HYDRODYNAMIC AND THERMODYNAMIC EFFECTS IN PHASE INVERSION EMULSIFICATION PROCESS OF EPOXY RESIN IN W

Chinese Journal of Polymer Science Vol. 24, No. 2, (2006), 155-161Chinese Journal ofPolymer Science02006 World- ScientificHYDRODYNAMIC AND THERMODYNAMIC EFFECTS IN PHASE INVERSIONEMULSIFICATION PROCESS OF EPOXY RESIN IN WATER'Yuan-ze Xu"*, Yu-zhe Wu and Jian-mao YangThe Key Laboratory of Molecular Engineering of Polymers, Ministry of Education,Department of Macromolecular Science, Fudan University, Shanghai 200433, ChinaAbstract The mechanism of phase inversion emulsification process (PIE) was studied for waterbormne dispersion of highlyviscous epoxy resin using non- ionic polymeric surfactants. Drop deformation and breakup, rheological properties,conductity, and particle size measurements reveal the micro-stuctural transtion amid emulsification. It is revealed thatstrong flow causes water drop to burst with the formation of droplets and huge interface. Phase inversion corresponds to anabrupt rheological transition from a type of viscous melt with weak elasticity to a highly elastic type of aqueous gel. Thisimplies that the phase inversion equivalent to a curvature inversion. Based on this, a geometric model is postulated tocorrelate process variables to the particle size. The coverage and conformation of the surfactant plays key role for the particlesize of the final emulsion. The interactions of thermodynamic and hydrodynamic effects are also discussed. It is concludedthat the thermodynamics control the PIE while the hydrodynamics drives the creation of interface and involves every stepofPIE.Keywords: Phase inversion; Emulsification; Rheology; Waterborne; Epoxy resin.INTRODUCTIONThe phase inversion emulsification (PIE) have been widely used in the industrial process of waterbormedispersions of polymer resins and in the preparation of homogeneous contollable micro-structures in scientificfrontierl-. It involves the dispersion of water into polymer resin by employing suitable surfactants underoptimal mixing conditions, up to certain temperature and by adding water, the W/O system is inverted to O/W,namely, the dispersion of polymer resin in water. Among various emulsification means, one advantage of PIEtechnique is that it will give very narrow particle size distribution at very high concentration. Process practiceshows that many processing parameters including temperature, shearing, dispersion time, water adding andmaterial parameters, especially, surfactant structure and concentration can significantly influence the quality ofthe final dispersion!2-7. It is desirable to have a clear physical picture and provide a basis for the optimization ofPIE process. So far the theories on PIE mechanism are developed along two separate approaches: thehydrodynamic and thermodynamic consideratiosl-5s. The hydrodynamic theorys9 suggests that thecompetition of drop breakdown and collapse under shearing determines the particle size distribution (PSD). So,the capillary number Cg the combination of interfacial tension, the viscosity ratio and the shear rate is decisive.However, the theoretical prediction leads to a PSD much broader than its distribution after PIE. Thethermodynamic considerations suggest that the mechanical work is required to increase interface. But for manyimmiscible systems PIE is esentially spontaneous with the help of surfatatsl,10-31. The interfacial bendingcaused by the surfactant structure will drive the phase inversin"4,19. This work will add more arguments to our'This project was supported by the National Natural Science Fou中国煤化工Major State BasicResearch Projects No.2003CB61 5604), Shengli Oilfield, SINOPEC..MYHCNMHG” Corresponding author: Yuan-ze Xu (许元泽), E-mail: yuanzexu@fdan.edu.cnReceived April 4, 2005; Revised April 25, 2005; Accepted April 28, 2005156YZ. Xu et al.recent efforts for clarifying the mechanism of PIE process of highly viscous resin and propose a unified pictureof PIE process.EXPERIMENTALMaterialsEpoxy resin (No. 638, Shanghai Resin Co. viscosity: 10 Pa●s) was used as the oil phase. Distilled water withtrace salts was used as water phase to maintain its electric conductivity. Emulsifier, the key component preparedin our lab, is a block copolymer of ABA type, where A is polyethylene glycol, (M = 10000) the hydrophilicsegment, and B is the hydrophobic segment made of bis-A