# Characterization and Separation of Traditional and Bio-Plastics by Hyperspectral Devices
## 1 Introduction
In order to obtain high quality secondary raw materials, the separation process should produce a pure product or several distinct pure products consisting of a single polymer type. Therefore, it is necessary to employ a robust methodology to correctly separate bio-plastics, which act as contaminants, from traditional plastics and to enhance the separation processes traditionally used within recycling plants. Traditional methods are based on the floatation principle, which separates polymers with respect to their density. This requires the density of the floatation medium to be adjusted by adding saline solutions or organic solvents [7]. Nowadays, a few advanced sorting technologies are recognized of being promising for the fast industrial identification and separation of solid plastic wastes [8]. Among them, separation processes exploiting the potential of the hyperspectral analysis are gaining increasing attention. In fact, these are considered to be alternatives for the classical methods, which require large amounts of chemical reagents and are both time consuming and more expensive [9]. In particular, Near InfraRed (NIR) spectroscopy presents several advantages such as remote high-speed measurements, high penetration depth of the NIR radiation and high signal-to-noise ratio [8]. Hyperspectral systems exploit the interaction of an object with a light source (natural or artificial) allowing the extraction of the object's spectral signature, which describes the percentage of incident radiation reflected from the surface of the object as a function of the wavelength. Since the spectral signature is a characteristic feature of each material (all materials have their own signature) this property can be exploited for distinguishing plastic samples of different typology. At the Laboratory of Hydraulics of DICEA-Sapienza University of Rome, a system for the acquisition of hyperspectral images based on the use of linear spectrometers and an effective methodology for the discrimination of materials based on their chemical constituents have been designed and realized. This system has been used for the characterization of two traditional polymers, polyethylene terephthalate (PET) and polyvinyl chloride (PVC), adopting two different strategies for this purpose, the first one based on the position and characteristics of the spectral signature absorption bands of each polymer and the second one on the correlation matrix analysis [10]. Ref [11] presents the application of principal component analysis and partial least squares-discriminant analysis on waste samples made of the most diffused plastic typologies, i.e., polyethylene, polypropylene, polyvinyl chloride, PET, and PS, achieving the complete classification of the polymer classes. Although their result is very interesting, the application of the method they developed for a real-time separation of plastic wastes appears to be impracticable. This paper presents the hyperspectral analysis of samples of three types of plastics, two of traditional type (PET and polystyrene (PS)) and one of bio-based and degradable origin (PLA). The purpose is to establish a procedure which could be successfully employed in recycling plant to achieve the real-time separation of the three typologies of plastics. PET, PS and PLA samples analyzed are collected at different stages of their life cycle (from virgin to urban plastic waste) and present different morphological (granules, flakes, pieces and the original shape of the waste) and dimensional (from 2 mm to roughly 25 cm) characteristics. Those polymers are chosen because of the occurrence in the food market of products with the same shape but realized by employing different plastic types, either traditional or bio. For instance, water bottles are made of both PET and PLA. Analogously, disposable dishes are available in both PS and PLA. As a matter of fact, traditional plastic waste may be contaminated with products realized with bio-plastics leading to a decreased quality of the recycled plastic stream in which the bio-plastics have ended up.