Stress-Strain Characteristics of VINYL ELASTOMERS M. C . REED Carbide and Carbon Chemicals Corporation, New York, N. Y.
E
LASTOMER" was sugThe stress-strain curves of typical plasticized vinyl strikes a reversing mechanism and returns to the zero polymers have been determined at loo, 25' and gested by Fisher (3) to designate those syn40' C. by an autographic stress-strain machine. or balance position at the thetic polymeric substances right. Another cable, atThe stress-strain diagram of these elastomers in tached to the load, operates extension is substantially a straight line up to 1000 which have rubberlike properover a set of pulleys against ties of extension and repounds per square inch during the first elongation traction. Many individual cycle, but is markedly concave toward the load axis a counterweight and moves products of this class have the pen across the chart over the second cycle and shows a reduction in been developed in the past laterally. The tank shown in hysteresis on repeated flexure. few years. Plasticized comthe background contains water Vinyl elastomers exhibit a greater increase in pounds made from polyvinyl which is circulated through stiffness with decrease in temperature than vulchloride, polyvinyl chloridethe cylinder and back through canized rubber and, for most commercial comacetate copolymer, and polyan overflow pipe. The tank pounds, a higher modulus of elasticity at room vinyl butyral constitute an is provided with an electrical temperature. In the case of vinyl chloride-acetate important group of elastomers, heater and with a removable copolymers, increasing the vinyl chloride content They resemble natural rublid so that ice may be added up to 95 per cent reduces the temperature sensiber to some degree but are to the water. tivity. Increase in vinyl chloride content above 95 characterized by higher hysThe travel of the load is per cent has no appreciable effect. Polyvinyl 17.25 inches which produces a teresis and greater change in butyral elastomers show greater stiffening at reflexibility with temperature stress of 1000 pounds per duced temperatures than uolvvinvl - - - chloride-acechanges. The stress-strain square inch on a specimen 0.005 tate copolymers. characteristics of some typisquare inch in cross section. cal compounds in this group For larger specimens additional weights are added to the beam to maintain a uniform scale on are the subject of this paper. The property of deformation under load and of recovery on the chart. The time required for the complete cycle is 150 removal of load is essential to certain applications such as seconds, 74 seconds to apply the load and 76 seconds to unflexible electrical insulation, waterproof clothing, protective load. The temperature was maintained within 0.2" C. of the sheeting, tubing, and many other articles. Quantitative stated value, and the thickness of the specimen was measured measurements of the relation of deformation and load under to the nearest 0.0005 inch with a dial gage having a 0.25-inch specified conditions are essential to a proper choice of elastocircular foot and a 3-ounce weight ( I ) . meric materials for technical applications. The specimens were cut 4.75 inches long and 0.5, 0.25, or I n the evaluation of rubber compounds, tensile strength, 0.125 inch wide, depending on the thickness. A special jig elongation a t break, and the stress-strain diagram are the was designed for this purpose, and a razor blade in a holder properties most commonly measured. Other tests, such as was used as the cutting tool. The specimens were doubled to those involving compression, shear, or bending, yield useful form a loop and fastened in a clamp, as shown on the table of the machine in Figure 1. information, but the stress-strain test was chosen for the investigation of these elastomeric materials primarily for the MATERIALS TESTED.Included in this study were typical simplicity of expression of results and its adaptability to vinyl elastomeric compounds designed for use as wire insulation, shoe-upper stock, aircraft paulins, cloth-coating comoperation a t various temperatures. Because of the importance of controlling the temperature of pound, safety-glass interlayer, and flexible tubing (Table I). the specimen during test, the ordinary vertical type of stressAll charts reproduced here were rotated 90" counterclockwise strain tester was unsuitable. A machine described by Wilfrom the position in which they were drawn by the machine. liams and Sturgis (4) appeared to fulfill the requirements of Thus, the curves may be viewed in the same aspect as they are usually drawn by hand from data obtained by other types the test, with certain minor changes (Figure 1). of machines. STRESS-STRAIN TESTING STRESS-STRAW UNDER VARIOUS CONDITIONS
