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Ind. Eng. Chem. Prod.
Res. Dev. 1002, 27, 686-690
Effect of Cross-Linked Polymeric Microparticles on the Rheology of High-Solids Coatings David R. Bauer, Llnda M. Brlggs, and Ray A. Dlckie' Engineering and Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
Conventional solvent-based coatings formulated for industrial spray application rely on solvent evaporation to achieve the control of flow necessary for proper appearance. Low-emission high-solids coatings contain less solvent and require additional means of flow control. One approach to flow control in high-solids coatings is through the addition of cross-linked polymeric microparticles. The addition of these particles greatly modifies coating rheology and in particular imparts an apparent yield stress to the coating. The size of the yield stress has been found to be a strong function of the concentration of the disperse particles. It has also been found to be proportional to the polymer solution viscosity. For sufficiently polar polymers, the yield stress has been found to be independent of polymer and solvent composition and polymer molecular weight (except as these parameters affect the solution viscosity). The yield stresses for solutions of nonpolar polymers were found to be substantially lower than those for solutions of polar polymers.
Introduction Spray application places severe limitations on the flow behavior of coating formulations (Schoff, 1976; Takahashi, 1980); these limitations are especially severe in the case of automotive topcoats. The coating formulation must be sufficiently fluid at high shear rates to atomize well. During film formation the coating must coalesce and level. However, after leveling, flow must rapidly cease to prevent sagging and, in the case of metallic coatings, reorientation of aluminum flake pigments. Low-solids solvent-based formulations conventionally used for industrial spray application typically achieve flow control through the controlled evaporation of solvent (Hansen, 1968, 1970; Sletmoe, 1966). Coating rheology is basically Newtonian, i.e., the viscosity is independent of the shear rate. As the solvent evaporates, the viscosity increases rapidly because the viscosity is a strong function of solids level. High-solids coatings contain less solvent and the viscosity increase on evaporation is much less (Mercurio and Lewis, 1975; Erickson, 1976). A typical example of the dependence of viscosity on polymer concentration (or coating solids level) is shown in Figure 1 for polymer solvent systems typical of low-solids and high-solids coating formulations. For coatings that are baked, the temperature dependence of the viscosity can also play a role in coating appearance. In general, soon after entering the bake oven nearly all the solvent is evaporated. As the temperature rises, the viscosity then falls until the coating cross-links sufficiently to gel. Since a high-solids coating requires much more cross-linking to occur before gelation (Bauer and Budde, 1981),a high-solids coating will have a greater tendency to sag in the bake oven than will a low-solids coating. This behavior is illustrated for typical low- and high-solids coatings in Figure 2. Similar results have also been obtained elsewhere (Oguri, 1978). Thus a high-solids coating has two major flow problems. First, the viscosity buildup after spraying is usually insufficient to achieve good appearance. Second, the high-solids coating has a much greater tendency to flow in the bake oven. The achievement of flow control in a high-solids coating requires different rheological behavior than that for a low-solids coating. Spray application is a high-shear process while sagging and aluminum reorientation are low-shear phenomena. Wu (1978a,b) has shown that in 0196-432 1/82/1221-0686$01.25/0
order to prevent sagging in coatings where there is no solvent evaporation, it is necessary that the coating either is pseudoplastic (viscosity decreases with increasing rate of shear) or has a yield stress. It has been found that the addition of disperse particles such as silicas, bentonite clays, or certain organic thixotropes can be used to achieve flow control in high-solids coatings (Schoff, 1976). In particular, it has been claimed that the addition of insoluble cross-linked polymeric microparticles to a coating formulation is especially effective in providing flow control in coatings (Makhlouf and Porter, 1979). In this paper, we present a detailed rheological study of polymer solutions containing cross-linked polymeric microparticle additives. It is found that these additives do indeed impart a yield stress to the polymer solution. The magnitude of the yield stress has been studied as a function of microparticle concentration, solution polymer composition, concentration, and molecular weight, solvent composition, and temperature. Experimental Section Materials. The solution polymers used in this study were acrylic copolymers which were prepared by conventional free radical polymerization. The polymers along with their molecular weights (as determined by gel permeation chromatography) are listed in Table I. The melamine formaldehyde cross-linkers which were blended with polymer 1in some solutions were obtained from the American Cyanamid Co. A cross-linked acrylic polymeric microparticle prepared by a nonaqueous dispersion (NAD) polymerization technique (Makhlouf and Porter, 1979) was used in this study. Microparticle concentrations are given by weight percent of the total solution. Microparticle particle size was determined by quasielastic light scattering to be -300 nm. Rheological Measurements. Rheological measurements were performed using a Contraves LS-30 low-shear couette viscometer. Shear stresses were measured over a shear rate range of 0.0127-1.27 s-l. Samples were preconditioned by shearing at 1.27 s-l shear rate for several minutes. The shear stress was then monitored as the shear rate was decreased. The results were found to be not sensitive to the rate of decrease of shear rate. For the experiments reported here a 3.3-min ramp was used. The cup containing the polymer solution was loosely capped 0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
687
Table I. Solution Polymers
/
polymer no.
