Surface Treatment by Hydrophobic Particles ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Surface Treatment by Hydrophobic Particles: Influence of Starch and Ionic Strength Frida Iselau,*,†,‡ Krister Holmberg,‡ and Romain Bordes*,‡,§ †

Kemira Kemi AB, Box 902, 251 09 Helsingborg, Sweden Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden § Vinn Excellence Center SuMo Biomaterials, Chalmers University of Technology, 412 96 Göteborg, Sweden ‡

S Supporting Information *

ABSTRACT: This paper deals with the colloidal behavior of mixtures of cationic polymer particles and anionic starch. Such mixtures are commonly used for the hydrophobization of paper. The effect of the concentration of anionic starch and of the presence of different electrolytes was assessed. In a previous study, oxidized starch had been found to induce aggregation of cationic hydrophobic nanoparticles, and in the study reported here it is demonstrated that this aggregated state is beneficial for the performance, enabling a substantial reduction of the amount of polymer nanoparticles needed to reduce the water uptake. It was found that when the particles were in a highly aggregated state, the water uptake by the paper surface was very small. The effect of mono- and divalent ions on the colloidal stability was also investigated. In general, it was found that an increased ionic strength gave a less hydrophobic paper surface. Na2SO4 was more detrimental than NaCl and CaCl2, which is explained by the valence of the anion. This implies that the hydrophobization can be tuned by controlling the aggregation of the polymer particles. KEYWORDS: Aggregation, Starch, Ionic strength, Anionic effect, Turbidity, Surface sizing, Salt

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INTRODUCTION

In previous work, we have observed that mixing of oxidized starch with cationic sizing particles (SP+) induced a loss of the colloidal stability of the system and gave rise to aggregates, while with anionic (SP−) and amphoteric sizing particles, the colloidal stability was unaffected.10 The nature of these aggregates was further investigated with advanced scattering techniques,11,12 and it was shown that the aggregation of the particle-polyelectrolyte system was partly reversible and that the aggregates were fractal species composed of amylopectin and the cationic particles, while the role of the amylose fraction of the starch was minor. The aggregation maximum was found to be at charge neutrality between the cationic particles and the negatively charged starch. The formed SP+/starch complexes were stabilized by an electrosteric mechanism. The adsorbed amylopectin contributed to both charge reversal and steric hindrance. Another factor influencing the behavior of the particles is the salts present during the application. The ions can screen the charges and thus affect the electrostatic interactions.13−17 The most common ions present in paper mills are chloride, sulfate, sodium, calcium, iron, and aluminum, originating from various treatments. The chemical composition of the white water from

With the growing need for improved cellulose-based packaging materials, especially products that are able to withstand exposure to water, there is an increasing interest in paper hydrophobization, often referred to as “sizing.” The trend is toward surface sizing rather than internal sizing. As the name implies, surface sizing is a coating procedure done as a separate step after the formation of the paper. This is very different from the traditional internal sizing, which is done by adding alkyl ketene dimer (AKD) or some other hydrophobic substance into the fiber/filler slurry during the paper making process.1 In surface sizing, a combination of hydrophobic nanoparticles and starch is applied after formation of the paper in the paper machine.2−5 The role of the starch is mainly to increase the surface strength of the paper.6,7 The hydrophobic nanoparticles typically consist of a hydrophobic core and a stabilizer. The hydrophobic core is usually a styrene−acrylate copolymer, and the stabilizer can be either starch or a synthetic polymer.5,7,8 The nature of the stabilizer determines the particle charge. Cationic, anionic, and amphoteric systems are used industrially. Prior to application on the paper web, the particle suspension is mixed with the starch solution.9 One of the current challenges is to further improve the particle efficiency in order to reduce the amount of fossil-based material used, i.e., the synthetic particles. © 2017 American Chemical Society

Received: March 31, 2017 Revised: May 9, 2017 Published: May 15, 2017 6107

DOI: 10.1021/acssuschemeng.7b00984 ACS Sustainable Chem. Eng. 2017, 5, 6107−6115

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ACS Sustainable Chemistry & Engineering five different paper mills has been analyzed, and the values obtained are given in the Supporting Information. Whether the presence of the SP+/starch aggregates is beneficial or not for the hydrophobization process is currently not clear, and no dedicated study to understand the role of aggregation on the surface sizing process has yet been reported. In this paper, we report the results of size press trials in which SP+/starch complexes in a highly aggregated state were compared to restabilized SP+/starch complexes. We also compare the results with a corresponding system based on anionic particles, SP−, which do not form aggregates with oxidized starch. Furthermore, the impact of the addition of salt to the formed SP+/starch complexes is also reported.

