Wettability Regulation Of Plastics Surface Towards Plastics Flotation

Plastic has become a significant material in modern life and industry due to its low cost,high stability,and strong flexibility.However,post-consumer plastics are easily aged and damaged yet difficult to disintegrate in nature.The incompatibility of short life cycle and long degradation time has hastened the development and accumulation of waste plastics.Recycling can help to alleviate the environmental pollution caused by waste plastics and improve resource conservation.Because complicated waste plastics obstruct recycling,separation becomes a preferred method to optimize the recycling of waste plastics.Based on surface molecules of plastics,this paper revealed an active plastics surface,studied the wettability difference of plastics surface in regulation systems including chemical oxidation and physical coating,and separated plastic mixtures using flotation technology.Floatability experiments were performed to explore the wettability difference of polyvinyl chloride（PVC）,polycarbonate（PC）,polystyrene（PS）,polyethylene terephthalate（PET）,and acrylonitrile-butadiene-styrene（ABS）in hydroxyl radical（^·OH）and sulfate radical(SO₄^-·)oxidation systems.The wettability of plastics surface was determined by oxidation activity,and its order was ABS>PS>PC>PET>>PVC.The surface morphology of plastics had a limited effect on regulating their surface wettability.However,the molecular structure was the fundamental reason for the difference in oxidation activity.ABS,PS,PC,and PET had active sites of nitrile groups,double bonds,backbone chains,and ester bonds,which caused complex chemical processes including oxidation,ester hydrolysis,chain scission,and chain crosslink.These processes introduced hydrophilic oxygen-containing end groups on plastics surface（C=O,O=C–O,and C–O）,caused the fluctuation of molecular weight distribution,and caused the exudation of oxidation intermediates.The abundance of hydrophilic functional groups was a predominant role in the floatability difference of waste plastics.Floatability experiments were performed to explore the difference in the surface wettability of PVC,PC,PS,PET,and ABS under calcium particles and mineral coating.The coating activity of waste plastics determined the wettability difference,whose order was PVC>>ABS>PET≈PC≈PS.Surface topography and crystallinity had an equivocal effect on the behavior of surface coating.Calcium ions(Ca²⁺)neutralizing the surface charge of plastic samples was a premise for wettability regulation.The fundament for preferential coating was chemical bonds with strong polarity on plastics surface.Ca²⁺and particles synergistically generated hydrophilic shells made up of well-crystallized particles on the plastics surface,a process known as endothermic heterogeneous adsorption with multi-molecular layers.The interface free energy demonstrated that the interaction between minerals（high-energy surfaces）and plastics（low-energy surfaces）in the aqueous medium without Ca²⁺was composed of electrostatic repulsion,hydrophobic attraction,and van der Waals forces.The introduction of Ca²⁺into coating system significantly reduced the electronic repulsion and weakened the energy barrier of interaction.The"P-C-S ternary bridging"model consisting of plastics,Ca²⁺,and particles was constructed to explain the wettability regulation for plastics surface.Based on plastic flotation,chemical oxidation and physical coating were suitable for the removal of hazardous PVC.Chemical oxidation introduced hydrophilic oxygen-containing groups on the surface of non-PVC plastics,and physical coating biulded hydrophilic shells on the PVC surface,leading to their specific wettability.Optimal conditions for plastic flotation in the^·OH oxidation system were H₂O₂ dosage of 2 m M,the molar ratio of H₂O₂/activator 65:1,oxidation time of 10 min,and p H=5.The floatation percentage,purity,and recovery of PVC were 100%,97.1%,and100%.Optimal separation in SO₄^-·oxidation system occurred under Na₂S₂O₈ dosage of 10 m M,the molar ratio of Na₂S₂O₈/activator 1000:1,oxidation time of 10 min,and p H=7.The floatation percentage,purity,and recovery of PVC were 100%,98.4%,and 100%.Under conditions of Ca CO₃ dosage 1.5 g/L,coating temperature 50℃,coating time 20 min,p H=9,and rotational speed 900 r/min,the sinking percentage,purity,and recovery of PVC were 100%,96.3%,and 100%.PVC was extracted by mineral coating under optimal conditions of calcite dosage 4.5 g/L,coating temperature 55°C,coating time 20 min,p H=9.3,and rotational speed 900r/min.The sinking percentage,purity,and recovery of PVC were 96.0%,93.2%,and 96.0%.Under ambient temperature,the improved process based on the in-situ synthesis of calcite substantially reduced energy consumption,achieving 100%,81.8%,and 100%of sinking percentage,purity,and recovery for PVC.The improved process was potential for industrial application.Molecular structures and functional groups on the plastics surface determined their oxidation activity.The introduction of hydrophilic functional groups created the specific wettability of plastics surface,thus forming the selectivity in the flotation process.The surface coating was based on the Ca²⁺status and the polarity of chemical bonds.Interfacial interactions were fundamental for selective coating,which protected the original surface of waste plastics.It had practical significance for the wettability regulation and plastic flotation.