V O L U M E 2 0 , NO. 8, A U G U S T 1 9 4 8 rhat the larger the noise burst the less likely is its occurrence, and that the longer the period of recording the more likely is the recording of a large burst. While the below figure is taken from a single record, it represents fairly closely the author's experience max. peak with niariy records. The ratio of = 2.3 is slightly lower r.1n.s. rhxrr that rtaported by 1,andon (3.4) ( 2 ) . Johnson noise (calculated) - 0 . 3 3 x lo-' volt R.m.s. noise (measured) =to.57 x 10-9 volt .\laximum peak noise (meamred) + 1 . 3 X 10-9 volt
In niakiiig record A the viave-length drive motor was turned off and the gain was set to give 0.1 microvolt for full-scale deflection. Record B was made with the gain reduced to that required to give full-scale deflection for 1.O microvolt, and the wave-length drivr motor was turned off. The maximum noise was about -0.27, of full scale (2.0 X volt). The percentage error in this raw appears to exceed that for record A because the potentiometer in the recorder is incapable of interpolating between 1-oltagrs smaller than 0.1% of full scale. The potentiometer slide Rire is helically wound and has about 1000 turns for full scale. The didiny contact must therefore operate in a stepwise fashion in tinits of about 0.1%. In view of this fart it is believed that the n~aximrimnoiw voltage rrrordrd at 1 microvolt for full scale is
713 eaaeiitially tht. rame a' that for recording a t 0.1 microvolt for full scale. Record C' was made undei the same conditions as record B except that the wave-length drive motor was turned on. Pickup from the drive motor approximatply doubles the noise voltage. Reference lines (broken) are ruled beside the recorded lines in records B, C, and D to aid in estimating noise voltage and in the case of record D to show linearity. Record D was made to show the action of the attenuator and fiducial marker. The record is very nearly linear and shows no stepwise variations attributable t o the attenuator. For this rerord R, m d Rs (see Figure 9,C) were 2000 and 0, respectively. LITERATURE CITED
(1) Johnson, J. B., f'hys. Reu., 32, 97 (1928). (2) L a n d o n , V. D., Proc. Inst. Radio Engrs., 24, 1515 (1936). (3) Liston, M . D., Q u i n n , C . E., Sargeant, W . E., and Scott. G, G., Rev. Sci. Instruments. 17, 194 (1946). (1)N y q u i s t , H., Phys. Reo., 32, 110 (1928). ( 5 ) Perkin-Elmer Corp.. Instruction Manual for Infrared Spectrometer ,Model 12.1, p. 10, 1945. (6) Tee], R . P.. private coinmuiriwt~ioiit o author. R r c E I r E n October 11, 1947
Instrument for Measuring Stress Relaxation of High Polymer Materials USHAKOFF W.S . Macdonald & Co., Cambridge, Mass.
\ 5 . S. 1ZACDONALD AND ALEXIS
4 compact instrunlent is described for measuring stress relaxation of high poljmer materials. It contains a minimum of moving parts, is essentiallj free from draft and vibration effects, and measures the relaxation characteristics of a subbtance under ronstant strain (constant sample deflection).
s
7 TA?;DAItD test methods in the field of stress relaxation of high polymer materials have been neglected partly because iatisfactory instrumentation has not been available. One inethod of measurement recommended by the A.S.T.M. (1) has been described as a compression set test, under which a sample of standard size and shape is distorted by clamping between metal plates, heated for a prescribed time in an oven, then unclaniped and allowed to resume its shape for a period of one hour. The ratio of the thickness of the sample before clamping to that after re1ea.x is used as a figurc- of inrrit of the material test e d . Thi. -1.S.T.hI. method does not, of coui'se, give a dynamic nieasurr:nient of stress relaxation under strain, and although the moasurenients provide an index by ivhich materials may bc graded, it is often desirable to obtain more accurate and detailed information and to he able to predict by extrapolation the expected future behavior of the material. Furthermore, as the sample is claniped before being heated, it is difficult to separate the effrcts of temperature change from thow raused by the fundsmcntal decay of the material. The authors were desirous of making rapid nieasuremeiits that would allow the classification of materials to a higher degree of accuracy than could be obtained through the use of the conipression set method. It was hoped that enough informatioil n-ould be obtained to enable predictions to be made of relaxation values as a function of time following the application of a given load t o the material. It was expected that this might be acconiplished by extrapolation of data taken in a short period of time with a suitable instrument.
