Reactive Polymers Fundamentals and Applications || Epoxy Resins

3

Epoxy Resins

Epoxy resins are formed from an oligomer containing at least two epoxidegroups and a curing agent, usually either an amine compound or a diacidcompound. A great variety of such resins is on the market. There are manymonographs on epoxy resins available.1, 2

3.1 HISTORY

N. Prileschajew discovered in 1909 that olefins can react with peroxybenz-oic acid to epoxides.3 Schlack claimed in 1939 a polymeric material basedon amines and multi functional epoxides.4 Castan∗, in the course of search-ing for dental materials claimed the preparation of bisphenol A diglycidylether (DGEBA).5, 6 A similar material, but higher in molecular weight,was invented by S. O. Greenlee.7 Epoxy resins came on the market around1947. The first major intended application was as coating material.

3.2 MONOMERS

3.2.1 Epoxides

Epichlorohydrin is the monomer used for the synthesis of glycidyl ethersand glycidyl esters. Epichlorohydrin (1-chloro-2,3-epoxypropane) is syn-thesized from propene via allyl chloride. A number of epoxides are shown

∗Pierre Castan, born in Bern 1899, died in Geneva 1985

139

140 Reactive Polymers Fundamentals and Applications

O

CH3

CH2 O C

O

O

H3C

O

O

O O

O

O O

Figure 3.1: Cycloaliphatic Epoxides

in Table 3.1. Reactive diluents, i.e. monofunctional epoxide compoundsare shown in Table 3.2. The curing of cycloaliphatic epoxides proceedseasily with anhydrides, but is too slow with amines. Synthetic proceduresfor including styrenic, cinnamoyl, or maleimide functionalities, into cyclo-aliphatic epoxy compounds, have been described.8

3.2.1.1 Epoxide Equivalent Weight

The equivalent weight of the epoxide used is an important parameter forthe amount of curing agent needed. The common method to determine theequivalent weight is the titration procedure with HBr in glacial acetic acid.However, a method for the determination of the epoxide equivalent weightin liquid epoxy resins using proton nuclear magnetic resonance (1H-NMR)spectroscopy has been described.9

3.2.2 Phenols

Bisphenol A is the most important ingredient in standard epoxy resins. It isprepared by the condensation of acetone with phenol. The latter two com-pounds can be prepared in the Hock process by the oxidation of cumene.

Phenolic products are shown among others in Table 3.3 and Figure3.2. The hydroxyl and amino functions are epoxidized with epichlorohy-drin.

Epoxy Resins 141

Table 3.1: Epoxides

Epoxide Remark/Reference

Epichlorohydrin Used for the formation of glycidylethers and esters

Butadiene diepoxide1,4-butanediol diglycidyl ether(1,4-BDE)

10

Glycerol diglycidyl ether1,3-Didodecyloxy-2-glycidyl-glycerol

Amphiphilic polymers, for potentialuse as emulsifiers and solubilizingagents11

Poly(butadiene) epoxides FlexibleVinylcyclohexene epoxide Both with vinyl and epoxy functionStyrene oxide ( = ethenylphenyloxir-ane)

Both with vinyl and epoxy function12

Glycidyl methacrylate (GMA) Both with vinyl and epoxy functionEpoxidized linseed oil 13

Epoxy methyl soyate 14

Epoxy allyl soyate 14

Vernonia oil Naturally epoxidized,E-12,13-epoxyoctadeca-E-9-enoicacid esters15–17

Triglycidyl isocyanurateTriglycidyloxy phenyl silane Flame retardant18

2-(6-Oxid-6H-dibenz[c,e][1,2]oxa-phoshorin-6-yl)-1,4-benzenediol

Flame retardant19

3,4-Epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate

Coatings

2,3,8,9-Di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane

Dental applications20

Bis(3,4-epoxy-6-methylcyclo-hexylmethyl)adipate

Dental applications20

Epoxidized cyclololefins Multifunctional, c.f. Figure 3.1Fluoro-epoxides 21

Biphenyl-based epoxies Liquid crystalline, c.f. Figure 3.3Terephthaloylbis(4-oxybenzoic) acidDGEBA adduct

Liquid crystalline22

Bis[3-(2,3-epoxypropyl thio)phenyl]-sulfone

Optical applications23

4,4′-Dihydroxychalcone-epoxyoligomer



Table 3.2: Reactive DiluentsReactive Diluent Remark/Reference

Phenyl glycidyl ether (PGE)Styrene oxideAllyl glycidyl etherTetraethyl orthosilicate caprolactonediol adducts

Cationic curable coatings25

2-Hydroxy-4(2,3-epoxypropoxy)-benzophenone

Reactive photostabilizer for wood26

exo-3,6-Epoxy-1,2,3,6-tetrahydro-phthalimidocaproic acid

Polymers show anticarcinogenic ac-tivity27

exo-3,6-Epoxy-1,2,3,6-tetrahydro-phthalic anhydride

Polymers show anticarcinogenic ac-tivity28, 29

Table 3.3: Compounds for Glycidyl Functionalization for Epoxide Resins

Compound a Remark/Reference

Bisphenol A Standard resinsBisphenol FPhenol novolakNaphthyl or limonene-modified Bis-phenol A formaldehyde novolak

Improved mechanical properties, re-duced water absorption30

Cresol novolakTetrakis(4-hydroxyphenyl)ethane Increases crosslinking densityp-Aminophenolb Higher reactivity at amine curingAminopropoxylate 31

4,4′-Diaminodiphenylmethane b

Hexahydrophthalic acid c

1,3-Bis(3-aminopropyl)tetramethyl-disiloxane

32

Tetrabromobisphenol A For flame retardant formulationsBishydantoinIsocyanurate Powder coatingsCresol Reactive diluent1,4-Butanediol Reactive diluenta: Compounds are epoxidized at the hydroxyl function with epichlorohydrinb: Compounds epoxidized at the amino function with epichlorohydrinc: Compounds epoxidized at the carboxyl function with epichlorohydrin

Epoxy Resins 143

CH2 OHHO

Bisphenol-F

HO NH2

p-Aminophenol

CH CH

OH

OHHO

HO

Tetrakis(4-hydroxyphenyl)ethane

CH2 NH2H2N

4,4’-Diaminodiphenylmethane

C

CH3

CH3

OHHO

Bisphenol-A

CH2 CH2 CH2

OH OH OH

Novolac

Figure 3.2: Compounds for Epoxide Resins


3.2.3 Specialities

3.2.3.1 Hyperbranched Polymers

Hyperbranched polymers are highly branched macromolecules that are pre-pared through a single-step polymerization process.33 Many polymers ofthis type are also known as dendrimers, because their structure resemblesthe branches of a tree. Also, star-like and comb-like polymers belong to theclass of hyperbranched polymers. However, hyperbranched polymers arebuilt up from dendritic, linear, and terminal units. They can be synthesizedvia three routes:

1. Step-growth polycondensation of ABx monomers,2. Self-condensing vinyl polymerization of AB∗ monomers,3. Multibranching ring-opening polymerization of latent ABx mono-

mers.

The methods of synthesis available allow a wide variety of differentpolymer types. Further special properties can be imparted by suitable endcapping reactions. This type of polymer has unique properties that arecharacteristic for dendritic macromolecules, such as low viscosity, goodsolubility, and a high functionality.

Dendrimers are used in medical fields, as carriers of organic com-pounds. Hyperbranched polymers are easier to synthesize in large quan-tities and are used as tougheners, plasticizers, antiplasticizers and cur-ing agents.34, 35 Hyperbranched polymers (HBP) with hydroxyl terminalgroups can initiate curing by a proton donor-acceptor complex. In cur-ing a tetrafunctional epoxy resin, the activation energy is lower than in anepoxy system with linear polymers.36 Hyperbranched polymers stronglyenhance the curing rate due to the catalytic effect of hydroxy groups.37

The gel time increases with increasing functionality from diglycidyl etherof bisphenol A (DGEBA) to tetraglycidyl-4,4′-diaminodiphenylmethane(TGDDM).38 A hydroxyl-functionalized HBP reduced the gel time of theblends because of the accelerating effect of -OH groups to the epoxy curingreaction.

Star-like epoxy polymers can be rooted from polyhydroxy fullerenewith a cycloaliphatic epoxy monomer.39 Around 8 to 10 epoxy units canbe attached to the fullerene core.

The addition of small amounts of hyperbranched polymer to an ep-oxy system enhances dramatically its toughness. The critical strain energy

Epoxy Resins 145

release rate DGEBF resin can be increased by a factor of 6 by the additionof only 5% of hyperbranched polymer.40 At higher concentrations, a phaseseparation is indicated by two glass transition temperatures.41

In composite materials, resins modified by hyperbranched polymersallow higher volume fractions of fibers for producing void-free laminatesin comparison to unmodified resins.42

3.2.3.2 Liquid Crystalline Epoxide Resins

Initially a few technical terms concerning liquid crystals are recalled. Thereare textbooks on liquid crystals, e.g., that of Collings and Hird.43

Liquid Crystal. Liquid crystals were discovered by the Austrian chemistand botanist Friedrich Reinitzer, who found that cholesterol benzoate didnot melt into a clear liquid, but remained turbid. On further heating theturbid liquid turned suddenly clear. This transition point is now calledthe clearing point. For this reason, in addition to the common states ofaggregation, the liquid crystalline state was established. The term liquid

crystal goes back to the German physicist Otto Lehmann.Liquid crystals are formed mostly by rod-like molecules. They are

sometimes addressed as mesomorphic phases. Materials that can form suchphases are called mesogens. An ordinary fluid is called isotropic, i.e., itsproperties are independent of direction. A liquid crystal is orientated, orlikewise an anisotropic liquid. This means that the molecules are orientedpreferably in a certain direction. Such an anisotropic fluid is a nematic liq-uid crystal. A liquid crystal more similar to a solid is a smectic phase. Herethe molecules are arranged in layers, but within the layers the moleculeshave no fixed positions.

Polymers. Liquid crystalline polymers exhibit a number of improvedproperties in comparison with traditional plastics, in particular increasedelastic moduli at high temperatures, reduced coefficients of thermal ex-pansion, increased decomposition onset temperatures, and reduced solventabsorption.

Suitable epoxide monomers are based on biphenyl moieties.44 Mono-mers for liquid crystalline epoxide resins are shown in Figure 3.3. It isbelieved that micro-Brownian motion in the polymer chain is increasinglysuppressed as the mesogen concentration increases. This effect causes an


OO CH2CH2 C CH2CCH2

O

HH

O

OO CH2CH2 C CH2CCH2

O

HH

O

CH3

CH3

H3C

H3C

CH2CH2

OO

(CH2)nCCH2

O

H

(CH2)n

O

H

CH2C

Figure 3.3: Monomers for Liquid Crystalline Epoxide Resins

increase in the thermal decomposition onset temperatures, a decrease ofthe coefficient of thermal expansion, and a decrease in water absorption.

When the diglycidyl ether of bisphenol A is cured with sulfanilam-ide, a crosslinked network with liquid crystalline properties is obtained.45

Sulfanilamide has two different amine functions of unequal reactivity. Thiscauses the formation of a smectic phase when it is used as a curing agent.Polarized optical microscopy indicates that the epoxy monomer does notshow a liquid cristalline (LC) phase. Also a mixture of sulfanilamide anddiglycidyl ether of bisphenol A does not show LC properties. An isotropicliquid is formed above the melting point. However, when the reaction be-tween epoxy and amine proceeds, an LC texture is developed, which islocked in the crosslinked network by the nematic arrangement.

3.2.4 Manufacture

3.2.4.1 Epoxides

Epoxides can be manufactured by the epoxidation reaction, in particular

1. By direct oxidation,2. Via peroxyacids,3. In-situ epoxidation,

Epoxy Resins 147

4. By hypochlorite reaction, and5. By reaction with fluoro complexes.

Direct Oxidation. Olefins can epoxidized by oxidizing them in the va-por phase in the presence of a silver catalyst. The catalyst is activated byadding small amounts of dichloroethane to the reaction mixture. The directoxidation with oxygen is less important for the synthesis of epoxies usedfor epoxy resins, in favor of peroxyacids.

Certain Schiff bases that are attached on polymers allow the directoxidation of olefins. A polymer bound Schiff base ligand is prepared frompoly(styrene) bound salicylaldehyde and glutamic acid. With complexes ofthese catalysts, cyclohexene, 1-octene, 1-decene, 1-dodecene and 1-tetra-decene can be oxidized by molecular oxygen.46

Peroxyacids. Also, organic peroxides can serve as an oxygen source.Unsaturated fatty acids and their esters are epoxidized with peroxyaceticacid. Originally peroxybenzoic acid was used, which is highly selective.However, this reagent is comparatively expensive. Several other peroxy-acids have been investigated; they are in general less efficient. The reactionof olefins with peroxyacids is a single-step reaction.

Hydrogen peroxide itself is a rather poor epoxidation oxidant, how-ever, it is used to generate the peroxyacids that are much more active. Theperoxyacids are prepared by reacting hydrogen peroxide with the corre-sponding acid. The reaction is an equilibrium reaction. Highly concen-trated peroxyacids can be obtained by adding anhydrides, or removingthe water by azeotropic distillation. Another route to prepare peroxyacidsstarts from the anhydride and sodium peroxide, in presence of an acid ascatalyst. There should not be even traces of heavy metals present that causea loss in activity of the hydrogen peroxide.

For technical synthesis, peroxyacetic acid is used most frequently,because it has a high equivalent weight, a high efficiency for epoxidation,and a sufficient stability.

In-Situ Epoxidation. The peroxyacids can be regenerated during theepoxidation reaction with hydrogen peroxide. In this way all the hazardsin preparation and handling of the peroxyacids as such are avoided. Thereaction is heterogeneous and the peroxyacid has to be regenerated underconditions that would result in ring opening of the epoxide. Therefore, only


fast epoxidation reactions can be conducted utilizing the in-situ technique.For this reason, the most reactive peroxyacids are also selected. These arein particular the 3-nitroperoxybenzoic acid and 4-nitroperoxybenzoic acid.

Less reactive olefins must still be epoxidized with the peroxyacidsformed in a previous step. The ring opening of the epoxide with the acidformed from the peroxyacid can be minimized, allowing the phases utmostseparation. This means there should be only small agitation. On the otherhand, with certain solvent combinations the epoxide and the acid are mu-tually insoluble.

Hypochlorite. Partially fluorinated epoxides can be prepared by the oxi-dation of the corresponding olefins by NaOCl or NaOBr with phase trans-fer catalysts, e.g., methyltricaprylylammonium chloride.47 For example,hexafluoroisobutene reacts with the solution of sodium hypochlorite in wa-ter at 0 to 10°C giving the corresponding epoxide in a yield of 65 to 70%.

Fluoro Complex. By reacting diluted fluorine with aqueous acetonitrile,a complex HOF × CH3CN is formed. This complex is a very efficientoxygen transfer agent. It was shown to be useful to obtain various typesof epoxides that are otherwise difficult to synthesize. The products can beobtained in a single-step reaction with high yield.48

3.2.4.2 Glycidyl Ethers

In the simplest case a glycidyl ether for an epoxy resin is prepared by thereaction of bisphenol A (and epichlorohydrin), as pointed out in Figure3.4. In the first step DGEBA is formed, however, the condensation canproceed further. The reaction proceeds in two steps. First the epoxide ringis opened and then the ring is formed again, as shown in Figure 3.5.

Hydrogen chloride is evolved during the condensation and capturedwith caustic soda. The ring opening occurs such that the primary carbonatom is attacked and thus a 1,2-chlorohydrin (ΦCH2CH(OH)CH2Cl) isformed, as shown in Figure 3.5.

However in a side reaction the secondary carbon atom is also at-tacked and thus a 1,3-chlorohydrin (HOCH2CH(Φ)CH2Cl) is formed. Ifthe degree of dehydrochloration is not complete, then 1,2-chlorohydrin endgroups also may be present.

