Polyethyleneimine–Oleic Acid Complex as a Polymeric Dispersant for

Article pubs.acs.org/IECR

Polyethyleneimine−Oleic Acid Complex as a Polymeric Dispersant for Si3N4 and Si3N4‑Based Multicomponent Nonaqueous Slurries Motoyuki Iijima,* Naoki Okamura, and Junichi Tatami Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogayaku, Yokohama, Kanagawa 240-8501, Japan ABSTRACT: Polymeric dispersants that adsorb onto various types of fine particles have been designed through complex formation between cationic polyethyleneimine (PEI) and anionic oleic acid (OA) to improve particle surface compatibility with nonaqueous solvents. Complex formation involves the OA-assisted dissolution of PEI in a nonaqueous solvent (toluene) by ultrasonication. While PEI itself is immiscible in toluene, various molecular weights (MWs) of PEI (Mw = 600, 1800, and 10 000) visibly dissolved when, in the presence of more than 5-mol % PEI (based on the ethyleneimine (EI) unit of PEI), a complex formed between the carboxyl group of OA and the amines of PEI in toluene. Si3N4 and one of the typical multicomponent compositions for Si3N4 ceramic fabrication, Si3N4−Y2O3−MgO, were chosen to demonstrate the usage of PEI−OA complexes for improving nonaqueous slurry stability. While nontreated Si3N4 particles rapidly formed dense slurry aggregates followed by solidification in toluene, PEI associated with 30 mol % OA effectively adsorbed on Si3N4 fine particles and drastically improved the flow properties with ∼1.8 mg/m2 treatment. Surprisingly, the flow properties were maintained even for dried and redispersed slurries. The effect of the MW of PEI−OA and its additive content on the stability of Si3N4/toluene slurries has been clarified, and their applicability toward Si3N4−Y2O3−MgO multicomponent systems is discussed.

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INTRODUCTION Controlling the stability of multicomponent nonaqueous slurries is an important technique for improving the properties of devices designed from combinations of various powder reagents, such as polymer composites,1,2 advanced ceramics,3,4 and functional electrodes,5,6 using a wet colloidal processing route. For instance, Si3N4 bulk ceramic is one of the promising materials used for end products that require high abrasion and/ or heat resistance, such as bearings, motor shafts, burner nozzles, and heat exchangers, because of its excellent mechanical strength, thermal shock resistance, and fracture toughness under ambient to relatively high temperatures.7,8 Because Si3N4 itself cannot be sintered because of its very low self-diffusivity, addition of sintering additives, such as MgO, Al2O3, and Y2O3, which assist the generation of a liquid phase at relatively low heating temperatures and promote liquid phase sintering, is required for processing Si3N4 sintered materials.7,8 During wet colloidal processing of Si3N4 ceramics, preventing severe aggregation of multicomponent slurries is strongly required in order to reduce the chance of slurry solidification and large pore generation in green compacts, as both factors can severely damage the final mechanical properties. Furthermore, it is preferable to process under nonaqueous solvents as that inhibits the generation of a surface oxidation layer of Si3N4 raw powders and the reaction of MgO with water, which also has a negative impact on the final mechanical properties.9−11 Addition of polymeric dispersants is one of the main routes of controlling the stability of fine particles in liquid media. It is well-known that the structure of functional groups and chains in polymers must be carefully selected for effective adsorption on particles and generation of repulsive forces, respectively, based on the combination of different types of fine particles and solvents, to achieve improved stability of slurries.12−26 © 2015 American Chemical Society

