Epoxy Resin Technology 0470113901, 9780470113905

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EPOXY RESIN TECHNOLOGY Edited by PAUL F. BRUINS

EPOXY RESIN TECHNOLOGY Edited by PAUL F. BRUINS, Polytechnic Institute of Brooklyn. CONTENTS:

General Chemistry of Bisphenol ABased Epoxy Resins (R. T. Dowd) Chemistry, Properties, and Applica¬ tions (John Stevens, Jr.) Chemistry, Properties, and Applica¬ tions of Epoxy Novolac, Flexible Epoxy, and Flame Retardant Epoxy Resins (Allan R. Meath) Commercial Epoxy Resin (Rafael J. Perez) Epoxy Resin: Formulating Tech¬ niques and Evaluation (John E. Magee) Safe Handling of Epoxy Resin Sys¬ tems (Lawrence G. Silverstein) Flooring (G. M. Scales) Epoxy Adhesives (D. A. Shimp) Epoxy Molding Compounds (Terry E. Steiner) Filament Reinforced Epoxy Design and Processes (Fred R. Pflederer) Epoxy Resins in the Electrical In¬ dustry (Leo S. Kohn) Ambient Curing Epoxy Resin Coat¬ ings (John M. Klarquist) Epoxy Resin Baking Finishes (George G. Velten) Polyamide/Epoxy Coating Systems (William E. Shackelford) Epoxies in Marine and Industrial Maintenance Fields (Harold H. Flegenheimer)

\

Author Index Subject Index A volume in the Polymer Engineering and Technology Series dmu.ac.uk RB-2790

S'Ky

LEICESTER

library

Epoxy Resin Technology

POLYMER ENGINEERING AND TECHNOLOGY Executive Editor: D. V. Rosato

,

,

,

Editors: R. B. Akin H. F. Mark J. J. Scavuzzo S. S. Stivala J. Zukor

, .L.

SYNTHETIC FIBERS IN PAPERMAKING Edited by O. A. Battista FILAMENT WINDING: Its Development, Manufacture, Applications, and Design D. Y. Rosato and C. S. Grove, Jr. REINFORCEMENT OF ELASTOMERS Edited by Gerard Kraus ENVIRONMENTAL EFFECTS ON POLYMERIC MATERIALS (2 volumes) Edited by D. V. Rosato and R. T. Schwartz FUNDAMENTAL ASPECTS OF FIBER REINFORCED PLASTIC COMPOSITES Edited by R. T. Schwartz and H. S. Schwartz MAN-MADE FIBERS: Science and Technology (3 volumes) Edited by H. F. Mark, S. M. Atlas, and E. Cernia THE SCIENCE AND TECHNOLOGY OF POLYMER FILMS (2 volumes) Edited by Orville J. Sweeting PLASTIC FOR ELECTRICAL INSULATION Edited by Paul F. Bruins EPOXY RESIN TECHNOLOGY Edited by Paul F. Bruins POLYURETHANE TECHNOLOGY Edited by Paul F. Bruins ADDITIONAL VOLUMES IN PREPARATION

Epoxy Resin Technology

Edited by Paul F. Bruins Polytechnic Institute of Brooklyn, New York

Interscience Publishers a division of John Wiley & Sons, New York

•

London

•

Sydney

•

Toronto

LEICESTER

POLYTECHNIC

uni: un

GITV OF LEICESTER POLYTECHNIC LIBRARY

Copyright © 1968 by John Wiley & Sons, Inc. All Rights Reserved. No part of this book may be reproduced by any means, nor transmitted nor translated into a machine language without the written permission of the publisher.

Library of Congress Catalog Card Number: 68-21489 SBN 470

113901

Printed in the United States of America

Epoxy resins are unique in the wide variety of curing agents and catalysts that may be employed in controlling the rate of cure and the final properties; in the many reactive and nonreactive diluents that may be employed; in the choice of fillers and reinforcing agents; and even in the choice of the type of epoxy resin! Epoxy resins are likewise unique in the variety of applications that have been developed for them ranging from coatings to adhesives, molding compounds, laminates, flooring, and electronic potting. In this constantly developing technology, it is timely and appropriate to review the state of the art. This has been done in this series of chapters authored by representatives of some of the leading manufacturers and users of epoxy resins, and first presented at a seminar held at the Polytechnic Institute of Brooklyn. The text also includes a review of epoxy curing agents based on a literature survey prepared by a graduate student while working for his Master’s Degree in Polymeric Materials at the Institute. The editor wishes to express his thanks especially to R. W. Boeker, Market Sales Manager of the Dow Chemical Company, and to R. J. Moore, Manager of the Resins Technical Center of Shell Chemical Company, for their help in organizing this seminar. F. Bruins Brooklyn, New York Paul

July, 1968

v

.

Authors R. T. Dowd

Shell Chemical Company, Resins Technical Center, Union, New Jersey Harold H. Flegenheimer

Celanese Coatings Company, Newark, New Jersey John M. Klarquist

Shell Chemical Company, Union, New Jersey Leo S. Kohn

Materials and Processes Laboratory, General Electric Company, Schenectady, New York E. Magee Shell Chemical Company, Resins Technical Center, Union, New Jersey

John

Allan R. Meath

Plastics Division, The Dow Chemical Company, Midland, Michigan Rafael J. Perez

Arizona Chemical Company, Stamford, Connecticut Fred R. Pflederer

Smith Plastics Division, A. 0. Smith Corporation, Little Rock, Arkansas G.M. Scales Ciba Products Company, Summit, New Jersey E. Shackelford General Mills, Incorporated, Kankakee, Illinois

William

D. A. Shimp

Celanese Resins Division of Celanese Coatings Company, Louisville, Kentucky G. Silverstein Biochemical Research Laboratory, The Dow Chemical Company, Midland, Michigan

Lawrence

E. Steiner Plastics Engineering Company, Sheboygan, Wisconsin

Terry

vii

John Stevens

Union Carbide Company, New York, New York George G.

Velten

Shell Chemical Company, Union, New Jersey

Contents General Chemistry of Bisphenol A-Based Epoxy Resins.

By R. T. Dowd.

1

Chemistry, Properties, and Applications of Cycloaliphatic Epoxy Resins.

By John J. Stevens, Jr.

11

Chemistry, Properties, and Applications of Epoxy Novo lac, Flexible Epoxy, and Flame Retardant Epoxy Resins.

By Allan R. Meath.

31

By Rafael J. Perez

45

Commercial Epoxy Resin Curing Agents.

Epoxy Resins: Formulating Techniques and Evaluation.

By John E. Magee.

Ill

Safe Handling of Epoxy Resin Systems.

By Lawrence G. Silver stein...

123

By G. M. Scales.

141

By D. A. Skimp.

159

By Terry E. Steiner.

185

Flooring.

Epoxy Adhesives.

Epoxy Molding Compounds.

Filament Reinforced Epoxy Design and Processes.

By Fred R. Pflederer.

203

By Leo S. Kohn.

219

By John M. Klarquist.

229

By George G. Velten.

239

Epoxy Resins in the Electrical Industry. Ambient Curing Epoxy Resin Coatings.

Epoxy Resin Baking Finishes.

Polyamide/Epoxy Coating Systems.

By William E. Shackelford.

257

Epoxies in Marine and Industrial Maintenance Fields.

By Harold H. Flegenheimer.

269

Subject Index.

277

IX

1 General Chemistry of Bisphenol A-Based Epoxy Resins R. T. Dowd Shell Chemical Company Resins Technical Center Union, New Jersey

CONTENTS I. Introduction.

1

II. Manufacture of Bisphenol A-Epichlorohydrin Resins. III. Curing of Epoxy Resins. IV. Aliphatic Amines.

2 4

References.

10

4

I. INTRODUCTION Twenty years ago epoxy resins were introduced commercially and since that time their use has grown until today nearly 130 million pounds are produced annually in the U.S.A. The major portion of these resins are derived from bisphenol-acetone and epichlorohydrin. Figure 1 shows the structure of a bisphenol A-epichlorohydrin resin as well as other types of epoxy resins. These resins are characterized by the reactive oxirane ring which can be reacted with curing agents or catalytically homopolymerized to form a crosslinked polymeric struc¬ ture. Their most outstanding property is their excellent adhesion which is due in part to the secondary hydroxy group located along the molecular chain. In addition, cured epoxy resins have good mechanical and electrical properties, superior dimensional stability, and good re¬ sistance to heat and chemical attack. These properties are derived mainly from the aromatic nature of the bisphenol A portion of the molecule and the excellent chemical and thermal stability of the phenolic ether linkage. Since no small molecule, such as water, is liberated during the curing of epoxy resins, they exhibit unusually low shrinkage and they can be formed and cured under contact or low pressure. Depending upon the choice of curing agent, these versatile resins can be made to cure, or harden, either slowly (several hours) or very quickly (less than 1 min) at room temperature or at elevated temperatures. l

2

R. T. DOWD

Versatility is also achieved in performance. Epoxy resins can be formulated to yield a variety of properties ranging from soft, flexible materials, to hard, tough, chemical resistant products. They can be modified into low viscosity liquids for easy pouring or converted to solid compositions for laminating and molding applications.

/\

0 —CH,—CH—CH2

'V

«Cr

O-CH-CH—CH;

(in)

/°\

CH-CH,

/°\

(v>

(vi)

Fig. 1. Chemical structures of epoxy resins.

II. MANUFACTURE OF BISPHENOL A-EPICHLOROHYDRIN RESINS The glycidyl ether resins vary in physical form from moderately viscous (ca. 50 poises) liquids to low melting (ca. 70°C) solids, these resins are manufactured by reacting epichlorohydrin and bisphenol A in the presence of aqueous caustic soda (Fig. 2). The reaction is always carried out with an excess of epichlorohydrin so that the resulting resin has terminal epoxy groups. Thus, by varying manufacturing condi¬ tions and the excess of epichlorohydrin, resins of low, intermediate, or high molecular weight may be produced. If a large excess of epichloro¬ hydrin (10-20 moles per mole of bisphenol A) is used, the likelihood of producing a low molecular weight resin, where n is equal to 0, is con¬ siderably improved. Even lower molecular weight resins can be ob¬ tained by fractionation of the liquid epoxies. This fractionation can be carried out by distillation.

BISPHENOL A-BASED EPOXY RESINS

3

~o—k-ct

-o-R-cr + 0"

O'

I

I

cich2-ch-ch2-o-r-o-ch2- ch-ch2ci

/°\

—►

“0—R—0~

CH2- CH- CH2—0—R—0- CH- CH- CH2

—-* ch2-ch-ch2ci

®

1

/°\ ?H ' ch2-ch-ch2- o-r-£o-ch2- -ch-ch2-o-r+o-ch2-c: ch-ch2

Figure 2

Typical properties of some bisphenol A-epichlorohydrin resins are shown in Table 1. It will be noted that as the molecular weight in¬ creases, the epoxide content is decreased and the hydroxyl content is increased.

TABLE 1.

Typical Properties of Glycidyl Ether Resins

Average

Viscosity,

Epoxy content

Hydroxyl value

mol. wt.

poise

equiv. 100 g

equiv./100 g

7

0.54

0.05

350

70

0.53

0.06

380

150

0.52

0.06

470

Solid

0.40

0.16 0.21

330

610

Solid

0.34

900

Solid

0.20

0.28

1400

Solid

0.11

0.32

2900

Solid

0.03

0.36

3750

Solid

0.03

0.40

4

R. T. DOWD

Flame retardant versions of the bisphenol A-epichlorohydrin resins are also available. These resins, which are available in varying mo¬ lecular weight, acquire their fire retardant characteristics through bromine substitution in the phenyl ring of the bisphenol A. In addition to the bisphenol A-based resins, there are several other types of epoxy resins of commercial significance (see Fig. 1); namely, the aliphatic epoxies, cycloaliphatic epoxies (IV, V, and VI), glycidated Novolacs (II), and the glycidyl ether of tetraphenylolethane (III).

III. CURING OF EPOXY RESINS Epoxy resins can be cured or crosslinked through the oxirane ring by a large variety of chemical compounds. For the purposes of this seminar, only the three most important classes of curing agents will be discussed. Namely, aliphatic and aromatic amines, acid anhydrides, and latent curing agents or catalysts. Other types of curing agents used in sig¬ nificant quantities in commercial applications are the phenols, carboxylic acids, ureas, inorganic acids and bases, and mercaptans. The high molecular weight epoxy resins are cured through the hydroxyl groups with phenols or ureas.

IV. ALIPHATIC AMINES Aliphatic amines react readily with glycidyl ether resins at room temperature. The most important aliphatic amines are diethylenetriamine (DTA), triethylenetetramine (TETA), diethylaminopropylamine (DEAPA), amine adducts, and amine-terminated polyamides. /\ —CH-CH.

H\ >N—R—N

fir -CH-CH.

V/

OH

OH

—CH—CH2v

CH-CH-

/N—R—N -ch-ch2

I

OH Figure 3

BISPHENOL A-BASED EPOXY RESINS

5

Figure 3 shows that a poly amine, in this case a diprimary amine, reacts with epoxide groups to form a crosslinked structure. Each of the four active hydrogens reacts with one epoxide group. Kinetic studies have shown that the conversion of primary amine to secondary amine proceeds at approximately the same rate as the'con¬ version of the secondary amine to the tertiary amine (Fig. 4). However, due to steric hindrance, the homopolymerization by the tertiary amine is a negligible reaction.

R—N

+

R-N

CH-CH-

V

/' \

CH,-CH-

X0X

OH ,H

OH

/

/

I ,CH2—CH—

/°\

'CH-CH-

-I-

r-n;

CH.-CH-

I

'CH2—CH—

OH OH Figure 4

Several workers have shown that hydroxy compounds or other ma¬ terials capable of hydrogen bonding accelerate the reaction of amines with glycidyl ether resins, whereas some polar, non-hydrogen bonding compounds actually retard the reaction. The specificity of hydroxyl groups in accelerating the amine-epoxide reaction is illustrated in Fig. 5. H

H R-N

i

H

I +

CH-Cff-

+

R-N-CH-CH-

HOX

h y

"V u

HOX '

H

H

I. R-N-CH-CH-

I H

.

