High temperature and alkali-resistant refractory fiber for reinforcing cementitious products

Abstract: A composition for forming a high temperature, alkali resistant refractory fiber. The compositional formulation, by weight, is 45-76% silica, 12-32% alumina, 5-30% zirconia with a silica to alumina ratio in the range from 1.8 to 4. These fibers have a maximum service temperature in the range of 1400°C (2550°F) to 1455°C (2650°F) and exhibit better shrinkage resistance than similarly rated commercially available refractory fiber.

Description: Background and Summary of the Invention To date, most of the work in development of a synthetic inorganic fiber suitable for use in reinforcing cementitious matrices has been performed in the glass area. As used here, the term "glass" refers not only to the vitreous, or non-crystalline, structure of the resultant fiber but, also, to a composition containing one or more fluxing agents, usually alkaline metal oxides. The function of these fluxing agents is to reduce the melting temperature of the batch mix making the composition more fiberizable by such methods as continuous pulling and rotary fiberization (extrusion through holes of a spinner disc). At the same time, however, these fluxing agents generally detract from the chemical inertness of the fiber and reduce the service temperature thereof. Inorganic fibers consisting essentially of silica, alumina and zirconia are generally known. The article "Refractory Glass Fibers" of GATES and LENT in "Ceramic Bulletin", Vol. 46(2), 1967, page 202 seq. for example discloses compositions of 50% SiO 2 , 22.5% and 27% Al 2 O 3 respectively, and 20% ZrO 2 for the production of "fairly fine long fibers, easy to fiberize" and "long flexible fine fibers" respectively. US-A-3,205,079, relating rather to semicrystalline ceramic bodies than to fibers, discloses among others the composition 55% SiO 2 , 20% Al 2 O 3 , and 20% ZrO 2 as being appropriate. A composition of 55% SiO 2 , 25% Al 2 O 3 , and 20% ZrO 2 is disclosed in US-A-2,873,197 as being suited for the production of "somewhat coarse fibers" with a high percentage of long fibers. It is noted that above references do not at all mention shrinkage properties. US-A-3,904,424 discloses fibers comprising as principal components SiO 2 , Al 2 O 3 , ZrO 2 , and MgO. The preferred ranges of named components are 35 to 50% SiO 2 , 25 to 45% Al 2 O 3 , 7 to 19% ZrO 2 , and 3 to 20% MgO. The corrosion resistance of the fibers against an alcaline substance is mentioned as being improved by the ZrO 2 component. In any case, however, the alumina content shall remain below 45%, otherwise a higher temperature would be required for melting the components and the yield of fibers would be reduced. Zirconia (ZrO 2 ) has been extensively used as an additive in glass formulations as a means of imparting alkali resistance. See for example US-A-3,859,106, US-A-3,966,481, US-A-4,036,654 and US-A-4,330,628. In addition, zirconia by itself or as a predominant component has been used to formulate refractory compositions with relatively high service temperatures. See for example US-A-2,873,197, US-A-2,919,944, US-A-3,035,929, US-A-3,754,950, US-A-3,793,041, US-A-4,053,321 and US-A-4,119,472. The scope of the present invention is as set out in the appended claims. The claimed refractory fibers are formulated by impinging a molten stream upon the surfaces of two rapidly rotating spinners. This process for manufacturing spun fibers is actually more efficient when the melt stream is in the range of 3000°F (1705°C), making the use of the fluxing agents noted above, undesirable. A number of prior art fibers are rated at 1425° C (2600° F) by their manufacturers. Comparative tests of shrinkage run against