Materials and Design 32 (2011) 1477–1484

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Materials and Design

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Structural, thermal, mechanical and dynamic mechanical propertiesof cenosphere filled polypropylene composites

Arijit Das, Bhabani K. Satapathy ⇑Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 May 2010Accepted 25 August 2010Available online 31 August 2010

Keywords:Polymer matrix compositesMechanicalX-ray analysis

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.08.041

⇑ Corresponding author.E-mail address: bhabaniks@gmail.com (B.K. Satapa

Polypropylene (PP)/cenosphere based composites were fabricated and characterized for their structural/morphological and mechanical properties such as tensile, flexural, impact and dynamic mechanical prop-erties such as storage and loss moduli as a function of temperature. The morphological attributes werecharacterized by scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD) whilethe thermal characterizations were done by conducting differential scanning calorimetry (DSC) andthermo-gravimetric analysis (TGA). The morphological investigations have revealed a uniformly distrib-uted/dispersed state of the cenosphere in the bulk PP matrix of the composites. The WAXD/DSC studieshave revealed a decrease in crystallinity of the composites with increase in cenosphere content. Dynamicmechanical analysis (DMA) revealed an enhancement in the energy dissipation ability of the compositewith 10 wt.% of cenosphere and an increase in the storage modulus up to �30% in the composites relativeto the soft PP-phase. The tensile modulus increased up to �43% accompanied by a nominal decrease intensile strength while the strain at break remained largely unaffected. The impact strength of the com-posites marginally reduced compared to PP indicating a low-cost material-concept with maximized stiff-ness–toughness combination. The theoretical modeling of the tensile data revealed appreciable extent ofphase-adhesion despite the cenospheres lack any surface modification indicating better extent ofmechanical interlocking and surface-compatibility between polymer and filler. Fractured surface mor-phology indicated that the failure mode of the composites undergoes a switch-over from matrix-con-trolled shear deformation to filler-controlled quasi-brittle modes above a cenosphere loading of10 wt.% in the composites.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The techno-commercial relevance of cenospheres as hollow alu-mino-silicate micro-spheres lies in their potential abilities to beused in innovating functional engineering materials including inor-ganic or organic binders based filled composites for various struc-tural and industrial applications. The varied set of properties incombination with variation in the diameter and the thickness ofthe wall of the cenospheres make them a unique class of multi-functional filler that not only facilitates fine dispersion and homo-geneity but also adds to the enhancement of inertness andchemical stability. Developing light and heat insulating polymercomposite material filled with cenospheres may also be realizedwith low density and spherical alumino-silicate structures whichmay offer other additional advantages like enhanced mechanicalproperties such as elastic modulus, toughness, high durabilityand increased isotropic-compression. Further cenospheres contain

ll rights reserved.

thy).

chemically active groups on the surface which makes it easy fortheir surface modification that may eventually promote the kineticcharacteristics of surface phenomena in both, static and dynamicmodes [1,2].

Flyash cenospheres are increasingly being used in plastic andrubber compounding applications since they contribute to weightreduction, shrinkage reduction, better surface finish, warpagereduction and resistance to water absorption [3]. Deepthi et al.have reported the mechanical and thermal properties of surfacetreated cenospheres with silane coupling agent as a filler inHDPE-g-dibutyl maleate as a compatibilizer. It was observed thatthe addition of silan treated cenospheres to HDPE with HDPE-g-dibutyl maleate as a compatibilizer substantially improves themechanical thermal properties [4]. Willis and Masters [5] haveinvestigated on the effect of filler loading on the flexural propertiesof cenospheres/polyester resin composites that revealed theincrease in the flexural modulus/stiffness with a decrease in theflexural strength. Fibre reinforcement in combination with ceno-sphere filling into the polyester resin may result in the productionof high strength-high modulus materials with filler contents up to75% without any loss in flexural strength. The tensile mechanical

Table 2aExtrusion temperature profile.

Zones Z � 1 Z � 2 Z � 3 Z � 4

Temperature (�C) 190 210 230 240

Table 2bTemperature profile set in injection molding machine.

Zones Feed Z � 1 Z � 2 Z � 3 Z � 4

Temperature (�C) 35 175 185 200 220

Table 2cProcessing parameters used in injection molding machine.

Process parameters Values

Injection pressure 50 barInjection speed 50 barHolding time 14 sBack pressure 5 barCooling time 18 s

1478 A. Das, B.K. Satapathy / Materials and Design 32 (2011) 1477–1484

properties, fracture toughness and the notched impact strength offlyash filled PP was investigated by Wong and Truss [6]. The addi-tion of 20 wt.% of flyash to PP and to an EPR leads to a small in-crease in modulus which however increased further by theaddition of 5 wt.% of PP-g-MA as a coupling agent. The fracturetoughness of the copolymer based composites was found to de-crease with flyash addition.

