Yanjie Wu,Changfa Xiao*,Hailiang Liu,Qinglin Huang

School of Textiles and State Key Laboratory of Separation Membranesand Membrane Processes,Tianjin Polytechnic University,Tianjin 300387,China

Keywords:Polyurethane(PU)Spongy Hollow fi ber membrane Pressure responsive Foaming

ABSTRACT Foam-like materials had attracted great interest as promising absorbent.In this study,thermoplastic polyurethane(TPU)block sponge was synthesized.Polyester(PET)braid tubular reinforced polyurethane(PU)spongy hollow fi ber membrane w as prepared by a concentric circular spinning method.The method w as woven from an outer coated w ater-blow n PU separation layer and inner PET braid tubular.We have developed a simple and useful preparation technique for the PUspongy hollow fi ber membrane.For the fi rst time,the PUspongy hollow fi ber membrane w as prepared using a coating and controlled foaming technique.The in fl uence of toluene isocyanate index on the physical properties,morphology,and structure of fl exible PU sponge was discussed in terms of water contact angle(CA),pure w ater fl ux(PWF),Fourier Transform Infrared Analysis(FTIR),pressure-responsive property,and pull-out strength.The morphologies of the membranes were investigated by scanning electron microscopy.We have characterized the foams from an intuitive point of view and demonstrated that the dimensional morphology of the membrane w as closely related to isocyanate index.The result show ed that the surface cell size of the PU sponge hollow fi ber membrane gradually decreased w ith an increase of the isocyanate index.Due to the elasticity of PUat room temperature,the pressure responsive characteristic of the membrane wasprepared.When isocyanate index was1.05,the interface bonding strength of PUspongy hollow fi ber membranes reached as high as 0.37 MPa,porosity and PWFw ere 71.5%and 415.5 L·m-2·h-1,respectively.

1.Introduction

Membrane separation processes had become one of the emerging technologies that have experienced rapid grow th over the past few decades[1-3].The permeability and selectivity of membrane w ere directly related to the microporous structure that includes pore size,shape,porosity,pore size distribution and arrangement of a threedimensional space[4-8].With the w ide application of membrane technology,the membrane manufacturing industry was rapidly expanding to meet the grow ing global market demand.

Traditional methods of forming the membrane pores mainly included the follow ing three ways:(1)Soluble additives of poreforming:soluble additive w as added to the casting solution or melt w hich functionsasa pore former.After the membrane formation,dissolution of the pore-forming agent produced a microporous structure in the membrane[9,10];(2)stretching:this was a solvent free technique,in w hich the polymer w asheated above themelting point and followed by stretching to make it porous.This technique w as suitable for the highly crystallinepolymersw herethecrystallineregionsof thepolymer provided strength and amorphous regions formed the porous structure[11,12].(3)Additive thermal decomposition:The non-soluble additives in thecasting solution or melt were acting aspore forming material[13,14].

The membrane manufacturing industry w as rapidly expanding to meet the grow ing global market demand.As we all know,membrane manufacturing itself w as quite far from being a green process.Phase transformations w ere usually carried out using toxic solvents such as N,N-dimethyl acetamide(DMAc),N-methyl-2-pyrrolidone(NMP)and N,N-dimethyl formamide(DMF)[15,16].These solvents w ere mixed w ith w ater during the formation of the polymer membrane,thus large amounts of solvent-containing w astew ater w ere generated.During the membrane preparation process,the coagulation bath,a w ashing tank and a w inding tank generated a large amount of w astewater containing organic solvent.The waste water must pass through the dispose to discharge.Wastew ater treatment methods w ere dif fi cult and complicated[17].The membrane manufacturing industry,universities,research funding agencies and governments all agree that w ater treatment w as one of the grand engineering challenges of today.

PU was a multi-block copolymer that usually consists of a polyester glycol-based soft segment and a glassy diisocyanate-based hard segment.The alternating structure of soft and hard chains gave the PU desirable properties,such as elasticity,resistance to abrasion,and excellent hydrolytic stability.The PU foaming method had many advantages,such as low preparation temperature,simple operation,easy to bond w ith other substances,no use of solvents,and low cost[18-20].

