Process of Investigation

1. Investigators can use the general pattern of burn damage to polymers to assess the point of origin of fire.

An example can be found in the case of a 1993 Ford F150 to be covered later in the course. If you want to see the relevant pages of the case study, click here.

2. Ignition temperatures of polymers are low enough for autoignition on a hot exhaust system. Fire can occur if wiring insulation or other polymeric parts contact hot exhaust system components during vehicle operation or crash deformation.

3. Differences in polymer properties may also complicate interpretation of the evidence. Sometimes the variation in burn damage of adjacent parts may be caused by differences in flame resistance of the polymers involved rather than the characteristics of the fire. Identification of such differences can be investigated through controlled tests or by research of manufacturering specifications.

4. Degradation of polymeric components can lead to fluid leakage, causing non-collision fires as well. Evidence of the original leak is likely to be destroyed early in the fire. Identification of degraded parts leading to fire may be aided by inspection of similar non-fire vehicles.

5. The temperature history of polymers (including paints) can sometimes be determined through laboratory analysis. For example, paints provide characteristic “signatures” when submitted to a process called Thermal Gravimetric Analysis (TGA) that may give a clue as to prior temperature exposure. This is because there are certain irreversible changes that the paint may undergo during heating, such as solvents that are driven off at particular temperatures. Similarly, other polymers (especially “thermoset” plastics) may suffer heat exposure degradation, which is not visually apparent. Analytical techniques such as Fourier Transform Infrared (FTIR) spectroscopy can assess the characteristic fingerprint of molecular bonding, often affected by heat exposure. Other methods, such as Differential Scanning Calorimeter (DSC), provide information on melt history of thermoplastic materials. In such cases, it is usually helpful to have an undamaged sample of the polymer for comparison in order to aid interpretation of the findings.