ac and dc dielectric properties of some polypropylene/calcium carbonate composites

AC and DC Dielectric Properties of Some Polypropylene/ Calcium Carbonate Composites

GYORGY BANHEGYI* and FRANK E. KARASZ

Polymer Science and Engineering Department University of Massachusetts

Amherst Massachusetts 01 003 and

ZORAN PETROVIC

Institute for Petrochemistry, Gas, Oil and Chemical Engineering Faculty of Technology Novi Sad, Yugoslavia

AC dielectric properties and thermally stimulated polarization (TSP) and depolarization (TSD) currents were studied in a series of CaC0,-filled polypropylene composites. The filler content (0 to 50 weight percent) and the average particle size (3.0 to 16.1 pm) at constant filler content (30 weight percent) were varied in separate groups of samples. In a third group of samples the filler (20 to 40 weight percent) was surface treated with stearates. The AC dielectric behavior of composites containing untreated fillers is largely determined by a small amount of adsorbed water. Upon heating, the dielectric properties show maxima (increasing with decreasing frequency) which disappear on cooling. In the case of stearate- treated fillers the dielectric loss level is higher, the dispersion and loss curves on heating reflect a combination of dipolar and protonic processes with water desorption. In the dry state the onset of an audio frequency relaxation process is observed in the pre-melting zone. The thermally stimulated currents of the composites containing treated and untreated fillers are also different. In the case of the untreated fillers the TSP curves show maxima indicating water desorption which are increasingly intense and roughly exponential with filler content. The high temperature conductivity and the intensity of the pre-melting depolarization peak pass through a minimum as a function of filler content. Above 20 weight percent filler content the activation energy of high temperature conductivity decreases. In the case of the surface treated samples, the thermally stimulated response is different for "wet" and dried samples. The dry samples exhibit a relaxation between the amorphous and crystalline transitions of the matrix polymer which is probably due to interfacial relaxation caused by the enhanced surface conductivity of the stearate-treated fillers.

INTRODUCTION

mprovements in the properties of low cost, large I volume commodity plastics by addition of mineral fillers has long been used in industry and recent technologies of surface treatment and chemical cou- pling have given properties to polyolefin composites comparable to those of engineering plastics. Because of their direct relevance to processing and perform- ance in structural applications, rheological and mechanical properties of mineral filled composites have

* Bio-Pharm. Ltd.. Research & Development, Konyves Kalman K r t . 76. 1087 Budapest, Hungary.

been widely studied ( 1). Electrical properties, however, also deserve attention in part because filled polymers are used as insulators and in part because dielectric relaxation provides further information about their structure.

In this paper the AC dielectric properties (permittivity and loss) and the thermally stimulated DC properties (thermally stimulated polarization, TSP, and thermally stimulated depolarization, TSD) will be described with special attention to adsorbed moisture, surface treatment, filler particle size, and volume fraction. The dynamic mechanical, thermal, and morphological properties of these systems will be described elsewhere (2).

374 POLYMER ENGINEERING AND SCIENCE, MARCH 1990, Vol. 30, No. 6

Polypropylene/Calcium Carbonate Composi tes

0

I I I 1 I

EXPERIMENTAL

Sample preparation

The polypropylene (PP) used in this study, HIPO- LEN-MA3 (Hipol, Odzaci, Yugoslavia), had a melt flow rate of 11 g/lO min. The polymer was mixed with four types of fillers. Three (Supermikrokalcit, Mik- rokalcit, and Kalcit) were not surface treated. The fillers were produced by Industrogradnja-Licko Lesce, Zagreb, Yugoslavia. The average particle sizes were: Supermikrokalcit (SMC), 3.5 pm: Mikrokalcit (MC), 6.6 pm: and 16.1 pm for Kalcit (C). Samples with 0, 5, 10, 20, 30,40, and 50 weight percent SMC were prepared. For comparison two composites with 30 weight percent MC and C fillers were also prepared.

The fourth filler, Polcarb S is an ultrafine calcium carbonate, with 80 percent of its particles finer than 2 pm and 40 percent finer than 1 pm. produced by ECC International Ltd., England. The particles were coated with 2 weight percent stearate. Samples containing 20, 30, and 40 weight percent Polcarb S were studied. The stearate coating usually improves the rheology of the filled polymer melt and facilitates the mixing process (3).

The fillers were mixed with the polymer at 180" for 5 min in a Haake Rheomix internal mixer, model EU- 5, at 45 rpm. The mixture was then compression molded into 0.5 mm by 3 mm thick sheets using the following protocol: preheating 2 min at low pressure, compression molding for 2 min at 230°C and with a pressure of 7 Mpa, cooling in 2 min, under a pressure of 12 Mpa. The electrode and the guard ring were deposited on 5.5 cm diameter by 0.5 mm thick sheets by vacuum deposition of aluminum.