METHOD. The test specimen is immersed in water in the glass cylinder shown at the left. The specimen is connected through a steel cable over the chart drum to the end of the load beam. As the specimen elongates, the drum rotates and moves the chart vertically. The load, shown near the middle of the beam, is moved laterally through a motor-driven screw toward the left or the outer end of the beam where it
Figure 2 shows the stress-strain diagram of several familiar materials a t 25" C. The unplasticized copolymer resin shows no elongation, as would be predicted from its modulus of elasticity, which is in the neighborhood of 400,000 pounds per square inch. Curve A , therefore, illustrates the accuracy of the machine when applied to a substantially unyielding 429
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1. Autographic Stress-Strain Tester
TABLE I. DESCRIPTION OF COMPOUNDS TESTED 1 2
3 4 5
6
7 8
9 10
11 12 13 14 15 16
Unplasticized vinyl chloride-acetate copolymer Rubber tire tread vulcanized 30 min. a t 140° C. UnfXled vulcanized rubber Vinyl chloride-acetate copolymer (87% vinyl chloride) plasticized Vinyl chloride-acetate copolymer (90% vinyl chloride) plasticized Vinyl chloride-acetate copolymer (95yo vinyl chloride) plasticized with 25 per cent Flexol plasticizer DOP Vinyl chloride-acetate copolymer (95% vinyl chloride) ulasticized with 30 Der cent Flexol ulasticiaer DOP Vinyl chloride-acetaie copolymer ;95% vinyl chloride) plasticized with 35 per cent Flexol plasticizer D O P Vinyl chloride-acetate copolymer (95% vinyl chloride) plasticized with 40 per cent Flexol plasticizer DOP Vinyl chloride polymer, plasticized with 35% Flexol plasticizer D O P Plasticized polyvinyl butyral Plasticized polyvinyl butyral, calendered, more highly plasticized than compound 11 Plasticized vinyl chloride-acetate copolymer, electrical insulating type Plasticized vinyl chloride-acetate copolymer, softer electrical insulating type Plasticized vinyl chloride-acetate copolymer, calendered, of type used for thin films Plasticized vinyl chloride-acetate copolymer, soft sheeting type
specimen. Curve B shows an extension and recovery cycle on a steel coil spring. The width of the line, as compared with the single lines of the lower curves, represents frictional losses of the machine rather than hysteresis of the spring. The divergence of the two lines is equivalent to approximately 3 per cent of elongation or 20 pounds per square inch as applied t o the usual test specimen. The units of load on the ordinate do not apply to curve B. Curve C1 represents the stress-strain diagram of a rubber tire tread stock ( 2 ) properly vulcanized, and stretched a t 25" C. through the first cycle. As soon as the weight returned t o the zero position, a second cycle mas started. The change in shape of the stressstrain curve is shown in curve Cz. The hysteresis of the
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second cycle was much less than that of the first. The hysteresis was found to decrease still further on repeated flexures, but the changes for each succeeding cycle became progressively smaller. This behavior of rubber is well known, but the writer believes that a study of compounding variables based on the properties of repeatedly flexed rubber, instead of on the first cycles, would yield valuable practical results. This should be particularly true of the new synthetic rubbers, many of which are reputed to have higher hysteresis than natural rubber, The vertical movement of the load beam was insufficient to show the stress-strain of unfilled rubber up t o 1000 pounds per square inch on the same basis as the other curves of these charts. By using a shorter test piece, however, materials having a high degree of extensibility can be stretched with autographic recording. Theeffect of temperature on the stressstrain properties of plasticized polyvinyl chloride-acetate copolymer resin is shown in Figure 3. First and second cycles are shown as in curve Cl and C2,Figure 2. The effect of temperature on the stress-strain is greater in compound 16 than in compound 14, as evidenced by comparison of the 10" and 40" C. charts. This is a fundamental difference between the two different resins in these compounds. Further data on this point appear in Table 11. Within the range of speed available with this machine, the effect of speed was slight, varying in one instance from 80 per cent elongation a t 140 seconds to 100 per cent a t 575 seconds. The shapes of the curves were substantially the same. For best results specimens must not be tested within 40 hours after molding. One compound yielded 110 per cent elongation 30 minutes after molding, 85 per cent after 50 hours of aging, and 80 per cent after 76 hours; the elongation
P E R CENT ELONGATION
Figure 2 . A.
