Kinetic Evaluation and Process Optimization Strategies for the Chemical Recycling of PET and PBT with Glass Fiber

Chemical recycling of pure and glass fiber reinforced PET and PBT via hydrolysis was evaluated using differential scanning calorimetry (DSC) with high-pressure crucibles as well as through batch reactor experiments. Initially, a kinetic study was performed using data from DSC and three different methods were compared, namely, Kissinger, KAS, and Friedman. Integral methods such as KAS and Kissinger provided activation energy values with low errors, matching the literature reports. Particularly, KAS offers the advantage of being capable of tracking shifts caused by secondary reactions. On the other hand, differential methods such as Friedman can detect activation energy fluctuations, making them effective for identifying side reactions. However, these methods are more susceptible to experimental noise, which can result in parameters with higher errors. The results suggest that glass fiber can impair the accessibility of water to the polymeric chain, resulting in diffusional limitations at the interface of polymer/glass fiber. Then, the depolymerization of these materials was carried out in batch reactors, covering a wide range of polymer conversions. Despite the effects observed during the kinetic evaluation, the difference between the conversion of polymer with and without glass fiber observed on batch experiments is smaller than 10%, without an impact on the recovery of the monomer. Regardless of the presence of glass fiber, the recovery of the diol monomer from PBT (1,4-butanediol (1,4-BD)) was lower than 10%. The main reason for this is the dehydration reaction of 1,4-BD forming tetrahydrofuran (THF). To address this issue, this work evaluated two different strategies to increase the monomer recovery from the aqueous phase for both polymers. First, to increase the 1,4-BD recovery for PBT, it is proposed to add KOH to prevent its dehydration into THF, with an additional effect of favoring polymer conversion and facilitating the separation of terephthalic monomers from the glass fiber. For PET+30%GF, it was demonstrated that recycling the process water can increase more than 3× the initial concentration of EG (facilitating thus the purification process) without an impact on polymer conversion. It is also suggested to associate this process with the washing of solid residues to maximize the recovery of EG.