ORIGINAL PAPER

Polysiloxane-based core-shell microspheres for tougheningof epoxy resins

Prasun Kumar Roy & Nahid Iqbal & Devendra Kumar &

Chitra Rajagopal

Received: 12 August 2013 /Accepted: 26 December 2013 /Published online: 5 January 2014# Springer Science+Business Media Dordrecht 2014

Abstract We present a simple procedure for preparation ofcore shell poly(dimethylsiloxane)–epoxy microspheres (CPR)by suspension polymerisation route and demonstrate its po-tential as effective toughener for thermosetting epoxy resin.The curing of siloxane macromonomer was performed in thepresence of platinum based hydrosilylation catalyst and theeffect of reaction parameters on the dimensions of thepolydimethylesiloxane (PDMS) based elastomeric micro-spheres was quantified, which could be varied from 90 to216 μ. CPR were prepared by coating the PDMS core withepoxy resin in an additional step. Composites containingvarying amounts of microspheres (3–10 % w/w) were pre-pared and the effect of their incorporation on quasi-static aswell as dynamic properties of epoxy resin was evaluated. Theglass transition temperature of the unmodified epoxy wasunaltered on blending with elastomeric microspheres, whichindicated its existence in a well separated phase. The presenceof an epoxy coating on the silicone core led to improveddispersion in the epoxymatrix, which was evident from higherimpact strength and fracture energies(GIC) as compared to itsuncoated analogues. The charpy impact strength and GIC

increased by 148 % and 70 % respectively on introductionof 5 % CPR. This was however accompanied with a reduction

in the tensile modulus and strength of the base epoxy. Excel-lent agreement was found between the experimentally mea-suredmodulae and the predictions made on the basis of HalpinTsai and Lewis-Neilson models. Post-mortem morphologicalstudies of the fracture surfaces revealed the presence of spher-ical cavities which substantiate the role of rubber cavitation asthe primary toughening mechanism in microsphere toughenedepoxy composites.

Keywords Core-shell . Toughening . Epoxy .Microsphere

Introduction

Blending of epoxy with elastomeric resins is routinelyemployed to improve the dynamic properties of the brittlethermoset [1]. In this context, conventional methods involveblending with liquid reactive rubbers [2,3], where the subse-quent curing of epoxy leads to the precipitation of the rubberas a separate phase, thereby forming toughened blends withimproved fracture properties. However, blends with liquidrubbers suffer from specific drawbacks including reducedelastic modulus, low glass transition temperatures and highwater absorption. To address these issues, blending with ther-moplastics [4,5] and nanofillers have been attempted [6–8],however these compositions are associated with processingdifficulties.

Interestingly, the properties of the above mentioned blendsare governed by the thermodynamically and kinetically con-trolled process of phase separation, a phenomenon whichbecomes increasingly difficult to control in fast curing epoxyresins. In view of the above, developing methodologies forpredicting and controlling the phase morphology has receivedenormous research interest lately [9,10]. In this regard, one ofthe most successful techniques which has evolved over theyears involve inclusion of preformed rubbers within the

Electronic supplementary material The online version of this article(doi:10.1007/s10965-013-0348-5) contains supplementary material,which is available to authorized users.

P. K. Roy (*) : C. RajagopalCentre for Fire, Explosive and Environment Safety, Timarpur,Delhi 110054, Indiae-mail: pk_roy2000@yahoo.com

P. K. Roye-mail: pkroy@cfees.drdo.in

N. Iqbal :D. KumarDepartment of Applied Chemistry and Polymer Technology, DelhiTechnological University, Delhi 110042, India

J Polym Res (2014) 21:348DOI 10.1007/s10965-013-0348-5

matrix [11–13] where the blend morphology becomes inde-pendent of the phase separation process, thereby resulting inpronounced improvements at comparatively lower loadings[14].

