mechanical and tribological properties of epoxy modified by liquid carboxyl terminated...

Mechanical and Tribological Properties of Epoxy Modifiedby Liquid Carboxyl Terminated Poly(butadiene-co-acrylonitrile) Rubber

Wunpen Chonkaew,1 Narongrit Sombatsompop2

1Department of Chemistry, Faculty of Science, King Mongkut’s University of Technology Thonburi,Thongkru, Bangkok, 10140, Thailand2Polymer Processing and Flow (P-PROF) Group, School of Energy, Environment and Materials,King Mongkut’s University of Technology Thonburi, Thongkru, Bangkok, 10140, Thailand

Received 21 July 2011; accepted 2 September 2011DOI 10.1002/app.35580Published online 19 December 2011 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin was modified using liquid carboxyl-termi-nated poly(butadiene-co-acrylonitrile) (CTBN) rubber. Theliquid CTBN contents used ranged from 2.5 to 20 parts perhundred parts of resin (phr). Mechanical properties of themodified resins were evaluated and the microstructures ofthe fracture surfaces were examined using SEM technique.The changes in storage modulus and the glass transitiontemperature were also evaluated using dynamic mechanicalanalysis (DMA). The tribological tests were performed usinga ball-on-disc tribometer. The worn surfaces and the ballcounter-mates after tribological tests were investigated usingoptical microscope technique. The results revealed the influ-ence of liquid CTBN content on mechanical and tribological

properties, and also microstructure of the modified epoxyresins. Impact resistance increased whereas the storage mod-ulus and the hardness decreased when the CTBN rubberwas introduced to the epoxy network. The coefficient of fric-tion of the CTBN-modified epoxy was lower than that of theneat epoxy. The CTBN content of lower than 10 phr was rec-ommended for improving the wear resistance of epoxyresin. Changes in tribological properties of the CTBN-modi-fied epoxy correspond well to those in mechanical changes,especially the toughness properties. VC 2011 Wiley Periodicals,Inc. J Appl Polym Sci 125: 361–369, 2012

Key words: thermosets; toughness; mechanical properties;rubber-modified epoxy resin; tribological properties

INTRODUCTION

Epoxy resin is one of the thermoset polymers thatcrosslink when mixed with a catalyzing agent or ahardener. Because of the highly crosslink character, theneat epoxy resin generally exhibits many favorableproperties including high modulus, relative high-chemical resistance, high-temperature performances,good adhesion, and bonding characteristics.1–3 Besides,epoxy performs the ease of manufacturing with addi-tives and has a lower price as compared to otheradvanced polymers such as poly(ether ether ketone)(PEEK) and polyimide (PI).4 These all make it one ofthe most extensive uses in tribological studies.5–14

However, the crosslinking structure of epoxy is knownto inhibit the effectively transfer-film formation, whichis one of the key for producing materials with lowcoefficient of friction.14 Moreover, the neat resin is rela-

tively brittle and has poor resistance to crack initiationand growth, resulting in a poor wear resistance. There-fore, almost all the epoxies used as high-performanceresins for many industrial applications are modifiedby adding other materials to make toughened epoxyresins or to make reinforced epoxy composites.Several methods15–35 have been proposed to increase

the toughness of epoxy resins. One of the most suc-cessfully methods involves the addition of modifiedreactive rubbers, e.g., carboxyl-terminated butadieneacrylonitrile copolymer (CTBN)15–23 the amine-termi-nated butadiene acrylonitrile copolymer (ATBN),19,24

acrylic rubber,25,26 and hydroxyl terminated polybuta-diene (HTPB),27,28 to the uncured epoxy resin and con-trolling the reaction to obtain the rubber particles dis-persed in the polymer matrix. Kunz and Beaumont15

found the improvement in toughness and mechanicalproperties of CTBN rubber-modified epoxy resinsin the temperature range �20 to þ110�C. Kinlochet al.16,17 found that the toughness of CTBN-modifiedepoxy resins increased, but the modulus and yieldstrength decrease slightly with volume fraction of thedispersed rubbery phase. Bartlet et al.18 found that theimpact strength behavior of epoxy networks enhancedwith the reactive rubber (CTBN), but it exhibited theoptimum for 15% CTBN.

Correspondence to: W. Chonkaew ([email protected]).

