INDUSTRIAL
March 1950
AND
=
mole fraction
y*
=
equilibrium vapor mole fraction at gas-liquid interface
ß
=
y
=
time, seconds parameter defined by Equation 7,
Subscripts A. B, C, etc. components A, B, i stagnant gas or gases =
a
ENGINEERING
function of y*
C, etc., of gas
o
gas-liquid interface
or
mixture
standard temperature and pressure
LITERATURE
475
(2) Arnold, J. H., Trans. Am. Inst. Chem. Engrs., 40, 361 (1944). (3) International Critical Tables, Vol. Ill, New York, McGrawHill Book Co., 1929. (4) Ibid., Vol. V. (5) Mack, J. Am. Chem. Soc., 47, 2473 (1925). (6) Maxwell, J. C., “Scientific Papers,” Vol. 2, p. 57, Cambridge, England, Cambridge University Press, 1890. (7) Serf ass, E. J., Ind. Eng. Chem., Anal. Ed., 13, 262-3 (1941). (8) Stefan, Sitzber. Akad. Wise. Wien, 63 (2), 63 (1871). (9) Ibid., 65 (2), 323 (1872). (10) Wilke, C. R., Chem. Eng. Prog., 46, 95-104 (1950).
= —
CHEMISTRY
CITED
(1) American Petroleum Institute Research Project 44, Table 5K, Washington, D. C., National Bureau of Standards, 1945.
Received July
5, 1949.
Low Temperature Stiffening of Elastomers S. D.
GEHMAN, P. J. JONES, C.
S.
WILKINSON, JR.,
AND D. E. WOODFORD
The Goodyear Tire and Rubber Company, Akron 16, Ohio
The generally observed stiffening of elastomers
at
low
of temperatures may be supplemented by the occurrence crystallization if the elastomers have sufficient regularity and other conditions are favorable. of molecular structure To study these effects, observations of the progressive stiffenings of elastomer compounds at low temperatures were of the relative torsional modulus made by measurements of test strips mounted in racks which were stored at low temperature. The periods of observation extended over 30 to 60 days with temperatures in the range from —59° to 18° C. A correlation was found between density changes due to crystallization as determined dilatometrically and torsional stiffness changes. Factors studied in addition to —
EXTENDING the low temperature range for useful applica-
the phenomena of spontaneous stiffening INtions of elastomers, other must taken into account.
causes be due to crystallization or Crystallization, accompanied by progressive stiffening, sometimes In general, this may be exoccurs over a long period of time. pected in the case of elastomers which have sufficient regularity of molecular structure to show the more familiar phenomenon of crystallization upon stretching. Because such elastomers include some of the most useful types of rubber, it is important to understand under what conditions spontaneous crystallization at low
temperatures is to be anticipated. Work reported from the National Bureau of Standards (10,11), dealing principally with the crystallization and melting of unvulcanized Hevea rubber, furnishes a useful background for the crystallization phenomena with vulcanizates. Observations of the progressive low temperature stiffening of a class of technical vulcanizates have been published by Gregory, Pockel, and Stiff (7). Forman (6) investigated the comparative cold hardening of neoprene, GR-S, and Hevea vulcanizates over a period of time at low temperatures. Conant and Liska (S) observed progressive stiffening with plasticized butadiene-acrylonitrile vulcanizates, which was attributed to low temperature incompatibility or crystallization of the plasticizer. Forman reported similar effects for plasticized compounds of Neoprene Type FR. Beatty and Davies (2) studied effects of time, temperature, and stress on the behavior of rubber at low temperatures. The present work comprises a description of what appears to be an improved technique for securing extensive data on the progressive stiffening of elastomers, a survey of the effects of
temperature included compounding variables such as cure and plasticizer content. Some data were obtained to show the acceleration of crystallization due to compressive stress. Progressive stiffening due to crystallization was observed for vulcanizates of Hevea, Neoprene Type GN, and Butyl rubber as well as for an 85-15 butadiene-styrene redox-type copolymer. The results were interpreted in accordance with concepts of crystal nucleation and growth with of necessary modifications due to the molecular structure elastomers. Although the phenomenon of spontaneous crystallization of elastomers was found to be complicated by many factors, a useful degree of generality appears in some of the results. temperature and other significant variables on the crystallization of typical vulcanizates of technically important elastomers, and a discussion of the results in order to reach an understanding of their significance in terms of general principles applicable to crystallization and the molecular structure of elastomers. EXPERIMENTAL
METHOD
A method of measuring the torsional stiffness of rubber samples previously described (1, 6) has been adapted for securing data on the stiffening due to prolonged exposures to low temperature. Figure 1 is a photograph of the assembled apparatus. The test strips were mounted in supplementary racks as shown in Figure 2. Ten of these racks were available for the investigation, so that fifty samples could be run concurrently. The torsional stiffness of each test piece was measured at room temperature and at the storage temperature. The racks were then stored in constant temperature cold boxes or cold rooms at the desired temperatures. It was found advantageous to invert glass beakers over the racks to minimize the chances of any temperature variation of the samples due to opening a cold box during the course of the tests. The racks were removed at suitable intervals and the test pieces were measured without changing their temperature, by transferring the rack to be measured into the appropriately precooled insulated cylinder of the apparatus and carrying the rack to the measuring stand where the storage temperature was maintained. After measurement, the rack was removed from the insulated cylinder in the cold box, if it was desired to continue the test.
