US3252818A - Thermosetting composition for exothermic hot tops - Google Patents

Description

United States Patent 3,252,818 THEOSETTING COMPOSITION FOR EXOTHERMIC HOT TOPS Ronald H. Cooper, Clare, and Robert D. Goodenough and Gerald M. Corbett, Midland, Mich, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed May 1, 1963, Ser. No. 277,231 2 Claims. (Cl. 106-217) This application is a continuation-in-part of application Serial Number 99,171, filed March 29, 1961, now abandoned.

This invention relates to improved compositions for use in porous refractory structures which will withstand heat at elevated temperatures such as are encountered upon contact with molten metals. More particularly it is concerned with improved compositions utilizing moldable exothermic compounds for the production of risers and hot tops used in the pouring of molten metals such as iron, ferrous alloys and steels into certain mold forms and to a method of fabricating such structures.

Hot tops and risers, as is well known in the art, are devices that often are employed with molds during the pouring of iron and steel to keep the metal molten in the upper portion of the ingot for as long a period of time as possible. These particularly are used in the pouring of killed steels which undergo substantial contraction upon solidification from the molten state. In use, the hot top which is affixed to the top of the mold acts as a feeder supply for the molten metal during its solidification and accompanying contraction in a mold. The resulting shrinkage cavity, or pipe as it is commonly called, thus occurs in the metal held in the hot top or in the upper portion of the ingot and thereby increases the ingot yield. In recent years, the use of exothermic hot topping compounds, as they have come to be known, has replaced for many operations the conventional insulating type refractory materials long used for these purposes.

It is a principal object of the present invention, therefore, to provide compositions for fabrication into exothermic hot tops, risers and the like porous structures which are readily moldable, have a relatively short curing time, yield a non-hygroscopic end product and are quite inexpensive.

It is also an object of the present invention to provide compositions for fabrication into exothermic hot tops and the like porous refractory structures wherein polymerization of the binder is achieved by an acidic agent which does not decompose upon heating at the curing temperatures nor liberates toxic materials at this temperature.

It is an additional object of the present invention to provide compositions which give ready ease of formation of strong and tough exothermic hot tops along with a minimum amount of after-cleaning of the fabricating apparatus.

It is a further object of the present invention to provide compositions for hot tops which, when used in metal pouring, maintain the metal in molten conditions as long as possible and thereby minimize the severity of the piping in the metal formed in the mold.

It is an additional object of the present invention to provide compositions for use in the preparation of fabri- "ice compositions which readily are blended into homogeneous mixtures for the fabrication of exothermic hot tops and the like refractory structures.

These and other objects and advantages will become apparent from the detailed description presented herein after and by reference to the accompanying figure.

The drawing in the figure shows schematically a unitary exothermic hot top device prepared from a composition of the instant invention.

In accordance with the present invention, the compositions for use in porous exothermic refractory structures which will withstand heat at elevated temperatures comprise a commonly used exothermic filler material such as a mixture of aluminum, oxidizing agents and refractory powders and a binder system built around novel sugar polymers. In the production of hot tops, for example, the filler can comprise from about to about weight percent or more of the composition with the composite binder system making up the balance.

Fillers useful in the present compositions are those particular materials conventionally used in the manufacture of exothermic hot tops and risers for ingot molds and castings. These materials include, for example, mixtures of aluminum, oxidizing agents such as thermally reducible metal oxides (manganese dioxide or iron oxide) and oxygen containing salts and refractory powders. Additionally, metallic halides such as potassium chloride, sodium chloride and the like can be incorporated into the hot topping compound if desired. These latter materials, if added, impart a fluxing action on the aluminum particles and promote good combustion of the exothermic mixture. The filler material will be in particulated form as is commonly used in preparing the exothermic fabrications. Another illustrative example of an exothermic hot topping composition known in the art is described in US. Patent No. 2,591,105. The material taught in this patent is a composition formed from an oxidizable metal such as aluminum (30-50 weight percent), metal oxide such as iron oxide or manganese oxide (5-20 weight percent) i.e. in an amount substantially less than required to completely oxidize the aluminum and a fluoride suchas an alkali fluoride or alkali aluminum fluoride (1-5 weight percent) to facilitate the start of the reaction upon contact with molten metal and to facilitate maintaining the reaction to completion. Additionally, this illustrative patented composition can contain sodium nitrate (0-10 weight percent), bentonite (0-2 weight percent), core gum (2-7 weight percent) and granulated grog (remainder).