Furthermore, the MWs of dispersants also need to be carefully considered based on particle sizes and/or their mean interparticle distances to generate effective steric repulsive forces without forming bridgelike networks of polymers when particles approach each other in slurries.12,16,24 Therefore, many polymeric dispersants with different molecular weights and structures, such as polyacrylates,12,13 polyacrylate-based block copolymers,14−17 polyethylene-glycol-grafted comb-type polymers,18−21 polyethyleneimine,22,23 functionalized polyethyleneimine,24,25 and other tailor-made substances26 have been synthesized based on polymer chemistry, leading to improved dispersion systems. Although the effect of polymer dispersant structures and additive conditions on the stability of fine particles in both aqueous and nonaqueous systems has been researched, some tasks still need to be completed for controlling the stability of multicomponent nonaqueous slurries. First, polymeric dispersants that can completely dissolve in low-polarity solvents must be selected or designed; however, the number of candidates for such dispersants is relatively limited compared with water-soluble polymer dispersants. Next, the selected or designed polymer dispersant must have the ability to adsorb on various types of raw particles with different surface characteristics. It would be also useful to have a simple protocol to control the structures of polymeric dispersants (i.e., type of functional group, functional group concentration in polymers, MWs, etc.) to enhance the solubility of polymers in the solvent, improve the adsorption on various particle surfaces, and control Received: Revised: Accepted: Published: 12847

October 1, 2015 December 3, 2015 December 7, 2015 December 7, 2015 DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854

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Industrial & Engineering Chemistry Research

transfrom infrared (FTIR) analysis performed on a JASCO FT/IR-6000 instrument. PEI−OA with 30 mol % (based on the number of monomer units of PEI) OA addition was further used as the dispersant for stabilizing Si3N4-based toluene slurries (denoted as PEI600−OA30, PEI1800−OA30, and PEI10000−OA30 for PEI−OA complexes prepared from PEI with MWs of 600, 1800, and 10 000, respectively). Characterization of the Adsorbed Content of PEI−OA on Si3N4 Particles. A 19.00 g sample of a toluene solution of PEI−OA (PEI600−OA30, PEI1800−OA30, or PEI10000− OA30) was prepared, wherein the concentration of PEI−OA was controlled to 0−3.0 mg/m2 based on the total surface area of particles added in the following process. Then, 1.00 g of Si3N4 were added to the prepared PEI−OA solution and the solution was stirred for 24 h. The adsorbed content of PEI−OA on Si3N4 was analyzed by determining the amount of unadsorbed, free PEI−OA in the slurries and subtracting it from the amount of PEI−OA originally added. After 24 hof adsorption, in order to characterize the free PEI−OA content in the slurries, the slurries were centrifuged for 5 min at 50 000g using a Beckman Coulter OPTIMA MAX-XP, and the particlefree supernatant solution was collected. A 10.0 g sample of supernatant solution was then mixed with 20.0 g of absolute ethanol, and its conductivity was analyzed using a Horiba LAQUA F-74. The concentration of PEI−OA in the supernatant solution was calculated based on the standard curve prepared by analyzing the conductivity of various known toluene/PEI−OA/absolute ethanol solutions. Stability Characterization of Si3N4/Toluene Slurries. Sedimentation tests of diluted slurries and flow curve measurements on dense slurries were conducted in order to characterize the effect of PEI−OA (PEI600−OA30, PEI1800− OA30, or PEI10000−OA30) addition on the stability of Si3N4/ toluene slurries. For the sedimentation tests, Si3N4/toluene slurries with various amounts of PEI−OA were prepared in a manner similar to that explained above for the samples prepared for adsorption content analysis. After 24 h of stirring, 3.0 mL of slurry was collected, diluted using 12.0 mL of toluene, and mixed thoroughly. Then 10 mL of the as-prepared 0.41 vol % slurry were poured into a 10 mL measuring cylinder and allowed to settle for 180 min. For flow curve measurements, 10−35 vol % Si3N4/toluene slurries with 0−3.0 mg/m2 PEI−OA were prepared by adding Si3N4 particles to a toluene solution of PEI−OA. The slurries were then mixed using a THINKY ARE-250 planetary mixer (20000 rpm, 60 s), and their flow curves were measured using a TA Instruments ARG2 rheometer. The shear stress was measured at 25 °C using a cone−plate apparatus while increasing the shear rate from 0 to 300 s−1 (in 90 s) and then decreasing from 300 to 0 s−1 (in 90 s). The flow curves of Si3N4/toluene slurries prepared after drying the original slurry were also measured. The original 20 vol % Si3N4/toluene slurries treated with PEI−OA were centrifuged, and the resulting Si3N4 fine particles were dried under vacuum at 90 °C. The dried powder cake was mixed with toluene to obtain a 20 vol % mixture, which was then gently stirred for 24 h before the flow curve measurements were performed. Note that no strong mechanical treatments, such as ultrasonication, were conducted to prepare the redispersed slurries. Application of PEI−OA as a Polymeric Dispersant for Si3N4−Y2O3−MgO/Toluene Slurries. The adsorption properties of PEI1800−OA on Y2O3 and MgO nanoparticles were analyzed using a process similar to that used for Si3N4. The

