Textured Cementitious Sheet

Abstract

The textured surface of an asbestos-cement sheet covered with steam cure bloom or normal cure bloom is abraded in a particular manner to remove bloom from certain areas of the surface to produce a highly attractive simulated stone decorative sheet.

Description

BACKGROUND OF THE INVENTION

Asbestos-cement panels and sheets have been in use for many years as architectural panels, siding panels, roofing and siding shingles, furniture table tops and other related products. Their color is usually produced by suitable coating material or by an integrally mixed pigment, and their exposed surfaces often are additionally decorated such as by a texturing operation.

A noticeable trend in design today is in the direction of simulating the appearance of natural products, and with regard to asbestos-cement products the trend is to provide a texture which simulates that of polished stone. It is difficult, however, to faithfully reproduce a stone texture on an asbestos-cement sheet because of the problem of bloom created during the curing operation. With regard to steam cured sheets, as the temperature within the autoclave is raised during a steam curing operation calcium silicate hydrate, which is present on the surface of the cementitious sheets in dissolved form, becomes less soluble and eventually is deposited on the surface of the sheets as a water insoluble precipitate of dull white appearance. This deposit, resembling a layer of white chalk dust, is known in the art as "steam cure bloom." In a normal cure operation, where the asbestos-cement sheets, after being formed, are exposed to air for a number of days (usually 28), calcium carbonate deposits form. They also can be made to form on normal cured asbestos-cement sheets by exposing them to a carbon dioxide atmosphere under pressure in the presence of moisture. Such deposits, also resembling a layer of white chalk dust, are known in the art as "carbonate bloom" or "normal cure bloom."

One way of preventing the formation of bloom is to treat the sheets, prior to curing them, with a suitable bloom inhibitor. Finished sheets treated in this manner do not give the appearance of stone, however, even though the surface texture may closely resemble the texture of stone, because the color or body or sheen of the surface is too uniform and sterile looking.

If bloom inhibitor is not used, the bloom covered sheet that results bears little or no resemblance to stone. Attempts to remove the bloom have involved the use of suitable solvents, such as, for example, hydrochloric acid or ammonium bifluoride, or mechanical methods involving sanding or sand blasting. While such methods of bloom removal have been satisfactorily employed on flat sheets, they are not well suited for treatment of texture sheets. It is virtually impossible from a practical standpoint to uniformly remove bloom by sanding without damaging the texture of the sheet. In addition, sanding raises asbestos fibers adjacent the surface to the point where they are discernable to the touch and interfere with the formation of a smooth surface coating which may subsequently be applied. Use of a suitable solvent is also objectionable because it tends to etch the surface of the sheet and raise and expose surface fibers. In addition, it is difficult to adequately reach the low portions of a textured surface with a solvent applicator. Aside from such objections to sanding or solvent removal of bloom, stone textured cementitious sheets that have been treated in this manner do not have the desired appearance. They are still too obviously artificial because of a lack of an almost undefinable combination of qualities that contribute to the overall appearance of natural stone.

OBJECTS OF THE INVENTION

The main object of this invention is to provide a method of producing a cementitious sheet, particularly an asbestos-cement sheet, having a textured surface that closely resembles the appearance of natural stone.

A further object is to provide such a method which can be readily incorporated into a conventional asbestos-cement sheet manufacturing operation with a minimum of difficulty and expense.

SUMMARY OF THE INVENTION

The above objects are achieved by removing only a portion of the bloom from a textured cementitious sheet, leaving some of the bloom to remain. Specifically, it is removed from the high portions of the textured surface of a cementitious sheet and, to a lesser extent, from the low portions, leaving significant amounts of bloom on the lower portions, especially in the small depressions and valleys of the textured surface. In addition, the textured surface is polished. The removal of bloom and the subsequent polishing of the surface remove only minute quantities of the cementitious composition, and where the sheet is comprised of asbestos-cement composition, the process does not noticeably raise or expose the asbestos fibers. This results in a sheet having a textured surface comprised of high and low portions, wherein the color of the high portions contrasts with the white bloom still remaining on substantial areas of the low portions, the bloom being more pronounced at the lowest parts of the textured surface. In the case of a sheet pigmented with a dark gray color to produce a slatelike appearance, the higher portions of the textured surface will be substantially the color of the base material while portions of adjacent valleys and other low portions of the textured surface will still be covered with white bloom. The colors of the bloom covered areas and the areas from which bloom has been removed blend gradually into one another where the low portions of the textured surface slope gently upwardly to meet the high portions, but provide a direct contrast to one another where a low-portion ends abruptly at a high portion, such as at a ridge, the effect of which is to heighten the similarity of the sheet to that of polished natural stone.

