Barrier Polymers and Structures

412 Pages·1990·6.48 MB·English

by  William J. Koros (Eds.)

423
A C S SYMPOSIUM S E R I E S
Barrier Polymers and
Structures
William J. Koros,
EDITOR
The University of Texas at Austin
Developed from a symposium sponsored by
the Division of Polymer Chemistry, Inc.
at the 197th National Meeting
of the American Chemical Society,
Dallas, Texas,
April 9-14, 1989
American Chemical Society, Washington, DC 1990
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Library of Congress Cataloging-in-Publication Data
Barrier Polymers and Structures
William J. Koros, editor
p. cm—(ACS Symposium Series, 0097-6156; 423).
"Developed from a symposium sponsored by The Division of
Polymer Chemistry, Inc. at the 197th Meeting of the American
Chemical Society, Dallas, Texas, April 9-14, 1989."
Includes bibliographical references.
ISBN 0-8412-1762-9
1. Food—Packaging—Congresses
Congresses.
I. Koros, William J., 1947- . II. Polymer Chemistry, Inc.
III. American Chemical Society. Meeting (197th : 1989 : Dallas,
Tex.) IV. Series.
TP374.B37 1990
668.4'9—dc20 90-275
CIP
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PRINTED IN THE UNITED STATES OF AMERICA
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
ACS Symposium Series
M. Joan Comstock, Series Editor
1990 ACS Books Advisory Board
Paul S. Anderson Michael R. Ladisch
Merck Sharp & Dohme Research Purdue University
Laboratories
V. Dean Adams
Tennessee Technological
University Robert McGorrin
Kraft General Foods
Alexis T. Bell
University of California— Daniel M. Quinn
Berkeley University of Iowa
Malcolm H. Chisholm Eisa Reichmanis
Indiana University AT&T Bell Laboratories
Natalie Foster C. M. Roland
Lehigh University U.S. Naval Research Laboratory
G. Wayne Ivie Stephen A. Szabo
U.S. Department of Agriculture, Conoco Inc.
Agricultural Research Service
Wendy A Warr
Mary A. Kaiser Imperial Chemical Industries
Ε. I. du Pont de Nemours and
Company Robert A Weiss
University of Connecticut
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Foreword
The ACS SYMPOSIUM SERIES was founded in 1974 to provide a
medium for publishing symposia quickly in book form. The
format of the Series parallels that of the continuing ADVANCES
IN CHEMISTRY SERIES except that, in order to save time, the
papers are not typeset but are reproduced as they are submitted
by the authors in camera-ready form. Papers are reviewed under
the supervision of the Editors with the assistance of the Series
Advisory Board and are selected to maintain the integrity of the
symposia; however, verbatim reproductions of previously pub
lished papers are no
research are acceptable, symposi y
types of presentation.
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Preface
BARRIER RESINS HAVE REVOLUTIONIZED the packaging industry and,
therefore, have been the focus of intense investigation. Numerous
interesting developments related to barrier applications have been
reported in journals and at various technical meetings Typically
however, the informatio
number of outlets. The time seemed right to publish a collection of
both fundamental and practical principles involved in making and using
barrier polymers and structures. The project took the form of this book,
which provides a comprehensive treatment of the state of science and
technology in the area of barrier polymers and barrier structures.
The topics covered will be of direct interest to industrial scientists
making or using barrier packaging. Moreover, government regulators
who work with the packaging industry should find the book useful in
indicating trends in materials and processes in the industry. Academic
researchers working in fundamentals of sorption and transport processes
will also find several good updates in these areas.
Acknowledgments
The National Science Foundation and the ACS are acknowledged for
partial support of my time in the preparation of the overview chapter
and the coordination of the book. It has been a sincere pleasure
working with Cheryl Shanks and Beth Pratt-Dewey of the ACS on this
project; their good nature and efficiency were examples for me. Thanks
are extended to the session chairs and participants in the symposium on
which the book is based. Finally, I express my deepest gratitude to the
authors whose work is presented in this volume and the many excellent
reviewers who helped perfect the papers with their thoughtful comments.
ix
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Dedication
This book is dedicated to Vivian T. Stannett, whose pioneering
contributions to understanding the fundamentals of barrier polymers
continue to inspire and motivate scientists in this field.
