Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 412-417
412
with increasing initiator concentration, stirring speed, and seed size. Linear regression done on the same latexes and morphological properties showed that (i) The ratio In (Ni/Nf) was related to the pH, initiator concentration, stirring speed, percent solids, ionic strength, and seed size. This ratio increased with increasing pH and decreased with seed size. The rest of the parameters entered as second-order interactions. (ii) The production of the acorn morphology was dependent upon the following variables: pH, initiator concentration, monomer feed rate, ionic strength, stirring rate, and seed size. The tendency to form acorns decreased with pH and increased with the other significant parameters. A significant amount of seed material was removable by treatment with ion-exchange resin. Polymerization onto this cleaned seed latex resulted in a lower amount of grafted PS and significantly less grafted PBA than did comparable polymerization onto an uncleaned seed latex. It is proposed that the major locus of the styrene polymerization is in the aqueous phase. The styrene material nucleates new particles and interacts both with surfactant and any water-soluble seed polymer. These particles are then captured by the seed. The capture of particles by the seed, coupled with polymerization in these seed particles, does not always result in an even shell. The PS acorn domains form early on, with an even shell growth taking precedence as the domains grow larger than 60-90 nm in diameter. The domains form due to (i) incompatibility during the polymerization of the monomer near/on the seed surface and (ii) an overall minimization of interfacial energy resulting from the different polymer/polymer and polymer/water interfacial tensions. Surfactant addition rate and concentration play a large role in the tendency to form acorns.
Registry No. SLS, 151-21-3; BA, 141-32-2; (PS/PBA) cop o l y m e r , 25767-47-9; styrene, 100-42-5.
Triton N - l O l / I g e p a l CO-730, 9016-45-9;
Literature Cited Chainey, M.; Hearn, J.; Wilkinson, M. C. Br. Polym. J. 1981, 73(3), 132. Chainey, M.; Hearn, J.; Wiikinson, M. C. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 171. Collins, E. A.; Bares, J.; Biilmeyer, F. W. "Experiments in Polymer Science"; Wiley: New York, 1973;p 334. Dickie, R. A.; Cheung, M. F.; Newman, S. J. Appl. Polym. Sci. 1973, 17,
65. Ferry, W. J.; Jones, D. R.; Graham, R. K. US. Patent 3965703, 1976. Fitch, R. M.; Tsai, C. H. "Polymer Colloids, ACS Symposium on Polymer Colloids"; American Chemical Society: Washington, DC, 1971;p 73. Gasperowicz, A.f Kolendowicz, M.; Skowronski, T. Polym. 1982, 23, 839. Gershberg, D. AIChE-I. Chem. E . Symp. Ser. No. 3 1985, 4. Hagiopoi, C.; Dimonie, V.; Georgescu, M.; Deaconescu, I.; Deleanu, T.; Marinescu, M. Acta Polym. 1981, 32(7)390. Hansen, F. K.; Ugelstad, J. J. Polym. Sci.. Polym. Chem. Ed. 1979, 17,
3033. Isaacson, W. Chem. Eng. 1970 (June), 69. Matsumoto, T.; Okubo, M.; Imai, T. Kobunshi Ronbunshu 1974, 37,5767. Matsumoto, T.; Okubo, M.; Onoe, S.Kobunshi Ronbunshu 1978, 5 , 771. Min, T. I.; Klein, A.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci. Polym. Chem. Ed. 1983, 21, 2645. Purvis, M. T.; Grant, R. P. U.S. Patent 3983296, 1976. Shaffer, 0.L.; El-Aasser, M. S.; Vanderhoff, J. W. "Proceedings of the 41th Annual Meeting of the Electron Mlcroscopy Society of America"; Electron Microscopy Society of America: Phoenix, AZ, 1983;p 19. Snuparek, J., Jr. J. Appl. Polym. Sci. 1979, 24, 909. Snuparek, J., Jr. Angew. Makromol. Chem. 1980, 88, 61. Snuparek, J., Jr.; Tutalkova, A. J. Appl. Polym. Sci. 1979, 2 4 , 915. Stabenow, V. J.; Haaf, F. Angew. Makromol. Chem. 1973 29/30(359),1. Stutman, D. Ph.D. Dissertation, Lehigh University, Bethlehem, PA, 1964. Tseng, C. M. Ph.D. Dissertation, Lehigh University, Bethlehem, PA, 1983. Vanderhoff, J. W. "Characterization of Latexes by Ion Exchange and Conductometric Titration"; Academic Press: New York, 1977;p 77. Yamazaki, S. Kobunshl Ronbunshu 1976, 1 1 , 1. Yamazaki, S. Shikizai Kyokaishi 1977 5 0 , 267. Yamazaki, S.;Hattori, S.:Hamashima, M. Kobunshi Ronbunshu 1978. 5 ,
662.
