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Spray applications: Part V. Influence of high solid compositions on coatings sprayability.

Introduction

Previous studies (1-4) from our group have examined fluid properties (surface tension, high and low shear viscosities, storage moduli (G') and dynamic uniaxial extensional viscosities (DUEVs) of water-soluble polymer blends and fully formulated waterborne coatings. After evaluating all of the parameters, sprayability and droplet size could be correlated only to DUEVs. The results are similar to past studies examining spatter in roll-applied coatings (5) and aqueous polymer blends. (6)

Waterborne latex coatings are an alternative to conventional high molecular weight resins dissolved in an organic solvent (e.g., nitrocellulose solubilized in organic solvents used in Henry Ford's model T automobiles and still used in nail polishes). High-solid (H-S) coatings that contain large amounts of oligomers and crosslinker and a lower level of solvent are another alternative to low concentration, high molecular weight polymers in organic media. For a pigmented primer, the percent solids can be as high as 50 NVV, and as high as 75 NVV for a clearcoat. (7) The protective coating is obtained by polymerization of the oligomers during and after application on the surface to provide thermosetting resins (Scheme 1). Organic component influences on the sprayability of high solids coatings is the focus of this study.

Mercurio and Lewis evaluated the effects of molecular weight on solids content and viscosity. (8) Their results indicated that there is a linear relationship between the log of the viscosity and the weight percent of the polymer solids. The increase in viscosity with increasing solids content is much lower with oligomers than with higher molecular weight polymers. Flory (9) recognized this well before high solid coatings became popular. Reduction of application viscosity and sagging of the applied fluid after leveling are two major rheological problems in high solid coatings. (10,11) Problems with sagging can be reduced by adding a thixotropic agent, such as polymeric microparticles (12,13) or clays. (10) Application viscosities are improved by using lower molecular weight oligomers, low-viscosity solvents, lowering functional group content, (14) minimizing resin-solvent interactions, (15) reactive diluents, (16) and heat (17,18) to control the viscosity of the high solids coatings. In general, the viscosity of high solid coatings decreases with increasing temperature, and the molecular weight of the oligomers. (19)

This study is devoted to taking a recommended high solid formulation* and observing how variations in its composition and storage effect its sprayability. Using the Particle Image Velocimeter (PIV) technique in waterborne coatings provided insights into performance properties with structural changes in the thickeners. Using this technique in the high solid coatings area seemed a logical followup to end our studies in this area.

Materials and methods

High solid coatings were formulated to 60 wt% solids with varying ratios of an acrylic oligomer (Desmophen 365, Bayer), polyester oligomer (Desmophen 670, Bayer), melamine formaldehyde crosslinker (Cymel 1168, Cytec), and 1 wt% sulfonic acid catalyst (Nacure 5414, King Industries) with xylene (Aldrich) as the solvent. The various formulations used in this study are listed in Table 1.

Velocity profiles

Spray droplet velocity profiles were acquired using the PIV technique, discussed in the previous manuscript. (4) The time between frames, [DELTA]t, was kept at 100 [micro]s. The interrogation region was a 64 x 64 mm square selected to give approximately 10-15 particles per interrogation region. Processing of the captured images to give velocity vectors was performed using Insight Software. Plotting of the velocity vectors to obtain the velocity profile was performed using TecPlot software.

Measurement of droplet size

The various formulations were air-sprayed (55 psi) with a fan-nozzle geometry (discussed below). The droplets were collected using a procedure outlined in references (1) and (2). The collected drop sizes were analyzed using Optimus 6.0 Image Analysis Software. Sauter Mean Diameters (SMD) were calculated based on the acquired drop size distribution. The SMD is defined as the diameter of a drop that has the same volume to surface area as the entire spray volume. The equation for SMD is given below.

SMD ([D.sub.vs]) = [SIGMA]([N.sub.i][D.sub.i.sup.3])/[SIGMA]([N.sub.i][D.sub.i.sup.2]) (1)

where: [N.sub.i]: is the numbers of droplets having a [D.sub.i] diameter.

