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.


Any shape, size up to 5” x 7” x 2”
Artificial weathering as defined by ASTM is:
The exposure of plastic to cyclic laboratory conditions involving changes in temperature, RH and UV radiant energy with or without direct water spray in an attempt to produce changes in the material similar to those observed after long term continuous outdoor exposure. A variety of light sources are used to simulate the natural sunlight. The artificial light sources include carbon arc lamps, xenon arc lamps, fluorescent sun lamps, and mercury lamps. These light sources, except the fluorescent, are capable of generating a much higher intensity light than natural sunlight. In the same wavelength band, xenon arc lamps can be operated over a wide range from below peak sunlight to twice the sunlight levels. Quite often, a condensation apparatus is used to simulate the deterioration caused by sunlight and water as rain or dew. Modern instruments have direct specimen spray on the front and/or back side of the specimen.
Most data on the aging of plastics are acquired through accelerated tests and actual outdoor exposure. The latter is a time-consuming method; accelerated tests are often used to expedite screening the samples with various combinations of additive levels and ratios. Though there is no precise correlation between artificial laboratory weathering and natural outdoor weathering the standard laboratory test conditions produce results which are in general agreement with data obtained from outdoor exposures. Moreover the conditions can be easily reproduced.

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.


Specific gravity is defined as the ratio of the weight of the given volume of a material to that of an equal volume of water at a stated temperature. The temperature selected for determining the specific gravity of plastic parts is 23°C. Specific gravity values represent the main advantage of plastics over other materials, namely, light weight. All plastics are sold today on a cost per pound basis and not on a cost per unit volume basis. Such a practice increases the significance of the specific gravity considerably in both purchasing and production control. Two basic methods have been developed to determine specific gravity of plastics depending upon the form of plastic material. Method A is used for a specimen in forms such as sheet, rods, tubes, or molded articles. Method B is developed mainly for material in the form of molding powder, flakes, or pellets.
Method A
This method requires the use of a precision analytical balance equipped with a stationary support for an immersion vessel above or below the balance pan. A corrosion-resistant wire for suspending the specimen and a sinker for lighter specimens with a specific gravity of less than 1.00 is employed. A beaker is used as an immersion vessel. The test specimen of any convenient size is weighted in air. Next, the specimen is suspended from a fine wire attached to the balance and immersed completely in distilled water. The weight of a specimen in water (and sinker, if used) is determined. The specific gravity of the specimen is calculated as follows:
Specific gravity = a (a + w) / b
a = weight of specimen in air;
b = weight of specimen (sinker, if used) and wire in water;
w = weight of totally immersed sinker (if used) and partially immersed wire.
Method B
This method, which suitable for pellets, flakes, or powder, requires the use of an analytical balance, a pycnometer, a vacuum pump, and a vacuum desiccator. The test is started by first weighing the empty pycnometer. The pycnometer is filled with water and placed in a water bath until temperature equilibrium with the bath is attained. The weight of the pycnometer filled with water is determined. After cleaning and drying the pycnometer, 1–5 g of material is added and the weight of the specimen plus the pycnometer is determined. The pycnometer is filled with water and placed in a vacuum desiccator. The vacuum is applied until all the air has been removed from between the particles of the specimen. Last, the weight of the pycnometer filled with water and the specimen is recorded. The specific gravity is calculated as follows:
Specific gravity = a (b + a / m)
a = weight of the specimen;
b = weight of the pycnometer filled with water;
m = weight of the pycnometer containing the specimen and filled with water.
If another suitable immersion liquid for the water is substituted, the specific gravity of the immersion liquid must be determined and taken into account in calculating the specific gravity.


