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Film coating technologies and adhesion
Hectrochimica km, Vol. 37, No. 9, pp. 1471~1478. 1992 Printed in Great Britain.
FILM COATING
0013-4686/92 SS.00 + 0.00 @J 1992. Pqamon Prts Ltd.
TECHNOLOGIES
AND ADHESION
GORDON P. BIERWAGEN North Dakota State University, Department of Polymers and Coatings, Fargo, ND 58105, U.S.A. (Received 20 December 1991) Abstract-Solid Polymer Electrolyte (SPE) films must be formed in a reproducible, controlled manner in order to construct devices which take advantage of their unique properties. Film coating processes that yield reproducible thin films will be reviewed vis-ri-vis the device requirements (eg thin film lithium batteries) of SPE films. Solution characteristics important for film coating processes will he presented and process control parameters outlined. Film defects and their control in processing are major problems which must be addressed in the choice of materials and coating process for thin film applications. Proper processing of SPE films will be a key to their utilization, and the criteria for correct choice of a coating process for this type of film will be examined. A further key to the proper utilization of SPE film will be their wetting and adhesion to device substrates, ie lithium, in the case of thin film batteries. Poly(ethylene oxide) SPE films may have advantages in this respect if their adhesion behavior is analogous to that of PEO-based polymers used in the protective organic coatings. Key words: polymer films, batteries, coating, processes.
INTRODUCTION Solid Polymer Electrolyte (SPE) films are receiving considerable attention as possible solid electrolytes in
lightweight, high power density batteries, especially those based on Li films[l-4]. To use these materials as battery electrolytes at ionic conductivities that are practical, requires that the SPE films be quite thin, and have exellent adhesion to the Li films. Also, there are the further practical requirements that the film thicknesses of both the Li and SPE are constant over the contact surface areas in the battery, have no defects such as film thickness fluctuations, holes, insoluble particles of contaminants, have uniform properties across the film, etc. Also, the chemical nature of the film must be constant across and through the film. For reasons of economics and function, the film must be capable of being cast at high speed and with close control of film properties[5]. This work will review studies in film coating science and technology pertinent to the creation and control of SPE films appropriate for battery use and suggest possible film coating methods that would be appropriate to SPE film formation. The science and technology of film coating processes has recently been undergoing an influx of attention from the engineering community, and the basics of designing the film precursor liquid and dry film characters are quite well established for most organic coatings[6]. These studies should be considered in order to properly take advantage of all of the inherent properties of SPE films for battery use. Also, the issues of polymer film adhesion to metal substrates have received considerable attention by workers in both the areas of organic coatings and adhesives[7, 81, and again those attempting to utilize SPE films over metals would be well served by considering the literature of these areas. The purpose of this paper is to review these areas with respect to requirements of SPE films in batteries and similar
devices, including the composite matrix cathodes based on SPEs[9], and make suggestions based on this review as to the design casting of SPE films and SPE-based matrix cathodes. The relationship of battery SPE film technology the organic coatings science is quite extensive, and the matrix insertion cathodes used in Li-SPE batteries are quite close in nature to pigmented organic coatings (ie paints). Many organic coatings are cast from liquid, as are most SPE films; most organic coatings are quite sensitive to variations in film thickness and quality, as are most SPE films; and many of the properties of organic coatings films are determined by the polymer portion of the film, as are most SPE films. It thus seems quite reasonable to consider carefully the literature on organic coatings design and coatings application processes as a guide to further studies and development of Li-SPE film batteries, and to the utilization of SPE films of all types and uses. The unique uses and requirements of SPEs as compared to most organic coatings require or yield specific properties usually not considered by coating scientists. Among these are the electrochemical transport properties, cycling lifetime and cycling behavior, environmental sensitivity, absolute long term stability, and chemical purity[lO]. An SPE film will be required to provide ionic conductivity that is stable to frequent and extensive recycling, inertness with respect to its substrate and sustainment of mechanical and thermal properties over periods of extended use[ 111. BASIC CONCEPTS OF FILM APPLICATION/FORMATION (a) Film casting/coating as a process
Large scale formation of a thin film on a substrate is an important industrial process that serves many
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G. P. BIERWAGEN
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Before Coating GAS
I
SUBSTRATE
1. Before coating process
During this step, most properties
2. Coating fluid displacing
deposited on substrate
4. Film becomes solid
Fig. 1. Schematic of coating process.
areas of technology[6]. In some cases, multiple films are cast sequentially to yield a multi-layer film of increased functionality and performance. Inherent in many continuous film casting operations are drying or curing steps which are required to change the liquid cast film in to a dry, solid film[l2]. Some processes such as vacuum deposition, plasma polymerization, extrusion, and variations, thereof yield dry films that need no further “curing”, but many continuous film application processes require drying and even extra energy to crosslink the applied film[6]. A schematic diagram of the coating process for the application of a liquid film to a solid substrate is shown in Fig. 1. The coating process is often done at high speed and with relatively good control of the film thickness and quality. However, there are many problems that can occur in high-speed continuous coating processes, and they are often very capital intensive processes. Many of the important parameters of the coating process are the properties of the liquid solution/ dispersion from which the film is cast. The user is often interested in the dry film properties of the solid film, but must also design and control the wet film properties very carefully to control the dry film quality. An introduction to coating as an engineering process is given in the September 1990 issue of Chemical Engineering Progress. There are quite a few specific coating processes, each designed and used for a particular dry film end utilization. A recent review[l3] of coating process from the view point of the paint/organic coatings scientist considers many of these individual processes. These and other continuous film-forming processes will be considered below with respect to their potential utility in the formation of useable SPE films for battery use. Comments will be made concerning their individual technical and economic merits, as well as the strengths and weaknesses of the individual processes. Generally, one can say that the faster the speed of the process and the wider the web or sheet to be coated, the greater the control and reproducibility problems. Table 1 gives a list of the coating processes that will be considered below.
(b) Carrier web issues
An important fact that must be considered along with these coating processes is the need for a carrier web in many of these coatings processes, especially the continuous, high speed processes. In the case of SPE electrolyte films, the film forming would ideally be done onto a carrier web of Li film, but this metal film would have insufficient tensile strength to allow its use in a continuous web process. Many film coating processes use carrier webs that become part of the final product, such as the polyester film that is used as the carrier for magnetic tapes or for photographic films. The carrier web may be later removed and the film transferred to another substrate such as in heat/pressure transfer films that are used for decorating plastics and fibre board[l4]. Paper and other polymers such as polyethylene or polypropylene are also used as carrier webs for film coating
Table 1, Characteristics of coating processes 1. Thickness of film h 2. Bulk viscosity TV -shear -extensional 3. Surface tension 0 4. Temperature T 5. Concentrations c -solvent -surfactants 6. Relative velocities v 7. Surface lifetime 5 8. Diffusion coefficients D 9. Edge radii of substrate r 10. da/dT Il. 12. 13. 14. 15. 16. 17. 18. 19. 20.
du/dc dT/dr Contact angle 0 Surface modulus da/dlnA Rate of surface area (A) formation d lnA/dt Surface viscosities r Radius of curvature at surface R Density p Surface area A Characteristic length I
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Film coating technologies and adhesion processes. The carrier web may be cast in situ by an extrusion process[ 151. The carrier web for a coating process must be stable to the chemicals of the liquid film, the temperatures of application and drying/curing, and the mechanical stresses of the film coating process. Its dimensions and properties must remain constant during all stages of the coating process. Further, the cost of the carrier web often is a significant part of the cost of the final film. The carrier web must be chosen carefully, for it may determine many parameters of the coating process. (c) Film drying and curing Another issue that must be treated in considering high-speed, continuous film coating processes is the drying/curing step in which the liquid film is converted to the solid film which provides the properties of interest. This step involves the removal of the solvent/carrier liquid from the film and/or the crosslinking/coalescence of the film polymer[l6]. This step must occur, in most cases, before the film is ready for use or for the application of the next film in a multi-film coating process[l2]. Often, the solvent or carrier liquid is an organic liquid that must be collected or incinerated after removal by the drying process, or the curing process generates a by-product vapor that must also be collected. The solvents are often hazardous or polluting materials that must be dealt with very carefully, making this stage of the coating process a major part of the process cost. (d) Properties properties
of the liquid coating vs. dry film
The important properties of the film during the coating process are those of the liquid state of the film forming material and the properties of the substrate, and thus involve many properties often not considered in the development of solid film properties. Table 2 lists many of these properties which must be controlled in the liquid film application process. This list is not all inclusive, but lists most of the important parameters which are important in the liquid film coating process.
