the mechanical behavior of springy polypropylene

The Mechanical Behavior of Springy Polypropylene S. L. CANNON* and W. 0. STATTON

University of Utah, Salt Lake City, Utah

and

J. W. S. HEARLE University of Manchester,

Institute of Science and Technology Munchester, Enghnd

To obtain further details of the unusual behavior of the %ard elastic,” “springy” form of polypropylene (SPP) , various loading conditions were studied with an Instron model 1130. S ecscally, the variation of the elongation at N ture with

The nature and degree of the specimen hysteresis resulting from various loading cycles have been characterized. The ability of this material to recover from large extensions custom- arily expected to be ermanent deformation has been identified

for strain levels of 10, 25, 50, and 100 percent of the original s a m le length. The effects of submitting the material to

various electrolyte concentrations and autoclaving for 30 min at 250°F and 15 psig have been studied. Characterization of the stress relaxation behavior of the polymer has been made. Fatigue behavior has been identified utilizing a novel testing apparatus which permits load-cycling of the fiber. Rheovibron work has facilitated calculation of the activation energy of the second order relaxation of SPP from tan 6 vs temperature data. Stoll abrasion testing results on SPP tubular knit fabric and multifilament yarn have been compared to normal polypropylene and Dacron@.

c K ange in gauge length and strain rate has been c f etermined.

and the time depen B ency of this recovery has been determined

con x ’tioning treatments, e.g., immersing in saline solutions of

INTRODUCTION

T he first preparation of a crystalline polymer which exhibited elastic recovery from very large

extensions was reported in 1966 by Kargin and Tsarevskaya (1). Preparation of polybutylene was effected by stereo-specific polymerization of film specimens from a melt which had been cooled at the rate of 2°C per minute from 160°C down to crystallization temperatures ranging between 85 and 95°C. The significance of this work lay in discover- ing this unusual elastic phenomena in crystalline polymers and in recognizing that the extension of individual crystallites under deformation was related to the extension of the whole assemblage.

A treatment in greater depth of another “hard elastic” material appeared in the U.S. patent litera- ture in 1966 (2). In this case the polymer was polypropylene in filament form and a detailed explana-

0 Present address: University of California, Los Angeles, California.

tion of the processing procedure was presented, e.g., spinning conditions, fiber orientation, and heat and tension treatments. Briefly, it appears that the elasticity characteristic of these fibers vis A vis the normal, highly crystalline counterpart results from a specific molecular orientation within the material. This arrangement has been referred to as “gamma” orientation and is easily identified by X-ray diffrac- tion techniques. The other requisite for achieving this novel elastic state is that the polymer must exhibit a “heat-stable orientation angle” of 10 to 55 deg. This angle is a parameter which relates to the degree of alignment of the molecular axes of the polymer (forming a filament) with respect to the fiber axis. Orientation angles may be determined according to the technique of Ingersol (3) which employs an instrument described by Owens and Statton (4).

Given a likely set of specifications of polymer type, spinneret orifice geometry, number of orifices, and material throughput, gamma orientation is achieved through temperature regulation and windup speed.

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1975, Vol. 15, No. 9 633

S. L. Cannon, W. 0. Statton, and 1. W. S. Hearle

The desired orientation angle is produced by specify- ing a suitable spin stretching (drawing) which in practice should not exceed 2.5 times.

Lastly, the final processing requirement essential to the development of the desired recovery properties involves heat treating. This annealing is car- ried out at temperatures ranging between 105 and 160°C usually for a period of 30 to 60 min.

This patent states that the fiber produced under specified conditioning will exhibit an elongation at break of from 100 to 700 percent while at the same time demonstrating a tensile recovery of at least 82 percent after sustaining a 25 percent elongation. Additional work on this material has been reported by others (5-8) which, among other things, has demonstrated that this form of polypropylene exhibits a porosity of a peculiar nature, i.e., the porosity may be vaned widely by extending to different strain levels. The practical implication of this property should indicate its usefulness as a candidate mem- brane for separation process studies.

The development of another fiber having excep- tionally high work recovery properties has been described by Knobloch and Statton (9). These mod- ified polypivalolactone fibers blend the attributes of high extension to rupture and large restoring force from deformation. Typical values of elongation at rupture range from 80 to 100 percent while the power of the reqtoring force is more than 20 times that of rubber and 10 times that of commercially available Spandex@ ( segmented polyurethane elastomeric) fibers. Since these properties are known to be related to the ability of a textile structure to recover from creasing ( l o ) , they are important factors in design and development of textile structures.

Another crystalline polymer, polyoxymethylene, has been shown to exhibit elastic properties similar to those of polypropylene and polypivalolactone (11). A list of proposed end uses for this form of polyoxymethylene appears in Table 1, and it is thought that these uses are common to the whole class of hard elastic fibers.

Thus, in recent years there has been a significant demonstration that a new combination of properties can be obtained from oriented fibers. They are crystalline and highly oriented, yet they extend and recover as if they were elastomeric. Their initial modulus of deformation is not like elastomers, but similar to normal crystalline fibers. This new combination of properties gives rise to a semantic prob- lem: the proper descriptive term “high modulus, high

Table 1. Applications involving Springy Fibrous Materials (11)

1. A core yarn to supplant the widely used Spandex yarn. 2. Fabrication of various stretch garments, e.g., ski outfits,

3. Sewing threads which would reduce seam puckering. 4. A stuffing material for upholstery and allied applications. 5. Construction of fabrics exhibiting an increase in porosity

6. Carpet base fabrics and pile yarns,

lingerie, and stockings.

when subjected to tensile stressing.

extension, high recovery” is too unwieldy. The term “hard elastic” is shorter and adequate but appears to be an anomalous contradiction. We propose the use of the simple word “springy” as an apt, descriptive, and concise term for this state.

