Epoxy

EPOXY RESINS 3 L.S. Penn and €3. Wang

3.1 INTRODUCTION

Epoxy resin systems have achieved acceptance as adhesives, potting compounds, molding compounds and as matrices for continuous fil- ament composites used in structural applications. In this chapter, we discuss epoxy resins in their role as matrices in fiber compos- ites. In this role, they possess several advantages over other types of polymers. The main advantages are:

0 inherently polar nature that confers excel- lent adhesion to a wide variety of fibers;

0 relatively low cure shrinkage that makes dimensional accuracy of fabricated struc- tures easier to obtain;

0 no volatile by-products of the curing reac- tion to cause undesired bubble or void formation;

0 crossIinked structure that confers excellent resistance to hostile environments, both aqueous and nonaqueous.

In addition to these advantages, epoxy resins have tremendous versatility because they can be formulated to meet a broad range of specific processing and performance requirements. To know how to take advantage of the formula- tion options, the engineer needs to have an elementary understanding of epoxy resin chemistry and structure-property relation- ships. This chapter attempts to provide that by presenting information about the resin system constituents, how they react together to form a

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

crossllnked network and how they lead to dif- ferent processing parameters and final properties in the formulated system. The chapter also describes the role of cure moni- toring and property evaluation in epoxy resin system technology.

3.2 GENERAL DESCRIPTION OF THERMOSETTING SYSTEMS

3.2.1 DEFINITIONS

According to common chemical practice, mol- ecules can be classified by the functional groups they contain. Thus, a molecule contain- ing the epoxide group (shown below) as part of its structure is called an epoxide, regardless of the remaining details of the molecule.

/O\ -c-c- I I

In practice, other types of molecules are added to the epoxide to formulate a thermosetting system, i.e. one that will undergo a curing reaction to harden into a rigid form. The con- fusing practice has been followed of referring to both the epoxide alone and the formulated system as 'epoxy resin'. One must determine which is truly intended by the context in which it is used. For the purposes of clarity in this chapter, we will use epoxide when refer- ring to the epoxide constituent alone and will

General descripfion of thermoseffing systems 49

use the term epoxy resin when referring to the uncured or cured formulated system.

3.2.2 THE THERMOSETTING (CURING) REACTION

The thermosetting reaction is the joining of many small molecules by chemical reaction to produce an extended network structure. Although this process is a polymerization, it is distinct from the type of polymerization that forms many individual long chains; the ther- mosetting reaction unifies all the constituent monomers into a single large molecule extend- ing to the boundaries of the material.

Epoxide molecules in the pure state at room temperature normally do not react with each other and can sit for years in a dry container without mutual reaction. The types of chemi- cals added to the epoxide to effect network formation fall into two categories: curing agents and catalysts. Curing agents, some- times called hardeners, are added in significant amounts to the epoxide and react with it to become a part of the crosslinked net- work. These curing agents can be aliphatic amines, aromatic amines, or anhydrides. Catalysts, on the other hand, are added in extremely small amounts to cause the epoxide molecules to react directly with each other, i.e. to homopolymerize. Sometimes the chemicals used as catalysts for homopolymerization can be used for another purpose; when added in small amounts to epoxide-curing agent mix- tures, they will accelerate the curing reaction. In this role they are called accelerators rather than catalysts.

In the sections below, we discuss the chem- ical reactions involved in network formation, both when the different curing agents are used and when homopolymerization occurs.

Amine curing

In amine curing agents, each hydrogen on an amine nitrogen is reactive and can open one epoxide ring to form a covalent bond 1-5.

When the amine nitrogen contains two hydro- gens, each reacts with a different epoxide ring. This scheme is shown in Fig. 3.1, where the developing network is evident. This scheme applies to both aliphatic and aromatic amine curing agents. The reaction between epoxide and amine produces a C-N bond, whose envi- ronmental resistance is good, but whose stability to elevated temperature is highly dependent on the adjacent molecular structure.

OH I

cH2-m-

- CH-CH2 CH,-CH-

- OH

/

OH I

CH-CH, \

OH I /N-R-y I

Fig. 3.1 Reaction scheme for the reaction between epoxide and amine curing agent. Each hydrogen reacts with an individual expoxide group. Thus the primary amine group acts bifunctionally and the secondary amine group acts monofunctionally. The epoxide group acts monofunctionally.

From the reaction scheme, it is obvious that the correct relative amounts of epoxide and amine curing agent must be used. If there is ,an imbalance, unreacted functional groups will be present and the full properties of a com- plete network will not be developed. The correct amounts by weight to combine are determined by computing the weight of cur- ing agent that contains one chemical equivalent of amine hydrogens and matching that with the weight of epoxide that contains one chemical equivalent of epoxide groups.

50 Epoxy resins

Anhydride curing functions as a catalyst to speed up the epox-

In anhydride curing agents, the anhydride groups themselves must be cleaved asymmet- rically to start the rea~t ion '~ ,~ . Initial cleavage is accomplished with the help of a small amount of accelerator, which is a chemical that

0 II 0-

I

NR3 (Accelerator)

C' / I + 0

0- I {c=o

L = o + /O\

0- I

f C = O

L = o +

I

CC'O

L = o + C c = O

'C= 0 +

0- 0- I I c= 0

I L o I

(c=o + L O +

0 0 I I

I I c= 0 c= 0

o-cH~-c------c-cH*-o

Fig. 3.2 Simplified reaction scheme for the reaction between epoxide and anhydride curing agent. After cleavage, each anhydride group reacts with two epoxide groups and each epoxide group reacts with two anhydride groups. Thus both the epoxide group and the anhydride group act bifunctionally.

ide-curing agent reaction. (Accelerators will be discussed later.) Figure 3.2 shows an idealized scheme, where the cleaved anhydride reacts with an epoxide ring carbon, opening the ring in the process. The negatively charged oxygen formed by the opening of the epoxide ring can proceed to react with a different anhydride group, perpetuating the reaction. In this ideal- ized developing network, each anhydride group is bifunctional, i.e. it links to two differ- ent epoxide molecules. In practice, the high temperatures required for anhydride cure, plus the presence of accelerator, provides conditions for some extent of epoxide homopoly- merization (described later) to take place, making the actual curing reaction much more complex than depicted in Fig. 3.2. The reaction between epoxide and anhydride produces pri- marily ester linkages, which have good stability to elevated temperatures and to most hostile environments except bases.

