Salt Mixtures

Article pubs.acs.org/IECR

Rheology of Poly(acrylic acid) in Water/Glycol/Salt Mixtures Yuchen Wang,† Richard A. Pethrick,‡ Nicholas E. Hudson,‡ and Carl J. Schaschke*,† †

Department of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, Scotland G1 1XJ Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, Scotland G1 1XL

‡

ABSTRACT: Aqueous mixtures of 1,2-propylene glycol with added poly(acrylic acid), neutralized and with added salt, are used for aircraft deicing. This study reports data on the effects of varying the salt concentration, architecture, and temperature of a series of different mixtures. For a deicing fluid to be effective, it is desirable that it be able to create a liquid layer that is stable under low-shear conditions yet can be completely removed during the initial stages of takeoff. A number of the mixtures examined show a viscosity profile that is either almost independent of temperature or exhibits a peak. To gain a greater understanding of the factors responsible for the observed rheological behavior, a theoretical model was fitted to oscillatory data. Using two fitting parameters, it was possible to describe the changes in the observed behavior, suggesting that, as the temperature is varied, the extent of shielding of carboxylic acid groups and the conformation of the chain balance one another to give an apparent temperature independence of the viscosity. The rheological data were used to interpret the boundary-layer displacement thickness data obtained from wind-tunnel measurements. molecules.28 In this work, we explore the application of this model to the description of a typical deicing formulation. Measurements of the performance of these fluids in a wind tunnel were carried out to explore the possible correlation between the measured rheological characteristics and the performance of the fluids in a simulated real situation.

1. INTRODUCTION Poly(acrylic acid) (PAA) is a hydrophilic associative polymer that has been used extensively as a viscosity thickener and gelforming medium and finds application in, for example, pharmaceuticals, water treatment, cosmetics, and controlled release of biologically active agents.1−7 In very dilute solutions, a large portion of the carboxylic acid groups of PAA will be ionized and repel each other, forming pseudoisolated polymer chains.7−13 As the polymer concentration increases, the extent of dissociation will decrease, leading to apparent changes in polymer size that are, in turn, influenced by pH and added salt concentration. These effects have been investigated extensively for aqueous solutions.14,15 PAA is widely used as a thickener for aircraft deicing fluids, which consist of mixtures of propylene glycol and water adjusted to have a suitably depressed freezing point, with PAA added to impart viscoplastic characteristics.15−26 In addition to being able to disperse ice that has been deposited on an aircraft, the deicing fluid is required to form a thin layer on the surface that protects the aircraft from further icing. If snow is deposited, it will partially melt but then forms a thickened fluid that is retained on the surface. During this period, the aircraft is standing still or taxiing to the holding area.23−26 Once the aircraft accelerates during the first 20 s before lift-off, the fluid layer must be cleanly sheared from the aircraft surface. The fluids are produced at a pH of approximately 7 for environmental reasons, and an inert electrolyte, such as sodium chloride, is added to suppress the viscosity and achieve the viscoplastic characteristics allowing the fluid layer to flow when the air velocity is increased. In this study, we investigate the influences of the electrolyte on the rheological properties of PAA-thickened glycol/water mixtures. In a previous work,27 we showed that it is possible to describe the shear-dependent viscosity of PAA in water in terms of a theoretical model that includes descriptions of Rouse-like behavior, combined with reptation and stick−slip motions, as well as the possibility of complexation between neighboring © 2012 American Chemical Society

2. EXPERIMENTAL DETAILS 2.1. Materials and Sample Preparation. Two poly(acrylic acid) polymers (carbomers A and B) were supplied by Lubrizol (Brussels, Belgium) as flocculated solid particles with a diameter of ∼0.2 μm and nominal molar masses of 4 × 106 and 5 × 106 g/mol, respectively. A detailed characterization of these materials was undertaken and reported previously.16 To achieve quick dissolution, the solid polymers were dispersed using a Silverson high-shear mixer placed in a 1 L beaker containing 600 g of deionized water and operated at 4800 rpm for 5 min; upon completion, the rotation speed was reduced to 3600 rpm, and stirring was continued for a further 55 min. A gel-like dispersion was produced that was mixed with a 1,2-propylene glycol/deionized water mixture to achieve a total polymer concentration of 0.30 g/dL. The ratio of glycol/water was 50:50 (wt/wt). Sodium chloride (5.0 M) was progressively added to the solution, and a series of samples were taken. Each sample was neutralized to a pH of ∼7.0, using 5.0 M potassium hydroxide aqueous solution. The electrolyte levels of all of the samples are presented in Table 1. 2.2. Rheology. Viscosities were measured using both a temperature ramp at fixed stress and a steady-shear-stress ramp at various temperatures. These measurements were carried out on a shear-stress-controlled Carri-Med CSL2500 rheometer (TA Instruments, Crawley, U.K.) using a 4-cm parallel plate Received: Revised: Accepted: Published: 594