Recently, another area of involvement for the springy materials has been identified ( 12) : their characterization and evaluation as candidate materials for prosthetic devices in the cardiovascular sys- tem. Recent descriptions have been given of the structural peculiarities of these materials, their surface and internal morphologies, and the deduced physical model of the polymer aggregates. However, the detailed characteristics of the mechanical behavior of these polymers must be determined in order to facilitate the design and development of cardiovascular prostheses. The following results show the unusual nature of this class of materials and extend our knowledge of their behavior.

EXPERIMENTAL A multifilament springy polypropylene yarn pro-

duced and provided by Celanese Research Com- pany, Summit, New Jersey, was the subject of all testing reported in this study. The manufacturer’s specilications of this yarn appear in Table 2 ( 13).

A description of the impurity content has been provided by Celanese. Prior to spinning, 3.5 percent of PCM-010, a polypropylene resin stabilizer con- centrate, was added to the Hercules Profax 6301 polypropylene flake used by Celanese. The stabilizer contains approximately 20 percent active ingredients, the remainder being polypropylene. These active ingredients are an ultraviolet absorber, Cyasorb 531, and a heat stabilizer. Cyasorb 531 is present in the fiber in the amount of 0.001 percent along with the customary spin finishes which may be removed by isopropanol extraction.

Density of this yam has been determined by a gradient column to be 0.9369 g/cc. The yam diameter is 13.0 mils while the filament diameter is 1.58 mils. These dimensions give resulting cross-sectional areas of 1.327 x lod4 in.2 and 1.96 x ina2 for the yarn and filament, respectively.

The mechanical testing of the sample specimens was effected by an Instron model 1130 operating at specified strain rates; a UMIST Fatigue Tester operating at 50 Hz; and a Rheovibron operating at 3.5, 11, 35, and 110 Hz. All testing was conducted at ambient temperature and relative humidity, i.e., 21

Table 2. Springy Polypropylene Yarn Specifications (13)

Denier per filament 13.7 Filaments per yarn 50 Elongation to break, percent 295 Tenacity, g/den 1.0 Init ial modulus, g/den 39 Elastic recovery, percent

from 50 percent extension 98.0 from 100 percent extension 96.5

Yarn twist, turns per in. 0.5 to 1.0

634 POLYMER ENGINEERING.AND SCIENCE, SEPTEMBER, 1975, Vol. 15, No. 9

The Mechanical Behavior of Springy Polypropylene

to 23°C and 25 to 30 percent, unless otherwise designated.

MECHANICAL PROPERTIES OF SPP IN THE DRY STATE

Initial Values of Elongation at Rupture, Young's Modulus, and Yield Stress

Figure I represents a typical stress-strain diagram for springy polypropylene (SPP). Upon loading, the material exhibits a Hookean behavior (line A B ) up to a point B most frequently referred to as a well- defined yield point. Path BCD represents the con- tinuation of sample deformation up to rupture (point D). The apparent percent elongation at rupture (segment A E ) is markedIy affected by the initial (gauge) length of sample specimen as shown by Figure 2. While these results would indicate that the fibers as now produced will be better suited in applications involving short lengths if high extensions are required, it must be remembered that a relation- ship of this type may be produced by extension of the material in the Instron jaws. Thus, the true ex-

D

FRACTURE STRAIN OF NORMAL POLYPROPYLENE

E 300 to fOO

ELONGATION, %

Fig. 1. TypicaZ stress-strain diagram for springy PO& PrOPYk?f=

EXTENSION AT RUPTURE, %

700

500

STRAIN RATE OF 400% PER MIN.

0 I 2 3 4 5 6

GAUGE LENGTH, in.

Fig. 2. Variation of elongation at rupture with gauge length.

tension at rupture may be deduced by determining the slope of a plot of apparent extension at rupture versus gauge length. The results of such a strategy appear in Fig. 3 and indicate a true extension at rupture of 309 percent.

Emphasis has been placed on determining the nature of the dynamic response of the fiber to various loading circumstances. Two factors of particular in- terest are the modulus of elasticity E and yield stress YS. Since both of these fiber properties are usually affected by rate of straining, data over a wide range of strain rates were taken from which E and YS values were calculated and plotted in Figs. 4 and 5. The yield stress appears to pass through a maximum at about 400 percent per minute after which it stead- ily declines. The modulus appears to decline smoothly with increasing rate of strain. This would indicate that in applications where easy extensibility

GAUGE LENGTH (inches)

Fig. 3. Relution between measured extm'm and m p k ? length.

Fig. 4. Elastic modulus versus strain rate.

635 POLYMER ENG/NEER/NG A N D SC/ENCE, SEPTEMBER, 1975, Vol. 15, No. 9

S. L. Cannon, W. 0. Statton, and J . W. S. Heark

d= YlEW STRESS, psi Table 3b. Initial Modulus and Yield Stress Versus Gauge

Length at Varying Strain Rates

6500 500 loo0 1500 zoo0

STRAIN RATE, PrCMl W mUIY1.

Fig. 5. Yield stress versus strain rate.

10.0 ' ' . I * * , * ' ' ' 1 1 J * . * ' ' " I * , , * ' ' " I * ' , " ' " I S

I00 10' lop 103 lo4 I00

Fig. 6. Maximum stress supported by fiber versus strain rate. STRAIN RATE, % per mln.

is desired it will be achievable by applying either a mild (less than 100 percent per min) or a more stringent (greater than 600 percent per min) rate of strain. This is a most unusual response to strain rate; to our knowledge no other polymer shows a peak of this type.

