First, selection of artificial accelerated aging test conditions
This problem is about which aging factors to simulate. Many factors, including the climate, may affect the aging of polymer materials in use. If we know the main factors of aging in advance, we can select the test method.
We can find the test method by looking at the material’s transport, storage, and use environments, and its aging mechanism. For example, manufacturers make rigid PVC profiles from PVC. It has stabilizers, pigments, and other additives. They are mainly used outdoors. Polyvinyl chloride (PVC) decomposes easily with heat due to its aging. Environmental concerns also limit its use. Oxygen, UV light, heat, and moisture age PVC profiles.
The national standard is for ‘doors, windows with unplasticised polyvinyl chloride (PVC-U) profiles’. It requires tests for light and oxygen aging. It also requires a ‘plastic laboratory light exposure test method Part II: Xenon Arc Lamp’ aging for 4,000h or 6,000h. This simulates outdoor factors like ultraviolet and visible light, temperature, humidity, and rainfall. It also tests for thermo-oxidative aging. The test is to heat the sample at 150 °C for 30 minutes. Then, check for bubbles, cracks, pockmarks, or separation to assess heat resistance. Another example is a product our country competes in: export shoes. They do well in the foreign trade market. The sun’s ultraviolet rays mainly cause shoes to discolour and fade. So, we must use a UV light box to test for yellowing resistance.
The yellowing resistance test chamber for shoes uses a 30W UV lamp. The sample is 20cm from the light source. After 3 hours of irradiation, observe any colour change. The heat and humidity in the container will discolour, spot, and damage the upper, sole, and glue. So, before loading and transporting, test for moisture and heat aging. Simulate the container’s high heat and humidity at 70℃ and 95% RH. After 48 hours, check for any appearance and color change.
Second, artificial accelerated aging light source selection
A laboratory light exposure test can simulate the environment in a test chamber. It should mimic factors like light, oxygen, heat, and rainfall. This test is a common method for artificially accelerating aging. Of these factors, the light source is the most important. Experience shows that sunlight breaks polymers. Its wavelengths are mainly in UV and some visible light.
Currently used artificial light sources are trying to match the sun’s spectrum. They aim to simulate and accelerate the sun’s spectrum. This is the main basis for choosing the artificial light source. After about a century of development, the laboratory light source has a closed carbon arc lamp, sunlight-type carbon arc lamps, fluorescent ultraviolet lamps, xenon arc lamps, high-pressure mercury lamps and other light sources to choose from. ISO technical committees on polymer materials recommend three light sources. They are: sunlight-type carbon arc lamps, fluorescent ultraviolet lamps, and xenon arc lamps.
01 Xenon arc lamp
Xenon arc lamps are now thought to be the most like sunlight among artificial lights. They are similar in their UV and visible light. Their spectral energy distribution matches it best. The right filter can block most of the short-wave sunlight that reaches the ground. Xenon lamps emit strong radiation in the 1000nm to 1200nm infrared range. They produce a lot of heat.
Therefore, it is necessary to choose a suitable cooling device to take away this part of the energy. There are two types of xenon lamp aging test devices: water-cooled and air-cooled. Water-cooled xenon lamps cool better than air-cooled ones. But, they are more complex and expensive. The xenon lamp’s UV part has less energy than the other two light sources. So, its acceleration rate is the lowest.
02 fluorescent ultraviolet lamp
Theoretically, 300nm ~ 400nm of short-wave energy is the main factor causing aging. If we increase this part of energy, we can achieve the effect of a rapid test. Fluorescent UV lamps emit most of their light in the UV range. You can achieve a high acceleration multiplier.
Fluorescent UV lamps increase the UV energy in natural daylight. They also emit radiant energy absent in natural daylight at the earth’s surface. This can cause unnatural damage. Also, the fluorescent light has no energy above 375 nm, except for very narrow mercury lines. So, materials sensitive to longer UV wavelengths may not change as they would if exposed to sunlight. As a result of these inherent shortcomings it can lead to unreliable results.
As a result, fluorescent UV lamps are poorly analogue. However, its high acceleration multiplier allows for rapid screening of specific materials. You can do this by selecting the right lamp.