Not surprisingly, the correct amount of anhydride curing agent relative to the epoxide must be used to obtain a well developed net- work and the associated good properties. The correct amounts by weight to combine are esti- mated by examining the reaction scheme and computing the weight of curing agent needed to react completely with a given weight of epoxide. The simplified reaction scheme of Fig. 3.2 proposes that one anhydride group reacts with two epoxide groups and one epox- ide group reacts with two anhydride groups, making the number of anhydride groups con- sumed equal to the number of epoxide groups consumed in the reaction.

Catalytic curing (homopolymerization)

The remaining route to formation of a crosslinked network from epoxide molecules requires homopolymerization'4~7. This can be brought about if small amounts of certain Lewis acids or Lewis bases are added. These operate as true catalysts by initiating a self- perpetuating cationic (Lewis acid) or anionic

Constituents used in formulated systems 51

/O\ +

/O\

R3N -I- CH2-m- + R,NCH2-CH-a2- I 0-

+ CH2-CH- + R3NCH2- CH- CH2-

cH2-CH-

I

P I 0-

Fig. 3.3 Reaction scheme for the homopolymerization of epoxide. After ring opening, each epoxide group reacts with two other epoxide groups. Thus the epoxide group acts bifunctionally.

(Lewis base) polymerization. A simplified reaction scheme initiated by a Lewis base is shown in Fig. 3.3. Homopolymerization results in the formation of a network of ether

3.3 CONSTITUENTS USED IN FORMULATED SYSTEMS

3.3.1 EPOXIDES

Although many different types of epoxides are available, only a few are favored for use in matrices in fiber compositess-lO. These are

linkages, which has outstanding elevated-tem- perature stability and resistance to hostile environments.

shown in Table 3.1.

Table 3.1 Structures and characteristics of commonly used epoxides

Epoxy equivalent Viscosity at 25"C, Comments weighf, g/eq Pa s (cP)

171-177

180-188

185-200

Diglycidyl ether of bisphenol A (DGEBA)

3.5-5.5 (3500-5500)

6.5-9.6 (6500-9500)

10.0-19.0 (10 000-19 000)

450-550 Melting point 65-76°C (149-167°F)

May crystallize on storage. Example: DER 332 (Dow). Contains small amount of higher polymer to prevent crystallization. Examples: DER 330 (Dow) Epon 826 (Shell). Contains small amount of higher polymer to prevent crystallizalion. Example: Epi-Rez 510 (Hi-Tek Polymers), DER 331 (Dow), and GY 6010 (Ciba). n = 2; used in prepregs. Examples: Epon 1001 (Shell) and DER 661 (Dow).

52 Epoxy resins

Table 3.1 Continued

Epoxy equivalent Viscosity at 25"C, Comments weight, g/eq Pa s (cP)

Diglycidyl ether of bisphenol F (DGEBF)

/O\ 0 - CH2 -CH- CH2

/O\ CH2- CH- CH2- 0

158-165 5.0-8.0 (5000-8000)

Isomeric mixture that will not crystallize on storage. Example: GY 281 (Ciba).

Polyglycidyl ether of phenol-formaldehyde Novolac

172-179 1 .l-1.7 (1100-1700) Example: DEN 431 (Dow).

176-181 20.0-50.0 (20 000-50 000) Example: DEN 438 (Dow).

191-210 4.0-10.0 (4000-10 000) Example: DEN 439 (Dow).

at 52°C (126°F)

at 52°C (126°F)

at 52°C (126°F)

200

225

230

235

Polyglycidyl ether of o-cresol-formaldehyde Novolac

Melting point 35°C (95°F)

Used for high-temperature service: R represents chlorohydrins, glycols, and/or polymeric ethers. Molecular weight = 540. Example: ECN 1235 (Ciba). Same as above, but molecular weight = 1080. Example: ECN 1273 (Ciba). Same as above, but molecular weight = 1170. Example: ECN 1280 (Ciba). Same as above, but molecular weight = 1270. Example: ECN 1299 (Ciba).

Melting point 73°C (163°F)

Melting point 80°C (176°F)

Melting point 99°C (210°F)

Constituents used in formulated systems 53

Table 3.1 Continued

Epoxy equivalent Viscosity at 25"C, Comments weight, g/eq Pa s (cP)

N, N, N', N', - Tetraglycidyl methylenedianiline

117-133 10.0-15.0 (10 000-15 000) Used for prepregs. Example: MY 720 (Ciba).

Triglycidyl p-aminophenol

/O\ O-CH2-CH- CH2

I

/O\ I

CH2-CH-CH2 CH2-CH- CH2 /O\ / N\

95-107 0.55-0.85 (550-850) Used extensively for prepregs and adhesives. Example: MY 0510 (Ciba).

Conspicuously, these epoxides all contain aro- matic rings in their structures. Aromatic rings confer mechanical rigidity and thermal stabil- ity to the crosslinked network. It is also worth noting that some of the epoxides in the table have two epoxide functional groups, while others have three or four or more. Whether a network is developed by mixing the epoxide with a curing agent or is developed from epox- ide alone by catalytic homopolymerization, a large number of the molecules in a given for- mulation must be able to react with more than two other molecules in order to form a crosslinked network instead of merely form- ing linear chains.

3.3.2 AMINE CURING AGENTS

Table 3.2 presents commonly used amine cur- ing agents. The temperatures required to

achieve cure with amine curing agents fall in a wide range, from 25°C to nearly 200"C, depending on the chemical structure of the amine. The first five entries in the table are aliphatic amines, which can cure epoxides at room temperature or only slightly above. Aliphatic amine-cured systems also tend to have low glass transition temperatures, T, (the temperature at which the mechanical behavior changes from rigid to rubbery) and cannot be used in composites that will experience high temperature use.

Most other entries in the table are aromatic amines, whose ring structures confer solidity and mechanical rigidity. These amine curing agents require elevated temperature cure, but the networks they produce have high glass transition temperatures and are suitable for use in composites that will be exposed to ele- vated temperatures in service.

54 Epoxy resins

Table 3.2 Structures and characteristics of commonly used amine curing agents

Amine hydrogen Viscosity at 25°C (77"F), Comments equiv. weight, gleq Pa s (cP)

Diethylenetriamine (DETA)

HzN-CH2-CH2-NH- CH2-CH2 -NH2

20.6 0.00554.0085 5.5-8.5)

Available from Dow as DEH 20.

24.4

Triethylenetriamine (TETA)

H2Nf (CH2)z-NH CH2-CH2 -NH;!

0.020-0.023 (20-23)

Available from Dow as DEH 24.

