INTRODUCTION

Physical evidence is a necessary element in any criminal investigation. They consist of all the objects that can prove that a crime has been committed or at least can provide a link between the crime scene, the victim, and the criminal. Often, small fragments of this physical evidence are present at the crime scene such as hair, fiber, or paint that might prove to be essential evidence for solving a crime. These tiny fragments of physical evidence are known as trace evidence, which is usually transferred when the crime is committed. This evidence although, small, possesses a great value in indicating that a person or an object was present at the crime scene.

Paint acts as important trace evidence as well as an investigative tool in establishing the crucial links in a criminal investigation. This is because paints can be easily transferred between two objects, two individuals, or between an object and an individual as can be seen in cases when a vehicle collides with another vehicle, a building, or an individual. If the object with which it collides also has a painted surface, it may also result in a double transfer with both the objects having paint from one another. Transfer of paint is also commonly seen in cases when an instrument is used to open a window or door that has a painted surface or when a person comes in contact with wet paint or between two individuals when they come in contact. Normally, a forensic scientist compares two or more samples of paint to determine if they have a common source.

Spray Paints are the third most important type of paint after automotive and architectural paint. The use of spray paints has increased drastically in recent times in almost all materials as a coat because of their exceptional qualities such as increased coverage due to high solid to solvent ratio, fast-drying, increased durability, and high flexibility to name some.

Scientists carefully analyze the physical, optical, and chemical properties of these spray paints while comparing two or more samples by using a variety of tools. This is because these properties are subjected to varying degrees of change caused by different factors such as changing environmental conditions.

SPRAY PAINT AND ITS COMPOSITION

Spray paint consists of aerosol so that it can be dispensed as a fine mist on the surface it is being sprayed. Spray painting is much faster and also helps in the uniform application of the paint as compared to the conventional painting methods. Special types of air compressors are used in industries that help in the application of the paint in the form of fine mist, whereas commercially available spray paints are self-contained aerosol cans. These aerosol cans utilize liquefied gases to atomize the paint.

Pigments

Pigments are the component of the spray paint that is used to provide the paint with its colour and opacity. Spray paints mainly consist of four broad types of pigment. The first one being the white pigments such as titanium dioxide which are utilized to scatter the light and to increase the opaqueness of the painted surface. The second one is the colour pigments, and as the name suggests, they are responsible for providing the paint with its colour and consists of various synthetic chemicals. The third is the inert pigments which function as fillers to change the film characteristics of the paint. Fourth is the functional pigments that provide the paint with specialized characteristics such as resistance from UV rays. Pigments should be carefully selected because they affect important properties of the paint such as viscosity.

Solvents

Solvents are another important component of spray paints and account for the liquids that carry the rest of the ingredients. The solvent should have the property of quick-drying, thus non-aqueous liquids are chosen. The solvent should be wisely chosen as they affect the stability of the dispersion of pigments. A good solvent help in stabilizing the dispersion by being absorbed onto the outer layer of the particle and cause it to swell. If a solvent completely prevents the interaction of particles, it will lead to destabilization of dispersion causing the degradation of the paint.

Propellants

Propellants are the gaseous components of spray paints that help in forcing the paint out of the can. They do so by expanding quickly as soon as the valve is opened. Earlier Chlorofluorocarbon gases were used as propellants, but after they were banned in 1978, many other alternatives were introduced such as butane and propane. But later in the 1980s with the determination of the fact that these hydrocarbons were a major cause of smog, they were replaced by a new type of propellants known as Hydrofluorocarbon. Some examples of hydrofluorocarbons are 1,1, -difluoroethane and 1, 1, 1, 2, -tetrafluoromethane.

Fig. 1- “This image is taken from http://www.madehow.com/images/hpm”

WEATHERING AND AGEING OF SPRAY PAINTS

The study of weathering of paint or aging of paint has always been of great significance to forensic science. It is a commonly known fact that all kinds of paint perform the function of a coating material providing some characteristic optical features like colour, luminosity, etc to the paint. But, apart from this function, paints have a much more important function i.e., to protect the surface of the object from varying environmental conditions. Yet, the external conditions are seen to have a harsh effect on the paint when many factors such as UV radiation, moisture, heat, etc. act simultaneously on the surface of the paint. These result in many chemical changes in the molecular structure of the paint such as the scission of chemical bonds, or the creation of cross-linking. Thus, it becomes necessary for the forensic scientist to take into account these changes while comparing samples during a criminal investigation.

Whenever a comparison case is being talked about, there is always a minimum of two samples, one being the known sample which usually has a known origin (for instance, the sample from a suspected vehicle in a hit-and-run case) and the other being the questioned sample which is the one that is recovered from the scene of the crime or the suspect (for instance, a paint chip collected from the scene where the hit-and-run happened). Even though it seems like a considerably direct procedure, the real-world samples undergo varying degrees of weathering which pose some challenges in the comparison process. Also, the samples recovered from the scene of the crime are usually small which adds to the challenge of interpreting the evidence. However, analysis of some unique features of the paint proves to be quite significant if similar results are obtained from both the known and questioned sample.

