SUMMARY OF THE INVENTION

[OOOl ^' l Described herein is an ethoxylated starch crossiinked with a cross linking a ents wherein the degree of ethoxy substitutio on the starch is from 1% to 3%; the crosslinking agent is from 0.02% to 0.05% of the weight of starch; and a 7% wt/wt aqueous solution of the crossiinked ethox starch exhibits a viscosity of at least 1600 cps at 95°C In a typical embodiment the ethoxylated crossiinked starch has a degree of substitution from 1.4% to 2.4%. Any crosslinking agent may be used, in exemplary embodiments the cross linking agent is epkhlorohydrm. In the most desirable embodiments the 7% solution exhibits a viscosity of 1700-1900 cps. In exemplary embodiments the 7% solution exhibits a viscosity of 1700-1800 cps.

[0002] Also described is the use of the ethoxylated crossiinked starch to form ceiling tiles from a mixture comprising the ethoxylated crossiinked starch combined with borax, stucco, steel wool in water. In a general embodiment with respect to the volume of water, the amount of borax is 0.1.5%-0.35% wt/vol, the stucco is 4%~5% wt/vol and the ethoxylated crossiinked starch is 3.25%-4,25% wt/ vol. In an more specific embodiment the borax is about 0,25%, wt/ ol and the stucco is about 4.5% wt/vol. In another more specific embodiment the crossiinked ethoxylated starch is 3.25% to 4% wt/vol.

|0003] The ethoxylated crossiinked starch of the present invention is also useful in hydraulic fracturing fluids as a viscosity enhancing agent BRIEF DESCRIPTIO OF THE DRAWINGS

[0004] Figure lis a graph illustrating the effects of crosslinking degree (expressed as % crosslinking on the x axis) on viscosity (expressed in centipoise on the y axis as measured using a 7% aqueous solution at 3°C) of ordinary corn, starch (♦ ) in comparison to ethoxylated com starch {« ) having a 1.8% degree of ethox substitution.

DETAILED DESCRIPTION

fOCXSJ The present disclosure is directed to the surprising discovery that ethoxylated starch having a certain defined degree of substitution combined with a crosslinklng agent within a certain defined range of

crosslinking, together operate to produce a crosslinked ethoxylated starch having a significantly higher viscosity than can be obtained using ordinary starch or ethoxy substituted starch alone. The degree of substitution of the ethoxy groups on the starch refers to the percentage of sugar residues in the starch that are ethoxylated. The degree of substitution, should be in the range of 1 % to 3%. in a more narrow range for particular embodiments the degree of substitution should, be in the range of 1.4% to 2,4%. A .1.3% degree of substitution represents a typical embodiment.

[0006] The degree of crosslinking should be in the range of 0.015% to 0.05%. Degree of crosslinking is defined as the ratio of crosslinking agent to starch on. a t/ et basis used in the crosslinking reaction. It is believed that all or nearly all of the crosslinking agent is converted in the reaction so that the degree of crosslinking is equal to the ratio of crosslinking agent to starch used in the reaction. However, it has not been chemically proven that all of the crosslinking agent is converted, hence, to resolve any ambiguity, the degree of crosslinking is hereby defined as the ratio of materials used, in the reaction, regardless of whether the ultimate crosslinked ethoxy starch product precisely has the same wt/ wt ratio of crosslinking agent to starch. [0007J Figure 1 illustrates the surprising effect of the combination of crosslinking and ethoxylatiori of starch on viscosity. The data shown in Figure 1 is based on comparing ordinary corn starch to ethoxylafc -d corn starch that had been substituted to a degree of 1.8%. Without any crosslinking agent the erhoxy a Ited starch with this degree of substitution is actually less viscous than ordinary starch by about --20%. However, when the ethoxylated starch is crosslinked to within the range of (1015% to 0.05% the viscosity is substantially increased. At a crosslinking ratio of 0.02% to 0.04% the viscosity surpasses that achievable by crosslinking ordinary starch by +16% to + 20%. Higher degrees of crosslinking however begin to reverse that trend, so eihoxyaited starch crosslinked to a ratio higher than 0.05% again demonstrates less viscosity than ordinary starch crosslinked to the same degree. The viscosi y measureme s depicted in Figure 1 were determined using a. Rapid Viseo Analyzer (RV'A) meter with, an aqueous solution containing 7% wt/vol of the starch or ethoxylated. starch and was measured at a temperature of 95°C

0008] Strikingly, the increase of viscosity exhibited by ethylene oxide cross linked starch reaching in the range of 1600 - 1900 cps is not observed with propylene oxide crosslinked starch, which is commonly used in. food

applications to increase the viscosity of food products such as yogurts, puddings and the like. The maximum viscosity achieved using propylene oxide r ther than ethylene oxide to crosslink the starch was about 1380 cps, which was achieved at a cross linking ratio of epichiorhydrin of 0.01%. Higher degrees of crosslinking up to 0.03% did not improve the viscosity of the propylene oxide cross linked starch, and in fact showed less viscosi y.