Lately flyash and flyash based cenospheres have already beentried in friction composites and their influence on the performanceproperties have been critically investigated by several researchers[7–9]. It was observed that addition of flyash/flyash cenospheresincreases the thermal conductivity and the friction-fade resistanceof the friction materials in addition to cost reduction of the prod-ucts. The use of PVA fibres with flyash via hygro-thermal process-ing technique has also been reported to facilitate brittle to toughtransition of the flaysh as filler [10]. Ahmed and coworkers havestudied the flexural responses of hybrid steel-polyethylene fibrereinforced cement composites containing high volume flyashwhere strain–hardening and multiple cracking behaviour aspectshave been reported to be enhanced by replacing cement withabout 50% of flyash [11]. The damping properties of flyash filledepoxy composites have been studied by Gu and co-workers[12,13] where the slow attenuation of loss-tangent with increasingfrequency has been reported indicating better damping character-istics. The non-linear creep performance of polyoxymethylene(POM) reinforced with micro-spheres derived from flyash has beenreported by Koszkul and Kwiatkowski [14]. It was reported that thenon-linear coefficient of retardation intensity decreased by 60%upon the addition of flyash into the polymer. The fracture behav-iour of flyash filled FRP composites have been extensively investi-gated by Srivastava and co-workers [15]. The fracture toughnessand fracture surface energy of epoxy based composites reinforcedwith glass and carbon fibres in combination with flyash has beenintensively investigated. It was observed that flyash plays a crucialrole in arresting the crack path and thereby improving the fractureproperties of the composites.

In view of the above literature reports, the present paper makesan attempt to understand the mechanical response and the damp-ing properties of cenosphere filled PP composites in relation to thestate of filler distribution in polymer matrix for exploring the po-tential of flyash based rigid hollow alumino-silicate spheres i.e.,cenospheres to be used in polymer composites as a sustainablelow-cost material for potential structural applications.

2. Experimental

2.1. Preparation of PP/cenosphere composites

The details of the materials selected and the processing condi-tions are given in Tables 1 and 2. The mixing of the two ingredientsfor the fabrication of the composites was carried out in a counter-rotating type (S.K. Dey Make-2008) twin-screw extruder. The con-tinuous strands thus obtained were later chopped in a granulatorand subsequently kept for drying in the oven prior to injectionmolding of the same to obtain test specimens conforming to vari-ous ASTM standards. The details of the composite designations aregiven in Table 3.

Table 1Data sheet of raw materials and their characteristics.

Raw materials Grade Supplier Characteristics

Polypropylenehomopolymer

REPOLH110MA

Reliance IndustriesLimited

Density (q) = 0.91

Cenosphere CS-300 Micro Minechem IndiaPvt. Ltd.

Particle density =water; Tm = 1300

2.2. Structural characterization/2D wide angle X-ray diffraction(WAXD)

Wide angle X-ray diffraction (WAXD) was carried out on the ex-truded granules (quasi-isotropic sample) to characterize crystallin-ity and orientation in the samples, apart from investigating thechanges in the peaks corresponding to different crystal planes asa function of cenosphere content. The measurements were doneon an X-pert PRO (Netherland) model-PW 3040-60 X-ray diffrac-tometer of PAN analytical using Ni filtered Cu Ka radiation of1.54 A�. The crystallinity was evaluated by applying the peak-areaintegration method in the range of 2h = 10–35� (as typical for PP)by applying an amorphous scattering curve that was realized byexperimental and theoretical experiences.

2.3. Thermal characterization

2.3.1. Differential scanning calorimetry (DSC)DSC measurements, to obtain information about the influence

of the alumino-silicate micro-spheres, i.e. cenospheres on the crys-tallisation behaviour of PP, were conducted with a Perkins Elmer(Model Pyris 6) instrument at a scan rate of ±10 K/min in a temper-ature range of�30–200 �C in N2 atmosphere. An initial heating wasdone to remove the residual thermal stress from samples. Appar-ent enthalpies of fusion were calculated from the area under theexothermic peaks. The percent crystallinity is determined follow-ing the equation Xc = (DHm/DHmo � w) � 100, where, DHm is theheat of fusion of PP in the PP-cenosphere composites and DHmo

is the heat of fusion of 100% crystalline PP taken as 138 J/g [6]and w is the weight fraction of PP.