P
%
symbol
1
b
30
1500
20
4200
30
4500
30
4000
20
8200
20
5900
7
30
1500
s b
30
1500
0
5000
0
3900
2
d
0
3
IO
20
I
1
1
I
I
1
30
40
50
60
70
80
%SOLIDS
Figure 1. Viscosity vs. percent solids for two acrylic polymers in 2-heptanone: (0) M,, = 8200; (0) M,, = 1500. The horizontal line indicates typical spray viscosity. Viscosities were determined over the shear rate range 0.1-10 s-l and found to be independent of shear rate over that range.
I
FLASH
0
+
BAKE O V E N
b
D-
-C
I
I
I
1
I
I
2
4
6
8
IO
12
TIME AFTER SPRAY
9 10
Polymer in t h e a % hydroxyethyl acrylate by weight. following solvents: xylene-heptane ( 50-50), xylene, Polybutanol, 2-heptanone, and butyl carbitol acetate. Polymer 1 mer 1 plus 30% fully alkylated melamine. plus 30% partially alkylated melamine.
GEL P O I N T
-2
M,
hydroxy
(mi" )
Figure 2. Viscosity vs. time after spray for conventional low solids coating (-) and high solids coating without flow control additive (-.-). Data above 1 Pes are extrapolated from data at lower viscosities. It should be noted that over the time scale involved, flow is negligible when the viscosity is greater than 1 Pes.
to prevent solvent evaporation. Results The effect on the rheology of adding different amounts of cross-linked polymeric microparticles to a polymer solution is shown in Figure 3. The addition of even a small amount of this additive introduces a marked yield stress to the polymer solution. The rheology and particularly the yield stress are strong functions of particle concentration. To evaluate the magnitude of the yield stress, it is necessary to extrapolate the shear stress to zero shear rate. It has been found (Figure 4) that Casson plots (Casson, 1959) provide the most convenient empirical method for obtaining the yield stress. In the Casson plot, the yield stress is obtained from the intercept. Yield stress measurements for any given formulation can be reproduced to within 5-15% with the uncertainty increasing as the microparticle concentration decreases, and the rheological
-3
A
-2
-I
0
LOG SHEAR R A T E (Sec-I)
Figure 3. Log (shear stress) vs. log (shear rate) for three different microparticle concentrations ( % by weight of total solution) in a 70% solution of polymer 1 in 2-heptanone.
behavior becomes increasingly more Newtonian. Possible errors due to differences in preconditioning and other time-dependent effects have been estimated to be 10-20%. Thus yield stresses can be measured to roughly *30%. The slope of the Casson plot gives an estimate of the high shear rate viscosity. Even though the yield stress (intercept) is a strong function of particle concentration, the slope is within experimental error independent of particle concentration. This implies that spray application will not be strongly affected by the presence of the microparticles. Figure 3 shows the effect of adding microparticles to a polymer solution whose total solids level including microparticle was fixed. To determine the rheological behavior of a coating as it dries, the rheology has to be de-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
58
32
-2
0
-I
LOG SOLUTION VISCOSITY (Po.
5 )
Figure 6. Log (yield stress) vs. log (solution viscosity) for different solution polymers. The microparticle concentration was 5.8%. See Table I for symbol key. 0
0
1
2
3
4
5
(SHEAR RATE )'I2 (Sec - ' h )
Figure 4. Casson plot of the solutions of Figure 3.