October 11, 2012 December 11, 2012 December 12, 2012 December 12, 2012 dx.doi.org/10.1021/ie302765j | Ind. Eng. Chem. Res. 2013, 52, 594−602

Industrial & Engineering Chemistry Research

Article

0, −5, −10, and −15 °C (not applicable for 50 vol % dilution because of freezing). During each run, the air velocity was increased, at a linear rate, from stationary to 65 m s−1 over a period of 25 s. Tests at each temperature were performed in triplicate.

Table 1. Electrolyte Levels of Sample Solutions sample PAA A1 A2 A3 A4 A5 A6 A7

A A A A A A A

NaCl (g/100 g of solution) 0 0.030 0.045 0.061 0.076 0.091 0.122

sample PAA B1 B2 B3 B4 B5

B B B B B

NaCl (g/100 g of solution) 0 0.030 0.061 0.091 0.122

3. RESULTS AND DISCUSSION The two carbomer polymers are poly(acrylic acid) materials: Carbomer A is essentially a linear polymer with a low level of anhydride linkages. Carbomer B is a copolymer that contains a low level of pentaerythritol to create a more branched chain structure. 3.1. Carbomer A (PAA-[A]). 3.1.1. Temperature-Ramp Test. Temperature-ramp tests confirmed that partially neutralized PAA has a significant thickening effect on the glycol/water mixture, and the viscosity increased steadily as the temperature was lowered (Figure 1). At a shear stress of 5 Pa (simulating conditions on taxiing or standing under a light wind), the water/glycol mixture had a viscosity of ∼0.0078 Pa s at 15 °C, rising to ∼0.046 Pa s at −15 °C. The viscosities of the thickened mixture were approximately 103 times higher (∼9.37 and ∼48.64 Pa s, respectively), indicating the thickening power of the PAA. This latter increase in viscosity as the temperature is lowered is consistent with an increase in the hydrodynamic radius. Decreasing temperature leads to population of the more extended conformations, which have lower energy. The viscosity levels at any value of temperature were found to be independent of whether the fluid was being heated or cooled. The addition of sodium chloride alters the interactions between the carboxylic groups of PAA by randomly blocking the interaction between neighboring groups, preventing the polymer from forming gel-like entities and resulting in a decrease in viscosity. Varying the sodium chloride level not only changes the magnitude of the viscosity but also the form of its temperature dependence (Figure 1). Unlike solutions with no electrolyte, those containing electrolyte exhibit a peak in viscosity that changes with changing temperature. For example, the solution with the highest level of electrolyte, A7 (cNaCl =

fitted with a solvent trap. Temperature was controlled by using the Peltier effect and an antifreezing bath maintained at 0 °C, enabling test temperatures between +15 and −15 °C. Temperature-ramp tests were carried out at a rate of 1.0 °C/ 2 min from +15 to −15 °C and then from −15 to +15 °C at shear stresses of 5 and 10 Pa. Steady-shear-stress tests were carried out at 0, −5, −10, and −15 °C at 0.1−100 Pa, with a 10fold increase in stress per 10 min and a subsequent decrease. 2.3. Wind-Tunnel Testing. The ability for the surface layer to be removed during the initial stages of takeoff was measured in wind-tunnel tests. The wind tunnel consists of a duct through which temperature-controlled air is blown. The effectiveness of the removal of the fluid was measured using pressure sensors to determine the boundary-layer displacement thickness (BLDT). The BLDT is a measure of the film thickness remaining on the surface during its continuous removal by an increasing flow of air. The acceleration of the air flow mimics the passage of air over the wing during the initial stages of takeoff. Tests were performed on carbomer A4 solution with an electrolyte concentration of 0.061 g of NaCl per 100 g of solution, as this system has characteristics similar to those found in commercial fluids. The BLDT values were measured for the pure fluid (100%) and 75 and 50 vol % dilutions with deionized water. The test fluid was initially applied as a coating on the floor of the duct at a thickness of 1.5 mm using a doctor blade, simulating the coating that would be created during spray application. The temperature of the test facility was thermostatically controlled, and tests were carried out at approximately

