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4.f Epoxy Resin Chemistry
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Epoxy Resin Chemistry
Ronald S. Bauer, EDITOR
Shell Development Company
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9- Based on a symposium sponsored
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10. Organic Coatings and Plastics
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79 | at the 176th Meeting of the
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er American Chemical Society,
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D Miami Beach, Florida,
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n September 11-15, 1978.
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ACS SYMPOSIUM SERIES 114
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D.C. 1979
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0 Epoxy resin chemistry.
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0. (ACS symposium series; 114 ISSN 0097-6156)
oi: 1 Includes bibliographies and index.
d 1. Epoxy resins—Congresses.
79 | I. Bauer, Ronald S., 1932- . II. American Chemi
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1 cal Society. Division of Organic Coatings and Plastics
3, Chemistry. III. Series: American Chemical Society.
er ACS symposium series; 114.
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e TP1180.E6E59 668'.374 79-21858
ec ISBN 0-8412-0525-6 ASCMC8 114 1-271 1979
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on Copyright © 1979
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blic American Chemical Society
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P All Rights Reserved. The appearance of the code at the bottom of the first page of each
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PRINTED IN THE UNITED STATES OF AMERICA
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ACS Symposium Series
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4.f M. Joan Comstock, Series Editor
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02 Advisory Board
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De Jeremiah P. Freeman F. Sherwood Rowland
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Robert A. Hofstader Aaron Wold
James D. Idol, Jr. Gunter Zweig
FOREWORD
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4.f The ACS SYMPOSIUM SERIES was founded in 1974 to provide
11 a medium for publishing symposia quickly in book form. The
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1 IN CHEMISTRY SERIES except that in order to save time the
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0 mitted by the authors in camera-ready form. Papers are re
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10. viewed under the supervision of the Editors with the assistance
oi: of the Series Advisory Board and are selected to maintain the
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9 | integrity of the symposia; however, verbatim reproductions of
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PREFACE
^ince their commercial introduction into the United States in the late
^ 1940's, epoxy resins have established themselves as unique building
blocks for high performance coatings, adhesives, and reinforced plastics.
Over the past five years epoxy resin sales have experienced an annual
growth rate of about 8%. In 1978, production in this country exceeded
310 million pounds, and domestic sales were greater than 280 million
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0 pounds. This sustained growth of epoxy resins is a result of the wide
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pr range of properties that can be achieved with these versatile materials;
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11 uses range from coatings for the ubiquitous beverage can to high per
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9- formance adhesions in spacecraft as well as for encapsulating their
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1 sophisticated electronic components.
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1/b Work still continues as pressures for more environmentally acceptable
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0 and energy efficient systems, and demands for even higher performance,
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10. challenge the polymer chemist. This volume covers some of the new
oi: resins, curing agents, and application techniques along with new ideas
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9 | about the chemistry and properties of existing epoxy resin systems.
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1 Although comprehensive coverage of this field is not possible in our
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er symposium, it is hoped the papers presented here will help stimulate
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ix
1
The Photoinitiated Cationic Polymerization of Epoxy
Resins
J. V. CRIVELLO and J. H. W. LAM
General Electric Corporate Research and Development Center, Schenectady, NY 12301
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4. The importance of epoxy resins in the fabrication of surface
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1 coatings is well recognized in the coatings and plastics industry.
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9- In fact, the chief use of epoxy resins, amounting to over 100
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19 million pounds last year, was for coating applications (1). Tra
bk- ditionally, these coatings have been applied from solvents and
21/ cured by a baking process. Current efforts to reduce the amount
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1 of energy required to carry out coating processes as well as an
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1 increasing concern for the environment have provided the impetus
oi: to begin the search for new curing chemistry and coating applica
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9 | tion techniques which circumvent these problems.
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9 Ultraviolet curing has emerged as a rapidly growing method
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3, for the fabrication of essentially pollution-free coatings, having
er only a fraction of the energy requirements of traditional ther
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m mally cured materials. At the same time, the generally excellent
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ec properties of the coatings which are obtained using uv curing
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e: resins together with their high application and cure speeds have
at made it attractive for many industries to install uv-cure lines.
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n While the bulk of the work in uv curing to date has involved
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ati the radical polymerization of vinyl compounds, during the past
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bli five years discovery of several new photoinitiator compounds now
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cationic process. With such systems, solventless liquid epoxy
resins can be cured continuously at very high line speeds using
approximately one tenth the energy which would be consumed by a
comparable thermal process. The first of these new photoiniti
ators are the aryldiazonium salts (I) (2_>3.»4,5j. when these com
pounds are irradiated using uv light, a fluoroaromatic compound,
nitrogen and the strong Lewis acid, boron trifluoride, are pro
duced (equation 1).