Received for reuiew D e c e m b e r 26, 1984 Accepted April 22, 1985
Latex Paint Rheology and Performance Properties Cheng-Fa Lu Hercules Incorporated, Wilmington, Delaware 19894
Viscoelastic measurements reveal a distinct difference in rheology between high and low PVC latex paints. For low PVC (pigment volume concentration) paints, the translational-rotational Brownian motion dominant regime in which G"(o) > G'(o) occurs at frequency w lower than 0.1 (or 0.01) rad/s. However, G"(o) > G'(o) for high PVC paints can appear at a much higher frequency ( > l o rad/s). The correlation between G"(w) at low o and paint leveling was found to be excellent. Murphy's equation, which describes the leveling mechanism, was tested. Lab results confirm its accuracy. Extensional viscosities of paints were obtained by using convergent flow analysis. I t was found that paints showing increasing extensional viscosity with applied stress have very poor spattering property.
Introduction The application of a paint to a substrate by brushing involves two rheological extremes. During the brushing process, the encountered shear rate is believed to be within the range from 5 000 to 20 000 s-l (Patton, 1979). After brushing, the leveling of brushmarks is brought about by the driving of surface tension at a corresponding shear rate less than 1.0 (Patton, 1979). A level film is important not only for its physical appearance (e.g., smoothness, gloss, and color) but also for 0196-432118511224-0412$01.50/0
its better protective properties (e.g., cracking and corrosion resistance (e.g, Patton, 1979; Lin, 1975). Quach (1973)gave a detailed review of many aspects of leveling. Most theorties consider that surface tension is the major driving force. Gravity is just a minor cause. In many cases, paints are supplied on a substrate by using a roller coater. During the application, numerous paint filaments are pulled out from the interstices among fibers on the coater, and this may result in paint spattering. In addition to shear deformation, uniaxial extension of the 0 1985 American
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I
-2R
Figure 2. Schematic diagram of convergent flow profile in the entry region. SUBSTRATE
Figure 1. Schematic diagram for striation cross section.
paint is encountered. Glass (1978) demonstrated that the extensional viscosity of a paint is related to the spattering property. In his study, he used a “spinning-fiber” apparatus to obtain the extensional viscosity. However, the availability of “spinning-fiber” technique is restricted to paints exhibiting high tensile tension. To overcome the measurement difficulty for common paints, we used convergent flow analysis, developed by Cogswell (1969,1972), to extract the extensional viscosity. Theory A. Paint Leveling. The brushing of a paint over a smooth surface leaves brushmarks due to the uneven force distribution. An idealized striation of a brushmark is shown in Figure 1. The striation curve is assumed to be sinusoidal. Smith et al. (1961) obtained the exerted stress distribution, which results from surface tension, on the sinusoidal surface by assuming that the paint is an elastic body. They found that the stress distribution is not a function of elasticity. On the basis of the obtained stress distribution and assuming constant viscosity with time, Smith et al. (1961) gave the following equation predicting the rate of decrease in brushmark amplitude for a Newtonian paint:
where a is the amplitude of brushmarks at time t , u is the surface tension of the fluid, 71 is the fluid viscosity, h is the mean film thickness, and X is the wavelength of the brushmark. Since most paints behave as non-Newtonian fluids, eq 1 is of little use in reality. To overcome this difficulty Murphy (1968) derived an equation for the rate of leveling by assuming that the paint can be described by the power law; this is a realistic assumption for low shear rates.