Measurements

AC dielectric measurements were performed in the 100 Hz to 100 kHz frequency range using a Hewlett- Packard 4274A type automatic RLC bridge: some measurements were made using a Polymer Labora- tories DETA system which contains a GenRad 1869 M type automatic bridge. The temperature of the sample holder could be controlled (the usual heating rate was 2"C/min) and was flushed with dry Ar or Nz gas. The thermally stimulated current curves were recorded with an amplifier system built by Hedvig (4). Currents between lo-@ and A could be detected with time constants of less than 1 s. Cur- rents and temperature were recorded with an X-Y recorder: the usual heating and cooling rates were f 2"C/min.

Results

Typical AC dielectric behavior of the PP/CaC03 composites containing untreated fillers is shown in Figs. 1 and 2 for a sample containing 40 weight percent SMC filler. At the highest measuring frequency, t' decreases in a roughly linear fashion with temperature, which is a consequence of the thermal dilatation of the sample. The dispersion (the fre-

I W

\ \ i

Y

W

0 . 0 0 6 I

I- 1 kl I7 \

\ \

Fig. 2. Dielectric loss curves of the 40 we igh t percent PP/ CaC03 s a m p l e in Fig. 1 .

POLYMER ENGINEERING AND SCIENCE, MARCH 7990, Yo/. 30, No. 6 375

G. Banhegyi, F. E. Karasz, and 2. Petrovic

0 005

- N 0004. - I Y - - - 0 0 0 3 - 0

0 0 0 2

quency dependence of the dielectric permittivity) is large at lower temperatures and practically dimin- ishes above about 70°C (see Fig. I ) . The dielectric loss shows a characteristic but unusual temperature dependence (Fig. 2): it passes through a maximum at a specific temperature and is nearly independent of the measuring frequency. The loss maximum, however, increases considerably with decreasing frequency. If the permittivity difference between two permittivity curves (which is a measure of dielectric dispersion) is plotted as a function of temperature (see Fig. 3). the qualitative picture is similar to that shown by the losses (d'). This plotting technique is applicable if the direct loss representation is not available or is disturbed by other factors (5). Figure 3 also shows the values obtained on cooling (dashed lines): the peaks observed on heating disappear. If the filler content dependence is studied in the series filled with the finest untreated (SMC) filler, then the high frequency limiting permittivity increases with the filler content, as does the dispersion peak around 40°C (see Fig. 4). The latter can hardly be seen in the sample containing 20 weight percent SMC filler. The loss curves are similar to the dispersion curves, but contain more noise. The dielectric loss curves of the

-

O o 7 I

I t \ I

20 30 LO 50 60 70 80 90 100 110 12( 0 I I I I I I I 20 30 LO 50 60 70 80 90 100 110 120

TEMPERATURE ( " C )

Fig. 3. Dielectric dispersion curves (difference between the lowest and highest measured frequency perrnittivi- ties) of the 40 weight percent PP/CaC03 sample shown upon subsequent cooling.

i -O

TEMPERATURE I OC 1 CaCO, WEIGHT %

Fig. 4. Dielectric dispersion (left) and high frequency limiting permittivity (right) at dgferent concentrations of the

finest size untreated filler.

samples rmtaining 30 weight percent untreated fillers with different average diameters are compared in Fig. 5. In the case of the smallest particles the loss peak is clearly visible, while for the largest particles the loss peak practically disappears.

The dielectric response of the samples containing stearate treated (Polcarb S) filler is characteristically different. This is shown in Figs. 6 and 7 for a sample filled with 30 weight percent Polcarb S. The permittivity and dispersion observed in the first heating of samples previously stored under ambient conditions are higher than in samples filled with 30 weight percent untreated filler. The temperature dependence of the dispersion is also different from that of the SMC composites. In the case of samples containing the Polcarb S filler there is no sudden drop in the dispersion above 70°C. On cooling, a continuously decreasing difference between the 120 Hz and 100 kHz permittivities can be observed. The temperature dependence of the loss curves is also unusual (Fig. 7). At the higher frequencies one broad maximum is observed, at lower frequencies two maxima, but the relative position does not correspond to that expected in the case of a dipolar relaxation process (4). On cooling the behavior is much simpler: the loss mon- otonically decreases at each frequency, the loss value is higher with lower frequency. Figure 8 compares the permittivity and loss curves of the samples filled with 20, 30, and 40 weight percent Polcarb S filler, measured at 1 kHz. The curves are qualitatively similar but the loss intensities and the permittivity differences between the heating and cooling curves are not a linear function of the filler content. The permittivity values observed on cooling do not depend appreciably on the temperature at 1 kHz, and they are comparable to the high frequency permittivities observed for samples containing similar amounts of untreated fillers (cf. Fig. 4, right).

The thermally stimulated DC properties are also of interest. Thermally stimulated polarization (TSP) curves (which are the superposition of ohmic conductivity and dipolar polarization) were measured in the following way: the samples were polarized for

0 006 I 'V 30% C a C 0 2

o 001 I I 1 I 1

20 40 6 0 80

TEMPERATURE ( 'C 1 Fig. 5. Dielectric loss curves measured at 1 kHz for sarn- ples containing 30 weight percent of untreated CaC03 fillers with 3.0. 6.6, and 16.1 pm average diameters.