The weathering or aging process can be seen either as an optical change such as colour fading, or a mechanical change such as the presence of cracks, delamination, etc, or even as a chemical change. Chemical changes usually cause a difference in results obtained from any analytical technique used for the comparison process. Thus, utilizing different analytical techniques for comparison becomes inevitable to avoid any false-negative results. Studies have shown that spray paints mainly are composed of alkyd modified polyester binders [6] and these alkyd-based binders were shown to give peaks around the 2800– 3400 cm-1regions of UV-light radiation [4].

Among all the different factors that lead to the weathering of spray paints, solar radiation is considered as the factor having the most impact. This is due to the UV radiation (295–400 nm) that has short wavelengths, and highly energetic radiations. These high energy radiations consist of photons with enough energy to catalyze the photo-oxidation process in the presence of oxygen.

Another important factor that leads to the weathering of spray paints is moisture content. Moisture affects both the paint and the support. The reaction of the support with the water molecules usually results in cracks and peelings of the paint [2]. Also, when there is a high concentration of oxygen in the atmosphere, hydrolysis can take place which results in chemical modification of the paint.

Some chemical and biological factors also cause the weathering of the paint. For instance, acid rain containing SO2 and H2SO4 can act as a catalyst for hydrolysis as well as photo-oxidation reactions. In the case of biological factors, the bird excrements are one of the most common factors acting as a catalyst for the fission reaction of ether-ester bonds, eventually leading to the aging of the paint.

It is necessary to understand the chemistry of spray paints well to correctly predict the course of degradation or weathering of these paints, and also to study the different variations the weathering process causes in the results of instrumental analysis.

Fig. 2- “This image is taken from Jost, C., Muehlethaler, C. and Massonnet, G., 2016. Forensic aspects of the weathering and ageing of spray paints” by Forensic science international

CHANGES IN VARIOUS INSTRUMENTAL ANALYSES DUE TO WEATHERING

PHYSICAL ASPECT

As we have seen above that the adherence of paint film on the support is dependent on the moisture content in the atmosphere. It is seen that cracks begin to appear on certain paint films (for instance, green paint containing Alkyd orthophthalic and nitrocellulose) after about a month of exposure to the natural environment. Usually, there is no significant fading of the paint in the natural environment for about 3 months at least, although fading can be brought about by exposing the paint to artificial weathering conditions in a Vo¨tsch VC3 climate chamber.

INFRARED SPECTRA

This analytical method plots a graph of photo-oxidation index vs degradation or weathering of the paint sample. Most plots of the different spray paints indicate that weathering is very rapid in the first two months of exposure to the natural environment, but as time passes by this degradation becomes stable and a linear trend is followed. Also, it should be noted that during the comparison process, the log function in the case of infrared spectral analysis shows accurate and better results than the linear function. This is due to the comparatively rapid degradation that is seen during the initial days of exposure to the natural environment.

Another observation is that infrared spectra of spray paints are more readily visible only during the initial months of exposure to the natural environment. This may be because the degradation of paint stops after a few months when all possibilities of oxidation reaction get exhausted. Another observation made is the comparatively greater percentage of degradation happening in artificially weathered paint samples (2%) in comparison to the naturally weathered paint sample (1.5%). Also, it should be noted that paint samples left in dark for a long period show values of weathering percentage in between that of a fresh sample and naturally weathered sample. This shows that it’s not just the solar radiations that affect photo-oxidation index but also other factors such as temperature, humidity, etc. Although, the effect of the other factors in weathering is much less than compared to the effect of solar radiation.

So, it can be stated that different spray paints behave differently to the ultraviolet radiations weathering, depending upon their chemical composition. Since spray paints mostly consist of alkyd polyesters, the more rapid decrease in a peak of methylenic as well as modification in peaks of magnesium silicate or nitrocellulose can be seen. But these changes in results of the infrared spectra be very minor and usually does not lead to any false exclusion of common source.

RAMAN SPECTRA

Raman spectroscopy mainly shows the changes in pigment composition or pigment degradation. Raman spectra generally show no difference in peak position and intensities. Also, it should be noted that the photo-oxidation index cannot be calculated for Raman spectra, so Principle component analysis is studied to find the difference in the extent of weathering.

Raman instruments are considered as very sensitive to the surrounding environment as well as any calibration procedure. And due to these reasons, measurements of Raman spectra taken even a few days apart may show some minor differences. Therefore, any changes in Raman spectra peaks that are observed are due to the possible errors in the analytical method and measurement condition, rather than due to a real weathering process.

