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

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1 Chinese Journal of Polymer Science Vol. 24, No. 2, (2006), 1 7 Chinese Journal of Polymer Science 2006 World-Scientific HYDRODYNAMIC AND THERMODYNAMIC EFFECTS IN PHASE INVERSION EMULSIFICATION PROCESS OF EPOXY RESIN IN WATER * Yuan-ze Xu **, Yu-zhe Wu and Jian-mao Yang The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai , China Abstract The mechanism of phase inversion emulsification process (PIE) was studied for waterborne dispersion of highly viscous epoxy resin using non-ionic polymeric surfactants. Drop deformation and breakup, rheological properties, conductivity, and particle size measurements reveal the micro-structural transition amid emulsification. It is revealed that strong flow causes water drop to burst with the formation of droplets and huge interface. Phase inversion corresponds to an abrupt rheological transition from a type of viscous melt with weak elasticity to a highly elastic type of aqueous gel. This implies that the phase inversion equivalent to a curvature inversion. Based on this, a geometric model is postulated to correlate process variables to the particle size. The coverage and conformation of the surfactant plays key role for the particle size of the final emulsion. The interactions of thermodynamic and hydrodynamic effects are also discussed. It is concluded that the thermodynamics control the PIE while the hydrodynamics drives the creation of interface and involving every step of PIE. Keywords: Phase inversion; Emulsification, Rheology; Waterborne; Epoxy resin. INTRODUCTION The phase inversion emulsification (PIE) have been widely used in the industrial process of waterborne dispersions of polymer resins and in the preparation of homogeneous controllable micro-structures in scientific frontier [1 7]. It involves the dispersion of water into polymer resin by employing suitable surfactants under optimal 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 PIE technique is that it will give very narrow particle size distribution at very high concentration. Process practice shows that many processing parameters including temperature, shearing, dispersion time, water adding and material parameters, especially. surfactant structure and concentration significantly can influence the quality of the final dispersion [2 7]. It is desirable to have a clear physical picture and providing a basis for the optimization of PIE process. So far the theories on PIE mechanism are developed along two separate approaches: the hydrodynamic and thermodynamic considerations [1, 8 15] [8, 9]. The hydrodynamic theory suggests that the competition of drop break down and collapse under shearing determines the particle size distribution (PSD). So, the capillary number C a, 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. The thermodynamic considerations suggest that the mechanical work is required to increase interface. But for many immiscible systems PIE is essentially spontaneous with the help of surfactants [1, 10 13]. The interfacial bending caused by the surfactant structure will drive the phase inversion [14, 15]. This work will add more arguments to our * This project was supported by the National Natural Science Foundation of China (No ), Major State Basic Research Projects (No. 2003CB615604), Shengli Oilfield, SINOPEC. ** Corresponding author: Yuan-ze Xu ( 许元泽 ), yuanzexu@fudan.edu.cn Received April 4, 2005; Revised April 25, 2005; Accepted April 28, 2005

2 2 Y.Z. Xu et al. recent efforts for clarifying the mechanism of PIE process of highly viscous resin and propose a unified picture of PIE process. EXPERIMENTAL Materials Epoxy resin (No. 638, Shanghai Resin Co. viscosity: 10 Pa s) was used as the oil phase. Distilled water with trace salts was used as water phase to maintain its electric conductivity. Emulsifier, the key component prepared in our lab, is a block copolymer of ABA type, where A is polyethylene glycol, (M = ) the hydrophilic segment, and B is the hydrophobic segment made of bis-a epoxy resin (No. 638). The synthesis of surfactant was carried out using BF 3 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 a convincible ABA structure (M ). Emulsification A lab-built emulsifier made of four-necked spherical glass flask with anchor stir was used. Temperature was well controlled using an oil bath. The conductivity change during emulsification was measured using an inductive sensor. Addition of water was accurately controlled using a metering pump. Rheology Rheological material functions were measured using a strain-controlled rheometer (ARES, TA Instruments) with a torque transducer capable of measurement in the range of g cm. Small-amplitude oscillatory shear and steady shear measurements were performed using a set of 25 mm diameter parallel-plate geometry in 20 on fresh samples from emulsifier at various stage 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 in Fig. 1. A servo-system allows keeping drop in the center of observation window to trace the deformation and break-up of single drop or the recombination of drops. The operation principle is similar to that published in literatures [16 18]. Fig. 1 Four-roll mill rheometers to study the drop deformation, break-up and collapse in flow fields a) Vertical view; b) Side view RESULTS AND DISCUSSION Hydrodynamic Effects The hydrodynamic, or kinetic models consider the balance of breakup and coalescence of droplets and predicts the inversion that occurs in certain viscosity ratio and interfacial tension. So, the capillary number is dominant in the process, but the formation of very fine droplets is to be clarified. Our four-roll mill experiments in Fig. 2 indicate the function of surfactant mentioned previously that surfactant adding reasonably increases the drop deformation, which is nearly proportional to the drop size and shear rate. Deformation (D) is defined as L B D = L + B