This process permits the formation of cementitious sheets without the use of bloom inhibitor and does not require expensive bloom removal operations, such as sanding or sand blasting operations which, as mentioned above, are highly impractical to use on textured surfaces. By taking the unorthodox approach of deliberately not removing all the bloom, the problem of bloom prevention or removal and the problem of forming a realistic appearing simulated stone surface have been solved. In practice, it is preferred that the bloom be removed by a fine abrasive material of sufficient resilience, compressibility and thickness such that the abrasive material contacts both the high and low portions of the textured surface but exerts more pressure on the higher portions than on the lower portions. The abrasive material itself carries out the polishing of the surface, to a more noticeable degree on the high portions than on the lower portions.

DESCRIPTION OF THE DRAWINGS

The nature of the invention will be more fully understood and other objects may become apparent when the following detailed description is considered in connection with the accompanying drawing, wherein:

FIG. 1 is a pictorial representation of a textured panel formed in accordance with the invention;

FIG. 2 is an enlarged partial plan view of a textured panel, the textured surface of which is covered with bloom;

FIG. 3 is a diagrammatic pictorial representation of an illustrative method of removing bloom in accordance with the invention;

FIG. 4 is an enlarged cross-sectional view taken on line 4-4 of FIG. 3; and

FIG. 5 is a partial plan view similar to that of FIG. 2, but showing the panel after portions of the bloom have been removed and the surface polished.

DETAILED DESCRIPTION OF THE INVENTION

The term "bloom," when used in the specification and claims, shall mean either a deposit of water insoluble calcium silicate hydrate produced by precipitation of such material on the surface of a cementitious composition during a steam curing operation or a deposit of water insoluble calcium carbonate produced by precipitation of such material on the surface of a cementitious composition during a normal curing operation or by exposure of the composition to a carbon dioxide atmosphere under pressure in the presence of moisture.

Referring to the drawing, FIG. 1 shows a typical asbestos-cement panel 10, containing a simulated stone textured surface consisting of irregularly formed high portions 12 and low portions 14. In some areas the surface is gradually sloped from a high portion to a low portion, as for example at 16. In other areas high portions are separated from low portions by more abruptly sloped connecting walls, as where valleys 17 and pockets or crevasses 19 are formed. As explained previously, even with a faithfully reproduced stone texture, such a surface does not give the impression of stone if the color is consistent throughout and the surface is not varied in regard to color, body and sheen.

A textured asbestos-cement sheet typically is formed from an asbestos-cement aqueous slurry which is introduced into the cavity of a die press. Apertures in the cavity permit water to be expressed during the pressing operation in order to consolidate the material into final shape and thickness and to permit it to be dry enough to hold its shape prior to the drying operation. A textured die plate presses the wet asbestos-cement mix with sufficient pressure to express the desired amount of water and to permanently imprint its texture in the surface of the asbestos-cement composition, after which the wet formed sheet is removed from the press, cured and dried.

As mentioned previously, the curing operation causes water insoluble calcium silicate hydrate, in the case of a steam cured product, or water insoluble calcium carbonate, in the case of a normal cured product, to be deposited on the surfaces of the asbestos-cement sheets as a light coating resembling chalk dust. A portion of such a sheet is illustrated in FIG. 2 wherein the entire sheet, including high and low portions, are coated with bloom 20. As indicated in this figure, the bloom prevents the sheet from giving the impression that it is formed from natural stone and for all practical purposes makes the sheet unsalable. In accordance with the invention, the bloom is not entirely removed, as the practice has been in the past, but is used as the medium for producing the desired final appearance.

