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Importance of Elongational Flows in the P e r f o r m a n c e o f W a t e r - B o r n e
Formulations Raymond H. Fernando, David J. Lundberg, and J. Edward Glass Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105
Applications in which dynamic uniaxial elongational viscosities (DUEVs) have been important to the performance of water-borne formulations are discussed. The achievement of high viscosities at low shear rates with minimum mechanical degradation of the water-soluble polymer and with minimum viscoelastic effects in applications that involved convergingflowsare discussed. These two factors serve as driving forces for the acceptance of hydrophobically modified water-soluble polymers. Complexities arising from the combined contribution of shear and elongational deformations in applications and in their measurement are discussed in thefinalsection.
DRAG-REDUCTION PHENOMENA WERE ACTIVELY INVESTIGATED in the 1960s to determine their relationship with the elongational behavior of fluids. Some doubt remains (1) as to the role of elongational viscosity in dragreduction phenomena, but the early interest resulted in a variety of experimental techniques (2) for estimating the dynamic uniaxial elongational viscosity (DUEV) of synthetic water-soluble polymers. Earlier studies (2-5) provided the framework for delineating the importance of elongational viscosities in the performance of many formulations. These areas will be highlighted in this chapter along with the reasons why hydrophobically modified water-soluble polymers provide greater latitudes to formulators in many applications. 0065-2393/89/0223-0245$06.00/0 © 1989 American Chemical Society
Glass; Polymers in Aqueous Media Advances in Chemistry; American Chemical Society: Washington, DC, 1989.
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Most applications using dispersed systems (e.g., coatings, cosmetics, petroleum, and textiles) require high viscosities at low deformation rates to inhibit settling of the disperse phases and moderate-to-low viscosity at high rates to minimize the energy required during application. These properties can be attained by using high molecular weight polymeric thickeners. The use of these thickeners, however, imparts viscoelastic properties to the dispersions that detract from optimum performance during application. In formulations to be used over multiple stress cycles, the degradation of high molecular weight polymers results in a marked decrease in the low-shearrate viscosities. Attempts can be made to overcome both of these limitations of high molecular weight polymers by using lower molecular weight, hydrophobically modified, water-soluble polymers. These associative polymers offer specific advantages in different applications, but greater mechanical stability and lower viscoelasticity in solutions with high viscosities at low shear rates (LSVs) comparable to higher molecular weight polymers are desired in all areas.
Mechanical Stability of Polymers The instability of high molecular weight synthetic polymers was one of the considerations in pioneering the synthesis of the hydrophobically modified styrene-maleic acid copolymers (3). " A n improved polymer system [that] would not degrade mechanically and [that] would retain [its] viscosity in the presence of electrolytes and heat was required" (4). The mechanical stability of polymers was related to the polymers conformation in some of the earlier drag-reduction studies. Above a critical stress, degradation was faster the more contracted and entangled the polymer's conformation (5-7). In petroleum applications the mechanical instability of synthetic relative to carbohydrate polymers was well-recognized. The relative stability problems (possibly related to D U E V s (8)) encountered in the use of high molecular weight hydrolyzed poly(acrylamide) (HPAM) led to the development of an inverse-emulsion polymerization technique (9). (Current research directions using this technique are discussed in Chapter 9.) The solution conformation of a macromolecule under deformation, and therefore its stability, is not always predictable, particularly for carbohydrate polymers. This aspect of carbohydrate polymer stability has been investigated and related to the D U E V s of the aqueous solutions (JO). Increasing entanglements at higher concentrations and higher shear deformations can also result in mechanical degradation (JJ).
Converging Flows Extensional deformations are imposed on fluids when they undergo converging flows (e.g., roll, spray, and penetration of porous media), and the
Glass; Polymers in Aqueous Media Advances in Chemistry; American Chemical Society: Washington, DC, 1989.
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elongational response of the fluid will play an important part in its behavior. Examples that shall be discussed include the roll application of trade-sale latex paints and water-reducible industrial coatings. Petroleum applications involving enhanced oil recovery and primary recovery areas such as drilling fluids are also considered. In spray applications, elongational viscosities are related to mechanical stability (as delineated in some of the studies just mentioned), to combustion efficiencies of coal-water slurries, and to spray-
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ability in interior can coatings. R o l l C o a t i n g A p p l i c a t i o n s . A n example of a classical approach to high viscosities at low shear rates (LSVs) in disperse systems is illustrated (Figure 1) in the shear-thinning behavior of a vinyl-acrylic latex thickened with (hydroxyethyl)cellulose ( H E C ) of various molecular weights and with a poly(oxyethylene) of high molecular weight. The viscosity-shear rate profiles of the variable molecular weight H E C s follow the melt viscosity trends observed by several investigators. Significant dissipation of the coating as spatter (Figure 2) is realized (12) when titanium dioxide and other ingredients commonly used in coatings formulations are added to this latex dispersion, and the higher molecular weight water-soluble polymers are used as thickeners. This phenomenon in a variety of commercial and experimental coatings did not correlate with any shear deformation response (e.g., shear
1 0 0
r
,
Figure 1. Vinyl acetate-acrylic latex variable aqueous thickener solutions. Key: o, MW HEC 800,000; o, M W HEC 650,000; A, MW HEC 300,000; O, MW HEC 70,000; •, MW POE 800,000.
Glass; Polymers in Aqueous Media Advances in Chemistry; American Chemical Society: Washington, DC, 1989.
