THE IMPROVEMENT OF THE
LOW-TEMPERATURE BEHAVIOUR OF
NATURAL RUBBER VULCANIZATES BY
CHEMICAL MODIFICATION
WITH THIOL ACIDS
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECH-NISCHE HOGESCHOOL T E DELFT, KRACHTENS ARTIKEL 2 VAN HET KONINKLIJK BESLUIT VAN
16 SEPTEMBER 1927, STAATSBLAD NR. 310 EN OP GEZAG VAN DE RECTOR MAGNIFICUS DR. O. BOTTEMA, HOOGLERAAR IN DE AFDELING DER ALGEMENE WETENSCHAPPEN, VOOR EEN COM-MISSIE UIT DE SENAAT TE VERDEDIGEN OP
WOENSDAG 27 JUNI 1956, DES NAMIDDAGS TE 2 UUR, DOOR
FRIDOLIN JACOB RITTER
GEBOREN TE SOERABAJA
DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR: PROF. DR. IR. A. VAN ROSSEM
[ 1
I
I
) IAan mijn Ouders Aan mijn Vrouw
C O N T E N T S
I N T R O D U C T I O N 1
C H A P T E R I. G E N E R A L L A B O R A T O R Y T E S T S F O R
L O W - T E M P E R A T U R E S E R V I C E A B I L I T Y O F E L A S T O M E R S . . 3
§ 1. T h e causes of failure of elastomers at low temperatures 3
§ 2. Stiffness tests 5 § 3. Brittleness tests 6 § 4. Elactic recovery tests 6 § 5. Miscellaneous tests 7
C H A P T E R II. T H E T E M P E R A T U R E - R E T R A C T I O N T E S T (TR T E S T ) 11 § 1. Historical development. Correlations with other test methods . . . 11
§ 2. Practical significance of the T R test for natural rubber 16 § 3. Limitations of the T R test. Influence of variations of test conditions 18
§ 4. Modifications of the T R test 22 § 5. Description of apparatus 27
C H A P T E R III. P O S S I B L E M E T H O D S F O R A L I M I T E D M O D I F I C A T I O N
O F N A T U R A L RUBBER 29 § 1. General considerations 29
§ 2. Cross-linking 31 § 3. Intramolecular reactions (isoraerizations) 32
§ 4. Branching 33 § 5. Modification by high energy irradiation 34
§ 6 . Other methods for introducing foreign groups (addition, substitution, etc.) 34
C H A P T E R IV. P R E P A R A T I O N O F T H I O L A C I D S 39 § 1. Aliphatic monothiol acids (monocarbothiolic acids, thiolic acids) 39
§ 2. Aromatic monothiol acids (monocarbothiolic acids) 41 § 3. Bisthiol acids (dicarbothiolic acids, dithiolic acids) 45 § 4. Dithio acids (carbodithio acids, thionthiolic acids) 46
C H A P T E R V . T H E M O D I F I C A T I O N O F N A T U R A L RUBBER IN
S O L U T I O N W I T H M O N O T H I O L ACIDS 48
§ 1. Historical survey 48 § 2. Preliminary experiments with thiolbenzoic acid ( T B ) . Evaluation of
the best reaction circumstances 51 § 3. Preparation and raw-polymer properties of high thiol acid rubbers . 52
§ 4. Preparation and raw-polymer properties of low thiol acid rubbers 56 § 5. Vulcanization of low thiolbenzoic acid rubbers. Physical properties
C H A P T E R VI. T H E M O D I F I C A T I O N O F N A T U R A L RUBBER I N T H E
D R Y S T A T E W I T H M O N O T H I O L A C I D S 62
§ 1. General considerations 62 § 2. Preparation and raw-polymer properties of low thiol acid rubbers 62
§ 3. Vulcanization of low thiol acid rubbers. T h e scorching effect, its
causes and its elimination 66 § 4. Low-temperature and other properties of the vulcanizates . . . . 69
A. "Pure gum" compounds 69 B. Carbon black compounds 81 § 5. T h e improvement of the low-temperature resistance of natural rubber
by the combined use of thiolbenzoic acid and plasticizers . . . . 87 C H A P T E R VII. T H E M O D I F I C A T I O N O F N A T U R A L RUBBER L A T E X W I T H M O N O T H I O L A C I D S 95 C H A P T E R VIII. C R O S S - L I N K I N G O F N A T U R A L RUBBER W I T H B I S T H I O L A C I D S 98 § 1. Historical survey 98 § 2. Cross-linking in solution 99 § 3. Cross-linking in dry rubber. Vulcanization with sulphur after
prevul-canization with a bisthiol acid 102 C H A P T E R IX. T H E O R E T I C A L C O N S I D E R A T I O N S A N D A D D I T I O N A L
E X P E R I M E N T S 105 § 1. Preparation and application of thiol acids containing radio-active sulphur 105
§ 2. T h e mechanism of the reaction between thiol acids and natural rubber.
Search for side reactions 110 § 3. Infra-red analysis of thiol acid rubbers 113
§ 4 . Investigation of the reactivity of dithio acids towards natural rubber 114 § 5 . X-ray analytical determinations on stretched vulcanizates . . . . 116
A P P E N D I X 118 Compounding formulae 118
Suggested recipes for practical application 119
S U M M A R Y 121 S A M E N V A T T I N G 126
I N T R O D U C T I O N
In present-day polymer chemistry the procedure of copolymerization of different monomers and its advantages are well-known. One of the most important examples is the copolymerization of butadiene and styrene, which process has opened up the possibility of a large-scale practical application of synthetic rubber (Buna S, GR-S, etc.). T h e copolymer often has properties which, in one or more ways, are superior to those of the corresponding homopolymers, and to mixtures of these homopoly-mers. By variation of the monomer ratio, series of polymers may be composed, each with its own field of application.
In the case of the "homopolymer" natural rubber this principle, of course, cannot be applied in an identical way, unless the rubber would first be depolymerised to isoprene and subsequently be polymerised with some other monomer. As there are cheaper ways of obtaining isoprene, this would be an unpracticable proposition.
There is an other possibility, however, of preparing copolymers, namely the chemical modification of some of the units of the finished polymer, for example the partial saponification of a polyester. A product thus obtained, may chemically be considered as a copolymer. Such a "quasi-copolymer" may, by partial modification, be synthesized from natural rubber too.
Many possibilities exist for converting the isoprene unit of the rubber molecule into some derivative. Most work of this kind has been directed, however, towards conversion of as large a number of isoprene units as possible. In this way new "homopolymers" are obtained, in which generally little is left of the rubbery properties. T h e hydrochlorination of natural rubber is an example of such a modification.
The purpose of the research described in the present thesis, was to examine whether some properties of natural rubber might be enhanced by synthesis of "quasi-copolymers" which contain the units
CHa C H j I I — CH2 — C = C H — C H 2 — and — C H 2 — C H — C H — C H 2 — I
s
Ic = o
I Rthe latter being obtained by the addition of the thiol acid group to part of the isoprene units. This addition reaction turned out to be suited to the purpose of an adjusted limited introduction of foreign groups into the rubber molecule, whilst retaining the elastic properties of the rubber. As is shown by the research to be described in the following pages, some properties of the rubber can indeed be improved in this way, namely the low-temperature properties. These properties have become of increasing importance in recent years, a growing demand for elastomers which remain serviceable under extremely severe conditions being observed. The main reason for this is the rapid development of aircraft flying at ever higher altitudes and the opening up of the arctic regions.
In this development of cold-resistant rubbers most attention up to now, has been paid to synthetic rubbers, as natural rubber is known to be in danger of crystallization at low temperatures, which may render it unserviceable under these circumstances. Increase of hardness, resp. stiffness, stress relaxation and decrease of volume may be the reasons for this lack of serviceability.
