Discussion on common problems in artificial weathering accelerated aging test
Polymer materials such as plastics, rubber, and paints may encounter aging problems during use. In order to evaluate the aging resistance of polymer materials, two types of aging test methods have been formed: one is the natural aging test method, that is, the aging test directly using the natural environment; the other is the artificial accelerated aging test method, that is, in the experiment The chamber uses an aging chamber to simulate aging tests conducted by certain aging factors of natural environmental conditions. Due to the diversity of aging factors and the complexity of the aging mechanism, natural aging is undoubtedly the most important and reliable method of aging test. However, due to the relatively long natural aging cycle, the differences in climatic conditions in different years, seasons and regions lead to incomparability of test results; and the artificial accelerated aging test simulations strengthen some important factors in the natural climate, such as sunlight. Temperature, humidity, rainfall, etc., shorten the cycle of the aging test, and the test results are reproducible due to the controllability of the test conditions. As an important supplement to natural aging, artificial aging is widely used in the research, development and testing of polymer materials.
In the process of artificial accelerated aging, people generally pay attention to the following questions: what kind of test conditions should be selected and how long to conduct tests; what indicators should be selected to evaluate the aging performance of the product. This paper attempts to explore some of the problems associated with artificial accelerated aging tests.
1
Selection of artificial accelerated aging test conditions
This problem can be understood as the aging factor that should be simulated. During the use of polymer materials, many factors in the climatic 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 from the aspects of transportation, storage, use environment and aging mechanism of the material. For example, a hard polyvinyl chloride profile is made of polyvinyl chloride as a raw material, and is added with additives such as stabilizers and pigments, and is mainly used for outdoor use. Considering the aging mechanism of polyvinyl chloride, PVC is easily decomposed by heat; considering the use environment; oxygen, ultraviolet light, heat and moisture in the air cause the aging of the profile.
Therefore, the national standard GB/T8814-2004 "unplasticized polyvinyl chloride (PVC-U) profiles for doors and windows", not only specifies the photo-oxidation aging test method, using GB/T 16422.2 "plastic laboratory light source exposure test method" The second part: Xenon arc lamp aging 4000h or 6000h, simulating outdoor ultraviolet light and visible light, temperature, humidity, rainfall and other factors, and also stipulated the thermal oxygen aging project: after heating, placed at 150 ° C for 30 min, visual observation whether Bubbles, cracks, pitting or separation phenomena appear to investigate the heat resistance of the profile. Another example is China's competitive products in the international market: foreign trade exports shoes. In the process of use, the ultraviolet rays in the sun are the main cause of discoloration and fading of the shoes. Therefore, it is necessary to test the yellowing resistance with an ultraviolet light box. The commonly used footwear yellowing test chamber uses a 30W UV lamp, the sample is 20cm away from the light source, and the color change is observed after 3 hours of irradiation. At the same time, during the transportation process, the harsh environment of the hot and humid inside the container may cause discoloration, spots and even deterioration of the upper, the sole and the glue. Therefore, before shipment, it is necessary to consider the heat and humidity aging test to simulate the high heat and high humidity environment in the container. Observe the appearance and color change after 48 hours test at 70 ° C and 95% relative humidity.
2
Artificial accelerated aging light source selection
The laboratory light source exposure test is a commonly used artificial accelerated aging test method because it can simultaneously simulate the light, oxygen, heat and rainfall in the atmosphere visible in a test chamber. Among these simulation factors, The light source is the most important. Experience has shown that the wavelengths that cause the polymer material to break in the sun 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. The simulation and acceleration magnification are the main basis for selecting artificial light sources. After about a century of development, the laboratory light source has a variety of light sources such as closed carbon arc lamps, sunlight-type carbon arc lamps, fluorescent ultraviolet lamps, xenon arc lamps, and high-pressure mercury lamps. The technical committees related to polymer materials in the International Organization for Standardization (ISO) mainly recommend the use of sunlight-type carbon arc lamps, fluorescent ultraviolet lamps, and xenon arc lamps.
