4
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951
The heat oi swelling of frozen rubber (sheets 52 T ) was determined as the average of ten measurements. Crystallized rubber, average Thawed rubber, average Crystallization
34.7 * 3 . 0 joules 4.9 * 1 . 2 joules 80 * 4.2joules
lkasureiiients at 11" C. carried out a t the University of Groningen with the same rubber gsve lower results. Swelling frozen sheets Swelling thawed sheets Crystalliiation energy
23.0
0.95 22
=t =t
2 . 0 joules 0.08 joule 2 joules
Taking the average heat of crystallization 25 joules per gram for the 38% crystallized sheets, the latent heat of crystallization for the crystallites is computed as 66 joules per gram. This may be compared with the heat of melting of isoprene a t 126" C., which amounts t o 70.9 joules per gram ( 3 ) . Taking into account the specific weight of the S and T vulcanizates and their crude rubber contents, total crystallization would develop 58 joules per ml. of vulcanizate. With this figure as a base, we can evaluate the energy effects in stretching mentioned in Table 111. In Table I V the degree of crystallinity calculated from Table I11 is compared with the figure determined by direct x-ray analysis taken from the investigation of these compomds by Arlman and Goppel(1,2). It is obvious from Table IV that there is fair agreement between the energy-computed crystallization figures and those measured by direct x-ray analysis, though the energy tends to yield somewhat lower values. As neither method gives very accurate figures, a better agreement is not to be expected.
Conelasion The degree of crystallization of stretched vulcanized pure gum compounds can be determined by calculating the differential and integral energy effects on stretching from the elastic tension us. temperature measurements, on the assumption that the plots represent equilibrium tension values. The percentage of crystallinity figures calculated in this way a t various elongations
365
Table IV. Degree of Crystallinity Computed by Energy Measurements and Determined by Direct X-Ray Analysis Elongation,
%
300
400 500
Method Energy X-ray Energy X-ray Energy X-ray
S Compound,
%
T Compound %
2.8 4
2.8 5
13 15
10 10.5
24
19 19-22
30
agrees fairly well with figures obt,ained by direct x-ray measurements.
Acknowledgment The author is indebted t o A. van Rossem for placing a t his disposal smoked sheets 35 years old, to E. H. Wiebenga and H. Benninga for calorimetric measurements at the Groningen University, and to J. J. Arlman for determination of degree of crystallinity by x-ray analysis.
Literature Cited (1) Arlman, J. J., and Goppel, J. M., Applied SCLResearch, AI, 347
461 (1949). (2) Arlman, J. J., and Goppel, J. M., Rubber Chem. and Technol., 23,306,310(1950). (3) Bekkedahl, Norman, and Wood, L, A., J . Reseawh Natl. Bur. Standards, 19,551(1937); RP 1044. (4) Elliott, D.R., and Lippmann, S. .4., J . Applied Phys., 16, 50 (1945). (5) Flory, P.J., J.Chem. Phys., 15,397(1947). (6) Meyer, K.H., and Ferri, C., Helv. Chim. Acta, 18,570 (1935). (7) Meyer, K.H., and van der Wijk, A. J. A,, Ibid., 29,1842 (1946). (8) Rossem, A.van, and Lotichius, J., Kautschuk, 5,2 (1929). (9) Roth, F. L., and Wood, L. A , , J . Applied Phys., 15, 749,781 (1944). (10) Wiegand, W. B., and Snyder, J. W., Trans. Inst. Rubber Ind., 10,234(1934). (11) Wildschut, A. J., "Technological and Physical Investigations on Natural and Synthetic Rubbers," pp. 89, 186, New York, Elsevier Publishing Co., 1946. (12) Wood, L. A., Bekkedahl, Norman, and Gibson, R. D., J . Chem. Phys., 13, 475 (1945).
RECEIVED October 2, 1950. Communication 143 of Rubber-Stichting, Delft
Primary Creep and Stiffness of Vulcanized Rubbers D. H.Cooper Dunlop Rubber Co., Ltd., BZrmZngham, England P
Because the stiffness of springs and mountings is important in determining how they will behave in service, many products are required to have a specified deflection under load. If the load on a rubber product is maintained continuously for a week or so, the deflection will increase slowly all the time; the extent of this deflection, known as creep, is given by formulas and graphs. The results take into account the magnitude and type of the stress. Compounds creep more in tension and less in compression than in shear. A black loaded compound creeps more than a gum stock having the same initial deflection, but less than one having the same stress. Results quoted are for natural rubber, Neoprene Type GN, and Hycar OR, and are formulated so that the deflection of a product in any compound may be predicted for 3 weeks after a test lasting 1 minute.