Epoxy Resins 149

CH2

O

CHCH2

CH2 CH

O

CH2 O

CH3

CH3

C O

CH2

OHCH

CH2

O O

CH3

CH3

C

n

C

CH3

CH3

OHHOCH2 CH

O

CH2 Cl +

Figure 3.4: Synthesis of an Epoxide Oligomer

O

OH

ClCH2CHCH2

O CH2CHCH2

O

-HCl

OH CH2 CH CH2 Cl

O

+

Figure 3.5: Formation of the Glycidyl Ether


Concerning the nomenclature, the situation is confusing. There aremany synonyms for the glycidyl ethers. The Chemical Abstracts name fordiglycidyl ether of bisphenol A (DGEBA) is 2,2′-[(1-Methylethylidene)-bis(4,1-phenyleneoxymethylene)]bis(oxirane), and there are some 12 othersynonyms of chemical names in use, besides the trade names.

We focus back to the main reaction. The newly formed epoxidegroups from the second step of the reaction may again undergo a reactionwith the phenolic group, and in the case of a bifunctional phenol, such asbisphenol A, the molecule grows. The degree of oligomerization (n−1 inFigure 3.4) can vary from 1 to approximately 25. The oligomer is liquid atroom temperature when n is smaller than one and becomes solid when n islarger than two.

The degree of polymerization that can be achieved depends on theratio of bisphenol A to epichlorohydrin. If epichlorohydrin is in excess,then the diglycidyl ether will be the main product. Impurities such as watercan substantially decrease the degree of polymerization by side reactions.Water reacts with epichlorohydrin to form a glycol.

3.2.4.3 Fluorinated Epoxides

The incorporation of fluorine enhances the chemical and the thermal sta-bility, the weathering resistance. Further the surface tension is lowered andthus the hydrophobicity is enhanced. Fluorinated epoxy monomers havebeen synthesized from fluorinated diols, such as 2,2,3,3,4,4,5,5-octaflu-oro-hexane-1,6-diol or 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluoro-dec-ane-1,10-diol by etherification with allyl chloride and subsequent oxida-tion of the allyl group.21 In UV curing, the monomers showed a higherreactivity than hexanediol diglycidyl ether.

The adduct of 2-chlorobenzotrifluoride and glycerol diglycidyl ether(DGEBTF) has been co-reacted with DGEBA using 4,4′-diaminodiphen-ylmethane as hardener.49 The introduction of the trifluoromethyl groupinto the chain of the epoxy resin results in an improvement of the dielec-tric and mechanical properties. Further the glass transition temperature islowered. The glass transition temperature of a pure DGEBA resin is 193°Cwhereas the glass transition temperature of the DGEBTF resin is 105 °C.This indicates that the introduction of fluorine enhances the mobility of thenetwork.

Epoxy Resins 151

Table 3.4: Toughening Agents for Epoxy Resins

Compound Class Reference

Poly(ethylene) phthalates 50

Poly(ethylene phthalate-co-ethylene terephthalate) 51

Hyperbranched aliphatic polyester 52, 53

Hyperbranched block copolyethers 54

Epoxidized soyabean oil 14, 55, 56

Copolymers of 2-ethylhexyl acrylate and acrylic acid 57

Methacrylic microgels 58

Terpolymers of N-phenylmaleimide, styrene and p-hydroxy-styrene

59

Triblock copolymer poly(styrene-b-ethylene-co-butene-b-styrene)

60

Poly(benzimidazole) 61

Poly(phenylene oxide 62

Silicon-modified polyurethane oligomers 63

Poly(dimethylsiloxane) polymersEpoxy-aminopropyltriethoxysilane 64

Poly(ether ether ketones) 65

Polyetherimides 66–68

Carboxylated polymers 69

Phenolic hydroxy-terminated polysulfones 70, 71

Liquid rubbers 72

Liquid rubbers carboxyl-terminated with poly(2-ethylhexylacrylate)

73–75

Poly(vinyl acetate) 76

Rubbery epoxy based particles 77

Glass beads 78, 79

3.3 SPECIAL ADDITIVES

3.3.1 Toughening Agents

Highly crosslinked epoxy resins are brittle. For various applications theyneed to be toughened. Toughening agents are summarized in Table 3.4. Ex-tensive literature on toughening of polymers is available.80–83 The tough-ening mechanisms of elastomer-modified epoxy systems are different fromflexibilized epoxy systems.

• Flexibilized epoxy systems reduce mechanical damage throughlowering modulus or plasticization; this allows stress to be relieved


through distortion of the material.84

• Elastomer-toughened epoxy systems in general maintain a largepercentage of the modulus and temperature resistance of the un-modified resin system. Stress is absorbed by cavitation of theelastomer particles and shear banding in the cavitated zone. Elas-tomer-toughened epoxy systems can tolerate a certain degree ofdamage by preventing growth of a crack. In this way the damagedregion remains local.85

When using thermoplastic-modified thermosets, compromises be-tween toughness and thermal stability associated with the rubber tough-ening of thermosets can be avoided. Another advantage of using the reac-tion induced phase separation procedure is that by the adequate selectionof cure cycles and initial formulations, a variety of morphologies can begenerated.

However, the fracture toughness is significantly improved with anonreactive thermoplastic, only, when bicontinuous or inverted phase struc-tures are formed. On the other hand, when the phase separation producesthermoplastic-rich particles that are dispersed in a continuous thermoset-rich matrix, little or no improvement of the fracture properties is obtained.This is mainly due to the poor adhesion between the phases.60

Basically, functionalized thermoplastics are capable of forming achemical linkage between the phases. This interphase bonding could im-prove the adhesion properties. However, the reactivity of the modifier canalso complicate the behavior and the control of the phase separation pro-cess.

3.3.1.1 Polyvinylic Compounds

Many polyvinylic compounds increase the flexibility and are used as tough-ening agents.

Poly(styrene). Blends of poly(styrene) (PS) with an epoxy monomer(DGEBA) and a tertiary amine, benzyldimethylamine (BDMA), are ini-tially miscible at 120°C. However, at very low conversions a phase separa-tion occurs. Here, at the cloud point, a sharp decrease of the light transmit-tance is observed. There is a significant difference between the refractiveindices of poly(styrene) and the DGEBA/BDMA solution. The refractiveindex of the epoxy network increases in the course of polymerization. Due

Epoxy Resins 153

to the continuous increase of the refractive index of the epoxy phase duringcuring, finally the refractive indices of both phases match, so that the finalmaterials at complete conversion appear transparent.86

Copolymers of Styrene and Acrylonitrile. In an epoxy system contain-ing tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and a 4,4′-di-aminodiphenylsulfone (DDS) hardener, blends with poly(styrene-co-acryl-onitrile) (SAN) up to 40 phr show complete miscibility over the entirerange.87 The glass transition temperature and the curing characteristicscan be modelled with various theories.88 In several systems autocatalyticcuring kinetics is observed.89–93

Copolymers of Phenylmaleimide, Benzyl methacrylate, and Styrene.

The vinylic compounds can be polymerized in-situ during the curing of theepoxy system.94 A suitable monomer system consists of three monomers:phenylmaleimide, benzyl methacrylate, and styrene. An advantage is thatby the admixing of the monomers the viscosity of the uncured resins dropssignificantly.

Graft Polymers of Ethylene/vinyl acetate to Methyl methacrylate. Agraft polymer synthesized by grafting ethylene/vinyl acetate (EVA) ontopoly(methyl methacrylate) thus resulting in a poly(ethylene-co-vinyl ac-etate)graft-poly(methyl methacrylate) exhibits a special performance. TheEVA moieties are initially immiscible in the uncured epoxide formulation.The PMMA moieties are initially miscible, however they separate duringcuring. Therefore, EVA-g-PMMA as modifier yields stable dispersions ofEVA blocks, favored by the initial solubility of PMMA blocks. So thePMMA acts initially as a compatibilizer for the epoxy moieties.95

Blends of Poly(methyl methacrylate) and Poly(ethylene oxide). Blendsof poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA)form a single phase in the melt. In solid mixtures of these polymers, phaseseparation is often observed. In blends of an epoxy resin with PMMA,PEO acts as a compatibilizer. The morphology of the resulting polymermixture may be changed dramatically by only small amounts of PEO. Thestiffness is controlled by the corresponding matrix of the ternary mixture,but both strength and fracture toughness are a function of the resultingmorphology.96


Poly(benzimidazole). The incorporation of poly(benzimidazole) into adifunctional epoxy resin matrix enhances both the glass transition temper-ature of the matrix and its toughness.61

Multilayer Particles. Multilayer particles of PMMA can be manufac-tured formed by emulsion polymerization. They consist of alternate glassyand rubbery layers. The outer layer bears glycidyl groups to allow a chem-ical bonding of the particles onto the cured resin. This type of toughen-ing particles is more effective than acrylic toughening particles or a liquidcarboxyl-terminated butadiene-acrylonitrile rubber.97

3.3.1.2 Polycondensates

Aromatic polyesters that are prepared from aromatic dicarboxylic acidsand 1,2-ethanediol are improving the toughness of bisphenol A diglycidylether epoxy resins. In particular, phthalic anhydride, isophthalic acid, tere-phthalic acid and 2,6-naphthalene dicarboxylic acid, and mixtures of thesecompounds are used. The aromatic polyesters are soluble in the epoxy re-sin without solvents and are effective modifiers for toughening the epoxyresins.50 The inclusion of 20% poly(ethylene phthalate) increases the frac-ture toughness of a cured resin by 130% with no loss of mechanical andthermal properties.51

Instead of 1,2-ethanediol, 1,4-cyclohexanedimethanol can be used toobtain poly(1,4-cyclohexylenedimethylene phthalate).98 Other flexibilityenhancers are polyamide, polyetherimides,66, 67 carboxylated polymers,69

phenolic hydroxy-terminated polysulfones,70 and fatty diamines.

Polyetherimide. In blends of an epoxy system of diglycidyl ether of bis-phenol A and nadic methyl anhydride, a phase separation occurs by theaddition of polyetherimide in the course of curing. The phase separation isnot observed without polyetherimide. By increasing the amount of poly-etherimide in the blends, the final conversion is decreased. This indicatesthat polyetherimide hinders the cure reaction between the epoxy and thecuring agent.99 Homogeneous structures are formed at low polyetherimideconcentration (5 phr).100

Poly(ether ether ketone). Poly(ether ether ketone) (PEEK) is a tough,semi-crystalline high performance thermoplastic polymer with good ther-

Epoxy Resins 155

O O C

OC

CH3

CH3H3C

PEEK-T

O C

O

C

O

O

C

OPEEK-C

Figure 3.6: Poly(ether ether ketone)s

mal and mechanical properties. Because of its semi-crystalline nature, it isdifficult to blend this material with epoxy resins.

Phenolphthalein poly(ether ether ketone) (PEEK-C) is miscible withTGDDM. Several methods, including dynamic mechanical analysis, Fouri-er-transform infrared spectroscopy, and scanning electron microscopy in-dicate that the cured blends are homogeneous. With increasing PEEK-Ccontent, the tensile properties of the blends decrease slightly. The fracturetoughness factor also decreases. This happens presumably due to the re-duced crosslink density of the epoxy network. Inspection of the fracturesurfaces of fracture toughness test specimens by scanning electron micro-scopy shows the brittle nature of the fracture for the pure epoxy resins andits blends with PEEK-C.101 A lower curing temperature favored the homo-geneous morphology in amine cured DGEBA+PEEK-C blends.102

In general, the processing of blends with PEEK should be easier, byusing PEEK with terminal functional groups and bulky pendant groups.However, poly(ether ether ketone) based on tertiary butyl hydroquinone(PEEK-T) showed a decreasing rate of reaction with increasing PEEK-Tcontent. The rate of reaction also decreased with the isothermal curingtemperature. This can be explained by the phase separation. As the curingreaction proceeds, the thermoplastic component undergoes a phase sepa-


ration. The separated thermoplastic could retard the curing reaction. Thedispersed particle size increases with the lowering of curing temperatureand with an increase in the thermoplastic material added.65 Poly(etherether ketone)s are shown in Figure 3.6.

Chain-extended Ureas. The synthesis of chain-extended ureas runs viaa two-stage process. In the first stage, a prepolymer with isocyanate endgroups is synthesized by the reaction of poly(propylene) glycol and tolu-ene diisocyanate. In the second step, the prepolymer is end-capped with di-methylamine or imidazole, to result in an amine-terminated chain-extendedurea (ATU) or an imidazole-terminated chain-extended urea, respectively,with flexible spacers.103 This type of toughening agent accelerates the cur-ing of the epoxide groups significantly because of the amino functions inthe molecule.

3.3.1.3 Liquid rubbers

The addition of elastomers to epoxy adhesives can improve peel strength,fracture resistance, adhesion to oily surfaces and ductility. Liquid rubbers,like carboxyl, amine, or epoxy-terminated butadiene/acrylonitrile rubbers,are used as toughening agents.72, 104 Liquid rubber modifiers are initiallymiscible with the epoxy resin. However, in the course of curing a phaseseparation takes place.

Carboxy-terminated butadiene/acrylonitrile copolymers (CTBN) areparticularly suitable because of their miscibility in many epoxy resins. Thecarboxyl group can react easily with an epoxy group. If a CTBN is notprereacted with an epoxy resin, the carboxylic acid groups can react duringcuring.

Solid acrylonitrile-butadiene rubbers (NBR), in particular with highcontent of acrylonitrile are also suitable tougheners.105 A high content ofacrylonitrile in the rubber imparts better compatibility between NBR andthe epoxy resin.

3.3.1.4 Silicone Elastomers

CTBN and amine-terminated butadiene-acrylonitrile elastomers (ATBN)lose the desired mechanical properties in the high temperature region andin the low temperature region. Silicone rubbers are superior in this aspect.However, silicone rubbers are completely immiscible with epoxy resins

Epoxy Resins 157

and cannot be used for this reason. The addition of a silicone grafted poly-(methyl methacrylate) is effective to stabilize the interface of the siliconerubber and the epoxy resin and helps to disperse the silicone rubber in theepoxide matrix in this way. The molecular weight of the silicone segmentstrongly affects the effectiveness of the compatibilizer. With increasingparticle diameter of the silicone the fracture toughness decreases and dropseventually below the unmodified resin.106

For a carboxyl-terminated dimethyl siloxane oligomer used as a rub-ber modifier, aramid/silicone block copolymers were used as compatibiliz-ers.107 The aramid-type blocks have phenolic groups on the aromatic rings.These groups can react with the epoxy resin to cause the compatibilization.

3.3.1.5 Rubbery Epoxy Compounds

Instead of liquid rubber, rubbery epoxy based particles obtained from analiphatic epoxy resin can be blended with another epoxy resin to act astoughening agents themselves.77 One of the limitations of epoxy-CTBNadducts is their high viscosity; however, there are also low-viscosity typesavailable.

3.3.1.6 Phase Separation

During curing of polymer resin blends, a phase separation occurs. Thephase separation can be characterized by

1. Small angle X-ray scattering,2. Light transmission,3. Light scattering,4. Transmission electron microscopy, and5. Atomic force microscopy.

