Understanding the Effect of Nano-modifier Addition Upon the Properties of Fibre Reinforced Laminates 2008

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Composites Science and Technology 68 (2008) 718–726

SCIENCE ANDTECHNOLOGY

Understanding the effect of nano-modifier addition upon theproperties of fibre reinforced laminates

Marino Quaresimin a, Russell J. Varley b,*

a Department of Management and Engineering, University of Padova, 36100 Vicenza, Italyb CSIRO Manufacturing and Materials and Technology, Clayton South 3168, Australia

Received 27 February 2007; received in revised form 23 August 2007; accepted 13 September 2007Available online 20 September 2007

Abstract

This work presents a survey of the effect of three different commercially available nano-modifiers on the mechanical properties of anepoxy/anhydride unidirectional carbon fibre reinforced laminates. The nano-modifiers consisted of an organo-modified layered silicate,vapour grown carbon fibre (VGCF) and a triblock copolymer (SBM). The work has shown that tensile modulus exhibited little differencebetween the unmodified laminates while a modest decrease was observed for the tensile strength. Properties related to the toughness ofthe matrix, demonstrated improvements compared to the unmodified laminate such as the notch sensitivity under compression, ILSS andGIIC performance. The improvement of the GIIC for the VGCF modifier in particular was found to be over 100%. It was suggested thathigh aspect ratio of the nano-additive helped to constrain the growth of the micro-cracks which in turn delayed failure. Mode I GIC

performance however, was found to decrease as a result of the fibre tows preventing optimum dispersion of the modifier. The effectof this was a very high ‘‘effective’’ clay concentration in the interlayer resin rich regions and a less than optimum fibre dispersion, resultsin promotion of the propagating crack rather than inhibition.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Particle reinforced composites; B. Fracture toughness; A. Layered Structures

1. Introduction

The use of nano-additives, such as organo-modified lay-ered silicates, carbon nanofibres or nanotubes and others,to reinforce epoxy resins has generated significant interestboth academically and commercially in recent times [1–4].This interest is primarily a result of the concurrentimprovements in mechanical properties such as toughness,strength and modulus, as well as improvements and ther-mal properties such as fire performance, degradation andglass transition temperatures (amongst others), at low lev-els of addition that allows the use of existing processingmethods. It has been well documented however, that toachieve these property improvements, the nano-additive

0266-3538/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2007.09.005

* Corresponding author. Tel.: +61 3 9545 2491; fax: +61 3 9544 1128.E-mail address: [email protected] (R.J. Varley).

must be sufficiently dispersed and compatible with theepoxy resin. Depending upon the nano-additive in ques-tion, this brings a range of associated challenges uniqueto the material. In the case of layered silicates, the lateraldimensions of the clay platelets are of the order of micronsin area, around 1 nm thick and arranged in stacks knownas tactoids [5]. Complete exfoliation, therefore requiresthe separation of the tactoids from the primary particle,followed by the destruction of the order or the clay plate-lets within the tactoids. Although it is widely reported thatfull exfoliation of the clay platelets will maximise theimprovement in strength, modulus and toughness [4,6], ithas also been suggested that a balance between an exfoli-ated and intercalated structure may be preferable to maxi-mise modulus and toughness enhancements in epoxy resinmatrices [7]. Boo et al. [8] reached similar conclusions thattoughening mechanisms in nano-platelet reinforcedcomposites, such as crack deflection or crack pinning, are

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promoted when intercalated tactoids or even agglomeratesare present. As well it was suggested that fully exfoliatednanocomposites would if fact possibly reduce fracturetoughness. Carbon based nanotubes are another class ofnano-additives which show significant potential providedthey can be adequately dispersed within an epoxy matrix.Difficulties in complete dispersion however, arise from elec-trostatic attractive forces between the tubes causing themto exist in agglomerated bundles. A variety of methods,reviewed elsewhere [9] have been investigated, which partic-ularly highlights the difficulty of reducing the electrostaticforces while not degrading the fibre and hence compromis-ing the composite properties. Other nano-additives thathave shown significant improvement in improving themechanical properties of epoxy resins, are triblock copoly-mers. These additives can self assemble through thermody-namic processes to form unique nanostructures withinreactive epoxy matrices [10,11] that promote unique tough-ening mechanisms arising from the unique morphologies.However, the effect upon viscosity and the tendency tomicro-phase separate tends to inhibit the benefits of thenano-additive.

