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Ind. Eng. Chem. Res. 2007, 46, 2212-2219
Hyperbranched Polymers as a Fixing Agent for Dissolved and Colloidal Substances on Fiber and SIO2 Surfaces Lars Wågberg,† Sedat Ondaral,*,‡ and Lars-Erik Enarsson† Department of Fibre and Polymer Technology, KTHsRoyal Institute of Technology, SE-10044 Stockholm, Sweden and Department of Pulp and Paper Technology, Faculty of Forestry, Karadeniz Technical UniVersity, 61080 Trabzon, Turkey
Hyperbranched polymers (polyesteramides) were used as a cationic fixing agent to remove dissolved and colloidal substances (DCS) from the water phase in a fiber suspension. The relative turbidity, electrophoretic mobility, and average diameter of the colloidal particles were determined as a function of polyelectrolyte concentration. The results indicated that maximum removal of DCS was achieved at about zero electrophoretic mobility of the suspension where the negative surface charges of particles were neutralized by the oppositely charged hyperbranched polymer. The amount of hyperbranched polymers needed to maximize DCS fixation on the fibers was higher than the amount of poly-DADMAC (diallyldimethylammonium chloride) needed to reach the same effect. This was found to be due to the lower molecular weight and lower charge density of hyperbranched polymers. The lower molecular mass allowed penetration of these polymers into pores of fibers that resulted in higher polymer consumption before removal of the dissolved and colloidal substances from the fiber suspensions. A lower charge density further resulted in a higher saturation adsorption of the hyperbranched polymer. Experiments with both DCS and model-latex particles showed that the initial increase in relative turbidity was due to the aggregation of particles before fixation to fibers. The results from quartz crystal microbalance with dissipation (QCM-D) experiments showed that the efficiency of hyperbranched polymer and poly-DADMAC was similar even if they had different structure. Therefore, this study highlights the importance of molecular mass and charge density of the polymers as well as the surface structure of polymer layers which in turn gives implications for development of new structures of fixing agents. Introduction In the production process of wood-containing papers, such as newsprint, super calendered, and lightweight coated magazine papers, the white water system contains dissolved and colloidal substances (DCS) from the wood raw material. These substances contain lipophilic extractives, lignin, neutral polysaccharides, charged polysaccharides, and proteins.1,2 They usually accumulate in the white water system in a paper mill due to the recirculation of water in the process. The increase of DCS concentration may lead to several operational problems, including increased corrosion, foaming and scaling problems, slime and biological growth, deposits of extractive materials of the DCS on wires and felts, higher incidence of sheet breaks, and increased consumption of cationic retention aids with alteration of the wet end chemistry, loss of paper brightness, formation of specks and holes, reduced sheet strength, burst, and breaking length.3-7 The DCS often forms polyelectrolyte complexes with cationic polyelectrolytes used to enhance the process efficiency in the papermaking process.8,9 This complexation significantly decreases the efficiency of the additives, and the complexes as such might cause disturbances in the papermaking processes by clogging felts and screens and depositing on different parts of the papermaking machinery. Furthermore, since the charge of these complexes is still negative, at low to moderate additions of the cationic polyelectrolytes it is not possible to adsorb these complexes to the anionic wood fibers used for paper production. * To whom correspondence should be addressed. Tel.: +90 462 377 3243. Fax: +90 462 325 74 99. E-mail:
[email protected]. † KTHsRoyal Institute of Technology. ‡ Karadeniz Technical University.
