US2204581A - Insulating material and its manufacture - Google Patents

Description

June 18, 1940. P. s. DEN-NING 2,204,581

INSULATING MATERIAL AND ITS MANUFACTURE Filed Jan. 26, 1938 .u'

7 J2 0672.882" Paul flervzir g 25/ 4% (g7 Patented June 18, 1940 H UNITED STATES PATENT OFFICE INSULATING MATERIAL AND ITS MANUFACTURE of Illinois Application January 26, 1938, Serial No. 187,038

Claims.

This invention relates to insulating material and its manufacture, and among other objects aims to provide an insulating material capable of resisting high temperatures and having im- 5 proved insulating efficiency.

The nature of the invention may be readily understood by reference to one product and its manufacture embodying the invention and shown in the accompanying drawing.

In said drawing:

Fig. 1 is a section on an enlarged scale of a portion of a slab or other molded shape of the material representing somewhat diagrammatically the arrangement of the ingredients comprising the material;

Fig. 2 is a sectional elevation of a mold for making fiat slabs of the material and of a portion of the apparatus associated therewith;

Fig. 3 is a transverse section taken on the plane 33, of Fig. .2;

Fig. 4 is a sectional elevation of another form of mold for making the slab; and

Fig. 5 is a section taken on the plane 5-5 of Fig. 4. 1 Most insulating materials which are efiicient at low temperatures cannot withstand high temperatures. Moreover, the qualities which render a material efficient as an insulator at low temperatures do not necessarily render it so at high temperatures where a much larger'proportion of the heat energy is represented by radiant heat. Radiant heat energy increases approximately as the cube of the temperature and, of course, readily penetrates air spaces which at low temperatures are eflicient insulators.

Exfoliated vermiculite is both refractory and an e men men a or a igh as well as low temperatures. It is an alteration product of certain icac is i r l such as biotite', which exfoliate or expand to many times eir original size upon the application of heat to produce a granular material weighing as little as four pounds per cubic foot and characterized by a multitude of extremely thin and polished laminae which are very slightly separated upon expansion or exfoliation of the material. The reflecting action of the multitude of highly polished laminae very materially contributes to the resistance which the material presents to the transmission of radiant heat. The slight separation of the laminae upon exfoliation gives the material its insulating efficiency at low temperatures and, of course, materially contributes to the insulating efliciency of the material at high tem- 55 peratures The efficient resistance to penetration of radiant heat is effective only in directions which are transverse to the polished laminae, but in a given layer of granules having random arrangement at least fifty per cent (50%) of the granules lie with their laminae approximately I transverse to the direction of the flow of heat.

It is difficult, however, to preserve the natural insulating efficiency of a layer of granules of exfoliated vermiculite if the latter be embodied in molded insulation. The use of bonding '10 cements and other bonding materials (for bonding the granules of vermiculite together) very greatly reduces the insulating efficiency of the material and, of course, greatly increases its weight. The molded insulation heretofore proposed has contained far too much bonding material to produce an eilicient insulation.

Exfoliated vermiculite, when bonded with magnesium oxysulphate cement, provides an insulating material of unexpectedly high efliciency. I have discovered that magnesium oxysulphate cement is unexpectedly quite refractory, being capable of resisting temperatures up to 1800 F.

It is much stronger than other refractory cements, thereby making it possible to use less 3 cement to secure the desired strength. Indeed the weight of the cement may be considerably less than one-third of the weight of the material. Magnesium oxysulphate cement is formed bycombining magnesium oxide with a solution of magnesium sulphate. The precise nature of the reaction between these substances is not known but probably there is a partial or progressive solution of the magnesium oxide in the magnesium sulphate solution which materially assists in obtaining a uniform distribution of the cement and the intimate contact thereof with the granules to produce a good bond. When uniformly distributed throughout a mass of vermiculite granules, a satisfactory bond may be produced by using from 4 to 8 pounds of 5- nesium oxide per 25 pounds of w miculite, a ratio by volume of about 1 to 25. Enough ma esium sul hat solution (of about 5 to 10 Eaumg concentration) is used to obtain the desired consistency of mixture. Excess magnesium sulphate solution may be permitted to drain away.

