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Influence of surface charge on maximizing solids loading in colloidal processing of alumina
October 2002
Materials Letters 56 (2002) 475 – 480 www.elsevier.com/locate/matlet
Influence of surface charge on maximizing solids loading in colloidal processing of alumina Bimal P. Singh *, Sarama Bhattacharjee, Laxmidhar Besra Regional Research Laboratory, CSIR, Bhubaneswar-751013, India Received 10 January 2002; accepted 22 January 2002
Abstract Stability of concentrated aqueous colloidal alumina powder suspensions with and without dispersant (albumin and dibasic ammonium citrate (DAC)) has been investigated by measuring surface charge at different solids loading and pH values. The isoelectric point (iep) of alumina powder was found to be pHiep=8.5. The surface charge of alumina powder changed significantly with anionic polyelectrolyte and iep shifted towards more acidic pH range under different dispersion conditions. The study illustrated that albumin is more effective as dispersant than DAC at all solid loading conditions. The computed specific 0 interaction energies DGSP of albumin (7.5) and DAC (3.5) indicate strong adsorption of albumin compared to DAC on alumina powder surface, leading to higher solid loading. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface charge; Iso-electric point; Solids loading; Interaction energy; Alumina powder
1. Introduction Many recently developed ceramic processing techniques including spray drying, slip casting, pressure casting, tape casting and gel casting employ welldispersed suspensions with very high levels of solid loading. For these forming techniques, the slurry density defines the green density of the formed ceramic material. Therefore, for such type of forming processes, it is critical that the process engineer be able to increase the solid loading in the slurry to as high a level as possible. In addition, high solids loading reduce the shrinkage during drying and firing and increase the green strength. Typical slurry volume fractions are between 40 and 80 vol.% depending on *
Corresponding author. Fax: +91-674-581637. E-mail address: [email protected] (B.P. Singh).
the process and the powder [1]. However, with increasing solid loading, processing of a suspension becomes increasingly difficult. Producing a free flowing, high solids slurry requires an overall good understanding of physical properties of the ceramic particles and precise control of the interactions among particles, the medium in which the particles are processed, particle – solvent interactions such as wetting and dispersion, slurry rheology and mixing techniques. In general, a high solids slurry displays several characteristics during the mixing operation to achieve a good slurry, or upon attempting to achieve a slightly higher volume fraction solids. These characteristics include colloidal instability, excessively high viscosity, difficulty in adding additional powder and dilatancy. Powder characteristic is a crucial parameter in controlling the slurry making process. In this connection, particle size and particle size distribution are the
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 5 3 3 - 5
476
B.P. Singh et al. / Materials Letters 56 (2002) 475–480
two most important parameters that need to be controlled. Selection of a dispersing technique and chemical dispersant system are also very important factors critical to achieving high solids slurry. The dispersion of powder depends on several factors: the solvent must wet it, the particles must become separated from one another and mixed with the solvent system, and they must remain separated and not re-agglomerate. Good dispersion at high solid loading is not only necessary but also pre-requisite for all slurry based forming processes. The dispersion quality of ceramic suspensions, prior to the forming process, therefore, must be controlled satisfactorily in order to maintain high standard of manufacturing and to obtain consistent/reproducible products. The dispersant screening process usually starts at low solids loading using either sedimentation or light scattering to determine the degree of dispersion of the particles in the solvent. However, studies need to be carried out at higher solid loading since it has been shown that a dispersant that provides the same at low solids concentration may not necessarily provide good dispersion at high solids concentration [1]. Many new techniques are now available to characterize the dispersion quality of suspensions, these include the rheological, sedimentation, adsorption, electrophoresis and charge quantity measurements. Rheology is perhaps the most frequently used probe to determine the agglomeration properties of concentrated ceramic powder suspensions. Rheology may also be used as an analytical tool for determining the optimum viscosity of suspensions. Usually this means the minimum viscosity for the maximum solids loading. In the present investigation, the surface charge quantity of concentrated slurries of alumina powder has been studied at varying dosing conditions with the aim of maximizing the solid loading by increasing overall charge on the system, and the resulting increase in mutual repulsion. The properties important in determining the rheological characteristics of concentrated colloidal suspensions are the charge density of the suspended particle and the ionic strength of the suspension. The magnitude of the surface charge density is also very important. When the relative magnitude of the surface charge density to ionic strength is high, the net particle interaction is repulsive and a non-plastic, low viscosity suspension is usually obtained.
