*2.5.2.1 Conventional SLE: shake-flask extraction and Soxhlet*

In a shake-flask extraction the polymer is mixed with the extraction solvent. The solvent will penetrate through the pores of the solid matrix and dissolve the polymer [38]. To speed up the extraction methods, the sample can be heated or refluxed [91]. After the extraction, the insoluble solid matrix can be separated from the solvent-containing analytes via decantation, filtration or centrifugation [38, 91]. The shake-flask method can be performed in batches and multiple extractions can be carried out [91]. The extraction efficiency is dependent on the type of solvent, extraction time, temperature, particle and pore size of the solid compound [38]. For example, Spell and Eddy [93] used the shake-flask technique to extract antioxidants (Ionol and Santonox) and a slip agent (oleamide) from PE at room temperature. For the extraction of these antioxidants with carbon disulphide, a time of 1–3 hours for Ionol and 50–72 h for Santonox at room temperature were necessary to achieve recoveries around 98%. Carbon tetrachloride was used for the extraction of the slip agent. After 21 h at room temperature, recoveries of 93 to 100% were obtained [38, 94]. The shake-flask method (which is basically a batch solid–liquid extraction) is a simple method, but it requires high residence



#### **Table 4.**

*Summary of solid–liquid extraction methods.*

times [38, 91]. Currently, there are no such industrial plants within the plastic recycling sector, but it has been applied in other sectors [38, 95].

The Soxhlet extraction method has been traditionally performed to extract polymer additives and residues with strong solvents at high temperatures [96]. The principle of the Soxhlet method is the continuous extraction of the solid target compound using a suitable solvent with repeated boiling-condensation cycles [91]. The extraction temperature is limited by the boiling point of the solvent used [97]. One of the disadvantages of this method is the time needed to extract the additives, that sometimes can go higher than 12 h [98]. For example, hindered amine light stabilizers (HALS) (Tinuvin 770 and Hostavin TMN 20) were extracted from a polyolefin with chloroform at 60°C for 16 h and around 96% recovery was obtained [38, 99]. The Soxhlet extraction method is not yet available on the market for plastics, but has been commonly applied in other sectors, such as agriculture [38, 100].