In this study,based on the existing block sponge formula,the formula modi fi cation of the hollow sponge w as studied including the change of the isocyanate content(isocyanate index),w ater,catalyst,and surfactant.We used the foaming method to prepare hollow fi ber membranes instead of block sponge w ithout producing w aste water.The isocyanate index was ranged from 0.95 to 1.10.The reinforced polyurethane(RPU)spongy hollow fi ber membranes were obtained throughout w ater-blow n PU spongy separation layer coating on the polyester(PET)tubular braid.This is a new process for producing a spongy hollow fi ber membrane w ith the water-blown method.The incorporation of the PET tubular braid into polyurethane foam yielded enhanced mechanical properties.The in fl uences of isocyanate index and formulations on structure and performance of RPU spongy hollow fi ber membranes w ere investigated by morphology,permeability and mechanical property.Furthermore,the interfacial bonding state of RPU spongy hollow fi ber membranes w as characterized.

2.Experimental

2.1.Materials

The polyether polyol(SEP-560D)used in this study was purchased from Tianjin Daqiuzhuang foamy factory(Tianjin,China).The viscosity at 25 °Cequals 400-700 m Pa·s w ith a typical hydroxyl number of 54-58 mg KOH·g-1of resin.The toluene 2,4-di-isocyanate(TDI)w as provided by Daqiuzhuang foamy factory(Tianjin,China)CangZhou Weida Hi-tech Co.,Ltd.Additional ingredients used for fl exible polyurethane foams w hich w ere purchased from CangZhou Weida Hi-tech Co.,Ltd.were catalysts(stannous caprylate,triethylene diamine)and surfactant(Siliconeoil 840).PETtubular braidsw ere purchased by Tianjin Boanxin Co.,Ltd.(Tianjin,China).Distilled water was made in the laboratory.All chemical reagents w ere of analytical grade,and there w as no further puri fi cation.

2.2.Membrane preparation

2.2.1.Reaction mechanism

PU foams w ere prepared by polyether polyols and TDIas the main ingredients,stannous caprylate and triethylene diamine as catalysts,silicone oil S840 acted as a surfactant,and w ater acted as a blow ing agent.In PU material,the contents of--NCOgroup must be appropriately adjusted according to the contents of--OH group.Free-rising polyurethane sponge material w as prepared by using the tw o reactions of isocyanate w ith polyether polyol matrix and w ater.First,isocyanate groups reacted w ith w ater to form an amino acid group that w as unstable and turned into an amine end-group molecular chain and carbon dioxide.The amount of carbon dioxide formation by this reaction greatly increased the porosity.These bubbles were stabilized due to increased viscosity caused by the condensation.Then,isocyanate groups reacted w ith hydroxyl groups of polyether polyol to form the PU chains.Finally,it show ed that the amine end-group reacted and an isocyanate end-group forms a urea linkage.That w as shown in Eq.(1).

According to Siggia[21],the time values for tack-free increased as isocyanate index increased.It w as possible to produce higher or low er molecular weight polymers.Because the ratio of[NCO]/[OH]affected the degree of reaction.Densities of the PU foam samples increased with the increase of the isocyanate index[22].In theory,when[NCO]/[OH]ratio w as equal to 1.0,the isocyanate w ill react completely w ith hydroxyl groups.

2.2.2.Membrane preparation

The fl exible PU sponge hollow fi ber membrane w as prepared in a laboratory scale in a straightforw ard procedure using a tw ocomponent(Aand B)system.The component Aconsisted of the proper amounts of polyol mixed w ith TDI,surfactant and stannous caprylate.First,an amount of polyether polyol matrix w as mixed with catalysts(triethylene diamine)and distilled w ater at a predetermined mass ratio,w hich w ere combined into component B.In the follow ing step,the residual polyether polyol w as mixed w ith surfactant and stannous caprylate,then this resultant mixture w as added to a speci fi c amount of TDIand stirred.The isocyanate index w as 0.95,1.00,1.05,1.10,and 1.15,respectively.

In order to keep the hollow fi ber membrane structure and improve thecompaction resistanceof PU,a PETbraided tubew asused assupport layer.The hollow fi ber membranes w ere spun via a concentric-circular process using the spinning system schematically as show n in Fig.1.The prepared components A and B w ere loaded into the spinning dope tank while the braided tube passed through the spinneret,respectively.Under the certain tension provided by a pay-off and take-up device,the hollow braided tube w ould locate in the center of the concentric circle spinning system w hich made the dope solution coated on the outer surface homogeneously.Part 3 used a humidi fi er to generate atomized water to cause a foaming reaction on the surface of the membrane.Water and atomized w ater w ere utilized as blow ing agent.In order to prevent the reaction betw een the A component and the B component in the tank,the cooling temperature of the tw o groups w as maintained at-10°C.After w hich,the PU spongy hollow fi ber membrane passed through the spin corridor at 170°Cand fi nally obtained the samethicknessof theseparation layer.The PUspongy hollow fi ber membranes were collected and kept in room temperature for at least 24 h.Finally,fi ve membranes w ith different isocyanate indices(0.95 wt%,1.00 w t%,1.05 wt%,1.10 wt%,and 1.15 wt%)were received,and labeled as membranes M0,M1,M2,M3,and M4,respectively.The compositions of the Acomponent and the Bcomponent were shown in Table 1,w hile the spinning process parameters of PU spongy hollow fi ber membranes w ere show n in Table 2.The digital photo of the PU spongy hollow fi ber membrane was shown in Fig.2.

