Investigation of the electret properties of PDLA/PEC porous composite films

The influence of both low pressure and time storage on the surface potential decay of high density polyethylene (HDPE) composite films with different weight concentrations of the zeolite particles - 0 wt.%, 2 wt.% and 6 wt.% were studied. The samples were charged in a corona discharge by means of a corona triode system for ' minute under room conditions. Positive or negative 5kV voltage was applied to the corona electrode and 1 kV voltage of the same polarity as that of the corona electrode was applied to the grid. After charging, the electret surface potential was measured by the method of the vibrating electrode with compensation. Two groups of tests were performed. In the first group after charging, the electrets were placed into a vacuum chamber where the pressure was reduced step by step in the range from 1000 mbar to 0.1 mbar. At each step the samples were stored for 1 minute. Then the electrets were removed from the vacuum chamber, the surface potential was measured again and the normalized surface potential was calculated. It was established that the low pressure had led to the surface potential decay of the electrets. The influence of the low pressure was analyzed by the equation that describes processes of desorption from the electret surface accompanied with surface diffusion. In the second group after charging, the electret surface potential was measured with the time of storage for two months. The experimental results obtained show a significant change in the electret behaviour of the composite films after the incorporation of zeolite particles with different concentration into the HDPE matrix. It was established that the surface potential decay depends on the corona polarity and the particle concentration.

In the present paper the influence of both relative humidity and low pressure on the surface potential decay of charged 20 μm polypropylene films was investigated. Sample charging was performed in a three-electrode corona discharge system. After charging the initial surface potential was measured using the method of the vibrating electrode with compensation. Two groups of tests were performed. In the first group, after charging the electrets were placed into a vacuum chamber under various low pressures (from 0.1 mbar to 1000 mbar) for 30 minutes. After that the electrets were removed from the vacuum chamber. The surface potential was measured again and the normalized surface potential was calculated. The influence of low pressure was analyzed by the equation that describes processes of desorption from the electret surface. In the second group after charging the electrets were put in desiccators at 0% and 100% relative humidity for 220 days at room temperature. After that the surface potential was measured periodically out of the desiccators. The results obtained were analyzed with the percolation 2D model. It was established that the higher values of the relative humidity led to a faster decay of the surface potential. We assume the presence of surface discharge processes.

The influence of low pressure on the surface potential decay of gamma irradiated polymer films of polypropylene (PP) and poly(ethylene terephtalate) (PET) were studied. Polymer film samples were subjected to integral irradiation doses (Ey = 1.25 MeV, 60Co source) of 5 kGy and 25 kGy accumulated in air at a dose rate of 0.26 Mrad/h. After irradiation, the samples were charged in a corona discharge by means of a corona triode system for 1 minute under room conditions. Positive or negative 5 kV voltages were applied to the corona electrode. Four different voltages of the same polarity as that of the corona electrode were applied to the grid. The electret surface potential V0 was measured by the method of the vibrating electrode with compensation. After charging the electrets were placed into a vacuum chamber as the pressure was reduced step by step in the range of 1000 mbar to 0.1 mbar. At each step the samples were stored for 1 minute and the surface potential V was measured again. Then values of the normalized surface potential V/V0 were calculated. Low pressure dependences of the normalized surface potential for positively and negatively charged PP and PET films were presented. It was established that the low pressure led to the surface potential decay of the electrets. The influence of low pressure was analyzed by the equation that describes processes of desorption from the electret surface accompanied with surface diffusion. The experimental results obtained show a significant change in the electret behaviour of the polimer films after gamma irradiation and storage at different low pressure. It was established that the surface potential decay depends on the corona polarity, gamma irradiation and the values of low pressure.

Influence of low pressure on electrets behaviour at various air gaps

Polyelectrolyte multilayers (PEMs) deposited on non-porous and porous blend substrates were studied. Films, prepared from two biodegradable polymers poly (D-lactic acid) (PDLA) and poly(ε-caprolactone) (PCL) and their blends were used as substrates in the present paper. All films were initially charged in a corona discharge (positive or negative corona). After charging, the initial surface potential of the samples V0 was measured and the normalized surface potential was calculated. The dependencies on time of the normalized surface potential for electrets, possessing either positive or negative charges, were studied. It was found that the steady-state values of the normalized surface potential for the porous substrates were higher than those of the non-porous ones, independently of material type and corona polarity. It was also shown that the values of the normalized surface potential for the PCL electrets were the highest and decreased when the content of PDLA increased. Scanning electron microscopy (SEM) was utilized for the determination of the substrates’ surface morphology. With the largest pore size, PCL substrates allowed for a greater capture of charges on their surface and facilitated the retention of said charges for prolonged periods of time. Differential scanning calorimetry (DSC) measurements were performed to determine the degree of crystallinity, which was very high for PCL substrates, when compared to the other investigated substrates. The wettability of the investigated substrates was measured using the static water contact angle method. The obtained results demonstrated that the created blends were more hydrophilic than the pure films. The two chosen polyelectrolytes were layered onto the surface of the substrates with the use of the layer-by-layer (LbL) technique and benzydamine hydrochloride was loaded in the multilayers as a model drug. Its loading efficiency and release profile were carried out spectrophotometrically. It was determined that for non-porous substrates, independently of the corona polarity, the best fitting model was Korsmeyer-Peppas, while for the porous substrates the best fitting model was Weibull.