these competitive fibers show the fibers of the present invention to be more thermal-resistant. In addition, this formulation produces a higher melt rate for the same energy, input and a greater percentage of recovered fiber per pound of melted batch than conventional alumina/silica refractory melts. These and other features, advantages and characteristics of the present invention will become better understood after a reading of the detailed description which follows. Brief Description of the Drawing FIG. 1 is a triaxial diagram depicting the claimed formulation of the high temperature refractory fiber composition of the present invention; and FIG. 2 is an enlargement of a portion of the triaxial diagram of FIG. 1 showing the pertinent area and a number of production runs for this high temperature formulation. Detailed Description of the Preferred Embodiments Fiber-reinforced cementitious products having a calcium silicate matrix are manufactured for a number of commercial applications requiring thermal resistance and structural strength. One such high density calcium silicate product is marketed under the trademark of MARINITE by Manville Building Materials Corporation. One use for MARINITE is the formation of molds for casting molten metals. At one time, these cementitious panels were reinforced with amosite asbestos fibers (see US-A-2,326,516 and US-A-2,326,517). The health problems, both real and imagined, associated with asbestos fibers have led to investigation of other fibers as suitable substitutes as reinforcing fibers for calcium silicate matrices. Suitable formulations, employing up to 40% by weight wollastonite fibers and up to 15% by weight of the panel alkali-resistant glass fibers, were developed in the mid to late 1970's (see US-A-4,111,712 and US-A-4,128,434). The use of these AR glass fibers which have an average fiber diameter of 12 (or 20) microns necessitated the inclusion of 5% organic fibrous material such as kraft as a processing aid to improve formability of the slurry mix, to slow drainage and to provide green strength. Curing of these panels sometimes requires autoclaving (a steam curing at 166° C ≙330° F and 6.9 bar ≙ 100 psi) to accelerate the silica/calcium hydroxide reaction. In addition, the organic fiber must be burned out by a 288° C (550° F), 482° C (900° F) or 593° C (1100° F) heat treatment. The commercially available alkali-resistant (AR) glass used to reinforce these panels has a formulation of SiO 2 - 61%, ZrO 2 -10.5%, Na 2 O-14.5%, K 2 O-2.5%, CaO-5%, TiO 2 -6.0%. Tests indicated that, following curing, the panels reinforced with AR glass fibers, in some cases, retained less than 70% of their modulus or rupture and/or their specific strength (modulus of rupture over the square of the density, MR/D 2 ). Such results indicated a break down of one of the fibrous reinforcement components and further analysis showed that it was the AR glass which failed. A chemically resistant refractory fiber was sought as a substitute for the AR glass. Initially, four compositions (Z1-Z4) were tried experimentally (see FIG. 1) seeking a fiber with the desired properties. Compositional formulations were targeted, the components added in the desired proportions to a 0.914 m (3 feet) diameter, 0.483 m (19 inch) deep research melter. The compositions were melted electrically with the melt stream exiting through an orifice and impinging on a pair of 0.203 m (8 inch) diameter spinners rotating at 12,000 r.p.m. This produces fibers which are generally 2-7 microns in diameter, 12.7 mm to 254 mm (0.5 to 10 inches) in length (50.8-76.2 mm ≙ 2-3 inch average), and having a varying shot content (35-45%, usually). The fibers were collected and analyzed to substantiate composition and 1 