Particle dimension of the elastomeric filler is primarily themost important parameter which influences its tougheningability. Previous studies have revealed that the particles shouldbe large enough to allow their deformation energy to be higherthan their interfacial bonding to the epoxy matrix to permitcavitation [14–16], and particles with diameter ~100–200 μmare reportedly most efficient [11]. However, due to the incom-patibility between the preformed rubbers and matrix, properdispersion of the former is an issue, which can be resolved bycoating these with a compatible layer, forming core-shellrubbers (CSR). Surprisingly, majority of relevant literature inthis field is confined to CSR’s in which the elastomeric core iscomposed of organic rubbers, which are not heat resistant,thereby restricting their usage only to applications which areless demanding [17,18]. In view of the excellent thermalstability of inorganic polymers, substitution of the organiccore with siloxanes can render them suitable for more chal-lenging applications [19–22].

At present, CSRs for the purpose of epoxy toughening aresynthesised by a two step emulsion polymerisation technique,but very few papers describe the synthesis of such materials inthe open literature [18,23,24]. We hypothesise that elastomer-ic particles of these dimensions can also be obtained bysuspension polymerisation route, which would permit tailor-ing of the particle dimensions by varying operating parameterslike stirring speed and concentration of the feed solution.

The aim of this work was to toughen epoxy resins byintroducing preformed siloxane based elastomeric micro-spheres. Our approachwas to synthesise the core poly(dimeth-ylsiloxane) (PDMS) microspheres by a simple suspensionpolymerization technique followed by coating with a layerof epoxy, which led to formation of PDMS-epoxy core-shellmicrosphere (CPR) thereby aiding its apt dispersion within thethermoset. The mechanical properties and fracture energy ofthese CPR toughened epoxy polymers were determined andthe results were compared with well established analyticalmodels. Post-mortem analysis of the fracture surface morphol-ogy of the toughened epoxy composites was performed toidentify the underlying toughening mechanisms.

Experimental

Materials

Silicone resin (Elastosil M4644) and the platinum based hard-ener was obtained from Wacker, Germany. An amine-curedepoxy resin (Ciba Geigy, Araldite CY 230; epoxy equivalent200 eq g−1) and hardener (HY 951; amine content 32 eq kg−1)

was used as received. PVA (Mol. wt. 14000, CDH) andchloroform (CDH) were used without any further purification.Distilled water was used throughout the course of study.

Preparation of elastomeric microspheres

The PDMS core was prepared by suspension polymerizationprocess. A feed solution was prepared by diluting the vinylterminated siloxane macromonomer with chloroform (30–60 % w/v) followed by addition of requisite amounts ofplatinum based hardener. The curing reaction was performedin reaction vessel under inert atmosphere where the feed wasintroduced through a hypodermic syringe into an aqueousPVA solution (1.5 % w/v), which was maintained at 45 °Cunder continuous stirring. The polymerisation reaction wasallowed to continue for 8 h under varying stirring speeds(500–700 rpm), after which the reaction mixture was cooledand filtered. The extent of conversion was measured gravi-metrically as the ratio of mass of microspheres obtained to theamount of macromonomer used for its preparation.

The obtained microspheres were coated with epoxy in aseparate step. For this purpose, a mixture of PDMS micro-spheres (15 g) and epoxy resin (7 g) was ultrasonicated for15 min prior to addition of stoichiometric amounts of harden-er. The mixture was diluted with small amounts of chloroform(5 ml), and the slurry was slowly introduced into a reactorcontaining aqueous PVA solution being stirred under theconditions mentioned previously.

Preparation of epoxy composites

Toughened epoxy composites with varying amounts of coatedand uncoated microspheres (3–10% w/w) were prepared. Themicrospheres were first sieved through a 60–80 mesh toobtain a particle size range of 250 to 177 μmwhich was addedto the epoxy prepolymer. The suspension was degassed undervacuum and transferred to greased silicone moulds, where thecuring reaction was allowed to proceed for 24 h at 30 °C. Neatepoxy specimens were also prepared in a similar manner andthe details of all the compositions prepared are listed inTable 1. The samples have been designated as EP followedby its concentration and by the type ofmicrosphere used for itspreparation, i.e. ‘R’ denotes uncoated PDMSmicrosphere and‘CPR’ denotes epoxy coated core-shell microspheres withPDMS core.