I OH OX'

R-N-CH-CH-

+ HOX

OH

Figure 5

Although most of the people who work with DTA and TETA have not suffered any ill effects, some find them to be skin irritants, especially if simple hygienic practices are not observed. Therefore, proprietary amine adducts which do not appear to be skin irritants and are less volatile than DTA and TETA have been developed. The amineethylene oxide adducts are one group of these curing agents. The

6

R. T. DOWD

amine-terminated polyamide resins are another group, although these curing agents also impart other desirable characteristics, such as flexibility and toughness to the final polymer. The aromatic diamines are less reactive toward the BPA based epoxy resins than aliphatic amines and require elevated temperature cures. The aromatic amines which are the most widely used for all types of epoxy resins are: ra-phenylenediamine (often symbolized as either Cl or MPDA); 4,4'-methylenedianiline (MDA); modified aromatic amines (as proprietary eutectic mixtures); and 4,4'-diaminodiphenyl sulfone (DDS). Heat deflection temperatures of 300-500°F are achieved with liquid epoxy resin-aromatic amine systems, indicating good retention of prop¬ erties up to this temperature range. Some recent work has shown that by using 75% of the stoichiometric amount of MPDA with a very pure, liquid, BPA-based epoxy resin and utilizing long postcures at elevated temperatures (>200°C), heat deflection temperatures as high as 260°C (500°F) have been realized (1). It is speculated that the hydro¬ gen in the 4-position of the MPDA ring reacts to increase the crosslink density. Aromatic amine cured glycidyl ether resin systems have excellent resistance to water, solvents, and alkaline solutions, as well as a range of good to excellent dilute acid resistance. Another important group of curing agents is the acid anhydrides. Some of the reasons for their importance are: ease of handling (especially for the liquid or low melting anhydrides which produce low viscosity mixtures), lower toxicity, compatibility with liquid and solid resins, long pot lives (depending upon the amount of accelerator used), and lower peak exotherm temperatures. In general, all anhydride blends with epoxy resins require heat cures. They provide better electrical prop¬ erties than the aromatic diamines. It is noteworthy that little or no reaction occurs between the pure epoxide and the pure anhydride (Fig. 6). However, in the presence of hydroxyl, for example, the secondary hydroxyl group of the conventional epoxy resins, the anhydride will react to form what is called a half-ester. The carboxylic acid portion of the half-ester then can react with an epoxy group to form an ester linkage and a hydroxyl. It has been found that unless a basic catalyst is used, etherification reactions also occur so that less than stoichiometric quantities of an¬ hydride are required to obtain optimum mechanical properties. Under basic conditions, 1 mole of anhydride per epoxide produces polymers of optimum properties.

BISPHENOL A-BASED EPOXY RESINS

.*0

/°N

-CH-CH-CH,

+

Ctc

C-° AO

C-OH | L-0—CH

0 ajO

+

0 0

little or no reaction

✓0

| +

C-OH , C-O-CH

HO-CH '

Sq ^0

7

'o

C—0—CH2—CH—CH2- - C-O-CH-

I

!0

O

I

C—0—CH2—CH—CH2~ •

?H

*'0

/°\ ch-ch-ch2-~

OH

I

1 -CH2

p C-O-CH-CH-O-Cp^N L-c-o-chho-c—AJ

s0

A

-CH—CH-CHj

-•ch2

,o

T

C—0—CH2—CH—0 - CH2—CH—CH2- C-0—CH—

^O

T

Figure 6

The use of varying proportions of initiators such as polycarboxylic acids, polyhydric alcohols and phenols, strongly accelerates the an¬ hydride-epoxide reaction. The properties of the resulting polymers vary considerably and are dependent on the chain length and function¬ ality of the initiators. With the highest molecular weight glycidyl ether resins in which the reactive groups are predominantly secondary hydroxyl groups, urea and phenol-formaldehyde resins are used as crosslinking agents (Fig. 7). The important reaction is the formation of ether linkages between the methylol groups of the urea and phenolic resins and the secondary hydroxyls of the epoxy resin. This reaction may be catalyzed by acids. In addition, the epoxide groups present may react with the phenolic hydroxyl groups of phenolic resins to form a more highly crosslinked polymer. Esterification reactions are associated with the conventional epozy resins of intermediate molecular weight and to some extent with the epoxidized olefins. Resinous polyesters may be formed by the reaction of the resin with organic unsaturated acids. Both the epoxide and the secondary hydroxyl groups of the glycidyl ether resins react, each epoxide group having the potential of being converted into two ester linkages.

8

R. T. DOWD OH HO-CH

OH

■CH-OH +

ch-ch-ch-o-r-o-ch-ch-ch2—

CH-OH

OH 0—ch2— ch—ch2—0 —R— 0—ch2— ch— ch2— ~ “ ** /u\

CH2-CH-CH2-

-o—ch—ch2— CH; 0 R 0

Figure 7

Since these reactions take place slowly at room temperature, elevated temperatures of 450-500°C are normally employed. Even under these conditions, the esterification reactions are often catalyzed by the addition of basic salts. The mechanism associated with these reaction conditions is shown in Fig. 8. Several workers have found that the first two reactions shown are the important reactions in the early stages of an uncatalyzed esterification. In fact, by the time the reaction mixture reaches 450-550°F, most of the epoxide groups have been consumed. Unless a basic catalyst is employed, the etherification reaction takes on great importance, resulting in the intermolecular reaction between epoxide groups with secondary hydroxyl groups to form higher molecu¬ lar weight materials, this causes premature gelation and high acid num¬ bers in the resulting ester. ' When the esterification is base-catalyzed, the rate of the first reaction is so enhanced that the etherification reaction takes on a minor role. When the epoxide groups have been consumed, the reaction between hydroxyl and carboxylic acid assumes the major role. In addition to these reactions, the olefinic portions of the unsaturated fatty acids crosslink, probably via a Diels-Alder type of addition, during esterification at elevated temperatures. This reaction leads to products of increasing viscosity, which are dependent on the time and temperature of reaction. In surface coating formulations, these un¬ saturated esters are further crosslinked by air drying in the presence of conventional metal driers by baking at elevated temperatures.

BISPHENOL A -BASED EPOXY RESINS

*0 -C-0H

0 II

1

—► -c-o-c

+

OH 1

1 o

iS

h2o

1

1 CH CH-CH-CH=CHCH-C-O-CHII 0

—►

CH CH-CH2-CH=CH1 CH CH-C-0-CH2\ / II QH 0

-aII o

CH II CH

1

1

o

1 CH II CH

+

1 HO-C 1

1

0 II -C-OH

o 1 o—

+

. 1

0 / \ CH-CH-

t

, 1 C-OH

OH ,0 I —C—0—CH2—CH—

/°\ CH,-CH-

+

9

6=0 1 0 1 qh2

I 0

ch2 Figure 8

Catalytic polymerization of epoxy resins is accomplished with Lewis acid type catalysts and also tertiary amines. In general, rapid cures can be achieved by the use of acid catalysts. Of the acid-type catalysts available, boron trifluoride, in the form of one of its complexes has been employed with varying degrees of success. In curing epoxy resins which contain two epoxide groups, this type of polymerization leads to a highly_ crosslinked structure. Contrary to the types of reactions shown above, which required stoichiometric or near stoichiometric quantities of the curing agents or coreacting resins, cured polymers with good properties can be obtained by using catalyst concentrations of from 1 to 5% (based on the resin). On the other hand, it has been found that tertiary amines do not produce polyether polymers with as high a crosslink density as those obtained with BF3 complexes, except for certain heterocyclic ring compounds such as 2-ethyl-4-methylimidazole (EMI-24). It appears that the reactivity of the amine is associated with its structure. Most amines containing two methyl groups on the nitrogen are fast curing agents. Benzyldimethylamine and trisdimethylaminomethylphenol are good examples. The reactions associated with catalytic polymerization are shown in Fig. 9. You will note that for the most part the homopolymerization reaction is the same for both the tertiary amine and the boron fluoride

10

R. T. DOWD

catalyzed polymerizations. Again, both require hydroxyl to initiate the polymerization reactions.

the

presence

of

0

"

R3N:

+

/°\ ch2-ch-

RO"

+

I

ROH

r3n-ch2-ch-

R3N— CH2—CH— 0 I CH-CH'H-70

A

+

+

RO"

"

CH,—CH—

OR

/✓\

7 ch2-ch-

CH-CH0 I CH2-CH0

I

ch2-chOR

A

CH2—

F-B-F

A -ch-ch2 t

—►

cr 1 CH-CH1 OR

0 /\

R

H Figure 9

Dicyandiamide is a useful curing agent which has found wide utility in laminates. Dicyandiamide (dicy) is most suitable with the high hydroxyl group containing epoxies and its cured properties appear to be related to a reaction between the cyano group of dicy and the hydroxyl group of the epoxy (2).

REFERENCES L

C;.Ilhnan’ “Cure of EP°xy Resins with Aromatic Amines-High Heat Distortion btuches, paper presented at the 22nd Annual Technical Conference of the Society ot Plastics Engineers, Montreal Quebec, Canada, March 7-10, 1966 2. T. R Saunders, M. F. Levy, and J. F. Serino, “A Mechanistic Study of the Dicyandiannde-Epoxy Curing Process,” paper presented at the National Meeting ot the American Chemical Society, Pittsburgh, Pa., March, 1966.

2

,

Chemistry, Properties and Applications of Cycloaliphatic Epoxy Resins John J. Stevens, Jr. Chemicals and Plastics Research and Development Department Union Carbide Corporation Bound Brook, New Jersey CONTENTS

I. II.

Introduction. H Manufacture and Structure of Peracetic Acid Based Cycloaliphatic Epoxies. 12 III. Reaction Chemistry. 13 IV. Catalyst Effects on Reactivity. 15 V. Liquid Resin Properties. 18 VI. Cast Resin Physical Properties. 19 VII. Cast Resin Electrical Properties. 22 VIII. Typical Applications. 24 References. 30

I. INTRODUCTION Cycloaliphatic epoxides were introduced in 1957. Most applica¬ tions then were experimental. Recently, new applications and an in¬ creasing demand for these resins have developed. Forseeing this, Union Carbide Corporation began building increased capacity for these resins in late 1965. This new plant in Taft, La., is now undergoing startup operations. This new family of epoxy resins has some interesting properties that complement, and in some cases, improve upon those of existing epoxy resins. Before detailing these advantages, a brief description of cyclo¬ aliphatic epoxide features should help acquaint us with these resins. Often in the literature, general statements are made which encompass the entire epoxy family. It is obvious that the wise epoxy user should discriminate among the resin types and pick the best for his specific needs. Let’s pinpoint some features of cycloaliphatic resins. They are outstanding in providing a combination of low viscosity, excellent high temperature ac loss properties and arc-track resistance under high voltage loading. 11

12

JOHN J. STEVENS, JR.

II. MANUFACTURE AND STRUCTURE OF PERACETIC ACID BASED CYCLOALIPHATIC EPOXIES How are these resins made? Figure 1 shows the process for ERL4221. 3-cyclohexenyl-methyl-3-cyclohexenecarboxylate, which we con¬ veniently call diene-221, is formed by the Tischenko condensation of tetrahydrobenzaldehyde. Under mild conditions, this diene is combined with 2 moles of peracetic acid in solution and the resultant epoxide, 3,4epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate is formed (1,2). The significant point in this process is not the formation of ERL4221, but rather, the chemical simplicity of the process. Virtually any olefin can be peracid epoxidized. These, of course, include epoxidized linear olefins as well as cyclo olefins; an example of the former is peracid epoxidized soybean oil offered as a vinyl plasticizer.

Fig. 1.

Basic manufacturing scheme for cycloaliphatic epoxy resins using peracetic acid.

Of the many epoxy structures synthesized by our research laboratories, cycloaliphatic diepoxides offered an interesting combination of proper¬ ties, and these structures were made commercially available. Figure 2 shows three general purpose cycloaliphatic epoxides. ERL-4221, an almost water while low viscosity liquid is used where good electrical loss properties, arc-track resistance, good weathering or high heat dis¬ tortion properties are needed. ERL-4289, as can be inferred from its structure, offers somewhat more flexible products than ERL-4221. The dilution properties of ERL-4206 are outstanding. In contrast to many diluents, ERL-4206 greatly cuts resin viscosity without detract¬ ing from cured resin properties. An interesting fact about this resin, is that both amines and anhydrides can be used to cure it. The epoxy¬ cyclohexane group is more reactive under acidic conditions, whereas the epoxy ethyl group is more responsive to amines.

CYCLOALIPHATIC EPOXY RESINS

13

O

ERL-4206 Fig. 2.

Some common cycloaliphatic epoxy resin structures. any aromatic rings.

Notice the absence of

III. REACTION CHEMISTRY Generally, cycloaliphatic resins are most easily cured by acids (8) or anhydrides (4). Amine curing agents, such as diethylene triamine, are less effective because of low reactivity and apparent aminolysis of the ester linkages at the high curing temperatures required. The following chemical reactions presented in Fig. 3, illustrate the curing mechanism with carboxylic acids, anhydrides and alcohols; furthermore, they indicate the method by which resin properties can be altered by choosing various combinations of materials containing these reactive groups. They also allow an explanation of why the optimum amount of curing agent depends upon the specific curing agent and type of catalyst. The reaction of alcohol and epoxide group (Reaction 1) is very slow in the absence of catalysts; however, when a carboxylic acid is present, this reaction is significant, particularly in cycloaliphatic epoxides, and results in polyether formation, thus reducing the amount of epoxide available for reaction with carboxylic acid groups. Reaction 1 also goes well in the presence of acid catalyst such as boron trifluoride and boron tri¬ fluoride amine complexes. Reaction 2 is rapid at moderate temperatures; however, the generated alcohol group is also reactive as indicated before. Both reactions are significant when cycloaliphatic epoxides are used and result in an opti¬ mum reactant ratio of 0.5-0.7 carboxyl groups for each epoxide group. Diepoxides and dicarboxylic acids, therefore, react to give thermoset

14

JOHN J. STEVENS, JR.

