I
R. H. CAREY Bakelite Co., Division of Union Carbide Corp., Bound Brook, N. J.
Creep and Stress-Rupture Behavior of Polyethylene Resins Here is a dramatic illustration of the versatility, as well as the limitations, of polyethylene as an engineering material. With continued developments in the art and science of rheology, more quantitative test methods will be devised
POLYETHYLENEminute. termined using a speed of 1 inch per Then the speed of the testing
plastic, because of varying degrees of crystallinity ( I , 77, 72), exhibits a wide range of mechanical behavior. A soft flexible polyethylene will elongate or deform easily under stress, but a highly crystalline polyethylene will exhibit less deformation and is susceptible to brittle failure. This diverse behavior causes difficulty when polyethylenes of low and higher density are compared. Density is used in this report as a measure of the degree of crystallinity. Other factors that compound the problems of reporting timedependent properties are secondary effects of factors such a fillers, special environment, and manner of stressing (2, 5-8). Testing Methods and Apparatus
Short-Time Tests. The short-time stress properties of polyethylene were measured by commonly accepted standard testing procedures (3, 4). Modulus of elasticity, yield point, tensile strength and elongation were determined at a number of ambient temperatures by methods similar to ASTM D 638-52T. An exception was made, in that a speed of 0.1 inch per-minute was used for moduli data, modulus being defined as the slope of a secant line drawn through the origin and the 1% strain coordinate. The yield point was de-
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A simple laboratory apparatus is used to determine creep and stress rupture
machine was increased to 20 inches per 'minute, and the tensile strength and elongation were observed. Two specimens are required to obtain all three parameters of the stress-strain curve. Time-Dependent Tests. A simple laboratory apparatus has been devised for determining creep and stress-rupture data. A multiplying lever applies a predetermined tensile stress to a tensile bar immersed in a water bath. With this apparatus, failure may occur in one of two ways-failure (ductile) by excessive elongation where the multiplying lever meets the limits of its travel (about 20%), and rupture by breaking or fracture of the specimen. The creep and stress-rupture properties are obtained by applying a dead weight load to a standard tensile bar. Three hypothetical stress rupture curves can result from such data. A : B, and C (Figure 1) represent, respectively, ductile failure, brittle failure, and a combination of ductile and brittle failure. Such stress-rupture curves are most useful for design in applications such as pipe. Combined creep and stress-rupture curves have been used (70), but they are more complex. By the standard short-time testing procedures, a typical high-density material has a static yield point of approximately 1800 p.s.i. at 60" C. At stresses of 1000 to 1400 p d . , the material creeps or elongates with time until the apparatus reaches the limit of its travel, 20% strain-"ductile" failures (Figure 2). At stresses of 600 and 800 p.s.i. however, the material does not creep to the limit of the apparatus but fractures at some strain less than 20~0--('brittle" failures. Except in a few instances, where temperatures were varied from 23 O to 100°C., tests were conducted a t 60" C. I n general, a n increase in testing temperature increases the rate of creep for any given applied stress and decreases the time to rupture (5).
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Short-Time Properties
The static tensile properties as measured by standard testing procedures show that the short-time strength and modulus are primarily a function of the crystallinity as measured by density (Figures 3 and 4). However, the tensile strength of low-density resins (0.914 to 0.925 gram per cc.) varied between 1200 and 2600 p.s.i. The log of tensile strength is approximately linear with the log of melt index. [Figures 3 and 4 are constructed from only two points. For more comprehensive data see ( I O ) ] . Time-Dependent Properties
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Low Density Polyethylene. Behavior of one typical resin (0.918 gram per cc.; melt index 1.2 decigrams per minute) was studied during one year at 40" and GO" C. because creep is a function of both temperature and stress. [Melt index is a capillary measure of melt viscosity (ASTM D 1238-52T). Correlation of this test with composition and structure has not been established.] At 40" C. (Figure 5,A) elongation is excessive (50%) after 1 year at 600 p.s.i., unduly large (15%) at 400 p.s.i., and less than 10% for stresses below 300 p.s.i. At 60" C. (Figure 5,B) the material ruptures a t 600 p.s.i. in 2 months after a n extremely large elongation (200%) ; at 400 and 300 p.s.i., failure occurs in a year after rather large elongations of 35 and 15%, respectively. At 200 p.s.i., failure occurs after 1 year and
Sfress is applied by a multiplying lever arm VOL. 50, NO. 7
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4 Figure 1. Hypothetical stress-rupture curves are useful in design
b Figure 2. A typical high-density polyethylene resin creeps with time TIME, HWRS
CREEP PIT 60°C
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Figure 3. Modulus of low-, medium-, and high-density resin is primarily a function of crystallinity as measured b y density
8% elongation. The approximate safe operating strain limit is 5%. .4t 40' C.: this is produced in a year by 200 p.s.i.; a t 60" C., by 100 p.s.i. A resin of similar density but lower melt index was also studied a t 40" and 60" C. Generally, the creep behavior is similar to that shown in Figure 5,A and B, but rupture is less prevalent. At 40" C. (Figure 5,C) elongation is 30, 10, and 5% after one year under stresses of 600, 400, and 300 p.s.i. At 60" C. (Figure 6,A), elongation is 60, 20, 10; and 5% a t stresses of 600, 400, 300, and 200 p.s.i. With the same strain criterion for failure (5%), this material can be used at 300 p.s.i. a t 40" C. and a t 200 p s i a t 60' C. This is in contrast to safe values of 200 and 100 p.s,i., deduced from Figure 5,A and B . Medium-Density Polyethylene. Medium-density resin exhibits good characteristics if the criterion for judgment is the magnitude of creep (Figure 6,B and C) where elongations are approximately equal. However, rupture occurs sooner
and a t a lower stress for resin of higher melt index (Figure 6,C). This behavior is difficult to prove conclusively because of the data scatter, characteristic of such testing. One approach to this problem is the use of a screening technique: If a criterion of failure is arbitrarily established at 5% strain, both intermediate density resins may safely be used a t a stress of 300 p.s.i. If a criterion of rupture is established, 300 p.s.i. is also a safe stress, but the safety factor is greater with the resin of low melt index. Testing, of course, is also inseparable from specimen preparation.
