Patent Publication Number: US-6699555-B2

Title: Bonding of thermal tile insulation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 09/676,682, filed Sep. 29, 2000, now U.S. Pat. No. 6,494,979. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to thermal insulation tiles and more particularly to a method for bonding thermal insulation tiles. 
     BACKGROUND OF THE INVENTION 
     Thermal insulation tiles, such as those used to insulate the space shuttle orbiter, are typically formed from low-density fibrous materials having extremely high temperature resistance and a relatively low coefficient of thermal expansion as compared to metals. These materials are well known in the art and include, for example, FRCI (fibrous refractory composite insulation) and AETB (alumina enhanced thermal barrier) materials. 
     In fabricating the tiles, fibers of an insulating material, such as silica, alumina boro-silicate and alumina, are mixed with water to form a slurry. The slurry is deposited into a casting tower where the water is drained and the silica fibers are subjected to compressive forces to form a raw block of insulation material having a cross-sectional area that may range from 144 square inches to almost 576 square inches depending upon the dimensions of the casting tower. The raw block is then dried in an oven and subsequently fired (sintered) to bond the fibers of the insulating material together. Thereafter, tiles are formed from the fired block through conventional machining processes wherein tiles of a desired shape are cut from the solid block. 
     One drawback associated with this process is the maximum size of the tiles that can be formed. As the surface of the space shuttle orbiter, for example, is relatively large, it is highly desirable to form the tile as large as possible so as to reduce the labor that is required to affix the tiles to the orbiter, as well as minimize the use of the material which bonds the tiles to the orbiter to thereby minimize the weight of the orbiter&#39;s thermal protection system. In covering a leading or trailing edge of a craft, a tile having a length in excess of 6 feet is highly desirable. 
     To some extent, the size of the tiles may be increased by enlarging the size of the casting tower. In practice, however, casting towers that produce raw blocks having dimensions greater than 22″×22″×7″ inches are not practical due to the increased rate at which defects and other problems are encountered in the manufacturing process. Problems such as weight associated with transporting a large block filled with water, the inability to completely dry very large raw blocks, overheating the exterior portion of the raw block during the firing operation and under heating the interior portion of the raw block during the firing operation frequently lead to defects such as shrinking, cracking and improper bonding of the fibers. As the material that is used to form the raw blocks is relatively expensive, the increased rate of defects renders the formation of relatively large fired blocks commercially impracticable. 
     Another drawback associated with the previously known methods of forming tiles concerns the manner in which tiles having a complex shape are formed. Tiles which are relatively flat and sized approximately equal to the cross-section of the fired block are relatively easy to machine with little waste. Tiles having a complex shape, however, are routinely carved from a fired block, with the remainder of the fired block being discarded as scrap. As mentioned above, the material that is used to form the raw blocks is relatively expensive. Consequently, tiles that are produced in a process wherein large amounts of the fired blocks are scrapped are extremely costly to produce. 
     Accordingly, there remains a need in the art for a method for forming relatively large insulation tiles. There also remains a need in the art for a method for forming a complex shaped insulation tile which produces relatively less scrap. There also remains a need in the art for a method for bonding insulation tiles together. 
     SUMMARY OF THE INVENTION 
     In one preferred form, the present invention provides an insulative body having first and second porous insulation members and a binder. Each of the first and second porous insulation members is formed of a fibrous, low-density silica-based material and cooperatively defines a joint. The binder is disposed between a pair of mating surfaces that form the joint. The binder couples the first and second porous insulation members together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of an insulative body formed in accordance with the teachings of the present invention; 
     FIG. 2 is a schematic illustration of the method of the present invention; 
     FIG. 3 is a side elevation view illustrating an alternate joint construction; 
     FIG. 4 is a perspective view of another insulative body formed in accordance with the teachings of the present invention; 
     FIG. 5 is a side elevation view of the leading edge of a craft formed from insulative materials in accordance with the teachings of the present invention; and 
     FIG. 6 is a side elevation view of a leading edge of a craft formed in accordance with the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1 and 2 of the drawings, an insulating body constructed in accordance with the teachings of the present invention is generally indicated by reference numeral  10 . Insulating body  10  is shown to include first and second tile members  14  and  16 , respectively, and a ceramic/organic thermal setting binder  20 . First and second tile members  14  and  16  are extremely porous, being constructed from a fibrous, low-density silica-based material. First and second tile members  14  and  16  are preferably formed from a homogeneous alumina enhanced thermal barrier (AETB) material which is well known in the art and more fully described in Leiser et al., “Options for Improving Rigidized Ceramic Heatshields”, Ceramic Engineering and Science Proceedings, 6, No. 7-8, pp. 757-768 (1985) and Leiser et al., “Effect of Fiber Size and Composition on Mechanical and Thermal Properties of Low Density Ceramic Composite Insulation Materials”, NASA CP 2357, pp. 231-244 (1984). As those skilled in the art will understand, however, first and second tile members  14  and  16  may be formed from other fibrous low-density silica-based materials including, for example, fibrous refractory composite insulation (FRCI), which is well known in the art and more fully described in U.S. Pat. No. 4,148,962, the disclosure of which is hereby incorporated by reference as if fully set forth herein. 
