Modification of aromatic diamine-cured epoxy resins by poly(oxymethylene) or hybrid modifiers containing poly(oxymethylene)

Polymer International 44 (1997) 125È133

Modification of Aromatic Diamine-curedEpoxy Resins by Poly(oxymethylene) or

Hybrid Modifiers ContainingPoly(oxymethylene)

Takao Iijima,* Atsushi Sugizaki, Wakichi Fukuda & Masao Tomoi

Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku,Yokohama 240, Japan

(Received 28 October 1996 ; revised version received 22 January 1997 ; accepted 7 February 1997)

Abstract : A copolymer comprising poly(oxymethylene) (POM, polyacetal) wasused to improve the fracture toughness of a resin based on diglycidyl ether ofbisphenol A (DGEBA) cured with 3,3@-dimethyl-5,5@-diethyl-4,4@-diaminodiphenyl methane. POM was a less e†ective modiÐer for epoxies and athird component was used as a toughener or a compatibilizer for POM. Thethird component includes polypropylene glycol-type urethane prepolymer (PU)and aromatic polyesters. The hybrid modiÐers composed of POM and PU weremore e†ective as modiÐers for toughening epoxies than POM alone. In theternary DGEBA/POM/PU (90/10/10 wt ratio) blend, the fracture toughness, KIC ,for the modiÐed resin increased 50% with retention of Ñexural properties and aslight decrease in glass transition temperature compared with those of the(T g)unmodiÐed epoxy resin. The aromatic polyesters include poly(ethylene phthalate)(PEP), the related copolyesters and poly(butylene phthalate). PEP was moste†ective of them as a third component in the hybrid modiÐer. In the ternaryDGEBA/POM/PEP (85/15/10) blend, for the modiÐed resin increased 70%KICwith medium loss of Ñexural strength and retention of The tougheningTg .mechanism is discussed in terms of morphological and dynamic viscoelasticbehaviour of the modiÐed epoxy resin systems.

Polym. Int. 44, 125È133 (1997)No. of Figures : 5. No. of Tables : 3. No. of References : 43

Key words : epoxy resin, 3,3@-dimethyl-5,5@-diethyl-4,4@-diaminodiphenyl-methane, polyacetal, polyurethane prepolymer, aromatic polyesters, fracturetoughness.

INTRODUCTION

Epoxy resins are among the most important of thehigher performance thermosetting polymers and havewide use as structural adhesives and matrix resins forÐbre composites, but the cured resins are often brittle,with poor resistance to crack propagation. The tough-ness of epoxy resins has been increased by blendingwith reactive liquid rubbers such as carboxyl-terminated butadiene acrylonitrile rubbers (CTBN)1 or

* To whom all correspondence should be addressed.

epoxide-containing acrylic elastomers.2h6 The use ofreactive rubbers leads to a reduction in the mechanicaland thermal properties. Furthermore, reactive rubbersare rather less e†ective in the highly cross-linkedepoxies.6 Engineering thermoplastics have proved to beinteresting materials as modiÐers for epoxy resins fromthe viewpoint of the maintenance of mechanical andthermal properties for the matrix resins. Engineeringthermoplastics which have been examined as modiÐersinclude poly(ether sulphone),7h9 polysulphones,10h12poly(etherimide),13h16 poly(ether ketone)s,17h19 poly-(phenylene oxide),20,21 poly(butylene terephthalate),22,23

1251997 SCI. Polymer International 0959-8103/97/$17.50 Printed in Great Britain(

126 T . Iijima et al.

aromatic polyesters such as poly(ethylene phthalate), allof which are soluble in epoxy resins without sol-vents,24h26 and N-phenylmaleimideÈstyrene copolymerand related copolymers.27h29 Recently we have reportedthat N-phenylmaleimideÈstyreneÈp-hydroxystyrene(PMSH) and N-phenylmaleimideÈN-(p-hydroxy)phenylmaleimideÈstyrene (HPMS) terpolymers, havingfunctionalities reacting with epoxide, were e†ectivemodiÐers for aromatic diamine-cured epoxy resins.30,31The hybrid modiÐers composed of PMS and PMSHwere also e†ective for epoxies.32,33

Poly(oxymethylene) (POM, polyacetal) is a semicrys-talline engineering plastic having excellent physical andmechanical properties. To date it has apparently notbeen used to modify thermosets, because POM is insol-uble in most solvents. Furthermore, only a few poly-mers, such as polyurethane, are known to blend withPOM.34h41 This paper reports the modiÐcation of thearomatic diamine-cured DGEBA epoxy resin bycopolymer-type POM and the hybrid modiÐers con-taining polyurethane prepolymer (PU) or aromaticpolyesters as a third component. The aromatic poly-esters include poly(ethylene phthalate) and the relatedcopolyesters and poly(butylene phthalate).