65

Diethylaminepropylamine (DEAPA)

CH3- CH2,

CH3- CH2 - (CHZ)3- NH2

50 Available from Union Carbide. (5000)

26-27

Tetraethylenepentamine (TEPA)

0.055 Available from Dow as DEH 26. (55)

77-82

Aliphatic polyether triamine (APTA)

H2C f-0 - CH2- CH-(CH3) NH2 I I

CH3 - CH2 - C - CH2 f-0 - CH2- CH-(CH,) NH2 Y

H2C f-0 - CH2- CH-(CH3) % NH2

0.072-0.080 x + y + z = 5.3. Available from Texaco as (72-80) Jeffamine T 403.

Constituents used in formulated systems 55

Table 3.2 Continued

Amine hydrogen Viscosity at 25°C (77"F), Comments equiv. weight, g/eq Pa s (cP)

50

27

4,4'-Methylenedianiline (MDA)

Melting point 89°C (192")

Available from Ciba as HT 972 and from Pacific Anchor as Ancamine DDM.

rn-Phenylenediamine (MPDA)

NH2 I

Melting point 60°C (140°F) Company.

Available from E.I. duPont de Nemours &

62

62

44' -Diaminodiphenylsulfone (DDS) 0

0

Melting point 170-180°C Used mainly in prepregs; yields good shelf life and

Ciba as HT 976. (338-356°F) high-temperature properties. Available from

3,3' -Diaminodiphenylsulf one

0

P- It . S

I1 0

Melting point 174178°C (345-352°F)

Used mainly in prepregs; reacts more slowly than 4,4' analog. Available from Ciba as HT 9720.

56 Epoxy resins

Table 3.2 Continued

Amine hydrogen Viscosity at 25°C (77"F), Comments equiv. weight, gleq Pa s (cP)

38

28

40% MPDA-60% MDA

1.50 Eutectic mixture. Available from UniRoyal (1500) as Tonox 60-40

Dicyandiamide (DICY)

Melting point 207-209°C Slow reacting. Used for prepregs. Available from (405408°F) Cytec Ind.

3.3.3 ANHYDRIDE CURING AGENTS

Table 3.3 presents commonly used anhydride curing agents. Their structures vary widely and some are liquid at room temperature whereas others must be heated to liquefy.

Elevated temperature, typically in the range 100-200°C, is required to achieve cure with anhydride curing agents. The glass transition temperatures of anhydride-cured systems are

high.

Table 3.3 Structures and characteristics of commonly used anhydride curing agents

Anhydride equiv. Melting point, "C ("F) Comments zueigkt, g/eq

Phthalic anhydride (PA)

148

0

130 (266) Available from Amoco.

Constituents used in formulated systems 57

Table 3.3 Continued

Anhydride equiv. Melting point, "C ( O F ) Comments weight, g/eq

Hexahydrophthalic anhydride (HHPA)

154 40 (104) Available from Pacific Anchor as Ancadride MHHPA and from Ciba as HT 907.

180

270

Nadic methyl anhydride (NMA) maleic anhydride adduct of methyl cyclopentadiene

Liquid at 25°C (77°F) (0.200 Pa s) 200 cP

Widely used for prepegs. Available from Pacific Anchor as Anhydride METHPA and from Ciba as HY 906.

Dodecenyl succinic anhydride (DDSA)

Liquid at 25°C (77°F) (0.200 Pa s) 200 cP

Available from Dixie Chemical and from Humphrey Chemical.

58 Epoxy resins

Table 3.3 Continued

Anhydride equiv. Melfing point, "C ( O F ) Comments weight, g/eq

Chlorendic anhydride (CAI

371 231-235 (448445)

Needs no cure accelerator, but is high melting and hard to handle; good fire retardant. Available from Velsicol.

193

Trimellitic anhydride (TMA)

,o

\\ 0

161 -1 64 Available from Buffalo Color. (322-327)

100 53 (127)

Maleic anhydride (MA)

0

\\ 0

Available from Amoco.

100

Succinic anhydride (SA)

/p

l0

120 (248) Available from Buffalo Color.

Constituents used in formulated systems 59

Table 3.3 Continued

Anhydride equiv. Melting point, "C ( O F ) Commen ts weight, g/eq

Methyltetrahydrophthalic anhydride

166 Liquid at 25°C (77°F) (0.06 Pa s) 60 cP

Available from Lindau.

161

3,3', 4,4' - Benzophenone-tetracarboxylic dianhydride (BTDA)

0 0 II 0

\\ o / c ~ c y & o \\

'C

0 //

0

221 (430) Used mainly in powder coatings; when used as minor component in fiber composite matrix, it improves high-temperature properties. Available from Allco.

3.3.4 CATALYSTS FOR HOMOPOLYMERIZATION

Table 3.4 presents some Lewis acids and Lewis bases that have been found effective as cata- lysts for homopolymerization of epoxides. Certain Lewis acids, such as boron trifluoride (BF,) produce rapid and very exothermic homopolymerization and need to be used in blocked form. BF,, when blocked with monoethyl amine to form the complex BF,MEA, is latent at room temperature and becomes active only above 90"C, the tempera- ture at which the complex separates. The epoxide homopolymerization that occurs above 90°C is rapid, but has controllable release of heat. Epoxides containing blocked Lewis acids as catalysts have been found to be

useful in prepregs that must be stored for some time without cure advancement prior to being used in fabrication of a structural com- ponent.

3.3.5 ACCELERATORS FOR CURING AGENT SYSTEMS

Some Lewis acids and Lewis bases can also be used as accelerators in epoxide-curing agent mixtures to speed up a sluggish reaction. They are added in small amounts, only a few weight per cent, empirically determined to give the best results. Most often they are used to speed up the curing reaction in epoxide-anhydride systems. Table 3.4 indicates which catalysts are also used as accelerators and lists additional chemicals used as accelerators only.

60 Epoxy resins

Table 3.4 Structures and characteristics of commonly used catalysts and accelerators

Amine hydrogen Melting point, "C ( O F ) Comments equiv. weight, g/e9

Benzyldimethylarnine (BDMA)

- Liquid at 25°C (77°F) (0.1 Pa s, 100 cP)

Lewis base used as an accelerator mainly for anhydride mixtures. Avalible from Ciba as DY 062.

2,4,6-Tris(dimethylaminomethyl)phenol

Liquid at 25°C (77°F) (0.3 Pa s, 300 cP)

Lewis base used as an accelerator for epoxide anhydride mixtures to provide room-temperature cure. Available from Rohm & Haas as DMP-30 and from Ciba as DY 064.

2-Ethyl-4-methylimidazole (EMI)

CH3- CH2 - C CH \ /

N H

Liquid at 25°C (77°F) (4-8 Pa s, 4000-8000 cP)

Lewis base used as an accelerator for epoxide- anhydride mixtures to provide long pot life and good elevated-temperature properties. Available from Air Products as EMI-24.