K and N are the power law parameters such than In q = In K + (N- 1) In T, in which i. is the shear rate. Integrating a over t for eq 2, we have
where a. is the initial brushmark amplitude. In this study we obtained K and N from the dynamic viscosity (71’). The dynamic viscosity is defined as G”(u)/u, where G”(o)is the loss modulus and o is the applied frequency of oscillation. B. Convergent Flow Analysis. Material flowing through an orifice undergoes a tensile deformation. On leaving the orifice the elastic component of this defor-
mation can be recovered as extrudate swelling. If the path through the orifice is short compared to the orifice diameter, the shear deformation can be ignored (Cogswell, 1972). The exact profile of the flow in the orifice entry region is unknown. However, Cogswell (1969) assumes that the flow profile is conical (see Figure 2) on the basis of the experimental observation: 4a ‘APo Cle = (4) i.o(ln A2 - In B2) where Po is the pressure across the orifice, qo is the shear rate at the wall of the orifice (To = 4Q/7rR3 where Q is the volumetric flow rate of fluid and R is the radius of the orifice), B is the extrudate swelling ratio, i.e., extrudate radius divided by the orifice radius, and a ’and A are shape factors defined by Cogswell (1969). Empirically, he found that a’= 114 and A = 3.5 give consistent results for a wide range of materials (e.g., polymer melts). Note that a’ h/R’and A = R’IR. As a result 3.5P0 (5) - T0(2.5 - In B2)
Experimental Methods A. Paint Leveling. Materials: Fifteen samples of paints thickened up to 90 KU (Stormer viscosity) were prepared, including gloss, semigloss, and flat paints. The gloss paints contain 26.4% acrylic latex solids and 27.4% Ti02 pigment, while the semigloss paints have 22.5% acrylic latex solids, 22.5% Ti02 pigments, and 2.3% silica particles. Flat paints contain about 9.4% vinyl acrylic latex, 15% Ti02 pigments, 10.7% CaC03 particles, 10.7% Iceberg clay, and 2.1 % silica. HEC (hydroxyethylcellulose) and polyacrylamide polymers were made by Hercules. Methods: Viscoelastic properties of paints were measured at room temperature with a Rheometrics mechanical spectrometer. A sensitive transducer was used to detect the rheological responses. Cone and plate geometry was used. Samples were placed in the cup and covered by a thin layer of silicone oil to prevent solvent loss. In order to avoid the uniformity problem, samples were presheared at 5 rad/s for 5 min and then relaxed for 5 min. The storage [G’(o)],loss [G”(o)] moduli, and complex viscosity [71*(w)] were measured as a function of frequency at 35% strain. Too much applied strain causes input overload. However, applying a very small strain can result in an inaccurate reading due to the generation of low torque on the drive shaft. In addition, the nonlinear viscoelastic response of concentrated dispersions usually occurs at small strain amplitudes (Milkie et al., 1982). Leveling of paints was rated with a standard Leneta leveling test blade. The best value of leveling for paints is 10, and the lowest value is 0. B. Convergent Flow Analysis. Materials: A schematic diagram of the experimental setup for convergent flow analysis aided with photographic technique is shown in Figure 3. The chamber in which materials are loaded is made of Plexiglass of 5-mm thickness. A stainless steel plate, 0.3 mm thick, for the bottom allows a minimum flow
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PRESSURE
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path through the orifice so that the resulting shear deformation may be ignored. The orifice diameter used in this study is 2.5 mm. The convergent flow chamber is a 47 mm X 47 mm square box, 150 mm high. Standard oil (viscosity = 105 CPat 25 OC)was used to test the accuracy of results obtained from the convergent flow analysis. Methods: Samples were first injected into the convergent flow chamber and then rested for 10 min to eliminate existing bubbles. The height of the liquid level and the supplied air pressure were recorded for the calculation of maximum stress across the orifice. The flow visualization of extrudate was photographed in order to measure the maximum extrudate diameter. The flow rate of the extrudate was also measured.