Figure 1. Temperature/viscosity plot for carbomer A, cPAA‑[A] = 0.30 g/dL, at a shear stress of 5 Pa. Electrolyte levels for all samples are given in Table 1. 595

dx.doi.org/10.1021/ie302765j | Ind. Eng. Chem. Res. 2013, 52, 594−602


Article

Figure 2. Temperature/viscosity plot for carbomer A, cPAA‑[A] = 0.30 g/dL, at a shear stress of 10 Pa. Electrolyte levels for all samples are given in Table 1.

level, while still retaining “viscostaticity” for deicing applications. 3.1.2. Steady-Shear Flow Tests. In the context of deicing, the film must be stable up to the hold point (at the end of the runway prior to takeoff). This is achieved by having a sufficiently high value of the viscosity. For the layer to be cleaned by shearing, it is essential to understand how the viscosity changes with increasing shear rate. Figure 3 shows the

0.122 g of NaCl/100 g of solution), had a viscosity of ∼0.29 Pa s at +15 °C and ∼0.26 Pa s at −15 °C, with a “peak” observed at around 2.3 °C. As the temperature changes, the equilibrium constant associated with binding of the salt to the carboxylic acid will vary, as will the conformation of the polymer. In the presence of the salt, the two effects appear to balance one another, leading to an apparent peak or a temperature independence of the viscosity. This is clearly a desirable phenomenon if the fluid is to perform adequately over a temperature range from +5 to −15 °C. A decrease in the ionic concentration results in the formation of globular structures that are considered precursors of gel formation. Collapse of the polymer chain to form a globular structure prior to gelation leads to the observed reduction in the viscosity as the solution is cooled below 2.3 °C. It is worth noting that the decrease in viscosity with electrolyte level also passed through a minimum (at around cNaCl = 0.08 g of NaCl/100 g of solution) before passing through a maximum. This phenomenon is under investigation. Measurements were also performed at a shear stress of 10 Pa (simulating conditions on the runway prior to takeoff) (Figure 2). Again, the viscosity levels at any value of temperature did not depend on whether the fluid was being heated or cooled (little or no hysteresis). In comparison to the solution without electrolyte, the fluids containing salt exhibited apparently constant viscosity over the temperature range studied. For sample A2, with cNaCl = 0.030 g of NaCl/100 g of solution, the peak appeared at around −3.6 °C for a shear stress of 5 Pa and at −6.9 °C for a shear stress of 10 Pa. Sample A3 (cNaCl = 0.045 g of NaCl/100 g of solution) peaked at −1.3 °C for 5 Pa and at −2.9 °C for 10 Pa, and sample A4 (cNaCl = 0.061 g of NaCl/100 g of solution) peaked at −0.9 °C for 5 Pa and at −3.2 °C for 10 Pa. Solution A6 (cNaCl = 0.091 g of NaCl/100 g of solution) exhibited a peak at 5.6 °C for 5 Pa and at 1.9 °C for 10 Pa; for sample A7 (cNaCl = 0.122 g of NaCl/100 g of solution) at 10 Pa, the viscosity peak shifted to −1.6 °C. Generally, the temperature at which the viscosity peak occurred increased with increasing electrolyte level. However, it decreased at higher shear stresses. Thus, the addition of the electrolyte allows the solution to have its viscosity adjusted to a desired

Figure 3. Shear stress/viscosity plots for A4 at various temperatures, cNaCl = 0.061 g of NaCl/100 g of solution.

dependence of viscosity on shear stress at various values of temperature. Similar characteristics were observed for solutions studied at various salt concentrations (Figure 4). As the shear rate increases, the ability of the polymer to respond decreases, and the viscosity also decreases. The viscosity/shear rate characteristics are a reflection of polymer mobility and change with the level of salt addition. At the lower-stress end of the stress/viscosity plot, the viscosity of a solution was found to drop by approximately an order in magnitude when the electrolyte level was doubled (compare sample A2 to sample A4 and sample A3 to sample A6); even at the higher-stress end, viscosity still decreased to half of its original value when the electrolyte concentration was doubled. 596



Article

Figure 4. Shear stress/viscosity plot for the PAA-[A] series at 0 °C. Electrolyte levels for all samples are given in Table 1.

