Abstract: Paint is important trace evidence encountered during the investigation of crime, especially in hit-and-run vehicular homicide cases, burglaries, and art forgeries. Paint flakes were collected at random from one hundred motor vehicles in the city of Kuala Lumpur, Malaysia. They were studied under a stereomicroscope (20 X to 115 X) for color and layer structure. Under microscopic examination, all the paint flake specimens were distinguishable from one another, based on layer structure cross-section. Thus, the layer structure of a paint flake is significant in the characterization of paint evidence.

Introduction

Paint is an important trace evidence material that can play a role in a crime investigation. It is usually associated with burglaries, automobile accidents, hit-and-run crimes, murder, and art forgeries. In the history of criminal investigation, many serious crimes have been successfully probed and solved using paint as evidence [1-5].

There are two types of paint that serve as evidence: household paint and vehicle paint. Such paint occurs as physical evidence in two forms: flakes or smears. Paint can be transferred during accidents at the point of impact. Flakes from a vehicle's surface may also be left at an accident scene or transferred to the clothing of the victim in an accident. Most paint specimens that are examined by forensic scientists are dry flakes with one or more layers [3].

Automotive finishes are the most complex in the paint industry. With advanced technology, more materials and intermediates are being used in the chemical industry [4]. Hence, this presents a tremendous challenge in forensic paint examination.

Forensic examination of paint uses microscopy to determine color and layer structure and uses instrumental methods for compositional information [5, 6]. Advanced instrumentation in the field of forensic paint examination has provided new developments and has shown greater differentiation capabilities [6]. However, the microscopic examination of paint flakes for color and other features is the most fundamental to characterize paint evidence [1-7]. This is because the evidence can be readily assessed by such examinations [7].

The microscopic examination of the layer structure provides useful information such as color, tint, texture, metallic dispersion, layer order and thickness, inclusion between layers, and sanding striations on the primer layers resulting from refinishing procedures [8]. The layer structure of paint flakes is considered significant because of the variations in car refinishing [3].

The study of paint layer structure is very important in establishing the distribution of paint among various vehicles in a particular state or area [3]. A number of efforts have been made in the past to assess the evidential value of automobile paint transfer [3, 5, 6]. The present work endeavors to provide information on paint layer structure of automobiles in Malaysia for a proper evaluation of the paint evidence.

Methods and Materials

Specimen Collection

During the month of November 2003, paint specimens from one hundred motor vehicles were collected from several workshops, auction yards that held damaged vehicles awaiting insurance investigations, salvaged used parts, and the police station's accident vehicles' yard in Kuala Lumpur. Vehicles that were ten years old or more were included, because the lifetime of vehicles in Malaysia is sometimes up to 20 years.

The specimens were collected following the standard procedure for paint collection [8]. A sharp razor blade was used to chip intact fragments of paint from the damaged area, including the front, right, left, and rear sides of the vehicles. The collected paint flakes were kept in white paper envelopes, which were sealed and labeled appropriately.

Examination Procedure

There are various methods that can be applied in order to distinguish paint flakes. These include microscopic examination, solvent tests, pyrolysis-gas chromatography, diamond-cell infrared spectrophotometry, emission spectroscopy, scanning electron microscopy, neutron activation analysis, and many others [6, 8]. However, the microscopic examination, which is the basic and most fundamental approach to distinguish paint flake structures [1-8], was followed in this study.

Specimen Preparation

The specimens were sorted out into groups according to their topcoat color and finish. A 45 degree angular cross-section was then made on each specimen. With the help of forceps, the specimen was mounted vertically, with the cut edge in plasticine, on a glass slide. The mounted specimens were then examined.

Microscopic Examination

A stereomicroscope was used to examine the paint flake's cross-section. All specimens were examined for color, tint, texture, metallic dispersion, layer structure and thickness, and inclusions between layers, if present [8].

Magnifications ranging from 20 X to 115 X were used to examine the paint flakes. (Magnification varied, depending upon the thickness of the paint flakes.) A vertical illuminator was used to distinguish the layers.

Results and Discussion

The results of the study that was carried out on the layer structures have shown some pertinent findings. The samples were sorted according to the vehicle year and manufacturer in order to evaluate the validity of the sampling procedure. The time frame of the samples that were collected was diverse, ranging from the 1970s through the year 2003. The majority of the samples fell in the 1989 to 1999 year of production.

