1, What is tape casting
The casting method involves adding binders, solvents, etc. to the powder, followed by ball milling, filtration, vacuum defoaming, and controlling the viscosity within a certain range. This viscous slurry, under constant pressure, flows through the gap between the slurry scraper and the silicone coated film tape running at a certain speed, and adheres to the film tape. After drying and trimming at intervals, the green tape and film tape are separated (or never separated) at the tail of the casting machine, and then wound in sequence to obtain a green tape. Then proceed to the next process, such as cutting, stacking, printing, sintering, etc., to produce the desired product, thus forming the casting preparation process.
The most critical aspects of the casting process are the preparation of the slurry and the casting molding process.
2, Application of tape casting
Engineering flowchart for the preparation of ceramic flakes by casting method
By using a series of processes such as ball milling to remove organic or inorganic residues from the slurry, a smooth and uniform casting slurry can be obtained. The prepared casting slurry is cast into shape on a casting machine, and can be completed through wetting, cutting, degreasing, and sintering.
3, Introduction to Common Additives in Slurry
1. Solvent
The important factors to consider when choosing a solvent are: (1) it must be able to dissolve other added components, such as dispersants, binders, and plasticizers; (2) It has a certain chemical stability in the slurry and will never undergo chemical reactions with the powder; (3) Easy to evaporate and burn off; (4) It is difficult to use safety, medical care, and environmental pollution.
The most common solvents are divided into two categories: organic solvents and water.
The slurry obtained from organic solvents has high viscosity, slow solvent evaporation, and long humidity time, so organic solvents are relatively more commonly used in the film casting method. Common ones include ethanol, methyl ethyl ketone, trichloroethylene, toluene, xylene, etc. However, organic solvents have the disadvantages of flammability and toxicity.
Water as a solvent also has advantages such as high cost, safe use, medical and convenient comprehensive production. Its disadvantages are: (1) it has less moisture resistance for powder particles, evaporates quickly and has a long moisture time; (2) It is difficult to remove gas from the slurry, and the presence of bubbles can affect the quality of the embryo membrane; (3) The remaining binder used in water-based slurry is an emulsion, which is difficult to produce in the market and limits the selection of binders. The importance of solvents for the moisture performance of powders is related to their surface tension. The higher the surface tension, the better the moisture performance of powder particles. The surface tension of organic solvents is much higher than that of water, so their moisture performance is better than that of water. The comprehensive properties such as surface tension and dielectric constant of mixed solvents are better than those of individual components, and their boiling point is high. However, their solubility for dispersants, binders, and plasticizers is also relatively poor. To ensure evaporation during the wet process, binary azeotropic mixtures are often used in the casting slurry. The most common ones are ethanol/methyl ethanol, ethanol/trichloroethylene, ethanol/water, and trichloroethylene/methyl ethyl ketone.
2. Dispersant
The concentration and uniformity of powder in the casting slurry indirectly affect the quality of the blank film, which in turn affects a series of characteristics such as material density, porosity, and mechanical properties. There are four common types of dispersants in the film casting method: non-ionic, anionic, cationic, and zwitterionic. Generally speaking, anionic surfactants are important for neutral or weakly alkaline slurries with positively charged particle surfaces, while cationic surfactants are important for neutral or weakly acidic slurries with negatively charged particle surfaces. Mikeska et al. found through experimental research on the concentration effect of 70 commercial dispersants that phosphoric acid esters, ethoxylated compounds, and fresh fish oil had the best concentration effect in ceramic powder slurry. The latter two are anionic surfactants, so fresh fish oil is definitely not a surfactant.
3. Adhesives and plasticizers
To facilitate separation and handling from the substrate material, the cast film must have certain strength, toughness, and ductility. As a result, binders and plasticizers must be added to the slurry. The factors to consider when choosing a bonding agent include: (1) the thickness of the blank film; (2) The selected solvent type and compatibility are conducive to solvent evaporation and the absence of bubbles; (3) It should be easy to burn and leave no residue; (4) Can play a role in stabilizing the slurry and suppressing particle settling; (5) A relatively high plastic transition temperature is required to ensure that no condensation occurs at room temperature; (6) Considering the properties of the substrate materials used, they must never adhere to each other or be easily separated. Adhesives are classified into three types based on the functional groups they act on: non-ionic, anionic, and cationic. The most commonly used binders in the casting process are anionic and non-ionic binders, including ethylene and propylene based binders. Common binders in non-aqueous slurries include PVB, methyl polyacrylate, and ethyl cellulose. Common binders in water-based media include polyvinyl alcohol, acrylic acid emulsion, and polyacrylamide salts. The main function of plasticizers in the slurry is to lower the plastic limit temperature Tg of the binder, so that the Tg reaches room temperature or below, thereby ensuring that the binder has good fluidity and does not set at room temperature. In addition, plasticizers also have a lubricating and bridging effect on powder particles, which is beneficial for the concentration and stability of the slurry. However, adding plasticizers will reduce the strength of the blank film. The most common plasticizers include polyethylene glycol, phthalates, ethylene glycol, etc., and their effects on the rheological properties of the slurry are not very similar. Phthalates can lubricate powder particles and reduce the viscosity of the slurry, so formaldehyde also forms organic bridges in the powder particles, which can increase the viscosity of the slurry.
4 Other additives
In addition, functional organic additives are often added during the preparation process of the casting slurry to produce specific slurry properties or perfect dry tape properties.
Defoamer: It is important to use in low molecular solution (PVA) or stable foam water medium where polymer concentration system is not easy to cause harm, especially in the process of stirring (such as using specific wax or using vacuum stirring). Preventing foam production should be more precise than eliminating foam. The common methods for removing bubbles are mechanical and chemical methods. Add foaming agent to the slurry and then perform vacuum stirring to remove gas. The common defoamer is a mixture of half butanol and half ethylene glycol.
Wetting agent: It is an important surface activator that is soluble in the liquid phase, used to reduce the surface tension of the liquid (especially water) and improve their wetting properties for powders and reactants. So, surfactants are also used as dispersants. Homogenizer: Used to increase the mutual solubility of components (such as cyclohexanone), thereby preventing peeling when wet. They also increase the density and tensile strength of the substrate.
Flow control agent: Sometimes a small amount is added to prevent the surface of the substrate from becoming too wet and slow, and to prevent cracking.
Flocculant: a reagent that prevents the formation of such low-density precipitates in a concentrated system.
4, Advantages of Casting Process
Low degree of automation and low production efficiency. Good product quality with low pass rate. Adapt to the production of large-scale and diversified substrates.
