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Shear-thickening in aqueous surfactant-associative thickener mixtures.

Abstract Associative thickeners represent an important class of rheology modifiers used in waterborne coatings. Understanding molecular level interactions between associative thickeners and surfactants has been the subject of a number of prior studies. Our recent studies focused on the behavior of a hydropho-bically modified, aminoplast ether (HEAT) associative thickener and a highly hydrophobic ethoxylated octyl-phenol surfactant in aqueous solution. Aqueous blends of these two materials exhibit shear-thinning, as well as rarely reported, transient, shear-induced thickening behavior. In addition, the same compositions exhibit both thixotropy and antithixotropy. The shear-induced thickening is shown to be the result of transient aggregated structures formed under shear. Addition of a third component, [beta]-cyclodextrin--a molecule known to disrupt hydrophobic associations--to the mixture helped us advance the understanding of the nature of associative thickener-surfactant interactions that cause the transient shear-thickening behavior. Results indicate that, while overall viscosity of the HEAT/surfactant mixtures is decreased by [beta]-cyclodex-trin, the shear-induced thickening is unaffected. An intermolecular interaction model to describe the transient thickening mechanism is presented.

Keywords Shear-thickening, Rheology modifier, Surfactants, Viscosity, Thickeners, Thickener-surfactant interactions, Thickening mechanisms, Associative thickeners, Cyclodextrin

Introduction

Associative thickeners are widely used as rheology modifiers in waterborne coatings and other aqueous systems. Introduction of these thickeners to the coatings industry about three decades ago enabled coating formulators to optimize rheology in ways that had not been possible before. Unlike conventional cellulosic thickeners that dissolve and increase the viscosity of the aqueous phase, associative thickeners thicken aqueous media through a complex mix of inter- and intra-molecular associations as well as interactions with the surfaces of latex polymer and inorganic colloidal particles. Presence of hydrophobic groups in the molecular structure is the main difference between associative thickeners and conventional cellulosic thickeners. Understanding the nature of associative thickener interactions and their effects on rheology has been the focus of numerous studies. (1-31) A vast majority of these studies focused on HEUR (hydro-phobically modified, ethoxylated urethanes), (1-19) HASE (hydrophobically modified, alkali-swellable emulsions), (1), (4), (10), (11), (19-26) and HMHEC (hydrophobically modified, hydroxyethyl cellulose) (1), (4), (27-31) associative thickener types. There is very little information available on another class of associative thickeners, HEAT (hydrophobically modified, ethoxylated aminoplast). (32) In our initial efforts to understand the behavior of this class of thickeners, (33), (34) rheological behavior of two HEAT type associative thickeners in aqueous solution was compared with the behavior of HEUR type thickeners. When each associative thickener was mixed in water with surfactants of varying CMC (critical micelle concentration) and HLB (hydrophilic-lypophil-ic balance) values, (35) their response varied widely. For example, more hydrophilic surfactants improved solubility of marginally soluble associative thickeners whereas a highly hydrophobic surfactant, Triton X-45, caused formation of gels and precipitates of associative thickeners. Those results highlighted the influence of the chemistry of associative thickeners and surfactants on the nature of intra- and inter-molecular interactions in the aqueous medium. During these early studies, (33), (34) one of the HEAT associative thickeners (Optiflo L-100, supplied by Southern Clay Products Company) exhibited unusual Theological behavior when mixed with the most hydrophobic surfactant, Triton X-45. Structures of the Triton X series of surfactants used in that study are shown in Table 1. They are ethoxylated octyl phenols having varying numbers of ethylene oxide units. The surfactants' CMC and HLB values are shown in Table 2. Generic structure of HEAT L-10032 is provided in Fig. la. The structure has been described as being essentially linear. (32) Examination of the patent literature related to this product class (36), (37) indicates HEAT thickeners are prepared by the reaction between multifunctional methylolated amines (e.g., melamine, glycoluril) and polyethylene glycols followed by the incorporation of long chain hydrocarbon hydrophobes. It is reasonable to conclude that the schematic four-member ring containing nitrogen is a glycouril unit (Fig. lb). This would minimize branching compared to a polymer based on, for example, methylolated melamine.