epoxy resin (No. 638). The synthesis of surfactantwas carried out using BF3 type catalyst, which catalyzed the reaction of end hydroxide group and epoxy group.Under well-controlled condition, the molecular weight distribution of the block copolymer was narrow with aconvincible ABA structure (M≈20000).EmulificationA lab-built emulsifier made of four-necked spherical glass flask with anchor stir was used. Temperature waswell controlled using an oil bath. The conductivity change during emulsification was measured using aninductive sensor. Addition of water was accurately controlled using a metering pump.RheologyRheological material functions were measured using a strain-controlled rheometer (ARES, TA Instruments) witha torque transducer capable of measurement in the range of 0.02- 2000 g cm. Small-amplitude oscillatory shearand steady shear measurements were performed using a set of 25 mm diameter parallel-plate geometry in 20°Con fresh samples from emulsifier at various stages of emulsification.The measurements of drop deformation, break- up were done in a four -roll mill rheometer built in our lab.The system creates shearing or elongational field by adjusting the rotation speeds of the four rollers as shown inFig. I. A servo-system allows keeping drop in the center of observation window to trace the deformation andbreak-up of single drop or the recombination of drops. The operation principle is similar to that published inliteratures'16-18]xrollerFig.1 Four-roll mill rheometers to study the drop deformation, break-up and collapse in flow fieldsa) Vertical view; b) Side viewRESULTS AND DISCUSSIONHydrodynamic EffectsThe hydrodynamic, or kinetic models consider the balance of breakup and coalescence of droplets and predictsthe inversion that occurs in certain viscosity ratio and interfacial tension. So, the capillary number is dominant inthe process, but the formation of very fine droplets is to be Cxperiments in Fig. 2indicate the function of surfactant mentioned previously that中国煤化工icrases the dropdeformation, which is nearly proportional to the drop size and s:rYHCNMHGlefinedas_ L-BD=-L+BPhase Inversion Emulsification of Epoxy Resin in Water157where L and B are the length and breadth of the drop suspended in the matrix respectively.0.28-■H2O + 0.02% sufacant0.40- b▲H2O0.360.24-od= 238 H2O0.32| o d= 285 0.02 surfactant0.20-▲0.16-0.24|00.160.12-。0.082003004005006007008009005010140180 220Diameter (μm)Extension rate (s5)Fig. 2 Effect of surfactant on drop deformationa) As a function of drop size; b) As a function of extension rateBeyond critical rate when the hydrodynamic force exceeds the interfacial tension, the drop bursts with aninteresting way, as shown in Fig. 3. In extensional flow of our four-roll mill, the water drop deforms (Fig. 3a),then two threads spin off from the drop and becoming thinner and thinner (Fig. 3b) and let flow stop (Fig. 3c),finally, forming fine droplets due to capillary wave. The droplet sizes depend on the thread diameter when itbreaks up as we can see in Figs. 3(d) and 3(e). The shearing can also cause this type of droplet formation, but atmuch higher strain rate. So, sufcient strong stiring is the first and necessary step for the emulsification.However, hydrodynamic force separated droplets always feature a relative broad sizc distribution as observed inFig. 3(e), and it is also predicted by hydrodynamic theory of PIEI8.9. The hydrodynamic models also ignore themultiple functions of emulsifiers, which cannot be simplified just as one parameter of surface tension.ad中国煤化工Fig. 3 Experimental results of water drop break-MYHCNMHG(viscosity: 42 Pa●s) in a four-oll milla)-e): Stand for the different time stages158Y.Z. Xuet al.Rheological EvidencesRheology, conductivity, and particle size measurements reveal the micro structural transition of the systemduring emulsification. Figure 4 describes the linear viscoelasticity change with the percentage of water. Theabrupt changes are observed in curves of storage modulus G', loss modulus G" and complex viscosity atinversion. This change corresponds to a percolation type increase of conductivity indicating the phase inversionfrom W/O to O/W. To study how sudden this change could be, we managed to measure the rheological changeat the inversion point. The viscosity curve changes from near Newtonian to severe shear thinning type and theelastic modulus jumps over 20 times. These all happen in short time by adding of 0.2% more water, as shown inFig. 