(339d) Investigation and Fabrication-Based Optimization of Polymeric Ultrafiltration Membrane Using Recycled PET and Green Solvent Components

Polymeric membranes are commonly fabricated using phase inversion techniques due to the relative ease in altering the permeance and selectivity via modification of fabrication parameters. In particular, nonsolvent phase-induced separation (NIPS) is regarded as the dominant phase inversion method, as polymeric membranes with a diverse porous structure and high selectivity can be casted at low operating temperatures. However, one critical drawback to NIPS and other phase inversion methods is the frequent use of petroleum-based solvents, including N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), and dimethylacetamide (DMAc). High levels of irritability, toxicity, flammability, and carcinogenicity of traditional solvents pose significant hazards to operating safety, as well as human health and the environment due to the leaching of solvents into membrane fabrication wastewater. Moreover, the petroleum-derived nature of traditional solvents increases the carbon footprint and overall environmental impact of membrane technology. As such, regulations on solvent use have recently increased and further motivate the need to develop polymeric membranes with green properties (i.e., non-hazardous, recyclable, bio-derived, and/or biodegradable characteristics).

Among potential green solvents, Methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (Rhodiasolv® PolarClean) shares the high thermal stability, chemical resistance, and mechanical properties of traditional solvents while being bio-derived from nylon 6,6 synthesis and having biodegradable, nonflammable, and non-toxic properties. Several studies have investigated the incorporation of PolarClean in phase inversion-formed polymeric membranes, including the fabrication of polysulfone (PSf), polyvinylidene fluoride (PVDF), and polyethersulfone (PES) membranes for microfiltration and ultrafiltration applications. In addition to homogeneity measured between the solvent and polymers during solution mixing, PolarClean-based membranes were reported to have higher water flux and rejection capabilities than traditional counterparts; whereas finger-like pores in the membrane morphology were reported for traditional solvent-based membranes, membranes derived from PolarClean exhibited a sponge-like structure that likely contributed to the higher performance parameters. However, the sustainability of PolarClean-based membranes can be further enhanced by using recycled materials, namely polyethylene terephthalate (PET), commonly utilized in synthetic film and plastic packaging manufacturing due to its excellent thermal and chemical resistance properties. While global PET production has grown and totaled 50 million metric tons in 2016, appropriate end-of-use remains challenging due to the low profitability of recycling and has resulted in substantial plastic pollution. However, the integration of PET into membrane fabrication could create a high-value niche application for PET recycling. In addition to improving the sustainability, there exists a need to further investigate optimization of polymeric membrane performance parameters. One parameter that is often adjusted is the amount of time the solution film is exposed to air before immersion in the precipitation bath. Known as evaporation time or an evaporation step, exposure to air causes partial evaporation of solvent from the top “skin” layer and increases the local polymer concentration; the altered skin layer acts as a resistance barrier between the nonsolvent bath and bulk membrane layers, thus limiting diffusion of nonsolvent into the membrane. Understanding the relationship between adjusting the evaporation time and performance parameters (e.g., permeability, rejection capabilities) can lead to the determination of an optimal evaporation time range for maximizing the performance of sustainable membranes.

In this study, recycled PET-PolarClean ultrafiltration membranes were fabricated via NIPS and optimized by altering the evaporation time. Kinetics of the polymer-solvent interactions were measured to determine the dope solution mixing. Membrane performance parameters, namely permeability and bovine serum albumin (BSA) rejection, were evaluated for recycled PET-PolarClean and compared to those exhibited by PSf-NMP and PSf-PolarClean-gamma valerolactone (GVL) membranes. Similar to related literature, the inclusion of PolarClean resulted in enhanced water flux, including an optimal flux value of 151 ± 0.37% LMH at an evaporation time of 15 seconds; trends in flux and rejection with respect to evaporation time indicated that 15 seconds produced an appropriate balance of performance parameters. We expect that fabricated PET-PolarClean membranes would exhibit similar, enhanced parameters. In addition, membrane morphology was examined via SEM imaging of the membrane surface and cross-sections; sponge-like pore structures were present in PolarClean membranes, thus likely contributing to the higher water flux. The end-products of this study are ultrafiltration membranes for water treatment applications with adequate performance parameters and green properties to reduce environmental impacts, as well as guidance for the fabrication and optimization of green polymeric membranes.