**2. Pre-treatments**

The aim of this chapter is to provide a review of the advances in pre-treatment proposed in literature for the purification of plastic waste. Thereto, technologies that are applied from lab-scale until industrial scale are discussed. The first section of this chapter focusses on the more conventional pre-treatment steps comprising sorting and washing of plastic packaging. Subsequently, the recent advances in deodorization, deinking and delamination of plastic packaging will be discussed in depth. In the last section, a detailed overview will be given on solvent-based methods, comprising dissolution-precipitation and extraction technologies.

#### **2.1 Conventional pre-treatment steps**

In Europe, extended producer responsibility (EPR) is set up for the collection and recycling of packaging waste. Via EPR a producer's responsibility for a product is extended to the post-consumer stage of a product's life cycle [9]. This is generally

#### *Recent Advances in Pre-Treatment of Plastic Packaging Waste DOI: http://dx.doi.org/10.5772/intechopen.99385*

translated to an environmental fee that producers need to pay to have their products managed through the so-called producer responsibility organization (PRO). A PRO is a collective entity set up by producers or through legislation, which becomes responsible for meeting the recovery and recycling obligations of the individual producers [10]. Depending on the country, an individual or co-comingled kerbside collection system can be implemented, a deposit-refund system might be organized or combinations thereof. In Germany, for instance, there are two separate collection systems for plastic packaging, namely a deposit system for PET bottles and the Dual System, where packaging and non-packaging wastes made of plastic, paper, metals and composite material are disposed for commingled collection [11].

Depending on the particular collection scheme, the waste stream will comprise a whole range of different packaging products and contaminants [12]. Due this compositional complexity, sorting of plastics in a Material Recovery Facility (MRF) is an essential step to provide high quality materials for the recycling industry [13]. An MRF comprises sorting units such as waste screening, air separation, ballistic separation, magnetic separation, eddy current separation, sensor-based sorting such as near infrared technology (NIR) and manual sorting in a certain configuration in order to generate bales as pure as possible. The output materials achieve purity levels up to more than 97% [13]. However, even after sorting, the generated plastic bales still contain certain substances, such as polymers, paper, and organic residue, present in levels between 10% and 20% of the total mass of the end-of-life packaging product [4].

These bales are sent to recycling factories, currently mainly mechanical recyclers, that wash and regranulate the plastics. **Figure 1** illustrates a typical process chain applied at a recycler's plant.

The recycling process starts by reducing the size of the plastic products via a crude shredder. Henceforth, the shredder particles are transported via a conveyor belt to a first washing step. This is, for instance, a rotating drum washer where rocks, metals and glass are separated gravitationally; a water flow provides the washing [1]. This step is followed by a second washing by means of a friction washer. The plastic surfaces are here cleaned of organic residues, adhesives and glued-on labels that stick to the plastics by intensive mechanical agitation. The plastic material is transported via a screw, while dissolved impurities, fines, and process water are discharged through sieves [15]. The plastic particles are now fed into a miller to further reduce the particle size. From this point, they proceed to the float-sink installation, where a density separation of polymers is performed. Polymers with a density lower than 1 kg/L will float (e.g., PE and PP), whereas polymers with a density higher than 1 kg/L will sink (e.g., PET, PS and PVC). As

**Figure 1.** *Generic process flow of a recycling process. Adopted from [14].*

this is a water-based technique, the flakes simultaneously undergo an extra washing [1]. After the float-sink step, the flakes are dried via a mechanical and/or a thermal dryer. Finally the plastics are sent to extrusion, which includes a melt filtration to remove fractions such as wood, paper, aged rubber particles and higher-melting polymers (e.g. PET in PP processed at 220°C) [16].

Most recycling plants use water to wash the plastics, whether or not at elevated temperature. In some cases sodium hydroxide (NaOH) or a detergent can be added as washing additives in order to reduce surface contamination such as dirt, labels and glue [17]. Logically, a water-based washing step cannot achieve 100% efficient removal of the heterogeneous substances that are present on a post-consumer plastic waste stream, even not when caustic or detergents are added. Therefore, more advanced purification steps are under developing at lab or pilot-scale or are being integrated before, during or after the extrusion step in order to provide recyclates with a higher purity. However, even for more advanced pre-treatment steps sorting and cleaning with an initial water-based washing step to remove organics, papers, and polymers e.g., with a friction washer or density separation, is probably still a key step within the plastic recycling chain.