EXPERIMENTAL

Materials

The epoxy resin was the liquid bisphenol-A diglycidylether (DGEBA; Epikote 828TM, Shell Chemical Indus-trial Corp., epoxy equivalent 190). The copolymer-typePOM was supplied by Mitsubishi Gas Chemical Co.,Inc. (Iupital F20-01TM ; melt index 9É0, 2É5 mol% oxy-ethylene units). 3,3@-Dimethyl-5,5@-diethyl-4,4@-diaminodiphenylmethane (Curehard MEDTM, IharaChemical Industry Inc. ; abbreviated as CMED) wasused as a curing agent. PU was provided by NipponPolyurethane Corp. (Colonate 4193TM ; toluenediisocyanate-terminated polyoxypropylene type, iso-cyanate units 1É33 meq/g, 1500) and used asM1 nreceived.The aromatic polyesters were prepared by the reactionof aromatic dicarboxylic acids and a,u-alkanediol asreported previously.24 Table 1 shows the characteristicsof the polyesters. The gel permeation chromatography(GPC) average molecular weight was used as a measureof the molecular weight in this study, as well as in theprevious papers,24h26 because the number averagemolecular weight is highly sensitive to the presence of asmall amount of lower-molecular-weight materials.Other reagents were used as received.

Measurements

1H nuclear magnetic resonance (NMR) spectra of thepolyesters were recorded on a 90 MHz instrument

TABLE 1. Characterization of aromatic polyesters

Entry no. Polyestera M1GPC

b M1n

b M1w/M1

nT

gc

composition (103) (103) (¡C)

(mol%)

PEP 1 – 9·6 6·7 1·6 26

2 – 12·1 8·1 1·7 –

3 – 12·4 7·5 1·8 –

PEPI IP 10 8·0 5·3 1·5 31

PEPT TP 10 6·9 5·1 1·5 –

PBP – 15·1 10·1 1·8 –

a IP, isophthalate unit ; TP, terephthalate unit.

b By GPC.

c By DSC.

(JEOL JNM-9MX 90) at 60¡C using as solventCDCl3and tetramethylsilane as internal standard. Molecularweights of the polyesters were measured by GPC(Shimadzu LC-5A instrument) using polystyrene stan-dards. The mechanical properties of the cured resinswere determined at ambient temperature with a Shi-madzu autograph AGS-500B universal testing machine.Flexural tests were carried out at a cross-head speed of2 mm min~1 (JIS K7203). The fracture toughness, KIC ,was measured in a three-point bending geometry at across-head speed of 1 mm min~1 (ASTM E-399). Theglass transition temperatures of both the polyesters(Tgs)and cured resins were measured as the onset tem-peratures by di†erential scanning calorimetry (DSC;Shimadzu DSC 41M) at a heating rate of 10 K min~1 inan atmosphere of nitrogen (c. 50 ml min~1). Scanningelectron micrographs (SEMs) were taken with a HitachiSEM S-2100A instrument using failed specimens fromthe tests. Dynamic viscoelastic analysis was per-KICformed with a Rheometrics RDS-II type (RheometricsCo.) between [ 150 and 250¡C at a heating rate of5 K min~1 at frequency of 1 Hz under nitrogen (c.50 ml min~1).

Curing procedure

When using POM alone as a modiÐer, the POM wasdissolved into DGEBA at 160È170¡C without solvents.CMED, the hardener, was then added to the mixture,which was kept at 160¡C. The resulting clean mixturewas poured into a mould preheated at 160¡C to obtainplagues 7 mm thick. The mould consisted of one pair ofupright and metal clip-held glass plates spaced by U-shaped silicone rubber. The resin plate obtained wasmachined using a diamond saw. The curing cycle was160¡C/1 h ] 180¡C/5 h in an atmosphere of air. CMEDwas used in stoichiometric amount to the epoxy resin.