The application of toughening polymer matrices to afibre reinforced composite however, creates further com-plexities in optimising the performance of the resin domi-nated composite properties. While the constrainingnature of fibre reinforcement can minimise the effectivenessof rubber additives in carbon fibre composites [12], it issuggested here that alternative toughening mechanisms ofnano-additives may overcome these limitations. It is impor-tant to note here that that comparatively weak out of planeinter-laminar properties have the greatest potential (andindeed need) for improvement, while it is unlikely (andequally less important) that the in-plane fibre dominatedproperties would be able to be significantly improved.The research performed to date, aimed at translating resinproperties to the fibre reinforced composite, has met withmixed results. Rice et al. [13] reported a 12% improvementin modulus at 2 wt% of organosilicate for aerospace com-posite materials while reporting no improvement in othermechanical properties. Timmerman et al. (2002) [14],reported negligible improvements in mechanical properties,although they did report a significant reduction in transver-sal microcracking during cryogenic cycling of nanoclaycomposites compared to traditional composites, and indi-cating the need for careful selection of nanoclay concentra-tion and surface modification. Becker et al. (2003) [15] haveshown that improvements in crack opening fracture tough-ness can be achieved at low levels of clay addition. This wasexplained as being due to the formation of a crack bridgingmechanism and the supplementary reinforcement providedby the clay. Although the matrix was not epoxy based, anAS-4/PEEK composite modified with SiO2 nanoparticleshas been shown by Jen et al. (2004) [16] to synergisticallyimprove mechanical properties in unidirectional laminates.They found that a 1 wt% addition of nanoparticlesimproved strength and modulus by 12% and 19%, respec-

tively in quasi-isotropic laminates while cross-ply laminatesproduced smaller increases. Additionally they found thatthe nano-modified laminates had little effect upon the ten-sile fatigue performance. The use of glass fibre as the rein-forcement however, has yielded improved results comparedto carbon. Improvements in strength (compressive [17] andflexural [18,19]) of the order of 25% have been reported atlow levels of addition. The work by Haque and Sham-suzzoha [18] also reported large improvements in the ILSSand fracture toughness. The level of improvement has beenattributed by Kornmann et al. (2005) [19] to the affinitybetween the silicate organoclay and the silicate glass fibresand enhancing the fibre matrix adhesion. It is of interest tonote that for the abovementioned reasons the substantialimprovements reported in the fibre dominated properties(i.e. tensile and bending) are considered to warrant moreresearch in this area.

This paper therefore seeks to further understand theeffect of nano-modifiers similar to those discussed aboveon the properties of an epoxy anhydride carbon fibre lam-inate. Some benefits of using nano-modifiers are high-lighted, while also presenting and justifying somedeleterious effects arising from specific difficulties in rela-tion to processing the composite. Three commerciallyavailable nanomodifers are used in this work, namely org-ano-modified layered silicate (nanoclay), vapour growncarbon fibre (VGCF) and an organic triblock copolymersystem. All three systems were fabricated into carbon fibrereinforced composites and a range of mechanical propertieswere determined. Scanning electron and optical micro-scopic techniques were used to provide an explanationfor the results obtained.

