress, solvent stress, or any of the causes mentioned. the testing temperature is increased, the rate of relative stress T o make such a study it was necessary t o develop a new relaxation increases rapidly so that a t 80" C. the stress falls method of measuring crazing. The equipment and methods used practically to 0 in less than 5 hours. Visual observations of the samples used for these tests indicated that a t some time during the tests crazing occurred and the surface became covered with a blush of small cracks. The \ i higher the testing temperature, the more severe the blush. The cracks all appeared in planes normal to the tensile stress and therefore reduced the area over which the stress was acting. This reduced the number of strained molecular chains causing the stress and thereby increased the stress relaxation. At least part of the increase in the rate of relaxation a t the higher temperatures is due to the appearance of crazing STANDARD AS,;T.M. TENSION SPECIMEN cracks, c The effect of elongation on the relaxation of relative stress is shown in Figure 2. If the mechanism of relaxaTEMPLIN GRIPS tion were only the slipping of the chains, we would expect little if any difference in the relaxation curves a t different elongations. Obviously this is not the SECTION C d case. The curves for relaxation a t various elongations show that a t higher LOAD elongations crazing takes place rapidly a t the beginning and reduces the relaFigure 3. Device for Measuring Crazing 1000
10000
-
n
P
T
1
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
1990
Vol. 41, No. 9
Tate-Emery load maintainer within =t2 pounds. Measurements of crazing were taken a t regular time intervals. T o prevent overheating of the test specimen, precautions were taken to avoid use of the light for more than 5 seconds in each minute.
CRAZING AT VARIOUS LOADS TEMF: 30'C. RATE OF LOADING400 POUNDS /MINUTE
EFFECT OF ELONG.ITIO?T
I
If a sample is placed under a constant load in tension, it will elongate or creep with time. The higher the load, the more rapid the elongation. Figure 4 shows the rate of crazing resulting from such tests, the units of crazing being the direct reading of the light meter in foot-candles. This graph indicates t h a t the time to craze decreases and the rate of crazing increases with increased loads, and consequently, crazing and elongation are definitely related. If the creep test temperature is increased, the rate of elongation will again be more rapid. Therefore the rate of crazing should increase with increased temperature and the time to the inception of crazing should decrease, if crazing and elongation are directly related. Figure 5 indicates that this is true.
0
10
20
30
TIME (MINUTES)
Figure 4.
40
50
6o
.
Rate of Crazing a t Various Loads, in Creep
previously in industry do not give quantitative results, and reproducible results are difficult to obtain. For this study a device was developed which accurately determines the inception of crazing and also the rate a t which crazing progresses. When a sample of polystyrene is loaded in tension, all the craze cracks appear as planes normal t o the tensile stress. If a light, A , is beamed on the side of the sample a t a n oblique angle as indicated in Figure 3, those rays which fall on the cracks will be reflected t o B. Those rays which do not fall on the cracks will pass through the sample in the direction of C. Therefore, the intensity of the light a t B will be a function of the total crazed areat h a t is, a function of the number of cracks times the average area of the individual cracks. The dimensions shown in Figure 3 are those of the actual apparatus used. The amount of sample exposed to the light is the maximum that the contour of the specimen mill permit. This gives as representative a test area as possible and averages the effects of local imperfections The light guides are made of phenolic paper laminate lined with black paper. ,4 Weston Model 603 illumination meter is used to measure the intensity of light at B . T h e light source is a Spencer microscope illuminator equipped with a General Electric 110-volt 100-watt projector lamp. A Variac (variable-voltage transformer) and voltmeter are used to give the lamp a constant 110-volt supplgr. The parallelism of the crazing cracks may be proved in the following way: If a pin-point source of light is used at A with a well crazed sample in the apparatus, a n image of the source can be found horizontally opposite A in the direction of B and at the same distance from the sample as the source-that is, the parallel crazing cracks focus the diverging beam of light from A into a n image of the source. Cast polystyrene sheets ' / 8 inch thick were cut into standard tensile test specimens. T o remove dirt and foreign matter from the surface and to protect the samples from fingerprints and solvents, the sheets were washed with cold water and then covered with masking tape until ready for testing. This protection by the masking tape prior to testing kept the surface as clean and as craze resistant as any form of heat treatment. Also, samples prepared in this manner gave the most reproducible results. T h e tests were performed in the Universal testing machine, with the sample and craze-measuring device enclosed in a thermostatically controlled air oven. After the sample was allowecl t o come to thermal equilibrium with the oven, the load was applied at the rate of 400 pounds per minute, time 0 being taken at the beginning of the loading. The required load was maintained by a
