Abstract:

Objective Air pollution is increasingly a serious global problem. Traditional filtration media are reported to have insufficient mechanical properties and low filtration efficiency. Therefore, in order to reduce the ecological and public health hazards of PM_0.3 suspended particles emitted from human production activities, the preparation and property evaluation of the nanofiber membrane with improved mechanical properties and filtration efficiency were reported in the paper.

Method Polyurethane (PU) with outstanding flexibility was selected as electrospinning polymer material and the solution mass fraction was fixed at 14%. Polyvinyl butyral (PVB) was used as additives to improve morphology, structure, and properties of PU nanofiber membrane. Different PU/PVB composite fiber membrane was fabricated by changing the mass ratios (8∶2, 7∶3, 6∶4) of PU and PVB. After a series of tests such as scanning electron microscope, Fourier transform infrared spectroscope, thermogravimetric analyzer, differential scanning calorimeter, stretching and filtration, the effects of PVB percentage on the morphological structure, chemical structure, mechanical properties, thermal properties, and filtration properties of PU/PVB composite nanofiber membranes were discussed.

Results The addition of PVB not only increased the spinnability of the fiber solution, but also improved the morphology of the nanofibers(Fig. 1). Under different ratio conditions (8∶2, 7∶3, 6∶4), the average diameter of the fibers was all less than 400 nm, with PU/PVB-8∶2 having the largest average diameter of 385 nm. PU/PVB composite nanofiber membranes presented similar characteristic peaks with PU nanofiber membrane, and the decrease of PU mass share in electrospinning solution leaded to a decreasing trend of the characteristic peaks at 3 318 cm^-1, 1 700-1 600 cm^-1 and 1 100 cm^-1(Fig. 2). PU/PVB composite nanofiber membranes also showed similar decomposition trends and characteristic peaks with those of PU membranes (Fig. 3). These indicated that the addition of PVB did not change chemical structure of PU in the composite membranes. The decomposition onset temperature of PU nanofiber membrane was 249.49 ℃, while the temperature of PU/PVB composite nanofiber membranes was higher than 280 ℃(Tab. 1), indicating that the addition of PVB increased the thermal stability of PU nanofiber membrane. At this time, the introduction of PVB effectively promoted the mixing of molecules within the blend and thus moderating the thermodynamic process and enhancing the thermal stability of the composite nanofiber membranes(Fig. 4). PU nanofiber membrane exhibited a fracture stress of 11 MPa and a fracture strain of 189%, while PU/PVB-8∶2 nanofiber membrane showed mechanical properties with a fracture stress of 16 MPa and a fracture strain of 148%. At this point, the elastic modulus of the composite nanofiber membrane reached a maximum of 8 MPa(Fig. 5), indicating that the mechanical properties of the composite nanofiber membrane were optimal at this mass ratio. The average pore size and porosity of PU nanofiber membrane was 7.24 μm and 55% separately. PU/PVB nanofiber membranes showed decreased pore size ranging from 1.57 to 2.95 μm and increased porosity ranging from 77% to 81% (Tab. 2). For various PU/PVB nanofiber membranes, the pore size of PU/PVB-8∶2 was only 1.78 μm and the pore size distribution was uniform. Compared to the permeability of PU fiber membrane ((63.39±1.83) mm/s), the permeability of the composite nanofiber membranes ranged from 29.04 to 37.57 mm/s. The Q_F values of PU/PVB composite nanofiber membranes were all greater than 0.02 Pa^-1, and the filtration efficiencies for PM_0.3 particles were all greater than 95%.

Conclusion The addition of PVB effectively reduces the diameter of the nanofiber and the pore size of nanofibers membranes, increases the porosity of the nanofiber membranes and improves the thermal, mechanical and filtration properties of the nanofiber membranes. When the mass ratio of PU to PVB is 8∶2, the average diameter of the fibers is 385 nm, the fracture stress is 16 MPa, the fracture strain is 148% and initial decomposition temperature is 289.37 ℃. The composite nanofiber membrane also shows the smallest pore size of 1.78 μm, a permeability of 29.37 mm/s, a filtration efficiency of 98.851% for PM_0.3, a resistance pressure drop of 181.7 Pa, and a quality factor of 0.024 6 Pa^-1,indicating that it is an ideal medium for microfiltration.

Key words: polyurethane, polyvinyl butyral, electrospinning, nanofiber, mechanical performance, air filtration