Further characterization of the mechanical behavior of springy polypropylene when subjected to dynamic loading indicates that the breaking stress (level CD of Fig. 1 ) increases with increasing rates of sample strain, cf. Fig. 6. This behavior is not unusual for fibrous materials although it is certainly an attribute .

The behavior of E with changing sample gauge length is displayed in Table 3a. These data indicate the modulus is only slightly affected by change in gauge length; whereas, the values in Table 3b sug- gest a decrease in E at smaller gauge lengths tested

Tabla 3a. Initial Modulus Versus Gauge Length at Constant Strain Rate

Strain rate, percentlmin Modulus, psi Gauge length, in.

100 382,000 2 50 394,000 4 100 397,000 5 50 410,000 10 100 405,000 10 100 411,000 20

Strain rate, Yield Gauge percentlmin Modulus, psi stress, psi length, psi

55.6 404,000 7550 9 83.3 415,000 7570 6 167.0 400,000 7600 3 500.0 257,000 7630 1

at high rates of strain; whereas, YS appears to be unaffected by changing gauge length and strain rate.

Values of Mechanical Properties After Cyclic Loading

In the case of cyclic loading at extension levels inadequate to produce rupture, SPP exhibits behavior unlike either highly oriented fibrous yams or elastomeric materials. Figure 7 is a reproduction of experimental data obtained for extension cycling. A number of interesting aspects are suggested by this figure, Firstly, path ABC represents the loading history of the sample up to 50 percent elongation, while CA' indicates the path taken as the sample is returned to its original gauge length. The permanent deformation (AA') experienced by the sample is much less than would be expected from the shape of the loading path ABC. This amazing ability of SPP to recover immediately ("elastically") from extensions of 100 to 3000 percent in excess of the "yield point" and even higher has been further researched. SPP specimens were extended to 25, SO, and 100 percent of the original (gauge) length and then allowed to recover. Resulting values of the recovered deformation as a percentage of the designated extension are listed in Table 4. That this fiber does not perform as a material which exhibits the classical stress-strain behavior is clear: an 85 to 90 percent recovery from 100 percent extension (roughly 1000 to 2000 percent of the yield point) is achievable.

A slightly different means of reducing the recovery data leads to a similar conclusion. In this case the value of the initial set (non-recovered deformation) is compared with the specimen gauge length, cf., Table 5. For example, autoclaved SPP subjected to a

CYCLE I - CYCLE 2 ---

C'

I I I 1 0 50 100 150

ELONGATION, %

Fig. 7. Cyclic loading to 50 percent extension.

636 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1975, Yo/. I S , No. 9

The Mechanicd Behavior of Springy Polypropylene

Table 4. Values of Recoverable Deformation as a Percentage of the Specified Extension

Extension, percent Treatment 25 50 100

Dry 92.2 94.3 90.2 0.15" NaCI* 87.2 92.0 85.5 0.90 N NaCI* 88.5 95.0 84.2 Autoclaved 93.8 93.4 87.5 lsoprapanol extracted 94.8 95.6 93.0 Ethanol extracted 94.9 96.2 92.8 Stress relaxed 600 sec 83.6 87.0 81.7

Table 5. Values of Initial Set (mils) Resulting from Various Specimen Treatments and Extension Levels