The SMD can be calculated from the transformation:

ln [D.sub.vs] = ln [D.sub.GM] + 2.5 * [ln.sup.2][S.sub.G] (2)

where: [D.sub.GM] = [D.sub.50%] and [S.sub.G] = [D.sub.84.14%]/[D.sub.50%]

Using this equation, the SMD can be calculated from a log-normal probability plot (using Minitab 1.0 statistical program) of the spray droplet size distribution.

Rheology measurements

Shear rate profiles from 0 [s.sup.-1] to 1000 [s.sup.-1](0-597 Pa shear stress), with a 2-min increasing shear cycle, were measured with a Carri-Med CSL100 controlled stress rheometer with a double-concentric cylinder ([R.sub.3] = 21.96 mm, [R.sub.2] = 20.38 mm, [R.sub.1] = 20.00 mm; cylinder immersed height = 20.50 mm; gap = 1000 [micro]m) geometry. All of the measurements were performed at 25[degrees]C.

Newtonian fluids do not exhibit elastic properties (e.g., storage moduli, First Normal Stress differences (5)) observed in polymer melts or polymer solutions, but Newtonian fluids possess extension viscosities. Uniaxial extension viscosities are simply a multiple of three times larger than a Newtonian shear deformation viscosity. (20,21)

Visual spray studies

Phenomena involved in the spray process and the various procedures involved in spraying a fluid have been discussed in great detail in a proceeding publication. (22) The internal shape of a fan nozzle is designed to cause the liquid to move in a single direction and to curve inwards, so that two streams of liquid forming a fan meet at an elliptical orifice. The shape of the orifice is particularly important in determining not only the amount of liquid emitted, but also the shape of the sheet emerging from it, particularly the spray angle. The type used in this study and the elliptical pattern generated is illustrated above. Nozzle openings in an air spray application range from 1 mm to 3.5 mm in diameter for various coating viscosities. The pattern is illustrated in Fig. 1.

[FIGURE 1 OMITTED]

Results and discussion

High solid coatings differ from conventional alkyd coating in that during application they are predominantly low molecular weight oligomers (Scheme 1). Unpigmented high solids coatings were formulated with different ratios of oligomers and crosslinker (Table 1). Their molecular weights were determined by Size Exclusion Chromatographs (Fig. 2). Melamine formaldehyde used as the crosslinker has a very low molecular weight. A polyester oligomer and an acrylic oligomer are the binders. The acrylic oligomer has a slightly larger [M.sub.n] (number average molecular weight) than the polyester, but the [M.sub.w] (weight average molecular weight), [M.sub.z], and [M.sub.z+1] (the latter reflecting the higher molecular weight fractions with greater elastic influences) of the acrylic oligomer is much larger than that of the polyester oligomer (Fig. 2) and it possesses a high molecular weight tail. In the spray figures, M:P:A will be used to reference the melamine formaldehyde, polyester, and acrylic weight percent ratios. The number average molecular weight and the weight average molecular weight of the blends were calculated (equations (3) and (4)), along with the uniaxial extensional viscosity (Table 2).

[ILLUSTRATION OMITTED]

PDI = polydispersability index, [M.sub.w]/[M.sub.n]

The number average molecular weight, [bar.[M.sub.n]], of the blend is calculated using equation (3).

[bar.[M.sub.n]] = [[w.sub.1] + [w.sub.2] + [w.sub.3]]/[[[w.sub.1]/[bar.[M.sub.n1]]] + [[w.sub.2]/[bar.[M.sub.n3]]] + [[w.sub.3]/[bar.[M.sub.n3]]]] (3)

where [bar.[M.sub.nj]] is the number average molecular weight of the individual components of the high solids and [w.sub.j] is the weight percent of each component. The weight average molecular weight of the blend, [bar.[M.sub.w]] was calculated using equation (4).

[bar.[M.sub.w]] = [[w.sub.1][bar.[M.sub.w1]] + [w.sub.2][bar.[M.sub.w2]] + [w.sub.3][bar.[M.sub.w3]]]/[[w.sub.1] + [w.sub.2] + [w.sub.3]] (4)

where [bar.[M.sub.wj]] is the weight average molecular weight of the individual components of the high solids. (20) The z-average molecular weight of the blend, [M.sub.z], can be calculated using a similar equation.

