bright multicolored composition of paints mixed together

Paint

Introduction

Paint is a common term for any liquid used to colour the surface of an item by covering it with a pigmented (coloured) coating. It is most often used to protect, colour, or texture items.

Zinc oxide, zinc sulphide, lithopone, and titanium dioxide are the most common white pigments. The majority of black pigments are made out of elemental carbon. The minerals iron oxide, cadmium, and cuprous oxide, as well as numerous synthetic organic pigments, are common red colours.

Components

Pigments, binders, solvents (liquids), and additives are the four primary components of all paints. Binders work to “bind” the pigment together and produce the paint film, while pigments give colour and camouflage.

Paint As An Evidence

Paint can be found at the site of a number of occurrences, such as a car hit-and-run, a break-in, an attack, and so on. Paint may be left on clothing, cars, or items as chips or stains, or it may be left loose at the site. Cross-paint transfers between two vehicles, a vehicle and an item, or two objects are also possible.

Types of Paint

Paints are classified for forensic applications based on their intended use. There are three fundamental kinds.

1.Automotive Paints: This is the most popular form of paint used in forensics. Automobiles are frequently utilised in criminal activity, and many accidents involve automobiles and trucks.

2. Structural Paints: These are used to paint structures such as houses and items such as mailbox boxes. They are utilised to provide both protection and colour.

3. Artistic Paints: These are the most traditional sorts of paints. They are made to last for a long period. The majority are derived from naturally occurring oils and pigments. Forgery is the most common forensic occurrence involving these paints.

Special purpose paints are also available for use as protection, colour, or for other purposes. Special paints, for example, are used to colour and seal concrete floors. Some road and warning signs now use fluorescent paints. Skid-resistant paints are utilised in public areas with a lot of foot activity.

Collection of Paint Evidence

All paint samples left at a location should be collected. Additional evidence may be present, and it is important to collect it. Automobile parts, glass, and plastic lens fragments from a hit-and-run, or plaster, wood, and safe insulation from a break-in, are examples of such evidence.

  • Separate samples into suitably sized containers (e.g., do not put extremely little objects into very big envelopes) to avoid breakage or loss.
  • Vials, metal or cardboard pillboxes, or paper bindles put in envelopes can all be used as containers.
  • Because of static electricity, plastic should not be utilised to pack small/loose trace evidence.
  • Packing cotton or other protective material that comes into direct contact with the object is not recommended.
  • To prevent evidence loss, all envelope edges/corners should be sealed.
  • Wet evidence should not be packed. Prior to packing, clothing or objects with paint traces should be air dried.
  • Items can be wrapped in paper before being placed in a suitable container.
  • Paint chips can be retrieved using tape lifts as a last option. If tape is required, use a kind with the least quantity of adhesive (e.g., magic tape). In clear page protector sheets, plastic, or Kapak bags, package tape lifts. Allowing tape to stick to itself or other packing materials such as brown paper bags or cardboard is not a good idea. Fingerprint lift tape should not be used.
  • Wrap objects on a table top only after the surface has been completely cleaned.
  • Cross contamination between evidence and reference samples should be avoided.

Analysis of Paint

Paint has a number of physical and chemical characteristics that may be used to compare and analyse different types of paint. Some emphasize on pigments, while others focus on binders. Others are carried out on the entire paint sample.

Physical Properties:

The colour layer pattern is the most essential aspect of paint as proof. Layers are found in automotive paints and some structural paints. In the case of automobile paints, each layer may be composed and coloured differently. If the colour layer sequences of an unknown paint sample and a known paint sample differ, the known can be ruled out as a source for the unknown. The connection between the bottom layer of paint and the bare metal is generally recognised to be the weakest in an automobile paint job. Paint chips may, however, fall off between layers of paint, and not all layers may be present. While all of the layers are missing in this scenario, the known and unknown may have a similar source. Using visible microspectrophotometry, the exact colours of each layer of paint may be verified. The layers will have to be evaluated individually in this scenario. This may be accomplished by creating “peels” and removing each layer with a sharp scalpel. A microtome may also be used to cut a cross section of the paint, allowing each layer to be examined independently.

Chemical Properties:

Smears of Paint

Paint smears may cause a lot of issues. They may be practically fused to the paint on another surface if they are transmitted to it by impact. They might be quite tough to get removed of.

SEM/EDX may be the best technique to describe the smear in many situations since it contains mainly pigment.

Visible microspectrophotometry can be used to assess the colours if it can be removed. If there is enough binder present, FTIR and PyGC can be used to do the analysis.

Chromatography

If paints are to be chromatographed, they must first be pyrolyzed. A paint chip is typically examined intact and pyrolyzed at 600°C–800°C. The pyrogram that results will be a combination of all the binders found in the paints. If the pigments are inorganic, they will not show in pyrolysis.

Solublity

Different binders can be found in automotive paints. This might be due to the manufacturer or the expense of the vehicle. Different binders may be soluble in different solvents or in no solvent at all. Under a stereomicroscope, a small paint chip is placed on a white spot plate. There is a drop of solvent added. It’s possible that the paint is insoluble, soluble, or partially soluble. Pigments seldom ever disintegrate.

Pigments

The majority of pigments are inorganic and some are coloured minerals. SEM/EDX (scanning electron microscopy with energy-dispersive x-ray analysis) is one of the most effective methods for analysing pigments. This will reveal which components are present and in what proportions. This technique is simple to use and nearly nondestructive. It is a good complement to colour analysis. Visible Microspectrophotometry may be used to examine the colour of peels or cross sections of paint chips.

Infrared Microspectrophotometry

There are two methods for obtaining infrared spectra of paints. The entire paint chip can be crushed into a powder and combined with potassium bromide before being pressed into a pellet. The overall transmission spectrum of the paint may then be determined. This spectrum will be a composite, containing peaks for all binders and dyes present in the paint, if any are present. This is useful for comparing paint chips, but not for determining the type of binder in a single binder. Finally, a cross section of the paint chip may be created, and each layer can be examined and studied using the FTIR’s microscope.

Making peels and running the transmission spectrum of each layer is another technique to get a paint spectrum.

References
  • Caddy, B. (ed.). (2001). Forensic Examination of Glass and Paint. Taylor & Francis, New York.
  • Robertson, J. and M. Grieve (eds.). (1999). Forensic Examination of Fibres, 2nd edn. Taylor & Francis, New York.
  • Thornton, J. L. (2002). Forensic paint examination, in Forensic Science Handbook, R. Saferstein, ed. vol. 1, 2nd edn. Prentice Hall, Upper Saddle River, NJ.