[FIGURE 1 OMITTED]

[TABLE OMITTED]
Table 2: CMC and HLB values of surfactants (33)

Surfactant    CMC (mM)  CMC (wt%)   HLB

Triton X-45     0.11     0.0045     9.8
Triton X-100    0.24     0.0150    13.4
Triton X-102    0.28     0.022     14.4
Triton X-405    0.81     0.16      17.6

CMC, critical micelle concentration


A number of studies cited above (1), (3-9), (13-15), (17), (19), (20), (22-25), (27-31) addressed the effect of surfactants on the behavior of HEUR, HASE, and HMHEC associative thickeners. In our studies of HEAT thickeners, the most interesting unusual rheological behavior exhibited by L-100/X-45 aqueous blends was the shear-thickening in approximately 10-100 [s.sup.-1] shear rate range. Detailed rheological analysis of shear-thickening of these blends and attempts to understand the mechanisms are the main objectives of the study reported here.

Addition of [beta]-cyclodextrin, a cyclic molecule having seven [alpha]-l,4-linked glucopyranose units, was one of the approaches utilized in our attempts to probe intermo-lecular interactions related to the shear-thickening mechanism observed in aqueous mixtures of L-100 and X-45. Reviews of the chemistry and industrial utility of cyclodextrins have been published. (38), (39) The overall conformation is that of a truncated cone with a tapered cavity of 7.9 [Angstrom] depth. For [beta]-cyclodextrin, the top and bottom diameters of the cavity are 6.0 and 6.5 [Angstrom], respectively. These dimensions are different for six-membered [alpha]-, eight-membered [gamma]-, and other cyclodextrins. The outer shell of the molecule is hydrophilic, while the interior cavity is hydrophobic. Figure 2 shows the structure of this molecule. Cyclodextrins are well known for disrupting hydrophobic interactions in aqueous media. (40-46) They have been utilized to probe hydrophobic interactions in aqueous solutions of HEUR, (43) HASE, (42) HMHEC, (45) and other (44), (46) polymers. These studies have shown that the inner cavity of a cyclodextrin molecule has a capping effect on the hydrophobe moieties of thickeners, reducing the network strength of linked polymers and lowering the viscosity of the solution. This mechanism has been exploited to suppress viscosity of concentrated L-100 solutions (47) as well as another class of associative thickeners, (48) hydrophobically modified polyacetal polyether (HM-PAPE).

[FIGURE 2 OMITTED]

Experimental materials and methods

HEAT thickener Optiflo L-100 (Lot No. 3726) was supplied by Southern Clay Products Company (formerly Sud-Chemie) at 20% solids by weight in water. Dilutions were made with deionized water to the following concentrations in separate glass bottles: 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, and 4.0% by weight. These solutions were allowed to equilibrate for 24 h before being subjected to Theological tests. To prepare aqueous blends, the surfactant, [beta]-cyclodextrin, or both, were added at 1.0% by weight to the various HEAT solutions. [beta]-Cyclodextrin is soluble in aqueous media up to 1.8% by weight concentration. Triton X-45 was supplied by Dow Chemical Company (100% active), and [beta]-cyclodextrin was purchased from Sigma-Aldrich in the form of a 98% pure solid powder. Upon addition to the Optiflo L-100 solutions, Triton X-45 and [beta]-cyclodextrin were allowed to mix and equilibrate for 30 min at room temperature. In situations where both Triton X-45 and [beta]-cyclodextrin were added to a single Optiflo L-100 solution, each component was allowed to individually mix and equilibrate for 30 min before the other was added. The surfactant HLB was varied by choosing four different Triton X series surfactants having identical hydrophobic groups but varying ethoxylate repeat units. In each case, the aqueous concentration of the polymeric surfactant component was held constant at 0.5% or 1.0% (by weight) while the surfactant concentration was varied within a broader range.

