Date of Award
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Chemical, Biological and Pharmaceutical Engineering
Kamalesh K. Sirkar
Costas G. Gogos
Microporous polymeric membranes are used in a variety of applications for separations, purification as well as barrier function. A major application is for microfiltration (MF). Changes in the properties of MF membranes exposed to acids, bases and organic solvents are of interest in semiconductor processing as well as in membrane contactor applications. Microfiltration membranes used for sterilization in beverage, biotechnology and pharmaceutical industries are sterilized by gamma radiation among others. Irradiation-induced degradation in membrane properties should be known. A variety of fluoropolymer-based microporous membranes are available with varying properties. Ethylene chlorotrifluoroethylene (ECTFE) membranes are a new addition and are of potential interest. Microporous membranes of ECTFE membranes subjected to caustic soaking, organic solvent soaking and γ-irradiation were characterized extensively and compared with widely-used polyvinylidene fluoride (PVDF) membranes for selected properties.
ECTFE membrane swellings by seven solvents including tri-n-octylamine (TOA) were much larger than those of nonporous ECTFE films. Scanning electron microscopy (SEM), atomic force microscopy (AFM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) indicated significant defects in TOA-soaked membranes. Bubble-point-pressure (BPP) based maximum pore diameters of selected solvent-soaked ECTFE membranes are in good agreement with the pore size distribution estimated from AFM. Fourier transform infrared and Raman spectroscopies were used to study the solvent-membrane interactions: TOA introduced C-H stretching and deformation. Thermogravimetric analysis (TGA) and DSC confirmed TOA presence in membrane pores. Solvents tetrahydrofuran, toluene, acetonitrile and TOA decreased Young’s modulus by 6 to 30%. ECTFE membranes resisted plasticization by these solvents: glass transition temperature variations were limited. In TOA-treated membranes, XRD indicated more significant defects in PVDF membranes. Treatment with NaOH solutions showed no effect on contact angle and BPP. Only 3M caustic solution reduced liquid entry pressure by 13.8 kPag. ECTFE membranes showed greater hydrophobicity, stronger wetting resistance and better ability to maintain hydrophobicity vis-à-vis PVDF membranes. ECTFE membranes subjected to γ-radiation (up to 45 kGy) showed almost no effect on morphology, porosity and Young’s modulus. Slight variations were observed in BPP, melting enthalpy obtained via DSC and energy loss measured in dielectric relaxation spectroscopy.
The solvent resistance of ECTFE membranes, especially to TOA, is important especially in membrane solvent extraction in the presence of diluents e.g., xylene. Many characterization techniques were employed to study solvent-treatment effects on ECTFE membranes exposed to ethanol, xylene, xylene80/TOA20 and pure TOA. Membrane-surface roughness of virgin, ethanol-soaked and TOA-soaked membranes indicated: TOA-soaked membranes were the roughest, followed by ethanol-soaked and virgin ones. Bubble-point-pressure based maximum pore diameters (dmax) of solvent-treated membranes were: dmax, TOA > dmax, Xylene/TOA > dmax, Xylene > dmax, Ethanol > dmax, Virgin. In FTIR and Raman spectra, TOA introduced extra peaks contributing to C-H stretching and deformation. Raman spectra of xylene80/TOA20-soaked membrane were a combination of those of xylene and TOA. The presence of a large amount of diluent reduces the impact of TOA on ECTFE membranes.
In dead-end MF, fouling mechanisms behaved differently for virgin and TOA-soaked membranes; filtrate particle size distributions agreed well with estimated pore sizes. The values of permeance (kg/m2-s-kPa) determined from the slope of the linear plot of filtration flux vs. the applied pressure difference across the membrane, were 0.39, 0.23 and 0.03 for methanol, ethanol and 2-propanol, respectively. In cross-flow MF using silica nanoparticles suspended in 25% ethanol solution, Particle agglomerates having less than 100 nm size can pass through the membrane; some fouling was observed. The governing fouling mechanisms for tests operated using 3.8 ppm at 6.9 kPag (1 psig) and 13.8 kPag (2 psig) were pore blocking; for tests conducted using 3.8 ppm at 27.6 kPag (4 psig ) and 1.9 ppm at 6.9, 13.8 and 27.6 kPag (1, 2 and 4 psig), the mechanism was membrane resistance controlled. Less particles got embedded in membrane pores in experiments operated using suspensions with lower concentrations or higher concentrations with a higher transmembrane pressure. This is in good agreement with the values of the shear rate in the pore flow and SEM images of the membrane after MF.
Yao, Na, "Characterization of microporous ECTFE membranes exposed to different liquid media and ?-radiation and nanoparticle microfiltration through such membranes" (2017). Dissertations. 49.