Review: Synchrotron FTIR Characterization of Automotive Primer Surfacer Paint Coatings for Forensic Purposes

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Emily C. Lennert

 

Category

Chemistry

Keywords

paint, primer, automotive primer, FTIR, Fourier transform infrared spectroscopy, chemometrics, synchrotron FTIR

Article Reviewed

  1. Maric, M.; van Bronswijk, W.; Lewis, S. W.; Pitts, K. Synchrotron FTIR characterization of automotive primer surfacer paint coatings for forensic purposes. Talanta. 2014, 118, 156-161.

Disclaimer

The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.

For more information on principal component analysis, refer to Review: Characterisation and classification of automotive clear coats with Raman spectroscopy and chemometrics for forensic purposes.

Summary

Automotive paint is a form of forensic evidence that may be encountered in vehicle collisions, hit and run accidents, or any other incident involving a vehicle. The Scientific Working Group for Materials Analysis (SWGMAT) has set guidelines in place for forensic paint analysis. These guidelines include several instrumental methods of analysis, such as Fourier transform infrared spectroscopy (FTIR). Currently, interpretation of the results is largely subjective and based on the analyst’s interpretation. However, there is a drive to introduce chemometrics to eliminate subjectivity in the interpretation of results. Chemometrics allows for statistical analysis of the data, which can be used to determine how similar or dissimilar samples are through a quantitative measure.

Synchrotron FTIR is a form of FTIR analysis that offers improved spatial resolution over traditional FTIR methods. Spatial resolution refers to the pixel size in an image; images with better spatial resolution will have smaller pixels which provides a clearer picture. The spatial resolution can be improved to 3 μm, as opposed to 25 μm that is obtained with traditional FTIR sources, i.e. globar sources. The infrared light source in synchrotron FTIR is 100-1000 times brighter than globar sources, which results in an improved signal to noise ratio. Automotive paint consists of a layer system, as seen in figure 1a within the study and in Figure 1 below. The layers typically consist of a clear coat, base coat, primer surfacer, and electrocoat primer. The authors of this study used synchrotron FTIR for the analysis of automotive paint primer coatings, and used chemometric methods to characterize the samples.

Synchrotron FTIR spectra were taken from the center of the primer surfacer area, where the area was most chemically uniform. The primer surfacer was selected to allow for analysis of an area that is not exposed to the environment, such as in the case of clear coat analysis since these coats can degrade over time with exposure. The dataset contained 450 spectra, obtained from 75 vehicles, covering 13 makes and 45 different models. Both Australian and international manufacturers were included in the study. Principal component analysis (PCA) was applied to the dataset to visualize groupings of the data based on how different the samples are from one another, i.e. variance in the dataset. For more information of PCA, refer to Review: Characterisation and classification of automotive clear coats with Raman spectroscopy and chemometrics for forensic purposes. A large portion of the variance, 73.5%, was accounted for in the first three PCs, indicating that a 3-D plot of PC1-PC3 would allow the visualization of most of the variance within the data, which can aid in identifying which samples are the most similar from those that are the most different. The 3-D PCA plot can be seen in Figure 2 within the study. In this figure, 14 distinct groups can be seen. The groups are displayed in Table 1 within the study. For example, Class 1 contained 3 samples representing Mitsubishi vehicles manufactured in Nagoya, Japan.

PC loadings can be examined to determine what causes the formation of groups in the PCA plot. For example, the loadings plot of PC1 showed regions of high positive correlation at peaks that were indicative of terephthalic alkyd resin. Therefore, separation of groups across PC1 is dependent on the presence or absence of terephthalic alkyd resin. Samples that did not contain terephthalic alkyd resin had negative scores on PC1. Similarly, the loadings plot for PC2 showed positive correlation at a peak indicative of melamine, and negative correlation at peaks indicative of barium sulphate. Therefore, relative abundance of melamine and barium sulphate will be the discriminating factor in PC2.

Of the 14 classes present in the dataset, 3 represented Holden vehicles manufactured by General Motors. Chemical differences were attributed to plant location and year of the vehicle. One class consisted of South Korea made Holden vehicles, while the other two classes were Australian made. Of the Australian made classes, one class consisted of vehicles manufactured up until 2005, whereas the other represented vehicles manufactured after 2005. Similarly, two Mitsubishi classes were formed and the differences were attributed to the manufacturing plant. One class consisted of Mitsubishi vehicles manufactured in Nagoya, while the other consisted of Mitsubishi vehicles manufactured in Mizushima. Another interesting set of classes was the two classes that formed for German manufactured Ford Focus. One class was found to consist of the typical four layer paint system, while the other class represented vehicles with a 6 layer paint system. The four layer system had much higher abundances of barium sulphate compared to the six layer system. Similarly, two Hyundai classes existed, which differed based on the number of paint layers. One class had an atypical three layer system, with no clear coat, while the other had the typical four layer paint system.

Based on these results, the authors state that the primer surfacer model may be used to obtain specific information regarding the potential vehicle manufacturer.

Scientific Highlights

  • Vehicles were successfully characterized by primer surfacer analysis via synchrotron FTIR spectroscopy.
  • The primer surfacer is not exposed to the environment and may therefore offer advantages over clear coat analysis, which may experience environmental degradation.

Relevance

Automobile paint may be a form of evidence available from hit and run accidents, vehicle collisions, or other incidents involving a vehicle. Using chemometrics, characteristics of the vehicle may be identifiable based on using other paint layers that are not subjected to environmental degradation like the clear coat that is commonly used.

Potential Conclusions

The primer surfacer may provide discriminatory information when analyzing forensic automobile paint samples.