2009 if With Epoxy ShearPeel Shenkar (2)

Journal of Adhesion Science and Technology 24 (2010) 1083–1095brill.nl/jast

The Effect of Tungsten Sulfide Fullerene-Like Nanoparticleson the Toughness of Epoxy Adhesives

Mark Shneider a, Hanna Dodiuk a,∗, Samuel Kenig a and Reshef Tenne b

a Shenkar College of Engineering & Design, Ramat Gan 52526, Israelb Weizmann Institute of Science, Rehovot 76100, Israel

Received in final form 17 April 2009; revised 26 October 2009; accepted 31 October 2009

AbstractIn this work the effect of inorganic fullerene-like (closed cages) nanoparticles of tungsten disulfide (IF-WS2)

on the mechanical properties and especially on the toughness of epoxy resins, was studied. The epoxy resinused was the well-known DGEBA (di-glycidyl ether of bis-phenol A) cured with polyamidoamine.

The epoxy/IF-WS2 nanocomposites were prepared by applying a high shear mixing to obtain a uniformdispersion and homogeneous distribution of the IF nanoparticles in the epoxy matrix. Two mixing proce-dures were used — a low shear of short duration and high shear with a long mixing time. The resultingepoxy nanocomposites were first characterized for their shear and peel strength using appropriate bondedjoints. The experimental results demonstrate that enhanced shear strengths and shear moduli were achieved,together with a significant increase in the peel strengths at low concentrations of the IF-WS2 nanoparticles(more than 100% increase at 0.5 wt% IF-WS2). Above the threshold value of 0.5% IF-WS2 the peel strengthdecreased sharply.

The fractured surfaces of the bonded joints were examined by transmission and scanning electron mi-croscopy in order to characterize the fracture mechanisms and analyze the dispersion level of the nanopar-ticles within the polymer. The electron micrographs indicated that the presence of the nanoparticles inthe epoxy matrix induced fracture mechanisms which were different from those observed in the pristineepoxy phase. These mechanisms included: crack deflection; crack bowing; and crack pinning. Evidence fora chemical interaction between the nanoparticles and the epoxy were obtained by infrared measurementsin the attenuated total transmittance mode. The data suggests the formation of new carbon–oxygen–sulfurbonds, which are most likely due to the reaction of the outermost sulfur layer of the IF nanoparticles withthe reactive epoxy groups. The observed simultaneous increase in both shear and peel strengths at very lowIF-WS2 concentrations, found in this work, could lead to the development of high performance adhesivesand to new types of structural and ballistic fiber nanocomposites.© Koninklijke Brill NV, Leiden, 2010

KeywordsEpoxy, nanoparticles, toughening mechanisms, nanocomposite

* To whom correspondence should be addressed. Tel.: 972-3-6110057, 972-3-6110110;Fax: 972-3-6110151; e-mail: [email protected]

© Koninklijke Brill NV, Leiden, 2010 DOI:10.1163/016942409X12584625925268

1084 M. Shneider et al. / Journal of Adhesion Science and Technology 24 (2010) 1083–1095

1. Introduction

Epoxy resins are commonly used as coatings, adhesives and matrices in fiber com-posites for a wide range of automotive, aerospace, marine and electronic applica-tions. The mechanical and thermal properties of epoxy resins can be altered bycontrolling the cross-linking density. Highly cross-linked epoxies exhibit a high de-gree of stiffness and strength. The increase in the cross-linking density results in adecrease in toughness and elongation. One of the primary aims of many researchersis to increase epoxy toughness without significantly sacrificing the glass transitiontemperature, the stiffness or the strength. These properties are required for a vari-ety of applications, such as adhesion, coating and polymer composites, and wouldaugment the excellent mechanical and thermal attributes of epoxy resins.

The commonly used toughening agents for epoxy resins are liquid rubbers [1],spherical rubber particles [2], core–shell particles [3, 4], glass beads [5, 6], hyper-branched polymers [7] and combinations of these [8]. Patterning the epoxy matrixwith microvoids is also used for the same purpose [9]. The toughening mechanismsof the different epoxy resins vary with the materials used. Rubber particles inducethe formation of microvoids, which can be responsible for yielding processes dueto the reduction of the plastic resistance of the material. In this case, a substantialamount of energy is dissipated within the plastic zone near the crack tip. Ther-moplastic particles promote delocalized microcracking as well as crack bridgingeffects [10]. In this case, the thermoplastic microparticles can avoid a decline in themodulus of the epoxy. In rigid particle filled epoxies the toughening mechanism isa combination of particle–matrix debonding, void formation around the particles,and yielding of the matrix [5, 11].

