A 1 1 1 3 DflS3flM

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CO Q* NBS SPECIAL PUBLICATION 260-58

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Standard Reference Materials:

SRM 1470:

POLYESTER FILM FOR OXYGEN

GAS TRANSMISSION

MEASUREMENTS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in

trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

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and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

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develops test methods and proposes engineering standards and code changes; develops and

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research and provides scientific and technical services to aid Federal Agencies in the selection,

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'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
JUL 9 ?97S

Standard Reference Materials:

SRM 1470:

POLYESTER FILM FOR OXYGEN

GAS TRANSMISSION MEASUREMENTS

John D. Barnes

Gordon M. Martin

Polymer Science and Standards Division

Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

% NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1979
Library of Congress Catalog Card Number: 79-600077

National Bureau of Standards Special Publication 260-58

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-58, 43 pages (June 1979)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979
PREFACE

Standard Reference Materials (SRM's) as defined by the

National Bureau of Standards are well -character i zed materials,

produced in quantity and certified for one or more physical

or chemical properties. They ara used to assure the accuracy

and compatibility of measurements throughout the Nation.

SRM's are widely used as primary standards in many diverse

fields in science, industry, and technology, both within the

United States and throughout the world. They are also used

extensively in the fields of environmental and clinical analysis. In many applications, traceability of quality control

and measurement processes to the national measurement system

are carried out through the mechanism and use of SRM's. For

many of the Nation's scientists and technologists it is

therefore of more than passing interest to know the details

of the measurements made at NBS in arriving at the certified

values of the SRM's produced. An NBS series of papers, of

which this publication is a member, called the NBS Special

Publication - 260 Series is reserved for this purpose.

This 260 Series is dedicated to the dissemination of

information on different phases of the preparation, measurement, certification and use of NBS-SRM's. In general, much

more detail will be found in these papers than is generally

allowed, or desirable, in scientific journal articles. This

enables the user to assess the validity and accuracy of the

measurement processes employed, to judge the statistical

analysis, and to learn details of techniques and methods

utilized for work entailing the greatest care and accuracy.

These papers also should provide sufficient additional information not found on the certificate so that new applications

in diverse fields not foreseen at the time the SRM was originally issued will be sought and found.

Inquiries concerning the technical content of this paper

should be directed to the author(s). Other questions concerned with the availability, delivery, price, and so forth

will receive prompt attention from:

Office of Standard Reference Materials

National Bureau of Standards

Washington, D.C. 20234

George A. Uriano, Acting Chief

Office of Standard Reference Materials

• • • 1 1
OTHER NBS PUBLICATIONS IN THIS SERIES

Catalog of NBS Standard Reference Materials

(1975-76 edition), R. W. Seward, ed., NBS

Spec. Publ. 260 (June 1975). $1.50* SN003-003-

01445-1

Michaelis, R. E., and Wyman, L. L., Standard

Reference Materials: Preparation of White Cast

Iron Spectrochemical Standards, NBS Misc.

Publ. 260-1 (June 1964). COM74-1 1061**

Michaelis, R. E., Wyman, L. L., and Flitsch, R.,

Standard Reference Materials: Preparation of

NBS Copper-Base Spectrochemical Standards,

NBS Misc. Publ. 260-2 (October 1964). COM74-

11063**

Michaelis, R. E., Yakowitz, H., and Moore, G. A.,

Standard Reference Materials: Metallographic

Characterization of an NBS Spectrometric LowAlloy Steel Standard, NBS Misc. Publ. 260-3

(October 1964). COM74-1 1060**

Hague, J. L., Mears, T. W., and Michaelis, R. E.,

Standard Reference Materials: Sources of

Information, NBS Misc. Publ. 260-4 (February

1965). COM74-11059

Alvarez, R., and Flitsch, R., Standard Reference

Materials: Accuracy of Solution X-Ray Spectrometric Analysis of Copper-Base Alloys, NBS

Misc. Publ. 260-5 (March 1965). PB 168068**

Shultz, J. I., Standard Reference Materials:

Methods for the Chemical Analysis of White

Cast Iron Standards, NBS Misc. Publ. 260-6

(July 1975). COM74-11068**

Bell, R. K., Standard Reference Materials:

Methods for the Chemical Analysis of NBS

Copper-Base Spectrochemical Standards, NBS

Misc. Publ. 260-7 (October 1965). COM74-

1 1067**

Richmond, M. S., Standard Reference Materials:

Analysis of Uranium Concentrates at the

National Bureau of Standards, NBS Misc. Publ.

260-8 (December 1965). COM74-1 1066**

Anspach, S. C, Cavallo, L. M., Garfinkel, S. B.,

Hutchinson, J. M. R., and Smith, C. N., Standard Reference Materials: Half Lives of Materials

Used in the Preparation of Standard Reference

Materials of Nineteen Radioactive Nuclides

Issued by the National Bureau of Standards,

NBS Misc. Publ. 260-9 (November 1965).

COM74-1 1065**

Yakowitz, H., Vieth, D. L., Heinrich, K. F. J.,aij 1 Michaelis, R. E., Standard Reference Materia

Homogeneity Characterization on NBS Specti

metric Standards II: Cartridge Brass and Lc

Alloy Steel, NBS Misc. Publ. 260-10 (Decern!

1965). COM74-11064**

Napolitano, A., and Hawkins, E. G., Standa

Reference Materials: Viscosity of Standa

Lead-Silica Glass, NBS Misc. Publ. 260-

(November 1966). NBS Misc. Publ. 260-11*:

Yakowitz, H., Vieth, D. L., and Michaelis, R.

Standard Reference Materials: Homogene

Characterization of NBS Spectrometric Star

ards III: White Cast Iron and Stainless St

Powder Compact, NBS Misc. Publ. 260-

(September 1966). NBS Misc. Publ. 260- 12* Spijkerman, J. I., Snediker, D. K., Ruegg, F. i|h

and DeVoe, J. R., Standard Reference Ma *

rials: Mossbauer Spectroscopy Standard for I ^ Chemical Shift of Iron Compounds, NBS Mil 4

Publ. 260-13 (July 1967). NBS Misc. Pu1

. *

260-13**

Menis, O., and Sterling, J. T., Standard Referei: to Materials: Determination of Oxygen in Fern ; ?

Materials - SRM 1090, 1091, and 1092, N i S Misc. Publ. 260-14 (September 1966). NBS Mi iR Publ. 260-14**

Passaglia, E., and Shouse, P. J., Standard Ref- ence Materials: Recommended Method of I: *k of Standard Light-Sensitive Paper for Calibr • Si ing Carbon Arcs Used in Testing Textiles f «

Colorfastness to Light, NBS Misc. Publ. 260 > (June 1967). (Replaced by NBS Spec. Pu m 260-41.)

Yakowitz, H., Michaelis, R. E., and Vieth, D. * »J Standard Reference Materials: Homogene |i o Characterization of NBS Spectrometric Stai- 'k ards IV: Preparation and Microprobe Charterization of W-20% MO Alloy Fabricated jf %

Powder Metallurgical Methods, NBS Sp. i Publ. 260-16 (January 1969). COM74-1106:*

Catanzaro, E. J., Champion, C. E., Garner, E. Marinenko, G., Sappenfield, K. M., and Shiei;. W. R., Standard Reference Materials: Bcc

Acid; Isotopic and Assay Standard Refereie

Materials, NBS Spec. Publ. 260-17 (Februy

1970). Out of Print

iv

m
i dler, S. B., Mantek, P. A., and Cleveland, N.

|i G., Standard Reference Materials: Calibra

,a j tion of NBS Secondary Standard Magnetic Tape

it i (Computer Amplitude Reference) Using the

it i Reference Tape Amplitude Measurement

Li

I "Process A," NBS Spec. Publ. 260-18 (November

i 1969). (See NBS Spec. Publ. 260-29.)

aule, R. C, and Mandel, J., Standard Reference

i4

j Materials: Analysis of Interlaboratory Measureit I ments on the Vapor Pressure of Gold (Certificai j tion of Standard Reference Material 745), NBS

I

i Spec. Publ. 260-19 (January 1970). PB190071**

I iaule, R. C, and Mandel, J., Standard Reference

u j Materials: Analysis of Interlaboratory MeasureIs

j ments on the Vapor Pressures of Cadmium and

S j Silver, NBS Spec. Publ. 260-21 (January 1971).

J | COM74-11359**

II akowitz, H., Fiori, C. E., and Michaelis, R. E.,

;

| Standard Reference Materials: Homogeneity

J j Characterization of Fe-3 Si Alloy, NBS Spec,

j \ Publ. 260-22 (February 1971). COM74-11357**

\ lapolitano, A., and Hawkins, E. G., Standard

p \ Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23

t ((December 1970). COM71-0O157**

r [tppenfield, K. M., Marineko, G., and Hague, J.

1

\ L., Standard Reference Materials: Comparison

\

| of Redox Standards, NBS Spec. Publ. 260-24

(January 1972). COM 72-50058**

I jicho, G. E., Yakowitz, H., Rasberry, S. D., and

I j! Michaelis, R. E., Standard Reference Materials:

[A Standard Reference Material Containing

«

j Nominally Four Percent Austenite, NBS Spec.

I

| Publ. 260-25 (February 1971). COM 74-1 1356**

f

f artin, J. F., Standard Reference Materials:

National Bureau of Standards-US Steel Corporj tion Joint Program for Determining Oxygen and

| Nitrogen in Steel, NBS Spec. Publ. 260-26

i (February 1971). 85 cents* SN003-O03-00786-2

, |iraer, E. L., Machlan, L. A., and Shields, W. R.,

^Standard Reference Materials: Uranium

j lsotopic Standard Reference Materials, NBS

(Spec. Publ. 260-27 (April 1971). COM74-

11358**

.unrich, K. F. J., Myklebust, R. L., Rasberry, S.

|D., and Michaelis, R. E., Standard Reference

Materials: Preparation and Evaluation of

jSRM's 481 and 482 Gold-Silver and GoldCopper Alloys for Microanalysis, NBS Spec.

Publ. 260-28 (August 1971). COM7 1-50365**

Geller, S. B., Standard Reference Materials: Calibration of NBS Secondary Standard Magnetic

Tape (Computer Amplitude Reference) Using

the Reference Tape Amplitude Measurement

"Process A-Model 2," NBS Spec. Publ. 260-29

(June 1971). COM71-50282**

Gorozhanina, R. S., Freedman, A. Y., and

Shaievitch, A. B. (translated by M. C. Selby),

Standard Reference Materials: Standard

Samples Issued in the USSR (A Translation

from the Russian), NBS Spec. Publ. 260-30 (June

1971). COM71-50283**

Hust, J. G., and Sparks, L. L., Standard Reference

Materials: Thermal Conductivity of Electrolytic

Iron SRM 734 from 4 to 300 K, NBS Spec. Publ.