gram samples of these various fibers were boiled in a 0.1 N solution of NaOH for one hour; dried, and weighed to determine % weight loss; the results of these tests are shown in TABLE 1. Composition (% wt) Fiber I.D. SiO 2 Al 2 O 3 ZrO 2 SiO 2 /Al 2 O 3 % wt loss Z1 50.0 43.0 6.7 1.16 8.2 Z2 47.3 40.1 12.2 1.17 4.6 Z3 50.1 34.6 15.0 1.45 3.3 Z4 59.1 25.9 14.6 2.28 2.2 2400°F Std RF 53.8 46.0 -- 1.19 7.8 In addition, thermal tests were run on these fibers to identify those candidates with the best refractory performance. Bulk fiber was vacuum formed into a felt for each formulation to be tested. Lengths of these fibrous felt samples were precisely measured, placed in a refractory kiln for a specified time and temperature, allowed to cool, and then remeasured. The results are shown in TABLE II along with pour rate (or rate of fiberization), average fiber diameter and shot content. These shrinkages will be lower than for production blanket since the felting has eliminated some of the interstitial shrinkage that will occur in production materials (i.e., these test samples have higher than normal densities). 2400F Std 2600F Std Z1 Z2 Z3 Z4 Pour Rate (Lb/hr) -- -- 1200 550 925 1000 AveFiberDia(Microns) 2.8 3.5 1.9 2.4 3.7 4.7 Shot content (%) 40-45 typ 40-45 typ 48.6 41.5 30.4 40.1 Linear Shrinkage (%) a. 2400F-112 Hrs 3.7 -- 3.2 2.7 2.2 -- b. 2600F-24 Hrs -- 3.15 4.15 3.5 2.3 4.15 c. 2600F-125 Hrs -- 3.70 4.2 3.5 2.3 4.25 d. 2700F-24 Hrs -- 6.1 7.3 6.8 3.2 9.2 e. 2700F-125 Hrs -- 10.1 8.0 7.6 3.7 -- The results of those tests indicated that Z3 had the best thermal performance and acceptable alkali resistance while Z4 had the best alkali resistance with acceptable (disregarding the 1482° C ≙ 2700° F reading) thermal performance. It was determined that two separate families of fibers should be pursued, one seeking to optimize thermal performance based on the Z3 formulation and the other seeking to optimize the alkali resistance in more moderate temperature environments (<1093° C ≙ 2000F), based on the Z4 formulation. These alkali resistant fibres are not part of the present invention. It was believed that increasing alumina and/or zirconia in the Z3 formulation would improve refractoriness and that increasing silica and/or zirconia in the Z4 formulation would enhance alkali resistance. Accordingly, a family of fibers (B1-B7) based on the Z3 formulation was produced using the 0.914 m (3 feet) research melter in accordance with the procedures outlined above. These formulations are depicted on the triaxial diagram in FIG. 1. These fibers were then subjected to a plurality of temperatures for various periods of time in the refractory kiln to determine refractoriness. The results of these tests and the formulations of the B1-B7 fibers appear in TABLE III. Z3 B1 B2 B3 B4 B5 B6 B7 Components (wt %) SiO 2 50.1 31.3 38.9 38.3 35.3 27.6 34.1 27.6 Al 2 O 3 34.6 53.2 45.5 41.3 48.2 53.1 55.0 58.4 ZrO 2 15.0 15.3 15.2 20.2 16.2 19.1 10.6 13.7 SiO 2 /Al 2 O 3 1.45 .59 .85 .93 .73 .52 .62 .47 SiO 2 /ZrO 2 3.34 2.046 2.559 1.896 2.179 1.445 3.217 2.015 Temp/Time Linear Shrinkage (%) 2200F/24 Hr 1.94 2.72 -- -- 2.92 3.01 3.52 2.83 2400F/24 Hr 2.47 2.83 -- -- 2.63 2.80 4.50 2.20 2600F/48 Hr 2.94 2.94 -- -- 2.90 3.64 4.69 3.23 2700F/24 Hr 3.42 4.07 5.94 6.34 4.79 4.22 3.68 3.06 2700F/100 Hr 4.7 5.1 10.4 9.6 7.1 4.8 6.8 3.9 2700F/260 Hr 5.4 6.0 14.5 12.7 9.3 5.2 8.8 4.9 2800F/24 Hr +50.0 13.8 19.4 19.6 20.0 14.2 19.3 10.5 Even though these tests succeeded in producing a fiber with approximately 10% shrinkage at 1538°C (2800°F) as opposed to 50+% for Z3, these tests did not result in the definition of a commercially viable fiber. First, all of these high alumina formulations (B1-B7) were significantly more difficult to fiberize