Polymers as Rheology Modifiers - American Chemical Society

Chapter 20 Dynamic Uniaxial Extensional Viscosity Response in Spray Applications 1

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David A. Soules , Gustav P. Dinga , and J. Edward Glass

Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch020

Polymers and Coatings Department, North Dakota State University, Fargo, ND 58105 The importance of dynamic uniaxial extensional viscosities in the spray application of interior can coatings and aerosol adhesive for mulations is addressed. Model systems are designed to approximate actual industrial formulations that cannot be examined with current dynamic uniaxial extensional viscometers. Increasing the cross-link density (and therefore the rigidity) of gel dispersions is observed to decrease the dynamic uniaxial extensional viscosities of thickened systems.

The addition of fillers to polymer melts and polymer solutions is a com mon practice for a variety of reasons: hiding and coloration, strength enhancement, rheology modification, and cost reduction. Polymer solu tions in the absence of fillers exhibit increasing non-Newtonian rheology with both increasing molecular weight and concentration. This is related to overlapping and entanglement of macromolecular chains (Chapter 1, this volume). The entanglements and the aggregates in filled systems are disrupted with increasing shear rate, η , and the viscosity of the solu tions, melts, or dispersions decreases. In melts or concentrated polymer solutions (i), particulate interactions are overshadowed by the medium viscosity and are generally neglected. Particle—particle interactions are important to the Theological properties of particle-loaded solutions at low polymer concentration. Three factors are primary in the shear deforma tion response of filled polymer solutions: the molecular weight of the polymer, the volume fraction of the filler, and the aspect ratio of the filler. The addition of fillers has a more dramatic effect on extensional

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Current address: Phillips Petroleum Research Center, Bartlesville, OK 74006 Current address: Department of Chemistry, Concordia College, Moorhead, MN 56560

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0097-6156/91/0462-0322$06.00/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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viscosities of suspensions than on shear rate viscosities. Weinberger and Goddard (2) were among the first to observe the effect of rod-shaped glass particles on the extensional viscosity of Newtonian fluids. The addi tion of high-aspect-ratio fibers greatly enhanced the axial stress (~10-fold increase) relative to the unfilled solution. Alignment of the fibers appears to occur at extension rates less than 1 s . The extensional viscosity of filled solutions that contain spherical beads is significantly different from that of fluids with needle-shaped fillers. Nicodemo, De Cindio, and Nicolais (3), using a tubeless syphon extensional viscometer, observed that the dynamic uniaxial extensional viscosity of the fluids decreases with increasing concentration of spherical particles (diameter between 40-50 microns). This behavior is in contrast to the shear viscosity of spherical dispersions and to the extensional viscosity of high-aspect-ratio particulates; the latter two viscosities increase with increasing particulate concentration. This chapter will address a slightly different area: the importance of dynamic uniaxial extensional viscosities in spray application of fluids as a function of polymer structure and in filled systems with decreasing deformability due to increasing cross-linked densities.


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Experimental Details Acrylamide monomer was purchased from Aldrich and was recrystallized from ethanol. ΛΓ,ΛΓ-Methylene bisacrylamide (BISAM) was electrophoresis-grade and was used as received. A hydrolyzed (23%) poly(acrylamide), an experimental polymer supplied by American Çyanamid, My = 6 χ 10 , was used in preparing the semi-interpenetrating polymer networks (semi-IPNs). Potassium persulfate, purchased from Aldrich, was the free-radical initiator. The gels were prepared by dissolving the acrylamide, BISAM and potassium persulfate in water or an aqueous poly mer solution (depending on whether a gel or an IPN was prepared) in a 500-mL, three-neck, round-bottom flask. The solutions were stirred for 30 min under an argon purge. The final solids at 12 h was 4 wt %. Four different BISAM concentrations were used: 1%, 2.5%, 5%, and 7.5% based on the total monomer weight. Two different linear polymers were used in preparing IPNs, the 23% hydrolyzed poly(acrylamide) and hydroxyethyl cellulose with two molecular weights, 9.5 χ 10 and 3.0 χ 10 . The gels were diluted to 2%, dispersed with a high-speed mixer to promote a more-uniform gel particle size, and then blended with a linear-polymer solution to obtain a final gel concentration of 1%. The extensional viscosity data were obtained with a suction fiber viscometer {4). 5