Characterisation

The effect of epoxy coating on the microsphere dimensionswas determined by a particle size analyser (DIPA 2000,Donner). FTIR spectra of samples were recorded in the wave-length range 4,000–600 cm−1 using a Thermo Fisher FTIR(NICOLET 8700) analyser with an attenuated total reflectance

348, Page 2 of 9 J Polym Res (2014) 21:348

(ATR) crystal accessory. The thermal behaviour was investi-gated using Perkin Elmer Diamond STG-DTA under N2 at-mosphere in the temperature range 50–800 °C. A heating rateof 10 °C/min and sample mass of 5.0±0.5 mg was used foreach experiment. For determination of Swelling Index (SI),the microspheres were placed in different mediums for 72 h,after which they were removed, tapped dry and re-weighed. SIwas quantified as the ratio of mass of solvent imbibed to theinitial mass of microspheres used for the experiment.

The glass transition temperature (Tg) of the epoxy basedsamples, as determined from the peak value of damping tan δ,was measured by dynamic-mechanical analyser (DMA8000,Perkin Elmer) using test specimens, which were subjected to aheating rate of 3 °Cmin−1 from 20 °C to 200 °C at a frequencyof 1 Hz.

The quasi-static mechanical properties, were determined asper ASTM method D638 using a Universal Testing System(International equipments) at ambient temperature. The dog-bone shaped specimens used for tensile testing were 165 mmlong, 3 mm thick, and 13 mm wide along the centre of thecasting for epoxy resin. The samples were subjected to a crosshead speed of 50 mm min−1. The notched Charpy impactstrength of the specimens was determined as per ASTM D256 using an impact strength testing machine (InternationalEquipments, India). Notched flexural testing of the sampleswas performed under three point single edge notch bendingmode using the same instrument. For this purpose, specimenswith requisite dimensions (127 mm length×12.5 mm width×3.5 mm thickness and 3 mm notch) were prepared and thesamples were subjected to a deformation rate of 2 mm min−1

while maintaining a span length of 60 mm. The mode I criticalstress intensity factor (KIC) of the samples was determined asper the following equation [25]:

KIc ¼ 3� P � L� a1=2

2� B� w2Y

a

w

� �ð1Þ

Where, P, L and B refer to the load at break, span length andsample thickness respectively. The geometry factor, Y a

w

� �, is

calculated as per the formula below, where a is the notchlength and w is the sample width.

Ya

w

� �¼ 1:93−3:07� a

w

� �þ 14:53� a

w

� �2−25:11

� a

w

� �3þ 25:8� a

w

� �4

ð2Þ

The KIC was used to estimate the fracture energy (GIc),which was calculated using the following equation

GIC ¼ KIC2 1−ν2ð ÞE

ð3Þ

where E is the flexural modulus of the polymer, and ν is thePoisson’s ratio of epoxy (0.35) [26]. For each composition, atleast five identical specimens were tested and the averageresults along with the standard deviation have been reported.

The surface morphology of samples was studied using aScanning Electron Microscope (Zeiss EVO MA15) under anacceleration voltage of 20 kV. Samples were mounted onaluminium stubs and sputter-coated with gold and palladium(10 nm) using a sputter coater (Quorum-SC7620) operating at10–12 mA for 120 s. The core shell structure of the CPR wasalso confirmed using energy dispersion analyser (EDS). Forthis purpose, a representative CPR was cut with a sharp razorblade and carefully mounted on a stub, followed by elementalcomposition determination in both core as well as the shellregion.

Results and discussion

In this study, a simple procedure for preparation of core shellsiloxane microspheres has been developed which have beensubsequently employed as toughening agents for epoxyresins.

