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IMBSIR 80-2008

Physical Testing of Polymers for

Use in Circulatory Assist Devices

Robert vV. Penn, Ph D., Prncioal Investigator

Gregory B McKenna. Ph 0., Project Leader

FrecWy A. Khoury. Ph D., Project Leader

Polymer Science and Standards Division

National Bureau of Standards

U S. Deoartrnent of Commerce

Washington, D.C, -20234

Second Annual Report

October 1979

Issued March 1 980

Reimbursable Agreement

Y01-HV-8-0003

Prepared for Devices and Technology Branch

Division of Heart and Vascular Diseases

tionai Heart, Lung, and Stood Institute

tionai Institutes of Health

I )

NBSIR 80-2008

PHYSICAL TESTING OF POLYMERS FOR

USE IN CIRCULATORY ASSIST DEVICES

MATrOHAL BUREAU

OE BTAKDARM

UBEABT

NOV 2 4 1980

ac.- : Q u I Oc-

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Robert W. Penn, Ph.D., Principal Investigator . Gregory B. McKenna, Ph.D., Project Leader

Freddy A. Khoury, Ph.D., Project Leader

Polymer Science and Standards Division

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

Second Annual Report

October 1979

Issued March 1980

Reimbursable Agreement

Y01-HV-8-0003

Prepared for Devices and Technology Branch

Division of Heart and Vascular Diseases

National Heart, Lung, and Blood Institute

National Institutes of Health

U.S. DEPARTMENT OF COMMERCE. Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
aA«x>rr*«

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Preface

This report presents results obtained in a research program on

Physical Testing of Polymers for use in Circulatory Assist Devices

carried out at the National Bureau of Standards with the support of the

National Heart, Lung and Blood Institute of the National Institutes of

Health. The report covers work performed in the period July 1, 1978 to

September 30, 1979.

1
Abstract

This is the second annual report on a project directed to the

development of accelerated physical testing procedures and to the evaluation

of candidate elastomers for use in circulatory assist devices. Three

candidate materials are under study along with a standard butyl rubber.

The three candidate materials are a polyolefin rubber, a urethane-silicone

copolymer, and a segmented polyurethane elastomer. During this report

period, all of these materials have become, or are becoming, available

in adequate supply.

We report results from both uniaxial and biaxial testing with both

static and dynamic stress histories. Our results show that in static

uniaxial loading the butyl rubber is superior to the polyolefin at short

times (high stresses) but that this behavior reverses at long times (low

stresses). Under dynamic (fatigue) loading the failure times decrease

with increasing test frequency, but not rapidly enough to say that the

samples fail after a constant number of cycles. Statistical • analysis of

the failure data shows that there is greater variability in the failure

times for the polyolefin rubber than for the butyl rubber in all tests

in which a comparison can be made.

Biaxial fatigue test results show that the urethane-silicone copolymer

lasts longer than the polyolefin rubber, which lasts longer than the butyl

rubber when they are compared at the same reduced pressures. For both the

polyolefin rubber and the butyl rubber, the dependence of lifetime on stress

is the same in static and fatigue loading. The butyl rubber in both cases

shows a higher stress dependence. In the case of both rubbers, the variability

in the failure times is greater in static testing than in dynamic testing.

Data for the butyl rubber and preliminary results for the urethane silicone

copolymer from biaxial fatigue tests at 23® and 37°C reflect activation
energies for the failure process of 22 kcal and 25 kcal respectively.

Creep data under uniaxial loading are reported for all four elastomers.

Creep rates range from 1.7% per decade of time for the segmented polyurethane

elastomer to 4% per decade for the polyolefin rubber.

Examination of the urethane-silicone copolymer by optical microscopy

shows heterogeneities on a coarser scale than has been previously reported.

These structures consist of flattened spheroids with their short axes

perpendicular to the plane of the sheet. Their sizes parallel to the plane

of the sheet vary from a few micrometers up to about 30 pm.
Table of Contents Page

Title Page

Preface i

Abstract ii

Table of Contents iv

List’ of Tables . v

List of Figures • vi

I. Introduction ”

1

II. Current Research 3

A. Test Materials 3

B. Material Failure Behavior 6

(1) Test Methodology 6

(2) Uniaxial Testing 8

(3) Biaxial Testing 19

i C. Creep Behavior of Candidate Elastomers 32

(1) Background 32

(2) Uniaxial Creep 35

(3) Equi-biaxial Creep 35

D. Morphological Characterization 45

III. References 52

iv
List of Tables

1. Test Materials Received

2. Time to Fail Parameters for Uniaxial Static Tests

3. Least Squares Parameters for Polyolefin Uniaxial Dynamic Tests

4. Time to Fail Parameters for Uniaxial Dynamic Tests

5. Regression Parameters for Biaxial Tests

6. Creep Exponent, n, for Elastomers in Uniaxial Creep

V
List of Figures

1. Time to Fail Versus Stress for Uniaxial Static Tests of Polyolefin and

NBS Butyl Rubbers.

2. Histogram for the Distribution of Deviations for NBS Butyl Rubber in Uniaxial

Static Tests.

3. Histogram for the Distribution of Deviations for Polyolefin Rubber in Uniaxial

Static Tests.

4. Time to Fail Versus Peak Stress for Uniaxial Dynamic Tests of Polyolefin Rubber

5. Time to Fail Versus Test Frequency for Uniaxial Dynamic Tests.

6. Histogram for the Distribution of Deviations for Polyolefin Rubber in Uniaxial

Dynamic Tests.

7. Apparatus for Static Biaxial Testing.

8. Time to Fail Versus Reduced Pressure for Biaxial Static Tests.

9. Histogram for the Distribution of Deviations for NBS Butyl Rubber Static

Biaxial Tests.

10. Histogram for the Distribution of Deviations for Polyolefin Rubber Static

Biaxial Tests.

11. Dynamic Biaxial Test Apparatus.

12. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests of

NBS Butyl Rubber at 23°C.

13. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests of

NBS Butyl Rubber at 37°C.

14. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests of

Polyolefin Rubber at 23®C.

15. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests of

Urethane Silicone Copolymer at 23°C.

16. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests of

Urethane Silicone Copolymer at 37®C.

17. Histogram for the Distribution of Deviations for NBS Butyl Dynamic

Biaxial Tests.

13. Histogram for the Distribution of Deviations for the Polyolefin Dynamic

Biaxial Tests.

19. Uniaxial Creep for NBS Butyl Rubber at 23®C.

20. Uniaxial Creep for Polyolefin Rubber at 23°C.

VI
21. Uniaxial Creep for Urethane Silicone Copolymer at 23°C.

22. Uniaxial Creep for Polyurethane Elastomer at 23°C.

23. Biaxial Creep for NBS Butyl Rubber at 23°C.

24. Biaxial Creep for Polyolefin Elastomer at 23°C.

25. Biaxial Creep Exponent, n, vs. Reduced Pressure for Polyolefin Rubber at 23°C.

26. Photomicrograph of Urethane Silicone Copolymer Cut Perpendicular to the

Surface of the Sheet (lOOx).

27. Photomicrographs of Urethane Silicone Copolymer. Higher Magnification of

Figure 26 (250x).

28. Photomicrographs of Urethane Silicone Copolymer. Higher Magnification of

Figure 26 (250x).

29. Photomicrograph of Urethane Silicone Copolymer Cut Parallel to the Surface

of the Sheet.

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I. Introduction

The National Heart, Lung and Blood Institute of the National Institutes

of Health has been supporting the development of artificial hearts and

related circulatory assist devices for approximately fifteen years. One

of the current goals of this program is to develop a human implantable

left ventricular circulatory assist device. This program has been

divided into three interrelated projects:

1. Pump Design and Development

2. Power Systems Development

3. Biomaterials Development.

Current configurations of the pump devices include a collapsing bladder

or diaphragm with a pusher plate mechanism for circulating the blood.

The bladders and diaphragms as well as tubulature and valves in these

devices are frequently constructed of elastomeric materials. These

materials must meet rigid requirements for thrombo-resistance, biocompatibility, and mechanical durability.

One part of the biomaterials effort is the preparation of new

materials with suitable blood compatibility. This report describes

efforts directed towards determining the mechanical durability of these

new biomaterials. In such applications, an elastomeric bladder or diaphragm

is expected to undergo cyclic stress or strain histories at a frequency of

approximately one hertz for periods of many years. Test methodologies for

characterizing the mechanical durability of such materials do not exist.

We are developing a methodology for describing the mechanical durability

of the candidate elastomers. The methodology is based on previous work

using a cumulative damage rule as a framework to describe time dependent

failure of glassy and semi crystal 1 ine polymers.

1
In the initial phase of this research, three materials were selected

for beginning the study. These were a polyolefin rubber, a segmented polyurethane elastomer, and a urethane silicone copolymer. In addition, we

prepared and distributed a quantity of NBS Standard Butyl Rubber to the

other contractors.

This report presents preliminary results directed towards the- objectives

t of developing test procedures and the evaluation of candidate materials for

use in circulatory assist devices. The test procedures should permit the

use of accelerated tests to predict and evaluate material performance over

long time intervals from relatively short time tests. As a basis for this

program, we use the concept of additive damage which was presented in the

pevious annual report (2). !|-

The additivity of damage framework suggests that the testing protocol

be designed to look at four aspects of failure behavior:

[

1. Failure times under static loads ;

2. Failure times under dynamic loads •

3. Frequency dependence of failure times in dynamic loads

4. Statistics of failure

We have been studying the failure behavior of blood pump elastomers in

both uniaxial and equibiaxial states of stress using a testing and analysis

protocol which includes the above four points.

Most of our data to date have been obtained for the polyolefin rubber

and the standard butyl rubber with work in progress on the other elastomers.

Our results show that the uniaxial failure behavior under static loads for

the butyl rubber is superior to that of the polyolefin rubber at short times,

but that the polyolefin rubber is superior at long times. Under fatigue

loading conditions, the failure times for both rubbers decrease with

2
increasing test frequency. The statistical analyses of the failure data

show that the variability in the failure times for the polyolefin rubber

is greater than that for the butyl rubber. Biaxial fatigue test results

show that the urethane silicone copolymer lasts longer than the polyolefin

rubber which lasts longer than the butyl rubber when the comparison is

made at the same reduced pressures. The variability in the failure times

for the polyolefin rubber is greater than that for the butyl rubber for

both static and dynamic tests.

Data for the butyl rubber and preliminary results for the urethane silicone

copolymer from fatigue tests at 23° and 37°C reflect activation energies

for the failure process of 22 kcal and 25 kcal respectively.

Creep data under uniaxial loading are reported for all four elastomers.

Creep rates range from 1.7% per decade of time for the segmented polyurethane

elastomer to 4% per decade for the polyolefin rubber. Under biaxial

loading, data have been obtained for the butyl rubber and the polyolefin

rubber. The creep rate for the butyl rubber is 9.1% per decade of time

and for the polyolefin rubber it varies with increasing pressure from

5.1% per decade to 14% per decade over the range of reduced pressures studied.