376 POLYMER ENGINEERING AND SCIENCE, MARCH 7990, Yo/. 30, No. 6

Pol ypropylene/Calcium Carbonate Composites

M wt %

100 k H r 1

2 5 L L I- L - 1. u- 20 30 10 50 60 70 80 90 1CO 110 120 130 li0

Fig. 6. Dielectric permittivity curves of a PPICaCO, composite containing 30 weight percent stearate-treated filler, between 120 H z and 100 kHz . The sample was stored under ambient conditions before the measure- ment. Dashed lines denote the values measured upon subsequent cooling.

010 - I

i t

005 -

I nr j-

\ \,,/' 1 kHz

"" 20 32 Lo 50 60 70 m 90 100 110 -20 -30

Fig. 7. Dielectric loss curves of the PPICaC03 sample containing 30 weight percent stearate-treated f i l ler shown in Fig. 6.

25- I " " " ' 1 ' 1 20 40 60 80 I00 20 40 60 80 100

TEMPERATURE ("C)

Fig. 8. Dielectric permittivity (c') and loss [c") curves measured at 1 kHz on heating (-1 and on cooling (- - - - -1 for samples containing dvferent amounts of stearate- treated filler. (All filler contents given in weight percent).

about 30 min at room temperature at a given voltage level, and then current was detected under the effect of this field for a constant heating rate (2"C/min). Figure 9 shows the results for samples filled with

c 250V, "dry"

0 100 I50 5 0 I , I I I , I J

50 100 150

TEMPERATURE ("C)

Fig. 9. Thermally stimulated polarization currents in reduced units measured under DC conditions, 2'C/min heating rate for composites containing different amounts of the finest untreated filler. Right: "wet" samples [previously stored under ambient conditions). left "dry" samples [dried at 125°C for 1 hj. Note the duerence in the polarization voltages in the two cases.

different amounts of the finest (SMC) untreated filler. The polarization currents are plotted in reduced units, which, in the case of ideal ohmic conductivity and linear dielectric relaxation is independent of the applied field strength. On the left hand side of Fig. 9 the results for the sample stored under ambient conditions are shown, the polarization voltage is 1 kV. On the right hand side results for some previously dried (1 h at 125°C) samples are shown at a lower polarization voltage (250 V). A reduction of the polarization voltage seemed to be necessary; when a 1 kV polarization voltage was applied to the dried samples, high electrical impulses, presumably due to partial discharges at the polymer/filler interface, appeared. If the thermally stimulated polarization curves of the "wet" (not dried) samples are compared, some tend- encies are clear: a) there is a current peak with exponentially increasing intensity between room temperature and 100°C in the samples containing more than 20 weight percent filler: b) the high temperature polarization current above 100°C suddenly drops from the value characteristic of pure PP to a much lower value on addition of 5 to 10 weight percent CaC03 filler: and c) there is a further drop and a change in the high temperature activation energy if the filler content is increased to 20 weight percent, but at higher filler loadings the high temperature polarization current again increases. In the case of the dried samples the peak below 100°C clearly disappears, the high temperature conductivity again goes through a minimum as a function of filler content. However, there are differences in the reduced current values at high temperature as compared to the "wet" samples, indicating nonlinear electrical behavior and/or conductivity mechanism differences arising from the presence or absence of even a very small amount of adsorbed water.

The thermally stimulated polarization curves of non-dried samples containing 30 weight percent filler of different average diameters are compared in

POLYMER ENGINEERING AND SCIENCE, MARCH 1990, Vol. 30, No. 6 377

G. B a n h e g y i , F. E. Karasz , a n d 2. Petrovik

3 2 -

- v) - 3 6 - - \ - - W

\ - 4 0 - W0

.- - v -

- 4 4

Fig. 10. The current peak can be observed at the two smaller diameter values only, in the third case only a change in the current slope is apparent in the semi- logarithmic representation. I t is interesting to ob- serve that the order of polarization current curves is reversed at the lowest and highest temperatures.

The thermally stimulated depolarization (TSD) curves were studied under the following conditions: the samples were heated to 125°C. and an electrical field of 1 kV was applied for 30 min. The samples were cooled to -50°C in the presence of a high voltage field at a rate of -2"C/min. The field was then re- moved and the short circuit current was detected under a constant heating rate (2"C/min). In the cooling cycle dry Ar gas was used to prevent moisture precipitation. The depolarization current curves obtained from samples containing different amounts of the finest (SMC) untreated filler are compared in Fig. 1 1 . There are two groups of transitions: one below room temperature and another around 100 to 120°C. The most conspicuous change upon the addition of fillers is the order of magnitude drop in the intensity of the high temperature relaxation peak at 5 to 10 weight percent filler contents. The intensity of the depolarization current around 100°C again increases on further addition of filler. The current intensity at lower temperatures increases continuously with filler addition both below room temperature and between room temperature and the high temperature peak. The depolarization curves of samples with 30 weight percent filler of different average sizes are compared in Fig. 12. The differences are insignifi- cant in the high temperature region but it appears that the lower temperature transition is less structured in the case of the coarse fillers.

