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Forming method of special ceramics, types and dosage of binders
Examples of adhesives used in the molding method
Polyvinyl butyral by thousand pressure method, etc. 1~5
Casting propylene-based resin 1~3
5~ 15 methyl cellulose is extruded.
Inject polypropylene, etc. 10~25
Preparation of ammonium polycarboxylate 0~3 by isostatic pressing.
Binders can be divided into lubricants, plasticizers, dispersants, surfactants (with dispersing and lubricating functions) and so on. In order to meet the molding needs, a combination of various organic materials is usually used. When choosing an adhesive, the following factors should be considered:
1) The binder must be wettable by powder. When the critical surface tension (yoc) or surface free energy (yos) of powder is greater than the surface tension (yoc) of adhesive, it can be well wetted.
2) A good binder is easy to be fully wetted by powder and has strong adhesion. When the adhesive is wetted by the powder, there is a gravitational interaction between intermolecular forces, and a red bond (primary bond) occurs between the adhesive and the powder. At the same time, due to the effects of orientation, induction and dispersion, cohesion (secondary combination) occurs in the adhesive molecules. Although water can fully wet poplar, it is not a good binder because of its high volatility, small molecular weight and small cohesive force. According to the combination order of various organic materials, they can be arranged as follows according to the basis:
A CONH a >; -con H2 & gt； ; A COOH & gt；; -Oh>；; -NO2 & gt； ; -cooc2h 5 & gt； ; COOCH5 & gt； -CHO & gt； = CO & gt； -CH3 & gt； = CH2 & gt； -CH2
3) The molecular weight of the binder should be moderate. In order to fully wet, the molecular weight is small, but the cohesion is weak. With the increase of molecular weight, the binding capacity is enhanced. However, when the molecular weight is too large, the surrounding cohesive force is too large to wet, which is easy to deform the green body. In order to help the movement of intramolecular segments, plasticizer should be added properly at this time to make the adhesive more flexible and easier to form while being easy to wet.
4) In order to ensure the quality of products, it is also necessary to prevent people from mixing impurities from binders, raw materials and preparation procedures, resulting in harmful defects of products.
In the preparation of raw materials, mechanical methods such as crushing and mixing are used to cooperate with binder and dispersant to realize dispersion, and agglomeration particles are not contained as much as possible. Binders are affected by the type and molecular weight, particle surface properties and solvent solubility. And is adsorbed on the surface of raw material particles, so that the agglomeration of powder raw materials can be prevented through the stereoscopic stability effect. In the molding process, the binder endows the raw materials with plasticity, has the function of water retention, and improves the strength and construction workability of the molded body. Generally speaking, the binder is decomposed and volatilized by heating during degreasing, because it hinders the sintering of ceramics. Therefore, it is necessary to choose organic materials that are easy to disperse and remove, and do not contain harmful inorganic salts and metal ions to ensure the quality of products. Special ceramic materials such as silicon nitride have excellent properties such as high strength, high wear resistance, low density (light weight), heat resistance and corrosion resistance, and are suitable for manufacturing automotive ceramic parts such as turbine feed impeller, rocker burner and auxiliary combustion chamber. These components require complex shapes, high-precision dimensions and high reliability. Internal defects (cracks, pores, foreign bodies, etc.). ) and surface defects are not allowed.
One of the molding technologies that can meet these quality requirements is ceramic injection molding. Ceramic injection molding technology comes from the injection molding of polymer materials, which is based on the characteristics of polymer melting at high temperature and curing at low temperature, and then the polymer is removed after molding. Compared with the traditional ceramic processing technology, it is much simpler, can manufacture high-precision ceramic parts with various complex shapes, and is easy for large-scale and automatic production.
CiM (Ceramic Colloid Injection Molding Method and Device) invented by Professor Yang Jinlong of Tsinghua University Department of Materials Science and Engineering is at the leading level in China.
Ceramic injection molding technology has many advantages. Using it to prepare ceramic components with complex shapes not only has high dimensional accuracy and good surface condition, but also saves post-treatment operation, reduces production cost and shortens production cycle. It also has the characteristics of high degree of automation and is suitable for mass production. The process generally includes the following steps: selection of ceramic powder, selection of binder, uniform mixing of ceramic powder and binder, injection molding, degreasing and sintering. Degreasing is the key.
The original ceramic molding injection technology is to mix a large amount of polymer resin with ceramic powder to get a mixture, then put it into an injection molding machine and inject it into the mold at a certain temperature, and then quickly condense and demould it to make a blank. This technology is suitable for preparing products with high green strength, high dimensional accuracy, less machining and uniform green body, and is suitable for mass production. It has obvious advantages for preparing products with complex shape and thin thickness. However, due to the existence of a large number of polymer binders, the degreasing of ceramic blanks has become an insurmountable problem, and the blanks are easy to deform and form pores.