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cell, and the viscosity was measured at a rotating speed of 60 rpm using spindle no 00. Dynamic Light Scattering, DLS. A Malvern Nano instrument was used for measuring the particle size by dynamic light scattering. The SP+ suspension, 0.1 wt %, was filtered with a 0.2 μm hydrophilic syringe filter before measurement. SP+/starch complexes were formed by the addition of a 3 wt % starch solution (filtered) to the SP+ 0.1 wt % sample rendering a starch to SP+ charge ratio of 1:1. The impact of salt was assessed by the addition of NaCl to the SP+/starch complexes described above yielding an ionic strength of 26 mM. Surface Sizing. The oxidized starch solution used for the surface sizing tests was prepared in a jet batch cooker where high temperature, high pressure, and high shear forces from the steam give a fully hydrated starch in solution.18 The starch solution was diluted to the required concentration and was held at approximately 70 °C during storage to prevent retrogradation2 and at around 60 °C when used in the size press trial. The hot starch solution was added to a specific amount of hydrophobic particle suspension to a final concentration of 0.03−0.5 wt % particles. The mixture was then applied in size press laboratory equipment from Mathis AG. During application, the liquid temperature was held above 60 °C. The roll speed was 1.8 m/min, and the roll pressure was 2 kg/cm2. The surface sized paper sheets were dried in a contact dryer held at 80 °C and were then stored in a climate room at 23 °C and 50 % relative humidity overnight. The hydrophobic resistance was evaluated after 24 h by the Cobb 60 test method19 according to the TAPPI standard method 441. The results from the Cobb tests are the water uptake of the paper presented as grams per square meter. A paper sheet with a water uptake of 30 g/m2 or below is usually regarded as a sufficiently hydrophobized surface. For the size press trials with increased ionic strength, the salt was added to the SP+/starch mixtures as aliquots of a stem solution, and corrections were made for the dilution. The SP+ concentration was held constant during the trials, and the aim was to have a starting point of around 60 g/m2 in water uptake in order to capture both increases and decreases in performance. Since subtle variations in the starch solution, the testing environment, and the base paper, among others factors, can cause variations in the water uptake measurements, inbetween trials variations of the starting point of the water uptake in the different trials were noted.