The viscosity-shear rate profiles (Fig. 3) of the different high solid formulations are essentially Newtonian; all of the components are oligomers, except for the higher molecular weight fractions of the acrylic. The viscosity of the blends increases with the amount and molecular weight of the components added.

The formulations varied in weight percent ratios of MF, polyester, and acrylic. The lowest viscosity contains the least amount of the acrylic component (3:3:1); increasing the acrylic and ester levels (1:4:2) increases the shear viscosity modestly (Fig. 3) and a difference in the spray angle is observed (Fig. 4). As the increment of acrylic is again increased (3:1:3) the spray angle of the fluid again narrows, and larger drop sizes are visibly evident in the upper frame panels distanced from the nozzle. The variations in viscosity are marked, compared to smaller variances in viscosity when the 3:1:3; 1:3:3 and 1:2:4 compositions (Table 2 and Fig. 3) are compared, but the variances in the spray pattern of the latter compositions are more dramatic (Fig. 5). The spray angle decreases markedly to "zero" representing the single stream of fluid in the 1:2:4 formulation that does not spray. High molecular weight fractions are known to markedly increase the extensional viscosity of higher-molecular weight polymers. (23-25)

[FIGURE 3 OMITTED]

In the 1:3:3 formulation, thicker ligaments and a greater number of larger drop sizes are evident, similar to the spray patterns seen in latex paints with moderate DUEVs. (4) The 1:2:4 formulation was not sprayable from a fan nozzle, similar to that observed (1,2,22) for latex coatings with high extensional viscosities. The spray behaviors in Figs. 4 and 5 are not indicative of a viscous fluid without elastic properties. At this point we should have demonstrated this by spraying such fluids as glycerol, as was done in roll applications of fluids, (5) but this was overlooked. The transition in spray patterns in Fig. 5 is likely reflecting more in extensional viscosity changes than is reflected in the traditional multiple of 3x a steady state shear viscosity of a Newtonian fluid. With increasing deformation rates, extensional viscosities are known to increase and if formulations around the 1:2:4 ratios had been studied, high extensional viscosities may have been measurable, despite its Newtonian shear viscosity behavior.

The particle velocity patterns for the four formulations that provided spray patterns are illustrated in Fig. 6. They follow the velocities profiles noted in latex paints with increasing DUEVs. The lower viscosity fluids (3:3:1 and 1:4:2) exit the nozzle at high velocities, dissipate into finer drops, and as discussed in the previous article4 in this series, the fine droplet sizes decrease in velocity as they distance from the nozzle, due to the greater aerodynamic drag on their greater specific surface area. The higher viscosity (3:1:3 and 1:3:3) fluids exit the nozzle at lower velocities, hold together in more elastic ligaments that do not dissipate as readily, and the larger particles do not notably decrease in velocity, as they distance from the nozzle.

For those who have followed all of the studies in this series, an attempt to spray the high solids (HS) formulations through cone nozzles was made. By any viscosity measure most of the HS coatings are more viscous than the latex coatings studied and the HS coatings and one formulation sprayed poorly. The others did not spray. This comparison of different nozzle effects is reflected in the angle of spray from the nozzle types in Fig. 7.

The SMDs of the droplets increase with an increase in viscosity (Fig. 8). Formulations with lower viscosities exit the nozzle at greater velocities with smaller SMDs. With higher surface area, they encounter more aerodynamic drag and lower droplet velocities (26,27) as they are projected farther from the nozzle. Good misting correlates to a low SMD, given that the particle sizes are not too small, with low velocities, to allow redirection of their flow before impacting the surface.

[FIGURE 6 OMITTED]

A high solids system is one primed to polymerize, and from the onset of polymer synthesis the fluidity will change dramatically. Thus storage at elevated temperature and/or an increase in the concentration of the catalyst for the synthesis ought to influence the sprayability of the formulation. The parameters are investigated below.