' To process control comes the possibility of greater accuracy and saving of time; for the designer additional information on how creep and stiffness are influenced by compounding and cure.
P
RIMARY creep is the subject of a comprehensive work by Leaderman ( S ) , in which there are numerous references to rubber, but time has not premitted the publication here of normalized creep and recovery curves on which further analysis along those lines of thought would depend. Leaderman includes typical creep curves for a hard and a soft rubber. This paper, on the other hand, shows that at room temperature the shape of the creep curve depends on some or all of the ingredients in the basic mix, probably including the polymer, but only to a very small extent on the reinforcing black.
366
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No 2
Figure 1. Creep Apparatus
The results form part of a program of development work on specimens, some have been loaded in tension and compreasion, the stiffness of rubber springs. The work is concerned with while others have been loaded in shear t o give values of the deflection after 1 minute and of the change of deflection during subthe accurate determination of load-deflection curves and with predicting the deflection and the subsequent slow change in sequent days. In short, results show the influence of most of the variables mentioned above on stiffness and primary creep. deflection which together result from the application of a steady load. Such data are required not only for the design of rubber Experimentd M-ork components but also for process control and acceptance tests on finished products. Compounding, Molding, and Curing. Four basic mixes were The gradual increase of deflection of spccimen under constant employed: (1) a highly resilient natural lubber mix cured with load is known commonly as creep, and that component of thedefection which is wholly recoverable after removal of the load has been called primary creep (5) 01 delayed elasticity. In the present work all the deformations have a recoverable component JThich OPRENL-SOFT is large compared with the nonrecoverable component or permanent set, and the latter has consequently been neglected in quoting results. Generally speaking, the deflection in a rubber spring under load n-ill depend upon the compounding and processing of the rubber, the temperature of the test, the shape and size of the test specimen, the geometry of loading--i.e., compression, tension, load hisshear, bending, etc.-the tory, and the previous history of the specimen. Experimental work has been carried out to qtudy the effect of these variables and of the interaction between them. The distinguishing feature of the I IO 100 IO00 10000 MINS main experiment lies in the fact that of a number of small cylindrical Figure 2. Ratio of Creep after t Minutes t Q Creep after 10 Minutes us. T i m e
February 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
sulfur and accelerated with Santocure (reaction product of cyclohexylamine and mercaptobenzothiazole) and tetramethylthiuram disulfide, (2) a natural rubber bonding mix, cured with sulfur and accelerated with mercaptobenzothiazole, (3) a Keoprene Type GN mix cured with zinc oxide and lightly calcined magnesia, and (4) a Hycar OR 15 mix cured with sulfur. A highly resilient natural rubber mix has been compounded with lampblack and the other mixes with equal quantities of gas black and lampblack (Table I). All compounds were mill-mixed; the hard and medium neoprene compounds were extruded as 0.5-inch bar first, but all samples were ultimately press-cured in a six-cavity compression mold. Each specimen was first cured for 20 minutes a t 153 C., rested for 3 days, and tested. Some specimens were then given a second cure of 20 minutes a t 153" C. and rested 3 days before testing. Temperature Control. All tests were carried out at a controlled temperature of 21' * 1" C. Shape and Size of Test Specimens. T o study the phenomenon of primary creep the load is required to change instantaneously from zero t o the required load-a condition t h a t can be approached in practice only by applying the load within a time interval which is short compared with the time that elapses before the first reading. Many engineering problems would arise in attempting to apply a load of, say, 150 pounds per square inch in 2 seconds, unless the specimens were limited to 0.5 square inch in cross section. A relatively long specimen is often unstable in compression and always subject to bending when loaded in shear, so t h a t it is desirable to keep the diameter-height ratio of a standard test specimen to above 1.5. T h e height of the specimen is therefore restricted t o about 0.5 inch and measurements to 0.0005 inch allow the strain to be measured to 0.1%.
367
Table I. Basic Mixes (Parts of carbon black per hundred parts of rubber) GECa Microncx MPC b Black Compound (Lamp) (Gas) Resilient natural rubber gum stocks 0 0 Resilient natural rubber soft 0.1 0 Resilient natural rubber medium 0.37 0 0.98 0 Resilient natural rubber hard Bonding natural rubber gum stock 0 0 Bonding natural rubber soft 0.209 0.209 Bonding natural rubber medium 0.36 0.36 0 82 Bonding natural rubber hard 0.82 Neoprene gum stock 0 0 Neoprene soft 0.07 0.07 Neoprene medium 0.256 0.256 0.432 Neoprene hard 0.432 0 Hycar gum stock 0 0.035 0.035 Hycar soft 0.28 0.28 Hycar medium 0.5 0.5 Hycar hard
1.