The solution properties of the liquid film are very important for the coating process, especially the surface and rheological properties of the coating fluid. If this fluid is a gas-in-liquid (foam), a liquid-inliquid (emulsion), or solid-in-liquid (colloidal) dispersion, specific properties of the dispersion system such as dispersed particle (bubble, globule) size distribution, dispersed phase volume concentration, dispersion (foam, emulsion) stability and concentration of stabilizing material (surfactant, dispersant, emulsifier). Further, these properties may be time dependent (surface tension, viscosity, surface concentration) or dependent on the history of the fluid (shear dependence, non-Newtonian viscosity, viscoelastic behavior)[ 13, 11.
FILM FORMING PROCESSES POTENTIALLY APPLICABLE TO CASTING SPE FILMS Below are described many of the currently extant methods of film coating operations along with a discussion of their individual potential applicability to SPE and SPE matrix cathode films. The target thickness used for an SPE film for lithium battery use is ca 10-50 pm[l8-201, with target thicknesses of lO-220pm for the composite matrix cathodes [18]. The Li film thickness for this use is estimated to he 25-75 pm. The desired accuracy of the film forming process for battery use is + 5% of the desired dimension[ 191. Some speculations have been made conceming appropriate film coating techniques for Li/SPE batteries[l8], but there has heen no comprehensive survey of film coating processes vis-ti-vis the film
requirements of this technology. Many of the more common film coating processes are summarized in Table 2. In all of the coating processes discussed below, proper wetting of the substrate is crucial to the control and success of the process, and requires careful control of the substrate surface cleanliness, etc. (a) Roll film coating processes Roll film coating processes are widely used industrially, ranging from the application of paint to metal
Table 2. Important coating liquid properties for coating processes Use/products
Coating process
Advantages
Disadvantages Problems in transfer from engraved roll Flow instabilities
Adhesive films, printing decorative films
Precision and spatial control, patterning
Adhesive films, coil coatings, etc.
Speed, precise metering, gap control
Dip coating
Rods, sheets, roll goods
Speed and simplicity
Film flow problems
Bead coating
Web fed roll stock Flat rigid and roll stock Photographic films, etc. General use Plastic films Electronic parts
Film thickness control Speed and control
Curtain breaks possible
Transfer coatings, and plastic films
Solvent free, high speed
Gravure coating-all Roll coating-all
types
types
Curtain coating Slide coating Spray coating Extrusion coating Spin coating Lamination coating and calendering
Multilayer, thin films Ease of use, simplicity Little solvent use Excellent tilm control
Flow instabilities
Flow instabilities Poor film control, batch High shear, temperature Batch only Thermoplastics only
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or vinyl siding, to the application of scratch resistant coatings to plastics, and to the application of coatings to polyester film to make magnetic tapes. They are processes that have been studied quite widely[21-241 and their strengths and weaknesses are given in Table 1. There are several configurations possible under the general heading of roll coating processes: forward, reverse, deformable rolls (squeeze coating), and metered with a doctor blade. These are discussed quite extensively in the studies of Coyle et aZ.[21,25]. Roll coating processes are almost exclusively used with a continuous web of rapidly moving substrate, and thus require a web of considerable strength and toughness, as well as temperature stability if there are drying and curing steps subsequent to the film coating process step. Roll coating processes can be used to accurately apply dry films of 10-25 pm at high speed and therefore must be considered as a candidate process for the casting of SPE films if a proper carrier web can be chosen within the economic constraints of the potential SPE film use. Webs of up to 60 in (1.524 m) in width can be coated in this manner, with speeds of up to 1100 ft S-I (335.4 ms-‘) for paper webs and 500 ft SK’ (152.4 ms-’ ) for forward roll coating. Thickness control is perhaps best in reverse roll coating processes with a metering roll and a doctor blade. Roll film coating processes can be used with a wide range of liquid properties, including rheological and surface properties. Multiple layer films can be created easily by inserting sets of coating and drying stations in series along the web handling line. The fluid flows in roll coating processes can be quite complex, and there are flow instabilities that can occur during the film application that lead to film imperfections. Among these are ribbing[21], film breaks due to wetting problems, “cascade” instability, and air entrainment with Newtonian fluids[25], while with non-Newtonian fluids, mottling and “stranding” of fluid filaments between roller and web can occur. Other imperfections, such as foaming due to air entrainment problems in the reservoir, problems in the wetting of the moving web, or cratering due to surface active impurities[ 131,can occur in this, as well as many other film coating operations[26,27]. Further, film defects can also occur during the drying/curing step for all coating processes[ 12,281. (b) Gravure coating processes
Gravure coating processes are another class of continuous web coating processes that are very similar to roll coating operations. The unique feature of gravure coating processes is that the transfer roll is an engraved cylinder which is wiped by a doctor blade so that liquid transfers to the web in the pattern of the engraved (recessed) areas of the transfer cylinder, and, thus, this method is used mainly for high speed printing[29]. If close geometric patterns of diamonds, dots, etc, are engraved on the transfer roller, the process can be used with liquids of sufficiently low viscosity to create thin continuous films due to reflow from the patterned film transferred from the engraved rol1[30]. Gravure is used in this manner to put on thin continuous films of adhesive, protective coatings, etc. Multiple station gravure printing can be used for multi-color printing or multiple layer films, and, in
combination with roll coating stations, for special multiple layer films. Gravure coating processes are of very specific interest to the printing and ink industries, but are used widely outside of these for creating well controlled thin films. Gravure coating processes have many of the same advantages that roll coating processes, plus it can be used to deliver a controlled pattern of materials, if so desired, to a moving web. They are high speed, continuous, and controllable processes, and have been used extensively for delivery of thin organic films and there are suppliers of this type of equipment available for consultation on proper equipment design. Prior to considerations of equipment, the coating fluids will have to be characterized in the manner discussed above with respect to their flow properties, etc. Gravure line speeds can be as high as 900 m min-’ for paper printing operations, and are ca 110 mmin-’ for adhesive tapes, etc, and line widths can be up to 2 m. This class of coating processes is not as well studied on a fundamental basis as roll coating, and therefore has a greater percentage of empiricism and/or trade secret process methods within its technology. Besides many of the defect problems of roll coating processes, the transfer of the coating fluid from the engraved cells on the transfer roll is somewhat of an ill understood process and the cause of many defects characteristic of gravure coating[25]. (c) Curtain coating processes Curtain coating processes are those in which a freely falling sheet of coating fluid is directed to a moving web or to rigid objects on a moving open conveyor belt[31]. This may be a “short curtain” falling from an inclined plane fed by a coating slot die[32, 331. This form of the process is closely related to slide coating, and with a multiple slot die configuration and with proper control of flows and viscosities, this process can be used to put several layers of coatings at one time on a moving web. This method of coating is often used in the manufacture of photographic films and paperboard[34]. Dry film thicknesses can be controlled, depending on the volume fraction solids of the coating fluid, from 20 to 100 pm in the short curtain process, and from 40 to 200 pm in the “long” curtain process. Line speeds can be as high as 600 m min-’ in photographic film processes. Curtain coating processes, especially the short curtain process, will allow high rate, controlled film thickness coating or moving webs, and curtain coating stations can be put in series with drying/curing ovens for multiple layer coating operations. They are reasonably well studied theoretically, and much of the success of an operation based on this method will depend on proper design of the die through which the coating fluid is pumped from the reservoir into the coating head. There are a broad range of fluids that can be used in these processes, and it can be used for narrow or wide webs. The long curtain method will be suitable for thicker polymer based films needed in battery manufacture. Curtain coating operations are susceptible to many of the defects discussed above for roll and gravure operations (not ribbing, however) and have problems specific to their freely falling film of liquid, that of