260-31 (November 1971). COM71-50563**

Mavrodineanu, R., and Lazar, J. W., Standard

Reference Materials: Standard Quartz Cuvettes,

for High Accuracy Spectrophotometry, NBS

Spec. Publ. 260-32 (December 1973). 55 cents*

SN003-O03-01213-1

Wagner, H. L., Standard Reference Materials:

Comparison of Original and Supplemental

SRM 705, Narrow Molecular Weight Distribution Polystyrene, NBS Spec. Publ. 260-33

(May 1972). COM72-50526**

Sparks, L. L., and Hust, J. G., Standard Reference

Materials: Thermoelectric Voltage, NBS Spec.

Publ. 260-34, (April 1972). COM72-50371**

Sparks, L. L., and Hust, J. G., Standard Reference Materials: Thermal Conductivity of

Austenitic Stainless Steel, SRM 735 from 5

to 280 K, NBS Spec. Publ. 260-35 (April 1972.)

35 cents* COM72-50368**

Cali, J. P., Mandel, J., Moore, L. J., and Young, D.

S., Standard Reference Materials: A Referee

Method for the Determination of Calcium in

Serum, NBS SRM 915, NBS Spec. Publ. 260-36

(May 1972). COM72-50527**

Shultz, J. L, Bell, R. K., Rains, T. C, and Menis,

O., Standard Reference Materials: Methods of

Analysis of NBS Clay Standards, NBS Spec.

Publ. 260-37 (June 1972). COM72-50692**

Richmond, J. C, and Hsia, J. J., Standard Refer- ence Materials: Preparation and Calibration of

Standards of Spectral Specular Reflectance,

NBS Spec. Publ. 260-38 (May 1972). COM72-

50528**

Clark, A. F., Denson, V. A., Hust, J. G., and

Powell, R. L., Standard Reference Materials:

The Eddy Current Decay Method for Resistivity

Characterization of High-Purity Metals, NBS

Spec. Publ. 260-39 (May 1972). COM72-50529**
McAdie, H. G., Gam, P. D., and Menis, O., Standard Reference Materials: Selection of Thermal

Analysis Temperature Standards Through a Cooperative Study (SRM 758, 759, 760), NBS

Spec. Publ. 26(M0 (August 1972.) COM-72

50776**

Wood, L. A., and Shouse, P. J., Standard

Reference Materials: Use of Standard LightSinsitive Paper for Calibrating Carbon Arcs

Used in Testing Textiles for Colorfastness to

Light, NBS Spec. Publ. 260^1 (August 1972)

COM72-50775**

Wagner, H. L., and Verdier, P. H., eds., Standard

Reference Materials: The Characterization of

Linear Polyethylene, SRM 1475, NBS Spec.

Publ. 260-42 (September 1972). COM72-

50944**

Yakowitz, H., Ruff, A. W., and Michaelis, R. E.,

Standard Reference Materials: Preparation and

Homogeneity Characterization of an Austenitic

Iron-Chromium-Nickel Alloy, NBS Spec. Publ.

260^3 (November 1972). COM73-50760**

Schooley, J. F., Soulen, R. J., Jr., and Evans, G. A.,

Jr., Standard Reference Materials: Preparation

and Use of Superconductive Fixed Point

Devices, SRM 767, NBS Spec. Publ. 260^4

(December 1972). COM73-50037**

Greifer, B., Maienthal, E. J., Rains, T. C, and

Rasberry, S. D., Standard Reference Materials:

Powdered Lead-Based Paint, SRM 1579, NBS

Spec. Publ. 260^5 (March 1973). COM73-

50226**

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference Materials: Austenitic Stainless Steel, SRM's 735 and

798, from 4 to 1200 K, NBS Spec. Publ. 260^6

(March 1975). SN003-003-0 1278-5

Hust, J. G., Standard Reference Materials: Electrical Resistivity of Electrolytic Iron, SRM 797,

and Austenitic Stainless Steel, SRM 798, from 5

to 280 K, NBS Spec. Publ. 260^7 (February

1974). COM74-50176**

Mangum, B. W., and Wise, J. A., Standard Refer- ence Materials: Description and Use of Precision

Thermometers for the Clinical Laboratory,

SRM 933 and SRM 934, NBS Spec. Publ. 260^8

(May 1974). 60 cents* SN003-003-0 1278-5

Carpenter, B. S., and Reimer, G. M., Standard

Reference Materials: Calibrated Glass Standards for Fission Track Use, NBS Spec. Publ.

260^9 (November 1974). SN003-003-0 1344-7

Hust, J. G., and Giarratano, P. J., Standard Refer ft ence Materials: Thermal Conductivity and Eleci I trical Resistivity Standard Reference Material! i Electrolytic Iron, SRM's 734 and 797 from 4 t I 1000 K, NBS Spec. Publ. 260-50 (June 1975 I 1.00* SN003-003-0 1425-7 ( Mavrodineanu, R., and Baldwin, J. R., Standar it Reference Materials: Glass Filters As a Standar i Reference Material for Spectrophotometry I Selection; Preparation; Certification; Use-SRI\ <

930, NBS Spec. Publ. 260-51 (November 1975 < $1.90* SN003-003-01481-8

Hust, J. G., and Giarratano, P. J., Standard Refei 1

ence Materials: Thermal Conductivity and Elec I trical Resistivity Standard Reference Materia! I 730 and 799, from 4 to 3000 K, NBS Spec. Pub I 260-52 (September 1975). $1.05* SNOO3-O0!

01464-8 | Durst, R. A., Standard Reference Material; $ Standardization of pH Measurements, NB \ Spec. Publ. 260-53 (December 1975, Revised $ $1.05* SN003-003-01551-2

Burke, R. W., and Mavrodineanu, R., Standar

Reference Materials: Certification and Use ( Acidic Potassium Dichromate Solutions as a Ultraviolet Absorbance Standard, NBS Spei

Publ. 260-54 (August 1977). $3.00* SN003-00:

01828-7

Ditmars, D. A., Cezairliyan, A., Ishihara, S., an Douglas, T. B., Standard Reference Material:

Enthalpy and Heat Capacity; Molybdenui

SRM 78 1

, From 273 to 2800 K, NBS Spec. Pub

260-55 (September 1977). $2.20* SN00300:

01836-8

Powell, R. L., Sparks, L. L., and Hust, J. G

Standard Reference Materials: Standar

Thermocouple Material, Pt.-67:SRM1967, NB

Spec. Publ. 260-56 (February 1978). $2.20

SN003-003-0 18864

Cali, J. P. and Plebanski, T., Guide to Unite

States Reference Materials, NBS Spec. Pub

260-57 (February 1978). $2.20* SN003-00.

01883-0

Barnes, J. D. and Martin, G. M., Standard Refe: ence Materials: Polyester Film for Oxygen Ga

Transmission Measurements. SRM 1470, NB

Spec. Publ. 260-58 (in press).

Chang, T. and Kahn, A. H., Standard Referenc

Materials: Electron Paramagnetic Resonanc

Intensity Standard: SRM 2601, NBS Spei

Publ. 260-59 (August 1978). $2.30* SN-00.'

003-01975-5

vi
felapoldi, R. A., Paule, R. C, Schaffer, R.,

Mandel, J., and Moody, J. R., Standard Refer- ence Materials: A Reference Method for the

Determination of Sodium in Serum, NBS Spec.

Publ. 260-60 (August 1978). $3.00* SN-003-003-

01978-0

'erdier, P. H. and Wagner, H. L., Standard Refer- ence Materials: The Characterization of Linear

Polyethylene (SRM 1482, 1483, 1484), NBS

Spec. Publ. 260-61 (December 1978). $1.70*

SN-003-003-02006-1

oulen, R. J. and Dove, R. B., Standard Reference

Materials: Temperature Reference Standard

for Use Below 0.5 K (SRM 768), NBS Spec.

Publ. 260-62 (April 1979). $2.30* SN003-003-

02047-8

'elapoldi, R. A., Paule, R. C, Schaffer, R.,

Mandel, J., Machlan, J. L., and Gramlich, J. W.,

Standard Reference Materials: A Reference

Method for the Determination of Potassium in

Serum, NBS Spec. Publ. 260-63 (in press).

* Send order with remittance to: Superintendent

of Documents, US Government Printing

Office, Washington, DC 20402. Remittance

from foreign countries should include an

additional one-fourth of the purchase price for

postage.

* May be ordered from: National Technical

Information Services (NTIS), Springfield,

Virginia 22151.

;

vii
TABLE OF CONTENTS

Page

1.

2. Selection of a Film for Use as an SRM for Oxygen Gas Transmission Rate . . 3

2.1 SRM 1470 - Provenance and Packaging •.•••I 4

2.2 Sampling for Characterization Purposes 5

2.3 Treatment of Specimens Prior to Testing 5

3. Data 6

4. Statistical Analysis of the Experimental Data 9

4.1 Models for Describing the Experimental Data 9

4.2 • 10

4.3 • 12

5. The • 16

5.1 • 18

5.2 • 18

6. How to Use the Certificate Values for SRM 1470 • 21

7. The Units of P • 2|

8. • 23

8.1 Effect of Thermal History ("Aging") • 23

8.2 • 25

8.3 • 26

8.4 • 26

9. • 2i

10.

n.

12. Appendix A

vi i i
LIST OF FIGURES

Figure Page

1. Cross Sectional View of the NBS Sample Cell 8

2. Scatter Diagram for the Specimen Effect on the Measured Permeance 13

3. Scatter Diagram for the Specimen Effect on the Time-Lag 13

4. Scatter Diagram Giving Specimen Effect on the Permeance as a Function of

Position Within the Stratified Sample ..... 14

5. Scatter Diagram for Time-Lag Values as a Function of Position of the

Specimen Within the Stratified Sample 14

6. Correlation Plot of the Specimen Effect for Permeance Versus that for

Time-Lag 17

7. DSC (Differential Scanning Calorimetry) Thermogram for SRM 1470 27

LIST OF TABLES

Table Page

1. Treatment Schemes Used to Measure Certificate Values for SRM 1470 6

2. Estimates from Multiple Linear Regression Analysis 11

3. Differences Between Samples from the Same Can 16

4. Quantities for Interl aboratory Comparisons 19

5. Units Related to Permeability 21

6. Effect of Thermal History on Permeability 24

7. Effect of Thermal History on Time-Lag 25

ix

SRM 1470: Polyester Film for Oxygen Gas Transmission

Measurements

John D. Barnes and Gordon M. Martin

Polymer Science and Standards Division

Center for Materials Science

National Bureau of Standards

Washington, D. C. 20234

Abstract

This report presents information which should be of interest to users of NBS

Standard Reference Material 1470. This SRM takes the form of 23 ym thick sheets of

poly(ethylene terephthalate) film. The gas transmission rates of these films with

respect to oxygen gas have been carefully analyzed. We describe where the film comes

from, how it is packaged, and how it should be conditioned prior to measuring. The

steps which were taken to characterize a random sample of sheets from the production

lot of the SRM are discussed in detail. The gas transmission rates and the time-lags

Df 22 films were measured using a state-of-the-art electronic manometric permeation

facility. The temperature dependence of the permeability was determined over the

temperature range 288 K to 310 K. A small pressure effect was found which is thought

to be an artifact. The statistical measure which were derived from the data are

Jiscussed in detail. It is concluded that the largest source of variability is from

)ne sample to another with a coefficient of variation amounting to 4 percent. A brief

liscussion of units for expressing permeabilities is given. Effects due to thermal

:ondi tioni ng ("aging") and outgassing are discussed.

fey Words: Gas transmission rate; manometric technique; oxygen; permeability; poly(ethyl

terephthalate); standard reference material; temperature coefficient; time-lag.