2.3.2. Determination of melt flow index of the compositesThe melt flow index (MFI) values for the different compositions

were determined from a Dynisco MFI apparatus following ASTM D

g/ml; Tm (�C) = 220 �C; MFI (g/10 min) = 11 @ 230 �C, 2.16 kg

0.45–0.80 g/ml; Particle size = 0–100 micron; Light grey powder; Non-soluble in–1500 �C

Table 3Details of the composition and designation of the PP-cenosphere composites.

SerialNo.

Compositiondesignation

Polypropylene(wt.%)

Cenosphere content(wt.%)

1 PPC-0 100 02 PPC-5 95 53 PPC-10 90 104 PPC-15 85 155 PPC-20 80 20

10 15 20 25 30 35

0

3000

6000

2 theta (deg)

........ Cenosphere PP PPC-5 PPC-10 PPC-20 PPC-15

a

b

d e

f

# *

$

^

c

Dif

frac

tion

Int

ensi

ty (

arbi

trar

y un

it)

0 5 10 15 2040

45

50

55

60

65

70

75

80

DSC Curve XRD Curve

Cenosphere content (wt %)

Cry

stal

linit

y (%

) ob

tain

ed f

rom

DSC

48

52

56

60

64

68

Cry

stal

linit

y (%

) ob

tain

ed f

rom

XR

D

(a)

(b)

Fig. 1. X-ray diffractograms (intensity versus 2h) of the composites (b) variation incrystallinity of the composites with cenosphere content (based on XRD and DSCdata).

A. Das, B.K. Satapathy / Materials and Design 32 (2011) 1477–1484 1479

1238. The MFI values normally characterize the flow behaviour ofthe polymer melt and also qualitatively highlight the state of filler–filler interaction.

2.3.3. Dynamical mechanical analysis (DMA)Dynamic mechanical analysis (DMA) measurements have been

carried out on the composite test specimens with dimensions of20 � 8 � 1 mm3 in tensile mode on an Q800 (TA Instruments,USA) to characterize the storage modulus, loss modulus and tand for qualitatively investigating the reinforcement effects andquantitatively ascertaining shift (if any) in glass transition temper-ature of the nanocomposites in the temperature range of �30–165 �C at a frequency of 1.0 rad/s and heating rate of 5 K/min.

2.3.4. Mechanical property investigationsTensile tests were performed according to ASTM D 638 norms

on a Zwick Z010 testing machine at room temperature with a testspeed of 50 mm/min. Three injection molded tensile bars weretested for each composition. Flexural tests were conducted accord-ing to the ASTM D 790 method-I in a three-point bending modeand notched Izod impact tests were conducted on specimen barsconforming to ASTM D 256 norms.

2.3.5. Fractured surface morphologyThe cryo-fractured surface morphologies of virgin PP, ceno-

sphere filled PP composites have been investigated using scanningelectron microscopy (SEM) on a Zeiss EVO-50 electron microscopeto analyze the associated failure-mechanisms and structural integ-rity of such multiphase micro-composite materials. The surfaces ofthe specimens were gold sputter coated prior to examination tomake the surfaces conductive.

3. Results and discussion

3.1. Structural characterization by 2D Wide angle X-ray diffraction(WAXD)

The structural interpretations of the cenosphere filled PP com-posites have been made from WAXD on extruded granules andSEM studies on cryo-fractured specimen surfaces. The XRD plotsin terms of I versus 2h measured in the range of �10–35� is shownin Fig. 1a. The pure PP is characterized by the intense and promi-nent peaks assigned as a, b, c, d and e with the peaks a and b accom-panied by low intensity shoulder peaks. Interestingly, cenosphereis characterized by the peak flanked by the peaks a and b corre-sponding to pure PP. Additionally two more cenosphere specificpeaks are seen surrounding the peak d. The most intense peak ofcenosphere is observed to be overlapping peak e of PP. Thus theincorporation of cenosphere may have caused possible structuralreorganization of the crystalline domains of PP, which is evidentfrom the appearance of distinct splitting of the intensity peaks band d accompanied by a simultaneous decrease in intensity withthe increase in cenosphere content. This evidently indicates thatthe crystalline phase/orientation (of the polymer chains) is appre-ciably affected due to the incorporation of cenospheres though it