II. Current Research

A. Test Materials

Specimens of four elastomers have been employed in this program. They

are designated as:

1 . Polyolefin Rubber

2. NBS Butyl Rubber

3. Urethane-silicone Copolymer

4. Segmented Polyurethane

3
Table 1

Materials Received

Material Date

Received

Number of

Sheets

Thickness

(mils)

Urethane-Silicone

Copolymer 5/9/79 12 25

" (reject) 5/9/79 10 25

Urethane-Silicone

Copolymer 6/27/79 15 15-18

II 9/25/79 35 15-18

II II 15 11-14

" (reject) 11 50 16

Polyolefin 9/31/78 3 40

II 10/9/78 4 40

II 11/29/78 11 40

II 9/24/79 6 40

Segmented

Polyurethane 3/30/79 2 10

II II 2 30

II 8/9/79 2 10

II II 4 30

12 X

6 X 6

6 X 6 6x6

I 6 X 6

7 X 1

7 X 1

II

4
Table 1 lists the materials received to date.

(1) Polyolefin Rubber

The polyolefin rubber is a carbon black filled poly(hexene) which

is sulfur vulcanized by the Goodyear Corporation under the trade name

"Hexsyn" and is subsequently extracted with an acetone-toluene mixture at

the Cleveland Clinic laboratories. In clinical applications, this material

is prepared with a porous surface to which is attached a blood compatible

"biolized" layer. The samples supplied to date have neither the porous

surface nor the blood compatible coating. Dr. Carl McMillin of Monsanto

Research Corporation has been assigned the responsibility of supplying

this material and further details of the preparation and characterization

of this material should be available in the annual report of that group.

(2) NBS Butyl Rubber

The NBS Butyl Rubber is a standard compound prepared according to

the ASTM D-3188-73 Formula lA, using NBS Standard Reference Material 388j

It is a sulfur vulcanized synthetic isoprene-isobutylene copolymer. This

formulation was chosen as a material with properties in the range of

those of the candidate CAD materials and because it has a known history of

good property uniformity and reproducibility. Four hundred 6-inch square

sheets 0.040" thick were prepared and distributed to the three physical

testing program contractors for interlaboratory comparisons.

(3) Urethane-Silicone Copolymer

The urethane-silicone copolymer is prepared by Avco-Everett Research

Laboratories under their trade name "Avcothane-51" . It is a block copolymer

of a poly-ether urethane and a poly-si loxane. It is prepared as a prepolymer

solution in a mixture of dioxane and tetrahydrofuran. Sheets of elastomer

are prepared by repeatedly dipping flat plate glass mandrels in the prepolymer
solution until the desired thickness is obtained. The sheets are then

exposed to atmospheric moisture which completes the curing of the elastomer.

We have distributed a total of 65 one-foot square sheets of material

which were certified by the Avco Everett Laboratory for chemical composition

and thickness uniformity. In addition, we have distributed another 60

sheets of; material which were rejected because of foreign inclusions,

streaks or uneven thickness. These sheets are believed to be chemically

satisfactory and could be used for small samples cut from unflawed areas.

(4) Segmented Polyurethane

The segmented polyurethane was prepared as a solution of polyether

urethane in dimethyl acetamide by Ethicon, Inc. under the trade name

"Biomer". It was fabricated into sheet form by centrifugal casting by

Thoratec Laboratories Corporation. We have received 10 sheets approximately

7 X 10 inches. Dr. John Kardos of Washington University has been assigned

the responsibility for acquiring and distributing this material and further

details of the fabrication and chemical characteristics of this material

should be available in the annual report of that group.

B. Material Failure Behavior

(1 ) Test Methodology

We have developed a useful methodology for describing the mechanical

durability of elastomers which are candidate materials for blood pump

applications. This methodology is based on previous work using a cumulative

damage rule as a framework to describe time dependent failure of both glassy

and semicrystalline polymers [1, 2, 3J.

The concept of cumulative damage depends on the assumption that material

failure is the result of damage accumulation due to stress. When the damage

reaches a critical value failure occurs. The Bailey [4] criterion and the

form of the rule which we will use assumes that the rate at which damage

6
accumulates is a function only of the current stress, a. The time to fail,

tn, is related to the stress history, a(^), by the following equation:

D

/

( 1 )

0

where Tg(a) is the time to fail in constant stress experiments. Thus, each increment

of time, dC during which the load is a(^), is weighted inversely as the lifetime,

Tg(a), which the specimen would have had under a constant stress, a, and the

sample should fail when the value of the integral reaches one.

The additivity of damage framework suggests that the testing protocol be

designed to look at four aspects of failure behavior:

1. Failure times under static loads

The failure times under static load determine the function rg(cr) which will

be used with equation 1 to predict failure behavior under other load histories.

2. Failure times under dynamic loads

The deviations of the observed failure times under dynamic loads

from those predicted by equation 1 and the static load data will provide

guidance for the prediction of endurance in applications.

3. Frequency dependence of failure times in dynamic loads

The additivity of damage rule predicts that failure time should be

independent of test frequency and frequency dependence data to test this

prediction are important in determining the validity of accelerated test

methods which use high frequency test data.

4. Statistics of failure

Previous results obtained for glassy and semi crystal line polymers

show that the variability in the failure times changes quite significantly

7
in going from one stress history to another. Various failure models do not

make this prediction. It is important to obtain data to check the

validity of these predictions.

(2) Uniaxial Testing

Uniaxial failure data have been obtained for NBS Butyl and Polyolefin

rubbers. Testing of the other two elastomers has been initiated. The

tests were conducted at 23 + 1°C in air. Each specimen's width and

thickness were measured prior to testing. Static data were obtained by

hanging weights on samples and observing the failure time. These tests

were conducted to provide a stress range of between 3 and 16 MPa. Time

to failure was recorded for each specimen.

Dynamic testing was conducted using an Instron* servo-controlled

hydraulic testing machine which is interfaced with a Hewlett-Packard*

minicomputer for control and data acquisition. The test machine is

commanded by voltages generated by the computer. Sinusoidal fatigue

tests were run by generating a sinewave of 200 steps per cycle for tests

_3 run at frequencies from 2 x 10 to 0.09 Hz. The fatigue tests

were conducted from zero to peak stresses. Peak stresses of from 6.2 to 10.3 MPa

were used for the polyolefin rubber. Tests on the butyl rubber were conducted at

5.5 MPa. Statistical analyses were performed on both constant stress

and fatigue data.

The static test results are depicted in a plot of the logarithm of

the time to fail versus stress in Figure 1. The data were fitted by

least squares to the model

iln tg = A + Ba (2)

*Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification

imply recommendation or endorsement by the National Bureau of Standards, nor does

it imply necessarily the best available for the purpose.