Contract grant sponsor: Thailand Research Fund;contract grant number: MRG-5380129.

Journal of Applied Polymer Science, Vol. 125, 361–369 (2012)VC 2011 Wiley Periodicals, Inc.

The size and the distribution of rubber particleswere reported to strongly influence the impact resist-ance of final products.19,29,30 Kunz and Beaumount19

compared the morphology and toughness of CTBNand ATBN-modified epoxy resins. They found thatATBN-modified epoxies had a diffuse-appearinginterface between the dispersed rubber phase and theepoxy matrix, in contrast to the sharp boundaries ofCTBN particle interfaces. However, both rubbers pro-duced essentially identical toughness values.

It has been reported in the literatures20,22,29,33 thatthe rubber particles could increase toughness byactivating cavitation, inducing crack bridging andpromoting shear yielding of the matrix. Pearson andYee29 explained that the toughening mechanisms ofrubber-modified epoxies involved both cavitation,which relieved the triaxial tension at the crack tip,and shear band formation, that created a large plas-tic zone. Besides, the compatibility between rubberand epoxy phases was reported to strongly affectthe toughening behavior of the rubber-modifiedepoxy resins.22,27 Chen and Jan22 found that the frac-ture energy of the CTBN-modified epoxy resins sig-nificantly increased if the rubber/matrix interfacialinteraction was enhanced by encapping CTBN withthe flexible diglycidyl ether of propylene glycol.They suggested that the interaction between cavita-tion and localized shear yielding should beaccounted for toughening mechanism of rubber-modified epoxy resin. Ozturk et al.27 performed theaddition of silane coupling agent (SCA) to enhancethe interaction between HTPB and epoxy matrix.They found the improvement in tensile strength andplastic deformation of the epoxy systems by pre-reacting HTPB with SCA and hardener. Thomaset al.28 investigated the miscibility, thermal and me-chanical properties of epoxy resins toughed with liq-uid HTPB. They found that the maximum impactproperties were observed as the amount of liquidHTPB was 10 parts per hundred (phr), but the poorflexural and other mechanical properties were founddue to the inclusion of rubber. Tripathi and Srivas-tava32 modified the epoxy resins with CTBN liquidrubber using an aromatic amine hardener. Theyfound that the impact strength increased with higherconcentration of CTBN, whereas the mechanicalproperties like tensile and flexural strength reduced.Tripathi and Vastava34 also investigated the mechan-ical properties and chemical resistance of the filmsof coating based on CTBN rubber-modified epoxyresins. They reported that the epoxy modified with15 wt % CTBN offered better chemical resistanceand thermal stability than other modified samples.

Although researchers have found that reactive liq-uid rubber could improve the toughness or resist-ance to crack of the thermoset epoxy, the effect ofliquid rubber on wear behavior of the thermoset ep-

oxy has been rarely known. One of the interestingwork done by Yu et al.12 was the use of rubbernanoparticles to prepare the epoxy composites. Theyfound that the 5 wt % content of the rubber nano-particles was the most effective in reducing the wearmass loss and coefficient of friction of the epoxy-piperidine system. The improvement of the wearresistance of epoxy nanocomposites was proposed tobe because of the improved toughness by the rubbernanoparticles. Ma et al.35 employed the rubber nano-particles to improve the mechanical properties andto toughen an epoxy thermoset matrix. They foundthat the rubber nanoparticles caused much lessYoung’s modulus deterioration but toughened epoxyresins in a similar degree in comparison with liquidrubber that formed microscale particles during cur-ing. Therefore, it is interesting to study the tribologi-cal behaviors of the epoxy resin modified with liq-uid rubber. In this work, the microparticle rubbertoughened epoxy resin was prepared. The reactiveliquid CTBN rubber was used as a tougheningagent. The influence of toughness enhancement onthe wear behaviors of epoxy resin was investigated.We attempt to improve wear resistance of diglycidylether of bisphenol A (DGEBA) epoxy resin usingcycloaliphatic amine as a hardener. The objectives ofthe present work were to investigate the tribologicaland mechanical properties of the CTBN-modified ep-oxy resins and to characterize the wear mechanisms.In addition, the relationship between the mechanicaland tribological performance of the CTBN-modifiedepoxy resins and their relevant microstructure werealso established.