Figure 5. Temperature/viscosity plot for the carbomer B series, shear stress 5 Pa, cPAA‑[B] = 0.30 g/dL. Electrolyte levels for all samples are given in Table 1.

3.2. Carbomer B (PAA-[B]). A previous study indicated that PAA-[B] has a more highly branched structure than PAA[A].27 Its glycol/water mixture-based solution is acidic (pH 3.61) at 20 °C. When neutralized to pH ∼7 using 5.0 M potassium hydroxide solution, a PAA-[B] solution becomes very viscous. The temperature/viscosity behavior for pH 7 sample solution B1 with varying electrolyte addition is shown in Figure 5. At the lower electrolyte concentrations, the curves exhibited hysteresis; the curve upon heating (upper) maintained a higher viscosity that was not recovered upon cooling until a temperature of −15 °C was achieved. The location of the peak was found to depend on the shear stress and the electrolyte concentration. With electrolyte addition, both an immediate drop in viscosity and a loss of the peak in the temperature domain were observed. Sample B2 had a sodium chloride concentration of 0.030 g of NaCl/100 g of solution (the same as PAA-[A] sample A2), but its viscosity values decreased to approximately 0.25 Pa s at 15 °C and 0.44 Pa s at −15 °C (compared to 1 and

2 Pa s, respectively, for sample A2) when 5 Pa of shear stress was applied. Under these conditions, sample A2 showed a peak in its viscosity of approximately 3.5 Pa s at −3.6 °C, whereas sample B2 showed little or no peak. PAA-[B] is much more sensitive to electrolyte in terms of viscosity level than PAA-[A]. The inclusion of branches in the chain increases the sensitivity to electrolyte and also leads to the observation of a lower viscosity. Branched chain polymers have a smaller hydrodynamic volume than linear polymers with an equivalent molecular mass, which, in part, explains some of the observed differences in behavior. However, the chain branches also inhibit extended elements of the chain from interacting with neighboring polymer molecules, and this will also contribute to differences in the observed behavior. The rheological behaviors of the two carbomers indicate that, even in dilute solutions of about 0.30 g/dL, significant intraand interchain interactions are occurring that can be mediated by the addition of salt. The salt is able to interact with the carboxylic acid groups and partially block these intra- and 597



Article

Figure 6. Model fitting for carbomer A solution A4 (cNaCl = 0.061 g per 100 g of solution) at 20 °C.

interchain interactions. The extent of these interactions varies with temperature and leads to a peak in the viscosity/ temperature plots for linear polymer systems but not for more branched (carbomer B, in this case) systems. 3.3. Modeling. In a previous work,27 a phenomenological model was developed to describe the dynamic response of a polymer in solution. The model successfully described the behavior of poly(acrylic acid) in aqueous solution and combines Rouse theory (which describes the short-range motions of polymers in solution29) with the reptation theory of Doi−Edwards and the stick−slip modification of Doi, thereby combining scaling parameters that allow for intra- and interchain interactions.27−32 The overall viscosity is described by the equation η1(ω) = ηs + +

+

+

6(η0 − ηs) π

2

Nc

1 8(ηe − η0) z1 2 π2

Ne p=1

π2

Ne

∑ p=1

p2 p4 + ω 2τb 2 Nd

∑ p=1

p2 p4 + ω 2τd12

⎤ ⎥ p4 + ω 2τd2 2 ⎥⎦ p2

3π 2kT

temperature (°C) 20 10 0 −10

(4)

ηs (Pa s) 6.8 9.7 1.6 3.1

× × × ×

10−3 10−3 10−2 10−2

η0 (Pa s)

ηe (Pa s)

0.35 0.50 0.56 0.72

500 1200 1800 1500

of ηs were obtained from measurements of the solvent mixture before the polymer was added. The values of ηe were obtained from the low-shear measurements, and the values of η0 were estimated from the concentration dependence of the viscosity reported previously.27 The molecular mass of the polymer was previously determined from intrinsic viscosity measurements and taken to be 1 × 106 g/mol, and a normal distribution was assumed for the molecular mass distribution. The main

6η0Mc π 2p2 ρRT

c0b2N 2Ne 2

Table 2. Values of the Viscosities Used in the Theoretical Fit of the Data.

