Weathering lifetime predictions

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. 

A correlation between natural and artificial weathering was considered for lifetime prediction in a short exposure time. It was found that the confidence level of predicting time on the basis of artificially accelerated exposure trials is dependent on many parameters which include time, material, equipment, etc. 

Therefore is must be concluded that enhanced weathering tests normally should be cyclic, simulating many practical parameters with their changes in time. Interpretation should be performed with extreme care. Improvements of last testing procedures, e.g., by incorporating electrochemical and dielectric sensors, are of extreme importance in order to arrive at laboratory-based lifetime prediction. 

Weathering Resistance Prediction of service life, although one of the most important areas of testing, is probably the most uncertain. Cable jackets of synthetic rubber used in the Distant Early Warning (DEW) Line in Northern Canada in the 1950s were forecast to have a 20-year lifetime. Most are stiU in service. Similar synthetic rubber wire coverings were used at that time for automotive wire harness when... 

Based on predicted weathering and erosion rates of the region, we estimate the profile to be several million years old. Because the soil has developed in situ, the topmost grains have reacted with water for the greatest extent of time. With depth, the total "lifetime" of the particles as soil decreases. This implies that the topmost quartz surfaces should be "reactively mature" (all fines removed, deep grown-together etch pits) and the bottom-most quartz surfaces should be "reactively young" (plentiful fines, fresh surfaces). ... 

In principle, the computational approach to the kinetics of the complex photooxidation process can give meaningful insight into the effects of outdoor weathering of hydrocarbon polymers. For clear amorphous linear polyethylene, the model suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which could trap peroxy radicals. An additive which decomposes hydroperoxides would also be effective but would require higher concentrations. The useful lifetime of unstabilized polyethylene is predicted to vary from a few months in hot weather (100°F) to almost two years in cool weather (45°F), which correlates well with experimental results and general experience. 

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. 

Lifetime of polymers exposed to natural euvironmeut cannot be predicted accurately because of complexity of the natural weathering phenomenon and multitude of weather parameters interacting simultaneously [7]. A brief discussion of the important factors that mainly contribute to degradative processes during natural weathering follows. 

The prediction of lifetime of greenhouse films is not possible without conducting the real-time weathering trials at locations where those films have to be used... 

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]. 

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. 

Weather Testing Experimental tests that aim to predict the lifetime of manufactured articles. 

A number of studies have been conducted to investigate how well SLP accomplishes these goals for selected polymer-based materials [1 ]. From these, reasonable estimates of lifetime were obtained as compared to actual values of time to failure observed for outdoor exposures. Some of the predictions covered the entire degradation pathway, for which changes in material properties generally followed a trend similar to that measured during outdoor weathering. The SLP methods employed varied somewhat from study to study and all used measured climate data to test their models. 

Real weathering loads on contemporary vehicles cannot be simulated in an accelerated test. Depending on the task, a large number of very different tests are applied. The most important test is artificial weathering and simulated solar irradiation, s. Section 2.2.4.2. Investigations for BMW, for example, showed that 240 hours of sunlight simulation are sufficient to obtain reliable predictions for component lifetime. However, it is decisive that the component test be performed under actual service conditions for the vehicle [203]. 

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