Degradation of respirable fibrils of poly(p-phenylene terephthalamide) by solar ultraviolet radiation

After approximately 160 days in the environment, the polymer in a respirable-size poly(p-phenylene terephthalamide) fibril (diameter = 0.3 μm) is expected to degrade, due to solar ultraviolet radiation, from an initial molecular weight of approximately 20,000 to less than 400. This estimate is based on ultraviolet absorption characteristics, photolytic degradation kinetics, and judgments regarding how these translate into actual time in the environment. Degradation to this molecular weight level will cause the fibril tensile strength and modulus to decrease to approximately 1% of their initial values. Mechanically, such a degraded fibril will be weak and brittle with properties similar to those of uncooked spaghetti.