In the present paper the influence of both time and storage temperature on the surface potential decay of polypropylene (PP) composite films was investigated. We have investigated the 200 μm PP composite films with different weight concentrations of the particles (ZnO or CuO) - 0 wt.%, 2 wt.% and 4 wt.%. The samples were charged in a corona discharge by means of a corona triode system for 1 minute under room conditions. Positive or negative 5 kV voltage was applied to the corona electrode and 1 kV voltage of the same polarity as that of the corona electrode was applied to the grid. After charging, the surface potential was measured using the method of the vibrating electrode with compensation. The results obtained show a significant change in the electret behavior of the composite films after the inclusion of ZnO or CuO particles with different concentration into the PP matrix. The possible surface potential decay mechanisms responsible for the electret’s behavior were discussed. It was established that the surface potential decay depended on the corona polarity, the particles concentration and the type of the particles. It has been experimentally demonstrated that the normalized surface potential of PP composite films with different concentrations of ZnO or CuO particles decays faster than the one of the pure PP films.

Harmonic voltage can accelerate the aging of insulating materials. In this paper, an ac voltage with low-order harmonic components was applied on oil-paper insulated bushing with a duration of one month. Oil-paper samples extracted from bushing were tested by Surface Potential Decay (SPD) after positive and negative corona charging. Experimental results of SPD illustrate that the surface potential of aged oil-paper sample drops faster than the pristine one. Surface trap distribution is obtained for both pristine and aged samples. Besides, the hole and electron motilities are calculated based on transit time and initial surface potential. The conclusion can be reached that during the harmonics aging experiment, the scission of cellulose occurred under the thermal, electrical and mechanical stress, inducing surface traps and leading to higher nobilities of electron and hole.

Investigation of PP and PTFE film electrets stored at low pressure

Electrophotography is the science of document copying by selective dissipation of charge of corona charged layers. The recent trend in this field is to develop binder-layer type xerographic plates [1, 2]. Copper-phthalocyanine (CuPh) is a well studied photosensitive material and some work on its electrophotographic use has been' done using silicone resin as a binder [3, 4]. In the present paper, electrophotographic characteristics of copper-phthalocyanine (CuPh) in polyvinyl cinnamate (PVCn) binder are described. PVCn is also a photosensitive polymer. CuPh and PVCn used in the present study were obtained from Fluk: A. Chemische Fabrik CH-9470 BUCTIS and Polysciences Inc (USA), respectively. The desired weight ratios of these materials were milled in a mechanical grinder using benzene as a thinner. The films were made by pouring this mixture on an aluminium substrate. The layers were dried in atmospheric benzene and later in an oven maintained at 60°C for 48 h to remove all traces of benzene. The films were positively charged by a d.c. corona unit for 15 sec and then the surface potential decay was measured in the dark as well as in the light. The various samples studied during the present investigation were 20 : 80, 30: 70, 40:60, 50:50 and 60:40 weight ratio of CuPh and PVCn. The film thickness was 100 pro. The light from a 60 W tungsten lamp of intensity 1200 lux was used for photodischarge studies. The room temperature (30 ° C) and relative humidity (55%) were maintained constant during the course of investigations. The grid voltage and charging time were optimized to 1.5 kV and 15 sec, respectively, which were maintained constant throughout the experiments. The corona voltage was 6 kV. The effect of charging time and grid potential on the initial surface potential of 50:50 CuPh: PVCn samples are shown in Figs. 1 and 2, respectively. It has been observed that initially the surface potential increases both with charging time and grid potential and finally attains a saturation value. These results are expected. The number of charge carriers becoming trapped during corona charging increases with charging time and grid potential and hence the surface potential increases with the increase in time and grid potential. But the detrapping and recombination of charge carriers takes place simultaneously along with trapping. When these two, i.e. trapping and detrapping and recombination become equal, further charging of the film stops and we observed a saturation effect [5, 6]. The difference in surface potential decay in the dark and the light determines the use of a material in electrophotography. A good electro

