As previously noted, many commercial plastic bottle sorting lines use near-infrared spectroscopy to accurately ID bottles by resin type at very high speed.
Compared to MIR, in addition to speed, NIR has the advantage that direct or close contact between the detector and the sample is not necessary . NIR instruments are also compatible with flexible fiber-optic probes .
NIR spectroscopy has been applied to the fully automated sorting of dismantled plastic parts on a conveyor belt (Fig. 14.12). An interesting evaluation of the technology can be found in a recent International Symposium on Electronics and the Environment (ISEE) conference paper . The point raised in the paper is that under optimum conditions sorting accuracy is at best about 98% and that 98% resin purity is unacceptable for higher value engineering polymers. Since the NIR-sorted parts would need to go through another separation and purification step, the paper points out that it may be more cost effective to go directly to this type of processing step if it can be run with sufficient accuracy and throughput. The point may be valid under some situations and should be part of any overall recycling technology assessment, especially in the case of mixed plastics obtained from end-of-life vehicles and electrical and electronic equipment. Such an assessment leads to consideration of whether plastics sorting should take place at the parts dismantling stage using instrumented ID methods to separate by resin type  or at the granulate/flake stage after size reduction [10, 49, 71, 72] where higher throughputs are in principle possible.
A related study compared recyclate quality and plastic separation costs for three alternative processing schemes . The first scheme was called a manual sort and involved bench-top spectroscopic (MIR or NIR) identification and manual presorting of plastic electronic product housings followed by dry and wet processing of the manually sorted plastic to obtain clean granulate. The second scheme was called automated ID and sort and used an automated NIR process to identify and sort the plastic housings on a conveyor belt followed by dry and wet processing. The third method used dry and wet processing to separate the plastics from a stream of mixed housings without going through the manual or automated presort of the housings. The conclusion of the study was that under most conditions using real-world feedstocks neither the manual or automated presort produced a final recyclate stream with adequate purity (99 + percent). In all cases, dry and wet processing that included a sophisticated flake sorting step was required to achieve high purity. Table 14.11 shows the comparative economics developed in the study. Information such as this will become more important as more and more end-of-life electronics are collected for recycling and both the public and private sectors work to minimize overall system costs and maximize material value.
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