aggregation of caco3 particles in pp composites: effect of surface coating

COMPOSITES

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Composites Science and Technology 67 (2007) 1574–1583

SCIENCE ANDTECHNOLOGY

Aggregation of CaCO3 particles in PP composites:Effect of surface coating

Attila Kiss a,b, Erika Fekete a,b, Bela Pukanszky a,b,*

a Department of Plastics and Rubber Technology, Budapest University of Technology and Economics, H-1521 Budapest, P.O. Box 91, Hungaryb Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary

Received 14 March 2006; received in revised form 7 July 2006; accepted 11 July 2006Available online 7 September 2006

Abstract

The occurrence and effect of aggregation in PP composites containing seven different precipitated CaCO3 fillers coated with stearicacid are described in this study. The particle size and specific surface area of the filler varied in a relatively wide range, the latter changedbetween 2 and 12 m2/g. The fillers were characterized by various methods including surface area, particle size and bulk density. PP com-posites were prepared in an internal mixer in the composition range of 0–0.3 volume fraction filler content and their structure was studiedby optical microscopy. The tensile and rheological properties of the composites were related to their structure. The results prove that theunambiguous detection of the presence of aggregation is difficult in particulate filled composites. The coating of CaCO3 fillers with asurfactant changes the behavior of the particles considerably, but does not eliminate aggregation completely. The association of fillerparticles depends on the relative magnitude of adhesion and separating forces. Although coating decreases the surface free energy ofthe filler significantly, gravitational forces are much smaller than adhesion between either uncoated or coated fillers thus powder particlesalways aggregate. Different forces act in suspensions used for the determination of the particle size distribution of the filler; the shape ofthe distribution may indicate the presence of aggregation. Coated fillers form much looser aggregates with more diffuse interphases, thanuncoated particles. Composite stiffness is completely insensitive to changes in structure or interaction, but the direct evaluation of othertensile properties may also lead to erroneous conclusions. Model calculations, oscillatory viscometry, as well as the proper representationof the results allow the unambiguous detection of aggregation.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. CaCO3 filler; B. Stearic acid coating; B. Particle characteristics; D. Optical microscopy; B. Aggregation

1. Introduction

The filler used in the largest quantities in particulatefilled composites is CaCO3 [1,2]. Its composites find a widerange of application like sewer pipes, garden furniture,breathable films, etc. The use of fillers offer a number ofadvantages from improved stiffness and dimensional stabil-ity to better thermal properties compared to the neat,unmodified polymer. The mechanical properties of particu-

0266-3538/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2006.07.010

* Corresponding author. Address: Department of Plastics and RubberTechnology, Budapest University of Technology and Economics, H-1521Budapest, P.O. Box 91, Hungary. Tel.: +36 1 463 4335; fax: +36 1 4633474.

E-mail address: [email protected] (B. Pukanszky).

late filled composites usually improve with decreasing sizeof the particles [3–5]. Although stiffness is modified onlyslightly [5,6], tensile yield stress and ultimate strengthincrease considerably as particle size becomes smaller [3–5].

However, all advantages offered by the use of fillers can beexploited only if the particles are distributed homogenouslyin the polymer matrix, if they do not form aggregates, whichare often detected in particulate filled polymers [6–10].Aggregation results in processing problems, deterioratedmechanical properties and poor aesthetics. The appearanceand extent of aggregation is determined by the relative mag-nitude of adhesive and shear forces prevailing in the polymermelt during compounding [6]. Large shear forces and smallsurface energy favors homogeneous distribution, while smallparticles have strong tendency to aggregate. One of the most

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A. Kiss et al. / Composites Science and Technology 67 (2007) 1574–1583 1575

efficient ways to hinder aggregate formation is the surfacecoating of the filler with a surfactant [11,12]. Surface treat-ment leads to the decrease of both particle/particle andmatrix/filler interaction. As a consequence, surface coatedfillers are used practically always for the production of par-ticulate filled thermoplastic products [1].

Although surface modification decreases the probabilityof aggregate formation, it does not guarantee completehomogeneity. The presence of aggregates is often not obvi-ous and only processing problems or the premature failureof the part indicates the formation of an inhomogeneousstructure. Various methods were proposed for the determi-nation of filler aggregation. The most often used and suc-cessful technique is optical microscopy, the measurementof the area of aggregates in thin slices [13–15]. Otherapproaches have been also published including the analysisof particle characteristics [6], the determination of sedimen-tation behavior [6,16], the use of X-ray diffraction [16] orthe measurement of composite properties [6,13,15,17].Since the extent of aggregation depends on a number offactors including filler content and processing conditions,the detection of aggregates is difficult and the opinionsabout their effect on properties are rather contradictory.