O /\

1.

OH

RCHCH2 + RT>H-^5^RCHCH2OR' 0

*.

OH

/X

I

RCHCH2 + R'C02H-»RCHCH202CR' O

3.

CO

/\

/

\

\

/

RCHCH2 + R'

O--> -f02CR'C02CH2CHA-*

CO

I

R

CO

4-

b/

^0 + R'OH->R'02CR"C02H

V' Fig. 3.

Competitive mechanisms that occur when cycloaliphatic epoxies react with alcohols and anhydrides.

resins since under these conditions, each epoxide group has a function¬ ality of about 1.5. In other words, polyether formation reduces the amount of carboxyl groups required to react with an epoxy system. Acidic catalysts, such as stannous octoate, increase the amount of ether and therefore, reduce the amount of carboxylic acid required. Basic catalysts, such as tertiary amines and sodium acetate, increase the amount of ester and therefore, the amount of carboxylic acid required. Acid anhydrides and epoxides do not react well unless an initiator, such as an alcohol (reaction 4), or catalyst, such as a tertiary amine is present. In fact, we feel reaction 3 is slow even if tertiary amine is present. Since this is so, ethylene glycol, or some other hydroxyl contributor is often added to the resin/anhydride system. These added hydroxyls react quickly with the anhydride to form the half ester. The resultant carboxylic acid group can react with epoxies by the mechanism presented in reaction 2. Thus, epoxide groups are bifunctional with anhydrides; this results in highly crosslinked resins having high deflection tempera¬ tures when diepoxides and dicarboxylic acid anhydrides are combined. Acidic and basic catalysts again affect the amount of anhydride necessary for optimum curing because of preferential ether or ester formation. Even the acidity of the dicarboxylic acid anhydrides has a great effect on the preferred ratios of anhydrides and cycloaliphatic resins. A carb¬ oxyl to epoxide ratio of 0.7 : 1 is recommended for maleic anhydride whose acid has a pKa of 1.0. On the other hand, hexahydrophthalic anhydride with a pKa of 4.4, has its best resin forming characteristics at a 1.6/1 carboxyl/epoxide ratio, more than double that of maleic.

CYCLOALIPHATIC EPOXY RESINS

15

IV. CATALYST EFFECTS ON REACTIVITY Low melting anhydrides such as hexahydrophthalic anhydride are particularly useful as epoxy hardeners since they can be blended at low temperatures thus allowing longer pot lives than most other anhydrides. Of course, the most convenient anhydride form is liquid, such as our ZZL-0334. Table I shows the optimum proportions of ZZL-0334 with a series of cycloaliphatic epoxy resins. These ratios represent an opti¬ mum combination of low weight loss at high temperatures, high heat distortion temperatures and good physical properties in the cured resins. The limits of anhydride are quite broad, generally lower levels give the lowest weight loss and higher levels give higher deflection temperatures and better mechanical properties after a modest cure. Note the differ¬ ence in ratios between tertiary amine and acidic catalyst; the carboxyl/ epoxide ratios change significantly as the ester producing tertiary amine is changed to the ether forming Sn (oct)2 catalyst. TABLE I.

PHR of ZZL-0334 with Various Epoxides'1 Sn(oct)2 No catalyst

BDMA catalyst

catalyst

ERL-4221

102

115

63

ERL-4289

63

72

40

ERL-4050

79

90

48

ERL-4052

62

69

39

Epoxide

a Notice the effect of catalyst type on the optimum proportions of anhydride. Ter¬ tiary amines like benzyldimethylamine promote esterification, thus more anhydride is needed than when the ether favoring stannous octoate is used.

ERL-4050 and ERL-4052, which appear in this table cannot be rep¬ resented by chemical structural entities; they are instead, long-chain alcohol modified cycloaliphatic resins. In addition to affecting the level of anhydride used, catalysts differ in their reactivity rates. Cycloaliphatic resins react more quickly under acidic conditions than basic conditions. Figure 4 illustrates these dif¬ ferences. Room temperature viscosity increases of an optimum mixture of ERL-4289 and hexahydrophthalic anhydride show stannous octoate as the most effective catalyst. With benzyldimethylamine (BDMA) and stannous octoate, the viscosity increased to 2,000 cps. after 1 day and 12 hr, respectively. In the absence of catalyst, this took about 3 days, thus long pot lives are possible with cycloaliphatic systems.

16

JOHN J. STEVENS, JR.

Fig. 4.

Effect of catalysts on viscosity in mixtures of ERL-4289 and hexahydrophthalic anhydride.

Even within a catalyst family, quite different effects are possible. For example in Fig. 5, both DMP-30 (2,4,6-tris(dimethyaminomethyl phenol) and BDMA act as basic catalyst for a silica filled ERL-4050/ hexahydrophthalic anhydride system, but there is a significant difference in reactivity data.

In a one pound mass the highest peak exotherm

temperature reached with DMP-30 catalyst was 172°C, whereas the BDMA systems reached 196°C.

In large castings, high exotherm tem¬

peratures can cause nonuniform and excessive shrinkage. is minimized when DMP-30 is the catalyst.

This problem

In addition to avoiding

excessive, localized heating and gelling, DMP-30 promotes a smoother transition from a liquid to a gelled state than BDMA.

By the way, our

work has not shown this reactivity difference with bisphenol resins, like ERL-2774. Because there are few hydroxyl groups present in cycloaliphatic resins, a hydroxyl contributor is often added to anhydride/epoxide systems to inciease reactivity as mentioned before.

Figure 6 shows that ethylene

glycol can significantly reduce the gel time of an ERL-4289/HHPA system,

dhe uncatalyzed, uninitiated reaction was very slow, even

at 160°C, however, the combined addition of ethylene glycol and catalyst was very effective in promoting the reaction at only 120°C. Note how little ethylene glycol is needed here.

Greater than 2 phr eth¬

ylene glycol has little effect on the gel time of the catalyzed system.

CYCLOALIPHATIC EPOXY RESINS

17

GEL TIME AT I20°C .MINUTES (FR0M80°C)

GEL TIME - PEAK EXOTHERM VS. CATALYST CONCENTRATION

Fig. 5.

Even within a catalyst family, note the effect these different tertiary amines have on cycloaliphatic epoxide/anhydride reactivity.

Fig. 6.

Effect of ethylene glycol and catalyst on gel time of resins from ERL-4289 and hexahydrophthalic anhydride.

18

JOHN J. STEVENS, JR.

V. LIQUID RESIN PROPERTIES Unfortunately the short gel times induced by ethylene glycol are also reflected in shorter pot lives. It seems that the ideal epoxy system would have an instantaneous gel time combined with an infinite pot life. The lower viscosity of cycloaliphatic resins give them an inherent ad¬ vantage over other epoxy types. Table II shows viscosities and epoxy equivalent weights of some basic cycloaliphatic resins. Although all the resins are low in viscosity, the value of 7 centipoise for ERL-4206 is remarkably low for an epoxy and indicates why it is often used as an epoxy diluent. ERRA-4205 is also used as a diluent. This is a mix¬ ture of solid and liquid isomers of a glycidyl ether, cycloaliphatic type resin. Since it is a substituted glycidyl ether, it is more reactive with amines. In fact, this resin does not react well with anhydrides; there¬ fore, aromatic amines are the hardeners recommended. High per¬ formance structural uses make up the major applications for ERRA4205 and its isomers. The lower viscosities of cycloaliphatic resins will sometimes permit processing cold and allow slightly higher filler loadings than other resin types. TABLE II.

Viscosity and EEW of Common Cycloaliphatic Epoxies Epoxide

ERL-4221 ERL-4289 ERRA-4205 ERL-4206

Viscosity, cps

Epoxy equivalent

at 25°C

weight

400 900 50 a 7

135 220 98 76

tt May crystallize out at room temperature.

For many applications, particularly electrical ones, complete deaera¬ tion of the liquid resin system is essential. Although air is soluble in epoxy resins, particularly cycloaliphatic types as reported by Mr. Hill and Dr. Cialdella of Hysol Corporation (6) the volatility of the resins is an important consideration, too. As an example, a 10 g mass of ERL4221 at 135°C in a forced air oven lost between 5^ and 8 wt % in 3 hr. On the other hand, ERL-4289 lost about 0.3% and ERL-2774 lost between 0.4 and 0.7 wt % under these conditions. So ERL-4289 behaves much like ERL-2774 under vacuum but ERL-4221 is more volatile. Hardeners like hexahydrophthalic anhydride which lose up to 90 wt % in this test of course can promote resin system loss too. Again we see where some cycloaliphatic systems must be handled differently than other resin types.

CYCLOALIPHATIC EPOXY RESINS

19

Acid anhydrides as mentioned earlier, are commonly used as curing agents. Since these generally require elevated temperature cures, some epoxy users will not tolerate this additional step. However, where optimum properties are desired, an oven cure is essential even with room temperature curing systems.

VI. CAST RESIN PHYSICAL PROPERTIES In addition to handling properties, performance properties are also very important to potential epoxy users. Often the heat distortion temperature (HDT) of cycloaliphatic sys¬ tems is higher than bisphenol based resin systems. On the other hand, as the HDT of a resin increases, the system generally becomes more brittle. Following from this many bisphenol resins are tougher than many cycloaliphatic resins. As Table III shows, hexahydrophthalic anhydride cured ERL-2774 offers a better combination of tensile strength and elongation properties than does any similarly cured cycloaliphatic epoxide. On the other hand, metaphenylene diamine cured ERRA4205 offers the ultimate in toughness. Note the heat distortions of the systems. Note, too, that the tougher resins obtained from ERL-4289 and 4052 are generally more useful where thermal shock resistance is needed. ERL-4221 and 4052 are compatible in all proportions and can be blended to give resins with properties intermediate to those shown. TABLE III.

Comparison of Some Cycloaliphatic Resin Physical Properties with

the Diglycidyl Ether of Bisphenol A, ERL-2774

Epoxide ERL-42218 ERL-42898 ERL-40528 ERRA-4205b ERL-27748

Tensile strength, psi 10,200 10,000 4,300 17,000 12,400

Properties, elong. %

Deflection temp., °C

2.7

196

5.1 16.7 6.0 6.1

100 44 200 133

“Cured with HHPA. bCured with MPDA.

Since cycloaliphatic resins are usually low in viscosity, they all act as good solvents for other resin families. This advantage has not been capitalized on fully by us or others in the field.

20

JOHN J. STEVENS, JR.

o

o

ol 2 UJ

cc o

R3N—CH2—CH—Ri + R'O O e

O H

COMMERCIAL EPOXY RESIN CURING AGENTS

49

The alkoxide R0e then attacks an epoxy and produces another alkoxide which in turn forms another link and produces more alkoxide. O e

/ \

RO + CH2—CHR—>ROCH2—CHR

I O

o ROCH2—CHR + CH:

I o

HR—>ROCH2—CHR O—CH2—CHR

O

e

e

The reaction rate is initially dependent on the hydroxyl equivalents present when only a small amount of non-hydroxyl-containing tertiary amine is present.

C.

Anhydride Cures Catalyzed by Tertiary Amine (35,36)

This reaction proceeds through the activation of the anhydride by the tertiary amine and the subsequent attack of this “salt” on the epoxy molecules.

()=(

:=0

()=C

nr3 +

O + CH2—CHR Can react with more epoxy as above

o o e

II II

NR3-^C C—OCH2—CHR

0

0 C C

0

*

o o II II

NR3—C C—OCH2—CHR

O

50

RAFAEL J. PEREZ

In anhydride cures other reactions are also possible and do take place. The most important is esterification of epoxy with alcohol groups which is catalyzed as in Section I-B by tertiary amines or even by acid. A large part of the reason why acid anhydrides can produce higher heat distortion temperatures than aromatic amines is that they are so much more efficient in causing epoxy polymerization as well as crosslinking through the reactive acid groups.

D.

Anhydride Cures in Conjunction with Alcohols (35,36) R H C

H2C—o O

OOO P

ROH+

I

P

0=C

E.

H

/

C—OCR

0

0

II

II

H

OH

U

H

0H

O

RCH—CH 20—C C—OCR

I

°

\

O

\y

X—^

/)+ H2C—-chr—> C \

->

Lewis Acid-Amine Complex Catalysis (1,37)

The long-standing theory that heating dissociates the complex and produces Lewis acid and amine to catalyze resin polymerization has been attacked in recent literature.* It is claimed that dissociation does not take place in amine BF3 complexes; thus it cannot be the primary re¬ action preceding epoxy resin cure. What is proposed is as follows: 0 R / \ R BF3NH2 + CH2— CHR-*F3BNH -0—CH2 H \l CH

I R

For a fluoroborate salt: O +

CH2

/ V

H+

/

BF-4RNH3 + CH2—CH-+BF-4RNH—o I

H CH

I R * See, for example, J. J. Harris and S. C. Temin, J. Appl. Polymer Sci., 10, 523534 (1966).

COMMERCIAL EPOXY RESIN CURING AGENTS

51

In each case the amine is more basic and will retain the hydrogen in spite of competition from the oxygen, thus CH2 CH2 R F3BN—H—O ->F-3BN—H+—O H H CH R R

T

“The reactivity of the resulting complex towards an attacking nucleo¬ phile, such as another epoxy ring, will depend upon the magnitude of charge transfer which will in turn depend upon the acidity of the hy¬ drogen atom which is attached to the amine. As the temperature is increased, some point is reached at which reaction becomes sufficiently rapid for curing.” CH2

R

CH,

+ /

/

F-3BN—H—O H \

+ 0 \ R

R

CH2 R+ / ^(F3BN)HOCH—ch2—0 HR \ CH R

Propagation takes place by attack of epoxy at the positive center. In the presence of alcohol the following takes place: CH2

R F3BN—H—O H N

+ R"OH^RNH2BF3 (regenerated) + HOCHCH2OR" R CH R

Now the new alcohol is the attacking species in propagation rather than an epoxy. The weaker the complexed base (more acidic hydrogen) the lower the curing temperature. Therefore, aromatic amines give the lowest tem¬ perature polymerizations followed by aliphatic primary amines.

