VINYL ELASTOMERS Low-Temperature Flexibility Behavior R. F. CLASH, JR.,
AND
R. lM,BERG
Carbide and Carbon Chemicals Corporation, New York, N. Y .
Other workers have approached this same problem, employing different means and with slightly different emphasis. Russell (4) studied the stress-strain diagrams of polyvinyl chloride, both unplasticized and plasticized, over a temperature range of -50” to f80” C., and found that these curves change in shape as the temperature is lowered, finally reaching a temperature a t which rupture occurs a t nearly zero elongation (his so-called brittle point). This brittle point he correlated with the temperature at which a plastic bar (25.4 X 0.97 X 0.97 cm.) failed a t low bending angles. Selker, Winspear, and Kemp (6) studied this same point in a
FIGURE 1. FLEXTESTAPPARATUS
A torsional apparatus has been developed and applied to the measurement of the lowtemperature flexibility of elastomers of vinyl copolymer resins. This measurement determines a composition’s “flex” temperature, T F , which is defined as the lower temperature limit of the compound’s usefulness as an elastomer. The data of single plasticizer additions are shown, and it is established that the flex temperature behavior of binary and ternary systems can be predicted on a simple additive basis.
T
H E physical properties of vinyl resin compounds are known to be sensitive to temperature change. In unplasticized or rigid compositions this effect is a peculiarity of the base resin used in the compound. When plasticizer is added, the physical characteristics are modified, and their dependence on temperature involves both the type and amount of plasticizer used. This modified functional form makes for greater complexity in defining the compound state, but it has the important advantage of permitting the development of compounds with widely different physical characteristics. Generally, the selection of the compound for a given job is predicated on a compromise between the important properties required by the application. These include flexibility, plasticizer permanence, electrical constants, abrasion and fatigue resistance, chemical properties, etc., a t various temperatures. The quantitative evaluation of these properties expedites this compromise and a t the same time points out product limitations in such a way as to define development objectives. This paper traces the development of a test designed to determine one of these properties-namely, lowtemperature flexibility. More specifically, this test defines a limiting temperature below which the composition has lost, to an arbitrary extent a t least, a wide degree of its elastomeric characteristics. This “flex” test, which determines the low-temperature limit of a compound’s elastomeric usefulness, is based on a simple torsional principle, and possesses the advantages of simple specimen preparation and ease of apparatus manipulation.
cantilever beam apparatus. However, Myers (3) showed that in bending tests of this type the fracture point depends not only on temperature but on the rate of load application. McCortney and Hendrick (g) experimented with the T-50 test which determines the temperature a t which a certain percentage of the elastic properties are recovered, and the durometer hardness; then they adopted a diaphragm msembly, similar to an actual service mechanism, to measure the cold resistance of natural and synthetic rubber parts for automotive use. Yerzley and Fraser (8), on the other hand, discarded a modified T-50 and an autographic torsion method 1218
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and set up a factor derivable from durometer hardness measurements as a criterion of the low-temperature behavior of various elastomers. The work reported here completes this cycle and places emphasis on the torsional method. Mechanical Details The flex test apparatus is shown schematically in Figure 1. Essentially it is a device in which a rectangular test specimen is mounted so that its lower end is clamped rigidly, while its upper part is attached to the shaft of a horizontal pulley to which controlled torque may be applied. The angular twist, 4, of the sample is read from -so the torque head which is calibrated in degrees of arc. During a test the above system is immersed in a coolant to a level above the test specimen. For temperatures above -50°C. a solution of 60 parts ethylene glycol and 40 parts water is used as the coolant, and for lower temperatures (down to -75" C.) a solution of 50 parts ethyl alcohol, 30 parts ethylene glycol, and 20 parts water is recommended. It was established for all practical purposes that neither coolant affected the stock chemically during the time interval involved. Solid carbon dioxide was used to cool all solutions. Figure 1 does not show heating, circulating, or thermometer members which were incorporated in the apparatus in a conventional manner.
4 0
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TEMPERATURE -DEGREES
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FIGURE 3. DATABY ORIGINALPROCEDURE
Figure 2 shows the stressed specimen in the apparatus at the instant the twist, 4, is 200" of arc. Experimental Procedure
A standard method of sample preparation was adopted and followed throughout this investigation in order to control processing variables. The general formula used was: Resin (vinyl ahloridevinyl acetate copolymer) Plasticizer Stabilizers and lubrioants
87
-a"
-
X%
100
FIGURE2 .