#### **2.2 Deodorization technologies**

One of the main hurdles towards high-end recycling of plastics is the presence of odorous constituents. Due to the persistent odor that remains after washing with specialized equipment such as friction washers, large volumes of plastic packaging waste are currently only suitable for downcycled applications, such as plant trays, compost bins, street or garden benches, etc. [18]. In several scientific papers, the odor profile of different plastic packaging products is characterized. Especially the heterogeneity of odor components on packaging material in terms of physicochemical properties is described. This is illustrated by **Figure 2**, which gives an indication of the abundance of chemical groups that were detected on post-consumer waste plastics.

In total, over 400 volatile organic components (VOCs) are detected, which are here divided into 19 subcategories. With 60 and 56 components, respectively, esters and alkanes are two of the main functional groups found on plastics. However, their physicochemical properties are fundamentally different. This physicochemical heterogeneity of odor components makes efficient deodorization very challenging. Yet, most recycling plants only apply a water-based washing step, in best case with

#### **Figure 2.**

*Number of VOCs detected on plastic materials, divided into subcategories based on their chemical structure. Adopted from [19].*


#### **Table 1.**

*Overview of scientific literature to evaluate odor removal from plastics achieved by going through a full-scale water-based industrial mechanical recycling facility and their respective main results.*

addition of a detergent. Logically, scientific and industrial experience have shown that the water-based washing media are insufficient to become recyclates with an acceptable odor threshold. **Table 1** shows an overview of different scientific studies that investigated the efficiency of industrial recycling plants in term of deodorization of plastic waste.

As water-based treatments are insufficient, the demand for new and improved deodorization technologies is increasing. Although, industrial application of such technologies remains relatively limited to date. An interesting option is the use of a solvent-based approach in order to remove the more hydrophobic constituents. A commercial solvent-based process applies hot ethyl acetate to clean polyolefinbased packaging products [23]. Scientific research at lab-scale has indicated that with a batch-wise extraction using ethyl acetate at 65°C, an average removal of analyzed odor components of 90% is feasible [8]. Taking into account the benefit that can be made with a continuous counter-current process, polymers with relatively high purity levels can be achieved.

Another solvent that is tested to remove, among others, odorous constituents from a HDPE waste stream is poly(ethylene glycol) (PEG). PEG is considered as a relatively eco-friendly solvent and is getting attention due to its low volatility and toxicity compared to conventional solvents, and its higher miscibility with organic compounds compared to water [24]. A batch lab-scale extraction with PEG has shown that the quantity of VOCs was reduced with 74% after PEG extraction at 100°C. Hence, PEG is considered to be a promising solvent towards deodorization [24]. A schematic representation of a deodorization by means of PEG can be found in **Figure 3a**.

Applying solvent-based technologies might be quite expensive, taking into account the extra capital expenditures (CAPEX) and operating expenditures (OPEX) that are typically linked to such technologies. This is often not preferable in plastic recycling, given the associated typically low profit margins [4]. Applying chemicals such as solvents and detergents on a relatively highly contaminated waste stream, comprising next to odorous constituents also glues, paper, inks, additives, degradation products, non-target polymers, etc., should preferably be able to remove a large range or even all of such substances in order to be interesting from economic perspective. In this perspective, such technologies are not only investigated towards removal of odor components, but also towards removal of a broader range of impurities. The application and development of more solvent-based

#### **Figure 3.**

*Schematic representation of different deodorization strategies. a) Deodorization by means of a solvent, b) deodorization by means of hot air stripping, and c) deodorization by means of steam stripping. Adopted from [24].*

technologies and their efficiency towards removal of different substances will be discussed more in detail in Section 2.5.