2. Experimental

2.1. Materials

The polymeric matrix consisted of a blend of the diglyc-idyl ether of bisphenol A (DGEBA) under the trade nameof D.E.R. 331 (DOW, USA) and the epoxy novolac resinunder the trade name of D.E.N. 431 (DOW,USA), Theanhydride hardener was a hexa-hydrophthalic anhydride(XD5200) supplied by Ciba Specialty Chemicals (Austra-lia). The composition of the base matrix formulation wasas follows: 29 phr of DER331 resin, 27 phr of DEN431resin, 44 phr of the hardener XD5200 and 0.5 phr of theaccelerator, 1-methyl-imidazole. The nanoclay used herewas Cloisite� 30B supplied by Southern Clay Products(USA). They are surface modified lamellae of montmoril-lonite 1 nm thick and with lateral dimensions from 70 to150 nm according to the product data sheet supplied. Thevapour grown carbon nanofibers (VGCF) were obtainedfrom Applied Sciences under the trade name of Pyrog-raf�-III PR24-PS. They have a diameter of 200 nm andtheir length can range between 30 and 100 lm, with a spe-cific surface area of 50–53 m2/g. The triblock copolymerused was an acrylic block copolymer poly styrene butadi-

720 M. Quaresimin, R.J. Varley / Composites Science and Technology 68 (2008) 718–726

ene methyl methacrylate (SBM), supplied by Arkema,under the trade name of AF-X E20. The unidirectional car-bon fibers Tenax� G30-700 12K HTA-7C by Toho-TenaxAmerica Inc. was used. In this work, the following lami-nates containing 5 wt% of nanoclay, 7.5 wt% of VGCFand 10 wt% of SBM were prepared for evaluation andcompared to an unmodified laminate. The concentrationsfor each additive were specifically chosen so as to havethe greatest potential for exhibiting improvement in thelaminate properties while minimising any deleteriousconsequences.

2.2. Nanocomposite formation

The nano-additives were dispersed in the resin systemsprior to application onto the unidirectional fibre as follows.The organoclay was dispersed through a shear mixing pro-cess where the epoxy resins were heated to 130 �C and theclay added while mixing at a rate of 1000 rpm. This wascontinued for 1 h until the hardener was added and theresin was cooled to room temperature. The VGCF was dis-persed in the epoxy resin system according to an attritionmilling process. The VGCF was placed in a container alongwith milling balls and solvent and stirred at 1000 rpm for1hr. After this time the balls were removed and the VGCFand solvent were added to the epoxy resin. The solvent wasconcentrated to a specific volume and then pre-pregged byapplying a known quantity of resin onto the carbon fibre.The triblock copolymer was formed by dissolving in ace-tone along with the epoxy resin formulation and thenpre-pregged in a similar manner to the VGCF.

2.3. Sample Fabrication

Prepregs were prepared according to a hand lay-up pro-cess at room temperature of around 22 �C and ambienthumidity. In the case of the unmodified and the nanoclaymodified resins, a solventless melt process was used toapply the resin evenly. In the case of the VGCF and tri-block copolymer systems, a solvent based process wherebythe solution was applied to the carbon fibre using a paintbrush. The solvent was allowed to evaporate overnightwhere the temperature varied from 22 �C to 12 �C andambient humidity before fabrication was commenced. Inall cases, care was taken to ensure that similar quantitiesof resin were applied to the fibre. The prepreg was thencut out and fabricated according to the test being per-formed. A teflon film of thickness 5 lm was inserted asthe crack starter for the mode I and II lay-up sequence.In the case of the mode I and II samples, [0]24 sequencewas used, a [0]10 sequence was used for the compressiveand tensile properties while a [0]6 sequence was used forthe impact testing. Curing for all systems unmodified andnano-modified was performed using a vacuum bag config-uration between a platen press and the cure profile was asfollows: 30–80 �C at 5 �C/min under vacuum and 1.4 barsof additional pressure, hold at 80 �C for 2 h and under vac-

uum and 1.4 bars of pressure for a further 70 min, releasevacuum while increasing pressure to 5.5 bars, heat from80 to 180 �C at 5 �C/min with pressure at 5.5 bars, holdat 180 �C at 5.5 bars for 2 h and finally cool down to roomtemperature overnight. The fibre volume fractions of thelaminates prepared for the unmodified resin, the clay, theVGCF and the SBM modified resin were found to be69 vol%, 63 vol%, 65 vol% and 54 vol%, respectively.

