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In the forming process, tape casting is used to form large-scale thin-plate ceramic or metal parts
.
Such parts are almost impossible or difficult to be molded by pressing or extrusion, but it is very easy to manufacture blanks of various sizes and shapes by casting, and the quality of the blanks can be guaranteed
.
Traditional casting technology uses organic solvents as solvents, which is also called non-aqueous casting.
However, because organic solvents can cause certain harm to the human body and the environment, it has been studied to use water or alcohol as solvents instead of toxic solvents.
Become a trend in coating online coatingol.
com
.
5 to 500 microns (0.
2 to 20 mils) thickness of green ceramic sheet casting
Suitable for solvent-based and water-based slurry casting (KEKO casting machine)
Many ceramic powders, such as Al2O3, ZrO2, SiC, Si3N4, BaTiO3, AlN, etc.
, can be molded by water-based tape casting
.
Compared with non-water-based casting, the water-based casting slurry system is more complicated
.
In water-based casting slurry, since polar molecular water is used as the solvent, and most of the powders are non-charged, barren powders, the water-based casting slurry is easy to precipitate and layer, and its rheology and stability are relatively high.
Poor, so additives such as dispersant, binder, and plasticizer must be added in the slurry configuration process to make the slurry meet the requirements of the casting process
.
Let's take a look at the additives that play a key role in the success or failure of water-based casting
.
01
Adhesive system
The adhesive mainly plays the role of linking ceramic powder particles, providing green strength, ensuring the formation of the green sheet, and controlling the rheological behavior in the casting process.
At present, the more commonly used adhesives in water-based casting systems are mainly as follows Species system
The factors that should be considered when selecting a binder are: 1) the thickness of the blank film; 2) easy to burn off without leaving residue; 3) can stabilize the slurry and inhibit particle settling; 4) have a better Low plastic transition temperature to ensure that no condensation occurs at room temperature; 5) Considering the nature of the substrate material used, it is easy to separate the green film
.
At present, there are two types of binders commonly used in water-based casting processes: cellulose and ethylene or acrylic polymers
.
Cellulose is usually divided into ionic and non-ionic types.
The former is a polyelectrolyte, which can act as a binder and a dispersant at the same time, while the latter mainly acts as a binder.
The ethylene additive is a CC long-chain structure with a terminal There are free radicals that can be combined with other atoms, and if two functional groups are combined, an acrylic structure is formed
.
Table 1: Commonly used binders for water-based casting
glue | Application characteristics |
Carboxylate monomer | Such as acrylic and so on |
Polyvinyl alcohol | The cast sheet has high strength, but the polyvinyl alcohol (PVA) binder system also has its shortcomings: that is, the viscosity of the slurry is large, the solid content of the ceramic powder is small, the volume fraction is generally below 35%, and the drying time is particularly Long, easy to deform after drying, etc. |
Acrylamide monomer | The gelation reaction can be initiated by adding catalysts and initiators, thereby acting as a binder. |
Cellulose | The disadvantage is that the solid content of the slurry is low, and the density of the cast sheet is low |
Latex system | The advantages are that the ceramic powder has high solid content, fast drying speed, the relative density of the cast sheet can reach 55%, and the organic content is low, and it is easy to burn off. |
02
Plasticizer
Since the addition of the binder can only ensure the strength of the green body but cannot make the green body have sufficient processing toughness, it will also make the glass transition temperature Tg of the organic matter higher than room temperature
.
In order to change this situation; adding a certain amount of plasticizer can reduce the glass transition temperature Tg of the binder, thereby reducing the glass transition temperature Tg of the entire slurry system, so that the binder has a good performance at room temperature or below.
Fluidity and film-forming properties; in addition, the plasticizer also plays a role in lubricating and bridging the powder particles, which is beneficial to the dispersion and stability of the slurry
.
The mechanism of action of the plasticizer is that the plasticizer molecules are inserted between the polymer molecular chains of the binder to weaken the stress between the polymer molecular chains, thereby increasing the mobility of the polymer molecular chains and reducing the crystallinity of the polymer molecular chains.
, To increase the plasticity of the polymer
Table 2: Commonly used plasticizers for water-based casting
Water-soluble plasticizers that can be used for water-based casting include alcohols such as glycerol (glycerin), polymers such as polyethylene glycol (PEG) or polyacrylic alcohol (PPG), and phthalic acid additives such as benzyl Butylphthalic acid (BBP) and dibutylphthalic acid (DBP)
.
03
Dispersant
The degree of dispersion of ceramic particles in the solvent has a very important influence on the microstructure of the green film
.
The dispersant used in the water-based casting process can be divided into two types: ionic (anionic, cationic and zwitterionic) and non-ionic according to molecular surface active groups and charging conditions
.
At present, the commonly used dispersant is polyelectrolyte, and its stabilization mechanism is mainly electrostatic-steric hindrance stabilization mechanism, such as NH4-PA, NH4-PMA and polyacrylic acid