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Results and Discussion Sprayability. Atomization of the fluid is the generally desired goal in spray applications. The critical breakup length of a filament from a spray noz zle has been described, over a decade ago, in terms of a dimensionless breakup length (5), L/2a, for HPAM-thickened fluids. Analysis of a


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filament's shape has been a standard technique for quantifying the dynamic surface tension (6) of surfactant solutions; recently this technique has been applied to estimate polymer dynamic uniaxial extensional viscosities (7). In our initial study of low-viscosity industrial coatings applied by spray, dynamic uniaxial extensional viscosities of formulations with different spray characteristics could not be measured because of sensitivity limitations of the viscometer. In convergent-flow behavior in roll applications (8), highmolecular-weight (10 ) poly(oxyethylene) (POE) was added to roll coating formulations to overcome the sensitivity limitation. The formulation exhi biting the greater rib development exhibited the greater dynamic uniaxial extensional viscosity. With additional improvement in the instrument's sensitivity, this correlation also was observed in formulations to which POE was not added. The technique of adding P O E and improved instru ment sensitivity, however, did not facilitate the measurement of the dynamic uniaxial extensional viscosities of the interior can coatings applied by spray. In a previous study of coal-water slurries (9), extension of such slur ries over very short fiber distances provided a response under extensional deformation. One would expect that a greater dynamic uniaxial exten sional viscosity would result in a more stable filament that would not mist. This was observed. The relative values of the two slurries were inversely related to differences in misting and combustion efficiencies (10). The dynamic uniaxial extensional viscosities, however, decreased with defor mation rate, indicating a significant shear viscosity contribution in the response. To ascertain the dominance of extensional vs. shear viscosities in stabilizing the fiber and thus inhibiting breakup of the spray filament, thickened aqueous solutions were studied. The thickeners, described below, are comparable to those used to stabilize coal-water slurries. The inverse relationship of a fluid's sprayability and dynamic uniaxial extensional viscosity is evident in thickened fluids containing water-soluble polymers of variable segmental and conformational flexibilities. Poly(oxyethylene), a segmentary and conformationally flexible polymer in aqueous solutions, contributes relatively little to the shear viscosity of aqueous solutions relative to fermentation carbohydrate polymers of com parable molecular weight, such as xanthan gum (XCPS) or scleroglucan (SGPS) (Figure 1). The reverse is true with respect to their contribution to the extensional viscosity of aqueous solutions (Figure 2). SGPS or XCPS solutions exhibit high shear viscosities but low dynamic uniaxial extensional viscosity and readily mist on exit from a spray gun (Figure 3). POE solutions, with notable dynamic uniaxial extensional viscosities even at low concentrations (1000 ppm), string rather than mist on exit from the spray nozzle (Figure 4). The antimisting phenomenon would appear to be related to the dynamic uniaxial extensional viscosities, as was the tendency to rib in roll applications.


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Sprayability of Gel-Containing Formulations. As noted earlier, particulate structures can exert different influences in the rheological response of



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SHEAR RATE (s ) 44

Figure 1. Dependence of viscosity (Pa s) on shear rate (s ) of 0.5 wt% aqueous solutions of poly(oxyethylene), My = 6 χ ΙΟ ( Δ ) ; hydroxyethyl cellulose (HEC), M = 9.5 χ 10 ( O ) ; d Xanthomonas campestris polysaccharide (XCPS), My = 2 χ 10 (•). (Reproduced with permission from reference 1. Copyright 1985 Wiley.) 6

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ELONGATION RATE

Figure 2. Dependence of viscosity (Pa s) on extension rate (s ) of 0.5 wt% aqueous solutions of poly(oxyethylene) [POE1, My = 6 χ ΙΟ ( Δ ) ; hydroxyethyl cellulose (HEC), My = 9.5 χ 10 ( O ) ; and Xanthomonas campestris polysaccharide (XCPS), My = 2 χ 10 (•). (Reproduced with permission from reference 1. Copyright 1985 Wiley.) 6

5

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Figure 3.

