Aging test is one of the important means to improve product reliability, and it cannot be replaced by other methods at present. Through the aging test, the problems and defects of the product under various environmental conditions can be exposed, and these problems can be repaired and improved, thereby improving the reliability and service life of the product. Commonly used reliability equipment include: UV aging test chamber, xenon lamp aging test chamber, etc.
Ⅰ. Selection of artificial accelerated aging test conditions
This question can actually be understood as what aging factors should be simulated. During the use of polymer materials, many factors in the climate environment may have an effect on the aging of polymer materials. If the main factors causing aging are known in advance, the test method can be selected in a targeted manner.
We can determine the test method by considering the transportation, storage, use environment and aging mechanism of the material. For example, rigid polyvinyl chloride profiles are made of polyvinyl chloride as raw material and added with additives such as stabilizers and pigments. They are mainly used outdoors. Considering the aging mechanism of PVC, PVC is easy to decompose when heated; considering the use environment, oxygen, ultraviolet light, heat and moisture in the air are all causes of profile aging.
Ⅱ . Selection of light source for artificial accelerated aging test
Laboratory light source exposure test: It can simultaneously simulate light, oxygen, heat, rainfall and other factors in the atmospheric visible environment in a test chamber. It is a commonly used artificial accelerated aging test method. Among these simulation factors, the light source is relatively important. Experience shows that the wavelengths in sunlight that cause damage to polymer materials are mainly concentrated in ultraviolet light and some visible light.
The artificial light sources currently used strive to make the energy spectrum distribution curve in this wavelength range close to the solar spectrum. Simulation and acceleration rate are the main basis for selecting artificial light sources. After about a century of development, laboratory light sources include closed carbon arc lamps, sunlight-type carbon arc lamps, fluorescent ultraviolet lamps, xenon arc lamps, high-pressure mercury lamps and other light sources to choose from. Technical committees related to polymer materials in the International Organization for Standardization (ISO) mainly recommend the use of three light sources: solar carbon arc lamps, fluorescent ultraviolet lamps, and xenon arc lamps.
01. Xenon arc lamp
It is currently believed that the spectral energy distribution of xenon arc lamps among known artificial light sources is most similar to the ultraviolet and visible parts of sunlight. By choosing an appropriate filter, most of the shortwave radiation present in sunlight reaching the ground can be filtered out. Xenon lamps have strong radiation in the infrared region of 1000nm~1200nm and generate a large amount of heat.
Therefore, a suitable cooling device must be selected to take away this energy. Currently, there are two cooling methods for xenon lamp aging test equipment on the market: water-cooled and air-cooled. Generally speaking, the cooling effect of water-cooled xenon lamp devices is better than that of air-cooled ones. At the same time, the structure is more complex and the price is more expensive. Since the energy of the ultraviolet part of the xenon lamp increases less than the other two light sources, it is the lowest in terms of acceleration rate.
02. Fluorescent UV lamp
Theoretically, shortwave energy of 300nm~400nm is the main factor causing aging. If this energy is increased, rapid testing can be achieved. The spectral distribution of fluorescent UV lamps is mainly concentrated in the ultraviolet part, so it can achieve higher acceleration rates.
However, fluorescent UV lamps not only increase the ultraviolet energy in natural sunlight, but also radiate energy that is not present in natural sunlight when measured on the earth's surface, and this energy can cause unnatural damage. In addition, except for the very narrow mercury spectral line, the fluorescent light source does not have energy higher than 375nm, so materials that are sensitive to longer wavelength UV energy may not change as they do when exposed to natural sunlight. These inherent flaws can lead to unreliable results.
Therefore, fluorescent UV lamps are poorly simulated. However, due to its high acceleration rate, rapid screening of specific materials can be achieved by selecting the appropriate type of lamp.
03. Sunlight carbon arc lamp
Sunlight-type carbon arc lamps are currently rarely used in our country, but they are widely used light sources in Japan. Most JIS standards use sunlight-type carbon arc lamps. Many automobile companies in my country that are joint ventures with Japan still recommend the use of this light source. The spectral energy distribution of the solar carbon arc lamp is also closer to that of sunlight, but the ultraviolet rays from 370nm to 390nm are concentrated and strengthened. The simulation is not as good as the xenon lamp, and the acceleration rate is between the xenon lamp and the ultraviolet lamp.
Ⅲ . Determination of artificial accelerated aging test time
1. Refer to relevant product standards and regulations
Relevant product standards have already stipulated the time for the aging test. We only need to find the relevant standards and execute them according to the time specified therein. Many national standards and industry standards have stipulated this.
2. Calculation based on known correlations
Research shows that the color stability of ABS is evaluated through changes in color and yellowing index. Artificial accelerated aging has a good correlation with natural atmospheric exposure, and the acceleration rate is about 7. If you want to know the color change of a certain ABS material after one year of outdoor use and use the same test conditions, you can refer to the acceleration rate to determine the accelerated aging time 365x24/7=1251h.
For a long time, a lot of research has been carried out on correlation issues at home and abroad, and many conversion relations have been derived. However, due to the diversity of polymer materials, differences in accelerated aging test equipment and methods, and differences in climate at different times and regions, the conversion relationship is complicated. Therefore, when selecting the conversion relationship, we must pay attention to the specific materials, aging equipment, test conditions, performance evaluation indicators and other factors that derive the correlation.
3. Control the total amount of artificially accelerated aging radiation to be equivalent to the total amount of natural exposure radiation
For some products that do not have corresponding standards and no reference for correlation, the radiation intensity of the actual use environment can be considered, and the total amount of artificially accelerated aging radiation should be controlled to be equivalent to the total amount of natural exposure radiation.
Example: How to control the total radiation amount of artificial accelerated aging
A certain plastic product is used in the Beijing area, and it is expected to control the total radiation amount of artificially accelerated aging to be equivalent to one year of outdoor exposure.
Step 1: Since this product is a plastic product and is used outdoors, choose Method A in GB/T16422.2-1996 "Plastic Laboratory Light Source Exposure Test Methods Part 2: Xenon Arc Lamp".
The test conditions are: irradiation intensity 0.50W/m2 (340nm), blackboard temperature 65°C, box temperature 40°C, relative humidity 50%, water spray time/no water spray time 18min/102min, continuous light;
Step 2: The total annual radiation in Beijing is about 5609MJ/m2. According to the international standard CIENo85-1989 (GB/T16422.1-1996 "Plastic Laboratory Light Source Exposure Test Methods" for comparing the spectral distribution of artificial light sources and natural sunlight) Part: Cited in "Xenon Arc Lamp"); of which the ultraviolet and visible regions (300nm~800nm) account for 62.2%, or 3489MJ/m2.
Step 3: According to GB/T16422.2-1996
When the 340nm irradiation intensity is 0.50W/m2, the irradiation intensity in the infrared and visible areas (300nm~800nm) is 550W/m2; the irradiation time can be calculated as 3489X106/550=6.344X106s, which is 1762h. According to this calculation method, the acceleration factor is about 5. Since natural aging is not a simple superposition of irradiation intensity, it is only determined that sunlight is causing the material.