Constituents used in formulated systems 61

Table 3.4 Continued

Amine hydrogen Melting point, "C ( O F ) Comments equiv. weight, g/eq

Boron trifluoride-monoethylene amine (BF,MEA) F

I I

F -B:NHz- CH2- CH,

F 85-90 (185-194) Blocked Lewis acid; used as an accelerator for

epoxide DDS systems in high temperature service; provides latency. Available from ATOTech.

3.3.6 DILUENTS epoxide groups. Some of these diluents are

For some types of processing the viscosity of the uncured resin system needs to be lowered. When it is not advisable, as in the case of wet filament winding or hand lay-up, to use volatile solvents as diluents, reactive diluents must be used. Table 3.5 lists some acceptable diluents, all low viscosity liquids containing

monofunctional, i.e. have only one epoxide group, so they cannot form crosslinks in the network like bi-, tri- and tetrafunctional mole- cules do. However, by chemically attaching to the network, the reactive diluents become a permanent and stable part of it. Other diluents are bifunctional and will form crosslinks.

Table 3.5 Structures and characteristics of commonly used, commercially available epoxide reactive dilutents

Epoxy equivalent Viscosify at 25"C, Comments weight, g/eq Pa s (cP)

Butyl glycidyl ether (BGE)

/O\ CH3-(CH2)3 -0 - CH2- CH-CH2

130-149 0.002-0.003 Example: RD-1 (Ciba). (2-3)

215-235

Octyl, decyl glycidyl ether blend

/O\ CH3-((CH2)7 - 0 - CH2- CH- CH2

0.005-0.015 Example: Epotuf 37-057 (Reichhold) and Dy 027 (5-15) (Ciba) .

62 Epoxy resins

Table 3.5 Continued

Epoxy equivalent Viscosity at 25"C, Comments weight, g/eq Pa s (cP)

~~ . _ _ _ _ ~ ~ ___

p- t - Butyl phenyl glycidyl ether

220-245

150

170-190

CH3 /O\

0 - CH2 - CH- CH2 I -

0.015-0.30 Example: Epi-Rez 5014 (Hi-Tek Polymers). (1 5-30)

Phenyl glycidyl ether (PGE)

/O\ 0 - CH2 -CH-CH2

0.006 Example: Heloxy WC-63 (Wilmington Chemical). (6)

Cresyl glycidyl ether (CGE)

0.005-0.050 (5-50)

Less volatile than BGE. Example: Epotuf 37453 (Reichhold), and by DY 023 (Ciba).

Diglycidyl ether of 1,4 - butanediol (BDE)

12@140 0.010-0.025 Example: RD-2 (Ciba); not a pure compound. (1 0-25)

~- ~- ~~~~ ~~ -~ _____

General principles of formulation 63

Table 3.5 Continued

weight, g/e9 Pa S ( c ~ ) Epoxy equivalent Viscosity at 25°C’ Comments

~

13CL145

175-205

305335

76

Diglycidyl ether of neopentyl glycol

CH3

0.005-0.015 Example: AZ epoxy N (AZS Corp.); not a pure (5-15) compound.

Diglycidyl ether of polypropylene glycol

r 1

0.30-0.060 (30-60)

0.55-0.100 (55-100)

n = 4. Example DER 736 (Dow).

n = 9. Example DER 732 (Dow).

0.20 (20)

Vinyl cyclohexene dioxide

/O\ A CH - C H ~

O ’ W

Example ERL 4206 (Union Carbide).

3.4 GENERAL PRINCIPLES OF FORMULATION

Epoxy resin systems must be formulated on a rational basis and the chemical structure of the constituents forms this basis. Many detailed structure-property relationships can be reduced to rules of thumb and some of those we present in this section. The reader should use these with caution, because often a more detailed examination of the chemical structure reveals conflicting trends and experimentation is needed.

First, the presence of significant amounts of aliphatic segments in the chemical structure of

the cured network results in lower rigidity and lower Tg as compared to aromatic rings, or even saturated rings. Thus, aromatic amine- cured systems or homopolymerized systems both have high stiffness and high Tg. whereas an aliphatic amine-cured system will have a lower stiffness and lower T,.

In terms of processing and cure, the flexibil- ity and mobility of aliphatic segments imparts low viscosity (if a liquid) or low melting point (if a solid). Constituents whose structures are mainly aliphatic react rapidly at room temper- ature. Thus, an aliphatic amine-cured system is recommended over an aromatic amine-cured

64 Epoxy resins

system if low viscosity for processing and a in all but a few cases; the anhydride-cured sys- room temperature cure are needed1’,12. tem is degraded in strong base due to basic Constituents with aromatic ring structures hydrolysis of its ester linkages and both the react sluggishly or not at all at room tempera- anhydride-cured and the homopolymerized sys- ture13J4. Thus aromatic-amine cured systems tems are vulnerable to swelling by the strong require elevated temperature cures. solvent, trichloroethylene.

As cited earlier in the discussion on cure reactions, the relative amount of curing agent to epoxide is important in achieving a well- developed network. The formulations presented in Table 3.6 are approximately what would be used for some specific epoxide-curing agent formulations. The reader should verify the correctness of the formulations by comput- ing the appropriate weight ratios from the molecular structures given in Tables 3.1, 3.2 and 3.3.

Table 3.6 Formulations for selected epoxy resin systems

Constituents Epoxide Curing agent PbV Pbw‘

Diglycidyl ether of bisphenol A triethylene tetramine 100 14

Diglycidyl ether of bisphenol A rneta- phenylene diamine 100 16

Diglycidyl ether of bisphenol A hexahydrophthalic anhydride 100 90

Parts by weight

Finally, through the chemistry of the constituents and the network they form, the formulation duences the environmental resistance of the resin. Thus the formulation must be selected with the future environmental exposures in mind. Figure 3.4 compares the environmental resistance of four important types of epoxy resin systems: aliphatic amine-cured, aromatic amine- cured, homopolymerized epoxide (BF,MEA catalyst) and anhydride-curedI5. The excellent resistance of the crosslmked network is evident

3.5 PROCESSING CONSIDERATIONS IN EPOXY RESIN COMPOSITES

The goals of the processing procedures used to make a good quality fiber composite are to ensure that the resin forms a void-free contin- uous phase, surrounds each filament, is evenly distributed, is present in the desired amount relative to the fiber and is fully cured. The ease with which these goals can be achieved is highly dependent on the rheological proper- ties of the resin as it progresses through its cure and on the engineer’s ability to evaluate rheology and degree of cure throughout the processing cycle.