In the presentation of the results, the ratio of the torsional modulus measured at various time intervals to the original
INDUSTRIAL
476
Table I. Materials Smoked sheet
Sulfur Zinc oxide Stearic acid -Captax
Phenyl-2-naphthylamine EPC black Paraflux
1
100
3.0 3.0 3.0 0.8 1.0 50.0 6.0
2
100
100
3.0
3.0 3.0
0.8 1.0
5
6
7
8
100
100
100
100
2.25 5.0 2.0
2.25 5.0
2.25 5.0
0.8
0.75
0.75
6.0
0.8 1.0 15.0 6.0
15
16
17
18
100
100
ióo
ióo
3.0 3.0
3.0
30,0
1.0
6.0
GR-S-10 Polybutadiene (5° C.) GR-S
2.0
Butyl
Captax
Tuads
Diphenylguanidine Stearic acid EPC black Gastex black Cumar CX Paraflux
2.'5 5.0
2.’5
2.0
0.75
0.75 0.25
i!o
i'.o
i'.o
40 ¡0
4010
5.0 2.0
0.75 2.0
100
2.25 0.0 2.0 0.75 2.0 50.0
0.0
2.25 5.0
2.25 5.0
0.75
0.75
0.75
!0
2.25
2.0
2.0 50.0
2.0
2.0 50.0
2.0 2.0
20 !o
30!
50 ¡0
19
20
21
22
23
24
25
26
27
28
25 .‘0
ÓÓ.’O
75.0
ióo 2.5
i!o
0! 75
1ÍÓ0
0.75 0.25
0.75 0.25
2 .’5
i'.o
i'.o
i'.o
40 !o
40'. 0
40* 0
i'.o
2.25
14
50‘.0
ióo
50.0 30
ióo
2 *1
0¡55
3.0
0¡64 2.0 50.0
6.0
Trioctyl phosphate
2.0 50.0
13
100
30.0
5.0
40!
0.75
12
100
20'. 0
5.0
0.25
2.0 50.0
11
100
10'.0
5.0
5.0
0.75
2.25 5.0 2.0
10
100
s'o
ióo
2.0 50.0
2.25 5.0 2.0
9
100
50.0
2.5 5.0
2.5
2.' o
Vol. 42, No. 3
2.0 50.0
2.0
Smoked sheet Neoprene Type GN
Sulfur Zinc oxide Magnesia
CHEMISTRY
Compound Formulas Used
3.0
3.0 3.0
Dioctyl sebacate Trioctyl phosphate
85/15 B/S (5° C.) 85/15 B/S (50° C.) Polybutadiene (50° C.)
4
3
100
ENGINEERING
AND
1Ó0
75.0
50.0
2‘ó
2.5
2.5
0.0
5.0
2.5 0.0
i'o
i'.o
i'.o
.0
5.0
i'.o
i'.o
i'.o
25.0
.0
50.0
ióo 5.0
i'.o
50.0
50.0
50.0
50.0
50.0
é'.o
é'.o
é'.o
é'.o
é’.o
ióo
2.0
5.0 4.0
i’.o
3.0
Ó.5 1.0
3¡0
45.0
é’.o 3
Rubber process oil
modulus at the storage temperature is plotted on a logarithmic scale against the time. Thus the plots present directly the relative change in modulus brought about by the storage at low temperature. The relative modulus at room temperature is represented on the graphs by points on the axis of ordinates, so that the over-all relative stiffening from room temperature to the storage temperature at any subsequent time also can be obtained. COMPOUNDS USED
The compound formulas which
were
used
are
given in Table I.
Figure
1.