It is this tendency to crystallize, which can be combated by the synthesis of "quasi-copolymers" of the type described above.
T h e author wishes to thank the Managing Directors of the Rubber-Stichting at Delft for their permission to publish this work and for their contribution towards the cost of printing this thesis.
CHAPTER I
G E N E R A L L A B O R A T O R Y T E S T S F O R L O W - T E M P E R A T U R E S E R V I C E A B I L I T Y O F E L A S T O M E R S
§ 1. The causes of failure of elastomers at low temperatures
Elastomers are macromolecular substances which show a rubber-like or long-range- ^ elasticity at room temperature. At higher temperatures many other polymers may show a rubber-like character and most of these polymers show some of the phenomena described hereafter for elastomers, although generally at higher temperatures. For the sake of brevity the following discussion will be restricted to elastomers only.
According to current theories, the rubber-like elasticity can be ascribed to the so-called "micro-Brownian movement"" of segments of macro-molecules, which can rotate or vibrate freely at room temperature. These thermal movements decrease when the temperature is lowered, until a point is reached at which the kinetic energy of these segments is so much decreased, that they are immobilized by the cohesive forces. During this process a gradual stiffening of the elastomer may be observed until at a certain temperature a more sudden stiffening takes place. This point, the vitrification point, glass transition point or second order transition point may be determined by plotting some physical property, like specific volume, heat capacity or modulus, against temperature and observing the temperature at which a change in slope of the curve occurs. This temperature is, however, dependent on the criteria used for its determination. Besides it is never very sharp, because the elastomer molecules will exhibit a whole spectrum of motions, which on cooling may gradually come to a standstill^. These complications may raise the question whether at all we are right in speaking about a transition point. A discussion of this subject lies beyond the scope of this thesis *. In spite of objections of this kind, the vitrification points have been helpful tools in the evaluation and classification of elastomers. It should be noted, however, that already far above this point simple temperature effects can be observed, such as increase in modulus, hardness and brittleness and loss in resilience. As the temperature is decreased, resilience will decrease to a minimum and then increase with a further decline in temperature. These phenomena are related to vitrification phenomena but they occur over a broad temperature range.
Stiffening effects due to phenomena of the kind described above, are to be found with all elastomers, if sufficiently cooled.
Some elastomers, however, which have molecules of a sufficiently regular structure, may show in addition a gradual stiffening on exposure to temperatures which may He considerably above the temperatures at which vitrification phenomena are becoming appreciable. Unlike these phenomena the gradual stiffening is usually not completed after the establishment of a temperature equilibrium. It may take hours, weeks or months before it is completed. From X-ray diffraction experiments ^ it has been shown that this type of low-temperature stiffening has to be associated with crystallization phenomena. If stresses are applied to these crystallizing elastomers, the molecules are oriented under the influence of the external forces and crystallization may be strongly accelerated. At low temperatures this may result in stress relaxation " and subsequent deformation set.
Changes in physical properties which are known to accompany crystallization of low-molecular compounds, are also found in the case of crystallizing elastomers. For instance, the onset of crystallization may be followed and measured by dilatometrical ^, calorimetrical *, refracto-metrical ^, spectroscopical'" and other optical **' methods. Reviews of some of these methods were given by Wood in 1946^^ and 1954^-.
Finally, it must be mentioned that some elastomeric compounds show a stiffening on exposure to low temperatures, which has to be attributed to the presence of plasticizers. These effects will in most cases have to be explained by a limited low-temperature solubility of the plasticizer in the elastomer ^^. Other cases will be discussed in Chapters III and V I of this thesis.
Although the changes in the physical properties of elastomers at low temperatures have been the subject of extensive scientific studies, the experimental techniques used in these studies were intended for theoretical purposes rather than for technological tests. For example, the methods for crystallization studies mentioned above, would have to be rigidly standardized to be of value in the classification and purchase specifications of elastomers.
Therefore, much attention has, in recent years, been paid to the development of practical low-temperature serviceability tests for elastomers. Annual and biannual reviews of these tests have been given by BekkedahP^, from 1950 onwards, and a discussion of present-day methods by Juve is to be found in Whitby's recent book on synthetic rubber^". As mentioned there, co-operative laboratory tests of four laboratories, in which the reproducibility and usefulness of several test methods were compared ^", led to the conclusion that at present the following standardized tests are the most acceptable:
1. the temperature-retraction test 2. the Gehman torsion method
In this Chapter attention will be paid to the Gehman torsion method and the compression set test. In addition, the brittleness tests will be described seperately, as several brittle point data determined by the author will be given in this thesis. In the next Chapter the temperature-retraction test will be discussed extensively.
§ 2. Stiffness tests
According to Conant ^*"= "Stiffness" is defined as the ease or difficulty with which a material can be deformed and is indicated by the stress required to produce a given strain.
For measuring the stiffness of elastomers at low temperatures, torsion tests are most frequently used. Other methods will be discussed in paragraph 5 of this Chapter. Yerzley and E r a s e r " , Clash and Berg ^*, Bilmes ^*, Mullins"" and Fletcher ^^ have described various torsion tests. Conant ^"'^ summarized the results of comparative low-temperature stiffness tests and concluded that there appears to be no "best" test. T h e most widely used procedure, however, seems to be that with the Gehman torsional a p p a r a t u s ^ . In this tester a torque is applied to one end of a rubber strip by means of a torsion wire which is attached to a torsion head. This head is turned 180 degrees and the degree of twist of the specimen, indicated by a pointer, is recorded after a period of ten seconds. A number of calibrated wires are available which allow comparable results to be made on different instruments.
After a room temperature reading is made, for which the wire in use shall give a deflection between 170 and 120 degrees, the readings are taken 5 minutes after temperature equilibrium has been established, first at the lowest temperature and then at 5 or 10° C intervals as the temperature is raised. The apparatus can be adapted to long-time cold-hardening tests ^ .
A plot is made of the angle of twist of the specimen against the temperature, as illustrated in Fig. 1. The torsional modulus of the
Twisr, dcg. 110 120 SO • 8 0 -60 -iO -20 0 20 iO Ttmp «C
specimen at any temperature is proportional to the quantity 180 deg. — twist
twist
T h e relative modulus at each test temperature, i.e. the ratio of the modulus at that temperature to the modulus at 25° C is determined. The temper-atures at which the elastomer obtains a relative modulus of 2, 5, 10 and 100 times the modulus at room temperature are reported and designated Ta, T5, Tio and Tjoo respectively.
§ 3. Brittleness tests
T h e brittle point of an elastomer is the lowest temperature at which the material withstands a sudden impact without fracturing.
As early as 1928 Kohman and Peek*-^ described a method by which a small test piece was bent quickly through 90° by a hammer-blow at a low temperature. Kemp^' determined by this method the brittle points of rubber, gutta-percha and related substances. Many other tests have been devised of which most are based on hand-operated and free-falling mechanisms.
The brittle point data given in this thesis have been determined by the method of Selker, Winspear and Kemp'-*". In this procedure rectangular test samples are cooled in an acetone-carbon dioxide bath and after attainment of temperature equilibrium brought sharply in contact with a fixed rigid arm by rapid manual rotation of a brass disk quadrant on which the samples are mounted.
If perfect reproducibility is desired, the deformation speed should be taken into account"^. Electric solenoids have been used to get impact velocities that can be better reproduced ^^. A motor-driven tester has been adopted as a tentative method by the American Society for Testing Materials-^» ( A S T M ) .
Smith and Dienes '"^ showed that some elastomers give a distribution of failures over a temperature range rather than a sharp brittle point. Usually the temperature of 50 % failures is taken as the brittle point.
