KR100776641B1 - Evaluation method on degradation of polymeric materials by using dielectric relaxation properties - Google Patents

Abstract

A method for evaluating degradation of a polymeric material by using dielectric relaxation properties is provided to predict the limited life span of the polymeric material through a non-destructive process by using the change of the dielectric relaxation properties. A method for evaluating degradation of a polymeric material includes the steps of: measuring the isolation transition temperature of the polymeric material; polarizing the polymeric material by changing the frequency and forming an AC electric field in the polymeric material; measuring the complex relative permittivity and the complex relative loss coefficient from the polarization; and calculating the dielectric alleviation strength from the measured complex relative permittivity and complex relative loss coefficient.

Description

Evaluation method on degradation of polymeric materials by using dielectric relaxation properties

1 is a schematic diagram of an apparatus for measuring dielectric properties used in measuring dielectric relaxation strength according to the present invention;

2 is a graph showing a change in temperature dependence of PVDF due to radiation deterioration according to an embodiment of the present invention;

3 is a graph showing the relationship between the complex relative dielectric constant-complex dielectric loss factor of PVDF due to radiation deterioration according to an embodiment of the present invention; And

4 is a graph showing a change in dielectric relaxation strength of PVDF due to radiation deterioration according to an embodiment of the present invention.

The present invention relates to a method for nondestructively evaluating the degree of degradation of a polymer material by heat and radiation by using dielectric relaxation properties.

The dielectric relaxation phenomenon refers to the time dependence of the polarization phenomenon. Here, the polarization phenomenon refers to a phenomenon in which the amount of charge is rearranged or displaced by an electric field, and the higher the dielectric constant, the more polarization is formed. Types of polarization include electron polarization, ion polarization, orientation polarization (dipole polarization), and interface polarization (space charge polarization). Polarization is closely related to frequency. For example, when the audio frequency exceeds 20 kHz, the interfacial polarization phenomenon disappears, the orientation polarization disappears in the radio frequency, the ion polarization in the infrared, and the electron polarization disappears in the ultraviolet. This polarization is characterized in that the development is completed after a certain time, rather than being formed or disappeared at the time of application or removal when voltage is applied or removed. Dielectric relaxation time is shorter at higher temperatures. Therefore, the electron relaxation or ion polarization has a very short dielectric relaxation time, and the relaxation time of orientation polarization and interfacial polarization is relatively long. When voltage is applied to the dielectric, instantaneously high current flows due to the instantaneous charging current due to electron and ion polarization, and it exponentially decreases due to the dielectric absorption current due to orientation and interfacial polarization, and reaches a certain value after a certain time. At this time, the current value is referred to as leakage current. This is why the measurement of insulation resistance is based on the value 1 minute after application of voltage. Dielectric loss is also due to dielectric relaxation.

On the other hand, the lifespan of all electrical and electronic products depends on the lifespan of the material, and is particularly attained by the breakdown of the insulation due to deterioration of the insulating material. Deterioration means that a material causes harmful changes in its chemical structure by heat or light, and in particular, a permanent change in physical properties causes the property to deteriorate. Degradation factors of electrical insulation materials include temperature, voltage, current, pressure, vibration, moisture, chemical reaction, radiation, and the like, and thermal degradation and radiation degradation have the greatest influence.

Thermal deterioration refers to a phenomenon caused by an increase in the leakage current as the temperature increases, thereby increasing the oxidative decomposition reaction rate. Radiation deterioration refers to a phenomenon in which crosslinking or decomposition occurs between molecules when radiation is applied to an organic polymer material and is caused by diffusion of internal oxygen atoms.

The stressors that promote deterioration are as follows.

First, there is deterioration due to heat stress. Temperature rises that promote chemical reactions are the most common deterioration factors that increase the rate of degradation and shorten the life of the material. The limit of temperature rise in equipment is often determined in terms of thermal degradation of the material.