(1)

where η1 is the viscosity of the solution at frequency ω, ηs is the solvent viscosity, ηe is the equilibrium viscosity measured at low strain, and η0 is the terminal viscosity corresponding to the Rouse relaxation process. The mode numbers p are modified to include polymer molecules having a normal molecular mass distribution. The Rouse relaxation time has the form

τr =

(3)

where c0 is the Rouse bead friction coefficient, b is the monomer Kuhn length, N is the total number of the monomer (Kuhn) units, and Ne is the number of monomers (Kuhn units) between entanglement points. To allow for intra- and intermolecular interactions, two scaling parameters were introduced, namely, h1 and z1. (Note that h2 = 1 − h1 and z2 = 1 − z1.) The parameter h1 allows for the degree of complexation, and z1 allows for the degree of interaction, and both are defined in full in the previous article.27 Carbomer A at an electrolyte concentration of 0.061 g per 100 g of solution (sample A4) was selected for dynamic oscillatory studies because it had a viscosity/temperature profile that would make it useful for deicing applications. Tests were performed at 20, 10, 0, and −10 °C, and the data obtained, together with the best theoretical fits to the data, are presented in Figures 6−9, respectively. The theoretical curves were fitted using the values of the viscosities given in Table 2. The values

p + ω 2τr 2

∑

⎡ 1 ⎢ 8(ηe − η0)h1 z2 2 ⎢⎣ π2 8(ηe − η0)h2

τd1 =

4

p=1

ρRT

where Ma is the molecular mass of the polymer. The stick−slip relaxation process has a relaxation time given by

p2

∑

ρ0 Ma 3

τb =

(2)

where Mc is the limiting molecular mass for entanglement, which is taken as having a value of 21000 g/mol; ρ is the density; R is the gas constant; and T is the absolute temperature. The reptation motion has a relaxation time of 598



Article

Figure 7. Model fitting for carbomer A solution A4 at 10 °C.

Figure 8. Model fitting for carbomer A solution A4 at 0 °C.

Figure 9. Model fitting for carbomer A solution A4 at −10 °C.

variables in the theory are the parameters h1 and z1 defining the extent of complexation of the polymers. It is assumed that the molecular mass remains unchanged, and therefore, the total number of modes within a polymer

chain, Kuhn units, and entanglement size remain unchanged. For a decrease in the electrolyte concentration, the coefficient for the “slip−coil” process needs to be increased from 0.015 to 0.018 to fit the experimental data, which suggests that, because 599



Article

Figure 10. BLDT for A4 (cNaCl = 0.061 g of NaCl per 100 g of solution): (+) undiluted, (□) 75% (wt/wt) concentration, (○) 50% (wt/wt) concentration, () acceptance. The dashed lines are guides to the eye.

the temperature is lowered, the ability for the salt to complex with the carboxylic acid groups of the polymer is decreased, and this ultimately leads to the formation of globular gel particles and the precipitation of the polymer. Changes in the degree of shielding of the interaction between groups will influence both the chain mobility and the extent to which inter- and intrapolymer interactions lead to complexation. These changes will influence the extent to which ladder and loop structures are created and, hence, the overall hydrodynamic volume of the polymer. 3.4. Wind-Tunnel Evaluation of Carbomer Fluids. For a fluid to have suitable characteristics to perform as a type II, III, or IV fluid, it is important that the film that is created during the deicing process be removed in the period from hold-up to rotation. From previous temperature-ramp tests, it was found that sample A4 (electrolyte concentration of 0.061 g of NaCl per 100 g of solution) has a viscosity profile that is similar to those obtained for commercial fluids. A wind-tunnel test simulates the acceleration of air flow that occurs as an aircraft increases its speed prior to achieving rotation. The ability for the layer deposited on the wing surface to be removed by the increasing air flow is a critical parameter in judging the performance of a fluid. The values of the boundary-layer displacement thickness (BLDT) are measured in terms of the film thickness remaining after 25 s, which is the rotation time for most small- to medium-sized aircraft.36 Values for the BLDT obtained for sample A4 are presented in Figure 10. Acceptable BDLT values for fluids are lower than 11.3 at −20 °C and lower than 8.75 at 0 °C.23 The performance of the sample fluid at 100% was found to lie approximately on the limiting line and remain stable across the investigated temperature range. The BLDT values measured around −10 °C were slightly higher than those measured at other temperatures. This corroborates the observation of a peak in viscosity observed in the temperature-ramp studies. The peak was shifted to lower temperature when higher shear stress was applied to the fluid, reflecting the ability to achieve (with a greater energy input) a different equilibrium conformation. In wind-tunnel tests, high air velocity resulted in a shear stress higher than 10 Pa; therefore, it is reasonable to assume that a viscosity peak would be exhibited at temperatures below −3.2

