Polyethylene/nanoclay specimens containing from 0 to 5% nanoclays were prepared from a commercially available premixed PE/nanoclay masterbatch containing 50% wt of nanoclay. The masterbatch was diluted to the desired concentration by adding PE along with various amounts of compatibilizer in order to achieve the best possible dispersion of the nanoclay platelets. The dielectric response of the compounded samples was investigated using a combination of time and frequency-domain spectroscopy in order to cover a wide frequency window. Both techniques were in good agreement when the time-domain data was transformed into frequency-domain data. Despite their low concentration, the addition of the dispersed nanoclays led to a significant alteration of the material dielectric response in the form of the appearance of various interfacial relaxation processes and an increase of charge carrier transport within the insulation material. Moreover, an onset of nonlinear charge transport process was observed at moderate fields for specimens containing a relatively low level of nanoclays. The high-field breakdown strength was shown to have been improved by the incorporation of the nanoparticles, particularly when the exfoliation was enhanced by the use of a maleic anhydride grafted polyethylene compatibilizer.
Certain properties of polymers have long been known to be favorably altered by a small addition of nanofillers [
In this paper, 1%, 3%, and 5% LLDPE/clay nanocomposites were prepared by melt compounding from a commercially available masterbatch whose polymer-clay compatibilization was facilitated by surface compatibilization based on ion-dipole interaction, which expands the nanoclay platelets to the point where individual platelets can be separated from one another by mechanical shear during material processing. The dielectric response both in the time and in the frequency domains and the breakdown strength of thin films were measured, and the results are reported and discussed in this paper.
Linear low-density polyethylene (LLDPE), with a melt flow index (MFI) of 0.1 g/min and a density of 0.917 g/cm3, was used to dilute a commercially available masterbatch of LLDPE/clay composite with 50% organomodified Montmorillonite (O-MMT) to the desired concentrations. Maleic anhydride grafted linear low-density polyethylene (LLDPE-g-MA), having a density of 0.940 g/cm3 and MFI of 2.5 g/min, was used as a compatibilizer. This material is referred to as M603 in this text.
The neat LLDPE, the masterbatch, and the compatibilizer were dried at 40°C in a vacuum oven for a minimum of 48 hours prior to extrusion. The melt compounding was achieved through an extrusion process using a corotating twin-screw extruder coupled with a metering feeder in order to control the feed rate. More details on the compounding process can be found elsewhere [
DSC results for pure LLDPE and LLDPE/O-MMT nanocomposites.
Sample identification | Composition | Melting point | Cristallinity |
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LLDPE | 100% LLDPE | 107.7 | 33.7 |
LLDPE/0/1 | 99% LLDPE |
108.0 | 36.8 |
LLDPE/0/3 | 97% LLDPE |
107.1 | 35.3 |
LLDPE/5/3 | 92% LLDPE |
||
LLDPE/10/3 | 87% LLDPE |
107.4 | 34.1 |
LLDPE/15/3 | 83% LLDPE |
107.1 | 35.2 |
LLDPE/0/5 | 95% LLDPE |
107.7 | 34.0 |
Frequency-domain measurements (FD) were conducted with a broadband dielectric spectrometer for a 10−3 to 106 Hz frequency range in a temperature-controlled chamber. A 3 V sinusoidal voltage was applied across the sample. Time-domain measurements (TD) were conducted by applying a step voltage across the samples in a two-active electrode setup. Both the polarization and the depolarization current were continuously monitored for 9000 s. Voltage steps from 400 to 1000 V were used, creating a nearly uniform electrical field ranging from 0.7 to 1.8 kV/mm.
The dielectric breakdown measurements were conducted according to the ASTM D149-09 standard and using a BAUR DTA 100 device to hold both the samples and the surrounding media. The specimens were placed between two 12.7 mm hemispherical electrodes (type 5) and held by a small applied pressure. The whole setup was immerged in a surrounding medium, as shown in Figure
Dielectric breakdown measurement setup.
The measurements were made using mineral oil (Voltesso 35, ESSO Imperial Oil) as a surrounding medium. The tested samples were labeled as listed in Table
According to the IEC/IEEE 62539 standard (based on IEEE Standard 930-2004), the dielectric strength data were treated by using the two-parameter Weibull statistical distribution, where the cumulative distributive function of dielectric strength
For each sample, the cumulative probability of failure,
The calculation of the estimator of the scale factor of the Weibull distribution for each set of data was done by assigning weightings corresponding to the White method as suggested in the IEC/IEEE 62539 standard. This estimator, as given in Table
Dissipation factor at power frequency (60 Hz) for LLDPE/nanoclay composites.
20°C | 40°C | 60°C | |
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LLDPE |
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LLDPE/0/1 |
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LLDPE/0/3 |
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LLDPE/10/3 |
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LLDPE/0/5 |
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Breakdown strength of various PE/nanoclay composites.