Second, there is deterioration due to electrical stress. This is due to the electric field applied to the device material and is caused by various causes such as conduction current, dielectric damage, electromagnetic force and electrostatic force or partial discharge. In addition to the thermal effect as Joule heat, the conduction current causes electrochemical effects in ionic conduction to degrade the material, and the dielectric loss occurs under an alternating electric field and causes thermal degradation. In addition, the electromagnetic force and the electrostatic force are mechanical forces generated by high voltages such as short-circuit and large currents, and have a mechanical effect. When a partial discharge of gas and liquid occurs in a high electric field, thermal action, particle impact action, and chemical action by excitation molecules or ions Rises and causes deterioration.

Third, there is deterioration due to mechanical stress. This is caused by mechanical stress, vibration, etc., and is caused by thermal distortion force due to a difference in coefficient of thermal expansion, electron stress due to short-circuit large current, etc. in addition to their conventional mechanical force.

Fourth, there is deterioration due to environmental stress. In high-energy radiation environments such as neutron beams, γ-rays, x-rays, electron beams, etc., caused by radioactive elements or particle accelerators, physical and chemical degradation are promoted. In addition, attention has been paid to hydrolysis by reactive substances, moisture absorption and erosion by microorganisms. Moreover, deterioration by strong ultraviolet irradiation is promoted even in a natural environment.

In general, however, these compounds are often combined with each other as compared with the deterioration of the above-mentioned factors acting alone. The deterioration at this time may be superimposed on the case where the deterioration is approximately alone, and the large deterioration may be promoted by the simple superposition when alone. In particular, when larger deterioration is accelerated, it is treated as a composite deterioration.

Meanwhile, in order to maintain the unique high stability and reliability, the cable for nuclear power plants maintains the cable performance during the life cycle of the power plant, radiation resistance, moisture resistance, heat resistance, ozone resistance, chemical resistance, high flame retardancy, and loss of cooling. Such cable characteristics are demanded. These cables are coated with a cable insulation material such as tin-plated twisted pair, EPR insulation, and synthetic rubber sheath structure, and are used for power, control, and instrumentation.

In the case of the insulation material of the cable, three types of power, communication, and control cable are connected to the reactor, and deterioration is caused by radiation and heat. In particular, when a nuclear accident occurs, deterioration of the cable may lead to deterioration of the cable, which may cause secondary accidents such as line short circuit or short circuit. Therefore, efforts are being made to prevent such accidents by periodically changing cables during operation.

In general, nuclear power plant cables are designed to last 40 years or more under normal operating conditions. However, cables in hot spots are expected to shorten life due to deterioration, and it is necessary to analyze the appropriate replacement time before design life. In particular, in case of cables in poor environmental conditions in case of design criteria accidents, the service life should be evaluated in consideration of the accident environmental conditions. Therefore, it is necessary to diagnose the exact deterioration state of the installed cable.

In the conventional deterioration evaluation method, a thermogravimetric analyzer (TGA) is used to record pyrolysis, by recording the weight change of a sample with time and temperature while maintaining the temperature or increasing the temperature of the material at a constant rate in a specific gas atmosphere. The method of calculating the material's intrinsic activation energy by analyzing the increase or decrease of weight due to sublimation, evaporation, and oxidation, is pulled until the tensile specimens specified in ISO37 and ISO / R527 are destroyed. How to measure the elongation by dividing the increase of the elongated length by the original length, 50% and 10% reduction, respectively, based on the dielectric breakdown strength and insulation resistance of the healthy materials specified in IEC 243 and IEC 167. There is a method of identifying the time point as a limit of the life, and a partial discharge measurement method.

According to Homma et al., The surface degradation of silicone rubber was measured with a thermogravimetric analyzer in an oxygen and nitrogen atmosphere to determine the chemical reaction due to the degradation of the siloxane matrix, but this is limited to surface degradation. Is heated from 50 ° C. to 700 ° C. at a rate of 10, 15, 20 ° C./min, the material may be destroyed during heating, and deterioration of a polymer material that is difficult to separate is difficult to measure (Proc. Of 1998 ISEIM, 631, 1998).

According to Anandakumaran et. Degradation and elongation and insulation resistance data are presented, but this should be accompanied by breakage of cable specimens (IEEE Trans. Dielect. Electr. Insul., 8, 5, 818, 2001).

