Effect Of Direct Fluorination Research Articles

Effect of Direct Fluorination on the Transport Properties and Swelling of Polymeric Materials: A Review

Fluorine-containing polymers occupy a peculiar niche among conventional polymers due to the unique combination of physicochemical properties. Direct surface fluorination of the polymeric materials is one of the approaches for the introduction of fluorine into the chemical structure that allows one to implement advantages of fluorinated polymers in a thin layer. Current review considers the influence of the surface interaction of the polymeric materials and membranes with elemental fluorine on gas, vapor and liquid transport as well as swelling and related phenomena. The increase in direct fluorination duration and concentration of fluorine in the fluorination mixture is shown to result mostly in a reduction of all penetrants permeability to a different extent, whereas selectivity of the selected gas pairs (He-H2, H2-CH4, He-CH4, CO2-CH4, O2-N2, etc.) increases. Separation parameters for the treated polymeric films approach Robeson’s upper bounds or overcome them. The most promising results were obtained for highly permeable polymer, polytrimethylsilylpropyne (PTMSP). The surface fluorination of rubbers in printing equipment leads to an improved chemical resistance of the materials towards organic solvents, moisturizing solutions and reduce diffusion of plasticizers, photosensitizers and other components of the polymeric blends. The direct fluorination technique can be also considered one of the approaches of fabrication of fuel cell membranes from non-fluorinated polymeric precursors that improves their methanol permeability, proton conductivity and oxidative stability.

DC breakdown and flashover characteristics of direct fluorinated epoxy/Al2O3 nanocomposites

The effect of direct fluorination on DC breakdown and flashover characteristics of epoxy/Al 2 O 3 nanocomposites was investigated. Pure epoxy resin and epoxy/Al 2 O 3 nanocomposites with 1, 3 and 5 wt% mass fractions were synthesized, and direct fluorination of the samples was carried out at 55 °C for 30 min using a F 2 /N 2 mixture with 20 vol% F 2 at 0.05 MPa. Fourier transform infrared spectroscopy and atomic force microscopy show that molecular chain scission occur during direct fluorination, which may significantly suppress surface charge accumulation while incorporating Al 2 O 3 nanoparticles into epoxy may restrict surface potential decay of fluorinated epoxy resin. The trap characteristics obtained by isothermal surface potential decay show that 1 wt. % sample has deeper traps and higher trap density. DC breakdown and flashover of samples were measured to evaluate the influence of nano filling and direct fluorination on electrical performance of epoxy resin. The results show that epoxy resin with higher DC breakdown and flashover strength can be obtained by simultaneous application of nano filling and direct fluorination. Compared to un-fluorinated epoxy resin, the DC breakdown and flashover voltage of fluorinated epoxy/Al 2 O 3 nanocomposites with mass fraction of 1 wt% improved 17 and 23%, respectively.

Effect of direct fluorination on surface and electrical properties of polyimide thin films

The influences of direct fluorination on the structural, morphological, surface free energy, thermal and electric properties of polyimide films were systematically investigated by ATR-IR, SEM, contact angles, thermo-gravimetric analysis, surface conductivity and potential decay, respectively. The results indicate that the fluorination leads to substantial changes in surface composition and structure, and a modification of the surface morphology. It is also observed that the fluorination also gave rise to an increase in surface hydrophilicity and a decrease in surface free energy. The thermal stability of fluorinated polyimide film seems to slightly decrease. The intrinsic increase in surface electrical conduction is considered to be a result of decrease in depth of charge traps.

Direct Fluorination Induced Variation in Interface Discharge Behavior Between Polypropylene and Silicone Rubber Under AC Voltage

This paper reports on the effect of direct fluorination on interface discharge characteristics between isotactic polypropylene (PP) and silicone rubber under AC voltage. The influence of fluorination time on discharge initiation and propagation stages has been estimated by measuring phase resolved partial discharge pattern and relationship between discharge rate ( $N$ ) and discharge magnitude ( $Q$ ). Attenuated total reflection Fourier transform infrared spectrum, water contact angle and surface roughness were also measured to gain a better understanding of the fluorination effect on surface chemical and physical structures of sample. Obtained results showed that with the increase of fluorination time from 0 to 15 min, the average surface roughness of PP increased from ~42 to ~65 nm, whereas the water contact angle decreased from ~88° to ~58°. In addition, the discharge initiation time increased from ~2 to ~4 s but the discharge propagation time decreased from ~14 to ~2 s, leading to the reduction in overall time of interface breakdown from ~16 to ~6 s. It is suggested that at the initiation stage the C–F bond introduced by direct fluorination with higher bond energy and deeper trap depth should be responsible for the alleviation of polymer degradation, while at the propagation stage the formation of larger void at the surface tends to accelerate the degradation of polymer. It could be concluded that although the direct fluorination has positive effect by delaying the appearance of discharge channel, it possesses negative effect by shortening the propagation process.