I have found that the use of a small amount of asbestos fiber greatly assists in maintaining the um"form distribution of the magnesium sulphate cement throughout the mass and particularly in preventing segregation of the cement in the material. Uniform distribution of the cement throughout the material and prevention of 5 segregation of the cement are very important in minimizing the amount of cement which must be used and in securing a lightweight but strong material of maximum insulating efficiency. The fiber is not used primarily as a reinforcing or strengthening agent for the material. Indeed, the proportion used is too small for it to function effectively in this respect. Its use is minimized to avoid unnecessary increase in the weight of the material and fiuxing action at high temperatures.

A thorough distribution of the cement throughout the mass as well as a substantial increase in strength of the resulting material may be effected by preparing the ingredients in the form of a slurry (i. e., a soupy or sloppy mixture) wherein they are thoroughly mixed and uniform-v ly distributed prior to deposit in a mold or other forming means. In the slurry the granules of vermiculite are relatively free to move, and during removal of the excess liquid they tend to orient themselves with their flat faces generally parallel to the face of the mold. Of course, not all of the granules so orient themselves, but there is a sufficient orientation of the granules to produce a very material increase in insulating efliciency of the material (over that of a random arrangement of granules) in a direction perpendicular to the face of the mold, this being the direction of travel of the heat when the material is placed in use. During the mixing of the material in the slurry, a substantial number of granules are split into individual laminae or into thin fiat plates. These tend to overlap with each other and with the granules in a sort of felting relationship which greatly increases the strength of the material. The insulating eificiency of the material is not reduced by the splitting of the granules into very thin laminae or by the reduction of the granules into a plurality of thin fiat plates.

During the removal of excess liquid from the molded material, the asbestos fiber (which is, of course distributed uniformly throughout the mold) not only tends to prevent migration of the cement with the liquid (i. e., segregation of the cement) but opens up the overlapping laminae and plates for the escape of liquid. The exact manner by which the asbestos fiber prevents segregation of the cement is not fully understood. The action of the fibers both as an aid in dewatering the material and in maintaining uniformity of distribution of the cement (i. e., preventing segregation) is much more pronounced than one would expect from the small amount of fiber employed.

An effort has been made to illustrate diagrammatically (in Fig. 1) the relation between the several constituents of the product, but obviously exact illustration is impossible. Considerable exaggeration is necessary to represent the fine asbestos fiber and its function in preventing segregation of the cement and in facilitating dewatering. As here shown, the granules 8 and the individual laminae 9 (and thin plates comprising a plurality of laminae) are felted together, i. e., overlapped, and a large proportion of them lie with their faces generally parallel to a face of the product. Of course, not all granules and platesare so arranged, but this parallel relation isvery substantially greater than would be the case if the arrangement were merely a random one. The asbestos fibers ID are shown in exaggerated proportion lying between the granules and plates and forming nuclei for the magnesium oxysulphate cement II which binds the granules and plates together. The fibers It, therefore, function to locate the cement where it can function with maximum efficiency as a bonding agent and to render the felting of the vermiculite effective in increasing the strength of the material.

The dewatering of the molded material may be greatly hastened and the qualities of the material improved by the application of suction to the mold. This quickly displaces the surplus magnesium sulphate solution without compressing the material, leaving the voids in the material fllled with air. The removal of the liquid by the use of air pressure produces a substantially different and greatly superior product to what would result from displacing excess liquid by use of a press which functions by compressing the material and reducing the voids therein.

Various forms of suction molds may be employed. In Figs. 2 and 3 is illustrated one form of apparatus for molding fiat slabs l2. It will be understood that the shape of the mold l3 depends upon the particular shape and proportions of the slab desired. For example, in making insulating pipe covering, the mold should have a semi-cylindrical or cylindrical contour. One face or bottom of the mold is perforated to allow the liquid to drain therefrom. In the present instance, the perforated bottom comprises a perforated metal supporting plate l4 upon which rests a screen cloth l5 of sufliciently fine mesh to prevent the escape of any of the solid constituents of the mixture. As here shown, the bottom of the mold is enclosed by a hopper or hood IE to which a suction line I1 is connected. To fill the mold, it is simply necessary to dip or immerse the same in the slurry until enough material has entered the mold or to apply and maintain suction until the desired amount of solid material is drawn into the mold. The mold is then withdrawn from the mixture and the suction continued until the slab has been adequately dewatered. After which, the insulating slab may be discharged by the application of air pressure from line l8. A two-way valve I8 is advantageously introduced between the suction and air pressure lines I! and I8 so that suction and pressure may be selectively applied to the mold.