particle interactions; this is currently carried out using precise polymer synthesis techniques. Herein we propose a simple protocol to design and tune the structure of polymeric dispersants that can dissolve in lowpolarity solvents and adsorb on various fine particles to control the stability of nonaqueous slurries through complex formation between cationic hydrophilic polyethyleneimine (PEI) and anionic oleic acid (OA). The cationic polymer and anionic surfactant combination itself has been previously prepared in an aqueous/alcohol-based solution to produce lamellar structured macromolecules and layers,27−34 although the concept used is different from that described in this article. In this article, surfactant design involves the dissolution of various molecular weights (MWs) of PEI (Mw = 600, 1800, and 10 000) in a nonaqueous solvent (toluene) by complex formation with OA during ultrasonication. The effect of the MW of PEI and additive content of custom-designed PEI−OA complexes on the stability of Si3N4/toluene slurries was investigated, and their applicability to Si3N4−Y2O3−MgO multicomponent systems was evaluated.

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EXPERIMENTAL SECTION Materials. Polyethyleneimine (PEI, average molecular weight 600, 1800, and 10 000), oleic acid (OA), toluene (99.5%), and ethanol (99%) were purchased from Wako Pure Chemical Industry Ltd., Japan. Si3N4 powder (9.42 m2/g analyzed by BET, SN-E10, >98%), MgO powder (35.8 m2/g analyzed by BET, 500A, > 99.9%), and Y2O3 powder (9.30 m2/ g analyzed by BET, RU-P, >99.9%) were purchased from UBE Industries, Ltd., Japan, UBE Material Industries, Ltd., Japan, and Shin-Etsu Chemical Co., Ltd., Japan, respectively. The sizes and morphologies of the raw powders are shown in Figure 1. All materials were used without further purification.

Figure 1. SEM images of (a) Si3N4, (b) MgO, and (c) Y2O3 powder.

Preparation and Characterization of the PEI−OA Complex. A 0.50 g sample of PEI (Mw = 600, 1800, and 10 000) and 0.066−3.3 g of OA (2−100 mol % of OA based on the number of PEI monomer units; calculated assuming that all amines were secondary amines) were mixed with toluene (to make a total of 10.0 g of solution) in a 50 mL glass vial and treated in an ultrasonic bath for 5 min. The toluene solution was then magnetically stirred for 24 h. The structure of the resulting PEI−OA complex was characterized by Fourier 12848

DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854

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Industrial & Engineering Chemistry Research

and CH2 scissoring (1462 cm−1) from oleic acid,36−38 and N− H bending of amines (1600 cm−1) and CH2 scissoring (1462 cm−1) of PEI.25 In the PEI−OA samples, peaks corresponding to free −COOH groups (1710, 1418 cm−1) on OA disappeared and new peaks corresponding to symmetric (1407 cm−1) and asymmetric (1543 cm−1) vibrations of −COO− (carboxylates)37 were formed along with signals from PEI and toluene at lower added OA contents. These peak changes strongly suggest complex formation between the −COO− groups of OA and the amine groups of PEI. When the OA content was increased to more than 50−60 mol %, peaks attributed to −COOH (1710, 1418 cm−1) started to appear again, which suggests the existence of free excess OA that did not form complexes with PEI. Titration curves for 0.5 g of PEI (Mw = 600, 1800, and 10 000) dissolved in 9.5 g of deionized water and titrated with 1.0 M HCl aqueous solution were prepared (Figure 4) to

availability of PEI−OA as a polymeric dispersant was tested by analyzing the flow curves of 15 vol % Si3N4−Y2O3−MgO (weight ratio for Si3N4:Y2O3:MgO was 93:5:2, which is a typical composition for Si3N4 bulk ceramics) multicomponent toluene slurries with 1.8 mg/m2 of added PEI1800−OA. The slurry preparation and flow curve characterization methods were the same for the Si3N4/toluene slurries.