Film coating technologies and adhesion “curtain breaks”[31] due to flow instabilities, surface active impurities[l3], or bubbles in the fluid. Again, defects associated with high speed drying/curing steps also may occur. (d) Slide coating processes
Slide coating is a process used in photographic film coating and related processes, and is quite similar to curtain coating process discussed the “short” above[35]. The coating flow process is flow of the coating fluid through a slot orifice down an incline “slide” to direct contact with the moving web at the end of the “slide”, with no free fall zone as in the short curtain process. This process has many of the same features in relation to line speed and defects discussed immediately above with curtain coating. (e) Dip coating processes One of the oldest coating processes is the dip coating process, a self-metering coating process[3 l] that uses a moving web continuously withdrawn from a reservoir of the coating fluid. It is commonly used in web wire, rod and fiber coating and has been extensively studied[32, 34,361. The method has no external devices supplemental to the moving web and the coating reservoir, relaying on control of web speed and coating fluid properties to control the film thickness deposited on the web. For a moving web it is a two-sided coating process that is dependent on proper wetting of the web for successful operating. The withdrawal from the reservoir may be vertical or horizontal[37], and the coating may have a wide range of properties. A stripping gas jet, known as an “air knife”, may be used as a film thickness control method for obtaining thinner films than at steady state under the influence of gravity or the surface tension and other properties of the coating fluid[38]. For further discussion of the strengths and weaknesses of this process, the reader is referred to the literature cited above. The process would seem to only apply to symmetrical simultaneous deposition of films on a series battery construct, somewhat limiting its use. (f ) Spray coating processes Spray coating processes are a common batch coating process for individual items such as automobiles and airplanes, or for continuously cast amorphous metal strips (spray metal casting) and for patterning as in ink jet printing[39]. The coating liquid is pumped through an orifice and broken up into droplets by pressure (airless spray painting) or by a coincident flow of air (air-assisted spray)[40], by an electric field from the nozzle to the workpiece (electrostatic spray), broken up by a vibrating reed at an orifice, broken up by centrifugal force and electrostatic forces on a rotating horizontal disk or a vertically rotating be11[41], or driven through an orifice by a propellant from an aerosol spray coating can. Spray coating process range from fairly unsophisticated hand held sprayers to technically sophisticated ink jet printers and coaters. At high rates of continuous substrate motion and for very close thickness control, as a class, spray coating operations may not be the most desirable, especially with the hazardous solvents required for PEO systems. For
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batch operations, they may prove useful. Most spray coating processes also require very low viscosity fluids. (g) Spin coating processes Spin coating is another batch coating process that is very useful for applying thin, uniform layers, extensively used in the electronics industry for polymer resist coatings, etc[42,43]. A fixed amount of coating fluid is dispensed onto the center of a round substrate, and the disk is rotationally accelerated to a preset speed, centrifugally moving and thinning the fluid to the desired thickness over the surface of the disk. It is a well analysed method capable of providing very close control of film thickness, but a batch process not well suited to large production rates. This process is well suited to the preparation of films of very closely controlled film thickness for lab studies and testing of various batches of polymer, but not large scale production of SPE films for battery use. (h) Extrusion coating processes
Polymer extrusion is a film forming process[l5] that is pertinent to SPE film formation, and one that forms films from thermoplastic solid polymers, avoiding the need for dissolving or dispersing the polymer in a solvent. It thus is a process that requires no drying/curing ovens and is thus low in pollution. The process is widely used for forming films of polymers for various uses, and has the requirement that the polymer be thermoplastic, stable to the heat and mechanical stresses of the extruder, and melt processable. Films of 25 km are frequently cast in this manner, and extrusion processing lends itself to multi-layer film forming processes(4.41. Extrusion is also used as a wire coating process[45], and multilayer coatings may be cast by the extrusion process as well. This would seem to be a very attractive process for SPE or polymer matrix cathode films if the solid polymer (eg the PEO/LiClO, ionic conducting SPE commonly considered for battery use) is stable to the processing conditions found in extruders, and the flow temperature is low enough for the use of this process. (i) Bead coating processes
Beat coating is the liquid version of extrusion coating in which the liquid is pumped through a slot to form a liquid bead that directly contacts a moving substrate web. It has been discussed somewhat in the literature (see, for example[35], and citations therein), but there has not been much description of its engineering parameters such as line speeds and web widths. (j ) Lamination /calendering Other related process that may be useful for multilayer SPE film formation are lamination and calendering processes[ 151, and film transfer processes[ 141. In these types of processes, the film is formed by another process, and then joined to another film or carrier web under pressure of heated rolls (lamination) or reduced in thickness and smoothed by pressure of heated roll (calendering)[ 151. Hypothetical uses of these process are illustrated in Figs 2 and 3, below, illustrating how multilayer films could be
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G. P. BIERWAGEN
I
I Film Casting Process Schematic Drawing
I
Lithium Film
_&Le \
Lithium Film
1
Step @
11 : 1 Heat +
IRoll. Gravure1
Process @ Step
I
Polyester Carrier Web Film
Fig. 2. Schematic drawing of possible film casting process. prepared from single layer processes plus lamination steps.
GENERAL COMMENTS ON FILM COATING PROCESSING The viability of Li battery designs based on SPE and SPE matrix cathodes may well depend on the availability of economical process methods to form coating films of these materials in well controlled, high rate processes. Choosing a process is one step, but control measurements must also be available for the process to ensure the proper thicknesses and electrochemical properties of the films. This will require some development work on measurement techniques, preferably on-line, for the characterization of film quality. As stated above, work will be necessary to characterize the liquid coating parameters appropriate to coating process control, as well as the proper choice of process steps to yield the solutions and dispersions used as process fluids. There is a good deal of literature on the preparation of dispersions[ 15,401 of the type necessary for the SPE matrix cathode film forming, and the general literature on organic coatings addresses the problems and methods of dealing with pigmented polymer systems (Ref. [40] is a good introductory source for the literature in this area). Care will have to be taken to account for the effects of dispersed phase volume effects in the poloymer matrix cathodes (PMCs), as they have been noted in pigmented coatings and other filled polymer systems[46,47]. The unique added effect in the PMCs is the Li ion injection swelling of the dispersed phase, which will have to be accounted for in the battery design.
Another area commented on above, but worth considering further, is the defect problems likely to be seen in film coating process for liquid polymer solutions and dispersions[l3]. Special care is required to avoid these problems in film coating processes. Further there may be polymer alignment effects at film surfaces during film casting processes.
FILM ADHESION Proper adhesion of the SPE film to Li and to and the PMC will be critical to the viability of the battery designs currently under consideration[2]. Experience in the organic coatings field has shown that epoxy related polymers and co-polymers have excellent adhesion to metal and metal oxide surfaces[48]. Adhesion of organic films to Li electrodes has not been explicitly studied as such, but experience with this situation Seems to indicate that as long as the surface of the Li is clean, the films adhere well[49]. References[l, 91 discuss the adhesion issues at metal polymers interfaces, and Ref. [l l] discusses some of the changes seen with battery cycling on Li films in contact with SPE films. The alignment of the coated film polymers[50] and the stresses[51] in the films induced during the coating and curing/drying processes are known to effect adhesion[52]. Battery cycling and operation at temperatures above the Tgof the SPE systems may relieve any stresses induced during film coating formation and thus may actually improve the adhesion as the system is used. As long as impurities do not migrate to the interface between the Li and the SPE during battery cycling, adhesion should be maintained during battery lifetime[53].