In addition, Hommer et al. Introduced the leakage current measurement method related to the partial discharge method in order to evaluate the deterioration of the polymer insulating material, but has a disadvantage in that the degree of degradation in the dielectric cannot be evaluated by measuring only the surface deterioration (Proc). EEIC / ICWA, 655, 1993).

The partial discharge measuring method detects the discharge signals from the deteriorated material and the deteriorated material to determine the type of deterioration source by the size and number of discharge peaks based on the phase of applied voltage. Although the merits of the position estimation of the power source can be found in theory, it is difficult to apply in the field due to the low signal / noise ratio due to the superposition of the ambient partial noise and the partial partial discharge signal generated in the defect in the partial discharge measurement environment. According to the current IEC 27 regulations, deterioration is compared only by the magnitude of discharge amount.

Therefore, while the present inventors are studying a method for accurately evaluating the deterioration state of a polymer material through a non-destructive process, the deterioration evaluation method using dielectric relaxation characteristics does not damage the material at all, and deterioration of a polymer material that is difficult to separate The present invention was confirmed to be suitable for measuring the present invention.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for predicting the life of a polymer material and measuring and diagnosing the deterioration of the polymer material through a non-destructive process by using a change in dielectric relaxation characteristics according to the progress of deterioration of the polymer material.

In order to achieve the above object, the present invention

Measuring the glass transition temperature of the polymer material (step 1); Polarizing the polymer material by forming an alternating electric field by applying an alternating current voltage to the polymer material at a glass transition temperature measured in step 1 while changing a frequency (step 2); Measuring a complex relative dielectric constant and a complex relative dielectric loss coefficient from the polarization made in step 2 (step 3); And calculating the dielectric relaxation strength from the complex relative dielectric constant and the complex relative dielectric loss coefficient measured in step 3 (step 4).

Hereinafter, the present invention will be described in detail step by step.

Step 1

Step 1 is a step of measuring the glass transition temperature (Tg) of the polymer material.

The glass transition temperature refers to the temperature at which the polymer molecules become active and move due to an increase in temperature. This is an important physical property of only a high molecular material which is distinguished from a low molecular material which forms a solid phase by crystallinity.

In general, the glass transition temperature may be a thermal analysis method of directly measuring the phase change of the polymer material by applying heat. As the thermal analysis method, a method of measuring using a differential scanning calorymeter (DSC) or a dynamic mechanical analyzer (DMA) is typical.

On the other hand, the glass transition temperature of the step 1 according to the present invention can select the temperature at which the absorption peak measured from the change in the complex dielectric constant and the complex dielectric constant loss coefficient according to the temperature change of the polymer material.

Permittivity is an important characteristic value indicating the electrical properties of a dielectric material, and a large permittivity can be understood to basically mean that electrical energy is well transmitted. This permittivity can be expressed as a value obtained by measuring the relative permittivity of each dielectric by setting the permittivity of air to 1, which is called relative permittivity.

Unlike in vacuum, when a substance in air reacts with an external field (generally an alternating current electric field), it is affected not only by its external field but also by its frequency. As a result, the relative dielectric constant is regarded as a complex function of the frequency of the applied external field, and the relative dielectric constant at this time is called a complex dielectric constant.

In addition, when the alternating current electric field is applied to the dielectric, when the electric polarization by the molecules constituting the material cannot be followed, the corresponding polarization amount is changed to heat. In this case, the amount of heat lost is called a complex dielectric loss, which can be generally expressed as a complex dielectric loss factor.

In general, the complex dielectric constant and the complex dielectric loss coefficient of the polymer material exhibit a temperature-dependent characteristic, and the dispersion and energy absorption of the complex dielectric constant and the complex dielectric loss coefficient appear in two kinds above the glass transition temperature of the polymer material. . When this is shown as a temperature change type with respect to a constant frequency in an external alternating electric field, dispersion and absorption appear on the high temperature side or the low temperature side, respectively, corresponding to the absorption of the low frequency side and the high frequency side.