In the early years of epoxy resin technol- ogy, processing procedures were developed by a trial and error approach. This is still a viable approach, especially for engineers skilled in the art of thermoset composite pro- cessing. However, in recent years, rheologica116-18 and chemorheologica119~20 models that relate viscosity, rigidity and degree of cure to time and temperature have been developed. These models predict rheo- logical changes during cure and can serve as an aid to processing and cure cycle develop- ment.

3.5.1 PROCESSING VARIABLES EARLY IN CURE

One of the major concerns in the early stages of composite processing is resin viscosity. Sometimes, the freshly mixed, uncured resin formulation is inherently fluid enough to pen- etrate the fiber bundle and surround each filament. (Tables 3.1, 3.2 and 3.3 give viscosity values for several epoxides and curing agents.) Often, however, the viscosity of the uncured resin system is too high and must be

Processing considerations in epoxy resin composites 65

100

75

50

ae I 25 ln m C .- c1 VI

9 0 $ Sodium hydroxide, 50% 0 82°C (180°F) E+ L 0 C 0 C W

.- c

w

E c 0 100 E 2 ; 75

- 3 c

-

W U -

50

25

0 Distilled water 54°C (13CPF)

Sulfuric acid, 25% 82°C (180°F)

Hydrochloric acid, 25% 82°C (180°F)

Trichloroethylene 54°C (130°F)

Exposure conditions

i Sodium hypochlorite, 6%

54°C (130°F)

Fig. 3.4 Environmental resistance of common cured epoxy systems as indicated by flexural modulus reten- tion after environmental exposure Is.

Aliphatic amine-cured (TETA), Homopolymerized (BF,MEA),

Aromatic amine-cured (MPDA), @ Anhydride-cured (PA), mrm reduced to achieve the desired flow require- the simplest approach, can result in bubble or ments. The two major approaches to reducing void formation within the composite if the viscosity are thinning the mixture with low solvent cannot escape completely. This could viscosity organic solvents and adding low vis- be a problem for component fabrication by cosity reactive diluents, such as those wet filament winding, where layers contain- presented in Table 3.5. ing the freshly mixed epoxy resin system are

placed sequentially on top of one another. Thinning with organic solvents, although

66 Epoxy resins

Filament winding processors do not use sol- vents to reduce viscosity, rather selecting lower viscosity resins, reactive diluents, or diluting with heat. On the other hand, the use of small amounts of solvent to reduce resin viscosity during fabrication of prepreg (pre-impregnated fiber) presents no prob- lems, since prepreg is made in the form of single, thin-layer sheet, tape or tow from which solvent can vaporize easily and the prepreg is heat treated to eliminate solvent and to advance cure.

- 1100 A 'T

- m IS00 -

i - .- ln 1100 - 0 0 ln .- , 1100 -

1000 - eo0 - 800 - 700 - 600 - 500 - 400 - $00 - zoo - 100 - Illlllrllll 0 10 20 30 4 0 SO 60 70 80 90 IOC

DILUENT, 'Io

By contrast, reactive diluents, being them- selves epoxides, chemically react to become a permanent part of the crosslinked network.

ity and the amount of diluent added to a viscous epoxide. Ideally, the engineer wants to use just enough diluent to lower viscosity as needed without dramatically altering the properties of the final cured network.

The length of time that an epoxy resin for- mulation remains fluid is important. Liquid-like flow becomes impossible once the gelation stage, marked by an abrupt increase in viscosity, is reached. Time to gelation is called gel time, or sometimes pot life. Aliphatic amine curing agents produce pot lives of the order of minutes or a few hours, while aromatic amine curing agents produce pot lives of 24 h or more13,22,23. Anhydride cur- ing agents typically produce very long pot lives (e.g. two months for NMA) when mixed with epoxides. This is because, as already mentioned, the anhydride group is not very reactive with epoxides unless it is cleaved with the aid of an accelerator molecule. Once the accelerator is activated, the pot life of the mix- ture will be shortened to a few hours. Pot life can be controlled over a wide range by careful use of accelerators.

Each resin formulation has a unique chem- istry that imparts a set of processing variables with unique values. Standard laboratory test methods for processing variables that are important early in cure are described by the American Society for Testing and Materials, Philadelphia, Pennsylvania. ASTM numbers of the procedures for determining resin viscosity, gel time and melt flow are listed in Table 3.7.

Figure 3.5 shows the relation between L '1SCOS- '

Table 3.7 Laboratory test methods for measuring processing variables

Processing variable Reference -

Viscosity

Melt flow

Fig. 3.5 Viscosity versus per cent of diluent in epox- ide-diluent mixturesz1. The epoxide is DGEBA and the diluent is BDE (diglycidyl ether of 1, 4-butane diol).

ASTM D2393 ASTM D2471

ASTM D3795, ASTM D4473 time (pot life)

Processing considerations in epoxy resin composites 67

1.24

1.22

1.20

1.18-

1.16

1.14-

\ El s c .- v)

3.5.2 MONITORING OF CURE

Once the cure is underway in an epoxy resin system, all of the properties of the system change rapidly until the final crosslinked net- work is reached. Not only is flow decreasing and rigidity increasing, but all other properties (electrical, chemical, optical, etc.) are chang- ing. When the curing epoxy resin system is subjected to temperature and pressure changes, the resin response will be characteris- tic of its degree of cure at that moment. Cure cycle events, such as temperature and pressure changes, need to be carefully timed with this in mind.

The degree of cure can be defined in terms of any one of a large number of chemical or physical (including mechanical) properties that change continuously during the curing reac- tion and reach a constant value at end of the cure. Evaluation of the degree of cure is usually based on one of these properties and full cure is then defined as the point at which this selected property reaches a constant value.

Originally, the only available methods for evaluating cure were off-line laboratory meth-

- + -

+ $$ + s t + + $ + + +

-

ods where samples were taken and tested at intervals in the cure cycle. The time needed to develop one data point depended on the par- ticular laboratory method. Off-line testing has been made more convenient with the develop- ment of continuous monitoring techniques (i.e. data points in real time) used on small dedicated specimens of resin or prepreg. Recently, real time, in situ methods where cure can be monitored in the composite structural component itself have been developed.