Extension, percent Treatment 25 50 1 00

~~~~ ~

Dry 18 28 97 0.15 N NaCl* 32 40 146 0.90 N NaCI* 28 24 168 Autoclaved 14 34 126 lsopropanol extracted 13 22 70 Ethanol extracted 76 115 430 Stress relaxed 600 sec 41 65 183

Gauge length 1.0 inch. Crosshead valocity 2.0 inches per minute. * Samples placed in wet cell filled with saline solutions and tested immediataly thereafter.

strain of 100 percent of its original length of 10oO mils would be expected (classically) to exhibit a set of about 950 mils. Table 5 shows the experimentally determined set to be 126 mils or ca. 1/8 of the "nor- mally" expected value. Of course, this argument as- sumes that an extension of 100 percent is attainable when in fact, and even more importantly, the "normal'' form of polypropylene ruptures at 31 percent extension (6.0 in. gauge length and cross-head velocity of 2 in. per min). The recovery ability of SPP is truly superlative to any highly-crystalline, oriented material.

Closer examination of Figs. 7-9 reveals another

CYCLE I - CYCLE 2----

I I 1 0 50 I00 I50

I f--7

CYCLE I - CYCLE 2----

I I 1 I50

ELONGATION, 96

Fig. 8. Cyclic loading to 100 percent extension.

I STRESS, psi

CYCLE I - CYCLE 2 ----

0 50 I00 I50 ELONGATION, Yo

Fig. 9. Cyclic loading to 150 per cent extension.

INITIAL STRAIN= 10% 25%

/ /',.

80

1 ' 1 1 , ' ' ' " C C J ' ' " S J ' ' ' 1 , J ' ' ' 1

10-1 I00 101 102 108 104 TIME, mln.

Fig. 10. Recouey of set, percent us time, min.

unusual property of springy polypropylene: the s e t arising from the initial loading cycle apparently decreases with time. Figure 10 elucidates the transient behavior of this set for four levels of initial strain. As is seen in the figure, a linear recovery of set with the logarithm of time after cycling exists although it is not clear why this phenomena occurs.

Further details of the nature of the elastic recovery (projection of A'C on abscissa in Fig. 7 as a percent of projection AC) appear in Table 6. The data of Table 6 indicate the recovery to be independent of strain rate. This factor further indicates the feasibil- ity of application of springy polypropylene in a mechanically dynamic environment.

Table 6. Elastic Recovery of SPP After Various Cyclic Extension Loadings Under Differing Strain Rates

Cyclic extension, percent 25 75 150

Strain rate, Elastic recovery, percent percentlmin

33 95.3 94.7 76.8 50 94.0 94.7 81.2

100 95.2 94.9 80.7 200 95.6 94.3 77.5 500 100.0 96.0 77.8

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1975, Yo/. 15, No. 9 637

S . L. Cannon, W. 0. Statton, and J. W. S . Hearle

I00

90

80

70

60

50

40

30 0 40 80 120 160 200 240

EXTENSION, %

Fig. 11. Variation of the relatice decrease in yield point for diflering cyclic strain levels.

Secondly, the subsequent loading cycle ( ADC') indicates a marked decrease in the value of the apparent yield stress D. Figures 7-9, and 11 show the variation of this decrease with differing cyclic strain levels; whereas, the data of Table 7 show the variation of this decrease as affected by changing strain rate. Also, after the initial loading cycle, it is clear from Fig. 7 that any specified extension can be at- tained with the application of a lesser load. The practical importance of this feature is that SPP may be mechanically conditioned to produce a range of yield and modulus properties.

Thirdly, the modulus of elasticity E may be calculated by determining the slope of segment AB of Fig. 7. Likewise, E for the second loading cycle is equivalent to the slope of segment AD which by inspection is less than that of the initial cycle. Fur- ther qualitative evidence of this behavior is demonstrated in Figs. 8 and 9. Alternatively, Fig. 12 depicts this increase over a wider range of cyclic

Table 7. Effect of Strain Rate on Decrease in Second Cycle Yield Point

Cyclic extension, percent 25 75 150

Decrease in second cycle Strain rate, percentlmin yield point, percent

33 33.6 57.8 87.1 100 32.1 54.5 91.5 200 31.2 53.5 91.0 500 30.0 53.0 90.2

Table 8. Effect of Strain Rate on Decrease in Second Cycle Modulus

Cyclic extension, percent

Decrease in second cycle 25 is 150

Strain rate, percentlmin modulus, percent

~

33 33.6 58.9 94.5 100 34.0 53.0 93.1 200 26.2 58.9 94.5 500 29.7 59.7 94.0

extension while maintaining a constant strain rate of 33 percent per min. It is interesting that the decrease is linear to above 90 percent (150 percent cyclic extension) although the phenomenological reason for this behavior is not known.

Further examination of the decrease in second cycle modulus as affected by variation in strain rate indicates that over a wide range of strain rates little or no significant change in the decrease of the second cycle modulus is noticeable, cf. Table 8.

The modulus values of Fig. 12 are replotted along with the percent decrease in second cycle yield point in Fig. 13. It would appear that the same trend exists for both parameters. In fact, the modulus points appear to bracket the yield point values even though the former exhibit a wider scatter. The rel- atively wide scatter in E is to be expected because of the difficulty in evaluating a derivative in the prox- imity of AB in Fig. 7.

Fourthly, the ultimate load C' ( F i g . 7) supported by the sample is only slightly less than that of the initial loading cycle. This decrease from C to C' has been determined for various extension cycling from

I SECOND CVUE MODULUS

~

0 50 loo I50 200

CYCLIC EXTENSION, %

Fig. 12. Decrease in second cycle modulus for varying levels of cyclic extension.

B I00 I I I I I I

DECREASE IN SECOND CYCLE MODVLUS PND YlEUl POINT, %

0 50 100 lx) 200

CYCLIC EXTENSION, X

Fig. 13. Decrease in second cycle modulus and yield point cyclic extension level.

638 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1975, Vol. 15, No. 9

The Mechanical Behauwr of Springy Polypropylene

9s n .

5.0 t

I

/

0.0 50 100 150 200 250

CYCLIC EXTENSION, %

Fig. 14. Maximum decrease in h d supporting ability, percent vs cyclic extension, percent.

