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Dispersion of Al2O3 suspension with acrylic copolymers bearing carboxylic groups
Colloids and Surfaces A: Physicochem. Eng. Aspects 212 (2003) 271 /283 www.elsevier.com/locate/colsurfa
Dispersion of Al2O3 suspension with acrylic copolymers bearing carboxylic groups M. Ramzi Ben Romdhane a, Sami Boufi b, Samir Baklouti a, Thierry Chartier c, , Jean-Franc¸ois Baumard c a Laboratoire de Chimie Industrielle, Equipe Ce´ramique, ENIS, BP W 3038 Sfax, Tunisia Laboratoire de Sciences des Mate´riaux et Environnement, Faculte´ des Sciences de Sfax, BP 802-3018 Sfax, Tunisia c Laboratoire de Science des Proce´de´s Ce´ramiques et de Traitements de Surface, SPCTS, UMR CNRS 6638, 47 Avenue Albert Thomas, 87065 Limoges Cedex, France b
Received 15 March 2002; accepted 2 July 2002
Abstract The interaction between acrylic copolymers carrying carboxylic groups and the surface of an alumina powder in water has been studied. The trends of the adsorption isotherms suggest the formation of a monolayer according to the Langmuir model. The quantity absorbed on the plateau increases when the fraction of carboxylic groups on the polymer decreases. The electrokinetic properties of the alumina suspensions were also analysed. Results indicate that the copolymer adsorption leads to a shift in the IEP towards acidic pH. Moreover, the density of charge of the surface of alumina particle at pH 8 /9 was found to depend on the quantity of adsorbed carboxylic groups and on the configuration of the adsorbed polymer. The stability of dispersions was investigated through particle size distribution and rheological properties after addition of various amounts of copolymers. The ability of the acrylic copolymer to stabilise alumina suspensions was found to be greatly affected by the content of carboxylic groups contained in the macromolecular chain. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Dispersion; Copolymers; Carboxylic groups; Alumina
1. Introduction Polymer adsorption at solid/liquid interfaces continues to receive considerable attention nowadays [1 /4]. Many processes in ceramic, paint, wastewater treatment technologies are closely
Corresponding author. Tel.: /33-5-5545-2222; fax: /33-55579-0998 E-mail address: [email protected] (T. Chartier).
connected to adsorption phenomena. During ceramic processing, various polymeric additives are used to confer desired properties to the media [5 /9]. Quite often, polyelectrolytes are added to the suspension to get a homogenous dispersion of the powder in the liquid phase, thus leading to both high solid loadings and low viscosities [10 / 13]. After adsorption onto the surface of the powder, polyelectrolytes create an excess of charge which results in the formation of an electrical
0927-7757/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 2 ) 0 0 3 2 7 - 8
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double layer around each particle. Interactions between double layers result in net repulsive electrostatic forces, that prevent the particles coming physically close enough for the attractive force active at small distances to cause flocculation [14,15]. In addition to this deflocculant, other polymers, which are generally water soluble and non-ionic, are also used as binders to enhance the green strength of pressed parts [16]. Among the products which prove to be convenient for this purpose, polyvinyl alcohol (PVA) is often employed for oxide powders [17,18]. However, the presence of several polymers in the same system may induce a competitive adsorption, which can in turn affect rheological properties and dispersion stability [19,20]. Indeed, the macromolecular chain of PVA only contains hydrocarbon and hydroxyl groups, the interaction of which with the oxide particle surfaces proceeds via dispersive forces and hydrogen bonding. Such interactions are weak compared with electrostatic interactions which are typically encountered for polyelectrolytes. This is the reason why the adsorption of PVA is limited in practice. As a consequence, a large amount of the binder remains in solution, where it contributes to increase the suspension viscosity, depending on the molecular weight of the polymer selected [21,22]. To circumvent this problem, Table 1 The three polyelectrolytes used
incorporation of a polyelectrolyte to get a high state of dispersion and of a binder to confer a sufficient cohesion in the green state within the same macromolecule chain could be a solution. In this context, the effect of copolymers bearing carboxylate and hydroxyl groups during the preparation of aqueous a-Al2O3 suspensions was studied. In the present work, we attempted to clarify the influence of the hydroxyl fraction on the polymer adsorption, and its effect on the stability and on the rheological properties of the suspensions. The information thus obtained will hopefully help for the design of a polymer suitable to substitute both the dispersant and the binder.
2. Experimental section 2.1. Starting materials An a-Al2O3 powder (P172SB, Pe´chiney-France) was used in all experiments. This powder was characterised by a mean diameter of 0.35 mm and a specific surface area (BET) of 10 m2 g 1. Four polyelectrolytes were used in this work. The first one is a commercial ammonium salt of polymethacrylic acid (Darvan-C† , Vanderbilt, UK). The three others (EH2A20, EH2A35 and EH2A55) are copolymers of ammonium acrylate and of 2-hydroxyethyl acrylate. All were prepared in the laboratory. The structure of the polyelectrolytes used in the present work and the corresponding molecular weights are presented in Table 1. 2.2. Copolymer synthesis
EH2A20, EH2A35 and EH2A55 are synthetised. PMA-NH4 is a commercial polymer (Darvan-C† , Vanderbilt, UK). a COO % is the molar fraction of carboxylic groups in the copolymer. bThe molecular weight (Mw) is determined by viscometry.
