Handbook of plastic foams 1995 landrock

HANDBOOK OF
PLASTIC FOAMS
Types, Properties,
Manufacture and Applications

Edited by

Arthur H. Landrock (ret.)
Plastics Technical Evaluation Center (PLASTEC)
Picatinny Arsenal
Dover, New Jersey

In

NOYES PUBLICATIONS
PI

Park Ridge, New Jersey,

U.S.A.


CopyrightQ 1995 by Noyes Publications
No part of this book may be reproduced or utilized in
any form or by any means, electronic or mechanical,
including phptowp@g, recording or by any information storage and retrieval system, without permission
in writing from the Publisher.
Library of Congress Catalog Card Number: 94-15236
ISBN: O-8155-1357-7
Printed in the United States
Published in the United States of America by

Noycs PubIications
MiII Road, Park Ridge, New Jersey 07656
10987654321

L.iirary of Congress Cataloging-in-Publication

Data

Handbook of plastic foams : types, properties, manufachue, and
applications / [edited by] Arthur H. Landrock.
p.
an.
Includes bibliographical references and index.
ISBN O-8155-1357-7
1. Plastic foams. I. J_androck, Arthur H.
TP1183.F6H35 1995
668.4’93~-dc.20
94-15236
CIP


Dedication
To my wife, Rose-Marie,
for her unfailing support and understanding


CONTRIBUTORS

Kaneyoshi Ashida
Polymer Institute

University of Detroit Mercy
Detroit, Michigan

Arthur H. Landrock (ret.)
PLASTEC
Picatinny Arsenal
Dover, New Jersey

Kurt C. Frisch
Polymer Technologies, Inc.
University of Detroit Mercy
Detroit, Michigan

Michael 0. Okoroafor
Technical Center
PPG Industries, Inc.
Monroeville, Pennsylvania

Kadzuo Iwasaki
Iwasaki Technical Consulting
Lab., Ltd.
Ohta City
Gumma-Prefecture, Japan

ix


Notice
To the best of our knowledge the information in this
publication is accurate; however, the Publisher does

not assume any responsibility
or liability for the
accuracy or completeness of, or consequences arising
from, such information. This book is intended for
informational purposes only. Mention of trade names
or commercial products does not constitute endorsement or recommendation
for use by the Publisher.
Final determination
of the suitability
of any information or product for use contemplated
by any
user, and the manner of that use, is the sole responsibility of the user. We recommend that anyone
intending to rely on any recommendation
of materials
or procedures mentioned in this publication should
satisfy himself as to such suitability, and that he can
meet all applicable safety and health standards.

X


CONTENTS

1. INTRODUCTION TO FOAMS AND FOAM
FORMATION .................................

1

Michael 0. Okoroafor and Kurt C. Frisch

Introduction ...............................
CFC Effects and Alternatives ...................
Fundamentals of Foam Formation ...............
References .................................

1

3
5
9

11

2. THERMOSE’ITING FOAMS .....................
Kaneyoshi Ashida and K&imo Iwasaki
Introduction (by Kaneyoshi Ashidu) ..............
Isocyanate-Based Foams (by Kaneyoshi Ashida)

11
.....

Introduction ..............................
Raw Materials for Isocyanate-Based Foams .......
Polyisocyanates .........................
Polyols ...............................
Blowing Agents .........................
Catalysts ..............................
Surfactants .............................
Epoxides ..............................
Flame Retardants ........................

Polyurethane Foams ........................
Preparation .............................
Processes of Urethane Foam Preparation ........
Flexible Urethane Foams .....................
Introduction ............................
Classification ...........................
Hand-Mixing Process .....................
Materials and Equipment ...................
Foaming Procedures ......................
Foam Properties and Testing Methods ..........
xi

13

13
16
16
21
24
30
38
39
39

40
40
42
46
46
46

47
47
47
49


xii

Contents
Applications of Flexible Urethane Foams ........
Slabstock Foams ..........................
Molded Flexible Urethane Foams ...............
Hot-Molded Foam and Cold-Molded Foam ......
High-Resilient Foam (HR Foam) .............
Dual-Hardness Molded Foam ................
Microcellular Urethane Elastomers ..............
Preparation of Microcellular Foams ............
Integral-Skin Flexible Urethane Foams ...........
Preparation of Integral-Skin Flexible Foams .....
Properties of Integral-Skin Flexible Urethane
Foams ..............................
Flame Retardant Flexible Foams ..............
Non-CFC-Blown Flexible Urethane Foams .......
Viscoelastic Foams and Energy-Absorbing Foams ...
Polyolefinic-Polyol-Based Flexible Foams ........
Semi-Rigid (or Semi-Flexible) Foams ...........
Manufacturing Process .....................
Applications ............................
Rigid Urethane Foams ......................