Results and Discussion A. Paint Leveling. We have measured rheologies of 15 various paints. Figures 4,5, and 6 show the typical rheological profiles of gloss, semigloss, and flat paints, respectively. The viscoelastic properties of these paints can be determined by G”(w) (loss modulus) and G T w ) (storage modulus). G’(w) is the elastic response, and G”(w) measures the viscous contribution at oscillation frequency w. The complex viscosity, ?*(a), is defined as [G’(w)’ + G ” ( W ) ~ ] ~(e.g., /~/W Milkie et al., 1982). For a low solid dispersion in a Newtonian medium without strong flocculation, G’(w) at low frequency is much smaller than GN(w) (Milkie et al., 1982). Therefore, ?*(a)should be approximately equal to $(o)at low frequency. However, at high solid concentration the complex viscosity of a dispersion may be much larger than ~ ’ ( w due ) to the existence of elasticity (Milkie et al., 1982). The viscous response of a dispersion results from the energy dissipation through individual particle motion and the “stretching” of particle aggregates in the shear field (Van de Ven and Hunter, 1977). However, elasticity of a dispersion arises from the loss of configurational entropy when the steric barriers adsorbed on particles are compressed owing to contact with one another (Milkie et al., 1982) as well as from the “stretching” of particle aggregates (Van de Ven and Hunter, 1977). A t low oscillation frequency the time scale is large, and particles can take up their equilibrium positions through Brownian motion. In this situation, the effect of the configurational entropy of the steric barriers adsorbed on particles is relatively small. As a result, G’(w) of a dispersion should be less than G”(w) at w below a specific
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value. For semigloss and gloss paints the Brownian motion dominant regime generally falls below 0.01 rad/s (see Figures 4 and 5). However, for flat paints we always see Gf(w) < G”(w) at w less than 10 rad/s (see Figure 6). This distinct difference can be attributed to the content of latex particles in paints. Both semigloss and gloss paints contain latex solids at concentrations of about 22 %, while flat paints consist of only 10% latex. All latices were sterically stabilized by short chain polymers (Craig, 1983). To further prove the effect of latex content on the viscoelaqtic property of paint, we changed the latex content to 30% of the flat paint while maintaining the total solid concentration. Viscoelastic properties of this paint are shown in Figure 7. Again, we see that the elastic contribution is greater than the viscous contribution over the measured frequency range. The type of latex usually does not alter the viscoelastic properties of a paint. The substitution of acrylic latices in the flat paint formulation did not affect the Brownian motion dominant regime (see Figures 6 and 8). Figures 9 and 10 show the correlation between G “and corresponding paint leveling at frequency 0.01 and 0.1 rad/s, respectively. These two frequencies correspond to
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 3, 1985 415
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,tE# G’(w) occurs a t a frequency w lower than 0.1 rad/s. However, the regime can appear at a much higher frequency (>lo rad/s) for low latex contents. The difference can be attributed to the larger loss of configuration entropy of steric barriers adsorbed on latices for dispersions containing higher latex concentration. The correlation between G”(w),at low w , and paint leveling was found to be excellent. However, the correlation of G’(w) with leveling does not appear to be good. This suggests that viscosity is the leveling-controllingparameter. Results confirm the validity of Murphy’s equation relating
