Measurement scheme for identifying waste plastics using near infrared spectroscopy

　　In order to recycle waste plastics, they must first be identified and classified because the waste plastics to be processed and recycled and the parts made of them are mixed, and many are made of composite materials, which include not only different types of plastics, but also various materials other than plastics (such as metals, textiles, etc.). In addition, some plastic parts are made of different layered materials. Therefore, the identification of waste plastics is the first step in recycling. In fact, this identification and separation operation are carried out simultaneously, that is, the identification system and the separation device constitute a fully automatic continuous separation production line.

　　In recent years, such production lines have been built and put into operation in some waste plastic recycling plants in Western Europe. There are many methods available for the identification of waste plastics, among which NIR is undoubtedly one of the most attractive advanced technologies and has been applied in industry. This article introduces the principles and advantages of NIR technology, especially the NIR system for identifying waste plastics that has been put into production in Europe.

　　NIR absorption is very suitable for analyzing transparent or light-colored polymers, and is quite fast and reliable. For some common waste plastics (such as PE, PP, PVC, PS, ABS, PET, PC, PA, PU, ​​etc.), their NIR spectra are different and easy to identify.

　　In the NIR spectrum range (wave number 14300-4000cm-1, wavelength 750-2500nm), the spectrum of large-volume and long-path samples (such as plastic bottles) can also be accurately recorded, which is of great practical significance for identifying waste plastics. The CH, OH, NH and CO bands in the NIR spectrum can be used to identify most commonly used plastics. For example, the NIR spectrum of HDPE shows a characteristic peak near 1200nm, while PET does not have this characteristic peak, and the absorption peak of PVC here is also very small. However, PET shows three characteristic peaks in the range of 2100-2200nm.

　　Advantages of Near Infrared Technology for Identifying Waste Plastics Using NIR technology for identifying waste plastics has a series of advantages.

　　*NIR photodetectors have short response times, high sensitivity, and deeper sample penetration than mid-infrared, and can use low-attenuation and affordable quartz fiber optical components. They are easy to use and can also be detected remotely.

　　*NIR spectrometer has no moving parts, is not affected by vibration and dust, and can work in harsh environments, so it is particularly suitable for waste plastic recycling systems. *NIR equipment is easy to maintain, has good reproducibility of results, and has almost no instrument drift.