II. DISCUSSION OF HARDENERS AND RESULTANT PROP¬ ERTIES In the previous section it was seen what types of materials cure epoxy resins and their cure mechanisms. In this section the commercial epoxy curing agents will be discussed from the standpoint of the properties they lend to the impure diglycidyl ethers of bisphenol A when added in the correct quantities and cured in the recommended manner.

52

A.

RAFAEL J. PEREZ

Aliphatic Amines (28,33,87)

The aliphatic amines, as a class, represent the most used epoxy hard¬ eners today. They are employed unmodified and also reacted with several agents to produce less volatility, toxicity, and speed of reaction with lower exotherms in the final product. Another very important function of reacting the primary alkyl amines partially before use as epoxy hardeners is to increase the ratio in which they are used to epoxy resin. The practical reasons for wanting the above improvements are usually found in production facilities. The short pot life of unmodified amines complicates the production parameters causing special techniques such as continuous mixing or batch replenishment to be used so that an eco¬ nomic throughput can be realized. High exotherms prevent the con¬ venient casting of large shapes. Unmodified amines are usually used at around stoichiometric ratio which amounts to 12 parts or less per hun¬ dred of liquid epoxy resin. This necessitates careful weighing opera¬ tions which are undesirable in production line operations when cured system parameters are critical. Unmodified amines are skin sensitizers. If they tend to be volatile, they also affect the respiratory system; thus use of unmodified aliphatic amines generally requires a great deal of careful attention to ventilation and personal hygiene. The Dow Chemical Co. brochure “Dow Liquid Epoxy Resins” desciibes aliphatic amines as follows: “The liquid aliphatic polyamines and their adducts are convenient to handle, give excellent cured resin physi¬ cal characteristics including chemical and solvent resistance and cure at ambient or moderately elevated temperatures. Good long term reten¬ tion of properties is possible at temperatures up to 100°C. Short term exposure at higher temperatures can be tolerated. Pot life is short and exotherm is high in thick sections and large masses.”

Z.

Unmodified Aliphatic Polyamines

The most common unmodified aliphatic polyamines used as epoxy re¬ sin hardeners are diethylene triamine (DETA) and triethylene tetramine (TETA). Two others of the same general series that have been used are ethylene diamine (EDA) and tetraethylene pentamine (TEPA). EDA is much too volatile for commercial use in the unmodified form and TEPA finds some usage in coatings. Manufacturers differ somewhat on the similarity and differences of castings prepared from epoxy resins cured with DETA and TETA. It

COMMERCIAL EPOXY RESIN CURING AGENTS

53

seems reasonable, however, to assume that, as one manufacturer claims (70), the two curing agents are very nearly interchangeable. Table I is a compilation of properties attainable when DETA and TETA are used at their optimum ratio to cure liquid epoxy resins. Many properties are reported as ranges because of the varying cure schedules that have been used that affect these properties. In addition, epoxy resin modi¬ fiers such as triphenyl phosphate (3) are often added to aliphatic amine cured systems. These change the properties of castings cured with aliphatic amines to a great extent. TABLE I. Comparative Mechanical Properties of DETA and TETA Cured Epoxy Castings (10,28,61,70) Values at 25°C

Property Heat deflection temperature, °C Flexural strength, psi Flexural modulus, X 10“5 psi Compressive strength, psi Tensile strength, psi Ultimate elongation, % Izod impact strength, ft-lb/in. of notch Hardness, Rockwell M

DETA 10-11 phr 95-124

TETA 13-14 phr 98-124

14,500-17,000 5.0-5.4

13,900-17,700 4.4-4.9

16,500 11,400 5.5 0.4 99-108

16,300 11,400 4.4 0.4 106

Systems cured with DETA and TETA generally have similar elec¬ trical properties. Dielectric constants vary from 3.4 to 4.0 depending on the test frequency, and dissipation factors vary from 0.007 to 0.03. The chemical properties are also quite similar. DETA and TETA cured castings have excellent resistance to aqueous sodium hydroxide even at 50% concentration and 180°F. Their resistance to 25% sulfuric acid, 25% hydrochloric acid, and 25% chromic acid is also very good up to 180°F. Strong organic acids such as acetic do attack DETA and TETA cured epoxy resins as well as strong (40%) nitric acid. Solvent resist¬ ance to intermittent wetting is excellent; however, long term immersion in any but aliphatic solvents such as kerosene is not recommended. Great care must be exercised in the use of DETA and TETA cured epoxy resins in load carrying uses under any chemical environment. Flexural modulus of these castings changes significantly under continuous im¬ mersion conditions in water, sodium hydroxide, 25% sulfuric acid, and other chemicals in which no attack on the castings is noticeable.

54

RAFAEL J. PEREZ

Another widely used amine curing agent that falls into the unmodified “aliphatic” category is diethylamino propylamine (DEAPA). This ma¬ terial is a reactive primary amine with two active hydrogens as well as tertiary amine with catalytic activity. Due to the activity of the un¬ blocked tertiary amine group, the quantity required for commercial resins is 4-8 parts per hundred. DEAPA is similar to DETA in many respects; however, it offers pot lives at room temperature of up to 3-4 hr and actually requires a little heat to fully cure at all. Full cure can take place in an unheated system due to the exotherm heat released during epoxy resin cure which may reach 170°C in the case of DEAPA systems. Generally, DEAPA cured castings have lower heat deflection temperatures and hardness than the DETA cured materials. DEAPA finds a large outlet as a curing agent for epoxy adhesives. Table II lists the attainable physical properties in well cured DEAPA-epoxy systems. Dielectric constant and dissipa¬ tion factor of DEAPA cured systems are superior to DETA systems for the frequency range between 102 and 106 cps. Dielectric constant varies between 3.75 and 3.6 and dissipation factor between 0.002 and 0.12. DEAPA does not produce as tightly crosslinked an epoxy as the TETA series of amines and thus the chemical properties of DEAPA systems are somewhat poorer. The catalytic effect of the tertiary amine portion of the DEAPA molecule produces a significant number of ether linkages in the cured epoxy. These crosslinks are more prone to attack by acids than the sterically hindered tertiary amines formed during pri¬ mary or secondary amine reactions with epoxies. The flexible ether linkage is also more prone to solvent attack than the highly hindered tertiary amines. TABLE II. Mechanical Properties of DEAPA Cured Epoxy Castings (28,33,57) Property Heat deflection temperature, °C Compressive strength, psi Tensile strength, psi Izod impact strength, ft-lb/in. of notch Hardness, Rockwell M

DEAPA, 7 phr 78-94 13,500-15,500 7,100-9,400

0.20-0.21 90-98

Although the possibilities for varying the type of unmodified aliphatic amine used for hardening epoxy resins are almost infinite, the foregoing examples make up practically all the market for unmodified amine hard-

COMMERCIAL EPOXY RESIN CURING AGENTS

55

eners. There are references in the literature to flexibilization with hexamethylene diamine and the use of 1,3-propyldiamine; however, no properties are given and there is no allusion to a commercial product. 2. Glycidyl Adducts of Aliphatic Amines (28,33,61,71)

An amine hardener such as diethylenetriamine can be partially reacted with an epoxy material to produce a low volatility adduct. The com¬ mon reaction product used by hardener manufacturers is that between DETA and the diglycidyl ether of bisphenol A. In a typical reaction the epoxy resin is added slowly to a large excess of diethylenetriamine. The reaction is maintained at approximately 75°C by cooling and agi¬ tation throughout the reaction to insure good contact and lessen con¬ centration effects that would lead to polymer and gel formation. At the end of reaction, excess DETA is vacuum distilled away from the adduct. The adducts yield properties to well cured epoxy systems that are the same as the amine adducted. Thus the above adduct produces cured castings with properties similar to DETA cured epoxies. The advan¬ tages of adducts over unmodified polyamines is their low volatility, the higher mixing ratio to epoxy resin and a faster cure. The low volatility produces advantages over DETA in fairly thick cross sections with poor heat transfer. In these types of uses, the high exotherm due to aliphatic amine reaction with epoxy causes DETA to volatilaze and form bubbles. The adduct, however, does not volatilize under conditions of high exo¬ thermic heat. The higher mixing ratio allows small errors in weighing to be made that would be critical in the use of DETA. Faster cure is an advantage in adhesive and laminating formulations, where thin sec¬ tions must be cured rapidly. The reasons for the shorter cure times with the adducts are that the adduct is already a partially reacted sub¬ stance, thus less reaction is required to reach gel; in addition, the pres¬ ence of hydroxyl groups causes an accelerating of the cure reaction. Typical commercial products in the adduct class are Shell’s Curing Agent U, Dow Chemicals DEH.52, and Union Carbide Corp. ERL-2793. The above have all the characteristics of the DETA diglycidylether of bisphenol A reaction products. Another possible adduct would be the reaction product of DETA with one of the common epoxy resin reactive diluents such as butylglycidyl ether. Union Carbide Corp. Patent No. 2,992,192 (July 1961) claims better solvent resistance from systems cured with the monophenyl glycidyl ether adduct of diethylenetriamine. No company literature hints at such an adduct in commercial usage but a fair candidate for this type of hardener would be Union Carbide’s ERL-2807.

56

RAFAEL J. PEREZ

Properties typically encountered in the adduct cured epoxies are listed in Table III. TABLE III. Mechanical Properties of Epoxy Resin DETA Adduct Cured Epoxy Castings (28,71,82) Property

Adduct, 20-30 phr

Heat distortion temperature, °C Compressive strength, psi Tensile strength, psi

80-105 15,000 7,000-8,200 2.4

Ultimate elongation, % Izod impact strength, ft-lb/in. of notch Rockwell M hardness

0.20-0.6 97-104

The adduct cured systems exhibit dielectric constants between 0.009 and 0.032 as the frequency varies from 60 to 106 cps. These values are very similar to DETA and TETA cured systems. The chemical resist¬ ance of these systems is identical with DETA systems for the same degree of cure. It is characterized by excellent alkali resistance, very good nonoxidizing mineral acid resistance and poor strong organic acid and solvent resistance in total immersion tests.

3.

Ethylene and Propylene Oxide Amine Adducts

Union Carbide Patent No. 2,901,461 (Aug. 1959) describes many of the procedures and properties involving the use of ethylene and pro¬ pylene oxide adducts of alkylene poly amines to cure epoxy resins. The reaction, like that of the adducts of the glycidyl epoxies derived from bisphenol A, is closely analogous to that which takes place during the cure of epoxy resins with primary and secondary amines. The alkylene oxide and amine are reacted in the presence of excess water and a great many adducts are described in the patent; however, in commercial prac¬ tice the ethylene oxide adducts of diethylene triamine are most used with one outstanding exception being Monolene (Wyandotte Chemical Corp.) which is the propylene oxide adduct of ethylene diamine; N(2-hydroxypropyl) ethylene diamine. Commercial products of Union Carbide are ZZL-0814 and ZZL-0816 and of Shell, Curing Agent T-l. These latter are believed to be normal ethylene oxide adducts of DETA. The advantages attained by hydroxyethylation of DETA are increased reaction speed, low irritation potential produced by the hydroxyethyl

COMMERCIAL EPOXY RESIN CURING AGENTS

57

group and low volatility combined with low viscosity. Physical prop¬ erties suffer somewhat due to the reduction of active amine hydrogens of DETA to four in the case of the monoadduct of ethylene oxide and to three when the bisadduct is formed. The compounds tend to be hygroscopic and must be stored in tightly closed containers. High humidity interferes with room temperature cures, particularly in films. The slow cures of films can be overcome by the addition of bisphenol A. The bisphenol acts as an acid accelerator which reduces cure times by 30-40%. The viscosity of the hardener is increased, however, from approximately 3 poises to 20-30 poises. Due to the high speed of cure of these products, they are generally not recommended for casting applications. Their quick cure, however, serves the purpose of laminates and auto body patching kits. Low temperature tooling is also performed with this type of resin hardener. Table IV lists some of the properties attainable with this type of hard¬ ener. Three different sources were used to make this table; the Shell brochure for Curing Agent T-l (a bisphenol modified ethylene oxide adduct of DETA), SP-23-C (72); Union Carbide Corp. Brochure No. J-2214-A (87) which describes properties attainable with curing agent ZZL-0816 that is believed to be the 85/15 mixture of the mono- and bishydroxy ethyl DETA adduct and ZZL-0814 that is believed to be a similar mixture or an all mono adduct containing bisphenol A; and Lee and Neville, Epoxy Resins (28).

TABLE IV.

Mechanical Properties of Ethylene Oxide Polyamine: Adduct Cured

Epoxy Resins Shell curing agent

T-l, Property

25 phr

Heat deflection temperature, °C

87-93

Flexural modulus, psi X 10-6 Flexural strength, psi Compressive strength, psi

13,000

Tensile strength, psi

9,500

U.C.C. ZZL-0814, 25 phr 98

U.C.C. ZZL-0816, 20 phr 87

4.0

4.2

16,000

14,800

Lee and Neville data, 20 phr 88-92

14,100-14,600 9,000

9,700

9,000-11,000

5.5

Ultimate elongation, % Izod impact strength, ft-lb/in. of notch Rockwell M hardness

0.5 83-94

58

RAFAEL J. PEREZ

The hydroxyethyl derivatives of DETA generally reduce relatively all properties of the epoxy resins with which they are compounded. The electrical properties such as dielectric constant will vary from 4.7 to 3.4 and the power factor from 0.0036 to 0.045 as the frequency changes from 60 to 106 cps. Large variations occur because of the hygroscopy of the curing agent; therefore, these materials are not generally used in elec¬ trical applications. The chemical properties of DETA and TETA cured liquid epoxies are superior to those attained with hydroxyethylated DETA. In the latter case the cured composites are less than adequate when in contact with most organic solvents except aliphatic hydrocarbons and lose much of their resistance to nonoxidizing mineral acid (30% sulfuric) as well as to caustic soda solutions. The apparent reason would be formation of ethers with the hydroxyl group in combination with less crosslinking because of the reduced reactivity of the amine from five to four or three active hydrogens.