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RESULTS AND DISCUSSION PEI−OA Complex Formation. Figure 2 shows photographs of three MWs of PEI dissolved in toluene with the

Figure 2. Photograph of PEI/toluene solution with 2 and 5 mol % added OA.

assistance of OA. Although PEI itself was not soluble and a highly viscous liquidlike material sediment was formed in toluene regardless of the PEI molecular weight, PEI started to disperse and a uniformly opaque toluene solution was formed with the addition of a small amount of OA (2.0 mol %); eventually, a transparent solution (visibly dissolved) was formed when the OA content was increased to 5.0 mol % based on the number of ethyleneimine units in PEI. To analyze the structure of PEI and OA in toluene, the FTIR spectra of PEI, OA, toluene, and PEI−OA complexes with various OA ratios in toluene were measured (Figure 3). From the FTIR spectra of the original reagents, signals are observed for the aromatic C−C stretching modes (1603 and 1496 cm−1), CH3 deformation vibrations (1460 and 1384 cm−1) from toluene,35 stretching vibrations of the hydrogen-bonded CO from −COOH (1710 cm−1), C−O−H bending (1418 cm−1)

Figure 4. pH value versus added 1.0 M HCl during the titration of aqueous PEI solutions.

characterize the number of base segments of PEI. The number of reactive base points for PEI to reach pH 7 were 13.4, 11.6, and 10.3 mmol/g-PEI for PEIs having MWs of 600, 1800, and 10 000, respectively. Using the molecular weight of ethyleneimine monomer units with secondary amine structures (−CH2CH2NH−, 43 g/mol), it can be estimated that 57, 50,

Figure 3. FT-IR spectra of toluene, OA, PEI, and PEI−OA with various added OA contents and PEI MWs ((a) Mw = 600, (b) Mw = 1800, and (c) Mw = 10 000). 12849

DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854

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Industrial & Engineering Chemistry Research and 44 mol % of monomer units can act as reactive base points within the total number of ethyleneimine units that constitute PEI. Because free OA was detected through FTIR analysis around the condition where the added OA content was equal to the number of base points, it can be concluded that the acid− base interaction is the major factor causing complex formation between the amines on PEI and the carboxyl groups on OA. Effect of PEI−OA Content on the Properties of Si3N4/ Toluene Slurries. To use the PEI−OA complex as a polymeric dispersant for Si3N4-based multicomponent toluene slurries, the adsorption properties of PEI−OA on Si3N4 fine particles in toluene have been characterized. PEI having various MWs (Mw = 600, 1800, and 10 000) complexed with 30 mol % OA, which is a favorable composition for the formation of a complex that can dissolve in toluene having residual uncapped free amines in PEI and no free OA in solution, has been focused on as polymeric dispersant in this work. In Figure 5, it

Figure 6. Effect of added PEI1800−OA30 on the sedimentation behavior of Si3N4/toluene slurries.

generated between the particles at full and saturated coverage of PEI−OA on the Si3N4 fine particles. When the content of PEI−OA increased beyond 1.8 mg/m2, the slurry stability slightly decreased because of a depletion effect or because of the formation of a polymer network due to the existence of an increased quantity of free polymers. Figure 7 shows the effect of added PEI1800−OA30 on the stability of dense Si3N4/toluene slurries (25 vol %). When the

Figure 5. Relation between the additive content and adsorbed content of PEI−OA (with various MWs of PEI and 30 mol % of OA) on Si3N4 fine particles in toluene.