remained inappreciably influenced by the content of cenosphere.Such observations apparently imply the tendency of the crystallinephases to be more disorganized causing a reduction in the intensitylevels. The crystallinity of the various composites and that of thevirgin PP was determined by area integration technique of thecrystalline and amorphous peaks and is shown in Fig. 1b. It wasfound that the percentage crystallinity systematically decreasedwith the increase in the cenosphere content indicating more offlexible amorphous content. Theoretically such an increase in theamorphous content may promote the ease in molecular-chain mo-tion due to an obvious increase in free volume space. The crystal-linity decreased by �15% in case of PPC-5 and PPC-10 whereason further increasing the cenosphere content up to 20 wt.% (PPC-20) the crystallinity suffered a reduction by �26% as compared topure PP matrix. Such a substantial reduction in the crystallinitymay be attributed to the fact that the cenospheres may have beeninefficient in acting as nucleation sites since the surface energydynamics may have been unfavorable for the PP-chains to alignin a thermodynamically favorable manner to cause stable crystal-lization process.

3.2. Thermal charaterization

The thermal analysis of the composites were carried out bydifferential scanning calorimetry (DSC) to characterize the

-25 0 25 50 75 100 125 150 1750

500

1000

1500

2000

2500

3000

3500

4000

4500

PPC 20

PPC 15

PPC 10

PPC 5

PP

PP PPC 5 PPC 10 PPC 15 PPC 20

Stor

age

Mod

ulus

(M

Pa)

Temperature (°C)

160

180

200 PP PPC 5 PPC 10a)

(a)

(b)

1480 A. Das, B.K. Satapathy / Materials and Design 32 (2011) 1477–1484

crystallization behaviour, thermo-gravimetric analysis (TGA) tostudy the degradation behaviour and melt flow index (MFI) mea-surements to qualitatively identify the influence of filler–fillerand filler–polymer interactions on flow behaviour of the compos-ites. The relevant DSC, TGA and MFI data are given in Table 4.The DSC data have clearly revealed that the onset melting and crys-tallization temperature remained nearly unaffected whereas theoverall crystallinity of the composites decreased due to cenosphereincorporation, an aspect which is also in agreement with theWAXD interpretations. The reduction in crystallinity is attributedto the fact that cenospheres inhibit the close packing of PP chains.Further, the inappreciable changes in the melting and the crystal-lization temperatures also indicate that the cenospheres do not actas nucleating agents in the investigated systems. The TGA analysishas revealed that there is no change in the onset temperature fordegradation. The shift in the onset temperature of the compositesrelative to the PP matrix was very marginal, i.e. by �5–8 �C. Thisindicates the absence of any surface/physical interactions/betweenthe cenospheres and the PP chains.

The variation of melt flow indices (MFI) measured at 230 �C andunder a load of 2.16 kg is given in Table 4. The melt flow indices ofthe cenosphere filled PP composites have been found to increaseby �40% with the incorporation of cenosphere till a concentrationof 15 wt.% (PPC-15). However, on further increasing the cenosphereloading to 20 wt.% (PPC-20) the MFI drops down a to value lowerthan that of pure PP, indicating filler induced effects caused by fil-ler-agglomeration. At 190 �C, i.e. in the molten stage, the physicaland mechanical restrictions due to entanglements are overcomecausing easier flow of the composite melt whereby the molten ma-trix phase acts as the carrier in which the perfectly spherical ceno-spheres with inherent inherently fluid-like flow behaviourpromote the MFI without any resistance. Such non-linear behaviourof MFI with increase in the filler content in polymer composites hasalso been reported incase of glass bead filled polypropylene and insilica filled polymers [16,17]. Further, at elevated temperaturesthe surface changes/adhesion increasingly becomes poorer andthereby helps overcoming the partial immobilization due to localconstraints of kinematics origin. Such a phenomenon causes therelaxation of the spatial-mobility-frustration.

3.3. Dynamic mechanical analysis

The results from solid state dynamic mechanical studies interms of variation of storage modulus (E0) and loss-modulus (E00)with temperature are shown in Fig. 2a and b. The variation of E00

with temperature as shown in Fig. 2b shows that the magnitudeof E00 increases dramatically with the incorporation of 10 wt.%(PPC-10) of cenospheres leading to a maximum, which however,gets subsequently decreased with further increase in cenospherecontent. In the entire composition range E” remained in betweenthat of the composite with 10 wt.% (PPC-10) cenosphere and purePP. This qualitatively indicates the enhancement in the energy dis-

Table 4Thermal characterization of PP-cenosphere composites.

SerialNo.

