Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2002-02-27
2004-08-03
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S379000, C427S419200, C427S430100, C427S443200
Reexamination Certificate
active
06770325
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a novel process of preparing chemically bonded composite hydroxide ceramics by exposing a thermally treated hydroxide ceramic to a phosphate reagent to produce a system and subsequently heat treating the system to initiate a rapid chemical bonding reaction.
BACKGROUND OF THE INVENTION
Chemical reactivity in systems containing phosphoric acid or various forms of phosphates have received attention in scientific and patent literature. Particularly, the refractory applications and dental cements applications of chemical bonding (CB) of ceramics through phosphating have been disclosed. (See D. Kingery, “Fundamental Study of Phosphate Bonding in Refractories, Part I,II,III”,
J. Am. Cer. Soc
. 33 (1950) 239-50; J. Cassidy, “Phosphate Bonding Then and Now”,
Am. Cer. Soc. Bull
. 56 (1977)640-43; J. Bothe and P. Brown, “Low Temperature Formation of Aluminum Orthophosphate”,
J. Am. Cer. Soc
. 76 (1993) 362-68; and J. Bothe and P. Brown, “Reactivity of Alumina towards Phosphoric Acid”,
J. Am. Cer. Soc
. 76 (1993) 2553-58.) For example, mixing aluminium oxide, or alumino-silicates, or zircon, or many other pure or mixed oxides (such as Cr
2
O
3
, ZrO
2
with phosphoric acid H
3
PO
4
(PA) or monoaluminum phosphate Al(H
2
PO
4
)
3
, (MAP) leads to reaction between the constituents and formation of chemical bond at relatively low temperatures of 200-400° C. These processes yield successful, commercial refractory materials. (See D. Kingery, “Fundamental Study of Phosphate Bonding in Refractories, Part I,II,III”,
J. Am. Cer. Soc
. 33 (1950) 239-50.) The objective of these inventions was to produce monolithic ceramic while avoiding the usual high-temperature treatment (or “sintering”) necessary to bond ceramic particles. Additionally, chemically bonded ceramics experience very small shrinkage processing, i.e. size and shape of the resulting chemically bonded component is approximately the same as those of the mixed and pressed powder component.
In another example of known prior art, zinc oxide mixed with zinc metal and aluminium hydroxide is further mixed with phosphoric acid. The mixture reacts and sets at room temperature yielding dental cement. The cementitious behaviour of phosphate-containing systems has been explored on a large scale if one of the oxides in the system exhibits substantial room temperature reactivity towards phosphates. For example, MgO rapidly reacts with monoaluminum phosphate to form hydrated magnesium phosphates that bond the aggregate components of the cold-setting concrete. Reaction bonding of alumina with phosphates has also been used to produce ceramics of controlled, fine pore structure such as molecular sieves U.S. Pat. No. 5,178,846. Phosphating of steel or aluminium produces a thin (1-10 &mgr;m) mildly protective layer which can be utilised as a bondcoat for subsequent application of organic paints, or other coatings, such as ceramic coatings.
In order to produce a phosphate-bonded ceramic, a chemical reaction is initiated between the phosphate-carrying reactant, for example orthophosphoric acid (H
3
PO
4
), and an oxide (such as alumina, zirconia, chromia, zinc oxide, and others). As a result, refractory phosphates, such as aluminium phosphate, are formed at relatively low temperatures. For example, for the system Al
2
O
3
—H
3
PO
4
—Al(H
2
PO
4
)
3
, the reaction starts at 127° C., and is complete at about 500° C. At higher temperatures, the resulting amorphous aluminophosphates undergo a chain of crystallization-phase transformations, to eventually decompose to P
2
O
5
and Al
2
O
3
above 1760° C. (See Bothe and P. Brown, “Low Temperature Formation of Aluminum Orthophosphate”,
J. Am. Cer. Soc
. 76 (1993) 362-68; and J. Bothe and P. Brown, “Reactivity of Alumina towards Phosphoric Acid”,
J. Am. Cer. Soc
. 76 (1993) 2553-58.)