Optiflo L-100 and Triton X-45 were characterized by an Agilent 6890 Gas Chromatograph equipped with MS and FID. Proton NMR spectra were obtained with a 300 MHz Bruker Spectrometer. FTIR spectra were obtained with a Thermo-Nicolet Nexus 470 ATR-FTIR Spectrometer. Optiflo L-100 molecular weight was characterized with an Agilent 1200 series GPC using tetrahydrofuran (THF) as the solvent.

A TA Instruments AR 2000 rheometer equipped with a 40 mm, 2[degrees] cone was used to measure viscosity and other rheological properties of all solutions. A 55 micron gap between the truncated tip of the cone and bottom Peltier plate was maintained. The rheometer has a low-end torque resolution of roughly 0.1 [mu]Nm, so data points with a torque reading below this value exhibited a large amount of error and were discarded. Air bearing and inertia calibrations were performed routinely. All experiments were performed at 25[degrees]C.

Results and discussion

Thickener and surfactant characterization

The proton NMR of Triton X-45 and Optiflo L-100 are shown in Figs. 3 and 4, respectively. The X-45 NMR shows excellent agreement with the structure provided by the supplier. The GC/MS and GC/FID results showed excellent agreement with the NMR data. Three different types of protons are detected in the L-100 spectrum. It is reasonable to conclude that protons with larger abundance are from aliphatic hydrocarbon groups (approx. 2.3 ppm) and ethoxylate groups (approx. 3.7 ppm). No evidence of aromatic protons is present (approx. 7.4 ppm peak is from chloroform impurity in deuterated chloroform used as the solvent). The FTIR spectrum of Optiflo L-100 shows the presence of a C=O group (approx. 1700 [cm.sup.-1]) supporting that the structure contains glycoluril groups. GPC results indicate the weight average molecular weight is about 40,000; however, it should be noted that no calibrations arc available for HEAT type molecules in THF, and therefore, molecular weight calculations were made on the basis of polystyrene calibration curves (Fig. 5).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Initial viscosity results

HEAT L-100, as supplied by the supplier at 20.0% concentration, is a viscous liquid. When diluted to 0.5% and 1.0% (by weight) in water the viscosity drops below 2.0 mPa.s. Plots of viscosity vs concentration for each individual surfactant in water up to surfactant concentrations of 10 wt% show that X-45 results in the highest viscosity for the same weight percent concentration of surfactants (Fig. 6). Visual descriptions of HEAT L-100 and various surfactant blends are shown in Table 3. HEAT L-100 in all blends is constant at 0.5 wt%. Aqueous blend prepared at a HEAT L-100 concentration of 1.0 wt% showed similar behavior.

[FIGURE 6 OMITTED]
Table 3: Visual description of surfactant/HEAT L-100 blends

Optiflo L-100 0.5 wt%                      Description

Triton X-45
  0.0%                  Clear homogeneous liquid
  0.25%                 Cloudy homogeneous liquid
  0.5%                  Cloudier and thicker, homogeneous
  1.0%                  Cloudier and thicker, homogeneous
  2.0%                  Very cloudy, very thick, appears homogeneous
  3.0%                  Very cloudy, very thick, appears homogeneous
  4.0%                  Cloudy liquid, white precipitate ~85% of bottle
  5.0%                  Cloudy liquid, white precipitate ~98% of bottle
Triton X-100 0.25-5.0%  Clear homogeneous liquid
Triton X-102 0.25-5.0%  Clear homogeneous liquid
Triton X-405 0.25-5.0%  Clear homogeneous liquid


