Monday, February 26, 2018

list of weather resistance test methods and outdoor weathering of plastic (ASTM D 1435)

The effects of weather on plastics can be predicted by any of the following methods:
1.       Outdoor weathering (ASTM D 1435)
2.       Accelerated weathering (ASTM G 23)
3.       Water cooled Xenon arc type (ASTM D 2565)
4.       Accelerated exposure to sunlight using Atlas type 18 FR Fade-O-meter  
Out of which the first is test procedure using natural weathering and remaining three uses artificial environmental conditions. This methods conclude that there is no substitute for natural weather.

Any standard molded specimen or cut pieces of sheet or machined sample.
Exposure test specimens of suitable shape or size are mounted in a holder directly applied to the racks. Specimens are mounted outdoor on racks slanted at 450 angle, facing south or facing the equator. Many other variations in the position of the racks are also employed, depending upon the requirements. The specimens are removed from the racks after a specified amount of time and subjected to appearance evaluation, electrical tests, and mechanical tests. The results are compared with the test results from control specimens. It is recommended that concurrent exposure should be carried out in many varied climates to obtain the broadest results. Since weathering is a comparative test, control samples are always utilized and retained at standard conditions of temperature and humidity. The control samples must also be covered with inert wrapping to exclude light exposure during the aging period. However, dark storage does not insure stability.
Since one quarter of all polymers end up in outdoor applications, outdoor weathering tests have become very popular. It is the most accurate method of obtaining a true picture of weather resistance but the test time required is of several years exposure.

The test is devised to evaluate the stability of plastic materials exposed outdoors to varied influences that comprise weather exposure conditions that are complex and changeable.
Factors affecting:
Climate, time of year, and the presence of industrial atmosphere. It is recommended that repeated exposure testing at different seasons and over a period of more than one year be conducted to confirm exposure at any one location. Test sites are selected to represent various conditions under which the plastic product will be used. Arizona is often selected for intense sunlight, wide temperature cycle, and low humidity. Florida, on the other hand, provides high humidity, intense sunlight, and relatively high temperatures.

Outdoor Accelerated Weathering
To accelerate outdoor weathering, a reliable method for predicting long-term durability in a shorter time frame had to be developed. The method employs Fresnel-reflecting solar concentrators that use 10 fl at mirrors to uniformly focus natural sunlight onto specimens mounted in the target plane. High-quality, first surface mirrors provide an intensity of approximately eight suns with spectral balance of natural sunlight in terms of ultraviolet integrity. The test method provides an excellent spectral match to sunlight, correlating well to subtropical conditions such as southern Florida as well as an arid desert environment such as Arizona.
The test apparatus is a follow-the-sun rack with mirrors positioned as tangents to an imaginary parabolic trough. The axis is oriented in a north–south direction, with the north elevation having the capability for periodic altitude adjustment. The target board, located at the focal line of the mirrors, lies under a wind tunnel along which cooling air is deflected across the specimens. A nozzle assembly is employed to spray the specimens with deionized water in accordance with established schedules. Nighttime spray cycles can be used to keep specimens moist during the non tracking portion of the test. The entire three-year real-time Florida exposure test can be carried out in just six months depending on the program start date. The test is widely used in automotive, agriculture, building, textile, and packaging industries.

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