1. 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.
Therefore, the national standard GB/T8814-2004 "Unplasticized polyvinyl chloride (PVC-U) profiles for doors and windows" not only stipulates the photooxygen aging test method, but also adopts GB/T16422.2 "Plastic Laboratory Light Source Exposure Test" Part 2 of the method: Xenon arc lamp aging for 4000h or 6000h, simulating factors such as outdoor ultraviolet light and visible light, temperature, humidity, rainfall, etc., and also stipulates the thermal oxygen aging items: state after heating, placed at 150°C for 30 minutes, visual observation Check whether there are bubbles, cracks, pitting or separation to examine the heat resistance of the profile. Another example is a product that my country has competitiveness in the international market: foreign trade export shoes. During use, ultraviolet rays in sunlight are the main cause of discoloration and fading of shoes. Therefore, it is necessary to use a UV light box to test their yellowing resistance.
The commonly used footwear yellowing resistance test chamber uses a 30W UV lamp. The sample is 20cm away from the light source. The color change is observed after 3 hours of exposure. At the same time, during transportation, the hot, humid and harsh environment in the container will cause discoloration, spots, and even deterioration of shoe uppers, soles, and glue. Therefore, before shipment, it is necessary to consider conducting a heat and humidity resistance aging test to simulate the high heat and high humidity environment in the container. Under the conditions of 70°C and 95% relative humidity, observe the appearance and color changes after 48 hours of testing.
2. 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.
3. 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℃, box temperature 40℃, 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.
4. Selection of performance evaluation indicators for artificial accelerated aging test
The selection of performance evaluation indicators is mainly considered from two aspects: the use of the material and the characteristics of the material itself.
1. Determine the evaluation index according to the use of the material. For the same material, due to its different uses, different evaluation indexes may be selected. For example, if the same paint is used for decoration, the change in its appearance must be considered. In GB/T1766-1995 "Rating of Aging of Paint and Varnish Coatings", the rating methods for various appearance changes such as gloss, color change, chalking, and gold-finishing are specified in detail.
For some functional coatings, such as anti-corrosion coatings, a certain degree of color and appearance changes are acceptable. At this time, when selecting evaluation indicators, the main considerations are its cracking resistance, degree of powdering, etc. It is also polyvinyl chloride (PVC). If it is used to make shoe uppers, its resistance to yellowing must be considered. If it is used in rain downpipes, the requirements for appearance changes are not high, and the physical and mechanical properties of the material change, such as pulling The change in tensile strength is the main assessment index.
2. Determine the evaluation index based on the characteristics of the material itself. For the same material, different properties decline at an unequal rate during the aging process. In other words, certain properties are sensitive to the environment and decline rapidly, which is the main factor causing material damage. When selecting evaluation indicators, these sensitive properties should be selected. Research shows that for most engineering plastics, the impact strength changes greatly and decreases significantly during natural aging tests.
Therefore, when conducting aging tests of engineering plastics, priority should be given to selecting the impact strength decrease as the evaluation index. Impact strength is also very sensitive to the aging of polypropylene and is the main indicator for assessing aging performance. For polyethylene materials, the decrease in elongation at break is more obvious and is the priority evaluation index. For polyvinyl chloride, both tensile strength and impact strength decrease relatively quickly, and one of them should be selected for evaluation based on the actual situation.
In the national standard GB/T8814-2004 "Unplasticized polyvinyl chloride (PVC-U) profiles for doors and windows", the impact strength retention rate after aging ≥ 60% is selected as the qualification indicator; in the light industry standard QB/T2480 -2000 Rigid polyvinyl chloride (PVC-U) rainwater pipes and fittings for construction, the tensile strength retention rate after aging is ≥80% as the qualification criterion.