Fig.1.The scheme of concentric-circle coating spinning process.1-component B,2-component A,3-atomized water,4-spin corridor.

2.3.Method and measurements

2.3.1.The pure water fl ux(PWF)

The pressuredifferenceacrossthemembranewas0.1 MPaunder the condition of external pressure.The pure w ater fl ux(PWF)of the membranes was generally calculated using Eq.(2).

w here V is the volume of permeation(L),A is the effective area of the membrane(m2),and t is the fi ltration time(h).

Fig.2.The digital photo of PUspongy hollow fi ber membrane.

2.3.2.Porosity

The porosity which determined the mass of liquid contained in the membrane pores w as assessed by the dry-w et gravimetric method[23].Formula(3)w asused to calculatetheporosity(ε)of the membrane.

w here w1is the mass of the w et membrane,w2is the mass of the dry membrane,D1is the n-butyl alcohol density(D1=0.8098 g·cm-3),and Dpis the polymer density(DPU=1.2 g·cm-3).

Table 1 Formulation of the dope solutions weight percentage

2.3.3.Morphology observation

The morphology of the obtained membranes w as observed with scanning electron microscopy(TM3030).The specimens w ere fi xed on the conductive adhesive.The surface of samples w as sprayed gold coating.

2.3.4.Fourier Transform Infrared Analysis(FTIR)

The chemical structure of the synthesized spongy membranes with different isocyanate indicesw asstudied by a Specac attenuated total refl ectance device(Golden Gate?ATRAccessory)and studied under the nitrogen atmosphere in the range of 4000-400 cm-1.

2.3.5.Contact angle

Thecontact anglesof all thesamplesw ere measured using an optical contact angle meter(Jinshengxin Inspection instrument Co.,Ltd.,model JYSP-180).A w ater droplet w as dropped on the surface of the sample.After a while,the contact angle was calculated using the projected drop image.Measurements w ere taken at fi ve different spots for each sample and then averaged.

2.3.6.Pull-out tests

The interfacial peel strength betw een the surface separation layer and thereinforcement of themembranewasevaluated by testingthepull-out strength.The membrane was half-inserted into the mixture of the epoxy resin and the curing agent.After the epoxy resin was completely cured,the sample asshown in Fig.3 w asprepared.The outer surface of the separation layer w as allow ed to contact w ith the epoxy resin,and the embedding length w as about 5 cm.The force required to pull the braided tube out of the separation layer wastested using an electronic tensile tester.The pull-out strength is calculated as follows:

where P isthe pull-out strength(MPa),F is the pull-out force(N),d isthe outer diameter of the PUspongy membrane(m),πis3.14 and l istheembedding length(m).

Fig.3.Schematic diagram of pull-out test.1-Pull-out force,2-Braid,3-Coating layer,4-Epoxy resin.

3.Result and Discussion

3.1.Morphologies

Morphologiesof the PUsponge hollow fi ber membraneswith different isocyanate indices w ere show n in Fig.4.As can be observed,the internal micrographs show ed that although the spongy hollow fi ber membranes w ere open celled,an apparent closed cell structure can be observed in Fig.4(D1-E1).The inner cellular structure w as relatively unaffected by altering the TDIindex.It could be found that the outer surface of the PU sponge hollow fi ber membrane M0 w as relatively loose and rough with obvious pores.With the increased of isocyanate index,the outer surface of the PU sponge hollow fi ber membrane changed signi fi cantly,w hich exhibited the gradual decline of cell size of the PU sponge hollow fi ber membrane.The reasons could be concluded that:1)the increase of isocyanate index resulted in an increase of the crosslink density in PU,w hich further brought about the reduction of the molecular mobility and restricted microphase separation;2)when the amount of catalyst was constant,the reaction of TDIand w ater w as accelerated w ith the increase of TDI,and the condensation reaction w asslow ed down by residual TDI,so that the foaming reaction and the gel reaction could not be matched,resulting in cell shrinkage closure.Therew eresigni fi cant differencesin morphology of theinternal and external surfaces.The oblique plane morphology of the PUsponge hollow fi ber membranes show ed that the osmotic quantity of casting solution increased and the interfacial bond strength w as enhanced in a higher isocyanate index,as show n in Fig.4(A2-E2).The results based on the phase separation process could be analyzed and interpreted.Thiscould be attributed to thelow er viscosity of the casting solution w hich makes it easier to penetrate into the braided tube.