The ends of the mold are preferably provided with trunnions by which it may be handled and inverted as desired. For example, the mold may be carried on a continuous conveyor or the like from the supply point to the discharge point and back again. As here shown, the mold may be advantageously carried in an inverted position, i. e., with its open face down. This considerably simplifies the filling of the mold which may be effected by simply immersing the mold until with the aid of suction the mold has been completely filled with solid material. Thereafter, the mold is withdrawn and when the slab has been adequately dewatered, it is placed against a supporting surface and air pressure applied to separate the slab from the mold without danger of cracking it in its weak condition. This procedure may be subject to some modification for large area molds wherein it might not be possible for the falling away of some solids if the mold be withdrawn in inverted position. In that event,

it is desirable to withdraw the mold from the slurry open side up and thereafter to invert it and discharge the slab in the manner just described.

The initial mixture is sufficiently fluid to allow the vermiculite laminae and granules to arrange themselves during the dewatering process. As the liquid is drained away, it carries the laminae and granules toward the perforated face, during which movement they tend to shift to bring their flat faces to rest parallel to the perforated face of the mold, and, of course, parallel to the face of the material. Such action causes a substantial felting or interlocking of adjacent laminae and granules. While the fiber in the mixture does not interfere with the arrangement and overlapping of the laminae, it does prevent the formation of a structure so impervious as to impede dewatering. At the same time, the fiber performs the important function of preventing migration of the cement with the liquid to the discharge face of the mold.-

The magnesium sulphate solution not only performs the function of providing the plasticity or fluidity necessary to effect uniform and thorough distribution of the various constitutents throughout the mass, but a portion of it actually goes into chemical combination with the magnesia to form the cement. This portion, therefore, does not leave any voids or fine pin holes upon drying of the cement which characterizes hydraulic cements, wherein the water used toprovide plasticity leaves pin holes (upon drying) which weaken the cement. Pin holes in magnesium oxysulphate cement are, therefore, greatly minimized and the resulting cement is much stronger.

It is not essential for the above-described action of the fiber that long fiber material be used. Indeed, a short fiber asbestos of --0-5-11 Canadian grading is fairly satisfactory for this purpose, although generally it is preferable to use a somewhat longer fiber since, for reasons not fully understood, the longer fibers seem to produce a more rapid dewatering, probably because they can anchor themselves more effectively in the mass in position to hold the granules and laminae open and are not carried with the liquid as it drains through the mass. The definition of the fiber length by reference to Canadian grading is a simple means for generally specifying the character of fiber. The grading apparatus comprises three superposed sieves of mesh, 4 mesh and 10 mesh, respectively. In testing the fiber, a quantity of 16 ounces of fiber is placed on the top or mesh sieve. The sieves are vibrated laterally rapidly for about two minutes after which the asbestos remaining on the successive sieves and that passing through all the sieves and into a receptacle below the bottom sieve are weighed. This is expressed in a series of four numerals indicating the ounces of asbestos remaining respectively on the sieves and in the receptacle. Thus in the specification given above the asbestos is of such character that none remains on the A or 4 mesh sieve, 5 ounces remain on the mesh sieve and 11 ounces pass through into the receptacle. A satisfactory long fiber asbestos is one having a 0--86-2 Canadian grading. If the slab be intended for high temperature use, the fiber used should, of course, be capable of safely withstanding the temperatures likely to be encountered.

