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answered by this research, but glass transition temperatures at water contents extrapolated to zero )3:(" suggest an ionic cross-linking mechanism. A steady upward progression of T: from 102 "C at a = 0 to 251 "C at a = 1 was noted by Eisenberg et al. (19711, with a monotonic increase with a. Since plasticization by water cannot possibly be a feature at zero water content, the basic structure of the film-former is made more rigid by neutralization. This practical benefit competes successfully with the increased water sensitivity only at low a. Conclusions 1. The amount of water evaporated from gelled solutions of polyacrylic acid is retarded in direct proportion to the extent of neutralization by alkali. The rate of evaporation is not. 2. Neutralization beyond the halfway point results in the retention of secondary waters of hydration by the electrostatically expanded polymer coils. 3. Secondary water is retained in an immobilized form, rheologically indistinguishable from primary water. 4. Molecular entanglements leading to gelation and the retention of water on drying are enhanced by increased degree of neutralization.
5 . Plasticization by entrained water lowers the rigidity of the film. A competing electrostatic effect of neutralization overrides at low a. Literature Cited Ferry, J. D., "Viscoelastic Properties of Polymers", 2nd ed, p 397, Wiley, New York, 1970. Eisenberg, A,, Matsura, H., Yokoyama, T., J . Polym. Sci., Part A-2, 9, 2131 (1971). Hughes, Fordyce, J . Polyrn. Sci., 22, 509 (1956). Leyte, J. C., Mandel M., J . Polym. Sci., A2, 1879 (1964). McSkimln, H. J., Heiss, J. H., Phys. Rev., 75, 936 Mason. W. P., Baker, W. 0.. (1949). Mathieson, A. R., McLaren, J. V., J . Polym. Sci., A3, 2555 (1965). Myers, R. R.. J . Polym. Sci. Parf C, 35, 3-21 (1971). Myers, R. R., Klimek, J., Knauss, C. J.. J. Coat. Techno/.,38 (500), 479 (1966). Myers, R. R., Knauss, C. J., J.,Coat. Techno/.,40 (523), 315 (1968). Myers, R. R.. Knauss, C. J., Film Formation from Polymer Colloids", In "Polymer Colloids",p 173, R. M.FRch, Ed., Plenum Press, New York, 1971. Myers, R. R., Schuitz, R. K., Off. Db. Fed. SOC.Paint Techno/.,34, 801 (1962). Myers, R. R., Schultz, R. K., J . Appl. Polym. Sci., 8, 755 (1964). Nagasawa, M., Hoitzer, A,, J . Am. Chern. SOC.,88, 538 (1964). Sakamoto, R., Yoshioka, K., Nippon Kagaku Zasshi, 83, 517 (1962). Wada, A,, Mol. Phys., 3, 409 (1960).
Received for review January 9, 1980 Accepted April 7, 1980 Presented at the 53rd Colloid and Surface Science Symposium, Rolla, Mo., June 1979. This research was supported by the Paint Research Institute.
111. Symposium on Mechanisms of Film Formation from Powders, Melts, and Solutions J. H. Lupinski, Chairman 179th National Meeting of the American Chemical Society, Houston, Texas, March 1980
Capillary-Fed Meniscus Coating Technique Carlyle S. Herrick General Electric Company, Corporate Research and Development, Schenectady, New York 1230 1
A new method can produce uniform continuous coatings without requiring precision equipment. In the laboratory uniform coatings can b e obtained from small volumes of coating solutions containing highly volatile solvents.
Introduction The practical use of low viscosity polymeric solutions to produce continuous coatings on moving belts, webs, or casting surfaces is usually accomplished with a roll coater or a doctor blade coater. Both techniques require precision machinery. In many cases substantial amounts of coating material must be expended before steady-state operation is attained accompanied by uniform coating and the desired coating thickness. In the laboratory the need occasionally arises to test the coating properties of new experimental materials which are available in quantities too small to permit steady-state operation with a roll coater or a doctor blade coater. In addition, high solvent volatility may render coating control difficult. 0196-4321/80/1219-0314$01 .OO/O
A New Coater A new technique called capillary-fed meniscus coating has been developed in this laboratory to meet this need. Figure 1 illustrates the principle of operation. A hollow coating head contains a reservoir of coating solution connected to a free meniscus by a series of capillary channels. These channels are angled so that the inlet ends are submerged below solution level while the outlet ends are elevated above solution level. Solution rises by capillary action to the elevated ends of the capillary tubes where a meniscus is produced by momentary contact with the tape to be coated. As solution is carried away from the meniscus by motion of the tape, it is replenished by the exact same amount of solution rising through the capillary 0 1980
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"''7 LIQUID
MENISCUS
I
COATING SOLUTION
Figure 1. In capillary-fed meniscus coating a reservoir of coating solution is interconnected with a liquid meniscus by a series of capillarysized holes.
Figure 3. Interference patterns indicate the flat central area of B thin polymer coating 10 mm wide.