Oxygen index is defined as the minimum concentration of oxygen, expressed as volume percent, in a mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature under specified conditions.
The oxygen index test is considered one of the most useful flammability tests because it allows one to precisely rate the materials on a numerical basis and simplifies the selection of plastics in terms of flammability. The oxygen index test overcomes the serious drawbacks of conventional flammability tests. These drawbacks are variation in sample ignition techniques, variation in the description of the endpoint from test to test, and operation of tests under non equilibrium conditions.
Test Procedures
The test determines the minimum concentration of oxygen in a mixture of oxygen and nitrogen flowing upward in a test column that will just support combustion. This process is carried out under equilibrium conditions of candle like burning. It is necessary to establish equilibrium between the heat removed by the gases fl owing past the specimen and the heat generated from the combustion. The equilibrium can only be established if the specimen is well ignited and given a chance to reach equilibrium when the percent oxygen in the mixture is near limiting or critical value. The equipment used for measuring the oxygen index consists of a heat-resistant glass tube with a brass base. The bottom of the column is filled with glass beads, which allows the entering gas mixture to mix and distribute more evenly. A specimen-holding device to support the specimen and hold it vertically in the center of the column is used. A tube with a small orifice having propane, hydrogen, or other gas flame, suitable for inserting into the open end of the column to ignite the specimen, is used as an ignition source. A timer, flow measurement, and control device are also used. The test specimen used in the experiment must be dry since the moisture content of some materials alters the oxygen index. Four different types of specimens are specified. They are physically self-supporting plastics, flexible plastics, cellular plastics, and plastic film or thin sheet. The dimension of the specimen varies according to the type. The specimen is clamped vertically in the center of the column. The flow valves are set to introduce the desired concentration of oxygen in the column. The entire top of the specimen is ignited with an ignition flame so that the specimen is well lighted. The specimen is required to burn in accordance with set criteria, which spell out the time of burning or the length of specimen burned. The concentration of oxygen is adjusted to meet the criteria. The test is repeated until the critical concentration of oxygen, which is the lowest oxygen concentration that will meet the specified criteria, is determined. 
The oxygen index is calculated as follows:
Oxygen index percent = (100 × O2)/ (O2 + N2
where O2 = volumetric flow of oxygen at the concentration determined; 
N2 = volumetric flow of nitrogen.
Factors Affecting the Test Results
1. Thickness of Specimen. As the specimen thickness increases, the oxygen index also increases steadily.
2. Fillers. Fillers such as glass fibers tend to increase the oxygen index up to a certain percentage loading. In case of polycarbonate, the oxygen index peaks at about 25 percent loading. Higher loading beyond this point subsequently decreases the oxygen index.
3. Flame Retardants. Flame retardants increase the oxygen index, making polymers more suitable for applications requiring improved flammability.


Environmental Stress Cracking Resistance (ASTM D 1693)
Specimen: LDPE measuring 1/8 x ½ x 3/2 inches, annealed in water or steam at 100 0 C for 1 hour and then conditioned at room temperature for 5 to 24 hours.
 The specimen is placed in an air circulating oven and then inserted in a test tube which is then filled with a fresh reagent [Igepal = RC6H4O(CH2CH2O)nCH2CH2OH  where R is C6H17 or higher homolog]. The tube is stoppered with an Aluminum covered cork and placed in a constant temperature bath at 50 0C. These are inspected periodically and any visible crack is considered as failure. The duration of the test is recorded along with the percentage of failure.
The cracking obtained in this test indicates what can be expected from a wide range of other stress cracking agents. Though the information cannot be translated directly into end use service prediction, but it serves to rank various types and grades of PE in categories of resistance to stress cracking. This test can also be used on high and medium density used on high and medium density material.

flow test and Spiral flow test, cup flow test

Flow Tests
The ability of the material to flow is measured by filling a mold with the plastics material under a specified condition of applied temperature and pressure with a controlled charge mass. The flow tests are used as a quality control test and as an acceptance criterion for incoming raw materials.
Factors Affecting Flow
Resin Types. All resins flow differently because of basic differences in the structure of the polymers. For example, melamine formaldehyde exhibits longer flow than urea formaldehyde. Phenolics, because of the variety of resin types, enable the molder to select the flow best suited for a particular design.
Type of Fillers. The small particle size of wood fl our, mica, and minerals creates less turbulence and less frictional drag during mold filling. The size of the glass fibers, short or long, can adversely affect the flow.
Degree of Resin Advancement. The degree of advancement is generally controlled by the resin manufacturers. Molders can advance resin polymerization with oven or radiant heat or electronic preheating.
Storage Time. All resins have a natural tendency to polymerize in storage, causing partial precure which reduces flow. An exception might be polyester in which catalyst decomposition slows or prevents curing, which increases flow duration.
Spiral Flow of Low-Pressure Thermosetting Compounds (ASTM D 3123)
The spiral flow of a thermosetting molding compound is a measure of the combined characteristics of fusion under pressure, melt viscosity, and gelation rate under specific conditions. The test requires a transfer molding press, a standard spiral flow mold, and a thermosetting molding compound. The molding temperature, transfer pressure, charge mass, press cure time, and transfer plunger speed are preselected as specified. The preconditioned compound is forced through a sprue into a spiral flow mold. Once the curing is complete, the part is removed and the spiral flow length is read directly from the molded specimen. Compounds are classified as low (1–10), medium (11–22), and high (23–40) plasticity.

Cup Flow Test (ASTM D 731)
Molding Index of Thermosetting Molding Powder. This test is primarily useful for determining the minimum pressure required to mold a standard cup and the time required to close the mold fully. The preconditioned and preweighed material is loaded into the mold. The mold is closed using sufficient pressure to form a required cup. The pressure is reduced step by step until the mold cannot close. The next higher pressure and time to close the mold is reported as the molding index of the material.