There are three major accelerated
weathering tests:
1. Exposure to carbon arc lamps
2.
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)
Specimen:
This method is applicable when
light and water exposure approximation are used for artificial weathering.
Procedure:
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.
Significance:
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.
