mechanical properties of plasma surface-modified calcium carbonate-polypropylene composites

Polymer International 42 (1997) 195È202

Mechanical Properties of PlasmaSurface-Modified Calcium

Carbonate–Polypropylene Composites

G. Akovali* & M. A. Akman¹

Departments of Chemistry and Polymer Science & Technology, Middle East Technical University, Ankara 06531, Turkey

(Received 9 July 1996 ; accepted 20 August 1996)

Abstract : Calcium carbonate was surface-modiÐed by plasma-polymerizedacetylene and the e†ect of surface modiÐcation on the mechanical properties ofcalcium carbonateÈpolypropylene composites was investigated. Two di†erentplasma polymerization conditions were selected and applied. Chemical structuresof plasma-polymerized acetylene products were identiÐed. Mechanical andthermal properties of the composites prepared were evaluated and the e†ects ofsurface modiÐcation on the extent of adhesion of Ðller to the matrix, as well ason polymer phase, were investigated with the help of scanning electron micros-copy. Some of the composite samples prepared with surface-modiÐed calciumcarbonate are found to yield higher percentage elongations and are mechanicallysuperior compared to those prepared with unmodiÐed Ðller.

Key words : plasma, surface modiÐcation, polypropylene composite

INTRODUCTION

Polymers are virtually never used alone, but always incombination with other materials. Materials added topolymers during processing may be Ðllers or other rein-forcing agents, stabilizers, plasticizers, pigments or otherproducts ; these are formulated into multicomponentcomposite systems with properties well suited for thespeciÐed application. Particulate Ðllers are added tothermoplastics, mainly to improve the physical proper-ties of virgin polymers, to help reduce the cost ofmoulded products and also to aid in tailoring thematerial to suit requirements. In general, particulateÐllers tend to reduce the toughness of the compound. Asthe degree of toughness is reduced, elongation at breakis, in general, also decreased.1,2 The properties of apolymer composite are mainly characterized by thematrix (polymer) and the Ðllers as well as by the inter-

* To whom all correspondence should be addressed.¤ Present address : TUBITAK (Turkish National and Scienti-Ðc Research Council), SAGEM Research Laboratories,Lalahan, Ankara, Turkey.

action between them. The nature of the interfaces andinterphases that can exist in these systems a†ects theperformance of the system as a whole,3 and their struc-ture is of the utmost importance.3h7 For example,proper surface treatment of Ðller particles can lead tobetter adhesion between polymer and Ðller and conse-quently to an improvement in the overall mechanicalbehaviour.

Incorporation of in polypropylene (PP) isCaCO3common practice in plastics technology to improve heatresistance, dimensional stability, sti†ness, hardness andprocessability of the matrix polymer. It has also beenshown that the same compound can help to avoidcatastrophic failure of the material at [30¡C.8 In theliterature, a number of di†erent coupling agents andmodiÐers are reported for this system, such as varioussilanes and titanates9 as well as stearic acid,10 EPDMrubber, polyethylene11 and others.12h18 All of theseshould be unique for the system, because when thee†ects of various coupling agents on mechanical proper-ties of highly Ðlled PP systems were investigated for dif-ferent inorganic Ðllers, i.e. for glass and CaCO3 ,di†erent results were obtained even if the same coupling

195Polymer International 0959-8103/97/$09.00 1997 SCI. Printed in Great Britain(

196 G. Akovali, M. A. Akman

agent was used with glass and systems.9CaCO3-ÐlledVirgin is shown to act as a weak nucleant inCaCO3PP, giving a higher fraction of lower melting point crys-tallites than unÐlled polymer, while an oligomer impu-rity at the polymerÈÐller interface lowers the nucleatingactivity.12

In this study, calcium carbonate was surface-modiÐedby plasma-polymerized acetylene, prior to compositeproduction, with the aim of increasing its wettabilityand surface adhesion to the PP matrix, and the e†ectsof this modiÐcation on both mechanical properties andcrystallinity were investigated.