For example, as shown in FIG. 2 , when the complex non-dielectric loss coefficient of radiation accelerated degradation polyvinylidene fluoride (PVDF) is shown with respect to temperature change, the complex non-dielectric loss coefficient of healthy PVDF at a frequency of 1 kHz is shown. Shows a sharp increase in the vicinity of -25 ° C and gradually increases after 25 ° C. Since the rapid increase in the complex dielectric loss coefficient is the same as the actual glass transition temperature of the PVDF, the temperature at which the complex dielectric loss coefficient is rapidly increased can be selected as the glass transition temperature of the polymer material.

를 인가한 경우, 재료 내부에 흐르는 전전류는 저항성분과 커패시턴스 성분에 의하여 커패시턴스 성분에 대해 일정각(δ)만큼의 위상차를 가지게 된다. 이때 전전류는 커패시턴스, 유전율, 비유전율, 인가전압에 대해

로 표시되며 다시

로 된다. 여기서

는 재료의 비유전율,

는 복소 비유전율이며, 복소 비유전율

로 나타낼 수 있으며,

는 비유전손실계수이다. 유전율, 비유전율, tanδ 등은 측정시 인가한 주파수 및 온도에 따라 변화하며, 특정온도에서 다양한 주파수 인가로 측정한 복소 비유전율 및 복소 비유전 손실 계수를 종축, 횡축으로 두면 디바이(Debye) 방정식에 따라 복소좌표에서 이론상 반원(半圓)의 형태로 나타나게 된다. 실제 고분자 재료에 있어서는 완벽한 반원이 아닌 아크로 나타나게 되며, 복소 비유전율의 양 교점 간 크기는 전계인가로 인한 재료 내에서 발생한 유전분극의 크기를 나타내는 값으로 유전 완화 강도라고 한다.AC power source with polymer material as parallel equivalent circuit of resistance and capacitance

When is applied, the electric current flowing inside the material has a phase difference of a certain angle (δ) with respect to the capacitance component by the resistance component and the capacitance component. At this time, the total current is the capacitance, dielectric constant, relative dielectric constant,

And again

It becomes here

Is the dielectric constant of the material,

Is the complex dielectric constant, and the complex dielectric constant

Can be represented by

Is the dielectric loss factor. Dielectric constant, relative dielectric constant, tanδ, etc. vary depending on the frequency and temperature applied at the time of measurement. Therefore, in complex coordinates, it appears as a semicircle in theory. In actual polymer materials, they appear as arcs, not perfect semicircles, and the magnitude between the intersections of the complex dielectric constants is the dielectric relaxation strength that represents the magnitude of dielectric polarization in the material due to the application of an electric field.

Step 2

Step 2 is a step of polarizing the polymer material by forming an alternating electric field by applying an alternating voltage to the polymer material at a glass transition temperature measured in step 1 while varying the frequency.

The AC voltage and frequency applied in step 2 according to the present invention may vary depending on the type of polymer material and its glass transition temperature. For example, the AC voltage may be applied while varying the frequency at a constant AC voltage in the range of several to several tens of volts and the frequency of several Hz to several hundred kHz. Preferably, the AC voltage may be applied at 0.5 to 10 V and the frequency at 1 Hz to 100 kHz.

The polarization generated in the molecule due to deterioration of the polymer includes atomic polarization, electron polarization, orientation polarization, space charge polarization, and the like. Among them, atomic polarization and electron polarization are observed in the high frequency region of 100 MHz or more, but because of their small size, it is difficult to observe them. The space charge polarization is relatively high because it can be observed in a low temperature region of high temperature and is caused by intramolecular motion such as ion conduction and impurity ions, but does not exhibit a specific tendency. On the other hand, the orientation polarization is shown in proportion to the number of dipoles in the molecule increased by the deterioration of the polymer material, and is easier to measure than other polarizations, and the size of the orientation polarization may be the best to show the effect of deterioration. Accordingly, the present invention is characterized by measuring the change in the orientation polarization of the polymer material by deterioration and calculating the degree of deterioration of the polymer material therefrom.

