**5. Hydrolysis kinetic mechanism**

126 Material Recycling – Trends and Perspectives

monomer. It is important to point out that after the tests at the highest conversion level (i.e. BPA yield >90% wt), the melting point of the product resulted over 145-150 °: this is another


**-1/T [K-1]**

excellent indication of the substantial purity of the obtained raw monomer.

Fig. 5. Yield of BPA versus time at different temperatures.

Fig. 6. Experimental kinetic constant (s-1) as a function of -1/T [K-1].


**ln(k) [ln(s-1)]**

It is interesting to observe that PC hydrolysis is mainly dominated by a six center concerted path reaction mechanism. By analogy with other reactions following the same mechanism, involving hydrolysis of esters, the kinetic constant suggested for every elementary depolymerization act is:

$$k = 10^9 \exp\left(-\frac{84000}{RT}\right) \qquad \text{l/mol/s} \tag{1}$$

This figure is coherent also with other activation energies as represented in Table 5.

Fig. 7. Hydrolysis mechanism.

Of course the radical reactions path becomes more important at higher temperatures (and of course in practical absence of water). In our conditions water becomes a powerful reactant. It has also to be pointed out that water approaching critical conditions has an increased solubility. Moreover, in this range of temperatures (>230-250 °C), the swelling of the molten polymer offers a large increase of diffusion coefficient into the polymer phase. Formally the homogeneous reaction can be schematized as shown in figure 7.

Poly(bisphenol A carbonate) Recycling: High Pressure Hydrolysis Can Be a Convenient Way 129

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**7. Conclusions** 

**8. References** 

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