The viscosity at the cloud point can have a strong effect on the finalmorphology and mechanical properties of the resin. The phase separationmechanisms are dependent on the initial modifier concentration and on theratio of the phase separation rate to the curing rate. The curing temperaturehas a strong effect on the extent of phase separation. Annealing allows thephase separation process to proceed further.67

The extent of phase separation depends on the cure cycle, as shownin blends of a standard epoxy resin and poly(methyl methacrylate). The


extent of phase separation can be diminished or suppressed by longer pre-curing times at lower temperatures, before the main curing is started.108

In addition, the phase separation can be controlled by the choice ofthe curing agents. In the case of poly(methyl methacrylate) as modifier, inan epoxy system, based on DGEBA some hardeners effect a phase separa-tion before gelation and others do not. For example, 4,4′-diaminodiphen-ylsulfone (DDS) and 4,4′-methylenedianiline (MDA) result in a phase sep-aration, but for 4,4′-methylene bis(3-chloro-2,6-diethylaniline) (MCDEA)no phase separation is observed.109

3.3.1.7 Preformed Particles

Preformed particles do not require phase separation and remain in thatshape in which they were added to the neat resin or composite. There-fore, these particles may be synthesized prior to the resin formulation andthen added to the thermosetting resin or formed in-situ, i.e., during theformulation of the resin, before the resin is cured.110

Prereacted urethane microspheres can be formed by dynamic vul-canization method in liquid diglycidyl ether of bisphenol A. The prereactedparticles are then added to an uncured epoxy resin system and cured. Themechanical and adhesion properties do not depend on any curing condi-tion of epoxy resin because the particles are stable, in contrast to a processwhere a phase separation occurs during curing.111

3.3.1.8 Inorganic Particles

In contrary to rubber, the toughening of inorganic particles is rather mod-est. However, the toughening by inorganic particles has an advantage in-sofar as it can also improve the modulus. Rubber toughens such that theincrease in toughness is accompanied at the expense of a decrease in themodulus.

The toughening of inorganic particles is explained by the crack frontbowing mechanism.112–114 A crack front increases its length by changingits shape when it interacts with two or more inhomogeneities in a brittlematerial. The inorganic particles inside the polymer matrix can resist acrack propagation.

When a crack propagates in a rigid particle filled composite, the rigidparticles try to resist. Because of this resistance, the primary crack front hasto change its direction between the rigid particles (bowing), thus forming

Epoxy Resins 159

a secondary crack front. The bowed secondary crack front now has moreelastic energy stored than the straight unbowed crack front. A crack frontstarts to bow out between particles, when it meets the particles.

Microcracking with debonding has been proposed as one of thetoughening mechanisms of glass bead-filled epoxies. Three types of mi-cro-mechanical deformations can be distinguished:78

1. Step formation2. Debonding of glass beads and diffuse matrix shear yielding3. Micro-shear banding

Among the micro-mechanical deformations, micro-shear banding isconsidered the major toughening mechanism for glass bead-filled epoxies.Step formation and combined debonding and diffuse matrix yielding aresecondary toughening mechanisms.79

3.3.2 Antiplasticizers

Antiplasticizers are additives for increasing the strength and modulus ofthe respective material. They act via strong interactions with the epoxidematrix. Epoxides with antiplasticizers characteristically115

1. Have a sufficiently high value of the glass transition temperatureas needed for the applications,

2. Exhibit a higher modulus and higher toughness around room tem-perature,

3. Exhibit a lower water uptake at equilibrium.

Antiplasticizers for epoxide resins are shown in Table 3.7. The addi-tion of the reaction product of 4-hydroxyacetanilide and 1,2-epoxy-3-phen-oxypropane (EPPHAA) to an epoxide resin increases the tensile strengthand the shear modulus of the cured system.116 The mechanism of antiplas-ticization can be formulated in terms of hindrance of the short-scale co-operative motions in the glassy state as a dynamic coupling between theepoxy polymer and the antiplasticizer molecule.117

In systems where the antiplasticizers have a poor affinity to the resin,a phase separation during curing occurs. The mobility of the constitutinggroups can be characterized by nuclear magnetic resonance techniques.118


O CH2 CH CH2 O NH CCH3

OOH

EPPHAA

O CH2 CH CH2 O CH3

OH

AM

O CH2 CH CH2 O

O

CH3

CH3

AO

Figure 3.7: Antiplasticizers for Epoxide Resins

3.3.3 Lubricants

In automotive, aviation and the related industries, there is a tendency to usemetallic materials with polymeric materials. For many parts in such appli-cations, good tribological properties are required.119 Fluorinated polymersare known as low friction materials. This property arises due to their lowsurface energies.

Fluorinated poly(aryl ether ketone) (12F-PEK) can be added to ep-oxy resins to improve the tribological properties. At low concentrationsof 12F-PEK, homogeneous systems are obtained after curing. Above 10%12F-PEK, a phase separation is observed. At still higher concentrations,an inversion of the morphology is observed. With fluoropolymer concen-trations of 10% 12F-PEK, a friction reduction of 30% can be obtained.120

3.3.4 Adhesion Improvers

Epoxy polyurethane hybrid resins are used in high strength adhesives. Elas-tomer-modified resins are used for adhesive formulations that cure underwater.

Epoxy Resins 161

Table 3.5: Reinforcing Materials for Epoxides

Material Remark/Reference

Glass fibers 121–123

Hollow glass fibers 124

Carbon fibers 125–127

Carbon nanotubes 128–131

Graphite 132–138

Aluminum 139, 140

BoronAluminum borate whiskers 141

PaperPoly(ethylene) fibersPolyaramid Fabric Low density and extremely high strengthCottonFlax 142

3.3.5 Conductivity Modifiers

To modify the thermal and electrical properties, thermally and electricallyconductive materials are added.

3.3.6 Reinforcing Materials

3.3.6.1 Composites and Laminates

Composites and laminates are made by reinforcing the polymers with con-tinuous fibers. About 1/4 of the epoxy resins are reinforced materials. Re-inforcing materials are shown in Table 3.5. Traditional composite struc-tures are usually made of glass, carbon, or aramid fibers. The advancesin the development of natural fibers in genetic engineering and in com-posite science offer significant opportunities for improved materials fromrenewable resources with enhanced support for sustainable applications.Biodegradable composites from biofibers and biodegradable polymers willserve to solve environmental problems.143

Often the surface of the fiber is chemically modified to increase theadhesion properties to the resin matrix. For example, glass fibers are coatedwith a silane coupling agent. The interfacial bonding between carbon fiberand epoxy resin can be improved by modification with poly(pyrrole). Poly-(pyrrole) (PPy) can be deposited on carbon fibers via the oxidation-poly-merization of pyrrole (Py) with ferric ions.144


Laminates are used for insulations. Impregnated sheets of wovenglass, paper, and polyaramid fabric or cotton are laminated in large presses.These sheets are used for printed circuit boards in the electronics industry.

3.3.6.2 Nanocomposites

Polymer nanocomposites, in particular polymer-layered silicate nanocom-posites, are a radical alternative to macroscopically filled polymers. Thepreparation of epoxy resin-based nanocomposites was first described byMessersmith and Giannelis.145 Extensive work on epoxy based nanocom-posites has been done and is reviewed among other polymers in the litera-ture.146, 147

Organoclays. Organoclays are used as precursors for nanocomposites inmany polymer systems. Usually montmorillonite is used for organoclays.Montmorillonite belongs to the 2:1 layered silicates. Its crystal structureconsists of layers of two silica and a layer of either aluminum hydroxideor magnesium hydroxide. Water and other polar molecules can enter be-tween the unit layers because of the comparatively weak forces betweenthe layers. Substitution of the ions originally in the layers by such ionswith different charges generates charged interlayers. The stacked array ofclay sheets separated by a regular spacing is addressed as gallery.

For true nanocomposites, the clay nanolayers must be uniformly dis-persed in the polymer matrix, to avoid larger aggregations. Small aggrega-tions are still addressed as nanocomposites, as intercalated nanocompos-ites, ordered exfoliated nanocomposites, and disordered exfoliated nano-composites.148 Originally, intercalation was the insertion of an extra dayinto a calendar year. Exfoliation refers to the peeling of rocky materialsinto sheets due to weathering.

Clay nanolayers in elastomeric epoxy matrices dramatically improveboth the toughness and the tensile properties.145, 149 The dimensional sta-bility, the thermal stability and the chemical resistance can also be im-proved with clay nanolayers.150

Exfoliated clays are formed when the clay layers are well separatedfrom one another and individually dispersed in the continuous polymer ma-trix. Since exfoliated nanocomposites exhibit a higher phase homogeneitythan the intercalated clays, exfoliated clays are more effective in improvingthe properties of the nanocomposites.

Epoxy Resins 163

Successful nanocomposite synthesis depends not only on the curekinetics of the epoxy system but also on the rate of diffusion of the cur-ing agent into the galleries, because it affects the intragallery cure kinetics.The nature of the curing agent influences these two phenomena substan-tially and therefore the resulting structure of the nanocomposite. The cur-ing temperature controls the balance between the extragallery reaction rateof the epoxy system and the diffusion rate of the curing agent into the gal-leries.151 It was found that the activity energy decreases with the additionof organic montmorillonite.152 Hexahydrophthalic anhydride (HHPA) isusually used for hot curing of epoxy resins. With an alkoxysilane, it alsoacts as a condensation agent.153 Hot curing of montmorillonite-layeredsilicates has been described with methyltetrahydrophthalic anhydride.154

An exfoliated epoxy-clay nanocomposite structure can be synthes-ized by loading the clay gallery with hydrophobic onium ions and thenallowing diffusion in the epoxide and a curing agent. The degree of ex-foliation increases with decreasing curing agent.155 Clays exert catalyticeffects on the curing of epoxy resins.156

An organically modified montmorillonite, prepared by a cation ex-change reaction between the sodium cation in montmorillonite and di-methyl benzyl hydrogenated tallow ammonium chloride is suitable for highdegrees of filling for epoxy resins.157 Nanocomposites exhibit a significantincrease in thermal stability in comparison to the original epoxy resin.158

Quaternary ammonium ions both catalyze the epoxy curing reac-tions and plasticize the epoxy material. This causes a large reduction inglass transition temperature and lowers the storage modulus. Plasticizationis small for aromatic epoxy resins, but large for aliphatic resins. There-fore, aromatic epoxy-clay systems may result in a complete exfoliationof the clay galleries, whereas mixtures of aliphatic and aromatic epoxymay produce intercalated systems.159 Poly(oxypropylene)amine interca-lated montmorillonite is highly organophilic and compatible with epoxymaterials.160

Star branched functionalized poly(propylene oxide-block-ethyleneoxide) was used with an organophilic modified synthetic fluorohectorite ascompatibilizer for nanocomposites. The polarity of the polyol could be tai-lored by the type of functionalization. A mixture of two epoxy resins, tetra-glycidyl 4,4′-diaminodiphenylmethane and bisphenol A diglycidyl ether,cured with 4,4′-diaminodiphenylsulfone, was used as matrix material.161

The hybrid nanocomposites were composed of intercalated clay particles


Table 3.6: Interpenetrating Polymer Networks

Epoxide Further Component Reference

Diglycidyl ether ofbisphenol A

Unsaturated polyesters 162

Aliphatic epoxide resin Vinylester resin (Bisphenol Aglycidylmethacrylate adduct instyrene with layeredsilicate nanoparticles)

10


Bisphenol A diacrylate 163

Epoxide bismaleimide resin Cyanate ester 164

Epoxide-amine network Silica 165


Hexakis(methoxymethyl)mel-amine

166

Novolak epoxy resin 2,2′-Diallyl bisphenol A (DBA) 167

Epoxy resin Polyaniline 168

as well as separated PPO spheres in the epoxy matrix. Phenolic alkylimid-azolineamides were also used to exchange the interlayer sodium cations ofthe layered silicates.169

Electric capacitors based on epoxy clay nanocomposites can be in-tegrated into electronic devices.170

3.3.7 Interpenetrating Polymer Networks

Interpenetrating polymer networks are ideally compositions of two or morechemically distinct polymer networks held together exclusively by theirpermanent mutual entanglements.171 In practice, interactions of both net-works beyond entanglement may occur, for instance, intercrosslinking.

In a simultaneous interpenetrating polymer network, the two net-work components are polymerized concomitantly. In a sequential inter-penetrating polymer network, the first network is formed and then swollenwith a second crosslinking system, which is subsequently polymerized.

Interpenetrating polymer networks are known to remarkably sup-press creep phenomena in polymers. The motion of the segments in inter-penetrating polymer networks is diminished by the entanglement betweenthe networks.

Interpenetrating polymer networks including epoxide resins as oneof the components are summarized in Table 3.6.

Epoxy Resins 165

3.3.7.1 Curing Kinetics

If a thermosetting system is cured at a temperature below its maximally at-tainable glass transition temperature, vitrification occurs during cure. Thevitrification slows down the reaction. The reaction may freeze beforereaching full conversion.

In contrast, in an interpenetrating network, if one component (I) re-acts more slowly than the other component (II), the former component (I)may act as a plasticizer of the polymeric component (II). This allows afaster reaction of the second component (II) and a more thorough curewithout vitrification.172

In the simultaneous curing of a vinylester resin (VER) and an epoxyresin a reduction in reaction rate due to the dilution of each reacting systemby the other resin components is observed.

The radical polymerization of an acrylate monomer is hardly af-fected by the oxygen inhibition effect, while the cationic polymerizationof an epoxy monomer is enhanced by the atmosphere humidity.173

The decomposition of peroxides is known to be accelerated by am-ines. In fact, if for the radical curing of the vinylester component peroxidesare used instead of azo compounds, a strong redox interaction between theperoxide and the amine used for curing the epoxide component is observed.In such systems the peroxide decomposes too quickly to develop its fullpower for curing the vinylester system.

Further, there is an interaction between the vinyl groups of the vinyl-ester system and the amine via a Michael addition. The curing performanceof the epoxide resin is less affected by the radical initiator.174

3.3.7.2 Unsaturated Polyesters

In mixtures of epoxy based on diglycidyl ether of bisphenol A and un-saturated polyesters, the curing monitored with differential scanning cal-orimetry indicated a higher rate constant than the pure epoxide resin. Itis believed that the hydroxyl end group of the unsaturated polyester in theblend provides a favorable catalytic environment for the epoxide curing.162

The interpretation of the viscosity development suggests that an in-terlock between the two growing networks exists that causes a retardedincrease of the viscosity.175 The introduction of unsaturated polyester intoepoxy resin improves toughness but reduces the glass transition tempera-ture.176


Functional Peroxides. Peroxy ester oligomers can be obtained by con-densation of anhydrides with poly(ethylene glycol)s and tert-butyl hydro-peroxide. Suitable anhydrides are pyromellitic dianhydride and the tetra-chloroanhydride of pyromellitic anhydride. The resulting esters containcarboxylic groups and peroxy groups. These compounds can be used ascuring agents for unsaturated polyesters as such and for hybrid resins con-sisting of an epoxy resin and an unsaturated polyester resin.177

3.3.7.3 Acrylics

For interpenetrating polymer networks consisting of diglycidyl ether ofbisphenol A (DGEBA) and bisphenol A diacrylate as radically polymeriz-able component, 4,4′-methylenedianiline and dibenzoyl peroxide are suit-able curing agents. The curing can be achieved between 65°C and 80°C.The kinetics of curing of the epoxide takes place as a combination of an un-catalyzed bimolecular reaction and a catalyzed termolecular reaction. Thekinetics of curing of the acrylate runs according to a first-order reaction.163

In the mixture, the rate constants are lower than in the separate sys-tems. Also the activation energies in the mixtures are higher. It is be-lieved that chain entanglements between the two networks cause a sterichindrance for the curing process. The vitrification restrains the chain mo-bility that is reflected as a decrease of the rate constants. The incorporationof the methacryloyl moiety in an epoxide resin improves the weatheringstability and the photostability of the system.178, 179

3.3.7.4 Urethane-modified Bismaleimide

Urethane-modified bismaleimide (UBMI) can be introduced and partiallygrafted to the epoxy oligomers by polyurethane grafting agents. After-wards, a simultaneous bulk polymerization technique can be used to pre-pare interpenetrating networks.180 The tensile strength increases to a max-imum value with increasing UBMI content, then decreases with further in-creasing UBMI content. If the polyurethane grafting agent contains poly-(oxypropylene) polyols the interpenetrating network shows a two-phasesystem, whereas in the case of poly(butylene adipate) a single phase sys-tem is observed. The better compatibility of poly(butylene adipate) basenetworks results in a higher impact strength.

