**4. Solid State Polymerization (SSP)**

Solid state polymerization (SSP) is a coupling reaction between POSTC-PET and extender that takes place in steel reactors, under high vacuum, at temperature above glass transition (Tg) and under melting temperature (Tm), in the catalysts presence (Karayannidis et al., 1991, 2003; Baldi et al., 2006; Flieger et al., 2003; Mano et el., 2004; Cangli et al., 2008; Bikiaris et al., 2003; Gantillon at al,1990, 2004; Rosu et al, 1999; Karayanidis at al., 1991; 1993, 2003). Usually the reaction occurs at temperatures ranging between 200 – 240 °C. These temperatures favour the SSP chain extension in detriment of the degrading ones. In SSP, the temperature control is essential because if the temperature is too low the extension lasts too long, and if the temperature is too high then the POSTC-PET flakes agglomerate and the extension can no longer happen evenly (Lee & Lichtenhan, 1999). In SSP the reaction time is too long (hours) because the reaction speed is controlled by the diffusion of the reaction byproduct and the diffusion of the end–groups into the reaction mass (Gantillon et al., 2004; Apoorva, 2002; Yong et al., 2008). A convenient growth of molecular weight is obtained after 8 hours at 230 0C (Karayannidis et all, 2003). The reactions speed can be increased by the presence of nanomaterials probably because of their nucleation effect (Huimin et al., 2004; Tannenbaum et al., 2002;) The volatiles are constantly removed from the reactor that must operate under vacuum or under an inert gas blanket (Awaja & Pavel, 2005).

To eliminate the negative influence of the residual impurities there exists an alternative solution according to which the POSTC-PET is dissolved first in a selected solvent, then the polymer is recovered by precipitation with methanol and finally the polymer is chain extended according to SSP methods (Karayannidis et al. 2003). SSP can be a proper method to prepare POSTC-PET nanocomposites (Bikiaris et al,2006; Apoorva, 2002).

Although, apparently SSP can be considered a good "bottle to bottle" recycling method, due to the longer reaction time and the high cost of the equipments and of the control devices, the procedure is considered unsuitable for industrial level (Martinez et al., 2008; Cavalcanti et al., 2007; Awaja & Pavel, 2005).