There are several earlier publications on the topic of fixation of DCS to the fibers using different types of polymers.8-15 In these earlier works the mechanisms for fixing of DCS to the fibers were identified as patch flocculation, charge neutralization, or nonequilibrium bridging flocculation. All of these studies were related to the interaction between commonly used polyelectrolytes and DCS. For the nonequilibrium type of mechanism it was suggested that the 3-D structure of the polyelectrolyte is of large importance and development of new types of polyelectrolytes would be interesting.8 To our knowledge there is, however, no earlier work available about use of hyperbranched polymers for fixation of DCS to the fibers. Hyperbranched polymers are highly branched macromolecules with threedimensional dendritic architecture. Due to their unique physical and chemical properties and potential applications in various fields from drug delivery to coatings, interest in hyperbranched polymers is growing rapidly.16-18 The aim of the present work is therefore to clarify the efficiency of hyperbranched polymer for DCS removal and determine the properties of the adsorbed layer of DCS on preadsorbed hyperbranched polymer layers on the SiO2 surface using QCM-D. Even though this study was focused on DCS from wood raw material it has a large general interest, for example, in wastewater treatment since these waters usually contain mixtures of both dissolved and colloidal material. Experimental Section Materials. Unbleached thermomechanical pulp (TMP, SCA Ortviken Mill, Sweden) was used to prepare the DCS. TMP was stored in the freezer to preserve its composition before use. In the experiments conducted to clarify polymer performance in the fixation of DCS on fibers, totally chlorine-free (TCF)
10.1021/ie061108b CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2213 Table 1. Charge Densities and Hydrodynamic Diameter of Hyperbranched Polymersa polymer
charge density (meq/g)
HA1 HA2
2.0 1.7
hydrodynamic diameter (nm) 2.3a 4.2a
3.4b 6.0b
a The hydrodynamic radius were measured by a Zetasizer Nano ZS system. b The hydrodynamic radius were measured by a BI-200SM goniometer system.
bleached chemical softwood fibers (SCA, O ¨ strand Pulp Mill, Sundsvall, Sweden) were used. This pulp was delivered in dry lap form. The hyperbranched polymers (Hybranes-polyesteramides) (HA1 and HA2) used in the experiments were purchased from DSM, Netherlands, without further purification. Their molecular weights were 2000 and 6000 Da, and they were specially substituted with 3.75 and 2.99 mmol/g polymer tertiary amine groups, respectively, and these were quarternized with dimethyl sulfate to 75%. The charge density of the hyperbranched polymers was determined by polyelectrolyte titration at pH 6.2 using potassium polyvinylsulfate (KPVS; Wako Pure Chemical Industries, Osaka, Japan) as the titrant and Ortho Toluidine Blue (OTB; VWR, Sweden) as an indicator. An end point was detected with a polyelectrolyte titrator from BASF AG., Ludwigshafen, Germany.19 The hydrodynamic diameters of HA1 and HA2 were determined by dynamic light scattering measurements with two different types equipment: Zetasizer Nano ZS from Malvern Instruments, Malvern, U.K., and a BI-200SM goniometer system connected to a BI-9000AT digital correlator from Brookhaven Instruments, Holtsville, NY, and a watercooled Lexel 95-2 laser, at an angle of 90°. The polymer solutions were prepared at 0.5 M NaCl after dialyzing the polymer against a solution of 0.5 M NaCl. The charge densities and hydrodynamic diameters are given in Table 1. Poly-diallyldimethylammonium chloride (poly-DADMAC) was used for common comparison representing commonly used polyelectrolyte fixing agents (Alcofix 111, Ciba Specialty Chemicals, Bradford, U.K.). It had a molecular weight of 624 kD according to Ciba Co. and a charge density of 6.18 meq/g according to polyelectrolyte titration and the theoretical chemical structure of the polymer. The latex used for comparison with DCS was composed of polystyrene microspheres purchased from Interfacial Dynamics Corp., Portland, OR. They had an average diameter of 0.21 µm and a surface charge density of 3.64 µeq/cm2. NaOH and NaCl used for pH adjustment were analytical grade. Water used in all experiments was deionized water with Millipore quality. Methods. Preparation of Fibers. Before using the totally chlorine-free (TCF) bleached chemical softwood fibers it was disintegrated according to the standard procedure (ISO 5263: 195). To convert the carboxyl groups on fibers to their sodium form and remove unwanted preadsorbed metal ions, fibers were treated in a washing procedure, first soaking with HCl at pH 2 for 30 min. Then the fibers were washed with deionized water several times. After treatment the fibers were treated with 10-3 M NaHCO3 for 30 min and washed with deionized water until the conductivity of water was less than 2 µS/cm through a wire having 75 µm openings to remove small fibrous fines. The water was removed by simple filtration, and the so-prepared fiber pads were stored in a refrigerator before use. Polymer Adsorption to Fibers. In the polymer adsorption measurements the concentration of the fiber suspension was adjusted to 5 g/L with deionized water. After polymer addition and pH adjustment to 6.2 the fiber suspension was mixed for 30 min at room temperature. Then fibers were then separated