APPARATUS CONTAINING TEE STRESSED SPECIMEN
The ingredients (500-gram batch) were thoroughly preblended in a kitchen-type mixer, fluxed, and milled for 5 minutes on a 16 X 8 inch (41 X 21 om.) two-roll mill at 140145" C. The milled sheet was molded to a thickness of 0.040 inch (0.1016 om.), and the sample aged a t least 48 hours at 25" C. before testing. Using special jigs, the molded piece was cut into strips 2.50 X 0.25 X 0.040 inch (6.35 X 0.635 X 0.1016 cm.) with mounting holes located 2 inches (5.08 cm.) apart in the test piece. This procedure was applied to a sample which had a 1.50-inch (3.81-cm.) span length (Figure 1). In earlier work a 3-inch (7.62-cm.) span w&s used with similar procedures. As originally conducted, the test procedure used a span length of 3 inches and called for immersion of the system in a coolant a t a temperature below -50" C. Circulation of the coolant was started, and the electric heater, which provided heat at the rate required to raise the temperature of the system approximately 2" C. per minute, was turned on. At the instant temperature T was a t some arbitrary value (say -50" C.), the torque (3.41 X lo5 dyne-cm.) was applied, and the angular twist, 4, of the torque pulley head was recorded as a function of temperature, the loading remaining constant. Figure 3 shows typical data obtained by this procedure. It is apparent that the curves of Figure 3 do not compare all the compounds on an equivalent basis. For example, the B-11 test was started a t -50" C. where the 4 values were already high, and the resulting 4 us. T curve was distorted considerably as compared with that of compound B-22. Thus, the curve obtained depended not only on the material but also on an arbitrarily selected starting temperature, an undesirable situation. At this point the effect of starting temperature was studied. Figure 4 shows the first effort along this line. Method 1
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materials under constant load and temperature into three portions: 1. When the stress is applied, there is an initial rapid elastic deformation. 2 , A "creep" or plastic flow period follows, during tvhich the rate of deformation changes with time.
Preferred Test Procedure The Dewar flask is filled with coolant (the temperature of which is a t the lower end of
followed t h e o r i g i n a l procedure d i s c u s s e d a b o v e , while method 2 was b a s e d o n socalled instantaneous deflect i o n s . B y the latter, the stop pin is released when the system has reached a g i v e n temperature and the "instantaneous" deflection is recorded. This deflection is usually reached in one second or
the range to be studied) and placed around the sample. At a given temperature the torque (5.68 X l o i dyne-cm.) is applied and the angular twist, $6, is noted after an interval of 5 seconds. The torque pulley is then returned to the zero position and the procedure repeated a t successively higher temperatures until a twist of approximately 400" of arc is attained. It is advisable to start a t a temperature which gives a deflection close to 50" of arc and to run a test about every 5" C. The same sample may be used for the entire test; for when the run is started a t the lowest temperature and succeeding data are taken at continually higher values, the results agree with those obtained if a new test specimen is used at each temperature. Typical data obtained by the preferred procedure are shown in Figure 6 for tricresyl phosphate in 20, 30, 40, and 50 per cent concentrations. A study of these curves reveals that the results obtained by this method are independent of the starting temperature; further, the indeterminacy of the instantaneous deflections method has been resolved.
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specimen (0.040 inch thick) reaches a deflection of 200" of arc in 5 seconds under t h e preferred test conditions. A 20 per cent d i - 2 - e t h y l hexyl p h t h a l a t e plasticized com.020 .030 .WO ,050 .OB0 pound has such an THICKNESS, INCH i n d e x a t about room temperature FIGURE7. RELATION RETWIEN p (Figure 10). BeAND SPECIMEN THICKNESS low this temperature the material may be considered as possessing very limited elastomeric properties. This conclusion has been confirmed by practical experience in the field on existing compounds in use. Theoretical Considerations The pure torsion problem, wherein 'la right cylinder or prism is twisted and held in equilibrium by means of couples applied a t its ends", has been solved rigorously 500 I for a limited number of , T H I C N N E I S CORRECTION sections. These include 9 FOR T, DETERMINATION the circle, ellipse, recVS. THICKNESS tangle, and equilateral triangle (7). The solution for the case of a rectangular cross section is applicable to the flex 4 test only as a first approximation. This restriction is imposed because of the failure of the test conditions to comply exactly with all of the boundary conditions set 04 up in the theoretical ap.030 ,040 ,050 ,060 proach to the problem. THICKNESS - INCH The s p e c i m e n u n d e r FIGURB 8 test conditions is not free to contract longit u d i n a l l y as i t 3 s twisted, and hence a tension stress is superposed on the distribution of stress described by the mathematical treatment. Despite this, the -2c agreement between theory and experiment is quite good, as will be shorn, and this indicates that constant span length during test does not change the functional form of the solution; or that the restoring torque due to the superposed tension stress is comparatively small for the 0 strains realized experimentally. The torque obtained from the mathematical development for the case of a right prism of rectangulaz section is given by