-2
-I75
-I 5
-I25
-I
L O G NAD CONCENTRATION
Figure 7. Log (yield stress/solution viscosity) vs. log (microparticle concentration). The slope of both lines is 3.1 f 0.3. 50
60
70
X SOLIDS
Figure 5. Log (yield stress) vs. percent solids of polymer 1. The initial microparticle concentrations (at 50% solids) were 4.2% (0) and 5.8% (0). The horizontal line indicates the yield stress necessary to prevent sagging in a 25-pm film.
termined as a function of total solids level of the polymer solution. As shown in Figure 5 , the yield stress is found to be a rapidly increasing function of coating (or polymer) solids level. As the solvent evaporates and the solids level increases, both the Newtonian viscosity of the polymer solution without microparticle (hereafter referred to as the solution viscosity) and the microparticle concentration as a function of the total solution increase. The effect of solution viscosity on yield stress at constant microparticle concentration is shown in Figure 6. Within experimental error the yield stress is simply proportional to the solution viscosity. For the polymers shown in Figure 6 no specific dependence on polymer molecular weight or composition was seen except as these parameters affect the solution viscosity. Similar results are obtained at all microparticle concentrations. The dependence of the yield stress on microparticle concentration can be obtained by plotting the yield stress divided by the solution viscosity vs. the microparticle concentration. As shown in Figure 7, the yield stress is a strong function of microparticle concentration. For the polymers studied in Figure 7 the yield
stresses fall on two parallel lines. The yield stress can be given by yield stress = A
X qsol x
[ m i c r o p a r t i ~ l e ] ~ . ~(1) *~.~
where A is a constant that depends only on the polymer polarity and the structure of the microparticle. The value of A is roughly 3 times lower for solutions containing polymers with no hydroxy functionality than for solutions containing polymers that comprise at least 20 % by weight hydroxy functional comonomer. As long as the amount of hydroxy functional comonomer is at least 20%, A is independent of polymer molecular weight, polymer composition, or solvent. Melamine cross-linkers can also be added to the polymer solution without affecting the magnitude of A. Since most acrylic copolymers used in high-solids coatings contain at least 20% hydroxy functional comonomer, it is possible to estimate the yield stress knowing only the solution viscosity and the microparticle concentration. The dependence of the yield stress on temperature is shown in Figure 8 for three different microparticle concentrations at constant solution viscosity. The yield stress (i.e., the value of A ) is found to be independent of temperature over the range 25-70 "C. Thus eq 1can be used to calculate the yield stress for coatings as they enter the bake oven as well as during the solvent flash.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 689
-
I
I0
% NAD
v)
1-
f
, 4 2 1
t a 9 -I
25
40 55 TEMPERATURE,
70 OC
Figure 8. Log (yield stress/solution viscosity) vs. temperature for solutions of polymer 2.
Discussion The yield stress is an important parameter in determining the extent of flow in these coatings. For example, if the yield stress is greater than about 0.2 Pa, a coating whose thickness is 25 pm and whose density is 1g/cm3 will not sag under gravitational stress. The strong dependence of yield stress on solution viscosity and microparticle concentration and the lack of a dependence on polymer and solvent composition and coating temperature have direct implications for high-solids coating formulation. The strong dependence of yield stress on concentration implies that optimum rheological properties will likely be found over a very limited range of microparticle concentration, The lack of specific dependences on polymer and solvent composition greatly simplifies coating formulation. Polymers and solvents can be varied within the range studied without much concern for their specific rheological interactions with the microparticle additive. Control of the yield stress can be achieved by controlling the rate of solvent evaporation since this affects the change with time of both the solution viscosity and the microparticle concentration. The lack of dependence of the yield stress on temperature suggests that microparticle additives can essentially eliminate flow in the bake oven. Consider for example, the high-solids coating of Figure 2. Assuming that the yield stress remains independent of temperature up to 100 "C, the yield stress at the solution viscosity minimum in the oven can be estimated from eq 1 to be some 100 times greater than that of the original formulation. The increase is due both to the fact that the solution viscosity at the minimum in the oven is over 10 times higher than the spray viscosity and to the fact that, due to solvent evaporation, the microparticle concentration is roughly twice that of the original formulation, causing a further 8-fold increase in the yield stress. Non-Newtonian flow behavior is complicated and nomenclature is similarly complicated and potentially confusing (Reiner and Scott Blair, 1967). The flow behavior described in this paper is shear thinning, i.e., a reduction of viscosity or consistency with increasing shear rate, Further, there exists an apparent yield stress; i.e., a minimum stress is required to produce flow. In practice, this flow behavior is often also accompanied by thixotropy, a phenomenon perhaps best characterized by the time-dependent recovery of viscosity upon cessation of flow. These phenomena, and especially thixotropy, have been studied in dispersions, including paints, for many years (Bauer and Collins, 1967). In fact, thixotropy was first reported in
-2 -2
-I 5 -I -5 LOG (PARTICLE CONCENTRATION)
Figure 9. Log (yield stress/solution viscosity) vs. log (particle concross-linked centration) for microparticle in melamine solution (0); polystyrene particles in polystyrene solution ( 0 ) . (Data estimated from Onogi et al. (1973) and Matsumoto et al. (1975) for a 300-nm particle in a 10% polystyrene solution) and a titanate fiber in polystyrene solution (A) (Onogi et al., 1977).
