Recycling of Bulk Polyamide 6 by Dissolution- Precipitation in CaCl2-EtOH-H2O Mixtures

Given the problems of virgin plastic production from fossil resources and the growing amount of plastic waste, a rapid transition towards a circular economy is pursued. For recycling, the separation of mixed plastics into pure fractions is of paramount importance to prevent downcycling. Here, experimental parameters for selective bulk dissolution of polyamide 6 (PA 6) filaments (1.75 mm diameter, 1 cm long) using CaCl2-EtOH-H2O mixtures (CEW) at 75 ◦C are investigated. They include the energy supply mode, dissolution time, CEW composition and the CEW:PA mass ratio. Energy supply with ultrasound instead of microwaves improved the yield of dissolved and recovered PA 6 after 5 hours from 31 to 52%. In total, the PA 6 yield after 3 hours of bulk dissolution could be increased from 18 to 69% by changing the energy supply mode from microwave to ultrasound and the H2O:EtOH molar ratio of CEW from 0.40 to 1.33 whilst maintaining an optimal CEW:PA mass ratio of 8.5. Additionally, master plot analysis suggests that dissolution by microwave energy supply follows a contracting cylinder model, while ultrasonic energy supply aligns with a 2D diffusion or third-order kinetic model. Microscopic observations suggest that, in the case of ultrasonic energy supply, oscillating bubbles on the particle surface enhance the dissolution rate of PA 6 filaments in CEW.