Weathering accelerated testing

The case of liquids is simpler than weathering in that it is essentially a two agent situation, liquid and temperature. As described in Sections 4.12.3 and 6.6.1, the rate of attack may be governed by the rate of diffusion of the liquid into the material, which may be slow in relation to the time scale of an accelerated test. Also, it is necessary to consider that there may be physical change (swelling) of the polymer as well as chemical degradation. 

Further uncertainty arises from the environmental exposure. For accelerated tests, repeatability can be estimated from exposure of replicate test pieces and minimised by control of the exposure conditions. Particular points to consider are spatial variation in temperature as well as mean temperature and air flow in ovens. In accelerated weathering apparatus, spatial variation and variation with time of light sources can be very significant. 

In another series of trials, geotextiles were exposed in two natural weathering sites one in southern France and the other in Berlin [13]. At the same time they were exposed to three methods of accelerated testing, with either xenon arcs or fluorescent tubes. 

Weathering is an example of where lifetime prediction is based largely on service life experience under normal or severe conditions as well as accelerated testing. If sufficient allowance is made for the variation between accelerated and natural weathering, and in natural weathering itself, the predictions can be regarded as very satisfactory. 

The mathematical methods used for interpolation and extrapolation of the data obtained from accelerated tests, as described in Chapters 8 and 9, include both the mechanistic and the empirical. Arrhenius formula, based on chemical rate kinetics and relating the rate of degradation to temperature, is used very widely. Where there are sufficient data, statistical methods can be applied and probabilities and confidence limits calculated. For many applications a high level of precision is unnecessary. The practitioners of accelerated weathering are only too keen to tell you of its quirks and inaccuracies, but this obscures... 

In an attempt to accelerate testing, higher irradiances were employed in the Weather-Ometer . Reciprocity is a necessary condition for this acceleration to be... 

Outdoor exposure of polymer samples, mounted at a 45° angle and facing south, has been used to measure the resistance of polymers to outdoor weathering (ASTM-D1345). Since these tests are expensive and time-consuming, tests such as ASTM-G23 have been developed in an attempt to gain accelerated test results. 

There are several accelerated tests which differ in the selection of light source and cyclic exposure to varying degrees of humidity. Some accelerated tests include salt spray, heat, cold, and other weather factors. 

Resistance to light and weather generally depends on the chemical composition, structure, defects, particle shape and size, and concentration of the pigment [1.53], However, these properties also depend on the medium in which the pigment is used. Testing is carried out by open-air weathering, accelerated weathering, and chemical test methods. 

Accelerated Tests. Weather resistance in an accelerated test is defined as the resistance of plastics towards changes caused by simulated open-air weathering (simulation of global radiation by means of filtered xenon arc radiation and periodic rain). After the weathering (measured by the product of intensity and duration), defined properties of the test sample are compared with those of an identical unweathered sample. Properties should be considered which are of practical importance, such as color or surface properties. For standards, see Table 1 ( Weathering in apparatus ). Apparatus test chamber, rain and air humidification equipment, air flow equipment, radiation measuring equipment. 

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