2. Experimental 2.1. Materials A high purity alumina powder with alumina content > 99.9% was used in this study. The average particle size (D50) and BET surface area of the powder were found to be 0.72 Am and 9.1 m2/g, respectively. D50 means that 50% of the particles by volume have sizes below that size. The chemical additives (dispersant) used in this investigation were dibasic ammonium citrate (DAC) supplied by Merck, Germany, and albumin prepared from fresh egg by mixing white portion of the egg with equal volume of double distilled water. Analytical grade HCl and NaOH were used to modify pH of the system. 2.2. Charge quantity measurement Measurement of charge quantity of the system was made with Particle Charge Detector (PCD 03-pH, Make Mutech Germany). The working principle of PCD and experimental details are described elsewhere [2,3]. 2.3. Measurement of dispersion characteristics Measurement of dispersion characteristics of the alumina suspension was carried out using the conventional sedimentation method in graduated cylinder. After addition of requisite amount of additive, the suspension was stirred thoroughly for proper mixing and allowed to stand in the graduated cylinder for 24 h. The sediment heights were then read directly from the graduated cylinder. The higher the sediment height, the more stable is the suspension.
3. Results and discussion 3.1. Surface charge and interaction of alumina powder with DAC/albumin The stability of alumina powder suspension in aqueous solution is closely related to its electrophoretic properties. Well-dispersed suspension can be obtained with high surface charge density to generate strong
B.P. Singh et al. / Materials Letters 56 (2002) 475–480
Fig. 1. Schematic representation of bridzing of alumina particle with water by H-bonding.
repulsive forces. When oxide powder is dispersed in water, it can adsorb water molecule and form a hydration layer. The surface chemical properties of the powder are determined by the H+ and OH ions adsorbed on the particle surface. Alumina particle at the natural pH (=8.1) is slightly positively charged in aqueous suspension. Fig. 1 shows schematic representation of bridging of alumina particle with water by hydrogen bonding. The hydrogen bonding makes the alumina particles interconnected with each other and consequently lead to agglomeration. Fig. 2 shows the surface charge of alumina powder as a function of pH with and without dispersant. In the case of alumina suspension without dispersant, the
477
surface charge varies from +1.12 C/g at pH 2 to 0.3 C/g at pH 11, with an iso-electric point or the point of zero mobility or zero net surface charge at pH 8.5. At pH below 8.5, the alumina powder is positively charged and above pH 8.5 it is negatively charged. The magnitude of negative surface charge increases with increasing pH above 8.5. This result suggests that the alumina suspensions should experience progressive deflocculation with increasing pH above 8.5. However, when the dispersants albumin and dibasic ammonium citrate are added at their optimum concentration (5250 and 150 ppm, respectively), it causes significant changes in alumina surface charge properties. In the case of albumin, surface charge changes from +5.3 C/g at pH 3.5 to 2.2 C/g at pH 10 with an iso-electric point at pH 5.5. Addition of albumin results in more negative surface charge than the untreated alumina. It may be due to dissociation of albumin in the solution producing COO groups adsorbed on the alumina particle, which in turn increases the net negative charge of powder surface and also increases the repulsive forces. Similarly, addition of DAC changes the surface charge from +1 C/g at pH 2.8 to 0.5 C/g at pH 10.5 with isoelectric point at pH 4.5. In this case, there is only a marginal change in surface charge in comparison to albumin, but a significant shift in iep towards acidic pH region. The shift in iep is more than three units with addition of albumin and more than four units with DAC addition. Further, the increase in magnitude
Fig. 2. Effect of pH on surface charge of 5% (w/v) alumina suspension.
