Near-infrared spectroscopy polymer identification

The first step in recycling plastics is to sort the plastics by their resin type, or resin identification code. The resin identification code is a number assigned to a plastic product (or container) according to the type of polymers it is made of. While it was once common to directly use this code to identify the types of polymer(s) present, there are now other methods, such as near-infrared spectroscopy or density sorting approaches, that are used to sort mass quantities of plastic samples for recycling. (See Polymer Chemistry for more information on resin identification codes.)... 

The technique commonly used to separate household packaging wastes is sensor-based near-infrared spectroscopy that is based on the fact that NIR spectra of different types of polymers are quite distinct. NIR is able to provide rapid and reliable identification of various polymers especially the polyolefin group, polyethylene (PE), and polypropylene (PP). However, the objects which are too small or too big are not suitable for this technology. Another drawback is its inability to detect dark polymers, like automotive parts and some of the electronics waste, since radiation is absorbed completely. In practice, for packaging polymers, around half of the total input materials end up in residuals. 

A brief introduction to conventional qualitative and quantitative analysis of water-based polymer dispersions is followed by a demonstration of the use of near-infrared spectroscopy for the rapid identification and determination of water content. Use of the BC AP chemo-metric software package is discussed for library searching to identify unknown tests. It is shown how within a few minutes various polymer dispersions (about 50%) can be identified and their water content determined to a plus or minus 0.2% degree of precision. 5 refs. 

IR is a nondestructive technique suitable for the analysis of formulated products, and gives a considerable amount of information about the compounds present. Near-infrared (from 13 000 to 4000 cm ) and Fourier transform infrared (FTIR) (from 4000 to 400 cm ) spectroscopies are used. Qualitative analysis of the ethanol soluble fraction allows the identification of functional group types such as hydrotropes (xylenesulfonate and toluenesulfonate). In addition, zeolite, alkalis, polymers, and builders may be identified in the insoluble ethanol fraction. For quantitative analysis, method development is slow because a great number of calibration standards... 

Near-infrared (NIR) is routinely used to qualify monomers prior to polymerization reactions. It is used to measure the kinetics of polymer onset and can be used to detect end-point completion and initiator compound levels in polymerization reactions. NIR spectro- scopy can also be used to sort polymers and to control the quality of incoming raw monomers and finished polymeric materials. Molecular spectroscopy using NIR and IR measurement techniques is often used for competitive analysis and to determine thermal or photo-induced oxidation or degradation reactions in polymers. In general, NIR spectroscopy is valuable for polymer identification, characterization, and quantitation. NIR spectroscopy can be completed for in situ process applications where no sample preparation is a requirement, and where rugged optical systems are a necessity. Some of the earliest work in applying IR and NIR spectroscopy to polymer characterization is found in References 1 to 11. 

Plastics identification by spectroscopic techniques has increasingly focused on the use of near-infrared and Raman spectroscopic techniques. LLA Instruments, in conjunction with Daimler-Chrysler [78] have developed a superfast near-infrared (NIR) sensor system that has been used to separate mixed plastics by type from shredded automotive parts. NIR spectroscopy uses the near infrared region of the electromagnetic spectrum (from about 800 to 2500 nm). Their two-phase process initially separates bright and colored polymers and black polypropylene from the mix. A second long-wavelength NIR sensor is employed to then separate black plastics such as PC, PMMA, ABS, PC/ABS blends, and others. 

Raman spectroscopy has been used to characterize organic fibers and films since the 1960s. Initially, Raman spectroscopy was used primarily to identify the material in the same way that infrared (IR) spectroscopy was used. Chemical identification and quantification are still used extensively to determine the type of polymer, the type and amount of comonomers, and the type and amount of pigments, dyes, or other additives. This has been used in forensic science, archaeology, competitive analysis, and quality control. The techniques are nearly identical to those used for the identification of other solids and liquids, with minor modifications required by the fibrous or filmlike nature of the materials. This application will be discussed in Section II,... 

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