Plastics, rubber, paints and other polymer materials will encounter aging problems during use
Polymers such as plastics, rubbers, and coatings will experience aging problems during use. In order to evaluate the aging resistance of polymer materials, two types of aging test methods have been gradually 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 the aging test of certain aging factors of the natural environmental conditions. Due to the diversity of aging factors and the complexity of aging mechanisms, natural aging is undoubtedly the most important and most reliable aging test method. However, due to the relatively long natural aging cycle, differences in climate conditions in different years, seasons, and regions have led to incomparable test results, and artificial accelerated aging test simulations have strengthened certain important factors in the natural climate, such as sunlight, Temperature, humidity, rainfall, etc. shorten the cycle of the aging test, and due to the controllability of the test conditions, the test results are reproducible. Artificial aging is an important supplement to natural aging and is widely used in the research, development and testing of polymer materials.
In the experimental process of artificial accelerated aging, people generally care about the following questions: what kind of test conditions should be selected and how long to conduct the test; what indicators should be selected to evaluate the aging properties of the product. This article attempts to address some of these issues on artificial accelerated aging test
1Selection of artificial accelerated aging test conditions
This problem 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 you know in advance the main factors that cause aging, you can have targeted test methods. We can determine the test method from the aspects of transportation, storage, use environment and aging mechanism of the material. For example, hard PVC profiles, using PVC as raw materials, added stabilizers, pigments and other additives processed, mainly for outdoor use. Considering the aging mechanism of polyvinyl chloride, polyvinyl chloride is easily decomposed by heat. Considering the use environment, oxygen, ultraviolet light, heat, and moisture in the air are the causes of the aging of the profile. Therefore, in GB/T8814-2004 Unplasticized Polyvinyl Chloride ( PVC-U ) Profiles for Doors and Windows , both the photo-oxygen aging test method and GB/T 16422.2 “Laboratory Laboratory Light Source Exposure Test Method†are used. The second part: Xenon arc lamp "aging 4000h or 6000h , simulated outdoor UV and visible light, temperature, humidity, rainfall and other factors, while the provisions of the thermal oxygen aging project: the state after heating, placed at 150 °C 30min , visually observed whether Bubbles, cracks, pits, or separations occur to examine the heat resistance of the profile. Another example is China's competitive product in the international market: foreign trade export shoes. During use, ultraviolet light in the sun is the main cause of discoloration and discoloration of the shoes. Therefore, it is necessary to perform yellowing resistance test using an ultraviolet light box. The commonly used footwear anti-yellowing test box uses a 30W UV lamp. The sample is 20 cm 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 hot, humid and harsh environment inside the container can cause discoloration, spots, and even deterioration of the upper, sole, and glue. Therefore, before shipment and transportation, it is necessary to consider the heat and moisture aging test, simulate the high-heat and high-humidity environment inside the container, and observe the appearance and color change after 48 hours 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 atmospheric visible environment in a test box. Among these simulation factors, The light source is the most important. Experience has shown that the wavelengths of damage caused by polymer materials in sunlight are mainly concentrated on ultraviolet light and part of visible light. Currently used artificial light sources are trying to make the 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 sources have been equipped with closed carbon arc lamps, solar carbon arc lamps, fluorescent ultraviolet lamps, xenon arc lamps, high-pressure mercury lamps, and other light sources. The technical committees related to polymer materials in the International Organization for Standardization (ISO ) mainly recommend the use of three types of light sources: solar carbon arc lamps, fluorescent ultraviolet lamps, and xenon arc lamps.
2.1 Xenon arc lamp
At present, it is believed that the spectral energy distribution of the xenon arc lamp in the known artificial light source is most similar to the ultraviolet and visible light portions in sunlight. By selecting a suitable filter, most of the short-wave radiation that reaches the ground in sunlight can be filtered out. The xenon lamp has a strong radiation peak in the near-infrared region of 1000 nm to 1200 nm , and generates a large amount of heat. Therefore, it is necessary to select a suitable cooling device to take away this part of energy. Currently, there are two types of cooling methods for xenon lamp aging test devices: water-cooled and air-cooled. In general, the cooling effect of a water-cooled xenon lamp device is better than that of an air-cooled type. At the same time, the structure is also more complicated and the price is relatively expensive. Since the energy of the ultraviolet part of the xenon lamp is less increased than that of the other two types of lamps, it is the lowest in accelerating magnification. 2.2 Fluorescent UV Lamp
Theoretically speaking, the short-wavelength energy of 300nm to 400nm is the main cause of aging. If we increase this part of energy, we can achieve rapid test results. The spectral distribution of the fluorescent UV lamp is mainly concentrated in the ultraviolet portion, and therefore, a high acceleration rate can be achieved. However, fluorescent UV lamps not only increase the UV energy in natural sunlight, but also the radiant energy that is not present in natural daylight when measured on the Earth's surface, and this part of energy can cause unnatural damage. In addition, the fluorescent light source has no energy higher than 375 nm except for a very narrow mercury spectral line , so that materials sensitive to longer wavelength UV energy may not change under the influence of natural sunlight. Because these inherent defects can lead to unreliable results. Therefore, fluorescent UV lamps have poor analogy. However, because of its high acceleration rate, rapid screening of specific materials can be achieved by selecting a suitable type of lamp.