2.
3.
4.
b
General Electric Co. lampblack. Micronex medium process carbon gas black.
The test specimens used throughout the main test were in fact cylinders approximately 0.5 inch in height and 0.5 square inch in area of cross section, having a metal end piece bonded to each end. To determine the influence of shape on stiffness, a few cylinders 2 inches in diameter were also used. Geometry of Loading. The specimens were of such proportions that they could be loaded in compression, tension, or shear and resulting deflections conveniently measured. Load History. I n the main experiment, the load w&s in t h e form of a hanging weight whose application required never more than 2 seconds; it was applied t o each specimen at zero time and maintained in most cases for one week.
507.
Figure 3.
Ten-Minute Creep Strain us. 1-Minute Strain for Soft Compounds
Figure 4.
Ten-Minute Creep Strain us. 1-Minute Strain for Medium Compounds
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
368
Vol. 43, No. 2
to right, carry one bonded specimen in compression, one in shear, one in tension, and one unbonded lubricated specimen in compression. A dial gage carried on a geometric mounting of the dimple, slot, and plane type enabled deflections to be read to 0.001 inch; the fourth place of decimals, however, could be estimated. In the photograph it is seen in contact with an anvil on \vhich its zero is checked. The height and diameter of the specimen were measured at the start of each test. One end of each bonded specimen was then bolted to a bracket on the framework and a stirrup was fixed on the other end. At zero time, a weight was lowered onto the specimen by means of the hand lever shown in the photograph. Readings on the dial gage were taken once before applying the load and afterward according to a time schedule of 0.3, 1, 3, 10, 30, etc., minutes up to approximately 30,000 minutes or 3 weeks.
Nomenclature t = time in minutes after application of load Yt = deflection in inches measured t minutes after application of load Y , = 1-minute deflection Y c - Y , = &minute creep r = - -Yt - 1' = relative creep Yl H = unstrained height of specimen Characteristic Creep Curves of Polymers
Figure 5.
Ten-Minute Creep Strain us. 1-Minute Strain for Hard Compounds
Previous History of Specimen ( 4 ) . In recognition of the possibility that specimens tested more than once might soften because of filler breakdown, this effect was eliminated by straining the specimens before the test slightly above the limits worked to during the test. I n earlier work it was found that this precaution enabled results to be repeated within the required accuracy.
When a number of different compounds were compared, it was found that the curve of deflection, Yt, against log t ha3 a characteristic shape for each polymer. Thus, the deflection of natural rubber test specimens is proportional to log t over a wide range of loads, and throughout all these tests no departure from proportionality was observed. The deflection of neoprene specimens is not proportional to log time, but the slope of the curve increases slightly with time, whereas that of the Hycar curve diminishes. I n order to compare the primary creep of one compound with another, it is convenient to use the I-minute deflection, 1'1, as the datum from which creep is measured, and to plot the ratio of creep during first t minutes to creep during first 10 minutes [ = (Yt - Y,)/(Y,o - Y , ) ] against t (log scalc) as in Figure 2.
Testing To begin with, each specimen was tested twice: first in tension and then in compression, or fmt in compression and then in tension, or twice in shear. Any small permanent extension which occurred during a prolonged test in tension was nullified during a similar test in compression, and vice versa. Permanent deformation in shear was reduced by ensuring that duriqg the second test the direction of the 10. d was reversed by rotating the specimen. Between tests specimens were allowed to recover. After being tested twice, certain specimens mere given an additional cure, rested again, and retested according t o the above plan. T h e equipment used for the application of the load and for the measurement of deflection has been deficribed (2), but Figure 1 shows four of the twenty-four heads available, which, reading from left
Figure 6.
Relative Creep us. Per Cent Black for 20-Minute Cure Curve 20 minutes at 153' C. 4- Nhtural rubher, resilient X Natural rubber, bonding 0 Neoprene Hyoar
February 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY Table 11.
369
Values of Load, 1-Minute Strain and 10-Minute Creep
Soft Compounds Medium Compounds Hard Compounds Gum Stock, Cure 20 Min. Cure 20 Min. Cure Cure 20 Min. Cure Cure 20 Min. Cure 10-min. 10-min. 20 20 10-min. 20 20 10-min. 20 20 Load/ l-min. relative Load/ l-min. relative , Min. Load/ l-min. relative Min. Load/ relative Min. Geometry original strain, creep, original strain, creep, 10-min. original strain, creep, 10-min. original strain, 10-min. of area I O O Y ~ / Y t - -1 area, I O O Y ~ / W relative area, I O O Y ~ /Z-55 relative area IOOY~/ relative Loading Ib./sq.'in. H Y ~ l o g t lb./sq.in. H Y ~ l o gt creep lb./sq. in. H Y ~ l o g t creep Ib./sq.'in. H Y I logt creep
+
+
+
2.LC
~VATURAL RUBBERS Compression Compremsion Tension Tension Shear Shear
... ... ... ... ...
