Dispersion of aqueous nano-sized alumina suspensions using cationic polyelectrolyte

Transcription

1 Materials Research Bulletin 41 (2006) Dispersion of aqueous nano-sized alumina suspensions using cationic polyelectrolyte Yu-Jen Shin, Chia-Chi Su, Yun-Hwei Shen * Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan ROC Received 1 November 2004; received in revised form 29 September 2005; accepted 24 January 2006 Available online 18 April 2006 Abstract In this paper, we investigate and report the effects of a cationic polyelectrolyte, polydiallydimethylammonium chloride (PDADMAC), on the stability of nano-sized alumina (a-al 2 O 3 ) suspension. Due to the static electrical repulsion interactions, PDADMAC show significant adsorption on a-al 2 O 3 only at alkaline ph. Considerable amount of non-adsorbed free PDADMAC exist in a-al 2 O 3 suspensions dispersed by PDADMAC at acidic and neutral ph. At 1.0 wt.% PDADMAC addition, a-al 2 O 3 became highly positively charged (>50 mv) in the entire ph range contrast with that the stability of a-al 2 O 3 suspension is enhanced only over the basic ph range in the presence of anionic polyelectrolyte. Free PDADMAC electrolyte in suspension results in increasing viscosity of a-al 2 O 3 suspensions dispersed with PDADMAC at alkaline and neutral ph. However, non-adsorbed free PDADMAC in suspension do not contribute to viscosity increase in acidic a-al 2 O 3 suspensions and a-al 2 O 3 suspensions dispersed with PDADMAC show the best flow behavior at acidic ph. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Oxides; D. Surface properties 1. Introduction During ceramic processing polyelectrolytes are usually added to the suspension to produce a homogenous and stable dispersion of the powder in the liquid phase, thus leading to both high solid loadings and low viscosities [1 6]. Polyelectrolytes are polymers that possess charges that are present along the length of the polymer chain. When adsorbed on ceramic powders, these species can impart electrostatic and steric stabilization to the suspension. The most extensively investigated polyelectrolytes for dispersion of alumina suspension are poly(acrylic acid) [1,3,7,8] and ammonium salt of polymethacrylic acid (Darvan-C) [2,9,10]. The dispersing ability of those two anionic polyelectrolytes is closely associated with the presence of carboxyl (-COOH) groups, as the carboxyl groups adsorb strongly on alumina [11 13] and adsorbed dissociated carboxyl groups increase the net negative charges of alumina. The surface charge of alumina is positive at low ph values, zero at ph about 8 and negative at higher ph values [8,14 16]. The addition of anionic polyelectrolyte leads to a shift of isoelectric point (IEP) to lower ph value, as well as a decrease in zeta potential, thus, the stability of alumina suspension can be enhanced generally over the basic ph range in the presence of anionic polyelectrolyte. The use of cationic polyelectrolytes as dispersants for alumina have not been frequently investigated * Corresponding author. Tel.: ; fax: address: yhshen@mail.ncku.edu.tw (Y.-H. Shen) /$ see front matter # 2006 Elsevier Ltd. All rights reserved. doi: /j.materresbull

