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How can I easily identify a plastic?

   

Frequently Asked Questions

One of the simplest ways to carry out a flame test is by cutting a sample from the plastic and igniting it in a fume cupboard. The colour of flame, scent and characteristics of burning can give an indication of the type of plastic:

  • Polyethylene (PE) - Drips, smells like candlewax
  • Polypropylene (PP) - Drips, smells mostly of dirty engine oil and undertones of candlewax
  • Polymethylmethacrylate (PMMA, "Perspex") - Bubbles, crackles, sweet aromatic smell
  • Polyvinylchloride (PVC-U, Unplasticised) - Self extinguishing flame
  • Polyvinylchloride (PVC, Plasticised) - Green tinge to flame and white acrid fume
  • Polyamide or "Nylon" (PA) - Sooty flame, smells of marigolds
  • Acrylonitrilebutadienestyrene (ABS) - Not transparent, sooty flame, smells of marigolds
  • Polycarbonate (PC) - No drips, phenolic smell
  • Polyurethane foam (PU) - Yellow flame, acrid smell, plastic crumbles
  • Polyethylene foam (PE) - Drips, smells of candlewax

A more scientific way of identifying plastic is the use of Fourier Transform Infrared Spectroscopy (FTIR), which is used for qualitatively and quantitatively determining the presence of specific chemical groups, e.g. C=O, or C-H, or C-CH3 in a plastic material. This is possible since each group has characteristic frequencies at which absorption (or conversely transmission) of infrared radiation occurs. By reference to an established collection of infrared spectra, it is possible to assign specific absorptions to particular groups to confirm the type of plastic. The Fourier Transform method has enabled the response of the material to a single excitation to be transformed into a spectral scan. The process is repeated many times and results averaged to attain high accuracy. The use of polarised infrared radiation allows a comment to be made concerning the direction of particular bonds in an organised polymer structure.

A more recent polymer characterisation technique is Nuclear Magnetic Resonance Spectroscopy (NMR) and is used to determine chain structures. A library of NMR spectra is available to help identify unknown materials.

See further information about plastics welding and testing or please contact us.

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contactus@twi.co.uk