Infrared Spectroscopy in Analysis of Plastics Recycling

Abstract

AbstractPlastics recycling recovers valuable materials to save natural resources which are limited in the long term. Reprocessing of the materials requires the identification of the various types of polymers. Difficulties in applying physical separation techniques led to the modification of methods of chemical analysis which have to be operated fast and reliably under harsh industrial environments favoring infrared (IR) methods.In general, the plastic products to be handled can be separated into two main fractions. Domestic wastes contain only five relevant polymers in large quantities. Technical products use a broad variety of polymers containing fillers and additives, e.g. flame retardants, which also have to be identified to allow economic recycling.Solid samples absorb IR radiation strongly, and the fundamental vibrations of the molecules are especially intense. The near‐infrared (NIR) spectral range with the overtone and combination bands is appropriate for analyzing bulky materials in reflection and transmission modes. The plastics from domestic wastes can be reliably identified by the first overtone of the CH bands between 1600 and 1800 nm. Extending this wavelength range to 1000 nm, technical non‐black plastics can also be treated. In the case of black plastics, which are often used in audio and video appliances, the mid‐infrared (MIR) spectral range has to be applied, owing to the reduced penetration depth of radiation into these materials. Also, larger amounts of additives such as plasticizers and flame retardants can be analyzed.The identification of plastics from household waste requires fast scanning techniques in the millisecond range, especially if many samples are to be taken for one identification on movement. Mainly acousto‐optic tunable filter (AOTF) spectrometers or grating spectrometers with diode arrays can be applied. Germanium (Ge) and indium gallium arsenide (InGaAs) detectors with extended wavelength sensitivity cover the spectral range of interest. The movement excludes Fourier transform infrared (FTIR) spectrometers with e.g. indium antimonide (InSb) and mercury cadmium telluride (HgCdTe) diodes, but its application to technical plastics is possible because these pieces are heavier and can be identified in a fixed position.Identification for recycling means a qualitative analysis. Simple algorithms such as those forming the differences of normalized spectra are sufficient for comparing measured with stored spectra. The performance of microprocessors also allows the application of more complex algorithms such as multivariate analysis or even neural networks. The use of NIR spectroscopy in industrial recycling started with application to sorting systems for plastics from household wastes or hand‐held sensors for cable recycling. The MIR technique still suffers from its sensitivity to harsh industrial environments and has been used only rarely and needs further development.