Theories of elasticity have been treated mathematically (A), hut the behavior of an elastic body over a period of time under constant strain has not been represented by an equation involving but one constant of proportionality. The apparatus developed by the authors to solve this problem consists basically of a mea8wing jig that applies a predetermined initial stress to the sample; the strain thus built up in the sample is then maintained essentially constant, and the stress is allowed to relax. The stress a8 a function of time is rerorded automatirally for the entire period of the test. Through experiment ai ion with this instrument it was found that the data obtairird by plotting stress idaxation against time at, constant temperature yield logarithmic curves. Therefore, plotting on semilog papc'r. gives straight lines, the slopes of which can be used as indexeh of the stress relaxation characteristic oi the materials. This is i n agreement with work done on creep tests of textiles (3). The straight lines obtained in tests of a few hours' duration apparent11 ma! be extrapolated to periods of a month or more; actual tests carried to over 1800 hours have indicated vel!. close adherence to the straight-line plot. Tests are currently being extended for several months to provide additional experimental vwification of the logarithmic stress relaxation behavior. REQUIREMENTS
The requirements for a suitable measuring jig are as follows:
It should be able t o apply in less than a second a predetermined initial stress of several hundred pounds per square inch t o a 88mple 1 square inch in crow section and 0.5 inch thick.
714
ANALYTICAL CHEMISTRY ples of varying thicknesses, and for predetermining the strain (and initial stress) to bo amlied t,o the samnln. The 'cam driv;' always moves 'the plunger through a fixed distance, and ~
~~~
lowm 'spring surfGe. R. is balanced and temporhture-corrected so that it holds its zero reading within 0.2% from 70" t o 180" F. under no-load conditions. Subsequently, there a calibration curve of load in pounds against millivolts pcr volt, applied to the bridge. A madificd Foxhoro Dynalog, C, is used for roeofding t,he output, of the
is;;;
Figure 1. Early Model oCJig The measuring spring deflection should he small (less than 5%
of the sample compression under load) in order that the spring may he affected only by relaxation, and that the sample map remain under nearly constant strain. The anvil upon which the sample t o be moasured is clamped should deflect less than 1%of the spring deflection under load. The measurhg element should he temperature-componsated, so that measurements can be taken a t an without instrument. sensitivity or zero shift It should he uossible to measure samnpk sion, and shear.. Provision should be mado for keeping tlic surmceb ui m e awriples from dipping in the clamps. The recorder should he able to record tho output of the measuring jig over periods of saveral days with an error of less than 0.2%
the voltage su& for the bridge in addition t o performing its main iunotion of recording output voltage. The modification consists of adding a rangecxtending switch t o s u p p r ~ s sthe zero of the insttrumcnt in thrco steps, thus changing the normal 4-inch dcfleetion rangc o i the instrument to 12 inches. The final version of t,he apparatus is illustrated in Figure 3. The measuring jig, Figure 2, a t.empcrature-regulated oven, and 8 Foxhoro recorder h a w been intogretcd into cumpact, unit. PROCEDURE
;iae selected for tests is t,he standard pellet of the ...... ...~ ./r.for Test,ing Materials, normally used for measuring ruhher compression set. This pellet measures 0.5 inch thick by 1.129 inch88 in diamcter (1 square inch in area). The standard 2-inch high plastic sample, however, may be used.