TIME (MINUTES1
Figure 5. Rate of Crazing at Various Temperatures, in Creep
To investigate further the relation between crazing and elongation, a sample was stressed t o a constant load (3500 pounds per square inch) for 20 minutes a t 50" C.; crazing and elongation were both recorded a t regular intervals. The load was then removed and the sample left free of applied stress for 20 minutes. The original load was again applied for 20 minutes and then removed. After the end of another 20-minute cycle of no load, the test ,chamber and sample were heated to 70' C. with no applied load. Figure 6 shows that, during the first 20 minutes under load, crazing and creep both progressed as would be expected. When the load was removed, a n elastic recovery took place immediately, together with a n immediate recovery in crazing. During the following 20 minutes an elastic after recovery took place in elongation and a corresponding delayed recovery in crazing. When the load was again applied, the strain jumped up immediately while the crazing also increased rapidly. It should be noted t h a t
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1949
I
i
6o
150
10
; i40
8
7;
w
-I
x
6
20
4 ;
IO
2 i j
a
0
L 0 82
(3
N O
20
2
5
z
%
0-
30
0
10
Figure 6.
20
30
40
50 60 70 80 TIME ( M I N . )
90
100
110
120'
2 W
Crazing and Elongation us. Time under Changing Load Cycles
1991
and immersed in the organic compound for 5 minutes before testing. Those compounds (including some constituent of the human fingerprints) which have an effect on the critical elongation reduce it to approximately one third or one half of its value for clean samples. Apparently npropyl alcohol has no effect. Other investigators (6) have found that, if polystyrene is stretched a t a temperature above its heat distortion point and allowed to cool in this oriented condition, the tensile strength will be greatly increased in the direction of orientation. This is attributed t o the lining up of the molecular chains parallel t o the direction of tensile stress. Conversely, if an oriented sample is subjected t o a tensile stress normal to the direction of orientation, its tensile strength is reduced. EFFECT OF ORIENTATION
a finite time was required t o attain the value of crazing reached at the end of the previous cycle. With the load again applied, crazing and creep continued as before. The removal of the load brought the same instant recovery of crazing followed by the delayed recovery. When the sample was heated with no applied load, crazing underwent thermorecovery. The similarity between this crazing curve and the typical deformation recovery curve is striking. We may conclude that crazing and deformation are definitely related. Two techniques have been developed for measuring the exact elongation a t which crazing starts. Method A involves loading the sample t o some small stress, letting it creep t o the critical elongation, and then measuring elongation and crazing simultaneously at regular time intervals. A plot of crazing against elongation will show the specific critical strain. Method B consists of straining the sample at a specific slow, constant rate, and then measuring strain and crazing simultaneously. Similar results are obtained by both methods. Table I shows t h a t the critical elongation for clean samples of polystyrene is independent of time and temperature over the range tested. The masking-tape cleaning technique and the annealing method give the same value of critical elongation within experimental error. Certain organic compounds, such as kerosene (a), accelerate the crazing. Tests were performed to determine whether compounds really accelerate crazing or merely lower the critical elongation. Table I1 shows these results. I n each case the samplw were annealed for approximately 15 hours at 80' 0.
To determine the effect of orientation on the rheology and crazing of polystyrene, sheets of cast material were given a 50% elongation at 120" C. and cooled under this strain. From these sheets samples were cut -both parallel and normal t o the direction of orientation. The samples oriented normal t o the direction of tensile testing crazed more quickly than unoriented samples. The samples oriented parallel to the direction of testing were tested at both 3500 and 5000 pounds per square inch. The results are shown in Figure 7 with the corresponding results on unoriented samples for comparison. The orientation causes a pronounced increase in the time for crazing t o start and a decrease in the rate a t which i t progresses. We may conclude t h a t the chains oriented in the direction of tension strengthen the sample against crazing; conversely, those oriented normal to the direction of tensile stress are less craze resistant. It has been stated that i t should be possible t o induce crazing by thermal shock. If a sample is heated t o some specific tem-
B
TABLE I. CRITICALELONGATIONS FOR CLEAN SAMPLES Temp., C. 30 40 50 60 B 26 Thermal shock
Method A A A A
Approx. Time to Craze, Min. 4 1.5 0.76 0.5 10.5 0
Remarks Masking tape Masking tape Masking tape Masking tape Annealed Masking tape
Critical Elongation, % 0.76 0.72 0.75 0.70 0.73 0.75approx.