4.

Cyanoethylated Polyamines (28)

Cyanoethylated polyamines are made by the addition of acrylonitrile to an amine. In the reaction, the double bond of acrylonitrile is sat¬ urated by addition of one of the amine hydrogens to one carbon and the nitrogen group to the other carbon atom. These hardening agents for epoxy resins were first patented by Union Carbide Corp. Patent No. 2,753,323 (July 1956) and this company seems to be the only one to have developed marketable products known as ZZL-0803 and ZZL-0812. The former is a high viscosity (90-130 poise) high modification (cyanoethylation) product of DETA in which approxi¬ mately 2 moles of acrylonitrile are reacted with 1 of DETA to form a trifunctional cyanoethylated amine. The latter is a medium viscosity cyanoethylated DETA in which 1.167 moles of acrylonitrile are reacted with one mole of DETA. Cyanoethylation of DETA reduces the reactivity of the hardener and produces materials with increased pot life. Viscosity is increased by increased cyanoethylation, but since equivalent weight is also increased, the mixtures with liquid epoxies of various degrees of cyanoethylation product will produce nearly the same viscosity in stoichiometric quan¬ tities. Increased modification by acrylonitrile reduces the peak exo¬ therm temperature of epoxy compounds, but at high modification a post cure at elevated temperature is required to develop good physical prop¬ erties.

COMMERCIAL EPOXY RESIN CURING AGENTS

59

DETA cyanoethylation products are best adapted to laminating and adhesive formulations. One good reason for this is that increased cyano¬ ethylation produces hardeners with much better wetting properties. Casting operations benefit from the reduced exotherms of the more highly modified hardeners. A product such as ZZL-0803 is valuable for vacuum casting because it has an extremely low vapor pressure com¬ bined with long pot life and low exotherm. This allows large pieces to be formed rather easily. Generally, cyanoethylated products produce well cured epoxies of poorer physicals than ethylene oxide adducts of alkylene poly amines. Table V lists some of these properties for ZZL-0803 and ZZL-0812.

TABLE V. Resins

Mechanical Properties of Cyanoethylated Amine (28) Cured Epoxy

Molar ratio of acrylonitrile to DETA Property

1.167/1

2/1

Heat distortion temp., °C

72-76

50-58

Compressive strength, psi

12,500

12,300-13,500

7,500-9,100

9,000-10,900

Izod impact strength, ft-lb/in. of notch

0.23-0.28

0.28-0.39

Rockwell M hardness

up to 100

78-96

Tensile strength, psi

Electrical properties of cyanoethylated amine cured epoxies fall off as the cyanoethylation on the amine is increased. The low modification produces dielectric constants between 4.2 and 3.6 and power factors be¬ tween 0.013 and 0.035 as the frequency increases from 60 to 106 cps. The high modification produces deilectric constants between 5.1 and 4.0 and power factors from 0.019 to 0.054. Chemical resistance of epoxy resins cured with cyanoethylated DETA is not hurt badly. Higher modification produces cured products with greater solvent resistance but somewhat less mineral acid resistance (30% sulfuric acid). Of particular interest is that resistance to chlo¬ rinated solvents is greatly increased by high modification.

5. Cycloaliphatic Polyamines (16,19,31,76,77)

Many applications have been found for linear aliphatic amines, but when large castings are desired with moderately high heat distortion

60

RAFAEL J. PEREZ

temperatures, they are generally found lacking. Aliphatic amines yield¬ ing good heat distortion products exotherm too violently for large cast¬ ings and, in addition, cure to thermoset materials too quickly. The modified aliphatics yield poor heat distortion temperatures. In those applications requiring intermediate properties between solid aromatic amines and aliphatics a few cyclic nonaromatic materials have found usage. The most important of these are piperidine, aminoethylpiperazine, and menthane diamine. Piperidine was one of the first amines to be used with epoxy resins on significant commercial scale. It was mentioned by the epoxy resin inventor Pierre Castan in his early U.S. Patent 2,444,333 (1948). Piperidine contains only one active hydrogen for combining; however, after the initial reaction, the molecule formed acts as a tertiary amine catalyst for the epoxy resin polmerization. In this manner, small amounts of piperidine (6 phr) will cure epoxy resins, but it is deficient in producing a high epoxy crosslink density. Epoxy groups are used up, but the cured product does not attain a high heat distortion temperature or a great degree of solvent resistance. The physical properties of a piperidine cured system fall into the same class as primary aliphatic polyamine cured systems. In spite of its drawbacks, piperidine con¬ tinues to be popular because of its long pot life and reduced exotherms. Its widest uses are found in electrical potting when a softer end product is sought and strength properties are not critical. Aminoethylpiperazine has gained prominence as an epoxy curing agent in applications where the superior impact resistance imparted by this hardener give it an advantage over DETA and TETA. AEP is a clear, high boiling liquid that demonstrates cure behavior in large masses, com¬ parable to DETA. Pot lives and exotherms are very similar to unmodi¬ fied aliphatic amines, but AEP alone will not give true room temperature cures to thin sections (under 1/8 in. thick). Usually unmodified formu¬ lations of AEP and epoxy require a postcure at 100-150°F to develop full properties. The addition of small concentrations at phenolic ma¬ terials, such as bisphenol A, as accelerators will provide adequate end polymer structures with ambient temperature cures. Aminoethyl piperazine has found applications in tooling, laminating, electrical, and casting end uses. Union Carbide Patent No. U.S. 2,897,179 (July 1959) describes the use of and properties of menthane diamine. This cycloaliphatic diamine offers the formulator of epoxy resins the high performance of aromatic amines together with the relative ease of mixing of the aliphatic amines. Menthane diamine provides low initial viscosity to epoxy mixtures and concurrent ease of mixing without having to heat. The amine has light

COMMERCIAL EPOXY RESIN CURING AGENTS 61 color and provides it to its epoxy mixtures both before and after cure. These properties are enjoyed with concurrent long pot life, finished castings of high heat distortion temperature and excellent chemical resistance. Castings based on menthane diamine and epoxy resins have been found to exhibit poorer elevated temperature strength reten¬ tion than resins cured with aromatic amines, and the amine itself must be handled carefully before use to avoid reaction with carbon dioxide in the air and subsequent bubbling in castings. Menthane diamine has found usage in many of the applications where aromatic amines held forth. These include casting, filament winding, electrical and laminat¬ ing end uses. In addition, some speciality uses in tooling where aliphatic amines cannot provide enough mechanical strength are open to this versatile material. Metaxylylene diamine (MXDA) is another cycloaliphatic amine hard¬ ener for epoxy resins which yields cured resin properties similar to men¬ thane diamine. MXDA has been used to produce “water white” cast¬ ings with Dow epoxy resin DER-332. This material tends to absorb carbon dioxide from the air and produce bubbles in castings if not pro¬ tected before use. Other less known materials exhibiting superior properties as epoxy resin hardeners and containing aliphatic primary amine reactivity in conjunction with ring structure are 7W-aminobenzylamine, p-aminobenzylamino, 3,5(diaminomethyl)aniline and triaminomethyl benzene. Compounds of this type produce faster cures with epoxy resins than purely aromatic amine structures, with properties very much superior to aliphatic amine systems. It is probable that materials of this type are used as modifiers in liquid aromatic systems for their nonstaining qualities and the enhancement of the curing reaction. Electrical properties of piperidine cured epoxy systems vary similarly to aliphatic amines. Dielectric constant and dissipation factor seem improved. The former varies from 3.0 to 3.6 and the latter from 0.002 to 0.025 over a frequency range from 60 to 106 cps. Aminoethyl pipera¬ zine cured epoxies exhibit dielectric constants and dissipation factors much the same as the aliphatic unmodified amines. Menthane diamine also yields electrical properties similar to aliphatic amines in its cured formulations with epoxies. This amounts to a slight improvement over some aromatic amines such as 7n-phenylenediamine whose dissipation factor varies more widely as frequency changes occur. The chemical resistance of the three cycloaliphatics vary greatly from one to the other. Piperidine acts more as a tertiary amine catalyst. Therefore, cured systems will contain many ether groups subject to acid and solvent attack. AEP acts as a trifunctional curing agent, but also

62

RAFAEL J. PEREZ

as a catalyst. The chemical resistance imparted by AEP to epoxies is, therefore, somewhat intermediate between a tight aliphatic cure and a catalytic cure, with the pronounced properties toward aliphatic cures. A good point of reference would be diethylaminopropylamine. AEP systems are more chemically resistant than DEAPA systems. Menthane diamine, on the other hand, lends epoxies good chemical resistance to acids and solvent resistance better than that imparted by aliphatic amines and slightly poorer than systems cured with aromatic amines such as m-phenylene diamine. Two examples that bring out the above point are 30% sulfuric acid and ethylene dichloride. In each case weight gain of menthane diamine cured epoxies was definitely greater than aromatic amine systems and less than aliphatic amine cured systems. The mechanical properties achievable with epoxy resins cured by the more popular cycloaliphatic amines are listed in Table VI. TABLE VI.

Mechanical Properties of Cycloaliphatic Amine Cured Epoxy Resins Aminoethyl piperazine (77)

Menthane diamine (76)

Metaxylylne diamine (16)

100-120

148-158

130-150

13,500-15,500 13,000-16,000 7,000-9,500 6.0-8.5

8,700 9,000 8.8

15,500-17,500 10,500 9,000 2.9

0.3-0.5 90-96

1.0-1.2 95-105

0.3-0.4 105

Piperidine (19,28) Heat deflection temp., °C Flexural modulus, psi X 10~6 Flexural strength, psi Compressive strength, psi Tensile strength, psi Ultimate elongation, % Izod impact strength, ft-lb/in. of notch Rockwell M hardness

B.

75-110

4.36 15,200 10,600 6.7

Aromatic Amines

The advantages most apparent of aromatic amines over aliphatic amines for curing epoxy resins are the development of higher heat dis¬ tortion temperatures in the cured products, longer pot lives of mixtures and greater chemical resistance after cure. Similarly to the aliphatic amines, aromatic amines produce high exotherms during cure and if heat is required for mixing, increase the dermatitis potential by the release of irritating vapors. Aromatic amines, modifications and mixtures thereof have, up to the near present, had to be cured by some heat. The aromatic amines are

COMMERCIAL EPOXY RESIN CURING AGENTS

63

much less reactive than aliphatics and will eventually “B” stage at room temperature, but will not fully cure for months. The B stage effect is taken advantage of for preparing solid molding compounds from aro¬ matic amines. These compounds often contain proprietary additives to increase stability and yield fast cures at molding temperatures. In the ensuing sections, a detailed discussion of the most used aromatic amines will be used to represent the class in general. In addition, a cursory look at less used products will also be attempted.

1.

Metaphenylene Diamine (25,28)

Metaphenylene diamine (MPDA) is probably the most popular of the aromatic amines used to cure epoxy resins. The amine is amber to very dark in color, depending on purity and degree of exposure to the atmos¬ phere; it is a solid that melts at 63°C; and in the molten condition stains the skin rather badly. Shell Chemical Co. (73) suggests the following mixing procedure with its Epon 828: Casting mixture: Epon 828, 100 parts; MPDA, 14 ± 0.5 parts. 1. Heat 15 parts of Epon 828 to 150°F. 2. Mix well with 14 parts of molten (150°F) MPDA. 3. Blend thoroughly with 85 parts of Epon 828 at room temperature. Room temperature pot life of 1 gal mix prepared as described above should be in the range of 7 hr. At 150°F the pot life would be 40-60 min. MPDA is often B staged before final cure for very large castings applications, for dry filament winding and in molding compounds. This is accomplished by allowing epoxy MPDA mixes to stand at room tem¬ perature until a solid gel is formed. The B staged resin is then cured, in the case of castings, at elevated temperatures between 175 and 300°F and postcured for maximum properties at 350°F after removal from the mold. In dry filament winding the B staged resin is dissolved in solvent and used to impregnate glass fiber; solvent is driven off and winding can then be accomplished dry before cure. Typical properties of MPDA cured systems are listed in Table VII along with those of other aromatic amines. 2.

P,Pr-Methylenedianiline (61,62)

P,P'-Methylenedianiline (MDA) is a solid diaromatic amine melting at 90°C. The stoichiometric quantity required for the most used com¬ mercial epoxy liquid resins is 27-30 phr. The mixing procedure for this

64

RAFAEL J. PEREZ

amine with epoxy resin is very similar to that employed for MPDA. The resin is raised to 90°C and the amine added at 100°C. After thor¬ ough mixing, the batch is cooled with additional resin and mixed well again. MDA gives high exotherms on cure (as does MDPA) and there¬ fore two-stage cures are recommended with this amine (B stage). MDA does not give quite the high temperature strengths of MPDA and its cured epoxy products do not quite have the same chemical re¬ sistance; but electrical properties such as dielectric constant and dissi¬ pation factor are enhanced by the low polarity of this curing agent. Table VII lists the properties attainable with this curing agent in epoxy resins. 3.