8
Time

to

Fail,

i I I I 0 5 10 15 20 25

Stress, MPa

Figure 1. Time to Fail vs. Stress for NBS Bu\vl and Polyolefin Rubbers. Uniaxial

Creep in Air at 23°C.

9
where is the time to fail in seconds and a is the stress in MPa. As

D can be seen, the fit is quite good over the stress ranges covered. The

parameters determined for the two rubbers are shown in Table 2 with

their correlation coefficients, R.

To illustrate the variability in the failure times, we assume that the

distribution of the deviations of the logarithm of the failure times about

the regression line is homoscedastic in the stress. Deviations from equation

2 are plotted as histograms in Figures 2 and 3, Both distributions appear

symmetrical. That for the polyolefin rubber is much broader than that

for the NBS Butyl rubber.

The assumption that the distribution of the deviations in the logarithm

of the failure times is homoscedastic was tested by individually fitting

the half of the data points having the highest stresses and the half having

the lowest stresses. The variances from the two fits were then compared

in an "F" test. We performed the "F" test for these uniaxial • static tests

and for all the other uniaxial and biaxial tests for which we had 17 or more

data points. In no case could the assumption of homoscedasticity be rejected

at the 95% confidence level.

The dynamic test data for the polyolefin rubber were fitted to equation

2 by the least squares technique. The resulting parameters and correlation

coefficients are displayed in Table 3. The results for the polyolefin rubber

are summarized in Figure 4. As can be seen, the dynamic test lifetimes are

much shorter than those predicted by the additivity of damage rule, and the

lifetimes are a strong function of test freouency which is contrary to the

additivity of damage prediction. We note that the slopes (B in eq. 2) for

the dynamic test data are considerably lower than those for the static tests.

To further illustrate the frequency dependence of the fatigue

lifetimes, we cross plot the dynamic test data in Figure 5 together with

the results for NBS Butyl rubber. The logarithm of the time to fail, tg,

is plotted versus the logarithm of the test frequency, oj. We show
Table 2

Time to Fail Parameters for Uniaxial Static Tests*

A B R N

NBS Butyl 22.05 -.951 -.962 39

Polyolefin 29.44 -1.848 -.877 20

*Fit to iln tn = A + Ba where tg is the time to fail in seconds,

and a is the stress in MPa, and R is the coefficient of correlation.

N is the number of samples.

Table 3

Least Squares Parameters for Polyolefin Uniaxial Dynamic Tests*

FREQUENCY, Hz A B R N

0.002 18.36 -.773 -.989 6

0.01 17.31 -.854 -.828 14

0.09 14.56 -.689 -.732 15

*Fit to £n A + Ba where tn = time to failure in seconds and a is the peak

stress in MPa.

R is the coefficient of correlation.

N is the number of samples.

11
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03 CNI CM

II II

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12

Figure

2.

Histogram

of

Deviations

from

Least

Squares

Line

for

NBS

Butyl

Rubber

in

Uniaxial

Creep

to

Failure

Tests,

In

Air

at

23°C.
CD

<N

O

CNJ

I

C

eaniiejj |o Aouenbejj

13

Figure

3.

Histogram

of

Distribution

of

Deviations

from

Least

Squares

Line

for

Polyolefin

Uniaxial

Creep

Data.
Time

to

Fall,

0 5 10 15

Peak Stress, MPa

Figure 4. Time to Fail vs. Peak Stress for Uniaxial Fatigue of Polyolefin Rubber

In Air at 23°C.

14
Time

to

Fall,

Figure 5. Time to Fail vs. Test Frequency for MBS Butyl and Polyolefin Rubbers.

Uniaxial Fatigue in Air at 23°C.

15
only the least squares lines. These dynamic (fatigue) data were fit to a

multiple regression model of the form

£n tg = A + B£n co + Co (3)

The statistics were used to examine the hypothesis that fatigue failure

in the two carbon black filled rubbers is a "cycle dependent process" (i.e.

fatigue failure is a result of the accumulation of damage due to cycling the

rubber and failure occurs at a constant number of cycles, N^, which is

independent of the test frequency. This is opposed to the additivity of damage

model v.'e have been examining in which damage accumulates with time at a rate

dependent upon the level of stress and for which fatigue failure would occur

at a constant time, tg). The coefficients from the model of equation (3)

are shown with their standard errors in Table 4. The standard error of estimate

for the fit is 0.391 with 32 degrees of freedom for the polyolefin data. Note tl

we obtain a slope of -.748 for the polyolefin frequency dependence. This

is different from a value of -1 at the 97.5% confidence level. For the NBS

Butyl data, we obtain a slope of -.644 which differs significantly from -1 at

the 99.9% confidence level based on the student's "t" test. The value of -1

would be obtained if the material failure occurred at a constant number of

cycles independent of test frequency (i.e. "cycle dependent fatigue"). These

slopes are also significantly different from zero which would be obtained

if the additivity of damage rule were valid. Obviously, the behavior of the

polyolefin and butyl rubbers is intermediate between that predicted by these two

model s

The distribution of the deviations of the logarithm of the failure

times for the polyolefin rubber about the model of equation 2 is shown in

Figure 6. The distribution is somewhat narrower than that from the static

uniaxial data and appears skewed to the left . Insufficient data were obtained

to construct a histogram from the NBS Butyl rubber uniaxial dynamic test results.
o

uoo

o

CO

o

CN

o

o

o

o

(N

o

CO

0jnijBj |o Aouenbejj

i 03

C

17

Figure

6.

Histogram

of

Distribution

of

Deviations

from

Least

Squares

Line

for

Polyolefin

Rubber

Under

Uniaxial

Fatigue

Loading.

