Emulsion Polymers and Emulsion Polymerization

596 Pages·1981·10.23 MB·English

by  David R. Bassett and Alvin E. Hamielec (Eds.)

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Emulsion Polymers and
Emulsion Polymerization
David R. Bassett, EDITOR
Union Carbide Corporation
Alvin E. Hamielec, EDITOR
1 McMaster University
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1- Based on a symposium
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8 and Polymer Chemistry at the
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Las Vegas, Nevada,
August 25-29, 1980.
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ACS S Y M P O S I UM SERIES
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D. C. 1981
Library of Congress CIPD ata
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0 Emulsion polymers and emulsion polymerization.
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w (ACS symposium series, ISSN 0097-6156; 165)
65.f "Symposium on Emulsion Polymerization"—Pref.
01 Includes bibliographies and index.
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8 1. Addition polymerization—Congresses.
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1 I. Bassett, David R., 1939- . II. Hamielec, Alvin
bk- E., 1935- . III. Symposium on Emulsion Polymeri
1/ zation (1980: Las Vegas, Nev.). IV. American Chemi
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0 cal Society. Division of Organic Coatings and Plastics
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0. Chemistry. V. American Chemical Society. Division of
oi: 1 Polymer Chemistry. VI. Series.
1 | d QISDB2N8 10.P-864E1427-0 642-2 547'.28 81A-1A0C8R232
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9 ASCMC 8 165 1-605 1981
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02 David L. Allara James P. Lodge
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James D. Idol, Jr. Gunter Zweig
FOREWORD
1 The ACS SYMPOSIUM SERIES was founded in 1974 to provide
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65.f format of the Series parallels that of the continuing ADVANCES
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1-0 IN CHEMISTRY SERIES except that in order to save time the
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9 papers are not typeset but are reproduced as they are sub
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bk- mitted by the authors in camera-ready form. Papers are re
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PREFACE
The symposium on Emulsion Polymerization held at the National Meet­
ing of the American Chemical Society in Las Vegas followed a similar
symposium held five years earlier at an ACS meeting in April, 1975. The
proceedings of the 1975 symposium, organized by I. Piirma and J. L.
Gardon, were subsequently published as Volume 24 in the ACS Symposium
Series. The remarkable growth in emulsion polymerization technology was
noted at that time. This growth has not only continued during the succeed­
ing half-decade, but has accelerated. The present volume documents
01 recent advances made by an international body of scientists working on a
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pr wide range of fundamental and applied problems in emulsion polymeriza­
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16 tion and emulsion polymers.
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81- In planning the program, it was felt that tutorial lectures by recognized
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k-1 authorities would be an appropriate way of reviewing the state of the art
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10 emulsion polymerization in preparation for the contributed papers to
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1 follow. Invited lectures (Chapters 1-6) included a treatment of several
doi: important aspects of emulsion polymer particles: their nucleation, their
1 | growth and stabilization, and their characterization by light scattering. A
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1 discussion of the synthesis and study of model polymer colloids revealed
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er their wide use in diverse applications. Also included were lectures on
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O tinuous latex reactors.
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at One area of intense current effort is the design and control of molec­
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on ular and particle structure in emulsion polymerization. At least eight of
cati the chapters touch on this subject. Recent advances include the use of
ubli novel reaction pathways to achieve nonuniform as well as uniform struc­
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tures when desired. Light scattering, electron microscopy, and NMR
techniques have been utilized to probe particle morphologies and molecular
architecture. Reaction kinetics has always been a central concern in
emulsion polymerization investigations. The effects of chain entanglement,
monomer diffusion, and nonreactive components on kinetics have been
treated. The engineering aspects of emulsion polymerization are closely
aligned with the foregoing subjects since the reaction process has such a
strong effect on the properties of emulsion polymers. Of particular interest
are methods for control of reactors and for real-time monitoring of
reactor dynamics.
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This collection of papers indicates the rapid advancement in our
understanding and use of the many facets of emulsion polymerization
and emulsion polymers. It also suggests likely advances during the next
five years: further development of analytical techniques for investigating
the surface chemistry, internal structure, and physical properties of emul­
sion polymers; inverse emulsion polymerization of water-soluble monomers;
development of a better understanding of coagulation—for particle size
control as well as for the prevention of reactor fouling; a more complete
understanding of high-conversion kinetics; and an increase in on-line
instrumentation to permit the use of continuous reactors to produce high-
quality emulsion polymers.
In addition to the support of the cosponsoring Divisions of Organic
Coatings and Plastics Chemistry and Polymer Chemistry, we gratefully
acknowledge financial contributions to the symposium from the following
corporations: Air Products and Chemicals, Diamond Shamrock, Dow
