The evolution of the space charge and conductivity with DC poling of two types of polypropylene-(PP-) based nanocomposites (PNCs) was investigated. The PNCs were manufactured with different concentrations of synthetic and natural organoclays. The optimal concentrations of nanofiller that can efficiently mitigate the space charge with DC aging time were 2-wt% for PP-natural-clay and between 2 and 4 wt% for the PP-synthetic-clay. Above these percentages charge transport through overlapping of nanoparticles can occur due to the interaction zone of double layers formed at the nanoparticle/host material interfaces. Under DC field the overlapping increases the conductivity of PNCs and minimizes the benefit of incorporating nanofillers into PP. The total charge stored in unfilled PP increased continuously with time reaching a maximum around 5000 h before decreasing but it also changed slightly in all filled specimens. It was perceived that the smaller the size of nanofiller platelets the more efficient the charge mitigation. The conductivity of specimens containing 6 wt% of natural clay and 8 wt% of synthetic clay reached
Polymers such as polypropylene (PP) are extensively used as electrical insulation in power capacitors, cable wraps, and phase separators for rotating electrical equipment. The impact of filling polymers with a small quantity of nanoparticles is currently the most active subject of research for universities, research centers, and industrial institutions. The main objective of these activities is to develop the next high-performance industrial materials with superior properties compared to unfilled polymers. This can only be realized if the resulting polymer nanocomposites (PNCs) possess as many as possible of the following features: more discharge endurance, better thermal and mechanical properties, lower quantity of accumulated space charges, longer lifetime, and so forth.
It was reported that the mechanical and thermal properties of polypropylene (PP) [
Less accumulation of space charge in the insulation material allows for applying higher field at the same insulation thickness, which could decrease the weight and cost of power equipment. Therefore, understanding the real impact of filling polymers with nanoparticles on space charge evolution with aging time is absolutely critical for developing new composites for high-voltage applications.
Compatibilizers are normally used for making the interaction of PP and nanoclay more favourable [
It was observed that the quantity of space charges was substantially reduced by adding a few wt% of nanoparticles to the polyethylene and polypropylene [
In this work the following two types of organoclay were incorporated in the isotactic polypropylene (PP): Topy synthetic tetrasilisic mica from Topy Co., Ltd. and Cloisite 20A powder of Wyoming natural montmorillonite clay from Southern Clay Products. The concentration of nanoparticles was between 1 wt% and 8 wt%. Measurements of the evolution of space charge density and conductivity during more than 11000 h of DC poling at −25 kV/mm were performed. This could lead us to define the optimal concentration of nanoclay at which the PNC space charge mitigation is at a maximum. This could set up the base for developing the next generation of advanced insulation materials with high performance for power capacitors and different high-voltage electrical applications.
The tested specimens were composed of two sets of PNCs. The first set was composed of an isotactic PP (Profax HL-451H from Basell) filled with different wt% of Topy synthetic tetrasilisic mica from Topy Co., Ltd. of Japan. This set was prepared for a collaboration project between the Industrial Materials Institute (IMI) and the Institute for National Measurement Standards (INMS) of the NRC to develop nanocomposite insulations for high-voltage capacitor applications. The second set was composed of the same PP material but filled with a different quantity of Cloisite natural montmorillonite clay from Wyoming. However, this set was manufactured for a project within the Versailles Project on Advanced Materials and Standards (VAMAS), Technical Working Area 33 (TWA-33) concerned with the development of test methods for chemical, morphological, mechanical, and electrical characterization techniques for polymer nanocomposites. VAMAS is an international organization that supports world trade in high technology [
The preparation of nanocomposites was carried out in the following steps [
The base PP material was melt compounded with a 1 : 1 mixture of two compatibilizers—PP grafted with maleic anhydride (PP-MA) from Eastman (Epolene 3015) and from Chemtura (Polybond 3150), antioxidant Irganox B-225 (from Ciba), and one of the organoclays. The compounding was carried out at 200°C under a blanket of dry nitrogen in a twin-screw extruder (TSE; Leistritz 34 mm,
PNC compositions for both sets of tested specimens. Synthetic for NPC filled with nanoclays from Topy and natural for PNC filled with Cloisite 20A.
Given name | PP (%) | Antioxidants (%) | Compatibilizer (%) | Nanoclay | |
---|---|---|---|---|---|
(%) | Type | ||||
PP1-0% | 82.4 | 0.2 | 17.4 | 0 | — |
PP2-0% | 87.0 | 0.2 | 12.8 | 0 | — |
PP1-S1% | 81.6 | 0.2 | 17.2 | 1 | Synthetic |
PP1-S2% | 80.7 | 0.2 | 17.1 | 2 | Synthetic |
PP1-S4% | 79.1 | 0.2 | 16.7 | 4 | Synthetic |
PP1-S8% | 75.8 | 0.2 | 16 | 8 | Synthetic |
PP2-N2% | 85.3 | 0.2 | 12.5 | 2 | Natural |
PP2-N6% | 81.8 | 0.2 | 12 | 6 | Natural |
Samples containing 0, 1, 2, 4, and 8 wt% of synthetic clay from Topy and 0, 2, and 6 wt% of natural clay (Cloisite 20A) were prepared. To ascertain the same compounding history of the samples all samples were extruded at 180°C under a blanket of dry nitrogen, using the same TSE with the same processing parameters as used for both MBs. Thus, PP + antioxidant was extruded twice. The MBs obtained in Step
The pelletized dried compositions obtained in Step
The percentage of nanofillers, the composition and names of each type of specimens used in this paper are given in detail in Table
The filled specimens are referred to in this paper as “PP1+S
The individual platelets’ aspect ratios were around 6000 and 286 for synthetic and natural clay, respectively [
Space charge measurements were performed using the Five Labs Pulse Electroacoustic (PEA) measurement system [
During the PEA and conductivity tests, a thin layer of silicon oil was applied on each side of the sample to ensure a good contact between the specimen and the electrodes. A 0.2 mm thick, 30 mm diameter semicon disk was inserted between the specimen and the upper brass electrode during DC poling. Specimen poling and conductivity measurements were performed at the same field, −25 kV/mm DC. However, the PEA measurements were performed at 4 kV/mm and performed first on fresh specimens (0 h of DC poling) to be used as a reference and then after 168 h, 336 h, 1008 h, 2016 h, 3024 h, …,11088 h of poling time.