WILLIAM J. KOROS
The University of Texas at Austin
Austin, TX 78712
January 25, 1990
x
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Chapter 1
Barrier Polymers and Structures:
Overview
William J. Koros
Department of Chemical Engineering, The University of Texas at Austin,
Austin, TX 78712
This introductory chapter provides an overview of the
papers presented at the symposium on Barrier Polymers
and Barrier Structures that was sponsored by the Polymer
Chemistry Division of the American Chemical Society at
the Spring 1989 meeting. A total of nineteen papers
from the symposium are included in this volume. Topics
covered include barrier transport fundamentals, advanced
composite structures, reactive surface treatments and
the effects of orientation on barrier properties.
Relationships between polymer molecular structure and
barrier efficacy are also treated in detail. Time and
history dependent phenomena associated with retorting of
barrier laminates are discussed from the standpoint of
theoretical modeling and experimental characterization
of the barrier layers. The effects of concentration
dependent diffusion, flavor scalping and nonFickian
transport phenomena are also discussed. The coverage,
therefore, is broad while providing sufficient depth to
provide a state-of-the-art update on the major technical
issues facing the barrier packaging field.
The development of efficient packaging materials and containers has
allowed the evolution of the modern market system based on
concentrated production facilities supplying individuals residing
far from the ultimate source of products. The goal of the packaging
industry has been to provide increasingly more cost effective means
of preserving the quality of materials with as close to their
as-produced natures as possible.
Modern packaging is a sophisticated technology rooted deeply
in fundamental polymer science. Nevertheless, the sheer size and
competitiveness of the packaging industry also makes it an extremely
practical field with an eye to applying polymer science to achieve
the bottom line of cost and barrier efficiency. This book seeks to
0097-6156/90/0423-0001$06.25/0
© 1990 American Chemical Society
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
2 BARRIER POLYMERS AND STRUCTURES
reflect the dual nature of this technology by presenting fundamental
principles along with complementing discussions of applications of
these principles by leaders in the field.
Many excellent reviews of the fundamental processes by which
small molecules penetrate between the segments comprising a
polymeric film are available (1-10). These reviews, coupled with
past symposia publications related to barriers and the allied field
of membranes (11-18) should allow a newcomer to the field to rapidly
gain the needed background to become an active participant in this
dynamic technology.
The chapters in the present book reflect the current research
and development directions of active university and industrial
participants in the barrier field. The overview provided by this
first chapter gives a framework for appreciating the more detailed
subsequent discussion of topics that are at the cutting edge of this
evolving technology. Therefore, i t is anticipated that this book
will be of interest to bot
the field.
Fundamentals
In addition to providing a container to prevent scattering and bulk
phase mixing of components, modern packages control the exchange
of components between the package contents and the external
environment. For instance, the protection from attack by oxygen is
among the most common function served in food packaging. Even this
general requirement, however, has manifold aspects. The degrees of
sensitivity of different materials to environmental effects are
clearly very different and the oxygen barriers required to
successfully store these products differ accordingly. Similar
considerations apply to water permeation, since foods and many
pharmaceuticals show varying degrees of stability in a dry state as
compared to that in the presence of water. Table I (19)
illustrates typical ranges of sensitivity to oxygen and water vapor
for a spectrum of common foods. The table also illustrates the
manifold requirements needed in terms of oil and flavor and/or aroma
component barriers that packaging must provide for the different
foods.
As packaging capabilities increase, the complexity of the
applications that can be treated also increases considerably,
thereby explaining the ever expanding markets for packaging
materials. An interesting example in which complex requirements
must be met involves the storage of blood platelets (20). Blood
platelets are living cells that both consume oxygen to live and
generate carbon dioxide as a metabolic byproduct. The generation of
carbon dioxide presents problems to viability, since i t tends to
cause undesirable changes in the natural pH unless i t can escape.
