Materials and Design 31 (2010) 802–807

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Materials and Design

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Fracture behavior dependence on load-bearing capacity of filler in nano- andmicrocomposites of polypropylene containing calcium carbonate

Amir Afshar, Iman Massoumi, Rasool Lesan Khosh, Reza Bagheri *

Polymeric Materials Research Group, Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., P.O. Box 11155-9466 Tehran, Iran

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

Article history:Received 25 May 2009Accepted 29 July 2009Available online 3 August 2009

Keywords:A. Polymer matrix compositeA. Nano materialsE. Deformation and fracture

0261-3069/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.07.054

* Corresponding author. Tel.: +98 21 66165207; faxE-mail address: rezabagh@sharif.edu (R. Bagheri).

The fracture toughness and deformation mechanism of PP/CaCO3 (15 wt.%) composites were studied andrelated to load-bearing capacity of the particles. To alter the load-bearing capacity of the particles, differ-ent particle sizes (0.07–7 lm) with or without stearic acid coating were incorporated. The fracture tough-ness of the composites was determined using J-Integral method and the deformation mechanism wasstudied by transmission optical microscopy of the crack tip damage zone. It was observed that theload-bearing capacity of the particles decreased by reduction of particle size and application of coating.A linear relationship between normalized fracture toughness and inverse of load-bearing capacity of par-ticles was found. The crack tip damage zone in composites, which consists in massive crazing, furthergrows by reduction in load-bearing capacity.

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1. Introduction

Polypropylene (PP) is widely used in automotive, appliance andpackaging industries. In order to improve stiffness, dimensionalstability and to reduce cost, different kinds of fillers are added toPP [1]. Among these fillers talc, calcium carbonate and glass fibersare mostly used. In general, such fillers have a negative effect onfracture toughness of PP [2]. However, in recent years, this under-standing has been changed to some extent and the toughening ef-fect of mineral fillers has been claimed [1,3–9]. Mineral fillers havehigh stiffness and can not be deformed by external forces, but theycan induce crazing and shear yielding mechanisms in the matrixvia debonding [8,9]. Particle debonding in filler-modified PP canact similar to rubber cavitation in rubber-modified PP [7]. Thismanner may result in a noticeable increase in fracture toughnessof these composites [7]. Particle size and aspect ratio, debondingof particles before yielding and uniform dispersion of particlesare the parameters which can affect the toughness of PP/CaCO3

composite [8]. The important role of particle size in mechanicalproperties and fracture toughness of PP has been noticed [10]. Ithas been claimed that composites with submicron particles givethe best combination of mechanical properties compared to thosehaving micron or nanoparticles [8,9,11]. Meanwhile, the coating ofthe particles can considerably affect dispersion and debonding ofparticles [12], and consequently, the fracture toughness and defor-mation mechanism of the composite.

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More recently, Weon et al. [13] studied the deformation mech-anism of PP filled with CaCO3 nanoparticles (�44 nm). Their resultsshowed the deformation mechanisms of PP in the vicinity of cracktip include massive crazing and shear banding. Seemingly, theCaCO3 particles are the nucleating sites for damage initiation. Onthe other hand, Zebarjad et al. [2,14] studied the deformationmechanism of PP filled with microscaled CaCO3 particles(�5 lm). Their study showed that shear banding was not presentinside the damage zone and the dominant deformation mechanismwas crazing.

The load-bearing capacity is the ability of particles to maintainintegrity of a composite without failure. It depends on both sizeand surface treatment of the particles (see Eq. (2)). For example,Demjen et al. [15] have studied the effect of different couplingagents on tensile properties of PP/CaCO3 composites. They quanti-fied the results by employing the load-bearing capacity (B param-eter, see Section 3.3) for the PP/surface treated CaCO3 compositesand classified the coupling agents into non-reactive (B < 1), inac-tive (1 < B < 2) and reactive (B > 2) categories. More recently, LesanKhosh et al. [3] and Kiss et al. [12] utilized the load-bearing capac-ity to evaluate the presence of agglomerates in PP/CaCO3 compos-ites. It seems that load-bearing capacity has an important role inthe fracture behavior of the composite.

