Question

You want to quantitatively match the suspect car colour to car paints from different car manufacturers. You have visually matched two samples from a range of “touch-up” colours at an auto supply store.

(iv) Discuss the spectroscopic method you will use to compare the colours of the two paint samples. What experimental parameters are essential to keep consistent when performing a comparative quantitative test? (Graduate Capabilities - Chemistry knowledge & skills; Inquiry /Research)

Answer 1

As we know paint consisits of different organic binders and inorganic fillers and pingments. So we need that spectroscopic methods or techniquesthat can analyze paints in these two areas. FTIR (Fourier Transform infrared spectroscopy) and EDS (Energy Dispersive X ray spectroscopy) are perfectly suited for this analysis. In FTIR, infrared light is passed through a organic material (such as paint), which has covalent bonds and hence results in characteristic The produced infrared spectrum is unique for a given organic material and known as materials “chemical fingerprint”.

Although IR Spectroscopy is not a particularly reliable technique for indicating the presence of a minor constituent of paint, it is excellent for determining the different organic binders in paint for example, Epoxy, Urethane, Acrylic etc. For an example, if three samples of red paint cannot be distinguished by color then it can be immediately distinguished by FTIR if they have a different organic binder. As FTIR was not particularly suited for indicating the presence of minor constituent of paint, such as an inorganic pigment or drying agent. So we use Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (EDS) which allows detailed elemental composition of any material. This combination is useful in studying the inorganic additives and other minor inorganic ingredients used in modern paint formulations. Just as FTIR gives a “fingerprint” of the covalent bonding paint molecules, EDS gives an “elemental fingerprint” of additives which often go undetected by FTIR. consider an example, if there are two acrylic automotive paints which match in color (say red) but have different red pigments Cadmium red and Iron oxide. These paints may even have equivalent FTIR scans and the only way to distinguish between these is by EDS, which can easily distinguish between Cadmium and Iron.