Compositiondesignation

DSC TGA MFI (g/10 min)@230 �C/2.16 kg

Meltingtemperature(�C)

Crystallizationtemperature(�C)

IDTa

(�C)FDTa

(�C)

1 PPC-0 163 121 448 480 112 PPC-5 163 121 451 486 143 PPC-10 163 121 453 487 134 PPC-15 163 122 447 488 145 PPC-20 163 122 455 491 9

a IDT-Initial degradation temperature; FDT- Final degradation temperature.

sipation ability of the composite with 10 wt.% (PPC-10) of ceno-sphere. Thus, it theoretically implicates a possible transition interms of their response to isotropic stress situation. The combina-tion of high modulus (�13–17 GPa) and microsphere dimensions(�100 lm) of cenospheres makes them potentially effective asreinforcement for polymers. The storage modulus (E0) versus tem-perature as plotted in Fig. 2a shows an enhancement of the modu-lus (stiffness) with the increase in the amount of cenosphere, anobservation which is well in accordance with theoretical expecta-tion that is typical for rigid spherical particle filled composites. Theextent of increase of storage modulus is relatively high withincreasing cenosphere content at lower temperatures (sub-zerotemperatures, i.e. in the range of �25–0 �C), whereas such an in-crease is not so pronounced at higher temperatures. For example,at room temperature the increase of E0 of the composite with20 wt.% cenosphere is �26% compared to pure PP whereas an in-crease of �31% in the E0 compared to pure i-PP has been observedat �10 �C.

3.4. Mechanical properties

The tensile stress (ry and ru) and Young’s modulus (E) as a func-tion of cenosphere content are shown in the inserts of Fig. 3. It isobserved that the elastic modulus (E) increased marginally tillthe composition PPC-15 whereas and beyond the same composi-tion E increased by �43% in the composite PPC-20 in contrast toPP. In contrast the ry suffered a decrease by �18% in the compos-ites with a cenosphere loading up to 10 wt.% (PPC-10) and on fur-ther increasing the filler content to 15 wt.% (PPC-15) or above (i.e.in PPC-15 and PPC-20) ry decreased further, i.e. by �30% whencompared to the matrix PP. This fundamentally indicates that the

-25 0 25 50 75 100 125 150 1750

20

40

60

80

100

120

140

PPC 20

PPC 15

PPC 10

PPC 5

pp

PPC 15 PPC 20

Los

s M

odul

us (

MP

Temperature (°C)

Fig. 2. Dynamic mechanical analysis plots of the composites (a) storage modulus(E0) and (b) loss modulus (E00) versus temperature.

0 5 10 15 201000

1100

1200

1300

1400

1500

1600

1700

Flexural modulus Flexural strength

Cenosphere content (wt %)

Fle

xura

l mod

ulus

(M

Pa)

30

35

40

45

Fle

xura

l str

engt

h (M

Pa)

Fig. 4. Flexural strength and flexural modulus of the composites as a function ofcenosphere content.

A. Das, B.K. Satapathy / Materials and Design 32 (2011) 1477–1484 1481

composites undergo mechanistically different modes of damage/fracture at the microscopic level under uni-axial tensile loadingconditions, i.e. a mechanism of deformation that is compositiondependent. On the other hand ru remained insubstantially affectedup to 10 wt.% (PPC-10) of cenosphere content indicating that ru iscontrolled by the matrix (PP) phase whereas when the cenospherecontent is increased to P15 wt.% (above PPC-15) a decrease in ru

by �16% is observed, theoretically implying a possible role of theinterface due to cenosphere–polymer adhesion or inter-particledistance related effects. The increase in modulus (E) with increasein the cenosphere content accompanied with a decrease instrength may be explained by the theory of mechanical fractureof composites. The modulus is related to the elastic regime of thestress–strain response of the composites prior to failure/fracturewhere the effects due to the rigid filler–matrix interface predomi-nate whereas the decrease in tensile strength ru are theoreticallyattributed to poor state of filler–matrix interfacial adhesion andthe presence of voids in the composites. In the investigated com-posites such poor interface characterized by easy debonding/dis-lodging of the spherical filler particles in combination with thepresence of voids at the interface promoting premature/early-frac-ture of the composites have also been observed from scanningelectron microscopy (SEM) and is discussed in the subsequent sec-tion. The addition of cenosphere causes a systematic increase inthe flexural modulus and a decrease in the flexural strength whilethe flexural strain at break broadly remained unaffected, as shownin Fig. 4. The Increase in modulus of the cenosphere filled compos-ites as a function of the extent of cenosphere loading displays theeffect of the inherent rigidity of cenospheres whereas the decreasein flexural strength apparently indicate the filler shape (spherical)determines the bending strength of these composites. The Izod im-pact strength of the composites remained marginally reduced ornearly unaffected when compared to the PP matrix as revealedfrom Fig. 5. It is interesting to note that the impact strength ofPPC10 is nearly identical to that of unfilled PP matrix quantita-tively. These observations may well be explained by the crystallin-ity data obtained from XRD/DSC measurements. The crystallinitydecreased progressively with the increase in cenosphere contentand thereby enhancing the amorphous volume fraction in thestructural form of the composites. The increase in the amorphouscontent obviously has an influence on the impact toughness in lim-iting the extent of lowering in the toughness due to enhancedmechanical restrictions posed by the spherical fillers to the molec-ular chain mobility/bulk matrix deformation. Furthermore, hollowcenospheres/spheres based particles induced toughening of ther-