Thus, it can be stated that no analytical differences are seen in Raman samples. Visual examination of Raman spectra is mostly stable and shows consistent ratios between the peaks observed. This indicates that pigments are much more resistant to weathering due to Ultraviolet radiation as compared to binders. This is because pigments do not consist of any reactive chemical groups or a cross-linked structure in its composition that aids in the weathering process. Fading of colour also does not produce any change in Raman spectra even after months.

CASE STUDIES

Case study 1:

A hit and run accident was reported in district Jind. One motorcycle was found in extremely damaged conditions at the scene of the crime. This accident had caused the death of the person riding the motorcycle. It was observed that the motorcycle had red transferred paint on the damaged battery cover. This transferred red paint was collected for comparison with the accused vehicle. Several vehicles were examined which were suspected to be involved in the accident. Out of these, one tanker truck was observed to be having traces of red and white transferred paint. This transferred paint was analyzed and it was found to be matching physically and chemically with the paint of the number plate of the motorcycle. The red paint found on the motorcycle was found to be matching physically and comically with the red paint of the tanker truck after analysis. Hence it was the paint scrapings that acted as physical evidence for linking the collision of the accused vehicle with the victim’s vehicle and hence the case was solved beyond any doubt.

Fig 3- “This image is taken from Kaur, G., 2020. Forensic Implications of Paint, It’s Use & Detection.” By Journal of Forensic Science and Criminal Investigation

 Case study 2:

One suspect was alleged of graffiti painting on a damaged train. Six paint control scrapings from the train were also submitted. The suspect was arrested and his clothing along with two cans of paint was seized. Both the paint cans were ‘Mtn Pintura paint spray hardcore’ cans, one was ‘mustard’ color and the other was ‘toasted brown’ color of 400ml capacity. They were both half full and both had nozzles in-situ. The paint controls that were submitted had one red scraping, 3 dark brown scrapings, and 2 light brown scrapings. Optical comparisons were conducted and the ‘toasted brown’ spray can is found to be different from the dark brown control sample. The spray can that contained ‘mustard’ was found to be indistinguishable in colour when compared with the light brown samples. The presence of yellow and orange particles in both the control paint and this spray paint was confirmed when examined with transmitted, polarized light. They both showed weak yellow fluorescence when observed under light reflected through blue and ultra-violet filters, and were indistinguishable when analyzed by FTIR. This finding provided scientific support for the fact that the can was used for the creation of this particular light brown graffiti. 

Case study 3:

A suspect was alleged of criminal damage done to a control box at the side of a London underground station. Thus, two control scrapings from the control box at the side of a London underground station were submitted for examination. The suspect was arrested while he was running from the scene where the spraying of graffiti had been witnessed. A hooded top was seized from him after his arrest. The seized hooded top was submitted to the laboratory and microscopic examination was conducted. The examination revealed that the top had a moderate concentration of paintballs on the outer surface, and most of them were located on the front of the right sleeve and cuff. Testing using FTIR showed that both paints were chemically different, even though they were optically indistinguishable from each other. Also, when tested microscopically, the red paint was indistinguishable from both the red control paint samples. Examination using transmitted, polarized light revealed that both the control paints and the recovered paintballs contained red and brown particles and did not fluoresce when observed under light reflected through blue and ultra-violet filters. It was also observed that the recovered paint was found to match with one of the submitted controls when analyzed by FTIR. The finding provided strong scientific support for the fact that the wearer of the top had been in the vicinity of this graffiti while it was being sprayed.

CONCLUSION

As we have seen above, the weathering of spray paints due to UV radiation as well as other factors such as moisture and temperature causes spectral variation in the case of FTIR spectra due to the presence of oxidation product, whereas, there is not much difference observed in case of Raman spectra. Spray paints are subjected to rapid weathering due to their simple composition, and this rapid weathering usually continues for two months, after which the weathering process becomes more stable. The changes in the results due to weathering can be determined using the photo-oxidation index. Thus, care should be taken when a forensic scientist compares two samples for determining the common origin, keeping in mind all the weathering and aging aspects of spray paints. This is important in explaining any significant changes in the spectral analysis that may appear between two paint samples of common origin.

The forensic paint examiner’s major task is to take into account the heterogeneity in the samples as well as any changes that might have occurred in the sample due to the natural weathering process so that any false exclusion of common source does not occur. These facts emphasize the importance of having an appropriate sample so that a correct comparison is made. Unfortunately, most, necessary samples are not made available and thus the determination of the extent to which the questioned sample has undergone weathering can be quite challenging for the forensic scientist. Sample size, sample heterogeneity, and varying degrees of weathering are some common challenges faced by the forensic examiner. These conditions as well as the unavailability of required databases for all different types of paint decrease the reliability of paint evidence. Thus, improvement in analytical techniques, as well as enhancement in the quality of databases on different types of paint, would help in making paint evidence unambiguous.

REFERENCES 

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