MATERIALS AND METHODS

Materials. The surface sizing particles, SP+ and SP−, were synthesized as described in a previous paper.10 In short, SP+ and SP− have the same hydrophobic core consisting of a copolymer of styrene and butyl acrylates but differ by the type of stabilizer that gives rise to a stable water suspension of the nanoparticles. SP+ has a synthetic, cationic stabilizer, and the SP− particles are stabilized by an oxidized, degraded starch. NaCl, CaCl2, and Na2SO4 were all from SigmaAldrich and used as purchased. The starch powder, oxidized potato starch from Avebe, The Netherlands, was used as received. Starch consists of repeating units of glucose that can form two types of polymer chains: amylose and amylopectin. Amylopectin is a highly branched, high molecular weight polymer, while amylose is a shorter and mostly linear polymer. Potato starch has a composition of 20 wt % amylose and 80 wt % amylopectin. The test paper used in the size press trials was a recycled paper grade from Mintec that consists of 100% recycled fibers with a basis weight of 140 g/m2. It was not internally sized. Methods. UV/vis for Turbidity Study. The turbidity measurements were performed on an Agilent Cary 60 UV/vis instrument or a HP8453 UV/vis instrument using disposable acrylic cuvettes. The baseline was recorded in Milli-Q water. The value of the absorbance at 400 nm was used as a measure of the turbidity. The particle suspension, 0.1 wt %, was filtered with a 0.2 μm hydrophilic syringe filter (Sartorius) before a 2 mL sample was transferred to the cuvette. The 3 wt % starch solution was also filtered with a 0.2 μm hydrophilic syringe filter before being added to the particle suspension. After the starch addition, the cuvette was shaken vigorously before being put into the UV/vis instrument. After an equilibration time, an aliquot of a salt solution was added to the SP+/starch mixture, and the cuvette was again shaken vigorously before being put into the instrument for recording the absorbance value. The importance of order of addition was assessed by a set of experiments where either the salt was added to an SP+/starch mixture or the starch was added to an SP+/salt mixture. Starch Preparation. A total of 3.5 g of starch powder was suspended in 100 mL of Milli-Q water under vigorous stirring. The suspension was allowed to boil heavily for 10 min, yielding a clear, slightly pale yellow starch solution. The cooled starch solution was diluted to a final concentration of 3 wt % calculated on the dry weight of the starch. Size Determination of the Aggregates. The size of the aggregates was determined by laser diffraction using a MasterSizer MicroPlus. A particle concentration of 0.4 wt % was used in order to achieve sufficient signal intensity. To the SP+ sample, NaCl/CaCl2/Na2SO4 was added to obtain an ionic strength of 80 mM. Initially, the salt addition was done in situ in the instrument mixing chamber under vigorous stirring. The stirring rate is related to the pump efficiency, which operated at 2000 rpm, which was believed to give sufficient circulation in the system. Another set of experiments was performed where the salt was added to the particle suspension prior to injection into the measurement chamber. Viscosity. The viscosity of the starch samples was measured with a Brookfield DV-III Ultra Viscometer equipped with a UL adapter, using a temperature controlled measuring cell: a concentric cylinder held at 25 °C. A total of 16 mL of the starch solution was transferred to the

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RESULTS AND DISCUSSION Effect of the Starch to Particle Charge Ratio on the Surface Sizing Performance. In our previous study, we reported that only the cationic particles, SP+, form large aggregates with the starch and that a maximum in aggregation was found at a charge ratio of around 1:1.11 It was therefore decided to use this starch to SP+ charge ratio also in this work. In the regular surface sizing procedure, the starch to SP+ charge ratio is typically in the range of 20 to 60, i.e., much higher than used in this work. To examine if the aggregate formation influences the surface sizing performance, a size press trial was performed using two different starch concentrations in combination with different dosages of SP+. The starch concentrations were 0.3 and 8 wt %, which correspond to starch to cationic particle charge ratios of around 1:1 (corresponding to maximum aggregation) and 1:20 and above (corresponding to the regular procedure), respectively. The paper surface hydrophobization effect, determined by the Cobb60 method, for SP+ and SP− in combination with either a 0.3 wt % or an 8 wt % starch solution is shown in Figure 1. As can be seen, the SP− particles, which do not aggregate with the negatively charged starch, gave poor hydrophobization regardless of the starch concentration. SP+ gave much better hydrophobization, and particularly good results were obtained with the 0.3 wt % starch concentration, 6108


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unexpected since anions are known to have a stronger influence than cations on the stability of colloidal systems20−22 and the valence of the anions is of particular importance. This is formulated in the modified Schulze−Hardy rule, which empirically correlates the ionic strength with the valence through (1/z)4.9.23 The actual value of the exponent for a specific system depends both on the surface charge density of the particles and on the nature of the co-ion of the added salt.23 The size of the formed flocs was determined by laser diffraction, which typically allows measurement of systems in the micron range. In order to overcome sedimentation, the samples were kept under flow in the measurement chamber. The trial was performed by adding salt to the particle suspension in the sample chamber with the aim of capturing the formation of the flocs. In contrast to the turbidity measurements, the introduction of salt did not immediately induce significant growth of the particles, which was unexpected. However, a sample withdrawn from the measurement chamber and allowed to stand in a cuvette for a couple of minutes exhibited a major change in appearance, as a result of growth of the flocs. The sample appearance was comparable with the samples shown in Figure S1. These results demonstrate that the formed flocs induced by the addition of salt are shear sensitive. A second trial in which the salts were added to the particle suspension and allowed to equilibrate for >1 h before measurement was therefore performed. The changes in the floc size distribution were monitored with time and are shown in Figure S2. The shear sensitivity of the aggregates was captured as a decrease in size with time. NaCl and CaCl2 gave similar results while Na2SO4 gave larger initial aggregate size, confirming the optical microscopy study. Effect of the Ionic Strength on the Surface Sizing Performance of SP+. A size press trial was performed where salt was added to the SP+ suspension at different ionic strengths in order to examine the effect on the surface sizing performance when the cationic particles are strongly aggregated. In this trial, no starch was used. The results are shown in Figure 2 where the water uptake values for SP+ in combinations with NaCl, CaCl2, or Na2SO4 are plotted versus ionic strength. As can be seen from the figure, the three salts had different impacts on the surface sizing performance of SP+. Whereas Na2SO4 had a detrimental effect and NaCl did not significantly affect the performance, CaCl2 had a positive effect,