03 Sunlight-type carbon arc lamp
China uses sunlight-type carbon arc lamps less than Japan. Most JIS standards use them. Many of our joint ventures with Japan’s auto firms still recommend this light source. A sunlight-type carbon arc lamp has a spectral energy distribution closer to sunlight. But, it has a weaker UV concentration at 370-390 nm than a xenon lamp. The acceleration multiplier is between the xenon and UV lamps.
Third, the determination of the test time
01Refer to the relevant product standards
The aging test has relevant product standards. We just need to find them. They specify a time for the test. Many organizations have established national and industry standards in this provision.
02 According to the known correlation projection
Research shows that: use the colour and yellowing index to test the colour stability of ABS. It has a better correlation with artificial aging and natural atmospheric exposure. The accelerated aging factor is about 7. To assess an ABS material’s outdoor use after a year of color changes, use the accelerated multiplier. It will give an aging time of 1251h, using the same test conditions. This is for 365 days at 24/7.
For a long time, many researchers have studied the link between the question. They have derived many conversion relations. However, the diversity of polymer materials and aging tests complicates the conversion relationship. Different equipment, methods, and climates add to the issue. So, when choosing the conversion relationship, one must consider the specific materials, aging equipment, test conditions, and performance indexes. These factors will help find the correlation.
03 Control the total amount of artificial accelerated aging radiation. It must match the total of natural radiation exposure.
For some, there are no relevant standard provisions. Consider the radiation from its use. Control the total of both artificial and natural radiation. They should be equivalent.
Example: How to control the total radiation of artificial accelerated aging
Someone uses a plastic product in Beijing. It must limit the total artificial, accelerated aging radiation to a year’s worth of outdoor exposure.
Step 1: The product is plastic and used outdoors. So, use the ‘plastic laboratory light source exposure test methods Part II: Xenon arc lamp‘ in the A method.
Test conditions: 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 radiation in a region for a year is about 5609MJ/m2. This compares artificial light sources to sunlight. It follows the CIENo85-1989 guidelines on light exposure tests of plastics. They cite the spectral distribution of radiation. The ultraviolet and visible parts (300nm ~ 800nm) accounted for 62.2%, or 3489MJ/m2. The 340nm irradiation is at 0.50W/m2. The infrared and visible (300nm-800nm) irradiation is at 550W/m2. We can calculate 3489X106/550 = 6.344X106s, or 1762h. Using this method, the accelerated multiplication rate is about 5. Natural aging is not just the result of sunlight. It’s not a simple irradiation intensity of the Iteration.
Fourth, the choice of performance evaluation index
The choice of a performance evaluation index is twofold. It must consider the materials used and their properties.
4.1 Determine the evaluation indexes based on the material’s use. For the same material, its different uses may require different indexes. If someone uses the same paint decoratively, its appearance must vary. The report, “Rating of Ageing of Colour Paint and Varnish Coatings,” gives methods to rate changes in appearance. This includes gloss, colour change, chalking, and gold flashing.
For some coatings, like anti-corrosion ones, some color change is fine. So, when choosing tests, focus on their resistance to cracking and chalking. If using the same PVC to make shoe uppers, it must resist yellowing. If using it for rainwater downpipes, the appearance isn’t as important. The main test is the change in tensile strength.
4.2 Determination of evaluation indexes based on the material’s traits. For the same material, different properties age at different rates. In other words, some properties are sensitive to the environment. They decline quickly, causing material damage. When choosing evaluation indexes, you should select these sensitive properties. Research shows that, for most engineering plastics, impact strength suffers most in aging tests. It decreases the most.
So, in aging tests of engineering plastics, use the decline in impact strength as the key measure. Impact strength is sensitive to the aging of polypropylene. It is the main index to assess aging performance. For polyethylene materials, the drop in elongation at break is more obvious. It is the most important evaluation index. For polyvinyl chloride, both strengths decline quickly. Choose one for testing based on the situation.
The national standard for PVC-U profiles for doors and windows is a quality standard. It requires a ≥60% retention of impact strength after aging. The light industry standard for PVC-U rainwater pipes and fittings requires a test. It must have a ≥80% retention of tensile strength after aging.
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