Finally the thermally stimulated DC response of samples filled with the stearate-treated Polcarb S filler was studied. Figure 1 3 shows the thermally stimulated polarization (TSP) curves of samples previously stored under ambient conditions ("wet" samples, dashed lines) together with those of samples

I , I I 1 I I -

16. I

/;.~ 6 6 .A 0

0 ' 0 - U ,e' a.-n,*- - 3.0-

IJm -

P:,' @,/ o;Y 4,'

- ,',* , Q' 6'

A' I K V "wet ' ' , 3 0 % .

I I 1 I I

- 2

h

In \ - -3 -

- Y \ .- Y

- 4 0 0 -

- 5

-6

TEMPERATURE ( "C Fig. 1 1 . Thermally stimulated depolarization currents measured in PP/CaC03 composites containing different amounts of the finest untreated filler, previously polarized at 125°C. 30 min, Up,,( = 1 kV.

- 5 t

T ("C) I I u

~ 50 0 K, 102 150

Fig. 12. Thermally stimulated depolarization currents measured in PP/CaC03 composites containing 30 weight percent untreated fillers of dqferent average sizes. Polar- ization conditions as in Fig. 1 1 .

dried at 125°C. 1 h. In the latter case lower polarization voltage was applied (250 V) to avoid partial discharges and the initial temperature of the TSP meas- urement was lower (-50°C). If one compares the polarization currents of "wet" samples to those of the samples containing similar amounts of untreated fillers (Fig. 9) one can see that current peaks of the samples filled with stearate-treated CaC03 are higher than those filled with equal amounts of untreated filler. The intensity of the peak observed for the 40

378 POLYMER ENGINEERING AND SCIENCE, MARCH 1990, Yo/. 30, No. 6

Polypropylene/Caiciurn Carbonate Composites

I I 1 I I 1 40 80 I20

TEMPERATURE ( OC

Fig. 13. Thermally stimulated polarization currents in PP/CaC03 composites containing dqferent amounts of stearate-treated filler. For the meaning of “wet” and “dry” states see the legend of Fig. 9.

weight percent Polcarb S sample corresponds approximately to that for the 50 weight percent SMC sample). The other important difference is that after reaching a maximum value, the current for the Pol- carb S filled samples drops continuously instead of exhibiting a minimum as in the SMC filled samples. The thermally stimulated polarization behavior of the dry samples is basically different from that observed in the “wet” samples; in this case two broad peaks can be observed, which are similar to the depolarization curves measured for the same samples after polarizing for 30 min at 125°C in a 1 kV field (see Fig. 14) . If the polarization and depolarization curves of the dry samples containing 20 and 40 weight percent Polcarb S filler are compared on a linear scale (Fig. 15) one can see that they are qualitatively similar for the same kind of sample but there are quan- titative differences. In the case of the 20 weight percent Polcarb S sample the TSP curve is more intense than the TSD one in the whole temperature range, while in the case of the sample containing 40 weight percent Polcarb S the TSP current is higher than the TSD in the case of the lower temperature peak and the situation is reversed at higher temperatures. This again indicates nonlinear field strength dependence or differences in the polarization and depolarization processes.

DISCUSSION

The AC dielectric properties of polypropylene have been extensively studied (6-8). PP is a n essentially apolar material with a low inherent dipole moment (4). exhibiting dielectric relaxation peaks mainly as a consequence of slight oxidation and/or the presence of small amounts of polar additives (8). In the audio frequency range, PP shows two dielectric loss peaks in the temperature range covered in the present study. They appear around 30 and 120°C and the loss maximum is on the order of The peak amplitude depends on the crystallinity, oxidation level, and on the concentration of polar impurities

1

40

\- - : I I I I 7

0 40 80 I 2 0

TEMPERATURE ( “C )

Fig. 14. Thermally stimulated depolarization currents measured in composites containing dijferent amounts of stearate-treated fillers. Polarization conditions as in Fig. 1 1 .

I O L I I I I I I I

TSD, I K V , 125°C

TSP, 250V 40 -- YO -

, I I 1 I 1 I I I 1

TEMPERATURE ( “ C 0 40 8 0 I 2 0

Fig. 15. Comparison of the measured TSP and TSD curves in reduced units on the linear scale for d r y composites containing 20 and 40 weight percent stearate- treated filler.

and additives in the sample. Unfortunately the in- strumentation used in the present measurements could not detect such low losses with sufficiently high precision and reliability so deviations from the pure PP sample in the composites could be detected only when the loss level reached about 5 X to 5 x lo-*. Below this range the frequency dependence of t‘ could be used for comparison.

The AC dielectric characteristics of mineral filled apolar polymers have been studied by only a few groups. Hakim and coworkers have investigated the frequency and volume fraction dependence of t ’ and t” in mineral filled apolar elastomers (9-1 l) , Wer-


G. Bdnhegyi, F. E. Karasz. and 2. Petrovic

theimer, et al. have studied mica/polyethylene composites in the frequency domain (12-14). while the present authors have performed some frequency and temperature domain measurements on CaC03/ polyethylene composites (1 5- 17). A common obser- vation in all these cases was the importance of adsorbed water in determining the dielectric properties. I t has been observed in all cases that drying the samples decreases the loss and dispersion of the composites. In some cases the loss shows a maximum as a function of frequency (9, 11). in other cases it is a superposition of a loss process and a dispersive transport process (1 3- 15). In still other cases no loss peak is detectable as a function of frequency (15- 17). only a dispersion and loss curve increasing with decreasing frequency is detected, which can be described by the “universal dielectric response function” (1 8). Electrical losses in wet heterogeneous systems may originate from several processes such as dipolar relaxation of the adsorbed water molecules, protonic hopping processes, and ohmic interfacial relaxation (see e.g. Ref. 19). In the presence of water- soluble ions the interfacial relaxation process can be complicated by diffusion-controlled relaxation of the ion-atmosphere at the interfaces (20).

