Detection of Obliterated Traceability Elements in Polypropylene through Strain-Induced Photoluminescence Variations

The use of polymeric materials in firearm, vehicle, and medical manufacturing has grown substantially over the years, highlighting the need for reliable methods to recover obliterated stamped alphanumerical characters. Among the available techniques, hyperspectral Raman imaging has emerged as a promising approach due to its sensitivity to residual strain. However, traditional analysis often involves a time-intensive acquisition procedure and a two-step fitting process, first identifying strain sensitive spectral features, then performing the curve fitting itself. Additionally, luminescence that arises from additives like optical brighteners and antioxidants commonly present in commercial polymers, often overwhelms the weaker Raman signal, complicating spectral interpretation. Given the proprietary nature of polymer formulations, optimizing Raman parameters remains a case-dependent challenge. To address these limitations, this study proposes a complementary approach by directly leveraging strain-induced photoluminescence variations (mechanochromic effects) caused by structural rearrangements within the polymer matrix or embedded additives. These effects are detectable using simple optical filters, eliminating the need for a complex apparatus and significantly reducing acquisition times as they benefit from a significantly larger cross-section compared to Raman scattering. Experimental results demonstrate enhanced visual contrast and reasonable statistical discrimination (i.e., 2.4σ separation between strained and unstrained pixels) of reconstructed marks in polypropylene compared to conventional techniques, specifically Raman spectroscopy. Possible explanations for the observed photoluminescence variations are discussed. This proof-of-concept highlights the potential for rapid and field-deployable forensic analysis of polymer-based traces.