Bipolar charge transport (BCT) model has been widely used to simulate time/space evolution of space charges in insulating materials. The BCT simulations are performed to investigate the relationships between space charge accumulation and conduction, electroluminescence (EL), charge packet formation, electrical breakdown, and surface potential decay (SPD) properties. Accordingly, the charge advection-reaction equation that contains shocks or high gradient regions should be solved by highly accurate and stable numerical methods to obtain high resolution. We use Runge-Kutta discontinuous Galerkin (RKDG) method and finite differential weighted essentially non-oscillatory (WENO) method to resolve the charge advection-reaction equation. Then, we calculate the SPD properties and space charge profiles of corona charged low-density polyethylene (LDPE) at various initial surface potentials. The simulated results of the two schemes are compared with analytical SPD results, and also compared with each other. It is found that the simulated SPD curves of RKDG and WENO in the case of single carrier injection are both consistent with the analytical results. Moreover, in the case of both single carrier injection and bipolar carrier injection, WENO scheme is more accurate than RKDG scheme at a given spatial discretization.

Nonlinear conductivity composites can be applied to modify the local electric field distribution and accelerate the surface and space charge dissipation in HVDC cables. This paper focuses on the effect of the nonlinear conductivity on the charge transportation under impulse superimposed on DC voltage. The surface-charge-accumulation property of silicone rubber (SiR)/SiC composites under impulse superimposed on DC voltage is studied. The surface potential decay (SPD) is measured after the sample is stressed by impulse superimposed on DC voltage. The effect of nonlinear conductivity on carrier mobility is analyzed. The results show that under DC voltage, the decay rate increases for the rise in the initial surface potential (ISP). Higher surface potential caused by higher impulse voltage leads to a larger nonlinear conductivity, which immediately dissipates the surface charge. The increase in DC voltage is shown to be more effective in accumulating surface charge than impulse. The vertical movement to the ground and the horizontal movement of surface charges are two factors affecting the ISP and SPD, which are both affected by the nonlinear conductivity of SiR/SiC composites.

This communication discusses styrene removal in air by positive and negative DC corona discharges. Experiments were performed with a wire-plate reactor and under a gas flow rate of 305 m3/h. In terms of averaged applied voltage and corona current, it is observed that the maximum negative corona current is always at least two times larger than the positive one at the same voltage level. At the same corona discharge energy density, however, the positive corona discharge produces around 2–6 times more ozone in comparison with the negative corona. For styrene removal, the positive corona processing is also around 2–6 times more effective than the negative corona. Humidity, an important and variable component of ambient air, affects the positive corona processing significantly. But it exerts a moderate effect on the negative corona. The differences between positive corona and negative corona discharges are attributed to their different discharge properties.

Polypropylene electrets 20 μm thick charged in a negative corona were studied. The samples were charged to different initial surface potentials and were stored under various pressures for 30 minutes. Then, the surface potential was again measured. The dependence of the electret surface potential on the pressure, under which the electrets were kept after charging, was investigated. As the pressure was lowered from 1000 mbar to 0.1 mbar a sharp decay of the surface potential occurred. It was established that a main parameter determining the surface potential decay is the ratio of the pressure to initial surface potential, and only one generalized curve describes all results obtained. Analyses of the equation described the results obtained was done.

Surface-potential decay characteristics of corona-charged insulators are analyzed theoretically. The effects of field-dependent drift mobility and injection efficiency, as well as time-dependent injection, are included. The theoretical results are compared with existing surface-potential decay data on polyethylene. At the highest initial surface potentials all the surface charge is injected into the insulator on a time scale which is small compared to that of the measurement, and the injected charge drifts under its own self-field to the collecting electrode by means of a field-dependent transport process without significant range limitation. At the lowest initial surface potentials little or no charge is injected, and the surface potential is constant in time. For intermediate initial surface potentials the injection process is time and field dependent, with the time dependence being stronger than an injection process associated with a surface charge density that decreases exponentially in time, but weaker than that for instantaneous complete injection.

Surface potential measurement provides a useful tool to gauge the electrical properties of materials. It has been observed that the potential of a sample with an initial high surface potential decays faster than that with an initial lower surface potential, known as the cross-over phenomenon. The phenomenon was found a few decades ago and various theories and models have been proposed. A common feature of the existing models is based on single charge carrier injection from a corona-charged surface. With our recent space charge measurement results on corona-charged samples, double injection from both electrodes has been verified. Based on this new fact, a new model based on bipolar charge injection is proposed and initial numerical simulation reveals that the surface potential cross-over phenomenon can occur under bipolar charge injection.