An intercrosslinked network of bismaleimide-modified polyureth-ane-epoxy systems was prepared from the bismaleimide having ester link-

Epoxy Resins 167

ages, polyurethane-modified epoxy, and cured in the presence of 4,4′-di-aminodiphenylmethane. Infrared spectral analysis was used to confirm thegrafting of polyurethane into the epoxy skeleton. The prepared matriceswere characterized by mechanical, thermal, and morphological studies.

The changes of the properties depend on the relative amounts of themoieties used. The incorporation of polyurethane into the epoxy skeletonincreases the mechanical strength and decreases the glass transition tem-perature, thermal stability, and heat distortion temperature. On the otherhand, the incorporation of bismaleimide with ester linkages into a polyur-ethane-modified epoxy system increases the thermal stability, tensile andflexural properties, and decreases the impact strength, glass transition tem-perature, and heat distortion temperature.181

3.3.7.5 Electrically Conductive Networks

Electrically conductive polymers could find use in rechargeable batter-ies, conducting paints, conducting glues, electromagnetic shielding, anti-static formulations, sensors, electronic devices, light-emitting diodes, coat-ings, and others. Low concentrations of polyaniline can make the poly-mer electrically conductive when a co-continuous microstructure could beachieved.

For the preparation of conductive polyaniline epoxy resin compos-ites, a doped polyaniline is blended with the epoxy resin. Plasticizers areadded to assist in the dispersion of the conductive polymer. The curingagent must be selected in order to avoid dedoping.168

The grafting onto the nitrogen of polyaniline was achieved by thering-opening graft copolymerization of 1,2-epoxy-3-phenoxypropane. Bythe degree of grafting, the solubility, the optical and the electrochemicalproperties of the grafted polyaniline can be tailored.182

3.3.8 Organic and Inorganic Hybrids

An organic-inorganic hybrid interpenetrating network has been synthes-ized from an epoxide-amine system and tetraethoxysilane (TEOS). Thekinetics of the formation of the silica structure in the organic matrix, and itsfinal structure and morphology, depend on the method of preparation of theinterpenetrating network. In the sol gel process, hydrolysis and polymer-ization of TEOS are performed at room temperature in isopropyl alcohol.The hybrid network can be prepared by two procedures.


In the one-step procedure, all reaction components are mixed simul-taneously. In the two-step procedure, TEOS is hydrolyzed in the first step,then mixed with the organic epoxy components and polymerized under theformation of silica and epoxide networks.

Large compact silica aggregates, with 100 to 300 nm diameter, areformed by the one-stage process of polymerization. In the two-stage pro-cess the partial hydrolysis of TEOS effects an acceleration of the gelation.This results in somewhat smaller silica structures. The most homogene-ous hybrid morphology with the smallest silica domains of size 10 to 20nm can be achieved in a sequential preparation of the interpenetrating net-work.165, 183 An increase in modulus by two orders of magnitude wasachieved at a silica content below 10%.184 Phenolic novolak/silica andcresol novolak epoxy/silica hybrids can be prepared in a similar mannerwith TEOS.185

3.3.9 Flame Retardants

Flame retardancy can be imparted by suitable monomers and curing agents.Flame retardants can be grouped into halogen-containing compounds, themost important being tetrabromobisphenol A, halogen free systems con-taining aluminum trihydrate with red phosphorus, and phosphate esters.186

Flame retardants that are used in epoxide resins are shown in Table 3.7.

Triglycidyloxy phenyl silane cured with 4,4′-diaminodiphenylmeth-ane and others gives highly flame retardant polymers.18 Heating in airindicates that a silicon-containing carbon residue formed is superior in pre-venting oxidative burning.

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) issynthesized by a multi step reaction from o-phenylphenol and phosphorustrichloride.

From this compound, an adduct with p-benzoquinone, 2-(6-oxid-6H-dibenz[c,e][1,2]oxa-phosphorin-6-yl)-1,4-benzenediol (ODOPB), canbe obtained. ODOPB can be used as a reactive flame-retardant in o-cresolformaldehyde novolak epoxy resins for electronic applications.19, 187 A re-lated compound, 2-(6-Oxid-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)meth-anol (ODOPM) can be used as flame-retardant hardener for o-cresol-form-aldehyde novolak epoxy (CNE) resin in electronic applications.188 Somephosphorous-containing flame retardants are shown in Figure 3.9.

Epoxy Resins 169

Table 3.7: Flame Retardants for Epoxide Resins

Compound Remark/Reference

Tetrabromobisphenol A-based epoxiesTriglycidyloxy phenyl silane 18

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO)

19, 189

10-(2,5-Dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DHPDOPO)

190

Bis(m-aminophenyl)methylphosphine oxide (BAMPO)

191

Bismaleimide(3,3′-bis(maleimidophenyl))phenylphosphine oxide (BMPPPO)

192

Bis(3-glycidyloxy)phenylphosphine oxide 193

Bis(4-aminophenoxy)phenylphosphine oxide (BAPP) 194

Tris(2-hydroxyphenyl)phosphine oxides 195, 196

Bis(3-diethylphosphono-4-hydroxyphenyl)sulfide (DPHS) 197

Benzoguanamine-modified phenol biphenylene components 198

Melamine phosphate 199

2,4,6-Tris(2,4,6-tribromophenoxy)-1,3,5-triazine 200 a

2,2′-[(1-Methylethylidene)bis[(2,6-dibromo-4,1-phenylene)oxy]]bis[4,6-bis[(2,4,6-tribromophenyl)oxy]]-1,3,5-triazine

200 a

Carbon black 201

a c.f. Figure 3.8


N N

N

O

OO

Br Br

BrBr

BrBr

Br

BrBr

N N

N

O

OO

Br Br

BrBr

BrBr

BrBr

C

Br Br

Br Br

Br Br

BrBr

O O

O

N

NN

CH3H3C

Figure 3.8: Top: 2,4,6-Tris(2,4,6-tribromophenoxy)-1,3,5-triazine,Bottom: 2,2′-[(1-Methylethylidene)bis[(2,6-dibromo-4,1-phenylene)oxy]]bis[4,6-bis[(2,4,6-tribromophenyl)oxy]-1,3,5-triazine200

Epoxy Resins 171

P OOCH2

OH

ODOPM

P OO

OHHO

ODOPB

Figure 3.9: 2-(6-Oxid-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methanol(ODOPM), 2-(6-Oxid-6H-dibenz[c,e][1,2]oxa-phosphorin-6-yl)-1,4-benzenediol(ODOPB)

Other phosphorus-containing epoxy resins can be obtained from theaddition reaction of DOPO and the glycidyl ether of cresol-formaldehydenovolak.202, 203 The cured products are highly flame resistant.

In the presence of a phosphorous-containing hardener, bis(m-amino-phenyl)methylphosphine oxide (BAMPO), the volatilization of the curedresin is reduced and aromatization is accelerated. This results in a largeryield of stable char. This behavior is attributed to the flame retardant ac-tion of BAMPO. However, at high content of BAMPO this effect is over-whelmed by flame quenching due to the volatilization of the phosphorus-containing moieties from BAMPO.191

Further, bismaleimide(3,3′-bis(maleimidophenyl))phenylphosphineoxide (BMPPPO), is a phosphorus-containing compound that is soluble inorganic compounds. Interpenetrating networks can be prepared by simulta-neously curing an epoxy/diaminodiphenylmethane system and BMPPPO.

The cured resin system exhibits a glass transition temperature around212°C, thermal stability at temperatures beyond 350°C, and excellent flameretardancy with a limiting oxygen index (LOI) of 40%.192

Phosphorous-containing diamines have been prepared that act ascuring agents for epoxy resins.204 The compounds and their synthesisare shown in Figure 3.10. When cured with phosphorus-containing cur-ing agents, the epoxy resins show extremely high LOI values of up to 49.

Amine-based curing agents destabilize a brominated epoxy resin bya mechanism of the nucleophilic substitution of bromine. As a result, abrominated epoxy resin releases products of pyrolysis about 100°C lowerthan a nonbrominated epoxy resin.205


P

ONH2H2N

BAPPPO

P

ONO2O2N

P

O

H2SO4/HNO3

SnCl2/HCl/EtOH

C NH2H2N

O

P OO

H

C NH2H2N

P

P

O

O

O

O

2-DOPO-A

+

Figure 3.10: 2-DOPO-A, Bis(4-aminophenyl)phenylphosphine oxide(BAPPO)204, 206

Epoxy Resins 173

Table 3.8: Global Production/Consumption Data of Important Monomersand Polymers207

Monomer Mill. Metric tons Year Reference

Ethylene oxide 14.7 2002 208

Ethyleneamines 0.248 2002 209

Epichlorohydrin 0.640 1999 210

Epoxy Resins 0.65 1999 211

3.3.10 Production Data

Global production data for the most important monomers used for unsatur-ated epoxy resins are shown in Table 3.8.

3.4 CURING

3.4.1 Initiator Systems

The epoxide group reacts with several substance classes. Only a few of thepossible reactions are used for curing in practice. Curing agents of epoxyresins can be subdivided into three classes:

1. Compounds with active hydrogens,2. Ionic initiators, and

3. Hydroxyl coupling agents.

The most commonly used curing reaction is based on the polyaddi-tion reaction, thereby opening the epoxide ring. The glycidyl group can becured by amines and other nitrogen-containing compounds such as poly-amides. Many of the amines effect curing at room temperature. This typeof curing is called a cold curing.

The reactivity of an epoxy compound with an amine depends on thestructure of the compounds. The relative reaction rates of the secondaryamine to the primary amine can be explained in terms of substitution ef-fects.212 Anhydrides are active only at elevated temperatures. This type ofcuring is addressed as hot curing.


CH2 CHNHR

OH

CH2 CH

O

CH2 CH

OH

NRCH2 CH

OH

CH2 CH

O

R NH2 CH2 CHNHR

OH

CH2 CH

O

R OH CH2 CHOR

OH

+

+

Figure 3.11: Reaction of the Glycidyl Group with an Amine and with a HydroxyGroup

3.4.2 Compounds with Activated Hydrogen

3.4.2.1 Amines

Both primary and secondary amines can be used. From a chemical point ofview, the active hydrogen attached to the nitrogen group effects an additionreaction, as the epoxide group is opened. The curing of the diglycidyloligomer with a diamine occurs in three stages:

1. Linear coupling of the oligomer,2. Formation of a branched structure, and3. Crosslinking.

The basic reaction between the glycidyl groups with a primary amine isshown in Figure 3.11. The first reaction in Figure 3.11 is the additionreaction of primary amine hydrogen with an epoxy group. The productof this reaction is a secondary amine. The secondary amine may reactwith another epoxy group to form a tertiary amine, as shown in the secondreaction, Figure 3.11. Usually the secondary amine is less reactive thanthe primary amine. The ratio of the kinetic constants is approximately 1/2.Both reactions are autocatalyzed by OH groups formed during the process.

The third reaction shown is the etherification reaction between ep-oxy functions and hydroxyl groups. In most systems, this reaction can

Epoxy Resins 175

Table 3.9: Amines Suitable for Curing

Compound Remarks

Ethylene diamine Fast curing, low viscosityDiethylenetriamine Fast curing, low viscosityTriethylenetetramine Fast curing, low viscosityHexamethylene diamine Slower curing, needs elevated temper-

ature, flexible materialsDiethylaminopropylamine Needs elevated temperature, good ad-

hesiveIsophorone diamine1,2-DiaminocyclohexaneBis-p-aminocyclohexylmethaneBisaminomethylcyclohexaneMenthane diamine Needs elevated temperature,

good potlifeN-aminoethyl piperazine Fast curingDiaminodiethyl toluene Mixture of 2,6-diamino-3,5-diethyl

toluene and 2,4-diamino-3,5-diethyltoluene

m-Phenylene diamine Chemical resistant materials4,4′-Diaminodiphenylmethane Chemical resistant materials3,3′,5,5′-Tetraethyl-4,4′-diaminodiphenylmethane

Flame retardant191

4,4′-Diamino-3,3′-dimethyldicyclohexylmethane (DCM)

Cycloaliphatic diamine213, 214

1,5-Naphthalene diamine

be neglected. However, it has been shown that this reaction takes placeusing 4,4′-methylene bis(3-chloro-2,6-diethylaniline) (MCDEA) as curingcatalyst. On the other hand, with 4,4′-diaminodiphenylsulfone (DDS) and4,4′-methylenedianiline (MDA) as catalysts the etherification was not ob-served.109, 215

Typical nitrogen compounds used for cold curing are shown in Ta-bles 3.9, and 3.10, and in Figures 3.12 and 3.13. There are many possibil-ities for formulating a curing system from primary and secondary amines,and also with tertiary amines.

Tertiary amines catalyze the reaction. Other catalysts are complexesof boron trifluoride complexes, quaternary ammonium salts, thiocyanocompounds, etc. Retarders are certain ketones and diacetone alcohol.


Table 3.10: Polymeric Amines and Hetero Functional Amines

Compound Remarks

Poly(propylene oxide)diamineTrimercaptothioethylamine Optical applications23, 216

Polymercaptopolyamines In combination with customary aminehardeners217

2,4-Diamino-4′-methylazobenzene(DMAB)


4,4′-Dithiodianiline Reversible crosslinking219

Dicyandiamide Common for adhesives4,4′-Diaminodiphenylsulfone Chemical resistant materialsbis(m-aminophenyl)methylphosphineoxide (BAMPO)

191

4,4′-Methylene bis[3-chloro-2,6-diethylaniline]

67

Olefin oxide polyamine adducts Fast curing, low toxicityGlycidyl ether polyamine adducts Fast curingDiamide of dimerized linoleic acidand ethlyene diamine

For adhesives

Ketimines Low viscosity, long potlife,latent hardening catalysts

2,5-Bis(aminomethyl)bicyclo[2.2.1]heptane di(methylisopropylketimine)

Norbornane diketimine220

Substituted imidazolines, e.g.,2-ethyl-4-methylimidazole, 1-meth-ylimidazole

Wide range in stoichiometry

Sulfanilamide 45, 221–223

Polysilazane-modified polyamines Thermal resistant224

Epoxy Resins 177

Diethylenetriamine

H2N CH2 CH2 NH CH2 CH2 NH2

H2N CH2 CH2 CH2 CH2 CH2 CH2 NH2

Hexamethylenediamine

NCH2

CH2CH3

CH3

CH2 CH2 CH2 NH2

Diethylaminopropylamine

CH2N

CH3

CH3

CH3

NH2

Menthanediamine

NCH2CH2H2N H

N-Aminoethyl piperazine

Figure 3.12: Aliphatic Nitrogen Compounds for Curing: Diethylenetriamine,Hexamethylene diamine, Diethylaminopropylamine, Menthane diamine, N-am-inoethyl piperazine


4,4’-Diaminodiphenylsulfone

S

O

O

NH2H2N

CH2 NH2H2N

4,4’-Diaminodiphenylmethane

NH2H2N

m-Phenyenediamine 1,5-Napthalene diamine

H2N

NH2

Figure 3.13: Aromatic Nitrogen Compounds for Curing: m-Phenylene diamine,1,5-Naphthalene diamine, 4,4′-Diaminodiphenylsulfone, 4,4′-Diaminodiphenyl-methane

Epoxy Resins 179

Certain cyclic amines, such as 1,2-bis(aminomethyl)cyclobutane andisomers of diaminotricyclododecane increase the pot life time. Polyaminesand dicyanamide are preferably used for adhesive formulations.

Phenolic hydroxyl groups exert autocatalysis at low conversions withrespect to the ring opening of the epoxide group, thereby adding the aminegroups. In the later stage of curing the amine groups are largely consumedand the phenolic hydroxyl groups start to react with the residual epoxidegroups.225 A suitable accelerator for adhesive formulations is 2,4,6-tris(di-methylaminomethyl)phenol.