Film coating technologies and adhesion
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Possible Transfer Film Casting Process Schematic Drawing
Lithium Film
Carrier Web with SPE and PMC Layers already on by Extrusion Coating
Laminating Polyester Carrier Web with SPE and PMC Layers already on by Extrusion Coating Fig. 3. Schematic drawing of possible transfer film casting process. SUMMARY In general, those film coating processes that can yield thin controllable films at high coating rates are viable as candidate processes for the SPE and PMC film casting for use in solid state battery devices.
Existing technology in roll coating, extrusion coating, and other coating processes most likely could be adopted to the unique demands of the SPE and PMC film casting needs. However, more attention needs to be given to the liquid film precursors and their fluid properties to enable proper process technology to be developed for these films. Not just the solid state properties of the SPE systems need to be characterized, but also the fluid properties of the solutions and dispersions that are used in their preparation must be studied. In the case of extrusion coating processes, the melt flow characteristics of the SPE systems would need characterization. Examination of the existing literature yields little on the species of adhesion of SPE films, but studies on related polymers indicates that this should not be a problem as long as the metal/SPE interfaces are free of contamination. Film formation induced stresses may cause some adhesion problems but film cycling and operation at temperatures above the Ts of the SPEs under consideration should alleviate thts effect. It should be considered in radiation crosslinked sys-
tems, however. REFERENCES 1. J. Owen, Chemistry and Industry, 71-74, Feb. 1 (1988). 2. M. Zafar, A. Munshi and B. B. Owens, IEEE Spectrum, 32-35, Aug. (1989).
3. M. Z. A. Munshi and B. B. Owens, in Proc. Symp. Electrochemistry Solid State Science Education at Graduate and Undergraduate Level (Edited by N. H.
Sinyrl and F. McLamon), Electrochem. Sot. (1987). 4. M. B. Armand, Solid St. Ionics 9/10, 745 (1983). 5. Lee, et al., U.S. Patent 4,830,939, May 16, 1989; Schwab, et al., U.S. Patent 4,792,504, Dec. 20, 1988; le Mehaute, et al., U.S. Patent 4,556,614, Dec. 3, 1985. 6. E: D. Cohen, E. J. Lightfoot and E. B. Cutoff, Chem. Eng. Progress, 30-36 Sept. (1990). 7. A. Banerjea, J. Ferrante and J. R. Smith, in Fundamentals of Adhesion (Edited by Lieng-Huang Lee), pp. 325-348. Plenum Press, New York (1991). 8. P. S. Ho, R. Haight, R. C. White, B. D. Silverman and F. Faupel, in Fundamentals of Adhesion (Edited by Lieng-Huang Lee), pp. 383-406. Plenum Press, New York (1991). 9. J. R. Owens, J. Drennan, G. E. Lagos, P. C. Spurdens and B. C. H. Steele, Solid St. Ionics 5, 343 (1981). 10. B. B. Owens, Technology Assessment of Ambient Temperature Lithium Secondary Batteries: Electric Utility and Vehicle Uses, EPRI AP-5218 Project 370-30, Final
Report, June 1987. 11. M. Z. A. Munshi and B. B. Owens, Solid St. Ionics 27, 251 (1988).
12. E. D. Cohen, E. B. Gutoff and E. J. Lightfoot, Thin Film Drying Technology Review, Intern. Symp. A.1.Ch.E. Spring Meeting Orlando, Fl., March (1990) (preprint). 13. G. P. Bierwagen, Prog. Organic Coatings 19, 59 (1991). 14. Proc. Regional Educational Technical Conference, Sot. Plastics Engineers, Decorating Division, October 24 and 25, 1989, Hyatt Regency, Dearborn, Michigan. 15. D. H. Morton-Jones, Polymer Processing. Chapman and Hall, London (1989). 16. Y.-O. Tu and R. L. Drake, J. Colloid Interface Sci. 135, 562 (1990).
17. R. F. Probstein, Physicochemical Butterworths, Boston (1989).
Hydrodynamics.
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18. M. Z. A. Munshi and B. B. Owens, Solid St. Ionics 26, 41 (1988).
Methoak in FIuia!s 4, 207 (1984).
19. M. Bauthier, A. Belanger, B. Kapfer, G. Vassort and M. Armand. “Solid Polvmer Electrolvte (SPE) Lithium Batteries”, Polymer klectrolyte R&iews, 2 (Edited by J. R. MacCallum and C. A. Vincent) pp. l-53. Elsevier Applied Science, London (1989). B. B. Owens, private communication. D. J. Coyle, C. W. Macosko and L. E. Striven, ACS Symposium Series 197, Computer Applications in Applied Polymer Science (Edited by T. Provder) Ch. 15, pp.
252-264 (1982). Amer. Chem. Sot., Washington, DC. 22. H. Benkreira and M. F. Edwards, J. Non-Newtonian Fluid Mech. 14, 377 (1984).
23. J. R. A. Pearson, J. Fluid. Mech. 7, 481 (1960). 24. T. Matsuda and W. H. Brendly, J. Coating Tech. 51, (658) 46 (1979).
25. D. J. Coyle, C. W. Macosko A.I.Ch.E.J.
33. S. F. Kistler and L. E. Striven, Int. J. Numerical
and L. E. Striven,
36, 161 (1990).
26. L. L. Kornum and H. K. Raaschou Nielsen, Prog. Org. Coatings 8, 275 (1980).
27. C. M. Hansen and P. E. Pierce, Ind. Eng. Chem. Prod. Res. Dev. 13, 218 (1974).
28. P. E. Pierce and C. K. SchotT, “Coating Film Defects”, in Federation Series on Coatings Technology (Edited by T. Miranda and D. Bresinski). Federation of Societies for Coating Technology, Philadelphia (1988). 29. F. R. Prankh and L. E. Striven, A.Z.Ch.E.J. 36, 587 (1990).
and H. F. George, Rotogravure Press Theory and Practice, Seminar Lecture Notes, Gravure Association of America (1989). 31. D. R. Brown, J. Fluid Mech. 10, 297 (1961). 32. K. J. Ruschak, Ann. Rev, Fluid Mech. 17, 65 (1985). 30. R. H. Oppenheimer
34. J. D. Coleman, “Curtain Coating Method”, U.S. Patent # 4,2242,423 Aug. 3, 1982. 35. S. F. Kistler, Ph.D. Thesis, Dept. of Chemical Engineering, Univ. Minnesota, 1983. 36. S. D. R. Wilson, A.I.Ch.E. J. 34, 1732 (1988). 37. S. Torza, J. uppl. Phys. 47, 4017 (1976). E. 0. Tuck, A.Z.Ch.E.J. 30, 808 (1984). :;. M. R. Keeling, Phys. Technol. 12, 196 (1981). 40: D. A. Ansdell, Paint and Surface Coatings (Edited by R. Lambourne) Ch. 10. Halsted Press, New York (1987). 41. T. C. Papanastatiou, A. N. Alexandrou and W. P. Graebel, J. Rheology 32, 485 (1988). 42. W. W. Flack, D. S. Soong, A. T. Bell and D. W. Hess, J. uppl. Phys. 56, 1199 (1984). 43. S. A. Jenekhe and S. B. Schuldt, Ind. Eng. Chem. Fundam. 23, 432 (1984).
44. M. Freedman, private communication. 45. C. D. Han and D. A. Rao, Polym. Engng Sci. 20, 128 (1980). 46. G. P. Bierwagen and T. K. Hay, Prog. Org. Coatings 3, 281 (1975).
47. F. DeCandia and A. Renzulli, J. appl. Polymer Sci. 41, 955 (1990).
48. R. A. Dickie, J. W. Holubka
and J. E. DeVries,
J. Adhesion Sci. Technol. 4, 57 (1990).
49. C. T. Havens, private communication. 50. W. M. Prest. Jr and D. J. Luca. J. aool. Phvs. 51. 5170 (1980). ’ 51. T. S. Chow, C. A. Liu and R. C. Penwell, J. Polymer Sci., Polymer Physics Edn 14, 1311 (1976). 52. D. Y. Perera, J. Oil Colour Chem. Assoc. 68,275 (1985). 53. E. Goren, 0. Chusid (Youngman) and D. Aurbach, J. electrochem. Sot. 138(S), L6, May (1991). _.
