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-FRB..4 ...
NCAR Facilities Report
Strength Characteristics of
DuPont "Surlyn A" Film/
National Center. for Atmospheric Research
Boulder, Colorado
i=,, : ' _ APRIL 1965 .... ^ 5 '"NCAR Library
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I~~~~~~~~~~~~~~~~~~~~~
STRENGTH CHARACTERISTICS OF DUPONT "SURLYN A" FILM
NCAR Facilities Report FRB-4
A test program conducted for the National Center forAtmospheric Research, by the Hauser Research and Engineering
Company, 2965 Peak Avenue, Boulder, Colorado
(Hauser Report No. 5034-65-05)
April 1965
I
ii
PREFACE
This report is one of a series prepared for the Materials Research
Project of the NCAR Scientific Balloon Facility. The Materials Research
Project is one of several related efforts undertaken by the National
Center for Atmospheric Research (NCAR) to increase the reliability,
and to extend the capabilities, of scientific ballooning.
NCAR was founded in 1960, to conduct basic research in the atmos-
pheric sciences, and to foster such research on the part of the
universities and research groups in the U.S.' and abroad. NCAR is
operated by the University Corporation for Atmospheric Research (UCAR),
and sponsored by the National Science Foundation.
The present report covers certain investigations performed by
Hauser Research and Engineering Company, Boulder, Colorado, under sub-
contract with UCAR. Other reports published in this balloon materials
research series include: FRB-1-64, Tests of Balloon Materials; FRB-2-64,
Standard Test Methods for Balloon Materials; and FRB-3-64, Non-Standard
Tests for Balloon Materials.
iii
SUMMARY
A new plastic film of the ionomer class, developed by DuPont,
and tradenamed Surlyn A, appeared to have possibilities as a balloon
material. Samples of the film were subjected to standard tests for
mechanical strength. The test data indicate that Surlyn A is comparable
in many of its material properties to a good polyethylene. However,
the samples tested carried high static charges which would make the
material unsuitable for use in balloons. If current development pro-
grams succeed in eliminating the static charge, Surlyn A may become
a good prospect for balloon use.
iv
CONTENTS
PREFACE. ............................. ii
SUMMARY ............. iii
LIST OF TABLES .......................... v
Section
I. INTRODUCTION ... ................. 1
II. MANUFACTURER'S SPECIFICATIONS .............. 2
III. DESCRIPTION OF SAMPLES ................. 4
IV. TESTS AND RESULTS -- MECHANICAL PROPERTIES OF SURLYN A . 6
V. CONCLUSIONS. ...................... 8
FIGURE AND TABLES. ........................ 9
REFERENCES ............................ 15
v
LIST OF TABLES
1. Summary of Tests.................. . 10
2. Test Data, 1-Mil Surlyn A . ................ 11
3. Test Data, 2-Mil Surlyn A .................. 12
4. Comparisons of Films Tested at 25C ....... .. 13
5. Comparisons of Films Tested at -80°C. ............ 14
1
I. INTRODUCTION
In September 1964 E. I. DuPont de Nemours & Company announced
the development of a new class of plastic films called "ionomers. "
Only one form of ionomer film, tradenamed "Surlyn A," is commercially
available at present.
The trade literature on Surlyn A indicated it might have value
as a balloon material. Under subcontract to the National Center for
Atmospheric Research, an investigation was begun in December 1964 to
determine the strength characteristics of this film.
One- and two-mil thicknesses of the film were subjected to
the standard tests used in earlier film test programs for NCAR. These
consisted of 160 tests to determine Surlyn A properties of ultimate
tensile strength, tensile yield strength, elongation at yield, ulti-
mate elongation, tensile modulus of elasticity, tear initiation
strength, and tear propagation strength.
2
II. MANUFACTURER'S SPECIFICATIONS
DuPont classifies their ionomers as thermoplastics -- materials
which soften when heated, harden when cooled, and can be reshaped many
times by alternate heating and cooling.
DuPont asserts(l) that Surlyn is stronger and tougher than unmodified
polyethylene, and that it has the stiffness of a medium-density poly-
ethylene with the high elongation and low-temperature flexibility of
many polyolefins. Because its structure consists of oxidized polymer
chains and inorganic cation groups, the attractive forces between oxidized
chains and cations give a partial cross-linking effect, as in thermoset
plastics. However, in thermosets such cross-linking is irreversible.
The ionic linking of the ionomers, on the other hand, is thermally
reversible. Thus, the ionomers can be worked like thermoplastics, but
offer some attributes of thermoset plastics.
The manufacturer claims that the incorporation of inorganic metallic
ions, such as sodium and potassium, increases the material's modulus of
elasticity and yield point, as well as its chemical resistance to oil
and solvents.
DuPont ascribes the following properties (at room temperature) to
Surlyn A: (
Specific gravity 0.93 -- 0.94
Tensile strength 3500 -- 5500 psi
Yield strength 2000 -- 2500 psi
Elongation 300 -- 400%
Modulus 28,000 -- 40,000 psi
Tear strengthElmendorf 20 -- 80 g/mil
3
The resin in bulk quantities costs 50¢/lb. Film of 1- or 2-mil
thickness costs $0.85 to 1.00/lb.
DuPont is continuing development on the ionomer family. They
expect that other polymers, such as polypropylene, with various cation
groups (sodium, potassium, magnesium, and zinc) will give ionomers
with a wide range of properties.
