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Effect of Moisture on Epoxy Resins and Epoxy Composites. 1) Bisphenol-A Based EpoxyCured with Amine-Amide Hardener.

M. C. M. Cucarella

Escuela de Metalurgia y Ciencia de los Mat er ialcs. U niversidad Central de Venezuela, Apartado 5 1717, CaracasI050¡\" Venezuela .

Thl'l,rkct "r moist ur« .uid ln.u 0"..1 l\i'phl'IlO: ,\ h."l'J l'POX\' rl',illlllrnl wit li « .unme-umid« h.rrdcncr i, rcporu-d. Scannim;L'IL'l t ro n a nd npllc.d 11111.. In,1!Llph:-- Illl..til.llL l h;,lt 1.1 i lu rc DL L L1!:-' hy •..r,u . .k propa~,il ion t h rough cr.iz c ma t t c r a nd that m o ist ur eCIIh.llllD lUZl' in it í.u ;1111.Tl'll,ik "rl'lI).: l halle! Y 1I111l,e's modulu- wcrc Iound to dClIc",c wi lh l'XpUSU rv while ul ti m.u e s t ra in\\;.1:-' n o t :--igllil"it.::llllh- changed .. \loi:-.IUfL" i.:-. lik clv lo ch;lnge t lu, :-.itc o l hond ru p t u rc du« t o hyJrol!cn b on d i ng.

1. INTRODUCTION

Epoxy resin and its composites sorbe waterwhich affects the physical and mechanical proper-tieso The basic physical phenornena that are inducedand / or modified by sorbed moisture are reviewed inreference [1]. Humídity has been found to depressthe Tg [2] and to influence the tensile strength ofepoxy composites [3]. Water sorbed by crosslinkedpolymers could be atracted to hydrophilic sites pro-bably breaking up hydrogen bonding and reducingintersegmental atraction[4]. Balan et al [5] proposethat the Tg and the tensile strength are loweredbecause of reduced intersegmental atractions whenpreferential hydrogen bonding to water oecurs.

Crack propagatian in thermoplastics is knownto occur by the breakdown of crazes [6]. Morgan andO'Neal [7] concluded the fracture of Dgeba-Detaepoxy glasses could be considered a crazing failureprocess in agreement with Lilley and Holloway [8].However Ya mini and young [9] consider unlikely infully cured materials that the amount oflocal tensiledrawing required to forrn a craze would be possible.They have found no evidence of craze debris on thefracture surface of the epoxy resin studied. To a simi-lar conclusion carne Racich and Koutsky [10]. Theauthors have also studied the effect of nodules uponthe properties of epoxy resins [10,11] and concludedthat the mechanical proper ties depend on nodulesize. It has been reported [9] that nodule size dependson cure temperature and that mechanical propertiesare affected by post-cure temperature [11]. The au t-hors point out that both variables may or may not beindependent. Even the existence of such nodules ismatter of controversy since Uhlmann [12] considersthem to be artefacts while Yeh [13] reports theyrepresent the true structure.

This is the first of a series of articles on the effectof moistu re on epoxy resins and their composites. Itsmain objetive is to determine the mechanism hy

which a Bisphenol-A based epoxy resin ruptures andthe effect of water sorption on this process.

2 . EXPERIMENTAL

2.1. M(¡feri(./!s and speamens preparation

The epoxy resin used was known comercially asGenep: oxy" 180 (Bisphenol A based) cured withGenamid* 250 (amineamide hardener). The ratioresin ro hardener was 5:2 wt. % which gave the hig-hest Tg and tensile strength. The resin and hardenerwere preheated a 70 "C for 3 hours in a vacuum charn-ber before mixing. After thorough mixing, the mate-rial was poured into moulds, left to gel and cured for24 h at 100 "c. The specimens were cooled at roomtemperature and removed from the moulds. The spe-cimens were 15 cm long, 1 cm wide and 0.3 cm thick.Before mechanical testing they were cut into dogbone shape with a gauge length of 2.5 cm.

2.2. Procedures

Speeimens were dried in dessieator for 24 h andweighed. 20% were left as reference, the rest wereintroduced in 100% moisture at 60 and 90°C up to216 h. The specimens were then removed from thebath, wiped dry and weighed. A tensile tester (Instron1130) was used to determine the tensile mechanicalproperties of the wet and dry materials at a cross-head rate of 10 cm min !.,

A scanning reflection electron microscope(Hitachi S-450) and optical microscope (VersamedUnion 5263) where used for fracture topography stu-dies after coating the fracture surfaces for the SEMwith gold.