MATERIALS AND METHODS

Materials

The epoxy used in this study was diglycidyl ether ofbisphenol A, DGEBA (EPON 828) with epoxy equiv-alent weight of 185–192 g/eq. The hardener usedwas cycloaliphatic amine (EPIKURE 3383) withequivalent weight of 114 g/eq. Both chemicals werefrom HexionTM Specialty Chemicals, USA. The reac-tive liquid rubber was carboxyl-terminated acryloni-trile butadiene copolymer (CTBN) with molecularweight (Mn) of 3800 containing acrylonitrile of 8–12wt % from Aldrich, Singapore. All chemicals wereused as received without further purification.

Preparation of rubber toughed epoxy resins

DGEBA epoxy was mixed with required quantitiesof liquid CTBN rubber at 80�C for 45 min whilemechanical stirring. The mixture was then degassedin vacuum oven and ultrasonicated to remove theentrapped bubbles. Once the mixture had cooled

362 CHONKAEW AND SOMBATSOMPOP

Journal of Applied Polymer Science DOI 10.1002/app

down at room temperature, the stoichiometric quan-tity of curing agent was then added prior to handmixing for 3–5 min. Finally, the mixture was pouredinto molds and cured at 25�C for 16 h and postcure at100�C for 2 h as recommended from a manufacturerfor the neat epoxy resin. The rubber-modified epoxyresins containing 2.5, 5.0, 10, 15, and 20 phr of CTBNrubber were named here as 2.5-CTBN, 5-CTBN, 10-CTBN, 15-CTBN, and 20-CTBN, respectively.

Impact testing

Izod impact tests were performed at room tempera-ture using pendulum impact tester (Yasuda SeikiSeisakusho 9347, Intro Enterprise) according to ASTMD256-10. At least five specimens were tested and theaverage results were reported with standard deviation.

Scanning electron microscope (SEM)

The fracture surfaces obtained from the impact testswere examined at magnification of 1500x using thescanning electron microscope (JEOL JSM-5800,Japan) with 10 kV working energy. All sampleswere gold coated to prevent the charging before thesurface observation.

Dynamic mechanical analysis (DMA)

Storage modulus (E/) and tan d were determinedusing dynamic mechanical analyzer (Mettler Teledo,DMA/SDTA 861e, Switzerland). Specimens of rec-tangular shape (63 � 12 � 3 mm3) were analyzed in3-point-bending mode. The measurement was car-ried out from �20�C to þ130�C with 3�C/min ofheating rate, at 1 Hz of frequency.

Tribological testing

Tribological test was carried out using a ball-on-disctribometer (CSM Instrument, model 18-272, German)at ambient condition. The sample surface wasrubbed against stainless steel ball with sliding veloc-ities of 0.10 m/s for 500 m under applied loads of 3and 5 N. The sliding ball and sample surface werethen cleaned using lens paper towels soaked in iso-propanol before testing. The wear track radius (R)and the ball end spherical radius (r) were 6.0 mmand 3.175 mm, respectively. The friction coefficient(l) was obtained using Amonton’s first law of fric-tion as expressed in eq. (1).

l ¼ F=W; (1)

where F is tangential force and W is normal force inthe unit of N. To access the wear damage and wearmechanisms, rubbed surfaces and stainless steel

balls from tribological testing were analyzed usingoptical microscope (OM). The wear volume loss (V)in the unit of cubic millimeter was calculated fromthe wear track width (d) in the unit of mm as givenin eq. (2).

V ¼ 2pRðr2 sin�1ðd=2rÞ � ðd=4Þð4r2 � d2Þ1=2Þ: (2)

The wear rate (k) in the unit of mm3/(Nm) wascalculated using eq. (3).

k ¼ V=ðWsÞ; (3)

where W is the normal force and s is sliding distancein the unit of m.

RESULTS AND DISCUSSION

Mechanical properties

Dynamic mechanical analysis of CTBN-modifiedepoxy resins was analyzed to investigate the influ-ence of liquid CTBN content on the stiffness and theglass transition temperature. The results are repre-sented in Figure 1.

Figure 1 Temperature dependence of (a) storage modu-lus and (b) tan d of the neat epoxy and the epoxies modi-fied with CTBN 2.5, 5, 10, 15, and 20 phr. [Color figurecan be viewed in the online issue, which is available atwileyonlinelibrary.com.]