10 to 200 percent and the results appear in Fig. 14. The decrease in load supporting ability increases to about 17 percent when cycling at 50 percent elongation but further decrease is not substantial and in fact lateaus at about 22 percent. Thus, the lowering of t fe yield stress does not mean a significantly weaker fiber. This characteristic is particularly re- markable when it is remembered that for normal polypropylene the decrease in load supporting ability is 100 percent at 31 percent elongation.

Tabb 9 shows that there exists no reduction in the elongation at rupture after extension cycling at levels up to 200 percent. Thus, there is no embrittlement or weakening of SPP after cycling at major extensions.

Characterization of Stress Relaxation Behavior of SPP

Stress relaxation is an important factor in predic- tion of polymer performance under sustained tensile loading. Figure 15 illustrates the stress-time behavior of SPP as a family of curves produced by varying the level of extension prior to the relaxation step. The relaxation is essentially independent of extension level when the latter is 100 or more percent. It is interesting that in each case the reduction of stress is approximately 40 percent of the initial value after 500 sec.

Figure 16 depicts the effect of changing strain rate upon the relaxation of stress. A 5 percent larger stress decay appears to result from increasing the

Table 9. Extension to Rupture After Cyclic Loading

Cy c I i c extension, Elongation at percent No. cycles rupture, percent

0 25 50 100 150 200 300

0 9

11 9 10 10 10

335 343 366 327 340 336 382

STRESS, psi. * I0,OOO

7,000

6,000

5,000

----___

4,000 200 300 400 500 0 100

TIME, sec. Fig. 15. Stress relaxation as afected by extension level.

strain rate from 2 to 5 in. per min but further increase in strain rate produces relaxation rates within an envelope described by the 2 and 5 in. per minute conditions.

A simple method of assigning a value for the time required for a polymer to relax (reduce) its internal stress by a factor of l/e is given by E q 1.

where u = value of stress at some time t (psi) u0 = initial value of stress (psi), t = time (min), and A = relaxation time ( min ) .

Determination of the relaxation time (A) provides knowledge of the time required for the reduction to 37 percent of the original stress resulting from an applied strain.

Figure 17 compares the relaxation times calculated from E9 1 for nylon 6, Dacron@ polyester, and springy polypropylene yarns. The relaxation time is seen to increase with the time held at a given initial extension which in this case was 10 percent. This approach showed SPP has significantly faster reduction of its internal stress than Dacron@ or nylon 6 fibers from a 10 percent strain.

Another more sophisticated approach to the prob- lem of understanding the effects of stress relaxation


S . L. Cannon, W . 0. Statton, and 1. W . S . Hearle

12,000

I 1,000

I0,OOO

9,000

8.000

7,000

6,000

GAUGE LENGTH 6.0 in.

EXTENSION : 100 %

I I I I I

0 100 200 300 400 500 TIME, sec.

F i g 16. Stress relaxation as afected by initial strain rate.

70 t 50 ““lr P

NYLON 6

DACRON POLYESTER

/ d 30L 20 SPRINGY POLYPROPYLENE

0 I I I I I I 0 2 4 6 8 10 12

TIME, min.

Fig. 17. Var’ation of relaxation time with the transient sampk stress level histo y.

involves the method of collocation (14) which makes use of the relation expressed in Eq 2.

where Eo, = instantaneous modulus (psi) at any time t, c ( ~ ) = value of stress at any time t , 6, = ratio of sample elongation to gauge length, A, = estimate of sample stress level at infinite time (psi), Ai = constants, ai = arbitrary constant chosen in this case to be l/2ti, t = time (sec), and n = one plus the number of the time intervals ti.

Relaxation data were obtained for elongation ra- tios of 10, 50, 100, 150, and 200 percent for strain rates of 2, 5, 10, and 20 in. per min. From these data were calculated the constants Ai which are listed in Table 10. By using these constants, the values of initial modulus, viz., the data appearing in Table 11, can be estimated with less than 3 percent deviation. In fact, this technique will predict the instantaneous modulus for up to three decades of time equally well. The value of these results is that fabrication of prod- uct devices will be facilitated with knowledge of how the material will be expected to respond to static and dynamic loading to extensions in excess of the yield point.

Analysis of Fatigue Behavior of SPP In the case of fibrous materials, there exists a

dearth of both theoretical and experimental informa- tion concerning the mechanics of fatigue failure. Only recently, Bunsell, Hearle, and Hunter (15) have developed a prototype unit enabling collection of S-N data for multifilament yarn and monofilament analogous to traditional fatigue studies of metals. This is the first apparatus for testing fibers under constant load, rather than the more usual (and less desirable) constant strain.

Figures 18 and 19 depict the basic apparatus. The novelty of this unit is the incorporation of a ceramic transducer which is lo00 or so times as sensitive as the common strain gauge counterpart. Although a traditional strain gauge is used to measure the static load component, it is the transducer which enables the monitoring of the oscillating component of the loading cycles. The test specimen is held by clamps one of which is fastened to a V1 vibration generator working on the loud speaker principle. This vibrator may operate between zero and 5000 Hz with a dis- placement of up to 6 mm. Thus by varying the sample length essentially any strain condition may be achieved.