3.2.FTIR

FTIRof fi ve PUspongy membranesin the400-4000 cm-1rangecannot be discriminated,so w e had investigated the differences betw een thedifferent groupsasshow n in Figs.5-7.For PUsamplesw ith different isocyanate indices,w e studied the difference in FTIRof the hard segment region due to the isocyanate constituting the hard segment.Fig.5 show ed the--C═O bidentate peak spectrum of M0,M1,M2,M3,and M4 sponge hollow fi ber membranes.The bidentate peak w as centered on about 1640 cm-1.The bidentate peak show ed that the orderingw ithin thehard segmentsgoesthrough amaximum asafunction of index similar to the behavior by Skorpenske et al.[24].The M2 exhibited a stronger bidentate peak than M1 and M0,w hich w as thought to be due to a concentration of highly w ell-ordered hard segment domains.This behavior strongly suggested that the development of the hard segment hasbeen further improved with initial increase of the isocyanate index.How ever,Excessive-NCOcan be further reacted w ith the carbamate,urea to form an allophanate and biuret crosslink,respectively.The cross-linking reaction increased the steric hindrance and destroys the ordering of the hard segments.So the bidentate peak absorbance for M3 or M4 w as much low er than that of M2.In other w ords,the higher isocyanate index increased the covalent crosslinking,and thecovalent crosslinking destroyed theregular ordering of thehard segments,making it more dif fi cult for the urea segments to fully integrated w ith the urea and other hard segments,thereby limiting the further re fi nement of the bidentate hydrogen bonding in the hard segment.The trend of the N--H absorbance region w as show n in Fig.6.The trend was similar to that of carbonyl region.The absorbance of N--H w as centered at about 3280 cm-1,and the N--H absorbance w as the highest and M4 w as the low est.As mentioned above,this w as due to the orderly arrangement of M2 in the hard segment region,and the covalent cross-linking in M4 destroyed the ordering of hard segment regions.Fig.7 show ed the FTIRspectra of the free isocyanate region of the fi ve spongy hollow fi ber membrane.As can be seen from Fig.7,w ith the increased of isocyanate index,the amount of unreacted isocyanate was also increased.

Fig.4.SEM images of PU spongy hollow fi ber membranes.A-M0,B-M1,C-M2,D-M3,E-M4,1-the surface,2-oblique plane,3-cross-section.

3.3.Hydrophobicity

The changes in the w ater contact angles of the PU sponge hollow fi ber membranes w ere displayed in Fig.8.The results showed that the average w ater contact angle increased from 84.4°to 102.4°as the isocyanate index increased from 0.95 to 1.15.The reasonscould be concluded that:1)the more TDIwas bene fi cial to the crosslink density in PU.Theincreaseof thecrosslink density reduced the molecular mobility and restricted microphase separation.With the increased of isocyanate index,the pore size of PUsponge hollow fi ber membrane decreased.As a result,the decreased in pore size on the surface of the membrane made the intrusion of w ater dif fi cult.The result is consistent w ith the results discussed above.2)As the isocyanate index increases,the toluene diisocyanate w as in excess,the end groups carried the isocyanate groups,and the contact angle increased.

Fig.5.In fl uence of isocyanate index on the bidentate absorbance.

Fig.6.In fl uence of isocyanate index on the NH absorbance.

Fig.7.In fl uence of isocyanate index on the free isocyanate absorbance.

Fig.8.Water contact angle of PUspongy hollow fi ber membranes.

3.4.Porosity

The trends of PUsponge hollow fi ber membranes'porosity w ith different isocyanate indices w ere show n in Fig.9.The porosity of the M0 membrane w as high.The porosity at fi rst only slightly decreased,and then signi fi cantly decreased w ith an increase in the isocyanate index.Decrease of porosity was owing to the increase in the isocyanate index,w hich resulted in an increase of crosslink density in PU.The increase of the crosslink density reduced the molecular mobility and restricted microphase separation.The observed decrease in porosity w hen the isocyanate index increased from 0.95 to 1.15 w as due to the closed pore produced by cross-linking.The results agreed w ell w ith the permeability results as discussed above.

Thepore sizedistribution of PUspongy hollow fi ber membraneswas show n in Fig.10.Ascan be seen from Fig.10,w hen the isocyanate index w as 1.05,the pore size and pore size distribution w ere averaged.The pore size decreased w ith the increased of the isocyanate index.This is consistent w ith the results of the porosity.