2 Y.-J. Shin et al. / Materials Research Bulletin 41 (2006) Fig. 1. Schematic representation of PDADMAC segment. presumably due to that the mutual electrostatic repulsion prohibited adsorption of cationic polyelectrolytes onto alumina surface. However, it is still interesting to understand the dispersive behavior of alumina in solution of cationic polyelectrolyte. In this paper, we investigate and report the effects of a cationic polyelectrolyte, polydiallydimethylammonium chloride (PDADMAC) (Fig. 1), on the stability of nano-sized alumina (a-al 2 O 3 ) suspension. The stability of the suspensions is investigated using adsorption isotherms, electrophoresis and rheological measurements. 2. Experimental 2.1. Materials A a-al 2 O 3 powder (Design & Quality chemicals Co., Taiwan) with a mean particle size of 0.15 mm and a specific surface area (BET) of 12 m 2 /g was used. The cationic polyelectrolytes used were highly charged polydiallydimethylammonium chlorides (PDADMAC) (20 wt.% in water) with low (approximate MW 70,000), medium (approximate MW 150,000) and high (approximate MW 300,000) molecular weight from Aldrich Chemical Co. Distilled water was used in all tests Zeta potential measurements The zeta potential of the a-al 2 O 3 in dilute suspension (0.01 vol.%) was measured with a Zetaplus analyzer (Zetaplus, Brookhaven, NY). The mixture was ultrasonicated for 15 min prior to measurements. The ionic strength was maintained at 10 3 M using NaCl. The ph of the suspension was adjusted using NaOH or HCl. For PDADMAC-adsorbed a-al 2 O 3 suspensions, a specific amount of a-al 2 O 3 powder was added into solutions containing specific amounts of PDADMAC. The suspensions were equilibrated for 1 h, and then ultrasonicated for 15 min prior to analysis Adsorption isotherms All adsorption tests were conducted for 10 wt.% a-al 2 O 3 suspensions. The suspensions for adsorption measurements were prepared by adding a known amount of a-al 2 O 3 into solution containing specific amounts of PDADMAC. Suspension ph was adjusted to the desired value with HCl or NaOH. The suspensions were mixed using a magnetic stirrer at 600 rpm for 48 h, a time period required for complete adsorption. The equilibrated suspensions were centrifuged at 8500 rpm for 60 min and the supernatant was analyzed for PDADMAC concentration using a total organic carbon analyzer (Shimadzu TOC-5000A). The adsorbed amount was calculated by subtracting the amount left in the supernatant from the amount initially added Dispersive properties The rheological behavior of the a-al 2 O 3 dispersions was measured in a Brookfield HBDV-III rheometer with coneplate adapter at 25 8C. The measurement required 0.8 ml of each dispersion. Suspensions of a-al 2 O 3 with 10 vol.% solid content in PDADMAC solution were prepared after ultrasonic disaggregation treatment for 30 min. Suspensions of 10 ml of 10 vol.% a-al 2 O 3 with varying amounts of PDADMAC were put in cylindrical tubes for 1 week before the final volume of the sediments were measured.

3 1966 Y.-J. Shin et al. / Materials Research Bulletin 41 (2006) Results and discussion 3.1. Adsorption isotherms Adsorption isotherms for PDADMAC on a-al 2 O 3 at three different ph values are presented in Fig. 2. The adsorption isotherms at acidic and neutral ph are of low affinity type and much free PDADMAC remains in the suspension. This is due to that static electrical repulsion between the powder surface and the PDADMAC molecular because they are both positively charged. It is interesting to observe that a small amount of PDADMAC was adsorbed on a-al 2 O 3 regardless of the unfavorable interaction. The isoelectric point (IEP) of the bare a-al 2 O 3 suspension used in this study occurs at about ph 8.4 as indicated in Fig. 4. In general for oxides, the relative charge density on the surface of the particles varies with ph [1,2,11,17]. For oxides at every ph, there are large numbers of negative, neutral, and positive surface sites. The zeta potential value gives the overall net charge density. For a-al 2 O 3 at ph < 8.4, positive sites are in the vast majority and, therefore, zeta potential is positive. At ph > 8.4, negative sites are in the majority, and zeta potential is negative. At the IEP, which occurs when the number of positive sites equals the number of negatives sites, zeta potential is 0. Thus, a small amount of PDADMAC was able to interact with few negative sites on a-al 2 O 3 at acidic and neutral ph. At ph > IEP the adsorption of positively charged PDADMAC on negatively charged powders is favored by the static electrical attraction. Adsorption isotherms at ph 11.9 for PDADMAC with different molecular weight on a-al 2 O 3 are presented in Fig. 3. In generally, the amount of adsorbed increased as the molecular weight of PDADMAC increase but the effect was not significant. The results of adsorption study indicated that large amount of free PDADMAC existed in the solution due to the low adsorption affinity of PDADMAC on a- Al 2 O 3 at acidic and neutral ph. This free electrolyte in the suspension may disturb the electrostatic forces within the system and hence increasing the viscosity [1,2,18] Zeta potential The effect of the addition of PDADMAC on the electrokinetic properties of a-al 2 O 3 suspensions is shown in Figs. 4 and 5. The isoelectric point (IEP) of the bare a-al 2 O 3 suspension occurs at about ph 8.4. The addition of 0.1 wt.% PDADMAC can push the IEP into the more alkali region, at about ph For an addition of 1.0 wt.% PDADMAC, a-al 2 O 3 became highly positively charged (>50 mv) in the entire experimental ph range. The significant increased of the measured zeta potential is caused by the adsorption of PDADMAC at alkaline ph, which reflects the fact that at ph > IEP the adsorption of positively charged PDADMAC on negatively charged powders is favored by the static electrical attraction. The zeta potential at acid ph is still as high as 50 mv for the PDADMAC-added alumina suspension regardless of the presence of free PDADMAC in suspension. As indicated in Fig. 5, the molecular weight of PDADMAC did not show a significant influence on the electrokinetic properties of a-al 2 O 3 and this result was in line with the result from PDADMAC adsorption data in Fig. 3. It is apparently that when cationic polyelectrolyte is Fig. 2. Adsorption isotherms of PDADMAC on alumina at different ph.