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B.P. Singh et al. / Materials Letters 56 (2002) 475–480
of the negative surface charge in the basic pH region is more with albumin addition than with DAC. This clearly shows a stronger electrostatic stabilization effect of albumin compared to DAC. The shift in iep on addition of albumin and DAC are due to the following reason: the negatively charged carboxylic group dissociated from dispersant is adsorbed on the positively charged alumina surface and consequently the surface is negatively charged. When albumin/DAC is adsorbed on the surface of the alumina particle, the hydroxyl groups associated with alumina particle react with the carboxyl groups of albumin/DAC and thus will result in a shift of iep to a lower value. Fig. 3 shows a schematic representation of the adsorption of albumin/DAC on an alumina surface. This arrangement is most favourable to have the alumina surface most negative surface charge. As stated earlier, both anionic chemical additives have active COOH group. In the case of albumin, it has amino group apart from carboxylic group. 3.2. Specific energy of interaction The specific energy of interaction between alumina powder surface and dispersant was calculated using the following equation [4,5] DpH iep ¼ 1:0396C0 expðDG0SP =RT Þ
ð1Þ
where DpHiep is the shift in the iso-electric point at the 0 dispersion concentration C0 and DGSP represents the corresponding specific energy of interaction between the alumina powder surface and the dispersant. R and T are the standard gas constant and temperature in
Table 1 Computed values of specific energy of interaction System
Dispersant concentration (ppm)
pH
DpHiep
0 DGSP (RT units)
No dispersant Albumin DAC
– 5250.0 150.0
8.1 9.0 9.2
– 3.0 4.0
– 7.5 3.6
Kelvin, respectively. Table 1 shows the computed data of specific energy of interaction between alumina powder surface and dispersant. It is obvious from the Table 1 that the computed 0 value of specific interaction energy DGSP is higher in the case of albumin than DAC. This clearly indicates stronger adsorption of albumin on alumina powder surface than DAC, which in turn increased effectiveness of albumin leading to higher solid loading. The surface charge vs. pH curve for alumina slurry (Fig. 2) without dispersant and with optimum Albumin and DAC concentrations indicates that the difference of DpH iep is more than 2.5 for both the dispersants with respect to pHiep of alumina powder. This suggests that both the dispersants yield welldispersed slurry [6]. Further, the higher value of 0 DGSP for albumin (7.5) as compared with DAC (3.6) suggests more electrical double layer repulsion resulting in better dispersion [4]. 3.3. Effect of pH on dispersion characteristics Fig. 4 depicts the sedimentation height plotted against the slurry pH in presence and absence of optimum dosages of two different dispersant. The
Fig. 3. Model for interaction of DAC on alumina surface.
B.P. Singh et al. / Materials Letters 56 (2002) 475–480
479
Fig. 4. Effect of slurry pH on dispersion characteristics of 5 vol.% alumina.
dispersants employed were albumin and DAC. In the absence of dispersant, increasing the pH value from acidic region upto pH 8.5 leads to decrease in the sediment height indicating that a more consolidated structure of the particulate is formed. Further increase in pH above 8.5 has no effect on the sediment height. As higher sediment height is a measure of more stability, the suspension in absence of any additive is more stable at very acidic pH (3.7). In the presence of DAC, increasing pH from acidic to alkaline increases sediment height, indicating more deflocculated state of suspension. After reaching a maximum height (11.5 cm) at pH around 8.5 – 9.0, further increase in pH decreases the sediment height appreciably. At pH 12, the sediment height is minimum (2.2 cm). Similar increase in sediment height and attainment of maximum (12.5 cm) at pH around 8.5 –9 is observed for albumin addition with increasing pH from 4.7 to 12. Further increase in pH above 8.5 decreases the sediment height significantly. Both dispersants follow the same trend but the sediment height achieved for albumin is comparatively higher than that for DAC. One of the reasons for this type behaviour is due to highly negative surface charge in the presence of albumin on the alumina particle in comparison to DAC (Fig. 2). This in turn suggests that albumin acts as a better dispersant in comparison to
DAC, and that maximum dispersion in either case is achievable in the natural pH range of 8.5 – 9. 3.4. Solid loading vs. surface charge Fig. 5 shows the variation of surface charge as a function of solid loading in presence and absence of dispersant. Increasing the solid loading in absence of any dispersant from 1.47 to 50 vol.% changes the surface charge from +0.02 to 0.0098 C/g. It is clear from this curve that there is not appreciable change in surface charge with increasing solid loading, although an increase in pH value from 8 –9 has been observed. In the presence of dispersant DAC at optimum dosages (105 ppm), the surface charge remains constant with increasing solid loading. It has been observed that pH also does not vary and remains constant at 8.7. However, the magnitude of surface charge in presence of DAC is shifted towards more negative value. Similarly, in the presence of albumin, although there is no variation in surface charge with increasing solids loading, the magnitude of negative surface charge is significantly higher in comparison to the magnitude of change with DAC. It may be concluded from this investigation that solid loading has no effect on optimum dosages of
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Fig. 5. Effect of solid loading on surface charge of alumina powder.
chemical additives is reproducible at higher solids loading too. Therefore, one can have first hand information at lower solid loading, which will be valid at higher solid loading also.