Traditional off-line methods

Intermittent off-line methods include chemical titration of the unreacted epoxide groups pre- sentz4jz5, specific gravity to measure densificationzh and differential scanning calorimetry to measure the residual cure e ~ o t h e r m * " ~ ~ ~ ~ ~ * . Figure 3.6 shows a plot of spe- cific gravity data obtained on specimens cured for increasing lengths of time at a single tem- perature. Figure 3.7 shows differential scanning calorimeter scans for two epoxy resin specimens of the same formulation, but with different degrees of cure.

Fig. 3.6 Specific gravity compared with cure time for an aromatic amine-cured epoxy system cured at 120°C 26. Volume reduction (densification) during early network formation is rapid and levels off as cure nears completion.

Exotherm A

Energy calls

1 Endotherm

Fig. 3.7 Differential scanning calorimeter scans for an epoxy resin system28. Scans for two different speci- mens, each having a different original degree of cure, are shown, with scan A displaced upward from scan B for graphical visibility. The height of the residual cure exotherm is inversely related to the original degree of cure. The higher exotherm peak in B indicates an original degree of cure lower than in A.

. I I I I I I 1

B

. I I I I I I I

Continuous off-line methods include infrared s p e c t r o s ~ o p y ~ , ~ ~ , ~ ~ , parallel plate-type bulk diele~trornetry~’,~~ and dynamic mechanical spe~trometry~~”~. Figure 3.8 shows the results of infrared monitoring for five different neat resin specimens, each cured at a different tem- perature and Fig. 3.9 shows data obtained by bulk dielectrometry.

The off-line methods, real time or not, are useful for developing a cure schedule for a new epoxy resin formulation, for optimizing processing variables and for quality control of incoming resins or prepregs. They have also been used successfully for the development of mathematical models of cure kinetics. However, because they are off-line, they can- not be used for process control.

Fig. 3.8 Degree of cure compared with time for an aromatic amine-cured epoxy systemz8. Data for five specimens, each cured at a different temperature, are presented.

Property data for cured epoxy resin systems 69

Decreasing molecular dipole mobility

Temperature profile

I \

increasing molecular dipole mobility ;re elevation

--- ----- No further I-

cure at temperature

I 1 I

200

- t I I I -

- % r C 0 c m v)

.- n .- .- v)

150

0

I

w 100 E F P E

50

0

I I f 1 f f

.1 I 1

0 2 4 6 8 Time - h

Fig. 3.9 Bulk dielectrometry data compared with time for an aliphatic amine-cured epoxy system28. Dissipation factor (dashed line) is inversely related to rigidity in the developing network. The applied tem- perature is shown by the solid line.

Modern in situ methods In situ methods for cure monitoring are real time methods that require sensors small enough to implant and leave in the composite itself. When the information from in situ mon- itoring is used as continuous input to an appropriate process model, it can be used in process control loops that adjust the process- ing conditions automatically.

One example of an in situ method is low fre- quency dielectrometry using a very small assembly of interdigitated electrodes called a fringe field sensor. This technique measures changes in ability of permanent dipoles within the resin chemical structure to align themselves with the applied oscillating electric field and also measures changes in the mobility of ions present as impurities in the resin3941. Both of these quantities correlate with resin viscosity in the early stages of cure and with mechanical rigidity in the later stages of cure39,40,42.

Other examples of in situ monitoring are based on fluorescence spectroscopy of tag molecules in the resin43,44 and on infrared45,46 and Raman47 spectroscopies of the resin mole- cules themselves. In these methods, optical fibers are the sensors that transmit the appro- priate wavelength of light into the curing resin and also transmit the spectral information back The spectral changes relate directly to the chemical changes that occur as the cur- ing reaction progresses to completion.

3.6 PROPERTY DATA FOR CURED EPOXY RESIN SYSTEMS

Property data for cured, unreinforced epoxy resin systems are needed for two purposes. First, they are useful when selecting the best fiber and matrix combination for a particular application. Resin system choices can be rapidly narrowed down to a few alternatives

70 Epoxy resins

when comparisons of key properties are made from existing data tables or manufacturers’ data sheets.

Second, epoxy resin data are required in micromechanics computations of composite properties. Elastic constants, thermal expansion coefficients, moisture absorption coefficients,

Table 3.8 An aliphatic amine-cured epoxy resin system, room-temperature curable12 28

and many other properties of the composite can be computed in advance, if one has the corre- sponding values for the fiber and the matrix.

Tables 3.8, 3.9 and 3.10 present property data for three major resin systems: aliphatic amine-cured, aromatic amine-cured and anhy- dride-cured. Property data for commonly used

Resin system constituents Parts by weight 100 45

Epoxy: DGEBA, eg., DER 332 (Dow) Curing agent: APTA, e.g. Jeffamine T-403 (Texaco)

Cure cycle: 16 h at 60°C (140°F) for improved properties over room temperature cure

Viscosity at 25°C (77°F) 0.8 Pa s (800 cP)

Density of cured resin at 25°C (77°F) 1.16 g ~ m - ~

Volumetric shrinkage After gelation After cure

4.4% l’

4.4%

Water absorption, wt. gain after 2 h in

Impact strength (Izod notched bar test)

Shear properties Failure stress Modulus of elasticity

Modulus of elasticity

boiling water 0.75% a

11.0 J m-’ of notch

61 MPa (8.85 ksi) 1.27 GPa (184 ksi)

3.24 GPa (470 ksi) Tensile properties

120 I I I I Cures: 16

r 16 h @ 60°C 4

Heat distortion temperature at

Coefficient of linear thermal expansion

Average specific heat from

Coefficient of thermal conductivity

1820 kPa (264 psi)

from 298 to 374 K 6 6 x “C-’

62°C (144°F)

286 to 367 K 1 . 7 5 ~ 1 0 ~ J k g - l K ~ ~

At 298 K 0.133 W m-’ K-’ a

At 318 K 0.174 Wm-’ K-’ a

At 336 K 0.210 W m-l K-’

Compressive properties Modulus of elasticity

3.48 GPa (504 ksi)

._ m Y

0 0.8 1.6 2.4 3.2 4.0

Compressive strain %

0 0 1 2 3 4 5

Tensile strain %

~~ -~ ~~~ __

a Cured for 24 h at 60°C (140°F) + 24 h at 77°C (171°F).