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0 Chemical, Eastman Kodak, Nalco Chemical, PPG Industries, SOHIO, and
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5.pr Union Carbide. We must also acknowledge the referees who reviewed
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k- D. R. BASSETT
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1 South Charleston, West Virginia 25303
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81 | A. E. HAMIELEC
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xii
1
Latex Particle Nucleation and Growth
ROBERT M. FITCH
Department of Chemistry and Institute of Materials Science,
The University of Connecticut, Storrs, CT 06268
Emulsion polymerization was first reported in 1927 by
Ray P. Dinsmore of the Goodyear Tire and Rubber Co. (1). He made
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00 aqueous emulsions of various methyl-butadienes using oleic acid
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5.c salts and casein or egg albumin as emulsifiers, and allowed them
16 to stand at 50°-70°C for six months to polymerize. Although not
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1- what we would consider a practical process today, it led over the
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9 next two decades to an entire industry. It was natural that the
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10 turned out, the appellation has been an unfortunate one in that,
10. except in rare circumstances, the mechanism does not involve
oi: polymerization in emulsified monomer droplets. It was observed
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98 latex particles could be formed in the absence of emulsifying
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7, agents from monomers of low water-solubility (2, 3). Since the
er particles were much smaller than the droplets of monomer which
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O eous nucleation of the polymer particles had occurred. In 1937
ate: Fikentscher showed that even in the presence of micellar emulsi
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n fier the "aqueous phase" was the principal locus of polymerization,
atio not the emulsified monomer droplets (4). Heller and Klevens
c reported in 1943-1945 on their quantitative studies on the strong
ubli influence of emulsifier concentration on the number of polymer
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particles formed both below and above the critical micelle con
centration (CMC) (5). Two years later Harkins published the
results of a series of quantitative investigations on the polymer
ization of styrene and isoprene both in the absence and presence
of monomer-swollen soap micelles (6). Harkins observed that
the rates of polymerization were much greater when micelles were
present and therefore proposed that these were the principal locus
of particle formation. This led to the landmark work of Smith and
Ewart, published the following year, which presented quantitative
theories for the prediction of the absolute particle concentra
tion, N, and for the absolute rate of polymerization (7). All of
the above studies involved monomers with very low solubilities in
water.
0097-6156/81/0165-0001$07.25/0
© 1981 American Chemical Society
2 EMULSION POLYMERS AND EMULSION POLYMERIZATION
At about the same time Baxendale, Evans and coworkers pub
lished work on a more water-soluble monomer, methyl methacrylate
(MMA), both in the absence and presence of a cationic surface-
active agent, and concluded that nucleation of the polymer
particles was by a homogeneous mechanism, in which soap micelles
played no role (8). Thus, two contending schools of thought were
established concerning the mechanism of particle formation, one
embracing the theory of homogeneous nucleation, the other, the
micellar, or heterogeneous, mechanism. The latter had much
greater success for many years, probably because of i ts applica
bility to an industry which was well developed (synthetic rubber),
and because it received experimental support in the work of several
investigators (9). This was, however, limited almost entirely to
styrene and a very few comonomers.
It is the purpose of this paper to present the case for the
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00 homogeneous nucleation school, which will be seen to apply to a
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5.c large number of monomers more water-soluble than styrene, as well
16 as to resolve the differences between the two schools. The
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1- nucleation period during an emulsion polymerization will be taken
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9 as the time during which the number of particles is changing,
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k- either increasing or decreasing. This involves processes which
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0 prediction of the final particle size and size distribution. The
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10. growth of particles will be discussed only to the extent that it
oi: affects the mechanism or kinetics of particle formation.
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98 General Nucleation Theory
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er Colloidal particles are formed from a homogeneous medium by
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ate: of repeat units of the same or different molecules. The specific
D surface area of such embryos is very great, and i ts creation
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cati A, times the interfacial free energy, y:
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AG = A#y joules. (1)
This energy is supplied by the condensation of hydrophobic units
from the aqueous medium and by the energy of polymerization,
AG and AG , both expressed per unit volume of polymer. For
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isotropic, amorphous polymer particles of radius r, the free
energy of formation of an embryo will be
AG^ = -|irr3 (AG + AG ) + 4irr2y (2)
f 3 v p 1
When the size of the particles is very small, the last term
dominates, AG^ is positive and the embryos are unstable, i.e.
they will disaggregate. When r reaches a critical size, r*, the
slope of dAG/dr becomes negative, so that further growth is
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favored. Thus chains, initiated by free radicals generated in
the continuous phase, must grow until they reach a critical size