Step 3

Step 3 is a step of measuring the complex relative dielectric constant and the complex relative dielectric loss coefficient from the polarization made in step 2.

When the AC voltage is applied in step 2, an AC electric field is formed, and the formed AC electric field polarizes the test piece with a polymer material, thereby causing a difference in phase angle between voltage and current. Therefore, in this step 3, the complex relative dielectric constant and the complex dielectric constant loss factor described in step 1 can be measured using the difference in the phase angles.

Step 4

Step 4 is a step of calculating the dielectric relaxation strength from the complex relative dielectric constant and the complex relative dielectric loss coefficient measured in step 3 above.

로 나타나며, β는 완화시간분포를 나타내는 파라미터이며, 완화강도

은

로 표현된다. 열화의 진행에 따라 유전 완화 특성의 변화는 주로 이온전도에 기인한 것으로 밝혀져 있으며, 이에 따라 완화강도는 증가하게 된다. 또한, 완화강도의 증가는 결정화도의 감소에 비례하는 것으로 알려져 있으며 일반적으로 결정화도는 열화의 진행에 따라 감소하게 된다. .The frequency characteristics of the complex dielectric constant of the polymeric material follow the Cole-Cole's circular arc law. The above rule

Where β is a parameter representing relaxation time distribution and relaxation intensity.

silver

It is expressed as As the deterioration progresses, it is found that the change in the dielectric relaxation characteristic is mainly due to the ion conductivity, and thus the relaxation strength increases. In addition, the increase in relaxation strength is known to be proportional to the decrease in crystallinity. Generally, the crystallinity decreases with the progress of deterioration. .

The dielectric relaxation intensity may be expressed as a difference between the complex dielectric constant of the instantaneous charge before polarization and the complex dielectric constant of the charge after polarization. At this time, the horizontal axis of the relationship curve between the complex relative dielectric constant of the material and the complex relative dielectric loss coefficient corresponds to the difference between the two intersection values.

In conclusion, the magnitude of the dielectric relaxation strength means the number of polarizations due to deterioration of the polymer material, and the size increases in proportion to the progress of the deterioration. Since the magnitude of the dielectric relaxation strength is particularly favorable for the progress of radiation and thermal degradation, the degradation of the polymer material can be evaluated through this.

1 shows a schematic diagram of an experimental apparatus for measuring dielectric properties used for measuring dielectric relaxation strength using the deterioration evaluation method according to the present invention. The device may comprise a control / analyzer, module processor, digital signal processor, analog / digital converter, frequency generator, response interface, electrodes, and the like.

In more detail, the experimental apparatus is equipped with a polymer material between two gold electrodes, and has a range of 1 kV to 100 mV (1 mW, 3 mW, 10 mW, 30 mW, 100 mW, 300 mW, 1 mW). , 3 ㎑, 10 ㎑, 30 ㎑, 100 ㎑) by applying a sine wave voltage of 1 V AC to form an alternating electric field to polarize the sample, and by using the difference in the phase angles of the resulting voltage and current Measure the total loss factor. The upper electrode uses a guard-ring shape, and the lower electrode includes a resistance temperature detector (RTD) for sensing the temperature of the specimen, and the specimen is fixed at a pressure of 300 N between both electrodes.

Hereinafter, the present invention will be described in detail by way of examples. However, the following examples are merely to illustrate the present invention, the contents of the present invention is not limited by the following examples.

Example

PVDF was used as the polymer material applied to the degradation evaluation. The change of the complex dielectric constant and the complex dielectric constant loss coefficient with the temperature change of the polymer material was measured, and the glass transition temperature at which the absorption peak occurred was determined. At the glass transition temperature, an alternating current of 1 V, a frequency of 1 Hz to 100 Hz is applied, an AC field is formed to polarize the specimen, and the complex relative dielectric constant and Complex non-dielectric loss coefficients were measured. In the relation between the measured complex dielectric constant and the complex dielectric constant loss coefficient, the dielectric relaxation strength was calculated as the absolute value of two intersections of the abscissa according to Cole-Col's arc law.