The copolymers were prepared by radical copolymerisation of ammonium acrylate and of 2hydroxyethyl acrylate. Copolymerisation was carried out in aqueous medium at 60 8C during 2 h with potassium persulfate (K2S2O8) as initiator and sodium metabisulfite (K2S2O5) as activator. After precipitation in acetone, the copolymer was isolated, dried under a nitrogen atmosphere, then dissolved in water to provide a concentrated solution. Owing to the structural similarity of the two monomers used, e.g. ammonium acrylate and
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Fig. 1. Adsorption isotherms of the various polyelectrolytes onto alumina as a function of initial amount of polyelectrolyte. The solid lines are fits based on the Langmuir adsorption model as explained in the text (pH 8.5 /9).
2-hydroxyethyl acrylate, the corresponding units are randomly distributed along the macromolecular chain [23]. The content of carboxylate groups / COO in the polymer was determined by colloidal titration [24] with a cationic polyelectrolyte (poly(chloride N ,N ,N -trimethyl ammonium ethyl acrylate) ‘PAD-Cl ’) studied in an early work [25]. 2.3. Adsorption isotherms Experiments concerning the adsorption of polymers were performed by adding given amounts of the polymer solution, at the pH required, to a 10 wt.% aqueous Al2O3 suspension. The pH adjustments were made using NaOH or HCl solutions. Then suspensions were sonicated for 15 mn under an output power of 200 W. Samples were then shaked for 24 h to reach adsorption equilibrium. The suspensions were centrifugated at 3000 rpm during 1 h. The supernatant was removed, and the amount of free polyelectrolyte in the solution was
determined by a colloid titration technique using the cationic PAD-Cl polymer and orthotoluidine blue as indicator [23]. This colloidal titration is based on the fact that oppositely charged polyelectrolytes form 1:1 complexes in a medium possessing a low ionic strength. The point of charge equivalence is determined by colour change of an appropriate indicator, as orthotoluidine blue in the present case. In cationic systems, this indicator become pink due to the formation of a complex with the anionic polymer. All titrations were here carried out under conditions of low ionic strength (lower than 10 3), obtained by dilution of the supernatant sample with distilled water. Blank tests conducted on PMA-NH4 and other commercial acrylic copolymers, made of acrylamide and N ,N ,N trimethyl ammonium ethyl acrylate chloride, have confirmed the validity of the method. Tests conducted on solution with known concentration have indicated that colloidal titration is sensitive up to 1 ppm (1 mg l 1).
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3. Results and discussion 3.1. Adsorption isotherms Adsorption isotherms at pH 8.5 /9 for PMANH4, EH2A20, EH2A35 and EH2A55 copolymers on Al2O3 particles are presented in Fig. 1. In all cases, the amount adsorbed increases as the polymer concentration increases, until a plateau was reached. The shape of these isotherms suggests a monolayer adsorption of the polyelectrolyte, independently of the polymer composition. Whatever the polyelectrolyte, the adsorption is complete for polyelectrolyte concentrations as small as 0.5 wt.%, as no polymer is detected in the supernatant until the plateau was reached. Such an observation indicates a high affinity type adsorption. The amount of adsorbed polyelectrolyte on the plateau decreases as the content of carboxylic groups in the polymer increases. The quantities adsorbed are about 0.36, 0.59, 0.72 and 1.1 mg m 2 for PMANH4, EH2A55, EH2A35 and EH2A20, respectively. Such values correspond to respective adsorbed surface molar amounts of 4.27 /10 3, 5.7 /10 3, 6.64 /10 3 and 9.85 /10 3 mol m 2 of polymer. In the case of PMA-NH4, the quantity adsorbed once the plateau was reached, i.e 0.36 mg m 2, (pH 9) is in agreement with that reported by Cesarano [26]. At pH 9, PMA-NH4 is fully ionised, and its adsorption on the surface of alumina proceeds in a relatively flat configuration [26]. However, the increase of the quantity adsorbed on the plateau as the carboxyl fraction group was reduced may be explained by two reasons. First a lower repulsion between ionisable groups allows a denser packing of the macromolecular chains on the alumina surface, secondly the formation of loops in the adsorbed state is expected to increase as the fraction of hydroxyethyl acrylate units in the macromolecular chain itself increases. Then, the surface area occupied per molecule diminishes, and more chains are required to get a saturated monolayer. The second hypothesis is privileged, since with non ionic hydroxyethyl acrylate segments the driving force for adsorption on the solid surfaces comes from hydrogen bonding, which is generally
Fig. 2. Langmuir adsorption isotherms of polymers PMANH4 , EH2A55, EH2A35 and EH2A20 onto alumina particles.