via filtration with a Buchner funnel fitted with filter paper (Munktel no. 3, Kebo-Grave, Sweden). The amount of polymer in the solution was determined by polyelectrolyte titration.19 KVPS and OTB were used as a titrant and color indicator, respectively. The amount of adsorbed polymer on the fibers was calculated by subtracting the amount of polymer remaining in solution from the added amount of polymer. Preparation of DCS. DCS was prepared from the frozen TMP. Following defrosting the pulp was diluted to a concentration of 10 g/L, stirred at 150 rpm for 1 h at 60 °C, and filtrated over a Buchner funnel fitted with coarse filter paper (Munktel no. 3, Kebo-Grave, Sweden) and with a large diameter in order to avoid formation of a thick filter pad of fibers during filtration. The filtrate was then used to reslush a new batch of TMP for preparation of a more concentrated DCS sample. That operation was repeated three times, and the final pulp suspension was centrifuged at 2800 rpm for 30 min at room temperature to remove very fine fractions after the last filtration of the DCS suspension. The concentration of DCS was determined after evaporating in a heated oven. Excess evaporation time was avoided in order to prevent material loss. Concentrated DCS was then stored in a refrigerator before use, and the stability was high enough for storage over at least 2 weeks. The charge density of DCS was calculated as 20 µeq/g by polyelectrolyte titration as mentioned before. Fixation of DCS and Latex onto Fibers. The fixation of DCS onto fibers induced by addition of cationic polyelectrolytes with and without NaCl was monitored via changes in relative turbidity as determined with a Hach Turbidometer (model 2100A, Hach Co.) borrowed from STFI-Packforsk, and the electrophoretic mobility was measured using the Zetasizer equipment. The concentrations of DCS and fibers were 200 mg/L and 5 g/L, respectively, in all experiments. DCS and fibers were added into a 200 mL flask, and deionized water was added to adjust the total volume to 100 mL. After a short period of shaking, polyelectrolytes were added and the sample was mixed for 30 min. Samples for both turbidity and zeta potential measurements were taken by a specially constructed syringe fitted with a screen (having 51 µm opening size) to exclude fibers. The average diameter of particles in this sample was measured using the Zetasizer equipment in order to determine how the aggregation changed during polymer addition. The turbidity of this sample was also determined with the Hach turbidimeter, and the relative turbidity (RT) of the suspension was calculated as
RT ) τ/τo
(1)
where τ and τo are turbidities of the suspensions with and without polyelectrolyte, respectively. The pH of the suspension was about 6.2 in all experiments. Fixation of latex particles on fibers by addition of HA2 and p-DADMAC was also studied for comparison with DCS. The experimental procedures and other conditions, pH, etc., were the same as those for DCS experiments, and the latex concentration was 200 mg/L. Quartz Crystal Microbalance (QCM) Experiments. Fixation of DCS on silicon oxide surfaces induced by a preadsorbed polyelectrolyte was followed using a quartz crystal microbalance instrument (QCM-D) from Q-Sense AB, Va¨stra Fro¨lunda, Sweden. QCM crystals coated with silica (QSX 303/50 SiO2) were also supplied by the same company. The adsorbed amount of polyelectrolytes and DCS on silicon oxide, ∆m, was calculated using the well-known Sauerbrey equation
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∆m ) -
C∆f n
(2)
where ∆f ) f - f0, f0 is the resonance frequency of the crystal before material adsorption, f is the resonant frequency when material adsorbs on the surface of the crystal, n is the overtone number, and C is a constant that describes the sensitivity of the crystal to changes in mass. For the crystals used, C ) -0.177 mg m-2 Hz-1 and n ) 1, 3, 5, or 7.20,21 When the driving power to the crystal is switched off there is decay in the oscillation due to frictional losses in the crystal, in the adsorbed layer and in the surrounding solution. The energy dissipation is characterized by a dissipation factor D which is related to the decay time constant τ as follows
D)
1 πfτ
(3)
Figure 1. Adsorption isotherm of polyelectrolytes onto cellulose fibers at pH 6.2 both without and with the addition of 0.01 M NaCl.