Fig.9.Effect of the isocyanate index on porosity.

Fig.10.The pore size distribution.

3.5.Interface bonding strength of the PUsponge hollow fi ber membrane

The interface bonding strength between thesurface separation layer and the reinforcement w asdue to the in fi ltration of the casting solution into the pores of the braided tube during the spinning process.When the viscosity of casting solution w as high,the fl uidity w as poor,and the in fi ltration of the casting solution on the surface of the braided tube w as limited,w hich resulted in poor interface combination.The effects of the isocyanate index on the pull-out strength of PU sponge hollow fi ber membranes w ere illustrated in Fig.11.It could be found that the interface bonding strength increased with the increasing isocyanate index.Thereasonscould be concluded that:1)theviscosity of polyether polyols w as much higher than the TDI.At the beginning of the reaction,the viscosity of casting solution decreased w ith the increase in TDIcontent(isocyanate index).The in fi ltration quantity of the casting solution into the braided tube w as more.So interface combination strength w as larger.2)Toluene diisocyanate w as a rigid segment,w ith the increased of TDI,rigidity of PU spongy hollow fi ber membrane increased,under the sameconditionsof the foaming casting solution,the interfacebonding strength increased.

Fig.11.Interface bonding strength of PU sponge hollow fi ber membranes w ith various isocyanate indices.

3.6.Pure water fl ux

The PWFof the PU spongy hollow fi ber membrane w ith different isocyanate indices w as presented in Fig.12.It could be seen that PWF improved w ith the decrease in isocyanate index.The PWFof the PU spongy hollow fi ber membrane signi fi cantly decreased w hen the isocyanateindex washigher than 1.05.Thiscould beexplained by the fi nding that the surface pore size of the PU spongy hollow fi ber membrane gradually decreased w ith the increase of isocyanate index.Increasing the isocyanate index resulted in low er porosity and size,and the porous PU spongy hollow fi ber membrane turned into a denser structure(Fig.4)which further induced lower PWF.

Fig.12.PWFof PUspongy hollow fi ber membrane.

The PWFof membranes could be described by the Hagen-Poiseuille equation[25]:

w hereεis the porosity,r is the radius of the pore,ηis the viscosity,τis thecurvature of thepore,Δx isthethicknessof the membrane,andΔP is the transmembrane pressure.When the feed liquid waspure water,assuming that K is a constant,w hen the osmotic pressure difference changed,it could beseen from formula(5)that thereisalinear relationship betw een the PWFand pressure.

How ever,asshow n in Fig.13,therelationship betw een the PWFand the operating pressure in the experiment w as nonlinear.It could be seen that it is a convex curve,so the K value increased w hen the pressure increased.Assuming that the deformation was small under low pressure conditions,the number of micropores in the hollow fi ber membrane is constant.It is assumed that the hollow fi ber membrane body is uniformly changed and the force around the membrane pore is uniform,so the porosity and pore tortuosity do not change during the pressure response.Our research is consistent with Hu's research[26].So the porosity and curvature of the spongy membrane w ere constant in the operating pressure range,the ratio of r2/Δx increased w ith pressureincreased.Thismembrane,w hich changed poresizeand thickness by the pressure change,w as called a pressure responsive membrane.It w as clear that the pressure responsive characteristics of the spongy membrane w as due to the elasticity of PUat room temperature.Fig.13 also showed that there is no overlap between the pressure increasing curve and the pressure decreasing curve and form the circle.This phenomenon indicated that the pores could not fully 100%recovery and probably due to the delay of strain after stress or plastic deformation.

Fig.13.The relationship of PWFand pressure of the PU spongy hollow fi ber membrane.

4.Conclusions

PU spongy hollow fi ber membranes w ere prepared by a concentriccircle coating method,by coating w ater-blow n PU spongy separation layer onto PET tubular braids.The concentric-circle coating method and controlled foaming techniques w ere used for the fi rst time to prepare a hollow fi ber membrane.They were fabricated from polyether polyols and TDIof various isocyanate indices and the following conclusions had been derived.With the isocyanate index increasing,the in fi ltration quantity of the casting solution into the PETbraided tube w as extended and caused enhancement in the interface bonding strength while the porosity and permeability of the membranes decreased obviously.The PU spongy hollow fi ber membrane w ith 1.05 isocyanate index w as found to provide an optimal performance.

Acknow ledgements

Thisstudy w asfunded by the National Natural Science Foundation of China(51673149,51603146)and Industrial innovation project of TJOA(BHSF2017-01).