The systems of particular interest include ceramic particles that are chemically bonded to form a protective film on metallic substrate. The films can be used for surface modification in preparation for deposition of subsequent coatings (e.g. phosphate treatment of metals before painting) or for added protection against corrosion and/or wear. However, due to substantial reactivity of phosphates, e.g. phosphoric acid, towards metals, the systems involving metals (e.g. phosphate-containing coatings on metals, or coatings that contain metallic particles) must include means of controlling reactivity of such system. One of such systems disclosed in the scientific and patent literature, is a protective coating for metals (e.g. steel) that contains simultaneously phosphoric acid and aluminium metal particles. In such coating particulate aluminium is combined with the phosphoric acid solution, applied to surface of metal, and heat treated at 250-550° C. to bond the metal particles together, and to the substrate base metal. In such a coating formulation aluminium must be protected from extensive, and possibly violent, reaction with the phosphate. One of the best-known systems that achieve this objective has been disclosed in U.S. Pat. No. 3,248,251, where chromates or molybdates were added to the solution to effectively protect aluminium metal from excessive reaction with the phosphate. These predominantly metallic coatings are still widely applied to protect ferrous metals form corrosion and oxidation. Another similar system has been disclosed in U.S. Pat. No. 3,395,027. In an attempt to eliminate the use of environmentally dangerous chromates or molybdates, formulations rich in dissolved aluminium ions, e.g. less reactive towards aluminium metals, have been proposed by Stetson et al in U.S. Pat. Nos. 5,279,649 and 5,279,650. These formulations contained numerous other substances that were supposed to inhibit reactivity of phosphates towards aluminium particles. Yet another attempt to produce “environmentally friendly” phosphate bonding composition suitable for coatings is disclosed by Mosser et al in a series of U.S. Pat. Nos. 5,478,413; 5,652,064; 5,803,990 and 5,968,240. All of these formulations include complex mixtures of ions (in addition to the phosphate ion solution) with the objective to control reactivity of phosphates in coatings application. In one variant of such coating system, disclosed in U.S. Pat. No. 4,544,408, a water/acid dispersion premix of hydrated alumina (e.g. boehmite or pseudoboehmite) is admixed into the usual chromate/phosphate or molybdate/phosphate coating composition. The patent teaches that mixing the two solutions leads to gelation of the hydrated alumina particles and, as a result of this process, a thixotropic mixture is formed. The thixotropic nature of the mixture allows deposition of uniform coatings in the spin coating process. It is disclosed that particles of alumina or aluminium improve performance of such coatings. It is further claimed in U.S. Pat. No. 4,838,942 that the coating system containing aluminium particles and a mix of chromic, phosphorous, phosphoric acids and aluminium phosphate can be cured at very low temperature of 150° C. to 190° C.
Another area pertaining to the present invention includes fully ceramic systems (i.e. no metals are present) where very fine particles (nanometer size) of hydroxide ceramic (HC), such as boehmite AlOOH, are mixed with calcined ceramic, such as alpha aluminium oxide. (See S. Kwon and G. L. Messing, “Sintering of Mixtures of Seeded Bohemite and Ultrafine Alpha Alumina”,
J. Am. Cer. Soc
. 83 (2000) 82-88; and M. Kumagai and G. L. Messing, “Controlled Transformation and Sintering of a Bohemite Sol-Gel by Alpha Alumina Seeding”,
J. Am. Cer. Soc
. 68 (1985) 500-505.) These systems are referred to in the present invention as composite hydroxide ceramic CHC. During heat treatment the nanometer-size particles of HC decompose, releasing water, and form very active nanometer-size particles of aluminium oxide. In these systems the very large surface area (in excess of 100 m
2
/gram), and thus high reactivity, of the thermally decomposed boehmite is utilised to accelerate sintering of the resulting aluminium oxid
Troczynski Tomasz
Yang Quanzu
Barr Michael
Oyen Wiggs Green & Mutala
The University of British Columbia
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