Viscosities (at 9.3 [s.sup.-1] shear rate) of HEAT L-100 (0.5 wt%) and surfactant blends are shown in Fig. 7. Blends containing more hydrophilic surfactants (X-100, X-102, and X-405) have very low viscosities whereas the highly hydrophobic X-45 surfactant and HEAT L-100 blend shows significant viscosity synergy. Above 3.0 wt% concentration these blends became insoluble in water and precipitated out of solution. This behavior can be contrasted from what is reported in literature regarding somewhat insoluble associative thickener and other polymeric surfactants when combined with relatively hydrophilic surfactants. In such combinations, the surfactant helps solubilize the associative thickener and leads to increased viscosities due to formation of mix micelles. However, at high surfactant concentrations where there is a stiochiomet-ric excess of surfactant molecules, the mix micelle network breaks up, leading to sharply reduced viscosities. (1), (13), (15), (19), (20), (22), (23), (25), (27), (28) This type of behavior was observed with a highly hydrophobic thickener, HEAT TVS, when combined with the relatively hydrophilic surfactants used in our studies. (33) With HEAT L-100 and Triton X-45 surfactant combination the situation is reversed. HEAT L-100 is soluble in water on its own. When combined with the highly hydrophobic X-45 surfactant, it appears they form interactions leading to an intermolecular network. Increase in viscosity seen in the blends as a function of X-45 concentration (Fig. 7) supports this point. However, at higher surfactant concentrations these interactions lead to insolubilization of the HEAT L-100 thickener.

[FIGURE 7 OMITTED]

Shear-thickening in HEAT L-100/X-45 aqueous blends

Viscosity dependence on shear rate of a series of HEAT L-100/X-45 blends is shown in Fig. 8. Each data point was collected after a 30 s equilibration time. An initial shear-thinning region followed by a shear-thickening region between 10 and 100 [s.sup.-1] shear rates is evident for L-100/X-45 blends containing up to 2.5 wt% X-45 concentration. Shear-thickening is a well-known phenomenon observed in colloidal dispersions of solid particles in liquids. Its mechanism in such colloidal systems has been reported a half century ago (49) and a recent article described a number of applications that utilize the shear-thickening phenomenon. (50) It appears, in the system represented in Fig. 8, that at lower shear rates weak inter- and intramolecular interactions are disrupted leading to shear-thinning, and higher stresses in the 10-100 [s.sup.-1] shear rate range are facilitating shear-induced intermolecular interactions. The critical range of shear rates shift to lower values as the surfactant concentration is increased. Shear-thickening is not apparent at the highest (i.e., 3.0 wt%) X-45 surfactant concentration. It appears that other viscosifying mechanisms that are more dominant in this system are shielding the shear-thickening.

[FIGURE 8 OMITTED]