4 Y.-J. Shin et al. / Materials Research Bulletin 41 (2006) Fig. 3. Adsorption isotherms of various PDADMAC on alumina at ph Fig. 4. Zeta potential of alumina as a function of ph without and with PDADMAC. Fig. 5. Effect of the molecular weight of PDADMAC on zeta potential of alumina.

5 1968 Y.-J. Shin et al. / Materials Research Bulletin 41 (2006) used as a dispersant to prepare aqueous a-al 2 O 3 suspensions, a stable suspension with high positive zeta potential may be prepared in both acidic and alkaline ph range. In contrast, it is difficulty to prepared aqueous a-al 2 O 3 suspensions with high negative zeta potential in acidic ph range when anionic polyelectrolyte is used Dispersive properties Fig. 6 shows the variation of the shear stress and the viscosity of 10 vol.% a-al 2 O 3 suspensions at ph 11.5 versus shear rate for various PDADMAC additions. When 0.05 wt.% PDADMAC is added at this ph, the viscosity increase due to that the adsorption of PDADMAC on negatively charged a-al 2 O 3 causes flocculation by charge neutralization. This flocculation leads to an increased viscosity of the suspension. Further addition of PDADMAC to 0.35 wt.% reverses the charge on a-al 2 O 3 and stabilizes the suspension by providing an electrostatic repulsion. At this PDADMAC dosage, a lowest viscosity is observed. When the amount of PDADMAC added is over this optimum, the amount of free PDADMAC in suspension increases. Free electrolyte in suspension disturbing the electrostatic forces within the system [1,2,18] and decreasing amount of solvent available hence results in increasing viscosity. All the curves in Fig. 6 exhibit shear-thinning characteristics, that is, the apparent viscosity decreases with increasing shear rate except that of 0.35 wt.% PDADMAC added suspension. A shear-thinning behavior usually results from flocculation of particles in suspension [19,20]. At high shear rates, the viscous force tends to reduce the size of agglomerates and release the water immobilized in the agglomerates, thus facilitating flow and shear-thinning occurs. The Newtonian behavior of a-al 2 O 3 suspensions prepared with addition of 0.35 wt.% PDADMAC is characteristics of a stable suspension that allows the particles in the suspension to move independently. Figs. 5 and 6 indicate that the presence of free PDADMAC in suspension causes a shear-thinning behavior of a-al 2 O 3 suspension even if the suspension is stabilize by large positive zeta potential on a-al 2 O 3. The variation of the shear stress and the viscosity of 10 vol.% a-al 2 O 3 suspensions at ph 7.7 versus shear rate for various PDADMAC additions were shown in Fig. 7.At Fig. 6. Rheogram of alumina slurry (ph 11.5) for various amount of PDADMAC added.