The temperature dependent dispersion and loss observed in the present PP/CaC03 composites with no surface treatment (Figs. 1 to 5) are similar to those observed for polyethylene/CaC03 systems (1 5- 17) where the filler was treated with apolar titanates. The isothermal dielectric response of these systems can be approximately described: (1 7)

t‘(w) - i t ” (w) = 6- + B(iT@)”-’ (1)

where tm is the high frequency limiting permittivity, B is a proportionality constant which increases with increasing adsorbed water content, w = 2av is the angular frequency, u is the frequency, T is a thermally activated time constant, 0 < n < 1 is a slowly varying function of frequency and water content. We have interpreted (1 7) this type of response in terms of the “anomalous low frequency dispersion” theory (2 1 ) but it must be mentioned that Goto and Kawai (22) explained similar phenomena observed in glass filled epoxies in terms of Sack’s theory (23, 4) which is based on proton hopping along hydrogen-bonded molecular chains of various length and orientation. At present it is difficult to decide between models because of the complex structure of the specimens and because of the lack of knowledge about microscopic surface conditions between the matrix and the filler. In the case of apolar matrices (such as PP) it is certain that polar water molecules can agglomerate along the matrix-filler interface only, since the solubility of water in the matrix is close to zero. The maximum of the temperature dependent loss (Figs. 2 and 5) and dispersion (Figs. 3 and 4 ) curves is the result of two opposite effects: T in Eq 1 decreases on heating (shift- ing the curve towards higher frequencies thus increasing t’ and 6”). while the desorption of water sharply decreases B which reduces dispersion and loss. Initially the first effect prevails, at higher tem-

380

peratures the second. This explains why the value of the maximum loss and not its position depends on the measuring frequency (see Fig. 2) as would be expected in the case of a normal dipolar mechanism (4). The dispersion and loss maxima, of course, disappear on cooling, because there is practically no adsorbed water left in the system. The original dielectric response again appears if the sample is stored under ambient conditions for a few days. A comparison of the dielectric data in composites containing different amounts of untreated filler with particles of the same average diameter (Fig. 4 ) shows that while the higher frequency limiting permittivity increases according to a moderately nonlinear function describable by heterogeneous mixture formulae (4, 16, 24), the dispersion [and loss) curves increase almost exponentially with the filler content. This is in qualitative agreement with our earlier findings (16) in PE/CaC03 composites. If the temperature dependent loss curves of composites containing the same amount of filler with different particle sizes are compared (Fig. 5) the result can be again qualitatively understood in terms of water adsorption: the lower the particle diameter, the higher the specific surface, the amount of adsorbed water, and, consequently, the greater the magnitude of the dielectric response.

The temperature dependent AC behavior of the composites containing stearate-treated Polcarb S filler (Figs. 6 to 8) is more complex. The role and desorption of water is obvious from the asymmetries appearing on the curves upon heating and cooling, but the electrical response is no longer describable by a simple expression such as Eq 1. The loss curves on heating reflect a complex combination of dipolar relaxation processes (presumably water and stearate groups) protonic and interfacial processes. The shape of the loss curves is characteristic and self-similar in samples containing 20, 30, and 40 weight percent stearate-treated filler (see Fig. 8) but interpretations based on this set of data is not possible. The AC dielectric data of the same samples upon cooling is much simpler. The loss and dispersion are fairly small a t room temperature and increase toward higher temperatures. Below the melting point of the polymer matrix no loss peaks are observed in the 100 Hz to 100 kHz frequency range, but the increasing dispersion at high temperatures clearly indicates that the increasing loss is not simply due to the onset of ohmic conduction. A relaxation may appear at higher temperatures which could not be detected because of the melting of the sample. This peak may be due either to an interfacial relaxation process (induced by enhanced surface conductivity of the surface- treated filler particles) or it may be related to the molecular relaxation of the polar molecules on the filler surface.