the polymer chain is shielded by the electrolyte, the polymer chain adopts a more extended structure and the contribution from the slip−coil process is increased.27 As the temperature decreases, the population of the lower-energy conformations will be increased, creating a more extended structure that undergoes a greater degree of polymer−polymer interaction. Changing the temperature will lead to salting out of the electrolyte, which, in turn, will increase the number of carbonyl groups that can undergo interactions through hydrogen bonding.33−35 The observation of a peak can be attributed to the balance of these two effects. At 20 °C (Figure 6), the viscosity decreases almost linearly (on the log/log plot), and it is not obvious how the balance of the various contributions would give rise to the observed behavior. However, at the lower temperature of 10 °C (Figure 7), a separation of the highershear-rate Rouse process from the lower-shear-rate processes begins to appear. The model indicates that the degree of complexation and the slip−coil coefficient are reduced to 0.86 and 0.01, respectively, leading to a lower contribution from the stick−slip motion of the loops formed within the chain. Decreasing the temperature to 0 °C (Figure 8) increases the separation between the Rouse type of motion and the slower processes, and the fitting parameters h1 and z1 rise to 0.92 and 0.012, respectively. The temperature-ramp test (Figure 1) indicated a peak in viscosity at approximately −0.9 °C, which suggests that the process of electrolyte salting out is causing an increase in carboxyl interactions and supports the theory in which both parameters are rising. Then, as the temperature is continuously lowered, a decrease in the ionic concentration finally leads to a state in which the polymer chains are so entangled that they are forming species that will eventually precipitate from solution and form a gel. Prior to this phase-separated state, collapse of the polymer chain to form a globular structure explains the observed reduction in the viscosity in the temperature-ramp tests.34,35 At −10 °C (Figure 9), h1 is reduced to 0.9 because of the formation of these globular structures. However, z1 has increased to 0.016, which suggests that the extent to which the polymer chains are forming loops is increasing. This analysis indicates that the desirable peak in the viscosity of PAA is achieved by a complex interplay of several factors. As 600



Article

Figure 11. Temperature−viscosity plot for A4 at 5 Pa applied shear stress. Concentrations (wt/wt, %) are indicated.

■

°C, which was the peak temperature for sample A4 at a shear stress of 10 Pa (Figure 2), and it would be normal for the fluid to have a higher BLDT value at around −10 °C. After dilution, the random-coil structure of the polymer chains will swell and form a more extended structure. With the extent of complex entanglements of polymer chains being reduced, the increased value of the BLDT would be in parallel with the increased viscosity as the temperature was lowered (Figure 11). As the solution was diluted with water, the values would decrease, but the lowest usable temperature would also increase. The ability to achieve the required values of BLDT are clearly closely connected to the observation of the peak in the viscosity, which is, in turn, related to the complex array of interand intrachain interactions that exist for PAA. The ability for the salt to shield the carboxylic acid interactions is critical to achieving the desired level of the viscosity, the creation of a peak in the viscosity in the temperature-ramp measurements, and the oscillatory shear response that determines the ability of the fluid to be released from the wing during the initial stages of takeoff.