• Suppl icd bv Venezolana de Resinas.

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Revista Latinoamericana de Metalurgia y Materiales, Vol. 4, N° 1, 1984

3. RESULTS AND DISCUSSION

3.1. Water sorption

Fig. 1 shows the sorbed water (as wt. %) as a func-tion of time for epoxy resin exposed at 100% humi-dity at 60 and 90 "C. There is a clear increase in watersorption at 90 0e. Fick's second law of diffusion isaccepted as valid for short times of exposure [14-21].For one-dimensional diffusion is given by

dc dc-=D-dt dx '

where e is the concentration, D is the diffusion coefi-cient and X is the distance the moisture has advancedfrom the boundary. This equation can be solved for aflat sheet sorbing moisture at both fases and gives

Mt = ~ (Dt)111M 2 tt

where Mt is the amount of moisture sorbed after atime t, M is the equilibrium amount of moisture sor-bed and L is the plate thickness [22]. Fig. 2 shows thepercent of moisture sorbed versus the square root oftime; these plots are linear for short times of expo-sure. the diffusion coefficients calculated using theslopes of the linear part of the curves are 33 X 10 9and 103 X 10 ')cm s I for the resin exposed at 60 and90 "C respectively. The increase in D with ternpera-ture is expected fram Arrhenius equation. The calcu-lated values are in the range of those reported forother epoxy resins [14, 23-25].

3.2. Fracture topograpbies

\ The fracture topographies of dry and wet epo-xies and composites were studied by optical, andscanníng electron microscopy. On the fracture surfa-ces of the epoxies it is possible to observe the initia-tion region, a mirror-like region and a rough regionas shown in Fig. 3. The initiation region is coarse andit is found in a comer. A typical topography observedin this region is shown in figs. 4 and y, very similar tothose illustrated by Morgan et al [7]~Dry samples pre-sent a bigger initiation area than wet ones. The cha-racteristics of this region have been explained byMorgan et al. in terms of a crazing failure process sug-gesting that the coarse in tia tia n region in epoxiesresults from vo id growth and coalescence throughthe center of a simultaneously growing, poorly deve-loped craze, which consist of coarse fibrils. The areaof the mirror-Iike region increases with water sorp-tion. This agrees with the faet observed by Owen andRase [26] that this area increases with flexibility andít is dependent on temperature, molecular weightand strain rate [27-29]. River markings are often

observed in this area(Eig . .5).The SEM figs. 6 (a and b)and 7 (a and b) illustrate two portions of the mirror-like regio n at high magnifications and their topo-graphic maps. Some irregularities are seen in Fig. 6while the topographic map of Fig. 7 illustrates analmost featureless area.

Fig. 8a shows conic markings produced by thedeviation of the fracture front from its normal planarpath due to microstructural features. Inhomogenei-ties cause secondary fractures which spreads in a cir-cular manner in aplane parallel to main fracture. Ifthe two fracture planes are clase the fronts will inter-sect along a small step of hight 02 having a locus inthe x-y plane given by,

x= vty= [n2f - (x., - X)2]lí2

which gives

where x., is the distance from the primary front to thecenter of the secondary Fracture at t = O, v and u arethe velocities of primary and second respectively[30]. This equation becomes quasiparabolic ifu= V.This marks are commonly observed in PMMA. Thedensity of secondary fracture fronts decreases withtime of exposure; another interesting feature obser-ved in the micrographs is the presence of interfe-rence colours. An analogous effect has been inter-preted for PMMA as fracture prppagation-on a broadfront leaving platically deformed material. Thecolours been caused by interference between lightreflected from the surface and light reflected by theinterface between the plastically deformed layer andthe undeformed substrate [31]. Fig. Sb shows thesame parabola area as Fig. 8a observed through cross __nicols with rests of craze like matrer, Sem Figs. 9 and10 show the initiation and middles part of the hackleregion. Figs. 11 and 12 show micrographs of a crackperpendicular to the applied stress. It is possible tosee rests of craze-like material.

