Plastics 197
Universal Indicator paper at the mouth of the tube. The results may be
interpreted as:
●pH 1–4 (Acidic): cellulose nitrate or acetate, polyester, polyurethane
and PVC
●pH 5–7 (Neutral): polyethylene, polystyrene, acrylics, polycarbonate,
silicones and epoxies
●pH 8–14 (Alkaline): nylon and formaldehyde plastics.The flammability of plastics can be a useful tool in their identification, but
the additives present have an influence on their ability to burn. A clear test to
indicate the presence of chlorine, mainly found in PVC and poly(vinylidene
chloride) is the Beilstein test. A clean, copper wire 30–40 cm long, with a cork
or other heat-insulating material at one end as a handle, is heated with a
Bunsen burner to clean it of residual impurities, heating it until the flame is
colourless. The hot, cleaned wire is placed in contact with the plastic to be
analysed so that a small piece is melted onto it. The wire is returned to the
flame and the colour noted; a green or blue–green flame denotes the presence
of chlorine while other colours suggest the plastic to be other than PVC or
poly(vinylidene chloride).
3.2 Analytical Techniques
Most simple tests can only indicate the group of plastics to which the object
belongs. In order to identify materials more precisely, it is necessary to use
instrumental analytical methods. Different techniques provide information
about which polymers and which additives are present and their state of degra-
dation, so it is usually necessary to use several in combination. For example,
Gas Chromatography–Mass Spectrometry (GC–MS) is a destructive technique
that allows identification of the organic components and volatile materials
present due to degradation. X-ray Fluorescence (XRF) spectroscopy is an
effective, non-destructive surface technique for identifying inorganic fillers,
pigments and metal components, but cannot detect polymers.
The most widely available technique for identifying mainly polymer, but
also additives in plastics, is Fourier Transform Infrared (FTIR) spectroscopy.
Samples are exposed to infrared light (4000–400 wavelengths per centimetre
or cm^1 ) causing chemical bonds to vibrate at specific frequencies, corres-
ponding to particular energies. In the last 5 years, an accessory for FTIR has
been developed, which enables non-destructive examination of surfaces and
so is ideal for analysis of plastics in museum collections. Attenuated Total
Reflection-FTIR (ATR-FTIR) requires samples to be placed on a diamond
crystal with a diameter of 2 mm through which the infrared beam is reflected