Space charge characteristics of fluorinated polyethylene: Different effects of fluorine and oxygen

Abstract

Direct fluorination are proved having obvious effect on space charge characteristics of polyethylene. It is believed that fluorine has a positive effect on suppressing space charge injection while oxygen impurity has a negative effect. However, the mechanism for the opposite effect of fluorine and oxygen is still not clear. In this paper, the different effects of fluorine and oxygen on space charge characteristics of fluorinated low density polyethylene (LDPE) are investigated on the basis of dielectric property, chemical constitutes and trap performance of surface fluorinated layers. The results show that direct fluorination has obvious effect on chemical constitutes and dielectric properties of surface fluorinated layer. Introduced fluorine is the main factor for suppressing charge injection from the electrodes, because it seriously changes the chemical constitutes and further the trap properties of the surface fluorinated layer. While introduction of oxygen results in heterocharges and makes space charge distribution complex, due to the ionization of generated small groups like C=O containing groups. Moreover, direct fluorination will result in cleavage of some LDPE molecules whatever there is oxygen impurity or not.

Direct Fluorination as Method of Improvement of Operational Properties of Polymeric Materials

“…Direct fluorination of PE usually leads to significant increase in the surface dielectric constant (Table 4) due to the appearance of strongly polar groups, like C=O containing and some of the -CF x groups, such as -C(O)F, -C(O)OH, -CHF-, -CF 3 , etc. [147,158,159]. The surface charge decay rate in PE does not improve, but rather slows down by fluorination [147,151,159,160], in contrast to polymers with the benzene ring or branches fracture in molecular chains [59].…”

“…[147,158,159]. The surface charge decay rate in PE does not improve, but rather slows down by fluorination [147,151,159,160], in contrast to polymers with the benzene ring or branches fracture in molecular chains [59]. For example, silicone rubber, first demonstrates a decrease in the dielectric constant, and then the increase to the initial values [51].…”

“…Besides improving wettability, adhesion and chemical stability, direct fluorination is effective approach to modulate the electrical properties for use as an insulating material for transformers, high and extra-high voltage cables, gas-insulated switchgears, inverter-fed motors etc. For the successful use of polymer dielectrics, it is necessary to control the main cause of aging and breakdown -the distribution of space charge in polymer materials [147,148].…”

“…On the one hand, since fluorine atoms are strongly electronegative and have a large atomic radius it is easy to capture electrons [147]. Also the substitution of H atoms by F atoms in the polymer chain results in shorter inter-chain distance, less amorphous space and more condensed molecular conformation, which therefore forms deeper electron traps, while holes, generally attracted by the intra-chain chemical defects with much stronger bonding have no clear tendency to change [57].…”

“…Also the substitution of H atoms by F atoms in the polymer chain results in shorter inter-chain distance, less amorphous space and more condensed molecular conformation, which therefore forms deeper electron traps, while holes, generally attracted by the intra-chain chemical defects with much stronger bonding have no clear tendency to change [57]. Therefore fluorination leads to the formation of deep electron traps in the surface layer, electric field at metal-insulator interface becomes lower, and the further charge injection from electrode is blocked by the screening effect of the trapped electrons charge layer [55,57,147].…”