than Z3. Secondly, these fibers all showed a low degree of thermal stability at temperatures above 1093°C (2000°F). Fibers B1, B5 and B7 devitrified and lost their fibrous nature between 1093° C (2000° F) and 1316°C (2400°F). Such behavior would preclude their use as an insulation, as a high-temperature reinforcing fiber or for any other industrial use. The degradation of fibers B2, B3, B4 and B6 was not as great; however, these fibers had linear shrinkages from 8 to 15% after 260 hours of exposure to 1482°C (2700°F). Such high shrinkages would make these fibers unacceptable, as well, for any commercial application. Note, also, each of these fibers (B2, B3, B4 and B6) failed to produce samples of less than 5.0% linear shrinkage for 100 hours of exposure to 1482° C (2700°F). In order to be rated at a particular temperature, a fiber sample of this type (i.e., felted) should exhibit no more than 5% linear shrinkage after 100 hours of heat soaking. This insures that a particular fiber gill not underto unacceptable levels of shrinkage (i.e.. exceeding 12%) when cycled up to its service temperature repeatedly throughout its service life. When the fiber samples are collected in a normal production run and needled into a blanket, rather than being pressed into a felt, the shrinkages seen in this type of test will be more akin to the maximum shrinkages desired during use. Production runs of various formulation chemistries were made to determine which formulations 1) fiberized well; 2) made fibers with acceptable levels of shrinkage; and 3) could achieve production results comparable to those achieved in the test melter. It was during these production runs that the advantages of these zirconia formulations became apparent. In a stabilized melter of the same configuration used formerly for silica/alumina melts (both top- and bottom-entry electrode), the zirconia formulation produced higher pour rates for the same energy input and significantly better percentages of recovered fiber per pound of melted batch (i.e., lower shot content and less rejected blanket). Samples of these various blanket formulations were analyzed for chemistry and tested for shrinkage as before. The amount of linear shrinkage is a time/temperature phenomenon. Accordingly, although this fiber will probably be targeted for usage in the 1400 to 1455°C (2550 to 2650°F) range, the samples were inserted in a kiln at 1482°C (2700°F) for four hours to 1) accelerate the tests thereby reducing the time required and 2) to insure that the blanket could withstand limited exposure to peak temperatures above those recommended for usage without catastrophic failure. The results of these tests appear in TABLE IIIA. % By Weight Sample No. SiO 2 Al 2 O 3 ZrO 2 SiO 2 /ZrO 2 Ratio % Linear Shrinkage 1 53.1 45.8 0.4 132.75 15.6 2 51.9 42.5 4.6 11.282 14.6 3 50.0 44.3 5.3 9.434 14.9 4 51.7 42.1 5.4 9.57 13.3 5 49.0 38.9 11.2 4.375 12.1 6 48.0 38.2 13.2 3.636 9.2 7 49.2 36.4 13.8 3.565 8.2 8 54.7 30.3 14.1 3.879 12.9 9 49.4 35.0 14.5 3.407 11.3 10 50.2 34.1 14.7 3.415 9.7 11 51.9 32.9 14.7 3.531 9.1 12 49.7 34.7 15.0 3.313 6.8 13 49.7 34.8 15.0 3.313 8.2 14 47.6 37.3 15.2 3.132 10.9 15 49.3 34.6 15.3 3.222 9.7 16 46.0 37.3 15.3 3.007 9.0 17 46.4 37.2 15.4 3.013 8.1 18 46.2 37.0 15.5 2.98 6.8 19 46.2 37.3 15.6 2.961 7.2 20 50.1 32.0 17.4 2.879 7.6 21 49.3 32.4 17.4 2.833 7.4 22 47.2 34.4 17.5 2.697 8.6 23 47.4 33.9 17.6 2.693 6.0 24 47.4 34.2 17.7 2.678 7.2 25 48.0 33.2 17.8 2.697 7.3 26 47.7 33.4 17.8 2.680 7.2 For these tests, an acceptable level of shrinkage is defined as 11.5%. While this may seem like an inordinately high level of shrinkage, it should be remembered that the amount of shrinkage has been intentionally aggravated by exposing these samples to