Effects of storage on spray behavior

A coating formulation with equal ratios of components (M:P:A = 1:1:1, at 60% solids and with 1% sulfonic acid catalyst) was heated to 60[degrees]C for 2, 4, 6, 8 and 10 h, and sprayed through the fan nozzle. Molecular weight growth and crosslinking of the coatings increases with storage time, as reflected in the viscosity increases, but remains, more or less, Newtonian (Fig. 9). Crosslinking is also witnessed by the discontinuities in the flow behavior at 8 and 10 h, suggesting some gel formation. It has been our experience (unreported) that the best way to approximate the viscosity of such "solutions" is to interpolate dynamic viscosities from oscillatory measurements at low frequency. Since the fluids remained Newtonian, such measurements were not attempted.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The high solids stored for 0, 2, and 4 h have similar spray profiles with good misting and wide spray angles. Formulations stored at 6, 8, and 10 h exhibit poorer misting, their spray angle became narrower, and there are more ligaments at the rim of the spray (the extreme comparisons are illustrated in Fig. 10). While exiting from the nozzle there does not appear to be a difference in particle velocities, unless one studies them in detail. A parallel response, consistent with those discussed in Fig. 6, is easily observed in the velocity profiles distance from the nozzle (Fig. 11).

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Catalyst concentration increases

Different weight percents of catalyst were added to the high solid coatings formulated with equal components ratios (1:1:1) at 60% solids. Increasing the catalyst (sulfonic acid) level at a standard 2 h heat treatment could be expected to follow the trend observed with increased storage levels at a lower catalyst concentration, and in general, this is observed. The viscosities are essentially Newtonian up to 6 wt% catalysis (Fig. 12). The formulation with the 8 wt% catalyst level gelled in 2 h. The sprayability with increasing sulfonic acid catalyst levels (heated to 60[degrees]C for 2 h) are illustrated in Fig. 13 for the transition region, 4-6 wt% catalyst. There is little difference in sprayability with the high solid formulations that contain 0%, 2%, and 4% catalyst formulation; however, the formulation that contained 6% catalyst sprayed poorly, there is little misting and a narrower spray angle. The spray pattern was different from other narrowing sprays patterns cited above. There were no differences in the particle velocity profiles (not illustrated).

[FIGURE 11 OMITTED]

Conclusions

In the 1980s, E.I. Dupont Corp. developed and commercialized a narrow molecular weight acrylic resin by a unique Group Transfer Polymerization catalyst. It was expected to be used in high solid coating formulations, probably because of the lack of broad formulation applications with transitional acrylics used in this report.

[FIGURE 12 OMITTED]

The formulations, with low levels of the traditional acrylic, with high molecular weight tails, and different concentrations of a crosslinker and oligomeric polyester, spray as would be expected for a shear viscous fluid with no elastic behavior. However, as the ratio of the traditional acrylic is increased (1:3:3 formulation) the spray pattern exhibits thicker ligaments and a greater number of larger drop sizes are evident, similar to the spray patterns seen in latex paints with moderate DUEVs. (4) The highest traditional acrylic level (1:2:4 formulation) was not sprayable from a fan nozzle. In all of our previous fan spray nozzle studies this was observed (1,2,23) only for a latex coatings with high extensional viscosities (containing high [M.sub.w] polyethylene oxide thickener). The transition in spray patterns in Fig. 5 is likely reflecting more in real extensional viscosity changes than is reflected in the traditional multiple of 3x a steady state shear viscosity of a Newtonian fluid or in the [M.sub.z] that is calculated for the blends (>60,000). With increasing deformation rates, extensional viscosities are known to increase exponentially in this molecular weight range.

The particle velocity patterns of the four formulations that provided spray patterns also follow the velocities profiles noted in latex paints with increasing DUEVs. The lower viscosity fluids (3:3:1 and 1:4:2), exit the nozzle at high velocities, dissipate into finer drops, and as discussed in the previous article (4) in this series, the fine droplet sizes decrease in velocity as they distance from the nozzle, due to the greater aerodynamic drag on their greater surface area. The higher viscosity (3:1:3 and 1:3:3) fluids exit the nozzle at lower velocities, hold together in more elastic ligaments that do not dissipate as readily, and the resulting larger particles do not notably decrease in velocity as they distance from the nozzle.