Although, there are many studies on the mechanical propertiesand fracture behavior of PP/CaCO3 composites, there is no attemptto relate the fracture toughness and deformation mechanism of thecomposite to load-bearing capacity of the particles. The presentstudy tries to clarify the dependence of fracture behavior of thepolypropylene composites on CaCO3 particle size and coating usingthe load-bearing capacity concept. CaCO3 particles with three dif-

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ferent sizes covering two orders of magnitude average diametershave been incorporated in a homo-polypropylene matrix. Stearicacid has been used for particle coating. Elastic–plastic fracturemechanics approach (J-integral) has been utilized to evaluate thefracture resistance of materials. Crack tip damage zones have beenstudied via transmission optical microscopy (TOM).

2. Experimental procedure

2.1. Sample preparation

Homo-PP with a melt flow index of 9 g/10 min and a density of0.9 g/cm3 was used. The precipitated CaCO3 nanoparticles (averagesize of 70 nm) with stearic acid coating and the ultrafine (0.9 lmaverage size) and microscaled (7 lm average size) CaCO3 particleswith and without stearic acid coating were incorporated.

Composites were made via melt compounding in a co-rotat-ing twin-extruder with length-to-diameter (L/D) of 20. The tem-

Fig. 1. Preparing a SEN-3 PB sample for TOM examination.

peratures of different zones in the extruder were varied from180 to 210 �C. Standard test specimens were made throughinjection molding. The temperatures at four successive zonesof the injection molding apparatus were set from 220 to240 �C. The samples made contained 5 and 15 wt.% CaCO3

particles.

2.2. Mechanical evaluation

To determine fracture toughness (Jc), single edge notched three-point bending (SEN-3PB) tests were carried out according to ASTMD6068 using a universal Hounsfield H10KS at 1 mm/min cross-head speed. The dimensions of the samples were 62.5 �12.6 � 4.7 mm. The 6.5-mm deep pre-cracks were produced usinga fine saw (4.5 mm) followed by sliding a sharp razor (2 mm).Stress whitening zone method was employed as crack length mea-surement criteria [16]. In this method, the growth of the stress-whitened zone at the crack-tip is employed in place of the cracklength increment. To measure the damage zone length, the sidesurface of specimen was polished down to half of the thicknessusing the common grinding and polishing techniques. The lengthof damage zone, i.e., crack extension plus the length of crack-tipdeformation zone was measured using Olympus DP12 opticalmicroscope at reflective/dark-field mode.

To determine the load-bearing capacity of the particles, theyield strength was obtained by the tensile test. The tensile testswere carried out according to ASTM D635 using the same appara-tus at 5 mm/min cross-head speed at ambient temperature.

2.3. Microscopic evaluation

The damage zone in front of SEN-3 PB crack tip was studiedby Olympus DP12 optical microscope at bright field and X-polar-ized modes. The samples were prepared by cutting along crackgrowth direction as shown in Fig. 1. One side of the samplewas grinded and polished to the center. The polished side wasthen attached on a flat support by transparent glue. Using com-mon grinding and polishing, the thickness of sample was re-duced to less than 100 lm for the ease of transmission opticalmicroscopy (TOM).

Scanning electron microscopy (SEM; Vega TESCAN) at 5 kV wasused to evaluate dispersion and debonding of CaCO3 particles. Thesamples were coated by a thin gold layer before performing themicroscopy.

Fig. 2. Normalized fracture toughness vs. particle size for PP filled with differentamounts of coated and uncoated CaCO3.

804 A. Afshar et al. / Materials and Design 31 (2010) 802–807

3. Results and discussion

3.1. Fracture toughness of the composites

Fig. 2 shows the effects of the particle size and coating on thefracture toughness of the PP/CaCO3 composites. The results arenormalized by that of the neat Homo-PP ðJc0

¼ 570 J=m2Þ. It canbe observed that addition of 5 wt.% of CaCO3 could not alter thefracture toughness of PP considerably. As seen in Fig. 2, however,addition of 15 wt.% nanoscaled CaCO3 particles to PP caused anoutstanding (about 180%) increase in the fracture toughness. Addi-tion of ultrafine coated particles could not change the fracturetoughness of PP effectively and adding microscaled coated parti-cles to PP caused a slight decrease in fracture toughness (Fig. 2).In order to explain the positive effect of nanoparticles on fracturetoughness, one may consider the stress concentration effect of deb-onded particles on crazing of the matrix. In other words, the deb-