EXPERIMENTAL

Materials

The polypropylene used (PP H 531F) was a product ofTiszai Vegyi Kombinat (Hungary) marketed under thetrade name of Tipplen.

The calcium carbonate used was ground limestonegrade of Tekno Kimya Koll.Sti (Turkey). It was dried at90¡C for 24 h and sieved in a shaker for 50 h ; theproduct was collected between 230 and 270 mesh sizes(53È63 km) before use.

Acetylene was purchased from Oxan AS and had99É9% purity. It was used without further modiÐcation.

Tensile tests and dynamic mechanical analysis werecarried out using an Instron TM 1102 and a DuPont981 unit, respectively. For di†erential scanning calorim-etry (DSC), a DuPont 910 system was used.

Sample preparation

Powder Ðller surfaces were modiÐed by use of a lowtemperature plasma system consisting of a Pyrex glassreactor (50 cm in length and 6 cm in diameter), and aPyrex glass sample tray (15 cm in length and 5É8 cm indiameter, shaped to just Ðt into the reactor) equippedwith a home-made magnetically operating mixingaccessory and two external copper electrodes(15 cm] 7É5 cm). The plasma system used (Tegalproduct, USA) had a Ðxed frequency (13É56 MHz) r.f.power source coupled with a matching network thatcould deliver up to 100 W. The system was equippedwith an electronic pressure manometer (Balzers TPG300 Total Pressure Controller), needle valves and Ñow-meters. A schematic presentation of the system is givingin Fig. 1.

For the plasma modiÐcation of calcium carbonatesurfaces with acetylene plasma, calcium carbonatepowder (c. 15 g) was spread in the Pyrex glass sampletray, keeping the average thickness at c. 2È3 mm. Priorto glow discharge, the pressure of the reactor wasdecreased to 0É2 mbar, followed by a Ñush of acetylenegas twice and a decrease of pressure again (to either 0É8

Fig. 1. Plasma system set-up.

POLYMER INTERNATIONAL VOL. 42, NO. 2, 1997

Mechanical properties of surface-modiÐed composites 197

Fig. 2. FTIR spectra of plasma-polymerized acetylene samples prepared under two di†erent plasma conditions.

TABLE 1. Conditions employed during plasma poly-

merization of acetylene

Condition 1 Condition 2

Acetylene flow rate (ml minÉ1) 110 250

Pressure (final) (mbar) 0·4 1·0

r.f. power applied (W) 6 8

Plasma duration (h) 2 2

or 1É8 mbar, respectively, for the two di†erent condi-tions employed ; Table 1). After reaching a steady Ñowregime of gas, a plasma was initiated. As soon as theglow was initiated, the pressure at the centre of thereactor was observed to decrease to values of 0É4 or1É0 mbar, respectively, and stayed constant at thesevalues throughout the process. The two di†erent steadyplasma conditions used were selected after initial opti-mization studies, and are presented in Table 1 as condi-tions 1 and 2.

During the plasma modiÐcation process, at intervalsof 20 min the glow discharge was cut o† and the Ðllerwas mixed magnetically from outside. During mixing,the Ñow of acetylene gas was allowed to continue.

After completion of the modiÐcation process 10, 20 or30% (w/w) calcium carbonate samples were loaded tothe PP. For this, PP was placed in a Brabender used ata rate of 60 rpm at 220¡C for 3 min and then calciumcarbonate was added and all were mixed for 7 min.Samples were compression premoulded at 210¡C for2 min at 30 atm and then used to prepare mouldedsamples at 210¡C for 4 min at 30 atm. All the samplesprepared were carefully annealed at 90¡C for 12 h. Thesame procedure was applied for all the other compositesamples prepared.

TABLE 2. Results of elemental analysis (% , w/w) of

plasma-polymerized acetylenes

Condition 1 Condition 2

C 76·5 ½0·3 57·7½0·3

H 8·0½0·3 5·5½0·3

N 0·0½0·3 0·0½0·3

O(Cal’d) 15·4 ½0·3 36·8½0·3

Fig. 3. Tensile stress versus strain plots of virgin PP and itscomposites with calcium carbonate.