Effect of direct fluorination on the mechanical and scratch performance of nitrile butadiene rubber

To explore the possibility of direct fluorination to enhance the scratch performance of rubber materials, the effect of direct fluorination on the mechanical and surface properties and scratch performance of nitrile butadiene rubber (NBR) was studied. The bulk mechanical performance of NBR showed no apparent change after fluorination except the fair alteration of the type B tear strength, while the surface properties experienced substantial variations. The surface of the fluorinated samples appears to be cleaner, accompanied with the decreased surface roughness. The friction coefficient decreased substantially with fluorination treatment for only 30minutes. Obvious changes were observed for the surface modulus and surface energy of the fluorinated NBR. The onset of the scratch groove was significantly delayed under certain fluorination conditions. However, the surface whitening did not show significant improvement. The correlations between the scratch resistance and the bulk mechanical and surface properties were analyzed systematically. The findings demonstrated the surface fluorination can be an efficient method to enhance the rubber's resistance to scratch groove.

Effects of direct fluorination on charge coupling behavior of oil-paper insulation under DC and pulse voltages

The oil-impregnated paper, which is the major section in converter transformer, plays a considerable role in the reliability and safety of power grid. However, the insulation degradation will be accelerated by the accumulation of charges that are on the surface of oil-impregnated paper. Therefore, the operating overvoltage occurs and irremediable damage will appear. Direct fluorination is an effective method of modifying the chemical components of polymer matrix so as to have effects on the surface characteristic of the insulating material without reforming the bulk properties. In this research, for studying effects of fluorination on charge behavior on the surface of oil-impregnated paper under the application of DC and impulse voltage, the F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> compound, which the volume fraction for the F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas occupied 10 % and the pressure was 0.05 MPa in a reaction kettle for 5, 30 and 60 min respectively, was applied. Meanwhile, the chemical bond groups and the surface morphology of the fluorinated specimen were tested. Besides, the DC voltage amplitude was 4 kV and the applied time was 4 min. The impulse frequency was 500 Hz and the impulse voltage amplitude was 1, 2, 3 and 4 kV, respectively. The charge properties under various situations were investigated. From the research under the application of different fluorination time, impulse voltage amplitudes and polarities, conclusions can be drawn that, fluorination has significant effects on the chemical structure properties and surface characteristic of oil-impregnated paper. Furthermore, the initial potential can be reduced and the decay process of surface charge can be accelerated with different fluorination time increasing from 0 to 30 min. Nevertheless, the tendency is the opposite when the fluorination time continuously extends from 30 to 60 min. The trap distribution also exhibits alternation through the fluorination.

Surface charge migration and dc surface flashover of surface-modified epoxy-based insulators

Epoxy-based model insulators were manufactured and fluorinated under a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for 15 min and 60 min. Surface charge accumulation and decay behavior were studied with and without dc voltage application. The effect of direct fluorination on surface charge migration as well as on flashover voltage was verified. The obtained results show that the charge decay of epoxy-based insulators is a slow process, but the decay rate increases when an outer dc electric field is applied. The surface charge distribution is changed when a streamer is triggered on the insulator surface. The existence of heteropolarity surface charges can decrease the dc surface flashover voltage to some extent, while the surface flashover voltage is almost unchanged when charges of the same polarity accumulate on the insulator surface. The short time fluorinated insulator can modify the surface resistivity, and the rate of surface charge dissipation is greatly increased under a dc electric field.

Surface charge and flashover characteristics of fluorinated PP under pulse voltage

Polypropylene (PP), with low loss and high dielectric strength, is widely used as dielectric for the metallised film capacitor (MFC). The electric ageing and dielectric breakdown of the dielectrics under high-voltage stress is a major concern in the research on polymer materials. Surface discharge and flashover may happen on the PP film between the metallised segments in MFCs, which can influence the dielectric property and cause the breakdown of insulation. Fluorination is effective on modifying the surface characteristics of the polymers, meanwhile retaining the bulk properties. The effects of direct fluorination on the surface charge and flashover characteristics of PP films under pulse voltage are experimented in this study. The samples with different fluorination time were prepared. Surface charging was performed by corona discharge using repetitive pulse power, which was applied to measure the surface flashover voltage as well. The experimental results indicated that the surface charge and flashover voltage were strongly correlated with the charge polarity, pulse frequency and fluorination time. It was suggested that the fluorination of certain time could significantly improve the decay rate of the surface charge as well as the surface flashover voltage of PP samples.

Effects of Direct Fluorination on Space Charge and Trap Distribution of PI Film in LN2

In recent years, high-temperature superconducting (HTS) cables are quickly being developed because of high efficiency and high current transportation capability. Compared with the HTS ac cable, the HTS dc cable has lower loss and stable properties. Under dc environment, the space charge formed in the polyimide (PI) film would distort the local electric field and further influence the characteristics of partial discharge and accelerate the insulation aging. Direct fluorination, as a solution to change the chemical component, can alter the electrical properties of the polymer surface. This paper concentrates on the effect of the fluorination on the space charge and trap distribution of the PI film in liquid nitrogen (LN 2 ). The polarization and leakage current of the PI film in LN 2 are measured under dc voltage. And then, the space charge mobility, amount, and the conductivity are calculated. Moreover, the trap distribution was obtained by the modified isothermal discharge current test. The results indicated that the fluorination improved the space charge mobility and conductivity. The appropriate fluorination suppressed the space charge accumulation in the PI film. However, excessive fluorination brought in more deep traps leading to significant amount of space charge accumulation in the bulk of polymer. As a consequence, the appropriate fluorination has a potential application for insulation in the HTS dc cable.