The temperature dependent DC conductivity (25- 28) and the short-circuit thermally stimulated depolarization behavior (29-35) of polypropylene have been studied previously. In step-response DC conductivity measurements around room temperature, the stationary, equilibrium conductivity level cannot be

POLYMER ENGINEERING AND SCIENCE, MARCH 7990, Vol. 30, No. 6

Polypropylene/Calciurn Carbonate Composites

reached in a few hours, the current shows a t-" type time dependence, Curie-von Schweidler law (18). The temperature dependence of the isochronal current values follows that of the thermally stimulated depolarization current (33). Changes in the activation energy of the conductivity were observed in some cases (25, 28, 34) around the mechanical softening point but the magnitude and even the sign of this change was not uniform among the various samples. Some authors interpreted the DC conductivity of PP as a result of impurity ion migration (25), while later measurements were interpreted in terms of an elec- tron-injection process from the electrodes (27, 28). Depolarization current measurements revealed the presence of three main relaxation regions in accord- ance with AC dielectric and mechanical results (4). The lowest temperature (so called y) relaxation appears below -1OO"C, which is attributed to local mo- tions of the oxidized chain or polar additives (used as antioxidants). The second, 0 transition around 0°C is interpreted as the glass transition of the amorphous phase of the semicrystalline polymer. Some authors have reported (30) that this transition can be resolved into two subcomponents. In the present case this relaxation in fact shows some structure in the pure PP sample and in the composites containing the finest (SMC) untreated filler (see Fig. 11). The intensity of the 0 relaxation peak can be enhanced by oxidation (29). The third, so-called a, relaxation is known as a crystalline or pre-melting relaxation process. As shown by morphological (36) and by combined mechanical and rheo-optical(37) measurements, in this temperature range the molecular mobility considerably increases, secondary crystallization, crystal- crystal transformation, spherulite growth, and crystal perfection become possible. Thus the morphology of the sample has a strong effect on the intensity of the ac peak as measured by the thermally stimulated current technique (31, 35). In fact, the a, peak itself seems to be a composite peak with at least two sub- processes: one around 80°C and another around 1 10°C. The relative intensity of these sub-processes is a function of crystallinity, spherulite size (31, 35), or processing conditions (34). It is usually assumed that the a, peak is due to the release of injected charge carriers from traps at crystalline defects or crystalline/amorphous interfaces.

In the present results, the TSP curve of the pure PP sample is comparable to the conductivity curves reported by other authors (25-28) taking into account the order of magnitude variation between different grades of polypropylene. The slope of the log Iltem- perature curve changes around 70"C, the apparent activation energy decreases. A comparison of the TSP and TSD curves in polypropylene shows that below the ac relaxation region the temperature dependence of the polarization and depolarization currents is similar and above the depolarization peak temperature the activation energy of the conductivity decreases. Our interpretation is that below the a, peak temperature the polarization curve is mainly gov- erned by the a, relaxation process and above this

POLYMER ENGlNEERING AND SCIENCE, MARCH 1990, Vol. 30, No. 6

temperature the charge transport process through the sample (with different activation energy] becomes dominant. According to some preliminary results (38) the application of blocking Teflon layers between the PP sample and the electrodes does not appreciably change the intensity of the a, peak in polypropylene, thus it is probable that it is caused by internal rather than injected charge carriers.

The thermally stimulated current peak appearing around 60 to 70°C upon polarizing the composites containing untreated filler particles (Fig. 9) can be understood in a manner similar to the low frequency dielectric relaxation peaks. The thermally stimulated current peak is a result of increasing mobility and gradual desorption of adsorbed water (1 7). In similar- ity to the AC dielectric dispersion and loss peaks (Fig. 4 ) the intensity of this thermally stimulated current peak increases roughly exponentially rather than linearly with filler content. A rather surprising effect is a drastic decrease in the high temperature conductivity on the addition of CaCO, filler to PP (see Fig. 9). At first (5 to 10 weight percent filler) the activation energy in the high temperature range does not change sharply, only the current drops by about a 1 to 1.5 order of magnitude. This is not expected, since the conductivity of imperfect ionic lattices is usually higher than that of apolar polymers, and even if the conductivity of the filler is lower than that of the matrix, at low concentrations a minor effect is expected. The only reason can be an interaction between the filler and the charge carriers either directly (e.g. by the adsorption of impurity ions) or indirectly (e.g. by changing the morphology of the sample thus changing the trap distribution and the mobility of charge carriers). Thermal and morphological studies on PP/chalk systems (39) indicate a nucleating activ- ity of CaC0, on PP, which can change trap distribution. The high temperature polarization current further drops at a 20 weight percent loading level (the activation energy also decreases) but at even higher filler contents the conductivity again increases. The picture is further complicated by the fact that the previously dried samples behave somewhat differ- ently. The effect of CaCO, filler on the thermally stimulated polarization currents of ultra-high molecular weight polyethylene is different (1 7). In this case a transient peak can also be observed due to water desorption but the high temperature conductivity consistently increases with filler content in the 0 to 50 weight percent range. If the polarization currents of composites containing fillers of different average sizes are compared (Fig. 10) at 30 weight percent filler loading then in the low temperature range (where the current is determined by the adsorbed water) the current decreases with decreasing surface area. At high temperatures, however, the finest filler causes significantly lower conductivity.