Fig. 4. Storage shear modulus, G@, versus temperature plots ofvirgin PP and its composites with calcium carbonate.

To avoid the e†ect of ageing, freshly modiÐed calciumcarbonate samples were used in the preparation of com-posite samples throughout this work.

RESULTS AND DISCUSSION

Plasma polymerization products obtained were in theform of Ðlms for both conditions 1 and 2, being muchdenser in the latter case. As indicated above, subsequentto plasma initiation ; the pressure of the systemdecreased appreciably for both conditions, which isusually taken as an indication of the polymerization ofgas fragments.19 In fact, plasma polymerization of

Fig. 5. Loss modulus versus temperature plots of virgin PPand its composites with calcium carbonate.

Fig. 6. Tensile stress versus strain plots of PP compositeswith modiÐed calcium carbonate (10%).

acetylene is reported to be very rapid, yielding a smallamount of hydrogen gas20 and resulting in appreciabledecrease of pressure in the system. Plasma-polymerizedacetylene was shown to be in lightly or highly cross-linked solid states, as well as in oligomeric liquid states,depending on the plasma operational parametersemployed.21 In addition, the plasma polymer product isexpected to contain both hydrophobic and hydrophilicgroups with properties similar to that of the poly-propylene matrix and to the Ðller surfaces, and hencecan be e†ective in improving interfacial/interphaseproperties between Ðller particles and the matrixpolymer.





The plasma-polymerized acetylene Ðlm deposited onthe walls of the reactor (between the electrodes) wasused for IR spectroscopic and elemental analysis of theproduct, the results of which are presented in Fig. 2 andTable 2, respectively. There were di†erences, both physi-cal and chemical, between the products obtained by thetwo conditions, which may be due to the di†erences inthe amplitude of r.f. power, reactor pressure and acety-lene gas Ñow rate during the course of the plasma poly-merization.21,22 One common feature for the productsobtained under conditions 1 and 2 is the existence of anOH peak in the IR spectra, which is most likely due toreaction with traces of oxygen and/or water vapour. Anattempt was made to avoid the e†ects of ageing in thisstudy, since freshly prepared plasma-polymerized acety-lene samples are known to have high concentrations of

Fig. 9. Loss modulus versus temperature plots of PP compos-ites with 30% modiÐed and unmodiÐed calcium carbonate.

trapped free radicals (2É8 ] 1020 spins cm~2) and maygive rise to reactions with oxygen and water vapourleading to development of carbonyl signals (at 1692È1700 cm~1) and stronger OH signals (at3450È3475 cm~1). These signals are both traceable inthe plasma polymers prepared in this study despite allprecautions employed. Their intensities were found toincrease with time, showing that they were active forlong durations (even for 15 months21). In addition tothese two characteristic peaks, peaks were also observedfor CxC groups (1636È1653 cm~1) for both samples, aswell as a unique peak (at 1075 cm~1 for oxygen incorp-orated in the main chain backbone ; CwOwC) forplasma-polymerized samples prepared by condition 1only.

The tensile stress versus strain plots of virgin PP as

Fig. 10. Enthalpy of fusion values of various PP composites.



Fig. 11. Scanning electron micrographs of PP composites prepared with 10% (a) unmodiÐed, (b) modiÐed (condition 1) and (c)modiÐed (condition 2) calcium carbonate.

well as PP with 10, 20 and 30% (w/w) virgin calciumcarbonate-loaded samples at room temperature are pre-sented together in Fig. 3. As seen from the Figure, anincrease in the amount of unmodiÐed Ðller causes amonotonic decrease in the toughness, yield strength andpercentage elongation values of the system at high load-ings, as expected.

The temperature dependence of storage shear andloss moduli of the same samples (in the temperaturerange between [ 80 and ] 80¡C, with a heating rate of5¡Cmin~1 at a frequency of 10 Hz) are presented inFigs 4 and 5.