The illustrative process materially contributes to the reduction in amount of cement required and the resulting increase in insulating efficiency. By way of illustration, a satisfactory material may be prepared by mixing the material in the proportion of 53 pounds of exfoliated vermiculite granules of a minus four to plus ten mesh size, about three pounds of long fiber asbestos, and from 4 to 8 pounds of caustic magnesium oxide. Sufficient magnesium sulphate solution is employed to provide a rather sloppy mixture. Magnesium sulphate solution of from 5 to 10 B. concentration is satisfactory but it is not essential that the concentration be kept within these limits. It will be understood that the expression minus 4 to plus 10 mesh size defines granular sizes which will pass through a four mesh screen and be retained on a ten mesh .10 screen.

The quality of the exfoliated vermiculite aggregate may be substantially improved if it be first floated upon a liquid constituent of the cement,

i. e., in this instance magnesium sulphate solu- .-15 tion. During the floating action, the heavy impurities, such as rock (which has not been previously eliminated during the expanding process) will sink to the bottom of the solution and only the vermiculite granules will remain floating on go the surface. Since the liquid floating vehicle is a constituent of the cement, it is unnecessary to effect a complete separation of the liquid and the vermiculite. It is necessary simply to collect the vermiculite from the liquid, allowing the latter 2,5 to drain away by gravity. What adheres by capillarity and otherwise to the vermiculite is simply carried into the cement, and the deficiency of the magnesium sulphate solution in the mixture is supplied by the addition of enough magnesium sulphate solution to produce a slurry of the proper consistency.

In Figs. 4 and 5 of the drawing is illustrated another apparatus for molding flat slabs. As there shown, the molds 23 have a perforated bottom comprising a perforated metal plate 24 upon which rests a screen cloth 25. Suction is applied to the perforated face of the metal by suction box 26 having adjacent its upper edge a seat 21 upon which the mold may rest during the application of suction. A gasket 28 serves to prevent leakage around the edge of the mold. The mold bottom is supported at intermediate points by plates 29 which are perforated to permit access of suction and withdrawal of the liquid. v

After dewatering, the suction valve 3| in suction line 30 is closed and the mold may be lifted off the suction box and inverted to discharge the dewatered slab.

After the slabs have been dewatered and removed from their molds, they are preferably dried in a moderate heat of from 400 to 600 F. Improvement in strength has been observed if the slabs be permitted to air cure for twenty- 355 four to forty-eight hours before introduction into a drier.

The slabs are advantageously initially made of substantial thickness, that is from 4 to 6 inches, and may be sawed longitudinally into two or more slabs of less thickness if desired. To secure close fitting joints between the slabs, it is preferable to saw the molded slabs so as to provide absolutely square and true faces. Waste may be minimized by making the slab initially 455 of sufiicient thickness to provide a plurality of these slabs of less thickness. Because of the arrangement of granules, the slabs split so perfectly under the saw as to leave a perfectly smooth sawed surface. The saw waste may be '70 used again as a part of the aggregate.

In most uses of the material, only one. face is subjected to high temperatures, and because of the eflicient insulating action of the vermiculite granules it is not serious if such temperatures actually reach the point of dehydration of the magnesium oxysulphate cement since the heat penetrates only the surface layer and the remainder of the slab is unaffected and serves to support the surface layer. However, where the slab is exposed to prolonged excessive temperatures, and particularly to so-called soaking temperatures, it is desirable to provide means for strengthening or hardening the slab throughout the temperature range wherein dehydration of the magnesium oxysulphate cement occurs.

I have discovered that the addition of a highly siliceous material comprising particles having a large surface area in relation to their volume, if brought into intimate contact with the m nesia cement, will harden the material and produce an excellent supplementary bond which preserves the strength and hardness of the material throughout the temperature range to which it may be subjected.

Diatomaceous earth, because of its large surface area in re a ion to volume of particles, has proved quite satisfactory in this connection. The relatively large surface area of the particles makes it possible to bring it into a much more complete and intimate contact with the magnesium oxysulphate cement than if they were roughly spherical in shape, with the result that when high temperatures are encountered, a reaction probably takes place between the magnesium of the cement and the particles of diatomaceous earth to form magnesium silicate, a highly refractory bonding agent. Such reaction takes place in the neighborhood of 1400 F., despite the fact that this temperature is far below the fusion points of either diatomaceous earth, magnesia, or the exfoliated vermiculite.