§ 4. Elastic recovery tests
From the various low-temperature elastic recovery tests described in the relevant literature, only procedures adopted as tentative A S T M methods and found acceptable by co-operative laboratory studies ^°^, will be discussed by the present author.
At present the most widely used recovery test for low-temperature serviceability of elastomers is the low temperature compression set test designated as A S T M method D1229-52T^^, an extension for low temperatures of A S T M method D 3 9 5 ^ ^ In method D 1229 cylindrical test specimens are compressed at room temperature between flat steel plates. The sample is given a compression of 40, 30, 25 or 20%, depending
on wheter the specimen has a durometer hardness up to 44, 45 to 64, 65 to 84 or 85 and over, respectively. The compression device is then placed in a low-temperature cabinet and maintained either for 22 or 94 hours at the test temperature. At the end of this period the compression clamps are released and the thickness of the samples is measured after 10 seconds and again after 30 minutes. The compression set is expressed as a percentage of the original deflection.
There are two regions in which a high compression set of natural rubber compounds is usually found ^^, namely a high temperature region, in which the set will have to be associated with oxidation and similar molecular changes and a low temperature region in which crystallization phenomena are usually observed. At high temperatures the set represents a permanent change, whereas the low-temperature set largely disappears if the temperature is raised.
As a second important low-temperature elastic recovery test the temperature-retraction test should be mentioned. This test, which has been one of the main research tools of the present author, will be discussed extensively in Chapter II of this thesis.
§ 5. Miscellaneous tests
A large number of widely divergent test methods for low-temperature testing of elastomers have been reported. For surveys the reader is referred to the reviews of Bekkedahl and Juve, mentioned in paragraph 1 of this Chapter. Some of these methods concern actual service tests, for example in the arctic regions^'. In this paragraph, however, only methods of general interest which can be conducted in ordinary laboratories, will be considered.
Low-temperature hardness tests are usually carried out with so-called indentometers or durometers. These instruments are based on a measurement of the penetration into the rubber test piece of a plunger or indentor of specified dimensions under the force exerted by a calibrated spring or a dead load. Usually the hardness is expressed in degrees which are related to the depth of indentation. Various types of durometers have been recommended. Labbe'^ summarized the results of a comparative study of 7 hardness testers with respect to efficiency and reproducibility at low temperatures. He concluded that, although spring-actuated instruments are desirable, none of the usual durometers of this kind yields adequate results over the complete range of temperatures studied. It appeared that the Material Laboratory Indentometer (a modification of the British Admiralty instrument) was the most satisfactory of the dead-weight load hardness testers for use at low temperatures.
Laine and Roux ^'^ were of the opinion that, although the results of hardness tests can be of practical interest, it is hardly possible to deduce
from the,m the true mechanical properties of the samples to be studied without resorting to other test methods.
Low-temperature modulus tests of widely divergent kinds have been compared and discussed by Conant ^'''^. The torsional stiffness tests described in paragraph 2 of this Chapter may be considered as a special class in the group of modulus tests. The other tests may be classified as tension, compression, flexure, or shear tests.
Conant analysed the results of Young's modulus measurements obtained from 17 laboratories using 8 different test methods. Young's modulus was chosen as a basis for comparison as it is generally believed to represent a basic material property which can be obtained by all the methods mentioned above. The variation in moduli was of such an extent, however, that Conant came to the conclusion that the term "Young's modulus" as applied to rubber-hke materials is rather vague. unless the method of obtaining that modulus is also given. From the results obtained it did not seem logical to choose a "best" test for determining low-temperature moduli •'"'=.
Low-temperature tensile tests have been carried out by adapting conventional tensile tests and similar procedures to low-temperature measurements ^^. In this way both modulus and ultimate tensile strength are obtained.
Impact tests for low-temperature rebound resilience have been made, using a modified Liipke pendulum ^**. On cooling, the resilience of natural rubber decreases from high values at room temperature to a low value, at which practically all the kinetic energy of the pendulum is absorbed. The resilience-temperature curve passes through a minimum, after which resilience increases on further reduction of the temperature. By free vibration^^ and forced vibration'^" methods similar "damping curves" may be obtained, together with dynamic moduli measured in compression or shear. An increase in the frequency of the applied force has the same effect as a decrease in temperature.
The effect of temperature on rubber subjected to a sinusoidal loading has been studied in compression over the frequency interval of 0.1 to 1000 vibrations per minute''^. Increasing the frequency over this range increases the temperature of the rapid rise in modulus by 30 to 35° C. Rubber which is partially crystallized at room temperature has a more rapid rise in modulus on exposure to low temperatures than an amor-phous sample.
Finally, it should be mentioned that the A S T M recently adopted a tentative recommended practice for standard test temperatures^^. A list is given of 13 test temperatures, of which the sub-zero temperatures are —55, —40, —24 and —10° C, with a maximum allowable variation of ± 2 ° C .
R E F E R E N C E S
la. H. L. Fisher, Ind. Eng. Chem., 31, 941, (1939).
b. P. J. Flory, "Principles of Polymer Chemistry", N e w York 1953, p. 432. 2a. K. H. Meyer, G. v. Susich and E. Valkó, KoUoid Z., 59, 208, (1932).
b. W . Kuhn, Kolloid Z., 87, 6, (1939).
3. R. Houwink, Chapter XIII of : H. R. Kruyt, "Colloid Science", Amsterdam 1949, Vol. II, p. 653.
4. R. F. Boyer and R. S. Spencer in : H. Mark and G. S. W h i t b y , "Advances in Colloid Science", New York 1946, Vol. II, p. 1.
5. J. R. Katz, Naturwiss., 13, 410, (1925).
6. A . N . Gent, Trans. Faraday S o c , 50, 521, (1954).
7a. N . Bekkedahl, J. Research Nat, Bur. Standards, 13, 410, (1934). b. L. A. W o o d and N . Bekkedahl, J. Appl. Phys., 17, 362, (1946). c. N. Bekkedahl J. Research Nat. Bur. Standards, 43, 145, (1949). d. E. W . Russell, Trans. Faraday Soc., 47, 539, (1951).
8a. A. V. Rossem and J. Lotichius, Kautschuk 5, 2, (1929).
b. J. P . Ehrbahr and Ch. G. Boissonnas. Helv. Chim. Acta, 38, 125, (1955). 9. L. R. G. Treloar, Trans. Faraday Soc., 37, 84, (1941).
10a. J. T . Maynard and W . E. Mochel, J. Pol. Science, 13, 235, (1954). b. J. B. Nichols, J. Appl. Phys., 25, 840, (1954).
11. L. A. W o o d i n : H. Mark and G. S. Whitby, "Advances in Colloid Science', New York 1946, Vol. II, p. 57.
12. .L. A. W o o d , Chapter X i n : G. S. W h i t b y , "Synthetic Rubber", New York 1954, p. 354.
13a. A S T M Standards on Rubber Products, 1954, p. 404. A S T M Designation D 8 3 2 ^ T , b. Anon., Materials and Methods, 38, 114, (1953).
c. D. B. Forman, Ind. Eng. Chem., 36, 738, (1944).
14. N. Bekkedahl and M. Thyron, Anal. Chem., 27, 595, (1955).
15. A. E. Juve, Chapter XII i n : G. S. Whitby, "Synthetic R u b b e r ' , New York 1954, p . 471.
16a. A. F. Helin and B. G. Labbe, India Rubber World, 126, 227, 365, (1952). See also :
b. R. A. Offner, Bibliog. Tech. Reports, 22, 11, (1954). c. F. S. Conant, A S T M Bull. N o 199, 67, (1954). d. B. G. Labbe, A S T M Bull. N o 199, 73, (1954).
