Monday, February 26, 2018

types of accelerated weathering test and their interpretations

There are three major accelerated weathering tests:
1. Exposure to carbon arc lamps
2. Exposure to fluorescent UV lamps
3. Exposure to xenon arc lamps
4. Accelerated exposure to sunlight using the Atlas type 18 FR Fade-O-Meter
The xenon arc, when properly filtered, most closely approximates the wavelength distribution of natural sunlight.

Fluorescent UV Exposure of Plastics (ASTM D4329)
This method simulates the deterioration caused by sunlight and dew by means of artificial ultraviolet light and condensation apparatus. Solar radiation ranges from ultraviolet to infrared. Ultraviolet light of wavelengths between 290 and 350 nm is the most efficient portion of terrestrial sunlight that is damaging to plastics. In the natural sunlight spectrum, energy below 400 nm accounts for less than 6 percent of the total radiant energy. Since the special fluorescent UV lamps radiate between 280 and 350 nm, they accelerate the degradation process considerably. The test apparatus basically consists of a series of UV lamps, a heated water pan, and test specimen racks. The temperature and operating times are independently controlled for both UV and the condensation effect. The test specimens are mounted in specimen racks with the test surfaces facing the lamp. The test conditions are selected based on requirements and programmed into the unit. The specimens are removed for inspection at a predetermined time to examine color loss, crazing, chalking, and cracking.

Xenon Arc Exposure of Plastics Intended for Outdoor Applications
(ASTM D 2526)
This method is applicable when light and water exposure approximation are used for artificial weathering.
A water-cooled xenon-arc-type light source is one of the most popular indoor exposure tests because Xenon arc have been shown to have a spectral energy distribution when properly filtered. This closely simulates the spectral distribution of sunlight at the surface of the earth. The xenon arc lamp consists of a burner tube and a light filter system consisting of interchangeable glass filters used in combination to provide a spectral distribution that approximates natural sunlight exposure conditions. The apparatus has a built-in recirculating system that recirculates distilled or deionized water through the lamp. The water cools the xenon burner and filters out long wavelength infrared energy. For air-cooled lamps, this is accomplished by the use of optical filters.
It is highly recommended that a controlled irradiance exposure system be used. This is best accomplished through the use of a continuously controlled monitor that can automatically maintain uniform intensity at preselected wavelengths or wavelength range, when broadband control is being used.
There is no precise correlation existing between the data obtained by Xenon arc method and outdoor weathering and other laboratory weathering devices because the emitted energy from Xenon lamps decay with time and the parameters of temperature and water do not represent specific known climatic conditions.

Accelerated exposure to sunlight using the Atlas type 18 FR Fade-O-Meter:
It is used to check and compare the color stability. Besides determining the ability of various pigments needed to provide both standard and custom colors, the Fade-O-Meter is helpful in studying various stabilizers, dyes and pigments compounded in plastics to prolong their useful life. It is for testing material to be used in articles subject to indoor exposure to sunlight.
It was extensively used in the development of UV absorbing acetate film for store windows to protect merchandise (good for sale) displayed in direct sunlight.
Exposure in the Fade-O-Meter cannot be directly related to the exposure in direct sunlight because other weather effects are always present in outdoors.   

Interpretations and Limitations of Accelerated Weathering Test Results:
There has been a severe lack of understanding on the part of users regarding the correlation between the controlled laboratory test and the actual outdoor test and application. The questions often asked are: “How many hours of exposure in a controlled laboratory enclosure is equal to one month of outdoor exposure?” “How do the results obtained from one type of weathering device compare to another type?” There is a general agreement among the researchers, manufacturers, and users that the data from accelerated weathering tests are not easily correlated with the results of natural weathering. However, accurate ranking of the weatherability of most material is possible using improved test methods and sophisticated equipment.
Accelerated weathering tests were devised to study the effect of actual outdoor weather in a relatively short time period. These tests often produce misleading results that are difficult to interpret or correlate with the results of actual outdoor exposure. The reason for such a contradiction is that in many laboratory exposures, the wavelengths of lights are distributed differently than in normal sunlight, possibly producing effects different from those produced by outdoor weathering. All plastics seem to be especially sensitive to wavelengths in the ultraviolet region. If the accelerated device has unusually strong emission at the wavelength of sensitivity of a particular polymer, the degree of acceleration is disproportionately high compared to outdoor exposure. The temperature of the exposure device also greatly influences the rate of degradation of a polymer. The higher temperature may cause oxidation and the migration of additives which, in turn, affects the rate of degradation. One of the limitations of accelerated weathering devices is their inability to simulate the adverse effect of most industrial environments and many other factors present in the atmosphere and their synergistic effect on polymers. Some of the newly developed gas-exposure cabinets have partially overcome these limitations. These units are capable of generating ozone, sulfur dioxide, and oxides of nitrogen under controlled conditions of temperature and humidity. Improved ultraviolet sources and more knowledge of how to simulate natural wetness now make it possible to achieve reliable accelerated weathering results if the following procedures are observed:
1. Include a material of known weather resistance in laboratory tests. If such a material is not available, use another similar product that has a history of field experience in a similar use.
2. Measure or estimate the UV exposure, the temperature of the product during UV exposure, and the time of wetness under service conditions of the product.
3. Do not use abnormal UV wavelengths to accelerate effects unless testing small differences in the same material. Evaluating two different materials by this technique can distort results.

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