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(12) Ivey, D. G., hlrowca, B. A., and Guth, E.. J . Applied Phys., 20.486 (1949). Juve; A. E., Fihding, J. H., and Graves, F.L., A . S . T . M . Bull. 146 (May 1947); I n d i a Rubber W o r l d , 116, 208 (1947). King, A. J., J . I n s t . Elect. Engrs., 93, 435 (1946). Kohlrausch, F., Pogg. Ann. Phys., 158, 337 (1876). Kosten. C. UT..Proc. Rubber Tech. Conf. London, 1938, 987. (17) Kraemer, E. O., “Recent Advances in Colloid Science,” p. 623, New Pork, Interscience Publishers. 1942. (18) Lafferty, R. E., Electronics, 19 (May 1946). (19) Mullins, L., T r a n s . I n s t . Rubber I n d . , 26, 27 (1950). ( 2 0 ) Naunton, W. J. S., and Karing, .J. R. S., Proc. Rubbei Tech. Con!. London, 1938, 805.
Vol. 43, No. 2
(21) Naunton, W. J. S.,and Waring, J. R. S., Trans. I n s t . Rubber I n d . , 14,340 (1939). (22) Nolle, A. W., J. Acoust. Sac. Am., 19, 194 (1947). (23) Oberto, S.,and Palandri, G., Rubber A g e (LV,Y.), 63, 725 (1948). (24) Rivlin, R. S., T r a n s . Inat. Rubber Ind., 26, 78 (1950). (25) Roelig, H., Proc. Rubber Tech. Conf. London, 1938,821. 126) “Rubber in Engineering,” g . 5, London, H.M. Stationery Office, 1946. (27) Ibid., p. 223. (28) Smith, H. DeW., and Eisenschitz, R., J . Teztile Inst., 22, 1701’ (1931). (29) Waring, J. R. S., Trans. I n s t . RiibherInd., 26,4(1950) RECGIT-RI) October 2, 1950.
Some Properties of uleanize Rubber under Strain DEGREE OF CRYSTALLIZATION AS CALCULATED FROM TEMPERATURE COEFFICIENT OF ELASTIC TENSION R. H. S. T. Hoonstru, Rnbbsr-Stiehting, Delft, Holland
To elucidate
the crystallization phenomenon in natural rubber and to investigate the applicability of thermodynamic calculation to measurements of the elastic tension as a function of temperature, i t seemed necessary to check whether crystallization determined bj x-ray analysis (and combined with density) lined up reasonably with the percentage o f crystallization computed from the energy change found by applying thermodynamics to stretched vulcanized rubber) on stretching. Calorimetric measureinents were desirable, as no accurate figures are availahle for the heat of crystallization of rubber crystalli tes. The heat of melting of rubber crystallites was determined to about 66 joules per gram, which is of the same order as that of isoprene. The spreading in the results
w-as large; the determination is based on the degree of crystallization found by x-ray analysis of raw rubher. The heat of Crystallization on stretching, found by thermodynamic evaluation of the elastic tension and its temperature coefficient, is comhined with the value of 66 joules for the heat of melting of the pure rubber crystallites. The degree of CrSstallization calculated in this way agrees reasonably well with the direct x-ray measurements of Goppel and Arlman. Crystallization as determined by x-ray analysis and that responsible for the energy change on stretching are much the same. This also means that thermodynamic evaluation of the change of stress with temperature is justified if suffcient relaxation of stress has taken place.
T
K i j k ( 7 ) mention that this corrcction should riot be applied in such a simple way, agreeing in t,hihiS respect, with Elliott, and Lippniann ( 4 ) . Accuratc measurements wit’h natural rubher and GIory,measurcnicnts of
(g)
m?re carried out
hy WildscliuC ( 1 1 ) . who first cordateti the drop in clastic force on crpstallizatjion with the degree oi crystallization. Later, a more accurate treatment of t,he correlation between degree of crystallinity and the resulting decrease in elastic tension n-as given by Flory ( 5 ) . Similar espcriment,s, described in the pr nt paper, mere carried out with two well defined pure gum vulcanizates at, different elongations. The crystallinity was estimated by determination of the heat of crystallization and change in internal energy.