(1) Chu, J. S.; Yu, D. M.; Amidon, G. L.; Weiner, N. D.; Goldberg, A. H. Viscoelastic Properties of Polyacrylic-Acid Gels in Mixed Solvents. Pharm. Res. 1992, 9, 1659. (2) Bhosale, P. S.; Berg, J. C. Poly(acrylic acid) as a rheology modifier for dense alumina dispersions in high ionic strength environments. Colloids Surf. A 2010, 362, 71. (3) Winnik, M. A.; Yekta, A. Associative polymers in aqueous solution. Curr. Opin. Colloid Interface Sci. 1997, 2, 424. (4) Glass, J. A perspective on the history of and current research in surfactant-modified, water-soluble polymers. J. Coat. Technol. 2001, 73, 79. (5) Viota, J. L.; Delgado, A. V.; Arias, J. L.; Duran, J. D. G. Study of the magnetorheological response of aqueous magnetite suspensions stabilized by acrylic acid polymers. J. Colloid Interface Sci. 2008, 324, 199. (6) Jones, D. S.; Muldocin, B. C. O.; Woolfson, A. D.; Sanderson, F. D. An examination of the rheological and mucoadhesive properties of poly(acrylic acid) organogels designed as platforms for local drug delivery to the oral cavity. J. Pharm. Sci. 2007, 96, 2632. (7) Bromberg, L.; Temchenko, M.; Alakhov, V.; Hatton, T. A. Bioadhesive properties and rheology of polyether-modified poly(acrylic acid) hydrogels. Int. J. Pharm. 2004, 282, 45. (8) Wang, J.; Li, L.; Ke, H. L.; Liu, P.; Zheng, L.; Guo, X. H.; Lincoln, S. F. Rheology control by modulating hydrophobic and inclusive associations of side groups in poly(acrylic acid). Asia-Pac. J. Chem. Eng. 2009, 4, 537. (9) Miquelard-Garnier, G.; Demoures, S.; Creton, C.; Hourdet, D. Synthesis and rheological behavior of new hydrophobically modified hydrogels with tunable properties. Macromolecules 2006, 39, 8128. (10) Guo, X. H.; Abdala, A. A.; May, B. L.; Lincoln, S. F.; Khan, S. A.; Prud’homme, R. K. Rheology control by modulating hydrophobic and inclusion associations in modified poly(acrylic acid) solutions. Polymer 2006, 47, 2976. (11) Bossard, F.; Sotiropoulou, M.; Staikos, G. Thickening effect in soluble hydrogen-bonding interpolymer complexes. Influence of pH and molecular parameters. J. Rheol. 2004, 48, 927. (12) Bromberg, L.; Temchenko, M.; Colby, R. H. Interactions among hydrophobically modified polyelectrolytes and surfactants of the same charge. Langmuir 2000, 16, 2609. (13) Foerster, S.; Schmidt, M.; Antonietti, M. Experimental and theoretical investigation of the electrostatic persistence length of flexible polyelectrolytes at various ionic strengths. J. Phys. Chem. 1992, 96, 4008. (14) Tunc, S.; Duman, O.; Uysal, R. Electrokinetic and rheological behaviors of sepiolite suspensions in the presence of poly(acrylic acid

4. CONCLUSIONS The rheological behaviors of carbomers A and B in propylene glycol/water mixtures at various temperatures illustrate the complex nature of the interactions that define their viscosity behavior. In comparison with the behavior found in solutions with no electrolyte,16 the viscosities were lower, but the shielding effects introduced by the salt created an additional complexity in the temperature dependence that allowed the viscosity to remain almost constant or exhibit a slight peak in the temperature range of interest for deicing fluids. The salting out of the electrolyte can create gels that have the potential to become insoluble. This problem is the subject of current investigations.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: 0044 (0) 141 548 2371. E-mail: carl.schaschke@strath. ac.uk. Notes