TABLE11. CRITICALELONGATIONS (BY METHODB) FOR SAMPLES IMMERSED IN ORGANIC CO~V~POUNDS Organio Compound Kerosene n-Propyl alcohol Oleio acid Cyclohexanol Fingerprints Fingerprints a
Remarks
Critical Elongationa, %
Cast Cast Cast Cast Compression-molded Cast
Critical elongation for clean samples equals approximately 0.75%.
TIME (MINUTES)
Figure 7.
Rate of Crazing of Oriented and Unoriented Polystyrene
1992
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 8.
voi. 41, No. 9
Crazing Cracks on Unoriented Polystyrene Irnrnediately (ZeJL) and 5 Minutes ( r i g h t ) after Thermal Shock
perature long enough to permit thermal equilibrium, the entire piece should be in an expanded condit,ion. If t,he outside of the piece is then suddenly cooled, it will contract while the inside is still expanded. The result will be a tensile stress in all directions on the surface and a compression in the int.erior. If this contraction of the surface is great enough, crazing should result. Since t,he stress will be uniform in all directions, the crazing cracks should appear in all directions if the sample is completely isot,ropic. Thermal shock tests of t'liis t,ype were performed on both oriented and unoriented samples by heating them in water t o 90" C. and then quickly transposing them to an ice-water bath. The photomicrographs of Figure 8 show t,he cracks immediately result,ing from a thermal shock on unoriented material and a photograph of the same spot 5 minutes later, when the cracks had almost disappeared. Figure 9 shows the cracks resulting from a thermal shock on oriented material; they are all parallel to the direction of orientation.
strain a t a hich ciazing takes place since the critical elongation is independent of temperature. The foregoing theorj does not explain why once a crack starts, it does not continue to grow until the sample fails but stops while new cracks appear near by. A possible explanation of this may be as follows: No matter what the fabricating process, the surface of the material becomes slightly oriented on a micromolecular scale. If the sample is molded from granules, the surface will be oriented in little patches a t random to one another as a result of the original granules having been oriented. ilpparently, then, each crazing crack starts in a region where the general orientation is normal t o the tensile stress, and this crack grows until it reaches a region where the general orientation is parallel t o the tensile stress This explains why once the cracks start, they do not necessarily continue t o grou until the sample breaks
COnCLUSIONS
The st,ress relaxation studies indicate that the molecular chains in polystyrene are prevented from slipping over one another by some relatively strong forces. The chemical structure of this material indicates that t,here should be only very weak secondary forces between the chains. Therefore, the chains must be prevented from slipping by mechanical hindrances. Among the more obvious conclusions to be derived from the crazing studies are: (I) The time to the inception of crazing decreases and the rate of crazing increases with temperature. (2) Crazing cracks tend to disappear with time aft,er the removal of stress; this recovery of crazing follows the typical form of a deformation recovery curve. (3) Orientation of the chains in the direction of tensile stress retards the rate of crazing. (4) Thermal crazing will appear as cracks in all directions in isotropic samples and as cracks parallel to t'he direction of orientation in oriented samples. All of these effects are really the result of exceeding the critical elongation (approximately 0.75%). We may postulate Lhat crazing is due to the application of stress to the material, which results in a local strain greater than the material can stand. In general, crazing cracks appear because the weak secondary forces between t,he chains have been broken. Some organic compounds tend to reduce these forces by plasticizer action. It is interesting to note t,hat temperature apparently does not reduce the
Figure 9. Crazing Cracks on Oriented Polystyrene Immediately after Thermal Shock
(6j Matheson, L. A., and Goggin; W.’C., IND.ENG.CHEM.,31, 334
ACKNOWLEDGMENT
IlO3O). \ - - - - ,
Grateful acknowledgment is made to A. V. Tobolsky and N. Vasileff for encouragement and assistance. The authors also %?ishto thank the members of the of the plastics Laboratory for helpful advice and H. R. Robinson for aSSiSt,ance in Preparing the graphs.