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:
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715
V O L U M E 20, N O , 8, A U G U S T 1 9 4 8
comparison betwoen the curves for certain samples of natural rubber, neoprene, and Buna S. Error in choosing the starting time at 0.01 hour has displaced the neoprene curve downward hut has not affected the slope which is the important paramet,er. Figure 5 shows the effect of ambient temperature on specimens of Buna S. Figure 6 shows the effect of humidity variation at constant temperature: 100% humidity was maintained after the sample had been soaked in wstor. Figure 7 shows the curves of other materials and suggests that the test is useful for a wide variety of plastics. All t,hese measurements were made with an initial stress of approximately 300 pounds per squarc inch. Additional measurements with initial stress up to 600 pounds per square inch in-
0. I
I
I.o
IO
i" Figure 3.
Final Apparatus
spring and the plunger adjusted by trial unt,il the pressure exerted by the spring on the specimen reaches the desired value. The expended specimen is then removed in each case and a now one of the same material subst.ituted beforo tho rccorder is st,artcd.
It has been found desirable to hold the temperature of thc testing jig constant during te&; othcmise thc volume expansion of the material produecs an error in reading. Although the expansion error might he corroeted, it is more st,raightforivard to control tho measuring jig temperaturo. Individual tests have covered periods ranging from 1how t,o 3 months. A t the end of each test a cheek reading of the insarument zcio mskcs certain that no drifts have occurred during the snalysis. The test data are analyzed by plotting the ratio bctween the stress a t each point on tho record curve and that a t a common referencc point as ordiiiate against, a lognrithmic time scale as abscissa. Because the rate 'of decay of stress is st f i s t in all instnnccn very rapid, it has been found impraclical to use t,he peak recorded stress as tho common denomiriator for tho interpretation of results. Accordingly t.here is selected the earliest point for which accurate measurements e m hc madc after the relaxation of the specimen has stabilized. In the work coverod by this report, this time was established RS 0.01 hour after the stress had been applied to the specimcn. After the above-defined stress ratios have been plotted against a logarithmic time scale for each test run, the negative slopes of the CurYeS are determined graphically. Figures 4, 5, 6, and 7 arc characteristic of' the plots obtained. Figure 4 shows 8.
09
IO
SUNA 5 . 8 5 F
%. 0.8 -
O 0.7
-'
L
IN[ .-I LOPRENE
Figure 4 (lower). Comparison of Natural Rubber, Neoprene, and Buna S. Figure 5 (second from hottom). Effect of Temperature. Figure 6 (second from top). Effect of Humidity. Figure 7 (upper). Mesonite, Leather, and Nylon
ANALYTICAL CHEMISTRY
716 Table I. Material Stock Numbers and Code Letters for Samples Illustrated in Figilres 4 through 8 F igiire
Du Pont dtocl, Yo
Cod?
AIatrr1nl
Ruhher
546-14 846-S-15',42
Neoprer1e
Buna E Buna P Buna 5 Biina S
846B-27 1333B-2.i 1333B-2 4
546B-17
Riihbrr
846-14
I .o
0.01
10
100
1000
'"bo.1
At the start of' each test a slight slippage of the material along the surface of the clamping plates is apparent. This effect 1ast)s for, varying lengths of time, depending on the material under test. Because it is also logarithmic in character, it appears in the final plot of t,he test results as an increase in the slope of the curve. The slippage, when it is not,iceable, seldom lasts for more than 0.1 hour and is easily separated from the stress relaxation of t,he spccimen, by the appearance of a discontinuity in the slope of the rurve. This may be seen clearly in Figure 9. The clamping 1)latesare roughened to reduce the slippage effect and avoid the ntwssity of cementing or vulcanizing the specimen to them. To give iurtlw experimental support to the method of extrapolating the data, several long-term tests were conducted on I3una S arid neoprene covering periods in excess of 60 days. These showed satisfactory linearity when plotted on semilog paper, and, therefore, agi,eed clowly with extrapolation from the early data of the respectivr tests. In connection with these and all other tests it vias also found that the linearity of the data improved with the care taken in performing the experiment. Such it,ems as stabilizing the oven and sample temperatures hefore test have relatively largtb influence upon the appearance of the resulting data. Specimens have also been tested in tension and shear by simple adaptation of the clamping device. The resulting curves have h w n indistinguishable from the ones taken in compression. APPLICATIONS
Figure 8.
