US3563790A - Open-top porous container having sealed exterior surface - Google Patents

Abstract

AN OPEN-TOP, POROUS CONTAINER MADE OF LIGHT WEIGHT AGGREGATE AND A BINDER SUCH AS SODIUM SILICATE OR PORTLAND CEMENT IS DIPPED IN AN AQUEOUS MEDIUM CONTAINING SODIUM SILICATE AND FINELY DIVIDED SILICA FILLER. THE DIPPING OPERATION IS CONDUCTED TO WET AND COAT THE EXTERIOR BUT NOT THE INTERIOR SURFACES TO THE CONTAINER AND AFTER HEAT CURING OF THE COATED CONTAINER THE EXTERIOR SURFACES ARE EFFECTIVELY SEALED WHILE THE INTERIOR SURFACES REMAIN POROUS. SUCH POROUS SURFACES CAN EXERT A WICKING ACTION WHEN A HYDROCARBON FUEL, E.G., WAX, IN THE CONTAINER IS BURNED TO SUPPLY HEAT, FOR INSTANCE, TO ORCHARDS OR OTHER CROPS IN COMBATING FROST.

Description

United States Patent 3,563,790 OPEN-TOP POROUS CONTAINER HAVING SEALED EXTERIOR SURFACE Richard J. Pratt, Flossmoor, Donald E. Saflford, Park Forest, and William S. Hoock, Flossmoor, I]l., assignors to Sinclair Research, Inc., New York, N.Y., a corporation of Delaware No Drawing. Original application Nov. 6, 1967, Ser. No. 680,955, now Patent No. 3,511,692. Divided and this application Oct. 23, 1969, Ser. No. 871,208

Int. Cl. B05c 8/00 U.S. Cl. 117-94 7 Claims ABSTRACT OF THE DISCLOSURE An open-top, porous container made of light weight aggregate and a binder such as sodium silicate or portland cement is dipped in an aqueous medium containing sodium silicate and finely divided silica filler. The dipping operation is conducted to wet and coat the exterior but not the interior surfaces of the container and after heat curing of the coated container the exterior surfaces are effectively sealed while the interior surfaces remain porous. Such porous surfaces can exert a wicking action when a hydrocarbon fuel, e.g., wax, in the container is burned to supply heat, for instance, to orchards or other crops in combating frost.

This is a division of my copending application, Ser. No. 680,955, filed Nov. 6, 1967, now Pat. No. 3,511,692.

This invention relates to the coating of the exterior surfaces of an open, porous block container. Particularly, this invention is concerned with the coating of porous fuel block heaters to prevent leakage of hot hydrocarbon fuel contained therein during long periods of firing.

Fuel block heaters can be used, for example, as a means for supplying heat to citrus groves, fruit orchards, and vegetable farms to protect the crop against damage caused by frost or freezing. Usually, the fuel block heater is comprised of a container having a solid or liquid fuel therein. In operation, a large number of the block heaters are positioned throughout the area to be heated and the fuel ignited. The addition of heat in this manner is based upon the principle of temperature inversion. On a clear, calm night there is a relatively thin layer of cold air near the ground and a gradual increase in air temperature up to a height of 300 to 800 feet. Hot gases from the burning fuel mix rapidly with the surrounding cold air to give a slightly increased ambient air temperature which rises slightly and acts as a roof over the area to be heated to retain the subsequently heated air.

A particularly useful fuel block heater comprises a normally solid hydrocarbon wax as the fuel component encased in an open top container. The container can be composed of a light weight, porous, predominantly inorganic aggregate material such as perlite or expanded vermiculite and a suitable binder, e.g., portland cement. The porous interior of the container functions as a Wick to burn the melted fuel therein.

In use, the fuel blocks may be fired for several hours, for instance, a period of up to about 12 hours or more. During such periods of sustained firing the liquid or molten fuel may penetrate the porous container walls 3,563,790 Patented Feb. 16, 1971 causing leakage and subsequent loss of fuel. It is therefore desirable to coat the exterior of the fuel blocks to prevent such leakage.

To be effective, a fuel block coating must remain stable during long periods of firing of the fuel block. Coatings which crack or disintegrate from the heat generated during sustained firing periods not only permit fuel loss, but also prohibit reusev of the container. Ideally, the coating should possess an affinity for the aggregate container, be composed of materials low in cost and of ready availability and be easy to handle and apply.