Diaminodiphenyl Sulfone (DDS) (30)

This is a tan colored solid melting at 170-180°C. It is used with liquid epoxy resin at 20 phr. Due to its high melting point, special mixing instructions are used for castings. Typical are those issued by Ciba (58) as follows: “Heat the resin to 135°C and then add the aromatic amine with stirring. Continue stirring until a homogeneous mixture is obtained. Cool the solution to 120°C, then pour.” Liquid epoxy-DDS mixtures are very viscous at low temperature; therefore, they are usually poured at 100°C or higher. Long pot lives are experienced when DDS is used as the epoxy resin hardener. These range up to 3 hr at 100°C. The big advantage of DDS over other aromatic amines is the very high heat distortion temperatures attainable with this curing agent. In addition, strength properties are maintained to higher temperatures than the other popular aromatic amines. Due to the use of BF3.MEA and its own polar sulfone group, DDS cured epoxies are not used often in electrical applications requiring high insulation performance, but they are used for laminated circuit boards for high temperature applications, prepreg, tooling and casting applications. Table VII lists the properties attainable with DDS cured epoxies.

4.

Eutectic Mixtures (6,28,87)

Several manufacturers supply proprietary aromatic amine mixtures that are liquid at room temperature. These generally calculate out to be 60/40 mixtures of MPDA and MDA. However, proprietary addi¬ tions are sometimes made to prevent crystallization. Commercial ex¬ amples include Shell Epon Curing Agent Z, Ciba Araldite Hardener 957,

COMMERCIAL EPOXY RESIN CURING AGENTS

65

and Union Carbide Corp. ZZL-0820. The eutectic mixtures can be mixed with epoxy resin without difficulty, and provide extended pot lives because of the low mixing temperatures. Properties attained by use of these eutectics are very similar to MPDA and MDA. Their big advantage is the relative ease of handling of a liquid. A similar product on the market is Uniroyal’s Tonox which is MDA and polymerized versions of it. This mixture has a lower melting point than MDA alone and produces cured products with properties typical for aromatic amines.

5.

Other Aromatic Amines

2,6-Diaminopyridine and 4-chloroorthophenylene diamine (MOCA) have also been used at one time or another for special reasons. MOCA (24,25) is a very slow acting curing agent and a solid to approximately 90°C. Cure schedules in the order of 24 hr at 160°C are used to cure MOCA epoxy resin mixes. In Table VII, the mechanical properties of MOCA cured systems are included.

TABLE VII.

Properties of Aromatic Amine Cured Epoxy Resins MPDA (25-73)

MDA

Property Heat deflection temp., °C

150

144

(61)

DDS (58,78)

Eutectic (75,87)

MOCA (24,25)

190

145 4.4

3.2

17,900

16,400

15,200

119

Flexural modulus, 4.0

3.9

Flexural strength, psi

15,500

17,500

Compressive strength, psi

10,500

10,500

8,000

8,100

8,550

8,000

8,350

3.0

4.4

3.3

4.8

2.2

Izod impact strength, ft-lb/in. of notch

0.2-0.3

0.3-0.5

0.5

0.25

Rockwell M hardness

108

106

psi X HR6

Tensile strength, psi Ultimate elongation, %

10,500

110

105-110

Aromatic amines generally impart some of the best chemical resist¬ ance properties to epoxy resins. Acetic acid resistance is greatly im¬ proved over aliphatic amines and resistance is developed to oxidizing mineral acid. Long term immersion in strong solvents cannot be toler¬ ated, although resistance is better for long periods than aliphatic amines.

66

C.

RAFAEL J. PEREZ

Anhydride Hardeners (19,43,59,84,87)

Acids and their anhydrides are the second most used reactants for curing epoxy resins. They give good overall properties to epoxy resins and seem to be especially suited for electrical applications. Anhydrides are used in most important applications; acids are restricted to surface coatings because of the water of condensation released by them on re¬ action and the greater difficulty of incorporating acids in the epoxy resins. Anhydrides are not skin-sensitizing agents although their vapors can be irritating. Solid anhydrides require heat and considerable mix¬ ing to incorporate well into liquid epoxies; however, no large exotherms are experienced in epoxy cures with them so no danger of runaway re¬ action is present. Liquid anhydrides are as easy to handle as the ali¬ phatic amines. Acid anhydrides first found use in Europe as curing agents for epoxy resins. Castan’s Patent No. U.S. 2,324,483 (1943) described their use and thus prevented commercial applications in the United States for a number of years. In 1956, a cross-licensing arrangement between most of the basic resin suppliers opened the anhydride curing agents to wide¬ spread application in the USA. The original patent was assigned to Ciba and that company became the outstanding proponent of anhydride hardener among basic epoxy resin producers in the United States. As depicted in the introduction, cured epoxy-acid structure contains ester-type bonds. This chemical linkage causes anhydride cured epoxy resins to be more caustic sensitive than amine cured systems. Acid anhydride cured systems are generally very stable thermally and chemi¬ cally; however, their low exotherms require that systems employing them, be cured for relatively long periods at elevated temperature. This involves some risk of vaporization with volatile anhydrides such as phthalic anhydride. The introduction of tertiary amine accelerators and the use of liquid anhydrides offsets many of the earlier difficulties encountered in the use of anhydride hardeners. Anhydride cures produce two competing reactions of the epoxy resins. They may form ethers under acid catalysis or they may react with acid to form ester bonds. It is generally true that higher cure tempera¬ tures produce increasing amounts of ester and decreasing ether forma¬ tion. It is easy to see, therefore, that the initial cure temperature of an anhydride epoxy system may affect the ultimate properties and also the optimum amount of curing agent. Since the etherification reaction does not take up acid, as little as 0.5 moles of acid carboxyls per epoxy may be sufficient; however, since the acid can also react with hydroxyl groups as well as epoxy groups, as much as 1.1 moles may be employed per epoxy equivalent.

COMMERCIAL EPOXY RESIN CURING AGENTS

67

Let us now specifically discuss each useful anhydride and then sum¬ marize the properties in Table VIII. 1.

Phthalic Anhydride (PA)

This anhydride is very inexpensive because it has become a commod¬ ity chemical in many resin syntheses. It is used with epoxy resins where cost is a primary factor and overall performance is secondary. It is employed chiefly for laminates, castings, and pottings with medium range heat distortion temperatures and used where the product volume justifies installation of suitable equipment for handling it. Unlike the other anhydrides, phthalic anhydride sublimes readily, if not quickly reacted with the epoxides. Formulations have useful but shorter pot lives than epoxy mixtures with other anhydrides. This is due to the mixing temperature which must be employed to get compatability of phthalic anhydride and epoxy resins. Systems employing this anhyd¬ ride have low exotherms so they permit the production of large castings. Generally, well cured products show excellent resistance to chemicals, except strong alkalies, and have very good electrical properties. Di¬ electric constant and power factor are unaffected by temperatures up to 100°C. Dielectric strength is also preserved over a wide range of tem¬ perature (to 175°C). 2.

Hexahydrophthalic Anhydride (HHPA)

HHPA is a low melting (36°C) solid which imparts excellent all-around properties to epoxy resin systems in which it is incorporated. It is widely accepted for many uses, including electrical and filament winding applications. It tends to add resilience to the cured products without any appreciable strength loss. Because it is liquefiable at temperatures of 50-60°C and does not sublime like phthalic anhydride, HHPA can be mixed very conveniently into epoxy resins. Viscosities of HHPA epoxy mixtures are lower than with any but the liquid anhydrides, and have long pot life and low cure exotherms. HHPA produces light colored cured products which are much more stable electrically than amine cured epoxies. HHPA cured epoxies are used in laminating, potting, tooling, and industrial coatings. It is used with other anhydrides to make low viscosity liquid eutectic mixtures. The chemical properties of HHPA-epoxy blends after cure are gen¬ erally equal to those of phthalic anhydride cured systems. These chemi¬ cal resistances may be listed briefly as follows: 1. Water resistance is excellent up to 70°C with very little effect up to a year immersion.

68

RAFAEL J. PEREZ

2. Resistance to sulfuric acid is excellent below 60°C and at 50% or less concentration. Hydrochloric and hydrofluoric acid also affect an¬ hydride cured systems to a minimal degree. 3. Oxidizing mineral acid such as nitric acid is very reactive toward anhydride cured epoxy above 10% concentration and causes spongy swelling in a few days. 4- Organic acids such as formic and acetic acid do not harm anhydride cured epoxies over long periods at room temperature but do attack at elevated temperatures. 5. Strong alkalies do attack anhydride cured systems. Aqueous am¬ monia at room temperature will badly swell anhydride cured epoxies. 6. Solvent resistance of anhydride cured epoxies is similar to some extent to amine cures. Aliphatic hydrocarbons and alcohols do not attack the converted products but chlorinated hydrocarbons and the stronger polar and nonpolar solvents do attack the cured epoxies after prolonged immersion.

3.

Nadic Methyl Anhydride (NMA)

This liquid anhydride is very widely used for electrical laminating and filament winding applications. It is compatible with liquid epoxy resins at room temperature, so it produces no mixing problems. In addi¬ tion, its mixtures with epoxy resins exhibit long pot life even when accel¬ erator is added. The cured products can be formulated to high heat distortion temperatures together with light color and excellent electrical properties including arc resistance. Castings can be made with minimal shrinkage and excellent high temperature aging characteristics. The electrical and chemical resistance properties of NMA cured epoxies are similar to HHPA. Electricals are excellent and are held to high temperatures, while chemical properties suffer in alkali resistance and strong solvent resistance mainly.

4.

Dodecenylsuccinic Anhydride (DDSA)

DDSA is another liquid anhydride that presents few mixing problems with epoxy resins due to its compatibility and long pot life. It has been reported to give the most outstanding electrical properties of any hard¬ ening agent and is excellent for imparting flexibility to the cured resins. Cured products are softer than with other anhydrides and the mixtures with liquid epoxy resins exhibit low viscosities, normal curing times and good clarity. DDSA is often used in admixture with other anhydrides

COMMERCIAL EPOXY RESIN CURING AGENTS 69 due to its high equivalent weight which precludes its use exclusively in many applications for economic reasons. The long hydrocarbon chain in this curing agent produces solvent sensitive cured epoxy systems where it is used. Therefore, systems employing this curing agent should not be in contact with most organic fluids.

5.

Tetrahydrophthalic Anhydride (THPA)

Tetrahydrophthalic anhydride gives properties very similar to HHPA. It must be handled carefully, however, or it will give much darker cured products. The widest use for this hardener is as a modifier of other anhydrides to lower the cost of the overall system.

6.

Maleic Anhydride (MA)

MA is used chiefly to obtain high compressive strength where some sacrifice can be made in the tensile and flexural properties. By itself, it produces brittle cured epoxies; however, when used with other hard¬ eners it acts as an accelerator of the epoxy cure and gives superior prop¬ erties to those obtainable with phthalic anhydride. 7.

Pyromellitic Dianhydride (PMDA)

PMDA is a white powder of very high melting point and insolubility. The high reactivity of this material for epoxy resins, together with the above-mentioned properties, make it somewhat difficult to incorporate. Curing agent systems composed of PMDA in combination with other organic anhydrides, such as phthalic anhydride and maleic anhy¬ dride, are easier to dissolve and more convenient to handle. Mixtures with MA in particular, have given cured epoxy resins with exceptional properties and with excellent retention of these properties at elevated temperatures. Heat distortion temperatures as high as 250°C are at¬ tainable when epoxy resins are cured with PMDA-MA mixtures. PMDA/MA mixtures give the best performance in uses as epoxy resin hardeners when used at 0.85 anhydride equivalents per epoxide equival¬ ent. High ratios of PMDA/MA give higher heat distortion tempera¬ tures but shorter pot lives. Convenient handling requires that a maxi¬ mum of 50% PMDA based on anhydride equivalents be used. The most common procedure for preparing epoxy-PMDA/MA cast¬ ing formulations is to add the finely ground acids to the epoxy resin at

70

RAFAEL J. PEREZ

about 70°C. With good agitation, the acids can be well dispersed quickly. The mixture is then heated with continued agitation to 120°C or until the anhydrides dissolve. If maximum pot life is desired, the casting formulation should then be cooled back to 90°C or to the lowest temperature giving a pourable viscosity. To avoid the settling of PMDA during the early stages of curing, preheated molds and small amounts of amine accelerator such as methylene dianiline should be employed. Harper (19) describes a PMDA system with an extremely short cure (15 min at 180°C) giving a cured epoxy with the same outstanding high temperature properties and with slightly better room temperature prop¬ erties than obtained with other PMDA epoxy systems. It is formulated

as follows: Parts by weight Common liquid epoxy resin (0.52 equiv./lOO g) PMDA Tetrahydrofurfuryl alcohol (THFA) containing 1% dicyandiamide

100

56 20

Mix by adding PMDA to the resin at 75°C and mixing thoroughly for 5 min. The THFA solution is stirred in at the same temperature. This mixture is poured into a mold at 180°C and allowed to cure. Pot life is 20 min at 180°C. The dianhydride structure of PMDA produces tightly crosslinked cured epoxies. This results in very high heat distortion temperatures and compressive strengths but rather low tensile and flexural strengths. The low strengths are offset by the fact that they are held to higher tem¬ peratures than with other curing agents. Chemical resistance of epoxies cured with PMDA is superior to that of other anhydrides. Nitric acid and caustic resistance are particularly improved although not to the point of aromatic amine cured epoxy resins. Electrical properties are good and are held to very high temperatures. 8.

Chlorendic Anhydride

Chlorendic anhydride is a highly chlorinated acid anhydride which yields epoxy resin systems curing to heat distortion temperature as high as 200°C. The high chlorine content also reduces the flammability of epoxy i esin systems in which it is employed. The stoichiometric amount of chlorendic anhydride for the common liquid epoxy resins is 185 phr, but the best properties are obtained when about 100-115 phr are used! Its nonflammability creates uses for chlorendic anhydride in casting and

COMMERCIAL EPOXY RESIN CURING AGENTS

71

laminating applications. Like PMDA, it holds good electrical and mechanical properties to high temperatures and is used in elevated tem¬ perature service. Chemical properties imparted by chlorendic anhy¬ dride to the resin systems it cures are not any better than the ordinary anhydrides. 9.

