Photodegradation in Ballistic Laminates: Spectroscopy and Lifetime Extension*

Abstract

Polycarbonate (PC), a critical component in ballistic laminates (BLs), is known to degrade upon exposure to ultraviolet (UV) light. For the purpose of reducing the photodegradation, a UV blocking chemical has been added to the adhesives used to join the layers of the BL. This report describes the development of a spectroscopic method for monitoring surface photodegradation of PC and the method's use in demonstrating the effectiveness of the UV blocker. Reports in the literature demonstrate that photodegradation in thin PC films may be detected by transmission infrared (IR) spectroscopy. The present work extends this approach to thick films, where small surface changes are detected by reflectance IR spectroscopy. We show that UV photodegradation of the PC surface produces a characteristic shift in the carbonyl absorption band at about 1775 cm⁻¹. This shift is consistently observed in PC samples that have been subjected to direct artificial exposure and in PC samples that have been subjected through the outboard layers of the BL to both natural and artificial exposure. When a UV blocker is incorporated into the adhesive layers of the laminate, no peak shift is observed in the carbonyl band after the equivalent of 10 years of exposure.