17. F . L. Yerzley and D . F. Eraser, Ind. Eng. Chem., 34, 332, (1942). 18a. R. F. Q a s h and R. M. Berg, Ind. Eng. Chem., 34, 1218, (1942).
b. A S T M Standards on Rubber Products, 1954, p. 460. A S T M Designation D1043-51. 19. L. Bilmes, J. Soc. Chem. Ind. .Trans., 63. 182, (1944).
20. L. Mullins, Trans. Inst. Rubber Ind., 21, 247, (1945). 21. W . P. Fletcher, Rubber Developments, 8, 2, (1955).
22a. S. D. Gehman, D. E. Woodford and C. S. Wilkinson, Ind. Eng. Chem., 39, 1108, (1947).
b. A S T M Standards on Rubber Products, 1954, p. 486. A S T M Designation D1053-54T. c. I. Kahn, India Rubber World, 126, 500, (1952).
23a. S. D. Gehman, P. J. Jones, C. S. Wilkinson and D. E. Woodford, Ind. Eng. Chem., 42, 475, (1950).
b . R. D. Juve and J. W . Marsh, Ind. Eng. Chem., 4 1 , 2535, (1949). 24. G. T . Kohman and R. L. Peek, Ind. Eng. Chem., 20, 81, (1928). 25. A. R. Kemp, J. Franklin Inst., 211, 37, (1931).
26. M. L. Selker, G. G. Winspear and A. R. Kemp, Ind. Eng. Chem., 34, 157, (1942). 27. R. E. Morris, R. R. James and T . A. Werkenthin, Ind. Eng. Chem., 35, 864, (1943).
28a. F . L. Graves,-India Rubber World, 111, 305, (1944).
b. H. G. Bimmerman and W . N . Keen, Ind. Eng. Chem., Anal. Ed., 16, 588, (1944). 29a. A. R. Kemp, F. S. Malm and G. G. Winspear, Ind. Eng. Chem., 35, 488, (1943).
b. A S T M Standards on Rubber Products, 1954, p. 340. A S T M Designation D746-54T. 30. E. F. Smith and G. J. Dienes, A S T M Buil. N o 154, 46, (1948).
31. A S T M Standards on Rubber Products, 1954, p. 570. A S T M Designation D1229-52T. 32. A S T M Standards on Rubber Products, 1954, p. 148. A S T M Designation D395-53T. 33. R. E. Morris, J. W . Hollister and P. A. Mallard, India Rubber World, 112,
455, (1945).
34a. R. E. Morris and A. E. Barrett, India Rubber World, 129, 773, (1954). b. H. L. Fisher, Ind. Eng. Chem., 46, 2067, (1954).
35. B. G. Labbe. A S T M Buil. N o 199, 73, (1954).
36. P. Laine and A. Roux, Rev. Gén. Caoutchouc, 21, 189, (1944). 37. F. L. Graves and A. R. Davis, India Rubber World, 113, 521, (1946). 38. L. Mullins, Trans. Inst. Rubber Ind., 22, 235, (1947).
39a. D. H. Cornell and J. R. Beatty, Rubber Age, N . Y . , 60, 679, (1947).
b. A S T M Standards on Rubber Products, 1954, p. 434. A S T M Designation D945-52T. 40a. C. W . Kosten, Proceedings Third Rubber Technol. Conf., London 1938, p. 987.
b. R. S. Marvin, Ind. Eng. Chem., 44, 696, (1952).
41. A. P. Aleksandrov and Y. S. Lazurkin, J. Technical Physics (U.S.S.R.), 9, 1249, (1939), Rubber Chem. and Technol., 13, 886, (1940).
42. A S T M Standards on Rubber Products, 1954, p. 586. A S T M Designation D1349-54T.
CHAPTER II
T H E T E M P E R A T U R E - R E T R A C T I O N T E S T ( T R T E S T ) § 1. Historical development. Correlations with other test methods If unvulcanized natural rubber is stretched at room temperature to a sufficiently large elongation and then cooled tot 0° C, it does not retract after release. This "ice water test" was, in its most primitive form, already carried out by Gough as long ago as 1805^.
If, however, the rubber is vulcanized, it does retract at 0° C. In this way a rough qualitative test can be made to distinguish between vulcanized and unvulcanized natural rubber^.
In 1933, Gibbons, Gerke and Tingey^ developed on this principle the "T 50 test", which was meant to provide a quantitative method to determine the state of cure. T h e cooling temperature was lowered to —50 to —70° C, after which the sample was gradually warmed up. T h e temperature at which the test piece had retracted to 50 % of its initial elongation (the " T 50 value") was used to characterize the state of cure. This test has found extensive application both for checking uniformity of cure in production and for experimental work where knowledge of the state of cure is important"*. It has been adopted as a tentative A S T M method °. The T 50 value drops with increasing time of cure and for natural rubber vulcanizates a good correlation is often found between T 50 and combined sulphur". The test is only suitable for elastomers which crystallize on stretching.
As the T 50 test is based on crystallization and melting phenomena it is not surprising that its fitness for examining the cold-resistance of elastomers was investigated.
In 1942 Yerzley and Eraser^, studying the effects of low temperature on the properties of neoprene, extended the T 5 0 test so as to record the entire process of retraction, beginning with the temperature at which 1 0 % of the initial elongation is recovered and proceeding until 9 0 % recovery is reached. An initial elongation of 1 7 0 % was adopted to permit the testing of compounds containing substantial amounts of carbon black, which are prone to break at higher elongations. As the authors state, the test was as yet inadequate as a standard test for low-temperature serviceability.
In 1951, however. Smith et al.* and Svetlik and Sperberg" demon-strated the existence of close correllations between the data obtained from the retraction test and those obtained from other low-temperature tests. They introduced the name of "Temperature-Retraction test" ( T R
test) and the symbols T R I O , T R 50, T R 7 0 , etc. which indicate the temperature of respectively 10, 50, 7 0 % retraction, etc. This test has been developed into a general test for evaluating the low-temperature characteristics of elastomers, which has recently been adopted as an A S T M method^".
Svetlik and Sperberg*, in their first description of the T R test^ advocate the utility of this test for the following purposes :
A. To establish an "extrapolated freezing point", corresponding witli the freezing point obtained with the Gehman torsional test by extra-polation of the straight vertical part of the stiffness curve (see Fig. 1, p. 5 ) . These points are related to the vitrification point and therefore also to the brittle point. From the T R curves the freezing points are obtained by extrapolating the sigmoid retraction curves to zero retraction
(Fig. 2, p. 13). The satisfactory agreement with Gehman freezing points is shown in Table I. The authors state that the freezing points thus. obtained are not appreciably affected by varying the test elongation.
TABLE I
Comparison of freezing points obtained with T/? test and torsional stiffness test
Freezing point, °C
TR method Torsion modulus method.
GR-I —51 - ^ 8 Hycar OR-15 —11 —12 GR-S —46 —45 Neoprene —35 —39 122° F Polybutadiene —70 —79 No. 1 Smoked Sheets —55 —60
B. To measure the Gehman low-temperature stiffness. The authors found that the temperatures at which the samples retract 50, 20, 10 and 1 % in the T R test, correspond, respectively, to the temperatures at which the torsion modulus has increased to 2, 5, 10 and 100 times the modulus at room temperature in the torsional test. These data indicate that the degree of stiffening is inversely proportional to the percentage retraction and on this basis the reciprocal of the retraction can be used as an index of stiffness.