Besides applying extraction techniques, also the use of air to remove odorous constituents, as shown in **Figure 3b**. A commercial available example is the ReFresher technology with the INTAREMA® TVEplus® machine (EREMA Engineering Recycling Maschinen und Anlagen Ges.m.b.H., Ansfelden, Germany). This equipment applies heated air directly to the extruded pellets for flushing out volatile contaminants and simultaneously removes the air via a degassing unit. Applying a hot air stream during a few hours can significantly reduce the overall odor intensity of recycled HDPE pellets with an efficiency varying between 51.0 and 99.3% [25]. A disadvantage of this technique is the relatively long contact time that is needed to achieve the maximum feasible removal efficiencies, typically between 4 and 7 hours. Depending on its size, the ReFresher has a capacity between 350 kg/h and 4000 kg/h.

Likewise, steam can be applied to remove VOCs from plastic materials (see **Figure 3c**). A study has shown that an increased VOC reduction is achieved via steam stripping compared to hot air stripping [24]. Thereto, post-consumer HDPE was treated at a lab-scale distillation unit for 2 h. The produced vapor flowed through the plastic bed and left on the top, passing to a condenser. An overall reduction of volatile components above 70% was reported.

A similar approach is the use of a degassing system during extrusion is investigated towards the removal of VOCs [19]. Different methods are used e.g. degassing by vacuum, thermal degassing or degassing with the help of ultrasound [26]. A study to the removal of VOCs from plastics via a vacuum degassing system showed that the odor concentration was reduced with around 37% after three degassing steps. Devolatilization is considered to be a complex process as the correct choice of temperature and shear profiles, along with screw configuration and placement of venting influence the removal efficiencies of VOCs. For instance, a higher temperature and pressure during extrusion can increase the volatility of the moisture content and permit water and/or other volatile materials to be released [27]. **Figure 4** shows a typical set-up for devolatilization of LDPE.

Another investigated method to minimize odor on post-consumer plastic packaging waste is through the use of a probiotic bacteria solution during the recycling process. In a recent study, a commercial probiotics formulation was investigated at a pilot scale before the washing step [29]. Significant differences in the overall odor intensity of the untreated reference sample and the sample treated with probiotic bacterial cultures were obtained by applying the probiotic bacterial cultures to the input material, followed by 40 days of storage. Especially those substances that most likely originated from microbial degradation of organic matter are reduced up to 70%. However, further improvement and investigation to industrial implementation of probiotic treatment is required.

*Recent Advances in Pre-Treatment of Plastic Packaging Waste DOI: http://dx.doi.org/10.5772/intechopen.99385*

#### **Figure 4.**

*A typical set-up for devolatilization of LDPE. 1) drive, 2) rear vent, 3) overheated solution with p > pvapour, 4) flash valve, 5) kneading section, 6) vacuum, 7) stripping agent, and 8) discharge/pelletizing. Adopted from [28].*

A more established option to remove VOCs from plastic waste is the addition of high specific surface adsorbents during the extrusion process [21]. A great variety of adsorbents are available on the market. As they are added to a polymer in melting phase, VOCs can adsorb on the adsorbing agents' surfaces. It is stated that adding 0.30 wt% of a certain adsorbents such as zeolite and activated silicate, can significantly reduce the amount of VOCs coming from post-consumer HDPE with approximately 50% [30]. A similar more recent development is the addition of reactive additives that undergoes a chemical reaction with the functional groups of odor-causing substances and, hence, convert them into non-volatile components [31]. Examples of such a commercial available additives are zinc ricinoleate (Tego Sorb PY 88; Evonik Industries AG, Essen, Germany) and Recycloblend 660 (PolyAd Services). However, scientific studies to quantify the effectiveness of such additives are scarce.