Over the last two decades nanocomposites based on inorganic nanoparticles andpolymeric resins having improved toughness, thermal, electrical and mechanicalproperties have been developed [12–14]. The main difficulties associated with thesynthesis of nanocomposites and systematic control of their properties is related tothe homogeneous dispersion of the nanoparticles and the avoidance of their agglom-eration [15]. Furthermore, a specific chemical interaction between the nanoparticlesand the epoxy matrix was accomplished by functionalization of the nanoparticlesurface [16].

Inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INT) ofWS2 and MoS2, were discovered in 1992 and 1993, respectively [17, 18]. Ex-tensive efforts were undertaken over the ensuing years to understand their growthmechanism and characterize their structures [19, 20]. Elucidation of the growthmechanism combined with the careful design of a new series of fluidized bed re-actors allowed for the scale-up of the IF-WS2 production and more recently ofINT-WS2 [21]. These nanoparticles were shown to exhibit improved tribologicalbehavior [22] and were recently successfully commercialized bearing the brandname ‘NanoLub’. Recent studies have also shown that the addition of a few weightpercent of IF-WS2 nanoparticles into various thermoplastic polymers, significantlyalter their structural, rheological, mechanical and tribological properties [23–26].

M. Shneider et al. / Journal of Adhesion Science and Technology 24 (2010) 1083–1095 1085

The mechanical properties of IF nanoparticles are comparable to those of WS2nanotubes which exhibit a high modulus (160 GPa), yield strength (16 GPa) andstrain (12%) [27]. Therefore, it can be expected that these nanoparticles may playthe role of pinning centers which prevent crack propagation in the nanocompos-ite. The objective of the present study is to investigate the effect of adding smallamounts of inorganic fullerene-like nanoparticles of tungsten disulfide (IF-WS2)

on the mechanical properties of thermosetting epoxy resins.

2. Experimental

The epoxy resin which has been used in this study is based on di-glycidyl etherof bis-phenol-A (DGEBA) (D.E.R. 331 product of Dow Chemical, Midland, MI,USA). The curing agent used was polyamidoamine (Versamide 140 product ofMiller Stephanson, Sylmar, CA, USA) with theoretical equivalent weight of 97.The mixing ratio was 70:30 w/w. The IF-WS2 nanoparticles with quasi-sphericalmorphology [19] were obtained from NanoMaterials Ltd. (Nes Ziona, Israel) Thenanoparticles consist of approximately 20–30 concentric closed molecular layersof WS2 having an average size of 100–120 nm and an empty hollow core whichoccupies about 30% of the nanoparticle volume.

In order to disperse the nanofiller in the epoxy resin two mixing procedures wereundertaken. In the first one, the dispersion was obtained by using a mechanicalshear mixer (Hsiangtai HG-300 mixer, Taipei Hsien, Taiwan) at a mixing rate of10 000 rpm for 15 min at room temperature. The temperature of the uncured epoxymixture during the mixing process with IF nanoparticles, was 50◦C. The curingagent was then added to the epoxy/IF-WS2 mixture and cured at 80◦C for 16 h. Inthe second method the dispersion was prepared with the same mixer at a mixingrate of 18 000 rpm for 4 h at room temperature. The high shear rate resulted ina temperature increase. To minimize the temperature increase a bath with flowingwater was used to cool the dispersion during the mixing process. The curing agentwas then added and mixed manually and the curing was carried out using the sameconditions as in the first method.

The adhesion properties of selected formulations were characterized in shearand peel tests and the adherents used for the specimens were aluminum 6061-T651and aluminum 1100-O alloy, respectively. The rigidity of the material allows it towithstand high bending loads which are produced during the shear test. This valuecan be compared to the Aluminum 1100-O which has a yield strength of 34 MPaand a hardness of 23 AA (brinell). That characterize by flexibility, the flexibility ofthe adherents produce the peeling mechanism.

Both adherents were treated with N-aminoethyl-3-aminopropyltrimethoxysilane(Dynasylan® DAMO, Degussa, Parsippany, NJ, USA). Single lap shear (SLS)joints were prepared and tested according to ASTM D-1002 at a loading rate of1.3 mm/min and adherent overlap of 25 mm. T-Peel joints were prepared and testedaccording to ASTM D-1876 at a loading rate of 127 mm/min. The resin thickness


between the aluminum adherents was 0.1 mm for both tests. The specimens weretested in a universal testing machine (Instron 4201, Grove City, PA, USA).