Fig. 3. (a, b) An agglomerate of uncoated CaCO3 particles in the PP matrix at thefracture surface of a SEN-3 PB sample.

onded nanoparticles may act as initiation sites for crazingprocess in the polymeric matrix. Possible massive crazing inducedby debonded nanoparticles at the crack tip might be responsiblefor fracture toughness improvement in the composite. Johnsenet al. [17] have reported similar results for epoxy filled with silicananoparticles. Their results showed that incorporation of 13 vol.%of silica nanoparticles resulted in 4.6-fold increase (from 100J/m2 to 460 J/m2) in the fracture energy of unmodified epoxy. Theyinvestigated the possibility of different toughening mechanismsresponsible for this observation. Using microscopic evaluationand theoretical models, they concluded that the most possiblemechanism is debonding of nanoparticles followed by plastic voidgrowth of the matrix [17]. In other words, the debonding of parti-cles is prerequisite to activation of the toughening mechanism andfracture toughness improvement. Large particles may not act thesame as nanoparticles, since larger voids formed by debonding oflarger particles may induce cracking of the matrix prior to massive

Fig. 4. Debonding of CaCO3 particles from PP matrix in composites containing(a) coated 7 lm and (b) uncoated 7 lm particles.

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crazing. Zuiderduin et al. [8] argued that voids larger than 5 lm arenot stable free volume and do not have positive effect ontoughness.

As shown in Fig. 2, introducing 15 wt.% of uncoated particles,both ultrafine and microscaled, reduced toughness of PP. As seen,in the case of both particle sizes mentioned, the composite con-taining uncoated particles have almost 25% lower fracture tough-ness compared to their coated counterparts. As shown in Fig. 3,this can be attributed to the agglomeration of particles in the ab-sence of stearic acid coating [12,18,19]. Large agglomerates facili-tate cracking in the matrix. As we show later (see Section 3.3), itseems that coated particles could debond from the matrix moreeasily. As shown in Fig. 4, however, it appears that debonding oc-curred in both composites containing coated and uncoated parti-cles. As a result, one may claim that the role of the coating onparticle–particle interaction may be more important than on parti-cle-polymer interaction. In other words, the main role of the coat-ing on fracture toughness of the composite is to improve theparticle dispersion.

3.2. Deformation mechanism of the composites

Fig. 5 shows the TOM micrographs of the damage zones underbright-field light condition. The dark areas seen in Fig. 5 are crazes

Fig. 5. Deformation mechanism at the crack tip of SEN-3 PB s

formed at the crack tips under bending condition. The two impor-tant factors should be taken into account in this figure include: (i)the extent of craze growth that is indicated by the damage zonesize and (ii) the intensity and number of crazes that are seen bythe darkness of damage zones. As illustrated in Fig. 5, introducing15 wt.% of uncoated CaCO3 to PP caused a slight rise in the inten-sity of the damage zone (compare Fig. 5c and e with a). Incorpora-tion of coated particles, however, in addition to increase in theintensity, enlarged the damage zone (compare Fig. 5b, d and f witha). This effect is maximized when nanoparticles are utilized(Fig. 5b) which is in agreement with the fracture toughness mea-surements (Fig. 2).

Comparing the results shown in Figs. 2 and 5, the hypothesismentioned earlier is reinforced and it can be concluded that fillerscan enhance fracture toughness of PP if they are fine enough, welldispersed and debond from the matrix prior to crack growth. Therole of stearic acid coating is to provide well dispersion of the par-ticles and possibly, control the interfacial adhesion to the matrix.

3.3. Fracture behavior dependence on load-bearing capacity

To quantify the dependence of the composite fracture tough-ness on particle size and coating of CaCO3, the load-bearing capac-ity of fillers (B) has been considered using the yield stress of the

pecimens of neat PP and PP/CaCO3 (15 wt.%) composites.

Fig. 7. Normalized fracture toughness vs. the inverse of load-bearing capacity ofCaCO3 particles for PP filled with 15 wt.% of filler.

Table 1Yield stress and B parameter for the materials used in this study.