#### **2.3 Deinking technologies**

Plastic packaging is typically heavily printed with inks for functional benefits such as including information about composition, presence of allergens and nutritional details, etc., but also for marketing purposes to make them more appealing to consumers. The main constituents of printing inks are resins, solvents, colorants and additives [32]. Resins are high molecular weight polymers constituting 15 to 50% of the composition of the ink and they act as binder for colorant stabilization [32, 33]. Solvent constitutes the largest part of the ink composition (up to 65%) [32]. Solvents are used to dissolve the resins and also to keep the ink liquid for supporting ink transfer [34]. Colorants used to give desired color to plastic packaging, constitute 5–30% of the ink composition [32, 33]. Colorants can be used as pigments or dyes. Pigments are insoluble solid fine particles which are dispersed in the binder, while dyes are substances that are completely soluble in the binder [35, 36]. In addition to colorants, lacquers or overprint varnishes are uncoloured forms of printing inks, which can be used to provide gloss and protective properties to the print [33]. Lastly, additives are generally used up to 10% in order to improve physicochemical properties of inks such as among, others, adhesion, slip and scratch resistance [37, 38]. The composition of these ink components can considerably differ depending on the printing process and also on the substrate. For example, for most substrates solventbased inks in which the resin is dissolved in a suitable solvent are used as they allow sufficient wetting and adhesion [34]. Compared to solvents, the evaporation rate of water is much slower, making the drying process of the ink energy intensive [34]. Therefore, water-based inks are generally used for substrates which can promote absorption mechanism such as paper and board [39]. In addition to solvent and water-based inks where drying of inks is performed through evaporation of the liquid medium, in UV-based inks UV radiation is used for drying, which allows the

ink to immediately form a three-dimensionally cross-linked film. UV-based inks require reactive resins such as acrylates which can react with free radicals created by UV radiation [33, 34].

Although inks are one of the necessary components of plastic packaging, they are a significant source of contamination in plastic recycling. As all printed plastic films are generally collected and processed together, a low quality brownish, grayish or black recyclate is obtained, making it only suitable for downcycled products [40]. The presence of ink also causes recycled films to be less stiff, weaker, and denser compared to the original material, thus its price is considerably lower than the price of films free of ink. Furthermore, during the processing or reprocessing, residual ink can also decompose and produce gases causing rancid odor formation and also decrease the physical properties of a raw material [41]. In order to eliminate these problems and obtain high quality recyclates, interest in deinking technologies is increasing. However, only a few deinking technologies are so far active in deinking of plastic packaging. For example, in the patented process known as known as the Nordenia Extraction and Cleaning process or NorEC (DE19651571A1), ethyl acetate was used as a solvent-based extraction medium to remove broad range of inks [42]. The NorEC process has currently been applied in an industrial packaging plant in the North of Germany with a capacity of 15000 tons [42]. In this plant industrial PE film waste is being shredded and treated with this extraction technology. It is stated that the NorEC process requires lower amount of energy compared to the conventional wet processing [43]. Furthermore, surfactants are extensively studied as a potential deinking medium [44–48]. Deinking mechanism by using a surfactant consists of four main steps: [1] adsorption of the surfactant on plastic surfaces, [2] solubilization of the binder in the surfactant aggregates so called micelles, [3] detachment of ink particles from the surface, and [4] stabilization of the detached ink particles (**Figure 5**) [48]. According to the study of Chotipong et al. [48], cationic surfactants such as cetyl trimethylammonium bromide (CTAB) were more effective to remove both water- and solvent-based inks. In addition, it is shown that critical micelle concentrations (CMC), pH of the medium, temperature and stirring are important parameters on deinking efficiency [48].

Furthermore, in the patented method of the University of Alicante (EP2832459B1), a surfactant is used to remove inks from plastic packaging [49]. This method has also a semi-industrial demonstration deinking plant with a treatment capacity of 100 kg/h [49]. In this closed-loop recycling plant, printed plastic films pass through several treatments such as grinding, deinking, washing, drying, and pelletizing in order to obtain ink free plastics with high optical quality [41]. As a water-based medium is used, the medium can go to wastewater treatment. The use of surfactants to deink plastic films was also described in another patent filed by Duchenaud Uniflexo (EP1419829A1) [50]. In this method, the deinking medium contains organic solvents and a non-ionic surfactant. Although high deinking efficiencies can be achieved via this method, it has been stated that physico-mechanical properties of recycling plastic substantially decrease [41]. Furthermore, the use of

**Figure 5.** *Four-step mechanism for removal of solvent-based ink from HDPE surface. Adopted from [48].*

dangerous products and the high cost of the process limit the potential scaling-up [41]. Similarly, the Italian company Gamma Meccanica uses mechanical brushes for deinking purposes, but again deinking is limited to packaging having the ink layer on the surface [51].