5. Figure 6 shows schematically the water droplets in oil (resin) phase before phase inversion, therheological model is a viscous fluid with weak elasticity, while after the inversion the system changes to anaqueous gel type of highly elasticity due to the entangling of long chains of PEO.100000。GI'A Eia'1000010001000个100g.Ela"10501.0Water (%)Frequency (rad/s)Fig.4 The changes of viscoelasticity versus water addingFig.5 Viscoclastic curves for the emulsion nearMeasured at 20°C, frequency 0.25 rad/sphase inversionWhite and black symbols correspond to 21.5% watercontent and 21.7%, respectively; Measured at 20°C,at frequency 0.25 rad/sSurfactant经ResinInversionModeling个RCWaterFig. 6 Schematic view of the inversion of PIE from W中国煤化工lelYHCNMHGThis very impressive sudden“catastrophic" PIE implies that the process will not allow much particle diffusionor material migration. The phase inversion can be simplified as an interface curvature inversion. If we assumePhase Inversion Emulsification of Epoxy Resin in Water159that phase inversion causes lite change of particle size distribution (PSD) from W/O to final O/W emulsion, wecan model the particle size based on simple geometric arguments.Let us consider a system of water drops in oil. Under the assumptions that: (1) all surfactant prefer to remain atthe iterface and (2) drops are uniform spheres, an equation may be deduced for spherical particle emulsion.The surfactant concentration n, is concentrated on the sphere shelln。= n(4rR)VAwhere n is surfactants/crm', np is drop/cm' , R is drop radius, A is the itrfacial arca per surfactant molecule.The water volume fraction isqw = n(4π/3)Rwhere Pw is water volume fraction.Combine the two equations, we haveR= 3qw(ngA)(1)If we use the ratio k = Rinv/R, where Rnv being the radius of emulsion particle after phase inversion.The particle size (PS) of an emulsion can be predicted byRmv = 3kqwW(nzA)(2)In the case of dense packing of water droplets near PI point, k≈1. This equation does reflect some importantfacts about the relationship between PS and process variables:(1) PS becomes larger as the addition of water at inversion increases. This“Iater" inversion causes theparticles to become larger when the surfactant is ineffective or insufficient.(2) PS becomes finer as the surfactant content increase. Substituting surfactant number density ng withweight concentration Wg= ns MwN, where Mw being the molecular weight of the surfactant and N, theAvogadro constant, a is the scaling factor of gyration radius of surfactant, so the scaling of A should be anexponent 2a of Mw, we can rewrite Eq. (2):Rimv∞qw(ws Mw2a-)(3)where Ws is the weight concentration of surfactant. Mw is the weight average molecular weight of PEO segments.This equation correctly predicts that the PS decreases by adding more sufactant. The one-dimensionalscaling index a ranges between 0.5 and I, depending on the interaction with water. Therefore, PS also decreasesas the molecular weight increases, especially when PEO becomes more hydrophilic at lower temperatures. Thisagrees with the industrial experiences of surfactant design, that ABA type surfactant is more efficient than ABtype. The following experimental results ilustrated in Table 1 agree qualitatively to the theoretical predictions.Table. 1 Average particle size in emulsions at different surfactant addingAt inversionFinal emulsionAmount of surfactant (wt%)Water adding (%) Av. particle size (micron) Av. particle size (micron)7.33273.72.225.80.711.3).90.5The average particle size in final emulsions are close to the particle size in inversion, so basically theparticle size remains constant at inversion. The system containi中国煤化工1 apparent particlesize in final emulsion, may be due to some micelles content ofe(3) The PS estimated by the theory is in the right range.YHCNMHGIf we use commonly measured values into Eq. (2): water volume fraction at inversion φw = 25%, surfactantconcentration = 9%, and assume the interfacial area per surfactant molecule A= 0.5 nm",k= 1, i.e, as inverted160Y.Z. Xu et al.PS being the same as that of water droplets near inversion, we obtain the particle size = 0.9 micron. It is close tothe PS of emulsion products.Therefore, the simple model predicts some important facts about the