When using the hybrid modiÐers, POM and the thirdcomponent (PU or polyesters) were dissolved intoDGEBA at 160È170¡C and then CMED was added at160¡C. The curing cycle was 160¡C/1 h] 180¡C/5 h in

POLYMER INTERNATIONAL VOL. 44, NO. 2, 1997

ModiÐcation of cured epoxy resins 127

TABLE 2. Physical properties of modified epoxy resinsa

Entry Resin compositions KIC

n b Flexural properties Tg

c (¡C)

no. (wt ratio) (MPa m1@2)Strength Modulus nb DSC DVA

DGEBA POM PU (MPa) (GPa)

Control – – – 0·78 À0·03 6 129 À4 2·62 À0·01 6 158 184

EP18 95 5 0 0·79 À0·02 6 119 À2 2·62 À0·04 8 – –

EP19 90 10 0 0·82 À0·02 5 125 À5 2·71 À0·02 7 – 180

EU01 95 0 5 0·70 À0·01 7 124 À1 2·61 À0·03 8 140 –

EU02 90 0 10 0·68 À0·02 6 120 À3 2·59 À0·03 8 132 169

EPU08 90 10 3 0·89 À0·02 8 123 À2 2·56 À0·03 8 – –

EPU03 90 10 5 0·86 À0·01 7 121 À6 2·63 À0·02 8 – –

EPU09 90 10 10 1·04 À0·03 9 121 À2 2·55 À0·03 7 – 155

EPU16 90 10 15 0·82 À0·03 6 88 À1 2·68 À0·03 7 – –

EPU10 85 15 10 1·10 À0·33 7 73 À25 2·62 À0·05 6 – –

a The Àx values show standard deviation.

b Number of specimens tested.

c By DSC or dynamic viscoelastic analysis (DVA).

an atmosphere of air. The resin compositions are shownin Tables 2 and 3.

RESULTS AND DISCUSSION

Mechanical and thermal properties of modified epoxyresins

4,4@-Diaminodiphenyl sulphone (DDS) and 4,4@-diaminodiphenyl methane (DDM) are widely used asaromatic diamine curing agents. DDS could not be usedin this study, because compatibility of POM and the

DGEBA/DDS matrix was poor and specimens couldnot be prepared. DDM displayed a higher compatibilitywith POM than DDS, but DDM is carcinogenic. Con-sequently, CMED has been commercialized as an alter-native to DDM and has higher compatibility withPOM than DDS. Thus, CMED was used as a curingagent in this study. Curing behaviour of the DGEBA/CMED resin was examined at a heating rate of10 K min~1 by DSC to obtain the optimum curing con-ditions ; the onset and peak temperatures in the exother-mic peak were 159 and 199¡C, respectively, which weresimilar to those of the DGEBA/DDS material (161 and

TABLE 3. Physical properties of modified epoxy resinsa

Entry no. Resin compositions (wt ratio) KIC

(MPa m1@2) n c Flexural properties Tg

d (¡C)

DGEBA POM polyesterb Strength Modulus n c DSC DVA

(MPa) (GPa)

Control – – – 0·78 À0·03 6 129 À4 2·62 À0·01 6 158 184

EP19 90 10 0 0·82 À0·02 5 125 À5 2·71 À0·02 7 – 180

EE01 90 0 10 (PEP2) 0·88 À0·05 5 128 À5 2·84 À0·01 6 160 –

EE02 80 0 20 (PEP2) 1·07 À0·03 6 129 À1 2·89 À0·05 7 144 178

EPE22 90 10 5 (PEP1) 0·93 À0·01 6 120 À3 2·68 À0·04 7 – –

EPE35 90 10 15 (PEP3) 1·16 À0·09 7 83 À2 2·68 À0·09 8 – –

EPE24 85 15 7·5 (PEP1) 1·17 À0·14 7 82 À3 2·68 À0·02 6 – –

EPE25 85 15 10 (PEP1) 1·35 À0·06 8 85 À15 2·69 À0·08 5 – ¿177

EPE31 85 15 15 (PEP3) 1·33 À0·08 7 83 À14 2·63 À0·11 8 – –

EPE28 85 15 10 (PEPI) 1·09 À0·11 9 69 À11 2·78 À0·04 5 – –

EPE29 85 15 10 (PEPT) 1·23 À0·17 8 69 À11 2·81 À0·03 5 – –

EPE17 90 10 15 (PBP) 1·04 À0·14 7 76 À19 2·58 À0·06 7 – –

a The Àx values show standard deviation.

b The entry number in parentheses is shown in Table 1.

c Number of specimens tested.

d By DSC or dynamic viscoelastic analysis (DVA).