Suspension curing of siloxane

In the present study, vinyl terminated methyl hydrosiloxane-dimethylsiloxane copolymer was cured at 45 °C in the pres-ence of a hydrosilylation catalyst [27] as per Scheme 1.

The effect of increasing the feed macromonomer solutionconcentration on the particle size distribution of the resultantmicrospheres is presented in Fig. 1. As is evident, the distri-bution shifts towards larger sized microspheres as the concen-tration of siloxane in the feed increases, which can be attrib-uted to the increased polymerisable content in the hydropho-bic dispersed droplets. Morphological investigations revealthe smooth texture of the siloxanemicrospheres (Fig. 1, Inset).

Table 1 Sample designation and compositional details

Sample designation Amount

Epoxy resin (g) Hardener (g) Microsphere (g)

EP 100 13 –

EP3x 100 13 3.3

EP5x 100 13 5.6

EP7x 100 13 7.9

EP10x 100 13 11.3

x denotes the type of filler (R: PDMS, CPR: coreshell PDMS-epoxy)

J Polym Res (2014) 21:348 Page 3 of 9, 348

In all cases, complete conversion (>98 %) could be achieved,as evidenced by gravimetric analysis.

The effect of varying the stirring speed on the particle sizedistribution of the microspheres is presented in Fig. 2, wherethe feed concentration was maintained at 60 % w/v. It can beseen that with increase in the stirring speed, the particle size

distribution shifts towards lower size, which could be attrib-uted to the shearing of the large oily droplets into smallermicrospheres, under the experimental conditions employed.

Effect of epoxy coating on microsphere dimensionsand morphology

The siloxane microspheres were coated with a layer of cyclo-aliphatic epoxy in a separate step in order to improve itscompatibility with the thermosetting epoxy resin. As a repre-sentative scenario, the size distribution and SEM image ofCPR microspheres prepared using 60 % w/v siloxane in thefeed, followed by epoxy coating is presented in Fig. 3. Asexpected, coating on the siloxane core led to an increase in theparticle dimensions from 198±56 μ to 242±44 μ, whichcorresponds to a coating thickness of ~20 μ. Interestingly, incomparison to that of neat PDMS, the surface of the CPR wasfound to be rather rough.

The elemental composition of the coated and uncoatedPDMS microspheres was determined by EDX, and the aver-age value over a specific square area within a microsphere arepresented in Fig. 4. As can be seen, the amount of siliconpresent in the shell is practically negligible, which furtherconfirms the coating of epoxy on the PDMS core.

Scheme 1 Platinum catalysed hydrosilylation of silicone

Fig. 1 Effect of feedconcentration on the particle sizedistribution and surfacemorphology of PDMSmicrospheres (stirring speed600 rpm). a30% b40 % c50% d60 %

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The additional coating of epoxy on the core siloxane re-sulted in a change in the FTIR spectra, as presented in Fig. 5.Characteristic absorption peaks at 802 and 1,258 cm−1 due tothe (CH3)2SiO group vibration and broad absorption at~1,000–1,130 cm−1, due to the Si-O-Si stretching couldbe observed [28]. Epoxy coated microspheres exhibitedadditional absorption bands at 1,100 cm−1 due to the asym-metrical stretching vibration of polyether [29]. Appearanceof absorption in the region 1,250–1,360 cm−1 due to νs (C-N) and 1,580–1,650 cm−1 due to νs (N-H) further confirmscoating of epoxy on the PDMS surface.

PDMS exhibits excellent thermal stability and the same isevident from the TG-DTG traces presented in Fig. 6. Theelastomer exhibits a single step decomposition initiating at~400 °C leaving behind a residual mass of ~65 %, which isconsistent with the results reported previously [30,31]. On theother hand, coating the PDMS microspheres with epoxy re-sulted in a change in the thermal degradation behaviour and atwo-step degradation was observed in CPR. The first step, dueto pyrolytic decomposition of epoxy occurs at much lowertemperatures (Tonset ~250 °C) followed by a subsequent py-rolytic step resulting from the degradation of the core PDMSsilicone.