coatings over 45 years ago (Pryce-Jones, 1936). The emphasis in most of the experimental (Bauer and Collins, 1967; Pierce and Donegal, 1966) and theoretical (Ritter and Govier, 1970) studies has been on the time dependence of the shear stress at different shear rates. In our studies the time dependence of the shear stress was relatively minor compared to the large variation in equilibriwyield stress. There have been relatively few studies of the equilibrium yield stress and to our knowledge no detailed studies of the yield stress developed in high solids coatings containing microparticle additives. Matsumoto et al. (1973 and 1975) and Onogi et al. (1973 and 1977) have studied the yield stress induced in polymer solutions by a number of different particles including carbon black, cross-linked polystyrene emulsion particles, and titanate fibers. The polymer solutions were typically high molecular weight polystyrene solutions whose viscosities were much higher than those of the present study. Nevertheless, if we assume that the observed proportionality of yield stress with solution viscosity is valid out to the viscosities used by Onogi et al., it is possible to compare their results with those of the present work. Figure 9 shows a plot of yield stress divided by solution viscosity vs. particle concentration for three different particles: the microparticle of the present work, here blended with a melamine resin instead of an acrylic copolymer, cross-linked polystyrene spheres (Onogi et al., 1973; Matsumoto et al., 1975), and titanate fibers (Onogi et al., 1977) both in polystyrene solutions. Despite vast differences between particle structure and the polymer solutions used, the concentration dependences of the yield stresses are virtually identical. Only the magnitude of the effect (the A factor of eq 1)is different. Matsumoto et al. (1975) also found the yield stress to be independent of temperature. This suggests that the existence of a yield stress in a dispersion is a general phenomenon and is not specific to one particular class of particles. The details of the specific interaction between particles determine the magnitude of the effect but not the qualitative behavior. The existence of a yield stress is generally taken to imply the existence of some sort of structure in the fluid, the nature of which is usually not known (Bauer and Collins,
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1967). In the case of microparticle dispersions, the structure is apparently caused by an attractive force between the particles. The existence of attractive forces in dispersions containing dissolved polymer has been demonstrated. Vincent et al. (1980) and Clarke and Vincent (1981) have found that small amounts of dissolved polymer can cause aggregation in dispersions. De Hek and Vrij (1981) have observed that settling of silica particles occurs over a limited range of solution polymer concentration. Asakura and Oosawa (1954, 1958) have argued that the configurationalentropy of a solution polymer is lower near a hard surface (such as that of a disperse particle). This leads to an attractive force between particles. This effect may be real, but it does not seem to account for the observed dependence of the yield stress on solution polymer polarity. Vincent et al. (1980) and Clarke and Vincent (1981) have argued that the degree of interpenetration between the dissolved polymer and the stabilizer polymer on the surface of the disperse particle controls the strength of the attractive force between particles. If the polymers do not interpenetrate, a concentration gradient of solution polymer causes the attractive force. Based on the method of preparation, the microparticle is thought to have a relatively nonpolar surface. The relatively polar solution polymers used here will not penetrate the surface of the particle. The attractive force induced by this does not cause settling. Instead, the interaction between the microparticles caused by the presence of solution polymer leads to the formation of a structure which produces a yield stress. Following this argument the yield stress is lower in solutions containing nonpolar polymers because they can penetrate the nonpolar surface layer reducing the concentration gradient and thus the force between particles. The yield stress should be zero when the microparticle is dispersed in a totally nonpolar medium. This is in fact observed to be the case. On the other hand, once the solution polymer is sufficiently polar so as not to penetrate the surface layer at all, increasing the polarity of the solution polymer does not change the strength of the interaction. This is consistent with the observation that the yield stress is independent of the acrylic copolymer hydroxy content above the 20% level. Although this explanation seems plausible, more work is required to verify the mechanism and to quantify the magnitude of the forces involved.