3 Phase Inversion Emulsification of Epoxy Resin in Water 3 where L and B are the length and breadth of the drop suspended in the matrix respectively. Fig. 2 Effect of surfactant on drop deformation a) As a function of drop size; b) As a function of extension rate Beyond critical rate when the hydrodynamic force exceeds the interfacial tension, the drop bursts with an interesting 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 it breaks up as we can see in Figs. 3(d), 3(e). The shearing can also cause this type of droplet formation, but at much higher strain rate. So, sufficient strong stirring is the first and necessary step for the emulsification. However, hydrodynamic force separated droplets always feature a relative broad size distribution as observed in Fig. 3(e), and it is also predicted by hydrodynamic theory of PIE [8, 9]. The hydrodynamic models also ignore the multiple functions of emulsifiers, which can not be simplified just as one parameter of surface tension. Fig. 3 Experimental results of water drop break-up in strong flow of epoxy resin(viscosity: 42 Pa s) in a four-roll mill a) e): Stand for the different time stages

4 4 Y.Z. Xu et al. Rheological Evidences Rheology, conductivity, and particle size measurements reveal the micro structural transition of the system during emulsification. Figure 4 describes the linear viscoelasticity change with the percentage of water. The abrupt changes are observed in curves of storage modulus G, loss modulus G and complex viscosity at inversion. This change corresponds to a percolation type increase of conductivity indicating the phase inversion from W/O to O/W. To study how sudden this change could be, we managed to measure the rheological change at the inversion point. The viscosity curve changes from near Newtonian to severe shear thinning type and the elastic modulus jumps over twenty times. These all happen in short time by adding of 0.2% more water, as shown in Fig. 5. Figure 6 shows schematically the water droplets in oil (resin) phase before phase inversion, the rheological model is a viscous fluid with weak elasticity, while after the inversion the system changes to an aqueous gel type of highly elasticity due to the entangling of long chains of PEO G' G" ETA* G' G" ETA* G' G" ETA * G',G", ETA* (N/m2) 1000 G', G", ETA* (N/m2) Water % Fig. 4 The changes of viscoelasticity versus water adding Measured at 20, frequency 0.25 rad/s Frequency (rad/s) Fig. 5 Viscoelastic curves for the emulsion near phase inversion White and black symbols correspond to 21.5% water content and 21.7%, respectively; Measured at 20, at frequency 0.25 rad/s Fig. 6 Schematic view of the inversion of PIE from W/O to O/W and its geometric model Modeling This very impressive sudden catastrophic PIE implies that the process will not allow much particle diffusion or material migration. The phase inversion can be simplified as an interface curvature inversion. If we assume