The effect of coating on the chemical resistance was mea-sured by gravimetric studies which reveal that PDMS

microspheres were chemically inert, with the mass loss beingpractically negligible when placed in contact with commonsolvents like methanol, DMF, THF, chloroform, water andtoluene. However, in the coated microspheres, the surfaceepoxy layer turned brittle on extended exposure to methanol,chloroform and toluene where a mass loss of 15±4 % wasobserved [32].

Epoxy-PDMS composites

Composites containing either PDMS or CPR microspherewere prepared by dispersing them within the epoxy resin byultrasonication followed by curing with the amine hardener inrequisite ratio.

Representative plots, exhibiting the variation of storagemodulus and tan δ as a function of temperature for neat epoxyand its composites with siloxane and CPR microspheres (5 %w/w) is presented in Fig. 7. The glass transition temperatures(Tg) and the storage modulus (G′) of the pristine cured epoxyresin was 66 °C and 2.73 GPa at 20 °C respectively. Interest-ingly, the introduction of the elastomeric microspheres wasfound to have no significant reduction on Tg values of the basepolymer, and the damping tan δ traces appear to overlap. Thisis in contrast with the results reported on epoxy blends withorganic liquid rubbers e.g. CTBN, where significant reductionin Tg has been reported [33]. Our studies clearly highlights thebenefit of employing preformed elastomeric microspheres asimpact modifiers as the second phase remains phase-separated, in view of which their plasticizing action is practi-cally negligible. As expected, the introduction of the softpolysiloxane elastomers led to a decrease in the rigidity ofthe epoxy polymers by an approximately constant amountover the whole temperature range below the Tg of the epoxyas shown in Fig. 7. The TG trace of epoxy and representativecomposites containing PDMS and core shell PDMS-epoxy ispresented in the supplementary section (Figure S1). It wasobserved that the introduction of elastomeric fillers do notaffect the thermal properties of the base resin and all thecompositions can be used in service till 250 °C, irrespectiveof the type of filler employed.

Fig. 2 Effect of stirring speed onthe particle size distribution andsurface morphology of PDMSmicrospheres. a 500 rpm b600 rpm c 700 rpm

Fig. 3 Particle size distribution of epoxy coated PDMS microspheres.SEM image is shown in the inset

J Polym Res (2014) 21:348 Page 5 of 9, 348

Figure 8 presents the variation in mechanical properties ofthe epoxy composites containing both neat PDMS as well ascore shell PDMS-epoxy microspheres. As expected, the in-troduction of elastomeric micropsheres led to a decrease in thetensile strength and increase in strain, the extent of which wasfound to be proportional to the amount of loading. It hashowever been reported that in comparison to other acrylatebased elastomers, silicones result in less severe loss in me-chanical properties, and this is one of the primay reasons foradvocating their potential in high end applications [34].

The values of the experimentally obtained modulae werecompared with two existing theoretical models, namelyHalpin-Tsai and the Lewis-Nielsen models, which predictthe modulus of a composite as a function of the modulus ofneat epoxy, Em, and that of silicone rubber filler, Ef [21,35].

According to the Halpin-Tsai model [34], the modulus ofthe epoxy composite is predicted as per the following equation

E ¼ 1þ ζηV f

1−ηV fEm ð4Þ

where ζ is the shape factor, Vf is the volume fraction of themicrosphere, and

η ¼E f

Em−1

� �

E f

Emþ ζ

� � ð5Þ

The shape factor in the Halpin-Tsai model is a function ofthe aspect ratio of the filler particles, which is usually calcu-lated as ζ=2w/t, where w and t are the length and thickness ofthe reinforcement respectively [36]. In view of the sphericalnature of the micropsheres used in the present work, a shapefactor of ζ=2 was used for the prediction of the modulus. Thevalue of modulae (Em) of polysiloxane elastomer has beenassumed to be 2.5 MPa, based on the previous studies [37].

