Discrimination of polymers by laser induced breakdown spectroscopy together with the DFA method

Laser Induced Breakdown Spectroscopy (LIBS) is used to analyze and identify six kinds of the most important recyclable polymers i.e high density polyethylene (HDPE), low density polyethylene (LDPE), polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC). Using a Nd:YAG laser with wavelength 1064, a plasma is created on the polymer surface. By analysis of spectral emission of plasma, some qualitative information about the plasma elements is obtained. The plasma spectra of polymers are similar and contain strong carbon and hydrogen spectral lines. Here, a statistical method called discriminant function analysis (DFA) is used to discriminate between the polymers by slight differences between the spectra. DFA establishes a model on the basis of input variables to predict group memberships of polymers. The spectral line ratios of C, CN, C2, N Cl, O and H are used as input variables in DFA. Results show that LIBS together with DFA has the ability to correctly classify 99% of the polymers.