COMPARISON OF VOLUME
AND STIFFNESS CHANGES
Because the stiffness changes observed in this work over extended periods of time were attributed primarily to crystallization, it was desirable to compare the course of the stiffening with direct measure of crystallization. For this purpose, a more simultaneously with one of the stiffness tests in a cold room, observations were made of the volume changes of pieces of the same compound in a mercury-filled dilatometer. The results of this comparison are shown in Figure 3. The similar shape of the two curves indicates a close córrela-
Torsional Flexibility Apparatus
March 1950
INDUSTRIAL
AND
ENGINEERING
tion between the increase in density of the rubber and the stiffening. The plot in Figure 4 shows, however, that the percentage increase in stiffness far exceeds the percentage increase in density. The stiffness is evidently very sensitive to crystallization. The structural picture which should be kept in mind to understand this result is that the density increase is concentrated in small crystallites, whereas only the average density is deduced from a dilatometer measurement. The crystallites stiffen the rubber not only because they are relatively stiff themselves but because they introduce fixed points in the molecular network structure, thus reducing the flexibility of the remaining amorphous material. It is interesting to note from the curves in Figure 4 that the effectiveness of the crystallites on a percentage basis for increasing the stiffness was about the same for both gum and tread stocks.
477
'
Figure 2. Test Racks called homogeneous if it
CHARACTER OF CRYSTALLIZATION
PROCESS IN ELASTOMERS
The fundamental thermodynamic theory of a phase change, the direction of which is controlled by the free energy, must, of course, apply to the spontaneous crystallization of elastomers. The rate of transformation is not given by simple thermodynamic theory, so that theories of the rate of crystallization become more elaborate and include such concepts as transition through an activated state. The problem in the development of the theory of crystallization of elastomers lies in the identification and formulation of the role which the various structural features play in the process. There is reason to suppose that, as in other media, the crystallization process involves the formation of crystal nuclei and subsequent growth from these centers (4, 8). The nucleation is
|Figure
CHEMISTRY
3.
Correlation of Density and Stiffness Changes
Figure 4. Relative Magnitude of Density and Stiffness Changes
occurs in the material itself and heteroforeign particles or an already existing is plausible in the case of elastomers, because of the unique character of the crystal structure, that the nucleation is homogeneous. The nuclei may consist of small groups of molecular segments which by chance thermal fluctuations have assumed relative positions and spacings approximating those of molecules in the final crystal lattice so that crystal growth can
if it interface. It geneous
occurs
on
proceed.
The induction period before crystallization starts, illustrated by the curves in Figure 3, represents the time required for the formation of nuclei. In a recent development of phase transformation theory (4), it is shown that the nuclei must exceed a critical size before continuous growth can occur. This is a consequence of the free energy change in the vicinity, which includes terms due to the interfacial free energy, the bulk free energy difference between the phases, and the strain energy. In the case of elastomers, the growth from a single nucleus is limited, and only small crystallites occur, even when the number
INDUSTRIAL
478
AND 20
ENGINEERING
/ 275
CHEMISTRY
Vol. 42, No. 3
TESTING TEMP
CURE
40/275
10/290
Figure
6.
Stiffening of Hevea Gum Compound at -35° C.
Figure 9. Contrast in Effect of Temperature on Stiffening of Hevea and Copolymer Tread Compounds
CURE
57-C. -35°C. -I8°C.«-L0NG TERM TESTING
Figure 10. Torsional Flexibility of Hevea and Copolymer Tread Compounds
Figure 7.
Stiffening of Copolymer Tread Compound
Figure 8. Stiffening of Copolymer Tread Compound with Higher Combined Sulfur at —57° C.
of crystallites is low. The limitation to the size of the crystallites probably arises from entanglements which are encountered in the network structure as the crystal lattice is propagated and which offer insurmountable barriers to the rotation or movement of additional molecular segments into the lattice. The average length of the crystallites might provide a measure of the average distance between such structural features as cross linkages, branches, or hard entanglements. The rapid increase in the degree of crystallinity after the induction period, such as is shown by the curves in Figure 3, implies that new nuclei are being formed at a rapid rate. This process is retarded again as the transformation becomes more complete. The curves in Figure 5 show the stiffening or crystallization for a series of cures of a Hevea tread stock at —35° and —57° C., respectively. As others have observed (7, 10), the process is dominated by the state of cure as indicated by the combined sulfur values shown. The per cent of combined sulfur is calculated on the amount of rubber in the compound. The cross linkages or other chemical irregularities introduced by the combined sulfur interfere with the crystallization to such an extent that 2.5% of combined sulfur appears to be sufficient practically to forestall spontaneous crystallization in natural rubber. This may be readily understood when the sulfur combination is viewed as bringing about the formation of a relatively immobile molecular network structure with fixed points which cannot be accommodated in a crystal lattice. Figure 6 shows the stiffening observed for a Hevea gum stock stored at —35° C. Only the two lower cures crystallized. This was also the case when the samples were stored at —29° C.