Figure 1. Surface sizing results using either 0.3 or 8 wt % starch together with either SP− or SP+. In some cases, the error bars are smaller than 3 % and are then not visible in the graph.

i.e., at a starch to cationic particles ratio that corresponds to charge matching and also to a maximum in turbidity. The surface sizing effect may not only depend on the ratio of the two components, oxidized starch and positively charged nanoparticles, it may also depend on the starch concentration. Starch is water-soluble and a larger amount of starch on the surface will render the surface more hydrophilic. On the other hand, one reason for using starch in surface sizing is that the starch film applied on the paper surface will decrease the surface roughness of the paper, which may be beneficial because decreased surface porosity usually means a lower water absorption rate.2 If this is a parameter of importance for the water uptake, it would be seen in the experiments with two very different starch concentrations. However, the results presented in Figure 1 clearly show that the starch concentration per se seems not to be important. For the experiments with SP− particles, the higher starch concentration gave somewhat lower water uptake than the lower starch concentration, but the difference was small. For the SP+ formulations, the lower starch concentration was by far the best. This is a clear indication that it is the aggregation behavior of SP+ with the starch and not the amount of starch in the formulation that is decisive for the surface sizing performance. Effect of Salt on SP+. The studies were performed in the presence of either NaCl, CaCl2, or Na2SO4, using concentrations covering the ranges reported in Table S1. Colloidal Behavior of SP+ at Increasing Ionic Strength. The colloidal behavior of suspensions of SP+ in combination with starch has previously been studied by measuring the absorbance at 400 nm, as a measure of turbidity. This approach is based on the assumption that even though the system aggregates, it does not sediment. However, this was not the case for a suspension of SP+ at 0.1 wt % in the presence of salt. The apparent, lower turbidity was due to sedimentation of the formed flocs. A visual inspection showed that addition of Na2SO4 caused formation of flocs at a lower salt concentration compared to the other salts. Increasing the salt concentration further led to flocculation, and Na2SO4 gave the most pronounced effect. Pictures of the particle suspensions after salt addition are shown in the Supporting Information, Figure S1. A microscopy study confirmed the visual inspection that the flocs formed in the presence of NaCl or CaCl2 had similar size, while the addition of Na2SO4 gave rise to larger flocs, which appeared to be loosely connected. This is not

Figure 2. Plot of water uptake versus ionic strength for NaCl, CaCl2, and Na2SO4. 6109