In earlier studies we investigated the effect of aggrega-tion on the properties of polypropylene composites[6,15,17]. We always used uncoated CaCO3 as filler andfound that aggregates appear at specific surface areas lar-ger than 6 m2/g. Strength and fracture resistance decreasedconsiderably in the presence of aggregates. Since surfacecoating is expected to influence aggregation significantly,the goal of this study was to determine this effect quantita-tively. We used various methods for the detection of aggre-gation and examined the sensitivity of different propertiesto the presence of aggregates.

2. Experimental

The fillers used in this study were coated precipitatedCaCO3 samples; Table 1 contains their most importantcharacteristics. The specific surface area of the fillers, whichchanged from 2 to 12.3 m2/g, was determined by nitrogenadsorption (BET method) using an Autosorb 1 (Quanta-chrome, USA) apparatus. Samples were degassed at

Table 1Particle and surface characteristics of the investigated fillers

Filler Specificsurface(m2/g)

ParameterC

Particlesize (lm)

Stearicacid(wt.%)

Surfacecoverage(%)

Surfacetension,cd

s

(mJ/m2)

D20 2.0 21.9 2.13 0.4 56 28.6D27 2.7 21.3 1.50 0.5 52D30 3.0 19.6 1.11 0.6 56D39 3.9 21.0 0.67 0.8 57D55 5.5 19.6 0.35 1.1 56D99 9.9 20.4 0.18 1.4 40 31.4D123 12.3 23.2 0.11 1.6 36 29.9

100 �C and 10�5 mmHg pressure for 24 h before the mea-surement. The initial section of the adsorption isothermwas measured at liquid nitrogen temperature in the relativepressure range of 0.05–0.35 at five points. Specific surfacearea was calculated from the BET equation using0.162 nm2 for the surface area occupied by a nitrogen mol-ecule. Parameter C of the BET equation, which is related tothe differences in adsorption enthalpies for the first and thefollowing adsorbed nitrogen layer, was also calculatedfrom the measurements. Our earlier results [18] proved thatparameter C is strongly related to the surface energy of fill-ers. The bulk density of the filler was measured accordingto the ISO 60 standard. The median particle size and sizedistribution of the fillers was determined using a Malvern2000 laser diffraction particle analyzer in water suspension.Homogeneous dispersion was achieved by ultrasonic agita-tion. All distributions are the average of three separateruns. The aspect ratio of the particles were close to 1, thustheir orientation did not influence properties. A SEMmicrograph taken from sample D20 is presented in Fig. 1to demonstrate this statement. The appearance of all fillersamples was practically the same as shown in the figure.

All samples were surface coated with stearic acid. Theamount of surfactant used for the treatment is also listedin the table, together with the dispersion component of sur-face tension measured for selected samples. This latterquantity was determined by infinite dilution inverse gaschromatography (IGC). The measurements were carriedout at 100 �C using a Perkin Elmer Autosystem XL appa-ratus with columns of 50 cm length and 6 mm internaldiameter. Vapor samples of 5–20 ll were injected into thecolumn and retention peaks were recorded by a FID detec-tor. High purity nitrogen was used as carrier gas and itsflow rate was 5 ml/min. Each reported value is the resultof three parallel runs. Before measurement, samples werepreconditioned in the column at 140 �C for 16 h.

The polymer used as matrix for composite preparationwas the Tipplen H543 F grade homopolymer producedby TVK, Hungary. Composition changed from 0 to30 vol.% filler content in 5 vol.% steps. The polymer andthe filler were homogenized in a Brabender W 50 EH inter-nal mixer at 190 �C, 50 rpm for 10 min. The homogenizedmelt was compression molded to 1 mm plates at 190 �Cin 5 min. Specimens were cut from the plates for furthertesting. Homogeneity of the samples was checked by opti-cal microscopy (OM). 40 lm thick slices were cut from thecompression molded plates and micrographs were takenfrom them by a Polaroid digital camera at a magnificationof 32. 10 mm wide dumbbell specimens were used for thedetermination of tensile properties. The stiffness of thesamples was measured at 0.5 mm/min, while other proper-ties were determined at 5 mm/min cross-head speed. Gaugelength was 60 mm in both cases. Tensile yield stress (ry)and yield strain (ey), as well as tensile strength (r) and elon-gation-at-break (e) were derived from recorded force vs.elongation traces. Rheological measurements were carriedout using a Paar-Physica USD 200 apparatus at 220 �C

Fig. 1. SEM micrograph of sample D20 to show the shape of the particles and their aspect ratio, which is close to 1.