     As best shown in FIG. 2, the ceramic/organic thermal setting binder  20  includes a ceramic binder  22  and a thermal set organic binder  24 . The ceramic binder  22  may be glass, such as Ferro Corporation&#39;s EG0002, EG1001 and EG2790 electronic glasses, or ceramic, such as an aluminum oxide and silica mixture, and is selected on the basis of its coefficient of thermal expansion. Preferably, the coefficient of thermal expansion of the ceramic binder  22  is about equal to the coefficient of thermal expansion of the first and second tile members  14  and  16 . 
     The thermal set organic binder  24  is an agent that aids in the processing of insulating body  10 . In this regard, the thermal set organic binder  24  is an agent that is employed to temporarily bond the first and second tile members  14  and  16  to one another. Additionally, the thermal set organic binder  24  is an agent that assists in the distribution of the ceramic binder  22  as will be discussed in greater detail, below. Examples of suitable thermal set organic binders  24  include epoxies and acrylics. 
     The thermal set organic binder  24  and ceramic binder  22  are combined and preferably thinned out (i.e., the thermal set organic binder  24  is at least partially dissolved) to a desired viscosity with a suitable solvent  26 . The ceramic/organic thermal setting binder  20  is then applied to the pair of mating surfaces  30  that form the joint  32  between the first and second tile members  14  and  16 . In applying the ceramic/organic thermal setting binder  20  to the joint  32 , it is highly desirable that the mating surfaces  30  are sufficiently wetted out so as to create a high quality bond. It is also desirable that the ceramic/organic thermal setting binder  20  not significantly wick into the porous and fibrous tile members  14  and  16  as relatively thinner bonds are more desirable (thinner bonds are lighter and less likely to affect the thermal and mechanical properties of the finished insulating body  10  as compared to relatively thicker bonds). 
     Once a desired amount of the ceramic/organic thermal setting binder  20  has been applied to the mating surfaces  30 , the mating surfaces  30  are placed in contact with one another, the thermal set organic binder  24  bonds the first and second tile members  14  and  16  together and a tile assembly is formed. Measures may be taken to ensure that the mating surfaces  30  will remain in constant and continuous contact with one another during the subsequent processing step. Such measures are well known in the art and are typically employed in the fabrication of epoxy composites. One such measure is the use of a vacuum bag wherein the first and second tile members  14  and  16  are placed in a vacuum bag, the vacuum bag is coupled to a vacuum source (e.g., a vacuum pump) and air is evacuated from the vacuum bag to permit the atmosphere to exert pressure onto the joint  32  to maintain the mating surfaces  30  in constant and continuous contact with one another. Other measures include the use of an adhesive tape or the application of a small weight across the length and width of the joint, so as to exert a force onto the joint which maintains the mating surfaces  30  in constant and continuous contact with one another. 
     The tile assembly is next placed into an oven and slowly heated. As the temperature of the thermal set organic binder  24  increases, its viscosity lowers, permitting it to flow into through the joint  32  and into any open pores in the first and second tile members  14  and  16 . As the thermal set organic binder  24  is mixed with the ceramic binder  22 , the flowing action of the thermal set organic binder  24  assists in the distribution of the ceramic binder  22  by carrying the ceramic binder  22  through the joint  32  and into the open pores. Depending upon the particular type of thermal set organic binder  24  that is used, the thermal set organic binder  24  will set up at about 200-500° F., thereby fixing the position of the particles of the ceramic binder  22  that are distributed throughout the joint  32  and temporarily bonding the first and second tile members  14  and  16  to one another. The tile assembly may then be removed from the oven to permit any vacuum bags, weights, tape, etc. to be removed. Thereafter, the tile assembly is placed in a furnace where it is slowly heated to a temperature of about 800° F. to about 1000° F. and held within this temperature range for an appropriate time such as one hour, for example, to permit the thermal set organic binder  24  to burn-out of the joint  32  so as not to affect the weight, strength, thermal properties or coefficient of thermal expansion of the joint  32 . Thereafter, the tile assembly is slowly heated to a temperature from about 1200° F. to about 2400° F. to permit the ceramic binder  22  to fuse the into the first and second tile members  14  and  16  and fixedly couple the mating surfaces  30  to one another. 