... ... ...
.... ....
... ...
...
..... ... .... ....
Compression Compression Tension Tension Shear Shear
26.8 51.5 47.7 51.5 12.37 26.8
8.05 15.81 24.4 28.1 23.05 52.4
0.0197 0.0174 0.02665 0.0242 0.0270 0.0228
. I .
69.2 130 72 137.2 16.4 33.6
16:l 27.9 28.1 82.4 23.45 46.25
21.2 43.2 43.2 64.8 21.6 43.2
10 13.2 24.15 26.8 58.6
0.0155 0.0150 0.0321 0.0320 0.0193 0.0211
.... .... .... .... ....
9.71 17.7 15.3 35.0 21.0 48.3
0.0216 0.022 0.0267 0.0378 0.0273 0.0248
....
....
72 137 75 138.5 26.8 58.0
.... .... ....
....
74.2 142 70 133.5 72 138
4.78 8.72 5.93 2.55 25.6 53.7
0.0348 0.0364 0.0388 0.0416 0.0464 0.0428
0.0337 0.0329 0.0458 0.0492 0.0385 0.0352
0.03055 0,0273 0.0528 0,0594 0.0356 0.0394
70 133.5 69.2 132 31.6 69.7
7.95 15.38 11.2 27.8 20.0 49.3
0.064 0.0558 0,0821 0.0797 0.063 0.068
0,0723 0.0536 0.0943 0,1026 0.0877 0,0753
68 128.7 68 130.5 68 130
1.942 3.7 1.87 4.79 11.18 29.2
0.1558 0.1437 0.2285 0.1887 0.1652 0.1380
5
YlO - Y1 Y1
YlO - Y1 Y1
YlO
....
- YI
YlO
Yl
....
.... .... , , , ,
....
....
0.271a 0.146 0.1865 0.1818 0.1850 0.1625
- YI Yl
NEOPRENEB I
Compression Compression Tension Tension Shear Shear
27.8 57.7 27.8 57.7 12.37 26.8
7.45 13.83 7.42 23.5 18.25 41.4
0.0912 0.0788 0.1008 0.1337 0.1130 0.1082
71.2 12.08 20.5 136 71.2 17.61 46 2 136 17.26 16 75 32.4 30 4
0.0703 0,0703 0,1068 0.1350 0.0844 0.0879
0.0637 0.0587 0.1064 0.13, 0.0759 0.0805
Compression 2 7 . 8 7.15 Compression 5 7 . 7 13.09 Tension 27.8 8.98 Tensio? 57.7 21.36 Shear 12.37 17.2 Shear 26.8 38.72 Improbable value.
0.0692 0.0700 0.0991 0.0994 0.0702 0.0719
72 134 71.5 139 17.4 32.7
0.0944 0.0752 0.162 0.1752 0.108 0.102
0.0622 0.0748
71.6 136.7 71.6 136.7 32.5 71.6
6.78 10.21 8.00 17.30 12.5 29.25
0.1162 0.1126 0.1517 0.1627 0.130 0.124
6.73 11.68 7.75 18.40 17.98 38.80
0.1772 0.1503 0.229 0.222 0.198 0.181
0.1012 0.1024 0.1364
....
0.1117
....
67.8 127.5 67.8 131.5 67.8 128
1.875 4.48 2.82 5.50 11.0 27.55
0.167 0.136 0.161 0.154 0.1465 0.1526
69.0 132.0 69.9 133.8 69.2 132.5
2.74 5.40 3.43 7.92 17.85 38.50
0.228 0.190 0.287 0.227 0.202 0.202
0.172 0,135 0.099
.... .... ,,.,
HYCARS
t, Rlin.
Log t
Yt
0 1 2 3 4
33-Lb. Tension 0.0862 0.0954 1.0 0.1065 2.207 0.1216 3.848 0.1419 6.054
10,000
0 1 2 3 4
8-Lb. Shear 0.0805 .., 0.0873 1.0 0.0953 2.176 0.1060 3.603 0.1172 5.397
1 10 100 1,000 10,000
0 1 2 3 4
1 10
0
I
10 100 1,000
100
1,000 10,000
1 10
100 1,000 10,000
1 2 3 4
0 1
2 3 4
0.1456 0.1214 0.1752 0,174
....
....
...