where f is the resonance frequency. The QCM-D instrument measures the change in the dissipation factor ∆D ) D - D0 during the adsorption process, where D0 is the dissipation factor of the pure quartz crystal immersed in the solvent and D is the dissipation factor when the mass is adsorbed.21 Before using the QCM crystal it was treated with piranha cleaning solution (H2SO4:H2O2 3:1) for 1 min, rinsed with deionized water, and dried with N2 gas. After mounting the quartz crystal in the QCM cell, it was exposed to an aqueous buffer solution containing 10-2 M NaCl (used for preparation of polymer and DCS solution) for establishing a baseline for the frequency and energy dissipation. Adsorption started by exchanging the buffer with polymer solution. Residual polymer was removed, after 15 min, by adding a fresh sample of the buffer solution. Then DCS solution was injected into the cell to initialize secondary adsorption. The Qsoft 3.01 software, from Q-Sense AB, recorded the changes in the properties of adsorbed polymer and DCS layers in terms of frequency and dissipation shifts at four different overtones during the adsorption process. The third overtone has been used for evaluation of the data due to its stability. The concentrations of polyelectrolytes and DCS were 100 and 200 mg/L, respectively, in all experiments. The experiments were conducted at a constant temperature of 24.3 oC. Results Adsorption of Polyelectrolytes and Fixation of DCS to Fibers. The saturation adsorption of both hyperbranched polymers and poly-DADMAC onto fibers at pH 6.2 is shown in Figure 1. The adsorbed amounts of hyperbranched polymers are about 15-20 times higher than poly-DADMAC. This large difference in the amount of adsorbed polymer is attributed to the smaller molecular size and lower charge density of HA1 and HA2. The polyelectrolytes with lower molecular weight can penetrate into the pores of the fibers.9 The influence of a simple background electrolyte, i.e., 10-2 M NaCl, on the adsorption of one of the hyperbranched polymers (HA2) and poly-DADMAC was also determined, and the results showed that the adsorbed amount of HA2 decreased in the presence of electrolyte while adsorption of poly-DADMAC increased. The change in the relative turbidity and electrophoretic mobility of filtrates from suspensions containing a mixture of DCS and fibers was measured to evaluate the performance of the two hyperbranched polymers regarding the DCS fixation to fibers. The relative turbidity (RT) and electrophoretic mobility
Figure 2. (a) Relative turbidity and (b) electrophoretic mobility of DCS particles, after removing fibers by filtration, as a function of polymer addition of HA1 and HA2 at pH 6.2 and different salt concentrations.
of suspension as a function of concentration of HA1 and HA2 are shown in Figure 2a and b. From these results it can be seen that the relative turbidity of the suspension initially increased at lower concentrations of hyperbranched polymers while maintaining a negative electrophoretic mobility. After addition of 175 mg/L of HA1 and HA2, charge neutralization was achieved according to Figure 2b, and this resulted in a large decrease in the relative turbidity. When
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Figure 4. Changes in the relative turbidity (RT) and electrophoretic mobility (EM) of the latex particles, after removing fibers by filtration, as a function of HA2 addition.
Figure 3. (a) Relative turbidity and (b) electrophoretic mobility of particles from the DCS as a function of poly-DADMAC concentration at pH 6.2.