The test conditions were as follows for springy polypropylene. The initial test specimen length was 5 mm and the vibration frequency was 50 Hz. The vibrator was adjusted to give a constant amplitude of oscillation to the lower jaw. Next the load setting was put to a given value so that the servo will operate to drive the top jaw upwards in an attempt to maintain the maximum load equal to the set point value. Strictly speaking, this maximum load is equivalent to the observed mean load plus the oscillating load although averaging complications may exist due to nonlinearity. The polypropylene extended rapidly during the initial stages of the test so that control of the maximum load was not assured until the work softening occurred. This condition is transient and

640 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1975, Vol. IS, No. 9


Table 10. Characterization of Stress Relaxation of Springy Polypropylene by the Method of Collocation

Strain Elongation rate, ratio,

in./min percent A1 A2 Aa A4 A5 Aa

10 50

2 100 150 200 10 50

5 100 150 200

10 50

10 100 150 200

10 50

20 100 150 200

3868.4 490.9 325.3 201.7 64.4

-369.3 1274.8 720.9 514.8 315.0

4823.2 1709.6 1032.3 714.1 502.9

4445.6 1837.7

-918.6 872.5 646.7

932.0 196.8 150.4 79.7 84.8

3921.6 474.1 260.2 142.5 137.9 842.4 340.4 334.6 208.6 141.7

1055.1 495.4

1966.4 50.9

129.8

5728.2 1951.1 1148.5 725.0 505.2

6211.4 2105.5 1351.2 918.4 670.0

6086.3 2215.0 1452.2 918.4 706.4

6261.9 2464.6 2083.4 1043.3 700.1

1762.3 258.0 370.1 205.0 176.0

2222.3 768.6 388.0 260.6 183.2

1973.6 696.8 391.4 260.6 181.1

1809.7 1201.5 463.0 265.4 192.4

9526.1 3252.8 1726.2 1306.0 910.9

8779.5 3173.3 2045.1 1181.2 885.9

8780.0 3493.9 1945.8 1181.2 848.7

8418.5 -269.0

1895.1 1164.7 842.8

9931.1 2781.1 1440.3 711.7 496.5

9932.5 2234.9 993.2 662.3 496.6

9931.0 1987.0 993.0 662.3 496.7

10164.5 10056.0

893.9 662.1 533.9

Table 11. Variation of the Instantaneous Modulus Ew' with Extension and Strain Rate

~~ ~

Extension. percent Strain rate,

in./min 1 10 50 100 150 200

2 343,300 70,400 19,200 10,800 7,200 5,300 5 270,200 69,000 19,700 11,200 7,400 5,500

10 248,200 69,200 20,200 11,600 7,500 5,600 20 2Q2,400 68,400 20,600 12,500 7,700 5,800

*E(t) has in all these cases been evaluated one second after the indicated extension was achieved.

soon settles down. After this interval, only occasional perturbations of small amplitude were noticeable. After this stage was reached the loads were re- corded.

The data acquired by this technique have been plotted in classical S-N form (stress level ratio versus corresponding number of cycles at rupture) and appear in Fig. 20. The indication is that the endurance limit of SPP is about 0.20; the material can survive essentially an infinite number of loading cycles so long as the loading amplitude is 20 or less percent of the tensile breaking strength.

Superficially, it might appear that this endurance limit is quite low, e.g., a 4340 (AISI designation) steel has an endurance limit of about 2.5 times that of SPP. However, the test conditions of the two cases are extremely different. The SPP fiber was cyclically loaded at an initial extension corresponding to about 50 percent of the specimen length, whereas the steel was loaded at only a few percent. Furthermore, the steel could never be extended to 50 percent.

The specification of the vibrational testing frequency of 50 Hz is another factor which tends to

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1975, Vol.

produce a conservative value of the endurance limit in Fig. 18 since the limit is generally known to be somewhat sensitive to strain rate at higher levels of amplitude. Even so, testing at this frequency is justi- fiable: testing at 1 to 3 Hz would require more than 25 times the machine time and therefore would not be economically feasible. The longest duration trial of Fig. 18 if run at 1 Hz would have required approximately 208 days.

In each of the SPP trials the break was a typical tensile fatigue break with a long tail stripped off one end. Remote from the break there was cracking on the fiber surface. This type of axial splitting is typical

Dial

--I_ _____._____

Fig. 18. Load cycling fatigue apparatus.

15, No. 9 641

S . L. Cannon, W. 0. Statton, and I. W. S . Hearle

- High input Oscillator

Process timer

Comparator with variable

r--- I I

I Motor control I I

I Feedback via test r ig and mater ial under test re loy

1 ------ + _ _ _ _ _ _ _ L _ _ _ _ _ - _ - f _ _ - _ _ _ _ _ _ _ _ _ - - - - - --

Fig. 19. Block circuit diagram.

10 , , , , , , , ~ , , , , , , , , , , , , , , , , , , , I

out u t I i P ~ stale buffer transducer -

LG+- STRESS RATIO

Pietoelect r ic resi st Once Rectifier + f i l t e r a n d - m

I I [oscilloscope ] " Summing LOW pass I * amplifier * f i l t e r +

I I

I I

t I

I Strain gauge Differential r - - - - bridge A amplifier -$iq

0.0 105 Id 10' 108 I 09

CYCLES TO RUPTURE

Fig. 20. Stress ratio versus cycles to rupture.

Maximum load

!I

of all fibrous materials which have been studied by this technique.

These initial results are gratifying in showing that springy polypropylene has no abnormal fatigue behavior. At moderate extensions it can survive in- definitely. At higher extensions, the reduction in strength due to fatigue is between one-half and two- thirds, as is found with other fibers.

Complex Dynamic Mechanical Behavior of SPP A number of measurement techniques can provide

increased understanding of the complex behavior of polymeric materials at elevated temperatures. For example, the temperature dispersion of the complex modulus of elasticity E" and the tangent 6 profile are recognized as being related to transition phenomena and describe various states of high polymers such as crystallized, or glassy, or rubbery. A convenient commercial instrument for obtaining these data is the Rheovibron, and it was used to study springy polypropylene.

Figure 21 depicts the profiles of tan 6 vs temperature at four vibrational frequencies which span from -20 to 30°C or so. The importance of identifying these temperature spans and their bounds lies in the knowledge that when the material is in service at these temperatures it absorbs strain energy but this

0-02 t - 110 HZ ---4 35 HI --.---a- I I HZ SPP

Q 3.5 Hz __-

0.00 -33 -20 -10 0 10 20 30 40

TEMPERATURE, O C