Introduction ............................
Preparation .............................
Production Technologies of Rigid Urethane
Foam ...............................
Properties of Rigid Urethane Foams ...........
Miscellaneous Urethane Foams ................
Isocyanurate-Modified Rigid Urethane Foams ....
Isocyanurate-Modified Flexible Urethane Foams .
Urethane-Based IPN Foams .................
Urethane-Based Hybrid Foams ...............
Urethane/Oxazolidone Foams ................
Polyisocyanurate Foams .....................
Introduction ............................
Principles of Urethane Modification ...........
Preparation .............................
Properties ..............................
Processing .............................
Oxazolidone-Modified Isocyanurate Foams .....
Amide-Modified Isocyanurate Foams .........
Carbodiimide-Modified Isocyanurate Foams ....
Imide-Modified Isocyanurate Foams ..........

51
51
56
58
60

63
63

63
64
64

65
66

67
68

69
69
69
71
71
71
71
78
78
85
85
. 85
85
86
88
88
88

91
97

99
99
105
109
110
111


Contents

Xl11
*.*

Filled Isocyanurate Foams .................
Polyurea Foams ..........................
Polycarbodiimide Foams ....................
Polyoxazolidone Foams .....................
Polyimide Foams .........................
Polyamide Foams .........................
References for Isocyanate-Based Foams ...........
Pyranyl Foams (by Kbneyoshi Ash&z) ...........
Introduction .............................
Chemistry of Pyranyl Foams .................
Raw Materials ...........................
Foam Preparation .........................
Properties of Pyranyl Foams .................
Mechanical Properties ....................
Thermal Conductivity ....................
Cell Structure and Permeability ..............
Dimensional Stability ....................

Thermal Stability .......................
Flame Retardance .......................
Chemical Resistance .....................
Possible Applications ......................
Advantages of Pyranyl Foams ................
Disadvantages of Pyranyl Foams ..............
References for Pyranyl Foams ..................
Syntactic Foams (by Kaneyoshi Ashida) ..........
Introduction .............................
Preparation of Hollow Microspheres ............
Matrix Resins ...........................
Thermosetting Resins ....................
Thermoplastic Resins ....................
Preparation of Syntactic Foams ...............
Epoxy Resin-Hollow Glass Microsphere
Syntactic Foam .......................
Phenolic Resin-Based Syntactic Foam .........
Polyimide-Based Syntactic Foam ............
Syntactic-Foam Prepregs ..................
Polystyrene-Epoxy Syntactic Foam ...........
Effect of Matrix Resins on Physical Properties ...
Properties of Syntactic Foams ................
Applications ............................
References for Syntactic Foams .................

111
114
115
117
117

120
122
140
140
140
140
142
142
143
143
144
145
145
145
145
146
146
147
147
147
147
148
154
154
154
154
154
155
155
156

156
157
157
162
162


xiv

Contents
Foamed Composites (by Kkneyoshi Ashida) .......
Introduction .............................
Raw Materials ...........................
Matrix Plastic Foams .....................
Reinforcing Materials ....................
Blowing Agents ..........................
Surfactants .............................
Preparation of Foamed Composites .............
Physical Properties ........................
Properties of Unidirectional Type Composites ....
Applications ............................
References for Foamed Composites ..............
Phenolic Foams (by Kudzuo IWLWZ~~)............
Introduction .............................
History ..............................
Classification ..........................
Chemistry ..............................
Material Chemistry ......................
Resol-Type Foam Chemistry ...............
Novolac-Type Foam Chemistry .............

Foaming Mechanism .....................
Raw Materials ...........................
Materials for Resol-Type Foams ............
Materials for Benzylic Ether-Type Foams ......
Materials for Novolac-Type Foam ...........
Foaming Processes and Facilities ..............
Foaming Process of Resol-Type Foam ........
Manufacturing Facilities ...................
Foaming Process of Novolac-Type Foam ......
Processing Facilities .....................
Properties ..............................
Foaming Characteristics ...................
General Properties .......................
Thermal Properties ......................
Flame Retardance .......................
Drying ...............................
Chemical Resistance .....................
Demand and Applications ...................
Demand ..............................
Applications ...........................
Conclusion ............................

163

163
164
164
166
166
167

167
171
173
179
180
183
183
183
183
184
184
185
188
190
191
191
195
195
197
197
200
203
204
204
204
206
209

211
212

212
214
214
214
218


Contents

xv

References for Phenolic Foams . . . . . . . . . . . . . . . . . 219
3. THERMOPLASTIC FOAMS ....................