(5) Albumin is more effective as dispersant than DAC, that is further corroborated by comparing 0 specific interaction energy (DGSP ) which is higher in magnitude in the case of albumin (7.5) than DAC (3.6).
4. Conclusions The following conclusions have been drawn on the basis of this investigation. (1) Surface charge measurement is one of the most effective ways to describe the state of ceramic powder in aqueous suspension. (2) The iep of alumina powder without dispersant is around pH 8.5. The iep of alumina powder shifted to more acidic pH (5.5 for albumin and 4.5 for DAC) with addition of anionic polyelectrolyte. The surface is negatively charged at a wide range of pH>4.5. Both polyelectrolytes change the surface charge appreciably. (3) Addition of small amounts of both anionic polyelectrolytes (albumin and DAC) increase the negative surface charge significantly, but the change is more pronounced with albumin. (4) The surface charge remains more or less constant at all conditions of solid loading. Therefore, optimisation of dispersant dosages at lower solidloading holds good for higher solid loading too.
Acknowledgements The authors are thankful to the Director of the Regional Research Laboratory, Bhubaneswar, for his kind permission to publish this paper. The authors are also thankful to Prof. Parag Bhargav, IIT and Kharagpur for fruitful discussion.
References [1] M.A. Janney, Attaining high solids in ceramic slurries, website, http://www.ornl.gov, 6/8/01. [2] B.P. Singh, L. Besra, S. Bhattacharjee, Colloids Surf. A: Physicochem. Eng. Aspects (Comm.). [3] B.P. Singh, S. Bhattacharjee, L. Besra, Ceram. Int. (Comm.). [4] Pradip, Trans. IIM 41 (1988) 15. [5] R.M. Anklekar, S.A. Borkar, S. Bhattacharjee, C.H. Page, A.K. Chatterjee, Colloids Surf. A: Physicochem. Eng. Aspects 133 (1998) 41. [6] S.G. Malghan, R.S. Premachandran, P.T. Pei, Powder Technol. 79 (1994) 43.
Materials Letters 56 (2002) 475 – 480 www.elsevier.com/locate/matlet
Influence of surface charge on maximizing solids loading in colloidal processing of alumina Bimal P. Singh *, Sarama Bhattacharjee, Laxmidhar Besra Regional Research Laboratory, CSIR, Bhubaneswar-751013, India Received 10 January 2002; accepted 22 January 2002
Abstract Stability of concentrated aqueous colloidal alumina powder suspensions with and without dispersant (albumin and dibasic ammonium citrate (DAC)) has been investigated by measuring surface charge at different solids loading and pH values. The isoelectric point (iep) of alumina powder was found to be pHiep=8.5. The surface charge of alumina powder changed significantly with anionic polyelectrolyte and iep shifted towards more acidic pH range under different dispersion conditions. The study illustrated that albumin is more effective as dispersant than DAC at all solid loading conditions. The computed specific 0 interaction energies DGSP of albumin (7.5) and DAC (3.5) indicate strong adsorption of albumin compared to DAC on alumina powder surface, leading to higher solid loading. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface charge; Iso-electric point; Solids loading; Interaction energy; Alumina powder
1. Introduction Many recently developed ceramic processing techniques including spray drying, slip casting, pressure casting, tape casting and gel casting employ welldispersed suspensions with very high levels of solid loading. For these forming techniques, the slurry density defines the green density of the formed ceramic material. Therefore, for such type of forming processes, it is critical that the process engineer be able to increase the solid loading in the slurry to as high a level as possible. In addition, high solids loading reduce the shrinkage during drying and firing and increase the green strength. Typical slurry volume fractions are between 40 and 80 vol.% depending on *
Corresponding author. Fax: +91-674-581637. E-mail address: [email protected] (B.P. Singh).