Binder can fill the powder into the expected shape, which has an important influence on the whole process. The ideal adhesive should have the following characteristics:
1) The viscosity of the pure binder is below1pa s at the molding temperature, and it does not separate from the powder when flowing, and it has sufficient strength and hardness after cooling;
2) It is inert and does not react with powder;
3) decomposing above the molding mixing temperature, and the decomposed products are non-toxic and non-corrosive, with less residual ash;
4) Low expansion coefficient and low residual stress caused by thermal expansion or crystallization;
5) It meets the requirements of environmental protection, is cheap, safe, moisture-free, contains no volatile components and has a long shelf life.
Most adhesives used can be divided into three categories: wax-based or oil-based adhesives, water-based adhesives and solid polymer solutions. Wax-based binders usually contain 3-4 components, and the polymer controls the flowing viscosity, the strength of the green body (green body before sintering) and the degreasing characteristics. Short molecular chains have good formability and can minimize orientation in molded components. Wax or oil is the main filler and is removed in the initial stage of degreasing. Surfactants are used to improve the compatibility between powder and binder. Plasticizers are used to adjust the flow characteristics of polymers. Water-based adhesives contain water-soluble polymers, gels or water glasses. This kind of binder usually adopts low pressure molding to avoid the separation of powder and binder, and reduce die wear and residual stress. Because water is easy to remove, it makes it possible to make thicker parts. The curing or gelation of the binder solution imparts strength to the green body. Before sintering, water is evaporated or sublimated from the green body to minimize deformation. A new adhesive formula of polystyrene solid polymer solution is adopted to avoid deformation. The main filler is removed by solution impregnation. Because the skeleton structure of polystyrene will not be weakened, the deformation of raw products is avoided. The main filler is small organic molecules, which have both benzene rings and polar groups. Benzene rings make it soluble in polystyrene when mixed, while polar groups make it soluble in solvents such as water or alcohol when degreased.
Common adhesives include polypropylene (PP), random polypropylene (APP), polyethylene (PE), ethylene-vinyl acetate copolymer (EVA), polystyrene (PS), acrylic resin and so on. Among them, PE has excellent formability; EVA has good compatibility, fluidity and moldability with other resins. APP has the characteristics of good compatibility with other resins, good fluidity and degreasing; PS has good liquidity. Additives include paraffin, microcrystalline paraffin, denatured paraffin, natural paraffin, stearic acid, compounding agent, etc. The fluidity of molding materials can be evaluated by using high-type flow point tester and melt index instrument. When there is a lot of binder, degreasing tends to decrease, and when there is more paraffin as additive, degreasing is good. If all organic materials can't fly away in a specific temperature region, it will affect the sintering of ceramics, so it needs to consider the thermal decomposition characteristics to choose. Cordierite is widely used as the carrier of automobile exhaust purification catalyst because of its excellent material properties such as heat resistance, corrosion resistance, porosity and low thermal expansion. Cordierite honeycomb uses the orientation of raw material particles to produce a honeycomb structure with low thermal expansion, which can be manufactured by extrusion molding.
According to the molecular composition of cordierite (2mo2al2o35sio2), talc, kaolin and alumina can be used as raw materials. The green soil for molding enters the cover from the feed hole in the cover, expands to the thin wall after subdivision, and then combines, thus obtaining the quality with good ductility and cohesiveness. In addition, as the extruded honeycomb, in order to keep the shape, the higher the yield value of the brick, that is to say, the choice of binder should optimize the fluidity and self-preservation of the brick.
Raw material powder, adhesive, additive (lubricant, surfactant, etc.). ) and water are mechanically kneaded, and then continuously extruded by a screw extruder or extruded by a hydraulic plunger extruder. Generally speaking, adhesives used in extrusion molding can show high viscosity as long as low concentration aqueous solution is used. Commonly used are methyl cellulose (MC), carboxymethyl cellulose (CMC), polyoxyethylene (PEO), polyvinyl alcohol (PVA), hydroxyethyl cellulose (HEC) and so on. MC is easily soluble in water and gels rapidly after heating. CMC is soluble in water and has high dispersibility and stability. PVA is widely used in various forms. Lubricant can reduce the friction between powders, and surfactant can improve the wettability of raw powder with water.
The lack of plasticity and expansion of adobe makes extrusion not smooth enough and increases surface defects. Therefore, the performance of the adhesive should be evaluated. The methods to evaluate the plasticity of backfill soil include applying torsion, compression, tension and other stresses, finding the relationship between stress and deformation, using capillary rheometer, viscoelasticity and so on. This method can be used to evaluate the self-preservation and fluidity of adobe. When evaluated by viscoelastic method, it can be concluded that when the amount of binder increases to a certain extent, self-preservation and fluidity will increase. In other words, the increase in the amount of binder contributes to the increase in the plasticity of raw materials.
Organic materials are the main binders of special ceramics, and reasonable selection of these organic materials is the key to ensure product quality. In production, comprehensive selection should be made according to the characteristics of powder, product shape and molding method.