Most low molecular amines are toxic and also sensitive to the carbondioxide in air. Therefore, the various adducts of the amines have beendeveloped to mitigate this drawback.

3.4.2.2 Ketimines

Ketimines form the active amine structure by addition of water; thus theyact as delayed-action catalysts.

3.4.2.3 Amino Amides

Amide-based compounds are used to achieve special properties and desiredcuring characteristics, such as lower toxicity, less sensitive final propertiesto the stoichiometry, lower peak temperatures for large castings. The activegroup in curing is not directly the amide group, but the attached primaryand secondary amino groups present in the molecule. The amide groupis helpful for achieving the other benefits, mentioned above. Examplesfor amino amides are adducts of polyamines with fumaric acid or maleicacid, or fatty acids. Similar to amines, in amine amides the reaction can beaccelerated with boron trifluoride complexes, Mannich bases, etc.

3.4.2.4 Metal salts

Zirconium tetrachloride catalyzes effectively the nucleophilic opening ofepoxide rings by amines. This has been used for the efficient synthesisof β-amino alcohols.226 Zinc bromide and zinc perchlorate are also activein this manner.227 However, it seems that this catalyst is not used for thecuring of epoxy resins.


Table 3.11: Anhydrides for Hot Curing

Anhydride Remark/Reference

Dodecenyl succinic anhydride LiquidHexahydrophthalic anhydride3-Methyl-1,2,3,6-tetrahydrophthalicanhydride (MeTHPA)

N,N-dimethylbenzylamine as acceler-ator228

Hexahydro-4-methylphthalicanhydride

Tetrahydrophthalic anhydrideMethyltetrahydrophthalic anhydridePhthalic anhydrideMethyl nadic anhydride LiquidHET anhydridePyromellitic dianhydride (PMDA)5-(2,5-Dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-di-carboxylic anhydride(DMCDA)

229

Glutaric anhydride Biodegradable Formulations230

Styrene-maleic anhydridecopolymers

Low molecular weight copolymers

3.4.2.5 Phenols

Bisphenol A is a main ingredient for the manufacture of glycidyl ethers.Polyfunctional phenols can be used to cure epoxy resins. This methoddid not find large commercial use, except in the development of highlychemically resistant coatings. The curing reaction is completely similar tothe curing reaction of amines.

Phenoplasts. Polyfunctional phenols can be applied as phenol/formalde-hyde condensates of the novolak-type. In this field a wide variety has beenexamined, including phenolic adducts of chloromethylated diphenyl oxide,tetrabrominated bisphenol, and phenol adducts of poly(butadiene)

3.4.2.6 Anhydride Compounds

Typical anhydride compounds used for hot curing are shown in Table 3.11and in Figure 3.14. Most anhydrides need elevated temperatures to beactive. The anhydride group is not active in the absence of acidic or basic

Epoxy Resins 181

O

O

O

C12H23

Dodecenylsuccinic anhydride Phthalic anhydride

O

O

O

O

O

O

Tetrahydrophthalic anhydride

O

O

O

Hexahydrophthalic anhydride

Methylnadic anhydride

O

CHO

O

H3C

OO

O

O

O

O

Pyromellithic anhydride

Figure 3.14: Anhydrides for Hot Curing


catalysts; instead the anhydride group must be converted into the carboxylgroup. This can be achieved by hydrolysis by natural occurring moisture,or by alcoholysis.

The reaction of an anhydride is accelerated by a tertiary amine or bycomplexes of metal salts, such as ferric acetylacetonate.231 The reactionof the anhydride group, as well as the acid group with the epoxide group,results in an ester linkage, with all the advantages and disadvantages of theester link.

Anhydrides are in some cases preferred over amines because theyare less irritating to the skin, have longer pot life times, and low peak tem-peratures. Aromatic and cycloaliphatic anhydrides find wide applicationsfor molding and casting techniques.

3.4.2.7 Polybasic Acids

The carboxyl group is capable of opening the epoxide group. Theoretically,the optimum stoichiometry is one acid group by one epoxide group. Inpractice an excess of acid is used.

3.4.2.8 Polybasic Esters

To obtain tough materials, the epoxides can be cured by the insertion re-action into ester groups. The curing agent is formed in-situ by the radicalpolymerization of N-phenylmaleimide and p-acetoxystyrene.232 2,5-Di-methyl-2,5-bis(benzoylperoxy)hexane is suitable, because its decomposi-tion temperature of 110°C is close to the desired cure temperature of 100°C.The two monomers copolymerize satisfactorily in the absence of the epoxycompound. The advantage of using the in-situ technique of polymerizationis that the initial composition has low viscosity.

The insertion mechanism is shown in Figure 3.15. Compared toepoxy systems cured with a phenol resin, the copolymer of N-phenylmale-imide and p-acetoxystyrene shows a significantly higher glass transitiontemperature.

3.4.3 Coordination Catalysts

Coordination catalysts consist of metal alkoxides, such as aluminum iso-propyloxide, metal chelates, and oxides. Coordinative polymerization re-sults in high molecular weight and stereospecific species.

Epoxy Resins 183

N OO

CH CH2

O CH2CHCH2

OC

O

H3C

N OO

CH CH2

OC

O

H3C CH2 CH CH2

O

+

Figure 3.15: Insertion of the Epoxide into a Pendent Ester Group

3.4.4 Ionic Curing

3.4.4.1 Anionic Polymerization

The anionic polymerization of epoxides can be initiated by metal hydrox-ides, and secondary and tertiary amines. The rate of curing is low in com-parison to other curing methods. Therefore, anionic polymerization has notfound wide industrial application. Moreover, the mechanical properties ofthe final materials are not satisfactory.

3.4.4.2 Cationic Polymerization

Cationic polymerization can lead to a crosslinking process if diepoxidesare taken as monomers. Thus, a wide variety of compounds can be usedcatalytically as cationic curing initiators for epoxy resins that act at a highrate. Moreover, their low initial viscosities and fast curing make them goodcandidates for rapid reactive processing.

Cationic polymerization is initiated by Lewis acids. A lot of metalhalogenides have been shown to be active, such as AlCl3, SnCl4, TiCl4,SbCl5 or BF3, but the most commonly used compound is boron trifluoride.In practice, boron trifluoride is difficult to handle and the reaction runstoo fast. Therefore, the compound is used in complexed form, e.g., asan ether complex or an amine complex. The strength of the ether andamine complexes can be related to the base strength of the ether and amine,


Table 3.12: Latent Catalysts

Compound Reference

N-Benzylpyrazinium hexafluoroantimonate 233

N-Benzylquinoxalinium hexafluoroantimonate 233

Benzyl tetrahydrothiophenium hexafluoroantimonate 234

o,o-Di-tert-butyl-1-piperidinylphosphonamidate 235

o-tert-Butyl-di-1-piperidinylphosphonamidate 235

o,o-di-tert-Butyl phenylphosphonate 236

o,o-Dicyclohexyl phenylphosphonate 236

respectively. Since the reactivity of a complex depends on the dissociationconstant, some predictions on the activity of the complex can be made.

Water or alcohols cause chain transfer reactions. The alcohol attacksthe positively charged end of the growing polymer chain and forms an etherlinkage or a hydroxyl group, respectively. The released proton can initiatethe growth of another polymer chain. Diols and triols yield polymers withpendent hydroxyl groups. Therefore, diepoxides or higher functional ep-oxides are polymerized in the presence of diols or triols, etc.; branched andcrosslinked products may appear.

In the cationic UV curing of an aliphatic epoxy compound it wasobserved that the polymerization rate decreased strongly after a conversionlevel of less than 10%. This effect was not caused by the glass transitiontemperature. However, the addition of 1,6-hexanediol (HD) raised the con-version at room temperature.237

There are photolatent and thermolatent catalyst systems. A great va-riety of those catalysts have been reviewed.238 Besides the direct thermo-lysis of the initiator, also indirect methods are viable. Table 3.12 providesa list of latent catalysts.

Spiroorthocarbonate. The cationic curing reaction of a bisphenol A-typeepoxy resin in the presence of a spiroorthocarbonate can be performed withborontrifluoride dietherate. The spiroorthocarbonate undergoes a doublering opening reaction.239 The conversion of the epoxy groups increases asthe content of the spiroorthocarbonate increases.

3,9-Di(p-methoxybenzyl)-1,5,7,11-tetra-oxaspiro[5.5]undecane, c.f.Figure 3.16 as spiroorthocarbonate, can be synthesized by the reaction of2-methoxybenzyl-1,3-propanediol with dibutyltin oxide.

Differential scanning calorimetry shows two peaks that are attributed

Epoxy Resins 185

3,9-Di(p-methoxy-benzyl)-1,5,7,11-tetra-oxaspiro(5,5)undecane

O

O

O

O

OO CH3H3C

O

O O

O

O

O

3,23-Dioxatrispiro[tricyclo[3.2.1.0<2,4>]octane-6,5’-1,3-dioxane-2’2"-1,3-dioxane-5",7’"-tricyclo[3.2.1.0<2,4>octane]

Figure 3.16: 3,9-Di(p-methoxybenzyl)-1,5,7,11-tetra-oxaspiro[5.5]undecaneand 3,23-dioxatrispirotricyclo[3.2.1.0[2.4]]octane-6,5′-1,3-dioxane-2′2′′-1,3-dioxane-5′′,7′′′-tricyclo[3.2.1.0[2.4]octane]

to the polymerization of the epoxy group, and to the copolymerization ofthe spiroorthocarbonate with epoxy groups or homopolymerization, re-spectively. Copolymers containing a spiroorthocarbonate are capable ofyielding a hard, non-shrinking matrix resin. Examples of these copolymersinclude a 2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecanespiroorthocarbonate, and 3,23-dioxatrispirotricyclo[3.2.1.0[2.4]]octane-6,5′- 1,3-dioxane-2′2′′-1,3-dioxane-5′′,7′′′-tricyclo[3.2.1.0[2.4]octane] andcis,cis-, cis,trans-, and trans,trans-configurational isomers of 2,3,8,9-di-(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]undecane.

These spiroorthocarbonates were determined to undergo an expansionof 3.5% during homopolymerization and demonstrated acceptable cytotox-icity and genotoxicity properties. These properties make them promisingcomponents of composite resin matrix materials.20

Trifluoromethanesulfonic acid salts. Triflic acid, i.e., trifluorometh-anesulfonic acid, CF3SO3H is a known strong acid. Lanthanide triflatesare Lewis acids and they maintain their catalyst activity even in aqueoussolution. The strong electronegativity of the trifluoromethanesulfonate an-ion enhances the Lewis acid character of the initiator. Therefore, lanthan-ide triflates are excellent catalysts in the ring opening of the epoxy com-pounds.240


Phosphonic Acid Esters. Phenylphosphonic esters decompose into phen-ylphosphonic acid and the corresponding olefins at 150 to 170°C. In thepresence of ZnCl2 they can initiate a cationic polymerization of glycidylphenyl ether (GPE) to molecular weights up to 2000 to 7000 Dalton.236

Examples are o,o-di-1-phenylethyl phenylphosphonate, o,o-di-tert-butyl phenylphosphonate, and o,o-dicyclohexyl phenylphosphonate. Thesecompounds can be synthesized from phenylphosphonic dichloride and thecorresponding alcohols.

Phosphonamidates. Phosphonamidates are thermally latent initiators,suitable for the polymerization of epoxides.235 These compounds, suchas o,o-di-tert-butyl-1-piperidinylphosphonamidate and further o-tert-but-yl-di-1-piperidinylphosphonamidate can be synthesized from phosphorusoxychloride and piperidine in the presence of triethylamine, followed bythe reaction with tert-butyl alcohol in the presence of sodium hydride. Nopolymerization of epoxide resins occurs below 110°C, whereas the curingproceeds rapidly above 110°C. At room temperature a mixture of epoxideand phosphonamidate is stable for months.

3.4.5 Photoinitiators

Photoinitiation is one of the most efficient methods for achieving very fastpolymerization. Often the reaction can be completed within less than onesecond.241 Curing with ultraviolet light has been developed for the coatingarea, printing inks and adhesives. The mechanism of photo curing consistsmostly of a cationic photopolymerization of epoxides. The kinetics of thephotoinduced reactions can be monitored by differential photocalorime-try.242 The major drawback of differential photocalorimetry is the ratherlong response time in comparison to the curing rate.

The well-known use of radical generating photoinitiators in vinyl-containing systems is not applicable in pure epoxy systems. There is anexception when the epoxide resin is mixed with a vinyl monomer that bearsthe hydroxyl functionality or the amide functionality. The radical generat-ing photoinitiator reacts then with the vinyl monomer.243

Common photoinitiators for epoxy systems are shown in Table 3.13.In the photoinduced curing of epoxides, the propagating polymer

cations cannot deactivate one another, but require a deactivation by anotherspecies present in the polymerization mixture. Therefore, after the light

Epoxy Resins 187

Table 3.13: Photoinitiators for Epoxides

Compound Reference

Aryl diazonium tetrafluoroborates4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)-ammonium hexafluoro antimonate)benzophenone

244

Calixarene derivatives 245

9-Fluorenyl tetramethylene sulfonium hexafluoroantimonate 246

Cyclopentadiene-Fe-arene hexafluorophosphate 247

is switched off, a pronounced postpolymerization reaction can be moni-tored.248 The conversion in the dark may contribute up to 80% of the totalcuring process. The overall polymerization quantum yield reaches ca. 200mol per photon.

It has been shown that polyglycols, i.e., polyols from 1,2-diols, slowdown the cationic polymerization, whereas polyols made from 1,4-diols donot show this effect.234 Also the addition of small amounts of crown ethers(12-crown-4 ether) retards the polymerization. This behavior is attributedto complexes that are formed only with glycol-like structures that reducethe effective concentration of cations available to initialize the polymeriza-tion.

3.4.5.1 Aryl Diazonium Tetrafluoroborates

The azo group in aryl diazonium tetrafluoroborates decomposes on ultravi-olet radiation into the aromatic compound, nitrogen and boron trifluoride.The latter compound initiates a cationic polymerization of the epoxide re-sin. The evolution of nitrogen limits the applications to thin films.

3.4.5.2 Aryl Salts

Other efficient photoinitiators are based on the photolysis of diaryliod-onium and triarylsulfonium salts, that when decomposed liberate strongBrønsted bases. These bases initiate the cationic polymerization.

It has been shown that diaryliodonium hexafluoroantimonate initial-izes photochemically the cationic copolymerization of 3,4-epoxycyclohex-ylmethyl-3′,4′-epoxycyclohexane carboxylate and triethylene glycol meth-ylvinyl ether.249 Epoxy-functionalized silicones can be synthesized by


Thioxanthone

S

O

Anthracene

Figure 3.17: Thioxanthone, Anthracene

rhodium-catalyzed, chemoselective hydrosilation of vinyl ethers with sil-oxanes or silane.250

Epoxidized soyabean oil accelerates the crosslinking reaction of aro-matic diepoxides in the presence of a triarylsulfonium photoinitiator.251

The photoinitiated copolymerization leads within seconds to a fully curedinsoluble material showing increased hardness, flexibility, and scratch re-sistance.

In interpenetrating networks, constructed by vinyl polymers and ep-oxides by photo curing, a mixture of a radically decomposing photoinitia-tor and a cationic photoinitiator is used. Examples are a mixture of a hy-droxyphenylketone and a diaryliodonium hexafluorophosphate salt. Dur-ing the UV curing of a mixture of acrylate and epoxide monomers, the ep-oxides react slower than acrylates.173 The low efficiency of the initiationprocess is caused by the low ultraviolet absorbance of cationic photoiniti-ators. However, photosensitizers can improve the performance.