0 5 10 15 20

280

320

360

400

440

480

520

Young modulus (E) Tensile (Yield) strength (σ

y)

Ultimate tensile strength (σu)

Cenosphere content (wt %)

You

ng m

odul

us (

MP

a)

20

24

28

32

36

Tens

ile s

tren

gth

(MP

a)

0 5 10 15 2020

24

28

32

36

Stra

in a

t br

eak

(%)

Cenosphere content (wt %)

Fig. 3. Tensile modulus (E), yield strength (ry) and ultimate tensile strength (ru) ofthe composites as a function of cenosphere content.

moplastic matrices is well known since the energy dissipation abil-ity of the hollow spheres are known to be elastically stable [18].

Several models for describing the mechanical properties of two-phase composites as a function of the filler content have been re-ported to be successful in theoretically predicting the influenceof the filler content on the Young’s modulus (E) and the yield stress(ry). For example, Guth–Smallwood equation, which is essentiallya modified Einstein’s equation has been used as a simple model fordescribing E while taking into account the matrix–filler adhesion.The equation can be stated as,

E ¼ Emð1þ 2:5/þ 14:1/2Þ

where / is volume fraction and Em is modulus of the polymer ma-trix. Similarly, for expansible polymers with rigid spherical particlesthe Kerner equation can be used to theoretically estimate E. Theequation can be given as,

E ¼ Em 1þ 15ð1� tÞ8� 10t

� /1� /

� �

where t is the Poisson ratio which is approximately assumed to be0.35 [19,20].

The enhancement of ensile elastic modulus of the compositefilled with hard spherical fillers may be theoretically estimatedand analyzed following the Halpin–Tsai equation. The Young’s

0 5 10 15 20

16

18

20

22

24

26

28

30

32

34

Impa

ct s

tren

gth

(J/m

)

Cenosphere content (wt %)

Fig. 5. Variation of Izod impact strength of the composites with cenosphere content.

0 5 10 15 20

16

20

24

28

32

36

Yie

ld S

tres

s (M

Pa)

Cenosphere content (wt %)

Experimental Nicolasis -Narkis Piggot -Leidner Nielsen Pukanszky

Fig. 7. Theoretical model-fits for high-strain mechanical response i.e., Yieldstrength (ry) of the composites.

Table 5Values of different phase adhesion parameters conforming to various theoreticalmodels.

Theoreticalmodels

Phase adhesionparameter

PPC-5

PPC-10

PPC-15

PPC-20

Averagevalue

Nicolais–Narkis

K 1.087 0.68 0.84 0.79 0.908

Piggot–Leidner

B 2.718 1.349 1.478 1.267 1.703

Nielsen a 2.986 1.478 1.729 1.517 1.93Pukanszky B 0.366 1.763 1.429 1.579 1.284

1482 A. Das, B.K. Satapathy / Materials and Design 32 (2011) 1477–1484

modulus of a filled composite material as per the Halpin–Tsaiequation may be given as,

Ec

Em¼

1þ fg/f

1� g/f

where g ¼ Ef

Em� 1

� �.Ef

Emþ f

� �.

Where Ec, Ef, Em are Young’s modulus of composites, inclusionsand polymer matrix, respectively. /f is the filler volume fractionand g is the shape parameter dependent on filler geometry loadingdirection. For the rigid hollow hard fillers based composites, f maybe taken as 1 since l/d = 1 for spheres [6,19–21].

