Compositions: ceramic – Ceramic compositions – Clay containing
Patent
1996-04-23
1998-02-10
Marcheschi, Michael
Compositions: ceramic
Ceramic compositions
Clay containing
501142, 501143, 501144, 501119, 501122, 501128, 501150, 428689, 428697, C04B 3304, C04B 3324
Patent
active
057168946
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of an International Application of PCT/GB94/01963, filed Sep. 9, 1994.
This invention relates to whiteware ceramic compositions, especially those which may be amenable to pressing, slip casting or injection moulding. The whiteware compositions of this invention comprise glasses and many of the same crystalline phases as conventional whiteware compositions, although the proportions of the phases, the compositions of the glassy phases or other aspects will differ. The whitewares covered by this invention include electrical porcelain, sanitaryware and tableware, including bone china and hotelware, but exclude e.g. steatites, aluminas and whitewares used for wall tiles. One whiteware made possible by the invention, a fine translucent whiteware based on anorthite, has no conventional equivalent, being composed mainly of anorthite, a major crystalline component of bone china, glass and small amounts of other crystalline phases such as mullite and possibly some composed of calcia-magnesia-silica. It contains reduced (preferably no) .beta.-tricalcium phosphate, the other major crystalline component of bone china, and so has a much lower coefficient of thermal expansion and an improved thermal shock resistance.
Conventional compositions ("body mixtures") apart from bone china comprise roughly: 1 part by weight of a filler, such as 10 .mu.m quartz, cristobalite, or a mixture of alumina and quartz; 1 part of flux, such as 10 .mu.m feldspar or nepheline syenite; and 2 parts of fines, such as china clay and/or ball clay, which are both mainly kaolinitic platey particles at least an order of magnitude smaller than the filler. (Sizes throughout the specification are median sizes, are somewhat approximate, and are taken as equivalent spherical diameter (e.s.d.) as measured by laser diffraction. Amounts throughout are by weight). The raw clay content of conventional whitewares of the types listed above, ranges from about 25% to greater than about 60%. In some compositions, small quantifies of coarsely ground prefired clay ("chamotte") or coarsely ground whiteware body ("pitchers") are added to reduce shrinkage. These are often termed grogs. The e.s.d. of this component may be up to 1 mm. A bone china composition might comprise 50% by weight of 5 .mu.m calcined bone (hydrated calcium phosphate), 20% of 10 .mu.m Cornish stone flux and 30% micron-to-submicron china clay. These compositions give rise to a reject rate which ought to be taken seriously, many of the faults such as a centre pip or mould lines being attributable to unwanted alignment of the platey clay particles.
However, the clays are present for good reason. They confer plasticity for forming, binding properties giving strength in the unfired state (green strength) and a high specific surface area to drive densification during firing. China clays and ball clays are indeed usually mixed to obtain the required properties. Ball clay particles are much finer, consequently increasing the ball clay content in the mixture improves the plasticity and increases both the green strength and driving force for densification. However, impurities such as iron and titanium which cause unwanted colouration in the fired product are higher in ball clay than in china clay. Occasionally, halloysite is employed as the clay, because it combines a low impurity content with a very fine particle size. Halloysite particles are tubular in shape.
Returning to consider the other components, flux produces a viscous liquid when its alkali oxides interact mainly with the clay and to some extent with siliceous fillers during firing. (Quartz and cristobalite fillers tend to dissolve in liquid flux). Densification occurs through a liquid-phase sintering process, in which viscous flow is the dominant mechanism.
Filler, so called even though it is not usually inert, acts as an antidistortion component by reducing the plasticity of the body in the unfired, wet state and during firing. Quartz and cristobalite when used as fillers also play an important role in increasing the t
REFERENCES:
patent: 2898217 (1959-08-01), Selsing
patent: 5143871 (1992-09-01), Leese et al.
patent: 5455210 (1995-10-01), Kreiner
patent: 5461015 (1995-10-01), Kinsman et al.
Database WPI Week 8459,Derwent Publications Ltd., London, GB, AN 84-311604, & SU A, 1 090 678, 7 May 1984, Abstract.
Chemical Abstracts #155329z, vol. 95 No. 18, 2 Nov. 1981, Columbus, Ohio, US. "Manufacture of Porecelain Products", p. 297.
Chemical Abstracts #160285c, vol. 107, No. 18, 2 Nov. 1987, Columbus, Ohio, US, p. 385.
Capoglu Ahmet
Jafari Majid
Messer Peter Francis
Noor Ahmad Fauzi Mohd
Okojie Henry Ehibhanre
British Technology Group Limited
Marcheschi Michael
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