Yt
.
Yt - Yl YIO YI
-
Yt
,,
,
,
.... .... ....
.... ....
Yt - Yl -lY1
YlO
NEOPRENE, MEDIUM 33-Lb. Comp. 63-Lb. Comp. 0.0327 , 0.0488 ... 0.0365 1.0 0.0543 1.0 0.0412 2.237 0.0607 2.164 0.0461 3.526 0.0685 3.582 0.0520 5.080 0.0780 5.309
NEOPRENE, HARD 33-Lb. Comp. 63-Lb. Comp. 0.0093 ... 0.0175 0.0109 1.0 0.0199 1:o' 1 0.0127 2.154 0.0225 2.113 0.0146 3.372 0.0255 3 274 0.0292 4.916
63-Lb. Tension 0.229 ... 0.260 1.0 0.297 2.19 0.346 3.78 0.394 5.34
33-Lb. Tension 0.0382 ... 0.0440 1.0 0.0511 2.222 0.0602 3.79 0.0710 5.66
63-Lb. Tension 0.083 0.0965 1:O' 0.114 2.29 0.137 4.00 0.167 6.22
33-Lb. Tension 0.0143 ... 0.0166 1.0 0.0191 2.087 0.0217 3.217 0.0244 4.39
63-Lh. Tension 0.0253 ,., 0.0292 1.0 0.0335 2,103 0.0394 3.615 0.0448 5.000
15-Lb. Shear 0.148 ... 0.161 1.0 0.1743 2.03 0.190 3.23 0,214 5.08
15-Lb. Shear 0.060 0.0678 1.0 0.0770 2.179 0.0890 3.718 0.1042 5.667
33-Lb. Shear 0.1410 ... 0.1585 1.0 0.1805 2,260 0.2145 4.19 0.2600 6.80
33-Lh. Shear 0.0545 0.0625 1.0 0.0713 2.10 0.0817 3.40 0.0950 5.06
63-Lb. Shear 0.1362 0.1570 1.0 0.1810 2.154
..
1 100
0.1082 0.0803 0.0767
70.0 133.5 71.2 135.5 31.9 70.2
Table 111. Values of t-Minute Creep/lO-Minute Creep Yt - Y1 Yt - Y1 Yt - Yl Yt YlO - Y1 Y1 YlO - Yl Yt YlO - YI
0 1 2 3 4
1,000 10,000
0.1061
Yt - Yl YlO - Y1
1 10 100 1,000 10,000
10
14.83 26.75 25.3 79.3 21.9 40.8
'
.
.
I
....
...
...
....
...
....
...
...
HYCAR,HARD 33-Lb. Comp. 63-Lb. Comp. 0.01340 , ,. 0.0263 ... 0.01646 1.0 0.0313 1.0 0.01920 1.895 0.0356 1.84 0.02162 2 686 0.0395 2.24 0 02370 3 366 0.0427 3.28 63-Lb. ~... 0.325 0.382
33-Lb. Tension 0.1218 0.1415 1.0 0.1562 1.746 0.1675 2.320 0.1765 2.775
0.422
8-Lh. Shear 0.1056 , .. 0.117 1.0 0.1257 1.763 0.1321 2.325 0.1364 2.702
15-Lb. Shear 0.1960 , , 0.2160 1.0 0.2298 1.690 0.2410 2.250 0.2495 2.675
....
....
Tension .
I
.
1.0
1,701
...
...
.
33-Lb. Tension 0.0375 0.0461 1:O' 0.0540 1.918 0.0605 2.674 0.0656 3.267
63-Lb. Tension 0.0892 ... 0.1090 1.0 0.1280 1.91 0.1440 2.68 0 1580 3.44
33-Lh. Tension 0.0167 ... 0.0215 1.0 0.0258 1.896 0.0292 2.604 0.325 3.290
63-Lh. Tension
15-Lb. Shear 0.0870 ... 0.1042 1.0 0.1200 1.92 0.1350 2.72 0.1483 3.48
33-Lh. Shear 0.188 ... 0.222 1.0 0.248 1.76 0.274 2.52 0.298 3.23
33-Lb. Shear 0.0870 ... 0.1045 1.0 0.1200 1.943 0.1350 2.743 0.1482 3.497
63-Lh. Shear 0.188 ... 0.226 1.0 0.257 1.81
0.0387 0.0475 0.0553 0.0630 0.0702
.... ....
,., 1.0
1.886 2.761 3.576
... ...
370
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Vol. 43, No. 2
Influence of Cure. The values of (Yt - Y,)/(Ylo - YJ,shoan in Table 111, are for specimens 0.2 cured for 20 minutes a t 153"
