Reciclyng of rigid PVC foam under glycolisis conditions

Rigid PVC foam is one of the main core materials used in wind turbine blade construction thanks to its excellent strength-to-weight ratios, low cost, and good chemical resistance properties.1 Despite its name, rigid PVC foam is actually a semi-interpenetrating hybrid of PVC, polyurethane and polyurea. It is made by reacting isocyanates and acidic anhydrides in the presence of PVC.2 The main current end-of-life options for decommissioned wind blades are landfilling and incineration, with only a minority of the material being mechanically recycled into low-value filler.3-4 Investigations into the chemical recycling of wind blades have thus far focused on high-temperature and high-pressure solvolysis of the entire blade, the core being treated together with the glass fiber reinforced polymer shell and have not thus far resulted in viable industrial processes due to energy costs and the low purity of the resulting materials.4 To our knowledge the chemical recycling of PVC foam by itself has not been investigated before. Here we report a recycling strategy to effectively depolymerize the isocyanate-derived crosslinked portion of the foam, preserving the PVC fraction from an extended degradation. The process is accomplished by glycolysis in the presence of potassium acetate or dibutyltin dilaurate (DBTL) as catalyst and thermal stabilisers. Ethylen glycol (EG), 1,4-butanediol (BD), diethylene glycol (DEG), dipropylene glycol (DPG) and polyethylene glycol with average molecular weight of 600 Da (PEG600) were tested to investigate the effect of the glycol structure on the process efficency as well as on the degradation of the PVC. The reaction time and temperature were varied from 8 min to 180 min and from 155 °C to 200 °C, respectively. Heavy depolymerization of the crosslinked fraction was accomplished by processing at T ≥165 °C for not less than 30 min. The disentangled PVC fraction, recovered after extraction with methanol, was characterized by DSC, FT-IR spectroscopy, solubility test in THF, elemental and rheological analyses. The recovered solid showed a solubility in THF strongly dependent on processing time, with the value affected by both crosslinked fraction residua and degradation processes. The maximum was 90-95% after processing at 175°C for 30 min in DEG or DPG. The complex viscosity by rotational rheometry at 170°C and 100 rad/s of the so recovered PVC was about 500 Pa s suggesting the possibility of processing it by injection moulding in conventional machines. The process cost, as well as the enviromental impact of the proposed process in perspective may be reduced by also exploiting the fraction soluble in methanol. Indeed, NMR, FT-IR and hydroxyl groups titration showed it is composed of hydroxyl-ended olygourethane and oligoureas that will be tested for polyuretane preparation.