2.4. Mechanical properties

Mechanical properties were evaluated using an MTS 809axial/torsion test system with load cell of 10/100 kN. Forthe tensile testing measurements, samples were machinedto the following dimensions: 2 · 15 · 110 mm. Tabs wereapplied to the specimens in order to improve the grip inthe clamps. The crosshead speed was set at 2 mm/minand three specimens were tested for each material configu-ration. An MTS 632.85F-14 extensometer was used fordetermination of tensile modulus. Compressive propertieswere investigated both on plain and notched samples toemphasise the matrix contribution. Plain samples werecut in the size of 2 · 24 · 110 mm and tested with an unsup-ported length of 20 mm. Open hole compressive (OHC)properties were determined using samples of dimensions2 · 36 · 300 mm with holes of different size, namely 4, 8and 12 mm, to investigate also the notch size effect. Holeswere carefully drilled undersize and then reamed to finaldimensions. To avoid sample buckling notched sampleswere fitted with an anti-buckling fixture according toASTM D6484. The crosshead speed used for all compres-sive samples was set at 2 mm/min and the test was repeatedon at least three specimens for each material configuration.

The interlaminar shear strength (ILSS) was determinedaccording to ASTM D2344 using samples of the followingdimensions, 12 · 4 · 30 mm. The crosshead speed was setat 1 mm/min and six specimens were tested for each mate-rial configuration.

Mode I testing was carried out according to ASTMD5528. Three double cantilever beam (DCB) specimensfor each material configuration were tested, with the fol-lowing dimensions: width 25 mm, thickness 4 mm, length75 mm. Steel hinges (25 · 50 mm) were applied to the spec-imens to ensure that the specimen is always verticallyloaded. An auxiliary load cell of 1 kN was fitted to testingmachine and the crack propagation was monitored visuallywith a travelling microscope. The crosshead speed was setat 0.5 mm/min. The reported GIC values were calculatedusing the compliance calibration method (MCC) and theinitiation values were determined by visual observation.

Mode II testing was determined using end notched flex-ure method (ENF) according to DIN EN6034. Specimendimensions consisted of width 25 mm, thickness 4 mmand length 130 mm. Three specimens were tested underthree point bending for each material configuration andthe crosshead speed was set at 1 mm/min. The distancebetween the lower rolls was 100 mm and the initial crack

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length 25 mm. Mode II strain energy release rate, GIIC, wascalculated according to beam theory.

The impact test was carried out according to ASTMD5628 standard using a CEAST Fractovis Plus 7520 fallingmass impact tester. A strain gauge striker with a hemi-spherical head of diameter 12.7 mm was used.

3. Results and discussion

3.1. Laminate morphology

A selection of SEM micrographs are shown in Fig. 1 atlow magnification to highlight the laminate morphologyproduced in each of the laminates prepared. The character-istic layering of the plies is observed with each of the lam-inates which highlights a thin strip of a resin rich layer inbetween the carbon fibre tows. It is worth noting here thatthe quality of the laminate of the SBM triblock system isclearly inferior to the other laminates with significant num-bers of voids apparent. In addition to this, in the resin richlayer it can be seen that phase separation of the SBM hasoccurred producing particles of the order of a few micronsin diameter. This suggests that in the case of the SBMnano-additve there has been little if any nanostructure for-mation. The reason for this is expected to be a result of theentrapment of residual solvent caused by the increase in

Fig. 1. Scanning electron micrographs at a magnification of ·200 showing thprepared for the (a) unmodified (b) nanoclay modified, (c) VGCF modified an

viscosity. This would be expected to impact the mechanicalproperties measured for this system.

3.2. Tensile Properties

The effect of the different nano-additives upon the com-posite tensile moduli are shown in Fig. 2. As can be seenthere is little effect upon properties as would be expectedgiven the fibre dominated nature of the test performedhere. The SBM system however, is seen to decrease signif-icantly, in accord with the reduced fibre volume fractionand lower quality of the laminate, although the elastomericnature of the additive may also be contributing to this largedecrease. The results of the tensile strength measurementsshown in Fig. 3, highlight a modest decrease in perfor-mance compared to the unmodified laminate for the rangeof nano-modified used here. Again, for the SBM system,lower volume fraction and laminate quality induced thelowest strength value.