C. To determine the tendency of elastomers to crystallize upon cold storage. W h e n first conditioned for 72 hours at 50 % elongation and 0 ° F , the T R curves of GR-S, Hycar OR-15 and 122° F polybutadiene are not changed in comparison with those of unconditioned samples. These elastomers all are known not to crystallize upon cold storage. T h e T R values of GR-I are somewhat increased by this conditioning, whereas
neoprene and natural rubber, both of which crystallize more readily, show markedly higher T R values in the region between 20 and 80 % retraction, after this conditioning. Using this procedure, the authors found polybutadienes prepared at low temperatures to crystallize much more readily than polybutadiene prepared at higher temperatures. These results were consistent with compression set data of the same polymers. D. To determine the tendency of elastomers to crystallize upon stretching.
By observing the T R curves after varying degrees of initial elongation, it was found that elastomers like natural rubber, neoprene and GR-I, which are known from X-ray studies to crystallize upon stretching, show at high initial elongations (400—500%) higher 'TR values than at low elongations. This was most pronounced for T R 50 and neighbouring T R values, whereas exceptions sometinies could be observed a t very low initial elongations ( 2 5 — 5 0 % ) . Elastomers which do not
crystallize upon stretching, do not show these phenomena. In this case a decrease in T R values with increasing initial elongation could even be observed. The T R test therefore can be used to detect the tendency to crystallize upon stretching. Fig. 3, p. 14 shows typical series of T R curves for natural rubber.
For the first three applications mentioned above, the authors preferably used low initial elongations ( 5 0 % ) . Evaluating various elastomers (all carbon-reinforced, except one neoprene compound), Svetlik and Sperberg claim that a complete characterization of the low-temperature properties can be made with the T R test in a shorter time than by any other test method or combination of methods. The test is said to be highly reproducible. 10 c o t^ 30 5 0 -c Ü CL 70 • . o _ . ^ I 9 0 | - ^ 0 C R - I G R - S P o l y b u t a d i e n e N a t u r a l rubber Neoprene Hycar OR-15 L -70 -60 -60 -40 -30 -20 -10 0 tlO —*' Temp.'C Fig. 2. Determination of extrapolated freezing points from T R curves ".
— " Temp.'C -70 -50 -30 -10 10 Z 10 i 20 Ï 40 u t 50 ^ 60 70 90 -70 -50 -30 -10 10 — " Tennp.''C « ?'^°f^F\ X 507o o 1 0 0 % a 2 0 0 % • 3 0 0 % A iOO% o 5 0 0 % ongation
Fig. 3. Determination of the tendency to crystallize upon stretching ^. T R curves of natural rubber at varying initial elongation.
Smith, Hermonat, Haxo and Meyer **, in their version of the T R test, used preferably large deformations, as these greatly accelerate crystal-lization and subsequent stiffening and therefore decrease the time necessary to conduct the test. The authors paid special attention to the T R 10 and T R 70 values, as these were found to be indicative of low-temperature stiffness and crystallization, respectively. Generally, their results correspond with those of Svetlik and Sperberg. They adopted, however, 250 % initial test elongation as a standard for carbon-loaded compounds, as they found that the T R 70 temperatures of both natural rubber and GR-S reach a practically constant value above 200 % initial elongation. As many experimental vulcanizates break at higher elongation, 250 % was found desirable from that standpoint too. For similar reasons 400 % initial test elongation was preferably used for "pure gum" natural rubber vulcanizates.
In contrast to the T R 7 0 , the T R I O value was found to be but slightly influenced by the test elongation. The authors observed the following correlations:
A. TR 10 as a measure of low-temperature flexibility, prior to low' temperature storage.
By comparing the T R 10 with the T 10 value from torsional modulus determinations (Chapter I, par. 2), the authors found the same hnear relationship as Svetlik and Sperberg, without any exception, for a large number of elastomers (Fig. 4 ) .
T 10,°C (Torsion modulus) - 2 0 - 3 0 -40 - 5 0 -60 -70 -60 -50 -^0 -30 -20 TR 10.'C (Retraction) Fig. 4. T 10 versus T R 10 of Butadiene-Styrene, Butadiene-Iso-prene, and Butadiene-Acrylonitrile
Vulcanizates *.
They therefore claim that the T R 10 can be used reliably as a figure of merit for low-temperature flexibility of crystalhzable elastomers for dynamic applications or for non-crystallizable elastomers under all low-temperature conditions.
B. TR 70 as a measure of the low-temperature compression set. The T R 70 data were examined as a possible means for indicating the low-temperature merit of an elastomer after a long period of low-temperature storage. It was found for various elastomers that stocks having equal compression sets, also had equal T R 70. If the compression set of these compounds is plotted against their T R 70, a smooth curve is developed (Fig. 5).
The T R 70 value therefore can be used as an index to estimate the compression set of vulcanizates. This is important, because the T R test can be performed within one hour, whereas 48 to 96 hours' storage is required to measure the compression set. It should be pointed out that in the opinion of the present author only a correlation is shown and for that reason it is at least risky to derive the low-temperature compression set from the T R 70 value. If, for natural rubber, the compression set measurement is made at too low a temperature, (—45° C ) , the corre-lation will disappear, because under these conditions the crystallization
• ButAdienc-styrene copolymers o Perbunan
• Hevea.(50 parts channel black) a Hevea (qum)
Fig. 5. Low-temperature compression set
versus T R 70 «.
of natural rubber is too slow to show the ultimate compression set within the time required for the test.
In the T R test, as it has been adopted by A S T M in 1954, the significance of the test is described as follows:
a. T R 70 correlates with low-temperature compression set.
b. T R 10 correlates with brittle points in vulcanizates based on polymers of similar type.
c. T h e difference between T R 70 and T R 10 increases as the tendency to crystallize increases.
d. In general the retraction rate is believed to correlate with low-temperature flexibility of both crystalhzable and non-crystallizable elastomers.
In the A S T M test 45 % of the elongation at break or any greater initial elongation may be employed.
§ 2. Practical significance of the TR test for natural rubber
From the facts mentioned above, it will be clear that the T R test may be a very versatile tool in low-temperature research for elastomers in general. For natural rubber this is especially true on account of the information which can so quickly be obtained about the tendency to crystallize, the main drawback of natural rubber in low-temperature applications. Thus, if this crystallization is inhibited, for instance by chemical modification, this fact will easily be detected by the T R test. If, however, this improvement should have happened at the expense of the properties associated with the vitrification point, this too would be detected.
Together with the T R 70 and T R 10 values, the A S T M method requires a report of T R 30 and T R 50 values. T h e last one is the
same as the T 50 value for the state of cure mentioned in § 1 of this Chapter. In many cases, especially in the case of natural rubber, in which the brittle point depends but little on vulcanization characteristics, the determination of the T R 50 value might replace ( T R 10—TR70) in the estimation of the tendency to crystallize. For these vulcanizates the precision of the T 50 as a measure of the state of cure and the utility of the T R 50 for a rough characterization of the T R curve, result from their relative position on this curve. Generally, they lie on that part of the curve for which a slight temperature rise causes a pronounced retraction. Thus the T R 50 value can be regarded as a kind of melting point of the frozen stretched rubber after a certain thermal and mechanical treatment. Actually, this melting point is always a melting range, which is the narrower in proportion as the structure of the rubber is more regular.
Irregularities, like cross-hnks, cause a broadening of the melting range and at the same time they shift it to lower temperatures. Fig. 6 gives two curves, one for unvulcanized rubber and the second for a vulcani-zate, the latter showing distinctly the two phenomena just mentioned. This resembles the phenomena to be observed in micromolecular com-pounds, where foreign substances may both decrease the melting point and change it into a melting range.
% Elong. 500 250 -60 -40 -20 0 20 «C Unvulcanized rubber. 111 11 Vulcanized rubber.
Pig. 6. Influence of vulcanization on T R curves.
In macromolecular compounds we must distinguish between the effects of physical admixture with diluents (plasticizers, etc.) and the effects of the introduction of compounds which chemically combine with the polymer.
