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1.1 Disadvantages of existing waterproof and breathable membranes
The elasticity of polytetrafluoroethylene (PTFE) microporous membrane is poor (~30%); the use of toxic perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) in the manufacturing process brings environmental problems.
1.2 Limitations of original polyurethane nanofiber membranes
The original PU nanofiber WBM has poor hydrophobicity; toxic solvents are used in the manufacturing process of PU nanofiber WBM, resulting in water air pollution and excessive residual solvents.
1.3 Advantages of waterborne polyurethane nanofiber membrane
Water is used as the solvent to avoid the use of toxic and harmful solvents; it has a stable porous structure and strong hydrophobicity; it has good tensile deformation characteristics and rebound performance; it has small pore size, high porosity, high waterproofness and breathability.
2.1 Preparation of waterborne polyurethane nanofiber membrane
The emulsion electrospinning technology combined with heat treatment was used to construct hydrophobic channels with small pore size and high porosity by in situ doping of water-based fluoropolymers and aziridine crosslinkers.
2.2 Experimental comparison
Spinning solutions with different contents of polyethylene oxide (PEO) and aziridine (TTMA) were prepared, and different proportions of water-based fluoropolymer (WFE) emulsion were added. The nanofibers were spun by electrospinning equipment and cross-linked by heat treatment.
2.3 Results Characterization
The prepared waterborne polyurethane nanofiber membrane was tested and characterized in terms of water pressure, gas permeability, water vapor permeability and tensile properties.
3.1 Waterborne polyurethane
The nanofiber membrane is prepared using water-based polyurethane, which has waterproof and breathable properties, and no toxic solvents are used in the production process.
3.2 Preparation of high-performance nanofiber membranes
Hydrophobic channels with small pore size and high porosity were constructed by combining emulsion electrospinning technology and thermal treatment.
3.3 Hydrophobic breathable structure
By adding water-based fluoropolymer and aziridine (TTMA) cross-linker into waterborne polyurethane, a nanofibrous membrane with a porous structure and strong hydrophobicity was constructed.
3.4 Good physical properties
The prepared waterborne polyurethane nanofiber membrane has excellent waterproofness (water pressure 74.3 kPa), air permeability (gas permeability 9.3 mm/s), water vapor permeability (12.8 kg/m^2·d) and high elasticity (67.4%).
Fig. 1. A schematic overview of the preparation procedure of environmentally friendly water-processed PU nanofibrous membranes with high waterproof and breathable performances.
With water-based PU as the polymer matrix, a small amount of PEO as the template polymer initiator, TTMA as the cross-linking agent, WFP as the hydrophobic dopant introduced into the spinning emulsion, and combined with heat treatment to prepare a waterproof and breathable membrane.
Fig. 2. SEM images of water-processed WPU/PEO membranes derived from different PEO concentrations of (a) 0.2, (b) 0.4, (c) 0.6, and (d) 0.8 wt%. (e) Dmax and porosity of the membranes. (f) Tensile stress–strain curves of the corresponding membranes. Schematic diagrams describing the formation mechanism of (g) WPU/PEO beads, beaded fibers, (h) and uniform fibers fabricated from the water-based emulsions with various PEO contents.
The WPU emulsion had many WPU beads due to lack of chain entanglement, and with increasing PEO concentration, the fiber structure became uniform without beads and bead-fibers, attributed to sufficient molecular chain entanglement of WPU and PEO. The porosity of the electrospun membrane decreased regularly from 70.7% to 59%, which may be attributed to the structural transformation from microbeads and microbead-like fibers to uniform fibers. The tensile stress and strain as well as toughness of the nanofibrous membrane were significantly improved, which was attributed to the reduction of beads and more uniform fibers.
Fig. 3. SEM images of the water-based WPU/PEO/TTMA membranes containing different TTMA contents: (a) 0, (b) 1, (c) 2, (d) 3, (e) 4 wt% after heating treatment.(f) The porosity of prepared nanofibrous membranes. (g) Crosslinking reaction of WPU and TTMA. (h) FTIR spectra of WPU, PEO, WPU/PEO, TTMA, and WPU/PEO/TTMA-3 membranes. (i) Tan δ of WPU/PEO/TTMA membranes with different TTMA concentrations versus temperature. (j) Tg and area under tan δ curves of WPU/PEO/TTMA membranes. (k) The toughness of the fabricated membranes.
Significant effect of TTMA content on the morphology and structure of WPU/PEO/TTMA films. Nanofibers without TTMA are very sensitive to high temperatures during heat treatment and they fuse to form a nearly dense film, resulting in an extremely low porosity of 5%. As TTMA concentration increases, the films exhibit much lower tensile strength and elongation as well as toughness due to the reduced adhesion area.
Fig. 4. (a) The SEM image and EDS mapping images of WPU/PEO/TTMA/WFE-2 membranes. (b) XPS spectra, (c) F atomic percentage, F/C ratio, and (d) WCA of WPU/PEO/TTMA/WFE membranes with various WFE concentrations. (e) The pore size distribution of the corresponding membranes. (f) Liquid-proof mechanism of nanofibrous membranes based on Laplace’s law. (g) The hydrostatic pressure of WPU/PEO/TTMA/WFE membranes with various WFE concentrations. (h) Self-cleaning effect of WPU/PEO/TTMA/WFE-2 membranes.
Fig. 5. (a) The plausible mechanism of air and moisture transmitting through the water-processed PU nanofibrous WBM. (b) The porosity of WPU/PEO/TTMA/WFE membranes with various WFE concentrations. (c) Air permeability and WVT rate of WPU/PEO/TTMA/WFE membranes. (d) Tensile loading and unloading stress–strain curves of WPU/PEO/TTMA/WFE-2 membranes at different maximum strains of 50, 100, 150, 200, and 250%. (e) Plastic deformation and elasticity of the membranes as a function of tensile stains. (f) Comparison of waterproof and breathable performances of WPU/PEO/TTMA/WFE-2 nanofibrous membranes and other microporous WBM. Photographs demonstrating (g) waterproof, air-permeable, and (h) moisture transmitting properties of the nanofibrous membranes. (i) Photographs of the membranes on a finger knuckle demonstrating superior stretchable and elastic properties.
With the increase of WFE concentration, the porosity of the membranes gradually decreased, which was attributed to the deposited nanofiber bonding structure, and the air permeability gradually decreased.
In this study, water-based emulsion electrospinning technology combined with heat treatment was used to fabricate environmentally friendly waterborne polyurethane nanofiber membranes with high water resistance and air permeability. Using emulsion electrospinning technology, hydrophobic channels of waterborne polyurethane nanofiber membranes with small pore size and high porosity were constructed by in-situ doping of water-based fluoropolymers and aziridine crosslinkers combined with heat treatment. The obtained water-treated PU nanofiber membranes showed good water and blood intrusion pressures of 74.3 and 44.9 kPa, respectively, air permeability of 9.3 mm s−1, WVT rate of 12.8 kg m−2 d−1, and high elasticity of 67.4% at a maximum strain of 250%.
Link to paper: https://doi.org/10.1016/j.cej.2021.130925