Temgeratnre CaeEficient of Elastie Tension ut Various Elongations Experimental Procedure. The mixes were vulcanized in a steam-heated press into sheets about 0.5 mm. thick. From these sheets 33.4 X 36.7 mm. rings were punched, and these were stretched over pulleys about 3 mm. in diameter. Rings proved to be preferable t o dumbbell samples, as the latter need constant adjustment., owing t o different contraction of the narrow
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1951
Figure 1. Tension-Temperature Relationship for Mix S
and broader part of the dumbbell when the temperature is increased. As with the rings described there is still about 10% difference between outer and inner diameter, another type of sample was tried-a ring 25 and 26.2 mm. thick and about 3.5 mm. broad. Although here the small difference between inner and outer diameter warrants a more homogeneous elongation, molding difficulties reduced the accuracy of the results with these samples t o about the same amount as with the flat rings. T h e apparatus consisted of a jacketed vertical glass tube in which the sample was fastened to the bottom and the other end is suspended from the beam of a laboratory balance with a sensitivity of about 0.05 gram. T h e rings were put in the apparatus at the highest temperature of the range of the measurement, stretched t o the desired elongation, and kept there until further stress relaxation during measurements was negligible. For elongations up t o 300y0 the highest temperature was 70" C. and the relaxation period 0.5 or 1 hour. After this period the temperature was lowered 0.5" C. per minute to the lowest temperature (usually -40' C,), and after that, increased again until the original value had been reached.
v
The experiment n-as considered satisfactory when the tension after this cycle amounted t o practically the value shown at the start. However, at elongations of 300% and higher the tensions of the return (increasing temperature) lay for an important temperature region at much lower values than for decreasing temperatures. The tension-temperature curves at decreasing temperature were used for the calculations, on the argument t h a t they must be nearer t o the true equilibrium than the ascending branch of the curve. T h e latter will contain less amorphous (mobile) rubber and therefore will be less capable of reaching equilibrium (between stress and orientation or crystallization) than the
363
more amorphous rubber which is present before the temperature was lowered. The measurements were repeated three or more times, and the average of the slopes and tension values is plotted in Figures 1 and 2. No corrections for thermal expansion were applied. Discussion of Results. At elongations below 3 o o ~ 0the relationship between tension and temperature is represented by a straight line; at 3oOy0 and higher elongations, however, two rather sharp bends become visible. One, occurring above zero, is found increasingly higher for higher elongations; the other, less sharp, is found below 0" C. and has the same trend but is less reproducible than the first. T h e straight lines at low elongations indicate the proportionality of the tension with temperature, and therefore must yield zero intercepts at absolute zero temperature. The same is valid at higher elongations for t h a t part of the line that lies above the temperature of the first discontinuity; here the rubber is wholly amorphous and its tension is proportional to the absolute temperature. When the temperature is lowered, crystallization sets in at a point T i (the upper break in the curve), the temperature of incipient crystallization. Then crystallization increases with further cooling and causes a steeper descent of tension with temperature. However, the relationship can still be represented by a straight line, which extrapolated exhibits a strongly negative intercept. It is remarkable t h a t the tension a t 500y0 elongation may become lower than a t 400% elongation a t certain temperatures. At still lower temperatures a second transition point is encountered; below this the decrease of
30
20
IO
-40 - 2 0
0
20
40
60
80
100°C
Figure 2. Tension-Temperature Relationship for Mix T
+-
Table I.
Test Mixtures S
Smoked sheeta Firbt latex crepe Zinc oxide Captax Di henylguanidine Alfl01 or-naphthylamine Tuads Stearic acid Sulfur Vulcanization a t 142" C. in press, min. Tensile strength kg./sq em. Elongation at rdpture. % Load a t 300% elongation, kg./sq. om. Hardness (Shore A) Elasticity (Shore B) Permanent set after 24 hours, 200% el ongation measured 24 hours after release, % Specific weight
100
... 5
T
...
100 5
0.8 0.5 1
... ...
1.75 20 320
...
... 1
680 22
38 97
1
0.97
... 3 1
30 265 755
10.5 29 90
0 0.96
tension with temperature resumes roughly itg original course, This can, however, not be accurately determined, as the measurements are less reproducible in this region and only a few points can be plotted. The second discontinuity would indicate that below this point crystallization no longer increases or increases only a t a much reduced speed. These low temperature discontinuities lie well above the second-order transition point of natural rubber (-72" C.) and even definitely above the brittle point (-58' C.). E n e r g y E f f e c t s on S t r e t c h i n g If a n approximate equilibrium is assumed a t the various tem-
peratures, the tension may be represented by:
INDUSTRIAL AND ENGINEERING CHEMISTRY
364
Vol. 43, No. 2
but for a whole range 01 temperatures, the breadth of which deDends on the elongation. At low elongations when no crystallization occurs, the first term on the right is practically zero or has a small positive value; a t higher elongatione this is valid only a t sufficiently high ternperatures (above the temperature of incipient crystallization).