5 Phase Inversion Emulsification of Epoxy Resin in Water 5 that phase inversion causes little change of particle size distribution (PSD) from W/O to final O/W emulsion, we can 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 at the interface and (2) drops are uniform spheres, an equation may be deduced for spherical particle emulsion. The surfactant concentration n s is concentrated on the sphere shell n s = n p (4πR 2 )/A where n s is surfactants/cm 3, n p is drop/cm 3, R is drop radius, A is the interfacial area per surfactant molecule. The water volume fraction is where φ w is water volume fraction. Combine the two equations, we have φ w = n p (4π/3)R 3 R = 3φ w /(n s A) (1) If we use the ratio k = R inv /R, where R inv being the radius of emulsion particle after phase inversion. The particle size (PS) of an emulsion can be predicted by R inv = 3kφ w /(n s A) (2) In the case of dense packing of water droplets near PI point, k 1. This equation does reflect some important facts about the relationship between PS and process variables: (1) PS becomes larger as the addition of water at inversion increases. This later inversion causes the particles to become larger when the surfactant is ineffective or insufficient. (2) PS becomes finer as the surfactant content increase. Substituting surfactant number density n s with weight concentration w s = n s M w /N a, where M w being the molecular weight of the surfactant and N a, the Avogadro constant, α is the scaling factor of gyration radius of surfactant,so the scaling of A should be an exponent 2α of M w, we can rewrite Eq. (2): R inv φ w /(w s M w 2α 1 ) (3) where w s is the weight concentration of surfactant. M w is the weight average molecular weight of PEO segments. This equation correctly predicts that the PS decreases by adding more surfactant. The one-dimensional scaling index α ranges between 0.5 and 1, depending on the interaction with water. Therefore, PS also decreases as the molecular weight increases, especially when PEO becomes more hydrophilic at lower temperature. This agrees with the industrial experiences of surfactant design, that ABA type surfactant is more efficient than AB type.the following experimental results illustrated in Table 1 agree qualitatively to the theoretical predictions. Amount of surfactant (wt%) Table. 1 Average particle size in emulsions at different surfactant adding At inversion Final emulsion Water adding (%) Av. particle size (micron) Av. particle size (micron) The average particle size in final emulsions are close the particle size in inversion, so basically the particle size remains constant at inversion. The system containing more surfactant shows low apparent particle size in final emulsion, may be due to some micelles content of excess surfactants. (3) The PS estimated by the theory is in the right range.

6 6 Y.Z. Xu et al. If we use commonly measured values into Eq. (2): water volume fraction at inversion φ w = 25%, surfactant concentration = 9%, and assume the interfacial area per surfactant molecule A = 0.5 nm 2, k = 1, i.e., as inverted PS being the same as that of water droplets near inversion, we obtain the particle size = 0.9 micron. It is close to the PS of emulsion products. Therefore, the simple model predicts some important facts about the relationship of working parameters and emulsion particle size distribution, i.e., particle becomes finer when more surfactant added; larger particle size corresponds to more water adding 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 than dense packing. Recent work by J.R. Xu, et al., revealed the cause the formation of percolation network of water drops at pre-inversion stage of epoxy resin emulsification [18]. The inversion point observed by means of conductivity 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 the local phase inverts and forms local phase-inverted domains. The growth of the domains consumes the released water from the broken water drops until all system converted. All these must proceed under high shearing. That s why in the emulsification practice, high shear is asserted at inversion point, while water adding stopped for some minutes until the inversion completed, then the subsequent dilution will complete the emulsion preparation. It is worth mentioning that in the case of highly viscous oil phase not only the shear rate alone, but also the total amount of shear strain to create the W/O interface is important for PIE. On the driving force of inversion Our discussion so far does not answer the key question of what is the driving force for the inversion. The classic thermodynamic theory for spontaneous emulsification of oil/water/surfactant system treats the problem as an equilibrium process of interfacial curvature inversion driven by the change of HLB value [1, 10 15]. No mechanical energy needs to be introduced. Even in the case of viscous resin, this thermodynamic mechanism still dominates the process. However, the industrial experiences of PIE for resin indicate that resin s viscosity has much to do with inverted particle size. High viscosity resin needs denser surfactant layer and more effective surfactant structure. For example, ABA type surfactant and longer A (PEO) segment tend to turn the interface towards O/W and is more effective than AB type. More quantitative frameworks were built for the prediction of emulsion s PS [13 15]. One problem with theory relating the monolayer bending energy to the free energy of the emulsion droplets is that the radius of curvature for surfactants (in nm range) is several orders of magnitudes smaller than that of a emulsion droplets ( in micron range), and in a surfactant scale the W/O or O/W interface are essentially planar. How that slight bending can assert strong force to realize so homogeneous droplets? Recently, some interesting papers [19, 20] on the drop stability have applied the surfactant monolayer bending elasticity to the kinetics of hole formation of W/O/W film between two contacting droplets, the key step of phase inversion. This inspires us that the surfactant structure and particle size may be more naturally correlated through the film stability, which is determined by the interfacial tension of planar film, the spontaneous curvature of surfactant, bending modulus, which is in turn related to surfactant structure and its packing density at the interface. The quantitative theory have to be formulated. We only emphasize here that the hydrodynamic force 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. REFERENCES 1 Lopez-Montilla, J.C., Herrera-Morales, P.E., Pandey, S. and Shah, D.O., J. Dispersion Science and Technology, 2002, 23: 219

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