1
1. INTRODUCTION

The oxygen gas transmission characteristics of polymer films used in packaging are frequently a primary determinant of the performance of packages made from such films.

Films which do not allow gases or vapors to permeate them are said to have good "barrier

properties." In the packaging industry there are cases where good barrier properties are desirable and other cases where a film with poor barrier properties is desirable. The

role of oxygen gas or water vapor [1,2]* in causing premature degradation of food or drug

products is obvious. Less obvious is the role that a controlled influx of oxygen can play

in maintaining the appearance ("bloom") of packaged meats [1]. There are a number of

applications of packaging materials in which films of two or more materials are laminated

to produce barriers with special properties. In all such applications it is necessary to

have means of monitoring the performance of the barrier materials.

Early in 1975, members of ASTM Committees F-2 (Flexible Barrier Materials) and D-20

(plastics) decided that there was a need to undertake the development of standards relatin

to the use of a commercially available measurement system for oxygen gas transmission

rates which is based on a coulometric detection principle [3]. In order to evaluate the

precision and accuracy of such measurements it was necessary to have available, for

interlaboratory testing, a quantity of a barrier material with known oxygen gas transmissi

properties. The similarity between this requirement and the criteria for establishment of

a Standard Reference Material (SRM) [4] led the Polymers Division to undertake studies aim

at the establishment of suitable SRM's. A market survey revealed that there was likely to

be enough demand on a continuing basis to justify SRM production.

Once the desirability of an oxygen gas transmission SRM was established, the three

primary technical questions which had to be answered were:

1) What properties of the SRM should be measured in order to meet the user's needs?

2) What substance possesses the most desirable properties for consideration as an

3) What is the state of the art needed to measure the desired characteristics of

In order to answer the first of these questions, let us consider a description of

transport of one substance through a matrix of a different substance [5]. We assume at

the outset that transport in a homogeneous membrane is described by values of the diffusic

coefficient, D, and the solubility, S, which are independent of permeant concentration ant

position within the matrix. We further assume that there is a linear relationship betweei

the external vapor pressure, p, and the equilibrium concentration, C, within the matrix

(Henry's Law; C=Sp). If we consider a membrane of thickness SL with a pressure p-j on one

side and a pressure (P2 < P]) on tne other side then gas will be transported across a

unit area of the membrane at a rate

1 Numbers in square brackets refer to the list of references at the end of this paper.

SRM?

the SRM?

2
F = p CP-, " P

2

) = P(p

q " P2

) (1)

It is this rate of gas transmission, F, which is the quantity of interest to workers

the packaging industry. In general, p-j is the partial pressure of the chosen permeant

ecies in the atmosphere and is zero. The combination of D, S, and £ can be measured

re accurately than any single component (see reference [6] for further discussion of

is question). For this reason, we elected to characterize P, the ratio between the

rmeant flux and the partial pressure difference across the membrane. We call this

antity the "permeance."

If the experiment to characterize the permeance2 is properly carried out, it is

ssible to characterize an additional quantity x, called the "time-lag." Under the

2 sumption of a homogeneous membrane the time-lag is given by t = £ /6D [5]. The time-lag

a measure of the length of time required for the permeant flux to reach a steady state

ter a step-function increase in the pressure is applied to the upstream side of a

nmbrane which is free of permeant. As will be discussed later, the time-lag is a useful

1ol in checking for pinholes in films.

The temperature and pressure variation of both the permeance and the time-lag should

characterized so that meaningful comparisons between measurements can be carried out

er a range of temperatures and pressures rather than at just one set of conditions.

Our examination of the existing permeation measuring methods demonstrated that the

st way to obtain a system with the required sensitivity, flexibility, and traceability

s to build it ourselves along classical lines [7]. The result was an automated partition

11 using a capacitance-operated manometer as the detector. The development and use of

is instrument are described elsewhere [6].

2. SELECTION OF A FILM FOR USE AS AN SRM FOR OXYGEN GAS TRANSMISSION RATE

Five characteristics which an SRM should possess are listed below:

1) The material should be homogeneous with respect to the property of interest

throughout a lot.

2) The size of the lot should be large enough to permit all interested parties to

obtain samples for a period of ten years or so.

3) The material should be stable under storage and shipment during the period it is

stocked.

4) It should be possible to fabricate test specimens without changing the characteristics of the material.

i the certificate for SRM 1470 (see Appendix A) this quantity is called the "permeability

efficient. " The matter of units is discussed more fully in Section 7.

3
5) The material should possess a value of the parameter under test which is repre- 1

sentative of the range of the apparatus used in industry to measure the property

A sizable segment of the packaging industry has already adopted poly(ethylene tere- I

phthalate) (PET) film approximately 25 urn in thickness as a de facto standard. In many

ways this is an excellent choice. The material is plentiful, thus guaranteeing an assured

supply. Dimensional stability at ambient temperature is good, indicating that any anneal V

effects [8] are taking place very slowly. Some annealing effects may be detectable over ; long period of storage.

PET is a "good" oxygen barrier. "Excellent" oxygen barriers possess gas transmissiot

values approximately 1/5 to 1/50 of the ones for PET. It is thought that the barrier

properties of PET are relatively insensitive to the presence of moisture or other extranet

influences but our knowledge in this area is inadequate. A limited attempt to estimate

the effect of "aging" on the film was made in the present study (see Section 7.1). It is

known that "cyclic trimer" and other low-molecular-weight species are present in the

material and the effect of these on the aging process or the barrier properties is poorly

known.

PET film in the 25 urn thickness range is relatively robust. Care must be taken to

avoid creasing the film as this is likely to produce pinholes. The usual dies, punches,

etc. can be used to cut out samples for measurement.

No other candidate film which was clearly superior to PET with respect to the above

characteristics was offered for consideration and it was decided to use a 23 urn PET film

on the basis of past experience.

2. 1 SRM 1470 - Provenance and Packaging

The polyester film from which SRM 1470 is made was obtained from E. I. DuPont de

Nemours and Company3 in the form of sheets approximately 21 x 27 cm with a nominal thickn.s

of 23 urn. The film is known to be biaxial ly drawn and has a nominal density of 1395

kg/m

3

.

The roll from which the sheets are cut is the middle third of a mill roll. Experien;

shows that the middle third is more uniform in thickness and has fewer flaws than the

outer parts. The roll of film was cut into sheets and stacked by a machine whose operatii

is such that every fifth sheet in the stack came from an adjoining part of the roll.

The sheets of film were packaged by NBS for distribution before the specimens used i

the random sample were drawn. Fifteen sheets of film are packaged in a cylindrical tinplate can approximately 10 cm in diameter and 25 cm long. The can has a friction fit lie

The fifteen sheets were simply taken consecutively from the lot of material as received.

Special care was taken to avoid creasing the film at all stages of the packaging and otht

handling. The packet of fifteen sheets is enclosed in a slip case made from a high qual'iy

paper with a low pH and a low sulphur content. This slip case is intended to protect the

film from damage caused by the metal can or the hands of the user.

3 Identif ication of the products of particular manufacturers is made purely for purposes <

identifying experimental conditions and does not imply endorsement by the National Bureai

of Standards.

4
12 Sampling for Characterization Purposes

Two goals of the experimental work were to provide characterization of the "true"

nan value of the permeability coefficient in the lot of material which makes up SRM 1470

li to provide meaningful measures of the variability that a user could expect from measure- mts on individual sheets of film or from different films within the small lot of fifteen

ssets issued by NBS.

A stratified random sample was chosen as the scheme for selecting films from the lot

If testing. This procedure offered the prospect of being able to characterize systematic

viations from one part of the lot to another. The scheme for sampling was as follows:

1) The population of 1120 cans of film was divided into 28 sequential groups of 40.

i 2) One can of the 40 was selected at random from each group.

3) Two films out of the fifteen in each can were selected at random. Thus, there were a total of 56 samples.

4) The samples were numbered from 1 to 56.

5) The order in which the samples were submitted to testing was determined with the

help of a random number table.

6) Measurements were terminated when enough degrees of freedom had been accumulated

I to adequately characterize the population of SRM films.

21 Treatment of Specimens Prior to Testing

Other workers have found that better results are obtained if films are preconditioned

b ore their gas transmission rate is measured. Care must be exercised in choosing proper

ciditions because it is possible to change the crystal 1 inity of PET by annealing it at

tiperatures not too far below its glass transition [8] (75 °C < T < 95 °C). We observed

Q

tit the films tend to outgas over long periods of time. This tendency is somewhat reduced

ithe film has been placed under a vacuum at a temperature in the neighborhood of 40 to

5 °C (i.e., well above the actual measurement temperature). The experimental results

i Section 8.1 indicate that the effect of thermal history on the permeation rate as

in sured by our method is small. It is possible that the leakage or background flux,

w ch is found to be strongly influenced by prior conditioning, interferes with measurements

ijsome cases. Those outgassing species which are removed by this rather mild treatment

a probably atmospheric gases such as ^ O2, CO2, ^0, etc. There is another, much more tacious, component which is probably made up mainly of low-molecular weight degradation

p ducts or oligomers of PET.

During a measurement, there is additional outgassing because the gas is in contact

wh the specimen only while measurements are actually being made. There may be delays of

Iki 2 to 8 hours between the time a fresh specimen cell is connected to the system and

I first measurements are made.

Users who make measurements within a short time after placing the specimens in their

insuring apparatus should expect short-term drifts. This is likely to be true in any

m-specific detection scheme where correction is not made for the background flux.
In order to be on the safe side and since we found that results were more reliable •

when the background flux was minimized, we subjected films to an outgassing cycle in an ! evacuated jar (i.e., a vacuum dessicator containing no dessicant) at 50 °C overnight, ill vacuum was applied continuously with a mechanical pump using a liquid cold trap in thl

line. It was necessary to filter the air which was admitted to the dessicator before

opening it. The films tended to attract dust and dirt from unfiltered air. An alpha

particle source placed in the dessicator with the films reduced problems with static

charges.

