## 5 In-Line Application of LIBS in Industrial Production
### Waste Polymers  
For the recycling of waste polymers and the use of polymer recyclates as secondary raw material, the identification of different types of polymers and the detection of contaminations such as surface layers and heavy metals is important. The polymer polyvinylchloride (PVC, monomer formula C\({}_{2}\)H\({}_{3}\)CI) is used in different segments such as building and construction, packaging, automotive, electrical and electronic, and textile. PVC is among the top three materials by market share accounting for 10% of the total European demand for plastics (Figure 4). More than 42% of the collected post-consumer plastic waste in Europe is used for energy recovery. For the energy recovery, the PVC fraction has to be sorted out from the polymer waste to avoid the formation of HCl and other detrimental or toxic substances. LIBS enables discriminating different types of polymers and detecting contaminations. PVC can be identified by measuring the Chlorine emission line, which is unique for this polymer type. However, this task is challenging because of the low emission rate (Figure 1) and rather high LOD values for Cl (Figure A4 in Appendix). The in-line measurement of waste polymers for the identification of PVC in an industrial waste materials sorting plant is shown in Figure 21a [343]. Material from municipal waste plastic collection containing different types of plastic pieces and impurities was measured on the conveyor belt. The LIBS spectra (100 measurements/sec) were evaluated in real-time comparing the NIR range with the Cl emission line at around 837.6 nm. LIBS spectra of two different polymer waste samples measured in-line and of a pure PVC reference sample are shown in Figure 21b. PVC pieces were identified by a high correlation of spectra of the waste and reference materials (e.g., sample 1). Waste polymers of low optical reflectivity are difficult to measure by standard NIR reflectance sensors, but they are easy to measure with LIBS as this signal is largely independent of the sample color. Similar measurements were performed on polymer recylate material to identify impurities such as PVC and surface  
Figure 21: LIBS in-line measurement of waste polymers. (**a**) Photograph of the LIBS sensor head mounted above a conveyor belt in a waste sorting plant. (**b**) LIBS spectra of two polymer waste samples and a pure PVC reference sample. Chlorine emission line at 837.6 nm used for PVC detection. Adapted from [343]. contaminations in recycled PET flakes [481]. LIBS analysis of heavy metals and halogens in waste polymers has been reported by several groups, e.g., [345; 347; 482; 483]. In the production of plastics, new polymers are often diluted with recycled material. The properties (mechanical, color, chemical) of the diluted polymer should be monitored to keep it within specifications. This is a challenging task as the chemical and mechanical properties are usually tuned via a huge variety of additives such as inorganic coloring pigments, flame retardants, and various thermal and photochemical stabilizers. LIBS can be used for the elemental monitoring of recycled plastics in the production process. A demonstration of LIBS monitoring in polymer production is illustrated in Figure 22[418]. The produced polymer material is measured at the extrusion orifice of an industrial extruder in a recycling plant (Figure 22a). In addition to the elemental analysis by LIBS, other parameters of the recycled material such as color and strength can also be measured. The monitoring of chemical composition of polymers is shown in Figure 22b. The measured LIBS intensities for Ti and Sb (from additives) are changing with extrusion time as the raw polymeric material (ABS) is increasingly replaced by recycled plastic material (granulate from casings of electronic waste) [418]. After completion of transition from raw to recycled material (at approx. 20 min time) the elemental signals reach a plateau level.