The basic Lewis-Nielsen model [38], takes into account theeffect of the adhesion between the polymer and the fillers byemploying the following equation to predict the modulus ofthe composite:

E ¼ 1þ kE−1ð ÞβV f

1−βμV f

� � Em ð6Þ

Fig. 4 Cut cross section of singlecoated microspheres a SEMimage, and EDX analysis (b) shell(c) core

Fig. 5 FTIR spectra of a poly(dimethylsilicone), b neat epoxy c PDMSepoxy core-shell microspheres

Fig. 6 TG-DTG traces of poly(dimethylsilicone) and PDMS-epoxycore-shell microspheres

348, Page 6 of 9 J Polym Res (2014) 21:348

Where, kE is the generalised Einstein coefficient and β andμ are constants. The constant β depends on the ratio of Ef andEm and is given by:

β ¼E f

Em−1

� �

E f

Emþ kE−1ð Þ

� � ð7Þ

The constant μ being dependant on the maximum volumefraction of the filler (Vmax) by:

μ ¼ 1þ 1−V f

� �Vmaxð Þ VmaxV f þ 1−Vmaxð Þ 1−V f

� �� ð8Þ

The values of Vmax for a range of particle shapes and typesof packing have been tabulated by Nielsen and Landel [39].

The morphological studies indicated that all the microsphereswere randomly dispersed, therefore a figure of 0.632 for Vmax

was employed for the present studies, which is representativeof random close-packed and non-agglomerated spheres [39].The value of the generalised Einstein coefficient, kE, report-edly varies with the Poisson’s ratio of the polymeric matrixand the degree of the adhesion of the polymer to the particles.Since the present investigation deals with epoxy resin (ν=0.35), where no slippage at the interface between the poly-meric matrix and the particles was evidenced from the mor-phological investigations, a value of kE=2.167 was used[26,39].

The experimentally observed values were compared withthe predictions based on the above mentioned models, theresults of which are presented in Fig. 9. Substantial agreementcan be seen, with the modulus of composites containing CPRmicrospheres was relatively higher. However, for both thecases, the experimental data was found to generally lie be-tween the Halpin-Tsai and Lewis-Nielsen predictions, wherethe Halpin-Tsai model yields the upper bound and the Lewis-Nielsen model gives the lower bound figure.

Interestingly, the introduction of elastomeric microspheresled to a remarkable improvement in the impact strength ofepoxy, as is evident from data presented in Fig. 10. The charpyimpact strength increased from 22.5 J m−1 (unfilled resin) to55.8 J m−1 on addition of 5 % w/w CPR (~148 % increase).Similar increase in the toughness at such low loadings hasbeen reported earlier [36]. In comparison, the improvement in

Fig. 7 Storage modulus and damping tan δ traces for representativeepoxy-composites

Fig. 8 Effect of microsphere loading on the tensile strength and strain ofepoxy composites

Fig. 9 Comparison of experimental modulus of toughened compositionswith Halpin-Tsai and Lewis-Nielson models

Fig. 10 Effect of elastomeric microspheres on impact strength of epoxy

J Polym Res (2014) 21:348 Page 7 of 9, 348

impact strength was less pronounced when uncoated PDMSwas used as the filler.

Flexural three point bending tests were also performed onspecimens, and as expected, the composites exhibited in-creased flexural strain in comparison to the pristine epoxy,which can be attributed to the high extensibility of the elasto-meric domain present in the composites. The CPR containingsamples could be flexed to a higher extent, which in turnreflected in larger values of critical stress intensity factor(KIC). A comparison of KIC and fracture energy (GIC) as afunction of composition type is presented in Fig. 11.