[FIGURE 13 OMITTED]

The GTP acrylics achieved commercial realization in printing inks but were not widely accepted in high solid coatings. Costs relative to traditional acrylics might be sighted as a determining factor, but perhaps not. The use of clay rheology modifiers and pigments such as alumina flakes could have negated differences seen in GTP and traditional acrylics. We leave this for others to investigate, as we leave this area of research.

References

1. Xing, LL, Structure and Rheology in the Spray Application of Complex Fluids, Ph.D. Dissertation. North Dakota State University (1998)

2. Xing, LL, Glass, JE, Fernando, RH, "Parameters Influencing the Spray Behavior of Waterborne Coatings." J. Coat. Tech., 71 (890) 37-50 (1999)

3. Elliott, PT, Steffenhagen, MJ, Glass, JE, "Spray Applications: Part III. Assessment of Viscosities at High Shear Rates and Dynamic Uniaxial Extensional Viscosities on Fan Nozzle Air-Sprayability." J. Coat. Technol. Res., 4 (4) 341-349 (2007)

4. Elliott, PT, Mahli, DM, Glass, JE, "Spray Applications: Part IV. Compositional Influences of HEUR Thickeners on the Spray and Velocity Profiles of Waterborne Latex Coatings." J. Coat. Technol. Res., 4 (4) 351-374 (2007)

5. Glass, JE, "Dynamics of Roll Spatter and Tracking Part III. Importance of Extensional Viscosities." J. Coat. Technol., 50 (641) 56 (1978)

6. Fernando, RH, Glass, JE, "Dynamic Uniaxial Extensional Viscosity (DUEV) Effects in Roll Application II. Polymer Blend Studies." J. Rheol., 32 (2) 199-213 (1988)

7. Wicks, ZW, Jones, FN, Pappas, SP, Organic Coatings: Science and Technology (2nd ed.). Wiley-Interscience, New York (1999)

8. Mercurio, A, Lewis, SN, "High Solids Emission Industrial Finishing." J. Coat. Technol., 47 (607) 37 (1975)

9. Flory, PJ, "Viscosities of Linear Polyesters. An Exact Relationship between Viscosity and Chain Length." J. Am. Chem. Soc., 62 (5) 1057 (1940)

10. Schoff, CK, "Recent Advances in the Rheology of High Solids Coatings." Prog. Org. Coat., 4 189 (1976)

11. Blank, WJ, Polym. Mater. Sci. Eng., 80, 307-308 (1999)

12. Bauer, DR, Briggs, LM, Dickie, RA, "Effect of Crosslinked Polymeric Microparticles on the Rheology of High-Solids Coatings." Ind. Eng. Chem. Prod. Res. Dev., 21 686 (1982)

13. Bauer, DR, Briggs, LM, "Rheological Models for Predicting Flow in High Solids Coatings." J. Coat. Technol., 56 (716) 87 (1984)

14. Hill, LW, Wicks, ZW Jr., "Design Considerations for High-Solids Coatings." Prog. Org. Coat., 10 (1) 55 (1982)

15. Erickson, JR, "Viscosity of Oligomer Solutions for High Solids and UV Curable Coatings." J. Coat. Technol., 48 (620) 58 (1976)

16. Slinckx, M, Henry, N, Krebs, A, Uytterhoeven, G, "High-Solids Automotive Coatings." Prog. Org. Coat., 38 (3-4) 163 (2000)

17. Antonelli, JA, "High Solids Coatings." Am. Paint Coat. J., 59 (42) 37 (1975)

18. Williams, ML, Landel, RF, Ferry, JD, "The Temperature Dependence of Relaxation Mechanisms in Amorphous Polymers and Other Glass-Forming Liquids." J. Am. Chem. Soc., 77 (14) 3701 (1955)

19. Frick, NH, "High Solids Applications, Properties and Performance." High Solids Coat., 4 (4) (1979)

20. Bird, RB, Armstrong, RC, Hassager, O (eds.) In: Dynamics of Polymeric Liquids, Fluid Mechanics. John Wiley & Sons, New York (1977)