Fig. 21. Tan u 11s temperature, "C.

absorption is not totally reversible, i.e., a portion of the strain energy is dissipated internally by frictional forces, resulting in the evolution of a quantity of heat. Therefore, under conditions of long-term cyclic stressing, it is advisable to refrain from subjecting the candidate material to service at these temperatures since the life expectancy of the specimen may be foreshortened.

Since the basic difference between SPP and the normal type polypropylene (NPP) is one of morphology and since morphological variation might be expected to promote varying energetic behavior to this type loading, comparison of the tan 6 profile of SPP and NPP was made at loading frequencies of 3.5, 11, and 110 Hz. Figures 22-24 indicate categori- cally the existence of a 10 to 15 deg difference in the tan 6 peak maximum between the two fibers. Also in each instance the NP exhibits the larger value. In addition, the peak breadth of the SPP appears to be characteristically more narrow at higher vibrational frequencies. In summary, the SPP fiber ir- reversibly absorbs strain energy at lower temperatures than NPP and the absorption occurs over a smaller band of temperatures.

The apparent activation energies ( A H " ) for the relaxation process were determined from a plot of

642 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1975, Vol. 75, No. 9


? 0.04

3.5 Hr

0.00 -30 -20 -10 0 10 20 30 40

TEMPERATURE, OC

Fig. 22. Tan u vs temperature.

0.00 -30 -20 -10 0 10 20 30 40

TEMPERATURE, O C


0 SPP

0 NFQ I - --

-30 -20 -10 0 lo 20 30 40 50

TEMPERATURE, 'C


log frequency ( f ) versus the reciprocal of the ab- solute temperature at the loss peak maximum using the following relation:

(3)

The resulting activation energies were 37.14 and 35.16 kcal/mole for SPP and NPP, respectively. These values are in the range expected for major glass transition relaxation phenomena.

Abrasion and Wear Resistance Standard testing procedures have been estab-

lished for the evaluation of abrasion and wear resistance of textile structures and proto-structures, d. ASTM Method D1379-64. The abrasion and wear resistance of a tubular knit fabric of springy polypropylene (SPP) have been compared to an identi- cal specimen of normal polypropylene (NPP). The results indicate that no significant Merence exists betwen the two: five Stoll Flex Tests on the SPP fabric averaged 310 cycles at rupture; whereas, six tests on the NPP fabric averaged 326 cycles at rupture. Stoll Testing of SPP, NPP, and Dacron@ yarns indicates no significant difFerence in abrasion resistance exists between SPP, NPP, and Dacron@ 68. Table 12 includes ,the results from these tests.

Mechanical Properties of SPP in the Wet State SPP samples have been conditioned in aqueous

and saline mediums for various intervals of time to determine the extent to which the mechanical properties, e.g., extension at rupture and yield point, have been altered. Figure 25 indicates that the extension at rupture of SPP is essentially unaffected by treatment with distilled water; whereas, conditioning in 0.9 N saline solution effects a decrease of some 200 percent in the elongation at rupture. Condition- ing the material in a 0.15 N saline solution gives results which indicate that no reduction in elongation has occurred. Also, the effect of conditioning in saline solutions upon the work softening characteristics of the polymer is not significant as shown by the results of Tables 13 and 14. There is negligible deviation of the experimental values from the control

Table 12a. Results of Flat Abrasion on SPP and NPP Tubular Knit Fabrics

Abrasion cycles SPP NPP

419 396 224 292 211

Averages 310 -

566 294 3 16 210 416 155 326

Table 12b. Results of Abrasion Testing of Yams

Abrasion cycles SPP NPP Dacron@ 88

8642 9011 8427 8359 9100 8535 - -

Averages 8679

8360 8614 8820 8643 9088 8622 - -

869 1

8780 8603 8900 9159 9946 8638 8973 9046 9006


S. L. Cannon, W . 0. Statton, and 1. W. S. Hearle

% x 10-2

5t 6 in. GAUGE

CONDITIONING MEDIUMS 0 DISTILLED WATER

0.1 5 N SALINE SOLUTION 0 0.90N SALINE SOLUTION 0 AUTO CLAMD

LENGTH 2 in./rnin. CROSSHEAD VELOCITY I

~

5 10 15 20 DAYS

0 4 8 12 16 20 24 28 C O N D I T I O N I N G T I M E , mtn. x 10-3

Fig. 25. Extension at rupture vs conditioning time.

Table 13. Mechanical Behavior of Springy Polypropylene After Various Treatments

~~ ~ ~

A. Extension at rupture: strain rate = 2 in./min, gauge length 1 in.

Treatment Extension at rupture, percent

Dry (untreated) 0.15 N NaCI* 0.90 N NaCI* Autoclaved lsopropa no1 extracted

446.0 448.7 460.7 429.0 381.5

B. Initial yield point decrease (percent) during initial loading cycle

Treatment Extension, percent

25 50 100

Dry (untreated) 29.6 42.9 66.2 0.15 N NaCI** 27.7 43.6 69.4 0.90 N NaCI** 29.0 45.6 67.9 Autoclaved 29.1 47.1 63,6 lsopropanol extracted 29.0 40.0 64.2 Ethanol extracted 33.4 42.1 66.5

Table 14. Percent Decrease of Yield Point Versus Conditioning Time

Conditioning time in minutes, days Cyclic extension, per- 0 1,440 2,880 5,760 7,200 48,960 56,160 cent Control (1) (2) (4) (5) (34) (39)

25 29.6 27.7" 31.2 30.4 26.4 27.8 - 50 42.9 43.6 43.2 43.1 38.0 40.5 -

100 66.2 69.4 62.6 66.4 64.3 63.6 -

29.0** - - - - 27.9

45.6 - - - - 46.1

67.9 - - - - 63.7

*Upper box values represent the results from conditioning in a 0.15 N saline solution. **Lower box values represent the results for conditioning in a 0.90 N saline solution. Control values are for the dry (untreated) fiber.

Table 15. Percent Decrease in Second Cycle Modulus Versus Cyclic Extension Level for Various

Conditioning Treatments

Treatment

Cyclic extension, percent

25 50 100 Decrease in modulus,

percent

Dry (control) 21.4 56.6 61.8 0.15 N NaCl for 24 hours 13.2 33.0 59.4 0.90 N NaCl for 24 hours 13.6 32.2 65.4 Autoclaved-30 minutes a t 250°F 23.5 42.8 70.3

Sample gauge lengths were 1.0 in. and crosshead speed was 2.0 in./min.

Table 16. Modulus and Breaking Stress of Samples Conditioned in 0.15 N Saline