6 Y.-J. Shin et al. / Materials Research Bulletin 41 (2006) Fig. 7. Rheogram of alumina slurry (ph 7.7) for various amount of PDADMAC added. this ph, large amount of free PDADMAC exists in the solution due to the low adsorption of PDADMAC on positively charged a-al 2 O 3 (Fig. 2) and hence increases the viscosity. All the curves in Fig. 6 exhibit shear-thinning characteristics presumably due to the presenceoffreepdadmacinsuspension.fig. 7 shows a typical increase of suspension viscosity with increasing PDADMAC addition. The flow curves for 10 vol.% a-al 2 O 3 suspensions at ph 2.5 for various PDADMAC additions were shown in Fig. 8. It is interesting to note that a-al 2 O 3 suspensions dispersed with PDADMAC show the best flow behavior at this acidic ph condition. All the curves in Fig. 8 exhibit Newtonian behavior indicating a stable colloid system regardless the presence of non-adsorbed free PDADMAC in suspension. In absence of PDADMAC addition, a-al 2 O 3 suspensions show relatively low viscosity presumably due to the large positive zeta potential at ph 2.5 for a-al 2 O 3 (Fig. 4). However, an optimum PDADMAC addition (0.38 wt.%) still exists for a-al 2 O 3 suspensions at ph 2.5. It is not clear why the non-adsorbed free PDADMAC in suspension do not contribute to the viscosity increase in acidic a-al 2 O 3 suspensions. A reviewer of this manuscript has suggested that the centrifugationusedbeforetocanalysismaydisturb weaker interactions between PDADMAC and a-al 2 O 3 surface, leading to chain desorption, that is, it is possible that the amount of PDADMAC chains that interact with a-al 2 O 3 is higher than that assumed from TOC analysis. In this case, the low viscosity observed at acidic ph could be explained. Evolution of the viscosity under a steady shear rate of 100 s 1 versus amount of PDADMAC addition is presented in Fig. 9. As reported early, PDADMAC dispersed a-al 2 O 3 suspensions at acidic ph exhibit the best flow behavior. The optimum PDADMAC dosages are about 0.35 wt.% for a-al 2 O 3 suspensions at both ph 11.5 and 2.5. Fig. 10 depicts the specific volume of sediment plotted against the concentration of PDADMAC for various ph values. The a-al 2 O 3 suspensions dispersed with PDADMAC at ph 7.7 and 11.5 show very little solid settling within suspension and high specific volume of sediment. This stability of a-al 2 O 3 suspensions is mainly a consequence of high suspension viscosity due to the presence of excess free PDADMAC in solution as discussed previously. The a-al 2 O 3

7 1970 Y.-J. Shin et al. / Materials Research Bulletin 41 (2006) Fig. 8. Rheogram of alumina slurry (ph 2.5) for various amount of PDADMAC added. suspensions dispersed with PDADMAC at ph 2.5 give relatively low specific volume of sediments and a minimum volume of sediment exists at 0.38 wt.% PDADMAC addition. This PDADMAC dosage agrees well with that found in rheology measurements (Fig. 8). As mentioned previously that non-adsorbed free PDADMAC in suspension do not contribute to the viscosity increase in acidic a-al 2 O 3 suspensions, thus, in this case the minimum volume of sediment may truly result from a a-al 2 O 3 suspension stabilized by electrostatic interaction. Fig. 9. Viscosity of 10 vol.% alumina slurry as a function of PDADMAC added (wt.%) at different ph.

