Forensic Analysis of Paints
Everyone has seen or used some kind of
paint during their lives. Anyone who has used it will realize that
the paint goes on in liquid form, then dries to form a solid coating. There
is much more to paints than meets the eye. Below are some definitions
which will help in deciphering exactly why forensic scientists sound so
sure of themselves when they match a paint chip from a crime scene to the
vehicle of a suspect.
- Paint- "Commonly known as a pigmented
- Coating- "A generic term for
paint, lacquer, enamel, or other liquid or liquefiable material that is
converted to a solid, protective, or decorative film or a combination
of these types of films after application." (9)
- Solvent- The liquid that holds
the sold particles in paint. This must be able to evenly carry the
paint particles for application and must provide adhesion to the desired
surface. The solvent will evaporate leaving behind the solid coating.
Different solvents can be used to get different surface effects.
- Binder-The binder holds the
pigment particles together into a solid film after the solvent has evaporated.
This is the most significant component of paints as it "determines
many of the necessary film properties such as adhesion, gloss level, hardness,
abrasion resistance, flexibility, speed of drying and durability." (10)
- Pigment-This is where the paint
gets its color. Pigment particles are very small and are suspended
in the binder and solvent system. They are not soluble in the
solvent and the binder holds them together to form the surface film. "Extender pigments are used in primers and primer surfacers
to enhance film properties and to determine the filling and sanding properties
of the paint film." (10)
- Additive- "Additives are the
specialist components of paint, they are used in small quantities to improve
production and storage properties of the liquid paint product as well
as application and other performance properties of the paint film." (10)
Additives may be used as corrosion inhibitors, plasticizers, driers, UV
absorbers and various other things to improve the performance of the paints.
Forensic paint matching has
many steps. The steps taken depend on the details of the situation.
What kind of paint is being matched? Is the unknown sample from
a smear or from a flake or was it transferred to another painted surface?
How much paint is available for matching? What is the reference
sample? Does the sample need to be preserved? All of these are questions
that must be asked by the forensic scientist. The FBI uses the following
flowchart as a guideline for testing the paints: (9)
Once it is decided that chemical analysis is necessary, methods must be
chosen to analyze the paint for the components used in the binders, pigments,
Notice that FTIR is one of the options
for each of these analyses. Raman is only listed with the pigments,
but it can be used for some of the additives as well.
Though FTIR methods have been used in analyzing paints for some time,
Raman is relatively new to this forensic application. (11,12)
Binders, pigments and additives contain specific types of molecules.
Though many of the molecules may be analyzed using either FTIR or
Raman spectroscopy, one of the methods may be more accurate than the other
in this application. Below is a summary of the types of molecules
found in each paint component. For detailed spectroscopic information,
click the icon to the left of each heading.
Typical binders consist of polyurethane, epoxy, acrylic or silicone
resins. Some inorganic silicates are also used. (10) Melamine
or styrene are often used to cross-link the binders. All of these
substances have vibrational frequencies that can be identified spectroscopically.
The most common pigment is rutile (TiO2). (10) This
is used in a majority of white paints and in many primers. Other
pigments are usually organic molecules, but several inorganic compounds
are used as extender pigments. Extender pigments are used in primers
to enhance the properties of the coating. Barium sulfate and calcite (CaCO3)
are typical extenders.
Additives can be used to gain a variety of properties. Silicone
is used to make the paint surface more resistant to marking and scratching.
Stabilizers like phthalate are used to prevent the aggregation of
pigment molecules while the paint is drying. (13) Anti-molding agents,
anti-corrosion agents, and drying agents are just a few more of the additives
used in modern paints.
In summary, both FTIR and Raman are necessary in order to best identify
an unknown paint sample. Much research is being conducted (5, 11, 12, 15,
16) which will hopefully enhance the scientists' ability to confidently
say, "it's a match"!