The pro p erty of deform ation under load and of recovery on removal of load is essential to certain applications such as flexible electrical insulation, w aterproof clothing, protective sheeting, tubing, and m any other articles. Q uantitative those involving compression, shear, or bending, yield useful inform ation, b u t th e stress-strain te st was chosen for th e in
vestigation of these elastom eric m aterials prim arily for the simplicity of expression of results and its ad aptability to operation a t various tem peratures.
Because of th e im portance of controlling th e tem perature of the specimen during te st, th e ordinary vertical ty p e of stress- to th e nearest 0.0005 inch w ith a dial gage having a 0.25-inch circular foot and a 3-ounce w eight (1).
T he specimens were cu t 4.75 inches long and 0.5, 0.25, or 0.125 inch wide, depending on th e thickness. A special jig was designed for this purpose, and a razor blade in a holder was used as th e cutting tool. T he specimens were doubled to form a loop and fastened in a clamp, as shown on th e ta b le of th e machine in Figure 1.
Materials Tested. Included in this stu d y were typical vinyl elastomeric com pounds designed for use as wire insula
tion, shoe-upper stock, aircraft paulins, cloth-coating com
pound, safety-glass interlayer, and flexible tub in g (Table I).
All charts reproduced here were ro ta te d 90° counterclockwise from th e position in which th e y were draw n b y th e m achine.
T hus, th e curves m ay be viewed in th e sam e aspect as th e y are usually draw n b y h and from d a ta obtained b y o ther types of machines.
S T R E S S - S T R A I N U N D E R V A R I O U S C O N D I T I O N S
Figure 2 shows th e stress-strain diagram of several fam iliar m aterials a t 25° C. T he unplasticized copolymer resin shows no elongation, as would be predicted from its m odulus of elasticity, w hich is in th e neighborhood of 400,000 pounds per square inch. C urve A , therefore, illustrates th e accuracy of
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y V o l. 35, N o . 4
F ig u re I . A u to g ra p h ic S tr e s s - S tr a in T e s te r
Ta b l e I. De s c r i p t i o n o p Co m p o u n d s Te s t e d
U nplasticized vinyl chloride-acetate copolymer Rubber tire tread vulcanized 30 min. at 140° C.
Unfilled vulcanized rubber
Vinyl chloride-acetate copolymer (87% vinyl chloride) plasticized
Vinyl chloride-acetate copolymer (90% vinyl chloride) plasticized
V inyl chloride-acetate copolymer (95% vinyl chloride) plasticized with 25 per cent Flexol plasticizer D O P Vinyl chloride-acetate copolymer (95% vinyl chloride)
plasticized with 30 per cent Flexol plasticizer D O P V inyl chloride-acetate 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 D O P V inyl chloride polymer, plasticized with 35% Flexol plasti
cizer D O P
Plasticized polyvinyl butyral
Plasticized polyvinyl butyral, calendered, more highly plasticized than compound 11
Plasticized vin yl chloride-acetate copolymer, electrical in
sulating type
Plasticized vinyl chloride-acetate copolymer, softer elec
trical insulating type
Plasticized vinyl chloride-acetate copolymer, calendered, of type used for thin films
Plasticized vinyl chloride-acetate copolymer, soft sheeting type
second cycle was m uch less th a n th a t of the first. T he hysteresis was found to decrease still fu rth er on repeated flexures, b u t th e changes for each succeeding cycle became progressively smaller. This be
havior of rubber is well known, b u t the w riter believes th a t a stu d y of com
pounding variables based on th e proper
ties of repeatedly flexed rubber, instead of on th e first cycles, would yield valu
able p ractical results. This should be par
ticularly tru e of th e new synthetic rubbers, m any of which are reputed to have higher hysteresis th a n n atu ra l rubber.
The vertical m ovem ent of th e load beam was insufficient to show th e stress-strain of unfilled rubber up to 1000 pounds per square inch on th e same basis as the other curves of these charts. B y using a shorter te s t piece, however, m aterials having a high degree of extensibility can be stretched w ith autographic re
cording.
The effect of tem perature on th e stress- strain properties of plasticized polyvinyl chloride-acetate copolymer resin is shown in Figure 3. F irst and second cycles are shown as in curve Ci and C2, Figure 2.
The effect of tem perature on th e stress-strain is greater in compound 16 than in com pound 14, as evidenced by com
parison of the 10° and 40° C. charts. T his is a funda
mental difference between th e tw o different resins in these compounds. F urther d ata on this p o int appear in T able II.
W ithin the range of speed available w ith this m achine, 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 th e same.
For best results specimens m ust n o t be te ste d w ithin 40 hours after molding. One com pound yielded 110 per cent elongation 30 m inutes after molding, 85 per cent after 50 hours of aging, and 80 per cent after 76 hours; th e elongation
P E R C E N T E L O N G A T I O N
F ig u re 2. S tr e s s - S tr a in D ia g ra m s o f F a m i li a r M a te r ia ls a t 25° C.