~~~~ ____

Breaking stress, Conditioning time, days Modulus, psi psi

~

0 141,000 12,400 0.2 109,000 12,300 1 152,000 12,300 4 189,000 12,400 5 124,000 12,000

34 153,000 12,200 201 312,000 12,200

*Samples placed in test cell containing saline (no prior conditioning). ** Samples conditioned one day prior to testing.

which was obtained for the yarn in the dry, untreated state. All these determinations were made using a 6-in. gauge length and a strain rate of 2.0 in. per min unless otherwise specified. Table 15 contains values of this second cycle modulus decrease after various conditioning treatments (select values of dry testing have been repeated to serve as a control). Table 16 further identifies the behavior of the modulus and breaking stress after sample conditioning in 0.15 N saline for up to 201 days. The magnitude of variability in determining these modulus values is

such that no significant difference exists between any entry in Table 16.

The data of Table 17 indicate that the unusual elastic behavior of SPP is not adversely affected by saline conditioning for extended periods of time.

It can be concluded that neither conditioning in 0.15 N saline nor sterilizing by autoclave significantly reduces the elastic modulus or breaking stress, or elastic recovery with comparison to the dry state (control) values. These results are important to consideration of SPP as a biomaterial in that blood is roughly this normality saline and autoclaving is a convenient means of sterilization.

644 POLYMER ENGINEERING A N D SCIENCE, SEPTEMBER, 1975, Vol. 15, No. 9


Tabk 17. Elastic Recovery from Various Extension Cycling After Conditioning in Saline Solutions

Cyclic extension, percent 25 50 lii6

Conditioning Treatment time, days Elastic recovery, percent

Dry 0 0 1 2

0.15 N NaCl 4 5

34 201

0 0.90 N NaCl 1

39

92.2 81.2 91.2 93.0 94.4 93.3 91.5 99.0 88.5 91.8 91.1

94.3 92.0 94.0 93.6 94.8 90.0 94.3 S.4 95.0

90.4 -

90.2 85.5 91.4 90.6 91.8 92.9 90.5 92.6 84.2 90.6 91.2

CONCLUSIONS 0 SPP is a high modulus fiber which may be ex-

tended several hundred percent of its original gauge length prior to rupture. The degree of this extension is dependent upon gauge length and, to a lesser extent, strain rate.

0 SPP fiber has demonstrated the amazing ability to recover immediately (“elastically”) from extensions of 100 to 3000 percent in excess of the “yield point”.

0 The “initial set” (nonrecovered deformation after extension beyond the yield point) of conditioned polymer after being subjected to extensions of 25, 50, and 100 percent of original length was in all cases approximately 10 percent.

0 The initial set decreases (“heals”) with time. A linear recovery of set with the logarithm of time after cycling was shown to exist.

0 SPP may be work softened by extension cycling. The degree of reduction in modulus and yield stress increases as the percent extension cycle increases; however, the concomitant change in the ultimate load supporting ability is slight.

0 Conditioning of SPP in 0.90 N NaCl for extended periods produces a reduction in elongation at rupture; otherwise, the various conditioning treatments produced no significant deterioration of mechanical properties.

0 The elongation at rupture of SPP is not in- fluenced by cyclic extension at any level up to 300 percent.

0 The method of collocation (14) adequately models the stress relaxation behavior of SPP.

0 SPP exhibits no abnormal fatigue behavior: the break was a typical tensile fatigue break with a long tail stripped off one end.

0 The complex thermomechanical behavior of SPP is different from the normal type of polypropylene and this difference is quite likely due to morphology.

0 The abrasion and wear resistance of SPP fabric and yarn is not significantly different from NPP and Dacron@ 68.

ACKNOWLEDGMENT This work has been conducted under the auspices

of the National Heart and Lung Institute, contract number NIH-N.HL1-73-2912.

We also acknowledge the contributions of the following individuals: Stephen D. Bruck, Jack D. Towery, William T. Petuskey, Robert Beckstead, Richard Morrison, and Mrs. Ludmilla Konopasek.

REFERENCES 1. V. A. Kargin and I. Yu Tsarevskaya, Vysokomol. Soedin.

2. A. J. Herman, (to E. I. duPont de Nemours and Com-

3. H. G. Ingersol, 1. Appl. Phys., 17, 924 (1946). 4. H. G. Owens and W. 0. Statton, Acta Cyst., 10, 560

( 1957). 5. R. G. Quynn, 7th Ann. Synthetic Fibers Symp., Tide-

water-Virginia Section Am. Inst. Chem. Eng., Williams- burg, Va., 1970, un ublished.

6. R. G. Quynn, et a!, 1. Macromol. Sd.-Phys., B4, 953 (1970).

7. E. S. Clark, Poly. Sci. and Tech., I , in “Structure and Properties of Polymer Films,” R. W. Lenz and R. S. Stein eds., p. 267, Plenum Press, N.Y. (1972).

8. B. S. Sprague, japan-United States Conf. “Polymer Solid State,” Case Western Reserve University, Cleveland, Ohio. 1972, to be published. 1. Macromnl. Sci.

9. F. W. Knoblach and W. 0. Statton, (to E. I. du Pont de Nemours and Company), U. S. Patent 3,299,171 ( 1967).

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8, 1455 (1966).

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13. H. D. Noether, private communication (Nov. 1972). 14. R. A. Schapery, Proc. 4th US. Nut. Cong. Appl. Mechs.,

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E : Sci. Instru., 4,868 (1971).


the mechanical behavior of springy polypropylene

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