considered as weak compared with the strong interaction taking place with /COO carboxyl groups. In the pH range between 8 and 9, electrostatic interaction is strong with alumina surface, through hydrogen bonding and chemisorption resulting from an exchange process between surface hydroxyl groups and /COO , that leads to the formation of Al /O /COO complexes [27]. According to Hidber [19], polymers bearing hydroxyl groups such as PVA are weakly adsorbed on the surface of alumina. Moreover, co-adsorption studies in binary systems containing polyacrylic acid and PVA revealed that the adsorption of PVA was significantly reduced by the presence of poly(acrylic acid). In the coil-like configuration of copolymer EH2A20, EH2A35 and EH2A55, loops are mainly formed of nonionic hydroxyethyl acrylate units, whereas carboxylic units act as anchoring groups. For the systems considered, the Langmuir model was found to be applicable in interpreting polyelectrolyte adsorption onto Al2O3 particles. The Langmuir adsorption isotherm is expressed by [28]: Cads
KCmax Csol (1 KCsol )
(1)
where Csol is the equilibrium concentration of the polymer in solution (mol l 1), Cmax the amount of
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Table 2 Maximum values of adsorption (Cmax determined from adsorption isotherm and C max determined from the Langmuir model) and adsorption equilibrium constant K for PMA-NH4 and the three EH2A copolymers Polymer
Cmax (mg m 2)
C max (mg m 2)
K (l mg 1)
K /10 6 (l mol 1)
PMA-NH4 EH2A55 EH2A35 EH2A20
0.38 0.59 0.72 1.1
0.37 0.57 0.71 1.05
486 20.1 19.9 16.7
435 2.1 2 1.8
adsorbed polymer for a monolayer coverage (mg m 2) and K a constant (l mol 1). This equation may be rewritten under a linearised form: Csol Cads
Csol Cmax
1 KCmax
(2)
The linearity of the variation of Csol/Cads in function of the added polymer for PMA-NH4, EH2A20, EH2A35, and EH2A55, shown in Fig. 2, confirms that the Langmuir model is an adequate choice. The adsorption constants K , as well as the concentrations of polymer adsorbed on the plateau, were calculated and the data obtained are
reported in Table 2. Large values of K , that confirm again the high affinity of polyelectrolytes with respect to alumina particles surface, suggest that the adsorption process is dominant with respect to desorption. Agreement of the adsorption characteristics with predictions made from the Langmuir model gives evidence of a monolayer polymer adsorption. As the pH is reduced, a significant increase of the amount adsorbed was observed regardless of the carboxyl fraction in the polymer Fig. 3. This phenomenon results probably from two effects when the pH decreases, viz. (i) a larger density of positive surface charges that contributes to anchor
Fig. 3. Adsorption isotherms for EH2A copolymers onto alumina vs. pH of suspension.
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Fig. 4. Electrokinetic properties of the suspension after addition of PMA-NH4 .
Fig. 5. Electrokinetic properties of the suspension after addition of EH2A20.
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more carboxylic groups and, (ii) a lower fraction of dissociated carboxyl groups that reduces the global charge of the polyelectrolyte chain, and lowers the electrostatic interaction between the surface and the polymer, and thus favours adsorption in a more coiled conformation.
3.2. Electrokinetic behaviour The effect of the addition of the different polyelectrolytes on the electrokinetic properties of alumina suspensions is depicted on Figs. 4 /6. Addition of PMA-NH4 shifts the IEP towards acidic pH. Beyond contents of 0.35 wt.%, the IEP stabilises close to 2.6 and no more evolution was observed. This feature is in agreement with the results of other studies [29,30]. After addition of polymers EH2A20 or EH2A55, the same trend was observed. However the asymptotic value of the IEP value is larger, and it amounts to four and three respectively for the two copolymers. Such values are obtained after addition of about 2 wt.% EH2A20 and 1 wt.% EH2A55. Below the IEP, positive ESA values indicate that particles are charged positively in the suspension.
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After addition of a polymer (PMA-NH4, EH2A20 or EH2A55), the ESA magnitude was lowered. This is due to adsorption of the polymer, the negative charges of which contribute to neutralise positive sites on the surface of particles, and then to decrease the net positive surface charge (Figs. 4/6). On the other hand, beyond the IEP, negative ESA values result from the effect of both AlO negative surface sites and adsorbed carboxylic groups of the polymer which are dissociated to a large extent above this pH. At pH 8.5 /9, after a threshold of polymer addition (about 0.35, 0.5 and 1 wt.% of PMA-NH4, EH2A55 and EH2A20), no more evolution of the ESA signal amplitude was observed, that suggests that the surface of alumina particles becomes saturated. This is in agreement with the maximum values of adsorption reported in Table 2. The magnitude (absolute value) of the ESA signal on the plateau is increasing from EH2A20, EH2A55 to PMA-NH4, then as the fraction of carboxylic groups is increasing. In order to investigate the reasons of this tendency, the evolution of the ESA signal at pH 8.5 /9 was plotted as
Fig. 6. Electrokinetic properties of the suspension after addition of EH2A55.
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carboxylic groups bound to the surface does not change. Nevertheless, the ESA amplitude decreases significantly as the fraction of hydroxyethyl acrylate units raises. It is likely that the loops of the polymer extending in the liquid result in the disruption and outward movement of the shear plane, the modified shear plane exhibiting a lower potential. The phenomenon will be enhanced if the thickness of the adsorbed layer increases. This result further reinforces the hypothesis of a coil-like conformation of the polymer adsorbed on the surface of alumina in the pH range 8.5 /9. 3.3. Particle size distribution
Fig. 7. Evolution of the ESA signal as a function of the adsorbed carboxylic amount (pH 8.5 /9).
a function of the amount of carboxylic groups bound to the surface for each polymer (Fig. 7), as determined from adsorption data. If the polymer configuration is not taken into account, almost the same ESA value is expected as the amount of
In order to evaluate the effect of polyelectrolyte addition on Al2O3 powder dispersion, the particle size distribution in suspensions containing the dispersants synthetised was analysed by means of an X-ray sedimentation technique. The cumulative grain size distributions obtained after addition of acrylic polyelectrolytes are reported on Figs. 8 and 9. PMA-NH4 is known to be an efficient dispersant of a-Al2O3. The best state of dispersion for
Fig. 8. Grain size distribution of alumina after addition of polymer EH2A20.
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Fig. 9. Grain size distribution of alumina after addition of polymer EH2A55.
P172 SB alumina was obtained after addition of an amount of 0.2 wt.% of dispersant with respect to the dry powder [17]. In absence of polymer, Al2O3 particles flocculate, as the pH of the native suspension, e.g. 8.5, is close to the IEP [31].