The binder used in the improved compositions of the instant invention is a polymeric composition resulting from the acid polymerization of monosaccharide sugars, preferably by employing alkali metal hydrogen sulfates as the acidic reaction promoter. These monosaccharides, such as glucose and fructose for example, can be blended directly with the filler. However, conveniently the monosaccharides are produced in situ by hydrolysis of readily obtainable inexpensive diand polysaccharide hexose sugars as are present in cane sugar, beet sugar, molasses and corn sugar. The hydrolysis results in utilizing the same acidic material as hydrolyzer which subsequently, at moderately elevated curing temperatures, acts to give polymerization of the monosaccharides. In the production of the exothermic hot top and riser fabrications the amount of saccharide to be employed will range from about 3 percent to about 14 percent by weight of theexothermic aggregate filler. Preferably the sugar concentration will range from about 6 to about 10 percent of the aggregate weight.

Use of a binder having a relatively high sugar content is not desirable for those structures which will be subjected to relatively high temperatures during metal pouring because of the problems that may be introduced by undesirable decomposition of the binder.

For example, hot top devices which contain more than 14 percent by weight of polymerized sugars, may fire and smoke excessively, cause intolerable degrees of metal splattering and give carbonization of the ingot. In fact, at such high sugar levels, the hot tops may not even be capable of the primary requisite to hold the hot metal during the .pouring of a heat. Also, at the higher sugar concentrations, the cured product displays a tendency to swell.

On the other hand, hot tops and the like structures prepared using relatively small amounts of sugars can result in refractory structures which do not have the requisite strength after curing.

Useful acidic materials for hydrolyzing and polymerizing the sugar saccharides are acidic compounds (i.e., those having a hydrogen ionization constant, Ka, greater than about 1x10 including both acids and acidic salts that are non-volatile at the elevated curing temperatures, which do not decompose within this range and which do not produce undesirable decomposition of the polymerformin-g constituents during the curing stage. Further, the acidic materials will be solids which are molten at the curing temperatures. Particularly useful materials are the alkali metal hydrogen sulfates, e.-g., sodium bisulfate (NaHSO potassium bisulfate (KHSO and lithium bisulfate (LiHSO Sodium bisulfate (NaHSO because of its ready availability, low cost and effectiveness as a hydrolyzer and polymerizing agent has been found to be particularly useful in the present polymer resin forming system.

Acid materials for use as hydrolyzers and polymerization promoters should be stable at the curing temperatures employed. Thus, relatively low boiling aqueous solutions of mineral acids such as hydrochloric, dilute nitric, dilute sulfuric, phosphoric, pyrophosphoric, acetic and the like generally are not useful in the present compositions as hydrolyzers and polymerization promoters as they are volatile below the curing temperatures employed. Therestore, they do not remain in contact with the mix components for a sufficiently long period to give the needed hydrolysis of polysaccharides and subsequent acid promoted polymerization of the resulting monosaccharides into the thermoset binder. Likewise, ammonium sulfate, ammonium persulfate, ammonium sulfamate, ammonium acid sulfate, zinc chloride, monoammonium phosphate and the like are not suitable for use in the present process and composition as all of these produce the corresponding volatile acids at the curing temperatures.

The amount of acid material to be used will vary depending upon the total amount and types of saccharide sugars present. Highly satisfactory polymeric binders result wherein the amount of acidic material is from about 1 to about 8 percent of the particulate exothermic hot topping compound filler. Lower amounts of acid polymerization promoter are employed advantageously when monosaccharides are used directly in the composition. Preferably, the amounts of acid will be somewhat less than the amount of sugar used; for example in an exothermic hot top utilizing from about 6 to about 10 percent sucrose correspondingly from about 1.5 to about 4.5 weight percent of NaHSO is used as hydrolyzing and monosaccharide polymerizing agent.

The actual binder formation is carried out in the presence of water as a fiuidizing medium. 'From about 3 to about 8 weight percent of water (based on total composition weight) is added to the exothermic filler-saccharideacid mixture. Preferably the amount of water will range from about 5 to about 7 weight percent of the total binderfiller composition. At higher concentrations, much above 9 weight percent, the structures tend to sag while in the green condition. Also, dimensional instability of the resulting 'hot top results as the product may undergo undesirable swelling accompanied by the formation of surface cracks during the curing operation. At water contents less than indicated, the cured product may be weak and friable.

The water can be satisfactorily added to a dry-blended mixture offiller and binder. Alternatively, an aqueous solution of the sugar-acid material or a solution of the sugar is used instead of first dry-blending the binder components with the aggregate and then tempering with water.

However, very-satisfactory hot tops are produced by first dry blending the ingredients and in most applications and operations this technique may be preferred. In preparing the dry-blends, the saccharide and acid binder components can be premixed and the blend then added to the filler. Alternatively all of the dry components can be blended simultaneously.