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ACS Sustainable Chemistry & Engineering i.e., reduced the water uptake. Overall, the sizing efficiency did not follow the trend in aggregate size, i.e., Na2SO4 > CaCl2 ≈ NaCl. In fact, the larger size of the aggregates obtained with Na2SO4 could be the reason for the poor surface sizing performance observed. A parameter that may play a role is the effect of shear. Whereas the laboratory experiments discussed above were made without applying a shear force, the size press trials were made under relatively high shear. High shear may lead to a breakdown of the flocs, which, in turn, may affect the sizing performance. The positive effect of CaCl2 is somewhat surprising. The floc formation and the shear sensitivity were similar for CaCl2 and NaCl, but CaCl2 was much more superior in providing hydrophobicity. A possible explanation of this effect is that the calcium ions interact with carboxyl groups present in the paper surface.24 This has been discussed by other authors,25,26 and it has been postulated that calcium ions can form chelates with surface carboxylate groups. Such interactions may contribute to the reduction in water uptake when CaCl2 is added. Effect of Salt on Starch. The starch used in this study was oxidized potato starch. The oxidation was done in order to decrease the molecular weight of the starch and thereby decrease the solution viscosity. The oxidation also generates carboxylate groups in the starch. The carboxylate charge is pH dependent, and we have previously reported a value of −176 μeq/g at pH 5.12 The scattering profile of starch is very weak, and it was not possible to monitor the aggregation behavior directly by following the turbidity. Instead, the viscosity for a 1 wt % starch solution was measured with and without the addition of NaCl corresponding to a concentration of 100 mM. To compensate for dilution, Milli-Q water in an equivalent amount was added to the sample without NaCl. The viscosity for the starch solution was found to decrease when NaCl was added, from the initial value of 2.4 mPas to 1.3 mPas. This can be explained as an effect of changes of conformation that the starch chains undergo when salt is added. The addition of salt decreases the entropy gain from the counterions,27 leading to contraction of the starch,28,29 which, in turn, results in a lower viscosity.30 Effect of Salt on the SP+/Starch Complexes. As demonstrated earlier, stabilization of the complexes formed by SP+ and starch is of electrosteric nature, which means that there are several parameters that can influence the stability of the system.12 In the current investigation, we have focused on the nature and the concentration of salt. The kinetics of aggregation were also considered. Effect of SP+/Starch Ratio and Salt Concentration on the Aggregation. When NaCl was added to the SP+/starch complexes having a starch to SP+ charge ratio of 0.7:1 or higher, the turbidity was found to change in an unexpected and interesting manner. The initial turbidity originating from the SP +/starch complexes was decreased when the salt was added. The turbidity was decreased to values similar to those obtained with the bare particles, and the effect was seen already at low ionic strength. The same was observed for all ratios, as seen in Figure S3. With time, at intermediate ionic strength, the turbidity started to increase, and after approximately 48 h, equilibrium was reached at turbidity values higher than those seen for the SP+/starch complexes only. This is illustrated in Figure 3 where the turbidity evolution with time is shown together with pictures of the samples at different times. We interpret this peculiar behavior result as superimposition of two

Figure 3. Evolution of turbidity with time for SP+/starch complexes at a starch to SP+ charge ratio of 0.7:1 in the presence of 26 mM NaCl.

phenomena related to the starch adsorption. The initial decrease in turbidity can be described as a “screening-reduced adsorption” that occurs when the attraction between the polymer and the particles is mainly of an electrostatic nature.13 The addition of salt screens the electrostatic attraction between patches on the cationic particles covered by the negatively charged starch and bare patches on the particles. The result is a decrease in turbidity. In parallel, the added salt has an impact on the starch. As discussed earlier, starch contracts when the ionic strength is increased, leading to a more coiled structure that gives a denser packing on the particle surface, i.e., an increased starch adsorption.13,31 A larger adsorbed amount usually promotes bridging flocculation.16,32 The net effect will depend on the salt concentration; at intermediate ionic strengths, as illustrated in Figure 3, there is a buildup of the turbidity with time. This can be seen as a “screening-enhanced” adsorption since now the starch chains are coiled and can pack closer on the particle surface; i.e., it behaves like an uncharged polymer where loops and tails are formed.16,17,33 Thus, the increase in turbidity is most likely due to additional bridging flocculation at this intermediate ionic strength.16,32 The degree of neutralization of the complexes had an effect on the maximum value of the turbidity, as well as on the ionic strength required to reach the maximum in turbidity. This effect of salt on the turbidity of colloidal systems has been reported in the literature.34 The higher the starch to SP+ charge ratio, the lower was the maximum in turbidity. A higher charge ratio also shifted the maximum toward higher ionic strength. This can be seen in Figure 4, where the turbidity curves for the higher starch to SP+ charge ratios with added NaCl after 48 h are shown. The samples were allowed to rest between measurements and shaken gently right before measurement. The turbidity increase is due to the formation of aggregates, and previous studies 11 have shown that the maximum in aggregation correlates with the aggregate size. The maximum in turbidity decreases with the increase in starch to SP+ charge ratio. This is likely due to the steric effect of the starch adsorbed on the particle surface;12 the higher the charge ratio, the more starch is adsorbed, contributing to steric stabilization, and thus smaller aggregates are formed. The starch to SP+ charge ratio also affects the onset of the aggregate formation. The ionic 6110


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Figure 5. Size distributions for SP+, for the SP+/starch complex and for the aggregate formed by SP+/starch at an ionic strength of 26 mM NaCl.