A preferred method of removing bloom in accordance with the invention is illustrated in FIG. 3 which shows a sheet 10, the surface of which has been textured and is covered with bloom, being carried along by a continuous conveyor 22, trained about end rolls 24 and 26. Mounted for rotation above the conveyor is a shaft 28 on which are mounted a number of ringlike mildly abrasive pads or segments 30. Suitable clamps 32 are located on shaft 28 adjacent the outermost pads 30 to hold the pads in place, thus forming an abrasive wheel 34 comprised of the pads 30 extending from one side edge of sheet 10 to the other. The abrasive material should be relatively fine in order both to remove bloom from portions of the sheet and also to polish the sheet, while leaving intact, or removing only very minute portions of, the asbestos-cement sheet. It is important not to remove significant amounts of asbestos-cement composition in order not to raise or expose asbestos fibers which are close to the surface of the sheet. The abrasive material must be of sufficient resiliency, compressibility and thickness as to be capable of contacting both the high and low portions of the textured surface of the sheet but with more pressure applied to the higher portions, thus removing more of the bloom from the higher areas than from the lower areas.

FIG. 4 illustrates the action of the abrasive wheel 34 more clearly. The outer peripheral work surface of each of the abrasive segments 30 is engaged with the textured surface of the sheet 10 both at the highest and lowest points of the surface. This is possible, even though the shaft on which the segments are mounted is positioned at a constant distance above the conveyor, because the abrasive material is compressed to a greater degree when it contacts the lower portions. More pressure is thus exerted by the abrasive wheels on the high portions of the textured surface than on the low portions, and more bloom is therefore removed from the high portions than from the low portions. The high portions thus are subjected to greater polishing or buffing action than are the low portions. It can be seen that a surface such as that illustrated at 16, which connects in a gradually sloping manner the high portion 12 with the valley 14, will be subjected to varying amounts of pressure, ranging from relatively great pressure at the high portion to relatively low pressure at the low portion. The result of this arrangement is to leave some bloom on the low portion, substantially no bloom on the highest portion, and gradually diminishing amounts of bloom between the high and low extremities of the surface 16.

The portion of the panel shown in FIG. 5 illustrates the effect of the abrading or buffing treatment of the sheet of FIG. 2. As shown in FIG. 5, the bloom has been removed from the high portions of the texture, has been substantially retained at the lower portions immediately adjacent the high portions, and has been gradually removed on surfaces connecting low points to high points, such as the surface illustrated by reference numeral 16. The resulting textured surface bears a striking resemblance to natural stone, particularly where the base color of the asbestos-cement mix is a natural stone color, such as gray or brown.

An example of a suitable abrasive material to be used in the combined bloom removing and buffing operation is a ringlike abrasive segment sold by Minnesota Manufacturing and Mining Company under the trademark "SCOTCHBRITE," which is comprised of a nylon web with abrasive particles such as aluminum oxide, silicone carbide or flint bonded to the fibers of the matrix. Fibrous material carrying abrasive particles is also available in belt form, an example of which is a product sold by Armour Abrasive Company under the trademark "MICROLON." The invention is not limited to the use of abrasive material of a particular grit size since the type of texture and surface finish desired to be produced may vary depending on the end use of the product as well as on individual tastes. Exemplary of a satisfactory abrasive is one of 280 grit size, which has been found to remove bloom and polish the cementitious material in a highly desirable manner. It is not intended to limit the invention to the use of abrasive material consisting of a fibrous matrix carrying bonded abrasive particles. The buffing operation can also be carried out with material which itself is abrasive as well as resilient and compressive, such as, for example, steel wool of finer grades. Because of the varied nature of suitable abrasives, the term "abrasive material" as used in the specification and claims includes abrasive particles bonded to a resilient, compressible matrix and resilient, compressible material which itself is an abrasive.