the system was recharged by addition of HA2, the relative turbidity was increased, while the same addition level of HA1 did not change the relative turbidity. The amount of HA1 and HA2 needed for maximum DCS removal was about 20 times higher than the amount of poly-DADMAC needed to reach the same RT value as shown in Figure 3a and b. The efficiency of poly-DADMAC is different compared to the hyperbranched polymers. After addition of 5 mg/L polyDADMAC the system recharged in terms of electrophoretic mobility and the relative turbidity started to increase again. For the HA2 polyelectrolyte, addition of an electrolyte to the fiber suspension increased the efficiency of the polymers and prevented recharging of the DCS colloids. Fixation of Polystyrene Latex onto Fibers. Fixation of latex colloids onto fibers, selected as a model system for the colloidal fraction of the DCS, was studied in order to check whether a similar behavior could be detected for these more wellcharacterized particles as for the DCS. In Figures 4 and 5 the changes in the relative turbidity and electrophoretic mobility of suspension are shown as a function of HA2 and polyDADMAC addition, respectively. Compared with the DCS results a small initial increase in the relative turbidity at lower polymer additions was observed for both poly-DADMAC and HA2 even though the increase was significantly lower for the HA2 polyelectrolyte and latex compared with DCS. Compared with the DCS, however, aggregation/fixation of the latex colloids starts at a clearly
Figure 5. Changes in the relative turbidity (RT) and electrophoretic mobility (EM) of the latex particles, after removing the fibers by filtration, as a function of poly-DADMAC addition.
negative electrophoretic mobility, which is typical for both patch and bridging types of flocculation. The HA2 is also efficient in aggregating/fixing the latex particles over a broad range of polyelectrolyte addition, while poly-DADMAC shows a narrower range of aggregation/fixation efficiency. This behavior is similar to the aggregation/fixation of the DCS material. Finally, an overcharging of the system leads to a restabilization of the system. DCS Adsorption onto Silicon Oxide Surfaces Pretreated with Cationic Polyelectrolytes. The QCM-D instrument was used to determine the adsorbed amount and properties of the deposited polyelectrolyte layers as well as DCS layers on silica surfaces. Figure 6 shows the change in the resonance frequency (∆f) vs time for the 15 MHz signal, i.e., the third overtone. The absolute frequency shift associated with polymer adsorption was greater for HA2 than for poly-DADMAC. After rinsing the crystal with a solution of 10-2 M NaCl to remove excess polyelectrolyte, DCS addition caused a significant drop in ∆f due to adsorption DCS onto the preadsorbed polyelectrolyte layer. Discussion Fixation of DCS and Polystyrene Latex to Fibers. The results from the experiments regarding DCS fixation with different types of polyelectrolytes showed a significant difference in efficiency between the hyperbranched polymers and
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Figure 6. Adsorption of DCS on silica induced by preadsorption of either HA2 or poly-DADMAC, respectively, at pH 6.2 and a salt concentration of 10-2 M NaCl.
Figure 7. Average diameter of particles in suspension as a function of polymer addition for the DCS material. pH ) 6.5, and no extra salt was added.
poly-DADMAC, but similar trends were nevertheless observed for the different polyelectrolytes. As can be seen in Figures 2a and 3a and 2b and 3b the removal process of DCS can be divided into three phases. In the first phase there is a slight increase in the relative turbidity up to an addition level of about 75 (HA1/HA2) and 3 mg/L (poly-DADMAC), respectively. This increase in turbidity has been detected earlier9,22 and ascribed to a precipitation of the dissolved fraction of the DCS since a pure fraction of dissolved material showed this behavior while a clean colloidal fraction did not. The hyperbranched polyelectrolytes also resulted in a significantly larger increase in turbidity compared with the poly-DADMAC, indicating a different size and/or different number of the formed aggregates by the two different polyelectrolytes. The electrophoretic mobility of particles in suspension was still clearly negative at this stage, indicating that the initially formed aggregates had a negative net charge; therefore, fixation to the negatively charged fibers is not expected. A further test of the hypothesis that the initial precipitation process creates colloidal particles that are about the same size and charge as the initially present particles can be achieved by measuring the size of colloidal particles in the suspension. Results from these measurements are shown in Figure 7, and as can be seen, there is no considerable change in the average diameter of particles in suspension at lower concentrations of HA1, HA2, and polyDADMAC.