Figure 4. Turbidity curves for starch to SP+ charge ratios of 0.7:1, 1.1:1, 1.8:1, and 3.6:1 when NaCl was added to the samples, measured 48 h after the salt addition. A shift in maximum toward higher ionic strength with higher starch to particle ratio can be seen.

With time, the hydrophobic interaction between the cationic particles will dominate when the electrostatic repulsion is screened and salt-induced flocs of SP+ will form. At a higher salt concentration, 80 mM, the sedimentation is immediate. It has been reported in many studies that the order of addition plays a role for the formation of aggregates of particles and polyelectrolytes.38−40 In our previous study on the formation of SP+/starch complexes, it was concluded that neither the order of addition nor the mixing speed had an impact on the aggregate formation. To assess this question in the presence of salt, an experiment was performed where NaCl was added to SP+ with subsequent addition of the starch solution in the same ratio as for the regular case when the salt was added to the SP+/starch complexes. It was found that the turbidity was the same irrespective of the order of addition, i.e., SP+/starch/NaCl or SP+/NaCl/starch, as illustrated in Figure S6. Effect of the Nature and the Valence of the Salt on the Aggregation. The same approach as for the addition of NaCl was used to study the effect of CaCl2 on formation of the SP+/starch aggregates. In general, it was found that CaCl2 influenced the colloidal behavior in the same way as NaCl. For the lowest starch to SP+ charge ratios, the turbidity evolution was the same, with an initial increase in turbidity and with a subsequent decrease in turbidity with time due to sedimentation of the formed flocs. The colloidal behavior at higher starch to SP+ charge ratios was also similar to when NaCl was added, with an initial decrease followed by an increase in turbidity to reach a maximum. The onset of aggregation was shifted toward higher ionic strength the higher the starch to SP+ charge ratio was, as shown in Figure S7. For the higher starch to SP+ charge ratios, the highest CaCl2 concentration gave rise to sufficient screening of the electrostatic repulsions. This led to floc formation as previously seen for the cationic particles alone and for the lowest starch to SP+ charge ratios. This was not seen in the experiments with NaCl because the NaCl concentration was not sufficiently high to induce flocculation. Interestingly, the more pronounced effect observed with CaCl2 compared to NaCl can be normalized to the anion concentration. The increased turbidity at intermediate ionic strength can be correlated to the chloride concentration for the two electrolytes; hence, it is a specific anion effect and not due the ionic strength per se. This is illustrated in Figure 6, where

strength needed for initiating aggregate formation was higher with the more starch that was adsorbed on the particle surface. With increasing ionic strength, the range within the different starch to SP+ charge ratios that gave rise to aggregation became broader, as shown in Figure S4. The widening of the instability regime with increased ionic strength has been described in the literature before35 and is explained by a decrease in the Debye length, which triggers patchwise aggregation.36 The rate of aggregation with increased ionic strength was low compared to the formation of the SP+/starch complexes, which reached a maximum in aggregation immediately after mixing. The maximum in turbidity at increased ionic strength was reached after more than 48 h. The electrostatic attraction between patches on the cationic particles covered by the negatively charged starch and bare patches on the particles was here partly screened by the increased ionic strength, causing a decrease of the aggregation rate.35,37 The steep decrease in turbidity for the starch to SP+ charge ratio of 0.7:1 above 50 mM in ionic strength was due to formation of larger flocs that settle, resulting in a decrease of the apparent turbidity. We used dynamic light scattering to examine the size distributions of SP+ and the SP+/starch complexes with and without NaCl. The starch to SP+ charge ratio used was 0.7:1, and the ionic strength for the SP+/starch/NaCl sample was set to 26 mM. This NaCl concentration corresponds to the maximum in turbidity. A bimodal distribution was found for the SP+/starch/NaCl complexes, clearly different from the monomodal distribution seen for SP+ and for the SP+/starch complex. A comparison of size distributions for SP+ solely, SP +/starch complex, and SP+/starch/NaCl aggregate is shown in Figure 5. After 14 days of storage, the samples with maximum turbidity had similar or slightly higher turbidity values compared to the 48 h values demonstrating the stability of the formed aggregates. The starch to SP+ charge ratios also influenced the kinetics of the aggregate formation, as can be seen from Figure S5. At low starch to SP+ charge ratios, there was an immediate increase in turbidity when the ionic strength was increased with NaCl up to 50 mM. The initial increase in turbidity is probably due to a bridging effect as described above, but the amount of starch was not sufficient to provide stability of the aggregates. 6111