B.
Stress-Strain Diagrams of Familiar 3Iaterials a t 25' C. Rigid specimen GI,G. First a n d second Steel spring
cycle, rubber tire tread
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T o m 11. ELONQATION AT 1000 POUNDS PER SQUARE INCH FOR FIRSTAND SECOND CYCLESAT THRBB TEMPERATURES Corn ound
N”,. a
...
1 2 3 4 5 6 7 8 9 10 11 13 14
;16:I% a
(L)
-
% ’ Elongation in Second
% Elongation in First
loo C.
Cycle 25OC.
10°C.
Cyole 25’C.
... ...
690 14 59 15
285 746 105 147 53
40° C .
... 251
213 600 10
0 240 660 80
i% 774
45 12
122 42
9s
25 56 96 57 13 92 97
57 113 178 98 57 225 230
124 208 315 186 208 315
...
31 68 115 GS 18 108 114
27 68 33 48 57
69 142 56 114 139
149 263 83 208 293
34 82 40 60 70
Stretched longitudinally. (TI
-
40’C.
...
878
iiS. 142 235
iii
238
... ...
82 164 65 140 160
174 304 95 284
...
stretched transversely.
k - l O O % ~ P E R CENT ELONGATION
did not change with further aging. .Heating of the aged sample to 100’ C. for 15 minutes just prior to testing brought the elongation up to 112 per cent or practically equal to the
I , 100% --I
Figure 4. Effect of Plasticizer Concentration on StressStrain Compounds 6,7,8, and 9 at 25’ C.
freshly molded piece. It was also noted that a specimen which had been tested several times came back to its original condition in 3 days of aging in an unstretched condition a t room temperature. The effect of calender grain or other manipulation which fixes strains in a sheet is to decrease the elongation along the strain axis and increase the elongation perpendicular to the strain axis. For this reason all tests were made on sheets molded a t a temperature high enough practically to eliminate the strains, except where otherwise noted. The elongations of several vinyl elastomers a t 1000 pounds per square inch are shown in Table 11. The method employed in gripping the specimens causes rupture at stresses below the normal tensile strength for these materials; hence elongations a t 1000 pounds per square inch could not be obtained in all instances. No attempt has been made to relate these elongation data t o tensile strength, brittleness st very low temperatures, or fatigue life. The detailed effect of kind and amount of plasticizer on elongations of plasticized vinyl copolymer resin is the subject of another study to be published. Variable amounts of any given plasticizer do, however, alter the stressstrain behavior of the compound. This is shown in Figure 4 where 25,30,35, and 40 per cent of a common plasticizer were added to an otherwise identical formulation. These charts were made a t the same temperature (25’ C.), and the effect of the increasing plasticizer content is obvious. ACKNOWLEDGMENT
The author wishes to express his appreciation to L. C. Hosfield of the National Carbon Company for designing and building the machine used in these tests and to J. H. Teeple, formerly of E. I. du Pont de Nemours & Company, Inc., from whom blueprints of the stress-strain testing machine were obtained. !-
100%
4
PER CENT ELONGATION
Figure 3. Effect of Temperature on Stress-Strain of Compound 16 (above) and 14 (below), Showing First and Second Cycles Sample broke at point A.
LITERATURE CITED
(1) Am. SOC.for Testing Materials, Method D412-41. ( 2 ) Davis, C. C., and Blake, J. T., “Chemistry and Technology of Rubber”, A. C. S. Monograph 74,p. 758,New York, Reinhold Publishing Gorp., 1937. (3) Fisher, H.L.,IND.ENQ.C ~ a w . 31, , 941 (1939). (4)Williams, I.,and Sturgis, B. M., Ibid.,31, 1303 (1939).
