A Binding Matter: The Role of Binders in the Ceramic Sintering Process
Poly(alkylene carbonate) binders have been shown to burn out cleanly and uniformly, while providing high green strength to refractory products.
It is widely understood that the use of additives and processing aids is critical in the forming stage for ceramic materials. These additives include binders, plasticizers, dispersants, surfactants and lubricants. Each has its own specific use in the ceramic system, but all are a potential source of contamination and therefore can affect the production cycle, rejection rates and properties of the finished product, and ultimately the overall manufacturing cost.
Binders have been called the most important processing additive of the ceramic sintering process, and along with plasticizers, account for the bulk of all additives used in the ceramic processing industry. An effective binder will hold dry powders or aggregate together during sintering, burning out cleanly and uniformly while providing exceptional green strength to the sintered parts.
Poly(alkylene carbonates)* are a family of organic polymers that possess a number of unique characteristics that make them ideal for use as binders for ceramic powders, especially alumina and silicon carbide, two well-known refractory-grade ceramic materials commonly formed by way of various pressing, extrusion, slip casting (tape casting) and powder injection molding (PIM) operations.
Properties of Poly(Alkylene Carbonate) Binders
The poly(alkylene carbonate) binders are synthesized through the reaction of carbon dioxide and epoxides. Two poly(alkylene carbonate) polymers are readily available as binders—poly(propylene carbonate) and poly(ethylene carbonate). Both have properties that translate into superior performance for advanced ceramics.
Decomposition of poly(alkylene carbonate) binders is achieved through three phases—solid, liquid and vapor. The poly(alkylene carbonate) binders decompose completely in air below 300°C, at temperatures at least 100°C less than conventional binders. Complete burnout in nitrogen and argon and reducing atmospheres that contain hydrogen is possible at temperatures as low as 360°C; under vacuum, poly(alkylene carbonate) burnout temperatures are even lower. Poly(alkylene carbonate) binders burn out completely, and the products of their combustion—carbon dioxide and water vapor—are non-toxic, non-flammable and environmentally safe.
Many refractory-type ceramics find end uses in applications in the electronics industry as capacitors, piezoelectrics, insulators and sensors, all requiring high purity. Poly(alkylene carbonate) binders have been shown to yield strong green sintered parts that are virtually contaminant-free. Ash residues are typically less than 50 parts per million (ppm) for the pure binder. Based on 3% binder use (a typical use amount), residual ash in finished parts is well under 2 ppm, suitable for applications where purity is essential.
The low sodium levels of the binders are also encouraging for those producing dielectric materials, which also have critical purity requirements. Based on 3% binder levels, poly(alkylene carbonate) binders exhibit less than 0.3 ppm residual sodium.
Unlike other binders, poly(alkylene carbonate) binders are also unique in that they burn out mildly, without violent gas formation. Therefore, fewer rejects due to cracking and variations in thermal expansion can be expected. Decomposition can be easily predicted, allowing for more reliable control.
Poly(propylene carbonate) binders are amorphous with an easily reached glass transition temperature (Tg) of 40°C. Poly(ethylene carbonate) binders are also amorphous, but with a Tg of 25°C. The Tg can be further lowered with the addition of propylene carbonate, a monomer co-produced during the polymerization process of poly(propylene carbonate).
Poly(alkylene carbonate) binders are soluble in a wide range of polar organic solvents, including:
- methylene chloride
- methyl ethyl ketone (MEK)
- ethyl acetate
- chlorinated hydrocarbons
The binders are insoluble in alcohol, ethylene glycol and aliphatic hydrocarbons. Although they are also insoluble in water, stable aqueous emulsions are available on a custom basis for use in water-based processes.
The poly(propylene carbonate) form of poly(alkylene carbonate) binders performed well in tests with numerous ceramic powders including the following:
- Alcoa alumina (A-12, A-14, A-16, SG)
- Coors Ceramics Co. (now CoorsTek) ADA-90 alumina
- Reynolds Metals Co. RC-HP-DBM alumina with MgO
- H.C. Stark B-10 silicon carbide
The poly(propylene carbonate) form of poly(alkylene carbonate) has shown significant benefits in ceramic applications. The use of poly(propylene carbonate) over conventional binders, such as polyvinyl alcohol (PVOH) and methylcellulose, in pressing, extrusion, tape casting and PIM operations has been successfully demonstrated.
During the pressing operation, samples coated with poly(propylene carbonate) had higher green densities relative to theoretical densities than those samples coated with no binder, with PVOH and with methylcellulose (see Table 1).
These four samples were sintered in four different atmospheres (air, argon, helium and nitrogen) at 1,500°C for 2 hrs at heating/cooling rates of 2°C/min. In all four atmospheres, the poly(propylene carbonate) burned out more completely than the other three samples (see Table 2).
By using a binder that decomposed cleanly and completely in inert atmospheres, the volume of gas products produced was dramatically reduced during sintering relative to combustion in air. Reducing the gas volume produced during sintering in this manner decreased the likelihood of flaw generation during sintering, thereby increasing the likelihood of crack-free ceramic parts being produced, and allowing for thicker ceramic parts to be manufactured.
Tape Casting and Injection Molding
The tape casting process is often used to form thin, flat ceramic parts, but this process requires that the chosen binder remain flexible, even after drying. One of the unique characteristics of poly(propylene carbonate) is its ability to be plasticized by its own monomer, propylene carbonate. Poly(propylene carbonate) binders also have excellent film forming and coating capabilities, making them an excellent choice for the tape casting process. Extensive work has been done to develop optimal formulations for binding alumina using poly(propylene carbonate) as a binder in MEK and MEK/toluene solvent mixtures. These formulations offer high green strength and density, efficient binder burn-off, and good ability for lamination. Tapes significantly thinner than 1 mil (.001 in.) and as thick as 50 mils (.050 in.) have been produced using poly(propylene carbonate).
Studies have also been carried out comparing a poly(propylene carbonate)-based alumina composition with a wax-based (based on paraffins and microcrystalline waxes) alumina composition in ceramic injection molding trials. Results from these trials suggest dramatic improvements in mean failure stress, from about 230 to over 300 Mpa, when going from the wax mix to the poly(propylene carbonate) mix. Overall, there were fewer flaws in the poly(propylene carbonate) bars than in the bars made from the wax mix composition. In the poly(propylene carbonate) bars, the flaws were limited to the contours of the molding defects that were knit lines in the thick sections. In the wax-mix bars, flaws were evenly distributed and more spherical in nature.
Poly(alkylene carbonate) binders have demonstrated their value commercially over a wide range of ceramic powder forming operations for over a decade. Their unique and valuable properties provide advantages for certain refractory-grade ceramics in pressing, extrusion, tape casting and injection molding processes. As novel and demanding applications involving refractory-type powders increase, binders such as poly(alkylene carbonate) will become even more critical.
For more information about poly(alkylene carbonate) binders, contact Empower Materials, Inc., 100 Interchange Blvd., Newark, DE 19711; (302) 452-6600; fax (302) 452-6610; or visit www.empowermaterials.com.
*Poly(alkylene carbonates) are available commercially as QPAC® binders, a registered trademark of Empower Materials, Inc., Newark, Del.