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corresponded to the NaCl and CaCl2 concentrations that give maximum turbidity. From the turbidity study, it was found that it was the chloride concentration that governed the aggregation behavior. The results from the size press trial when NaCl or CaCl2 was added to the SP+/starch mixture are shown in Figure 7a, where the water uptake values are plotted versus the chloride ion concentration. It can be seen that the addition of either of the two salts to the SP+/starch mixture was detrimental for the performance since the water uptake increased with an increase in salt concentration. Thus, the ionic interaction with the SP+/starch complex that gave an increased turbidity influenced the surface sizing performance in a negative manner. This anion effect is also clearly seen in the size press trial when either NaCl or Na2SO4 was added, see Figure 7b. The negative impact is much more pronounced for Na2SO4 than for NaCl, demonstrating the anion effect according to the Hofmeister series and also demonstrating the importance of the valence of the anion. No changes in the water uptake values for paper surfaces treated with only starch could be found when the ionic strength was varied (data not shown). This can be explained by the paper sized with only starch being fully saturated with water in the Cobb60 test; therefore, additional water uptake cannot take place. Size press trials with the same cation, sodium, but with different anions were performed using NaCl, NaBr, and Na2SO4 as electrolytes. In this study, two starch concentrations were used, one corresponding to the more aggregated state, i.e., 0.3 wt %, and one to the starch concentration normally used in the size press, i.e., 8 wt %. The size press results are shown in Figure 8. As can be seen, all three salts gave a pronounced increase of the water uptake, and the effect increased in the order NaCl < NaBr < Na2SO4. The fact that Na2SO4 gave the strongest effect is probably related to the divalent anion’s stronger ability to flocculate cationic particles, as discussed above. The larger impact on the increase in water uptake with NaBr compared to NaCl is in accordance with the indirect Hofmeister series that positively charged particles follow.41 With the higher starch concentration, 8 wt %, the water uptake also increased for all three electrolytes, but the effect of the salts was much less pronounced than for the low starch concentration. Obviously, the effect of added electrolyte

Figure 6. Plot of chloride ion concentration needed to reach a maximum in turbidity for different starch to SP+ charge ratios.

the anion concentration to reach maximum aggregation is plotted as a function of the starch to SP+ charge ratio. The curves align well, illustrating the negligible effect of the cation. Different monovalent salts were tested in order to investigate how the nature of the ion affects the aggregation behavior. It was found that the effect of both KCl and NaBr was very similar to that of NaCl. Effect of the Ionic Strength on the Surface Sizing Performance of SP+/Starch. Size press trials were performed in which SP+ were mixed with starch and the ionic strength subsequently increased by the addition of either NaCl, CaCl2, or Na2SO4. The starch concentration used was 0.3 wt %, corresponding to the starch to SP+ ratio that gives a maximum surface sizing effect. The equilibration time for the aggregate formation when salt was added to the SP+/starch complex was found to be more than 48 h, as observed in the turbidity study. This is a very long waiting time compared to the time scale of the size press trial, which is a continuously running process. Taking into account that the size press trial is performed at elevated temperatures (to prevent retrogradation of the starch) and that the aggregation kinetics have previously been found to be temperature dependent (the higher the temperature, the shorter the time to reach equilibrium),12 2 h of equilibration time was used. The ionic strength used in the size press trial

Figure 7. (a) Water uptake results for SP+/starch when NaCl or CaCl2 was added, plotted versus chloride ion concentration. (b) Water uptake results for SP+/starch when NaCl or Na2SO4 was added, plotted versus ionic strength. In some cases, the error bars are smaller than 3 % and are then not visible in the graph. 6112