3.3. Mechanical properties

The tensile strength, decrease of tensile strength,decrease in Young' s modulus and ultimate strain forsamples exposed at 60 "C and 90 "C are shown in Figs.13-16. Wet samples have lower tensile streangth andmoduli than dry samples. The rate of degradatíon ofthese mechanical properties is h,igher at 90 °e than a t60 °e in both cases. The main effect is produced afterfew hours of exposure despite the [act water sorptionequilíbriurn has not been reached. Ultimate strain isneither affected by time nor temperature of ex-posure.

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Latin American [ournal o/ Metallurgy and Materials, Vol 4, N° 1, 1984

3.4. Effect on sttucture

According to the suppliers the formulas of theepoxy and the hardener are as shown in Fig. 16, withan epoxy equivalent of180-195 and an amine equiva-lent of 425-450. After crosslinking the structure canbe represented by

CH3 OH RzI I IRl-0~-T-o-°CH2-CH-CHz-N-

CH3

with R1 from the epoxy chain and R2 from the harde-ner chain where the weakest bond has been calcula-ted to be the C-C bond in the (-CH(OH) -CH2-O-@-) sequence [32], since the normal bon-ding energy, Eb, value for a C-C bond is reducedabout 10% due to the -OH group effect and appro-ximately 25% due to the resonance of the -0-@-group. The Eb for C-N is 55;5 Kcal/mol.When molecules of water are present however, thehydrogen bond formed on the N will reduce conside-rably the C- N Eb value (aproximately by 20%). Thepresence of water is then likely to shjft the site ofbond rupture and simultaneously low:er the neces-sary energy.

4. CONCLUSIONS

Optical and scanning electron microscopy indi-cate that the mechanism of failure of the epoxy resinsystem studied is by crack propagation through craz ematter and that moisture enhances this process. Ten-sile strength and Young's modulus decreased withmoisture exposure while ultimate strain was notaffected.lt seems likely that moisture changes thesite of bond rupture in the matrix by selectíve bon-ding of the water molecules.

REFERENCES

1. Morgan, R.]. and O'Neal,j. E., Polymer Plast. Techn. s: Eng.10 (1978) 49.

2. McKaguc, E. L., Reynolds, ]. D. and Halkias.]. E.]. Appl.Polym. Scí, 22 (1978) 1643.

3. Vinson,]. R, Pipea, R. B., Walker, W.]. and Ulrich, D. L.,"The effeets of Relatíve Humidity and Elevated Tempera-ture on Composite Structur es", AFOSR· 76-2966, Air ForceOffice of Scientific Research, Washington, D. C. (1976).

4. Barrie,]. A., in 'Diffusion in Polymers",]. Crank and G. S.Parks, Eds. Academic, London (1968) 259.

5. Bolon, D. A., Lucas, G. M., 0150n, D. R. and Webb, K. K.,].Appl. Polym. Sci., 25 (1980) 543.

6. Marshall, G. P., Coutts, L. H. and Williams,]. G.,]. Mat. Sci. 9(1974) 1409.

7. Morgan, R. and O'Neal,].,J. Mat. Sci. 12(1977) 1966.

8. Lillcy,]. and l lrIloway, D. G., Phil. Mag. 28(1973) 215.

9. Yamini, S.. ,,,,,: Young, R,]. Mat. Sei. 15(1980) 1823.

10. Racich,]. and Koutsky, ]. A., J. Appl. Polymer. Sci. 20(1976) 211l.

11. Yamini, S. and Young, R].,]. Mat. Sci. 14 (1979) 1609.

12. Uhlmann, D. R., Discuss. Faraday Soc. 68 (1978).

13. Yeh, G. S., Crt. Rey. Maeromol. Chem., 1(1972) 173.

14. Morgan, R. ]., O'Neal,]. E. and Fanter, D. L.,]. Mal. Sci.,15(1980) 751.

15. Mehta B. S., DiBenedetto, A. T. and Kardos,.J. L.,.J. Appl.Polym. Sci. 21(1977) 3111.

16. McKague, E. L., Reynolds,]. D. and Halkias,]. E., Trans.Amer. Soco Mech. Eng., S. H., 98 (1976) 92.

17. Manson,]. A. and Chiu, E. H., ). Polym. Sei. C(1973)654.

18. Shen, C. H. and Springer, G. S., ). Composite Mater 10(1976) 2.

19. McKague, E. L., Halkias, j. E. and Reynolds, ]. D., Ibid.9(1975) 2.

20. Wilins, D.]., in "Cornposíte Materials: Testing and Des ign"(4th Conf.) ASTM STP 617 (American Soc. for Test ing andMat.), M. Park, Ohio(1977) 497