3. DATA COLLECTION FOR CERTIFICATE VALUES

It was decided to characterize the following properties of SRM 1470:

1) The permeance P(Tq,Pq), at 23 °C and one atmosphere pressure differential,

2) The temperature coefficient of the permeance, and

3) The effect of pressure on permeance (if one was found).

The time-lag and its temperature and pressure dependence were also characterized. The

range of values over which these were to be characterized was limited to that expected '

the normal laboratory environment.

In order to characterize the above quantities it was necessary to subject each samfa

to a number of different levels of the variables in the experiment. We elected to use ; 3x3 factorial design as shown in Table I. There are, therefore, nine different treatments to be applied to each sample. It is possible that the order in which the treatmeis

are applied could alter the observations (through aging). Thus, there may be a need to

randomize the order in which the treatments are applied.

Counterbalancing this need is the fact that the user of the film will generally be

interested only in making his measurement under standard conditions (23 °C, 0.1013 MPa).

For this reason, in our measurements we always applied the standard treatment first.

Table I

Treatment Schemes Used to Measure Certificate

Values for SRM 1470

Scheme A

Scheme B

P

0

"AP

Po

P

0+Ap

P

0

-AP

Po

P 0+AP

T

Q

-AT

9

7

8

T

Q

-AT

6

4

5

3

1

2

T

(

3

1

2

TQ+AT

6

4

5

TQ+AT

9

7

8

6
The second factor influencing the choice of the order in which treatments were to be

iplied was the fact that it was much easier to change the inlet pressure than to change

lie temperature. Thus, a particular temperature level was selected and the tests were

irried out at the three different pressures before moving on to another temperature

1

ivel. Complete randomization of the order of all treatments after the first, including

le constraint that all three pressure levels had to be tested before moving on to a new

imperature level, leads to 144 different orders in which to carry out the 8 measurements

n'ch remain.

It was felt that any effect of pressure on aging would be minimal. The order in

n'ch the pressure treatments were applied was, therefore, fixed. This reduced the number

: treatment schemes to 2. Both schemes are shown in Table I.

True replication would require that the specimen be removed from the cell and reinserted

itween measurements. This was not practical in our case. In practice, however, a certain

lount of replication was provided by repeating parts of a treatment sequence at a given

imperature before going on to the next temperature. After enough data had been collected

i characterize the temperature coefficient of permeance the measurement was further

mplified by collecting only data from treatments 1, 2, and 3.

The NBS automated manometric permeation measuring facility and its operation are

scribed in detail elsewhere [6]. An abbreviated description of the time-lag mode of

eration used in these experiments is given below.

The specimen is first placed in a cell as shown in Figure 1. The fitted glass

ipports constrain the film and prevent distortion caused by an applied pressure gradient.

ie function of the microporous cellulose acetate cushions is to minimize damage to the

lm from loose glass particles or sharp edges in the cell. It is also thought that

rect contact between the film and the impermeable glass of the rigid substrate may cause

rors in the effective area through which the film is transmitting gas. The highly

rmeable cushion allows gas to diffuse away from the downstream side of the film before

y localized excess of gas could accumulate.

After the specimen cell is connected into the measuring system, both the upstream

nlet) and the downstream (outlet) sides of the cell are evacuated. This provides an

ditional opportunity to eliminate gases sorbed in the film. The goal of removing all

ses which could interfere with the measurement is never achieved. For this reason, the

rst step in the measuring process is to estimate the rate at which the film is outgassing.

order to accomplish this, the outlet side of the cell is isolated from the vacuum pumps

ile the. inlet side of the cell remains under a vacuum. A small pressure rise, due to

ntinued outgassing from the film, is observed in the measuring head. The apparent gas

ux due to this source is measured. This flux is referred to as the "background" or

eakage" flux. Large values of the leakage flux often meant that the experiment was

bject to a systematic error.

At an appropriate time (which is a new zero value for elapsed time) the gas pressure

the inlet side of the film is quickly raised to the value desired for measuring purposes,

essure measurements on the outlet side of the membrane are made at closely spaced intervals

ea!

t!

7
CROSS SECTIONAL VIEW OF NBS SAMPLE CELL

GLASS FRIT

CUSHION

'O" RINGS

CUSHION

GLASS FRIT

VACUUM AND OUTLET

PRESSURE MEASUREMENT

INLET

PLATE

POLYESTER

SPECIMEN

GUARD RING

OUTLET

PLATE

UUM

Figure 1. Cross Sectional View of NBS Sample Cell. This exploded view

of the cell depicts the parts of the assembly that "sandwich"

the specimen between the inlet (upstream) and outlet (downstream)

chambers. The fritted glass supports provide necessary rigid

support. The guard ring serves to trap gases that would otherwise

enter the downstream chamber by leaking along the downstream

face of the specimen.

8
until a time corresponding to 2 or 3 times the expected time-lag has elapsed. The interval

between pressure measurements is then increased until the total elapsed time has exceeded

the time-lag by a factor of 6 or 8. By this time the permeant flux will have attained a

steady state. Values of pressure as a function of time were recorded on a magnetic tape

cassette for subsequent computer analysis.

The analysis of the data consisted of extracting values of the permeance P, and the

time-lag, t, from readings which had been corrected for background flux and calibration

errors. An iterative nonlinear least squares technique was used.

The mean of the squared residuals between the model obtained by non-linear leastsquares and the data was called and was used as a criterion for rejecting bad experimental runs. In all cases where was found to be greater than 0.3 Pa a systematic

error was found in the raw data. In general it was found that the time-lag values from

such sets of data were also meaningless.

Estimates of the errors in the parameters can also be obtained from the least-squares

fits. Such estimates should be considered as measuring how well a given set of data fits

the equation describing the pressure rise as a function of time. The precision with which

P and t are determined on a given specimen is more properly assessed by replicate experiments.

4. STATISTICAL ANALYSIS OF THE EXPERIMENTAL DATA

For the purpose of an SRM certificate, the experimentally determined quantities must

be reduced to estimates of values which describe the whole population of possible values

in terms of simple mathematical expressions.

|4. 1 Models for Describing the Experimental Data

Since the steps which were taken to accomplish this were the same for measurements of

both P and t, we will use the symbol Q to designate either one of these quantities in the

ensuing discussion. We choose to label the specimen number by the symbol v and the

experiment number by the index i. If the temperature during the i'*' experiment is T and

the pressure is p^., where 1 < j < 3, we can describe the result of a measurement by Q.j(T,

p., v). It is reasonable to describe the temperature influence by means of an activation

energy:

J Q (T, p, v) = Q (T

Q

, p, v) exp (j- - j)] (2)

With only three levels of the pressure available, it appears better to describe the

pressure influence in terms of a discrete variable. The model used to describe the

pressure effect is:

Q (T, p, v) = Q (T, p, v) e ^

(3)
with I <J>. = 0. It is expected that $ will be small. It is also expected that the influence

of the specimen on Q (T, p, v) can be represented by a deviation from a mean value which

is a random function of v. This sample effect can be represented as:

Q (T, p, v) = Q (T, p) e (4)

where I = 0. The values of <|)j and are really coefficients of variation which measure

the percentage variation due to the two effects. If we combine Eqs. 2, 3 and 4 and take

the logarithms of both sides we can write our statistical model in the form:

h_

£n Q

i (T, p, v ) = u (T

0

, p) + P

v + <t)j + RTTq

(T - T

Q

) + 6. (5)

where e. is the random error in the observation. In Eq. 5 all interaction terms between

factors have been omitted. A search was made for such effects but they were found to be

negligible. Thus the e. may contain a contribution from the neglected interactions in

addition to reflecting the random experimental error. The presence of the T - Tq term and

the fact that the design is not balanced (due to the rejection or absence of some observations) led us to use multiple linear regression techniques [9] for obtaining estimates

of p v , <b., and E fl in Eq. 5. The vehicle used was the FIT command of the OMNITAB computer

J " program [10]. The distribution of the residuals was examined and it was found that the

e- were distributed normally.

4. 2 The Significance of the Regression Estimates

The parameter estimates derived from fitting the permeance and time-lag data to

Eq. 5 are listed in Table II.

The units of are pmol/(m^'S*Pa) for the permeance and seconds for the

time-lag. The activation energy E^ is given in kJ/mol. The quantities § and 0 are dimensionless.

The parameters ty^ t and §^ are the first parameters obtained from the regression

analyses on the experimental data that we wish to discuss. Non-zero values of the

<t>'s imply that there is a change in the permeability coefficient (or the time-lag) as the

inlet pressure is changed. Indeed it appears that the permeance increases with increasing

pressure (t^X^X)^ ) while the time-lag is unaffected ((|>j(T) = 0).

The only mechanism known to produce a variation of P and t with pressure when one is

considering dilute gases in semicrystal 1 ine polymers is partial immobilization [11]. If

this mechanism were the source of the observed variation the trend of the (|)(P) would have

to be reversed. It is also to be expected that <t>(t) ¥ 0 if partial immobilization is at

work. Data of other workers [12] indicates that partial immobilization does not operate

in the system 0^ - PET. We are, therefore, left with the conclusion that partial immobilization is unlikely to be the source of the pressure effect on the permeance and the

10
Table II

Estimates from Multiple Linear Regression Analysis

Permeance Time- Lag

Parameter Val ue Est. S.D.

(s

K (P))

Value Est. S.D

(s

K

(x))

M(T

0

,P 0

) -1. 128 0.004 5.840 0.006

eu + (J) 3 0.3302 0.0014 343.8 1.9

27.2 0.3 -38.9 0.8

<| >1 (p=.0872) -.028 .003 0.000 .007

<|> 2 (p= 1028) .008 .003 0.000 .007

<t> 3 (p=.1155) .020 .003 0.000 .007

APi -.050 .009 -.029 .022

A

P 2 -.007 .009 .071 .022

A

P 3 -.064 .010 -.023 .024

A

P 4 .027 .011 -.002 .026

A

P 5

' -.004 .011 -.001 .026

AP 6 .040 .011 -.130 .026

A

P 7 .025 .009 -.028 .020

APs .018 .010 .010 .023

A

P 9 -.042 .010 .021 .023

PlO .025 .009 .059 .022

APll -.059 .010 .065 .023

APl2 .013 .009 -.017 .022

APl3 .019 .010 .014 .023

APl4 -.024 .010 .027 .023

APis .010 .007 .011 .016

Pl6 .004 .008 .034 .019

APl7 -.025 .008 .005 .018

Pl8 .028 .016 -.040 .039

A.Pl9 .019 .016 -.022 .039

AP20 .038 .014 .028 .034

AP21 .029 .012 -.011 .028

AlP22 -.020 .013 -.013 .031

11
effect is, therefore, an artifact. The most likely source of this artifact is contaminatior

of the upstream gas reservoir by ^ which leaks in (from the atmosphere along with O^)

when the reservoir is evacuated and refilled. Confirmation for this interpretation is

provided by the fact that the effect is also observed with He gas. The reservoir is being

rebuilt to eliminate leakage and the operating pressure range of the system is being

extended.