Property data for cured epoxy resin systems 71

Table 3.9 An aromatic amine-cured epoxy resin systemI3

Resin system constituents Parts by weight 100 25 29

Epoxy: DGEBA, e.g. Epon 826 (Shell) Diluent: BDE, e.g. RD-2 (Ciba-Geigy) Curing agent: MDA-MPDA eutectic, e.g. Tonox 6040 (UniRoyal)

Cure cycle: 3 h at 60°C (140°F) + 2 h at 120°C (248°F)

Viscosity at 25°C (77°F)

Time to reach 2.0 Pa s Gel time for a 30-g mass at

Density at 25°C (77°F)

25°C (77°F)

Uncured Cured

Volumetric shrinkage After gelation After cure

Tensile properties Modulus of elasticity

1.2 P a s (1200 CP)

6 h

23 h

1.15 Mg m-3 1.21 Mg m-3

3.7% 5.4%

2.68 GPa (389 ksi)

Water absorption, wt. gain

Glass transition temperature

Heat distortion temperature at

after 6 h in boiling water 0.93%

130°C (266°F)

121°C (250°F) 1820 kPa (264 psi)

Coefficient of linear thermal expansion

Specific heat At 363 K At 424 K

from 298 to 3755 K 6.81 x 10-5oc-1

1.54 x lo3 J kg-' K-' 1.71 x lo3 J kg-'K-'

Coefficient of thermal conductivity

100

80

h 60 H

E 5 40

VI In

20

0

At 325 K At 356 K

0:243 W m-' K-' 0.244 W rn-' K-'

I I I I - At 389 K 0.256 W m-'K-' 14

Shear properties Failure stress 52 MPa (7.54 ksi)

._ Maximum stress 111 MPa (16.1 ksi)

VI Modulus of elasticity 2.9 GPa (420 Ksi) E - 6 G

- 4

- 2

- 10 Compressive properties

VI - 8 % Strain at maximum stress 8.0 %

0 2 4 6 a i o - Strain YO

epoxy resin formulations are often available from resin suppliers. Data for new or unusual formulations must be generated by the user. Whether the data are generated by the resin supplier or the user, it is important that stan- dard test procedures be followed. This will ensure that the resin systems can be compared on an equal basis. Where standardized test

procedures exist, e.g. as from the American Society for Testing and Materials, they should be followed. Where they do not exist, litera- ture references are helpful. Table 3.11 lists some commonly tested properties and the standard methods (American Society for Testing and Materials) describing the tests.

72 Epoxy resins

Table 3.10 An anhydride-cured epoxy resin systemI5

Resin system constituents Parts by weight 100

Curing agent: NMA 90 Accelerator: BDMA 1

Epoxy: DGEBA, e.g. Epon 828 (Shell)

Cure cycle: 3 h at 120°C (248°F) + 24 h at 150°C (302°F)

Viscosity at 27°C (81°F) Time to reach 100 Pa s (1000 cP) Pot life of a 500 g mass at 23°C (73°F) Heat distortion temperature at 1820 kPa (264 psi)

Solvent absorption, wt. gain

1.78 Pas (1780 cP) 556 days 4-6 days

121°C (250°F)

After 24 h in boiling water After 3 h in boiling acetone

0.67% 1.9%

Tensile properties At 23°C (73°F) At 100°C (212°F) Maximum stress 72.4 MPa (10.5 ksi) 46.2 MPa (6.70 ksi)

Modulus of elasticity 1.38 GPa (200 ksi) Strain at maximum stress 2.7% 7.2%

3.45 GPa (500 ksi)

Table 3.11 Standard test methods for cured epoxy resin systems

Property ASTM

Physical and chemical properties:

Standard Method ___

Specific gravity D792 Chemical resistance D543 Water absorption D570 Light and water exposure D1499

Volume resistivity D257 Surface resistivity D257 Dielectric strength D149 Dielectric breakdown voltage D149 Permittivity, dielectric constant D150 Dielectric loss D150

Heat deflection temperature D648 Glass transition temperature D4065 Coefficient of linear thermal expansion D696 Coefficient of thermal conductivity C177

Tensile modulus and strength D638 Compressive modulus and strength D695 Flexural modulus and strength D790 Impact resistance D256

Storage modulus D4065 Loss modulus D4065 Transition temperature D4065

Electrical properties:

Thermal properties:

Mechanical properties:

Dynamic mechanical properties:

References 73

REFERENCES 17. Lee, W.I., Loos, A.C. and Springer, G.S. ’Heat of - - reaction, degree of cure and viscosity of Hercules 3501-6 resin,’ J. Compos. Mater., 1. Lee, H. and Neville, K. Handbook ofEpoxy Resins,

Classical Re-issue, McGraw-Hill, New York, 1982.16.510-520. , , 1982. 2. Goodman, S’H. ‘Epoxy Resins’y ch’ in

Handbook of Thermoset Plastics, (S.H. Goodman,

Jersey, 1986, pp. 132-182.

18. Springer, G.S., ‘Compaction and consolidation of thermoset and thermoplastic composites,’ Compos. Polym,, 1990, 3, 155-175,

ogy of an amine-epoxy system of the type used in advanced composites,’ J. Compos. Mater.,

Ed.), Noyes Publications~ Park New 19. Mijovic, J. and Ott, J. ’Modeling of chemorheol-

3. Tanaka, Y. and Bauer, R.S. ’Curing Reactions,‘ ch.3 in Epoxy Resins: Chemistry and Technology, 1989,23,163-185.

Dekkerr In‘., New 20, Mijovic, J. and Wang, H,T, ‘Cure kinetics of neat 1988, pp. 285463. and graphite-fiber-reinforced epoxy formula-

4. Fischerf M.y F. and Schmid7 R’ tions,’ J. Appl. Polym. Sci., 1989,37,2661-2673. ’Struktureller Aufbau and Physikalisches 21. Product data sheet for Araldite RD-2, Ciba- Verhalten venetzter Epoxiharze,‘ Makromol. Chem., 1980,181,1251-1287.

resins: model reactions of curing with amines,’ J.

6. Fisch, W. and Hofmann, W. ’ m e r den

Geigy, Ardsley, New York, 1979. 22. Penn, L.S. and Chiao, T.T. ‘A long pot-life epoxy

the 7th National SAMPE Technical Conference,

1975, vel. (1975), pp. 177-187.

5. Dusek* R. and B1ehar M. ‘Curing Of system for filament winding,’ in Proceedings of

POlym. SCi., Polym. Chem. Ed., 1977~15,2393-2400. Albuquerque, N~~ ~ ~ ~ i ~ ~ , October 14-16,

Hartungsmeehanismu. der AthexY1i*arze,’ J. 23. Chiso, T.T., Jessop, E.S. and Perm, L.S., Polym. Sci. 1954,12,497-502. ‘Screening of epoxy systems for high perfor-

mance filament winding applications,~ in 7. Morgan, R.J. and Mones, E.T. ’Cure reactions, netWork structure, and Proceedings of the 7th National S M P E Technical diamine sulfonecured tetraglycidyl Conference, Albuquerque, New Mexico, October 4,4‘-diamino methane epoxy matri- 1&16,1975, Val. 7 (1975), pp. 167-175.

response Of

ces,’ J. Appl. Polym. Sci., 1987,33,999-1020.