a temperature (1432° C ≙ 2700° F) exceeding its recommended service temperature (1399-1454°C ≙ 2550-2650°F). Secondly, much of the blanket made using this formulation will find its way into insulation modules of the type described in US-A-4,001,996. In such a modular configuration, the blanket will undergo less shrinkage since only a small portion of its surface is exposed to the internal furnace temperatures. Further, a major portion of any gap which might form as a result of such shrinkage will be filled as the compressed modules expand. From TABLE IIIA, it appears that the formulations producing fiber with acceptable shrinkage level have the following compositions: SiO 2 ranging from 46.0 to 52%, Al 2 O 3 ranging from 32 to 38%, and ZrO 2 ranging from 13 to 18% (all percentages are percent by weight). As a further restriction on this family of formulations, the silica to zirconia ratio appears to be significant and should not exceed 3.8. From TABLE III, the SiO 2 /ZrO 2 ratio for the defined zirconia range (13-18%) has a minimum of 2.6. Within this family, a more preferred formulation (based on shrinkages) is SiO 2 from 46.4 to 50.1%, Al 2 O 3 from 32 to 37.3%, ZrO 3 from 15 to 18%, and a SiO 2 to ZrO 2 ratio of between 2.6 and 3.32. The results from TABLE IIIA are depicted graphically in FIG. 1 with the family of formulations shown in solid line. FIG. 2 is an enlargement of a portion of the triaxial diagram of FIG. 1 (shown in dotted line) to enable the plotted formulations to be seen more clearly. Even in the enlarged view of FIG. 2, two points (samples 13 and 19) were too close to other formulations to be plotted as distinctly separate points. Both the fiber in bulk form and in blanket are usable to 1426°C (2600°F). In order to compare this fiber to other fiber rated to 1426°C (2600°F), blanket samples of the disclosed fiber (having a formulation of 49.7% SiO 2 , 34.7% Al 2 O 3 and 15.0% ZrO 2 ), were tested for shrinkage in side-by-side furnace tests to 1) a commercially available refractory fiber blanket having the formulation Al 2 O 3 -54%, SiO 2 -46%; b) a commercially available refractory fiber blanket which has undergone a heat treatment (i.e., has been preshrunk) and has a composition of 51% alumina, 49% silica; and c) a commercially available refractory fiber blanket with the formulation of SiO 2 -51.6%, Al 2 O 3 -47.7%, the blanket having subsequently received a surface treatment of 1/2% chromium oxide. Strips of these four blanket samples were tested for shrinkage in a manner performed in the previous tests. Measured lengths of blanket were placed in a kiln at 1400°C (2550°F) and remeasured after each of 25, 50 and 75 hours percent shrinkage was computed by dividing the change in length by the original length and multiplying by 100. Due to the relatively small amount of shrinkage change between the 50 and 75 hour measurements, subsequent tests run at 1426°C (2600°F) and 1482°C (2700°F) were run only to 50 hours. The results of these tests appear in TABLE IIIB. % Linear Shrinkage Sample 25 Hr 50 Hr 75 Hr 25 Hr 50 Hr 25 Hr 50 Hr Disclosed fiber 3.0 3.1 3.1 3.2 3.2 5.7 5.7 54/46 formulation 4.2 4.9 5.0 5.0 5.7 4.8 5.7 51/49 formulation 3.4 3.6 3.6 4.6 4.6 6.7 6.9 Chrome treated 3.6 3.9 4.0 4.1 4.3 10.5 11.2 These tests show the superiority of the disclosed fiber to these three other commercially available fibers. When taken in conjunction with the difficulties of fiberizing a high alumina composition, problems associated with chromia treatment, the aforestated improvements in pour rate and fiber recovery rate as against other silica/alumina melts, the high temperature formulation of the present invention is clearly superior to other known refractory fiber compositions. A production fiber was manufactured in a full sized melter, P1 based on Z3. The composition varied slightly from the Z3 formulation to determine what effect these compositional changes might have on fiber characteristics. 