TECHNICAL REVIEW
Polymer Coatings Physics and Mechanics of Leveling A. Quach j 1
1 A. QUACHis a Senior Research Chemist at PPG Industries, Coatin@ and Resins Division, Research and Development Center, Springdale, Pa. E e received his B.S. degree from Nalional Taiwan University in Taiwan (1966),
I ne appearance a n a perrormance UI ~ U L L S auu cuitunga greatly depend on flow and leveling properties. Brush marks and striations, as well as orange peel arising from brushing, rolling, or spraying applications, often give rise to unpleasant optical effects. Deviations from planarity also constitute sites of potential weakness in the coating and thereby harm its performance. Fortunately, the importance of the flow and leveling properties has long been recognized in the coatings industry and the rheological properties of paints and coatines have been subjects of considerable studies. The flowout or leveling of surface irregiilarities in coatings has been studied since the 1920's. but aavances m unaerLitanding the mechanics and the various factors governing the 1xocess have been slow. For example, it was only recently ;hat viscoelasticity was recognized as an influential factor. n~~~~~~~ ~tL>.. .I.oherty aiid €€urd, 1958; Fischer, 195Oa,b). Iloliert'y and IIurd (1958) also studied the effect of t,nlc particle size on tlie thixotropic recovery rate and observed that sinal1 particles give faster recovery rates. They also found that a high zero-shear viscosity (high level of thixotropy) prevents pailit from flowiiig too quickly into tlie pores of a substrate. Garrett, et al. (1959), showed that higher levels (e.g., 57,) of thickener in pailits generally give lower yield values than 1% t,liickeiier aiid t h a t high levels of thickener would give slower wicking rates. I h t ~ o m a c e o u ssilica paints were shown generally to have lower yield values than calcium carbonate paints. Asbeck (1961) explained thixotropy in terms of equations derived by Casson. Kreider (1964) carried out ail important, study 011 the factors coiitrolliiig latex paint rheology. The interaction bet w e i i tlie t'liickener (ii-at,er-soluble macromolecules) and the latex aiid:'or pigment part,icles was investigated. I t was found that the primary factor affecting the rheology is the standoff distance between part'icles ant1 that the conipositioii of the latcs particles has little or no effect. If tlie distance between partic*les is less than the length of a n expaiided macromoled e , a link occurs forming a netIvork which is rigid (ie., having a yield point,) but which can be broken under a minimum shear to a l l o ~flow. The time interval for re-formation of the network after cessation of shear was thought to deterriiiiie the degree of paint flow and leveling. A method em-
(A)
GOOD
LEVELLING
(B)
BAD
LEVELLING
Figure 3. Models of good and b a d leveling paints
ployiiig a Brookfield viscometer was described to study the rate of re-formatioil and the yield point. Times varying fro? 20 see for a 1800-A latex to more than 180 sec for a 7000-A latex were found. Siiice only relatively mild shear was used by Kreider (15 see in a Osterizer a t low spceti), it is coilceivable that, longer t,iInes would be observed should the paints be subjected to higher shear rates comparable t o brushing or rolling. Kreider suggested that the standoff tlistaiice be controlled by choice of particle size of pigments and by adjust,ing part'icle size of the latex aiid the per cent film-forming solids by volume in the paint. Garrett (1960) drew similar conclusions, stat'iiig that good leveling lies in the direction of low solids as well as high thickener level. Effects of wettiiig and dispe iiig agents and fine particle size extenders, as \vel1 as bacteri des anti fungicides, were also subjects of study by Iireider. Kreicler (1969) Iat,er investigated the displacement of hydrophilic colloitls from particle interfaces through the use of specific surfactants in an effort to reduce the bridging among particles and thickeners. Noriionic surfactants appear to be ineffective while cert,aiii anionic types have a marked effect on reduction of polymerparticle iiiteract,ioas. This fact has a bearing 011 pigment dispersiori as