Trimellitic Anhydride (TMA) (44)

This is a very reactive acid anhydride due to the free carboxyl group which tends to accelerate cures with epoxy resins. TMA is normally used with solid resins for prepreg laminating. It is generally post cured at temperatures around 180°C and yields heat distortion points on the order of 200°C. The chemical and electrical properties are very good for systems in which TMA is used. Amoco Chemicals produces TMA as well as TMX anhydrides. TMX220 is a commercial grade of ethyleneglycol bistrimellitate anhy¬ dride and TMX330 is a commercial grade of glycerol tristrimellitate anhydride. The TMX ester grades are lower melting than pure TMA and more compatible with epoxy resins. Therefore, their handling char¬ acteristics are much better although the payment for this is lower flexural strength and impact resistance. In addition, the esters must be used in larger quantities (56 and 66 phr vs. 33 phr for TMA) and produce cast¬ ings with poorer alkali resistance. The tighter crosslinking resulting from the polyanhydride nature of the TMX materials produces cured epoxy systems of improved solvent resistance and heat distortion tem¬ peratures over 200°C. TMA cured epoxies attain flexural strengths of 15,900 psi and flexural modul of 446,000 psi. Impact strengths are low (0.15) and Rockwell hardness is 113 M. 10.

Eutectic Anhydride Mixtures (23,43,80)

Since most of the anhydrides used to cure epoxy resins are solids, it is often difficult to mix them into liquid epoxies because they require ele¬ vated temperatures and then produce irritating fumes as they are stirred into the epoxies. To obtain more easily used anhydride systems, eutec¬ tic mixtures have been prepared that are liquid at low or room tempera¬ ture. One of those most popular is the 70/30 mixture of chlorendic anhydride and hexahydrophthalic anhydride. The physical properties of epoxy resin in which 70-100 phr of the above is used suffer a little in comparison to pure chlorendic anhydride, although mixing is infinitely simpler. Some other eutectic mixtures with low melting points are:

72

RAFAEL J. PEREZ

6°C 26°C 26°C 0°C 0°C 0°C 0°C

30/70 15/85 15/85 75/25 75/25 50/50 50/50

maleic anhydride/hexahydrophthalic anhydride phthalic anhydride/hexahydrophthalic anhydride tetrahydrophthalic/hexahydrophthalic Nadic methyl/maleic Nadic methyl/tetrahydrophthalic Nadic methyl/Hexahydrophthalic Dodecenylsuccinic/hexahydrophthalic

A more complete listing can be found in Allied Chemical Plastics Div. Brochure 81-112-5M “Anhydride Hardeners for Epoxy Resins.” An interesting material that may be used in eutectic anhydride epoxy hardening mixtures to increase flexibility and improve electrical prop¬ erties is polyazelaic-polyanhydride (PAPA) (5). By itself, it is recom¬ mended that 70 phr be used with epoxy resin. Eutectics melting below room temperature include: NMA/PAPA 50 or more/less than 50 DDSA/PAPA 70 or more/less than 30 HHPA/PAPA 60/40 A maleic anhydride modified tung oil has been tested as a liquid poly¬ anhydride curing agent for epoxy resins (34). This material, like PAPA, offers improved electrical properties and a flexible cure to epoxy formu¬ lations in which it is included. In Table VIII a summary of mechanical properties for the typical anhydrides are listed. It may be assumed that similar structures will produce similar results.

D.

Catalytic Curing Agents and Latent Hardeners

Catalytic curing agents promote self-polymerization of epoxy resins. They cause the epoxy ring to open and attack other epoxy groups on other molecules or hydroxyl groups which are ever present in epoxy resins to some degree. The catalytic curing agents do not themselves participate in the epoxy polymerization reactions. They act merely as initiators and promoters of epoxy resin curing reactions. I he most popular catalytic curing agents are tertiary amines, amine salts, boron trifluoride complexes, and amine borates. The pot life of formulations containing tertiary amine as the sole curing promoter varies from 2-24 hr. Boron trifluoride amine complexes provide pot lives on the order of 6 months as does a latent catalyst, dicyandiamide. Catalyst amounts vary depending on the end use. Tertiary amines as acid anhydride cure accelerators are rarely used at more than \ ]/2 parts

COMMERCIAL EPOXY RESIN CURING AGENTS

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Concrete Subsoil

Figure 5

Laitance is present to a depth of 0.05 in. or so and is weak in almost evety respect. Unless it is removed, it will limit the performance, and perhaps cause the failure, of anything bonded to its surface. 2. Curing Compounds

Curing compounds are often sprayed onto wet concrete to act as a membrane to retard water evaporation while the concrete is curing. They are almost invisible when cured. The chemicals used for such compounds are frequently fatty oils or resinous materials of types that can act as parting agents for any subsequently applied topping. That is, they are likely to prevent good adhesion. If possible, prohibit the use of such compounds in the specification for the concrete because, if they have been used, they must be removed before any subsequent topping is applied. If in doubt as to the surface condition, place a test patch and check the adhesion.

FLOORING

149

3. Concrete Setting Time

Since new concrete contains much water, some of which is surplus, it must be permitted to cure (age) for as long as possible before an epoxytopping is applied. This is for two reasons: to reduce the moisture content, and to permit the major part of slab shrinkage to take place. The initial water content of the slab, its thickness, and the prevailing climatic conditions will all have great effect on the aging time required. The aging time should be as long as possible, with 30 days being re¬ garded as the minimum.

4. Steel

Don’t expect good results over a rusted surface. Any exposed steel over which a floor topping is to be placed must be sandblasted and vacuum cleaned just prior to placing the topping. Where sandblasting is not possible, the steel surface should be de¬ greased and ground or mechanically abraded (sanding or wire brush) to reveal continuous bright metal.

5. Wood-

Wood is sometimes found as part of the subfloor.

Sanding or sand¬ blasting followed by vacuum cleaning are recommended as cleaning methods. Should the floor have been creosoted, an adhesion test should be made. Fresh creosote acts as a parting agent, but fair adhesion may be obtained with an old creosoted surface that has been well cleaned. An old wooden floor is not easy to clean, and it may be cheaper to sand flat and overlay the surface with exterior glued plywood of Yi in. minimum thickness. For the best results this should be five or more plies tongued and grooved at the edges, edge bonded and nailed. Edge bonding should be with a thermosetting gap filling glue such as epoxy or resorcinol. Nails should be of the barbed type or galvanized in sea¬ shore areas. Smooth joints by sanding when glue is hard. Clean off dust by vacuum (see Fig. 6). The same applied to new floors, the object being to reduce edge movement and make underlayment seams less noticeable.

6. Bituminous Asphalt

This is not recommended as a subfloor for the same reasons that the application of a hard paint is not advisable over a weak surface.

150

G. M. SCALES

7. Tile

The treatment of tile will vary according to the type of tile. Usually an acid etch gives the best results but on some extremely acid resistant tile a sandblast and vacuum cleaning technique may be the only work¬ able method.

8. Old Terrazzo

Treat as for old concrete with the preferred method being mechanical scarification by grinding followed by vacuum cleaning.

B. The Preparation of New and Old Concrete Floor 1. Precleaning

All heavy deposits of dirt, asphalt, oils, or greases must be removed before the final surface preparation. The best way of removing such deposits is by mechanical means, aided by grease cutting detergent of the nonionic type. In particular stubborn instances, solvents such as perchloroethylene or high flash naphtha may be useful aids if the appropriate safety pre¬ cautions are observed. Care should be taken that the use of such solvents does not spread the contaminant over a larger area. The detergent should be flushed off very thoroughly. 2. Preparation

The preferred methods are mechanical such as: (а)

Grinding

(б)

Sandblasting

(c)

Mechanical scarification

FLOORING

151

so that the surface is removed to a noticeable depth. After any of these preparations all dust and loose particles must be removed entirely. Heavy duty industrial vacuum cleaning is the preferred method. Should mechanical cleaning prove impracticable, then the surface may be prepared by acid etching then flushing off with high pressure water and drying. The following details of acid etching are given as it is a method very frequently used. Alshough not as effective as mechanical methods, it is a satisfactory way for concrete subfloor preparation but it must he carried out properly otherwise poor results can be expected. 3. Alternate Method (Acid Etching)

After the surface has been precleaned the following solution is spread by brush or spray over the concrete surface. Commercial muriatic acid (20° Baume HC1) Water

1 part by volume 2 parts by volume

Total

3 parts by volume

This is approximately a 10% solution of HC1. In our opinion, the acid etching solution should be premixed so that the workers do not have any chance of obtaining a solution of wrong strength. They should be supplied with an approved acid etching solution and not a formula such as above. The application rate required is about: 1 pint per square yard for a 15% solution 1.5 pints per square yard for a 10% solution The acid solution should be worked into the surface by hard-bristled brooms until complete wetting and coverage is obtained. The acid will react with the concrete surface and bubble vigorously for a few minutes. During this time, brushing should continue. After 10 or 15 min the bubbling will have subsided and a slurry will be left on the surface. This must be removed. The most effective way is by high pressure water hosing. The more force from the water jet the better for removing semi-loosened and fine particles as well as the acid residue. There have been instances where some acidity has been known to re¬ main inspite of apparently adequate rinsing. This can be checked by moistened litmus paper placed on the concrete surface. It is good practice, but not essential, to neutralize any possible acid surface

152

G. M. SCALES

condition, which can impair adhesion, by a final rinse of a 1% solution of ammonia in water. Again, flush with water and allow to dry. The surface must be completely dry before any epoxy topping is placed. After acid etching, test patches can be placed to check adhesion. Any clean surface can be easily contaminated not only visibly, but by exposure to heavily contaminated air in industrial areas. Therefore if there is any undue time delay before placing the epoxy topping, the acid etch treatment should be made again.

4. Powdered Acid Concentrates

Occasionally the situation arises where the delivery of liquid acids to the job site is either impracticable, disproportionately expensive or just plain inconvenient. In such situations the use of a powdered acid concentrate which can be dissolved in water to obtain the desired concentration on site is convenient. DuPont’s Sulfamic Acid is readily dissolved in water and, used in a concentration oi 10% (by weight), is equally effective in the acid etching of concrete as is a 10% solution of HC1. Sulfamic acid is readily avail¬ able and even obtainable at hardware stores.

C. The Repair of Old Concrete Subfloors As only poor results can be expected from placing a strong epoxy topping over a weak subfloor, a few remarks on subfloor repair may be of interest.

1. Cracking

Cracks are most apparent in cold weather when they are at their widest due to thermal contraction of the slab. Cracks are most noticeable when a floor has been washed and dried but the cracks are still damp. But, it takes considerable experience as a concrete specialist to determine what caused them. We make no attempt here to discuss the causes of cracks. Clearly, knowing the cause of the crack is of great importance when deciding the best method of remedial treatment. Without expert knowledge, there is probably only one way for the “amateur” to seal a crack wisely—that is, with a flexible sealing compound.

FLOORING

153

There are two types of cracks: (a) (b)

Moving cracks (which expand and contract) Nonmoving cracks

a. Moving Cracks.

Crack Preparation. Clean the cracked surfaces. Dust and dirt should be blown out with dry compressed air so that the surfaces are clean and dry. Often the sides of the crack are weakened and, as the edge strength of the jointing compound will be only as good as the tensile strength of the cracked edge, it is advantageous to reinforce this surface with an im¬ pregnating, thermo setting primer. These materials are usually low solids content epoxy adhesives capable of penetrating into porous and weak concrete. Therefore: Prime the crack with an epoxy adhesive primer mixed and applied generously in accordance with the manufacturer’s instructions. Fill the crack with expansion joint compound preferably of the thermo¬ setting type—after the solvent of the primer has evaporated, but before the primer has lost its tackiness. When filling the crack with expansion joint sealing material, there are several things to bear in mind—they are the conditions described in Figs. 7-9.

(a)

(b)

Figure 7

Clearly, it is easiest to fill such a crack in the winter (Figs. 7a and 7b) but it should be borne in mind that much of the material (in Fig. 8a) will be squeezed out by midsummer (Fig. 8b).

(a)

(b) Figure 8

If the material cannot exude upward because of a floor topping, then (with a little bit of luck) it will be expressed downwards (Fig. 9a). When making such winter repairs care should be taken to ensure that

154

G. M. SCALES

there is such downward escape, for if there is not, then subsequent blistering of the floor topping will almost certainly result (Fig. 96).

Figure 9

b. Expansion Joints. It is poor technique to place an almost rigid floor topping over an expansion joint as it will normally either crack as in (Fig. 10a) or blister as shown in (Fig. 106), depending on whether the topping is in tension (Fig. 10a) or compression (Fig. 106).

^-

->

->

(°)

(b) Figure 10

The correct technique is to end the epoxy topping at the end of the slab and treat both as a monolithic block by filling up with expansion joint material. Expansion joint sealing materials are elastomeric and made purposely of low tensile strength. Even the best quality of such materials is unlikely to have chemical resistance of the same order as the epoxy topping and therefore expansion joints should be designed into high points of the floor so that liquids will drain away from them. D. The Factor of Shape The shape of the expansion joint material is important. If the shape is incorrect it can cause undue distress in the expansion joint material and premature failure. The shape is also dependent on relative dimensions: the general rule being that the depth of the joint should not be greater than one-half of its width.

FLOORING

155

It would be ideal but rarely possible in practice to pour the jointing material at a time when the slab is at the middle point between maxi¬ mum and minimum dimension. In this instance the joint will look like Fig. 11a.

Flexible joint filler

^-Compressible filler

Figure 11a

This implies that the joint will change dimension with seasonal tem¬ perature changes and should then look like Fig. lib,c.

Summer

Winter

Figures 116 and 11c

In order to facilitate the pouring in the correct shape and the subse¬ quent correct movement a compressible cheap filler is frequently placed in the required shape first (Fig. 12a) and then over it is placed a material that the joint sealer won’t stick to, to act as a bond breaker and ease the movement of the upper sealer away from the lower filler (Fig. 126).