.
,
FILM COATING
0013-4686/92 SS.00 + 0.00 @J 1992. Pqamon Prts Ltd.
TECHNOLOGIES
AND ADHESION
GORDON P. BIERWAGEN North Dakota State University, Department of Polymers and Coatings, Fargo, ND 58105, U.S.A. (Received 20 December 1991) Abstract-Solid Polymer Electrolyte (SPE) films must be formed in a reproducible, controlled manner in order to construct devices which take advantage of their unique properties. Film coating processes that yield reproducible thin films will be reviewed vis-ri-vis the device requirements (eg thin film lithium batteries) of SPE films. Solution characteristics important for film coating processes will he presented and process control parameters outlined. Film defects and their control in processing are major problems which must be addressed in the choice of materials and coating process for thin film applications. Proper processing of SPE films will be a key to their utilization, and the criteria for correct choice of a coating process for this type of film will be examined. A further key to the proper utilization of SPE film will be their wetting and adhesion to device substrates, ie lithium, in the case of thin film batteries. Poly(ethylene oxide) SPE films may have advantages in this respect if their adhesion behavior is analogous to that of PEO-based polymers used in the protective organic coatings. Key words: polymer films, batteries, coating, processes.
INTRODUCTION Solid Polymer Electrolyte (SPE) films are receiving considerable attention as possible solid electrolytes in
lightweight, high power density batteries, especially those based on Li films[l-4]. To use these materials as battery electrolytes at ionic conductivities that are practical, requires that the SPE films be quite thin, and have exellent adhesion to the Li films. Also, there are the further practical requirements that the film thicknesses of both the Li and SPE are constant over the contact surface areas in the battery, have no defects such as film thickness fluctuations, holes, insoluble particles of contaminants, have uniform properties across the film, etc. Also, the chemical nature of the film must be constant across and through the film. For reasons of economics and function, the film must be capable of being cast at high speed and with close control of film properties[5]. This work will review studies in film coating science and technology pertinent to the creation and control of SPE films appropriate for battery use and suggest possible film coating methods that would be appropriate to SPE film formation. The science and technology of film coating processes has recently been undergoing an influx of attention from the engineering community, and the basics of designing the film precursor liquid and dry film characters are quite well established for most organic coatings[6]. These studies should be considered in order to properly take advantage of all of the inherent properties of SPE films for battery use. Also, the issues of polymer film adhesion to metal substrates have received considerable attention by workers in both the areas of organic coatings and adhesives[7, 81, and again those attempting to utilize SPE films over metals would be well served by considering the literature of these areas. The purpose of this paper is to review these areas with respect to requirements of SPE films in batteries and similar
devices, including the composite matrix cathodes based on SPEs[9], and make suggestions based on this review as to the design casting of SPE films and SPE-based matrix cathodes. The relationship of battery SPE film technology the organic coatings science is quite extensive, and the matrix insertion cathodes used in Li-SPE batteries are quite close in nature to pigmented organic coatings (ie paints). Many organic coatings are cast from liquid, as are most SPE films; most organic coatings are quite sensitive to variations in film thickness and quality, as are most SPE films; and many of the properties of organic coatings films are determined by the polymer portion of the film, as are most SPE films. It thus seems quite reasonable to consider carefully the literature on organic coatings design and coatings application processes as a guide to further studies and development of Li-SPE film batteries, and to the utilization of SPE films of all types and uses. The unique uses and requirements of SPEs as compared to most organic coatings require or yield specific properties usually not considered by coating scientists. Among these are the electrochemical transport properties, cycling lifetime and cycling behavior, environmental sensitivity, absolute long term stability, and chemical purity[lO]. An SPE film will be required to provide ionic conductivity that is stable to frequent and extensive recycling, inertness with respect to its substrate and sustainment of mechanical and thermal properties over periods of extended use[ 111. BASIC CONCEPTS OF FILM APPLICATION/FORMATION (a) Film casting/coating as a process
Large scale formation of a thin film on a substrate is an important industrial process that serves many
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G. P. BIERWAGEN
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Before Coating GAS
I
SUBSTRATE
1. Before coating process
During this step, most properties
2. Coating fluid displacing
deposited on substrate
4. Film becomes solid
Fig. 1. Schematic of coating process.
areas of technology[6]. In some cases, multiple films are cast sequentially to yield a multi-layer film of increased functionality and performance. Inherent in many continuous film casting operations are drying or curing steps which are required to change the liquid cast film in to a dry, solid film[l2]. Some processes such as vacuum deposition, plasma polymerization, extrusion, and variations, thereof yield dry films that need no further “curing”, but many continuous film application processes require drying and even extra energy to crosslink the applied film[6]. A schematic diagram of the coating process for the application of a liquid film to a solid substrate is shown in Fig. 1. The coating process is often done at high speed and with relatively good control of the film thickness and quality. However, there are many problems that can occur in high-speed continuous coating processes, and they are often very capital intensive processes. Many of the important parameters of the coating process are the properties of the liquid solution/ dispersion from which the film is cast. The user is often interested in the dry film properties of the solid film, but must also design and control the wet film properties very carefully to control the dry film quality. An introduction to coating as an engineering process is given in the September 1990 issue of Chemical Engineering Progress. There are quite a few specific coating processes, each designed and used for a particular dry film end utilization. A recent review[l3] of coating process from the view point of the paint/organic coatings scientist considers many of these individual processes. These and other continuous film-forming processes will be considered below with respect to their potential utility in the formation of useable SPE films for battery use. Comments will be made concerning their individual technical and economic merits, as well as the strengths and weaknesses of the individual processes. Generally, one can say that the faster the speed of the process and the wider the web or sheet to be coated, the greater the control and reproducibility problems. Table 1 gives a list of the coating processes that will be considered below.
(b) Carrier web issues
An important fact that must be considered along with these coating processes is the need for a carrier web in many of these coatings processes, especially the continuous, high speed processes. In the case of SPE electrolyte films, the film forming would ideally be done onto a carrier web of Li film, but this metal film would have insufficient tensile strength to allow its use in a continuous web process. Many film coating processes use carrier webs that become part of the final product, such as the polyester film that is used as the carrier for magnetic tapes or for photographic films. The carrier web may be later removed and the film transferred to another substrate such as in heat/pressure transfer films that are used for decorating plastics and fibre board[l4]. Paper and other polymers such as polyethylene or polypropylene are also used as carrier webs for film coating
Table 1, Characteristics of coating processes 1. Thickness of film h 2. Bulk viscosity TV -shear -extensional 3. Surface tension 0 4. Temperature T 5. Concentrations c -solvent -surfactants 6. Relative velocities v 7. Surface lifetime 5 8. Diffusion coefficients D 9. Edge radii of substrate r 10. da/dT Il. 12. 13. 14. 15. 16. 17. 18. 19. 20.
du/dc dT/dr Contact angle 0 Surface modulus da/dlnA Rate of surface area (A) formation d lnA/dt Surface viscosities r Radius of curvature at surface R Density p Surface area A Characteristic length I
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Film coating technologies and adhesion processes. The carrier web may be cast in situ by an extrusion process[ 151. The carrier web for a coating process must be stable to the chemicals of the liquid film, the temperatures of application and drying/curing, and the mechanical stresses of the film coating process. Its dimensions and properties must remain constant during all stages of the coating process. Further, the cost of the carrier web often is a significant part of the cost of the final film. The carrier web must be chosen carefully, for it may determine many parameters of the coating process. (c) Film drying and curing Another issue that must be treated in considering high-speed, continuous film coating processes is the drying/curing step in which the liquid film is converted to the solid film which provides the properties of interest. This step involves the removal of the solvent/carrier liquid from the film and/or the crosslinking/coalescence of the film polymer[l6]. This step must occur, in most cases, before the film is ready for use or for the application of the next film in a multi-film coating process[l2]. Often, the solvent or carrier liquid is an organic liquid that must be collected or incinerated after removal by the drying process, or the curing process generates a by-product vapor that must also be collected. The solvents are often hazardous or polluting materials that must be dealt with very carefully, making this stage of the coating process a major part of the process cost. (d) Properties properties
of the liquid coating vs. dry film
The important properties of the film during the coating process are those of the liquid state of the film forming material and the properties of the substrate, and thus involve many properties often not considered in the development of solid film properties. Table 2 lists many of these properties which must be controlled in the liquid film application process. This list is not all inclusive, but lists most of the important parameters which are important in the liquid film coating process.
