Natural weathering tests laboratory testing

The standard tests for assessing the durability of polymers in outdoor exposure (and other degradative environments) have been listed and discussed by Brown [5]. He concluded that the standard tests are of little value in predicting service lifetime and discussed some of the reasons for this. He also noted the lack of correlation between the results of natural weathering and laboratory weathering, a topic also discussed by other authors [2, 4, 5]. 

Many accelerated laboratory tests have been devised to determine the susceptibility of paint films to breakdown by atmospheric weathering, however, the demand for a generally applicable test exists. In this study different typical paint systems have been subjected to various natural environments and laboratory tests (DEF-1053 Method No. 26,... 

It has been found that many of the reactive dyes have not the highest degree of fastness to natural weathering. When exposed over prolonged periods to the atmosphere they fade more rapidly than normal laboratory fastness tests would predict. Procion Yellow H-A, Procion Yellow M-6G, Procion Yellow M-R, and Procion Yellow H-SG are all deemed to be of borderline fastness to weathering. They are, nevertheless, in many cases superior to vat dyes because they do not induce accelerated oxidation of the cellulose on exposure to light. The deeper the shade the less apparent is the alteration due to weathering. 

Comparisons of the results of laboratory corrosion tests with damage sustained in use have revealed differences in corrosion and corrosion products. For example, the constant salt spray fog test leads to a different type of rust formation than that produced by natural weathering, with its alternating damp and dry periods. Test cycles have therefore been developed in which the individual stresses are applied in sequence. A good correlation with natural corrosion in automobiles was demonstrated in the VDA alternating test. The sample is subjected to salt spray fog for 24 h, followed by four 24-h cycles each consisting of 8 h at 40°C and 100% relative... 

In laboratories, coatings are artificially weathered in specially designed apparatus to simulate or measure the aging processes that occur during natural weathering. Artificial weathering involves a smaller number of parameters than natural weathering but can be controlled more uniformly and allows accelerated test conditions [9.49]-[9.51]. 

Experimental assessments of the effects of surface pretreatment are of limited value using mechanical tests unless environmental exposure is included. It is very sound policy to collect and examine information on joints loaded and exposed to natural weathering conditions rather than depend solely on laboratory experiments. It is clear that water is the substance which causes most problems in attaining environmental stability of bonded joints interfacial failure generally indicates that a better surface pretreatment would impart improved joint performance. 

Information on the natural weathering behaviour of joints is very useful. By combining this information with data from accelerated laboratory tests, some realistic predictions of service-lifetime may be made. Theoretical models of the pattern of bondline saturation of joints as a function of time of environmental exposure provide a useful appreciation of the possible extent of problems (e.g. Fig. 4.21). The process of joint failure, as observed in practice or in the laboratory, is frequently non-diagnostic i.e. it rarely reveals the true cause, or the series of stages, leading to deterioration or failure. 

Weatherability of plastic materials can be investigated under natural or accelerated laboratory exposure. Typically, standard ASTM (dog-bone shaped) test pieces of the plastic of interest are exposed outdoors in natural weathering exposure sites. Ideally, locations of high insolation (such as AZ and FL) are selected to ensure fast degradation. The samples exposed outdoors, usually on South-facing racks, are periodically removed and tested to determine their extent of degradation. 

The UV energy in sunlight varies in wavelength distribution and intensity with the time of day and year as well as latitude and elevation of exposure. The presence of water also depends on the region, time of year, etc. Therefore, predicting natural weather resistance from laboratory tests can be difficult. 

Laboratory accelerated weathering devices have been used for more than 80 years with increasing importance concomitant with the development of more weather-able materials and the need to determine in a short time the effects of natural exposures over prolonged periods. The importance of these devices lies in their ability to accelerate the weathering processes imder controlled and reproducible conditions. They are particularly useful in research and development of new polymeric formulations. They are also used for quality control and specification testing. However, their application to prediction of service life under use conditions is still under development (see section on Laboratory Accelerated Versus Natural Weathering). 

Laboratory accelerated weathering tests have played an important role in development of polymeric materials with highly improved weatherability. However, since all stresses present in an outdoor exposure cannot be simulated in a laboratory accelerated test, the latter cannot replace natural exposure. It is a complimentary technique, the usefulness of which largely depends on how closely it reproduces the chemistry and weathering effects caused by the slower outdoor exposures. Thus, correlation is a fundamental issue which must be considered when selecting a laboratory accelerated weathering method. It depends on reasonably representing... 

Acceleration factors are material dependent and can be significantly different for each material and for different formulations of the same material. Therefore, it is erroneous to attempt to establish a single acceleration factor for a laboratory accelerated test to be used to predict lifetimes under natural weather conditions for a variety of materials and formulations. Because of the complex nature of the interaction of the combined weather stresses with a material, there is presently no simple way to estimate the acceleration factor for a material. Increase in irradiance cannot be equated with acceleration of degradation. For most polymeric materials, the rate of degradation is not simply a linear function of the level of irradiance. Also, it does not take into account the effect of temperature, moisture, and other weather factors. Thus, there is no substitute for determining the acceleration factor for a given material experimentally. 

D-1828. Practice for Atmospheric Exposure of Adhesive-Bonded Joints and Structures. Whereas the three preceding standards utilize controlled laboratory conditions, this standard (a) defines classes of natural exposure and (b) outlines a weathering test practice for measuring strength loss as a function of time in those exposures. Several, somewhat extreme, natural environments and their associated geographic locations are mentioned. 

Natural weathering is invariably a combination of UV and oxidation from wetness. In laboratory tests, UV or water can be excluded to define the problem area. Do not over look this technique. 

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