It can be seen that the critical stress intensity factor (KIC) ofthe epoxy polymer increased with the addition of the elasto-meric microspheres, from 2.17 MPa m1/2 for the unmodifiedepoxy polymer to 2.81 MPa m1/2 for composites containing5 % w/w of the CPR microspheres, which corresponds to anincrease of ~30 %. The mean fracture energy also increasedsubstantially on addition of elastomeric microspheres andEP5CPR was found to exhibit significant improvement(~70 %) in fracture energy as compared to the neat resin.

Toughening mechanisms

The morphology of the fractured surface was studied by SEMimaging and the secondary electron images are presented inFig. 12. As can be seen, the fractured surface of epoxy issmooth and featureless and shows no sign of plastic deforma-tion, which is typical of a brittle thermosetting polymer [34].In comparison, the surface of the fractured composite is ratherrough and shows the presence of holes, formed as a result ofrubber cavitation [6,37,40].

The primary criteria which needs to be met by the elasto-meric microspheres in order to exhibit their full potential asimpact modifiers is their random dispersion in the matrix as awell separated phase. It appears that the surface layer of epoxyin the CPR strongly interacts with the epoxy during the curingprocess, which leads to its better dispersion as compared to itsuncoated analogue, and the same is evident from the SEMimages. However, with increased loadings, agglomeration ofthese microspheres was observed which was responsible forthe decrease in the mechanical properties.

The foremost contributing micromechanism behind theimproved toughenability of silicone-epoxy composites ap-pears to be “rubber cavitation”. It has been reported thatduring the tensile loading, the plastic region surrounding themicrospheres dilates, and the siliconemicrospheres, in view oftheir large extensibility (ν=0.499), tend to cavitate from with-in [41]. The role of the particle cavitation, therefore, is torelieve the plane strain constraint from the surrounding matrixand allow plastic deformation within the matrix [42]. This

Fig. 11 Increase in the critical stress intensity factor, KIC, and fractureenergies, GIC, of epoxy due to introduction of elastomeric microspheres

Fig. 12 SEM image of thefractured surface of epoxycomposite a EP5CSR b neatepoxy c EP5R

348, Page 8 of 9 J Polym Res (2014) 21:348

mechanism can be used explaining the presence of voids withdimensions much larger than that of the microspheres thatwere introduced initially during blending.

Another mechanism which can be used to explain thetoughening in these toughened composites is that of particleyielding induced shear banding [43]. The siloxane basedmicrosphere filler tends to yield, thereby producing significantstress concentration, which in turn initiates shear banding inthe matrix. The rough texture of the fracture surface canmainly be attributed to crack path deflection andmicrocracking [44]. As a result of these phenomena, thesurface area of the crack increases and the mode I characterof the crack opening is reduced, thereby resulting in increasedenergy for crack propagation.

Conclusion

Core-shell PDMS epoxy microspheres (CPR) were preparedfor use as impact modifier for thermosetting epoxy resin. Thecore elastomeric beads of PDMS microspheres were preparedby suspension polymerization process, which involved theaddition reaction of silicone macromonomer in the presenceof platinum based hydrosilylation catalyst. The effect of oper-ating parameters like stirring speed and feed concentration onthe particle size distribution of the resultant microspheres wasquantified. The microspheres were subsequently coated withan epoxy layer in a separate step to prepare core shell rubbers,to improve their compatibility with the epoxy thermoset in thesubsequent step of composite preparation. Both uncoated aswell as epoxy coated PDMS microspheres were dispersed inepoxy matrix to prepare toughened composites (3–10 % w/w).The effect of addition of these fillers on mechanical properties(both quasi static and dynamic) was studied. The presence of asuperficial epoxy coating on the elastomeric core seems to playan important role in dispersion of the microspheres, as evidentfrom the improved mechanical properties in CPR loaded com-positions. Impact strength and fracture energy increased by~148 % and 70 % respectively on addition of 5 % w/w CPR.The reduction in the experimentally observed tensile moduluswas compared with two analytical models which revealedexcellent agreement. Spherical cavities were observed in thefracture surface of composites which substantiate the role ofrubber cavitation as the primary toughening mechanism inmicrosphere toughened epoxy composites.

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