PROPERTIES OF EPOXY MODIFIED BY CTBN RUBBER 363


Figure 1(a) shows the temperature dependence ofstorage modulus (E/), which measured the stiffnessand load bearing capability of the epoxy resin. Thestorage modulus of all the studied resins decreasedas the temperature increased. One sharp drop instorage modulus, indicated the glass transition tem-perature (Tg), was observed over the temperaturerange of this study (from �20�C to þ130�C). It wasfound that the storage modulus of an epoxy resinbelow and above its Tg decreased when the rubberwas added. To facilitate the comparison amongthose rubber-modified epoxy resins, the E/ atthe below and at the above Tgs are summarized inTable I. The E/ at the below Tg zone (at 25�C) isreferred as the storage modulus at glassy region,while the modulus at the above Tg zone (at 120�C) isreferred as the rubber plateau storage modulus. Itwas found that the glassy storage moduli of all themodified epoxy resins were lower than that of theneat epoxy resin. The higher amount of the CTBNrubber, the lower the stiffness of the resin would be.The storage modulus was by 48.5% reduction as the20-phr-CTBN rubber was added. The decreases inthe moduli were attributed to the presence of lowmodulus rubber particles in the epoxy matrix. Thesimilar effect was also found for the storage modulusat 120�C. It decreased with the liquid CTBN loading.The significant reduction of the storage modulus at120�C with addition of the liquid rubber wasobserved (i.e., by 14.6%, 21.3%, 23.3%, 37.3%, and39.2% reduction for the modified resin containing the2.5, 5, 10, 15, and 20 phr-liquid CTBN, respectively).From the theory of the rubber elasticity,36 the plateaustorage modulus (E0

plat) at a given temperature aboveTg is proportional to the crosslink density. Thus, itcould be inferred from the reduction of the E0

plat thatthe presence of the liquid CTBN rubber reducedthe crosslink density at the interface of the modifiedepoxy resins. The explanation, offered by Thomaset al.,28 was that during the curing of epoxy resin thephase separated rubber domains occupied the spacein between the reaction sites, thereby impairing thecrosslink reaction at a particular site.

Figure 1(b) shows the damping properties of theneat and the rubber-modified epoxy resins. The

glass transition temperatures (Tg) identified fromtan d at the peak maxima of the neat and the modi-fied epoxy resins are summarized in Table I. Theneat resin illustrated the highest value. Only the min-imal changes in the Tg toward the lower temperaturerange from that of the neat epoxy was observed. Tg

dropped with incorporation of 2.5 and 5 phr-CTBNrubbers but then increased slightly with further addi-tions. Similar results were reported by othersresearchers.21,23 At least four possible effects were re-sponsible for this observation. One of the effects wasthe reduction in crosslink density at interface due tothe presence of rubber particles as previously dis-cussed. The second effect was the plasticizing actionof small levels of rubber dissolved in the matrix.Additionally, the Tg of the rubbery CTBN phase wasalso lower than the Tg of the neat CTBN.23 Verchereet al.21 gave the alternative explanation. An increasein free volume caused by the thermal hydrostaticstresses applied to the rubber inclusions wasaccountable for the Tg lowering.21 Lastly, the stericeffects arising from the presence of the rubber par-ticles led to the suppressed relaxation mobility in theepoxy segments. For the modified resins with CTBNlarger than 5 phr, the steric effects were possiblyenhanced and brought on the slight increase of Tg ascompared to those at lower rubber loading.The hardness results are given in Figure 2. It was

found that the neat epoxy resin had the highest hard-ness, and the hardness decreased with the increasedCTBN content. The results agree with the moduliobtained from DMA. The reduction in hardness wasattributed to the lowered crosslink density of theresin in the presence of rubber particles and/or theplasticization effect of the CTBN rubber partially dis-solved in epoxy matrix as already discussed.Figure 3 shows the impact strength of the liquid

CTBN-modified epoxy resins as a function of liquidCTBN content. The improvement in impact strengthcould be correlated with toughness enhancement.All rubber-modified epoxy resins showed higher

TABLE IStorage Modulus and Tg of the CTBN-Modified

Epoxy Resins

SampleStorage modulusat 25�C/MPa

Storge modulusat 120�C/MPa Tg (

�C)