Spray misting behavior of XCPS, 0.5 wt % aqueous solution.



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Figure 4. Spray cobwebbing behavior of POE, 0.15 wt % aqueous solu tion.



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fluids and this may be related to various formulations used in different applications. For example, polymeric gels are added to high-solids coat ings (11). The initial components of the coating are very low in molecu lar weight, and the formulations are very low in viscosity. The polymeric gels increase the low-shear-rate viscosity and thereby prevent sagging dur ing oven curing (12). Antimisting of the formulation during spray appli cation is not observed, presumably because of the spherical shape and rigi dity of the gel. Gels are used also for strength enhancement in adhesives. The gels tend to be lightly cross-linked to provide tactility to the adhesive. Many of the adhesives are applied by aerosol spray and the addition of the gels, prepared in the presence of the adhesive often do not provide misting to the aerosol adhesive. To model this system, a series of semiinterpenetrating network gels, prepared in the presence of linear polymers, was synthesized. At low cross-link (1-5%) levels, increasing the concen tration of the difunctional monomer effects a smaller pore size (13). The semi-IPNs were prepared with acrylamide cross-linked with N,W-methylene bisacrylamide. The free-radical polymerization was conducted in the pres ence of 0.3 wt % of the hydroxyethyl cellulose with molecular weight of 10 . The gel was blended into 0.75 wt % of hydroxyethyl cellulose with molecular weight of 10 . The extensional viscosity of any of the filled fluids is lower than that of the unfilled hydroxyethyl cellulose fluid (Figure 5). As the cross-link density of the semi-IPN is increased, the dynamic uniaxial extensional viscosities of the resulting filled fluids decrease. In view of the prior art, it would appear that the weakly cross-linked IPN is deformable. With increasing cross-link density, the rigidity of the gel increases and lowers the dynamic uniaxial extensional viscosity of the thickened dispersion, similar to the influence on the extensional viscosity of rigid glass beads (3). No evidence of polymer fragments on the surface of the IPN is evident in systems polymerized in the presence of hydroxy ethyl cellulose of variable molecular weight. The semi-IPNs prepared without the carbohydrate polymer gave responses similar to those prepared in the presence of hydroxyethyl cellulose in systems using a 2.5 wt % BISAM level. Hydrolyzed poly(acrylamide) (HPAM), unlike hydroxyethyl cellulose, is not prone to degradation in the presence of free radicals. Semi-IPNs were prepared in the presence of 0.02 wt % HPA M and then added to 0.01 H P A M solutions. HPA M polymers of My = 1.6 χ 10 have a much greater extensional viscosity at lower concentrations compared with car bohydrate solutions (4). The dynamic uniaxial extensional viscosities of 1 wt % semi-IPN slurries prepared without 0.02 wt % H P A M solutions are presented also in Figure 6. There appears to be a lower extension rate required to reach the plateau with decreasing cross-linker concentration. 6

6

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Commercial Adhesive Hydrocarbon Formulations The

dynamic uniaxial

extensional

viscosities

of

commercial adhesive


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Figure 5. Dependence of dynamic uniaxial extensional viscosity (DUEV, Pa s) on extension rate (s ) for a series of fluids containing 1% acrylamide (AM) gels suspended in 0.75 wt % hydroxyethyl cellulose (HEC), My = 9.5 χ 10 . The gels were prepared in the presence of 0.75 wt % H E C and with the following levels of Ν,Ν'methylenebisacrylamide (BISAM): 1 wt %, O ; 2.5 wt %, Δ ; 5 wt %, • ; and 7.5 wt %, 0 . The D U E V of H E C in the absence of the AM/BISAM gel is noted by · . _1