2.1
Xenon arc lamp
It is currently believed that the spectral energy distribution of a xenon arc lamp in a known artificial light source is most similar to the ultraviolet and visible light portions of sunlight. By selecting the appropriate filter, most of the short-wave radiation present in the surface sunlight can be filtered out. The xenon lamp has a strong radiation peak in the near-infrared region of 1000 nm to 1200 nm, which generates a large amount of heat. Therefore, you must choose the appropriate cooling device to take this part of the energy. At present, there are two cooling methods for the xenon lamp aging test device on the market: water-cooled and air-cooled. In general, the cooling effect of the water-cooled xenon lamp device is better than that of the air-cooled type, and the structure is also complicated and the price is relatively expensive. Since the ultraviolet light energy of the xenon lamp is less increased than the other two light sources, it is the lowest in terms of the acceleration magnification. 2.2 fluorescent UV lamp
Theoretically, short-wave energy from 300 nm to 400 nm is the main cause of aging. If you increase this part of the energy, you can achieve the effect of rapid test. The spectral distribution of the fluorescent UV lamp is mainly concentrated in the ultraviolet light portion, so that a higher acceleration magnification can be achieved. However, fluorescent UV lamps not only increase the UV energy in natural daylight, but also the radiant energy that is not found in natural daylight when measured on the Earth's surface, and this part of the energy causes unnatural damage.
In addition, the fluorescent light source has no energy higher than 375 nm except for the narrow mercury spectral line, so that materials sensitive to longer wavelength UV energy may not change as exposed to natural sunlight. These inherent defects can lead to unreliable results. Therefore, the fluorescent ultraviolet lamp is inferior in simulation. However, due to its high rate of acceleration, rapid screening of specific materials can be achieved by selecting the appropriate type of lamp.
2.3
Sunlight carbon arc lamp
Sunlight-type carbon arc lamps are currently used less in China, but they are widely used in Japan. Most JIS standards use sunlight-type carbon arc lamps. Many of China's joint ventures with Japanese companies still recommend the use of such light sources. The spectral energy distribution of the sunlight-type carbon arc lamp is also closer to that of sunlight, but it is concentrated at 370 nm-390 nm, and the simulation is inferior to that of the xenon lamp. The acceleration ratio is between the xenon lamp and the ultraviolet lamp.
3
Determination of test time
3.1
Refer to relevant product standards
The time of the aging test has been stipulated in the relevant product standards. We only need to find the relevant standards and execute it according to the time specified in it. This is stipulated in many national standards and industry standards. Table 1 lists the aging time provisions in some common product standards. 3.2 Based on known correlations
Studies have shown that the color stability of ABS is evaluated by the change of color and yellowing index, and the 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 an ABS material after one year of outdoor use, the same test conditions can be used to refer to the acceleration rate to determine the accelerated aging time 365x24/7=1251h.
For a long time, a lot of researches have been carried out on related sexual issues at home and abroad, and many conversion relationships have been obtained. However, due to the diversity of polymer materials and the different aging test equipment and methods, the differences in climate at different times and regions have led to a complicated conversion relationship. Therefore, when selecting the conversion relationship, it is necessary to pay attention to the specific materials, aging equipment, test conditions, performance evaluation indicators and other factors of the correlation. 3.3 Control the artificial accelerated aging radiation total and natural exposure
The total amount of radiation is quite
For some products that have no corresponding standard regulations and no reference correlation, the radiation intensity of the actual use environment can be considered, and the total amount of artificial accelerated aging radiation is controlled to be equivalent to the total amount of natural exposure radiation. Table 2 lists the solar radiation intensity in different regions of China [2].