3.3. Compressive properties

The compressive properties determined here are shownin Fig. 4. The poor performance of the SBM system isclearly evident and can again be attributed to the extentof voids in the laminates prepared. In fact, the reduction

e consolidation and layered morphology of the unidirectional laminatesd (d) SBM modified laminates.

60000

70000

80000

90000

100000

110000

120000

130000

140000

150000

un-modifiedresin

organo-clay SBM VGCF

Ten

sile

mo

du

lus

E [

MP

a]

Fig. 2. Effect of nano-modifier addition upon the tensile modulus.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

unmodifiedresin

organo-clay SBM VGCF

Ten

sile

str

eng

th [

MP

a]

Fig. 3. Effect of nano-modifier addition upon the tensile strength.

-800

-700

-600

-500

-400

-300

-200

-100

0plain 4mm 8 mm 12 mm

Co

mp

ress

ive

Str

eng

th (

MP

a)

un-modified resinnano-claySBM

VGCF

Fig. 4. Effect of nano-modifier addition upon the compressive strength forplain and notched laminates.

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35diameter/width

no

tch

ed/p

lain

co

mp

ress

ive

stre

ng

th (

MP

a)

un-modified resin

VGCF

nanoclay

Fig. 5. Effect of nano-modifier addition on the normalised compressivestrength as a function of the normalised notch size.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

un-modifiedresin

organo-clay SBM VGCF

GIC

[kJ

/m2 ]

initiation

propagation

Fig. 6. Effect of nano-modifier addition upon the initiation and propa-gation mode I interlaminar fracture toughness (DCB).

722 M. Quaresimin, R.J. Varley / Composites Science and Technology 68 (2008) 718–726

in strength for SBM laminates is far larger than thatexpected on the basis of the lower content of reinforce-ment. Furthermore, although in the presence of a signifi-cant scatter, the results indicate a trend of relativeimprovement in strength for both the VGCF and nanoclayas the hole size increases. This result is highlighted furtherin Fig. 5 which plots the normalised strength compared to

the plain specimens against normalised hole diameter. Theresults obtained from the SBM laminates are not presenteddue to the poor compressive values for the plain sampleswhich makes the comparison unrealistic. It can be sug-gested therefore that the addition of both VGCF and nano-clay provide the laminate with an increased resistance tothe presence of notches compared to the unmodified baselaminates. The brittle nature of epoxy resins, and their cor-responding notch sensitivity is an inherent aspect of epoxyresin which is always taken into account during design. Thereduction in notch sensitivity for nanoclay and VGCFmodified systems therefore, has the potential to increaseflexibility during the design of epoxy composites. Theseresults complement those of Subramaniyan and Sun(2006) [17,20] who observed maximum improvements incompressive strength to be of the order of 25% for a wetlay-up method similar to that used here. When they useda VARTM methods however, they reported a decreasein compressive strength, and attributed this to the

Fig. 7. Optical micrographs at two magnifications, illustrating the filtering effect of the carbon fibre tows preventing nanomodifer from penetrating thefibre rich regions for the (a) nanoclay (on the left) and (b) VGCF (on the right).

0 0.5 1 1.5 2 2.5

E [keV]

Co

un

ts

Fiber-rich region

Resin-rich region (interlayer)

C

O Al

Si

Fig. 8. Electrodispersive spectroscopic elemental analysis of the nanoclaycomposite quantifying the inhomogeneous dispersion of the layeredsilicate in the interlayer region and between the fibers.

0.0

0.4

0.8

1.2

1.6

2.0

un-modifiedresin

organo-clay SBM VGCF

GIIC

[kJ

/m2 ]

Fig. 9. Effect of nano-modifier addition upon the mode II fracturetoughness (ENF).

