Abstract:

Objective In recent years, with the rapid development of modern industry, the amount of oily wastewater discharged from industrial production and oil spills caused by frequent maritime accidents have become an important source of water pollution, causing catastrophic damage to the environment. Membrane separation technology is an excellent choice for separating oil-water mixture and emulsion due to its high efficiency, low energy consumption and environmental protection. In this research, reinforced hydrophobic PP hollow fiber membranes were prepared using a braided tube reinforcement/thermally induced phase separation coupling method. This study provides a reference for the preparation of PP/GE composites with high performance.

Method A hydrophobic and oleophilic polypropylene (PP) hollow composite fiber membrane with a dual continuous pore structure was prepared by simple blending modification using a combination of braided tube reinforcement and thermally induced phase separation (TIPS) method. The effect of graphene (GE) doping on the morphology, mechanical properties, and permeability of hollow fiber membranes was studied using PP as the film-forming polymer, soybean oil as the diluent, silica (SiO₂), and two-dimensional layered structure GE as hydrophobic dopants.

Results A small amount of GE doping was used as a non-homogeneous phase nucleating agent, providing more nucleation sites for the uniformly dispersed GE and making the membrane spherical structure more homogeneous. With increasing GE content, the spherical crystal structure of the membrane became less, and more branch-like structures appeared. With increasing GE content, the membrane demonstrated opposite trend to the above, and the interconnected porous structure got decreased. This is due to the increase in the viscosity of the casting fluid and the deterioration of the fluidity. The introduction of GE into the PP membrane-forming system was demonstrated through infrared and Raman testing. The mechanical properties of PP hollow fiber membranes showed a slight improvement with the doping of G, while the porosity and average pore size were seen to increase and then decrease. GE doping improved the hydrophobic and lipophilic properties of the membrane. In the pure oil flux and oil-water separation test, significant improvement was witnessed. The kerosene flux of the optimal M4 membrane reached 110 L/(m²·h), and the kerosene in water, soybean oil in water, and n-hexane in water emulsion fluxes were 34.8 L/(m²·h), 29.6 L/(m²·h), and 52.3 L/(m²·h), respectively, and the separation efficiency for the three situations was 98.9%, 98.4%, and 98.6%, respectively.

Conclusion In this study, soybean oil was used as a diluent, while SiO₂ and two-dimensional layered GE were applied as hydrophobic dopants. TIPS braided tube reinforcement technology was used to successfully prepare PET-braided tube reinforced PP hollow fiber membranes. The effects of different GE contents on membrane morphology, pore size distribution, oil-water separation, and other aspects were studied. The results indicate that the doping of GE can alter the pore structure, hydrophobicity, and permeability of PP hollow fiber membranes. With the increase of GE doping amount, the water contact angle and roughness of the membrane gradually increase, and the average pore size, oil flux, and oil-water separation performance show a trend of first increasing and then decreasing. When the GE doping amount is 0.5%, the prepared M4 film has a water contact angle of 132.4 °, exhibiting excellent hydrophobic and lipophilic properties. The kerosene flux was further measured to be 110 L/(m²·h), and the flux of kerosene in water, soybean oil in water, and n-hexane in water emulsion were 34.8 L/(m²·h), 29.6 L/(m²·h), and 52.3 L/(m²·h), respectively. The separation efficiency was 98.9%, 98.4%, and 98.6%.

Key words: thermal induced phase separation, polypropylene, graphene, reinforced hollow fiber, oil-water separation