Conclusion The addition of cross-linked polymeric microparticles to polymer solutions typical of high-solids coating formulations has been found to impart a yield stress to those solutions. The magnitude of the yield stress was found to be simply a function of the viscosity of the polymer solution without microparticle and the microparticle concentration. For polymers typically used in coating formulations, no specific dependence on composition was found. However, it was found that very nonpolar polymer solutions had lower yield stresses than that found in polar polymer solutions. This suggests that the mechanism responsible for the yield stress involves an attractive force between the microparticles which is controlled by the interaction of the solution polymer and the surface layer of the microparticle. Literature Cited Asakura, S.; Oosawa, F. J . Chem. Phys. 1954, 2 2 , 1255. Asakura, S.; Oosawa, F. J . Polym. Sci. 1958, 33, 183. Bauer, D. R.; Budde, G. F. Ind. Eng. Chem. Prmf. Res. Dev. 1981, 20, 674. Bauer, W. H.; Collins, E. A. "Rheology", Voi. 4, Eirich, F. R., Ed.; Academic Press: New Ywk, 1967; p 423. Casson, N. "Rheology of Disperse Systems", C. C. Mill. Ed.; Pergamon Press: London, 1959, p 84. Clarke, J.; Vincent, B. J . Chem. Soc., Faraday Trans. I , 1981, 77, 1831. De Hek, H.; Vrij, A. J . CoU. Int. Sci. 1981, 8 4 , 409. Erickson. J. R. J . Coat. Techno/. 1976, 48, (620), 58. Hansen, C. M. J . Oil Colour Chem. Assoc. 1968, 5 1 , 27. Hansen, C. M. Ind. Eng. Chem. Prod. Res. Dev. 1970, 9 , 282. Makhlouf, J. M.; Porter, S. J., Jr. US. Patent 4 147688. Apr 3, 1979. Matsumoto, T.; Segawa, Y.; Warashima, Y.; Onogi, S. Trans. SOC.Rheol. 1973, 17, 47. Matsumoto, T.; Hitomi, C.; Onogi, S. Trans. SOC. Rheol. 1975, 19, 541. Mercurio, A.; Lewis, S. N. J . Palnt Techno/. 1975. 4 7 , 37. Oguri, H. Toso @/ufsu Zalro to Seko 1978, 17(215), 112. Onogi. S.; Matsumoto, T.; Warashima, Y. Trans. Soc. Rheol. 1973, 17, 175. Onogi, S.; Mikami, Y.; Matsumoto, T. Polym. Eng. Sci. 1977, 17, 1. Pierce, P.; Donegal, J. J . faint Technol. 1968, 38(492), 1. PryceJones, J. J . 011 Colow Chem. Assoc. 1936, 19, 295. Reiner, M.; Scott Blair, G. W. "Rheology", Voi. 4, Eirich, F. R., Ed.; Academic Press: New York, 1967; p 461. Rltter. R. A,; Govier, G. W. Can. J . Chem. Eng. 1970, 48, 505. Schoff, C. Prog. Org. Coaf. 1978, 4 , 189. Sletmoe, G. M. J . Paint Technol. 1985, 38, 642. Takahashi, M. Polym. Pkst. Technol. Eng. 1980, 15, 1. Vincent, 8.; Luckham, P. F.; Wake, F. A. J . Colloid Interface Sci. 1980, 73, 508. Wu, S. J . Appl. Polym. Sci. 1978, 2 2 , 2769. Wu, S . J . Appi. Polym. Sci. 1978, 2 2 , 2783.
Received for review May 10, 1982 Accepted August 6, 1982