In

Air

at

23°C.
Table 4

Time to Fail Parameters for Uniaxial Dynamic Tests*

Polyolefin Rubber, N = 35

Parameter Value

's

A 13.244 + .928

B -.748 + .107

C -.755 + .101

NBS Butyl Rubber, N = 11

Parameter Value

's

A 7.41 ± 0.187

B -.644 i .519

C

*Fit to Jin tn = A + BJln oo + Ca, where tn = time to fail in seconds, oo = frequency,

in Hz and a°= peak stress in MPa.

°

is the standard error of the coefficient and N is the number of data points. |

18
(3) Biaxial Testing

Biaxial static test data have been obtained for the NBS Butyl and

the polyolefin rubbers at room temperature in air. Testing of the other two

elastomers has been initiated. Biaxial dynamic data have been obtained for

the NBS Butyl and polyolefin rubbers and for the urethane-silicone copolymer

at 23°C and at 37°C in normal saline solution. Testing has been initiated

on the segmented polyurethane elastomer.

The static data were obtained by inflating a circular membrane which

was clamped at its edge using constant regulated air pressure. The apparatus

is shown in Figure 7. Time to failure was recorded for each specimen. The

static test results are depicted in a plot of the logarithm of time to fail

versus reduced pressure (pressure divided by specimen thickness) in Figure 8.

The data were fitted by linear regression to the model:

£n tg = A + B P/h (4)

where tg is the time to fail in seconds and P/h is the reduced pressure in kilo

pascals per millimeter. The parameters determined for the two rubbers are

included in Table 5 with the standard errors from the regression line.

To illustrate the statistics of failure in these tests, we show histograms

of the deviations from the model (equation 4) in Figures 9 and 10. The

distribution for the polyolefin rubber is significantly broader than that

for the NBS Butyl rubber. However, some of the spread of the polyolefin

data might be attributable to different lots of rubber. This effect was

tested by fitting the data to the model:

£n tg = A + B-p + CN

where N is the lot number 1 or 2. The parameter C was found to be insignificantly different from zero.

The dynamic data were obtained by inflating a circular membrane which

was clamped at its edge between two glass flange joints using pressure pulses

varying between zero and a peak pressure P at a frequency of three Hertz.

19
POLE

<

UJ

00

CD

(C

20

Figure

7.

Schematic

of

Apparatus

Used

for

Equi-biaxial

Creep

and

Failure

Measurements

by

Inflation

of

Rubber

Membrane.
Figure 8. Time to Fail Versus Reduced Pressure for Biaxial Static Tests.

21
I

1

'

o

3aniivddo AGN3no3ad

22

Figure

9.

Histogram

of

Distribution

of

Deviations

from

Least

Squares

Line

for

NBS

Butyl

Rubber

Static

Biaxial

Data.

In

Air

at

23°

C.
o

3jn|!8;j |o Aou8nbdJj

k

23

Figure

10,

Histogram

of

Distribution

of

Deviations

from

Least

Squares

Line

for

Polyolefin

Rubber

Biaxial

Static

Data.

In

Air

at

23°C,
X

Figure 11. Dynamic Biaxial Test Apparatus. A, Air Supply. F, Filter. PT

,

Pressure Transducer. R1 , Primary Pressure Regulator. R2, Press

Regulator. VI, Solenoid Valve. V2, Motor Driven Spool Valve.

SW, Condom Activated Microswitch. T, Timer.

24
The cells formed by the glass flange joints were filled with physiological

saline solution. A diagram of the apparatus is shown in Figure 11. One

side of the test cell is alternately connected to a regulated air pressure

line and vented to the atmosphere by means of a motor driven spool valve.

The peak air pressure in the cell is monitored with a pressure transducer

interfaced to a rectifier circuit with a large time constant. Upon sample

failure, a latex condom inflates to trip a micro-switch which turns off the

running time meter and a solenoid valve in the pressure supply line. The latex

condoms also serve as barriers to retard evaporation of the physiological

saline solution. Time to fail is recorded for each specimen. A foamed polystyrene float is placed in the low pressure side of the cell to prevent sloshing

of the saline solution. Measurements of the peak height of this float permit

the calculation of the volume of the inflated membrane to estimate the stress

and strain in the specimens as discussed in reference 2. Six of these test

cells were operated in air at 23®C and six more were immersed in a water

bath maintained at 37°C. :

The dynamic test results are depicted in plots of the logarithm of the

time to fail versus reduced pressure in Figures 12 through 16.

The data were fitted to equation 4 by linear regression analysis. The

resulting parameters are shown in Table 5. Several significant features

may be observed in these statistics:

(1)

, The slopes (or pressure dependences) are not significantly different

in static and dynamic testing. This is in contrast to the results obtained

in uniaxial testing on the polyolefin rubber.

(2)

, The slopes are not significantly different at 23 and 37°C for

the two materials for which data are available. This fact allows us to calculate activation energies for the fatigue failure process. For the NBS Butyl

rubber, we obtain an activation energy of 22 kcal/mo^e and for the urethanesilicone copolymer we obtain 25 kcal/mole.

(3)

, The slopes are significantly different for the different

25
10

YEAR

NBS BUTYL

BIAXIAL FATIGUE

23 X. 3 Hz

2 -

IL

0

L

100

REDUCED PRESSURE

KPa

200

igure 12. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests

of NBS Butyl Rubber at 23°C.

26
REDUCED PRESSURE

KPa

Figure 13. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests

of NBS Butyl Rubber at 37°C.

27
10

Figure 1

9-

8 -

7 -

6 -

= 5

YEAR

MONTH

- DAY

CDO

POLYOLEFIN

BIAXIAL FATIGUE

23 ®C. 3 Hz

HOUR

100

REDUCED PRESSURE

KPa

200

4. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests

Polyolefin Rubber at 23°C.

28
10

9“

_ 7-

6 -

CDO

YEAR

MONTH

5- DAY

HOUR

URETHANE-SILiCONE

COPOLYMER

2 -

IL

0

I 100

REDUCED PRESSURE

KPa

200

Figure 15. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Te

Urethane-Silicone Copolymer at 23°C.