Of significant importance is the manner in which the coating is applied to the fuel block heaters. A desirable method is one that applies a coating which is nearest the minimum, effective amount, that is, the least amount sufficient to prevent substantial loss of fuel upon firing, yet leaves the interior of the block porous to effect a wicking action when the block is fired. A block to which has been applied a heavy amount of coating may be leak-free, yet inoperative if the interior is not left porous, Similarly, a coated block possessing a porous interior and an insufficient amount of coating is of no use. A desirable coating method further should produce coated blocks at a commercially-acceptable speed.

We have now found a coating for the exterior of open, porous containers such as fuel block heaters which possesses the above desirable characteristics. In making the containers We apply to the exterior surfaces, but not the interior surfaces, of the container an aqueous composition of a water-soluble sodium silicate binder and an inert, finely-divided silica filler. The container which is preferably as dry as practical, can be dipped, open end up, up to approximately or just below the rim of the open end, in the aqueous composition for a time sufficient to effectively coat the exterior of the container, that is, for a time sufiicient to provide an exterior coating which, after subsequent heat curing, gives the desired leak-protection, yet leaves the interior of the container substantially porous to efiect wicking action. Usually the block is dipped for at least about 5 seconds up to about 1 minute or longer. A vacuum, for example, of up to about 15, preferably about 0.5 to 2, inches of mercury, can be applied to the cavity or interior of the block to hasten the dip coating operation. With a vacuum applied, the dipping time usually need not exceed about 30 seconds. The viscosity of the aqueous composition is often about 1000 to 1600, preferably from about 1200 to 1300, centipoises at F. during the dipping operation. After removal of excess coating, by, for example, draining, brushing or wiping with a doctor blade, etc., the container is subjected to a heat treatment, e.g., at a temperature of about 250 to 700 F. to cure the coating. A suitable curing time is in the range of about 0.25 to 2.5 hours or more. Preferably, the coated block is subjected initially to a heat treatment at a temperature of about 250 to 350 F. for about 0.25 to 0.75 hour to set the coating and then heated at a higher temperature, say about 500 to 600 F. for about 0.75 to 1.75 hours, to cure the coating. The container can be filled with a solid or liquid fuel providing a fuel block heater having a cured coating which only partially penetrates the exterior of the walls of the heater which coating will not leak when the heater is fired.

We have found that a single or double dip in the coating composition with a short interval between dips for removal of excess coating or for briefly drying with warm air provides excellent results. Further, dip coating from tanks provides for high speed coating of the blocks and minimizes material loss.

The amounts of sodium silicate, silica filler and water employed in the coating composition may vary depending upon the porosity and moisture content of the blocks coated. In general, the coating composition applied to the blocks contains about to 40, preferably about 9 to 30, wt. percent of sodium silicate, about to 60, preferably about 30 to 50, wt. percent of silica filler of defined particle size and about 25 to 50, preferably about 30 to 45, wt. percent of water. The composition may further contain, for instance, about 2 to 5 weight percent of an inorganic pigment, for example, colored iron oxides, etc., if desired. The amount of solids in the aqueous composition, however, is preferably such that the viscosity of the composition is in the range given above during the dipping operation. Generally, a solids system of about 45 to 70 wt. percent can be employed. The higher solids systems of about 55 to 65% are preferred for very porous or moist blocks (15-30% moisture) while lower solids systems of say about 45 to 55% solids content are preferred for blocks of lower porosity and lower moisture content.

Suitable sodium silicates include water-soluble, sodium silicates of about to 45, preferably of about 35 to 41 Baum. The silica filler is finely divided and has a particle size predominantly in the range of about 150 to 350 mesh, preferably about 250 to 350 mesh. The silica filler is relatively non-porous as compared with materials such as diatomaceous earth. Moreover, the aqueous coating composition can contain a minor amount, for instance, about 1 to or more wt. percent of other fillers such as talc, diatomaceous earth, asbestos, bentonite, silica of larger particle size than about 150 mesh. On a cured basis the coating contains about 1 to 8, preferably about 1 to 6, parts by weight of sodium silicate, about 3 to 12, preferably about 6 to 10 parts by weight of the silica filler of about 150 to 350 mesh size, and, if desired, a minor amount, e.g., about 0.2 to 5 parts by weight of other filler.

