Grafted hydrogel with switchable surface wettability based on temperature responsive poly(N-isopropylacrylamide) for oil/water separation

Abstract

The ever-increasing oily wastewater from industry and frequently oil spill accidents have fueled the demands for special wettable materials that can effectively separate oil-water mixtures. Therefore, this study emphasized on the development of temperature responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogel grafted onto the polyethylene terephthalate (PET) nonwoven textile (PNIPAAm-g-PET) via ultraviolet light emitting diodes (UV LED) photopolymerization system. In the first part, the effect of surface functionalizations and grafting onto unmodified PET (UPET) and functionalized PET textiles via carboxylation (CPET) or hydrolysis (HPET) were explored. The investigations were conducted by using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, atomic force microscopy and surface wettability. These analyses have shown that the HPET surface exhibited high accessibility of the –OH groups, more porous and rougher compared to the CPET and UPET surfaces, which yielded to be more efficient for grafting. Due to these reasons, PNIPAAm-g-HPET textile possesses better wettability, anti-oil staining property and reversible switchability functions from hydrophilic/oleophobic to hydrophobic/oleophilic. In addition, the functionalized PNIPAAm-g-HPET textile exhibited considerable permeability (1600 L/m2.hr.bar) and good selectivity (~95%), and therefore, was selected for the next study. In the second part of this study, the PNIPAAm-g-HPET textiles were optimally synthesized via response surface method (RSM) approach. Good models with R-squared values between 0.94 to 0.99 in the RSM analysis revealed that the optimal conditions were achieved as follows: the immersion of activated HPET textiles for 7 minutes in the pre-gel solution containing 20 wt% N-isopropylacrylamide monomer and 5.53 wt% N,N'-methylenebisacrylamide crosslinker (PN20C5.53.E7) and 20 minutes exposure to UV light irradiation. The responses obtained from this study for the degree of grafting, oil/water permeability and oil rejection were best predicted at 31.3%, 1559 L/m2.hr.bar and 96.23%, respectively. With respect to the actual oil/water separation performances, the permeation of the PNIPAAm-g-HPET-PN20C5.53E7 textile was reduced by 8-folds but remarkably improved the oil rejection (~97%). In addition, the PNIPAAm-g-HPET consistently maintained satisfactory results by achieving considerable flux recovery ratio values, ~80% by solely gravity-driven even after 7 cycles of measurements. Besides, this grafted PET textiles also exhibited unique self-cleaning property as oil can be repelled instantaneously (~30 seconds) from the textile surface. Lastly, the synthesized PN20C5.53E7 textile was further subjected to separation experiments by using cooking oil, crude oil and oily machinery waste. The overall oil rejection results of the PN20C5.53E7 textiles were >90% when temperature at below LCST and rapidly decreases to <60% when temperature rises above the LCST. Importantly, the PNIPAAm-g-HPET textile has possessed higher surface hydrophilicity, stability and recyclability. As a conclusion, the innovation of PNIPAAm-g-HPET nonwoven textile has a bright prospect for oil and water separation, owing to its switchability mechanism triggered by temperature change.