221

Arthur H. La&rock
.............................
Introduction
Structural Foams (Rigid Foams) ...............
Introduction .............................
Structural-Foam Types .....................
Phenylene Oxide Alloys (Modified Polyphenylene Oxide) ......................
Polycarbonate ..........................
Acrylonitrile-Butadiene-Styrene
(ABS) ........
Acetal ...............................
Thermoplastic Polyester (Polybutylene
Terephthalate) (PBT) ...................
Polyetherimide .........................

Polystyrene (PS) ........................
Additional Rigid-Foam Types ................
Semi-Rigid Foams .........................
Polyolefin Foams .........................
Low-Density Polyethylene Foams ...........
High-Density Polyethylene Foams ...........
Polypropylene Foams ....................
Cross-Linked Foamed Resins ...............
Ionomer Foams .........................
Polystyrene Foams (Low-Density) .............
Extruded-Polystyrene Foam ................
Expandable Polystyrene (EPS) for Molded
Foam ..............................
Vinyl Foams ............................
Open-Cell Vinyl Foams ..................
Closed-Cell Vinyl Foams .................
Cross-Linked Vinyl Foams ................
Miscellaneous Foams ......................
Cellular Cellulose Acetate (CCA) ............
Polysulfone Foams ......................
References ...............................

4. ELASTOMERIC

221
221
221
223
223
225

225
226
227
227
228
228
228
228
230
232
232
233
234
235
235
236
239
239
240
241
241
241
242
243

. . . . . . . . . . . . . . . . . . . . . .

246

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

246

FOAMS

Arthur H. Landrock

Introduction


xvi

Contents
General ................................
Sponge Rubber ..........................
Cellular Rubber ..........................
Comparison of Cellular-Rubber Products
Types of Elastomeric Foams ..................
Neoprene ...............................
Silicone Foams ..........................
Silicone Rubber Sponge ...................
Room-Temperature-Foaming
Silicone
Rubbers ............................
References ...............................

246
246
247
247

248
248
249
249

........

250
25 1

5. MISCELLANEOUS AND SPECIALTY FOAMS:
(Epoxy Foams, Polyester Foams, Silicone Foams,
Urea-Formaldehyde Foams, Polybenzimidazole,
Foams, Polyimide Foams, Polyphosphazene Foams,
and Syntactic Foams) . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Arthur H. La&rock

Epoxy Foams . . . . . . . . . . . . . .
Polyester Foams . . . . . . . . . . . .
Silicone Foams . . . . . . . . . . . . .
Urea-Formaldehyde (UF) Foams
Polybenzimidazole (PBI) Foams
Polyimide Foams . . . . . . . . . . .
Polyphosphazene Foams . . . . . .
Syntactic Foams . . . . . . . . . . . .
References . . . . . . . . . . . . . . . .
6.

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SOLVENT CEMENTING AND ADHESIVE
BONDING OF FOAMS ........................

253
254
255
256
258
259
261
263
264

267

Arthur H. Lmdrock

Introduction ..............................
Solvent Cementing .........................
Thermoplastic Foam Substrates ...............
Cellular Cellulose Acetate .................
Acrylonitrile-Butadiene-Styrene
(ABS) ........
Acetal Homopolymer (DELRINB) ...........
Acetal Copolymer (CELCON@) .............
Polyvinyl Chloride (PVC) .................
Polycarbonate ..........................
Polystyrene ...........................

267
267
268
268
269
269
269
269
269
269


Contents
Polysulfone ...........................
Modified Polyphenylene Oxide (NORYLQ) .....
Polybutylene Terephthalate (PBT) ............

Polyetherimide (ULTEMQ) ................
Adhesive Bonding .........................
Thermoplastic Foam Substrates ...............
Acetal Copolymer (CELCONB) .............
Acetal Homopolymer (DELRIN@) ...........
Acrylonitrile-Butadiene-Styrene
(ABS) ........
Cellular Cellulose Acetate .................
Polyvinyl Chloride (PVC) .................
Polycarbonate ..........................
Modified Polyphenylene Oxide (NORYL@) .....
Polystyrene ...........................
Polyethylene and Polypropylene .............
Ionomer ..............................
Nylons (Polyamides) .....................
Polyetherimide .........................
Polybutylene Terephthalate (PBT) ............
Polysulfone ...........................
Thermosetting Foam Substrates .............
Polyurethanes ..........................
Epoxies ..............................
Polyester .............................
Phenolic .............................
Silicone ..............................
.....................
Urea-Formaldehyde
Syntactic Foams ........................
References ...............................

xvii

270
270
271
271
271
271
271
272
272
272
272
273
273
273
273
273
273
274
274
274
274
274
275
275
275
275
275
275
275

...