An added requirement enters because the aqueous solution containing
the platelets must not lose a significant amount of water by
permeation. This case, therefore, illustrates the need for an
advanced "controlled atmosphere" package, or "smart package" that is
able to allow relatively free exchange of oxygen and carbon dioxide
with the external environment while essentially preventing outward
permeation losses of water. Similar considerations apply
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
1. KOROS Overview 3
Table I: Permeation protection required for various foods and beverages for a
one year shelf life at 25°C f
Estimated Estimated High oil High volatile
oxyge gain,
Food or Beverage ppm wL percent
Canned milk, meats lto5 -3 yes -
Baby foods lto5 -3 yes yes
Beer, ale, wine ltt>5* -3 - yes
Instant coffee lto5 +2 yes yes
Canned vegetables,
soups, spaghetti lto5 -3 -
Canned fruits 5 to 15 -3 yes
Nuts, snacks 5 to 15 +5 yes -
Dried foods 5 to 15 +1 - -
Fruit juices, drinks 10 to 40 -3 - yes
Carbonated soft drinks 10 to 40* -3 - yes
Oils,shortenings 50 to 200 +10 yes -
Salad dressings 50 to 200 +10 yes yes
Jams, pickles, vinegars 50 to 200 -3 - yes
Liquors 50to200 -3 - yes
Peanut butter 50 to 200 +10 yes -
* Less than 20% loss of CC>2 is also required,
t (Data taken from ref. 19.)
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
4 BARRIER POLYMERS AND STRUCTURES
to the packaging of fruits and vegetables that require inward
permeation of carbon dioxide and respiration of oxygen without loss
of water (21).
Modern packages typically regulate the contact between the
contents and the environment by a process known as permeation.
The first good description of the permeation process in polymers
dates back to 1831 when Mitchell (22) noted that natural rubber
membranes allowed the passage of carbon dioxide faster than
hydrogen under equivalent conditions. Mathematically, one can
describe the permeation process in terms of Eq(l) using a
permeability coefficient of component i , P-^,
Pi = (steady state flux of ii (D
A /C
Pi
The permeability is defined in terms of the steady state
permeation rate of componen
normalized partial pressur
across the membrane of thickness, t (see Fig. 1) (23,24) .
Accurate description of barrier films and complex barrier
structures, of course, requires information about the composition
and partial pressure dependence of penetrant permeabilities in each
of the constituent materials in the barrier structure. As
illustrated in Fig. 2 (a-d), depending upon the penetrant and
polymer considered, the permeability may be a function of the
partial pressure of the penetrant in contact with the barrier layer
(15). For gases at low and intermediate pressures, behaviors shown
in Fig. 2a-c are most common. The constant permeability in Fig.2a
is seen for many fixed gases in rubbery polymers, while the response
in Fig. 2b is typical of a simple plasticizing response for a more
soluble penetrant in a rubbery polymer. Polyethylene and
polypropylene containers are expected to show upwardly inflecting
permeability responses like that in Fig. 2b as the penetrant
activity in a vapor or liquid phase increases for strongly
interacting flavor or aroma components such as d-limonene which are
present in fruit juices.
The permeability vs pressure response in Fig. 2c is found for
most gases in glassy polymers (24-27). In general, the magnitude of
the decline in permeability with pressure depends upon the glass
transition temperature of the polymer and the critical temperature
of the gas. This type of response is related to intersegmental
packing defects present in glasses due to their hindered mobilities,
and is discussed in Chapter 2 (28) under the heading of "dual mode"
sorption theory.
At sufficiently high penetrant partial pressures in glassy
polymers, the onset of plasticization produces the upturn in
permeability seen in Fig. 2d. While this behavior is shown for
the acetone-ethyl cellulose system, i t is also typical of many
other systems. In fact, if one extends the pressure range of the
measurements in Fig. 2c to 900 psia, an upturn in permeability like
that seen in Fig. 2d is observed (29). This response is, therefore,
a combination of the dual mode response in Fig. 2c and the
plasticization response in Fig. 2b at high sorbed concentrations.
In Barrier Polymers and Structures; Koros, W.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.