If contamination of the inlet reservoir is the source of the pressure effect P should

approach its correct value asymptotically from below as p.- nlQ+ is raised. On this basis,

( +fh ^

I'll I St

the value e^ *3. should be interpreted as the best (although biased) estimate for the

permeability coefficient which can be derived from the present data. In any case, the §

values represent a relatively small effect.

The values of permeance and diffusion constant found in this work are both lower than

those observed by Michaels, et al. [13]. This indicates that SRM 1470 posesses a morpholog;

which tends to impede the diffusion of oxygen molecules. This might be attributable

either to crystal 1 i nity or to orientation.

The temperature effect is much more important than the pressure effect and considerabT

care should be taken to ascertain the temperature in any permeance measurement using SRM

1470. Differences between the experimental temperature, T, and the reference temperature,

Tg, can be accounted for by means of the following equation:

Q (T) = Q (T

Q

) exp (T - T

Q

)} (6)

0

The approximate temperature interval over which was obtained is from 288 K to 310 K.

The use of Eq. 6 for correction outside this interval is not justified (see Section 5.2).

The values of E^ found in this work are somewhat lower than those found by Michaels et al.

[13]. Such differences are not too surprising because the specimen morphology is known to

influence the activation energies.

4. 3 Specimen- to-Specimen Variabi 1 ity

The parameters p-j through p„o represent effects due to changing from one specimen to

another. Thus, if all data are corrected for temperature and pressure effects, there

would still be significant differences between data taken on different specimens. The source of these differences may be in differences in the physical state between specimens

or in some uncontrolled feature of the experiment which varies from one specimen to

another. Figures 2 and 3 are scatter diagrams for PV (P) and Pv (t) respectively. While

the scatter diagrams do not clearly demonstrate that the parameter values are significantly

different from zero the appropriate F tests show that the p terms contribute significantly

to reduction of variation. The absence of a trend in these plots suggests that there was

no influence attributable to a systematic factor whose value changed during the course of

the experiments. The plots in Figures 4 and 5 indicate that there was no systematic trend

from one part of the lot of material to the other which would give rise to the observed

12
>

ca.

gure 2.

6 8 10 12 14 16 18 20 22 24 26 28

SEQUENCE IN MEASUREMENT

Scatter Diagram for the Specimen Effect on the Measured Permeance. The absence

of a trend suggests that any cause of the specimen-to-specimen variability did

not change during the sequence of measurements.

.04

.02

0.00

Q_

~> -.02

ca.

-.04

-.06

i—i—i—

i

j—

I

DCO• ui <

>cc

b

CM

J I I I I L

>

CO. cco

<

>cc

0

CNI

I <

Q_

>-

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

SEQUENCE IN MEASUREMENT

gure 3. Scatter Diagram for the Specimen Effect on the Time-Lag. The absence of a trend

suggests that no systematic changes were made in the technique for measuring the

time-lag as the experiments progressed.

13
0.04

0.02

0.00

ca. -0.02

-0.04

-0.06

1 7

T —i

1

1 1

i

1

1 1

1 1 1 1 1 1

1 1 1 1 1

• •

• i • •• > • LU • (—

• b

CNI •

#

<C_5 — Q_ - • —

>- •

• 1— •

—

• —

•

•

1

1 1 1

1 1

1 1 1 1

Figure 4.

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

SEQUENCE IN STRATIFIED SAMPLE

Scatter Diagram Giving Specimen Effect on the Permeance as a Function of Position

Within the Stratified Sample. The absence of a trend indicates that properties do

not change from one part of the batch of material to another.

0.08

0.04

>

CO. -0.04

-0.08

-0.12

I 1 1 1 1 I.I 1 1 1 1 1 • •

_ i <

>

QC • LU

1— •

• 2j • • •

•

CNI • —1 •

•

• • <C_3 •

•

Q_ • —

>- L 1—

•

1 ! 1

| III! I 1 I I I

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

SEQUENCE IN STRATIFIED SAMPLE

Figure 5. Scatter Diagram for Time-Lag Values as a Function of Position of the Specimen

Within the Stratified Sample. The precision of the time-lag data is less than

that of the permeance data. This accounts for the fact that more of the points

fall within the two-si gma limits.

14
variation. The P v are important because, as will be seen in Section 5.2, specimen-tospecimen variability is a primary source of differences among measurements made on SRM

1470. Since we do not now have a way of estimating p for a given specimen from additional

characterization measurements we have to be content with treating the specimen-to-specimen

effect as a random perturbation on the experimental measurements. It is reasonable to assume that the values of p are independent of one another and distributed normally with

2 2 mean 0 and variance aft (we shall use the notation p n(u, oR

) to indicate that a random

P P

2 variable, p in this case, is normally distributed with mean u and variance a ).

2

In order to construct tolerance intervals (see Section 5.2) we need to estimate aQ . P Now, if we recall that the p in Table II are estimates (which contain a random error introduced by the estimation process) of the actual values of p we can write p = p + e , 2 where e is the random error of estimation. We can assume that e ~ n(0, a ). Thus

ap

2 = 0^2

- a2.. The estimated standard deviations of the p as derived from the regression

analysis can be used to estimate a . Note that the standard deviations all lie between

0.007 and 0.016, the larger values being derived from experiments using a truncated design

(one temperature, T = Tq). The observed values of p form a sample of size 22 from the

popul ation of al l sheets of film and the sample standard deviation can be estimated by

SI =\*p2/(n-l) = 0.032. If we use 0.008 as a lower bound for a then aD = [(0.032)

2

- p 2 1/2 p

j (0.008) ] = 0.031 is an upper bound for from the permeance data. A similar analysis

of the time-lag data yields 0^(1) = 0.039.

It is reasonable to postulate that a portion of the specimen effect is attributable

to manufacturing variability and another portion of this effect is attributable to random

errors introduced in mounting the specimen in its holder. It is also possible that there

, are systematic differences from one cell to another which are not taken care of by the

calibration step. Unfortunately the design of the present experiment confounds the three

effects. The estimated variance of the sample effect appears to be consistent with what

is known about the process for manufacturing PET film.

The p values do not seem to vary in any systematic way from one sample cell to

another. It is, therefore, reasonable to discount any systematic cell-to-cell variability.

Given that the 15 sheets of film in a given can come from the same part of the

original roll of film, one might hope that the specimen-to-specimen variability within a

given can is lower than that over the lot as a whole. Unfortunately, it appears that two

different sheets taken from the same can should be considered as two completely independent

specimens. One basis for this statement is shown in Table III where we consider data from

the cases where measurements were made on two samples from the same can. If the two

measurements are considered as representative, the range R in Table III as determined from

a random sample of size 2 can be used as an estimate of aR for the population of specimens

I p in a single can. The fact that the mean value for the six observations of the range is

not greatly less than the value of aD estimated above leads us to reject the hypothesis of

P reduced variation within a can. The test described above is very crude and we have not attempted to estimate its

power. A more detailed characterization of variability with respect to wi thin-can and

15
within-lot components is prohibitive and the effort would be better spent improving the

overall precision and seeking simple estimators for p(P) and P(t).

In the analysis of the original experimental data, we found that cases where p(P)

and p(t) both lay considerably outside the appropriate tolerance intervals were indicative

of situations where pinholes were permitting anomalously high transmission through the

membranes.

This observation led us to look for a correlation between p v (P) and Pv(x). If, for

example, the specimen-to-specimen variability could be attributed to variations in thickness

one would expect to find p v (t) = - 2 p v (P). The scatter diagram of Figure 6 shows that

any such association between Pv (t) and PV (P) is unlikely.

Table III

Differences Between Samples from the Same Can

Permeability Time- Lag

CAN h KR

Pi R

234 0.019 0.029 0.010 0.041 -0.022 0.019

524 .013 .025 .012 -.017 .059 .076

583 -.059 -.007 .052 .065 .071 .006

660 -.024 .038 .062 .027 .028 .001

785 -.025 .004 .029 .005 .034 .029

1080 -.050 .018 .068 -.029 .010 .039

(ZR)/6 .039 .028

.031 .039

Definitions: Let 6-, and ^ De the result from measurements on two separate sheets from

the same can of film. Then = min , f^)* P

u = max (0S » f^' anc* ^ = "

^SL'

The lack of precision of the p values renders attempts to find suitable estimators

difficult. For example, a precision of better than one percent would be necessary if

thickness measurements were to be used to develop estimates for Pv based on variability

from this source. Thickness measurements with the needed precision are extremely difficult

(if not impossible) to make on specimens 23 urn in thickness.

Other quantities which should be investigated in an effort to develop estimators for

P include density, crystal linity, and crystallite orientation. Experiments to investigate

specimens with different morphologies are presently being prepared.

5. THE ACCURACY OF THE MEAN VALUES OF P AND t

The extent to which parameters derived from the regression analysis represent accurate

estimates of the true mean values of P and t is dependent on our success in providing

16
.08

.04

.02

.04

.06

.08

.10

.12

.14

I I I I I •

I JI I I I

•

1 1

—

• _ •

•

w —

•

•

•

•

• •

•

•

•

—

• •

•

i

i

i

i

I I I I I ii i ii r i i -.06 -.04 -.02 0.0 .02 .04 CO

Pv(P)

Figure 6. Correlation Plot of the Specimen Effect for Permeance Versus that for Time-Lag.

The absence of clustering around a trend line suggests that these two properties

are uncorrelated in any one piece of film.

17
traceability and eliminating bias. We have already pointed out that there is a possible

bias in the P value due to contamination of the upstream pressure reservoir. As far as

is known, this is the only source of bias.

The permeance is a function of the following quantities which must be measured [6]:

1. The rate of pressure rise

2. The downstream cell volume

3. The effective area of gas transmission

4. The inlet reservoir pressure

5. The absolute temperature

A systematic error in any one of these quantities could introduce a bias into the results.

The manner in which we have pursued these sources of error is discussed elsewhere [6].

For our present purposes it is sufficient to say that item 3 above is the most troublesome.

The time-lag values are subject to a somewhat different set of errors. Items 2, 3,

and 4 do not influence the time-lag measurements. If the precision of the time-lag

measurements could be improved and a similarly precise set of sorption measurements could

be carried out it should be possible to make an independent determination of the effective

area [14].

