PoreformationofPolyurethane-basedhollowfibermembranepreparedbymelt-spinning【精品论文大全】.doc

精品论文推荐pore formation of polyurethane-based hollow fiber membrane prepared by melt-spinninghu xiaoyu，xiao changfa *，liang haixian，an shulinkey laboratory of hollow fiber membrane materials and membrane process，ministry ofeducation，tianjin polytechnic university，tianjin（300160）email: abstracta novel pore formation process consisting of drawing, water cooking and secondary drawing was used to obtain the melt spinning polyurethane / poly(vinylidene difluoride) / poly(ethylene glycol) (pu/pvdf/peg)blend hollow fiber membranes. the pore formation of the membrane after each step was analyzed by observing the morphology of the pores. the permeability of the membranes was alsostudied by measuring the change in pure water flux (pwf) with pressure. results show that the pore formation of the membrane was related not only to the miscibility and the rheological behavior, butalso to the different thermo-shrinkage and mechanical properties of the components. thepu/pvdf/pegblendhollowfiberswithpermeableporeshavingthepropertyof pressure-responsibility were finally prepared after the process of drawing, water cooking and secondarydrawing in turn.keywords：pu/pvdf/peg blend；hollow fiber membrane；pore formation1.introductionpolyurethane(pu)-based membranes have drawn great interests as an efficient material for separation because of pus good elasticity, good physical and chemical properties such as high tensile strength, abrasion, oil resistance and so on 1-3. and hollow fiber membranes prepared by melt-spinning usually have larger strength than those prepared by both dry-wet spinning and wet spinning. however, reports about melt-spinning pu-based hollow fiber membranes used for water treatment is relatively few for the difficulty of permeable pore formation.in this article, pvdf which has excellent chemical resistance and thermal stability 4-6 wasblended in pu with peg (relative molecular weight 20000) which is always used to adjust membrane pore size 7. after melt spinning, the pu/pvdf/peg blend hollow fibers as spun were obtained, a novel pore formation process consisting of drawing, water cooking and secondary drawing in turn was used and analyzed to obtain pores permeable. the pressure-responsibility of the hollow fiber membranes was also found by studying the effect of working pressure on the pure water flux (pwf).2experimental2.1 materialspupolyether-type,themolecularweightofpolyetheris1000,fibergrade, (polyether/diisocyanate (mdi)/butanediol (bdo) =1/2/1) was purchased from tianjin daqiuzhuang foamy factory (tianjin, china). pvdf (w#1300 powders) was purchased from kureha chemical industrial co.ltd (tokyo, japan). poly(ethylene glycol) (the molecular weight is 20000) was purchased from tianjin chemical agent co.ltd (tianjin, china).2.2 preparationblend of pu, pvdf and peg was prepared in a twin-screw extruder (d: 20 mm, l/d ratio: 42). hollow fibers were spun using the same extruder equipped with spinneret at 170 and cooled in water at room temperature for rolling and getting rid of peg on the surface. then drawing, water cooking* corresponding author.- 6 -and secondary drawing were used to treat the hollow fibers orderly to make the pores of the hollow fiber membranes permeable, and samples after each process were kept for characterization.2.3 characterizationsthe plasticizing curves were obtained by using xly-rheometer (china).the morphologies of the hollow fiber membranes were observed with fei quanta 200 (netherlands) scanning electron microscope (sem), and the samples were all sputtered with gold. pure water flux (pwf) of the membranes was determined by the following equation:j=v (at)(1)where j is pwf (l.m-2.h-1), v is the quantity of the permeate (l), a is the membrane area (m2) and t is the testing time (h). the pwf measurement apparatus was showed in fig.1.fig.1 schematic diagram of membrane measurement apparatus(1) feed tank; (2) pump; (3) manometer; (4) module; (5) permeate through; (6) valve3.results and discussionaccording to the basic principle of rheology, during the melt spinning of polymer blend, the components with lower viscosity would centralize on the surface of the fiber. fig.2 shows the plasticizing curves of pu and pvdf. the last inflection point always indicates the flowing temperatureof a polymer. in fig.2, the flowing temperature of pu is about 160, while that of pvdf is about180.because the melt-spinning of peg 20000 was 75, the pu and peg would easily flow while the flowability of pvdf was relatively worse under the spinning temperature (170) in the study.therefore, pvdf would mainly present in the middle of the hollow fiber wall almost like particles and the components in the middle of fiber walls were mainly pvdf/pu blend. the pu/peg blend would present on the outer layer and peg would mainly present on the outermost layer of the hollow fiber.deformationpu75 100 125 150 175deformationpvdf120 140 160 180 200temperature/fig.2 plasticizing curves of pu and pvdfthe solubility parameter of pu and pvdf is 1= 20.46(j/cm3)1/2and 2=23.50(j/cm3)1/2, respectively. according to the solution theory, since |1-2|1.0 (j/cm3)1/2, the compatibility of pu and pvdf would be bad, and the chains of the two polymer would exclude each other, thus the interfacial micro-void (ifm) would be easily formed.fig.3 shows the morphology of the pu/pvdf/peg blend hollow fiber as spun. since the solubility parameter of peg (3) was 19.20 (j/cm3)1/2 and |3-1|=1.26 (j/cm3)1/2 which is approach to1.0 (j/cm3)1/2, and for the melt was mixed by twin-screw extruder, the mixing