From a practical point of view, the T R test has proved to be a sensitive tool in the comparative evaluation of the efficiency of various substances in reducing the tendency to crystallize of natural rubber, as will be demonstrated in the following Chapters.
§ 3. Limitations of the TR test. Influence of variations of test conditions
Although, as has been shown in the preceding paragraph, the T R test has proved to be of great value in making a quick estimate of various low-temperature properties, it should be realized that the correlations described were of a purely empirical nature. Therefore, caution should always be exercised about the validity of these correlations and in several cases reservations considering the applicability and interpretation of T R data will have to be made, as is illustrated by the following examples:
A. The extrapolation in the determination of the "freezing point" by the method of Svethk and Sperberg (Fig. 2, p. 13) will sometimes be rather arbitrary, as a really linear part of the curve is not always present. B. Although the T R 10 value generally is much less dependent on the presence or absence of crystallization than the T R 50 and T R 70 values, a small increase of the T R 10, due to crystallization, may sometimes be observed, for example with natural rubber at high initial elongations. In these cases a perfect correlation between the T R 10 value and the T 10 value of the Gehman stiffness test can no longer be expected. Therefore, if estimations of the low-temperature stiffness are to be made from T R 10 values, the elongation should not be too high.
C. If the T R test is used to estimate the crystallization upon cold storage, it should not be forgotten that the orientation of the rubber in the T R test greatly facilitates crystallization and therefore suffers from the danger of exaggerated conclusions. It would, in the extreme case, be conceivable that some polymer (e.g. a butyl rubber!) would never show crystalhzation phenomena upon cold storage in the unstretched condition, whereas it does crystallize when oriented. Generally, however, it seems reasonable to presuppose a connection between the crystallization in the stretched and in the unstretched condition, the difference being manifested mainly in the rate of crystallization.
D. Difficulties of a special kind may arise if conclusions are to be drawn from T R curves of vulcanizates containing low-temperature plasticizers. These agents may on the one hand decrease the "melting point" (e.g. the T R 50), but on the other hand increase the rate of crystallization at low temperatures by increasing the mobility of the crystallizing rubber molecules. This adverse effect will not be detected when the usual T R procedure is followed, as the crystallization upon stretching at room temperature is effected within such a short time-*^, both for plasticized and unplasticized natural rubber, that differences in rate of crystalhzation can hardly be detected. In the next paragraph a procedure will be described by which differences in rate of crystalli-zation of stretched rubbers at low temperatures can be detected, by using a modified T R test.
E. The interpretation of the T R curves is hampered by the absence of a thermodynamical equilibrium during the test. As set forth in the preceding paragraph, the T R 50 may, in some cases, be considered as a melting point. Now it has been known for a long time that the melting point of polymers and especially of natural rubber is greatly dependent on their thermal history ^-. For a frozen stretched rubber the mechanical history is of as much importance. This "memory effect" was demonstrated by the present author by comparing the TR curve of a natural rubber vulcanizate after it was brought at —50° C and 400 % elongation along two different ways, namely b y : A. first stretching it at room temperature and then cooling it to—50° C (Fig. 7a).
VoElong. 400 300 200 100 - 6 0 -30 -10 +10 "C Fig. la. Normal T R curve.
J O'C
and B. first cooling it to —50° C and then stretching it to 400 % elongation (Fig. 7b). In Fig. 7b the elongated sample was conditioned for 5 minutes at —50° C before it was released and the retraction curve was determined. From the curve obtained it is obvious that in Fig. 7b the stretching temperature was too low and the racking time too short for appreciable crystallization to occur. If the racking time is prolonged to 6 hours, the occurrence of crystalhzation is manifested in the T R
jlQ min. fixed ' at ^OO'/o.-BO' "/oElong. 400-300-1
t|
200-[ 1 100-1 1 n-l 0 '• 5 min. fix I at 400O/.. i~T-t-^ -40 Fig. lb ed -50 20 »C 1 •/.I -1Ï
1 0 Elong. I6hr.fix ' J --40 Fig. edV ^
•20 Iccurve (Fig. 7c) but still the "melting point" (resp. the T R 5 0 ) , is quite different from that obtained after stretching at room temperature.
These results demonstrate that the "melting point" of a frozen stretched rubber is dependent not only on the conditions at the initial point of the T R curve (test elongation and freezing temperature), but also on the path along which this point is reached.
Because this "memory effect" is so pronounced, it seemed of importance to know the influence of the separate factors of the testing conditions more in detail. The investigation, carried out with natural rubber, led to the following results.
A. Influence of initial elongation.
An unstretched natural rubber vulcanizate at room temperature is amorphous, according to its X-ray diagram. W h e n stretching the rubber in the X-ray camera, the appearance of a diffraction pattern is usually observed at about 250 % elongation with "pure gum" compounds, indicating the formation of crystal lattices to a sufficient extent to yield perceptible reflections. The higher the elongation, the sharper the X-ray reflections. This is due to an increase in the degree of crystal-linity^^. Above a certain elongation, further extension has httle influence.
If we now determine the T R curve of a natural rubber vulcanizate at various initial elongations, we obtain results which completely fit into the picture obtained from X-ray analyses at room temperature. In the T R test the condition which existed at room temperature has been
•/o Retraction 10 30 50 70 •70 -60 -50-40 -30 -20 -10 0 Temp.°C 5 0 0 % E l o n g . 5 0 0 % „ 4 0 0 % _ 3 5 0 % 3 0 0 % 2 00 7o 1 0 0 % VoEl 600 500 4ÜU 350 300 2ÜÜ 100 0 -7 ong. r ^ »^ ^ ^ \ = - ^ " " - - ^ \ - . ^ " N X f i l l I I I -0 -50 -30 -10 0 Temp.'C Fig. 8b. Fig. 8a.
frozen-in. The results are shown by Fig. 8a and are in agreement with those of Svetlik and Sperberg (see Fig. 3, page 14). It is more convenient and it often yields clearer results if the elongation instead of the retraction is plotted versus the temperature, as is done in Fig. 8b.
At low elongations the rubber retracts almost completely at tempera-tures near the brittle point. T h e higher the elongation, the higher the T R values. This is true only to some extent for the T R 10 values but very pronounced for the values round the T R 50. At the same time we see at higher elongations the melting range becoming more and more a melting point. W e must therefore conclude, in agreement with Smith et al. ^, that high initial elongations promote reproducible results.
B. Influence of racking-(freezing-) temperature.
It has been found that the temperature at which the rubber is frozcn-in is of httle importance as long as it is not too far above the vitrification point. This is not surprising because, as we have seen before, the course of the curve depends largely upon the situation (degree of crystallinity) at room temperature. "/oElongation 500 p i : ' : : ' . ^ : - _ • ' _ _ : 250 lO'/oRetr. 30°/oRetr. •. 5 0 % R e t r . •.70''/oRetr. 0 20 Temp.''C Fig. 9. Influence of stretching temperature on the T R curves of an unvulcanized dried latex film.
C. Influence of stretching temperature.
Contrary to the freezing temperature the stretching temperature is of much importance. This is illustrated in Fig. 9, showing the T R curves of an unvulcanized latex film and their dependence on the stretching temperature. For vulcanized natural rubber a similar dependence has been found. W e may conclude that stretching at lower temperatures results in lower retraction temperatures ("melting points"), which is in agreement with the results of Bekkedahl and W o o d for unstretched natural rubber, where they found that lower crystallization temperatures
result in lower melting points ^^. The explanation of the "memory effect" described before (Figs. 7a and b) can be reduced to the problem of the influence of the stretching temperature on the T R curve. Fig. 7b representing the special case in which the stretching temperature is equal to the freezing temperature.