Addition of EH2A20 improves the dispersion state of the suspension (Fig. 8). However, some agglomeration still takes place even if the amount of added polymer is increased up to 2 wt.%. According to the results depicted in Fig. 5, the polymer
Fig. 10. Influence of the concentration of dispersant (PMA-NH4 ) on the viscosity of alumina suspension (27 vol.%, pH 8.5 /9).
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Fig. 11. Influence of the concentration of dispersant (EH2A55) on the viscosity of alumina suspension (27 vol.%, pH 8.5 /9).
Fig. 12. Influence of the concentration of dispersant (EH2A20) on the viscosity of alumina suspension (27 vol.%, pH 8.5 /9).
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Fig. 13. Variation of the viscosity of the alumina suspension versus concentration of the copolymer dispersant at a shear rate of 22 s 1.
EH2A20 does not generate a sufficient surface charge density to prevent solid particles from agglomeration. On the other hand, EH2A55 is a more efficient dispersant (Fig. 9). A state of dispersion similar to that obtained with 0.2 wt.% PMA-NH4 was attained after addition of about 0.5 wt.% EH2A55. This indicates that an optimum content of carboxylic groups in the copolymer EH2A is required to get a good state of dispersion and to prevent alumina particles from flocculation.
3.4. Rheological properties Viscosity measurements were performed to evaluate the state of dispersion of the suspensions prepared with the various copolymers. Figs. 10/12 report the flow curves for slurries containing 27 vol.% Al2O3 after addition of various quantities of polymer. In absence of any polyelectrolyte, a shear thinning behaviour was observed, that results from flocculation of alumina particles. Entrapped liquid within the agglomerates results in a larger appar-
ent volume fraction of solid, that is responsible for the large viscosity of the suspension [32 /34]. Addition of 0.27 wt.% PMA-NH4 induces an abrupt change from shear thinning to Newtonian behaviour (Fig. 10). The viscosity becomes low (h /13 mPa s 1) and almost independent of the shear rate. In presence of EH2A55, a similar trends was observed with a Newtonian-like behaviour for contents of polymer larger than 0.6 wt.%, with a viscosity of about 16 mPa s 1 (Fig. 11). The Newtonian behaviour of suspensions prepared with addition of PMA-NH4 and EH2A55 is characteristics of a stable system, already suggested by the large negative zeta potential resulting from the polymer adsorption, that allows to the particles in the suspension to move independently. On the other hand, with the copolymer containing a large content of hydroxylethyl acrylate moieties (EH2A20), the suspension exhibits a significant shear thinning behaviour with a yield stress whatever the amount of added EH2A20 (Fig. 12). In this case, the charge density on the surface of alumina is not large enough, even at pH
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9, to maintain repulsion (Figs. 5 and 6) and some interactions between particle likely lead to a yield stress. This is in agreement with the results of the analysis of particle size distribution (Fig. 9). Evolution of the viscosity under a steady shear rate of 22 s 1 versus amount of polymer added is presented in Fig. 13. As reported previously, the viscosity first decreases to reach a minimum at a given amount of dispersant. The optimum concentrations are about 0.27 and 0.6 wt.% for PMANH4 and EH2A55, respectively. According to the adsorption data plotted in Fig. 1, these concentrations closely correspond to the saturation of the particle surface. The viscosity does not seem to be strongly affected by the addition of EH2A20 and one does not observe any clear optimum. Two remarks can be pointed out from inspection of Fig. 13, (i) the minimum of viscosity is shifted towards larger polymer concentrations as the fraction of hydroxylethyl acrylate groups is increasing or as the fraction of carboxylic groups is decreasing. This can be attributed to the loop-tail conformation of the polymer, that must be compensated by a larger polymer content for an equivalent surface
coverage and/or to a lower negative charge bringing by carboxylic groups and, (ii) the minimum value of the viscosity increases from PMA-NH4 to EH2A55 and EH2A20. First, the density of charge increases from EH2A20 to EH2A55 and PMA-NH4 (Fig. 7) and, secondly a larger fraction of hydroxyethyl acrylate groups in the macromolecular chain induces a thicker layer of adsorbed polymer, that in turn may increase electroviscous effect due to particle double layer overlap [35]. In presence of EH2A20, the large viscosity and the poor state of dispersion (Figs. 9 and 12) suggest that poly (hydroxyethyl acrylate) segments do not contribute efficiently to steric stabilisation. Study is in progress to get a better understanding of the effect of the non-ionisable groups in the copolymer structure on the stabilisation process. Finally, the efficiency of the EH2A55 copolymer containing 55% of carboxylic groups is close to that of the classical PMA-NH4 dispersant, as shown by the evolution of the viscosity of alumina suspensions in function of the powder loading (Fig. 14).
Fig. 14. Variation of the viscosity of alumina suspension in function of the powder loading for 1.25 wt.% EH2A20, 0.6 wt.% EH2A35 and 0.2 wt.% PMA-NH4 (alumina powder basis).
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4. Conclusion Acrylic copolymers prove to be efficient dispersants for aqueous suspensions of alumina when the fraction of carboxylic groups is larger than 50% within the macromolecular chain. Adsorption of acrylic copolymers takes place due to carboxylic groups which anchor the polymer chain on specific surface sites, namely Al /(OH2). However, the hydroxyethyl acrylate moieties promote a coil-like adsorbed configuration, the thickness of which increases with the content of such moieties. Moreover, the amount of carboxylic groups controls the effective charge density of the particle surface and the copolymer aptitude to be used as a dispersant. Addition of a copolymer containing 55% of carboxylic groups, at an optimum amount of 0.6 wt.%, leads to stable suspensions which display a Newtonian behaviour with a fairly low value of viscosity (16 mPa s 1). Hydroxyl groups in the copolymer are expected to act as a binder after forming of alumina by drypressing. The effect of these copolymers on the cohesion and mechanical properties of green products is presently under investigation and the results will be the subject of a forthcoming paper.