lime-Dependent Behavior of a Spectrum of Polyethylenes A number of experimental resins of different density and different melt index were screened by a single-point creep test a t 60" C. and 600 p s i . Figure 7 and Table I show creep to approach an asymptotic value of approximately 10% strain a t a density of 0.930. Creep, in the density range of 0.915 to 0.920. is very pronounced, rising as high as 707, Figure 5. Modulus of tow-, medium-, and high-density resin i s primarily a function of crystallinity as measured b y density
b 6. Creep at 60" C. for 1.2 melt index, low. density resin
b C.
Creep at 40' C. for 0.3 melt index, lowdenrily resin
4 A.
Creep
at
40" C. for 1.2 melt index, low density resin
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0
10
20
0 40 TEMPERhTURE 'C
50
60
70
Figure 4. Short-time strength of low-, medium-, and hiah-density resin is a function of crystallinity
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in 30 days Qualitative observations indicated that densin alone \vas not sufficient to describe the behavior of the materials. Furthermore, materials of high melt index failed prematurely without necessarily undergoing excessive elongation The same evperlmental materials listed in ordei of incieasing melt index (Table I) show That materials of high melt index break piematurely but
POLYETHYLENE RESINS Figure 6. Modulus of low., medium-, and high-density resin is primarily a function of crystallinity as measured by density
not necessarily after high strain. Thus, it is concluded that crystallinity primarily regulates the degree of creep or elongation, but melt index or some molecular weight property related to melt index regulates the fracture properties. From experience, it is possible to draw a line in Table I a t a melt index of approximately 1.8: Materials with a melt index greater than 1.8 are susceptible to rupture or fracture prematurely; materials with a melt index of less than 1.8 are less likely to fracture. A number of low-, medium-, and highdensity polyethylenes were evaluated in
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b Creep at 60' C. for 0.3 melt index, lowdensity resin
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Creep at 60' C. fori1.87 melt index, medium-density resin
terms of stress-rupture curves (Figure 8 ) . At durations of about 100 hours or less, failure occurs by elongation-a ductile failure; at greater times, by fracture or rupture with little elongation. The intersection of the creep-failure curve and the stress-rupture-failure curve is called the "induction" point. The induction period, or the time required for a material to reach failure by rupture rather than by creep, may be used to compare materials. Materials of lower melt index had a longer induction period and, thus, may be considered more resistant to the effects of stress and time. Low-density polyethylenes are not characterized by induction points. At a melt index of approximately 0.1, 1000 hours is required to develop the induction period (Figure 9) ; brittle fractures occur in an induction period of less than 2 hours a t a melt index of 30. Creep and stress-rupture characteristics of high-density polyethylenes are affected by changes in testing temperature. In addition to curves a t 40°, 60°, 80°, and 100' C., Figure 10 also shows a Larson-Miller curve ( 5 ) . This correlation (Larson-Miller) for interchanging time and temperature appears satisfac-
C.
Creep at 60' C. for 9.5 melt index, medium-density resin
tory for a short time but does not apply after the induction period. Secondary Effects An attempt was made to investigate the relative importance of flaws (9). A drilled hole showed no effect on the stress-rupture characteristics of the resin (Figure 11). These materials apparently are unaffected by accidental stress concentrations of tbe magnitude present in this test. Furthermore, the data verify the reproducibility of this procedure. In considering the effect of fillers on the creep properties of polyethyleqe, samples of filled polyethylene (7) were all hung in a 60" C. testing chamber and subjected to a stress of 600 p.s.i. (Table 11). Without exception, all the fillers improved the inherent creep elongation characteristics under test conditions used.
Table 1. Stress Rupture of Polyethylenes Materials of high melt index break prematurely but not necessarily after high strain
Melt Index, Dg./min.
(23' C.),
G./cc.