Ar-N+EN BF" — > Ar-F + N + BF^ (1)
4 2
I
If the photolysis of an aryldiazonium salt is conducted in the
0-8412-0525-6/79/47-114-001$05.00/0
© 1979 American Chemical Society
2 EPOXY RESIN CHEMISTRY
presence of an epoxy resin, boron trifluoride catalyzes the
cationic polymerization of the resin. Due to the production of
nitrogen as a byproduct of the photolysis of aryldiazonium salts,
the major uses of coating systems employing these photoinitiators
lies primarily in thin film applications such as container coat
ings [6) and photoresists (7J.
Recently, we have reported that diaryliodonium salts (II)
are a second class of highly efficient photoinitiators for the
Ar-I-Ar'
X" = BF", AsF , PF , SbCl , etc.
4 6 6 6
II
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h0 cationic ring opening polymerization of epoxy resins (8,9J.
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4. Parallel work in two other laboratories has led to the same con
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01 clusion (10, ]V). Mechanistic studies have shown that on photol
79- ysis, diaryliodonium salts liberate strong Br0nsted acids of the
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1 type, HX, which subsequently initiate cationic polymerization.
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2 which has led to the development of triarylsulfonium salts (III)
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doi: 1 pzaotliyomn erainzda tiino n paofr tiecpuolxaird, esd. escribe their application to the
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n Results and Discussion
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bli In our laboratory, we have found that triarylsulfonium
Pu salts (III) in which the anions are of the type BF " AsF", PF",
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SbF ~ etc., are excellent photoinitiators for the polymerization
of epoxy resins as well as a variety of other monomers (12, 13).
Similar results have also been reported by another group of
investigators (14).
Triarylsulfonium salts may be conveniently prepared via a
number of synthetic routes. In equations 2, 3 and 4 are shown
three of the most direct preparative methods.
6ArH + SC1 + 3C1 A1C13 ^ 2(Ar)S+Cl" + 6HC1 (2)
2 2 2 3
Ar Ar
^S-* 0 + Ar'-MgBr benzene ^+. - " + gBrOH (3)
> s Ar x M
Ar' ref 1 ux Ar
1. CRIVELLO AND LAM Photoinitiated Cationic Polymerization 3
Cu11 (cat)
(Ar)S + (Ar')I+ X" > ArAr»S+ X" + Ar'I (4)
2 2 2
Although the reaction shown in equation 2 is simple and direct,
the products which are obtained are generally impure (15). The
condensation of diarylsulfoxides with Grignard reagents (equation
3) gives pure triarylsulfonium salts; however, the overall yields
are generally of the order of 20-30% (16). In a very recent
publication from this laboratory,we described a general synthesis
which affords these compounds in high yields (17_). This new
route involves the copper (II) catalyzed arylation of diaryl
iodonium salts with diarylsulfides (equation 4).
Both of the methods shown in equations 2 and 3 give rise to
01 triarylsulfonium halides which are inactive as photoinitiators
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h for cationic polymerization. These salts must, therefore, be
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4. converted to the corresponding salts in which the anion is of
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01 the type X" = BF4", ASF5", PFc", etc. This conversion may be
79- accomplished using either of the two methods shown in equation 5.
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3, Using the above synthesis, a wide variety of triarylsulfon
ber ium salt photoinitiators can be prepared. In Table 1 are shown
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e some representative triarylsulfonium salt photoinitiators which
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De were prepared during the course of this research. All the com
e: pounds in this table are well characterized crystalline com
Dat pounds with well defined melting points, elemental analyses,
on ultraviolet, proton, and 1 3C nmr spectra (1_7). The ability
ati of triarylsulfonium salts bearing non-nucleophilie counterions to
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bli serve as photoinitiators is completely general and includes all
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as well as polynuclear and heterocyclic salts shown in Table 1.
In contrast to radical photoinitiators which are also gen
erally thermally unstable, triarylsulfonium salts display a sur
prising level of thermal stability. Figure 1 shows the thermo-
gravimetric analysis curves for triphenylsulfoniurn hexafluoro-
arsenate performed in air and nitrogen at a heating rate of
10°C/minute. In both cases, thermal decomposition is not ob
served below 350°C. This high level of thermal stability is
reflected in the extremely long shelf lifes of epoxy resins
sensitized with triarylsulfonium salt photoinitiators. Figure 2
shows the plots of the increase in solution viscosityversus
+
time in months for various concentrations of (Cs^JqS ASF5" in
the highly reactive bisepoxide, 4-vinylcyclohexene dioxide. Only
a slight increase in the overall solution viscosity was noted
