Study on High Accelerated Life Aging Test Conditions

During the processing, storage, and use of polymer materials, due to the combined effects of internal and external factors, their physical, chemical, and mechanical properties gradually fail, resulting in the ultimate loss of use value. This phenomenon is known as material aging. In order to evaluate the aging resistance of polymer materials, there are mainly two types of aging test methods: one is the natural aging test method; The second is the artificial accelerated aging test method. Natural aging test is undoubtedly the most important and reliable test method, but due to its long testing cycle and different environmental stresses in different regions, the test results lack comparability. As a supplement to natural aging test, artificial accelerated aging test greatly shortens the aging test cycle by strengthening environmental stress and increasing the controllability of environmental stress, which is widely used in the research of polymer materials Testing and development in progress.

However, currently, artificial accelerated aging tests, such as ultraviolet irradiation tests, damp heat tests, and wet freezing tests, have test cycles ranging from hundreds of hours to thousands of hours, which cannot meet the requirements of quickly and efficiently judging the reliability of materials. On this basis, people have introduced high accelerated life aging tests, such as HALT tests, HASS tests, HAST tests, and PCT tests. By increasing environmental stress, the test cycle is tens of hours, Achieve the expected experimental results.

As HALT test and HASS test are test methods for design quality verification and manufacturing quality verification extended by the US military, they are currently mainly used for the verification of high-precision electronic and electrical equipment. For the photovoltaic testing field, the common high accelerated life aging tests are mainly HAST test (product level) and PCT test (pressure cooker test), whose main role is to test the ability of products to withstand moisture under certain pressure conditions. From the perspective of test environment control, the difference between HAST test and PCT test mainly lies in the control method of environmental pressure. The temperature, pressure, and humidity parameters of HAST test are randomly set according to user test conditions, and are not affected by saturated steam pressure and saturated steam humidity. They are under unsaturated control. The PCT test can only set the temperature and humidity parameters of the test environment, and the ambient pressure is the saturated vapor pressure at the corresponding temperature.

The standards used for HAST testing mainly include IEC, JIS, JEITA, JEDEC, and JPCA. The test standard requirements are shown in Table 1.

Table 1 Reference Standards for HAST Test

The standard source for PCT testing in the photovoltaic detection field is UL1703, which does not explicitly specify the parameter settings for the experiment. Therefore, under the conditions that meet the physical properties of the material within its scope of application, the commonly used test parameters are 121 ℃ and 100% RH, and the test duration varies. Under this experimental condition, it can be seen from the "Saturated Vapor Pressure Temperature Correspondence Table" of water that at 121 ℃, the saturated vapor pressure of water is 204.85 KPa, which is about two standard atmospheric pressures (101.325 KPa).

For high accelerated life testing, the most critical part is to require that the failure mechanism of materials under artificial accelerated aging testing be the same as that of materials under natural conditions, followed by an appropriate acceleration factor. Therefore, the corresponding acceleration factor must be calculated when selecting experimental conditions. Currently, the Arrhenius equation is used to calculate the acceleration factor under the high temperature and humidity environment model, and its formula is as follows:

AF=exp{(Ea/k)*[(1/Tu)-(1/Ts)]+(RHsn-RHun)}

Note:

AF - Accelerated Factor;

Ea - Starting energy of material aging reaction (eV);

K - Boltzmann constant (k=8.617 × 10-5 eV/K）；

T - absolute temperature (subscript u refers to normal state, subscript s refers to accelerated state);

RH - Relative humidity (subscript u refers to normal state, subscript s refers to accelerated state, and n refers to failure factor)

Detected acceleration ratio=10 × AF；

Based on the above acceleration factors, it can be calculated that when the acceleration conditions are 121 ℃, 100% RH, and two atmospheric pressures, the accelerated aging of polymer materials under this environmental condition for 48 hours can characterize their life cycle of withstanding about 24.8 years under outdoor conventional conditions, and 48 hours is also the most widely used accelerated test cycle in the industry.