The solution properties of the liquid film are very important for the coating process, especially the surface and rheological properties of the coating fluid. If this fluid is a gas-in-liquid (foam), a liquid-inliquid (emulsion), or solid-in-liquid (colloidal) dispersion, specific properties of the dispersion system such as dispersed particle (bubble, globule) size distribution, dispersed phase volume concentration, dispersion (foam, emulsion) stability and concentration of stabilizing material (surfactant, dispersant, emulsifier). Further, these properties may be time dependent (surface tension, viscosity, surface concentration) or dependent on the history of the fluid (shear dependence, non-Newtonian viscosity, viscoelastic behavior)[ 13, 11.
FILM FORMING PROCESSES POTENTIALLY APPLICABLE TO CASTING SPE FILMS Below are described many of the currently extant methods of film coating operations along with a discussion of their individual potential applicability to SPE and SPE matrix cathode films. The target thickness used for an SPE film for lithium battery use is ca 10-50 pm[l8-201, with target thicknesses of lO-220pm for the composite matrix cathodes [18]. The Li film thickness for this use is estimated to he 25-75 pm. The desired accuracy of the film forming process for battery use is + 5% of the desired dimension[ 191. Some speculations have been made conceming appropriate film coating techniques for Li/SPE batteries[l8], but there has heen no comprehensive survey of film coating processes vis-ti-vis the film
requirements of this technology. Many of the more common film coating processes are summarized in Table 2. In all of the coating processes discussed below, proper wetting of the substrate is crucial to the control and success of the process, and requires careful control of the substrate surface cleanliness, etc. (a) Roll film coating processes Roll film coating processes are widely used industrially, ranging from the application of paint to metal
Table 2. Important coating liquid properties for coating processes Use/products
Coating process
Advantages
Disadvantages Problems in transfer from engraved roll Flow instabilities
Adhesive films, printing decorative films
Precision and spatial control, patterning
Adhesive films, coil coatings, etc.
Speed, precise metering, gap control
Dip coating
Rods, sheets, roll goods
Speed and simplicity
Film flow problems
Bead coating
Web fed roll stock Flat rigid and roll stock Photographic films, etc. General use Plastic films Electronic parts
Film thickness control Speed and control
Curtain breaks possible
Transfer coatings, and plastic films
Solvent free, high speed
Gravure coating-all Roll coating-all
types
types
Curtain coating Slide coating Spray coating Extrusion coating Spin coating Lamination coating and calendering
Multilayer, thin films Ease of use, simplicity Little solvent use Excellent tilm control
Flow instabilities
Flow instabilities Poor film control, batch High shear, temperature Batch only Thermoplastics only
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G. P. BIERWAGEN
or vinyl siding, to the application of scratch resistant coatings to plastics, and to the application of coatings to polyester film to make magnetic tapes. They are processes that have been studied quite widely[21-241 and their strengths and weaknesses are given in Table 1. There are several configurations possible under the general heading of roll coating processes: forward, reverse, deformable rolls (squeeze coating), and metered with a doctor blade. These are discussed quite extensively in the studies of Coyle et aZ.[21,25]. Roll coating processes are almost exclusively used with a continuous web of rapidly moving substrate, and thus require a web of considerable strength and toughness, as well as temperature stability if there are drying and curing steps subsequent to the film coating process step. Roll coating processes can be used to accurately apply dry films of 10-25 pm at high speed and therefore must be considered as a candidate process for the casting of SPE films if a proper carrier web can be chosen within the economic constraints of the potential SPE film use. Webs of up to 60 in (1.524 m) in width can be coated in this manner, with speeds of up to 1100 ft S-I (335.4 ms-‘) for paper webs and 500 ft SK’ (152.4 ms-’ ) for forward roll coating. Thickness control is perhaps best in reverse roll coating processes with a metering roll and a doctor blade. Roll film coating processes can be used with a wide range of liquid properties, including rheological and surface properties. Multiple layer films can be created easily by inserting sets of coating and drying stations in series along the web handling line. The fluid flows in roll coating processes can be quite complex, and there are flow instabilities that can occur during the film application that lead to film imperfections. Among these are ribbing[21], film breaks due to wetting problems, “cascade” instability, and air entrainment with Newtonian fluids[25], while with non-Newtonian fluids, mottling and “stranding” of fluid filaments between roller and web can occur. Other imperfections, such as foaming due to air entrainment problems in the reservoir, problems in the wetting of the moving web, or cratering due to surface active impurities[ 131,can occur in this, as well as many other film coating operations[26,27]. Further, film defects can also occur during the drying/curing step for all coating processes[ 12,281. (b) Gravure coating processes
Gravure coating processes are another class of continuous web coating processes that are very similar to roll coating operations. The unique feature of gravure coating processes is that the transfer roll is an engraved cylinder which is wiped by a doctor blade so that liquid transfers to the web in the pattern of the engraved (recessed) areas of the transfer cylinder, and, thus, this method is used mainly for high speed printing[29]. If close geometric patterns of diamonds, dots, etc, are engraved on the transfer roller, the process can be used with liquids of sufficiently low viscosity to create thin continuous films due to reflow from the patterned film transferred from the engraved rol1[30]. Gravure is used in this manner to put on thin continuous films of adhesive, protective coatings, etc. Multiple station gravure printing can be used for multi-color printing or multiple layer films, and, in
combination with roll coating stations, for special multiple layer films. Gravure coating processes are of very specific interest to the printing and ink industries, but are used widely outside of these for creating well controlled thin films. Gravure coating processes have many of the same advantages that roll coating processes, plus it can be used to deliver a controlled pattern of materials, if so desired, to a moving web. They are high speed, continuous, and controllable processes, and have been used extensively for delivery of thin organic films and there are suppliers of this type of equipment available for consultation on proper equipment design. Prior to considerations of equipment, the coating fluids will have to be characterized in the manner discussed above with respect to their flow properties, etc. Gravure line speeds can be as high as 900 m min-’ for paper printing operations, and are ca 110 mmin-’ for adhesive tapes, etc, and line widths can be up to 2 m. This class of coating processes is not as well studied on a fundamental basis as roll coating, and therefore has a greater percentage of empiricism and/or trade secret process methods within its technology. Besides many of the defect problems of roll coating processes, the transfer of the coating fluid from the engraved cells on the transfer roll is somewhat of an ill understood process and the cause of many defects characteristic of gravure coating[25]. (c) Curtain coating processes Curtain coating processes are those in which a freely falling sheet of coating fluid is directed to a moving web or to rigid objects on a moving open conveyor belt[31]. This may be a “short curtain” falling from an inclined plane fed by a coating slot die[32, 331. This form of the process is closely related to slide coating, and with a multiple slot die configuration and with proper control of flows and viscosities, this process can be used to put several layers of coatings at one time on a moving web. This method of coating is often used in the manufacture of photographic films and paperboard[34]. Dry film thicknesses can be controlled, depending on the volume fraction solids of the coating fluid, from 20 to 100 pm in the short curtain process, and from 40 to 200 pm in the “long” curtain process. Line speeds can be as high as 600 m min-’ in photographic film processes. Curtain coating processes, especially the short curtain process, will allow high rate, controlled film thickness coating or moving webs, and curtain coating stations can be put in series with drying/curing ovens for multiple layer coating operations. They are reasonably well studied theoretically, and much of the success of an operation based on this method will depend on proper design of the die through which the coating fluid is pumped from the reservoir into the coating head. There are a broad range of fluids that can be used in these processes, and it can be used for narrow or wide webs. The long curtain method will be suitable for thicker polymer based films needed in battery manufacture. Curtain coating operations are susceptible to many of the defects discussed above for roll and gravure operations (not ribbing, however) and have problems specific to their freely falling film of liquid, that of
Film coating technologies and adhesion “curtain breaks”[31] due to flow instabilities, surface active impurities[l3], or bubbles in the fluid. Again, defects associated with high speed drying/curing steps also may occur. (d) Slide coating processes