Corporation, Hawthorne, New York, 1990. 9. Epoxy Resin Manual: Industry Edition, Dow

Chemical Company, Midland, Michigan, 1988.

Resins and Epon Curing Agents, Shell Chemical Company, Houston, Texas, 1990. 26. Chou, C.T. and Penn, L.S. ’Mechanism by which

11. Chiao/ T.T.r JessoP, E.S. and NeweYf H.A. ‘A an orthocarbonate reduces residual stress in a moderate- temperature-curab1e epoxy for composite,’ J. Compos. Mater., 1992,26,171-184. advanced composites,’ SAMPE Quart., 1975, 6, 27. Carpenter, J.F. and Bartels, T.T., 3842. ’Characterization and control of composite

prepregs and adhesives,’ in Proceedings ofthe 7th ture-curable epoxy for advanced fiber National SAMPE Technical Conference,

24. Jay, R.R. ’Direct titration of epoxy compounds

25. Jahn, H. and Goetzky, P. ’Analysis of epoxides and epoxy resins,, ch. 13 in E~~~~ ~ ~ ~ i ~ ~ : Chemistry and Technology, (C.A. May, Ed.),

1049-1087.

8. Resins and Hardeners Product Guide, Ciba-Geigy and Aziridines,r ~ ~ ~ 1 , (-hem, 1964, 36, 667468.

10. Specification Guide for Epon and EpOnol Epoxy Marcel Dekker, Inc. New Y’rk, 1988, pp.

12. Chiao, T.T. and Moore, R.L. ‘A roOm-tempera-

composites,’ 29th Ann‘ ‘ O n f . I Albuquerque, New Mexico, October 14-16, PlasticslComposites Institute, SPI, Washington, 1975, vel, (1975), pp. 43-52. D.C., February 5-8,1974, section 16-B. 28. Penn, L.S. and Chiao, T.T. ‘Epoxy resins,’ ch. 15

in ~ ~ n d b o o k of Composites, (George Lubin, Ed.), 13. Chiao, T.T., Jessop, E.S. and NeweY H.A. ‘An epoxy system for Quart., 1974, 6,28-32. 57-88.

Van Nostrand Reinhold, New York, 1982, pp.

14. Product data sheets, Ciba-Geigy, Hawthorne,

15. Epoxy Resins for Casting, Shell Chemical

29, Damenberg, H. ‘Determination of functional groups in epoxy resins by near infrared spec- troscopy; SPE pans., 1963,3, 78-88.

30. Morgan, R. ’Structure-property relations of epoxies used as composite matrices,’ Adv. Polym, sei,, 1985, 72, 143.

New York, 1993.

Company, New York, 1967. 16. Springer, G.S., ’Resin flow during the cure of

fiber reinforced composites,’ J. Compos. Mater., 1982,16,400410.

74 Epoxy resins

31. Yalof, S. and Wrasidlo, W. 'Crosschecking between dielectric measurements, DTA, and other methods of thermal analysis in research and production,' J. Appl. Polym. Sci., 1972, 16,

32. Delmonte, J. 'Electric properties of epoxy resins during polymerization,' J. Appl, Polym. Sci. 1959,

33. Lewis, A. 'Dynamic mechanical behavior dur- ing the thermosetting curing process,' SPE Trans., 1963, 3,201-212.

34. Kreahling, R.P. and Kline, D.E. 'Thermal con- ductivity, specific heat, and dynamic mechanical behavior of diglycidyl ether of bisphenol A cured with m-phenylene diamine,' J. Appl. Polym. Sci., 1969, 13,2411-2425.

35. Arridge, R. and Speake, J. 'Mechanical relax- ation studies of the cure of epoxy resins: 1. Measurement of cure,' Polymer, 1972, 13, 443449.

36. Kline, D.E. 'Dynamic mechanical properties of epoxy resins during polymerization,' J. Appl. Polym. Sci., 1960,4, 123.

37. Babayevsky, P. and Gillham, J. 'Epoxy ther- mosetting systems: dynamic mechanical analysis of the reactions of aromatic diamines with the diglycidyl ether of bispenol A,' J. Appl. Polym. Sci., 1973,17, 2067-2088.

38. Enns, J.B. and Gillham, J.K. 'The time-tempera- ture transformation (TTT) cure diagram: modeling the cure behavior of thermosets,' J. Appl. Polym. Sci., 1983,28,2567-2591.

39. Senturia, S.D. and Sheppard, N.F. 'Dielectric analysis of thermoset cure,' Adv. Polym. Sci., 1986, 80, 147 .

2159-21 73.

2, 108-113.

40. Day, D.R., Lewis, T.J., Lee, H.L. and Senturia, S.D. 'The role of boundary layer capacitance at blocking electrodes in the interpretation of dielectric cure data in adhesives,' J. Adhesion,

41. Micromet Instruments, Inc., Cambridge, Massachusetts, Technical literature, 1991.

42. Ciriscioli, P.R. and Springer, G.S. 'Dielectric cure monitoring - a critical review,' S A M P E J., 1989, 25, 3542.

43. Sung, C.S.P., Pyun, E. and Sun, H.L. 'Characterization of epoxy cure by UV-visible and fluorescence spectroscopy: azochromic labeling approach,' Macromolecules, 1986, 19,

44. Sung, C.S.P. and Mathisen, R. 'Cure characteri- zation of an epoxy network by fluorescence behavior of trans-diaminostilbene,' Polymer,

45. Compton, D.A., Hill, S.L., Wright, N.A. et al. 'In situ FTIR analysis of a composite curing reac- tion using a Mid-Infrared Transmitting Optical Fiber,' Appl. Spectros. 1988,6,972-979.

46. Young, P.R., Druy, M.A., Stevenson, W.A. and Compton, D.A. 'In situ composite monitoring using infrared-transmitting optical fibers,'

47. Myrick, M.L., Angel, S.M., Lyon, R.E. and Vess, T.M. 'Epoxy cure monitoring using fiber-optic Raman spectroscopy,' S A M P E J., 1992, 28, 3742.

48. Mijovic, J., Kenny, J.M., Nicolais, L. and Pejanovic, S. 'Present and future trends in in- si tu monitoring of processing of advanced composites,' SAMPE J., 1992, 28, 3946.

1985,18,73-90.

2922-2932.

1987,28,941-945.

S A M P E J., 1989,25,11-16.