Neat EP 3851 28.7 83.52.5-CTBN 2664 24.5 77.85-CTBN 2500 22.6 73.310-CTBN 2248 22.0 78.315-CTBN 2204 18.0 80.920-CTBN 1982 17.3 82.1

Figure 2 Hardness of the rubber-modified epoxy resinsas a function of the CTBN content.



impact strength than the neat resin. However, theimpact strength reached the maximum at 5 phr-CTBN rubber, and above this optimum rubber con-tent a falling tendency was observed. The modifiedepoxy containing 5-phr CTBN had the impactstrength of about 67% higher than the neat resin. Forepoxy containing 20-phr CTBN, the impact strengthwas about 22% lower than the maximum. One of thepossible reasons for the increased impact strength ofthe rubber-modified epoxy resin in comparison tothat of neat epoxy was the ductility of the epoxymatrix attained by the incorporation of rubber. Theductility enhancement was due to the reduction incrosslink density and/or the plasticization effect aspreviously discussed. The effective stress concentra-tion of the phase separated rubber-rich particles hasbeen known to amplify plastic deformation of ahighly brittle matrix to a certain extent, leading tothe increases in values upon the optimum level ofrubber content.37 The other reason was the cavita-tions or crack bridging induced by the rubber phase.On the other hand, comparatively lower perform-ance of the present rubber at high content wasattributed to poor interaction between the epoxymatrix and the rubbers.27 In addition, the agglomer-ation of rubber particles as the CTBN contentincreased was also responsible for the decreasingtendency of impact strength after an optimum levelof 5-phr rubber inclusion. This will be evidenced bymicrostructure investigations.

Microstructure of fracture surface

To understand how CTBN rubber particles affect theimpact strength and fracture behavior, the fracturesurfaces of samples obtained from the impact testswere studied. Figure 4 shows the SEM micrographsof the fracture surfaces for the neat epoxy and therubber-modified epoxy resins. The neat epoxy exhib-ited the fracture behavior of brittle material in whichwas evidenced by very smooth fracture surface [Fig.

4(a)]. The micrographs of the modified epoxy matrix[Fig. 4(b,c)] exhibited two phases. The CTBN rubberphases appeared as spherical particles with averagediameters in a range of 0.87–4.70 lm, and they werewell-dispersed in the epoxy matrix. The maximumrubber particle size of the modified epoxy resin atlow CTBN content (5-phr) was lower than that ofthe modified resin at high-CTBN content (20-phr).The average diameter of rubber particles increasedsteadily with the amount of CTBN (see Table II).

Figure 3 Impact strength of the rubber-modified epoxyresins as a function of the CTBN content.

Figure 4 Scanning electron micrographs of the fracturesurface of (a) the neat epoxy; (b) the rubber-modified ep-oxy resin containing 5 phr CTBN (5-CTBN); and (c) therubber-modified epoxy resin containing 20 phr CTBN (20-CTBN).



The increase in domain size with the incorporationof rubber was attributed to the agglomeration of thedispersed rubber particles. These data are well corre-lated with the impact strength results. For the epoxyresin modified with low CTBN content, it was foundfrom Figure 4(b) that a large number of white defor-mation lines developed through the rubber particles.No rubber particles were pulled out from the epoxymatrix, but only internal cavitation was observed.Also there was no obvious phase separation betweenthe rubber and epoxy. The results indicated the pres-ence of chemical bond formation and strong interac-tion between the rubber particles and the epoxymatrix.27 By contrast, at the high-CTBN content[Fig. 4(c)] most of the rubber particles were pulledout, the void formation occurred, and the particle

deformation was observed. It was found fromFigure 4(b,c) that the interparticle distance betweenthe neighboring particles decreased with inclusion ofrubber. When the interparticle distance decreased,the stress concentration effect increased. Conse-quently, the epoxy matrix supported the average loadhigher than the applied load and yielded.37 This maybe responsible for the debonding around the rubberin the epoxy modified with high-rubber content. Thesimilar observation was also found by Thomas et al.28

The poor interaction between the particles and theepoxy matrix was found in high-modified epoxies ascompared to those in low modified epoxies.