Figure 2. Interference patterns indicate uniform thickness of a polymer coating.
tubes from the reservoir in the coating head. Exact replenishment means that the inventory of solution in the meniscus is constant. This automatically constant meniscus inventory plus the closed solution reservoir are unique to this coating technique and are responsible for its desirable features. Performance With this technique, coating thickness depends only on solution parameters, tape speed, and coating head angle. It is independent of the thickness of the meniscus for thicknesses typically between 50 and 500 pm. Uniform coating thickness can he obtained therefore without the need for precise mechanical specifications or adjustments. Flow resistance in the capillaries usually limits coating speeds to less than 1m/s. Greater capillary flows may he obtained by pressurizing the reservoir by the desired amount. Coatings may he done in a hatch mode hy closing both ends of the reservoir in Figure 1,or they may he continuous hy arranging for the reservoir to be replenished as needed. Under laboratory conditions the capillary meniscus coater produces coatings of superior flatness and uniformity. Figures 2 and 3 are examples of small lot coatings made by this technique. Figure 2 shows the interference pattern generated hy a polymer coating approximately 7 pm thick when viewed under sodium light. Neighhoring fringes of corresponding kind may represent the same thickness or a thickness difference of pm. The subject coating is the central 16-mm portion of a coating that was initially applied to he about 26 mm wide on a 35 mm wide helt. The perforations were created later. Clearly a very thin and very flat coating was obtained, Figure 3 shows the interference pattern for a second polymer coating 7 pm thick applied between the perforations on a piece of 16-mm film base. Corresponding kinds of fringes have the same significance as before in Figure
Figure 4. A small coating head made from 3.8 cm Teflon rod.
Figure 5. A mall coating head for unattended or remote operation.
2. The coating has very thick edges and a discontinuity on each side of a flat central portion. A very flat uniform coating was obtained just 5 mm in from the edge of the meniscus. Coating Heads Figure 4 shows a coater head similar to that used to produce the coating of Figure 3. It was fashioned from a
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Res. De".
Figure 6. This large meniscus coating apparatus is 38.1 em wide.
solid block of TFE Teflon. The meniscus contact area which is 17 mm wide has been artificially blackened to outline the exit ends of the capillaries. Figure 5 shows a second Teflon coating head with the meniscus contact area 25 mm long and blackened as before. It is fitted atop a glass jar solution reservoir and is surmounted by a 3-rpm motor driving a chain pump to maintain a constant solution level in the coater head. This was the most important component in a tape coating system designed to operate a t elevated temperature in a
1980, 19. 316-319
laboratory vacuum oven without any manual manipulations or adjustments. Figure 6 shows a capillary fed meniscus coating head on a 38.1 cm (15 in.) wide film casting machine built in the laboratory. The head is rotated about 90" up from its normal operating position to permit this photograph. It is made of normal 2-in. IPS pipe with a flat surface milled on one side completely through the pipe wall. A bar of Teflon containing the capillaries is fastened to that opening by a series of spring-loaded clamps. An inflow of solution to the coating head reservoir takes place at the center of concentric channels a t either end of the pipe. The excess solution overflows into the 3/rin. IPS end pipes and then returns to storage. The capillaries were 0.76 mm (30 mil) in diameter and equispaced at 5.9/cm (15/in.). Conclusion A new coating technique has been demonstrated in which the coating thickness is substantially independent of the liquid meniscus thickness. Flat and uniform coatings are therefore possible without high precision coating machinery. A totally enclosed coating solution reservoir also contributes to high-quality coatings by eliminating solvent losses. Reproducible coatings can be made with small amounts of coating solution. Received for review April 14, 1980 Accepted June 10,1980 Presented at 179th National Meeting of the American Chemical Society, Houston,TX, Mar 25,1980,Division of Organic Coatings and Plastics Chemistry.
Polydimethylsiloxane-Poly(alky1ene oxide) Block Copolymers as Flow-Out Additives for Epoxy Resin Powder Coatings Michael P. Hill Dow Corning Ltd., Bany, United Kingdom
Michael J. Owen* Dow Corning Corporation, Midland, Michigan 48640
Block cop0 ymers of polydimethylsiloxaneand poly(ethyleneoxide) are effective in promoting the flow-out during men'ng of epoxy resin powder coatings. The effect of copolymer compos:tion was investigated Lsing a liquid epoxy resin on an aluminum suostrate. Two properties were studied as a function of time, the surface tension of the resin and the contact angle with the substrate. A direct correlaton is obtained between mese two properties implying that the lowering of the surface tension is the mechanism whereby the flow-out improvement is effected. The 50-60% pa ysiloxane content copolymers have the desired oalance of surfaceactivity and soluoility that gives the best performance. Homopolymeric polydimethylsiloxaneis incompatiole and inhibits flow-out. The inclusion of titania f ller markedly compiicates the behavior of the addt've and indicates specific interaction between copolymer and filler.
Introduction These studies were prompted by the need for additives to improve the flow-out on melting of powder coatings. The principal powder coatings used are epoxy resins. Preliminary studies with one such material, Shell DX55, revealed that polydimethylsiloxane (PDMS)-poly(ethy1ene
oxide) (PEO) block copolymers were effective in this application. A small piece of Shell DX55 containing the additive was placed in an oven a t 195 " C for 10 min and the contact angle (0) so produced was measured a t room temperature. Significant contact angle lowerings were produced by certain of the PDMS-PEO block copolymers.
0 1980 American Chemical Society 0196-4321/80/1219-0316$01.00/0
