Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Shaping by extrusion
Patent
1996-11-04
1998-06-30
Derrington, James
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Shaping by extrusion
264639, 264645, 264669, 264670, 264432, C04B 3806
Patent
active
057729530
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the production of ceramic articles by making shaped bodies from a monomer containing dispersion of ceramic particles, polymerising to form a green body which is then dried and sintered. In particular to a method of producing ceramic articles of high density by a process called gel casting.
Gelcasting is a technique for moulding ceramic powders and is outlined in articles in J Amer Ceram.Soc 74(3) 612-18 (1991) and Ceramic Bulletin 70(10) 1641-1649 (1991); and in a paper "Forming of Silicon Nitride by Gelcasting", Soc. Automotive Engineers, Proc. Annual Automotive Technology Development Contractors Coordination Meeting pages 243 and 245-251, 1991. In this method an article is cast from a concentrated aqueous suspension of a ceramic powder in a solution of a polymerisable monomer and then polymerising. Two systems have been proposed: one based on acrylamide and the other based on acrylates. In the first case an acrylamide undergoes a vinyl polymerisation and the product is a strong crosslinked polymer-water gel filled with ceramic powder. This is dried and processed further. The polymerisation of the acrylamide is carried out using an initiator and catalyst, together with a crosslinking compound such as N,N'-methylenebisacrylamide. Ammonium persulphate can be used as the initiator and tetramethylethylenediamine as the catalyst. Such a system is the subject of U.S. Pat. Nos. 5,028,362, 5,082,607 and 5,145,908.
The second proposal is based on the use of acrylates. The paper in Ceramic Bulletin Vol 70(1), 1991 reports extensively on different acrylate systems tested and on page 1645 concludes: drawbacks. (1) They are not purely aqueous systems as a cosolvent for example NMP in addition to water is needed. (2) The premix solution inevitably produced an incomplete gel either as a result of solvent overloading or syneresis. (3) The incomplete gelation of the premix solution extended to the slurry mixture, which also only gelled partially. The presence of initiators led to unpredictable gelling reactions even at room temperature as the premix aged. (4) The dispersion of RC-152 in this system was poor. At 50% solids loading only DARVAN C gave a slurry that would just flow. Even then the slurry was dilatant. Because of the problems encountered in the acrylate systems there was therefore a need to develop a process that used only water as the solvent and that did not possess the limitations of the acrylate system. The acrylamide-based system was developed to meet this need."
It has now been discovered that by selection of a class of acrylates the drawbacks indicated above can be side-stepped, allowing gelcasting to take place effectively and to produce articles of notably high density. The acrylates in question are those which are essentially water soluble and which leave no residues after heat treatment.
According to the invention in one aspect there is provided a method of making ceramic article, the method comprising the steps of material, and as a polymerisable monomer an ammonium acrylate and/or ammonium methacrylate, particles and the water ceramic particles in a matrix of the polymer, and densification of the formed ceramic article.
The selected polymerisable monomer will provide a matrix, e.g. a gelled network when polymerised, to stabilise the ceramic particles before and after removal of the water. In contrast to acrylamide the water soluble acrylate monomers are of low toxicity. The acrylate monomer is selected to be substantially or wholly water soluble so that co-solvents are avoided and a dispersion and matrix can form which is why water-soluble ammonium acrylate and ammonium methacrylate are used.
It is preferred to form a premix solution comprising the monomer (or monomers), a crosslinking agent, e.g. N,N'-methylenebisacrylamide and water (preferably deionized). The polymerisation may be initiated by a suitable initiator such as ammonium persulphate or the like; and catalysed by a suitable catalyst such as triethanolamine or tetramethyethylenediamine or heat generated, e.g. by
REFERENCES:
patent: 3998917 (1976-12-01), Adelman
patent: 4587068 (1986-05-01), Borase
patent: 4894194 (1990-01-01), Janney
patent: 4906424 (1990-03-01), Hughes
patent: 4978643 (1990-12-01), Venkataswamy
patent: 5028362 (1991-07-01), Janney
patent: 5082607 (1992-01-01), Tanse
patent: 5145908 (1992-09-01), Janney
Schildknecht, Calvin E., Vinyl and Related Polymers, John Wiley & Sons, New York, 1952, p. 299.
Sambrook Rodney Martin
Smith Robert Terence
Derrington James
Dytech Corporation, Ltd.
LandOfFree
Production of ceramic articles does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Production of ceramic articles, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Production of ceramic articles will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1855936