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Figure 8. (a) Water uptake results for the size press trial with SP+ when 0.3 wt % starch was used and with NaCl/NaBr/Na2SO4 used as electrolytes. (b) Water uptake results for the size press trial with SP+ when 8 wt % starch was used and with NaCl/NaBr/Na2SO4 used as electrolytes. In some cases, the error bars are smaller than 3 % and are then not visible in the graph.

nanoparticles and anionic starch is important for surface sizing performance. It is also shown that salt addition can drastically affect the colloidal behavior. When the cationic particles were combined with the anionic starch in a ratio corresponding to a highly aggregated state, a very efficient hydrophobization was obtained. The aggregates exhibited very good colloidal stability. The effect of electrolytes on the surface sizing performance depended on the salt type. Whereas the addition of Na2SO4 reduced the sizing efficiency, the addition of CaCl2 was beneficial. Addition of NaCl did not affect the performance significantly. The superior efficacy of CaCl2 might be attributed to the formation of chelates between calcium ions and carboxylate groups present on the fibers. The flocs formed from SP+ in the presence of salts were found to be shear sensitive. The size press trials with SP+ in combination with starch showed that the addition of salt reduces the sizing efficiency. Na2SO4 in particular turned out to have a negative impact on the performance. No difference in performance was seen between NaCl and CaCl2, indicating that the salt impact was mainly an anion effect. This study underlines the importance of the aggregation state for the efficiency of surface sizing of paper and suggests a new approach for reducing the use of material based on nonrenewable resources in surface modification by colloidal systems.

becomes relatively small when starch is used in a large excess. This behavior is most likely related to the reduced tendency for flocculation of the system at high starch concentration, see Figure 4. The Nature of the Aggregates. On the basis of the above results, it is evident that, in the presence of salt, the SP+/starch complexes behave differently compared to SP+ alone. This is already reflected in the colloidal behavior of the systems. It is generally believed that to achieve proper hydrophobization of a paper surface, the hydrophobic particles used as sizing agents should be small. Only small particles with a large surface to volume ratio are able to undergo film formation upon drying, which is seen as an essential step in the sizing process. It is therefore surprising that the formation of aggregates of the cationic nanoparticles with the anionic starch was found to be beneficial for the surface sizing performance. One may speculate that the aggregates formed will have better retention due to the large size and/or better surface affinity due to attractive interaction with the starch compared to the nonaggregated negatively charged particles, SP−. Nevertheless, the picture should not be oversimplified: “the bigger the better” does not hold. When extensively aggregated by the addition of salt, the efficiency of SP+ alone was mostly lost, and the resulting aggregates exhibited mechanical weakness. The addition of salt to the SP+/starch complexes influenced the electrostatic contribution to the stabilization of the aggregates, and the starch conformation was also affected. Further aggregation and formation of larger flocs were obtained, and a specific anionic effect could be seen. At a starch to cationic particle charge ratio corresponding to maximum aggregation, these large aggregates gave poor sizing performance. The experimental findings from this work seem to indicate that formation of complexes between the hydrophobic nanoparticles and the anionic starch is beneficial as long as the aggregates do not become too large. However, we do not have enough data to quantify this statement; i.e., we cannot suggest an optimal size of the aggregates. Most likely, the ability of the aggregates to undergo film formation during the drying process is also an important parameter.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00984. A table with the ion types and abundance and additional experimental data from the study of aggregation at increased ionic strength (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected]. *E-mail: [email protected].

CONCLUSION The results presented in this paper demonstrate that the colloidal behavior of aggregates composed of hydrophobic

ORCID

Frida Iselau: 0000-0002-2192-4734 6113


Research Article


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Romain Bordes: 0000-0002-0785-2017 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swedish Research Council is gratefully acknowledged. The authors thank SuMo Biomaterials for economic and scientific support. Kerstin MalmborgNyström is acknowledged for performing the size press trials.

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ACS Sustainable Chemistry & Engineering (40) Štajner, L.; Požar, J.; Kovačević, D. Complexation between lysozyme and sodium poly(styrenesulfonate): The effect of pH, reactant concentration and titration direction. Colloids Surf., A 2015, 483, 171−180. (41) Oncsik, T.; Trefalt, G.; Borkovec, M.; Szilagyi, I. Specific ion effects on particle aggregation induced by monovalent salts within the Hofmeister series. Langmuir 2015, 31, 3799−3807.

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