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EXTENSION RATE

1000

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Figure 6. Dependence of D U E V on extension rate for a series of fluids containing 1% acrylamide (AM) gels suspended in 0.0001 wt % hydro lyzed (23%) acrylamide, M^ = 1.6 χ 10 . The gels were prepared in the presence of 0.02 wt % HPAM and with the following levels of BISAM: 1 wt %, · ; 2.5 wt %, Δ ; 5 wt %, • ; and 7.5 wt %, Ο . The D U E V of H P A M in the absence of the AM/BISAM gel is noted by *. The closed symbols represent gel responses in the absence of HPAM. 7

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100 1000 ELONGATION RATE (β" ) Figure 7. Dependence of D U E V on extension rate for a series of com mercial gel (20 wt %) adhesive formulations. Key: 0> formulation that misted on spray; Δ , formulation that cobwebbed on spray. 4

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formulations, supplied by the 3M Corporation, were examined. The "good" adhesive provided a mist during spray application from a pressurized con tainer; the "poor" formulation did not mist. Like the coal—water slurries, the viscosity response of commercial formulations reflected a significant shear deformation response (Figure 7), and the responses were within experimental error. The hydrocarbon adhesives were in a poor solvent for the gel, possibly decreasing the difference between the gels under the measurement conditions. The model acrylamide gels and semi-IPNs were all in a good solvent. The commercial adhesive had a gel content much higher than the gel concentration studied with the aqueous model systems; the total solids content of the adhesive was ~20%. Our measurements of these hydrocarbon gels were made at 8 wt %. The extensional viscometer used in this study, as well as other techniques used in the past or in this period, is not capable of properly addressing the problem of complex for mulations of this type. This is true of many commercial formulations util izing polymers as rheological modifiers. Acknowledgments The financial support of this study by the 3M Corporation is gratefully acknowledged.

Literature Cited 1. Metzner, A. B. J. Rheol. 1985, 29(6), 739. 2. Weinberger, C. B.; Goddard, J. D. Int.J.Multiphase Flow 1974, 1, 465. 3. Nicodemo, L.; De Cindio, B.; Nicolais, L. Polym. Eng. Sci. 1975, 15(9), 679. 4. Soules, D. Α.; Fernando, R. H.; Glass, J. E. J. Rheol. 1988, 32(2), 181-198. 5. Gordon, M.; Yerushalmi, J.; Shinnar, R. Trans. Soc.Rheol.1973, 17, 303. 6. Adamson, A. W. Physical Chemistry Surfaces, 4th Edition; Interscience Publishers, Inc., 1982. 7. Schummer, P.; Tebel, R. H. J. Non-Newtonian Fluid Mech. 1983, 12, 331. 8. Fernando, R. H.; Glass, J. E. J. Rheol. 1988, 32(2), 199-213.

9. Fernando, Raymond H.; Lundberg, David J.; Glass, J. E. In Poly mers in Aqueous Media: Performance through Association; Gl Ed.; Advances in Chemistry 223; American Chemical Society: Wash ington, DC, 1989; Chap. 12.


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10. Rakitsky, W. G.; Knell, E. W.; Murphy, T. J. Proc. 11th Int. Conf. Slurry Technol. 1986, 137.

11. Lambourne, R. Paint and Surface Coatings: Theory and Prac Halsted Press (Wiley): New York, 1988. 12. Bauer, D.; Briggs, L. M.; Dickie, R. A. Ind. Engin. Chem.,Prod.Res. Dev. 1982, 21(4), 686.


13. Allen, R. C.; Saravik, C. Α.; Maurer, H. R. Gel Electrophoresis and Isoelectric Focusing of Proteins; deGruyter: Berlin, 1984; Chap. 1. RECEIVED March 11, 1990


Polymers as Rheology Modifiers - American Chemical Society

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