Particle size(lm)

Coated with stearicacid

Yield stress (MPa) Bparameter

5 wt.% 15 wt.%

Neat PP – 28.1 ± 0.2 –0.07 Yes 27.0 ± 0.1 25.1 ± 0.1 1.41 ± 0.030.9 Yes 27.5 ± 0.2 25.7 ± 0.1 1.86 ± 0.06

No 27.7 ± 0.1 25.9 ± 0.1 2.28 ± 0.257 Yes 27.4 ± 0.1 25.6 ± 0.1 1.92 ± 0.11

No 27.7 ± 0.1 26.2 ± 0.1 2.46 ± 0.14

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composites. Eq. (1) was employed to evaluate the yield stress of theparticulate filled composite (ry) as a function of the volume frac-tion of the filler (u) [20]:

ry ¼ ry0

1�u1þ 2:5u

expðBuÞ ð1Þ

where ry0is the yield stress of unfilled matrix. In other words, this

model describes the yield stress dependence on the compositionand contains the parameter, B, characterizing the interaction. Theresults of ry measurements and the calculated B values are summa-rized in Table 1. Fig. 6 shows the relation between the normalizedyield stress and B for PP filled with 5 and 15 wt.% of CaCO3 particles.As seen in this figure, yield stress increases by B. The influence ofload-bearing capacity on the yield stress can be explained by meansof ease of particle/matrix debonding at lower B values.

Fig. 7 shows the normalized fracture toughness (Jc=Jc0) vs. 1/B.

This figure shows that increase in 1/B, i.e. decrease in interactionbetween the particles and the matrix, enlarges the fracture tough-ness. A linear relationship between Jc=Jc0

and 1/B can be postulated(Fig. 7).

B is a parameter which depends on the specific surface area (Af)and density of filler (qf), as well as, thickness (l) and yield stress ofthe interphase formed around the particles (ryi) [12]. Eq. (2) showsthis correlation:

B ¼ ð1þ Af qf lÞ ln ryi

ry0

ð2Þ

The interphase is the region that lies between the reinforcementand the bulk polymer and may have different properties from thebulk because of the interaction between polymeric chains and thereinforcements. The size of the particles and coating them withstearic acid can affect Af and ryt. According to the results of thisstudy, decrease in CaCO3 particle size and coating particles withstearic acid, reduced the magnitude of B (Table 1). It is worthwhile

Fig. 6. Normalized yield stress vs. load-bearing capacity of CaCO3 particles for PPfilled with different filler contents.

to note that B > 2 for uncoated and <2 for coated particles. Compar-ing the results to that of Demjen et al. [15], it can be postulated thatinactive agents (1 < B < 2) may have better effects on fracturetoughness of the PP/CaCO3 composites than reactive agents(B > 2). One can speculate that as B decreases (1/B increases) deb-onding of particles from matrix becomes easier, due to less interac-tion of particles and matrix. The debonded particles then facilitateplastic deformation at the crack tip.

4. Conclusions

1. While addition of 15 wt.% of CaCO3 nanoparticles amelioratedthe fracture toughness of PP profoundly, the ultrafine andmicroscaled particles did not alter the fracture toughnessconsiderably.

2. The fracture toughness of PP filled with coated CaCO3 parti-cles was higher than PP filled with uncoated particles dueto better dispersion of the particles and easier debondingfrom the matrix which resulted in further growth of the dam-age zone.

3. A linear relationship between normalized fracture toughnessand inverse of load-bearing capacity of particles was observedfor PP filled with 15 wt.% of CaCO3 particles. It seems that asB decreases, debonding of particles from matrix becomes easierdue to less interaction of particles and matrix which can facili-tate plastic deformation at the crack tip. The load-bearingcapacity of the particles is reduced by reducing particle sizeand application of coating.

Appendix A

List of Symbols

Af specific surface area of the filler B load-bearing capacity of the filler particles Jc J-integral fracture toughness of the composite Jc0 J-integral fracture toughness of the unfilled PP l thickness of the interphase formed around the filler particles u volume fraction of the filler qf density of the filler ry yield stress of the composite ry0 yield stress of the unfilled PP ryi yield stress of the interphase formed around the filler particles

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