5. 1 Variances of the Mean Values

The mean values quoted in Table IV represent the result of a large number of measurements carried out under carefully controlled conditions. Each of these measurements has a 2

random error, e, associated with it. This random error is ^ n(0,a ). Contributors to

2 e XJ c*e include random measurement errors in the instrumentation for recording data and controllin

the experimental parameters. This source of error can, therefore, be expected to vary

2 from one measuring facility to another. The best estimate of a

g

is the mean-squared

residual which remains after fitting the model of Eq. 5 to the data (assuming that the

model is not deficient in some systematic fashion). The mean values of InQ determined

from our experiments differ from the true population means by a random amount which we

2 2 assume is ~ n(0,a ). The variance of e , a = V(e ), is estimated by the square of S

u MM M from the first line

p 2.2

of Table IV. The relationship between a and a is a =

CT 2 pep e . The value of Neff is less than N, the total number of measurements, because the

ef f

variance - covariance matrix is non-diagonal because of some factor, such as a lack of

balance, in the statistical design of the experiment. N was 200 and N ^ was = 153 for

the data as presented.

Thus, in order to improve our estimates of the means we can take more measurements or

reduce the error variance. The observed magnitudes of the error variances are consistent

with estimates obtained by analyzing the variances of the contributing factors. This

indicates that there is more to be gained from improving the precision and accuracy of

the measuring apparatus than from improving the statistics.

5. 2 The Use of SRM 1470 in Interlaboratory Comparisons

Facilitating interlaboratory comparisons of measurement methods and materials propertie:

is certainly a primary objective of the Standard Reference Materials Program of the National

Bureau of Standards [15]. In order to accomplish this goal, it is necessary to know what

18
coc

o

CO

S-fOD_eo

c_>£

o

p

> re

s_o

o> to

re S- 1— cu pc

S-o

4-

cu p

-pc

TO3c

05

TO O COo

00 o

LO

o

o

lia^—

cn

co + H

CO.

co

'—>

+

s

cn.

*—>

+ o00

en

i

CM

co >

+

00.

co >

+

. s CD.

o

>>

CM

00

COO

CM

coo

/ s CO. Q- -—

U >

tr> + + o CL CnJ CM o ca.

II o

_Q a + CO CO

TO 00 3- >- * o o CU o o

E St- *—

S-cu oo

1 CO

> + CM CO. (0 + 00

/—

N

1— >

o < CM ill 1— o O V (II a I— LU 1— CO. n. oo LU I— 1— CO > *—

> O0

cu P h- a

i qntrll ca <

r— -P CU i

S- 4-

4- cu o O TO s- E P CO -p P

T3 O •!- CU P ro TO

S- 4- 4— _J 00 S- cu a Q

cr <T3 O cu 4- P

c a S- CU cu C_3 O cz Q O0 OO c CU o u cu 99

>> (0 o •i- c: c a CU E CU c p 4— -p r— <1) cu (J CU u cu <o O oo p D.TD 5- re c S- c E o p fD •r— «r- cu TO TO -r- "O P

c * f— P 4- 4- 5- CO •r- U •r~

<tJ p > i— C 4- cu S- to S- CU CO P

3 cu CO cu 3 O •i

co to cu TO CL CU o- s: LU Q Q > s: > OO o;

cu i— CM CO
quoted on SRM certificates mean and, perhaps even more important, what a measurement in a given laboratory actually estimates.

Table IV presents information which can be used for this purpose. The data on lines

1, 2, and 3 can be combined to give the usual confidence interval estimates in the form

*n Q(T

0

) = u Q

(T

0

) ± la

S

p

(7)

For the present discussion, we have chosen to consider quantities which are natural

logarithms of ones which are actually measured. This means that terms which are added to

the logarithms are actually coefficients of variation and can be easily converted to

percentages. The interval given by Eq. 7 is an interval within which the true value of

Uq(Tq) will be found 100 x (1-a) % of the time. The confidence interval represented by

Eq. 7 above will generally not be approached by the average user of SRM 1470 because he is

usually not prepared to make the large number of measurements on the large number of

specimens that are necessary.

An experimenter who makes a measurement on a sheet of SRM 1470 can expect to obtain a permeance value (or a time-lag) which is different from the population mean value for four

reasons:

1) his measurement temperature may be different from Tq by an amount AT,

2) the specimen he tests is likely to have a non-zero value of p,

v 3) his test method may have a built-in bias,

4) his measurement will contain some random error.

The first effect can be accounted for by using the measured activation energy to

correct all readings to T = Tq. The second and third effects can only be accounted for by

making measurements on a large number of sheets of film. A bias will be revealed as a non-zero value for the mean of p. Contributions due to random experimental errors must,

as usual, be estimated from replicate measurements. All of the above factors must be

considered when a user asks whether his measurement falls within an acceptable tolerance

interval (see Section 6). Recourse must be taken to the Analysis of Variance [16] to

construct intervals for more complicated sequences of measurements involving replication

and multiple specimens.

Measurements made close to Tq have minimum variance. The variance increase slowly as

[AT I >0 (at a AT of 15 °C this term constitutes (0.016)

2

to the overall variance). A much

more important term, over which the experimenter has no control, is V(p). We have seen earlier that this is a characteristic of the population of specimens. Indeed, this term

is the limiting factor on interlaboratory comparisons because it is necessary to make

measurements on a larger number of specimens in order to see if there is a bias present.

Further work is needed to provide for greater uniformity in specimens for a gas transmission

rate SRM. Alternatively, it may be possible to develop relationships which will permit us

to use the results of other measurements, such as thickness or density, to predict values

for p.

20
QUANTITY

Quantity of Matter

Table V

Units Related to Permeability

SI UNIT COMMON UNIT

(abbreviation)

mole (mol)

Thickness

Pressure

meter (m)

pascal (Pa)

Area

Time

Example:

cm Hg).

square meter (m )

second(s)

cm (STP)

gram

grain

centimeter

mi 1

pound per square

inch

atmosphere

centimeter of

mercury (cm Hg)

100 square inches

square centimeter

day

CONVERSION FACTOR

(common to SI)

4.462 x

10" 5

depends on molecular weight

depends on molecular weight

1.000 x 10" 2

2.540 x 10~ 5

6.895 x 10

3

1.013 x 10

5

1.333 x 10

3

6.452 x 10" 2

1.000 x 10" 4

8.640 x 10

4

I

- 1 0 3

A widely known unit of permeability is the barrer which is 10 cm (STP)/(cm s

The table above provides a series of conversion factors as follows:

1 barrer = 10~ 10x 4.462 x 10

_5/(10" 2 x 1.000 x 1.333 x 10

3

)

= 3.347 x

10" 16 mol/(m s Pa) = .3347 fmol/(m s Pa)

-15

where we have used the prefix for 10 (femto or f) to eliminate unnecessary

powers of 10.

One cause of results which are far outside the interval defined by Eq. 7 is pinholes

in the specimen. Pinholes (if they are prominent enough) give rise to large values of P,

low values of t, and large errors in fitting the time-lag model to the raw data. A user

vho encounters a reading of this kind should examine his data very carefully. Another

-est for pinholes is discussed in Section 8.

6. HOW TO USE THE CERTIFICATE VALUES FOR SRM 1470

Contacts with some of the first persons to use SRM 1470 indicate that there is some

iifficulty in adapting to the set of units we have specified on the certificate which
accompanies each purchase of SRM 1470 (see Appendix A). As noted in Section 7 of this

report, we have taken a departure from existing unit systems. The following information

is provided to help users make conversions from the units they use to SI.

P(T,p) is the ratio between the oxygen flux across the barrier and the pressure

difference (in oxygen partial pressure) which exists across the membrane during the

measurement process. Most of the standard measurement techniques measure the amount of

permeant which flows across a well-defined area in unit time. This rate of flow can be

converted to a flux by normalizing the result to a unit area of membrane. It is this flux

which we have designated as the gas transmission rate, G. Table I in Appendix A is meant to be used for converting gas transmission rates among

different unit systems. In order to use Table I in Appendix A it is first necessary to

convert the result of the user's measurement to a gas transmission rate. On directindicating devices this step is taken care of by internal calibration devices. The gas

transmission rate should then be divided by the partial pressure difference across the

membrane to obtain the measured value of P(T,p).

2

Example : A user measures a gas transmission rate of 63.8 scc/m 24 hr) on a directreading gas transmission device. The pressure difference is one atmosphere and the

film is at a temperature of 23 °C. Using Table I of Appendix A, we find that the

2 2 3 conversion factor between scc/(m 24 hr) and pmol(m s) is 0.5160 x 10 . If we

remember that one atmosphere is 0.1013 x 10^ Pa, we find that

3

n , T s 63.8 x 0.5160 x 10 n 001- // 2 n

>. P(T,p) = r = 0.325 mol/(m s Pa)

0.1013 x 10

b

The example given above is appropriate to the coulometric method of detection [3],

equations suitable for calculating P(T,p) using other measurement methods are cited in

[6]. Those who have become accustomed to using units which are not found in Table I of

Appendix A can derive appropriate conversion factors by using the base units listed in

Table V and forming the appropriate functions of conversion factors between base units.

7. THE UNITS OF P

The usual textbook definition of permeability [5,18,19] treats it as a property of

bulk material and defines as the product of D and S, which are properties which can be

2 defined on a microscopic (or molecular) scale for any material. D has units of m /s and

3

S has units of mol/(m -Pa) so that the permeability, P, (so designated in order to distinguish it from the P used elsewhere in this paper to designate the permeance) has unit

o of mol/(m-Pa-s). P has units of mol/(m «Pa-s). The extra power of m in the units for P

arises because P refers to a unit thickness of material.

The choice between P and P as a means of describing permeation data is largely a

question of whether one wishes to characterize a material or a product made from a

22
f material. If the product is made from a material which is homogeneous throughout and

whose dimensions can be accurately measured then P is a good way to express results which

can be expected to be useful under a wide range of end-use conditions. The vagaries of

polymer processing, on the other hand, are such that one cannot always be certain that a product, such as a thin film, is homogeneous nor can the thickness always be ascertained

accurately. In the case of SRM 1470 normalizing the data to unit thickness would probably

introduce more scatter into the results because of the difficulty of measuring the film

thickness to better than 3 or 4 percent, which is the accuracy to which the permeation

flux through a given film could be measured. Eq. 1 in Section 1 is the basis for determining

P and we see that the determination requires only that the ratio between the permeation

flux and the pressure difference across the barrier be measured.

If one ignores units, P is defined by:

p _ quantity of matter

unit area-unit time-unit pressure difference (8)

ilTable V shows a variety of ways for expressing each of the quantities in Eq. 8. None of

the customary sets of units for expressing permeabilities [19] conform to SI throughout.

