Electrets corona charging

Keywords Charge storage properties Commodity polymers Corona charging Electret materials High performance polymers Physical aging Polymer films... 

In this chapter we have presented concepts and methods to improve polymer electret materials. Corona charging was employed for film charging since this method possesses the advantages of being cost-effective, easy to perform, and applicable in short periods of time. 

A very simple but yet time-consuming treatment to improve amorphous electret materials is physical aging prior to corona charging. For instance, the charge retention ofUltem 1000 films after 24h at 120 °C can be raised from 18% to 95% when the films were physically aged at 200 °C for 4 days. 

Electret is an important cross-scientific subject of dielectric physics, material science, sensor engineering, medical and bio-engineering (Sessler G., Sessler et al., Nalwa H., Goel M.)-Over the years, considerable interest has been shown in the surface potential decay of corona-charged polymeric materials. Besides the electrets material and conditions of producing electrets the surface potential decay depends on number of factors under which the electrets have been stored or used, for example temperature, humidity, pressure etc. (Zhongfu et al, Gang-Jin et al., Ribeiro et al.). 

The present paper reports the results of investigations on the low paessure effect on the behaviour of corona charged PP electrets. The PP films are charged in a positive or a negative corona at different charging temperatures T=20°C and T=90°C. In the vacuum chamber some electrets were stored between two plate shorted electrodes with various air gaps and the other ones were put only onto a metal electrode. The time and temp>erature dependences of the surface potential of electrets were also studied. 

The influence of the charging time on the surface potential of the electrets was studied. For that purpose the PP samples were corona charged using a three-electrode system (Fig. 1) at different charge time - 1, 5,10, 20 and 30 minutes. The voltage = -5kV was applied to the corona electrode and Ug = -650V to the grid. 

Electrets obtained in a corona charging and stored at low pressures between two short-circuited piate eiectrodes with various air gaps between the electrodes... 

Time dependence of the surface potential of electrets obtained by corona charging and stored at low pressures in the absence of a second electrode... 

Temperature dependence of the surface potential of corona charged electrets... 

Wild, J. Stranathan, J. (1957). Influence of External Treatments on Electret Behavior. The Journal of Chemical Physics, Vol.27, Nq5, pp. 1055-1059 Yovcheva, T. Avramova, L Mekishev, G. Marinova, T. (2007). Corona-charged polypropylene electrets analyzed by XPS. Journal of Electrostatics, Vol.65, pp. 667-671 Zhongfu, X. Yuda, W. Guamao, Y. Ximin S. (1991). Corona Charging at Elevated Temperature and Charge Transport for Mylar PETP Foils. 7th Int Symp. on Electrets... 

The non-woven mats were able to retain up to 60% of the initial surface potential after several days of annealing at temperatures up to 140 °C. This is higher than in the case of corona-charged compact films. These properties make the materials ideal candidates for electret filter and sensing applications [55]. 

The particle collection efficiency of these air filters can be enhanced by the static charging of the fibres, " and this can be realised by using mixed fibres of various layouts of electret filters " such additional filtration mechanism is more useful for filters used in respiration devices.The ionic species generated by the negative polarity corona (CO, ) seem to be more effective for corona charging of a PP nonwoven... 

Erhard DP, Lovera D, Giesa R, Alstadt V, Schmidt HW (2010b) Influence of physical aging on the performance of corona-charged amorphous polymer electrets. J Polym Sci B Polym Phys 48 990-997... 

Normalized surface potential V / Vg vs normalized p / Pg pressure for samples charged in a positive or a negative corona respectively at room temperature are presented in Fig. 6 and Fig. 7, while the samples in Fig. 8 and Fig. 9 are charged in a positive or a negative corona respectively at temperature higher than the room one. As the electrets charged at 75°C and 90°C have the same behaviour the only presented results are the ones obtained at 90°C. 

The storage time dependence on the decrease of the surface potential of PP electrets charged in negative corona at room temperature and at 90°C was studied. Voltage of Uc = -5kV was... 

The results presented in Fig.14-16 show that irrespective of the corona polarity and the charging temperature the Friondlih and Lengmuir equations do not describe very well the results obtained in our experiments as the determination factor is 0.36 2 0.85. That is why we assumed that the desorption process might run together with another process like charge movement along the surface, for example. Our assumption is based on the results reported in various papers (Protodyakonov et al., Baum et al., Karmazova et al., Atkinson et al., 1976, Atkinson et al., 1980,1, Atkinson et al., 1980, II) and some additional experiments we have carried out with electrets with an island charge distribution. It is reported in... 

Fig. 17. Set-up for obtaining electrets with island charge distribution 1 - grounded plate electrode 2 - metal mask 3 - apertures 4 - corona electrode 5 - electrets Uc - corona voltage Ug - metal mask voltage.

Equations of this type (Eqs. 26 and 27) describe reasonably well the discharge in the range of corona electrets - in particular in polyethylene (Berlepsch 1991). This model, however, disagrees considerably with experimental data obtained for nonpolar fluoropolymers, notably those charged in positive corona. In this case the model does not take into account homocharge trapping in surface and near-metal layers. 

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