Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-07-12
2001-10-23
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S407100
Reexamination Certificate
active
06306465
ABSTRACT:
This invention is concerned with a method of coating an uncured mineral substrate. More particularly, though not exclusively, this invention concerns a method of coating an uncured cementitious substrate, such as a concrete roof, floor or wall tile or a concrete house siding, to give the substrate a high gloss finish.
Precast concrete roof tiles are typically produced from a concrete mortar whose consistency is such that it can be molded whilst in its green state into a design having features of shape, configuration, pattern or ornament which it will retain when cured. The tiles are usually formed of a layer or layers of green concrete molded to correspond to the desired surface appearance of the tiles. The tiles may be formed of a single layer of concrete or they may be formed of multiple layers of concrete, for example as disclosed in U.S. Pat. Nos. 4,789,319, 4,986,744 and 5,017,320.
Where a smooth finish on the upper surface of the tile is desired, the tile usually comprises at least two layers, the upper most layer being formed from a green concrete or cementitious slurry, comprising hydraulic cement, fine-grained aggregates, pigments and dispersing agents. This upper most layer has a thickness typically from 0.05 mm to no more than 3 mm and may be applied to the coarser base layer(s), for example, by a process such as described in any of the above U.S. patents or by curtain coating the formed tile.
Concrete roof tiles tend to be susceptible to efflorescence when exposed to atmospheric moisture or precipitation. To suppress efflorescence in concrete roof tiles, it has been common practice to coat the upper side of the cured or uncured concrete roof tiles with an aqueous paint or clear coat composition which, when dry, provides a waterproof coating. Such aqueous paint compositions will typically comprise from 30 to 50% by weight of polymer dispersion, for example an aqueous, acrylate-based polymer dispersion, from 20 to 60% by weight fine inorganic filler, for example chalk, barytes and/or silica, and optionally from 2 to 10% by weight iron oxide pigment. Such aqueous clear coat compositions typically consist essentially of a 100% acrylic dispersion (e.g. 30-50% solids). Coatings formed from such paint and clear coat compositions tend to have a thickness on the tile of no greater than 0.15 mm. Improved efflorescence suppression has been proposed through the use of relatively more complex copolymer dispersions, such as those described in DE-A-4341260 and U.S. Pat. No. 5,215,827, as replacements in whole or in part for the more conventional simple acrylate-based polymer dispersions.
In U.S. Pat. No. 4,177,232 there is disclosed a method of making a concrete roof tile which has improved efflorescence resistance. In this method, a formed tile body is coated with a cementitious slurry and before that slurry layer has cured, an aqueous emulsion of a film forming polymeric material is applied thereto. The fully cured tiles formed by this method exhibit low gloss finishes.
It is often desirable for a concrete roof tile to exhibit a high gloss finish. A high gloss finish is typically obtained by a process of applying a coating of an aqueous clear coat composition to the uppermost surface of the tile after the tile has been dried and cured in an oven or autoclave, and then drying the coating. Alternatively, a high gloss finish may be obtained by a process of applying a coating of a non-aqueous clear coat composition to the uppermost surface of the tile either before or after the tile has been dried and cured.
In WO-98/52698 a fast-setting coating material is prepared by contacting a stable aqueous dispersion of a polymer that contains strong cationic groups and weak acid groups onto a substrate that is basic, such as cementitious materials, or rendered to be basic. As an alternative to using a basic substrate, it is disclosed that the quick setting coating material can be prepared by contacting in either order or concurrently, the surface of a substrate with two separate polymers, one of which contains strong cationic groups, and the other of which contains weak acid groups. Though it is disclose in WO-98/52698 that the aqueous dispersions may be used, amongst other applications, to coat cured or uncured cement to reduce water evaporation, thereby to improve the properties of the final concrete, there is no disclosure or suggestion for using the aqueous dispersions to provide a high gloss finish on a concrete substrates.
It is an object of the present invention to provide a finished mineral substrate, such as a roof tile, which exhibits a high gloss finish but which has been manufactured by a method which either is quicker and potentially more energy efficient than previously known methods employing aqueous coatings or which does not rely upon the use of solvents used in non-aqueous coatings for making the same.
In accordance with the present invention, there is provided a method of coating a mineral substrate with at least one coating composition comprising a film forming polymeric binder, which method comprises the following process steps:
i) forming said substrate from an uncured mineral composition, preferably a green concrete, clay or fiber cement;
ii) forming a continuous layer of film-forming polymeric binder over a surface of said substrate formed in i) by:
a) applying to said surface a first coating composition comprising film-forming polymeric binder, and
b) destabilizing said first coating composition prior to application of, upon contact of, or subsequent to application of said first coating composition on said surface;
iii) applying to the coated surface formed in ii) a second coating composition comprising a film forming polymeric binder; and
iv) drying and curing said coated substrate formed in iii), for example in an oven or autoclave.
In accordance with a second aspect of the present invention, there is provided a method of improving the gloss finish of a mineral substrate formed by a process comprising the following process steps:
i) forming said substrate from an uncured mineral composition, preferably a green concrete, clay or fiber cement;
ii) forming a continuous layer of film-forming polymeric binder over a surface of said substrate formed in i) by:
a) applying to said surface a first coating composition comprising film-forming polymeric binder, and
b) destabilizing said first coating composition prior to application of, upon contact of, or subsequent to application of said first coating composition on said surface;
iii) applying to the coated surface formed in ii) a second coating composition comprising a film forming polymeric binder; and
iv) drying and curing said coated substrate formed in iii), for example in an oven or autoclave.
Surprisingly, finished mineral substrates coated by the method of the present invention exhibit a high gloss finish which is comparable with finished substrates formed by conventional processes. The method of the present invention enables high gloss finished substrates to be manufactured more quickly, and potentially in a more energy efficient manner, than similar finished substrates manufactured by conventional processes using aqueous coatings, where an aqueous coating is applied to the substrate after the substrate body has been dried and cured.
To assist further understanding of the present invention, but without wishing to be limited by it, the inventor offers the following as a simplistic explanation of the mechanism considered key to the success of the present invention: in the coatings formulation art it is known that the components of a stable coating composition can rapidly drop out of suspension or solution when that composition is suitably destabilize. Accordingly, when a coating composition is applied as a coating on an uncured mineral substrate and appropriately destabilized, the film-forming polymeric binder in the coating composition will drop out of dispersion or solution and the particles or precipitate of film-forming polymeric binder will begin to build over the surface of the substrate. Eventually, sufficien
Cameron Erma
Rohm and Haas Company
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