In the depolarization regime (Fig. 1 1 ) the intensity of the aC peak also passes through a minimum around 10 weight percent filler content. Above 20 weight percent filler content the intensity of the depolarization current in the 0 relaxation range consistently

38 1

G. Banhegyi, F. E. Karasz, and 2. Petrouid

increases. The changing slope of the semilogarith- mically plotted depolarization current-temperature curves in the a, peak region is consistent with the changing activation energies observed on the polarization current curves, thus they also reflect the change of the trap-distribution. In earlier TSD studies of mica filled epoxy resin (40), mica filled polystyrene (41). glass filled polypropylene (32), silica and zeolite filled ethylene-propylene rubber (42), and in CaC03 filler polyethylene (1 7) the depolarization current proved to be higher in the composite than in the matrix resin. This phenomenon has been explained in terms of the interfacial polarization effect (40, 41) and is due to the conductivity difference between the components. This effect, however, cannot explain the decrease of conductivity and depolarization currents in the a, range, observed in our systems containing untreated filler. These data indicate that there are probably two, competing effects: one is decreasing the high temperature conductivity and the intensity of the aC peak (this is related to the number and mobility of the charge carriers) and the other (possibly the interfacial polarization mechanism) increases these parameters. At low filler contents the first effect is more important, while at higher loading levels the second takes precedence. Nevertheless, both effects are present simultaneously and because of possible morphological changes in the polymer matrix they probably cannot be combined linearly.

The thermally stimulated DC behavior of composites containing stearate-treated fillers (Figs. 13 to 15) seems to be even more complex. The first com- plication is the qualitative difference between the thermally stimulated polarization behavior of the samples in the “wet” and dry states (Fig. 13). First the normal TSP curves starting from room temperature were studied. Here (as in the case of the untreated fillers) a current peak appears which might be associated with water desorption, but in this case the peak intensity is higher than in the composites containing similar amounts of untreated fillers. The other important difference is that after reaching the peak, the polarization current decreases continuously up to 100°C and above: there is no “conductivity tail” as in the case of untreated fillers (cf. Fig. 9). If, however, the polarization begins in the dry state, the samples exhibit two polarization peaks which are very similar to the corresponding depolarization current peaks (cf. Fig. 14). The higher temperature peak seems to be structured, i.e., it has two components around 80 and 120°C which is probably related to the a, relaxation of the PP matrix. The relative intensity of the sub-processes changes with the filler content. The lower temperature peak around 30°C can be due either to a dipolar relaxation process of the stearate groups or to a n interfacial relaxation process caused by the enhanced surface conductivity of the stearate-treated filler. Because of its higher intensity it almost totally suppresses the p process of the polypropylene matrix. This peak cannot be observed inthe “wet” state probably because in the presence of water the low frequency relaxation process is domi-

382

nated or altered by the protonic relaxation. The low temperature relaxation peak was not observed in the AC dielectric spectra of the dry Polcarb S filled samples but this can be understood when the large effective frequency difference is taken into account. The effective frequency range of the thermally stimulated current technique is to [see Ref. 4). so assuming an activation energy of 40 to 50 kJ/mole for the relaxation process (which is reasonable if low molecular weight compounds are involved) the transition would shift from 20 to 30°C (observed in TSD) to the melting range (1 40 to 1 6OoC, observed in the audio-frequency AC measurements).

CONCLUSIONS

The dielectric relaxation study of a series of CaC03 filled polypropylene samples has revealed some characteristics common for mineral-filled nonpolar polymers, such as:

the composites tend to adsorb moisture which in the case of untreated fillers results in a dielectric response described by the “universal response function” (1 8) (see Eq 1 ) ;

0 upon heating, water first becomes more mobile, then desorbs leading to a maximum both in loss (t”) and dispersion (6’ - tm) and to a peak in the thermally stimulated polarization current:

0 these maxima increase with decreasing effective frequency and with increasing water content (i.e. at constant specific surface with filler content, at constant filler content with specific surface).

The thermally stimulated current studies, however, have shown some features specific to the PP/untreated CaC03 filler system. These are:

0 the high temperature conductivity and the intensity of the depolarization current in the ac region pass through a minimum as a function of filler content:

0 the activation energy of conduction at high temperatures decreases at filler loadings higher than 20 weight percent.

The composites containing stearate-treated CaC03 filler usually show a higher loss level than their counterparts containing the same amounts of untreated filler. In the wet states the AC behavior is determined by a n unusual combination of molecular and protonic relaxation processes and by the desorption of water. In the dry samples at low effective frequencies (TSD) a new relaxation mechanism appears between the p and ac relaxations of the matrix polymer, which is probably related to the presence of the stearate groups at the matrix-filler interface.

In considering the complicated morphology of filler semi-crystalline polymers and the lack of knowledge about the thermally stimulated a, relaxation process in general and about the structure of charge traps in particular, the explanations presented are necessar- ily tentative. Further research is suggested in the following areas:

0 the effect of mineral fillers on the morphology (crystallinity, crystalline size, and spherulite size distributions, etc.) of polyolefins;

POLYMER ENGINEERING AND SCIENCE, MARCH 1990, Vol. 30, No. 6

P o l y p r o p y l e n e l C a l c i u l

0 the effect of surface treatments and specific surface area on the water-adsorption characteristics and dielectric properties of fillers and composites.

ACKNOWLEDGMENT

One of us (FEK) would like to acknowledge support from AFOSR, grant W38-011.