1576 A. Kiss et al. / Composites Science and Technology 67 (2007) 1574–1583

in oscillatory mode in the frequency range between 0.1 and600 sec�1 on disks of 25 mm diameter. The amplitude ofdeformation was 5%, which was in the linear elastic regionas confirmed by an amplitude-sweep with controlled sheardeformation.

3. Results and discussion

Experimental results are reported in several sections.First the surface characteristics of the coated filler are pre-sented, followed by the discussion of the particle character-istics of the studied samples. The observations related tothe structure of the composites are discussed subsequently,and then the effect of particle size on the mechanical andrheological properties of the composites is described. Theresults obtained on coated fillers are always compared todata measured on composites containing uncoated fillers.

3.1. Surface characteristics, coverage

Adhesive forces acting among the particles depend onthe surface free energy of the filler [19]. The coating ofCaCO3 with stearic acid leads to a significant decrease insurface tension [20–26], which results in decreased interac-tions and hopefully to limited aggregation. We have threesources of information related to the surface characteristicsof the fillers: the amount of stearic acid used for treatment,parameter C of the BET equation related to interactionand surface tension. The samples were coated during prep-aration with amounts leading to approximately the samesurface coverage. According to our approach 100% surfacecoverage is the amount necessary to cover the filler with a

monolayer surfactant as determined by the dissolutionmethod described elsewhere [27,28]. The amount of stearicacid used for coating is listed in Table 1. It increases withincreasing specific surface area to achieve the same cover-age in each case. The surface coverage of the filler was cal-culated by using the reference line determined earlier by usfor fillers covering a range of specific surface areas [28]. Ascolumn 6 of Table 1 shows, apart from the two fillers withthe largest specific surface areas, coverage is very similar,around 55%. The two other fillers were covered by the acidto a considerably lower extent, which may lead to highersurface energies and stronger aggregation tendency.

Parameter C of all fillers is rather similar indicating verysimilar interaction between nitrogen and the surface. Thevalues are rather small; they are around 20 showing weakinteraction and considerable coverage of the filler. The C

values of uncoated montmorillonite and CaCO3 arearound 150–250 [18], which clearly shows the effect of coat-ing and forecasts weak interaction among particles andwith the matrix as well. It is rather surprising that eventhe two fillers for which smaller coverage was calculatedhave practically the same C values. Considering the simi-larity of surface coverage and C parameters, we selectedonly three samples to determine their surface tension: thesample with the smallest and the two fillers with the largestspecific surface area for obvious reasons. Surface tensionsare compiled in the last column of Table 1. They confirmthe results of BET measurements and the conclusionsdrawn form the evaluation of parameter C; the surfacecharacteristics of the fillers including the dispersion compo-nent of their surface tension cd

s are very similar for all thestudied fillers.

0

250

500

750

1000

1250

1500

Bul

k de

nsity

(g/

l)

Specific surface area (m2/g)

0 5 10 15 20 25

Fig. 3. Effect of specific surface area on the bulk density of CaCO3 fillers.(h) Uncoated, (d) coated filler.


In order to put these results into a wider perspective andsee their possible consequence on the aggregation of the fil-ler, we plotted the surface tension of CaCO3 as function ofthe extent of surface coverage in Fig. 2 using also some ear-lier data [22]. We can see that the dispersion component ofsurface tension decreases steeply at small surface coverages,it reaches a minimum value at monolayer coverage (100%)and then increases again as a second layer of surfactantadsorbs on the filler [22]. The values obtained for the pre-cipitated calcium carbonates of this study fit the correlationvery well. Although they are somewhat larger than the min-imum achieved at monolayer coverage, the surface tensionof the samples is rather small. As a consequence, we mayexpect that the aggregation tendency of these fillers will dif-fer considerably from that of uncoated fillers.