Slide coating is a process used in photographic film coating and related processes, and is quite similar to curtain coating process discussed the “short” above[35]. The coating flow process is flow of the coating fluid through a slot orifice down an incline “slide” to direct contact with the moving web at the end of the “slide”, with no free fall zone as in the short curtain process. This process has many of the same features in relation to line speed and defects discussed immediately above with curtain coating. (e) Dip coating processes One of the oldest coating processes is the dip coating process, a self-metering coating process[3 l] that uses a moving web continuously withdrawn from a reservoir of the coating fluid. It is commonly used in web wire, rod and fiber coating and has been extensively studied[32, 34,361. The method has no external devices supplemental to the moving web and the coating reservoir, relaying on control of web speed and coating fluid properties to control the film thickness deposited on the web. For a moving web it is a two-sided coating process that is dependent on proper wetting of the web for successful operating. The withdrawal from the reservoir may be vertical or horizontal[37], and the coating may have a wide range of properties. A stripping gas jet, known as an “air knife”, may be used as a film thickness control method for obtaining thinner films than at steady state under the influence of gravity or the surface tension and other properties of the coating fluid[38]. For further discussion of the strengths and weaknesses of this process, the reader is referred to the literature cited above. The process would seem to only apply to symmetrical simultaneous deposition of films on a series battery construct, somewhat limiting its use. (f ) Spray coating processes Spray coating processes are a common batch coating process for individual items such as automobiles and airplanes, or for continuously cast amorphous metal strips (spray metal casting) and for patterning as in ink jet printing[39]. The coating liquid is pumped through an orifice and broken up into droplets by pressure (airless spray painting) or by a coincident flow of air (air-assisted spray)[40], by an electric field from the nozzle to the workpiece (electrostatic spray), broken up by a vibrating reed at an orifice, broken up by centrifugal force and electrostatic forces on a rotating horizontal disk or a vertically rotating be11[41], or driven through an orifice by a propellant from an aerosol spray coating can. Spray coating process range from fairly unsophisticated hand held sprayers to technically sophisticated ink jet printers and coaters. At high rates of continuous substrate motion and for very close thickness control, as a class, spray coating operations may not be the most desirable, especially with the hazardous solvents required for PEO systems. For
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batch operations, they may prove useful. Most spray coating processes also require very low viscosity fluids. (g) Spin coating processes Spin coating is another batch coating process that is very useful for applying thin, uniform layers, extensively used in the electronics industry for polymer resist coatings, etc[42,43]. A fixed amount of coating fluid is dispensed onto the center of a round substrate, and the disk is rotationally accelerated to a preset speed, centrifugally moving and thinning the fluid to the desired thickness over the surface of the disk. It is a well analysed method capable of providing very close control of film thickness, but a batch process not well suited to large production rates. This process is well suited to the preparation of films of very closely controlled film thickness for lab studies and testing of various batches of polymer, but not large scale production of SPE films for battery use. (h) Extrusion coating processes
Polymer extrusion is a film forming process[l5] that is pertinent to SPE film formation, and one that forms films from thermoplastic solid polymers, avoiding the need for dissolving or dispersing the polymer in a solvent. It thus is a process that requires no drying/curing ovens and is thus low in pollution. The process is widely used for forming films of polymers for various uses, and has the requirement that the polymer be thermoplastic, stable to the heat and mechanical stresses of the extruder, and melt processable. Films of 25 km are frequently cast in this manner, and extrusion processing lends itself to multi-layer film forming processes(4.41. Extrusion is also used as a wire coating process[45], and multilayer coatings may be cast by the extrusion process as well. This would seem to be a very attractive process for SPE or polymer matrix cathode films if the solid polymer (eg the PEO/LiClO, ionic conducting SPE commonly considered for battery use) is stable to the processing conditions found in extruders, and the flow temperature is low enough for the use of this process. (i) Bead coating processes
Beat coating is the liquid version of extrusion coating in which the liquid is pumped through a slot to form a liquid bead that directly contacts a moving substrate web. It has been discussed somewhat in the literature (see, for example[35], and citations therein), but there has not been much description of its engineering parameters such as line speeds and web widths. (j ) Lamination /calendering Other related process that may be useful for multilayer SPE film formation are lamination and calendering processes[ 151, and film transfer processes[ 141. In these types of processes, the film is formed by another process, and then joined to another film or carrier web under pressure of heated rolls (lamination) or reduced in thickness and smoothed by pressure of heated roll (calendering)[ 151. Hypothetical uses of these process are illustrated in Figs 2 and 3, below, illustrating how multilayer films could be
1476
G. P. BIERWAGEN
I
I Film Casting Process Schematic Drawing
I
Lithium Film
_&Le \
Lithium Film
1
Step @
11 : 1 Heat +
IRoll. Gravure1
Process @ Step
I
Polyester Carrier Web Film
Fig. 2. Schematic drawing of possible film casting process. prepared from single layer processes plus lamination steps.
GENERAL COMMENTS ON FILM COATING PROCESSING The viability of Li battery designs based on SPE and SPE matrix cathodes may well depend on the availability of economical process methods to form coating films of these materials in well controlled, high rate processes. Choosing a process is one step, but control measurements must also be available for the process to ensure the proper thicknesses and electrochemical properties of the films. This will require some development work on measurement techniques, preferably on-line, for the characterization of film quality. As stated above, work will be necessary to characterize the liquid coating parameters appropriate to coating process control, as well as the proper choice of process steps to yield the solutions and dispersions used as process fluids. There is a good deal of literature on the preparation of dispersions[ 15,401 of the type necessary for the SPE matrix cathode film forming, and the general literature on organic coatings addresses the problems and methods of dealing with pigmented polymer systems (Ref. [40] is a good introductory source for the literature in this area). Care will have to be taken to account for the effects of dispersed phase volume effects in the poloymer matrix cathodes (PMCs), as they have been noted in pigmented coatings and other filled polymer systems[46,47]. The unique added effect in the PMCs is the Li ion injection swelling of the dispersed phase, which will have to be accounted for in the battery design.
Another area commented on above, but worth considering further, is the defect problems likely to be seen in film coating process for liquid polymer solutions and dispersions[l3]. Special care is required to avoid these problems in film coating processes. Further there may be polymer alignment effects at film surfaces during film casting processes.
FILM ADHESION Proper adhesion of the SPE film to Li and to and the PMC will be critical to the viability of the battery designs currently under consideration[2]. Experience in the organic coatings field has shown that epoxy related polymers and co-polymers have excellent adhesion to metal and metal oxide surfaces[48]. Adhesion of organic films to Li electrodes has not been explicitly studied as such, but experience with this situation Seems to indicate that as long as the surface of the Li is clean, the films adhere well[49]. References[l, 91 discuss the adhesion issues at metal polymers interfaces, and Ref. [l l] discusses some of the changes seen with battery cycling on Li films in contact with SPE films. The alignment of the coated film polymers[50] and the stresses[51] in the films induced during the coating and curing/drying processes are known to effect adhesion[52]. Battery cycling and operation at temperatures above the Tgof the SPE systems may relieve any stresses induced during film coating formation and thus may actually improve the adhesion as the system is used. As long as impurities do not migrate to the interface between the Li and the SPE during battery cycling, adhesion should be maintained during battery lifetime[53].
Film coating technologies and adhesion
1477
Possible Transfer Film Casting Process Schematic Drawing
Lithium Film
Carrier Web with SPE and PMC Layers already on by Extrusion Coating
Laminating Polyester Carrier Web with SPE and PMC Layers already on by Extrusion Coating Fig. 3. Schematic drawing of possible transfer film casting process. SUMMARY In general, those film coating processes that can yield thin controllable films at high coating rates are viable as candidate processes for the SPE and PMC film casting for use in solid state battery devices.
