Study of dielectric strength in EPDM by nondestructive dynamic mechanical analysis in high electrical field

Abstract

In the present work the value of the degree of the area swept by the polymer chain due to an electrical force for a given mesostructure was related to the corresponding value of the dielectric strength. This value was deduced from the electric inclusion formalism applied to dynamic mechanical analysis (DMA) studies conducted under high electric field, which were performed in commercial ethylene-propylene-diene M-class rubber (EPDM); used for the housing of polymeric electrical insulators. EPDM samples with different arrangements of the polymer chains and crystalline degree, promoted by controlled neutron irradiation were studied. Several characterization techniques, as infrared absorption spectroscopy (IR), differential scanning calorimetry (DSC), positron annihilation lifetime spectroscopy (PALS) and dielectric strength (DS) were also used. The relationship between the DS and the degree of movement of polymer chains promoted by electrical forces coming from the electric field applied in a non destructive test as the DMA was successfully established. In fact, a larger empty space in the sample leads to larger areas swept by the polymer chains during bending under the application of the field strength in the dynamic mechanical analysis tests. Therefore, an increase in the capability of movement of charges occurs, corresponding to smaller dielectric strength values. Crystallinity improves the dielectric strength due to the increase in the internal stresses which decreases the capability of movement of the polymer chains and electric carriers by electric forces.

Electric memory effects in styrene-butadiene rubber, containing electric inclusions of highly aromatic oil

“…2,3,16,17,23,24 In order to explore the behavior of the internal stresses during the field increase and after it is switched off, the behavior of β coefficient and the ratio W z m =W z T , as a function of the applied electric field, calculated by means of Eqs. (5) and (6), respectively are shown in Fig. 12.…”

“…4,16 The behavior of tan( Þ curves as a function of the applied electric field for SBR 1712 composite samples follows the usual trend earlier reported for DMA experiments conducted under high electrical field. [4][5][6] Thus, the higher the increase of modulus values as the field strength increases, the higher the decrease of tan( Þ values. This behavior of tan( Þ is in agreement with the above mentioned regarding the development of internal stresses in the polymer matrix promoted by the electric field.…”

“…The increase in internal stresses in the polymer matrix acts like obstacles which difficult the movement of its chains leading both to the decrease in the damping values and to the increase in the modulus values. 1,[4][5][6]17,23,24 The viscosity of HAO (type DEA) oil increases as the strength in the field increases due to the stretching, rotation and rearrangement of dipoles located in the oil. As said before, the electric dipoles in a highly aromatic mineral oil are mainly located in the zone of the bonding between the hydrocarbonated chains and the aromatic rings.…”

“…[4][5][6] The model allows determining the behavior of internal stresses promoted by the electrostriction phenomenon by monitoring the behavior of the misfit coefficient and the transfer process of elastic energy. [4][5][6] In fact, the electric inclusion model considers that applying an electric field to a dielectric material leads to the stretching of dipoles giving rise to the appearance of inclusions. They promote the development of internal stresses in the dielectric material, whose electromechanical equilibrium condition is determined from the inclusion formalism for continuous media.…”

“…They promote the development of internal stresses in the dielectric material, whose electromechanical equilibrium condition is determined from the inclusion formalism for continuous media. [4][5][6] In the present paper, it was discovered that styrenebutadiene rubber (SBR), containing highly aromatic oil (HAO), under electric field, transforms to a composite involving electric inclusions; which exhibits memory effects. It gives rise to dynamic elastic modulus, damping and internal stresses degree which can be controlled depending on the applied electric field strength.…”