the process and the powder [1]. However, with increasing solid loading, processing of a suspension becomes increasingly difficult. Producing a free flowing, high solids slurry requires an overall good understanding of physical properties of the ceramic particles and precise control of the interactions among particles, the medium in which the particles are processed, particle – solvent interactions such as wetting and dispersion, slurry rheology and mixing techniques. In general, a high solids slurry displays several characteristics during the mixing operation to achieve a good slurry, or upon attempting to achieve a slightly higher volume fraction solids. These characteristics include colloidal instability, excessively high viscosity, difficulty in adding additional powder and dilatancy. Powder characteristic is a crucial parameter in controlling the slurry making process. In this connection, particle size and particle size distribution are the
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 5 3 3 - 5
476
B.P. Singh et al. / Materials Letters 56 (2002) 475–480
two most important parameters that need to be controlled. Selection of a dispersing technique and chemical dispersant system are also very important factors critical to achieving high solids slurry. The dispersion of powder depends on several factors: the solvent must wet it, the particles must become separated from one another and mixed with the solvent system, and they must remain separated and not re-agglomerate. Good dispersion at high solid loading is not only necessary but also pre-requisite for all slurry based forming processes. The dispersion quality of ceramic suspensions, prior to the forming process, therefore, must be controlled satisfactorily in order to maintain high standard of manufacturing and to obtain consistent/reproducible products. The dispersant screening process usually starts at low solids loading using either sedimentation or light scattering to determine the degree of dispersion of the particles in the solvent. However, studies need to be carried out at higher solid loading since it has been shown that a dispersant that provides the same at low solids concentration may not necessarily provide good dispersion at high solids concentration [1]. Many new techniques are now available to characterize the dispersion quality of suspensions, these include the rheological, sedimentation, adsorption, electrophoresis and charge quantity measurements. Rheology is perhaps the most frequently used probe to determine the agglomeration properties of concentrated ceramic powder suspensions. Rheology may also be used as an analytical tool for determining the optimum viscosity of suspensions. Usually this means the minimum viscosity for the maximum solids loading. In the present investigation, the surface charge quantity of concentrated slurries of alumina powder has been studied at varying dosing conditions with the aim of maximizing the solid loading by increasing overall charge on the system, and the resulting increase in mutual repulsion. The properties important in determining the rheological characteristics of concentrated colloidal suspensions are the charge density of the suspended particle and the ionic strength of the suspension. The magnitude of the surface charge density is also very important. When the relative magnitude of the surface charge density to ionic strength is high, the net particle interaction is repulsive and a non-plastic, low viscosity suspension is usually obtained.
2. Experimental 2.1. Materials A high purity alumina powder with alumina content > 99.9% was used in this study. The average particle size (D50) and BET surface area of the powder were found to be 0.72 Am and 9.1 m2/g, respectively. D50 means that 50% of the particles by volume have sizes below that size. The chemical additives (dispersant) used in this investigation were dibasic ammonium citrate (DAC) supplied by Merck, Germany, and albumin prepared from fresh egg by mixing white portion of the egg with equal volume of double distilled water. Analytical grade HCl and NaOH were used to modify pH of the system. 2.2. Charge quantity measurement Measurement of charge quantity of the system was made with Particle Charge Detector (PCD 03-pH, Make Mutech Germany). The working principle of PCD and experimental details are described elsewhere [2,3]. 2.3. Measurement of dispersion characteristics Measurement of dispersion characteristics of the alumina suspension was carried out using the conventional sedimentation method in graduated cylinder. After addition of requisite amount of additive, the suspension was stirred thoroughly for proper mixing and allowed to stand in the graduated cylinder for 24 h. The sediment heights were then read directly from the graduated cylinder. The higher the sediment height, the more stable is the suspension.
3. Results and discussion 3.1. Surface charge and interaction of alumina powder with DAC/albumin The stability of alumina powder suspension in aqueous solution is closely related to its electrophoretic properties. Well-dispersed suspension can be obtained with high surface charge density to generate strong
B.P. Singh et al. / Materials Letters 56 (2002) 475–480
Fig. 1. Schematic representation of bridzing of alumina particle with water by H-bonding.
