**Abstract**

There is an urgent need to close the loop of plastic waste. One of the main challenges towards plastic packaging waste recycling is the presence of a variety of contaminants. These contaminants include organic residues, additives, labels, inks and also other plastic types that can be present in the waste stream due to missorting or in multimaterial structures (e.g. multilayer films in packaging). In this context, pre-treatment processes are a promising route to tackle the difficulties that are encountered in mechanical and chemical recycling due to these contaminants. This chapter gives better insight on the already existing pre-treatment techniques and on the advances that are being developed and/or optimized in order to achieve closed-loop recycling. Some of these advanced pre-treatments include chemical washing to remove inks (deinking), extraction methods to remove undesired plastic additives and dissolution-based pre-treatments, such as delamination and dissolution-precipitation techniques.

**Keywords:** Recycling, Plastic Packaging, Circular Materials Economy, Pre-treatment, Chemical cleaning

### **1. Introduction**

The largest share of post-consumer plastic waste is plastic packaging, [1, 2] which is typically composed of among others low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), poly(ethylene terephthalate) (PET) and polystyrene (PS) [2]. The European Commission has set recycling objectives to move towards a more sustainable plastic economy, namely: (i) achieve 55% recycling of plastic packaging by 2030, (ii) increase fourfold the sorting and recycling capacity (iii) and produce only reusable or recyclable plastic packaging [3].

The main challenges faced during the recycling processes of plastic materials is the complexity of this waste stream. Plastic packaging waste streams typically consist of a mixture of polymers and contaminants, such as paper, organic residues, odor constituents, adhesives and inks containing halogens and metals [4]. The presence of additives that are incorporated in the plastics during the manufacturing process also bring complications during mechanical and chemical recycling [1, 5]. The most used additives in packaging materials are: plasticizers, antioxidants, acid scavengers, light and heat stabilizers, lubricants, pigments, antistatic agents, slip compounds and thermal stabilizers [5]. However, during mechanical recycling

these additives are blended in the recycled materials, which might be a potential health risk for consumers, especially in food contact materials [4]. For example, flame retardants such as persistent organic pollutants (POPs), phosphorous flame retardants and phthalates have been found in children toys [5]. Brominated flame retardants were also detected in food contact materials and household products [5]. Some additives can also have a direct impact on the recyclability of the plastics and even lead to the degradation of the plastics. This is the case for metal-containing additives, such as metals salts or oxides like Fe2O3, CuxO and ZnO, that will form pro-oxidants and photo-oxidation catalysts, promoting the degradation of the plastics during the reprocessing phases in mechanical recycling [5]. Furthermore, leaching to the environment of substances such as metals, volatile organic compounds, phthalates, polycyclic aromatic hydrocarbons, among others, is also a problem faced during different stages of mechanical recycling [5]. Another recycling issue is the presence of multilayer materials that typically consist of immiscible polymers (e.g. PET and polyethylene (PE)) that lead to recyclates with low mechanical properties [6]. Complications also arise during chemical recycling when e.g. halogenated compounds are released during the polymer decomposition, which can cause corrosion of the process equipment and reactors [4, 7]. Also for chemical recycling technologies, multilayer materials might cause complications as they typically contains polymers such as PET layers, which lead to the release of oxygenated compounds [4, 6].

With the currently applied pre-treatment technologies, such as sorting, washing, float-sink and grinding, there are still contaminants present in the post-consumer plastic waste, [4] as these techniques are not able to thoroughly clean the polymers and remove the impurities embedded in the polymer structure [8]. A more circular economy for plastics would need more advanced pre-treatments such as chemical washing technologies, deodorization, deinking, delamination and solvent-based extraction methods. Deodorization and deinking techniques enable the removal of odor constituents and inks present in the plastic waste, respectively. Delamination can tackle the issue of multilayer materials by selective decomposition of polymer layers and/or adhesives [6]. Solvent-based extraction methods are able to remove several additives from the polymer matrix. Solvent-based extraction methods can be divided into two groups: dissolution-precipitation and solid–liquid extraction methods, which include ultrasonic extraction, supercritical fluid extraction, microwave-assisted extraction and accelerated solvent extraction.