At higher concentration of HA1, HA2, and poly-DADMAC (the second phase of the fixation/aggregation process) the relative turbidity of the suspension started to decrease. This is attributed to DCS removal from suspension via fixation to the fibers, and the difference in the fixation ability of the HA1 and HA2 polyelectrolytes can most probably be ascribed to the difference in charge density of these two polyelectrolytes. Although the maximum in fixation of colloidal particles to the fibers was achieved around zero in electrophoretic mobility, the RT of the suspension started to decrease at lower polyelectrolyte concentration, suggesting that patch flocculation was the dominating mechanism responsible for this fixation of DCS to the fibers for HA1 and HA2. In this interval of polyelectrolyte concentration under the point of recharge, DCS aggregated from its initial average size to larger aggregates between 1.5 and 2.5 nm (Figure 7), and fixation of these aggregates to the fibers decreased the DCS concentration in the filtrate sample and, therefore, lowered the turbidity of the suspension. For poly-DADMAC, in deionized water, there is a close correlation between zero electrophoretic mobility and the minimum in turbidity, and therefore, charge neutralization is suggested to be the active flocculation mechanism for this type of system. At the last phase of the aggregation/fixation process, beyond zero electrophoretic mobility, higher additions of HA2 and polyDADMAC induced an increase in the relative turbidity. This also correlates with the transition to positive electrophoretic mobility, clearly indicating that these polyelectrolytes are able to recharge the DCS and fibers leading to a renewed electrostatic stabilized system. As clearly shown in Figure 2b, HA1 is not able to recharge the system, and this is probably the reason for the lack of restabilization with this polyelectrolyte. As shown in Figure 7 there is a decrease in the average size of the DCS aggregates as polyelectrolyte addition is increased, even though the RT is still low for the HA1-treated suspension. This indicates a change in the interaction between the colloids and naturally between the colloids and the fibers. There might naturally be several reasons to this phenomenon, but since the colloids and fibers are being totally covered with HA1 at higher additions there will be a lack of any type of patch type of interactions. This will naturally decrease the aggregation between the DCS colloids since the collision efficiency factor between the particles will be decreased but the lower surface potential of the colloids at higher HA1 additions results in a significantly lower stability of the slightly recharged colloids. In general, large colloidal particles are more easily deposited on fibers, and this will lead to a fraction of DCS, leaving the fraction of small colloids in solution. Such a fractionation process is described in hydrodynamic terms. The larger DCS colloids, which have higher inertia, will still be able to collide with the fibers, and therefore, the fibers will be able to work as collectors for the larger DCS colloids.23 The smaller DCS colloids will follow the streamlines around the fibers and will not be able to pass the layer of water following each fiber due to the low inertia of these colloids. This might give rise to a fractionation resulting in a lower average size of the colloids remaining in the suspension. Another interesting result with the HA1 and HA2 polyelectrolytes is the large range of efficiency of these polyelectrolytes in removal of dissolved and colloidal material compared with poly-DADMAC in deionized water. The exact reason for this is not known, but it can be suggested that the size of the polyelectrolytes in comparison to the DCS colloids plays an
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important role. In deionized water both the size and the charge of the poly-DADMAC molecules and the DCS colloids are of the same order of magnitude whereas the HA1 and HA2 molecules are much smaller than the DCS colloids. Using a molecular mass of 624 kDa and a charge of 6.18 meq/g for poly-DADMAC and a charge of the DCS of 20 µeq/g in combination with a radius of 100 nm (density of 1000 kg/m3) for the DCS it could be estimated that about 13 molecules of poly-DADMAC are needed to neutralize the DCS colloids. In summary, this means that only a few polyDADMAC molecules will be needed to neutralize the DCS colloids and it is not really meaningful to discuss a distribution of the molecules, as mentioned earlier, on the surface of the DCS colloids. Therefore, a charge neutralization of the DCS colloids will be dominating, and since the poly-DADMAC will easily recharge the colloids, there is a narrow concentration range for