ABSTRACT
The resistance of sealants to weathering is a major factor in their suitability for use in outdoor applications that depend on their durability. Therefore, determining the weatherability of sealants is an essential step in development of new and improved products. Weathering may be dened as irreversible changes in the chemical and physical properties in a direction that usually shortens the useful life of the sealant. Changes in appearance and mechanical properties result from modication of the chemical structures of the sealants due to complex interactions with environmental elements, primarily solar radiation, heat and cold, moisture (solid, liquid, and vapor), oxygen, and atmospheric contaminants, such as acid rain. Oxygen, because of its high chemical reactivity, is by far the most important atmospheric constituent responsible for material degradation. Photooxidation, which results from the effect of absorbed solar actinic radiation by the sealants in the presence of oxygen, accounts for most failures that occur during outdoor exposure [1]. In many of the applications of sealants, cyclic movements are imposed by climatic and construction factors, which, in combination with environmental stress factors, synergistically accelerate their degradation. Although all weather factors, particularly in combination with one another, play a very important role in the deteriorating effect of the environment on materials, the absorbed actinic radiation of the sun, that is, the spectral region responsible for the damage, is the critical factor. It has sufcient energy to break many types of chemical bonds, thus initiating the reactions that result in chemical and physical changes. The other weather factors promote degradation through their inuence on the secondary reactions that follow the breaking of bonds. The rate of the secondary reactions is strongly inuenced by temperature, being more rapid at higher temperatures. The signicance and effect of each of the weather factors on the degradation of sealants is described in this chapter and in Chapter 6.