278

Introduction .............................
Antistats (Antistatic Agents) ..................
Blowing Agents Foaming Agents) ..............
General ................................
General Production Methods for Blowing Foams ...
Chemical Blowing Agents (CBAs) .............
Physical Blowing Agents ...................
ChlorofluorocarbonLiquids (CFCs) ............
Carbon Dioxide (CO,) ....................

278
279
280
280
281
282
283
284
287

7. ADDITIVES, FILLERS AND REINFORCEMENTS
Arthur H. Landrock


...

XV111

Contents
Flexible Foams .........................
Rigid Foams ..........................
Catalysts ................................
General ................................
Rigid Urethane Foams .....................
Flexible Urethane Foams ....................
............
Fire Retardants (Flame Retardants)
General ................................
Additive Fire Retardants ....................
Reactive Fire Retardants ....................
Uses of Fire Retardants in Specific Foam Types ....
Rigid Polyurethane Foams .................
Flexible Polyurethane Foams ...............
Polystyrene Foams ......................
Polyolefin Foams .......................
Polyvinyl Chloride (PVC) Foams ............
Phenolic Foams ........................
Urea-Formaldehyde Foams ................
Mold-Release Agents (Parting Agents) ..........
General ................................
External Mold Releases .....................
Paraffins, Hydrocarbon Waxes ..............
Polyethylene Waxes .....................
Water-Base Mold Releases ................
Semi-Permanent Mold Releases .............
Nucleating Agents (Nucleators) ................

...........................
Reinforcements
Urethane Foams ..........................
Thermoplastic Structural Foams ...............
Stabilizers ...............................
..............................
Surfactants
General ................................
Flexible Foam Surfactants ...................
Rigid Foam Snrfactants .....................
References ...............................

8. METHODS OF MANUFACTURE

................

288
289
293
293
294
296
297
297
297
299

300
300
301

301
302
302
302
302
303
303
303
303
304
304
304
304
306
306
306
308
308
308
308
309

310
316

Arthur H. Landrock
.............................
Introduction
Molding .................................
Reaction Injection Molding (RIM) .............

316
316
318


Contents
Liquid Injection Molding (LIM) ...............
Slabstock Molding (Free-Rise Foaming) .........
Spraying ................................
Frothing ................................
Laminating ..............................
Structural Foam Preparation .................
Structural Foam Molding ...................
Structural Foam Extrusion ...................
Syntactic Foam Preparation ..................
Foam-in-Place (Foam-in-Bag) Techniques ......
References ...............................
9.

SOURCES OF INFORMATION ..................

xix
318
319
320
322
324
325
325

327
327
328
329

332

Arthur H. Lmdrock

Introduction .............................
Journals and Other Periodicals ................
Books ..................................
Conferences, Proceedings, Technical Bulletins,
and Technical Reports .....................
10. TEST METHODS

............................

332
332
340
349

354

Arthur H. Landrock

354
Introduction .............................
355

Compilation of Standard Test Methods ..........
371
Discussion of Selected Test Methods ............
Combustion Properties (Fiammability) (Smoke
376
Evolution) .............................
376
ASTM D 2843 for Smoke Density ...........
ASTM D 41OO-Gravimetric Determination of
377
Smoke Particulates .....................
ASTM E 662 NBS (NISI) Smoke Density Test . _ 377
378
ASTM D 2863 Oxygen Index Test ...........
ASTM D 3014 Flame Height, Time of Burning,
and Loss of Weight of Rigid Thermoset
Cellular Plastics, in a Vertical Position
379
(Butler Chimney Test) ..................
ASTM D 3675 Surface Flammability of Flexible
Cellular Materials Using a Radiant Heat
379
Energy Source ........................
379
ASTM D 3894 Small Comer Test ............
ASTM D 3574 Methods of Testing Flexible