repulsive forces. When oxide powder is dispersed in water, it can adsorb water molecule and form a hydration layer. The surface chemical properties of the powder are determined by the H+ and OH ions adsorbed on the particle surface. Alumina particle at the natural pH (=8.1) is slightly positively charged in aqueous suspension. Fig. 1 shows schematic representation of bridging of alumina particle with water by hydrogen bonding. The hydrogen bonding makes the alumina particles interconnected with each other and consequently lead to agglomeration. Fig. 2 shows the surface charge of alumina powder as a function of pH with and without dispersant. In the case of alumina suspension without dispersant, the
477
surface charge varies from +1.12 C/g at pH 2 to 0.3 C/g at pH 11, with an iso-electric point or the point of zero mobility or zero net surface charge at pH 8.5. At pH below 8.5, the alumina powder is positively charged and above pH 8.5 it is negatively charged. The magnitude of negative surface charge increases with increasing pH above 8.5. This result suggests that the alumina suspensions should experience progressive deflocculation with increasing pH above 8.5. However, when the dispersants albumin and dibasic ammonium citrate are added at their optimum concentration (5250 and 150 ppm, respectively), it causes significant changes in alumina surface charge properties. In the case of albumin, surface charge changes from +5.3 C/g at pH 3.5 to 2.2 C/g at pH 10 with an iso-electric point at pH 5.5. Addition of albumin results in more negative surface charge than the untreated alumina. It may be due to dissociation of albumin in the solution producing COO groups adsorbed on the alumina particle, which in turn increases the net negative charge of powder surface and also increases the repulsive forces. Similarly, addition of DAC changes the surface charge from +1 C/g at pH 2.8 to 0.5 C/g at pH 10.5 with isoelectric point at pH 4.5. In this case, there is only a marginal change in surface charge in comparison to albumin, but a significant shift in iep towards acidic pH region. The shift in iep is more than three units with addition of albumin and more than four units with DAC addition. Further, the increase in magnitude
Fig. 2. Effect of pH on surface charge of 5% (w/v) alumina suspension.
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B.P. Singh et al. / Materials Letters 56 (2002) 475–480
of the negative surface charge in the basic pH region is more with albumin addition than with DAC. This clearly shows a stronger electrostatic stabilization effect of albumin compared to DAC. The shift in iep on addition of albumin and DAC are due to the following reason: the negatively charged carboxylic group dissociated from dispersant is adsorbed on the positively charged alumina surface and consequently the surface is negatively charged. When albumin/DAC is adsorbed on the surface of the alumina particle, the hydroxyl groups associated with alumina particle react with the carboxyl groups of albumin/DAC and thus will result in a shift of iep to a lower value. Fig. 3 shows a schematic representation of the adsorption of albumin/DAC on an alumina surface. This arrangement is most favourable to have the alumina surface most negative surface charge. As stated earlier, both anionic chemical additives have active COOH group. In the case of albumin, it has amino group apart from carboxylic group. 3.2. Specific energy of interaction The specific energy of interaction between alumina powder surface and dispersant was calculated using the following equation [4,5] DpH iep ¼ 1:0396C0 expðDG0SP =RT Þ
ð1Þ
where DpHiep is the shift in the iso-electric point at the 0 dispersion concentration C0 and DGSP represents the corresponding specific energy of interaction between the alumina powder surface and the dispersant. R and T are the standard gas constant and temperature in
Table 1 Computed values of specific energy of interaction System
Dispersant concentration (ppm)
pH
DpHiep
0 DGSP (RT units)
No dispersant Albumin DAC
– 5250.0 150.0
8.1 9.0 9.2
– 3.0 4.0
– 7.5 3.6
Kelvin, respectively. Table 1 shows the computed data of specific energy of interaction between alumina powder surface and dispersant. It is obvious from the Table 1 that the computed 0 value of specific interaction energy DGSP is higher in the case of albumin than DAC. This clearly indicates stronger adsorption of albumin on alumina powder surface than DAC, which in turn increased effectiveness of albumin leading to higher solid loading. The surface charge vs. pH curve for alumina slurry (Fig. 2) without dispersant and with optimum Albumin and DAC concentrations indicates that the difference of DpH iep is more than 2.5 for both the dispersants with respect to pHiep of alumina powder. This suggests that both the dispersants yield welldispersed slurry [6]. Further, the higher value of 0 DGSP for albumin (7.5) as compared with DAC (3.6) suggests more electrical double layer repulsion resulting in better dispersion [4]. 3.3. Effect of pH on dispersion characteristics Fig. 4 depicts the sedimentation height plotted against the slurry pH in presence and absence of optimum dosages of two different dispersant. The
Fig. 3. Model for interaction of DAC on alumina surface.