Extended Period Test
dicated variatious iri the slopes smaller than inherent variations among individual sample pellets of any one material. Although cylindrical samples ere u e d , the shape factor is not important as long as comparisons are made hetween speriinens of identical geometry. The nature of the stress-tinw decay curvw as illustrated in Figures 4 t,o 8 (see Table 1 ) is apparent,ly that of a logarithmic relaxation in stress with respect to timr. Figure 8 shows an extrnded period test with measurements up to 14*5hours, in n h i c t i
.o I T= 8 2 " F.
RUBBER
0.90 n DC "IOJ-
I
I
A strcss relaxation measurement, makiiig possible the estrapolat,ion to a period of months of a curve requiring but a short test to establish, lends itself admirably to industrial problems involving gaskets, shock mount,s, retaining bands, supports, etc. In an effectively rigid system the high polymer member may be adjusted to ensure proper pressures or positions in the future. A s the shape of the sample is not a limiting factor, jigs can be aclapt,ed to an unlimited number of articles to be tested. Thus, the guesswork of choosing a material best suited to a given set of conditions is reduced onre a set of thest: testb$has been performed. Curves may be run on materials that are i n process of curing, aging, and dehydrating. Extrapolations of purely theoretical interest niiiy also be carried out on samples whose properties are curves taken on a not stable over a long period. Sur series of supposedly identicd sarnpl indicate the effect of a proccw, whether it tJ0 natural or the result of a predeterof imposed c*onditions;. Controlled atmospheres cmontaining moisture, ozoncl, or solvents niay be introduced. The instrument can be used for qualit,y control. Samples from produrtion lines may bcx trsted to be sure that the effects of plasticiaor. arrelt~rator,or solvtsnt remairi constant, variations i n
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Figure 9.
EKect of Slippage
no1
0.1
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IO
I
I
I
100
1000
10000
thc logarithmic stress relayation is very closely maintained. 011 the basis of t,hese data the pressure exerted on relatively stiff clamping members by a plastic sample at times greatei than 0.01 hour may be calculated from the equation:
(S,/SO) = 1
-
K(l0g t,lta)
n-htw S = clamping pressure at time t So = clamping pressure at t,ime h f d = 0.01 hour K = slope of time decay curves The case for which to is less than 0.01 hour 1s mcluded, because the instrument does not lend itself to ansly& over such qhovt periods.
I
Figirrp 10.
I
Example of Stress Relaxation of Gasket
V O L U M E 20, NO. 8, A U G U S T 1 9 4 8
717 Conduit De
I
'Clamping
stant >train teats R hich have necrssitated cunibei5onie eyulpment. The instrunient ubed in the experiments described above iir a compact unit, measuring 2 feet 8 inches X 1 foot 4.5 inches X 1 foot, containing a minimum of moving parts, essentially free from draft and vibration effects, and with the advantage that i t measures the relaxation characteristics of a substance under constant strain (constant sample deflection). As identical curves have resulted from samples in tension and from those under compression, the compreasion test is generall? used because of its relative simplicity. The relaxation characteristics of a number of iiiaterials may be specified by one constant when tested under given temperature and humidity conditions and for a specified sample shape. Because all characteristic c u r v ~ are s straight lines on semilog paper, the suggested ideal parameter is the slope K of the characteristic curve as indicated by the equation
Bolt
(S/So) = 1 - K log (tlloi Flange
Figure 11.