Hereinafter, the embodiment according to the present invention will be described in more detail.

1. Changes in temperature dependence of PVDF with deterioration

Polyvinylidene fluoride (PVDF) was used to accelerate degradation using radiation to compare with the material in a healthy state. Radiation accelerated degradation was achieved at a dose rate of 5 kGy / hr at room temperature and in the air using a ⁶⁰ Co γ-ray source and irradiated at 400 kGy and 1,000 kGy. The change in the complex non-dielectric loss coefficient with the temperature change of the accelerated deteriorated PVDF was measured, the glass transition temperature was measured, and the results are shown in FIG. 2 .

As shown in Figure 2 , it can be seen that the complex non-dielectric loss coefficient with temperature shows a peak due to energy absorption in the vicinity of -25 ℃.

2. Determination of the complex dielectric constant and the complex dielectric constant of PVDF due to radiation deterioration

A complex dielectric constant and a complex dielectric constant of the PVDF accelerated and deteriorated in the same manner as in (1) were measured, except that the PVDF was irradiated with ⁶⁰ Co γ-ray sources at 200, 400, 600, 800, and 1,000 kGy. the results are shown in Fig.

As shown in FIG . 3 , it can be seen that the relaxation strength obtained by the call-call arc law of PVDF according to the radiation acceleration deterioration increases with the increase of the radiation dose.

3. Comparison of dielectric relaxation strength of polymers due to deterioration

To compare the dielectric relaxation strength of polymers due to deterioration, From the results of the measurement of the complex dielectric constant and the complex dielectric constant loss factor (2), the value of the dielectric relaxation strength of PVDF in each case was calculated according to the call-call arc law and the results are shown in Table 1 and FIG. 4 .

Psalm Dielectric relaxation strength  Health PVDF 1.778 ⁶⁰ Co γ-rays 200 kGy Irradiated PVDF 1.972 ⁶⁰ Co γ-ray 400 kGy Irradiated PVDF 2.113 ⁶⁰ Co γ-ray 600 kGy Irradiated PVDF 2.204 ⁶⁰ Co γ-ray 800 kGy Irradiated PVDF 2.367 PVDF irradiated with ⁶⁰ Co γ-ray 1,000 kGy 2.525

As shown in Table 1 above, it was found that the dielectric relaxation strength increased with respect to the progress of thermal or radiation degradation in PVDF materials. In addition , as shown in FIG . 4 , it can be seen that degradation can be evaluated regardless of the degree of radiation deterioration by showing linearity with respect to deterioration. Therefore, the degradation of the polymer material can be evaluated by measuring the dielectric relaxation strength.

As described above, since the degradation evaluation method according to the present invention can evaluate the degradation of the polymer material only by the change of the frequency of application of the low voltage power source by measuring the dielectric relaxation strength, it does not damage the material at all and is difficult to separate. Since it is possible to predict the replacement cycle and failure in the installation state of the polymer material, the present invention can predict the life expectancy of the polymer material installed in the industrial and large factory facilities, etc., and can be usefully used to diagnose the deterioration state.

Claims (5)

Measuring the glass transition temperature of the polymer material (step 1); Polarizing the polymer material by forming an alternating electric field by applying an alternating current voltage to the polymer material at a glass transition temperature measured in step 1 while changing a frequency (step 2); Measuring a complex relative dielectric constant and a complex relative dielectric loss coefficient from the polarization made in step 2 (step 3); And Calculating dielectric relaxation strength from the complex dielectric constant and the complex dielectric constant loss coefficient measured in step 3 (step 4) Degradation evaluation method of a polymer material using a dielectric relaxation property comprising a. 2. The dielectric relaxation according to claim 1, wherein the glass transition temperature of step 1 selects the temperature at which the absorption peak measured from the change of the complex dielectric constant and the complex dielectric constant loss coefficient according to the temperature change of the polymer material is generated. Evaluation method of polymer material deterioration using characteristics. delete The method of claim 1, wherein the AC voltage is 0.5 to 10 V, and the frequency is 1 Hz to 100 kHz. delete

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