The authors declare no competing financial interest. 601



Article

sodium salt)s, polyacrylamides, and poly(ethylene glycol)s of different molecular weights. J. Appl. Polym. Sci. 2008, 109, 1850. (15) Coffey, D. A.; Nieh, E. C. Y.; Armstrong, R. A. (Texaco Inc.). Aircraft Deicing Fluid with Thermal and pH-Stable Wetting Agent. U.S. Patent 5,389,276, 1995. (16) Jenkins, R. D.; Bassett, D. R.; Lightfoot, R. H.; Boluk, M. Y. (Union Carbide Chemicals & Plastics Technology Corporation). Aircraft anti-icing fluids thickened by associative polymers. U.S. Patent 5,681,882, 1997. (17) Jenkins, R. D.; Bassett, D. R.; Lightfoot, R. H.; Boluk, M. Y. (Union Carbide Chemicals & Plastics Technology Corporation). Aircraft anti-icing fluids. World Patent 9,324,543, 1995. (18) Ross, F. (Kilfrost Limited). Aircraft De-/Anti-Icer. U.S. Patent 7,875,202, 2011. (19) Switzenbaum, M. S.; Veltman, S.; Mericas, D.; Wagoner, B.; Schoenberg, T. Best management practices for airport deicing stormwater. Chemosphere 2001, 43, 1051. (20) Kent, R. A.; Andersen, D.; Caux, P. Y.; Teed, S. Canadian water quality guidelines for glycolsAn ecotoxicological review of glycols and associated aircraft anti-icing and deicing fluids. Environ. Toxicol. 1999, 14, 481. (21) Cancilla, D. A.; Martinez, J.; Van Aggelen, G. C. Detection of aircraft deicing/antiicing fluid additives in a perched water monitoring well at an international airport. Environ. Sci. Technol. 1998, 32, 3834. (22) Klecka, G. M.; Carpenter, C. L.; Landenberger, B. D. Biodegradation of Aircraft Deicing Fluids in Soil at Low Temperatures. Ecotoxicol. Environ. Safety 1993, 25, 280. (23) Standard Specification for Reagent Water; ASTM D1193-06; ASTM International: West Conshohocken, PA, 2011. (24) Fluid, Aircraft Deicing/Anti-Icing, Non-Newtonian (Pseudoplastic), SAE Types II, III, and IV; SAE AMS1428; SAE International: Warrendale, PA, 2010. (25) Standard Test Method for pH of Aqueous Solutions with the Glass Electrode; ASTM E70-07; ASTM International: West Conshohocken, PA, 2007. (26) Deicing/Anti-Icing Fluid, Aircraft, SAE Type I; SAE AMS1424; SAE International: Warrendale, PA, 1993. (27) Wang, Y.; Pethrick, R. A.; Hudson, N. E.; Schaschke, C. J. Ind. Eng. Chem. Res. In press. (28) Hudson, N. E.; Ferguson, J.; Warren, B. C. H. Polymer Complexation Effects in Extensional Flows. J. Non-Newtonian Fluid Mech. 1988, 30, 251. (29) Rouse, P. E. A Theory of the Linear Viscoelastic Properties of Dilute Solutions of Coiling Polymers. J. Chem. Phys. 1953, 21, 1272. (30) Doi, M.; Edwards, S. F. Dynamics of concentrated polymer systems. Part 1.Brownian motion in the equilibrium state. J. Chem. Soc., Faraday Trans. 2 1978, 74, 1789. (31) Rahalker, R. R.; Lamb, J.; Harrison, G.; Barlow, A. J.; Hawthorn, W.; Semlyen, J. A.; North, A. M.; Pethrick, R. A. Viscoelastic Studies of Linear Polydimethylsiloxanes. Proc. R. Soc. London A 1984, 394, 207. (32) Doi, M. Molecular Rheology of Concentrated Polymer Systems.1. J. Polym. Sci. B: Polym. Phys. 1980, 18, 1005. (33) Staikos, G.; Sotiropoulou, M.; Bokias, G.; Bossard, F.; Oberdisse, J.; Balnois, E. Hydrogen-Bonded Interpolymer Complexes Soluble at Low pH. In Hydrogen-Bonded Interpolymer Complexes: Formation, Structure and Applications; Khutoryanskiy, V. V., Staikos, G., Eds.; World Scientific: Singapore, 2009; p 23. (34) Volk, N.; Vollmer, D.; Schmidt, M.; Oppermann, W.; Huber, K. Conformation and Phase Diagrams of Flexible Polyelectrolytes. In Polyelectrolytes with Defined Molecular Architecture II; Schmidt, M., Ed.; Advances in Polymer Science Series; Springer: Berlin, 2004; p 29. (35) Bertini, V.; Pocci, M. Functional Polymers for Selective Flocculation of Minerals. In Desk Reference of Functional Polymers: Syntheses and Applications; Arshady, R., Ed.; American Chemical Society: Washington, D.C., 1997. (36) Standard Test Method for Aerodynamic Acceptance of SAE AMS 1424 and SAE AMS 1428 Aircraft Deicing/Anti-icing Fluids; SAE AS5900B; SAE International: Warrendale, PA, 2007.

602


Salt Mixtures

Recommend Documents