can be seen that, when added in low quantities to the slurries, almost all PEI−OA (with 30 mol % OA) effectively adsorbed on the Si3N4 fine particles and, regardless of the MW, eventually reached a saturated adsorbed content when the additive content was ∼1.8 mg/m2. In our previous paper, we reported that nanoparticles whose surface was modified with PEI followed by an anionic surfactant can be effectively adsorbed on the surface of unmodified bare submicrometersized particles in toluene through the PEI segment protruding from the anionic surfactant layer.39 Similarly, we expect PEI− OA to strongly adsorb on Si3N4 fine particles through the residual uncapped free amines of PEI. Next, the effect of added PEI−OA and its MW on the stability of Si3N4/toluene slurries was characterized based on sedimentation tests and flow curve measurements. Figure 6 shows the settling-time-dependent sedimentation behavior of 0.41 vol % dilute Si3N4/toluene slurries with various amounts of added PEI1800−OA30. Although Si3N4 fine particles rapidly formed a sediment within 30 s because of aggregation when the quantity of PEI1800−OA30 was less than 1.4 mg/m2, the stability of the slurry began to improve when the quantity of additive reached 1.4 mg/m2. Moreover, stability was achieved for a longer duration when the additive content reached 1.8 mg/m2. We believe that the hydrophilic Si3N4 fine particles became hydrophobic and effective repulsive forces were

Figure 7. Effect of PEI1800−OA30 content on the flowing properties of Si3N4/toluene slurries (25 vol %).

PEI1800−OA30 content had not reached 1.0 mg/m2, the surface of the Si3N4 fine particles was still relatively hydrophilic and not compatible with toluene because of a very small amount of absorbed PEI−OA; consequently, the Si3N4/toluene slurries were completely solidified because of the formation of 12850

DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854

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Industrial & Engineering Chemistry Research aggregates and the flow curves were not measurable. When the PEI1800−OA30 content was increased to 1.0−1.4 mg/m2 and partial surface modification had occurred, the Si3N4/toluene slurries underwent high-shear-stress flow with strong hysteresis flowing properties. Although the wettability of the Si3N4 surface with toluene increased and slurry solidification was prevented as the content of PEI−OA increased, the possibility of forming network structures between the PEI−OA fixed surface and the bare surface was high because the adsorbed content of PEI−OA had not yet reached the additive saturation point. The hysteresis properties of the measured flow curve are attributed to rapid network formation and simultaneous aggregation deformation of this network structure due to high shear forces. As the additive PEI1800−OA30 content reached 1.8 mg/m2, which is the additive condition required for achieving saturated adsorption, the viscosity of the Si3N4/toluene slurries was drastically reduced and no hysteresis flowing properties were observed. Full coverage of PEI−OA on Si3N4 fine particles was essential for improving toluene wettability and generating effective steric repulsive forces to stabilize the dense Si3N4/ toluene slurries. Effect of the MWs of PEI−OA and the Drying Process on the Properties of Si3N4/Toluene Slurries. The settlingtime-dependent sedimentation behavior of 0.41 vol % Si3N4/ toluene slurries containing various MWs of PEI−OA is shown in Figure 8. The additive content of PEI−OA was fixed at 1.8 mg/m2, which is the condition required for achieving saturated adsorption. While the Si3N4/toluene slurries with PEI1800− OA30 showed improved stability, which was maintained for 3 h, it was observed that the sedimentation rate due to flocculant formation increased and the transparent supernatant solution started to appear after 30 min and several minutes for PEI600− OA30 and PEI10000−OA30, respectively. The stabilities of the dense Si3N4/toluene slurries (15−35 vol %) were also characterized through flow curve measurements (Figure 9). Regardless of the MWs of PEI−OA, it can be seen that all measured flow curves did not possess hysteresis properties, although flocculant formation was accelerated under the conditions used for PEI600−OA30 and PEI10000−OA30 in the sedimentation tests. Considering that the sedimentation test is conducted under static (without shear) conditions, it can be assumed that the flocculation formed upon addition of PEI600−OA30 and PEI10000−OA30 is quite weak and can therefore be maintained in a dispersed state during flow curve measurements when shear is applied. From Figure 9, it can be seen that 35 vol % Si3N4/toluene slurries containing PEI10000−OA30 had completely solidified and could not flow, whereas those with PEI600−OA30 and PEI1800−OA30 could flow. To understand the effect of solid concentration and the MW of PEI−OA on slurry viscosity, the viscosity at 300 s−1 under various solid concentrations was plotted and is shown in Figure 10. The experimental points were also fit to the Krieger−Dougherty (KD) model, which is expressed in eq 1: ηr = {1 − (Φ /Φm)}−n

