A Practical Aqueous Tape Casting Binder

May 1, 2007
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An optimized aqueous binder system for tape casting offers advantages over other aqueous systems, and also outperforms typical solvent-based systems like polyvinyl butyral (PVB) and acrylics.

SEMs of green tape showing 100 nm barium titanate completely dispersed at 63 volume % using WB binder. Photos courtesy of TPL, Inc., Albuquerque, N.M.

Tape casting is a popular and economical means of producing flexible green sheets of ceramic in a wide range of thicknesses. These green sheets can be fired as single sheets, or they can be laminated into complex structures that often incorporate different ceramic or conductive metal layers.

Fired tape cast parts are typically flat pieces, but other shapes-including tubes-are possible. Thicknesses of tape-cast ceramics can range from several microns to over a millimeter. Tape casting is commonly used in the manufacture of electronic and high-tech ceramic products like multilayer ceramic capacitors (MLCCs), substrates and fuel cells, as well as low-temperature co-fired ceramic (LTCC) and high-temperature co-fired ceramic (HTCC) packages, to name a few.

Tape casting involves the dispersion of a ceramic powder in a liquid, which typically also contains polymeric binders, dispersants, plasticizers and other additives to achieve certain characteristics. The final liquid ceramic dispersion is very similar to paint, wherein the ceramic is the pigment. This paint, or slip, as it is commonly called in the ceramic industry, is then processed into final dry films through the use of a tape casting machine.

In the arena of home and industrial paints and coatings, aqueous-based paints have steadily increased in market share at the expense of their solvent-based counterparts, due mainly to a variety of safety, economic and governmental pressures. Aqueous binders have been used in tape casting for many years, but they have not reached a high level of popularity due to real or perceived problems in use or performance.

Water-based binders used in tape casting are typically emulsion- or water- soluble types. Developed in the mid-1990s and available commercially in 2001, one aqueous binder system, termed WB, is based on variably soluble acrylic polymers.*

*WB4101, developed by Polymer Innovations, Inc., Vista, Calif.

Aqueous Binders

Emulsion binders are typically stable suspensions of submicron particles of insoluble polymers (most commonly acrylic) in water. The stability of these emulsions can be destroyed by a number of common stresses, including freezing or excessive heat during shipping or storage; mechanical stresses similar to those encountered in the dispersion of the ceramic powder; and contact with solvents, additives and various ions that may dissolve from the ceramic powder itself. The result is the irreversible coagulation of the insoluble polymer into a large lump, making it useless as a ceramic binder.

During the drying process, water is removed from the emulsion and dispersed ceramic, which then allows the water-insoluble submicron polymer particles to coalesce into a final dry film containing ceramic particles. Typically, the final dry cast tape is not soluble in water, which makes it difficult to re-dissolve and recast any tape scrap.

The other binder group is water soluble, such as polyvinyl alcohol (PVA). In this system, the polymer is actually dissolved in water with the dispersed ceramic. Although this system is typically stable to mechanical and thermal stresses, one disadvantage is the relatively high slip viscosity, which requires lower ceramic loading in the liquid phase. Other problems are related to the hydroxyl groups in the polymer being reactive to certain common ceramic additives, such as boron, and being prone to hydrogen bonding effects.

The result can be a tape that requires very severe lamination conditions and a narrow range of usable ceramic powders. Since the polymer in these tapes is water soluble, there is the potential for re-dissolving and re-casting scrap tapes. However, this also means that the properties of these cast tapes are sensitive to the humidity in the air, which can cause variability in downstream processes.

A Better Binder

The WB binder system offers advantages over other aqueous systems, and also outperforms typical solvent-based systems like polyvinyl butyral (PVB) and acrylics. The system is a high-molecular-weight acrylic polymer with copolymerized active groups. The molecular weight, acrylic monomer composition and number of active groups have been optimized for tape casting, and can be used in both pure aqueous and aqueous/solvent hybrid systems.

The base polymer is not soluble in water until the active groups are activated via minute amounts of base, such as ammonium hydroxide. After activation, the binder can dissolve in water and be used to cast tape. During drying, this small amount of ammonium hydroxide evaporates with the water, yielding water-insoluble and relatively humidity-insensitive ceramic tape. Yet the re-addition of a small amount of ammonium hydroxide and water allows the tape to be re-dissolved and cast, allowing effective reworking of scrap tape.

The incorporation of the active groups in the binder molecule also provides the ability to control the viscosity of the binder and slip solutions, and allows the binder itself to become a very strong ceramic dispersant. This strong dispersing property enables higher green tape densities than those that can be achieved with most other aqueous or solvent tape casting binders. It also allows complete dispersion of nanoparticles to their base particle size, as well as high loading levels typical of much coarser powders.

The binder and tape properties are further influenced by the type of base used to activate the active groups of the binder. The result is a wide range of additives that have been developed to manipulate binder properties for a variety of special applications, such as ultra-thin and thick casting, tape tack and lamination, nanopowders, and the utilization of a wide range of ceramic powders typically too reactive for other solvent or aqueous binders. Table 1 shows how the different groups of binders compare to one another.

The WB system has eliminated many of the common problems associated with available aqueous binder systems, but how does it compare to solvent-based processing? In general, aqueous solvents enjoy many advantages over organic solvents, including non-flammability, low hazard, low cost, ease of transportation and safe water cleanup.

In the past, it was felt that aqueous binder performance was generally inferior to solvent-based systems, and was incapable of making dependable ultra-thin ceramic tapes. However, the performance improvements of the WB system have met and exceeded the tape performance typical of the most popular PVB solvent-based systems. In particular, higher loading with dramatically better tacking and lamination characteristics can be achieved with WB (see Table 2).

Important Considerations

Some complications related to water in comparison with solvents do require consideration. For example, while water has a set drying rate, the evaporation of a solvent can be modified for different drying rates simply by changing the solvents or solvent mix. Other considerations include water's difficulty in wetting low-surface-energy casting substrates (such as silicone-treated PET film), and the increased stabilization of foam in water, which can be created in the handling of the slip.

However, the drying rate when using a water-based system can be easily controlled by adjusting the drying temperature. In addition, the WB system's special binder formulation, combined with the high level of available technical assistance, ensures that these complications are all easily manageable.

For more information regarding binders for tape casting, contact Polymer Innovations, Inc., 2426 Cades Way, Vista, CA 92081; (760) 598-0500; fax (760) 727-3127; e-mail mark@polymerinnovations.com; or visit www.polymerinnovations.com.

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