(g)T
The term
is equal to the intercept of the tension axis
a t absolute zero temperature.
r
elonaation
The curvcs in Figure 3 for
($):
us.
elongation may lie
integrated up t o 300, 400, and 500%, respectively. tegral is proportional t o the value of
/ ($)
This in-
dL for these cases
-i.e., the actual change in int'crnal energy on stretching. This energy calculated from the surface area, say to 500% elongation, is the latent heat of crystallization developed when the sample is stretched 500y0,provided that other energy effects of positive value due to increase in interatomic distances or distortion of valence angles remain small. At about 20" c. t.he values for S and T compounds are those listed in Table 111.
Table 111. Crystallization Energy Developed on Stretcliing to Various Elongations (Values in joules per ml.) Elongation. % 300 400 1.6 7 5 T compound 1.6 5.8
S compound
I
I
50t 60
h
x\
If the heat of crystallization of pure rubber is known, t h r degree of cryst'allization a t these thrce elongations can be estimated in this way.
I
I \
701.
The values for
Latent Heat of Crystallizationand Degree of Crystallization at Various Elongations
(g)T
that are actually found for the plot
above the temperature of incipient crystallization are given in Table 11.
Table 11.
Experimental Values for above Ti
Elongation, % 8 compound, kg./sq. om. T compound, kg.,/sq. cm.
300 0 0
rg)
360 -1
-
TatTemperatures
400 -4 -2
500
-12 -7
550
-
-15
These data show that only for 300yo elongation t8he rubber above Ti behaves approximately as an ideally elastic material within the errors of the measurements (about *l kg. per sq. cm.). If we are really determining equilibrium values, this means t h a t above 300% elongation and a t temperatures above Ti there are still effects which result in decrease of energy on stretching. Such effects have been called procrystallization, as probably some crystallites are formed during the cooling in this temperature region. At Ti crystallization sets in on a larger scale and corisequeutly the tension decreases more rapidly wit,h temperature. This has been shown clearly before by several investigators. I t is interesting t,o mark the value of
in the tempera-
ture region of increasing crystallization. Here the intercept is strongly negative because of the heat of crystallization n-hich develops. I n Figure 3 the value of elongations.
500 14 0 10 8
(g)T
is computed for various
This curve holds not for one definite temperature
I n 1929, van Rossem and Lotichius (8) determined the heat oi' crystallization of "frozen" first latex sheets which had been stored for between 10 and 14 years in a cool cellar. The measurement,s were carried out a t 0 " C. by swelling "frozen'! and "thawed" sheet,s in toluene in an ice calorimeter; the difference in heat of swelling of the two specimens is the heat of crystallization. Thc value found by these authors was about 5.0 calories per gram. This has been confirmed by experiments of Wood, Bekkedahl, and Gibson ( 1 2 ) , who determined t'he heat of fusion by measwing thc rise of the melting point through pressures of about 1200 Irg. per sq. cni. Applying Clapeyron's equation, they found a value of about 4 calories per gram for a stark rubber which t,hey considered not crystallized t o its final value. The degree of crystallinity of frozen sheets, such as those studied by van Itosseni and Lotichius, can be estimated to 30 t o 35y0 according t o x-ray analysis f ~ yArlman and Goppel ( I ) , which agrees xyith their density measurements. This would indicate a heat of cryst,allization of the crystallites of 14.5 to 16.8 calories per gram (60 to 70 joules per gram). Experimental. B s no accurate measurements of the heat of crystallization of rubber with a known degree of crystallinity are available, the author determined this property with smoked sheets about 3.5 years old, which had been stored in the cellar of the Rubber lnstitute a t Delft. The degree of crystallinity of this sample as determined by x-ray analyis and density measurements amounted to about 38 * 2%. The calorimet,er was of the twin DDPWI~. flask type and the compensation heating method ivas used in most of the experiments. The temperature indicator was a mirror galvanometer semitive to about lo-* ampere, in combination with a tenfold copperconst,antar: thermocouple series. About 1 gram of frozen rubber was swollen in 25 ml. of toluene and the decrease in temperature was compensated by electrical heat,ing. The experiments were repeated with amorphous rubber, obtained by heating the frozen sheets t o about 70" C. for about 15 minutes. The thermostat, containing t'he Dewar flasks was kept constant a t 0" C. with ice w t e r .
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
13
5
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.C o o p e r 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.