The various models explained above have been analysed interms of their utility to PP-cenosphere composite system and thefits of the experimentally determined E value as a function of cen-osphere content are shown in Fig. 6. It is found that the theoreticalvalues of E based on Kerner, Guth–Smallwood and Halpin–Tsaimodels are well above the experimental values though the closestpossible approximation is exhibited by the predictions of Kernermodel. Since Kerner model takes into account of rigid sphericalparticles filled in expansible polymer systems the close proximityof this theoretical model can well be comprehended and the devi-ation may well be attributed to the fact that censopheres are hol-low spherical particles with an appreciable level of rigidity. Onthe other hand the large deviation of the theoretically predictedE values from Guth–Smallwood and Halpin–Tsai models inevitablylead to the assumption that these models are unsuitable for PP-cenosphere composites. This may be attributed to the fact thatthe Young’s modulus of the cenosphere, size and geometrical con-siderations viz. aspect ratio (l/d) etc. of the filler-phase (ceno-sphere) are ignored in these equations in spite of the fact thatcenospheres have appreciably high Young’s modulus and are hol-low perfect spheres capable of withstanding very high stresses incompressive/bending/tensile loading modes.

The yield stress (ry) was analysed using the two phase compos-ite models viz. Nicolais–Narkis, Piggot–Leidner, Nielsen and thePukanszky model to estimate the theoretical yield stress. A charac-teristic of all theoretical approaches is a relationship between vol-ume fraction (/) and projected area fraction of the particulateinclusions [19–21]. The Nicolas–Narkis equation is a two-thirdpower law function with K as a parameter for filler–matrix adhe-sion and rm as yield stress of the polymer matrix:

r ¼ rmð1� K/2=3Þ

As an example, the theoretical value of K for poor Adhesion is 1.21.Piggot and Leidner used a first-power relationship with is similar

0 5 10 15 20

300

400

500

600

700

800

You

ng M

odul

us (

MP

a)

Cenosphere content (wt %)

Experimental Guth-Smallwood Kerner Halpin-Tsai Inverse rule of mixture

Fig. 6. Theoretical model-fits for low-strain mechanical response i.e., elasticmodulus (E) of the composites.

parameter B like in equation 10 which is attributed to weaknessin the structure due the stress concentration [19–21].

r ¼ rmð1� B/Þ

The Nielsen equation based on a two-phase system with pooradhesion as a matrix with voids and leads to the expression:

r ¼ rm expð�a/Þ

where the factor a is taken as stress concentration parameter. Ahigh value means high stress concentration or rather poor adhesion[21].

Similarly Pukanszky et al. [21] equation can be stated as,

ryt ¼ rym1� /f

1þ 2:5/fexpðB/f Þ

where B is an empirical parameter characterizing the degree of fil-ler–matrix interaction. The value of the parameter B depends on all

-25 0 25 50 75 100 125 150 175

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

PPC 20

PPC 15

PPC 10

PPC 5PP

PPPPC 5 PPC 10PPC 15 PPC 20

Tan

delt

a

Temperature (°C)

Fig. 8. Loss tangent (tan d) versus Temperature for the composites.

Ductile deformation Formation of Juxtaposed shear-lips

Dislodged cenosphres

A good state of distribution of cenosphres

Shear deformation induced ridge formation transverse to the fracture planes

Dislodged cenospheres

Cenosphere wall breakage

Apparent disappearance of plastic flow induced shear deformation Dislodged

cenospheres

Broken cenosphere

Tightly embedded cenosphres in PP matrix

Mechanically interlocked cenosphere particles

Voids at the filler-polymer interfacial curvature

(a) (b)

(d)(c)

(e)

Fig. 9. Bulk morphology as observed by SEM of the cryo-fractured surfaces of the composites (a) PP, (b) PPC-5, (c) PPC-10, (d) PPC-15 and (e) PPC-20.

A. Das, B.K. Satapathy / Materials and Design 32 (2011) 1477–1484 1483

factors influencing the load bearing capacity i.e. strength of theinterface and size of the interface. The model was developed forcomposites containing spherical particles however, anisotropic fill-ers can lead to increase of the load-transfer efficiency and hence to ahigher B value.

The fit of the yield stress and the experimentally determinedvalues are shown in Fig. 7 and gives positive K, B and a-valuesfor Nicolais–Narkis, Piggot–Leidner, Nielsen-model. The phaseadhesion parameters corresponding to the various models are gi-ven in Table 5. These inevitably lead to the assumption that funda-mentally these models reveal an approximate validity in case ofthe investigated cenosphere filled PP composites. The appreciablesignificance of the parameters based on these models is attributed

to the fact that the dynamics of polymer–filler interaction has afundamental resemblance to those assumptions behind the differ-ent theoretical models proposed for micro-composites. In case ofPP-cenosphere composites the extent of adhesion of polymerchains that forms the interface does not really alter the physicalsignificance of polymer–filler interaction at the molecular levelsince structural organisation of the matrix at the interface maypractically be assumed to be identical to that in the bulk. However,this may be theoretically valid in composites with the exception atlow cenosphere loading levels where the effective restrictions dueto physical/mechanical/kinematical constraints may be apprecia-bly higher whereas above a certain critical level of cenosphereloading such effects may get overshadowed by the randomly

1484 A. Das, B.K. Satapathy / Materials and Design 32 (2011) 1477–1484

localised cenospheres in the form of quasi macro-agglomeratesenhancing the incompatibility between the host PP matrix andthe spherical particle reinforcement phase. This is also evidentfrom the tan d plots obtained from DMA (Fig. 8) where the Tg in-creased with the incorporation of 5 wt.% (PPC-5) of cenospherewhereas above the same level of filler loading the Tg followedapparently no defined trend for any meaningful correlations toemerge.