The topcoat color and finish gives an initial differentiation, because it is the most eye-catching component of vehicle paint [5]. It provides an indication of the frequency of various paints (Chart 1). It was found that silver/gray topcoat color had the highest occurrence and purple topcoat color had the least. Hence, a pair of matching purple topcoat paint chips (one from the scene and the other from the suspect car) would hold more evidential weight than a pair of matching silver/gray paint chips. This type of information is useful in evaluating a case where only a topcoat color is present. Therefore, the idea of prevalence of a particular color is required [8].

Chart 1
Distribution of samples by topcoat color and type of finish.

Further, the study showed that about 66% of the vehicles had a metallic finish (Chart 1). Metallic finishes are preferred as exterior topcoats because they give a glossy look and the paint does not easily chip off. However, nonmetallic finishes contain aluminum flakes [4]. Therefore, a nonmetallic topcoat holds a better evidential value, because the occurrence of nonmetallic paint is less common.

Out of the one hundred specimens that were examined microscopically for layer structures, 98 specimens were differentiated according to their color and layer structure. The remaining two paint flakes, which were of silver topcoat color, shared a common layer sequence. However, these two specimens differed in their layer thickness. In one paint flake, one layer measured 20 micrometers, and the other layer measured 30 micrometers. In the other flake, one layer measured 40 micrometers, and the other layer measured 60 micrometers. These two paint flakes originated from two Honda cars that were manufactured in 1994 and 1997. The differences in the thicknesses of the layers of the two paint flakes could not be explained satisfactorily. Differences in the layer thickness of paint flakes are usually noticed when the corners and other similar parts of automobiles have been subjected to damage and, consequently, have been repainted. In the present instance, the paint coatings appeared to be the original finish in both cars and hence the conspicuous difference in thickness in the layers [4] could be construed to be of some significance. The two paint flakes could therefore be reasonably differentiated by their thickness. Thus, all of the specimens that were collected were distinguishable from one another, depending on the characteristics exhibited by their cross-sections. Significantly, no two paint flakes shared a common layer structure.

The layer distribution of the specimens is shown in Table 1. The layer distribution ranged from a single layer to multiple layers. The maximum number of layers encountered was 19 (Figure 1). The variations in the layer structures are caused by the fact that the specimens came from new cars as well as old ones. Based on the information obtained from the specimens, 21% had three layers. This is a common number of layers a car would have if it was not involved in an accident or repainting process.

About 10% of the specimens had more than nine layers. These can be classified as complex multilayered paint structures, having greater evidential value. The higher the number of paint layers observed, the more distinguishable the paint will be.

Conclusions

The one hundred paint specimens that were collected at random from Kuala Lumpur, Malaysia, were distinguishable by standard forensic microscopic examination. Significantly, about 57% of the specimens had a multilayered structure. A paint flake possessing a multilayered structure possesses greater evidential value and could serve as a "fingerprint" of a particular vehicle.

The results of this pilot study could be used to plan a national study on the frequency of layer structures for better evaluation of this important trace evidence, thus initiating the Malaysian automotive paint database.

Acknowledgments

The authors wish to thank Professor Norazmi Mohd. Nor (Chairman, Forensic Science Program, Universiti Sains Malaysia) for his keen interest and support; the police officers from the Accident Vehicles' Yard; the workshop owners for permitting paint specimen collection; Mrs. Hafizah Harun, Mrs. Rosniah, and Mr. Nik Fakuruddin (Scientific Officers, Universiti Sains Malaysia, Kelantan, Malaysia) for their laboratory assistance; Mr. Alwi Abdul Manap (father of A. Reeza) for his assistance during the collection of the paint specimens from the vehicles; and the reviewers for their constructive comments leading to an improved presentation of the work.

The present work forms a part of the dissertation carried out under the project "Studies on the Layer Structure of Paint Flakes Collected from Motor Vehicles in Kuala Lumpur City for Evaluating Their Evidential Significance" in the School of Health Sciences, Universiti Sains Malaysia, Kelantan, Malaysia.

Table 1
Layer distribution of total samples.

Figure 1
Photomicrograph (50 X) of a paint flake showing
a complex nineteen layers, varying in thickness.

For further information, please contact:


Aedrianee Reeza Alwi
Chemistry Department of Malaysia
Jalan Sultan
46661 Petaling Jaya
Selangor Darul Ehsan
Malaysia
forex275@lycos.com

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