In particular the mole has not found wide acceptance as representing the quantity of

matter. The ISO [20] went so far as to promulgate a new gas transmission unit whose units

are fm/(Pa-s). These units were obtained from our units for P by cancelling cm^(STP) in

2

the numerator of the expression against m in the denominator. It is clearly wrong to

make such a cancellation and the confusion could easily have been avoided if the quantity

j of matter had been expressed correctly.

In our work the units in which P is expressed are completely those of the System

Internationale (SI) [21] and thus represent the metric way of expressing the results of

measurements.

Appendix A contains a table which can be used to convert gas transmission rates among

different sets of units. (See example in Section 6).

8. MISCELLANEOUS ASPECTS OF SRM 1470

In the following sections we present additional observations which may be of interest

to purchasers of SRM 1470. The use of polymeric materials as standards of measurement is

still in its infancy and a great deal remains to be learned about their properties when

hused in this way.

I 8.1 Effect of Thermal History ("Aging")

In order to see if irreversible changes could be caused by thermal treatment at,

temperatures below Tg a series of experiments were carried out as listed in Table VI. The

specimens were not pretreated. Permeability measurements were carried out in an up and

down temperature cycle beginning at 23 °C, going to 37 °C, back to 23 °C, up to 50 °C, and

back to 23 °C. Measurements were made at a given temperature for 24 hrs. before going to

the next temperature.

23
Table VI

Effect of Thermal History on Permeability

Sample A Sample B Sample C

Expt Temp p P(T_,p) Temp p P(T.,p) Temp p P("L,p)

No

0 0 0

1 1 .379 1 .380 1 .370

2 295. 8K 2 .386 296. 3K 2 .383 296. 9K 2 .372

3 3 .375 3 .370

4 1 .384 1 .385 1 .374

5 310. 9K 2 .388 310. 9K 2 .387 310. 9K 2 .380

6 3 .377 3 .380 3 .353

7 1 .384 1 .384 1 .371

8 295. 5K 2 .388 295. 9K 2 .386 296. 7K 2 .364

9 3 .387 3 .377 3 .344

10 1 .384 1 .386

11 1 .396 1 .398 1 .385

12 323. 2K 2 .404 322. 8K 2 .403 322. IK 2 .395

13 3 .393 3 .392 3 .368

14 1 .400 1 .395 1 .385

15 295. 9K 2 .399 296. 4K 2 .395 297. OK 2 .386

16 3 .397 3 .389 3 .355

p, = .1028 MPa

p' = .1185 MPa

P3 = .0872 MPa

Note:

9

Units of P(T

Q ,p) are pmol/(m -s-Pa)

The results in Table VI have all been normalized to 23 °C (P(T

Q ,p)) using the activation energy experimentally determined. There appears to be an increase of about .005 in

the value of P(T

Q

,p) on first going from 23 °C to 37 °C. Upon going back to 23 °C the

original value is recovered. Increasing the temperature to 50 °C gives rise to an increase

of about 0.012 in P(Tq,p) which is retained upon lowering the temperature back to 23 °C.

These changes are barely larger than our confidence interval on the mean value of P(Tq,p)

but the repeatability of measurements on a single sample shown in Table VII is much better

than this (on the order of 0.003) so that the changes noted above are probably real.

24
Table VII

Effect of Thermal History on Time-Lag

Sample A Sample B Sample C

Expt

No

12

3

Temp

295. 8K

12

3

t L (T

0 ,p) Temp p t l (Tq ,p)

379.

383.

363.

1 296. 3K 2

3

359.

342.

321.

Temp

296. 9K

p Wp)

12

379.

392.

4

5

6

7

8

9

10

1 310. 9K 23

295. 5K

356.

344.

349.

359.

369.

372.

330.

310. 9K 2

3

1 295. 9K 2

3

1

328.

325.

330.

379.

371.

377.

364.

310. 9K

296. 7K

12

3

12

3

355.

359.

351.

339.

364.

351.

1112

13

323. 2K 2

3

321.

323.

326.

1 322. 8K 2

3

332.

312.

322.

322. IK

12

3

389.

325.

326.

14

15

16

295. 9K

348.

349.

348.

1 296. 4K 2

3

354.

354.

357.

297. OK

12

3

357.

358.

361.

Note:

Units of T. are seconds.

The time-lag data from these experiments (See Table VII) yield a mixed result when

analyzed in the same way. This is probably due to scatter in the time-lag data. There is

evidence that the morphology of the PET film changes with time (annealing) [8]. For this

reason it is desirable to maintain the film as far below its glass transition temperature

(see Section 8.3) as is practical. On the other hand refrigerating the film is not

desirable because moisture could condense on cold films exposed to the atmosphere and lead

to changes in permeability which may be more serious than those caused by aging at 23 °C.

The aging effect of long-term storage at room temperature remains to be evaluated.

8.2 Significance of the Leakage Flux

The leakage flux decreases with time at a given temperature. This is evidence for an outgassing effect. Further proof that the leakage flux has its origins in outgassing from

the film is provided by experiments in which the polyester film is replaced by an aluminum

foil barrier. The pressure rise with the aluminum barrier in place is one fifth to one

p. = . 1028 MPa

p' = .1185 MPa

P3 = .0872 MPa

25
I

tenth that found with the polyester film (in both experiments the upstream side of the

cell is under a vacuum). Correlation of the leakage flux with other factors remain to be

investigated.

The leakage flux is an important effect because its presence contributes to serious

errors (as much as 10 to 20 percent) in the Linde Cell and Dow Cell procedures [22] which

do not separate this flux from the true permeant flux. Experiments in which the outgassing

species are trapped for subsequent analysis would be worthwhile. Some of the variation in

the measured values of the time-lag and the permeability may be traceable to variations in

outgassing behavior. The flux from outgassing species is not necessarily constant during

the course of a given measurement and it is reasonable to believe that the correction for

leakage flux (which assumes that the flux remains constant) is itself a source of error

after a while.

It may be possible to eliminate some of the outgassing by proper conditioning of the

film prior to measurement. It appears that the leakage flux is reduced if the film is

measured at a low temperature after outgassing at a higher temperature. When choosing an

outgassing temperature one must take care to ensure that the film is not annealed. In

our experience a 24 hour outgassing at 50 °C did not prove to be too severe. It is also

possible that a portion of the outgassing arises from degradation of the polymer to maintain

an equilibrium between high and low molecular weight species. In this latter instance the

effect is intrinsic to the polymer and cannot be eliminated.

8.3 Glass Transition Behavior in SRM 1470

•> In view of the fact that glassy polymers are known to undergo slow changes in thermodynamic state at temperatures below their glass transition temperature, T [8,23], we

y decided to try to characterize the glass transition behavior of SRM 1470. The results are

shown in Figure 7. The curve labelled Q is for a sample which had been rapidly cooled

after being melted and curve A is for a sample as received. Curve Q shows a welldeveloped glass transition region between 345 K and 360 K. There is also a prominent

crystallization exotherm centered around 427 K. The area of this exotherm provides a

measure of the amount of the material which is in the amorphous state. In curve A the

crystallization feature is essentially absent and the feature associated with the glass

transition is very weak. The evidence is that there is very little possibility for

reorganization of the molecular chains.

This is not surprising in view of the relatively high crystal 1 inity of the materials

3 as revealed by its density (1396 kg/m ). It is also known that this material is biaxial ly

cold-drawn when it is made. Both the drawing process and the crystal 1 inity lead to constraints on the ability of the chains to reorient. On this basis we expect that SRM 1470

should be relatively immune to annealing effects in the temperature range of 23 °C to

50 °C. This temperature range represents the normal conditions which the film would be

subjected to during postal shipment and conditioning prior to measurement.

8.4 Another Method for Assessing Pinholes

In earlier sections we have seen that pinholes in the films being tested manifest

themselves through anomalous values of the time-lag and the permeability as well as through

26
Figure 7. DSC (Differential Scanning Calorimetry) Thermogram for SRM 1470. The upper (A)

trace was made on an as-received specimen of SRM 1470. The lower (Q) trace was made after specimen A had been cooled rapidly from above T .

27
poor fits between the theoretical curves and the experimental data. PET films are sensitive

to mishandling. If a film is creased several pinholes will almost always appear. Ordinary

care such as avoiding dirt particles and creases will almost always serve to keep pinholes

away from the area being measured.

In spite of all precautions a pinhole will appear sooner or later. If a film is

observed to have an exceptionally short time-lag and an exceptionally high transmission

rate, it should be removed from the sample holder and checked for pinholes. A simple

visual inspection sometimes reveals a crease or a prick mark on the film. These are the

most likely locations for pinholes. Once the visual inspection is complete the film

should be placed on a grounded aluminum plate. The surface of the plate should be smooth

and free of projections. A hand-held Tesla coil should then be passed over the film far

enough away from it so that no sparks penetrate the film. As the Tesla coil is moved

closer to the film surface some small number of sparks will begin to penetrate the film.

The locations where the sparks penetrate the film should be circled with a china-marking

pencil for later examination. We have found that a pinhole was almost always in the

vicinity of a preexisting flaw.

The technique described above is cheap and simple but somewhat subjective. It is

easy to practice by using a sheet of film which has been deliberately creased or pricked.

On a fresh film sparks will travel for 2 or 3 cm along the film to penetrate it at a

pinhole.

9. CORRECTIONS FOR NON-STANDARD TEMPERATURE AND PRESSURE

Once the user's measurement is converted to SI units it can be corrected to the

standard temperature by multiplying by exp[-l 1

.

06(T-Tq)/T] (this expression is slightly

more accurate than exp[-0. 038(T-Tq)] as shown on the certificate). We advise against

using the pressure correction shown on the certificate (which was printed before we discovered that the pressure coefficient was an artifact). Once the user's measurement result

is converted to SI and corrected to standard temperature it can be compared with the value

given on the certificate. In Section 4.2 we stated that we feel that the correct value of

2

the permeability coefficient is closer to 0.330 pmol/(m s Pa). This value was obtained

with the pressure on the upstream gas reservoir above atmospheric pressure and is, therefore, less likely to be influenced by N£ leakage into the reservoir. With this change in

the mean value the two-sided tolerance interval within which 95% of all measurements will

fall with 95% probability [17] becomes 0.330 ± 0.028 pmol/(m2

s Pa). This represents a variability of about 8 percent, which corresponds roughly to 2.7 times the standard

deviation of the sample-to-sample variability, p. In order to obtain a better comparison

of the performance of his measuring system the user will have to make measurements on a

number of samples so that he can make comparisons between his own estimate of the population

mean value of the permeability and the value given by the 95% confidence interval derived

2 from our measurements (0.330 ± 0.004 pmol/(m s Pa)).

28
10. ACKNOWLEDGEMENTS

Many members of ASTM Committees F-2 (Flexible Barrier Materials) and D-20 (Plastics)

assisted this work through their contributions of ideas, experience, and moral support.

