:I ' 1. I III IUWlII III IIII11 1 I IIIIII Strength Characteristics of DuPont "Surlyn A" Film/ NCAR Facilities Report

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1 -FRB NCAR Facilities Report Strength Characteristics of DuPont "Surlyn A" Film/ National Center. for Atmospheric Research Boulder, Colorado i=,, : ' _ APRIL ^ 5 '"NCAR Library :I ' 1 I III IUWlII III IIII11 1 I IIIIII

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3 STRENGTH CHARACTERISTICS OF DUPONT "SURLYN A" FILM NCAR Facilities Report FRB-4 A test program conducted for the National Center for Atmospheric Research, by the Hauser Research and Engineering Company, 2965 Peak Avenue, Boulder, Colorado (Hauser Report No ) April 1965

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5 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 atmospheric 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 subcontract 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.

6 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 programs succeed in eliminating the static charge, Surlyn A may become a good prospect for balloon use.

7 iv CONTENTS PREFACE.... ii SUMMARY... LIST OF TABLES... iii v Section I. INTRODUCTION II. MANUFACTURER'S SPECIFICATIONS... 2 III. DESCRIPTION OF SAMPLES... 4 IV. TESTS AND RESULTS -- MECHANICAL PROPERTIES OF SURLYN A. 6 V. CONCLUSIONS FIGURE AND TABLES REFERENCES... 15

8 v LIST OF TABLES 1. Summary of Tests Test Data, 1-Mil Surlyn A Test Data, 2-Mil Surlyn A Comparisons of Films Tested at 25C Comparisons of Films Tested at -80 C

9 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, ultimate elongation, tensile modulus of elasticity, tear initiation strength, and tear propagation strength.

10 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 polyethylene 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 Tensile strength psi Yield strength psi Elongation % Modulus 28, ,000 psi Tear strength Elmendorf g/mil

11 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 considered as possible balloon materials.

12 4 III. DESCRIPTION OF SAMPLES The 1-mil sample of Surlyn A was shipped on two rolls designated as: Surlyn 'A' ER mil The rolls, each 24 in. wide and 50 ft long, were received in excellent condition in December (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 concentration. 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

13 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 manufacturing problems. It may also present considerable restraint in unfolding any large balloon made of Surlyn. We cannot at present ascertain 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 Surlyn A Consolidated GF19X Consolidated GF19X Visqueen 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 in., interpolating to in. Ten readings were taken on each sample. The 1-mil sample had as average thickness of mil with a variation -+J 8%. The 2-mil sample had an average thickness of mil, with extremes of 2.19 and 2.01 mil. Personal communication from J.P. Broussard, Industry Representative, Polyolefins Division, DuPont, 18 December 1964.

14 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: C ( F) and C ( F). The coldenvironment 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 description 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.

15 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 0 to, or (at -80 C) slightly inferior to, Visqueen.

16 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.

17 10, C II/,8 0 0 C _ Surlyn A, I mil c, ^^ /. Visqueen, 1.5 mil _ /._. _ Consolidated GF 19X, 1.5mil - /,,~... Visqueen (adjusted to I mil) --» i /,, 4 (n I, (/) 25 C 04 1 S ' ^ Strain Fig. I--Typical Modulus Curves (machine direction)

18 Table 1 SUMMARY OF TESTS (5 samples each in machine and transverse directions, on Dillon Model M Tester) Test Method Sample Size Crosshead Rate or Type Strain Rate (in./min) Tensile ASTM D882 61T 1" x 6" 0.5a Strength method B (4" gage length) Ultimate Ultimatie l(determined from tensile strength test) Elongation b I Tensile ASTM D882 61T 1" x 12 ' (10" 1 o Modulus gage length) Yield ieldgh l(determined from tensile modulus test graphs) Strength Tear ASTM D die-cut specimens 2 Initiation Graves tear test Tear ASTM D1004 die-cut specimens 2 Propagation Hauser-modified -- 1/16" slit in./in./min strain rate Autographic stress-strain curves made of load vs elongation. Pendulum-head travel corrected.

19 Table Z TEST DATA, 1-MIL SURLYN A Direction Ultimate Ultimate Tensile Tensile Elongation Tear Tear Tensile Elongation Modulus Yield at Yield Initiation Propagation Strength (%) (psi) Strength (%) Strength Strength (psi) (psi) (lb/in.) (lb/in.) -25 C Machine , , , , , average , Transverse , , , , , average , C Machine , , , , , average , Transverse , , , , , average ,

20 Table 3 TEST DATA, 2-MIL SURLYN A Direction Ultimate Ultimate ' Tensile Tensile Elongation Tear Tear Tensile Elongation Modulus Yield at Yield Initiation Propagation Strength (%) (psi) Strength (%) Strength Strength (psi) (psi) (lb/in.) (lb/in.) -+25 C Machine , , , , , average , Transverse , , , , , average , C Machine 12, , , , , , , , , , average 11, , Transverse , , , , , average ,

21 Table 4 COMPARISONS OF FILMS TESTED AT 250C (elongation at yield, 8% for all samples; no deviation) Film Weight Direc- Ultimate Tensile Ultimate Tensile Tear Tear (lb/1000 tion Tensile Yield Elongation Modulus Initiation Propagation sq ft) Strength Strength 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.) Surlyn M , A mil T , Surlyn M , A mil T , Winzen Strato- M , film 320 3, mil T , Winzen Strato- M , film mil T , Winzen Strato- M , film mil T , Consolidated M , GF19X 4.8 I-mil T , Consolidated M , GF19X mil T , Visqueen M , A mil T , deviation coefficient Source: Data from Hauser Research & Engineering testing programs

22 Table 5 COMPARISONS OF FILMS TESTED AT -80 C (elongation at yield, 3% for all samples; no deviation) Film Weight Direc- Ultimate Tensile Ultimate Tensile Tear Tear (lb/1000 tion Tensile Yield Elongation Modulus Initiation Propagation 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.) Surlyn M , A mil T , Surlyn M 11, , A mil T , Winzen Strato- M , film mil T , Winzen Strato- M , film mil T , Winzen Strato- M , film mil T , Consolidated M 10, , GF19X mil T , Consolidated M , GF19X mil T , Visqueen M , A mil T , deviation coefficient Source: Data from Hauser Research & Engineering testing programs

23 15 REFERENCES 1. "New Tough Transparent Plastics," Materials in Design Engineering 60, No. 3, , September Surlyn A Ionomer Resin, Brochure A-38352, Plastics Dept., Polyolefins Div. E. I. DuPont de Nemours & Co., Wilmington, Del., undated. 3. Tests of Balloon Materials, NCAR Facilities Report FRB-1-64, National Center for Atmospheric Research, Boulder, Colo., November Standard Test Methods for Balloon Materials, NCAR Facilities Report FRB-2-64, National Center for Atmospheric Research, Boulder, Colo., November Non-standard Tests for Balloon Materials, NCAR Facilities Report FRB-3-64, National Center for Atmospheric Research, Boulder, Colo., November 1964.

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