Figure 8. Sedimentation behavior of Si3N4/toluene slurries treated with 1.8 mg/m2 of PEI600−OA30, PEI1800−OA30, and PEI10000− OA30.

is a factor that can be expressed as [η] Φm, where [η] is the intrinsic viscosity ([η] = 2.5 for spherical particles). Fitting to the KD model gave us values for of n, Φm, and [η] parameters for slurries using the three MWs of PEI−OA, as shown in Figure 10. The [η] values for the Si3N4/toluene slurries using PEI600−OA30, PEI1800−OA30, and PEI10000−OA30 were 12.7, 12.1 and 10, respectively, which are larger than that for an ideal spherical particle but similar to the values obtained for some previously reported dispersion systems using commercial powders.41−43 Because it is well-known that the intrinsic viscosity of slurries is affected by the shape and polydispersity of the dispersed particles, we assumed that these large values originated from the nonspherical shape of Si3N4 fine particles and the locally aggregated structures of Si3N4 particles in the slurry. It was also observed that the Φm for PEI10000−OA30 (Φm = 0.33) was quite different from those of PEI600−OA30 (Φm = 0.49) and PEI1800−OA30 (Φm = 0.47). This difference can be explained by the difference in the effective volume of the PEI− OA layer formed on the Si3N4 fine particles. According to the literature,44 the effective volume fraction Φeff (with the particle and polymer layer) can be expressed as

(1)

where ηr, Φm, and Φ are the relative viscosity, maximum volume fraction, and volume fraction of the slurries, respectively.40 The highest possible packing fraction at a high shear rate is theoretically reported to be 0.74, while the value obtained with the random packing of hard spheres and the experimental values are 0.64 and around 0.50, respectively.41 The value for n

Φeff = Φ{1 + (ρs Swd /1000)}

(2)

where ρs (g/cm ), Sw (m /g), and d (nm) are the density of the particle, specific surface area, and polymer thickness, 3

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DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854

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Figure 9. Flow curves of Si3N4/toluene slurries (15−35 vol %) treated with 1.8 mg/m2 of (a) PEI600−OA30, (b) PEI1800−OA30, and (c) PEI10000−OA30.

Figure 10. Effect of PEI−OA MWs and Si3N4 particle concentrations on the viscosity of Si3N4/toluene slurries. The broken lines show the KD model fitting results (parameters defined in the figures).

Figure 11. Flow curves of the original and redispersed (after drying process) Si3N4/toluene slurries with 1.8 mg/m2 addition of various PEI−OA MWs.

respectively. Based on the very rough assumption that particles were dispersed as spheres with hard polymer layers (which may not be suitable for systems having high [η] values), the polymer thickness required for reaching Φeff = 0.64, which is the theoretical value for the maximum volume fraction for the random packing of hard spheres, can be calculated to be 9, 11, and 29 nm using the Φm values for PEI600−OA30, PEI1800− OA30, and PEI10000−OA30, respectively, as Φ. Although the adsorbed content of PEI−OA was similar for the series with different molecular weights, it can be assumed that PEI10000− OA30 has a thicker (and sparser) layer of PEI−OA due to its high molecular weight, while PEI600−OA30 and PEI1800− OA30 formed thinner (and relatively dense) layers. The thicker adsorbed layer of PEI10000−OA30 increased the effective volume content (volume of particles and adsorbed polymers) in the slurries and resulted in an increase in the suspension viscosity compared with Si3N4 at the same particle volume concentration. Figure 11 shows the flow curves for 20 vol % Si3N4/toluene slurries treated with 1.8 mg/m2 of PEI−OA and those redispersed in toluene to make 20 vol % samples after a simple drying process. Regardless of the MW of PEI−OA, it is surprising to observe that Si3N4 fine particles covered with a saturated amount of PEI−OA can be completely redispersed in toluene without forming strong aggregates and possess flowing properties similar to the original slurries (before drying) after only a gentle stirring process. It is strongly expected that PEI−

