Impact of High Temperature on Developing Fingerprints Deposited on Typical Thermoplastics

In arson incidents, suspects may employ plastic items such as lighters and containers for accelerants as tools, which are prone to retaining fngerprint evidence. However, the high-temperature environment of a fre scene can cause plastics to undergo a series of changes including softening, deformation, melting, pyrolysis, and combustion, with the composition of fingerprints also being altered. Therefore, studying the evolution of the fingerprint-bearing substrate under high-temperature conditions holds significant importance for fngerprint development. This study focused on PET plastic mineral water bottles and ABS plastic lighter shells as research subjects. Initially, the thermal properties of these two plastic materials were analyzed to set the heating temperature and duration that simulate fre scene conditions. Subsequently, the latent fngerprints on the heated plastics were developed using the fuorescent powder method, and the evolution of the plastic substrate and fngerprint patterns was observed in real-time via a hot-stage microscope. The mechanism of how high temperature affect fngerprint development on the surfaces of thermoplastic plastics was explained in conjunction with the phase behavior of the substrate materials. The results indicate that the effective ness of fngerprint development on thermoplastic surfaces is predominantly infuenced by the intrinsic properties of the substrate. Before the pyrolysis of thermoplastic materials, the fuorescent powder method can effective ly develop fngerprint, but the ridges will be noticeably distorted due to plastic softening, and the subsequent melting process further severely interferes with fngerprint comparison and identification. Specifically, the PET plastic bottle heated at 200 ℃ for 10 to 30 minutes can still exhibit fngerprint patterns, but prolonged heating exacerbates the distortion. When the temperature rises to 250 ℃, the plastic bottle melts completely, rendering latent fngerprints invisible. For the ABS plastic lighter shell, fngerprint ridges can be revealed but are severely distorted at 250 ℃; when the temperature reaches 300 ℃, the lighter shell undergoes severe deformation, and fngerprints cannot be revealed. The results of this study provide invaluable scientific data to support fngerprint development on thermoplastic plastics under high-temperature fre scene conditions.