Tribological properties

Figure 5 shows the coefficients of friction as a func-tion of sliding distance under the applied loads of3.0 and 5.0 N. The coefficient of frictions of the neatand the modified epoxy resins rose at the run-inperiod (approximately before 40 m), and then asteady state was reached. It was found that the coef-ficients of friction of the neat and the modifiedepoxy resins under the applied load of 3.0 N wereslightly lower than those of 5.0 N. Meanwhile, thecoefficients of friction of all CTBN-modified epoxyresins were lower than those of the neat epoxyunder both applied loads. To facilitate the compari-son among the modified epoxy resins, average val-ues of the steady state coefficients of friction werecalculated in a range of 400–500 m. The results aregiven in Table III. Under the applied load of 3.0 N,the steady state coefficient of friction tended todecrease with increasing CTBN loading. Under the

TABLE IIRubber Particle Size in the Matrix of the CTBN-Modified Epoxy Resins

Sample Maximum diameter (lm) Minimum diameter (lm) Average diameter (lm) Standard deviation

2.5-CTBN 2.97 0.25 0.87 0.6385-CTBN 5.00 0.45 1.17 0.92310-CTBN 6.43 0.39 1.73 1.2515-CTBN 10.31 0.74 2.73 1.8120-CTBN 11.94 1.52 4.70 2.53

Figure 5 Coefficients of friction of the neat epoxy andthe rubber-modified epoxy resins (2.5-CTBN, 5-CTBN, 10-CTBN, 15-CTBN, and 20-CTBN) as a function of slidingdistance at the applied loads of (a) 3.0 N; and (b) 5.0 N.[Color figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

TABLE IIICoefficient of Friction of the Neat Epoxy and theCTBN-Modified Epoxy Resins at the Testing

Loads of 3.0 and 5.0 N

Sample

Steady state coefficient of friction

Load 3 N Load 5 N

Neat EP 0.711 6 0.020 0.565 6 0.0192.5-CTBN 0.610 6 0.023 0.529 6 0.0115-CTBN 0.540 6 0.013 0.499 6 0.00810-CTBN 0.562 6 0.028 0.489 6 0.00515-CTBN 0.560 6 0.010 0.525 6 0.00820-CTBN 0.550 6 0.013 0.525 6 0.006



applied load of 5.0 N, the coefficient of frictiondecreased with the addition of CTBN upon 10 phrbut then increased gradually with further additions.

To elucidate the effects of CTBN rubber on thewear behavior, the sliding-ball countermates andwear tracks from tribological tests were investigatedusing optical microscope. The results obtained areshown in Figures 6 and 7. In Figure 6(a), transferfilm did not occur on the surface of the sliding ballfor the neat epoxy resin, but only loose debris(chips) was formed with no tendency to adhere tothe ball countermate. Thus, the wear mechanismwas mainly the abrasion. Larsen et al.4,10 suggestedthe formation of the transfer film in the modifiedepoxy. The formation of the transfer films indicatedthat the dominant wear mechanism changed to theadhesion. It was found from Figure 6(b–d) that theaddition of CTBN led to the formation of transferfilms on the sliding-ball surface. The thermal soften-ing due to the frictional heat4,15 and the decrease incrosslink density which enhanced the deformationwere responsible for the transfer-film formation. Thepresence of the transfer film on the ball counter-mates could be associated with the lower coefficientof friction of CTBN-modified epoxy resins as com-

pared to the neat epoxy. In addition, the formationof transfer film could greatly affect the sliding char-acteristics of the modified epoxy resins.38 As shownin Figure 6(d), thicker film was formed on the sur-face of ball sliding on the 20-CTBN modified resin.When the thick and lumpy transfer film was created,an abrupt movement or even a seizure of the slidingcontact could occur.38 This resulted in a higher fric-tion as observed when adding CTBN rubber oflarger than 10 phr (see Table III).Figure 7(a) shows the worn surface of the neat ep-

oxy resin after performing the tribological test at theapplied load of 5.0 N. Small cracks across the weartrack and materials wave which occurred due to thebrittle nature of the epoxy39 were observed. Someploughing grooves due to three body abrasivenessleft traces on the worn surface of the neat resin. Theresults suggested that the wear process was gov-erned by the material deformation and microplough-ing type of abrasive wear. Figure 7(b–d) shows theworn surface of the CTBN-modified resins. It wasfound from Figure 7(b) that the worn surface of the2.5-CTBN modified resin exhibited the scale-likedamage pattern. This damage pattern could befound to be generated under a repeating load during