The equilibrium time for data point collection had a significant effect on the magnitude and shear rate range of shear-thickening effect. This point is evident in the data shown in Fig. 9 for 1.0% L-100/1.5% X-45 blend. When data point equilibration time is increased from 30 s to 90 s, shear-thickening shifts to a lower shear rate range (from 20 to 10 [s.sup.-1], approximately). In addition, the extent of shear-thickening is greater, indicating antithixotropic (rheopectic) behavior. The drop in viscosity at the low shear rate end is equally impressive, and indicates a high degree of thixotropy. The fact that the fluid represented in Fig. 9 (i.e., 1.0% L-100/1.5% X-45 aqueous blend) exhibits at least four rhcological complex phenomena--shear-thinning, shear-thickening, thixotropy, and rheopexy is remarkable. These results indicate that there are two different types of interactions between L-100 and X-45 molecules. First, there are weak interactions leading to increased viscosities at rest. They are weak and can be disrupted by very low stresses associated with low shear rates (0.01-2.0 [s.sup.-1] range). The second type of interactions leads to shear-thickening. These appear to be between X-45 hydrophobes and hydrophobes in L-100 molecules that are not accessible at rest or under very low shear rates. It appears that the deformations of L-100 molecules within a critical shear rate range change their dynamic conformations in such a way that certain hydrophobes inaccessible at lower shear rates become accessible. Figure 10 shows results for higher concentration blends obtained at different data point equilibration times. Results of viscosity measurements as a function of increasing shear rates (ramp-up) followed by decreasing shear rates (ramp-down) for a 1.0% L-100/1.0% X-45 blend and 2.5% L-100/2.5% X-45 blend are shown in Fig. 11. It is interesting to note that the ramp-down viscosity curves are very different from the ramp-up curves. These results indicate that the intermolecular interactions responsible for shear-thickening during the ramp-up step of the experiment are stable enough to cause increased viscosities at low shear rates during the ramp-down step. The results of dynamic oscillatory experiments are shown in Figs. 12 and 13. The complex viscosity of 1.0% L-100/1.5% X-45 blend does not indicate a thickening region as a function of the oscillatory frequency. This observation is consistent with prior literature on other polymeric surfactant systems.27 The dynamic oscillatory experiment conditions do not provide sufficient shearing time in one direction to facilitate the interactions necessary for thickening to take place. Complex viscosities of two additional L-100/X-45 blends can be seen in Fig. 13. None of these curves exhibit a thickening region.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Effects of [beta]-cyclodextrin

Results discussed so far strongly indicate that there are two different types of hydrophobic interactions between X-45 surfactant and L-100 associative thickener. The interactions leading to shear-thickening appear to be between X-45 hydrophobes and L-100 hydrophobes that are only accessible when the L-100 conformation is deformed within a critical shear rate range. To further probe the nature of these interactions, [beta]-cyclodextrin was added to L-100/X-45 aqueous blends. As mentioned earlier, [beta]-cyclodextrin and its derivatives have been shown to disrupt hydrophobic interactions in solution. (38-44) In our studies, [beta]-cyclo-dextrin was added at 1.0 wt% concentration to a series of solutions having HEAT L-100 between 0.1 and 4.0 wt% concentration. Viscosity vs shear rate results of these solutions is shown in Fig. 14.

[FIGURE 14 OMITTED]

HEAT L-100 shows Newtonian behavior at all concentrations shown, and as expected, with increasing concentration of HEAT L-100, viscosity increases as well. It should be noted that all L-100 solutions show signs of weak shear-thickening within about 10-20 [s.sup.-1] shear rate range. The effect of [beta]-cyclodextrin is one of uniform viscosity reduction. The degree of viscosity reduction is greater as the L-100 concentration increases. These results are consistent with the well-established hydrophobe disruption mechanism of [beta]-cyclodextrin. Very similar results for HEUR thickener solutions have been reported. (41) It is worth noting that [beta]-cyclodextrin does not eliminate the weak shear-thickening observed in L-100 solutions shown in Fig. 14.

The next step of our investigation involved aqueous blends of HEAT L-100/X-45 with and without [beta]-cyclodextrin. Figure 15a compares the effect of [beta]-cyclodextrin on the 1.0%/1.0% blend of L-100 and X-45, whereas Fig. 15b compares similar results at a 2.0% L-100 concentration. For each of the different concentrations, the Optiflo L-100 solution with 1% Triton X-45 shows the distinct shear-thickening region around 10 [s.sup.-1] shear rate, as shown earlier. However, the shear-thickening becomes much more pronounced upon the addition of 1% [beta]-cyclodextrin. What is unique is that, unlike how cyclodextrin uniformly reduces viscosity of Optiflo L-100 on its own, when paired with X-45, it creates an uneven reduction in viscosity that appears to make the shear-thickening region more distinct. Addition of [beta]-cyclodextrin suppresses the viscosity in the low and high shear rate regions, amplifying the relative shear-thickening observed around 10 [s.sup.-1] shear rate.

