Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2002-04-19
2003-10-28
Barr, Michael (Department: 1762)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S457000, C427S397800, C427S294000, C427S295000, C106S038300
Reexamination Certificate
active
06638628
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to coating compositions, methods of preparing such compositions and surfaces coated with such compositions. In particular it relates to coatings useful in protection of articles produced by powder metallurgy techniques and otherwise-untreated galvanized articles from corrosion.
BACKGROUND OF THE INVENTION
While one aspect of preparing a coating for a surface is to provide a decorative finish, even more important is to provide a resistance to environmental and atmospheric conditions. This is particularly true of coatings for metallic surfaces where materials present in the atmosphere, including both natural components such as water vapor and pollutants such as acids and other corrosive materials, can damage the surface of the metal. Many of the most commonly used coating materials contain organic resin materials to provide binding for the pigment and other corrosion resistant materials in the composition. Such organic pigments frequently require organic solvents to ensure their dispersion in the composition, which can create major pollution problems.
A particularly valuable type of anti-corrosion composition is one containing zinc. The outstanding corrosion resistance afforded by galvanizing has made it one of the most effective means for long term protection of steel from oxidation (rusting), and subsequent corrosion. It is the conventional method of providing protection for guard rails, transmission towers, light poles, electrical equipment and dozens of other specific applications. A coating of 5 mils or 125 microns of a galvanizing composition can protect its exposed equivalent for a period in excess of 20 years. Galvanizing coatings can be applied both by hot dipping and electroplating but these techniques have their limitations, particularly in regard to the shape of the article during installation. Typically galvanized products are then post treated to avoid the formation of “white rust”. In the past such treatments involved use of chromic acid or chromates. Use of these materials is, however, becoming recognized as creating unnecessary environmental hazzards. Today, use of polymer coatings is becoming more common to address this problem Alternatively in some situations, such as water treatment plants, corrosion inhibiting chemicals are being added to water that contacts the galvanized parts. Alternative ways of dealing with the problem are desirable. Coating compositions, sometimes known as zinc rich primers, have been considered to be optimum anti-corrosion coatings on iron or steel substrates. The zinc inhibits rust by reason of an electro-chemical interaction between the zinc and steel substrate. It is desirable to avoid too great a binder component to avoid insulating zinc particles from each other and the substrates but this can mean there are difficulties in application because of settling of the composition. There is a very extensive art on the use of zinc in coating compositions alone or in combination with other components such as glass microspheres—see for example U.S. Pat. No. 5,580,907 Ronald R. Savin.
The use of silicate bonded compositions particularly compositions containing zinc has been suggested for various types of surfaces.
In U.S. Pat. No. 5,792,803 there is described the use of alkyl silicates in an anhydrous alkyl alcohol in combination with epoxy or other polymeric resins as a binding agent.
U.S. Pat. No. 5,888,280 (Montes) describes a composition suitable for forming a protective coating on steel substrates comprising zinc dust, a group 1A metal silicate (preferably potassium silicate), a colloidal silica ingredient modified with a group 1A metal (preferably lithium polysilicate) and a carbonate-containing internal hardener.
U.S. Pat. No. 6,071,624 (Hubbard) describes an aqueous barrier coating for polymerizing substrates comprising a lithium-potassium copolysilicate.
U.S. Pat. No. 4,917,960 (Homberger) describes several different types of coating compositions for coating porous substrates on a metal base particularly for use in heat exchangers. Such substrates may include heat exchangers including a porous coating of bonded metal particles. The coatings applied to such substrates include fugitive materials to ensure that the coating applied to the bonded metal particle layer is also porous. Such fugitive materials maybe comprised of various bonding agents such as sodium or potassium silicates or lithium polysilicate. There is, however, no teaching of the use of a mixture of these materials. The coating may also contain metal particles, preferably particles of the same metal as is used from the substrate, such as aluminum.
British Patent 1,095,191 (Esso Research and Engineering) discloses coating compositions for metal comprising zinc dust and a lithium-sodium silicate solution.
U.S. Pat. No. 4,162,169 (Schutt, assigned to NASA) describes a paint binder for use in protection of ferrous articles or aluminum alloys which comprises a potassium or sodium silicate dispersion having a silicon dioxide to alkali metal molar ratio of 4.8:1 to 6.0:1 which is obtained by using silicon dioxide as a hydrogel and incorporation of a silicone.
It is desirable to use silicates which are soluble in water as distinct from organic silicates which create environmental problems because of the organic solvents.
Potassium silicates have been proposed. Thus for example corrosion-resistant coating can be prepared from zinc and potassium silicate. A powdered material comprising zinc dust, with a small content of mica and zinc oxide (5% or less each) is mixed with a water solution of potassium silicate (20% by weight), potassium hydroxide (0.5 to 5% by weight) and lithium polysilicate (5% by weight) with amorphous silica (5% by weight) and the resulting blend is sprayed onto the surface to be protected. The use of this primarily potassium silicate product has the disadvantage that there is a relatively slow development of water resistance.
The addition of the lithium polysilicate in proportions of 5% to 25% by weight to potassium silicate although found to improve the properties of potassium silicate indicated the contents of any silicate including the lithium polysilicate should be maintained relatively low.
SUMMARY OF THE INVENTION
It has now been determined that if sodium silicate or potassium silicate is combined with lithium polysilicate in a composition containing relatively high proportions or both sodium silicate and/or potassium silicate and lithium polysilicate there is obtained a silicate primer system which is both inexpensive and rapidly develops initial impressive water resistance. Lithium polysilicate (SiO
2
20%, lithium 2%) is extremely effective as a binder for zinc rich coatings over blasted steel and becomes extremely insoluble in a very short period of time. Its adherence to cold rolled steel, galvanized products and other porous substrates is very poor. When subjected to baking for industrial application with or without zinc dust or zinc flake, lithium polysilicate will not bond satisfactorily to unblasted metallic substrates. There is obtained a coating composition which cured within a reasonable time and provided a composition which was then extremely resistant to corrosion even at relatively elevated temperatures.
According to the invention therefore there is provided a coating composition with a silicate binder in which the silicate binder comprises a sodium silicate or potassium silicate with a relatively high proportion of lithium polysilicate. As an embodiment of the invention there is provided a coating composition comprising zinc in particle form together with, as binding component, sodium or potassium silicate together with a significant proportion of lithium polysilicate composition all being dispersed in an aqueous medium.
Such compositions are of particular use in coating powdered metal artefacts and galvanized products that have not been passivated or pretreated, for example with a chromate or equivalent pretreatment.
DETAILED DESCRIPTION OF THE INVENTION
In a particularly preferred embodime
Barr Michael
Ladas & Parry
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