From the stress versus strain plots of composites pre-

pared with 10, 20 and 30% modiÐed Ðllers under thetwo di†erent conditions and compared with similarsamples containing unmodiÐed (Figs 6, 7 andCaCO38), it is seen that condition 1 is much more successful inincreasing the values of percentage elongation at breakin all cases (it is more than 50% for the case of 10%loading). There are some gains in the tensile strengthvalues as well. The glass transition temperatures ofcomposites abstracted from DSC scans (run with aheating rate of 5¡Cmin~1 which are not included here)seemed to be una†ected by surface modiÐcation of theÐllers (Fig. 9).

To gain some insight on the e†ect of modiÐcation of



Fig. 12. Scanning electron micrographs of PP composites prepared with (a) 20% unmodiÐed, (b) 20% modiÐed (condition 1), (c)20% modiÐed (condition 2) and (d) 30% modiÐed (condition 1) calcium carbonate.

surfaces on the crystallinity of matrix PP, a series ofDSC experiments were performed, details of which havebeen published elsewhere.23 Although no signiÐcantchange was observable in crystalline melting points ofunÐlled, unmodiÐed and modiÐed calcium-carbonate-Ðlled PP samples, as presented in Table 3, there weresome changes in the heat of fusion values obtained. Thetrend of decrease in enthalpy of fusion, which is the casefor unmodiÐed and modiÐed (condition 1) calcium car-bonate samples, is not unexpected owing to the possi-bility of distortion in the crystalline structure of PP

matrix resulting in the possible change of orientation ofcrystallites with added amount of Ðller, (Fig. 10).However the increase of enthalpy of fusion observed forthe systems Ðlled with calcium carbonate modiÐed bycondition 2 is opposite to this, which is a new Ðnding.One can speculate that this results by renucleation bythe new surfaces created. X-ray studies of the compos-ites and the e†ects on crystalline structure and textureare underway. The results will be presented in anothercommunication.

Scanning electron microscopy was used to investigate



TABLE 3. Values of the heat and temperature of

fusion

Compound Heat of fusion Temperature

(J gÉ1) (¡C)

Virgin PP 1166 160

Unmodified

10% CaCO3

1067 159

20% CaCO3

1156 158

30% CaCO3

1102 159

Modified (condition 1)

10% CaCO3

1100 159

20% CaCO3

1132 159

30% CaCO3

1164 159

Modified (condition 2)

10% CaCO3

1237 158

20% CaCO3

1200 158

30% CaCO3

1186 159

the tensile fracture surfaces of the composites and arepresented in Figs 11 and 12. As can be seen, the surface-treated calcium carbonate is covered better with thematrix, while untreated calcium carbonate is displacedby the matrix. Comparison of the performance of com-posites prepared by use of Ðllers surface-modiÐed underthe two di†erent plasma conditions shows that thematch with the matrix is better in the case of Ðllerstreated under condition 1.

CONCLUSIONS

The following conclusions may be drawn.

(1) The surfaces of calcium carbonate powder canbe modiÐed by plasma-polymerized acetylene,which can improve certain mechanical propertiesof Ðlled polypropylene.

(2) Depending on the di†erences in plasma-operational parameters used, surfaces with di†er-ent characteristics can be obtained, which cana†ect the crystal structure of the matrix in di†er-ent ways : they can distort the crystal structure,or even increase the percentage of crystallinity.

(3) One of the conditions (condition 1) applied tothe Ðller surface during plasma polymerizationyielded a better composite system with increasedtoughness compared with the unmodiÐed Ðller.

ACKNOWLEDGEMENTS

We gratefully acknowledge Tekno Kimya AS/Tu� rkiye(Mr S. Berkman, manager) for the generous supply ofcalcium carbonate and polypropylenes used in thisseries of studies. The partial support of METU-APFproject No. 93.01.03.07 is also gratefully acknowledged.

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mechanical properties of plasma surface-modified calcium carbonate-polypropylene composites

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