D. Influence of rate of heating.
If the rate of heating during the determination of the T R curve is too large, the measured retraction of the test piece at a certain tem-perature will be too small. It would even be conceivable that at any constant temperature, retraction would take place continuously. That this is not the case, however, to any appreciable extent, was demon-strated with a natural rubber vulcanizate by a retraction measurement in which the temperature was not continuously raised, but kept constant for a considerable time (up to one hour) at certain predetermined elongations. It was found that a constant elongation is usually obtained within one minute. Only in the initial stages of the retraction (after release of the test piece), this period may be somewhat longer. Usually, however, these stages are of little importance and therefore a heating rate of 1 ° C per minute, as adopted in the A S T M method, may be considered adequate.
§ 4. Modifications of the TR test
For special purposes it may sometimes be convenient to modify the T R test by adopting divergent test procedures:
A. In the "rate of retraction test" or "tension recovery test", the sample is elongated by 50 % and conditioned for the desired time at the test temperature. One end of the test specimen is then released, and the retraction at definite time intervals is noted. T h e results are said to correlate with those of the compression set test ^°. One of the most conspicuous differences with the T R test is the fact that retraction occurs at constant temperatures. It should therefore be run at several temperatures for complete polymer evaluation.
B. Radi and Britt ^" based their low-temperature tests also on measurements of retraction at constant temperatures. In their procedure the samples are elongated by 300 % and placed in the low-temperature cabinet where they are held in the extended state for twenty hours at the test temperature to induce a maximum degree of crystallization *. After the twenty hours the samples are allowed to retract freely while the temperature remains constant. Twenty hours after release of the specimens their set is supposed to have reached a constant value. The
* According to the present author we can only speak of a sufficient, but not of a maximum degree of crystallization in this case.
Percent retraction at constant temp. 100
J 80
60 40 20 -60 -40 -30 0 - 6 0 - 4 0 - 3 0 0 Retraction temp..''C Fig. 10. Effect of elastomer type on theC T R curve of Radi and Britt.
corresponding "constant temperature retraction" ( C T R ) values are determined at each of the test temperatures desired. W h e n plotting these C T R values against their corresponding temperatures, curves are obtained which are sigmoid for non-crystallizing elastomers but deviate from the sigmoidal form in the case of crystallizing elastomers (Fig. 10). The lowest point in the dip or deviation has been designated as T j or temperature of maximum deviation. It is considered to be the temperature most favourable for crystallization and would indicate the temperature range to be avoided in prolonged low-temperature storage. T h e authors state that the magnitude of deviation is a measure of the degree of crystallization and can be a measure of the rate of crystalliza-tion as well.
In fact, the method of Radi and Britt comprises a series of determinations of the tension set at several low temperatures after a certain pre-treatment. The information which it is said to yield about the optimum crystallization temperature and the rate of crystallization would be a positive improvement in comparison with the usual T R test. T h e following objection can, however, be made to the conclusions of Radi and Britt:
In their procedure, as well as in the original T R test, the sample is first stretched at room temperature and subsequently transferred to a bath of a lower temperature. From the "memory effect" described before and from the demonstrated dependence of the T R 50 on the stretching temperature we know that the "melting point" is greatly dependent on the thermal and mechanical history of the sample. From the curve obtained by Radi and Britt for natural rubber (Fig. 10), it appears that the "melting point" after their particular pre-treatment was about —30° C. In that case the temperature of maximum rate of crystallization (T^j) was —39° C. It might be doubted, however, whether this maximum would remain the same if the stretching temperature and
thereby the "melting point" would be varied. Moreover the degree of elongation will influence the data mentioned above. It therefore should be recognized that the temperature of the maximum rate of crystalh-zation in actual service might be quite different from that obtained by the method of Radi and Britt.
C. In order to obtain information about the optimum crystallization temperature and the rate of crystallization of stretched natural rubber which can be better interpreted, the present author has devised a new test procedure which contains some features of the normal T R test and of the methods of Radi and Britt but differs from them in several essential respects.
The following procedure was adopted:
First the test specimens are cooled to the desired test temperature (—10 to —60° C) for two minutes and then stretched at this tem-perature to the elongation chosen (usually 4 0 0 % ) . This procedure is largely the same as that of Fig. 7b and c, (p. 19). Retraction, however, is now determined at constant temperatures, as a function of time (CTR curve), until the elongation remains essentially constant, and subse-quently at increasing temperatures, as a function of temperature (ITR curve). Figs. 11 a and b illustrate the results obtained by this method.
°/o E long. 400 •/o E long. 300 200 100 \ Ret Xco, Retraction at constant temp of -SO'C Retraction at increasing temp. ( J T R ) 10 20 -30 Time.mln 0 10 Temp.,'C Fig. 11a Fig. lib
Retraction of a natural rubber vulcanizate after stretching at —30° C and conditioning for 6 hours at the same temperature.
From the C T R curve the low-temperature tension set can be obtained
(l.t.t.s. in Fig. 11a). Moreover, the rate of retraction gives an impression of the stiffness of the sample at the test temperature.
T h e "melting point" of the stretched rubber after crystalhzation at the test temperature is found from the ITR curve (Fig. l i b ) .
If the test is run at several test temperatures and, in addition, the racking time of the stretched sample at the test temperature is varied, the following additional information can be obtained:
Low-temp. tension set 400Vo|-300»/. Test temp.-30''C
tW
Racking time. mm. 100 200 300 Fig. 12. Dependence of the low-temperature tension set on the racking time at —30° C.Natural rubber vulcanizate.
a. By plotting the 1.1.1. s. values for a certain temperature versus the racking time (Fig. 12), curves are developed which are reminiscent of the sigmoid crystallization curves which are obtained when the rate of crystallization is determined from dilatometrical methods^'.
These curves show how the irreversible crystallization of the stretched rubber proceeds at the test temperature. (Here the term "irreversible" is not used in the thermodynamic sense, but implies that crystallization does not disappear on removal of the external forces).
The rate of crystallization can be characterized by the half-values ( t j ) , which represent the time necessary for the achievement of half the ultimate tension set (Fig. 12). Similar characterizations of the rate of crystallization have been used in dilatometrical and in stress-relaxation procedures ^^. L.t.t.s.after 30 min racking 4 0 0 V o 300% --60 Fig. Determination of -40 -30 -20 Test temp.,'C 13. optimum crystallization -40 -30 Test temp..'C Fig. 14.
temperature from the low-temperature tension set and from the 11 value. Natural rubber vulcanizate.
b. By plotting either the low-temperature tension set, or the t i versus the test temperature, the optimum crystallization temperature at the elongation in question can be found. This optimum is unambiguous, as the whole procedure of stretching, crystallization and retraction has been carried out at the same constant temperature for each of the test temperatures (Fig. 13 and 14).
From Figs. 13 and 14 it appears that the natural rubber vulcanizate in question (a gum compound containing a low-temperature plasticizer) had, at 400 % elongation, an optimum crystallization temperature of —40 tot —50° C. Similar results have been obtained with other natural rubber vulcanizates (see Chapters III and V I ) .
From these results it follows that the optimum crystallization tem-perature which is known to be about —25° C both for unvulcanized'" and vulcanized^''' natural rubber in the unstretched state and which therefore has been advocated for crystallization studies on natural rubber in generaV^^, may be quite different from —25° C in the case of natural rubber in the stretched state. Therefore, comparative determinations of the rate of crystallization of different rubbers in the stretched state, by measurements of volume change or stress relaxation, might possibly be carried out more efficiently at temperatures below —25° C.
The procedure described above has been used in the present investigation in those cases where more detailed information on the rate of crystallization at low temperatures seemed desirable (Chapter III and V I ) .