The dry blend of binder-filler can be used directly as mixed, or, if desired, can be stored prior to use. The dry binder-alkali metal bisulfite mix also can be stored prior to its use in a refractory composition.

In either of the above suggested methods of dryblending, it is essential to obtain a substantially uniform and thorough dispersion and interblending of all the ingredients.

The curing of formed refractory structures prepared from the instant compositions is achieved by heating the compositions at temperatures from about 350 to about 450 F., and perferably at about 400 F. for a period of from about 5 to about 30 minutes. The time of cure varies directly, both on the temperature of cure and the thickness and/ or size of the device being cured. For conventional hot top fabrications the cure time ordinarily will range from about 8 to about 15 minutes.

Curing the fabrications prepared using the present compositions at thermosetting temperatures below that indicated can yield a relatively weak structure as Well as cause difiiculties in the decomposition of the polymeric binder upon contact with heat from the molten metal or other sources. Thus, such improperly cured hot top devices may evolve considerable quantities of fire and smoke upon contact with the molten metal and may severely carbonize the resultant solidified metal in a manner similar to that seen when excessive amounts of binder are employed.

At curing temperatures higher than that given herein, degradation and some premature burnout of the polymer with subsequent loss in binding strength may be encountered.

Additionally, if desired, small amounts, from about 0.5 to about 2 weight percent refractory type minerals, iron oxides and other similar materials can be incorporated into the mix to give added thermal shock resistance to the cured product. Also, glues, molasses, pitch, cereals, bentonites, silica flour, sea coal and the like when added in similar amounts can act as thixotropic agents to give better green strength to the formed, uncured device. Small amounts from about 0.25 to about 1.0 percent by weight of thermosetting synthetic resins, e.g., phenolformaldehyde resins, when incorporated into the blend can serve as strength extenders.

To illustrate the utility of the present new and novel compositions, an exothermic hot top was prepared. In

preparing this device, a commercially available exothermic aggregate was coated with about 8 percent (on the weight of the aggregate) of sucrose, about 3 weight percent NaHSO, and the mixture tempered with about The wet, plastic green composition was dropped into a mold cavity of size as set forth above, rammed lightly and the top surface of the resulting formed product trowelled.

7 weight percent water. The coating was accomplished 5 The specimens were cured at a predetermined temperaby adding the dry binder components to the filler and mixture for a given period of time after which the compresing for about 5 minutes. The water then was added and sion strength and surface hardness of the product samples the mixing continued for an additional 5 minutes. The were determined. resulting composition, as freshly prepared, was a damp, Table I, which follows, summarizes the results of a pliable and flowable mixture of binder coated filler. number of tests run using various mix compositions and This material was pressed by ramming into a unitary process variables.

TABLE I Binder Composition 1 (weight percent) Cure Minimum Test Cure Time Temperature, Tensile Surface 4 Remarks N 0. (Min.) F. Strength, Hardness Sucrose NaHSO; H1O lbs/sq. in.

8.0 3.0 7. 0 10 400 170 90-93 Very good mix and core. 8.0 4.0 7.0 8 400 150 95-97 Excellent mix and core. 8.0 2. 0 7. 0 14 400 140 90412 Good mix and core. 6. 0 3. 0 7. 0 12 400 100 90-92 Fair mix and core. 8.0 3.0 8.0 10 450 130 90-95 Good mix and core. 8.0 3.0 7.0 10 400 150 85-87 Goog mix, not as tacky 8.0 2.0 6.0 12 400 135 85-87 G d mix and core. 8.0 2.0 6. 0 14 350 140 85-87 Good mix and fair core. 8. 0 1.0 5.0 16 350 130 85-88 Good mix and good core. 8.0 4. 0 7.0 12 350 120 85-87 Good mix and fair core. 8. 0 3. 0 7. o 12 350 110 85-87 Do. 8. o a. 0 7. o 10 400 100 75-80 10.0 3.0 7.0 12 400 125 90-92 Good mix. 10. 0 2. 3 s. 0 400 120 85-87 Do.

39% sodium silicate (control sample) 80-85 1 Based on weight of aggregate. 2 Commercial dried corn syrup was used in place of sucrose. 3 Refined dextrose, or corn sugar, was used.