3.2. Particle characteristics

As shown in one of our earlier studies [6], the bulk den-sity of fillers depends strongly on their specific surface area;it decreases with decreasing particle size. Particles interactwith each other in the powder and form aggregates. Theonly force separating them is gravitation, while adhesiveforces, mainly secondary van der Waals forces, keep themtogether. Since coating with stearic acid decreases the sur-face free energy of the filler considerably, we expected theformation of less aggregates in coated fillers and largerbulk density compared to the uncoated ones. The depen-dence of bulk density on specific surface area is presentedin Fig. 3. Rather surprisingly, coating does not influencebulk density at all, all values fall onto the same correlation.As mentioned earlier, aggregation is determined by the rel-

20

30

40

50

60

70

Surf

ace

tens

ion,

γSd (

mJ/

m2 )

Surface coverage (%)

0 40 80 120 160

Fig. 2. Dependence of the dispersion component of surface tension ofCaCO3 fillers coated with stearic acid on surface coverage. (h) Previousresults, (d) this work (see Table 1).

ative magnitude of adhesive and separating forces. Theselatter must be significantly weaker than adhesive forcesboth for the uncoated and for the coated samples, in spitethe fact that coating decreases adhesive forces significantly.

Interactions are more complicated in suspensions usedfor the determination of particle size distribution. Besidesparticle/particle interaction also the interaction of waterused in the measurement must be taken into account bothwith the filler and the surfactant. Since the surface of thefiller is not covered completely with the surfactant, watermay adsorb on it and alter aggregation tendency. The factthat ultrasonic agitation and surfactants are used practi-cally always to facilitate dispersion indicates also thataggregation may occur under such conditions.

The particle size distribution of the filler with the small-est specific surface area is presented in Fig. 4 together withthat of an uncoated filler with approximately the same sur-face area. The distributions are regular without tails orshoulders and they are very similar to each other. The lar-ger specific surface area, i.e. smaller particle size, of the pre-cipitated and coated sample is indicated by a slight shift ofthe distribution towards smaller particle sizes.

Unfortunately not all distributions have the same regu-lar appearance as the one shown in Fig. 4. The particle sizedistribution of three coated fillers is presented in Fig. 5.The filler with the largest specific surface area exhibits theusual features of fillers with small particle size. The averagesize of this filler is around 0.1–0.2 lm as determined bymeasurement (see Table 1) or by calculation from theBET surface. The size distribution does not even show suchsmall particles, only larger ones even up to 30 lm. The dis-tribution has a shoulder at small particle sizes, which prob-ably belong to particles with the corresponding size, while

0.1 1 10 1000

2

4

6

8

10

Freq

uenc

y (v

/v%

)

Particle size (μm)

Fig. 4. Particle size distribution of two fillers with relatively small specificsurface area. (—) Coated filler (Af = 3.9 m2/g), (–––) uncoated filler(Af = 3.5 m2/g).

0.1 10 1000

2

4

6

8

10

Freq

uenc

y (v

/v%

)

Particle size ( m)

1

Fig. 5. Irregular particle size distribution of coated fillers with variousspecific surface areas. (—) Af = 12.3 m2/g, (–––) Af = 2.7 m2/g, (� � �)Af = 2.0 m2/g.


the tail at large particle sizes definitely indicates aggrega-tion. This result clearly proves that we could not disinte-grate all aggregates prior the measurement of thedistribution. Interestingly the other two fillers showingthe largest irregularity in their distribution are those withthe smallest specific surface area. Both possess a small par-

ticle size tail, which indicates the non-uniformity of theproduction technology and/or the presence of small parti-cles. These latter might aggregate during composite prepa-ration. The study of particle characteristics indicate that inspite of surface coating, aggregation of filler particles takesplace under the conditions of the measurements, thus wemust assume that this may occur in some extent also duringcomposite preparation.

3.3. Structure, optical microscopy

Optical microscopy done on thin slices of composites isone of the most frequently used methods to characterizestructure and detect aggregation. Since the extent of aggre-gation depends also on filler content [15], we studiedmainly the structure of composites with the largest fillercontent, with 30 vol.% loading. The optical micrographtaken from the composite containing the coated filler withthe largest particle size is presented in Fig. 6a. The struc-ture is fairly homogeneous, large aggregates definitely arenot present in it at all. On the other hand, the micrographof the sample containing the coated filler with 9.9 m2/g spe-cific surface area offers a completely different picture(Fig. 6b). Lighter and darker areas can be seen in themicrograph indicating areas with varying density, i.e. thefiller definitely forms aggregates in this composite. InFig. 6c we present the micrograph of a composite preparedwith an uncoated filler. The specific surface area of the filleris 9.0 m2/g in this case. Although the area of dark aggre-gates might not be larger in Fig. 6c than in Fig. 6b, butthe difference in contrast is striking. The phase boundariesbetween the aggregates and the matrix are much sharperwhen the uncoated filler is used indicating the formationof denser aggregates, probably with larger aggregatestrength. On the other hand, the aggregates of the coatedfiller are much looser, the contact surface with the matrixis larger and aggregate strength should be also weaker thanfor the uncoated filler.