The following example shows how to control the total amount of radiation accelerated by artificial acceleration:
A plastic product is used in the Beijing area, and it is expected that the total amount of artificially accelerated aging radiation will be equivalent to one year of outdoor exposure.
The first step: Since the product is a plastic product and is used outdoors, it is selected to adopt the A method in GB/T16422.2-1996 "Plastics Laboratory Light Source Exposure Test Method Part II: Xenon Arc Lamp". The test conditions are: irradiation intensity 0.50W/m2 (340nm), blackboard temperature 65°C, cabinet temperature 40°C, relative humidity 50%, water spray time/no water spray time 18min/102min, continuous illumination;
Step 2: From Table 2, the total amount of radiation in Beijing in one year is 5609 MJ/m2, according to the international standard CIE No 85-1989 for comparing the spectral distribution of artificial light source and natural sunlight radiation (see Table 3, GB/T16422.1- 1996 "Plastics laboratory light source exposure test method Part I: Xenon arc lamp" cited;) The ultraviolet region and visible region (300nm-800nm) accounted for 62.2%, namely 3489MJ/m2.
The third step: According to GB/T 16422.2-1996, when the 340nm irradiation intensity is 0.50 W/m2, the irradiation intensity of the infrared region and the visible region (300nm～800nm) is 550 W/m2; the irradiation time can be calculated as 3489 X 106/550=6.344 X 106s, ie 1762h. According to this calculation method, the acceleration magnification is about 5. Since natural aging is not a superposition of simple irradiance, only when it is determined that sunlight is causing the material
This calculation method can only be used when the main factor of the damage is broken and the test time cannot be determined by other methods.
4
Selection of performance evaluation indicators
The selection of performance evaluation indicators is mainly considered from the aspects of the use of materials and the characteristics of the materials themselves. 4.1 Determine evaluation indicators based on material use
For the same materials, the evaluation indicators may be chosen differently depending on their uses. For example, the same is the paint, if it is used for decoration, it must focus on the change of its appearance. In GB/T 1766-1995 "Classification of Paint and Varnish Coating Aging", the rating methods for various appearance changes such as gloss, color change, chalking, and pan-gold are specified in detail.
For some functional coatings, such as anti-corrosion coatings, a certain degree of color and appearance change is acceptable. At this time, when selecting the evaluation index, the crack resistance and the degree of chalking are mainly considered. The same is polyvinyl chloride (PVC), if used to make the upper, it must consider its yellowing resistance, and if it is used for 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 indicator. 4.2 Determine the evaluation index according to the characteristics of the material itself
For the same material, the decline in performance during aging is not constant. In other words, some properties are sensitive to the environment and fall the fastest, which is the main factor causing material damage. These sensitive properties should be selected when selecting evaluation indicators. Studies have shown that for most engineering plastics, the impact strength is the most significant change and the most obvious decline in the natural aging test. Therefore, in the aging test of engineering plastics, priority should be given to selecting the drop in impact strength as an evaluation index. Impact strength is also quite sensitive to the aging of polypropylene [4], which is the main indicator for assessing aging performance. For polyethylene materials, the drop in elongation at break is the most obvious and is a preferred indicator of evaluation. For polyvinyl chloride, the tensile strength and impact strength are reduced relatively quickly, and one of them should be selected according to 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 is selected to be ≥60% as the qualified judgment index; in the light industry standard QB/T2480 - 2000 for construction
In the rigid polyvinyl chloride (PVC-U) rainwater pipe and fittings, the tensile strength retention rate after aging is selected to be ≥80% as the qualified judgment index.
5
Conclusion
The artificial accelerated aging test has been rapidly developed due to the rapid evaluation of the weather resistance of materials. As an important supplement to natural aging, it is widely used in the research, development and testing of polymer materials. The selection of test conditions, the choice of light source, the determination of test time, and the selection of performance evaluation indicators are frequently encountered in artificial accelerated aging tests. This article discusses the above aspects and puts forward some ideas for solving the problem.