D. The use of the (TR 70—TR 10) value as a measure for the tendency of an elastomer to crystallize has two obvious drawbacks :
a. T o observe crystallization effects, the T R test should be carried out at high initial elongation. At this elongation not only the T R 70 value, but sometimes also the T R 10 value (which should be independent of elongation) may be increased as a consequence of the crystallization of the rubber. In that case the value found for ( T R 70—TR 10) is too small.
b. Even in the case of elastomers which cannot crystallize at all, the difference between the T R 70 and T R 10 values is still larger than zero. This difference is then only a measure for the temperature-dependence of the stiffness.
To eUminate both of these drawbacks it is sufficient to replace the T R 10 value by a T R 70 value determined at low initial elongation. A t this elongation no crystallization effects should be observable. As an index to the "tendency to crystallize" we may then use the general formula:
§ 5. Description of apparatus
The apparatus (Fig. 15) used by the present author for the majority of the T R tests described in this thesis was, for the greater part, constructed on similar lines as the "type A " T-50 apparatus of A S T M method D599^='^
In a testing bath (1 in Fig. 15) a liquid coolant (acetone or methanol) was brought at the desired temperature by circulating it by means of a pump (4) through a refrigerating system (3), in which the liquid passed a copper coil which was cooled by a mixture of dry ice and acetone. The apparatus was provided with a conditioning bath (2) similar to the testing bath.
The main differences distinguishing it from the "Type A " T-50 apparatus w e r e :
a. The replacement of the immersion heater by a heating-wire (5) which was wound round the inlet tube of the testing bath.
b. The construction of the specimen rack, which was built in such a way as to allow the stretching and the release of six test specimens to occur simultaneously.
The temperature was measured by means of two completely immersed thermometers, which had previously been gauged with a thermo-couple. T h e reproducibility of the various T R values obtained with this apparatus has been examined with various vulcanizates. It turned out that the TR values normally agree within 1 ° C if samples are taken from the same homogeneous vulcanizate.
For some preliminary investigations a device was used consisting simply of a testing bath which was cooled to the freezing temperature by adding dry ice directly to the coolant. In it six individual racks were present and therefore the stretching and the release of the test specimens had to be carried out separately. It was mainly advantageous when specimens requiring different degrees of elongation had to be tested simultaneously. Its reproducibility, however, was less satisfactory than that of the first mentioned apparatus.
R E F E R E N C E S
1. J. Gough, Memoirs of the Manchester Lit. and Phil. S o c , 2°^ series. Vol. 1, p. 288, (1805).
2. B. S. Garvey and W . D. White, Ind. Eng. Chem., 25, 1042, (1933).
3. W . A. Gibbons, R. H. Gerke and H. C. Tingey, Ind. Eng. Chem., Anal. Ed., 5, 279, (1933).
4a. W . F. Tuley, India Rubber World, 97, 39, (1937). b. N . Bergem, "Vulcanization", Oslo 1948.
5. A S T M Standards on Rubber Products, 1954, p. 267, A S T M Designation D 599-40 T .
6. A. Chiesa, Rubber Chem. and Technol., 27, 648, (1954).
8. O . H. Smith, W . A. Hermonat, H. E. Haxo and A. W . Meyer, Anal. Chem., 23, 322, (1951).
9. J. F. Svetlik and L. R. Sperberg, India Rubber World, 124, 182, (1951). 10. A S T M Standards on Rubber Products, 1954, p. 579, A S T M Designatioa
D l 329-54 T .
11, L. A. W o o d in H. Mark and G. S. W h i t b y , "Advances in Colloid Science", N e w York 1946, Vol. II, p. 65.
12a. L. A. W o o d in H. Mark and G. S. W h i t b y , "Advances in Colloid Science', N e w York 1946, Vol. II, p. 57.
b. H. A. Stuart, Kunststoffe, 44, 286, (1954).
13. J. M. Goppel, „Quantitatieve Röntgenografische onderzoekingen aan rubber", Diss., Delft 1946.
14. N . Bekkedahl and L. A. W o o d , Ind. Eng. Chem., 33, 381, (1941). 15. A. F. Helin and B. G. Labbe, India Rubber World, 126. 229, (1952). 16. L. J. Radi and N. G. Britt, Ind. Eng., Chem., 46, 2439, (1954).
17. L. A . W o o d and N. Bekkedahl, J. Research Nat. Bur. Standards, 36, 489, (1946). 18. A. N . Gent, Trans. Faraday. S o c , 50, 521, (1954).
19. L. A. W o o d and N. Bekkedahl. J. Applied Phys., 17, 362, (1946). 20. E. W . Russell, Trans. Faraday S o c , 47, 539, (1951).
21. A S T M Standards on Rubber Products, 1954, p. 404, A S T M Designation D 832-46 T .
C H A P T E R III
POSSIBLE M E T H O D S F O R A L I M I T E D M O D I F I C A T I O N O F N A T U R A L RUBBER
§ 1. General considerations
If the tendency of natural rubber to crystallize has to be reduced, we may try to achieve this either by mixing the rubber with substances which are inert towards rubber or by reducing the regularity of the rubber molecule by modifying its chemical structure.
Bergem, in a study on vulcanization ^, investigated the effects of the admixture into natural rubber of various substances. He found that inert fillers generally increase the T 50. At a fixed percentage of bound sulphur only plasticizers decrease the T 50. W e must, however, bear in mind that plasticizers, which may reduce the "melting point ", might nevertheless be able to promote crystallization at low temperatures by increasing the mobility of the rubber molecules (see p. 18). By means
— — W i t h o u t p l a ï t l c i z e r W i t h t r i bu t y l p h o s p h a t e Fig. 16, Influence of tributyl phosphate on the C T R and ITR curves of a natural rubber vulcanizate.
of the modified T R test, devised by the present author (see p. 24), it was demonstrated that the introduction of a low-temperature plasticizer (tributyl phosphate) indeed increases the rate of crystallization of a stretched natural rubber vulcanizate at low temperatures.
of the "pure gum" vulcanizate A 1* and a corresponding compound A 5* which contained tributyl phosphate. It appears that after 5 minutes racking at 400 % elongation and —50° C, the rubber containing tributyl phosphate shows the largest tension set, although the rate of retraction during the first minutes is somewhat increased by the plasticizer. Similar results have been obtained with another low-temperature plasticizer, viz. T P 90 B, a polyether of the Thiokol Corp. In Figs. 17 and 18 the low-temperature tension set after 30 minutes' racking and the t j values (see p. 25) respectively, are plotted against the test temperature, for the "pure gum" vulcanizate A 1 * and the tributyl phosphate containing compound A 6 *.
L.L.L.S.,after 3 0 m i n . r a c K i n g ^ 0 0 % • NF^. v u l c a n i z a t e A I Cno p l a s t i c i z . e r ) O N R . v u l c a n i z . a i t e A 2 1 W i t h L r l b u t y l ' p h o < > p K a t e 2 0 0 % - 5 0 -hO - i o T e a t t e m p . j ' C
Fig. 11. Determination of optimum crystallization tem-perature from low-temtem-perature tension set.
It will be clear from Figs. 16, 17 and 18 that tributyl phosphate increases the rate of crystallization of stretched natural rubber at low temperatures. Therefore, it must be concluded that low-temperature plasticizers, although they might decrease both the vitrification point'' and the melting point of polymers, are not satisfactory agents for improving the low-temperature serviceability of natural rubber. Moreover, Fig. 17 suggests that the optimum crystallization temperature is increased by the presence of the plasticizer. This, too, would be an undesirable effect in actual practice.
If we now consider the alternative method, i.e. the chemical modifica-tion of the rubber molecules, it appears that there are a great number of possibilities. In 1950, Le Bras and Delalande^, in their book on the chemical derivatives of natural rubber, gave extensive surveys on many of these compounds. T h e reader is referred to this book for detailed