degree of pu with a little peg (for most of the peg were washed off in water during spinning) would be relatively better,therefore no apparent pores could be seen on the outer or inner surface. and the two sides seem similar except that the inner surface is relatively rougher. this is due to the different cooling rate of solidification process of the two sides. during the solidification process, the cooling rate of the inner surface is relatively higher, thus the coarsening time would be slightly longer, which may cause the phase separation time longer and thus led to the roughness. in this study, because the inner surface would exhibits similar changes as the outer one after the same post treatments, the discussion below would be focused on the change of the outer one and the cross section.a80b outer side 1200b inner side 1200fig.3 morphologies of pu/pvdf/peg blend hollow fiber as spunthe hollow fiber as spun shows no flux because pores could not be seen on the surface of them(see fig.3). in order to obtain pores permeable, it needs to make defects on the surface. after the pegon the surface was washed off, there were many micro-traces left on the hollow fiber surface (shown in fig.3), and due to the stress concentration of polymers, micro-gaps would generate after drawing. so the hollow fibers above were then drawn in the air at room temperature. fig.4 shows that more pores generated from micro-traces on the surface after drawing with draw ratio of 5 (here draw ratio indicates the handle ratio), though the quantity and the shape of pores on the cross section did not change too much. the pores all distribute along the drawing direction and seem different from the ifms which seem like eyes in the hollow fiber wall.a cross section 1200b surface 1200fig.4 morphology of pu/pvdf/peg blend hollow fiber after drawing (draw ratio 5)however the hollow fibers obtained after drawing still have no flux. so the permeability of the pores needs to improve. the hollow fibers were then put into water bath at 75 for 10min then cooledin room temperature. since pu and peg are all hydrophilic polymers, water would penetrate easily from the surface pores to the hollow fiber wall, peg left in the fiber wall would be easily washed off.simultaneously, during the temperature changing from 75 to room temperature, the quantity of ifmbetween pu and pvdf increased (see fig.5a) for different shrinkage between the two polymers. while the shrinkage of pu also led to the pore size decrease especially on the hollow fiber surface (see fig.5b), the hollow fibers after this treatment still have no flux.a cross section 1200b surface 1200fig.5 morphology of pu/pvdf/peg blend hollow fiber from last step after water cooking (75)in order to open the pores, the hollow fibers was drawn again, which is called here secondary drawing. during this process, both the surface pore size and the ifms in the hollow fiber wall opened (see fig.6). and measurements proved the permeability of the hollow fiber membrane.a cross section 1200b surface 1200fig.6 morphology of pu/pvdf/peg blend hollow fiber finally obtained after secondary drawing (draw ratio 2)fig.7 shows the pwf changing with pressure. usually the pwf of membrane can be described by the hagen-poiseuille equation 8:j=r 2 p8 xwhere is surface porosity, r the radius of the pore, the viscosity, the pore tortuosity，xthe membrane thickness and p the transmembrane pressure (called pressure in this paper). when the feed is pure water, the osmotic pressure difference is zero, and r and x are assumed to be constant,the pwf can be described as j=kp, which is a linear relationship. however, in fig.7, the real curves are all nonlinear, which indicates that both r and x of the blend membrane changed during thechange in pressure. this phenomenon is called here pressure-responsibility and is due to not only the different mechanical properties, but also the elasticity of pu at room temperature.8060pwf/l/(m2*h)402000.040.080.120.16pressure/mpafig.7 the change in pwf of membrane finally obtained with pressurefig.7 also exhibits non-coincidence between the increasing pressure curve and the decreasing pressurecurve in a circle. the phenomenon indicates that the pores are unable to attain 100% recovery, and this may due to the delay of strain after stress.4. conclusionthe pore formation process of pu/pvdf/peg blend hollow fiber membrane prepared by melt-spinning was analyzed in this article. results show that the pores on the surface of the hollowfiber membrane and in the hollow fiber wall are different. the surface pores are mainly due to the micro-traces after peg 20000 washing off and drawing, while the eye-like pores in the fiber wall are mainly due to both the miscibility of pu and pvdf and the worse flowability of pvdf under the spinning temperature. for pu and pvdf have different hydrophilic property, different thermal behavior and different mechanical properties, the permeability of the interfacial microvoids could be improved by water-cooking then cooling. but the shrinkage of pu after water-cooking also decreased the pore size both on the surface and in the hollow fiber wall. thus the hollow fiber membranes with permeable pores were finally obtained by secondary drawing, and the pwf measurement indicates that this kind of hollow fiber membrane has the property of pressure-responsibility.5. acknowledgementsthe article comes from the project: study on smart hollow fiber membrane with property of pressure-responsibility (no.20040058002), which is financially supported by specialized research fund for the doctoral program of higher education.referen