Fig. 1. SEMS of fracture surfaces of the ternary DGEBA/POM/PU blend system: (A) control ; (B) binary DGEBA/PU (90/10)blend ; (C) binary DGEBA/POM (90/10) blend ; (D) ternary DGEBA/POM/PU (90/10/10) blend.

207¡C) and higher than those of the DGEBA/DDMcuring system (120 and 148¡C). In order to avoid solidi-Ðcation of POM during curing, this was carried out at160¡C for 1 h followed by 180¡C for 5 h. Table 2 showsthe results for the modiÐcation of the epoxy resin withPOM or the hybrid modiÐers composed of POM andPU. The unmodiÐed epoxy resin was transparent, whilethe POM (10 wt%)-modiÐed resin became opaqueduring curing. When using 15 wt% POM, the modiÐedresin phase-separated macroscopically to a considerableextent and specimens could not be prepared. The use ofPOM (10 wt%) alone as a modiÐer led to only a smallincrease in the fracture toughness, for the modiÐedKIC ,resin (Table 2, entry no. EP19). Unexpectedly, POMwas less e†ective as a modiÐer for epoxies ; a third com-ponent was therefore used as a toughener or a compat-ibilizer for POM. To date there have been only a fewstudies on blends of POM and other polymers.34h41Polyurethane is one of few polymers compatible withPOM and is used as a toughener for it.34,35,38,40 Hence,

hybrid modiÐers composed of POM and polyurethane(PU) prepolymer were examined for improvement of thetoughness of epoxies : the PU-modiÐed resins weretransparent and the use of PU alone was less e†ective(Table 2, EU01 and EU02). The values for theKICmodiÐed resins increased and then decreased withincreasing PU content in the ternary epoxy/POM/PUblend system; in the resin composition of DGEBA/POM/PU (90/10/10 wt ratio), for the modiÐed resinKICincreased 50% with retention in Ñexural properties,compared with those of the unmodiÐed epoxy resin(Table 2, EPU09). In the ternary DGEBA/POM/PU(85/15/10) blend, the modiÐed resin phase-separatedmacroscopically (Table 2, EPU10). The for theTgunmodiÐed epoxy resin was 158¡C and the POM usedin this study had an onset temperature for melting at159¡C, by DSC. The for the modiÐed resins couldTgsnot be determined because of overlapping of the twoendothermic peaks. Consequently, the for the modi-TgÐed resin was obtained by dynamic viscoelastic analysis,



as shown later ; the incorporation of PU led to adecrease in the for the modiÐed resin.Tg

Some kinds of aromatic polyesters have been used astougheners for POM.42 We have already reported thataromatic polyesters, including poly(ethylene phthalate)(PEP), poly(butylene phthalate) (PBP) and the relatedpolyesters, are e†ective modiÐers for epoxies.24h26 Thus,aromatic polyesters were used as a third component ofthe hybrid modiÐer. The polyesters used include PEP,poly(ethylene phthalate-co-ethylene isophthalate)(10 mol% isophthalate unit) (PEPI), poly(ethylenephthalate-co-terephthalate) (10 mol% terephthalate unit)(PEPT) and PBP. The characteristics of the polyestersare shown in Table 1. In this study, PEP1, 2 and 3 wereused as equivalent modiÐers. Table 3 shows the modiÐ-cation results for the ternary blends of DGEBA/POM/polyesters. The toughness in the ternaryDGEBA/POM/PEP blend was improved more signiÐ-cantly than in the other ternary blends. for theKICmodiÐed resin increased 70% with 35% loss of Ñexuralstrength and with retention in in the ternaryTgDGEBA/POM/PEP (85/15/10) blend (Table 3, EPE25).In the resin composition of DGEBA/POM/PEP (85/15/15), the modiÐed resin phase-separated macroscopically(Table 3, EPE31). The use of the other polyesters as thethird component led to macroscopic phase-separationof the modiÐed resin.