When additional ionomer films are developed, they will be con-
sidered as possible balloon materials.
4
III. DESCRIPTION OF SAMPLES
The 1-mil sample of Surlyn A was shipped on two rolls designated
as:
Surlyn 'A'ER 16011 mil
The rolls, each 24 in. wide and 50 ft long, were received in excellent
condition in December 1964. (We do not presently know if this film is
available in lay-flat tubing or other production widths.)
The 2-mil sample was shipped later and bore a similar notation.
It was also received in good condition. This roll was 24 in. wide; its
length was not measured.
Each of the three rolls of film had slight ripples parallel to
the machine direction. These ripples may have collected as a result of
uneven rolling following fabrication. The rippling seemed to have no
effect, except in the transverse-direction tensile modulus tests.
This effect, consisting of a small inflection, was virtually negligible.
(An example of the effect can be discerned in Fig. 1, near the origin
of the curve for the -80 C test of the Surlyn film.)
Close inspection of the film revealed that it contained many gel
granules. During examination of a few tensile specimens by polarized
light, these particles were observed to serve as centers of stress con-
centration.
In handling the film to prepare test specimens, a high static charge
was noted on its surface, similar to that which can be observed on Saran
5
Wrap. We did not make a quantitative measure of this force. It was
present to such a degree, however, that it may cause balloon manu-
facturing problems. It may also present considerable restraint in
unfolding any large balloon made of Surlyn. We cannot at present ascer-
tain whether buildup of high static charges might also cause perforations
in this material. Surlyn is very resistant to corona degradation
but perforation cannot be ruled out. DuPont engineers are working on
the static problem and may develop more satisfactory materials in the
near future.
The film was weighed, and can be compared in this respect with
films tested previously:
Material Nominal Thickness Weight (lb/1000(mil) sq ft)
Surlyn A 1 5.2
Surlyn A 2 10.3
Consolidated GF19X 1 4.8
Consolidated GF19X 2 9.9
Visqueen 1.5 7.4
Film thickness was measured around 2 ft-square samples from 1- and 2-mil
rolls, and was found to be quite uniform. The dial gage used was accurate
to 0.0001 in., interpolating to .00001 in. Ten readings were taken on
each sample. The 1-mil sample had as average thickness of 1.005 mil
with a variation -+J 8%. The 2-mil sample had an average thickness of 2.068
mil, with extremes of 2.19 and 2.01 mil.
Personal communication from J.P. Broussard, Industry Representative,Polyolefins Division, DuPont, 18 December 1964.
6
IV. TESTS AND RESULTS -- MECHANICAL PROPERTIES
OF SURLYN A
The tests were standard procedures, described in Refs. 3 and 4.
They are summarized in Table 1.
The tests were conducted under two temperature environments:
25 - 10C (73.4 - 1.80 F) and -79 - 2 C (-110.2 + 3.60F). The cold-
environment chamber used for the latter tests is described on p. 5 of
Ref. 3.
Humidity in the testing area varied from 20% to 50%. Samples were
preconditioned in the 50% humidity chamber. DuPont's technical descrip-
tion of Surlyn indicates that moisture absorption should not be a problem
with this type of film.
The results of the tests are presented on Tables 2 and 3 (pp. 10
and 11). Tables 4 and 5 summarize these data and compare them with test
results on other films.
The methods used for deriving these test results and deviation
coefficients are described on p. 24 of Ref. 5.
For all tests, our data show lower strengths than those reported
by DuPont. This is because our tests are run at slower rates
(0.125 in./in./min) than are most commercial tests.
In ultimate strength, Surlyn A film is slightly lower than Visqueen
at 25 C, and comparable at -80 C. The rather large value (.220) of the
deviation coefficient for I-mil Surlyn A at 25°C in the transverse direction
is due to an average of two high and three low values for this series.
We could assign no cause for this variation.
7
The test values at 25°C for tensile yield strength and tensile modulus
are superior to those of any other film listed in Table 3. At -80 C these
properties are comparable to, or slightly lower than, those of other
films. Surlyn A thus appears to have better creep resistance than standard
polyethylene balloon films.
In Fig. 1, typical tensile modulus test curves are plotted for 1-mil
Surlyn A, and for two other films, using data from Tables 4 and 5 and
from Ref. 3.
In tear initiation and propagation strengths, Surlyn is comparable
0to, or (at -80 C) slightly inferior to, Visqueen.
8
V. CONCLUSIONS
On the basis of the samples tested, Surlyn A is comparable in many
of its material properties to a good polyethylene. At room temperature
it is superior to most of the other films tested in tensile yield strength
and modulus. At low temperature it appears slightly inferior in modulus
and tear properties.
The film should be evaluated for sealing qualities and brittleness
at low temperature. No tests were made of these properties during the
program reported here.
Surlyn film may become a good prospect for balloon use if it can
be treated to eliminate its static charge.
We believe our data to be representative of the samples received.
These same data may not be representative of commercial production.
10,
8 10 -800C
II/ ,8 0 0 C _ Surlyn A, I milc , ^^ / __.___ Visqueen, 1.5 mil
sq ft) Strength Stren th _____________________________average d.c.* average d.c. average d.c. average d.c. average d.c. average d.c.(psi) (psi) (%) (psi) (lb/in.) (lb/in.)