29
10

9-

8 -

YEAR

<^ 6 o

C3O

MONTH

h DAY

HOUR

URETHANE-SILICONE

COPOLYMER

BIAXIAL FATIGUE

11 ! 0 100 200

REDUCED PRESSURE

KPa

Figure 16. Time to Fail Versus Peak Reduced Pressure for Dynamic Biaxial Tests of

Urethane-Silicone Copolymer at 37®C.
Table 5 Regression Parameters for Biaxial Tests*

A B is

N

NBS Butyl

Static Biax. 23°C 24.62 -.154 + .863 25

Polyolefin

Static Biax. 23°C 27.20 -.118 + 1.55 17

NBS Butyl

Biax. Fatigue 23°C 19.68 -.153 + .619 33

NBS Butyl

Biax. Fatigue 37°C 18.33 -.156 + .491 10

Polyolefin

Biax. Fatigue 23°C 20.94 -.110 + 1.01 19

US Copolymer

Biax. Fatigue 23°C 27.79 -.136 + .980 4

US Copolymer

Biax. Fatigue 37°C 22.52 -.104 + 1.34 7

*Fit to £n tp = A + B P/h, where tp is the time to fail in seconds and P/h is the

reduced pressure in kPa. E is“the standard error from the regression line

and N is the number of data points.

31
material s

.

(4)

, The lifetimes are very much shorter in dynamic testing than in

static testing when they are compared at the same reduced pressure.

This seriously conflicts with the concept of simple linear additive

damage.

(5)

, The variances of the distribution of deviations are significantly

different at the 95% confidence level between static and dynamic loading

for both the NBS Butyl and the Dolyolefin rubbers in biaxial testing.

To illustrate the statistics of failure in these tests, we show

histograms of the deviations from equation 4 for the NBS Butyl rubber

and the polyolefin rubber at room temperature. These histrograms are

shown in Figures 17 and 18.

C. Creep Behavior of Candidate Elastomers

(1) Background

One aspect of the suitability of elastomers for blood pump applications

is their resistance to time dependent deformation i.e., creep. In this

program, we have been studying the creep behavior of the four candidate

elastomers under uniaxial and eaui-biaxial load conditions. The data

which we have obtained to date are from tests conducted at 23°C in air. Work

has recently been initiated at 23°C and 37°C in saline solution. Future

work will include testing at 50°C in saline solution.

There are reasons for characteri zing the material thoroughly at

ambient conditions (i.e. 23°C in air). The first is that it is easier

to characterize the material behavior at ambient conditions and,

therefore, establish a data base for understanding the basic material

creep behavior. Second, by studying changes in material behavior upon

changing conditions to saline solution, higher temperature, etc, one can

obtain a separate measure of chemical and mechanical effects on the deformation

of the rubber. Such comparisons and their significance have been extensively

32
Huniivd do AONdnodyd

33

Figure

17.

Histogram

of

Deviations

from

Least

Squares

Line

for

NBS

Butyl

Rubber

Fqul-blaxlal

Fatigue

Data.

In

Saline

Solution

at

23

C.
o

0jn|!Bj p Aouanbajj

c

34

Figure

18.

Histogram

of

Distribution

of

Deviations

from

Least

Squares

Line

for

Polyolefin

Rubber

Biaxia
discussed by Wood [5] in his work on natural rubber^

(2) Uniaxial Creep

Uniaxial creep of the four candidate elastomers is being measured

by applying a constant force to dumbbell specimens and measuring the

separation of gage marks on the specimens with a cathetometer. As shown

in Figures 19 through 22, the creep of the four candidate elastomers

plots as nearly straight lines on log strain vs. log time plots.

Further, we see that the lines are parallel for different stress levels.

Thus, one can easily compare the deformation behavior of the four elastomers.

Since the data are straight lines on a log c vs. log t 'plot, we fit

least squares lines through the data of the form £n c = A +n X-nt. Since n is

the slope of the log c-log t plot it represents the rate of creep, and

so serves as a useful representation of the resistance of each material

to time dependent deformation. Table 6 tabulates the average values of

n obtained for each material. As can be seen, the polyolefin shows the

highest creep rate and the polyurethane the lowest.

Future work in this area will include obtaining data at longer

times for the materials at room temperature in air, testing in saline solution at

room temperature, 37°C and SC^C. We remark, again, that comparisons of

behavior with room temperature, air testing will provide a basis for

understanding how the saline environment is affecting the candidate

elastomers.

(3) Equi-biaxial Creep

Equi -biaxial creep data are being obtained on the candidate elastomers

by applying an internal pressure to inflate a membrane. Figure 7 shows

a schematic drawing of the device. The state of deformation at the pole

of the membrane is one of equi-biaxial stretch. At the edge of the

35
100.0

°i/n v=3 'Nivais

caa

vnO

o

COO

</i

ooCO

CNJ

4-> (O

s-

*r“ c

c

36

Figure

19.

Uniaxial

Creep

of

NBS

Butyl

Rubber.
100.0

ooo

o

too

UOo

o

COo

CMo

o“i/-iv =3 ‘uiejis

(/> ©

E

K

37

Figure

20.

Uniaxial

Creep

of

Polyolefin

Rubber.

In

Air

at

23°C.
100.0

00o

o

<oo

LOo

«TO

COo

evjo

o

o

oT“d

o

d

°1/TV=3 ‘UjBJiS

(A©

38

Figure

21.

Uniaxial

Creep

of

Urethane-Silicone

Copolymer.

In

Air

at

23°C.
100.0

GOo

r-o

(Oo

o

o

C*5O

CMO

°inV=3‘UI84S

'.i

(Ji

<uE

P

)

39

Figure

22.