In order to evaluate the distribution of the IF nanoparticles in the epoxy ma-trix the epoxy/IF-WS2 samples were sectioned into thin (50–70 nm) slices usinga diamond knife (Micro Star technologies 45, Texas, USA) and a Leica ultraUCT ultramicrotome (Wetzlar, Germany). The thin sections were placed on car-bon/collodion coated Cu TEM grids. Transmission electron microscope (TEM)analysis was carried out by a Phillips CM120 TEM (FEI, Eindhoven, Ntherland)operating at 120 kV. The fractured surfaces were also examined by scanning elec-tron microscopy (SEM, Zeiss LEO Supra 55VP and Zeiss ULTRA 55 (Oberkochen,Germany)). A 2 nm thick chromium coating was applied on the surface to preventthe electronic charging of the non-conductive specimen. The accelerating voltageof the beam in SEM experiments was between 5–15 kV.

Fourier transform infrared spectroscopy (FT-IR) in the attenuated total transmit-tance mode (Alpha-T Optic GMBH Bruker, Billerica, MA, USA) was used in orderto analyze the possible existence of chemical interactions at the interface betweenthe nanoparticles and the epoxy matrix. For the transmission test, a thin (0.1 mmthick) nanocomposite (epoxy containing 20 wt% IF) layer was prepared. The scan-ning frequency was 375–4000 cm−1 with a phase resolution of 32. The number ofsample scans was 16.

3. Results and Discussion

Figure 1(a) shows a typical SEM micrograph of the IF-WS2 powder produced in afluidized bed reactor [28]. A TEM picture of a typical IF-WS2 nanoparticle is alsoshown in Fig. 1(b). The closed multiwall (onion-like) structure of such nanoparti-cles can be seen. The uniformity of the dispersion of the nanoparticles in the matrixis a key factor in obtaining improved mechanical properties of the nanocomposites[15, 16, 29, 30]. The IF-WS2 nanoparticles tend to form agglomerates of thousands

Figure 1. SEM micrograph of IF-WS2 powder (a) and TEM micrograph of a single IF-WS2 nanopar-ticle (b).


of nanoparticles, which are each larger than 10 microns in size. Therefore, in orderto obtain a well dispersed nanocomposite matrix, a thorough mixing procedure wasundertaken.

Figure 2(a) depicts the dispersion of the nanoparticles using the first set of mix-ing conditions (10 000 rpm for 15 min). Relatively large agglomerates, consistingof a few tens of IF nanoparticles each, are observed in the matrix. By using moreintensive mixing conditions (18 000 rpm for 4 h) the dispersion was significantlyimproved. Smaller clusters of the IF nanoparticles well dispersed in the solid ma-trix, were observed. A typical example showing the improved dispersion of thenanoparticles in the epoxy matrix is shown in Fig. 2(b). The TEM analysis wasrepeated with a number of slices which were prepared in the same way and verysimilar results were obtained. Uniform dispersion of the IF nanoparticles blendedin various thermoplastic polymers have also been reported [23–26].

To estimate the nanoparticles distribution in the epoxy matrix semi-quantitatively,various microtome samples were prepared and four representive TEM pictures wereanalyzed. Statistical analysis of the number density per given surface area ratio, in-dicated that, on the average, 11 and 5 particles per 400 µm2 (projected area asobserved by TEM) could be detected for the high and low shear mixing experi-ments, respectively. Therefore, we infer the a long mixing time at a higher mixingrate leads to a reduction of agglomeration and a more uniform dispersion of theindividual nanoparticles.

The effect of rigid micro-size particulate fillers on the stress–strain behavior ofpolymers is well known. The addition of microfillers, like calcium silicate and alu-mina trihydrate, generally increases the stiffness and simultaneously reduces thestrain to break off the polymer matrix [31, 32]. Furthermore, the value of the lapshear strength of microparticle filled composites is reduced with increasing fillercontent [5, 33, 34]. In nanocomposites the behavior can be different due to the

Figure 2. TEM micrograph of the epoxy nanocomposite containing 0.5 wt% IF-WS2.


Figure 3. Dependence of the lap shear strength (left) and shear modulus (right) of epoxy nanocom-posites on the concentration of the IF-WS2 nanoparticles. Mixing conditions were 18 000 rpm for 4 hat a temperature of 50◦C.