System for Testing Gasket
The unit requires a 110-volt alternating current supply but le voltage-stabilized t o withstand normal industrial line voltage variations. 4CKNOWLEDGWEh'I
performance beyond those permissible for the process would he quickly recognized and call for corrective action. EXAMPLE
.1 problem of the stress relaxation of a gasket will serve as ail example of the use of information obtained in the manner described above. Assume that a neoprene gasket must be compressed to 500 pounds per square inch in order t o prevent leakage in a pipe joint. Three arbitrary formulations are available: A, B, and C (Figure 10). The mechanics of the system (Figure 11) limit the maximum gasket stress to 700 pounds per square inch. Which composition may be used to prevent leakage over a period of a thousand hours? If all three gaskets are originally tightened to 700 pounds pel square inch, gaskets -4, B, and C will in 10 hours relax to 640, 610, and 570 pounds per square inch, respectively. At the end of 100 hours they will have relaxed to 628, 580, and 525 pounds per square inch, respectively. Gasket C will fail a t 400 hours, but a t 1000 hours gaskets A and B will still be exerting safe loads of 600 and 545 pounds per square inch. CONCLUSIONS
Up to now little use has been made of the strain gage as tt means of measuring stress, although the idea is not original and has been used recently in textile and building materials laboratories. Chain balances (6), sliding balance weights actuated by wires ( 6 ) , Ltnd servo-mechanisms ( 2 ) have been incorporated in most con-
The materials reported on in this paper were furnished through the courtesy of the General Tire & Rubber Company and E. I. du Pont de Semours & Company. The authors wish to acknowledge the help and encouragemelit of W. E. C. Eustis of the Saval Ordnance Laboratory who first presented the problem and 31. J. Sanger of the General Tire & Rubber Company and W. 11.Keen of E. I. du Pont de Xemours & Company who furnished samplrs for this work. A commercial model of the apparatus described has recently been made available by the Haird Associates, Inc., Cambridge, Xiass, under thc commercial name Hi-Po-Log LITERlTURE CITEI) (1) din. Soc. Testing Materials, Standards, p. 1770, 1544. (2) Cayce, K., thesis, Massachusetts Institute of Technology, 1944. (3) Fried, R. P., "Load Relaxation and Recovery in Certain Textile
Yarns," M.S. thesis, Textile Engineering Department, Masse chusetts Institute of Technology, 1947. (4) Guth, Eugene, and James, H. B., I n d . Eng. Chem., 33, 624 (1941). (5) Mooney, M.,Wolstenholrn, W. E., and Wllars, D. S., J . Applied Phvs., 15, 324 (1944). (6) Tobolsky, A . V., Prettyman, I. B., and Dillon. J. H., Ibid., IS, 380 (1544). RECEIVK:D -4ugust 14, 1947. Presented before the Division of Cellulose Chemijtry, High Polymer Forum, at the 112th Meeting of the . % V E R I C l N CHEMICAL S O ~ I F Y Sew Y . York, S . Y.
Determination of Free Carbon in Compounded Rubber and Synthetic Elastomers GEORGE D. LOUTH, The Firestone Tire and Rubber Company, Akron, Ohio
T
HERE appeaici to be no iecold in the chemical liteiatuie of a
general method for the estimation of carbon black in both natural rubber and all types of synthetic elastomer compounds The A.S.T.M. method for the chemical analysis of synthetic elastomers (1 ) states that the carbon black in elastomers containing polyisobutylene cannot be determined by the method described, but it may be determined in the decantate from the polvisobutylenr estimation, provided the polrisobutylene con-
tent IS not over 25.';. This method, thus, cwuld not he used for straight Butyl rubber compounds. b3cCready and Thompson (6) have modified the A.S.T.M. method for rubber compounds t o include the estimation of free carbon in Butyl rubber reclaim. The essential change is the subqtitution of a hot digestion in mineral-seal oil in place of the hot nitric acid digestion of the sample. These authors, however, recommend further checking of this method before applying it t o
