*2.5.1 Dissolution-precipitation technique*

The dissolution-precipitation technique allows the removal of contaminants and additives from polymers. The principle of this technique is the dissolution of the polymer in a suitable solvent followed by solid–liquid separation steps for removal of contaminants. Next, the polymer can be recovered by rapid evaporation of the solvent or by adding a proper antisolvent that will make the polymer to precipitate [77]. Drying of the polymer grains and recovery of the solvent and antisolvents are the final steps [78]. Advantages of the dissolution of plastics in a suitable solvents are (i) the decrease in the bulk volume of the plastics, (ii) the precipitated polymer is in a more acceptable form for reuse, (iii) insoluble contaminants can be removed with a solid–liquid separation process and (iv) the final product might be competitive with virgin material in terms of quality [79, 80].

Furthermore, dissolution-based processes can also be used for the separation of mixed plastic wastes based on the selective dissolution (**Figure 7**) [78]. Selective dissolution-precipitation has been applied in a laboratory scale to different plastics, namely PP pipes, PVC bottles, PS waste foam, LDPE film and HDPE bottles from agrochemical packaging [77]. Pappa et al. [77] studied the selective dissolutionprecipitation techniques at a laboratory scale and pilot-scale for the two-component mixture LDPE/PP. The solvent/antisolvent used was xylene/i-propanol in a 3:1 ratio and the dissolution was performed at different temperatures in the range of 85 to 135°C, depending on the polymer. The recovery of the two polymers was higher than 99%.

The choice of the solvent (S) and antisolvent (AS) is dependent on the solubility of the polymer and/or polymers. The amount of solvent used for dissolution also plays an important role, as low concentrated solution will lead to low viscous fluids, but will require higher amounts of antisolvent, since normally the added ratio of S/ AS is 3:1 [81, 82]. On the other hand, concentrated polymer solutions lead to very high viscous fluids, which are hard to process [82]. Therefore, typical recommended concentrations are in the range of 5–15 wt% [81]. Papaspyrides et al. [82] studied the dissolution-precipitation technique for LDPE pellets using xylene and toluene as solvents at 85°C. Toluene proved to be the most suitable solvent, as it permitted to

achieve higher concentrations (0.30 kg/L) and remaining within the limit of viscosity. The choice of the antisolvent is also important since it influences the form of the precipitated polymer. Some antisolvent may lead to gelly polymers while other permit the precipitation in the form of powder or grains [78, 82]. Papaspyrides et al. [82] concluded that acetone as antisolvent was the most successful one for the LDPE-toluene solution as the polymer was precipitated in the form of powder without forming gelling lumps. **Table 3** summarizes solvent/antisolvent systems that have been proposed in literature for different polymer types.

Currently, there are some pilot plants on the market for plastics. The CreaSolv® technology, patented by Fraunhofer Institute IVV, consists of a dissolution-precipitation technique that is able to remove additives, for instance plasticizers, from different polymers, such as polyolefins, PS and PET in scrap packaging, among others [87]. The APK's Newcycling® technology is designed to separate different polymer types such as PE and polyamide (PA) from multilayer plastic waste via a chemical dissolution process [38, 88]. The PureCycle TechnologiesSM process is designed to remove contaminants and purify PP, [89] and Polystyvert developed a dissolution process for recycling all types of PS [90].

#### *2.5.2 Solid – liquid extraction methods*

Next to the conventional solid–liquid extraction (SLE) methods such as the shake-flask and Soxhlet extraction method, [91] alternative techniques like ultrasonic extraction, supercritical fluid extraction (SFE), microwave-assisted extraction (MAE) and accelerated solvent extraction (ASE®) have been developed [91, 92]. Compared to the traditional SLE, these techniques reduce the amount of solvent and shorten the residence time [92]. Further advantages of increasing the temperature and pressure during extraction is the performance enhancement due to the increase of solubility, mass transfer effects and disruption of the surface equilibria [91]. **Table 4** summarizes the advantages and disadvantages of these techniques.


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

#### **Table 3.**

*Solvent/Antisolvent systems proposed in literature for several polymers typically found in plastic packaging waste streams.*