Combinations of photo curing and thermal curing in interpenetratingnetworks of a vinyl polymer and an epoxide are possible. Such a comb-ination of crosslinkable resins allows the partial or complete cure of eachcomponent independent of the other.252

3.4.5.3 Photosensitizers

Photosensitizers can be used to improve characteristics of photo curing forpigmented materials. These photosensitizers exhibit significant UV ab-sorption in the near UV and transfer the absorbed energy to a cationic pho-toinitiator.253 Examples for photosensitizers are anthracene and thioxan-thone derivatives, such as 2,4-diethylthioxanthone, isopropylthioxanthone,c.f. Figure 3.17. Photoinitiators are iodonium salts that exhibit a compara-

Epoxy Resins 189

CH2

CH2

CH2CH2

CH2

CH2

CH3

CH3

CH3

CH3

H3C

OH

OH

OH

OH

HO

HO

H3C

Figure 3.18: p-Methylcalix[6]arene

tively low triplet state energy.

3.4.5.4 Calixarenes

Calixarenes are by-products in the phenol/formaldehyde condensation toprepare bakelite. They found attention for their application as surfactants,chemoreceptors, electrochemical and optical sensors, solid-phase extrac-tion phases, and stationary phases for chromatography.254

The hydroxyl groups in calixarenes (c.f. Figure 3.18) can be pro-tected with tert-butoxycarbonyl groups, trimethylsilyl groups, and cyclo-hexenyl groups, respectively. In this way the hydroxyl group does not reactwith an epoxide group. The phenol groups can be restored if a compoundis present that generates acids photolytically.245

3.4.6 Derivatives of Michler’s Ketone

4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propenyl)ammonium hexa-fluoro antimonate)benzophenone (MKEA) is synthesized from 4,4′-bis(di-methylamino)benzophenone (Michler’s ketone) and ethyl α-(bromometh-yl)acrylate, c.f. Figure 3.19. MKEA initiates cationic photopolymerizationof cyclic ethers, like cyclohexene oxide (CHO) via a conventional addition


C

O

N+N+

CH3

CH3

CH3

CH3

CH2CH2 CC CH2H2C

CC OO

OO

CH2 CH2

CH3 CH3

SBF6- SBF6

-

MKEA

C

O

NNCH3

CH3

H3C

H3C

CH2 C CH2

C O

O

CH2

CH3

Br+

Figure 3.19: Synthesis of 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarbonyl-1-propen-yl)ammonium hexafluoro antimonate)benzophenone (MKEA)

fragmentation mechanism. MKEA belongs to the group of addition-frag-mentation catalysts.

The mechanism of initiation of MKEA is shown in Figure 3.20. Thisinitiator does not require supplementary free radical sources. It is sug-gested that radicals stemming from the photoinduced hydrogen abstractionparticipate in addition fragmentation reactions to yield reactive species ca-pable of initiating cationic polymerization.244

Monomers with strong electron donors such as N-vinyl carbazole,isobutyl vinyl ether, and n-butyl vinyl ether undergo explosive polymer-ization upon illumination of light. In the case of cyclohexene oxide thereis an induction period, owing to the trace impurities present, but afterwards,the polymerization proceeds readily.

3.4.6.1 Photoinitiator Systems

Visible light photoinitiator systems include an iodonium salt, a visible lightsensitizer, and an electron donor compound.20

Epoxy Resins 191

N+

CH3

CH3

CH2CH2C

CO

O

CH2

CH3

R*

N+

CH3

CH3

CH2C*H2C

CO

O

CH2

CH3

R*

N*+

CH3

CH3

CH2C*H2C

CO

O

CH2

CH3

R*

+

+

Figure 3.20: Mechanism of Initiation of 4,4-Bis(N,N-dimethyl-N-(2-ethoxycarb-onyl-1-propenyl)ammonium hexafluoro antimonate)benzophenone (MKEA)


Examples of useful aromatic iodonium complex salt photoinitiatorsinclude diaryliodonium hexafluorophosphates and diaryliodonium hexaflu-oroantimonates, such as (4-(2-hydroxytetradecyloxyphenyl))phenyliodo-niumhexafluoroantimonate, (4-octyloxyphenyl)phenyliodonium hexaflu-oroantimonate (OPIA), and (4-(1-methylethyl)phenyl)(4-methylphenyl)-iodonium tetrakis pentafluorophenylborate. These salts are more thermallystable, promote faster reaction, and are more soluble in inert organic sol-vents than are other aromatic iodonium salts of complex ions. Diphenyliodonium hexafluoroantimonate has a photoinduced potential greater thanN,N-dimethylaniline.

The second component in the photoinitiator system is the photo-sensitizer. Desirably, the photoinitiator should be sensitized to the visiblespectrum to allow the polymerization to be initiated at room temperatureusing visible light. The sensitizer should be soluble in the photopolymeriz-able composition, free of functionalities that would substantially interferewith the cationic curing process, and capable of light absorption withinthe range of wavelengths between about 300 and about 1000 nanometers.Suitable sensitizers include compounds in the following categories:

• α-Diketones• Ketocoumarins• Aminoarylketones• p-Substituted aminostyrylketones

For applications requiring deep cure (e.g., cure of highly filled com-posites), it is preferred to employ sensitizers having an extinction coeffi-cient below about 1000 lmol−1cm−1 at the desired wavelength of irradia-tion for photopolymerization, or alternatively, the initiator should exhibita decrease in absorptivity upon light exposure. Many of the α-diketonesexhibit this property, and are particularly preferred for dental applications.A suitable photosensitizer is camphorquinone.

The third component of the initiator system is an electron donorcompound. The electron donor compound should be soluble in the poly-merizable composition. Further, suitable compatibility and interplay withthe photoinitiator and the sensitizer and other properties, like shelf stabil-ity, should be fulfilled. The donor is typically an alkyl aromatic polyetheror an alkyl, aryl amino compound wherein the aryl group is optionally sub-stituted by one or more electron withdrawing groups. Examples of suitableelectron withdrawing groups include carboxylic acid, carboxylic acid ester,

Epoxy Resins 193

ketone, aldehyde, sulfonic acid, sulfonate, and nitrile groups.In practice, the following compounds find application:

4,4′-Bis(diethylamino)benzophenone,4-Dimethylaminobenzoic acid (4-DMABA),Ethyl-4-dimethylamino benzoate (EDMAB),3-Dimethylaminobenzoic acid (3-DMABA),4-Dimethylaminobenzoin (DMAB),4-Dimethylaminobenzaldehyde (DMABAL),1,2,4-Trimethoxybenzene (TMB), andN-Phenylglycine (NPG).

3.4.7 Epoxy Systems with Vinyl Groups

Besides pure epoxy systems, mixed systems such as epoxy acrylates arein use. These systems can be cured with radical photoinitiators. Examplesfor such initiators are 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (BDMB), 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one (TPMK), 2,2-dimethoxy-1,2-diphenylethan-1-one (BDK),and hydroxy-2-methyl-1-phenyl-propanone.255

3.4.8 Curing Kinetics

There are various methods to investigate the kinetics of curing, including

1. Viscometry,2. Differential scanning calorimetry,3. Modulated differential scanning calorimetry,4. Dielectric analysis,5. Dynamic mechanical analysis,6. In-situ Fourier transform infrared spectroscopy, and7. Fluorescence response.

3.4.8.1 Viscometry

In the course of curing, the crosslinking density and the viscosity as wellas the modulus of the resin system increase. The viscoelastic propertiescan be measured in a torsional motion.256


3.4.8.2 Differential Scanning Calorimetry

Differential scanning calorimetry is the only direct reaction rate methodwhich operates in two modes: constant temperature or linear programmedmode. Several methods to evaluate the data obtained by differential scan-ning calorimetry are available.257 The isoconversional method258 is fre-quently used to calculate the energies of activation and evaluating the de-pendence of the effective activation energy on the extent of conversion.259

Relations are available between the degree of conversion, the timedependence of the conversion, and the direct measurable parameters, i.e.,viscometry, differential scanning calorimetry and dynamic mechanical ana-lysis. The equation is always second-order although the coefficients to thisequation are different for the individual methods. The DSC technique be-comes insensitive at conversions shortly after the gel point.260 However,changes in the heat capacity can be indicators of the onset and the finishingof the vitrification.214

Differential scanning calorimetry allows statements concerning thereaction mechanism of curing. The ring opening reaction between phenylglycidyl ether and aniline was investigated by DSC. The reaction resem-bles the diepoxy-diamine cure mechanism. However, it was detected thatbesides that from the epoxy ring opening reaction, another exothermic pro-cess at the last stages of the reaction takes place. It was concluded that thereaction of epoxy ring opening by aniline occurs by two concurrent path-ways,261, 262 an uncatalyzed one and an autocatalyzed one.

3.4.8.3 Temperature Modulated Differential Scanning Calorimetry

In temperature modulated differential scanning calorimetry (TMDSC), thesample is subjected to a sinusoidal temperature change. The instrumentsare called differential AC-calorimeters. This particular method can mea-sure the storage heat capacity and the loss heat capacity, i.e., the reversiblepart of heat that can be withdrawn again by cooling, and a part of heatconsumed by chemical reaction. A complex heat capacity with a real part(storage heat capacity) and an imaginary part (loss heat capacity) can bedefined.263 The treatment is similar to other complex modules in mechan-ics.

During the curing, the glass transition temperature rises steadily.The reaction induced vitrification takes place when the glass transitiontemperature rises above the curing temperature. This transition can be fol-

Epoxy Resins 195

lowed simultaneously with the reaction rate in TMDSC.264, 265

Modulated differential scanning calorimetry allows detecting of re-action induced phase separations. The apparent heat capacity changes, asphase separation occurs. The cloud point can be determined with opti-cal microscopy, and there is a correspondence between the optical methodand the calorimetry method.266, 267 In an amine curing system, a com-plex formed from the primary amine and the epoxide was postulated thatinitiates the curing reaction. The reactions of the primary amine and thesecondary amine with an epoxy-hydroxyl complex are comparatively slowand thus rate determining during the whole curing process.264, 268 In anepoxy-anhydride system some complications have been elucidated.269

Temperature modulated DSC can be used with advantage during iso-thermal curing of semi-interpenetrating polymer networks.270

3.4.8.4 Dielectric Analysis

Dielectric analysis271 is based on the measurement of the dielectric per-mittivity ε′ and the dielectric loss factor ε′′ in the course of curing. Thecomplex dielectric constant ε∗ may be expressed by

ε∗ = ε′ − iε′′ (3.1)

The permittivity is proportional to the capacitance and depends onthe orientation polarization. The orientation polarization results from thechange in the dipole moment due to the chemical reaction and also fromthe change of the concentration of dipoles due to the volume contractionduring the curing reaction. The loss factor corresponds to the energy loss.

Both dielectric and mechanical measurements are suitable tech-niques for monitoring the curing process. Also, phase-separation processescan be monitored by dielectric analysis, because dielectric measurementsare sensitive to interfacial charge polarization. Dipolar relaxation indicatesthe vitrification through the α-relaxation process in both phases.272 Fur-ther, dielectric sensor measurements have the advantage that they can bemade in the laboratory as well as in-situ in the fabrication tool in a produc-tion line.273 A relation between the dielectric response and other methodsmeasuring the gel point has been established in epoxy systems.214

Dielectric analysis, in combination with other experimental tech-niques, can be used to establish a time-temperature-transition (TTT) di-agram. The curing must be measured in a series of experiments at differ-


ent temperatures. In such a diagram gelation, vitrification, full cures, andphase-separation are marked.274

A technique involving simultaneous dielectric and near infrared mea-surements has been used for monitoring the curing of blends of a diglycidylether bisphenol A epoxy resin with a 4,4′-diaminodiphenylmethane hard-ener and various amounts of poly(methyl methacrylate) as modifier.275

3.4.8.5 In-Situ Fourier Transform Infrared Spectroscopy

During the curing reaction, the appearance or disappearance of variouscharacteristic infrared bands can be monitored. This method yields moreinformation than a single parameter, e.g., as obtained from a DSC measure-ment. However, there is more work needed to calibrate the system properlythan in a DSC experiment. Multivariate analysis, in particular alternatingleast squares (ALS), allows calculation of the concentration profiles andthe spectra of all species involved in the reaction of curing epoxy resins.276

During curing, the intensity of the epoxy group, at 789 to 746 cm−1

decreases.277 For example, based on such experiments, in the curing of adicyanate ester (1,1-bis(4-cyanatophenyl)ethane) with a bisphenol A epox-ide, the formation of an oxazoline structure has been proposed.278

3.4.8.6 Fluorescence Response

Fluorescence is a very sensitive and non-destructive technique to monitorthe curing. The fluorescence response from chemical labels and probesenables the changes to be followed in the surroundings of the chemicallabel. In the curing process, the viscosity may change about six orders ofmagnitude.

A change in the viscosity of the medium leads to a decrease in thenon-radiative decay rate and consequently a change in the fluorescencequantum yield. The reaction medium acts as a thermal bath for the ex-cited fluorescent molecule. When the monomers become fixed in forminga crosslinked polymer, a reduction of translational, rotational, and vibra-tional degrees of freedom in the bath takes place. Therefore, a reductionin the number of non-radiative deactivation pathways and an increase influorescence intensity occurs.

1-Pyrenesulfonyl chloride (PSC) was used as a chemical label forsilica epoxy interfaces, the surface coated with (3-aminopropyl)triethoxy-silane, because it reacts easily with amine groups, yielding sulphonamide

Epoxy Resins 197

NH3C CH3

S OO

NHCH2CH2

NH2

DNS-EDA

COHO

9-Anthroic acid

Figure 3.21: 9-Anthroic acid, 5-dimethylaminonaphthalene-1-(2-aminoethyl)-sulfonamide (DNS-EDA)

derivatives.279 Also 9-anthroic acid, its ester derivatives and 5-dimethyl-aminonaphthalene-1-(2-aminoethyl)sulfonamide (DNS-EDA), c.f. Figure3.21 are common fluorescence dyes.280, 281

3.4.9 Thermal Curing

By investigating the curing of a commercial epoxy prepolymer with imid-azole curing agents, it has been verified that the cure schedule influencesthe properties of the end product. The highest thermal stability of the poly-mers can be achieved by isothermal cure schedules. Samples cured by atemperature program showed lower glass transition temperatures. In a se-ries of temperature programmed curing experiments, a lower heating rateresulted in higher transition temperatures and superior thermal stability.The initial and postcure schedules are thus of critical importance for thefinal properties of the polymer.282

3.4.10 Microwave Curing

Due to increasing application in the aerospace and microelectronics indus-tries the demand for accelerated curing has emerged. In particular, for themicroelectronics industry, the curing of thermoset systems has become abottleneck of the whole production process. Besides photo curing, curingwith γ-rays and electron beams is an alternative.

Microwave curing of materials has the potential to deliver severalmajor advantages over conventional thermal processing. One of these is


a decrease in the time necessary for manufacture since another potentialadvantage is that the power is directed to the sample. The microwave en-ergy is absorbed throughout the body of the material rather than relyingon thermal conduction and convection. Therefore, the energy consumed isless than thermal curing.

Experiments with the diglycidyl ether of bisphenol A and three typesof curing agents, i.e., 4,4′-Diaminodiphenylsulfone, 4,4′-Diaminodiphen-ylmethane, and m-phenylene diamine with various energies of microwaveenergy showed that in comparison to thermal curing microwave curing isfaster. The glass transition temperatures are somewhat lower in the caseof the products cured with microwave technology in comparison to thosecured by thermal methods.283 However, the curing performance is stronglydependent on the curing agent used.284 The interfacial shear strengths inthose composites cured with microwave techniques are comparable withbeing thermally postcured.285

3.5 PROPERTIES

Mechanical properties of epoxy resins can be correlated and traced backto the constituting monomers. The mechanical properties of epoxy resinsdepend on the flexibility of the segments and on the crosslinking density.Epoxy resins shrink in the course of curing less than vinyl resins.

It is important to distinguish between the shrinkage that occurs be-fore gelling and after gelling. Only a shrinkage that occurs after gellingresults in residual stress in the final product.