3.4. Mechanical properties

Properties of particulate filled polymers depend on theparticle characteristics of the filler used [1,5]. Modulusdepends only slightly on particle size [6], but yield stressand strength are influenced more strongly by the specificsurface area of the filler [3,4]. Aggregation is expected tochange structure-property correlations, thus offering a pos-sibility for the detection of its occurrence by the measure-ment of various composite properties. The Young’smodulus of composites containing 25 vol.% filler is plottedagainst the specific surface area of the filler in Fig. 7.Results obtained with uncoated fillers are also shown asreference. The correlation presented in the figure stronglysupports our previous statement: the stiffness of particulatefilled composites is not sensitive either to structure or inter-action. We know that uncoated CaCO3 fillers with specificsurface areas larger than 6 m2/g aggregate [6], but this does

Fig. 6. Various degree of aggregation in PP/CaCO3 composites detectedby optical microscopy. (a) Coated filler, Af = 2.0 m2/g, u = 0.30, (b)coated filler, Af = 9.9 m2/g, u = 0.25, (c) uncoated filler, Af = 9.0 m2/g,u = 0.20.

20 250

1

2

3

4

5

6

You

ng's

mod

ulus

(G

Pa)

Specific surface area (m2/g)0 5 10 15

Fig. 7. Effect of coating and specific surface area of the filler on thestiffness of PP/CaCO3 composites. (h) Uncoated, (d) coated filler.

12 1515

20

25

30

35

Ten

sile

yie

ld s

tres

s (M

Pa)

Specific surface area (m2/g)0 3 6 9

Fig. 8. Dependence of the tensile yield stress of PP/CaCO3 composites oncoating and on the specific surface area of the filler. (h) Uncoated, (d)coated filler.


not have any effect on the correlation. Coated fillers fit per-fectly the general trend of uncoated fillers. Although thestiffness of composites containing the coated filler seemsto be located slightly below the correlation obtained withuncoated CaCO3, this might be a result of the differentmatrix or slightly dissimilar processing conditions.

The yield stress of PP composites prepared from coatedand uncoated fillers at 20 vol.% filler content is presented inFig. 8 as a function of the specific surface area. We had toselect smaller filler loading for comparison, since compos-ites with larger filler content do not exhibit a yield stress.

Composites prepared with uncoated fillers of large specificsurface area did not possess a yield stress even at this smal-ler filler loading, thus the corresponding values are missingfrom the plot. Larger difference exists in the yield stress ofcomposites containing coated and uncoated fillers, respec-tively, than in their stiffness. The smaller yield stresses mea-sured in the presence of coated fillers is the result ofdecreased matrix/filler interaction. Apart from the smallervalues, practically no other difference can be observed in


the correlations obtained for the two kinds of fillers; wecannot draw any conclusions about aggregation from theseresults.

Unlike yield stress, the tensile strength of all compositescan be measured and the results are plotted in Fig. 9 as afunction of specific surface area. The strength of the com-posites containing the uncoated filler increases stronglywith increasing specific surface area due to the formationof a hard interphase and its reinforcing effect. However,strength does not increase, but decreases above 6 m2/g spe-cific surface area because of the formation of aggregates.Debonding of large structural units is easier and also thefracture of aggregates may occur above this surface area.On the other hand, the strength of composites containingthe coated fillers increases continuously with decreasingparticle size. The lack of any change in the correlation indi-cates the complete absence of aggregates, which is quitesurprising in view of the results obtained by opticalmicroscopy.

3.5. Discussion, detection of aggregation

We could draw contradictory conclusions from the eval-uation of particle characteristics and structure, on the onehand, and from mechanical properties, on the other. Wemight conclude from the latter results that aggregation orat least its effect is negligible in composites containing thecoated filler. This conclusion might be supported by thelack of any drastic change in properties when plotted asa function of specific surface area and also from the diffuseinterfaces presented in Fig. 6b. Nevertheless, further mea-surements and analysis is required in order to eliminatethe possibility of aggregate formation.

