Addition of tracers into the polypropylene in view of automatic sorting of plastic wastes using X-ray fluorescence spectrometry

Abstract

This study focused on the detection of rare earth oxides, used as tracers for the identification of polymer materials, using XRF (X-ray fluorescence) spectrometry. The tests were carried out in a test system device which allows the collection of static measurements of the samples’ spectrum through the use of energy dispersive X-ray fluorescence technology.

A sorting process based on tracers added into the polymer matrix is proposed in order to increase sorting selectivity of polypropylene during end-of-life recycling. Tracers consist of systems formed by one or by several substances dispersed into a material, to add a selective property to it, with the aim of improving the efficiency of sorting and high speed identification.

Several samples containing rare earth oxides (Y₂O₃, CeO₂, Nd₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃ and Yb₂O₃) in different concentrations were prepared in order to analyse some of the parameters which can influence the detection, such as the concentration of tracers, the acquisition time and the possible overlapping among the tracers. This work shows that by using the XRF test system device, it was possible to detect 5 of the 7 tracers tested for 1 min exposure time and at a concentration level of 1000 ppm. These two parameters will play an important role in the development of an industrial device, which indicates the necessity of further works that needs to be conducted in order to reduce them.

Introduction

Thanks to their technical and economical performances, plastics cover today a large and varied range of uses (packaging, building, automobile, electrical and electronic equipments, etc.). Once they have completed their task, the objects become wastes and so it is necessary to manage their future.

In order to reduce the growing volume of plastic waste, the management of their treatment has become a high priority on the political, economical and environmental agenda of all industrialized nations.

To complete the European Commission directives, dealing with the management of the end-of-life post-consumer products (European Parliament, 2000, European Parliament, 2003), concerned manufacturers are encouraged to develop solutions which reduce the impact of their end-of-life products, and to have a real policy of raw resource management based on a relevant recycling plan. Furthermore, the European legislations and directives (Zoboli, 1998) require producers to use a fraction of their recycled material in their final product.

Currently, the energetic recovery of plastic wastes is achieved through easy options such as incineration, which is often neither economically profitable nor respectful to the environment (Olofsson et al., 2005, Patel et al., 1998). However, it is generally difficult to recycle these materials because of their contamination with other plastic materials that are incompatible with each other. Therefore, it is necessary to improve technologies such as the sorting of polymer materials so as to make their recycling profitable. For an economically efficient recycling of polymer materials, waste plastics need to be sorted cheaply and automatically into individual types and grades (Froelich et al., 2007).

Techniques based on optical spectroscopy, such as infrared reflection/absorption (Alam et al., 1994, Florestan et al., 1994, Moore, 1999) have reached their limits. This technique is not applicable to dark plastics of automotive parts; it cannot identify different grades of the same polymers and cannot be used if the surface of the plastic wastes is wet.

The technique of high resolution imaging using X-rays is limited to the separation of PVC from PET (Kenny and Bruner, 1994) and the elimination of PVC and brominated aromatic compounds contained in electronic waste and substituted combustibles. In the automobile recycling industry this technique is limited to the identification of heavy metals contained in aluminium.

All the above methods do not use a tracer in order to facilitate detection and sorting. In early 1992, Ahmad (1992) developed a new concept of identification of plastics by incorporating a fluorescent tracer into the polymers and giving them fluorescence signatures in Ultra Violet spectroscopy. Recently, he also described a new technology for automatic sorting of plastics, based upon optical identification of fluorescence signatures of dyes, incorporated into such materials in trace concentrations (Ahmad, 2004). The conclusion of this research was that the speed and purity of sorting was limited by the mechanical singulation inadequacy in the conveyor system at high speeds.

The use of tracers is also found in the companies specializing in magnetic sorting. For example, Eriez has filed a patent (Mankosa and Luttrell, 2005) on a magnetic sorting process of polymers in which a magnetic substance is incorporated. The advantage of the magnetic detection is the lack of sensitivity regarding the additives contained in the polymers. However, the magnetic tracer system required amounts of tracers which are of the order of the percent and these quantities could engender homogenisation problems and affect the mechanical properties of the polymer.

The challenge of overcoming the inefficiency of existing sorting technologies associated with the identification of the black plastics has led to the development of new methods of identification of polymeric materials.

The principal objectives of this work are to prove the detection of tracers (rare earth oxides) added to polypropylene through the use of X-ray fluorescence spectrometry and to evaluate some of the parameters which can influence the detection, such as the concentration of tracers, the time of detection and the possible overlapping interferences among the tracers. Dispersion of such selected tracers in thermoplastic polymers, during the compounding process, will, therefore, allow automatic sorting and increase sorting selectivity of the plastic wastes during end-of-life recycling.