well. Khaniia (1969) follonecl the work of Kreider (1964) and proposed a "dynamic network" model which is very similar to t'hat of Kreider (1964). The model is represented in Figure 3. The filled aiid open circles represent the latex and pigment particles. The water-soluble niacromolccules are symbolized by the zigzag lilies. I n model h,tlie standoff distance is large and there is little bridging of part'icles and polymers. This represents a system of low-yield point and long thixot,ropic recovery time and should give good leveling qualities. In model 13, however, the particles are flocculated. The small staridoff distance between part'icles enhances the number of polymer-particle linkages, causing a high-yield value and poor leveling properties. Khaiiiia also reported that systems showing increasing iiormal stress as shear rate increases have better leveling properties. The measurement of normal stress difference gives an indication of t'lie elasticity of the system concerned. The observation is in line with model A since its network structure should be able to store more elast'ic energy than model B. €€e also suggested that the excellent sag resistance of a paint having a good leveling can be due to the pronounced viscoelastic behavior of the paint a t low shear rates rather than to t,he thixotropy. Therefore, he questioned the concept of thixotropy as a necessary factor to compromise bet'ween sagging and leveling. Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 2, 1973
1 15
leveling of Powder Coatings
This review would not be complete without mentioning bhe leveling of powder coatings. However, little work has been published. Van Oene (1972) investigated the rheology of sintering, spreading, and leveling of powder, and Orchard’s theory of leveling has been used and modified for studies of these three processes. Wolpert and Wojtkowiak (1972) studied the leveling of a n acrylic thermoplastic pigmented powder and concluded that Orchard’s equat’ion describes the data to a t least the correct order of magnitude. However, the viscoelastic effects have been neglected. It was mentioned earlier that the time constant(s) of a viscoelastic fluid always has (have) a negative effect on the rate of leveling (Biermann, 1968). This might have caused the discrepancy observed by Wolpert and Wojtkowiak. It appears, therefore, that the modified Orchard equation derived by Biermann should be a good starting point for investigating the leveling of powder coatings. Acknowledgment
The author wishes to thank Drs. P. E. Pierce and C. M. Hansen for their suggestions and discussions.
Garrett, B. S.,Off.Dig., Fed. SOC.Paint Technol., 32, No. 424, m n iiw,n) Garrett, B. S.,Prentiss, W. S., Scott, J. D., Off. Dig., Fed. Paint Varn. Prod. Clubs,31, N o . 409,213 (1959). Grimshaw, F. P., Pateman, R. A. W., J . Oil. Colour Chem. Ass,, 43.34 (19601. Hansen, C , M:, Off. Dig., Fed. SOC.Paint Technol., 37, 57 (1965). Hansen, C. M., J . Paint Technol., 44, 61 (1972a). Hansen, C. M.,Znd. Eng. Chem., Prod. Res. Develop., 11, 426 (1972h) ,- - .- ,. Hansen, C. M., Pierce, P. E., ibid., 12, 67 (1973). Haslam, G. S., Grady, L. D., Jr., Anal. Chem., 2, 66 (1930). Hoagland, S., “The Rheology of Surface Coatings,” R-B-H Dispersions, Inc., N. J., 1946. Hess, &I.,et al., “Paint Film Defects,” 2nd ed, Reinhold, New York, N. Y., 1966. Jarrett, M.E. D., J . Oil Colour Chem. Ass., 31, 357 (1948). Khanna, R. K., Paint Technol., 33, No. 1, 23 (1969). Kreider, It. W., Of. Dig., Fed. SOC.Paint Technol., 36, 1244 (1964). Kreider. R. W.. J . Polzlm. Sci.. Part C. No. 27.,~ 275- il96Q) \-__I,_ Liberti,’F., J . Paint Technol., 43, No. ’553, 72 (1971). hlchlillen, E. L., Ind. Eng. Chem., 23,676 (1931). Rlill, C. C., J . 0 2 1 Colour Chem. Ass., 50, 396 (1967). Mill, C. C., South, G. R.. J . Fluid Mech.. 28. Part 3. 523 11967). ~, Orchard, S.’E., Appl. Sci.’Res., Sect. A , 11, 451 (1962). Patton, T. C., “Paint Flow and Pigment Dispersion,” Chapter 5, Interscience. New York. N . Y.. 1964. Patton, T. C., J . Paint Tichnol., 38; No. 502, 656 (1966). Pearson, J. R. A,, J . Fluzd Mech., 7,481 (1960). Pierce, P. E., Donegan, V. A., J . Paznt Technol., 38, No. 492, 1 -
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