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Figure 2. Spatter generation via fiber dissipation. Horizontal rollout: 37 ft Is.
thinning, thixotropy, storage or complex modulus, and first normal stress difference (13)). Cinematographic studies revealed that the flows were extensional in nature. When parallel components are used (e. g., an H E C steriestabilized latex) and the molecular weight of a nonassociating thickener such as H E C is varied, the spatter behavior and extensional viscosity have been approximated by first normal stress difference (IV ) by using the Maxwell-Oldroyd equation (14). x
Recent improvements in the sensitivity (15) of the vacuum-suction fiberdrawing technique for elongational viscosity measurements have permitted the evaluation of cellulose-thickened coatings with a small-particle latex (16). The data (Table I) indicate that the D U E V involves contributions not only from the thickener but also from the components of the formulation. O n the basis of earlier observations (17), this contribution was assumed to arise from high-aspect ratio flocculents formed by the latex and pigment in extension. Trade-sale coatings are applied at low peripheral velocities. Most industrial coatings ( for paper, metal, or other substrates) are applied at high roll velocities (e.g., 300 vs. 2 ft/s). Lower viscosity, water-reducible coil coatings applied at greater roll velocities can exhibit finer fiber dissipations, and result in "misting". Such coatings applied to metal substrates (file cabinets, etc.) that were used in this study were provided by a commercial manufacturer. One coating exhibited little tendency to mist ("good"); the other exhibited a greater tendency to mist ("fair"). The rib sizes of these
Glass; Polymers in Aqueous Media Advances in Chemistry; American Chemical Society: Washington, DC, 1989.
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Table I. Dynamic Uniaxial Extensional Viscosities and Shear Viscosities of Aqueous H E C Solutions and HEC-Thickened Trade-Sale Paints DUEV Shear Thickener M of HEC Solution Concentration (wt %) Viscosity (Pa · s) (Pa · s) (x JO ) v
5
Trade-sale paints 80KU 105KU Aqueous solution
7.0 9.5
0.29
0.72 1.14 1.00
$71
NOTES: M is viscosity-average molecular weight. Deformation rate is 100 s" . The percent pigment to disperse component volume concentration is 35; the volume fraction of dispersed components is 32; the latex median particle size is 117 nm; the concentration of H E C is based on the aqueous phase. 1
V
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1.10 5.35 1.22
α 52
Figure 3. Comparison of ribbing in in dustrial coatings. "Good" paint (little misting), DUEV 0.1 Pa · s. "Fair" paint (adequate misting), DUEV 0.58 Pa · s.
unpigmented formulations are illustrated in Figure 3. The formulations with the greater D U E V exhibited a larger rib size in our studies and the greatest misting in industrial applications (16). High molecular weight thickeners are not employed in water-reducible coatings. The trade-sale coating formulations studies discussed contained cellulose ethers as thickeners. Hydrophobic (e.g., C H - u n i t ) modification of H E C will effect the higher LSVs realized with higher molecular weight H E C but with lower solution viscoelasticity. Our extensional viscosity studies of hydrophobically modified water-soluble polymers are now in a preliminary stage; lower N values, which parallel, in homologous component formula tions, lower spatter observations (18) in a comparative study (discussed in Chapter 18), were obtained with the hydrophobically modified H E C . A d ditional benefits associated with the use of associative thickeners in coatings when other structural features are present are discussed in Chapter 26. 16
33
x
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P e t r o l e u m A p p l i c a t i o n s . The relative drilling rates of clay, modified clay, and polymer-only thickened fluids were cited (19) and related (20) to their viscoelastic behavior through N measurements. At that stage of our studies, there was insufficient sensitivity in the D U E V measurement procedure to quantify differences in elongational viscosities. In applications involving mobility control for improved recovery from "depleted" reservoirs (21, 22), there were differing opinions regarding the role of D U E V s in flowthrough porous media. In studies that involved continuous converging flow sequences, Chauveteau (22) concluded that the relaxation time of the deformed polymer was increased, and therefore, the coil-stretch transition, which led to increased resistance factors (pressure differential measurements taken during core testing) occurred at a much lower deformation rate in continuously converging flows pattern (see Figures 14 and 15 in ref. 22).
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l
Recently (23), comparison of elongational viscosity of an H P A M solution before and after passing through a glass bead pack at a high strain rate revealed an elongational viscosity drop from 13,300 to 339 cP, although the maximum resistance factor of the sheared H P A M decreased only 33% relative to the original solution. These studies were interpreted in light of other investigations (24) that concluded, for cones and wedges, that converging flow of a viscoelastic fluid with a high extensional viscosity-shear viscosity ratio could be approximately described as a core of fluid in a pure extensional flow that is lubricated by a thin layer of shear flow at the confining walls of the convergence. If a similar fluid flow distribution occurs in converging pore throats, the weak elongational viscosity dependence of flow resistance observed might be explained. The shear rate in a thin shear layer would be very high, although high extensional flows occur in the core at relatively low strain rates. In this case, the pressure drop would be dominated by shear flow even though the overall flow pattern is dictated by the high extensional viscosity-shear viscosity ratio (23). Previous studies (25) revealed ways in which blending water-soluble polymers of different segmental rigidities can effect fluids that vary in D U E V characteristics while maintaining equivalent solution viscosities over a broad shear-rate range (Figure 4). Use of these polymer blend fluids permitted delineation of the role of elongational viscosities in the roll application of coating formulations. Similar blends were used to assess the role of D U E V s in mobility control (Figure 5). A proper analysis of the data would necessitate corrections for adsorption and degradation of the polymers in the core (26). The following statement is made in the concluding sections of Chapter 26: "despite our lack of understanding of associative thickeners in coatings technology, these products have gained commercial acceptance." A classic parallel in the use of polymers in mobility control buffer applications exists. Xanthan gum was used successfully in pilot tests of the low-permeability (