3.5. Fractured surface morphology

The SEM investigations have demonstrated the state of disper-sion and distribution of the cenospheres at the micro-structural le-vel (Fig. 9). The general surface morphological features indicatethat with the increase in the cenosphere content the number ofvisible cenosphere particles on the scanned surface increasedapparently indicating the good state of distribution while at higherloadings of cenosphere contents the presence of cenospheres beingtightly embedded and mechanically interlocked by the resinaround indicate a good state of dispersion of the particles in thebulk of the matrix. In case of PP the surface revealed ductile defor-mation patterns characterized by the formation of distinctly juxta-posed shear lips as shown in Fig. 9a. The micrograph shown in Fig9b corresponding to PPC-5 shows that cenospheres have a nearuniform distribution in the PP matrix and the extent of shear-lipsformation is lower. Additionally dislodging of cenospheres parti-cles are also discernible on the surface indicating apparently a poorstate of polymer–filler adhesion. In Fig. 9c corresponding to PPC-10more of cenospheres distributed across the cryo-fractured surfaceare visible. The surface features also reveal that shear-deformationis substantially arrested possibly due to enhance mechanical con-straints imposed by cenosphere particles. Strikingly, on furtherincreasing the cenosphere loading to 15 (PPC-15) and 20 wt.%(PPC-20) more intense dislodging of the spherical fillers have beenobserved (Fig. 9d and e). The features also include broken ceno-spheres indicating a shift from ductile plastic deformation modein PP and lightly filled polymer composites to brittle failure mech-anism in moderately filled composites. This may be due to the factthat with the increase in the cenosphere content the inter-particledistances decreases and thereby restricting the polymer phase tocontribute to the fracture mode via homogeneous deformation.The increasing extent of composition induced deformation in-homogeneity originates from the enhanced matrix/filler phaseincompatibility which may have a qualitative correlation to themodulus mismatch between the two phases. This leads to theinterface controlled fracture mode initiation as in Fig. 9c and d.Thus the bulk morphological features clearly demonstrates thatup to a filler loading of�10% (PPC-10) the deformation is more ma-trix controlled whereas above the said level of cenosphere loadingthe fracture is primarily interface-controlled as is clearly seen inthe extensive debonding of the cenospheres from the matrix.

4. Conclusions

XRD studies have convincingly indicated that the crystallinephase/orientation (of the polymer chains) is appreciably affecteddue to the incorporation of cenospheres though it remained inappre-ciably influenced by the content of cenosphere. XRD/DSC studieshave showed that the crystallinity marginally reduced with the in-crease in cenosphere content while TGA revealed that the onset ofdegradation temperature remained unaffected. DMA investigationsshowed that the damping properties of cenosphere filled compositeis enhanced and is particularly observed in the composite with10 wt.% of cenosphere. The storage modulus increased up to �30%in the composites relative to the soft PP-phase. The tensile modulus

increased up to�43% accompanied by a nominal decrease in tensilestrength while the strain at break remained largely unaffected. Theimpact strength of the composites marginally reduced comparedto PP indicating a low-cost material-concept with maximized stiff-ness–toughness combination. The theoretical modeling of the ten-sile data revealed appreciable extent of phase-adhesion despitethe cenospheres lack any surface modification indicating better ex-tent of mechanical interlocking and surface-compatibility betweenpolymer and filler. The study, further, has demonstrated the differ-ential nature of deformation of PP-cenosphere composites as a func-tion of the extent of filler loading. It was observed from SEM analysisof fractured surface morphology that the matrix behaviour predom-inates up to a filler loading of �10 wt.% whereas above 10 wt.% theinterfacial/reinforcing effects control the deformation mode of thecomposites, an observation which needs further investigation tounderstand the underlying mechanistic details preceding suchmaterial failure modes.

Acknowledgements

Authors gratefully acknowledge Mr. Ratnesh Jain of MicroMinechem India Pvt. Ltd. for providing the flyash based ceno-spheres used in this study.

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