Figure 4. Strain at break for epoxy nanocomposites vs IF-WS2 content. Mixing conditions were18 000 rpm for 4 h at a temperature of 50◦C.

large interfaces between the nanoparticles and the hosting matrix. Figure 3 showsa near linear increase of the lap shear modulus with filler content. Furthermore, asexpected, the shear strength of the nanocomposite also increases.

The simultaneous increase in stiffness and shear strength, as observed for theIF-WS2 nanocomposites, indicates that stresses are efficiently transferred via thematrix/nanoparticle interface [5, 33, 35]. Furthermore, the strain at break usuallydeclines with rising filler content. However, as Fig. 4 shows, in the present case thestrain to break remains unchanged with increasing the nanoparticles loading in theIF-WS2 nanocomposites.

It was suggested that under certain conditions the nanoparticles may induce ma-trix yielding, and act as a stopper to crack propagation by pinning, bowing [36–38],debonding [39–41] and deflection [42–45]. Although the effect of these mecha-


Figure 5. SEM micrograph of shear-failed epoxy/0.5 wt% IF-WS2: crack deflection (A) and crackbowing (B). Epoxy prepared at 18 000 rpm for 4 h at a temperature of 50◦C.

Figure 6. (a) SEM micrograph of shear-failed untreated (neat) epoxy. Cohesive failure (A), the mor-phology of the adhesive failure (B) follows the aluminum substrate roughness; (b) SEM micrographof shear-failed epoxy/0.5 wt% IF-WS2. Mixing conditions were 18 000 rpm for 4 h at a temperatureof 50◦C.

nisms (besides debonding) in the case of nanoparticles is small [15, 46, 47], it canbe significant enough to make a difference. Figure 5 shows that the main mecha-nisms for energy absorption in the epoxy-IF nanocomposites, are crack deflectionand bowing. It should be emphasized that debonding was not observed. The ap-pearance of such mechanisms at the nanoscale was manifested as roughness in themacroscale leading to cohesive failure. Figure 6(a) presents a SEM micrograph ofan untreated (neat) epoxy resin fractured under shear in the joint. This figure showsthat large flat fracture planes are dominant which reflects a brittle behavior. Thisfigure is indicative of a mixed (adhesive and cohesive) failure mode. On the otherhand, the shear fractured nanocomposite exhibits a very rough surface suggestingdeflection and crack bowing mechanisms (Fig. 6(b)).


Figure 7. Peel strength of aluminum (1100-O series aluminium alloy) epoxy/IF-WS2 joints as a func-tion of nanoparticles content in the epoxy matrix.

Figure 7 compares the peel strength of joints produced with IF-epoxy nanocom-posites which were prepared using a low mixing rate for 15 min (A) with thoseprepared with a high mixing rate for 4 h (B). The peel strength of the joint preparedwith the untreated epoxy was 0.0012 kN/m. The peel strength, of the joint producedwith a high rate of mixing for the epoxy resin containing 0.5 wt% of the nanopar-ticles, was double that value. At higher concentrations the peel strength decreased.In the case of the low mixing rate process, an even better peel strength value wasobtained and, again, the maximum was found at 0.5 wt% of IF-WS2 nanoparticles.The fact that the epoxy mixed at lower shear rate exhibited a better peel strengththan the nanocomposite mixed at higher shear rates and for longer period, cannoteasily be explained. It may be attributed to the lack of complete reproducibility inthe preparation of the specimen even though the dispersion did not seem to have aneffect on the lap shear and modulus measurements. The shear and the peel strengthmeasurements were supplemented by extensive electron microscopy analysis.

The surface of the fractured sample prepared with untreated epoxy (Fig. 8(a))shows planes with a flat and smooth appearance which is typical of brittle mate-rials [31]. The fractured surface of the epoxy containing 0.5% IF-WS2 exhibits adifferent morphology (Fig. 8(b)). The surface appears very rough suggesting thatthe crack deflection mechanism was operating here. Figure 9 shows that pinningand bowing mechanisms also appear during the peel experiment in the joint madewith the 0.5 wt% loaded IF nanocomposite. It should be noted that these mech-anisms were not reported by Kinloch et al. in joints produced with silica/epoxynanocomposites [46]. This difference could be attributed to the different interfacialinteractions present in the IF-WS2 nanoparticles and in the silica nanoparticles.

The interface between the IF-WS2 nanoparticles and the epoxy was investi-gated using transmission electron microscopy on carefully microtomed samples.