The materials used for making the containers coated in the present invention are light weight predominantly inorganic aggregates with a density, for instance, up to about 90 lbs./ft. preferably no greater than about 70 lbs./ft. The light weight of the aggregates can be achieved by screening out the fines to reduce the weight and using aggregates with a high porosity. The aggregate preferably passes an 8 size screen (Tyler). The porosity is contributed both by the pores of the aggregate and the space between aggregate particles. Hence, porosity may be varied by aggregate choice and by screen size selection. The aggregates include such light Weight materials as perlite, pumice, scoria tuff, cinders, expanded shale, expanded slag, expanded clay, expanded slate, exfoliated vermiculite and mixtures thereof, with expanded shale or exfoliated vermiculite being particularly advantageous. Exfoliated or expanded vermiculite is produced when sheets of mined vermiculite are exposed to temperatures of about 2000 F. which removes a substantial amount of moisture causing the sheets to separate and move apart. At the same time the granules of vermiculite expand 1215 times their original size forming thousands of tiny cells of dead air. There are several advantages of containers made from light weight aggregates as compared with those composed of metal and other materials previously suggested in the prior art. These light weight aggregates are inexpensive and therefore can be discarded after burning the fuel or in some instances may be crushed and used as a soil conditioner. The containers are also non-combustible, highly resistant to extreme temperature conditions, and as an essential feature possess a low bulk density, permanently porous structure which acts as a wick in cooperating with the solid fuel to assure relatively constant heat from ignition until the flame is extinguished. The fuel block containers can be formed by compression and thus their porosity may depend to a considerable extent on formation pressures. The closed side walls and, bottom of the container may frequently have thicknesses of about 4 to 2 inches, preferably /2 to 1 inch. Frequently the container is an open top square or rectangular box with essentially solid walls and a bottom but other shapes can be employed.

A binder material is added to the light weight aggregate of the present invention before the container is formed, and is especially useful in molding the containers. Suitable inorganic binders include sodium silicate and portland cement. The preferred binder is portland cement. While portland cement has a cost advantage over the other binder, it may have a weight limitation. The use of sodium silicate as a binder gives a container having an appreciable weight advantage over one made with cement.

20 This weight difference is exemplified by comparing the following two formulae:

Material required for 3-gal1on container:

Weight, pounds Another mixture suitable for forming the container has about 40 weight parts of expanded shale, 15 weight parts of portland cement and 11 weight parts of water.

In producing the present containers, a volume ratio of light weight aggregate to binder of about 2:1 to 14:1 can be used, often about 5 to 10:1. The containers of the present invention can be formed by compressing the container material mixed with the desired binder until a container of desired strength is obtained, often at least about 1000 p.s.i. The curing temperature may vary depending upon which binder is used. For example, if sodium silicate is selected as the binder, a curing temperature of about 600 C. for about thirty minutes gives a desired container which has a compressive strength of about 200 p.s.i. In using cement as the binder material for the container, it was found desirable to cure under high humidity conditions, for example, using a steam kiln for about 8-16 hours or longer at a temperature of about to F. to produce a strong container having a compressive strength of at least about 1000 p.s.i. The binder material selected for use with the light weight aggregate in forming the container of the present invention may depend on how the burner is to be used and the purpose for which it is used. For example, if a light container is desired, sodium silicate would serve as a better binder o ver portland cement.

The following example is included to further illustrate the present invention and is not intended to be limiting.

EXAMPLE Several compositions listed in Table I were prepared and a number of concrete fuel block containers were coated using various methods to apply the coating. The coated blocks were dried at 350 F. for 30 minutes unless indicated otherwise. Table II describes the type of blocks coated as well as the methods of coating employed and the results obtained with each method.