the polyelectrolytes to perform effectively. For HA1 and HA2 the distribution of the polyelectrolytes on the colloids will be important, and despite a correlation between the rapid decrease in RT and zero mobility it is very likely that there will be a strong contribution from a patch type of flocculation due to an uneven distribution of these polyelectrolytes on the surface of the DCS colloids. These hypotheses are also supported from the model experiments with the latex particles. As seen in Figures 4 and 5 the HA2 polyelectrolyte has a relatively broader range of efficiency compared with poly-DADMAC also for the latex particles. However, for both types of polyelectrolytes there is significant particle removal before zero mobility, indicating the large contribution from a patch type of flocculation for both types of polyelectrolytes. For HA2 this is plausible considering the charge and size relationship between the polyelectrolytes and the colloids.24 However, as discussed earlier, only a few molecules of poly-DADMAC will be needed to neutralize the latex particles, and therefore, the patch type of flocculation behavior for the latex/poly-DADMAC system has to be taken with great caution. On the contrary, poly-DADMAC will have a stretched conformation in deionized water, and a model where the colloids will be oriented along the polyelectrolyte molecule in a pearl necklace type of orientation25 is much more likely. The results also show that the latex is removed at a much lower polymer addition compared with the DCS. The most obvious explanation for this difference between the latex and the DCS is the different charge density of these materials. Experiments showed that the latex had a charge density of 3.6 µeg/g whereas the DCS had a charge density of 20 µeq/g. As shown in Figures 2 and 3 the presence of electrolytes in suspension enhanced removal of DCS. The relative turbidity of suspension with 10-2 M NaCl started to decrease at lower polymer concentration, and the suspensions were not restabilized at higher polyelectrolyte concentrations. This is most likely linked to compression of the electrostatic double layer around the particles, thereby screening the electrostatic repulsion between negatively charged particles.26 As can be seen in Figures 2b and 3b, the electrophoretic mobility will, as expected, be decreased, indicating a lower surface potential of the colloids and, hence, a lower electrostatic repulsion barrier between the colloids. This means that less polyelectrolyte will be needed to decrease the electrostatic repulsion between the particles to allow for aggregation of the colloids and fixation to the fibers. Additionally, electrolyte can also cause a slight aggregation of
Table 2. Charge Ratio for Hyperbranched Polymers and Poly-DADMAC
polymer
adsorbed amount (mg/g fiber)
adsorbed charge (µeq/g fiber)
charge ratio
HA1 HA2 HA2:10-2 M NaCl poly-DADMAC poly-DADMAC:10-2 M NaCl
17 22 17.5 0.8 2
34 38 31 4.9 12.4
0.8 0.9 0.7 0.1 0.3
particles,15,27 changing the collision frequency between the particles and the polyelectrolytes. The lack of restabilization of the colloids at 10-2 M NaCl is also suggested to be linked to compression of the double layers and the decrease of the surface potential of the colloids. As shown in Figures 2b and 3b the electrophoretic mobility of the recharged colloids is much lower than the initial electrophoretic mobility of the colloids even in deionized water, and in 10-2 M NaCl the polyelectrolytes are not able to recharge the colloids and more polyelectrolyte is needed to reach zero mobility. More model experiments are needed to clarify the exact mechanism behind the lack of charge reversal at 10-2 M NaCl, but it has to be stressed that this phenomenon is very important for practical use of these polymers. An important difference between the polyelectrolytes regards the additions required for effective DCS fixation. A comparison between Figures 2 and 3 shows that much less poly-DADMAC is needed to reach the maximum change in relative turbidity as well as the point of zero electrophoretic mobility compared to corresponding amounts of HA1 and HA2. The adsorption isotherms of the polyelectrolytes in Figure 1 showed that the saturation adsorption of HA1 and HA2 was much higher than the saturation adsorption of poly-DADMAC. Additionally, when the charge ratio (i.e., the adsorbed amount of polymer charge/ total anionic charges on the fiber (taken to be 44 µeq/g28)), which is a measure of how deep into the porous structure of the fibers the polymers will reach, is considered, it can be seen that the hyperbranched polymers and poly-DADMAC show different behavior as summarized in Table 2. The higher charge ratio for hyperbranched polymers is most likely linked to the hydrodynamic dimension