REFERENCES

1. L. E. Nielsen, Mechanical Properties of Polymers a n d Composites, Marcel Dekker, New York (1974).

2. Z . S. Petrovii: and J . Budinski-Simendic, to be pub- lished.

3. H. S. Katz and J . V. Milewski, Eds., Handbook of Fillers a n d Reinforcements f o r Plastics, Van Nostrand Reinhold, New York (1978).

4. P. Hedvig, Dielectric Spectroscopy of Polymers, Adam Hilger, Bristol (1977).

5. G. Banhegyi, P. Hedvig, and F. E. Karasz, J . Appl. Polym. Sci. , 35, 679 (1988).

6. H. Kramer and K. E. Helf, Kolloid 2.. 180, 114 (1962). 7. E. 0. Forster. 1973, Annu. Rep. CEIDP, National Acad-

e m y of Sciences, pp. 447-55. Washington, D.C., (1 974).

8. T. Umemura, T. Suzuki, and T. Kashiwazaki, IEEE Trans. Electr. Insul., EI-17, 300 (1982).

9. I. K. Hakim and F. F. Hanna, Br. Polym. J., 8, 87 (1 976).

10. I. K. Hakim and A. M. Bishai, Polym. Photochem.. 4, 373 (1 984).

11. I. K. Hakim, Plaste und K a u t s c h u k , 34,225 (1987). 12. M. R. Wertheimer, L. Paquin, and H. P. Schreiber, J .

Appl. Polym. Sci., 20, 2675 (1976). 13. M. R. Wertheimer. L. Paquin, H. P. Schreiber, and S.

A. Boggs, IEEE Trans. Electr. Insul., EI-12, 137 (1977). 14. L. Paquin, H. St-Onge, and M. R. Wertheimer, IEEE

Trans. Electr. Insul., EI-17. 399 (1982). 15. G. Banhegyi and F. E. Karasz, J. Polym. Sci., Polym.

Phys. Ed., 24, 209 (1986). 16. G. Banhegyi and F. E. Karasz. Colloid & Polym. Sci.,

265, 394 (19871. 17. G. Banheai . P. Hedvig, and F. E. Karasz, Colloid &

Polym. Sci., 266, 701 (1988).

n Carbonate Composites

18. A. K. Jonscher, Dielectric Relaxat ion i n Sol ids , Chel- sea Dielectrics Press Ltd., London (1983).

19. J. B. Hasted, Aqueous Dielectrics, Chapman and Hall. London (1973).

20. S. S. Dukhin and V. N. Shilov, Dielectric Phenomena a n d the Double Layer i n Disperse S y s t e m s a n d Polyelectrolytes, Wiley, New York (1974).

21. L. A. Dissado and R. M. Hill, J. C h e m . SOC. Faraday Trans. 280, 291 (1984).

22. K. Goto and R. Kawai, Kobunshi Ronbunshu, 42,559 (1 985).

23. R. A. Sack, Aus t . J . Scien. Res.. 5, 135 (1952). 24. G. Banhegyi, Colloid & Polym. J., 264. 1030 (1986). 25. R. A. Fuoss and W. Dannhauser, J . Appl . Polym. Sci.,

7, 1015 (1963). 26. D. K. Gupta and K. Joyner, J. Phys . D., 9,2041 (1976). 27. H. P. Singh and D. Gupta, Ind ian J . Pure Appl . Phys..

23, 35 (1 985). 28. H. P. Singh and D. Gupta, Ind ian J . Pure Appl. Phys. ,

24, 444 (1986). 29. G . Weber, Prog. Colloid Polym. Sci., 66, 125 (1979). 30. M. Jarrigeon. B. Chabert, D. Chatain, C. Lacabanne.

and G. Nemoz, J . Macromol. Sci.-Phys.. B17, 1 (1980). 31. P. Myslinski and M. Kryszewski. Polym. Bull., 2, 761

(1980). 32. M. Goel. V. B. Gupta, and P. K. C . Pillai. Polym. Bull..

7, 103 (1982). 33. J. M. Keller and S. C. Datt. Phys . Stat. Sol. A,. 91, 205

(1985). 34. B. Fruth, H. J. Richter, and D. Meurer, IEEE Trans.

Electr. Insul., EI-21,327 (1986). 35. P. Myslinski and M. Kryszewski, Acta Polymerica, 38,

253 (1987). 36. M. Pluta and M. Kryszewski, Acta Polymerica, 37, 726

(1986). 37. M. Pluta and M. Kryszewski, Acta Polymerica, 38, 42

(1986). 38. G. Banhegyi, P. Hedvig, and F. E. Karasz, unpublished

results. 39. T. Kowalewski and A. Galeski, J . Appl . Polym. Sci..

32, 2919 (1986). 40. T. Tanaka, S. Hayashi, and K. Shibayama, J. Appl.

Phys. , 48,3478 (1977). 41. T. Tanaka, S. Hayashi, S. Hirabayashi. and K. Shibay-

ama, J. Appl . Phys. , 49, 2490 (1978). 42. K. Kudo, J. Tanaka, and D. Damon. 1987 Annu. Rep.

CEIDP, IEEE Service Center , pp. 209-214, Piscata- way, N.J . (1 987).


ac and dc dielectric properties of some polypropylene/calcium carbonate composites

Documents