relationship of working parametersand emulsion particle size distribution, i.e, particle becomes finer when more surfactant added; larger particlesize corresponds to more water being added at inversion; the effect of surfactant molecular weight is estimated;the particle dimension is in the right range. However, the actual water adding at inversion is much lower thandense packing. Recent work by J.R. Xu, et al, revealed the cause- the formation of percolation network ofwater drops at pre inversion stage of epoxy resin emulsification"8. The inversion point observed by means ofconductivity and rheological measurements is actually the percolation point of O/W domains. Under shearing,the chain structures break down to local clusters, within which dense packing of water drops may occur, then thelocal phase inverts and forms local phase-inverted domains. The growth of the domains consumes the releasedwater from the broken water drops until all system converted. All these must proceed under high shearing. Thatis why in the emulsification practice, high shear is asserted at the inversion point, while water adding stoppedfor some minutes until the inversion has completed, then the subsequent dilution will complete the emulsionpreparation. It is worth mentioning that in the case of highly viscous oil phase not only the shear rate alone, butalso the total amount of shear strain to create the W/O interface is important for PIE.On the driving force of inversionOur discussion so far does not answer the key question of what is the driving force for the inversion. The classicthermodynamic theory for spontaneous emulsification of oil/watersurfactant system treats the problem as anequilibrium process of interfacial curvature inversion driven by the change of HLB valul:10-151. No mechanicalenergy needs to be introduced. Even in the case of viscous resin, this thermodynamic mechanism still dominatesthe process. However, the industrial experiences of PIE for resin indicate that resin's viscosity has much to dowith inverted particle size. High viscosity resin needs denser surfactant layer and more effective surfactantstructure. For example, ABA type surfactant and longer A (PEO) segment tend to turm the interface towardsO/W and is more effective than AB type. More quantitative frameworks were built for the prediction ofemulsion's PSIB 5. One problem with theory relating the monolayer bending energy to the free energy of theemulsion droplets is that the radius of curvature for surfactants (in nm range) is several orders of magnitudessmaller than that of a emulsion droplets ( in micron range), and in a surfactant scale the W/O or O/W interfaceare essentially planar. How that slight bending can assert strong force to realize so homogeneous droplets?Recently, some interesting papersl9.201 on the drop stability have applied the surfactant monolayer bendingelasticity to the kinetics of hole formation of W/O/ w film between two contacting droplets, the key step ofphase inversion. This inspires us that the surfactant structure and particle size may be more naturally correlatedthrough the flm stability, which is determined by the interfacial tension of planar film, the spontaneouscurvature of surfactant, bending modulus, which is in turn related to surfactant structure and its packing densityat the interface. The quantitative theory has to be formulated. We only emphasize here that the hydrodynamicforce not only creates the interface, preparing the condition for the realization of thermodynamic dominant PIE,but may also involve in the key step of phase inversion.ACKNOWLEDGMENTS The authors wish to thank Yuntao Hu for the help in the setup of the four roll mill.REFERENCESlLopez-Montilla, J.C.. Herrera-Morales, P.E, Pandey, S. and Shah, DO 1 Dicnereinn Srience and Technology, 2002,23: 219中国煤化工2 Yang, z.Z., Xu, Y.Z., Xu, M, Zhao, D.L., Polymer Bulletin (in CITYHCNMHG3 Yang, 2.Z, Xu, Y.Z, Zhao, D.L, Xu, M., Acta Polymerica Sinica (in Chinese), 1998,(1): 784 Yang, Z.Z, Xu, Y.z., Zhao, D.L., Xu, M., Chemical Joumal of Chinese Univrsties (in Chinese), 1999, 20: 809Phase Inversion Emulsification of Epoxy Resin in Water1615 Yang, z.Z., Xu, Y.Z, Zhao, D.L. and Xu M., Chemical Jourmal of Chinese Universities (in Chinese), 1997, 18: 15686 Yang, Z.Z, Xu, Y.Z., Zhao, D.L., Xu M., Colloid Polym. 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