Figure 6 Optical micrographs of the stainless steel balls after sliding against (a) the neat epoxy; (b) the rubber-modifiedepoxy 2.5-CTBN; (c) the rubber-modified epoxy 5-CTBN; and (d) the rubber-modified epoxy 20-CTBN.



sliding.13 For the worn surface of the 5-CTBN modi-fied epoxy resin [see Fig. 7(c)], the ploughinggrooves were found besides the scale pattern. Theformation of such grooves was frequently observedwhen adhesive wear occurred. The possible mecha-nism of these grooves involved ploughing the epoxysurface by the work-hardened transfer particles.38 Asthe CTBN content increased to 20 phr [see Fig. 7(d)],more severe surface damages caused by the adhe-sive wear were observed. The surface delaminationwas found instead of the ploughing grooves. Itshould be noted that discussion on the tribologicalreaction mechanism was not given in this work, butwill be considered for further investigations. How-ever, it was worth mentioning by selected litera-tures12,40,41 that the tribochemical reaction mechanismcan be induced by thermal and mechanical effects.The thermally induced reaction at the surface asperitywas due to flash temperature, leading to thermal deg-radation and/or oxidation,12,40 whereas the mechani-cally induced reaction was associated with therepeated action of the shear stress within the three-body agglomerates which led to the chain scission.41

Figure 8 shows the wear rates (k) as a function ofCTBN content under the applied loads of 3.0 and

5.0 N. It was found that the wear rates of the neatand the modified epoxy resins increased withincreasing applied loads. At the applied load of3.0 N, the wear rate decreased with the addition ofliquid CTBN rubber upon 5 phr and then increasedwith further increasing CTBN content. One possiblereason for the improved wear resistance at lowCTBN content was toughness enhancement as indi-cated in Figure 2. Another reason was the formation

Figure 7 Optical micrographs of worn surfaces of (a) the neat epoxy; (b) the rubber-modified epoxy 2.5-CTBN; (c) therubber-modified epoxy 5-CTBN; and (d) the rubber-modified epoxy 20-CTBN.

Figure 8 Wear rate of the rubber-modified epoxy resinsas a function of the CTBN content.



of transfer film. The transfer film formed on the ballsurface probably acted as protective layers whichcould not allow a direct contact between the hardball and the epoxy surface, and then softened thewear. At the applied load of 5.0 N, the addition of2.5-phr CTBN rubber showed the most effective toreduce the wear rate of epoxy, but a higher wearrate than the neat epoxy was found when the CTBNwas further added. The result was possibly becausethe poor adhesion between rubber particles andepoxy matrix, leading to an ease for polymer removalduring wear process. In addition, the ductilityenhancement which was responsible for the toughen-ing mechanism became a key part when CTBN wasmore added. The high-rubber-modified epoxy was sosoft that it was readily to worn away. This resultdoes not agree with that reported by Yu et al. whoinvestigated the wear behavior of epoxy modifiedwith the rubber nanoparticles.12 Unlike the rubbernanoparticles, the presence of the large amount ofrubber microparticles could not improve the wear re-sistance of epoxy resin. It could be postulated fromthe results that the improved material toughnessalone was not always the requirement for tribologicaloptimization. The high-wear resistance of the modi-fied epoxy resins would exhibit only if the increasingtoughness compensated the decreasing stiffness.

CONCLUSIONS

The effect of CTBN rubber particles on the mechani-cal and tribological properties was studied. Theresults suggested that impact resistance of epoxyresin enhanced whereas the storage modulusreduced, when CTBN rubber was incorporated tothe epoxy resin. These results were because the pres-ence of the liquid CTBN rubber reduced the cross-link density at the interface of the modified epoxyresins, and hence increased the ductility. The coeffi-cient of friction of the modified epoxy resins waslower than that of the neat epoxy due to the forma-tion of transfer film. However, the addition of CTBNrubber of less than 10 phr was recommended forreducing the wear rate of the epoxy. The combina-tion effects of increasing toughness and formation oftransfer film were responsible for the wear behaviorof the CTBN-modified epoxy resins.

The authors are grateful to the East Asiatic (Thailand) PublicCo., Ltd. for supplying epoxy system and to National ResearchUniversity (NRU) by Office of Higher Education Commission(OHEC) for providing supports throughout thiswork.

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