Effects of direct fluorination on space charge accumulation in HTV silicone rubber

HTV silicone rubber (SiR), due to its better electrical insulation, elasticity, temperature resistance, is used in cable accessories for HVDC cables. The space charge, formed under dc stress in SiR, would distort the local electric field, and furtherly influence the characteristics of partial discharge and accelerate the insulation aging. Direct fluorination, as a solution to change the chemical component, could alter the electrical properties of the polymer surface layer. This paper focuses on the effect of the fluorination on the space charge accumulation in HTV SiR. In this investigation, the samples were fluorinated at 25 ° using a F2/N2 mixture gas with 20 % F2 by volume at 0.05 MPa for 10, 20, 30 and 40 minutes. The space charge behavior of the fluorinated samples was observed by PEA method and the trap level distribution was obtained by modified isothermal discharge current (MIDC) test. The results indicated that the fluorination for 20 minutes effectively suppressed the space charge injection in SiR. However, excessive fluorination (over 30 minutes) brought in abundant deep traps, and led to significant amount of space charge accumulation in the bulk of polymer. As a consequence, the appropriate fluorination has significant effect on the space charge accumulation in HTV SiR and has a potential application for inner insulation in premolded accessories of HVDC cable.

Effect of direct fluorination on partial discharge characteristics of polyimide film used as magnet wire insulation of generator

Polyimide (PI) film is widely used as the insulation between coils in generator, where partial discharge (PD) may occur to cause the insulation breakdown. Fluorination is considered as an effective method for surface modification of polymer materials to change its electrical properties. In this study, to study the effect of fluorination on the PD behaviour of PI film used as magnet wire insulation, the samples of magnet wire were covered by PI film. Samples were fluorinated in a reaction kettle using the F2/N2 mixture for different time. The non-fluorinated and fluorinated samples were subjected to partial charges under alternating electric field. A wideband detection system was used to detect PD waveform and record discharge phenomenon. The experimental results indicate that the direct fluorination has great effects on the surface charge accumulation and the decrease in PD intensity. The magnet wire fluorinated for 45 min shows the highest PD inception voltage, the lowest intensity of successive discharges and the smallest number of discharge pulses.

Effect of direct fluorination on surface charge of polyimide films using repetitive pulsed power

As one of the basic insulating materials, polyimide films have been widely used in turn to turn insulation and turn to ground insulation in wind turbine generators. However, the pulse voltage from the converter often causes overvoltage and localized high electrical field in the insulation system. Surface charge could affect the dielectric property and cause the breakdown of insulation. Direct fluorination is a method to modify the surface layer of the polymers without changing the bulk properties. As a result of the fluorination, hydrogen atoms are replaced by the fluorine atoms and the component of the polymer surface is changed, which can influence the surface charge accumulation and affect the electrical properties. The experiment in this paper was focused on the effect of direct fluorination on surface charge of polyimide films under pulse voltage. The polyimide films with the fluorination time of 15, 30, 45 and 60 min were prepared, together with the unfluorinated one as a comparison. Surface charging was performed by corona discharging using repetitive pulse power at the temperature of 25 °C with the relative humidity of ~45%. The rising time of the pulse was 25 μs and the frequency was set at 200, 400, 600 and 1000 Hz, respectively. The surface charge decay was measured using a specific probe coupled with an electrostatic voltmeter. The results showed that the surface charge behavior was affected by the fluorination, corona discharging time, pulse voltage amplitude and frequency.

Effect of direct fluorination on surface partial discharge characteristics of polyimide films

Surface charge accumulation induced by partial discharge has a great effect on breakdown characteristics of polymer insulation. By changing the chemical component in the surface layer of insulation material, fluorination will cause the corresponding change in electrical property. This paper presents a study aimed at clarifying the effect of fluorination time on the partial discharge of fluorinated polyimide (PI) film. PI samples were fluorinated in a reaction kettle at about 328 K (55 °C) using a F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> blend with volume fraction of 20% nitrogen and 0.05 MPa (500 mbar) for respectively 15, 30, 45, 60 and 90 min. The partial discharge experiment was tested at room temperature with the relative humidity of ~ 40%. A wideband detection system and a video recorder were employed to detect the partial discharge current and record discharge images in a shielding darkroom. Obtained results show that the conductivity of polyimide film becomes large after the fluorination. The corona resistance time increases with the fluorination time increases from 0 to 45 min, while it decreases when the fluorination time increases from 45 to 90 min. Recurrent plot (RP) analysis of discharge current shows that the change of chemical component in surface layer of polyimide film by fluorination makes the partial discharge become more stable and regular.