20 250

10

20

30

40

Ten

sile

str

engt

h (M

Pa)


0 5 10 15

Fig. 9. Effect of coating and specific surface area of the filler on the tensilestrength of PP/CaCO3 composites. (h) Uncoated, (d) coated filler.Drastic influence of aggregation.

The simple model developed earlier for the quantitativeevaluation of the composition dependence of tensile yieldstress in particulate filled composites [3,4,29] offers a conve-nient way to detect the appearance and effect of aggrega-tion. According to the model, tensile yield stress ofcomposites is determined by three components, the yieldstress of the matrix, effective load-bearing cross-sectionand interaction:

ry ¼ ry0

1� u1þ 2:5u

expðBuÞ ð1Þ

where ry and ry0 are the yield stress of the composite andthe matrix, respectively, u is the volume fraction of the fil-ler in the composite and B is a parameter related to theload-bearing capacity of the filler. This latter depends onthe size of the contact surface between the polymer andthe filler and on the properties of the interphase formed i.e.

B ¼ ð1þ Afqf‘Þ lnryi

ry0

ð2Þ

where Af and qf are the specific surface area and density ofthe filler, while ‘ and ryi are the thickness and yield stress ofthe interphase. Aggregation decreases the surface availablefor the polymer, thus the value of B should also decrease asa result. Any deviation in the slope of the B vs. Af correla-tion would be an indication of aggregation.

Parameter B can be determined from the linear form ofEq. (1). Relative yield stresses can be expressed by the rear-rangement of the equation

ryrel ¼ry

ry0

1þ 2:5u1� u

¼ expðBuÞ ð3Þ

and its natural logarithm should depend linearly on com-position with a slope of B and an intersection of zero.Apart from calculating B, the linear plot offers a possibilityto check the validity of the model and the presence ofaggregation. Any deviation from a straight line indicatesthe presence of structural effects.

The tensile yield stress of composites containing three ofthe studied coated fillers is plotted in Fig. 10 in the linearform. Two of the lines are perfectly linear indicating theabsence of aggregation. However, for the filler with thelargest specific surface area two points deviate from the lin-ear correlation indicating the appearance of some struc-tural effect, probably aggregation. This result contradictsthe conclusion drawn from the direct evaluation of primarydata of mechanical properties. The slope of the linesincreases with decreasing particle size as expected; this alsoverifies the validity of the model.

Moreover, if we plot B against the specific surface areaof the filler (Fig. 11), our doubts raised by Fig. 10 aboutpossible aggregation are further strengthened. Resultsobtained for uncoated and coated fillers are shown in thefigure. For uncoated fillers parameter B increases linearlyat small specific surface areas as predicted by Eq. (2). Atlarger surface areas, above 6 m2/g, B does not changeany more. Fillers form aggregates and a majority of their

10 100 1000

100

1000

10000

100000

Pro

pert

y, G

', G

" (P

a),

η * (

Pas)

Frequency (s-1)

0.1 1

Fig. 12. Effect of particle size on the linear viscoelastic characteristics ofPP/CaCO3 composite melts. (,) G 0, (n) G00, (m) g* at A f = 2.0 m2/g, (h)G 0, (s) G00, (d) g* Af = 12.3 m2/g.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

ln(r

elat

ive

yiel

d st

ress

)

Volume fraction of filler

0.0 0.1 0.2 0.3 0.4

Fig. 10. Tensile yield stress of PP/CaCO3 composites plotted in the linearform of Eq. (3). Coated fillers: (s) Af = 2.0 m2/g, (n) Af = 5.5 m2/g, (h)Af = 12.3 m2/g. Deviation from linearity at large specific surface area andfiller content.