U.A . R ig id s p e c i m e n
S te e l s p r i n g C i, C i . F i r s t a n d s e c o n d c y c le , r u b b e r t i r e t r e a d
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 rath er 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 ap
plied to the usual te st specimen. The units of load on the ordinate do not apply to curve B. Curve Ci 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 to the zero position, a second cycle was started. The change in shape of the stress- strain curve is shown in curve C2. The hysteresis of the
A p r il, 1 9 4 3 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 4 3 1
Table II. Elongationat 1000 Poundspek Square Inchfor Firstand Second Cyclesat Three Temperatures C o m p o u n d
N o .“
21
3 4 56
78 109
11
12a (L) 12 (T ) 13 14 15 (L) 15 (T ) 16
% E l o n g a ti o n i n F i r s t C y c le _________
% E l o n g a ti o n in S e c o n d C y c le ___________
doo
25° C. 40° C. 10° C. 25° C. o O O
0 ..
213 240 280 2 5 Î 283 330
600 660 774 690 746 878
10 80 . .. 14 105
45 122 59 147
12 42 '9 8 15 53 Ü 5
25 57 124 31 68 142
56 113 208 68 133 235
96 178 315 115 211
57 98 186 68 115 ¿ie
13 57 208 18 73 238
92 225 315 108 271
97 230 . . . 114 290 . . .
27 69 149 34 82 174
68 142 263 82 164 304
33 56 83 40 65 95
48 114 208 60 140 234
57 139 293 70 160
« (L) = s tr e t c h e d lo n g i tu d in a lly . (T ) = s tr e t c h e d tr a n s v e r s e ly .
did not change w ith further aging. H eating of the aged sam ple to 100° C. for 15 m inutes ju st prior to testing brought the elongation up to 112 per cent or practically equal to the
F ig u r e 3. E ffe c t o f T e m p e r a t u r e o n S t r e s s - S t r a in o f C o m p o u n d 16 ( a b o v e ) a n d 14 ( b e l o w ) , S h o w in g F ir s t a n d
S e c o n d C y c le s S a m p l e b r o k e a t p o i n t 4.
F ig u r e 4. E ffe c t o f P la s tic iz e r C o n c e n t r a t io n o n S t r e s s - S tr a in C o m p o u n d s 6, 7, 8, a n d 9 a t 25 ° C.
freshly molded piece. I t was also noted th a t a specimen which had been tested several tim es came back to its original condition in 3 days of aging in an unstretched condition a t room tem perature.
T he effect of calender grain or other m anipulation which fixes strains in a sheet is to decrease th e elongation along th e strain axis and increase the elongation perpendicular to the strain axis. F or this reason all tests were m ade on sheets molded a t a tem perature high enough practically to elim inate th e strains, except where otherwise noted.
T he elongations of several vinyl elastomers a t 1000 pounds per square inch are shown in Table I I . T he m ethod em
ployed in gripping th e specimens causes ru p tu re a t stresses below the norm al tensile strength for these m aterials; hence elongations a t 1000 pounds per square inch could n o t be ob
tained in all instances.
N o a tte m p t has been m ade to relate these elongation d ata to tensile strength, brittleness a t very low tem peratures, or fatigue life. T he detailed effect of kind and am ount of plas
ticizer on elongations of plasticized vinyl copolymer resin is th e subject of another study to be published. Variable am ounts of any given plasticizer do, however, alter th e stress- strain behavior of th e com pound. This is shown in Figure 4 where 25, 30, 35, and 40 per cent of a common plasticizer were added to an otherwise identical form ulation. These charts were m ade a t th e same tem p eratu re (25° C.), and th e effect of th e increasing plasticizer content is obvious.
a c k n o w l e d g m e n t
T he author wishes to express his appreciation to L. C. Hos- field of th e N ational C arbon Com pany for designing and building th e m achine used in these te sts and to J. H . Teeple, form erly of E . I. du P o n t de N em ours & Com pany, Inc., from whom blueprints of th e stress-strain testing m achine were obtained.
L I T E R A T U R E C I T E D
(1) Am. Soc. for Testing Materials, Method D412-41.
(2) Davis, C. C., and Blake, J . T „ "Chemistry and Technology o f
Rubber” , A. C. S. Monograph 74, p. 758, New York, Reinhold Publishing Corp., 1937.
(3) F is h e r , H . L ., In d. En g. Ch e m., 31, 941 (1939).
(4) Williams, I., and Sturgis, B. M., Ibid., 31,1303 (1939).
F i g u r e 1 . Ex t e n t o f F o u l i n g a t D a y t o n a Be a c h i n 2 . 5 M o n t h s