21. Bohlmann, R. E. and Derby, E. A., AIAA (1977) Pupcr N"77-339.

22. Van Amerongen, C. j., Rubber Chem. Technol. 37(1964)871.

23. Br.owning, C. E., Ph. D. Thesis, University of Dayton (1976)cited by Morgan in ref. 14.

24. Young, H. L. and Brecver, W. L., 6th Sto Louis Symposium,Cornposíte Mat. in Eng. (I972) St Louis.

25. Greever, W. L., "High Temperature Strength Degradationof Cornposites during Ambient Aging" Hereules, Inc, FinalReport, C. N" 70-000-22, Convair Contract NAS 8-27435(1972).

26. Owen, M. ]. and Rose, R G.,). Mat. Sei., 10(1975) 1711.

27. Bir d, R.)., Mann,]. Rogany, G. and Rooney, G., Polymer7(1966) 307.

28. Newman, S. B. and Wolock, I.). Appl. Phys. 29(1958) 49.

29. Wolock,1. and Newman, S. B. in" Fracture Process in Poly-meric So lids", Edi. B. Rosen (Interscience), (1964) Ch.Ile.

30. Andrews, E. H. in "fracture in Polyrncr s", Oliver and BoydLtd., Eds., Aberdeen University Press, London (1968).

31. Berry,]. P., Nature, 185(1960) 91.

32. Kíng, N., Ph. D. Thesis, Univ. of London, England (1976).\

FOOTNOTES OF FIGURES

1) Weighr pereent of moisture sorbed at 100% r.h. at O 60 "Cand O 90 "C versus time of exposure.

2) Weight pcrcent of moisture sorbed at 100% L h. at O 60 "Cand O 90°C versus the square root of time of exposure.

3) Optical micrograph of a fracture surface of a sample exposedat 100% r.h. and 90 "C for 48 h. Bar 0.5 mm.

4) Scanning elecrron micrograph of the initiation rcgion of asample exposed at 100% r.h. at 60°C for 12 h.

5) Optical mierograph of an unexposed sarnple, Bar 0.4 mm.

6) a) Scanning electron micrograph of the mirror-like regionof a sample exposed at 100% r.h, at 90°C for 48 h and b) itstopographic map.

7) Same as 6 of an adja cc nr regio n.

8) a) Optical micrograph of the parabo la region and b) sarnearca as observcd through cross nicols. Bar 0.1 111m.

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Revista La tinournerica na de Metalurgia y Materiales, Vul. 4, N" 1, 1984

9) Scanning cl ecr ro n micrograph of a crack propagating per-pcndicu lar to .ipp licd stress in an u nex poscd sarnple.

lO) Sa m c as 9 at highcr magnification.

11) Bcgning ofr hc rough rcgion in a s.unplc cxp oscd a t 100% r. h.at 60 "c: for 24 h.

12) Rough rcg io n s.un e sa mplc as 11.

1») Dccr cuse in te nsil c strength as el function of time uf exposurea t 100% r.h. for O 60 "C and O 90 "c.

14) Decrease in Young' mo dulus as a function o l time of ex po-sur e ato 100% r.h. Io r O 60 "C and O 90 "c.

1 5) Ult imu t c s't rai n as a function of time 01" exposure a 100% r.h.for O 6ü "C and O 90 "c.

16) Chc m ica l structure 01" the epo xy and b) of hardener.

..__ .....L__----I......._._

12.0 14,0

ac

6.0

o:

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LatinAmertúm [ourna! o/ Metu!llIrgy UI/tI Muta/uls. Vol 4. N" J. 1984

Fig.4

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Revista Latinoamericana de Metalurgia y Materiales, Vol. 4, N° 1, 1984

Fig. 6

Fig. 7

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LatinAmerican Journal o/ Metallurgy and MaterialJ, Vol 4, N° l, 1984

a)

b)

Fig. 8

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Revista Latinoamericana de Metalurgia y Materiales, Vol. 4, N° 1, 1984

Fig. 9 Fig. 10

Fig. 11 Fig. 12

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Fig. 15

Fig. 14

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0.1

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Revista Latinoamericana de Metalurgia y Materiales, Vol. 4, N° 1, 1984

Fig. 16

a)

b)

H2N - C2H4 - NH- C2H4 - ~ - ~ - C34H68 - ~ - ~ - C2H4 - NH - C2H4H O O H

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41