xx

Contents
Cellular Materials .....................
ASTM E 84 Steiner Tunnel Test .............
ASTM E 162 Radiant-Panel Test ............
ASTM E 286 Eight-Foot Tunnel Test .........
ASTM E 906 Heat and Visible-Smoke-Release
Rate Test ...........................
UL 94 Appendix A-Horizontal Burning Test
for Classifying Foamed Materials 94HBF,
94 RF-l, or 94 BF-2 ..................
BS 5946 Punking Behavior of Phenol-Formaldehyde Foam ........................
Compression/Deflection Properties .............
Constant-Deflection-Compression-Set
Test .....
Indentation-Force-Deflection (IFD) (ILD) Test
(to Specified Deflection) .................
Indentation-Force-Deflection (IFD) (ILD) Test
(to Specified Force) ....................
Compressive Properties of Rigid Cellular
Plastics .............................
MIL-HDBK-304-Chapter 6 ...............
Fatigue ................................
Fragmentation (Friability) (Dusting) ............
Flexibility (of Cushioning Materials) ...........
Flexural Properties ........................
Fungal Resistance .........................
Hydrolytic Stability .......................
Impact Strength (Brittle Strength) ..............
Open-Cell Content ........................
Resilience (Ball-Rebound Test) ...............

Tear Resistance (Tear Strength) ...............
Tension Test ............................
Water Absorption .........................
Water-Vapor Transmission ..................
Special Non-Standardized Test Methods .........
Thermal Analysis .........................
Differential Themal Analysis (DTA) ..........
References ...............................

380
380
380
380
381

381
381
381
382
382
382
382
382
383
384
384
384
385
385
385

385
386
386
387
387
388
388
389
389
393

11. STANDARDIZATION DOCUMENTS . . . . . . . . . . . . . . 395
Arthur H. Lmdrock
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395


Contents

Industry Standards

........................

American Society for Testing and Materials .......
ASTM Practices, Definitions, Abbreviations,
Guides, Classifications, etc. ................
Underwriters Laboratories (UL) ...............
.....................
Military Specifications

Military Standards .........................

Military Handbooks ........................
Federal Specifications .......................
Federal Standards
British Standards

IS0 Standards
References

.........................
.........................
............................

...............................

xxi
400
400

419
425
425

436
437
439
442
442

447
454

GLOSSARY . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . 456
Arthur H. Landrock
References
. . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . 479

INDEX

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482


1

INTRODUCTION
FORMATION

TO FOAMS AND FOAM

Michael 0. Okoroafor and Kurt C. Frisch

INTRODUCTION
Cellular plastics or plastic foams, also referred to as expanded or
sponge plastics, generally consist of a minimum of two phases, a solidpolymer matrix and a gaseous phase derived from a blowing agent. The
solid-polymer
phase may be either inorganic, organic or organometallic.
There may be more than one solid phase present, which can be composed
of polymer alloys or polymer blends based on two or more polymers, or
which can be in the form of interpenetrating polymer networks (IPNs)
which consist of at least two crosslinked polymer networks, or a pseudoor semi-IPN formed from a combination of at least one or more linear
polymers with crosslinked polymers not linked by means of covalent

bonds.
Other solid phases may be present in the foam in the form of
fillers, either fibrous or other-shaped fillers which may be of inorganic
origin, e.g. glass, ceramic or metallic, or they may be polymeric in
nature. Foams may be flexible or rigid, depending upon whether their
glass-transition
temperatures are below or above room temperature,
which, in turn, depends upon their chemical composition, degree of
crystallanity, and degree of crosslinking.
Intermediate between flexible
and rigid foams are semi-rigid
or semi-flexible
foams.
The cell

1


2

Handbook

of Plastic Foams

geometry, i.e. open vs. closed cell, size and shape, greatly affect the foam
Thus, closed-cell
foams are most suitable for thermal
properties.
insulation, while open-cell foams are best for acoustical insulation.
Plastic foams can be produced in a great variety of densities,

ranging from about 0.1 lb/f? (1.6 kg/m3) to over 60 lb/ft3 (960 kg/m3) (1).
Since the mechanical-strength
properties are generally proportional to the
foam densities, the applications of these foams usually determine which
range of foam densities should be produced. Thus, for rigid foam, loadbearing applications require high densities and (or) fiber-reinforced
foams, while low densities are usually used for thermal insulation.
The production of polymeric-foam
materials can be carried out
by either mechanical, chemical, or physical means. Some of the most
commonly used methods are the following (2):
1.

2.

3.

4.

5.
6.

7.

Thermal decomposition of chemical blowing agents generating either nitrogen or carbon dioxide, or both, by
application of heat, or as the result of the exothermic
heat of reaction during polymerization.
Mechanical whipping of gases (frothing) into a polymer
system (melt, solution or suspension) which hardens,
either by catalytic action or heat, or both, thus entrapping
the gas bubbles in the polymer matrix.