INDUSTRIAL
March 1950
Figure
AND
ENGINEERING
CHEMISTRY
479
Stiffening of Series of Hevea Compounds
11.
Amount of carbon black varied
At —57° C. there was no evidence of crystallization for any of the cures. In comparing the crystallizing tendencies of the gum and tread stock, it appears that these tendencies were much more pronounced in the latter. Most of this effect is due to the difference in the combined sulfur values for the same time of cure. But the 60-minute cure of the tread stock with 2.37% combined sulfur definitely crystallized, whereas the 40-minute cure of the gum stock with 2.38% did not. Either more combined sulfur was required for the rubber in the tread stock than in the gum stock to prevent crystallization, or the combined sulfur values as ordinarily determined are not strictly accurate in reflecting the combination of sulfur with rubber. There may be side reactions in the vulcanization system. On the other hand, the increased tendency of the tread stock to crystallize might be evidence of at least some heterogeneous nucleation. In contrast to these effects with long-term exposures to low temperatures, short-term tests did not show any systematic The dependence on the combined sulfur for this range of cures. short-term torsional flexibility test {1) provided the temperatures, or T values, at which a sample was 2, 5, 10, and 100 times as stiff as at 25° C. These temperatures for the different cures did not differ by more than 2° or 3°. The molecular basis for the short-term stiffening is, of course, very different from crystallization, so that this result is not surprising. The average T values for all the cures are listed in Table II.
Table II.
Average T Values for Stocks Compound. Tread, 0 C.
r,
-38 -51
T, 0
Tm
-55
-64
1,
Hevea Tread
and
Gum
Compound 4, Gum, ° C.
-58 -61 -62 -67
The higher T values for the tread stock imply some significant alteration in either the secondary valence forces or the segmental molecular mobility due to the introduction of carbon black into the compound. The carbon black does not interfere with the molecular motion necessary for spontaneous crystallization; hence the mobility of the molecular segments does not seem to be seriously impaired. The increased sensitivity of the secondary valence forces to temperature when the carbon black is present is rather difficult to understand. It is fairly certain that the carbon black particles are attached to the rubber molecules by secondary valence forces. Thus some of the secondary valence forces
Figure 12.
Stiffening of Series of Hevea Tread Compounds
Amount of plasticizer varied
between rubber molecules in the gum stock are replaced in the tread stock by secondary valence forces between carbon and rubber molecules. It is possible that these forces have a greater temperature dependence than the mutual forces between rubber molecules. Another possibility is that changes in temperature set up a system of microstresses in the tread stock due to the difference in the thermal coefficients of expansion of carbon black and rubber. These internal stresses might lead to a greater temperature sensitivity for the tread stock as compared to the gum stock. Figures 7 and 8 show stiffening curves for tread stocks of an 85-15 butadiene-styrene polymer, the polymerization having been carried out at 5° C. In comparing the curves at —57° C., it is at once apparent that the higher state of cure produced by formula 15 as compared to formula 16 results in less crystallization. The crystallizing tendencies of Hevea and the copolymer at 35° C. as compared to —57° C. show a marked contrast, which is brought out by the curves in Figure 9. The Hevea tread stock crystallized much more readily at —35° C. than at —57° C., whereas the reverse is true of the tread stock of the redox copolymer. To try to reach an understanding of this fact, Figure 10 gives curves showing the relative flexibility of these compounds in a short-term torsional flexibility test. The ordinate is the twist of the test piece corresponding to 180° twist of the tor—
sion head. The Hevea compound crystallized readily at the long-term test temperatures of —18° and —35° C., but at —57° C., which is a considerable distance below the bend in the twist-temperature curve, its crystallization was greatly reduced. The copolymer tread stock showed no tendency to crystallize at —18° C., a slight amount of crystallization at —35° C., and pronounced crystallization at —57° C. The fact that the flexibility of the copolymer was but slightly impaired at this lowest test tern-
INDUSTRIAL
480
AND
ENGINEERING
CHEMISTRY
Figure 15.
Vol. 42, No. 3
Stiffening of Hevea-GR-S Blends (Tread Stocks)
TIME (DAYS)
Figure 13. Effect of Temperature of Stiffening of High and Low Plasticized Compounds 57*C.
Figure
14.
-18·