B.P. Singh et al. / Materials Letters 56 (2002) 475–480
479
Fig. 4. Effect of slurry pH on dispersion characteristics of 5 vol.% alumina.
dispersants employed were albumin and DAC. In the absence of dispersant, increasing the pH value from acidic region upto pH 8.5 leads to decrease in the sediment height indicating that a more consolidated structure of the particulate is formed. Further increase in pH above 8.5 has no effect on the sediment height. As higher sediment height is a measure of more stability, the suspension in absence of any additive is more stable at very acidic pH (3.7). In the presence of DAC, increasing pH from acidic to alkaline increases sediment height, indicating more deflocculated state of suspension. After reaching a maximum height (11.5 cm) at pH around 8.5 – 9.0, further increase in pH decreases the sediment height appreciably. At pH 12, the sediment height is minimum (2.2 cm). Similar increase in sediment height and attainment of maximum (12.5 cm) at pH around 8.5 –9 is observed for albumin addition with increasing pH from 4.7 to 12. Further increase in pH above 8.5 decreases the sediment height significantly. Both dispersants follow the same trend but the sediment height achieved for albumin is comparatively higher than that for DAC. One of the reasons for this type behaviour is due to highly negative surface charge in the presence of albumin on the alumina particle in comparison to DAC (Fig. 2). This in turn suggests that albumin acts as a better dispersant in comparison to
DAC, and that maximum dispersion in either case is achievable in the natural pH range of 8.5 – 9. 3.4. Solid loading vs. surface charge Fig. 5 shows the variation of surface charge as a function of solid loading in presence and absence of dispersant. Increasing the solid loading in absence of any dispersant from 1.47 to 50 vol.% changes the surface charge from +0.02 to 0.0098 C/g. It is clear from this curve that there is not appreciable change in surface charge with increasing solid loading, although an increase in pH value from 8 –9 has been observed. In the presence of dispersant DAC at optimum dosages (105 ppm), the surface charge remains constant with increasing solid loading. It has been observed that pH also does not vary and remains constant at 8.7. However, the magnitude of surface charge in presence of DAC is shifted towards more negative value. Similarly, in the presence of albumin, although there is no variation in surface charge with increasing solids loading, the magnitude of negative surface charge is significantly higher in comparison to the magnitude of change with DAC. It may be concluded from this investigation that solid loading has no effect on optimum dosages of
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B.P. Singh et al. / Materials Letters 56 (2002) 475–480
Fig. 5. Effect of solid loading on surface charge of alumina powder.
chemical additives is reproducible at higher solids loading too. Therefore, one can have first hand information at lower solid loading, which will be valid at higher solid loading also.
(5) Albumin is more effective as dispersant than DAC, that is further corroborated by comparing 0 specific interaction energy (DGSP ) which is higher in magnitude in the case of albumin (7.5) than DAC (3.6).
4. Conclusions The following conclusions have been drawn on the basis of this investigation. (1) Surface charge measurement is one of the most effective ways to describe the state of ceramic powder in aqueous suspension. (2) The iep of alumina powder without dispersant is around pH 8.5. The iep of alumina powder shifted to more acidic pH (5.5 for albumin and 4.5 for DAC) with addition of anionic polyelectrolyte. The surface is negatively charged at a wide range of pH>4.5. Both polyelectrolytes change the surface charge appreciably. (3) Addition of small amounts of both anionic polyelectrolytes (albumin and DAC) increase the negative surface charge significantly, but the change is more pronounced with albumin. (4) The surface charge remains more or less constant at all conditions of solid loading. Therefore, optimisation of dispersant dosages at lower solidloading holds good for higher solid loading too.
Acknowledgements The authors are thankful to the Director of the Regional Research Laboratory, Bhubaneswar, for his kind permission to publish this paper. The authors are also thankful to Prof. Parag Bhargav, IIT and Kharagpur for fruitful discussion.
References [1] M.A. Janney, Attaining high solids in ceramic slurries, website, http://www.ornl.gov, 6/8/01. [2] B.P. Singh, L. Besra, S. Bhattacharjee, Colloids Surf. A: Physicochem. Eng. Aspects (Comm.). [3] B.P. Singh, S. Bhattacharjee, L. Besra, Ceram. Int. (Comm.). [4] Pradip, Trans. IIM 41 (1988) 15. [5] R.M. Anklekar, S.A. Borkar, S. Bhattacharjee, C.H. Page, A.K. Chatterjee, Colloids Surf. A: Physicochem. Eng. Aspects 133 (1998) 41. [6] S.G. Malghan, R.S. Premachandran, P.T. Pei, Powder Technol. 79 (1994) 43.