Volatilization of low-boiling
liquids such as fluorocarbons or methylene chloride within the polymer mass
as the result of the exothermic heat of reaction, or by
application of heat.
Volatilization of gases produced by the exothermic heat
of reaction during polymerization such as occurs in the
reaction of isocyanate with water to form carbon dioxide.
Expansion of dissolved gas in a polymer mass on
reduction of pressure in the system.
Incorporation of hollow microspheres into a polymer
mass. The microspheres may consist of either hollow
glass or hollow plastic beads.
Expansion of gas-filled beads by application of heat or
expansion of these beads in a polymer mass by the heat
of reaction, e.g. expansion of polystyrene beads in a
polyurethane or epoxy resin system.

The production of foams can take place by many different techniques.
These may include (3):


Introduction to Foams and Foam Formation

3

1. Continuous slab-stock production by pouring or impingement, using multi-component foam machines.
2. Compression molding of foams.
3. Reaction-injection molding (RIM), usually by impingement.
4. Foaming-in-place by pouring from a dual- or multicomponent head.
5. Spraying of foams.

6. Extrusion of foams using expandable beads or pellets.
7. Injection molding of expandable beads or pellets.
8. Rotational casting of foams.
9. Frothing of foams, either by introduction of air or of a
low-boiling volatile solvent (e.g. dichlorodifluoromethane, F-12).
10. Lamination of foams (foam-board production).
11. Production of foam composites.
12. Precipitation foam processes where a polymer phase is
formed by polymerization or precipitation from a liquid
which is later allowed to escape.
It should be recognized that almost every thermoplastic and
thermoset resin may be produced today in cellular form by means of the
mechanisms and processes cited above. The physical properties of the
foams reflect in many ways those of the neat polymers, taking into
account the effects of density and cell geometry.
There are numerous books, chapters in books, and reviews
published on foams, covering a wide spectrum of cellular plastics. Some
of these are listed in references 1-15.
In addition, two journals (in English) deal exclusively with plastic
foams.
These are the “Journal of Cellular Plastics” (Technomic
Publishing Co.) and “Cellular Polymers” (RAPRA Technology Ltd.). A
valuable source of information for foamed plastics has been the annual
proceedings of various technical organizations such as the Society of the
Plastics Industry (SPI); the German FSK and others.

CFC EFFECT3 AND ALTERNATIVES
A search for alternate blowing agents for urethane foams became
necessary in 1987 following the Montreal Protocol, which mandated the
development of foams with substantially reduced CFC content by 1995.

4

Handbook of Plastic Foams

CFC’s or chlorofluorocarbons are chemicals that cause ozone depletion in
the stratosphere as well as the “Greenhouse Effect”. They have been
typically employed as blowing agent in foams.
Since the initial
proclamation, the mandate has been revised several times to accelerate the
CFC phaseout schedule, with the latest revision resulting from the
Copenhagen agreement in November 1992 where 87 nations resolved to
move up total CFC phaseout by four years in January 1996. The recent
Copenhagen revision induced major CFC manufacturers to accelerate their
phaseout time table. DuPont announced recently that it plans to stop
CFC production by 1994, almost 2 years ahead of plan.
Some countries have independently banned CFC use. For
example, Sweden banned the use of CFC’s in 1991, followed by Switzerland in 1992. In Europe Rigid Foam manufacturers are using hydrocarbon blowing agents, such as cyclopentane as an alternative to CFC.
The U.S. Environmental Protection Agency issued a final rule
banning the use of CFC’s in flexible plastics and packaging foams, among
other uses, after February 15, 1993. Exceptions are CFC-11 and CFC13 which can be used, temporarily, in mold release agents and the
production of plastic and elastomeric materials. However, in 1994, no
CFC’s will be allowed in flexible foams in the U.S., and a tax will be
levied on other CFC uses. Total CFC phaseout is mandated in the U.S.
for 1995.
Users of CFC’s in foam applications are, for the time being, able
to employ alternative blowing agents (ADA’s) available to them. CFC11, the workhorse of the foam industry can now be replaced with a
hydrochlorofluorocarbon, namely HCFC-141b. Although HCFC-141b
and other HCFC’s are not considered drop-ins for CFC-11, the use of

foam additives, such as surfactant and softening agents, has made it
possible to achieve comparable insulation value in rigid foams blown
with HCFC’s.
Total U.S. HCFC use is to be phased out by 2005; however
current trends indicate HCFC’s may be dropped in some industries as
early as 1997. Even though eventual phaseout of all HCFC substitutes
is expected to start by the year 2003, because of its ozone depleting
potential, foam manufacturers, especially, rigid foam blowers, are
committed to it in the short term.
For flexible foams in which CFC’s have typically been employed
as auxiliary blowing agents, entirely water-blown foams can be achieved
with the performance additives.