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Dispersion of Al2O3 suspension with acrylic copolymers bearing carboxylic groups M. Ramzi Ben Romdhane a, Sami Boufi b, Samir Baklouti a, Thierry Chartier c, , Jean-Franc¸ois Baumard c a Laboratoire de Chimie Industrielle, Equipe Ce´ramique, ENIS, BP W 3038 Sfax, Tunisia Laboratoire de Sciences des Mate´riaux et Environnement, Faculte´ des Sciences de Sfax, BP 802-3018 Sfax, Tunisia c Laboratoire de Science des Proce´de´s Ce´ramiques et de Traitements de Surface, SPCTS, UMR CNRS 6638, 47 Avenue Albert Thomas, 87065 Limoges Cedex, France b
Received 15 March 2002; accepted 2 July 2002
Abstract The interaction between acrylic copolymers carrying carboxylic groups and the surface of an alumina powder in water has been studied. The trends of the adsorption isotherms suggest the formation of a monolayer according to the Langmuir model. The quantity absorbed on the plateau increases when the fraction of carboxylic groups on the polymer decreases. The electrokinetic properties of the alumina suspensions were also analysed. Results indicate that the copolymer adsorption leads to a shift in the IEP towards acidic pH. Moreover, the density of charge of the surface of alumina particle at pH 8 /9 was found to depend on the quantity of adsorbed carboxylic groups and on the configuration of the adsorbed polymer. The stability of dispersions was investigated through particle size distribution and rheological properties after addition of various amounts of copolymers. The ability of the acrylic copolymer to stabilise alumina suspensions was found to be greatly affected by the content of carboxylic groups contained in the macromolecular chain. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Dispersion; Copolymers; Carboxylic groups; Alumina
1. Introduction Polymer adsorption at solid/liquid interfaces continues to receive considerable attention nowadays [1 /4]. Many processes in ceramic, paint, wastewater treatment technologies are closely
Corresponding author. Tel.: /33-5-5545-2222; fax: /33-55579-0998 E-mail address: [email protected] (T. Chartier).
connected to adsorption phenomena. During ceramic processing, various polymeric additives are used to confer desired properties to the media [5 /9]. Quite often, polyelectrolytes are added to the suspension to get a homogenous dispersion of the powder in the liquid phase, thus leading to both high solid loadings and low viscosities [10 / 13]. After adsorption onto the surface of the powder, polyelectrolytes create an excess of charge which results in the formation of an electrical
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double layer around each particle. Interactions between double layers result in net repulsive electrostatic forces, that prevent the particles coming physically close enough for the attractive force active at small distances to cause flocculation [14,15]. In addition to this deflocculant, other polymers, which are generally water soluble and non-ionic, are also used as binders to enhance the green strength of pressed parts [16]. Among the products which prove to be convenient for this purpose, polyvinyl alcohol (PVA) is often employed for oxide powders [17,18]. However, the presence of several polymers in the same system may induce a competitive adsorption, which can in turn affect rheological properties and dispersion stability [19,20]. Indeed, the macromolecular chain of PVA only contains hydrocarbon and hydroxyl groups, the interaction of which with the oxide particle surfaces proceeds via dispersive forces and hydrogen bonding. Such interactions are weak compared with electrostatic interactions which are typically encountered for polyelectrolytes. This is the reason why the adsorption of PVA is limited in practice. As a consequence, a large amount of the binder remains in solution, where it contributes to increase the suspension viscosity, depending on the molecular weight of the polymer selected [21,22]. To circumvent this problem, Table 1 The three polyelectrolytes used
incorporation of a polyelectrolyte to get a high state of dispersion and of a binder to confer a sufficient cohesion in the green state within the same macromolecule chain could be a solution. In this context, the effect of copolymers bearing carboxylate and hydroxyl groups during the preparation of aqueous a-Al2O3 suspensions was studied. In the present work, we attempted to clarify the influence of the hydroxyl fraction on the polymer adsorption, and its effect on the stability and on the rheological properties of the suspensions. The information thus obtained will hopefully help for the design of a polymer suitable to substitute both the dispersant and the binder.
2. Experimental section 2.1. Starting materials An a-Al2O3 powder (P172SB, Pe´chiney-France) was used in all experiments. This powder was characterised by a mean diameter of 0.35 mm and a specific surface area (BET) of 10 m2 g 1. Four polyelectrolytes were used in this work. The first one is a commercial ammonium salt of polymethacrylic acid (Darvan-C† , Vanderbilt, UK). The three others (EH2A20, EH2A35 and EH2A55) are copolymers of ammonium acrylate and of 2-hydroxyethyl acrylate. All were prepared in the laboratory. The structure of the polyelectrolytes used in the present work and the corresponding molecular weights are presented in Table 1. 2.2. Copolymer synthesis
EH2A20, EH2A35 and EH2A55 are synthetised. PMA-NH4 is a commercial polymer (Darvan-C† , Vanderbilt, UK). a COO % is the molar fraction of carboxylic groups in the copolymer. bThe molecular weight (Mw) is determined by viscometry.
The copolymers were prepared by radical copolymerisation of ammonium acrylate and of 2hydroxyethyl acrylate. Copolymerisation was carried out in aqueous medium at 60 8C during 2 h with potassium persulfate (K2S2O8) as initiator and sodium metabisulfite (K2S2O5) as activator. After precipitation in acetone, the copolymer was isolated, dried under a nitrogen atmosphere, then dissolved in water to provide a concentrated solution. Owing to the structural similarity of the two monomers used, e.g. ammonium acrylate and
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Fig. 1. Adsorption isotherms of the various polyelectrolytes onto alumina as a function of initial amount of polyelectrolyte. The solid lines are fits based on the Langmuir adsorption model as explained in the text (pH 8.5 /9).