2.3 sunlight type carbon arc lamp
Solar-type carbon arc lamps are currently used in China less, but it is a widely used light source in Japan, and most of the JIS standards use solar-type carbon arc lamps. Many of our country's joint venture with Japan's auto companies still recommend the use of this light source. Sunshine carbon arc light type of the spectral energy distribution is closer to the sunlight, but the 370nm - 390nm UV reinforcing concentrated, the analog of less than a xenon lamp, a xenon lamp accelerated ratio between the UV lamp and
3 Determination of test time
3.1 Reference to relevant product standards
The relevant product standards have already stipulated the time of the burn-in test. We only need to find the relevant standards and execute them within the specified time. This is stipulated in many national standards and industry standards. Table 1 lists the provisions of aging time in some common product standards. 3.2 Based on known correlations
Studies have shown that the color stability of ABS is evaluated by changes in color and yellowing index , and that artificial accelerated aging has a good correlation with natural atmospheric exposure with an acceleration rate of approximately 7 . If you want to understand the color change of an ABS material after one year of outdoor use, using the same test conditions, you can refer to the acceleration rate and determine the accelerated aging time 365x24/7=1251h .
For a long time, a large amount of research has been carried out on correlation problems at home and abroad, and many conversion relations have been drawn. However, due to the diversity of polymer materials and the differences in accelerating aging test equipment and methods, the differences in climate at different times and regions have led to the complexity of the conversion relationship. Therefore, when selecting the conversion relationship, it is necessary to pay attention to the specific materials, aging equipment, test conditions, and performance evaluation indicators of the correlation. 3.3 Control Artificial Accelerated Aging The total amount of radiation is equivalent to the total amount of naturally exposed radiation. For some products that do not have the corresponding standards, and there is no reference to the correlation, the radiation intensity of the actual use environment can be considered to control the total amount of artificial accelerated aging radiation. It is equivalent to the total natural radiation exposure. Table 2 lists the solar radiation intensity in different regions of China [2] .
Here is an example of how to control artificially accelerated aging total radiation :
A certain plastic product is used in the Beijing area, and it is expected that the total amount of artificial accelerated accelerated aging will be equivalent to one year of outdoor exposure.
The first step: Since this product is a plastic product and used outdoors, the method A in GB/T16422.2-1996 "Method of Exposure of Plastic Laboratory Light Source Exposed to Part II: Xenon Arc Lamp" was selected. The test conditions were as follows: irradiation intensity 0.50W/m2 ( 340nm) , blackboard temperature 65 °C, cabinet temperature 40 °C, relative humidity 50 %, spraying time / no spraying time 18min/102min , continuous light;
Step 2: From Table 2, we can see that the total radiation in Beijing for a year is 5609 MJ/m2 , based on the international standard CIE No 85 -1989 comparing the spectral distribution of artificial light sources and natural sunlight (see Table 3 , GB/T 16422.1- 1996 "Plastic Laboratory Light Source Exposure Test Method Part I: Xenon Arc Lamp" (cited by Xenon Arc Lamps); the ultraviolet and visible regions ( 300nm - 800nm ) account for 62.2% , ie 3489MJ/m2 .
The third step: According to GB/T 16422.2-1996 , when the 340nm irradiation intensity is 0.50 W/m2 , the infrared region and the visible region ( 300nm to 800nm ) irradiation intensity 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 rate is about 5 . Since natural aging is not simply a superposition of irradiation intensity, this calculation method can be used to simulate only when it is determined that sunlight is the main factor causing material breakage and the test time cannot be determined by other methods.
4 Selection of performance evaluation indicators
The selection performance evaluation index mainly considers the use of the material and the characteristics of the material itself. 4.1 Determine evaluation indicators according to the use of materials
For the same material, because of its different uses, different evaluation indicators may be selected. For example, the same paint, if used for decoration, must focus on changes in its appearance. In GB/T 1766-1995 " Grading of paint and varnish coating ageing", rating methods for various appearance changes such as glossiness, color change, chalking, and pan-gold are specified in detail.
For some functional coatings, such as anti-corrosion coatings, a certain degree of color, appearance changes are acceptable, then, when selecting evaluation indicators, the main consideration is its resistance to cracking, chalking and other aspects. The same is polyvinyl chloride (PVC) . If it is used for the manufacture of uppers, it must be considered for its resistance to yellowing. If it is used for raining down pipes, the requirements for appearance change are not high, and the physical and mechanical properties of the material change, such as pulling. Extensive strength changes are the main assessment indicators. 4.2 According to the characteristics of the material itself to determine the evaluation index
With regard to the same material, the decline in different properties during aging is unequal. In other words, some properties are sensitive to the environment and the fastest decline is the main factor causing material damage. When selecting evaluation indicators, these sensitive properties should be selected. Studies have shown that for most engineering plastics, the impact strength is the largest and most significant decline in the natural aging test. Therefore, in the aging test of engineering plastics, priority should be given to selecting the impact strength as an evaluation index. The impact strength is also quite sensitive to the aging of polypropylene [4] , and it is the main indicator for evaluating aging performance. For polyethylene materials, the decrease in elongation at break is the most obvious and is a preferred evaluation indicator. For polyvinyl chloride, the tensile strength and the impact strength both decrease rapidly, and one of them should be evaluated according to the actual situation. In the national standard GB/T8814-2004 "unplasticized polyvinyl chloride ( PVC-U ) profiles for doors and windows ", the retention rate of impact strength after aging is ≥ 60 % as a qualified determination index; in the light industry standard QB/T2480 - In 2000 , the PVC-U rain-falling water pipe and fittings for construction use the retention rate of tensile strength ≥ 80 % after aging as a qualified criterion. 5 Conclusion
Artificial accelerated aging test has been rapidly developed due to the need for rapid assessment of the weatherability 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, selection of light sources, determination of test time, and selection of performance evaluation indexes are problems often encountered in artificial accelerated weathering tests. This article discusses the above aspects and puts forward some ideas for solving problems.
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