30 days
60 days
0.35 0.40 0.82 1.4 1.6 1.9
0.926 0.918 0.922 0.922 0.931 0.931
17 54 18 54 13 11
19 55 19 58 13 11 (failure 100
1.95
0.918
1.96
0.914
2.0 3.0 4.7
0.928 0.924 0.929
5.2 7.25
0.919 0.929
9.5
0.9295
9.6
0.920
Conclusions A considerable volume of stress-rupture data on high-density polyethylene indicates that materials with a melt index greater than approximately 0.2 have a 1000-hour stress of less than 800 p.s.i. a t
Density
yo Creepa
days) Failed in 14 and 112 days at 170% Failed in 24 hours at 300% 12 30
13 33 Failed in 1 and 3 days at 12% 67 69 Failed in 1 and 2 days at 10% Failed in 3 and 6 days at 10% Failed in 5 and 21 days at 62%
At 60' C. and 600 p.8.i.
b y I I I I I Figure 8. Stress-rupture curves for low-, medium-, and high-densityresins, At 100 hours or less failure occurs by elongation
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S O U D P01nlS. OUCIlLI FAILURES
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Figure 7. Single-point creep test (30 days at 60' C., and 600 p.s.i.) shows that creep approaches asymptotic value
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Melt index
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b ; Figure 10. Temperature affects creep and stress-rupture characteristics of high-density
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Figure 11. Flaws are comparatively unimportant
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60’ C. With a melt index of less than 0.2, a n extended period of time, possibly 5000 hours or more, is required to evaluate a n engineering design stress. T h e time-dependent behavior may be a function of the molecular weight and the molecular weight distribution. No complete or definite engineering answers can be provided until testing of high-density materials is extended to a t least 5000 hours, and until the slope of the stresstime curve is determined with sufficient accuracy to allow reasonable confidence in a n extrapolation of the data. Many materials, particularly plastics, glass, lead, and asphalt, have short-time strengths considerably greater than their long-time strengths. Crystallinity governs the creep or elongation characteristics, but a low creep rate does not necessarily guarantee a long life. Materials having a low melt index (high molecular weight) have a greater probability of survival. Table 11.
Figure. 12. In typical low-density resin failure i s based on creep rather than rupture
T h e long-time creep data may be summarized in a stress-strain curve. This technique is particularly useful with the low-density polymers, where the criterion for failure is based on creep rather than rupture (Figure 12). Any desired elongation may be selected as the limiting value. If 5% strain limit is chosen, this particular material is useful a t 23’ C. for a stress of 450 p.s.i.; at 40’ C. for 300 p s i . ; and a t 60’ C. for 200 p.s.i. The techniques described are not readily adaptable to quality control testing, because they are laborious and time-consuming. Creep and stress-rupture tests indicate trends and are particularly useful in engineering problems involving structural applications.
Acknowledgment The author wishes to thank E. A. Rogers and B. B. Pusey for constructing the apparatus and obtaining the data.
Creep of Filled Polyethylene Compounds
fillers improved creep elongation characteristics (At 60° C. and 600 p.s.i.; melt index 1.2; density, 0.918) mt. % Time t o Filler Filler Fail, Hr. 33 25 Beaded channel black (control) 33 108 Fibrous magnesium silicate 50 120 Calcined clay 33 400 Wet-ground calcium carbonate 33 1390 Diatomaceous earth 33 1680 Glass fibers, l/,-inch 33 2300 Precipitated hydrous silica 50 2520 Glass fibers, l/a-inch 33 s Short asbestos fibers No failure at 7200 hr. and 16%, All
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Literature Cited (1) Bunn, C. W., Alcock, T. C., Trans. Faraday Sac. 41, 317-25 (1945). (2) Carey, R. H., A S T M Bull. No. 167, 56 (Julv 1950). Carey; R.‘ H., J. SOC. Plastics Engrs. 10, s o . 4,12 (1954). Carey, R. H., Dienes, G. J., Schulz, E. F., IND.ENG.CHEM.42, 842-7 (1950). Carey, R. H., Oskin, E. T., J . SOC. Plastics Engrs. 12, 21 (March 1956). Cumrnings, J. D., Ellis, W. C., A S T M Bull., NO. 178,47-50 (1951). Frissell. W. J.. Plastics Technol. 2. 723 ’(19561. ’ (8) Gohn, ‘G. R . , Cummings, J. D., Ellis, W. C., Am. SOC.Testing M a t e rials Proc. 49. 1139 11949’1. Griffiths, A. A., Phii. Trans. Roy. Soc. London A221, 163 (1920). Manufacturing Chemists’ Assoc., Washington, D. C., “Technical Data on Plastics.” Richard, K., Diedrich, G., Kunstftojje 45,429-33 (1955). Richards, R. B., J . Appl. Chem. 1, 370-6 (1951). Richards, R. B., T r a n s . Faraday SOG. 42, 27 (1946). RECEIVED for review December 16, 1957 ACCEPTED March 25, 1958 Divisions of Paint, Plastics, and Printing Ink Chemistry and Industrial and Engineering Chemistry, Symposium on Chemical Engineering Aspects of Polyethylene and Urethane Production, 132nd Meeting, ACS, New York, N. Y . ,September 1957.