(7) Tobolsky, A. V., Prettyman, I. B., and Dillon, J. H., J. Applied Phys., 15, 380 (1944). RECEIVED August 3. 1948. The work described here was sponsored by the U. S. Army Signal Corps, Bureau of Ships, Bureau of Aeronautics, Bureau of Ordnance, and Office of Naval Research, under Contract W-36-039-SC-
32011.
Properties of Technically Important Hexavalent Chromium Compounds WINSLOW H. HARTFORD Mutual Chemical Company of iimerica, Baltimore, M d . Data are lacking on many physical properties of th,e principal industrial chromium compounds and their aqueous solutions, and are based on impure material or obsolete technique in other cases. Questionable published values have been checked, and many new data are presented in this paper to give reliable information on the following properties of sodium, potassium, and ammonium dichromates, sodium and potassium chromates, and chromium trioxide (chromic acid): physical appearance and crystal structure, density of the solid, melting point of the solid, transition points between hydrates, eutectic with water, solubility in water, freezing point of aqueous solutions, density of aqueous solutions and its variation with temperature, pH of solutions, heat capacity of solutions, and viscosity of solutions. A comprehensive bibliography is included.
T
HE following chromium compounds are produced in quantity and their properties are considered in this paper. Sodium dichromate, Nadh07.2HzO Potassium dichromate, K2Cr~07 Ammonium dichromate, (NH4)&rzO1 Sodium chromate, Na2Ci-04 Potassium chromate, KzCrOa Chromium trioxide (chromic acid), ‘ 2 1 . 0 3
.
pounds are now produced in grades of comparable purity, so that the availability of pure material is no longer a problem. Investigation of the literature indicates t h a t much of it is old and t h a t some earlier workers employed techniques which are now obsolete. The more reliable of these data, after checking, were combined with unpublished measurements in 1941 to give a fairly complete privately printed source of information ( 4 8 ) on solubility, density, pH, freezing and boiling points of solutions, and various properties of the solids. A similar publication appeared in 1933 (68)but was much less comprehensive. With the availability of pure materials and improved techniques, i t appeared advisable to check the evisting data and t o supplement them with additional measurements. The following properties have been critically examined and new data obtained wherever it appeared necessary: Physical a pearance and crystal structure Density o&he solid Melting point of the solid Transition points between hydrates Eutectic with water D a t a for the system salt-water (solubility, freezing point) Density of solutions, and its variation with temperature p H of solutions Heat capacity of solutions 10. Viscosity of solutions 1. 2. 3. 4. 5. 6. 7. 8. 9.
METHODS EMPLOYED
The most important of the six compounds listed is sodium dichromate, b u t the usual reference works (26, 35, 38, 62, 7 2 ) contain sketchy and inaccurate notes on many of its properties because a t the time much of the classical work was being done, sodium dichromate was not available in a state sufficiently pure for investigation. The present technical grade may have the following analysis: Total Cr as NazCr20,.2H20 c1
so4
% of Cr present as -\u’a#2rOa
99.8% 0 06% 0.20% 0.20%
and a C.P. grade is regularly produced, in which impurities are reduced to the amount required by AMERICAN CHEMICAL SOCIETY specifications for potassium dichromate ( 4 ) . The other com-
APPEARANCE AND CRYSTAL STRUCTURE.Little experimental work was required here. Descriptions of the various materials have been revised t o conform t o present production standards and t o eliminate statements obviously based on examination of impure materials. Some crystal constants have been recalculated t o conform t o a uniform convention. DENSITY OF THE SOLID.This constant was determined by pycnometer measurements a t laboratory temperature, using as the immersion liquid, toluene or refined kerosene, both of which are inert toward most hexavalent chromium compounds a t ordinary temperatures. MELTING POINT OF THE SOLID. These data were determined by observing thermal breaks as the material cooled and heated through the melting point, using a calibrated thermocouple and recording potentiometer. I n the case of chromic acid, supplemental data were obtained from manufacturing experience. TRANSITION POINTS. Transition points between hydrates in aqueous solution were determined by measuring the thermal