10 15 200

1

2

3

4

Para

met

er B


0 5

Fig. 11. Dependence of the accessible surface (load bearing capacity) ofthe filler in PP/CaCO3 composites on its specific surface area. (h)Uncoated, (d) coated filler; influence of coating.


surface is not accessible by the polymer. A somewhat differ-ent behavior is shown by B parameters derived from themechanical testing of composites containing coated fillers.At large particle sizes B increases linearly also in this case,but the slope of the straight line is slightly smaller than foruncoated fillers. The decrease of slope can be explained bythe effect of ‘ and ryi, both of which change with interac-

tion, with the decrease of surface free energy. Above a cer-tain specific surface area a deviation from this initialstraight line can be observed here, too. However, insteadof reaching a plateau, B increases continuously even at lar-ger surface areas. This continuous increase indicates thatalthough some aggregates form at small particle sizes, thesurface of the filler accessible for the polymer also increasescontinuously. This increase led to the lack of change in thecorrelation presented in Fig. 9 and to the almost straightlines in Fig. 10. The less compact structure of the aggre-gates may also play a role in the better performance ofcomposites containing the coated fillers.

Rheology is often used for the characterization of poly-mer melts and for the detection of various structures inthem. The formation of aggregates might be detected bydynamic viscometry, by the measurement of the complexviscosity and other linear viscoelastic characteristics ofthe melt as a function of frequency [30,31]. Complex viscos-ity and the components of complex modulus (G 0 and G00)are presented in Fig. 12 for composites containing thecoated filler with the largest and the smallest specific sur-face area in 25 vol.%. The frequency dependence of rheo-logical properties clearly differs for the two samples. Bothoptical microscopy and the evaluation of parameter B indi-cated the occurrence of aggregation in the sample contain-ing the small particles. The increase of complex viscosity atsmall frequencies clearly supports these observations.However, it is very difficult to draw quantitative conclu-sions from the figure about the presence or extent of aggre-gation in the sample containing the large particles.

A different representation of the results, the so calledCole–Cole plot was shown to detect very sensitively the for-mation of higher order structures in polymer melts [30,31].

0 3000 6000 9000 12000

0

2000

4000

6000

Vis

cosi

ty,

η" (

Pas)

Viscosity, η' (Pas)

Fig. 13. Cole–Cole representation of the dynamic viscosity of PP/CaCO3

melts containing fillers with various sizes at 25 vol.%. (d) Coated,Af = 2.0 m2/g, (m) coated, Af = 12.3 m2/g, (s) uncoated, Af = 5.0 m2/g.


In this representation the imaginary part is plotted againstthe real part of a complex elastic property. The plot shouldbe a perfect arc if higher order structures are absent and therelaxation behavior of the melt can be described by a singlerelaxation time [32,33]. A broad relaxation time spectrumleads to the flattening of the arc, while structural effectsresult in the appearance of a second arc, a tail or anincreasing correlation [31]. The dynamic viscosity of threecomposites is plotted in this way in Fig. 13. The Cole–Coleplot is arc like for the composite containing the largestcoated particles, but deviates very strongly from an arcfor the composites prepared with the smallest particles.We can see that the upper part of the Cole–Cole plot ofthe viscosity of the composite containing medium sizeduncoated particles is similar to the one containing the smallcoated particles, i.e. the extent of aggregation is consider-able in this composite. These results confirm our conclu-sion presented above saying that considerableaggregation takes place in composites containing smallparticles even when they are coated with a surfactant. Rhe-ology is a very sensitive tool to detect aggregation, but fur-ther study and analysis is required for the determination ofquantitative relations among specific surface area, surfaceenergy and aggregation.

4. Conclusions

The results presented above prove that the unambiguousdetection of the presence of aggregation is difficult in par-ticulate filled composites. Coating of CaCO3 fillers with asurfactant changes the behavior of the particles consider-ably but does not eliminate aggregation completely. The

association of filler particles depends on the relative magni-tude of adhesion and separating forces and any results canbe explained only by the thorough consideration of theirrelationship. Gravitational forces are much smaller thanadhesion between either uncoated or coated particles, inspite of the fact that coating decreases the surface freeenergy of the filler significantly. Different forces act in sus-pensions used for the determination of the particle size dis-tribution of fillers; the shape of the distribution mayindicate the presence of aggregation. Coated fillers formmuch looser aggregates with more diffuse interphases, thanuncoated particles. Composite stiffness is completely insen-sitive to changes in structure or interaction, but the directevaluation of other tensile properties may also lead to erro-neous conclusions. Model calculations, oscillatory viscom-etry, as well as the proper representation of the resultsallow the unambiguous detection of aggregation.

Acknowledgements

The authors are indebted to Janos Moczo for supplyingdata on uncoated CaCO3. The research on heterogeneouspolymer systems was partly financed by the National Scien-tific Research Fund of Hungary (OTKA Grant No.T043517).

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