在一起交通肇事逃逸案件中,一辆摩托车在行驶过程中被同向行驶的大货车撞倒,摩托车驾驶员当场死亡,肇事大货车逃逸,后办案人员根据证人证言和视频监控查获一嫌疑车辆。嫌疑车右侧车门上有一道约3.5 cm×1.0 cm的黑色划痕,根据痕迹高度、形成方向判断该划痕可能来源于被撞摩托车把套,而摩托车把套也有相应的擦划痕迹。从摩托车把套擦划痕迹周围提取的黑色塑料与嫌疑车划痕处的黑色附着物均为聚氯乙烯（PVC）,但红外谱图有明显差异。提取摩托车把套擦划痕迹处的黑色塑料熔球进行检验,其红外谱图与嫌疑车划痕处的黑色附着物完全相同,有力支持了大货车与摩托车发生接触的事实。摩托车把套不同部位提取塑料的红外谱图存在明显差异,其原因是由于摩托车与大货车碰撞过程中摩擦产生高温导致塑料中的部分组分发生变化所致。本案提醒实验室检验人员在提取比对样本时应充分考虑高温作用的影响,避免因提取部位不当得出错误的检验结论。在不破坏痕迹特征或痕迹检验已完成的情况下,可直接在擦划痕迹处取样检验。

Plastic is an important kind of evidence in hit-and-run accidents. Transfer of plastic substance occurs frequently in the striking process between the involved vehicles. Plastic samples, collected from vehicles involving in hit-and-run accidents and other investigated vehicles, are routinely examined and compared in present forensic laboratories. Fourier Transform Infrared Spectroscopy (FT-IR) is the most frequently used method for plastic analysis. Polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and acrylonitrile butadiene styrene (ABS) are the most commonly used polymers of vehicles. Some plasticizers and inorganic fillers, added into the plastic, can also be analyzed by FT-IR. This paper introduced an examination of smeared plastic sample collected from the suspected truck and the known plastic sample from the handlebar of involved motorcycle. The main component of the plastic sample from the suspected truck is PVC with calcium carbonate as the filler and a small amount of dimethyl-phthalate as the plasticizer. In order to avoid destroying the scratched mark, the compared sample was collected near the scratched mark of the handlebar of the involved motorcycle. By FT-IR analysis, the compared sample showed stronger bands than the plastic smears from the suspected truck at 744, 1074,1125,1287 and 1726 cm^-1 which were assigned to dimethyl-phthalate. The results indicated that the sample contained more plasticizer. It is necessary to perform further examination since high temperature in the friction process may affect the component of PVC plastic. Under the stereomicroscope, we found molten plastic balls on the position of the scratched mark of the motorcycle’s handlebar. However, there was no difference found by the FT-IR method between the molten balls and the plastic smears of the suspected truck. Nevertheless, the SEM/EDS elemental analysis showed the plastic smears on the suspected truck was much likely to come from the involved motorcycle. Thus, it can be concluded that the weak FT-IR spectra of PVC from the plastic smear were caused by the decreased amount of PVC transferred in the friction between the suspected truck and the motorcycle. In conclusion, it is important to consider the influence of high temperature on the compared sample together with careful selection of appropriate collection places. Only can sample collection be directly performed on the position of the scratched mark when not damaging the mark characteristics and/or its inspection.