Microstructures of the modified epoxy resins

The morphologies of the cured resins were investigatedby scanning electron microscopy (SEM). The unmod-iÐed cured epoxy resin had only one phase and a fea-tureless morphology (Fig. 1(A)). When using PU(10 wt%) alone, the modiÐed resin had no distinct mor-phology (Fig. 1(B)). Inclusion of POM led to phaseseparation. When using 10 wt% of POM, the modiÐedresin had a two-phase morphology with POM-richspherical particles dispersed in the epoxy-rich matrix(Fig. 1(C)) ; the average diameter (D) of the particles was1É9 km and the volume fraction (0É073) of the POM-(Vf)rich particles was equal to the feed weight fraction(0É075) of POM. In the ternary DGEBA/POM/PU (90/10/10) blend, the modiÐed resin also had a particulatemorphology (D 3É4 km, 0É081) (Fig. 1(D)), which indi-Vfcates that some of the PU was incorporated into thePOM-rich particles. The SEMs under higher magniÐ-cation are instructive for considering the fractureprocess (Fig. 2). In the binary DGEBA/POM blend, thePOM particles seem to be brittle at fracture (Fig. 2(A)).In the ternary DGEBA/POM/PU blends, ductiletearing of the particles is clearly observed at fracture(Figs 2(B and C)). Toughening of epoxies in the epoxy/POM/PU ternary blend could be achieved because ofthe particulate phase structures, where PU acts as atoughener for POM.

In the modiÐcation with PEP (MW 12 100), the resin

Fig. 2. SEMs of fracture surfaces under higher magniÐcationin the ternary DGEBA/POM/PU blend system: (A) binaryDGEBA/POM (90/10) blend ; (B) ternary DGEBA/POM/PU(90/10/5) blend ; (C) ternary DGEBA/POM/PU (90/10/10)

blend.

had a particulate morphology for 20 wt% addition (D0É47 km, 0É17) (Fig. 3(A)). In the ternary blend con-Vftaining PEP, morphologies of the modiÐed resins



Fig. 3. SEMs of fracture surfaces in the ternary DGEBA/POM/PEP blend system: (A) binary DGEBA/PEP (80/20) blend ; (B)ternary DGEBA/POM/PEP (90/10/15) blend ; (C) ternary DGEBA/POM/PEP (85/15/7É5) blend ; (D) ternary DGEBA/POM/PEP

(85/15/10) blend ; (E) ternary DGEBA/POM/PEP (85/15/10) blend under higher magniÐcation.

changed drastically, depending on the resin composi-tion. In the resin composition of DGEBA/POM/PU(90/10/15), the resin had a particulate morphologyhaving irregular-shaped large particles (c. 100 km in

diameter) dispersed in the epoxy matrix ; the particleswere composed of clusters of smaller particles (Fig.3(B)). In the ternary DGEBA/POM/PEP (85/15/7É5)blend, the modiÐed resin had a cocontinuous phase



structure, which phase-separated macroscopically con-sidering the low magniÐcation (]40) of the photograph(Fig. 3(C)). In the ternary DGEBA/POM/PEP (85/15/10) blend, the modiÐed resin had a phase-inverted struc-ture : dark, somewhat irregularly shaped epoxy-richdomains were surrounded by light, thin, continuousphases (Fig. 3(D)). Figure 3(E) shows the SEM of thesame sample under higher magniÐcation, where thelight, continuous phases consisted of clusters of par-ticles, and some small particles (c. 0É55 km in diameter)were dispersed in the dark epoxy-rich domains. Thetoughness of epoxies could be achieved because of thephase-inverted structure in the epoxy/POM/PEPternary blend ; the decrease in Ñexural strength could beattributed to the macroscopic phase-separation struc-ture of the modiÐed resins.