Compressible filler

156

G. M. SCALES

E. The Sealing of Fine Cracks On very fine cracks which move slightly, the following techniques is often successful and may even prevent a small subfloor crack from being reflected through the epoxy topping. Cut a “V” in the upper part of the crack. This allows a greater width for placing the joint sealing material and therefore lowers its percent elongation or compression.

(b) Figure 13

Now place the joint sealing material in the “V” groove and crack and place the epoxy topping over it.

(b) Figure 14

F. Unsound Concrete All faulty concrete must be removed. Care should be taken during removal to avoid damaging the surrounding concrete. Such damage can occur if heavy pneumatic hammers are used. Damage may be cut out by saw and light hammers or by routing machines designed for such a purpose. All dust should be removed by vacuum, or if this is not possible, blown away by compressed air.

FLOORING

157

G. Patching Conventional concrete patches should be used to repair the prepared areas. If the patches are thin, an epoxy concrete bonding adhesive should be used to ensure good adhesion. High early strength concrete may be used for the patches. No chemical curing membranes should be used over the patches. An adequate hardening and maturing time should be allowed for the concrete patch. It is most important that the patch should be dry before placing an epoxy topping. Should there not be enough time for adequate drying of a concrete patch, then an epoxy aggregate filler type or patch should be used. This patch can be of a sand filled flooring type of material but due to reason of exotherm should be placed and allowed to harden before the floor topping is placed.

-

Epoxy Adhesives D. A. Shimp Celanese Resins Division of Celanese Coatings Co. Louisville, Kentucky

CONTENTS I. Introduction. 159 II. Performance Data and Applications. 160 A. B. C. D. E. F.

Weather ability. Chemical Resistance. High Temperature Formulations. High Peel Strength Modifications. Fast Setting Adhesives. Electrical Insulating Properties.

160 162 163 163 166 168

G. H. I. J.

One Package Formulations. Adhesives for Concrete. Effect of Low Cost Extenders on Adhesion. Adhesion to Room Temperature Cured Epoxy Formulations.

169 175 178 179

K. Adhesion to Asphalt. 180 L. Flame Retardant Adhesives. 182 References. 183

I. INTRODUCTION Epoxy resins have captured a share of the adhesives market largely through qualifying for new applications with performance requirements exceeding the capabilities of the long-established phenolic thermosetting adhesives and thermoplastic solution or emulsion adhesives. Typical of the new adhesive applications made feasible by epoxy resin develop¬ ments are pressure grouting repair of fractured concrete and rock structures, and aluminum skin-honeycomb core composites for aircraft construction. The remarkable bonding strength of epoxy adhesives to a wide variety of structural materials, including metals, wood, glass, plastics, ceramics, concrete, asphalt, and rock, is attributed to three basic properties: 1. Good wetting characteristics combined with repeating polar sites, particularly hydroxyl groups generated by condensation of epoxides with active hydrogen containing curing agents, establish strong van der Waals forces and induce hydrogen bonding with some surfaces. Primary 159

160

D. A. SHIMP

valence bonds may be formed with active hydrogen-containing sub¬ strates, such as nylon and various cellulosics 2. Low chemical condensation shrinkage minimizes stresses developed at the adhesive interface, a factor which is particularly important in preserving the bond strength of rigid formulations. Linear shrinkage values ranging from 0.05 to 1.0% are typical of unmodified epoxy sys¬ tems cast in Yi in. thickness. Appreciably less shrinkage is obtained with filled or extended modifications and when the compositions are cast in thinner sections, due to lower exothermic temperature rise and consequent reduction of thermal shrinkage. Unfortunately no test method for measuring the shrinkage in adhesive bond line thicknesses is in general use 3. High physical strengths characteristic of most epoxy systems (tensile strengths commonly in the range of 8,000-14,000 psi and ultimate compressive strengths as high as 38,000 psi) assure that cohesive failure is not a limiting factor in the overall performance of the adhesive In addition to possessing these three properties essential to good adhesion, epoxy resin systems contribute numerous bonus features: J+. Chemical resistance 5. Weatherability 6. Electrical resistance and low dielectric loss 7. Versatility of modifications (a) High temperature strength and stability (b) Peel strength (c) Wide range of reactivity (d) Cure over wide ambient temperature range (e) Bond to damp surfaces (/) Flame retardancy (g) One package (latent cure) or two component formulations The remainder of this chapter is devoted to performance data and specific examples of end use applications intended to illustrate the versatility and potential of epoxy resin adhesives. The sample formu¬ lations divulged are only starting point suggestions and are not opti¬ mized with respect to curing agent concentrations, fillers, thixotropic agents, and other modifications. Finished epoxy adhesives are avail¬ able from a large number of reputable compounding houses. II. PERFORMANCE DATA AND APPLICATIONS A. Weatherability Although epoxy surface coatings undergo ultraviolet initiated surface attack with consequent reduction of gloss and discoloration, maintenance

EPOXY ADHESIVES 161

Fig. 1. Tensile shear strength retention of weathered aluminum and copper test speci¬ mens.

of adhesion during exterior exposure is excellent. A combination of good water resistance and thermal shock resistance account for the durability of the epoxy adhesive bond even under tropical and arctic weathering conditions (1). Figure 1 illustrates the retention of tensile shear strength of copper and aluminum strips bonded with a polyamidoamine cured adhesive after 2 years weathering in a temperate climate. Adhesive Formulation Parts by weight 100 100 20 20

Epi-Rez 510 Epi-Cure 8525 Alumina T-60 Asbestos shorts

Clean, untreated glass surfaces present an unusual adhesion problem in that the high affinity of water for glass results in preferential wetting of the glass by moisture during exterior exposure, with subsequent debonding of the epoxy adhesive. This condition is fully corrected in

162

D. A. SHIMP

fiberglass-epoxy composites by pretreatment of the glass with silane coupling agents or chrome-methacrylate complexes. Incorporation of silane coupling agents, such as glycidoxypropyltrimethoxysilane into the epoxy adhesives is also effective in providing good weatherability for composites such as decorative glass-epoxy matrix building panels and slab glass windows. B. Chemical Resistance

The accepted uses of epoxy adhesives in wet cell batteries and for assembly of reinforced thermoset plastic pipe used in chemical plants illustrate the good chemical resistance of even room temperature curing formulations. Table I presents some acid resistance data compiled in an accelerated test used to screen battery adhesives. Adhesive formu¬ lations exhibiting less than 4% weight gain generally outlast the life of the battery under actual service conditions. TABLE I.

Battery Acid Resistance of Epoxy Adhesive Formulations

Composition (parts by weight)

1

2

100 18

3

4

100

100

100

18 73

18

_

81

Resin Epi-Rez 510 (DGEBA) Converter Portion Epi-Cure 874 (accelerated aliphatic amine) Stygene R-2 (petroleum derived extender) CP-524 (coal tar extender)

— —

Epi-Cure 8494 (accelerated aromatic amine)

45

Cure schedule Percent weight gain after immersion in 33% H2S04 for 2 weeks at 150°F

1 week at 77 °F 2.9

3.6

3.9

1.5

Accelerated aliphatic poly amines are frequently employed as curing agents in battery adhesive formulations, and up to 40% coal tar or high aromaticity petroleum extender oils can be incorporated without adversely affecting acid resistance. Accelerated aromatic amine curing agents capable of practical cure i ates at ambient temperatures as low as 40°F are promising candidates for use m assembly of reinforced plastic pipe and couplings. The wide spectrum of chemical resistance, particularly at elevated temperatures, which is characteristic of aromatic amine cured compositions far exceeds the resistance properties of other fast setting adhesives based on accel¬ erated aliphatic polyamine and polymercaptan curing agents. The

EPOXY ADHESIVES

163

rapid, dependable, on-site assembly made possible with epoxy adhesives constitutes a major economic advantage of reinforced plastic pipe over thinner walled stainless steel pipe (2). C. High Temperature Formulations

Novolac epoxy resins and purified bisphenol-based epoxy resins in combination with anhydride, phenolic, or aromatic amine curing agents are commonly selected for formulation of high temperature resistant adhesives. Imidazole catalysts are a more recent class of converter showing promise in this usage. Although useful shear strengths at temperatures as high as 600°F for short periods of time can be obtained with these formulating approaches, long-term service temperatures are limited by thermal degradation processes to 400-450°F for anhydride and phenolic curing agents, and to 300-350°F for aromatic amines. Stabilizing additives such as zinc naphthenate (3) are sometimes incorporated to increase these service temperatures modestly. Figure 2 illustrates the tensile shear strength versus test temperature relationships for five representative formulations. Whereas the incorporation of mineral fillers often improves tensile shear strengths by decreasing shrinkage, there are indications that certain nonfibrous fillers, such as alumina, function as reinforcing agents when incorporated into imidazole catalyzed formulations. For ex¬ ample, composition C of Fig. 2 exhibits an increase in heat distortion temperature of 180°F and 1500 psi more flexural strength at 500°F than the unfilled base system, composition D. The interaction of silica, alumina, and barium sulfate fillers with epoxy resin/imidazole binders is not paralleled by any significant improvement in high temperature properties of similarly modified anhydride, aromatic amine, or tertiary amine cured compositions. D. High Peel Strength Modifications

A number of flexibilizing techniques are effective in distributing over a larger area the localized stresses imposed by peel forces. The following modifications have been successfully utilized in formulating epoxy adhesives with 90° peel strengths as high as 30 lb/in. width: 1. Higher molecular weight resins (lower crosslink density) 2' Polymerized fatty acid modifications of resins and/or cuiing agents 3. Polyalkylene glycol modifications of resins and/or curing agents 4. Low crosslink density dimercaptan curing agents 5. Thermoplastic polyamide (nylon) modifications

164

D. A. SHIMP

Composition (parts by weight) Epi-Rez 508 (purified DGEBA) Epi-Rez 5155 (Novolac epoxy) Epi-Rez 510 (DGEBA) Epi-Rez 5108 (purified DGEBA) Cyclan 330 (a trianhydride) Hexahydrophthalic anhydride Diethylaminoethanol Cyclopentanetetracarboxylic dianhydride 2-Ethyl, 4-Methylimidazole Epi-Cure 841 (aromatic amine) Alumina T-60 Asbestos shorts Colloidal silica Aluminum powder

A 50 50

B

_

C

__ _

D

_

—

100

—

_ _

—

—

100

100

33 53 0.5 — —

—

—

_ _

_ _

—

—

—

50 0.2

_

__

2

2

—

—

_

—

—

—

_

_ 179 _

2

2

3

50

50

E

_ 100

—

_

22.5 20 20

—

—

Figure 3 illustrates the effect on 90° peel strength and tensile shear stiength of increasing Epi-Rez 505 (flexibilizing diepoxide resin) substi¬ tution into a rigid aliphatic amine cured adhesive. The somewhat unexpected accompanying increase in tensile shear strength measured

EPOXY ADHESIVES

165

at room temperature is attributed to more effective distribution of bend¬ ing stresses concentrated at the overlap junctions of the ASTM D-1002 single lap shear specimens. The use of the double lap shear specimen, which confines applied stress to purely tensile shear, would have resulted in highest values for the rigid base composition. However, since minor bending force components as well as fatigue producing load cycles, vibration, and thermal shock are often encountered in adhesive joints designed primarily for shear loading, flexibilizing modifications are often desired to increase safety factors in adhesives used for other than peel force applications.

X I—

Q

cn UJ Q-

□

CL

X

I—

13 UJ

cr

i—

U") UJ CL O

a cr>

Fig. 3. Effect of Epi-Rez 505 modification on adhesive properties.

Basic formulation Epi-Rez 510: 100 parts by weight Epi-Cure 874: 19 parts by weight ASP 101 (clay): 60 parts by weight Cure schedule: 2 weeks at 77°F Test temperature: 77°F Shear coupons: 2024T3 aluminum Peel specimens: 0.005 in. aluminum to 0.125 in. aluminum

166

D. A. SHIMP

Table II summarizes many of the diverse types of adhesive formula¬ tions used for applications requiring high peel strengths. The chief limitation of high peel strength adhesives is loss of shear strength at moderately elevated temperatures. The hard resin solution/dicyandiamide compositions in Table II maintain useful shear strength values at temperatures in the 200-250°F range and are among the more versatile in this respect.

E. Fast Setting Adhesives

Highly reactive, two-component adhesives capable of curing in bond line thicknesses to handling strength within minutes at normal room temperatures, and of developing useful bond strengths at ambient temperatures as low as 0°F are currently being formulated. Uses for these adhesives are manifold:

1. Bonding difficult-to-clamp fixtures such as surface mounting elec¬ trical receptacles, brackets, and racks to masonry walls

2. Rapid production line assembling, such as bonding together sec¬ tions of reinforced plastic car bodies (4)

3. Bonding traffic buttons to highways when minimum traffic delay is important (change over from line paint to buttons on estab¬ lished roads, bridges and tunnels)

4- Emergency repair of boat leaks o. Cold weather construction uses of epoxy adhesives, such as pres¬ sure grouting of fractured concrete, pipe joint sealing, bonding piecast, bridge deck beams together, etc. Less than two years ago, practical cure rates of epoxy adhesives were limited to a minimum ambient temperature of 55-60°F. Figure 4 compares the pot life/bond line gel time relationship of the highest reactivity formulations representing three classes of curing agent: Lewis acid, accelerated polymercaptans, and accelerated ali¬ phatic polyamines. The unusually low ratio of 2:1 exhibited by the polymercaptan cured composition is very desirable in that a larger pro¬ portion of the adhesive set time can be used for mixing and applying the adhesive. Unlike Lewis acid catalyzed formulations which will not bond effectively to alkaline surfaces such as concrete, gypsum board and rubber containing amine antioxidants, the polymercaptan cured compositions will bond well to practically anything except untreated polyolefin thermoplastics.

EPOXY ADHESIVES

167

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