Uniaxial

Creep

of

the

Segmented

Polyurethane

Elastomer.
1

I

CREEP EXPONENT, n,

ELASTOMER

NBS Butyl

Polyolefin

Urethane-Silicone Copolymer

Polyurethane Elastomer

*From least squares fit of form

TABLE 6

FOR ELASTOMERS IN UNIAXIAL CREEP

n*

0.0339 + .0055

0.0399 + .0055

0.0208 + .0058

0.0177 + .0025

£ne = A + nX,n t. Average value for a 1 measurements

40
membrane (grips) the state of deformation is one of pure shear. Analysis

of the inflated membrane is complicated by the fact that the stress at

the pole of the membrane is dependent not only on the pressure and

deformation (as measured by the stretch, X) at the pole, but also on the

radius of curvature of the inflated "bubble". The radius of curvature

depends on the material modulus values as well as the internal pressure and

varies with time during the test. Thus, for a simplified data presentation,

we have chosen to use the internal pressure, p, divided by the membrane

thickness, h, as our "engineering stress", p/h.

Our work, to date, has been limited to the- polyolefin rubber and the

NBS Butyl rubber at room temperature. Figure 23 shows typical behavior

for the butyl rubber. Unlike the uniaxial data, which is nearly straight

on a log c - log t plot, the butyl rubber data shows a curvature upwards

towards the end of the plot in biaxial creep. Also, there is considerably

more scatter in the data than in the uniaxial case due to the imprecision

of measurement of the strain at the pole of the sample. In spite of the

curvature, we decided to fit a least squares line of the form £ne = A + n£nt

as we did for the uniaxial data. Since the curves are nearly parallel and

the exponent n was independent of the. magnitude of p/h, we averaged the data

and found that n is 0.0908 + .0160 which is considerably greater than the

value obtained in uniaxial creep testing.

Figure 24 shows typical biaxial creep behavior for the polyolefin

elastomer. In this instance, the data plot in relatively straight

lines, but are not parallel. This indicates that the creep rate is

dependent on p/h. Figure 25 shows how the creep exponent, n, varies

with the magnitude of p/h. Although there is considerable variability

41
0

001

00o

<0 CL

h-o

CO

.i: C)

< CM ^ " 2 H

3

CQ

CO 012

Cl E * 5 ° I

°

<

<

COo

too

CAoTE

CJ) oro

Csl -M

fT3

S- <

c

«TO

X

X

<3<

X O <

X o

(9 O.S

COcd

X o <

X o < — X o

COo

O X o <

a

a X o <

a X o < o

c=>

CNo

o

CO

CO

CO

I. -Y ‘UjBJlS

42

Figure

23.

Equi-biaxial

Creep

of

NBS

Butyl

Rubber.
100.0

00o

r^o

o

o

IT)o

o

COo

CMo

I. -Y ‘ujejis

<A

43

Figure

24.

Equi-biaxial

Creep

of

Polyolefin

Rubber

in

Air

at

23°C.
Polyolefin

Rubber ooCO

(NJ

s_c

44

Figure

25.

Creep

Exponent

vs.

Reduced

Pressure

for

Polyolefin

Rubber

in

Equi-biaxial

Stress.
in the data, a line of the form £n(n) = 0.0262 + .00819 (p/h) fits the

data quite well. Note that at the lowest values of p/h, the n is approximately

0.05 while at the higher p/h values n is approximately 0.14. This

compares with the uniaxial value of 0.0399.

It is interesting to note that both the polyolefin rubber and the

butyl rubber creep faster under equi-biaxial stress than under uniaxial

stress, and points up that, in pump design, such phenomena should be

taken into consideration.

During the upcoming year, this work will be extended to cover the

urethane-silicone copolymer and polyurethane elastomers. In addition,

work will be performed to study creep of the blood pump elastomers at

higher temperatures and in saline solution.

D. Morphological Characterization

Evidence pointing to the manifestation of phase separation in the

wall (200 pm thick) of an intra-aortic balloon made from a urethanesilicone copolymer was discussed in an earlier report (2). This earlier

work suggested that phase separation on a scale coarse enough to be

readily detectable under a light microscope at relatively low magnifications

may be a characteristic feature of the type of urethane-silicone copolymer

used in heart assist devices. Accordingly, the urethane-silicone copolymer

sheets (625 ym thick) used in the mechanical testing program described

elsewhere in this report were examined under an optical microscope. The

observations described below illustrate the heterogeneous nature of the

internal structure of these copolymer sheets.

Both transverse cross-sections, cut at right angles to the plane of

the sheets, as well as thin slices cut parallel (or closely parallel) to

the plane of the sheets were examined under a light microscope using

45
conventional transmission optics. The transverse cross-sections and the

aforementioned slices were approximately '^^100 pm thick.

The photomicrograph (magn. xlOO) in Fig. 26 illustrates a transverse

cross-section of a sheet. The letters A and B in that figure identify

respectively reference points situated at, or very near to, the upper

and lower surfaces of the sheet. The corresponding points are identified

in Figs. 27,28 which show portions of Fig. 26 as they appear at a higher

magnification (magn. x250). The regions depicted in Figs. 27,28 respectively

were photographed in each case at two different levels of focus. The

photomicrograph in Fig. 29 illustrates a portion of a slice which was

cut essentially parallel to the plane of the copolymer sheet; the arrows

in this figure delineate the edges of the slice which was somewhat

thinner in the lower right hand region than elsewhere.

Heterogeneities of varying sizes may be seen dispersed throughout

all the sample regions shown in Figs. 26-29 in which, due to the thickness

of the samples, all the heterogeneities are not in focus. It is evident

from these micrographs that phase separation occurs on a gross scale

during the preparation and (manufacture) of the copolymer sheets and

that the sheets consist of a matrix throughout which regions of another

phase are dispersed. The chemical composition of the matrix and dispersed

phases in these sheets is not known. Although the regions of the dispersed

phase vary in size. Figs. 26-29 clearly reveal that all regions of this

phase exhibit a similarity in shape, namely their appearance corresponds

roughly to that of squashed spheres which are oriented with their largest

dimension (corresponding to the diameter of the circular profile of the

squashed spheres) parallel to the plane of the sheet. Deduction of the

shape of the regions of the dispersed phase and the orientation of these

regions relative to the plane of the sheet is based on (a) the characteristical ly 1

46
f

(

Figure 26: Transverse cross-section of a urethane-silicone copolymer sheet as seen under a light microscope using conventional transmission optics.