£0009] Crosslinked ethoxylated starch with the increased viscosity characteristics described herein are particularly useful for the formation of industrial materials molded in a process that requires drying aqueous solutions of reagents in a form. One particular embodiment where the crosslinked ethoxylated starch was shown to he particularly useful is in the formation of ceiling tiles, Ceiling tiles are made by formin a mixture of borax, (0.!5%~0.35% wt/ vol) _s t _i¾ cco (4%~5% wt/vol) and starch (4 %~5% wt/vol) in water with steel wool that is cooked at a temperature of 180-210°F then poured into an open face mold and dried, h the most common formulations the borax is about 0.25%,. the stucco is about 4,5% and the starch is about 4.5%. The purpose of the starch, is to increase the viscosity of the mixture so that the mold holds its form along the edges when dried. The higher viscosity also permits easier imprinting of texture features on the front face of the ceiling tiles by running them through a stamping die. Use of ethoxylated cross! inked starch in lieu of starch for such a process can provide the same levels of viscosity using 5% to 20% less amounts of the ethoxylated starch than would be required using ordinary starch.

[0010] Accordingly another aspect of the present disclosure is a mixture tor forming ceiling comprisin , at least 90% wt/vol water, O.15%~035% wt/vol borax, 4%-5% wt/ vol stucco, steel wool, and 3.25%~4.25% wt/vol crosslinked ethoxy substituted starch, wherein the degree of ethoxy substitution on the starch is from 1 % to 3%; the crosslinking agent is from 0.02% to 0.05% of the weight of starch; and a 7% wt/ wt aqueous solution of the crosslinked ethoxy starch exhibits a viscosity of at least 1600 cps at 95°C In preferred formulations the amount, of the ethoxy substituted starch is starch is 3.25% to 4% wt/vol

[0011| Another application for ethoxylated starch is in hydraulic f racturing fluids (tracking fluids) used to release hydrocarbons from shale and other mineral deposits. Bracking fluids typically utilize a viscosity enhancing agent in a water slurry with popping agents, gelling agents and other materials. Use of ethoxylated crosslinked starch as a viscosit enhancing agent provides greater latitude in adjusting the viscosity of such fluids. Accordingly, another aspect of the present inventio is a tracking fluid containing a viscosity enhancing agent comprised of ethoxy substituted starch, wherein the degree of ethoxy substitution on. the starch is from 1% to 3%; the crosslinking agent is .from 0.02% to 0.05% of the weight of starch; and a 7% wt/ wt aqueous solution of the crosslinked ethoxy starch exhibits a viscosity of at least 1600 cps at 95 ^* C. [0012] A method of making the ethoxyiated starch compound having such a high viscosity includes contacting a suspension of starch in a 1 -5% solution of NaC! in water with ethylene oxide at a wt/wt ratio of ethylene oxide to starch of 0.06 to 0.09, at an alkaline pH from 10-12. The salt at this con en ration inhibits gelatiniz tion of the starch that would otherwise occur at the reaction temperature, which should be in. the range of 26.7°C to 48.9°C (i.e., 80 20^ ^' } for a period of 10 to 12 hours. Epichlorohydrin is subsequently added to the reaction mixture at 0.01% to 0.1% for a period of about 2 to 10 hours at the same temperature. The amount of crosslinking agent relative to starch is critical to obtain a. required viscosity of at least 1600 cps, and most preferably 1700 to 1900 cps. The amount of crosslinking agent should be 0.02 to 0.05 % the weight of the starch. Too little crosslinking agent will not increase the viscosity of the ethoxyiated starch to the desired lev ls and too much crosslinking agent will actually decrease the viscosity obtained with lower amounts of crosslinking agent.

[0013] Suitable crosslinking agents include epichlorohydrin, organic diacids and triacids such as succinic acid, malic, fumaric, or citric acid, acryiates and amides thereof, such as acrylic acid, aeroleion, acrylamide and bis acrylamide. Epichlorohydrin is a preferred crosslinking agent.

[0014] h exemplary methods a 3% solution of NaCl adjusted to pH 11- 11.5 with NaOH is mixed with starch to form a suspension containing 40% wt/wt starch, and ethylene oxide is added to bring it to 2.9% wt/wt in the .mixture. The mixture is reacted for 12 hours at 40.6°C (i.e., 106 ^e F) followed by addition of 0.02% to 0.05% epichlorohydrin whi h was reacted for a further five hours. This yields an ethoxyiated starch, which in a 7% aqueous solution, will have a peak viscosity of 1600-1900 cps at 95 ^C C. In one example, when the amount of epichlorhydrio was 0.025% he weight of the starch and the peak iscosity was 1740 cps for the 7% solution measured, at 95 ^ft C

|0015] Any starch may be used to make the crosslinkeo ethoxyiated starch compositions described herein. Typical starches are corn and whea starch each being about 25 % amylose nd 75% amylopec&n, potato and rice starch ~ each being about 20% amylose and 80% amyiopecitin, cassava (tapioca) starch being about 15-18% amylose and 82-85% amylopecticn and waxy- versions of the same. The waxy starch versions are those comprised of less than 10% amylose and at least 90% a ylopectic. Ethoxylated and crosslinked wax starches having the same degree of substitution and crosshnking described herein are expected to perform even better as viscosity enhancing agents due to the larger network of branching provided by the amylopectin molecule

Accordingly, lower amounts of crosslinked ethoxylated waxy starches are expected to provide the same improvements in viscosity than higher amounts of the crosslinked and ethoxylated ordinary starch varieties.