M. Quaresimin, R.J. Varley / Composites Science and Technology 68 (2008) 718–726 723

consequences of the nanoclay being filtered during the pro-cess and therefore not being evenly dispersed.

3.4. Fracture properties

Given the positive effect upon notch sensitivity of nano-clay and VGCF it was expected that the mode I crackopening strain energy release rate GIC, would also beimproved. Fig. 6, which shows the initiation and propaga-tion GIC results, all show large decreases in fracture tough-

ness, particularly for the organo-clay and the VGCFadditives. The poor performances particularly for the org-ano-clay and the VGCF can be explained by examiningboth the SEM images shown in Fig. 1 as well as the opticalmicrographs in Fig. 7. Fig. 7 shows that for both theVGCF and nanoclay systems, even though apparently welldispersed within the interlayer resin rich region, there isnegligible infusion of nano-additives into the fibre richregions. As a result the nano-additive is not evenly dis-persed throughout the composite and effectively filteredout by the fibres. In addition, the micrographs shownin Fig. 1, show that this resin rich region is actually a

0

10

20

30

40

50

60

un-modifiedresin

organo-clay SBM VGCF

ILS

S [

MP

a]

Fig. 10. Interlaminar shear strength (ILSS), as determined for thedifferent nano-modified laminates.

724 M. Quaresimin, R.J. Varley / Composites Science and Technology 68 (2008) 718–726

relatively small fraction of the area of the composite com-pared to the fibre rich region. The result of this is that theconcentration of the nano-additive is therefore muchhigher than the nominal 5 and 7.5 wt% of nanoclay andVGCF, respectively. At such high concentrations the qual-ity of the dispersion is compromised, increasing theagglomeration and clustering in the resin rich regions such

Fig. 11. Comparison of the fracture surfaces at magnifications of ·1500 for thlaminates.

that the nano-modifier actually provides points of weak-ness rather than reinforcement, and thus decreasing tough-ness [21,22]. Fig. 8 provides further evidence of thisfiltration of the nano-modifier from the fibre rich regionsfor the nanoclay system, showing electro-dispersive spec-troscopy analysis of the interlayer region and between thefibres. A lack of any Si and Al between in the fibre richregion is a clear indication of the lack of any nanoclay pres-ent. Again the poor performance of the SBM laminate canbe attributed to the lower quality of the laminates.Although the SBM GIC is poor, it remains significantlyhigher than the VGCF and nanoclay systems. This comple-ments the above discussion in that no filtering occurs forthese samples and the SBM GIC is well above that of theVGCF and nanoclay. The actual extent to which a nano-composite was formed either during the prepreg fabrica-tion stage or after cure, was not specifically measured inthis work. Thus this result highlights that the observedimprovements in properties, or in this case the deleteriouseffects that can arise when the additive is acting as a filler.

In contrast to the poor performance of the mode I crackopening behaviour, failure through shear loading displayedimprovements with the addition of VGCF and nanoclaycompared to the unmodified laminates. The results are