E. I. DuPont de Nemours and Company donated some 12,500 sheets of polyester film.

Dr. D. H. Reneker of NBS was responsible for pointing the authors along the research

path taken in this work. William P. Reed of the Office of Standard Reference Materials

was a constant source of help and encouragement. .

29
11 . REFERENCES

[I] Sacharow, S. , "Packaging Materials" in Encyclopedia of Polymer Science and Technology , Volume 9, pp. 709-714 (Interscience , New York, 1968).

[2] Yasuda, H. and Stannett, V. , "Vapor Barriers" in Encyclopedia of Polymer Science and

Technology Volume 2, pp. 316-327 (Interscience, New York, 1965).

[3] ASTM Section D. 20. 70. 07, "Standard Method of Test for Oxygen Transmission of Plastic

Films and Sheeting Using a Coulometric Sensor," Standard under development (number

X70-078-1), ASTM, Philadelphia.

[4] Michael is, R. E. , "The SRM Story at NBS" in Standard Reference Material s and Meaningful

Measurements , pp. 246-257, NBS Special Publication 408 (U.S. Govt. Printing Office,

Washington, 1975).

[5] Crank, J. and Park, G. S. , Eds. Diffusion i_n Polymers (Academic Press, New York,

1968). See especially Chapters 1, 2, and 6.

[6] Barnes, J. D. and Martin, G. M. , "An Automated Manometric Permeation Measuring

Facility," to be published.

[7] Barrer, R. M. and Skirrow, G. , "Transport and Equilibrium Phenomena in Gas-Elastomer

Systems. I. Kinetic Phenomena," J. Polymer Sci. 3, pp. 549-563 (1948).

[8] Siegmann, A. and Turi , E. , "Structural Changes in Glassy Poly(Ethylene Terephthalate),"

J. Macromol. Sci. Phys. , B10 (4), pp. 689-708 (1974).

[9] Draper, N. and Smith, H. , Appl ied Regression Analysis , (John Wiley and Sons, New

York, 1966).

[10] Hogben, D. T. , Peavy, S. T. and Varner, R. N. , Omni tab II User' s Reference Manual , NBS Technical Note 552 (U.S. Govt. Printing Office, Washington, 1971) and Hogben, D.

T. and Peavy, S. T. , OMNITAB H User's Reference Manual 1977 Supplement , NBSIR 77-

1276 (National Technical Information Service, Springfield, Va. , 1978).

[II] Paul, D. R. and Koros, W. J., "Effect of Partially Immobilizing Sorption on Permeability

and the Diffusion Time-Lag," J. Polymer Sci., Polymer Physics Ed., J4, 675-685

(1976).

[12] Michaels, A. S. , Vieth, W. R. and Barrie, J. A., "Solution of Gases in Polyethylene

Terephthalate," J. Appl. Phys., 34, pp. 1-12 (1963).

[13] Koros, W. J., private communication.

[14] Michaels, A. S. , Vieth, W. R. , and Barrie, J. A., "Diffusion of Gases in Polyethylene

Terephthalate," J. Appl. Phys., 34, pp. 13-20 (1963).

[15] Mandel, J., "Statistics and Standard Reference Materials" pp. 146-160 in Standard

Reference Material s and Meaningful Measurements , Op. Cit.

[16] Anderson, V. L. and McLean, R. A. , Design of Experiments A Realistic Approach

(Marcel Dekker, New York, 1974).

[17] Natrella, M. G. , Experimental Statistics , NBS Handbook 91, U.S. Government

Printing Office, Washington, D.C., 1966.

[18] Stern, S. A., "The 'Barrer' Permeability Unit," J. Polymer Sci., Part A-2, 6, pp.

1933-1934 (1968).

30
[19] Yasuda, H. , "Permeability Constants" in Polymer Handbook , Ed. J. Brandrup and E. H.

Immergut (Interscience, New York, 1966).

[20] "Plastics: Determination of the Gas Transmission Rate of Films and Thin Sheets Under

Atmospheric Pressure-Manometric Method", International Standard 2556 (ISO 2556-

1974(E)), International Organization for Standardization, Geneva, 1974.

[21] The International System of Units (SI) , NBS Special Publication 330 (1977 Edition)

(U.S. Govt. Printing Office, Washington, D.C. 1977).

[22] "Standard Methods of Test for Gas Transmission Rate of Plastic Film and Sheeting",

Designated Dl 434-75 , ASTM Annual Book of Standards , Part 35, American Society for

Testing and Materials, Philadelphia, Pa.

[23] Moore, R. S. and O'Loane, J. K. , "Review of Continuum and Molecular Aspects of Volume

and Enthalpy Recovery in Glassy Polymers," ACS Organic Coatings and Plastics Chemistry

Preprints, 39, pp. 224-229 (1978).

v

ii

31
M - >' U.S. Department of Commerce

Juanita M. Kreps

Rational BuroaAi of Standards

Ernest Ambler, Act in* Director

APPENDIX A

Rational J&mtm of js»tandard$

Certificate

Standard Reference Material 1470

Polyester Plastic Film for Oxygen Gas Transmission

John D. Barnes and Gordon M. Martin

This Standard Reference Material is intended for use in the measurement of the oxygen gas transmission rate of

thin polymeric films using the manometric [1], volumetric [1], or coulometric [2] methods. The permeability

coefficient (P) at 0.10133 MPa (one standard atmosphere) and 296. IS K of 24 films selected at random was

determined [3]. Of the 24 samples one was found to have pin holes. Of the remaining 23, one yielded a permeability coefficient which was incompatible with the other 22 and was not included in the calculations. From these

determinations the following value for the permeability coefficient was established with the uncertainty

estimated at the 95% confidence level:

P(T0, po) = 0.325 ± 0.004 pmol/(m2s Pa)

A two-sided tolerance interval within which 95% of all measurement values will fall with 95% probability [4] is given by:

P(T0, po) = 0.325 ± 0.028 pmol/m2 s Pa)

The experimental measurements were performed over a range of pressures between 0.085 and 0. 1 17 MPa and

over a range of temperatures between 291 and 302 K. Temperature and pressure coefficients (Jiy) of permeability were determined and can be used to calculate the permeability coefficient under conditions different from

the standard ones by using:

P(T,p) = P(T0, po) eP<T-T0 ) e7,pp o>

where T0 = 296.15 K, p0 = 0.10133 MPa, 0 3.80 x 10

-2 /K, and y = 1.4 x

10"6 /Pa. Extrapolation to other

temperatures and pressures should be limited to the range of 290 to 305 K. and .085 to .1 17 MPa, respectively.

Values for the permeability coefficient extrapolated beyond these temperatures and pressures are not certified.

The gas transmission rate, G, is obtained by multiplying P(T,p) by p and has units of pmol/(m s). Table I lists some conversion factors between the various units commonly, used to express G.

Table I

G desired

2 pmol/(m s) cm

3 /(m2day) (m

3 /(100 in2day)

G Given Multiply G Given by

pmol/m 2 /s

1

* 3,-2, -1 •cm /m day

cm3 / 100 in

2 /day

1.000

0.5160 x 10

3

0.7998 x 10

4

0.1938 x

10"2

1.000

15.50

1.2503 x

10"4

0.06452

1.000

cm - a cubic centimeter of gas at standard temperature and pressure (sec).

The overall coordination and evaluation of data leading to certification of this Standard Reference Material was performed by J. D. Barnes.

The technical and support aspects involved in the preparation, certification, and issuance of this Standard

Reference Material were coordinated through the Office of Standard Reference Materials by W. P. Reed.

Washington, D.C. 20234

January, 1978 32

J. Paul Cali, Chief

Office of Standard Reference Materials
APPENDIX A (Cont'd.

)

USE and STORAGE

This Standard Reference Material has been carefully packaged to avoid damage, however, occassional pin holes or other defects may be present. If, during the measurement process, values outside the certified uncertainties are encountered, a new portion of the SRM should be used.

Measurements indicate this material to be stable when stored under normal laboratory conditions. Temperatures in excess of 50° C should be avoided. While long term stability of the SRM at room temperature has not

been determined, it is being monitored. If the certified value changes beyond the stated uncertainty, NBS will notify purchasers.

References

[1] ASTM D1434-75 - Standard Methods of Test for Gas Transmission Rate of Plastic Film and Sheeting.

Part 35 of Annual Book ofASTM Standards] American Society for Testing and Materials, Phila., Penna.

[2] Private Communication, Modern Controls Corp., Minneapolis, Minn., R. A. Wood.

[3] NBS Special Publication 260- to be published.

[4] Experimental Statistics, NBS Handbook 91 by Mary G. Natrella, U.S. Govt. Printing Office, Washington,

D.C. 1966.

33
NBS-U4A (rev. 9-78)

U.S. DEPT. OF COMM. 2*Gov't Accession

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP 260-58

fa Accession No.

4. title and subtitle standard Reference Materials:

SRM 1470:

Polyester Film for Oxygen Gas Transmission Measurements

5. Publication Date June 1979

$. Performing Organization Code

7. AUTHOR(S)

John D. Barnes & Gordon M. Martin

8. Performing Organ. Report No

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

Froject/Task/Work Unit No.

11. Contract/Grant No.

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

Same as 9.

13. Type of Report & Period Covered NA

soring Ageaoy Code

15. SUPPLEMENTARY NOTES -

Library of Congress Catalog Card Number; 79-600077

I | Document describes a computer program; SF-185, Fl PS Software Summary, is attached.

16. ABSTRACT (A 200-word or leaa tactual summary of most significant infonnation. If document includes a significant bibliography or

literature survey, mention it here.)

* This report presents information which should be of interest to users of NBS

Standard Reference Material 1470. This SRM takes the form of 23 ym thick sheets of

poly(ethylene terephthalate) film. The gas transmission rates of these films with

respect to oxygen gas have been carefully analyzed. We describe where the film come< from, how it is packaged, and how it should be conditioned prior to measuring. The

steps which were taken to characterize a random sample of sheets from the production

lot ©f the SRM are discussed in detail. The gas transmission rates and the time-lag;

of 22 films were measured using a state-of-the-art electronic manometric permeation

facility. The temperature dependence of the permeability was determined over the

temperature range 288 K to 310 K. A small pressure effect was found which is though

to be an artifact. The statistical measures which were derived from the data are

discussed in detail. It is concluded that the largest source of variability is from

one sample to another with a coefficient of variation amounting to 4 percent. A bri

discussion of units for expressing permeabilities is given. Effects due to thermal

conditioning ("aging") and outgassing are discussed.

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)

Gas transmission rate; manometric technique; oxygen; permeability; polyjethylene

terephthalate); standard reference material; temperature coefficient; time-lag.

18. AVAILABILITY [X Unlimited

I | For Official Distribution. Do Not Release to NTIS

IY1 ^

Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-O2O77-O

I | Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF

PRINTED PAGE

43

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$2.00


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