     Preferably, a surface hardening agent  40  is applied to the mating surfaces  30  of the joint  32  and cured prior to the application of the ceramic/organic thermal setting binder  20 . Examples of suitable surface hardening agents  40  include silica sol and alumina sol. The cured surface hardening agent  40  is operable for partially filling the voids in the mating surfaces  30  to thereby limit the ability of the ceramic/organic thermal setting binder  20  to wick into the first and second tile members  14  and  16 . If silica sol is employed as the surface hardening agent  40 , it preferably includes small silica particles in the size range of from about 4 nanometers to about 150 nanometers. The silica particles are mixed with a carrier liquid, such as water with a small amount of ammonia such that the silica particles are present in an amount of from about 15 parts by weight to about 50 parts by weight of the mixture of silica and liquid, producing a mixture having the consistency of water. One operable silica sol of this type is commercially available as Nalco 2327 manufactured by Nalco Chemical Company. Curing is accomplished by heating the first and second tile members  14  and  16  to an elevated temperature, such as 300° F. until the liquid carrier has completely evaporated. If desired, a pore-obstructing material  41 , such as cordierite or mullite, may be applied to the mating surfaces  30  of the joint  32  prior to the application of the surface hardening agent  40  to limit the depth with which the surface hardening agent  40  is permitted to penetrate. 
     Also preferably, the first and second tile members  14  and  16  are formed with mitered end portions  50  so that the mating surfaces  30  are angled with respect to the exterior surfaces  60  of the first and second tile members  14  and  16 . Construction of insulating body  10  in this manner increases the surface area of joint  32  as compared to a conventional butt joint  32  as illustrated in FIG. 3, to thereby increase the strength of insulating body  10  in the area of the joint  32 . Also advantageously, the angling of the mating surfaces  30  relative to the exterior surfaces  60  permits any adverse effects of the ceramic/organic thermal setting binder  20  to be distributed over a path that is skewed to the direction through which thermal energy is transmitted through insulating body  10 . 
     The following non-limiting examples describe the invention further and represent best modes for practicing the invention. 
     EXAMPLE I 
     Tile members  14  and  16  are formed from AETB with mitered end portions  50 . A silica sol having silica particles that are mixed with a carrier liquid, such as Nalco 2327 which is commercially available from Nalco Chemical Company and  27  percent by weight of cordierite powder from Ferro Corporation, is applied to the mating surfaces  30  that are formed into the mitered end portions  50  of the tile members  14  and  16 . The tile members  14  and  16  are dried at about 300° F. for about 30 minutes to harden the mating surfaces  30  and to substantially reduce the porosity of the mating surfaces. An ceramic/organic thermal setting binder  20  consisting of about 90 percent by weight of a ceramic binder  22  and about 10 percent by weight of a thermal set organic binder  24 , such as an acrylic is provided. In the particular example provided, the ceramic binder  22  has a composition of about 0 percent by weight to about 90 percent by weight of aluminum oxide and 100 percent by weight to about 10 percent by weight of silica and preferably about 83.3 percent by weight of aluminum oxide and about 16.6 percent by weight of silica. The ceramic/organic thermal setting binder  20  is mixed with an acetone solvent to form a liquid having the consistency of milk. One or more coats of the ceramic/organic thermal setting binder  20  are applied to each of the mating surfaces  30  and the mating surfaces  30  are placed in contact with one another. The tile assembly is placed in a vacuum bag and a source of vacuum is applied to the vacuum bag to remove the air therefrom. The tile assembly is heated slowly in an oven to first 180° F. then to about 350° F. to cause the acrylic to temporarily bond the tile members  14  and  16  to one another. The tile assembly is removed from the oven, the vacuum bag is removed from the tile assembly and the tile assembly is thereafter heated slowly in a furnace such that the transition between approximately 800° F. to approximately 1000° F. is made in about one hour to permit the acrylic to burn out of the joint  32 . The tile assembly is thereafter fired in the furnace at a temperature of about 1200° F. to about 2400° F., and preferably at about 2000° F., to fuse the ceramic binder ( 22 ) to the mating surfaces and fixedly couple the tile members  14  and  16  together. 
     EXAMPLE II 
     Tile members  14  and  16  are formed from AETB with mitered end portions  50 . An ceramic/organic thermal setting binder  20  consisting of about 95 percent by weight of a ceramic binder  22 , such as Ferro Corporation electronic glass EG0002, EG1001 or EG2790, and about 5 percent by weight of a thermal set organic binder  24 , such as ethocellulose, is mixed with a butyl carbitol acetate solvent to form a paste that is somewhat wetter than commercially available tomato paste. A first coat of the ceramic/organic thermal setting binder  20  is applied to each of the mating surfaces  30  and permitted to slightly wick into the tile members  14  and  16 . A second coat of the ceramic/organic thermal setting binder  20  is thereafter applied to the mating surfaces  30  and the mating surfaces  30  are placed in contact with one another. A small weight is applied to the assembly to ensure that the mating surfaces  30  remain in constant contact during the subsequent step. The tile assembly is then heated slowly to about 350° F. to set the ethocellulose and temporarily bond the tile members  14  and  16  to one another. The weight is thereafter removed and the tile assembly is heated slowly in a furnace such that the transition between approximately 800° F. to approximately 1000° F. is made in about one hour to permit the ethocellulose to burn out of the joint  32 . The tile assembly is thereafter fired in the furnace at a temperature of about 2000° F. to about 2400° F. to fuse the electronic glass to the mating surfaces and fixedly couple the tile members  14  and  16  together. 