TABLE I.COAIING FORMULATIONS Calcined 40 sodium silicate I I diatomaceous Slhca 200 1 earth Water Other Solids Weight, lbs. Percent Water weight dry Weight Weight added, percent Weight Weight Wet Dry weight Lbs. percent Lbs. percent lbs. total Lbs. percent percent Formulation:

A 12 4. 5 10. 4 20 46. 1 4.0 9. 2 7. 3 34. 3 None 65. 7 12 4. 5 9.3 20 41.4 4.0 8.3 12.3 41. d0- 59.0 12 4.5 9.5 20 42.1 4.0 8.4 11.5 40 do 60 14 5. 2 10.3 10 19. 8 4.0 7. 9 12. 42. 2 Silica 80 Z 10 57. 8 12 4. 5 9. 5 45. 3 4. 0 10.6 11. 5 37. 6 None 62. 4 14 5.2 9.6 20 37.1 4.0 7.4 16.0 45.9 o 54.1 15 20. 3 17 29. 8 None None--. 43. 8 do 56. 2

1 Silica 300 is 300 mesh silica. Z Silica 80 is 80 mesh silica.

TABLE II.-RESULTS OF COATING BLOCK Weight 3 Number Type of Coating pickup of blocks block Pretreatment 1 mixture Coating method 2 (lbs.) Filling 4 Firing 5 Runs 6 X-2 None D 4 blocks vacuum dipped 1 block 0.8-0.9 Light (lt.) to very No change.

single and 1 block double dipped light (v. lt.) sweat. with no vacuum. 2 6 T-block .....do E 8 4 blocks vacuum dipped 0. 8-0.9 lVlloderate,1t. and v.

t. 1 block single and 1 block double 0.8 Moderate light dipped with no vacuum. 0.9 Double dip no vacuum 0. 6-1. 0 Very light Do. Vacuum during 20-30 seconds 01 dip 2. 7 No sweat D0. Dipped 10 sec., vacuum see. (1 Very light sweat D0.

had cracked bottom, leaked on filling). 6 1 X-2 Surface brushed B 9 Vacuum 10 sec 3. 5 Light sweat, pittcd Not fired.

with water to increase (0.1-0.2 lb.) water. 7 4 X-2 None C Dipped 10 sec., vacuum 25 sec 1. 6-1. 8 Light to very it. No change.

sweat.

1 Blocks were conditioned outside under a tarpaulin. Before applying the coating the blocks were moved into a F. room and left overnight. Blocks contained about 2-3 lbs. of moisture when coated.

2 Vacuum (1 inch of mercury) when used was applied to block before and during dipping (1 minute).

3 Weight Pickup, weight increase due to wet coating.

4 Appearance of coating aiter filling heated block with hot wax. Heavy sweat, all sides wet and some buildup of wax on lower half of block; Moderate sweat, all sides wet but no buildup; Light sweat, hall of total outslde area wet with wax; Very light sweat, less than one-fourth of total area wet with 5 N 0 change, no degradation of coating. Loss of wax less than lb. Block with large cracks or holes was not fired. 6 Rectangular shaped block 12% x 7% x 7%:400 square niches, 1% thick, weight, about 20 lbs., made from 6 parts by volume of expanded shale aggregate and 1 part by volume of Portland cement.

7 Square block, dimensions 9% x 9% x 7%=300 square inches, thick. weight about 14 lbs.

8 About lb. of water was added to bath after dipping each block to maintain original bath composition. 9 The B treated blocks were dried in 300 F. oven overnight Whlle the G treated blocks were dried at 300600 F. for 1.25 hours.

Average. 11 Not measured.

It is claimed: 1. An open-top container comprising a light weight sisting of sodium silicate and portland cement in an aggregate to binder volume ratio of about 2:1 to 14:1, the exterior surfaces of said container but not the interior surfaces thereof being sealed with a heat-cured coating comprising about 1 to 8 parts by weight sodium silicate and about 3 to 12 parts by weight silica of about to 350 mesh size.

2. The container of claim 1 wherein the coating has about 6 to 10 parts by weight of said silica.

3. The container of claim 2 wherein the coating has a minor amount of diatomaceous earth.

4. The container of claim 2 wherein the binder is portland cement.

5. The container of claim 1 wherein the aggregate to binder ratio is about 5:1 to 10:1 and the coating comprises about 1 to 6 parts of sodium silicate and about 6 to 10 parts by Weight of said silica.

6. The container of claim 5 wherein the binder is portland cement.

7. The container of claim 6 wherein the coating has a minor amount of diatomaceous earth.

References Cited UNITED STATES PATENTS 3,036,929 5/1962 Kawashima et a1. 11794X 0 ALFRED L. LEAVITT, Primary Examiner WM. E. BALL, Assistant Examiner U.S. Cl. X.R.