The Study and Determination of the Aging Life of New Environmentally Friendly Polypropylene Insulation Materials

Introduction

With the rapid development of industrial and agricultural production in China, the modernization of cities and the demand for electricity are increasing. Power cables, as an indispensable and important part of transmission lines, have a working reliability that is directly related to the stability and safety of the power grid as a whole. Today, the concept of environmentally friendly energy conservation has become a global trend, and the power cable industry has also begun to attach importance to the development and research of environmentally friendly cables. Polypropylene (PP), as an environmentally friendly thermoplastic material, has good recyclability and processability, and meets the requirements of low-carbon environmentally friendly materials. At the same time, the insulation made of PP material has many advantages, such as high breakdown field strength, high insulation strength, low dielectric loss, low heat generation, and high-temperature resistance. Therefore, PP insulation is also widely used in the research and development design of power cables and is a major trend in the development of power cables [1,2].

However, PP, like other polymer materials, has the problem of easy aging. Since there are a large number of tertiary carbon atoms on the PP macromolecular chain, it is prone to oxidation under the action of light and heat when exposed to the environment, resulting in the breaking of the macromolecular chain and the generation of active free radicals. Therefore, in the application of PP insulation, it is important to reasonably estimate its service life and prevent serious economic losses due to insulation aging. Studying the aging characteristics of PP and predicting its service life can not only provide a reference for the effective service life of products but also help improve the performance and service life of PP cables. In this study, two types of self-made PP insulation materials with better performance were selected to address the problem of PP insulation material life prediction. Based on the Arrhenius equation and the inverse power law (IPM model), the model parameters were obtained by using the linear regression parameter solution method and the least squares estimation method based on accelerated tests. The consistency of the test aging mechanism was verified, and the thermal oxygen aging life and electrical aging life of PP insulation materials were studied and measured.

Experimental Materials and Methods

Experimental Materials

Two types of self-made PP insulation materials, PP-6 (PP6) and PP-S (PPS), as well as PPS cable insulation that had been extruded and formed (PPS cable), were selected for the aging test.

Main Equipment and Preparation Methods

Thermal Oxygen Aging Test

The main equipment included an RL45 oven, a PolyLab torque rheometer, and a DSC 200 F3 differential scanning calorimeter.

The preparation method was as follows: the PP granules were pretreated in a vacuum drying oven for 24 hours, and the temperature in the oven was maintained at 50 °C. Then, the granules were poured into a torque rheometer for stirring and blending. The screw speed was set to 60 r/min, and the kneading time was set to 20 minutes. Finally, the obtained PP material was fully dried for later use. The obtained PP material was cut into flakes of approximately the same shape and volume with a knife, accurately weighed to 6.5 mg, and placed in a differential scanning calorimeter (DSC) special aluminum crucible for later use. PPS cable: A 0.5 mm thick sample was prepared by a ring cutter, dried by the above method, and then cut into flakes of the same shape and volume as the granule sample for later use.

Electrical Aging Test

The main equipment included an oven, a torque rheometer (same model as the thermal oxygen aging test), a ZG flat vulcanizing machine, and a ZJC-50 kV AC breakdown test system.

The preparation method was as follows: the PP mixture was made into a circular PP film with a diameter of 64 mm and a thickness of 0.2 mm by a mixing and pressing method. To ensure consistent test conditions, all test preparation processes were the same.

Thermal Oxygen Aging Life of Polypropylene Insulation

Oxidation Induction Period Test and Thermal Oxygen Aging Life Prediction Model

The thermal oxygen aging life of a material is inferred from the validity period of the antioxidant remaining in the PP material, that is, the oxidation induction period (OIT). The effective concentration of antioxidants varies with temperature. Measuring the OIT at a single temperature by DSC may be affected by the change in antioxidant concentration caused by sample volatilization, resulting in incorrect results. Therefore, based on the OIT measured at different temperatures, the service life at other temperatures is estimated, which can eliminate the influence of the thermal stability of the antioxidant. In this study, OIT tests were carried out on three test materials at six different temperatures. Under the protection of nitrogen atmosphere, the temperature was raised to 180, 190, 200, 210, 220, and 230 °C at a rate of 10 °C/min, and the temperature was kept constant. After 5 minutes, the oxygen atmosphere was switched until obvious oxidation exotherm occurred (oxidation peak appeared on the test interface). The time from the starting point of switching the oxygen atmosphere to the critical position of the oxidation peak was taken as the OIT, and the critical point of the oxidation peak was taken as the end point of the life. The OIT test results of three test materials at six different temperatures are shown in Table 1. Where “-” means that no oxidation peak occurred within 1000 minutes at this temperature.

Table 1 OIT test results of three materials

It can be seen from Table 1 that the OIT of each test material decreases with the increase of temperature, indicating that the life of the PP material shortens with the increase of aging temperature. According to the Arrhenius equation in formula (1), the specific calculation and derivation process of the thermal oxygen aging life model fitting relationship is as follows:

Z=Aexp(−RTEa) (1)

In the formula: Ea is the activation energy, kJ/mol; T is the aging temperature, K; R is the ideal gas constant, J/(mol·K); A is the exponential factor, min; Z is the reaction rate factor related to T, min.

P=ZY (2)

In the formula: P is the material reaction function; Y is the reaction time, that is, OIT, min.

When P is at the failure critical point, Z and P are constants. Substituting formula (2) into formula (1) and taking the logarithm on both sides, formula (3) can be obtained at the failure critical point.

lnY=a+b/T (3)

Where: a is a constant, that is, lnP−lnA; b is the solution coefficient.

According to the OIT test results, the relationship curve between lnY and T−1 is shown in Figure 2.

Figure 2 Fitting curve of lnY and T−1