     EXAMPLE III 
     Tile members  14  and  16  are formed from FRCI with mitered end portions  50 . A silica sol having silica particles that are mixed with a carrier liquid, such as Nalco 2327 which is commercially available from Nalco Chemical Company, is applied to the mating surfaces  30  that are formed into the mitered end portions  50  of the tile members  14  and  16 . The tile members  14  and  16  are dried at about 300° F. for about 30 minutes to harden the mating surfaces  30  and to substantially reduce the porosity of the mating surfaces. An ceramic/organic thermal setting binder  20  consisting of about 90 percent by weight of a ceramic binder  22 , such as Ferro Corporation electronic glass EG0002, EG1001 or EG2790, and about 10 percent by weight of a thermal set organic binder  24 , such as epoxy, is mixed with an acetone solvent to form a liquid having the consistency of milk. One or more coats of the ceramic/organic thermal setting binder  20  are applied to each of the mating surfaces  30  and the mating surfaces  30  are placed in contact with one another. The tile assembly is placed in a vacuum bag and a source of vacuum is applied to the vacuum bag to remove the air therefrom. The tile assembly is heated slowly in an oven to about 350° F. to cause the epoxy to temporarily bond the tile members  14  and  16  to one another. The tile assembly is removed from the oven, the vacuum bag is removed from the tile assembly and the tile assembly is thereafter heated slowly in a furnace such that the transition between approximately 800° F. to approximately 1000° F. is made in about one hour to permit the epoxy to burn out of the joint  32 . The tile assembly is thereafter fired in the furnace at a temperature of about 2000° F. to about 2400° F. to fuse the electronic glass to the mating surfaces and fixedly couple the tile members  14  and  16  together. 
     EXAMPLE IV 
     Tile members  14  and  16  are formed from AETB with mitered end portions  50 . A ceramic organic setting binder  20 , such as Nippon Electronic Corporation electronic glass GA-13 is mixed with a butyl carbitol acetate solvent to form a paste that is somewhat wetter than commercially available tomato paste. A coat of the ceramic organic setting binder  20  is applied to each of the mating surfaces  30  and permitted to slightly wick into the tile members  14  and  16 . A small weight is applied to the assembly to ensure that the mating surfaces  30  remain in constant contact during the subsequent step. The tile assembly is then heated slowly to about 200° F. to about 400° F. to evaporate the solvent and temporarily bond the tile members  14  and  16  to one another. The weight is thereafter removed and the tile assembly by heated slowly in a furnace such that the transition between approximately 800° F. to approximately 1000° F. is made in about one hour to permit the thermal set organic binder  24  to burn out of the joint  32 . The tile assembly is thereafter fired in the furnace at a temperature of about 1300° F. to about 1800° F. to fuse the electronic glass to the mating surfaces  30  and fixedly couple the tile members  14  and  16  together. 
     While the insulating body  10  has been illustrated thus far as being a planar insulative tile formed from several substantially planar tile members, those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. For example, insulating body  10 ′ may be formed from a plurality of fired blocks  70  of porous, fibrous, low-density silica-based material as illustrated in FIG.  4 . In this arrangement, raw blocks formed in a casting tower are dried and fired in a furnace to produce fired blocks  70  in a process that is well known in the art. The fired blocks  70  are thereafter bonded together with a ceramic/organic thermal setting binder  20  of the type and in the manner disclosed above to produce a block assembly. The block assembly may thereafter be machined as desired. The bonding of fired blocks  70  is advantageous in that it substantially reduces the processing time associated with the preparation of the mating surfaces  30 , the application of the ceramic/organic thermal setting binder  20 , the curing of the thermal set organic binder  24  and the subsequent firing to set the ceramic binder  22 . 
     Another arrangement is illustrated in FIG. 5 wherein a pair of tile members  14 ″ and  16 ″ are coupled to one another to form the leading edge  80  of a craft. Construction of the leading edge  80  from a plurality of planar tile members as illustrated in FIG. 5, as opposed to the prior art method of carving the leading edge  80   a  from a monolithic block  84  as illustrated in FIG. 6, is both extremely efficient and less costly due to a substantial reduction in the amount of waste that is generated to machine the leading edge  80 . 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.