[FIGURE 15 OMITTED]

In preparing the L-100/X-45/[beta]-cycIodextrin blends represented in Fig. 15, the order of addition was dilute L-100, add [beta]-cyclodextrin, equilibrate, and then add X-45. Figure 16 compares the effect of changing this order of addition for a mixture containing 1.0% L-100/ 1.0% [beta]-cyclodextrin/1.0% X-45 when X-45 is added before and after the addition of [beta]-cyclodextrin. There is a clear difference in the two viscosity curves indicating strong kinetic factors that must be considered in interpreting the results. This point was directly evident in viscosity results for L-100/X-45 presented in Figs. 9 and 10.

[FIGURE 16 OMITTED]

In order to determine the viscosity equilibration time in the shear-thickening region, aqueous 2.0 wt% L-100 and its blends with X-45, [beta]-cyclodextrin, and X-45 plus [beta]-cyclodextrin were subjected to constant shear at 10 [s.sup.-1] rate for an extended period of time. Results of this experiment are illustrated in Fig. 17. HEAT L-100 alone and Optiflo L-100 with 1% [beta]-cyclodextrin yield no unusual behavior. On the other hand, both solutions containing Triton X-45 display antithixotropic or rheopectic behavior. In the presence of 1% X-45, HEAT L-100 does not begin to attain equilibrium viscosity until after roughly 2-3 min of constant shear. It is clear that a 30 s or 60 s sample period is insufficient to fully achieve steady state.

[FIGURE 17 OMITTED]

Conclusions

This study focused on the behavior of the HEAT class of thickeners that have been rarely reported on in the literature. The behavior of HEAT L-100 in the presence of a highly hydrophobic octylphenol surfactant (X-45) proved to be extremely interesting from a rheological standpoint. HEAT L-100 and X-45 blends in water showed evidence of hydrophobic interactions commonly reported for HEUR, HASE, and HMHEC associative thickeners and other hydrophobic-hydro-philic polymers. This resulted in a synergistic increase in the viscosity of L-100/X-45 blends. However, the viscosity tests showed that most of the hydrophobic interactions are rather weak causing shear-thinning behavior. HEAT L-100/X-45 blends exhibited shear-thickening behavior in about 10-100 [s.sup.-1] shear rate range. Although such behavior is common in colloidal dispersions, it has been rarely observed in associative thickener-surfactant blends. Our attempts to disrupt the hydrophobic interactions in these blends with [beta]-cyclodextrin, viscosity profiles obtained under extended equilibration times indicated that there are two kinds of hydrophobes in HEAT L-100: those that are easily accessible and others that are accessible only when the molecule is deformed under critical shear rate conditions. One possibility is that the easily accessible hydrophobes are at the terminal positions of the L-100 molecules and others are more toward the middle of the molecule. The shear-thinning behavior exhibited above 100 [s.sup.-1] indicates that the interactions that led to shear-thickening are not strong enough to survive under the high shear rate conditions.

Acknowledgments The authors thank Dr. Alan Steinmetz of Southern Clay Products Company (formerly Stid-Chemie) for his help in supplying samples and for his helpful discussions. Authors thank Southern Clay Products and Dow Chemical companies for providing associative thickener and surfactant samples for the study. Also, authors acknowledge the help of Professors Dane Jones and Phillip Costanzo of Cal Poly in obtaining spectroscopy and chromatography data of the materials.

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S. J. Manion, L. L. Johnson, R. H. Fernando (*)

California Polytechnic State University, San Luis Obispo, CA 93407, USA

e-mail: rhfernan@calpoly.edu

URL: www.polymerscoatings.calpoly.edu

Scan J. Manion, Laura L. Johnson, Raymond H. Fernando

[C] ACA and OCCA 2011

DOI: 10.1007/S11998-011-9320-7
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Author:Manion, Scan J.; Johnson, Laura L.; Fernando, Raymond H.
Publication:JCT Research
Date:May 1, 2011
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