Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Shaping by extrusion
Patent
1997-03-26
1999-07-13
Derrington, James
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Shaping by extrusion
264639, 264650, 26417711, 26421111, 264443, 264 41, 264109, C04B 35622
Patent
active
059222727
DESCRIPTION:
BRIEF SUMMARY
The invention relates to ceramic articles and in particular to a method of making a mouldable ceramic composition that can be moulded into a ceramic article of high density.
Present methods for making such articles by, for example extrusion or injection moulding, use a dispersion of suitable ingredients and a polymer binder system. Unfortunately, a high polymer content is required and this leads to problems when the article is fired following moulding as the removal of the polymer by combustion can create defects in the product as well as causing pollution.
It is an object of the invention to provide an improved method of making a mouldable ceramic composition from ceramic powder particles.
Accordingly in one aspect the invention provides a method of making a ceramic article by moulding, the method comprising coating particles of ceramic powder by mixing them with an aqueous solution of an electrically charged polymeric material such that the particles acquire an electric charge and a suspension is formed; coagulating the coated particles in the suspension to form a moulding composition; moulding the composition to form the article in the green state; and then dRying and sintering.
In another aspect the invention provides a method of making a mouldable ceramic composition from ceramic powder, the method comprising:
1. Mixing water and electrically charged polymer to form a solution, then adding the ceramic powder to the solution, whereby the ceramic particles acquire an electrically charged polymeric coating and so form a suspension.
2. Modifying the surface charges on the coated particles to cause coagulation.
Preferably the composition is in paste form. By the method of the invention, a ceramic composition may be moulded using established techniques to form ceramic articles of high density and with little or no strength defects while avoiding the release of pollution combustion products.
The invention includes a ceramic article moulded from a ceramic moulding composition and having a high density.
The ceramic particles may be oxides, silicides, carbides, nitrides and the like. Specific materials include alumina, silica, zirconia, barium titanate, silicon carbide; and the like.
The effect of the charged polymer is to disperse the ceramic particles by coating them with a surface charge which has the effect of repelling adjacent particles. The content of the charged polymer is preferably less than about 1 wt % relative to the total weight, most preferably below about 0.5 wt % for charged polymers of average molecular weight of about 3500. Contents of up to 5 wt % or more may be required for higher molecular weight charged polymers. Excessive concentrations of charged polymer may lead to depletion flocculation, and so should be avoided. The polymer material may be ammonium polyacrylate, a polyacrylate derivative, a cellulose derivative, or the like
The water to which the charged polymer is added is preferably deionised.
When the ceramic powder is added to the polymer solution, it may be in sufficient proportion to produce a highly concentrated suspension, which is subsequently subjected to ultrasonic agitation. Alternatively, the suspension may be prepared as above but with a lower powder concentration, to enable the fractionation of hard agglomerates. The suspension is then consolidated to a high concentrated suspension.
If necessary a small quantity of polymeric binder may be added to provide adequate green strength to the moulded article and prevent water migration during the forming process. The polymeric binder material may be used, for example, in an amount of up to 2% of the total suspension weight. The binder provides green strength to the material and can prevent excessive migration of water during the shaping process. The polymeric binder material is preferably selected to be one which does not interact with the charged polymer. It may be, for example, polyvinyl alcohol, polyethylene glycol or a cellulose derivative; or the like.
The coagulation step is achieved by modifying the charge on the surfa
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Portions of a book entitled Organic Additives and Ceramic Processing with Applications in Powder Metallurgy, Ink, and Paint by Daniel J. Shanefield.
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Binner Jon
Davies Jason
McDermott Andrew
Sambrook Rodney Martin
Derrington James
Dytech Corporation Limited
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