Compositions: coating or plastic – Coating or plastic compositions – Alkali metal silicate containing
Reexamination Certificate
2000-07-19
2002-08-06
Wood, Elizabeth D. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Alkali metal silicate containing
C106S629000, C427S397800, C428S426000, C428S427000, C428S428000, C428S432000, C428S688000, C428S689000
Reexamination Certificate
active
06428616
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to inorganic and organic-modified inorganic curable coating compositions. More particularly, the invention provides aqueous curable coating compositions comprising a mixture of a water-soluble alkali metal silicate and a water-soluble crosslinking agent. The compositions are particularly useful for coating glass, ceramic, concrete and metal articles.
BACKGROUND OF THE INVENTION
Aqueous alkali metal or ammonium silicate compositions containing curing agents are well known, e.g., see U.S. Pat. No. 4,288,252.
In general, the prior art uses alkali metal silicates that are cross-linked with B-form aluminum trimetaphosphate. Cross-linking of the alkali metal silicate is attained by heating an article coated with the aqueous composition to a temperature of 205° C. or higher.
The aqueous prior art composition is typically made by mixing an alkali metal silicate with fillers and/or pigments and placing the mixture in a sealed container. The B-form aluminum trimetaphosphate is mixed in water with or without pigments and fillers and stored in a second sealed container. Prior to use, the materials in each container are mixed together. The separate containers are required since the B-form aluminum trimetaphosphate and alkali silicates undergo a non-cross-linking reaction even at room temperature to produce a solidified mass within about one day. Once the materials from the separate containers are mixed, the resultant composition must be used within eight hours. Thus, this system must be prepared as a two-pack system to prevent this undesirable reaction.
An aqueous, heat-curable coating composition may also be prepared in accordance with U.S. Pat. No. 5,498,284. In the '284 patent, the composition comprises a mixture of a water-soluble alkali silicate and an alkali aluminum phosphate crosslinking agent. However, the crosslinking agent in the '284 patent is difficult and time-consuming to prepare and it is not water-soluble, leading to variable results upon curing due to lack of homogeneity of distribution of the crosslinking agent in the mixture. Furthermore, the coating composition in the '284 patent must be heat-cured at temperatures of up to about 700° C. In contradistinction thereto, the crosslinking agents employed in the present invention are easily prepared, they are water-soluble thus leading to a clear solution upon admixture with the alkali metal silicate, and the coating composition may be readily cured by air drying or by moderate heating, i.e., temperatures above about 240° C. are not necessary for heat curing.
OBJECTS OF THE INVENTION
It is an object of the invention to prepare an aqueous coating composition that is curable by drying in air or by moderate heating, at temperatures of up to about 240° C. or higher if desired.
It is a further object of the invention to prepare an aqueous coating composition that is a clear solution such that the alkali metal silicate and the crosslinking agent are both water-soluble and are fully compatible with one another, thereby avoiding problems associated with a lack of homogeneity of distribution of the crosslinking agent in the alkali metal silicate.
It is yet a further object of the invention to prepare a crosslinking agent by a simple, highly efficient process that will result in a water-soluble material that will be fully compatible with the alkali metal silicate.
These objects and other objects have been achieved by the present invention that is described hereinbelow.
SUMMARY OF THE INVENTION
The present invention is directed to an aqueous, curable coating composition comprising a mixture of a water-soluble alkali metal silicate and a water-soluble crosslinking agent.
The present invention avoids the use of B-form aluminum trimetaphosphate hardener or other curing or hardening agents that have been previously employed and that have an undesirable low temperature reaction with the alkali silicates. In the present invention, it is possible to formulate the composition as a one-pack system since there is no significant low temperature, e.g., room temperature, reaction between the alkali silicate and the crosslinking agent employed in the mixture. This permits the formulation of curable coating compositions as a single container system with a long shelf life. The container should, of course, be sealed until the coating composition is to be applied to the substrate since the composition is capable of being cured at room temperature by air drying (as well as by moderate heating).
Since the coating composition of the present invention employs the use of a water-soluble crosslinking agent, the problems associated with the composition of the '284 patent discussed above may be avoided.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides aqueous, curable coating compositions comprising at least one water-soluble alkali metal silicate and a water-soluble crosslinking agent. The present invention also provides the method for producing and using such coating compositions. The invention further provides the water-soluble crosslinking agents and the method for their production. Additionally, the invention provides substrates that are coated with the coating composition and subsequently cured.
The aqueous, curable coating compositions of the invention comprise, in addition to water as the only necessary solvent, a mixture of:
(a) at least one water-soluble alkali metal silicate; and
(b) an effective amount of a water-soluble crosslinking agent comprising an alkali metal aluminophosphate, an alkali metal aluminoborophosphate, an ammonia aluminophosphate, an ammonia aluminoborophosphate, an alkali metal ammonia aluminophosphate, an alkali metal ammonia aluminoborophosphate or a mixture of two or more of the foregoing phosphates.
The alkali metal silicates employed herein are those that have been previously known for use in the formulation of silicate coating compositions. See for example, U.S. Pat. No. 4,288,252. Although any alkali metal may be employed for the preparation of the silicate and the crosslinking agent, it is preferred that the alkali metal of the water-soluble alkali metal silicate and each of the alkali metals of the water-soluble crosslinking agent independently are sodium, potassium or lithium.
The water-soluble crosslinking agent employed in this invention may be prepared as follows:. An aluminum phosphate, e.g., aluminum dihydrogen phosphate, is reacted in water with at least one alkali metal phosphate (e.g., sodium phosphate, potassium phosphate, lithium phosphate or mixtures thereof) at room temperature. To prepare the ammonia aluminophosphate, ammonia is substituted for the alkali metal phosphate. To prepare the alkali metal ammonia aluminophosphate, a mixture of the alkali metal phosphate(s) and ammonia is used to react with the aluminum phosphate. To prepare the corresponding alkali metal aluminoborophosphate or the corresponding ammonia aluminoborophosphate, a suitable boron compound such as boric acid is also employed as a reactant.
The reaction of the aluminum phosphate with the alkali metal phosphate and/or ammonia as well as the reaction of the aluminum phosphate with the alkali metal phosphate and/or ammonia and the boron compound, in water proceeds rapidly, e.g., in 1-5 minutes, at ambient temperature. The crosslinking agent preferably is not isolated, i.e., the aqueous solution of the crosslinking agent is used as is for subsequent mixing with the water-soluble alkali metal silicate. The water-soluble alkali metal silicate may be mixed as a solid or as an aqueous solution with the aqueous solution of the crosslinking agent to thereby form the aqueous, curable coating composition of the invention.
The amounts of the reactants employed in the preparation of the aqueous solution of the crosslinking agent will be approximately stoichiometric in nature. However, it is preferred that the amounts of the reactants be such that the pH of the aqueous solution of the crosslinking agent will be above about 5.0, preferably above 7.0. It is also preferr
Matalon Jack
Neely Industries, Inc.
Wood Elizabeth D.
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