Epoxy resins can exhibit several thermal transition regions, depend-ing on the chemical nature of the monomers. These transitions influencethe curing. If a glass transition occurs during curing at the temperature ap-plied, the individual reactive parts of the pendant molecules can no longermove sufficiently and the curing reaction freezes at this conversion. How-ever, raising the temperature effects further curing.

Cycloaliphatic epoxy resins have a low viscosity. The cured resinsexhibit a high glass transition temperature. On the other hand, they exhibitlow break elongation and toughness because of their high crosslinking den-sity.

Epoxy resins show good electrical properties. Of course, the elec-trical properties are affected by the moisture content. On the other hand,the resins can be made electrically conductive, by metal particles such as

Epoxy Resins 199

Table 3.14: Epoxies Based on Hybrid Polymers

Compounds Remark/Reference

Siloxane polymer with pendant epoxide rings 286–288

Epoxy polyurethane hybrid resinsMaleimide-epoxy resins 289

silver and copper. Epoxy resins adhere by forming strong bonds with themajority of surfaces, therefore, an important application is in adhesives.Epoxy resins have excellent resistance to acids, bases, organic and inor-ganic solvents, salts, and other chemicals.

3.5.1 Hybrid Polymers and Mixed Polymers

Hybrid polymers and mixed polymers are summarized in Table 3.14. Theseinclude silicone-epoxy hybrid polymers, urethane-epoxy hybrid polymers,and maleimide-epoxy polymers.

3.5.1.1 Epoxy-Siloxane Copolymers

A siloxane polymer with pendant epoxide rings on the side chain ofthe polysiloxane polymer backbone, when blended with diglycidyl bis-phenol A ether and cured, increases the mobility of the crosslinked networkand the thermal stability. Graft siloxane polymer with pendant epoxiderings can be synthesized by the hydrosilylation of poly(methylhydrosilox-ane) with allyl glycidyl ether.286

Aminopropyl-terminated poly(dimethylsiloxane) blended in an ep-oxy resin shows an outstanding oil and water repellency in coatings.290 Thepeel strength of a pressure-sensitive adhesive affixed to the modified epoxyresin also decreases. Polyether/poly(dimethylsiloxane)/polyether triblockcopolymers added in amounts of 5 ca. phr, efficiently reduce the staticfriction coefficient of the cured blends upon steel.291

Silsesquioxanes are organosilicon compounds with the general for-mula [RSiO3/2]n, c.f. Figure 8.1, at page 323. Silsesquioxane (SSO) solu-tions were reacted with diglycidyl either of bisphenol A with 4-dimethyl-aminopyridine as initiator, to result in SSO-modified epoxy networks. Themodification with SSO increased the elastic modulus in the glassy state.This is explained by an increase in the cohesive energy density.292


3.5.1.2 Maleimide-Epoxy Resins

Maleimide-epoxy resins are based on N-(p-carboxyphenyl)maleimide andallyl glycidyl ether.289 The resin can be cured thermally and is suitable asone component resin.

3.5.2 Recycling

3.5.2.1 Solvolysis

The recycling of wastes of epoxy resins is very difficult, because of theinherent infusibility and insolubility of the materials. Often the compositematerials contain reinforcing fibers, metals, and fillers.293

Efficient destruction of the organic material in composites can beachieved by thermolysis processes or by incineration processes. Thesemethods yield considerable amounts of non-combustible residues or de-composition products that are not attractive for further use.

Valuable recycled materials can be obtained by solvolysis methods.Here, the depolymerization products and reinforcing fibers can be retri-eved.

By glycolysis with diethylene glycol, the ester linkages of a bis ep-oxide (diglycidyl ether of bisphenol A) that is cured with a di anhydride,are cleaved. The transesterification is catalyzed by titanium n-butoxide.

In the case of 70% glass fiber/epoxide-anhydride composites, theglass fibers can be recovered. The liquid depolymerization products maybe converted to polyols, components for unsaturated polyester resins, etc.

The glycolysis of amine cured epoxide resins shows no volatile ni-trogen compounds. The most favored path of degradation is the decompo-sition of the ether linkage of bisphenol A to yield oligomers with phenolgroups.294 The separation of the phenolic compounds from the glycolysisproducts can be achieved by liquid-liquid extraction.

The glycolysis products can be basically used as a polyol in produc-tion of polyurethane. However, the hydroxyl value is much too high forpolyurethane production.

It has been suggested to use the solvolysis products from epoxideresins in combination with other solvolysis products, e.g., solvolysis prod-ucts from wastes from PET for semi interpenetrating networks based onPET hydrolyzate and epoxies.295

Epoxy Resins 201

3.5.2.2 Reworkable Epoxies for Electronic Packaging Application

Epoxy resins show excellent longevity and resistance to ageing. This is dueto the formation of an insoluble and infusible crosslinked network duringthe cure cycle. This property is sometimes seen as a drawback from therepairability standpoint.

During the manufacture of expensive electronic parts, such as multichip modules, several chips are mounted onto one high density board. Ifone chip is damaged, then the whole board will become useless. The sameis true if some special electronic parts in a board need to be modified be-cause of progress in technology.

Therefore, the availability of a reworkable material, that is, one thatundergoes controlled network breakdown, expands the potential routes torepairing, replacing, or removing assembled structures and devices. Imple-menting reworkable materials early could expand recycling concerns thatcould be faced in the near future.

An effective solution is to use thermally reworkable epoxide resinsfor underfilling.296, 297 In such systems, the cured epoxy network can bedegraded by locally heating to a suitable temperature, and the faulty chipcould be replaced.

Commercial cycloaliphatic epoxides degrade at about 300°C. Ep-oxides with secondary or tertiary ester bonds (as shown in Figure 3.22)have been demonstrated to decompose at temperatures between 200°C and300°C.216, 298 The epoxides are cycloaliphatic compounds and can be bas-ically derived by the esterification of cyclohexenoic acid with α-terpineolwith subsequent epoxidation. Diepoxides with carbamate and carbonategroups299 also degrade in this temperature range. In comparison to chem-ical degradation methods, heat to degrade the network can be localizedmore easily in the rework process, thereby allowing for more precise con-trol over the region of the board that will be reworked.

Instead of branched ester structures, ether structures, c.f. Figure3.22, bottom, are also suitable candidates for thermolabile linkages in ep-oxides.231

Thio links can be used to form a reversible network.219 Further di-epoxides connected via acetal links can be used for the introduction of re-versible chemical links.300 This type of network can be degraded in acidicsolvents.


OO

CH CH2

OOCH3

1,2-Bis (2,3-epoxycyclohexyloxy) propane

OO

CHCO

O CH3

CH3

CH3

2-Methyl-2,4-bis (2,3-epoxycyclohexyloxy) pentane

O

O

OO

CH3

CH3

O

O

OO

CH3

CH3

CH3

O

O

OO

O

O

OO

CH3

Figure 3.22: Epoxides with Thermally Cleavable Groups for Controlled NetworkBreakdown: Top Esters, Bottom Ethers. 1,2-Bis(2,3-epoxycyclohexyloxy)prop-ane, 2-Methyl-2,4-bis(2,3-epoxycyclohexyloxy)pentane

Epoxy Resins 203

3.6 APPLICATIONS AND USES

3.6.1 Coatings

The largest applications of epoxy resins are in coatings. Epoxy resin coat-ings have excellent mechanical strength and adhesion to many kinds ofsurfaces. They are corrosion resistant and resistant to many chemicals.Coatings find applications in various paints, white ware, and automotiveand naval sectors, for heavy corrosion protection of all kinds. Epoxy coat-ing formulations are available both as liquid and solid resins. Epoxy acrylichybrid systems are used as coatings for household applications, e.g., indoorand outdoor furniture and metal products.

Waterborne coatings are dispersions of special formulations of theresins with suitable surfactants. These materials can be applied by elec-trodeposition techniques. Powders can be applied as coatings by fluidized-bed techniques.

3.6.2 Foams

Epoxy resins can be fabricated to make foams. Foamable compositionshave been described from a novolak resin, an epoxy resin, and a blowingagent. Water can act as a blowing agent, especially when higher densityfoams are required. Novolak resins are typically suspended in an aqueoussolution, that is the blowing agent.301 Encapsulated calcium carbonate oranhydrous sodium bicarbonate are suitable blowing agents.302 Phosphoricacid is used to catalyze the polymerization of resin and it also reacts withthe carbonate core to generate a blowing gas to form voids.

3.6.3 Adhesives

Approximately 5% of total epoxy resin production is used in adhesive ap-plications. Epoxide resin adhesives are formulated two-component sys-tems that cure at room temperature, and as hot curing systems in the formof films and tapes. Among others, acrylates are suitable modifiers for ep-oxy adhesives.

3.6.4 Molding Techniques

Epoxy resins are used in all known reactive molding techniques. Non-re-inforced articles can be molded with aluminum molds. This is used for


electrical coil covering, etc. In electronic industries various embedmenttechniques, i.e., casting and potting, and impregnation are important ap-plications. Laminated sheets are used for the fabrication of printed circuitboards in the electronics industry.

Pultrusion and lamination are common techniques. Laminated arti-cles are also used in building constructions for concrete molds, honeycombcores, reinforced pipes, etc. Epoxy resins are superior to polyesters whereadhesion and underwater strength are important.

3.6.5 Stabilizers for Polyvinyl Chloride

Epoxy resins with monofunctional epoxy groups in the prepolymer are ef-fective in stabilizing polyvinyl chloride against dehydrochlorination duringprocessing and use, in comparison to tribasic lead sulphate. Lead-basedstabilizers for polyvinyl chloride are mostly banned and only allowed fora few applications. For example, the replacement of lead-based stabiliz-ers by epoxy stabilizers will improve the environmental toxicity of lead inwater flowing through PVC pipes.303

3.7 SPECIAL FORMULATIONS

3.7.1 Development of Formulations

In practice, epoxy resins are composed of a wide variety of individual com-ponents. To obtain a composition with the desired properties, a great dealof know-how is required.

A solid knowledge of the structure-property relationships can serveas a valuable tool for the art of formulation.304

On the other hand, there are methods that are helpful in the devel-opment of formulations. In particular, statistical methods can save time.An overview of such methods is given in the standard book of Box andHunter.305 Instead, most studies on epoxy formulation are done by the“one variable at a time” method. This means that only one parameter ofinterest is changed while the other remaining parameters are kept constant.This strategy provides admittedly valuable information, however, it doesnot allow a good insight into mutual interactions of formulation parame-ters. The usefulness of statistical methods in the field of formulation ofepoxy adhesives has been demonstrated in the literature.306

Epoxy Resins 205

3.7.2 Restoration Materials

A variety of epoxy resins are used for the consolidation of stone monu-ments in an outdoor environment. For these applications good weatheringresistance and sufficient penetration depth is mandatory.

A suitable epoxy monomer for restoration materials is 3-glycid-oxypropyltrimethoxysilane (GLYMO) and amine curing agent is (3-am-inopropyl)triethoxysilane (ATS). This monomeric composition penetratesdeep enough to exceed the maximum moisture zone and creeps beyond thedamaged layers.

The alkoxysilane groups are hydrolytically unstable and generatesilanol groups which may crosslink with one another, and also form bondsto the hydroxyl groups present in the stone, thus anchoring the organicpolymer onto the lithic matrix.307, 308

The curing kinetics of hybrid materials prepared from diglycidylether of bisphenol A and GLYMO has been investigated using poly(oxy-propylene)diamine as a hardener.309 The total conversion of epoxy groupswas found to decrease with increasing content of GLYMO. The experimen-tal data scattered, which was attributed to an uncontrolled initial hydrolysisof GLYMO caused by the varying air humidity during the sample prepara-tion.

3.7.3 Biodegradable Epoxy-polyester Resins

Biodegradable epoxy-polyester resins consist of polyesters with pendentepoxidized allyl groups.230 These polyesters are synthesized from succinicanhydride and allyl glycidyl ether and butyl glycidyl ether with benzyltri-methylammonium chloride as catalyst.

The butyl glycidyl ether acts as a diluent for the allyl functionalities,in order to reduce the amount of pendant allyl groups in the chain. Theepoxidation of the polyesters is achieved by m-chloroperbenzoic acid. Theepoxy-polyester resins can be cured with glutaric anhydride.

3.7.4 Swellable Epoxies

Hydrophilic polymers find applications in medicine and agriculture, owingto their biocompatibility.310

Crosslinked structures, prepared from sucrose and 1,4-butanediol di-glycidyl ether (1,4-BDE) are hydrogels with water regain values of 30%.311


The crosslinking is achieved with triethylamine or sodium hydroxide. Tri-ethylamine gives rise to end-capped diethylamine groups. By this reactionthe ethyl group is left behind as ethyl ether in the sucrose.

The ring-opening polymerization of epoxy end-terminated poly(eth-ylene oxide) (PEO) can serve to synthesize crosslinked materials with anexceptional swelling behavior.312 These gels have attracted interest for useas drug delivery platforms.

3.7.5 Reactive Membrane Materials

Reactive membrane materials can be prepared from 2-hydroxyethyl meth-acrylate and glycidyl methacrylate by radical photopolymerization.

Enzymes, such as cholesterol oxidase, can be directly immobilizedon the membrane by the reaction of the amino groups of the enzyme andthe epoxide groups of the membrane. The immobilization improves the pHstability of the enzyme as well as its thermal stability. The immobilizedenzyme activity remains quite stable.313

Poly(2-hydroxyethyl methacrylate) membranes can be also activatedby direct treatment with epichlorohydrin. On such materials poly(L-lysine)could be immobilized.314 Such a membrane with immobilized poly(L-lysine) can be utilized as an adsorbent for DNA adsorption experiments.

3.7.6 Controlled-release Formulations for Agriculture

In order to introduce pendant dichlorobenzaldehyde functionalities as ac-etals, the epoxide functionalities in linear and crosslinked poly(glycidylmethacrylate) are hydrolyzed to diol groups. In the second step the pendantdiol groups in the polymers are acetalized by dichlorobenzaldehyde.315 Di-chlorobenzaldehyde is a bioactive agent that is slowly released under vari-ous conditions.

3.7.7 Electronic Packaging Application

In flip-chip manufacturing, filled polymers serve as underfilling. Underfill-ing is the plastic material which is inserted in the gap between integratedcircuit and the substrate. The gap is approximately 50 to 75 µm wide. Theunderfilling is used to couple the chip and the substrate mechanically. It de-creases the residual stress in the solder joints caused by thermal expansion.

Epoxy Resins 207

The materials used for underfilling should have good wetting character-istics, significant adhesion, high conductivity, and should not form voids.The prevention of void formation is essential for thermal conductivity. Lowviscosity of the monomeric resin is essential to achieve void-free underfill-ings. A resin with a lower viscosity allows the addition of a greater amountof filler. The viscosity of a benzoxazine resin can be reduced by the incor-poration of a low-viscosity epoxy resin. The benzoxazine resin imparts alow water uptake, a high char yield, and mechanical strength. The epoxyresin reduces the viscosity of the mixture and results in higher crosslinkingdensity and improved thermal stability of the material. A melt viscosity ofabout 0.3 Pas at 100°C can be achieved.316

3.7.8 Solid Polymer Electrolytes

The interest in solid polymer electrolytes arises from the possibility of ap-plications of polymer ionic conductors in energy storage systems, electro-chromic windows, and fuel cells or sensors operating from subambient tomoderate temperatures.317

Hosts for solid polymer electrolytes are poly(ethylene oxide) (PEO),segmented polyurethanes with poly(ethylene oxide)/ poly(dimethylsilox-ane)318and with poly(ethylene oxide)/perfluoropolyether319 blocks, respec-tively, as well as crosslinked epoxy-siloxane polymer complexes.320, 321

The copolymers are immersed in a liquid electrolyte (1 M LiClO4 in prop-ylene carbonate) to form gel-type electrolytes.