Dynamic viscoelastic analysis can give informationon the microstructure of cured resins. Figure 4 showsthe storage moduli, G@ and tan d curves for the unmod-iÐed and modiÐed resins in the ternary blend systemDGEBA/POM/PU. When using 10 wt% of POM, thepeak position of the a-relaxation in the tan d curveshifted slightly towards lower temperatures and wasaccompanied by an increase in magnitude of the tan dcurve in the temperature range between 30 and 110¡C.It is believed that this phenomenon represents the a-relaxation of POM. The dynamic mechanical behaviourof POM was investigated in detail by Braun & Ross ;43the a-relaxation in the tan d curve changed in the tem-perature range 110 to 118¡C, depending on the oxyethy-lene unit content in the POM. When using 10 wt% of

PU, the peak position of the a-relaxation in the tan dcurve shifted towards lower temperatures and its shapebecame broader, because of PU incorporation into theepoxy network. It was also apparent that the b-relaxation of the epoxy network decreased. In a prelimi-nary experiment, DGEBA and PU reacted c. 80% in 3 hat 180¡C by IR spectroscopy, where the CH stretchingband at 2980 cm~1 was used as a reference band andthe CxN stretching band at 2275 cm~1 for the iso-cyanate groups. In the ternary DGEBA/POM/PU (90/10/10) blend, the peak position of the a-relaxation in thetan d curve shifted further towards lower temperaturesand an increase in magnitude of the tan d curve betweenroom temperature and the a-relaxation temperaturebecame more signiÐcant, because of various kinds ofrelaxations. These relaxations indicate the complexity ofthe matrix structure and serve to improve the toughnessof the epoxies ; the importance of the relaxations nearroom temperature for toughening of epoxies has alreadybeen reported in detail in previous papers.25,26 Thestorage modulus at room temperature observed for theternary blend was equal to that for the unmodiÐedresin.

Figure 5 shows the results of dynamic viscoelasticanalysis in the ternary DGEBA/POM/PEP blendsystem. In the binary DGEBA/PEP (80/20) blend, thepeak position of the a-relaxation in the tan d curveshifted slightly towards lower temperatures and a newrelaxation peak (a@-relaxation) was observed at 38¡C,compared with the dynamic viscoelastic behaviour ofthe unmodiÐed epoxy resin. The appearance of the a@-

Fig. 4. Dynamic viscoelastic analysis in the ternary DGEBA/POM/PU blend system: ÈÈ, 0 wt% (control) ; È È È, binary DGEBA/POM (90/10) blend ; È È È, binary DGEBA/PU (90/10) blend ; È È ÈÈ, ternary DGEBA/POM/PU (90/10/10) blend.



Fig. 5. Dynamic viscoelastic analysis in the ternary DGEBA/POM/PEP blend system: ÈÈ, 0 wt% (control) ; È È È, binaryDGEBA/PEP (80/20) blend ; È È È, ternary DGEBA/POM/PEP (85/15/10) blend.

relaxation peak indicates the existence of a phase-separated structure for the modiÐed resin. In the ternaryDGEBA/POM/PEP (85/15/10) blend, the specimen wasfractured at 177¡C and an a@-relaxation peak wasobserved at 41¡C. The fracture of the specimen in thecourse of the analysis could be due to the POMmelting ; the modiÐed resin has a phase-inverted struc-ture and the continuous phase is composed of clustersof the POM-rich particles (Fig. 2(C)). The storagemodulus at room temperature for the ternary blend wasequal to that for the unmodiÐed resin.

CONCLUSIONS

POM was used as a modiÐer for a commercial epoxyresin based on DGEBA. The use of POM alone toimprove the fracture toughness of the epoxies was(KIC)rather ine†ective. However, the increased in theKICternary blends of DGEBA, POM and PU or PEP. Theimprovement in toughness of the ternary DGEBA/POM/PU blend may be achieved because of the parti-culate morphology. When using a ternaryDGEBA/POM/PU (90/10/10) blend, for the modi-KICÐed resin increased by 50%, with retention in Ñexuralproperties and a slight loss of In a ternary DGEBA/Tg .POM/PU (85/15/10) blend, the for the modiÐedKICresin increased 70%, with 35% loss of Ñexural strengthand retention of The improvement in toughnessTg .

of the ternary DGEBA/POM/PEP blend has been(KIC)attributed to the development of a phase-inverted struc-ture in the Ðnal resin.

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