Introduction

to Foams and Foam Formation

5

FUNDAMENTALS OF FOAM FORMATION
The preparation of a polymeric foam involves first the formation
of gas bubbles in a liquid system, followed by the growth and
stabilization of these bubbles as the viscosity of the liquid polymer
increases, resulting ultimately in the solidification of the cellular resin
matrix.
Foams may be prepared by either one of two fundamental
methods. In one method, a gas such as air or nitrogen is dispersed in a
continuous liquid phase (e.g. an aqueous latex) to yield a colloidal system
with the gas as the dispersed phase. In the second method, the gas is

generated within the liquid phase and appears as separate bubbles
dispersed in the liquid phase. The gas can be the result of a specific gasgenerating reaction such as the formation of carbon dioxide when
isocyanate reacts with water in the formation of water-blown flexible or
rigid urethane foams. Gas can also be generated by volatilization of a
low-boiling
solvent (e.g. trichlorofluoromethane,
F-11, or methylene
chloride) in the dispersed phase when an exothermic reaction takes
places. (e.g. the formation of F-11 or methylene chloride-blown
foams).
Another technique to generate a gas in the liquid phase is the
thermal decomposition of chemical blowing agents which generate either
nitrogen or carbon dioxide, or both.
Saunders and Hansen (3) have treated in detail the colloidal
aspect of foam formation utilizing blowing agents. The formation of
internally blown foams takes place in several stages. In the first stage the
blowing agent generates a gas in solution in a liquid phase until the gas
reaches a saturation limit in solution, and becomes supersaturated.
The
gas finally comes out of solution in the form of bubbles. The formation
of bubbles represents a nucleation process since a new phase is formed.
The presence of a second phase which may consist of a finely divided
solid, e.g. silica, or some finely dispersed silicone oils, or even an
irregular solid surface such as an agitator or wall of a vessel, may act as
a nucleating agent.
The factors affecting the stability and growth of bubbles in
aqueous foams have been reviewed in depth by deVries (3). In order to
disperse a given volume of gas in a unit volume of liquid, one must
increase the free energy of the system by an amount of energy AF as
follows;

AF = yA
where y is the surface tension and A is the total interfacial

area.

When


6

Handbook of Plastic Foams

the surface tension of the liquid is lowered, either by heat or by the
addition of a surfactant, the free-energy increase associated with the
dispersion of the gas will be reduced and will aid in the development of
fine cells which corresponds to a large value of A.
According to classical theory, the gas pressure in a spherical
bubble is larger than the pressure in the surrounding liquid by a
difference Ap, as shown in the following equation:

where R is the radius of the bubble. Hence, the gas pressure in a small
bubble is greater than that in a large bubble.
In the case of two bubbles of radii R, and &, the difference in
pressure Ap’, is given by the equation:

Therefore, in a liquid system, a diffusion of gas takes place from
the small bubbles into the large bubbles, resulting in the disappearance
of the small bubbles, while the large bubbles grow in size with time. It
is also apparent that low values of y, e.g. by addition of a surface-tension
depressant such as a silicone surfactant, reduce the pressure differences

between bubbles of different sizes and hence lead to better bubble
stability and small average cell size.
In the formation of polymeric foams, a number of the
relationships described below are applicable, at least to some extent, when
the polymer phase is still a liquid. In order to form a stable foam, there
must be at least two components, one which is preferentially absorbed at
the surface. The Gibbs theorem teaches that the surface tension is
dependent upon the type and amount of absorbed solute, as follows:

dy = XI’dp
where P is the surface excess of a component with a chemical potential
p. This relationship explains the resistance to an increase in the surface
area or a thinning of the cell membrane. Due to the fact that membranes


Introduction to Foams and Foam Formation

7

tend to rupture more easily the thinner they are, this resistance to thinning
helps to stabilize the cell.
When a membrane expands and the concentration of a surfactant
at the interface decreases, there exist two mechanisms to restore the
The first mechanism, termed the
surfactant surface concentration.
“Marangoni effect” (16), refers to the fact that the surface flow can drag
with it some of the underlying layers, i.e. the surface layer can flow from
areas of low surface tension, thus restoring the film thickness. It is also
a source of film elasticity or resilience.
In the second mechanism, the “Gibbs effect,” the surface