2-hydroxyethyl acrylate, the corresponding units are randomly distributed along the macromolecular chain [23]. The content of carboxylate groups / COO in the polymer was determined by colloidal titration [24] with a cationic polyelectrolyte (poly(chloride N ,N ,N -trimethyl ammonium ethyl acrylate) ‘PAD-Cl ’) studied in an early work [25]. 2.3. Adsorption isotherms Experiments concerning the adsorption of polymers were performed by adding given amounts of the polymer solution, at the pH required, to a 10 wt.% aqueous Al2O3 suspension. The pH adjustments were made using NaOH or HCl solutions. Then suspensions were sonicated for 15 mn under an output power of 200 W. Samples were then shaked for 24 h to reach adsorption equilibrium. The suspensions were centrifugated at 3000 rpm during 1 h. The supernatant was removed, and the amount of free polyelectrolyte in the solution was
determined by a colloid titration technique using the cationic PAD-Cl polymer and orthotoluidine blue as indicator [23]. This colloidal titration is based on the fact that oppositely charged polyelectrolytes form 1:1 complexes in a medium possessing a low ionic strength. The point of charge equivalence is determined by colour change of an appropriate indicator, as orthotoluidine blue in the present case. In cationic systems, this indicator become pink due to the formation of a complex with the anionic polymer. All titrations were here carried out under conditions of low ionic strength (lower than 10 3), obtained by dilution of the supernatant sample with distilled water. Blank tests conducted on PMA-NH4 and other commercial acrylic copolymers, made of acrylamide and N ,N ,N trimethyl ammonium ethyl acrylate chloride, have confirmed the validity of the method. Tests conducted on solution with known concentration have indicated that colloidal titration is sensitive up to 1 ppm (1 mg l 1).
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3. Results and discussion 3.1. Adsorption isotherms Adsorption isotherms at pH 8.5 /9 for PMANH4, EH2A20, EH2A35 and EH2A55 copolymers on Al2O3 particles are presented in Fig. 1. In all cases, the amount adsorbed increases as the polymer concentration increases, until a plateau was reached. The shape of these isotherms suggests a monolayer adsorption of the polyelectrolyte, independently of the polymer composition. Whatever the polyelectrolyte, the adsorption is complete for polyelectrolyte concentrations as small as 0.5 wt.%, as no polymer is detected in the supernatant until the plateau was reached. Such an observation indicates a high affinity type adsorption. The amount of adsorbed polyelectrolyte on the plateau decreases as the content of carboxylic groups in the polymer increases. The quantities adsorbed are about 0.36, 0.59, 0.72 and 1.1 mg m 2 for PMANH4, EH2A55, EH2A35 and EH2A20, respectively. Such values correspond to respective adsorbed surface molar amounts of 4.27 /10 3, 5.7 /10 3, 6.64 /10 3 and 9.85 /10 3 mol m 2 of polymer. In the case of PMA-NH4, the quantity adsorbed once the plateau was reached, i.e 0.36 mg m 2, (pH 9) is in agreement with that reported by Cesarano [26]. At pH 9, PMA-NH4 is fully ionised, and its adsorption on the surface of alumina proceeds in a relatively flat configuration [26]. However, the increase of the quantity adsorbed on the plateau as the carboxyl fraction group was reduced may be explained by two reasons. First a lower repulsion between ionisable groups allows a denser packing of the macromolecular chains on the alumina surface, secondly the formation of loops in the adsorbed state is expected to increase as the fraction of hydroxyethyl acrylate units in the macromolecular chain itself increases. Then, the surface area occupied per molecule diminishes, and more chains are required to get a saturated monolayer. The second hypothesis is privileged, since with non ionic hydroxyethyl acrylate segments the driving force for adsorption on the solid surfaces comes from hydrogen bonding, which is generally
Fig. 2. Langmuir adsorption isotherms of polymers PMANH4 , EH2A55, EH2A35 and EH2A20 onto alumina particles.
considered as weak compared with the strong interaction taking place with /COO carboxyl groups. In the pH range between 8 and 9, electrostatic interaction is strong with alumina surface, through hydrogen bonding and chemisorption resulting from an exchange process between surface hydroxyl groups and /COO , that leads to the formation of Al /O /COO complexes [27]. According to Hidber [19], polymers bearing hydroxyl groups such as PVA are weakly adsorbed on the surface of alumina. Moreover, co-adsorption studies in binary systems containing polyacrylic acid and PVA revealed that the adsorption of PVA was significantly reduced by the presence of poly(acrylic acid). In the coil-like configuration of copolymer EH2A20, EH2A35 and EH2A55, loops are mainly formed of nonionic hydroxyethyl acrylate units, whereas carboxylic units act as anchoring groups. For the systems considered, the Langmuir model was found to be applicable in interpreting polyelectrolyte adsorption onto Al2O3 particles. The Langmuir adsorption isotherm is expressed by [28]: Cads
KCmax Csol (1 KCsol )
(1)
where Csol is the equilibrium concentration of the polymer in solution (mol l 1), Cmax the amount of
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Table 2 Maximum values of adsorption (Cmax determined from adsorption isotherm and C max determined from the Langmuir model) and adsorption equilibrium constant K for PMA-NH4 and the three EH2A copolymers Polymer
Cmax (mg m 2)
C max (mg m 2)
K (l mg 1)
K /10 6 (l mol 1)
PMA-NH4 EH2A55 EH2A35 EH2A20
0.38 0.59 0.72 1.1
0.37 0.57 0.71 1.05
486 20.1 19.9 16.7
435 2.1 2 1.8
adsorbed polymer for a monolayer coverage (mg m 2) and K a constant (l mol 1). This equation may be rewritten under a linearised form: Csol Cads
Csol Cmax
1 KCmax
(2)
The linearity of the variation of Csol/Cads in function of the added polymer for PMA-NH4, EH2A20, EH2A35, and EH2A55, shown in Fig. 2, confirms that the Langmuir model is an adequate choice. The adsorption constants K , as well as the concentrations of polymer adsorbed on the plateau, were calculated and the data obtained are
reported in Table 2. Large values of K , that confirm again the high affinity of polyelectrolytes with respect to alumina particles surface, suggest that the adsorption process is dominant with respect to desorption. Agreement of the adsorption characteristics with predictions made from the Langmuir model gives evidence of a monolayer polymer adsorption. As the pH is reduced, a significant increase of the amount adsorbed was observed regardless of the carboxyl fraction in the polymer Fig. 3. This phenomenon results probably from two effects when the pH decreases, viz. (i) a larger density of positive surface charges that contributes to anchor
Fig. 3. Adsorption isotherms for EH2A copolymers onto alumina vs. pH of suspension.