21. Walters, K., Rheometry, Chapter 7. John Wiley and Sons, New York (1975)

22. Xing, L-L, Glass, JE, Fernando, RH, "Spray Applications of Water-Borne Coatings." In: Glass, JE (ed.) ACS Symposium Series 663: Technology for Water-Borne Coatings, Chapter 15. American Chemical Society, Washington, DC (1997)

23. Munstedt, H, "Dependence of the Elongational Behavior of Polystyrene Melts on Molecular Weight and Molecular Weight Distribution." J. Rheol., 24 (6), 847-867 (1980)

24. Kurzbeck, S, Oster, F, Munstedt, H, J. Rheol., 43 (2), 359 (1999)

25. Sugimoto, M, Masubuchi, Y, Takimoto, Koyama K, "Melt Rheology of Polypropylene Containing Small Amounts of High-Molecular Weight Chain. 2. Uniaxial and Biaxial Extensional Flow." Macromolecules, 34 (17), 6056-6063 (2001)

26. Tuck, CR, Ellis, MC, Miller, PCH, "Techniques for Measurement of Droplet Size and Velocity Distributions in Agricultural Sprays." Crop Prot., 16 (7), 619-628 (1997)

27. Lefebvre, AH, Senser, DW, Ind. Finish., 6, 16-19 (1990)

[c] FSCT and OCCA 2008

Presented at the 2005 Western Coatings Societies Symposium in Las Vegas, NV, October 2005 and the 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 3-5, in New Orleans, LA.

D. M. Mahli, M. J. Steffenhagen, J. E. Glass ([mailing address])

Coatings Plus Department, University of Lake Wobegon, 1751 S. 23rd Street, Fargo, ND 58103, USA

e-mail: eglass@fargocity.com

* Appreciation is express to Philip V. Yaneff of E.I. DuPont for suggesting the general formulation and components used in this study.
Table 1: High solid formulations used in this study

Component Melamine Polyester Acrylic
M:P:A formaldehyde oligomer oligomer Xylene
ratios (wt%) (wt%) (wt%) (wt%)

1:3:3 8.6 25.7 25.7 40
3:1:3 25.7 8.6 25.7 40
3:3:1 25.7 25.7 8.6 40
1:4:2 8.6 34.4 17.2 40
1:2:4 8.6 17.2 34.4 40
1:1:1 20 20 20 40

Molecular weight of acrylic and polyester

[M.sub.n] [M.sub.W] [M.sub.Z] [M.sub.Z+1] PDI

3397 19,911 66,383 117,900 5.86
2221 4918 9173 13,712 2.21
 657 797 1021 1333 1.21

Fig. 2: Size exclusion chromatographs of oligomers in high solid spray
study

M:P:A 3:3:1 1:4:2 3:1:3

[M.sub.w] 5245 8680 9592
[[eta].sub.e] 0.26 0.38 0.66

Fig. 4: Air-spray pattern (fan nozzle, 55 psi) of 60 wt% high solid
coatings with different weight percent ratios of MF crosslinker, ester
oligomer, and acrylic oligomers

Table 2: [M.sub.n], [M.sub.w], shear viscosity, and uniaxial extensional
viscosity for the different high solid formulations

 Shear [eta] Uniaxial
 [M.sub.n] [M.sub.w] (Pa.s at extensional
MF:polyester:acrylic blend blend 2 [s.sup.-1]) [eta] (Pa.s)

1:2:4 2099 12,067 0.293 0.879
1:3:3 1874 10,769 0.241 0.723
3:1:3 1185 9,592 0.220 0.660
1:4:2 1793 8,680 0.127 0.381
3:3:1 1124 5,245 0.088 0.264

M:P:A 3:1:3 1:3:3 1:2:4

[M.sub.w] 9592 10,769 12,067
[[eta].sub.e] 0.66 0.72 0.88

Fig. 5: Air-spray pattern (fan nozzle, 55 psi) of 60 wt% high solids
coatings with low levels of MF crosslinker, and different weight percent
ratios of polyester oligomer and acrylic oligomers
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Author:Mahli, David M.; Steffenhagen, Mark J.; Glass, J. Edward
Publication:JCT Research
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Date:Mar 1, 2008
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