Existing technology in roll coating, extrusion coating, and other coating processes most likely could be adopted to the unique demands of the SPE and PMC film casting needs. However, more attention needs to be given to the liquid film precursors and their fluid properties to enable proper process technology to be developed for these films. Not just the solid state properties of the SPE systems need to be characterized, but also the fluid properties of the solutions and dispersions that are used in their preparation must be studied. In the case of extrusion coating processes, the melt flow characteristics of the SPE systems would need characterization. Examination of the existing literature yields little on the species of adhesion of SPE films, but studies on related polymers indicates that this should not be a problem as long as the metal/SPE interfaces are free of contamination. Film formation induced stresses may cause some adhesion problems but film cycling and operation at temperatures above the Ts of the SPEs under consideration should alleviate thts effect. It should be considered in radiation crosslinked sys-
tems, however. REFERENCES 1. J. Owen, Chemistry and Industry, 71-74, Feb. 1 (1988). 2. M. Zafar, A. Munshi and B. B. Owens, IEEE Spectrum, 32-35, Aug. (1989).
3. M. Z. A. Munshi and B. B. Owens, in Proc. Symp. Electrochemistry Solid State Science Education at Graduate and Undergraduate Level (Edited by N. H.
Sinyrl and F. McLamon), Electrochem. Sot. (1987). 4. M. B. Armand, Solid St. Ionics 9/10, 745 (1983). 5. Lee, et al., U.S. Patent 4,830,939, May 16, 1989; Schwab, et al., U.S. Patent 4,792,504, Dec. 20, 1988; le Mehaute, et al., U.S. Patent 4,556,614, Dec. 3, 1985. 6. E: D. Cohen, E. J. Lightfoot and E. B. Cutoff, Chem. Eng. Progress, 30-36 Sept. (1990). 7. A. Banerjea, J. Ferrante and J. R. Smith, in Fundamentals of Adhesion (Edited by Lieng-Huang Lee), pp. 325-348. Plenum Press, New York (1991). 8. P. S. Ho, R. Haight, R. C. White, B. D. Silverman and F. Faupel, in Fundamentals of Adhesion (Edited by Lieng-Huang Lee), pp. 383-406. Plenum Press, New York (1991). 9. J. R. Owens, J. Drennan, G. E. Lagos, P. C. Spurdens and B. C. H. Steele, Solid St. Ionics 5, 343 (1981). 10. B. B. Owens, Technology Assessment of Ambient Temperature Lithium Secondary Batteries: Electric Utility and Vehicle Uses, EPRI AP-5218 Project 370-30, Final
Report, June 1987. 11. M. Z. A. Munshi and B. B. Owens, Solid St. Ionics 27, 251 (1988).
12. E. D. Cohen, E. B. Gutoff and E. J. Lightfoot, Thin Film Drying Technology Review, Intern. Symp. A.1.Ch.E. Spring Meeting Orlando, Fl., March (1990) (preprint). 13. G. P. Bierwagen, Prog. Organic Coatings 19, 59 (1991). 14. Proc. Regional Educational Technical Conference, Sot. Plastics Engineers, Decorating Division, October 24 and 25, 1989, Hyatt Regency, Dearborn, Michigan. 15. D. H. Morton-Jones, Polymer Processing. Chapman and Hall, London (1989). 16. Y.-O. Tu and R. L. Drake, J. Colloid Interface Sci. 135, 562 (1990).
17. R. F. Probstein, Physicochemical Butterworths, Boston (1989).
Hydrodynamics.
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18. M. Z. A. Munshi and B. B. Owens, Solid St. Ionics 26, 41 (1988).
Methoak in FIuia!s 4, 207 (1984).
19. M. Bauthier, A. Belanger, B. Kapfer, G. Vassort and M. Armand. “Solid Polvmer Electrolvte (SPE) Lithium Batteries”, Polymer klectrolyte R&iews, 2 (Edited by J. R. MacCallum and C. A. Vincent) pp. l-53. Elsevier Applied Science, London (1989). B. B. Owens, private communication. D. J. Coyle, C. W. Macosko and L. E. Striven, ACS Symposium Series 197, Computer Applications in Applied Polymer Science (Edited by T. Provder) Ch. 15, pp.
252-264 (1982). Amer. Chem. Sot., Washington, DC. 22. H. Benkreira and M. F. Edwards, J. Non-Newtonian Fluid Mech. 14, 377 (1984).
23. J. R. A. Pearson, J. Fluid. Mech. 7, 481 (1960). 24. T. Matsuda and W. H. Brendly, J. Coating Tech. 51, (658) 46 (1979).
25. D. J. Coyle, C. W. Macosko A.I.Ch.E.J.
33. S. F. Kistler and L. E. Striven, Int. J. Numerical
and L. E. Striven,
36, 161 (1990).
26. L. L. Kornum and H. K. Raaschou Nielsen, Prog. Org. Coatings 8, 275 (1980).
27. C. M. Hansen and P. E. Pierce, Ind. Eng. Chem. Prod. Res. Dev. 13, 218 (1974).
28. P. E. Pierce and C. K. SchotT, “Coating Film Defects”, in Federation Series on Coatings Technology (Edited by T. Miranda and D. Bresinski). Federation of Societies for Coating Technology, Philadelphia (1988). 29. F. R. Prankh and L. E. Striven, A.Z.Ch.E.J. 36, 587 (1990).
and H. F. George, Rotogravure Press Theory and Practice, Seminar Lecture Notes, Gravure Association of America (1989). 31. D. R. Brown, J. Fluid Mech. 10, 297 (1961). 32. K. J. Ruschak, Ann. Rev, Fluid Mech. 17, 65 (1985). 30. R. H. Oppenheimer
34. J. D. Coleman, “Curtain Coating Method”, U.S. Patent # 4,2242,423 Aug. 3, 1982. 35. S. F. Kistler, Ph.D. Thesis, Dept. of Chemical Engineering, Univ. Minnesota, 1983. 36. S. D. R. Wilson, A.I.Ch.E. J. 34, 1732 (1988). 37. S. Torza, J. uppl. Phys. 47, 4017 (1976). E. 0. Tuck, A.Z.Ch.E.J. 30, 808 (1984). :;. M. R. Keeling, Phys. Technol. 12, 196 (1981). 40: D. A. Ansdell, Paint and Surface Coatings (Edited by R. Lambourne) Ch. 10. Halsted Press, New York (1987). 41. T. C. Papanastatiou, A. N. Alexandrou and W. P. Graebel, J. Rheology 32, 485 (1988). 42. W. W. Flack, D. S. Soong, A. T. Bell and D. W. Hess, J. uppl. Phys. 56, 1199 (1984). 43. S. A. Jenekhe and S. B. Schuldt, Ind. Eng. Chem. Fundam. 23, 432 (1984).
44. M. Freedman, private communication. 45. C. D. Han and D. A. Rao, Polym. Engng Sci. 20, 128 (1980). 46. G. P. Bierwagen and T. K. Hay, Prog. Org. Coatings 3, 281 (1975).
47. F. DeCandia and A. Renzulli, J. appl. Polymer Sci. 41, 955 (1990).
48. R. A. Dickie, J. W. Holubka
and J. E. DeVries,
J. Adhesion Sci. Technol. 4, 57 (1990).
49. C. T. Havens, private communication. 50. W. M. Prest. Jr and D. J. Luca. J. aool. Phvs. 51. 5170 (1980). ’ 51. T. S. Chow, C. A. Liu and R. C. Penwell, J. Polymer Sci., Polymer Physics Edn 14, 1311 (1976). 52. D. Y. Perera, J. Oil Colour Chem. Assoc. 68,275 (1985). 53. E. Goren, 0. Chusid (Youngman) and D. Aurbach, J. electrochem. Sot. 138(S), L6, May (1991). _.
.
,