8 Y.-J. Shin et al. / Materials Research Bulletin 41 (2006) Fig. 10. Specific volume (S.V.) of sediment for 10 vol.% alumina slurry as a function of PDADMAC dosage at various ph. 4. Conclusions A cationic polyelectrolyte, PDADMAC, is investigated as a dispersant for nano-sized a-al 2 O 3. Due to the static electrical repulsion interactions, PDADMAC show significant adsorption on a-al 2 O 3 only at alkaline ph. Considerable amount of non-adsorbed free PDADMAC exist in a-al 2 O 3 suspensions dispersed by PDADMAC at acidic and neutral ph. The zeta potential of a-al 2 O 3 is increased by the positively charged PDADMAC at all ph range. At 1.0 wt.% PDADMAC addition, a-al 2 O 3 became highly positively charged (>50 mv) in the entire ph range contrast with that the stability of a-al 2 O 3 suspension is enhanced only over the basic ph range in the presence of anionic polyelectrolyte. Free PDADMAC electrolyte in suspension disturbing the electrostatic forces within the system and decreasing amount of solvent available for dispersion hence results in increasing viscosity of a-al 2 O 3 suspensions dispersed with PDADMAC at alkaline and neutral ph. However, non-adsorbed free PDADMAC in suspension do not contribute to viscosity increase in acidic a-al 2 O 3 suspensions and a-al 2 O 3 suspensions dispersed with PDADMAC show the best flow behavior at acidic ph. Although the high viscosity of a-al 2 O 3 suspensions dispersed with PDADMAC undermines the opportunity for preparing a-al 2 O 3 slurry with high solid content, the good stability of suspension over a wide range of ph is advantageous in many colloidal processing techniques. References [1] J. Cesarano III, I.A. Aksay, A. Bleier, J. Am. Ceram. Soc. 71 (1988) 250. [2] J. Cesarano III, I.A. Aksay, J. Am. Ceram. Soc. 71 (1988) [3] P.C. Hidber, T.J. Graule, L.J. Gauckler, J. Am. Ceram. Soc. 79 (1996) [4] L. Guo, Y. Zhang, N. Uchida, K. Uematsu, J. Eur. Ceram. Soc. 17 (1997) 345. [5] L.M. Palmqvist, F.F. Lange, W. Sigmund, J. Sindel, J. Am. Ceram. Soc. 83 (2000) [6] J. Davies, J.G..P. Binner, J. Eur. Ceram. Soc. 20 (2000) [7] B.J. Briscoe, A.U. Khan, P.F. Luckham, J. Eur. Ceram. Soc. 18 (1998) [8] Y. Liu, L. Gao, L. Yu, J. Guo, J. Colloid Interf. Sci. 227 (2000) 164. [9] M.R.B. Romdhane, S. Boufi, S. Baklouti, T. Chartier, J.F. Baumard, Colloid Surf. A 212 (2003) 271. [10] Y. Liu, L. Gao, Mater. Chem. Phys. 82 (2003) 362. [11] L.T. Lee, P. Somasundaran, Langmuir 5 (1989) 854. [12] C.R. Evanko, R.F. Delisio, D.A. Dzombak, J.W. Novak, Colloid Surf. A 125 (1997) 95. [13] P.C. Hidber, T.J. Graule, L.J. Gauckler, J. Eur. Ceram. Soc. 17 (1997) 239. [14] K.F. Tjipangandjara, P. Somasundaran, Colloid Surf. 55 (1991) 245. [15] X. Yu, P. Somasundaran, J. Colloid Interf. Sci. 177 (1996) 238. [16] L. Jiang, L. Gao, Mater. Chem. Phys. 80 (2003) 157. [17] J.F. Kelso, T.A. Ferrazzoli, J. Am. Ceram. Soc. 72 (1989) 625. [18] E. Laarz, L. Bergstrom, J. Eur. Ceram. Soc. 20 (2000) 431. [19] J.C. Chang, F.F. Lange, D.S. Pearson, J. Am. Ceram. Soc. 77 (1994) [20] A.K. Nikumbh, H. Schmidt, K. Martin, F. Porz, F. Thummler, J. Mater. Sci. 25 (1990) 15.