While other finely divided materials, such as silica sand, would produce a similar reaction, it would be necessary to use it in so much greater quantities to secure the intimate contact necessary for chemical reaction, that the weight of the material would be objectionably increased. Most siliceous particles have a roughly spherical contour which has a small surface area in relation to volume. The peculiar origin of diatomaceous earth (skeletons of diatoms) is responsible for the very high surface area of the particles in relation to their volume. Therefore, with a relatively small amount of diatomaceous earth a relatively extensive contact with the magnesia cement may be effected, resulting in the development of a very high proportion of ceramic bonding material (probably magnesium silicate) in relation to the amount of diatomaceous earth employed. In this connection, it should be remembered that the extent of the chemical reaction which occurs at elevated temperatures depends upon the extent of contact between the reacting materials.

The aforesaid intimate contact between the magnesia and the diatomaceous earth is greatly promoted by the circumstance that the magnesium sulphate is employed in the form of a solution and can, therefore, much more intimately and completely contact with the particles of diatomaceous earth than solid particles of magnesia.

Other siliceous materials having a large surface area in relation to the volume of particles, such as a siliceous volcanic ash, would produce similar resu s. e particles of siliceous material should not, however, be of colloidal size since they would tend to make the material impervious and retard the dewatering operation.

The amount of diatomaceous earth employed may be considerably varied. I have found that if the weight of diatomaceous earth is about of that of exfoliated vermiculite, a very satisfactory product is produced. When made by the process above disclosed, the same reduction in amount of cement is possible. About 8 pounds of magnesium oxide are required for a weight of 60 pounds of exfoliated vermiculite and diatomaceous earth.

This application is a continuation in part of my copending application, Serial No. 75,789, filed April 22, 1936.

Obviously the invention is not limited to the details of the illustrative embodiment thereof herein disclosed since these may be variously modified. Moreover, it is not indispensable that all features of the invention be used conjointly since various features may be used to advantage in different combinations and sub-combinations.

Having described my invention, I claim:

1. An unfired refractory insulation efiicient at high temperatures against radiant heat comprising, in combination, particles of exfoliated vermiculite having shiny laminae capable of reflecting radiant heat and arranged so that a major portion thereof are generally parallel to each other and overlap each other in felting relationship to strengthen the material, and magnesium oxysulphate cement bonding the vermiculite particles together, said cement being used in a proportion insumcient to provide adequate strength without the added strength provided by the felting of said vermiculite particles.

2. A molded refractory insulation efficient at high temperatures against radiant heat comprising, in combination, particles of exfoliated vermiculite having shiny laminae capable of refiecting radiant heat and arranged so that a major portion thereof lie with their fiat faces in overlapping and felting relationship and generally parallel to a broad face of the insulation designed to be exposed to high temperatures, a small amount of refractory cementitious material between the particles of vermiculite binding the same together, the amount of said cementitious material being insufficient to provide adequate strength for the insulation without the added strength provided by the felting of said vermiculite, and a small amount of asbestos fiber or the like between the faces of said vermiculite to render the insulation somewhat pervious.

3. A molded refractory insulation efficient at high temperatures against radiant heat comprising, in combination, particles of exfoliated vermiculite having shiny laminae capable of reflecting radiant heat and arranged so that a major portion thereof are generally parallel to each other and overlap in felting relation to strengthen the material, said vermiculite being bonded together with magnesium oxysulphate cement aggregating by volume about & of that of said vermiculite.

4. A molded refractory insulation efiicient at high temperatures against radiant heat com prising, in combination, particles of exfoliated vermiculite having shiny laminae capable of refleeting radiant heat and arranged so that a major portion thereof lie with their flat faces in overlapping and felting relation, said vermiculite being bonded together with a cement formed with magnesium oxide and magnesium sulphate solution, and a quantity of highly siliceous material in intimate contact with the magnesium sulphate solution to react at elevated tempera- 10 lite being bonded together with a cement formed with magnesium oxide and magnesium sulphate solution, and a quantity of diatomaceous earth whose particles have large surface area in relation to their volume distributed throughout the insulation and in intimate contact with said 5 cement to react therewith at elevated temperatures to form magnesium silicate as a supplementary bonding agent.

PAUL S. DENNIN G.

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