4 As determined using a Dietert A.F.S. approved hardness Tester.

hot top mold form under a pressure of about 80 pounds In a manner similar to that described for the foregoing per square inch. The resulting satisfactory green hot example, exothermic risers, unitary hot tops and sectional top was cured to hydrolyze and to polymerize the binder hot tops and the like can be prepared using any of the components at a temperature of about 400 F. for about following as a binder system; glucose-potassium hydrogen 10 minutes. After this time, the thermoset hot top in sulfate, ralfinose-sodium bisulfate, beet sugar-potassium the form of a strong rigid shape was removed from the bisulfate, maltose-lithium hydrogen sulfate, fructosecuring oven. potassium hydrogen sulfate, sucrose-sodium hydrogen sul- The cured exothermic hot top was then employed in fate and the like. The binder-filler mix concentrations conjunction with an ingot mold during the pouring of and techniques to be used with these compositions are to hot top steel ingots. The heat from the molten steel be those as set forth hereinbefore. being poured decomposed and burned out the cured It is understood that any of the useful saccharide sugars polymeric sugar based resin and left a strong and unican be satisfactorily polymerized by any of the listed formly porous refractory structure which provided an acidic materials. easy escape for the gases evolved from the molten metal. Various modifications can be made in the present in- The structure, after burnout of the binder, still was vention without departing from the spirit or scope thereof strong enough to withstand the ferrostatic head developed for it is understood that we limit ourselves only as defined by the charge of molten metal. in the appended claims.

After the ingot had been poured and solidified, the We claim: hot top was readily broken away and stripped from the 1. In a thermosetting composition for the fabrication ingot. Substantially none of the burned out. structure of exothermic hot tops characterized in being thermosetadhere to the solidified metal. ting when heated to a temperature between about 350 In a similar manner, sectionalized hot tops and risers d about 5 F- and consisting essentially of a particalso can be fabricated, ulated exothermic hot topping compound aggregate filler The following example will serve to further illustrate and a binder the improvement which comprises a binder the present invention but is not meant to limit it thereto. as based on the filler weight of from about 6 to about 10 percent of a hexose saccharide selected from the group Example consisting of hexose monosaccharides and acid hydrolyzable polysaccharides and from about 1.5 to about 4.5 Test l 5 X 5 2% Were P p 118mg a percent of an alkali metal acid sulfate selected from the Commerclal xothefmlc hot PP g P group consisting of sodium hydrogen sulfate, potassium The flggr'egate Was dry blwded Wlth sugijlf-sodlum acld hydrogen sulfate and lithium hydrogen sulfate, and, from S111fate @lflder Components al'ld the resultlng blend about 4 to about 8 percent water, the water weight being PF Wlth Wale}? The coating of f aggrflg'ate y based on the total weight of said filler-binder. b d r wa carr ed out by mechanically Il'llXlIlg the m- 2. In a thermosetting composition for the fabrication gredlents 111 a mlx'mullef for about 5 q of exothermic hot tops characterized in being thermoset- T Sodium acid Sulfate WEIs ground, 8 a IIllCfO- ting when heated to a temperature between about 350 pulverizer and passed through an 0.027 inch screen. The and about 450 F. and consisting essentially of a parsucrose was a commercial product and ground through ticulated exothermic hot topping aggregate filler and a a micro-pulverizer mill before incorporation into the binder, the improvement which comprises a binder based blend. on the weight of said aggregate of about 8 percent sucrose,

about 3 percent sodium hydrogen sulfate and about 7 percent by weight water.

References Cited by the Examiner 2,996,759 8/1961 Smith 264-109 3,021,566 2/1962 Sommer 264109 FOREIGN PATENTS UNITED STATES PATENTS 5 785,984 11/1957 Great Britain.

9/1943 Happe 10640 10/1956 Gielow et 1 ALEXANDER H. BRODMERKEL, Primary Examiner.

7/1957 Pletsch et a1 22-147 3/1958 Jones ALFRED L. LEAVITT, Exammer 2/ 1959 Gogek 106 38,4 10 P- E- ANDERSON, D. ARNOLD, Assistant Examiners.

Claims (1)

1. IN A THERMOSETTING COMPOSITION FOR THE FABRICATION OF EXOTHERMIC HOT TOPS CHARACTERIZED IN BEING THERMOSETTING WHEN HEATED TO A TEMPERATURE BETWEEN ABOUT 350 AND ABOUT 450*F. AND CONSISTING ESSENTIALLY OF A PARTICULATED EXOTHERMIC HOT TOPPING COMPOUND AGGREGATE FILLER AND A BINDER THE IMPROVEMENT WHICH COMPRISES A BINDER AS BASED ON THE FILLER WEIGHT OF FROM ABOUT 6 TO ABOUT 10 PERCENT OF A HEXOSE SACCHARIDE SELECTED FROM THE GROUP CONSISTING OF HEXOSE MONOSACCHARIDES AND ACID HYDRO-

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