deficiency is replenished by diffusion from the interior and the surface
tension is lowered to obtain a desirable level. For the best stabilization
of a foam, an optimum concentration of surfactant as well as an optimum
rate of diffusion is desirable (3).
Another factor which affects the bubble stability is temperature,
since an increase in temperature reduces both surface tension and
viscosity, which results in thinning of the cell membrane and may
promote cell rupture.
Still another factor in cell stability is the drainage of the liquid in
the bubble walls which is due to gravity and capillary action. This
drainage from both capillary action and gravity can be retarded by an
increase in viscosity, especially at the film surface. This is particularly
important in primarily thermoset systems which involve simultaneous
polymerization and foaming of the liquid components.
A balance
between the viscosity and gas evolution must be provided in order to
obtain not only a stable foam, but also one with the highest foam volume
possible. It is obvious that if the viscosity increases too rapidly (as the
result of too fast a polymerization) the gas evolution will eventually cease
before reaching its desired foam volume, especially for the production of
low-density foams. On the other hand, if the viscosity is too low, when
most of the foam evolution occurs, foam stabilization may be very
difficult and may result in foam collapse (3).
The proper balance between viscosity and gas evolution can be
controlled by a number of factors such as a suitable type and
concentration of catalyst and surfactant, the presence of a nucleating
agent (not always necessary) (17,18) and control of reaction temperature
(or exotherm). Additional factors that must be considered are the use of
a suitable chemical blowing agent, which is especially important for the
production of thermoplastic foams, and the formation of oligomers

(prepolymers) which exhibit higher viscosities than monomers in the
preparation of thermoset foams (e.g. polyurethane foams).


8

Handbook

of Plastic Foams

of
The “electrical double layer” effect, i.e. the orientation
electrical charge on each film surface due to the use of ionic emulsifiers,
is generally more important in aqueous foams than in organic polymeric
foams. The stability effect arises from the repulsion of the electrical
charges as the two surfaces approach each other, thus limiting the
thinning of the film (cell walls) (3).
The morphology of cellular polymers ‘has been studied in great
detail by numerous investigators, in particular by Hilyard (5), Gent and
Thomas (19), Harding (5), Meinecke and Clark (20), and others.
The markets for plastic foams have been growing worldwide with
North America, the E.E.C., and Japan as the leading producers and
consumers of foams. However, the Comecon countries, Latin America,
especially Brazil, Argentina, and Mexico, and Asian countries, (other than
Japan) such as Taiwan, South Korea and India, are rapidly developing
foam markets and production facilities. Many developing countries in all
continents are using foams at an ever-increasing
rate by starting foam
production employing either imported or locally produced raw materials,
with major efforts being expended in utilizing certain domestic plant or

forest products, especially for foam composites.
The major industries which utilize flexible or semi-flexible foams
are:
Furniture
Transportation
Comfort cushioning
Carpet underlay
Packaging
Textiles
Toys and novelties
Gasketing
Sporting goods
Shock (vibration) and sound attenuation
Shoes
Rigid foam markets include the following
Thermal insulation
Building and construction
Appliances
Tanks/pipes
Transportation
Packaging

industries:


Introduction to Foams and Foam Formation

9

Furniture

Flotation
Moldings (decorative)
Business-machine housings
Food-and-drink containers
Sporting goods
Sound insulation
Surveys of foam markets are frequently prepared by raw-material
suppliers as well as various marketing-research organizations. A very
useful publication is the U.S. Foamed Plastics Markets 62 Directory,
published annually by Technomic Publishing Co., Lancaster, PA 17604.

REFERENCES
1.

Plastics Engineering Handbook, Society of the Plastics Industry,

Inc., 5th Edition, ed. M.L. Berins, Van Nostrand Reinhold, N.Y.,
1991.
2.

Frisch, K.C., in Plastic Foams. Vol. 1, eds. by Frisch, K.C. and
Saunders, J.H., Marcel Dekker, N.Y., (1976).

3.

Saunders, J.H., and R.H. Hansen in Plastic Foams, Vol. 1, eds.
Frisch, K.C. and Saunders, J.H., Marcel Dekker, N.Y ., (1972)
Chapter 2.

4.

Benning, C.J., Plastic Foams, Vols. 1 and 2, Wiley-Interscience,
New York (1969).

5.

Mechanics of Cellular Plastics, ed. by Hilyard, N.C., Macmillan,

New York, (1982).
6.

Shutov, F.A., IntegrallStructural Polymer Foams, eds. HenriciOliv, G., and Oliv, S., Springer, Berlin, (1985).

7.

Berlin, A.A., Shuto, F.A., and Zhitinkina, A.K.. Foams Based on
Reactive Oligomers, Technomic Publishing Co., Lancaster, PA
(1982).

8.

Polyurethane Handbook, ed. by Oertel, G., Hanser, and distrib.