T where T 2a
2b @
G
L 4
= abspGg
(1)
tor ue or moment of couple in plane any cross section = specimen width = specimen thickness = J(bl4 = modulusof rigidity of samplematerial = specimen length = angle of twist of torque head =
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Figure 7 shows ,u plotted as a function of specimen thickness, 2b, for a sample width, 2a, of 0.25 inch. These values are by St. Venant (5). The linearity of this relation does not apply to all values of sample thickness outside the range shown in Figure 7. Flex temperature TFhas been defined as the temperature at which a specimen 1.50 X 0.25 X 0.040 inch is twisted through 200" of arc in 5 seconds under an applied torque of 5.68 X lo5 dyne-cm. (two 50-gram weights, torsion head diameter 11.6 cm.). From Equation 1this is found to correspond to a modulus of rigidity, G, of 3.1 X lo9 dyne-cm.-Z (4.5 X lo4poundinches+). On this basis the flex temperature can be defined as the temperature a t which the specimen material has this standard modulus of rigidity. Using this definition of TF, Equation 1may be used to calculate C#I as a function of specimen thickness for G = 3.1 X 109 dynes-cm.-2. This provides a thickness correction for the flex temperature which is given in Figure 8, where the C#I value for TF determination is defined as B function of specimen thickness. Flex ternperaturcs determined by this method agree well with those obtained using an esperimentally determined thickness correction curve (Figure 9). This is shown in Table I which compares the end results calculated by both methods. Experience has indicated that the standard modulus method is applicable most reliably within the thickness interval 0.035 to 0.045 inch, whereas use of the empirical correction should be confined to the range 0.040 to 0.060 inch within which it is essentially linear. TABLE I. COMPARISON OF THICKNESS CORRECTION METHODS Flex Temp., TF,* C. Compound
No.
Thicknass, T e m p . a t +r In. 2000,o c . 19 -19 -23 -29.5 -41 -47.5 -48
-
-
Using stand- Usingempiriard modulus oal aor. cor. -16 18 20 20 -29 -27 -32 -31 -46 44
-
-51 -53
--50
-52
The degree of approximation, to the actual test, achieved by Equation 1 has not been investigated outside the functional manner in which it predicts the thickness correction men-
+zc
01
5 PLCl M E N TH ICKNES 5.1 NCH
CORRECTION CUR^ OBT.4INED BY SUPERPOSITTON OF DATA FOUR TYPICAL VINYL ELASTOMERS COWRINGA WIDB RAXGEOF TIPVALUES
FIGURE 9. FROM
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Tp FOR BINARY PLASTICIZER SYSTEMS
-70
20
10
PLASTICIZER CONTENT, P E R C E N T
30
PERCENT OF SECOND P L A S T I C I Z E R
FIQURE 10
FIGURE
11
tioned above. It appears reasonable to assume that modulus of rigidity values so calculated are correct, at least as regards their order of magnitude.
Experiments The flex test was designed to evaluate the low-temperature properties and limitations of natural and synthetic elastomers. It has been used in a study of vinyl copolymer resin compositions containing the more common and interesting plasticizers in primary, binary, and ternary mixtures. Figure 10 shows flex temperature TF plotted ab a function of concentration for three typical plasticizers. Tricresyl phosphate imparts the poorest flexibility a t low temperatures; di-Zethyl hexyl phthalate is moderately good, while triethylene glycol cli-2-ethyl hexoate is best. Figure 11 illustrates the T ~ v a l u eobtained s by adding di-2-ethyl hexyl phthalate and triethylene glycol di-2-ethyl hexoate to tricresyl phosphate to form binary mixtures of 40 per cent total plasticizer content. The TRlCRESYL PHOSPHATE linearity of the graphs is evidence of the fact that flex temperatures are essentially addiF I G U R E 12. COMP.4RlSON O F EXPERIMENTAL AND PREDICTED FLEX TEMPERATURE VALCESFOR A TERNARY PLASTICIZER SYSTEM O F 40 PERCENTCONCINtive for these combinations. TRATIOS IN VIKYL COPOLYMER RESIN The tricresyl phosphate-di-2-ethyl hexyl phthalate-triethylene gl y c o I d i-2-et h y l hexoate ternary system is shown in Figure 12. was responsible for the apparatus design and construcExperimental points appear as numbers (flex temperature values) in their proper plasticizer-ratio location. The broken tion. lines represent equiflex temperature contours calculated from Literature Cited the T F values of the compounds shown at the three vertices
assumed in calculating the e;ui-TP lines.
Aclmowledgment The authors are indebted to A. P. Wangsgard, who originated the idea of the torsional test described, and
Carbon ’Chemicals Gorp. (4) Russell, IND. ENQ.CHEM.,32, 509 (1940). ( 5 ) St Venant, “De la torsion des prisms”, Chap. IX (1855). (6) Selker, Winspear, and Kemp, IND.EXG.CHEW., 34, 167 (1942). (7) Trayer and March, Nat,. Advisory Comm. Aeronaut., Rept. 331 (1929).
(8) Yereley and Fraser, IND. ENG.CHEM.,34,332 (1942).