Sample | Average thickness (mm) |
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LLDPE—lab#1 | 0.59 | 60.7 | 25.5 | 59.7–61.6 |
LLDPE—lab#2 | 0.59 | 61.0 | 29.7 | 60.1–61.8 |
LLDPE—lab#2 | 0.61 | 58.8 | 17.1 | 57.4–60.1 |
LLDPE—lab#1 | 0.53 | 72.9 | 18.8 | 71.3–74.3 |
LLDPE/0/1—lab#1 | 0.59 | 65.8 | 14.6 | 63.9–67.5 |
LLDPE/0/1—lab#1 | 0.53 | 72.3 | 19.6 | 70.8–73.7 |
LLDPE/0/1—lab#2 | 0.59 | 66.1 | 17.2 | 64.5–67.5 |
LLDPE/0/1—lab#2 | 0.58 | 62.9 | 17.9 | 61.4–64.2 |
LLDPE/0/3—lab#1 | 0.58 | 69.6 | 19.7 | 68.1–70.9 |
LLDPE/0/3—lab#1 | 0.53 | 72.1 | 19.3 | 70.6–73.5 |
LLDPE/0/3—lab#2 | 0.58 | 69.8 | 18.1 | 68.2–71.2 |
LLDPE/0/3—lab#2 | 0.57 | 66.6 | 18.2 | 65.1–68.0 |
LLDPE/10/3—lab#2 | 0.56 | 69.6 | 17.9 | 68.0–71.1 |
In the time domain, the behavior of a linear insulating system is characterized by its conductivity
The last term of the second equation in (
An analytical calculation of the electrical field in a composite material can only be made if the minority phase is present in a small concentration and for regular shape inclusions. A number of results can be found where an exact solution for several matrix systems with periodic arrangements of regular inclusions is obtained [
Combining (
In this study, clay dispersion was mostly analyzed using X-ray diffractometery, Scanning Electronic Microscopy (SEM), Thermogravimetric Analysis (TGA) [
X-ray diffractograms of various LLDPE/M603/O-MMT nanocomposites.
TEM images were obtained using a JEOL JEM-2100F microscope operating at a 200 kV accelerating voltage. The TEM samples were cut to thin sections (50–80 nm thickness) at −120°C, using an ultramicrotome with diamond knife. Figure
TEM pictures of (a) LLDPE/0/3 and (b) LLDPE/10/3 samples.
TEM picture of a LLDPE/10/3 specimen.
Figure
Real and imaginary parts of the complex permittivity of LLDPE and LLDPE/clay nanocomposites at 20°C.
Real and imaginary parts of the complex permittivity of a 5% wt LLDPE and LLDPE/clay nanocomposites at 23°C: (a) time domain results; (b) combined time and frequency domain results.
At 20°C, the dielectric responses of the 1, 3, and 5 wt% LLDPE/O-MMT nanocomposites are somewhat similar, showing a broad interfacial relaxation peak around 102 Hz and an increase of both real and imaginary permittivity toward low frequencies. This behavior is known as low frequency dispersion, which in this case, is most probably related to ionic conductivity leading to electrode polarization. Extending the measurements down to 10−5 Hz allows an observation of a second relaxation peak in the vicinity of 10−3 Hz, as shown in Figure
Figure
Real and imaginary parts of the complex permittivity for a (a) 3 wt% LLDPE/clay nanocomposite and a (b) 3 wt% LLDPE/clay nanocomposite with 10% of M603 compatibilizer for temperatures from 0 to 60°C.
To relate the PE/clay nanocomposites dielectric response to their microstructure, the mean field theory (see (
Imaginary part of the complex permittivity at 20°C for: a (a) 3 wt% LLDPE/clay nanocomposite and a 3 wt% LLDPE/clay nanocomposite with 10% of M603 compatibilizer compared with theoretical prediction from (
An alternative way to describe the broad relaxation peak observed in the experimental data is in terms of a set of Debye functions with a continuous relaxation time distribution. The relaxation time distribution function,
(a) Modeling of the dielectric response of a 87/10/3 nanocomposite using (
(a) Modeling of the dielectric response of a 97/0/3 nanocomposite using (
For a possible future application as groundwall insulation in power cables, only the dielectric response at power frequencies (60 Hz in North America) really matters. Table
The calculated estimators
Weibull probability plot of dielectric strength of LLDPE with 0%, 1%, and 3% wt of O-MMT with their 90% confidence intervals.
To investigate the relation between the microstructure and the breakdown strength of PE/nanoclay composites, the same measuring procedure was used for four new samples, LLDPE, LLDPE/0/1, LLDPE/0/3, and LLDPE/15/3, with a 0.61 mm thickness. The calculated estimators
Weibull probability plot of the dielectric strength of pure LLDPE, LLDPE/0/1, LLDPE/0/3, and LLDPE/10/3.
A low level addition of nanoclay in a LLDPE matrix was found to significantly alter the material dielectric response, leading to a broad main interfacial relaxation peak. When a compatibilizer was used to improve the dispersion of the clay platelets, this peak was found to shift towards low frequencies. In addition, a second interfacial relaxation peak at a much lower frequency was observed for the samples with poor exfoliation, due to the anisotropy of the conductivity of clay tactoïds, which are much more conductive along the clay platelets than across. At power frequencies, the incorporation of a small percentage of clay leads to an increase of roughly two orders of magnitude of the material dielectric losses. However, the dissipation factor of PE/clay nanocomposites still remains low enough that it does not have a real impact for its future use as an insulation wall for power cables.
The observed increase of the dielectric breakdown with the addition of nanoclay inside a PE matrix reported in this paper is similar to what has been reported previously [
The financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and Hydro-Québec is greatly acknowledged. C. Daran-Daneau would also like to thank Hydro-Québec and the Fonds de Recherche-Nature et Technologie (FRQNT) for their financial support. The authors would also like to acknowledge the assistance of Abdellah Ajji, Weawkamol Leelapornpisit, and Ahmad Zohre Vand from Department of Chemical Engineering of the École Polytechnique de Montréal for the sample preparation and the TEM micrographs.