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...polymeric materials has great practical importance, as does the ability to relate laboratory performance of a polymeric system to its performance in the field. At present, the full potential of these materials is inhibited by the absence of a methodology for accurately predicting their service life during exposure in their end-use environments. A reliability-based methodology utilizing total effective dosage as an exposure metric has been proposed as an alternative to the conventional methodology for carrying out laboratory and outdoor weathering studies. The use of this proposed methodology involves not only a shift in paradigm but also in the way that laboratory and outdoor weathering experiments are carried out. The concepts of reciprocity and additivity, which were first introduced by the photographic community and then studied by the medical and biological communities, are also proposed as models for synthetic polymeric systems.
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INTRODUCTION
Synthetic and natural polymeric materials play an increasingly important role in today's society, and the versatility of these materials has been demonstrated through the breadth of their applications in almost every sector of the economy. The full potential of these materials is inhibited by the absence of a methodology for accurately predicting their service life during exposure in their end-use environments. This deficiency has been noted by industry and emphasized in a number of recent National Research Council (NRC) reports. (1,2)
Over the last several decades, both industry and consumers have identified service life prediction of polymeric materials as a high priority research need. This high priority is justified given the high economic risk of introducing a poorly performing product into the market place. The number of class action suits initiated by product failures has escalated sharply since the 1960s, (3,4) thus making it riskier for manufacturers to provide service life estimates and warranty coverage, and/or introduce new products into the market.
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Polymeric materials used in an outdoor environment are subjected to a wide variety of environmental factors, including solar ultraviolet (UV) radiation, temperature, and moisture. The physical and chemical degradation resulting from outdoor exposure is known in the materials community as weathering. Understanding the factors that govern the weathering performance of commercial polymeric materials has great practical importance, as does the ability to link laboratory performance of a polymeric system to its performance in the field. When this linkage can be accurately made, then the stage will be set for implementing a quantitative, scientifically based service life prediction methodology.
ACCELERATED LABORATORY VS. OUTDOOR WEATHERING EXPERIMENTS
For commercially viable materials, weathering is usually a slow and lengthy process, often taking five years or longer before failure occurs. These long testing periods carried out in real time are costly to the manufacturer and are also a major barrier to innovation and the timely introduction of new products into the market. A need exists for exposure strategies that can accelerate weathering, generate reliable performance data, demonstrate a linkage between laboratory and field exposure data, and permit valid extrapolations or predictions of service life from accelerated levels to in-service exposure conditions.
In recent years, the materials industry has made tremendous strides in terms of laboratory equipment for accelerated weathering and analytical techniques for characterizing the physical and chemical properties of polymeric systems. In terms of service life prediction capabilities, however, the current situation is not greatly changed from the previous century. At present, a commonly used materials weathering strategy consists of assigning specimens randomly between accelerated laboratory and outdoor exposure experiments. The accelerated laboratory testing conditions are adjusted to "simulate" as closely as possible the conditions encountered in the particular outdoor exposure, and the two sets of specimens are exposed for some predetermined amount of time. Within each study, the specimens are then ranked by performance (however that may be defined). The rankings of the two sets of samples are then correlated through a nonparametric statistic such as the Spearman Rank Correlation...
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