Quantification of time-dependent deterioration in mechanical property and ballistic performance through Cunniff Number of poly (p-phenylene terephthalamide) fibers under UV irradiation

Poly (p-phenylene terephthalamide) fibers, a crucial constituent of soft body armor, experience significant deterioration in mechanical and ballistic performance due to UV exposure. However, photo-degradation by UV radiation involves complex chemical reactions and the difficulty in estimating the extent of damage that restricts the development of the predictive aging models or kinetics in such fibrous materials. Hence in this work, the photo-degradation kinetics of poly (p-phenylene terephthalamide) or KF-fibers is quantified by correlating the degradation behavior of different properties with time, thereby assessing the useful performance span of the UV aged (UV-KF) fibers. The experimental trends indicate three stages of deterioration of any property (modulus, extension at break, Cunniff Number and crystallinity), i.e., an exponential decline followed by an increase over a short duration and finally, a steady and continuous drop. The suitable performance period of UV-aged fibers is estimated based on the initial two stages and presented as 'favorable property region' or 'FPR-zone.' Changes in V50 ballistic limit, theoretically derived from Cunniff Number, are also estimated, which show a good agreement with the literature data. It reveals that the energy absorption capacity of UV-KF fibers corresponds to initial two years as a span of considerable ballistic protection before continuing to function at 50% of its original protection level for lower aging temperatures. Besides, the apparent activation energy for all the studied properties is correlated to the microstructural and physico-chemical aspects. It indicates that relative changes in molar mass by bond scission/rearrangement and alterations in crystallinity are proportionally related to the degree of variation in modulus and extension at break, thus affecting the wave-velocities in terms of Cunniff Number. The proposed degradation model further can be extended to determine the rate of degradation and lifetime of various ballistic materials under other environmental conditions.