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Fig. 4. Electrokinetic properties of the suspension after addition of PMA-NH4 .
Fig. 5. Electrokinetic properties of the suspension after addition of EH2A20.
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more carboxylic groups and, (ii) a lower fraction of dissociated carboxyl groups that reduces the global charge of the polyelectrolyte chain, and lowers the electrostatic interaction between the surface and the polymer, and thus favours adsorption in a more coiled conformation.
3.2. Electrokinetic behaviour The effect of the addition of the different polyelectrolytes on the electrokinetic properties of alumina suspensions is depicted on Figs. 4 /6. Addition of PMA-NH4 shifts the IEP towards acidic pH. Beyond contents of 0.35 wt.%, the IEP stabilises close to 2.6 and no more evolution was observed. This feature is in agreement with the results of other studies [29,30]. After addition of polymers EH2A20 or EH2A55, the same trend was observed. However the asymptotic value of the IEP value is larger, and it amounts to four and three respectively for the two copolymers. Such values are obtained after addition of about 2 wt.% EH2A20 and 1 wt.% EH2A55. Below the IEP, positive ESA values indicate that particles are charged positively in the suspension.
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After addition of a polymer (PMA-NH4, EH2A20 or EH2A55), the ESA magnitude was lowered. This is due to adsorption of the polymer, the negative charges of which contribute to neutralise positive sites on the surface of particles, and then to decrease the net positive surface charge (Figs. 4/6). On the other hand, beyond the IEP, negative ESA values result from the effect of both AlO negative surface sites and adsorbed carboxylic groups of the polymer which are dissociated to a large extent above this pH. At pH 8.5 /9, after a threshold of polymer addition (about 0.35, 0.5 and 1 wt.% of PMA-NH4, EH2A55 and EH2A20), no more evolution of the ESA signal amplitude was observed, that suggests that the surface of alumina particles becomes saturated. This is in agreement with the maximum values of adsorption reported in Table 2. The magnitude (absolute value) of the ESA signal on the plateau is increasing from EH2A20, EH2A55 to PMA-NH4, then as the fraction of carboxylic groups is increasing. In order to investigate the reasons of this tendency, the evolution of the ESA signal at pH 8.5 /9 was plotted as
Fig. 6. Electrokinetic properties of the suspension after addition of EH2A55.
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carboxylic groups bound to the surface does not change. Nevertheless, the ESA amplitude decreases significantly as the fraction of hydroxyethyl acrylate units raises. It is likely that the loops of the polymer extending in the liquid result in the disruption and outward movement of the shear plane, the modified shear plane exhibiting a lower potential. The phenomenon will be enhanced if the thickness of the adsorbed layer increases. This result further reinforces the hypothesis of a coil-like conformation of the polymer adsorbed on the surface of alumina in the pH range 8.5 /9. 3.3. Particle size distribution
Fig. 7. Evolution of the ESA signal as a function of the adsorbed carboxylic amount (pH 8.5 /9).
a function of the amount of carboxylic groups bound to the surface for each polymer (Fig. 7), as determined from adsorption data. If the polymer configuration is not taken into account, almost the same ESA value is expected as the amount of
In order to evaluate the effect of polyelectrolyte addition on Al2O3 powder dispersion, the particle size distribution in suspensions containing the dispersants synthetised was analysed by means of an X-ray sedimentation technique. The cumulative grain size distributions obtained after addition of acrylic polyelectrolytes are reported on Figs. 8 and 9. PMA-NH4 is known to be an efficient dispersant of a-Al2O3. The best state of dispersion for
Fig. 8. Grain size distribution of alumina after addition of polymer EH2A20.
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Fig. 9. Grain size distribution of alumina after addition of polymer EH2A55.
P172 SB alumina was obtained after addition of an amount of 0.2 wt.% of dispersant with respect to the dry powder [17]. In absence of polymer, Al2O3 particles flocculate, as the pH of the native suspension, e.g. 8.5, is close to the IEP [31].
Addition of EH2A20 improves the dispersion state of the suspension (Fig. 8). However, some agglomeration still takes place even if the amount of added polymer is increased up to 2 wt.%. According to the results depicted in Fig. 5, the polymer
Fig. 10. Influence of the concentration of dispersant (PMA-NH4 ) on the viscosity of alumina suspension (27 vol.%, pH 8.5 /9).