Letters A and B identify points situated near opposite surfaces of

the sheet. These points correspond to the similarly denoted ones

in Figs. 27, 28. Even at this low magnification, elliptically

shaped heterogeneities of varying sizes can be discerned over the

whole cross-section. These heterogeneities are all oriented with

their long axis parallel to the plane of the sheet (horizontal in

the field of view). Magn. xlOO.

47
elliptical profile exhibited by the heterogeneities seen in transverse

t cross-sections in which these heterogeneities are characteristically

oriented with their long axes parallel to the plane of the sheet (see

<h> in Figs. 27,28), and on (b) the circular or essentially circular

profiles exhibited by the heterogeneities when they are examined along

an axis perpendicular to the plane of the sheet (Fig. 29).

The diameters of the squashed-sphere-1 ike shaped regions of the

dispersed ‘phase, i.e. their sizes parallel to the plane of the sheets

vary widely from a few micrometers up to 30ym. The average elliptical

aspect ratio [long axis (i.e. diameter)/short axis] exhibited by the

regions of the dispersed phase specifically identified (<h>) in Fig. 27

is 2.1.
Figure 27: Portion of the transverse cross-section of the urethane-silicone

copolymer sheet shown in Fig. 26 as seen at a higher magnification.

The arrow A in each of the above figures serves to identify the area shown in relation to the corresponding area in Fig. 26. (a)

and (b) were recorded at different levels of focus. Note the

essentially elliptical shapes exhibited by the heterogeneities

<h> which vary in size and which are oriented with their long

axis parallel to the plane of the copolymer sheet. Magn. x250.

49
Figure 28: Portion of the transverse cross-section of the urethane-silicone

copolymer sheet shown in Fig. 26 as seen at a higher magnification.

The arrow B in each of the above figures serves to identify the area shown in relation to the corresponding area '.n Fig. 26. (a)

and (b) were recorded at different levels of focus. Note the

essentially elliptical shapes exhibited by the he ierogeneities

which vary in size and which are oriented with their long axis

parallel to the plane of the copolymer sheet. Magn. x250.

50
Figure 29: Portion of a slice cut paral 1 e1 to the plane of the same urethanesilicone copolymer sheet from which the transverse cross-section

shown in Figs. 26 to 28 was obtained. The arrows delineate the

edges of the slice. Note the circular or essentially circular

profile of the heterogeneities.

51
References

(1) McKenna, G. B. and Penn, R. W., "Time Dependent Failure Behavior of

Poly(methyl methacrylate) and Polyethylene", Polymer (in press).

(2) Penn, R. W. and McKenna, G. B., "Physical Testing of Polymers for Use

in Circulatory Assist Devices", First Annual Report, July, 1978,

NBSIR #79-1741

.

(3) Penn, R. W. and McKenna, G. B., "Fatigue Effects in Poly(methyl methacrylate^!

in Durability of Macromolecular Materials , ed. by R. K. Eby, ACS Symposium

Series, 95, ACS, Washington, D. C., 1979.

(4) Bailey, J., "An Attempt to Correlate Some Tensile Strength Measurements on Glass: III", Glass Industry, 94 (1939).

(5) Wood, L. A., Bullman, G. W. and Roth, F. L., "Long-Time Creep in a Pure-Gum

Rubber Vulcanizate: Influence of Humidity and Atmospheric Oxygen", J. Res.,

NBS A, 78A, 5, Sept. -Oct., 1974, pp. 623-629.

52
NBS«114A (REV. 9-781

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. MBSIB 30-2008

Z* RBCIfttertf & Ac(Se$$c(oa Mo..

4. TITLE AND SUBTITLE

PHYSICAL TESTING OF POLYMERS FOR USE IN CIRCULATORY

ASSIST DEVICES

5. Publication Date March 1980

s.

7. AUTHOR(S)

Penn, R. W. , McKenna, G. B., Khoury, F. A.

8. Performing Organ. Report No.

9. PERFORMING: ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, state. ZIP)

Devices and Technology Branch

Division of Heart and Vascular Diseases

National Heart, Lung and Blood Institute

National Institutes of Heal th/Bethesda , MD 20014

13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Softwrare Summary, is attached.

16. ABSTRACT (A 200»word or less (actual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here,) The mechanical durability of an elastomer is a critical factor in its suitability for

blood pumpi appl ications . In such applications, an elastomeric bladder is expected to

undergo cyclic stress or strain histories at a frequency of approximately 1 Hz for

periods of: several years. Test methodologies for characterizing the mechanical durability of such materials do not exist.

In this report, we describe a test methodology which we have developed for the characterization of the durability of elastomers which are candidate materials for blood

pump applications. This framework is based upon a cumulative damage rule which we have

used to describe the time dependent failure of glassy and semicrystall ine polymers.

Within this framework, we are in the process of studying the durability of a polyolefin

rubber, a urethane-silicone copolymer elastomer and a segmented polyurethane elastomer

which are candidate materials for blood pump applications. We have also prepared a standard butyl rubber to use for inter laboratory comparisons with other labs working

on this contract.

The cumulative damage framework suggests that the testing protocol be designed to

examine four aspects of material failure: (1 )failure under static loads ,(2)fail ure under

dynamic loads , (S)frequency dependence of dynamic failure, (4)statistics of failure. We

report on our results to date on the uniaxial and equibiaxial failure of the blood pump

elastomers using a testing protocol which inr,ludp«=; thp ahovp four pf^irrts.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Elastomers; blood pump materials, biomaterials, urethane-silicone copolymer;

Butyl Rubber; segmented polyurethane; time dependent failure; polyolefin rubber.

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