It is not practical to assign limits to the pressure at which the abrasive and buffing operation is carried out since the pressure can vary considerably depending on the type of abrasive material employed, the texture, the cementitious composition, and personal taste. For example, it may be desirable to use an extremely fine abrasive material in order to produce a desired effect, which in turn may require a greater pressure to be applied than if a coarser abrasive were used. In addition, as previously mentioned, the pressures exerted over the face of the sheet during an abrading operation vary according to the texture; that is, greater pressures are applied at the high portions than at the low portions. It is contemplated that various pressures can be used and, as mentioned above, various grades of abrasives, so long as the abrasive material is resilient and compressible and thick enough to penetrate down to the lower portions of the textured surface.

As an example of a practical manner of carrying out the invention, asbestos-cement sheets containing by dry weight, approximately 40 percent Portland cement, 35 percent asbestos fibers and 25 percent silica flour were formed from an aqueous mixture of about 25 percent solids content using a die press having a textured die plate designed to produce a slatelike appearance. The sheets were made 1/8 inch oversize in thickness to allow for level sanding of the backs before being steam cured. The sheets were air cured for 7 days, then trimmed to size and backsanded to remove 1/16th inch of material. They were then placed on a pallet with protective material, such as soft foam, between adjacent sheets in order to protect the texture from being scuffed and compressed out of shape by the weight of the sheets above. The sheets were steam cured in an autoclave at 100 p.s.i.g. for 16 hours, followed by a drying period of 8 hours at 250.degree. F. After being dried the back surfaces of the sheets were sanded to remove the additional 1/16th inch and to make them level, after which the textured surfaces of the sheets were subjected to the action of a segmented abrasive wheel comprised of "SCOTCHBRITE" material formed from a nylon web to which aluminum oxide of 280 grit size was bonded. The resulting appearance of the asbestos-cement sheets bore a striking resemblance to charcoal colored slate and was highly desirable for use in both interior and exterior installations. It was estimated that about 90-95 percent of the bloom was removed, the remaining small amount being retained on the sheets to enhance their natural stone appearance.

In many cases, it will be desirable to coat the textured surface with a clear lacquer finish in order to protect it from the weather if used in an exterior environment and from scuffing and spilling of materials if used in an interior environment. An example of a suitable lacquer coat which can be sprayed on the sheets is cellulose acetate butenate (CAB) lacquer, diluted with two parts of methyl ethyl ketone (MEK) solvent for each volume of lacquer. The amount of lacquer sprayed onto the textured surface of course may vary according to personal preference. It has been found that a coverage of about 10 grams per square foot of surface is adequate.

While the invention has been explained primarily with respect to asbestos-cement products, it should be understood that it can be used equally as well with other types of cementitious sheets so long as the surface of the sheet is coated with bloom and is textured to permit varying removal of bloom by the abrasive operation described. The depth of the texture may vary according to taste, although it is preferred that the difference in height between the higher portions of the texture and the lowest portions of the texture be no more than about 1/4 inch. This permits the buffing or abrasive device to readily reach the low portions of the textured surface without being unduly compressed as it contacts the high portions.

It should now be apparent that this invention, by making use of the very material that was heretofore responsible for difficulties in producing suitable textured designs in cementitious sheets, has enabled the elusive combination of characteristics that makes up a polished stone appearance to be captured and duplicated simply and inexpensively.

Claims

What we claim is:

1. A cementitious sheet having a decorative surface comprising

a. a textured surface having high and low portions of varying height,

b. bloom deposits being present on substantial areas of the low portions of the textured surface and being more pronounced on the lowest portions thereof,

c. the color of the base cementitious sheet being readily apparent on the high portions of the textured surface and contrasting with the bloom present on low portions thereof, and

d. the areas of bloom terminating relatively abruptly adjacent some high portions of the textured surface and gradually diminishing in other portions of the textured surface to produce a textured appearance of varying color.

2. A cementitious sheet as recited in claim 1, wherein the composition of the sheet comprises asbestos fibers and the hydrothermal reaction product of hydraulic cement, silica and lime.

3. A cementitious sheet as recited in claim 1, wherein the textured surface has a polished appearance, particularly on the high portions thereof.

4. A cementitious sheet as recited in claim 3, wherein the texture of the sheet simulates that of stone and the varying shades of color of the base cementitious sheet and the varying degrees of polished surface contribute significantly to the appearance of polished stone.