the change of brushmark amplitude with time. Convergent flow analysis aided by photographic technique was applied to determine the extensional viscosity of paints. The accuracy of the rheological measurement with this device was found to be satisfactory. For a Newtonian fluid the determined extensional viscosity obtained from convergent flow analysis is approximately equal to 3 times the shear viscosity. For paints thickened with certain polymers, such as polyacrylamides, the extensional viscosity was found to increase with the applied stress. It was found that paints showing increasing extensional viscosity with applied stress have very poor spattering property. Literature Cited Bird, R. B.; Armstrong, R. C.; Hassager, 0.“Dynamics of Polymeric Liquids”; Why: New York, 1977; Voi. I , p 187. Cogswell, F. N. Rheol. Acta 1069, 8 , 187. Cogswell, F. N. Po/ym. Eng. Scl. 1072, 72(1), 64. Coiclough, M. L.; Smith, N. D.P.; Wright, T. A. J . Oil Colour Chem. Assoc. 1980, 63, 183. Craig, D. H., personal communication, 1983, Hercules Inc. Glass, J. E. J . Coat. Technol. 1078. 50(641), 56. Lin, 0.C. C. CHEMTECH 1075, 5 , 51. Milkie, T.; Lok, K.; Croucher, M. D. Colloid Po/ym. Scl. 1982, 260, 531. Murphy, J. PRA internal report, RS/T/31/68. Patton, T. C. “Paint Flow 8 Pigment Dispersion”, 2nd ed.; Wiiey: New York, 1979; Chapter 28. Quach, A. Ind. Eng. Chem. Prod. Res. D e v . 1073, 72, 110. Smith, N. D. P.; Orchard, S. E.; Rhind-Tutt, A. J. J . Oil Colour Chem. Assoc. 1061, 4 4 . 618. Smith, R. E. J . Coat. Technol. 1982, 54(694), 21. Van de Ven, T. 0. M.; Hunter, R. J. Rheol. Acta 1977, 76, 534. Vinogradov, G. V.; Malktn, A. Ya. “Rheology of Polymers”; Springer-Veriag: New York, 1980; Chapters 1, 7.
Received f o r review October 1, 1984 Accepted April 29, 1985
The Effects of Metals and Inhibitors on Thermal Oxidative Degradation Reactions of Unbranched Perfluoroalkyl Ethers Wllllam R. Jones, Jr.,” Kazlmlera J. and Relnhold H. Kratzert
L. Paclorek,t David H. Harris,+Mark E. Smythe,t James H. Nakahara,’
NASA Lewis Research Center, Cleveland, Ohio 44135
Thermal oxidative degradation studies were performed on unbranched perfluoroalkyl ethers at 288 O C in oxygen in the presence of metals and alloys. The pure metals, titanium and aluminum, promoted less degradation than Ti(4A1,4Mn) alloy. The two inhibkors Investigated (a (perfluoropheny1)phosphlneand a phosphatKiazine) reduced degradation rates by several orders of magnitude: both were effective for the same duration (75-100 h).
Introduction
Unbranched perfluoroalkyl ethers developed by Montedison (Sianesi et al., 1973), in view of their remarkable temperature-viscosity properties (Snyder et al., 1981), nonflammability (Snyder et al., 1982),and thermal stability (Jones et al., 1983), offer great potential as high-temperature lubricants. Unfortunately, these materials, as reported earlier (Jones et al., 1983; Snyder et d., 1981), were found to undergo decomposition in oxidizing atmospheres below 288 OC. This reaction was greatly Ultrasystems, Inc., Irvine, CA 92714. O196-4321/05/ 1224-O417$01.50/0
accelerated by the presence of alloys M-50 and Ti(4A1,4Mn). On the other hand, low concentrations of additives were found to arrest the process virtually entirely. Both for basic understanding and for practical applications, it is necessary to know the effect of different metals and alloys on the stability of these fluids. It has been established (Paciorek et al., 1979) that in the case of commercially available poly(hexafluoropropene oxide) fluids (Krytox series, product of Du Pont) additives are effective for limited time only. Thus, the objective of the current study was to evaluate the action of pure metals on the unbranched perfluoroalkyl ethers and to assess the effectiveness of the degradation inhibitors with respect to 0 1985 American Chemical Society