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Fig. 11. Influence of the concentration of dispersant (EH2A55) on the viscosity of alumina suspension (27 vol.%, pH 8.5 /9).
Fig. 12. Influence of the concentration of dispersant (EH2A20) on the viscosity of alumina suspension (27 vol.%, pH 8.5 /9).
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Fig. 13. Variation of the viscosity of the alumina suspension versus concentration of the copolymer dispersant at a shear rate of 22 s 1.
EH2A20 does not generate a sufficient surface charge density to prevent solid particles from agglomeration. On the other hand, EH2A55 is a more efficient dispersant (Fig. 9). A state of dispersion similar to that obtained with 0.2 wt.% PMA-NH4 was attained after addition of about 0.5 wt.% EH2A55. This indicates that an optimum content of carboxylic groups in the copolymer EH2A is required to get a good state of dispersion and to prevent alumina particles from flocculation.
3.4. Rheological properties Viscosity measurements were performed to evaluate the state of dispersion of the suspensions prepared with the various copolymers. Figs. 10/12 report the flow curves for slurries containing 27 vol.% Al2O3 after addition of various quantities of polymer. In absence of any polyelectrolyte, a shear thinning behaviour was observed, that results from flocculation of alumina particles. Entrapped liquid within the agglomerates results in a larger appar-
ent volume fraction of solid, that is responsible for the large viscosity of the suspension [32 /34]. Addition of 0.27 wt.% PMA-NH4 induces an abrupt change from shear thinning to Newtonian behaviour (Fig. 10). The viscosity becomes low (h /13 mPa s 1) and almost independent of the shear rate. In presence of EH2A55, a similar trends was observed with a Newtonian-like behaviour for contents of polymer larger than 0.6 wt.%, with a viscosity of about 16 mPa s 1 (Fig. 11). The Newtonian behaviour of suspensions prepared with addition of PMA-NH4 and EH2A55 is characteristics of a stable system, already suggested by the large negative zeta potential resulting from the polymer adsorption, that allows to the particles in the suspension to move independently. On the other hand, with the copolymer containing a large content of hydroxylethyl acrylate moieties (EH2A20), the suspension exhibits a significant shear thinning behaviour with a yield stress whatever the amount of added EH2A20 (Fig. 12). In this case, the charge density on the surface of alumina is not large enough, even at pH
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9, to maintain repulsion (Figs. 5 and 6) and some interactions between particle likely lead to a yield stress. This is in agreement with the results of the analysis of particle size distribution (Fig. 9). Evolution of the viscosity under a steady shear rate of 22 s 1 versus amount of polymer added is presented in Fig. 13. As reported previously, the viscosity first decreases to reach a minimum at a given amount of dispersant. The optimum concentrations are about 0.27 and 0.6 wt.% for PMANH4 and EH2A55, respectively. According to the adsorption data plotted in Fig. 1, these concentrations closely correspond to the saturation of the particle surface. The viscosity does not seem to be strongly affected by the addition of EH2A20 and one does not observe any clear optimum. Two remarks can be pointed out from inspection of Fig. 13, (i) the minimum of viscosity is shifted towards larger polymer concentrations as the fraction of hydroxylethyl acrylate groups is increasing or as the fraction of carboxylic groups is decreasing. This can be attributed to the loop-tail conformation of the polymer, that must be compensated by a larger polymer content for an equivalent surface
coverage and/or to a lower negative charge bringing by carboxylic groups and, (ii) the minimum value of the viscosity increases from PMA-NH4 to EH2A55 and EH2A20. First, the density of charge increases from EH2A20 to EH2A55 and PMA-NH4 (Fig. 7) and, secondly a larger fraction of hydroxyethyl acrylate groups in the macromolecular chain induces a thicker layer of adsorbed polymer, that in turn may increase electroviscous effect due to particle double layer overlap [35]. In presence of EH2A20, the large viscosity and the poor state of dispersion (Figs. 9 and 12) suggest that poly (hydroxyethyl acrylate) segments do not contribute efficiently to steric stabilisation. Study is in progress to get a better understanding of the effect of the non-ionisable groups in the copolymer structure on the stabilisation process. Finally, the efficiency of the EH2A55 copolymer containing 55% of carboxylic groups is close to that of the classical PMA-NH4 dispersant, as shown by the evolution of the viscosity of alumina suspensions in function of the powder loading (Fig. 14).
Fig. 14. Variation of the viscosity of alumina suspension in function of the powder loading for 1.25 wt.% EH2A20, 0.6 wt.% EH2A35 and 0.2 wt.% PMA-NH4 (alumina powder basis).
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4. Conclusion Acrylic copolymers prove to be efficient dispersants for aqueous suspensions of alumina when the fraction of carboxylic groups is larger than 50% within the macromolecular chain. Adsorption of acrylic copolymers takes place due to carboxylic groups which anchor the polymer chain on specific surface sites, namely Al /(OH2). However, the hydroxyethyl acrylate moieties promote a coil-like adsorbed configuration, the thickness of which increases with the content of such moieties. Moreover, the amount of carboxylic groups controls the effective charge density of the particle surface and the copolymer aptitude to be used as a dispersant. Addition of a copolymer containing 55% of carboxylic groups, at an optimum amount of 0.6 wt.%, leads to stable suspensions which display a Newtonian behaviour with a fairly low value of viscosity (16 mPa s 1). Hydroxyl groups in the copolymer are expected to act as a binder after forming of alumina by drypressing. The effect of these copolymers on the cohesion and mechanical properties of green products is presently under investigation and the results will be the subject of a forthcoming paper.
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