of hyperbranched polymers and polyDADMAC. Since the sizes of the HA1 and HA2 molecules are on the order of a few nanometers, see Table 1, they will be able to penetrate into pores in the fiber wall whereas polyDADMAC will be limited to the external surface of the fibers. Addition of 10-2 M NaCl to the fiber suspension increased the charge ratio for poly-DADMAC due to increased coiling of polymer.26 Conversely, the salt addition decreased the charge ratio when the HA2 was used, and this is suggested to be due to a shielding of the interaction between the charges on the fibers and polyelectrolytes. Since the polyelectrolyte is of such a low molecular mass the number of charges per polymer will be low, and therefore, the salt addition will have a high influence on the interaction. DCS Adsorption on Silica Surface. The QCM-D study showed that a preadsorbed layer of HA2 on the silica surface adsorbed DCS as efficiently as a poly-DADMAC layer. The responses in ∆f, ∆D, and ∆m (adsorbed amount of material, calculated by using eq 2) due to addition of polyelectrolyte and DCS are summarized in Table 3. The ∆D values showed that the adsorbed polyelectrolyte layer had different properties for each polyelectrolyte. Poly-DADMAC adsorption on the SiO2 surface caused an increase in ∆D, while there was no change in ∆D by addition of HA2. This indicates that poly-DADMAC formed a less dense adsorbed layer on the SiO2 surface than
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Table 3. Changes in Resonance Frequency (∆f) and Dissipation (∆D), as Well as the Calculated Adsorbed Mass (∆m), Due To Adsorption of Polyelectrolyte and DCS Adsorption at 15 MHz (10-2 M NaCl) addition of polymer
addition of DCS
polymer
∆f (Hz)
∆D (10-6)
∆m (mg/m2)
∆f (Hz)
∆D (10-6)
∆m (mg/m2)
HA2 poly-DADMAC
-18,1 -9.3
0.0 0.4
1.1 0.6
60.4 -68.5
3.2 2.9
3.6 4.1
the hyperbranched polymer, although both polymers have different layer properties and gave different amounts of adsorbed charge on silica, 1.9 µeq/m2 for HA2 and 3.7 µeq/m2 for poly-DADMAC. Despite the difference in adsorbed charge, HA2 was almost as efficient as poly-DADMAC in fixating DCS. The average adsorbed layer thicknesses of HA2 and poly-DADMAC for full coverage of the polymer on the surface are 1.1 and 0.6 nm, respectively, by means of dividing the adsorbed amount of polymer by the assumed density of polymer layer (1 g/cm3).29 The thicker the polymer layer formed on the surface, the more DCS particles can be retained onto surface. This indicates that hyperbranched polyelectrolytes have a large collector efficiency once adsorbed to a solid substrate despite the small size of these polyelectrolytes. Conclusion The efficiency of hyperbranched cationic polyesteramides (HA1 and HA2) regarding fixation of DCS to wood fibers was investigated and compared with poly-DADMAC. HA1 and HA2 were found to be effective in the removal of DCS from fiber suspension. The amount of HA1 and HA2 needed for maximum fixation of DCS onto fibers was much higher than the amount of poly-DADMAC required for reaching the same degree of fixation. This difference was attributed to the lower molecular weight and smaller molecular size of the hyperbranched polymers in combination with their lower charge density. From the turbidity measurements in deionized water the hyperbranched polymers were found to be efficient over a much larger range of addition levels in comparison with poly-DADMAC. Since this removal started before neutralization of the system by addition of HA1, HA2, and poly-DADMAC, i.e., before the isoelectric point, the mechanism behind the aggregation of DCS was defined as the patch flocculation for both DCS particles and latex model experiments. Additionally, the smaller size of the hyperbranched polymers was suggested to be the main reason for the insufficient restabilization of DCS found in the experiments. The QCM-D experiments showed the importance of polyelectrolyte conformation on a solid substrate for DCS fixation and how this conformation was affected by charge density and molecular weight. Results indicated that the thicker polyelectrolyte layer on the SiO2 surface is, the higher the efficiency for fixation of the DCS. Three-dimensional hyperbranched polymers were found to be as effective as poly-DADMAC in these experiments. This means that a 3-D structure of the polyelectrolytes, such as hyperbranched polymers, in combination with a higher molecular mass would be ideal for fixation of DCS in water purification. Acknowledgment The authors would like to thank Ph.D. student Andrew Horvath for assistance with the dynamic light scattering
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ReceiVed for reView August 22, 2006 ReVised manuscript receiVed January 16, 2007 Accepted January 17, 2007 IE061108B