Solvent-free solid polymer electrolytes are based on polyether epoxycrosslinked with poly(propylene oxide) polyamines.322 The crosslinkedpolyether networks are doped with LiClO4. The network is prepared bymixing epoxy monomer, the curing agent dissolved in acetone and LiClO4.To obtain films the mixture is poured on plates and cured at elevated tem-peratures. The electric conductivity of the polymer electrolyte is dependenton interactions between ions and the host polymer.

3.7.9 Optical Resins

3.7.9.1 Lenses

In comparison to glasses, plastics have low density, i.e., comparative lowweight, are fragmentation-resistant and can be easily dyed. Therefore, op-tical materials made from organic polymers are attractive for optical ele-


ments such as lenses of eyeglasses and cameras. However, the refractiveindex of the standard resins is relatively small. Therefore, there is a needto use materials with high refractive index and low chromatic aberration.

The introduction of sulfur into the monomers raises the refractive in-dex. Sulfur-containing resins have a high refractive index, low dispersion,and a good heat stability.23, 216 Components for epoxy resin with high re-fractive index are obtained from bis(3-mercaptophenyl)sulfone (BEPTPhS)and epichlorohydrin.

A sulfur-containing curing agent is trimercaptotriethylamine(TMTEA) which can be obtained from triethanolamine. Besides sulfur-containing epoxies, with tailor-made polyphosphazenes, refractive indicesranging from 1.60 to 1.75 can be achieved.323

3.7.9.2 Liquid Crystal Displays

In liquid crystal displays (LCDs), control of the alignment of the liquidcrystal (LC) molecules is one of the most important issues with respectto the quality of LCDs. The rubbing method does not satisfy the recentdemands for alignment quality. The photoalignment method reduces con-taminations that lower the contrast ratio and electrostatic buildup that cancause failure of thin film transistors.324

Nematic liquid crystalline materials can be aligned homogeneouslyon a photoreactive polymer film when exposed to linearly polarized light.Thermal stability and photostability of the alignment layer is a crucial pa-rameter and the alignment layer must be transparent in the visible regionfor a display device. Certain photocrosslinkable polymer systems meetthese demands. Derivatives of cinnamic ester and cinnamic acid are suit-able candidates for phototransformations. In particular, the anisotropic[2+2] cycloaddition of the cinnamate moiety can induce an irreversiblealignment of a low molecular weight liquid crystal. Polymers with thechalcone group in the side chain react in a similar way. A chalcone-epoxycompound can be synthesized from 4,4′-Dihydroxychalcone and epichlo-rohydrin in the same way as with bisphenol A. In this photoreactive epoxyoligomer, the photosensitive unsaturated carbonyl moieties are located inthe main chain. For the polymerization of the epoxy groups, triarylsulfon-ium hexafluoroantimonate (TSFA) is a suitable photoinitiator.

The photodimerization of the chalcone precedes the photopolymer-ization of the epoxy groups. Under continuous irradiation, the anisotropic

Epoxy Resins 209

C

O

CH

CH OH

HO

1,3-Bis-(4-hydroxy-phenyl)-propenone

Figure 3.23: 4,4′-Dihydroxychalcone

photocrosslinked chain molecules can be frozen by the photopolymeriza-tion of the epoxy groups at both ends of the compound. Without a photo-initiator, the end groups of the oligomer are not fixed. Therefore, there aretwo kinds of photochemical reactions that enhance the photostability of theinduced optical anisotropy.24

3.7.9.3 Holography

Materials for high-resolution holograms, which can be used on holographicoptical elements such as heads-up display, consist of a bisphenol-type ep-oxy resin and a radically polymerizable aliphatic monomer. A diaryliod-onium salt and 3-ketocoumarin (KCD) are used as a complex initiator. Theformation of the image is based on the radical polymerization of the mono-mer initiated by a holographic exposure, followed by the cationic polymer-ization of the epoxy resin by UV-exposure after post-exposure baking.325

3.7.9.4 Nonlinear Optical Polymers

Second-order nonlinear optical (NLO) polymeric materials are of interestbecause of their potential applications in integrated optical devices, such aswaveguide electro-optic modulators, switches, and optical frequency dou-bling devices. The interest in these polymeric materials is mainly due totheir large optical nonlinearities, low dielectric constants, and ease of pro-duction. For practical use, the poled polymers must possess large second-order optical nonlinearities which should be sufficiently stable at ambienttemperature for a long period of time.

A high crosslinking density and stiffness makes interpenetrating net-works attractive for such applications. The possibility of introduction ofchromophores that impart the nonlinear optical properties is essential.


An example for an NLO active interpenetrating polymer network isan epoxy prepolymer and a phenoxy-silicon polymer. 4,4′-Nitrophenyl-azoaniline (Disperse Orange 3) functionalized with crosslinkable acryloylgroups is incorporated into the epoxy prepolymer. The epoxy prepolymerforms a network through acryloyl groups which are reactive at high tem-peratures without the aid of any catalyst or initiator. The phenoxy-siliconpolymer is based on an alkoxysilane dye made of (3-glycidoxypropyl)tri-methoxysilane and Disperse Orange 3, and 1,1,1-tris(4-hydroxyphenyl)-ethane, as a multifunctional phenol. The two networks are formed simul-taneously and separately at 200°C.326

Interpenetrating polymer networks based on crosslinked polyureth-ane/epoxy based polymer can be obtained by simultaneously crosslinkingphenol-capped isocyanates with 2-hydroxypropyl acrylate and curing ep-oxy prepolymers. To each of these components phenylazo-benzothiazolechromophore groups are linked. The crosslinked polyurethane and the ep-oxy based polymer show glass transition temperatures of 140 and 178°C,respectively, whereas the interpenetrating network shows two Tg’s at 142and 170°C. Thin, transparent poled films of the crosslinked polymers canbe prepared by spin-coating, followed by thermal curing and corona polingat 160°C. The polymers exhibit a long-term stability of the dipole align-ment at 120°C.327

3.7.10 Reactive Solvents

Polymers can be processed more easily by using solvents. The disadvan-tage the necessary removal of the solvent. This might be tedious and atime consuming step. Also, environmental hazards may arise. Reactivesolvents are those that polymerize after the molding process. In this case,no removal is necessary. Accordingly, intractable polymers can be pro-cessed by the utilization of reactive solvents. The polymers are dissolvedin a liquid curable resin. Then the homogeneous solution is transferred intoa mold. The curing of the reactive solvent takes place in the mold.

In the course of curing, molecular weight of the resin increases. Aphase separation and phase inversion are likely to take place. The dissolvedpolymer should become the continuous matrix, and the reactive solvent isdispersed as particles in the matrix. So the final properties of the systemare dominated by the properties of the thermoplastic phase.

The main advantage of this procedure is a lower processing temper-

Epoxy Resins 211

ature because of decrease with viscosity. There is no need to remove thesolvent which is bounded to the polymer.

3.7.10.1 Poly(butylene terephthalate)

Although poly(butylene terephthalate) can be relatively easily processed,a further improvement of the processing is required when a difficult flowlength or mold geometry has to be mastered.328

3.7.10.2 Poly(phenylene ether)

Poly(2,6-dimethyl-1,4-phenylene ether) (PPE) can be dissolved at elevatedtemperatures in an epoxy resin and the solution can be easily transferredinto a mold or into a fabric.329

During the curing of epoxy resin, a phase separation and a phaseinversion occurs. The originally dissolved PPE then becomes the contin-uous phase. The dispersed epoxy particles become an integral part of thesystem and act as fillers or as toughening agents, depending on the type ofepoxy resin. An important parameter for the final physical and mechanicalproperties is the size of the dispersed particles.

The size of the dispersed phase is governed by the competition be-tween the coalescence of dispersed droplets, and the vitrification or gela-tion rate, respectively, induced by the curing process. For the coalescence,the viscosity of the system plays an important role which is dependent onthe curing temperature. The viscosity can be further controlled by addinganother thermoplastic material such as poly(styrene).

Blends of poly(phenylene ether) and an epoxy resin cured with di-cyandiamide materials show a two-phase morphology. To improve the uni-formity and miscibility, triallyl isocyanurate (TAIC) can be used as an in-situ compatibilizer.330 Also the fracture toughness of the modified systemsis improved by adding TAIC.

3.7.11 Encapsulated Systems

Photopolymerizable liquid encapsulants (PLE) for microelectronic devicesmay offer important advantages over traditional transfer molding com-pounds. A PLE is comprised of an epoxy novolak-based vinylester resin,fused silica filler, a photoinitiator, a silane coupling agent, and optionallyof a thermal initiator.331


3.7.12 Functionalized Polymers

The epoxy group can be used to functionalize various polymers, to achievecertain desired properties.

3.7.12.1 Tougheners

Vinylester-urethane hybrid resins (VEUH) can be toughened by function-alized polymers.332 Suitable basic materials for toughening are nitrile rub-ber, hyperbranched polyesters, and core/shell rubber particles. These ma-terials can be functionalized with vinyl groups, carboxyl groups, and epoxygroups.

Toughness is improved in VEUH when the functional groups of themodifiers react with the secondary hydroxyl groups of a bismethacryloxyvinylester resin and with the isocyanate groups of the polyisocyanate com-pound. Functionalized epoxy and vinyl hyperbranched polymers are lessefficient as toughness modifiers in comparison to functionalized liquid ni-trile rubber. They show no adverse effect on the mechanical properties.

3.7.13 Epoxy Resins as Compatibilizers

Most polymers are not miscible with one another. This lack of miscibilityresults in poor properties of polymeric blends. However the properties canbe improved by adding compatibilizers. Due to the inherent reactivity ofthe epoxy group, an interfacial chemical bonding can be achieved whichresults in small particle sizes of the blend. This enhances the properties ofthe blends. Some compatibilizers based on epoxy compounds are shownin Table 3.15.

3.7.13.1 Polyamide Blends

Blends of polyamide 6 and epoxidized ethylene propylene diene (e-EPDM)can improve the toughness of polyamide 6. The particle size of e-EPDMis much smaller than that of unepoxidized EPDM (u-EPDM) rubber in apolyamide 6 matrix. It is believed that the epoxy group in e-EPDM re-acts with the polyamide 6 to form a graft copolymer. Thus an interfacialcompatibilization takes place.333

Styrene/glycidyl methacrylate (SG) copolymers are miscible withsyndiotactic poly(styrene) (s-PS). In blends of polyamide 6 (PA6) withsyndiotactic poly(styrene), the epoxide units in the SG phase are capable

Epoxy Resins 213

Table 3.15: Compatibilizers Based on Epoxy Compounds for VariousPolymers

Polymer 1 Polymer 2 Compatibilizer

PA6 PS Styrene/glycidyl methacrylate copolymersPA6 ABS Glycidyl methacrylate/ methyl methacrylate co-

polymers (GMA/MMA)PA6 PP Poly(ethylene) functionalized with maleic anhy-

dridePBT PPE Low molecular weight epoxy compoundsPBT SAN Terpolymers of methyl methacrylate, glycidyl

methacrylate (GMA), and ethyl acrylate

PA6 Polyamide 6PS Poly(styrene)PBT Poly(butylene terephthalate)ABS Acrylonitrile butadiene styrene (ABS) copolymersPP Poly(propylene)SPE Poly(phenylene ether)SAN Styrene/acrylonitrile copolymers

of reacting with the PA6 end groups. Copolymers of styrene/glycidyl meth-acrylate are effective in reducing the s-PS domain size and improving theinterfacial adhesion. The best compatibilization is found with a content of5% glycidyl methacrylate (GMA) in the SG copolymer. Both the strengthand modulus of the blend are improved by the addition of the SG copoly-mers. However, a loss in toughness is observed at loadings of copolymer.The addition of SG copolymer to the blend has little influence on the crys-tallization behavior of the polyamide component. The crystallinity of s-PSis reduced.334

Blends of nylon 6 with acrylonitrile/butadiene/styrene (ABS) co-polymers and with styrene/acrylonitrile copolymers (SAN) can be preparedusing glycidyl methacrylate/methyl methacrylate (GMA/MMA) copoly-mers as compatibilizing agents.335

Known compatibilizers for blends of low density poly(ethylene)(LDPE) and polyamide 6 (PA6) are ethylene/acrylic acid copolymers(EAA), maleic anhydride functionalized polyethylenes, and an ethylene-/glycidylmethacrylate copolymer (EGMA). The effectiveness of EGMAas a reactive compatibilizer is comparable to that of the EAA copolymers.However the effectiveness is lower than that of poly(ethylene) functional-ized with maleic anhydride. A possible reason is the reaction of the pendent


epoxy groups with the amide groups that attach the polyamide moleculestogether and hinder the dispersion in this way.336

In blends of poly(propylene) and polyamide 6, poly(ethylene) func-tionalized with maleic anhydride showed better compatibilization thanglycidyl methacrylate. The compatibilizing effect of the PP-MA for thePP/Ny6 blends was more effective than poly(propylene) functionalizedwith glycidyl methacrylate.337

Glycidyl methacrylate copolymers are miscible with SAN. The ep-oxide unit can react with the polyamide end groups. The compatibilizercan form graft copolymers at the polyamide/SAN interface during meltprocessing. Incorporation of the compatibilizer does not significantly im-prove the impact properties of nylon 6/ABS blends.

The direct mixing of polyamide and poly(propylene) leads to incom-patible blends with poor properties. Poly(propylene) functionalized withglycidyl methacrylate can be used as a compatibilizer in the blends of PPand nylon 6.338

3.7.13.2 Poly(butylene terephthalate)

Poly(butylene terephthalate) (PBT) and poly(phenylene ether) (PPE) canbe compatibilized by low molecular weight epoxy compounds.339 Ter-polymers of methyl methacrylate, glycidyl methacrylate (GMA), and ethylacrylate are effective reactive compatibilizers for blends of (Poly(butyleneterephthalate) (PBT) with styrene/acrylonitrile copolymers (SAN) or ABSmaterials.340 During melt processing, the carboxyl end groups of PBT re-act with epoxide groups of GMA to form a graft copolymer.

In blends of poly(butylene terephthalate) with an ethene/ethyl acryl-ate copolymer (E/EA), which show the general features of uncompatibi-lized polymer blends, such as a lack of interfacial adhesion and a relativelycoarse unstabilized morphology, no evidence of transesterification reac-tion was found. In contrast, blends containing both virgin and modifiedE/MA/GMA terpolymers show a complex behavior. Two competitive re-actions take place during the melt blending:

1. Compatibilization due to interfacial reactions between PBT chainends and terpolymer epoxide groups, resulting in the formation ofE/MA/GMA/PBT graft copolymer, and

2. Rapid crosslinking of the rubber phase due to the simultaneouspresence of hydroxyl and epoxide groups on E/MA/GMA chains.

Epoxy Resins 215

The competition reactions between compatibilization and crosslink-ing are dependent on the type of the terpolymer, since the modified E/-MA/GMA contains hydroxyl groups before mixing. The in-situ compati-bilization reaction of the pendent epoxy groups with PBT causes the for-mation of E/MA/GMA hydroxyl groups.341

The concentration of carboxyl groups at the PBT chain ends influ-ences the rate of compatibilization but not the final morphology. The lowerthe concentration, the slower the morphology development. Ternary blendsof PBT/(E-MA-GMA/E-MA) exhibit a very fine morphology. Here the de-velopment of the morphology is mildly influenced by the crosslinking rateof the rubber phase caused by the shear rate in the mixing chamber.342

3.7.14 Surface Metallization

Established methods for the metallization of a polymer surface are343

1. Electroless plating,2. Vacuum deposition or metal spraying, and3. Coating using a metallic paint.

A more recent method has been described that utilizes the reductionof metal ions incorporated directly in the polymer. It has been shown thatcobalt or nickel ions integrated in an epoxy network could be reduced tothe pure metal by dipping the film in an aqueous NaBH4 solution.229

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Reactive Polymers Fundamentals and Applications || Epoxy Resins