e (a) unmodified laminate (b) VGCF modified laminate and (c) nanoclay

M. Quaresimin, R.J. Varley / Composites Science and Technology 68 (2008) 718–726 725

shown in Figs. 9 and 10 for the mode II ENF and interlam-inar shear (ILSS), respectively. During the interlaminarshear tests all the specimens failed by delamination. Rea-sonably similar trends in the mode II and ILSS resultscan be seen, although it is clear that the effect of VGCFaddition is significantly higher than that of the nanoclay.An interesting aspect of this work therefore is the decou-pling of the relationship between the tensile (mode I) fail-ure and failure through shear processes. It is typicallyexpected that if the toughness of a polymer composite isincreased, improvements in both mode I and II deforma-tions should be evident. However here, the filtration effectthat reduces the crack opening GIC has not had the sameeffect for the shear failure mode GIIC. Mode II occurs viathe development of a series of sigmoidal shaped matrixmicro-cracks ahead of the crack tip with an orientationof approximately 45� to the fibre direction. In a brittlematrix, these micro-cracks grow during loading and even-tually coalesce and then fail catastrophically. The fracturesurface contains many ‘‘zipper’’ looking facets, usuallycalled hackles as can be seen from the SEM fracture sur-faces shown in Fig. 11a for the unmodified laminate at amagnification of ·1500. Fig. 11b and c show scanning elec-tron micrographs of the fracture surfaces of the VGCF andnanoclay modified systems at ·1500, respectively. Themicrographs show clear evidence of modification of themechanism by both of the nano-additives. For bothnano-modifiers, the hackles are less well defined comparedto the unmodified laminate and appear closer together orsmaller. This has lead us to suggest that improved shearperformance may be related to ability of the high aspectratio additives to effectively ‘‘anchor’’ the micro-crackstogether, (despite their effective higher concentration) bythe VGCF fibres (in particular) delaying their growth, coa-lescence and thus laminate failure. Therefore, the betterperformance of the VGCF is likely to be a result of theirhigher aspect ratio and the presence of more well dispersedfibres compared to the nanoclay. Again in both ILSS and

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

un-modifiedresin

organo-clay SBM VGCF

abso

rbed

to

imp

act

ener

gy

rati

o

Fig. 12. Comparison of the absorbed to impact energy ratio forunmodified and nano-modified laminates prepared.

mode II failure, the poor results obtained from the SBMlaminates, can be related to the lower quality of the lami-nate prepared, since in these cases the results are mainlymatrix-controlled and the fibre content has less influence.

3.5. Impact

The effect of nano-addition upon the energy absorptioncapability of the laminates under impact loading has beendetermined here. Although the [0]6 sequence is not idealfor impact design application, it was chosen to emphasisethe contribution of the matrix in terms of energy absorp-tion. Therefore the results of the impact testing in Fig. 12are represented as the ratio of the absorbed to impactenergy. As can be seen, the absorption capability of boththe organoclay and the VGCF are above that of theunmodified laminate. The observation that damage mainlyoccurred through splitting, confirms the primary contribu-tion of the matrix to the increased performance. In addi-tion, the similarity of the impact behaviour to the GIIC

suggests it can be used as a representative parameter todescribe the matrix contribution to impact performance.

4. Conclusions

In summary, this paper has presented a short survey ofthe effect of three different nano-modifiers upon keymechanical properties of carbon fibre reinforced laminates.With the exception of the triblock SBM laminates, goodquality laminates were prepared. The higher void contentwas discussed in terms of the viscosity increase arising fromthe use of SBM and the corresponding entrapment of sol-vent. This in turn had a deleterious effect upon mechanicalproperties.

The work has shown that tensile modulus exhibited littledifference between the unmodified laminates while a mod-est decrease was observed for the tensile strength for theVGCF and nanoclay modified systems. In contrast how-ever, properties dominated by the matrix contribution,demonstrated significant improvements compared to theunmodified laminate. The notch sensitivity under compres-sion was shown to be improved when nanoclay and VGCFwas added. The shear behaviour as evidenced, via ILSS andGIIC was also improved, particularly for the VGCF modi-fier which was found to improve GIIC by over 100%. It wassuggested that high aspect ratio of the nano-additive helpedto constrain the growth of the micro-cracks which in turndelayed failure. This decrease in brittle behaviour evi-denced by improvements in notched sensitivity, GIIC andILSS, were not however, translated into improved GIC per-formance as may have been expected. Substantial decreasesin mode I crack opening behaviour were observed and wereshown to be a result of the fibre tows preventing optimumdispersion of the modifier by filtration. The result beingthat the ‘‘effective’’ clay concentration in the interlayerresin rich regions was much higher than the nominalnano-additive concentration.

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Acknowledgements

The activity was carried out in the frame of the project‘‘A methodology for the integrated design and develop-ment of nanocomposite products’’ CPDA055157 fundedby University of Padova, Marino Quaresimin warmlyacknowledge the financial support. Russell Varley wouldalso like to gratefully acknowledge the support of the Aus-tralian Academy of Science in carrying out this work.

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