Figure 8. (a) SEM micrograph of the surface of a fractured sample of untreated (neat) epoxy afterpeel test; (b) SEM micrograph of the surface of a fractured sample of epoxy/0.5 wt% IF-WS2 afterpeel test. Adhesion failure (A). Mixing conditions were 18 000 rpm for 4 h at a temperature of 50◦C.

Figure 9. SEM micrograph of the surface of a fractured sample of epoxy/0.5 wt% IF-WS2 after peeltest showing crack pinning (A) and crack bowing (B). Mixing conditions were 18 000 rpm for 4 h at atemperature of 50◦C.

Figure 10(a) indicates that a gradient in electron absorption density exists betweenthe fullerene surface and the bulk epoxy resin. In some parts along the fullerene sur-face strong adhesion to the epoxy matrix was observed (Fig. 10(b)). The thicknessof the interfacial layer was estimated to be in the range of 20–40 nm.

The good adhesion between the IF-WS2 nanoparticles and the epoxy promptedthe investigation into a possible chemical reaction between the surface of theIF-WS2 nanoparticle and the epoxy resin. For this purpose a special sample hav-ing a concentration of 20 wt%. IF-WS2 was prepared. The high concentration wasselected in order to increase the resolution of the FT-IR using the ATR mode.

The FT-IR spectra (ATR mode) in Fig. 11(a) shows a newly formed strong andnarrow peak at 1249 νmax/cm−1 which is typical of R–SO3 or R–SO3H bonds. In


Figure 10. TEM micrograph of microtomed IF-WS2 particles/epoxy resin. Mixing conditions were18 000 rpm for 4 h at a temperature of 50◦C, (a) detachment of the fullerene from the matrix, (b) strongadhesion between the particle and the matrix.

addition a wide and medium absorption peak was obtained around 1543 νmax/cm−1

indicative of derivatives of the CO–NH–R bond. The other absorptions around 1650and 3317 νmax/cm−1 are indicative of OH and NH moieties (Fig. 11(b)).

From these results it appears that in the curing process a nucleophilic attack tookplace between the outermost sulfur atoms of the nanoparticles and the oxa-cyclo-propane group of the epoxy to form a covalent bond between carbon and sulfur thatcan be oxidized to sulfonic or sulfuroxide. This C–S species may reduce the abilityof the epoxy to react with the amide group from the curing agent and, thus, hinderthe forming of cross-linked bonds.

4. Conclusions

Epoxy nanocomposites based on IF-WS2 were prepared using high shear mix-ing in order to disperse IF-WS2 nanoparticles in the epoxy resin. Shear and peeltests of the IF-epoxy nanocomposite were carried out in bonded joints followed bytransmission and scanning electron microscopy of the failed surfaces. It was foundthat long mixing times and high sheering levels reduced the agglomeration of theIF-WS2 and resulted in a homogeneous dispersion of the nanoparticles.

It was also found that only at low concentrations of the IF-WS2 (0.5 wt%) didthe composite material exhibited high shear and peel strength. At concentrationsabove this threshold, the peel strength decreased sharply. The SEM analysis of thefractured surfaces indicated that a variety of energy absorbing mechanisms tookplace leading to these somewhat unexpected results. FT-IR analysis verified that thesulphur in the outermost layer of the nanoparticles reacted with the epoxy group toform a C–S bond.


Figure 11. (a) Region of 1000–1700 cm−1; (b) 2800–3600 cm−1 in the FT-IR spectra of the untreatedand mixed epoxy films. Mixing conditions were 18 000 rpm for 4 h at a temperature of 50◦C.

The simultaneous increase of both shear and peel strength at very low concen-trations of the IF-WS2 nanoparticles, may find applications in high performanceadhesives and coatings as well as in structural and ballistic fiber composites.

Acknowledgements

We are grateful to the staff of the Plastics and Rubber Center at Shenkar College ofEngineering & Design for their assistance and to NanoMaterials, Ltd. for providingthe IF-WS2 nanoparticles. We also thank Dr. R. Popovitz-Biro (Weimann Institute)for the assistance in the TEM analysis. We acknowledge the Irving and Cherna


Moskowitz Center for Nano and Bio-Nano Imaging at the Weizmann Institute ofScience and the H. Perlman, Horowitz and the Gurwin Foundations. Reshef Tenneis the Drake Family Chair Professor of Nanotechnology and director of the Helenand Martin Kimmel Center for Nanoscale Science.

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2009 if With Epoxy ShearPeel Shenkar (2)