OA segments fixed on Si3N4 fine particles have high wettability with toluene. Therefore, toluene could easily diffuse among the dried Si3N4 fine particles covered with a saturated amount of PEI−OA. From the sedimentation tests and flow curve measurements for the Si3N4/toluene slurries with saturated amounts of added PEI−OA, it was found that PEI−OA can improve the dispersion stability and the wettability of Si3N4 particles with toluene. PEI1800−OA30 was suitable for generating effective repulsive forces between Si3N4 fine particles and stabilizing them in toluene to achieve long-term stability; in contrast, PEI10000−OA30 has large polymer layers that form weak polymer networks under static conditions, which results in flocculation and/or solidification at high concentrations. The repulsive forces of PEI600−OA30 may be too weak to maintain particle stability under static conditions. Stabilizing the Si3N4-Based Multicomponent Slurries. To use PEI−OA as a polymeric dispersant for multicomponent slurries, the adsorption behavior of PEI1800−OA30 on Y2O3 and MgO nanoparticles was measured (Figure 12). Similar to the case for Si3N4 fine particles, PEI1800−OA30 showed effective adsorption properties; all dispersants added in low quantities adsorbed on the surface of Y2O3 and MgO nanoparticles and reached saturation when the additive content was 1.8 mg/m2. Figure 13 shows the flow curve for Si3N4− Y2O3−MgO multicomponent toluene slurries with 1.8 mg/m2 of PEI1800−OA30. While the multicomponent slurry without 12852

DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854

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saturated adsorption of PEI1800−OA30 drastically improved the particle wettability with toluene and enhanced suspension stability, while nontreated Si3N4/toluene slurries rapidly formed aggregates and solidified under concentrated conditions. The MWs of PEI−OA were also found to have an effect on the slurry stabilities; PEI600−OA30 was suspected to have weaker repulsive forces that led to the formation of weak flocs under static conditions and PEI10000−OA30 to have large polymer adsorption segments that resulted to solidification of concentrated slurries at relatively lower particle concentration. Because PEI−OA effectively adsorbed on Y2O3 and MgO, it was also possible to stabilize Si3N4−Y2O3−MgO multicomponent toluene slurries.

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Figure 12. Relation between the additive content and adsorbed content of PEI1800−OA30 on Y2O3 and MgO fine particles in toluene.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone and Fax: +81-45-339-3958. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Nippon Sheet Glass Foundation for Materials Science and Engineering (14-005). The authors also gratefully thank the support from the Instrumental Analysis Center of Yokohama National University for the use of FT-IR equipment.

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Figure 13. Flow curve of Si3N4−Y2O3−MgO/toluene multicomponent slurries treated with 1.8 mg/m2 of PEI1800−OA30.

PEI−OA completely solidified, it became fluid in the presence of PEI1800−OA30 and showed no hysteresis properties. This observation indicates that PEI1800−OA30 can also be used for the stabilization of multicomponent toluene slurries. Because PEI−OA has the ability to adsorb onto various types of particles and it is expected that complex formation of PEI can be expanded to various types of anionic surfactants, we strongly believe that the proposed preparation and usage of PEI− surfactant complexes will be a powerful tool for tuning the dispersion stability of various multicomponent nonaqueous slurries.

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CONCLUSION A complex of polyethyleneimine (PEI) with different MWs (600, 1800, and 10 000) and oleic acid (OA) has been successfully prepared as a polymer dispersant for multicomponent nonaqueous slurries through the dissolution of PEI in a nonaqueous solvent (toluene) aided by OA addition. The analysis confirmed the complex formation between the −COO− of OA and amines of PEI, and the prepared PEI−OA complex became soluble in toluene when the additive OA content was greater than 5 mol % based on the number of ethyleneimine units. It was demonstrated that PEI complexed with 30 mol % OA effectively adsorbed on various particles, such as Si3N4, Y2O3, and MgO, regardless of the molecular weight of PEI, and the adsorption reached saturation when the additive content was increased to 1.8 mg/m2. When the Si3N4/ toluene slurries were investigated, it was found that the 12853

DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854

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DOI: 10.1021/acs.iecr.5b03696 Ind. Eng. Chem. Res. 2015, 54, 12847−12854