Up to three specimens were tested for each condition, and the average of the conductivity and the total charge
The figures representing the distribution of space charge depict the cathode, to which the −25 kV/mm DC field was applied, on the right side and the anode on the left side.
The double injection of charges (homocharges) is clearly observed in Figure
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP1-0%.
Figure
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP2-0%.
The two types of base materials PP1-0% and PP2-0% only differ from each other by the percentage of compatibilizers (see Table
Just by loading PP1-0% with 1 wt% of synthetic clay (PP1-S1%), the value and profile of the charges are dramatically changed, see the distribution of charges in Figures
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP-S1%.
Two wt% of synthetic clay, PP1-S2%, was able to mitigate the majority of charges in the central zone of the specimens as shown in Figure
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP-S2%.
Increasing the synthetic nanofiller content to 4 wt% further decreases the amount of charge, especially in the central area (Figure
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP-S4%.
When the PP1-0% is loaded with 8 wt%, sample PP1-S8%, significant charge build-up was observed in the central area, see Figure
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP-S8%.
Hence, from the results shown above, the optimal percentage for the synthetic filler seems to be between 2 and 4 wt%.
The addition of 2 wt% of natural clay, PP2-N2%, results in decreasing the charges all over the sample bulk as depicted in Figure
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP-N2%.
Loading PP with 6 wt% of natural clay as shown in Figure
Effect of the poling time at −25 kV/mm DC field on space charge distributions in PP-N6%.
Electric field distributions after 5040 h of aging with −25 kV/mm DC are depicted in Figure
Electric field distributions after 5040 h of DC poling at −25 kV/mm.
Figure
Space charge distributions after 66 W of DC poling at −25 kV/mm for the unfilled and filled with 2 wt% of natural and synthetic clay.
As the concentration of nanofillers increases over the percolation threshold the interaction zones, or Gouy-Chapman-Stern layers, which are highly conductive, could overlap, and the overall conductivity of the composites considerably increases [
The integral of the absolute value of charges measured with the PEA technique between both electrodes gives the total charge,
Total charge,
Figure
The effect of nanoparticle concentration DC conductivity measured at −25 kV/mm.
The lowest value of the conductivity was observed for PP2-0% despite having the highest quantity of accumulated charge, which could be due to deeply trapped charges. The applied field of −25 kV/mm DC during the conductivity measurements would not be high enough to free these charges from their deep traps.
It is to be noted that the conductivity tends to increase after 5000 h, and more tests and longer poling time are required to better understand the effect of DC field on space charge effects in polymer nanocomposites.
The high conductivity of the PP-S8% and the PP-N6% could be due to overlapping of the interaction zones around nanoparticles, which could lead to a dominant charge transport process throughout the system via the charge double layers [
In summary, from all results presented above it is clear that the clay platelets limit the molecular motion in the amorphous phase. This happens because of the interaction between the platelets and PP matrix [
The evolution of space charge density and quantity as well as the conductivity with DC poling in PP-based nanocomposites containing natural and synthetic organoclays was investigated.
PP material can be affected by the manufacturing process where the percentage of compatibilizers can lead to different space charge evolution.
It was observed that loading PP with certain low percentages per weight of nanoclay can efficiently mitigate space charges. The optimal concentration of clay was 2 wt% for the PNCs with natural clay and between 2 and 4 wt% for PNCs with synthetic clay.
The accumulated space charge in PP1-0% and PP2-0% increases continuously with time of DC poling until 5000 h. In contrast, both nanofilled composites are almost independent of poling time.
At least 1000 h of DC poling is needed to clearly see the benefit of charge mitigation in PNC specimens.
Samples containing natural clay had less space charge than the samples containing synthetic clay. This behaviour could be related to the difference in the individual platelets’ aspect ratios of the synthetic and natural organoclays.
The high conductivity of PP1-S8% and PP2-N6% could be related to excessive nanoparticle concentrations that may increase the overlapping of the diffused double-layer charge clouds and promote charge transport throughout those layers. To obtain the best results of filling PP with nanoclays, the nanoclay’s wt% incorporated in the PP materials should be less than the percolation threshold.
The authors express many thanks to Dr. L. A. Utracki of the Industrial Materials Institute of NRC Canada for providing nanocomposite specimens for this work. They also gratefully acknowledge the technical help of Mrs. Y. Chen of the Institute for National Measurement Standards, NRC Canada.