3,243.2 kg (7150 pounds) of P1 fiber was produced having a planned formulation of 49% SiO 2 , 37% Al 2 O 3 and 14% ZrO 2 . The melt was made in a top-entry electrode, open-topped melter generally of the type described in U.S. patent 3,983,309. P1 has a liquidus temperature of 1760° C (3200° F) and was fiberized at an average melt rate of 491.2 kg/h (1083 lb/hr) using 800 kw of power producing a stream temperature of 1843° C (3350°F). While the actual fiber composition for P1 varied considerably from the planned formulation, analysis showed some of the fiber produced to have the following composition: 49.2% SiO 2 , 36.5% Al 2 O 3 and 13.6% ZrO 2 . This formulation compares favorably with the original Z3 fiber having a 50.1/34.6/15.0 composition. These two formulations showed only .9/1.9/1.4% deviation from each other. Yet, the linear shrinkage for the P1 samples varied from 7.0 to 9.6 following only 24 hours exposure to 1482°C (2700°F). This suggests that the Z3 formulation must be controlled quite closely ( ± 1%) in order to produce an acceptable 1482° C (2700° F) fiber. In formulating these various compositions, it is necessary to use ingredients that have a certain degree of purity. Of course, the inclusion of CaO or MgO in the Z3 formulation would adversely affect the high temperature performance of that fiber. In fact, in order to insure the high temperature performance of this fiber, impurity levels, particularly for alkali metal oxides and alkaline earth oxides, must be kept at or below levels normally desired for high temperature silica-alumina refractory fibers. Suitable batch ingredients include Zircon sand (-325 mesh flour) available from Continental Minerals as 582 W.S. Zircon, a 200 mesh ground silica available from Wedron Silica Company, and any number of 200 mesh aluminas, for example, normal soda grade alumina such as C1 grade available from Kaiser Corporation or A-1 grade available from Aluminum Company of America. Various changes, alternatives and modifications will, be apparent following a reading of the foregoing specification. For example, it is contemplated that the addition of up to 2.5% chromia to the Z3 formulation may be beneficial in improving refractoriness. Also, although only one fiberization technique has been discussed, this refractory fiber may be formed using any other commercial technique, such as blowing, for example. Accordingly, it is intended that all such changes, alternatives and modifications as come within the scope of the appended claims be considered part of the present invention.

Claims: A high temperature refractory glass fiber, said fiber in bulk or blanket form exhibiting superior shrinkage resistance of less than 11.5% when subjected to temperatures up to 1482°C (2700°F) for about four hours, said refractory fiber having a composition consisting of in percent by weight: SiO 2 46 - 52% Al 2 O 3 32 - 38% ZrO 2 13 - 18% and having a silica to zirconia ratio in the range of from 2.6 to 3.8 but not including the composition: SiO 2 50% Al 2 O 3 35% ZrO 2 15% The high temperature refractory glass fiber of claim 1 which consists of in percent by weight: SiO 2 46.4 - 50.1% Al 2 O 3 32.0 - 37.3% ZrO 2 15.0 - 18.0% and has a SiO 2 to ZrO 2 ratio of between 2.6 and 3.32.

Inventors: CHAUDHURI ASIT RANJAN, EKDAHL WENDELL GRAYDON, MIILLER WILLIAM CLYDE, CHAUDHURI, ASIT RANJAN, EKDAHL, WENDELL GRAYDON, MIILLER, WILLIAM CLYDE
Applicants: MORGAN CRUCIBLE CO, THE MORGAN CRUCIBLE COMPANY PLC
Application year: 1984
Main classification: C03C13
IPC codes: C03C13, C04B35